How increasing proton and electron conduction benefits electrocatalytic CO2 reduction

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<h2>Summary</h2> 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.