Kinetic Behavior of Metal Hydride Electrode by Means of AC Impedance

A mathematical model f or the electrochemical impedance spectroscopy of a metal-hydride electrode was developed. The model was used to study the effect of various parameters on predicted kinetic behavior. The simulations obtained using the model show that the first arc appearing in the higher frequency range is due to a charge-transfer reaction, the second arc in the middle frequency range represents the hydrogen transfer between the absorption and adsorption, and the third arc (or curve) at low frequency corresponds to the diffusion of absorbed hydrogen in the alloy. As the rate constant of the charge-transfer reaction, the rate constant of hydrogen transfer, and the diffusion coefficient increase, all of the three arcs moved toward higher frequency range and the diameter of the arcs decreases. The model simulations also show that the state of discharge plays an important role in the Nyquist plot of the metal-hydride electrode. The kinetic parameters used in the model predictions are those for TiMni5Hr Introduction Recently, nickel/metal hydride (Ni/MET) batteries have been developed and commercialized because of their high energy density, high dischargeability, long charge-discharge cycle life, and environmental cleanliness.1 The electrode reactions at the negative and positive electrode in Ni/MET1 batteries can be expressed by M + H20 + e fl MH + OW Ni(OH)2 + OW fl NiOOH + H20 + e In charge/discharge processes, the concentration of H20 and OHis constant. Extensive work has been carried out on the cathodic hydrogen evolution reaction on hydrogen-absorbing metals.28 By considering that the electrochemical sorption/desorption of hydrogen by and from metallic hosts is diffusion limited, Conway and Wojtowicz5 performed numerical calculations for the time scales of the hydrogen desorption. Apart from the electrochemical and diffusion step, Yang et al.1 also considered hydrogen transfer through the interface and established a general relationship between the overpotential and the rate of hydrogen diffusion in the electrodes, as well as the kinetic parameters characterizing the transfer of hydrogen from the absorbed to the adsorbed state. Lei et al.8 first presented a mathematical model for the hydride electrode, i.e., the dependence of discharge capacity on discharge current and cycle life by considering the phase transformation and oxide film growth on the surface of metal particles during hydriding/dehydriding processes. Electrochemical impedance spectroscopy (ElS) is an effective technique for analyzing the mechanisms of hydriding/dehydriding reaction for a hydride electrode.9 Lim and Pyun'° derived a faradaic admittance of the hydrogen absorption reaction (HAR) on a Pd membrane electrode and analyzed the diffusion-controlled HAR for fast rate hydrogen transfer and the interface-controlled HAR for slow rate hydrogen transfer from bulk to Pd surface. Kuriyama * Electrochemical Society Active Member. et al.11'12 used the ac impedance method to investigate the degradation mechanism of metal hydride electrodes and activity of alloys. Using the electrochemical impedance spectroscopy (ElS) technique, Zhang et al.9 analyzed the mechanism of the hydriding/dehydriding reaction on the hydride electrode, and proposed an equivalent circuit for the MET1 electrode. However, in Zhang's equations, the diffusion of hydrogen and the phase transformation was not considered, which was a very important factor in the electrochemical hydriding/dehydriding reaction for the hydride electrode. Thus to date, the electrode kinetics of the hydriding/dehydriding reaction have not been well elucidated. In this present paper, ac impedance involving HAR for hydride electrodes with different spherical geometry is theoretically derived by considering the phase transformation in hydriding/dehydriding processes. The factors affecting kinetics of the reaction have been analyzed. Some equations deduced by Zhang et al.9 are also included here for completeness. AC Impedance Modeling Basic assumptions and model development—The electrochemical charge-discharge test and ac impedance measurement were carried out in a sealed standard threeelectrode cell, in which the counter electrode was nickel oxyhydroxide, the reference electrode was Ag/AgCl, and the electrolyte used in all experiments was 6 M KOH solution. The electrode is considered initially to be uniformly charged with hydrogen with a bulk concentration Camax for solid solution phase electrode and C5 for the hydride electrode, expressed in mole hydrogen concentration stored in the host metal. Thus, the molar concentration quotient in the bulk can be expressed according to Wagner'3 as .r = Ca/N, where N is the total molar number of interstitial sites available for hydrogen per unit volume of the host metal, and x, denotes the molar occupation quotient of absorbed hydrogen in the near-surface. The hydriding reaction involves a charge-transfer step followed by the surface transition from the adsorbed site on the electrode surface to the absorbed site in the near-surface [1]