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(NH₂BH₃) 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_0.5Li_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 ¹¹B and ¹H 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).