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 cm(2)) 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%, 4.1x10(- 3) min( -1)) > Cu2O particle (38%, 2.6x10(- 3) min( -1)) > Cu2O sheet (30%, 1.8x10(- 3) min( -1)). Interestingly, ammonia oxidation to N-2 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.