Self-supported NiO/CuO electrodes to boost urea oxidation in direct urea fuel cells

Direct urea fuel cells (DUFCs) can be used to treat residual wastewater while generating electricity. Herein, self-supported electrodes consisting of NiO nanosheets vertically grown on CuO nanowires are synthesized to realize the urea oxidation reaction (UOR). Such electrodes show excellent UOR performance requiring 1.39 V vs. RHE to achieve 100 mA/cm2. Besides, DUFCs provide open circuit voltages and power densities up to 0.88 V and 11.35 mW/cm2. Electrochemical characterization and Raman spectroscopy prove the formation of NiOOH to enable the UOR. Mott-Schottky analysis and UV photoelectron spectroscopy show the NiO/CuO p-p heterostructure to facilitate the charge transfer from CuO nanowires to NiO nanosheets. Besides, at a local level, density functional theory calculations show that the presence of CuO modulates the electronic states of Ni at the very NiOOH/CuO interface, which results in stretched Ni-O bonds and a uniquely elongated N-H bond of urea that favors its oxidation.