Structural diversity and trends in properties of an array of hydrogen-rich ammonium metal borohydrides

DOI: 10.1021/acs.inorgchem.0c01797

Authors: Jakob B. Grinderslev (Aarhus University) , Lars H. Jepsen (Aarhus University) , Young-su Lee (Korea Institute of Science and Technology) , Kasper Moller (Aarhus University; Curtin University) , Young Whan Cho (Korea Institute of Science and Technology) , Radovan Cerny (University of Geneva) , Torben Jensen (Aarhus University)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Inorganic Chemistry

State: Published (Approved)
Published: August 2020
Diamond Proposal Number(s): 12714 , 14015

Abstract: Metal borohydrides are a fascinating and continuously expanding class of materials, showing promising applications within many different fields of research. This study presents 17 derivatives of the hydrogen-rich ammonium borohydride, NH4BH4, which all exhibit high gravimetric hydrogen densities (>9.2 wt % of H2). A detailed insight into the crystal structures combining X-ray diffraction and density functional theory calculations exposes an intriguing structural variety ranging from three-dimensional (3D) frameworks, 2D-layered, and 1D-chainlike structures to structures built from isolated complex anions, in all cases containing NH4+ countercations. Dihydrogen interactions between complex NH4+ and BH4– ions contribute to the structural diversity and flexibility, while inducing an inherent instability facilitating hydrogen release. The thermal stability of the ammonium metal borohydrides, as a function of a range of structural properties, is analyzed in detail. The Pauling electronegativity of the metal, the structural dimensionality, the dihydrogen bond length, the relative amount of NH4+ to BH4–, and the nearest coordination sphere of NH4+ are among the most important factors. Hydrogen release usually occurs in three steps, involving new intermediate compounds, observed as crystalline, polymeric, and amorphous materials. This research provides new opportunities for the design and tailoring of novel functional materials with interesting properties.

Journal Keywords: Hydrogen; Anions; Metals; Crystal structure; Cations

Subject Areas: Chemistry


Instruments: I11-High Resolution Powder Diffraction

Other Facilities: BM01 at ESRF; I-711 at MAXlab