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

Abstract

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.

Exploring the Kinetic and Thermodynamic Relationship of Charge Transfer Reactions Used in Localized Electrodeposition and Patterning in a Scanning Bipolar Cell

“…Indeed, voltammetry in ascorbic acid free electrolyte when pH is controlled by additions of sulfuric acid (Figure 4C) shows a potential shift of the spike by only 122 mV/pH. This value is comparable to a similar study observing a voltammetric spike attributed to OH − generation by Equation 8, reporting a 110 mV/pH unit shift of the voltammetric feature (Ritzert and Moffat, 2016). Both potential shifts of the spike exceed that expected by thermodynamics for the 2 electron water reduction reaction, possibly indicating a more complicated mechanism for formation of the nickel hydroxide surface species than simply…”

Electrocatalytic hydrogen evolution reaction activity comparable to platinum exhibited by the Ni/Ni(OH) 2 /graphite electrode

“…The reduction of hydronium ions at the catholyte leads to the codeposition of Ni(OH) 2 along with Ni. The origin of Ni(OH) 2 generation has been explained earlier (25,29,31). Compared with direct current deposition (DP), cycling potentiostatic and galvanostatic pulses in PP changes the morphology of nickel deposits from pure nickel to layered Ni/colloidal Ni(OH) 2 deposits on the graphite substrate to a greater extent (Fig.…”

On the Control and Effect of Water Content during the Electrodeposition of Ni Nanostructures from Deep Eutectic Solvents

“…We believe that the formation of NiO x (OH) 2(1−x) is responsible for the growth self-termination, similarly to that reported in aqueous solutions. 5,6,40,41 Furthermore, we believe that the reduction of choline cations could also be responsible for the growth self-termination. Figure 10(a)).…”