The polyoxometalates mediated preparation of phosphate-modified NiMoO4−x with abundant O-vacancies for H2 production via urea electrolysis

It is urgent to develop non-noble metal electrocatalysts with both excellent activity and durable stability for H2 production via water electrolysis. Electric energy is mainly consumed by the sluggish anodic oxygen evolution reaction (OER). The electrocatalytic urea oxidation reaction (UOR) has been regarded as a promising reaction to replace OER because of its small thermodynamic oxidation potential. However, developing a facile and large-scale preparation method for bifunctional hydrogen evolution reaction (HER) and UOR electrocatalysts is still challenging. Herein, phosphate-modified (4.46 atomic%) NiMoO4-x net-like nanostructures are formed on Ni foam (NF) via H3PMo12O40 etching strategy at room temperature (denoted as NF/P-NiMoO4-x). The etched NF can directly serve as HER electrode, and delivers overpotential of 116 mV at current density of 10 mA/cm2 with Tafel slope of 77.5 mV/dec. Furthermore, it displays excellent UOR activity with potential of 1.359 V at current density of 10 mA/cm2 and Tafel slope of 19.3 mV/dec. The apparent activation energy of NF/P-NiMoO4-x is 20.6 kJ/mol, lower than that of NF (37.7 kJ/mol), indicating smaller apparent barrier for CN bond cleavage in urea. The cell voltage of urea electrolysis is around 1.48 V for H2 production to deliver current density of 10 mA/cm2, and better long-term stability for 50 h than that of Ir/C||Pt/C. The etching solution can be recycled for five times by addition of H2O2, turning heteropoly blue into its original state. This work develops a facile and large-scale method to prepare bifunctional HER and UOR electrocatalysts for H2 production in a less-energy saving way via urea electrolysis.