A Model for the Galvanostatic Deposition of Nickel Hydroxide

A mathematical model is presented for the galvanostatic deposition of Ni(OH)2 films in stagnant Ni(NO3)2 solutions. The objective is to quantify the anomalous deposition behavior reported previously in which the utilization of the electrochemically generated OH– species decreased drastically as the concentration of Ni(NO3)2 increased beyond 0.1 M. For example as the Ni(NO3)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\u27s law indicates that only 10% of the OH– species generated at the surface led to deposition. It has been proposed that the inefficient use of electrochemically generated OH– species is due to the presence of Ni4(OH) as an intermediate in the deposition process. As the bulk Ni(NO3)2 concentration increases, the concentration of Ni4(OH) 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 OH– from the electrochemical reduction of nitrate to ammonia and the diffusion and migration of Ni2+, NO, OH–, H+, 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(NO3)2 concentrations. The present work confirms the role that Ni4(OH) plays in the deposition process and provides a fundamental framework for understanding the electrochemical impregnation of nickel electrodes