Transition-Metal Cation Effects on Electrodeposited Ni(OH)2 for Urea Electrolysis

<jats:p>Urea electrolysis couples the anodic urea oxidation reaction (UOR) with the cathodic hydrogen evolution reaction (HER), making the balance between the two half-reactions critical to bifunctional electrode selection. Here, Ni(OH)2 supported on nickel foam (NF) was systematically compared with electrode series prepared from baths containing Fe3+, Zn2+, Co2+, or Mn2+ under a common synthesis and electrochemical testing protocol. XRD patterns were dominated by the NF substrate, consistent with a low-crystallinity and/or low-diffracting-volume deposited phase, while SEM revealed pronounced cation-dependent changes in nanosheet morphology. In 1 M KOH/0.33 M urea, EUOR,100 ranged from 1.435 to 1.512 V vs RHE, with the lowest value obtained for the 4% Co:Ni(OH)2/NF and 2% Mn:Ni(OH)2/NF. HER followed a distinct trend, where 2% Fe:Ni(OH)2/NF and 8% Fe:Ni(OH)2/NF exhibited the lowest ηHER,10 of 188 mV, compared with 213 mV for Ni(OH)2/NF. In symmetric two-electrode urea electrolysis, cell voltages ranged from 1.57 to 1.63 V at 10 mA cm-2 and from 1.84 to 1.94 V at 100 mA cm-2. UOR, HER, and full-cell performance therefore follow distinct cation-dependent rankings. These results establish cation identity as an important design variable for balancing the two half-reactions and overall urea-electrolysis performance in electrodeposited Ni(OH)2 systems.</jats:p>