ABO3 pervskite oxides as candidate materials for hydrogen storage

Finding an alternative energy source to fossil fuels is a priority among the goals of our society. In fact, fossil fuels provide over 80% of all energy consumed globally and, in particular, the transport sector accounts for almost 60% of the world energy consumption. [1] Among the most sustainable sources hydrogen is the favourite because it has high energy storage capacity, equal to 120 MJ/kg (= 33.33 kWh), and its only reaction product is H2O. However, hydrogen has an extremely low density (0.089 kg/m3), hence for its storage the solid state would be the most effective solution. [2] Metal hydrides have been widely investigated to this aim due to their high hydrogen storage densities, but they are also characterized by slow kinetics and are unable to release hydrogen at low temperatures, as the applications require. [3] As candidate materials for hydrogen storage we are evaluating ABO3 perovskite oxides because these materials have a high thermal stability, they are relatively active, they allow the exploitation of a great variety of elements in the composition while maintaining the basic structure unchanged and, to improve its efficiency, it is also possible to easily vary the stoichiometry.[4] Among the different materials investigated CaMnO3 (CM) seems to be the most promising. This material was synthesized by Pechini method and characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) microscopy analyses. Moreover, to determine the theoretical maximum hydrogen amount that can be stored in the structure of CM structural evaluations were applied. Due to orthorhombic structure of this compound and considering the presence of octahedral voids (whose dimension is 2.82 Å3), we evaluated the fitting of both molecular hydrogen and hydride ion. So, approximating the species to a sphere, we can observe that hydrogen as H-, with an ionic radius of 1.34 Å cannot fit into the material structure considered, while molecular hydrogen, with a radius of 0.74 Å, can theoretically fit into an octahedral void. Then the H2 adsorption/desorption measurements as a function of pressure (till 40 bar of H2) were carried out with a High Pressure Gas Sorption Analyzer (i-Sorb Anton Paar), a volumetric system, at different temperatures. The achieved results have shown that the nonstoichiometric amount of oxygen affects the hydrogen storage capacity. In particular, the structures with an excess of oxygen exhibit a higher H2 absorption capacity: at 100 °C, CM absorbs 0.44 kg/m3 of H2 while the uptake of H2 for CMO3±δ is 0.98 kg/m3 (Figure 1). In order to enhance the storage capacity of these oxides, we are also investing the addition of 1% wt. Pd0, which should promote the dissociation of hydrogen and, therefore, improve the ad/desorption kinetics of this gas. As a result, we observed that, at the same experimental conditions, the materials in presence of Pd0 absorbs a significantly larger amount of H2 12.88 kg/m3 and 44.95 kg/m3 respectively (Figure 2).What is more, they are able to retain it for future utilization. Further studies are ongoing to investigate the gas release conditions and eventual structural modifications to better understand the mechanism by which hydrogen is stored and consequently improve the storage efficiency. References [1] International Energy Agency (IEA). Key World Energy Statistics, 2020. [2] D. J. Durbin, C. Malardier-Jugroot, International journal of hydrogen energy, 2013, 38(34), 14595- 14617. [3] M. S. El-Eskandarany, RSC advances, 2019, 9(18), 9907-9930. [4], S. M. A. A. Ibrahim, Korean J. Chem. Eng., 2014, 31, 1792-1797.