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

Abstract

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.

Tip‐like Fe−N4 Sites Induced Surface Microenvironments Regulation Boosts the Oxygen Reduction Reaction

“…The OH À adsorbed on the curved structure surface can transfer electrons to FeN 4 sites, promoting the adsorption and activation of O 2 . [30] As for OER, local high hydroxide concentration in the catalyst surface and inner channels could realize enhanced OER activity. [8b] The surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS) was performed to further investigate the water structure change.…”

Gas Diffusion Electrodes (GDEs) for Carbon Dioxide (CO2) Reduction in Microfluidic Cells: Towards a Fluid Dynamics Assisted Rational Design

“…[101][102][103] Numerous mechanistic explanations of this effect have arisen with authors proposing a local pH influence of the cation near the surface of the catalyst, 104,105 the repulsive cation-cation interaction with the surface at the outer Helmholtz plane (OHP) that modifies the interfacial electric field and the corresponding charge density, 25,58,106 or the direct stabilization through coordination of the cation with intermediates. 107,108 To date, agreement has not been reached on which of these models ultimately explains the experimental observations.…”

“…Thenceforth, the selectivity of the catalyst for a certain product in the CO 2 RR can give insight to the possible intermediates involved and it is a first step to elucidate the most likely mechanism involved. However, a multitude of products from one carbon (C 1 ) species (like methane 109,110 and methanol 111,112 ) to C 2+ species (like ethene, ethane [113][114][115] and ethanol 116,117 ) can be produced depending on: (i) intrinsic properties of the catalyst (e.g., gold usually generates C 1 products due to the ease of desorption that CO has on the surface of the metal); 108,118 (ii) the crystalline structure of the catalyst (e.g., some facets of copper-based catalysts have shown preference towards formation of selected C 2+ products 113,117,119 ), and (iii) experimental conditions (e.g., the identity of the cation and anion in the electrolyte 58,108 or the presence of auxiliary reactants that have shown improvement in the reaction rates and the selectivity of the CO 2 RR 110 ). Therefore, due to the varied nature of factors that influence the CO 2 RR in different electrochemical systems, the simple identification of reaction products and selectivity does not give sufficient evidence to propose a reliable reaction mechanism.…”

“…This approach allowed to systematically investigate the role and influence of the potential configurations that the OHanion could adopt within the mechanism. 108 While AIMD is a powerful tool for the analysis of interface reactions as shown previously, it heavily favours the stabilization of the intermediates through direct interaction since calculations tend to focus in searching for a minimum geometry configuration to analyse the thermodynamics of each step. In addition, this approach only allows following the system for a short time (in the range of picoseconds, with timesteps of femtoseconds) with relatively small amounts of solvent.…”

Solvothermal Preparation of Oxygen-Vacancy-Rich Nano-ZnO for Electrocatalytic CO2 Reduction in a Flow Cell

“…On the other hand, a relatively weak binding energy favors the dissociation of *CO resulting in the generation of CO. Therefore, designing catalysts with a weak binding affinity for *CO is paramount in the electrocatalytic reduction of CO 2 to CO. Noble metal-based catalysts such as Au, Ag, and Pd have strong interactions with CO 2 but weak coordination with *CO, allowing highly selective electrocatalytic CO 2 RR toward CO. − However, the high cost and limited reserve restrict their industrial application. In addition to the abundance and environmentally friendly characteristics, zinc-based catalysts are a potential alternative considering their excellent CO 2 activation capability.…”