Deciphering mass transport behavior in membrane electrode assembly by manipulating porous structures of atomically dispersed Metal-Nx catalysts for High-Efficiency electrochemical CO2 conversion.

[Display omitted] • A systematic study for the effect of porous structures on CO production was performed. • Different performance trends between H-cell and full cell analyses were observed. • A thin catalyst layer possessing mesopores promoted CO production in MEA condition. • The optimized catalyst exhibited the highest energy efficiency (55 %) at 265 mA cm−1. The introduction of a porous structure is a promising approach to promote the electrochemical reaction of catalysts, which can maximize the utilization of catalytic active sites and enhance mass transport. To fully understand the role of the porous structure, parallel studies on both half-cell and full-cell environments must be performed; however, few studies have reported electrochemical CO 2 conversion in a full-cell operation. In this work, we fabricated four types of porous Ni N C model catalysts designed to systematically investigate the relationship between porous structures and catalytic performances in a membrane electrode assembly (MEA) based catholyte-free CO 2 electrolyzer. The performance degradation of the microporous catalyst in the MEA resulted from low CO 2 accessibility due to small openings (<2 nm), and the absence of meso- or macropores that can facilitate mass transport in the catalyst layer. A thick catalyst layer developed a region in which H 2 evolution was dominant; the formation of this region degraded the CO 2 reduction efficiency in the MEA based on the macroporous catalyst. Consequently, mesoporous Ni-N x catalysts with small, uniform particles exhibited the highest efficiency in MEAs, because their appropriate pore size and catalyst layer thickness facilitated mass transport. The optimized catalyst achieved industry-relevant performance for CO production (265 mA cm−2 at 2.3 V) with a state-of-the-art energy efficiency of 55 % and excellent long-term stability in a full cell. [ABSTRACT FROM AUTHOR]