Biradical-mediated synergistic electrocatalysis using metal-free redox molecular catalysts

Domain: physics
Published: 2025-12-13
Editorial outcome: major_revision

Abstract
Nature’s redox enzymes achieve remarkable selectivity by organizing active sites with precise spatial control, an ability difficult to replicate in synthetic systems. Inspired by this, we report a class of stable, metal-free bicarbenium-based molecular catalysts that undergo electrochemical two-electron reduction to form biradical intermediates. These biradicals are confined within a rigid xanthene bridge (~4.3 Å), creating a spatially defined pocket that engages paramagnetic substrates, such as O₂ and NO, resembling nature’s synergistic catalysis. This biradical-mediated synergistic catalysis enables highly selective two-electron oxygen reduction (99.3% H₂O₂ selectivity, 96.8% Faradaic efficiency, 2.21 mol g⁻¹ h⁻¹ productivity) and three-electron nitric oxide reduction (NH₂OH as major product, 87.2% Faradaic efficiency, 1.68 mol g⁻¹ h⁻¹ productivity). Experimental and computational studies confirm the sustained redox cycling, supported by favorable spin-pairing and substrate binding geometries, which enhance both catalytic selectivity and efficiency. This work presents a blueprint for pathway-specific, radical-mediated catalysis, offering new design principles for metal-free electrocatalytic platforms that exploit open-shell reactivity.

Introduction
Nature has evolved remarkably efficient redox catalytic systems, where spatially organized active centers cooperatively mediate complex multi-electron transformations that are difficult to replicate synthetically1–3. For instance, cytochrome c oxidase (CcO) catalyzes the four-electron oxygen reduction reaction (ORR) in mitochondria via a binuclear metal center that stabilizes reactive intermediates (Fig. 1a)4. In contrast, NADPH oxidase selectively promotes the one-electron ORR to generate superoxide by integrating a flavin cofactor and two spatially separated metal sites to achieve selective oxygen activation (Fig. 1a)5. These biological systems exemplify how synergistic catalysis enables exceptional control over redox processes.

Despite efforts to emulate these biological principles, synthetic cooperative catalysts often suffer from limited structural precision, structural heterogeneity and poorly defined site distributions hinder the identification and isolation of the truly active motifs. Moreover, controlling the electronic and spatial interactions between adjacent sites is critically challenging, often leading to compromised selectivity and efficiency11–13. Molecular catalysts offer atomic-level tunability, enabling systematic reactivity modulation and precise mechanistic investigation14–16. While metal-centered designs remain dominant due to their redox versatility, they face issues such as stability, limited synthetic flexibility, and challenges in fine-tuning selectivity17–19. In this context, metal-free catalysts are emerging as promising alternatives, offering high structural tunability and potential for sustainable catalysis20–24.

Radicals play a central role in many biological and synthetic redox systems, enabling mechanistic pathways inaccessible to closed-shell species25–27. It has been demonstrated that triaryl carbon radicals can effectively activate O₂ via spin-pairing interactions, leveraging the biradical triplet nature of O₂ (Fig. 1b)28–32. Moreover, two such radicals can cooperatively bind O₂ to form a peroxide-linked intermediate. The inherent instability of such intermediate allows for multiple pathways, including reversion to O₂, generation of H₂O₂, or O-O bond cleavage under further reduction. These features make carbon-centered radicals particularly attractive for the activation and transformation of open-shell substrates.

A full catalytic cycle can be envisioned for triaryl carbon radicals in ORR (Fig. 1c, using ORR as the example hereafter): two radicals bind O₂ to form a peroxide intermediate (i), which undergoes stepwise protonation to release H₂O₂ and generate the carbenium (ii). Carbenium species can receive electrons to regenerate the active radicals, completing the cycle (iii). However, multiple degradation pathways, such as self-dimerization (iv), reductive protonation (v), unwanted side reaction and/or hydrolysis of intermediates (vi–vii)33,34, can compete, necessitating molecular designs that stabilize various intermediates to ensure selective and efficient turnover.

The binding geometry of O₂ at active sites further influences catalytic outcomes. Three conventional modes, Griffiths, Yeager, and Pauling types, govern selectivity and efficiency (Fig. 1d)35. Griffiths and Yeager involve side-on adsorption and tend to promote O-O bond cleavage, favoring the 4e- pathway, but often suffer from selectivity. Pauling-type end-on binding better preserves the O-O bond and thus supports the 2e- pathway, but it faces the issue of insufficient activation. Therefore, both active site architecture and electronic structure must be tailored to balance substrate binding, activation, and product selectivity.

Guided by principles of enzymatic dual-site synergy, carbenium/radical reactivity and substrate binding geometries, we designed a class of persistent bicarbenium scaffolds featuring a rigid xanthene bridge that positions the two carbenium centers ~4.3 Å apart (Fig. 1e). Upon electrochemical 2e- reduction, these scaffolds generate biradical intermediates that form a spatially confined pocket, ideally suited for engaging open-shell paramagnetic diatomic substrates such as O₂ and NO. These metal-free catalysts promote highly selective and efficient 2e- ORR (up to 99.3% selectivity, 96.8% Faradaic efficiency (FE) and 2.21 mol g⁻¹ h⁻¹ H₂O₂ productivity) and 3e- NORR (NH₂OH as major product with 87.2% FE and 1.68 mol g⁻¹ h⁻¹ productivity). This work demonstrates the power of biradical-mediated synergistic catalysis enabled by robust bicarbenium architecture and offers a general design strategy for next-generation, metal-free electrocatalysts, especially for redox transformations of open-shell substrates.

Results
Synthesis and structural characterization
The dibromoxanthene bridge (1) was synthesized following reported procedures36. Three anthrone analogues (2–4), with heterocycles introduced to modulate electronic properties via inductive and resonance effects, thereby stabilizing both the carbenium and key catalytic intermediates, were prepared (Fig. 2a). Lithiation of 1, followed by nucleophilic addition to the anthrones, yielded dihydroxyl precursors (C-C-2OH, O-C-2OH, and S-C-2OH). The lower yield of S-C-2OH likely arises from sulfur’s more substantial electron-donating effect as well as more effective lone pair electron delocalization across the anthrone unit, which significantly reduces the electrophilicity of the carbonyl (resisting nucleophilic attack). The dihydroxyl precursors were dissolved in acetonitrile and treated with aqueous HBF₄ to afford the corresponding bicarbenium compounds via acid-mediated dehydration. The reaction progress was monitored by UV-vis spectroscopy, where characteristic absorption bands associated with carbenium species emerged and stabilized within 1 h (Supplementary Figs. 28–30)37–39. After solvent removal and recrystallization from diethyl ether, the bicarbenium compounds, C-C²⁺ (brownish yellow), O-C²⁺ (brownish red), and S-C²⁺ (brownish red), were isolated in nearly quantitative yields.

The chemical structures were confirmed by 1H, 13C, and 19F NMR spectroscopy, as well as high-resolution mass spectrometry (Supplementary Figs. 1–15 and 20–25). The single crystals of C-C²⁺ were obtained by slow evaporation from their acetonitrile solution, while single crystals of O-C²⁺ and S-C²⁺ were obtained via slow diffusion of diethyl ether into their acetonitrile solutions (Supplementary Figs. 60–62 and Supplementary Tables 8–10). X-ray crystallographic analysis revealed a cofacial spatial arrangement of the two carbenium centers in all three bicarbeniums (Fig. 2b). S-C²⁺ exhibits a more coplanar configuration due to additional sulfur-sulfur interactions. Intramolecular distances between the two carbenium centers were determined to be 4.61, 4.43, and 4.29 Å for C-C²⁺, O-C²⁺, and S-C²⁺, respectively, which are suitable for the activation of diatomic molecules. In the solid-state packing structures (Fig. 2c), the BF₄⁻ counter-ions are positioned in the intermolecular voids, leaving the bicarbenium sites sterically unencumbered and accessible for interaction with substrates. Notably, the arrangement of bicarbenium molecules indicates that each pair of intramolecular carbenium centers is well isolated from those neighboring molecules. This minimizes intermolecular interference between active sites and thus can effectively preserve the integrity of the intramolecular dual-site architecture and promotes synergistic catalysis at the molecular level.

These bicarbenium compounds show stable behavior under ambient conditions and in water, underscoring their suitability for catalytic applications in practical environments. To evaluate their robustness, a systematic pH-dependent stability study was conducted by adjusting the pH of their acetonitrile solutions using aqueous HCl or NaOH (Supplementary Figs. 31–33). The results revealed that C-C²⁺ remains stable only under acidic conditions (pH ≤ 6), while undergoing hydrolysis near neutral pH. In contrast, O-C²⁺ and S-C²⁺ displayed significantly enhanced tolerance, retaining structural integrity up to pH 9 and pH 12, respectively. The stability of these tertiary carbenium centers is initially supported by charge delocalization into adjacent aryl rings, while the use of O-xanthene and S-xanthene backbones provides additional stabilization via inductive donation effect and more effective charge delocalization through extended π-conjugation (Supplementary Fig. 65). In particular, sulfur, with its stronger electron-donating character and greater ability to engage in π-conjugation, confers higher stability to S-C²⁺.

Electronic and electrochemical properties
C-C²⁺, O-C²⁺, and S-C²⁺ exhibit intense absorption features in the UV-vis region (Fig. 3a). Specifically, C-C²⁺ displays a dominant absorption band with a maximum (λmax) at 407 nm, whereas O-C²⁺ and S-C²⁺ show two major absorption bands at 360/454 nm and 376/496 nm, respectively. The progressive red shift in absorption as well as emergence of new absorption band from C-C²⁺ to S-C²⁺ can be attributed to both a stronger inductive donation effect and enhanced π-conjugation within the xanthene-based carbenium, induced by O- and S-heterocycles. Based on their optical transitions, the optical band gaps of C-C²⁺, O-C²⁺, and S-C²⁺ were approximately 2.52, 1.83, and 1.72 eV, respectively. Cyclic voltammetry (CV) measurements in argon-saturated acetonitrile solution revealed that all three bicarbeniums exhibited two well-defined, reversible redox waves (Fig. 3b). When the first electron occupies the LUMO of the bicarbenium, it forms a radical cation, making subsequent reduction more difficult, thus the second redox wave appeared at lower potential. The first reduction onset potentials were measured at −0.05 V, −0.25 V, and −0.28 V (vs. Fc/Fc⁺) for C-C²⁺, O-C²⁺, and S-C²⁺, respectively, corresponding to the single-electron reduction from the bicarbenium (C⁺⁺C) state to the radical carbenium (C⁺•C) intermediate. The second reduction waves, observed at −0.22 V, −0.45 V, and −0.47 V, reflect the further one-electron reduction to the biradical (C••C) state. The stability of these diradical intermediates was monitored by ESR spectroscopy (Supplementary Figs. 108–110). The good reversibility of both redox steps underscores the high electrochemical stability of these molecular systems in all redox states. The frontier molecular orbital energy levels (HOMO and LUMO) were estimated to be −4.75/−2.50 eV for C-C²⁺, −4.55/−2.72 eV for O-C²⁺, and −4.52/−2.80 eV for S-C²⁺ (vs. vacuum, Fig. 3c). The rather low LUMO level of C-C²⁺ suggests a strong tendency to accept electrons, which may raise concerns of undesirable side reactions (like water oxidation reaction) under certain conditions40. This issue is addressed in detail in a later section.

In contrast, CV measurements in O₂-saturated acetonitrile revealed irreversible redox waves, suggesting that the reduced biradical intermediates readily react with O₂ (Supplementary Figs. 34–36). This observation supports the potential of these bicarbeniums to electrochemically activate molecular oxygen. To further investigate the reduction process and subsequent O₂ activation, chemical reduction using zinc powder was employed as a rapid electron source. The resulting biradical intermediates (C••C) were characterized by electron spin resonance (ESR) spectroscopy. ESR spectra of the reduced biradicals exhibited intense signals with g-values of 2.0028, 2.0031, and 2.0038 for C-C²⁺, O-C²⁺, and S-C²⁺, respectively (Fig. 3d, Supplementary Figs. 37 and 39). (Please also refer to the stepwise generation of mono-radical and diradical species via in situ ESR measurements, Supplementary Fig. 107) The higher g-values for S-C²⁺ suggest spin delocalization into the sulfur atoms, consistent with calculated spin density distributions (Supplementary Fig. 65). Variable-temperature (VT) ESR measurements from 273 to 153 K showed an increase in signal intensity with decreasing temperature (Fig. 3e), and the product of the ESR intensity (I) and temperature (T) remained nearly constant (Fig. 3f), suggesting negligible magnetic coupling between the two radical centers41. Similar observations were also found for C-C²⁺ and O-C²⁺ (Supplementary Figs. 38 and 40). This behavior is indicative of a singlet-triplet bistable state, in which the energy gap between spin states is minimal. Such spin flexibility is particularly advantageous for the activation of substrates with a triplet ground state.

Upon bubbling O₂ into the acetonitrile solution of the biradical intermediates, precipitates gradually formed, indicating the generation of COOC peroxide intermediates, which exhibit poor solubility in acetonitrile. These solids were collected by filtration and subsequently redissolved in deuterated DCM for further 1H NMR and mass spectrometry analysis. Taking S-COOC as an example, the 1H NMR spectrum appeared complex (Fig. 3h), likely due to multiple possible orientations of the peroxide bridge42 and concurrent reactions such as protonation and partial desorption (Supplementary Fig. 91). Despite the complexity, mass spectrometry provided clear evidence for the formation of the COOC peroxide intermediate (Fig. 3j). Besides, no obvious mass peaks corresponding to non-cooperative or single-site catalysis were detected in the higher mass range. To probe the catalytic cycle, excess aqueous HBF₄ as a proton source was added to the S-COOC solution, immediately restoring the characteristic dark red color of S-C²⁺. This regeneration was further confirmed by 1H NMR (Fig. 3g, i), indicating release of peroxide product and recovery of the bicarbenium. Similar catalytic cycling behavior was observed for O-C²⁺ (Supplementary Figs. 16, 17 and 26), whereas the higher relative mass spectrometric intensity of the peroxide-linked intermediate detected for S-C²⁺ highlights its enhanced capacity to stabilize this key species (Supplementary Figs. 26, 27). In contrast, isolation of the peroxide intermediate for C-C²⁺ proved difficult, likely due to its lower stabilization towards the intermediates.

Nevertheless, it offered a valuable opportunity to investigate the potential catalyst decomposition/competing pathways, given the low LUMO level of C-C²⁺ and its limited ability to stabilize reaction intermediates. To investigate this, two acetonitrile solutions of C-C²⁺ (each containing one drop of pH 7 pure water for accelerating side reactions) were prepared. One was stored under an argon (Ar) atmosphere, while the other was exposed to air. Both were left to evaporate slowly over several days (about 10 days for the Ar atmosphere sample and 3 days for the air-exposed sample). In the Ar-protected sample, the solution faded to pale yellow, while the air-exposed sample became colorless, both indicating decomposition of C-C²⁺. Single crystals were successfully isolated from both cases. X-ray crystallographic analysis revealed the formation of a protonated C-C-2H product under Ar, whereas the air-exposed sample yielded the corresponding hydroxyl precursor C-C-2OH (Supplementary Figs. 62, 63 and Tables 11, 12). These findings suggest the presence of a potential decomposition/competing pathway, although it occurs more slowly and at higher pH. Under inert conditions, C-C²⁺ underwent reductive protonation, with water acting as the electron donor and becoming oxidized. In the presence of O₂, the formation of a transient COOC peroxide intermediate likely underwent product desorption with water as both the electron and proton source. Once OH⁻ is accumulated, C-C²⁺ undergoes hydrolysis, resulting in the dihydroxyl precursor. The same trials for O-C²⁺ and S-C²⁺ reflected that their structures remained intact. Overall, these results revealed competing reductive protonation and hydrolysis pathways that may gradually deactivate the carbenium species for the case of C-C²⁺ due to a lack of additional stabilization as compared to O-C²⁺ and S-C²⁺.

Electrocatalysis
Having developed a comprehensive understanding of the molecular structures and redox properties of these bicarbenium compounds, we next evaluated their electrocatalytic performance toward the reduction of O₂ and NO. Catalyst inks of C-C²⁺, O-C²⁺, and S-C²⁺ were prepared by dispersing the compounds with carbon black and Nafion binder to enhance electrical conductivity and compatibility with aqueous electrolytes for heterogeneous electrocatalysis43.

ORR electrocatalysis. CV measurements were first conducted on a rotating disk electrode (RDE) in both Ar- and O₂-saturated 0.1 M HClO₄ (Supplementary Fig. 52). In Ar-saturated conditions, no apparent redox features were observed in the range of 0.2–1.0 V (vs. RHE). In contrast, well-defined cathodic waves appeared at ~0.4-0.7 V (vs. RHE) under O₂-saturation, highlighting the electrocatalytic activity of all three catalysts toward the ORR. To further assess their performance, linear sweep voltammetry (LSV) curves were recorded at varying rotation rates (400–2400 rpm) in O₂-saturated electrolyte (Supplementary Figs. 53–55). The increasing current density with higher rotation rates reflects improved mass transport through convective diffusion, supporting faster reaction kinetics. Their electrocatalytic properties were further probed using rotating ring-disk electrode (RRDE) measurements to determine the onset potentials, Tafel slopes, electron transfer numbers (n), and H₂O₂ selectivity at 1600 rpm and a scan rate of 10 mV s⁻¹ (Fig. 4a). The onset potentials for C-C²⁺, O-C²⁺ and S-C²⁺ were determined to be 0.665, 0.686, and 0.696 V (vs. RHE), respectively (defined as the potential at which the current density reaches 0.1 mA cm⁻²). The more positive onset potential or small overpotential for S-C²⁺ catalysts suggests that S-C²⁺ initiates ORR more readily. Tafel analysis revealed slopes of 63.7, 59.7, and 53.1 mV dec⁻¹ for C-C²⁺, O-C²⁺, and S-C²⁺, respectively (Fig. 4b), with the lower slope of S-C²⁺ indicating faster reaction kinetics. At 0.2 V (vs. RHE), the current densities reached approximately 1.77, 2.27, and 2.88 mA cm⁻², for C-C²⁺, O-C²⁺, and S-C²⁺, respectively, in line with their relative catalytic activities. The LSV comparison with commercial carbon black and Pt/C and the electrochemical surface area (ECSA) measurements revealed a 2e- reaction pathway feature for these bicarbenium catalysts and their intrinsic catalytic activity can be attributed to the bicarbenium sites (see Supplementary Figs. 105, 106). The electron transfer numbers (n) remained relatively constant between 0.2–0.6 V (vs. RHE, Fig. 4c), with values of 2.32, 2.12, and 2.03 for C-C²⁺, O-C²⁺, and S-C²⁺, respectively, suggesting that these catalysts predominantly follow a two-electron ORR pathway to produce H₂O₂. Correspondingly, the H₂O₂ selectivity across the same potential window was 93.2%, 96.8%, and 99.3%, respectively (Fig. 4c). These findings demonstrate that all three bicarbenium systems exhibit high selectivity toward the two-electron ORR pathway, with S-C²⁺ showing the highest catalytic activity and product selectivity, followed by O-C²⁺ and C-C²⁺.

An H-type cell was assembled to quantitatively evaluate the H₂O₂ production rate and the corresponding FE. H₂O₂ quantification was performed using the conventional Ce(SO₄)₂ titration method44, calibrated via UV-vis spectrophotometry (Supplementary Fig. 43). The electrocatalysis reaction was conducted over 6 h, with aliquots of the catholyte collected hourly for analysis. All three catalysts, C-C²⁺, O-C²⁺, and S-C²⁺ demonstrated continuous H₂O₂ generation, with total yields of 668.7 μmol, 762.4 μmol, and 1076.5 μmol, respectively, after 6 hours (Fig. 4d and Supplementary Figs. 44, 46). The corresponding H₂O₂ production rates (κ) and FE were calculated at each time point (Fig. 4e and Supplementary Figs. 47–49). Both parameters remained stable throughout the experiment, with production rates of 1.36, 1.56, and 2.21 mol g⁻¹ h⁻¹ and FEs of 92.6%, 94.0%, and 96.8% for C-C²⁺, O-C²⁺, and S-C²⁺, respectively. (Please also refer to the performance and discussion in flow cell using the best performed S-C2+ catalyst, Supplementary Fig. 111) The catalyst durability was investigated at 0.4 V vs. RHE by chronoamperometric method (Fig. 4f). While both O-C²⁺ and S-C²⁺ exhibited good stability, with only 5.1% and 1.5% loss of current density respectively, after 60,000 s of operation, the stability of C-C²⁺ raised some concerns. Its current density showed a noticeable decline in the early stage, followed by a slight decrease in the later stage. As identified in the previous section, the presence of a competing reaction (reductive protonation and hydrolysis) could be responsible for the observed loss in catalytic activity. Nevertheless, the hydrolyzed species may gradually revert to carbeniums in the acid media, thus the loss of current density becomes less significant in a later stage. These results highlight the effective electrocatalytic performance of the bicarbenium catalysts, with performance competitive to that of previously reported catalysts under similar electrocatalytic conditions (Supplementary Table 6).

NORR electrocatalysis. Nitric oxide (NO) features a doublet ground state with a single unpaired electron whose spin density is primarily on the nitrogen atom but also delocalized over oxygen (about 65% vs 35%)45. This electronic structure suggests that our catalysts may be well-suited for the nitric oxide reduction reaction (NORR). Given its high activity in oxygen reduction and enhanced ability to initiate reactions and stabilize key intermediates, S-C²⁺ was chosen for detailed investigation of NORR. LSV in Ar- and NO-saturated 0.1 M HClO₄ revealed a pronounced increase in both onset potential and current density in the presence of NO (Fig. 4g), indicating efficient NO electroreduction. To estimate the electron transfer number, LSV curves were compared with those obtained using a polycrystalline Pt electrode under identical conditions, where Pt is known to reduce NO selectively to N₂O via a one-electron pathway at 0.2 V vs. RHE46. The diffusion-limiting current density (jd) of Pt was measured at 1.64 mA cm⁻², consistent with literature values, while S-C²⁺ exhibited a significantly higher jd of 5.02 mA cm⁻². This ~3.06-fold increase suggests a 3e⁻ transfer pathway for our bicarbenium catalyst in NORR. (Please refer to the SI for the NORR performances of O-C2+ and C-C2+, Supplementary Fig. 93)

An H-type cell was assembled to identify the reduction products and quantify the reaction performance. Based on the inferred electron transfer number, the likely products were narrowed to hydroxylamine (NH₂OH/NH₃OH⁺) and ammonia (NH₃/NH₄⁺). NH₂OH was quantified using a colorimetric method involving 8-quinolinol oxidation to a strongly absorbing dimer at 705 nm, while NH₃ was quantified using Nessler’s reagent at 420 nm, both calibrated via UV–vis spectrophotometry (Supplementary Figs. 50, 51)47. For safety, the electrolysis was carried out over 3 h, with hourly aliquots collected for analysis. S-C²⁺ showed continuous NH₂OH production with a total yield of 206.2 μmol (Fig. 4h), a steady rate of 1.68 mol g⁻¹ h⁻¹, and a FE of 87.8% (Fig. 4i). These findings underscore the high selectivity and efficiency of such bicarbenium catalysts for NORR, showing competitive performance compared with previously reported catalysts under comparable conditions (Supplementary Table 7).

Compared with the monocation S-C⁺ catalyst (see detailed characterization data in Supplementary Figs. 94–100), which was prepared using the same synthetic strategy, the bicarbenium catalysts exhibited enhanced selectivity and catalytic performance due to the synergistic mechanism.

Mechanism study
ORR mechanism. Comprehensive in situ spectroscopy and density functional theory (DFT) calculations were conducted to elucidate the catalytic activity and selectivity of the bicarbenium catalysts. In situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-IR) was employed to probe the reaction pathway, with spectra collected across a potential range of 1.0 to 0.2 V vs. RHE (Fig. 5a and Supplementary Figs. 41, 42). Corresponding vibrational assignments were guided by the literature precedents and computed IR spectra (Supplementary Figs. 70–81)24,29,48,49. Taking S-C²⁺ as a representative example (Fig. 5a), several characteristic peaks were identified (Supplementary Table 4): the peak at 917 cm⁻¹ corresponds to O-O symmetric stretch in the *C-OO-C* peroxide-linked intermediate; The band at 1334 cm⁻¹ appeared at low potential is assigned to the C-O-O⁻ anion stretch from *COO⁻ intermediate (to form the *COOH intermediate by protonation). Peaks at 1183 and 1210 cm⁻¹ are attributed to the C-O symmetric stretch in the *C-OO-C* peroxide linkage and the C–OOH asymmetric stretch in the protonated *COOH intermediate, respectively. Besides, the broad signal at 3533 cm⁻¹ corresponds to O-H stretching in the *COOH intermediate. These spectral signatures suggest an ORR pathway wherein O₂ is first captured and reduced to form a *C-OO-C* peroxide-linked intermediate, followed by stepwise protonation to yield the *COOH species and ultimately release H₂O₂.

To complement the spectroscopic insights, DFT calculations were performed to elucidate the proposed reaction mechanism (Fig. 5b). Starting from the bicarbenium precursor, electron transfer from the electrode generates the key biradical intermediate. Geometry optimizations of both singlet and triplet states reveal minor energy differences of −0.38, −0.41, and −1.04 kcal mol⁻¹ for C-C²⁺, O-C²⁺, and S-C²⁺, respectively, suggesting a singlet-triplet bistable state with a slightly thermodynamic preference for the triplet ground state (Supplementary Table 13). These biradicals are highly reactive toward triplet-state O₂ by spin-pairing interactions that facilitate its reduction. Calculations indicated that the synergistic binding of O₂ via two radical centers to form a peroxide-linked intermediate is energetically more favorable than their non-cooperative, single-site interactions (Supplementary Figs. 68, 69). The first protonation step occurs with near-equal preference at either terminal site due to the nearly symmetric structure. This generates the *COOH intermediate, which upon further protonation releases H₂O₂ and regenerates the bicarbenium species for the next catalytic cycle. Similar pathways were observed for O-C²⁺ and C-C²⁺, as supported by both in situ ATR-FTIR and DFT results (Supplementary Figs. 41, 42). Free energy profiles constructed for C-C²⁺, O-C²⁺, and S-C²⁺ (Fig. 5c) illustrate a common ORR mechanistic framework: the bicarbenium-to-biradical transformation is endothermic and identified as the rate-determining step (RDS), whereas all subsequent steps, including formation of the peroxide-linked intermediate, first protonation to the *OOH intermediate, and second protonation for H₂O₂ release and catalyst regeneration, are exothermic. The computed energy barriers for biradical formation are 6.0, 19.2, and 12.9 kcal mol⁻¹ for C-C²⁺, O-C²⁺, and S-C²⁺, respectively. Although C-C²⁺ features the lowest energy barrier, attributable to its exceptionally low LUMO level (Fig. 3c), its overall catalytic performance is compromised by a competing reductive protonation pathway, as revealed by the corresponding energy profile (Supplementary Figs. 67, 112). A more systematic comparison provided by our microkinetic analysis (Supplementary Note 18) further demonstrates that, when both activation and quenching channels are considered, the apparent catalytic activity follows the experimental order of C-C²⁺ < O-C²⁺ < S-C²⁺.

NORR mechanism. Building on the mechanistic understanding established for the ORR catalysis, similar in situ spectroscopic and computational approaches were employed to elucidate the NORR pathway using the most promising S-C²⁺ catalyst. Specifically, in situ ATR-IR spectroscopy was conducted over a potential range of 0.3 to −0.2 V vs. RHE (Fig. 5d), revealing distinct vibrational features associated with various reaction intermediates. These spectral features were interpreted with the aid of computed IR spectra and referenced vibrational assignments (Supplementary Fig. 82-90)46,47,50. Several key bands were identified, supporting the stepwise transformation of NO during the catalytic cycle. The N•-O stretch at 1357 cm⁻¹ corresponds to the formation of a *C-N•O-C* nitroxide-linked intermediate. The subsequent protonation steps were evidenced by peaks at 1042 and 1122 cm⁻¹, attributed to the C-NHO-C asymmetric stretch in the monoprotonated *C-NHO-C* species and the C-NHOH stretch in the diprotonated *C-NHOH* intermediate, respectively. Additionally, a band at 1583 cm⁻¹, which emerged at more negative potentials, could be assigned to the C-NH-O⁻ anion stretch in the *C-NH-O⁻* intermediate (to form the *C-NH-OH* intermediate by protonation). The bands at 1206 and 1264 cm⁻¹, increasing in intensity from 0.1 V vs. RHE, are attributed to C-O and C-N stretches in various intermediates. Lastly, the broad absorption around 3560 cm⁻¹ is characteristic of O-H and N-H stretching modes in protonated species such as *C-NHO-C* and *C-NHOH*. These findings strongly suggest a reaction pathway in which NO is first captured via a reduction step to form the nitroxide intermediate, followed by sequential protonation leading to the release of NH₂OH.

DFT calculations were also performed to gain deeper insight into the proposed NORR mechanism catalyzed by S-C²⁺ (Fig. 5e). Similar to the ORR pathway, the reaction initiates with electron transfer from the electrode to the bicarbenium precursor, generating a biradical intermediate. This biradical, with high reactivity toward triplet-like NO, facilitates its reduction to form a C-N•O-C nitroxide intermediate. Spin density analysis indicated a delocalized unpaired electron, mostly residing on nitrogen, mirroring the electronic structure of free NO molecule (Supplementary Fig. 66). Therefore, the following protonation step is more favorable at the nitrogen atom, forming the monoprotonated *C-NHO-C* intermediate. The second protonation is energetically preferred at the oxygen atom, yielding the C-NHOH species. The final protonation releases NH₂OH and regenerates the original bicarbenium scaffold, completing the catalytic cycle (across S-C²⁺, O-C²⁺, and C-C²⁺, transition-state analyses show that C–O cleavage to form NH₂OH is kinetically favoured over N–O cleavage to form NH₃, explaining the high NH₂OH selectivity (87.8% FE) and only trace NH₃ observed experimentally, as summarized in Supplementary Figs. 113-114). The constructed free energy profile (Fig. 5f, the calculations were also performed for C-C²⁺ and O-C²⁺ for comparison) revealed that both the bicarbenium-to-biradical transition and the biradical-to-nitroxide linkage formation are endothermic. In contrast, all subsequent protonation steps are exothermic, facilitating smooth progression toward product formation and catalyst regeneration. The results reaffirm that the bicarbenium-to-biradical transition is the RDS, similar to the ORR process, with an energy barrier of 12.9 kcal mol⁻¹. However, the formation of the C-N•O-C nitroxide intermediate also presents a comparable barrier of 10.9 kcal mol⁻¹, giving rise to the relatively lower catalytic efficiency of NORR compared to ORR. The discrepancy can be attributed to the intrinsic doublet electronic structure of the NO molecule. Although its unpaired electron can delocalize between the N and O atoms, giving it a semblance of triplet character, it is inherently less compatible than triplet O₂ in engaging with the biradical intermediate. This diminished spin compatibility weakens the spin-pairing interactions, which likely contributes to the lower catalytic efficiency observed in NORR. Nevertheless, S-C²⁺ still exhibited good NORR performance, achieving high efficiency and selectivity despite the additional energetic demand. We also carried out the same microkinetic analysis for the NORR pathway (Supplementary Note 18), and the apparent activity trend remains C–C²⁺ < O–C²⁺ < S–C²⁺. Supplementary Videos 1 and 2 are provided to visualize the proposed ORR and NORR processes.

Electrocatalysis for closed-shell substrates. Beyond ORR and NORR, the electrocatalytic potential of S-C²⁺ was further explored with a range of commonly studied yet more inert closed-shell molecules. LSV measurements were conducted for the electrochemical reduction of N₂, CO₂, NO₂⁻, and NO₃⁻ (Supplementary Figs. 56–58). In each case, S-C²⁺ exhibited decreased onset potentials and elevated current densities, indicative of its ability to activate these closed-shell strongly bonded molecules51–53, likely facilitated by the interactions between the radical intermediates and the vacant orbitals of those molecules. The observed variations in activity likely stem from differences in molecular size and intrinsic electronic structure. While these preliminary results highlight the broader catalytic potential of bicarbenium systems, further studies are warranted to elucidate the underlying activation pathways and identify the resulting products.

Methods
General methods
All reagents and starting materials were obtained from commercial suppliers and used without further purification, unless otherwise noted. Anhydrous THF was distilled from sodium-benzophenone solution immediately prior to use. Column chromatography was performed on silica gel 60 (Merck 40–60 nm, 230–400 mesh). The 1H, 13C and 19F NMR spectra were recorded in solution of CDCl₃/CD₃CN on Bruker DPX400 or DPX500 NMR spectrometers. All chemical shifts are quoted in ppm, relative to tetramethylsilane (TMS) with the residual solvent peak as a reference standard. High resolution mass spectrometry (MS) measurements were performed on a Finnigan TSQ7000 triple stage quadrupole mass spectrometer by atmospheric pressure chemical ionization (APCI) or on a Bruker MALDI-TOF Autoflex. UV-vis-NIR absorption spectra were recorded on a Shimadzu UV-3600 spectrophotometer. Electrochemical measurements were conducted using an Autolab PGSTAT302N workstation.

Synthesis of bicarbenium catalysts
The respective dihydroxyl precursors (50 mg), C-C-2OH, O-C-2OH or S-C-2OH, were dissolved in acetonitrile (5 ml) under argon. Tetrafluoroboric acid solution (48 wt% in H₂O, 0.5 ml) was slowly added and the suspension immediately turned into a dark color solution. After stirring for 2 h, the solvent was removed under reduced pressure. The crude product was recrystallized from diethyl ether, filtered and dried under reduced pressure to afford the bicarbenium compounds: C-C²⁺ (brownish yellow), O-C²⁺ (brownish red) and S-C²⁺ (brownish red), all in nearly quantitative yield. The more detailed synthesis and characterization data are included in Supplementary Information.

X-ray crystallography
The single crystals of C-C²⁺ were obtained by slow evaporation from its acetonitrile solution, while single crystals of O-C²⁺ and S-C²⁺ were obtained via slow diffusion of diethyl ether into their acetonitrile solutions. The single crystals of C-C-2H and C-C-2OH were obtained by slow evaporation from acetonitrile solutions of C-C²⁺ with 1 drop of pH 7 water in argon atmosphere and air-exposed condition, respectively, after several days (about 10 days for the Ar atmosphere sample and 3 days for the air-exposed sample). Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 2448050 for C-C2+, 2448051 for O-C2+, 2448052 for S-C2+, 2448053 for C-C-2H and 2448055 for C-C-2OH. The X-ray intensity data were measured at low temperature (T=100 K), using a four circles goniometer Kappa geometry, Bruker AXS D8 Venture, equipped with an APEX-II detector. Frames were integrated with the Bruker SAINT software package. Data were corrected for absorption effects using the multi-scan method (SADABS)54. Molecule was solved with the software SHELXT55, using a Dual Space method. Refinement of the structure was performed by least squares procedures on weighted F² values using the SHELXL-version 2014/6 module56 in olex257.

ESR and VT-ESR
ESR spectra were recorded using a JEOL FA200 spectrometer operating at X-band frequency (~9.4 GHz). The respective samples were packed into quartz ESR tubes under an inert atmosphere. VT-ESR measurements were conducted from 273 K to 153 K with 10 K intervals, controlled by a variable temperature liquid nitrogen cryostat. Instrument parameters, including microwave power, modulation amplitude, and scan width, were carefully optimized for each sample to ensure accurate signal detection without saturation. Spectra were calibrated against an internal standard for g-factor determination.

Electrochemistry
Cyclic voltammetry (CV) measurements were performed in acetonitrile with a three-electrode cell, using 0.1 M tetrabutylammonium hexafluorophosphate (nBu₄NPF₆) as supporting electrolyte, Ag/AgCl as reference electrode, gold disk as working electrode, Pt wire as counter electrode, with a scan rate of 50 mV s⁻¹. The gold disk electrode was cleaned and polished with Al₂O₃ powders prior to use.

ORR electrocatalytic tests: The catalytic activity of catalysts was measured in 0.1 M HClO₄ electrolyte solution (pH=1). An ink of the respective catalyst was prepared by mixing 1.7 mg of catalyst and 1.3 mg carbon black with 250 μL of a 0.5 wt% Nafion solution (4/1 volume ratio between ethanol and 0.1 M HBF₄ water solution for stabilization), and further placed in an ultrasonic bath for 1 h. 12 μL of the solution was loaded onto the electrode and dried in air. The RRDE measurements were performed in a three-electrode, one-compartment cell at room temperature (25°C), equipped with a Pt wire counter electrode and an Ag/AgCl reference electrode. O₂ gas was purged into the 0.1 M HClO₄ electrolyte solution for 30 min. Linear sweep voltammetry (LSV) was performed in O₂-saturated 0.1 M HClO₄ electrolyte at an electrode rotation rate of 1600 rpm and a scan rate of 10 mV s⁻¹, with a ring potential held at +1.4 V vs. RHE. All potential scales were converted to RHE using:

E_RHE = E_Ag/AgCl + 0.059 × pH + 0.197 (1)

The H₂O₂ selectivity and electron transfer number were calculated using the following equations based on LSV curves:

H₂O₂ (%) = I_r / N / (I_d / N + I_r / N) × 2 × 100 (2)

n = 4 × I_d / (I_d + I_r / N) (3)

where I_r is the ring current, I_d is the disk current and N is the collection efficiency (0.29 after calibration).

The H₂O₂ production process was monitored using a self-assembled H-type cell with a three-electrode system with 0.1 M HClO₄ solution as both the catholyte and anolyte, which were separated by a Nafion 115 membrane (1.5 × 1.5 cm² DuPont). A carbon paper (1 × 1 cm²) with a microporous layer, spayed-coated with catalyst ink, was used as the working electrode. An Ag/AgCl and a carbon rod were applied as reference and counter electrode, respectively. The reaction was monitored under potentiostatic mode at 0.4 V vs. RHE for a consecutive 6 hours, with aliquots of the catholyte collected hourly for analysis. H₂O₂ quantification was performed using the conventional Ce(SO₄)₂ titration method, calibrated via UV–vis spectrophotometry. The production rate and FE for H₂O₂ generation were calculated based on:

rate: mol g_cat⁻¹ h⁻¹ = n_H2O2 / (L × t) (4)

FE (%) = 2 × n_H2O2 × F / Q × 100 (5)

where n_H2O2 is the quantity of H₂O₂ generated (mol), L is the catalyst loading (g), t is the duration (h), F is the Faraday constant, and Q is the total charge amount (C) derived from the current-time curve.

NORR electrocatalytic tests: the characterization of NORR followed similar procedures to ORR with some modifications. The electron transfer number was estimated by comparing the LSV current density of a polycrystalline Pt electrode at about 0.2 V vs. RHE with the current density of catalyst electrode at about −0.2 V vs. RHE under same conditions (0.1 M HClO₄ electrolyte and pH=1, 1600 rpm, and 10 mV s⁻¹). NH₂OH was quantified using a colorimetric method involving 8-quinolinol oxidation to a strongly absorbing dimer at 705 nm, while NH₃ was quantified using Nessler’s reagent at 420 nm, both calibrated via UV–vis spectrophotometry. For safety reasons, the electrolysis was carried out over 3 h, with hourly aliquots collected for analysis.

All reported potential and voltage values were iR-compensated (85%) using the automatic iR compensation function of the Autolab (Metrohm) electrochemical workstation with NOVA software.

In situ ATR-IR measurements
In situ attenuated total reflection Fourier transform infrared (ATR-IR) spectroscopy was conducted using a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific). A silicon prism coated with a thin gold film and the catalyst ink served as the working electrode, with a platinum mesh as the counter electrode and an Ag/AgCl electrode as the reference. Measurements were carried out in 0.1 M HClO₄ electrolyte. Prior to data acquisition, the electrolyte was saturated with high-purity O₂ (99.995%) by bubbling for 30 min. Spectra were recorded over a potential range of 0.2 to 1.0 V vs. RHE. Similar procedures were employed for nitric oxide (NO, 99.9%), with spectra collected over a potential window of −0.2 to 0.3 V vs. RHE.

Computational details
All density functional theory (DFT) calculations were carried out using the Gaussian 16 software58 package at the ωB97XD59/6–31G(d,p)60,61 level of theory. Solvent effects were incorporated using the SMD implicit solvation model with water as the solvent62. Electrostatic potential (ESP) maps and spin density were analyzed using Multiwfn version 3.863–65.

The Gibbs free energies of all reactants and intermediates are calculated by the equation:

G = E_elec + G_corr + RT ln(C_aq / C_gas) (6)

Where E_elec is the electronic energy of the species obtained from the self-consistent field (SCF) calculation, and G_corr represents the thermal correction derived from frequency analysis, which includes the zero-point energy (ZPE) as well as thermal contributions to enthalpy and entropy. The final term, RT ln(C_aq / C_gas), accounts for the standard-state conversion from the gas-phase reference state (1 atm) to the aqueous-phase reference state (1 mol L⁻¹), contributing an additional +1.89 kcal/mol.

The Gibbs free energy of the solvated proton (H⁺) in aqueous phase was taken as −270.29 kcal/mol. This value was obtained from the following equation:

G_H+ = G_gas_H+ + G_solv_H+ + RT ln(C_aq / C_gas) (7)

Where G_gas_H+ is the gas-phase Gibbs free energy of the proton (−6.28 kcal/mol)66, G_solv_H+ is the solvation free energy (−265.9 kcal/mol)67, and the standard state correction term RT ln(C_aq / C_gas) accounts for the transition from the gas-phase standard state (1 atm) to the aqueous-phase standard state (1 mol/L), which contributes +1.89 kcal/mol.

The Gibbs free energy of a free electron referenced to the reversible hydrogen electrode (RHE) was calculated based on the RHE work function, using the relation:

E_RHE = E_SHE + 0.059 × pH (8)

The absolute potential of the standard hydrogen electrode (SHE) was taken as 4.44 eV68. At pH=1, this yields an RHE-referenced electron energy of 4.499 eV. Since electrons contribute to the Gibbs free energy as negative chemical potentials, this corresponds to −103.75 kcal/mol, calculated using the relation μ_e = −e × Φ, with 1 eV ≈ 23.06 kcal/mol.

Data availability
All data supporting the findings of this study are provided in the article and its Supplementary Information. Crystallographic data for the structures reported in this article have been deposited in the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 2448050, 2448051, 2448052, 2448053 and 2448055. Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. The authors declare that the data supporting the findings of this study are available within the paper, the source data and Supplementary Information files. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this study, inspired by redox enzymes, the authors innovatively designed a class of stable, metal-free molecular catalysts based on bicarbenium that can confine free radicals within the molecular structure. This biradical-mediated synergistic catalysis achieved two-electron (2e⁻) oxygen reduction reaction (ORR) of 99.3% H2O2 selectivity, 96.8% Faraday efficiency and 2.21 mol g⁻1cat h⁻¹ productivity and three-electron nitric oxide reduction of NH2OH as the main product, 87.2% Faraday efficiency and 1.68 g⁻1cat h⁻¹. This study combines experiments and theoretical calculations to confirm the sustained redox cycling supported by favorable spin-pairing and substrate binding geometries, providing new design principles for metal-free electrocatalytic platforms utilizing open-shell reactivity. However, additional explanations need to be provided before publication.\n1. In the process of material synthesis, the yield of S-C-2OH (5%) is significantly lower than that of C-C-2OH (50%) and O-C-2OH (40%). The difference has not been further explained in the main text. It is suggested to supplement the discussion on the influence of sulfur doping on this type of reaction.\n2. After synthesizing the bicarbenium compounds, relevant characteristic absorption bands were detected using UV-vis spectroscopy. Please provide additional relevant literature to support this result.\n3. This work utilizes the method of cultivating single crystals and X-ray crystallographic analysis. In all three bicarbeniums, two carbenium centers are arranged in planar space. Can the material also maintain a stable coplanar state in the electrolyte?\n4. The catalyst used in this work was compared with other works under similar conditions, but it is evident that many catalysts were tested under different conditions. It is recommended to readjust the comparison conditions.\n5. The 2e⁻ selectivity of S-C2⁺ is 99.3%. However, in the S-COO-C structure, it is assumed that the occurrence of side reactions leads to complex peak shapes in the 1H NMR spectrum, but there is no theory or testing to determine the specific problem.\n6. The abstract mentions that: \"Nature’s redox enzymes achieve remarkable selectivity by organizing active sites with precise spatial control, an ability difficult to replicate in synthetic systems.\" but does not specifically explain how the structural characteristics of natural enzymes inspired the design of this article. Suggest adding an explanation of the ideas and material design for this work.\n7. In the 2e⁻ pathway of oxygen reduction reaction (ORR), volcano plots can quantify the nonlinear relationship between OOH* adsorption strength and activity, transforming the search for highly selective catalysts\" into an optimization problem of ΔGOOH*. In this work, double free radicals are used to confine oxygen molecules within the molecule, forming intermediates. Please provide additional calculations for the adsorption energy of this binding. Suggested reference: 10.1038/s41467-024-53714-3. and 10.1002/adma.202500063.\n8. C-C2+ has a lower LUMO level. Can this trigger other side reactions? It is recommended to conduct performance testing on this material in the NORR electrocatalysis section.\n9. This work emphasizes the advantage of the dual radicals in \"synergistically binding substrates\", but no monoradical analogues (such as derivatives containing only one carbon cation site) were set as controls. It is suggested to supplement the ORR/NORR performance data of single-site catalysts to directly prove the enhancement of selectivity (such as 99.3% H₂O₂) and activity (such as 2.21 mol g⁻¹cat. h⁻¹ yield of S-C²⁺) by \"dual-site synergy\".\n10. This work explains the stability through the current density decay (Figure 4f), but does not verify the structural integrity of the catalyst after long-term reaction. It is suggested to supplement the characterization of the catalyst after 6 hours of catalysis (such as ¹H NMR, XRD or XPS) to confirm whether there is degradation (such as the hydrolysis of C-C²⁺) or agglomeration, and directly correlate \"structural stability\" with \"activity retention rate\". Suggested reference: 10.1002/anie.202500945.\n11. Currently, only tested in 0.1 M HClO₄ (acidic) solution, but actual H₂O₂ production may require neutral/alkaline conditions (such as avoiding equipment corrosion). It is recommended to supplement the performance of the catalyst in neutral (0.1 M PBS) and alkaline (0.1 M KOH) solutions, combined with pH stability data (Supplementary Figures 31-33, such as S-C²⁺ stabilizing to pH 12), to discuss its practical application potential.\n12. Table 3 shows that the H₂O₂ yield of S-C²⁺ (2.21 mol g⁻¹cat. h⁻¹) is superior to most metal-based catalysts, but the core reason for the performance advantage has not been analyzed, is the double free radical mechanism enhancing the intrinsic activity, or is the carbon black carrier enhancing conductivity/mass transfer? It is recommended to supplement the electrochemical active surface area (ECSA) test of the catalyst, compare the performance of \"pure catalyst\" and \"catalyst-carbon black composite\", and eliminate the interference of the carrier contribution.", "reviewer_2": "This study synthesized three well-defined metal-free molecular catalysts, among which S-C2+ exhibited excellent performance in both two-electron oxygen reduction and three-electron nitric oxide reduction. Interestingly, the authors proposed a biradical-mediated reaction mechanism. However, the confirmation of this mechanism lacks sufficient experimental evidence (see Q14). Additionally, the electrochemical performance of the catalyst was primarily evaluated in an H-cell (or RDEs), and data on its application in membrane electrode assemblies (MEAs) or flow cells are still lacking, limiting the impact of this research. Furthermore, while the authors synthesized three molecular catalysts, the differences in their electrochemical behavior were explained solely by the electronic effect on reaction energy barriers, which appears oversimplified. If the novelty of this work mainly stems from the biradical-mediated reaction mechanism, the inclusion of O-C2+ and C-C2+ may not be strictly necessary.\nMy specific comments are listed as follows:\n1. The electronic structure depiction of the trityl radical in Figure 1b is incorrect. It is erroneously shown as having two electrons with parallel spins in two orbitals. This figure requires revision.\n2. In the Supporting Information (SI), there is a lack of detailed explanations and descriptions for the spectra in Supplementary Figs. 1–26. The authors should provide a thorough analysis of each peak signal to allow readers to better understand the structural details of the synthesized molecules. Given that the successful synthesis of these molecular catalysts is a major innovation of this work, such comprehensive explanation is essential.\n3. The mass spectrometry data provided by the authors should include a broader *m/z* range to allow readers to better assess the purity of the synthesized molecules. The authors do not appear to report the purity levels of the three catalysts. Although single-crystal structures of the corresponding molecules were obtained, the experimental section does not explicitly state whether the single-crystal samples were directly used for electrochemical testing. If not, the samples employed in the electrochemical measurements may contain impurities. The nature and concentration of such impurities should be reported, as this information is critical for evaluating whether the observed electrochemical behavior is influenced by these impurities.\n4. In Supplementary Figs. 31-33, the authors investigated the structural stability of the molecular catalysts using UV-Vis spectroscopy, which is clearly insufficient. While changes in UV-Vis spectra can indicate structural instability, demonstrating molecular stability requires additional structural characterization data, such as mass spectrometry (MS) or nuclear magnetic resonance (NMR) spectroscopy.\n5. In Fig. 3a, the UV-Vis spectra of O-C and S-C exhibit two distinct peaks, while C-C shows only one. The authors should provide a clear explanation for this observed difference.\n6. The assignment of the (C+/+C) state to (C+/*C) and subsequently to (C*/*C) in the CV (Fig. 3b) requires experimental validation. Structurally, the two C+ moieties exhibit symmetry—why then do they display distinct redox potentials? The authors must provide a convincing rationale for this observation. One feasible approach would involve monitoring EPR signals at varying potentials: if their assignment is correct, distinct radical species should be detectable at different applied potentials. Given that this work proposes a radical-involved electrocatalytic mechanism, the generation and evolution of radical intermediates are critical aspects that demand rigorous experimental proof.\n7. If radicals truly participate in the ORR process, the reaction onset potential should closely align with either the first or second radical formation potential. However, the polarization curves reveal that the ORR onset potential lags behind the second CV peak by at least 0.3-0.4 V (Supplementary Figs. 34-36), which appears inconsistent with the proposed reaction pathway. Another intriguing observation is that under O₂ atmosphere, the higher-potential redox signal (first peak) nearly disappears, while the lower-potential redox peak remains largely intact. A deeper analysis of this phenomenon could provide valuable mechanistic insights. Additionally, the ORR polarization curves lack a critical reference dataset (e.g., Pt/C) for performance benchmarking, which is essential for evaluating the catalysts' practical relevance.\n8. The authors assign the ESR signal at g = 2 to a C*-*C radical. However, the spectra lack hyperfine splitting. Carbon-centered radicals typically exhibit hyperfine coupling to nearby 1H nuclei. Furthermore, the schematic suggests conjugation with phenyl rings, where delocalization could also cause splitting. Why is the observed ESR signal a singlet rather than showing splitting? A convincing explanation for this apparent lack of hyperfine structure is required.\n9. In Figs. 3g-3i, the authors present NMR spectra of the initial sample, the sample after O₂ purging, and the sample after HBF₄ treatment. However, prior to O₂ exposure, the system should also include the state after Zn powder addition. Shouldn't the corresponding NMR spectrum for this intermediate state be provided as well?\n10. If the ESR measurement was performed ex situ? If a radical trapping agent (e.g., a spin trap) applied to stabilize and detect this radical signal? if no trap was used, data showing the stability of this radical ESR signal intensity over time are crucial to verify its stability during the measurement window and any subsequent experiments (e.g., variable-temperature ESR), ruling out signal changes due to radical decay.\n11. There appears to be a contradiction between two key experimental observations: (1) C-C exhibits the highest radical formation potential (Fig. 3b), yet (2) demonstrates the lowest ORR activity (Fig. 4a). This inverse relationship challenges the proposed radical-driven mechanism. While the authors attempt to explain this discrepancy through computational studies (suggesting competitive hydrogenation pathways for C-C), such interpretation lacks robust experimental validation.\n12. The assignment of the peak at ~917 cm⁻¹ in the in-situ ATR-IR spectra to the O-O stretching vibration requires stronger validation. The peak is weak, and its origin cannot be definitively distinguished from potential reduction features of the molecular catalyst itself at low potentials. Performing a control in situ ATR-IR experiment under an inert Ar atmosphere (non-reactive conditions) is necessary. Additionally, conducting an experiment using ¹⁸O₂ isotope labeling would provide much more compelling evidence for the assignment and the reaction mechanism. If correctly assigned to O-O, the vibrational peak should exhibit a predictable isotopic shift.\n13. Post-reaction structural characterization of the catalysts should be rigorously performed, as radical-mediated processes may induce catalyst degradation.\n14. The core mechanistic claim—that confined radicals within the rigid xanthene bridge facilitate O₂/NO reduction—requires rigorous validation of two critical aspects: (1) electrochemical generation of C*-*C radicals, and (2) formation of C-O-O-C intermediates. While the observed EPR signal (g=2.00338) after Zn reduction suggests radical formation, this non-electrochemical approach cannot confirm operational relevance; in-situ EPR measurements under applied potentials (without reactants) are essential to establish the potential-dependent radical generation. For the proposed C-O-O-C intermediate, the current FTIR evidence remains inconclusive due to the absence of isotopic labeling (e.g., ¹⁸O₂) experiments and background spectra for definitive peak assignment.", "reviewer_3": "In the manuscript titled \"Biradical-mediated synergistic electrocatalysis using metal-free redox molecular catalysts,\" the authors report a class of metal-free bicarbenium-based molecular catalysts that undergo electrochemical two-electron reduction to form biradical intermediates, enabling highly selective oxygen and nitric oxide reduction to H2O2 and NH2OH, respectively. The authors combine synthesis, spectroscopy, electrocatalysis, and DFT calculations to explain the role of spatially confined biradicals in promoting cooperative substrate activation.\nFirst of all, my expertise is in computational/theoretical chemistry, and I understand that I was invited to review this aspect. Regarding the computational (DFT) parts of this manuscript, I have several critical concerns, outlined below:\n1. Although the DFT-computed reaction energy profiles show that the C-C2+ shows the minimal overpotential, while the O-C2+ shows the largest value, the experimental ORR trend (shown in Fig. 4a) follows the order of C-C2+ < O-C2+ < S-C2+. This discrepancy implies the possibility that the reaction path that the authors investigated is not responsible for the actual experimental situations. A direct comparison between the computed reaction energy profiles and experimental results is required.\n2. While the DFT pathway toward NH2OH is proposed, the manuscript does not address whether continued protonation to NH3 is kinetically or thermodynamically less favorable. Since this is central to understanding the observed selectivity (87.8% FE to NH2OH), the authors should clarify whether NH3 formation was computationally examined or ruled out based on energetics.\n3. The computational methods are not described with sufficient detail. Most critically, the manuscript lacks an information about the treatment of entropy and/or zero-point energy (ZPE). The authors should describe how the ZPE and thermal corrections were evaluated to compute the free energy quantities.\n4. DFT often fails to accurately describe the spin states and related energetics of open-shell species. Some benchmarking of the chosen functional, especially for spin-sensitive energetics, is necessary to ensure reliability.\n5. (General comment) The reaction energetics support the mechanistic proposal qualitatively, but the analysis lacks depth expected for a high-impact submission. The current DFT analysis remains largely descriptive.\nIn summary, this work presents an exciting new direction for metal-free electrocatalysis using biradical intermediates. The DFT results are encouraging and generally align with the experimental observations. However, to match the rigor of the synthetic and spectroscopic work, the computational section requires clarification, benchmarking, and deeper mechanistic analysis."}, "author_response": "Point-by-point response to the reviewers’ comments\nReviewer #1 (Remarks to the Author):\nIn this study, inspired by redox enzymes, the authors innovatively designed a class of stable, metal-\nfree molecular catalysts based on bicarbenium that can confine free radicals within the molecular\nstructure. This biradical-mediated synergistic catalysis achieved two-electron (2e⁻) oxygen\nreduction reaction (ORR) of 99.3% H O selectivity, 96.8% Faraday efficiency and 2.21 mol g⁻1cat\n2 2\nh⁻¹ productivity and three-electron nitric oxide reduction of NH OH as the main product, 87.2%\ncalculations to confirm the sustained redox cycling supported by favorable spin-pairing and\nsubstrate binding geometries, providing new design principles for metal-free electrocatalytic\nplatforms utilizing open-shell reactivity. However, additional explanations need to be provided\nbefore publication.\nResponse: Thank you very much for your recognition and positive comments about our work. We\nhave carefully addressed the points you raised, and these revisions have indeed enhanced the depth\nof discussion in the current manuscript.\n1. In the process of material synthesis, the yield of S-C-2OH (5%) is significantly lower than that\nof C-C-2OH (50%) and O-C-2OH (40%). The difference has not been further explained in the\nmain text. It is suggested to supplement the discussion on the influence of sulfur doping on this\ntype of reaction.\nPage 1 of 59\nResponse: Thank you for the suggestion. The lower yield of S-C-2OH (5%) likely arises from\nsulfur’s more substantial electron-donating effect as well as more effective lone pair electron\ndelocalization across the anthrone unit, which significantly reduces the electrophilicity of the\ncarbonyl (resisting nucleophilic attack). We have included this discussion in the revised\nmanuscript.\n2. After synthesizing the bicarbenium compounds, relevant characteristic absorption bands were\ndetected using UV-vis spectroscopy. Please provide additional relevant literature to support this\nresult.\nResponse: The formation of carbon cation/radical in triaryl or conjugated aromatic systems is\nusually accompanied by a noticeable change in color due to arise of new absorption bands at longer\nwavelengths, and UV-Vis spectroscopy is thus a convenient and reliable technique to monitor the\ncation/radical formation. We have included three references in the revised manuscript (references\n37, 38, and 39) to support this statement: Angew. Chem. Int. Ed. 2025, 64, e202416833; CCS\nChem. 2022, 4, 3190–3203; Chem. Eur. J. 2022, 28, e202200687.\n3. This work utilizes the method of cultivating single crystals and X-ray crystallographic analysis.\nIn all three bicarbeniums, two carbenium centers are arranged in planar space. Can the material\nalso maintain a stable coplanar state in the electrolyte?\nResponse: The catalyst molecules were designed specially to have two very bulky anthrone units\nbridged by a rigid xanthene unit; therefore, movement of the two carbenium carrying units is\nlargely restricted, so the two carbenium centers could still remain in a co-facial manner in the\nPage 2 of 59\nelectrolyte. Moreover, the molecules also maintained a stable coplanar state in the SMD implicit\nsolvent calculations.\n4. The catalyst used in this work was compared with other works under similar conditions, but it\nis evident that many catalysts were tested under different conditions. It is recommended to readjust\nthe comparison conditions.\nResponse: Thank you for the suggestion. We have included various representative types of\ncatalysts involving metal, metal-free and metal-hybridized catalysts tested under acidic, basic and\nneutral conditions, and summarized the results in Supplementary Tables 3 and 4 for ORR and\nNORR, respectively.\n5. The 2e⁻ selectivity of S-C2⁺ is 99.3%. However, in the S-COO-C structure, it is assumed that the\noccurrence of side reactions leads to complex peak shapes in the 1H NMR spectrum, but there is\nno theory or testing to determine the specific problem.\nResponse: The complexity of 1H NMR spectrum for S-COO-C intermediate is mainly due to\nmultiple possible orientations of C-O-O-C bridge as well as concurrent reactions such as\nprotonation and partial desorption (Figure R1). As the sp2 carbenium centers convert to sp3 carbon\ncenters, the C-O-O-C bridge could adapt to different orientation in space, thus can cause 1H NMR\nshift variations. Additionally, the concurrently occurring reactions such as protonation and partial\ndesorption, may further complicate the intermediate contents. All these made it challenging to\nobtain the S-COO-C intermediate in pure single-oriented form (we would also like to mention that\nthe S-COO-C intermediate is not a stable intermediate for isolation, as doing so would likely stop\nPage 3 of 59\nthe catalytic reaction and result in the loss of its catalytic feature). As a result, the 1H NMR\nspectrum is complex. We have included the additional figures and these discussions in the revised\nSI as Supplementary Figure 91.\nFigure R1. A few possible orientations of the C-O-O-C bridge and concurrently proceeding partial\ndesorption and protonation.\n6. The abstract mentions that: \"Nature’s redox enzymes achieve remarkable selectivity by\norganizing active sites with precise spatial control, an ability difficult to replicate in synthetic\nsystems.\" but does not specifically explain how the structural characteristics of natural enzymes\ninspired the design of this article. Suggest adding an explanation of the ideas and material design\nfor this work.\nResponse: Thank you for the suggestion. We have now included relevant discussion on how\nmaterial design is more closely correlated to nature’s enzyme in the revised manuscript: “These\nPage 4 of 59\nbiradicals are confined within a rigid xanthene bridge (~4.3 Å), creating a spatially defined pocket\nthat engages paramagnetic substrates like O and NO, resembling nature’s synergistic catalysis.\nintroduction section.”\n7. In the 2e⁻ pathway of oxygen reduction reaction (ORR), volcano plots can quantify the nonlinear\nrelationship between OOH* adsorption strength and activity, transforming the search for highly\nselective catalysts\" into an optimization problem of ΔG . In this work, double free radicals are\nOOH*\nused to confine oxygen molecules within the molecule, forming intermediates. Please provide\nadditional calculations for the adsorption energy of this binding. Suggested reference:\n10.1038/s41467-024-53714-3. and 10.1002/adma.202500063.\nResponse: Thank you for the suggestion. In the conventional two-electron ORR pathway toward\nhydrogen peroxide, the overall reaction is:\n𝑂 +2𝐻+ +2𝑒− → 𝐻 𝑂\n2 2 2\nThis process is typically described by two elementary steps:\n∗+𝑂 +𝐻+ +𝑒− → ∗𝑂𝑂𝐻 (1)\n2\n∗𝑂𝑂𝐻+𝐻+ +𝑒− → ∗+𝐻 𝑂 (2)\n2 2\nThe free energy of adsorption of the *OOH intermediate, ΔG (*OOH), is the key descriptor\ngoverning the activity and selectivity toward H O . The catalytic performance is commonly\n2 2\nrationalized by a volcano-type relationship, following the Sabatier principle: as the adsorption free\nenergy of *OOH varies, the rate-determining step shifts. On the weak adsorption side, activity is\nPage 5 of 59\nlimited by O adsorption and activation, whereas on the strong adsorption side, activity is\n2\nconstrained by subsequent hydrogenation or product desorption. Thus, the position of a catalyst\non the *OOH-based volcano plot provides a direct measure of its suitability for efficient H O\n2 2\nproduction. However, in our proposed reaction mechanism, the two-electron ORR proceeds via\nfour distinct steps:\n∗+2𝑒− → ∗2∙ (3)\n∗2∙+𝑂 → ∗𝑂𝑂 (4)\n2\n∗𝑂𝑂+𝐻+ → ∗𝑂𝑂𝐻 (5)\n∗𝑂𝑂𝐻+𝐻+ → ∗+𝐻 𝑂 (6)\n2 2\nIn contrast to the conventional mechanism, our mechanism does not follow the Sabatier principle\nframework. The rate-determining step is fixed at the initial two-electron reduction step (3), which\ndoes not involve adsorbed oxygenated intermediates (Figure R2). As a result, the system does not\nexhibit the scaling relations with ΔG (*OO), ΔG (*OOH). Consequently, the conventional\nvolcano-plot representation, which relies on adsorption-energy descriptors, is not applicable in this\ncase.\nWe also carried out additional attempts. To facilitate comparison with the conventional 2e- ORR\npathway, we considered merging the first three steps of our mechanism (step 3-5) into a single\noverall step (7). Then the reaction steps become:\n∗+2𝑒− +𝑂 +𝐻+ → ∗𝑂𝑂𝐻 (7)\n2\n∗𝑂𝑂𝐻+𝐻+ → ∗+𝐻 𝑂 (8)\n2 2\nPage 6 of 59\nThis treatment is not equivalent to the conventional pathway: in our system, both electrons are\nconsumed entirely in the first step (7), rather than being transferred sequentially. If lumped into a\nsingle step, the overall process appears strongly exergonic (Figure R2). However, such a\nrepresentation neglects the crucial activation barrier required to generate the reactive intermediate,\nthereby concealing the true kinetics. This assumption is thus unrealistic and inappropriate, and a\nconventional volcano plot cannot be constructed for this mechanism.\nFigure R2. Free energy diagrams for ORR. The black arrow indicates that combining steps (3) -\n(5) into one step (7) gives an artificially exergonic pathway without explicitly considering the RDS,\ni.e., step (3).\nWe have included the additional figures and these discussions in the revised SI as Supplementary\nFigure 92. Although the volcano plot is not applicable to our system, we found these two recent\nliteratures you mentioned interesting and related to our study, thus we have added these two\nreferences in the introduction section as references 23 and 24 in the revised manuscript.\nPage 7 of 59\n8. C-C2+ has a lower LUMO level. Can this trigger other side reactions? It is recommended to\nconduct performance testing on this material in the NORR electrocatalysis section.\nResponse: Thank you for the suggestion. The better ORR performance of S-C2+ inspired us to\nconduct its NORR investigation. As described in the manuscript, the better radical stability, greater\nability to stabilized reaction intermediates and less tendency to undergo competing side reactions\nmade S-C2+ a promising catalyst also for “triplet-like” nitrogen monoxide (NO) molecule. We\nhave included the NORR investigation for both C-C2+ and O-C2+ for your reference (Figure R3).\nFigure R3. LSV curves of C-C2⁺, O-C2⁺ and S-C2⁺ catalysts measured in NO-saturated 0.1 M\nHClO electrolyte (LSV curve of polycrystalline Pt catalyst in NO-saturated 0.1 M HClO\n4 4\nelectrolyte was measured for comparison).\nIt can be observed that all three bicarbenium catalysts demonstrated NORR activity. While the\nonset potentials exhibited similar trends to ORR (S-C2⁺ showed slightly higher onset potential,\nfollowed by O-C2⁺ and C-C2⁺ catalysts), the current density of O-C2⁺ and C-C2⁺ catalysts\ncorresponding to NORR at -0.2 V vs. RHE is smaller than that of S-C2⁺. The results imply\ncompromised reaction selectivity and/or presence of competing reactions for O-C2⁺ and C-C2⁺\nPage 8 of 59\ncatalysts, making the reaction deviate from a 3e- reduction. The estimated electron transfer number\nwith reference to Pt catalyst is about 2.73 and 2.84 for O-C2⁺ and C-C2⁺, respectively, indicating\na major 3e- NORR pathway (similar to S-C2⁺). The NORR reaction pathways with lower electron\ntransfer numbers (1 and 2) usually involve 1e- reduction to N O and 2e- reduction to N , which are\n2 2\nnot industrially useful but could be potentially useful in dealing with NO gas pollution for\nenvironmental protection (e.g. dealing with exhaust of cars). We appreciate the reviewer’s\ninsightful comment. Unfortunately, we were unable to perform a quantitative analysis of the\ngaseous products due to the lack of a gas chromatography–mass spectrometer (GC–MS) that can\nbe directly coupled with the electrochemical workstation. Nevertheless, we note that the more\nselective and dominant formation of NH₂OH on the S-C2⁺ catalyst highlights the potential of this\nsystem for electrocatalytic production of industrially valuable chemicals, analogous to the case of\nH₂O₂ in the 2e⁻ ORR pathway. The new figure and discussion have been included in the revised\nmanuscript and SI (Supplementary Figure S93).\n9. This work emphasizes the advantage of the dual radicals in \"synergistically binding substrates\",\nbut no monoradical analogues (such as derivatives containing only one carbon cation site) were\nset as controls. It is suggested to supplement the ORR/NORR performance data of single-site\ncatalysts to directly prove the enhancement of selectivity (such as 99.3% H₂O₂) and activity (such\nas 2.21 mol g⁻¹cat. h⁻¹ yield of S-C²⁺) by \"dual-site synergy\".\nResponse: Thank you for the suggestion. We have synthesized the mono-cation S-C⁺ by analogy\nto S-C2⁺, using a similar synthetic strategy to that described in the manuscript and Supplementary\nInformation. Please see Figures R4-7 below for the high-resolution MS, 1H, 13C and 19F NMR\nspectra, which provide structural confirmation of mono-cation S-C⁺.\nPage 9 of 59\nFigure R4. HR mass spectrum of compound S-C+.\nFigure R5. 1H NMR spectrum of compound S-C+ (600 MHz, CD CN).\n8\n7\n6\n5\n4\n3\n2\n1\n×\n×\n×\n×\n×\n×\n×\n×\n1\n1\n1\n1\n1\n1\n1\n1\n0\n0\n0\n0\n0\n0\n0\n0\n5\n5\n5\n5\n5\n5\n5\n5\n05\n1 5 5\nC\n1 6\no u n\n5\n5\nt s\n1\n1\n7\n7\nv s\n. 3\n.\n0\n5\n1\n1\na\n8\n8\ns\n.\ns\n3\n-\n2\n5\n5\nt o\n1\n1\n-\n9\n9\nc\n.\nh\n3\na\n2\n5\nr\n2\ng\n0\ne ( m\n5\n/\n2\nz\n1\n)\n5 2 2\nFigure R6. 13C NMR spectrum of compound S-C+ (500 MHz, CD CN).\nFirstly, we measured the CV of this mono-cation S-C+ (Figure R8). Comparing with the CV of\ndicarbenium structure in the main manuscript (Figure 3b), it can be observed that only one redox\nwave can be observed with an onset potential at -0.35 V vs. Fc/Fc+. This potential is between the\ntwo onsets for the dicarbenium S-C2+, which are -0.28 and -0.47 V. The dicarbenium, carrying two\npositive charges, is more electron deficient; thus, the first reduction occurs earlier. However, after\nthe first reduction, the subsequent reduction becomes slightly more challenging due to coulombic\nrepulsion, thus the subsequent reduction occurs later. The ORR performance of S-C+ was\ninvestigated in O -saturated 0.1 M HClO electrolyte using RRDE (Figure R9, please note the ring\n2 4\ncurrent and disk current were plotted not on the same scale for clarity). Unlike the dicarbium cases,\nthe LSV curve plateaued soon at a lower potential range, indicating a 4e- ORR reduction character.\nThe calculated corresponding electron transfer number was close to 4 (about 3.7-3.8, Figure R10),\nconfirming the 4e- reduction pathway for such mono-cation. This result was found to be similar to\nour previous publication (Angew. Chem. Int. Ed. 2025, 64, e202424449), in which we\ndemonstrated that triaryl mono-radical sites will significantly promote 4e- ORR due to more\npolarized O-O bonds, thus assisting O-O bond cleavage to adapt to a 4e- ORR pathway. However,\nin our new dication system, the generated diradical binds to both sides of the dioxygen, making it\neasier to undergo protonation-assisted desorption to generate H O . These results highlighted the\n2 2\nsynergistic highly selective production of H O by those dicarbenium catalysts. The performance\n2 2\nof S-C+ for NORR was also investigated (Figure R9). Comparing with the dicarbenium catalyst,\nthe mono-cation exhibited lower onset potential as well as lower current density at -0.2 V vs. RHE.\nThese results suggested the poor activation of S-C+ for NORR due to the lack of our proposed\nsynergistic character. We have included these data in the revised manuscript and SI as\nSupplementary Figures S94-100.\nPage 12 of 59\nFigure R8. The cyclic voltammogram of S-C+ was measured in acetonitrile with nBu NPF as\n4 6\nthe supporting electrolyte. The potential was calibrated by Fc/Fc+ (Fc: ferrocene). The onsets of\nreduction waves are labelled.\nFigure R9. RRDE LSV curve of S-C+ measured in O -saturated 0.1 M HClO electrolyte and its\n2 4\ncorresponding electron transfer number (the ring current and disk current were plotted not on the\nsame scale for clarity).\nPage 13 of 59\nFigure R10. LSV curves of S-C⁺ and S-C2⁺ catalysts measured in NO-saturated 0.1 M HClO\n4\nelectrolyte (LSV curve of polycrystalline Pt catalyst in NO-saturated 0.1 M HClO electrolyte\n4\nwas measured for comparison).\n10. This work explains the stability through the current density decay (Figure 4f), but does not\nverify the structural integrity of the catalyst after long-term reaction. It is suggested to supplement\nthe characterization of the catalyst after 6 hours of catalysis (such as ¹H NMR, XRD or XPS) to\nconfirm whether there is degradation (such as the hydrolysis of C-C²⁺) or agglomeration, and\ndirectly correlate \"structural stability\" with \"activity retention rate\". Suggested reference:\n10.1002/anie.202500945.\nResponse: Thank you for the suggestion. We have measured their NMR spectra before and after\ncatalysis. It can be observed that the NMR spectra before and after catalysis did not show\nsignificant differences, except that the recovered catalysts showed more broadened peaks instead\nPage 14 of 59\nof sharp peaks with clear coupling features for freshly prepared samples, which could be due to\npresence of large amount of acid and/or hydrogen peroxide during catalysts recovery using\nacetonitrile. We have included these data in the revised SI as Supplementary Figures S101-103.\nFigure R11. ¹H NMR spectra of C-C2+ before and after catalysis.\nFigure R12. ¹H NMR spectra of O-C2+ before and after catalysis.\nPage 15 of 59\nFigure R13. ¹H NMR spectra of S-C2+ before and after catalysis.\n11. Currently, only tested in 0.1 M HClO₄ (acidic) solution, but actual H₂O₂ production may require\nneutral/alkaline conditions (such as avoiding equipment corrosion). It is recommended to\nsupplement the performance of the catalyst in neutral (0.1 M PBS) and alkaline (0.1 M KOH)\nsolutions, combined with pH stability data (Supplementary Figures 31-33, such as S-\nC²⁺ stabilizing to pH 12), to discuss its practical application potential.\nResponse: Thank you for the suggestion. As described in the main manuscript during the pH\nstability tests (Supplementary Figs. 31-33), these catalysts are more stable in acidic conditions and\nare prone to hydrolysis in base. The results revealed that C-C2⁺ remains stable only under acidic\nconditions (pH ≤ 6), while undergoing hydrolysis near neutral pH. In contrast, O-C2⁺ and S-C2⁺\ndisplayed significantly enhanced tolerance, retaining structural integrity up to pH 9 and pH 12,\nrespectively. Since it is not feasible to conduct ORR in 0.1 M KOH electrolyte. We only included\ntheir ORR performance in 0.1 M PBS solution (Figure R14). It can be observed that all three\nPage 16 of 59\ncatalysts, though exhibited ORR activity, the current density significantly reduced compared with\ntheir performance in 0.1 M HClO because: (1) These catalysts are more persistent in acid (C-C2+\n4\nonly stable below pH 6 while the pH of 0.1 M PBS is about 7.4); (2) The protonation-assisted\ndesorption process to produce H O and the regeneration of dicarbenium are largely restricted in\n2 2\nPBS conditions compared to those in acid conditions. We have included these data in the revised\nSI as Supplementary Figure S104.\nFigure R14. LSV curves measured in O -saturated 0.1 M PBS electrolyte for C-C2+, O-C2+, and\n12. Table 3 shows that the H₂O₂ yield of S-C²⁺ (2.21 mol g⁻¹cat. h⁻¹) is superior to most metal-\nbased catalysts, but the core reason for the performance advantage has not been analyzed, is the\ndouble free radical mechanism enhancing the intrinsic activity, or is the carbon black carrier\nenhancing conductivity/mass transfer? It is recommended to supplement the electrochemical\nactive surface area (ECSA) test of the catalyst, compare the performance of \"pure catalyst\" and\nPage 17 of 59\n\"catalyst-carbon black composite\", and eliminate the interference of the carrier contribution.\nResponse: Thank you for the recommendation. To illustrate the ORR activity, we included the\nLSV curves of carbon black and Pt/C for reference (Figure R15). Carbon black showed a much\nlower ORR onset potential and smaller current density while commercial Pt/C displayed a standard\n4e- ORR curve feature (high onset potential and large current density); therefore, the higher ORR\nactivity of these dicarbenium catalysts can be attributed to their active sites (the carbenium sites).\nFigure R15. Comparison of LSV curves for C-C2+, O-C2+, and S-C2+ catalysts, as well as pure\ncarbon black and commercial Pt/C catalysts.\nMeanwhile, we have measured the ECSA of pure carbon black and catalyst-carbon black\ncomposite (Figure R16). The results showed that the ECSA increased after the addition of catalysts,\nas these redox-active catalysts increase the electrochemically active sites. The three catalyst-\ncarbon black composites did not show a significant difference in ECSA values because each type\nof catalyst carries the same number of active sites. Since these molecular catalysts are essentially\nnon-conductive, we believe the role of carbon black is to enhance both conductivity and mass\nPage 18 of 59\ntransfer while the dicarbenium sites provide their intrinsic catalytic activity. We have included\nthese data in the revised SI as Supplementary Figures S105-106.\nFigure R16. The ECSA determined for carbon black and catalyst-carbon black composites.\nReviewer #2 (Remarks to the Author):\nThis study synthesized three well-defined metal-free molecular catalysts, among which S-C2+\nexhibited excellent performance in both two-electron oxygen reduction and three-electron nitric\noxide reduction. Interestingly, the authors proposed a biradical-mediated reaction mechanism.\nHowever, the confirmation of this mechanism lacks sufficient experimental evidence (see Q14).\nAdditionally, the electrochemical performance of the catalyst was primarily evaluated in an H-cell\n(or RDEs), and data on its application in membrane electrode assemblies (MEAs) or flow cells are\nstill lacking, limiting the impact of this research. Furthermore, while the authors synthesized three\nmolecular catalysts, the differences in their electrochemical behavior were explained solely by the\nPage 19 of 59\nelectronic effect on reaction energy barriers, which appears oversimplified. If the novelty of this\nwork mainly stems from the biradical-mediated reaction mechanism, the inclusion of O-C2+ and\nC-C2+ may not be strictly necessary. My specific comments are listed as follows:\nResponse: Thank you very much for your recognition and comments about our work. We have\ncarefully addressed the points that the reviewer raised, and these revisions have indeed enhanced\nthe depth of discussion in the current manuscript.\nThe suggestions regarding flow cells and the electrochemical properties of the three catalysts are\nincluded as responses to your specific comments below. The synthesis of this series of dicarbenium\nmolecules had gone through a long process. In the original experimental design, C, N, O, S, Se\nwere proposed in order to explore their effect on stabilizing the carbenium ion. While the synthesis\nis challenging for N, S, and Se due to a more substantial electron-donating effect as well as more\neffective lone pair electron delocalization across the anthrone unit, which significantly reduces the\nelectrophilicity of the carbonyl (resisting nucleophilic attack), we eventually managed to obtain\nthe C, O and S materials in pure form. As illustrated in our experiment, it is crucial to balance the\ncarbenium reactivity and stability. If over reactive, it is more prone to trigger side reactions (e.g.\nwater oxidation) and compromise ORR activity. As a holistic study, the inclusion of O-C2+ and\nC-C2+ can provide not only materials design strategy regarding reactivity and stability but also\nprovide insights into the catalytic mechanism study and catalyst deactivation/decomposition/re-\nactivation study.\n1. The electronic structure depiction of the trityl radical in Figure 1b is incorrect. It is erroneously\nshown as having two electrons with parallel spins in two orbitals. This figure requires revision.\nPage 20 of 59\nResponse: We have revised Figure 1b as shown below as Figure R17. For one triaryl radical, it is\nessentially a doublet. The idea we intend to convey is to demonstrate that two radical centers\ncollectively engage one oxygen molecule. Therefore, it should be more appropriate to show two\nidentical triaryl radicals in the revised figure.\nFigure R17. Catalyst design strategy. a, The representative enzymes, cytochrome c oxidase and NADPH\noxidase, for efficient and selective ORR pathways. b, The spin-pairing interactions between triaryl radical\nand open-shell substrates. c, The envisioned radical-mediated catalytic cycle (i-iii) and associated potential\ndestructive pathways (iv-vii). d, The conventional substrate binding configurations and the proposed\ncofacial-cooperative type. e, The proposed bicarbenium scaffolds as precatalysts and the corresponding\ncatalytic cycle.\n2. In the Supporting Information (SI), there is a lack of detailed explanations and descriptions for\nthe spectra in Supplementary Figs. 1–26. The authors should provide a thorough analysis of each\npeak signal to allow readers to better understand the structural details of the synthesized molecules.\nGiven that the successful synthesis of these molecular catalysts is a major innovation of this work,\nsuch comprehensive explanation is essential.\nPage 21 of 59\nResponse: We have revised the Supplementary Figs. 1–26. All the proton peaks have now been\naddressed (Figure R18-23). For the case of dihydroxyl molecules where structural isomers can be\npresent due to different orientations of -OH groups and restricted rotation, the NMR spectra could\nshow some small or shoulder peaks. After the following carbenium generation reaction, the clear\n1H, 13C and 19F NMR spectra of the final compounds (the catalysts) indicated high purity.\nMeanwhile, the purity can also be perceived from the wider range mass spectra (please refer to\nyour comment 3 and our response). Those NMR spectra were updated in the revised SI.\nFigure R18. ¹H NMR spectrum of C-C-2OH with peak assignment (500 MHz, CDCl ).\nFigure R19. ¹H NMR spectrum of C-C2+ with peak assignment (500 MHz, CD CN).\nFigure R21. ¹H NMR spectrum of O-C2+ with peak assignment (500 MHz, CD CN).\nFigure R23. ¹H NMR spectrum of S-C2+ with peak assignment (500 MHz, CD CN).\n3\n3. The mass spectrometry data provided by the authors should include a broader *m/z* range to\nallow readers to better assess the purity of the synthesized molecules. The authors do not appear\nto report the purity levels of the three catalysts. Although single-crystal structures of the\ncorresponding molecules were obtained, the experimental section does not explicitly state whether\nthe single-crystal samples were directly used for electrochemical testing. If not, the samples\nemployed in the electrochemical measurements may contain impurities. The nature and\nconcentration of such impurities should be reported, as this information is critical for evaluating\nwhether the observed electrochemical behavior is influenced by these impurities.\nResponse: The high-resolution MS spectra we measured earlier were usually obtained at a narrow\nrange. We have also measured the wider nominal mass spectra for all compounds for your\nPage 25 of 59\nreference. Based on these mass spectra with broader range (Figure R24-29), it can be observed\nthat the final compounds are pure since there are no significant peaks indicating impurities (in\ngeneral, only fragmental peaks can be observed; or in a few cases, some OH adduct was also\nobserved). Meanwhile, please also refer to Figures R18-23 for the response to reviewer’s\ncomments 2, where those clear 1H, 13C, and 19F NMR spectra indicate high purity of these\ncompounds. We have included these wider mass spectra in the revised SI as Supplementary\nFigures S20-25.\nFigure R24. HR mass spectrum (APCI) and wide-range mass spectrum of compound C-C-2OH.\nFigure R25. HR mass spectrum (APCI) and wide-range mass spectrum of compound C-C2+.\nPage 26 of 59\nFigure R26. HR mass spectrum (APCI) and wide-range mass spectrum of compound O-C-2OH.\nFigure R27. HR mass spectrum (APCI) and wide-range mass spectrum of compound O-C2+.\nPage 27 of 59\nFigure R28. HR mass spectrum (APCI) and wide-range mass spectrum of compound S-C-2OH.\nFigure R29. HR mass spectrum (APCI) and wide-range mass spectrum of compound S-C2+.\n4. In Supplementary Figs. 31-33, the authors investigated the structural stability of the molecular\ncatalysts using UV-Vis spectroscopy, which is clearly insufficient. While changes in UV-Vis\nspectra can indicate structural instability, demonstrating molecular stability requires additional\nPage 28 of 59\nstructural characterization data, such as mass spectrometry (MS) or nuclear magnetic resonance\n(NMR) spectroscopy.\nResponse: The monitoring of carbon cation/radical in triaryl or conjugated aromatic systems is\nusually accompanied by a noticeable change in color due to the rise of new absorption bands at\nlonger wavelengths, and UV-Vis spectroscopy is thus a convenient and reliable technique to\nmonitor the cation/radical formation and decomposition. Regarding the stability concern and\ninvestigation, we used NMR spectroscopy to evaluate the catalyst’s stability by comparing its\nNMR spectra before and after 6 hours of electrocatalysis (Figures R11-13). It can be observed that\nthe NMR spectra before and after catalysis did not show significant differences, except that the\nrecovered catalysts showed more broadened peaks instead of sharp peaks with clear coupling\nfeatures for freshly prepared samples, which could be due to the presence of a large amount of acid\nand/or hydrogen peroxide during catalyst recovery using acetonitrile. We have included these data\nin the revised SI as Supplementary Figures S101-103.\nFigure R11. ¹H NMR spectra of C-C2+ before and after catalysis.\nPage 29 of 59\nFigure R12. ¹H NMR spectra of O-C2+ before and after catalysis.\nFigure R13. ¹H NMR spectra of S-C2+ before and after catalysis.\n5. In Fig. 3a, the UV-Vis spectra of O-C and S-C exhibit two distinct peaks, while C-C shows only\none. The authors should provide a clear explanation for this observed difference.\nPage 30 of 59\nResponse: While O-C2+ and S-C2+ exhibited two major absorption bands in UV-Vis spectra, C-\nC2+ shows only one major absorption band. This is mainly due to both a stronger inductive\ndonation effect and enhanced π-conjugation within the xanthene-based carbenium, induced by O-\nand S-heterocycles. In the C-C2+ structure, the conjugation is broken at the top carbon position.\nWhereas O and S can participate in conjugation (S is more significant than O), and thus show\ndifferent UV-Vis absorption behavior as compared to C-C2+ (additional adsorption band and more\nred-shifted spectra). We have included this discussion in the revised manuscript when introducing\nFig. 3a.\n6. The assignment of the (C+/+C) state to (C+/*C) and subsequently to (C*/*C) in the CV (Fig.\n3b) requires experimental validation. Structurally, the two C+ moieties exhibit symmetry—why\nthen do they display distinct redox potentials? The authors must provide a convincing rationale for\nthis observation. One feasible approach would involve monitoring EPR signals at varying\npotentials: if their assignment is correct, distinct radical species should be detectable at different\napplied potentials. Given that this work proposes a radical-involved electrocatalytic mechanism,\nthe generation and evolution of radical intermediates are critical aspects that demand rigorous\nexperimental proof.\nResponse: Thank you for the suggestion. Although the two C+ sites are equal in the dicarbenium\nform, reduction of such a molecule is usually progressive instead of a one-time two-electron\nreduction. When the first electron occupies the LUMO of the dicarbenium, it now becomes a\nradical cation, and this makes it more difficult for the subsequent electron to undergo further\nreduction. It might be easier to think of this process the other way around since the redox waves\nare reversible. For the oxidation of the diradical (looking t at the CV curve from low potential to\nPage 31 of 59\nhigh potential), when the first radical is oxidized to form a radical cation, it is more difficult to\noxidize the other radical site because creating two positive charge with such close distance is\nhindered by coulombic repulsion. In our case, the energy required for the second redox process is\nlarge enough to be resolved in cyclic voltammetry, therefore it exhibited two successive redox\nwaves. We have included this discussion in the revised manuscript for clearer explanation.\nTo compare the synergistic catalytic mechanism, we have included the synthesized mono-cation\nproduct (S-C+) using a similar synthetic strategy as described in the manuscript and SI. Please see\nFigures R4-7 below for the high-resolution MS, 1H, 13C, and 19F NMR for the structural\nconfirmation of mono-cation S-C⁺.\nFigure R4. HR mass spectrum of compound S-C+\nPage 32 of 59\n8\n7\n6\n5\n4\n3\n2\n1\n×\n×\n×\n×\n×\n×\n×\n×\n1\n1\n1\n1\n1\n1\n1\n1\n0\n0\n0\n0\n0\n0\n0\n0\n5\n5\n5\n5\n5\n5\n5\n5\n05\n1 5 5\nC\n1 6\no u n\n5\n5\nt s\n1\n1\n7\n7\nv s\n. 3\n.\n0\n5\n1\n1\na\n8\n8\ns\n.\ns\n3\n-\n2\n5\n5\nt o\n1\n1\n-\n9\n9\nc\n.\nh\n3\na\n2\n5\nr\n2\ng\n0\ne ( m\n5\n/\n2\nz\n1\n)\n5 2 2\nFigure R5. 1H NMR spectrum of compound S-C+ (600 MHz, CD CN).\nFigure R7. 19F NMR spectrum of compound S-C+ (500 MHz, CD CN).\ndicarbenium structure in the main manuscript (Figure 3b), it can be observed that only one redox\nwave can be observed with an onset potential at -0.35 V vs. Fc/Fc+. This potential is between the\ntwo onsets for the dicarbenium S-C2+, which are -0.28 and -0.47 V. The dicarbenium, carrying two\npositive charges, is more electron deficient; thus, the first reduction occurs earlier. However, after\nthe first reduction, the subsequent reduction becomes slightly more challenging due to coulombic\nrepulsion, thus the subsequent reduction occurs later. For the catalytic performance of this mono-\nsite cation catalyst, we have included the relevant data and discussion under response for your\ncomments 14.\nPage 34 of 59\nFigure R30. ESR spectra monitoring the electrochemical reduction of S-C2+ for the formation of\nmonoradical and diradical species.\nThe chemical reduction using reactive metals has often been used as a straightforward and fast\nchemical reduction strategy, as the reaction can be completed in a rather short time and usually\nwill not trigger side reactions; therefore, we employed this strategy in the original experimental\ndesign. Nevertheless, we have now also performed in situ ESR to monitor the formation of mono-\nradical and diradical (also related to your comment 14). According to the CV (Figure 3b in the\nmain manuscript), when the applied potential is about -0.4 V vs Fc/Fc+, only the first reduction can\noccur. At 0 s (no applied external potential), no ESR peak was observed. After applying a\npotential of -0.4 V vs Fc/Fc+, a resonance peak with a g-value of 2.0035 corresponding to a carbon\nradical emerged (Figure R30). The spectra were recoded for several time intervals from 0 to about\n1500 s, and the peak intensity (integral) was plotted against time. As the duration of applied\npotential increases, the peak intensity (the integral) increases and then gradually plateaus, since\nmost dications are reduced to radical cations. We then applied a more negative potential, -0.8 V\nvs Fc/Fc+, to the working electrode, and the intensity (integral) immediately increased, implying\nthe further reduction of the radical cations to dications. After about 2700s, O gas was bubbled\n2\ninto the diradical solution. After another 1300s, the ESR peak disappeared due to the binding to oxygen\nmolecules of the radical sites. These results indicated the stepwise generation of monoradical and\ndiradical controlled by applied potential as well as the reactivity between the radical species and\noxygen molecules. We have included these results in the updated manuscript\nand SI as Supplementary Figure S107.\n7. If radicals truly participate in the ORR process, the reaction onset potential should closely align\nwith either the first or second radical formation potential. However, the polarization curves reveal\nthat the ORR onset potential lags behind the second CV peak by at least 0.3-0.4 V (Supplementary\nFigs. 34-36), which appears inconsistent with the proposed reaction pathway. Another intriguing\nobservation is that under O₂ atmosphere, the higher-potential redox signal (first peak) nearly\ndisappears, while the lower-potential redox peak remains largely intact. A deeper analysis of this\nphenomenon could provide valuable mechanism insights. Additionally, the ORR polarization\ncurves lack a critical reference dataset (e.g., Pt/C) for performance benchmarking, which is\nessential for evaluating the catalysts' practical relevance.\nResponse: Thank you for the suggestion. The potential of CV peaks (the x-axis) measured in\nacetonitrile in Fig. 3b in the manuscript refers to Fc/Fc+ while the potential of the ORR polarization\ncurved measured in 0.1M HClO aqueous solution in Fig. 4a is referenced to RHE. Converting\nvalues, but we can still do a rough approximation. The theoretical difference between Fc/Fc+ and\nRHE is about 0.63 V. If the pH of the organic phase is deemed as 7 (pH of 0.1 M HClO is 1), then\n4\nthis will contribute to another 0.059*6 = 0.354 V difference. Taking S-C2+ as an example, its two\nredox waves are about -0.47/-0.28 V vs Fc/Fc+ in acetonitrile, and thus it is about 0.51-0.70 V vs\nPage 36 of 59\nRHE. The onset for the S-C carbenium sample in the ORR polarization curve is 0.696 V vs RHE,\ndemonstrating excellent consistency with the first reduction potential. Meanwhile, these results\nalso suggested that the ORR reaction took place near the first redox wave, which can also explain\nwhy the first redox peak signal decreased faster in the presence of O . The observation for O-C2+\nis also consistent. As for C-C2+, it should be 0.76-0.93 V vs RHE, which showed a discrepancy.\nAs observed during experiment and discussion in the manuscript, the too-high reactivity could\nhave hindered the ORR process due to presence of side reaction like water oxidation reaction, and\nwe have indeed isolated the reduced hydrogenated C-C-2H (Supplementary Figure S62/Figure\nR31) during catalyst stability and mechanism study. The study of radical formation in Fig. 3b was\nconducted in dry acetonitrile, where the competing water oxidation reaction caused minimal\ninterference. However, the latter ORR study was performed in aqueous conditions where the\ninterference from water cannot be ignored. To the best of our knowledge, at higher potentials, the\ncarbenium or radicals tend to oxidize water more easily; whereas ORR dominates at lower\npotentials (but could still face competing water oxidation), which could explain the lower ORR\nactivity of C-C2+. In the case of O-C and S-C, the carbenium and radicals are significantly more\nstabilized; consequently, they exhibit much greater resistance to reacting with water, leading to a\nmore dominant engagement with oxygen.\nMeanwhile, please also refer to our response to your comment 6 for the electrochemical reduction\n(Figure R30). After the generation of radical species, the ESR signal disappeared upon the addition\nof O , which indicates the intrinsic reactivity between radicals and oxygen. We hope this explains\n2\nyour concerns. These discussions have been included in the revised SI.\nIn addition, to illustrate the ORR activity, we included the LSV curves of carbon black and Pt/C\nfor reference (Figure R15). Carbon black showed a much lower ORR onset potential and smaller\nPage 37 of 59\ncurrent density, while commercial Pt/C displayed a standard 4e- ORR curve feature (high onset\npotential and large current density); therefore we believe the ORR activity of these dicarbenium\ncatalysts can be attributed to their active carbenium sites.\nFigure R15. Comparison of LSV curves for C-C²⁺, O-C²⁺, and S-C²⁺ catalysts, as well as pure\ncarbon black and commercial Pt/C catalysts.\nFigure R31. Top view and side view of compound C-C-2H.\nPage 38 of 59\n8. The authors assign the ESR signal at g = 2 to a C*-*C radical. However, the spectra lack\nhyperfine splitting. Carbon-centered radicals typically exhibit hyperfine coupling to nearby 1H\nnuclei. Furthermore, the schematic suggests conjugation with phenyl rings, where delocalization\ncould also cause splitting. Why is the observed ESR signal a singlet rather than showing splitting?\nA convincing explanation for this apparent lack of hyperfine structure is required.\nResponse: Indeed, carbon-centered radicals can show hyperfine splitting, especially when there is\nan α-proton that is directly bonded (strong coupling that can be resolved). In our case, there is no\nα-proton. In the case of radical delocalization, we would like to mention that although there is a\npresence of spin delocalization, the majority of spin density is still on the center carbon\n(Supplementary Figure 65). Additionally, delocalization spreads over many atoms and each\nindividual nucleus then has only a small spin density for a small hyperfine constant, which could\nmake the splitting unresolvable. Besides, the measurement was done in the solid phase, which\nusually causes signal broadening. We hope these explanations address your concerns. These\ndiscussions have been included in the revised SI.\n9. In Fig. 3g-3i, the authors present NMR spectra of the initial sample, the sample after O₂ purging,\nand the sample after HBF₄ treatment. However, prior to O₂ exposure, the system should also\ninclude the state after Zn powder addition. Shouldn't the corresponding NMR spectrum for this\nintermediate state be provided as well?\nResponse: The radical compounds are paramagnetic; therefore, they are practically NMR silent\ndue to significant NMR signal broadening and weakening. Thus, radical species are usually\ncharacterized by ESR instead of NMR.\nPage 39 of 59\n10. If the ESR measurement was performed ex situ? If a radical trapping agent (e.g., a spin trap)\napplied to stabilize and detect this radical signal? if no trap was used, data showing the stability of\nthis radical ESR signal intensity over time are crucial to verify its stability during the measurement\nwindow and any subsequent experiments (e.g., variable-temperature ESR), ruling out signal\nchanges due to radical decay.\nResponse: We understand that radical scavengers (like DMPO and TEMPO) were often used to\ntrap and monitor possible radical species (like H, OH, OOH radical adducts) generated during the\nin situ investigation of ORR electrocatalysis. However, the diradical intermediates in our systems\nserved as the active sites to bind to O ; thus, adding radical scavengers will directly form adducts\n2\nto the diradical catalysts (causing catalyst deactivation). In fact, the isolated diradical intermediates\nwere reasonably stable due to effective spin delocalization, and the sulfur atom can further enhance\nradical stability because of the heavy atom effect (its empty d-orbitals). Nevertheless, we attempted\nto use ESR to monitor the stability of these diradical intermediates (Figures R32-34). The\nrespective diradicals generated from C-C2+, O-C2+, and S-C2+ were kept in both argon and air\natmosphere, and ESR spectra were recorded from Day 0 (as-prepared) to Day 5. The results\nrevealed that both O-C and S-C diradicals were stable in an argon atmosphere without significant\npeak intensity decay. However, the samples kept in the air decayed fast due to reaction with oxygen.\nThese results further confirmed the good ORR activity for O-C2+ and S-C2+ catalysts. The C-C\ndiradicals, on the other hand, exhibited significantly faster decay in both Argon and air, which can\nbe attributed to their poor spin delocalization and the absence of additional stabilization by\nheteroatoms such as O-C and S-C, making them less prone to sustaining. We have included these\nnew data and discussions in the revised manuscript and SI as Supplementary figures S108-110.\nPage 40 of 59\nFigure R32. S-C diradical intermediate stability monitoring by ESR in both Argon and air\natmosphere, with intensity-time plots (intensity was relative to day 0 as 100%).\nFigure R33. O-C diradical intermediate stability monitoring by ESR in both Argon and air\natmosphere, with intensity-time plots (intensity was relative to day 0 as 100%).\nFigure R34. C-C diradical intermediate stability monitoring by ESR in both Argon and air\natmosphere, with intensity-time plots (intensity was relative to day 0 as 100%).\nPage 41 of 59\n11. There appears to be a contradiction between two key experimental observations: (1) C-C\nexhibits the highest radical formation potential (Fig. 3b), yet (2) demonstrates the lowest ORR\nactivity (Fig. 4a). This inverse relationship challenges the proposed radical-driven mechanism.\nWhile the authors attempt to explain this discrepancy through computational studies (suggesting\ncompetitive hydrogenation pathways for C-C), such interpretation lacks robust experimental\nvalidation.\nResponse: Please also refer to comment 7 for this discrepancy. As illustrated in our experiment,\nit is crucial to balance the carbenium reactivity and stability. If over-reactive, it is more prone to\ntrigger side reactions and compromise ORR activity; As observed during the experiment, the too-\nhigh reactivity could have hindered the ORR process due to the presence of side reactions like\nwater oxidation reaction, and we have indeed isolated the reduced hydrogenated C-C-2H (Figure\nR31/Supplementary Figure 62) during catalyst stability and mechanism study. The study of radical\nformation in Fig. 3b was conducted in dry acetonitrile, where the competing water oxidation\nreaction caused minimal interference. However, the latter ORR study was performed in aqueous\nconditions where the interference from water cannot be ignored. To the best of our knowledge, at\nhigher potentials, the carbenium or radicals tend to oxidize water more easily; whereas ORR\ndominates at lower potentials (but could still face competing water oxidation), which could explain\nthe lower ORR activity of C-C dicarbenium. In the case of O-C and S-C, the carbenium and\nradicals are significantly more stabilized; consequently, they exhibited much greater resistance to\nreact with water, leading to a more dominant engagement with oxygen. We hope this explains your\nconcerns. These discussions have been included in the revised SI.\nPage 42 of 59\nFigure R31. Top view and side view of compound C-C-2H.\n12. The assignment of the peak at ~917 cm⁻¹ in the in-situ ATR-IR spectra to the O-O stretching\nvibration requires stronger validation. The peak is weak, and its origin cannot be definitively\ndistinguished from potential reduction features of the molecular catalyst itself at low potentials.\nPerforming a control in situ ATR-IR experiment under an inert Ar atmosphere (non-reactive\nconditions) is necessary. Additionally, conducting an experiment using ¹⁸O₂ isotope labeling would\nprovide much more compelling evidence for the assignment and the reaction mechanism. If\ncorrectly assigned to O-O, the vibrational peak should exhibit a predictable isotopic shift.\nResponse: We have added the ATR-IR spectrum under inert Ar atmosphere (Fig. 5a). Although\nthe peak at ~917 cm⁻¹ appeared very weak, it is not observable under Ar atmosphere. Therefore, it\ncould be assigned to the C-O-O-C stretch with reference to the theoretically simulated or\nreferenced IR (Supplementary Figure S80, Supplementary Table S1). Unfortunately, we were\nunable to provide the 18O isotope experiment, reason being that: (1) For safety reasons, all gas use\n2\nat our institution must follow pre-installed gas supply lines, and there is no line for 18O ; (2) We\n2\nhave tried to contact local gas suppliers in Singapore and found that 18O is not commercially\n2\navailable due to its rare use here. Nevertheless, we believe the current ATR-IR data, together with\nPage 43 of 59\nother experimental analysis and calculations, are sufficient to illustrate most of the information\nrequired for proposing the ORR mechanism in our cases. Meanwhile, the suggestion to add\nspectrum under Ar condition also supported that the peak arises after applying potential in O\n2\ncondition. We have amended accordingly in the revised manuscript.\n13. Post-reaction structural characterization of the catalysts should be rigorously performed, as\nradical-mediated processes may induce catalyst degradation.\nResponse: We have measured their NMR spectra before and after catalysis. It can be observed\nthat the NMR spectra before and after catalysis did not show significant differences, except that\nthe recovered catalysts showed more broadened peaks instead of sharp peaks with clear coupling\nfeatures for freshly prepared samples, which could be due to presence of large amount of acid\nand/or hydrogen peroxide during catalyst recovery using acetonitrile. We have included these data\nin the revised SI as Supplementary Figures S101-103.\nFigure R11. ¹H NMR spectra of C-C2+ before and after catalysis.\nPage 44 of 59\nFigure R12. ¹H NMR spectra of O-C2+ before and after catalysis.\nFigure R13. ¹H NMR spectra of S-C2+ before and after catalysis.\nPage 45 of 59\n14. The core mechanistic claim—that confined radicals within the rigid xanthene bridge facilitate\nO₂/NO reduction—requires rigorous validation of two critical aspects: (1) electrochemical\ngeneration of C*-*C radicals, and (2) formation of C-O-O-C intermediates. While the observed\nEPR signal (g=2.00338) after Zn reduction suggests radical formation, this non-electrochemical\napproach cannot confirm operational relevance; in-situ EPR measurements under applied\npotentials (without reactants) are essential to establish the potential-dependent radical generation.\nFor the proposed C-O-O-C intermediate, the current FTIR evidence remains inconclusive due to\nthe absence of isotopic labeling (e.g., ¹⁸O₂) experiments and background spectra for definitive peak\nassignment.\nResponse: To further validate our claim for diradical-mediated catalysis, we have now included\nthe synthesis of a mono-cation for comparison, as well as in situ electrochemical generation of\nmonoradicals and diradicals. Referring to comment 6, we have synthesized the mono-cation S–C⁺\nby analogy to S–C2⁺, using a similar synthetic strategy to that described in the manuscript and\nSupporting Information. Please see Figures R4-7 below for the high-resolution MS, 1H, 13C, and\n19F NMR spectra, which provide structural confirmation of mono-cation S-C⁺. Unlike the\ndicarbium cases, the LSV curve plateaued soon at a lower potential range, indicating a 4e- ORR\nreduction character. The calculated corresponding electron transfer number was close to 4 (about\n3.7-3.8, Figure R8), confirming the 4e- reduction pathway for such a mono-cation. This result was\nfound to be similar to our previous publication (Angew. Chem. Int. Ed. 2025, 64, e202424449), in\nwhich we demonstrated that triaryl mono-radical sites will significantly promote 4e- ORR due to\nmore polarized O-O bonds, thus assisting O-O bond cleavage to adapt to a 4e- ORR pathway. However,\nin our new dication system, the generated diradical binds to both sides of the dioxygen, making it\neasier to undergo protonation-assisted desorption to generated H O . These results\n2 2\nPage 46 of 59\nhighlighted the synergistic highly selective production of H O of those dicarbenium catalysts. The\n2 2\nperformance of S-C+ for NORR was also investigated (Figure R10). Comparing with the\ndicarbenium catalyst, the mono-cation exhibited lower onset potential as well as lower current\ndensity at -0.2 V vs. RHE. These results suggested the poor activation of S-C+ for NORR due to\nthe lack of our proposed synergistic character. We have included these data in the revised\nmanuscript and SI as Supplementary Figures S94-100.\nFigure R9. RRDE LSV curve of S-C+ measured in O -saturated 0.1 M HClO electrolyte and its\n2 4\ncorresponding electron transfer number (the ring current and disk current were plotted not in the\nsame scale for clarity).\nPage 47 of 59\nFigure R10. LSV curves of S-C⁺ and S-C2⁺ catalysts measured in NO-saturated 0.1 M HClO\n4\nelectrolyte (LSV curve of polycrystalline Pt catalyst in NO-saturated 0.1 M HClO electrolyte was\n4\nmeasured for comparison).\nFigure R30. ESR spectra monitoring the electrochemical reduction of S-C2+ for the formation of\nmonoradical and diradical species.\nPage 48 of 59\nThe presence of the C-O-O-C intermediate has been demonstrated by mass spectroscopy (Figure\n3j in the revised manuscript). The C-O-O-C intermediate is not a stable intermediate for isolation,\nor else the catalytic reaction will likely stop here, losing its catalytic feature.\nThe in situ electrochemical generation has been demonstrated in response to your comment 6.\nWe also like to mention that the chemical reduction using reactive metals has often been used as a\nstraightforward and fast chemical reduction strategy, as the reaction can be completed in a rather\nshort time and usually will not trigger side reactions; therefore, we employed this strategy in the\noriginal experimental design. Nevertheless, we have also performed in situ ESR to monitor the\nformation of mono-radicals and diradicals. According to the CV (Figure 3b in the main\nmanuscript), when the applied potential is about -0.4 V vs Fc/Fc+, only the first reduction can\noccur. At 0 s (no applied external potential), no ESR peak was observed. After applying a potential\nof -0.4 V vs Fc/Fc+, a resonance peak with a g-value of 2.0035 corresponding to carbon radical\nemerged (Figure. R30). The spectra were recoded for several time intervals from 0 to about 1500\ns and the peak intensity (integral) was plotted against time. As the duration of applied potential\nincreases, the peak intensity (the integral) increases and then gradually plateaus, since most\ndications are reduced to radical cations. We then applied a more negative potential, -0.8 V vs\nFc/Fc+, to the working electrode, and the intensity (integral) immediately increased, implying the\nfurther reduction of the radical cations to dications. After about 2700 s, O gas was bubbled into\n2\nthe diradical solution. After another 1300 s, the ESR peak disappeared due to the binding to oxygen\nmolecules of the radical sites. These results indicated the stepwise generation of monoradical and\ndiradical controlled by applied potential as well as the reactivity between the radical species and\noxygen molecules. We have included these results in the revised manuscript and SI.\nPage 49 of 59\nUnfortunately, we were unable to provide the 18O isotope experimentas we explained above.\nanalysis, and theoretical calculations are sufficient to support our core claim for the biradical-\nmediated synergistic catalysis.\nThis work aims to illustrate a mechanism study and bring new insights (biradical mediated\nsynergistic catalysis) for selective electrocatalysis. To address your general comments on using a\nflow cell for a more practical investigation of the catalyst, we have employed a self-assembled\nflow cell system to investigate the performance of the S-C2+ catalyst since it has demonstrated\nbetter ORR performance. The performances at higher current density were carried out in a\ncommercial three-phase flow cell (TPFC) device using the galvanostatic method at each fixed\ncurrent density. The working electrode was prepared by drop coating the catalyst ink onto the\nhydrophobic carbon paper, which served as the gas diffusion layer (GDL), with an effective area\nof 1 cm2 and a catalyst loading of 0.6 mg cm-2. A cation exchange membrane (Nafion 117\nmembrane) was applied to separate anolyte and catholyte, which are both 0.1 M HClO , and were\n4\nrecycled through each respective compartment at the flow rate of 10 mL min-1. The O was fed at\n2\nthe rate of 20 mL min-1 to the cathode and was flown from the back of the carbon paper through\nthe catalyst layer. The results (Figure R35) demonstrated that the production rate increased with\nhigher current density. However, both the production rate and Faradic efficiency dropped at higher\ncurrent density. For such a molecular catalyst, a series of issues could be present using a high\ncurrent flow cell test, including poor conductivity, weak catalyst-support interaction, limited mass\ntransport, catalyst ink/film instability, etc, which would cause deviated performance at higher\ncurrent density. Therefore, our future efforts lie in the materials modification, including covalently\ngrafting the catalyst molecules onto graphene, CNT, or similar supporting materials to improve\nPage 50 of 59\ntheir performance under high current density practical conditions. We have included these data\nand discussion in the revised manuscript and SI as Supplementary Figure 111.\nLastly, thanks again for your valuable suggestions, which have significantly enhanced the current\nversion of the manuscript, making it more comprehensive and holistic. We hope the revisions\nabove have addressed your concerns.\nFigure R35. The H O production rate and Faradic efficiency of S-C2+ measured using flow cell.\n2 2\nReviewer #3 (Remarks to the Author):\nIn the manuscript titled \"Biradical-mediated synergistic electrocatalysis using metal-free redox\nmolecular catalysts,\" the authors report a class of metal-free bicarbenium-based molecular\ncatalysts that undergo electrochemical two-electron reduction to form biradical intermediates,\nenabling highly selective oxygen and nitric oxide reduction to H2O2 and NH2OH, respectively.\nThe authors combine synthesis, spectroscopy, electrocatalysis, and DFT calculations to explain the\nrole of spatially confined biradicals in promoting cooperative substrate activation.\nPage 51 of 59\nFirst of all, my expertise is in computational/theoretical chemistry, and I understand that I was\ninvited to review this aspect. Regarding the computational (DFT) parts of this manuscript, I have\nseveral critical concerns, outlined below:\nResponse: Thank you very much for your recognition and positive comments about our work. We\nhave carefully addressed the points you raised, and these revisions have indeed enhanced the depth\nof discussion in the current manuscript.\n1. Although the DFT-computed reaction energy profiles show that the C-C2+ shows the minimal\noverpotential, while the O-C2+ shows the largest value, the experimental ORR trend (shown in Fig.\n4a) follows the order of C-C2+ < O-C2+ < S-C2+. This discrepancy implies the possibility that the\nreaction path that the authors investigated is not responsible for the actual experimental situations.\nA direct comparison between the computed reaction energy profiles and experimental results is\nrequired.\nResponse: Thank you for this insightful comment. We agree that a direct comparison between\ntheory and experiment is necessary. In the revised manuscript, we (1) report free energy diagrams\nfor C-C2⁺, S-C2⁺, and O-C2⁺, (2) include a competing H+/e−-quenching pathway that forms the\nCHHC by-product and passivates the site, and (3) compare the resulting apparent activities with\nthe experimental data in Fig. 4a.\nThe energy profiles show that C-C2⁺ exhibits the most favorable reaction energetics, followed by\nthe S-C2⁺ and O-C2⁺, indicating an activity trend of C-C2⁺ > S-C2⁺ > O-C2⁺. However, when the\ndeactivation branch is included (Figure R36; Supplementary Figure 67), the thermodynamic\npreference for CHHC formation relative to the ORR product,\nPage 52 of 59\nΔΔG ≡ ΔG(CHHC) − ΔG(target),\nis C: −7.7 kcal/mol, S: +0.4 kcal/mol, and O: +9.5 kcal/mol. Thus, C-C2⁺ strongly partitions to the\ndeactivation pathway, while ORR remains favored at S-C2⁺ and O-C2⁺. Moreover, the ORR\nactivation barrier at S-C2⁺ is ~6.3 kcal mol⁻1 lower than at O-C2⁺, implying approximately 104-fold\nrate advantage at 298 K, which accounts for S > O among the non-quenched systems. Together,\nstrong deactivation at C and the lower kinetic barrier at S yield the apparent activity order C-C2⁺\n< O-C2⁺ < S-C2⁺—in agreement with experiment.\nTo summarize, in the revised manuscript and SI, we have now added:\n• Supplementary Figure 112: free-energy diagrams for both ORR and CHHC paths, ΔΔG\nvalues, and a table of key barriers;\n• A panel directly comparing computed apparent limiting potentials (including deactivation)\nwith the experimental trend in Fig. 4a.\nFurthermore, formation of the CHHC by-product is observed in our single-crystal measurements\n(Figure R36/Supplementary Figure 62), supporting the proposed deactivation mechanism. We\nhave incorporated these analyses and figures into the revised manuscript and SI.\nPage 53 of 59\nFigure R36. Free energy diagrams to compare the ORR and production of CHHC by-product.\nFigure R31. Top view and side view of compound C-C-2H.\n2. While the DFT pathway toward NH OH is proposed, the manuscript does not address whether\n2\ncontinued protonation to NH is kinetically or thermodynamically less favorable. Since this is\n3\ncentral to understanding the observed selectivity (87.8% FE to NH OH), the authors should clarify\n2\nwhether NH formation was computationally examined or ruled out based on energetics.\nResponse: Thank you for raising this point. We have now explicitly examined the continuation\nfrom NH OH to NH and report both thermodynamics and kinetics. As shown in Figure R37, we\n2 3\nevaluated two competing pathways on all three catalysts (S-C2⁺, O-C2⁺, C-C2⁺): (i) Pathway 1\n(NH OH formation): C–O bond cleavage from the NHOH+C intermediate, and (ii) Pathway 2 (NH\n2 3\nformation): N–O bond cleavage from the NH+HOC intermediate (the positively charged N center\nstabilized by interaction with a nearby C atom).\nFigure R38 showed the optimized TS geometries for all three catalysts (S-C2+, O-C2+, and C-C2+).\nWe then compared the relative TS energies between the two pathways, defined as: ΔΔG‡ =\nΔG‡(Pathway 2) − ΔG‡(Pathway 1). The calculated ΔΔG‡ values are +7.0, +6.1, and +7.5 kcal/mol,\nrespectively, for S-C2+, O-C2+, and C-C2+. These positive values demonstrate that Pathway 1\n(NH OH formation) is consistently more favorable than Pathway 2 (NH formation). Hence, NH\n2 3 3\nformation is kinetically less preferred, which explains the experimentally observed high selectivity\n(87.8% FE) toward NH OH. Consistent with these theoretical results, only trace NH is detected\n2 3\nexperimentally (Fig. 4h–i), whereas NH OH reaches 87.8% FE.\nFigure R37. Competing reaction pathways towards NH OH (Pathway 1) and NH (Pathway 2).\n2 3\nPage 55 of 59\nFigure R38. Transition state geometries for Pathway 1 and Pathway 2 on S-C2+, O-C2+ and C-\nC2+ catalysts.\n3. The computational methods are not described with sufficient detail. Most critically, the\nmanuscript lacks an information about the treatment of entropy and/or zero-point energy (ZPE).\nThe authors should describe how the ZPE and thermal corrections were evaluated to compute the\nfree energy quantities.\nResponse: We have substantially expanded the Methods and SI to discuss more details on our\ncomputations of ZPE and thermal corrections. In brief, all reported free energies are built from\nelectronic energies plus vibrational thermochemistry obtained from analytic frequency\ncalculations at the same level of theory as the geometry optimization (software: Gaussian). Minima\nPage 56 of 59\nhave no imaginary frequencies, and transition states have exactly one, which follows the reaction\ncoordinate.\n4. DFT often fails to accurately describe the spin states and related energetics of open-shell species.\nSome benchmarking of the chosen functional, especially for spin-sensitive energetics, is necessary\nto ensure reliability.\nResponse: Thank you for the insightful comment regarding spin-sensitive energetics. To address\nthis concern, we benchmarked our chosen functional (ωB97X-D) against four widely used\nalternatives (B3LYP, M06-2X, MN15, and PBE0) using the same basis set (6-31G**) and the\nsolvation model (SMD, water). For the spin-sensitive first activation step of S-C2+ to S-C2, we\nperformed the single-point and frequency calculations on ωB97X-D-optimized geometries to\nminimize the influence of structural differences.\nAs summarized in Table R1, all functionals except B3LYP predict reaction free energies within 2–\n3 kcal·mol⁻¹ of ωB97X-D, which demonstrates that our conclusions are not strongly dependent on\nthe functional. The larger deviation observed with B3LYP is consistent with the well-documented\nself-interaction error and delocalization tendency of this functional in radical systems, which often\nleads to artificial stabilization of delocalized radical states (J. Chem. Phys. 2005, 122, 224103).\nWe also examined spin contamination by monitoring the ⟨S2⟩ values for the triplet states. For a\ntriplet with total spin quantum number S=1, the theoretical expectation value is\n⟨𝑆2⟩ = S(S+1) = 2\nAs shown in Table R1, the computed ⟨S2⟩ values across all five functionals are very close to 2.00,\nconfirming negligible spin contamination.\nPage 57 of 59\nTable R1. Benchmarking of the first activation step of S–C2+.\nMethod Energy Profile Relative Difference S2 for triplet\nSMD ωB97X-D 6-31G** 12.9 0.0 2.0069\nSMD B3LYP 6-31G** 19.7 6.8 2.0019\nSMD M06-2X 6-31G** 12.3 -0.6 2.0015\nSMD MN15 6-31G** 15.4 2.6 2.0033\nSMD PBE0 6-31G** 14.7 1.8 2.0043\nIn summary, this benchmark study confirms that ωB97X-D provides reliable results for the spin-\nsensitive energetics considered in this work. We have included these data and discussion in the\nrevised SI as Supplementary Table 11.\n5. (General comment) The reaction energetics support the mechanism proposal qualitatively, but\nthe analysis lacks depth expected for a high-impact submission. The current DFT analysis remains\nlargely descriptive.\nResponse: We sincerely thank you for highlighting the need for greater depth in our computational\nanalysis. Following your suggestions, we have substantially strengthened the mechanism\ndiscussion:\n1. Clarification of experiment–theory differences.\nBy explicitly comparing the calculated and experimental trends, we clarified that the strong\ndeactivation pathway at the C site and the lower kinetic barrier at the S site together rationalize the\nobserved order of activity (C-C2+ < O-C2+ < S-C2+), consistent with experimental observations.\n2. Benchmarking of the chosen functional.\nPage 58 of 59\nTo ensure reliability, we benchmarked our selected functional (ωB97X-D) against four widely\nused alternatives (B3LYP, M06-2X, MN15, and PBE0) using the same basis set. All functionals,\nexcept B3LYP, yielded reaction free energies within 2–3 kcal/mol of each other, confirming that\nour conclusions are not sensitive to the choice of functional.\n3. Transition-state analysis and selectivity.\nWe performed TS calculations to examine the possible pathway toward NH formation. The results\n3\nshow that further protonation to NH is kinetically less favorable than NH OH formation, thereby\n3 2\nsupporting the observed selectivity and reinforcing the mechanism picture.\nIn summary, this work presents an exciting new direction for metal-free electrocatalysis using\nbiradical intermediates. The DFT results are encouraging and generally align with the experimental\nobservations. However, to match the rigor of the synthetic and spectroscopic work, the\ncomputational section requires clarification, benchmarking, and deeper mechanism analysis.\nResponse: Overall, these additions provide a more rigorous and quantitative basis for our\nmechanism proposal. Importantly, the reviewer’s suggestions prompted us to carry out additional\nanalyses that not only confirm the robustness of our conclusions but also deepen our mechanism\ninsight into this biradical-mediated electrocatalysis. We are grateful that these comments have\nsignificantly improved the overall quality of the work.\nPage 59 of 59\nPoint-by-point response to the reviewers’ comments\nReviewer #1 (Remarks to the Author):\nThe authors have revised the questions properly. It can be accepted in the current\nversion.\nWe sincerely thank the referee for reviewing our paper and offering valuable comments\nthat have enhanced the quality of our manuscript.\nReviewer #2 (Remarks to the Author):\nThe authors have made meticulous revisions, but prior to publication, the following\nissues should be addressed as thoroughly as possible:\nWe are grateful to the reviewer for the careful evaluation and constructive suggestions.\nWe have carefully revised the manuscript to address the points raised as thoroughly as\npossible.\nReply to Q3: It is recommended to present the purity of the three molecules in a\ncomparative table, listing the purity values of the target products and impurities.\nResponse: As shown in Table R1, all the compounds used in our electrocatalysis\nexperiments are of high purity. The three ¹H NMR spectra (Figures R1–R3) display\nonly the characteristic peaks of the catalysts, with no detectable impurities. Both the\npeak shapes and integration values are consistent with the expected structures.\n1 | Pa ge\nFurthermore, the high-resolution mass spectra (HRMS) (Supplementary Figures 23–\n25) generally show only fragment peaks, with occasional observation of OH adducts,\nfurther confirming the high purity of these compounds.\nTaking together, these analyses provide strong evidence that the compounds are\nsufficiently pure (> 95%, which is the upper limit that can be reliably determined by 1H\nNMR analysis). We hope this addresses your concerns regarding the purity of the\nmaterials.\nTable R1. Types and purities of the catalysts.\n2 | Pa ge\nFigure R1. ¹H NMR spectrum of S-C2+ with peak assignment (500 MHz, CD CN).\n3\n3 | Pa ge\nFigure R3. ¹H NMR spectrum of C-C2+ with peak assignment (500 MHz, CD CN).\nthrough spectral simulations or by referencing spectra of similar structures reported in\nthe literature.\nResponse: Thank you very much for your valuable comments. We have performed\ntime-dependent DFT (TD-DFT) for the simulation of UV-vis absorption spectra, and\nthe calculated results are generally in good agreement with the experimental data, with\nthe only noticeable difference being an overall redshift (Figures R4-9). The emergence\nof new absorption peaks at higher wavelengths for dicarbenium compounds compared\nto dihydroxyl precursors are due to the combined effect of extended conjugation, charge\ndelocalization and donor-accept effect (from O/S to the carbenium sites). Based on the\n4 | Pa ge\nresults (Table R1-3), the longest-wavelength absorption peak can be mainly ascribed to\nmultiple transitions from HOMO → LUMO, HOMO → LUMO+1, HOMO-1 →\nLUMO, and HOMO-1 → LUMO+1 (noted the HOMO/HOMO-1 and\nLUMO/LUMO+1 are degenerated respectively due to the dimeric molecular design).\nThis redshift can be attributed to differences in how TD-DFT handles electron\ncorrelation effects in the excited states and functional selection (e.g. overestimation of\nconjugation and charge delocalization effect, thus exhibiting red-shifted UV-vis spectra\nduring simulation). We have included these new data and discussions in the revised SI\nas Supplementary Figures S28-30 and Tables S1-3.\nFigure R4. Calculated and experimental UV-vis absorption spectra of compound S-C-\n2OH and S-C2+ in DCM and acetonitrile, respectively.\n5 | Pa ge\nFigure R5. Calculated and experimental UV-vis absorption spectra of compound O-C-\n2OH and O-C2+ in DCM and acetonitrile, respectively.\nFigure R6. Calculated and experimental UV-vis absorption spectra of compound C-C-\n2OH and C-C2+ in DCM and acetonitrile, respectively.\nReply to Q12: Regarding the assignment of the IR peak, although the authors have\nsupplemented spectra under inert atmosphere and explained the difficulties in procuring\n¹⁸O₂ gas, scientific rigor necessitates isotopic labeling experiments to confirm this peak.\nUnless the assignment of this IR peak is not a core aspect of the study—i.e., it is not\ncritical to the understanding of the mechanism.\n6 | Pa ge\nResponse: As noted, the peaks at 1210 and 1183 cm⁻¹ are clearly observed, which can\nbe assigned to the asymmetric stretching modes of -COOC- and -COOH, in agreement\nwith previous reports (Angew. Chem. Int. Ed. 2025, e202424449, Angew. Chem. Int.\nEd. 2024, 63, e202410719, Nat. Commun. 2024, 15, 6098). In general, these\nasymmetric vibrations are more pronounced than symmetric analogues, due to the\nstronger dipole moment changes associated with asymmetric stretching. These\nstretching modes are more direct evidence for the formation of -COOC- and -COOH in\nour proposed 2e- ORR pathway. Nevertheless, we also detect a weak band at 917 cm⁻¹,\nwhich corresponds to the symmetric vibration of -COOC-. This is reasonable, as\nasymmetric stretching generally appears at higher wavenumbers than symmetric\nstretching due to stronger coupling and larger dipole moment changes. The observation\nfurther supports the formation of the COOC intermediates and is consistent with our\ntheoretical simulations (Supplementary Figure S80 and Table S1).\nWe fully acknowledge that 18O₂ isotopic labeling is a powerful and effective tool for\npeak assignment and mechanistic studies. As stated in our previous revision,\nunfortunately, we were unable to provide the 18O isotope experiment, reason being that:\n2\n(1) For safety reasons, all gas use at our institution must follow pre-installed gas supply\nlines, and there is no line for 18O ; (2) We have tried to contact local gas suppliers in\n7 | Pa ge\nanalysis and calculations, are sufficient to illustrate most of the information required\nfor proposing the ORR mechanism in our cases.\nThe authors emphasize in the conclusion: \"This study demonstrates how diradical\nintermediates drive highly efficient and selective redox transformations, providing a\nfoundational framework for harnessing open-shell reactivity in electrocatalysis.\" This\nstatement is overly broad and lacks specificity. The authors should clarify what specific\nreferences and guidance are provided.\nWhat puzzles the reviewers is: While the study demonstrates a radical synergistic\nreaction mechanism and emphasizes that balancing the reactivity and stability of\ncarbenium ions is crucial—as excessive reactivity tends to trigger side reactions (e.g.,\nwater oxidation) and compromise ORR activity. This appears to be a relatively well-\nestablished point in the current field. It seems to lack substantial contributions to\nsubsequent electrocatalytic research. What impressed the reviewers was the\ncontrollable synthesis and manipulation of molecules, but no significant guidance or\ninsights directly relevant to electrocatalysis were obtained. If possible, the authors are\nencouraged to provide further discussion and refinement on this aspect.\nResponse: We sincerely thank the reviewer for the insightful comments. We have now\nrevised the expression in a more specific way: “This study demonstrates that diradical\nintermediates enhance efficiency and selectivity in redox transformations through\nstabilized spin-pairing interactions and favorable binding geometries. These findings\n8 | Pa ge\nprovide design principles for tuning electronic structures to promote diradical character,\nthereby guiding the rational development of electrocatalysts with open-shell reactivity.”\nWe note that, although balancing reactivity and stability is a recognized principle, our\nstudy primarily aims to explore a stable bicarbenium framework capable of highly\nefficient and selective catalysis of paramagnetic gases such as O and NO, driven by a\n2\nunique synergistic mechanism. This represents a distinct structural and mechanistic\ninsight that goes beyond previously established concepts.\nImportantly, our findings provide new guidance for the design of non-metal molecular\ncatalysts, highlighting how stable dual-cation architectures can harness and fine-tune\nradical reactivity to achieve selective activation of difficult small molecules and,\nconsequently, promote efficient electrocatalysis, even for less reactive substrate like\nCO2 and N2 in the future. We hope that our explanation addresses your concerns.\nReviewer #3 (Remarks to the Author):\nI appreciate the authors for substantially revising the manuscript. I now believe it\nincludes much more detailed discussions and results that better corroborate the\nexperimental findings. However, I still have one critical and one minor comment:\nResponse: We sincerely thank the reviewer for the positive evaluation of our revised\nmanuscript and for acknowledging the improvements made. We greatly appreciate the\nreviewer’s continued constructive feedback, including the critical and minor comments,\n9 | Pa ge\nwhich will further help us to refine and strengthen the work.\n1. (Regarding Comment #1)\nThe authors rationalize why C-C2+ does not show the best activity (particularly\ncompared to S-C2+) by pointing to its preferred energetics toward the inactivation\nprocess, forming CHHC. While this explanation is possible, it is not fully convincing.\nThe reason is that S-C2+ also shows a nearly isoenergetic path toward CHHC formation,\nwhich could lead to substantial deactivation. In other words, the current argument may\nappear somewhat selective: the comparison between S-C2+ and O-C2+ is based on ORR\nenergetics, while the comparison between S-C2+ and C-C2+ is based on CHHC\nenergetics. To fully support their claim, the authors should provide a more quantitative\ndiscussion of these competing processes (e.g., via microkinetic modeling) and also\ninclude experimental evidence showing that the deactivation process is progressively\nsuppressed in the order O-C2+ < S-C2+ < C-C2+.\nResponse: Thank you for your insightful comment. We have analyzed all three\ncatalysts within a unified microkinetic framework, in which the ORR activity at site X\nis expressed as 𝐴 ∝ 𝑘 (𝑋)/[1+𝐾 ]. Here, 𝑘 represents the common activation\n𝑋 act 𝑋 act\nstep, and 𝐾 quantifies the competition between the CHHC quench and ORR branches.\n𝑋\nUsing the activation free energies from Fig. 5c (Δ𝐺 = 6.0, 19.2, 12.9 kcal mol⁻1\nact\nfor C, O, S) together with the branching free energies for ORR and CHHC from\nSupplementary Fig. 112, and applying a standard BEP relation (𝑎 = 0.5, 𝑇 = 298K),\n10 | Pa ge\nwe obtain 𝐴 < 𝐴 ≲ 𝐴 . A complete step-by-step derivation is presented below\nC O S\n(Supplementary Note R1).\nSupplementary Note R1:\nTo rationalize the observed catalytic trend in our ORR system, where the activity\nfollows C-C2⁺ < O-C2⁺ < S-C2⁺, we constructed a microkinetic framework consisting\nof three elementary steps: catalyst activation (Eq. 1), followed by branching either to\nthe ORR pathway or a competing CHHC (quenching) channel (Eq. 2).\n∗\n𝑘\n→\n𝑎 𝑐 𝑡 (𝑋 )\n𝐶 ··𝐶 (1)\n𝑘𝑂𝑅𝑅 (𝑋)\n→ 𝑂𝑅𝑅+∗\n𝐶 ··𝐶 { (2)\n𝑘 (𝑋)\n𝑞𝑢𝑒𝑛𝑐ℎ\n→ 𝐶𝐻𝐻𝐶\n𝑋 ∈ {𝐶,𝑂,𝑆}\nFrom Fig. 5c and Supplementary Fig. 112, the activation free energy barriers ΔG are\nact\n6.0, 19.2, and 12.9 kcal/mol for the C, O, and S catalysts, respectively. The ORR\npathway exhibits a uniform reaction free energy of ΔG = −18.1 kcal/mol across all\nORR\ncatalysts. The quenching pathway has ΔG values of −25.8, −8.6, and −17.7\nquench\nkcal/mol for the C, O, and S catalysts, respectively.\nUnder initial (short-time) conditions, the ORR rate at site X is\n𝑅 (𝑋) = 𝜃𝑘 (𝑋)𝑆 (𝑋) (3)\n𝑂𝑅𝑅 𝑎𝑐𝑡 𝑂𝑅𝑅\n11 | Pa ge\nwhere θ accounts for time-dependent site loss if passivation is irreversible, and\n𝑘 (𝑋)\n𝑂𝑅𝑅\n𝑆 (𝑋) = (4)\n𝑂𝑅𝑅 𝑘 (𝑋)+𝑘 (𝑋)\n𝑂𝑅𝑅 𝑞𝑢𝑒𝑛𝑐ℎ\ndenotes the ORR selectivity. We define the activity proxy\n𝑘 (𝑋)\n𝑎𝑐𝑡\n𝐴 ∝ 𝑅 (𝑋) ∝ (5)\n𝑋 𝑂𝑅𝑅 1+𝐾\n𝑋\n𝑘 (𝑋)𝑎\n𝑞𝑢𝑒𝑛𝑐ℎ 𝐻+\n𝐾 = (6)\n𝑋 𝑘 (𝑋) 𝑎\n𝑂𝑅𝑅 𝑂2\nHere, A represents the catalyst activity, while K measures the relative competition\nX X\nbetween quenching and the ORR reaction at site X, including contributions from pH\nand O activity.\ntriplet-triplet interactions with O , and the CHHC quenching pathway proceeds through\n2\nsequential addition of H+/e− (C··C → CH·C → CHHC), direct transition state\ncalculations for these processes are either inapplicable or highly challenging. We,\ntherefore, in this microkinetic treatment, approximate the kinetics using the reaction\nfree energies (ΔG) via a Brønsted–Evans–Polanyi (BEP) relation: ΔG‡ ≈ aΔG + b, with\na slope of a = 0.5 (J. Catal. 2001, 197, 229. J. Catal. 2004, 224, 206–217). Since the\nintercept b cancels when computing relative rate ratios, it is omitted. All evaluations\nare performed at T = 298 K, corresponding to RT = 0.593 kcal/mol.\nActivation factors. From Eq. 6,\n12 | Pa ge\n𝑎∆𝐺 (𝑋)\n𝑎𝑐𝑡\n𝑘 (𝑋) ∝ 𝑒𝑥𝑝[− ] (7)\n𝑎𝑐𝑡 𝑅𝑇\nNumerically, C: exp[−0.5⋅6.0/0.593] = 6.35×10−3, O: exp[−0.5⋅19.2/0.593] = 9.32×10−8,\nS: exp[−0.5⋅12.9/0.593] = 1.89×10−5.\nBranching competition. Taking O as the reference,\n𝑘 (𝑋)\n𝑞𝑢𝑒𝑛𝑐ℎ\n𝐾 𝑋 = 𝑘 𝑂𝑅𝑅 (𝑋) = 𝑒𝑥𝑝[− 𝑎(∆𝐺 𝑞𝑢𝑒𝑛𝑐ℎ (𝑋)−∆𝐺 𝑞𝑢𝑒𝑛𝑐ℎ (𝑂)) ] ≡ Λ ,Λ = 1 (8)\n𝐾 𝑘 (𝑂) 𝑅𝑇 𝑋 𝑂\n𝑂 𝑞𝑢𝑒𝑛𝑐ℎ\n𝑘 (𝑂)\n𝑂𝑅𝑅\nSince ΔG (X) = ΔG (O) = −18.1 kcal/mol, these two parts cancel with each other.\nORR ORR\nThe factor Λ is the passivation strength amplification factor relative to the O site,\nX\ndetermined solely by the difference in ΔG . Thus,\nquench\n𝐾 = 𝐾 Λ (Λ = 1) (9)\n𝑋 𝑂 𝑋 𝑂\nUsing the values above, Λ = exp[−0.5(−25.8+8.6)/0.593] = 2.00×106, Λ = 1, Λ =\nC O S\nexp[−0.5(-17.7+8.6)/0.593] = 2.15×103.\nCombining Eqs. 5 and 8 gives the working expression:\n𝑘 (𝑋)\n𝑎𝑐𝑡\n𝐴 ∝ (10)\n𝑋 1+𝐾 Λ\n𝑂 𝑋\nAt pH = 1, a(H+) = 10−1 = 0.1, while under O -saturated or air conditions, a(O ) ≈ 1.\n2 2\nTherefore, the prefactor a(H+)/a(O ) alone makes K ~ 0.1. Moreover, at the O site, the\ncomputed free energies for CHHC and ORR are of comparable magnitude (−8.6 vs.\n−18.1 kcal/mol), so their intrinsic rate constants are expected to be of similar order.\n13 | Pa ge\nHence, K naturally falls in the range 0.05–0.1. We take a moderate passivation K =\nO O\n0.1 for evaluation.\nResulting activities. From Eq. 10, we obtain:\nA ∝ 6.35×10−3 × (1/(1+0.1×2.00×106)) = 3.17×10−8,\nC\nA ∝ 9.32×10−8 × (1/(1+0.1×1)) = 8.47×10−8,\nO\nA ∝ 1.89×10−5 × (1/(1+0.1×2.15×103)) =8.75×10−8.\nS\nTherefore, the predicted ORR ordering is A < A ≲ A .\nC O S\nWe also extended this microkinetic framework to the NORR pathway, which involves\nthe same activation and quenching steps as ORR. In our calculations, the reaction\nfree energies are identical across the catalysts (ΔG (X) = ΔG (O) = −17.3 kcal/mol),\nNORR NORR\nso these contributions cancel when comparing relative rates. As a result, the apparent\nactivity trend remains unchanged and follows C–C2+ < O–C2+ ≲ S–C2+.\nBeyond this microkinetic analysis, the formation of the C-C-2H by-product is observed\nin our single-crystal measurements (Figure R7), supporting the strong deactivation\nprocess in C-C2+. In addition, we attempted to quantify CHHC by-products for all three\ncatalysts after electrocatalysis using ¹H NMR. However, the spectra before and after\ncatalysis (Figures R8-10) showed no significant differences, suggesting that the\namounts of impurities and by-products generated is too small to be clearly detected,\n14 | Pa ge\nand that the catalyst molecules remain the dominant species. As for experimentally\ndistinguishing the deactivation process between S-C2⁺ and O-C2⁺, such measurements\nwould require an exceptionally long time due to their very high stability, and is\ntherefore not feasible within the current study.\nFigure R7. Top view and side view of compound C-C-2H.\nFigure R8. ¹H NMR spectra of C-C2+ before and after catalysis.\n15 | Pa ge\nFigure R9. ¹H NMR spectra of O-C2+ before and after catalysis.\nFigure R10. ¹H NMR spectra of S-C2+ before and after catalysis.\nAccordingly, we have added the microkinetic analysis rationalizing the C-C2⁺ < O-C2⁺\n< S-C2⁺ activity trend in the revised SI as the Supplementary Note 18, and included the\nsingle crystal data and 1H NMR spectra as Supplementary figures S62, S101-103.\n2. (Regarding Comment #3)\nAlthough the revised Supporting Information now includes computational details, it\nstill lacks the required citations for the computational methods and software. In addition,\nsome acronyms (e.g., SCF) are used without definition.\n16 | Pa ge\nResponse: Thank you for pointing this out. The revised manuscript now includes\nappropriate citations for the computational methods and software employed. These\nwere not repeated in the Supplementary Information in order to minimize overlap and\npreserve conciseness. In addition, we have defined all acronyms (e.g., SCF) at their\nfirst occurrence to ensure clarity.\n17 | Pa ge"}, {"round": 2, "reviewer_comments": {"reviewer_1": "the authors have revised the questions properly,It can be accepted in the current version.", "reviewer_2": "The authors have made meticulous revisions, but prior to publication, the following issues should be addressed as thoroughly as possible:\nReply to Q3: It is recommended to present the purity of the three molecules in a comparative table, listing the purity values of the target products and impurities.\nReply to Q5: The authors should provide more convincing explanations, potentially through spectral simulations or by referencing spectra of similar structures reported in the literature.\nReply to Q12: Regarding the assignment of the IR peak, although the authors have supplemented spectra under inert atmosphere and explained the difficulties in procuring ¹⁸O₂ gas, scientific rigor necessitates isotopic labeling experiments to confirm this peak. Unless the assignment of this IR peak is not a core aspect of the study—i.e., it is not critical to the understanding of the mechanism.\nThe authors emphasize in the conclusion: \"This study demonstrates how diradical intermediates drive highly efficient and selective redox transformations, providing a foundational framework for harnessing open-shell reactivity in electrocatalysis.\" This statement is overly broad and lacks specificity. The authors should clarify what specific references and guidance are provided.\nWhat puzzles the reviewers is: While the study demonstrates a radical synergistic reaction mechanism and emphasizes that balancing the reactivity and stability of carbenium ions is crucial—as excessive reactivity tends to trigger side reactions (e.g., water oxidation) and compromise ORR activity. This appears to be a relatively well-established point in the current field. It seems to lack substantial contributions to subsequent electrocatalytic research. What impressed the reviewers was the controllable synthesis and manipulation of molecules, but no significant guidance or insights directly relevant to electrocatalysis were obtained. If possible, the authors are encouraged to provide further discussion and refinement on this aspect.", "reviewer_3": "I appreciate the authors for substantially revising the manuscript. I now believe it includes much more detailed discussions and results that better corroborate the experimental findings. However, I still have one critical and one minor comment:\n1. (Regarding Comment #1)\nThe authors rationalize why C-C2+ does not show the best activity (particularly compared to S-C2+) by pointing to its preferred energetics toward the inactivation process, forming CHHC. While this explanation is possible, it is not fully convincing. The reason is that S-C2+ also shows a nearly isoenergetic path toward CHHC formation, which could lead to substantial deactivation. In other words, the current argument may appear somewhat selective: the comparison between S-C2+ and O-C2+ is based on ORR energetics, while the comparison between S-C2+ and C-C2+ is based on CHHC energetics. To fully support their claim, the authors should provide a more quantitative discussion of these competing processes (e.g., via microkinetic modeling) and also include experimental evidence showing that the deactivation process is progressively suppressed in the order O-C2+ < S-C2+ < C-C2+.\n2. (Regarding Comment #3)\nAlthough the revised Supporting Information now includes computational details, it still lacks the required citations for the computational methods and software. In addition, some acronyms (e.g., SCF) are used without definition."}, "author_response": "Point-by-point response to the reviewers’ comments\nReviewer #1 (Remarks to the Author):\nThe authors have revised the questions properly. It can be accepted in the current\nversion.\nWe sincerely thank the referee for reviewing our paper and offering valuable comments\nthat have enhanced the quality of our manuscript.\nReviewer #2 (Remarks to the Author):\nThe authors have made meticulous revisions, but prior to publication, the following\nissues should be addressed as thoroughly as possible:\nWe are grateful to the reviewer for the careful evaluation and constructive suggestions.\nWe have carefully revised the manuscript to address the points raised as thoroughly as\npossible.\nReply to Q3: It is recommended to present the purity of the three molecules in a\ncomparative table, listing the purity values of the target products and impurities.\nResponse: As shown in Table R1, all the compounds used in our electrocatalysis\nexperiments are of high purity. The three ¹H NMR spectra (Figures R1–R3) display\nonly the characteristic peaks of the catalysts, with no detectable impurities. Both the\npeak shapes and integration values are consistent with the expected structures.\n1 | Pa ge\nFurthermore, the high-resolution mass spectra (HRMS) (Supplementary Figures 23–\n25) generally show only fragment peaks, with occasional observation of OH adducts,\nfurther confirming the high purity of these compounds.\nTaking together, these analyses provide strong evidence that the compounds are\nsufficiently pure (> 95%, which is the upper limit that can be reliably determined by 1H\nNMR analysis). We hope this addresses your concerns regarding the purity of the\nmaterials.\nTable R1. Types and purities of the catalysts.\n2 | Pa ge\nFigure R1. ¹H NMR spectrum of S-C2+ with peak assignment (500 MHz, CD CN).\n3\n3 | Pa ge\nFigure R3. ¹H NMR spectrum of C-C2+ with peak assignment (500 MHz, CD CN).\nthrough spectral simulations or by referencing spectra of similar structures reported in\nthe literature.\nResponse: Thank you very much for your valuable comments. We have performed\ntime-dependent DFT (TD-DFT) for the simulation of UV-vis absorption spectra, and\nthe calculated results are generally in good agreement with the experimental data, with\nthe only noticeable difference being an overall redshift (Figures R4-9). The emergence\nof new absorption peaks at higher wavelengths for dicarbenium compounds compared\nto dihydroxyl precursors are due to the combined effect of extended conjugation, charge\ndelocalization and donor-accept effect (from O/S to the carbenium sites). Based on the\n4 | Pa ge\nresults (Table R1-3), the longest-wavelength absorption peak can be mainly ascribed to\nmultiple transitions from HOMO → LUMO, HOMO → LUMO+1, HOMO-1 →\nLUMO, and HOMO-1 → LUMO+1 (noted the HOMO/HOMO-1 and\nLUMO/LUMO+1 are degenerated respectively due to the dimeric molecular design).\nThis redshift can be attributed to differences in how TD-DFT handles electron\ncorrelation effects in the excited states and functional selection (e.g. overestimation of\nconjugation and charge delocalization effect, thus exhibiting red-shifted UV-vis spectra\nduring simulation). We have included these new data and discussions in the revised SI\nas Supplementary Figures S28-30 and Tables S1-3.\nFigure R4. Calculated and experimental UV-vis absorption spectra of compound S-C-\n2OH and S-C2+ in DCM and acetonitrile, respectively.\n5 | Pa ge\nFigure R5. Calculated and experimental UV-vis absorption spectra of compound O-C-\n2OH and O-C2+ in DCM and acetonitrile, respectively.\nFigure R6. Calculated and experimental UV-vis absorption spectra of compound C-C-\n2OH and C-C2+ in DCM and acetonitrile, respectively.\nReply to Q12: Regarding the assignment of the IR peak, although the authors have\nsupplemented spectra under inert atmosphere and explained the difficulties in procuring\n¹⁸O₂ gas, scientific rigor necessitates isotopic labeling experiments to confirm this peak.\nUnless the assignment of this IR peak is not a core aspect of the study—i.e., it is not\ncritical to the understanding of the mechanism.\n6 | Pa ge\nResponse: As noted, the peaks at 1210 and 1183 cm⁻¹ are clearly observed, which can\nbe assigned to the asymmetric stretching modes of -COOC- and -COOH, in agreement\nwith previous reports (Angew. Chem. Int. Ed. 2025, e202424449, Angew. Chem. Int.\nEd. 2024, 63, e202410719, Nat. Commun. 2024, 15, 6098). In general, these\nasymmetric vibrations are more pronounced than symmetric analogues, due to the\nstronger dipole moment changes associated with asymmetric stretching. These\nstretching modes are more direct evidence for the formation of -COOC- and -COOH in\nour proposed 2e- ORR pathway. Nevertheless, we also detect a weak band at 917 cm⁻¹,\nwhich corresponds to the symmetric vibration of -COOC-. This is reasonable, as\nasymmetric stretching generally appears at higher wavenumbers than symmetric\nstretching due to stronger coupling and larger dipole moment changes. The observation\nfurther supports the formation of the COOC intermediates and is consistent with our\ntheoretical simulations (Supplementary Figure S80 and Table S1).\nWe fully acknowledge that 18O₂ isotopic labeling is a powerful and effective tool for\npeak assignment and mechanistic studies. As stated in our previous revision,\nunfortunately, we were unable to provide the 18O isotope experiment, reason being that:\n2\n(1) For safety reasons, all gas use at our institution must follow pre-installed gas supply\nlines, and there is no line for 18O ; (2) We have tried to contact local gas suppliers in\n7 | Pa ge\nanalysis and calculations, are sufficient to illustrate most of the information required\nfor proposing the ORR mechanism in our cases.\nThe authors emphasize in the conclusion: \"This study demonstrates how diradical\nintermediates drive highly efficient and selective redox transformations, providing a\nfoundational framework for harnessing open-shell reactivity in electrocatalysis.\" This\nstatement is overly broad and lacks specificity. The authors should clarify what specific\nreferences and guidance are provided.\nWhat puzzles the reviewers is: While the study demonstrates a radical synergistic\nreaction mechanism and emphasizes that balancing the reactivity and stability of\ncarbenium ions is crucial—as excessive reactivity tends to trigger side reactions (e.g.,\nwater oxidation) and compromise ORR activity. This appears to be a relatively well-\nestablished point in the current field. It seems to lack substantial contributions to\nsubsequent electrocatalytic research. What impressed the reviewers was the\ncontrollable synthesis and manipulation of molecules, but no significant guidance or\ninsights directly relevant to electrocatalysis were obtained. If possible, the authors are\nencouraged to provide further discussion and refinement on this aspect.\nResponse: We sincerely thank the reviewer for the insightful comments. We have now\nrevised the expression in a more specific way: “This study demonstrates that diradical\nintermediates enhance efficiency and selectivity in redox transformations through\nstabilized spin-pairing interactions and favorable binding geometries. These findings\n8 | Pa ge\nprovide design principles for tuning electronic structures to promote diradical character,\nthereby guiding the rational development of electrocatalysts with open-shell reactivity.”\nWe note that, although balancing reactivity and stability is a recognized principle, our\nstudy primarily aims to explore a stable bicarbenium framework capable of highly\nefficient and selective catalysis of paramagnetic gases such as O and NO, driven by a\n2\nunique synergistic mechanism. This represents a distinct structural and mechanistic\ninsight that goes beyond previously established concepts.\nImportantly, our findings provide new guidance for the design of non-metal molecular\ncatalysts, highlighting how stable dual-cation architectures can harness and fine-tune\nradical reactivity to achieve selective activation of difficult small molecules and,\nconsequently, promote efficient electrocatalysis, even for less reactive substrate like\nCO2 and N2 in the future. We hope that our explanation addresses your concerns.\nReviewer #3 (Remarks to the Author):\nI appreciate the authors for substantially revising the manuscript. I now believe it\nincludes much more detailed discussions and results that better corroborate the\nexperimental findings. However, I still have one critical and one minor comment:\nResponse: We sincerely thank the reviewer for the positive evaluation of our revised\nmanuscript and for acknowledging the improvements made. We greatly appreciate the\nreviewer’s continued constructive feedback, including the critical and minor comments,\n9 | Pa ge\nwhich will further help us to refine and strengthen the work.\n1. (Regarding Comment #1)\nThe authors rationalize why C-C2+ does not show the best activity (particularly\ncompared to S-C2+) by pointing to its preferred energetics toward the inactivation\nprocess, forming CHHC. While this explanation is possible, it is not fully convincing.\nThe reason is that S-C2+ also shows a nearly isoenergetic path toward CHHC formation,\nwhich could lead to substantial deactivation. In other words, the current argument may\nappear somewhat selective: the comparison between S-C2+ and O-C2+ is based on ORR\nenergetics, while the comparison between S-C2+ and C-C2+ is based on CHHC\nenergetics. To fully support their claim, the authors should provide a more quantitative\ndiscussion of these competing processes (e.g., via microkinetic modeling) and also\ninclude experimental evidence showing that the deactivation process is progressively\nsuppressed in the order O-C2+ < S-C2+ < C-C2+.\nResponse: Thank you for your insightful comment. We have analyzed all three\ncatalysts within a unified microkinetic framework, in which the ORR activity at site X\nis expressed as 𝐴 ∝ 𝑘 (𝑋)/[1+𝐾 ]. Here, 𝑘 represents the common activation\n𝑋 act 𝑋 act\nstep, and 𝐾 quantifies the competition between the CHHC quench and ORR branches.\n𝑋\nUsing the activation free energies from Fig. 5c (Δ𝐺 = 6.0, 19.2, 12.9 kcal mol⁻1\nact\nfor C, O, S) together with the branching free energies for ORR and CHHC from\nSupplementary Fig. 112, and applying a standard BEP relation (𝑎 = 0.5, 𝑇 = 298K),\n10 | Pa ge\nwe obtain 𝐴 < 𝐴 ≲ 𝐴 . A complete step-by-step derivation is presented below\nC O S\n(Supplementary Note R1).\nSupplementary Note R1:\nTo rationalize the observed catalytic trend in our ORR system, where the activity\nfollows C-C2⁺ < O-C2⁺ < S-C2⁺, we constructed a microkinetic framework consisting\nof three elementary steps: catalyst activation (Eq. 1), followed by branching either to\nthe ORR pathway or a competing CHHC (quenching) channel (Eq. 2).\n∗\n𝑘\n→\n𝑎 𝑐 𝑡 (𝑋 )\n𝐶 ··𝐶 (1)\n𝑘𝑂𝑅𝑅 (𝑋)\n→ 𝑂𝑅𝑅+∗\n𝐶 ··𝐶 { (2)\n𝑘 (𝑋)\n𝑞𝑢𝑒𝑛𝑐ℎ\n→ 𝐶𝐻𝐻𝐶\n𝑋 ∈ {𝐶,𝑂,𝑆}\nFrom Fig. 5c and Supplementary Fig. 112, the activation free energy barriers ΔG are\nact\n6.0, 19.2, and 12.9 kcal/mol for the C, O, and S catalysts, respectively. The ORR\npathway exhibits a uniform reaction free energy of ΔG = −18.1 kcal/mol across all\nORR\ncatalysts. The quenching pathway has ΔG values of −25.8, −8.6, and −17.7\nquench\nkcal/mol for the C, O, and S catalysts, respectively.\nUnder initial (short-time) conditions, the ORR rate at site X is\n𝑅 (𝑋) = 𝜃𝑘 (𝑋)𝑆 (𝑋) (3)\n𝑂𝑅𝑅 𝑎𝑐𝑡 𝑂𝑅𝑅\n11 | Pa ge\nwhere θ accounts for time-dependent site loss if passivation is irreversible, and\n𝑘 (𝑋)\n𝑂𝑅𝑅\n𝑆 (𝑋) = (4)\n𝑂𝑅𝑅 𝑘 (𝑋)+𝑘 (𝑋)\n𝑂𝑅𝑅 𝑞𝑢𝑒𝑛𝑐ℎ\ndenotes the ORR selectivity. We define the activity proxy\n𝑘 (𝑋)\n𝑎𝑐𝑡\n𝐴 ∝ 𝑅 (𝑋) ∝ (5)\n𝑋 𝑂𝑅𝑅 1+𝐾\n𝑋\n𝑘 (𝑋)𝑎\n𝑞𝑢𝑒𝑛𝑐ℎ 𝐻+\n𝐾 = (6)\n𝑋 𝑘 (𝑋) 𝑎\n𝑂𝑅𝑅 𝑂2\nHere, A represents the catalyst activity, while K measures the relative competition\nX X\nbetween quenching and the ORR reaction at site X, including contributions from pH\nand O activity.\ntriplet-triplet interactions with O , and the CHHC quenching pathway proceeds through\n2\nsequential addition of H+/e− (C··C → CH·C → CHHC), direct transition state\ncalculations for these processes are either inapplicable or highly challenging. We,\ntherefore, in this microkinetic treatment, approximate the kinetics using the reaction\nfree energies (ΔG) via a Brønsted–Evans–Polanyi (BEP) relation: ΔG‡ ≈ aΔG + b, with\na slope of a = 0.5 (J. Catal. 2001, 197, 229. J. Catal. 2004, 224, 206–217). Since the\nintercept b cancels when computing relative rate ratios, it is omitted. All evaluations\nare performed at T = 298 K, corresponding to RT = 0.593 kcal/mol.\nActivation factors. From Eq. 6,\n12 | Pa ge\n𝑎∆𝐺 (𝑋)\n𝑎𝑐𝑡\n𝑘 (𝑋) ∝ 𝑒𝑥𝑝[− ] (7)\n𝑎𝑐𝑡 𝑅𝑇\nNumerically, C: exp[−0.5⋅6.0/0.593] = 6.35×10−3, O: exp[−0.5⋅19.2/0.593] = 9.32×10−8,\nS: exp[−0.5⋅12.9/0.593] = 1.89×10−5.\nBranching competition. Taking O as the reference,\n𝑘 (𝑋)\n𝑞𝑢𝑒𝑛𝑐ℎ\n𝐾 𝑋 = 𝑘 𝑂𝑅𝑅 (𝑋) = 𝑒𝑥𝑝[− 𝑎(∆𝐺 𝑞𝑢𝑒𝑛𝑐ℎ (𝑋)−∆𝐺 𝑞𝑢𝑒𝑛𝑐ℎ (𝑂)) ] ≡ Λ ,Λ = 1 (8)\n𝐾 𝑘 (𝑂) 𝑅𝑇 𝑋 𝑂\n𝑂 𝑞𝑢𝑒𝑛𝑐ℎ\n𝑘 (𝑂)\n𝑂𝑅𝑅\nSince ΔG (X) = ΔG (O) = −18.1 kcal/mol, these two parts cancel with each other.\nORR ORR\nThe factor Λ is the passivation strength amplification factor relative to the O site,\nX\ndetermined solely by the difference in ΔG . Thus,\nquench\n𝐾 = 𝐾 Λ (Λ = 1) (9)\n𝑋 𝑂 𝑋 𝑂\nUsing the values above, Λ = exp[−0.5(−25.8+8.6)/0.593] = 2.00×106, Λ = 1, Λ =\nC O S\nexp[−0.5(-17.7+8.6)/0.593] = 2.15×103.\nCombining Eqs. 5 and 8 gives the working expression:\n𝑘 (𝑋)\n𝑎𝑐𝑡\n𝐴 ∝ (10)\n𝑋 1+𝐾 Λ\n𝑂 𝑋\nAt pH = 1, a(H+) = 10−1 = 0.1, while under O -saturated or air conditions, a(O ) ≈ 1.\n2 2\nTherefore, the prefactor a(H+)/a(O ) alone makes K ~ 0.1. Moreover, at the O site, the\ncomputed free energies for CHHC and ORR are of comparable magnitude (−8.6 vs.\n−18.1 kcal/mol), so their intrinsic rate constants are expected to be of similar order.\n13 | Pa ge\nHence, K naturally falls in the range 0.05–0.1. We take a moderate passivation K =\nO O\n0.1 for evaluation.\nResulting activities. From Eq. 10, we obtain:\nA ∝ 6.35×10−3 × (1/(1+0.1×2.00×106)) = 3.17×10−8,\nC\nA ∝ 9.32×10−8 × (1/(1+0.1×1)) = 8.47×10−8,\nO\nA ∝ 1.89×10−5 × (1/(1+0.1×2.15×103)) =8.75×10−8.\nS\nTherefore, the predicted ORR ordering is A < A ≲ A .\nC O S\nWe also extended this microkinetic framework to the NORR pathway, which involves\nthe same activation and quenching steps as ORR. In our calculations, the reaction\nfree energies are identical across the catalysts (ΔG (X) = ΔG (O) = −17.3 kcal/mol),\nNORR NORR\nso these contributions cancel when comparing relative rates. As a result, the apparent\nactivity trend remains unchanged and follows C–C2+ < O–C2+ ≲ S–C2+.\nBeyond this microkinetic analysis, the formation of the C-C-2H by-product is observed\nin our single-crystal measurements (Figure R7), supporting the strong deactivation\nprocess in C-C2+. In addition, we attempted to quantify CHHC by-products for all three\ncatalysts after electrocatalysis using ¹H NMR. However, the spectra before and after\ncatalysis (Figures R8-10) showed no significant differences, suggesting that the\namounts of impurities and by-products generated is too small to be clearly detected,\n14 | Pa ge\nand that the catalyst molecules remain the dominant species. As for experimentally\ndistinguishing the deactivation process between S-C2⁺ and O-C2⁺, such measurements\nwould require an exceptionally long time due to their very high stability, and is\ntherefore not feasible within the current study.\nFigure R7. Top view and side view of compound C-C-2H.\nFigure R8. ¹H NMR spectra of C-C2+ before and after catalysis.\n15 | Pa ge\nFigure R9. ¹H NMR spectra of O-C2+ before and after catalysis.\nFigure R10. ¹H NMR spectra of S-C2+ before and after catalysis.\nAccordingly, we have added the microkinetic analysis rationalizing the C-C2⁺ < O-C2⁺\n< S-C2⁺ activity trend in the revised SI as the Supplementary Note 18, and included the\nsingle crystal data and 1H NMR spectra as Supplementary figures S62, S101-103.\n2. (Regarding Comment #3)\nAlthough the revised Supporting Information now includes computational details, it\nstill lacks the required citations for the computational methods and software. In addition,\nsome acronyms (e.g., SCF) are used without definition.\n16 | Pa ge\nResponse: Thank you for pointing this out. The revised manuscript now includes\nappropriate citations for the computational methods and software employed. These\nwere not repeated in the Supplementary Information in order to minimize overlap and\npreserve conciseness. In addition, we have defined all acronyms (e.g., SCF) at their\nfirst occurrence to ensure clarity.\n17 | Pa ge"}, {"round": 3, "reviewer_comments": {"reviewer_2": "This reviewer has no more questions.", "reviewer_3": "The authors fully responded to my previous concerns. Particularly, the kinetic analysis is very useful. Therefore I agree to the publication of this manuscript."}, "author_response": ""}]

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