Hetero-interface triggering dynamic Ni(3+) active species in reduced Ru-doped NiCo-MOF to boost urea-assisted overall water splitting

Abstract

Urea-assisted water electrolysis is a significant and sustainable strategy to optimize electrolyte system to promote the efficiency of hydrogen producing in overall water splitting. Here we construct excellent catalysts (NiCoRu-X) by introducing Ru into reduced dual metal-organic frameworks (MOFs) via a mild chemical reduction method, which preserves multi hetero-interfaces that are spontaneously formed. Interestingly, excellent performances in urea oxidation reaction (UOR) can be observed on all NiCoRu-X samples but without obvious difference in the activity, while the optimal catalyst can be distinguished by testing their catalytic activity in hydrogen evolution reaction (HER). Dependent density functional theory (DFT) calculations are applied to identify the potential activity sites of different heterostructure in NiCoRu-X, and reveal that the hetero-interface promotes the formation of dynamic Ni³⁺ actives like NiOOH, which benefit for UOR. In the urea-assisted electrolytes, the reaction potential difference by coupling the LSV curves of HER and UOR is 155 mV lower than that in the electrolyte without urea at 20 mA cm⁻², and 70 mV lower than that in overall water splitting. It reaches a high faraday efficiency (FE) of 98.5 % at 2.0 V in urea-assistant water electrolysers, higher than that without urea assistant (92.9 %). This work highlights the possibility of using urea oxidation to decrease the reaction overpotential and to further improve the efficiency of producing hydrogen in overall water splitting by introducing urea in electrolyte system.