Discovering and engineering new materials with fast oxygen surface exchange kinetics and robust long‐term stability is essential for the large‐scale, economically viable commercialization of solid oxide fuel cell (SOFC) technology. The perovskite catalyst material BaFe0.125Co0.125Zr0.75O3 (BFCZ75), predicted to be promising from recent density functional theory (DFT) calculations and unconventional due to its extremely high Zr content and low electronic conductivity, exhibits oxygen reduction reaction surface exchange rates on par with Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) and excellent stability at typical operating temperatures. New composite electrodes are engineered by integrating BFCZ75 with commercial electrode materials La1–xSrxMnO3 (LSM) and La1–xSrxCoyFe1–yO3 (LSCF) and achieve high performance as measured by low area specific resistance (ASR) values, with the LSCF/BFCZ75 ASR values comparable to top performing noncomposite electrode materials such as SrCo0.8Sc0.2O3–δ, BaNb0.05Fe0.95O3–δ and BaCo0.7Fe0.22Y0.08O3–δ. The use of BFCZ75 as a composite with LSCF achieving low ASR values shows that BFCZ75 is highly active and can easily integrate into existing SOFC material supply chains, lowering the barrier for potential commercial application of new electrode materials. Finally, these findings point to a broader unexplored class of perovskite materials with high fractions of redox inactive species (e.g., Zr, Nb, and Ta) that may unlock new pathways to realizing improved commercial SOFCs.

In solid oxide device operation, delamination and cracking cause more catastrophic damage to cell life than other degradation mechanisms, leading to more rapid cell failure. The electrochemical performance degradation caused by electrode/electrolyte interface delamination and active layer cracking were investigated through in-house developed three-dimensional multiphysics simulations. The multiphysics simulations, fully calibrated against experimental datasets for button cells in a previous study, were extended to quantitatively investigate the effects of electrode/electrolyte interface delamination and active layer cracking. The simulated polarization curves and impedance behavior with various levels of delamination and cracking provided insights of the performance degradation. Furthermore, the local current distributions within the cell were analyzed for three different operating modes: constant voltage, constant current, and constant power. Finally, a practical strategy was proposed to mitigate the performance degradation in the realistic cell operations, extending overall cell lifetime.

Long-lasting solid oxide fuel cells (SOFC) rely upon an intact ionically conducting network in both the electrolyte and electrode. Understanding the impact of the high-temperature electrochemical operations on the possible nanostructure evolutions of ionic conductors is indispensable for the development of durable SOFC. Here we present possibly the first experimental evidence of space charge layer evolution of grain boundaries (GBs) of ionic conducting yttria-stabilized zirconia (YSZ) in a Ni/YSZ composite anode of SOFCs. Cells with identical Ni/YSZ composite anodes are either thermally treated or electrochemically operated using either dry or humidified H2 fuel for up to 3,349 h. When operating under humidified H2 fuel, a discrete core-shell structure is found to develop exclusively along the YSZ/YSZ GB planes. The core-shell structure is elliptically shaped elongated along the GB planes. The core has a lower mass with a Y-depleted ZrO2-x with a cubic structure. The shell has a constant thickness of ∼1 nm and is Y-enriched. The nucleation and growth mechanisms of the core-shell structure are proposed in terms of the space charge layer evolution at YSZ GBs. The effect of the humidity in the fuel, the cell operating temperatures, and the current density on the formation of the core-shell structure is investigated.

Temperature gradients resulting from local electrochemical reactions, current distribution and geometry of gas flow channels in solid oxide fuel cells (SOFCs) create thermal stresses, localized thermophysical property gradients and uneven property evolution, contributing to SOFC degradation. This paper presents a new method to perform temperature measurements (up to 800 degrees C) at high spatial resolutions to monitor the operation of SOFCs. Using femtosecond laser irradiation, distributed fiber sensors were hardened for high temperature environment applications. Distributed fiber sensors were embedded in interconnected plates using an additive manufacturing method to perform temperature measurements with 4-mm spatial resolution during the operation of a planar fuel cell. The measurement revealed the impact of various H-2 fuel concentrations and current loads have on temperature profiles of the SOFC tested. Temperature variation on the anode side was found to be less than 5 degrees C, and 3 degrees C on the cathode side. The measurements were compared to results from a multiphysics fuel cell performance model simulating similar conditions. These simulations predicted similar temperature gradients, indicating the experimental data obtained is reasonable. The model also predicts that the effect of the embedded sensor has on the local temperature will be minimal and that the gradient of temperature in the gas channels will be captured despite the separation between the sensor and the gas flow. The high spatial resolution data harnessed by these distributed fiber sensors provides experimental support for model-based design and optimization to improve the operational efficiency and longevity of solid oxide fuel cells and fuel cell assemblies.

The oxygen surface exchange and bulk diffusivity of BaCo0.4Fe0.4Zr0.1Y0.1O3-delta were determined by electrical conductivity relaxation (ECR) in order to quantitively assess cathode performance for protonic ceramic fuel cells (PCFCs). The measurements were performed at 600 degrees C, following pO2-step changes between 80 ppm to pure oxygen. The apparent value of surface exchange coefficient (k) and diffusion coefficient D, indicates BaCo0.4Fe0.4Zr0.1Y0.1O3-delta is a good oxygen conductor. k was found to vary with pO(2) and surface area while the chemical diffusion coefficient D remained invariant with pO(2) and surface coating. The fitted kinetic parameters obtained from ECR with reduction process (high to low pO(2)) were compared with that of the oxidation process (low to high pO(2)). In the presence of H2O, oxygen exchange kinetics was suppressed because of the competitive adsorption relationship between oxygen and H2O. Single cell measurements employing single phase BaCo0.4Fe0.4Zr0.1Y0.1O3-delta cathode and an anode prepared by phase inversion tape casting in a configuration of 40 wt% BCZYSm13 + 60 wt% NiO | BCZYSm13 | BCFZY(0.1) exhibited a power density of similar to 200 mW/cm(2) at 600 degrees C.

While performance degradation for solid oxide fuel cells (SOFCs) comes in many forms, one important contributor is cation transport during long-term operation. This transport alters the chemical composition at electrode/electrolyte interfaces. These interfaces are carefully engineered to provide the critical reaction sites for oxidation and reduction reactions that regulate SOFC electrical production. Therefore, fundamental understanding of the chemical evolution of these interfaces is critical. The current study coordinates scanning transmission electron microscopy (STEM) with atom probe tomography (APT) in order to probe composition at the nanoscale across cathode/electrolyte interfaces for anode-supported commercial SOFCs. These SOFCs contain a porous composite cathode layer, consisting of sintered (La0.8Sr0.2)0.95MnO3 cathode particles and yttria-stabilized zirconia (YSZ) electrolyte particles. SOFCs are operated up to 500 hours in duration at a current density of 0.75 A/cm2 (or at open circuit) and at an operation temperature of 800°C. Measured cell voltage increases over the first 100 hours of operation, followed by a steady and linear drop in cell voltage that translates to 5.35% performance loss per 1000 hours. STEM-based energy dispersive spectroscopy (EDS) indicates nanoscale Mn-oxide formation at LSM particle surfaces after 500 hours of operation. Compositional profiles acquired by APT across LSM/YSZ particle interfaces indicate as-sintered interfaces are chemically well-defined, but La and Mn penetrate up to 5 nm into YSZ particles over the course of 500 hours. Additionally, as-sintered LSM particles exhibit measurable Mn enrichment within 20 nm of an adjacent YSZ particle. After 100 hours of operation, A-site deficiency is restored for LSM particle surfaces adjacent to YSZ. Meanwhile, the YSZ composition adjacent to LSM exhibits depletion in Y content over the first 100 hours of operation. These results suggest that initial cell voltage increase within 100 hours of operation corresponds to both Mn and Y transport at LSM/YSZ interfaces. STEM-based electron energy loss spectroscopy (EELS) provides additional insight regarding local cation valence states in relation to measured composition variations for these LSM/YSZ interfaces.

Electrical conductivity relaxation (ECR) is a widely adopted technique for determination of oxygen surface exchange coefficient (k(chem)) and chemical oxygen diffusivity (D-chem) of mixed ionic and electronic conductors (MIECs). However, it has been argued that the fitting process of determining two kinetic parameters from a single conductivity relaxation curve inevitably leads to high error in the determined values. In this research, we demonstrate experiment-based analytical approaches to overcome the issue and obtain highly reliable kinetic parameters using ECR for the case of (La0.6Sr0.4)(0.95)Co0.2Fe0.8O3-delta (LSCF), a representative MIEC. As a baseline, kinetic parameters of a standard LSCF bar sample with the thickness close to the critical thickness are obtained using a conventional ECR method along with error range and sensitivity analysis at oxygen partial pressures of 0.2-3.125 x 10(-3) atm. D-chem with improved accuracy is obtained by ECR under primarily bulk diffusion-controlled condition, which is achieved by either increasing the thickness or coating porous LSCF on the baseline sample. With the D-chem determined, the baseline data are further refined to obtain k(chem) as the sole fitting parameter and the error range is reduced more than 70% of the original value under all test conditions. With the significantly improved accuracy and sensitivity of both k(chem) and D-chem, this demonstrated method is proven to be a practical approach to advance the application of ECR.

In this study, the performance of solid oxide cells (SOCs) under both electrolysis mode and fuel cell mode is investigated via in-house developed high fidelity multiphysics simulations. The full parameter space with various fuel/steam supply conditions is explored to investigate the trends of button cell performance under different working loads. For each specific working loads, a global minimum resistance is found, but the conditions for the global minimum resistance shift for different working loads under different working modes. The trends are also verified by the good agreements between simulations and experiments. Furthermore, the performance degradation due to Ni redistribution in the active layer of hydrogen/steam electrode is also investigated by implementing the microstructural properties change into the developed model. The results show that main performance degradation occurs on the high frequency range (> 1000 Hz), indicating that the Ni redistribution inside the active layer mainly affects the charge transfer processes in the hydrogen/steam electrode. This study can provide guidance for the design of reversible solid oxide fuel cell (r-SOFC) system as well as performance stability improvements.

The DOE Office of Fossil Energy Solid Oxide Fuel Cell (SOFC) Program is interested in the near-term commercialization of high-temperature SOFC and solid oxide electrolyzer cell (SOEC) technologies that are robust, reliable, and resilient. A recent report delivered to the United States Congress highlighted several development recommendations including the design of pilot-scale units, continued early stage research and development, increased industrial engagement, and the exploration of reversible operation of SOFC technology. The National Energy Technology Laboratory (NETL) SOFC research group currently addresses most of these recommendations. NETL’s in-house research efforts focus on the characterization, simulation, and mitigation of high temperature degradation of fuel cell components. The capstone of these efforts is NETL’s SOFC degradation modeling framework, which uses NETL supercomputing facilities to simulate SOFC performance degradation of thousands of possible electrode configurations experiencing multiple simultaneous degradation modes under a broad array of relevant operating conditions. The models for the different degradation modes are based upon experimental data (1) generated in-house, (2) referenced from available scientific publications, and (3) shared from collaborations with other industrial, academic, and national laboratory partners within the SOFC Program. The team then employs techniques in data analytics to select optimal electrodes to maximize the SOFC performance for given operating conditions. These modeling efforts guide electrode engineering efforts in-house and through external collaborations by identifying (1) which degradation modes contribute the most to overall performance degradation for given operating conditions and (2) which electrode features will have the greatest impact on lowering cell degradation and system costs. Additionally, the development on non-invasive in situ high temperature fiber optic sensors for temperature and gas composition measurement provides valuable data for inclusion in degradation models as well as informing technology development at the commercial scale. Finally, the wealth of experience gained in development degradation models and materials for reducing the cost of SOFC technology is being readily applied to reducing the cost of SOEC technology. NETL will report on its most recent progress in the field of SOFC and SOEC development, including degradation modeling, in situ fiber optic sensor development, electrode engineering, and relevant systems-level analyses.

Interconnected networks of 10-30 nm yttria-stabilized zirconia (YSZ) nanoparticles dramatically enhance both the electrocatalytic activity and bulk charge transport of commercial lanthanum strontium manganite (LSM)-YSZ solid oxide fuel cell (SOFC) cathodes. The improvement in both electrode functions increases the maximum power density of the commercial SOFC by 90%. In comparison, modifying cathodes with lanthanum strontium cobalt ferrite (LSCF) and praseodymium barium cobaltite (PBC) nanoparticles, highly active catalysts with mixed ionic-electronic conductivity (MIEC), only enhances electrocatalytic activity. The combination of dual enhanced electrode functions with nanoYSZ results in a maximum power density that is 50% and 11% higher than LSCF and PBC, respectively. Finally, the performance stability over time is highest for nanoYSZ modified cells. (c) 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.

The electrochemical performance of solid oxide fuel cell (SOFC) cathodes was improved via integration of ultra-high surface area ceramic nanoparticles, up to 115 m2·g-1, generated with a novel processing method herein introduced. The processing method generates the high surface area nanoparticles at traditional SOFC sintering temperatures in two steps. In the first step, a hybrid inorganic-organic material comprising the ceramic precursors is sintered in an inert atmosphere at any temperature between 850°C-1350°C. During this step, an amorphous carbon template is generated in situ, preventing coarsening of the ceramic nanoparticles. The carbon template is then removed during the second step by a mild calcination in air at 700°C. Yttria-stabilized zirconia (YSZ), lanthanum strontium cobalt ferrite (LSCF), gadolinium-doped ceria (GDC), and strontium titanate (STO) have all been successfully prepared by this method. YSZ nanoparticles (nYSZ) were incorporated into a lanthanum strontium manganite-YSZ (LSM-YSZ) cathode of a commercial cell, resulting in a 90% increase in maximum power density. Impedance spectroscopy indicates the large improvement in power density was attributed to a combination of significant improvement in polarization resistance, a 45% decrease, and ohmic resistance, a 35% decrease. Remarkably, the performance of the cell modified with nYSZ was higher than cells modified with two mixed ionic electronic conductors (MIEC): PrxBa1-xCo3-δ (PBC) and LaxSr1-xCoyFe1-yO3-δ (LSCF). Both MIECs enhanced the density of active sites, but, unlike nYSZ, did not significantly improve ohmic resistance. The results demonstrate a novel pathway to achieve high performance in commercial SOFCs.

A one-step phase inversion method was applied to fabricate an optimized anode structure for protonic ceramic fuel cells (PCFCs). The phase inversion process utilized raw starting chemicals, instead of crystalline BaCe0.7Zr0.1Y0.1Yb0.1O3.delta (BCZYYb) powder in an energy and time saving process. The resulting large and fingerlike pores exhibited enhanced performance as compared to the disordered pores produced by conventional preparation of anode structures using dry pressing methods. The electrochemical performance of the rational designed anode supported cell were 491, 402, 302 and 200 mW cm(-2) at 700, 650, 600 and 550 degrees C, respectively, which was nearly twice than the cell with dry pressing anode. An equivalent circuit modeling method was used to separate the anode polarization resistance from the single cell, confirming that the overall cell performance improvements were attributed to microstructural modifications of the anode by the phase inversion process. The one-step phase inversion method demonstrated great promise for improved processing of fuel cells and separation membranes.

The National Energy Technology Laboratory (NETL) Solid Oxide Fuel Cell (SOFC) Team performs fundamental SOFC technology evaluation, enhances existing SOFC technology, and develops advanced SOFC concepts in support of the U.S. Department of Energy SOFC Program. Program targets include a reduction in performance degradation to 0.2% per 1000 hours and a system cost of $900 per kilowatt. Research and development is essential to meet these targets. Research efforts are broadly focused on an investigation of cell and stack degradation, electrode engineering, grid integration challenges, and system analysis. The research approach is targeted to specifically address SOFC program technology development goals, especially regarding reducing stack costs, increasing cell efficiency, and increasing cell reliability and robustness. The goal of these efforts is to transfer technology that facilitates commercial acceptance of SOFC technology. This is accomplished through close collaboration with SOFC commercial developers, national laboratories, and academic institutions. NETL system-level analysis has shown that a critical consideration for reducing the cost of SOFC technology is the enhancement of electrode performance and longevity through materials and microstructure engineering. Within this scope is the special consideration of cell production costs and operating temperature. Specifically, at NETL, nano-electrocatalyst infiltrations have been a successful operation by scaling up the patented technology to a commercially-relevant scale, partnering with industrial manufacturers. The effort has continued to the construction of a scaffold microstructure that is optimized for surface modification and the development of gel-derived high surface area electrodes. Electrode materials selection has been made through a computational approach that uses electronic structure and energetics in conjunction with thermodynamics and statistical physics, identifying multicomponent oxides with high oxygen reduction reaction activity and stability. Highly reliable electrical conductivity relaxation analysis confirmed the predicted electrode materials’ superior surface exchange property when compared to the state-of-the-art lanthanum strontium cobalt ferrite. The developed electrode engineering techniques have been applied for reversible solid-oxide cell operation as well as single mode solid-oxide cells to stabilize electrodes while maintaining improved performance and longevity.

Real time gas sensing in high temperature energy conversion devices can enable optimal and efficient operation at both component and system levels, and the optical fiber based sensing platform shows significant advantages for harsh environment applications. In this research, (La0.8Sr0.2)0.95MnO3-δ (LSM), (La0.8Sr0.2)0.95CoO3-δ (LSC) and (La0.8Sr0.2)0.95Co0.2Fe0.8O3-δ (LSCF) films with thicknesses of several tens of nm are integrated with the optical fiber sensing platform as a functional sensor layer using a finely tuned-RF sputtering system designed for the fiber substrate deposition. Oxygen sensitivities, stabilities and overall feasibilities of these representative perovskite materials on the optical fiber platform are evaluated in the solid oxide fuel cell operational temperature regime at the oxygen concentration up to 19%, relevant for in-cell cathode stream gas composition sensing through optical transmission measurement which covers visible and near infrared wavelength ranges. Various sensitivity comparisons are carried out as a function of thickness, oxide composition, and deposition conditions. In general, the LSM sensor shows a stepwise absorption response to increasing levels of O2 in a N2 background, but also exhibits relatively slow kinetics including a continuous baseline drift. In contrast, LSCF based sensors exhibited enhanced transmittance responses in O2 containing gas and a more rapid recovery and response, presumably due to the enhanced oxygen ion diffusion kinetics as compared to LSM. The results presented here are promising for the broad application areas of high temperature O2 sensor research and a concomitantly wide range of energy related applications including combustion, solid oxide fuel cells, and others.

The interfaces between the different phases and the associated triple phase boundaries (TPBs) in solid oxide fuel cell (SOFC) cathodes are critical for the oxygen reduction reaction, and their degradation impacts SOFC performance and durability. This work examines nanostructure degradation of composite LSM/YSZ cathodes induced by electrochemical operation in humidified air. Three commercial button cells operated in humidified air for various durations at 800 degrees C exhibited more severe performance degradation than the cell operated in dry air. Microscopy imaging reveals nanostructure degradation within the cathode active layer, especially in the regions nearest to the electrolyte. Newly formed Mn-enriched nano-precipitates accompanied by nano-voids initiate at the original TPBs and propagate along the LSM/YSZ interface. The abundance of the nanoprecipitates at LSM/YSZ interfaces increases with operation time, and prolonged operation in humidified air further promotes the formation of the Mn-enriched precipitates along YSZ/YSZ grain boundaries. The formation mechanism of those precipitates and their influence on the cathode performance are discussed.

Electrode performance degradation is a major obstacle for the development of SOFC technologies. Among different causes of electrode performance degradation, variation of operating gas environment could impact durability of SOFCs and lead to degradation. For the cathode, certain degree of humidity is always present in ambient air. It has been consistently reported that the degradation rate is accelerated when the cell is operated in the humidified air. This work presents nanostructure analyses of operated commercial SOFCs with either LSM/YSZ cathode or with mixed conducting LSCF/SDC cathode exposed to humidified air. For the cell with LSM/YSZ cathode, three commercial button cells were operated in humidified air for various time at 800 °C. Microscopy work reveals nanostructure degradation occurred within the cathode active layer, and it is most pronounced in the region next to the electrolyte. Newly grown Mn-rich nano-precipitates accompanied with nano-voids and cracking propagate at the LSM/YSZ interface. Additionally, prolonged operation in humidified air promotes the formation of the Mn-enriched precipitates at the YSZ/YSZ grain boundaries. By contrast, for the cell with LSCF/SDC cathode, upon long term operation of 2600 hours, Co and Fe enriched Spinel nano-grains were found accumulated at the original pore region of the cathode. The formation mechanism of Mn-enriched nano-precipitates and Co-enriched nanograins for the LSM/YSZ and LSCF/SDC cathode are discussed.

The long-term performance degradation of LSM/YSZ composite cathodes is investigated via calibrated multiphysics simulation with amultistep oxygen reduction reaction (ORR) model and structural coarsening data from a phase field study. Multiphysics simulations, including a multistep ORR mechanism, are first developed for better understanding of the limiting processes in solid oxide fuel cells. The multistep ORR mechanism includes two parallel pathways, surface pathway and bulk pathway, which consist of elementary reaction steps. Themultiphysics model is simultaneously calibrated with experimental polarization curves and impedance behavior for various air/fuel supply conditions. To our knowledge, this is the first time that a multistep ORR model is calibrated with such datasets. Next, the calibrated simulations are utilized to simulate a 2D half-cell constructed with measured microstructural data and random heterogeneity. Finally, the long-term performance degradation of the half-cell is predicted by the calibrated multiphysics model coupled with structural coarsening trends simulated using a phase field-based coarsening model. Degradation of both polarization curves and impedance behavior is investigated. Thorough analyses, including changes of contributions from different pathways, the resistance components, and overall reaction order, are performed to provide more insights into cathode performance degradation due to grain coarsening phenomena. (C) The Author(s) 2019. Published by ECS.