Theoretical Insights into the Effects of KOH Concentration and the Role of OH^– in the Electrocatalytic Reduction of CO₂ on Au

The active and selective electrochemical reduction of CO2 to value-added chemical intermediates can offer a sustainable route for the conversion of CO2 to chemicals and fuels, thus helping to mitigate greenhouse gas emissions and enabling intermittent energy from renewable sources. Alkaline solutions are often the preferred media for the electrocatalytic CO2 reduction reaction (CO2RR) as they provide high current densities and low overpotentials while suppressing the hydrogen evolution side reaction. Recent experiments carried out on Au and Ag in KOH, as well as other electrolytes, including KHCO3, K2CO3, and KCl, showed that increasing electrolyte concentration lowered onset potentials, increased Faradaic efficiencies to CO, and improved current densities. Herein, we carry out potential-dependent ab initio molecular dynamic (AIMD) simulations along with density functional theory (DFT) calculations using explicit KOH electrolyte and H2O solution molecules to examine the influence of OH– anions and the KOH electrolyte on the elementary steps and their corresponding energetics in the mechanism for CO2 reduction. The simulations indicate that the first electron transfer step to CO2 to form the adsorbed *CO2(•−) radical anion is rate-limiting, while the subsequent proton and electron transfer steps are facile and downhill in energy at reducing potentials. The OH– anions present in the solution can adsorb on the Au cathode down to potentials as low as ∼ −3 V (SCE). This enables the OH– anions to transfer electrons to the Au cathode and into antibonding 2π* orbitals of CO2, thus facilitating the rate-determining adsorption and electron transfer to CO2 to form the adsorbed *CO2(•−) radical anion. Increasing the concentration of the K+OH– electrolyte reduces the barrier for the electrocatalytic reduction of CO2 and thus improves the current density, consistent with the reported experimental results. The *CO2(•−) radical anion that forms subsequently undergoes facile proton transfer from a vicinal water molecule in solution to form the hydroxy carbonyl (*HOCO) intermediate that readily undergoes subsequent proton and electron transfer from a second water molecule to form CO and OH– at a potential of ∼ −1.2 V SCE. While the formation of formate (HCOO–) is thermodynamically favorable, the direct hydrogenation of *CO2(•−) as well as the intramolecular proton transfer via *HOCO to form HCOO– are kinetically unfavored. The presence of OH– anions near the surface also facilitates the formation of bicarbonate (HCO3–) at lower potentials. The bicarbonate that forms can be converted to the reactive *HOCO intermediate at more negative potentials that subsequently reacts to form CO and regenerate OH–. The results discussed herein help provide a more detailed understanding of the interplay between the OH–, K+, H2O, and reaction intermediates on the Au surface in the electric double layer and their influence on the onset potential, electrocatalytic activity, and selectivity for CO2RR

Nanoporous Copper–Silver Alloys by Additive-Controlled Electrodeposition for the Selective Electroreduction of CO₂ to Ethylene and Ethanol

Electrodeposition of CuAg alloy films from plating baths containing 3,5-diamino-1,2,4-triazole (DAT) as an inhibitor yields high surface area catalysts for the active and selective electroreduction of CO₂ to multicarbon hydrocarbons and oxygenates. EXAFS shows the co-deposited alloy film to be homogeneously mixed. The alloy film containing 6% Ag exhibits the best CO₂ electroreduction performance, with the Faradaic efficiency for C₂H₄ and C₂H₅OH production reaching nearly 60 and 25%, respectively, at a cathode potential of just −0.7 V vs RHE and a total current density of ∼ – 300 mA/cm². Such high levels of selectivity at high activity and low applied potential are the highest reported to date. <i>In situ</i> Raman and electroanalysis studies suggest the origin of the high selectivity toward C₂ products to be a combined effect of the enhanced stabilization of the Cu₂O overlayer and the optimal availability of the CO intermediate due to the Ag incorporated in the alloy

Elucidation of Critical Catalyst Layer Phenomena toward High Production Rates for the Electrochemical Conversion of CO to Ethylene

This work utilizes EIS to elucidate the impact of catalyst–ionomer interactions and cathode hydroxide ion transport resistance (RCL,OH–) on cell voltage and product selectivity for the electrochemical conversion of CO to ethylene. When using the same Cu catalyst and a Nafion ionomer, varying ink dispersion and electrode deposition methods results in a change of 2 orders of magnitude for RCL,OH– and ca. a 25% change in electrode porosity. Decreasing RCL,OH– results in improved ethylene Faradaic efficiency (FE), up to ∼57%, decrease in hydrogen FE, by ∼36%, and reduction in cell voltage by up to 1 V at 700 mA/cm2. Through the optimization of electrode fabrication conditions, we achieve a maximum of 48% ethylene with >90% FE for non-hydrogen products in a 25 cm2 membrane electrode assembly at 700 mA/cm2 and RCL,OH– is translated to other material requirements, such as anode porosity. We find that the best performing electrodes use ink dispersion and deposition techniques that project well into roll-to-roll processes, demonstrating the scalability of the optimized process