Interfacial design strategies for selective electrocatalytic CO2 reduction through control of proton coupled electron transfer

The aim of this project was to construct and study a novel copper-based electrocatalyst embedded in a microenvironment with well-defined hydrophobicity to control proton activity and improve CO2 mass transport to modify the selectivity of the CO2 reduction reaction (CO2RR). Specifically, anodized aluminium oxide (AAO) membranes were used to grow and confine Cu nanowires (CuNWs) and the hydrophobicity of the CuNW-AAO composite electrode was increased by reaction with various silane molecules. Uniformly distributed CuNWs were successfully prepared in a commercial AAO porous membrane by square-wave pulsed electrodeposition. The length and width of CuNWs can be controlled by adjusting deposition time and applied current density. Analysis of the electrochemical behaviour of the composite CuNW-AAO electrode using capacitance measurements and a methylviologen redox probe, shows that the electrochemical surface area (ECSA) and roughness factor are reflective of electrolyte penetration throughout the CuNW-AAO electrode, supporting electrocatalysis along the length of the CuNW’s surrounded by AAO. Next, the hydrophobicity of the CuNW-AAO electrode was increased by reaction with various silane molecules. Contact angle measurements were used to quantify hydrophobicity as a function of silane and study the stability of the silane coating to electrolyte and under electrochemical conditions. Analysis showed that silane modified CuNW-AAO electrodes maintain large ECSA and roughness factor, and that addition of the silane coating etches the CuNWs surface, presumably via generation of HCl, increasing the electrode surface roughness. Control experiments using Cu foil electrodes exposed to silane, show that Cu can also be directly modified with silane. CO2RR reactions of silane modified Cu foil electrodes, show promotion of CO2 to C2+ product. Contact angle measurements after reaction showed a reduction in hydrophobicity due to partial loss of the silane coating. CO2RR using silane modified and unmodified CuNW-AAO electrodes resulted in mechanical instability due to hydrogen bubble formation behind the AAO membrane.