Production of hydrogen during electro-oxidation of urea using Ni-based catalysts modified with Mn and Zn supported on a Vulcan carbon matrix

Urea electrooxidation offers a promising alternative for hydrogen generation, applicable in electrolysis and direct urea fuel cells (DUFC), typically using noble metal electrocatalysts. In this study monometallic (Ni/C) and bimetallic (NiMn/C and NiZn/C) catalysts synthesized via a deposition-impregnation method were evaluated. Structural and physicochemical characterization through XRD, TEM, SEM-EDS, and XPS confirmed the presence of semispherical metal and metal oxide nanoparticles within a highly dispersed carbon matrix. Catalytic performance was assessed using cyclic voltammetry (CV), revealing high faradaic currents (∼5 mA) linked to nickel redox processes and urea oxidation. Electrochemical impedance spectroscopy (EIS) indicated that NiMn/C exhibited a lower charge-transfer resistance at 0.6 V vs. SHE, effectively reducing oxidation potential. In-situ electrochemical Raman spectroscopy identified intermediate species during anodic polarization, with intensified peaks above 0.6 V indicating NiOH/NiOOH redox processes and the formation of carbonates and nitrogen oxides. Differential Electrochemical Mass Spectrometry (DEMS) detected ionic currents corresponding to various species, including NO ₂, NO, N ₂, NH ₃, N ₂O, and O ₂. Selectivity studies showed that Ni/C achieved a 93% conversion to molecular nitrogen, while Ni ₂₅Zn ₇₅/C reached 68% conversion to ammonia, and Ni ₇₅Mn ₂₅/C favored nitrogen oxide production. Hydrogen generation was higher at the cathode under acidic conditions compared to alkaline. Stability tests via chronoamperometry at 0.65 V vs. SHE revealed a stable current (∼0.4 mA), particularly for Ni ₇₅Mn ₂₅/C, indicating enhanced tolerance to reaction intermediates due to improved bimetallic interactions.