Enhanced Urea Oxidation Reaction by Mn-Doped Nickel Hydroxide From Wastewater

Integrating urea electrolysis into wastewater treatment systems represents a promising strategy to couple hydrogen production with environmental remediation. Nickel-based materials are among the most efficient non-precious catalysts for the urea oxidation reaction under alkaline conditions, yet Ni scarcity and its potential toxicity call for more sustainable alternatives. After demonstrating that Ni hydroxide recovered from wastewater is a good urea oxidation reaction catalyst, in this work, we report a fundamental and systematic study about urea oxidation reaction catalysts based on manganese-doped nickel hydroxide obtained through a synthesis route that mimics Ni recovery from wastewater to enhance the urea oxidation reaction performance. Thus, it offers both catalytic performance and circular-material benefits. A comprehensive characterization of the Ni(OH)2-based materials by experimental techniques, such as X-ray diffraction, scanning electron microscopy, X-ray absorption, and X-ray photoemission spectroscopy, combined with DFT theoretical calculations has revealed that Mn incorporation modifies the hydroxide structure, introduces ions in the +3 oxidation state, and promotes the formation of catalytically active NiOOH species. Mn dopants induce a favorable electronic effect through an indirect participation in the urea oxidation reaction mechanism involving oxidation state variation but not direct chemical bond formation. The reaction intermediates are in part different from those reported in previous literature. At a given potential, Mn doping enhances the overall urea oxidation rate, resulting in increased hydrogen evolution at the cathode. These findings point to a novel mechanistic understanding of the Mn-dopant role through its ability for an easier change to higher oxidation states than Ni ions, which reflects in a more favorable energetics of NiOOH formation and an improved urea oxidation reaction catalysis based on circular earth-abundant elements.