Modeling of hydrogen storage in hydride-forming materials: Equilibrium gas-phase kinetics

Abstract

A kinetic model has been developed, describing the kinetics of the hydrogen storage reactions in hydrideforming materials under equilibrium conditions. Based on first principles chemical reaction kinetics and statistical thermodynamics, the model is able to describe the complex processes occurring in hydrogen storage systems, including phase transitions. A complete set of equations, governing pressure-composition isotherms in both solid-solution and two-phase coexistence regions has been obtained. General expressions for rate constant dependencies have been proposed, using well-defined phase-dependent Hamiltonians for the hydrogen energy state at the surface and in the bulk of hydride-forming materials. The characteristics of both model ͑LaNi y Cu 1.0 ͒ and commercial, misch-metal-based AB 5 -type materials at different compositions and temperatures have been simulated. Good agreement between experimental and theoretical results for the pressurecomposition isotherms has been found in all cases.

Modeling of Electrochemical Hydrogen Storage in Metal Hydride Electrodes

“…After the charge-transfer reaction at the electrode/electrolyte interface of the hydride-forming material ͑M s ͒ has been initiated, atomic hydrogen is chemically adsorbed ͑H ad ͒ at the electrode surface and is, subsequently, converted into the absorbed state ͑H abs ͒ and transported to the bulk of the hydride-forming material ͑M b ͒ by conventional solid-state diffusion. 9,12,21 In Fig. 1, the normalized hydrogen concentration in the bulk of the MH electrode is represented by x, and the normalized surface concentration is represented by , which is also known as the surface coverage.…”

“…12 All above models have been successfully applied to simulate the isotherms of various hydride-forming materials, including AB 5 -based and Mg-based alloys. [6][7][8][9][11][12][13] In the present paper, a further extension of the EKM is proposed to include the dynamic electrochemical ͑de͒hydrogenation kinetics, i.e., to include the exchange current density of the charge-transfer reaction, electrical double-layer charging, and ͑instantaneous͒ phase transition. The electrode/electrolyte interface properties are described by conventional electrochemical parameters, i.e., by the exchange current density and specific electrical double-layer capacitance.…”

“…12 This model was proposed as an extension of the KM. 9 It is also based on the first principles of electrochemical reaction kinetics and statistical thermodynamics and describes the complex, multistage, electrochemical ͑de͒hydrogena-tion process. A complete set of equations has been derived, describing the equilibrium hydrogen partial pressure and equilibrium electrode potential as a function of hydrogen content in both the solidsolution and two-phase coexistence regions.…”

“…10 Combining the thermodynamics and kinetics offered an interesting opportunity to estimate not only the hydrogen storage-related thermodynamic parameters but also the relevant kinetic reaction rates and rate constants. 9,11 Finally, an electrochemical kinetic model ͑EKM͒ was developed, describing the electrochemical hydrogen storage in hydride-forming materials under equilibrium conditions. 12 This model was proposed as an extension of the KM.…”

“…9 KM is based on the principles of statistical thermodynamics, described in LGM, [6][7][8] and also takes into account the complex gas-phase ͑de͒hydrogenation kinetics. The advantage of this kinetic approach is that it can, in principle, describe both equilibrium and nonequilibrium ͑dynamic͒ conditions.…”