High Oxygen Reduction Reaction Activity (Sm_0.6Sr_0.4)_n+1(Ni_0.6Fe_0.4)_nO_3n+1 (n = 1, 2, 3) Cathode with a Ruddlesden–Popper Structure for Low-Temperature Solid Oxide Fuel Cells

The slow reaction kinetics of the oxygen reduction reaction (ORR) is one of the important challenges facing the development of a cathode in low-temperature solid oxide fuel cells (LT-SOFCs). Herein, we developed the (Sm 0.6 Sr 0.4 ) n +1 (Ni 0.6 Fe 0.4 ) n O 3 n +1 [ n -SSNF ( n = 1, 2, 3)] cathode with a Ruddlesden–Popper (R–P) structure to investigate its oxygen ion conduction mechanism and cathodic ORR activity. Consequently, the cell device of the n -SSNF ( n = 3) cathode accomplished an impressive peak power density of 658 mW·cm –2 . Simultaneously, the device also delivered better low-temperature characteristics and can still operate at 400 °C with a maximum power density of 298 mW·cm –2 . The excellent performance is mainly attributed to the higher cathodic ORR activity, which benefits from the unique R–P structure with the AO rock salt layer and perovskite layer. The AO rock salt layer possess a large number of interstitial oxygen ions, while the perovskite layer can offer a certain amount of oxygen vacancy, which provides a fast channel for oxygen ion transport. Additionally, it exhibits a reduced R p as low as 0.41 Ω·cm 2 and has a smaller activation energy (0.31 eV) and bandgap (0.92 eV). The analysis results of time-domain distribution of relaxation times and frequency-domain electrochemical impedance spectroscopy are consistent. These findings open up directions for the design of high-order R–P-structured cathodes for LT-SOFCs.