•SrNbxFe1−xO3−δ perovskites were evaluated as cathodes for solid oxide fuel cells.•Oxygen nonstoichiometry in SrNbxFe1−xO3−δ was correlated with the ORR performance.•The peak power density of fuel cell with SNF0.1 achieved 810 mW cm−2 at 700 °C.The development of high performance perovskite cathode for solid oxide fuel cells (SOFCs) relies upon the knowledge and understanding of the interplay between the metal oxide components, structure, redox properties and conductivity. In this work, we partially substituted Fe on SrFeO3−δ with Nb. In particular, 3 Nb-doped compositions were prepared, e.g. SrNb0.05Fe0.95O3−δ (SNF0.05), SrNb0.1Fe0.9O3−δ (SNF0.1) and SrNb0.2Fe0.8O3−δ (SNF0.2). Mössbauer spectroscopy revealed decreasing ratio of Fe4+ to Fe3+ at the higher Nb doping content which translates to the gradual decrease of the average Fe oxidation state from 3.403 (for SNF0.05) to 3.375 (for SNF0.1) and to 3.291 (for SNF0.2). Likewise, the oxygen desorption process and the thermal expansion coefficients decreased with increasing Nb content, therefore providing evidence on their correlation with the thermal reduction of Fe4+. The temperature-dependent oxygen nonstoichiometry displayed two different regimes separated by a transition temperature of 625 °C, below which SNF0.2 showed the highest nonstoichiometry and above which SNF0.05 provided the highest nonstoichiometry. The analogous shifting in trends was reproduced for oxygen reduction reaction (ORR) performance which signifies oxygen nonstoichiometry as the main variable affecting ORR performance.

•Sr4Fe6O13−δ cathode exhibits single phase and low thermal expansion between room temperature and 1050 °C.•Sr4Fe6O13−δ layered structure restricts the oxygen ionic transport in the ac plane along the layered oxide (FeO5) layer only.•The ORR activity can be optimized by tuning the phase composition of Sr4Fe6O13−δ using oxygen deficient calcination process.A cobalt-free layered oxide-Sr4Fe6O13−δ is synthesized and characterized for application as a cathode of intermediate temperature solid oxide fuel cells. High temperature powder x-ray diffraction, oxygen temperature programmed desorption and electrochemical impedance spectroscopy are employed to evaluate the temperature-dependent crystal structure and oxygen reduction reaction (ORR) activity of Sr4Fe6O13−δ. The oxide exhibits a single phase between room temperature and 1050 °C with low thermal expansion coefficient. The drawback of this oxide lies on its low ORR activity which is likely due to its layered structure which favors oxygen ionic transport two-dimensionally in the ac plane along the layered oxide layer. We show here that the original ORR activity can be improved by adjusting the phase compositions through an oxygen-deficient calcination process. An area specific resistance of 0.139 Ω cm2 at 700 °C is attained by calcining Sr4Fe6O13−δ at 900 °C with ∼66.2 wt% of perovskite phase in the composite material.Cobalt-free layered perovskite oxide Sr4Fe6O13−δ which shows good phase stability (e.g. retain single phase between room temperature and 1050 °C in air) and low thermal expansion was evaluated as an oxygen reduction component in intermediate-temperature solid oxide fuel cells. We showed that the drawback of this layered oxide in terms of its low oxygen reduction reactivity (due to the restriction of oxygen transport in the ac plane along the layered oxide (FeO5) layer) can be circumvented by tuning the phase composition (perovskite to layered oxide phases) through an oxygen deficient calcination process. The optimum performance was attained by calcining Sr4Fe6O13−δ at 900 °C; leading to an area specific resistance (ASR) of 0.139 Ω cm2 at 700 °C.

Cobalt-free perovskite BaNb0.05Fe0.95O3−δ (BNF) is synthesized and characterized towards application as a cathode material for intermediate temperature solid oxide fuel cells. In situ X-ray diffraction and transmission electron microscopy are applied to study the crystal structure and thermally induced phase transformation. BNF exists as a multiphase structure composed of a monoclinic phase and a cubic phase at room temperature, and then undergoes a phase transformation to a cubic structure starting at ∼400 °C, which is maintained at temperatures up to 900 °C during a thermal cycle between room temperature and 900 °C; while it retains the cubic perovskite lattice structure on cooling from 900 °C to room temperature. Oxygen temperature-programmed desorption, combined thermal expansion and thermo-gravimetric analysis are used to clarify the thermal reducibility of BNF. A relatively good stability of BNF is demonstrated by electrical conductivity and electrochemical impedance spectroscopy measurements. The activity of BNF for oxygen reduction reaction is probed by symmetrical cell and single fuel cell tests. Favorable electrochemical activities at intermediate temperature, e.g. very low interfacial resistance of only ∼0.016 Ω cm2 and maximum power density of 1162 mW cm−2 at 750 °C, are demonstrated, which could be attributed to the cubic lattice structure of BNF within the temperature range of cell operation.

•Provide a thermal inkjet printing technique for the fabrication of electrolytes.•Stable YSZ and SDC inks with high solids contents are prepared.•The YSZ layers are prepared with precise thickness control.•The Z values of the prepared inks fit well within the printable range.In this study, we report the facile fabrication of thin-film yttria-stabilized zirconia (YSZ) electrolytes and Sm0.2Ce0.8O1.9 (SDC) buffering layers for solid oxide fuel cells (SOFCs) using a thermal inkjet printing technique. Stable YSZ and SDC inks with solids contents as high as 20 and 10 wt.%, respectively, were first prepared. One single printing typically resulted in an YSZ membrane with thickness of approximately 1.5 μm, and membranes with thicknesses varied from 1.5 to 7.5 μm were fabricated with multiple sequential printing. An as-fabricated cell with a La0.8Sr0.2MnO3 (LSM) cathode delivered a peak power density (PPD) of 860 mW cm−2 at 800 °C. The SDC layer prepared using the inkjet printing method exhibited enclosed pores and a rough surface, which was, however, ideal for its application as a buffering layer. A cell with a dense 7.5-μm-thick YSZ layer, a 2-μm-thick SDC buffering layer and a Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) cathode was fabricated; this cell delivered a PPD of 1040 mW cm−2 at 750 °C and a high open circuit voltage (OCV) of approximately 1.10 V. The described technique provides a facile method for the fabrication of electrolytes for SOFCs with precise thickness control.

Various Ni–LaxCe1−xOy composites were synthesized and their catalytic activity, catalytic stability and carbon deposition properties for steam reforming of methane were investigated. Among the catalysts, Ni–La0.1Ce0.9Oy showed the highest catalytic performance and also the best coking resistance. The Ni–LaxCe1−xOy catalysts with a higher Ni content were further sintered at 1400 °C and investigated as anodes of solid oxide fuel cells for operating on methane fuel. The Ni–La0.1Ce0.9Oy anode presented the best catalytic activity and coking resistance in the various Ni–LaxCe1−xOy catalysts with different ceria contents. In addition, the Ni–La0.1Ce0.9Oy also showed improved coking resistance over a Ni–SDC cermet anode due to its improved surface acidity. A fuel cell with a Ni–La0.1Ce0.9Oy anode and a catalyst yielded a peak power density of 850 mW cm−2 at 650 °C while operating on a CH4–H2O gas mixture, which was only slightly lower than that obtained while operating on hydrogen fuel. No obvious carbon deposition or nickel aggregation was observed on the Ni–La0.1Ce0.9Oy anode after the operation on methane. Such remarkable performances suggest that nickel and La-doped CeO2 composites are attractive anodes for direct hydrocarbon SOFCs and might also be used as catalysts for the steam reforming of hydrocarbons.Highlights► Ni–LaxCe1−xOy (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) cermets were synthesized. ► Ni–La0.1Ce0.9Oy showed the best catalytic activity and highest coking resistance. ► Ni–La0.1Ce0.9Oy anode also showed excellent operational stability. ► High power output of 850 mW cm−2 was obtained with methane–steam as fuel at 650 °C.

Cobalt-free perovskite BaNb0.05Fe0.95O3−δ (BNF) is synthesized and characterized towards application as a cathode material for intermediate temperature solid oxide fuel cells. In situ X-ray diffraction and transmission electron microscopy are applied to study the crystal structure and thermally induced phase transformation. BNF exists as a multiphase structure composed of a monoclinic phase and a cubic phase at room temperature, and then undergoes a phase transformation to a cubic structure starting at ∼400 °C, which is maintained at temperatures up to 900 °C during a thermal cycle between room temperature and 900 °C; while it retains the cubic perovskite lattice structure on cooling from 900 °C to room temperature. Oxygen temperature-programmed desorption, combined thermal expansion and thermo-gravimetric analysis are used to clarify the thermal reducibility of BNF. A relatively good stability of BNF is demonstrated by electrical conductivity and electrochemical impedance spectroscopy measurements. The activity of BNF for oxygen reduction reaction is probed by symmetrical cell and single fuel cell tests. Favorable electrochemical activities at intermediate temperature, e.g. very low interfacial resistance of only ∼0.016 Ω cm2 and maximum power density of 1162 mW cm−2 at 750 °C, are demonstrated, which could be attributed to the cubic lattice structure of BNF within the temperature range of cell operation.