As a value of hydrogen absorption constant you can consider properties of various low-temperature metal hydrides candidates for stationary hydrogen storage applications. This is theor. H2 capacity, theor. kg-H2/m3 wrt to hydride. See
Article Application of hydrides in hydrogen storage and compression:...
Nam, Jinmoo, Johan Ko, and Hyunchul Ju. "Three-dimensional modeling and simulation of hydrogen absorption in metal hydride hydrogen storage vessels." Applied energy 89.1 (2012): 164-175.
I accidentally found your post. The robot does not send a message unless the name is clicked with "Mention". The constant Ca is determined experimentally in each particular experiment.