Citric Acid-Assisted Engineering of Carbon-Supported Cobalt Sulfide for Efficient Urea-Assisted Hydrogen Production

Developing earth-abundant electrocatalysts capable of replacing the sluggish oxygen evolution reaction (OER) with energy-efficient urea oxidation reaction (UOR) is an effective strategy for sustainable hydrogen production. Herein, we demonstrate the pivotal role of citric acid in engineering carbon-supported cobalt sulfide (C-CoS) catalysts through a facile sol-gel assisted pyrolysis route. During synthesis, citric acid functions simultaneously as a chelating agent, carbon precursor, and microstructure-directing additive, promoting homogeneous cobalt dispersion, in situ carbonization, and the formation of an electronically coupled CoS/Co9S8 electrocatalyst. The optimized C-CoS (1:6) catalyst exhibits the highest electrochemical active surface area with accelerated charge-transfer kinetics, and superior bifunctional electrocatalytic activity towards hydrogen, oxygen evolution reaction and urea oxidation reaction in alkaline media. As an anodic catalyst, C-CoS (1:6) requires only 1.61 V for overall water electrolysis and 1.35 V for urea-assisted electrolysis at 50 mA cm–2, corresponding to an energy saving of approximately 16% by replacing OER with UOR. The catalyst further demonstrates excellent operational durability with minimal performance degradation during prolonged electrolysis. Post-electrolysis X-ray photoelectron spectroscopic study reveals the in situ surface reconstruction of cobalt sulfide into catalytically active CoOOH species. This study demonstrates the role of citric acid in controlling carbon incorporation and the structural characteristics of C-CoS electrocatalysts, and provides an efficient, scalable, and noble-metal-free strategy for energy efficient alkaline hydrogen production.