The options for storing hydrogen can be divided into physical methods (increase of volumetric energy density by changing physical parameters such as pressure or temperature) and material-based methods (increase of volumetric energy density by binding to certain materials or forming new chemical compounds). An overview is given in the table below.

The following graph shows the volumetric and gravimetric energy densities that can be achieved with the different storage methods. The squares symbolise liquid storage media, the diamonds represent compressed gases and the triangle the storage in a solid medium. 

It becomes clear that physical storage methods increase the volumetric energy density, while the gravimetric energy density of the stored hydrogen (33,33 kWh/kg) is not changed. In the case of material-based storage options, due to the formation of the new chemical compounds the volumetric energy density increases, but the gravimetric energy density decreases.