Ingsman Energy and Fuel Cell Research Organization

Hydrogen is clearly known as a promising energy carrier for the future due to the depletion of fossil fuels.It has a potential for fuel-efficient, pollution free vehicles to circumvent the global warming.Hydrogen can be generated from variable sources of energies like wind, solar, geothermal with water being the only raw material needed. Hydrogen can be used as a fuel in the internal combustion engine and fuel cell for the production of energy with water as by product.

Nevertheless, many critical technical challenges need to be overcome before a hydrogen-based economy could become viable. On-board hydrogen storage is one of the most challenging technical barriers that restrict the implementation of the hydrogen-based economy. Achievement of high fuel efficiency and less cargo space, weight and volume for on-board hydrogen storage system is inadequate.Hence the U.S. Department of Energy’s Freedom Car Program targets indicate that the gravimetric density of a hydrogen storage system should be at least 6 wt.% (i.e., 6 kg H₂ in a 100 kg tank), and the volumetric density should be at least 45 g H₂/L by 2010.The targets for 2015 are 9 wt.% for gravimetric density and 81 g H₂/L for volumetric density.

Hydrogen has been stored in compressed form in tanks, hydrogen-storing alloys and activated carbon or nano-scale materials such as carbon nanotubes. However, none of these methods are suitable for vehicle and portable applications due to the low volumetric and gravimetric efficiency of hydrogen storage as well as the associated safety issues.

In addition to above mentioned hydrogen storage methods, liquid fuels (methanol, ethanol, gasoline, etc.) and chemical hydrides (NaBH₄, KBH₄, LiH, NaH, etc.) could be employed as hydrogen sources. However, in the case of liquid fuels, high-temperature reforming processes are too complex to satisfy the requirements of vehicle applications. Among the chemical hydrides, sodium borohydride (NaBH₄) has been intensively studied as a hydrogen storage material because of its advantages of nonflammability and stability in air, easily controlled hydrogen generation rate, recyclable side product and high H₂ storage efficiency. Base-stabilized NaBH₄ solution hydrolyzes to hydrogen and sodium metaborate (NaBO₂) only when in contact with specific catalysts. The reaction equation is as follows.

Without a catalyst, the reaction proceeds very slowly and essentially stops with the addition of a few percentage of sodium hydroxide to the solution (For long-term storage of sodium borohydride, a pH near 14 is desired). Therefore, sodium borohydride can be dissolved in water and transported stably as an aqueous solution. The solution can be used to generate hydrogen when pumped through a catalyst bed. The reaction products could then be transported to central facilities for regeneration. In general, sodium borohydride systems are promising candidates for ultra safe on-board hydrogen storage.Borohydride though seemed to be costly at present, it will be very economical and efficient due the recycling and identification of new methods for low cost production (Recently such method has been identified by Millennium cell, USA).