Two Level Model of Hydrogen Diffusion in Nanoporous Solids with Strongly Bound Adsorption States

Abstract

A detailed understanding of hydrogen adsorption and diffusion in nanoporous solids is essential to design high-density storage materials. A two level lattice model with a dynamics of thermally activated molecular jumps, inspired by experimental evidence on cubic nitroprussides, is proposed for solids with strongly and weakly bound states coexisting in nanopores. The model provides a simple, unified theoretical framework to study both the thermodynamics and kinetics of hydrogen adsorption–desorption and to calculate, with minimum computational effort, quantities which can be directly compared to experimental results. In the linear and continuous limit, retarded diffusion is described by an integro-differential equation and a frequency-dependent diffusion coefficient. The transport diffusion coefficient shows a very strong loading dependence, with a high coverage behavior determined by intermolecular interactions. Desorption kinetics at constant external pressure and temperature is obtained from the analytical solution of the diffusion equation and compared to the results obtained from the numerical solution of nonlinear balance equations and from Monte Carlo simulations. The relevance of nonlinear, grain-size, and fluctuations effects is discussed. Fluctuations inside pores lead to fast, subdiffusive growth of desorption times with grain size in the nanometric range.