Influence of the Metal Center in M–N–C Catalysts on the CO2 Reduction Reaction on Gas Diffusion Electrodes

Revealing OH species in situ generated on low-valence Cu sites for selective carbonyl oxidation

Catalytic oxidation through the transfer of lattice oxygen from metal oxides to reactants, namely the Mars-van Krevelen mechanism, has been widely reported. In this study, we evidence the overlooked oxidation route that features the in situ formation of surface OH species on Cu catalysts and its selective addition to the reactant carbonyl group. We observed that glucose oxidation to gluconic acid in air (21% O2) was favored on low-valence Cu sites according to X-ray spectroscopic analyses. Molecular O2 was activated in situ on Cu0/Cu+ forming localized, adsorbed hydroxyl radicals (*OH) which played the primary reactive oxygen species as confirmed by the kinetic isotope effect (KIE) study in D2O and in situ Raman experiments. Combined with DFT calculations, we proposed a mechanism of O2-to-*OH activation through the *OOH intermediate. The localized *OH exhibited higher selectivity toward glucose oxidation at C1HO to form gluconic acid (up to 91% selectivity), in comparison with free radicals in bulk environment that emerged from thermal, noncatalytic hydrogen peroxide decomposition (40% selectivity). The KIE measurements revealed a lower glucose oxidation rate in D2O than in H2O, highlighting the role of water (H2O/D2O) or its derivatives (e.g., *OH/*OD) in the rate-determining step. After proving the C1-H activation step kinetically irrelevant, we proposed the oxidation mechanism that was characterized by the rate-limiting addition of *OH to C1=O in glucose. Our findings advocate that by maneuvering the coverage and activity of surface *OH, high-performance oxidation of carbonyl compounds beyond biomass molecules can be achieved in water and air using nonprecious metal catalysts.

Iron Triad-Based Bimetallic M-N-C Nanomaterials as Highly Active Bifunctional Oxygen Electrocatalysts

The use of precious metal electrocatalysts in clean electrochemical energy conversion and storage applications is widespread, but the sustainability of these materials, in terms of their availability and cost, is constrained. In this research, iron triad-based bimetallic nitrogen-doped carbon (M-N-C) materials were investigated as potential bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The synthesis of bimetallic FeCo-N-C, CoNi-N-C, and FeNi-N-C catalysts involved a precisely optimized carbonization process of their respective metal-organic precursors. Comprehensive structural analysis was undertaken to elucidate the morphology of the prepared M-N-C materials, while their electrocatalytic performance was assessed through cyclic voltammetry and rotating disk electrode measurements in a 0.1 M KOH solution. All bimetallic catalyst materials demonstrated impressive bifunctional electrocatalytic performance in both the ORR and the OER. However, the FeNi-N-C catalyst proved notably more stable, particularly in the OER conditions. Employed as a bifunctional catalyst for ORR/OER within a customized zinc-air battery, FeNi-N-C exhibited a remarkable discharge-charge voltage gap of only 0.86 V, alongside a peak power density of 60 mW cm-2. The outstanding stability of FeNi-N-C, operational for about 55 h at 2 mA cm-2, highlights its robustness for prolonged application.

Transition-Metal Porphyrin-Based MOFs In Situ-Derived Hybrid Catalysts for Electrocatalytic CO2 Reduction

Excellent electrocatalytic CO2 reduction reaction activity has been demonstrated by transition metals and nitrogen-codoped carbon (M-N-C) catalysts, especially for transition-metal porphyrin (MTPP)-based catalysts. In this work, we propose to use one-step low-temperature pyrolysis of the isostructural MTPP-based metal-organic frameworks (MOFs) and electrochemical in situ reduction strategies to obtain a series of hybrid catalysts of Co nanoparticles (Co NPs) and MTPP, named Co NPs/MTPP (M = Fe, Co, and Ni). The in situ introduction of Co NPs can efficiently enhance the electrocatalytic ability of MTPP (M = Fe, Co, and Ni) to convert CO2 to CO, particularly for FeTPP. Co NPs/FeTPP endowed a high CO faradaic efficiency (FECOmax = 95.5%) in the H cell, and the FECO > 90.0% is in the broad potential range of -0.72 to -1.22 VRHE. In addition, the Co NPs/FeTPP achieved 145.4 mA cm-2 at a lower potential of -0.70 VRHE with an FECO of 94.7%, and the CO partial currents increased quickly to reach 202.2 mA cm-2 at -0.80 VRHE with an FECO of 91.6% in the flow cell. It is confirmed that Co NPs are necessary for hybrid catalysts to get superior electrocatalytic activity; Co NPs also can accelerate H2O dissociation and boost the proton supply capacity to hasten the proton-coupled electron-transfer process, effectively adjusting the adsorption strength of the reaction intermediates.

Coordination of copper within a crystalline carbon nitride and its catalytic reduction of CO2

Inherently disordered structures of carbon nitrides have hindered an atomic level tunability and understanding of their catalytic reactivity. Starting from a crystalline carbon nitride, poly(triazine imide) or PTI/LiCl, the coordination of copper cations to its intralayer N-triazine groups was investigated using molten salt reactions. The reaction of PTI/LiCl within CuCl or eutectic KCl/CuCl2 molten salt mixtures at 280 to 450 °C could be used to yield three partially disordered and ordered structures, wherein the Cu cations are found to coordinate within the intralayer cavities. Local structural differences and the copper content, i.e., whether full or partial occupancy of the intralayer cavity occurs, were found to be dependent on the reaction temperature and Cu-containing salt. Crystallites of Cu-coordinated PTI were also found to electrophoretically deposit from aqueous particle suspensions onto either graphite or FTO electrodes. As a result, electrocatalytic current densities for the reduction of CO2 and H2O reached as high as ∼10 to 50 mA cm-2, and remained stable for >2 days. Selectivity for the reduction of CO2 to CO vs. H2 increases for thinner crystals as well as for when two Cu cations coordinate within the intralayer cavities of PTI. Mechanistic calculations have also revealed the electrocatalytic activity for CO2 reduction requires a smaller thermodynamic driving force with two neighboring Cu atoms per cavity as compared to a single Cu atom. These results thus establish a useful synthetic pathway to metal-coordination in a crystalline carbon nitride and show great potential for mediating stable CO2 reduction at sizable current densities.

Nickel Single Atom Density-Dependent CO2 Efficient Electroreduction

The transition metal-nitrogen-carbon (M─N─C) with MNx sites has shown great potential in CO2 electroreduction (CO2RR) for producing high value-added C1 products. However, a comprehensive and profound understanding of the intrinsic relationship between the density of metal single atoms and the CO2RR performance is still lacking. Herein, a series of Ni single-atom catalysts is deliberately designed and prepared, anchored on layered N-doped graphene-like carbon (x Ni1@NG-900, where x represents the Ni loading, 900 refers to the temperature). By modulating the precursor, the density of Ni single atoms (DNi) can be finely tuned from 0.01 to 1.19 atoms nm-2. The CO2RR results demonstrate that the CO faradaic efficiency (FECO) predominantly increases from 13.4% to 96.2% as the DNi increased from 0 to 0.068 atoms nm-2. Then the FECO showed a slow increase from 96.2% to 98.2% at -0.82 V versus reversible hydrogen electrode (RHE) when DNi increased from 0.068 to 1.19 atoms nm-2. The theoretical calculations are in good agreement with experimental results, indicating a trade-off relationship between DNi and CO2RR performance. These findings reveal the crucial role of the density of Ni single atoms in determining the CO2RR performance of M─N─C catalysts.

Life cycle of single atom catalysts: a Mössbauer study on degradation and reactivation of tetrapyrrolic Fe-N-C powders

The degradation of a single-site atomically dispersed, model Fe-N-C powder catalyst with high activity is investigated using cryo-Mössbauer spectroscopy. The results indicate a degradation initiated by an Fe2+ to Fe3+ oxidation due to coordination of oxygen to tetrapyrrolic Fe-N4 sites at atmospheric conditions (change between characteristic doublets) before iron(III) oxide is formed (sextet). Thermal reactivation can be used to restore substantial catalytic activity of aged Fe-N-C powders.

Operando Spectroscopic Analysis of Axial Oxygen-Coordinated Single-Sn-Atom Sites for Electrochemical CO2 Reduction

Sn-based materials have been demonstrated as promising catalysts for the selective electrochemical CO₂ reduction reaction (CO₂RR). However, the detailed structures of catalytic intermediates and the key surface species remain to be identified. In this work, a series of single-Sn-atom catalysts with well-defined structures is developed as model systems to explore their electrochemical reactivity toward CO₂RR. The selectivity and activity of CO₂ reduction to formic acid on Sn-single-atom sites are shown to be correlated with Sn(IV)-N₄ moieties axially coordinated with oxygen (O-Sn-N₄), reaching an optimal HCOOH Faradaic efficiency of 89.4% with a partial current density (j_HCOOH) of 74.8 mA·cm^-2 at -1.0 V vs reversible hydrogen electrode (RHE). Employing a combination of operando X-ray absorption spectroscopy, attenuated total reflectance surface-enhanced infrared absorption spectroscopy, Raman spectroscopy, and ¹¹⁹Sn Mössbauer spectroscopy, surface-bound bidentate tin carbonate species are captured during CO₂RR. Moreover, the electronic and coordination structures of the single-Sn-atom species under reaction conditions are determined. Density functional theory (DFT) calculations further support the preferred formation of Sn-O-CO₂ species over the O-Sn-N₄ sites, which effectively modulates the adsorption configuration of the reactive intermediates and lowers the energy barrier for the hydrogenation of *OCHO species, as compared to the preferred formation of *COOH species over the Sn-N₄ sites, thereby greatly facilitating CO₂-to-HCOOH conversion.

Substituent Effects in Iron Porphyrin Catalysts for the Hydrogen Evolution Reaction

For a future hydrogen economy, non-precious metal catalysts for the water splitting reactions are needed that can be implemented on a global scale. Metal-nitrogen-carbon (MNC) catalysts with active sites constituting a metal center with fourfold coordination of nitrogen (MN₄ ) show promising performance, but an optimization rooted in structure-property relationships has been hampered by their low structural definition. Porphyrin model complexes are studied to transfer insights from well-defined molecules to MNC systems. This work combines experiment and theory to evaluate the influence of porphyrin substituents on the electronic and electrocatalytic properties of MN₄ centers with respect to the hydrogen evolution reaction (HER) in aqueous electrolyte. We found that the choice of substituent affects their utilization on the carbon support and their electrocatalytic performance. We propose an HER mechanism for supported iron porphyrin complexes involving a [Fe^II (P⋅)]^- radical anion intermediate, in which a porphinic nitrogen atom acts as an internal base. While this work focuses on the HER, the limited influence of a simultaneous interaction with the support and an aqueous electrolyte will likely be transferrable to other catalytic applications.

Highly selective CO2 electroreduction to CO by the synergy between Ni-N-C and encapsulated Ni nanoparticles

Efficient catalysts are highly desirable for the selective electrochemical CO₂ reduction reaction (CO₂RR). Ni single-atom catalysts are known as promising CO₂RR catalysts, while Ni NPs are expected to catalyze the competing HER. In this work, we have modified the Ni NPs by encapsulating them into porous Ni-N-C nanosheets (Ni@Ni-N-C), to boost the synergy between Ni NPs and dispersed Ni-N species towards CO₂RR. The CO faradaic efficiency (FE_CO) reached 96.4% at -0.9 V and retained over 90% in a wide potential window. More importantly, FE_CO values of over 94% have been obtained from -50 to -170 mA cm^-2 with a peak FE_CO of 99% in a flow cell. Our work demonstrates that the surface modification of Ni NPs can inhibit the unexpected HER and activate the surface sites, offering a practical design strategy for CO₂RR catalysts.

Si Doping-Induced Electronic Structure Regulation of Single-Atom Fe Sites for Boosted CO2 Electroreduction at Low Overpotentials

Transition metal-based single-atom catalysts (TM-SACs) are promising alternatives to Au- and Ag-based electrocatalysts for CO production through CO₂ reduction reaction. However, developing TM-SACs with high activity and selectivity at low overpotentials is challenging. Herein, a novel Fe-based SAC with Si doping (Fe-N-C-Si) was prepared, which shows a record-high electrocatalytic performance toward the CO₂-to-CO conversion with exceptional current density (>350.0 mA cm^-2) and ~100% Faradaic efficiency (FE) at the overpotential of <400 mV, far superior to the reported Fe-based SACs. Further assembling Fe-N-C-Si as the cathode in a rechargeable Zn-CO₂ battery delivers an outstanding performance with a maximal power density of 2.44 mW cm^-2 at an output voltage of 0.30 V, as well as high cycling stability and FE (>90%) for CO production. Experimental combined with theoretical analysis unraveled that the nearby Si dopants in the form of Si-C/N bonds modulate the electronic structure of the atomic Fe sites in Fe-N-C-Si to markedly accelerate the key pathway involving *CO intermediate desorption, inhibiting the poisoning of the Fe sites under high CO coverage and thus boosting the CO₂RR performance. This work provides an efficient strategy to tune the adsorption/desorption behaviors of intermediates on single-atom sites to improve their electrocatalytic performance.

Identification of the Catalytically Dominant Iron Environment in Iron- and Nitrogen-Doped Carbon Catalysts for the Oxygen Reduction Reaction

For large-scale utilization of fuel cells in a future hydrogen-based energy economy, affordable and environmentally benign catalysts are needed. Pyrolytically obtained metal- and nitrogen-doped carbon (MNC) catalysts are key contenders for this task. Their systematic improvement requires detailed knowledge of the active site composition and degradation mechanisms. In FeNC catalysts, the active site is an iron ion coordinated by nitrogen atoms embedded in an extended graphene sheet. Herein, we build an active site model from in situ and operando ⁵⁷Fe Mössbauer spectroscopy and quantum chemistry. A Mössbauer signal newly emerging under operando conditions, D4, is correlated with the loss of other Mössbauer signatures (D2, D3a, D3b), implying a direct structural correspondence. Pyrrolic N-coordination, i.e., FeN₄C₁₂, is found as a spectroscopically and thermodynamically consistent model for the entire catalytic cycle, in contrast to pyridinic nitrogen coordination. These findings thus overcome the previously conflicting structural assignments for the active site and, moreover, identify and structurally assign a previously unknown intermediate in the oxygen reduction reaction at FeNC catalysts.

Highly efficient and stable indium single-atom catalysts for electrocatalytic reduction of CO2 to formate

Indium single-atom catalysts display a large total current density (38.94 to 81.08 mA cm^-2) over an extensive potential window (-0.91 to -1.41 V vs. RHE) for electrocatalytic CO₂ reduction to formate with high selectivity (85.2% faradaic efficiency at -1.31 V).

Control over Electrochemical CO2 Reduction Selectivity by Coordination Engineering of Tin Single-Atom Catalysts

Carbon-based single-atom catalysts (SACs) with well-defined and homogeneously dispersed metal-N4 moieties provide a great opportunity for CO2 reduction. However, controlling the binding strength of various reactive intermediates on catalyst surface is necessary to enhance the selectivity to a desired product, and it is still a challenge. In this work, the authors prepared Sn SACs consisting of atomically dispersed SnN3 O1 active sites supported on N-rich carbon matrix (Sn-NOC) for efficient electrochemical CO2 reduction. Contrary to the classic Sn-N4 configuration which gives HCOOH and H2 as the predominant products, Sn-NOC with asymmetric atomic interface of SnN3 O1 gives CO as the exclusive product. Experimental results and density functional theory calculations show that the atomic arrangement of SnN3 O1 reduces the activation energy for *COO and *COOH formation, while increasing energy barrier for HCOO* formation significantly, thereby facilitating CO2 -to-CO conversion and suppressing HCOOH production. This work provides a new way for enhancing the selectivity to a specific product by controlling individually the binding strength of each reactive intermediate on catalyst surface.

Sulfur-Dopant-Promoted Electroreduction of CO2 over Coordinatively Unsaturated Ni-N2 Moieties

Atomically dispersed nickel-nitrogen-carbon (Ni-N-C) moieties are promising for efficient electrochemical CO₂ -to-CO conversion. To improve the intrinsic electrocatalytic activity, it is essential but challenging to steer the coordination environment of Ni centers for promoting the CO formation kinetics. Here, we introduce alien sulfur atoms to tune the local electronic density of unsaturated NiN₂ species. A coordinated structure evolution is detected and S vacancies are generated at high overpotentials, as confirmed by X-ray absorption spectroscopy. The sulfur dopants enhance CO selectivity and activity over normal unsaturated NiN₂ structure, reaching a high CO Faradaic efficiency of 97 % and a large CO current density of 40.3 mA cm^-2 in a H-cell at -0.8 V and -0.9 V (vs. RHE), respectively. DFT calculations reveal both doped S atoms and evolved S vacancies in the NiN₂ coordination environment contribute to the reduced energy barriers for CO₂ electroreduction to CO.