SOFC Stack Research Articles

Исследование наборов твердооксидных топливных элементов в пакетном исполнении

The stressed state of fuel cells in a package is considered. It was found that for αi=1.2×10−5k−1, the rational geometric characteristic for a planar SOFC in a batch design is γi=6×10−2. It is concluded that if the relative thickness of the edge element of the SOFC stack is of planar design γi>6×10−2, then the resulting deformation complication will be characterized by the loss of stability of the structure. Otherwise, i. e. at γi<6×10−2, stacked SOFC elements can lose stability until plasticity appears in their materials. Consequently, only at γi=6×10−2, the use of the potentials of structures can be achieved both in terms of the stability of its elements and the strength of their materials.

Solid Oxide Cell Materials Development at Pacific Northwest National Laboratory

Pacific Northwest National Laboratory (PNNL), a U.S. Department of Energy (DOE) laboratory, has worked with government agencies and industrial collaborators to accelerate the commercialization of SOFC power systems for distributed and central power applications. This presentation will highlight work performed for the US DOE Office of Fossil Energy’s Solid Oxide Fuel Cell program. Topics to be covered include effects of contaminants and their mitigation on electrode performance; investigating volatility from cathode materials; improved cell materials; Co-free protective interconnect/BOP coatings development; capabilities of the SOFC stack test platform; and progress on industrial collaborations including seal development in electrolyzer systems fabricated using advanced manufacturing techniques such as 3D printing and rapid electromagnetic sintering.

Reconstruction and optimization of LSCF cathode microstructure based on Kinetic Monte Carlo method and Lattice Boltzmann method

Solid phase sintering is a critical process for fabricating mixed ionic and electronic conductivity (MIEC) electrodes. In this study, the microstructures of MIEC electrodes are numerically reconstructed by a Kinetic Monte Carlo method. The performance of the reconstructed MIEC electrodes is then evaluated by a pore scale Lattice Boltzmann model. The present study provides the first comprehensive assessment of local O2 partial pressure on electrode performance. It is found that ohmic loss tends to play remarkable roles at a low O2 partial pressure of pO2<0.1bar. As insufficiency of O2 is almost unavoidable in the SOFC stack, the influence of local O2 partial pressure on ionic conductivity should be considered in LSCF modeling. Another important finding is that the initial states of compact powder have a profound impact on the electrode performance. Small initial grain size and irregular particles both contribute to generate large reaction area after sintering thereby decrease activation loss. It is also found that compact powder consistency even plays a more important role in electrode performance than particle size. The study also provides deep insight into influence of sintering process. The effective conductivity of electrode is mainly controlled by the enhancement of electrode connectivity. Subsequently, nanostructured SOFC electrodes by infiltration/impregnation are reconstructed evaluated numerically. The infiltrated electrodes demonstrate improved performance and significantly promote uniformity of reaction rates. The present study forms a solid foundation for optimization of the fabrication procedures to improve the fuel cell performance.

Effect of flow modes on operation characteristics of three‐dimensional non‐isothermal solid oxide fuel cell stack

The three-dimensional non-isothermal model of anode-supported planar solid oxide fuel cell was established. In order to reduce the temperature rise of SOFC in the process of operation and improve the output performance of the stack, this paper studied and compared the operation of SOFC in four different flow modes. The study found that the flow mode has a great influence on the temperature distribution of the SOFC stack. In co-flow, the temperature increases continuously along the channel length. In the case that oxygen and hydrogen of each cell are in co-flow, by reversing the flow direction between adjacent cells, the temperature rise of the SOFC stack along the channel direction can be reduced by 26 K, which decreases from 41.2 K to about 15 K. At the same time, the output current density and power density of the SOFC stack are also improved to some extent.

Effect of interconnect pre‐oxidation on high‐temperature wettability and mechanical properties of glass seals in SOFC

AbstractMetallic interconnect oxidation has a significant influence on shear strength between interconnect and sealing glass in SOFC stack operation. This is attributed to high‐temperature mutual wettability. In this work, the glass having different crystallization kinetics was chosen to evaluate wettability and shear strength on Fe–16Cr alloy, which was heated treated at 750°C for 0, 50, and 100 h, forming oxide scale with a varying rough surface. Visual observation was used to quantify equilibrium contact angles between glass and substrates. The results illustrated that alloy oxidized for 50 h exhibited better wettability and shear strength, implying that thickness and roughness of the oxide scale are critical to enhancing interface joint strength. Long‐term testing indicated that thermally stable glass possesses higher joint shear strength and more consistent properties. It was found that precipitated crystalline phases limited improvement of glass wettability, resulting in interfacial delamination of the glass/alloy layer over long‐term operation.

Degradation Analysis of SOFC Performance for Long-Term Operation with High Fuel Utilization

A national project in Japan for SOFC development has been initiated from FY2020 to FY2024 in NEDO’s project in order to establish the method for degradation diagnosis of SOFC stacks with durability and reliability for 130,000 hours or the method for evaluation of expanding application. Central Research Institute of Electric Power Industry (CRIEPI) has operated three types of SOFC stacks, which are developed by KYOCERA, MORIMUA SOFC TECHNOLOGY, and DENSO and investigated their long-term durability in this project. We have developed the electrode polarization model for SOFC performance which is able to express the SOFC performance such as Nernst loss, cathode overvoltage, anode overvoltage, and internal resistance (IR) loss under various conditions with high prediction accuracy. Each SOFC stack has been operated with high fuel utilization (Uf = 80% and 85%) as a severe condition for high efficiency towards 20,000-hours operation, and degradation factors for SOFC durability has been analyzed by using our electrode polarization model. Analysis results until now showed that dominant degradation factors for both Uf = 80% and 85% were increase of cathode overvoltage or IR loss as same as that of the SOFC stacks in the previous term [1]. Although anode overvoltage was a minor degradation factor, increase rate of anode overvoltage under Uf = 85% was relatively larger than that under Uf = 80%. In addition, thermal property testing will be conducted for each SOFC stack in order to include temperature term into our electrode polarization model. This new model can predict SOFC performance more accurately under various operating conditions. This work has been carried out under the project of the New Energy and Industrial Technology Development Organization (NEDO), Japan as a contract research. We would like to express our gratitude to NEDO, KYOCERA, MORIMUA SOFC TECHNOLOGY, Denso as well as to university partners and National Institute of Advanced Industrial Science and Technology (AIST).Reference[1] A. Ido et al., ECS Transactions, 91(1), 801-808 (2019).

Investigation of the Glass-Ceramic Sealants in SOFC Stacks

The stack sealing is one of the most important factors in planar solid oxide fuel cell (pSOFC) applications. Two sealing approaches were used for the short stacks working under practical conditions, which are rigid bonded sealing with SiO2-BaO-based glass sealant and compressive sealing with ZrO2-based ceramic gasket. The prepared glass was characterized by inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), X-ray diffraction (XRD), and dilatometry to determine its component and structural and thermal properties. Once the stack system shut down, the bubble test was used to detect and locate the leak of the stack. Then post-test characterization was conducted to analyze the failure of the sealing. SEM-EDS and XRD were used to analyze the microstructure and the crystallization of the sealant inside and its interface with the interconnect. It was observed that the crystal was formed inside the glass sealant, which strengthened the material. Good adhesion and chromium diffusion were found between the glass sealant and the metal interconnect. Moreover, the lose the sealing glass under overheating was found close to the fuel inlet, which was suggested to lead to the failure of the stack. For the ZrO2-based ceramic sealing, the crack was observed inside the sealant and the airway was obstructed by the sealing material under the compressive force. In summary, the glass sealing was more controllable than the compressive ceramic gasket. However, the glass sealant should be protected from local overtemperature.

Electrically Conductive and Protective Coating Reaction with LSCF Material

Ferritic stainless steel interconnect materials are used in intermediate or high-temperature SOFCs because of their oxidation resistance, high formability, and low cost. Their corrosion resistance is due to the formation of a protective chromium oxide scale during operation. However, the protective oxide layer produces Cr-containing volatile species at SOFC operating temperatures and conditions, which can cause cathode poisoning. Electrically conducting spinel coatings have been developed to prevent cathode poisoning and maintain an electrically conductive pathway through SOFC stacks. Generally, Mn-Co-O spinel is used for this protective coating in SOFCs. To remove the Co and enhance the electrical conductivity, PNNL has developed a Mn-Cu-O spinel for this protective coating. This paper will focus on the electrically conductive coating’s compatibility with LSCF in high temperature operating environments.

Highly Efficient SOFC System with Mk35x-Stack Module Operated with Gasified Biomass - HieffBioPower -

A stationary SOFC system utilizing gas from a biomass updraft gasifier is under development within the project “HiEffBioPower” by MAWERA, BIOS, Bosal, AVL, Calida Cleantech, Fraunhofer IKTS, Wuppertal Institute and University Utrecht (European Union Horizon2020 No. 727330). The system is based on Mk352-CFY-stack with Chromium based (CFY) interconnects and electrolyte supported cells. These stacks are robust, redox stable and demonstrated low degradation rates. The stacks are arranged to modules of eight 30-cell stacks and are electrically connected in series. The stack module was pre-tested at the test rig at IKTS and showed performance in accordance with system requirements. Stationary power points were measured to compare the behaviour of the stack module with the system tests at MAWERA. AVL designed and installed the overall SOFC system (heat exchangers, stack, catalytic afterburner, process control and visualisation) with a nominal capacity of 6 kWel. After shipping all parts of the SOFC system to MAWERA in Hard (AT), the coupling of the biomass gasifier, the gas cleaning unit and the SOFC system was conducted. The system is operated by an automated process control. The highest efforts needed the novel gas cleaning unit, consisting of a high temperature filter, a desulphurisation unit as well as a novel catalytic gas reforming stage developed by BIOS.The biomass gasifier has been operated with wood chips of different quality. The high tar contents of the product gas make a tar cracking essential to reach a high calorific value of the gas for the SOFC system. The novel gas cleaning system shows good performance and reaches the defined SOFC requirements. The SOFC system developed and operated by AVL reached stable conditions for several days. The operating results of the stack module at lab conditions and at the real system are in a good agreement. The SOFC stack module performance in terms of cell voltages, stack temperature distribution and fuel utilization under biomass reformate fuel will be shown and supported by several simulation to get an overall energy balance. Examples of optimization for several components by CFD simulation and component tests will be given.

Numerical Study on Effects of Structure Parameters and Operating Conditions on the Micro-Tubular SOFC Stack Performance

A multi-physical field coupled three dimensional model of large-scale micro-tubular solid oxide fuel cell (MTSOFC) stacks was established in this study. The effects of structural parameters and operating conditions, namely configuration of the tube array, diameter, length, distance between tubes, the flow rate and flow path of air, on the stack performance were studied. The triangular array is more compact in space and can achieve higher volume specific power than the square array. However, the more concentrated arrangement also results in a higher local temperature and temperature difference, which is challenging for long-term stability. According to the simulation results, smaller tube diameter and distance between tubes lead to higher volume specific power, with the former having a marginal effect. A longer tube can reduce the axial temperature gradient, which is conducive to long-term operation stability. Increasing the air flow rate of the cathode is the most effective method to reduce the highest temperature. Furthermore, the co-flow arrangement is found to be more helpful to take away the heat generated by the stack than the counter-flow arrangement. Based on the simulation results, the structural parameters and operation conditions suitable for a large-scale micro-tubular SOFC stack are decided, considering the output power, long-term stability, and mechanical strength. The model is demonstrated as a powerful tool for the design and optimization of MTSOFC stacks.Key words: Micro-tubular SOFC Stack; Array Configuration; Structure Parameter; Operation Condition; Volume Specific Power

Recent Highlights of Solid Oxide Fuel Cell and Electrolysis Research at DTU Energy

A successful Green Transition requires efficient, and cost competitive technologies for converting & storing energy from renewable sources and for providing energy to the stationary and the transport sector. Solid oxide fuel cells and electrolysis (SOFC, SOE - SOC) provide high efficiencies for electricity generation and production of storage media / fuels, respectively. In addition, they are flexible and versatile, which makes them attractive in many different application areas and energy scenarios.Research on solid oxide concepts has a long tradition at the Department of Energy Conversion and Storage (DTU Energy) of the Technical University of Denmark. It spans from fundamental topics such as materials & electrode development over methodology development for example in micro structural & electrochemical characterization, up to the applied level, where integration of SOC with up- and down-stream processes becomes important. Significant funding from National and European programs supported this research. The presentation will introduce highlights of recent achievements in the areas of (i) materials & electrode development to reach higher activity and better durability, (ii) cell & stack development for lowering costs and increasing robustness, (iii) power to X concepts with special emphasis on X= fuels, (iv) lifetime & diagnostics approaches for accelerating durability studies, and (v) modelling from materials to stack levels aiding lifetime evaluation and designing of stacks & systems.(i) Materials & electrode development: With the aim to reduce use of critical raw materials in SOCs, Co-free oxygen electrodes were developed based on lanthanum, strontium, and iron containing perovskite materials (LSF) applying a nano engineered hybrid catalyst coating via co-infiltration. The electrodes were integrated into cells and a high electro catalytic activity was achieved leading to power densities of ca. 1 W/cm2 in SOFC mode. Such cells with CGO infiltrated Ni/YSZ fuel electrode allowed for a stable electrolysis operation at 650 oC and -0.5 A/cm2 with a voltage close to 1.3 V.(ii) Cell & stack development: A novel, metal-based monolithic concept for SOFC stacks was developed. It is based on tape casting & lamination to build a monolithic structure thereby combining interconnect and gas distribution into one layer. Electro catalytic materials are infiltrated after sintering. This concept allows for an increase of the power density >5 kW/l, a decrease of materials costs by a factor of ca. 4-5 as compared to conventional stacks, and robustness for cycling operations.(iii) Power to X concepts: A novel and efficient route for producing green methanol was demonstrated by use of residue from the agriculture. A 6 kW SOEC stack unit produced hydrogen, which was combined with syngas from a thermal gasifier operating on straw in a down-stream methanol reactor. Methanol is considered a promising green fuel for the shipping sector. The same applies for ammonia. SOFC anodes catalyze the cracking of ammonia. This endothermal process can be integrated and tailored with the exothermal hydrogen oxidation in the electrode. The cracking of ammonia was studied on materials and stack level.(iv) Lifetime & diagnostics approaches: The long lifetimes –both required and achieved - of mature SOC generations make it necessary to develop new approaches for degradation studies and lifetime assessment. With the aim to harvest gained results from single cell durability testing over the last ca. 20 years at DTU Energy, a database was established allowing for a new evaluation of durability behavior. The database makes it possible to apply machine learning routines to reveal linear and non-linear correlations and single & combined effects of operating parameters on durability over a large range of data. The aim is to identify accelerating testing approaches and establish lifetime prediction.(v) Modelling: To further optimize the operation of an SOC and avoid degradation, a time dependent 3D stack model describing the degradation over the entire lifetime was developed. For this to be computational feasible, the model was built using the multi-scale, multi-physics homogenization concept, making it possible to simulate 40,000 hours of operation for a full 3D stack in one hour and 15 minutes. Same modelling concept was also utilized to describe local mechanical failures by use of a sub-model for the mechanical stress concentrations near assembly points inside the stack.

Self-Supported Solid Oxide Fuel Cells by Multimaterial 3D Printing

Nowadays, renewable energy increases its contribution to the present energy scenario. In this regard, the implementation of energy storage and sources as the electrolysers and fuel cells gives the opportunity to transform the surplus of renewable energy to the form of hydrogen, by water electrolysis and the use of this hydrogen as a fuel for energy generation by the reverse mode of the same devices, in the so-called power to gas and gas to power routes. Among the different technologies of fuel cells and electrolysers, the high temperature technology based on ceramic solid oxides presents the best efficiencies. However, the fabrication of this devices implies several manufacturing procedures and multiple intermediate steps of fabrications, such as tape-casting, screen-printing, annealing, manual stacking, joining, sealing, etc. As a result, the SOC production process becomes more expensive and time-consuming. Recently, it has been demonstrated that the application of additive manufacturing (AM) technologies for functional ceramics fabrication, specially to SOC production, leads to significant improvement of the process. AM represents freedom of design, that allows to enhance the performance of the SOC-based device, an increase of manufacturing speed and productiveness, while the waste material is reduced. As a final result, one-step printed monolithic SOFC stack could be produced combining different AM technologies in a single printing process.For this purpose, hybrid 3D printing technology which combines stereolithography (SLA) and robocasting (direct ink writing, DIW) has been developed. Thus, the technique uses 8YSZ for the electrolyte fabrication by SLA and deposition of electrodes and interconnectors by DIW. The elaborated hybrid multimaterial 3D printing technology renders possible complete SOC stack fabrication as a single-step process. Complete fully printed SOFC cells, their co-sintering process and their characterization are here presented while the advantages and limitations of the fabrication process are discussed.

Joint Strength of a Braze Sealant with Metallic Interconnect in Solid Oxide Fuel Cell

To meet the quick start-up characteristics of metal-supported solid oxide fuel cell (MS-SOFC), brazing seals have been used in many MS-SOFC systems. During the cyclic operation of SOFC, thermal stresses are unavoidably generated due to temperature gradient and mismatch of coefficient of thermal expansion (CTE) between components. Excessive thermal stresses may damage the structural integrity of an SOFC stack, which in turn reduces the efficiency and lifetime of the SOFC. Therefore, it is essential to study the mechanical properties of the joint between braze sealants and metallic interconnects at room temperature (RT) and operating temperatures for developing a reliable MS-SOFC stack. The aim of this study is to investigate the mechanical strength of a braze sealant/metallic interconnect joint in SOFC. The braze sealant used is a silver-based alloy which is developed at the Institute of Nuclear Energy Research (INER) for MS-SOFC. The metallic interconnect used is a commercial ferrite stainless steel (Crofer 22 H). Two types of sandwich-like joint specimens are made to determine the shear and tensile joint strength at RT and 750 °C. Experimental results indicate that a bonding time of 30 min produces better joint strength at RT than 20 and 40 min, and it is thus selected as the bonding time for the specimens used in the subsequent tests. For tensile loading mode, the joint strength at RT and 750 °C is of 30.63 ± 3.2 MPa and 13.43 ± 0.99 MPa, respectively. For shear loading mode, the joint strength at RT and 750 °C is of 40.02 ± 2.88 MPa and 10.06 ± 0.36 MPa, respectively. Fractography analysis reveals that regardless of testing temperature, fracture of the tensile specimens occurs at the interface between the braze sealant and the Cr2O3 oxide layer and occasionally within the Cr2O3 oxide layer. When the testing temperature of the shear specimens is increased from RT to 750 °C, the location of rupture changes from the interface between the braze sealant and Crofer 22 H to the interface between the braze sealant and the Cr2O3 oxide layer.

Investigation of the Glass-Ceramic Sealants in SOFC Stacks

The stack sealing is one of the most important factors in planar solid oxide fuel cell (pSOFC) applications. In this study, the barium-silicate glass with addition of Al2O3 were analyzed by ICP, DSC, and SEM. Differential thermal analysis was utilized to design the heat-treatment of the glass to obtain a glass-ceramic for sealing. The glass material and sealing process were verified on the 114×114 mm SOFC stack. A tight stack was obtained with a leak rate much better than the desired 0.004 SCCM/cm, which is equivalent to 0.001 MPa/s. A good electric performance was also found for a 6-layer sealed stack. Post-characterization was conducted for the seal dismounted from the stack via the methods of leakage inspection, SEM-EDS, and XRD. A good interaction was found between the glass-ceramic seal and the steel interconnect, due to the diffusion of aluminum from the glass component.

Optimization of Contact Cathode Composition Based on La0.8Sr0.2MnO3±δ for SOFC Stacks

The formation of stable contact layers with minimum electrical resistivity at the electrode/interconnector interface is an important task for planar SOFC stack fabrication. This work was centered on the search for optimum processing conditions of the La0.8Sr0.2MnO3±δ (LSM) based contact pastes without inorganic additives. The ball-milling conditions and contact paste composition were optimized taking into account the LSM particle size, sintering, a possibility of partial phase decomposition, and electrical conductivity of the resultant cathode contacts. Testing of the model SOFC stack with Crofer 22H interconnectors revealed sufficient adhesion, porosity, conductivity, and stability of porous LSM contact layers.

Degradation Analysis of SOFC Performance for Long-Term Operation with High Fuel Utilization

In a NEDO’s project from FY2020 to FY2024, Central Research Institute of Electric Power Industry (CRIEPI) has operated three types of SOFC stacks, which are developed by KYOCERA, MORIMURA SOFC TECHNOLOGY (MST), and DENSO in order to clarify the technical issues for high efficiency and toughness of SOFC stacks and establish the method for degradation diagnosis of SOFC stacks. Each SOFC stack has been operated with the high fuel utilization (Uf = 80 and 85%) as a severe condition for high-efficiency and degradation factors for SOFC durability have been analyzed by using the electrode polarization model which we have developed. Analysis results until now showed that dominant degradation factor was increase of cathode overvoltage or IR loss as same as that in the previous term. Moreover, thermal property testing has been conducted to apply the effect of temperature to our polarization model. It was clarified that internal resistance and electrode resistance coefficients can be expressed by Arrhenius equation.

Electrically Conductive and Protective Coating Reaction with LSCF Material

Ferritic stainless-steel interconnect materials are used in intermediate or high-temperature SOFCs because of their oxidation resistance, high formability, and low cost. Their corrosion resistance is due to the formation of a protective chromium oxide scale during operation. However, the protective oxide layer produces Cr-containing volatile species at SOFC operating temperatures and conditions, which can cause cathode poisoning. Spinel coatings have been developed to prevent cathode poisoning while maintaining an electrically conductive pathway through SOFC stacks. Generally, Mn-Co-O spinel has been the material of choice, but PNNL has developed an Mn-Cu-O spinel protective coating to remove the costly Co and enhance the electrical conductivity. This paper will focus on the electrically conductive coating’s compatibility with LSCF in high-temperature operating environments.

An Investigation of the Redox Stability of an Anode-Supported SOFC Stack Using Acoustic Emission Monitoring

The mechanical stability of an SOFC stack is compromised when its cells operate in oxidation-reduction (Redox) environments. Previous studies, based mainly on voltage-current or electrochemical impedance measurements, showed that cell performance degraded upon redox cycling above 600°C. Therefore, the purpose of this work is to investigate the Redox process and the stability of a SOFC stack applying the Acoustic Emission (AE) monitoring technique. The investigation includes the interpretation of AE events and signals considering their waveform and frequency content.

Development Progress at Hexis SOC Activities

HEXIS is developing and manufacturing SOFC-based micro-CHP systems for houses and/or small companies. This paper will present the long-term experiences on long-term and cyclic testing of fuel cell modules in lab tests as well as results on robustness and performance in the field. It was observed that degradation rates as low as approx. 0.3 %/kh, i.e. 13 mΩ cm²/kh on the short-stack level for over 50,000 h can be reached. These stacks have been analyzed with electrochemical impedance spectroscopy (EIS) at regular intervals, which can provide valuable insights in understanding fundamental degradation mechanisms for long term operation of SOFC stacks. Moreover, the paper will include highlights of new developments such as improvements on cell and stack level as well as new applications for the Hexis technology in general. Finally, the paper will outline some of the future challenges in Hexis’ development activities.

Field Test Result of Residential SOFC CHP System Over 10 Years, 89000 Hours

The maxmum operating time without stack replacement, residential SOFC power generation unit (manufactured by Kyocera in 2009), which was installed by Osaka Gas in a house in 2009, has exceeded over 10 years, 89000 power generating hours. In 2004, Osaka Gas and Kyocera started joint development of residential SOFC CHP system and from 2007, we conducted series of field tests in the SOFC demonstration project operated by NEDO and NEF. At that time, various technical issues remained, and after joint development with Aisin and Toyota Motor, it was mass-produced and commercialized in 2012. We also mention the current status of residential SOFC (Manufactured by Aisin, SOFC stack supplied by Kyocera). Osaka Gas has sold more than a total of 88,000 residential SOFC CHP systems as of October 2020.