In many experimental investigations of thermodynamic equilibrium or kinetic properties of series of similar reactions it is found that the enthalpies and entropies derived from Van ′t Hoff or Arrhenius plots exhibit a strong linear correlation. The origin of this Enthalpy-Entropy compensation, which is strongly related to the coalescence tendency of Van ′t Hoff or Arrhenius plots, is not necessarily due to a physical/chemical/biological process. It can also be a merely statistical artefact. A new method, called Combined K-CQF makes it possible both to quantify the degree of coalescence of experimental Van ‘t Hoff lines and to verify whether or not the Enthalpy-Entropy Compensation is of a statistical origin at a desired confidence level. The method is universal and can handle data sets with any degree of coalescence of Van ‘t Hoff (or Arrhenius) plots. The new method requires only a standard least square fit of the enthalpyΔH versus entropy ΔS plot to determine the two essential dimensionless parameters K and CQF. The parameter K indicates the position (in inverse temperature) of the coalescence region of Van ‘t Hoff plots and CQF is a quantitative measure of the smallest spread of the Van ‘t Hoff plots. The position of the (K, CQF) couple with respect to universal confidence contours determined from a large number of simulations of random Van ‘t Hoff plots indicates straightforwardly whether or not the ΔH-ΔS compensation is a statistical artefact.

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.

Magnesium thin films covered with a layer of Pd absorb hydrogen at much higher pressures than bulk Mg. Such an effect was originally explained as a consequence of elastic clamping on Mg by the capping Pd layer. An alternative interpretation later suggested that the pressure increase could originate from simple alloying between Mg and Pd. Here we resolve this controversy by measuring the hydrogenation and dehydrogenation isotherms of Mg-Pd thin film alloys over a wide range of compositions. Our results disentangle the effects of elastic clamping and alloying and highlight the role of plastic deformations.

We tune the thermodynamics of hydrogen absorption in Mg by means of elastic clamping. The loading isotherms measured by hydrogenography show that Mg films covered with Mg-alloy-forming elements, such as Pd and Ni, have hydrogen plateau pressures more than 2 orders of magnitude higher than bulk Mg at the same temperature. An elastic model allows us to interpret the Mg thickness dependence of the hydrogen plateau pressure. Our results suggest an alternative route for the development of new hydrogen storage materials with optimized thermodynamic properties.

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.

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.

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.

The reflection and transmission spectra of Pd capped Mg_y Ti_{1 - y} thin films (y = 0.7, 0.8 and 0.9) are measured in the 0.5-5.5 eV energy range, both in the as-prepared and hydrogenated states. Upon hydrogenation these films switch reversibly from a shiny metallic state into a "black" absorbing one. The composition and thicknesses can be tailored to achieve high solar absorptance and low thermal emittance in the hydrogenated state. The combination of these two characteristics is interesting for the application of this material as switchable absorber in solar collectors. The use of a Mg_y Ti_{1 - y} switchable absorber in solar collectors allows to lower the stagnation temperature from 180 to 80 °C. The collector efficiency is affected only minimally.

X-Ray diffraction and electrochemical (de)hydrogenation were performed in situ to monitor the symmetry of the unit cells of Mg_yTi _100-y thin film alloys (70 ≤ y ≤ 90 at.% Mg) along the pressure-composition isotherms at room temperature. The diffraction patterns show that the crystal structures of all as-deposited alloys have a hexagonal closed packed symmetry. Inserting hydrogen transforms the structure to a body centered tetragonal structure for Mg₉₀Ti₁₀, whereas the unit cells of Mg₇₀Ti₃₀ and Mg₈₀Ti₂₀ transform into a face centered cubic symmetry. The structural change of the hydrides along with the ability to rapidly (de)hydrogenate the films emphasize the influence of the symmetry of the host lattice on the hydrogen transport properties. The lattice spacings of the unit cell of Mg₉₀Ti ₁₀H_x as a function of hydrogen content do not change much in the phase transformation region, indicating that only the fractions of the phases change. Remarkably, the lattice spacings found for Mg₇₀Ti ₃₀H_x in the two phase coexistence region reveal that not only the fractions but also the Mg-to-Ti ratio of both phases continuously change. Evidently, there is a large spread in the thermodynamic stability of the available sites for hydrogen. Since the X-ray diffraction patterns rule out large scale segregation, the results imply a nanostructured alloy with Ti-poor and Ti-rich regions and illustrate that the Mg and Ti atoms in Mg ₇₀Ti₃₀ are not randomly distributed.

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.

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.

We report on the implementation of Pd-capped chemo-chromic metal hydrides as a sensing layer in fiber optic hydrogen detectors. Due to the change in optical properties of Mg-based alloys on hydrogen absorption, a drop in reflectance by a factor of 10 is demonstrated at hydrogen levels down to 15% of the lower explosion limit. The switching takes place in only a few seconds. Comparing Mg-Ni and Mg-Ti based alloys, we find that the latter has superior optical and switching properties. Using a 50 nm thick Mg₇₀Ti₃₀ film capped with a 30 nm Pd catalytic layer as the sensing layer, we report on the sensitivity of the fiber optic detector for hydrogen both in argon and oxygen, the temperature behavior and the reversibility.

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.

Mg_y Ni (2 ≤ y ≤ 10) thin films covered with a Pd cap layer are hydrogenated in 1 0⁵ Pa H₂ between room temperature and 80 ° C and their dielectric function over(ε, ̃) is determined from reflection and transmission measurements. The hydrogenated Mg_y NiH_x thin films show a continuous shift of the optical absorption towards higher photon energies with increasing y. Comparison of the obtained dielectric functions with predictions from an effective medium theory show that a considerable doping of the Mg₂ NiH₄ host takes place at least for y ≤ 3.5 while no signature of MgH₂ is observed in that composition range in the optical spectra. This is in contrast to the predictions from the bulk phase diagram where a mixture of semiconducting Mg₂ NiH₄ (energy gap E_g = 1.6 eV) and MgH₂ (E_g = 5.6 eV) is expected.