Understanding the Effects of Anion Interactions with Ag Electrodes on Electrochemical CO₂ Reduction in Choline Halide Electrolytes

Abstract

Interactions of electrolyte ions at electrocatalyst surfaces influence the selectivity of electrochemical CO2 reduction (CO2R) to chemical feedstocks like CO. We investigated the effects of anion type in aqueous choline halide solutions (ChCl, ChBr, and ChI) on the selectivity of CO2R to CO over an Ag foil cathode. Using an H‐type cell, we observed that halide‐specific adsorption at the Ag surface limits CO faradaic efficiency (FECO) at potentials more positive than −1.0 V vs. reversible hydrogen electrode (RHE). At these conditions, FECO increased from I−<Br−<Cl−, that is, in the opposite order to the strength of specific adsorption of the halide ions (Cl−<Br−<I−). At potentials of −1.0 to −1.3 V vs. RHE, restructuring of the Ag surface in ChI and ChCl via dissolution and re‐electrodeposition led to more CO‐selective Ag facets ([220], [311], and [222]) than in ChBr. This mechanism allowed very high faradaic efficiencies for CO of 97±2 % in ChI and 94±2 % in ChCl to be achieved simultaneously with high current densities at −1.3 V vs. RHE. We also demonstrate that high selectivity to CO (FECO>90 %) in ChCl (at −0.75±0.06 Vvs. RHE) and ChI (at −0.78±0.17 V vs. RHE) could be achieved at a current density of 150 mA cm−2 in a continuous flow‐cell electrolyser with Ag nanoparticles on a commercial gas diffusion electrode. This study provides new insights to understand the interactions of anions with catalysts and offers a new method to modify electrocatalyst surfaces.

The role of electrode wettability in electrochemical reduction of carbon dioxide

“…This description originates from (1) the fact that CO2RR requires the CO2 and protons as reactants and a sink for liquid products and (2) the common observation of enhanced CO2RR reactivity and selectivity over CLs with a moderate increase of hydrophobicity. [126][127][128][129][130] For example, Shi et al 137 Nevertheless, this work is a step forward in understanding the ideal wetting conditions of CL for CO2RR experimentally. Nevertheless, the triple-phase boundaries' role in CO2RR or even the TPB locations in the GDEs (i.e., either within the CL or at the CL -GDL interfaces) are not explicitly described.…”

“…Section 3.1 provides a detailed discussion of the effects of liquid properties such as pH and ion types on the electrode wetting conditions. Additionally, an electric field can effect surface chemistry 125,126 and morphology [127][128][129][130][131][132] of the solid phase, which will impact wetting conditions. Yang et al's 75 reported such effects observed in experiments with X-ray photoelectron spectra to detect the loss of C-F bonds and increase in the concentration of oxygen species at the GDL surface after exposure to cathodic conditions, and this effect may explain the loss of the wettability in the CLs.…”

“…These trade-offs are usually responsible for the observations that a moderate level of hydrophobicity is usually optimal for CO2RR. 127,136,145 In summary, we envisage the ideal wetting conditions at the CL should allow fast liquid and electron transport, maximize CO2RR-active and selective sites and optimize catalyst local environments to thereby achieve a CO2RR with low overpotential and high product selectivity and stability at commercially relevant current densities. This means that the electrode structure should be precisely constructed with a combination of the materials with different wettability and functionality, and provide multiple transport pathways to maintain a sufficient supply of gaseous CO2, protons, and electron, and shortened travel distances of CO2, electrolyte, and products (i.e., OHand CO2RR products) in the liquid phase.…”

“…This effect has been widely exemplified by the enhanced hydrophilicity for carbon surfaces with a high oxygen coverage. 127,173 Organic additives -binders and ionomers Fluorinated polymers such as PTFE 74,76,174,175 and fluorinated silane 137 are the most commonly used materials to increase the hydrophobicity of GDLs and CLs because of their high hydrophobicity and chemical stability. Its high hydrophobicity originates from the fluorine's low polarizability and low London dispersion force, and its high chemical stability arises from fluorine's high electronegativity that makes C -F bonds strong.…”

Effects of the Catalyst Dynamic Changes and Influence of the Reaction Environment on the Performance of Electrochemical CO2 Reduction

“…introduced different halide solutions as the catholyte for CO 2 R toward CO over polycrystalline Ag foil. [ 197 ] It turned out that the FE of CO increases in the order of Cl − > Br − > I − at the potential window of > −1.0 V versus RHE. Besides the electronic effect, halide ions can also induce significant surface restructuring to the metal surfaces during CO 2 R, and further lead to changes in the reaction activity and selectivity.…”