The Impact of the Layer Thickness on the Thermodynamic Properties of Pd Hydride Thin Film Electrodes

Abstract

Recently, a lattice gas model was presented and successfully applied to simulate the absorption/desorption isotherms of various hydride-forming materials. The simulation results are expressed by parameters corresponding to several energy contributions, e.g., interaction energies. However, the use of a model system is indispensable in order to show the strength of the simulations. The palladium-hydrogen system is one of the most thoroughly described metal hydrides found in the literature and is therefore ideal for this purpose. The effects of decreasing the thickness of Pd thin films on the isotherms have been monitored experimentally and subsequently simulated. An excellent fit of the lattice gas model to the experimental data is found, and the corresponding parameters are used to describe several thermodynamic properties. It is analyzed that the contribution of H-H interaction energies to the total energy and the influence of the host lattice energy are significantly and systematically changing as a function of Pd thickness. Conclusively, it has been verified that the lattice gas model is a useful tool to analyze thermodynamic properties of hydrogen storage materials.

Mg-Based Thin Films as Model Systems in the Search for Optimal Hydrogen Storage Materials

“…The large impact on the thermodynamics was observed for rigid substrates which cause resistance toward volume expansion [29,30]. The increase of peq due to the stresses caused by capping layer or a substrate would be very desirable with respect to practical application if these stresses would not relax after few hydrogen absorption/desorption cycles drawing back the peq to its original value.…”

Modeling of Electrochemical Hydrogen Storage in Metal Hydride Electrodes

“…[6][7][8] Many microscopic parameters, such as interaction energies between absorbed hydrogen atoms, can be obtained by fitting the experimental data. The LGM agrees well with the experimental results of various hydride-forming materials.…”

“…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.…”

“…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.…”

“…As has been theoretically shown and experimentally proven, the charge-transfer kinetics is a function of the hydrogen content inside the solid. 14,15 The performance of the model is illustrated with results obtained with thin-film Pd electrodes, 7 which were used before to identify the parameters of the EKM.…”