Materials-Based Hydrogen Storage
There are presently three generic mechanisms known for storing hydrogen in materials: absorption, adsorption, and chemical reaction.
- Absorption. In absorptive hydrogen storage, hydrogen is absorbed directly into the bulk of the material. In simple crystalline metal hydrides, this absorption occurs by the incorporation of atomic hydrogen into interstitial sites in the crystallographic lattice structure.
- Adsorption. Adsorption may be subdivided into physisorption and chemisorption based on the energetics of the adsorption mechanism. Physisorbed hydrogen is more weakly and energetically bound to the material than is chemisorbed hydrogen. Sorptive processes typically require highly porous materials to maximize the surface area available for hydrogen sorption to occur and to allow for easy uptake and release of hydrogen from the material.
- Chemical reaction. The chemical reaction route for hydrogen storage involves displacive chemical reactions for both hydrogen generation and hydrogen storage. For reactions that may be reversible on-board a vehicle, hydrogen generation and hydrogen storage take place by a simple reversal of the chemical reaction as a result of modest changes in the temperature and pressure. Sodium alanate-based complex metal hydrides are an example. In many cases, the hydrogen generation reaction is not reversible under modest temperature/pressure changes. Therefore, although hydrogen can be generated on-board the vehicle, getting hydrogen back into the starting material must be done off-board. Sodium borohydride is an example.
DOE's materials-based storage activities are categorized as follows:
- Metal hydrides—reversible solid-state materials that can be regenerated on-board
- Chemical hydrides—hydrogen is released via chemical reaction (usually with water); the "spent fuel" or byproduct is regenerated off-board
- Carbon-based materials—reversible solid-state materials that can be regenerated on-board