Electrochemical and gas phase parameters of cathodes for intermediate temperature solid oxide fuel cells

A series of cyclic voltammetry, chronoamperometry and electrochemical impedance experiments have been carried out in order to investigate the effect of cathode composition and porosity on the electrochemical characteristics of strontium-doped lanthanum, praseodymium and gadolinium cobaltite cathodes. The impedance responses at different electrode potentials of the half cell and symmetric single cell setups are compared and analyzed by the equivalent circuit modeling method. The deconvolution of impedance spectra for single cell cathode and anode reactions contributions based on the results of simultaneous analysis of half cells and symmetric single cells has been made by differential impedance real part vs. ac frequency plot analysis method. Noticeable influence of cathode chemical composition, meso-porosity and macro-porosity on the electrochemical activity of the oxygen electroreduction has been demonstrated. Seeming activation energy values have been calculated and discussed.     

Effect of optimal GDC loading on long-term stability of La0.8Sr0.2Co0.2Fe0.8O3-δ/Gd0.2Ce0.8O1.9 composite cathodes

Three types of La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ/Gd_0.2Ce_0.8O_1.9 (LSCF/GDC) composite cathodes with different optimal GDC loading are fabricated through electrospinning, screen printing and solution infiltration method. Constant current polarization with current density of 100 mA cm^?2 at 750°C is applied to test the stability of LSCF/GDC composite cathodes. After constant current polarization for 144 h, the polarization resistance (R_p) of 280 nm-nanofiber skeletal LSCF/GDC composite cathode after pore-forming exhibits the minimum increase, from 0.062 Ω cm² to 0.098 Ω cm². Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) results show that the microstructure and surface chemical composition of the cathode maintain stable during the constant current polarization. Combined with the X-ray diffraction (XRD) result, a relationship among GDC loading, stress, Sr surface segregation and long-term stability is established.     

Review of composite cathodes for intermediate-temperature solid oxide fuel cell applications

A composite cathode exhibits low activation polarisation by spreading its electrochemically active area within its volume. Composite cathodes enable the development of high-performance electrodes for solid oxide fuel cells (SOFCs) at intermediate temperatures (600 °C – 800 °C) because of their significant role in determining the kinetics of oxygen reduction reaction (ORR). Few anions O²⁻ are transferred through the electrolyte component when the ORR is low, thereby lowering the reaction with cation H⁺ from an anode side to transfer electrons along the outer circuit to the cathode side to participate in ORR. The resistance to the ORR at the cathode is minimised, thereby contributing to performance degradation and efficiency loss in existing SOFCs, especially at intermediate temperatures. The suitability and compatibility of the cathode and electrolyte are crucial in the development of cathodes and electrochemical reactions. The intercomponent compatibility is important to ensure the robustness and durability of SOFCs, especially at an operating temperature around 800 °C, at which the components experience extreme thermal and mechanical stresses. Composite cathodes are used to improve cathode performance. These composite cathodes help enhance the properties of mixed electronic–ionic conductors and the intercomponent compatibility. Herein, we reviewed historical data of composite-cathode development for SOFCs, including its basic principle and criteria. The overall performance of as-synthesised composite cathodes in terms of microstructure, electrochemical reaction and intercomponent compatibility is briefly discussed.     

Nano-sized Sm0.5Sr0.5CoO3−δ as the cathode for solid oxide fuel cells with proton-conducting electrolytes of BaCe0.8Sm0.2O2.9

Nano-sized Sm_0.5Sr_0.5CoO_3−δ (SSC) was fabricated onto the inner face of porous BaCe_0.8Sm_0.2O_2.9 (BCS) backbone by ion impregnation technique to form a composite cathode for solid oxide fuel cells (SOFCs) with BCS, a proton conductor, as electrolyte. The electro-performance of the composite cathodes was investigated as function of fabricating conditions, and the lowest polarization resistance, about 0.21 Ω cm² at 600 °C, was achieved with BCS backbone sintered at 1100 °C, SSC layer fired at 800 °C, and SSC loading of 55 wt.%. Impedance spectra of the composite cathodes consisted of two depressed arcs with peak frequency of 1 kHz and 30 Hz, respectively, which might correspond to the migration of proton and the dissociative adsorption and diffusion of oxygen, respectively. There was an additional arc peaking at 1 Hz in the Nyquist plots of a single cell, which should correspond to the anode reactions. With electrolyte about 70 μm in thickness, the simulated anode, cathode and bulk resistances of cells were 0.021, 0.055 and 0.68 Ω cm² at 700 °C, relatively, and the maximum power density was 307 mW cm⁻² at 700 °C.     

Synthesis and characterization of fibrous La0.8Sr0.2Fe1-xCuxO3-δ cathode for intermediate-temperature solid oxide fuel cells

Aiming to offer a high-performance Co-free cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs), a series of La_0.8Sr_0.2Fe_1-xCu_xO_3-δ (LSFCux, x = 0.0–0.3) nanofiber cathodes were synthesized by the electrospinning method. The effects of various Cu doping amounts on the crystal structure, fiber morphology, and electrochemical performance of LSF nanofiber cathode materials were investigated. The results indicate that after being calcined at 800 °C for 2 h, the perovskite structure samples with a high degree of crystallinity are obtained. The morphology of electrospun nanofibers is continuous, and the average diameter of nanofibers is about 110 nm. In addition, the La_0.8Sr_0.2Fe_0.8Cu_0.2O_3-δ (LSFCu2) fiber cathode displays the optimal electrochemical performance, and the polarization resistance (R_p) is 0.674 Ω cm² at 650 °C. The doping of Cu transforms the main control step of the low-frequency band from dissociation of oxygen molecules to charge transfer on the electrode, and the maximum power density (P_m) of the Ni-SDC/SDC/LSFCu2 single cell reaches 362 mW cm^-2 at 650 °C.     

Bismuth oxide-coated (La,Sr)MnO3 cathodes for intermediate temperature solid oxide fuel cells with yttria-stabilized zirconia electrolytes

Taking Y₂O₃ stabilized Bi₂O₃ (YSB) as an example, bismuth oxide-added (La,Sr)MnO₃ (LSM) is evaluated as a cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs) with 8 mol% Y₂O₃ stabilized ZrO₂ (YSZ) electrolytes. YSB was added to LSM cathodes using an impregnation method, dramatically improving the electrode performance. The interfacial polarization resistance R_p, at 700 °C for the electrode coated with 50 wt.% of YSB is 0.14 Ω cm², which is only 0.2% of the value for a pure LSM electrode. The high oxygen ionic conductivity and the catalytic activity of YSB, as well as the favorable electrode microstructure are likely reasons for the dramatic reduction of R_p. The YSB-added LSM cathodes also exhibited lower overpotential and higher exchange current density than the pure LSM cathode. Moreover, these electrodes show much lower R_p than that of parallel-fabricated LSM electrodes with samaria-doped-CeO₂ as well as other LSM-based electrodes reported in the literature, demonstrating the superiority of the of YSB as the ionic conduction component in composite LSM electrodes. The superior performance of the single cell further demonstrates that the bismuth oxide-added LSM cathode is an excellent candidate for IT-SOFCs.     

Nano-structured composite cathodes for intermediate-temperature solid oxide fuel cells via an infiltration/impregnation technique

Solid oxide fuel cells (SOFCs) are high temperature energy conversion devices working efficiently and environmental friendly. SOFC requires a functional cathode with high electrocatalytic activity for the electrochemical reduction of oxygen. The electrode is often fabricated at high temperature to achieve good bonding between the electrode and electrolyte. The high temperature not only limits material choice but also results in coarse particles with low electrocatalytic activity. Nano-structured electrodes fabricated at low temperature by an infiltration/impregnation technique have shown many advantages including superior activity and wider range of material choices. The impregnation technique involves depositing nanoparticle catalysts into a pre-sintered electrode backbone. Two basic types of nano-structures are developed since the electrode is usually a composite consists of an electrolyte and an electrocatalyst. One is infiltrating electronically conducting nano-catalyst into a single phase ionic conducting backbone, while the other is infiltrating ionically conducting nanoparticles into a single phase electronically conducting backbone. In addition, nanoparticles of the electrocatalyst, electrolyte and other oxides have also been infiltrated into mixed conducting backbones. These nano-structured cathodes are reviewed here regarding the preparation methods, their electrochemical performance, and stability upon thermal cycling.     

Novel CO2-tolerant Co-based double perovskite cathode for intermediate temperature solid oxide fuel cells

For the commercial application of solid oxide fuel cells (SOFCs), CO₂-tolerant cathode materials with high electrochemical activity are required. Here, we discuss the performance of double perovskite Pr_0.2Sr_1.8CoTiO_6?δ (P02STC) as a potential cathode material for SOFCs. P02STC has a cubic structure and keeps lattice structure stable in the CO₂ atmosphere. The average thermal expansion coefficient is 17.8 × 10^–6 K^–1 at 30–900 °C in air. The P02STC cathode exhibits good electrochemical performance with a low polarization resistance of 0.080 Ωcm² at 700 °C. The P02STC cathode shows good structure stability, electrochemical performance stability, and excellent tolerance to CO₂ poisoning for the symmetrical cells based on the 350 h stability test in air and the 150 h stability test in O₂ containing 5%CO₂ at 700 °C. The electrolyte-supported single cell with a P02STC cathode shows a maximum power density of 677 mW cm^? 2 at 800 °C. The single cell operates stably for 250 h at a constant current of 0.3 A/cm² without obvious degrading performance. According to all of the experimental results, the P02STC sample might be a promising candidate cathode for SOFCs.     

Film percolation for composite electrodes of solid oxide fuel cells

A film percolation model is proposed for composite electrodes of solid oxide fuel cells (SOFCs). The model is developed to predict the percolation properties of 2D-infinite structures which represent the structural characteristics of composite electrodes of electrochemical devices such as SOFCs. The model can be used to estimate electrode properties, such as percolation probability, active three-phase boundary length and interfacial polarization resistance. Compared with the classic percolation theory, which is particularly applicable to 3D-infinite bulks, the model can explicitly capture the effects of thinly layered nature of composite electrodes, and describes a cross-over between 2D-infinite films and 3D-infinite bulks. It also permits the prediction within whole electrode composition range, and can be easily applied in SOFC modeling.     

Oxygen and chlorine evolution on RuO2 + TiO2 + CeO2 + Nb2O5 mixed oxide electrodes

A systematic investigation was conducted on the mechanism and electrocatalytic properties of O₂ and Cl₂ evolution on mixed oxide electrodes of nominal composition: Ti/[Ru_(0.3)Ti_(0.6)Ce_(0.1−x)]O₂[Nb₂O₅]_(x) (0 ≤ x ≤ 0.1). For the oxygen evolution, a 30 mV Tafel slope is obtained in the presence of CeO₂, while in its absence a 40 mV coefficient is observed. The intrinsic electrocatalytic activity is mainly due to electronic factors, as result of the synergism between Ru and Ce oxides. For chlorine evolution, the Tafel slope (30 mV) is independent on oxide composition. The best global electrocatalytic activity for ClER was observed in the absence of Nb₂O₅ additive. Variation of the voltammetric charge throughout the experiments confirms high CeO₂ content compositions are fragile, due mainly to the porosity caused by CeO₂ presence. On the other hand, Nb₂O₅ addition decreases considerably this instability.     

Synthesis and characterization of Ca3-xLaxCo4-yCuyO9+δ cathodes for intermediate temperature solid oxide fuel cells

The calcium cobaltite Ca_3-xLa_xCo_4-yCu_yO_9+δ with x and y = 0 and 0.1 were synthesized and the electrical, thermal, and catalytic behaviors for the oxygen reduction reaction (ORR) for use as air electrodes in intermediate-temperature solid oxide fuel cells (IT-SOFCs) were evaluated. X?ray diffraction confirms the Ca_3-xLa_xCo_4-yCu_yO_9+δ samples were crystallized in a monoclinic structure and scanning electron microscopic image shows lamella-like grain formation. Introduction of dopants decreases slightly the loss of lattice oxygen and thermal expansion co-efficient. The Ca_3-xLa_xCo_4-yCu_yO_9+δ samples exhibit good phase stability for long-term operation, thermal expansion, and chemical compatibility with the Ce_0.8Gd_0.2O_2-δ electrolyte. Among the studied samples, Ca_2.9La_0.1Co₄O_9+δ shows a maximum conductivity of 176 Scm^?1 at 800 °C. Although the doped samples exhibit a higher total electrical conductivity, an improved symmetrical cell performance is displayed by the undoped sample. Comparing the sintering temperatures, the composite cathode Ca₃Co₄O_9+δ + Ce_0.8Gd_0.2O_2-δ sintered at 800 °C exhibit the lowest area specific resistance of 0.154 Ω cm² at 800 °C in air. In the Ca_3-xLa_xCo_4-yCu_yO_9+δ + GDC composite cathodes, the charge-transfer process at high frequencies presents a major rate limiting step for the oxygen reduction reaction.     

Rapid degradation phenomenon of NiCu–Ce0.8Gd0.2O1.9 anode at high p(H2O) in different concentrations of dry methane

Degradation phenomena of Ni_0.5Cu_0.5–CGO (Ce_0.8Gd_0.2O_1.9) bimetallic anode in different concentrations of dry methane were studied in LSM ((La_0.75Sr_0.25)_0.95MnO_3−δ)–CGO cathode supported SOFCs. According to XRD, SEM and EIS analyses, the degradation of the Ni_0.5Cu_0.5–CGO bimetallic anode could be mainly attributed to re-oxidation of Ni by H₂O and/or by rapid sintering of Ni due to a high p(H₂O) appeared at a high current density. Furthermore, it is found that the degradation of anode resulted by the re-oxidation of Ni and/or by rapid sintering of Ni in the as-prepared cells generally occurred when the partial pressure ratio of the produced H₂O and CH₄ at the outlet of anode increased above a threshold value in different concentrations of dry methane.     

Optimization of the microstructure of porous composite cathodes in solid oxide fuel cells

A comprehensive micromodel to predict the electrochemical performance of porous composite LSM‐YSZ cathodes in solid oxide fuel cells (SOFCs) is developed. The random packing sphere model is used to estimate the cathode microstructural properties required for the micromodel. The micromodel developed takes into account the complex interdependency among the mass transport, electron and ion transports, and the electrochemical reaction, and can be used for optimization of the microstructure of porous LSM‐YSZ composite cathodes. It is shown that the electrochemical performance of these cathodes depends on the microstructural variables of the cathode porosity, thickness, particle size ratio, and size and volume fraction of LSM particles. The effect of these microstructural variables on the cathode total resistance, as the objective function to achieve the optimum microstructure for the cathode, is studied through computer simulation. The results indicated that for a LSM‐YSZ cathode operated at the average temperature of 1073.15 K, bulk oxygen partial pressure of 0.21 atm, and total current density of 5000 Am^?2, and constrained to the minimum value of 1 μm for the size of LSM particles and 0.25 for the cathode porosity, the optimum microstructure is obtained at the particle size ratio of unity, LSM particle size of 1 μm and volume fraction of 0.413, porosity of 0.25, and thickness of 60 μm. The cathode total resistance corresponding to the cathode optimized is estimated to be 0.291 Ω cm². © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Dependencies of the open-circuit voltage of Au|YSZ|Pt single-chamber fuel cell on the composition of the uniform CH4 + O2 gas mixture with solid carbon deposition

Single-chamber solid oxide fuel cell is a device where two electrodes of different materials contacting a solid oxide ionic conductor, may generate a considerable potential difference and electrical power, when supplied by a common fuel + oxidant gas mixture. The Au|YSZ|Pt system in the CH₄ + O₂ gas mixture is one of the simplest examples of such a cell. In this article the open-circuit voltage (OCV) of this cell, supplied with the gas mixture xO₂ + aCH₄ + (1 − x − a)Ar (where a = 0.01, 0.1 or 0.5), is investigated. On the basis of the obtained results, as well as those for the xCH₄ + (1 − x)(0.2O₂ + 0.8Ar) (0 ≤ x ≤ 1) gas mixture, reported in our previous work [Electrochim. Acta, 50 (2005) 2771], we postulate that the OCV of the above system arises as a result of electrode modification resulting from solid carbon deposition in the cell. After oxidation of the carbon deposit, the system, once treated by the gas mixture enabling the formation of the carbon phase, shows more and more tendency to generate the OCV. The open-circuit potential of the Au electrode depends only on the O₂ concentration in the initial gas mixture, whereas in the case of the Pt electrode it becomes dependent on chemical equilibria determining the O₂ content in the converted gas mixture. Our results reveal that the OCV achieves a reproducible limiting value of ∼650 mV, which is lower by ∼400 mV than the calculated equilibrium value.     

Layered NdBaCo2−xNixO5+δ perovskite oxides as cathodes for intermediate temperature solid oxide fuel cells

The effect of Ni substitution on the crystal chemistry, thermal and electrochemical properties, and catalytic activity for oxygen reduction reaction of the layered NdBaCo_2−xNi_xO_5+δ perovskite oxides has been investigated for 0 ≤ x ≤ 0.6. The oxygen content (5 + δ) and oxidation state of the (Co, Ni) ions in the air-synthesized NdBaCo_2−xNi_xO_5+δ samples decrease with increasing Ni content, accompanied by a structural transition from tetragonal (0 ≤ x ≤ 0.4) to orthorhombic (x = 0.6). Similarly, the thermal expansion coefficient (TEC) and electrical conductivity also decrease with increasing Ni content. The x = 0.2 and 0.4 samples exhibit slightly improved performance as cathodes in single cell solid oxide fuel cell (SOFC) compared to the x = 0 sample, which is in accordance with the ac-impedance data. Among the samples studied, the x = 0.4 sample exhibits a combination of low thermal expansion and high catalytic activity for the oxygen reduction reaction in SOFC.     

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.     

A highly conductive Ce0.8Nd0.2O1.9 electrolyte with double sintering aids for intermediate-temperature solid oxide fuel cells

In this paper, the influence of Bi/Zn mass ratio on the phase composition, microstructure, sintering properties, and electrical properties of Bi/Zn co-added Nd_0.2Ce_0.8O_1.9 (NDC) used for intermediate-temperature solid oxide fuel cells (SOFCs) was investigated. At 700 °C, the total conductivity of the NDC-based electrolyte (3Bi/1Zn-NDC) with the mass ratio 3:1 for Bi₂O₃ and ZnO was as high as 5.89 × 10^?2 S cm^?1, 4.60 and 4.51 times higher than the single addition of 4 wt% Bi₂O₃ and 4 wt% ZnO, respectively. In addition, the 3Bi/1Zn-NDC electrolyte exhibited a good physical and chemical compatibility with the electrode materials. The open circuit voltage (OCV) of the cell supported by the 3Bi/1Zn-NDC electrolyte was 0.67 V, and the output power density could reach 402.25 mW cm^?2 at 700 °C. It showed stable power output and OCV in the long-term stability test within 50 h. Overall, the combination of 3 wt% Bi₂O₃ and 1 wt% ZnO was a very effective dual sintering aid for NDC electrolyte.     

Composite cathodes of La0.9Ca0.1Ni0.5Co0.5O3-Ce0.8Sm0.2O1.9 for solid oxide fuel cells

The mixed ionic and electronic conductors of La_0.9Ca_0.1Ni_0.5Co_0.5O₃-Ce_0.8Sm_0.2O_1.9 (LCNC-SDC) are investigated systematically for potential application as a cathode for solid oxide fuel cells based on a Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte. The electrochemical impedance spectroscopy (EIS) measurements are performed in air over the temperature range of 600-850 °C to determine the cathode polarization resistance. The exchange current densities for oxygen reduction reaction (ORR), determined from the low-field cyclic voltammetry, high-field cyclic voltammetry, and EIS data are systematically investigated. The activation energies (E_a) for ORR determined from the slope of Arrhenius plots are in the range of 102.33-150.73 kJ mol⁻¹ for LCNC-SDC composite cathodes. The experimental results found that LCNC-SDC (70:30) composite cathode has a maximum exchange current density and a minimum polarization resistance of 0.30 Ω cm² for 850 °C among LCNC-SDC composite cathodes.     

Development of LSM/YSZ composite cathode for anode-supported solid oxide fuel cells

This paper presents the effect of (La,Sr)MnO₃ (LSM) stoichiometry on the polarization behaviour of LSM/Y₂O₃-ZrO₂ (YSZ) composite cathodes. The composite cathode made of A-site deficient (La_0.85Sr_0.15)_0.9MnO₃ (LSM-B) showed much lower electrode interfacial resistance and overpotential losses than that made of stoichiometric (La_0.85Sr_0.15)_1.0MnO₃ (LSM-A). The much poorer performance of the latter is believed to be due to the formation of resistive substances such as La₂Zr₂O₇/SrZrO₃ between LSM and YSZ phases in the composite electrode. A slight A-site deficiency (∼0.1) was effective in inhibiting the formation of these resistive substances. A power density of ∼1 W cm⁻² at 800 °C was achieved with an anode-supported cell using an LSM-B/YSZ composite cathode. In addition, the effects of cathodic current treatment and electrolyte surface grinding on the performance of composite cathodes were also studied.     

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).