Modeling the kinetic behavior of the Li-RHC system for energy-hydrogen storage : ( I ) absorption

The Lithium-Boron Reactive Hydride Composite System (Li-RHC) (2 LiH + MgB$_{2}$ / 2 LiBH$_{4}$ + MgH$_{2}$) is a high-temperature hydrogen storage material suitable for energy storage applications. Herein, a comprehensive gas-solid kinetic model for hydrogenation is developed. Based on thermodynamic measurements under absorption conditions, the system's enthalpy $\Delta$H and entropy $\Delta$S are determined to amount to -34 $\pm$ 2 kJ $\cdot$ mol H$_{2}^{-1}$ and -70 $\pm$ 3 J $\cdot$ K$^{-1}$ $\cdot$ mol H$_{2}^{-1}$, respectively. Based on the thermodynamic behavior assessment, the kinetic measurements' conditions are set in the range between 325 {\deg}C and 412 {\deg}C, as well as between 15 bar and 50 bar. The kinetic analysis shows that the hydrogenation rate-limiting-step is related to a one-dimensional interface-controlled reaction with a driving-force-corrected apparent activation energy of 146 $\pm$ 3 kJ $\cdot$ mol H$_{2}^{-1}$. Applying the kinetic model, the dependence of the reaction rate constant as a function of pressure and temperature is calculated, allowing the design of optimized hydrogen/energy storage vessels via finite element method (FEM) simulations.