Preparation and properties of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ as novel oxygen electrode for reversible solid oxide electrochemical cell

In this work, double perovskite-type oxide PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) is synthesized by the conventional wet chemical method and firstly characterized as the oxygen electrode for reversible solid oxide electrochemical cells (RSOCs). The microstructure and electrochemical performance of RSOCs based on this oxygen electrode are investigated. The maximum power density of the cell reaches 986 mW/cm² at 800 °C and the cell has good stability in short-term test in fuel cell (SOFC) mode. In electrolysis cell (SOEC) mode, it displays an electrolysis current density as high as 1.3 A/cm² when the temperature, absolute humidity (AH) and applied voltage are 800 °C, 50 vol % and 1.3 V, respectively. The cells also exhibit excellent durability of 120 h in SOEC mode and present good reversibility. The results suggest that the RSOCs based on this oxygen electrode has a very promising prospect.     

Characterization of La0.5Sr0.5Co0.5Ti0.5O3−δ as symmetrical electrode material for intermediate-temperature solid-oxide fuel cells

La_0.5Sr_0.5Co_0.5Ti_0.5O_3−δ perovskite oxide has been prepared as polycrystalline powder, characterized and tested as cathode and anode material for solid-oxide fuel cells. The oxidized material is suggested to present mixed ionic-electronic conductivity (MIEC) from “in-situ” neutron powder diffraction (NPD) experiments, complemented with transport measurements; the presence of a sufficiently high oxygen deficiency, with large displacement factors for oxygen atoms suggest a large lability and mobility combined with a semiconductor-like behaviour with a maximum conductivity of 29 S cm⁻¹ at T = 850 °C. A complete reversibility towards reduction–oxidation processes has been observed, where the reduced Pm-3m perovskite with La_0.5Sr_0.5Co_0.5Ti_0.5O_2.64 composition has been obtained by topotactical oxygen removal without abrupt changes in the thermal expansion. The oxidized material shows good performance working as a cathode with LSGM electrolyte, yielding output power densities close to 500 mW/cm² at 850 °C. At intermediate temperatures (800 °C) it may be used as a cathode or as an anode, yielding power densities of 220 and 170 mW/cm², respectively. When used simultaneously as cathode and anode a maximum power density of 110 mW/cm² was obtained. Therefore, we propose the La_0.5Sr_0.5Co_0.5Ti_0.5O_3−δ composition as a promising candidate for symmetrical electrode in intermediate-temperature SOFC.     

Cathode reaction mechanism of porous-structured Sm0.5Sr0.5CoO3−δ and Sm0.5Sr0.5CoO3−δ/Sm0.2Ce0.8O1.9 for solid oxide fuel cells

The cathode reaction mechanism of porous Sm_0.5Sr_0.5CoO_3−δ, a mixed ionic and electronic conductor (MIEC), is studied through a comparison with the composite cathode Sm_0.5Sr_0.5CoO_3−δ/Sm_0.2Ce_0.8O_1.9. First, the cathodic behaviour of porous Sm_0.5Sr_0.5CoO_3−δ and Sm_0.5Sr_0.5CoO_3−δ/Sm_0.2Ce_0.8O_1.9 are observed for micro-structure and impedance spectra according to Sm_0.2Ce_0.8O_1.9 addition, thermal cycling and long-term properties. The cathode reaction mechanism is discussed in terms of frequency response, activation energy, reaction order and electrode resistance for different oxygen partial pressures p(O₂) at various temperatures. Three elementary steps are considered to be involved in the cathodic reaction: (i) oxygen ion transfer at the cathode-electrolyte interface; (ii) oxygen ion conduction in the bulk cathode; (iii) gas phase diffusion of oxygen. A reaction model based on the empirical equivalent circuit is introduced and analyzed using the impedance spectra. The electrode resistance at high frequency (R_c,HF) in the impedance spectra represents reaction steps (i), due to its fast reaction rate. The electrode resistance at high frequency is independent of p(O₂) at a constant temperature because the semicircle of R_c,HF in the complex plane of the impedance spectra is held constant for different values of p(O₂). Reaction steps (ii) and (iii) are the dominant processes for a MIEC cathode, according to the analysis results. The proposed cathode reaction model and results for a solid oxide fuel cell (SOFC) well describe a MIEC cathode with high ionic conductivity, and assist the understanding of the MIEC cathode reaction mechanism.     

Preparation and characterization of new cobalt-free cathode Pr0.5Sr0.5Fe0.8Cu0.2O3−δ for IT-SOFC

A new cobalt-free perovskite oxide Pr_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ (PSFC) has been synthesized and evaluated as cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The chemical compatibility of PSFC with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte has be proven by XRD, and its electrical conductivity reaches the maximum value of 264.1 S cm⁻¹ at 475 °C. Symmetrical cells with the configuration of PSFC/SDC/PSFC are used for the impedance study and the polarization resistance (Rp) of PSFC cathode is as low as 0.050 Ω cm² at 700 °C. Single cells, consisting of Ni–YSZ/YSZ/SDC/PSFC structure, are assembled and tested from 550 °C to 800 °C with wet hydrogen (∼3% H₂O) as fuel and static air as oxidant. A maximum power density of 1077 mW cm⁻² is obtained at 800 °C. All the results suggest that the cobalt-free perovskite oxide PSFC is a very promising cathode material for application in IT-SOFC.     

Synthesis and characterization of Ba0.5Sr0.5Co0.8Fe0.1Ni0.1O3-δ cathode for intermediate-temperature solid oxide fuel cells

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.1Ni_0.1O_3-δ (BSCFNi) oxide is synthesized and characterized as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The X-ray diffraction (XRD) spectra show that BSCFNi is chemical compatible with La_0.9Sr_0.1Ga_0.83Mg_0.17O_2.865(LSGM) electrolyte below 950 °C, but weak reaction is observed between BSCFNi cathode and Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte after calcined at 950 °C for 10 h. The XPS results indicate that transition metal cations in BSCFNi sample exist two different valence states, i.e., Co^4+/3+, Fe^4+/3+ and Ni^3+/2+. The average thermal expansion coefficient (TEC) of BSCFNi is 18.7 × 10^?6 K^?1 between 200 °C and 850 °C in air. The maximum electrical conductivity reaches 35.3 Scm^?1 at 425 °C in air. The polarization resistance of BSCFNi cathode on LSGM and SDC electrolytes are 0.033 and 0.066 Ωcm² at 800 °C, respectively. The maximum power density of LSGM electrolyte-supported single cell with BSCFNi cathode reaches 690 mWcm^?2 at 800 °C. These primarily results indicate that BSCFNi is a candidate cathode material for IT-SOFCs.     

Palladium and ceria infiltrated La0.8Sr0.2Co0.5Fe0.5O3−δ cathodes of solid oxide fuel cells

La_0.8Sr_0.2Co_0.5Fe_0.5O_3−δ (LSCF) cathodes infiltrated with electrocatalytically active Pd and (Gd,Ce)O₂ (GDC) nanoparticles are investigated as high performance cathodes for the O₂ reduction reaction in intermediate temperature solid oxide fuel cells (IT-SOFCs). Incorporation of nano-sized Pd and GDC particles significantly reduces the electrode area specific resistance (ASR) as compared to the pure LSCF cathode; ASR is 0.1 Ω cm² for the reaction on a LSCF cathode infiltrated with 1.2 mg cm⁻² Pd and 0.06 Ω cm² on a LSCF cathode infiltrated with 1.5 mg cm⁻² GDC at 750 °C, which are all significantly smaller than 0.22 Ω cm² obtained for the reaction on a conventional LSCF cathode. The activation energy of GDC- and Pd-impregnated LSCF cathodes is 157 and 176 kJ mol⁻¹, respectively. The GDC-infiltrated LSCF cathode has a lower activation energy and higher electrocatalytic activity for the O₂ reduction reaction, showing promising potential for applications in IT-SOFCs.     

Resisting coking and sulfur poisoning of double perovskite Sr2TiFe0.5Mo0.5O6–δ anode material for solid oxide fuel cells

Development of high performance anode materials with resisting coking and sulfur poisoning is significant for the practical application of solid oxide fuel cells (SOFCs). Here we report a nickel-free double perovskite anode material Sr₂TiFe_0.5Mo_0.5O_6–δ (STFM05), which exhibits excellent electrochemical performance using H₂ as fuel and acceptable electrochemical stability in syngas fuel containing H₂S. At 800 °C, the STFM05 anode shows the electrical conductivity value of 22.3 S cm^?1 in H₂ and the maximum power densities of 547 and 215 mWcm^?2 with electrolyte-supported cell using H₂ and syngas as fuels, respectively. The single cell with STFM05 anode shows a relatively stable electrochemical performance in syngas at 750 °C, and no carbon deposition is detected by Raman spectroscopy. In addition, STFM05 anode is thermally and chemically compatible with the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3?δ and Ce_0.8Sm_0.2O_1.9 electrolytes in reducing atmosphere. These preliminary results suggest that the STFM05 double perovskite is a promising candidate of anode material with coking resistance and sulfur tolerance for use in SOFCs.     

Synthesis,electrical and electrochemical properties of Ba0.5Sr0.5Zn0.2Fe0.8O3−δ perovskite oxide for IT-SOFC cathode

The Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF) complex oxide with cubic perovskite structure was synthesized and examined as a new cobalt-free cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The electrical conductivity was relatively low with a peak value of 9.4 S cm⁻¹ at about 590 °C, which was mainly caused by the high concentration of oxygen vacancy and the doping of bivalent zinc in B-sites. At 650 °C and under open circuit condition, symmetrical BSZF cathode on Sm-doped ceria (SDC) electrolyte showed polarization resistances (R_p) of 0.48 Ω cm² and 0.35 Ω cm² in air and oxygen, respectively. The dependence of R_p with oxygen partial pressure indicated that the rate-limiting step for oxygen reduction was oxygen adsorption/desorption kinetics. Using BSZF as the cathode, the wet hydrogen fueled Ni + SDC anode-supported single cell exhibited peak power densities of 392 mW cm⁻² and 626 mW cm⁻² at 650 °C when stationary air and oxygen flux were used as oxidants, respectively.     

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.     

Properties and performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ + Sm0.2Ce0.8O1.9 composite cathode

The properties and performance of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) + Sm_0.2Ce_0.8O_1.9 (SDC) (70:30 in weight ratio) composite cathode for intermediate-temperature solid-oxide fuel cells were investigated. Mechanical mixing of BSCF with SDC resulted in the adhesion of fine SDC particles to the surface of coarse BSCF grains. XRD, SEM-EDX and O₂-TPD results demonstrated that the phase reaction between BSCF and SDC was negligible, constricted only at the BSCF and SDC interface, and throughout the entire cathode with the formation of new (Ba,Sr,Sm,Ce)(Co,Fe)O_3−δ perovskite phase at a firing temperature of 900, 1000, and ≥ 1050 °C, respectively. The BSCF + SDC electrode sintered at 1000 °C showed an area specific resistance of ∼0.064 Ω cm² at 600 °C, which is a slight improvement over the BSCF (0.099 Ω cm²) owing to the enlarged cathode surface area contributed from the fine SDC particles. A peak power density of 1050 and ∼382 mW cm⁻² was reached at 600 and 500 °C, respectively, for a thin-film electrolyte cell with the BSCF + SDC cathode fired from 1000 °C.     

Effect of iron substitution content on structure, thermal expansion behavior and electrochemical properties of La0.5Ba0.5Co1−yFeyO3−δ

Cubic perovskite oxides La_0.5Ba_0.5Co_1−yFe_yO_3−δ (LBCF-y) with Fe content y = 0.1–0.7 were synthesized by sol-gel method and were studied with respect to their oxygen content, defect chemistry, thermal expansion behavior, and electrical and electrochemical properties. LBCF-y had increased oxygen content and cell volumes with higher Fe content. The chemical defects at B-sites of LBCF-y are Fe⁴⁺, Co⁴⁺ and Co³⁺ ions for y = 0.1–0.3, and Fe⁴⁺, Fe³⁺ and Co³⁺ ions for y ≥ 0.5 respectively. Thermal expansion coefficients of LBCF-y oxides firstly increased to a maximum at y = 0.3 then decreased gradually with bigger y. Conductivities of LBCF-y decreased with higher Fe content, and the maximum conductivity was 800 S cm⁻¹ at 500 K for LBCF-0.1 sample. Fe substitutions for Co in LBCF-y cathodes increase the high-frequency resistance associated with oxygen ionic diffusion process while hardly influence the low-frequency gas diffusion process. Very low area-specific resistances, <0.1 Ω cm² at 923 K, were obtained for LBCF-y (y = 0.1–0.7) oxides, demonstrating their potential applications as cathode materials for intermediate-temperature solid oxide fuel cells.     

Progress in understanding and development of Ba0.5Sr0.5Co0.8Fe0.2O3−δ-based cathodes for intermediate-temperature solid-oxide fuel cells: A review

Solid-oxide fuel cells (SOFCs) convert chemical energy directly into electric power in a highly efficient way. Lowering the operating temperature of SOFCs to around 500-800 °C is one of the main goals in current SOFC research. The associated benefits include reducing the difficulties associated with sealing and thermal degradation, allowing the use of low-cost metallic interconnectors and suppressing reactions between the cell components. However, the electrochemical activity of the cathode deteriorates dramatically with decreasing temperature for the typical La_0.8Sr_0.2MnO₃-based electrodes. The cathode becomes the limiting factor in determining the overall cell performance. Therefore, the development of new electrodes with high electrocatalytic activity for oxygen reduction becomes a critical issue for intermediate-temperature (IT)-SOFCs. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) perovskite oxide was first reported as a potential IT-SOFC cathode material in 2004 by Shao and Haile. After that, the BSCF cathode has attracted considerable attention. This paper reviews the current research activities on BSCF-based cathodes for IT-SOFCs. Emphasis will be placed on the understanding and optimization of BSCF-based materials. The issues raised by the BSCF cathode are also presented and analyzed to provide some guidelines in the search for the new generation of cathode materials for IT-SOFCs.     

Investigation of electrospun Ba0.5Sr0.5Co0.8Fe0.2O3−δ-Gd0.1Ce0.9O1.95 cathodes for enhanced interfacial adhesion

Fibrous Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ-Gd_0.1Ce_0.9O_1.95 (BSCF-GDC) composite cathodes are fabricated by a facile electrospinning method. However, the electropun BSCF-GDC cathode shows poor adhesion to a GDC electrolyte because of the high shrinkage rate of the electrospun BSCF-GDC cathode during sintering. To solve this adhesion issue, mixed BSCF fiber-GDC powder cathode is investigated. As a result, mixed BSCF fiber-GDC powder cathode with an enhanced adhesion is successfully fabricated. This improvement can be attributed to the modified microstructure with the GDC powder that joins the BSCF fibers to the GDC electrolyte at the cathode and electrolyte interface. The polarization resistance of the mixed BSCF fiber-GDC powder cathode is 0.10 Ω cm², which is lower than 0.13 Ω cm² of conventional BSCF-GDC powder cathode at 700 °C. It is attributable to the improved oxygen gas and lattice oxygen diffusion, and the surface exchange of the mixed BSCF fiber-GDC powder cathode. The single cell with a mixed BSCF fiber-GDC powder cathode show 500 mW cm^?2 at 700 °C, which is 25% higher than conventional BSCF-GDC powder cathode.     

Fabrication and characterization of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ—Gadolinia-doped ceria cathode for an anode-supported solid-oxide fuel cell

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and gadolinia-doped ceria (GDC) were synthesized via a glycine-nitrate process (GNP). A cubic perovskite of BSCF was observed by X-ray diffraction (XRD) at a calcination temperature above 950 °C. An anode-supported solid-oxide fuel cell was constructed from the porous NiO + YSZ as the anode substrate, the yittria-stabilized zirconia (YSZ) as the electrolyte, and the porous BSCF-GDC layer as the cathode with a GDC barrier layer. For the performance test, the maximum power density was 191.3 mW cm⁻² at a temperature of 750 °C with H₂ fuel and air at flow rates of 335 and 670 sccm, respectively. According to the AC-impedance data, the charge-transfer resistances of the electrodes were 0.10 and 1.59 Ω cm², and the oxygen-reduction and oxygen-ion diffusion resistances were 0.69 and 0.98 Ω cm² at 750 and 600 °C, respectively. SEM microstructural characterization indicated that the fuel cell as fabricated exhibited good compatibility between cathode and electrolyte layers.     

Sm0.5Sr0.5CoO3–Ce1.8Sm0.2O1.9 electrodes enhanced by Sm0.5Sr0.5CoO3 impregnation for proton conductor based solid oxide fuel cells

Sm_0.5Sr_0.5CoO_3−δ–Ce_0.8Sm_0.2O_2−δ (SSC–SDC) composites, which are often used as the cathodes for solid oxide fuel cells (SOFCs) with oxygen-ion conducting electrolytes, have been recently shown to be also applicable in SOFCs based on proton conductors such as BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY). The electrochemical performances of blank SSC–SDC electrodes on BZCY electrolytes are substantially improved in this work by impregnating SSC nanoparticles additionally. When the loading increases, the interfacial polarization resistance of the symmetric cell decreases gradually at first, notably when it exceeds 14 wt.%, and to the lowest value at about 22 wt.%. Furthermore, impregnating SSC reduces the low-frequency-arc resistance that corresponds to the surface exchange step. In addition, impregnating SSC reduces the activation energy for oxygen reduction from 1.14 to 0.70 eV, thus resulting in significantly improvement on electrode performance at the reduced temperatures for SOFCs based on proton conductors.     

Preparation and characterization of Ba0.5Sr0.5Fe0.9Ni0.1O3−δ–Sm0.2Ce0.8O1.9 compose cathode for proton-conducting solid oxide fuel cells

A cobalt-free Ba_0.5Sr_0.5Fe_0.9Ni_0.1O_3−δ–Sm_0.2Ce_0.8O_1.9 (BSFN–SDC) composite was employed as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs) using BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) as the electrolyte. The chemical compatibility between BSFN and SDC was evaluated. The XRD results showed that BSFN was chemically compatible with SDC after co-fired at 1100 °C for 5 h. The thermal expansion coefficient (TEC) of BSFN–SDC, which showed a reasonably reduced value (16.08 × 10⁻⁶ K⁻¹), was effectively decreased due to Ce_0.8Sm_0.2O_1.9 (SDC) added. A single cell of Ni–BZCY/Ni–BZCY/BZCY/BSFN–SDC with a 25-μm-thick BZCY electrolyte membrane exhibited excellent power densities as high as 361.8 mW cm⁻² at 700 °C with a low polarization resistance of 0.174 Ω cm². The excellent performance implied that the cobalt-free BSFN–SDC composite was a promising alternative cathode for H-SOFCs.     

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.     

Physico-chemical properties of Ln0.5A0.5Co0.5Fe0.5O3−δ (Ln: La, Sm; A: Sr, Ba) cathode materials and their performance in electrolyte-supported Intermediate Temperature Solid Oxide Fuel Cell

Oxides with perovskite structure and composition: La_0.5Sr_0.5Co_0.5Fe_0.5O_3−δ, La_0.5Ba_0.5Co_0.5Fe_0.5O_3−δ, Sm_0.5Sr_0.5Co_0.5Fe_0.5O_3−δ and Sm_0.5Ba_0.5Co_0.5Fe_0.5O_3−δ were synthesized by a sol-gel EDTA based method. Their physico-chemical properties were evaluated by structural (XRD), transport (electrical conductivity, Seebeck coefficient), and high temperature oxygen nonstoichiometry measurements (TG, δ). A distorted perovskite structure was observed for all of the samples, varying with A-site average radius of cations and tolerance factor t. TG measurements, which were performed in air and in reducing atmosphere allowed to determine the initial, as well as the high temperature dependence of the oxygen nonstoichiometry δ for all materials. At high temperatures the electrical conductivity of the measured samples showed a characteristic maximum and corresponding increase of the Seebeck coefficient. Both effects can be interpreted as a result of a formation of the oxygen vacancies. Apart from Sm_0.5Ba_0.5Co_0.5Fe_0.5O_3−δ composition, all other materials possess very high electrical conductivity at high temperatures, well exceeding 100 S cm⁻¹. A custom made IT-SOFC cells were constructed with Ce_0.85Gd_0.15O_1.925 sinters as a support. Their performance was evaluated in 600-800 °C range. Despite rather similar transport properties of La_0.5Sr_0.5Co_0.5Fe_0.5O_3−δ, La_0.5Ba_0.5Co_0.5Fe_0.5O_3−δ and Sm_0.5Sr_0.5Co_0.5Fe_0.5O_3−δ perovskites, the best electrochemical properties were recorded in case of the cell with La_0.5Sr_0.5Co_0.5Fe_0.5O_3−δ based cathode.     

Electrochemical properties of ceramic membranes based on SrTi0.5Fe0.5O3−δ in reduced atmosphere

The present work aims at the investigation of the electrochemical properties of SrTi_0.5Fe_0.5O_3−δ as a membrane material for hydrogen production via electrochemical reforming. The dependence of the electrical conductivity on the oxygen partial pressure, as well as the oxygen permeability in the range of 10⁻²⁰ atm ≤ pO₂$p_{{\text{O}}_2}$ ≤ 10⁻¹⁴ atm is examined. The oxygen permeability is measured by an electrochemical method. The dependences of ion current as a function of the electromotive force (EMF) at various temperatures, oxygen partial pressures and the membrane surface conditions (rough and activated by PrO_x) are studied. Finally, the values of hydrogen flux at different temperatures are calculated and a long term investigation during 600 h at pO₂$p_{{\text{O}}_2}$ = 10⁻¹⁹ atm, T = 1173 K is carried out. According to the present results, the permeation current increases with the increase of temperature, oxygen partial pressure gradient and activation by PrO_x. The long term investigation shows that the electrical resistance of the SrTi_0.5Fe_0.5O_3−δ ceramic membrane increases by 10%, possibly due to the formation of micro-domains into the material's volume and the decrease in the grain boundary conductivity, because of the segregation of dopant-rich layers near the grain boundaries.     

Electrode performance and analysis of reversible solid oxide fuel cells with proton conducting electrolyte of BaCe0.5Zr0.3Y0.2O3−δ

Reversible solid oxide fuel cells (R-SOFCs) are regarded as a promising solution to the discontinuity in electric energy, since they can generate electric powder as solid oxide fuel cells (SOFCs) at the time of electricity shortage, and store the electrical power as solid oxide electrolysis cells (SOECs) at the time of electricity over-plus. In this work, R-SOFCs with thin proton conducting electrolyte films of BaCe_0.5Zr_0.3Y_0.2O_3−δ were fabricated and their electro-performance was characterized with various reacting atmospheres. At 700 °C, the charging current (in SOFC mode) is 251 mA cm⁻² at 0.7 V, and the electrolysis current densities (in SOEC mode) reaches −830 mA cm⁻² at 1.5 V with 50% H₂O-air and H₂ as reacting gases, respectively. Their electrode performance was investigated by impedance spectra in discharging mode (SOFC mode), electrolysis mode (SOEC mode) and open circuit mode (OCV mode). The results show that impedance spectra have different shapes in all the three modes, implying different rate-limiting steps. In SOFC mode, the high frequency resistance (R_H) is 0.07 Ωcm² and low frequency resistances (R_L) are 0.37 Ωcm². While in SOEC mode, R_H is 0.15 Ωcm², twice of that in SOFC mode, and R_L is only 0.07 Ωcm², about 19% of that in SOFC mode. Moreover, the spectra under OCV conditions seems like a combination of those in SOEC mode and SOFC mode, since that R_H in OCV mode is about 0.13 Ωcm², close to R_H in SOEC mode, while R_L in OCV mode is 0.39 Ωcm², close to R_L in SOFC mode. The elementary steps for SOEC with proton conducting electrolyte were proposed to account for this phenomenon.