Characterizations of composite cathodes with La0.6Sr0.4Co0.2Fe0.8O3?δ and Ce0.9Gd0.1O1.95 for solid oxide fuel cells

Composite cathodes with La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) and Ce_0.9Gd_0.1O_1.95 (GDC) are investigated to assess for solid oxide fuel cell (SOFC) applications at relatively low operating temperatures (650–800 °C). LSCF with a high surface area of 55 m²g⁻¹ is synthesized via a complex method involving inorganic nano-dispersants. The fuel cell performances of anode-supported SOFCs are characterized as a function of compositions of GDC with a surface area of 5 m²g⁻¹. The SOFCs consist of the following: LSCF-GDC composites as a cathode, GDC as an interlayer, yttrium stabilized zirconia (YSZ) as an electrolyte, Ni-YSZ (50: 50 wt%) as an anode functional layer, and Ni-YSZ (50: 50 wt%) for support. The cathodes are prepared for 6LSCF-4GDC (60: 40 wt%), 5LSCF-5GDC (50: 50 wt%), and 4LSCF-6GDC (40: 60 wt%). The 5LSCF-5GDC cathode shows 1.29 Wcm⁻², 0.97 Wcm⁻², and 0.47 Wcm⁻² at 780 °C, 730 °C, and 680 °C, respectively. The 6LSCF-4GDC shows 0.92 Wcm⁻², 0.71 Wcm⁻², and 0.54 Wcm⁻² at 780 °C, 730 °C, and 680 °C, respectively. At 780 °C, the highest fuel cell performance is achieved by the 5LSCF-5GDC, while at 680 °C the 6LSCF-4GDC shows the highest performance. The best composition of the porous composite cathodes with LSCF (55 m²g⁻¹) and GDC (5 m²g⁻¹) needs to be considered with a function of temperature.     

Highly efficient La0.8Sr0.2MnO3-δ - Ce0.9Gd0.1O1.95 nanocomposite cathodes for solid oxide fuel cells

La_0.8Sr_0.2MnO_3-δ-Ce_0.9Gd_0.1O_1.95 (LSM-CGO) nanostructured cathodes are successfully prepared in a single process by a chemical spray-pyrolysis deposition method. The cathode is composed of nanometric particles of approximately 15 nm of diameter, providing high triple-phase boundary sites for the oxygen reduction reactions. A low polarization resistance of 0.046 Ω cm² is obtained at 700 °C, which is comparable to the most efficient cobaltite-based perovskite cathodes. A NiO-YSZ anode supported fuel cell with the nanostructured cathode generates a power output of 1.4 W cm^?2 at 800 °C, significantly higher than 0.75 W cm^?2 for a cell with conventional LSM-CGO cathode. The results suggest that this is a promising strategy to achieve high efficiency electrodes for Solid Oxide Fuel Cells in a single preparation step, simplifying notably the fabrication process compared to traditional methods.     

Electrochemical characteristics of electrospun La0.6Sr0.4Co0.2Fe0.8O3−δ-Gd0.1Ce0.9O1.95 cathode

The poor activity of cathode materials for electrochemical reduction of oxygen in intermediate and low temperature regime (<700 °C) is a key obstacle to reduced-temperature operation of solid oxide fuel cells (SOFCs). In our previous work, the direct methane fuel cell exhibits approximately 1 W cm⁻² at 650 °C in hydrogen atmosphere without any functional layers when the electrospun LSCF–GDC cathode was applied into the La₂Sn₂O₇–Ni–GDC anode-supported cell, which is approximately two times higher performance than 0.45 W cm⁻² of the cell with the conventional LSCF–GDC cathode. For detailed analysis of the fibrous cathode, the symmetrical cells with the electrospun and conventional LSCF–GDC cathode are fabricated, and then their electrochemical characteristics are measured by using electrochemical impedance spectroscopy (EIS). Each resistance contribution is determined by equivalent circuit consisting of a series resistance (R_s) and three arcs to describe the polarization resistance of the cathode. Total polarization resistance of the electrospun LSCF–GDC cathode is approximately two times lower than that of the conventional LSCF–GDC cathode at 650 °C, which is attributed to fibrous microstructures and large amount of pores in 100–200 nm. The results correspond to the difference in the cell performances obtained from our previous work.     

Structure and thermal properties of La0.6Sr0.4Co0.2Fe0.8O3−δ-SDC carbonate composite cathodes for intermediate- to low-temperature solid oxide fuel cells

The structure and thermal properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ-SDC carbonate (LSCF-SDC carbonate) composite cathodes were investigated with respect to the calcination temperatures and the weight content of the samarium-doped ceria (SDC) carbonate electrolyte. The composite cathode powder has been prepared from La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ and SDC carbonate powders using the high-energy ball milling technique in air at room temperature. Different powder mixtures at 30 wt%, 40 wt% and 50 wt% of SDC carbonate were calcined at 750-900 °C. The findings indicated that the structure and thermal properties of the composite cathodes were responsive to the calcination temperature and the content of SDC carbonate. The absence of any new phases as confirmed via XRD analysis demonstrated the excellent compatibility between the cathode and electrolyte materials. The particle size of the composite cathode powder was ∼0.3-0.9 μm having a surface area of 4-15 m² g⁻¹. SEM investigation revealed the presence of large particles in the resultant powders resulting from the increased calcination temperature. The composite cathode containing 50 wt% SDC carbonate was found to exhibit the best thermal expansion compatibility with the electrolyte.     

PrBa0.5Sr0.5Co2O5+δ layered perovskite cathode for intermediate temperature solid oxide fuel cells

Layered perovskite oxides have ordered A-cations localizing oxygen vacancies, and may potentially improve oxygen ion diffusivity and surface exchange coefficient. The A-site-ordered layered perovskite PrBa_0.5Sr_0.5Co₂O_5+δ (PBSC) was evaluated as new cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The material was characterized using electrochemical impedance spectroscopy in a symmetrical cell system (PBSC/Ce_0.9Sm_0.1O_1.9 (SDC)/PBSC), exhibiting excellent performance in the intermediate temperature range of 500-700 °C. An area-specific-resistance (ASR) of 0.23 Ω cm² was achieved at 650 °C for cathode polarization. The low activation energy (Ea) 124 kJ mol⁻¹ is comparable to that of La_0.8Sr_0.2CoO_3−δ. A laboratory-scaled SDC-based tri-layer cell of Ni-SDC/SDC/PBSC was tested in intermediate temperature conditions of 550 to 700 °C. A maximum power density of 1045 mW cm⁻² was achieved at 700 °C. The interfacial polarization resistance is as low as 0.285, 0.145, 0.09 and 0.05 Ω cm² at 550, 600, 650 and 700 °C, respectively. Layered perovskite PBSC shows promising performance as cathode material for IT-SOFCs.     

LaBaCuFeO5+δ-Ce0.8Sm0.2O1.9 as composite cathode for solid oxide fuel cells

The performance of the LaBaCuFeO_5+δ-Ce_0.8Sm_0.2O_1.9 (LBCF-SDC) composite cathodes was studied in this paper. Electrical conductivity, thermal expansion and electrochemical properties were investigated by four probing DC technique, dilatometry, AC impedance and polarization techniques, respectively. The thermal expansion coefficients of the LBCF-SDC were between (16.3 and 13.4) × 10⁻⁶ K⁻¹ from 30 to 850 °C, which was lower value than LBCF (17.0 × 10⁻⁶ K⁻¹). AC Impedance spectroscopy measurements of LBCF-SDC/SDC/LBCF-SDC test cell were carried out. Polarization resistance values for the LBCF-SDC10 cathode was as low as 0.097 Ω cm² at 750 °C.     

Investigation of rhombohedral-cubic phase transition of La0.58Sr0.4Co0.2Fe0.8O3-δ using high temperature XRD

The phase transition of the La_0.58Sr_0.4Co_0.2Fe_0.8O_3-δ oxygen transport membrane material has been investigated by X-ray diffraction at elevated temperatures. It has a rhombohedral symmetry at room temperature that transforms to a cubic symmetry at operation relevant higher temperatures, which was investigated in detail in the current work on the basis of bar type and powder specimens and discussed in terms of correlations with mechanical behavior.     

Sintering and oxygen permeation studies of La0.6Sr0.4Co0.2Fe0.8O3−δ ceramic membranes with improved purity

High purity raw materials are used for synthesizing La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF6428) powders to reduce the effect of impurity phases on oxygen permeability of the corresponding membranes. The as-synthesized LSCF6428 powders require a sintering temperature above 1180 °C to achieve membrane density over 90%. Ball milling of the powders increases the membrane sintering. It also increases oxygen permeation flux from 0.37 to 0.43 ml cm⁻² min⁻¹ at 950 °C for the membranes sintered at 1100 °C. A decrease in oxygen permeation fluxes with the further increase in sintering temperature is observed for the membranes with ball-milled starting powders, accompanied by an obvious increase in grain size. It suggests, at low level of impurity phases, the grain boundaries facilitate the oxygen diffusion. The combination of ball milling of the starting powders and a sintering temperature of 1100 °C is optimal to achieve high oxygen permeability of LSCF6428 membranes with improved purity.     

Fabrication of ultrathin La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fibre membranes for oxygen permeation

An ultrathin La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) hollow fibre membrane for enhanced oxygen permeation flux was fabricated using a wet spinning/sintering method. The membrane exhibits a highly asymmetric structure comprising of a very thin dense outer layer supported by finger-like structures that are fully open on the inner surface. Oxygen permeation measurements were conducted using sweep gas as an operating mode. Effects of operating temperatures and flow rates of the sweep gas on the oxygen permeation fluxes were investigated in details. The highest oxygen permeation flux, i.e. 0.096 cm³/cm² s (5.77 cm³/cm² min) was obtained from the ultrathin hollow fibre membrane at 1323 K (1050 °C) and the sweep gas flow rate of 2.42 cm³/s. The results indicate that the oxygen permeation flux obtained is much higher (4.9-11.2 times) than that obtained from conventional LSCF hollow fibre membranes mainly due to the reduced thickness of the membrane as well as the porous surface on the permeate side. In addition, despite a very thin dense layer, the LSCF hollow fibre membrane possessed a reasonable mechanical strength (113.22 MPa).     

Electro-spinning Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3−δ nanofibers infiltrated with Gd0.2Ce0.8O1.9 nanoparticles as cathode for intermediate temperature solid oxide fuel cell

Pr_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3−δ (PSCFN) nanofibers and their corresponding Pr_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3−δ–Gd_0.2Ce_0.8O_1.9 (PSCFN–GDC) composites have been synthesized and applied as cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this paper, PSCFN nanofibers were obtained through electro-spinning and the following pyrolysis process. The resultant PSCFN nanofibers were infiltrated with GDC precursor to prepare nanofiber-structured PSCFN–GDC composite cathodes. The optimal PSCFN: GDC mass ratio of 1: 0.10 was identified to possess the lowest interfacial polarization resistances of 0.264, 0.155, 0.039 and 0.018 Ω cm² at 650, 700, 750 and 800 °C, respectively, lower than those of the PSCFN–GDC nanoparticle-structured composite cathode. The PSCFN–GDC (1: 0.10) shows an excellent stability of electrochemical activity under a current density of 200 mA cm⁻² for 100 h at 800 °C. All results proved that the nanofiber-structured PSCFN–GDC composite could act as a highly efficient cathode candidate for the IT-SOFCs.     

Electrochemical characterization of gradient Sm0.5Sr0.5CoO3−δ cathodes on Ce0.8Sm0.2O1.9 electrolytes for solid oxide fuel cells

This study investigates Sm_0.5Sr_0.5CoO_3−δ (SSC)-Ce_0.8Sm_0.2O_1.9 (SDC) composite cathodes with a gradual change in composition from electrolyte to the cathode in an attempt to discover a potential approach applicable to solid oxide fuel cells (SOFCs). The gradual change in composition from electrolyte to cathode shows the decline in charge transfer resistance (R₂) and gas phase diffusion resistance (R₃). Because the value of R₃ is always larger than R₂ and R₃ significantly dominates the total cathode polarization resistance (R_P) at temperatures within the range of 750-850 °C, i.e., in this temperature range, the rate-determining step is dominated by the diffusion or dissociative adsorption of oxygen. The functionally gradient cathode with a graded interface between cathode and electrolyte reveals both a higher exchange current density (i₀) and a lower activation energy for oxygen reduction reaction (ORR), which suggests that the ORR kinetics can be improved by using the configuration of a functionally gradient cathode.     

Characterization of La0.58Sr0.4Co0.2Fe0.8O3−δ–Ce0.8Gd0.2O2 composite cathode for intermediate temperature solid oxide fuel cells

La_0.58Sr_0.4Co_0.2Fe_0.8O_3?δ–Ce_0.8Gd_0.2O₂ (LSCF–GDC) composite cathodes with various weight ratios 90%, 70% and 50% of LSCF were prepared. Mechanical properties, thermal expansion properties and electrical properties were measured for potential applications in solid oxide fuel cells (SOFCs) with graded cathodes. LSCF and GDC as pure cathode and electrolyte materials were characterized as reference. The absence of new phases as confirmed by X-ray diffraction (XRD) analysis demonstrated the excellent compatibility between the cathode and electrolyte materials. Mechanical properties such as hardness and fracture toughness were measured by the micro-indentation technique, while hardness and elastic modulus were measured by the nano-indentation technique. Thermal expansion behavior was recorded by a dilatometer. Electrical conductivity was measured by the four probe DC method. The 50% LSCF–GDC composite has the lowest relative density among all the samples. Thermal expansion coefficients (TECs) and electrical conductivity increased with addition of LSCF contents in the composite, while mechanical properties depended more on the density than the LSCF content.     

Pr0.6Sr0.4CoO3−δ electrocatalyst for solid oxide fuel cell cathode introduced via infiltration

Effects of infiltrated Pr_0.6Sr_0.4CoO_3−δ (PSCo) electrocatalyst on SOFC cathode performance have been studied. Nano-sized particulate catalysts, deposited on surfaces of a composite cathode of Sm₂O₃ doped CeO₂ (SDC) and La_1−xSr_xCo_1−yFe_yO_3−δ (LSCF), are assumed to effectively widen active sites, or triple phase boundaries, for the oxygen reduction reaction. Area specific resistance of commercially available cells has been decreased by 36–40% with the addition of 23 wt% PSCo electrocatalyst on cathode. Analysis of the impedance spectra demonstrates that PSCo electrocatalyst plays a significant role in dissociation of oxygen molecules and adsorption of oxygen atoms into the cathode. A total of 200 h operation of the cells demonstrated that catalytic activity of PSCo has not been significantly degraded. Simultaneous operations of multiple cells using a parallel-cell testing system have made it possible to compare the performance of several cells with high reliability.     

固体氧化物燃料电池阴极材料La1.2Sr0.8Co0.8Ni0.2O4+δ的研究

以聚丙烯酸、柠檬酸和尿素三种复合配体作为络合剂,采用溶胶凝胶法制备成La1.2Sr0.8Co0.8Ni0.2O4+δ前驱物,经800℃煅烧获得阴极粉体。利用丝网印刷法将LSCN和GDC制备成复合阴极。利用XRD、TEM、ED、SEM、压汞仪和电化学工作等分别对LSCN粉体的物相、结构和电极的形貌、孔结构和电性能等进行了表征。研究结果表明,800℃煅烧获得了颗粒尺寸约为80nm、正交晶型结构的La1.2Sr0.8Co0.8Ni0.2O4+δ阴极粉体,单电池以含水蒸气的H2为燃料在750℃得到了稳定的电性能。     

Infiltrated La0.5Ba0.5CoO3-δ in La0.8Sr0.2Ga0.8Mg0.2O2.8 scaffolds as cathode material for IT-SOFC

The electrochemical response of infiltrated La_0.5Ba_0.5CoO_3-δ (LBC) in porous La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 (LSGM) has been investigated. The thermal expansion coefficient (TEC) of the resulting electrode was measured, obtaining α?=?12.5?×?10^?6 K^?1, a value similar to that of LSGM. The polarization resistance (Rp) and the processes involved in the oxygen reduction reaction (ORR) for the new electrode were studied and analyzed through complex impedance spectroscopy measurements as a function of temperature and oxygen partial pressure (pO₂), using a symmetrical cell. The value of Rp for the infiltrated LBC turned out to be lower than that measured for an electrode prepared with a composite LBC-LSGM (1:1?wt%) by an order of magnitude, for the temperature range 750?°C ≤ T?≤?900?°C, and about 5 times lower for the temperature range 450?°C≤ T?≤?650?°C. At 600?°C, the LBC infiltrated cathode exhibits a polarization resistance R_p =?0.22?Ω?cm², in air. The complex impedance spectra show two processes, one identified as low frequency (LF),with a characteristic frequency of 10?Hz, and the other as intermediate frequency (IF), with a range between 0.05 and 2000?Hz. The LF process could be associated to the diffusion of oxygen in the gas phase through the pores of the electrode. Its resistance, R_LF =?0.01?Ωc?m²_, was found to be independent of the temperature and half of that obtained for the LBC composite cathode. On the other hand, the IF process is related to charge transfer at the electrode surface and the electrode-electrolyte interface. The LBC cobaltite infiltrated in the LSGM scaffolds offers an adequate thermal expansion coefficient and good electrocatalytic activity for the ORR.     

Influence of small amounts of gallium oxide addition on ionic conductivity of La0.9Sr0.1Ga0.8Mg0.2O3-δ solid electrolyte

The effects of small amounts of gallium oxide on intragrain and intergrain conductivity of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ are investigated by impedance spectroscopy in the 280–420 °C range. Bulk specimens with 0.5, 1.0 and 1.5 mol% gallium oxide are prepared by solid state reaction at 1350 °C. All specimens achieved relative density values higher than 95%. The additive promotes grain growth indicating solid solution formation. A small fraction of the additive remains at grain boundaries and increases the fraction of the gallium-rich, LaSrGa₃O₇, impurity phase. The intragrain conductivity of gallium oxide containing specimens is higher than that of the parent solid electrolyte. Similar effect is found for the intergrain conductivity, which is maximum for 1 mol% gallium oxide addition.     

固体氧化物燃料电池复合电解质粉体BaCe0.8Y0.2O3–δ–Ce0.8Gd0.2O1.9的制备及表征

用分步硝酸盐–柠檬酸凝胶燃烧法制备新型混合离子传导复合电解质粉体BaCe0.8Y0.2O3–δ–Ce0.8Gd0.2O1.9[BCY–GDC,摩尔比n(BCY):n(GDC)=1:1],研究不同烧结温度下制备的BCY–GDC电解质片的性能,以及以BCY–GDC为电解质的单电池的电性能。结果表明:采用该法可以获得分布均匀的BCY–GDC粉末,平均晶粒尺寸约为40nm;BCY–GDC复合电解质的相对密度随着烧结温度的升高而不断增大;1450℃烧结5h的电解质片基本达到完全致密,且BCY、GDC两相分布均匀,并且随着烧结温度的升高,复合电解质片的电导率增大;以BCY–GDC为电解质的单电池在450℃的开路电压达到0.95V,700℃的最大功率密度达到0.4W/cm2。     

Eggshell-membrane-templated synthesis of hierarchically-ordered NiO–Ce0.8Gd0.2O1.9 composite powders and their electrochemical performances as SOFC anodes

Hierarchically-ordered NiO-Ce_0.8Gd_0.2O_1.9 (GDC) composite anode powders were synthesized using eggshell membranes as biotemplates. The morphology of the as-synthesized powders depended on the kind of Ni precursor and the use of EDTA as a chelating agent. Hierarchically-ordered anode powders were obtained from Ni chloride and Ni acetate precursors with EDTA. The Ni–GDC anode synthesized from Ni chloride precursor with EDTA exhibited the lowest polarization resistance at 800 °C and an activation energy of 0.01 Ω cm² and 0.74 eV, respectively, in humidified H₂. In accordance with the polarization resistance results, the 0.5-mm thick GDC electrolyte-supported single cell with the Ni–GDC anode synthesized from Ni chloride precursor with EDTA showed a maximum power density of 0.34 W cm⁻² at 800 °C with humidified H₂ fuel.     

Highly active and stable BaCo0.8Zr0.1Y0.1O3-δ cathode for intermediate temperature solid oxide fuel cells

Developing cathode material with high performance and excellent stability is the ultimate goal for solid oxide fuel cells (SOFCs). Based on this consideration, we design a new simple perovskite oxide BaCo_0.8Zr_0.1Y_0.1O_3-δ (BCZY) as the cathode material of SOFC without any further modification, which has good oxygen reduction reaction (ORR) activity and excellent stability in air and CO₂ at an intermediate temperature range of 600 ℃? 800 ℃. The area specific resistance (ASR) of symmetrical cell with BCZY cathode is 0.041 Ω cm² at 700 ℃, moreover, BCZY cathode keeps good structural and catalytic stability during 100 h test in air. The electrolyte-supported single cell fabricated with BCZY as cathode delivers a maximum power density of 460 mW cm^?2 and a superior steady operation over 200 h at 700 ℃. The good thermal physical structure stability of BCZY is further demonstrated by in-situ X-ray diffraction (XRD), good ORR activity and excellent CO₂ tolerance are further confirmed by density functional theory (DFT) calculations. These results indicates that BCZY maybe a potential cathode material for intermediate temperature SOFCs (IT-SOFCs).     

Liquid-phase synthesis of SrCo0.9Nb0.1O3-δ cathode material for proton-conducting solid oxide fuel cells

A novel liquid-phase synthesis strategy is demonstrated for the preparation of the Nb-containing ceramic oxide SrCo_0.9Nb_0.1O_3-δ (SCN). In comparison with the traditional solid-state reaction (SSR) method, the liquid-phase synthesis route offers a couple of advantages, including a lower phase formation temperature and a smaller particle size of the SCN materials that are beneficial for applications as proton-conducting fuel cell cathode. With BaCe_0.4Zr_0.4Y_0.2O_3-δ (BCZY442) as the electrolyte and the SCN synthesized in this work as the cathode, a proton-conducting solid oxide fuel cell (SOFC) shows a peak power density of 348 mW cm^?2 at 700 °C, significantly higher than that of a SOFC fabricated with SCN cathode prepared using the SSR method, which can only deliver 204 mW cm^?2 at the same temperature. Additionally, this new synthesis strategy allows impregnation of Sr²⁺, Co³⁺and Nb⁵⁺ on the solid backbone in aqueous solution, further improving cell performance to reach a peak power density of 488 mW cm^?2 at 700 °C.