La substituted Sr2MnO4 as a possible cathode material in SOFC

Sr_2−xLa_xMnO_4+δ (x = 0.4, 0.5, 0.6) oxides were studied as the cathode material for solid oxide fuel cells (SOFC). The reactivity tests indicated that no reaction occurred between Sr_2−xLa_xMnO_4+δ and CGO at annealing temperature of 1000 °C, and the electrode formed good contact with the electrolyte after being sintered at 1000 °C for 4 h. The total electrical conductivity, which has strong effect on the electrode properties, was determined in a temperature range from 100 to 800 °C. The maximum value of 5.7 S cm⁻¹ was found for the x = 0.6 phase at 800 °C in air. The cathode polarization and AC impedance results showed that Sr_1.4La_0.6MnO_4+δ exhibited the lowest cathode overpotential. The area specific resistance (ASR) was 0.39 Ω cm² at 800 °C in air. The charge transfer process is the rate-limiting step for oxygen reduction reaction on Sr_1.4La_0.6MnO_4+δ electrode.     

Fabrication and evaluation of a Ni/La0.75Sr0.25Cr0.5Fe0.5O3−δ co-impregnated yttria-stabilized zirconia anode for single-chamber solid oxide fuel cells

In this study, an anode-supported solid oxide fuel cell (SOFC) has been prepared using a porous yttria-stabilized zirconia (YSZ) anode matrix. The anode was prepared by impregnating the sintered porous YSZ matrix with a nitrate aqueous containing La³⁺, Sr²⁺, Cr³⁺, Fe³⁺, Ni²⁺ and urea. The formed anode exhibited high surface area and porosity, and had fast path for the transportation of oxygen ion and electron, as well as resulting in high three-phase boundaries (TPBs). Single-chamber fuel cell test was conducted in a methane-oxygen gas mixture using an YSZ membrane as the electrolyte and La_0.8Sr_0.2MnO_3−δ (LSM) as the cathode. The influences of environmental temperature and gas composition on the cell performance were also investigated. Under the optimized gas composition (CH₄/O₂ = 2/1) and furnace temperature (800 °C) conditions, a maximum power density of 214 mW cm⁻² was achieved. The test results demonstrated good cell stability and indicated that the perovskite oxide-based anodes were quite robust with redox cycling.     

Electrical conduction behavior of La,Co co-doped SrTiO3 perovskite as anode material for solid oxide fuel cells

The effects of La- and Co-doping into SrTiO₃ perovskite oxides on their phase structure, electrical conductivity, ionic conductivity and oxygen vacancy concentration have been investigated. The solid solution limits of La in La_xSr_1 − xTiO_3 − δ and Co in La_0.3Sr_0.7Co_yTi_1 − yO_3 − δ are about 40 mol% and 7 mol%, respectively, at 1500 °C. The incorporation of La decreases the band gap and thus increases the electrical conductivity of SrTiO₃ remarkably. La_0.3Sr_0.7TiO_3 − δ shows an electrical conductivity of 247 S/cm at 700 °C. Co-doping into La_0.3Sr_0.7TiO_3 − δ increases the oxygen vacancy concentration and decreases the migration energy for oxygen ions, leading to a significant increase in ionic conductivity but at the expense of some electrical conductivity. The electrical and ionic conductivities of La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ are 63 S/cm and 6 × 10⁻³ S/cm, respectively, at 700 °C. Both La_0.3Sr_0.7TiO_3 − δ and La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ show relatively stable electrical conductivities under oxygen partial pressure of 10⁻¹⁴–10⁻¹⁹ atm at 800 °C. These properties make La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ a promising anode candidate for solid oxide fuel cells.     

Fabrication and evaluation of anode and thin Y2O3-stabilized ZrO2 film by co-tape casting and co-firing technique

Co-tape casting and co-firing of supporting electrode and electrolyte layers could drastically increase productivity and reduce fabrication cost. In this study, Ni-YSZ anode supporting electrode and the YSZ electrolyte with the size of 6.5 cm × 6.5 cm have been successfully fabricated by co-tape casting and co-firing technique. The cell with 1.5 mm anode and 10 μm electrolyte is flat without warping, cracks or delaminations. The power density reaches 661, 856, 1085 mW cm⁻² at 0.7 V and 750, 800 and 850 °C, respectively. The EIS results demonstrate that the cathodic electrochemical resistance is 0.0680 Ω cm², about twice of the anode's which is 0.0359 Ω cm². SEM images show the dense YSZ film had a crack free of surface morphology. The anode and cathode layers are well-adhered to the YSZ electrolyte layer. The La_0.8 Sr_0.2 MnO_3−δ particles do not form a continuous network. Optimization of finer cathodic microstructure and anodic porosity are underway.     

A symmetrical solid oxide fuel cell prepared by dry-pressing and impregnating methods

In this study, a simple and cost-effective dry-pressing method has been used to fabricate a symmetrical solid oxide fuel cell (SOFC) where the dense yttria-stabilized zirconia (YSZ) electrolyte film is sandwiched between two symmetrical porous YSZ layers in which La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) based anode and cathode are incorporated using wet impregnation techniques. The maximum power densities (P_max) of a single cell with 32 wt.% LSCM impregnated YSZ anode and cathode reach 333 and 265 mW cm⁻² at 900 °C in dry H₂ and CH₄, respectively. The cell performance is further improved with additional impregnation of a small amount of Sm-doped CeO₂ (SDC) or Ni. When 6 wt.% Ni as catalyst is added to both the anode and cathode, P_max values of 559 and 547 mW cm⁻² can be achieved, which are better than with SDC. The effect of Ni on the cathode performance is also investigated by impedance spectra analysis.     

La1−xSrxCoO3 (x = 0.1-0.5) as the cathode catalyst for a direct borohydride fuel cell

Direct borohydride fuel cells (DBFCs), with a series of perovskite-type oxides La_1−xSr_xCoO₃ (x = 0.1-0.5) as the cathode catalysts and a hydrogen storage alloy as the anode catalyst, are studied in this paper. The structures of the perovskite-type catalysts are mainly La_1−xSr_xCoO₃ (_x = 0.1-0.5) oxides phases. However, with the increase of strontium content, the intensities of the X-ray diffraction peaks of the impure phases La₂Sr₂O₅ and SrLaCoO₄ are gradually enhanced. Without using any precious metals or expensive ion exchange membranes, a maximum current density of 275 mA cm⁻² and a power density of 109 mW cm⁻² are obtained with the Sr content of x = 0.2 at 60 °C for this novel type of fuel cell.     

Sr2Fe4/3Mo2/3O6 as anodes for solid oxide fuel cells

Sr₂Fe_4/3Mo_2/3O₆ has been synthesized by a combustion method in air. It shows a single cubic perovskite structure after being reduced in wet H₂ at 800 °C and demonstrates a metallic conducting behavior in reducing atmospheres at mediate temperatures. Its conductivity value at 800 °C in wet H₂ (3% H₂O) is about 16 S cm⁻¹. This material exhibits remarkable electrochemical activity and stability in H₂. Without a ceria interlayer, maximum power density (P_max) of 547 mW cm⁻² is achieved at 800 °C with wet H₂ (3% H₂O) as fuel and ambient air as oxidant in the single cell with the configuration of Sr₂Fe_4/3Mo_2/3O₆|La_0.8Sr_0.2Ga_0.83Mg_0.17O₃ (LSGM)| La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF). The P_max even increases to 595 mW cm⁻² when the cell is operated at a constant current load at 800 °C for additional 15 h. This anode material also shows carbon resistance and sulfur tolerance. The P_max is about 130 mW cm⁻² in wet CH₄ (3% H₂O) and 472 mW cm⁻² in H₂ with 100 ppm H₂S. The cell performance can be effectively recovered after changing the fuel gas back to H₂.     

Preparation and characterization of Pr1−xSrxFeO3 cathode material for intermediate temperature solid oxide fuel cells

A kind of cathode material of Pr_1−xSr_x FeO₃ (x = 0–0.5) for intermediate temperature solid oxide fuel cells (IT-SOFCs) was prepared by the coprecipitation method. Crystal structure, thermal expansion, electrical conductivity and electrochemical performance of the Pr_1−xSr_xFeO₃ perovskite oxide cathodes were studied by different methods. The results revealed that Pr_l−xSr_xFeO₃ exhibited similar orthorhombic structure from x = 0.1 to 0.3 and took cubic structure when x = 0.4–0.5. The unit cell volume decreased and the thermal expansion coefficient (TEC) of the materials increased as the strontium content increased. When 0 < x ≤ 0.3, the samples exhibited good thermal expansion compatibility with YSZ electrolyte. The electrical conductivity increased with the increasing of doped strontium content. When x = 0.3–0.5, the electrical conductivities were higher than 100 S cm⁻¹. The conductivity of Pr_0.8Sr_0.2FeO₃ was 78 S cm⁻¹ at 800 °C. Compared with the La_0.8Sr_0.2MnO₃ cathode, Pr_0.8Sr_0.2FeO₃ showed higher polarization current density and lower polarization resistance (0.2038 Ω cm²). The value of I₀ for Pr_0.8Sr_0.2FeO₃ at 800 °C is 123.6 mA cm⁻². It is higher than that of La_0.8Sr_0.2MnO₃. Therefore, Pr_1−xSr_xFeO₃ can be considered as a candidate cathode material for IT-SOFCs.     

Enhanced performance of a single-chamber solid oxide fuel cell with an SDC-impregnated cathode

The electrochemical performance of anode-supported single-chamber solid oxide fuel cells (SC-SOFCs) with and without SDC-impregnated cathodes was compared in a diluted methane–oxygen mixture. These cells were made of conventional materials including yttrium-stabilized zirconia (YSZ) thin film, a Ni + YSZ anode and a La_0.7Sr_0.3MnO₃ (LSM) cathode. Our results showed that the cell performance was greatly enhanced with the SDC-impregnated LSM cathode. At a furnace temperature of 750 °C, the maximum power density was as high as 404 mW cm⁻² for a CH₄ to O₂ ratio of 2:1, which was 4.0 times higher than the cell with a pure LSM cathode (100 mW cm⁻²). The overall polarization resistance of the impregnated cell was 1.6 Ω cm², which was much smaller than that of the non-impregnated one (4.2 Ω cm²). The impregnation introduced SDC nanoparticles greatly extended the electrochemical active zone and hence greatly improved the cell performance.     

Fe-based perovskites as electrodes for intermediate-temperature solid oxide fuel cells

A solid-oxide fuel cell (SOFC) based upon Fe perovskites, has been designed and tested. Materials with nominal compositions Sr_0.9K_0.1FeO_3−δ (SKFO) and Sr_1.6K_0.4FeMoO_6−δ (SKFMO) with perovskite structure have been prepared and characterized as cathode and anode, respectively. The anode material exhibits high electrical conductivity values of 407-452 S cm⁻¹ at 750-820 °C in pure H₂. In the test cells, the electrodes were supported on a 300-μm-thick pellet of the electrolyte La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM). The single SOFC cells gave a maximum power density at 850 °C of 937 mW cm⁻² with pure H₂ as a fuel. Sizeable power densities were also observed with alternative fuels: 694 and 499 mW cm⁻² with H₂ containing 5 parts per million of H₂S and CH₄, respectively, at 800 °C. Moreover, only a slight degradation of about 3.6% of the power density has been obtained after 65 different cycles of fuel-cell test in H₂ at 750 °C and 14% at 850 °C in 50 cycles using H₂-H₂S. This remarkable behavior has been correlated to the structural features determined in a neutron powder diffraction experiment in the usual working conditions of a SOFC for a cathode (air) and an anode (low pO₂). On the one hand, the cubic Pm-3m Sr_0.9K_0.1FeO_3−δ cathode material is an oxygen deficient perovskite with oxygen contents that vary from 2.45(2) to 2.26(2) from 600 to 900 °C and high oxygen isotropic thermal factors (4.17(8) Å²) suggesting a high ionic mobility. On the other hand, the actual nature of the anode of composition Sr_1.6K_0.4FeMoO_6−δ has been unveiled by neutron powder diffraction to consist of two main perovskite phases with the compositions SrMoO₃ and SrFe_0.6Mo_0.4O_2.7. The association of two perovskites oxides, SrMoO₃ with high electrical conductivity, and SrFe_0.6Mo_0.4O_2.7 with mixed ionic-electronic conductivity has resulted in an extraordinarily performing anode material for SOFCs.     

Performance of double-perovskite Sr2−xSmxMgMoO6−δ as solid-oxide fuel-cell anodes

Double-perovskite Sr_2−xSm_xMgMoO_6−δ (SSMM, 0 ≤ x ≤ 0.8) is investigated as a possible anode material for solid-oxide fuel cells on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolytes. Single-phase SSMM samples with 0 ≤ x ≤ 0.4 are prepared. At x ≥ 0.6, a small amount of SrMoO₄ and Sm₂O₃ impurities are observed. The Mg/Mo ordering in SSMM decreases with increasing Sm content. Substitution of Sm for Sr significantly improves the electrical conductivity of SSMM. At x = 0.6, the sample yields the highest conductivity, with values reaching 16 S cm⁻¹ in H₂ at 800 °C. The maximum power densities of single cells achieved with x = 0.0, 0.2, 0.4, 0.6, and 0.8 anodes on a 300 μm-thick LSGM electrolyte are 693, 770, 860, 907, and 672 mW cm⁻², respectively, in H₂ at 850 °C. The SSMM sample with x = 0.4 is considered as the best anode candidate because of the impurity formation seen in x ≥ 0.6 samples. The x = 0.4 sample not only has a thermal-expansion coefficient closer to that of the LSGM electrolyte but also exhibits good electrochemical performance and stability in commercial city gas containing H₂S, where the maximum power density achieved is 726 mW cm⁻² at 850 °C.     

Combustion synthesis and properties of highly phase-pure perovskite electrolyte Co-doped La0.9Sr0.1Ga0.8Mg0.2O2.85 for IT-SOFCs

The highly phase-pure perovskite electrolyte, La_0.9Sr_0.1Ga_0.8Mg_0.115Co_0.085O_2.85 (LSGMCO), was prepared by means of glycine–nitrate process (GNP) for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The perovskite phase evolution, sintering, electrical conductivity and electrochemical performance of LSGMCO were investigated. The results show that the highly phase-pure perovskite electrolyte LSGMCO can be obtained after calcining at 1150 °C. The sample sintered at 1450 °C for 20 h in air exhibited a better sinterability, and the relative density of LSGMCO was higher than 95%. The stoichiometric indexes of the elements in the sintered sample LSGMCO determined experimentally by EDS were in good agreement with the nominal composition. The electrical conductivities of the sample were 0.094 and 0.124 S· cm⁻¹ at 800 °C and 850 °C in air, respectively. The ionic conduction of the sample was dominant at high temperature with the higher activation energies. While at lower temperature the electron hole conduction was predominated with the lower activation energies. The maximum power densities of the single cell fabricated with LSGMCO electrolyte with Ce_0.8Sm_0.2O_1.9 (SDC) interlayer, SmBaCo₂O_5+x cathode and NiO/SDC anode achieved 643 and 802 mW cm⁻² at 800 °C and 850 °C, respectively.     

A modified suspension spray combined with particle gradation method for preparation of protonic ceramic membrane fuel cells

In order to prepare a dense proton-conductive Ba(Zr_0.1Ce_0.7)Y_0.2O_3−δ (BZCY7) electrolyte membrane, a proper anode composition with 65% Ni₂O₃ in weight ratio was determined after investigating the effects of anode compositions on anode shrinkages for co-sintering. The thermal expansion margins between sintered anodes and electrolytes, which were less than 1% below 750 °C, also showed good thermal expansion compatibility. A suspension spray combined with particle gradation method had been introduced to prepare dense electrolyte membrane on porous anode support. After a heat treatment at 1400 °C for 5 h, a cell with La_0.5Sr_0.5CoO_3−δ (LSCO) cathode was assembled and tested with hydrogen and ammonia as fuels. The outputs reached as high as 330 mW cm⁻² in hydrogen and 300 mW cm⁻² in ammonia at 700 °C, respectively. Comparing with the interface of another cell prepared by dry-pressing method, this one also showed a good interface contact between electrodes and electrolyte. To sum up, this combined technique can be considered as commercial fabrication technology candidate.     

Electrocatalytic activity of perovskite La1−xSrxMnO3 towards hydrogen peroxide reduction in alkaline medium

Perovskite-type series of compounds La_1−xSr_xMnO₃ are synthesized by a sol-gel method using Chitosan as the gelling agent. Their catalytic activity for hydrogen peroxide electroreduction in 3.0 mol dm⁻³ KOH at room temperature is evaluated by means of cyclic voltammetry and chronoamperometry. Effects of annealing temperature and the ratio of La to Sr of La_1−xSr_xMnO₃ on their catalytic performance are investigated. Among this series of compounds, La_0.4Sr_0.6MnO₃ calcined at 650 °C exhibits the highest activity, which is comparable with Co₃O₄. An aluminum-hydrogen peroxide semi-fuel cell using La_0.4Sr_0.6MnO₃ as cathode catalyst achieves a peak power density of 170 mW cm⁻² at 170 mA cm⁻² and 1.0 V running on 0.6 mol dm⁻³ H₂O₂.     

Synthesis and properties of La0.6Sr0.4CoO3−δ nanopowder

Uniform nanopowders of La_0.6Sr_0.4CoO_3−δ (LSC) were synthesized by the combined citrate–EDTA method. The precursor solution was prepared from nitrates of the constituent metal ion, citric acid and EDTA with a pH value controlled by ammonia. The obtained product was characterized by TG/DTA, XRD, SEM, and BET measurements. The single perovskite phase could form completely after sintering at the temperature of 900 °C. There was no significant effect of the precursor solution pH value on the perovskite phase formation temperature; however, LSC powders prepared from the precursors with different pH values showed specific shapes. The morphology of La_0.6Sr_0.4CoO_3−δ powder was also optimized with proper surfactant addition. The sintered La_0.6Sr_0.4CoO_3−δ bulk samples exhibited an electrical conductivity of 1867 S cm⁻¹ in air at 800 °C. The impedance spectra of a symmetric LSC cathode on a GDC electrolyte substrate were measured and polarization resistance (R_p) values of 0.17 Ω cm² at 700 °C and 0.07 Ω cm² at 750 °C in air were obtained.     

High-performance PrBaCo2O5+δ-Ce0.8Sm0.2O1.9 composite cathodes for intermediate temperature solid oxide fuel cell

PrBaCo₂O_5+δ-Ce_0.8Sm_0.2O_1.9 (PBCO-SDC) composite material are prepared and characterized as cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The powder X-ray diffraction result proves that there are no obvious reaction between the PBCO and SDC after calcination at 1100 °C for 3 h. AC impedance spectra based on SDC electrolyte measured at intermediate temperatures shows that the addition of SDC to PBCO improved remarkably the electrochemical performance of a PBCO cathode, and that a PBCO-30SDC cathode exhibits the best electrochemical performance in the PBCO-xSDC system. The total interfacial resistances R_p is the smallest when the content of SDC is 30 wt%, where the value is 0.035 Ω cm² at 750 °C, 0.072 Ω cm² at 700 °C, and 0.148 Ω cm² at 650 °C, much lower than the corresponding interfacial resistance for pure PBCO. The maximum power density of an anode-supported single cell with PBCO-30SDC cathode, Ni-SDC anode, and dense thin SDC/LSGM (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ)/SDC tri-layer electrolyte are 364, 521 and 741 mW cm⁻² at 700, 750 and 800 °C, respectively.     

Pd-impregnated SYT/LDC composite as sulfur-tolerant anode for solid oxide fuel cells

An Sr_0.88Y_0.08TiO_3−δ (SYT)/La_0.4Ce_0.6O_1.8 (LDC) composite impregnated with Pd was evaluated as a sulfur-tolerant anode for the La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM)-supported cells. The impregnation of Pd into the porous SYT/LDC anode was found to significantly enhance the anode performance. With the addition of 1.5 wt.% Pd into the anode, the anodic overpotential was reduced to about half of the original value. The single cell with the Pd-impregnated SYT/LDC anode showed a maximum power density of 1006 and 577 mW cm⁻² at 850 and 800 °C in dry H₂, respectively, which was more than twice of that prior to impregnation. The Pd-impregnated composite anode exhibited good tolerance to sulfur, with essentially no decay in performance in H₂ containing up to 50 ppm H₂S.     

Fabrication and performance of membrane solid oxide fuel cells with La0.75Sr0.25Cr0.5Mn0.5O3−δ impregnated anodes

La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCrM)-impregnated anodes have been fabricated by infiltrating 70% porous yttria-stabilized zirconia (YSZ) matrixes with an LSCrM solution. In these anodes, LSCrM is a primary electronic conducive phase while the well-sintered YSZ provides an ionic-conducting pathway throughout the electrode. The maximum power densities of a single cell with YSZ + 35 wt.% LSCrM composite anode reach 567 and 561 mW cm⁻² at 850 °C in dry H₂ and CH₄, respectively. Further, Ag and Ni are added via nitrate impregnating method for improving electronic conductivity and catalytic activity. With the addition of 6 wt.% Ni and 2 wt.% Ag to the YSZ + 32 wt.% LSCrM composite anode, the maximum power densities at 850 °C increase to 1302 mW cm⁻² in dry H₂ and 769 mW cm⁻² in dry CH₄. No carbon deposition is detected in the tested anodes, even with the presence of Ni.     

Sr2CoMoO6 anode for solid oxide fuel cell running on H2 and CH4 fuels

The double perovskite Sr₂CoMoO_6−δ was investigated as a candidate anode for a solid oxide fuel cell (SOFC). Thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) showed that the cation array is retained to 800 °C in H₂ atmosphere with the introduction of a limited concentration of oxide-ion vacancies. Stoichiometric Sr₂CoMoO₆ has an antiferromagnetic Néel temperature T_N ≈ 37 K, but after reduction in H₂ at 800 °C for 10 h, long-range magnetic order appears to set in above 300 K. In H₂, the electronic conductivity increases sharply with temperature in the interval 400 °C < T < 500 °C due to the onset of a loss of oxygen to make Sr₂CoMoO_6−δ a good mixed oxide-ion/electronic conductor (MIEC). With a 300-μm-thick La_0.8Sr_0.12Ga_0.83Mg_0.17O_2.815 (LSGM) as oxide-ion electrolyte and SrCo_0.8Fe_0.2O_3−δ as the cathode, the Sr₂CoMoO_6−δ anode gave a maximum power density of 1017 mW cm⁻² in H₂ and 634 mW cm⁻² in wet CH₄. A degradation of power in CH₄ was observed, which could be attributed to coke build up observed by energy dispersive spectroscopy (EDS).     

Electrical conductivity and cell performance of La0.3Sr0.7Ti1−xCrxO3−δ perovskite oxides used as anode and interconnect material for SOFCs

The anode materials La_0.3Sr_0.7Ti_1−xCr_xO_3−δ (LSTC, x = 0, 0.1, 0.2) with cubic structure were prepared via solid state reaction route. The influence of Cr content on the properties of LSTC as anode and interconnect materials for solid oxide fuel cells (SOFCs) was investigated. The Cr-doping decreased the lattice parameter while increased the sinterability of LSTC materials. The total electrical conductivity decreased with Cr doping level, from 230 S cm⁻¹ for x = 0 to 53 S cm⁻¹ for x = 0.2. The total electrical conductivity exhibited good stability and recoverability in alternative atmospheres of air and 5% H₂/Ar, showing excellent redox stability. The cell testing showed that the anode performance of LSTC was enhanced somewhat by Cr doping. The present results indicated that the prepared La_0.3Sr_0.7Ti_1−xCr_xO_3−δ can be potential anode and interconnect materials for SOFCs.