Cu-Doping Effect on the Electrocatalytic Properties of Self-Supported Cu-Doped Ni₃S₂ Nanosheets for Hydrogen Production via Efficient Urea Oxidation

The urea oxidation reaction (UOR) is considered as a substitutable oxidation process to supplant the oxygen evolution reaction (OER) for pure and clean hydrogen generation because of its much lower theoretic thermodynamic onset potential. Preparing heteroatomically doped and self-supported three-dimensional (3D) catalysts has become an efficient pathway to promote the electrochemical performance of catalysts. Recently, Cu-based complexes have been investigated as OER catalysts and exhibited improved catalytic activities. However, Cu-doped composites as UOR catalysts have been rarely reported, and the effect of Cu remains unclear in the UOR process. The present work exhibits a self-supported electrocatalyst of Cu-doped Ni₃S₂ nanosheets supported on Ni foam (NF) synthesized via a direct one-step hydrothermal sulfuration method and unlocks the effect of Cu on the relationship between the catalyst structure and UOR performance. The doping of the Cu element transformed the morphology of Ni₃S₂ from nanoparticles to nanosheets, increasing the active surface area. Meanwhile, the Cu dopant regulated the electronic structure of Cu-doped Ni₃S₂ by promoting electron transport from the Ni atom to the Cu dopant, stimulating the formation of active Ni sites with a high valency during UOR. Moreover, the doping of the Cu element optimized the Gibbs adsorption energies of the pivotal intermediates during urea oxidation. Remarkably, as-prepared Cu-doped Ni₃S₂/NF required only 1.30 V vs RHE toward UOR and an overpotential of 188 mV toward the hydrogen evolution reaction (HER) to deliver 10 mA cm^–2 with outstanding electrochemical durability. Besides, the overall urea electrolyzer constructed using Cu-doped Ni₃S₂/NF as the UOR and HER catalyst needed only 1.57 V to deliver 10 mA cm^–2 with stable durability during a long-term test. The present research offers novel insights into the research for designing and preparing efficient and durable electrodes in urea oxidation applications.