We examine the pathways to form either N₂ or NO₂ ⁻/NO₃ ⁻ on a Ni(OH)₂ surface using density functional theory. Our calculations show support for two major experimental observations: (1) N₂ favours an intramolecular mechanism, and (2) NO₂ ⁻/NO₃ ⁻ are formed in a 1 : 1 ratio with OCN⁻.

Abstract

Developing electrocatalysts for urea oxidation reaction (UOR) works toward sustainably treating urea-enriched water. Without a clear understanding of how UOR products form, advancing catalyst performance is currently hindered. This work examines the thermodynamics of UOR pathways to produce N₂, NO₂ ⁻, and NO₃ ⁻ on a (0001) β-Ni(OH)₂ surface using density functional theory with the computational hydrogen electrode model. Our calculations show support for two major experimental observations: (1) N₂ favours an intramolecular mechanism, and (2) NO₂ ⁻/NO₃ ⁻ are formed in a 1 : 1 ratio with OCN⁻. In addition, we found that selectivity between N₂ and NO₂ ⁻/NO₃ ⁻ on our model surface appears to be controlled by two key factors, the atom that binds the surface intermediates to the surface and how they are deprotonated. These UOR pathways were also examined with a Cu dopant, revealing that an experimentally observed increased N₂ selectivity may originate from increasing the limiting potential required to form NO₂ ⁻. This work builds towards developing a more complete atomic understanding of UOR at the surface of NiO _x H _y electrocatalysts.