A Computational Investigation of the Urea Oxidation Reaction Mechanism Using Density Functional Theory: Promoting the NiOOH Active Phase By Introducing Effective Dopants

Urea electrochemical oxidation is of general interest for water treatment in agriculture, but also a possible fuel for urea fuel cells. Urea oxidation is also of fundamental interest for amide chemistry. The urea oxidation reaction (UOR) in alkaline medium neutralizes urea into relatively benign products including H₂, N₂, and CO₂, rather than harmful NO_x compounds. Previous studies have suggested direct and indirect reaction routes for this full oxidation. The presence of metallic ions such as Fe, Co, and Cu in the nickel hydroxide surface may improve selectivity towards the indirect pathway where the Ni(OH)₂/NiOOH redox couple plays an important role. Since NiOOH is the active phase for UOR, the first step of transferring Ni(OH)₂ to NiOOH is crucial. In this work, we investigate the effect of Fe dopant on the Ni hydroxide transformation. Performing DFT calculations, as implemented in the Vienna Ab initio Simulation Package, the thermodynamics of the nickel dihydroxide to nickel oxyhydroxide mechanism was calculated. We found that doping the β-Ni(OH)₂ surface with Fe significantly reduces the free energy change for sequential surface dehydrogenation to β-NiOOH, from 2.27 eV to 0.93 eV, favoring the creation of UOR active sites. This indicates promising potential for the improvement of urea oxidation performance with Fe doping.