摘要
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, Cu <sub>2</sub> O with three types of morphologies, including flower, particle, and sheet, were electrodeposited on Ni foam substrates (Cu <sub>2</sub> O/NF) for direct AEO. The Cu <sub>2</sub> O flower-deposited NF electrode showed the largest electrochemical surface area (19.6 cm <sup>2</sup> ) and the smallest Cu <sub>2</sub> O size (200–600 nm) among all Cu <sub>2</sub> O/NF electrodes. In the ammonia solution, the peak current densities of direct AEO from cyclic voltammograms (CVs) at 10 mVs <sup>−1</sup> on the Cu <sub>2</sub> O flower, Cu <sub>2</sub> O particle, and Cu <sub>2</sub> O sheet-deposited NF electrodes reached 4.6, 3.1, and 2.2 mAcm <sup>−2</sup> , 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 Cu <sub>2</sub> O morphologies with respect to ammonia removal efficiency and rate is: Cu <sub>2</sub> O flower (52%, 4.1×10 <sup>−3</sup> min <sup>−1</sup> ) > Cu <sub>2</sub> O particle (38%, 2.6×10 <sup>−3</sup> min <sup>−1</sup> ) > Cu <sub>2</sub> O sheet (30%, 1.8×10 <sup>−3</sup> min <sup>−1</sup> ). Interestingly, ammonia oxidation to N <sub>2</sub> on the Cu <sub>2</sub> O sheet-deposited NF electrode exhibited the highest selectivity (SN <sub>2</sub> : 60%), which was much higher than the Cu <sub>2</sub> O flower-deposited NF electrode (SN <sub>2</sub> : 31%). The high SN <sub>2</sub> of Cu <sub>2</sub> O 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 NO <sub>3</sub> <sup>-</sup> . Our findings demonstrate that the surface morphologies of Cu <sub>2</sub> O deposited on the NF electrode show significant stability difference of Cu(I), which determines reactivity and N selectivity during direct AEO.