Controllable synthesis of M and Fe co-doped Ni3S2 (M = Mo, Cr, Co, Ni, Cu) catalysts for efficient urea oxidation and hydrogen production in urea electrolyte

Abstract

Urea electrolysis for hydrogen production can both alleviate the energy crisis and protect the environment. However, the oxidation of urea at the anode requires the transfer of six electrons, which greatly limits the rate of urea electrolysis. In this paper, a variety of M and Fe co-doped Ni₃S₂ (M = Mo, Cr, Co, Ni, Cu) materials were in situ prepared on Ni foam through a one-step hydrothermal reaction process. Their hydrogen evolution reaction (HER) performance and urea oxidation reaction (UOR) performance were systematically investigated in 1 M KOH + 0.5 M urea solution. Compared with other control group materials, the Mo, Fe–Ni₃S₂ material exhibits excellent hydrogen production performance (overpotential of 60 mV @ 10 mA cm⁻²) and urea oxidation performance (potential of 1.39 V @ 10 mA cm⁻²). When it is used as both the cathode and anode material simultaneously, the assembled two-electrode system also exhibits excellent urea electrolysis performance (cell voltage of 1.42 V @ 10 mA cm⁻²), showing one of the best catalytic activities among the catalysts known to date. A series of experiments has proven that the increase in activity is attributed to the doping of molybdenum, which regulates the electron state density of adjacent atoms and improves the electron transfer rate. Density functional theory analysis further confirms that the introduction of molybdenum optimizes the hydrogen Gibbs free energy and improves the electrical conductivity of the electrode. This work provides a novel idea for reducing energy consumption and improving efficiency in hydrogen production by electrolyzing urea.