Cu2O nanocubes favorably electroreduce CO2 to Methane: The interplay of appropriate electrochemical protocol

Nowadays, electrochemical CO₂ reduction reaction (CO₂RR) to value-added products represents one of the major challenges in electrocatalysis. Copper-based nanocubes (Cu NCs) have been proposed as the front-runner's catalyst for the production of C₂₊ products at the industrial level. However, their selectivity (C₁ vs. C₂ product distribution) is rather complex depending on the dynamic structural transformations, the presence of mixed Cu⁺/Cu⁰ states, the microenvironment, and nanocatalyst-support interactions. Commonly, electrochemically-grown Cu NCs are prepared in the presence of chlorides that acts as a shaping agent. In this study, an optimized electrodeposition method for the synthesis of Cl^--free Cu₂O nanocubes on a glassy carbon substrate with uniform size, shape, and loading is established. The successful preparation of chloride-free cuprous oxide nanocubes (Cu₂O NCs) was confirmed with X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) analyses. We report how the electrochemical double-layer capacitance (EDLC) method for electrochemical surface area (ECSA) determination with(out) subsequent return to the open-circuit potential (OCP) conditions before electrolysis influences the CO₂RR activity/selectivity. When Cu₂O NCs are subjected to the EDLC method  (often considered a non-invasive method) and exposed to the OCP before electrolysis, they become active for methane (CH₄) formation. Moreover, the influence of the potential window width (i.e. 200 and 400 mV) in which the EDLC-ECSA is employed and its correlations with the selectivity is presented. We underline the importance of the ECSA determination method and OCP on/off state as a triggering factor for reactivity/selectivity of particular Cu₂O NCs for CO₂RR and further emphasize the reconstructive nature of Cu₂O NCs under CO₂RR relevant conditions.