Chemical Hydrogen Storage
The term 'chemical hydrogen storage' is used to describe storage technologies in which hydrogen is generated through a chemical reaction. Common reactions involve chemical hydrides with water or alcohols. Typically, these reactions are not easily reversible on-board a vehicle. Hence, the 'spent fuel' and/or byproducts must be removed from the vehicle and regenerated off-board.
Hydrolysis Reactions
Hydrolysis reactions involve the oxidation reaction of chemical hydrides with water to produce hydrogen. The reaction of sodium borohydride has been the most studied to date. This reaction is:
In the first embodiment, a slurry of an inert stabilizing liquid protects the hydride from contact with moisture and makes the hydride pumpable. At the point of use, the slurry is mixed with water and the consequent reaction produces high purity hydrogen.
NaBH4 + 2H2O = NaBO2 + 4H2
The reaction can be controlled in an aqueous medium via pH and the use of a catalyst. While the material hydrogen capacity can be high and the hydrogen release kinetics fast, the borohydride regeneration reaction must take place off-board. Regeneration energy requirements, cost and life-cycle impacts are key issues currently being investigated.
Millennium Cell has reported that their NaBH4-based Hydrogen on Demand™ system possesses a system gravimetric capacity of about 4 wt.%. Similar to other material approaches issues include system volume, weight and complexity and water availability.
Another hydrolysis reaction that is presently being investigated by Safe Hydrogen, is the reaction of MgH2 with water to form Mg(OH)2 and H2. In this case, particles of MgH2 are contained in a non-aqueous slurry to inhibit premature water reactions when hydrogen generation is not required. Material-based capacities for the MgH2 slurry reaction with water can be as high as 11 wt.%. However, similar to the sodium borohydride approach, water must also be carried on-board the vehicle in addition to the slurry and the Mg(OH)2 must be regenerated off board.
Hydrogenation/Dehydrogenation Reactions
Hydrogenation and dehydrogenation reactions have been studied for many years as a means of hydrogen storage. For example, the decalin-to-naphthalene reaction can release 7.3 wt.% hydrogen at 210ºC via the reaction:
C10H18 = C10H8 + 5H2
A platinum-based or noble metal supported catalyst is required to enhance the kinetics of hydrogen evolution.
Recently, a new type of liquid phase material has been developed. This material, developed by Air Products and Chemicals, Inc., has shown 5-7 wt.% gravimetric hydrogen storage capacity and a greater than 0.050 kg/L hydrogen volumetric capacity. Future research is directed at lowering dehydrogenation temperatures. The advantages of such a system are that, unlike other chemical hydrogen storage concepts, the dehydrogenation does not require water. Since the reaction is endothermic the system would use waste heat from the fuel cell or internal combustion engine to produce hydrogen on-board. Furthermore, liquids lend themselves to facile transport and refueling. There are also no heat removal requirements during refueling since regeneration would take place off-board the vehicle. Thus, the replenished liquid must be transported from the hydrogenation plant to the vehicle filling station. Off-board regeneration efficiency and cost are important factors.
New Chemical Approaches
New chemical approaches are needed to help achieve the 2010 and 2015 hydrogen storage targets. The concept of reacting lightweight metal hydrides such as LiH, NaH, and MgH2 with methanol and ethanol (alcoholysis) has been put forward. Alcoholysis reactions are said to lead to controlled and convenient hydrogen production at room temperature and below. However, as is the case with hydrolysis reactions, alcoholysis reaction products must be recycled off-board the vehicle. The alcohol must also be carried on-board the vehicle and this impacts system-level weight, volume, and complexity.
Another new chemical approach may be hydrogen generation from ammonia-borane materials by the following reactions:
NH3BH3 = NH2BH2 +H2 = NHBH + H2
The first reaction, which occurs at less than 120ºC releases 6.1 wt.% hydrogen, while the second reaction, which occurs at approximately 160ºC, releases 6.5 wt.% hydrogen. Recent studies indicate that hydrogen release kinetics and selectivity are improved by incorporating ammonia-borane nanosized particles in a mesoporous scaffold (T. Autrey, Pacific Northwest National Laboratory, "Chemical Hydrogen Storage: Control of H2 Release From Ammonia Borane", Poster presented at the 2004 DOE Hydrogen Program Review, May 2004, Philadelphia).
Learn about DOE's Chemical Hydride Storage R&D Activities.