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. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/12044 |
| Date | 27 August 2020 |
| Creators | Aiyejuro, Victor Omoatokwe |
| Contributors | Harrington, David A. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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