CaMnO3: a promising material for low temperature hydrogen storage

To our days, the hydrogen storage, especially in the solid state, is a very interesting and challenging research topic. Hydrogen, in fact, is one of the most favourite candidates to replace fossil fuels, that are not sustainable and renewable energy sources.1 This gas has a high energy density (of 120 MJ/kg = 33.33 kWh), and its only reaction product is H2O. However, due to the extremely low density of hydrogen (0.089 kg/m3) solid-state storage systems are one of the most viable solutions. Particularly, we are studying ABO3 perovskite oxides, that have a high thermal stability, they are relatively active and they allow the exploitation of a great variety of elements in the composition while maintaining the basic structure unchanged.2 We are focused on CaMnO3 that is a promising material for hydrogen storage at low temperature. This perovskite oxide, whose constituent elements are earth-abundant and non-toxic, was synthesized by a modified 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. Then the hydrogen storage capacity was tested as a function of pressure with a High Pressure Gas Sorption Analyzer volumetric system (i-Sorb, Anton Paar). Specifically, we are considering the storage capacity of this material up to 40 bar of H2 at 100 °C. In these conditions CaMnO3 shows a storage capacity of 0.02 wt% (0.89 kg/m3). In order to improve the hydrogen sorption for this material, Pd0 was added (1 wt%.), since it can help the hydrogen molecule dissociation into atoms and therefore promote its absorption. CaMnO3/Pd 1%, in fact, in the same condition of pressure and temperature, has a hydrogen storage capacity equal to 0.86 wt% (38.70 kg/m3). The sorption kinetics of this material was investigated considering different equilibration times (5, 60, 120 minutes, and until equilibrium condition is reached) for each adsorption step (from 0 up to 40 bar). The maximum adsorption recorded at 120 min of time limit is comparable with the longest equilibration time, indicating that 2h is an optimum compromise for the equilibration time for this hydrogen storage material. With respect to other materials, such as metal hydrides, despite a relatively lower gravimetric storage capacity but a comparable volumetric capacity, perovskite oxides enable the hydrogen storage at significantly lower temperature, namely 100 °C vs 300 °C required for hydrides. 3 Moreover CaMnO3/Pd 1% can retain hydrogen even by decreasing pressure and release it when required at low temperatures. The evaluation of the storage kinetics model is ongoing. References: [1] J. O. Abe, A. P. I. Popoola, E. Ajenifuja, O. M. Popoola, International Journal of Hydrogen Energy 2019, 44, 15072- 15086. [2] S. M. A. A. Ibrahim, Korean J. Chem. Eng. 2014, 31, 1792-1797. [3] N.Z.A.K. Khafidz, Z. Yaakob, K. L. Lim, S. N. Timmiati, International Journal of Hydrogen Energy 2016, 41b (30), 13131-13151.