The Department of Energy (DOE) issued a challenge for hydrogen storage related technologies during 2003 to compliment existing programs on Hydrogen Production and Fuel Cell development. Part of the challenge involved proposed project milestones in calendar years 2010 and 2015 for the development of new materials and technologies relating to storing hydrogen for use as vehicle energy sources. Four technologies for storing hydrogen are under investigation in the technical art: (1) storage as simple metal hydrides, e.g., MgH2, (2) storage on carbon materials, including single-walled carbon nanotubes, (3) storage as complex metal hydrides, e.g., NaAlH4, and (4) chemical hydrogen storage, e.g. NHxBHx, where x=1 to 4. The technical developments related to chemical hydrogen storage technology are discussed further hereafter. Baitalow et al. have shown the potential for use of N—B—H compounds including ammonia borane (AB), NH3BH3, as a hydrogen storage material. Jaska et al. report hydrogen formation in AB is likely to occur by an intermolecular dimerization pathway as shown in reaction [1], although a two-step mechanism shown in reactions [2] and [3] is not ruled out:2NH3BH3→NH3BH2—NH2BH3+H2  [1]NH3BH3→NH2═BH2+2H2  [2]2NH2═BH2→NH3BH2—NH2BH3  [3]
Each step that forms a new B—N bond also forms hydrogen, as illustrated in reactions [4] and [5]:NH3BH3+NH3BH2—NH2BH3→NH3BH2—NH2BH2—NH2BH3+H2  [4]NH3BH3+NH3BH2NH2BH2—NH2BH3→NH3BH2—(NH2BH2)2—NH2BH3+H2  [5]
Baitalow et al. further report at temperatures greater than 150° C., additional hydrogen may be released as illustrated in reactions [6] and [7]:(NH3BH2NH2BH2—NH2BH3)n→(NH3BH2NH2BH═NHBH3)n+H2  [6](NH3BH2NH2BH═NHBH3)n→(NH3BH═NHBH═NHBH3)n+H2  [7]
However, it is well known in the art that release of hydrogen from neat AB occurs at temperatures at which undesirable side reactions occur thereby generating products that contaminate and decrease the purity of the released hydrogen for use as fuel. For example, formation of cyclic borazine, c-(NHBH)3, an inorganic analog of benzene, is one such contaminating product reported by Wideman et al., illustrated in reaction [8]:(NH3BH═NHBH═NHBH3)n→n(NHBH)3+H2  [8]
Raissi et al. have reviewed data for hydrogen release from the neat solid AB. The reaction of NH3BH3 that yields NH3(BH2—NH2)nBH3+free nH2 releases hydrogen at temperatures near 115° C. in reactions that are comparatively slow and that again have a high potential for forming borazine. At even moderate reaction temperatures (e.g., >150° C.), borazine yields are significant. Borazine is damaging to fuel cells. Thus, its presence means the purity of released hydrogen remains questionable and thus unsuitable for use.
As the current state of the art shows, use of AB materials remains problematic due to: 1) relatively high reaction temperatures required for hydrogen release, 2) slow rates for release of hydrogen, and 3) presence of contaminants like borazine that contaminate the hydrogen released from the source materials that complicate their use as a fuel source.
Accordingly, there remains a need to: 1) decrease temperatures at which hydrogen is released to meet proposed guidelines for bulk hydrogen fuel storage and use; 2) improve rates for hydrogen release; and 3) minimize side reactions that generate undesirable and contaminating products thereby increasing the purity of hydrogen that is available as a fuel.
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