Karol J. Fijalkowski

Substantial emission of NH3 during thermal decomposition of sodium amidoborane, NaNH2BH3

Sodium amidoborane (SAB, NaNH2BH3), an ammonia borane (AB, NH3BH3) derivative, was recently reported as a novel solid-state hydrogen storage material. We reinvestigate its thermal decomposition using the best grade commercially available AB (98%) and technical quality (90%) substrates. We characterize evolved gases with mass and FT-IR spectroscopy and observe emission of hydrogen contaminated with significant amounts of ammonia making it undesirable for low-temperature fuel cell applications. We propose a reaction scheme which explains the evolution of NH3 from SAB.

A multifaceted approach to hydrogen storage

The widespread adoption of hydrogen as an energy carrier could bring significant benefits, but only if a number of currently intractable problems can be overcome. Not the least of these is the problem of storage, particularly when aimed at use onboard light-vehicles. The aim of this overview is to look in depth at a number of areas linked by the recently concluded HYDROGEN research network, representing an intentionally multi-faceted selection with the goal of advancing the field on a number of fronts simultaneously. For the general reader we provide a concise outline of the main approaches to storing hydrogen before moving on to detailed reviews of recent research in the solid chemical storage of hydrogen, and so provide an entry point for the interested reader on these diverse topics. The subjects covered include: the mechanisms of Ti catalysis in alanates; the kinetics of the borohydrides and the resulting limitations; novel transition metal catalysts for use with complex hydrides; less common borohydrides; protic-hydridic stores; metal ammines and novel approaches to nano-confined metal hydrides.

Facile formation of thermodynamically unstable novel borohydride materials by a wet chemistry route.

A novel wet synthetic method utilizing weakly coordinating anions that yields LiCl-free Zn-based materials for hydrogen storage has recently been reported. Here we show that this method may also be applied for the synthesis of the pure yttrium derivatives, M[Y(BH4)4] (M = K, Rb, Cs). Moreover, it can be extended to the preparation of previously unknown thermodynamically unstable derivatives, Li[Y(BH4)4] and Na[Y(BH4)4]. Importantly, these two H-rich phases cannot be accessed by standard dry (mechanochemical) or solid/gas synthetic methods due to the thermodynamic obstacles. Here we describe their crystal structures and selected important physicochemical properties.

Complete Series of Alkali-Metal M(BH3NH2BH2NH2BH3) Hydrogen-Storage Salts Accessed via Metathesis in Organic Solvents

We report a new efficient way of synthesizing high-purity hydrogen-rich M(BH3NH2BH2NH2BH3) salts (M = Li, Na, K, Rb, Cs). The solvent-mediated metathetic synthesis applied here uses precursors containing bulky organic cations and weakly coordinating anions. The applicability of this method permits the entire series of alkali-metal M(BH3NH2BH2NH2BH3) salts (M = Li, Na, K, Rb, Cs) to be obtained, thus enabling their comparative analysis in terms of crystal structures and hydrogen-storage properties. A novel polymorphic form of Verkades base (C18H39N4PH)(BH3NH2BH2NH2BH3) precursor was also characterized structurally. For all compounds, we present a comprehensive structural, spectroscopic, and thermogravimetric characterization (PXRD, NMR, FTIR, Raman, and TGA/DSC/MS).

Chemically driven negative linear compressibility in sodium amidoborane, Na(NH2BH3).

Over the past few years we have been witnessing a surge of scientific interest to materials exhibiting a rare mechanical effect such as negative linear compressibility (NLC). Here we report on strong NLC found in an ionic molecular crystal of sodium amidoborane (NaAB) – easily-accessible, optically transparent material. In situ Raman measurements revealed abnormal elongation of B-N and N-H bonds of NaAB at pressure about 3 GPa. Ab initio calculations indicate the observed spectroscopic changes are due to an isostructural phase transition accompanied by a stepwise expansion of the crystal along c axis. Analysis of calculated charge density distribution and geometry of molecular species (NH2BH3) univocally points to a chemically driven mechanism of NLC – pressure-induced formation of hydrogen bonds. The new H-bond acts as a “pivot screw” coupling N-H covalent bonds of neighbor molecular species – a system resembling a two-lever “jack device” on a molecular scale. A mechanism based on formation of new bonds stands in apparent contrast to mechanisms so far reported in majority of NLC materials where no significant alteration of chemical bonding was observed. The finding therefore suggests a qualitatively new direction in exploration the field towards rational design of incompressible materials.

Hydrogen-mediated affinity of ions found in compressed potassium amidoborane, K[NH2BH3]

The paper reports on the experimental and theoretical investigation of bonding properties of potassium amidoborane, (K[NH2BH3]), which is one of the most promising compounds for hydrogen storage material among metallated derivatives of ammonia borane (NH3BH3). For this purpose, in situ Raman spectroscopy, synchrotron X-ray diffraction measurements and complementary ab initio calculations study have been performed under static pressure conditions in the range from ambient pressure up to 25 GPa. Unusual interplay between strong electrostatic and weak dispersive interactions has been revealed, resulting in experimental observation of pressure induced formation of relatively strong conventional hydrogen bonding between negatively charged molecular ions. This finding provides new insight for tailoring materials with desirable properties for various uses.

Reconnaissance of reactivity of an Ag(II)SO4 one-electron oxidizer towards naphthalene derivatives

We test divalent silver sulphate, Ag(II)SO4 as a novel reagent for oxidative coupling of aromatic hydrocarbons under ambient temperature conditions. The applicability of the C(sp2)–C(sp2) coupling protocol is illustrated for naphthalene and its 1-substituted derivatives containing either electron donating (e.g. Me, MeO, or Ph) or electron-withdrawing groups (X = F⋯I), leading to 4,4′-disubstituted-1,1′-binaphthyls. Coupling of 2-bromo-naphthalene yields a mixture of 2,2′-, 2,7′-, and 7,7′-dibromo-1,1′-binaphthyls together with their trimeric and tetrameric analogues. The coupling of strongly electron-withdrawing 1-CF3-naphthalene provides the 5,5′-disubstituted-1,1′-binaphthyl derivative. The new method does not require the presence of halogen substituents, in contrast to most of the known C–C coupling methods, and it preserves them, if present. Ag(II)SO4 may be easily electrochemically regenerated from the Ag(I)HSO4 byproduct. However, the C–C coupling method currently suffers from low yields, up to 17%, and it requires further optimization.