The electrical conductivity (σ) and oxygen diffusivity of the typical Ruddlesden-Popper oxide Sr3Fe2O7-δ were investigated with the variation of oxygen partial pressure, P(O2), and temperatures, and thus the results were discussed based on its defect structure. The σ increases with the increase of P(O2) and a positive slope of log σ depend on P(O2) is close to 1/4 with the small polaron conduction, where the mobility μP are between 0.01 and 0.02 cm2 V−1 s−1 regardless of temperature and P(O2). Oxygen diffusivity derived from the electrical conductivity relaxation (ECR) after an abrupt change of P(O2) increased with the increase P(O2) and temperature. A new pulse isotope 18O-16O exchange (PIE) at 623–773 K was carried in order to rapidly determine the tracer oxygen surface reaction coefficient. The numerical relationship of oxygen diffusivity measured by ECR and PIE measurements was successfully established by the ambipolar diffusion theory and defect chemical analysis.

•Enhanced performance of symmetrical solid oxide fuel cell (SSOFC) is reported by using a doped ceria buffer layer.•Cobalt-free La0.8Sr0.2FeO3-δ (LSF) is applied as a novel stable electrode material for SSOFC.•GDC buffer layer dramatically enhances the electrochemical performance by more than 90% at 700 °C.•Maximum power density of 387 mW/cm2 for YSZ-based SSOFC is obtained at 800 °C.•GDC buffer layer leads to improved long-term stability for SSOFC.Enhanced performance of symmetrical solid oxide fuel cell (SSOFC) is reported by using a doped ceria buffer layer, which can solve the issues during the operation of traditional solid oxide fuel cells, such as carbon deposition and sulfur poisoning. In this work, cobalt-free perovskite oxide La0.8Sr0.2FeO3-δ (LSF) is applied as a novel stable electrode material for symmetrical solid oxide fuel cell. The electrical conductivities of LSF are 141.1 Scm−1 and 0.138 Scm−1 in air and humidified H2 (3% H2O) at 800 °C, respectively. Gadolinium doped ceria (GDC) buffer layer is fabricated by screen printing onto the YSZ electrolyte, which dramatically enhances the electrochemical performance by more than 90 percent at 700 °C. The improvement of SSOFC performance is attributed to the elimination of reactivity and the optimization of interface between YSZ electrolyte and LSF electrode. These results demonstrate that the doped ceria buffer layer provides a highly repeatable route for further improving the performance of YSZ-based SSOFC, with potentially important implications for developing cost-effective SSOFCs with huge application opportunities.

A layered perovskite oxide Y0.8Ca0.2BaCoFeO5+d (YCBCF) was synthesized as a novel cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by citric acid-nitrates self-propagating combustion method. The phase and microstructure of YCBCF were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The average thermal expansion coefficient (TEC) of YCBCF was 14.6×10−6 K−1, which was close to other materials of SOFC at the range of RT-1000 °C. An open-circuit potential of 0.75 V and a maximum output power density of 426 mW/cm2 were obtained at 650 °C in a Sm0.2Ce0.8O1.9 (SDC)-based anode-supported SOFC by using humidified (∼3% H2O) hydrogen as fuel and static air as oxidant. The results indicated that the YCBCF was a promising cathode candidate for IT-SOFCs.Electrochemical performance of YCBCF/SDC/Ni-SDC at different temperatures

•Mixed rare-earth LSPF superiorly electrical conductivity has been demonstrated.•Proton-blocking LSPF–SDC was employed as cathode for proton-conducting SOFC.•Excellent chemical compatibility between LSPF and SDC up to 1000 °C.•The LSPF–SDC cathode was mechanically compatible with protonic BZCY electrolyte.•High power density reach 488 mW cm−2, low polarization resistance of 0.071 Ω cm2.Mixed rare-earth (La, Pr)0.8Sr0.2FeO3−δ–Sm0.2Ce0.8O2−δ (LPSF–SDC) composite cathode was investigated for proton-conducting solid oxide fuel cells based on protonic BaZr0.1Ce0.7Y0.2O3−δ (BZCY) electrolyte. The powders of La0.8−xPrxSr0.2FeO3−δ (x = 0, 0.2, 0.4, 0.6), Sm0.2Ce0.8O2−δ (SDC) and BaZr0.1Ce0.7Y0.2O3−δ (BZCY) were synthesized by a citric acid-nitrates self-propagating combustion method. The XRD results indicate that La0.8−xPrxSr0.2FeO3−δ samples calcined at 950 °C exhibit perovskite structure and there are no interactions between LPSF0.2 and SDC at 1100 °C. The average thermal expansion coefficient (TEC) of LPSF0.2–SDC, BZCY and NiO-BZCY is 12.50 × 10−6 K−1, 13.51 × 10−6 K−1 and 13.47 × 10−6 K−1, respectively, which can provide good thermal compatibility between electrodes and electrolyte. An anode-supported single cell of NiO-BZCY|BZCY|LPSF0.2–SDC was successfully fabricated and operated from 700 °C to 550 °C with humidified hydrogen (∼3% H2O) as fuel and the static air as oxidant. A high maximum power density of 488 mW cm−2, an open-circuit potential of 0.95 V, and a low electrode polarization resistance of 0.071 Ω cm2 were achieved at 700 °C. Preliminary results demonstrate that LPSF0.2–SDC composite is a promising cathode material for proton-conducting solid oxide fuel cells.

A cobalt-free Ba0.5Sr0.5Fe0.9Ni0.1O3−δ–Sm0.2Ce0.8O1.9 (BSFN–SDC) composite was employed as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs) using BaZr0.1Ce0.7Y0.2O3−δ (BZCY) as the electrolyte. The chemical compatibility between BSFN and SDC was evaluated. The XRD results showed that BSFN was chemically compatible with SDC after co-fired at 1100 °C for 5 h. The thermal expansion coefficient (TEC) of BSFN–SDC, which showed a reasonably reduced value (16.08 × 10−6 K−1), was effectively decreased due to Ce0.8Sm0.2O1.9 (SDC) added. A single cell of Ni–BZCY/Ni–BZCY/BZCY/BSFN–SDC with a 25-μm-thick BZCY electrolyte membrane exhibited excellent power densities as high as 361.8 mW cm−2 at 700 °C with a low polarization resistance of 0.174 Ω cm2. The excellent performance implied that the cobalt-free BSFN–SDC composite was a promising alternative cathode for H-SOFCs.

A cobalt-free Sm0.5Sr0.5FeO3−δ–BaZr0.1Ce0.7Y0.2O3−δ (SSF–BZCY) was developed as a composite cathode material for proton-conducting solid oxide fuel cells (H-SOFC) based on proton-conducting electrolyte of stable BZCY. The button cells of Ni-BZCY/BZCY/SSF–BZCY were fabricated and tested from 550 to 700 °C with humidified H2 (～3% H2O) as a fuel and ambient oxygen as oxidant. An open-circuit potential of 1.024 V, maximum power density of 341 mW cm−2, and a low electrode polarization resistance of 0.1 Ω cm2 were achieved at 700 °C. The experimental results indicated that the SSF–BZCY composite cathode is a good candidate for cathode material.Highlights► The button cells were fabricated with the thickness of BZCY membrane only 20 μm. ► The maximum peak power density was 341 mWcm−2 and the interfacial polarization resistance Rp was 0.1 Ωcm2 at 700 °C. ► The activation energies of composite cathode for Rp, RH and RL are calculated as 0.66, 0.60 and 0.67 eV, respectively.

A high-performance solid oxide fuel cell La1−xSrxMnO3 (LSM) cathode/metallic interconnect contact material Ni1−xCoxO, added with the mixed ionic-electronic conducting Sm0.2Ce0.8O2−δ (SDC), was proposed as a novel composite cathode for proton-conducting solid oxide fuel cells (H-SOFCs) with BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) as the electrolyte. The X-ray diffraction (XRD) results indicated that the maximum doped ratio of Ni1−xCoxO was Ni0.7Co0.3O (NC3O), also shown that NC3O was chemically compatible with SDC at temperatures up to 1400 °C. The TEC of NC3O was also measured to check its thermal compatibility with other components. Laboratory-sized tri-layer cells of NiO–BZCYYb/BZCYYb/NC3O-SDC were fabricated and tested with humidified hydrogen (∼3% H2O) as fuel and static air as oxidant, respectively. A maximum power density of 204 mW cm−2 and a low interfacial polarization resistance Rp of 0.683 Ω cm2 were achieved at 700 °C. The results have indicated that the NC3O-SDC composite is a simple, stable and cost-effective cathode material for H-SOFCs.

•A new A-site excessive strategy is reported to improve performance of layered perovskite cathode for IT-SOFCs.•A new A-site excessive Gd1.05Ba1.05Co2O5+δ layered perovskite was investigated as cathode materials for IT-SOFCs.•A-site excess remarkably improves the electrical conductivity owing to the higher concentration of electron hole.•A-site excess dramatically enhances the electrochemical performance at intermediate temperatures.•Maximum power density of 830 mW cm−2 for YSZ-based IT-SOFC is obtained at 800 °C.A new A-site excessive strategy is reported to improve performance of layered perovskite cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). A new A-site excessive Gd1+xBa1+xCo2O5+δ layered perovskite is investigated as cathode materials for IT-SOFCs. With an excess of A-site content, the crystal lattice and the thermal expansion coefficient increase slightly. The oxygen nonstoichiometry (δ) increases, while the oxygen vacancy decreases by defect chemistry analysis. A-site excessive Gd1.05Ba1.05Co2O5+δ (GBC1.05) is well compatible with Gd0.2Ce0.8O2-δ at 1000 °C for 10 h. The electrical conductivity of GBC1.05 increases dramatically owing to the higher concentration of electron hole and the lower oxygen vacancy concentration. The electrochemical performance towards oxygen reduction reaction can be enhanced from 0.246 Ω cm2 to 0.146 Ω cm2 by introducing A-site excessive content of Gd and Ba. Consequently, GBC1.05 cathode exhibits a good cell performance of 830 mW cm−2 at 800 °C, indicating that A-site excessive Gd1.05Ba1.05Co2O5+δ layered perovskite is a potential cathode material for IT-SOFCs due to its high electrochemical performance.