Electrochemical nitrate reduction as affected by the crystal morphology and facet of copper nanoparticles supported on nickel foam electrodes (Cu/Ni)

Cu/Ni composite electrodes were prepared and studied for the electrochemical reduction of nitrate in aqueous solutions. Electrodeless plating technique, with tartrate as chelatant and formaldehyde as reducing agent, enabled the in-situ incorporation of Cu nanoparticles into the open-pore structured Ni foam to form Cu-Ni composite electrodes. X-ray diffractometer (XRD) and scanning electron microscopy (SEM) revealed that the crystal facet and grain morphology of Cu nanoparticles was closely controlled by the plating time and played a significant role in nitrate reduction and nitrogen selectivity. Cyclic voltammetry provided information on the electron transfer between surface nitrogen species and Cu/Ni electrodes. Electrochemical nitrate reduction was initiated at the onset potential of Cu(0)/Cu(I) redox reaction over a potential window of −0.6 V to −1.2 V. The preferential Cu{1 1 1} facet orientation improved the electron transfer process. Batch kinetics studies at constant current and potential showed that specific adsorption of nitrate and nitrite on the Cu{1 1 1} facet was critical to efficient electrochemical nitrate reduction. Moreover, the conversion of nitrogenous byproduct was potential-dependent. Results showed that N₂ selectivity was high (55.6%) at low overpotential (i.e., ⩾−0.6 V vs. Hg/HgO. At high overpotential (i.e, <−0.6 V) there was complete of NO₃ ⁻ reduction with NH₄ ⁺ as major byproduct.