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 CO 2 to value-added chemical intermediates can offer a sustainable route for the conversion of CO 2 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 CO 2 reduction reaction (CO 2 RR) 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 KHCO 3, K 2 CO 3, 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 H 2 O 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 CO 2 reduction. The simulations indicate that the first electron transfer step to CO 2 to form the adsorbed *CO 2 (•−) 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 CO 2, thus facilitating the rate-determining adsorption and electron transfer to CO 2 to form the adsorbed *CO 2 (•−) radical anion. Increasing the concentration of the K + OH – electrolyte reduces the barrier for the electrocatalytic reduction of CO 2 and thus improves the current density, consistent with the reported experimental results. The *CO 2 (•−) 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 *CO 2 (•−) 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 (HCO 3 – ) 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 +, H 2 O, and reaction intermediates on the Au surface in the electric double layer and their influence on the onset potential, electrocatalytic activity, and selectivity for CO 2 RR.

Topics & Concepts