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

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posted on 14.11.2016, 00:00 by Nicole L. Ritzert, Thomas P. Moffat
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 (UMEs) coupled with optical imaging, generation/collection scanning electrochemical microscopy, 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 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.