Electrodeposition of porous CuNi alloys and their use as cathodes for nitrate reduction in alkali

The physico-chemical properties of Cu and Ni (same FCC crystal structure, similar valence, electronegativity and atomic radii) allow electrodeposition of the binary alloys over the entire composition range. Given the advantages of CuNi alloys in electrocatalysis - the components are common, relatively cheap and low toxicity metals - it is appropriate to test their performances in a process of current interest like nitrate electroreduction. We have recently described the results obtained with smooth CuNi layers deposited potentiostatically from a citrate bath [1]. In this communication we present the galvanostatic deposition of CuNi alloys from modified citrate baths, using large current densities (ca. 3 A cm-2) that impose a regime of vigorous hydrogen evolution in which the bubbles act like a dynamic template. As shown by SEM images (left figure below), the alloy layers are deposited with a bimodal porosity - pits with a diameter of tens of ?m and a spongy structure resulting from countless tiny dendrites with submicron dimensions. Voltammograms of nitrate reduction in alkaline solutions recorded at the porous alloy electrodes show (right figure below), in comparison with experiments at smooth electrodes, a less negative onset potential and a substantial increase of the maximum current: the spongy material shows higher activity at moderate overpotentials and a larger active surface due to easy transport in the macropores.