Relationship Between Equilibrium Hydrogen Pressure and Exchange Current for the Hydrogen Electrode Reaction at MmNi[sub 3.9−x]Mn[sub 0.4]Al[sub x]Co[sub 0.7] Alloy Electrodes

Abstract

Nickel-metal hydride (Ni-MH) batteries have attracted significant attention during the last decade because of their applications in consumer electronics, electric vehicles (EVs), and hybrid electric vehicles (HEVs). Research and development of the negative electrode materials are, however, still essential for further improvement of energy density, power density, and rate capability. 1,2 The exchange current (density), I 0 , for the hydrogen electrode reaction is one of the most important parameters for understanding electrode kinetics and is a measure of electrocatalytic activity of the negative electrode. The exchange current is influenced by changing the alloy composition, by mixing the alloy with other metals or oxides, or by adding chemical reducing agents to the electrolyte in order to activate the electrode surface. [3][4][5][6][7] Equations for the exchange current for metal hydride electrodes have been proposed by many research groups, and the electrode reaction mechanisms have been discussed extensively. 3,4,[8][9][10][11] Enyo and Maoka 8 investigated the detailed mechanism of the hydrogen electrode reaction by overpotential transients on a Pd hydrogen electrode and presented individual rates of the elementary steps and the activity of hydrogen adatoms on the electrode surface. Yayama et al. 9 proposed a simple theoretical model to describe the dependence of the equilibrium potential and exchange current on the hydrogen content of crystalline TiMn 1.5 H x . The experimental data could, however, only fit the model at low H content. Yang et al. 10 derived a modified model for this dependence and obtained several parameters, such as the equilibrium constant of the hydrogen transfer process, the maximum value of the storable hydrogen concentration, and the reaction order of the electrode reaction, from the fit of a model with no H-H interaction to Yayama's experimental data. Wakao and Yonemura 11 investigated the rate determining process in the electrode reaction on hydrogen-absorbing metal electrodes, such as LaNi 5Ϫx Al x , by anodic polarization measurements, and determined the charge-transfer coefficient for selected electrodes. Furthermore, one of the present authors 3,4 derived an expression for the exchange current involved in the electrochemical formation/decomposition process, by considering the influence of electrode blocking by adsorbed hydrogen.The equilibrium hydrogen pressure (P H2 ) is another important parameter for hydrogen storage alloys to predict the electrochemical discharge capacity of the negative electrodes because the capacity could be calculated from the hydrogen storage capacity (H/M) between P H2 ϭ 5 and 0.1 atm based on pressure-composition isotherm (PCT) curves at 45ЊC. 12 For high energy density and high power density of Ni-MH batteries, it is essential that the negative electrode material combines an appropriate plateau pressure with excellent electrocatalytic activity for the hydrogen electrode reaction. The purpose of the present study is to propose a theoretical model to d...

Modeling of Electrochemical Hydrogen Storage in Metal Hydride Electrodes

“…6A ͑curve a͒ as a function of equilibrium potential of the MH electrode, agrees well with the experimental and theoretical results reported before by Notten et al 14 and Senoh et al 15 for AB 5 -type materials. ␣ and ⌸ ca are calculated to be 0.27 and 0.545 eV, respectively.…”

“…The charge-transfer coefficient ␣ was 0.35, which is quite close to the values ͑0.4-0.6͒ reported in Ref. 15. The potential barrier ⌸ ca that the system has to overcome to facilitate the chargetransfer reactions ͑Eq.…”

Electrochemical Hydrogen Storage Characteristics of Thin Film MgX (X=Sc, Ti, V, Cr) Compounds

“…The results are listed in Table I and are given as gravimetrical storage capacity in both ͓mAh/g͔ and ͓wt % H͔. The measured hy- 4,18 If the charging curves are compared, measured when hydrogenating the thin films for the first and second time, other interesting facts come to light ͓see Fig. 2-5, curves ͑a͒ and ͑d͔͒.…”

“…4 Recently, Notten et al showed that MgSc compounds exhibit very high storage capacities of up to six times that of the mischmetal-based materials. [5][6][7] Although this material can reversibly store a large amount of hydrogen, the main drawback is the very high cost of Sc.…”

Modelling of rechargeable NiMH batteries

“…Theoretical model energy storage density and can be operated at high current densities and low temperatures. The NiMH battery consists of two electrodes immersed of the NiCd battery [1,2]. This makes these two different in an alkaline electrolyte, which provides ionic conducbattery systems very compatible.…”