Cathode For Intermediate Temperature Solid Oxide Research Articles

Electrochemical performance of La2NiO4+δ cathode for intermediate-temperature solid oxide fuel cells

The application of the La2NiO4+δ (LNO), one of the Ruddlesden–Popper series materials, as a cathode material for intermediate temperature solid oxide fuel cells is investigated in detail. LNO is synthesized via a complex method using ethylenediaminetetraacetic acid (EDTA) and citric acid. The effect of the calcination temperature of the LNO powder and the sintering temperature of the LNO cathode layer on the anode-supported cell, Ni–YSZ/YSZ/GDC/LNO, is characterized in view of the charge transfer resistance and the mass transfer resistance. Charge transfer resistance was not significantly affected by calcination and sintering temperature when the sintering temperature was not lower than the calcination temperature. Mass transfer resistance was primarily governed by the sintering temperature. The unit cell with the LNO cathode sintered at 1100°C with 900°C-calcined powder presented the lowest polarization resistance for all the measured temperatures and exhibited the highest fuel cell performances, with values of 1.25, 0.815, 0.485, and 0.263Wcm−2 for temperatures of 800, 750, 700, and 650°C, respectively.

The effect of calcium doping on the improvement of performance and durability in a layered perovskite cathode for intermediate-temperature solid oxide fuel cells

The partial replacement of Sr by Ca in PrBa0.5Sr0.5Co2O5+δ layered perovskite provides a breakthrough for achieving reliable operation of high performance IT-SOFCs.

BaFe0.6Ce0.1Co0.3O3-δ as a high-performance cathode for intermediate-temperature solid oxide fuel cells

The search for cathode materials with high oxygen reduction reactions activity is a key step toward reducing the operation temperature of solid oxide fuel cells. We report a novel perovskite-type BaFe0.6Ce0.1Co0.3O3-δ (BFCC) cathode, which exhibits outstanding activity for oxygen reduction reactions and yields low interfacial resistances, e.g., 0.113Ωcm2 at 650°C. The SEM results show that BFCC is chemically, thermally compatible with Ce0.8Sm0.2O2-δ (SDC) at operation temperatures. The tri-layer single cells of Ni–SDC|SDC|BFCC are operated from 500 to 650°C with humidified hydrogen (∼ 3% H2O) as fuel and oxygen as oxidant. It shows excellent power densities of 1422mWcm−2 at 650°C and 1086mWcm−2 at 600°C. Preliminary results demonstrate that the BFCC is a promising cathode material for reduced-temperature solid oxide fuel cells.

Characterization of SrCo0.7Fe0.2Nb0.1O3−δ cathode materials for intermediate-temperature solid oxide fuel cells

A new cubic perovskite oxide, SrCo0.7Fe0.2Nb0.1O3−δ (SCFN), is investigated as a cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). XRD results show that there are no serious reactions between SCFN and Sm0.2Ce0.8O1.9 (SDC) except a slight peak shift. XPS analysis shows that the transition-metal cations in the SCFN exist in two different valence states, i.e., [Sr2+][Co3+/Co4+]0.7[Fe3+/Fe4+]0.2[Nb4+/Nb5+]0.1O3−δ. The electrical conductivity of the SCFN sample reaches a maximum 304 S cm−1 at 350 °C in air. In order to optimize thermal expansion coefficients (TECs) and electrochemical performance of the SCFN cathode, we fabricate SCFN–xSDC (x = 0, 20, 30, 40, 50, 60, wt%) composite cathodes. The thermal expansion behavior shows that the TECs value of SCFN cathode decreases greatly with SDC addition. The SDC addition reduces the polarization resistance, and the lowest polarization resistance 0.0255 Ω cm2 is achieved at 800 °C for SCFN–50SDC composite cathode. For SCFN–xSDC (x = 0, 40, 50, 60) composites, the maximum power densities of single-cells with SCFN–xSDC cathodes on 300 μm thick SDC electrolyte achieve 417, 557, 630 and 517 mW cm−2 at 800 °C, respectively. These results indicate that SCFN–50SDC composite is a potential cathode material for application in IT-SOFCs.

Preparation and electrochemical performance of cobalt-free cathode material Ba0.5Sr0.5Fe0.9Nb0.1O3−δ for intermediate-temperature solid oxide fuel cells

Perovskite oxide Ba0.5Sr0.5Fe0.9Nb0.1O3−δ(BSFN) as a cobalt-free cathode for intermediate-temperature solid oxide fuel cells(IT-SOFCs) on the Ce0.8Sm0.2O1.9(SDC) and La0.9Sr0.1Ga0.8Mg0.2O3−δ(LSGM) electrolytes was prepared and investigated. The single phase BSFN oxide with a cubic perovskite structure and relatively high electrical conductivities was obtained after sintering at 1250 °C for 10 h in air. The BSFN cathode exhibited excellent chemical stability on the SDC and LSGM electrolytes at temperatures below 950 °C. The area specific resistance of the BSFN cathode on the SDC and LSGM electrolytes were 0.024 and 0.021 Ω·cm2 at 800 °C, respectively. The maximum power densities of the single cell with BSFN cathode in 300 μm-thick SDC and LSGM electrolytes achieved 414 and 516 mW/cm2 at 800 °C, respectively. These results show that the BSFN material is a promising cobalt-free cathode candidate to be used in IT-SOFCs. A combination of the BSFN cathode and LSGM electrolyte is preferred owing to its excellent electrochemical performance.

A novel composite cathode for intermediate temperature solid oxide fuel cell

Mixed transition-metal oxide Ni0.79Co0.2Zn0.01O (NCZ) powders have been prepared by the modified Pechini method and studied as cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The Ni-Ce0.8Sm0.2O1.9(SDC)|SDC|SDC-NCZ single cell achieves 326 mW cm−2 at 600 °C. The output power has been further improved by partial substituting Li for Ni at Ni sites. The Ni-SDC|SDC|SDC-Li0.2Ni0.79Co0.2Zn0.01O2 (L2NCZ) cell with the SDC film of 30 μm in thickness shows maximum power outputs of 1.32, 0.98 and 0.62 W cm−2 at 650, 600 and 550 °C, respectively. The impedance measurements at open circuit conditions show that the overall electrode polarization resistances is 0.033, 0.112, 0.351 Ω cm2 at 650, 600 and 550 °C, respectively. The primary results indicate a new family of potential cathode material compatible with the SDC electrolyte for IT-SOFCs.

Impregnated Nd2NiO4+δ- scandia stabilized zirconia composite cathode for intermediate-temperature solid oxide fuel cells

Here we developed a novel Nd2NiO4+δ (NNO) impregnated SSZ composite cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The area specific polarization resistance of the composite cathode for oxygen reduction can be as low as 0.04 Ω cm2 at 800 °C. The anode supported SOFC with the structure of Ni-YSZ anode, SSZ electrolyte and impregnated NNO-SSZ composite cathode was prepared by the tape casting, co-firing and impregnation method. The resulting fuel cell exhibits maximum power densities of 1.26 and 0.73 W cm−2 at 800 and 700 °C, respectively when operated in hydrogen and air. Additionally, the electrical conductivity of the NNO cathode and the chemical compatibility with the electrolyte material were also studied.

Nd1.8Ce0.2CuO4+δ:Ce0.9Gd0.1O2−δ as a composite cathode for intermediate-temperature solid oxide fuel cells

The (100 − x)Nd1.8Ce0.2CuO4+δ:(x)Ce0.9Gd0.1O2−δ (x = 00, 10, 20 and 30 vol.%) composite systems are obtained by impregnating a stoichiometric solution of cerium and gadolinium nitrates followed by sintering at 900 °C for 4 h. Impregnating the Ce0.9Gd0.1O2−δ not only inhibits the growth of the host Nd1.8Ce0.2CuO4+δ grains during sintering but also enlarges the oxygen reduction reaction zone by introducing a nanosized phase that is ionically conductive, which significantly decreases the electrode polarization resistance of the composite cathode. A minimum polarization resistance value of 0.23 ± 0.02 Ω cm2 is obtained at 700 °C for a (80)Nd1.8Ce0.2CuO4+δ:(20)Ce0.9Gd0.1O2−δ composite cathode, and this value is attributed to the optimal dispersion into the porous Nd1.8Ce0.2CuO4+δ matrix. The impedance spectra are modeled using an electrical equivalent model that consists of a mid-frequency ZR1–CPE circuit (parallel combination of R1 and constant phase element (CPE)) and a low-frequency Gerischer impedance. The Gerischer impedance decreases significantly when Ce0.9Gd0.1O2−δ infiltrates the Nd1.8Ce0.2CuO4+δ matrix. The oxygen partial pressure-dependent polarization study suggests a medium-frequency response, which is due to charge transfer step; however, the low-frequency response corresponds to the non-charge transfer oxygen adsorption–desorption and the diffusion process during the overall oxygen reduction reaction process.

The characteristic of strontium-site deficient perovskites SrxFe1.5Mo0.5O6−δ (x = 1.9–2.0) as intermediate-temperature solid oxide fuel cell cathodes

As the cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs), A-site deficient SrxFe1.5Mo0.5O6−δ (x = 1.9–2.0) (SxFM) materials have been successfully synthesized using the sol–gel combustion method. In the perovskite structure of these oxides, the unit cell varies from pseudocubic to cubic with increasing deficiency. Thermal expansion coefficient of SxFM has also been measured and compared with that of Scandium-stabilized zirconium (ScSZ) electrolyte. X-ray photoelectron spectroscopy (XPS) results indicate that the Sr-deficiency has changed the proportion of Fe2+/Fe3+ and Mo6+/Mo5+ ratios, which directly influences the conductivity of SxFM materials. S1.950FM possesses the largest electrical conductivity and the lowest polarization resistance (Rp) among all the samples. The maximum power densities of a single cell with the S1.950FM cathode reaches 1083 mW cm−2, and the area specific resistance value is 0.17 Ω cm2 at 800 °C. These results indicate that the A-site deficiency could promote the electrochemical performance of SFM materials as cathodes for IT-SOFCs.

A novel cobalt-free perovskite La0.6Sr0.4Fe0.9Mo0.1O3−δ cathode for intermediate-temperature solid oxide fuel cells

A novel cobalt-free perovskite oxide La0.6Sr0.4Fe0.9Mo0.1O3−δ (LSFM10) was prepared by a citric acid-nitrate process and investigated as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). It was discovered that the doping with Mo for Fe-site distinctly improved the electrical conductivity of the La0.6Sr0.4FeO3−δ sample. The maximum conductivity of LSFM10 reached 159Scm−1 at 300–800°C in air. A single cell with the LSFM10 cathode and the proton-conducting BaZr0.1Ce0.7Y0.2O3−δ (BZCY) electrolyte membrane (~35μm thickness) supported by a NiO–BZCY composite anode was fabricated and tested at 500–700°C with H2 as fuel and air as oxidant. The peak power density of the cell reached 496mWcm−2 and the interfacial polarization resistance was 0.15Ωcm2 at 700°C. The experimental results indicated that La0.6Sr0.4Fe0.9Mo0.1O3−δ is a suitable cathode material for the proton-conducting IT-SOFCs.

Effect of Cu doping on YBaCo2O5+δ as cathode for intermediate-temperature solid oxide fuel cells

Layered-perovskites YBaCo2-xCuxO5+δ (YBCCO, x=0.0, 0.2, 0.4, 0.6, 0.8) are synthesized by the sol-gel method and are assessed as potential cathode materials for utilization in intermediate-temperature solid oxide fuel cells (IT-SOFCs) on the La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM) electrolyte. With increasing x from 0.0 to 0.8, the structure does not change but the lattice parameters and unit-cell volumes increase. From X-ray photoelectron spectra, it can be seen that both Cu and Co exist in a mixed oxidation state. Cu ions are prone to low oxidation state (Cu+ and Cu2+), while Co ions basically exhibit high oxidation state (Co3+ and Co4+). The conductivity of the YBCCO samples decreases with increasing Cu content, and the maximum conductivity of YBCCO is 143 S cm−1 at 300°C for the x=0.0 sample. The thermal expansion coefficient of samples significantly decreases from 17.8×10−6K−1 for x=0.0 to 13.4×10−6K−1 for x=0.8 in the temperature range of 30–850°C. With increasing Cu substitution content for Co, the electrode polarization resistance (Rp) increases. The Rp values of the YBCCO (x =0.6) cathode on the LSGM electrolyte are 0.125Ω cm2 and 0.076Ω cm2 at 700°C and 750°C, respectively. The maximum power densities of the single cells fabricated with the LSGM electrolyte, Ce0.8Gd0.2O1.9 (GDC) interlayer, NiO/GDC anode and YBCCO (x=0.6) cathode reach 815, 642 and 479 mW cm−2 at 850, 800 and 750°C, respectively. This study suggests that the layered-perovskites YBCCO can be potential candidates for utilization as IT-SOFC cathodes.

Layered perovskite PrBa0.5Sr0.5CoCuO5+δ as a cathode for intermediate-temperature solid oxide fuel cells

Layered perovskite PrBa0.5Sr0.5CoCuO5+δ (PBSCCo) oxide is synthesized by EDTA–citrate complexing method and investigated as a novel cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). X-ray diffraction results show that PBSCCo is chemical compatible with Gd0.1Ce0.9O1.95 (GDC) electrolyte below 950°C. The thermal expansion coefficient of PBSCCo is 17.58×10−6K−1 between 30°C and 900°C. The maximum electrical conductivity of PBSCCo is 483Sсm−1 at 325°C. The polarization resistance of PBSCCo cathode on GDC electrolyte is as low as 0.06Ωcm2 at 800°C. The maximum power density of the electrolyte-supported single cell with PBSCCo cathode achieves 521mWcm−2 at 800°C. Preliminary results indicate that PBSCCo is a potential cathode material for application in IT-SOFCs.

Improvement of a GDC-based composite cathode for intermediate-temperature solid oxide fuel cells

La0.84Sr0.16MnO3−δ - Ce0.8Gd0.2O2-δ (LSM-GDC) composite cathodes were fabricated by impregnating the LSM matrix with both LSM′ (La0.84Sr0.16MnO3−δ) and GDC (or only GDC), and the ion-impregnated LSM-GDC composite cathodes showed excellent performance. At 750 °C, the value of the cathode polarization resistance (Rp) was only 0.058 Ω cm2 for an ion-impregnated LSM-GDC composite cathode which was impregnated with both LSM′ and GDC. For the performance of the single cell with the same cathode, the maximum power density was 1.1 W cm−2 at 750 °C. The long-term test of the cell was carried out at 700 °C with a constant load of 0.3 A cm−2 and the output voltage was stable on the whole. The results demonstrated that LSM-GDC fabricated by impregnating the LSM matrix with both LSM′ and GDC was a promising composite cathode material for the intermediate-temperature solid oxide fuel cells.

Evaluation of layered perovskites YBa1−xSrxCo2O5+δ as cathodes for intermediate-temperature solid oxide fuel cells

This study is focused on the structural characteristics, oxygen nonstoichiometry, electrical conductivity, electrochemical performance and oxygen reduction mechanism of YBa1−xSrxCo2O5+δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5). The high oxygen nonstoichiometry, δ = 0.18–0.43 at 700 °C, indicates the large oxygen vacancy concentrations in oxides. The electrical conductivity is improved due to the greater amount of electronic holes originated from the increased interstitial oxygen, and the conductivities of all samples are above 100 S cm−1 at 400–700 °C in air. The results demonstrate the promising performance of YBa1−xSrxCo2O5+δ cathodes at intermediate temperatures, as evidenced by low area-specific resistances (ASRs) e.g. 0.21–0.59 Ω cm2 at 700 °C. The lowest ASR, 0.44 Ω cm2, and the cathodic overpotential, −40 mV at a current density of −136 mA cm−2, are obtained in YBaCo2O5+δ cathode at 650 °C. The dependence of polarization resistance on oxygen partial pressure suggests that the charge transfer process is the rate-limiting step for oxygen reduction reaction in YBaCo2O5+δ cathode.

A collaborative study of sintering and composite effects for a PrBa0.5Sr0.5Co1.5Fe0.5O5+δIT-SOFC cathode

Recently, a novel cathode material PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) has been proposed as a solution to overcome the drawbacks of a conventional cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Here we report systematic procedures to optimize the sintering temperature and the composite for PBSCF as an IT-SOFC cathode. For optimization of the heat treatment conditions for a PBSCF composite cathode, the effects of sintering temperature on the microstructure and electrical transport properties of the material are examined. We also suggest the optimization processes to effectively expand the electrochemical reaction zone based on a combination of a mixed ionic and electronic conductor (MIEC) electrode and an ionically conducting phase (PBSCF-Ce0.9Gd0.1O1.95 (GDC)x, x = 0, 20, 40, 50, and 60 wt%). The optimal intersection point between these two processing systems is revealed to be 50 wt% of GDC containing a composite cathode sintered at 950 °C for 4 h. The area specific resistance (ASR) of PBSCF-GDC50 sintered at 950 °C for 4 h reaches a minimum value of 0.052 Ω cm2 at 600 °C, which is consistent with the electrochemical performance results representing peak power density of ∼2.0 W cm−2 at 600 °C.

Cobalt-free Sr0.7Y0.3CuO2+δ as a cathode for intermediate-temperature solid oxide fuel cell

Cobalt-free oxide Sr0.7Y0.3CuO2+δ (SYCu) with one-dimensional structure has been investigated as potential cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC) applications. The crystal structure, chemical compatibility, thermal expansion and electrochemical performance were examined by X-ray diffraction technique, electrochemical workstation and thermal dilatometer. One-dimensional structure Sr1−xYxCuO2+δ and Sr2−xYxCuO3+δ phases appeared as the main parts after calcination above 900 °C. The copper based oxide SYCu showed a thermal expansion coefficient (TEC) about 11.1 × 10−6/°C at 25–800 °C, exhibiting good physical compatibility with samarium doped cerium (SDC) electrolyte. High electro-catalytic performance was obtained for SYCu cathode in a symmetrical cell with a polarization resistance (Rp) of 0.029 Ω cm2 and an overpotential of 4.9 mV at 100 mA/cm2 at 800 °C, showing great promising use as cathode materials for IT-SOFCs. In addition, the polarization resistance of SYCu cathode remain constant after operation at 800 °C for 100 h, showing excellent long-term stability at operation temperature.

Theoretical study on SmxSr1−xMnO3 as a potential solid oxide fuel cell cathode

Cubic perovskite SmxSr1−xMnO3 (SSM) surface and bulk models have been constructed to simulate the oxygen reduction reactions by employing first-principles calculations. The results demonstrate that oxygen vacancies can be formed easily in Sm0.5Sr0.5MnO3 (SSM50). The oxygen migration barrier in bulk SSM50, which is predicted by the nudged elastic band (NEB) method, is the lowest, while the adsorption energy of O2 molecular on SSM50 (100) surface is the lowest among the considered doping systems, indicating the potential application of SSM50 as a cathode for intermediate-temperature solid oxide fuel cell (IT-SOFC). The reaction mechanisms of oxygen reduction on SSM50 (100) surface have also been studied.

Cobalt-free polycrystalline Ba0.95La0.05FeO3−δ thin films as cathodes for intermediate-temperature solid oxide fuel cells

Ba0.95La0.05FeO3−δ (BLF) thin films as electrodes for intermediate-temperature solid oxide fuel cells are prepared on single-crystal yttria-stabilized zirconia (YSZ) substrates by pulsed laser deposition. The phase structure, surface morphology and roughness of the BLF thin films are characterized by X-ray diffraction, scanning electron microscopy and atomic force microscopy. X-ray photoelectron spectroscopy is used to analyze the compositions of the deposited thin film and the chemical state of transition metal. The dense thin film exhibits a polycrystalline perovskite structure with a low surface roughness and a high oxygen vacancy concentration on the surface. Ag (paste or strip) and Au (strip) are applied on both surfaces of the symmetric cells as current collectors to evaluate electrochemical performance of the thin films. The electrode polarization resistances of the symmetric cells are found to be lower than those of most cobalt-free thin-film electrodes, e.g., 0.437 Ω cm2 at 700 °C and 0.21 atm. The oxygen reduction reaction mechanism of the BLF cathode in symmetric cells is studied by electrochemical impedance spectroscopy thanks to the equivalent fitting analysis. Both the oxygen surface exchange reaction and charge transfer are shown to determine the overall oxygen reduction reaction.

Effect of Bi2O3 on the electrochemical performance of LaBaCo2O5+δ cathode for intermediate-temperature solid oxide fuel cells

The LaBaCo2O5+δ−x wt.% Bi2O3 (LBCO-xBi2O3, x=10, 20, 30, and 40) were prepared as composite cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs) via the conventional mechanical mixing method. The effect of Bi2O3 on polarization resistance, overpotential, and long-term stability of the LBCO cathode was investigated. An effective sintering aid for LBCO cathode, Bi2O3 not only lowers its sintering temperature by ~200°C, but also improves the electrochemical performance within the intermediate temperature range of 600–800°C. Electrochemical impedance spectroscopy measurements showed that the addition of 20wt% Bi2O3 to LBCO exhibited the lowest area-specific resistance of 0.020Ωcm2 at 800°C in air, which was about a seventh of that of the LBCO cathode at the same condition. At a current density of 0.2Acm−2, the cathodic overpotential of LBCO-20Bi2O3 was about 12.6mV at 700°C, while the corresponding value for LBCO was 51.0mV. Compared to B2O3–Bi2O3–PbO frit, the addition of Bi2O3 significantly improved the long-term stability of cathode. Therefore, LBCO-20Bi2O3 can be a promising cathode for IT-SOFCs.

Synthesis and electrochemical characterization of Sr2Fe1.5Mo0.5O6–Sm0.2Ce0.8O1.9 composite cathode for intermediate-temperature solid oxide fuel cells

Nanoporous composite oxides Sr2Fe1.5Mo0.5O6–Sm0.2Ce0.8O1.9 (SFM–SDC) have been prepared by a facile one-step method as cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The SFM–SDC composite materials have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and electrochemical impedance spectroscopy (EIS). The EIS results exhibit that SFM–SDC40 (wt% 60:40) cathode has encouraging electrochemical performance with low polarization resistance (Rp) on YSZ (Y2O3-stabilized ZrO2) electrolyte. Subsequently, bi-layer cathodes SDC/SFM–SDC are fabricated, and excellent electrochemical performance of such composite cathodes are observed. We demonstrate that the SDC interlayer significantly decreases the Rp of cathode and accelerates the charge transfer process. As a result, the Rp of the SDC/SFM–SDC40 bi-layer cathodes is almost 50% less than that of SFM–SDC40 cathode on YSZ electrolyte at 800 °C, and Rp is only 0.11 Ω cm2. Compared with single cells without an interlayer, the anode-supported single cells with SDC interlayer exhibit enhancement in overall power performance.