The standard approach for the search of new hydrogen-storage materials is to synthesize bulk samples and to use volumetric [1,2] or gravimetric [3] techniques to follow their hydrogenation reaction and to record pressure-concentration isotherms (p-c isotherms). The equilibrium pressure of the metal-to-hydride transition is determined from the plateau of the p-c isotherm. The enthalpy of hydride formation is extracted from the temperature dependence of the equilibrium pressure, by means of the Van 't Hoff relation [4] lnwhere DH is the enthalpy of formation in kJ (mol H 2 ) -1 , DS 0 is the entropy of formation in JK -1 (mol H 2 ) -1 at standard pressure, R the gas constant, the absolute temperature, p 0 = 1.013 × 10 5 Pa the standard pressure, and p eq the H 2 equilibrium plateau pressure of the p-c isotherm. The great disadvantage of this approach is that a bulk sample is needed for each investigated chemical composition. Thin films provide an interesting alternative to bulk, as their nanostructure is controlled by the deposition conditions. Because of the small amount of material and large surfaces present, diffusion and local heating issues are minimized, the kinetics are fast, and the measurement time is reduced drastically. [5] Moreover, a large number of different chemical compositions can be deposited on a single substrate in a combinatorial way. The fact that hydrogen absorption in a metal leads to large optical changes [6,7] is the basis of a new combinatorial method that we call hydrogenography. With a straightforward optical setup, hydrogenography makes it possible to monitor hydrogen ab-and desorption simultaneously on thousands of samples under exactly the same experimental conditions. [8][9][10] We show here that hydrogenography is much more than a monitoring technique, as it also provides a high-throughput method to measure quantitatively the key thermodynamic properties (enthalpy and entropy) of hydride formation. We describe the essential ingredients of hydrogenography with the Mg-Ti-H system and demonstrate its combinatorial power with the Mg-Ti-Ni-H system. We show in particular that there is a relatively narrow range of compositions in the ternary Mg-Ti-Ni phase diagram with a remarkable combination of favorable properties for light-weight hydrogen storage. Pure MgH 2 would in principle be an attractive system for hydrogen storage as it can contain as much as 7.6 wt % of hydrogen. However, its large negative enthalpy of formation (-74 kJ (mol H 2 ) -1

Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. The structural, optical, and electrical transformations induced by hydrogen absorption and/or desorption in Mg-Ti thin films prepared by co-sputtering of Mg and Ti are investigated. Highly reflective in the metallic state, the films become highly absorbing upon H absorption. The reflector-to-absorber transition is fast, robust, and reversible over many cycles. Such a highly absorbing state hints at the coexistence of a metallic and a semiconducting phase. It is, however, not simply a composite material consisting of independent MgH 2 and TiH 2 grains. By continuously monitoring the structure during H uptake, we obtain data that are compatible with a coherent structure. The average structure resembles rutile MgH 2 at high Mg content and is fluorite otherwise. Of crucial importance in preserving the reversibility and the coherence of the system upon hydrogen cycling is the accidental equality of the molar volume of Mg and TiH 2 . The present results point toward a rich and unexpected chemistry of Mg-Ti-H compounds.

The hydrogenation of the CaH 2 +MgB 2 composite and the dehydrogenation of the resulting products are investigated in detail by in situ time-resolved synchrotron radiation powder X-ray diffraction, high-pressure differential scanning calorimetry, infrared, and thermovolumetric measurements. It is demonstrated that a Ca(BH 4 ) 2 +MgH 2 composite is formed by hydrogenating a CaH 2 +MgB 2 composite, at 350 °C and 140 bar of hydrogen. Two phases of Ca(BH 4 ) 2 were characterized: Rand β-Ca(BH 4 ) 2 . R-Ca(BH 4 ) 2 transforms to β-Ca-(BH 4 ) 2 at about 130 °C. Under the conditions used in the present study, β-Ca(BH 4 ) 2 decomposes first to CaH 2 , Ca 3 Mg 4 H 14 , Mg, B (or MgB 2 depending on experimental conditions), and hydrogen at 360 °C, before complete decomposition to CaH 2 , Mg, B (or MgB 2 ), and hydrogen at 400 °C. During hydrogenation under 140 bar of hydrogen, β-Ca(BH 4 ) 2 is formed at 250 °C, and R-Ca(BH 4 ) 2 is formed when the sample is cooled to less than 130 °C. Ti isopropoxide improves the kinetics of the reactions, during both hydrogenation and dehydrogenation. The dehydrogenation temperature decreases to 250 °C, with 1 wt % of this additive, and hydrogenation starts already at 200 °C. We propose that the improved kinetics of the above reactions with MgB 2 (compared to pure boron) can be explained by the different boron bonding within the crystal structure of MgB 2 and pure boron.

IntroductionThe Mg-Ti-H system is currently attracting a lot of attention, with potential applications in very different fields. Niessen et al. [1] proposed the use of Mg-Ti thin films as high-capacity hydrogen storage materials for batteries. By means of electrochemical loading of Mg 0.8 Ti 0.2 thin films they found a gravimetrical storage capacity of 6.53 wt% H: w4 times higher than the commercially available NiMH batteries. In our group we demonstrated the possibility of gas loading of Pdcapped Mg y Ti 1Ày thin films. These films, when exposed to hydrogen gas exhibit fast and reversible transitions from the metallic to the hydrogenated state. The Ti doping of Mg greatly enhances the kinetics of hydrogen uptake and release. A 65 nm thick film of Mg 0.7 Ti 0.3 , exposed to 5% H 2 in Ar at room temperature, hydrogenates completely in w10 s; if it is then exposed to 20% O 2 in Ar, it fully desorbs in less than 3 min, returning to its original metallic state [2]. The role of Ti is to favor the formation of a face-centered cubic hydride phase of Mg y Ti 1Ày H x (for y < 0.87 [3]) instead of the tetragonal MgH 2 -like phase.The hydrogenation of Mg y Ti 1Ày thin films is also accompanied by dramatic optical changes, which make them suitable for application as hydrogen sensors [4] and smart coating for solar collectors [5]: when exposed to hydrogen gas, they exhibit fast and reversible (>100 cycles) optical transitions

Hydrogenography, an optical high-throughput combinatorial technique to find hydrogen storage materials, has so far been applied only to materials undergoing a metal-to-semiconductor transition during hydrogenation. We show here that this technique works equally well for metallic hydrides. Additionally, we find that the thermodynamic data obtained optically on thin Pd-H films agree very well with Pd-H bulk data. This confirms that hydrogenography is a valuable general method to determine the relevant parameters for hydrogen storage in metal hydrides. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2821376͔ Energy storage, and particularly storage of hydrogen as an energy carrier, is a major issue for the possible implementation of a "hydrogen economy." 1 Metal hydrides provide an attractive option to store hydrogen reversibly. The standard approach for the search of hydrogen storage materials is to synthesize bulk samples and to use volumetric, gravimetric or calorimetric techniques to follow the hydrogen evolution in the sample. The great disadvantage of this approach is that a bulk sample is needed for each investigated chemical composition. Thin films provide an interesting alternative to bulk, as heat and hydrogen diffusion issues are minimized by the thin film geometry. When thin film metal alloys are deposited in a combinatorial way, the surface science and optical techniques allow for a fast screening of the hydrogen absorbing compositions. 2 Recently we have determined the enthalpy and entropy of hydrogen absorption of thousands of different Mg-Ni-Ti compositions simultaneously by hydrogenography, our optical combinatorial technique. 3 While the asdeposited alloys are metallic, the different Mg, Mg-Ni, and Mg-Ti hydrides formed upon hydrogenation are all heavily doped semiconductors. This results in the opening of an optical bandgap. The large optical contrast in transmission in the visible range between the metals and the semiconductor hydrides facilitates the optical measurements. However, as some potential hydrogen storage materials may not undergo a metal-to-semiconductor transition, it is important to demonstrate the applicability of hydrogenography to metallic hydrides. For this we choose the archetypal Pd-H system that has been extensively studied in bulk, 4-6 cluster, 7,8 or thin film form. 9,10 The pressure-concentration isotherms ͑PCI͒ of Pd-H exhibit wide plateaus where the low hydrogen ␣-PdH x phase coexists with the nonstoichiometric ␤-PdH x hydride phase. The 3.54% lattice mismatch 11 ͑11% in volume͒ between the ␣ and the ␤ phases results in an expansion upon hydrogenation that generates large stresses and plastic deformation. This means that, upon cycling, the macroscopic Pd decrepitates into grains of a few micrometer size. While very thin films ͑thickness ഛ10 nm͒ remain clamped to the substrate, 12 in thicker films most of the hydrogen compressive stress is released by the production of networks of large buckles ͑30-50 times the film thickness͒. 13 In this letter, we present a metho...