Unprecedented urea oxidation on Zn@Ni-MOF with an ultra-high current density: understanding the competition between UOR and OER, catalytic activity limitation and reaction selectivity

Abstract

Urea oxidation reaction (UOR) has been extensively studied as an alternative to the sluggish oxygen evolution reaction (OER) for energy-efficient hydrogen generation. However, the detrimental competition between the UOR and OER limits the UOR current density to less than 500 mA cm⁻² and ultimately switches the reaction toward the OER. In this study, we attempted to gain a fundamental understanding of the catalytic activity limitation for the UOR and the possible factors influencing the reaction selectivity employing Ni-MOF as an example. The study showed that upon doping the Ni-lattices of the framework with Zn, the factors influencing the detrimental competition, such as the mass and charge transport ability of the MOF catalyst could be enhanced and the formation of the catalytically active Ni³⁺-OOH phase could be accelerated. This populated Zn@Ni-MOF with Ni³⁺-OOH sites, and subsequently prevented the detrimental competition between the anodic reactions. Consequently, Zn@Ni-MOF demonstrated an outstanding ultra-high UOR current density of 1780 mA cm⁻² at a low electrode potential of 1.52 V vs. RHE and the benchmark current density of 10 and 100 mA cm⁻² at a lower electrode potential of 1.31 and 1.32 V vs. RHE, respectively, hence outperforming most of the high-performance UOR catalysts.