Perovskite Oxides: an Alternative Route for Reversible Solid-State Hydrogen Storage

Perovskite oxides, with their general formula ABO3, possess a unique combination of structural and electronic properties that make them highly versatile. Their functionality can be tuned by substituting various elements at the A and B cation sites, and their ability to exhibit mixed-valence states and oxygen non-stoichiometry, in the form of oxygen vacancies, is particularly noteworthy. These vacancies are not merely defects; they are crucial for ion mobility and catalytic activity, which are essential for applications in solid oxide fuel cells, sensors, and advanced electronic components. The use of perovskite oxides for hydrogen storage is especially compelling because they can be composed of abundant, non-toxic elements, offering a sustainable and economically viable alternative to traditional materials. In fact, the development of a hydrogen-based economy requires safe, efficient, and cost-effective solid-state hydrogen storage solutions. For this purpose, the most investigated materials are metal hydrides. However, their slow kinetics, poor reversibility, and the use of rare or toxic elements pose significant challenges. This doctoral thesis explores the potential of perovskite oxides as a new class of materials for energetic field applications, focusing on solid-state hydrogen storage, an area that has been largely underexplored from an experimental standpoint. The research project involved the synthesis and characterization of several perovskite oxides, including CaTiO3, BaMnO3, LaNiO3, LaFeO3, and CaMnO3, along with related structures like La2NiO4 and Ca2AlMnO5. The materials were prepared using established techniques such as solid-state synthesis and the Pechini method. X-ray diffraction (XRD) and Rietveld refinements were used to confirm the formation of single-phase perovskite structures, while scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) verified their morphological and chemical purity. Thermal stability was assessed through thermogravimetric analysis (TGA), which confirmed the stability of these materials under high temperatures and an inert atmosphere, further supporting their potential for energy applications. Initial hydrogen storage tests were conducted under moderate pressure at room temperature. Most of the pure materials showed minimal sorption capacity, indicating a weak physisorption mechanism. However, CaMnO3 stood out with a slightly higher storage capacity, leading to its selection for more in-depth investigation. The choice of CaMnO3 was motivated by its intrinsic properties, including structural flexibility, oxygen non-stoichiometry (with a determined δ of 0.03), and the multivalent nature of manganese, all of which are critical for creating active sites for hydrogen interaction. To enhance the hydrogen storage capability of CaMnO3, an optimization strategy was pursued by incorporating metallic palladium to promote H2 dissociation. Structural and morphological characterization with XRD and SEM confirmed the successful dispersion of Pd0 without altering the main perovskite structure. Subsequent hydrogen storage tests were performed on both pure CaMnO3 and the optimized CaMnO3/Pd 1% composite at three different temperatures (5 °C, 30 °C, and 100 °C) under a hydrogen pressure of 40 bar. The results demonstrated a significant enhancement in storage capacity (0.9 wt%) with the addition of palladium. These results were then confirmed at the DLR (Deutsches Zentrum für Luft und Raumfahrt) in Stuttgart, Germany, validating the reproducibility and scalability of the findings. Further analyses using TGA and temperature-programmed desorption (TPD) revealed the desorption behavior of the most promising material. The measurements showed that CaMnO3/Pd 1% releases hydrogen in two distinct steps, occurring at approximately 360 °C and 560 °C. This finding suggests a chemisorption mechanism, where hydrogen atoms are tightly bound within the material's structure. Finally, this thesis successfully demonstrates that perovskite oxides, particularly CaMnO3/Pd 1%, are promising materials for solid-state hydrogen storage. The achieved capacity of 0.9 wt% at 100 °C is a significant finding that positions this material as a viable alternative to existing technologies. Unlike physisorption materials such as MOFs, which require stringent cryogenic temperatures, or traditional metal hydrides, which face challenges related to safety, cost, and kinetics, the CaMnO3/Pd 1% composite operates at more practical temperatures and is composed of abundant, non-toxic elements. So, this research bridges the gap between theoretical proposals and experimental evidence, providing a new direction for the design of sustainable hydrogen storage solutions. The results highlight the potential of perovskite oxides to offer a balanced combination of good storage capacity and operational feasibility, paving the way for future advancements in hydrogen-based energy technologies.