Elaboration of Cu−Pd Films by Coelectrodeposition: Application to Nitrate Electroreduction

In this study, nanocrystalline copper−palladium films were synthesized over a wide range of compositions by coelectrodeposition of Pd and Cu in a 1 M NaCl solution containing both CuCl2 and PdCl2 in various proportions. The deposition potential was fixed at −0.5 V versus a saturated calomel electrode (SCE). These coatings were characterized by scanning electron microscopy coupled to energy dispersive X-ray analysis (SEM−EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These analyses revealed a fine and homogeneous distribution of Pd and Cu within and over the whole surface of the film. Depending upon the Cu(II)/Pd(II) ratio in solution, monophased Pd-rich films (Pd95Cu5 or Pd88Cu12 alloys) or biphased films (containing Pd80Cu20 and Cu phases in different proportions) were obtained. Theses materials were tested as electrocatalysts for nitrate reduction in alkaline media. Electrochemical measurements showed that biphasic (Pd80Cu20 + Cu) materials displayed the best electrocatalytic activity toward nitrate reduction. Results of prolonged electrolysis also proved that the selectivity of the modified electrodes clearly depends not only on the applied potential but also on their structure and chemical composition. At −1.3 V versus Hg/HgO, all the electrodes (except pure palladium, which is inactive for nitrate reduction) mainly produced ammonia. However, at −0.93 V versus Hg/HgO, biphasic Cu−Pd electrode composed of 77% Pd80Cu20 + 23% Cu successfully reduced nitrate to nitrogen with a current efficiency approaching 76%.