A-Site Cation-Control in Ruddlesden-Popper Perovskite Anode for Boosting Exsolution of Fe Nanoparticles and Formation of Oxygen Vacancies for High-Performance Direct-Ammonia Solid Oxide Fuel Cells

Direct ammonia solid oxide fuel cells (DA-SOFCs) present a promising pathway for efficient energy conversion for green ammonia utilization. However, challenges such as nitridation and agglomeration of Ni-based anodes have restricted their widespread application. This study explores the application of A-site cation control by partially substituting Sr with Ba in the Ruddlesden-Popper (RP) perovskite. This substitution enhances exsolution of Fe nanoparticles and formation of oxygen vacancies by inducing the lattice expansion in the crystal structure, thereby improving ammonia decomposition and the overall electrochemical performance of DA-SOFCs.<br/>The incorporation of Ba into the RP lattice modifies the crystal structure, facilitating metal nanoparticle exsolution and increasing basic properties of anode material. These modifications are crucial for enhancing nitrogen recombination and desorption during ammonia decomposition, thereby maintaining active catalyst sites and improving resistance to the nitridation of exsolved Fe nanoparticles. This study focuses on the in-situ exsolution of Fe nanoparticles and the partial Ba substitution, both of which significantly enhance catalytic activity and electrochemical performance while improving durability under operational conditions.<br/>Advanced characterization techniques, including X-ray diffraction with Rietveld refinement and high-resolution electron microscopy, were employed to elucidate the lattice changes and phase transitions induced by partial Ba substitution in the A-site of RP perovskite. These modifications promote the exsolution of Fe nanoparticles, which are essential for enhancing the electrochemical activity. Electrochemical assessments demonstrate that the novel La_1.2Sr_0.4Ba_0.4Mn_0.4Fe_0.6O_4-δ with exsolved Fe nanoparticles (R-LSBMF) as an anode material achieves nearly complete ammonia decomposition at 650^oC and maintains this activity over extended periods, indicating excellent durability. The in-situ reduced LSBMF, combined with GDC (Ce_0.9Gd_0.1O₂) as a DA-SOFC anode, exhibits a maximum power density of 1.019 W cm^-2 and a notably low ohmic resistance of 0.154 Ω cm² at 800 ^oC.<br/>This research advances the understanding of A-site cation substitution in RP perovskite anode materials and highlights the potential of RP perovskite with exsolved metal nanoparticles for high-efficiency and durable DA-SOFC anodes. Furthermore, it demonstrates the practical feasibility of using ammonia as an effective and sustainable fuel for SOFCs, driving the technology towards eco-friendly energy solutions. The exceptional catalytic and durability features of the R-LSBMF anode material pave the way for the commercialization of DA-SOFCs, which is crucial for addressing global environmental challenges.