Molybdenum-doped Ni–S nanostructure towards energy saving for hydrogen production coupling urea oxidation reaction

Abstract

The growing trend of using energy from fossil fuel sources and the gradual reduction of these sources have caused concerns about future energy. Therefore, the need to use clean and renewable sources such as hydrogen has increased. The synthesis and design of effective electrocatalysts are important to generate hydrogen via urea electrocatalysis with the minimum required energy. In the present research, Ni–S nanostructures doped with molybdenum (Mo) supported on nickel foam substrates were synthesized by the facile one-step cyclic voltammetry (CV) method, and some of the efficient factors such as scan rate, number of cycles, and molybdenum concentration were optimized to generate the minimum required overpotential. The proposed electrode was used as a working electrode for urea-assisted electrochemical hydrogen production. The activity and electrocatalytic stability of the prepared electrodes were measured by linear sweep voltammetry (LSV), CV, Tafel polarization curve, and chronopotentiometry (CP) in 1 M NaOH + 0.33 M urea electrolyte. The results showed that the optimal electrode (50 mV/s scan rate and 10 cycles) has excellent electrocatalytic activity for hydrogen evolution reaction (HER) equal to −142.2 mV overpotential at a current density of 10 mA/cm2. In addition, the optimal electrode showed outstanding activity for the urea oxidation reaction (UOR) with a potential of 1.36 V (vs. RHE) to achieve a current density of 10 mA/cm2, which is due to the high surface-to-volume ratio, maximizing the number of active sites, good electrical conductivity, and synergistic effect of elements. On the contrary, to reach 10 mA/cm2, OER requires 1.55 (vs. RHE). In addition, an overall urea splitting cell voltage of only 2.52 and 2.76 V (vs. RHE) is needed to obtain 10 mA/cm2 current in urea and without urea-containing electrolytes, respectively. Therefore, replacing UOR with OER reduces the energy consumption of hydrogen production.