Interconversion of nickel hydroxides studied using dynamic electrochemical impedance
Date
2020-08-27
Authors
Aiyejuro, Victor Omoatokwe
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Abstract
The interconversion of α- and β-Ni(OH)₂ was studied using cyclic voltammetry and dynamic electrochemical impedance (dEIS). Holding experiments were done at 0.5 V, 0.6 V, 0.8 V and 1.0 V while subsequent cathodic holds were applied in selected experiments at -0.1, -0.2, -0.25 V. The number of thickness of Ni(OH)₂ formed increased with increasing anodic potential.
After α-Ni(OH)₂ was formed (< 0.5 V), it was easily reduced by sweeping down to -0.15 V. However, sweeping further (> 0.5 V) resulted in its "irreversible" conversion to β-Ni(OH)₂. Since β-Ni(OH)₂ was not reduced by sweeping to -0.15 V, the current, capacitance and the conductance at the α-Ni(OH)₂ peak (at 0.2 V) decreased as a result.
However, β-Ni(OH)₂ was shown to be reducible during potential holds at -0.2 V or lower. In contrast, holding at -0.1 V only resulted in partial reduction. Eventually, a link was established between the reduction of β-Ni(OH)₂ and hydrogen evolution. The relatively slow reduction of the β-Ni(OH)₂ to metallic nickel appears to inhibit the capacitance increase at -0.15 V which occurs when the potential is kept under 0.5 V. The retention of a low capacitance while β-Ni(OH)₂ persists suggests a blocking mechanism. A concerted adsorption-desorption step which generates adsorbed hydrogen prior to hydrogen evolution was proposed.
An exponential increase in current and capacitance occurred during the potential hold at -0.2 V. The capacitance increase suggests a reversal of the blocking (low capacitance at -0.15 V) caused by the persistence of β-Ni(OH)₂.
Additionally, the exponential current decay during the hold at -0.2 V was significantly slower than the conversion of α- to β-Ni(OH)₂ at 0.8 V. This further demonstrates the possibility of a slow step involving surface blocking during the reduction of β-Ni(OH)₂.
These observations provide new information on the mechanism and kinetics of the interconversion of α-Ni(OH)₂ into β-Ni(OH)₂ and the interaction of the latter in the hydrogen evolution reaction.
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Keywords
Impedance, Nickel hydroxide, Interconversion of Nickel hydroxides, Dynamic Electrochemical Impedance, Interconversion of Nickel Hydroxides Studied using Dynamic Electrochemical Impedance, Dynamic Impedance, dEIS, Nickel hydroxides studied using Dynamic Electrochemical Impedance, using Dynamic Electrochemical Impedance, Nickel