Ultrafast synthesis of an efficient urea oxidation electrocatalyst for urea-assisted fast-charging Zn-air batteries and water splitting

The urea oxidation reaction (UOR) efficiently treats urea-containing wastewater while replacing the high theoretical potential of the oxygen evolution reaction (OER), thereby enabling wastewater valorization. Traditional UOR catalysts are limited by sluggish reaction kinetics and high energy barriers due to non-optimized structures with insufficient active sites and poor charge transfer. Additionally, their complex synthesis increases costs and limits scalability for industrial applications. We addressed these challenges by introducing a 2-second, room-temperature synthesis method for sulfur-doped nickel-iron layered double hydroxide (S-NiFe-LDH). The catalyst's nanostructured surface enhanced mass transfer, and its synthesis required minimal energy and cost. Sulfur doping lowered the catalyst's onset potential, stabilized active sites, and improved charge transfer, significantly enhancing urea oxidation efficiency. Given the critical role of the OER in both water electrolysis and zinc-air battery systems, we applied the catalyst to these two systems, substituting the traditional OER with the UOR. In UOR-assisted water electrolysis, the catalyst achieved a sustained high current density of 100 mA cm-2 at just 1.47 V over 288 h, demonstrating an energy conversion efficiency in which the electrolyzer consumed only 3.52 kW h of electricity to produce 1 m3 of hydrogen. Additionally, fast-charging UOR-assisted Zn-air batteries maintained stability for over 1931 h. The S-NiFe-LDH catalyst effectively removed urea, mitigating eutrophication from agricultural and industrial effluents. This dual functionality of energy-efficient urea degradation and wastewater purification aligns with global sustainability goals, particularly in terms of clean water access and renewable energy development, providing a scalable and cost-effective solution for clean water and energy.