Hierarchically nanostructured Ni(Mo,Co)-WOx electrocatalysts for highly efficient urea electrolysis

Abstract

Urea electrolysis (UE) represents a promising path for hydrogen production as an alternative to water electrolysis with a low thermodynamic potential of 0.37 V and the additional function of urea pollutant degradation. However, the sluggish kinetics caused by the six-electron processes increase the barrier energy for the urea oxidation reaction (UOR). In this work, hierarchical NiMo-WOx and NiCo-WOx nanowires are newly synthesized as hydrogen evolution reaction (HER) and UOR electrocatalysts, respectively, via a facile two-step process involving thermal evaporation and electrodeposition. Due to the higher surface exposure of active sites and greater electrical conductivity of the hierarchical structure, NiMo-WOx exhibits high HER activity with an overpotential of 17 mV at 10 mA cm-2 and a Tafel slope of 87 mV dec- 1, which is similar to the activity of a Pt. Hierarchical NiCo-WOx exhibits high UOR activity, requiring only 1.38 V to drive 100 mA cm-2, which is 0.28 V lower than the oxygen evolution reaction (1.69 V). Assembling NiMo-WOx and NiCo-WOx as the HER and UOR electrodes, respectively, for UE requires a potential of only 1.39 V for 10 mA cm-2 and a Faradaic efficiency of 98.7 %. An unassisted hydrogen production system is also demonstrated by combining UE and perovskite photovoltaic cells, producing a solar-to-hydrogen efficiency of 11 %.