The Key to the Problem of Reversible Chemical Hydrogen Storage and Reversible Energy Storage Alike is 12 kJ (mol H₂)^-1

This article outlines a potent theoretical formalism illuminating the boundaries to reversible solid hydrogen storage based on the ideal gas law and classic equilibrium thermodynamics. A global picture of chemical reversible hydrogen sorption is unveiled including a thermodynamic explanation of partial reversibility. This is utilized to elucidate a multitude of issues from metal hydride chemistry (as ESI): Highlights are explanations why the substitution of a mere 4 mol % Na by K in Ti-doped NaAlH₄ raises the reversible storage capacity by 42 % and elaboration of the utmost probable reaction pathway in (Rb/K)H-doped Mg(NH₂)₂/2LiH. The ESI further contains a demonstration of relevance to electrochemistry by means of the NAS-battery cell, concisely predicting the starting point of the cell voltage drop where experiment shows it to be. The findings of this work allow for a change of paradigm towards the understanding of reversible chemical energy storage and provide a hitherto sorely missing tool of tremendous analytic and predictive power, complementary to experiment.