The Effect of Zincate Ions in Alkaline Solution on the Electrochemical Behavior and Crystal Structure of Nickel Cathode
Authors: Matsuike Ai , Yamada Hirohisa , Katakura Katsumi 
Published: December 2020

Introduction
Zn is known as one of the most promising anode candidates for aqueous rechargeable batteries because it is a low cost and an abundance, and shows a comparatively higher hydrogen overpotential. When Zn is used with nickel cathode to construct a rechargeable alkaline Zn/ Zn/NiOOH battery, it shows high specific energy of 55-85 Whkg-1 and excellent specific power of more than 200 Wkg-1 1). However, a rechargeable alkaline Zn/NiOOH battery has not been commercialized yet because dendrite formation and shape change of Zn during charge-discharge cycles shorten the lifetime of a Zn anode, and the ZnO dissolved in a concentrated alkaline solution might affect the charging and discharging behaviors of the NiOOH cathode. Though numerous works have been done to prevent the dendritic formation and shape change of Zn anode during charging and discharging cycles,2) the works on the effect of dissolved Zn on NiOOH electrode of the cathode of Zn/NiOOH battery are relatively small, and the effect is unclear. For example, migration of dissolved Zn to the NiOOH electrode might cause the capacity loss of the cathode and, zinc-additive on Ni(OH)2 increases the cathode potential3,4). In this study, we have investigated the effect of Zn dissolved in a concentrated alkaline solution on redox reaction and crystal structures of NiOOH cathode to obtain an indication for designing the cathode.
Experimental
Ni(OH)2 is electrochemically precipitated on a Pt sheet with the constant current density of -1.6 mA cm-2 for 10 min from 0.02 M Ni(NO3)2 aqueous solution at room temperature. Cyclic voltammetry was adopted to study the redox behaviors of the Ni(OH)2/Pt in ZnO saturated or ZnO free 6 M KOH at the scan rate of 1 mV s-1 using a pair of Pt sheets counter electrodes and a 1 M Hg/HgO reference electrode. XRD and ICP and SEM/EDX were used to characterize the nickel oxides sample on a Pt sheet at the charged and the discharged states.
Results and Discussion
In the cyclic voltammograms of Ni(OH)2/Pt in ZnO free 6 M KOH, two oxidation peaks at around 350 mV and 400 mV, which are ascribed to oxidation of Ni(OH)2, were observed during anode sweep and a reduction peak at around 210 mV, which is ascribed to the reduction of NiOOH, was observed during cathode sweep. While the oxidation peak currents at 350 mV gradually decreased with increasing the cycle number and those at 400 mV increased with the cycle numbers, the reduction peaks gradually shifted towards an anodic direction about 40 mV after 20 cycles. In ZnO sat. 6M KOH, on the other hand, only one oxidizing peak and one reduction peak were observed at around 420 mV and 280 mV, respectively. Though the anodic potential sifted 20 mV towards anodic direction after 20 cycles, no significant changes in both potentials and shapes were observed. Degradation in the discharge capacities of NiOOH/Pt during the CV cycles in ZnO sat. KOH was smaller than that in ZnO free KOH. The results indicated that the charge and discharge stability Ni(OH)2/NiOOH was improved by the presence of zincate ions in the electrolyte. The redox peak potentials in ZnO sat. KOH is similar to those of Zn doped Ni(OH)2
4)

Regardless of cycle number, the XRD patterns of the nickel oxides on a Pt sheet after CV cycles in ZnO sat. 6M KOH indicated that the peaks for the reduced and the oxidation states are in good agreement with those of Zn substituted α-Ni(OH)2 and γ- NiOOH, respectively 5,6). The distance corresponds to (003) of Zn substituted α-Ni(OH)2 is approximately 8.4 Å. When as prepared Ni(OH)2/Pt was immersing in ZnO sat. 6M KOH for 4 hours at room temperature, the XRD patterns coincide with those of Zn substituted α-Ni(OH)2, while α-Ni(OH)2 is assumed to be preferentially formed.
In addition to the above results ICP and EDX measurement indicated that Zn might be incorporated into Ni(OH)2 when it is immersing in ZnO sat. 6 M KOH. Therefore, the formation of Zn substituted α-Ni(OH)2 in ZnO sat. 6 M KOH has a possible advantage on the stabilization of the redox reaction of Ni(OH)2/NiOOH.
References
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(3) W.H. Zhu, Journal of Power Sources, 56,75,(1995)
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(5) C. Tessier et al, Solid State Ionics, 133, 11 (2000)
(6) C. Tessier et al, J. Mater. Chem, 10, 1185 (2000)