Oxygen-Containing Anions Promote the Electrocatalytic Activity of Ni(OH)2/NiOOH for The Urea Oxidation

The electrocatalytic oxidation of urea (EOU) is an important half-reaction in water splitting, urea fuel cells, and urea wastewater treatment. The anions of nickel-based catalysts can be leached severely in the oxidation potential range. Therefore, the influence of anion on the electrocatalytic performance of Ni(OH) 2 /NiOOH for urea oxidation has not been systematically studied. The present experimental results show that the structure reconstruction of catalyst is a complicated process, specifically including deep penetration by alkaline electrolyte, leaching-induced oxidation by phosphorus or sulfur anions, transformation or cracking of the original structure, and formation of an amorphous layer on the surface. The reconstructed NF-NiP x -R or NF-NiS x -R has a three-layer core-shell structure, with a nickel foam skeleton in the innermost layer, an amorphous NiP x or NiS x layer in the middle layer, the outermost layer of oxygen-containing anion (PO x or SO x ) coordinated Ni(OH) 2 . In the prepared catalyst, the Ni(OH) 2 /NiOOH redox is the real catalytic species for the urea oxidation. It is found that the oxygen-containing anion can enhance the catalytic performance of the Ni(OH) 2 /NiOOH redox, and PO x anion has the best enhancing effect. It is found that the urea molecules are oxidized on the electrocatalyst surface mainly through an indirect pathway.