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

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(NH2BH3) salts are predominated by ionic conductivity, which at room temperature ranges from 5.5 mu S cm(-1) (M = Li) to 2.2-3.0 mS cm(-1) (Na, Na0.5Li0.5), 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 B-11 and H-1 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).