Solar-driven hydrogen generation coupled with urea electrolysis by an oxygen vacancy-rich catalyst

Abstract

Urea, an environmental pollutant for both soil and water, is widely present in wastewater. On the other hand, a strategy utilizing renewable electricity to decrease the cost of green hydrogen, which holds the key to a sustainable energy future, is promising but challenging. Gas crossover that generates explosive hydrogen–oxygen mixture becomes very serious with intermittent renewable power source (partial load issue). Herein, we address these issues in one device, i.e., a hybrid electrolyzer where water oxidation that produces oxygen is replaced by urea oxidation which generates inert gases. A self-supported electrocatalyst of nitrogen-doped nickel-iron oxyhydroxide derived from waste rusty iron foam is synthesized via an in situ ‘waste-to-value’ synthetic route followed by an ammonia/argon plasma treatment, which reconstructs the surface of the catalyst to a 3D nanosheet-like porous network with abundant oxygen vacancies. The as-prepared catalyst shows a small potential of 1.45 V vs. RHE at 500 mA cm−2 for urea oxidation. Overall water-urea electrolysis only requires 1.58 V to deliver 100 mA cm−2, which is 0.33 V less than that in urea-free water splitting, and thus lowers the overall energy consumption by 17.3%. Without oxygen evolution, the hybrid electrolysis does not suffer from the safety hazard caused by explosive hydrogen–oxygen mixture. We demonstrate the safe production of green hydrogen (3.1% oxygen in the gaseous product) in the hybrid electrolysis powered by solar energy via a photovoltaic panel. Our work provides a method to address the urea-caused environmental issues and simultaneously generate green hydrogen safely using solar energy.