Hydrogen (whether it is liquid, compressed gas at 700 bar, or compressed gas at 350 bar) has a gravimeter energy density of 120 MJ/kg H2.

However its volumetric energy density (measured in MJ/L) varies: For liquid H2 it is about 9 MJ/L, for compressed H2 gas at 700 bar, it is about 4.5 MJ/L, and for compressed H2 gas at 350 bar, it is about 3 MJ/L.

Because H2 gas is very light with mass density of about 0.09 g/L (i.e., 9.0E-5 kg/L), its gravimeter energy density is higher than its volumetric energy density. For this reason, H2 gas is compressed to 350 bar or 700 bar for on-board light-duty vehicles application in order to reduce the footprint of the on-board H2 storage tanks

For additional information you're welcome to download my published manuscript on hydrogen storage for on-board light duty vehicles from my profile under ResearchGate.net

Hope this helps answer your question and good luck!

Gravimetric energy density of pure hydrogen is constant – 142.8 MJ/kg, but the volumetric energy density is low (only 12.8 kJ/L at NTP and near 10 MJ/L in liquid phase) due to very low density of hydrogen gas. This is the key problem for storing hydrogen gas in quantities, necessary for many applications, where hydrogen can be used as fuel.

The volumetric density of hydrogen can be essentially increased if it will be stores in solid phase, for example, in the form of metal hydrides and/or metallic complexes. The volumetric energy density of stored this way hydrogen will appear higher, too (TiH2 – 21.375 MJ/L, MgH2 – 15.7 MJ/L, LiBH4–17.5 MJ/L). However, the gravimetric energy density of these storage materials obviously will be lower than for pure hydrogen (TiH2 – 5.7 MJ/kg, MgH2 – 10.85 MJ/kg, LiBH4 –26.25 MJ/kg). Moreover, if in calculations we account also the weight of storage containers, then their gravimetric energy densities will appear about 20-50% lower.