This communication studies the CO 2 reduction reaction in H 2 O/CH 3 CN mixtures on nanostructured copper. It was found that nanostructured copper electrode presents a welldefined voltammogram in acetonitrile, where it can be seen three signals related to adsorbed or surface attached (thin films) species. Also, it was found that the current density of CO 2 reduction in mixtures H 2 O/CH 3 CN on nanostructured copper electrodes with a molar fraction around 0.25 is higher than those observed with

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... molar fractions lower than 0.15 or higher than 0.35. Finally, nanostructured Cu electrodes show higher catalytic activity towards the CO 2 reduction than copper electrode. ACCEPTED MANUSCRIPT electro-reduction process is the Cu(211) facet. Also, it has been shown that nanostructured Cu electrode and nanoparticles have many advantages as electrocatalysts for CO 2 reduction. The enhanced electrocatalysis is usually explained taken into account changes on the surface area and morphology of the material [14] . It is well known that the reduction of CO 2 in water is not very efficient due to the low solubility of CO 2 and the high H 2 production. Acetonitrile is often used in electrochemistry due to its high dielectric constant and potential window, but a proton source, like water, has to be added in order to obtain better products, like methane or methanol, at low overpotentials [15] [16] . This communication is focused on the study of a suitable catalyst based on nanostructured Cu electrodes for the electrochemical reduction of CO 2 towards compounds that can be used as alternative fuels (like alcohols or methane). Taken into account that water has a high dielectric constant, it is a weak acid and miscible with acetonitrile, we decided to study the CO 2 reduction in mixtures of H 2 O/CH 3 CN by cyclic voltammetry. Materials and methods Electrodeposition of copper was done using a two-electrode cell. A copper disk (99,9 %) was used as working electrode. It was polished with 3 µm alumina powder, and rinsed with deionized water prior to use. A copper foil polished with 600-grade sandpaper was used as counter electrode. Nanostructured Cu deposits were prepared using reported methods [17] [18] [19] . The electrodes were immersed in a 0.2 M CuSO 4 (99,99%, Aldrich) and 1.5 M H 2 SO 4