Electrochemical CO₂ Reduction at Silver from a Local Perspective

Abstract

The electrochemical reduction of CO2 to chemical fuels and value-added chemicals is a viable pathway to store renewably generated electrical energy and to mitigate the negative impact of anthropogenic CO2 production. Herein, we study how the local reaction environment dictates the mechanism and kinetics of CO2 reduction to CO at an Ag electrode. The local reaction environment is determined using a hierarchical model that accounts for multistep reaction kinetics, specific surface charging state at a given electrode potential, and mass transport phenomena. The model reveals vital mechanistic insights into the reaction behavior. The increasing Tafel slope with overpotential is seen to be influenced by the surface charging relation and mass transport effects. In addition, the decrease of the CO current density at high overpotentials is found to be caused not only by the decrease in CO2 concentration due to mass transport, surface charge effects, and pH increase but also by lateral interactions between HCOOad, COOHad, and Had. Moreover, we explore how the electrolyte properties, including bicarbonate concentration, solvated cation size, and CO2 partial pressure, tune the local reaction environment.

Mechanistic Insight into Formic Acid/Formate Oxidation at the Au(111) Electrode: Implications from the pH Effect and H/D Kinetic Isotope Effect

“…The potential plateau in the E –pH s plots only exists in the range of 5 < pH s < 6 (Figure c), instead of what covers a range of 5 < pH < 10 before correction (Figure a). The apparent volcano-shaped j –pH b plot with a plateau on top of the volcano in the range of 6 < pH b < 10 experimentally observed in solutions free of buffer is a false impression, and all of the above indicate that the surface charge effect or poisoning effect is not the key factor, which leads to the apparent plateau in the bulk pH range of 6–10, as suggested recently. ,, This is also supported by the fact that with the increase of solution pH b , the onset potential for the anion adsorption shifts positively; as a result, the poisoning effect of such anions should be more pronounced in solutions of lower pH b , provided that other conditions are identical. , …”

“…The apparent volcano-shaped j−pH b plot with a plateau on top of the volcano in the range of 6 < pH b < 10 experimentally observed in solutions free of buffer is a false impression, and all of the above indicate that the surface charge effect or poisoning effect is not the key factor, which leads to the apparent plateau in the bulk pH range of 6−10, as suggested recently. 15,24,29 This is also supported by the fact that with the increase of solution pH b , the onset potential for the anion adsorption shifts positively; as a result, the poisoning effect of such anions should be more pronounced in solutions of lower pH b , provided that other conditions are identical. 30,31 On the other hand, after the correction of pH s drift, the intrinsic pH dependence of the peak current for FAOR still shows a volcano shape, although the width of the plateau narrows to a range of 5 < pH s < 6 (Figure 3d).…”

“…As for the parameter β, it is a phenomenological parameter that represents the effect of electrostatic interaction between the charged adsorbate and the electrode to the reaction energy barrier. , To properly choose the value of β and make a better connection between the β and microstructural parameters, the contribution of electrostatic interaction between the charged adsorbate and the electrode to the reaction energy barrier will be discussed in detail using the Bronsted–Evans–Polanyi (BEP) relation in the following.…”

Electrolyte Effects on CO2 Electrochemical Reduction to CO

“…Multiscale modeling by Ringe et al elucidated this duality in the mechanism interpretation in terms of the variations in the adsorption energy of reaction intermediates as a function of the applied potential. 17 This was further supported by Zhu et al 35 The potential shift of the RDS from PCET to ET may also clarify why at a less negative potential (−0.78 Vvs SHE) a reaction order in bicarbonate of ∼1 was measured on oxide-derived Au nanoparticles. 36 The difficulty in measuring the intrinsic kinetics of CO2RR resides not only in its potential dependence but also in the convolution with mass transport limitations, 2,17,35 hindering the identification of AH in the electroreduction of CO 2 .…”

Size-modified Poisson-Nernst-Planck approach for modeling a local electrode environment in CO2 electrolysis

“…CO 2 reduction on an In catalyst results in the formation of HCOO − and trace amounts of CO. The first step of the reduction process, namely the interfacial charge transfer reaction to form the intermediate radical anion (the rate determining step) depends on the CO 2 concentration at the reaction plane (OHP) [46]. Hence, if the current densities for these products match the experimental data, this would suggest an accurate estimation of CO 2 concentration.…”