Proton Diffusion in Nickel Hydroxide Films: Measurement of the Diffusion Coefficient as a Function of State of Charge

Electrochemical impedance spectroscopy (EIS) was used to measure the solid-state diffusion coefficient of protons in nickel hydroxide films at room temperature as a function of state of charge (SOC). A model for the complex faradaic impedance of the nickel hydroxide active material is presented and used to extract the diffusion coefficient of protons from the EIS data. Impedance data over a range of frequencies can be used to extract a constant diffusion coefficient without the knowledge of the initial mobile proton concentration or the form of the charge-transfer kinetic expression. The proton diffusion coefficient is a strong function of SOC and decreases approximately three orders of magnitude from 3.4 × 10–8 to 6.4 × 10–11 cm2 s–1 as the electrode discharges from the completely charged to the completely discharged state. The measurements were performed on well-conditioned nickel hydroxide films and therefore it is likely that the diffusion coefficients measured correspond to the -phase of the active material. The diffusion coefficient of protons was measured for three different film thicknesses, 1.5, 1.2, and 1.0 µm. The diffusion coefficient is independent of the thickness of the film as predicted by theory. The three orders of magnitude decrease in the diffusion coefficient of protons can be explained on the assumption that the protons move predominantly through the oxidized phase [NiOOH] which is interdispersed along with the reduced phase [Ni(OH)2] in the active material

Proton Diffusion in Nickel Hydroxide: Prediction of Active Material Utilization

Galvanostatic charge and discharge experiments reveal that the active material in nickel electrodes cannot be fully accessed at high currents or for thick films. It has been proposed that the utilization of the active material is controlled by the diffusion rate of protons through the film. This hypothesis is supported by the good agreement between mathematical simulations of material utilization and experimental data over a range of charge and discharge currents and film thicknesses. Furthermore, the fraction of material utilized is larger on charge than on discharge. The asymmetry on charge and discharge is due to a diffusion coefficient that is a function of the state-of-charge of the active material. The mathematical model is used to perform a parametric study of material utilization as a function of charge and discharge currents, and material loading (i.e., film thickness, concentration of nickel sites) in order to improve battery design and operation

The Effect of Temperature and Ethanol on the Deposition of Nickel Hydroxide Films

The objective of this work was to determine the effect of the temperature and the ethanol content of the Ni(NO3)2 solution on: (i) the efficiency of electrochemical deposition of nickel hydroxide; and (ii) the molecular weight of the deposited film. An electrochemical quartz crystal nanobalance (EQCN) was used to measure the mass of films electrochemically deposited from Ni(NO3)2 solutions and constant current discharges were used to determine the electrochemical capacity of the films. The data indicates that increasing the temperature increases both the efficiency of the deposition reaction and the molecular weight of the deposited film. The increased efficiency at higher temperatures is attributed to a decrease in the concentration of a nickel complex at the surface of the electrode. The lower complex concentration decreases the diffusion rate of this species away from the electrode surface and hence increases the rate at which the complex precipitates from the solution. The increase in the molecular weight at higher temperature is attributed to a combination of increased rate of deposition and an increase in the lattice spacing of the active material. The data also indicate that increasing the ethanol content of the solution had no noticeable effect on the efficiency of deposition, when water was present. In pure ethanol, however, the chemistry of deposition seemed to change considerably. However, increasing the ethanol content of the solution resulted in an increase of the molecular weight of the film. Increase in the molecular weight with an increase in the ethanol content of the solution is due to an increase in the relative percentage of ethanol incorporated in the active material. The data also indicate that the number of electrons in the discharge reaction is approximately 1.4 electrons per nickel atom

The Effect of Current and Nickel Nitrate Concentration on the Deposition of Nickel Hydroxide Films

An electrochemical quartz crystal nanobalance (EQCN) has been utilized to measure the mass of Ni(OH)2 films electrochemically deposited from Ni(NO3)2 solutions. The objective of this work was to quantify electrochemical deposition as a function of deposition conditions. The changing mass recorded on the EQCN was demonstrated to be the result of Ni(OH)2 deposition. Deposited mass was observed to increase proportionally with applied charge as suggested by previous investigators. Most significantly, the rate of deposition was found to decrease more than an order of magnitude as the Ni(NO3)2 concentration increased from 0.2 to 2.0M. The effect of concentration is shown to be related to Ni(II) concentration as opposed to solution pH or NO concentration. An empirical correlation is given to predict deposition rates in solutions ranging from 0.1 to 3.0M Ni(NO3)2 and at current densities ranging from 0.5 to 5.0 mA/cm2. The decreased deposition rates in concentrated Ni(NO3)2 are attributed to the formation of intermediate species [e.g., NiOH+ or Ni4(OH)] which diffuse away from the reaction interface before deposition can occur