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

Abstract

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 un-derstand 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 CO2 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 CO2 electrolyzer. The performance degradation of the microporous catalyst in the MEA resulted from low CO2 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 H2 evolution was dominant; the formation of this region degraded the CO2 reduction efficiency in the MEA based on the macroporous catalyst. Consequently, mesoporous Ni-Nx 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.