Publications
Synthesis and thermal decomposition properties of hydrogen-rich phosphorus salts
Complex metal hydrides continue to be investigated as solid-materials for hydrogen storage. Traditional interstitial metal hydrides offer favorable thermodynamics and kinetics for hydrogen release but do not meet energy density requires. Anionic metal hydrides, and complex metal hydrides like magnesium borohydride have higher energy densities compared to interstitial metal hydrides, but poor kinetics and/or thermodynamically unfavorable side products limit their deployment as hydrogen storage materials in transportation applications. Main-group anionic materials such as the bis(borane)hypophosphite salt [PH2(BH3)2] have been known for decades, but only recently have we begun to explore their ability to release hydrogen. We have developed a new procedure for synthesizing the lithium and sodium hypophosphite salts. Routes for accessing other metal bis(borane)hypophosphite salts will be discussed. A significant advantage of this class of material is the air and water stability of the anion. Compared to metal borohydrides, which reactive violently with water, these phosphorus-based salts can be dissolved in protic solvents, including water, with little to no decomposition over the course of multiple days. The ability of these salts to release hydrogen upon heating has been assessed. While preliminary results indicate phosphine and boron-containing species are released, hydrogen is also a major component of the volatile species observed during the thermal decomposition. Additives such as NaH or KH mixed with the sodium salt Na[PH2(BH3)2] significantly perturb the decomposition reaction and greatly increase the mass loss as determined by thermal gravimetric analysis (TGA). This symbiotic behavior has the potential to affect the hydrogen storage ability of bis(borane)hypophosphite salts.