Materials for hydrogen storage can be separated into different classes based on how they will be used in applications. In this sense there are three categories generally based on the energetics of storing hydrogen which we will designate as “Physisorption”, “On-board Reversible Hydrides”, and “Off-board Regenerable Hydrides” materials.
The relative strength of the interaction of hydrogen with a storage material greatly influences the kinetics and thermodynamics of the hydrogen uptake and release. Hydrogen binding mechanisms differ for physisorption storage and hydride storage materials, and the strength of the hydrogen binding affects the overall manner in which a hydrogen storage material will be used in a real application.
Materials for hydrogen storage can be separated into different classes based on how they will be used in applications. In this sense there are three categories generally based on the energetics of storing hydrogen which we will designate as “Physisorption”, “On-board Reversible Hydrides”, and “Off-board Regenerable Hydrides” materials. While not completely correct these are generically referred to as “Sorption”, “Metal Hydride”, and “Chemical Hydrogen” storage materials respectively.
The basic distinction these three classes of hydrogen storage materials are:
1) Physisorption materials involve weakly bound molecular hydrogen that is on-board reversible but generally require operating at liquid nitrogen temperatures (room-temperature atomic adsorption in the form of spill-over may be an exception).
2) On-board Reversible Hydride materials include the various metal hydrides described above, and some of the complex metal hydrides and amides…) that release hydrogen endothermically. Endothermic release allows for thermodynamically favorable, exothermic rehydrogenation during on-board recharging of the hydrogen storage materials under reasonable temperature ( < 300°C) and pressure ( < 200 bar) conditions.
3) Off-board Regenerable Hydride materials either release hydrogen exothermically, and/or involve complex chemical regeneration schemes that cannot be performed on board a vehicle. Examples in this class are the hydrocarbons, ammonia borane, and alane. The hydrocarbons and alane release hydrogen endothermically, but high hydrogen pressures or the complexity of their rehydrogenation most likely will require an off board process. Ammonia borane and other materials that release hydrogen rather exothermically cannot be rehydrogenated readily at common pressures and temperatures. This holds for compounds that release hydrogen with a free energy of greater than about 1kcal/mole exergonic.
