Heterostructured Ni₃S₂–Ni₃P/NF as a Bifunctional Catalyst for Overall Urea–Water Electrolysis for Hydrogen Generation

Urea oxidation reaction (UOR) has been proposed to replace the formidable oxygen evolution reaction (OER) to reduce the energy consumption for producing hydrogen from electrolysis of water owing to its much lower thermodynamic oxidation potential compared to that of the OER. Therefore, exploring a highly efficient and stable hydrogen evolution and urea electrooxidation bifunctional catalyst is the key to achieve economical and efficient hydrogen production. In this paper, we report a heterostructured sulfide/phosphide catalyst (Ni3S2–Ni3P/NF) synthesized via one-step thermal treatment of Ni(OH)2/NF, which allows the simultaneous occurrence of phosphorization and sulfuration. The obtained Ni3S2–Ni3P/NF catalyst shows a sheet structure with an average sheet thickness of ∼100 nm, and this sheet is composed of interconnected Ni3S2 and Ni3P nanoparticles (∼20 nm), between which there are a large number of accessible interfaces of Ni3S2–Ni3P. Thus, the Ni3S2–Ni3P/NF exhibits superior performance for both UOR and hydrogen evolution reaction (HER). For the overall urea–water electrolysis, to achieve current densities of 10 and 100 mA cm–2, cell voltage of only 1.43 and 1.65 V is required using this catalyst as both the anode and the cathode. Moreover, this catalyst also maintains fairly excellent stability after a long-term testing, indicating its potential for efficient and energy-saving hydrogen production. The theoretical calculation results show that the Ni atoms at the interface are the most efficient catalytically active site for the HER, and the free energy of hydrogen adsorption is closest to thermal neutrality, which is only 0.16 eV. A self-driven electron transfer at the interface, making the Ni3S2 sides become electron donating while Ni3P sides become electron withdrawing, may be the reason for the enhancement of the UOR activity. Therefore, this work shows an easy treatment for enhancing the catalytic activity of Ni-based materials to achieve high-efficiency urea–water electrolysis.