Oxidized Copper and Molybdenum Species Exclusively Boosting Electrocatalytic Hydrogen Evolution in Non-Extreme pH Carbonate Buffer Electrolyte

Abstract

Toward large-scale deployment of water electrolysis, non-extreme pH solution is a promising reaction medium because its mild condition can avoid the use of expensive corrosion-tolerant materials in the electrolyzer. In these electrolytes, however, the electrocatalytic performance was lower than the extreme pH counterparts. This study reports on oxidized copper and molybdenum species that catalyze the hydrogen evolution reaction (HER) in non-extreme pH carbonate buffer electrolytes. Electrocatalytic testing in the carbonate buffer electrolyte revealed that Cu addition reduced the overpotential at −1 A cm −2 by ca. 100 mV from the benchmark nickel−molybdenum oxide (NiMoO x ). Combined with a nickel−ironbased anode, overall water electrolysis was demonstrated in the dense carbonate buffer electrolyte at a non-extreme pH of 10.5, which exhibited a cell performance competitive with those of top-class commercial alkaline electrolyzers. Subsequent analysis of NiMoO x (Cu) and MoO x (Cu) elucidated that the improvements in the performance are due to the enlarged surface area, enhanced hydrophilicity, and presence of oxidized copper and molybdenum species during the HER. Notably, MoO x (Cu) required large overpotentials at alkaline pH where the oxidized copper was easily reduced, indicating the significant role of oxidized copper in non-extreme pH HER. Our findings illustrate the significance of catalyst design integrated with electrolyte conditioning and represent the potential for the non-extreme pH water electrolyzer on an industrial scale.