How increasing proton and electron conduction benefits electrocatalytic CO2 reduction

The renewably powered electrochemical conversion of CO2 into fuels and chemicals offers a pathway to decarbonized industrial processes that are difficult to abate with electricity alone. Electrocatalytic activity, selectivity, reaction kinetics, and stability are critical factors for the development of electrochemical CO2 reduction technology. While the activity and selectivity of various electrocatalysts have been reported in a number of publications, the understanding of kinetic issues of mass and charge transport from a perspective of materials science and engineering is lacking. This work covers the critical role of the triple-phase boundary of CO2, protons, and electrons in CO2 conversion. Proton and electron conduction in electrocatalysts are discussed as composite materials and single-phase materials, respectively. Relatively established composite proton- and electron-conducting electrocatalysts (PECEs) are reviewed from state-of-the-art research, and suggestions are given for further potential improvement. In contrast, as the development of single-phase PECEs for electrochemical CO2 reduction is still nascent, perspectives for future development are proposed. Metal-organic framework materials are suggested and discussed as one of the ideal material candidates for single-phase PECEs with strategies to increase proton and electron conductivity.