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.

Mg-Ti-H thin films are found to have very attractive optical properties: they absorb 87% of the solar radiation in the hydrogenated state and only 32% in the metallic state. Furthermore, in the absorbing state Mg-Ti-H has a low emissivity; at 400 K only 10% of blackbody radiation is emitted. The transition between both optical states is fast, robust, and reversible. The sum of these properties highlights the applicability of such materials as switchable smart coatings in solar collectors. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2212287͔ Many metal hydrides behave as switchable mirrors ͑i.e., their optical properties switch from reflective in the metallic state to transparent in the hydrogenated state͒. [1][2][3][4] In addition, magnesium-rare-earth ͑Mg-RE͒ and magnesium-transitionmetal ͑Mg-TM͒ switchable mirrors also exhibit an intermediate highly absorbing optical state on hydrogenation. The possibility to switch a film from a reflective state to an absorbing state suggests that such materials might be interesting for smart solar collectors, which absorb light in normal operation condition and switch to a reflective state to avoid overheating. Limiting the stagnation temperature of the solar collector makes it possible to use cheap materials such as plastics ͑generally not designed for high temperatures͒.In this letter we show that Mg y Ti 1−y thin films prepared by dc magnetron co-sputtering of Mg and Ti at room temperature ͑on quartz and CaF 2 ͒ satisfy the following requirements for a smart solar coating: ͑i͒ high absorption in the solar regime ͑0.5Ͻប Ͻ 4 eV͒, ͑ii͒ low emissivity in the thermal regime ͑ប Ͻ 0.5 eV͒, and ͑iii͒ reversibility. Three compositions are studied in detail: y = 0.70, y = 0.80, and y = 0.90. Typical deposition rates are 2 Å / s for Mg ͑150 W rf power͒, 0.1-1 Å / s for Ti ͑25-160 W dc power͒, and 1.3 Å / s for Pd ͑50 W dc power͒. All the films are covered with a Pd layer ͑10-50 nm͒ to promote dissociation of H 2 and to prevent oxidation of the underlying film.Reflection ͑R͒ and transmission ͑T͒ spectra are measured simultaneously during hydrogenation ͑pressures up to 1 bar H 2 ͒ in a Perkin Elmer Lambda 900 diffraction grating spectrometer ͑0.495Ͻប Ͻ 6.51 eV͒ and a Bruker IFS 66 Fourier transform infrared spectrometer ͑0.2Ͻប Ͻ 1.1 eV͒. The R-T spectra are measured through the transparent substrate at near normal incidence of the incoming beam. Figures 1͑a͒ and 1͑b͒ show the reflection and transmission spectra measured for 200 nm Mg y Ti 1−y / 10 nm Pd films ͑y = 0.90, 0.80, and 0.70͒ in the as-prepared and hydrogenated states ͑in 1 bar H 2 at room temperature͒. In the metallic state ͓Fig. 1͑a͔͒ all the films have a relatively high and featureless reflection that decreases with increasing Ti content.After hydrogen absorption, the reflection is low for all compositions, whereas significant transmission is observed only for the y = 0.90 sample. The combination of low reflection and low transmission in the hydrogenated state ͑y = 0.80 and 0.70 samples͒ gives rise to a highly a...

appear to be nicely spherical, whereas the surface layer has been oxidized. A native oxidation layer grows on the surface of individual Si nanoparticles when they are exposed to traces of air after synthesis. The presence of such limited pacifying oxide layer appeared of advantage for the further processing in air. The average thickness of the oxidation layer is around 1.2 nm, and it is amorphous when observed by X-ray diffraction (XRD) (Figure 1 c), i.e., only peaks corresponding to crystalline Si are visible. For a particle with a size of 20 nm Si in diameter and 1.2 nm outer layer of SiO 2 , the volume percentage of SiO 2 is 28.8%. Raman spectra (Figure 1 d) on the sample report that both crystalline and amorphous Si exist and the amount of amorphous Si is signifi cant (c-Si:a-Si = 0.39:0.61; quantitative analysis in the Supporting Information). To determine the amount of oxygen in the sample, thermogravimetric analysis (TGA) is carried out by heating the Si NP sample under a mixture of O 2 /Ar gas and fully oxidizing Si into SiO 2 . The result indicates that the amount of Si accounts for 69.0 wt% of the sample ( Figure S2, Supporting Information), i.e., Si:SiO 2 = 0.83:0.17 in mole. Meanwhile, the volume fraction from the estimated mass ratio above is 28.2% and is in good quantitative agreement with the one estimated from TEM.Galvanostatic tests on the Si NP electrode are performed using different dis-/charge currents between applied potentials of 0.01 and 2.8 V. In this paper, all specifi c currents applied are calculated with respect to the mass of Si. De-/sodiation capacities of Si stated in this paper are the capacities after subtracting the capacity of the super P carbon black ( Figure S3, Supporting Information), and excluding inactive SiO x inside the sample.Figure 2 a demonstrates an initial sodiation capacity of 1027 mAh g −1 for Si at 20 mA g −1 , which is higher than the theoretical capacity (954 mAh g −1 for NaSi). A large part of this initial capacity is attributed to the irreversible formation of a solid electrolyte interface (SEI) layer on the surface of Si in combination with some decomposition of electrolyte, and possibly also the irreversible formation of sodium silicate from reaction with SiO 2 . The subsequent Na ion extraction process achieves a capacity up to 270 mAh g −1 , indicating that a signifi cant Na fraction is stored reversibly, next to the large irreversible part. For the subsequent few cycles the sodiation capacity decreases from above 410 mAh g −1 to around 300 mAh g −1 but the desodiation capacity is relatively stable around 260 mAh g −1 . The Coulombic effi ciency grows gradually to >90%, after which the de-/sodiation capacity becomes relatively stable. After 100 cycles the reversible capacity retention reaches 248 mAh g −1 , which is 92% of the fi rst desodiation capacity; and the Coulombic efficiency declines slowly to 87% in this cycle test. Additionally, Figure 1 e shows that after charge/discharge for 100 cycles Si particles in this electrode got fractured into small g...

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