Perovskite Cathode Materials Research Articles

Role of silver current collector on the operational stability of selected cobalt-containing oxide electrodes for oxygen reduction reaction

The long-term stability for oxygen reduction reaction (ORR) of two typical perovskite cathode materials of SOFCs, i.e., Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and Sm0.5Sr0.5CoO3−δ (SSC), is investigated in a symmetric cell configuration under air condition at 700°C using Sm0.2Ce0.8O1.9 (SDC) electrolyte substrate and silver current collectors. Moreover, two different methods of silver current collection are tested, i.e., whole electrode surface deposited with a diluted silver paste (CC-01) and a mesh-like current collector using concentrated silver paste (CC-02). Electrochemical impedance spectra are applied for stability investigations. With the CC-01 current collector, the performance of the electrode deteriorates significantly, although the initial performance is good. By contrast, fairly stable performance is obtained from symmetric cells with either BSCF or SSC+SDC electrodes using the CC-02 current collector, even though a phase transition is observed for BSCF. For instance, after approximately 800h of continuous stability testing, the area-specific resistance of the BSCF electrode retains a value of approximately 0.065Ωcm2, except for a slight fluctuation within the range of 0.06–0.07Ωcm2. These findings reveal that both BSCF and SSC can be stably operated for ORR under symmetric cell conditions; however, an appropriate current collection method is crucial to achieving stable performance.

Characterization of Ba0.5Sr0.5M1−xFexO3−δ (M = Co and Cu) perovskite oxide cathode materials for intermediate temperature solid oxide fuel cells

Ba0.5Sr0.5Co1−xFexO3−δ (x=0.2, 0.6, and 0.8) and Ba0.5Sr0.5Cu1−xFexO3−δ (x=0.6 and 0.8) perovskite oxides have been investigated as cathode materials for intermediate temperature solid oxide fuel cells. All the samples synthesized by a citrate–EDTA complexing method were single-phase cubic perovskite solid solutions. Then, the thermal expansion coefficient, electrical conductivities, the oxygen vacancy concentrations, the polarization resistances (Rp), and the power densities were measured. An increase in the Co content resulted in a decrease in the polarization resistance, the electrical conductivities at low temperatures, and the inflection point of the thermal expansion coefficient, but it led to an increase in the electrical conductivities at high temperatures, the oxygen vacancy concentrations, and the maximum power densities. The Cu-based system has similar behavior to the Co-based system; yet, in terms of the electrical conductivities, high Cu content gave a better result than low content for the entire range of temperatures.

Perovskites in Solid Oxide Fuel Cells

Perovskite oxides comprise large families among the structures of oxide compounds, and several perovskite-related structures are also known. Because of their diversity in chemical composition, properties and high chemical stability, perovskite oxides are widely used for preparing solid oxide fuel cell (SOFC) components. In this work a few examples of perovskite cathode and anode materials and their necessary modifications were shortly reviewed. In particular, nickel-substituted lanthanum ferrite and iron-substituted strontium titanate as cathode materials as well as niobium-doped strontium titanate, as anode material, are described. Electrodes based on the modified perovskite oxides are very promising SOFC components.

Structural changes and voltage output in defect perovskite cathode materials

Structural changes and voltage output in defect perovskite cathode materials

Preparation and Evaluation of Ba<sub>x</sub>Sr<sub>(x-1)</sub>Co<sub>y</sub>Fe<sub>(y-1)</sub>O<sub>3−δ</sub> Nanomaterial for SOFC Cathode by Oxalate Co-Precipitation (x=0.2, y=0.8)

A recently reported promising new perovskite oxide cathode material, Ba0.2Sr0.8Co0.8Fe0.2O3−δ, (BSCF) (with x = 0.2 and y = 0.8) of high purity for intermediate-temperature solid oxide fuel cells (IT-SOFCs) was synthesised in the current work by using the co-precipitation method. The result indicated a precursor with a well-defined composition of fine particle size, high homogeneity, and high reactivity. After calcining has been developed at 900°C, the individual oxides from ammonium oxalate were alloyed into nanostructured perovskite (with x = 0.2 and y = 0.8) Ba0.2Sr0.8Co0.8Fe0.2O3 of high purity. The thermal properties, phase constituents, surface area and microstructure of the samples were characterised by TGA, XRD, BET, SEM and EDX techniques respectively. The results show that the BSCF powders have cubic perovskite-type structure with fine particle size, high surface area and high homogeneity. The current method employed is found to be very reliable for the synthesis of BSCF.

Solution Precursor Plasma Spray of Porous La1−xSrxMnO3 Perovskite Coatings for SOFC Cathode Application

The deposition of porous La1−xSrxMnO3 (LSM) perovskite cathode materials by conventional plasma spray has been a challenge because of the decomposition of perovskite materials to their suboxides at high temperature. In this paper, the solution precursor plasma spraying (SPPS) process, in which solution precursors of the desired resultant materials are fed into a direct current plasma jet by atomizing gas, was used to simultaneously synthesize LSM perovskite and deposit porous cathode coatings. The experimental results show that process parameters have a significant effect on the fabricated coatings. The perovskite coatings consist of porous agglomerates of small particles with rounded features and local denser regions referred to as thick flakes. The small particles and thick flakes were held together by the previously molten material. There are two kinds of pores in the fabricated coatings: large pores located between the agglomerates and fine pores inside the agglomerates. The porous LSM cathode coatings have 20–40 area % of desirable homogeneous pores determined by several processing parameters. X-ray diffraction of sintered coatings shows that no suboxides of La1−xSrxMnO3 perovskite appear. The results of this project indicate that the SPPS is a potential process to produce high quality cathodes for solid oxide fuel cell application.

Synthesis and properties of Ba 0.5Sr 0.5(Co 0.6Zr 0.2)Fe 0.2O 3− δ perovskite cathode material for intermediate temperature solid-oxide fuel cells

A highly stable perovskite cathode material, Ba 0.5Sr 0.5(Co 0.6Zr 0.2)Fe 0.2O 3− δ (BSCZF) for intermediate temperature solid-oxide fuel cells (IT-SOFCs) was synthesized via the improved EDTA–citric acid complexing technique combined with high-temperature sintering. The product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectra (EIS) measurements. An electrolyte-supported BSCZF/SDC/Ni-SDC fuel cell was fabricated to evaluate the performance of the material. The XRD study indicates that the sintering temperature higher than 950 °C is sufficient to the formation of clean single BSCZF perovskite phase. Due to the incorporation of Zr ions, BSCZF perovskite exhibit lower electrical conductivity with higher activation energy but higher structural stability than the Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ (BSCF) parent oxide. The maximum electrical conductivity of BSCZF attains 16.9 S cm −1 at around 540 °C. Impedance spectra showed that the ASRs of BSCZF cathode on samaria doped ceria (Ce 0.8Sm 0.2O 1.9, SDC) electrolyte are low but are still slightly larger than those of BSCF at similar conditions. The BSCZF/SDC/Ni-SDC cell exhibited a stable output with the maximum power densities of 30, 75, 139 and 241 mW cm −2 at 550, 600, 650 and 700 °C, respectively. Due to the high electrochemical performances as well as the excellent stability, BSCZF perovskite may be an attractive cathode material for IT-SOFCs.

Electrochemical Investigation of LSGMC-Composite Cathodes on LSGM-Substrate

As potential intermediate-temperature SOFC cathodes, perovskite composite cathode materials were evaluated in combination with a La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) electrolyte. Various cathode materials La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), and Pr0.3Sr0.7Co0.8Fe0.2O3-δ (PSCF) were compounded with La0.8Sr0.2Ga0.8Mg0.1Co0.1O3-δ (LSGMC) as a mixed conducting composite element. All powders were synthesized by the glycine-nitrate process (GNP). XRD analysis found resistive phases of LaSrGaO4 and LaSrGa3O7 present in the calcined layer of composite cathodes on LSGM. A bulk ohmic resistance of 3.81 Ω•cm2 and an interfacial resistance value of 0.16 Ω were measured for LSCF/LSGMC composite cathode on LSGM substrate at 800ºC in air. Low-frequency secondary arcs in the impedance spectra of LSCF/LSGMC cathode coupled with low current density and low cathode porosity suggested possible mass transfer limitations. The LSCF/LSGMC composite electrode performed best at 800ºC (0.11 A/cm2 at 0.045 V overpotential) while the BSCF/LSGMC performed best at the lowest temperature investigated (0.1 A/cm2 at 700ºC and 0.04 V overpotential).

Development of lanthanum strontium manganite perovskite cathode materials of solid oxide fuel cells: a review

The high-temperature solid oxide fuel cell (SOFC) is the most efficient and environmentally friendly energy conversion technology to generate electricity from fuels such as hydrogen and natural gas as compared to the traditional thermal power generation plants. In the last 20–30 years, there has been significant progress in the materials development and stack technologies in SOFC. Among the electrode materials, lanthanum strontium manganite (LSM) perovskites, till today, are the most investigated and probably the most important electrode materials in SOFCs. The objective of this article is to review and update the development, understanding, and achievements of the LSM-based materials in SOFC. The structure, nonstoichiometry, defect model, and in particular the relation between the microstructure, their properties (electrical, thermal, mechanical, chemical, and interfacial), and electrochemical performance and performance stability are critically reviewed. Finally, challenges and prospects of LSM-based materials as cathodes for intermediate and low-temperature SOFCs are discussed.

Optimization on fabrication and performance of A-site-deficient La0.58Sr0.4Co0.2Fe0.8O3-δ cathode for SOFC

A-site-deficient perovskite cathode material La0.58Sr0.4Co0.2Fe0.8O3 − δ (L58SCF) is coated on the yttria-stabilized zirconia electrolyte by screen-printing technique. Several key fabrication parameters including selection of additives (binder and pore former), effect of coating thickness, sintering temperature and time on the microstructure, and electrochemical performance of cathode are investigated by scanning electron microscopy and electrochemical impedance spectroscopy. We study the microstructure and the electrochemical property of the cathode with different kinds of additives. Results show that the cathode possesses fine microstructure, enough porosity, and ideal electrochemical property when polyvinyl butyral serves as both binder and pore former in the cathode. The cathode with three screen-printing coats (thickness 28 ± 7 µm, weight 6.07 ± 0.72 mg cm−2) sintering at 1,000 °C for 2 h shows lower polarization resistance of 0.183 Ω cm2 at 800 °C. Based on the optimized parameters, the polarization resistances of the L58SCF–Ce0.8Gd0.2O1.9 – δ composite cathode display the Rp values of 0.067 Ω cm2 at 800 °C, 0.106 Ω cm2 at 750 °C, 0.225 Ω cm2 at 700 °C, and 0.550 Ω cm2 at 650 °C.

Nd 2 − xLa xNiO 4 + δ, a mixed ionic/electronic conductor with interstitial oxygen, as a cathode material

The performance of Nd 2 − x La x NiO 4 + δ as a cathode material for solid oxide fuel cell (SOFC) was evaluated in this paper. Substitution of smaller Nd 3+ ion for La 3+ in La 2NiO 4 + δ increased the retention of mobile interstitial oxygen defects (O i″ ions). The concentration of the O i″ ions in Nd 2NiO 4 + δ was stabilized at δ ≈ 0.12 at 800 °C in air after thermal cycling. As a mixed oxide-ion/electronic conductor (MIEC), Nd 2NiO 4 + δ gave a performance comparable to that of the best perovskite cathode material, SrCo 0.8Fe 0.2O 3 − δ (SCF), for the Sm-doped ceria (SDC) electrolyte-supported SOFC. However, the Nd 2NiO 4 + δ cathode exhibited a higher overpotential with La 0.8Sr 0.2Ga 0.83Mg 0.17O 2.815 (LSGM) as the electrolyte, presumably due to the interdiffusion of La 3+ and Nd 3+ ions across the electrolyte/cathode interface and subsequent formation of ion-blocking phases at the interface. La 1.2Nd 0.8NiO 4 + δ , on the other hand, provided La isoactivity across the LSGM/cathode interface and therefore an improved performance with the LSGM electrolyte. Since Nd 2 − x La x NiO 4 + δ contains neither Sr nor Co, as a cathode, it might be compatible with the electrolytes such as yttria-stabilized zirconia without the need of a buffer layer.

Synthesis and Characterization of La<SUB>0.8</SUB>Sr<SUB>0.2</SUB>MO<SUB>3–δ</SUB> (M = Mn, Fe, or Co) Cathode Materials by Induction Plasma Technology

In this work, nanopowders of perovskite cathode materials (La0.8Sr0.2MnO3−δ, La0.8Sr0.2FeO3−δ, and La0.8Sr0.2CoO3−δ), for use in solid oxide fuel cells (SOFC), were successfully synthesized, using induction plasma techniques. Their compositions, structures, morphology, particle size distributions, and BET specific surface areas were determined for comparison with their counterparts prepared by the Pechini method and by the glycine-nitrate combustion (GNC) technique. The particle sizes of the plasma-synthesized powders are mostly around 63 nm. These plasma-synthesized powders are generally globular, their BET specific surface areas being about 26 m2g−1, approximately twice those of powders prepared by the GNC and Pechini methods. These plasma-synthesized powders are readily reproducible and are not agglomerated. Their individual particle sizes and distributions are very independent of their composition.

Processing of Ce<sub>1-x</sub>Gd<sub>x</sub>O<sub>2-δ</sub> (GDC) Thin Films from Precursors for Application in Solid Oxide Fuel Cells

Extensive interfacial reactions are known to occur between Fe-Co based perovskite cathode materials and the standard solid oxide fuel cell (SOFC) yttria stabilized zirconia (YSZ) electrolyte. Thin films of gadolinia doped ceria (GDC) could be used as a diffusion barrier between the cathode and the electrolyte. The present work investigates spin coating thin diffusion reaction inhibiting films onto SOFC electrolytes. The chemical and structural evolution of ethylene glycol based precursor solution is studied by means of rheology, x-ray diffraction (XRD), high temperature XRD (HT-XRD), Fourier-transformed infrared spectroscopy (FTIR) and differential thermal analysis (DTA). The studies show that cerium formate is formed as an intermediate resin. Thin films, up to 500 nm thick, of gadolinia doped ceria (GDC) are successfully produced by multiple spin coating of polymerized ethylene glycol derived solutions on 200 1m thick YSZ tapes. The GDC and YSZ interfacial surface morphology and film thickness are studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). These films are shown to successfully prevent the creation of non-conducting reaction phases at the cathode-electrolyte interface by blocking interdiffusion.

Characterization of perovskite powders for cathode and oxygen membranes made by different synthesis routes

Strontium lanthanum manganite (LSM) and lanthanum ferrite (LSF) perovskite cathode and oxygen membrane materials were synthesized using different techniques: spray pyrolysis, a modified citrate route, oxalate and carbonate co-precipitations. The use of Ca, a cheaper substituent on the A-site, was explored along to the substitution of La by Pr. The differently sourced powders were characterized by TG/DTA, XRD, ICP, TEM, XPS, PSD and BET. The co-precipitation of La, Ca and Fe was also possible using the cyanide route. This complexation method allowed the precipitation of a crystalline phase as evidenced by XRD. Among all methods, the cyanide and carbonate co-precipitation allowed the lowest perovskite phase transformation for LSF and LCF, followed by the nitrate (i.e. ‘ spray pyrolysis’). These phase transformation differences affected much the particle size distribution and the surface areas of these materials, the carbonate and the cyanide routes giving rise to very fine powders in the nm range. XPS and TEM analyses indicated uneven composition distributions. These different powder characteristics are expected to affect the catalytic and electrochemical properties of these materials.

Evaluation of La2Ni1−xCoxO4±δ as a SOFC Cathode Material

Recent work has identified fast oxide ion conductivity in the K2NiF4 structured oxides. In particular, the La2NiO4+δ material has been found to possess diffusion coefficients comparable with the known perovskite cathode materials. Coupled with the reasonable electronic conductivity of ~ 100 Scm−1 at 650°C, these materials would appear to be promising candidates for new SOFC cathodes.Preliminary measurements of the La2NiO4+δ composition as a symmetrical cell on a CGO electrolyte indicated that the overall performance was below that expected from the Adler model. Subsequent work has been carried out by Bassat and co-workers investigating the Ln2NiO4+δ materials (Ln = La, Nd, Pr) on an YSZ electrolyte. The work reported here investigated La2Ni1−xCoxO4±δ cathodes on both CGO and LSGM electrolytes and compared the ASR data from a series of compositions. It has been found that the materials deposited on LSGM gave the best ASR data.

Polarisation Behaviour of Mixed Conducting Perovskite Cathode Materials.

Electrode materials with improved polarization characteristics are required for a reduction in the operating temperature of solid oxide fuel cells (SOFC) to be technologically feasible. Oxide cathode materials that possess a high degree of oxide ion conductivity as well as electronic conductivity are expected to exhibit lower values of electrode polarization than current cathode materials with negligible ionic conductivity. Some perovskite oxides based on the system La{sub 1{minus}x}Sr{sub x}Fe{sub 1{minus}y}Co{sub y}O{sub 3} have been shown to possess a significant amounts of ionic conductivity by {sup 18}O/{sup 16}O isotopic exchange and dynamic secondary ion mass spectrometry (SIMS) investigations. AC impedance spectroscopy has been used to examine the electrode polarization behavior of these oxides as a function of temperature and oxygen partial pressure. This behavior has been modelled and attributed to definite ionic charge and mass transfer mechanisms.

Power Enhancement of Sealed-off CO2 Lasers by Incorporating Catalysts

Various catalysts were tested and evaluated for enhancing the output power of sealed-off CO2 lasers by suppressing the CO2 dissociation in the discharge tube. They include (i) two kinds of distributed heterogenous catalysts, i. e. sputtered Pt layer and ion-exchanged Pt on MnO2 layer both on the internal surface of laser tube, (ii) homogeneous H2 catalyst generated from the equilibrium dissociation of Nb Hx and finally (iii) catalytic cathode made of La1-xSrx CO3 perovskite oxide.Among them, the La0.7 Sr0.3 CO3 cathode sintered at 1150°C acted satisfactorily producing the output power of 52W/m, which was almost equal to that of a gas flowing laser. The longest life of this laser was 4400h at the discharge current of 15mA. This excellent performance resulted from the simultaneous presence of metallic conductivity, catalytic activity, oxygen emissivity and low sputtering rate in the perovskite cathode material.