Validation of Hydrogenography for the search of promising hydrogen storage materials

Hydrogenography is a combinatorial optical thin film technique to study the thermodynamic properties of metal hydride storage materials. It allows to study thousands of compositions simultaneously with exactly the same experimental conditions. Hydrogenography can pin point the most interesting regions/compositions in binary, ternary, quaternary etc. M-H phase diagrams. This valuable information can then be used in bulk experiments to measure other parameters such as the hydrogen volumetric density. The main advantage of Hydrogenography comes from the possibility to create thin film compositional gradients, allowing for fast and efficient screening. The main drawback of this approach is the presence of the rigid substrate, used to support the metal thin films. During (de-)hydrogenation reaction the substrate exerts a force, due to the clamping, leading to a modification of the thermodynamics properties as measured by Hydrogenography. Therefore, before drawing conclusions on the corresponding bulk materials, the Hydrogenography experiment should be properly analyzed. First, it is important to verify whether the film remains clamped to the substrate or has a tendency to delaminate. Delaminated or buckled films, as in the case of Pd, exhibit thermodynamic properties similar to bulk materials; in this case no correction to the enthalpies or entropies should be applied, i.e. Hfilm?Hbulk, Sfilm?Sbulk. Similar conclusion holds for other metal hydride systems, if the film under investigation is protected from the interaction (alloying) with the capping layer (Pd, Ni) by means of a buffer layer. Otherwise, a larger influence of the substrate on the thermodynamics is expected. Second, one needs to be aware of an expanded hysteresis and, consequently, affected thermodynamic parameters when the film remains in full contact with the substrate. The enthalpy of formation- which is always modified by a bulk hysteresis term- obtains an additional component due to the clamping by the substrate (Hfilm?Hbulk+Hclamping). The proper way to estimate the thermodynamics of the bulk from the thermodynamics of the clamped film, is then to perform a stress-strain analysis at several temperatures. The stress-strain model, developed in this thesis, is able to quantitatively predict the clamping effect. It postulates that the increase of the hysteresis in thin films is taking place due to severe ‘reversible’ plastic deformations. Bulk metal hydrides disproportionate into small grains, when the hydrogenation stresses become higher than the Yield stress, thus releasing most of the stresses. This behavior is not possible in films due to presence of the substrate. The outcome of the stress-strain analysis is Ghyst parameter, which represents an additional mechanical work, performed by the film during the (de-)hydrogenation cycle due to the expansion constraints. This mechanical work increases when increasing the ‘complexity’ of the hydrogen storage system. For the metal hydrides studied in this thesis it ranges from 1.69 kJ/mol H for PdH0.6, 2.93 kJ/mol H for MgH2, 3.52 kJ/mol H for Mg2NiH4 to 4.25 kJ/mol H for YH3. Ghyst can be used to recalculate back the equilibrium pressures of the free-standing film (film without a substrate). From the linear fit to the new pressure values the enthalpy and entropy of the corresponding bulk materials can be evaluated. This approach, however, is only valid for metal hydrides with no structure transformation (Pd case) or for metal hydrides which become amorphous on subsequent (de-)hydrogenation, such as MgH2. When the material cycles between preferentially oriented crystalline metal and metal hydride states, as in the case of YH3 and Mg2NiH4, the equilibrium pressures become affected by an additional parameter, Gstr. It represents the energy involved in the structure transformation and should be added to Ghyst in order to fully describe the hysteresis behavior in these films. The Gstr equals to 5.2 kJ/(mol H) and 4.5 kJ/(mol H) for YH3 and Mg2NiH4, respectively, which is a quite substantial amount of energy which cannot be disregarded. Thus, any other metal hydride system, possessing a structure transformation and remaining crystalline on cycling, will require this parameter, which can be measured via calorimetry or calculated via first principles methods. An alternative approach to correct for the clamping effect is to subtract the energetic value of the clamping effect, Hclamping (the change in enthalpy due to clamping in kJ/mol H), from the measured thin film enthalpy: Hbulk ? Hfilm -Hclamping. This, however, requires knowledge of the Hclamping in advance, which, again, can be estimated from the stress-strain analysis. The problem with this approach is that the entropy cannot be recalculated in this way. Hydrogenography allows for comparison of thousands of compositions under exactly the same conditions. As a result, by measuring the Mg-Ti-Al-H system we identified an enthalpy-entropy compensation effect. Whenever we saw an increase of the enthalpy, simultaneously the entropy values were increasing (both in absolute sense). As a result, the equilibrium pressures remain almost the same. We can exclude trivial reasons related to the conditions of measurement, presence of the substrate etc. The origin of this effect remains unclear and requires further study. It shows, however, that the analysis of absorption/desorption equilibrium pressure at the relevant temperature is required in addition to the estimated thermodynamic parameters in order to evaluate different metal hydrides as hydrogen storage systems.DelftChemTechApplied Science