Electrochemical characteristics of LSCF-SDC composite cathode for intermediate temperature SOFC

A kind of composite cathode, La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ-Ce_0.8Sm_0.2O_2−δ (LSCF-SDC), was presented in this paper. The electrochemical performance of the cathode on the electrolyte of SDC and YSZ coated with a thin SDC (YSZ/SDC) layer was studied by electrochemical impedance spectroscopy (EIS) and cathodic polarization techniques for their potential utilization in the intermediate temperature solid oxide fuel cell (IT-SOFC). Also studied was the relationship between the electro-catalytic characteristics and the electrode microstructure. Results showed that the LSCF-SDC composite electrode performed better on the SDC electrolyte than on the electrolyte of YSZ/SDC. The polarization resistance, R_p, of the cathode on the SDC electrolyte was 0.23 Ω cm² at 700 °C and 0.067 Ω cm² at 750 °C, much lower than the corresponding R_p of the same cathode on the YSZ/SDC electrolyte. At 750 °C, the cathodic overpotential of the composite cathode on the SDC electrolyte was 99.7 mV at the current density of 1.0 A cm⁻².     

Composite Cathodes for Proton Conducting Electrolytes

Different composite materials made of mixed protonic/electronic conductors, SrCe_0.9Yb_0.1O_3–δ (10YbSC) or BaCe_0.9Yb_0.1O_3–δ (10YbBC), and a mixed oxygen‐ion/electronic conductor, La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF), were investigated for cathode application in intermediate temperature solid oxide fuel cells (IT‐SOFCs) using a high temperature proton conducting BaCe_0.8Y_0.2O_3–δ electrolyte. Only the LSCF/10YbBC composite was found to be chemically stable. Area specific resistance (ASR) measurements were performed in wet air for LSCF/10YbBC cathodes, changing the weight ratio between the phases and the sintering procedure. The best performance was obtained for the composite cathode containing 50 wt.‐% of LSCF and 50 wt.‐% of 10YbBC, sintered at 1,100 °C. Electrochemical impedance spectroscopy (EIS) measurements of the tested cathodes showed two depressed semicircles in the middle and low frequency range, respectively. Performing ASR measurements at different p allowed us to attribute the two semicircles to charge transfer and oxygen diffusion processes, respectively. The microstructure of the LSCF/10YbBC(1:1) composite cathode was optimised changing the ratio of the particle sizes between the two phases. The lowest ASR values (0.14 Ω cm² at 700 °C) were observed for the LSCF/10YbBC(1:1) composite cathode with different particle size (sub‐micrometer particles for LSCF and nanometer particles for 10YbBC). Fuel cell polarisation curves demonstrated superior performance of the LSCF/10YbBC (1:1) cathode with respect to Pt.     

Preparation of thin electrolyte film via dry pressing/heating /quenching/calcining for electrolyte-supported SOFCs

The development of technologies used to prepare thin electrolyte films will stimulate the application of electrolyte-supported SOFCs since thin electrolyte films typically have low ohmic resistances and good electrochemical performance. This paper presents a novel method for the preparation of thin electrolyte films for yttria-stabilized zirconia (YSZ)-supported solid oxide fuel cells (SOFCs) via dry pressing/heating/quenching/calcining. The thicknesses of the as-prepared YSZ films were as low as 78?μm, which is significantly thinner than those prepared using a traditional method (greater than 200?μm) via dry pressing/calcining/polishing. More importantly, the preparation process was quicker. Using this novel method, a YSZ-supported cell with a configuration of (La_0.6Sr_0.4)_0.9Co_0.8Fe_0.2O_3-δ (LSCF)–Ce_0.8Sm_0.2O_2-δ(SDC)/SDC/YSZ/SDC/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ(BSCF)–SDC was fabricated and tested. The results showed promising electrochemical performance and a peak power density of 0.64?W?cm^?2 at 850?°C was obtained, which was much higher than the cell fabricated using the traditional method (0.29?W?cm^?2). The ohmic resistance (R_O) at 850?°C is 0.19?Ω?cm², which is much lower than that of the cell fabricated using the traditional method (0.33?Ω?cm²) at an identical temperature. The modified method described in this work is shown to be a promising technique to prepare thin electrolyte films for high-performance, electrolyte-supported SOFCs.     

Polarization properties of La0.6Sr0.4Co0.2Fe0.8O3-based double layer-type oxygen electrodes for reversible SOFCs

We have developed double layer-type (catalyst layer/current collecting layer) oxygen electrodes (DLE) for reversible SOFCs. As the catalyst layer (cathode for SOFC and anode for steam electrolysis) interfaced with a samaria-doped ceria [(CeO₂)_0.8(SmO_1.5)_0.2, SDC] interlayer/YSZ solid electrolyte, mixed conducting La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and SDC particles were employed. The current collecting porous LSCF layer was formed on the catalyst layer. By controlling the SDC content, as well as the thickness and porosity of the catalyst layer, the gas diffusion rate and the conduction networks for electrons and oxide ions were optimized, resulting in a marked reduction of the overpotential. The LSCF + SDC/LSCF DLE exhibited higher performance than single-layer electrodes of LSCF + SDC or LSCF; the IR-free anode potential vs. an air reference electrode was 0.12 V (corresponding to an overpotential of 0.08 V) at 0.5 A cm⁻² and 900 °C under an atmosphere of O₂ (1 atm).     

Electrochemical properties of LaBaCo2O5+δ-Sm0.2Ce0.8O1.9 composite cathodes for intermediate-temperature solid oxide fuel cells

The electrochemical properties of LaBaCo₂O_5+δ-xSm_0.2Ce_0.8O_1.9 (LBCO-xSDC, x = 20, 30, 40, 50, 60, wt%) were investigated for the potential application in intermediate-temperature solid oxide fuel cells (IT-SOFCs). The LBCO-50SDC composite cathode exhibited the best electrochemical performance in the LBCO-xSDC cathodes. With x = 50 wt%, the ASR was 1.308 Ω cm² at 500 °C (0.267 Ω cm² at 600 °C and 0.052 Ω cm² at 700 °C). The maximum of exchange current density i₀ was 0.5630 A cm⁻² at 700 °C. The improved electrochemical properties of LBCO-50SDC were ascribed to the porous structures of the cathode and more cathode/electrolyte/gas triple phase boundary (TPB) areas.     

Structure and thermal properties of La0.6Sr0.4Co0.2Fe0.8O3−δ-SDC carbonate composite cathodes for intermediate- to low-temperature solid oxide fuel cells

The structure and thermal properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ-SDC carbonate (LSCF-SDC carbonate) composite cathodes were investigated with respect to the calcination temperatures and the weight content of the samarium-doped ceria (SDC) carbonate electrolyte. The composite cathode powder has been prepared from La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ and SDC carbonate powders using the high-energy ball milling technique in air at room temperature. Different powder mixtures at 30 wt%, 40 wt% and 50 wt% of SDC carbonate were calcined at 750-900 °C. The findings indicated that the structure and thermal properties of the composite cathodes were responsive to the calcination temperature and the content of SDC carbonate. The absence of any new phases as confirmed via XRD analysis demonstrated the excellent compatibility between the cathode and electrolyte materials. The particle size of the composite cathode powder was ∼0.3-0.9 μm having a surface area of 4-15 m² g⁻¹. SEM investigation revealed the presence of large particles in the resultant powders resulting from the increased calcination temperature. The composite cathode containing 50 wt% SDC carbonate was found to exhibit the best thermal expansion compatibility with the electrolyte.     

Effect of optimal GDC loading on long-term stability of La0.8Sr0.2Co0.2Fe0.8O3-δ/Gd0.2Ce0.8O1.9 composite cathodes

Three types of La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ/Gd_0.2Ce_0.8O_1.9 (LSCF/GDC) composite cathodes with different optimal GDC loading are fabricated through electrospinning, screen printing and solution infiltration method. Constant current polarization with current density of 100 mA cm^?2 at 750°C is applied to test the stability of LSCF/GDC composite cathodes. After constant current polarization for 144 h, the polarization resistance (R_p) of 280 nm-nanofiber skeletal LSCF/GDC composite cathode after pore-forming exhibits the minimum increase, from 0.062 Ω cm² to 0.098 Ω cm². Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) results show that the microstructure and surface chemical composition of the cathode maintain stable during the constant current polarization. Combined with the X-ray diffraction (XRD) result, a relationship among GDC loading, stress, Sr surface segregation and long-term stability is established.     

A bi-layer cathode based on lanthanum based cobalt- and iron-containing perovskite and gadolinium doped ceria for thin yttria stabilized zirconia electrolyte solid oxide fuel cells

Composite cathodes based on La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) are investigated for lower operating temperature (<750 °C) applications of a solid oxide fuel cell (SOFC). To enhance a charge transfer, a bi-layer SOFC cathode is proposed, which has a LSCF–Ce_0.9Gd_0.1O_1.95 (GDC) composite layer and a pure LSCF layer. The bi-layer cathode SOFC shows a current density of 0.65 A cm⁻² at 0.8 V and 660 °C, which is higher than a LSCF–GDC composite single-layer cathode SOFC cell of 0.35 A cm⁻². The charge transfer polarizations in the bi-layer cathode SOFC are 0.14 Ω cm² and 0.35 Ω cm² at 760 °C and 660 °C, respectively, which are lower than those in the single-layer cathode cell of 0.23 Ω cm² and 0.66 Ω cm². The impedances characterized with a fitting model show that the lowered charge transfer polarization in the bi-layer cathode is a dominant factor in reducing the total polarization of SOFC.     

Enhancement of the interfacial polarization resistance of La0.6Sr0.4Co0.2Fe0.8O3-δ cathode by microwave-assisted combustion method

Thermogravimetry, phase formation, microstructural evolution, specific surface area, and electrical properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathode were studied as functions of its preparation technique. The pure perovskite LSCF cathode powder was synthesized through glycine–nitrate process (GNP) using microwave heating technique. Compared with conventional heating technique, microwave heating allows the rapid combustion to occur simultaneously between the nitrates and glycine in a controllable manner. The resulting powder is a single-phase nanocrystallite with a mean particle size of 113 nm and a high specific surface area of 12.2 m²/g, after calcination at 800 °C. Impedance analysis indicates that microwave heating has significantly reduced the polarization resistance of LSCF cathode. The area specific resistance (ASR) value of 0.059 and 0.097 Ω cm² at 800 °C and 750 °C, respectively, were observed. These values were twofold lower than the corresponding ASR of the cathode (0.133 and 0.259 Ω cm² at 800 °C and 750 °C, respectively) prepared through conventional heating. Results suggest that the microwave heating GNP strongly contributes to the enhancement of the LSCF cathode performance for intermediate temperature solid oxide fuel cells.     

Characterization of La0.6Sr0.4Co0.2Fe0.8O3–δ + La2NiO4+δ Composite Cathode Materials for Solid Oxide Fuel Cells

The structure, electrical conduction, thermal expansion and electrochemical properties of the La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ + La₂NiO_4+δ (LSCF‐LNO) composite cathodes were investigated with regard to the volume fraction of the LNO composition. No chemical reaction product between the two constituent phases was found for the composite cathodes sintered at 1,400 °C for 10 h within the sensitivity of the XRD. Compared to the performance of the LSCF cathode, the LNO composition in the composite cathode plays a role in deteriorating both electrical conductivity and electrochemical properties, however, improving the thermal expansion properties. The trade‐off between electrical conducting and thermal expansion classifies the composite cathode containing 30 volume percent (vol.%) LNO as the optimum composition. For characterizing cathode performance in a single cell, a slurry spin coating technique was employed to prepare a porous cathode layer as well as a YSZ/Ce_0.8Sm_0.2O_3–δ (SDC) electrolyte. The optimum conditions for fabricating the YSZ/SDC electrolyte were investigated. The resulting single cell with 70 vol.% LSCF‐30 vol.%LNO (LSCF‐LNO30) cathode shows a power density of 497 mW cm^–2 at 800 °C, which is lower than that of the cell with a LSCF cathode, but still within the limits acceptable for practical applications.     

Image analysis of the porous yttria-stabilized zirconia (YSZ) structure for a lanthanum ferrite-impregnated solid oxide fuel cell (SOFC) electrode

Image analysis and quantification were performed on porous scaffolds for building SOFC cathodes using the two types of YSZ powders. The two powders (U1 and U2) showed different particle size distribution and sinterability at 1300?°C. AC impedance on symmetrical cells was used to evaluate the performance of the electrode impregnated with 35-wt.% La_0.8Sr_0.2FeO₃. For example, at 700?°C, the electrode from U2 powder shows a polarization resistance (R_p) of 0.21?Ω?cm², and series resistance (R_s) of 8.5?Ω?cm² for an YSZ electrolyte of 2-mm thickness, lower than the electrode from U1 powder (0.25?Ω?cm² for R_p and 10?Ω?cm² for R_s) does. The quantitative study on image of the sintered scaffold indicates that U2 powder is better at producing architecture of high porosity or long triple phase boundary (TPB), which is attributed as the reason for the higher performance of the LSF-impregnated electrode.     

Enhancing the performance and stability of solid oxide fuel cells by adopting samarium-doped ceria buffer layer

In this work, the Sm_0.2Ce_0.8O_1.9 (SDC) buffer layer was used to replace the Gd_0.1Ce_0.9O_1.95 (GDC) buffer layer to improve the long-term stability and performance of the solid oxide fuel cells (SOFCs) in the intermediate temperature (550–750 °C). The buffer layer was prepared by screen printing method. The micromorphology of the SDC buffer layer and the cell structures was observed by scanning electron microscopy (SEM). The electrochemical impedance spectroscopy (EIS) results showed that the polarization resistance (R_P) of the cell with SDC buffer layer was smaller than that of the cell with GDC buffer layer, reducing the R_P values by 43.52% and 43.33%, respectively (SDC-cell: 0.12 Ω cm² at 650 °C and 0.27 Ω cm² at 600 °C). The maximum power density of the cell with SDC buffer layer is 560 mW cm⁻² at 650 °C, which was 25% higher than that with GDC buffer layer. The long-term durability of the cell with SDC buffer layer was better than that of the cell with GDC buffer layer. These provide an excellent prospect for utilizing SDC buffer layer.     

Cathodic polarisation of composite LSCF-SDC IT-SOFC electrode synthesised by one-step microwave self-assisted combustion

Nano-sized composite La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF)-Ce_0.8Sm_0.2O_1.9 (SDC) powder was synthesised by one-step microwave self-assisted combustion synthesis and characterized by X-ray diffraction and transmission electron microscopy. Electrochemical performance of composite LSCF-SDC cathode in contact with Ce_0.8Gd_0.2O_1.9 (GDC) solid electrolyte under steady state polarisation was studied employing electrochemical impedance spectroscopy in 3-electrode cell configuration. The evolution of elementary steps of oxygen reduction reaction was analysed under cathodic polarisation in potentiostatic mode (up to −0.8 V) in the temperature range 650–750 °C. The composite LSCF-SDC cathode prepared by one-step microwave combustion route demonstrated a lower overall polarisation resistance when compared to literature reports of similar cathodes produced by other synthesis routes.     

Evaluation of Fe and Mn co-doped layered perovskite PrBaCo2/3Fe2/3Mn1/2O5+δ as a novel cathode for intermediate-temperature solid-oxide fuel cell

A B-site cation-deficient double-perovskite oxide, PrBaCo_2/3Fe_2/3Mn_1/2O_5+δ (PBCFM2), was successfully synthesized by a sol-gel method and systematically investigated as an efficient cathode for IT-SOFC. The PBCFM2 exhibits good thermally stability and broad chemical compatibility at high temperature. Appropriate substitution of Mn and Fe for Co dramatically decreases the thermal expansion coefficient (TEC) from 21.5 × 10^–6 K^–1 for PrBaCo₂O_5+δ to 17.8 × 10^–6 K^–1 to PBCFM2 at a temperature range of 30–1000 °C. The temperature dependence of conductivity of the PBCFM2 was tested from 300 °C to 850 °C and then confirmed using the p-type small-polaron transport mode. The maximum conductivity value was 72 S cm^–1 at 600 °C. When using 300 µm of Sm_0.2Ce_0.8O_1.9 (SDC) as an electrolyte, the area specific resistance (ASR) and peak power density values were 0.028 Ω cm² and 588 mV cm^–2 at 800 °C, respectively. The activity and performance of the PBCFM2 cathode are further improved by impregnation with nano-sized SDC particles. The composite cathode with two times impregnation provided the optimal nano-scale SDC loading and microstructure where the ASR and peak power densities were 0.023 Ω cm² and 621 mV cm^–2 at 800 °C, respectively. Our preliminary results lead us to propose that PBCFM and its composite cathodes are good candidate cathodes for IT-SOFC.     

Composite cathodes of La0.9Ca0.1Ni0.5Co0.5O3-Ce0.8Sm0.2O1.9 for solid oxide fuel cells

The mixed ionic and electronic conductors of La_0.9Ca_0.1Ni_0.5Co_0.5O₃-Ce_0.8Sm_0.2O_1.9 (LCNC-SDC) are investigated systematically for potential application as a cathode for solid oxide fuel cells based on a Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte. The electrochemical impedance spectroscopy (EIS) measurements are performed in air over the temperature range of 600-850 °C to determine the cathode polarization resistance. The exchange current densities for oxygen reduction reaction (ORR), determined from the low-field cyclic voltammetry, high-field cyclic voltammetry, and EIS data are systematically investigated. The activation energies (E_a) for ORR determined from the slope of Arrhenius plots are in the range of 102.33-150.73 kJ mol⁻¹ for LCNC-SDC composite cathodes. The experimental results found that LCNC-SDC (70:30) composite cathode has a maximum exchange current density and a minimum polarization resistance of 0.30 Ω cm² for 850 °C among LCNC-SDC composite cathodes.     

Determination of the rate-determining step of the oxygen reduction reaction of La0.8Sr0.2MnO3(LSM)- 8mol% yttria-stabilized zirconia(YSZ): Composition and microstructure

In this study, LSM-YSZ composite cathodes were analyzed by changing the firing temperature, composition, and operating temperature to determine the contribution of each step of the oxygen reduction reaction (ORR). The overall resistance of the cathode reaction was characterized by fitting the AC impedance spectra with an equivalent circuit model. It was found that initial reactions of ORR (dissociative adsorption) are the main rate-determining step (RDS) regardless of operating or sintering temperature, while reactions on LSM surface become the main RDS when the ratio of LSM catalysts has a relatively small proportion. The [LSM-YSZ]_5:5 cathode fired at 1100 °C showed the best microstructure and lowest resistance in the ORR at an operating temperature of 700 °C (R_HF: 0.18 Ωcm², R_MF: 0.20 Ω cm², R_LF: 0.25 Ωcm², R_cathode: 1.14 Ωcm²). This demonstrates the potential use of LSM-YSZ cathodes for IT/LT-SOFC without the use of expensive materials, such as LSCF and BSCF.     

Enhanced performance and durability of lanthanum strontium cobalt ferrite by in-situ solvothermal modification

Due to the limitations of single material, the oxygen reduction kinetics may be slow, which is considered as a key challenge for developing the high-performance cathodes. In this work, an effective strategy is proposed, employing the in-situ solvothermal method, in which the LSCF, one of the most promising cathode materials, is decorated with Co₃O₄. After being treated at 180 °C in a neutral solution, the R_p of N-?Co₃O₄ @LSCF electrode is decreased by 37%, exhibiting reasonable stability for 120 h. Utilizing the DRT technique, finding the decorated Co₃O₄ mainly promotes the charge transfer and oxygen dissociation processes. The single cell with the N-?Co₃O₄ @LSCF cathode achieves a maximum power density of 803 mW cm^-?² at 750 °C, which is 25% higher than that for the untreated LSCF electrode. Our work demonstrates that such Co₃O₄ decorated composite obtained by the in-situ solvothermal treatment is a promising cathode material for solid oxide fuel cells.     

Ce0.9Gd0.1O1.95 supported La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes for solid oxide fuel cells

Lanthanum-based iron- and cobalt-containing perovskite has a high potential as a cathode material because of its high electro-catalytic activity at a relatively low operating temperature in solid oxide fuel cells (SOFCs) (600–800). To enhance the electro-catalytic reduction of oxidants on La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF), Ga doped ceria (Ce_0.9Gd_0.1O_1.95, GDC) supported LSCF (15LSCF/GDC) is successfully fabricated using an impregnation method with a ratio of 15 wt% LSCF and 85 wt% GDC. The cathodic polarization resistances of 15LSCF/GDC are 0.015 Ω cm², 0.03 Ω cm², 0.11 Ω cm², and 0.37 Ω cm² at 800 °C, 750 °C, 700 °C, and 650 °C, respectively. The simply mixed composite cathode with LSCF and GDC of the same compositions shows 0.05 Ω cm², 0.2 Ω cm², 0.56 Ω cm², and 1.20 Ω cm² at 800 °C, 750 °C, 700 °C, and 650 °C, respectively. The fuel cell performance of the SOFC with 15LSCF/GDC shows maximum power densities of 1.45 W cm^?2, 1.2 W cm^?2, and 0.8 W cm^?2 at 780 °C, 730 °C, and 680 °C, respectively. GDC supported LSCF (15LSCF/GDC) shows a higher fuel cell performance with small compositions of LSCF due to the extension of triple phase boundaries and effective building of an electronic path.     

La0.6Sr0.4Co0.2Fe0.8O3-δ oxygen electrodes for solid oxide cells prepared by polymer precursor and nitrates solution infiltration into gadolinium doped ceria backbone

Infiltration is a method, which can be applied for the electrode preparation. In this paper oxygen electrode is prepared solely by the infiltration of La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ (LSCF) into Ce_0.8Gd_0.2O_2-δ (CGO) backbone. The use a polymer precursor as an infiltrating medium, instead of an aqueous nitrate salts solution is presented. It is shown that the polymer forms the single-phase perovskite at 600 °C, contrary to the nitrates solution. As a result, obtained area specific resistance (ASR) is lowered from 0.21 Ω cm² to 0.16 Ω cm² at 600 °C. More than 35% of LSCF in the oxygen electrode decreases the performance.     

Advances in novel SDC@Al2O3 core−shell electrolyte for low-temperature solid oxide fuel cell

The preparation of electrolyte with excellent ionic conduction is an important development direction in the practical application of solid oxide fuel cell (SOFC). Traditional methods to improve ion conduction was structure doping to develop electrolyte materials. In this work, the ionic conductor Ce_0.8Sm_0.2O_2-δ (SDC) was modified by insulator Al₂O₃ to enhance ion conduction and apply as electrolytes for the SOFC. The transmission electron microscopy (TEM) characterization clearly clarified that a thin Al₂O₃ layer in the amorphous state coated on SDC to form the SDC@Al₂O₃ core−shell structure. The SDC@Al₂O₃ electrolyte with the core−shell structure possesses a super ionic conductivity of 0.096 S cm⁻¹ and results in advanced cell performance of 1190 mW cm⁻² at 550°C. The X-ray photoelectron spectroscopy (XPS) analysis revealed that the concentration of oxygen vacancies in the SDC@Al₂O₃ core–shell structure significantly improved in comparison with pure SDC, the newly produced oxygen vacancies can promote the oxygen ion transport. Moreover, the interface between SDC and Al₂O₃ provides a fast channel for the proton transport. In addition, the SDC-based SOFC was usually suffered from the reduction of the SDC electrolyte and the accompanying generated electron conduction should deteriorate the cell performance, this is the main challenge for the SDC electrolyte application. In our case, the Al₂O₃ shell on the SDC surface not only can avoid the contact between SDC and hydrogen to eliminate the reduction of SDC but also can restrain electron conduction due to the electron insulation characteristic of the Al₂O₃ shell. This work demonstrates an efficient approach to develop the advanced low-temperature SOFC technology from material fundamentals.