The polyoxometalates mediated preparation of phosphate-modified NiMoO_4-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%) NiMoO_4-x net-like nanostructures are formed on Ni foam (NF) via H₃PMo₁₂O₄₀ etching strategy at room temperature (denoted as NF/P-NiMoO_4-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-NiMoO_4-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.