Hydrogen Storage in a Potential Well for Room-Temperature Applications

Hydrogen has a gravimetric energy density nearly 3 times that of gasoline, and yet does not emit carbon dioxide at all in burning. However, since hydrogen exists in a bulky gas form at ambient conditions, the problem in the safe and efficient storage of hydrogen is a serious obstacle to the commercial use of hydrogen as a fuel. There is a recent proposal that, if we store the molecular hydrogen gas in a potential well created inside a material, the storage density increases by a Boltzmann factor which can be enormously large with a moderate potential depth even at room temperature. In the present study, we carry out a theoretical analysis for the storage at room temperature. Starting from a realistic description of hydrogen gas using the van der Waals equation of state, the expression for the density enhancement in the potential well is obtained. Especially, the first-order expansion of the equation provides us with an analytic solution which correctly shows the effects of the excluded volume and the dispersion interaction. We interpret and discuss the results taking into account the limitations of the van der Waals model. Keywords-hydrogen storage; potential well; van der Waals equation