Ultramicroelectrode Studies of Self-Terminated Nickel Electrodeposition and Nickel Hydroxide Formation upon Water Reduction.

The interaction between electrodeposition of Ni and electrolyte breakdown, namely the hydrogen evolution reaction (HER) via H3O+ and H2O reduction, was investigated under well-defined mass transport conditions using ultramicroelectrodes (UME's) coupled with optical imaging, generation/collection scanning electrochemical microscopy (G/C-SECM), and preliminary microscale pH measurements. For 5 mmol/L NiCl2 + 0.1 mol/L NaCl, pH 3.0, electrolytes, the voltammetric current at modest overpotentials, i.e., between -0.6 V and -1.4 V vs. Ag/AgCl, was distributed between metal deposition and H3O+ reduction, with both reactions reaching mass transport limited current values. At more negative potentials, an unusual sharp current spike appeared upon the onset of H2O reduction that was accompanied by a transient increase in H2 production. The peak potential of the current spike was a function of both [Ni(H2O)6]2+(aq) concentration and pH. The sharp rise in current was ascribed to the onset of autocatalytic H2O reduction, where electrochemically generated OH- species induce heterogeneous nucleation of Ni(OH)2(ads) islands, the perimeter of which is reportedly active for H2O reduction. As the layer coalesces, further metal deposition is quenched while H2O reduction continues albeit at a decreased rate as fewer of the most reactive sites, e.g., Ni/Ni(OH)2 island edges, are available. At potentials below -1.5 V vs. Ag/AgCl, H2O reduction is accelerated, leading to homogeneous precipitation of bulk Ni(OH)2·xH2O within the nearly hemispherical diffusion layer of the UME.