Physical and electrochemical properties of (La0.3Sr0.7)0.93TiO3–δ synthesized by Pechini method as an anode material for solid oxide fuel cells

The lanthanum strontium titanate (LST) has to be calcined at significantly high temperature (above 1,300 °C) to obtain its pure perovskite structure when synthesized by conventional solid-state method, which is main reason for reducing active surface area. In this study, A-site deficient (La_0.3Sr_0.7)_0.93TiO₃ was synthesized by Pechini method. Although the prepared powders were calcined at 600 °C, the pure perovskite structure can be obtained without any secondary phase such as TiO₂. Moreover, the porosity and surface area are 6 times and one order of magnitude higher in the LST powders synthesized by Pechini method than in the powders synthesized by solid-state method. Based on these results, the LST electrode (Pechini) leads to two times lower electrode resistance than the LST electrode (solid-state). Thus, the LST powders synthesized by Pechini can contributes to saving the energy needed for calcination process as well as increasing the porosity and active surface area, enhancing physical and electrochemical properties in SOFC anode.     

Cobalt-free cathode material SrFe0.9Nb0.1O3−δ for intermediate-temperature solid oxide fuel cells

A cobalt-free cubic perovskite oxide, SrFe_0.9Nb_0.1O_3?δ (SFN) was investigated as a cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). XRD results showed that SFN cathode was chemically compatible with the electrolyte Sm_0.2Ce_0.8O_1.9 (SDC) for temperatures up to 1050 °C. The electrical conductivity of SFN sample reached 34–70 S cm^?1 in the commonly operated temperatures of IT-SOFCs (600–800 °C). The area specific resistance was 0.138 Ω cm² for SFN cathode on SDC electrolyte at 750 °C. A maximum power density of 407 mW cm^?2 was obtained at 800 °C for single-cell with 300 μm thick SDC electrolyte and SFN cathode.     

Preparation and electrochemical properties of Ba0.8La0.2FeO3-δ cathode for intermediate-temperature solid oxide fuel cells

Journal of Sol-Gel Science and Technology - Perovskite Ba0.8La0.2FeO3-δ was synthesized via a glycine–nitrate process as the cathode material for intermediate-temperature solid oxide...     

The physical properties and electrochemical performance of Ca-doped Sr2MgMoO6-δ as perspective anode for solid oxide fuel cells

Journal of Solid State Electrochemistry - The double perovskites Sr2-xCaxMgMoO6–δ with x = 0, 0.25, and 0.5 were obtained by combustion of organometallic precursors and were...     

Novel BaCo0.7Fe0.3−yNbyO3−δ (y = 0–0.12) as a cathode for intermediate temperature solid oxide fuel cell

The BaCo_0.7Fe_0.3?yNb_yO_3?δ oxides (BCFNy, y = 0.00–0.12) were synthesized by the conventional solid state reaction process and investigated as a novel cathode for intermediate temperature solid oxide fuel cells(IT-SOFCs). Cubic perovskite, with enhanced phase stability at higher Nb concentration, was obtained at y ? 0.04. The unit cell volumes increased with y, reached a maximum at y = 0.10, and then decreased. The niobium doping concentration also had a significant effect on the electrochemical performance of BCFNy materials. Among the various BCFNy oxides tested, BCFN0.10 possessed the smallest interfacial polarization resistance (R_p). The R_p was as low as 0.9406, 0.1300, 0.0211, and 0.0082 Ω cm² at 500, 600, 700, and 800 °C, respectively. With a 220 μm-thick Sm_0.2Ce_0.1O_1.9 (SDC) as electrolyte and BCFN0.10 as the cathode, a fuel cell provides maximum power densities of 202, 350, 569, 820, and 1006 mW cm^?2 at 600, 650, 700, 750, and 800 °C, respectively. The encouraging results suggested that BCFN0.10 was a very promising cathode material for IT-SOFCs.     

Influence of cerium oxide on properties of glass–ceramic sealants for solid oxide fuel cells

The influence of the cerium oxide concentration on the properties of glasses and glass ceramics of the SiO₂–Al₂O₃–CaO–Na₂O–MgO–K₂O–B₂O₃–CeO₂ system as potential adhesive and sealing materials for solid oxide fuel cells was studied. According to the data of differential scanning calorimetry, variation of the CeO₂ concentration does not appreciably influence the glass transition and crystallization temperatures of glasses. As the cerium oxide concentration is increased, the linear thermal expansion coefficient increases for the glasses but decreases for the partially crystalline samples. The gluing temperature of the glass sealants prepared allows their use for joining YSZ solid electrolytes with interconnectors of Crofer22APU type in solid oxide fuel cells..     

Modeling current–voltage relationships of mixed conducting cathode materials for solid oxide fuel cells

In recent decades, high-temperature oxygen reduction reaction on mixed conducting cathodes were investigated intensively by many researchers. Computational approaches as well as electrochemical and spectroscopic studies have been made to elucidate the kinetics. Contribution of oxygen vacancy to the reaction rate was suggested in multiple reports, and plausible reaction pathways were proposed based on density functional theory (DFT) calculations. The picture of oxygen reduction reaction has become clearer in these years. However, there still is a discussion about a credible formula that represents the current–voltage relationships. Discrepancies are found among the reported data on the magnitude of the rate constant and on its dependencies on partial pressure and temperature. The difference is significant between a model electrode and a practical porous electrode. Comparison of the results suggests the existence of series reaction barriers, that is, the surface reaction and subsurface transport, which should be considered for consistent representation of the total electrode process.     

Functional properties and electrochemical performance of Ca-doped Sr2-xCaxFe1.5Mo0.5O6-δ as anode for solid oxide fuel cells

Journal of Solid State Electrochemistry - Performance of Ca-doped strontium ferrite-molybdate Sr2-xCaxFe1.5Mo0.5O6-δ (x = 0, 0.05, 0.15, 0.3, 0.5) has been studied under reducing...     

Investigation of Pr2NiMnO6‐Ce0.9Gd0.1O1.95 composite cathode for intermediate-temperature solid oxide fuel cells

Journal of Solid State Electrochemistry - The electrochemical performance of Pr2NiMnO6 (PNMO)-xCe0.9Gd0.1O1.95 (CGO) (x = 0–40 wt%) composite oxides as...     

Study of Nd2–x Ce x CuO4?±?δ (x?=?0.1–0.25) as cathode material for intermediate-temperature solid oxide fuel cells

The solid solubility limit of Ce in Nd_2–x Ce _x CuO_4 ± δ , prepared by sol–gel process, is established up to x = 0.2. The transition from negative temperature coefficient to positive temperature coefficient, within the solid solubility region, is observed at 620 °C. The area-specific-resistance (ASR) is optimized for electrochemical cell sintered at 800 °C. ASR enhances with increase in sintering temperature of cell. ASR value of 0.93 ohm cm² at 700 °C, determined by electrochemical impedance spectroscopy is comparable against that by voltage versus current (V–I) characteristics at 0.98 ohm cm² at the same temperature. Electrochemical performance and ASR of Nd_1.8Ce_0.2CuO_4 ± δ is improved when prepared by sol–gel route over solid-state reaction, which is attributed to uniform size and shape of nanocrystalline grains.     

Transport and electrocatalytic properties of La0.3Sr0.7Co0.8Ga0.2O3−δ membranes

Incorporation of gallium into the perovskite lattice of La_0.3Sr_0.7CoO_3– leads to increasing unit cell volume and to decreasing thermal expansion, total conductivity and oxygen permeability. At 973–1223 K, the oxygen permeation fluxes through La_0.3Sr_0.7Co_0.8Ga_0.2O_3– ceramics with 96.5% density are determined by the bulk ionic conduction and surface exchange rates. The total conductivity of La_0.3Sr_0.7Co_0.8Ga_0.2O_3–, predominantly p-type electronic, exhibits an apparent pseudometallic behavior due to oxygen losses on heating, whereas the p(O₂) dependencies of the conductivity and Seebeck coefficient suggest a small-polaron mechanism of hole transport. The average thermal expansion coefficients in air are 15.9×10^–6 K^–1 at 360–710 K and 27.9×10^–6 K^–1 at 710–1030 K. On decreasing oxygen pressure down to 4–30 Pa at 973–1223 K, perovskite-type La_0.3Sr_0.7Co_0.8Ga_0.2O_3– transforms into a brownmillerite-like modification, whose electrical properties are essentially p(O₂) independent. Further reduction results in the decomposition of the brownmillerite into a multiphase oxide mixture at p(O₂)=8×10^–10–3×10^–4 Pa, and then in the segregation of metallic cobalt. Due to surface-limited oxygen transport, La_0.3Sr_0.7Co_0.8Ga_0.2O_3– membranes are, however, kinetically stable under an air/CH₄ gradient up to 1223 K. The conversion of dry methane in model membrane reactors increases with oxygen permeation flux and temperature, but yields high CO₂ concentrations (>90%), indicating a dominant role of complete CH₄ oxidation on the membrane surface.     

Evaluation of manganese and oxygen content in La0.7Sr0.3MnO3−δ and correlation with the thermodynamic data

In order to understand how the thermodynamic properties are related to the oxygen and manganese content in the Sr-doped lanthanum manganites, nonstoichiometric perovskite phases La_0.7Sr_0.3MnO_3−δ have been investigated by using solid state electrochemical techniques, as well as wet chemical methods. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with solid state electromotive force measurements (EMF). The results were correlated with the average Mn valence values as determined by a chemical method based on two independent iodometric titrations, with amperometric dead-stop end point detection. New features related to the relationship between the average Mn valence, the oxygen nonstoichiometry variation, and the thermodynamic behavior were evidenced.     

Improvement of the stability of NiO–YSZ anode material for solid oxide fuel cell

Improvement of long-term stability of 40vol.%NiO–60vol.% yttria-stabilized zirconia (YSZ) anode material in reducing atmosphere and under exposure to thermal shock through the modification of vacancy concentration and pore shape has been investigated for a solid oxide fuel cell. We varied the amount of Y₂O₃ additives from 8 to 10 mol% in YSZ and the type of carbon pore former, from plated activated carbon to spherical carbon black, to improve the strength and the stability of porous NiO–YSZ anode materials. Modifications by varying the amount of Y₂O₃ additives and carbon pore former result in a highly stable anode, even upon exposure to a reducing atmosphere for 1,200 h. In particular, the strengths of the new anode materials are markedly improved at the same porosity level. Higher strengths do not degrade during a longtime durability test in a reducing atmosphere or upon thermal shock testing. The relatively smaller degradation of electrical conductivity of the new anode material is discussed in terms of the possibility of suppression of the disconnectivity of Ni phases during operation of a solid oxide fuel cell.     

Novel SrSc0.2Co0.8O3−δ as a cathode material for low temperature solid-oxide fuel cell

The SrSc_0.2Co_0.8O_3−δ (SSC) perovskite was investigated as a cathode material for low temperature solid-oxide fuel cell. The material showed an almost linear thermal expansion from room temperature to 1000 °C in air with the average thermal expansion coefficient of only 16.9 × 10⁻⁶ K⁻¹. The Sc-doping made the absence of Co⁴⁺ in SSC, which resulted in not only dramatically reduced thermal expansion coefficient but also extremely high oxygen vacancies concentrations in the lattice at low temperature. The area specific polarization resistance was 0.206 Ω cm² for SSC at 550 °C, which is about 52% lower than the value of a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ-based cathode. A peak power density as high as 564 mW cm⁻² was obtained at 500 °C based on a 20 μm thick Sm_0.2Ce_0.8O_1.9 electrolyte by adopting SSC cathode.     

Surface modification of La0.3Sr0.7CoO3−δ ceramic membranes

The dependence of oxygen permeability of dense La_0.3Sr_0.7CoO_3−δ ceramics on membrane thickness indicates significant surface exchange limitations to the permeation fluxes, which suggests a possibility to increase membrane performance by surface activation. The cobaltite membranes with various porous layers applied onto the permeate-side surface were tested at 850–1120 K. Silver-modified La_0.3Sr_0.7CoO_3−δ membranes showed enhanced permeation at temperatures above 950 K; deposition of porous layers of PrO_x and Pr_0.7Sr_0.3CoO_3−δ had no positive effect. The maximum oxygen permeability at 850–1120 K was observed in the case of porous La_0.3Sr_0.7CoO_3−δ layers with surface density about 10 mg cm⁻². These results suggest that the surface exchange of lanthanum–strontium cobaltite membranes under an oxygen chemical potential gradient is limited by both oxygen sorption at the surface and ion diffusion through the surface oxide layers. Oxygen permeability of La_0.3Sr_0.7CoO_3−δ ceramics was found to increase with increasing grain size due to decreasing grain-boundary resistance to ionic transport.     

Modeling investigation of Sm0.5Sr0.5CoO3−δ–Ce0.8Sm0.2O2−δ cathode for proton-conducting ceramic fuel cell

Journal of Solid State Electrochemistry - Simulation of proton ceramic fuel cell is of great importance in understanding their working mechanisms. Cathode activation polarization is a critical...     

A mixed-conducting BaPr0.8In0.2O3 − δ cathode for proton-conducting solid oxide fuel cells

A mixed ionic and electronic conductor, BaPr_0.8In_0.2O_3 − δ (BPI), was synthesized and examined as a cathode material for proton-conducting solid oxide fuel cells (H-SOFCs). X-ray diffraction analysis revealed that BPI had a perovskite structure and showed satisfactory tolerance to CO₂ and H₂O and good chemical compatibility with BaZr_0.1Ce_0.7Y_0.1 Yb_0.1O_3 − δ (BZCYYb) electrolyte. Test cells with a single-phase BPI cathode exhibited excellent electrochemical performances, demonstrating a peak power density of ~ 688 mW cm^− 2 at 750 °C. Furthermore, the cells with a BPI cathode showed very stable power output at a cell voltage of 0.7 V at 600 °C over 100 h, suggesting that BPI is a promising alternative cathode for H-SOFCs.     

A cobalt-free electrode material La0.5Sr0.5Fe0.8Cu0.2O3 − δ for symmetrical solid oxide fuel cells

Cobalt-free perovskite oxide La_0.5Sr_0.5Fe_0.8Cu_0.2O_3 − δ (LSFC) was applied as both anode and cathode for symmetrical solid oxide fuel cells (SSOFCs). The LSFC shows a reversible transition between a cubic perovskite phase in air and a mixture of SrFeLaO₄, a K₂NiF₄-type layered perovskite oxide, metallic Cu and LaFeO₃ in reducing atmosphere at elevated temperature. The average thermal expansion coefficient of LSFC in air is 17.7 × 10^− 6 K^− 1 at 25 °C to 900 °C. By adopting LSFC as initial electrodes to fabricate electrolyte supported SSOFCs, the cells generate maximum power output of 1054, 795 and 577 mW cm^− 2 with humidified H₂ fuel (~ 3% H₂O) and 895, 721 and 482 mW cm^− 2 with humidified syngas fuel (H₂:CO = 1:1) at 900, 850 and 800 °C, respectively. Moreover, the cell with humidified H₂ fuel demonstrates a reasonable stability at 800 °C under 0.7 V for 100 h.     

An insight into the electrocatalytic properties of porous La0.3Sr0.7Fe0.7Cr0.3O3−δ electrodes towards oxygen reduction reaction

Journal of Solid State Electrochemistry - The electrocatalytic properties of porous La0.3Sr0.7Fe0.7Cr0.3O3−δ electrodes towards oxygen reduction reaction were investigated as a function...