Conventional SOFC Research Articles

(Invited) Metal-Supported Solid Oxide Fuel Cells with High Power Density

Metal-supported solid oxide fuel cells (MS-SOFC) display a number of advantages over conventional all-ceramic SOFCs, including low-cost structural materials (e.g. stainless steel), mechanical ruggedness, excellent tolerance to redox cycling, and extremely fast start-up capability. Challenges for MS-SOFCs include: oxidation of the metal support, especially at 800°C and above; the possibility of the stainless steel exacerbating Cr poisoning of the cathode catalyst; fabrication and materials set restrictions arising from the requirement that stainless steel be sintered in reducing atmosphere; and, only moderate performance and lifetime have been demonstrated to date. These advantages and challenges suggest that MS-SOFCs may be well-suited for portable, ruggedized, fast-start, intermittent-fuel, or other unique and innovative applications. Nissan has developed the world’s first light-duty vehicle with a conventional SOFC stack for traction power, fueled by bio-ethanol reformate. Because of the desire for rapid-start capability, developing MS-SOFC cell and stack technology for this vehicular application is a priority. The small volume allowance for the SOFC stack on-board a small vehicle furthermore demands high power density from the MS-SOFC. In this work, we seek to maximize MS-SOFC power density. This work is an extension of our previous efforts focusing on co-sintered, YSZ-based MS-SOFCs with porous metal supports on both anode and cathode sides, with catalysts deposited into both electrodes via infiltration. These features provide for a mechanically rugged cell that can be processed with low-cost scalable techniques, and high surface-area catalysts that provide excellent performance by avoiding interdiffusion or coarsening during cell sintering. To support the vehicular traction application, in this work we further develop catalyst infiltration processing for MS-SOFCs to dramatically increase power density. The goal is to increase cell performance by improving infiltration of conventional catalyst compositions (lanthanum strontium manganite (LSM), Ni, doped ceria) into electrode backbones of conventional yttria-stabilized zirconia (YSZ). Various aspects of the infiltration procedure were optimized, including: precursor composition and dilution with water; catalyst loading; and, crystallization temperature. Figure 1 shows the resulting high performance. Peak power of 1.1 and 1.9 W/cm2 is achieved at 700°C and 800°C, respectively, and the 800°C performance is the highest reported for stainless steel-supported MS-SOFCs to date. Figure 2 shows initial stability of the MS-SOFC. Testing is ongoing, and operation for >1000 h and post-mortem analysis will also be presented. Demonstration of redox cycling and rapid-start tolerance will be discussed. Acknowledgements Funding for this work was provided by Nissan Motor Co., Ltd. through Strategic Partnership Projects Agreement FP00004436. This work was funded in part by the U.S. Department of Energy under contract no. DE-AC02-05CH11231. Figure 1

Metal-Supported Solid Oxide Fuel Cell with High Power Density

Metal-supported solid oxide fuel cells (mSOFC) display a number of advantages over conventional all-ceramic SOFCs, including low-cost structural materials (e.g. stainless steel), mechanical ruggedness, excellent tolerance to redox cycling, and extremely fast start-up capability. Challenges for mSOFCs include: oxidation of the metal support, especially at 800°C and above; the possibility of the stainless steel exacerbating Cr poisoning of the cathode catalyst; fabrication and materials set restrictions arising from the requirement that stainless steel be sintered in reducing atmosphere; and, only moderate performance and lifetime have been demonstrated to date. These advantages and challenges suggest that mSOFCs may be well-suited for portable, ruggedized, fast-start, intermittent-fuel, or other unique and innovative applications. Nissan has developed the world’s first light-duty vehicle with a conventional SOFC stack for traction power, fueled by bio-ethanol reformate. Because of the desire for rapid-start capability, developing mSOFC cell and stack technology for this vehicular application is a priority. The small volume allowance for the SOFC stack on-board a small vehicle furthermore demands high power density from the mSOFC. In this work, we seek to maximize mSOFC power density. This work is an extension of our previous efforts focusing on co-sintered, YSZ-based mSOFCs with porous metal supports on both anode and cathode sides, with catalysts deposited into both electrodes via infiltration. These features provide for a mechanically rugged cell that can be processed with low-cost scalable techniques, and high surface-area catalysts that provide excellent performance by avoiding interdiffusion or coarsening during cell sintering. To support the vehicular traction application, in this work we further develop catalyst infiltration processing for mSOFCs to dramatically increase power density. The goal is to increase cell performance by improving infiltration of conventional catalyst compositions (lanthanum strontium manganite (LSM), Ni, doped ceria) into electrode backbones of conventional yttria-stabilized zirconia (YSZ). Various aspects of the infiltration procedure were optimized, including: precursor composition and dilution with water; catalyst loading; and, crystallization temperature. Figure 1 shows the resulting high performance. Peak power of 1.1 and 1.9 W/cm2 is achieved at 700°C and 800°C, respectively, and the 800°C performance is the highest reported for mSOFCs to date. Cell stability in the operating temperature range 600 to 750°C will also be presented. Acknowledgements Funding for this work was provided by Nissan Motor Co., Ltd. This work was funded in part by the U.S. Department of Energy under contract no. DE-AC02-05CH11231. Figure 1

Comparative Study of Metal Impregnated Lanthanum Doped Strontium Titanate Anodes and Conventional Nickel Oxide Anode for Solid Oxide Fuel Cell Application

In conventional SOFC, composite anode made up of nickel oxide (NiO) and yttria stabilized zirconia (YSZ) are used [1]. NiO is an excellent anode for SOFC application because of its high electrochemical activity towards H2 oxidation. But the amount of NiO used is around 30- 40 wt% to reach a percolation threshold [2]. Agglomeration of nickel particles due to sintering is a serious concern during SOFC operation at elevated temperatures (800 ˚C). In order to address the above mentioned issue, we have developed doped strontium titanate (SrTiO3) based perovskite anodes. Perovskite anodes are mixed ionic and electronic conductors (MIEC) providing enhanced triple phase boundary (TPB) which lowers the over-potential losses compared to pure conductors [3]. To increase the electrical conductivity, lanthanum (La) has been doped at A-site of SrTiO3 using solid state route (SSR), to form La0.3Sr0.7TiO3+δ (LST). Physical characterizations like field emission scanning electron microscope (FESEM), x-ray diffraction (XRD) and image analysis (ImageJ) were carried out before and after reduction of anodes in H2 environment to analyze their microstructure, phase stability and particle sizes as shown in Figure 1 and 2. Conductivity measurements were carried out using two probe conductivity measurement system to determine the activation energy (Ea) required in reducing the anodes in H2 environment. Along with physical characterizations, electro-chemical characterizations were carried out as described. The halfcell (LST/ YSZ) gave a high polarization resistance (Rp) of 62 Ω cm2, low current density of 20 mA cm-2 and power density of 5 mW cm-2 at 800 ˚C. We impregnate the porous anode with varying NiO content using wet impregnation technique and observed that LST impregnated with 4 wt% NiO forms a percolated network and provided enhanced performance. A decrease in Rp to 4 Ω cm2 and increase in current density to 156 mA cm-2 in 4 wt% NiO impregnated LST halfcells is seen at 800 ˚C. The performance increased sharply upto 900 mA cm-2 in case of halfcell with LST anode when impregnated with both nickel(4%) and ceria(6%) with low Rp of 0.5 Ω cm2, giving a maximum power density of 200 mW cm-2 at 800 ˚C as shown in Figure 3(a, b). The above obtained power density is better in performance compared to the one given by half cells of NiO-YSZ anodes [2]. Thus, metal impregnated A-site doped perovskite anodes can be potential substitute for NiO-YSZ anodes for SOFC application. Key words:A-site doped perovskites, Mixed ionic and electronic conductor, Activation energy and half-cell study.

Development of High Efficiency Steel Cell Technology for Multiple Applications

Ceres Power’s unique SteelCell low temperature metal-supported SOFC technology has demonstrated best-in-class robustness to cyclic operation, along with efficiency comparable with more conventional SOFC technology, and intrinsically low cost at the 1kWe scale.There is also considerable work being undertaken to develop SteelCell technology to be used for higher power applications. Multiple development streams are being pursued to achieve the objectives of high efficiency (60%) and long life needed for prime power applications. This paper will discuss the progress being made to demonstrate the improvements to the fuel cell design to enhance efficiency, life, volumetric power density and fuel utilisation consistent with the requirements of larger power generation units.

Contribution of Triple-Phase Boundary Reaction in Cathodic Reaction of Solid Oxide Fuel Cell

In a conventional SOFC cathode using mixed ionic and electronic conductor (MIEC), the electrochemical reaction proceeds not only at triple-phase boundaries (TPB: gas/electrode/electrolyte) but also on double-phase boundaries (DPB: gas/electrode). In the MIEC cathode, electrochemical reaction generally occurs within a limited area from electrode/electrolyte interface because the resistance for oxide ion conductivity becomes significant compared to that for surface reaction as the diffusion length of oxide ion becomes longer. Our research group has experimentally clarified the reaction area in a porous MIEC cathode by operando micro X-ray absorption spectroscopy (XAS) measurements (1). In a porous La0.6Sr0.4CoO3-δ electrode under a constant cathodic overpotential of 140 mV at 873 K in 10-2 bar of oxygen partial pressure P(O2), it was found the electrochemical reaction proceeded within 2 µm from the electrode/electrolyte interface. In the higher P(O2), the reaction area would become even shorter. It is in general believed that the total electrochemical reaction in a porous MIEC cathode is dominated by the reaction on DPB and the contribution of the reaction at TPB is insignificant. However, when the electrochemical reaction proceeds only in the vicinity of the electrode/electrolyte interface, as mentioned above, it is considered that the electrochemical reaction at TPB cannot be ignored. In this study, for investigating the contribution of the TPB reaction, patterned thin film electrodes with/without TPB, which were kinds of columnar electrodes, were fabricated. The distributions of electrochemical reaction in the model electrodes were evaluated by operando micro XAS measurements. In a La0.6Sr0.4CoO3-δ patterned electrode without TPB, the electrochemical reaction proceeded within 60 µm from the electrode/electrolyte interface under a constant cathodic overpotential of 190 mV at 973 K in 10-1 bar of P(O2). On the other hand, in the patterned electrode with TPB, the electrochemical reaction proceeded within 40 µm from the interface under the similar experimental conditions. The distribution of electrochemical reaction was decreased by the introduction of TPB, which demonstrated the contribution of the TPB reaction even in the MIEC cathode. The distribution of electrochemical reaction was also evaluated by isotopic exchange experiments and following secondary ion mass spectrometry (SIMS) measurements (2). 18O/16O isotopic exchanges were performed with/without cathodic overpotential, and the isotope exchange profiles were analyzed by SIMS measurements. The electrochemically incorporated 18O was evaluated as the difference between the isotope exchange profiles with/without cathodic overpotential. In the presentation, the contribution of the TPB reaction in the MIEC cathode will be discussed based on the results obtained by isotopic exchange measurements. 1. Y. Fujimaki, H. Watanabe, Y. Terada, T. Nakamura, K. Yashiro, S. Hashimoto, T. Kawada and K. Amezawa, ECS Trans., 2013, 57 (1), 1925-1932. 2. Y. Fujimaki, T. Nakamura, K. Develos-Bagarinao, K. Yamaji, K. Yashiro, T. Kawada, F. Iguchi, H. Yugami and K. Amezawa, ECS Trans., 2015, 68 (1), 623-630.

Comparison Between Conventional Design and Cathode Gas Recirculation Design of a Direct-Syngas Solid Oxide Fuel Cell–Gas Turbine Hybrid Systems Part I: Design Performance

In this paper, a conventional SOFC–GT hybrid system and a SOFC–GT hybrid system with cathode gas recirculation system fueled with syngas as the main source of energy were analyzed and their performances were compared. In the conventional SOFC–GT hybrid system, the incoming air to the cathode was heated at air recuperator and air preheater to meet the required cathode inlet temperature. In the SOFC–GT hybrid system with cathode gas recirculation, besides air recuperator and air preheater, the recirculation of the cathode exhaust gas was also used to meet the required cathode inlet temperature. The system performances have been analyzed by means of models developed with the computer program Cycle–Tempo. A complete model of the SOFC–GT hybrid system with these two configurations evaluated in terms of energy and exergy efficiencies and their performance characteristics were compared. Simulation results show that the electrical energy and exergy efficiencies achieved in the cathode gas recirculation plant (64.76% and 66.28%, respectively) are significantly higher than those obtained in the conventional plant (54.53% and 55.8%). Article History: Received Feb 23rd 2017; Received in revised form May 26th 2017; Accepted June 1st 2017; Available onlineHow to Cite This Article: Azami, V, and Yari, M. (2017) Comparison between conventional design and cathode gas recirculation design of a direct-syngas solid oxide fuel cell–gas turbine hybrid systems part I: Design performance. International Journal of Renewable Energy Development, 6(2), 127-136.https://doi.org/10.14710/ijred.6.2.127-136

Understanding the Cathode Reaction Process for Proton Conducting Intermediate Temperature Solid Oxide Fuel Cells Using Comparison Between BSCF and Other Cathode Materials

Intermediate temperature (~400-600oC) solid oxide fuel cells (IT-SOFC) have drawn growing attention due to the possibility to use cheaper sealing and interconnect materials and the potential to improve system stability and reliability comparing with conventional SOFC that operate at ≥~750oC. For IT-SOFC, proton conducting oxides such as acceptor-doped Ba(Ce, Zr)O3 are often preferred over oxide ion conductors as the electrolyte materials due to their high ionic conductivities and low electronic leakage at intermediate temperature. In this study, the electrochemical behaviors of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), which is one of the most active cathode materials for SOFC and shows significant hydration to become proton conductive in humidified atmosphere, is studied in comparison with other oxide (e.g., Pr2NiO4+δ) as well as metal (e.g., Ag) cathode materials at intermediate temperatures of ~350-650oC. In particular, the influences of different testing conditions including cell configuration (cathode symmetrical cells versus full cells), proton conducting electrolyte thickness, cathodic bias voltage, oxygen concentration, and the presence of air impurities such as water vapor (H2O) and carbon dioxide (CO2) on the electrochemical performance are systematically investigated. The implication of the experimental observations especially in terms of the similarity and differences in electrochemical behaviors between different electrode materials to the overall cathode reaction mechanism for proton conducting IT-SOFC is discussed. Finally, the directions for further improving the cathode performance for proton conducting IT-SOFC will be pointed out.

(Invited) High-Temperature Electrosynthesis of Hydrogen and Chemicals

High-temperature electrochemical cells based on solid electrolytes have been considered for synthesis and production of hydrogen and a variety of other fuels and chemicals. Solid oxide electrolysis cells (SOECs) for example, are being developed for hydrogen production (from water), syngas production (from mixtures of water and carbon dioxide) and oxygen recovery (from water or carbon dioxide for space applications). The main attractive features of the SOEC are its simple chemistry, environmental compatibility and high electrical-to-hydrogen efficiency. Solid oxide fuel cell (SOFC type cells have been investigated for production of chemicals and cogeneration of electricity and chemicals (in this case, the cell is referred to as CSOFC); examples include production of C2 hydrocarbons and syngas from methane. The key features of the CSOFC include its high selectivity and the possibility to produce valuable chemicals such as hydrocarbons using methane-rich gases from non-fossil origins. Current state-of-the-art SOECs and CSOFCs, both fundamentally derived from the more advanced conventional SOFC technology are still in their feasibility/development stage. This paper discusses certain aspects in selected materials, performance and operation of the SOEC for hydrogen production from water and the CSOFC for production of C2 hydrocarbons from methane.

Effect of reverse Boudouard reaction catalyst on the performance of solid oxide carbon fuel cells integrated with a dry gasifier

A solid oxide carbon fuel cell (SO-CFC) integrated with a dry gasifier was operated on activated carbon fuel and the effect of adding a reverse Boudouard gasification catalyst on the performance and long-term operation characteristics of the SO-CFC was investigated. The reactivity of the carbon fuels for the Boudouard gasification reaction was analyzed by a thermal analysis at various operating conditions. The SO-CFC was then operated on gasified fuel gas consisting of CO2 and CO obtained from the integrated dry gasifier. The SO-CFC operated on activated carbon fuel with 5wt.% K2CO3 achieved a maximum power density of 202, 262, and 271mW/cm2 at 750, 800, and 850°C, respectively; the SO-CFC fueled with activated carbon fuel without a catalyst meanwhile yielded maximum power density of 168mW/cm2 at 850°C. By using electrochemical impedance spectroscopy, the effect of adding the catalyst on the gasification products and subsequently on the performance of the SO-CFC was studied. A long-term degradation test was conducted by continuously operating the SO-CFC at 50mA/cm2 for 518h at 750°C. During the long-term degradation test, the average degradation rate of the SO-CFC was found to be 183mV/kh. The post-mortem SEM and XRD analyses of the SO-CFC after the long-term test revealed the presence of carbon deposits and oxidation of Ni at the anode, causing a relatively higher degree of degradation in the SO-CFC integrated with the dry gasifier during the long-term operation. The addition of the K2CO3 based dry gasification catalyst significantly enhances the performance of the SO-CFC integrated with dry gasification, but during long-term operation, the degradation rate is found to be higher than that of conventional H2 fueled SOFCs.

Influence of a Ni Metal Phase and Geometrical Structures of YSZ in Ni-YSZ SOFC Anode to Its Mechanical Properties

Conventional SOFCs, which adopt an anode supported type cell design, use the porous cermet of Ni and YSZ as the anode. On the viewpoint of mechanical design, the anode is an important component next to the electrolyte because it prevents the electrolyte from destruction caused by thermal stress, chemical stress, and outer force. However, the anode also show uncertainly in the response to outer force, which is most fundamental property for mechanical design. This uncertainly is due to the existence of Ni metal phase and complex geometrical structure in the anode (Ni-YSZ). To maintain electrical conductivity, gas permeability, and catalytic activity, the anode consist of Ni, YSZ and porosity, which volumatic ratio are almost same, and each phase are continuously connected in the anode, in other words, three networks co-exist in the anode. At high temperature close to ½ of melting temperature of Nickel(Tm=1455oC), Nickel is easily deformed plastically. By contrast, YSZ(Tm=2700oC) is still brittle. In addition, each network are non-uninform. So, stress concentration and bending occur by outer force applying. These things make behavior more complex. However, the response of outer force and its relationship between the response to microstructure have not been well studied. Yield strength of nickel (ca.150MPa) at room temperature is close to fracture strength of YSZ(ca. 200MPa). Therefore, at room temperature, the anode cermet show brittle behavior because YSZ network is fractured at the same time of nickel network yields. However, at SOFC operating temperature, nickel yield strength becomes very low (ca.20MPa@800oC) . In this situation, nickel network yields much smaller stress than fracture of YSZ network. Geometrical structure in cermet is categorized to two types. First type is often used for high speed tools, and, guest material shows isolated distribution in host material. Second type is used for the anode, and host and guest materials make continuous networks in cermet, respectively. Response to outer force in first type are dominated by host materials. However, that in second type (SOFC Ni-YSZ anode) are not studied well.Therefore, we studied creep curves (response to small stress), stress-strain curves (response to large stress) in Ni-YSZ. Furthermore, we evaluated the influence of volume ratio between Ni and YSZ to fracture toughness. Based on the results, the influence of a Ni metal phase and geometric structures of YSZ to mechanical properties were discussed.Three chemical compositions (Ni vol% 30, 44, 58 : Ni30, Ni44, Ni58) of Ni-YSZ were tested. Both creep curves and stress-strain curves showed different behavior to outer forces from porous Ni metals or porous YSZ. In the case of creep test, activation of creep rate varied from that of YSZ to that of Ni metal, and this variation was suggested to be related to accumulated strain in Ni-YSZ specimens. Ni-YSZ specimens showed different fracture modes to outer forces from different directions. Under uniaxial compressive force, Ni44, Ni58 specimens was squished at the strain range over 1%, and Ni44 specimens were torn under bending force. These results suggested that mechanical properties were strongly influenced by the rigidity of YSZ network in Ni-YSZ. In addition, we discuss about the rigidity of YSZ networks at the presentation.

イットリアとすずをドープしたバリウム・ジルコニア電解質を用いた中温度プロトン伝導体固体酸化物燃料電池

In comparison with the conventional SOFC based on the conduction of oxide ion, the proton-conducting SOFC can be facilitated to high output power densities at significantly reduced operating temperatures between 600-800°C. Moreover, with the advantage of proton-conducting in this type of SOFC, a new alternative power generation system by integrating the proton-conducting SOFC with biomass gasification is very interesting in the near future. In this study a proton-conducting SOFC using BaZr0.7Sn0.1Y0.2O3-δ (BZSY) proton conductor was investigated. The solid oxide fuel cell with BZSY thin electrolyte was fabricated via simple and cost-effective dipping and sintering process. Although there was some cross-over of the single cell, the dense electrolyte was successfully developed with reduced sintering temperature at 1500°C by employing small amount of nickle oxide (NiO) as a sintering aid.

Performance Factors and Sulfur Tolerance of Metal Supported Solid Oxide Fuel Cells with Nanostructured Ni:GDC Infiltrated Anodes

Two metal supported solid oxide fuel cells (active area 16 cm2) with nanostructured Ni:GDC infiltrated anodes, possessing different anode and support microstructures were studied in respect to sulfur tolerance at an operating temperature of 650°C. The studied MS-SOFCs are based on ferretic stainless steel (FeCr) and showed excellent performance characteristics at 650°C with fuel utilization corrected area specific resistances of 0.35 Ωcm2 and 0.7 Ωcm2 respectively. The sulfur tolerance testing was performed by periodic addition of 2, 5, and 10 ppm H2S in hydrogen based fuel under galvanostatic operation at a current load of 0.25 Acm−2. The results were compared with literature on the sulfur tolerance of conventional SOFC Ni/YSZ cermet anode. The comparison in terms of absolute cell resistance increase and relative anode polarization resistance increase indicates, that the nanostructured Ni:GDC MS-SOFC based anode is significantly more sulfur tolerant than the conventional Ni/YSZ cermet anode. Furthermore, it was shown that the believed extension of the electrochemical three-phase-boundary reaction zone in the presence of GDC must be very limited and cannot account for the higher sulfur tolerance of GDC modified SOFC anodes.

Laser induced densification of cerium gadolinium oxide: Application to single-chamber solid oxide fuel cells

In single-chamber solid oxide fuel cells (SC-SOFC), anode and cathode are placed in a gas chamber where they are exposed to a fuel/air mixture. Similarly to conventional dual-chamber SOFC, the anode and the cathode are separated by an electrolyte. However, as in the SC-SOFC configuration the electrolyte does not play tightness role between compartments, this one can be a porous layer. Nevertheless, it is necessary to have a diffusion barrier to prevent the transportation of hydrogen produced locally at the anode to the cathode that reduces fuel cell performances. This study aims to obtain directly a diffusion barrier through the surface densification of the electrolyte Ce0.9Gd0.1O1.95 (CGO) by a laser treatment. KrF excimer laser and Yb fiber laser irradiations were used at different fluences and number of pulses to modify the density of the electrolyte coating. Microstructural characterizations confirmed the modifications on the surface of the electrolyte for appropriate experimental conditions showing either grain growth or densified but cracked surfaces. Gas permeation and electrical conductivities of the modified electrolyte were evaluated. Finally SC-SOFC performances were improved for the cells presenting grain growth at the electrolyte surface.

Design of a dual-layer ceramic interconnect based on perovskite oxides for segmented-in-series solid oxide fuel cells

A segmented-in series (SIS) SOFC consists of segmented unit cells connected in electrical series and shows improved stack efficiency over conventional SOFCs. In this design, a thin interconnect film provides both electrical contact and sealing between the anode of one cell and the cathode of the next; thus, it should have high conductivity and chemical/structural stability in both reducing and oxidizing atmospheres as well as impermeability to gases. Here, we report a dual-layer interconnect film for SIS–SOFCs comprising perovskite-type oxides, Sr0.7La0.2TiO3 (exposed to a reducing atmosphere) and La0.8Sr0.2FeO3 (exposed to an oxidizing atmosphere). The interconnect film is not only very dense but also highly conductive and stable under SOFC operating conditions; in particular, it shows an area-specific resistance of 19.6 mΩ cm2 at 800 °C, which is much lower than the generally accepted limit for SOFCs. A flat-tubular SIS–SOFC fabricated using these interconnect films exhibits a power density as high as 340 mW cm−2, which proves the feasibility of the dual-layer interconnect design.

Study of Water Gas Shift Reaction over Ceria Based Catalysts in Solid Oxide Fuel Cells

The production of hydrogen with low CO concentration from ethanol steam reforming for use in solid oxide fuel cell has been studied. The performance of tubular SOFC with internal reforming of ethanol in the anode side integrated with water gas shift reaction which is situated at low temperature side of tubular SOFC is studied to compare with a conventional tubular SOFC. The preparation method of Ni supported ceria based catalysts SOFC anode is also taken into consideration. The structural properties, particle size changes and the microstructure of the anodes are investigated using X-ray diffraction and scanning electron microscopy as well as the performance investigation. The Thermogravimetric and SEM analyses are employed to mainly characterize the carbonaceous deposits formed.

Performance Factors and Sulfur Tolerance of Metal Supported Solid Oxide Fuel Cells with Nanostructured Ni:GDC Infiltrated Anodes

In recent years, there has been a growing interest in developing metal supported solid oxide fuel cells (MS-SOFCs). MS-SOFCs are interesting as they potentially offer some advantages compared to conventional electrode and electrolyte supported SOFCs, such as low materials cost, better thermal conductivity and ductility of the support. The two later aspects improve the shock resistance and lower internal gradients within the stacks. This enables fast start-up and provides higher tolerance towards operation under transient conditions that are particularly desirable for APU applications. Today’s commercially available and relevant SOFC fuels such as natural gas and diesel etc. contain trace amounts of sulfur. Thus, tolerance towards sulfur poisoning is desirable. Ceria and gadolinium doped ceria (GDC) have been reported in the literature to have a beneficial effect on the tolerance towards sulfur poisoning. The ceria can be incorporated as a Ni:GDC cermet anode, but also via infiltration of ceria and doped ceria into the conventional Ni:YSZ cermet anode. Both approaches have been reported to improve the tolerance towards sulfur poisoning [1-3]. In the present study we report the performance and sulfur tolerance of MS-SOFCs with two different microstructures of the support and the anode functional layer (AFL). The MS-SOFCs of the present study are based on ferritic stainless steel (FeCr) with an aimed operating temperature of 650ºC. This lower operating temperature compared to electrode and electrolyte supported SOFC (750ºC-850ºC) is expected to favor sulfur adsorption and hence increase the impact of sulfur poisoning. The AFL was infiltrated with Ni-GDC precursor solution and subsequent heat treated resulting in a nanostructured coating of the electrocatalyst. The two studied MS-SOFC single cells (active area 16 cm2) have initial area specific resistances (ASR) at 650ºC of 0.35 Ωcm2 and 0.7 Ωcm2 respectively. It was found that the porosity of the support and the AFL seems to have a pronounced effect on the resulting performance. The MS-SOFCs were subjected to technologically relevant H2S concentrations of 2, 5 and 10 ppm in hydrogen fuel, during galvanostatic operation at a current load of 0.25 Acm-2. The results are illustrated and compared with the conventional SOFC Ni:YSZ cermet anode in figure 1, where the relative increase in anode polarization resistance as a function of Ni sulfur coverage is shown. The comparison indicate the MS-SOFC anode of the present study to be more tolerant towards sulfur poisoning than the conventional Ni:YSZ cermet anode. [1] K. Sasaki et al., J. Electrochem. Soc., 153, A2023–A2029 (2006). [2] L. Zhang et al., International Journal of Hydrogen Energy, 35, 12359–12368 (2010). [3] C. Xu, P. Gansor, J. W. Zondlo, K. Sabolsky, and E. M. Sabolsky, J. Electrochem. Soc., 158, B1405–B1416 (2011). Figure 1

Kinetics of Internal Methane Reforming on the Anodes of Low Temperature Ceres Power Steel Cell SOFCs

Ceres Power has developed a unique low-temperature metal supported SOFC design (the ‘Steel Cell’) based predominantly around the use of ceria. This unique design architecture offers greatly enhanced robustness to real-world operating conditions at a lower cost than conventional SOFC designs, whilst retaining the advantages of fuel flexibility, high efficiency and low degradation. A widely quoted benefit of SOFCs is the ability to carry out a significant percentage of the reforming of hydrocarbon fuels to syngas within the anode of the cells, simplifying the design of the fuel processing system by comparison with other fuel cell types. In addition performing the endothermic reforming reaction within the stack significantly reduces the requirement for air cooling, which improves the efficiency of a typical SOFC system by reducing the parasitic power demand from the cathode air blower. It has historically been viewed that a disadvantage of low temperature SOFC operation is that significant levels of internal methane reforming are no longer possible due to poor kinetics and thermodynamically limited methane conversion. The Ceres steel cell technology has been developed to perform relatively high levels (40-60%) of internal methane reforming. In this paper a method for approximately estimating the kinetics of methane reforming on Steel Cell anodes will be described, coupling experimental measurements to a mathematical model.

Quantitative Evaluation of Electrochemically Active Area in an SOFC Cathode by Oxygen Isotopic Exchange Measurements of a Model Patterned Electrode

In a conventional SOFC porous electrode consisting of mixed ionic and electronic conductor, the electrochemical reaction does not homogenously occur, and the reaction faradaic current decreases with the distance from the electrode/electrolyte interface. It is generally believed that the reaction distribution in the electrode is basically determined by the material properties, e.g. chemical diffusion coefficient D and surface reaction rate k, as well as the electrode microstructure, e.g. porosity, tortuosity, and particle size. However, so far, the contribution of D and k to the formation of the electrochemically active area was not quantitatively confirmed in an experimental manner. In this work, we experimentally evaluated the electrochemically active area in an La0.6Sr0.4CoO3-δ (LSC64) electrode by using 18O/16O isotopic exchange experiments and the following second ion mass spectroscopy (SIMS) analysis, and the contribution of D and k was discussed. Conventional SOFC porous electrodes have very complicated microstructures. For quantitative evaluation of the electrochemically effective area while eliminating the influence of structural complexities of the electrode, a columnar-type electrode (Fig. 1 (a)) is useful. However, this columnar electrode is difficult to be fabricated in practice. Therefore, we proposed the patterned film electrode, which is schematically shown by Fig. 1 (b). In this electrode, an Al2O3 insulating layer is partially inserted between electrode and electrolyte, and oxide ions can pass only through the electrode/electrolyte interface where the insulating layer does not exist. 16O/18O isotopic exchange experiments were performed with the patterned thin film electrode. The electrode was heat-treated at 973 K under P(16O2) = 1 bar for 1 hour and thereafter under the same pressure of 18O2 for 5 minutes. The dashed curve in Fig. 2 shows the observed profile of the oxygen isotopic ratio as a function of the distance from the electrode/electrolyte interface. The remarkable decrease of 18O concentration was found near the interface. This is because 16O can diffuse from electrolyte to electrode while surface-exchanged 18O can diffuse from electrode to electrolyte through the interface. Similar isotope exchange experiments were performed with the patterned thin film electrode under polarization. Cathodic bias of 220 mV were applied under P(O2) = 1 bar at 973K. The observed profile of the oxygen isotopic ratio was given by the solid curve in Fig. 2. The 18O concentration was gradually increased with approaching the electrode/electrolyte interface and drastically decreased in the vicinity of the interface. The increase of 18O was observed within about 20 μm from the interface. Such an increase of 18O near the interface is considered to be caused by electrochemical oxygen incorporation. This suggests that the electrochemically active area was approximately 20 μm from the electrode/electrolyte interface in the case of the LSC64 patterned thin film electrode under P(O2) = 1 bar at 973 K. In the presentation, the results obtained under various P(O2) and temperatures will be also presented, and the contribution of D and k to the formation of electrochemically active area will be discussed. Figure 1

Oxygen Nonstoichiometry and Cathodic Property of Ce0.6Mn0.3Fe0.1O2-δ for High Temperature Steam Electrolysis Cell Using LaGaO3-Based Oxide Electrolyte

The reverse operation of SOFCs, so called solid oxide electrolysis cells (SOECs), can efficiently produce hydrogen from high temperature steam. Ni-based metal, which are widely used for anode of the conventional SOFCs, are also widely used for the cathode of SOECs due to its high catalytic activity and high electronic conductivity. However, Ni-based cathodes frequently showed low stability against oxidation resulting in deactivation under SOECs condition. As a result, Ni-based electrodes are easily aggregated and also delaminated from electrolyte. Therefore, it is essentially required to recycle a part of produced hydrogen to the Ni based electrodes for preventing the oxidation of Ni, however, this recycle of the part of hydrogen formed requires the complicated system and also decrease the electrolysis efficiency. In order to solve these problems, recently, oxide cathode have been attracting much attention for the cathod of SOECs instead of Ni-based metal. Since oxide cathode is stable in redox cycles, recycles of hydrogen is not required. However, oxide cathode have some disadvantages, for instance, low catalytic activity and also low electronic conductivity. In this study, Mn- and Fe-doped CeO2 (Ce0.6Mn0.3Fe0.1O2, CMF) was investigated as a new oxide cathode for SOECs. CMF is a mixed ionic and electronic conductor (MIEC), and has been reported as a potential anode catalyst for SOFCs. Although its electrical conductivity is lower than that of the typical perovskites oxide electrode materials (10-1 S/cm at 900 °C), its catalytic activity for electrode reaction was enough high when it is used in SOFCs. Oxygen nonstoichiometry in this CMF was studied by thermogravimetry. It was found that higher oxidation state of Mn and Fe was observed resulting in hole conduction. The origin of this hole conduction was further studied by XPS measurement. On the other hand, in reducing atmosphere, because of the reduction of Ce, n-type semiconduction is observed. Therefore, oxygen nonstoichiometry of CMF is discussed in details from valence number of Ce, Mn, and Fe. On the other hand, CMF cathode showed almost the same performance with that of Ni-SDC cathode in steam electrolysis and hydrogen evolution rate well obeys the Faraday’s law in temperature range of 900-700°C and stable electrolysis performance was observed. The XRD pattern and the SEM images of CMF cathode after the electrolysis measurement revealed that the CMF phase is stably remained and aggregation or delamination were hardly observed. These results suggest that CMF is highly promising as oxide cathode for SOECs

Performance Factors and Sulfur Tolerance of Metal Supported Solid Oxide Fuel Cells with Nanostructured Ni:GDC Infiltrated Anodes

Two metal supported solid oxide fuel cells (active area 16 cm2) with nanostructured Ni:GDC infiltrated anodes, but different anode and support microstructures were studied in respect to sulfur tolerance at the aimed operating temperature of 650ºC. The studied MS-SOFCs are based on ferretic stainless steel (FeCr) and showed excellent performance characteristics at 650ºC with area specific resistances (ASR) of 0.35 Ωcm2 and 0.7 Ωcm2 respectively. The sulfur tolerance testing was performed by addition/removal of 2, 5, and 10 ppm H2S in hydrogen based fuel under galvanostatic operation at a current load of 0.25Acm-2. The results were compared with literature on the sulfur tolerance of the conventional SOFC Ni/YSZ cermet anode. The comparison in terms of absolute cell resistance increase and relative anode polarization resistance increase indicate, that the nanostructured Ni:GDC MS-SOFC based anode is significantly more sulfur tolerant than the conventional Ni/YSZ cermet anode.