Sodium Persulfate Pre‐treatment of Copper Foils Enabling Homogenous Growth of Cu(OH)2 Nanoneedle Films for Electrochemical CO2 Reduction

Abstract Oxide‐derived copper (OD−Cu) catalysts have received widespread attention for their ability to produce energy‐dense multicarbon products. Within this class of materials, nanostructured copper hydroxide (Cu(OH) 2 ) has shown excellent catalytic properties, but its synthesis requires complex pre‐treatment steps of the Cu surface. In this study, we have developed a simple two‐step synthesis method for homogenous Cu(OH) 2 nanoneedle films using a sodium persulfate pre‐treatment step prior to anodization. The Cu(OH) 2 nanoneedle films show drastically enhanced uniformity after the pre‐treatment due to improved current distribution and can be grown over large surface areas (63 cm 2 ). As a catalyst for CO 2 reduction, the Cu(OH) 2 favours ethylene formation, with a near total suppression of methane production. A peak faradaic efficiency (FE) of 36.5 % is found at −1.0 V vs. the reversible hydrogen electrode (RHE), and the catalyst remains stable while providing an ethylene to methane ratio of 27.8 after 6 h of reaction.