An optical method to determine the thermodynamics of hydrogen absorption and desorption in metals

Abstract

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...

High capacity hydrogenstorage materials: attributes for automotive applications and techniques for materials discovery

“…76,77 High-throughput materials screening, a promising technique commonly used in the pharmaceutical industry, couples the steps of synthesis, processing, and characterization to accelerate the discovery of hydrogen storage materials. 78,79 V. Materials attributes and methods for their evaluation…”

Enhancement of Destabilization and Reactivity of Mg Hydride Embedded in Immiscible Ti Matrix by Addition of Cr: Pd-Free Destabilized Mg Hydride

“…The hydrogenation and dehydrogenation properties of our thin films were evaluated by means of an optical technique called hydrogenography which allows to measure the P (pressure)-T (optical transmission) isotherms. [8][9][10] According to Lambert-Beer the relation ln(T/T 0 ) is proportional to C H d, where T 0 is the initial transmission, C H is the hydrogen content in the film, and d is the film thickness. 11 The detailed procedures were described in previous reports.…”

Thermodynamic Properties, Hysteresis Behavior and Stress-Strain Analysis of MgH2 Thin Films, Studied over a Wide Temperature Range

“…After deposition, the metal films were transferred to the hydrogenography setup to study their (de-)hydrogenation properties. A more detailed explanation of the hydrogenography technique and the experimental procedure can be found elsewhere [53].…”