The capacity of metal hydrides (e.g., lithium hydride, magnesium hydride, aluminum hydride, etc.) in storing hydrogen is based on a mass basis, This is called the gravimeter capacity (mass of H2 per unit mass of the hydride).

Please note that about 1/3 of the energy is lost in liquifing H2 gas and, hence, it is an energy intensive venue compared to storing H2 in the hydride form. Also, liquid H2 suffers from the boil off issue which is a safety concern.

Please see my uploaded papers on metal hydrides using the following URL:

https://www.researchgate.net/profile/Yehia_Khalil3

Hope this helps answer your question.

Professor Yehia Khalil, Yale University, USA

Fellow of the University of Oxford, UK

 Thanks for your rely

But your response doesn't cover my question.

I am curious about the situation in volumetric capacity

Qualitatively speaking, the actual volume that is occupied by molecules/atoms, depends on their chemical surrounding and/or steric conditions. For example, mixtures of alcohol and water obtain less space then the separated liquids. More or less, this is the answer you gave yourself already.

So to speak, also liquid water contains 110 g/L of hydrogen (chemically bound, of course). 

To give a quantitive answer, one would have to understand how and where exactly in the solids the hydrogen is bound.  

Thanks!!!!! your kind answer!!!!!!