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

Abstract

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.