Insights from impedance spectroscopy into the mechanism of thermal decomposition of M(NH2BH3), M = H, Li, Na, Li0.5Na0.5, hydrogen stores

Abstract

We report the first solid-state impedance study of hydrogen-rich ammonia borane, AB, and its three alkali metal amidoborane derivatives. Temperature-dependent impedance spectra of solid M(NH(2)BH(3)) salts are predominated by ionic conductivity, which at room temperature ranges from 5.5 μS cm(-1) (M = Li) to 2.2-3.0 mS cm(-1) (Na, Na(0.5)Li(0.5)), while the activation energy for conductivity is rather high (140-158 kJ mol(-1)). Variation of conductivity with time can be used to extract information about the evolution of the system during thermal decomposition. By using a combination of impedance spectroscopy, thermogravimetric analysis, scanning calorimetry, evolved gas analysis, infrared absorption spectroscopy as well as (11)B and (1)H MAS NMR, we were able to reconfirm the complex pathway of thermal decomposition of amidoboranes postulated by two of us earlier (J. Mater. Chem. 2009, 19, 2043).

Mono- and Bimetalic Amidoboranes

“…The first step of the most often cited mechanism of hydrogen evolution is identical to that determined for ammonia borane [99], and it relies on intermolecular recombination of protonic and hydridic hydrogen atoms to form dihydrogen molecule [18,23,24].…”

“…A number of papers were published reporting alkali metal amidoboranes as sources of pure hydrogen, e.g., LiAB [18,19,86], NaAB [18,86] and KAB [25]. On the other hand, several other groups reported for these materials the desorption of hydrogen contaminated with ammonia: α-LiAB [20,25,39,89,90], β-LiAB [20] and NaAB [23,24,87,88,98]. Some amidoborane salts (KAB [25,89], Mg(AB) 2 [29], Al(AB) 3 [35], and LiAl(AB) 4 [35,40]) were reported to desorb pure hydrogen and, currently, there are no contradictory reports.…”

“…Reproduced from reference [22] with permission from The Royal Society of Chemistry. ); and (b) Fijalkowski et al [24] reporting contamination of hydrogen with ammonia (Right. Reproduced from reference [24] with permission from the PCCP Owner Societies.).…”

“…); and (b) Fijalkowski et al [24] reporting contamination of hydrogen with ammonia (Right. Reproduced from reference [24] with permission from the PCCP Owner Societies.). Xiong et al [22] have not presented any data related to ammonia evolution.…”

“…Because of high reactivity against moisture and water, amidoborane compounds are typically stored and processed under inert atmosphere, mostly common argon, nitrogen or helium. Some of the compounds seem to be unstable at room temperature, thus they should be cooled down and stored at low temperature to preserve their properties [24].…”