Tomasz Jaroń

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.

Phase transition induced improvement in H2 desorption kinetics: the case of the high-temperature form of Y(BH4)3

The high-temperature (HT) phase of Y(BH(4))(3) has been prepared by heating of the as mechanochemically synthesised low-temperature (LT) phase of Y(BH(4))(3) to 194-216 °C and subsequent rapid cooling to ambient temperature. Although the differences in the crystal structure and vibrational spectra for these closely-related polymorphs are rather small, yet the NMR MAS (1)H and CP MAS (89)Y spectra reveal clear differences in the chemical shifts for both nuclei. The thermal decomposition process of both forms differs noticeably below 260 °C, decomposition being faster and more facile for the HT phase. The activation energy for thermal decomposition, calculated according to the Kissinger equation, is nearly three times lower for the HT than for the LT polymorph for the first step of the thermal decomposition signalling giant improvement of kinetics of H(2) desorption.

KAgF3, K2AgF4 and K3Ag2F7: important steps towards a layered antiferromagnetic fluoroargentate(II),

Crystal structure and magnetic properties of K2AgF4, related to recently studied Cs2AgF4, have been scrutinized. It crystallizes orthorhombic (Cmca No.64) with a = 6.182(3) A, b = 12.632(5) A, c = 6.436(3) A (Z = 4, V = 502.6(7) A3). K2AgF4 exhibits slightly puckered [AgF2] sheets and a compressed octahedral coordination of Ag(II) and it is not isostructural to related Cs2AgF4. Violet–coloured K2AgF4 orders ferromagnetically below 26 K. The DFT calculations reproduce semiconducting properties and ferromagnetism of K2AgF4 at the LSDA + U level but only if substantial values of Mott–Hubbard on-site electron–electron repulsion energies for Ag and F are used in calculations. We have also succeeded to solve the crystal structure of a brown KAgF3 (1D antiferromagnet below 64 K; GdFeO3–type, PnmaNo. 62, a = 6.2689(2) A, b = 8.3015(2) A, c = 6.1844(2) A, Z = 4, V = 321.84(2) A3) and to prepare K3Ag2F7, a novel KAgF3/K2AgF4 intergrowth phase and a member of the Ruddlsden–Popper KnAgFn+2 series (n = 1.5). Dark brown K3Ag2F7 crystallizes orthorhombic (K3Cu2Cl7-type, CccaNo. 68, setting 2) with a = 20.8119(14) A, b = 6.3402(4) A, c = 6.2134(4) A (Z = 4, V = 819.87(9) A3).

Prediction of thermodynamic stability and electronic structure of novel ternary lanthanide hydrides

We theoretically examine four hypothetical ternary lanthanide hydrides, CsLnIIH3 and Cs2LnIIH4, where LnII = Yb, Tm. We optimize their crystal unit cells in the BaTiO3 and K2NiF4 structures, respectively, and compute their electronic band structures. Our calculations indicate that the novel hydrides should be unstable with respect to decomposition into binaries (CsH and LnIIH2); ternaries would form only under elevated pressure (>7–22 GPa). We predict that significant perturbation of the electronic and magnetic properties of CsYbIIH3 and Cs2YbIIH4 will take place via a progressive exchange of f14 YbII for f13 TmII, while promoting magnetic ordering and valence fluctuations. Analysis of the phonon dispersion for these hydrides suggests that metallic forms of doped CsLnIIH3 and Cs2LnIIH4 would exhibit substantially high Debye temperatures of ∼1800 K. This is likely to prompt moderate-TC superconductivity in these as yet unknown materials.

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).

Tetra­methyl­ammonium borohydride from powder data

In the crystal structure of the title compound, C4H12N+·BH4 −, the tetramethylammonium cations are situated on special positions with site symmetry m2. The borohydride anions are situated on special positions with 4mm site symmetry and show rotational disorder around the fourfold axis.

Persistence of Mixed and Non-intermediate Valence in the High-Pressure Structure of Silver(I,III) Oxide, AgO: A Combined Raman, X-ray Diffraction (XRD), and Density Functional Theory (DFT) Study

The X-ray diffraction data collected up to ca. 56 GPa and the Raman spectra measured up to 74.8 GPa for AgO, or AgIAgIIIO2, which is a prototypical mixed valence (disproportionated) oxide, indicate that two consecutive phase transitions occur: the first-order phase transition occurs between 16.1 GPa and 19.7 GPa, and a second-order phase transition occurs at ca. 40 GPa. All polymorphic forms host the square planar [AgIIIO4] units typical of low-spin AgIII. The disproportionated Imma form persists at least up to 74.8 GPa, as indicated by Raman spectra. Theoretical hybrid density functional theory (DFT) calculations show that the first-order transition is phonon-driven. AgO stubbornly remains disproportionated up to at least 100 GPa-in striking contrast to its copper analogue-and the fundamental band gap of AgO is ∼0.3 eV at this pressure and is weakly pressure-dependent. Metallization of AgO is yet to be achieved.

High-Pressure Behavior of Silver Fluorides up to 40 GPa

A combined experimental-theoretical study of silver(I) and silver(II) fluorides under high pressure is reported. For AgI, the CsCl-type structure is stable to at least 39 GPa; the overtone of the IR-active mode is seen in the Raman spectrum. Its AgIIF2 sibling is a unique compound in many ways: it is more covalent than other known difluorides, crystallizes in a layered structure, and is enormously reactive. Using X-ray diffraction and guided by theoretical calculations (density functional theory), we have been able to elucidate crystal structures of high-pressure polymorphs of AgF2. The transition from ambient pressure to an unprecedented nanotubular structure takes place via an intermediate orthorhombic layered structure, which lacks an inversion center. The observed phase transitions are discussed within the broader framework of the fluorite → cotunnite → Ni2In series, which has been seen for other metal difluorides.

Ag2S2O8 meets AgSO4: the second example of metal–ligand redox isomerism among inorganic systems

Valence (redox) isomerism based on electron exchange between a metal and a ligand is immensely rare in purely inorganic systems, with only one documented case, that of PbS2 which adopts two polymorphic forms corresponding to Pb(iv)(S2-)2 and Pb(ii)(S22-). Here we have taken advantage of metathetic reactions using salts of weakly coordinating anions and we have prepared for the first time Ag(i)2S2O8, silver(i) peroxydisulphate. The title compound crystallizes in the non-centrosymmetric Cc space group with partial disorder of the anionic sublattice. Ag(i)2S2O8 is a highly thermally unstable diamagnetic and colourless valence isomer of the antiferromagnetic and black Ag(ii)SO4, described by us in the past.