Electrodeposited Cu Nanofoam Structures for Electrochemical CO₂ Reduction ^†

An exclusive Cu surface feature has the ability to convert CO₂ into hydrocarbons with significant Faradaic efficiency [1]. The catalytic activity of metal is highly sensitive to electrolysis conditions including surface structure, morphology and real surface area ($S_R$). Simple polycrystalline Cu electrodes possess a rather small surface area; therefore, their efficiency is low. Three-dimensional nanoramified Cu electrodes or foams can be produced via metal electrodeposition with intensive hydrogen evolution [2]. An accurate estimation of the porous electrode $S_R$ is of high importance, as precise knowledge of this parameter is crucial for comparison of the behaviour of various catalytic systems. The aim of this study was to evaluate the suitability of the known Cu real surface area determination methods [3] for their application for Cu 3D nanostructures. To reach this goal, the following design was created. The initial Cu electrode with a known $S_R$ value was employed as a basis for Cu 3D structure electrodeposition from an acidic sulphate solution. Electrochemical methods employing the underpotential deposition of Tl and Pb, as well the double-layer capacitance measurements, applying cyclic voltammetry and electrochemical impedance spectroscopy were applied for Cu 3D structure $S_R$ evaluation. The obtained results imply that non-porous Cu electrodes are not sensitive to the applied $S_R$ determination method, while this parameter for Cu 3D structures depends significantly on the evaluation mode. The most reliable data for Cu foam characterization were obtained with double-layer capacity measurements, while all other applied methods yielded inaccurate results. The electrodeposited Cu 3D layer structure and hence $S_R$ depend on the plating solution composition [4]. An attempt has been made to investigate the influence of HCl additives on deposited Cu foam $S_R$ values. The obtained results indicate that the addition of HCl increases Cu $S_R \mathrm{t}\mathrm{h}\mathrm{r}\mathrm{e}\mathrm{e}\mathrm{f}\mathrm{o}\mathrm{l}\mathrm{d}$.