Regulating electronic microenvironment of porous Ni(OH)2 by Ce and N codoping and its bifunctional catalyst for efficient urea-assisted water splitting

Abstract

Urea-assisted water splitting that involves urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) can purify urea-rich wastewater and produce hydrogen, but the lack of efficient catalysts hampers its application. Ni(OH)₂ is recognized as one of the most active electrocatalysts for UOR but the performance is hindered by the limited active sites with intrinsic activity. Herein, porous Ni(OH)₂ nanosheets with Ce and N codoping (Ce, NNi (OH)₂) are designed to regulate electrochemical microenvironment and used as a bifunctional catalyst for urea-assisted water splitting. The porous structure enhances the exposure of rich active sites, while the Ce, N-codoping optimizes the d band center of active Ni species and improves electron transport. In-situ Raman spectroscopy and theoretical calculation disclose the real active sites are γ-NiOOH species generated during the UOR process and the regulated microenvironment promotes a shift in the rate-controlling step of the UOR from CONNH* dehydrogenation to CONHNH* dehydrogenation with a reduced thermodynamic barrier. As a result, the Ce, N-Ni(OH)₂ catalyst delivers excellent UOR activity with a low potential of 1.39 V at 100 mA cm⁻² and a small Tafel slope of 18.4 mV dec⁻¹, along with superior HER activity. The assembled urea-assisted electrolyzer achieves a low cell voltage of 1.49 V at 10 mA cm⁻² and maintains robust operational stability over 70 h. This work demonstrates a synergistic design strategy of porous structure and dual-element modulation, offering a new avenue for developing advanced catalysts in sustainable energy and environmental applications.