A Dynamic Impedance Study of the Initial Stages of Nickel Oxidation

The nature of the species that form on oxidizing Ni electrode in alkaline solution has been studied for a long time. Bode summarized the formation and interconversion of these hydroxide and oxyhydroxide phases in terms of time and potential [1], but their structure and kinetics remain an active area of study [2]. The initially formed oxide is believed to be a hydrated hydroxide, α-Ni(OH)2, which irreversibly interconverts to a less hydrated β-Ni(OH)2 at higher potentials. The formation and interconversion of these hydroxide phases is studied here by dynamic electrochemical impedance spectroscopy (dEIS) [3,4], which applies a multi-frequency waveform during a cyclic voltammetry sweep or during a chronoamperometry experiment. It enables the study of non-steady states of the surface that cannot be accessed by conventional potentiostatic EIS. It is particularly suited to study the irreversible α-Ni(OH)2 to β-Ni(OH)2 interconversion. An electrochemical polishing procedure was used to remove the oxide layer in order to have a clean and oxide-free surface before doing the experiment. This step consisted of galvanostatic holding in phosphoric acid solution. Impedance data were collected during voltammograms at different slow sweep rates, with selected hold periods before, during, or after the peaks, and were correlated to charges under the voltammograms. The impedance data were fitted to equivalent circuits. In most cases the faradaic impedance approximated a charge-transfer resistance, but the use of complex capacitance plots showed evidence for additional structure at low frequencies. Comparison of the the polarization resistance determined from the slope of the voltammograms with the estimated low-frequency intercept of the impedance showed a discrepancy in the peak region, which also suggests that a slow process exists that whose kinetics is not determinable by dEIS. The sweep-hold experiments followed the conversion of the α phase to the β phase, and found that the oxidation is not complete in the potential range of 0.2-0.3 V. Voltammograms of the admittance at selected frequencies were found to be sensitive to the surface condition, and showed that the surface condition did not return to the same value after cycling. This research was conducted as part of the Engineered Nickel Catalysts for Electrochemical Clean Energy project administered from Queen’s University and supported by Grant No. RGPNM 477963-2015 under the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Frontiers Program. [1] H. Bode, K. Dehmelt and J. Witte. Electrochim. Acta., 11, 8 (1966). [2] M. Alsabet, M. Grden and G. Jerkiewicz, Electrocatalysis, 2, 4 (2011). [3] R.L. Sacci, F. Seland and D.A. Harrington, Electrochim. Acta., 131, 13 (2014). [4] R.L. Sacci and D.A. Harrington, ECS Transactions, 19, 31 (2009).

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