Simultaneously Boosting Direct and Indirect Urea Oxidation of Nickel Hydroxide via Strategic Yttrium Doping.

Urea electrolysis can address pressing environmental concerns caused by urea-containing wastewater while realizing energy-saving hydrogen production. Highly efficient and affordable electrocatalysts are indispensable for realizing the great potential of this emerging technology. Among the numerous candidates, α-Ni(OH) 2 has the merits of good electrocatalytic activity, adjustable heteroelement doping, and low cost; consequently, it has received tremendous attention in the electrolytic fields. Herein, a Y 3+ -doping strategy is developed to effectively enhance the catalytic performance of nickel hydroxide in the urea oxidation reaction (UOR). Our results show that Y 3+ incorporation successfully modulates the electronic structure of α-Ni(OH) 2 by inducing Ni 3+ formation in the crystal lattice to initiate direct UOR, facilitates the Ni 3+ /Ni 2+ redox transition with higher current responses to promote indirect UOR, and maintains the structural stability of YNi-10 (Ni 2+ /Y 3+ molar ratio = 1:0.1) during long-term UOR operation. Owing to these features, the obtained YNi-10 sample exhibits a higher current density (127 vs 79 mA cm -2 at 1.5 V), a lower Tafel slope (48 vs 75 mV dec -1 ), a larger potential difference between the UOR and oxygen evolution reaction (OER, 0.26 vs 0.22 V at 80 mA cm -2 ), a higher reaction rate constant (1.1 × 10 5 vs 3.1 × 10 3 cm 3 mol -1 s -1 ), and a reduced activation energy of UOR (2.9 vs 14.8 kJ mol -1 ) compared with the Y-free counterpart (YNi-0). This study presents a promising strategy to simultaneously boost direct and indirect UORs, providing new insights for further developing high-performance electrocatalysts.