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(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. Introduction Electrochemical impregnation is one of the main processes for the production of nickel hydroxide electrodes, because it yields superior electrodes with longer life.' This process involves the electrochemical reduction of NO; within a porous nickel sinter and the subsequent generation of OH-. Although any or all of reactions I-i through I-S shown in Table I could be involved, the primary result is the production of OH-, which may react further with Ni2t species to form Ni(OH), according to reaction 1-6. Recently, Streinz et al.2 reported the anomalous deposition behavior of Ni(OH), wherein they observed a drastic decrease in deposition rates as Ni(NO2)2 concentration increased from 0.1 to 2.0 M. They measured deposition rates for different currents and Ni(NO,)2 concentrations using an electrochemical quartz crystal microbalance (EQCM). In 2.0 M Ni(NOj, solutions, they observed only 10% utilization of the electrochemically generated OW species, as compared to almost 100% utilization in 0.1 M solutions at 2.5 mA/cm2. They concluded that the diffusion of an intermediate, Ni,(OH)r species was responsible for this inefficiency. Baes and Mesmer' also mention that for high Ni't concentrations > 0.1 M), the Ni4(OH) species is formed predominantly. Based on these observations, Streinz et al.' postulated a two-step deposition mechanism shown by reactions I-? and 1-8. The deposition occurs at the surface of the electrode which is saturated in hydroxyl ions and where the pH is between 6.5 and 8. The pH at which deposition begins is a strong function of Ni(NO,), concentration, as shown in Fig. la. * Electrochemical Society Student Member. * * Electrochemical Society Active Member. Figure la shows that pH affects the equilibrium distribution of nickel species for reactions 1-7 through I-il. (The calculations used for this figure are discussed in Appendix A.) The pH at which Ni(OH), forms decreases from 7.2 for 0.5 M Ni(NO,), to 6.8 for 4 M solutions. Figure la also shows that, in 4 M solutions, about 75% of the nickel exists as Ni,(OH) prior to the deposition of Ni(OH),. This maximum decreases to 65 and 40% for 2 and 0.5 M solutions, respectively. Figure la shows that at a pH less than S almost all the nickel exists as its divalent ion. These calculations are consistent with our measurements of the pH for various temperatures and concentrations of Ni(NO3), shown in Fig. lb. (See Appendix B for a discussion of the experimental technique.) Figure lb shows that the pH of Ni(NO,), solutions drops from 3.75 for 0.5 M to 1.2 for 4.0 M (at 25°C). A similar trend is also observed at higher temperatures. The change in pH is attributed to the generation of Ht mainly due to the formation of hydrolysis products like NiOHt according to reaction I-il. We calculate that, for 4.0 M solutions, about 2% of Ni2t is bound to OW generated from the water equilibrium reaction, and this results in a pH of 1.2. However, this value is difficult to see on the scale in Fig. la. Note that a comparison of the equilibrium constants for reactions 1-10 and 1-7 indicates that the concentration of NiOHt is small in the region where Ni,(OH)r exists. The purpose of this paper is to test the hypothesis of Streinz et al.2 by developing a model and comparing the predictions with their experimental results. Other workers'' have deposited films on planar electrodes but made no attempt to quantify the deposition process over a range of deposition conditions. For example, Cordoba-Torresi Table I. Elecfrochemical and chemical reactions considered in the model. (All values correspond to standard conditions at 25°C.) No; + H,O + 2e -. NO; + 2 OHNO; + 511,0 + 6e -*NH, + 7 OHNO; + 611,0 + 8C -' NH, + 9 OH2NO; + 4H,O + 6e -. N, + 8 OH2N0; + 311,0 + 4e -' N,O + 6 0H Ni' + 2 0H '-' Ni(OH),l Ni't + OH'-° 1/4 Ni,(OH)t 1/4 Ni,(oH)r + 0H '-'Ni(OH,)l Ht + 0H '-° 11,0 Ni" + OH '-' NIOHt Ni't + H,O '-' NiOH + Ht U' = 0.01 V Li' = —0.165 V U' = —0.12 V U' = 0.406 V LI' = 0.15 V K,.,,, = 1.6 X 10" (mol/cm')' (see Ref. 12) K,,, = 2.63 x 10" (mol/cm')"4 K,,, = 3.3 x 10—" (mol/cm')" b K.,,,, = 1 x 10'° $mol/cm')2 K,, = 1.38 x 10 cm'/mol d K,, = 1.38 X 10" mol/cm' (see Ref. 13) [1-11° [I-2]° [1-31° [1-4] [1-5] [1-6] [1-7] [1—8] [1-9] [1-10] [I-li] ° Equation1-3 used in the model is a combination of I-i and 1-2. Calculated from K,,,, and K,,, of reactions 1-6 and 1-7. ° Calculated by using K,,,, and reported value of log K,,, = —27.32 from Ref. 9. Calculated using K,,,, and K,,. J. Elect rochem. Soc., Vol. 143, No.7, July 1996 The Electrochemical Society, Inc. 2319 J. Electrochem. Soc., Vol. 143, No. 7, July 1996 © The Electrochemical Society, Inc.