Nanostructuring copper thin film electrodes for CO₂ electroreduction to C₂₊ products

The electrochemical CO₂ reduction reaction (CO₂RR) is a promising approach for achieving carbon-neutral processes in the chemical industry. In this context, various nanostructures have been reported to enhance the C₂₊ selectivity of Cu-based catalysts. Here, we prepared Cu nanoneedles (NN) from 300 nm sputtered Cu thin films through anodization under various conditions and investigated their performance in terms of C₂₊ product selectivity. Various combinations of anodization potentials (+0.75 V_RHE, +0.85 V_RHE, and +0.95 V_RHE) and KOH electrolyte concentrations (0.1 M, 0.5 M and 1.0 M) allow the tailoring of the NN length and density that are linked to their CO₂RR product selectivity at -1.0 V_RHE. The best performance using the C₂₊ : C₁ ratio was achieved with a high NN surface density. A detailed analysis using high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure spectroscopy of the best performing sample shows that the anodization of a Cu thin film produces NNs composed of a uniform 3D network of 2 nm hydroxide nanoparticles (NPs) and reconstructs into a rougher metallic Cu NP network after the CO₂RR. A high density of NNs with this inner structure may lead to an increase in the local CO concentration and thus to C₂₊ products. This systematic work demonstrates that nanostructuring the surface of copper thin film electrodes can enhance the CO₂RR selectivity to C₂₊ products while the correlation between the NN morphology and their inner structure strengthens further their applications as CO₂ electrocatalysts.