Sviluppo di dispositivi reversibili a base di Celle ad Ossido Solido funzionanti con ammoniaca

The transition to sustainable energy systems requires carbon-free fuels that can be efficiently produced, stored, and converted. Ammonia (NH₃) represents a particularly attractive candidate, owing to its high hydrogen density, global infrastructure, and dual role as both an energy vector and a potential product of electrochemical synthesis. Protonic reversible solid oxide cells (rSOCs), operating at intermediate temperatures (400–700 °C), offer a unique platform to exploit ammonia’s potential: in fuel cell mode they can directly utilize NH₃ without external reforming, while in electrolysis mode they can enable the renewable synthesis of ammonia from N₂ and H₂O. However, the deployment of such systems is limited by the lack of cost-effective and stable electrode materials capable of catalyzing the challenging reactions involved in ammonia decomposition and nitrogen reduction. This thesis addresses these challenges by systematically designing, synthesizing, and evaluating perovskite-based electrodes incorporating earth-abundant transition metals (Ni, Cu, Fe, Mo) as alternatives to scarce and costly noble metals such as Ru. Two complementary strategies were investigated: (i) intrinsic catalytic perovskites via B-site doping, exemplified by the double perovskite BaSrFe1.5Mo0.5O6 (BSFM), inspired by the FeMo cofactor of nitrogenase; and (ii) impregnation of transition metals onto BaCexZr0.9-xY0.1O₃₋δ (BCZY) supports, leveraging the chemical stability and proton conductivity of the backbone. The BSFM system was successfully synthesized, demonstrating phase stability under redox cycling, compatibility with BCY electrolytes, and promising mixed ionic–electronic conductivity. Structural analyses (XRD, SEM, XPS, TPR) revealed stable double-perovskite formation, controlled porosity, and Fe-assisted nitrogen adsorption under H₂/N₂ atmospheres. In SOEC mode, moreover, it proved to be able to synthesize ammonia at rates comparable to the best literature results. Complementary impregnation studies highlighted contrasting behaviors: Ni and Cu were relatively straightforward to stabilize, whereas Mo and especially Fe strongly interacted with Ba, forming persistent molybdate and ferrite phases. Optimization of synthesis parameters (precursor choice, calcination conditions, gas atmospheres, and Ce/Zr ratios) was critical to mitigate these reactions, with Zr-rich BCZY supports showing enhanced robustness. Catalytic testing at the powder level confirmed the hierarchy Ni > Fe > Mo > Cu for NH₃ decomposition, consistent with literature benchmarks. In SOEC-like conditions, however, no catalyst showed measurable thermal NH₃ synthesis, underscoring the necessity of electrochemical driving forces. Electrochemical evaluation in symmetrical cells further demonstrated that Ni–BCZY offered the lowest polarization resistances in H₂ and NH₃, while Cu suffered from severe kinetic limitations. Mo–BCZY was diffusion-limited at low temperatures but competitive at ≥700 °C, and Fe–BCZY displayed strong sensitivity to humidity but recovered good performance in dry atmospheres at high temperature. Finally, SOEC-mode chronoamperometry confirmed the direct electrochemical synthesis of ammonia, with Fe– and Mo-containing electrodes showing the most promising activity for nitrogen reduction. Overall, this thesis demonstrates that protonic rSOCs can be effectively operated with ammonia as both fuel and product when supported by carefully tailored perovskite-based electrodes. The results establish Ni as a reliable anode benchmark, highlight the potential of Fe and Mo for nitrogen activation, and clarify the limitations of Cu. By integrating structural design, catalytic screening, and electrochemical validation, this work provides a pathway toward sustainable, earth-abundant catalysts for green ammonia production and utilization in protonic solid oxide technologies.