Composite (La,Sr)MnO3–YSZ cathode for SOFC

(La, Sr)MnO₃ (LSM)–Y doped ZrO₂ (YSZ) composite was prepared using YSZ colloidal suspension (initial YSZ particle size ∼100 nm), YSZ and LSM polymer precursors on dense substrates at 800 °C annealing temperature. The results of a symmetrical LSM–YSZ composite cell test showed the area specific resistance for overpotential of 0.14 Ω cm² at 800 °C, which indicated that the LSM–YSZ composite could be a potential candidate for cathode in SOFCs. The performance of the cell with the LSM–YSZ composite cathode and Ni-YSZ anode was investigated and the power density of about 0.26 W cm^− 2 was obtained at 850 °C using hydrogen fuel.     

Reaction mechanisms of the copper-manganese spinel cathode in a solid oxide fuel cell

Cu_1.25Mn_1.75O₄ spinel (CMO) was studied as a potential solid oxide fuel cell (SOFC) cathode material at intermediate temperatures. The reaction mechanism of a composite cathode consisting of Cu_1.25Mn_1.75O₄ and yttria-stabilized zirconia (YSZ) was investigated by impedance spectroscopy. The influence of the CMO/YSZ ratio, time exposed to current passage and temperature on the impedance spectra was examined. Activation energy of the corresponding processes was calculated to be near 1 eV and between 1.32 and 1.96 eV for the high and low frequency arcs in the impedance spectra. Comparison between CMO-YSZ and Sr-doped LaMnO₃ (LSM)-YSZ composite cathodes showed they had similar reaction mechanisms. The transport or transfer of oxygen intermediates or oxide ions between the catalyst and electrolyte was suggested to be the rate determining steps between 700 and 800 °C, whereas dissociative adsorption, mass transfer and surface diffusion were rate controlling between 600 and 700 °C.     

8YSZ/(La0.8Sr0.2)0.95MnO3-δ cathode performance at 1-3 bar oxygen pressures

Pressurised operation of solid oxide fuel cells (SOFC) has been shown to significantly improve their performance (Singhal, 2000) [1], however little work has been done on the effects of pressure on SOFC cathodes. The effect of pressurised oxygen on the area specific polarisation resistance (ASRp) of (La_0.8Sr_0.2)_0.95MnO_3-δ/8YSZ SOFC cathodes was determined by electrochemical impedance spectroscopy (EIS). Pellets of 8YSZ were pressed and sintered at 1350 °C, and screen printed layers of LSM/8YSZ cathode and LSM current collector were applied and sintered at 1300 °C and 1200 °C respectively. EIS was carried out between 1 and 3 bar oxygen at 800-1000 °C. One process dominated the spectra, and was identified as process C, (Jorgensen and Morgensen, 2001) [2] by comparison of measured and reference frequency maxima, the dependence of polarisation resistance on P_O2, the capacitance, and the activation energy. It is suggested that this represents the physical process of dissociative adsorption of oxygen at the triple phase boundaries of the electrode. A second process, with a magnitude almost independent of P_O2, is observed, which may be process B [2], related to transport of oxygen ions in the YSZ.     

Thin porous Ni-YSZ films as anodes for a solid oxide fuel cell

Porous Ni-YSZ (YSZ—yttria-stabilized zirconia) films were fabricated by reactive co-sputtering of a Ni and a Zr-Y target, followed by sequentially annealing in air at 900 °C and in vacuum at 800 °C. The Ni-YSZ films comprised small grains and pores that were tens of nanometers in size. The porous Ni-YSZ films were used as an anode on one side of a YSZ electrolyte disc and a La_0.7Sr_0.3MnO₃ thick film was used as a cathode on the other side of the disc to form solid oxide fuel cells (SOFCs). The voltage-current curves of the SOFCs with single- and a triple-layered porous anodes were measured in a single-chamber configuration, in a mixture of CH₄ and air (CH₄:O₂ volume ratio=2:1). The maximum power density of the SOFC using the single-layered porous Ni-YSZ thin films as the anode was 0.38 mW cm⁻², which was lower than that of 0.76 mW cm⁻², obtained using a screen-printed Ni-YSZ thick anode. The maximum power density of the SOFC with a thin anode was increased, but varied between 0.6 and 1.14 mW cm⁻² when a triple-layered porous Ni-YSZ anode was used.     

Measurement of electrode overpotentials for direct hydrocarbon conversion fuel cells

Cathodic and anodic overpotentials were measured using current interruption and AC impedance spectroscopy for two separate solid oxide fuel cells (SOFCs). The fuel cells used yttria-stabilized zirconia (YSZ) as the electrolyte, strontium-doped lanthanum manganite (LSM) as the cathode, and a porous YSZ layer impregnated with copper and ceria as the anode. The Cu/CeO₂/YSZ anode is active for the direct conversion of hydrocarbon fuels. Overpotentials measured using both current interruption and impedance spectroscopy for the fuel cell operating at 700 °C on both hydrogen and n-butane fuels are reported. In addition to providing the first electrode overpotential measurements for direct conversion fuel cells with Cu-based anodes, the results demonstrate that there may be significant uncertainties in measurements of electrode overpotentials for systems where there is a large difference between the characteristic frequencies of the anode and cathode processes and/or complex electrode kinetics.     

Preparation of thin film SOFCs working at reduced temperature

SOFCs are expected to become competitive devices for electrical power generation, but successful application is dependent on decreasing working temperature from 1000 to 800 °C, without detrimental effects on resistance and on electrode processes. This requires a reduction of the stabilised zirconia electrolyte thickness and an optimisation of the electrodes and interfaces. We have studied the preparation of a thin film SOFC device working at intermediate temperatures (less than 850 °C). The electrolyte must be very dense and as thin as possible to avoid ohmic losses. Electrodes, anode and cathode, must be porous enough to enable the gas fluxes to go through. Ni/YSZ cermet anodes have been prepared by a conventional ceramic method and support the cells. Thin Yttria Stabilised Zirconia electrolytes (YSZ) have been deposited by RF sputtering, DC sputtering and spray pyrolysis onto a Ni/YSZ cermet. Thin film La_0.7Sr_0.3MnO₃ (LSM) cathodes have been deposited onto the electrolytes by a spray-pyrolysis method. We present here the preparation and the characterisation of each component and the electrochemical performance of such cells at 850 °C. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Characterization of doped La0.7A0.3Cr0.5Mn0.5O3 − δ (A = Ca,Sr, Ba) electrodes for solid oxide fuel cells

Doped lanthanum manganese chromite based perovskite, La_0.7A_0.3Cr_0.5Mn_0.5O_3 ? δ (LACM, A = Ca, Sr, Ba), on yttria-stabilized zirconia (YSZ) electrolyte is investigated as potential electrode materials for solid oxide fuel cells (SOFCs). The electrical conductivity and electrochemical activity of LACM depend on the A-site dopant. The best electrochemical activity is obtained on the La_0.7Ca_0.3Cr_0.5Mn_0.5O_3 ? δ/YSZ (LCCM/YSZ) composite electrodes. The conductivity of LCCM is 29.9 S cm^? 1 at 800 °C in air, and the electrode polarization resistance (R_E) of the LCCM/YSZ composite cathode for the O₂ reduction reaction is 0.5 Ω cm² at 900 °C. The effect of Gd-doped ceria (GDC) impregnation on the LCCM cathode polarization resistances is also studied. GDC impregnation significantly enhances the electrochemical activity of the LCCM cathode. In the case of the 6.02 mg cm^? 2 GDC-impregnated LCCM cathode, R_E is 0.4 Ω cm² at 800 °C, ~ 60 times smaller than 24.4 Ω cm² measured on a LCCM cathode without the GDC impregnation. Finally the electrochemical activities of the doped lanthanum manganese chromites for the H₂ oxidation reaction are also investigated.     

Preparation of YSZ films by magnetron sputtering for anode-supported SOFC

YSZ films for anode-supported SOFCs were prepared by reactive sputtering method. It was found that the surface morphology of anode substrate has a very important effect on the quality of sputtered films. By applying an anode functional layer and making the anode surface smooth, dense and uniform YSZ films of 10 µm in thickness were successfully fabricated. The sintering behaviors of the sputtered YSZ films were also discussed. It is suggested that the optimized densification condition for the deposited YSZ films is sintering at 1250 °C for 4 h. Single cells with sputtered YSZ film as electrolyte and LSM-YSZ as active cathode materials were tested. 1.08 V open circuit voltage and a 700 mW/cm² maximum power density were achieved at 750 °C by using humidified H₂ as fuel and air as oxidant.     

Time-dependent performance of mixed-conducting SOFC cathodes

The time-dependent degradation of anode-supported Solid Oxide Fuel Cells (SOFCs) with La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathodes has been studied. Eight SOFCs have been tested over a period of 1000 h under different operation conditions to investigate the influence of different operation parameters on the degradation of the electrochemical performance. The cells were tested at 700 or 800 °C, at 0.3 or 0.6 A/cm² and with 21% or 5% O₂ at the cathode side and showed performance losses of 2–4% per 1000 h. While an elevated temperature and an elevated oxygen partial pressure had a negative influence on long-term performance, the current density did not have a clear effect. Material analysis of the cells showed a formation of SrZrO₃ at the interface of the Ce_0.8Gd_0.2O_2−δ interlayer and the yttria stabilized zirconia (8YSZ) electrolyte during sintering of the cathode. There are indications of a further formation of this phase during the electrochemical characterization obtained from X-ray diffraction analysis on LSCF–YSZ powder mixtures that were exposed to 800 °C for 200 h.     

Microstructural studies on degradation of interface between LSM–YSZ cathode and YSZ electrolyte in SOFCs

The changes in the cathode/electrolyte interface microstructure have been studied on anode-supported technological solid oxide fuel cells (SOFCs) that were subjected to long-term (1500 h) testing at 750 °C under high electrical loading (a current density of 0.75 A/cm²). These cells exhibit different cathode degradation rates depending on, among others, the composition of the cathode gas, being significantly smaller in oxygen than in air. FE-SEM and high resolution analytical TEM were applied for characterization of the interface on a submicron- and nano-scale. The interface degradation has been identified as the loss of LSM coverage and the loss of three-phase-boundary (TPB) length. Firstly, the degradation is caused by the size reduction of the individual LSM/YSZ electrolyte contact points (areas) that are initially of 100–200 nm in diameter. Quantitative microstructure evaluation shows that in the cell tested in air this mechanism contributes to an estimated overall reduction in the LSM coverage and the TPB length by 50 and 30%, respectively. For the cell tested in oxygen the corresponding values are 10 and 4%. Secondly, in the cell tested in air the LSM coverage and the TPB length appear to decrease further due to the more pronounced formation of insulating zirconate phases that are present locally and preferably in LSM/YSZ electrolyte contact areas. The effects of the cathode gas on the interface degradation are discussed considering the change of oxygen activity at the interface, possible changes in the Mn diffusion pattern as well as the LSM/YSZ reactivity. Finally, based on thermodynamic calculations a T–p(O₂) diagram predicting the safe and risky operation conditions in terms of the zirconate formation is presented and compared with the experimental observations.     

Fe alloying effect on the performance of the Ni anode in hydrogen fuel

Composite materials of YSZ (yttria-stabilized zirconia) with various Ni–Fe alloys were synthesized and evaluated as the solid oxide fuel cell (SOFC) anode using a 200-µm thick YSZ electrolyte as support and YSZ +La_0.8Sr_0.2MnO₃ (LSM) as cathode. The single cell with the YSZ + Ni_0.75Fe_0.25 anode exhibited the highest performance among all the investigated cells, e.g. a peak power density of 403, 337, 218 and 112 mW/cm² was achieved with H₂ fuel at 900, 850, 800 and 750 °C, respectively. The composite anode with the Ni_0.75Fe_0.25 alloy also had the lowest polarization resistance of 0.55 Ω·cm² at 800 °C among all the alloy compositions, indicating that this specific alloy offered a better anode composition than pure Ni. The possible mechanism for the improved performance of Ni with the Fe alloying addition towards H₂ oxidation was discussed.     

Electrochemical characterization of mixed conducting and composite SOFC cathodes

The electrochemical performance of La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (L58SCF), La_0.9Sr_1.1FeO_4−δ (LS2F) and LSM (La_0.65Sr_0.3MnO_3−δ)/LSM–YSZ (50 wt.% LSM–50 wt.% ZrO₂ (8 mol% Y₂O₃)) cathode electrodes interfaced to a double layer Ce_0.8Gd_0.2O_2−δ (CGO)/YSZ electrolyte was studied in the temperature range of 600 to 850 °C and under flow of 21% O₂/He mixture, using impedance spectroscopy and current density–overpotential measurements. The L58SCF cathode exhibited the highest electrocatalytic activity for oxygen reduction, according to the order: LS2F/CGO/YSZ ≤ LSM/LSM–YSZ/CGO/YSZ < L58SCF/CGO/YSZ.     

High temperature oxidation behavior of interconnect coated with LSCF and LSM for solid oxide fuel cell by screen printing

The current study examined the effect of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and La_0.7Sr_0.3MnO₃ (LSM) coatings on the electrical properties and oxidation resistance of Crofer22 APU at 800 °C hot air. LSCF and LSM were coated on Crofer22 APU by screen printing and sintered over temperatures ranging from 1000 to 1100 °C in N₂. The coated alloy was first checked for compositions, morphology and interface conditions and then treated in a simulated oxidizing environment at 800 °C for 200 h. After measuring the long-term electrical resistance, the area specific resistance (ASR) at 800 °C for the alloy coated with LSCF was less than its counterpart coated with LSM. This work used LSCF coating as a metallic interconnect to reduce working temperature for the solid oxide fuel cell.     

Characterization of YSr2Fe3O8 − δ as electrode materials for SOFC

YSr₂Fe₃O_8 − δ was prepared by traditional solid state reaction method and characterized by X-ray diffraction, ac impedance, dc conductivity, dilatometry and thermogravimetric analysis for possible use in solid oxide fuel cells (SOFCs). YSr₂Fe₃O_8 − δ crystallizes with tetragonal symmetry in the space group P4/mmm and found to be stable at high temperatures under H₂ and air. Four probe dc electrical conductivity measurements show that the conductivity increases up to 745 K and then decreases with temperature; the highest conductivity σ_745K = 43.5 S cm^− 1. The n-type conductivity at low oxygen partial pressure (pO₂) changes to p-type at high pO₂. Polarization behavior was investigated measuring the ac impedance response in symmetrical cell arrangements in air with YSZ and GDC electrolytes. Cathodic area specific resistance (ASR) varies with firing temperature. The lowest area specific resistance was observed with a GDC electrolyte fired at 1000 °C. In case of YSZ, ASR increases and in case of GDC, ASR decreases in air when electrode firing temperature decreases. At 800 °C ASRs are 0.20 Ω cm² and 0.65 Ω cm² with GDC and YSZ electrolytes, respectively, in air. Fuel cell measurements with symmetrical electrodes were performed using a thin YSZ electrolyte under H₂ at anode and air at cathode, show that the power density is about 0.035 W/cm² at 900 °C.     

Electrochemical properties of nanostructured lanthanum strontium manganite cathode fabricated by electrostatic spray deposition

Electrostatic spray deposition was applied to prepare nanoporous lanthanum strontium manganite (LSM) films with high specific surface area (37.34 m²/g) for the cathode application in solid oxide fuel cell (SOFC). The electrochemical characteristics were investigated at a temperature range from 546 to 777 °C and oxygen partial pressure from 0.01 to 1.0 atm. The diffusion of atomic oxygen and oxygen ion transfer from three-phase boundary to the YSZ electrolyte were found to be the rate-determining steps for oxygen reduction reaction on LSM cathode. The polarization resistance of the LSM prepared using electrostatic spray deposition decreased from 15 to 1.2 Ωcm² with increasing temperature from 546 to 777 °C and the activation energy was 0.81 eV. It was demonstrated that the ESD method offers a promising approach for the preparation of electrochemically active nanoporous layers, particularly applicable for solid oxide fuel cells.     

Fabrication and electrochemical performance of La<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript> impregnated porous YSZ cathode

La_0.5Sr_0.5CoO₃-yttria-stabilized zirconia (LSCO-YSZ) composite cathode for solid oxide fuel cell (SOFC) has been fabricated by wet impregnation method. Nitrate precursors of La, Sr, and Co have been impregnated into the pre-sintered porous YSZ matrix, which is converted into LSCO phase after calcination at 850 °C in the presence of glycine as confirmed from X-ray diffraction. LSCO of 5, 7, and 10 wt% impregnated porous YSZ have been electrochemically characterized using 2-probe AC conductivity method. Maximum ionic conductivity of 0.27 S/cm at 800 °C and activation energy of 0.15 eV between 600 and 800 °C have been observed for 10 wt% LSCO-YSZ cathode. Area-specific resistance of 1.01 Ω cm² at 800 °C is estimated for the electrolyte-supported half-cell (10 wt% LSCO-YSZ/YSZ). After testing the LSCO-YSZ cathode matrix, the electrolyte-supported full cell (10 wt% LSCO-YSZ/YSZ/NiO-YSZ) has been tested and produced maximum power density 51.12 mW/cm² (109.38 mA/cm²) at 800 °C. The electrolyte-supported full cell exhibited 6 Ω cm² electrode polarization at 800 °C in H₂, which is in higher side leading to low performance. LSCO-YSZ/YSZ/NiO-YSZ SOFC found to give stable performance up to 2 h and scanning electron microscopy analysis has been carried out before and after cell testing to assess the morphological changes.     

Influence of YSZ electrolyte thickness on the characteristics of plasma-sprayed cermet supported tubular SOFC

Novel Ni–Al₂O₃ cermet-supported tubular SOFC cell was fabricated by thermal spraying. Flame-sprayed Al₂O₃–Ni cermet coating played dual roles of a support tube and an anode current collector. Y₂O₃-stabilized ZrO₂ (YSZ) electrolyte was deposited by atmospheric plasma spraying (APS) to aim at reducing manufacturing cost. The gas tightness of APS YSZ coating was achieved by post-densification process. The influence of YSZ coating thickness on the performance of SOFC test cell was investigated in order to optimize YSZ thickness in terms of open circuit voltage of the cell and YSZ ohmic loss. It was found that the reduction of YSZ thickness from 100 μm to 40 μm led to the increase of the maximum output power density from 0.47 W/cm² to 0.76 W/cm² at 1000 °C. Using an APS 4.5YSZ coating of about 40 μm as the electrolyte, the test cell presented a maximum power output density of over 0.88 W/cm² at 1030 °C. The results indicate that SOFCs with thin YSZ electrolyte require more effective cathode and anode to improve performance.     

Spray pyrolysis of electrolyte interlayers for vacuum plasma-sprayed SOFC

The effects of gadolinia-doped ceria (CGO, Ce_0.8Gd_0.2O_1.9−x) and yttria-doped zirconia (8YSZ, Zr_0.92Y_0.08O_2−x) interlayers prepared by spray pyrolysis between vacuum plasma-sprayed 8YSZ electrolytes (8YSZ–VPS) and screen-printed (La_0.8Sr_0.2)_0.98MnO₃ cathodes (LSM) on the power output of solid oxide fuel cells (SOFC) are investigated. Amorphous thin films are deposited and then converted to nanocrystalline electrolyte–cathode interlayers during the first heat-up cycle of a SOFC to the operating temperature. CGO thin films between the YSZ plasma-sprayed electrolyte and the LSM cathode increased the power output by more than 20% compared to cells without interlayers, whereas YSZ films degraded the power output of cells. It is assumed that CGO improves the charge transfer at the electrolyte–cathode interface and that the CGO layer prevents the formation of undesirable insulation of La-zirconate at the interface 8YSZ/LSM.     

Electrical properties of thin yttria-stabilized zirconia overlayers produced by atomic layer deposition for solid oxide fuel cell applications

Thin films of yttria-stabilized zirconia (YSZ) electrolyte were prepared by atomic layer deposition at 300 °C for solid oxide fuel cell (SOFC) applications. YSZ samples of 300-1000 nm thickness were deposited onto La_0.8Sr_0.2MnO₃ (LSM) cathodes. A microstructural study was performed on these samples and their electrical properties were characterised between 100 and 390 °C by impedance spectroscopy. A remarkable feature is that the as-deposited layers were already crystalline without any annealing treatment. Their resistance decreased when reducing the layer thickness; nevertheless, their conductivity and activation energy were significantly lower than those reported in the literature for bulk YSZ.     

Zirconia stabilized by Y and Mn: A microstructural characterization

Cubic stabilized ZrO₂ with 8 mol% Y₂O₃ (YSZ) is commonly used as an electrolyte in solid oxide fuel cells (SOFC). One of the most promising cathode materials is La,Sr-manganite (LSM). During manufacture and operation of the SOFC, Mn diffuses from the LSM into YSZ. The structural changes caused by the presence of Mn in the electrolyte under various oxygen partial pressures have been examined by X-ray diffraction. Microstructures of YSZ containing Mn have been examined by transmission electron microscopy and surface changes of the electrolyte due to Mn diffusion were studied by scanning electron microscopy. The results are discussed with respect to properties and stability of the electrolyte and the SOFC. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995