Phase Transition Study of Ammonia Borane/Polymer Composites: Potential Hydrogen Storage Systems

Ammonia Borane (NH$_{3}$BH$_{3})$, a potential hydrogen storage material exhibits a phase transition at T$_{\mathrm{p}}$ $\sim$ 223 K but the underlying mechanism is unclear. Ammonia borane (AB) blended with polymers (e.g. polyacrylamide) significantly improves the dehydrogenation kinetics and suppression of byproducts/impurities. We carried out heat capacity measurements over a temperature range of 180--300 K, and detected an anomaly at around 223 K, indicating a first-order transition. The change in enthalpy and entropy was calculated for AB as well as the composites and compared. The transition enthalphy and entropy revealed a decrease by increasing the polymer content and the transition temerature showed a small shift to lower temperatures for AB/Polymer composites both of which could be due to the interaction between polyacrylamide and AB after blending. This phenomena was also supported by infrared measurements.