ASR Values Research Articles

High Performance LaCo<sub>0.6</sub>Ni<sub>0.4</sub>O<sub>3-</sub><sub>δ</sub> Cathode Contact Material for Intermediate Temperature Solid Oxide Fuel Cells

LaCo0.6Ni0.4O3-δ (LCN) as cathode contact material (CCM) for application in solid oxide fuel cell (SOFC) is studied in this paper. LCN is prepared through the polymeric steric entrapment precursor route with polyvinyl alcohol (PVA) as the entrapment agent and then calcined at 800 °C and 1100 °C for 3 h respectively. Polished SUS430 pellets heated at 800 °C for 6 days as well as the untreated ones are used as interconnect. The assembly of LCN calcined at 800 °C with pretreated SUS430 at 800 °C for 6 days exhibit the best performance of a low and stable ASR value about 112 mΩ cm2 at the end of the 200 h test. The calcination temperature and the pretreatment of interconnect are considered to be crucial factors affecting the ASR property of LCN CCM for application in stack.

Effects of Pr3+-deficiency on structure and properties of PrBaCo2O5+δ cathode material–A comparison with Ba2+-deficiency case

Double-layered perovskite oxides of Pr1−yBaCo2O5+δ (P1−yBCO) with A-site Pr3+-deficiency contents of y = 0.00–0.10 have been studied with respect to phase structures, oxygen content, high-temperature chemical stabilities as well as electrical and electrochemical properties as cathode materials of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The Pr3+-deficiency content in P1−yBCO is limited by ∼8 mol%, and the Pr3+-deficiency hardly changes lattice parameters of P1−yBCO. Content of oxygen vacancies increases while that of Co4+ decreases with the higher Pr3+-deficiency content. P1−yBCO is chemically stable with the Gd0.1Ce0.9O1.95 (GDC) electrolyte at 1100 °C and below in air. Introduction of Pr3+-deficiency decreases electrical conductivities and significantly improves electrochemical performance of P1−yBCO. Among the studied oxides, P0.95BCO with 5 mol% Pr3+-deficiency shows the best electrochemical performance with low ASR values of 0.113 Ω cm2 at 600 °C, 0.054 Ω cm2 at 650 °C and 0.028 Ω cm2 at 700 °C respectively, demonstrating it a promising cathode material of IT-SOFCs. The results of P1−yBCO have also been compared with those of Ba2+-deficient PrBa1−xCo2O5+δ (PB1−xCO, x = 0.00–0.10) oxides and major differences have been found in lattice parameters, oxygen content, chemical defects, electrical conductivities and ASR results. Factors contributing to these differences have been discussed.

Electrical characterization of co-precipitated LaBaCo2O5+δ and YBaCo2O5+δ oxides

RE BaCo 2 O 5+ δ layered perovskite oxides ( RE = Rare Earth) are promising cathodes for IT-SOFCs. In this work, a simple co-precipitation synthesis in aqueous medium was applied to prepare LaBaCo 2 O 5+ δ (LBC) and YBaCo 2 O 5+ δ (YBC) cathodes. The chemical and electrochemical properties of both materials were characterized via XRD, SEM, TPO, TG–DTA, 4-probe conductivity measurement, and EIS tests on symmetric cells. The coprecipitation synthesis revealed a promising preparation route: the measured ASR values of both materials were well comparable with the literature ones. A kinetic investigation of the O 2 reduction process was performed on LBC (600–800 °C, 5–100% O 2 , v/v), whose results were analyzed with equivalent circuits. The main steps were identified (oxygen diffusion and charge transfer at high frequency, O 2 chemisorption at medium frequency), and their activation energy and reaction order were quantified. Aging tests (500 h time on stream, 500–800 °C) revealed quick deactivation for YBC and good stability for LBC.

Electrochemical properties of B-site Ni doped layered perovskite cathodes for IT-SOFCs

Offering fast oxygen reduction kinetics and high electrical conductivities relative to those of ABO3-type perovskite oxides, layered perovskite cathode materials have received considerable attention for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, we have investigated the effects of Ni substitution for the Co site in SmBa0.5Sr0.5Co2O5+δ in terms of structural characteristics, electrical properties, and electrochemical performance of SmBa0.5Sr0.5Co2−xNixO5+δ (x = 0, 0.1, and 0.2) in relation to its application as an IT-SOFC cathode material. At the given temperature, the electrical conductivities of SmBa0.5Sr0.5Co2−xNixO5+δ (x = 0, 0.1, and 0.2) decrease with increasing Ni content. For x = 0.1, the lowest ASR value, 0.125 Ω cm2, and the highest single cell performance, 1.78 W cm−2 at 600 °C, are obtained.

Preparation, characterization, and electrochemical properties of YBaCo3.4Al0.3Ga0.3O7+δ and YBaCo3.2Al0.4Ga0.4O7+δ cathodes for IT-SOFCs

Layered cobalt oxide materials YBaCo3.4Al0.3Ga0.3O7+δ (YBCAG33) and YBaCo3.2Al0.4Ga0.4O7+δ (YBCAG44) were prepared and the electrical conductivity, the thermal expansion coefficient and electrochemical performance were investigated as potential cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The highest conductivity of 6.2Scm−1 is found for YBCAG33 at 900°C in air. The average thermal expansion coefficients of YBCAG33 and YBCAG44 in the range of 30–850°C are 9.3×10−6K−1 and 9.0×10−6K−1, respectively. The ASR value of YBCAG33 and YBCAG44 cathodes on LSGM electrolyte is as low as 0.080Ωcm2 and 0.120Ωcm2 at 800°C, respectively. In an electrolyte-supported single test cell, the YBCAG33 cathode exhibited maximum power densities of 392, 292, 203, and 129mWcm−2 at 800, 750, 700, and 650°C, respectively. Preliminary results demonstrated that the YBCAG33 can be a promising alternative cathode materials in IT-SOFCs in terms of electrochemical performance.

Comparison Between Anode‐Supported and Electrolyte‐Supported Ni‐CGO‐LSCF Micro‐tubular Solid Oxide Fuel Cells

AbstractTwo types of micro‐tubular hollow fiber SOFCs (MT‐HF‐SOFCs) were prepared using phase inversion and sintering; electrolyte‐supported, based on highly asymmetric Ce0.9Gd0.1O1.95(CGO) HFs and anode‐supported based on co‐extruded NiO‐CGO(CGO)/CGO HFs. Electroless plating was used to deposit Ni onto the inner surfaces of the electrolyte‐supported MT‐HF‐SOFCs to form Ni‐CGO anodes. LSCF‐CGO cathodes were deposited on the outer surface of both these MT‐HF‐SOFCs before their electrochemical performances were compared at similar operating conditions. The performance of the anode‐supported MT‐HF‐SOFCs which delivered ca. 480 mW cm–2 at 600 °C was superior to the electrolyte‐supported MT‐HF‐SOFCs which delivered ca. six times lower power. The contribution of ohmic and electrode polarization losses of both FCs was investigated using electrochemical impedance spectroscopy. The electrolyte‐supported MT‐HF‐SOFCs had significantly higher ohmic and electrode polarization ASR values; this has been attributed to the thicker electrolyte and the difficulties associated with forming quality anodes inside the small (<1 mm) lumen of the electrolyte tubes. Further development on co‐extruded anode‐supported MT‐HF‐SOFCs led to the fabrication of a thinner electrolyte layer and improved electrode microstructures which delivered a world leading 2,400 mW cm–2. The newly made cell was investigated at different H2 flow rates and the effect of fuel utilization on current densities was analyzed.

Effect of the deposition temperature on the electrochemical properties of La0.6Sr0.4Co0.8Fe0.2O3−δ cathode prepared by conventional spray-pyrolysis

La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF) cathodes have been deposited by conventional spray pyrolysis on Ce0.8Gd0.2O1.9 (CGO) electrolytes at different temperatures between 250 and 450 °C, obtaining electrodes with different microstructure and porosity. Highly porous and macroporous electrodes are obtained at deposition temperatures of 250 °C and 450 °C, respectively, with an average grain size of 30–50 nm. The influence of the post-annealing treatment on the microstructure and on the electrochemical properties is investigated by scanning electron microscopy and impedance spectroscopy in air and as a function of the oxygen partial pressure to identify the different contributions to the polarization. Samples annealed at 650 °C show similar values of area specific resistance 0.04–0.06 Ω cm2 at a measured temperature of 650 °C. However, after annealing the samples at 850 °C, the ASR values increase up to 0.1–0.6 Ω cm2 with the lowest value corresponding to the film deposited at 250 °C due to the large porosity and surface area of this film. The performance degradation upon annealing is attributed to decreasing reaction sites induced by grain growth and densification.

Nano Coated Interconnects for SOFC (NaCoSOFC)

The NaCoSOFC project is focused on the development of nano coatings for SOFC interconnects. The project is sponsored by the Nordic Top Level Research Initiative and has four project partners: Sandvik Materials Technology which is producing coated interconnects, Chalmers University of Technology and the University of Oslo that characterize samples with respect to e.g. corrosion, Cr evaporation and ASR as well as Topsoe Fuel Cell that is testing the developed interconnects in its stacks. The developed coatings are based on a combination of Co with RE elements and exhibit high corrosion resistance, 10 fold decrease in Cr evaporation and ASR values that are approximately 50% of the uncoated material.

Advanced Electrodes for Intermediate Temperature Proton Conducting Fuel Cell

In order to develop efficient Protonic Ceramic Fuel Cell, different composite electrodes made of BaCe0.9Y0.1O3 and NiO or mixed ionic-electronic conductor (BSCF) were investigated as anode and cathode respectively. Ni-BCY composite synthesized by combustion offers sufficient open porosity, shows an electrical conductivity superior to 1000 S.cm-1 and a very low ASR value (0.06 Ω.cm2 at 600°C). The electrochemical properties of BSCF-BCY composite cathode have been studied and the obtained ASR value (0.53 Ω.cm2 at 600 °C) is lower compare to pure BSCF. An anode supported single cell (25 mm in diameter) consisting of a bi-layer Ni-BCY cermet, a dense 15 mm BCY electrolyte and a BCY-BSCF composite cathode were assembled and tested from 550 to 700 °C. Open circuit voltages of 1.055 V and 1.026 V were achieved at 600 °C and 700 °C. Maximum power densities of 156 and 293 mW.cm-2 were obtained at 600 and 700 °C.

Electrospinning La0.8Sr0.2Co0.2Fe0.8O3−δ tubes impregnated with Ce0.8Gd0.2O1.9 nanoparticles for an intermediate temperature solid oxide fuel cell cathode

In order to reduce the polarization resistance of the cathode, we have developed one-dimensional (1D) nanostructured La0.8Sr0.2Co0.2Fe0.8O3−δ (LSCF) tubes/Ce0.8Gd0.2O1.9 (GDC) nanoparticles composite cathodes for solid oxide fuel cell. Uniform LSCF/PVP composite nanofibers have been firstly synthesized by a single-nozzle electrospinning technique, followed by firing at 800 °C for 2 h to form one-dimensional LSCF tubes. Subsequently, the GDC phases were introduced into tube structured LSCF scaffold pre-sintered on a GDC pellet by a multi-impregnation process. Electrochemical Impedance spectra reveal that nanostructured LSCF tubes/GDC nanoparticles composite cathodes have a better electrochemical performance, achieving area-specific resistances of 4.70, 1.12, 0.27 and 0.07 Ω cm2 at 500, 550, 600 and 650 °C for the composite of GDC and LSCF in a weight ratio of 0.52:1. The low ASR values are mainly related to its optimal microstructure with larger triple-phase boundaries and higher porosity. These results suggest that LSCF tube/GDC nanoparticle composite can be an alternative cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC).

Synthesis and Characterization of Nanosized (DyO1.5)x(WO3)y(BiO1.5)1-x-y for Lower Temperature SOFC Application

The high ionic conductivity (DyO1.5)x(WO3)y(BiO1.5)1-x-y (DWSB) material was synthesized using a coprecipitation method. Adding the acid solution containing Dy(NO3)3 and Bi(NO3)3 to the basic solution containing (NH4)10W12O41•5H2O, a DWSB precursor was formed. Upon calcination, nano-sized (DyO1.5)x(WO3)y(BiO1.5)1-x-y powder with the appropriate fluorite phase is generated. This nanopowder was incorporated into a composite cathode with La0.80Sr0.20MnO3-δ (LSM) using the glycine-nitrate process to coat the DWSB with the LSM. The two phases are mixed at the nano-scale and no reactivity between the two phases is observed. The ASR values for this cathode (0.17 Ω-cm2 at 600⁰C) is reduced by a half order of magnitude compared with the same composition mixed with solid state methods (0.45 Ω-cm2 at 600⁰C). Furthermore, the ASR values are significantly reduced from composite cathodes of LSM and common ionic conductors, namely LSM-GDC (1.3 Ω-cm2 at 600⁰C) and LSM-YSZ (1.5 Ω-cm2 at 600⁰C).

Development and performance of MgFeCrO4 – based electrodes for solid oxide fuel cells

Composite anodes for solid oxide fuel cells (SOFC) developed on yttria stabilised zirconia (YSZ) porous supports by infiltration of electrode materials has been successfully applied for various anode and cathode compositions, resulting in high performance SOFC devices. The focus of this study is the performance of the chromium-rich spinel (MgFeCrO4) as an electrode support material when used alone or impregnated. The composite anodes were prepared by aqueous infiltration of nitrate salts to produce (La0.75Sr0.25)0.97Cr0.5Mn0.5O3−δ, Ce0.9Gd0.1O2−δ, CeO2 or Pd into a MgFeCrO4 scaffold with 45% porosity and studied by electrochemical impedance spectroscopy in symmetrical cell configuration. The performance was evaluated in humidified 5% H2/Ar in order to quantify their stability and performance up to 850 °C with respect to the MgFeCrO4 porous substrate. It was found that all the impregnated phases adhere very well to the spinel and considerably enhance performance and stability to a level required for SOFC applications. MgFeCrO4/LSCM/CGO and MgFeCrO4/LSCM/CGO/Pd showed the most substantial improvement in comparison to the scaffold’s performance, with ASR values of 1.74 Ω cm2 and 0.91 Ω cm2, respectively.

Metallic Interconnects for Solid Oxide Fuel Cell: Performance of Reactive Element Oxide Coating During 10, 20 and 30 Months Exposure

One of challenges in improving the performance and cost-effectiveness of SOFCs (Solid Oxide Fuel Cells) is the development of suitable interconnect materials. Chromia-forming alloys and especially ferritic stainless steels, like Crofer22APU, are considered to be among the most promising candidate materials as interconnects in SOFC stacks. However, the performance of chromia-forming materials can be limited by the low electronic conductivity of the oxide scale (high ASR—area specific resistance—value). Such degradation is unacceptable regarding the long-term operation (>40,000 h). A previous study demonstrated that in air, the addition of a nanometric reactive element oxide (La2O3) layer applied by Metal Organic Chemical Vapour Deposition (MOCVD) drastically improved both corrosion rate and electrical properties of Crofer22APU for 100 h at 800 °C. In the present study, coating performances were checked after 10 months (7,700 h), 20 months (15,400 h) and 30 months (23,100 h) exposure in air at 800 °C. The corrosion products were characterized by SEM, EDX and XRD. ASR measurements are realized after long time exposure. The study demonstrates that the Crofer22APU alloy has a good oxidation resistance after 23,100 h in air but this alloy has a high ASR value equal to 0.410 Ω cm2. The coatings composed of thin reactive element oxides made of La2O3 or Y2O3 resulted in an important improvement in the high temperature oxidation resistance; the ASR values are equal to 0.180 and 0.160 Ω cm2 respectively.

Development of Long-Term Stable and High-Performing Metal-Supported SOFCs

Metal-supported SOFCs are believed to have high potential for commercialization due to lower material costs and higher robustness in fabrication and operation. However, the development of the cell is challenged by the metal properties during fabrication, and the necessary lower operating temperatures, while retaining both the energy output and the stability. The metal-supported SOFC design developed at Riso̸ DTU has been optimized to an ASR value of 0.62 Ωcm2 at 650 °C, and a steady degradation rate of 1.0% kh-1 demonstrated for 3000 h on a 16 cm2 active cell level. Additional improvement in the performance has been demonstrated possible with cobalt-based cathode materials in combination with a magnetron sputtered CGO cathode barrier layer. Initial ASR values down to 0.27 Ωcm2 at 650 °C and power densities up to 1.14 Wcm-2 was observed with this design on a 0.5 cm2 active cell level.

Microstructural Aspects on the Performance of LSCF Cathodes for SOFCs

The electrode reaction of oxygen for various porous microstructures of La0.6Sr0.4Co0.8Fe0.2O3-δ cathode was investigated. The particle size distribution of the powder and the sintering parameter were changed to attain different porosity. The electrochemical performance was investigated by impedance spectroscopy in static air from 500°C up to 900°C. It was found that the increase of the total pore surface area resulted in a decrease of the average pore size leading to an increase in the concentration polarization associated with oxygen diffusion limitation. Above 600°C, this was attributed to a low frequency arc on the impedance spectra, contributing to the total cathode ASR up to 42% at 800°C. Consequently, the lower obtained ASR value (0.33Ω.cm2 at 800°C) was not correlated with microstructure having the highest pore surface area. Development of microstructure mitigating oxygen diffusion limitation and having high pore surface area is thus a major issue for high performance LSCF cathodes.

Electrochemical Oxidation at SOFC Anodes: Comparison of Patterned Nickel Anodes and Nickel/8YSZ Cermet Anodes

Patterned Ni anodes are a common method to determine elementary kinetic reaction mechanism at the anode of Solid Oxide Fuel Cells. Their well defined Triple-Phase-Boundary (TPB) of electrode, electrolyte and gas phase, where the electrochemical oxidation takes place, is easily accessible. However, the direct transfer of knowledge gained with patterned Ni anodes to technical Ni/8YSZ cermet anodes is doubtful. For the first time a comparison of the electrochemical performance of both anode concepts is performed by means of (i) transmission line model calculations and (ii) performance simulations using a 3D FEM microstructure model. The good agreement observed between the performance of patterned Ni anodes and Ni/8YSZ cermet anodes indicates similar reaction mechanisms at the TPB of both anode concepts. Especially in H2-H2O atmosphere, the agreement of (i) absolute ASR values and (ii) parameter dependencies is very good and shows the applicability of patterned Ni anodes for the investigation of the electrochemistry of technical Ni/8YSZ cermet anodes.

Optimisation of the Solid Oxide Fuel Cell (SOFC) cathode material Ca 3Co 4O 9− δ

This paper focuses on the electrochemical potentialities of the 2D misfit compound Ca 3Co 4O 9− δ , so far mainly investigated for its thermoelectric properties. Its expansion coefficient (TEC = (9–10) × 10 −6 °C −1) and its chemical stability are compatible with standard CGO IT-electrolyte and the first optimisation steps of the deposited cathode have been performed with the aim to minimise the ASR and increase the cell durability. Particular attention has been paid on the effect of thickness and microstructure for pure and composite cathodes. The electrode reaction was performed on symmetrical cells. The preliminary results presented here show that the composite (70 wt.% Ca 3Co 4O 9− δ –30 wt.% CGO) gives the lowest ASR values compare to single-phased electrodes. Strikingly, the ASR values increase for thinner deposited layers. The effect of various current collectors (gold grid vs. platinum paste) has been also checked.

Metallic interconnects for solid oxide fuel cell: Performance of reactive element oxide coating during long time exposure

AbstractOne of challenges in improving the performance and cost‐effectiveness of SOFCs (solid oxide fuel cells) is the development of suitable interconnects materials. Chromia‐forming alloys and especially ferritic stainless steels, like Crofer22APU, are considered to be among the most promising candidate materials as interconnects in SOFC stacks. However, the performance of chromia‐forming materials can be limited by the low electronic conductivity of the oxide scale (high ASR – area specific resistance – value). Such degradation are unacceptable regarding the long‐term operation (>40 000 h). A previous study 1 demonstrated that in air, the addition of a nanometric reactive element oxide (La2O3) layer applied by metal organic chemical vapor deposition (MOCVD) drastically improved both corrosion rate and electrical properties of Crofer22APU and Haynes230 alloys for 100 h at 800 °C. In this present study coating performances were checked after 10 months (7500 h) and 20 months (15 000 h) at 800 °C in air. The corrosion products were carefully analyzed by SEM, EDX, and XRD. ASR measurements are realized after long time exposure. This study demonstrates that the Crofer22APU alloy has a good oxidation resistance after 15 000 h in air but this alloy has an ASR value equal to 0.370 Ω cm2. The coatings composed of a thin reactive element oxide such as La2O3 resulted in an important improvement in the high temperature oxidation resistance; the ASR values are equal to 0.154 Ω cm2. Haynes230 alloy has a better oxidation resistance but the formation of an insulating Al2O3/SiO2 layer could be detrimental.

Microstructure-Electrical Properties of Original LSCF Films Deposited by ESD for IT-SOFCs

Original and different microstructures of La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) films were successfully deposited by electrostatic spray deposition (ESD) varying the process conditions. The correlation between deposition parameters and resulting morphology is discussed in detail. SEM/EDS analyses have shown that large arrays of different LSCF film microstructures were obtained, ranging from dense to reticulated to porous, with thicknesses ranging from 3 to 10 µm. Single phased LSCF with typical perovskite structure, were successfully deposited for the first time by ESD on CGO substrates. ASR values of 0.8 ohm.cm² for porous films were determined from impedance spectroscopy measurements at 600 ºC. This paper successfully shows that electrochemical performances are strongly dependent on the cathode microstructure for a fixed composition.

Area specific resistance of oxide scales grown on ferritic alloys for solid oxide fuel cell interconnects

Planar solid oxide fuel cells (SOFC) are considered to be power generators with high efficiency and low emission at small power units (1–200 kW el). Many prototype systems are already successfully realized. For mass production the costs have to be reduced and the long-term stability has to be enhanced. Power losses <0.5%/1000 h is the target value for stacks in stationary SOFC-based power systems. To reach this goal, the factors influencing degradation have to be found and reduced. In this work the interaction between interconnect and different ceramic materials such as perovskites (La 0.8Sr 0.2(Mn ,Co)O 3, La 0.65Sr 0.3MnO 3, La 0.65Sr 0.3(Mn,Co)O 3) and spinels (Mn(Co,Fe)O 4, (Cu,Ni)Mn 2O 4) was investigated on the cathode (air) side of conventional ferritic interconnect materials (CroFer22APU, ITMLC, ZMG232L). The method to determine the value of the area specific resistance between interconnect and contact layer ( R #ICC) within a tolerance of 10% has been developed to provide reliable data for ASR values and their degradation. The R #ICC-value increases with annealing time. The degree of this increase depends on used materials and their combination. The spinel contact layers form a thin dense ceramic layer at the beginning of the annealing process. This layer reduces the oxidation rate of the alloy. Because of this protection layer a thinner oxide scale grows and the ASR aging rate is much lower (0.4–0.9 mΩ cm 2/1000 h). The comparison of the aging rates of different alloys with La 0.8Sr 0.2(Mn,Co)O 3 contact layer reveals remarkable differences: 3.1 mΩ cm 2/1000 h for CroFer22APU, 10.9 mΩ cm 2/1000 h for ITMLC and 21.2 mΩ cm 2/1000 h for ZMG232L. The degradation in a stack has been determined from the R #ICC-values and geometric factors. The impact of oxidation at the cathode side of interconnect is about one third of the total stack degradation. The method opens the possibility for comparing area specific resistances of special material combinations with high accuracy. By optimized material combinations the degradation in stacks can be reduced to <0.5%/1000 h.