Enthalpy-entropy compensation (EEC) is very often encountered in chemistry, biology and physics. Its origin is widely discussed since it would allow, for example, a very accurate tuning of the thermodynamic properties as a function of the reactants. However, EEC is often discarded as a statistical artefact, especially when only a limited temperature range is considered. We show that the likeliness of a statistical origin of an EEC can be established with a compensation quality factor (CQF) that depends only on the measured enthalpies and entropies and the experimental temperature range. This is directly derived from a comparison of the CQF with threshold values obtained from a large number of simulations with randomly generated Van ‘t Hoff plots. The value of CQF is furthermore a direct measure of the existence of a genuine isoequilibrium or isokinetic relationship.

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We calculate from first principles the optical spectra of the hydrides in the sodium alanate hydrogen storage system: NaH, NaAlH4, and Na 3AlH6. In particular we study the effects of systematic improvements of the theoretical description. To benchmark the calculations we also measure the optical response of a thin film of NaH. The simplest calculated dielectric functions are based upon independent electrons and holes, whose spectrum is obtained at the G0W0 level. Successive improvements consist of including partial self-consistency (so-called GW 0) and account for excitonic effects, using the Bethe-Salpeter equation (BSE). Each improvement gives a sizable blue shift or red shift of the dielectric functions, but conserves the trend in the optical gap among different materials. Whereas these shifts partially cancel at the highest (GW 0-BSE) level of approximation, the shape of the dielectric functions is strongly modified by excitonic effects. Calculations at the GW 0-BSE level give a good agreement with the dielectric function of NaH extracted from the measurements. It demonstrates that the approach can be used for a quantitative interpretation of spectra in novel hydrogen storage materials obtained via, e.g., hydrogenography.@en

We investigated the influence of the substrate on the thermodynamic properties of metal hydride thin films by hydrogenography, using PdH_x as a model system. After appropriate hydrogen cycling, reproducible hydrogenation properties are found at the same equilibrium pressure for all substrates studied. Comparing these thin films with free-standing films-measured both by hydrogenography and by Sievert's method-we find a very similar behavior. Hence, thin films can be used to study the hydrogenation behavior of the corresponding bulk materials.

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Hydrogenography is a new optical thin film combinatorial method that follows hydrogenation and determines its associated thermodynamic properties. Due to clamping to the substrate, stresses generated in thin films are larger than in bulk. This must be taken into account for a comparison between these two types of systems. In this article, we follow the microstructure, surface morphology and in-plane stress changes of thin polycrystalline PdH_x films upon several hydrogen ab/desorption cycles and correlate them to the evolution in shape and hysteresis of pressure-optical transmission isotherms (PTIs) recorded by hydrogenography. The in-plane stress in the first instance is relaxed inhomogeneously by buckling, and a more complete, homogeneous relaxation is only reached after the creation of a buckle-and-crack network that is the two-dimensional analogue of bulk decrepitated grains. This sequence of changes is clearly visible in the PTIs, demonstrating another useful facet of hydrogenography for characterizing metal-hydrogen systems.

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A detailed structural analysis of Mg-Ti-H thin films reveals the presence of a chemically partially segregated but structurally coherent metastable phase. By combining X-Ray Diffraction and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy on Mg_yTi_1-yH_x thin films we find non-zero Chemical Short-Range Order (CSRO) parameters for all the compositions measured. Despite the positive enthalpy of mixing of Mg and Ti the degree of ordering does not increase upon loading and unloading with hydrogen. The robustness of this system and the fast and reversible kinetics of hydrogen loading and unloading are caused by the formation of nanoscale compositional modulations in the intermetallic alloy. This microstructure is responsible for the exceptional properties of Mg_yTi_1-yH_x thin films. It also shows that reversible metastable metal-hydrides offer new possibilities for hydrogen storage, beyond the limits imposed by thermodynamic equilibrium.

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A multisite lattice gas approach is used to model pressure-optical- transmission isotherms (PTIs) recorded by hydrogenography on Mgy Ti1-y Hx sputtered thin films. The model reproduces the measured PTIs well and allows us to determine the chemical short-range order parameter s. The s values are in good agreement with those determined from extended x-ray absorption fine structure measurements. Additionally, the PTI multisite modeling yields a parameter L that accounts for the local lattice deformations with respect to the average Mgy Ti1-y lattice given by Vegard's law. It is thus possible to extract two essential characteristics of a metastable alloy from hydrogenographic data.@en

MgHx thin films are grown by activated reactive evaporation in a Molecular Beam Epitaxy system fitted with an atomic hydrogen source. During deposition the electrical and optical properties are measured in-situ. The structural properties are determined ex-situ by Atomic Force Microscopy. These measurements confirm the growth of the MgH2 phase, however the presence of 10 vol.% of metallic Mg cannot be prevented. The metallic Mg grains cause an optical absorption edge at 2.0 eV, which has a completely different origin than the observed band gap of MgH2 at 5.6 eV. The observed optical spectra can be modelled using an effective medium theory. The Mg hydride films are electrically insulating despite the presence of metallic Mg particles. Upon re-hydrogenation of a de-hydrogenated in-situ grown MgHx thin film, the absorption edge at 2.0 eV disappears and the resistivity decreases to values normally observed for ex-situ hydrogenated films.@en

Using hydrogenography, we recently mapped the thermodynamic properties of a large range of compositions in the quaternary Mg-Ti-Ni-H system. The enthalpy of hydride formation of Mg-Ni alloys is significantly altered upon Ti doping. For a small range of compositions, we find a hydrogenation enthalpy ΔH=-40 kJ (mol H2) -1, which is the desired enthalpy for hydrogen storage at moderate temperature and pressure. This enthalpy value is surprising since it is significantly less negative than the ΔH of the Mg-Ni and Mg-Ti hydrides. The nanostructure of the Mg-Ti-Ni-H films hinders a direct determination of the hydride phases involved by x-ray diffraction. Using density functional theory calculations for various hydrogenation reaction paths, we establish that the destabilization of the Mg-Ni-H system by Ti doping is due to the formation of Mg2 Ni and Ti-Ni intermetallics in the as-deposited state, which transform into a metastable Ti-doped Mg2 Ni H4 phase upon hydrogenation. The Ti-doped Mg2 Ni H4 phase can be considered as a heavily doped semiconductor.@en

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 Mg H2 and Ti H2 grains. By continuously monitoring the structure during H uptake, we obtain data that are compatible with a coherent structure. The average structure resembles rutile Mg H2 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 Ti H2. The present results point toward a rich and unexpected chemistry of Mg-Ti-H compounds.

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The search for new lightweight metal hydride storage materials is essentially like looking for a needle in a haystack. Over the years, a number of combinatorial methods have been developed to scan the properties of materials in an efficient way. We demonstrate that combinatorial techniques are also applicable for the search of suitable hydrogen storage materials. This applies especially to hydrogenography, a novel optical screening method that measures simultaneously the enthalpy of hydride formation of thousands of materials on a single thin film wafer.

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Hydrogenography is an advanced combinatorial and standard method used for the search of new hydrogen-storage materials to synthesize bulk samples and to use volumetric or gravimetric techniques to follow their hydrogenation reaction. Hydrogenography, with a straightforward optical setup, makes it possible to monitor hydrogen absorption and desorption simultaneously on thousands of samples under exactly the same experimental conditions. Hydrogenography is much more than a monitoring technique, as it also provides a high-throughput method to measure quantitatively the key thermodynamic properties of hydride formation. The continuous change of optical transmission with hydrogen concentration was used to measure the pressure-concentration isotherms and determine the enthalpy of hydride formation. Hydrogenography is valuable for the search for catalytic caplayers promoting hydrogen uptake, electrode materials for batteries, and smart coatings for adaptive solar collectors.

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The hydrogenation of metallic Mg2 Ni films was shown to proceed via a self-organized double layering of transparent Mg2 NiH4 and metallic Mg2 NiH0.3. For stoichiometries departing from Mg2 Ni we conclude from optical reflection, transmission and electrical measurements that the hydrogenation process involves two competing effects: (i) the nucleation of the initial Mg2 NiH4 layer near the substrate and the ensuing growth of this layer and (ii) the slow random nucleation of the same phase within the remaining part of the film. Which process dominates depends critically on the stoichiometry of the parent metal alloy.@en

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.@en

Thin film composition spreads of Mg-Ni were deposited by co-sputtering on micromachined Si-cantilevers. The investigated compositions range from about Mg ₆₀ Ni ₄₀ to about Mg ₈₀ Ni ₂₀ . Structural properties as well as mechanical stress before and after hydrogenation were measured with X-ray diffraction (XRD) and laser profilometry, respectively. The composition spreads were hydrogenated in a special pressure vessel, which allows measuring optically the hydrogen-induced deflection (stress-change) of 16 cantilevers as a function of hydrogen pressure and/or temperature. It was found that the hydrogen-induced stress is correlated with the composition and microstructure of the films. Highest hydrogen-induced stress changes were found for compositions close to the crystalline Mg ₂ Ni phase.

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The optical properties, the switching kinetics and the lifetime of hydrogen switchable mirrors based on Mg-Ni alloys are determined with particular regard to the composition of the optically active metal-hydride layer in combination with the thickness of the catalytic capping layer. For this, a high-throughput experiment is introduced. The switching kinetics and the reversibility of switchable mirrors are strongly thickness dependent, though the details hinge on the fine structure of the clustered capping layer. Therefore, the kinetics is correlated with the surface structures of Pd on Mg y Ni 1-y as investigated by scanning tunneling microscopy. The results are explained by the so-called strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the capping layer clusters by oxidized species originating from the support. The SMSI-effect is less important with increasing Pd-layer thickness, and is suppressed by a good wetting of the Pd-clusters on the optically active film. This explains the critical thickness for the catalyzed hydrogen uptake observed in many switchable mirror systems. Moreover, the degradation of the kinetics during cycling is found to depend on the Pd-layer thickness and on the gas environment. Only films, covered with at least 15 nm Pd, show small degradation caused by the SMSI-effect. The SMSI-effect is partly reversible: after changing the gas environment from hydrogen to oxygen, the oxide on the Pd-clusters can be partly removed.@en

The structural, optical and dc electrical properties of Mg _xAl_1-x (0.2 ≤ x ≤ 0.9) gradient thin films covered with Pd/Mg are investigated before and after exposure to hydrogen. We use hydrogenography, a novel high-throughput optical technique, to map simultaneously all the hydride forming compositions and the kinetics thereof in the gradient thin film. Metallic Mg in the Mg_xAl_1-x layer undergoes a metal-to-semiconductor transition and MgH₂ is formed for all Mg fractions x investigated. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH₂ and Al occurs. This uncommon large-scale segregation is evidenced by metal and hydrogen profiling using Rutherford backscattering spectrometry and resonant nuclear analysis based on the reaction ¹H(¹⁵N, αγ)¹²C. Besides MgH₂, an additional semiconducting phase is found by electrical conductivity measurements around an atomic [Al]/[Mg] ratio of 2 (x = 0.33). This suggests that the film is partially transformed into Mg(AlH₄)₂ at around this composition.

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Mg-Al thin films with a compositional gradient are co-sputtered from off-centered magnetron sources and capped with a thin Pd layer. We study their hydride formation by monitoring their optical transmission during hydrogenation under defined pressure and temperature conditions. We find that Mg(AlH ₄)₂ is already formed from the elements at p(H2)=1 bar and T=100 ° C. A thin layer of Ti acts as a catalyst. Doping of Mg-Al with Ti has a negative influence on hydrogen absorption.

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