The Roles of Alkali/Alkaline Earth Metals in the Materials Design and Development for Hydrogen Storage

Abstract

ConspectusHydrogen storage for onboard applications has been recognized as a grand challenge for the large-scale implementation of hydrogen fuel cell vehicles. Tremendous research efforts have thus been devoted to the design and development of hydrides of lightweight elements (HLEs). A prominent feature of these materials is the indispensable ingredient of alkali/alkaline earth cations. Alkali alkaline earth metals (AMs) are highly reactive and have a rich coordination chemistry. As a matter of fact, an AM cation can form a complete range of compounds with hydrogenous anions, such as H-, [NH2]-, [BH4]-, [AlH4]-, [NH2BH3]-, [TMHX]-, [R-CH2-NH]-, [R-CH2-O]-, etc., and, thus, tune the Al-H, N-H, B-H, and C-H bond strengths for hydrogen storage.In this Account, our research efforts in the development of AM amide-hydride composites, AM amidoboranes, and metalorganic hydrides for hydrogen storage are reviewed. A partial substitution of the H in NH3 by AM gives rise to solid AM amides or imides. Those compounds can react with AM hydride (AMH) to produce H2. This is driven by the redox reaction between a protic H (N) and hydridic H (AM). A variety of amide-hydride composites holding promise for hydrogen storage were thus developed, including LiNH2-hydride, Mg(NH2)2-hydride, and complex amide-hydride composites. For amidoboranes, the substitution of H (N) in ammonia borane (AB) by AM transforms the molecular crystal AB into amidoboranes with an ionic crystal structure, leading to significant changes in terms of charge distribution, bond length, intermolecular forces, and so on, which in turn results in enhanced dehydrogenation properties. For metalorganic hydrides, through reacting AM compounds (usually AM hydride) with aliphatic, carbocyclic, or heterocyclic organic hydrides having "reactive protic H", corresponding metalorganic hydrides are formed. Because of the electron-donating nature of AM, the strengths of the C-H bond in metal-organic hydrides can be modulated.For each material system, we will introduce the synthesis of materials, show their performances, correlate the hydrogen storage property to a crystal and/or electronic structure, and especially highlight the functions of AM in tuning the thermodynamic and/or kinetic properties of HLEs. At the end of the Account, challenges and a future research direction of the hydrogen storage field are discussed.