Low-voltage anodizing of copper in sodium bicarbonate solutions

Abstract

The low-voltage (< 5 V) anodization of copper in aqueous solutions of sodium bicarbonate was studied for the first time. As demonstrated, this method leads to the formation of microstructures on a copper surface, that are composed of malachite (CuCO₃·Cu(OH)₂). Moreover, by tuning the operating conditions, i.e., applied cell voltage and electrolyte concentration, different surface morphologies can be grown. As shown by electron microscopy investigation, clusters of ribbons corrosion pits or nonuniformly located powdery precipitates are formed when the low anodizing voltage is applied. Anodization at 1.0 V in 0.4 M sodium bicarbonate solution led to the formation of a velvet-like, deep black anodic layer that covered the whole metal surface with ribbon-resembling structures. A thorough investigation of the obtained anodic layers with X-ray diffraction (XRD), X-ray adsorption (XAS), Raman, and X-ray Photoelectron Spectroscopy (XPS) uncovered the mixed crystalline-amorphous nature of the anodic copper species. Besides dominating the crystalline malachite phase, the amorphous cupric oxide was also identified. This composition offers promising features for catalytic applications, hence, low-voltage anodized copper was tested in an electrochemical CO₂ reduction reaction to explore one possible application of the presented material. The current density of 4.7 mA cm⁻² was registered for the selected sample.