In Situ Quartz Crystal Microbalance Studies of Nickel Hydrous Oxide Films in Alkaline Electrolytes

Abstract

The microgravimetric characteristics of electrodeposited nickel and composite Ni/Co hydrous oxide films on Au electrodes in KOH electrolytes were examined in situ with a combination of quartz crystal microbalance (QCM) and voltammetric techniques. In the case of freshly prepared a-Ni(OH)2 hydrous oxide films, denoted as a-Ni(OH)2(hyd), the mass increased during oxidation and then decreased upon subsequent reduction. As the redox cycling was continued, however, this behavior gradually reversed, i.e., the mass decreased upon oxidation and then increased following further reduction. This unique effect could be correlated with a shift in the position of the oxidation peak maximum (E°°)in the voltammogram toward more positive potentials, which has been ascribed to the transformation of a-Ni(O)2(hyd) into 3-Ni(OH)2(hyd). Based on these findings, it can be concluded that the mechanism of ionic charge transport in these two types of lattices is markedly different. In contrast, composite Ni/Co (9:1) hydrous oxide films displayed qualitatively the same behavior as pure freshly prepared Ni(OH)2(hyd) in the same alkaline media regardless of the extent of cycling. This observation provides evidence that the incorporation of cobalt into Ni(OH)2(hyd) modifies the structural properties of the lattice during the oxidation/reduction process.

A nonstoichiometric structural model to characterize changes in the nickel hydroxide electrode during cycling

“…Again, the symbols are a mean of 3 -4 data sets, and the error bars represent the high and low values. These data exhibit three characteristics that are consistent with previous literature observations: (a) approximately 1.67 electrons per nickel are transferred in the first charge [22]; (b) 1.0 electron per nickel is transferred during the subsequent discharge and charge [22]; and (c) the following discharge and charge capacities steadily decrease with cycling [8,15]. It should be noted that the less than 1 electron estimated in this figure cannot be explained by the traditional α−γ and the β−β cycles as both these reaction suggest electron transfers ≥ 1.0.…”

“…The authors ignore experimental data from the first charge, the formation process. Finally, using n 1 =1 with a defect content of 0.25, the maximum oxidation state for nickel is limited to 3.33, which is well below the reported literature value of 3.67 [15,22]. This prevented them from understanding any process involving an oxidation state above 3.33, e.g.…”

“…The literature has shown that capacity and mass can be measured simultaneously using an electrochemical quartz crystal microbalance (EQCM) [7,8,[13][14][15][16]. However, in the previous work, capacity could not be meaningfully linked with mass changes, because no account was made for the oxygen evolution reaction that is simultaneous with the nickel redox reaction.…”

“…In addition, atomic force microscopy (AFM) imaging [17,18] suggests that electrolyte, presumably within the pores or between the crystallites, is exchanged during charge/discharge. Recognizing that such exchanges can be studied using an EQCM, numerous researchers have provided mass change data under various conditions (e.g., charge/discharge, [13,14] and long-term cycling [7,8,15]. )…”

“…However, the interpretation provided by most authors is qualitative [8,15]. While some authors provide quantitative interpretation of the data (see Bernard et al.…”