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) HO and HO 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 NiCl + 0.1 mol/L NaCl, pH 3.0, electrolytes, the voltammetric current at modest overpotentials, , between -0.6 V and -1.4 V vs. Ag/AgCl, was distributed between metal deposition and HO reduction, with both reactions reaching mass transport limited current values. At more negative potentials, an unusual sharp current spike appeared upon the onset of HO reduction that was accompanied by a transient increase in H production. The peak potential of the current spike was a function of both [Ni(HO)] concentration and pH. The sharp rise in current was ascribed to the onset of autocatalytic HO reduction, where electrochemically generated OH species induce heterogeneous nucleation of Ni(OH) islands, the perimeter of which is reportedly active for HO reduction. As the layer coalesces, further metal deposition is quenched while HO reduction continues albeit at a decreased rate as fewer of the most reactive sites, , Ni/Ni(OH) island edges, are available. At potentials below -1.5 V vs. Ag/AgCl, HO reduction is accelerated, leading to homogeneous precipitation of bulk Ni(OH)·HO within the nearly hemispherical diffusion layer of the UME.