Types, Fabrication, and Characterization of Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs), as highly efficient energy conversion devices, are anticipated to play a significant role in the realization of a low-carbon society. ENE-FARM type S (EF), equipped with SOFC stack, has been commercialized since 2012 by Osaka Gas. To further expand the market of EF, higher electrical efficiency, higher durability, lower cost of SOFC stack, and downsizing of EF are needed. For realizing these issues, two model changes were conducted in 2016 and 2020. The SOFC stack installed in the current EF, which is composed of Anode-supported SOFC (ASC), has established durability of over 12 years. For further widespread adoption in the EF market, we have focused on Metal-supported SOFCs (MSC), which have the potential to reduce costs by using fewer expensive ceramics.However, the use of a metal substrate in MSCs introduces unknown degradation phenomena that differ from those in ASCs. These phenomena include the degradation of the metal substrate and the interfacial reaction between the metal substrate and anode due to interdiffusion from the metal substrate and anode. Understanding and addressing the degradation phenomena originating from a metal substrate are significant challenges for the commercialization of MSCs. In this study, we investigated the degradation phenomena of MSCs, especially at the metal substrate/anode interface.The degradation phenomena are influenced by evaluation conditions such as durability time, temperature, and gas composition. The concentration of water vapor at the fuel gas outlet is much higher than at the fuel gas inlet throughout electrochemical reactions. To confirm the differences in the degradation phenomena due to gas composition, two MSC pieces were prepared. MSCs were fabricated at a lower temperature compared to ASCs, utilizing ceramic electrodes and electrolytes on a 16 cm2 ferritic stainless-steel substrate. Single cell tests were conducted using air at the cathode and fuel at the anode. The operating conditions for the evaluation were as follows: current density was varied from 0 to 0.42 A/cm², with H₂/H₂O used as the anode gas at two different dew points (DP). Test 1 was conducted at a DP of 30°C, and Test 2 was conducted at a DP of 88°C, while air served as the cathode gas. Both the anode gas and air flow rates were fixed during the evaluation. The operation was conducted with a fuel utilization factor (Uf) of approximately 10% and an air utilization factor (Ua) of approximately 20% at 0.42 A/cm².Figure 1 shows the results of the durability tests of 16 cm2-MSC cells conducted at 800°C. The change in voltage from the initial value is denoted as ΔV. The reduction in voltage on the fuel outlet side was approximately 25 mV greater than that on the fuel inlet side after 1000 hours. During these durability tests, the IR and electrode overvoltage (η) were measured by the current interruption method. The open circuit voltage remained stable without significant changes throughout the 1000 hours durability tests for both the fuel inlet and outlet sides. Figure 2 shows the results of IR and η during the durability tests conducted at 800°C. The observed reduction in voltage was attributed to increases in both IR and η, with similar increments noted for both the fuel inlet and outlet sides in relation to IR. It was found that no abnormally high resistance layer formation or significant increase in oxidation layer occurred on the outlet side after 1000 hours at 800°C.The more pronounced voltage drop observed at the outlet side is primarily due to a more significant increase in η. Considering that the evaluation was conducted under uniform conditions, except for the water vapor concentration on the anode gas side, it is plausible that the factors contributing to the increase in η are associated with the sintering or poisoning of the anode. These findings suggest that the reduction in reaction area and decreased gas diffusivity are significant contributors to the observed effects. In the presentation, we will discuss the insights derived from the electrochemical measurement results along with detailed cross-sectional analyses conducted after the durability tests. Figure 1

Direct internal reforming (DIR) operation provides many benefits for solid oxide fuel cells (SOFCs) by eliminating the need for an extra reformer. Anode-supported SOFCs (AS-SOFCs) are commonly used for DIR operations. However, the strong endothermic reforming reactions at the cell entrance may result in large thermal stresses and lead to premature failure. In this study, a two-dimensional model was developed to analyze the differences between the AS-SOFC and the new generation metal-supported SOFC (MS-SOFC) in DIR operations. The model considered the coupling effects of the chemical/electrochemical reactions, the mass/charge/heat transport processes and the thermal mechanical stress. The distributions of temperature, species composition, current density and thermal stress were determined and compared for both structures. The failure probability of the cell was estimated by stress distributions. The results showed that the MS-SOFC led to a gentler temperature distribution and a smaller failure probability with slight performance increase compared to the AS-SOFC.

Metal-supported solid oxide fuel cells (SOFCs) have the advantage of the thermal shock resistance and higher mechanical strength compared with anode-supported SOFCs. In this study, Ni-Fe metal was used as supporting metal. The supporting metal was initially prepared in oxide form of NiO-Fe2O3, and co-sintered with Gd-doped ceria electrolyte at 1350ºC for 3h in air and later reduced to Ni-Fe metal. Two different cells were prepared and compared to see the effect of the thickness of the electrolyte and the role of the electrode on the performance of the cell.

Reviewing metal supported solid oxide fuel cells for efficient electricity generation with biofuels for mobility

Development and testing of anode-supported solid oxide fuel cells with slurry-coated electrolyte and cathode

Three-dimensional computational fluid dynamics modeling of anode-supported planar SOFC

A systematic study was initiated of anode-supported solid oxide fuel cells (SOFCs) with Nd2NiO4 cathodes. This type of cathode, a mixed conducting K2NiF4-type material, can be considered as a candidate for SOFC applications. In this study, the influence of (1) the presence of a Ce0.8Gd0.2O1.9 (CGO) interlayer between the electrolyte and the cathode, (2) sintering conditions, and (3) the grain size of the powder on the performance of SOFCs with Nd2NiO4 cathodes was investigated in more detail. Results from current density-voltage characteristics and permeation and gas diffusion measurements showed that the electrochemical performance was promising for SOFCs including a CGO interlayer and a Nd2NiO4 cathode (prepared with a powder with a d50 of 0.5A∕cm2 or 0.8μm) sintered at 1100°C or higher; current density at 800°C and 700mV between 1.2 and 1.3A∕cm2. The microstructure of the cathode was such that no gas diffusion problems occurred.

Microtubular anode-supported solid oxide fuel cells (μt-SOFC) were created with a metallic coil embedded in the anode to act as current collector. The electrochemical performance was experimentally examined by comparing the power density (PD) of μt-SOFC with embedded coils of different turns per unit length and composition (nickel and palladium). It is shown that an increase in the turns per unit length results in a proportional current density increase and in a quadratic increment of PD. Additional performance improvement is found for the cell with palladium current collector due to the higher catalytic activity for hydrogen oxidation.

In order to develop solid oxide fuel cells (SOFCs) running on hydrocarbon fuels, we have focused on a new method of improving electrode performance and reducing carbon deposition by coating thin films of samaria-doped ceria (SDC) within the pores of electrode by a sol-gel coating technique. The SDC coating on the pores of anode made it possible to have a good stability for long-term operation due to low carbon deposition and nickel sintering. In this study, we demonstrated a new method of improving electrode performance and reducing carbon deposition by coating thin films of samaria-doped ceria and applied the modification technique to two different types of fuel cell structures, anode-supported SOFC and comb-shaped SOFC. From our results, the maximum power density of an anode-supported cell (electrolyte; 8 mol% YSZ and thickness of 30μm, and cathode; La0.85Sr0.15MnO3) with the modified anode was ∼300mW∕cm2 at 700°C in the mixture of methane (25%) and air (75%) as the fuel, and air as the oxidant. The cell was operated for 500hr without significant degradation of cell performance. For the comb-shaped SOFCs operated in the mixed-fuels fuel cell conditions, the cell performance was 40mW∕cm2 at 700°C in the CH4∕O2 ratio of 1.

In this paper, a new approach for metal-supported Solid Oxide Fuel Cell (SOFC) fabrication was investigated. Single cell with NiO and Fe2O3 (Ni:Fe = 1:1) oxide support, NiO-Gd0.1Ce0.9O1.95 (GDC) anode, GDC electrolyte and La0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (LSCFN) cathode was prepared through traditional ceramic processing methods in air. Subsequently, the as-prepared whole cell was annealed in the humidified 5% H2-95% N2 at 700°C for 1 hour to get the metal oxide support reduced. Results show that after being annealed in the reducing atmosphere at 700°C for 1 hour, the oxide support was mostly reduced into metal, while the LSCFN cathode maintained its perovskite structure with slightly oxygen loss which was proved redox reversible in air at the typical cell test temperature of 700°C. Fully reduced Ni-Fe metal support could be obtained during the cell test condition with humidified H2 sweeping at the anode side. Furthermore, electrochemical impedance spectroscopy (EIS) results show that the polarization resistance of LSCFN symmetrical cell in air decreased after one redox cycle between the humidified 5% H2-95% N2 and air, suggesting the performance optimization of LSCFN cathode after the possible rearrangement of mixed-valence Fe3+/Fe4+ and Co2+/Co3+ redox couples and distribution of oxygen vacancies.

Coal gas fuel utilization effects on electrolyte supported solide oxide fuel cell performance

This review critically evaluates recent advancements in electrocatalytic technologies aimed at enhancing the efficiency of metal-supported Solid Oxide Fuel Cells (SOFCs) for biofuel-powered mobility applications. The study aims to elucidate the impact of these innovations on the performance, durability, and stability of SOFCs in transportation and portable energy systems. By integrating experimental findings, computational simulations, and practical applications, this work highlights the pivotal role of advanced electrocatalysts in optimizing SOFC functionality. Key developments, such as the incorporation of perovskite-based materials and exsolved nanoparticle catalysts, have demonstrated remarkable improvements in electrochemical performance and operational longevity. Specifically, lanthanum-strontium cobalt ferrite (LSCF)-based cathodes demonstrated a 30% increase in power output and a 25% enhancement in long-term stability under biofuel operating conditions. Furthermore, computational modeling has played a crucial role in refining catalyst designs, achieving a 45% reduction in degradation rates. These advancements underscore the potential of biofuel-driven SOFCs as a sustainable energy solution for transportation. However, future research must address challenges related to scalability, cost-effectiveness, and economic competitiveness to fully realize their practical implementation.

As a promising clean energy conversion device, solid oxide fuel cells (SOFCs) could efficiently utilize multiple fuels and have been applied in stationary power generation and other fields. However, the high operating temperature limits the use of SOFCs in the transportation sector. Consequently, reducing the operating temperature is a key focus in SOFCs development. In recent years, metal-supported SOFCs (MS-SOFCs) have received significant attention because of intermediate operating temperatures. MS-SOFCs offer advantages such as simple and reliable sealing, rapid start-up, and high power density, making MS-SOFCs a promising option for transportation applications. However, for commercial applications, the methods to enhance performance, improve durability and mitigate degradation need to be further studied. This paper comprehensively reviews the state of the art approaches to performance improvement, durability under various conditions and different fuel reforming methods. Finally, it highlights the prospects and challenges for future advancements in this field.

Long term durability was demonstrated on high-performance direct ethanol solid oxide fuel cell operating in gradual internal reforming. Anode supported single cells, fabricated at Forschungszentrum Jülich, composed of (La,Sr)MnO3 cathode, 8 mol% yttria-stabilized zirconia (YSZ) electrolyte, and YSZ-Ni anode were used for long term testing on anhydrous (direct) ethanol. Gadolinia-doped ceria (CGO), with 0.1 wt.% of Ir, catalyst was produced by the impregnation of Ir salt in commercial CGO (Praxair) and mixed with organic additives. The CGO-Ir suspension was deposited onto the surface of the YSZ-Ni anode support with an air brush. Such a catalytic layer acts both as a physical barrier to avoid direct contact of the fuel with Ni-based anode and to promote the gradual internal reforming of ethanol. The CGO-Ir catalyst was designed for the reforming of the main compounds resulting from the thermal decomposition of ethanol at high temperature and to inhibit the formation of ethylene, which is a known source of carbon deposits. After deposition the sample was heated at 850ºC in inert atmosphere to ensure good adhesion between the catalytic layer and the anode. Gold grid and paint were used as current collectors in both cathode and anode sides. The single fuel cells were mounted in the test apparatus, sealed with gold rings and alumina cement and heated to the measuring temperature (850°C) under inert atmosphere. After gas tightness was checked by flowing air on the cathode side and argon on the anode side, the anode was reduced under hydrogen. The fuel cell was kept at 0.6 V while monitoring the current drained from the cell at 850 °C. Samples were initially operated in hydrogen for a few hours and fuel was switched to dry ethanol, carried by nitrogen. It is important to mention that no oxidizing agent was added to the fuel and that water produced by the electrochemical reaction of hydrogen at the anode promoted the reforming of the ethanol in the CGO-based layer, in the so-called gradual internal reforming. In such conditions the current output was recorded and observed to not be significantly different form that under hydrogen. Moreover, the direct ethanol fuel cell was continuously operated for 650 hours without major losses due to carbon formation. The experimental results demonstrate that the water released by the electrochemical oxidation of reformate hydrogen at the electrolyte/anode active interfaces is sufficient for the reforming of the primary fuel in the catalytic layer, provided that a minimum fuel utilization is respected. After the durability tests, energy dispersive X-ray spectroscopy and scanning electron microscopy analyses of the samples revealed no carbon deposit formation in the anode. The experimental results provided compelling evidence for the viability of gradual internal reforming of ethanol in high performance solid oxide fuel cells.

The objective of this work was to investigate how fuel utilization and method of fuel delivery for a large planar fuel cell with co-flow configuration will affect the degradation rate and mechanism of phosphine poisoning of a solid-oxide fuel cell (SOFC). Coal syngas, a potential fuel source for SOFCs, contains gas phase impurities such as PH3, which rapidly degrade Ni-based SOFC anodes. Researchers have shown significant reconstruction of Ni-anodes in button cell configurations with ~0.5 mV hr-1 degradation rates, but it is not evident that these rates will occur in actual stack applications. A singular planar stack repeat unit was constructed using a Haynes 242 interconnect manifold with a cobalt-oxide coating. The cell was operated at 800°C with 10 ppm PH3 in dry H2. Cell performance was evaluated over 500 hours by means of voltage-current measurements and impedance spectroscopy. PH3 was measured entering the fuel cell and then exiting the fuel cell in the exhaust. The post-run material analysis of the contaminated cell was conducted via x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and scanning electron micrographs (SEM). In this work, there was no appreciable degradation attributable to PH3 poisoning of the anode. No reconstruction of the Ni-anode was observed. This result is in contrast to the many papers published on the subject, where anode-supported SOFC in a button cell configuration degraded rapidly. It is believed that the increased fuel utilization of the large planar cell compared to the published literature using button cells contributed to increased H2O generation which led to side reactions that prevented the phosphorus from interacting with the Ni-anode; platinum components may have catalyzed these reactions.