A Model for the Galvanostatic Deposition of Nickel Hydroxide

Abstract

A mathematical model is presented for the galvanostatic deposition of Ni(OH)2 films in stagnant Ni(NO,), solutions. The objective is to quantify the anomalous deposition behavior reported previously in which the utilization of the electrochemically generated OW species decreased drastically as the concentration of Ni(N03)2 increased beyond 0.1 M. For example, as the Ni(NO,)2 concentration increased from 0.1 to 2.0 M, the deposition rate decreased by a factor of ten at 2.5 mA/cm2. At this high ratio of concentration to current density, a comparison with Faraday's law indicates that only 10% of the OW species generated at the surface led to deposition. It has been proposed that the inefficient use of electrochemically generated OW species is due to the presence of Ni4(OH)r as an intermediate in the deposition process. As the bulk Ni(N0,), concentration increases, the concentration of Ni4(OH)r at the electrode surface increases. A high concentration of the intermediate results in an increase in the diffusion rate of the species away from the electrode surface and thus a decrease in the deposition rate. Here, this hypothesis is tested by developing a model which includes the generation of OW from the electrochemical reduction of nitrate to ammonia and the diffusion and migration of Ni2t, No;, OW, Ht, and Ni4(OH). The model predictions agree well with previously reported mass deposition data collected using an electrochemical quartz crystal microbalance at different currents and over a range of Ni(N03)2 concentrations. The present work confirms the role that Ni,(OH) plays in the deposition process and provides a fundamental framework for understanding the electrochemical impregnation of nickel electrodes.

Effect of Surface pH on Electrodeposited Ni Films

“…The concentration of NiOH + is always small enough to be neglected within the range of pH examined in this paper, which coincides with Murthy et al 28 The diffusion flux of 4D Ni 4 ͑OH͒ 4 4+͑ ‫ץ‬C Ni 4 ͑OH͒ 4 4+ /‫ץ‬x͒ x=0 potentially contained in Eq. 8 includes C Ni 2+ through i Ni as follows…”

“…The pH at the cathode surface remains an interesting subject for the researchers to examine the electrodeposition mechanism of transition metals. 28,[35][36][37] The pH value at the cathode surface changes due to the overall reactions…”

“…It was reported that Ni 4 ͑OH͒ 4 4+ is predominant rather than NiOH + among the Ni 2+ complexes in the concentrated solution. 27,28 The first species stems from H 2 SO 4 as the pH adjuster only when the pH is set to 1.5. Thus, n, H n A, and A n− represent one, HSO 4 − , and SO 4 2− , respectively.…”

Electrochemical studies on electrolytic preparation of battery grade nickel hydroxide – Effect of (OH)− to Ni2+ ratio

“…For the sake of argument it may further be assumed that Ni 2þ is not available in the bath to consume OH À by reacting with it. It is then the diffusivity, D, which is 5.6 Â 10 À5 cm 2 s À1 would play a vital role [21]. Hence, J/D is 0.46 M cm À4 and by common analogy (from Ficks Law and units of the terms), it equals to the concentration gradient through unit distance.…”

“…The XRD pattern shows a reasonably prominent peak of a-nickel hydroxide in the sample precipitated at lower nickel concentration and [OH À ]/[Ni 2þ ] ratio as high as 6.72. This may be due to the low diffusivity value (0.69 Â 10 À5 cm 2 s À1 ) of Ni 2þ [21]. Although the [OH À ]/[Ni 2þ ]ratio is above 6, due to the low concentration of Ni 2þ with low diffusivity, the nickel ions do not take positions quickly and hence, crystal growth does not proceed at a speed to avoid H 2 O intercalation.…”