Understanding the Mechanism of Urea Oxidation from First-Principles Calculations.

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) ß-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 NiOxHy electrocatalysts.

Nickel-Catalyzed Urea Electrolysis: From Nitrite and Cyanate as Major Products to Nitrogen Evolution.

The electrochemical urea oxidation reaction (UOR) to N2 represents an efficient route to simultaneous nitrogen removal from N-enriched waste and production of renewable fuels at the cathode. However, the overoxidation of urea to NOx - usually dominates over its oxidation to N2 at Ni(OH)2 -based anodes. Furthermore, detailed reaction mechanisms of UOR remain unclear, hindering the rational catalyst design. We found that UOR to NOx - on Ni(OH)2 is accompanied by the formation of near stoichiometric amount of cyanate (NCO- ), which enabled the elucidation of UOR mechanisms. Based on our experimental and computational findings, we show that the formation of NOx - and N2 follows two distinct vacancy-dependent pathways. We also demonstrate that the reaction selectivity can be steered towards N2 formation by altering the composition of the catalyst, e.g., doping the catalyst with copper (Ni0.8 Cu0.2 (OH)2 ) increases the faradaic efficiency of N2 from 30 % to 55 %.

Inductive effects in cobalt-doped nickel hydroxide electronic structure facilitating urea electrooxidation.

Electrochemical oxidation of urea provides an approach to prevent excess urea emissions into the environment while generating value by capturing chemical energy from waste. Unfortunately, the source of high catalytic activity in state-of-the-art doped nickel catalysts for urea oxidation reaction (UOR) activity remains poorly understood, hindering the rational design of new catalyst materials. In particular, the exact role of cobalt as a dopant in Ni(OH)2 to maximize the intrinsic activity towards UOR remains unclear. In this work, we demonstrate how tuning the Ni:Co ratio allows us to control the intrinsic activity and number of active surface sites, both of which contribute towards increasing UOR performance. We show how Ni90Co10(OH)2 achieves the largest geometric current density due to the increase of available surface sites and that intrinsic activity towards UOR is maximized with Ni20Co80(OH)2. Through density functional theory calculations, we show that the introduction of Co alters the Ni 3d electronic state density distribution to lower the minimum energy required to oxidize Ni and influence potential surface adsorbate interactions.