Tuning Cu₂O morphologies of Cu₂O/Ni foam electrodes for the control of reactivity and nitrogen selectivity in direct ammonia electrooxidation reaction

Direct electrochemical oxidation of ammonia (AEO) using non-precious metal catalysts is an energy-efficient and low-carbon-footprint method for ammonia removal from wastewater. In this work, Cu₂O with three types of morphologies, including flower, particle, and sheet, were electrodeposited on Ni foam substrates (Cu₂O/NF) for direct AEO. The Cu₂O flower-deposited NF electrode showed the largest electrochemical surface area (19.6 cm²) and the smallest Cu₂O size (200–600 nm) among all Cu₂O/NF electrodes. In the ammonia solution, the peak current densities of direct AEO from cyclic voltammograms (CVs) at 10 mVs⁻¹ on the Cu₂O flower, Cu₂O particle, and Cu₂O sheet-deposited NF electrodes reached 4.6, 3.1, and 2.2 mAcm⁻², respectively. The result showed that, at an initial pH of 11 and an applied potential of 0.95 V vs. Hg/HgO, the order of three types of Cu₂O morphologies with respect to ammonia removal efficiency and rate is: Cu₂O flower (52%, 4.1×10⁻³ min⁻¹) > Cu₂O particle (38%, 2.6×10⁻³ min⁻¹) > Cu₂O sheet (30%, 1.8×10⁻³ min⁻¹). Interestingly, ammonia oxidation to N₂ on the Cu₂O sheet-deposited NF electrode exhibited the highest selectivity (SN₂: 60%), which was much higher than the Cu₂O flower-deposited NF electrode (SN₂: 31%). The high SN₂ of Cu₂O sheets was ascribed to the presence of the stable Cu(I) sheet structure during direct AEO, confirmed by X-ray photoelectron spectroscopy analysis. On the other hand, the Cu(II)-rich flower structure facilitated the formation of NO₃^-. Our findings demonstrate that the surface morphologies of Cu₂O deposited on the NF electrode show significant stability difference of Cu(I), which determines reactivity and N selectivity during direct AEO.