Operando nuclear magnetic resonance decodes alkali-tuned proton-electron relay boosting CO_2-to-formate conversion

Domain: physics
Published: 2026-01-20
Editorial outcome: major_revision

Abstract
The proton-coupled electron transfer (PCET) kinetics plays a critical role in governing the CO2-to-formate conversion efficiency during CO2 eletroreduction reaction. While alkali metal cations are known to influence the reaction pathway, elucidating how trace doping modifies the catalytic sites remains a key challenge. Here we show that incorporating Li into bismuth oxycarbonate (BOC-Li) induces structural modifications that optimize the PCET process at bismuth-active sites, thereby boosting CO2-to-formate conversion. By employing dual-isotope (2H/13C) operando nuclear magnetic resonance (NMR) to track the formation of 1H13COO−/2H13COO−, combined with kinetic isotope effect, Tafel analysis and in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy, we observe a more efficient proton-electron transfer pathway. Density functional theory (DFT) calculations suggest that Li doping is associated with enhanced activity of Bi sites, potentially strengthening H2O/CO2 adsorption and reducing the O–H activation energy. Collectively, this work highlights alkali doping as a promising strategy for structurally engineering catalytic sites to improve PCET kinetics.

Introduction
The carbon dioxide reduction reaction (CO2RR) to value-added chemicals and fuels, particularly when powered by renewable energy sources, offers a sustainable pathway to mitigate fossil fuel dependence and address escalating greenhouse effects1–3. However, the CO2RR process involves intricate multi-step proton-electron transfer steps4–6. The proton-coupled electron transfer (PCET) mechanism is pivotal in the electrochemical CO2RR7, which relies critically on the dynamic balance among proton supply pathways, electron transfer synergy and intermediate stabilization. Recent optimization strategies for PCET have evolved from singular catalyst design to synergistic regulation of electronic structures and microenvironment engineering. However, challenges persist in dynamically resolving proton sources and transfer pathways, as well as precisely controlling intermediate stability in complex reaction networks. Early theoretical studies revealed the pH- and potential-dependent nature of PCET pathways. For instance, Göttle et al.8 presented that the CO2RR pathway of cobalt porphyrin (CoP) catalysts shifts with pH. Specifically, acidic conditions (pH~3.5) favor a sequential proton-electron transfer (SPET) mechanism forming sequential intermediates, while alkaline conditions (pH≥8.3) promote a concerted proton-electron transfer (CPET) process that directly stabilizes key intermediates without discrete protonation steps. Dunwell et al.9 propose that optimizing experimental conditions, such as employing low overpotentials and minimizing cation-related mass transport limitations, can promote PCET pathways during CO2RR. Within the kinetically controlled regime, PCET acting as the rate-limiting step enhances the efficiency of coupled proton-electron transfer processes and improves CO2 selectivity. These studies, however, did not explicitly address the influence of proton sources on pathway selection. Subsequent research by Li et al.10 achieved 98.1% CO selectivity and presented HCO3−-involved PCET as the RDS for CO2RR to CO in aqueous KHCO3 electrolytes, and Zhang et al.11 lowered the PCET barrier of *OCHO intermediates via Ce-doped Bi by tuning p-band electronic structures. Despite these advances, most studies relied on macroscopic electrochemical data, lacking in-situ tracking of proton transfer dynamics.

To address these limitations, recent efforts have integrated advanced characterization techniques to resolve PCET dynamics. For example, Mukhopadhyay et al.12 employed operando attenuated total reflection infrared reflection-absorption spectroscopy (ATR-IRRAS) to directly observe the characteristic vibration peak of the *OCHO intermediate (1380 cm−1) within a MOF reservoir (UiO-66-CN) and confirmed through DFT calculations that nitrile groups stabilize intermediates via secondary coordination effects, although proton transport efficiency remained constrained by spatial hindrance in MOF pores. Wang et al.13 applied in situ attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) to track the sequential PCET pathway in cobalt phthalocyanine-based COFs, revealing that the polarizing effect of hydrated K+ ions ([K(H2O)n]+) weakens the O–H bonds of coordinated water. However, proton sources still predominantly originated from bulk electrolytes. These advancements underscore the critical need to clarify proton origins (e.g., adsorbed water, electrolyte ions, or catalyst active sites) and their transfer pathways.

Alkali metal cations, widely recognized for their promotional effects in CO2RR, have been studied as electrolyte additives14,15. However, recent advancement has shifted focus toward integrating these alkali cations directly into catalyst architectures for performance optimization. For instance, Ding et al.16 engineered K+-doped CuSe (K11.2%-CuSe), where embedded K+ ions stabilized Cu+ sites, significantly enhancing ethanol selectivity by promoting *CO dimerization. Similarly, Peng et al.17 demonstrated that S and Na co-doping on bismuth (Bi(110)-S-Na) synergistically localized electrons at *OCHO intermediates while accelerating H+ generation from water activation, thereby boosting formate production. These studies establish alkali cation-incorporated catalysts as an effective strategy to address the selectivity and activity bottlenecks in CO2RR systems, thereby offering an effective approach for PCET research.

In this work, we introduce isotope-labeled operando nuclear magnetic resonance (NMR) coupled with a trace Li-incorporated bismuth oxycarbonate (BOC-Li) electrocatalyst to mechanistically probe PCET dynamics in CO2RR. By employing a dual-tracer approach, using 13C-labeled 13CO2 and 2H-based electrolyte (K1H12CO3/2H2O), our operando 13C NMR protocol reveals distinct 1H13COO− and 2H13COO− signals, enabling direct discrimination between HCO3−- and solvent-derived proton pathways. Through Li modification the selectivity of formate has been increased from 13.69% to 95.52% at 300 mA cm−2. Besides, the FE HCOO- reached ~90% even at 1.5 A cm−2. Comparative isotopic analysis between BOC-Li and undoped BOC reveals marked differences in proton transfer mechanisms, elucidating how alkali metal integration influences interfacial PCET kinetics through its effect on the electronic properties of Bi sites. DFT calculations suggest that Li doping on the Bi2O2CO3 (161) surface may enhance CO2RR toward HCOOH by potentially stabilizing four-coordinated Bi sites and promoting the formation of oxygen vacancies (Ov), which could strengthen the adsorption of CO2 and H2O. The Li-Ov configuration shows relatively favorable catalytic activity in simulations, with a predicted H2O dissociation barrier of 0.24 eV and a theoretically favorable pathway for HCOOH formation via the *HCOO intermediate. These results indicate possible synergistic effects in reactant adsorption, proton transfer kinetics, and reaction energetics.

Results
Synthesis and characterization of BOC-Li catalyst
Herein, we drew inspiration from Li’s unique hydration characteristics among alkali metals14 to integrate lithium species into the catalyst architecture leading to the creation of the BOC-Li electrocatalyst. The synthesis employed an optimized microwave-assisted protocol18 followed by sequential LiOH and CO2 treatments in a controlled sealed environment (see “Methods” section for details). Inductively coupled plasma mass spectrometry (ICP-MS) shows 0.0065 wt% of Li loading (Supplementary Table 1). For comparison, the Li-free BOC was used as the control catalyst.

As depicted in Fig. 1a, the X-ray diffraction (XRD) patterns of both BOC-Li and BOC samples are well-indexed to the orthorhombic phase of Bi2O2CO3 (BOC) with the Pna21 (33) space group. The (161) plane is identified as the predominant facet. Unlike BOC, the XRD pattern of BOC-Li exhibits noticeable peak broadening, suggesting reduced crystallinity19. Furthermore, the enlarged XRD patterns presented in Supplementary Fig. 1 demonstrate a minor angular shift of the BOC-Li diffraction pattern toward higher angle. It may attribute to the doping of Li due to its smaller atomic radius than Bi20. The scanning electron microscopy (SEM, Supplementary Fig. 2a) and transmission electron microscopy (TEM, Supplementary Fig. 2b) images show a nanosheet morphology of BOC-Li. Energy-dispersive X-ray (EDX) spectra (Supplementary Fig. 3) confirm the uniform distribution of Bi, O, and C elements throughout the material21,22. The high-resolution TEM (HRTEM) image (Fig. 1b) of BOC-Li displays lattice fringes with spacings of 0.295 nm, corresponding to the (161) lattice plane of orthorhombic Bi2O2CO3, respectively. In addition, the HRTEM image of BOC-Li reveals a significant presence of lattice distortions and dislocations, which can be attributed to the incorporation of Li23–26. For comparison, the HRTEM image of BOC (Fig. 1c) shows well-defined lattice fringes with a spacing of 0.295 nm, corresponding to the (161) lattice plane of BOC. X-ray photoelectron spectroscopy (XPS) was employed to examine the electronic structures of BOC-Li and BOC. The Bi 4f XPS spectra (Fig. 1d) for both BOC-Li and BOC exhibit two distinct peaks. The peaks centered at higher binding energies, ~164.49 and 164.2 eV, are attributed to the 4f 5/2 orbital of Bi3+. The peaks at 159.19 and 158.9 eV correspond to the 4f 7/2 orbital of Bi3+18. The peaks of BOC-Li shift 0.29 eV toward higher binding energies, indicating a slightly higher oxidation state27. The C 1s XPS spectrum (Fig. 1e) reveals C–C and C–O peaks at 284.8 and 286.4 eV respectively and that the C=O peak of BOC-Li at 288.9 eV is 0.4 eV higher than that of BOC7,28.

To resolve the atomic-scale local environment of the incorporated Li species, we employed solid-state 7Li magic-angle spinning NMR (MAS NMR) spectroscopy. As shown in Fig. 1f, Supplementary Fig. 4 and Supplementary Note 1, the deconvoluted spectrum exhibit two resolved resonances at δ = −0.16 ppm (Site I) and δ = −0.53 ppm (Site II), separated by a chemical shift difference (Δδ) of 0.37 ppm (85.30 Hz), confirming the existence of distinct Li coordination geometries. Quantitative integration of these signals yields a population ratio of 40.2% (Site I) to 59.8% (Site II), corresponding to a ratio of 2:3, which is consistent with the predicted dual Li-coordination environments in the crystalline framework. To further elucidate the local coordination of Li dopants, we performed variable-speed 7Li MAS NMR spectroscopy. As shown in Fig. 1g and Supplementary Note 2, increasing the MAS rate from 8 to 20 kHz results in line narrowing and the emergence of a dominant peak with a discernible shoulder. This spectral evolution confirms that the Li dopants are structurally immobilized within well-defined, yet slightly distinct, lattice sites. The absence of high Li mobility and the discrete nature of these sites strongly support our assignment of Li incorporation into the crystal lattice.

Furthermore, the Fourier transform infrared (FTIR) spectrum of BOC-Li (Fig. 1h) resembles that of BOC, exhibiting key vibrational modes including Bi-O stretching vibrations at 548.5 cm−1, out-of-plane bending modes of υ CO32− at 848.5 cm−1, and C–O stretching vibrations of (CO32−) at 1391 and 1465 cm−1. A notable difference is observed at the 1465 cm−1 peak, where BOC-Li displays a more pronounced absorption compared to BOC29. Comparative analysis of UV–Vis absorption spectra reveals a difference between BOC and BOC-Li systems (Fig. 1i). While pristine BOC shows no absorption in this region, BOC-Li displays a pronounced spectral feature centered at ~400 nm, likely corresponding to the emergence of a Li-associated coordination complex species.

CO2RR performance evaluation and mechanism investigation
The CO2RR performance was initially evaluated in a standard three-electrode H cell using CO2-saturated 0.5 M KHCO3 electrolyte. The gaseous and liquid products were quantified by gas chromatography (GC) and NMR spectroscopy, respectively6,30. As shown in Fig. 2a and Supplementary Fig. 5, BOC-Li exhibits higher selectivity toward formate than the pristine BOC catalyst, achieving high faradaic efficiency (FE) across a wide potential window while effectively suppressing the formation of CO and H2. Linear sweep voltammetry (LSV) measurements (Supplementary Fig. 6) indicate that BOC-Li attains a higher current density (−295 mA cm−2 at −2.0 V vs. RHE, RHE calibration in Supplementary Fig. 7, without iR compensation) compared to BOC (−200 mA cm−2). Electrochemical impedance spectroscopy (EIS) was employed to probe the interfacial charge transfer kinetics. The Nyquist plots in Fig. 2b show a discernibly smaller semicircle diameter in the high-frequency region for BOC-Li, indicating a decrease in charge transfer resistance. Together, these results—demonstrating higher selectivity, activity, and charge transfer efficiency—collectively verify the better CO2-to-formate conversion capability of BOC-Li in the H-cell setup.

To probe the origin of the enhanced catalytic performance, we investigated the reaction kinetics. The Tafel slope for formate production on BOC-Li is 64.0 mV dec−1, substantially lower than the 85.7 mV dec−1 value measured for undoped BOC (Fig. 2c). A Tafel slope near 60 mV dec−1 is consistent with a concerted proton-electron transfer (CPET) process. This interpretation is further supported by a deuterium kinetic isotope effect (KIE), wherein the formate production yield decreases monotonically as the 2H2O content in the electrolyte increases (Fig. 2d), confirming that proton transfer is involved in the rate-determining step6. Furthermore, this enhanced kinetics is facilitated by an improved reactant affinity, as solution-phase 13C NMR measurements revealed a stronger CO2 adsorption capacity for BOC-Li compared to BOC (Supplementary Fig. 8 and Supplementary Note 3).

To gain further mechanistic insight into the PCET process, we performed in situ ATR-SEIRAS. The spectra show characteristic peaks at 1438 and 1600 cm−1, which are assigned to the adsorbed *OCHO intermediate31–37. Additional spectral features are identified as follows: peaks at ~1272 cm−1 correspond to *CO37,38, the bands at ~1358 and ~1390 cm−1 are attributed to bicarbonate (HCO3−) and carbonate (CO32−), respectively39–42, and the peak at ~1638 cm−1 is assigned to the O–H bending mode of water33,40,43–45, based on our control experiments (Supplementary Figs. 9 and 10) and literature references. On BOC-Li, the *OCHO-related peaks at 1438 and 1600 cm−1 emerge and intensify at more negative potentials (Fig. 2e), whereas these features remain weak on undoped BOC (Fig. 2f). The potential-dependent formation and stabilization of this key intermediate indicate that the local environment of BOC-Li is favorable for the PCET step. Together, these spectroscopic data provide direct evidence that Li+ doping appears to promote formate production by facilitating a concerted PCET mechanism at the Bi sites.

Unraveling reaction mechanisms with operando NMR
The electrochemical reduction of CO2 to formate is known to proceed through competing pathways. However, their mechanistic details—particularly concerning protonation sources and oxygen transfer dynamics—remain contentious. Early studies have proposed two possible routes, namely the *OCHO and the *COOH pathways. The former proceeds through an oxygen-bound intermediates with sequential PCET, while the latter requires direct formation of a carbon-bound intermediate46,47. To contribute to this discussion, we employed a multimodal operando NMR approach integrating 1H, 17O, and 13C/2H isotopic tracking, focusing on a comparison between BOC-Li and undoped BOC. Our goal is to examine how Li-induced modifications to the electronic structure of Bi sites influence water-mediated oxygen transfer and protonation dynamics. This is achieved by correlating real-time intermediate speciation with interfacial kinetics under operational CO2RR conditions.

Operando 1H/17O NMR reveals H2O-oxygen activation on BOC-Li
Catalytic performance was evaluated in a custom three-electrode NMR microreactor (Supplementary Fig. 11) under continuous CO2 flow (1.5 standard cubic centimeters per minute, 1.5 sccm) at 298 K, using an isotopically enriched electrolyte (450 μL 2H216O, 50 μL 1H217O and 25.16 mg K1H12CO3)47. The working electrode was prepared by loading the catalyst onto carbon microfibers, with Pt wire as the counter/reference electrode.

We leveraged synchronized operando 1H/17O NMR to track water-derived oxygen. This enabled the simultaneous quantification of formate (1H NMR) and oxygenated species (17O NMR), thereby linking product formation to oxygen transfer events. As shown in Fig. 3a, in situ 1H NMR quantification during 3 h electrolysis at −1.18 V revealed that BOC-Li produced 21 times more formate than the undoped BOC. The formate peak also exhibited a downfield chemical shift of 3.50 Hz (from 8.467 ppm for BOC to 8.472 ppm for BOC-Li). The corresponding 17O NMR spectrum of BOC-Li (Fig. 3b) revealed distinct signals from key oxygenated species. Compared to undoped BOC, these signals exhibited significant downfield shifts: hydrated CO2 (79.381 ppm, +2.37 Hz), bicarbonate (173.21 ppm, +10.44 Hz), and 17O-formate (288.737 ppm, +25.14 Hz). Moreover, the signal intensity for 17O-formate was greater in BOC-Li.

To track oxygen involvement, we analyzed the 17O distribution in the hydrated CO2 and formate. The absence of detectable C-17OH signals in formates suggests rapid hydroxyl exchange between reaction intermediates and the solvent water. The transient nature of hydroxyl-bound 17O atoms explains their lack of detection by NMR. Consequently, the 17O NMR resonance at 288.737 ppm is collectively assigned to all 17O-enriched isotopologues (1HC17O16O−, 1HC17O17O−), serving as a unified spectroscopic marker.

The relationship between oxygen transfer dynamics and catalytic stability was investigated through a multimodal operando NMR approach. By performing synchronized 1H/17O spectroscopy with time-resolved spectral acquisition (30-min intervals over 180 min), we tracked oxygen incorporation pathways. As shown in Fig. 3c and Supplementary Fig. 12, BOC-Li exhibits rapid HCOO− signal emergence (τ onset < 30 min) with linear concentration growth, indicative of stable redox-active sites and efficient C–H bond formation. In contrast, formate signals for BOC were near the background level and accumulated slowly, indicating its lower activity.

Complementary operando 17O NMR analysis (Fig. 3d and Supplementary Fig. 13) provides insight into the oxygen transfer efficiency underlying these performance differences. For BOC-Li (left panel), distinct 17O-formate resonances (288.737 ppm) appear within 30 min and intensify linearly over 3 h, consistent with rapid oxygen activation and sustained interfacial stability. In contrast, BOC (right panel) shows negligible 17O-formate signals with minimal growth, indicating sluggish oxygen incorporation. This direct temporal correlation between persistent 17O-formate accumulation in BOC-Li and the stagnant signal in BOC aligns with the 21-fold productivity difference (Fig. 3a), strongly ascribing the enhancement to Li doping.

To probe oxygen incorporation pathways during CO2 electroreduction, we quantified the C=17O isotopologues in the formate products using a published method47. The quantitative analysis indicated that 17O-labeled formate constituted 2.46 mol% of the total produced by BOC-Li after 3 h of operation. This value is notably below the theoretical maximum for direct CO2-derived oxygen, supporting the dominance of the *OCHO pathway over the *COOH route47. In contrast, undoped BOC yielded only 1.50%. This enhanced isotopic incorporation in BOC-Li, resolved by operando 1H/17O NMR, confirms that Li doping promotes water-derived oxygen participation by modifying interfacial oxygen transfer dynamics.

Operando 13C NMR reveals the proton source of HCOO−
The electrochemical reduction of CO2 to formate is known to proceed through competing pathways. However, their mechanistic details—particularly concerning protonation sources and oxygen transfer dynamics—remain contentious. Early studies have proposed two possible routes, namely the *OCHO and the *COOH pathways. The former proceeds through an oxygen-bound intermediates with sequential PCET, while the latter requires direct formation of a carbon-bound intermediate46,47. To contribute to this discussion, we employed a multimodal operando NMR approach integrating 1H, 17O, and 13C/2H isotopic tracking, focusing on a comparison between BOC-Li and undoped BOC. Our goal is to examine how Li-induced modifications to the electronic structure of Bi sites influence water-mediated oxygen transfer and protonation dynamics. This is achieved by correlating real-time intermediate speciation with interfacial kinetics under operational CO2RR conditions.

To probe proton sourcing, we employed a dual-isotope (13C/2H) operando NMR approach. This method distinguishes protonation pathways by utilizing 2H-13C J-coupling in the 2H13COO− product, which discriminates between deuterium from water and protons from bicarbonate under 13CO2RR conditions. The operando 13C NMR experiments were conducted in a three-electrode setup with a BOC-Li-modified carbon fiber working electrode, Pt counter/reference electrodes, and a deuterated electrolyte (500 μL 2H2O, 25.16 mg K1H12CO3). Prior to electrolysis, the electrolyte was pre-saturated with 13CO2, which was then continuously supplied at 1.5 sccm during electrolysis at −1.28 V to maintain a constant concentration at the gas-liquid interface at 298 K.

As shown in Supplementary Fig. 14, 1H-decoupled 13C NMR spectroscopy clearly showed signals for solvated 13CO2 (δ = 124.62 ppm), bicarbonate (H13CO3−, δ = 160.23 ppm), and the dominant formate derivatives (δ = 170~172 ppm). High-resolution spectral deconvolution in 2H2O (Fig. 3e, blue bottom trace and Supplementary Note 4) revealed two distinct formate configurations. A diagnostic 1:1:1 triplet (δ = 170.89 ppm, purple triangles, 2J[2H-13C] = 29.83 Hz) was assigned to 2H13COO−, arising from scalar coupling between 13C and the bonded deuterium (n = 1, 2H, I = 1). Concurrently, a singlet at δ = 171.04 ppm (golden star) corresponds to 1H13COO−. The triplet serves as a spectroscopic fingerprint for α-carbon-selective deuteration, while the singlet indicates the coexistence of protonated isotopologues. This dual-isotope strategy effectively distinguishes protonation pathways: deuterium from interfacial water (2H13COO−) versus protons from bicarbonate (1H13COO−), providing strong evidence for deuteration dynamics and proton transfer selectivity.

The key finding is that formate formation is dominated by protons derived from water, as unambiguously resolved by their distinct J-coupling signatures. To gain further insight into interfacial proton transfer dynamics, we performed controlled 13C NMR in a mixed 2H2O/1H2O electrolyte (1:1 v/v, 25.16 mg K1H12CO3, 13CO2-saturated; Fig. 3e and Supplementary Note 4). Firstly, the diagnostic 2H13COO− triplet (δ = 170.89 ppm, purple triangles, 1:1:1, 2J[2H-13C] = 29.83 Hz) remained well-resolved, confirming persistent 2H-13C coupling. Secondly, in the decoupled spectra (pink middle trace), the 1H13COO− singlet (δ = 171.04 ppm, golden star) intensified relative to pure 2H2O conditions, consistent with greater proton availability from enhanced 1H2O auto-protolysis and K1H12CO3 dissociation in the mixed phase. Finally, in the coupled spectrum (red top trace), the 1H13COO− signature appeared as a 1:1 doublet (δ = 171.04 ppm, golden stars, 1J[1H-13C] = 194.79 Hz), demonstrating the distinct NMR signature of spin-1/2 protons (doublet). The simultaneous detection of deuterated (triplet) and protonated (doublet/singlet) adducts showcases the power of this dual-isotope operando NMR approach for tracking proton transfer pathways and interfacial deuteration kinetics.

The catalytic selectivity and proton transfer dynamics of CO2RR were probed using a multimodal NMR approach integrating 1H, 13C, and 2H isotopic tracing. For BOC-Li, operando 1H NMR revealed a characteristic α-proton doublet for 1H13COO− (δ = 8.465 ppm, 1J[1H-13C] = 194.79 Hz; Supplementary Fig. 15), confirming high isotopic fidelity. Complementary 13C NMR (Supplementary Fig. 16) and 2H NMR (Supplementary Fig. 17) spectra differentiated protonation pathways: the 13C NMR spectrum resolved the 2H13COO− triplet (from water) and the 1H13COO− singlet, while the 2H NMR spectrum showed the corresponding 2H13COO− doublet (2J[2H-13C] = 29.83 Hz, from bicarbonate). This pronounced isotopic discrimination, which correlates with BOC-Li’s higher performance, indicates that Li doping tunes the electronic structure of Bi active sites, thereby modulating proton transfer kinetics.

This interpretation is further supported by controlled electrolysis in 0.5 M K1H12CO3 (Fig. 3f). Spectral deconvolution showed that the 1H13COO−/2H13COO− ratio was lower for BOC-Li (8.5%) than for BOC (10.3%), indicating a relative decrease in bicarbonate-mediated protonation and a correspondingly greater reliance on water-derived protons for BOC-Li. This isotopic preference, which aligns with BOC-Li’s higher Faradaic efficiency, indicates that Li doping modifies the electronic structure of Bi sites, thereby promoting CO2 pre-activation and stabilizing the *OCHO intermediate.

The proton transfer pathways and carbon speciation during 13CO2RR were tracked by synchronized operando 1H and 13C NMR at −1.28 V over 3 h. Comparative 1H NMR (Supplementary Figs. 18 and 19) showed persistently higher and faster formate production for BOC-Li (signal growth at δ = 8.465 ppm). Complementing this, time-resolved 13C NMR (Fig. 3g, h, and Supplementary Fig. 20) clearly distinguished the 2H13COO− triplet from the 1H13COO− singlet, revealing a systematically stronger progression of the water-derived 2H13COO− signal for BOC-Li. This consistent kinetic advantage is attributed to Li-mediated tuning of the Bi sites, which enhances water-derived proton transfer. Thus, this multimodal approach not only distinguishes catalytic pathways but specifically reveals how Li doping steers the PCET mechanism toward a water-mediated route.

DFT calculation
To elucidate the Li-mediated activity of HCOOH formation, first-principles DFT calculations were systematically performed. As depicted in Fig. 4a, the predominant Bi2O2CO3 (161) surface identified through XRD analysis comprises four/three-coordinated Bi (Bi4c/Bi3c) and four/three-coordinated oxygen (O4c/O3c) atoms. Computational simulations suggested two distinct Li-doping sites on this surface, both of which induced spontaneous oxygen vacancy (Ov) formation, as illustrated in the structural models (Supplementary Fig. 21). The computational analysis indicates Li dopants tend to stabilize at four-coordinated sites (Li4c), with a thermodynamic advantage of 0.32 eV over three-coordinated sites (Li3c). It is worth noting that Li doping promotes the formation of surface Ov, which are predicted to act as new reaction centers (Supplementary Fig. 22). Additionally, Bader charge analysis suggests that Li4c exhibits a relatively lower charge state (+0.90|e|) compared to Li3c (+0.92|e|). These theoretical insights offer a mechanistic rationalization for the experimentally observed observations of dual Li species populations.

To elucidate the role of Li doping in catalytic activity, we systematically investigated CO2RR with H2O as the proton source. On the pristine Bi2O2CO3 (161) surface, H2O adsorbs at Bi4c and Bi3c sites with comparable energies of −0.77 eV and −0.79 eV, respectively (Fig. 4b, and Supplementary Fig. 23). In contrast, Li-doped configurations exhibit stronger H2O adsorption strength at Ov sites, with the adsorption energies of −0.80 eV (Li3c-doped) and −0.91 eV (Li4c-doped) (Fig. 4d). For CO2, the molecule preferentially binds at the Bi3c-O3c-Bi3c bridge site on the pristine surface, adopting a bent configuration and an adsorption energy of −0.54 eV (Fig. 4c, and Supplementary Table 2). Upon Li doping, CO2 adsorption is also enhanced, with adsorption energies of −0.75 eV (Li4c-doped) and −0.69 eV (Li3c-doped) compared to the undoped surface (Fig. 4d, and Supplementary Fig. 24), suggesting stronger surface interactions. This progressive increase in adsorption strength appears to correlate with improved CO2 surface coverage, suggesting that Li incorporation may facilitate CO2RR by optimizing the thermodynamics of reactant adsorption.

Moreover, H2O dissociation appears to be facilitated on the Li-doped surface. As shown in Fig. 4e and Supplementary Fig. 25, the energy profiles reveal a marked reduction in the activation energy (Ea) of O–H bond cleavage at Ov sites on the Li-doped surfaces, decreasing to 0.34 eV for Li4c-doped and 0.24 eV for Li3c-doped configurations, compared to 0.40 eV and 0.36 eV at Bi4c and Bi3c sites on the pristine surface. Notably, the Li3c-doped configuration, in particular, exhibits a more thermodynamically favorable H2O dissociation, with an exothermic energy of −0.43 eV, outperforming other reactive sites. We also compared the H2O dissociation at non-oxygen-vacancy sites on Li-doped Bi2O2CO3 (161) surfaces, further confirming the active Li-doped surface compared to clean surface (Supplementary Fig. 26, and Supplementary Note 5). These results suggest that Li incorporation may promote H2O dissociation, potentially increasing proton availability for subsequent catalytic steps.

To elucidate the origin of the lower activation barrier for water dissociation on the Li3c-doped surface, electronic structure analyzes were performed. The projected density of states (pDOS) analysis shows that for the clean surface, OH− adsorption induces a narrow peak slightly below the conduction band minimum (CBM), whereas on the doped surface a broader peak appears further below the CBM, indicating stronger orbital hybridization with Bi and enhanced bonding stability (Fig. 4f). In addition, the p-band center of surface Ov on the doped surface, particularly for Li3c doping, is shifted upward relative on the clean surface, which enhances H+ adsorption and further stabilizes the reaction intermediates (Fig. 4g). These results collectively suggest that Li doping modifies the surface electronic structure in a way that stabilizes both OH and H species, thereby facilitating water dissociation. It is also worth noting that the stronger binding of H+ could suppress the H–H coupling process, making the formation of H2 less thermodynamically favorable and consequently reducing its selectivity, which is consistent with the experimental observation.

To further evaluate the effect of Li-doping, the CO2RR to HCOOH was examined on pristine, Li4c-, and Li3c-doped surfaces. Free energy profiles for HCOOH formation via the *OCHO pathway (Fig. 4h, Supplementary Fig. 27, and Supplementary note 6) indicate that the initial hydrogenation step (*CO2 + *H+/e− → *OCHO) requires a noticeable energy input, followed by an exothermic hydrogenation step leading to HCOOH formation. Comparative analysis suggests that the Li3c-doped surface appears to exhibit the lowest overall energy requirement for HCOOH production among the three configurations, implying a potentially higher catalytic efficiency. These results point to Li3c-doping as a potentially favorable configuration for promoting HCOOH synthesis. More broadly, these findings support Li incorporation as a promising strategy to modulate CO2RR activity, with site-specific doping suggested as a possible route for tuning catalytic performance.

Flow-cell CO2RR performance
To assess the commercial potential of BOC-Li for CO2-to-formate conversion, we evaluated its performance across a range of current densities in a flow cell reactor (Fig. 5a) in 1.0 M KOH electrolyte48. LSV shows that BOC-Li delivers a 28% higher current density than BOC (−1762 vs. −1381 mA cm−2 at −4.97 V vs. RHE, Supplementary Fig. 28). This is accompanied by a doubled double-layer capacitance (Cdl), with measured values of 9.34 vs. 4.39 mF cm−2 for BOC-Li and BOC, respectively (Supplementary Fig. 29), indicating a likely increase in the electrochemical active surface area. This is further supported by a lower charge transfer resistance, as evidenced by the smaller semicircle in the Nyquist plot (Fig. 5b).

BOC-Li exhibits better CO2-to-formate performance across a wide current density range (100~1500 mA cm−2), sustaining a FE of ~90% with marginal decay at industrially relevant current densities (Fig. 5c). Notably, it achieves an FE of 95.5% at 300 mA cm−2 and a low potential of −1.50 V, whereas the undoped BOC catalyst only reaches 13.7% at a more negative potential of −2.07 V under the same condition (Fig. 5d). This performance disparity underscores the critical role of Li doping in enhancing both activity and selectivity. Furthermore, BOC-Li demonstrates robust operational stability over 289 h at 100 mA cm−2 with the FE HCOO- over 95% (Fig. 5e), and its overall performance is highly competitive among state-of-the-art formate-selective electrocatalysts (Fig. 5f, and Supplementary Table 3).

Post-reaction characterization confirms the structural stability of the BOC-Li catalyst. SEM imaging shows that the nanosheet morphology is preserved after electrolysis (Supplementary Fig. 30). XRD patterns demonstrate that the crystal structure of orthorhombic Bi2O2CO3 remains intact (Supplementary Fig. 31). XPS analysis indicates that the Li-modified oxidation state of Bi is maintained (Supplementary Fig. 32). Furthermore, ex situ 7Li MAS NMR spectra confirm that the two distinct Li sites remain structurally stable (Supplementary Fig. 33). These results collectively illustrate the structural and compositional integrity of BOC-Li under CO2RR conditions.

Extension to other alkali dopants
To systematically explore the broader potential of our approach, we extended the doping strategy to include Na+ and K+. Under identical synthesis conditions, both dopants were successfully incorporated into the BOC structure, as confirmed by comprehensive characterization. Similar to the Li+-modified catalyst, these alkali-doped variants exhibited universally enhanced electrochemical properties, including increased active surface area and improved charge transfer kinetics compared to pristine BOC (Supplementary Figs. 34–52). This common improvement translated directly to catalytic performance, with BOC-Na and BOC-K achieving high FE formate at low-to-medium current densities. Operando NMR spectroscopy further verified that both dopants promoted the water-derived protonation pathway, confirming that the enhanced PCET mechanism represents a general phenomenon across alkali-modified catalysts.

The investigation reveals that while the fundamental enhancement in PCET kinetics is a general characteristic of alkali doping, the specific choice of dopant determines the operational stability under practically relevant conditions. Although all doped catalysts performed comparably at moderate current densities, BOC-Li demonstrated better performance maintenance, sustaining >90% FE formate across an extended operating range up to 1100 mA cm−2, while the performance of BOC-Na and BOC-K began to decay beyond 500 mA cm−2. This systematic study establishes alkali doping as a promising strategy for modulating PCET kinetics in CO2-to-formate conversion, with Li+ emerging as the optimal dopant for applications requiring sustained high-current operation.

Methods
Chemicals
The following chemicals were acquired from their respective suppliers for the project: Bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, 99.0%) and a 5% Nafion 117 solution (5% in a mixture of lower aliphatic alcohol and water) were sourced from Shanghai Macklin Biochemical Technology Co., Ltd. Dicyandiamide (DCD, 99%) was provided by J&K Scientific Ltd. Shanghai Titan Scientific Co., Ltd was the supplier for Ethyleneglycol (EG, 99%), potassium hydroxide (KOH, 90.0%), lithium hydroxide (LiOH, 99%), potassium bicarbonate (KHCO3, 99.5%), and ethanol (EtOH, 99.7%). Deuterium oxide (2H2O, 99.9%) was obtained from Qingdao Tenglong Microwave Technology Co., Ltd, and isopropanol (PrOH, 99.7%) was supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. All chemicals were used as received, without further purification. Deionized (DI) water for our experiments was provided by the LICHEN System (UPTA-10).

Synthesis of Bi-based precursor
The Bi-based precursor was synthesized using a refined microwave-assisted ultrasonic method18. Initially, 2 mmol of Bi(NO3)3·5H2O and 30 mmol of dicyandiamide were dissolved sequentially in 100 mL of ethyleneglycol (EG) under ultrasonic agitation to form a clear solution. This was performed in a specialized three-neck flask.

The flask was then connected to a microwave/ultrasonic synthesizer (XH-300A, Beijing Xianghu Science and Technology Development Co., Ltd), equipped with a condensation setup. The reaction was conducted under a constant temperature mode, with microwave and ultrasonic powers both set to 500 W. The temperature was ramped to 150°C within 10 min and maintained for 20 min using an automated program.

The resulting white precipitate was collected by centrifugation and washed thoroughly with deionized water and ethanol to remove impurities. The final product was obtained as a white powder after drying at 60°C for 12 h in a vacuum oven.

Synthesis of BOC-Li
The synthesized Bi-based precursor was subjected to further modification using carbon dioxide (CO2) as both a reactant and a means to create a high-pressure environment, in conjunction with a lithium hydroxide (LiOH) solution. The process involved adding 300 mg of the Bi-based precursor into a 20 mL reaction vessel containing 2 mL of a 0.5 M LiOH solution. The vessel was then sealed and charged with 50 mL of CO2 gas to facilitate the reaction under high-pressure conditions. After 3 h of ultrasonication to promote complete mixing and reaction, the white precipitate formed was collected by centrifugation. It was subsequently washed thoroughly with deionized water and ethanol to remove any remaining impurities. The BOC-Li was then obtained as a white powder following a drying process at 60°C for 12 h in a vacuum oven.

Characterization
X-ray diffraction (XRD) analysis was executed on a PANalytical B.V. Rayon X-ray Diffraction System to ascertain the crystal structure of the synthesized electrocatalyst. The surface morphology was examined with precision using a Hitachi S4800 Scanning Electron Microscope (SEM) and a JEM-F200 Transmission Electron Microscope (TEM). Energy-dispersive X-ray spectroscopy (EDS) was utilized with a JED-2300T Analysis Station to evaluate the elemental composition of the samples.

X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Scientific Nexsa system, powered by an Al Kα X-ray source (1486.6 eV), to obtain intricate details of the chemical composition. The C 1s peak, positioned at 284.8 eV, served as a reference for the calibration of binding energies. Fourier-transform infrared spectroscopy (FTIR) analysis was conducted using a Thermo Fisher Nicolet iS50, while in situ FTIR measurements were captured with a Nicolet iS50 MCT A Detector. Ultraviolet-visible (UV–Vis) spectroscopy was carried out with a UV 3600I plus instrument. Inductively coupled plasma mass spectrometry (ICP-MS) utilizing an Agilent 7700 system was employed for a comprehensive evaluation of trace Li elements.

In situ ATR-SEIRAS measurements were performed in an external reflection configuration using a custom-built accessory coupled to an FTIR spectrometer (PerkinElmer Spectrum 100 detector). Spectra were collected as an average of 16 scans with a spectral resolution of 8 cm−1. A thin gold film (~10 nm) was deposited onto a silicon prism via electroless deposition. The catalyst ink was deposited onto the Au film by drop-casting 25 μL of the ink twice, allowing it to air-dry after each deposition. The catalyst-coated Si prism served as the working electrode in a three-electrode spectroelectrochemical cell, with a platinum wire as the counter electrode and an Ag/AgCl electrode (3.5 M KCl) as the reference. The cell was filled with CO2-saturated 0.5 M KHCO3 electrolyte prior to each experiment. Electrochemical control was maintained using a CHI760E/CHI660E potentiostat.

Solid state Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) experiments of the 7Li state in BOC-Li sample were conducted on a Bruker 600 MHz AVANCE III operating at larmor frequency of 600.18 MHz for 1H and 233.25 MHz for 7Li. Instrument was equipped with H/X DVT MAS probe, supporting MAS rotor with 4.0 mm outer diameter. All the experiments conducted on Bruker 600 MHz AVANCE III were with MAS frequency of 8 kHz. Besides, we have supplemented the 7Li NMR at different spinning rate (8k, 15k and 20 kHz) on Bruker AVANCE NEO 400 WB spectrometer operating at 155.55 MHz for 7Li, equipped with 3.2 mm double-resonance MAS probe.

Operando 1H, 17O and 13C NMR experiments were carried out on the Varian 700 MHz with an Agilent 5-mm z-axis pulsed-field gradient BB5 resonance probe at a 1H Larmor frequency of 699.79 MHz and 17O Larmor frequency of 94.87 MHz and 13C Larmor frequency of 175.96 MHz. Operando 1H NMR spectra were acquired using the s2pul selective excitation pulse sequence. Operando 17O NMR spectra were acquired using the s2pul pulse sequence with proton decoupling. Operando 13C NMR Spectra were acquired using the s2pul selective excitation pulse sequence with composite pulse 1H decoupling.

Quantitative analysis of the liquid-phase products 2H, 17O and 13C spectra and 13CO2 adsorption experiments were acquired on Bruker AVANCE III HD 500 MHz spectrometer with a 5 mm broadband probe at a 1H Larmor frequency of 500.17 MHz, 2H Larmor frequency of 76.78 MHz and 13C Larmor frequency of 125.77 MHz. The NMR experiments were conducted at 298 K.

Products quantification
The reaction products were analyzed using a combination of online gas chromatography (GC) and offline 1H NMR spectroscopy. Gaseous products generated during CO2 electrolysis were monitored in real time with a GC system (Ruimin GC-2060), equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). Liquid products were quantified by offline 1H NMR. The Faradaic efficiency (FE) for each product was calculated using the following equation:

FE = F × n × Z / Q × 100% (1)

F is Faraday constant (96,485 C mol−1), n is the mole of product, Z is the quantity of transferred electrons for each product, Q is total transfer charge.

Electrochemical measurements
Electrochemical measurements were performed using a CHI760E/CHI660E potentiostat (equipped with a 10A current booster for high current densities tests) at an ambient temperature (298 K) and pressure. The flow rate of high-purity CO2 in our research was controlled by a mass flow controller (Alicat Scientific).

Reference electrode calibration. All applied cathode potentials values were recorded without iR compensation. All potentials in this work were referenced to the reversible hydrogen electrode (RHE) scale. To eliminate errors associated with liquid-junction potentials, the Ag/AgCl reference electrode (3.5 M KCl) was calibrated in each electrolyte used for testing. A commercial RHE electrode (Dongping Electrochemical Instrument) with exchangeable internal electrolyte was employed as the primary standard. Before each calibration, the RHE body was filled with the same electrolyte as the test solution (1 M KOH or CO2-saturated 0.5 M KHCO3) and activated in situ by applying a cathodic potential (−9 V vs. a Pt counter) to generate a stable hydrogen layer on its internal Pt catalyst. The open-circuit potential difference between the activated RHE and the Ag/AgCl reference electrode was determined via cyclic voltammetry (CV, Supplementary Fig. 7) at a scan rate of 1 mV s−1 using a CHI 660E electrochemical workstation. The steady-state potential was taken as the average of the two intercepts where the current crossed zero during the forward and reverse sweeps49. The measured offset was negative in both electrolytes, confirming that the RHE potential lies more negative than that of Ag/AgCl under the present conditions. In 1 M KOH, the average offset (Eoffset) was −0.9894 V, giving the conversion formula E(vs. RHE) = E(vs. Ag/AgCl) – Eoffset = E(vs. Ag/AgCl) + 0.9894 V. In CO2-saturated 0.5 M KHCO3, the Eoffset was −0.628 V, yielding E(vs. RHE) = E(vs. Ag/AgCl) – Eoffset = E(vs. Ag/AgCl) + 0.628 V.

The polarization curves were meticulously recorded across a potential spectrum from 0 V to −5.0 V (vs. RHE), employing a scan rate of 10 mV/s. The electrochemical active surface area (ECSA) was estimated from double-layer capacitance measurements. Cyclic voltammetry (CV) was performed at sequentially increasing scan rates (40, 80, 120, 160, and 200 mV s−1) within a non-faradaic potential window of 0.551 to 0.651 V vs. RHE without iR compensation. The capacitive current density (Δj) at 0.601 V vs. RHE was calculated for each scan rate using the following formula:

Δj = ja − jc / 2 (2)

Where ja and jc represent the anodic and cathodic current densities of catalysts, respectively. The calculated Δj values were then plotted against the corresponding scan rates and fitted linearly. The slope of this linear fit corresponds to the double-layer capacitance (Cdl).

Electrochemical impedance spectroscopy (EIS) measurements were performed at a constant potential of −0.37 V over a frequency range of 100 kHz to 1 mHz with an AC amplitude of 5 mV. The Nyquist plots were analyzed using the equivalent circuit model shown in the insets of Figs. 2b and 5b, and Supplementary Figs. 41 and 50, which consists of a solution resistance (Rs) in series with a parallel combination of a constant-phase element (CPE) and a charge-transfer resistance (Rct).

Electrolysis in an H cell. The working electrode was prepared by loading catalyst onto carbon paper (SCP130N). Specifically, 10 mg of catalyst and 50 μL of Nafion solution were dispersed in 1000 μL of isopropyl alcohol, followed by ultrasonication for at least 30 min to form a homogeneous ink. A total of 100 μL (50 uL on each side) of the ink was evenly applied to both sides of the carbon paper and allowed to dry naturally, resulting in a catalyst loading (The weighted catalyst loading normalized by the geometric area) of ~1 mg cm−2 over a fixed area of 1×1 cm2 on a 1×2 cm2 carbon paper. A platinum plate (1.5×3 cm2) served as the counter electrode. A Nafion 117 proton exchange membrane (3×3 cm2, 183 μm thick) was used to separate the compartments. The membrane was pretreated by first boiling in 5 wt% H2O2 at 80°C for 1 h, followed by soaking in deionized water for 30 min. It was then boiled in 5 wt% H2SO4 at 80°C for 1 h and finally soaked again in deionized water for 30 min before use. The electrolyte employed was 0.5 M KHCO3, prepared by dissolving 50.31 g of KHCO3 and diluting to 1 L with distilled water. In the electrochemical cell, 40 mL of this solution was employed as the anolyte in the anode chamber, whereas 40 mL of the CO2-saturated electrolyte (pH = 7.27 ± 0.04, n = 3) was employed as the catholyte in the cathode chamber.

Electrolysis in a flow cell. A custom three-compartment flow cell was employed for the electrolysis. The compartments were separated by an anion exchange membrane (Fumasep FAB-PK-130, 3×3 cm2). The membrane, with a dry-state thickness of 110–150 μm, was pretreated prior to use by soaking in 0.5 M NaCl solution at 298 K for 72 h, with the solution being replaced several times during this period. The gas diffusion electrode (GDE) was fabricated by spray-coating catalyst ink onto a gas diffusion layer (GDL, Sigracet 28BC or 600 nm Ag/PTFE), with catalyst ink containing 25 or 50 mg catalyst sprayed onto a 3×6 cm2 gas diffusion layer to achieve a loading of at least 1 mg cm−2 or 2 mg cm−2, respectively. Plastic plate with a 0.5 cm2 window and silicone gaskets was used for structuring and sealing. A treated nickel foam (3×3 cm2) served as the counter electrode. Both chambers were filled with 30 mL of 1 M KOH electrolyte (pH = 13.98 ± 0.03, n = 3). For stability test, both chambers were filled with 1 L of 1 M KOH electrolyte. The electrolyte was prepared by dissolving 62.34 g of KOH in distilled water and diluting to a final volume of 1 L, and was circulated at a rate of 15 mL min−1.

All electrolytes mentioned above were sealed and stored in sealed polypropylene bottles at room temperature (298 K), and used within 1 week. pH measurements were performed with a LeiCi PHS-25 pH meter.

Electrochemical operando NMR setup
The operando NMR experiments were conducted using a custom-designed micro-reactor integrated within a 5 mm diameter NMR tube. The setup comprised a three-electrode electrochemical cell coupled with a plastic capillary for continuous CO2 gas delivery (Supplementary Fig. 11), where both the counter and reference electrodes are platinum wires, and CO2 is delivered via a PTFE capillary. The working electrode was fabricated from a bundle of ~3000 carbon microfibers (individual filament diameter of 6.9 µm). A catalyst ink was prepared by dispersing 10 mg catalyst in a mixture of 500 µL isopropanol and 100 µL Nafion 117 solution, followed by ultrasonication for at least 30 min to form a homogeneous suspension. The catalyst layer was loaded onto the carbon fiber bundle via a repeated dip-coating process: the fiber bundle was immersed in the ink, withdrawn, and dried thoroughly in an oven at 60°C, with this cycle repeated multiple times to achieve the desired loading (~3 mg cm−2). A single catalyst ink batch (prepared from 10 mg of catalyst) was used to fabricate three working electrodes simultaneously. Both the counter and reference electrodes were platinum wires with diameters of 0.3 mm. CO2 gas was introduced into the electrolyte through a PTFE capillary (inner diameter: 0.5 mm) at a controlled flow rate of 1.5 sccm, regulated and measured using a mass flow controller (Alicat Scientific). All three electrodes and the gas capillary were assembled within the 5 mm NMR tube. Before the operando experiment, the electrode was rinsed repeatedly with deuterated water and dried; this procedure ensured the complete removal of any residual protic solvent, which is crucial for obtaining a clean NMR background.

Electrolyte preparation
Electrolytes for the operando NMR experiments were freshly prepared for each use according to the following procedures:

For operando 17O NMR: The electrolyte consisted of 450 μL of deuterated water (2H2O) and 50 μL of H217O (enriched water, 35~40% isotopic purity), with 25.16 mg of KH13CO3 added as the bicarbonate source. The total volume was 500 μL.

For operando 13C NMR: The electrolyte was 500 μL of 2H2O with 25.16 mg of KH12CO3. During the experiment, 13CO2 gas was continuously bubbled at a flow rate of 1.5 sccm to provide the 13C-labeled substrate.

DFT calculations
Calculation details. All the spin-polarized calculations were conducted using Vienna Ab-initio Simulation Package (VASP)50,51. The DFT functional was applied at the Perdew–Burke–Ernzerhof (PBE) level. The project-augmented wave (PAW) method was used to describe the core-valence electron interactions, with the valence electronic states expanded in plane wave basis set using an energy cutoff of 450 eV52. The force convergence criterion was set as 0.05 eV/Å. The transition states were searched using the constrained optimization scheme. TSs were located by varying this fixed distance, which are confirmed when (i) all atomic forces on the relevant atoms vanish and (ii) the total energy is a maximum along the reaction coordinate, while being a minimum with respect to other degrees of freedom53–55. To well describe the reaction system, the DFT-D3 method with the van der Waals dispersion effect included was also used56–58.

In our calculations, the modeled surface is the (161) facet, which is consistent with the experimental observations confirmed by XRD and HRTEM (Fig. 1a, b). On the (161) surface, the exposed terminations consist of four- and three-coordinated Bi atoms (Bi4c/Bi3c) together with four- and three-coordinated O atoms (O4c/O3c). The Bi2O2CO3 (161) surface was modeled by four-layer p(1×2) periodical slabs with the volume of 15 Å (Supplementary Fig. 21). A corresponding 1×1×1 k-points mesh was consistently used during the structural optimizations to ensure reliable convergence of the calculations. For the doped surface, the introduction of low-valence Li species facilitates the formation of oxygen vacancies (Ov) on the Bi2O2CO3 (161) surface. This is evidenced by the significantly reduced Ov formation energies of −0.77 eV and −0.13 eV for Li3c- and Li4c-doped surfaces, respectively, compared to 3.61 eV for the pristine surface (Supplementary Fig. 22). These results clearly indicate that Li doping lowers the energetic barrier for vacancy generation, thereby making the surface more defect-rich and catalytically active under realistic reaction conditions.

In this work, the key processes under investigation mainly occur at the catalyst surface, where water dissociation and subsequent reactions are governed by the local electronic structure of surface atoms. Although the electrolyte environment can influence the energetics, an explicit solvation model would require tens or even hundreds of water molecules to faithfully capture the hydrogen-bonding network, which makes the computational cost prohibitively high for our large surface models. Therefore, in this work, we adopted an implicit solvation model, which can effectively account for the screening of the aqueous environment while maintaining computational feasibility.

The adsorption energy of surface species is defined as Eads(X) = E(X/sur) − E(sur) − E(X), where E(sur), E(X) and E(X/sur) are the energies of the catalysts surface, X in the gas phase and X adsorbed on the catalyst surface, respectively. The more negative Eads(X) is, the more strongly the species X binds to the surface.

Calculation of free energy
To ensure that the calculated energetics are comparable to realistic electrochemical interfaces, we performed energy corrections within the computational hydrogen electrode (CHE) framework. For instance, considering a representative elementary step,

A* + H* + e− → AH* + H*

the corresponding free energy change (ΔG) is evaluated as:

ΔG = ΔE + Δ(pV) − TΔS + ΔEZPE (4)

where ΔE is the reaction energy, Δ(pV) is typically minimal before and after the reaction, and thus is not considered in this study, TΔS is the entropy change and can be obtained from the Handbook of Chemistry and Physics, and ΔEZPE is the zero-point energy, which can be obtained through the vibrational frequency calculations. Importantly, the reaction energy ΔE is corrected relative to the applied electrode potential. Specifically, the work function (Φ) of the catalyst surface is obtained as the difference between the vacuum level (Evac) and the Fermi level (EF). The electrode potential versus the reversible hydrogen electrode (URHE) is then determined by:

U = Φ − ΦSHE / e (5)

where ΦSHE is the work function of the standard hydrogen electrode (4.60 eV is adopted in this work for Bi2O2CO3)59–61, and e is the elementary charge. The corrected reaction energy can thus be expressed as:

ΔE = ΔEcal + e(U − Ucal) (6)

where ΔEcal is the raw DFT energy obtained from VASP calculations, and U is the experimentally relevant potential (−1.18 V in this work). This correction enables our free energy profiles to properly reflect the influence of the electrochemical field, thereby bridging the gap between theoretical modeling and real electrochemical conditions.

Notably, for surface reactions on the Bi2O2CO3 surface, the free energies of intermediates include zero-point energy corrections. Since both the reactants and products remain adsorbed on the catalyst surface, the change in entropy is expected to be very small and was therefore neglected in these cases.

In contrast, for adsorption/desorption processes, and entropy of reactant/product was fully considered. However, it is challenging to directly obtain accurate entropy values for CO2 and HCOOH using periodic DFT package (such as VASP) with explicit aqueous models. To address this, we adopted entropy values reported in the literature, specifically S = 231.6 J/mol/K for CO2 and 129.0 J/mol/K for HCOOH62, which correspond to TΔS terms of 0.72 eV and 0.40 eV, respectively. This correction approach is commonly used in free energy calculations and has been shown to provide a reliable estimation for such systems63–65.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In the manuscript, the authors report an isotope-labeled operando NMR methodology for resolving proton-coupled electron transfer (PCET) pathways in CO2 reduction reactions (CO2RR). The methodology addresses a significant challenge, as the detection of proton pathways or sources remains difficult due to limitations in analytical techniques for light elements and the inherent complexity of distinguishing such pathways via conventional 1H NMR spectroscopy. The authors employ dual-isotope operando NMR (2H/13C) to differentiate between formate species (1H13COO- vs. 2H13COO-). The evidence presented is robust, and the conclusions are well-supported. The method and findings are promising and worth publishing. I would like to suggest the following revision comments for the authors to further improve their manuscript.\n1. The authors report that Li-modified BOC (BOC-Li) exhibits enhanced CO2RR performance for formate production. Concerns regarding the structural stability of BOC-Li itself warrant additional evidence, such as characterization of the material after CO2RR testing, to substantiate its stability.\n2. The authors state that BOC-Li exhibits stronger CO2 adsorption than BOC, as demonstrated by DFT calculations, and propose this as one of the reasons for its enhanced CO2RR performance. Considering the significance of this claim, it is recommended that the authors provide additional experimental evidence to support this conclusion.\n3. The authors demonstrate that BOC-Li significantly enhances CO2RR performance for formate production. To establish the uniqueness of Li modification, it would be valuable to clarify whether other alkali metals yield similar effects through relevant comparative experiments.\n4. It would be valuable to explore whether the methodology extends to other liquid products (e.g., C2H5OH, CH3COOH). Additionally, investigating whether dual-isotope labeling of such C2 products yields insightful results could be considered for future work.\n5. Several inconsistencies between the manuscript and Supplementary Information require verification. Specifically, the coupling constant 2J[2H-13C] is reported as 29.87 Hz in the manuscript (Lines 346, 350, 368) but appears as 29.83 Hz in Supplementary Fig. 10, 11, and 13. Additionally, discrepancies exist in the legend of Supplementary Fig. 10: the figure uses purple triangles and golden stars to denote 2H13COO- and 1H13COO- respectively, whereas the accompanying legend describes pink triangles and grey stars. These inconsistencies must be verified and corrected to ensure data accuracy and documentation consistency.\n6. The manuscript requires verification for minor typographical errors, particularly spacing omissions. Specific examples include \"4f 7/2orbital\" (Line 137) and \"-0.53ppm\" (Line 147), both of which lack required spaces. These errors should be corrected to maintain formatting consistency.\n7. The authors attribute the facilitated H2O dissociation to the presence of oxygen vacancies introduced by Li doping. However, have the authors considered whether the Li dopant might also influence H2O dissociation at other surface sites that are not directly associated with the oxygen vacancy? It would be helpful to clarify whether the observed enhancement is a localized effect at the vacancy site or if Li doping induces more general changes in surface reactivity.\n8. The authors state in the \"Computational details\" in Supplementary that the TS were searched by a constrained optimization scheme. Here they need to state more details on this method, such as the selected coordinate for the optimization, to allow for reproducibility.”", "reviewer_2": "In the present manuscript entitled “Operando NMR Decodes Alkali-Tuned Proton-Electron Relay Boosting CO2-to-Formate Conversion”, a dual isotope-labeled operando NMR methodology for resolving proton-coupled electron transfer (PCET) pathways in CO2RR is reported. This technique is reported to enable direct discrimination between formate species (1H13COO- vs. 2H13COO-) through distinct 13C NMR chemical shifts, moving beyond reliance on peak intensity variations. This investigation holds considerable significance for the CO2RR research community, particularly given the growing emphasis on elucidating the proton source and transfer mechanism. In light of the demonstrated novelty and substantive contributions, the manuscript is recommended for publication in Nature Communications after revisions. The detailed comments are listed as follows:\n1) The enhanced CO2RR performance for formate production using BOC-Li is reported, with *HCOO (Fig. 4f, also denoted as *OCHO) as the key intermediate. However, independent verification of the operando 1H/17O NMR findings is limited by the absence of supporting in situ spectroscopic evidence (e.g., FTIR or Raman spectroscopy). Acquisition of such complementary data is thus recommended to strengthen the mechanistic conclusions. Additionally, consistent use of the conventional notation “*OCHO” instead of “*HCOO” is suggested.\n2) A dual-isotope operando NMR (2H/13C) methodology is presented, enabling discrimination between 1H13COO- and 2H13COO- species based on distinct 13C NMR chemical shifts. However, observed discrepancies in the chemical shifts of these formate species across experimental datasets, specifically between Fig. 3e and 3f, as well as Supplementary Figures 10 and 13, require further clarification. A detailed explanation of this variation’s origin is essential for a comprehensive understanding of the spectral differentiation mechanism.\n3) The stability assessment of BOC-Li (Fig. 2e) during CO2RR requires enhancement. It is recommended to extend electrolysis duration (>24 hours) with periodic monitoring of formate Faradaic efficiency decay.\n4) Trace-level lithium loading (0.0065 wt%) is documented in Supplementary Table 1 (ICP-MS). However, exceptional signal-to-noise ratios at this ultralow concentration are exhibited in the 7Li NMR spectra presented in Fig. 1g and Supplementary Fig. 4. The origin of this enhanced detectability requires elucidation.\n5) In Figure 4f, the reaction free energy profile is presented. However, the manuscript does not provide sufficient information regarding how entropy contributions were treated in the calculation of the free energy changes. Could the authors clarify the methodology used to evaluate entropy for both adsorbed and gas-phase species? If any approximations (e.g., harmonic oscillator model for adsorbates or standard gas-phase entropy values) were applied, please specify and justify their applicability to this system.\n6) The proposed reaction mechanism involves the dissociation of H2O prior to its reaction with CO2. Have the authors considered and tested a direct reaction pathway between molecular CO2 and H2O without prior H2O dissociation? If so, please provide a comparison of the energetics. If not, please clarify why this alternative pathway was not considered and whether it could be relevant under the studied conditions.\n7) Verification is required for inconsistency between the manuscript and the Supplementary Information. For example, a discrepancy is noted where the 2J[2H-13C] is stated as 29.87 Hz in Lines 346 and 350 of the manuscript, while being listed as 29.83 Hz in Supplementary Figs. 10, 11, and 13.", "reviewer_3": "General Comments:\nThis manuscript presents a mechanistic investigation into Li-doped bismuth oxycarbonate catalysts for CO2 electroreduction, highlighting the role of alkali metals in modulating proton-coupled electron transfer (PCET) dynamics. The authors use dual-isotope (2H/13C) operando NMR and DFT calculations to support their claim of an alkali-tuned PCET relay that boosts formate production. While the integration of isotope labeling and spectroscopy is interesting and the catalytic performance is commendable, the central mechanistic claims—especially the synchronized PCET relay—are not convincingly demonstrated. Several aspects of the experimental design, theoretical support, and data interpretation remain weak or speculative. Therefore, I recommend reconsideration after major revisions. Detailed critiques are provided below.\nSpecific Comments:\n1. The authors introduced trace amounts of Li+ (only 0.0065 wt%) and characterized its presence solely before electrolysis. However, they did not provide sufficient justification for how such a minimal Li+ concentration—which could potentially leach out during the reaction—could significantly influence the reaction kinetics. A more plausible explanation, supported by prior studies (e.g., Nature Catalysis, 1, 111–119, 2018), is that trace dopants like Li+ may modify the intrinsic structure of active sites rather than directly steering kinetics through bulk-phase interactions.\n2. Central mechanistic claim lacks kinetic validation. The core hypothesis that Li+ facilitates synchronized PCET is not experimentally proven. There is no kinetic data—such as Tafel slopes, KIE measurements, or EIS—to evaluate whether proton and electron transfer are indeed coupled or accelerated. Isotope-resolved NMR only shows the presence of HCOO- species and their isotopic labeling, which does not suffice to establish PCET synchronization.\n3. Excessively strong or misleading terminology. Phrases such as 'decoding the PCET relay', 'synchronized transfer', or 'mechanistically unlocked' are overstated relative to the evidence provided. These claims should be toned down unless supported by more rigorous experimental validation.\n4. Dual-isotope NMR lacks direct correlation to PCET dynamics. The use of 2H and 13C labeling confirms the origin of atoms in formate but does not allow resolution of when or how these species form during PCET. The data shows that Li+ leads to more 2H-HCOO-, but this could arise from surface affinity differences or water structure changes, not necessarily PCET coupling.\n5. Missing critical control experiments. Key controls are absent—such as isotope labeling on undoped BOC, on Bi powder, or in the presence of Na+/K+. These comparisons are essential to isolate the unique role of Li+ in the proposed mechanism.\n6. Theoretical model lacks clarity. The manuscript does not specify surface terminations, defect models, solvation or field effects in DFT simulations. These omissions raise questions about how relevant the computed energetics are to real electrochemical interfaces.\n7. Figure presentation and experimental detail need improvement. Some spectra lack sufficient resolution and annotation (especially in the NMR section). Experimental conditions for NMR—such as method, electrolyte volume, or operating test setup configuration—are not described in enough detail to ensure reproducibility.\n8. The reported stability of the system falls significantly short compared to similar studies in literature (e.g., Nature Communications, 16, 1754 (2025); Nature, 626, 86-91 (2024)). Could the authors improve the current long-term stability performance?\n9． The mechanistic insights of CO2 reduction to formate presented in the manuscript are primarily supported by operando NMR analyses in an H-cell-like setup. However, the electrochemical experiments were conducted in a flow-cell configuration. These two systems differ significantly in terms of mass transport, catalytic site evolution, and interfacial dynamics, which may influence reaction pathways and intermediate behavior. Therefore, it is not sufficiently rigorous to extrapolate mechanistic conclusions from the NMR setup to the flow-cell environment. The authors are encouraged to justify the relevance of the operando NMR experiments to the actual reaction conditions and to provide experimental evidence validating the applicability of these mechanistic observations to the flow-cell system.", "reviewer_4": "This study systematically elucidates how trace Li incorporation into the BOC structure modulates the electronic configuration and reconstructs the interfacial reaction kinetics, thereby demonstrating that Li doping can effectively enhance both the selectivity and efficiency of CO2-to-formate conversion. A high formate Faradaic efficiency (FEhcoo-) of up to 90% was achieved at a current density of 1.5 A cm-2 accompanied by excellent operational stability. Moreover, the application of dual-isotope operando nuclear magnetic resonance (2H/13C NMR) enables real-time tracking of proton sources and transfer pathways during CO2 electroreduction, offering strong mechanistic insights into the proton-coupled electron transfer (PCET) process in CO2RR. Nevertheless, the catalyst assessed in the article was only evaluated over a fleeting time scale of 8.5 hours, lacking long-term stability data and post-structural analysis, which is insufficient to support its claim of industrial applicability. So, I regret to give my opinion that this paper is not suitable for publication in Nature Communications. The following comments should be considered before submitting it to another journal.\n1. The paper points out that the Li content is only 0.0065 wt%, which has a significant impact on performance. However, currently, only XPS and 7Li ssNMR provide indirect structural information (Fig. 1f–g). It is recommended to supplement with synchrotron radiation XAS to further verify the Li doping sites and bonding environment.\n2. Although it is pointed out that Li3c has the optimal water dissociation activation energy (0.24 eV), its electronic structure origin (such as PDOS or charge density difference map) is not explained in depth. It is recommended to supplement the analysis with electronic state density (DOS/PDOS) to intuitively show the effect of Li regulation on the electronic structure.\n3. The study only used undoped BOC as the control group and lacked a lateral comparison with other alkali metal (such as K+, Na+) doping systems. It is recommended to add comparisons with other alkali metal-regulated PCET systems to enhance the persuasiveness of Li's advantages.\n4. Stability was only evaluated for 8.5 hours at a current density of 200 mA cm-2, which is insufficient to fully demonstrate its industrial applicability. It is recommended to provide structural comparison data of the catalyst before and after deactivation (e.g., SEM/XRD/XPS).\n5. Figure 3e introduces multiple spectra to analyze different proton sources, but the main text is too long and dense with terminology. It is recommended to add a section titled “Key Information Summary” in the figure caption or main text."}, "author_response": "Point-to-Point Responses to Reviewers’ Comments\n1\n(Manuscript ID: NCOMMS-25-43176)\n2\n6 methodology for resolving proton-coupled electron transfer (PCET) pathways in CO\n2\n7 reduction reactions (CO RR). The methodology addresses a significant challenge, as\n2\n8 the detection of proton pathways or sources remains difficult due to limitations in\n9 analytical techniques for light elements and the inherent complexity of distinguishing\n10 such pathways via conventional 1H NMR spectroscopy. The authors employ dual-\n11 isotope operando NMR (2H/13C) to differentiate between formate species (1H13COO-\n12 vs. 2H13COO-). The evidence presented is robust, and the conclusions are well-\n13 supported. The method and findings are promising and worth publishing. I would like\n14 to suggest the following revision comments for the authors to further improve their\n15 manuscript.\n18 of our manuscript and for the constructive recommendations that have helped us\n19 significantly improve our work. We sincerely appreciate the recognition of our isotope-\n20 labeled operando NMR methodology as an innovative approach for resolving proton-\n21 coupled electron transfer pathways in CO RR.\n2\n23 comprehensive revisions to strengthen key aspects of our study. We have provided\n24 robust experimental evidence for the structural stability of our Li-doped catalyst\n25 through detailed post-reaction characterization, confirming that BOC-Li maintains\n26 excellent structural integrity after electrolysis. Furthermore, we conducted solution-\n1 / 160\n27 phase 13CO NMR measurements that offer experimental validation of the enhanced\n2\n2\n30 catalyst with Na- and K-doped analogues through comprehensive characterization.\n32 doped catalyst demonstrates superior CO RR performance and high-current stability,\n2\n33 highlighting the distinct advantage of Li in promoting efficient reaction kinetics.\n35 applicability to other reaction products. Using ethanol as a representative C product,\n2\n36 we successfully showed that our 1H-decoupled 13C NMR approach can clearly\n37 distinguish between protonated and deuterated species, confirming its potential for\n38 probing proton transfer in more complex reaction networks.\n40 providing more detailed analysis of water dissociation mechanisms on Li-doped\n41 surfaces, and have meticulously corrected all identified inconsistencies while\n42 expanding methodological descriptions to ensure full reproducibility.\n44 manuscript and fully addressed the reviewer’s valuable comments. The reviewer’s\n45 expert guidance has been instrumental in this improvement process, and we once again\n46 extend our sincere thanks for the thorough and constructive evaluation of our work.\n47\n2\n50 performance for formate production. Concerns regarding the structural stability of\n2\n2 / 160\n55 structural stability of our Li-modified BOC catalyst, which has guided us to\n56 significantly strengthen the experimental evidence supporting the durability of our\n57 material. In response to this comment, we have conducted post-reaction\n58 characterizations to evaluate the structural and chemical stability of BOC-Li after\n59 electrolysis.\n2\n61 after which the material was carefully collected and characterized. As shown in Fig. R\n62 1, the SEM image of BOC-Li after electrolysis maintains the nanosheet morphology of\n64 characteristic diffraction peaks of orthorhombic Bi O CO , with no observable phase\n2 2 3\n65 transformation or structural degradation, confirming excellent crystallographic stability\n66 under operating conditions.\n68 after reaction (Fig. R 3a) remain at higher binding energies compared to undoped BOC,\n69 consistent with pre-reaction observations and indicating the preserved higher oxidation\n70 state of Bi. The C 1s spectrum (Fig. R 3b) shows minimal changes, suggesting\n71 maintained local carbon environment. Importantly, the Li 1s spectrum (Fig. R 3c)\n72 continues to display two distinct peaks corresponding to the two crystallographic Li\n73 sites, confirming the retention of Li species in the structure after electrolysis.\n75 and compositional stability during CO RR, with nearly no detectable changes in crystal\n2\n76 structure or surface chemical states. Following the reviewer’s valuable suggestion, we\n77 have incorporated these results and corresponding discussions in the revised manuscript\n78 (yellow highlight, page 15, paragraph 3, lines 438-447) and Supplementary\n3 / 160\n80\n81\n2\n83 remaining nanosheet morphology.\n2\n85 collected before and after electrolysis at 100 mA cm-2 for 1800 s show identical\n4 / 160\n86 characteristic diffraction peaks, confirming the exceptional structural stability of the\n87 catalyst under operating conditions.\n89 s: (a) Bi 4f XPS, showing a 0.21 eV positive shift for BOC-Li compared to BOC; (b) C\n90 1s XPS, displaying binding energy differences attributed to Li doping; (c) Li 1s\n91 spectrum of BOC-Li after electrolysis, maintaining the two characteristic peaks\n92 corresponding to distinct Li sites.\n93\n95 “Besides, post-reaction characterization confirms the exceptional structural\n5 / 160\n96 stability of the BOC-Li catalyst. The SEM image in Supplementary Fig. 27 shows that\n97 the nanosheet morphology of BOC-Li is well preserved after electrolysis at 100 mA\n98 cm-2 for 1800 s. XRD patterns further demonstrate that the characteristic peaks of\n99 orthorhombic Bi O CO remain unchanged, with no evidence of phase degradation\n2 2 3\n100 (Supplementary Fig. 28). XPS analysis reveals that the higher oxidation state of Bi\n101 induced by Li doping is maintained after the reaction, and importantly, the two distinct\n102 crystallographic Li sites remain occupied (Supplementary Fig. 29). These results\n103 collectively illustrate the structural and compositional integrity of BOC-Li under\n2\n105\n2\n108 demonstrated by DFT calculations, and propose this as one of the reasons for its\n109 enhanced CO RR performance. Considering the significance of this claim, it is\n2\n110 recommended that the authors provide additional experimental evidence to support this\n111 conclusion.\n114 the experimental validation of CO adsorption. Following the reviewer’s insightful\n2\n115 guidance, we performed 13CO NMR measurements using high-pressure NMR tubes to\n2\n116 directly compare the CO adsorption capabilities of BOC-Li and BOC under solution-\n2\n117 phase conditions.\n119 2H O in a 5 mm high-pressure NMR tube. Then, 10 mL of 13C-labeled 13CO gas was\n2 2\n120 injected into the tube, and the system was allowed to equilibrate. 13C NMR spectra were\n121 acquired on a Bruker 500 MHz spectrometer under identical conditions.\n6 / 160\n2\n123 concentration in the liquid phase. The BOC sample exhibits a strong characteristic\n124 13CO peak, suggesting relatively weak CO adsorption and thus more CO remaining\n2 2 2\n125 in solution. By contrast, the sample containing BOC-Li shows a pronounced decrease\n126 in the 13CO signal intensity relative to the BOC sample, indicating enhanced CO\n2 2\n127 adsorption by BOC-Li due to effective uptake of CO from the solution.\n2\n128\n2\n2\n131 of 13CO in the presence of BOC and BOC-Li catalysts. The measurements were\n2\n132 conducted to compare the CO adsorption capabilities of the two materials under\n2\n133 solution-phase conditions. Specifically, 10 mg of each catalyst was dispersed in 500 μL\n134 of 2H O in a high-pressure NMR tube, followed by the injection of 10 mL of 13CO gas.\n2 2\n136 significantly lower signal intensity of dissolved 13CO observed for the BOC-Li sample,\n2\n137 compared to the BOC sample, indicates that a greater amount of CO was adsorbed by\n2\n138 the BOC-Li catalyst, directly demonstrating its enhanced CO adsorption capacity.\n2\n7 / 160\n139\n141 provide compelling experimental support that Li doping significantly enhances the CO\n2\n142 adsorption capacity of the catalyst, thereby providing strong experimental\n143 corroboration for our DFT calculations. We have incorporated the corresponding\n144 discussion and data into the revised manuscript (yellow highlight, page 8, paragraph\n145 1, lines 198-201) and the Supplementary Information (yellow highlight,\n147 recommended.\n148\n150 “Furthermore, this enhanced kinetics is facilitated by an improved reactant affinity,\n151 as solution-phase 13C NMR measurements revealed a stronger CO adsorption capacity\n2\n152 for BOC-Li compared to BOC (Supplementary Fig. 8 and Supplementary Note 3).”\n153\n155 “Supplementary Note 3. Solution-phase 13CO NMR Spectroscopy for Evaluating\n2\n2\n2\n158 using solution-phase 13CO NMR spectroscopy. For each measurement, 10 mg of\n2\n159 catalyst was dispersed in 500 μL of 2H O within a 5 mm high-pressure NMR tube. The\n2\n160 system was charged with 10 mL of 13C-labeled CO gas and allowed to reach\n2\n161 equilibrium prior to spectral acquisition. All 1H-decoupled 13C NMR spectra were\n162 acquired using a Bruker AVANCE III HD 500 MHz spectrometer with parameters :\n163 spectral width of 236.6 ppm, acquisition time of 2.0 s, relaxation delay of 30 s, and 40\n164 scans. Composite-pulse 1H decoupling (zgpg) was applied during acquisition.\n2\n166 liquid phase. Under identical conditions, the sample containing the BOC catalyst\n167 exhibited a strong characteristic 13CO peak. In contrast, the sample with BOC-Li\n2\n168 showed a pronounced reduction in the 13CO signal intensity relative to the BOC sample.\n2\n2\n2 2\n171 adsorption capacity compared to the undoped BOC material. This experimental\n172 observation provides direct support for the enhanced CO affinity of the lithium-doped\n2\n173 catalyst, which is consistent with the trends predicted by our computational\n174 calculations.”\n175\n2\n178 for formate production. To establish the uniqueness of Li modification, it would be\n179 valuable to clarify whether other alkali metals yield similar effects through relevant\n180 comparative experiments.\n183 the uniqueness of Li modification, which has guided us to undertake a systematic and\n184 comprehensive comparison with other alkali metals. Following the reviewer’s\n185 insightful suggestion, we have conducted extensive comparative experiments with Na\n186 and K dopants, accompanied by detailed characterization of the resulting materials to\n187 provide a thorough understanding of the distinctive role of Li in modulating the\n188 catalytic properties.\n2 2 3\n9 / 160\n191 structure (Pna21(33) space group). Enlarged XRD patterns (Fig. R 5b) exhibits\n192 noticeable peak broadening, indicating reduced crystallinity and possible lattice strain\n193 induced by Na doping. The negligible peak shift suggests minimal lattice parameter\n194 change, consistent with the similar ionic radius of Na+ compared to Bi3+.\n196 elemental mapping in Fig. R 6b demonstrates homogeneous distribution of Bi, O,\n198 without phase segregation.\n10 / 160\n2\n200 indexing to the orthorhombic phase of Bi O CO (BOC) with the Pna21(33) space\n2 2 3\n201 group. BOC-Na shows widened peaks, with negligible shift.\n11 / 160\n203 (b) corresponding elemental mapping demonstrating the uniform distribution of Bi, O,\n12 / 160\n205\n207 oxidation state of BOC-Na. (b) C 1s XPS spectra of BOC-Na and BOC, showing a\n208 slightly lower binding energy for the C-O peak and a slightly higher C=O peak in BOC-\n212 compared to BOC, indicating electron transfer from Bi to Na and a higher oxidation\n213 state of Bi species. The C 1s spectra (Fig. R 7b) display a slightly lower binding energy\n214 for the C-O peak and a higher C=O peak in BOC-Na, suggesting modified carbonate\n13 / 160\n215 coordination environments. Na 1s XPS (Fig. R 7c) shows a single peak at 1071.4 eV,\n216 characteristic of oxidized Na⁺ species1.\n219 existence of one dominant coordination environment for the doped sodium ions.\n221 broadened resonance at δ = -4.9 ppm. The observation of a single peak confirms that\n222 the doped Na+ ions occupy a single, dominant type of crystallographic site within the\n223 structure.\n224\n14 / 160\n226 comparison shows a slightly intensified absorption feature at 1465 cm-1 in the BOC-Na\n227 sample relative to the undoped BOC, indicating doping-induced changes in the\n228 vibrational modes of the material.\n15 / 160\n230 vis absorption spectrum of BOC-Na exhibits a weak but distinct absorption shoulder at\n231 approximately 400 nm, which is absent in the pristine BOC. This feature indicates that\n233\n235 compared to pristine BOC, indicating significant modifications in the local\n236 coordination environment of carbonate (CO 2-) groups within the bismuth oxycarbonate\n3\n237 lattice. Concurrently, UV-vis absorption spectra (Fig. R 10) demonstrate the emergence\n238 of a distinct absorption shoulder at approximately 400 nm in BOC-Na, which is absent\n239 in the undoped counterpart. This spectral feature suggests the formation of sub-bandgap\n240 states likely induced by Na doping. Together, these spectroscopic results provide\n241 complementary evidence that Na incorporation not only alters the structural\n242 configuration of carbonate species but also modifies the electronic structure of the\n243 catalyst, creating localized electronic states that may facilitate enhanced charge transfer\n244 during electrocatalytic processes.\n16 / 160\n246 ranging from 0.04 to 0.20 V s-1 (in increments of 0.04 V s-1) for both (a) BOC-Na and\n247 (b) BOC samples in 1 M KOH electrolyte. The ECSA (c) values, derived from C\ndl\n248 measurements, revealed an enhancement in the Na-modified sample. Specifically, the\ndl\n250 2 for BOC.\n251\n253 enhances catalytic properties. CV measurements at varying scan rates (Fig. R 11) show\n254 that BOC-Na exhibits substantially higher electrochemical active surface area (ECSA\n255 = 8.46 mF cm-2) compared to BOC (4.39 mF cm-2, Fig. R 11c), representing a ∼93%\n256 increase. EIS analysis (Fig. R 12) reveals reduced charge transfer resistance for BOC-\n2\n17 / 160\n258 performance of BOC-Na, achieving ~90% formate Faradaic efficiency at 100~500 mA\n259 cm-2, though performance decreases to ~83% at 600 mA cm-2, demonstrating inferior\n260 high-current stability compared to BOC-Li. In situ 13C NMR spectra of BOC-Na and\n262 provides evidence that Na doping alters the reaction microenvironment, favoring proton\n263 transfer from water (2H O) over other potential proton donors.\n2\n266 measurements show that the charge transfer resistance (R ) of BOC-Na is significantly\nct\n267 lower than that of the undoped BOC catalyst. This decreased resistance demonstrates\n268 that Na modification effectively facilitates electron transfer at the electrode-electrolyte\n269 interface, thereby enhancing the overall CO reduction reaction kinetics.\n2\n18 / 160\n271 moderate high-current stability. The CO RR performance plot shows that BOC-Na\n2\n272 maintains a high Faradaic efficiency for formate (FE~90%) across a wide current\n273 density range of 100 to 500 mA cm-2. Although the FE drops to ~83% at 600 mA cm-2,\n274 this performance still demonstrates a significant improvement over the undoped BOC\n275 catalyst.\n19 / 160\n278 ratio for BOC-Na compared to BOC. This indicates a greater relative incorporation of\n279 deuterium from the 2H O electrolyte, providing evidence that Na doping alters the\n2\n280 reaction environment to favor proton transfer from water dissociation over alternative\n281 sources (e.g., bicarbonate).\n20 / 160\n2\n283 indexing to the orthorhombic phase of Bi O CO (BOC) with the Pna21(33) space\n2 2 3\n284 group. BOC-K shows widened peaks, with negligible shift.\n21 / 160\n285\n287 patterns (Fig. R 15a) reveal that BOC-K maintains the orthorhombic Bi O CO\n2 2 3\n288 structure (Pna21(33) space group). Enlarged XRD patterns (Fig. R 15b) exhibits\n289 noticeable peak broadening, indicating reduced crystallinity and possible lattice strain\n290 induced by K doping. The negligible peak shift suggests minimal lattice parameter\n291 change.\n293 elemental mapping in Fig. R 16b demonstrates homogeneous distribution of Bi, O, C,\n294 and K elements throughout the material, confirming successful K incorporation without\n295 phase segregation.\n298 compared to BOC. The C 1s spectra (Fig. R 17b) display a slightly lower binding\n299 energy for the C-O peak and a higher C=O peak in BOC-K, suggesting modified\n300 carbonate coordination environments. K 2p XPS (Fig. R 17c) shows a single peak,\n301 characteristic of oxidized K⁺ species.\n22 / 160\n303 (b) corresponding elemental mapping demonstrating the uniform distribution of Bi, O,\n23 / 160\n306 oxidation state of BOC-K. (b) C 1s XPS spectra of BOC-K and BOC, showing a slightly\n307 lower binding energy for the C-O peak and a slightly higher C=O peak in BOC-K. (c)\n309\n24 / 160\n311 comparison shows a slightly intensified absorption feature at 1465 cm-1 in the BOC-K\n312 sample relative to the undoped BOC, indicating doping-induced changes in the\n313 vibrational modes of the material.\n25 / 160\n315 vis absorption spectrum of BOC-K exhibits a weak but distinct absorption shoulder at\n316 approximately 400 nm, which is absent in the pristine BOC. This feature indicates that\n318\n320 compared to pristine BOC, indicating significant modifications in the local\n321 coordination environment of carbonate (CO 2-) groups within the bismuth oxycarbonate\n3\n322 lattice. Concurrently, UV-vis absorption spectra (Fig. R 19) demonstrate the emergence\n323 of a distinct absorption shoulder at approximately 400 nm in BOC-K, which is absent\n324 in the undoped counterpart. This spectral feature suggests the formation of sub-bandgap\n325 states likely induced by K doping. Together, these spectroscopic results provide\n326 complementary evidence that K incorporation not only alters the structural\n327 configuration of carbonate species but also modifies the electronic structure of the\n26 / 160\n328 catalyst, creating localized electronic states that may facilitate enhanced charge transfer\n329 during electrocatalytic processes.\n331 ranging from 0.04 to 0.20 V s-1 (in increments of 0.04 V s-1) for both (a) BOC-K and\n332 (b) BOC samples in 1 M KOH electrolyte. The ECSA (c) values, derived from C\ndl\n333 measurements, revealed an enhancement in the K-modified sample. Specifically, the\ndl\n335 2 for BOC.\n336\n338 enhances catalytic properties. CV measurements at varying scan rates (Fig. R 20) show\n339 that BOC-K exhibits substantially higher electrochemical active surface area (ECSA =\n340 10.49 mF cm-2) compared to BOC (4.39 mF cm-2, Fig. R 20c), representing a ∼139%\n341 increase. EIS analysis (Fig. R 21) reveals reduced charge transfer resistance for BOC-\n2\n27 / 160\n343 performance of BOC-K, showing performance degradation from 500 mA cm-2,\n344 demonstrating inferior high-current stability compared to BOC-Li. In situ 13C NMR\n345 spectra of BOC-K and BOC (Fig. R 23) indicates reduced 1H/2H ratios of formate in\n347 microenvironment, favoring proton transfer from water (2H O) over other potential\n2\n348 proton donors.\n349\n352 measurements show that the charge transfer resistance (R ) of BOC-K is significantly\nct\n353 lower than that of the undoped BOC catalyst. This decreased resistance demonstrates\n354 that K modification effectively facilitates electron transfer at the electrode-electrolyte\n355 interface, thereby enhancing the overall CO RR kinetics.\n2\n28 / 160\n2\n357 degradation in formate Faradaic efficiency at current densities above 500 mA cm-2,\n358 highlighting the limited stability of the K-doped catalyst under demanding operational\n359 conditions.\n29 / 160\n362 ratio for BOC-K compared to BOC. This indicates a greater relative incorporation of\n363 deuterium from the 2H O electrolyte, providing evidence that K doping alters the\n2\n364 reaction environment to favor proton transfer from water dissociation over alternative\n365 sources (e.g., bicarbonate).\n367 incorporated all relevant detailed characterization data, comprehensive experimental\n368 procedures, and extended analytical results in the revised manuscript (yellow highlight,\n369 page 15, paragraph 4 - page 16 paragraph 2, line 448-470) and Supplementary\n371\n373 “Extension to other alkali dopants\n375 doping strategy to include Na+ and K+. Under identical synthesis conditions, both\n30 / 160\n376 dopants were successfully incorporated into the BOC structure, as confirmed by\n377 comprehensive characterization. Similar to the Li+-modified catalyst, these alkali-\n378 doped variants exhibited universally enhanced electrochemical properties, including\n379 increased active surface area and improved charge transfer kinetics compared to pristine\n381 catalytic performance, with BOC-Na and BOC-K achieving high formate Faradaic\n382 efficiencies of approximately 90% and 85%, respectively, at low-to-medium current\n383 densities. Operando NMR spectroscopy further verified that both dopants promoted the\n384 water-derived protonation pathway, confirming that the enhanced PCET mechanism\n385 represents a general phenomenon across alkali-modified catalysts.\n387 kinetics is a general characteristic of alkali doping, the specific choice of dopant\n388 determines the operational stability under practically relevant conditions. Although all\n389 doped catalysts performed comparably at moderate current densities, BOC-Li\n390 demonstrated superior performance maintenance, sustaining >90% Faradaic efficiency\n391 across an extended operating range up to 1100 mA cm-2, while the performance of BOC-\n393 alkali doping as a versatile strategy for modulating PCET kinetics in CO -to-formate\n2\n394 conversion, with Li+ emerging as the optimal dopant for applications requiring\n395 sustained high-current operation.”\n396\n399 products (e.g., C H OH, CH COOH). Additionally, investigating whether dual-isotope\n2 5 3\n400 labeling of such C products yields insightful results could be considered for future\n2\n401 work.\n404 suggestion, which has guided us to explore the broader applicability of our\n31 / 160\n405 methodology beyond the current system. Following this invaluable guidance, we have\n406 initiated exploratory studies focusing on ethanol as a representative C product. To\n2\n407 assess the applicability of our dual-isotope NMR approach, we performed a series of\n408 1H-decoupled 13C NMR experiments using ethanol with different isotopic labeling.\n6 2\n410 spectrum), (ii) EtOH-2H (100 μL fully deuterated ethanol + 400 μL 2H O, blue\n6 2\n411 spectrum), and (iii) a 1:1 mixture of EtOH-1H and EtOH-2H in 2H O (100 uL pure\n6 6 2\n412 ethanol + 100 uL 2H-labeled ethanol + 300 uL 2H O, red spectrum).\n2\n414 unambiguously identify different isotopic species (protonated and deuterated ethanol)\n415 in a mixture without spectral overlap. 1H-decoupled 13C NMR spectra of three ethanol\n416 solutions: EtOH-1H (black trace), EtOH-2H (blue trace), and their 1:1 mixture EtOH-\n6 6\n417 1H & 2H (red trace). The spectra of the pure compounds show distinct signatures:\n6 6\n418 singlets for the -13C1H (C , 57.20 ppm) and -13C1H OH (C , 16.66 ppm) groups in\n3 1 2 2\n6\n420 groups -13C2H (C , 56.36 ppm) and -13C2H O2H (C , 15.57 ppm) in EtOH-2H , arising\n3 1 2 2 6\n421 from 2H-13C J-coupling. The spectrum of the mixture is a superposition of both sets of\n422 signals, demonstrating the simultaneous detection of different protonation states\n32 / 160\n423 without spectral overlap.\n424\n6\n426 exhibits two singlets at 57.20 ppm (-13C1H , C ) and 16.66 ppm (-13C1H OH, C ),\n3 1 2 2\n427 respectively. In contrast, EtOH-2H6 (blue spectrum) shows characteristic multiplets: a\n428 quintet at 56.36 ppm (-13C2H , C ) and a septet at 15.57 ppm (-13C2H O2H, C ), resulting\n3 1 2 2\n429 from 2H-13C J-coupling. Notably, in the mixed sample (red spectrum), both the singlets\n430 (60.95 ppm and 20.63 ppm) and multiplets (quintet: 60.13 ppm; septet: 19.56 ppm) are\n431 clearly resolved without signal overlap.\n433 between protonated and deuterated carbon environments in C products such as ethanol.\n2\n435 elucidate proton transfer pathways in the formation of more complex CO reduction\n2\n436 products.\n438 suggestion. Following the reviewer’s insightful guidance, we have incorporated the\n439 exploratory investigation on C liquid products—using ethanol as a model system—\n2+\n440 along with the corresponding 13C NMR analysis into the revised manuscript (yellow\n441 highlight, page 17, paragraph 1, lines 491-502). The full set of 1H-decoupled 13C\n444 50 and Supplementary note 7 on page 57). This addition, inspired by the reviewer’s\n445 suggestion, significantly strengthens the methodological scope of our work and\n446 provides an important foundation for future studies on complex reaction pathways.\n447\n449 “Looking forward, the multimodal operando NMR methodology established here\n33 / 160\n450 could offer a versatile approach for deciphering complex reaction networks beyond\n451 formate production. Our preliminary 13C NMR analysis on ethanol models\n452 (Supplementary Fig. 49 and Supplementary Note 7) indicates that the diagnostic\n453 capability of isotopic labeling—specifically, the distinct NMR signatures of 1H- and\n454 2H-labeled terminal groups—is preserved in mixed isotopologue solutions. This\n455 observation in a model C system highlights the potential of our approach to track\n2+\n456 proton transfer pathways and carbon atom provenance in the formation of multi-carbon\n457 products (e.g., ethanol, acetate). Extending this dual-isotope probing strategy to actual\n2 2+\n459 which could contribute to understanding the intricate PCET and C-C coupling\n460 mechanisms at catalytic interfaces.”\n461\n463 “Supplementary Note 7. Validation of the NMR Methodology for Isotopic\n466 evaluate the applicability of our 1H-decoupled 13C NMR methodology for analyzing\n467 different isotopic species in C products, using ethanol as a model system. Three\n2\n468 samples were prepared in deuterated water (2H O): (i) 100 μL of fully protonated\n2\n469 ethanol (EtOH-1H ) in 400 μL 2H O; (ii) 100 μL of fully deuterated ethanol (EtOH-2H )\n6 2 6\n470 in 400 μL 2H O; and (iii) a 1:1 mixture containing 100 μL of each ethanol variant in\n2\n471 300 μL 2H O. 1H-decoupled 13C NMR spectra were acquired using a Bruker AVANCE\n2\n473 acquisition time of 2.0 s, relaxation delay of 30 s, and 40 scans. Composite-pulse 1H\n474 decoupling (zgpg) was applied during acquisition.\n6\n476 ppm and 16.66 ppm, assigned to the methyl (-13C1H ) and methylene (-13C1H OH)\n3 2\n34 / 160\n477 carbons respectively. The EtOH-2H spectrum exhibited a quintet at 56.36 ppm for the\n6\n478 -13C2H carbon and a septet at 15.57 ppm for the -13C2H O2H carbon, patterns consistent\n3 2\n479 with 2H-13C J-coupling. The spectrum of the 1:1 mixture showed both the singlets and\n480 multiplets at their respective chemical shifts without overlap.\n482 distinguish between protonated and deuterated carbon environments in C product\n2\n483 mixtures. The methodology may provide a useful approach for future dual-isotope\n484 labeling studies investigating reaction mechanisms in multi-carbon product formation.”\n485\n488 require verification. Specifically, the coupling constant 2J[2H-13C] is reported as 29.87\n492 1H13COO- respectively, whereas the accompanying legend describes pink triangles and\n493 grey stars. These inconsistencies must be verified and corrected to ensure data accuracy\n494 and documentation consistency.\n497 identifying the inconsistencies between the manuscript and Supplementary Information.\n499 reliability of our work. In response, we have systematically verified and corrected all\n500 the identified issues:\n502 the manuscript. The corrections have been made (yellow highlight, pages 10, 11,16,\n35 / 160\n503 and 29, lines 293, 307, 327, 479 and 778).\n505 manuscript (yellow highlight, page 29, paragraph 1, lines 782-786) and the revised\n508 identified as purple triangles for 2H13COO- and golden stars for 1H13COO-.\n510 the main text and the Supplementary Information to ensure full consistency. We\n511 sincerely appreciate the reviewer’s diligence, which has significantly improved the\n512 consistency and quality of our documentation.\n513\n515 “triplet (δ = 170.89 ppm, purple triangles, 2J[2H-13C] = 29.83 Hz) assigned to 2H13COO-”\n516 “diagnostic 2H13COO- triplet (δ = 170.89 ppm, purple triangles"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have made necessary revision in the manuscript. I have no more questions, and think the revised manuscript is now ok to be accepted.", "reviewer_2": "I am happy with the revisions and response from the authors to my comments. Publication of the revised manuscript is now recommended.", "reviewer_3": "The authors have fully addressed my concerns. This work is now ready for publishing.", "reviewer_4": "the authors have addressed all comments well, i would like to suggest the acceptance of this work."}, "author_response": "Point-to-Point Responses to Reviewers’ Comments\n(Manuscript ID: NCOMMS-25-43176)\nResponse to Reviewer #1\nGeneral Comment:\nThe authors have made necessary revision in the manuscript. I have no more\nquestions, and think the revised manuscript is now ok to be accepted.\nResponse:\nWe thank the reviewer for their positive assessment and for recommending our\nmanuscript for acceptance.\nResponse to Reviewer #2\nGeneral Comment:\nI am happy with the revisions and response from the authors to my comments.\nPublication of the revised manuscript is now recommended.\nResponse:\nWe are grateful to the reviewer for their supportive comments and are pleased that\nthey recommend publication.\nResponse to Reviewer #3\nGeneral Comment:\nThe authors have substantially addressed my previous major concerns. These\nrevisions are largely satisfactory and strengthen the overall evidence chain. I am\ninclined to recommend acceptance after minor revision.\nThat said, I encourage the authors to further clarify one remaining issue regarding\nthe assignment of their operando infrared peaks:\nHave the authors considered the possibility that the bands they attribute to *OCHO\nmight instead originate from interfacial bicarbonate or carbonate species, rather than\ngenuine adsorbed reaction intermediates? Bicarbonate-related bands are commonly\nreported in the 1460 cm-1 region, as documented in ACS Catal. 2020, 10, 8049-8057.\nMoreover, does the proposed *OCHO species exhibit a Stark shift——i.e., a\npotential-dependent peak position change-consistent with a specifically adsorbed\nintermediate?\nIn addition, a peak is observed around 1660 cm-1, which typically corresponds to\nthe O-H bending mode of water. Clarification is needed to determine whether this\nfeature is indeed related to *OCHO or merely originates from interfacial water.\nResponse:\nWe sincerely thank the reviewer for their positive and constructive feedback on\nour revised manuscript. We are particularly grateful for the opportunity to further clarify\nthe assignment of the operando infrared peaks, which has prompted us to perform\nadditional control experiments and refine our interpretation.\nRegarding the possibility that the bands attributed to *OCHO might originate from\ninterfacial bicarbonate or carbonate species, we conducted reference measurements\nusing standard solutions of K CO and KHCO in both H O and D O. These ATR-\n2 3 3 2 2\nSEIRAS measurements, as shown in Fig. R1 and Fig. R2, indicate that bicarbonate\n(HCO -) in H O exhibits characteristic bands at ~1358 cm-1, while carbonate (CO 2-)\n3 2 3\nappears near ~1390 cm-1. The peaks we observed at 1438 cm-1 and 1600 cm-1 on the\nBOC-Li electrode under operando conditions are distinct from these reference bands\nand are consistent with reported vibrational modes for adsorbed *OCHO intermediates.\nFurthermore, the potential-dependent intensification of these bands specifically on\nBOC-Li, which correlates with enhanced formate production, supports their assignment\nas a reaction intermediate.\nConcerning the question of a Stark shift for the *OCHO species, we note that a\npronounced potential-dependent peak shift is not uncommon for certain adsorbed\nintermediates like CO, but appears to be less characteristic for *OCHO in the formate\npathway. A survey of literature on operando ATR-SEIRAS studies of formate-\nproducing catalysts, including Bi1, 2, 3, 4, 5, 6, In7, 8, 9, Sn10, 11, 12, and Pb13, 14-based\nmaterials, frequently reports potential-dependent band intensification as the primary\nsignature of *OCHO, without emphasizing a Stark shift. This observed behavior may\nbe attributed to the adsorption configuration of *OCHO, which is often proposed to be\nbidentate, potentially resulting in a smaller change in the dynamic dipole moment\nnormal to the electrode surface compared to linearly-adsorbed species. Thus, the strong\npotential-dependent intensity increase of the bands at 1438 and 1600 cm-1 on BOC-Li\nis considered a reliable indicator of the adsorbed intermediate in this context.\nRegarding the feature around 1660 cm-1, we agree with the reviewer that this\nregion is typically associated with the O-H bending mode of water. Based on our\nreference spectra in H O, we have reassigned this feature to the δ (H O) mode at\n2 2\napproximately 1638 cm-1. This band is therefore not associated with *OCHO or\ncarbonate species.\nWe believe these clarifications, now incorporated into the revised manuscript and\nSupplementary Information, address the reviewer’s comments. We thank the reviewer\nagain for their thoughtful suggestions, which have helped us improve the presentation\nof our spectroscopic data. We have included these results and a corresponding\ndiscussion in the revised manuscript (yellow highlight, page 7, paragraph 4 - page 8,\nparagraph 1, lines 194-208; Fig 2e, f on page 26) and Supplementary Information\n(yellow highlight, Supplementary Fig. 8 on page 9 and Supplementary Fig. 9 on\npage 10).\nFig. R 1 ATR-SEIRAS spectra (a) and enlarged ATR-SEIRAS spectra (b) of standard\nsamples in D O with the concentration of 0.5 mol/L.\nsamples in H O with the concentration of 0.5 mol/L.\n“To gain further mechanistic insight into the PCET process, we performed in situ\nAttenuated Total Reflection - Surface-Enhanced Infrared Absorption Spectroscopy\n(ATR-SEIRAS). The spectra show characteristic peaks at 1438 and 1600 cm-1, which\nare assigned to the adsorbed *OCHO intermediate1, 3, 4, 5, 8, 9, 15. Additional spectral\nfeatures are identified as follows: peaks at ~1272 cm-1 correspond to *CO -2, 15, the\n2\nbands at ~1358 and ~1390 cm-1 are attributed to bicarbonate (HCO -) and carbonate\n3\n(CO 2-), respectively16, 17, 18, 19, and the peak at ~1638 cm-1 is assigned to the O-H\n3\nbending mode of water1, 7, 10, 17, 20, based on our control experiments (Supplementary\nFig. 9 and Supplementary Fig. 10) and literature references. On BOC-Li, the *OCHO-\nrelated peaks at 1438 and 1600 cm-1 emerge and intensify at more negative potentials\n(Fig. 2e), whereas these features remain significantly weaker on undoped BOC (Fig.\n2f). The potential-dependent formation and stabilization of this key intermediate\nindicate that the local environment on BOC-Li is favorable for the PCET step. Together,\nthese spectroscopic data directly support the conclusion that Li+ doping promotes\nformate production by enhancing a concerted PCET mechanism at the Bi sites.”\nResponse to Reviewer #4\nGeneral Comment:\nThis study systematically deciphers the mechanism by which Li-doped bismuth\noxycarbonate promotes proton-coupled electron transfer (PCET) during CO\n2\nelectroreduction, utilizing dual-isotope operando NMR spectroscopy. The authors have\ndemonstrated a high level of commitment in addressing the previous reviewers’\ncomments, undertaking comprehensive revisions with substantial effort. The response\nis detailed, incorporating extensive new experimental data, theoretical calculations,\nmethodological expansions, and textual refinements. Most revisions directly and\neffectively address the core concerns raised. However, a few points require further\nclarification from the authors before the manuscript can be recommended for\npublication.\nResponse:\nWe sincerely thank the reviewer for their thorough assessment of our revised\nmanuscript and for their valuable suggestions. Their expert guidance has helped us\nfurther improve the clarity and robustness of our work. In direct response to the points\nraised, we have provided additional clarifications and supporting evidence, while\nmaintaining a concise and focused presentation as recommended. We believe these final\nrevisions have considerably strengthened the manuscript and hope it now meets the\nhigh standards for publication in Nature Communications.\nOriginal Comment 1:\nThe authors’ response regarding the “relevance of Operando NMR to the flow-cell\nsystem” remains insufficiently convincing. More robust experimental design or\nscenario simulations should be provided to better demonstrate the transferability of the\nproposed mechanisms and conclusions from the H-cell to the flow-cell configuration.\nResponse:\nWe thank the reviewer for this insightful comment, which provides us with an\nopportunity to further clarify the connection between our operando NMR studies and\nthe catalyst’s performance in different electrochemical configurations. We appreciate\nthe reviewer’s emphasis on establishing the relevance of our mechanistic findings\nacross reactor geometries.\nIn our previous revision, we directly addressed this point by supplementing\ncomprehensive electrochemical data from the identical H-cell setup used for our\noperando NMR investigations. These results demonstrated that BOC-Li exhibits\nsuperior formate selectivity, current density, and charge transfer kinetics compared to\npristine BOC within this same configuration. This creates a direct link: the performance\nenhancement is intrinsic to the catalyst and is observed in the exact environment where\nthe mechanism was probed.\nThe key insight from our operando NMR study is that Li doping modifies the local\nenvironment of Bi active sites, which in turn facilitates the proton-coupled electron\ntransfer process at these sites. The potential-dependent behavior of reaction\nintermediates observed via NMR aligns perfectly with the potential window for high\nformate production in both H-cell and flow-cell systems. This correlation provides\nstrong evidence that the fundamental reaction mechanism operates consistently across\ndifferent reactor geometries.\nTherefore, the operational mechanism we uncovered is inherently linked to the\ncatalyst’s intrinsic activity. Its subsequent manifestation as high performance in the\nflow cell—a configuration optimized for mass transport—logically follows from this\nintrinsic superiority. This approach of using H-cells to decouple and identify intrinsic\nmechanisms, while using flow cells to demonstrate performance under industrially\nrelevant conditions, represents a well-established paradigm in electrocatalysis. This\nmethodology has been systematically employed across diverse catalytic systems,\nincluding Cu-based catalysts for multi-carbon products21, 22, 23, 24, 25, 26, CO-generating\nsystems27, 28, 29, methanol-producing catalysts 30, 31, and extensively validated in Bi-\nbased1, 2, 3, 6, 16, 32, 33, 34 and other formate-producing electrocatalysts35.\nWe are confident that the convergent evidence from our H-cell electrochemical\nperformance, operando spectroscopy, and theoretical calculations provides a robust\nfoundation for our proposed mechanism. The reviewer’s comment has been invaluable\nin helping us articulate these connections more clearly, and we hope this explanation\nsatisfactorily addresses the concern regarding the transferability of our mechanistic\ninsights.\nOriginal Comment 2:\nThe explanation of Li’s role, primarily relying on post-reaction XPS to confirm its\nstability and proposing electronic structure modification of active sites, is incomplete.\nIt is recommended that the authors supplement this with techniques such as synchrotron\nradiation studies to probe the catalyst’s behavior under operando conditions and\nprovide a more comprehensive interpretation.\nResponse:\nWe thank the reviewer for raising this insightful point regarding the interpretation\nof Li’s role. In direct response to this valuable suggestion, we have performed\nadditional ex situ 7Li Magic-Angle Spinning (MAS) NMR spectroscopy. The key result,\nas shown in Fig. R3, is that the characteristic 7Li NMR profile of the pristine BOC-Li\ncatalyst is consistently retained in the electrodes after 5 (Fig. R3a), 10 (Fig. R3b), and\n30 minutes (Fig. R3c) of electrochemical CO reduction. The persistence of this distinct\n2\nspectral signature under operating conditions provides complementary, molecular-level\nevidence that the local chemical environment of Li remains largely unaltered\nthroughout the reaction. This finding significantly strengthens the discussion of Li’s\nrole in our proposed mechanism. We have incorporated these new data and the\naccompanying discussion in the revised manuscript (yellow highlight, page 15,\nparagraph 2, lines 436-444; Fig 2e, f on page 26) and Supplementary Information\n(yellow highlight, Supplementary Fig. 31 on page 32). We sincerely appreciate the\nreviewer’s guidance, which has undoubtedly strengthened this aspect of our work.\nFig. R3 Ex situ 7Li MAS NMR spectra of post-electrolysis BOC-Li catalyst, obtained\nafter electrolysis at -0.9 V vs. RHE in CO -saturated 0.5 M KHCO for (a)"}]

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