•NiTiO3-Sm0.2Ce0.8O1.9 (NTO-SDC) was tested as an active anode functional layer.•Carbon-tolerant NTO anode can be totally reduced to Ni and TiO2−δ.•NTO-SDC functional layer greatly enhances the electrochemical performance.•No obvious degradation with discharging at 0.5 V was observed.A highly active anode functional layer NiTiO3-Sm0.2Ce0.8O1.9 (NTO-SDC) is proposed to improve electrochemical performance of intermediate temperature solid oxide fuel cells (IT-SOFCs) with carbon-tolerant Ni-TiO2−δ anode. When exposing to the H2 atmosphere, NTO in both anode support and anode functional layer can be totally reduced to Ni and TiO2−δ. Meanwhile, a small amount of Sm2Ti2O7 pyrochlore structure phase owning acceptable oxygen-ion conduction can be observed after high temperature sintering at 1250 °C. Maximum power densities of anode-supported NTO/NTO-SDC/SDC/La0.8Sr0.2Co0.2Fe0.8O3−δ (LSCF)-SDC single cells, are 338 mWcm−2 and 223 mWcm−2 with wet H2 and methane fuels at 700 °C, respectively. Short-time stability test for anode-supported cells with NTO-SDC anode functional layer shows no obvious degradation with discharging at 0.5 V in wet methane fuel. Preliminary results have demonstrated that fabricating an anode functional layer of NTO-SDC is a facile and efficient strategy to improve electrochemical performance of IT-SOFCs with carbon-tolerant anode.

•The electron-blocking effects of bi-layer electrolyte configurations are compared.•The oxygen partial pressure distribution under the open circuit displays the “S” type in barrier layers.•The activation greatly effects open circuit voltage and oxygen partial pressure distribution.•Electrochemical property can be optimized by balancing the open circuit voltage and ohm loss.A numerical model for ceria-based solid oxide fuel cells (SOFCs) with bi-layer electrolyte is proposed to evaluate the internal short circuit by the comparison of two cell configurations: the electronic barrier electrolyte adjacent to cathode and anode, respectively. In this model, the activation polarization of the electrode reaction and the charge transport of the electrolyte with both n/p-type electronic and oxygen ion conductivity are considered. The activation polarization and the charge transport are described by the Butler-Volmer equation and the Nernst-Planck equation, respectively. Parametric simulations are performed to compare the two bi-layer electrolyte configurations in terms of the open circuit voltage, I–V relationship, leakage current density, power density at 0.7 V, oxygen partial pressure distribution and electrochemical efficiency as functions of the temperature and thickness ratio of the electronic barrier electrolyte. From our modeling results, the cell configuration of which the barrier electrolyte is adjacent to cathode has significant p-type leakage current, leading to the lower open circuit voltages and electrochemical efficiency than the other one. The oxygen partial pressure distribution under the open circuit displays the “S” type in the barrier layer, which is related to the change of the n/p-type conductivity of the barrier layer. Besides, the activation polarization greatly influences the open circuit voltage and the oxygen partial pressure distribution between boundaries of electrolytes under open circuit. It is also found that the thickness ratio of the electronic barrier electrolyte can be optimized to maximize the electrochemical performance by balancing the open circuit voltage and ohmic polarization loss.

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.

High-temperature proton conductors gained ever-increasing interest as electrolyte materials alternative to oxygen-ion conductors due to their high conductivity associated with low activation energy at reduced temperatures. In this study, we reported our findings on chemically stable, easily sintered and highly proton-conductive BaZrO3 oxides with Bi2O3 and In2O3 co-addition as electrolyte materials for proton conducting solid oxide fuel cells (H-SOFCs). Among the composition series, BaZr0.75In0.2Bi0.05O3−δ (BZIB5) exhibited the improved sinterability and good conductivity. Correspondingly, high-temperature gravimetry results indicated that the concentrations of protonic defect and oxygen vacancy strongly depended on P(H2O) and temperature rather than P(O2). Importantly, single cells with 12-μm-dense BZIB5 electrolyte films were successfully fabricated, and achieved high performance with the maximum power density of 0.34 Wcm−2 at 700 °C. The encouraging results demonstrated that BZIB5 is a promising candidate as the electrolyte material for high performance H-SOFCs.

•Antimony is doped to barium strontium ferrite to produce novel cathodes.•δ, TECs and σ are evaluated as a function of antimony content.•The electrochemical performance is substantially improved with antimony doping.Antimony was doped to barium strontium ferrite to produce ferrite-based perovskites with a composition of Ba0.5Sr0.5Fe1−xSbxO3−δ (x = 0.0, 0.05, 0.1) as novel cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The perovskite properties including oxygen nonstoichiometry (δ), mean valence of B-site, tolerance factors, thermal expansion coefficient (TEC) and electrical conductivity (σ) are explored as a function of antimony content. By defect chemistry analysis, the TECs decrease since the variable oxygen vacancy concentration is decreased by Sb doping, and σ decreases with x due to the reduced charge concentration of Fe4+ content. Consequently, the electrochemical performance was substantially improved and the interfacial polarization resistance was reduced from 0.213 to 0.120 Ωcm2 at 700 °C with Sb doping. The perovskite with x = 1.0 is suggested as the most promising composition as SOFC cathode material.

Reversible solid oxide cells (RSOCs) can generate electricity as solid oxide fuel cells (SOFC) facing a shortage of electricity and can also store the electricity as solid oxide electrolysis cells (SOEC) at the time of excessive electricity. The composite Sr0.95Y0.05TiO3−δ–Sm0.2Ce0.8O1.9 (SYT–SDC) as the hydrogen electrode provides a promising alternative for a conventional Ni/YSZ. The possible charge compensation mechanism of SYT is described as Sr0.95Y0.05Ti0.95−2δ4+Ti2δ+0.053+O3−δ. The Ti3+ is approximately 11.73% in the reduced SYT by XRD Rietveld refinement, electron paramagnetic resonance (EPR) and thermogravimetry (TG) analysis. Voltage–current curves and impedance spectra are measured as a function of applied voltages to characterize the cells. The bulk resistance (Ro) and the electrode polarization resistance (Rp) at open circuit voltages (OCV) at 750 °C are 9.06 Ω cm2 and 10.57 Ω cm2, respectively. The Ro values have a small amount of changes with small slopes both in the SOFC (−0.29 Ω cm2 V−1) and SOEC mode (0.5 Ω cm2 V−1), whereas the Rp values decrease all the time with the increasing voltages at both the SOFC (−2.59 Ω cm2 V−1) and SOEC mode (−9.65 Ω cm2 V−1), indicating that the electrical conductivity and electro-catalytic property of the SYT-based hydrogen electrode can be improved under the SOEC mode.

Y0.8Ca0.2BaCo4O7+δ (YCBC4) was synthesized and investigated as a cathode for proton-conducting intermediate temperature solid oxide fuel cells (IT-SOFCs). Low thermal expansion coefficient (TEC) (~9.9×10−6 K−1) can provide good thermal expansion compatibility with the standard SOFC electrolyte materials. Laboratory-sized quad-layer cells, consisting of NiO–BaZr0.1Ce0.7Y0.2O3−δ (BZCY)/NiO–BZCY/BZCY/YCBC4, were fabricated by a one-step dry-pressing/co-firing process and tested from 550 to 700 °C with humidified hydrogen (~3% H2O) as the fuel and static air as the oxidant, respectively. An excellent power density of 472 mW cm−2 and a low electrode polarization resistance of 0.121 Ω cm2 were obtained at 700 °C. Preliminary results demonstrated that the YCBC4 can be a promising cathode material for proton-conducting IT-SOFCs.

•BaZr0.7Pr0.1Y0.2O3-δ (BZPY) powders were synthesized by an EDTA-citrate complexation process.•BZPY exhibits excellent chemical stability in atmospheres containing CO2 and water vapor.•LSCF-BZPY and LSCF-SDC composite cathodes are fabricated and evaluated for IT-SOFCs.•Proton electron mixed conducting composite cathodes are promising and beneficial.High-temperature proton conductor BaZr0.7Pr0.1Y0.2O3-δ (BZPY) was investigated as electrolyte for intermediate temperatures (400-800 °C) solid oxide fuel cells (IT-SOFCs), which exhibited excellent chemical stability in atmospheres containing CO2 and water vapor. In addition, both La0.4Sr0.6Co0.2Fe0.8O3-δ-BaZr0.7Pr0.1Y0.2O3-δ (LSCF-BZPY) and La0.4Sr0.6Co0.2Fe0.8O3-δ-Ce0.8Sm0.2O1.9 (LSCF-SDC) composite oxides were fabricated and evaluated as working cathodes for anode-supported IT-SOFCs based upon thin BZPY electrolytes. The single cells with LSCF-BZPY cathode showed the maximum power density of 86.7 mW cm−2 at 700 °C and the calculated activation energy was 91.19 kJ mol-1. While the cells with LSCF-SDC cathode exhibited the higher maximum power density of 112.4 mW cm−2 at 700 °C and the calculated activation energy was 101.61 kJ mol-1. The experimental results indicated that proton electron mixed conducting composite cathodes are more promising and beneficial than oxygen-ion electron mixed conducting composite cathodes for proton-conducting SOFCs during actual low operating temperatures.

•A single-phase cathode was investigated for IL-SOFCs.•The high-valence Mo ions doped enhance the structural stability.•The cell exhibited a high performance and a low total resistance.Cobalt-free perovskite Ba0.5Sr0.5Fe0.9Mo0.1O3−δ (BSFMo) was investigated as a single-phase cathode for intermediate-to-low-temperature solid oxide fuel cells (IL-SOFCs). The X-ray diffraction (XRD) Rietveld refinement, electrical conductivity, thermogravimetric (TG) measurements, the phase reaction were investigated. The doping of high-valence Mo cations into Fe-site obviously enhanced the electrical conductivity of BSFMo sample with the maximum value of 174 S cm−1. XRD results showed that BSFMo cathode was chemically compatible with the BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) electrolyte for temperatures up to 1000 °C. Laboratory-sized tri-layer cells of NiO-BZCYYb/BZCYYb/BSFMo were operated from 550 to 700 °C with humidified hydrogen (～3% H2O) as fuel and the static air as oxidant, respectively. An open-circuit potential of 1.001 V, the maximum power density of 428 mW cm−2, and a low electrode polarization resistance of 0.148 Ω cm2 were achieved at 700 °C. The experimental results indicated that the single-phase BSFMo is a promising candidate as cathode material for IL-SOFCs.

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 cobalt-free composite Sm0.5Sr0.5Fe0.8Cu0.2O3−δ–Ce0.8Sm0.2O2−δ (SSFCu–SDC) is investigated as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs) in intermediate temperature range, with BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) as the electrolyte. The XRD results show that SSFCu is chemically compatible with SDC at temperatures up to 1100 °C. The quad-layer single cells of NiO–BZCYYb/NiO–BZCYYb/BZCYYb/SSFCu–SDC are operated from 500 to 700 °C with humidified hydrogen (∼3% H2O) as fuel and the static air as oxidant. It shows an excellent power density of 505 mW cm−2 at 700 °C. Moreover, a low electrode polarization resistance of 0.138 Ω cm2 is achieved at 700 °C. Preliminary results demonstrate that the cobalt-free SSFCu–SDC composite is a promising cathode material for H-SOFCs.Research highlights▶ The quad-layer single cells of NiO–BZCYYb/NiO–BZCYYb/BZCYYb/SSFCu–SDC show an excellent power density of 505 mW cm−2 at 700 °C. ▶ A low electrode polarization resistance of 0.138 Ω cm2 is achieved at 700 °C, which exhibits high electrochemical activity for operation at intermediate temperature range. ▶ The XRD spectrum of the SSFCu–SDC composite cathode indicates an excellent chemical compatibility between SSFCu and SDC at temperatures up to 1100 °C. ▶ Adding Ce0.8Sm0.2O2−δ (SDC) electrolyte which is widely used for SOFCs based on SDC oxide ion conductors, into SSFCu cathode to prepare the composite cathode for H-SOFCs. Preliminary results demonstrate that the cobalt-free SSFCu–SDC composite is a promising cathode material for H-SOFCs.

A cobalt-free cubic perovskite oxide SrFe0.9Sb0.1O3−δ (SFSb) is investigated as a novel cathode for proton-conducting solid oxide fuel cells (H-SOFCs). XRD results show that SFSb cathode is chemically compatible with the electrolyte BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) for temperatures up to 1000 °C. Thin proton-conducting BZCYYb electrolyte and NiO–BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (NiO–BZCYYb) anode functional layer are prepared over porous anode substrates composed of NiO–BZCYYb by a one-step dry-pressing/co-firing process. Laboratory-sized quad-layer cells of NiO–BZCYYb/NiO–BZCYYb/BZCYYb/SFSb are operated from 550 to 700 °C with humidified hydrogen (∼3% H2O) as fuel and the static air as oxidant. An open-circuit potential of 0.996 V, maximum power density of 428 mW cm−2, and a low electrode polarization resistance of 0.154 Ω cm2 are achieved at 700 °C. The experimental results indicate that the cobalt-free SFSb is a promising candidate for cathode material for H-SOFCs.

A cobalt-free cubic perovskite oxide Sm0.5Sr0.5Fe0.8Cu0.2O3−δ (SSFCu) was investigated as a novel cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The thermal expansion coefficient (TEC) of SSFCu was close to that of Sm0.2Ce0.8O1.9(SDC) electrolyte and the electrical conductivity of SSFCu sample reached 72–82 S cm−1 in the commonly operated temperatures of IT-SOFCs (400–600 °C). Symmetrical electrochemical cell with the configuration of SSFCu/SDC/SSFCu was applied for the impedance study and area specific resistance (ASR) of SSFCu cathode material was as low as 0.085 Ω cm2 at 700 °C. Laboratory-sized tri-layer cells of NiO-SDC/SDC/SSFCu were operated from 450 to 700 °C with humidified hydrogen (∼3% H2O) as fuel and the static air as oxidant. A maximum power density of 808 mW cm2 was obtained at 700 °C for the single cell.