Characterization and mechanistic studies of the dehydrogenation of NHxBHx materials

In this review we cover the recent developments providing insight into the chemical and physical properties for a series of hydrogen-rich nitrogen–boron–hydrogen materials that are of interest as energy storage media for fuel cell power applications. These materials, ammonium borohydride (ABH2, [NH4][BH4]; 240 g H2/kg; 165 g/l), ammonia borane (AB, [NH3BH3]; 196 g H2/kg; 147 g/l) and diammoniate of diborane (DADB, [NH3BH2NH3][BH4]; 196 g H2/kg; ca. 151 g/l), release hydrogen by a series of moderately exothermic reaction pathways. The advantage of these materials is that hydrogen release is kinetically controlled and occurs at relatively low temperatures and moderate pressures. The challenges are devising economical pathways to regenerate the fully charged hydrogen storage materials off board and understanding and controlling the formation of volatile impurities that decrease the purity of the hydrogen available for polymer electrolyte membrane fuel cell applications. The focus of this review is on the solid phase ABH2, AB and DADB materials to complement the coverage of AB in other recent review articles (Stephens et al. [1]; Marder [2]). Additional discussion is given on the decomposition products of these materials, polyaminoborane (PAB, [NH2BH2]n) and polyiminoborane (PIB, [NHBH]n). The article is organized into three sections: (i) Synthesis and structural characterization; (ii) Kinetics and thermodynamics of hydrogen release and (iii) Outstanding challenges for breakthroughs.