Molecular cobalt catalysts for electrocatalytic acetylene semi-hydrogenation

Domain: chemistry
Published: 2026-04-13
Editorial outcome: major_revision

Abstract
Electrocatalytic acetylene semi-hydrogenation is an attractive route for ethylene production, but competing side reactions like hydrogen evolution, over-hydrogenation, and C–C coupling severely compromise its industrial viability. Here, we present a carbon-nanotube-supported metal phthalocyanine platform, MPc/XCNT (M = Cu, Co, Ni, Fe; X = O, N, S), to investigate the roles of metal centers and local coordination environments. Among these catalysts, CoPc-based structures exhibit markedly enhanced water dissociation kinetics and elevated C–C coupling energy barriers compared to conventional CuPc-based catalysts, thereby effectively suppressing undesired C by-products. Furthermore, molecular regulation of the Co center optimizes active hydrogen adsorption and utilization, mitigating both hydrogen evolution and over-hydrogenation. As a result, the optimized CoPc/NCNT catalyst delivers competitive performance under both high current densities and ethylene-rich conditions. At an industrially relevant −500 mA cm−2 under a pure ethylene feed, it achieves 86.7% Faradaic efficiency with a turnover frequency of 7019 min−1. Under simulated industrial crude conditions, it maintains 99.7% conversion and 99.5% selectivity during 110-hour continuous operation. This work provides a well-defined molecular strategy for advancing selective electrocatalytic transformations.

Introduction
Ethylene (C2H4) is a vital feedstock in the chemical industry for synthesizing most commodity chemicals, including ethanol, glycols, and vinyl chloride1-3. Currently, most C2H4 is obtained by energy-intensive steam cracking of petroleum, and the C2H4 streams unavoidably contain trace acetylene (C2H2, 0.5−2 vol%) that poisons Ziegler−Natta catalysts in downstream polymerization and must be removed4-6. An alternative industrial route produces C2H4 by semihydrogenation of C2H2 derived from natural gas or coal7,8. In both cases, efficient and highly selective conversion of C2H2 to C2H4 is indispensable to meet the requirement of polymer-grade ethylene, where the residual C2H2 content must be below 10 ppm9-11. Nowadays, conventional thermocatalytic hydrogenation can reach over 99% C2H2 conversion and 90% C2H4 selectivity12-14. However, this process requires an excess of hydrogen (molar ratio of H2 to C2H2 >2), elevated temperatures (<100 °C), as well as high pressures (10-30 atm), which increases energy consumption and favours over-hydrogenation to ethane (C2H6)15,16. Electrocatalytic acetylene semi-hydrogenation (EASH) has recently emerged as a more sustainable alternative that uses water as hydrogen source and operates close to ambient conditions, and in principle can be coupled to renewable electricity and upstream ethylene production8,17-20. C2H4 yield and selectivity, however, remain limited by competing reaction pathways, including conventional over-hydrogenation to C2H6, carbon-carbon coupling to C4 olefins, and the emerging hydrogen evolution reaction (HER)21. Therefore, developing efficient catalysts that enhance selectivity, suppress side reactions, and operate at industrially relevant current densities is crucial for making this process viable.

As the EASH typically proceeds via the addition of adsorbed atomic hydrogen (*H, *corresponds to an adsorption site) generated from water dissociation, selective C2H4 production depends on both an appropriate hydrocarbon (i.e., C2H2, C2H4) adsorption affinity and efficient *H formation and storage22. Currently, Cu-based homogeneous and heterogeneous catalysts have dominated the field23,24. To achieve an optimal hydrocarbon adsorption affinity that is sufficient to induce the reaction while still allowing easy desorption and avoiding overhydrogenation or C–C coupling, the local structure around homogeneous electrocatalysts has been modulated, such as surface orientation, cluster modification, heteroatom doping, et al., while for heterogeneous electrocatalysts, strain, alloy composition, coordination structure, and heteroatom functional groups have been engineered25-29. On the other hand, downsizing catalysts to the nanoscale to isolate active sites has emerged as an effective strategy to suppress competitive HER by limiting *H coupling. More recently, Zhang et al. demonstrated that nanosized Cu with rich unsaturated sites exhibits higher energy barriers for overhydrogenation, C-C coupling, as well as *H coupling compared to bulk Cu, enabling enhanced ethylene selectivity in EASH30. Yet even with these advances, complete suppression of C–C coupling at low overpotential remains difficult. A key limitation is that water dissociation on Cu surfaces is intrinsically sluggish and has recently been identified as the rate-determining step of EASH31. Thus, the rational design of catalysts that can balance selective hydrogenation and side reactions suppression remains an unmet challenge.

Metal phthalocyanines (MPcs), featuring a well-defined metal coordination environment, have been extensively studied for CO2 reduction, oxygen reduction, and HER32-34. However, their application in EASH remains underexplored. To date, only pioneering work by Zhang group35 firstly adopted bare CoPc and CuPc as electrocatalysts for EASH. Despite this initial progress, the influence of metal identity on EASH performance has been largely overlooked, leading most EASH systems to remain centered on Cu. Moreover, although the local coordination environment of MPcs is known to regulate their electronic structure, its role in governing EASH performance has yet to be systematically explored. To address these challenges, the MPcs supported on carbon materials provide a promising molecular platform, as their nearly square planar MN4 coordination unit offers a well-defined, tunable metal centre, and the macrocyclic ligand can be modified and immobilised on conductive supports while retaining a clear molecular structure.

In this work, we develop carbon nanotube-supported metal phthalocyanines, MPc/XCNT (M = Cu, Co, Ni, Fe; X = O, N, S), as a model platform to disentangle and exploit the function of the metal identity and local coordination environment in the EASH. Within the comparative MPc/OCNT series, CoPc/OCNT, NiPc/OCNT, and FePc/OCNT all exhibit significantly faster water-dissociation kinetics than the conventional CuPc/OCNT, thereby effectively suppressing undesired C–C coupling. This trend underscores the critical role of metal identity in facilitating water activation, which is essential for strongly suppressing C4 formation during acetylene semi-hydrogenation. Among these catalysts, Co-based catalysts exhibit the most effective suppression of C–C coupling but still suffer from severe HER at high current densities, which motivates further optimization of the Co sites. Guided by this insight, we then regulate the coordination environment of Co phthalocyanine through molecular interactions with nitrogen, oxygen, and sulfur functional groups on the CNT surface, where electron-withdrawing nitrogen functional group modulates the Co d-band center and generates optimized *H adsorption energy, elevating *H coupling barriers, and suppressing over-hydrogenation pathways, thereby mitigating HER and over-hydrogenation. The obtained CoPc/NCNT achieves >86.7% Faradaic efficiency (FE) for ethylene across a broad current density range (−50 to −500 mA cm−2), with ethane FE below 0.01% and no detectable 1,3-butadiene byproducts. Under simulated industrial conditions (5000 ppm C2H2 in crude ethylene), this catalyst maintains 99.7% acetylene conversion and 99.5% ethylene selectivity during 110-hour continuous operation, demonstrating good stability for scalable ethylene purification.

Results
EASH performance comparison between metal phthalocyanines

The EASH performance of metal phthalocyanines was evaluated in a normal flow cell device with an electrode surface area of 1.0 cm2 under a 100% C2H2 feed, in which Hg/HgO and Pt foil served as the reference and counter electrodes, respectively, and an anion exchange membrane separated two chambers filled with 1.0 M KOH aqueous solution (Supplementary Fig. S1). The electrocatalytic acetylene hydrogenation performance of CoPc/OCNT and CuPc/OCNT was initially evaluated. Those two structurally defined catalyst models were rationally constructed following previously reported work (Fig. 1a and Supplementary Fig. S2)36. Under an Ar atmosphere in 1.0 M KOH electrolyte, the LSV curves of the CoPc/OCNT catalyst exhibited an onset potential of −0.26 V vs. reversible hydrogen electrode (VRHE), approximately 30 mV more positive than the −0.29 V of CuPc/OCNT, and also delivered a higher current density across the HER polarization range (e.g., −87.5 mA cm−2 vs. −23.4 mA cm−2 at −0.6 VRHE), indicating its good water dissociation capability for generating reactive hydrogen species (Supplementary Fig. S2a); thus, under the C2H2 atmosphere, a more positive onset potential (approximately +100 mV) and higher current density of the EASH were observed on CoPc/OCNT as well (Supplementary Fig. S2b). The EASH process was then analyzed in the current density range from −50 to −500 mA cm−2 (Fig. 1b and c). For the CuPc/OCNT catalyst, the FE of C2H4 remained below ~50%, with C4 byproduct formation persisting at ~1.5%. In contrast, CoPc/OCNT achieved a significantly higher C2H4 FE of up to 90% with almost no detectable C4 byproducts, underscoring its superior ability to suppress C–C coupling (Fig. 1b).

To elucidate the reaction mechanisms, we performed online differential electrochemical mass spectrometry (DEMS). The C4H6 (m/z = 54) signal was absent on CoPc/OCNT, in sharp contrast to the prominent C4H6 signal observed on CuPc/OCNT, directly corroborating the C–C coupling effective inhibition of Co active sites (Fig. 1d and e). The H2 (m/z = 2) was detected at –0.28 V RHE on CoPc/OCNT, appearing 40 mV earlier than on CuPc/OCNT, while the C2H4 (m/z=28) signal for CoPc/OCNT emerged at –0.2 V RHE, 120 mV more positive than that of CuPc/OCNT. This trend is consistent with the ~30 and ~100 mV positive shifts in the HER and EASH onset potentials, respectively, from CoPc/OCNT to CuPc/OCNT observed in the LSV measurements. (Supplementary Fig. S2a). Moreover, one can see for CoPc/OCNT that the C2H4 signal emerged 80 mV earlier than the H2 signal, whereas for CuPc/OCNT, both signals appeared simultaneously, demonstrating the competitive advantage of CoPc/OCNT for EASH over HER. Kinetic isotope effect (KIE) studies (H2O/D2O) further validated accelerated water dissociation on CoPc/OCNT (kH/kD =1.48 vs. 1.62 for CuPc/OCNT), promoting more *H transfer to adsorbed *C2H3 intermediates rather than H2 formation (Fig. 1f)37. According to the following theoretical calculations, the d-band center of Co in CoPc/OCNT was calculated to shift closer to the Fermi level compared with that of CuPc/OCNT, likely strengthening the adsorption of critical intermediates (i.e., *H and *C2H3) and potentially facilitating rapid EASH (Fig. 1g). Subsequent analysis of water dissociation barriers suggests that CoPc/OCNT exhibited a theoretically lower barrier for H2O dissociation than CuPc/OCNT (Fig. 1h and Supplementary Fig. S3). Therefore, the calculated facilitation of H2O dissociation not only could lead to higher HER activity under Ar, as the H2O dissociation-associated Volmer step is generally considered rate-determining, consistent with earlier LSV and DEMS results, but also, under C2H2, the theoretically rapidly generated *H is proposed to be preferentially consumed by hydrogenation of adsorbed C2H2, which, together with the elevated C–C coupling barrier and *H–*H coupling barrier (Fig. 1i, Supplementary Fig. S3 and S4), may explain the high C2H4 selectivity and the absence of detectable C4 products experimentally observed on CoPc/OCNT.

Accordingly, CuPc/OCNT and CoPc/OCNT follow distinct catalytic behaviors during the EASH process. For CuPc/OCNT, the rate-determining step is water dissociation, resulting in insufficiently active *H species, and its high susceptibility to HER further exacerbates this limitation. Therefore, both HER and *C2H3 coupling into C4 products inevitably occur, and HER becomes the dominant pathway even from relatively low current densities (> 50 mA cm−2, Fig. 1c). In contrast, CoPc/OCNT enhances *H availability through efficient water dissociation, and prioritizes EASH over HER, thus fully suppressing C4H6 formation. To assess the generality of this water-dissociation-governed suppression of C–C coupling, we also synthesised NiPc/OCNT and FePc/OCNT and evaluated them under identical conditions. LSV curves and Tafel slopes under Ar indicate that both NiPc/OCNT and FePc/OCNT promote water dissociation more efficiently than CuPc/OCNT (Supplementary Fig. S6). Quantitative product analysis at −100 mA cm⁻2 shows that the Faradaic efficiencies of C4H6 on FePc/OCNT and NiPc/OCNT are both below 0.15%, much lower than that of CuPc/OCNT (1.40%), which is consistent with the view that accelerated water dissociation helps to alleviate C–C coupling and C4 formation (Supplementary Fig. S7). Therefore, within this MPc/OCNT series, CoPc/OCNT exhibited the most effective suppression of C4 formation while maintaining high C2H4 selectivity. However, despite its high selectivity, CoPc/OCNT still suffered from significant HER side reactions at high current densities (> 300 mA cm-2), which limit its industrial applicability (Fig. 1b).

Tunable molecular catalyst design and characterization

To address the limitations imposed by competing HER, we introduce a tunable molecular strategy that electronically modulates the Co center to optimize its catalytic performance. Specifically, silane coupling agents bearing diverse functional groups, i.e., -NH2 and -SH, were grafted onto hydroxyl-rich oxidized carbon nanotubes through reactions with surface hydroxyls38. These agents follow the general formula X-R-Si-Y3, where Y represents hydrolyzable groups forming silanol, X denotes metal-coordinating functional groups, and R is the carbon chain linking X and Si. Thanks to these grafted agents, functionalized CNTs (XCNT, X=O, N, and S) could coordinate with metal atoms through strong electronic interactions mediated by the incorporated heteroatoms (Fig. 2a). Fourier-transform infrared (FT-IR) spectroscopy confirmed the presence of abundant hydroxyl groups on OCNT and characteristic -NH2 and -SH on NCNT and SCNT, respectively (Supplementary Fig. S8). Additionally, energy-dispersive spectroscopy (EDS) mapping demonstrated the uniform distribution of these functional groups across the XCNT surface (Supplementary Figs. S9–S11). These heteroatom functional groups, serving as axial ligands, could electronically regulate the Co center in CoPc through strong d–p orbital hybridization, leading to a uniform distribution of CoPc. The X-ray diffraction (XRD) patterns of CoPc/XCNT exhibited characteristic peaks of amorphous graphite, with no diffraction peaks corresponding to crystalline species of CoPc, indicating that there is no agglomeration of CoPc on the nanotube walls (Supplementary Fig. S12). Similarly, the Raman spectra of CoPc/XCNT showed the absence of CoPc vibrational bands in the 600–700 cm⁻1 range, further confirming the high dispersion of CoPc molecules (Supplementary Fig. S13)39. According to high-resolution transmission electron microscopy (HRTEM) characterization, the nanotube structure of XCNT was retained after CoPc loading, with no significant aggregation observed as well (Fig. 2b). The spherical aberration-corrected scanning transmission electron microscope (AC-STEM) image displayed the isolated bright spots on the walls of XCNT, confirming the successful loading of CoPc (Fig. 2d and Supplementary Fig. S14). The elemental mapping via scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM-EDS) further demonstrated the uniform distribution of all elements in CoPc/XCNT (Fig. 2c and Supplementary Figs. S15 and S16). The inductively coupled plasma optical emission spectrometry (ICP-OES) revealed comparable Co loadings on CoPc/OCNT, CoPc/NCNT, and CoPc/SCNT, measuring 0.55 wt%, 0.47 wt%, and 0.57 wt%, respectively (Supplementary Fig. S17 and Table S1).

The electronic regulation of the Co center in CoPc achieved through this molecular strategy was investigated using X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS) characterizations. The O K-edge X-ray absorption near-edge structure (XANES) spectrum of CoPc/OCNT demonstrated a cobalt-oxygen bond feature at 532.8 eV40,41, which was absent in bare OCNT (Supplementary Fig. S18a). In CoPc/NCNT and CoPc/SCNT spectra, the cobalt-nitrogen and cobalt-sulfur bonds were observed at 399.7 eV and 168-185 eV, respectively (Supplementary Fig. S18b and S18c)42,43. The presence of these bonds clearly indicates that Co atoms in CoPc coordinate with X heteroatoms in XCNT through strong d-p orbital interactions. Furthermore, the high-resolution Co 2p XPS spectra exhibited slight shifts in both Co 2p3/2 and Co 2p1/2 peaks compared to the bare CoPc (Fig. 2e). In particular, the Co 2p3/2 binding energies followed the order CoPc/SCNT (779.1 eV) < CoPc (779.8 eV) < CoPc/NCNT (780.8 eV) < CoPc/OCNT (780.9 eV), indicating a gradual increase in Co valence from S- to N- to O-coordinated CoPc. This trend was consistent with the increasing electronegativity and decreasing electron-donating ability of the axial ligands from S to N to O, confirming that the Co electronic structure can be precisely tuned by the heteroatom-doped CNT support. To further elucidate the local coordination environment of Co in CoPc/XCNT, Co K-edge XANES and extended X-ray absorption fine structure (EXAFS) analyses were performed. As shown in Fig. 2f, the near-edge absorption intensity of the Co K-edge for CoPc-XCNT was between those of Co foil and Co3O4, confirming that Co was in a positively charged state. All CoPc/XCNT catalysts exhibited a distinct pre-edge feature at ~7715 eV, which was characteristic of the 1s→3d transition of the square-planar Co–N4 center, demonstrating that the in-plane Co–N4 configuration of CoPc was retained on the CNT supports. Notably, the Co K-edge and pre-edge features exhibit a systematic energy shift that correlates with the specific heteroatom in the CNT support. Relative to pristine CoPc (7750.5 eV), the absorption edge of CoPc/OCNT shifts toward higher energy (7751.4 eV), whereas a significant shift toward lower energies is observed for CoPc/NCNT (7745.7 eV) and CoPc/SCNT (7744.0 eV). This energy hierarchy (CoPc/OCNT > CoPc > CoPc/NCNT > CoPc/SCNT) underscores the potent electron-withdrawing nature of axial O relative to N and S, which induces a higher Co oxidation state in CoPc/OCNT and a progressively more reduced state in the N- and S-coordinated analogues. These observations are in agreement with the XPS results. The Fourier-transformed EXAFS (FT-EXAFS) spectra of CoPc/XCNT revealed the absence of the Co–Co scattering path (2.2 Å) observed in Co foil, signifying that the Co species existed as isolated single-atom sites (Fig. 2g)44. For CoPc/OCNT and CoPc/NCNT, broad peaks were observed at 1.5 Å, attributed to the Co-N or Co-O bonds, whereas CoPc/SCNT exhibited two distinct peaks at 1.5 Å and 1.9 Å, corresponding to Co-N and Co-S bonds, respectively, revealing the dual-coordination structure. Additionally, wavelet-transform (WT) contour plot analysis corroborated the absence of the Co–Co coordination in CoPc/XCNT (Fig. 2i and Supplementary Fig. S19). Quantitative EXAFS curve fitting results for CoPc/NCNT showed two paths at 1.92 and 2.18 Å, attributed to the Co–N and Co–N scattering paths, respectively (Fig. 2h and Supplementary Fig. S21 and Table S2). Moreover, the Co–N4O1 and Co–N4S1 configurations were confirmed for CoPc/OCNT and CoPc/SCNT, respectively. Overall, the above characterizations reveal the CoPc-like Co-(pyrrolic N)4 structure with X-atom modification in CoPc/XCNT, which significantly regulates the electronic distribution at the Co center.

EASH performance of molecular catalysts

The EASH performance of CoPc/XCNT was evaluated in a normal flow cell device with an electrode surface area of 1.0 cm2 under a 100% C2H2 feed. Initial performance evaluations in acidic, neutral, and basic electrolytes indicated that the alkaline environment is preferable (Supplementary Fig. S22), and thus, a 1.0 M KOH aqueous solution was chosen as the electrolyte for further studies unless otherwise stated. Compared with the CoPc supported on bare commercial carbon nanotubes (CoPc/CNT), most CoPc/XCNT catalysts significantly suppress side reactions and enhance selectivity, indicating the effectiveness of molecular regulation in enhancing EASH performance (Supplementary Fig. S23). The CoPc/NCNT showed a higher current density over the potential range from 0 V to −0.6 VRHE, as well as notably lower Tafel slope compared with CoPc/OCNT and CoPc/SCNT, indicating enhanced EASH kinetics over the molecular regulation (Fig. 3a and 3b). The product distribution of acetylene selective hydrogenation on CoPc/NCNT and CoPc/SCNT was also given across a current density range of −50 to −500 mA cm−2 (Fig. 3c-d and Supplementary Fig. S24). Note that the selectivity of C2H4 exceeded 99% on all CoPc/XCNT in the investigated current density range, verifying again that the isolated Co atomic site effectively suppressed C-C coupling processes. On CoPc/NCNT, the FE for C2H4 remained ~90%, reaching a maximum of 96.6% at a current density of −100 mA cm−2, while the byproduct C2H6, the product of the overhydrogenation reaction, had FE below 0.2%, and no C4H6 was detected. Impressively, at an industrial-level current density of −500 mA cm−2, the FE for C2H4 remained above 85%, continuing to fall within the profitable region and indicating the good economic potential of the CoPc/NCNT catalyst towards EASH19; in contrast, the FE for C2H4 on CoPc/OCNT and CoPc/SCNT dropped significantly to 17.5% and 4.7%, respectively. The variations in polarization curves and selectivity clearly indicated that the EASH kinetics and reaction pathway on CoPc/XCNT could be effectively tuned by regulating the electronic distribution at the Co center through X-atom modification. As a result, the CoPc/NCNT exhibited a high overall turnover frequency (TOF) value of 7019 min−1 at −500 mA cm−2 under pure C2H2 flow (Supplementary Note 1), which was increased by more than one order of magnitude in comparison with those of previously reported electrocatalysts (Supplementary Table S3). In addition, the CoPc/NCNT showed good catalytic stability. As shown in Fig. 3e, within 60 h of continuous operation at −100 mA cm−2, both the potential and the corresponding FE for C2H4 remained stable. Under a more demanding current density of −500 mA cm−2 in 1 M KOH, CoPc/NCNT still sustains an FE(C2H4) above 80% over 10 h of continuous operation (Supplementary Fig. S25). Post-electrolysis XPS and AC-STEM characterisation (Supplementary Fig. S26) showed nearly unchanged Co 2p features and only isolated Co sites without detectable nanoparticles or clusters, indicating that the CoPc-like single-atom configuration was preserved under EASH operating conditions. To rule out its potential contribution, the so-called electrochemical active surface area (ECSA) of CoPc/NCNT was evaluated. The calculated electrochemical double layer capacitance (Cdl) value of CoPc/NCNT was 19.3 mF cm−2, slightly higher than 16.6 mF cm−2 of CoPc/OCNT and 11.5 mF cm−2 of CoPc/SCNT, indicating its greater number of active sites. However, after normalizing by the obtained ECSA values, the EASH kinetic activity still followed the order of CoPc/NCNT > CoPc/OCNT > CoPc/SCNT, demonstrating that the performance enhancement primarily originated from the intrinsic activity of the active sites (Supplementary Fig. S27). In addition, the influence of CoPc loading was also examined (Supplementary Figs. S28 and S29). Notably, CoPc/NCNT with 5 wt% CoPc exhibited the highest EASH activity across various current densities. However, further increasing the loading to 10 wt% led to a decline in catalytic activity, likely due to CoPc aggregation. For flow cells, optimizing catalyst loading on the gas diffusion electrode (GDE) is essential to ensure sufficient acetylene reactants and active site availability. After adjusting the CoPc/NCNT loading on electrodes, the optimal EASH performance was achieved at a catalyst loading of 0.4 mg cm−2 (Supplementary Fig. S30). Therefore, unless otherwise stated, CoPc/XCNT with 5 wt% CoPc and a loading of 0.4 mg cm−2 was used in subsequent studies.

To validate the intrinsic catalytic sites, bare CoPc as well as CoPc-free OCNT, NCNT, and SCNT were employed for comparative analysis. The corresponding Faraday efficiency demonstrated much faster reaction kinetics for bare CoPc than OCNT, NCNT, and SCNT. The FE for C2H4 on bare CoPc was about 41.6% at −100 mA cm−2, much higher than 34.1% on OCNT, 6.2% on NCNT, and 12.3% on SCNT, respectively (Supplementary Figs. S31 and S32). Compared with bare CoPc, the XCNT substrates exhibited negligible acetylene semi-hydrogenation activity in both kinetics and selectivity, underscoring the pivotal role of molecularly dispersed CoPc sites in driving catalytic efficiency for EASH. This key role of CoPc was further supported by the LSV curves of NCNT and CoPc/NCNT under pure C2H2 flow, in which the markedly reduced performance in the absence of CoPc gave direct evidence of its catalytic contribution (Supplementary Fig. S33). When comparing the performance of bare CoPc with CoPc/XCNT catalysts, CoPc and CoPc/SCNT exhibited comparable reaction kinetics, both much lower than those of CoPc/OCNT and CoPc/NCCNT; the FE for C2H4 obtained at −100 mA cm‒2 followed the order: CoPc/SCNT (26.1%) < CoPc (41.6%) < CoPc/OCNT (91.2%) < CoPc/NCNT (96.6%). This performance trend is consistent with the regulation degree determined by the X heteroatom electronegativity (Fig. 2e and 2f). This consistency not only suggests that CoPc serves as the main active site while XCNT acts as its electronic modulator, but also highlights that the EASH performance is governed by the electronic structure of CoPc, which can be finely tuned through molecular regulation.

Mechanism of the high selectivity for ethylene

Before discussing the mechanistic role of molecular regulation, we first examined whether the enhanced activity of CoPc/NCNT could simply originate from changes in the physical properties of the carbon support. Four-probe tests show that the resistance increases from 0.018 Ω∙cm for CoPc/OCNT to 0.029 Ω∙cm for CoPc/NCNT and 0.086 Ω∙cm for CoPc/SCNT (Supplementary Table S4), indicating a slight increase in overall resistance. Contact angle measurements reveal that the surface becomes more hydrophobic, with the contact angle increasing from 65.5° (CoPc/OCNT) to 92.1° (CoPc/NCNT) and 96.4° (CoPc/SCNT) (Supplementary Fig. S34). C2H2-temperature programmed desorption results further show a gradual increase in C2H2 adsorption capacity (Supplementary Fig. S35). However, the EASH activity of the bare OCNT, NCNT, and SCNT supports remains low and in some cases even decreases after functionalisation (Supplementary Fig. S32). These results indicate that the high performance of CoPc/NCNT cannot be rationalised solely by favourable conductivity, hydrophilicity, or adsorption of the support, and instead mainly arises from the modified coordination environment and electronic structure of the Co centres induced by heteroatom doping.

To further elucidate the impact of molecular regulation on the reaction process, the EASH mechanisms of CoPc/XCNT catalysts were explored through a series of operando experiments. Operando Co K-edge XANES was used to probe the possible dynamic evolution of Co sites in CoPc/NCNT under working conditions (Supplementary Fig. S36). The spectrum at open-circuit potential resembles that of molecular CoPc and lies between Co foil and CoO, indicating a Coδ⁺ state between metallic Co and Co(II). As the applied potential becomes more negative, the absorption edge shifts progressively to lower energy and the white-line intensity decreases (Fig. 4a), showing that the Co centres become partially reduced during operation. Together with the negligible EASH activity of bare NCNT and the pronounced activity enhancement after introducing CoPc, it is safe to conclude that CoPc-derived Co centres serve as the primary active sites in CoPc/NCNT8. Subsequently, the EASH pathway on CoPc/NCNT and the associated interfacial water structure were investigated using potential-dependent in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. In the O–H stretching region, the broad band between 2900 and 3700 cm−1 can be deconvoluted into contributions from 4-coordinated hydrogen-bonded water (4-HB·H2O), 2-coordinated hydrogen-bonded water (2-HB·H2O), and K+-hydrated water (K·H2O). With increasingly negative potentials, the intensity of the 4-HB·H2O component decreases, while that of 2-HB·H2O increases, suggesting that the interfacial hydrogen-bond network becomes more weakly bound and that water dissociation is facilitated45, thereby increasing the availability of *H for the semi-hydrogenation of acetylene. In the lower-wavenumber region, positive bands at 1650 and 1559 cm−1 can be assigned to adsorbed *C2H2 and the generated *C2H3 intermediate on CoPc/NCNT, respectively. The intensity of the band corresponding to *C2H4 grows continuously with potential, consistent with the high ethylene selectivity observed on CoPc/NCNT (Fig. 4b). To determine the hydrogenation pathway in the EASH process, we systematically evaluated the origin and transfer of hydrogen through electrolyte-dependent experiments, including pH variation, scavenger tests, and cation effects. If the rate-determining step followed a proton-coupled electron transfer pathway of the form *C2HxHy + H2O + e- → *C2HxHy+1 + OH−, the EASH activity would display a pronounced sensitivity to pH or OH− concentration. The polarization curves and the C2H4 FE of CoPc/NCNT were then recorded in KOH solutions ranging from 0.5 to 5.0 M (pH≈13.7 to 14.7). With increasing pH, the polarization current density increases gradually, which can be mainly attributed to improved electrolyte conductivity and mass transport (Supplementary Fig. S37)8. In contrast, the onset potentials and the FE for C2H4 show no significant changes across this one-order-of-magnitude variation in OH− concentration (Supplementary Fig. S38). Such behaviour is not typical of a step controlled by proton-coupled electron transfer, and suggests that the key hydrogenation step does not directly depend on the bulk proton or hydroxide activity under the present conditions. The role of *H intermediates was further probed by adding tert-butanol (t-BuOH), a radical scavenger that deactivates *H via 2-methyl-2-propanol formation. Upon addition of t-BuOH, the polarization current density and C2H2 conversion decrease markedly (Supplementary Fig. S39). The combination of a weak pH dependence and a strong sensitivity to *H scavenging indicates that surface-bound *H, rather than a direct proton-coupled electron-transfer step, serves as the key reactive intermediate in the electrocatalytic hydrogenation of C2H2 on CoPc/NCNT46. Quasi-in situ electron paramagnetic resonance (EPR) spectroscopy further reveals the formation of carbon- and hydrogen-centred radicals on CoPc/NCNT under EASH-relevant potentials, whereas no radical signal is detected at open-circuit potential (Fig. 4c). In strongly alkaline media, the proton concentration is negligible, which disfavors a proton-coupled electron transfer pathway. Taken together with the pH variation and radical scavenger experiments, these results are consistent with a hydrogen atom transfer (HAT) mechanism. Specifically, water-derived *H intermediates are preferentially consumed via sequential hydrogen transfer to adsorbed C2H2, which mitigates competitive HER pathways (in particular the Heyrovsky-type route) and enables selective semi-hydrogenation in the low-to-intermediate current-density regime. To further probe this water dissociation–hydrogen transfer pathway, we tuned the hydrated cations M+(H2O)n by introducing different cations. Large hydration numbers and effective ionic radii are expected to weaken specific interactions between cations and Co active sites. This effect was examined by substituting compact K+(H2O)7 (n=7, r=3.3 Å) and Na+(H2O)13 (n=13, r=3.6 Å) with more strongly hydrated Li+(H2O)n (larger hydration shell, r=3.8 Å) and bulky TMA+ (n=18, r=4.5 Å) in 1.0 M KOH electrolytes (Supplementary Fig. S40). Under these conditions, the EASH kinetics follow the sequence K+ > Na+ > Li+ > TMA+, with C2H2 conversion decreasing from 30.3% (K+) to 10.4% (TMA+)47. This trend can be attributed to the ability of the cations to perturb and disrupt the interfacial hydration shell. Namely, the relatively compact K⁺(H2O)7 cluster likely polarises interfacial H2O networks more effectively than more strongly hydrated or bulkier cations, thereby facilitating water dissociation and the formation of reactive *H species on CoPc/NCNT.

Accordingly, a reaction mechanism is proposed (Supplementary Fig. S41) that involves coupled H2O dissociation and C2H2 hydrogenation. First, C2H2 adsorbs on the CoPc/NCNT surface in a flat configuration. In parallel, K⁺(H2O)7 promotes rapid water dissociation at the CoPc/NCNT interface to generate active hydrogen species. The adsorbed *C2H2 then couples with *H to form the partially hydrogenated intermediate *C2H3. A second hydrogenation step converts *C2H3 into *C2H4, which subsequently desorbs from the CoPc/NCNT surface, regenerating the catalytic sites for the next reaction cycle.

The rate-determining step in the EASH process was further analysed by combining kinetic isotope effect measurements with HER kinetics under Ar. The H/D KIE, defined as the ratio of C2H4 formation rates in H2O and D2O (Fig. 4d), is 1.48 for CoPc/OCNT and 1.73 for CoPc/SCNT, indicating that water dissociation remains strongly involved in the overall rate on these two catalysts. In contrast, CoPc/NCNT exhibits a smaller KIE of 1.15, which implies a reduced sensitivity to H/D substitution and that water dissociation is no longer the dominant kinetic limitation48. LSV and Tafel analysis in Ar saturated electrolyte (Fig. 4e) yields a Tafel slope of 94 mV dec⁻1 for CoPc/NCNT, lower than those of CoPc/OCNT (127 mV dec⁻1) and CoPc/SCNT (158 mV dec⁻1), a trend typically associated with more facile water activation in alkaline media (Supplementary Fig. S42). Together, these results show that CoPc/NCNT possesses the fastest water dissociation kinetics among the three CoPc/XCNT catalysts and shifts the kinetic bottleneck away from the initial water splitting step. The following isotopic labeling experiments detected deuterium in both C2H4 and H2 when using D2O, further verifying water as the exclusive hydrogen source (Fig. 4f). To monitor hydrogen species utilization during the EASH process, online DEMS was then performed under LSV mode (Fig. 4e, g, and h). Under C2H2 feed, CoPc/NCNT exhibited a more negative onset potential for H2 evolution than CoPc/OCNT and CoPc/SCNT, which, in combination with the KIE results, suggested that CoPc/NCNT enabled faster *H generation and more efficient transfer to C2H2, resulting in the exceptional HER suppression and improved hydrogenation kinetics.

According to the above experimental observations, targeted DFT calculations were also conducted to provide insights into the possible molecular regulation of CoPc in facilitating the EASH process. Models of CoPc/XCNT were constructed based on the EXAFS fitting results. Stabilization of CoPc on functionalized graphene surfaces was suggested by negative adsorption energies, with axial X heteroatom (O, N, S) spontaneously coordinating Co via strong d-p orbital hybridization (Supplementary Fig. S43). Charge density difference analysis indicated that the modeled CoPc/NCNT exhibited moderate charge depletion at the Co center compared to CoPc/OCNT and CoPc/SCNT (Fig. 4i). Due to its varied electronegativity, i.e., O > N > S, axial X heteroatoms modulated d-orbital occupancy through electron withdrawal/donation, redistributing electron density, and spin polarization. PDOS calculations suggested a volcano-shaped relationship between Co d-band center positions and ligand electron-withdrawing capability, which likely influences reaction intermediate adsorption strength (Fig. 4i)49. Given the predominant competition from the HER during EASH over CoPc/XCNT catalysts, the adsorption behavior of hydrogen intermediates (*H) at Co centers was identified as the critical factor of catalytic efficiency. Aligning with the Sabatier principle, excessively strong *H adsorption increases surface hydrogen coverage, blocking C2H2 access to active sites and impairing hydrogenation activity50. Conversely, overly weak *H adsorption fails to sustain C2H2 hydrogenation kinetics while favoring HER dominance. To quantify these interactions, crystal orbital Hamiltonian population (CCOHP) analysis was employed to probe the proposed bonding states of adsorbed *H on CoPc/XCNT (Fig. 4j). The *H adsorption strength correlated positively with the d-band center of Co in CoPc/XCNT. The CoPc/NCNT, with an intermediate d-band center, showed an optimal *H bonding energy, which could facilitate rapid hydrogen transfer during EASH while avoiding HER-driven deactivation caused by extreme *H coverage. Calculated H2O dissociation barriers and *H-binding Gibbs free energies further provided insights into potential mechanistic distinctions (Fig. 4k). Among the modeled CoPc/XCNT catalysts, CoPc/NCNT demonstrated a 3.20 eV lower H2O dissociation barrier alongside a 1.24 eV higher *H combination barrier. This dual energetic modulation is proposed to favor efficient *H generation via accelerated water activation while kinetically suppressing H2 formation. Consequently, abundant *H intermediates could participate selectively in C2H2 semi-hydrogenation, which aligns with the experimentally observed 86.7% FE and 99.99% selectivity for C2H4 at −500 mA cm⁻2, with complete suppression of C4 byproducts. Furthermore, a comprehensive theoretical analysis of the C2H2 hydrogenation process was then investigated by the Gibbs free-energy diagram for each elementary step (Fig. 4l and Supplementary Fig. S44). The first hydrogenation step (*C2H2 → *C2H3) of CoPc/OCNT, CoPc/NCNT, and CoPc/SCNT catalysts delivered limiting barriers of 0.61, 0.58, and 0.51 eV, respectively. This barrier hierarchy aligned with the onset potentials for C2H4 generation observed in online DEMS, supporting the idea that molecular regulation can act as an effective strategy to modulate EASH activity. Despite the more positive C2H4 generation potential of CoPc/OCNT, its EASH performance was limited by insufficient *H supply and unavoidable HER competition. In contrast, CoPc/NCNT achieved high activity through enhanced *H availability and utilization efficiency, while CoPc/SCNT suffered from severe HER interference. The calculated high energy barrier for *C2H4 → *C2H5 conversion (>0.2 eV) across CoPc/XCNT catalysts could explain the negligible C2H6 content experimentally observed in products (<0.6%), suggesting robust suppression of over-hydrogenation. These results collectively imply that the molecular strategy regulated EASH kinetics by tailoring the d-orbital electron occupancy of Co centers. CoPc/NCNT, with its intermediate d-band center, optimized *H adsorption and utilization, likely contributing to the highly efficient and selective acetylene reduction to ethylene over a wide current range.

Importantly, a similar coordination regulation also improves the performance of Cu phthalocyanine. Guided by the CoPc/NCNT design, we prepared an N-coordinated CuPc/NCNT analogue by immobilising CuPc on amino-functionalised CNTs using the same procedure as for CoPc/NCNT. In Ar saturated 1 M KOH, CuPc/NCNT displays a more positive HER onset potential and higher current density than CuPc/OCNT, which is consistent with more facile water dissociation at the Cu sites (Supplementary Fig. S45). Under EASH conditions, this enhanced water dissociation translates into higher ethylene selectivity and suppressed side reactions. At 100 mA cm⁻2, the Faradaic efficiency for C2H4 increases from 52.2 to 75.9% when moving from CuPc/OCNT to CuPc/NCNT, while the C2H6 FE decreases from 1.40 to 0.80% (Supplementary Fig. S46). Taken together with the CoPc/XCNT series, these observations indicate that axial coordination regulation provides a general route to improve water dissociation and to suppress competing HER and carbon–carbon coupling in MPc-based EASH catalysts, rather than being limited to the specific case of CoPc/NCNT.

Direct electroreduction of acetylene to high-purity ethylene

Motivated by its high FE for C2H4 and the selectivity, CoPc/NCNT catalysts were used for the direct conversion of C2H2 to high-purity C2H4 under practical operating conditions, including an ethylene-rich environment and a large work electrode area. Considering the low concentration of acetylene impurities in practical ethylene streams from cracked gas, the influence of acetylene concentration in the gas mixture on the EASH performance of CoPc/NCNT was first investigated in a 1.0 M KOH aqueous solution using a feed gas composed of argon and acetylene. The LSV curves of CoPc/NCNT exhibited a distinctly positive shift with increasing acetylene concentration (Supplementary Fig. S47). The FE for C2H4 at −100 mA cm-2 decreased from 96.6% under pure acetylene feed to 80.4% under 10% acetylene feed, resulting from the enhanced competition from HER caused by insufficient *H utilization for EASH (Fig. 5a). To satisfy the requirements for the industrial production of polymer-grade ethylene, the EASH performance was subsequently assessed under realistic conditions with ⁓0.5% acetylene impurities (5000 ppm) in crude ethylene. Lower inlet flow rates correspond to longer gas residence times on the catalyst surface. As shown in Fig. 5c, with a decrease in inlet flow rate, the increasing residence time of gas on CoPc/NCNT contributed to a higher conversion of C2H2. When the inlet flow rate was 20 sccm, the ethylene-specific selectivity was approximately 99%, nearly meeting the requirements of polymerized ethylene. Furthermore, during a 110 h stability test in a crude ethylene flow, both the acetylene conversion and specific selectivity of ethylene over CoPc/NCNT remained higher than 99.7% and 99.5%, respectively (Fig. 5b, Supplementary Fig. S48). This good stability indicated the efficient EASH capability of CoPc/NCNT, even in a C2H4-rich environment with a low C2H2 partial pressure. The direct electroreduction of acetylene to a high concentration of ethylene was performed using a homemade tandem device consisting of a 1.0 cm2 flow cell and a 25 cm2 flow cell (Supplementary Fig. S49). The 1 cm2 flow cell was designed to deal with high-concentration C2H2 feed gas, and the 25 cm2 flow cell was used to convert residual C2H2 at low concentrations. Initially, pure C2H2 was pumped into the 1.0 cm2 flow cell, and the outlet gas was then immediately fed into the 25 cm2 flow cell without any purification (Fig. 5d). At the optimal flow rate of 4 mL min−1, the single-pass C2H2 conversion exceeded ~99%, and C2H4 selectivity surpassed 98%, respectively. A preliminary techno-economic analysis based on recently reported EASH models suggests that, under literature-consistent assumptions, this tandem configuration can operate within an economically reasonable regime (Supplementary Note 2). Taken together, these results underscore the practical potential of CoPc/NCNT for selective acetylene semi-hydrogenation and highlight the importance of molecular regulation in tuning the performance of molecular electrocatalysts.

In summary, a carbon nanotube-supported metal phthalocyanines, MPc/XCNT (M = Cu, Co, Ni, Fe; X = O, N, S), is developed as a useful platform to reveal the function of the metal identity and local coordination environment in the EASH. Comparative studies within the MPc/OCNT series demonstrate the key role of metal identity in enabling water activation to suppress C4 formation, with CoPc-based catalysts standing out for effectively promoting this step and achieving high ethylene selectivity. Building on this, a tunable molecular strategy is further adopted to modulate the coordination environment of CoPc-based catalysts. The optimized molecular regulation fine-tunes the d-band center of Co sites, modulating *H intermediate generation and utilization to mitigate HER and over-hydrogenation. The resulting CoPc/NCNT delivers competitive performance, achieving 86.7% ethylene FE with negligible ethane production (<0.01%) and no detectable C4H6 even at a large current density of 500 mA cm-2 under a pure C2H2 feed, and reaching 99.7% C2H2 conversion with up to 99.5% C2H4 selectivity under a simulated industrial crude C2H4 atmosphere. This work not only establishes the great capability of cobalt to reconcile C–C coupling suppression with selective hydrogenation but also provides a universal molecular strategy for tailoring the local coordination environment of molecular catalysts to advance electrocatalytic transformations.

Methods
Chemicals and materials

Commerical CNT powder, cobalt phthalocyanine (CoPc, 99.5%), phthalocyanine (Pc, 93.0%), N, N-dimethylformamide (DMF, 99.0%), anhydrous ethanol (CH3CH2OH, 99.9%), tert-butanol (99.5%), tetramethylammonium hydroxide (95%), deuterium oxide (99.0%), carbon nanotube (95.0%), (3-aminopropyl) triethoxysilane (95.0%), (3-mercaptopropyl)trimethoxysilane (95.0%), Nafion solution (5 wt%) were purchased from Aladdin Ltd. (Shanghai, China). Potassium hydroxide (KOH, 95.0%), sodium hydroxide (NaOH, 95%), potassium chloride (KCl, 99%), vitriol (H2SO4, 99%), were purchased from Sinopharm Co. Ltd. The GDL (SGL, 28BC), Pt foil (0.4-mm thickness), anion exchange membrane (fumasep FASB-PK-130) was purchased from Fuel Cell Store. The deionized water (>18.25 MΩ) was used for the experiments. All chemicals were used without further purification.

Preparation of functionalized carbon nanotubes

The hydroxyl-modified carbon nanotubes (OCNT) material was synthesized by chemically introducing oxygen functional groups onto the surface of the original carbon nanotubes (CNT) through an oxidation treatment using concentrated nitric acid (HNO3). In a typical procedure, 2.0 g of CNT powder was uniformly dispersed in 100 mL of 68 wt% HNO3 while being continuously stirred magnetically. The oxidation process was carried out at 140 °C for 12 hours, followed by thorough washing with water and subsequent freeze drying. It is crucial to manage the evaporation of concentrated HNO3 and any decomposition products using an ice water condenser. During this process, potential metal impurities present on the CNT surface were effectively removed through intensive acid etching. Amino-modified carbon nanotubes (NCNT) were synthesized as follows: 0.5 g of the prepared OCNT was dissolved in 100 mL of mixed solvent (Vethanol/Vwater = 9/1). After achieving good dispersion, 0.75 mL of (3-aminopropyl) triethoxysilane was added dropwise under vigorous stirring. After that, the black solution was kept at 60 °C for 10 h. The product was washed with deionized water several times and freeze-dried for 24 h. Sulfhydryl-modified carbon nanotubes (SCNT) were synthesized using the same method by replacing (3-Aminopropyl)triethoxysilane with (3-mercaptopropyl)trimethoxysilane.

Preparation of catalysts

CoPc/NCNT and Pc/NCNT catalysts were synthesized using a simple impregnation method at 25 °C. Typically, NCNT (100 mg) and CoPc (5 mg) were dispersed in a strong polar solvent of DMF solution (100 mL) and stirred for 20 h, followed by separation and freeze-drying treatments. By adjusting the mass percent ratio of CoPc and NCNT using 1, 5, or 10 mg CoPc with fixed 100 mg NCNT, CoPc/NCNT-1, CoPc/NCNT-5, and CoPc/NCNT-10 were prepared. Similarly, FePc/OCNT, NiPc/OCNT, CuPc/OCNT, CuPc/NCNT, CoPc/OCNT, and CoPc/SCNT were also synthesized in parallel by the same method as the CoPc/NCNT catalyst, except for replacing NCNT with OCNT and SCNT, respectively. Meanwhile, CoPc/CNT was prepared using the original CNT as support.

Fabrication of electrodes

The electrodes used in this work were prepared using a traditional drop-coating technique. 6.0 mg of the catalyst powder was dispersed in 5 mL of an ethanol-water mixture (Vethanol/Vwater = 4/1) with sonication for 1 h. Then, 20 μl of Nafion solution (5 wt%) was added to the solution for another 0.5 h. Subsequently, the commercial GDE (SGL, 28BC) was cut into a rectangular shape with a size of 1 × 3 cm2 to serve as the electrode substrate. The influence of mass loading per geometric area on acetylene-to-ethylene performance was assessed by varying the catalyst loading on the GDL to 0.2, 0.4, and 0.6 mg cm−2 (Supplementary Figs. S22). Unless otherwise specified, the catalyst loading of 0.4 mg cm−2 on the GDL was used for the MPc/XCNT catalysts in subsequent experiments; the mass loading of bare CoPc on the GDL was matched to that of CoPcs in the CoPc/XCNT catalysts on the GDL, specifically 0.06 mg cm−2. The as-prepared electrodes were dried at 25 °C.

Electrochemical measurements in the flow cell

Electrochemical measurements were performed in a divided three-compartment electrochemical cell with a window area of 1 cm2 (2 × 0.5 cm2), which consists of a gas chamber, cathodic chamber, and anodic chamber. Generally, GDE with a catalyst as the working electrode, Pt foil as the counter-electrode, and Hg/HgO as the reference electrode constituted an electrocatalytic system using the Gamry 5000 electrochemical workstation. The cathodic and anodic compartments, both containing 1.0 M KOH electrolyte, were separated by an anion exchange membrane (FAB-PK-130). The 1.0 M KOH electrolyte was freshly prepared using ultrapure water and stored in a sealed polyethylene bottle. The cell resistance was determined prior to the electrochemical measurements using the built-in high-frequency impedance technique of the electrochemical workstation. The electrochemical surface area (ECSA) was estimated using the double-layer capacitance (Cdl) method. Cyclic voltammetry (CV) measurements were conducted in a non-faradaic region from −0.05 to 0.05 V vs. RHE at varying scan rates (20, 40, 60, 80, 100, 120, and 140 mV s−1). The Cdl values were determined by plotting the charging current density difference (Δj = ja –jc) at 0 V vs. RHE against the scan rates, where the slope of the linear fit represents twice the value of Cdl.

All LSV curves were iR compensated with a compensation level of 70%. All potentials were converted to the RHE scale according to Eq. (1).

E = EHg/HgO + 0.098 + 0.059 × pH (1)

For the three-electrode flow cell system operated under pure C2H2, a reactor with an electrode area of 1 cm2 was used. The cathode gas chamber was supplied with pure C2H2 at a flow rate of 20 ml min−1, while 1 M KOH (PH=14.0) was pumped around both the cathode and anode sides at a flow rate of 20 ml min−1. The potentials presented in this system were cell voltages (Ecell).

Three-electrode flow cell systems (5 × 5 cm2), which consist of CoPc/NCNT GDE, Pt foil, and Hg/HgO electrodes, were used with 15 mL of 1 M KOH. Chronoamperometry was performed at −40 mA cm−2 and 10 mL min−1 to purify ethylene from ethylene-rich simulated gas (0.5% C2H2, 20% C2H4 balanced with Ar). Electrolyte flow rate set at 5 mL min−1. Prior to quantitative analysis, the electrochemical semihydrogenation of acetylene was stabilized for 5–10 minutes under varying current densities. Following this stabilization, the effluent from the flow cell was directly introduced into a gas chromatograph for product analysis.

In the tandem flow cell configuration, comprising a fore-cell (1 cm2) and a post-cell (25 cm2), 100% C2H2 gas entered the fore-cell and was partially converted to a C2H4 and C2H2 mixture at −500 mA cm⁻2 with a flow rate of 4 mL min⁻1. The fore-cell’s outlet gas was immediately fed into the post-cell via a short connection to minimize dead volume. In the post-cell, residual C2H2 was converted to C2H4 at −40 mA cm−2 and 4 mL min⁻1. Individual pumps provided 1 M KOH electrolyte to the tandem system at 10 mL min−1. All the electrochemical performances are presented without the iR compensation except for LSV tests.

Electrochemical operando online DEMS analysis

In-situ differential electrochemical mass spectrometry (DEMS) analysis was performed using a Linglu Instruments QAS 100LiA system (Iridium filament, 5*10‒4 A/mbar). A detailed schematic of the DEMS electrochemical cell is provided in Fig. 4e. Given that all products of the EASH process are gaseous, this setup enabled real-time monitoring of product distribution, thereby directly elucidating selectivity. The single-cell reactor was coupled to the DEMS via a quartz capillary inserted into its outlet, which interfaced with a PTFE membrane (Gore-Tex, 0.22 μm) to selectively transfer the gaseous effluent into the negatively pressurized DEMS gas circuit. Electrochemical performance was evaluated using a CHI760E workstation, with linear sweep voltammetry (LSV) conducted from 0 to −0.5 V vs. RHE. The mass-to-charge ratios (m/z) of 2, 26, 28, 30, and 54 were assigned to H2, C2H2, C2H4, C2H6, and C4H6, respectively. Raw ion currents are plotted on absolute intensity scales without mathematical correction or deconvolution, as data collection began only after baseline stabilization.

Electrochemical in situ FTIR spectra measurements

In situ FTIR measurements were conducted using a Nicolet IS50 spectrometer equipped with a liquid-nitrogen-cooled MCT-A detector. A gold-coated Ge prism served as the working electrode, prepared via sputter-coating (Quorum SC7620) at 30 mA for 60 s to ensure a uniform conductive surface. Before each experiment, a CoPc/NCNT ink was deposited onto a gold film that was pre-coated on the Ge prism. A standard three-electrode configuration was employed, utilizing a Pt wire counter electrode and a Hg/HgO reference electrode. The measurements were performed in a 1.0 M KOH electrolyte, which was pre-saturated and continuously purged with C2H2. Potentiostatic IR spectra were acquired using a CHI760E workstation over a potential range from the open-circuit potential to −1.2 V. Spectra were synchronously collected from 4000 to 650 cm−1 with a 4 cm−1 resolution.

Electrochemical in situ XANES spectra measurements

In situ XANES measurements at the Co K-edge were performed using an ultra-high flux benchtop X-ray absorption fine structure spectrometer (RapidXAFS 2M, Anhui Absorption Spectroscopy Analysis Instrument Co., Ltd.). The instrument was operated at 20 kV and 40 mA and equipped with a Si (533) spherically bent crystal analyzer (radius of curvature: 500 mm), which serves for energy selection of the incident X-ray beam and enables rapid acquisition of XANES spectra under operando conditions. The measurements were conducted using a specially designed H-type electrolytic cell, with electrode configurations and reaction conditions identical to those in the performance tests. The spectra were acquired under a continuous gas flow. Initially, data were collected at the open-circuit potential (OCP) after purging the electrolyte with Ar or acetylene for 40 minutes. Subsequently, spectra were recorded after applying a series of potentials, each for a duration of 10 minutes.

Electrochemical quasi-in situ EPR measurements

Quasi-in situ electron paramagnetic resonance (EPR) spectra were recorded at 25 °C on an X-band Bruker EMX microspectrometer. The instrument was configured with the following parameters: center field = 3500 G, sweep width = 100 G, sweep time = 5 min, modulation amplitude = 1 G, and microwave attenuation = 20 mW. For electrochemical EPR measurements, the 1 cm2 flow cell integrated a three-electrode system, separated by a proton exchange membrane (FAB-PK-130), with Hg/HgO and Pt foil serving as the reference and counter electrodes, respectively. Potentials were applied using a Gamry 5000 electrochemical workstation. Sample solutions containing DMPO were loaded into quartz tubes for analysis, while the electrolyte was purged with ultrapure Ar for at least 30 minutes before measurements to ensure an anoxic environment.

Performance assessment

Gas products (C2H4, C2H2, C4, H2) were analyzed through gas chromatography (Fuli, 9790PLUS) coupled with a FID detector and a TCD detector. The Plot Al2O3 column separated ethylene, acetylene, and C4, while the Molecular Sieve 5 A columns separated H2. An external standard method was applied to estimate the concentration of the component of gas products (Supplementary Fig. S50). The calculation of the C2H2 conversion and the evolution rate of different products was performed using equations (2) and (3), and the FEs of different products were calculated using equation (4). Turnover frequency (TOF) for C2H4 (−500 mA cm−2) was calculated using equation (5):

Conversion (%) = (cfeed - c') / cfeed × 100% (2)

Selectivity (%) = (cfeed - c') / (cfeed ‒ c' + cC2H6 +2 × cC4) × 100% (3)

FE (%) = (n × m × F) / (I × t) × 100% (4)

TOF (min−1) = (amount of C2H4 molecules produced per min) / (amount of Co sites) (5)

Where, cfeed: concentration of C2H2 feed gas, c', cC2H6, cC4: the concentration of C2H2, C2H6, C4 in gas products. n: number of transferred electrons, m: amount of substance, F: Faraday’s constant, I: total current, t: electrolysis time. All data points with error bars represent the mean ± standard deviation from three independent measurements.

Characterization

The phase structure was characterized by X-ray diffraction (XRD, X’Pert PRO MPD) using Cu Kα radiation (λ= 1.518 Å). The morphology of the catalysts was measured by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Thermo Scientific Talos F200X). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were carried out on a spherical aberration-corrected microscope (Titan 80-300). The chemical constituents of the catalysts were studied by X-ray photoelectron spectroscopy (XPS) with an Mg Kα radiation (1486.6 eV) monochromatic X-ray source (Thermo Scientific Escalab 250XI). The cobalt content was detected by an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) (Agilent 5110). The hydrogen temperature-programmed desorption (H2-TPD) was conducted on a TP-5080 apparatus equipped with a thermal conductivity detector. X-ray absorption fine structure (XAFS) was carried out at 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). The acquired EXAFS data were processed and analyzed using the ATHENA module of the IFEFFIT software package following standard procedures. Co foil, CoO, and Co3O4 were used as reference standards to determine the valence state and coordination environment of Co in the catalysts.

Density functional theory (DFT) calculations

All density functional theory (DFT) calculations were executed utilizing the Vienna Ab initio Simulation Package (VASP)51. The electron exchange and correlation effects were treated through the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional. A plane-wave kinetic energy cutoff was set to 450 eV. For the CoPc surface models, the Brillouin zone was sampled using a 3×3×1 Monkhorst-Pack k-point mesh, selected based on the supercell dimensions. A vacuum region of more than 15 Å was incorporated along the z-direction to prevent spurious interactions between periodic slabs. Structural relaxations were carried out until the electronic energy and the Hellmann-Feynman forces on each atom converged to within 10⁻5 eV and 0.01 eV/Å, respectively. These specific parameters were finalized following rigorous total energy convergence tests52. Transition state (TS) searches were performed using the Dimer method in the VTST package53. This involved generating initial guess geometries with the climbing-image nudged elastic band (CI-NEB) method, followed by convergence to the saddle points using the Dimer method. All optimized structures were verified to have atomic forces less than 0.1 eV Å−1. The optimized atomic coordinates structures are provided as Supplementary Data 154.

The adsorption energy (Eads) of reaction intermediates and the free energy change can be computed using equations (6)-(7):

ΔEads = Esys – Esurface – Eadsorb (6)

ΔG = ΔE + ΔEDFT + ΔEZPE – TΔS (7)

where Esys is the energy of the system with surface and adsorbate, Esurface is the energy of the catalyst surface, and Eadsorb corresponds to the energy of the isolated adsorbate. ΔG, ΔE, ΔEDFT, ΔEZPE, and TΔS stand for, respectively, the free energy, total energy from DFT calculations, zero-point energy, and entropic contributions.

The C2H2 hydrogenation process to C2H4 can be delineated by equations (9)-(11):

* + C2H2 → *C2H2 (9)

*C2H2 + H+ + e− → *C2H3 (10)

*C2H3 + H+ + e− → *C2H4 (11)

The hydrogen combination proceeded through equations (12)-(13):

* + 2H → 2*H (12)

*H + *H → *H2 (13)

The C-C coupling proceeded through this equation (14):

2*C2H3 → *C4H6 (14)

While the water dissociation proceeded through equations (15)-(18):

* + H2O → *H2O (15)

*H2O → *H‒OH (16)

*H‒OH → *H + *OH (17)

*H + *OH → *H + OH (18)

Data availability

The Source Data file and Supplementary Data 1 have been deposited in Figshare under accession code DOI link54. https://doi.org/10.6084/m9.figshare.31622623.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript titled “Molecular Cobalt Catalysts for Highly Selective Electrocatalytic Acetylene Semi-Hydrogenation” investigates the electrocatalytic semi-hydrogenation of acetylene using cobalt phthalocyanine (CoPc) supported on differently functionalized carbon nanotubes (CNTs). The authors compare CoPc with CuPc catalysts, and systematically study the effect of O-, S-, and N-doped CNT supports on catalytic activity. To elucidate the reaction mechanism of CoPc/NCNT, they employ a broad range of techniques, including in situ XANES, FTIR, EPR, DEMS, and DFT calculations. Additional characterization methods such as TEM, Raman spectroscopy, and kinetic isotope effect studies are also used. Finally, they demonstrate the direct electroreduction of acetylene to high concentrations of ethylene in a homemade tandem device, combining a 1 cm2 flow cell with a 25 cm2 flow cell. Overall, the study is interesting and represents a meaningful contribution to the field of electrocatalytic acetylene semi-hydrogenation. However, the manuscript currently suffers from missing experimental details, insufficient depth in some aspects of the data analysis, and a lack of clarity in the presentation of certain results and writing. Before the manuscript can be considered for publication in Nature Communications, the following comments must be thoroughly addressed:\n1. On page 4, at the beginning of the Results and Discussion section, additional experimental details are required. For example, the electrolyte conditions should be specified when introducing the LSV data (“Under an Ar atmosphere, the linear sweep voltammetry (LSV) curves of the CoPc/OCNT demonstrated both a more positive onset potential and higher current density across the HER polarization range compared with those of CuPc/OCNT”). The writing should also be more quantitative: rather than stating “a more positive onset potential,” the authors should report the exact numerical difference. Similarly, when presenting selectivity values, the amounts of by-products (e.g. C4 species) should be given. On page 6, the phrase “consistent with the earlier onset potentials observed in LSV measurements” should be clarified by specifying the actual onset potential shift.\n2. Throughout the manuscript, the authors tend to draw conclusions too rapidly, and the hypotheses should be formulated more carefully. For example, on page 5 the statement that CoPc/OCNT exhibits “facilitated water dissociation and thus enhanced EASH performance” is not sufficiently supported, as LSV data alone only demonstrate higher or earlier activity and do not provide direct evidence of water dissociation. Similarly, on page 10 the claim that “the variations in polarization curves and selectivity clearly indicated that the ESAH kinetics and reaction pathway on CoPc/XCNT could be effectively tuned by modulating the X heteroatom electronegativity” is overstated, since the correlation with electronegativity remains speculative without additional mechanistic evidence.\n3. On page 6, the discussion of water dissociation barriers and Gibbs free energy for *H binding is confusing. The authors state that CoPc/OCNT exhibits stronger H2O dissociation and a higher barrier for *H coupling, resulting in slower HER kinetics (Figure 1h). However, this appears inconsistent with the earlier LSV and DEMS results, where CoPc/OCNT was described as facilitating the reaction. The authors should reconcile these two observations and clarify how they are connected.\n4. Regarding the characterization, for Figure S8, reference pattern should be provided for the XRD data. In addition, there is an inconsistency in Figure S9: the main text refers to FTIR, while the SI shows a Raman spectrum. Also, according to the literature, CoPc can typically be detected in the lower wavenumber region (600–700 cm-1) with Raman, which does not overlap with CNT vibrations (see J. Am. Chem. Soc. 2025, 147, 14, 12298–12307, Fig. S13). The authors should carefully examine this region. For Xanes, reference spectra of bare CoPc in Figure S13 should be provided for comparison.\n5. For the AC-STEM images (Figures 2d and S12), the legend for the 3D heat map is missing and should be provided with length-scale if possible.\n6. In general, it would be important to include error bars, for example in the ICP-OES data, to highlight the reproducibility of the synthesis and Co loading. Likewise, Table S2 for the EXAFS Fits should report explicit error values rather than only providing a general description below the table.\n7. For the explanation of the XANES-data in Figure 2f the authors should provide a clearer explanation of the significance of the pre-edge shift observed and how this correlates with the different CNTs.\n8. The SI should also include the measured electrode potentials. In Figure 3e, it is unclear whether Ecell refers to the working electrode potential or the total cell voltage, this should be specified. In the Methods section, all potentials are described as iR-corrected, yet in the SI no such correction appears to be applied. Please provide the typical ohmic drop values. Additionally, in many current density plots in the SI (e.g. Figure S17), the authors note “The results are presented without iR compensation”, but since no potential values are shown in the graphs, this statement is not meaningful.\n9. On page 10, the statement “Impressively, at an industrial-level current density of 500 mA cm-2, the FE for C2H4 remained above 85%, continuing to fall within the profitable region and indicating the excellent economic potential of the CoPc/NCNT catalyst towards EASH (Figure S18)” requires clarification. How is the “profitable region” defined, and what is the energy efficiency of the reaction under these conditions? What metrics are truly required for industrial application? Moreover, Figure S18 shows GC data, which does not support this economic claim. On page 11, the stability discussion is also incomplete. The authors show that at 100 mA cm-2 the catalyst remains stable for 60 h (Figure 3e), but no data are provided at 500 mA cm-2, which would be necessary to support claims of industrial relevance.\n10. The authors compare CoPc with supported CoPc on OCNT, SCNT, and NCNT. However, a direct comparison with CoPc on bare CNTs is missing. Including this control is important to clearly demonstrate the necessity and benefit of CNT functionalization.\n11. In Figure S22, the data for the lowest loading and lowest current appear significantly different and may represent an outlier. Is this result reproducible? On page 11, the authors state that CoPc/NCNT with 5 wt% CoPc and a loading of 0.4 mg cm-2 was used and gave the best electrocatalytic performance. How does this loading on CNT and catalyst loading compare with other reports on CoPc–CNT systems, for example in CO2RR to CO or methanol? Is the chosen loading consistent with the literature, or does it need to be specifically adjusted for the present reaction? What kind of GDE is used?\n12. Regarding the change in Co oxidation state observed by in situ XANES: can the authors provide clearer insights into the specific oxidation state transition (e.g., from which to which state)? The statement “This valence reduction was correlated with electron transfer from Co to adsorbed C2H2 species, facilitating rapid acetylene reduction and clearly confirming Co as the primary active site” is difficult to follow. Is this conclusion directly supported by the presented data, or is it primarily drawn from the literature?\n13. The peak assignments of the in situ FTIR data in Figure 4b do not align with the cited reference. The authors assign the bands at 1650 and 1559 cm-1 to the *C≡C bond of C2H2 and the generated *C=C bond adsorbed on CoPc/NCNT, respectively. However, according to the reference, adsorbed *C2H2 species can be detected at 1641 cm-1 over ED–Cu NPs, while the band at 1577 cm-1 corresponds to *C2H3, the key intermediate of C2H2 hydrogenation. Therefore, a more consistent assignment according to that reference would be 1650 cm-1 to *C2H2 and 1559 cm-1 to *C2H3. This interpretation would also better match the observed peak evolution, where the 1650 cm-1 band increases at higher overpotentials. Are these overpotential-dependent changes correlated with product selectivity? Furthermore, do the relative intensity changes of these two peaks align with the expected reaction pathway? The authors should also examine whether additional informative spectral regions around 2000 cm-1 are present in the FTIR spectra.\n14. On page 13, the discussion of pH variation and radical scavenger experiments is too brief and the conclusions are drawn too quickly. The pH was only varied slightly (from ~13.7 to ~14.3), which corresponds to a relatively small change in OH- concentration. The authors should also specify which concentrations of KOH corresponds to which pH. In addition, when introducing t-BuOH as a radical scavenger, the exact concentration used (50 mM) should be reported in the main text. This is a relatively high concentration, and it should be clarified whether the observed effect can be attributed specifically to radical scavenging or to other interactions with the system.\n15. The reported EASH reaction kinetics follow the trend K+ > Na+ > TMA+, with C2H2 conversion decreasing from K+ to 10.4% TMA+. How does this trend compare with previous literature? Have larger or smaller cations such as Cs+ or Li+ been tested? It is worth noting that molecular catalysts often exhibit very different cation-dependent effects compared to metallic catalysts (see Yu, S., Yamauchi, H., Wang, S. et al. Nat. Catal. 7, 1000–1009 (2024)).\n16. More details about the gas feed are required. Was the acetylene introduced in ethylene or in argon? More details about the gas feed mixing is required. Additional clarification is also needed on the 1 cm2 and 25 cm2 flow cell setups, how exactly are they configured and operated? In the text, the authors state: “To satisfy the requirements for the industrial production of polymer-grade ethylene, the EASH performance was subsequently assessed under realistic conditions with 0.5% acetylene impurities (5000 ppm) in crude ethylene (Figure S33).” However, Figure S33 shows LSV curves of different gas concentrations.\n17. The Methods section requires substantial revision, as both the main text and the Supporting Information currently provide too few details to ensure reproducibility. The experimental procedures should be described comprehensively, including complete information on catalyst synthesis and functionalization (precursors, solvents, conditions, purification), electrode preparation and loading (mass loading, electrode area, binder composition, deposition procedure), and electrochemical testing protocols (cell configuration, electrolyte composition and preparation, gas feed composition and flow rates, reference and counter electrode details, iR compensation). In addition, the in situ and operando experiments (XANES, FTIR, EPR, DEMS) must be described in sufficient detail, including setups, acquisition parameters, and data analysis methods. Calibration and quantification procedures (e.g., GC, ICP-OES) should also be reported, together with error analysis and reproducibility tests. Without this level of detail, the results cannot be reliably reproduced, and the section must therefore be carefully revised and expanded to meet the standards of Nature Communications.\nMinor comments:\n-Page 5, ESAE should be ESAH.\n-The authors should write the minus sign for the current densities (I assume negative currents were applied).\n-Hard to distinguish the colors (such as blue and green) in many of the plots.", "reviewer_2": "In this work, the authors report a molecular cobalt phthalocyanine (CoPc) catalyst platform that promotes electrocatalytic acetylene semi-hydrogenation (EASH) reaction activity through electronic regulation. They also investigate the performance modulation strategies and reaction mechanisms of the corresponding molecular catalysts. The utilization of analogous molecular catalysts (CoPc and CuPc) for EASH catalysis have been previously reported [10.1016/j.cej.2021.134129]. Moreover, the authors' research and conclusions on the performance enhancement mechanism of CoPc/XCNT are not sufficiently clear. Therefore, I do not recommend publishing this work in Nature Communications.\n1.Apart from the already reported feasibility of metal phthalocyanines in the EASH process, the employed CoPc/OCNT and CuPc/OCNT are also widely used in various electrocatalytic reactions including CO2RR, ORR, HER, and etc. (Angew. Chem. Int. Ed. 2024, 63, e202310623; Chem. Soc. Rev. 2021, 50, 12985; Nature, 2019, 575, 639). By simply combining the two points together, the present work is lack of innovation to be published in Nature Communications.\n2.Except for the novelty issue, the data and analysis is not adequately to support their hypothesis and conclusions. For example, to exclude the CPET pathway, the author claimed that “the pH was varied from ~13.7 to ~14.3, negligible changes were observed in the polarization curves and FE for C2H4 on CoPc/NCNT”, which is contradictory to the data shown in Figure S27.\n3.In Page 13, the authors claimed that a hydrogen transfer mechanism could bypass the HER competitive, which is groundless. It has been widely reported that H2 could also obtained by the combination of two water-derived *H, called Tafel step (J. Phys. Chem. C, 2018, 122, 23943.). Thus, the real reason for the improved C2H4 selectivity is not the claimed mechanism, which needs further discussion.\n4.The observed performance disparities between CoPC/NCTN and other samples are predominantly attributable to the difference in water dissociation ability. However, this seems inconsistent with the kinetic isotope experiment results for catalysts. The LSV or kinetic isotope experiment results for catalysts under argon gas should be provided for further comparison.\n5.The as-reported TOF value is too high to convince the reviewer. According to the data in the previous literature (Chem. Eng. J. 2022, 431, 134129), the bare CoPc only exhibited a TOF value of 0.29 min-1 at a current of 100 mA. How can the TOF of the as-reported CoPc/NCNT reach 9700 min-1 at a current of 500 mA? More details are needed for the TOF calculation. Besides, the claimed 24 h continuous operation under ethylene-rich simulated gas flow could not be regarded as an advancement compared to the recent progress (Nat. Catal.2021, 4, 557; 10.31635/ccschem.024.202404849; 10.1002/anie.202509501; 10.31635/ccschem.022.202101750;)\n6.In Figure 1g, comparing the d-band centers of different metals is quite uncommon and meaningless. Meanwhile, in Figures 1h, I and 4k, the author employed unusual plotting methods to present the reaction diagrams, which resulted in poorer data readability and a higher likelihood of misinterpretation. For example, in the conventional *H generation process, it is necessary to calculate the steady-state *H2O and *H+*OH as the initial and final states of the water dissociation process, as well as the corresponding transition states, while considering the *OH desorption process. It is very incomplete for the author to only calculate the energy barrier from *H2O to *H+*OH. The generation processes of H2 and C4 also face similar issues.\n7.The manuscript does not present any calculation methods, and lacks the display of relevant and representative adsorption models. For single-atom catalysts, the selection of adsorption sites is crucial and needs to be demonstrated. In the processes of water dissociation and hydrogenation, it is necessary to clarify whether C and O species occupy the metal sites, and where *H is adsorbed.", "reviewer_3": "In this work, the authors report a CoPc-OCNT catalyst towards EASH, achieving FE of 86.7% under 500 mA cm-2. The manuscript attributes the enhanced activity to the excellent hydrogenation ability of CoPc, but similar arguments were already reported in prior studies (Chem. Eng. J. 2022, 431: 134129). Therefore, the reviewer considers the current form of this work lacks of novelty and not suitable for publication in Nature Communications.\n1. The authors choose the CoPc/OCNT as the model catalyst due to its better ability of water dissociation. How about NiPc/OCNT and FePc/OCNT, which has been proved the great activity on water dissociation. Besides, the FE of C2H4 decreased and FE of H2 increased sharply under high current densities, suggesting the inferior ability of CoPC/OCNT to tune the *H. The authors propose the CoPc/OCNT can inhibit C-C coupling to generate C4 byproducts, while the key Co sites may have no activity on C-C coupling.\n2. The authors use the different carbon substrate (N-CNT, S-CNT, O-CNT) to tune coordination of Co center and optimize its catalytic performance. How about the performance of CoPc/CNT without doping? The CoPc/NCNT achieves the better performance. The authors should prove the increased performance is not originate the change of physical properties (conductivity, hydrophilicity, adsorption capacity).\n3. The most electrochemical results presented in this study lack error bars, which may raise concerns about the reproducibility and reliability of the reported data.\n4. The in-situ FT-IR data presented in this study are underexplored. The analysis of water peaks range from 3000-4000 cm-1 may be benefit of revealing the tune of water dissociation.\n5. The selectivity performance using t-BuOH in electrolyte should be provide to the demonstrate the role of *H, while the LSV curves are not enough.\n6. The authors design a tandem system to generate high-purity C2H4. However, the reviewer wants know if such a design has practical significance. Whether the added big-area reactor is better than other methods for removing low concentrations of C2H2. The technical economic analysis may be better to demonstrate the necessity of the design.\n7. Some typographical typos exist in the manuscript and the authors should carefully check it (e.g., affinity—strong, Y-axis title of Figure 5d, the calculation of FE in Performance assessment).", "reviewer_4": "This manuscript reports the synthesis of carbon nanotube-supported cobalt phtalocyanine (CoPc) complexes through silanoxyl links, and their electrocatalytic activity during the selective semi-hydrogenation reaction of acetylene to ethylene, mainly in pure form but also in ethylene-rich streams (actual commercial process). The characterization of the materials is comprehensive, the electrocatalytic results are well executed and the mechanistic part is sound. The results demonstrate that the axially NH2-complexed CoPc material is much more selective than the others (O- or SH-) due to the electronics of the coordination sphere, which allows a faster H2O dissociation and slower undesired C-C coupling. The fact that such a subtle change in the coordination of the supported Co cation enables a significant increase of the selectivity without compromising the catalytic activity is remarkable, moreover in a solid-supported catalyst, and will guide the design of future solid catalysts for this reaction. Therefore, publication in Nat. Commun. is suitable and recommended. However, it must be said that many electrocatalysts for this reaction has been recently published (many of them cited in the text but not compared in Table S3, which is clearly uncomplete), thus some work must be done to really contextualize the new catalyst. Some comments follow:\n- The key finding here is the action of the N-axial ligand on the metal Pc. This is the point to highlight. I would prepare the N-axial ligand/Cu-Pc material and confirm that the catalytic activity improves in the same way (no need for a deep characterization, the ligation should be similar to the Co complex).\n- The catalyst must be characterized after reaction. Is the complex stable? Evolves to an active species?\n- With the better numbers obtained here, could the process substitute the current thermocatalytic process in any way?\n- The conversion and selectivity numbers given in the introduction for the industrial semi-hydrogenation reaction of acetylene in ethylene streams are clearly underestimated, since a >99.99% selectivity for acetylene conversion is required (<10 ppm in the final stream). See and cite if appropriate: Nat. Catal., 2024, 7, 452–463. Indeed, the reaction conditions are also exaggerated, the industrial process operates at <100 ºC.\n- The necessity or not of carbon nanotubes to support the complexes must be justified. If a carbon support with dangling hydroxyls could also do the job, this should be commented in the text, since that would give a much more affordable catalyst (perhaps worth of study in a different study). Other hydroxyl-containing supports could also be suitable, please read and cite if appropriate the following examples on semi-hydrogenation reaction of acetylene in ethylene streams under industrial conditions: ACS Catal. 2017, 7, 3721−3729; J. Am. Chem. Soc. 2018, 140, 8827−8832.\n- Table S3 must be properly completed, or removed. The comparison between electro and thermocatalysts is perhaps unnecessary, otherwise the Table can be huge (see SI in the above-suggested references)."}, "author_response": "Dear Editors and Reviewers:\nWe would like to greatly appreciate you giving us the opportunity to revise our manuscript.\nWe carefully studied the reviewer’s comments and tried our best to revise the manuscript\naccording to the comments. The manuscript has been revised following the comments and\nsuggestions of the reviewers, and point by point replies are given below. Thanks again to the editors\nand reviewers for their hard work.\nMolecular Cobalt Catalysts for Highly Selective Electrocatalytic Acetylene Semi-\nHydrogenation\nResearch Article, No. NCOMMS-25-62672A, Nature Communications\nResponse to Reviewer #1:\nGeneral Comments:\nThe manuscript titled “Molecular Cobalt Catalysts for Highly Selective Electrocatalytic Acetylene\nSemi-Hydrogenation” investigates the electrocatalytic semi-hydrogenation of acetylene using cobalt\nphthalocyanine (CoPc) supported on differently functionalized carbon nanotubes (CNTs). The\nauthors compare CoPc with CuPc catalysts, and systematically study the effect of O-, S-, and N-\ndoped CNT supports on catalytic activity. To elucidate the reaction mechanism of CoPc/NCNT, they\nemploy a broad range of techniques, including in situ XANES, FTIR, EPR, DEMS, and DFT\ncalculations. Additional characterization methods such as TEM, Raman spectroscopy, and kinetic\nisotope effect studies are also used. Finally, they demonstrate the direct electroreduction of acetylene\nto high concentrations of ethylene in a homemade tandem device, combining a 1 cm2 flow cell with\na 25 cm2 flow cell. Overall, the study is interesting and represents a meaningful contribution to the\nfield of electrocatalytic acetylene semi-hydrogenation. However, the manuscript currently suffers\nfrom missing experimental details, insufficient depth in some aspects of the data analysis, and a lack\nof clarity in the presentation of certain results and writing. Before the manuscript can be considered\nfor publication in Nature Communications, the following comments must be thoroughly addressed:\nResponse: We appreciate the reviewer’s thoughtful summary and constructive suggestions. These\nopinions help to improve the academic rigor of our article. In response, we have carefully revised\nthe manuscript and Supplementary Information, incorporating new experimental data, figures,\nreferences, and discussion to address each of the reviewer’s comments point by point. Detailed\ndescriptions of these amendments are provided below. We believe that these revisions have\nsubstantially improved the overall quality of this work.\nSpecific Comments:\nComment 1: On page 4, at the beginning of the Results and Discussion section, additional\nexperimental details are required. For example, the electrolyte conditions should be specified when\nintroducing the LSV data (“Under an Ar atmosphere, the linear sweep voltammetry (LSV) curves of\nthe CoPc/OCNT demonstrated both a more positive onset potential and higher current density across\nthe HER polarization range compared with those of CuPc/OCNT”). The writing should also be more\nquantitative: rather than stating “a more positive onset potential,” the authors should report the exact\nnumerical difference. Similarly, when presenting selectivity values, the amounts of by-products (e.g.\nC species) should be given. On page 6, the phrase “consistent with the earlier onset potentials\n4\nobserved in LSV measurements” should be clarified by specifying the actual onset potential shift.\nResponse: We acknowledge the reviewer’s helpful comments and suggestions. We have revised the\nmanuscript accordingly to provide more comprehensive experimental details and quantitative\ncomparisons. The main revisions are summarized as follows:\nPage 5-6: “The EASH performance of metal phthalocyanines was evaluated in a normal flow cell\ndevice with an electrode surface area of 1 cm2 under a 100% C H feed, in which Hg/HgO and Pt\n2 2\nfoil served as the reference and counter electrodes, respectively, and an anion exchange membrane\nseparated two chambers filled with 1.0 M KOH aqueous solution (Supplementary Fig. S1).”\nPage 6: “Under an Ar atmosphere in 1.0 M KOH electrolyte, the linear sweep voltammetry (LSV)\ncurves…”\nWe appreciate the suggestion to increase the quantitative rigor of the manuscript. Accordingly,\nwe have revised the manuscript throughout to provide explicit numerical values wherever\ncomparative statements were previously qualitative. Specifically:\nPage 6: “…the CoPc/OCNT catalyst exhibited an onset potential of −0.26 V vs. reversible hydrogen\nelectrode (V ), approximately 30 mV more positive than the −0.29 V of CuPc/OCNT, and also\nRHE\ndelivered a higher current density across the HER polarization range (e.g., −83.8 mA cm−2 vs. −33.9\nmA cm−2 at −0.8 V )…”\nRHE\nPage 6: “…with C byproduct formation persisting at ~1.5%”\nonset potentials, respectively, from CoPc/OCNT to CuPc/OCNT observed in the LSV\nmeasurements.”\nComment 2: Throughout the manuscript, the authors tend to draw conclusions too rapidly, and the\nhypotheses should be formulated more carefully. For example, on page 5 the statement that\nCoPc/OCNT exhibits “facilitated water dissociation and thus enhanced EASH performance” is not\nsufficiently supported, as LSV data alone only demonstrate higher or earlier activity and do not\nprovide direct evidence of water dissociation. Similarly, on page 10 the claim that “the variations in\npolarization curves and selectivity clearly indicated that the EASH kinetics and reaction pathway on\nCoPc/XCNT could be effectively tuned by modulating the X heteroatom electronegativity” is\noverstated, since the correlation with electronegativity remains speculative without additional\nmechanistic evidence.\nResponse to comment 2: We thank the reviewer for this thoughtful suggestion.\nWe agree that, in the original version, the discussion on water dissociation and heteroatom\neffects could have been presented more cautiously. In the revised manuscript, we have carefully\nrefined the wording and improved the logical continuity between the hypotheses and the subsequent\nmechanistic evidence.\n(1) Clarification of the water dissociation part on Page 5:\nOn page 5, the wording that CoPc/OCNT exhibits “facilitated water dissociation and thus\nenhanced EASH performance” based solely on LSV data was indeed premature. As the review noted,\nLSVs reflect only apparent activity and onset behaviour, not the elementary water-dissociation step\ndirectly. In the revised version, we have relocated this statement to Page 6, following direct evidence\nfrom isotope-labeling experiments and DFT calculations, which confirm that CoPc/OCNT\nsignificantly lowers the energy barrier for the *H O → *OH + *H step compared with CuPc/OCNT.\nthan on CuPc/OCNT, while the C H (m/z=28) signal for CoPc/OCNT emerged at –0.2 V , 120\n2 4 RHE\nmV more positive than that of CuPc/OCNT.”\n(2) Clarification of the heteroatom effect description on Page 10:\nWe recognize that our conclusion was drawn somewhat early. Since a more detailed\nmechanistic discussion is provided later in the manuscript, we have rephrased the statement to\nenhance logical coherence and scientific rigor. The revised text now reads: “Overall, the above\ncharacterizations reveal the CoPc-like Co-(pyrrolic N) structure with X-atom modification in\nComment 3: On page 6, the discussion of water dissociation barriers and Gibbs free energy for *H\nbinding is confusing. The authors state that CoPc/OCNT exhibits stronger H O dissociation and a\n2\nhigher barrier for *H coupling, resulting in slower HER kinetics (Figure 1h). However, this appears\ninconsistent with the earlier LSV and DEMS results, where CoPc/OCNT was described as\nfacilitating the reaction. The authors should reconcile these two observations and clarify how they\nare connected.\nResponse to comment 3: We sincerely thank the reviewer for this insightful comment and for\npointing out this important issue, which is indeed crucial for understanding the catalytic mechanism.\nInspired by this suggestion, we have re-examined the HER mechanisms to gain a deeper\nunderstanding. In alkaline media, HER typically proceeds via Volmer-Heyrovsky and/or Volmer-\nTafel steps, in which Volmer step, Heyrovsky step, and Tafel step refers to water dissociation\n(Volmer: H O + e− + * → H* + OH−), electrochemical desorption (Heyrovsky: H* + H O + e− → H\n2 2 2\n+ OH⁻), and chemical recombination (Tafel: H* + H* → H ), respectively. As shown in Figure R1,\n2\nthe calculated Tafel slopes are 153 and 282 mV dec−1 for CoPc/OCNT and CuPc/OCNT, respectively,\nboth higher than 120 mV dec−1, indicating that the Volmer step is the rate-determining step (Sci.\nRep., 5, 13801 (2015); Angew. Chem. Int. Ed. 64, e202504667 (2025)). Therefore, we have\ncalculated the energy barrier for the water dissociation associated Volmer step. As shown in Figure\n1h, CoPc/OCNT exhibits a substantially lower energy barrier for H O dissociation than CuPc/OCNT,\n2\nsuggesting that the Volmer step is much more favorable on CoPc/OCNT. Therefore, CoPc/OCNT\ndisplays higher HER activity than CuPc/OCNT, consistent with the LSV and DEMS results.\nIn addition, we would like to note that the high HER performance of CoPc/OCNT does not\nimply a higher H generation rate during EASH. In contrast, under EASH conditions, the H\n2 2\ngeneration on CoPc/OCNT is in fact lower than that on CuPc/OCNT. A similar phenomenon was\nalso observed in previous work on EASH systems (Nat. Commun. 14, 8384 (2023)). This\nphenomenon can be attributed to the competition between EASH and HER on the catalyst surface.\nNamely, the *H produced through the H O dissociation is quickly captured by adsorbed C H to\n2 2 2\nform C H , which reduces the surface *H level and thus slows down HER. In most cases, the\n2 4\nbyproduct H is generated via chemical recombination between *H rather than electrochemical\n2\ndesorption (H* + H O + e− → H + OH⁻). Therefore, to explain this phenomenon, calculations of *H\n2 2\ncoupling were also performed. One can see that CoPc/OCNT gives a slightly higher energy barrier\nfor *H coupling, thereby suppressing the byproduct H formation under EASH. We acknowledge\n2\nthat our original phrasing might have been misleading, and we have now revised the text to clarify\nthe relationship between the theoretical and experimental results.\nTo avoid misunderstanding, we have revised the text on page 6 as follows:\nPage 6-7: “Subsequent analysis of water dissociation barriers shows that CoPc/OCNT had a\nmarkedly lower barrier for H O dissociation than CuPc/OCNT (Fig. 1h and Supplementary Fig. S3).\n2\nresults, but also, under C H , the rapidly generated *H is preferentially consumed by hydrogenation\n2 2\nof adsorbed C H , which, together with the elevated C–C coupling barrier and *H–*H coupling\n2 2\nbarrier (Fig. 1i, Supplementary Figs. S3 and S4), accounts for the high C H selectivity and the\n2 4\nabsence of detectable C products on CoPc/OCNT.”\nfor the XRD data. In addition, there is an inconsistency in Figure S9: the main text refers to FTIR,\nwhile the SI shows a Raman spectrum. Also, according to the literature, CoPc can typically be\ndetected in the lower wavenumber region (600–700 cm-1) with Raman, which does not overlap with\nCNT vibrations (see J. Am. Chem. Soc. 2025, 147, 14, 12298–12307, Fig. S13). The authors should\ncarefully examine this region. For Xanes, reference spectra of bare CoPc in Figure S13 should be\nprovided for comparison.\nResponse to comment 4: We thank the reviewer for these valuable comments.\nFollowing this suggestion, the reference diffraction patterns of CoPc and CNT have been added\nin Fig. S8 for comparison (Fig. R1). No characteristic diffraction peaks of CoPc were observed in\nCoPc/XCNT, indicating that CoPc is well-dispersed on the functionalized CNT surface.\nThe inconsistency in Figure S9 has been corrected (the spectrum is indeed Raman rather than\nFTIR). Following the reviewer’s suggestion, we carefully re-examined the 600–700 cm⁻1 region of\nthe Raman spectra (Fig. R2). For the optimized CoPc/XCNT catalyst, no CoPc-related Raman peaks\nare detected in this region, which is consistent with the XRD results and further supports the high\ndispersion of CoPc. To further certify this, we also measured a sample with higher CoPc loading\n(CoPcNCNT-10, CoPc/NCNT=1:10, m/m). In this case, clear CoPc characteristic peaks appear in\nthe 600–700 cm⁻1 and 1500-1600 cm⁻1 regions of the Raman spectrum, further confirming that their\nabsence in CoPc/XCNT arises from the high dispersion of CoPc. The reference (J. Am. Chem. Soc.\n2025, 147, 12298–12307) has also been included in the revised manuscript for comparison and\ndiscussion.\nRegarding the XANES data, the soft XANES spectra (O, N, and S K-edge) in Fig. S13 were\nused to probe the electronic interaction between the functionalised CNT supports and the Co atom\nin CoPc. To assess the influence on CoPc, hard Co K-edge XANES of bare CoPc has already been\nprovided as a reference in the main text (Fig. R3), where a detailed comparison of the Co electronic\nstructure in CoPc and CoPc/XCNT is presented.\nFig. R1 | XRD patterns of OCNT, CoPc, CoPc/OCNT, CoPc/NCNT, and CoPc/SCNT.\nFig. R2 | Raman spectra of CoPc/OCNT, CoPc/NCNT, and CoPc/SCNT.\nFig. R3 | XANES spectra of the Co foil, Co O , CoPc, CoPc/OCNT, CoPc/NCNT, and CoPc/SCNT.\n3 4\nComment 5: For the AC-STEM images (Figures 2d and S12), the legend for the 3D heat map is\nmissing and should be provided with length-scale if possible.\nResponse to comment 5: We appreciate the reviewer for pointing this out.\nThe missing legend for the 3D intensity heat map has now been added to Fig. R4 (Revised\nManuscript Fig. 2d) and R5 (Revised Supplementary Fig. S14), together with the corresponding\ncolor scale and length-scale bar, as shown below.\nFig. R4 | AC-STEM images and 3D atom-overlapping Gaussian-function map for CoPc/NCNT.\nFig. R5 | AC-STEM images and 3D atom-overlapping Gaussian-function map for (a) CoPc/OCNT\nand (b) CoPc/SCNT.\nComment 6: In general, it would be important to include error bars, for example in the ICP-OES\ndata, to highlight the reproducibility of the synthesis and Co loading. Likewise, Table S2 for the\nEXAFS Fits should report explicit error values rather than only providing a general description\nbelow the table.\nResponse to comment 6: We appreciate the reviewer for this valuable comment.\nThe ICP-OES data have been updated based on three independent synthesis batches, and the\ncorresponding standard deviations are now included in Table R1, as shown below. These results\nconfirm the good reproducibility of the synthesis and Co loading.\nTable R2 has been revised to explicitly list the fitting uncertainties for bond length and Debye–\nWaller factor, rather than providing only a general note.\nWe have added these results to the revised Supplementary Information (Page S28)\nTable R1 | Co mass loading of CoPc/OCNT, CoPc/NCNT, and CoPc/SCNT.\nCatalysts Loading (wt. %) Standard Deviation (%)\nCoPc/OCNT 0.55 ±0.01\nCoPc/NCNT 0.47 ±0.055\nCoPc/SCNT 0.57 ±0.026\nTable R2 | Co K-edge EXAFS curve-fitting parameters of CoPc/XCNT and reference samples.\nSample Path CN R (Å) σ2 (10−3Å2） ∆E (eV) R factor\nCo-N 4.1 1.91±0.007 3.3±1.2\nCoPc/OCNT 5.0±2.58 0.017\nCo-O 1.1 1.96±0.03 2.3±4.8\nCo-N1 4.0 1.92±0.02 3.4±1.1\nCoPc/NCNT -0.47±3.25 0.014\nCo-N2 1.1 2.13±0.04 1.0±0.5\nCo-N 4.3 1.90±0.05 5.7±4.4\nCoPc/SCNT 0.02±7.29 0.015\nCo-S 0.8 2.18±0.03 1.6±5.8\nCN: coordination numbers; R: bond distance; σ2: Debye-Waller factors; ΔE : the inner potential\n0\ncorrection. R factor: goodness of fit. Ѕ 2 (0.8) was obtained from the experimental EXAFS fit of Co\n0\nfoil reference by fixing CN as the known crystallographic value and was fixed to all the samples.\nComment 7: For the explanation of the XANES-data in Figure 2f the authors should provide a\nclearer explanation of the significance of the pre-edge shift observed and how this correlates with\nthe different CNTs.\nResponse to comment 7: We thank the reviewer for this helpful comment.\nIn the revised manuscript, we have clarified the overall characterization sequence and the\nspecific significance of the Co K-edge pre-edge feature in Figure 2f, as well as its correlation with\nthe different CNT supports.\nFirst, soft X-ray absorption spectroscopy (O, N, and S K-edge XANES) was carried out at a\nsynchrotron beamline to probe the electronic interaction between CoPc and the heteroatom-doped\nCNT supports. These spectra identify the formation of axial Co–O, Co–N, and Co–S bonds in\nCoPc/OCNT, CoPc/NCNT, and CoPc/SCNT, respectively, confirming strong d–p orbital coupling\nbetween CoPc and the XCNT supports. Next, Co 2p XPS was used to evaluate the Co valence state.\nThe Co 2p binding energy follows the order CoPc/SCNT < CoPc < CoPc/NCNT < CoPc/OCNT,\n3/2\nindicating that the Co valence increases from S- to N- to O-coordinated CoPc, in line with the\nincreasing electronegativity (and decreasing electron-donating ability) of the axial ligands. Finally,\nCo K-edge XANES and EXAFS were employed to resolve the fine local coordination structure of\nCo in CoPc/XCNT. In Fig. 2f, all CoPc/XCNT samples display a distinct pre-edge peak at ~7715\neV, which originates from the 1s → 3d transition of the square-planar Co–N center and thus\n4\nconfirms that the in-plane Co–N configuration of CoPc is preserved on the CNT supports. Moreover,\n4\nthe pre-edge (and edge) position shows a systematic shift that correlates with the heteroatom in the\nCNT: compared with bare CoPc, CoPc/OCNT shifts to higher energy, whereas CoPc/NCNT and\nCoPc/SCNT shift to lower energy. This energy sequence (CoPc/OCNT > CoPc > CoPc/NCNT >\nCoPc/SCNT) reflects the stronger electron-withdrawing character of axial O compared with N and\nS, leading to the highest Co valence in CoPc/OCNT and progressively lower Co valence in\nCoPc/NCNT and CoPc/SCNT. This trend is fully consistent with the Co 2p XPS results and\ndemonstrates that the heteroatom-doped CNTs can regulate the Co 3d electronic structure via axial\ncoordination.\nWe have added the above results and discussions to the revised manuscript (Page 10).\nPage 10: “In particular, the Co 2p binding energies followed the order CoPc/SCNT (779.1 eV) <\n3/2\nCoPc (779.8 eV) < CoPc/NCNT (780.8 eV) < CoPc/OCNT (780.9 eV), indicating a gradual increase\nin Co valence from S- to N- to O-coordinated CoPc. This trend was consistent with the increasing\nelectronegativity and decreasing electron-donating ability of the axial ligands from S to N to O,\nconfirming that the Co electronic structure can be precisely tuned by the heteroatom-doped CNT\nsupport. To further elucidate the local coordination environment of Co in CoPc/XCNT, Co K-edge\nXANES and extended X-ray absorption fine structure (EXAFS) analyses were performed. As shown\nin Fig. 2f, the near-edge absorption intensity of the Co K-edge for CoPc-XCNT was between those\nof Co foil and Co O , confirming that Co was in a positively charged state. All CoPc/XCNT catalysts\n3 4\nexhibited a distinct pre-edge feature at ~7715 eV, which was characteristic of the 1s→3d transition\nof the square-planar Co–N center, demonstrating that the in-plane Co–N configuration of CoPc\n4 4\nwas retained on the CNT supports. Notably, the pre-edge (and edge) position exhibited a systematic\nshift that correlates with the heteroatom in the CNT: compared with bare CoPc, CoPc/OCNT showed\na shift to higher energy, while CoPc/NCNT and CoPc/SCNT shift to lower energy. This energy\nsequence (CoPc/OCNT > CoPc > CoPc/NCNT > CoPc/SCNT) reflected the stronger electron-\nwithdrawing character of axial O relative to N and S, leading to a higher Co valence in CoPc/OCNT\nand a progressively lower valence in CoPc/NCNT and CoPc/SCNT, fully consistent with the XPS\nresults.”\nComment 8: The SI should also include the measured electrode potentials. In Figure 3e, it is unclear\nwhether E refers to the working electrode potential or the total cell voltage, this should be\ncell\nspecified. In the Methods section, all potentials are described as iR-corrected, yet in the SI no such\ncorrection appears to be applied. Please provide the typical ohmic drop values. Additionally, in many\ncurrent density plots in the SI (e.g. Figure S17), the authors note “The results are presented without\niR compensation”, but since no potential values are shown in the graphs, this statement is not\nmeaningful.\nResponse to comment 8: We thank the reviewer for these careful comments on the electrochemical\ndata presentation and fully agree that the potential definitions and iR treatment should be clearly and\nconsistently stated.\nFirst, we apologize for the inaccurate wording regarding iR correction in the original Methods\nsection. After carefully re-checking all datasets, we confirm that all electrochemical data in this work\nare presented without iR compensation. This has now been explicitly stated in the Methods and\ncorrected consistently throughout the main text and Supplementary Information.\nSecond, in Figure 3e, E refers to the total cell voltage (i.e., the full cell voltage between\ncell\ncathode and anode measured by the potentiostat), not the single working-electrode potential. We\nhave revised the Figure 3e caption to clearly specify that E is the overall cell voltage and that it is\ncell\nshown without iR correction. Following the reviewer’s suggestion, we have also added the measured\ncell voltages at different current densities to the revised Supplementary Information (Fig. R6), so\nthat the operating potentials of the electrodes are explicitly documented.\nRegarding the ohmic drop, because of the specific flow-cell design used in this work, we were\nnot able to obtain reliable solution resistances from conventional EIS measurements. Instead, we\ndetermined (i) the intrinsic resistance of the catalyst by a four-point probe setup and (ii) the overall\nresistance of the electrocatalytic system using the potentiostat. As summarised in Table R3, the\nresistances of the three CoPc/XCNT electrodes vary only slightly, indicating that the ohmic losses\nare very similar across these samples and do not account for the observed differences in activity and\nselectivity. In line with this, we have simplified and corrected the captions of the relevant SI figures\n(e.g., the original Figure S17) by removing redundant statements such as “without iR compensation”\nand we now state once in the SI that all potentials in this work are reported without iR correction\nunless otherwise noted.\nFig. R6 | The obtained cell voltage in C H flow over CoPc/OCNT, CuPc/OCNT, CoPc/NCNT, and\n2 2\nCoPc/SCNT under different current densities.\nTable R3 | The catalyst resistances and system resistances of measured at working conditions.\nCatalyst resistances\nSample System resistances (Ω)\n(Ω)\nCoPc/OCNT 0.018±0.003 1.9±0.1\nCoPc/NCNT 0.029±0.011 2.0±0.2\nCoPc/SCNT 0.086±0.009 2.2±0.2\nComment 9: On page 10, the statement “Impressively, at an industrial-level current density of 500\nmA cm-2, the FE for C H remained above 85%, continuing to fall within the profitable region and\n2 4\nindicating the excellent economic potential of the CoPc/NCNT catalyst towards EASH (Figure S18)”\nrequires clarification. How is the “profitable region” defined, and what is the energy efficiency of\nthe reaction under these conditions? What metrics are truly required for industrial application?\nMoreover, Figure S18 shows GC data, which does not support this economic claim. On page 11, the\nstability discussion is also incomplete. The authors show that at 100 mA cm-2 the catalyst remains\nstable for 60 h (Figure 3e), but no data are provided at 500 mA cm-2, which would be necessary to\nsupport claims of industrial relevance.\nResponse to comment 9: We thank the reviewer for this visionary comment.\nIn the revised manuscript, we clarify that the term “profitable region” is adopted from the\ntechno-economic analysis reported in Nat. Sustain. 6, 827–837 (2023), where the cost of\nelectrocatalytic acetylene semi-hydrogenation was evaluated under defined boundary conditions,\nincluding capital, operating, and materials costs. In that work, operation at about 0.4 A cm⁻2 with\nFE(C H ) ≥ 70% was identified as a regime in which the process can be economically viable (Fig.\n2 4\nR7). In our system, CoPc/NCNT reaches FE(C H ) >85% at 0.5 A cm−2 in 1 M KOH, which lies\n2 4\nwithin and in fact exceeds the current-density/Faraday-efficiency window identified in that work.\nFurthermore, we have conducted a full process‑level techno‑economic assessment based on the same\nframework (Nat. Sustain. 6, 827–837 (2023)). The results demonstrate that C H production using\n2 4\nCoPc/NCNT can attain a gross margin of approximately 19.7%, underscoring its strong potential for\nindustrial implementation (Revised Supplementary Note 2, Page 32). Following the discussion in\nNat. Sustain. 6, 827–837 (2023), we think that relevant industrial metrics include high FE(C H ) at\n2 4\ndifferent current densities, low cell voltage, and high energy efficiency, long-term stability at the\ntarget current density, high single-pass acetylene conversion, and polymer-grade ethylene purity, as\nwell as favourable system-level costs.\nRegarding stability, we agree that data at 500 mA cm−2 are important to support any statement\nabout industrial relevance. In addition to the 60 h test at 100 mA cm−2 shown in Fig. 3e, we have\nnow carried out a stability test at 500 mA cm−2 in 1 M KOH. As shown in Fig. R8, CoPc/NCNT\nmaintains FE(C H ) above 80% with almost no decay in performance over 10 h of continuous\n2 4\noperation at 500 mA cm−2.\nThe related contents have been included in the revised manuscript and supplementary\ninformation (Page 12, Supplementary Fig. S26 and Note 2 in Page S13 and S32, respectively).\nPage 12: “Under a more demanding current density of −500 mA cm⁻2 in 1 M KOH, CoPc/NCNT\nstill sustains an FE(C H ) above 80% over 10 h of continuous operation (Supplementary Fig. S25).”\n2 4\nFig. R7 | Techno-economic analysis results under different current densities from 0.4 A cm-2.\nFig. R8 | Long-term stability using CoPc/NCNT at a constant current density of −500 mA cm−2.\nComment 10: The authors compare CoPc with supported CoPc on OCNT, SCNT, and NCNT.\nHowever, a direct comparison with CoPc on bare CNTs is missing. Including this control is\nimportant to clearly demonstrate the necessity and benefit of CNT functionalization.\nResponse to comment 10: We appreciate the reviewer for this valuable comment.\nIn the revised manuscript, we have now included CoPc supported on bare commercial CNTs\n(CoPc/CNT) as an additional control to directly assess the effect of CNT functionalization. As shown\nin the revised Fig. R9a, the presence of oxygen functionalities still leads to a higher polarization\ncurrent density in the LSV of CoPc/OCNT compared with CoPc/CNT. The product distribution at\n100 mA cm−2 (Fig. R9b,c) further highlights the benefit of functionalization. CoPc/CNT exhibits\nnotable parasitic reactions, with Faradaic efficiencies of 11.7% for H , 0.47% for over-\n2\nhydrogenation to C H and 0.44% for C–C coupling products. In contrast, CoPc/OCNT shows\n2 6,\nFaradaic efficiencies of 8.1% for H , 0% for C H and C products, indicating strongly suppressed\n2 2 6 4\nside reactions and improved selectivity. These new results further demonstrate that CNT\nfunctionalisation is essential for optimising the intrinsic performance of molecular CoPc in EASH.\nThe related contents have been included in the revised manuscript and supplementary\ninformation (Page 11, Supplementary Fig. S23, Page S12).\nPage 11: “Compared with the CoPc supported on bare commercial carbon nanotubes (CoPc/CNT),\nmost CoPc/XCNT catalysts significantly suppress side reactions and enhance selectivity, indicating\nthe effectiveness of molecular regulation in enhancing EASH performance (Supplementary Fig.\nS23).”\nFig. R9 | (a) LSV curves and (b) Faraday efficiency of the obtained products using CoPc/CNT (b) and\nCoPc/OCNT (c) at various current densities.\nComment 11: In Figure S22, the data for the lowest loading and lowest current appear significantly\ndifferent and may represent an outlier. Is this result reproducible? On page 11, the authors state that\nCoPc/NCNT with 5 wt% CoPc and a loading of 0.4 mg cm-2 was used and gave the best\nelectrocatalytic performance. How does this loading on CNT and catalyst loading compare with\nother reports on CoPc–CNT systems, for example in CO RR to CO or methanol? Is the chosen\n2\nloading consistent with the literature, or does it need to be specifically adjusted for the present\nreaction? What kind of GDE is used?\nResponse to comment 11: We appreciate the reviewer for this insightful comment.\n1. Reproducibility of the lowest-loading data in Figure S22\nThank you for pointing this out. We have now repeated the measurements of the varied loading\nexperiments at least three times in independent experiments under identical conditions (CoPc/NCNT\nwith 5 wt% CoPc in 1 M KOH). The new results are reproducible and exhibit the same trend in the\nloading effects as observed previously, in which the CoPc/NCNT with the loading of 0.4 mg cm-2\nexhibits the best EASH performance (Fig. R10). Error bars have been included in Fig. S29 in the\nrevised Supplementary Information.\n2. Comparison of CoPc/NCNT loading with literature and rationale for our choice\nAs stated on page 11, we finally used CoPc/NCNT with 5 wt% CoPc and a total catalyst loading\nof 0.4 mg cm-2 on the gas diffusion electrode, which corresponds to an effective CoPc loading of\n~0.02 mg cm-2. In this work, these values were chosen following the systematic investigation in the\ncorresponding parameter experiments: we systematically varied both the CoPc content and the total\nloading and identified 5 wt% and 0.4 mg cm-2 as the best compromise between high FE for C H\n2 4\nand high (partial) current density for EASH in 1 M KOH (Fig. R10).\nTo directly address the reviewer’s concern, we also compared our loading with four\nrepresentative reports on CoPc–CNT systems used in gas-diffusion configurations for CO RR to CO\n2\nor methanol electrocatalysis (Nat. Nanotech. 20, 515-522 (2025); Nat. Comm. 16, 7359 (2025); Nat.\nComm. 8, 14675 (2017); Nat. Comm. 7, 987-999 (2024)). In those studies, the total catalyst loading\nis typically on the order of 0.4–1.2 mg cm-2, with CoPc contents from a few to several tens of wt%.\nOur electrode (5 wt% CoPc, 0.4 mg cm-2) lies well within the range used in the literature and, in\nterms of CoPc amount per geometric area, is in many cases slightly lower (Table R4). Meanwhile,\nwe have to note that the optimal loading window is unavoidably reaction- and setup-dependent. For\nthe present acetylene hydrogenation in 1 M KOH, the screening shown in Figure S22 indicates that\nthe chosen loading is specifically suited to balance intrinsic activity and mass transport in our GDE\nflow cell.\n3. Type of GDE\nThe gas diffusion electrode used in this study is a commercial carbon-paper GDE, Sigracet\n28BC (SGL Carbon), with a microporous layer, purchased from Fuel Cell Store. This information\nhas been added to the Experimental section in the revised manuscript (Page 21).\nPage 21: “The GDL (SGL, 28BC), Pt foil (0.4-mm thickness), anion exchange membrane (fumasep\nFAB-PK-130) was purchased from Fuel Cell Store.”\nFig. R10 | Faraday efficiency of the obtained products over (a) CoPc/NCNT-1, (b) CoPc/NCNT-5,\nand (c) CoPc/NCNT-10 at various current densities.\nTable R4 | Benchmarking of CoPc loading and total catalyst loading against literature.\nCoPc loading Catalyst loading\nCatalyst Reference\n(wt%) (mg cm−2)\nCoPc/NCNT 5 0.4 This work\nCoPc-NH2/CNT 6.5 1.2 Nat. Nanotech. 20, 515-522 (2025).\nCoPc/CNT(III) 6.67 0.4 Nat. Comm. 16, 7359 (2025).\nCoPc-CN 6 0.4 Nat. Comm. 8, 14675 (2017).\nCoPc/CNT 5 0.4 Nat. Comm. 7, 987-999 (2024).\nComment 12: Regarding the change in Co oxidation state observed by in situ XANES: can the\nauthors provide clearer insights into the specific oxidation state transition (e.g., from which to which\nstate)? The statement “This valence reduction was correlated with electron transfer from Co to\nadsorbed C H species, facilitating rapid acetylene reduction and clearly confirming Co as the\n2 2\nprimary active site” is difficult to follow. Is this conclusion directly supported by the presented data,\nor is it primarily drawn from the literature?\nResponse to comment 12: We thank the reviewer for the valuable suggestion.\nIn the Co K-edge XANES of CoPc/NCNT, the spectrum at open-circuit potential lies between\nCo foil and CoO, indicating a Coδ+ state between metallic Co and Co(II). Under EASH-relevant\ncathodic potentials, the absorption edge shifts to lower energy and the white-line intensity decreases,\nshowing that the Co centres become partially reduced during operation. The original sentence stating\nthat “this valence reduction was correlated with electron transfer from Co to adsorbed C H species,\n2 2\nfacilitating rapid acetylene reduction and clearly confirming Co as the primary active site” was a bit\nstrong and has been revised. In the updated text, the in situ XANES data are presented as evidence\nthat the Co centres in CoPc/NCNT are electronically responsive under EASH conditions and directly\nparticipate in the electrochemical process. The idea of charge redistribution between metal sites and\nadsorbed C H is guided by the Cu-based EASH study in Nat. Commun. 12, 7072 (2021), where\n2 2\nC H adsorption was shown to change the apparent oxidation state of Cu owing to electron transfer\n2 2\nbetween Cu and the antibonding π orbital of *C H . In our case, the conclusion that Co provides the\n2 2\nmain active sites is based on the negligible EASH activity of the bare NCNT supports, the\npronounced increase in activity after introducing CoPc, and the potential-dependent changes\nobserved at the Co K-edge under working conditions, interpreted together with this literature\nprecedent, rather than on the XANES data alone.\nWe added the above discussions to the revised manuscript (Page 15).\nPage 15: “The spectrum at open-circuit potential resembles that of molecular CoPc and lies between\nCo foil and CoO, indicating a Coδ⁺ state between metallic Co and Co(II). As the applied potential\nbecomes more negative, the absorption edge shifts progressively to lower energy and the white-line\nintensity decreases (Fig. 4a), showing that the Co centres become partially reduced during operation.\nTogether with the negligible EASH activity of bare NCNT and the pronounced activity enhancement\nafter introducing CoPc, it is safe to conclude that CoPc-derived Co centres serve as the primary\nactive sites in CoPc/NCNT8.”\nComment 13: The peak assignments of the in situ FTIR data in Figure 4b do not align with the cited\nreference. The authors assign the bands at 1650 and 1559 cm-1 to the *C≡C bond of C H and the\n2 2\ngenerated *C=C bond adsorbed on CoPc/NCNT, respectively. However, according to the reference,\nadsorbed *C H species can be detected at 1641 cm-1 over ED–Cu NPs, while the band at 1577 cm-\n2 2\n1 corresponds to *C H , the key intermediate of C H hydrogenation. Therefore, a more consistent\n2 3 2 2\nassignment according to that reference would be 1650 cm-1 to *C H and 1559 cm-1 to *C H . This\n2 2 2 3\ninterpretation would also better match the observed peak evolution, where the 1650 cm-1 band\nincreases at higher overpotentials. Are these overpotential-dependent changes correlated with\nproduct selectivity? Furthermore, do the relative intensity changes of these two peaks align with the\nexpected reaction pathway? The authors should also examine whether additional informative\nspectral regions around 2000 cm-1 are present in the FTIR spectra.\nResponse to comment 13: We appreciate the reviewer for this valuable comment.\nIn light of the cited reference and the reviewer’s comment, the bands at 1650 and 1559 cm−1 in\nFigure 4b have been reassigned to adsorbed *C H and *C H , respectively (Fig. R11). This\n2 2 2 3\nassignment is consistent with the reported values for *C H (~1640 cm−1) and *C H (~1570–1580\n2 2 2 3\ncm−1) in the referenced work (Nat. Sustain. 6, 827-837 (2023)), and with recent in situ FTIR data\nfor EASH reported in Angew. Chem. Int. Ed. 64, e202423381 (2025).\nRegarding the potential-dependent evolution of these bands, we note that the in situ FTIR\nmeasurements were performed in an H-type cell, where C H reaches the catalyst surface mainly via\n2 2\ndissolution and diffusion in the liquid phase. Under these conditions, increasing cathodic\noverpotential enhances the accumulation and adsorption of C H at the electrode–electrolyte\n2 2\ninterface, resulting in the growth of the 1650 cm−1 (*C H ) band at more negative potentials. The\n2 2\n1559 cm−1 band, assigned to *C H , appears at intermediate potentials and then gradually increases,\n2 2\nwhich is consistent with *C H → *C H → C H as the main hydrogenation sequence. In the flow\n2 2 2 3 2 4\ncell used for performance testing, gas-phase feed and mass transport conditions are different, and\nthe dependence of product selectivity on potential is less pronounced.\nThe relative intensity changes of the two bands are in line with the expected reaction pathway.\nAt low overpotentials, the *C H band is weak, and the *C H band is barely detectable, indicating\n2 2 2 3\nlimited activation of C H . With increasing overpotential, the *C H band at 1650 cm−1 increases,\n2 2 2 2\nand the *C H band at 1559 cm−1 grows and then tends towards saturation, which is consistent with\n2 3\n*C H acting as a transient intermediate that is continuously converted to C H . This behaviour\n2 2 2 4\nagrees with the intermediate evolution reported in Angew. Chem. Int. Ed. 64, e202423381 (2025),\nand supports our proposed EASH mechanism as well.\nFinally, the FTIR spectra were re-examined in the region around 2000 cm−1. No additional well-\nresolved bands attributable to C≡C stretching or metal–hydride species were observed in this range;\nthe signal is essentially featureless apart from background contributions. For this reason, this region\nis not shown in the main figure, and it does not alter the assignments or mechanistic interpretation\nbased on the 1650 and 1559 cm−1 bands.\nThe related contents have been included in the revised manuscript (Page 15).\nPage 15: “Subsequently, the EASH pathway on CoPc/NCNT and the associated interfacial water\nstructure were investigated using potential-dependent in situ attenuated total reflectance Fourier\ntransform infrared (ATR-FTIR) spectroscopy. In the O–H stretching region, the broad band between\n2900 and 3700 cm−1 can be deconvoluted into contributions from 4-coordinated hydrogen-bonded\nwater (4-HB·H O), 2-coordinated hydrogen-bonded water (2-HB·H O), and K+-hydrated water\n2 2\n(K·H O). With increasingly negative potentials, the intensity of the 4-HB·H O component decreases,\n2 2\nwhile that of 2-HB·H O increases, suggesting that the interfacial hydrogen-bond network becomes\n2\nmore weakly bound and that water dissociation is facilitated45, thereby increasing the availability of\n*H for the semi-hydrogenation of acetylene. In the lower-wavenumber region, positive bands at 1650\nand 1559 cm−1 can be assigned to adsorbed *C H and the generated *C H intermediate on\n2 2 2 3\nCoPc/NCNT, respectively. The intensity of the band corresponding to *C H grows continuously\n2 4\nwith potential, consistent with the high ethylene selectivity observed on CoPc/NCNT.”\nFig. R11 | In situ electrochemical FT-IR spectra of CoPc/NCNT electrode for acetylene semi-\nhydrogenation in acetylene-saturated 1 M KOH solution.\nComment 14: On page 13, the discussion of pH variation and radical scavenger experiments is too\nbrief and the conclusions are drawn too quickly. The pH was only varied slightly (from ~13.7 to\n~14.3), which corresponds to a relatively small change in OH− concentration. The authors should\nalso specify which concentrations of KOH corresponds to which pH. In addition, when introducing\nt-BuOH as a radical scavenger, the exact concentration used (50 mM) should be reported in the main\ntext. This is a relatively high concentration, and it should be clarified whether the observed effect\ncan be attributed specifically to radical scavenging or to other interactions with the system.\nResponse to comment 14: We acknowledge the reviewer’s constructive comments and suggestions.\nIn the revised manuscript, we specify the KOH concentrations and corresponding pH values\nand extend the pH window. The measurements were carried out in 0.5, 1.0, 2.0, and 5.0 M KOH (pH\n≈ 13.7, 14.0, 14.3, and 14.7, respectively; see Fig. R12). Across this wider range of OH−\nconcentration, the polarization curves and FE for C H on CoPc/NCNT change only marginally,\n2 4\neven though the OH− concentration varies by almost two orders of magnitude. This behaviour is\ndifficult to reconcile with a dominant electron–proton coupled pathway of the form *C H + H O +\nx y 2\ne− → *C H + OH−, for which a pronounced pH dependence would be expected. In the text, we\nx y+1\nnow avoid the wording “effectively ruling out” and state more cautiously that an electron–proton\ncoupled mechanism is unlikely to be the main pathway under our conditions.\nFor the radical scavenger experiment, we now explicitly report in the main text that t-BuOH\nwas added at a concentration of 50 mM. Although this is a noticeable amount, it is still below the\n0.1–1.0 M range commonly used for alcohol-based *H scavengers in related electrochemical studies\n(Adv. Mater. e15346 (2025); Adv. Energy Mater. 15, e03613 (2025); ACS Nano 19, 8189−8199\n(2025); Angew. Chem. Int. Ed. e202518122 (2025)). To further address the reviewer’s concern, we\nperformed additional measurements at a lower t-BuOH concentration of 20 mM (Fig. R13). After\nadding 20 mM or 50 mM t-BuOH, both the current density and the C H conversion decrease\n2 2\nmarkedly. This consistent suppression, together with the established role of t-BuOH as a *H\nscavenger, supports our assignment that the observed effect mainly arises from quenching of *H\nintermediates rather than from unspecific interactions of t-BuOH with the system.\nWe added the above results and discussions to the revised manuscript and supplementary\ninformation (Pages 15, Page S20, Supplementary Fig. S37-S39).\nPage 15: “If the rate-determining step followed a proton-coupled electron transfer pathway of the\nform *C H + H O + e- → *C H + OH−, the EASH activity would display a pronounced\nx y 2 x y+1\nsensitivity to pH or OH− concentration. The polarization curves and the C H FE of CoPc/NCNT\n2 4\nwere then recorded in KOH solutions ranging from 0.5 to 5.0 M (pH≈13.7 to 14.7). With increasing\npH, the polarization current density increases gradually, which can be mainly attributed to improved\nelectrolyte conductivity and mass transport (Supplementary Fig. S37). In contrast, the onset\npotentials and the FE for C H show no significant changes across this one-order-of-magnitude\n2 4\nvariation in OH− concentration (Supplementary Fig. S38). Such behaviour is not typical of a step\ncontrolled by proton-coupled electron transfer, and suggests that the key hydrogenation step does\nnot directly depend on the bulk proton or hydroxide activity under the present conditions. The role\nof *H intermediates was further probed by adding tert-butanol (t-BuOH), a radical scavenger that\ndeactivates *H via 2-methyl-2-propanol formation. Upon addition of t-BuOH, the polarization\ncurrent density and C H conversion decrease markedly (Supplementary Fig. S39). The combination\n2 2\nof a weak pH dependence and a strong sensitivity to *H scavenging indicates that surface-bound *H,\nrather than a direct proton-coupled electron-transfer step, serves as the key reactive intermediate in\nthe electrocatalytic hydrogenation of C H on CoPc/NCNT46.”\n2 2\nFig. R12 | The effect of the KOH concentration on the (a) LSV curves and (b) Faraday efficiency\nof the obtained C H over CoPc/NCNT.\n2 4\nFig. R13 | (a) LSV curves and (b) C H conversion over CoPc/NCNT in 1.0 M KOH solution with\n2 2\nand without 20 mM t-BuOH.\nComment 15: The reported EASH reaction kinetics follow the trend K+ > Na+ > TMA+, with C H\n2 2\nconversion decreasing from K+ to 10.4% TMA+. How does this trend compare with previous\nliterature? Have larger or smaller cations such as Cs+ or Li+ been tested? It is worth noting that\nmolecular catalysts often exhibit very different cation-dependent effects compared to metallic\ncatalysts (see Yu, S., Yamauchi, H., Wang, S. et al. Nat. Catal. 7, 1000–1009 (2024)).\nResponse to comment 15: We appreciate the reviewer’s valuable comment.\nIn the original submission, we reported a cation dependence of K⁺ > Na⁺ > TMA⁺, with C H\n2 2\nconversion dropping to 10.4% in the presence of TMA⁺. In the revised version, we have added\nmeasurements in LiOH; the overall trend now becomes K⁺ > Na⁺ > Li⁺ > TMA⁺ in both current\ndensity and C H conversion. This sequence is consistent with the cation effect reported for\n2 2\nelectrocatalytic acetylene semi-hydrogenation in Nat. Commun. 14, 8384 (2023), where K⁺ is also\nthe most promoting alkali cation, and large organic cations strongly suppress the reaction. The sub-\nseries K⁺ > Na⁺ > Li⁺ further agrees with the trend Cs⁺ > K⁺ > Na⁺ > Li⁺ observed by Yu et al for\nmolecular CO -to-CO electrocatalysts (Nat. Catal. 7, 1000–1009 (2024)), which we now explicitly\n2\ncite in the revised manuscript. These trends can be understood in terms of ionic radius, hydration,\nand interfacial accumulation: moderately hydrated K⁺ can approach the inner Helmholtz plane and\nmore effectively modulate the local electric field and water structure, whereas the more strongly\nhydrated Na⁺ and Li⁺ remain further from the interface, and the bulky TMA⁺ cation perturbs the\ninterfacial region and severely suppresses EASH activity.\nWe added the above results and discussions to the revised manuscript (Page 16) and\nSupplementary Information (Page S21).\nPage 16: “Large hydration numbers and effective ionic radii are expected to weaken specific\ninteractions between cations and Co active sites. This effect was examined by substituting compact\nK+(H O) (n=7, r=3.3 Å) and Na+(H O) (n=13, r=3.6 Å) with more strongly hydrated Li+(H O)\n2 7 2 13 2 n\n(larger hydration shell, r=3.8 Å) and bulky TMA+ (n=18, r=4.5 Å) in 1 M KOH electrolytes\n(Supplementary Fig. S40). Under these conditions, the EASH kinetics follow the sequence K+ >\nNa+ > Li+ > TMA+, with C H conversion decreasing from 30.3% (K+) to 10.4% (TMA+). This trend\n2 2\ncan be attributed to the ability of the cations to perturb and disrupt the interfacial hydration shell.\nNamely, the relatively compact K⁺(H O) cluster likely polarises interfacial H O networks more\n2 7 2\neffectively than more strongly hydrated or bulkier cations, thereby facilitating water dissociation\nand the formation of reactive *H species on CoPc/NCNT.”\nFig. R14 | (a) LSV curves and (b) C H conversion in 100% C H flow over CoPc/NCNT with 1 M\n2 2 2 2\nKOH, NaOH, LiOH, and TMAH electrolyte.\nComment 16: More details about the gas feed are required. Was the acetylene introduced in ethylene\nor in argon? More details about the gas feed mixing is required. Additional clarification is also\nneeded on the 1 cm2 and 25 cm2 flow cell setups, how exactly are they configured and operated? In\nthe text, the authors state: “To satisfy the requirements for the industrial production of polymer-\ngrade ethylene, the EASH performance was subsequently assessed under realistic conditions with\n0.5% acetylene impurities (5000 ppm) in crude ethylene (Figure S33).” However, Figure S33 shows\nLSV curves of different gas concentrations.\nResponse to comment 16: We thank the reviewer for this important question and for the opportunity\nto clarify the experimental details.\nIn the 1 cm2 flow cell, the inlet gas was 100% acetylene, used to evaluate the intrinsic catalytic\nactivity toward acetylene semi-hydrogenation. Since the industrial scenario typically involves\nremoving trace acetylene impurities from ethylene feed gas, we further examined the performance\nunder different acetylene concentrations (original Fig. S33). These various concentrations were\ngenerated using a custom-built gas-mixing system, in which high-purity argon was used to dilute\npure acetylene to the desired ratios.\nBecause acetylene is only sparingly soluble in the electrolyte, the semi-hydrogenation activity\ndecreases significantly at lower acetylene concentrations, as shown in Fig. S33. To better simulate\nindustrial conditions and validate the scalability, we designed a 25 cm2 large-area flow cell, which\nwas tested using a commercially purchased mixed gas (0.5% C H , 20% C H , balanced with Ar)\n2 2 2 4\nrepresenting a realistic crude ethylene stream. Under these conditions, the CoPc/NCNT catalyst\nexhibited excellent stability over 110 hours of continuous operation, demonstrating strong potential\nfor industrial application.\nFurthermore, we constructed a two-stage reactor system by coupling the 1 cm2 and 25 cm2 flow\ncells in series. The small cell was used to pre-convert pure acetylene to a low-acetylene mixture,\nwhich was then fed into the large-area cell to produce high-purity ethylene directly. This\nconfiguration highlights the practical relevance and scalability of our catalyst system.\nAll corresponding descriptions have been added to the revised manuscript for clarity (Methods,\nPage 22).\nComment 17: The Methods section requires substantial revision, as both the main text and the\nSupporting Information currently provide too few details to ensure reproducibility. The experimental\nprocedures should be described comprehensively, including complete information on catalyst\nsynthesis and functionalization (precursors, solvents, conditions, purification), electrode preparation\nand loading (mass loading, electrode area, binder composition, deposition procedure), and\nelectrochemical testing protocols (cell configuration, electrolyte composition and preparation, gas\nfeed composition and flow rates, reference and counter electrode details, iR compensation). In\naddition, the in situ and operando experiments (XANES, FTIR, EPR, DEMS) must be described in\nsufficient detail, including setups, acquisition parameters, and data analysis methods.\nCalibration and quantification procedures (e.g., GC, ICP-OES) should also be reported, together with\nerror analysis and reproducibility tests. Without this level of detail, the results cannot be reliably\nreproduced, and the section must therefore be carefully revised and expanded to meet the standards\nof Nature Communications.\nResponse to comment 17: We sincerely thank the reviewer for emphasizing the importance of\nmethodological transparency and reproducibility. We have thoroughly revised and expanded the\nMethods section in the revised manuscript to provide comprehensive experimental details that fully\nmeet Nature Communications standards. The revisions are briefly summarized as follows.\nCatalyst synthesis and functionalization: We have added full descriptions of precursor\nidentities, solvent types, reaction concentrations, temperatures, times, and purification steps. The\nprocedures for CNT functionalization and CoPc immobilization are now described step-by-step to\nensure reproducibility.\nElectrode preparation: We have included detailed information on ink formulation (binder\ntype and ratio, solvent, concentration), catalyst mass loading, electrode area, and drop-casting\nprocedure, along with the drying conditions.\nElectrochemical testing: We have provided complete descriptions of the cell configuration\n(three-electrode setup, electrode arrangement, gas-diffusion electrode use), electrolyte composition\nand pretreatment, gas flow rate and composition, reference and counter electrode specifications, and\nclarified that all potentials are reported without iR compensation.\nIn situ and operando characterizations (XANES, FTIR, EPR, DEMS): We have expanded\nthe corresponding subsections with detailed information on the experimental setups, detection\nmodes, data acquisition parameters, and data processing procedures.\nQuantification and calibration: We have added descriptions of GC calibration using standard\ngases, ICP-OES calibration curves, and error ranges, and reproducibility tests based on three\nindependent synthesis batches.\nAll newly added details have been highlighted in the revised manuscript (Methods, Pages 21-\n25). We appreciate the reviewer’s comment, which has significantly improved the completeness,\nrigor, and reproducibility of our experimental section.\nComment 18: Minor comments:\n-Page 5, ESAE should be EASH.\n-The authors should write the minus sign for the current densities (I assume negative currents were\napplied).\n-Hard to distinguish the colors (such as blue and green) in many of the plots.\nResponse to comment 18: We thank the reviewer for these careful observations. The typo on page\n5 has been corrected from ESAE to EASH. The minus signs for current densities have been added\nthroughout the manuscript and the Supporting Information to accurately indicate the applied\ncathodic currents. In addition, the color schemes of all relevant plots have been adjusted to improve\ncontrast and readability, particularly for the blue and green traces.\nResponse to Reviewer #2:\nGeneral Comments:\nIn this work, the authors report a molecular cobalt phthalocyanine (CoPc) catalyst platform that\npromotes electrocatalytic acetylene semi-hydrogenation (EASH) reaction activity through electronic\nregulation. They also investigate the performance modulation strategies and reaction mechanisms\nof the corresponding molecular catalysts. The utilization of analogous molecular catalysts (CoPc\nand CuPc) for EASH catalysis have been previously reported [10.1016/j.cej.2021.134129].\nMoreover, the authors' research and conclusions on the performance enhancement mechanism of\nCoPc/XCNT are not sufficiently clear. Therefore, I do not recommend publishing this work in Nature\nCommunications.\nResponse: We thank the reviewer for the overall assessment and for carefully reading our\nmanuscript.\nWe fully acknowledge that CoPc- and CuPc-based molecular catalysts for EASH have been\nreported previously (Chem. Eng. J. 431, 134129 (2022)), and we agree that the original version did\nnot clearly enough articulate how our work goes beyond these studies in both concept and\nperformance. In the revised version, we emphasise that our contribution is not the use of CoPc or\nCuPc per se, but the establishment of a comparative MPc/XCNT platform and a coordination-\nengineering strategy that together link water dissociation, *H management, and C–C coupling:\n1. MPc/OCNT platform and water-dissociation–C–C coupling relationship\nInstead of focusing on one CoPc system, we construct a comparative MPc/OCNT platform (M\n= Cu, Co, Ni, Fe) under identical conditions. Within this platform, CoPc/OCNT is identified as the\ncomposition that combines faster water dissociation with a more suitable adsorption environment\nfor key intermediates, leading to strongly suppressed C–C coupling compared with CuPc/OCNT.\nThe same “water-dissociation-assisted suppression” of C–C coupling is also observed on\nNiPc/OCNT and FePc/OCNT, for which the C H Faradaic efficiency remains below 0.15%.\n4 6\nHowever, the side reactions differ: NiPc/OCNT suffers from severe HER, while FePc/OCNT is\nlimited by pronounced over-hydrogenation to C H . These comparisons indicate that accelerating\n2 6\nwater dissociation is a general lever to mitigate C–C coupling within the MPc/OCNT family, but the\nmetal centre determines whether the generated *H is channelled into selective semi-hydrogenation,\nHER, or over-hydrogenation.\n2. Axial coordination engineering on CoPc and CuPc for high-current EASH\nWhile CoPc/OCNT already suppresses C formation, it suffers from increasing HER at current\n4\ndensities above ~200 mA cm−2. Guided by the mechanistic insight from the MPc/OCNT series, we\nintroduce axial coordination control by immobilising CoPc on nitrogen- and sulfur-functionalised\nCNTs to obtain CoPc/NCNT and CoPc/SCNT. This allows us to tune water-dissociation kinetics and\n*H utilisation at the Co centre. Among these catalysts, CoPc/NCNT exhibits the fastest water\ndissociation and the most favourable *H utilisation, which alleviates HER at high current densities\nand completely suppresses detectable C–C coupling, with no C products observed over the entire\n4\ntested current range. To test whether this concept is general, we also synthesised CuPc/NCNT.\nCuPc/NCNT shows enhanced water-activation behaviour compared with CuPc/OCNT, and at 100\nmA cm−2 the C H Faradaic efficiency decreases from 1.40% on CuPc/OCNT to 0.80% on\n4 6\nCuPc/NCNT. This marked reduction in C formation confirms that axial N-coordination can also\n4\nmitigate C–C coupling on CuPc, indicating that the coordination strategy is transferable within the\nMPc framework rather than being specific to CoPc.\n3. Quantitative performance benchmark against the prior CoPc system\nUsing the TOF definition from Nat. Sustain. 6, 827–837 (2023) and updated ICP-OES and gas-\nflow data, CoPc/NCNT reaches a TOF of 7019 min−1 at 500 mA cm−2, far exceeding the performance\nof the homogeneous CoPc electrode (TOF = 0.29 min−1 at 100 mA cm−2) reported in Chem. Eng. J.,\neven though ~11 times less CoPc is used in the current work. Under crude-ethylene-like feeds,\nCoPc/NCNT maintains >98% acetylene conversion and >95% C H selectivity for 110 h. We now\n2 4\npresent these numbers explicitly as a direct comparison, and we have softened overly strong wording,\npositioning our data as a substantial step toward industrially relevant EASH rather than as an\nabsolute record.\nWe appreciate the reviewer’s concerns and, in response to the detailed comments, have added\nNi/Fe/Cu controls, extended stability data, and refined mechanistic analysis. We hope that the\nrevised manuscript makes it clearer that our work uses the MPc/XCNT platform and axial\ncoordination engineering to establish and exploit a general link between water dissociation, C–C\ncoupling, and EASH performance, rather than simply combining known CoPc/OCNT and\nCuPc/OCNT motifs.\nSpecific Comments:\nComment 1: Apart from the already reported feasibility of metal phthalocyanines in the EASH\nprocess, the employed CoPc/OCNT and CuPc/OCNT are also widely used in various electrocatalytic\nreactions including CO RR, ORR, HER, and etc. (Angew. Chem. Int. Ed. 2024, 63, e202310623;\ntogether, the present work is lack of innovation to be published in Nature Communications.\nResponse to comment 1: We thank the reviewer for this important comment.\nWe fully agree that metal phthalocyanines, including CoPc and CuPc, are now a well-\nestablished molecular platform in electrocatalysis, with extensive work in EASH, CO RR, ORR,\nand have re-positioned our contribution accordingly.\nOur intention is not to claim novelty in using CoPc or CuPc per se, but rather in how we use a\nfamily of MPc/OCNT catalysts as a mechanistic platform and then translate the insight into a\ntargeted coordination-engineering strategy. In particular:\n1. From a single CoPc catalyst to a comparative MPc/OCNT study of water dissociation and\nC–C coupling\nThe previous EASH work demonstrated that CoPc and CuPc are feasible molecular catalysts.\nBuilding on this, we intentionally construct a broader MPc/OCNT series (M = Cu, Co, Ni, Fe) and\nuse it as a controlled platform to dissect how different metal centres regulate water dissociation, *H\ncoverage, and C–C coupling under the same support and morphology. Within this series, we find\nthat CoPc/OCNT balances fast water activation with moderated *H binding, thereby suppressing C\n4\nformation relative to CuPc/OCNT. We then show that the same water-dissociation-assisted\nsuppression of C formation appears on NiPc/OCNT and FePc/OCNT, where the C H FE remains\n4 4 6\nbelow 0.15%. However, NiPc/OCNT is dominated by HER, and FePc/OCNT suffers from strong\nover-hydrogenation to C H . These comparisons reveal that enhancing water dissociation is a\n2 6\ngeneral way to mitigate C–C coupling and improve EASH activity within the MPc/OCNT family,\nwhile the specific metal centre determines how the generated *H is distributed among EASH, HER,\nand over-hydrogenation. We regard this “water dissociation–C–C coupling” relationship, established\nacross a controlled MPc/OCNT platform, as going beyond simply showing that CoPc/OCNT or\nCuPc/OCNT can catalyse EASH.\n2. Axial coordination engineering on CoPc and CuPc for highly selective EASH\nGuided by the above findings, we deliberately modify the molecular environment via axial\ncoordination on amino- and thiol-functionalised CNTs, generating CoPc/NCNT and CoPc/SCNT.\nThis tuning adjusts the Co electronic structure, water-dissociation kinetics, and *H binding, and leads\nto CoPc/NCNT as an optimised composition that combines rapid water dissociation with controlled\n*H coverage, thereby easing HER at high current densities and completely suppressing detectable\nC–C coupling (no C products observed under our conditions). The optimised CoPc/NCNT delivers\n4\n86.7% C H FE and 99.99% C H selectivity at 500 mA cm−2, complete suppression of C products,\n2 4 2 4 4\na TOF of 7019 min−1 with ~11-fold lower CoPc usage than the Chem. Eng. J. system, and 110 h\nstability under crude-ethylene-like feed. To test generality on Cu, we additionally synthesised\nCuPc/NCNT. Compared with CuPc/OCNT, CuPc/NCNT shows enhanced water-activation\nbehaviour, and at 100 mA cm−2 the C H FE decreases from 1.40% on CuPc/OCNT to 0.80% on\n4 6\nCuPc/NCNT. This clear drop in C H formation demonstrates that axial N-coordination on CuPc\n4 6\nalso suppresses C–C coupling, indicating that the coordination strategy derived from the Co platform\nis transferable to other metal–phthalocyanine catalysts.\nIn light of the broader MPc literature, we have revised the Introduction and Discussion to\nclearly state that the main contribution of this work lies in (i) using a comparative MPc/OCNT\nplatform to connect metal-centre identity, interfacial water activation, and C–C coupling in EASH,\nand (ii) translating this mechanistic understanding into an axial coordination strategy (with both N-\nand S-functionalised CNTs, and applied to both CoPc and CuPc) that delivers highly selective EASH\nat high current densities. We hope this makes it clearer that our study goes beyond a simple\nrecombination of previously reported CoPc/OCNT and CuPc/OCNT motifs. And the work on\nMPc/OCNT has been cited in the introduction of our revised manuscript to highlight our\ncontributions.\nComment 2: Except for the novelty issue, the data and analysis is not adequately to support their\nhypothesis and conclusions. For example, to exclude the CPET pathway, the author claimed that\n“the pH was varied from ~13.7 to ~14.3, negligible changes were observed in the polarization curves\nand FE for C\n4\non CoPc/NCNT”, which is contradictory to the data shown in Figure S27.\nResponse to comment 2: We acknowledge the reviewer’s kind comments and suggestions.\nFor CPET-type EASH, Nat. Commun. 12, 7072 (2021) has shown that lowering pH (increasing\nproton availability) typically leads to a clear increase in C H Faradaic efficiency (Fig. R15), while\n2 4\nthe overall current density decreases due to lower KOH concentration (reduced conductivity and\nslower mass transport). This behaviour (monotonic enhancement of C H selectivity with decreasing\n2 4\npH, accompanied by a drop in limiting current) is a characteristic signature of a proton-coupled\nelectron transfer process. In contrast, non-CPET behaviour has been systematically discussed in Nat.\nCommun. 14, 8384 (2023), where the C H selectivity shows no systematic dependence on pH,\n2 4\nwhile the LSV limiting current density still decreases at lower KOH concentration because of\nincreased resistance and mass-transport limitations (Fig. R16). In other words, in a non-CPET\nregime, the pH mainly affects the solution environment (conductivity, diffusion) rather than the\nintrinsic surface kinetics of the rate-determining step.\nOur updated pH-dependent measurements on CoPc/NCNT are consistent with this second\nscenario. As shown in the revised Fig. R17, decreasing KOH concentration (pH from 14.3 to 13.7)\nleads to a gradual decrease in the limiting current density in the LSV curves, which we now explicitly\nattribute to increased ohmic and mass-transfer limitations. However, the C H Faradaic efficiency\n2 4\nin this pH window does not exhibit a clear monotonic trend and fluctuates within the experimental\nuncertainty, rather than increasing systematically at lower pH as would be expected for a CPET-\ncontrolled process. We have extended the pH range to 13.7–14.7 and still do not observe a clear\ncorrelation between pH and C H selectivity. Together with the literature benchmarks, these revised\n2 4\ndata and analyses support our conclusion that the rate-determining step on CoPc/NCNT does not\nfollow a classical CPET pathway in the studied pH window.\nAll updated pH-dependent data and detailed discussion have been included in the revised\nmanuscript (Page 15) and Supplementary Information (Page S20, Supplementary Fig. S37-S38).\nPage 15: “If the rate-determining step followed a proton-coupled electron transfer pathway of the\nform *C H + H O + e- → *C H + OH−, the EASH activity would display a pronounced\nx y 2 x y+1\nsensitivity to pH or OH− concentration. The polarization curves and the C H FE of CoPc/NCNT\n2 4\nwere then recorded in KOH solutions ranging from 0.5 to 5.0 M (pH≈13.7 to 14.7). With increasing\npH, the polarization current density increases gradually, which can be mainly attributed to improved\nelectrolyte conductivity and mass transport (Supplementary Fig. S37)8. In contrast, the onset\npotentials and the FE for C H show no significant changes across this one-order-of-magnitude\n2 4\nvariation in OH− concentration (Supplementary Fig. S38). Such behaviour is not typical of a step\ncontrolled by proton-coupled electron transfer, and suggests that the key hydrogenation step does\nnot directly depend on the bulk proton or hydroxide activity under the present conditions.”\nFig. R15 | The EASH performance of the Cu/GDL-CP under different KOH concentrations.\nFig. R16 | The effect of the KOH concentration on the (a) LSV curves and (b) FE over Cu-F.\nFig. R17 | The effect of the KOH concentration on the (a) LSV curves and (b) Faraday efficiency of\nthe obtained products over CoPc/NCNT.\nComment 3: In Page 13, the authors claimed that a hydrogen transfer mechanism could bypass the\nHER competitive, which is groundless. It has been widely reported that H could also obtained by\n2\nthe combination of two water-derived *H, called Tafel step (J. Phys. Chem. C, 2018, 122, 23943.).\nThus, the real reason for the improved C H selectivity is not the claimed mechanism, which needs\n2 4\nfurther discussion.\nResponse to comment 3: We thank the reviewer for this important comment and fully agree that\nour original wording was misleading.\nAs correctly pointed out, in alkaline media H can be formed not only via a Heyrovsky-type\n2\nelectrochemical step, but also via the Tafel recombination of two water-derived *H species, as widely\nreported in the literature (e.g., J. Phys. Chem. C 122, 23943 (2018)). Our intention was not to claim\nthat HER is completely bypassed, but rather that under low to intermediate current densities the\nwater-derived *H is preferentially channelled into the hydrogenation of *C H , thereby mitigating\n2 2\nthe competitive Heyrovsky-type HER pathway. Indeed, as shown in Fig. R18, the Faradaic\nefficiency of H₂ remains below 5% at low current densities, while at 500 mA cm−2 it increases to\n13.3%. This rise in H FE at high current density is consistent with accelerated water dissociation\n2\nand a high surface coverage of *H, which favours H–H recombination via the Tafel step. Thus, even\nthough the Heyrovsky route can be effectively suppressed in the presence of abundant *C H ,\n2 2\nthe Tafel-type pathway remains operative when *H accumulates at very high current densities.\nTo avoid overstatement, we have revised the text on Page 16 as follows:\n“Specifically, water-derived *H intermediates are preferentially consumed via sequential hydrogen\ntransfer to adsorbed C H , which mitigates competitive HER pathways (in particular the Heyrovsky-\n2 2\ntype route) and enables selective semi-hydrogenation in the low-to-intermediate current-density\nregime.”\nFig. R18 | Faraday efficiency of the obtained products at various current densities over CoPc/NCNT.\nComment 4: The observed performance disparities between CoPC/NCNT and other samples are\npredominantly attributable to the difference in water dissociation ability. However, this seems\ninconsistent with the kinetic isotope experiment results for catalysts. The LSV or kinetic isotope\nexperiment results for catalysts under argon gas should be provided for further comparison.\nResponse to comment 4: We thank the reviewer for this helpful comment.\nIn the original manuscript, the kinetic isotope effect was evaluated under C H by replacing\n2 2\nH O with D O, and we found that the KIE (H/D) value of CoPc/NCNT is smaller than that of\n2 2\nCoPc/OCNT and CoPc/SCNT, which is consistent with a lower barrier for water dissociation and\nhence faster water-activation kinetics on CoPc/NCNT. To further support this conclusion, we have\nperformed LSV measurements under Ar. As shown in the revised Fig. R19, CoPc/NCNT exhibits a\nTafel slope of 121 mV dec−1, notably smaller than those of CoPc/OCNT (156 mV dec−1) and\nCoPc/SCNT (206 mV dec−1). In alkaline media, such a decrease in Tafel slope is commonly\nassociated with facilitated water dissociation in the Volmer step. The Ar-saturated LSV/Tafel data\ntherefore agree well with the KIE results and together confirm that CoPc/NCNT possesses the fastest\nwater-dissociation kinetics among the three catalysts.\nThese additional data and the corresponding discussion have been included in the revised\nmanuscript (Page 17) and Supplementary Information (Page S22, Supplementary Fig. S42).\nPage 17: “LSV and Tafel analysis in Ar saturated electrolyte (Fig. 4e) yields a Tafel slope of 121\nmV dec⁻1 for CoPc/NCNT, lower than those of CoPc/OCNT (156 mV dec⁻1) and CoPc/SCNT (206\nmV dec⁻1), a trend typically associated with more facile water activation in alkaline media\n(Supplementary Fig. S42).”\nFig. R19 | (a) LSV curves and (b) Tafel plots under an Ar feed in 1 cm2 flow cell.\nComment 5: The as-reported TOF value is too high to convince the reviewer. According to the data\nin the previous literature (Chem. Eng. J. 2022, 431, 134129), the bare CoPc only exhibited a TOF\nvalue of 0.29 min-1 at a current of 100 mA. How can the TOF of the as-reported CoPc/NCNT reach\n9700 min-1 at a current of 500 mA? More details are needed for the TOF calculation. Besides, the\nclaimed 24 h continuous operation under ethylene-rich simulated gas flow could not be regarded as\nan advancement compared to the recent progress (Nat. Catal. 2021, 4, 557;\n10.31635/ccschem.024.202404849; 10.1002/anie.202509501; 10.31635/ccschem.022.202101750;)\nResponse to comment 5: We thank the reviewer for carefully checking our TOF analysis and\nstability data. We agree that the very high TOF value initially reported requires clear justification,\nand we have revisited both the calculation and the long-term operation experiments in the revised\nmanuscript.\nFor the TOF calculation, we follow the definition used in Nat. Sustain. 6, 827–837 (2023):\nTOF (min-1) = amount of C2H4 molecules produced per min\namount of Co sites\nIn Chem. Eng. J. 2022, 431, 134129, bare CoPc was used at a loading of 0.2 mg cm−2 and a\ncurrent density of 100 mA cm−2. In our work, the CoPc/NCNT loading is 0.4 mg cm−2 at 500 mA\ncm−2 under otherwise comparable conditions. However, according to ICP–OES, the Co content in\nour CoPc/NCNT electrode (0.4 mg × 0.47 wt% = 0.00188 mg Co per geometric electrode) is about\n11 times lower than the CoPc amount in the Chem. Eng. J. study, while the current density is five\ntimes higher. At 500 mA cm−2, the acetylene conversion and C H selectivity are 30.4% and 99.99%,\n2 4\nrespectively. Using an inlet gas flow of 20 mL min−1 and the measured outlet flow of 16.5 mL min−1\n(corrected in the revised manuscript), and a molar gas volume of 22.4 L mol−1, the TOF is calculated\nas:\nTOF (min−1) = 16.5×0.304×0.9999/22.4 = 7019 min−1\n0.00188/58.93\nThis updated value (7019 min−1) is lower than the original 9700 min−1 due to the corrected gas-\nflow treatment and Co content, but it still reflects the combination of a much lower Co loading and\na higher geometric current density compared with the homogeneous CoPc system in Chem. Eng. J.\nAll assumptions, parameters, and intermediate steps used in this calculation have now been added\nto the Supplementary Information for transparency. Regarding stability, we agree that the 24 h\ncontinuous operation originally shown under ethylene-rich simulated gas flow is not sufficient to\nclaim an advance over the recent state-of-the-art. In response to the reviewer’s comment, we have\nextended the durability tests of CoPc/NCNT under a crude acetylene stream. The revised data show\nthat CoPc/NCNT can operate continuously for 110 h while maintaining an acetylene conversion\nabove 98% and a C H selectivity above 94% (Fig. R20). These conversion, selectivity, and stability\n2 4\ndata metrics rank among the most competitive values reported in the recent literature (Table R5).\nWe added these figures and discussions to the revised manuscript (Page 12 and 20, Fig. 5b) and\nSupplementary Information (Page S29 and S31, Supplementary Table S3 and Supplementary Note\n2).\nFig. R20 | Long-term operation of ESAE over CoPc/NCNT under ethylene-rich environment,\nmeasured using a three-electrode flow cell (25 cm2) in 1 M KOH at room temperature under\nethylene-rich simulated gas (0.5% C H , 20%C H balanced with Ar) flow (20 mL min−1).\n2 2 2 4\nTable R5 | Performance summary of recently reported acetylene semi-hydrogenation\nelectrocatalysts.\nSV (mL Selectivity\nEntry Material Flow composition TOF (min-1) Stability (h) Ref.\ngcat -1 h-1) (%)\nNat. Sustain. 6, 827-837\n(2023)\nNat. Commun. 14, 8384\n(2023)\nNat. Commun. 12, 7072\n(2021)\nNat. Commun. 12, 6574\n(2021)\nNat. Catal. 4, 557-564\n(2021)\nNat. Catal. 4, 565-574\n(2021)\nCCS Chem. 5, 200-208\n(2023)\nNat. Commun. 14, 2137\n(2023)\nNat. Commun. 14, 2137\n(2023)\nAdv. Mater. 36, 2408681\n(2024)\nAngew. Chem. Int. Ed.\ne202307848 (2023)\nCuBcpi– 200 (MEA Angew. Chem. Int. Ed.\nBuOK electrolyzer) 64, e202509501 (2025)\nCCS Chem. 7, 2844–\n15 v-Cu NCs C2H2/C2H4=1/99 / / 97.4 50\n2853 (2025)\nComment 6: In Figure 1g, comparing the d-band centers of different metals is quite uncommon and\nmeaningless. Meanwhile, in Figures 1h, i and 4k, the author employed unusual plotting methods to\npresent the reaction diagrams, which resulted in poorer data readability and a higher likelihood of\nmisinterpretation. For example, in the conventional *H generation process, it is necessary to calculate\nthe steady-state *H O and *H+*OH as the initial and final states of the water dissociation process,\n2\nas well as the corresponding transition states, while considering the *OH desorption process. It is very\nincomplete for the author to only calculate the energy barrier from *H O to *H+*OH. The generation\n2\nprocesses of H and C also face similar issues.\n2 4\nResponse to comment 6: We thank the reviewer for these constructive suggestions. We agree that\nthe original electronic descriptors and reaction profiles may not be reader-friendly, and we have\nsubstantially revised the relevant figures and text accordingly.\nFirst, regarding Figure 1g, our intention was not to compare d-band centres in an abstract way,\nbut to highlight how the electronic structure of different metal centres governs their interaction with\nacetylene and hydrogen-derived intermediates in EASH. Since the semi-hydrogenation reaction\npredominantly occurs at the metal sites, the position of the metal d-band relative to the adsorbate p/s\norbitals is a key factor in determining adsorption strength and activation behaviour. In particular,\nZhang et al. (Small 19, 2205845 (2023)) reported that the energy gap (Δε) between the metal d-band\ncentre and the π orbital of acetylene can serve as an effective descriptor for acetylene electrocatalytic\nactivity. Inspired by this, we use the d-band centre as a compact descriptor to rationalise the trends\nacross the MPc/OCNT series.\nSecond, we acknowledge that the reaction diagrams in Figures 1h, 1i, and 4k were not presented\nin a sufficiently standard format and could lead to misinterpretation. Following the reviewer’s advice,\nwe have reprocessed all DFT data and redrawn the energy profiles in a more conventional way (Fig.\nR21). For water dissociation, we now explicitly show *H O and *H + *OH as the initial and final\n2\nstates on the reaction coordinate, include the corresponding transition state, and add the subsequent\n*OH desorption step, so that the full elementary sequence *H O → *H + *OH → * + OH− is\n2\nrepresented. For H evolution, we now provide the complete pathway from *H formation to H–H\n2\ncoupling and H desorption, including all relevant intermediates and transition states. For C\n2 4\nformation, we have similarly expanded the energy profile to cover the key C–C coupling and\nsubsequent hydrogenation steps, rather than only showing a single barrier.\nThe full computational methods, adsorption-site selection, and representative adsorption\nconfigurations for all intermediates are now described in detail in the revised Methods section, and\nSupplementary Information, and the updated reaction diagrams follow the conventions used in\nrecent literature. We note that, after this more complete treatment, the overall trends and mechanistic\nconclusions remain consistent with those in the original manuscript. We are grateful to the reviewer\nfor pointing out these issues, which have helped us to improve both the rigour and the clarity of our\ntheoretical analysis and data presentation.\nPage 25: “The C H hydrogenation process to C H can be delineated by equations (9)-(11):\n2 2 2 4\n* + C H → *C H (9)\n2 2 2 2\n*C H + H+ e− → *C H (10)\n2 2 2 3\n*C H + H+ e− → *C H (11)\n2 3 2 4\nThe hydrogen combination proceeded through equations (12)-(13):\n* + 2H → 2*H (12)\n*H + *H → *H (13)\n2*C H + *C H → *C H (14)\n2 3 2 3 4 6\nWhile the water dissociation proceeded through equations (15)-(18):\n* + H O → *H O (15)\n2 2\n*H O → *H‒OH (16)\n2\n*H‒OH → *H + *OH (17)\n*H + *OH → *H + OH (18)”\nFig. R21 | (a) Free energy diagram for the *H generation and H generation process of CoPc/OCNT\n2\nand CuPc/OCNT. (b) Corresponding free energy diagram of *C H to C H . (c) Free energy diagram\n2 3 4 6\nfor *H generation and H formation over CoPc/OCNT, CoPc/NCNT, and CoPc/SCNT.\nrelevant and representative adsorption models. For single-atom catalysts, the selection of adsorption\nsites is crucial and needs to be demonstrated. In the processes of water dissociation and\nhydrogenation, it is necessary to clarify whether C and O species occupy the metal sites, and where\n*H is adsorbed.\nResponse to comment 7: We thank the reviewer for this important comment.\nIn the revised manuscript, we now provide a detailed description of our DFT methods\n(including model construction, exchange–correlation functional, k-point sampling, and convergence\ncriteria) and explicitly display representative adsorption configurations relevant to water\ndissociation and EASH. The structural models of the CoPc/XCNT catalysts are built on reported in\nrecent work (Nat. Catal. 7, 987–999 (2024); Angew. Chem. Int. Ed. 64, e202517033 (2025)), and\nare further constrained by our own XAFS, XPS, and AC-STEM analyses, which point to isolated\nCo centres with a well-defined coordination environment. The selected adsorption geometries for\nreactants and intermediates are guided by both these experimental results and previous\ncomputational studies on related metal sites (e.g. Angew. Chem. Int. Ed. 62, e202307848 (2023);\nAdv. Mater. 35, 2303818 (2023)).\nWe have also clarified how adsorption sites are chosen and how the key species are distributed\nduring water dissociation and hydrogenation. Our in situ ATR-FTIR, in situ XAFS, and control\nexperiments on the bare XCNT supports show that the carbon framework alone is essentially\ninactive and that Co is the dominant active centre, with no clear evidence for a second, highly active\nsite. Consistent with this, our DFT screening indicates that both H O and C H preferentially interact\n2 2 2\nwith the Co site (Fig. R22-R24). Along the water dissociation pathway, H O adsorbs at Co and splits\n2\nto yield *OH and *H, with Co-bound configurations being energetically favoured over adsorption on\nadjacent C or N sites. During the subsequent hydrogenation steps, *H is mainly stabilised on Co and\nreacts with Co-bound *C H (and *C H ) to form *C H , rather than involving *H adsorbed on the\n2 2 2 3 2 4\ncarbon support. These findings support the picture that the isolated Co centres act as the primary\nsites for both water activation and hydrocarbon hydrogenation, while O-containing species do not\npermanently block the Co sites under the relevant potentials.\nThe expanded computational methodology and representative adsorption models are now\nincluded in the revised manuscript (Page 25) and supplementary information (Supplementary Figs.\nS3, S4, and S44).\nFig. R22 | Theoretical adsorption models H O dissociation and H generation over the surface of\n2 2\nCoPc/OCNT.\nFig. R23 | Theoretical adsorption models of *C H to C H over the surface of CoPc/OCNT.\n2 3 4 6\nFig. R24 | Theoretical adsorption models of the hydrogenation process of C H over the surface of\n2 2\nCoPc/NCNT.\nResponse to Reviewer #3:\nGeneral Comments:\nIn this work, the authors report a CoPc-OCNT catalyst towards EASH, achieving FE of 86.7% under\n500 mA cm-2. The manuscript attributes the enhanced activity to the excellent hydrogenation ability\nof CoPc, but similar arguments were already reported in prior studies (Chem. Eng. J. 2022, 431: 134129).\nTherefore, the reviewer considers the current form of this work lacks of novelty and not suitable for\npublication in Nature Communications.\nResponse: Thank the reviewer for these comments and valuable suggestions, which are helpful for\nus to improve the manuscript quality.\nWe fully agree that the work of Liu et al. (Chem. Eng. J. 2022, 431, 134129) represents an\nimportant milestone, as it first identified CoPc as an efficient EASH catalyst and attributed its\nperformance to favourable acetylene adsorption/hydrogenation behaviour. At the same time, several\nkey challenges remain: CoPc was used as a homogeneous molecular catalyst at relatively high\nloading, which increases catalyst cost; the activity and durability were limited (TOF of 0.29 min−1\nat −100 mA cm−2 and stability of ~40 h); and the performance was demonstrated only at\ncomparatively low current densities. These factors constrain the practical applicability of that system\nfor industrial ethylene purification and motivate the search for catalysts that deliver substantive\nimprovements under more realistic operating conditions. CoPc itself, however, offers the advantage\nof a well-defined coordination environment, which is highly attractive for precise structural design\nand mechanistic analysis.\nIn the revised manuscript, we clarify that our study is designed to address these open issues and\nto go beyond the Chem. Eng. J. work along three main lines:\n1. Comparative MPc/OCNT platform and Co vs Cu mechanistic insight\nDuring our preliminary exploration of EASH catalysts, we found that Co-based systems can\nmatch or even surpass the performance of Cu-based analogues, which contrasts with the current\nliterature where roughly 60–70% of reported EASH studies focus on Cu. This discrepancy motivated\nus to construct a CNT-supported metal phthalocyanine platform, MPc/OCNT (M = Fe, Ni, Cu, Co),\nto compare their behaviour under strictly identical conditions. Within this series, CoPc/OCNT\nexhibits faster water dissociation and a more favourable adsorption environment for key\nintermediates, which correlates with a markedly lower tendency towards C–C coupling. This model\nplatform allows us to disentangle how different metal centres regulate water dissociation, *H\nmanagement and C–C coupling, rather than attributing the performance difference simply to the\nintrinsic hydrogenation ability of CoPc.\n2. Water dissociation and C–C coupling across different metals\nTo examine whether enhanced water dissociation can generally mitigate C–C coupling, we\nsynthesised NiPc/OCNT and FePc/OCNT with the same structural motif. At 100 mA cm−2, the C H\n4 6\nFaradaic efficiencies of NiPc/OCNT and FePc/OCNT are both below 0.15%, showing that strong\npromotion of water dissociation indeed suppresses C formation within this molecular platform. This\n4\nextends the CoPc concept to other metal centres and provides a mechanistic link between interfacial\nwater activation and C–C coupling behaviour, which was not explored in the Chem. Eng. J. study.\n3. Axial coordination engineering of CoPc for industrially relevant performance\nWhile CoPc/OCNT already suppresses C–C coupling, it still suffers from pronounced HER at\ncurrent densities above 200 mA cm−2, which is unfavourable for industrial operation. Guided by the\nmechanistic insights from the MPc/OCNT series, we introduced axial coordination control by\nimmobilising CoPc on amino- and thiol-functionalised CNTs (NCNT and SCNT). This strategy\nallows us to tune water dissociation kinetics and *H utilisation more precisely. Among these catalysts,\nCoPc/NCNT exhibits faster water dissociation and more efficient *H use, strongly suppressing HER\nat high current densities. As a result, CoPc/NCNT achieves a C H Faradaic efficiency of 86.7% at\n2 4\n−500 mA cm−2 with 99.99% C H selectivity, complete suppression of C products, a turnover\n2 4 4\nfrequency of 7019 min−1 (about five orders of magnitude higher than the 0.29 min−1 at −100 mA\ncm−2 reported in Chem. Eng. J.), an eleven-fold lower CoPc usage, and an extended stability from\n40 h to 110 h. To test generality on Cu, we additionally synthesised CuPc/NCNT. Compared with\nCuPc/OCNT, CuPc/NCNT shows enhanced water-activation behaviour, and at −100 mA cm−2 the\nC H FE decreases from 1.40% on CuPc/OCNT to 0.80% on CuPc/NCNT. This clear drop in C H\n4 6 4 6\nformation demonstrates that axial N-coordination on CuPc also suppresses C–C coupling, indicating\nthat the coordination strategy derived from the Co platform is transferable to other metal–\nphthalocyanine catalysts.\nWe have revised the manuscript to present these points more clearly and to position our\ncontribution as (i) establishing a comparative MPc/OCNT platform that links water dissociation, *H\nregulation, and C–C coupling for different metal centres, and (ii) translating these mechanistic\ninsights into axial coordination engineering of CoPc/NCNT to move EASH performance closer to\nindustrial requirements, in clear contrast to the earlier CoPc system.\nSpecific Comments:\nComment 1: The authors choose the CoPc/OCNT as the model catalyst due to its better ability of\nwater dissociation. How about NiPc/OCNT and FePc/OCNT, which has been proved the great\nactivity on water dissociation. Besides, the FE of C H decreased and FE of H increased sharply\n2 4 2\nunder high current densities, suggesting the inferior ability of CoPC/OCNT to tune the *H. The\nauthors propose the CoPc/OCNT can inhibit C-C coupling to generate C byproducts, while the key\nResponse to comment 1: We thank the reviewer for this thoughtful comment.\nAt the outset of our work, we have observed that Co-based catalysts can deliver higher EASH\nactivity than Cu-based analogues. This is intriguing given that the majority (≈60–70%) of reported\nEASH studies still focus on Cu-based systems, and it motivated us to probe whether Co and Cu\nfollow distinct reaction pathways. To do so in a controlled manner, we selected CNT-supported metal\nphthalocyanines (MPc/OCNT, M = Cu, Co) as a model platform. Within this series, CoPc/OCNT\nemerged as a promising model because it combines relatively fast water dissociation with a\nfavourable adsorption environment for key intermediates, which correlates with a reduced tendency\ntoward C–C coupling.\nMotivated by the reviewer’s comment, we have extended the study of CoPc/OCNT and\nCuPc/OCNT to a broader comparative MPc/OCNT platform (M = Cu, Co, Ni, Fe). This expansion\nallows us to systematically investigate the effect of the metal center, thereby further strengthening\nthe novelty of our work. We have synthesised NiPc/OCNT and FePc/OCNT and evaluated them\nunder identical conditions. As shown in Fig. R25, LSV curves and Tafel slopes under Ar indicate\nthat both NiPc/OCNT and FePc/OCNT promote water dissociation more efficiently than\nCuPc/OCNT. Quantitative product analysis at −100 mA cm−2 shows that the FEs of C H on\n4 6\nFePc/OCNT and NiPc/OCNT are both below 0.15%, much lower than that of CuPc/OCNT (1.40%),\nconsistent with the idea that accelerated water dissociation helps to alleviate C–C coupling and C\n4\nformation (Fig. R26). However, FePc/OCNT suffers from pronounced overhydrogenation, and\nNiPc/OCNT exhibits a strong HER contribution. Therefore, CoPc/OCNT exhibits the most effective\nsuppression of C formation while maintaining high C H selectivity, which is why it was chosen as\n4 2 4\nthe model system in this study.\nRegarding the behaviour of CoPc/OCNT at different current densities, we agree that its ability\nto manage *H becomes limited at high currents. At low current densities (50–100 mA cm−2),\nCoPc/OCNT delivers excellent EASH performance, with C H FE above 90% and low H FE,\n2 4 2\nindicating efficient utilisation of water-derived *H for semi-hydrogenation. When the current density\nis increased to higher values (e.g. 300–500 mA cm−2), the C H FE decreases while the H FE rises\n2 4 2\nsharply. This trend is consistent with accelerated water dissociation at high overpotential, leading to\na high surface coverage of *H; a substantial fraction of this *H then recombines to H via HER rather\n2\nthan being fully consumed by C H hydrogenation. Our DFT results support this proposal: although\n2 2\nCoPc/OCNT has a higher H O dissociation barrier than CoPc/NCNT, it exhibits a lower H–H\n2\ncoupling barrier than CoPc/NCNT, indicating that at high *H coverage, CoPc/OCNT more readily\nchannels *H into H formation. In contrast, CoPc/NCNT maintains a higher barrier for H–H coupling\n2\nwhile still facilitating water dissociation, which improves *H utilisation for EASH and alleviates\nHER at high current densities.\nRegarding the possibility that Co sites might be intrinsically inactive for C–C coupling, our\nproduct analysis shows that primitive CoPc still generates a measurable amount of C H (FE = 0.12%\n4 6\nat −100 mA cm−2, Fig. R27). This suggests that Co centres can mediate C–C coupling. But the water-\ndissociation kinetics and intermediate binding energetics in CoPc/OCNT favour the semi-\nhydrogenation pathway to C H over further C–C coupling. We also note that cobalt complexes have\n2 4\nbeen reported to form C products in CO electroreduction, including Co corrole catalysts that yield\n2 2\nethanol as a major product and Co-phthalocyanine-based conjugated polymers that selectively\nreduce CO to ethanol (Nat. Commun. 10, 3864 (2019); Mater. Rep. Energy 3, 100176 (2023)).\nEASH system the reaction environment created by CoPc/OCNT steers the reaction toward selective\nC H production with strongly suppressed C by-products.\n2 4 4\nWe added these figures and discussions to the revised Manuscript (Page 7) and Supplementary\nInformation (Supplementary Fig. S6 and S7).\nPage 7: “To assess the generality of this water-dissociation-governed suppression of C–C coupling,\nwe also synthesised NiPc/OCNT and FePc/OCNT and evaluated them under identical conditions.\nLSV curves and Tafel slopes under Ar indicate that both NiPc/OCNT and FePc/OCNT promote\nwater dissociation more efficiently than CuPc/OCNT (Supplementary Fig. S6). Quantitative product\nanalysis at −100 mA cm⁻2 shows that the Faradaic efficiencies of C H on FePc/OCNT and\n4 6\nNiPc/OCNT are both below 0.15%, much lower than that of CuPc/OCNT (1.40%), which is\nconsistent with the view that accelerated water dissociation helps to alleviate C–C coupling and C\n4\nformation (Supplementary Fig. S7). Therefore, within this MPc/OCNT series, CoPc/OCNT\nexhibited the most effective suppression of C formation while maintaining high C H selectivity.”\n4 2 4\nFig. R25 | (a) LSV curves and (b) Tafel plots in Ar. (c) LSV curves under C H flow.\n2 2\nFig. R26 | Faraday efficiency of the obtained products over (a) FePc/OCNT, (b) NiPc/OCNT, and\n(c) CuPc/OCNT.\nFig. R27 | Faraday efficiency of the obtained products over CoPc at various current density.\nComment 2: The authors use the different carbon substrate (N-CNT, S-CNT, O-CNT) to tune\ncoordination of Co center and optimize its catalytic performance. How about the performance of\nCoPc/CNT without doping? The CoPc/NCNT achieves the better performance. The authors should\nprove the increased performance is not originate the change of physical properties (conductivity,\nhydrophilicity, adsorption capacity).\nResponse to comment 2: We appreciate the reviewer’s insightful comment.\nIn the revised manuscript, we have now included CoPc supported on bare commercial CNTs\n(CoPc/CNT) as an additional control to directly assess the effect of CNT functionalization. The\nelectrochemical performance of CoPc/CNT is compared with that of CoPc/OCNT in Fig. R28. As\nshown by LSV curves (Fig. R28a), the presence of oxygen functionalities leads to a higher limiting\ncurrent density in the CoPc/OCNT compared with CoPc/CNT. The product distribution at 100 mA\ncm−2 (Fig. R28b, c) further highlights the benefit of functionalisation: CoPc/CNT exhibits notable\nparasitic reactions, with Faradaic efficiencies of 11.7% for H , 0.47% for over-hydrogenation to\n2 6, 4 6\ndensity shows FE values of 8.1% for H , and 0% for C H and C products, indicating strongly\n2 2 6 4\nsuppressed side reactions and improved selectivity. These new data, now highlighted in the main\ntext, demonstrate that CNT functionalisation is necessary to be beneficial for optimising the intrinsic\nperformance of molecular CoPc in EASH.\nAs the reviewer correctly points out, functionalising CNTs with different heteroatom-\ncontaining groups inevitably affects their physical properties. In our work, we first oxidised\ncommercial CNTs to obtain OCNT and then grafted amino and thiol groups via silane coupling to\nprepare NCNT and SCNT, respectively. These O, N and S functionalities were introduced with the\nexplicit aim of electronically interacting with CoPc and tuning the coordination environment of the\nCo centre, but they may also lead to changes in conductivity, wettability, and gas adsorption. To\ndisentangle these effects, we kept the CoPc loading essentially identical across CoPc/OCNT,\nCoPc/NCNT, and CoPc/SCNT (CoPc/NCNT is even slightly lower, Table R6) and systematically\ncharacterised the physical properties of both the catalysts and the corresponding bare supports. Four-\nprobe measurements show that the resistance increases from CoPc/OCNT to CoPc/NCNT and\nfurther to CoPc/SCNT (Table R7), and contact-angle measurements reveal that the surface becomes\nmore hydrophobic along the same series (CoPc/OCNT < CoPc/NCNT < CoPc/SCNT, Fig. R29).\nC H -temperature programmed desorption results indicates a gradual increase in C H uptake from\n2 2 2 2\nCoPc/OCNT to CoPc/NCNT to CoPc/NCNT (Fig. R30). If the activity gain were dominated by such\nphysical factors, one would expect CoPc/SCNT, with the highest resistance, largest contact angle,\nand highest C H uptake, to show the best performance. In contrast, the observed catalytic trend is\n2 2\nCoPc/NCNT > CoPc/OCNT > CoPc/SCNT, which does not follow the simple order of conductivity,\nhydrophilicity, or adsorption capacity. To further separate the role of the carbon substrates from that\nof the molecular CoPc sites, we also evaluated the EASH activity of the bare OCNT, NCNT, and\nSCNT supports under identical conditions (Fig. R31). All three substrates exhibit very low activity,\nand NCNT and SCNT even perform worse than OCNT, despite their more pronounced changes in\nresistance, wettability and C H adsorption. This again suggests that the improved performance of\n2 2\nCoPc/NCNT cannot be ascribed to the physical properties of the functionalised carbons alone.\nTaken together, these results indicate that the enhanced EASH activity of CoPc/NCNT is not\nsimply a consequence of favourable conductivity, hydrophilicity, or adsorption of the carbon support.\nInstead, they support our conclusion that the dominant effect arises from the modified coordination\nenvironment and electronic structure of the Co centres induced by the N-containing surface groups,\nas corroborated by the spectroscopic and DFT analysis in the revised manuscript.\nWe added these figures and discussions to the revised Manuscript (Pages 11 and 13) and\nSupplementary Information (Supplementary Figs. S23, S34, and S35).\nPage 11: “Compared with the CoPc supported on bare commercial carbon nanotubes (CoPc/CNT),\nmost CoPc/XCNT catalysts significantly suppress side reactions and enhance selectivity, indicating\nthe effectiveness of molecular regulation in enhancing EASH performance (Supplementary Fig.\nS23).”\nPage 13: “Before discussing the mechanistic role of molecular regulation, we first examined whether\nthe enhanced activity of CoPc/NCNT could simply originate from changes in the physical properties\nof the carbon support. Four probe tests show that the resistance increases from 0.018 Ω∙cm for\nCoPc/OCNT to 0.029 Ω∙cm for CoPc/NCNT and 0.086 Ω∙cm for CoPc/SCNT (Supplementary Table\nS4), indicating a slight increase in overall resistance. Contact angle measurements reveal that the\nsurface becomes more hydrophobic, with the contact angle increasing from 65.5° (CoPc/OCNT) to\n92.1° (CoPc/NCNT) and 96.4° (CoPc/SCNT) (Supplementary Fig. S34). C H -temperature\n2 2\nprogrammed desorption results indicates a gradual increase in C H uptake from CoPc/OCNT to\n2 2\nCoPc/NCNT to CoPc/NCNT (Supplementary Fig. S35). Besides, the EASH activity of the bare\nOCNT, NCNT, and SCNT supports remains low and in some cases even decreases after\nfunctionalisation (Supplementary Fig. S32). These results indicate that the superior performance of\nCoPc/NCNT cannot be rationalised solely by favourable conductivity, hydrophilicity, substrates, or\nC H adsorption, and instead mainly arises from the modified coordination environment and\n2 2\nelectronic structure of the Co centres induced by heteroatom doping.”\nFig. R28 | (a) LSV curves and Faraday efficiency of the obtained products using (b) CoPc/CNT\nand (c) CoPc/OCNT at various current densities.\nFig. R29 | Water droplet contact angle measurements of (a) CoPc/OCNT, (b) CoPc/NCNT, and (c)\nCoPc/SCNT.\nFig. R30 | TPD curves of C H on CoPc/OCNT, CoPc/NCNT, and CoPc/SCNT.\n2 2\nFig R31 | Faraday efficiency of the obtained products over OCNT (a), NCNT (b), and SCNT (c) at\nvarious current densities.\nTable R6 | Co mass loading of CoPc/OCNT, CoPc/NCNT, and CoPc/SCNT.\nCatalysts Loading (wt. %) Standard Deviation (%)\nCoPc/OCNT 0.55 ±0.01\nCoPc/NCNT 0.47 ±0.055\nCoPc/SCNT 0.57 ±0.026\nTable R7 | The catalyst resistances and system resistances of measured at working conditions.\nCatalyst resistances\nSample System resistances (Ω)\n(Ω cm)\nCoPc/OCNT 0.018±0.003 1.9±0.1\nCoPc/NCNT 0.029±0.011 2.0±0.1\nCoPc/SCNT 0.086±0.009 2.1±0.1\nComment 3: The most electrochemical results presented in this study lack error bars, which may\nraise concerns about the reproducibility and reliability of the reported data.\nResponse to comment 3: We thank the reviewer for raising this important point.\nIn the revised manuscript, we have systematically reassessed all electrochemical data and added\nerror bars to the key quantitative results, including Faradaic efficiencies, conversion, and selectivity\nvalues. These data now represent the mean ± standard deviation of at least three independent\nmeasurements using separately prepared electrodes. We believe these additions more clearly\ndemonstrate the reproducibility and reliability of the reported electrochemical performance.\nComment 4: The in-situ FT-IR data presented in this study are underexplored. The analysis of water\npeaks range from 3000-4000 cm-1 may be benefit of revealing the tune of water dissociation.\nResponse to comment 4: We thank the reviewer for this valuable suggestion.\nIn the revised manuscript, we have further analysed the in situ ATR-FTIR spectra in the O–H\nstretching region to better elucidate the role of water dissociation. Specifically, the broad O–H\nstretching band of interfacial H O between 2900 and 3700 cm−1 was deconvoluted into three\n2\ncomponents, which can be assigned to 4-coordinated hydrogen-bonded water (4-HB·H O, low-\n2\nwavenumber component), 2-coordinated hydrogen-bonded water (2-HB·H O, main component) and\nCoPc/NCNT display a gradual decrease in the 4-HB·H O component accompanied by an increase\n2\nin the 2-HB·H O contribution, which becomes the dominant interfacial water species (Fig. R32).\n2 2\ndissociated to release protons, this evolution of the interfacial water structure indicates that\nCoPc/NCNT promotes H O dissociation and enhances the supply of *H for EASH.\nPage 15: “In the O–H stretching region, the broad band between 2900 and 3700 cm−1 can be\ndeconvoluted into contributions from 4-coordinated hydrogen-bonded water (4-HB·H O), 2-\n2\ncoordinated hydrogen-bonded water (2-HB·H O), and K+-hydrated water (K·H O). With\n2 2\nincreasingly negative potentials, the intensity of the 4-HB·H O component decreases, while that of\n2\n2-HB·H O increases, suggesting that the interfacial hydrogen-bond network becomes more weakly\n2\nbound and that water dissociation is facilitated, thereby increasing the availability of *H for the semi-\nhydrogenation of acetylene.”\nFig. R32 | In situ electrochemical FT-IR spectra of CoPc/NCNT electrode for acetylene semi-\nhydrogenation in acetylene-saturated 1 M KOH solution.\nComment 5: The selectivity performance using t-BuOH in electrolyte should be provide to the\ndemonstrate the role of *H, while the LSV curves are not enough.\nResponse to comment 5: We appreciate the reviewer’s valuable suggestion.\nIn addition to the LSV curves, we have now evaluated the catalytic performance in the presence\nof t-BuOH and quantified the product distribution (Fig. R33). When t-BuOH is added to the\nelectrolyte, the LSV limiting current density decreases markedly, and both the Faradaic efficiencies\nof the gaseous products and the acetylene conversion exhibit a pronounced decline. These results\nindicate that t-BuOH perturbs the formation and/or coverage of proton-derived hydrogen\nintermediates on the catalyst surface, thereby suppressing the electrocatalytic hydrogenation of\nacetylene. This behavior supports the crucial role of surface-adsorbed *H in driving the semi-\nhydrogenation reaction rather than a pathway that is insensitive to the availability of hydrogen\nintermediates.\nThe corresponding data and discussion have been added to the revised manuscript (Page 15-\n16) and Supplementary Information (Supplementary Fig. S39).\nPage 15-16: “Upon addition of t-BuOH, the polarization current density and C H conversion\n2 2\ndecrease markedly (Supplementary Fig. S39). The combination of a weak pH dependence and a\nstrong sensitivity to *H scavenging indicates that surface-bound *H, rather than a direct proton-\ncoupled electron-transfer step, serves as the key reactive intermediate in the electrocatalytic\nhydrogenation of C H on CoPc/NCNT.”\n2 2\nFig. R33 | (a) LSV curves and (b) C H conversion over CoPc/NCNT in 1 M KOH solution with\n2 2\nand without 20 mM t-BuOH.\nComment 6: The authors design a tandem system to generate high-purity C H . However, the\n2 4\nreviewer wants know if such a design has practical significance. Whether the added big-area reactor\nis better than other methods for removing low concentrations of C H . The technical economic\n2 2\nanalysis may be better to demonstrate the necessity of the design.\nResponse to comment 6: We thank the reviewer for pointing out this issue.\nIn our preliminary tests, we first evaluated the catalysis performance of CoPc/NCNT in a 1 cm2\nflow cell at different C H concentrations. As shown in the revised Fig. R34, when the C H content\n2 2 2 2\nis reduced to the 10%, the Faradaic efficiency for C H drops sharply. We attribute this mainly to\n2 4\nthe very low solubility of C H in alkaline electrolyte and to mass-transport limitations in bringing\n2 2\nhighly dilute C H through the gas-diffusion layer to the catalyst surface. Under such conditions, a\n2 2\nlarger geometric electrode area at similar local current density is required to maintain both high\nsingle-pass conversion and high selectivity. Using a large-area electrolyser for the low-concentration\nstage is therefore a practical way to compensate for these mass-transport limitations, and is in line\nwith common practice in recent EASH studies (Nat. Catal. 4, 565-574 (2021); Nat. Catal. 4, 557-\n564 (2021)).\nTo ascertain the economic potential of electrochemical acetylene semihydrogenation by our\ndesigned tandem system, we carried out a techno-economic analysis based on the model reporte\nrecently1-3. The electricity price used in this study was adopted from recently published literature,\nwhile the cell voltage was determined based on experimental data obtained from our system.\nFurthermore, the stack cost of 275.55 $ kW−1 was recalculated based on the stack cost from DOE\nCurrent Central H2A, 460 $ kW−1 according to the literature. The electrolyzer cost per unit area is a\nfunction of current density and operating cell voltage3. The costs of coal-derived acetylene was\nconfirmed as 767.74 $ ton−1 based on the recently published literature4. Accordingly, the costs\nconsist of three components, namely the capital costs, operating costs and materials costs.\nThe specific sample for the calculation of ESAE for ethylene under optimistic case assumptions:\nTaking the 10000 kg daily capacity of C H as an example at a current density of 0.5 A cm−2 (small\n2 4\nelectrolyzer) and 0.040 A cm−2 (big) with 99% selectivity of C H as an example.\n2 4\nThe specific assumptions made for the TEA are listed as follows:\n1. The electricity price was considered to be 0.03 $ 𝑘𝑊ℎ.\n2. The operating voltages were set as 2.1 V and 2.5 V.\n3. The electrolyzer cost per area was calculated as follows:\nElectrolyzer cost per area (small)= 275.55 $ 𝑘𝑊−1 × 0.5 A × 2.1 𝑉 × 104cm2 × kW\ncm2 m2 1000\n= 2893.28 $ m2\nElectrolyzer cost per area (big)= 275.55 $ 𝑘𝑊−1 × 1 A × 2.5 𝑉 × 104cm2 × kW\n25 cm2 m2 1000\n= 275.55 $ m2\n4. The anticipated service life of the electrolyzer was 20 years and the working time per year was\n350 days. The power of separation equipment was 0.25 kWh/m3\nThus, the parameters needed in the cost calculation are determined as follows：\nThe required total current is calculated as:\n𝑇𝑜𝑡𝑎𝑙 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 = 10000 kg × day × 1000 g × mol × 2 e− × 96500 C × 1 = 805843 A\nday 86400 s kg 28 g mol 0.99\nThe required C H feedstock inlet with 99% single-pass conversion was calculated as:\n2 2\nC H Inlet = 805843 A × 1 × 1 × 0.026 kg × 86400 s × 1 = 9475.7 kg day−1\n2 2 2e- 96485 C mol-1 mol day 0.99\nThe outlet gas at the cathode side consisted of C H , C H , and H :\n2 2 2 4 2\nC H Outlet Flow Rate = 10000 × kg × m3 × day = 333.33 m3 h−1\n2 4\nday 1.25 kg 24 h\nH out𝑙𝑒𝑡 = 814066 A × (1-0.99) × m3 × 1 × 1 × 0.002 kg × 86400 s × day = 3.42 m3\n2 0.089 kg 2e- 96485 C mol-1 mol day 24 h\nh−1\nTotal Outlet Flow Rate = (333.33 + 3.42) m3 h−1= 336.75 m3 h−1\nThe needed electrolyzer area was determined as:\nElectrolyzer Area (small) = 805843 A × 1 × m2 = 161.17 m2\n0.5 A cm-2 104 cm 2\nElectrolyzer Area (big) = 805843 A × 1 × m2 = 2014.60 m2\n0.04 A cm-2 104 cm 2\nThe input power was calculated as follows:\nW\nPower = (2.1 V +2.5 V) × 805843 A × =3.79 MW\nThe total cost of the proposed ESAE process consisted of capital cost, operating cost, and\nmaterial cost.\nCapital cost:\n1. Electrolyzer:\nElectrolyzer Cost per ton Ethylene\n= (161.17 m2 × 2893.28 $ m2 + 2014.60 m2 × 275.55 $ m2) × 1 × year × day = 14.59\n20 year 350 day 10 ton\n$ ton −1\nC2H4\n2. Balance of Plant:\nBalance of Plant Cost per ton Ethylene\n= 14.59 $ ton −1 × 0.39/0.67 = 8.49 $ ton −1\nC2H4 C2H4\n3. Separation Equipment (calculated as the reported reference)1:\nSeparation Equipment Cost per ton Ethylene\n= 1989043 $ × ( 336.75 m3 h-1 )0.7 × 1 × year × day = 13.26 $ ton −1\n1000 m3 h-1 20 year 350 day 10 ton C2H4\nOperating Cost:\n1. Electricity:\n3.79 MW × 1000 kW × 24 h × 0.03 $ × 1 day = 272.88 $ ton −1\nMW day kWh 10 ton\nC2H4\n2. Separation:\n336.75 m3 h−1 × 0.25 kWh × 0.02 $ × 24 h × 1 day = 6.06 $ ton −1\nm3 kWh day 10 ton C2H4\nMaterials Cost:\n1. The cost of coal-derived C H : 767.74 $ ton −1\n2 2 C2H4\n2. Water:\n1 1 0.18 kg 0.2642 gal 86400 s $ 1 day\n805843 A × × × × × × 0.0054 × = 0.46\n4e- 96485 C mol-1 mol kg day gal 10 ton\n$ ton −1\nC2H4\nThe total cost:\nTotal cost = (14.59 + 8.49 + 13.26 + 272.88 + 6.06 + 767.74 + 0.46) = 1083.48 $ ton −1\nC2H4\nTake the above together, the total production cost for C H at a rate of 10,000 kg per day is\n2 4\n$1083.48 per ton. Given the current market price of C H at $1,350 per ton, this yields a gross profit\n2 4\nof approximately $266.52 per ton and a gross margin of about 19.7%. Within this commonly used\nTEA framework, the analysis suggests that employing a tandem electrolyser to produce C H is both\n2 4\ntechnically reasonable and potentially compatible with economically viable operation, while\nrecognising that more detailed process engineering and techno-economic studies would still be\nrequired for an industrial implementation.\nThe corresponding data and discussion have been added to the revised manuscript (Page 20)\nand Supplementary Information (Page S32, Supplementary Note 2).\nPage 20: “A preliminary techno-economic analysis based on recently reported EASH models\nsuggests that, under literature-consistent assumptions, this tandem configuration can operate within\nan economically reasonable regime (Supplementary Note 2).”\nFig. R34 | The Faradaic efficiency and specific selectivity of ethylene versus acetylene concentration\nat −100 mA cm−2 in 1 cm2 flow cell.\nReference:\n1. Jouny, M., et al. General techno-economic analysis of CO electrolysis systems. Ind Eng. Chem.\n2. Lum, Y., et al. Tuning OH binding energy enables selective electrochemical oxidation of\nethylene to ethylene glycol. Nat. Catal. 3, 14-22 (2020).\n3. Shin, H., et al. Techno-economic assessment of low-temperature carbon dioxide electrolysis.\nNat. Sustain. 4, 911-919 (2021).\n4. Zhao, B. et al. Economically viable electrocatalytic ethylene production with high yield and\nselectivity. Nat. Sustain. 6, 827-837 (2023).\nComment 7: Some typographical typos exist in the manuscript and the authors should carefully\ncheck it (e.g., affinity—strong, Y-axis title of Figure 5d, the calculation of FE in Performance\nassessment).\nResponse to comment 7: We thank the reviewer for carefully checking the manuscript and pointing\nout these typographical issues. We have thoroughly rechecked the entire text and corrected all typos.\nIn particular, the expression involving “affinity” in the Introduction has been revised for grammatical\ncorrectness and clarity (Page 2), the Y-axis title of Figure 5d (Fig. R35) has been corrected, and the\ndescription and formula for the Faradaic efficiency calculation in the “Performance assessment”\nsection have been revised to ensure accuracy and consistency (Page 24). We have also carefully\nproofread the manuscript to minimize any remaining typographical errors.\nn × m × F\nPage 24: “FE (%) = × 100% (4)”\nI × t\nFig. R35 | Electrosynthesis of high purity ethylene under 100%C H by the tandem flow cells.\n2 2\nResponse to Reviewer #4:\nGeneral Comments:\nThe revision has fulfilled this Reviewer´s requirements. The manuscript seems ready for publication:\nResponse: We would like to thank the reviewer again for the considerable time and effort dedicated\nto our work."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have adequately addressed the concerns raised in the previous review. I recommend acceptance of the manuscript for publication in Nature Communications.", "reviewer_2": "The authors have made substantial improvements to the work in response to the previous round of review. The revised version is significantly more polished and addresses many of the concerns raised in the initial evaluation. Based on the current quality and scientific merit of the study, I, in principle, recommend that this work be considered for publication in Nature Communications.\nHowever, while the overall structure, methodology, and presentation of results are now much stronger, there are still several details that require careful revision and clarification before the manuscript can be accepted for publication. These include, but are not limited to, the following points:\n1. Physical quantities such as E, G, T, S, etc., should be in italic format.\n2. The abbreviation for \"generalized gradient approximation\" should be checked. It is usually GGA, not GCA.\n3. The chemical reaction equations should be checked for correctness. For example:\n*C2H2 + H+ e− → C2H3 (10)\n2*C2H3 + *C2H3 → *C4H6 (14)\n4. Transition states, such as *H–OH, are generally not considered stable states and should be clearly indicated.\n5. Software tools mentioned in the manuscript, such as VTST, should be properly cited.", "reviewer_3": "The reviewer appreciates the authors’ efforts to revise the manuscript and respond to the comments. However, I regret that the manuscript is still not suitable for publication in this journal. A major concern regarding the novelty of this work is that the CoPc catalytic system has already been reported (Chem. Eng. J. 2022, 431: 134129). A critical issue is that the CoPc catalyst itself does not reach the performance level previously reported in the literature. Consequently, the apparent benefit of introducing NCNT cannot be reliably attributed to a true support effect, and the work fails to demonstrate that NCNT provides a meaningful or generalizable advantage for CoPc-based catalysts. Regarding the tandem system, feeding the exhaust from a small electrolyzer into a larger-area reactor does not convincingly demonstrate practical relevance. Achieving higher conversion in a single appropriately scaled reactor would be more rational and informative. Overall, despite the authors’ considerable experimental effort, the manuscript does not demonstrate sufficient novelty or advancement for publication in Nature Communications.", "reviewer_4": "The revision has fulfilled this Reviewer´s requirements. The manuscript seems ready for publication."}, "author_response": "Response to Reviewers’ Comments\nDear Editors and Reviewers:\nWe are grateful for the opportunity to submit a revised version of our manuscript for further\nconsideration.\nWe have carefully studied the reviewer’s comments and tried our best to revise the manuscript\naccordingly. The manuscript has been revised following the comments and suggestions of the\nreviewers, and point-by-point replies are given below. We sincerely thank the editors and reviewers\nfor their time, effort, and constructive feedback, which have significantly improved the quality and\nclarity of the manuscript.\nMolecular Cobalt Catalysts for Highly Selective Electrocatalytic Acetylene Semi-\nHydrogenation\nResearch Article, No. NCOMMS-25-62672B, Nature Communications\nResponse to Reviewer #1:\nGeneral Comments:\nIn the revised manuscript, many of the comments have been addressed. However, several key\nconcerns remain:\nResponse: We appreciate the reviewer’s thoughtful summary and constructive suggestions. These\nopinions help to improve the academic rigor of our article. In response, we have carefully revised\nthe manuscript and Supplementary Information, incorporating new experimental data, figures,\nreferences, and discussion to address each of the reviewer’s comments point by point. Detailed\ndescriptions of these amendments are provided below. We believe that these revisions have\nsubstantially improved the overall quality of this work.\nSpecific Comments:\nComment 1: Figure captions throughout the manuscript still frequently lack essential details. For\nexample, in Fig. R2, the inclusion of a higher CoPc loading should be explicitly explained in the\ncaption. Similarly, in Fig. R3, it would be helpful to specify that the data correspond to the Co K-\nedge. This issue occurs repeatedly across the manuscript, and more detailed figure captions would\ngreatly improve clarity and readability.\nResponse: We appreciate the reviewer’s constructive suggestion and agree that several figure\ncaptions in the original submission lacked essential experimental/contextual details, which could\naffect clarity and readability. In the revised manuscript, we have systematically enriched the figure\ncaptions throughout to ensure that key information is explicitly stated. Specifically, for Fig. R2, we\nhave added a clear explanation for the higher CoPc loading: CoPc/NCNT-10 is now defined in the\ncaption as the sample with a CoPc-to-NCNT mass ratio of 1:10 (Fig. R1). For Fig. R3, as suggested,\nwe have clarified in the revised manuscript that the presented XAFS results correspond to the Co K-\nedge (Page 10). Moreover, we have expanded most figure captions to include crucial experimental\nparameters and key results. These changes are highlighted throughout the revised manuscript, but,\nfor the sake of brevity, are not listed individually here. We believe these enhancements significantly\nimprove the clarity and consistency of the work.\nPage 10: “Notably, the Co K-edge and pre-edge features exhibit a systematic energy shift that\ncorrelates with the specific heteroatom in the CNT support. Relative to pristine CoPc (7750.5 eV),\nthe absorption edge of CoPc/OCNT shifts toward higher energy (7751.4 eV), whereas a significant\nshift toward lower energies is observed for CoPc/NCNT (7745.7 eV) and CoPc/SCNT (7744.0 eV).\nThis energy hierarchy (CoPc/OCNT > CoPc > CoPc/NCNT > CoPc/SCNT) underscores the potent\nelectron-withdrawing nature of axial O relative to N and S, which induces a higher Co oxidation\nstate in CoPc/OCNT and a progressively more reduced state in the N- and S-coordinated analogues.\nThese observations are in excellent agreement with the XPS results.”\nFig. R1 | Raman spectra of CoPc/OCNT, CoPc/NCNT, CoPc/NCNT-10 (CoPc/NCNT=1:10, m/m),\nand CoPc/SCNT.\nComment 2: In Fig. R5, the newly added colour scale is still not visible.\nResponse to comment 2: We apologize for the omission. We have updated Fig. R5 (Fig. R2) in the\nrevised manuscript to include a visible color scale.\nFig. R2 | AC-STEM images and 3D atom-overlapping Gaussian-function maps for (a) CoPc/OCNT\nand (b) CoPc/SCNT. The red dashed frames delineate the regions used to generate the 3D atom-\noverlapping Gaussian-function maps.\nComment 3: For Tables R1 and R2, the authors should clearly state how the error bars were obtained.\nIn addition, the magnitude of the error bars should be consistent with the reported values. In Table\nR2, some errors are larger than the corresponding values, which suggests that there may be an issue\nwith the data analysis or error estimation.\nResponse to comment 3: We thank the reviewer for raising this point. The metal loadings were\ndetermined by ICP–OES (Agilent 5110) using three independently synthesized batches of\nCoPc/OCNT, CoPc/NCNT, and CoPc/SCNT. The values presented in Table R1 represent the mean\n± standard deviation (s.d.) for n=3 independent batches, and the error bars correspond to the\nstandard deviation across these batch-to-batch measurements.\nFor Table R2, the uncertainties represent the 1σ statistical errors estimated from the covariance\nmatrix during the EXAFS refinement process in Artemis. In our previous fit, some parameters were\ninsufficiently constrained (i.e., choice of paths/model and parameter correlations), which inflated\nthe formal uncertainties and, in a few cases, made them even larger than the fitted values. To address\nthis, we refined the EXAFS fitting using optimized coordination models and more appropriate\nscattering-path selections, retaining the solutions that yielded stable convergence and physically\nreasonable parameters. The updated fitting results, now provided in Table R2, exhibit uncertainty\nmagnitudes consistent with established literature standards (Adv. Mater. 2023, 35, 2303818; Adv.\nMater. 2022, 34, 2204021; Angew. Chem. Int. Ed. 2022, 61, e202209849; Nat Commun 2022, 13,\n723; Adv. Funct. Mater. 2022, 32, 2110857).\nTable R1 | Cobalt mass content in CoPc/OCNT, CoPc/NCNT, and CoPc/SCNT. Error bars represent\nthe standard deviation derived from three independently synthesized batches of CoPc/OCNT,\nCoPc/NCNT, and CoPc/SCNT.\nCatalysts Loading (wt. %) Standard Deviation (%)\nCoPc/OCNT 0.55 ±0.01\nCoPc/NCNT 0.47 ±0.055\nCoPc/SCNT 0.57 ±0.026\nTable R2 | Co K-edge EXAFS curve-fitting parameters of CoPc/XCNT and reference sample (Co\nfoil).\nSample Path CN R (Å) σ2 (10−3Å2） ∆E (eV) R factor\nCo-N 4.2 1.91±0.02 2.6±1.2\nCoPc/OCNT 7.8±1.96 0.010\nCo-O 0.9 1.96±0.03 8.8±7.2\nCo-N1 4.0 1.92±0.01 2.4±1.0\nCoPc/NCNT 4.9±1.98 0.010\nCo-N2 1.0 2.18±0.03 9.1±5.7\nCo-N 4.1 1.90±0.02 3.2±1.3\nCoPc/SCNT 1.08±0.66 0.018\nCo-S 0.9 2.19±0.02 5.5±2.8\nComment 4: I am concerned that iR compensation has not been applied. Without proper iR\ncorrection, it is difficult to meaningfully compare the LSV behavior at higher current densities across\ndifferent catalysts.\nResponse to comment 4: We agree with the Reviewer that iR compensation is essential for a\nmeaningful comparison of electrocatalytic activity, particularly at high current densities.\nAccordingly, we have now applied iR correction to all polarization curves. The ohmic resistance (R )\nS\nwas determined via an electrochemical workstation under operating conditions, and the potentials\nwere corrected using the formula, E . = E - iR . The revised figures are now reflected in the revised\ncorr S\nmanuscript.\nComment 5: The Methods section still lacks important experimental details. For example, if a\nsputtered gold film was required for the FIT measurements, the sputtering procedure should be\ndescribed. In addition, it is unclear whether the XANES measurements were performed using a\nlaboratory setup or at a synchrotron facility. The use of “RapidXAFS 2M” is mentioned, but this\ntechnique/setup is not sufficiently explained and is unfamiliar without further clarification.\nResponse to comment 5: We appreciate the reviewer’s comment regarding missing methodological\ndetails. To address this concern, we have updated the Methods section to provide a detailed account\nof the gold sputtering process for in situ FTIR measurements. Specifically, we have specified the\ninstrumentation, deposition duration, and applied current used to ensure the formation of a high-\nquality Au film. We have also clarified that the Co K-edge in situ XANES data were acquired using\na laboratory (bench-top) XAFS instrument (RapidXAFS 2M, Anhui Absorption Spectroscopy\nAnalysis Instrument Co., Ltd.), rather than at a synchrotron facility. To support this, we have added\na comprehensive description of the RapidXAFS 2M configuration (including the crystal analyzer)\ntogether with key acquisition parameters and the operando electrochemical protocol. These details\nhave been included in the revised manuscript (Page 23).\nPage 23: “In situ FTIR measurements were conducted using a Nicolet IS50 spectrometer equipped\nwith a liquid-nitrogen-cooled MCT-A detector. A gold-coated Ge prism served as the working\nelectrode, prepared via sputter-coating (Quorum SC7620) at 30 mA for 60 s to ensure a uniform\nconductive surface.”\n“In situ XANES measurements at the Co K-edge were performed using an ultra-high flux benchtop\nX-ray absorption fine structure spectrometer (RapidXAFS 2M, Anhui Absorption Spectroscopy\nAnalysis Instrument Co., Ltd.). The instrument was operated at 20 kV and 40 mA and equipped with\na Si (533) spherically bent crystal analyzer (radius of curvature: 500 mm), which serves for energy\nselection of the incident X-ray beam and enables rapid acquisition of XANES spectra under\noperando conditions.”\nResponse to Reviewer #2:\nGeneral Comments:\nThe authors have made substantial improvements to the work in response to the previous round of\nreview. The revised version is significantly more polished and addresses many of the concerns raised\nin the initial evaluation. Based on the current quality and scientific merit of the study, I, in principle,\nrecommend that this work be considered for publication in Nature Communications.\nHowever, while the overall structure, methodology, and presentation of results are now much stronger,\nthere are still several details that require careful revision and clarification before the manuscript can\nbe accepted for publication. These include, but are not limited to, the following points:\nResponse: We thank the Reviewer for the thoughtful evaluation and constructive feedback. These\ninsights have been instrumental in enhancing the academic rigor of our work. In response, we have\ncarefully revised the manuscript and Supplementary Information, incorporating new experimental\ndata, figures, references, and discussion to address each of the Reviewer’s comments point by point.\nDetailed descriptions of these amendments are provided below. We believe that these revisions have\nsignificantly strengthened the quality and clarity of our study.\nSpecific Comments:\nComment 1: Physical quantities such as E, G, T, S, etc., should be in italic format.\nResponse to comment 1: We appreciate the reviewer’s attention to detail regarding the formatting\nof physical quantities. We have thoroughly reviewed the manuscript to ensure that all variables and\nthermodynamic symbols are consistently typeset in italics, in accordance with standard scientific\nnotation and the journal’s style guidelines.\nComment 2: The abbreviation for \"generalized gradient approximation\" should be checked. It is\nusually GGA, not GCA.\nResponse to comment 2: We thank the reviewer for pointing this out. The abbreviation for\n“generalized gradient approximation” was incorrectly written as “GCA” in the original manuscript.\nIt has now been corrected to “GGA” throughout the revised manuscript (Page 25).\nPage 25: “All electron exchange-correlation functionals were calculated using the generalized\ngradient approximation (GGA) with the revised Perdew-Burke-Ernzerhof (PBE) method using a\ncutoff energy of 450 eV.”\nComment 3: The chemical reaction equations should be checked for correctness. For example:\n*C H + H+ e− → C H (10)\n2 2 2 3\n2*C H + *C H → *C H (14)\n2 3 2 3 4 6\nResponse to comment 3: We thank the Reviewer for the meticulous review and attention to detail.\nIn the revised manuscript, we have corrected the two equations accordingly: Eq. (10) has been\nrevised to “*C H + H+ + e− → *C H ”, and Eq. (14) has been revised to “2*C H → *C H ”, ensuring\n2 2 2 3 2 3 4 6\nconsistent surface-adsorbed notation and correct stoichiometry. In addition, we have carefully\nrechecked all other reaction equations throughout the manuscript to confirm their correctness and\nconsistency.\nComment 4: Transition states, such as *H–OH, are generally not considered stable states and should\nbe clearly indicated.\nResponse to comment 4: We thank the reviewer for this helpful comment. In the revised manuscript,\nwe have avoided using transition-state-like notations as if they were stationary surface states.\nSpecifically, we rewrote these descriptions in terms of the underlying elementary processes: *H–OH\nis now described as the water dissociation step, and *H–H is described as hydrogen coupling to form\nH (Fig. R3). Related statements and labels were adjusted accordingly throughout the manuscript\n2\nand figure captions to ensure that transition-state configurations are clearly distinguished from stable\nintermediates.\nFig. R3 | Free energy diagram for the *H generation and adsorption, and H generation process of\nComment 5: Software tools mentioned in the manuscript, such as VTST, should be properly cited.\nResponse to comment 5: We appreciate the reviewer’s suggestion. In the revised manuscript,\nwe have added the appropriate citations for the software/tools used in our calculations (References 51\nand 52), including VTST. We also reviewed the Methods/Computational Details section to ensure\nthat all other software packages mentioned in the manuscript are properly referenced and cited where\nthey first appear.\nReferences:\n51. Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations\nusing a plane-wave basis set. Phys. Rev. B 54, 11169 (1996).\n52. Henkelman, G. & Jónsson, H. A dimer method for finding saddle points on high dimensional\npotential surfaces using only first derivatives. J. Chem. Phys. 111, 7010–7022 (1999).\nResponse to Reviewer #3:\nGeneral Comments:\nThe reviewer appreciates the authors’ efforts to revise the manuscript and respond to the comments.\nHowever, I regret that the manuscript is still not suitable for publication in this journal. A major\nconcern regarding the novelty of this work is that the CoPc catalytic system has already been\nreported (Chem. Eng. J. 2022, 431: 134129). A critical issue is that the CoPc catalyst itself does not\nreach the performance level previously reported in the literature. Consequently, the apparent benefit\nof introducing NCNT cannot be reliably attributed to a true support effect, and the work fails to\ndemonstrate that NCNT provides a meaningful or generalizable advantage for CoPc-based catalysts.\nRegarding the tandem system, feeding the exhaust from a small electrolyzer into a larger-area reactor\ndoes not convincingly demonstrate practical relevance. Achieving higher conversion in a single\nappropriately scaled reactor would be more rational and informative. Overall, despite the authors’\nconsiderable experimental effort, the manuscript does not demonstrate sufficient novelty or\nadvancement for publication in Nature Communications.\nResponse: We thank the reviewer for the feedback on our revised manuscript. We appreciate the\nconstructive critique regarding the work novelty, the benchmarking of the CoPc system, the\ninterpretation of the NCNT effect, and the practical relevance of the tandem configuration. We have\naddressed these concerns through additional analysis and targeted revisions, as detailed in our point-\nby-point responses below.\nComment 1: The reviewer appreciates the authors’ efforts to revise the manuscript and respond to\nthe comments. However, I regret that the manuscript is still not suitable for publication in this journal.\nA major concern regarding the novelty of this work is that the CoPc catalytic system has already\nbeen reported (Chem. Eng. J. 2022, 431: 134129).\nResponse to comment 1: We appreciate the reviewer’s perspective regarding the novelty of our\nwork relative to the CoPc system reported by Liu et al. (Chem. Eng. J. 2022, 431, 134129). We fully\nacknowledge this pioneering study on CoPc-based EASH, which established metal phthalocyanines\nas active catalysts and analysed acetylene conversion and selectivity mainly from the viewpoint of\nacetylene adsorption and hydrogenation. Our work is conceptually distinct in that it treats the\nhydrogen source from water electrolysis as an explicit mechanistic and design variable. Specifically,\n(i) we are, to our knowledge, the first to identify a direct link between water-dissociation–derived\nhydrogen supply and selective acetylene hydrogenation, which was not addressed in the Chem. Eng.\nJ. work; (ii) using a unified MPc/XCNT platform, we show that promoting water activation provides\na general route to suppress C–C coupling; and (iii) we apply axial-coordination engineering to\nregulate water dissociation and interfacial *H transfer, thereby mitigating severe HER at high current\ndensities and achieving a 24071-fold enhancement in single-site EASH activity compared with the\nChem. Eng. J. benchmark.\n(1) Water-dissociation–derived hydrogen supply as a controlling step for EASH selectivity.\nIn our study, we explicitly decompose the alkaline EASH process into two coupled steps: water\ndissociation at the catalyst–electrolyte interface to generate reactive hydrogen species, and the\nsubsequent adsorption and hydrogenation of acetylene on the active sites. Using the MPc (M = Cu,\nCo, Ni, Fe)/OCNT platform, we correlate water-dissociation behaviour and subsequent *H utilisation\nwith the full product distribution and show that they are decisive in setting selectivity, including the\nbalance between desired C H formation, C–C coupling, and HER. To our knowledge, this is the\n2 4\nfirst direct confirmation that controlling water dissociation and the ensuing hydrogen supply,\nrather than acetylene binding alone, is a primary lever for tuning EASH selectivity.\n(2) General suppression of C–C coupling by accelerating water activation\nBuilding on this picture, we examine a series of MPc/OCNT catalysts under identical conditions\nto test how universal the role of water activation is. We find that the product distribution follows the\nability of the metal centre to promote water dissociation and utilise interfacial *H: catalysts with\nfaster water activation show a markedly lower tendency toward C–C coupling and C formation.\nleading to strongly suppressed C products, while NiPc/OCNT and FePc/OCNT display similarly\n4\nlow C yields at 100 mA cm⁻2. These trends indicate that enhancing water activation provides a\n4\ngeneral route to suppress C–C coupling in EASH, rather than being a catalyst-specific detail, which\ngoes beyond what can be inferred from the MPc focus of the Chem. Eng. J. study.\n(3) Axial-coordination regulation of water dissociation and *H transfer to suppress HER at\nhigh current densities.\nGuided by this mechanistic insight, we develop an axial-coordination strategy in which CoPc is\nanchored on amino- and thiol-functionalised CNTs (NCNT and SCNT), using the surface groups as\nwell-defined axial ligands that tune the Co coordination environment, water-dissociation kinetics,\nand partitioning of *H between HER and acetylene hydrogenation. As a result, CoPc/NCNT delivers\na C H Faradaic efficiency of 86.7% at –500 mA cm–2 under a 100% C H feed, with 99.99% C H\n2 4 2 2 2 4\nselectivity and complete suppression of detectable C products. The corresponding turnover\n4\nfrequency reaches 7019 min⁻1, representing a ≈24071-fold enhancement over the 0.29 min⁻1\nreported for CoPc at −100 mA cm⁻2 in Chem. Eng. J. 2022, 431, 134129, while using approximately\neleven times less CoPc and extending the operational stability from 40 h to 110 h. These data indicate\nthat the axial-coordination regulation of water dissociation and *H transfer to suppress HER at high\ncurrent densities, which far exceeds the CEJ study in both performance as well as the understanding\nof EASH.\nComment 2: A critical issue is that the CoPc catalyst itself does not reach the performance level\npreviously reported in the literature. Consequently, the apparent benefit of introducing NCNT cannot\nbe reliably attributed to a true support effect, and the work fails to demonstrate that NCNT provides\na meaningful or generalizable advantage for CoPc-based catalysts.\nResponse to comment 2: We have to note that this performance gap is attributed to differences in\nthe mass loading of bare CoPc, and when the loading is increased to levels consistent with the\nprevious study, comparable activity of the bare CoPc is achieved. In our main dataset, the CoPc\nbenchmark was prepared so that the amount of CoPc on the gas diffusion electrode matched the\nCoPc content in the CoPc/XCNT catalysts (≈0.06 mg cm⁻2). This configuration was chosen to allow\na direct comparison of the effect of CNT functionalisation at a fixed CoPc loading. As now shown\nin the revised manuscript, when the CoPc loading on NCNT is systematically increased, the EASH\nactivity first rises and then approaches a plateau, indicating a clear dependence of activity on loading\n(Figure S28). When we increased the CoPc loading on the gas diffusion electrode to the same level\nas in Chem. Eng. J. 2022, 431, 134129, the EASH performance at 50 and 100 mA cm⁻2 was in good\nagreement with the values reported in that work (Figure R4). The lower absolute activity observed\nin our main comparison, therefore, arises from the lower CoPc loading used for the\nsupport/coordination study, rather than from an intrinsic discrepancy in CoPc performance relative\nto the literature.\nMoreover, we can clearly see that bare CoPc suffers from severe hydrogen evolution at current\ndensities exceeding 100 mA cm-2 in both our study and the previous study, but the CoPc/NCNT\nsystem maintains exceptional stability and selectivity even at 500 mA cm-2, achieving a C H\n2 4\nFaradaic efficiency of 86.7% with 99.99% C H selectivity and complete suppression of detectable\n2 4\nC products. This stark contrast in performance clearly validates the efficacy of our support-based,\n4\ntunable molecular strategy in regulating the coordination environment of CoPc catalysts to steer\nreaction pathways even under extreme industrial conditions.\nTo further validate the efficacy of this strategy and elucidate the underlying mechanism, we\nsystematically evaluated the influence of the NCNT support by maintaining a fixed CoPc loading\nacross the various support platforms. SCNT and NCNT introduce distinct axial coordination\nenvironments at the Co centre. On SCNT, the weakened water dissociation and lower H–H coupling\nbarrier favour HER, leading to lower EASH activity and more severe hydrogen evolution. On NCNT,\naxial N coordination accelerates water dissociation and raises the barrier for H–H recombination, so\nthat a larger fraction of the *H pool is directed into C H hydrogenation rather than HER. Under\n2 2\nthese conditions, CoPc/NCNT maintains high activity, completely suppresses detectable C products,\n4\nand strongly mitigates over-hydrogenation. This behaviour reflects a pronounced support effect,\nindicating that the NCNT environment alters the electronic structure of CoPc and thereby its\ncatalytic behavior.\nTo demonstrate the universality of this strategy, we extended our investigation to a CuPc-based\nsystem. At comparable CuPc loadings and under identical EASH conditions, replacing OCNT with\nNCNT lowers the water-dissociation barrier, enhances the overall EASH activity, and decreases the\nC H Faradaic efficiency from 1.40% on CuPc/OCNT to 0.80% on CuPc/NCNT at 100 mA cm⁻2.\n4 6\nTaken together with the CoPc results, this behaviour shows that NCNT consistently improves both\nactivity and suppression of C–C coupling across different MPc catalysts. These observations support\nthe conclusion that NCNT provides a meaningful and transferable coordination environment for\nMPc-based EASH catalysts.\nFig. R4 | (a) Faradaic efficiencies at different current densities over CoPc, adapted from literature.\nReprinted with permission from Liu et al. (Chem. Eng. J. 2022, 431: 134129) Copyright 2022\nElsevier. (b) Faraday efficiency at different current densities over CoPc in this work.\nComment 3: Regarding the tandem system, feeding the exhaust from a small electrolyzer into a\nlarger-area reactor does not convincingly demonstrate practical relevance. Achieving higher\nconversion in a single appropriately scaled reactor would be more rational and informative.\nResponse to comment 3: We thank the reviewer for this comment.\nWe agree that performance in a single appropriately scaled reactor is an important benchmark,\nand we would like to clarify that the staged configuration in this work is not introduced to\naccommodate a laboratory test using pure acetylene. Rather, it is motivated by a practical process\nrequirement, namely the near-complete removal of acetylene to meet high-purity ethylene\nspecifications. In practice, even when a catalyst remains intrinsically selective, approaching\nquantitative conversion in a single electrochemical cell becomes increasingly constrained by mass\ntransport once acetylene is depleted to trace levels, because delivery of dilute C H through the gas\n2 2\ndiffusion layer becomes rate-limiting. This transport limitation also explains why, to the best of our\nknowledge, reported single-cell systems have not exceeded 99% single pass conversion when\nacetylene approaches low concentration.\nTo address this concern directly, following the reviewer suggestion, we have operated an enlarged\n25 cm2 flow cell. Under a pure C H feed, a single 25 cm2 cell reaches a maximum single-pass\n2 2\nconversion of 96.4% with 99.9% C H selectivity. More importantly, under an industrially relevant\n2 4\ncrude ethylene stream containing 0.5% C H and 20% C H (Ar balance), a single 25 cm2 cell\n2 2 2 4\ndelivers >99% acetylene conversion with >99% ethylene selectivity, demonstrating that high\nconcentration ethylene can be obtained in one reactor under realistic compositions.\nFor the more stringent regime in which acetylene becomes highly dilute, and transport dominates,\nwe implement a tandem flow cell scheme in which the first stage reduces 100% C H to a lower\n2 2\nconcentration stream and the second stage serves as a polishing step with an enlarged electrode area.\nIncreasing the electrode area lowers the local flux of dilute C H per unit area and increases its\n2 2\ncontact probability with active sites, enabling deeper depletion of residual acetylene while\nmaintaining high selectivity and reducing the burden on downstream separation. We also carried out\na detailed techno-economic assessment in response to the reviewer’s request, which indicates that\nthis approach is economically viable when the cost of removing trace acetylene by conventional\nseparation is taken into account.\nResponse to Reviewer #4:\nGeneral Comments:\nThe revision has fulfilled this Reviewer´s requirements. The manuscript seems ready for publication:\nResponse: We would like to thank the reviewer again for the considerable time and effort dedicated\nto our work."}]

Decision letter