The usable capacity of porous materials for hydrogen storage

A large number of different porous materials has been investigated for their hydrogen uptake over a wide pressure range and at different temperature. From the absolute adsorption isotherms, the enthalpy of adsorption is evaluated for a wide range of surface coverage. The usable capacity, defined as the amount of hydrogen released between a maximum tank pressure and a minimum back pressure for a fuel cell, is analyzed for isothermal operation. The usable capacity as a function of temperature shows a maximum which defines the optimum operating temperature. This optimum operating temperature is higher for materials possessing a higher enthalpy of adsorption. However, the fraction of the hydrogen stored overall that can be released at the optimum operating temperature is higher for materials with a lower enthalpy of adsorption than for the ones with higher enthalpy.