Self-regulating surface chemistry for more robust highly durable solid oxide fuel cell cathodes

Solid oxide fuel cells (SOFCs) which can directly covert the chemical energy of fuel to electric energy have received as one of alternative energy sources because of their high energy conversion efficiency, fuel flexibility and low emission of pollutant to the environments. However, there are still several technical issues for making commercialization of SOFCs such as limited durability and long start-up time. Hence, recent studies of the SOFCs have focused on improving performance of SOFCs in the intermediate temperature (IT) to achieve technical goals such as high stability, low-cost component materials, easier sealing and quicker start-up time. In particular, stable long-term performances of SOFCs are one of the essential factors for commercialization of fuel cell systems. However, the sudden unstable situations such as frequent electrical load on-off and high load can often occur during the SOFCs operations, and thus, these unexpected situations can affect the durability of SOFCs systems. Therefore, we need to investigate performance change of SOFCs under various electrical load conditions including the constant current, current on-off cycles and irregular current change. In addition, post-mortem analysis of SOFCs are necessary to find the main degradation mechanisms after various durability tests. In this work, Ni-cermet anode supported cells are fabricated by the tape-casting method with neodymium doped ceria and double-perovskite structure based cathode materials (NdBa0.5Sr0.5Co1.5Fe0.5O5+δ). The performance of the cells are investigated by I-V measurement and the durability test also conducted by using load trip (0~200 mAcm-2) and load cycle (120~200 mAcm-2) techniques with controlled current condition at 650 oC. For electrochemical analysis, electrochemical polarization and characterizations are conducted using an electrochemical impedance spectroscopy (EIS) with a Potentiostat/Galvanostat instrument using a frequency range from 1 MHz to 1Hz. The several physicochemical analyses are also investigated using field emission-scanning electron microscopy (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). References E. D. Wachsman and K. T. Lee, Science 334 (2011) 935-939.S. B. Adler, Chem. Rev. 104 (2004) 4791-4843.E. Park, S. Taniguchi, T. Daio, J. T. Chou and K. Sasaki, Solid State Ionics 262 (2014) 421-427.B. C. H. Steele and A. Heinzel, Nature 414 (2001) 345-352.S. Omar, E. D. Wachsman and J. C. Nino, Solid State Ionics 177 (2006) 3199-3203.M. Kahlaoui, S. Chefi, A. Inoubli, A. Madani and C. Chefi, Ceram. Int. 39 (2013) 3873-3879. Keywords : Solid oxide fuel cells; intermediate-temperature; durability; perovskite.

Solid Oxide Fuel Cell (SOFC) cathode materials rely on the catalytic processes at their surfaces to reduce and incorporate oxygen. For many common cathode materials surface passivation occurs through 'A site' segregation at common operating temperatures [1]. These processes have a negative effect upon the oxygen reduction rates and therefore overall cell performance. Quantifying the true extent these segregation effects have on the exchange properties has not been fully explored but it is clear that the properties of the surface are essential. Many techniques have been employed to study cell properties in-situ, however, surface studies are typically restricted to post mortem or ex-situ analysis due to the necessity of a high vacuum. In this study High Temperature Environmental Scanning Electron Microscopy (HT-ESEM) has been employed to analyse the morphology changes of common SOFC cathode materials in-situ. Samples of La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and La0.6Sr0.4CoO3-x (LSC) were annealed from room temperature to 1000oC at pressure ranges between 3 – 6mbar in atmospheres of Oxygen and Water. Using secondary electron imaging during the thermal annealing cycle morphological and compositional changes were able to be documented at temperature for the first time (Figure 1). This technique has enabled detailed analysis of the surface degradation that occurs during SOFC fuel cell operation. Secondary phase precipitation rate and formation behaviour has been quantified over a range of temperatures and time scales furthering the understanding of SOFC cathode surfaces significantly. [1] M. Kubicek, A. Limbeck, T. Fromling, H. Hutter, and J. Fleig, "Relationship between cation segregation and the electrochemical oxygen reduction kinetics of La0.6Sr0.4Co0.2Fe0.8O3 thin film electrodes" Journal of The Electrochemical Society, vol. 158, no. 6, pp. B727–B734, 2011. Figure 1 - Showing in-situ growth of secondary phase particulates on a polished surface of LSCF Figure 1

Influence of carbon deposition on the anode performance of solid oxide fuel cells

Solid oxide fuel cells (SOFCs) attract attentions as high-efficiency energy-conversion devices. In order to establish high-performance SOFCs, the slow electrode kinetics, which is often a main cause of the energy loss, should be improved. In present SOFCs, mixed ionic-electronic conducting (MIEC) oxides, such as (La,Sr)CoO3 -δ and (La,Sr)(Co,Fe)O3 -δ , are typically used as the cathode materials. In such an MIEC cathode, it is generally believed that electrochemical oxygen reduction reaction takes place mainly at the oxide surfaces (double phase boundary, DPB) and the contribution of the reaction at the electrode/electrolyte/gas interface (triple phase boundary, TPB) is insignificant. Therefore, researches on the kinetics of SOFC cathodic reaction have mostly concerned to investigate the surface reaction on the electrode oxides. In our previous works, the reaction distributions in MIEC SOFC electrodes were experimentally evaluated by using operando micro X-ray absorption spectroscopy (operando μ-XAS) [1, 2]. For more quantitative and precise investigation of the reaction distribution, we proposed a patterned thin film electrode, which is a kind of the columnar electrode simplifying the microstructures of a porous electrode [2]. However, the patterned thin film electrode in our previous work did not have triple phase boundaries, and it was thus impossible to discuss the contribution of TPB reaction to the total reaction. In this work, a patterned thin film electrode having triple phase boundaries was fabricated. (La,Sr)CoO3- d (LSC) was chosen as the model MIEC oxide for SOFC cathodes. The reaction distribution in this new type of the patterned thin film electrode was experimentally determined by using operando μ-XAS, and the results were compared with ones obtained with the old type of the patterned thin film electrode without TPB. It was found the effective reaction area tended to shrink by the introduction of TPB to the patterned thin film electrode. This fact might suggest that the reaction at TPBs in SOFC cathodes is not negligible even when MIEC oxides are used as electrode materials.

Solid oxide fuel cells (SOFCs) are getting more importance with their promising future, due to their high-energy conversion efficiency, low-emissions and flexibility of usable fuel type. The performance and the lifetime of SOFCs are keenly dependent on electrode microstructure. In order to recognize the microstructural evolution and its degradation kinetics in SOFC cermet cathodes during long exposure time (up to 20 000 h) under realistic operating conditions ( T= 850 °C, J= 190-250 ), investigations on porous LSM/YSZ cathodes were conducted. The 3D-tomography technique (FIB/SEM) offers extensive data about the microstructures of various cathode aged during different operating times (2 500 h, 15 000 h and 20 000 h), which allows an exact quantification of particle size distribution, phase-connectivity, tortuosity factor and Triple Phase Boundary Length (TPBL). With the increasing operating time no significant 3D microstructural changes in the cathode were noticed in the obtained data. However, additional qualitative X-ray fluorescence measurement, indicate a clear presence of chrome contamination on aged cathodes, which may be the main degradation mechanism in the SOFC cathode. Keywords: chrome poisoning, long operating time, particle size distribution, SOFC, triple phase boundary, SEM, XRF

A dynamic infiltration technique to synthesize nanolayered cathodes for high performance and robust solid oxide fuel cells

The solid oxide fuel cells (SOFCs) have been considered as important alternative energy technology, due to a variety of advantages, such as their high energy conversion efficiency, free noxious emissions, and direct use of a wide range of hydrocarbon fuels. Especially, the reverse operation of SOFCs, which is called solid oxide electrolysis cell (SOEC), is currently one of the most promising technologies for fuel production. However, their high-temperature operation (typically >700°C) leads to the rapid degradation of electrodes and necessitates a high balance of plant costs. To make SOFCs more economically feasible and viable, further reduction of operating temperature is inevitable. To this end, the utilization of proton-conducting oxides, which exhibit relatively high ionic conductivity with the low activation energy (<0.5eV), enables protonic ceramic fuel cells (PCFCs) to operate at an intermediate temperature regime (IT, 400–600°C), where the aforementioned benefits of SOFCs can also be accomplished. Moreover, PCFCs prevents the dilution of the fuel during the operation and provides higher resistance of carbon cocking on hydrocarbon fuels.Although the PCFCs have distinct advantages against traditional oxygen conducting SOFCs, the lower-than-predicted performance of PCFCs and scalability issues caused the considerable skepticism of the successful applicability of PCFCs. Most importantly, unlike traditional SOFCs, the development of mixed oxide ion and electron conductors (MIECs) for oxygen reduction reaction (ORR) at the cathode of PCFCs has shown marginal success due to the absence of protonic conduction and still needs to be improved. At the cathode, the ORR is limited to triple phase boundary (TPB), where the electrolyte, gas, and cathode mutually contact, and its geometrical enhancement at the interface area between cathode and electrolyte layers is crucial to improve the performance of PCFCs. Therefore, electrode-electrolyte interface architecting via micro-patterning is one of the most attractive approaches, since it significantly expands the catalytic active sites of TPB near the interface, where ORR preferentially occur. Moreover, this architecting approach also facilitates the use of currently viable state-of-art cathode materials of SOFCs; Lanthanum Strontium Cobalt Ferrite (LSCF), Barium Strontium Cobalt Ferrite (BSCF). Therefore, patterns with a high aspect ratio, which maximize the interface area, are required for significant enhancement of electrochemical performance.Here, for the first time, we describe a three-dimensionally micro-architecting process for anode-supported PCFCs, demonstrating dramatic performance enhancement from the expanded interface area between cathode and electrolyte. The various patterns with high ratios were exactly transferred from the original PUA (polyurethane acrylate) molds to the thin and dense protonic conducting electrolyte layer. The patterns maintained the same aspect ratios during the sintering process and finally increased the interface area of more than 30%. The morphology of the patterned surface was characterized by scanning electron microscope (SEM) and atomic force microscopy (AFM). The AC electrochemical impedance spectroscopy was utilized to examine the electrochemical performances of patterned cells. Our unique patterning method opens the new pathway for further performance enhancement of PCFCs. Figure 1

Solid oxide fuel cells (SOFCs) have aroused worldwide attention for their high conversion efficiency, zero emissions, and fuel flexibility. Generally, a SOFC consists of an anode, a cathode and a electrolyte. The electrolytes usually exert a crucial role in the performance of SOFCs. The normal electrolyte materials could be generally divided into two subclasses: oxygen ion and proton-conducting electrolyte ones, based on their different types of conduction mechanisms. In this chapter, we describe the requirements for the materials as the electrolytes of SOFCs and the recent developments of oxygen-ion conducting electrolyte, proton-conducting electrolyte and dual-phase composite electrolyte materials. Major attention is paid to the influences of structures, preparation and modified methods on the ion conduction properties of fluorite and perovskite electrolyte materials of SOFCs.

Solid oxide fuel cell (SOFC) technology requires further maturations to become commercialized broadly. Many existing problems are expected to be solved by intermediate temperature SOFC (IT-SOFC). On the other hand, electrochemical resistances of cathode materials increase rapidly at lower temperature. So it is necessary to understand the underlying mechanisms of oxygen reduction reaction and develop new materials. Although many experimental and theoretical researches for this purpose are in progress, traditional trial-and-error materials search must be inefficient and take too long time. Considering the above situation, we explore critical factors that can explain the oxygen exchange rate of perovskite oxide cathode in SOFCs. It has been said that the bulk ionic conductivity is strongly related to the oxygen exchange rate, while the electronic conductivity is not as far as it is high enough. However, we found that the combination of ionic and electronic conductivities shows much stronger correlation than ionic conductivity alone while analyzing reported experimental data on 18 materials in figure 1. Materials which have high oxygen exchange rate can be explained by the multiplication of bulk ionic conductivity and electronic conductivity while others explained by the ionic conductivity. Fig. 1 Relationship between oxygen exchange rate and bulk electronic conductivity, ionic conductivity, and the model function with both conductivities. Figure 1

The high-temperature solid oxide fuel cells (SOFCs) are the most efficient and green conversion technology for electricity generation from hydrogen-based fuel as compared to conventional thermal power plants. Many efforts have been made to reduce the high operating temperature (>800 °C) to intermediate/low operating temperature (400 °C<T<800 °C) in SOFCs in order to extend their life span, thermal compatibility, cost-effectiveness, and ease of fabrication. However, the major challenges in developing cathode materials for low/intermediate temperature SOFCs include structural stability, catalytic activity for oxygen adsorption and reduction, and tolerance against contaminants such as chromium, boron, and sulfur. This research aims to provide an updated review of the perovskite-based state-of-the-art cathode materials LaSrMnO3 (LSM) and LaSrCOFeO3 (LSCF), as well as the recent trending Ruddlesden-Popper phase (RP) and double perovskite-structured materials SOFCs technology. Our review highlights various strategies such as surface modification, codoping, infiltration/impregnation, and composites with fluorite phases to address the challenges related to LSM/LSCF-based electrode materials and improve their electrocatalytic activity. Moreover, this study also offers insight into the electrochemical performance of the double perovskite oxides and Ruddlesden-Popper phase materials as cathodes for SOFCs.

Review: 469 refs.

Numerical analysis of the effects of interconnector design and operating parameters on solid oxide fuel cell performance

Introduction With the recent enhancement of renewable energy utilization, large scale energy conversion and storage become more important to control the surplus electric power due to the unsteady power generation. Reversible hydrogen energy storage system via solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) are expected as a high efficient device because of its high operating temperature (600℃~1000℃). It is possible to unify them to one device as reversible system (RSOFC/EC), but it is necessary to understand the mechanism of FC/EC reactions and design the device with a good balance between FC and EC in order to apply them for an actual energy system. While SOFC reaction models and many electrochemical data of electrodes have been developed and reported by many researchers, there are few reports to investigate the reaction model of SOEC. Thus, it is possible to predict the performance of SOEC based on a comprehensive SOFC/EC models and experimental data of SOFC, because the reactions of SOFC and SOEC are a reversible relationship and the equal value of IR loss can be applied. However, the comprehensive SOFC/EC electrochemical model has not been established. A series of experimental data of anodic and cathodic current density vs. the potential should be investigated at various conditions to develop the SOFC/EC model. In this work, a series of anodic and cathodic electrode performance was taken using a SOFC in various conditions. Furthermore, attempting to predict SOEC performance from SOFC performances, we proposed and discussed a new model of electrolysis based on our developed SOFC model. Experimental The Disks of 8 mol% Y2O3-ZrO2 (YSZ; 0.25 mm thickness, 20 mm diameter) were used as the electrolyte and the anode was a Ni/YSZ or Ni/GDC composite, and the cathode was a La0.8Sr0.2MnO3/ScSZ composite. We measured the dependence of the current density of anodes on oxygen activity with wide range to get both power generation and electrolysis data for various gas composition (PH2=0.3~0.99 atm, PH2O=0.01, 0.03, 0.05 atm). Results and Discussion 1. Method to fit parameters on the SOFC/EC model We have developed the electrochemical reaction model on the fuel electrode (=anode) based on the equilibrium of competitive adsorption and oxygen activity, ao , specified by anode potential (Eq.1, Table.1) [1]. The SOFC model was constructed by assuming competitive adsorption equilibrium of H2, H2O and O on Ni surfaces at three-phase boundary and the rate-determining step to be Langmuir-type reactions of H with O (Table.1). We obtained an equation of the electrical current density, i, as a function of the oxygen activity (ao ) calculated from the anode potential which is simply measured from experiments. To confirm effectiveness of the model, we fitted the model to actual experimental data described by i vs. ao . From the equations of model on Table.1, each current density of SOFC (ia ) and SOEC(ic ) are described as Eq.2. To draw the plot of i vs. ao on the model, the rate constants (ka, kc ) and the equilibrium constants (KO, KH, KH2O ) need to be defined as reliable value. Thus, from the equilibrium potential vs. 1 atm O2 (Eq.3), we defined the rate constant of kc as Eq.4. When the kc became a dependent variable of ka , then we fitted ia to power generation experimental data described by i vs. ao so that the values of KHKO/KH2O were fitted approximately. Finally, i c model could be drawn on the plot of i vs. ao and the maximum value of i c was obtained by Eq.5 which is independent of ao and Eq.6 was obtained using Eq.4. 2. Experimental results and parameter fitting on the SOFC/EC model As the expreimental results, the maximum power density of SOFC with Ni/YSZ was 588.5 mW/cm2 for FC mode at PH2=0.99 atm, PH2O=0.01 atm, and ic(max) was 0.285 A/cm2 for EC mode at PH2O=0.03 atm. However, ic(max) calculated by the model (Eq.6) was 0.050 A/cm2. Also, in case of SOFC with Ni/GDC, ic(max) was 0.400 A/cm2 for EC mode at PH2O=0.03 atm, while ic(max) calculated by the model (Eq.6) was 0.055 A/cm2. The electrolysis current density measured by our RSOFC/EC showed 6 times higher than model values approximately. This result suggested the possibility that additional parallel reaction path was existed. In this presentation, we will discuss the possible electrochemical paths to explain the disagreement between the model and the experimental data. Finally, the comprehensive SOFC/EC model will be proposed to predict EC performance using SOFC data.

Integrated gasification fuel cell (IGFC) systems combining coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally friendly utilization of coal for power production. Most IGFC system analyses performed to-date have used nondimensional thermodynamic SOFC models that do not resolve the intrinsic constraints of SOFC operation. In this work a quasi-two-dimensional (2D) finite volume model for planar SOFC is developed and verified using literature data. Special attention is paid to making the model capable of supporting recent SOFC technology improvements, including the use of anode-supported configurations, metallic interconnects, and reduced polarization losses. Activation polarization parameters previously used for high temperature electrolyte-supported SOFC result in cell performance that is much poorer than that observed for modern intermediate temperature anode-supported configurations; thus, a sensitivity analysis was conducted to identify appropriate parameters for modern SOFC modeling. Model results are shown for SOFC operation on humidified H2 and CH4 containing syngas, under coflow and counterflow configurations; detailed internal profiles of species mole fractions, temperature, current density, and electrochemical performance are obtained. The effects of performance, fuel composition, and flow configuration of SOFC performance and thermal profiles are evaluated, and the implications of these results for system design and analysis are discussed. The model can be implemented not only as a stand-alone SOFC analysis tool, but also a subroutine that can communicate and cooperate with chemical flow sheet software seamlessly for convenient IGFC system analysis.

Integrated gasification fuel cell (IGFC) systems combining coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally friendly utilization of coal for energy production. Most IGFC system analyses performed to date have used non-dimensional thermodynamic SOFC models that do not resolve the intrinsic constraints of SOFC operation. In this work, a one-dimensional finite volume model for planar SOFC is developed and verified using literature data. Special attention is paid to making the model capable of supporting recent SOFC technology improvements, including the use of anode-supported configurations, metallic interconnects, and reduced polarization losses. Results are presented for SOFC operation on humidified hydrogen and methane-containing syngas, under co-flow and counter-flow configurations; detailed internal profiles of species mole fractions, temperature, current density and electrochemical performance are obtained. The effects of performance, fuel composition and flow configuration on SOFC performance and thermal profiles are evaluated, and the implications of these results for system design and analysis are discussed.