A Theoretical Study on Doped-NiOOH Catalysts for Enhanced Urea Oxidation Reaction

Abstract

Urea is a common waste specie in agricultural runoff water and has been considered as a promising oxidizable molecule for substitution of oxygen evolution reaction (OER) to save energy. However, the overpotential of electrochemical urea oxidation reaction (UOR) is still high due to the complicated six-electron transfer process on most metal catalysts and the competition with OER further limits catalyst options for UOR. The most promising and studied catalysts for UOR are Ni-based catalysts. Previous theoretical and experimental research works have focused on NiOOH step-edges as active sites for UOR. In this work, we focus on gaining atomic-level insight into the role of basal-type sites in UOR, which represent the largest surface area of NiOOH catalysts. We study the effects of vacancies as well as metal doping on the transformation from Ni(OH)2 to NiOOH active phase and the UOR pathway using density functional theory (DFT) calculations. By investigating various Ni(OH)2 and NiOOH (001) surfaces with vacancies and dopants such as Fe, Co and Cu, we are able to examine how the active sites influence the Ni-catalysts transformation and the urea oxidation. This work sheds light on the structure-property relationship of Ni(OH)2/NiOOH in urea oxidation and offers design principles for functional Ni-based materials, which will help accelerate the development of efficient UOR catalysts.