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

This article outlines a simple 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. The general applicability to experimental reality is demonstrated with pinpoint-accuracy by means of worked examples from metal hydride chemistry and electrochemistry (see ESI). Highlights of the metal hydride cases are why the substitution of 4 mol % Na by K in Ti-doped sodium alanate raises the reversible storage capacity from 3.3 to 4.7 % w/w H and the outlining of the additional reaction pathway in Mg(NH₂)₂/2LiH when doped with (Rb/K)H, increasing the reversible storage capacity from 3.6 to 4.4 % w/w H. The electrochemical case study derives the theoretical specific energy threshold for Li-batteries (274 Ah kg^-1) from the standard hydrogen electrode potential and then figures for a practical example the reversible specific energy (a T5 NAS-battery cell by NGK Insulators Ltd.).