Craig M. Jensen

Hydrogen cycling behavior of zirconium and titanium-zirconium-doped sodium aluminum hydride

The dehydrogenation kinetics of NaAlH4 are significantly enhanced upon doping with zirconium through homogenization with 2 mol% Zr(OPr)4. TPD measurements show that zirconium is inferior to titanium as a catalyst for the dehydriding of NaAlH4 to Na3AlH6 and Al but a superior catalyst for the dehydriding of Na3AlH6 to NaH and Al. The benefit of both catalytic effects can be realized in materials containing a combination of both titanium and zirconium catalysts. After the initial dehydriding/rehydriding cycle, NaAlH4 which is doped with titanium and/or zirconium is stabilized with a greater than 4 wt% cyclable hydrogen capacity. The onset of rapid dehydriding occurs in the titanium containing materials at temperatures below 100°C.

Advanced titanium doping of sodium aluminum hydride:: segue to a practical hydrogen storage material?

The dehydrogenation kinetics of NaAlH4 have been enhanced far beyond those previously achieved upon titanium doping of the host hydride. Homogenization of NaAlH4 with 2 mole % Ti (OBun) 4 under an atmosphere of argon produces a novel material which contains only traces of carbon. TPD measurements show that the dehydrogenation of this material occurs about 30°C lower than that previously found for NaAlH4 doped with titanium through wet chemistry methods. The novel titanium containing material can be rehydrided under 1600 psi of hydrogen pressure at 200°C. In further contrast to wet doped NaAlH4, the dehydrogenation kinetics observed for the novel material are undiminished over several dehydriding/hydriding cycles.

Catalyzed alanates for hydrogen storage

The discovery that hydrogen can be reversibly absorbed and desorbed from complex hydrides (the alanates) by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. Unlike the interstitial intermetallic hydrides, these compounds release hydrogen through a series of decomposition/recombination reactions e.g.: NaAlH4⇔1/3Na3AlH6+2/3Al+H2⇔NaH+Al+3/2H2. Initial work resulted in improved catalysts, advanced methods of preparation, and a better understanding of the hydrogen absorption and desorption processes. Recent studies have clarified some of the fundamental material properties, as well as the engineering characteristics of catalyst enhanced sodium alanate. Phase transitions were observed real-time through in situ X-ray powder diffraction. These measurements demonstrate that the decomposition reactions occur through long-range transport of metal species. SEM imaging and EDS analysis verified the segregation of aluminum to the surface of the material during decomposition. The equilibrium thermodynamics of decomposition have now been measured down to room temperature. They show a plateau pressure for the first reaction of 1 bar at 33°C, which suggest that, thermodynamically, this material is ideally suited to on-board hydrogen storage for fuel cell vehicles. Room temperature desorption with slow but measurable kinetics has been recorded for the first time. Studies at temperatures approaching that found in the operation of PEM fuel cells (125–165°C) were performed on a scaled-up test bed. The bed demonstrated surprisingly good kinetics and other positive material properties. However, these studies also pointed to the need to develop new non-alkoxide based catalysts and doping methods to increase the capacity and reduce the level of hydrocarbon impurities found in the desorbed hydrogen. For this reason, new Ti–Cl catalysts and doping processes are being developed which show higher capacities and improved kinetics. An overview of the current state-of-the-art will be presented along with our own studies and the implications for the viability of these materials in on-board hydrogen storage applications.

A highly active alkane dehydrogenation catalyst: stabilization of dihydrido rhodium and iridium complexes by a P–C–P pincer ligand

The novel P–C–P pincer complex, [IrH2{C6H3(CH22PBut2)2}] has long-term stability and 200 ° C and catalyses the transfer dehydrogenation of cyclooctnane to cyclooctene at the rate of 12 turnovers min1 min–1.

Engineering considerations in the use of catalyzed sodium alanates for hydrogen storage

Abstract The hydrogen storage properties of catalyzed NaAlH 4 (and associated Na 3 AlH 6 ) were studied in relation to various practical engineering considerations. Properties measured were cyclic capacity, charging and discharging rates, thermal effects, gaseous impurities, volume changes, low temperature plateau pressures and detailed isothermal desorption kinetics over the temperature range 23–180°C. Two materials were evaluated, one mechanically milled with the liquid alkoxides Ti(OBu n ) 4 and Zr(OPr i ) 4 and one milled with dry TiCl 3 as catalyst precursors. The alkoxide-catalyzed materials had low reversible capacities and released significant levels of hydrocarbon impurities during H 2 discharge. These problems were virtually eliminated with the inorganic TiCl 3 catalyst precursor. The NaAlH 4 and Na 3 AlH 6 decomposition kinetics of TiCl 3 -catalyzed Na-alanate conform to Arrhenius behavior with activation energies of 79.5 and 97 kJ/mol H 2 , respectively. Measured absorption and desorption kinetics were surprisingly good and it is shown that 3–4.5 wt.% H 2 can be stored and recovered in reasonable times at 100–125°C. It may even be ultimately possible to use the NaAlH 4 decomposition reaction to provide 3 wt.% H 2 at room temperature for low-rate applications.

X-ray diffraction studies of titanium and zirconium doped NaAlH4: elucidation of doping induced structural changes and their relationship to enhanced hydrogen storage properties

Abstract X-ray diffraction patterns of NaAlH4 doped with up to 10 mol.% of either titanium or zirconium do not contain Bragg peaks for the bulk metals or their aluminum alloys. Instead the hydride lattice parameters a and c undergo significant contraction upon 2 mol.% doping and then expand as the doping level increases from 2 to 5 mol.%. These results are explained by a model that entails substitution of sodium cations by variable valence transition metal cations and the creation of Na+ vacancies in the bulk hydride lattice.

LiSc(BH4)4: A Novel Salt of Li + and Discrete Sc(BH4)4 - Complex Anions

LiSc(BH4)4 has been prepared by ball milling of LiBH4 and ScCl3. Vibrational spectroscopy indicates the presence of discrete Sc(BH4)4(-) ions. DFT calculations of this isolated complex ion confirm that it is a stable complex, and the calculated vibrational spectra agree well with the experimental ones. The four BH4(-) groups are oriented with a tilted plane of three hydrogen atoms directed to the central Sc ion, resulting in a global 8 + 4 coordination. The crystal structure obtained by high-resolution synchrotron powder diffraction reveals a tetragonal unit cell with a = 6.076 A and c = 12.034 A (space group P-42c). The local structure of the Sc(BH4)4(-) complex is refined as a distorted form of the theoretical structure. The Li ions are found to be disordered along the z axis.

Structure and hydrogen dynamics of pure and Ti-doped sodium alanate

We use ab initio methods and neutron inelastic scattering (NIS) to study the structure, energetics, and dynamics of pure and Ti-doped sodium alanate (NaAlH_4), focusing on the possibility of substitutional Ti doping. The NIS spectrum is found to exhibit surprisingly strong and sharp two-phonon features. The calculations reveal that substitutional Ti doping is energetically possible. Ti prefers to substitute for Na and is a powerful hydrogen attractor that facilitates multiple Al--H bond breaking. Our results hint at new ways of improving the hydrogen dynamics and storage capacity of the alanates.

The synthesis and hydrogen storage properties of a MgH2 incorporated carbon aerogel scaffold

A new approach to the incorporation of MgH2 in the nanometer-sized pores of a carbon aerogel scaffold was developed, by infiltrating the aerogel with a solution of dibutylmagnesium (MgBu2) precursor, and then hydrogenating the incorporated MgBu2 to MgH2. The resulting impregnated material showed broad x-ray diffraction peaks of MgH2. The incorporated MgH2 was not visible using a transmission electron microscope, which indicated that the incorporated hydride was nanosized and confined in the nanoporous structure of the aerogel. The loading of MgH2 was determined as 15-17 wt%, of which 75% is reversible over ten cycles. Incorporated MgH2 had >5 times faster dehydrogenation kinetics than ball-milled activated MgH2, which may be attributed to the particle size of the former being smaller than that of the latter. Cycling tests of the incorporated MgH(2) showed that the dehydrogenation kinetics are unchanged over four cycles. Our results demonstrate that confinement of metal hydride materials in a nanoporous scaffold is an efficient way to avoid aggregation and improve cycling kinetics for hydrogen storage materials.

Microstructural characterization of catalyzed NaAlH4

Abstract A number of laboratories have now demonstrated that catalyst-assisted NaAlH 4 can reversibly absorb and desorb hydrogen in the solid state at moderate temperatures. An understanding of the mechanisms by which bulk decomposition and reformation of the compound can occur in the presence of a surface catalyst is important to improving the kinetic and thermodynamic properties of alanates for use in hydrogen storage applications. Using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), we have examined the microstructure and elemental composition of Na alanate samples, doped using a liquid Ti/Zr catalyst precursor, for a number of conditions. First, microscopy and compositional analyses were performed at different stages of the decomposition process within the first desorption cycle. Second, the material was characterized after multiple absorption/desorption cycles (five cycles). Finally, the effects of the catalyst doping procedure on particle size, surface morphology and surface composition were examined. Significant changes in particle morphology and in elemental distribution were found to be induced by the desorption and cycling processes. Importantly, our measurements indicate that the initial dehydriding reactions were accompanied by significant enhancement of Al concentration toward the surface of particles and that elemental segregation occurred with repeated absorption/desorption cycles.