Performance Of Solid Oxide Fuel Cells Research Articles

Total Energy and Total Power for the SOEC: Critical Role of Area Specific Resistance in Hydrogen Production Rate

This paper develops the governing Total Energy (TE) (kilowatt-hours per kilogram hydrogen) and Total Power (TP) equations for Solid Oxide ElectrolyzerCells (SOECs) and Solid Oxide Fuel Cells (SOFCs). The TE equation includes heat input, exergetic flows, enthalpy of vaporization, pressurization, heat loss, area specific resistance (ASR), etc. The TE equation developed, as it would happen, correlates well with the Idaho National Laboratory (INL) proven SOEC performance of 45 kilowatt-hours per kilogram hydrogen at 20 bars and 725 K. TE is the key performance equation necessary for designing, predicting, and planning for SOEC and SOFC performance and cost. The ASR has a critical role in SOEC TE and TP. The ASR and the targets for ASR necessary to meet important DOE performance targets are discussed.

Distribution of Relaxation Times of Fuel Electrodes for Reversible Solid Oxide Cells Fabricated Under Various Conditions

Reversible solid oxide cells (r-SOCs) are electrochemical energy devices that can reversibly switch between power generation by solid oxide fuel cells (SOFCs), and hydrogen production by solid oxide electrolysis cells (SOECs) the reverse operation of SOFCs. For the development of high-performance and durable r-SOCs, it is essential to understand not only the I-V characteristics but also the electrode reaction processes systematically. Here in this study, Ni-GDC cermet fuel electrodes, a composite of Ni and mixed-conducting Gd-doped ceria (GDC), were prepared at different sintering temperatures and electrode thicknesses. Electrochemical impedance measurements and distribution of relaxation times (DRT) analysis were performed in both SOFC and SOEC modes to investigate the influence of fabrication conditions on the fuel electrode reaction processes.

Numerical Investigation of the Impact of the Partial Anode Channel Blockage on the Enhanced Mass Transfer and Performance of a Solid Oxide Fuel Cell

This study aims to numerically reveal and assess the influence of inserts of different shapes partially obstructing the anode flow channel on flow characteristics, heat and mass transfer, and planar solid oxide fuel cell (SOFC) performance. To this end, a lattice‐based Boltzmann method is taken. Within this setting, trapezoidal, circular, and triangular shapes and inserts’ heights have been deemed. It turns out that trapezoidal and circular obstacles increase the fuel mass transport to the anode/electrolyte interface and effectively improve the SOFC's power density. As for triangular obstacles, their impact is negligible in terms of performance. Through the findings, it can be stated that it is the trapezoidal design that is better than the other designs. Further, the effect of the inserts’ height has also been assessed to set the optimal performance. It is found that the SOFC performance increases by 4% when using five trapezoidal obstacles with a 90% blocking rate compared to the unobstructed channel. However, in the case of the circular design with five blocks and a 90% blocking rate, the power density improves only about 2.3%. To sum up, improving SOFCs’ power density by designing the anode channel can be adopted to achieve optimal operations.

Evaluation of the environmental sustainability of SOFC-based cogeneration systems in commercial buildings

In order to achieve the European Union (EU)’s climate targets, efficient and sustainable solutions must be developed. In this context, the building sector has a key role to play in the transition to a decarbonized society. Cogeneration is considered an efficient solution that enables better use of fuel energy to supply buildings with heat and electricity. In particular, fuel cell-based cogeneration systems have shown promising performance, thus attracting considerable attention in recent years. The main goal of this study is to analyze the environmental impact of Solid Oxide Fuel Cell (SOFC) cogeneration systems, focusing on commercial buildings. The analysis was performed in the framework of the H2020 Comsos project, whose aim is to investigate the techno-economic and environmental feasibility of SOFC-based systems in non-residential buildings. An MILP-based optimization framework was developed to address the optimal design and operation of energy systems with the inclusion of SOFC-based cogeneration solutions. Electrical and thermal efficiency curves, derived from real SOFC operation, were used to improve the accuracy of the SOFC performance simulation. A sensitivity analysis on the carbon intensity of electricity was carried out to explore the role of the SOFC technology under different levels of electrical grid decarbonization. It was found that natural gas-fed SOFC systems become environmentally advantageous (in terms of CO2 emissions) when the carbon intensity of electricity is higher than ∼ 300 gCO2/kWh.

Feasibility study on supercritical fuel cooled solid oxide fuel cell stack with internal reforming

Solid oxide fuel cell (SOFC) is regarded as a promising energy conversion device because of its advantages such as high efficiency and fuel flexibility. With the advent of aviation electrification era, SOFC has been gradually studied as aviation power system. However, the high temperature operation and temperature gradient of SOFC bring challenges to practical application. Nowadays SOFC systems often use excessive air as coolant to reduce the temperature gradient of SOFC, which leads to the increases of the fuel consumption and reduction of the power generation efficiency. In order to maintain a lower temperature gradient with less air, a supercritical fuel cooled SOFC scheme is proposed in this paper. By establishing a one-dimensional model of conventional air-cooled fuel cell and fuel cell with fuel cooling channel. Through calculating the fuel cell temperature gradient under different working conditions, the results show that when the excess air coefficient (EAC) is 3, the temperature gradient of air-cooled SOFC reaches 22.7K/cm, while the temperature gradient of SOFC with fuel channel decreases to 8.35K/cm. Therefore, the SOFC with fuel channel can effectively reduce the use of air, reduce parasitic power consumption and improve the efficiency of the system while maintaining a lower temperature gradient.

Multiphysics Simulation for the Performance of Tubular Solid Oxide Fuel Cells Operated on Coal Syngas

Solid oxide fuel cells (SOFCs) can be operated directly on coal syngas, but the various contaminants in coal syngas affect the performance and durability of the device. In the present study, three-dimensional multiphysics simulations were developed to investigate the performance degradation of tubular SOFCs operated on coal syngas. The numerical model includes charge conservation, species transport within the electrodes and the channels, and heat transfer within the device. These physical processes are coupled by the chemical/electrochemical reactions, which are represented by a Butler-Volmer type formula. Based on the thermodynamic database about impurities interaction with the Ni-YSZ composite anode, the contaminants are assumed to be adsorbed on the anode surface and form secondary phases. These formed phases affect the microstructural properties, electrical conductivity, and the number of active sites available for electrochemical reactions (i.e., hydrogen and carbon monoxide oxidation). During the cell operation, the contaminants diffuse from the fuel channel/anode interface to the anode/electrolyte interface, causing further irreversible degradation. The in-house developed multiphysics simulation is designed to be robust for prediction of interaction between anode materials with different types of contaminants (e.g., PH3, H2S, H2Se, AsH3, and user-defined). With the model parameters calibrated in the previous study, the performance of the tubular SOFCs with different levels/types of contaminants is predicted. The tolerance limits of the SOFC cell on single and multiple contaminants are also predicted with the simulations. Furthermore, the degradation resulted from Ni coarsening and redistribution, are also implemented into the model to mimic the performance of realistic SOFCs operating on coal syngas. The performance degradation investigation in this study provides guidance for experimental testing with syngas exposure, which is eventually beneficial to the cost reduction of the coal syngas clean-up technology.

Experimental Evaluation of SOFC System Exposed to Marine Inclination Conditions

To comply with future emission regulations, the marine industry needs to reduce emissions. Marine actors are showing an increased interest in the application of Solid Oxide Fuel Cells (SOFCs) for ocean going ships, because of their high conversion efficiency, low pollutant emissions, and fuel flexibility. However, operation in ships is fundamentally different from stationary applications, since the installation rooms accelerate and incline due to wave-induced ship motions. This could lead to pressure variations in the fuel streams or detrimental mechanical stresses. So far it has not been extensively investigated what the consequences of inclinations and motions are on the operation of SOFCs. The goal of this research is to evaluate the influence of marine conditions in terms of static and dynamic inclinations on the operation, safety, and lifetime of SOFC systems. Ship motions are simulated with a one-axial oscillation platform. The inclination conditions the SOFC system is exposed to are derived from ship motion studies and marine regulations. The test campaign consists of two phases. In this paper, the results of the first test phase will be explained in detail. Moreover, the learned lessons and their implementation into the plan of the second test phase will also be shared.In the first phase of the test campaign, a 1.5kW SOFC system is inclined around two horizontal axes of rotation while producing power at nominal conditions. During the static experiments, the SOFC system is operated at fixed inclinations from -30 degrees to 30 degrees with steps of 5 degrees. In the dynamic experiment, the system is oscillated up to an angle of 30 degrees with frequencies of 0.025, 0.033, 0.05, and 0.1 Hz. The experiment showed no gas leakages or other significant safety risks during the experiment. However, for slow oscillations, relatively large deviations in several system parameters (e.g., fuel flow, stack voltage, and fuel utilisation) were found compared with the non-inclined operation. This indicates the natural frequency behaviour of particular components in the system. Although the deviations were much larger than in regular operation, the early stage indication was that these deviations do not result in safety issues or decreased system lifetime.The second phase of the test campaign was focused on including a larger range of possible ship motions and increasing test durations. An additional aim was to find the physical cause of the deviations in system parameters during oscillations. Additionally, a 100h test on one oscillation condition is included to evaluate whether the ship motions could cause increased cell degradation. Furthermore, several part-load conditions are included in the experiment.The results of the experiments are used to propose design improvements for SOFC systems that require to be inclined or accelerated. These proposals contribute to the marinization of SOFC systems. Furthermore, the results will be used to help class societies defining proper regulations for SOFC implementation in seagoing vessels. Figure 1

Reversible SOFC/SOEC System Development and Demonstration

The OxEon Energy team continues its 30+ year solid oxide fuel cell (SOFC) development history with the design, fabrication, and installation of two reversible solid oxide electrolysis (SOEC)/SOFC demonstration modules (rSOC), at Idaho National Laboratory (INL) and a private, stand-alone microgrid, scheduled for installation and commissioning in early 2023. OxEon’s SOEC/SOFC technology builds on the success of the SOEC stack installed on NASA’s Mars Perseverance Rover that has produced high-purity O2 by electrolyzing Mars atmosphere CO2 nine times to date.OxEon Energy’s technology space integrates cross-sector coupling to produce hydrogen or syngas from SOEC, electricity via SOFC, and transportation fuels from syngas through Fischer-Tropsch synthesis. A low energy plasma reformer provides an alternative approach of producing syngas from low value hydrocarbons. OxEon’s four complementary technologies enable a flexible approach to leveling fluctuating energy from renewables and converting it to accessible, storable, and higher value fuels and chemicals. The reversible SOEC/SOFC systems described in this work demonstrate the opportunity to generate and store H2 fuel as a method to stabilize and capture excess production from renewable or nuclear energy sources.The two demonstration units described in this work integrate OxEon’s reversible SOEC/SOFC stacks with an effective and reliable balance of plant (BOP) system. The high temperature electrolysis (HTE) systems produce hydrogen through electrolysis using solid oxide cell (SOC) technology derived from OxEon’s heritage stack technology and the advancements made during the development of stacks for NASA’s Mars2020 mission.The two demonstration units described in this work use the same modular system design based on 4-stack quad assemblies. The INL system consists of three 4-stack quad assemblies to meet the 30 kW SOEC/ 10 kW SOFC target. OxEon also designed the manifold and plenum assembly to interface with INL’s existing 50 kW test stand and scaled the hot section unit (HSU) to enclose the system. Pressure drop across the system is minimized by supplying even flow to each of the three stack quads, and allows for air delivery in SOFC mode with a blower rather than an air compressor. INL system installation and testing is scheduled for early 2023. A previous 10 kW SOEC system demonstration at INL exceeded project objectives with 14.5 kW system power output, with uniform performance measured from each of 4 stacks.OxEon is scheduled to deliver a 20 kW SOEC/ 10 kW SOFC system to the private microgrid at Stone Edge Farm in early 2023. The system is comprised of 2 quad modules and BOP that will connect with onsite hydrogen storage and renewable energy generation plant. The system will generate hydrogen in SOEC mode using renewable energy supplied by the farm’s solar array. Hydrogen produced in SOEC mode will be compressed and stored by a system designed by HyET Hydrogen B.V. During times of low renewable power generation, the SOFC system will use stored hydrogen to generate power.The Stone Edge Farm system includes two heat exchangers (one for air, one for fuel) that raise the gas feeds to within 50 ⁰C of operating conditions, and minimize pre-heating required for operation. Pre-heating is accomplished with heaters in the HSU enclosure. The feed path is routed to use a portion of the exotherm generated in SOFC mode. The air heat exchanger is oversized for SOEC mode but sized to accommodate the excess flow required for cooling in SOFC mode. The fuel heat exchanger is sized appropriately to deliver H2 in SOFC operation and steam in SOEC operation.Both systems apply mechanical compression to the stacks outside of the HSU enclosure. This design produces greater force than if the springs are enclosed in the hot zone and reduces the insulation envelope size. The end load is applied through a loading rod, an upper load plate, layers of insulation, and an additional outer load plate, placing the springs outside the insulation package that surrounds the hot region where the stacks are located. Low thermal conductivity ceramic rods minimize heat loss through the load transmission path.The materials set used in the rSOC systems uses a scandia-stabilized zirconia electrolyte-supported cell design with nickel-cermet fuel electrode and perovskite air electrodes. Green electrolyte is tape cast, cut, and fired to produce a dense electrolyte of about 250 microns thickness. Electrode inks are applied via screen printing, then fired to form porous electrode layers. Recent advancements in the air side electrode barrier layer, air electrode layers, and fuel electrode catalyst have improved stack performance and stability. Figure 1

Reversible Solid Oxide Cells: Cycling and Long-Term Durability of Air Electrodes

Introduction Reversible solid oxide cell (r-SOC) enables both power generation and electrolysis, when the cell can be operated as a solid oxide fuel cell (SOFC) and as a solid oxide electrolysis cell (SOEC). Many studies are being conducted to improve their performance and durability (1,2).Lanthanum strontium cobalt ferrite (LSCF) is an air electrode material with a perovskite-type crystal structure which exhibits high electronic and ionic conductivity, oxygen diffusivity, and electrocatalytic activity (3). However, performance and durability of the cells with Sr-containing oxide air electrodes have to be carefully analyzed, as Sr ions tend to easily diffuse during sintering and in long-term operation, reacting with Zr ions from the solid electrolyte to form a highly resistance reaction layer such as SrZrO3 (4-6). While the performance and durability of LSCF-based air electrodes and the Sr diffusion in SOFC operation have been studied, there are still limited number of studies on the SOEC operation, and especially on the r-SOC operation. The diffusion of constituent ions such as Co and Fe may not be negligible in SOCs.Here in this study, we fabricate YSZ electrolyte-supported r-SOCs and conduct durability studies in both SOFC and SOEC modes. STEM-EDS observation of the air electrode materials is also made to investigate the durability and diffusion of various elements in the LSCF-based air electrode of r-SOCs. Experimental R-SOCs with yttria stabilized zirconia electrolytes (YSZ, 200 µm thick) were prepared. Ni-GDC co-impregnated fuel electrode was prepared, where a mixture of La0.1Sr0.9TiO3 (LST) and Gd0.1Ce0.9O2 (GDC) at a volume ratio of 50:50 was used as the porous electrode backbone, onto which Ni-containing solution of 1 µL was impregnated for Ni loading of 0.167 mg cm-2 (7). Gd0.1Ce0.9O2 (GDC) buffer layer was prepared between the electrolyte and the air electrode to prevent elemental diffusion (8). La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) was used for the air electrode. In addition, a Pt reference electrode was deposited onto the electrolyte on the fuel electrode side. The voltage measurement terminals of the electrochemical measurement setup were connected between the reference electrode and the air electrode to evaluate the air electrode potential. 50%-humidified hydrogen (100 ml min-1) was supplied to the fuel electrode, and air (150 ml min-1) was supplied to the air electrode. Negative current density means the value in the SOEC mode, while positive current density in the SOFC mode. Results and discussion As shown in Fig. 2 describing one cycle, current density was varied at 800°C for the 1000-cycle durability tests in the r-SOC operation. The range of current density was between -0.2 A cm-2 and 0.2 A cm-2. The air electrode potential at different current densities and the electrode impedance in every 100 cycles were measured. For the 1000-hour electrolysis durability test in the SOEC mode, current density of -0.2 A cm-2 was applied for 1000 hours at 800°C. The air electrode potential and the impedance in every 100 hours were measured (9). It has been found that a gradual degradation of the r-SOCs associated with an increase in air electrode potential in the SOEC mode and a decrease in the potential in the SOFC mode could be distinguished. However, such degradation tended to be stabilized with the number of cycles.STEM-EDS observations were performed to analyze the elemental distribution around the air electrode and the electrolyte in the cell. It was found that such elemental diffusion could occur during sintering and durability experiments.AcknowledgmentsA part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Project No. JPNP20005). Collaborative support by Prof. H. L. Tuller, and Prof. B. Yildiz at Massachusetts Institute of Technology (MIT) for their continuous support is gratefully acknowledged. Figure 1

Total Energy for the SOFC and SOEC

The Solid Oxide Fuel Cell (SOFC) Program, managed by the U.S. Department of Energy (DOE) Office of Fossil Energy and Carbon Management (FECM) is currently developing low-cost SOFC and SOEC systems. This paper develops the governing Total Energy (TE) (kilowatt-hours per kilogram hydrogen) equation for the SOEC. The eight-component TE equation includes heat input, exergetic flows, enthalpy of vaporization, pressurization, heat loss, area specific resistance, etc. The eight-component TE equation developed, as it would happen, correlates well with the Idaho National Laboratory (INL) proven SOEC performance of 45 kilowatt-hours per kilogram hydrogen at 20 bars and 725K. TE is the key performance equation necessary for designing, predicting, and planning for SOEC and SOFC performance and cost. Important ramifications of TE are discussed.

Degradation Analysis of SOFC Performance for Long-Term Operation with High Fuel Utilization (2)

Solid oxide fuel cells (SOFCs) are a power generation technology with its high efficiency and low environmental impact, and the most important issue for SOFC for practical use is to improve reliability as well as high efficiency. A national project on SOFC development in Japan, “Development of advanced evaluation and analysis technologies for the durability of Solid Oxide Fuel Cells stacks (2020-),” has been initiated as a NEDO project in order to clarify their long-term durability and find dominant degradation factor for each SOFC stacks. Central Research Institute of Electric Power Industry (CRIEPI) has operated three kinds of different SOFC cell/stacks (flat-tubular and two kinds of planer) for durability under NEDO’s project, which have been developed by KYOCERA, MORIMURA SOFC Technology, and DENSO.Long-term durability tests of 20,000 hours have been conducted under high fuel utilization of 80% and 85%, which is close to the operating limit of each SOFC cell/stack for high efficiency, to evaluate the effects on the performance and degradation behavior of the cell/stack. In previous projects, we have developed an electrode polarization model that can accurately represent SOFC performance, including cell voltage and each overvoltage, under a variety of conditions. The degradation factors for durability of each SOFC cell/stack have been analyzed by using this model. The analysis revealed that the dominant degradation factors were increased cathode overvoltage and IR loss, similar to the SOFC cell/stacks that were modified from the previous project to improve their long-term durability. On the other hand, the degradation rate of cell voltage was higher at 85% than at 80% fuel utilization in one cell/stack, and the performance degradation factor analysis revealed that this difference was due to an increase in anode overvoltage at 85% than at 80% fuel utilization, where anode overvoltage was not the primary degradation factor.This work has been carried out under the projects (JPNP13001 and JPNP20003) of the New Energy and Industrial Technology Development Organization (NEDO), Japan as contract research. We would like to express our gratitude to NEDO, many SOFC developers, as well as to university partners and National Institute of Advanced Industrial Science and Technology (AIST).

Computational Modeling for Cr and S Poisoning Pathways on La0.6Sr0.4Co0.2Fe0.8O3 Surfaces

Perovskite oxygen electrode degradation caused by cation segregation sets the limit of performance and lifetime of solid oxide fuel cells (SOFC) [1][2]. For the state-of-art perovskite oxygen electrode, (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ (LSCF), the commonly known degradation mechanism is through Sr segregation, which can be further poisoned by Cr and as well as by S impurities to form insulating layers, limiting the oxygen evolution reaction (ORR) [3][4]. In order to advance our understanding of Cr and S poisoning pathways on LSCF surfaces, we will combine ab initio density functional theory (DFT) reaction energetics, defect chemistry, and microkinetic modeling to establish a quantitative model to find the deposition reaction on different termination slabs. To predict the reaction mechanism and kinetics of Cr and S poisoning reactions, we will study CrO3 and SO2 molecules deposition pathways on pristine LSCF, and surface-modified (eg. Hf, Zr, Pr, Nd, Ce surface-doped) LSCF slabs. I will correlate adsorption energies with LSCF surface characteristics to explain the origin of the deposition, and use these characteristics to evaluate the effectiveness of the surface infiltration. Ab initio molecular dynamics (AIMD) will be applied at different ensembles to investigate further steps of the reaction, and show how the surface modification will inhibit poisoning reactions. These findings will help to guide the design of better performance and more durable oxygen electrodes for SOFCs.[1] Neagu, D., Tsekouras, G., Miller, D. N., Ménard, H., & Irvine, J. T. (2013). In situ growth of nanoparticles through control of non-stoichiometry. Nature Chemistry, 5(11), 916–923.[2] Lee, K. T., & Wachsman, E. D. (2014). Role of nanostructures on SOFC performance at reduced temperatures. MRS Bulletin, 39(9), 783–791.[3] Jiang, S. P., & Chen, X. (2014). Chromium deposition and poisoning of cathodes of solid oxide fuel cells – a review. International Journal of Hydrogen Energy, 39(1), 505–531.[4] Chen, K., & Jiang, S. P. (2020). Surface segregation in solid oxide cell oxygen electrodes: Phenomena, mitigation strategies and electrochemical properties. Electrochemical Energy Reviews, 3(4), 730–765.

Highly Connected Oxygen Ion Conduction Pathways for Solid Oxide Fuel Cells Operating in Intermediate Temperatures with Fuel Flexibility

Solid oxide fuel cells are promising eco-friendly power generating devices directly utilizing various fuels such as hydrogen, methane, and carbon dioxide. However, a technical breakthrough is required for further commercialization by lowering the high operating temperature to the intermediate temperature regime. Introducing the anode functional layer (AFL) between the electrolyte and anode is one of the crucial methods in the development of high performance solid oxide fuel cells by maximizing the triple phase boundary (TPB) sites. To activate the TPB sites, ensuring the continuous oxygen ion conduction from the electrolyte to the TPB sites is essential to maximize their utilization for hydrogen oxidation reactions (HORs). In this study, we modify the connectivity of oxygen ion conduction pathways in the AFL by controlling the microstructure in AFLs. We calculated active reaction site using image processing of cross-sectional scanning electron microscopy (SEM) image and the strong correlation between the electrochemical performance and calculated active reaction site is revealed. The modified AFL with highly connected oxygen ion conduction pathways exhibits substantially higher maximum power density (MPD) compared with conventional AFL: ~1.7-fold higher MPD of 1.51 Wcm-2 at 550 ℃ with hydrogen and ~3.5-fold higher MPD of 1.11 Wcm-2 at 550 ℃ with methane and carbon dioxide, surpassing previously reported values. Moreover, excellent carbon tolerance is observed in the modified AFL, exhibiting nearly no degradation at 550 ℃ for 130 h. This result substantiates the role of connectivity of the oxygen ion conduction pathways in the HOR and carbon tolerance in AFLs. Figure 1

Degradation of Solid Oxide Fuel Cell Using Thin Film Electrodes Fueled By Carbon Monoxide-Hydrogen Syngas

According to the recent development trend of the smart farm industry, solid oxide fuel cells are emerging as a sustainable power source due to their high fuel efficiency and fuel flexibility which includes carbon fuels such as carbon monoxide. This study conducted a series of experiments to secure an operation strategy for a solid oxide fuel cell operating at the intermediate temperature of 650~800C. Although it is known that when a fuel cell is operated using carbon-based fuel at an intermediate temperature under 800C, fatal degradation occurs due to carbon coking, widely used commercial fuel cell stacks were manufactured not to exceed that temperature. To simulate a commercial fuel cell stack, the experiment was conducted in the commercial operating temperature range, and nickel-yttria stabilized zirconia was used as anode material. Additionally, ceria-based anode material was applied, and compared its single-cell performance with that of the yttria-based anode material. Carbon monoxide-hydrogen mixed fuel, which can be produced through torrefaction and fuelization from agricultural by-products, was supplied. To this end, a YSZ-based electrolyte-supported cell using Ni-YSZ/Ni-GDC and LSCF-GDC as anode and cathode materials, respectively, was fabricated by the sputtering method. The cell performance and degradation phenomenon were measured under the two temperature ranges and two fuel supply conditions. For structural analysis, FE-SEM and FIB-SEM methods were conducted, and techniques such as the i-V-P curve, EIS, and DRT were used for their electrochemical analysis. The relationship between the nano-scaled structural change caused by the carbon coking phenomenon and the cell performance was derived. Especially, EDS mapping and XPS analyses were conducted to quantify the carbon coking phenomenon. Through this, the degradation mechanism mainly caused by carbon coking was analyzed when syngas was used as a fuel. The optimal operating condition for durable fuel cell performance was confirmed for the stack-sized operation in the smart farm industry.

Effects of Transition Metal Elements on Ni Migration in Solid Oxide Cell Fuel Electrodes

Solid oxide cells (SOCs), both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC), have emerged and attracted more and more attentions due to its high energy conversion efficiency and fuel flexibility. However, degradation of fuel electrodes after long-term operation remains as one of the main challenges for their commercial application. Two major types of microstructure evolution, nickel (Ni) migration and Ni coarsening in Ni - yttria-stabilized zirconia (Ni-YSZ) fuel electrodes have been widely reported, which have strong impacts on both cell performance and durability. Therefore, materials-driven research has focused on developing more robust fuel electrodes with good electrocatalytic ability. Utilization of Ni alloys has immersed as a promising concept to enhance the performance and robustness of conventional Ni/YSZ. In the present study, Ni-M (M = Fe, Cu) bimetallic fuel electrodes are applied to investigate the effects of transition metal elements on the morphological evolutions under SOFC and SOEC operations. Ni-Fe, Ni-Cu and pure Ni patterned fuel electrodes are sputtered on YSZ pellets. The electrochemical performance of these Ni-M bimetallic fuel electrodes decreases compared with pure Ni fuel electrode, while the performance degradation rate of Ni-Fe fuel electrodes is smaller than others. Besides, the spreading of Ni film on YSZ surface is observed for all samples under anodic polarization and such Ni migration is suppressed by Fe doping, whereas enhanced by Cu doping. On the other hand, the adhesion is weakened at the electrode / electrolyte interface for Ni-Cu and pure Ni fuel electrodes under cathodic polarization, while good adhesion at the interface is maintained for Ni-Fe, which correlates with the smaller performance degradation rate.

Model Based Comparative Analysis of GDC and YSZ Based Solid Oxide Fuel Cells

Designing solid oxide fuel cell (SOFCs) stacks for transportation applications places new challenges on SOFC architecture and operation with respect to power density, dynamic response, and thermal management. Recent developments in SOFCs with either thin-film yttria stabilized zirconia (YSZ) electrolytes (thickness ≤ 5 μm) or highly conductive, gadolinia-doped ceria (GDC) electrolytes (thicknesses ≥10 μm) enable thigh-power-density stack operation at inlet temperatures well below 700ºC with the potential for using compliant seals and lower cost ferrous interconnect materials. Applications, such as hybridized aircraft engines, can use such SOFC stacks if they meet strict requirements with respect to specific power and thermal management. Highly conductive GDC-based electrolytes offer pathways to high power-density SOFCs at operating temperatures below 650ºC, but the rise in mixed ionic electronic conductivity (MIEC) above 650ºC reduces open circuit voltages and thus fuel cell efficiency [1,2]. This necessitates thermal management to maintain operating temperatures and mitigate thermo-mechanical stresses within an SOFC. Thin-film YSZ electrolytes offer lower power densities below 650ºC, but avoid the penalties associated with MIEC behavior at higher temperatures and therefore reduce the challenges associated with thermal management at high power densities. In this work, a model-based approach is used to compare SOFC stack operation with thin-film YSZ electrolytes and with GDC-based electrolytes for operation on hydrogen and reformate fuels at high-power densities relevant for transportation applications.To calibrate models to experimental cell performance, polarization curves of a commercially available YSZ-based cell operating between 600-750ºC and of a high-power-density GDC cell operating between 500-650ºC are used to fit electrochemical model parameters for both electrolytes. Modeling of the YSZ based cell can neglect electronic charge transfer through the membrane electrolyte, but GDC-cell modeling must account for the multiple charge carriers (ceria polarons and oxygen vacancies) in the electrolyte [3]. Electrochemical parameters for ionic conductivities and electrochemical reaction rate parameters for oxide cathodes and nickel-electrolyte-composite anodes are fit to the experimental polarization data over the broad range of operating temperatures. Global rate expressions to account for internal reforming on the active nickel surfaces of anode are adopted from literature [4,5]. The calibrated cell model for each cell type is adopted to a down-the-channel discretized cell simulation to predict heat loss and temperature gradients at high current densities.This presentation compares GDC- and YSZ-based SOFCs in high power-density applications from fundamental and practical perspectives. The open-circuit voltages for GDC-based cells drops with increasing temperature to about 0.85 V/cell at 650°C which is lower than the Nernst open-circuit voltage of YSZ-based cells (≥ 1.05 V/cell) at similar conditions. Both types of cells can sustain power densities around 1.0 W/cm2 with high excess cathode air ratios ≥ 5 to keep temperature gradients near 10°C/cm. These results show that GDC-based cells support higher power densities at lower cell flow inlet temperatures < 600°C, whereas thin-film YSZ cells provide superior performance at flow inlets that drive cell temperatures > 600°C. Higher stack power densities > 1.0 W/cm2 must be supported by stack designs that alleviate high thermal stresses and allow steeper gradients of temperature. A summary of stack design and operational strategies that meet the stringent requirements of high power-density operation will be presented in the light of the modeling resuls..

Transient Solid Oxide Cell Reactor Model Used in rSOC Mode-Switching Analysis and Power Split Control of an SOFC-Battery Hybrid

Defossilization of the global energy system requires a transition towards intermittent renewable energy sources and approaches that enable efficient conversion of primary energy sources into electrical energy. Due to their high efficiency in converting chemical into electrical energy and vice versa, solid oxide cell (SOC) systems provide solutions for both of these aspects. Within this contribution, two researched cases utilizing SOC's are presented, based on simulation studies and experiments.Characteristically, SOC reactors produce hydrogen from steam in solid oxide electrolysis (SOE) mode, or electricity from reformates in solid oxide fuel cell (SOFC) mode. An application of both modes as reversible solid oxide cell (rSOC) is to balance a renewable power supply with storage and power production, leading to high utilization of the same equipment. An application in SOFC mode is the maritime transportation powertrain. In both cases, transient operation is needed whenever mode transitions occur. In particular, switching between rSOC modes implies transitioning exothermal SOFC, endothermal, thermoneutral and exothermal SOE operation. Similarly, supplying the power demand of a maritime drivetrain in SOFC mode leads to various exothermic levels, as pertinent to part or full load operation. Operating strategies are needed to suppress potentially damaging thermal stresses during these transitions in the electrochemical SOC reactors. In order to identify such operating strategies, experiments have been carried out and a transient model has been developed for the analysis of rSOC mode-switching and SOFC drivetrain power supply, which are presented in this study. The 1D+1D SOC dynamic multi-reactor model includes the individual SOC reactors, piping and insulation, and is implemented in the in-house developed transient energy process system simulation framework TEMPEST [1,2]. The model couples the transient balances of mass and energy with electrochemistry, internal reforming kinetics, heat transfer, and flow distribution. As a result, temperature and voltage characteristics at cell, stack, and module levels are obtained to analyze for e.g. unwanted thermal stress. In the EU project SWITCH [3], experiments were performed at DLR with a Large Stack Module (LSM) from SolydEra (formerly SOLIDpower) to validate the model in transient 75 kW electrolysis mode, 25 kW fuel cell mode and mode-switching operation between electrolysis and polygeneration mode. The so called polygeneration mode refers to simultaneous generation of hydrogen and electricity at partial fuel utilization with natural gas, biogas or e-methane. Simulative studies of mode-switching procedures from SOE to SOFC-mode polygeneration show that drawing fuel cell current soon after reaching open circuit voltage and sufficiently in advance of the methane ramp completion leads to a reduced temperature decrease at the inlet of the cell without reaching oxygen to carbon ratios low enough to favor carbon deposition. In the EU project NAUTILUS [4], the mismatch between the transient response possibilities of SOFC systems and the power demand of a ship is addressed by connecting Li-ion batteries to the powertrain. Batteries respond to highly transient ship load demand changes, while the SOFC’s provide base part load to full load, according to a power split control strategy. A battery model developed and parametrized by the Chair for Electrochemical Energy Conversion and Storage Systems of RWTH Aachen University [5] was added to TEMPEST and validated using DLR experiments with a 40 kWh Li-ion battery. Simulation results of the SOFC-battery hybrid in Figure 1 show that a rule-based power split control strategy [6] ensures that the power demand of the ship is met at all times while the battery state of charge (SoC) remains within specified range, and the SOFC power is drawn at one of three fixed power levels for reduced thermal stress. An experimental campaign to test this and other control strategies with a 32 kW LSM from SolydEra and the 40 kWh battery is in progress at DLR. Acknowledgements Project SWITCH has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now Clean Hydrogen Partnership) under Grant Agreement No 875148. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation program, Hydrogen Europe and Hydrogen Europe Research. Project NAUTILUS has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 861647.

Engineered Nanofunctional Thin Films as Interfacial Layers to Enhance Performance and Durability of SOFCs

A strategy to improve the performance and durability of solid oxide fuel cells (SOFCs) is to increase the cathodic activity and decrease the interfacial resistance between the cathode and electrolyte. Pulsed laser deposition (PLD) has shown to be a promising method to engineer functional interlayers to enhance the cell's performance. In the present study, a bilayer consisting of Sm0.2Ce0.8O2-δ (SDC) barrier layer (BL) and a self-assembled nanocomposite consisting of SDC-La0.8Sr0.2MnO3-δ (SDC-LSM) employed as a cathode functional layer were deposited by PLD in an anode supported SOFC. The presented thin and dense layers are shown to be an effective strategy for the fabrication of high-performance fuel cells (power density of 1.0 W·cm−2 at 0.7 V and 750 °C). Durability tests carried out for up to 1500 h demonstrated the excellent stability of the fuel cell. Post-test analyses confirmed the stability of the microstructure of the developed bilayer.

Performance and Durability of Metal SOFCs in Alternate Fuels

Electricity generation from solid oxide fuel cells (SOFCs) based power generator are in great demand for both stationary and automotive use. SOFC is considered as one of the best energy conversion devices that is available to demonstrate Nissan’s carbon neutrality goal by 2050. In particular, SOFCs are known for their fuel-flexible operations, for example, SOFCs can operate on renewable fuels such as ethanol, natural gas, jet-fuel, propane. Unlike H2 based polymer fuel cell, the possibility of utilizing liquid fuels directly in SOFCs can pave the way to adopt the existing fuel infrastructure and can limit the greenhouse gas emissions significantly. This abstract presents the potential of using externally supported SOFCs, such as metal based SOFCs (metal-SOFCs) as the highly performing and durable power generator. It is identified that the metal-SOFC technology is a closest viable solution for using SOFCs in stationary and automotive applications.

Understanding Temperature Stability of Surface Cation Dopants on LSCF Electrodes

The widespread application of solid oxide fuel cells (SOFCs) is primarily inhibited by degradation issues impacting the stability and oxygen exchange kinetics of the oxygen electrode. State-of-the-art SOFC materials such as La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) boast fast oxygen ion and electron conduction as well as high oxygen reduction activity. Furthermore, materials such as LSCF provide a potential pathway to reduce the operating temperature of SOFCs from 800-100 ° C to 400-800 ° C. But degradation issues continue to impede LSCF, primarily through strontium segregation to the electrode surface and subsequent passivating poisoning reactions due to chromium- and sulfur-containing species.In this work, we investigate a solution infiltration technique to introduce cation dopants to the surface of LSCF porous electrodes on symmetrical cells to improve the stability and performance through suppression of Sr segregation. We have previously demonstrated that introducing more oxidizable transition metal cations on the LSC electrode surface improves both the stability and oxygen exchange kinetics. Others have demonstrated that lanthanides can electrochemically stabilize the LSCF surface as well (Yu Chen, et al. Energy Environ. Sci., 2017, 10, 964-971). However, the temperature dependence of these cation dopants on electrode surface composition and electrochemical stability is not well understood. Thus, we are interested in exploring more oxidizable transition metal and lanthanide cations as infiltrated dopants on the porous LSCF electrode as a function of temperature.Preliminary experiments demonstrate that both transition metal and lanthanide cation dopants diffuse into the LSCF lattice at SOFC operating temperatures (above 700 ° C) and Sr segregates to the surface with increasing temperature. We do also observe some fundamental differences between the transition metal and lanthanide cations, such as the temperature dependence of A:B site ratio. Our initial electrochemical measurements indicate that these cation dopants are found to improve the stability of LSCF electrodes when tested at SOFC operating temperatures (800 ° C in ambient air) despite having largely diffused into the lattice at these temperatures.These preliminary results indicate a potential electrode surface reconstruction as a function of cation type and temperature. We aim to elucidate 1) the temperature stability of these cations at the surface, 2) the impact of these cations on the electrode surface composition and cell electrochemical stability as a function of temperature, and 3) the mechanism for electrochemical influence of these sub-surface cations.We utilize electrochemical impedance spectroscopy and chronoamperometric stability studies to characterize the impact of infiltrated cations on electrode activity and stability. We investigate the stability of cation dopants and reasons for observed electrochemical changes through x-ray photoelectron spectroscopy and x-ray diffraction studies. This work aims to elucidate the impact of transition metal and lanthanide cation dopants on electrode stability and performance as well as the stability of the dopants at the surface at different operating conditions, with a focus on developing a mechanistic understanding of sub-surface dopant influence on electrochemical performance.