Dazo Water Fuel Technology

1.3.3 Hydrogen Storage in Materials

There are a number of other ways to store hydrogen in solid or liquid materials, for release on demand. The two most studied approaches are adsorption of hydrogen into solid metal hydrides and “chemical” storage as part of a chemical hydride. Both of these approaches are inherently safer than storing hydrogen as a high-pressure gas or a cryogenic liquid, and the process of releasing the hydrogen from the storage medium is less complex than reforming of hydrocarbon fuels. At present, these systems are heavy and bulky and require further development to be practical.

Metal hydride storage systems are based on the fact that some metals can adsorb significant amounts of hydrogen under high pressure and moderate temperatures. The hydrogen is either adsorbed onto the surface of the metal or actually incorporated into the crystalline lattice of the solid metal. When heated to some higher temperature at low pressure, the hydrogen is released from the metal. In a vehicle hydrogen storage system, waste heat from the fuel cell or ICE engine would typically be used to release the hydrogen (DOE, n.d.). Such a system could potentially be “re-fueled” onboard the vehicle by connecting it to a high-pressure hydrogen source.

Chemical hydrides are compounds that include significant numbers of hydrogen atoms chemically bound to other types of atoms, for example, sodium borohydride, which is composed of one sodium atom, one boron atom, and four hydrogen atoms (NaBH4). In a hydrogen storage system based on a chemical hydride, the hydrogen is released on demand through a chemical reaction with either water or an alcohol. The solid hydride is made into a slurry with an inert liquid, and when hydrogen is required, water is added, releasing hydrogen (DOE, n.d.). Unlike metal hydrides, chemical hydrides cannot be regenerated on the vehicle; after releasing all of its hydrogen the spent slurry must be removed and regenerated off-site.

Current metal and chemical hydride fuel storage systems are heavy and bulky; they can only store and release 6 percent or less of their weight as hydrogen (DOE, n.d.) (i.e., only 6 percent of the total weight of the system is the hydrogen fuel; the rest of the weight is the container). These systems have even lower energy densities than compressed gaseous hydrogen storage systems. More work is required to develop truly practical storage systems for vehicles based on these technologies.