Crystal Structures and Energy Storage Properties of Ammine Sodium Decahydro-closo-decaboranes (Na2B10H10 center dot nNH(3), n=1, 2)

Abstract

Metal closo-boranes show remarkable thermal and chemical stabilities, making them appealing candidates for a wide range of applications, such as electrolytes in electrochemical batteries and ammonia storage for indirect hydrogen storage. Furthermore, owing to the large size and nonspherical geometry of the anion (e.g., B10H102- or B12H122-) metal closo-boranes display a rich structural diversity and thermal polymorphism. Here, we present the synthesis, characterization, and structural determination of the ammoniated metal closo-boranes, Na2B10H10 center dot nNH(3) (n = 1, 2). The thermal decomposition of Na(2)B(10)H(10 center dot)2NH(3) was investigated with synchrotron radiation in situ powder X-ray diffraction and simultaneous thermogravimetric analysis, differential scanning calorimetry, and mass spectrometry, revealing a reversible ammonia storage capacity of 15 wt % below 150 degrees C. Additionally, ionic conductivities of 2.7 X 10(-8) (RT) and 4.7 X 10(-8) S/cm (30 degrees C) for Na2B10H10 center dot 2NH(3) and Na2B10H10 center dot NH3, respectively, were measured with electrochemical impedance spectroscopy. A lower Na+ conductivity compared to the parent compound, Na2B10H10, is explained by an anchoring effect of ammonia in the rigid framework of the B10H102- anions.