Nano-Coral-Like Ni3S2/CoSe2 Composite with p–n Heterojunction to Boost Urea Oxidation in Direct Urea Fuel Cells

Direct urea fuel cells (DUFCs), as an emerging power generation technology, can efficiently convert the chemical energy stored in urea into electrical energy. The urea oxidation reaction (UOR) at the DUFC anode is the key reaction determining its power output. In particular, when wastewater containing urea is used as fuel, the slow kinetics of the UOR severely limit the power output. Here, a Ni3S2/CoSe2-NF p-n heterojunction with a nano-coral structure was fabricated by in situ growing n-type Ni3S2 nanosheets onto p-type CoSe2 nanoneedle arrays on nickel foam (NF). Mott-Schottky analysis, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy reveal that the p-n heterostructure facilitates charge transfer from Ni3S2 nanosheets to CoSe2 nanoneedles. Furthermore, density functional theory calculations indicate that the p-n heterostructure modulates the electronic states, significantly optimizes the d-band center, and promotes the adsorption of urea molecules on the catalyst surface. Compared to Ni3S2 and CoSe2, the p-n heterojunction alters the rate-determining step (RDS) of UOR from the *CO(NH2)NH → *CO(NH)N pathway to the *CO(NH)N → *CONN pathway, significantly lowering the Gibbs free energy of the RDS and enhancing the UOR kinetics. Therefore, the Ni3S2/CoSe2-NF composite electrode achieves a current density of 100 mA cm-2 at only 1.33 V vs RHE, and at 1.35 V vs RHE with human urine as the electrolyte. Notably, the DUFC employing the Ni3S2/CoSe2-NF composite electrode delivers a maximum power density (Pmax) of 27.52 mW cm-2, and retains a Pmax of 19.68 mW cm-2 even with human urine as the fuel.