Effect of electrochemical surface area on carbon dioxide electrolysis using anionic electrolyte membrane electrode assembly

•The ECSA using metal UPD or bilayer capacitance was evaluated.•The ECSA area does not always correlate with particle size and cell performance.•Graphitization decreased the ECSA and affected larger than the hydrophilization. Synthesizing useful chemicals by electrolysis of carbon dioxide (CO2) is considered a crucial strategy for recycling CO2 in a carbon–neutral and green manner. However, generally, the catalysts used for CO2 electrolysis have low activity, stability, and product selectivity. Herein, we evaluated the effect of the surface area, determined by electrochemical methods, on the performance of a CO2 electrolysis cell using an anionic electrolyte membrane electrode assembly (MEA). The electrochemical surface area (ECSA) of the active metals supported on the carbon of the cathode side catalyst layer was analyzed via copper underpotential deposition (Cu-UPD) and cyclic voltammetry (CV). The ECSA was multiplied by a factor of approximately 0.6 when the particle size was increased from 2 nm to 4 nm. However, it increased by a factor of approximately 2 for Cu-UPD and no change for CV when particle size changed from 4 nm to 12 nm. The performance was evaluated using a CO2 electrolysis zero-gap cell with an anionic electrolyte MEA. The ECSA was correlated to Faraday's efficiency in CO and H2 production during CO2 electrolysis. However, the ECSA measured by the electrical double-layer capability showed a lower correlation with Faraday efficiency when particle size changed from 4 nm to 12 nm. The findings obtained in this study provide guidelines for improving the catalytic performance of CO2 electrolysis cells using anionic electrolyte MEAs, and these results will promote efforts toward achieving carbon neutrality.