Electro-synthesis of tungsten carbide containing catalysts in molten salt for efficiently electrolytic hydrogen generation assisted by urea oxidation

Abstract

Energy-efficient production of hydrogen through urea electrolysis is still challenging due to the lack of satisfactory catalysts for urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) in urea containing solution. In this study, Ni–WxC/C (x = 1,2) composite with high activity for urea electrocatalysis was prepared by direct electro-reduction of affordable feedstock of NiO–CaWO4–C in molten CaCl2–NaCl at 873–973 K. The addition of graphite in precursor decreases the particle size of Ni. Introducing WxC into Ni particles can reduce the overpotential for UOR. As a result, the obtained Ni-WxC/graphite composite exhibits high current density for urea oxidation, which is about 11-folds and 52-folds higher than that of Ni/graphite and Ni (@1.53 V vs. RHE), respectively. After changing the carbon source from graphite to CNTs, the anodic current density was further increased by 43%, reaching 50.31 mA cm−2. Moreover, the cathodic catalyst WxC/CNTs obtained by the same preparation process exhibits high performance towards HER, with a low onset potential of 131.5 mV and a Tafel slope of 69.5 mV dec−1. Assembling an electrolyzer using Ni-WxC/CNTs as anode and WxC/CNTs as cathode can yield a current density of 10 mA cm−2 at merely 1.65 V in 1 M KOH/0.33 M urea aqueous solution, with excellent long-term electrochemical durability. The environmental-friendly production process uses affordable feedstocks for the synthesis of efficient catalysts toward urea electrolysis, promising an energy-saving hydrogen production as well as waste treatment.