Anode-supported Cells Research Articles

Cathode-supported SOFCs enabling redox cycling and coking recovery in hydrocarbon fuel utilization

Although cathode-supported solid oxide fuel cells (SOFCs), characterized by significantly thinner anode layers than anode-supported SOFCs, are known to have unique advantages in ensuring stability and reliability against redox operating conditions and carbon deposition, they have received relatively less attention due to their lower performance. Furthermore, there is a lack of in-depth theoretical and experimental studies on their outstanding redox cycle characteristics than the anode-supported cells. Here, applying a sintering aid to the electrolyte considerably reduces the sintering temperature of the electrolyte, with the shrinkage and microstructure of the cathode effectively controlled to lower the diffusion resistance. Consequently, the operating temperature was lowered by more than 100 °C compared to previously reported cathode-supported SOFCs; moreover, at 650 °C, a maximum power density more than twice that reported in previous reports. The redox stability of the cathode-supported cells was demonstrated experimentally and theoretically through a redox cycling test and a 3-D finite element method-based SOFC structure model. Moreover, the outstanding redox cycle property of cathode-supported cells has been experimentally demonstrated for the first time to have the potential to mitigate carbon deposition that occurs when utilizing methane fuel. Hence, this study presents new insights for achieving stable and reliable SOFC operation.

Enhanced densification of screen-printed GDC interlayers for solid oxide fuel cells using nitrate-based precursor in various chelating agents as pore-filling solution

The Gd-doped CeO2 (GDC) diffusion barrier between the zirconia-based electrolyte and the LSCF-based cathode in a solid-oxide fuel cell (SOFC) is essential for preventing the formation of Sc2ZrO3 as a secondary phase. However, the reaction between ceria and zirconia at high temperatures (>1300 °C) impedes the formation of a highly dense GDC layer. Herein, we present a cost-effective approach for fabricating a highly dense GDC diffusion barrier layer at lower sintering temperatures by filling the pores of the GDC skeleton using a gelatin or polyvinylpyrrolidone (PVP) solution as a chelating agent for GDC cations. We investigated the interaction between the cations and the chelating agent molecules and its effect on the diffusion barrier. In addition, the flow behavior of the pore-filling solution was evaluated to determine its penetrability. The proposed method yielded a pore-filled GDC (PF-GDC) interlayer with enhanced density at 1000 °C, a remarkable 250 °C below the conventional sintering temperature for a porous GDC interlayer. The effectiveness of the PF-GDC was investigated by analyzing the performance of electrolyte-supported cells (ESCs) and anode-supported cells (ASCs). On ASCs, the observed peak power density at 800 °C was enhanced 1.5-fold, from 1.91 W⋅cm−2 (porous GDC sintered at 1250 °C) to 2.61 W⋅cm−2 (PF-GDC sintered at 1200 °C). These findings highlight the potential for pore-filling methods to improve the performance of SOFCs.

Mechanistic Insights on Direct Ammonia Solid Fuel Cell Using Ni-YSZ Anode: Experimental and Density Functional Theory Studies

While Ni-Yttria-stabilized zirconia (Ni-YSZ) cermet exhibits a great potential to realize high-efficiency ammonia to power using direct ammonia solid oxide fuel cell (DA-SOFC), the mechanisms of NH3 cracking and H2 oxidation reactions as well as direct electro-oxidation of ammonia are still unclear. In this context, with the motivation, the DA-SOFC performance of fuel electrode (Ni-YSZ) using conventional anode-supported cell (Ni-YSZ|YSZ|GDC-LSCF) is studied through microscale observation coupled with density functional theory (DFT) calculations to unravel the reaction mechanisms on Ni-YSZ(111) anode. The performance of DA-SOFC is examined using NH3, H2 and N2/H2 mixture as fuel at the temperature range of 700-800℃. In addition to XRD and SEM-EDX characterizations, the electrochemical characterization is performed using the impedance spectroscopy (EIS) measurements at three different temperatures of 700, 750 and 800 °C. Interestingly, the experimentally observed trend in Open-circuit voltage (OCV) implies that direct electro-oxidation of ammonia is the dominant reaction mechanism. Notably, polarization resistance is primarily contributed by ammonia anodic reaction and secondarily by the gas diffusion impedance. DFT results revealed that ammonia adsorbs favorably on the Ni cluster, and hydrogen follows a dissociative mode of adsorption on the Ni cluster. The activation barriers of elementary steps involved in the proposed mechanisms for fuel cracking and oxidation reactions are computed. This work is anticipated to provide valuable insights into the rational design of Ni-YSZ-based fuel electrodes. Keywords: ammonia, hydrogen, SOFC, fuel cells, DFT, reaction mechanism

Transitioning Low-Temperature GDC Socs: Lab to Commercialization Scale

Solid oxide fuel cells (SOFCs) electrochemically convert hydrogen and oxygen into electricity, heat, and water with high efficiency (~55%) [1], while in reverse operation, water and electricity are converted to hydrogen and oxygen in solid oxide electrolysis cells (SOECs). Appropriately designed solid oxide cells (SOCs) can operate in both SOFC and SOEC mode.Conventional SOCs typically demand elevated operating temperatures (800-1000°C), resulting in higher cost and accelerated degradation issues. Lowering operating temperatures below 600°C reduces thermal stresses, shortens start-up times, and broadens applications [2].Ce1-xGdxO2-δ (GDC), due to its superior oxygen ion conductivity at reduced temperatures, has lower ohmic losses than competing electrolyte materials. Thus, our 1.2 cm diameter circular anode supported GDC cell with 0.31 cm2 active area exhibits exceptional power densities, reaching approximately 3 watt per square centimeter at 650°C. Through optimal design, these cells exhibit exceptional performances in fuel cell mode, significantly enhancing efficiency and effectiveness when operating as an electrolyzer for fuel generation. This advance in performance is a result of mitigating the leakage current issues typical of ceria-based electrolytes through precise engineering of the cathodic microstructure and surface chemistry modification [3].The transition from lab-scale to large-scale commercialization (with active areas of 16 cm² &; 81 cm²) presents significant challenges such as tailoring material properties for large-cell versus button-cell synthesis while maintaining high performance levels.In this work we explore the challenges encountered during the scaling-up process. We report on fabrication strategies employed to achieve cell flatness, maintain high efficiency, ensure cost-effectiveness, and achieve long-term stability. Specifically, we have fabricated graded anode-supported cells [4] by developing an innovative lamination strategy involving tapes oriented perpendicular to each other. This approach notably contributed to achieving superior electrolyte densification and resulted in significantly flatter (low curvature values) with respect to both horizontal and vertical axes in cartesian coordinate space. When compared to the flatness values of 10x10 cm2 commercially available cells, our cells demonstrated remarkably flatter curvatures as shown in figure 1. This characteristic has proven highly beneficial for stack assembly, minimizing issues such as cell cracking and reducing sealing-related problems. By optimizing the cathode scaffold and infiltration process on larger area cells through a spray-coating machine, scaling up challenges are effectively mitigated compared to the button cell approach, where scaffold preparation and infiltration were performed using blade coating and a hand pipette, respectively. This approach effectively tackles electronic conduction issues in ceria-based electrolytes, leading to higher Open Circuit Voltages (OCVs) in fuel cell mode and increased efficiencies in reversible mode.

A Self-Assembled Multiphasic Thin Film as an Oxygen Electrode for Enhanced Durability in Reversible Solid Oxide Cells.

The implementation of nanocomposite materials as electrode layers represents a potential turning point for next-generation of solid oxide cells in order to reduce the use of critical raw materials. However, the substitution of bulk electrode materials by thin films is still under debate especially due to the uncertainty about their performance and stability under operando conditions, which restricts their use in real applications. In this work, we propose a multiphase nanocomposite characterized by a highly disordered microstructure and high cationic intermixing as a result from thin-film self-assembly of a perovskite-based mixed ionic-electronic conductor (lanthanum strontium cobaltite) and a fluorite-based pure ionic conductor (samarium-doped ceria) as an oxygen electrode for reversible solid oxide cells. Electrochemical characterization shows remarkable oxygen reduction reaction (fuel cell mode) and oxygen evolution activity (electrolysis mode) in comparison with state-of-the-art bulk electrodes, combined with outstanding long-term stability at operational temperatures of 700 °C. The disordered nanostructure was implemented as a standalone oxygen electrode on commercial anode-supported cells, resulting in high electrical output in fuel cell and electrolysis mode for active layer thicknesses of only 200 nm (>95% decrease in critical raw materials with respect to conventional cathodes). The cell was operated for over 300 h in fuel cell mode displaying excellent stability. Our findings unlock the hidden potential of advanced thin-film technologies for obtaining high-performance disordered electrodes based on nanocomposite self-assembly combining long durability and minimized use of critical raw materials.

Recycling and reuse of ceramic materials from components of waste solid oxide cells (SOCs)

As one of the main waste streams of the manufacturing process of solid oxide cells (SOCs), scrap half-cells - discarded before the deposition of the barrier layer and the air electrode - imply the loss of significant quantities of ceramic composite material containing critical and hazardous elements, namely yttria-stabilized zirconia (YSZ) and NiO. These materials represent more than 90 wt% of an entire anode-supported cell. In this contribution, scrap half-cells were fragmented and subjected to three subsequent milling and sieving (below 25 μm) steps to recover composite NiO-YSZ powders with the appropriate specifications for reuse in SOC manufacturing. The overall characterization in terms of specific surface area, particle size distribution and crystalline phase retention confirmed the suitability of the recovered powders. Half-cells were manufactured including 30 wt% of recovered NiO-YSZ composite powders in their fuel electrode support, produced via tape-casting in combination with 70 wt% of virgin materials. The obtained cells were morphologically, mechanically and electrochemically tested and compared against standard cells constituted by 100 wt% of virgin materials. The electrochemical characterization highlighted 0.68 A cm−2 for the standard and recycled cells, at 600 °C and 0.7 V, while at 650 °C and same voltage both cells measured 1.12 A cm−2. This demonstrates the feasibility of recovering scrap SOCs through the presented method.

A comprehensive comparison of performances of anode-supported, cathode-supported, and electrolyte-supported solid oxide fuel cells during warming-up process

Although the influence of supporting layer features on the steady performance of solid oxide fuel cells (SOFCs) has been discussed in previous studies, the literature shows a lack of similar studies for unsteady performance. The present study numerically assesses the effects of the type and thickness of the supporting layer on the warming-up process of an SOFC. The warming-up process is desired to be rapid; however, the rapidness can impose harmful temperature gradients. The warming-up process is also energy-demanding. Therefore, all the mentioned factors are considered in the present study, and an optimal set of supporting layer features is proposed using a multi-criteria decision analysis approach. The results indicate that the warming-up durations of anode-supported and cathode-supported cells are close to each other but considerably lower than that of the electrolyte-supported cell. Furthermore, the warming-up duration, which is directly proportional to the supporting layer thickness, is more sensitive to the electrolyte thickness than the anode and cathode thicknesses. On the other hand, the temperature gradients inside the SOFC, which are inversely proportional to the supporting layer thickness, are more affected by the electrolyte thickness than the anode and cathode thicknesses. In terms of energy usage, the electrode-supported cells are seen to be superior to the electrolyte-supported cells. Considering all the concerns simultaneously, the anode-supported cell with a supporting layer thickness of 1.5 mm is proposed as the best trade-off solution.

A highly oxygen reduction reaction active and CO2 durable high-entropy cathode for solid oxide fuel cells

One big obstacle for the oxygen reduction reaction (ORR) electrode in solid oxide fuel cells (SOFCs) is the poor reaction activity and fast degradations caused by CO2 poisoning. Here we report our design of an active A-site Ca-rich high-entropy Pr0.1875Ba0.1875Sr0.1875La0.1875Ca0.25CoO3-δ (PBSLC25C) electrode, guided by the O p-band theory. When applied as a cathode in solid oxide fuel cells (SOFCs), it demonstrates high ORR activity and excellent CO2 tolerance under realistic operating conditions. Ni-YSZ-based anode-supported cells with PBSLC25C cathodes demonstrate excellent peak power densities of 1.14 W cm−2, 1.04 W cm−2, and 0.77 W cm−2 in the air with 1%, 5%, and 10% CO2, respectively, at 750 °C. The engineered high-entropy PBSLC25C effectively diminishes the CO2 poisoning effect and maintains active surfaces for fast oxygen exchange, as confirmed by the cell durability test in air containing CO2 (5 and 10 vol%), Raman spectroscopy, and density functional theory calculations.

Defect-induced improved structural and electrical properties of Gd-Sr co-doped ceria for solid oxide fuel cell applications

Compositions of Ce1-x-yGdxSryO2-δ (x = 0.1, 0.2, and y = 0.025, 0.05, 0.075) are prepared using the sol-gel auto combustion method. The as-prepared powders are calcined at 500 °C for 2 h, followed by pelletization and sintering at 1450 °C for 4 h. Phase identification, defect distribution, microstructural analysis, and conductivity are examined using X-ray diffraction, Raman spectroscopy, scanning electron microscopy and impedance spectroscopy, respectively. A single crystalline phase is formed, and the lattice parameter variation with dopant concentration exhibits anomalous behavior. All the samples show relative densities higher than 92 %. The partial liquid phase sintering is also observed. In co-doped samples, the doping of 10 % Gd with 2.5 % Sr exhibits the highest conductivity, i.e., 1.20 × 10−2 S cm−1 at 600 °C. The developed materials are used in anode-supported cell testing. It shows a maximum power density of 112.7 mW cm−2 and a current density of 288.7 mA cm−2 at 600 °C with wet hydrogen and air on the anode and cathode side, respectively.

An excellent bismuth-doped perovskite cathode with high activity and CO2 resistance for solid-oxide fuel cells operating below 700 °C

Lowering the operating temperatures of solid-oxide fuel cells (SOFCs) is critical, although achieving success in this endeavor has proven challenging. Herein, Bi0.15Sr0.85Co0.8Fe0.2O3−δ (BiSCF) is systematically evaluated as a carbon dioxide (CO2)-tolerant and highly active cathode for SOFCs. BiSCF, which features Bi3+ with an ionic radius similar to Ba2+, exhibits activity (e.g., 0.062 Ω cm2 at 700 °C) comparable to that of Ba0.5Sr0.5Co0.8Fe0.2O3−δ and PrBaCo2O5+δ, while demonstrating a considerable advantage over Bi-doped cathodes. Moreover, BiSCF exhibits long-term stability over a period of 500 h, and an anode-supported cell with BiSCF achieves a power density of 912 mW cm−2 at 650 °C. The CO2-poisoned BiSCF exhibits quick reversibility or slight activation after returning to normal conditions. The exceptional CO2 tolerance of BiSCF can be attributed to its reduced basicity and high electronegativity, which effectively restrict surface Sr diffusion and hinder subsequent carbonate formation. These findings highlight the substantial potential of BiSCF for SOFCs operating below 700 °C.

Phase Transformation of YSZ Electrolyte in Anode-Supported SOFCs

Cubic-stabilized zirconia doped with 8 mol% Y2O3 (8YSZ) is a highly popular electrolyte material for solid oxide fuel cells (SOFCs) due to its exceptional oxide ionic conductivity, which remains consistent across a broad range of oxygen partial pressures. However, previous studies [1,2] have shown that the addition of nickel oxide (NiO) to 8YSZ can expedite the phase transformation process (from cubic to tetragonal) when exposed to a reducing atmosphere at relatively high temperature as 900℃. This phase transformation is closely related to the degradation of 8YSZ's conductivity. Therefore, it is crucial to investigate whether NiO, which should dissolve into the YSZ electrolyte during the co-sintering process with the NiO-YSZ anode at high temperatures, can lead to a similar phenomenon in anode-supported cells (ASCs). Hence, investigating the NiO solid solution phenomenon and its effect on the phase transformation in the YSZ electrolyte of ASCs can offer insights into its degradation behavior.In this study, NiO-YSZ anode support and YSZ electrolyte were co-sintered at 1370℃, 1450℃, and 1550℃, respectively, for 2 hours to fabricate the half-cell samples. Subsequently, these samples were reduced at 900℃ and 700℃, respectively, with a flow of 50ccm dry H2. High-resolution secondary ion mass spectrometry (SIMS) was used to detect the presence of NiO in YSZ, and Raman spectroscopy was used to detect the phase transformation (cubic to tetragonal) occurring in the YSZ.The SIMS results showed that the amount of NiO in the YSZ electrolyte increased with higher sintering temperatures. Moreover, after reducing the cells at 900℃, phase transformation was detected in all samples. However, there was no significant difference among those cells in terms of the degree of phase transformation, which indicated that phase transformation was not accelerated by the increase of dissolved NiO in YSZ. On the other hand, no phase transformation was observed for the cells reduced at 700℃. The mechanism of phase transformation of YSZ in anode-supported SOFCs and its related conductivity degradation will be discussed in the presentation.

(Invited) Materials Stability in Anode-Supported Solid Oxide Cells

Professor Anil Virkar has been a pioneer in the development and characterization of high-performance anode-supported solid oxide fuel cells, and in understanding degradation processes in such cells operated in electrolysis mode. This talk reviews methods for improving anode-supported cells, particularly for yielding high performance at reduced operating temperature. The understanding of degradation mechanisms occurring during solid oxide electrolysis cell operation will be discussed.

Sr-deficient medium-entropy Sr1-xCo0.5Fe0.2Ti0.1Ta0.1Nb0.1O3-δ cathodes with high Cr tolerance for solid oxide fuel cells

In this work, Sr-deficiency is proposed as an effective strategy to improve the oxygen reduction reaction (ORR) activity and Cr tolerance of medium-entropy Sr1-xCo0.5Fe0.2Ti0.1Ta0.1Nb0.1O3-δ (S1-xCFTTN, x=0–0.15) cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). With proper Sr-deficiency, S0.95CFTTN possesses the highest electrical conductivity and fastest oxygen exchange kinetics within tested samples. The optimized cathode yields outstanding ORR activity with a polarization resistance of 0.024 Ω cm2 at 700 °C, much lower than 0.037 Ω cm2 for SCFTTN cathode. Based on these, an anode-supported cell with S0.95CFTTN cathode exhibits a peak power density of 1153.5 mW cm−2 at 800 °C, much higher than that with SCFTTN cathode. Moreover, the S0.95CFTTN cathode displays dramatically enhanced Cr tolerance ability, which is due to the Sr-deficient diminishes surface Sr segregation and subsequent the formation of SrCrO4. Our results indicate appropriate Sr-deficiency is an effective approach to enhance the ORR activity and stability of SOFC cathodes.

Phase Transformation of YSZ Electrolyte in Anode-Supported SOFCs

The addition of nickel oxide (NiO) to 8 mol% yttria-stabilized zirconia (8YSZ) accelerates the phase transformation from cubic phase to tetragonal phase when exposed to a reducing atmosphere at relatively high temperatures such as 900 ℃, resulting in the degradation of oxide ion conductivity.In this study, NiO-related phase transformation of 8YSZ electrolyte is investigated for anode-supported cells (ASCs). 8YSZ electrolyte co-sintered with NiO-YSZ anode at high temperatures exhibited NiO dissolution into the 8YSZ electrolyte during the sintering process and the amount of dissolved NiO increased with increased sintering temperatures. After reducing the cells at 900 ℃, phase transformation is observed in the 8YSZ electrolyte and resulted in a decrease of the oxide ion diffusion coefficient (D*). On the other hand, no phase transformation is observed for the cells reduced at 700 ℃for 5 hours.

Self-Construction of Efficient Interfaces Ensures High-Performance Direct Ammonia Protonic Ceramic Fuel Cells.

Direct ammonia protonic ceramic fuel cells (PCFCs) are highly efficient energy conversion devices since ammonia as a carbon-neutral hydrogen-rich carrier shows great potential for storage and long-distance transportation when compared with hydrogen fuel. However, traditional Ni-based anodes readily suffer from severe structural destruction and dramatic deactivation after long-time exposure to ammonia. Here a Sr2 Fe1.35 Mo0.45 Cu0.2 O6-δ (SFMC) anode catalytic layer (ACL) painted onto a Ni-BaZr0.1 Ce0.7 Y0.1 Yb0.1 O3- δ (BZCYYb) anode with enhanced catalytic activity and durability toward the direct utilization of ammonia is reported. A tubular Ni-BZCYYb anode-supported cells with the SFMC ACL show excellent peak power densities of 1.77 W cm-2 in wet H2 (3% H2 O) and 1.02 W cm-2 in NH3 at 650 °C. A relatively stable operation of the cells is obtained at 650 °C for 200 h in ammonia fuel. Such achieved improvements in the activity and durability are attributed to the self-constructed interfaces with the phases of NiCu or/and NiFe for efficient NH3 decomposition, resulting in a strong NH3 adsorption strength of the SFMC, as confirmed by NH3 thermal conversion and NH3 -temperature programmed desorption. This research offers a valuable strategy of applying an internal catalytic layer for highly active and durable ammonia PCFCs.

Design of High Specific Power Solid Oxide Fuel Cells

The development of solid oxide fuel cells (SOFCs) has progressed from the early Electrolyte Supported Cell (ESC) to state-of-the-art Anode Supported Cell (ASC), which allows the implementation of thinner electrolyte layer (less than 10 micron) and reduction in the operating temperature (below 800°C). The various components of SOFC are constantly being optimized by many research groups to further improve the overall stack performance, including specific power, power density, thermal shock resistance, rapid start-up, and thermal cycling exposure.Precision Combustion, Inc. (PCI) has been working on an advanced electrochemical-based energy storage and power generator design for meeting aggressive specific-power and specific-energy targets required for mobile and transportation applications, such as all-electric propulsion of ground and air vehicles. The key drivers are an exceptionally power-dense solid oxide fuel cell design operating with energy-dense liquid fuels and a hybridized system architecture that maximizes component efficiencies for ultra-high system efficiency. This effort builds on PCI’s experience in developing groundbreaking SOFC generators for mobile and space applications.PCI has developed and demonstrated a novel design of fuel cells with high specific power, that also allows for direct internal reforming of methane and hydrocarbon fuels within a SOFC stack. The system consists of interconnected internal reforming Microlith® metal mesh catalysts strategically embedded within the solid oxide stack. The fuel cell configuration was optimized to reduce CTE mismatch and to improve area specific resistance. The resulting enhanced heat transfer design of the combined components offers the potential for higher overall system efficiency and system simplification as well as enables further compactness and weight reduction of the fuel cell while improving the conditions for long system life. The approach also offers the potential to operate with a wide range of input fuels (i.e., high hydrocarbons as well as various levels of CO2 and water in the fuel inlet) without forming carbon. Carbon formation within the stack system has been known to be detrimental to the performance and durability. Our approach allows for the operation of a modular, advanced SOFC stack for generating power directly from hydrocarbon fuels. Computational design, process simulation, and qualification testing were performed to validate the concept.In this presentation, the results from a breadboard SOFC stack system operation will be highlighted, including the power density, efficiency, and durability. An improvement in the performance as a result of the design advancement, which could improve the stack lifetime, will also be described. The carbon avoidance in the stack while operating with hydrocarbon fuels and with a low water requirement was confirmed. Sensitivity study of the concept to temperature, flow rate, and anode fuel composition was performed and will be highlighted. Finally, preliminary results from the fuel cell design optimization activities will be summarized.

Characterization of a Nickel / Gadolinia Doped Ceria Fuel Electrode for Co-Electrolysis

Modelling of the high temperature co-electrolysis process requires understanding of the underlying reaction pathways. These include the electrochemical steam reduction, the electrochemical CO2 reduction and their coupling via the catalytic (reverse-)water-gas-shift reaction (RWGS). The assumption of a very fast water-gas-shift reaction and therefore neglectable CO electro-oxidation is widely used. The validity of this assumption has been previously investigated and proven to be sufficient by Kromp et al. [1] for an anode supported cell (ASC) with a nickel / yttria-stabilized zirconia (Ni/YSZ) fuel electrode.Within this contribution we aim to extend the approach by Kromp et al. [1] towards an electrolyte-supported cell (ESC) with a nickel / gadolinium-doped ceria (Ni/CGO) fuel electrode. Previously, it has been proposed, that the electro-oxidation /-reduction of CO / CO2 can be present under reformate gas mixtures [2,3] on Ni/CGO fuel electrodes. However, a detailed comparison of pure activation resistance contributions during different operating modes has not been performed yet.We present results from a complex variation of operating parameters (such as various gas compositions, exemplarily shown in fig. 1) for process identification by the use of electrochemical impedance spectroscopy and the subsequent impedance analysis by the distribution of relaxation times (DRT). Visualization and deconvolution of gas diffusion impedance contributions is done via the evaluation of the polarization resistance differences with different inert gases. This has been successfully applied for binary gas mixtures of hydrogen and steam [4] and also CO2 and CO. [5] The applicability of this method towards more complex multicomponent gas mixtures incorporating hydrogen, steam, CO2 and CO will be shown. By subtracting the gas diffusion impedance contributions, it is possible to evaluate reaction kinetics of the underlying activation polarization and a meaningful comparison of operation modes is facilitated. The ratio of direct CO2electro-reduction and CO2-conversion by RWGS will be discussed for different compositions and temperatures to show a non-neglectable influence of the CO2 electroreduction at even lower pCO2 / pH2 ratios than previously reported. Furthermore, we compare our results with ones obtained from Ni/YSZ fuel electrodes, to investigate the influence of fuel electrode structure and material.

(High-Temperature Energy, Materials, & Processes Division Subhash Singhal Award) From Electrochemical to Mechanical Modeling of SOFCs and Their Experimental Validation

Due to the asymmetric structure with multiple materials, SOFCs are subjected to various modes of mechanical stress, resulting in a risk of failure. Of particular importance are thermal and chemical strains, which depend on the micro- and macroscopic distribution of electrochemical reaction sites. Therefore, SOFC modeling must include electrochemistry, mass and charge transport, thermo-fluid dynamics, and structural analysis. In addition, appropriate experiments must be performed to validate the modeling results. This research group is working to develop a comprehensive environment for modeling and validation in order to evaluate the mechanical reliability of SOFCs.Electrochemical modeling of cathode was made based on the defect chemistry and mass transport of (La,Sr)CoO3 (LSC) and (La,Sr)(Co,Fe)O3 (LSCF). An empirical equation for the surface reaction of the porous electrode was determined by electrochemical analysis of a dense film electrode taking into account the difference between the dense film and porous electrodes [1]. Anode reaction rate on a unit length of triple phase boundary of Ni-YSZ-gas was determined using circular Ni model electrodes of various size [2]. Evaluation of 3D oxygen potential distribution inside the constituent oxides was made using an in-house code called SIMUDEL where mixed ionic/electronic conduction and oxygen nonstoichiometry were taken into consideration [3,4]. It was combined with commercial software to make structural analysis. Mechanical properties used for calculation were obtained from measurements by the resonance method and the small punch method or collected from the literature. Non-linear stress-strain behaviors were observed with Ni-YSZ cermet [5] and inelastic behaviors were found in most nonstoichiometric oxide materials. Large ferro-elasticity emerged in the LSC and LSCF [6] and in Sc stabilized ZrO2 electrolyte at elevated temperatures. Chemical strain in vacancy formation was formulated with a linear function of the defect concentration as an independent factor from the thermal expansion. In the structural analysis, it was treated as the additional temperature change of the local material. Volume change of Ni-YSZ cermet on red-ox operation was a difficult factor to formulate because of its complex dependence on composition, temperature, and atmosphere. As expected, Ni-YSZ showed expansion when oxidized, but under certain conditions at an intermediate temperature range, it rather contracted on oxidation [7].In reality, the calculated stress distribution does not always match the actual behavior of the cell since there often are unconsidered factors. Thus, experimental validation is essential. For this purpose, we have developed methods for in-situ or operando measurements of cell shape and the residual stress. Long-focus laser profilometer was used to measure the warping of a planar cell during heating, operating, and cooling operations. For the change in the diameter of a tubular cell, the laser projection type dimension meter was employed. Residual stress measurements were made with a handy X-ray analyzer (μ-X360s, Pulstec Industrial. Co. Ltd.) combined with a newly developed one- or two-chamber cell holder [8,9]. Anode-supported cells from multiple sources were analyzed, showing different stress histories in red-ox operations depending on the microstructure of the anode support layer.Currently, we are developing a protocol for testing robustness of planar SOFCs. Here, temperature distribution is intentionally applied on a cell with segmented heater allays on the top and the bottom of the cell. Severe condition tests will be performed, and the results will be analyzed with the above-mentioned simulation.AcknowledgementThis study was supported by the New Energy and Industrial Technology Development Organization (NEDO) .[1] T. Kawada, Current Opinion in Electrochemistry, 21, 274-282 (2020) DOI: 10.1016/j.coelec.2020.03.016[2] M. Takeda, Master’s thesis, Graduate School of Environmental Studies, Tohoku University (2016) : M. Takeda, et al. Submitted to J. Electrochem. Soc.[3] K. Terada, et. al., ECS Trans. 35(2 part2), 923-933 DOI: 10.1149/1.3570073[4] M. Sato, et. al., Trans. Jpn. Soc. Comp. Eng. Sci. 2017, 14 (2017) DOI: 10.11421/jsces.2017.20170004[5] S. Watanabe et al., J. Mater. Sci. 55, 8679-8693 (2020) DOI: 10.1007/s10853-020-04624-4[6] Y. Kimura et al., J. Electrochem. Soc. 161(11), F3079- F3083 (2014) DOI: 10.1149/2.0131411jes[7] Y. Morishita et al. , ECS Trans., 91(1) 1979-1984 (2019). DOI: 10.1149/09101.1979ecst[8] K. Yashiro et al., submitted to J. Power Sources[9] K. Oshima et al.,ECS Trans. 103(1), 1251-1260 (2021) DOI: 10.1149/10301.1251ecst

La1.5Sr1.5Mn1.5Ni0.5O7 ±δ Vs La1.5Sr1.5Co1.5Ni0.5O7 ±δ: Exsolved Materials as Anodic Layers for Direct Biogas-Fueled SOFC

Currently, one of the main problems in commercial SOFC systems fueled directly with hydrocarbon compounds, such as biogas, is the high risk of blocking the active phase on the anode side (e.g. Ni-YSZ) due to carbon formation/deposition during the conversion of fuel into power energy. This issue causes a lowering in the overall performance and durability of the cell. Therefore, to overcome this deactivation mechanism, it was successfully demonstrated that an additional anodic active layer in commercial SOFC using materials based on exsoluted perovskites could be an interesting and viable alternative. This asseveration is based on the fact that using this kind of material is possible to get heterogeneous surface systems with highly stable and electrocatalytically active embedded nanoparticles uniformly distributed on the surface with a high carbon coking tolerance in a hydrocarbon fuel atmosphere.This study aimed to evaluate and compare the electrocatalytic behaviour of two new catalytic materials with Ni exsolution for direct dry biogas-fueled SOFC. The starting materials were Ruddlesden-Popper-type based on a nickel manganite (La1.5Sr1.5Mn1.5Ni0.5O7±δ or LSMN) and nickel cobaltite (La1.5Sr1.5Co1.5Ni0.5O7±δ or LSCN). Both materials have been synthesized by the Pechini method using stoichiometric amounts of precursors as nitrates. Once the respective gels have been formed, they were treated in the air at two dwell temperatures, 300°C for 2 h and 500°C for 3 h, to ensure the total elimination of the organic compounds. Finally, the resulting powders were treated in air at 1300°C for 12h and then, physicochemically characterized. For the electrochemical characterization, the as-treated powders were mixed individually with Gd0.1Ce0.9O2 (CGO) in a weight ratio of 70:30 using a ball milling for 6h. Finally, to get the slurry for the coating layer, each mixture (LSMN+CGO and LSCN+CGO) was ground for an additional 2h in the presence of 8 wt % of triethanolamine, 2 wt % polyvinyl butyral resin (BUTVAR B-98) and 2-propanol. Commercial button SOFC cells by InDEC® (anode-supported cell Ni-YSZ/YSZ/LSM) were painted on the anode side getting an active area of 2 cm2. The experiments were carried out at 800°C with pre-conditioning using diluted H2 and then, with simulated dry biogas. A Biologic tool was used as a device for the electrochemical measurements.The purpose of this communication is to present the results of experiments with two button cells derived from the same large area commercial cell (anode supporting cell) coated with the two electrocatalysts developed in this work. The electrochemical test carried out for more than 200 h demonstrated that this external functional layer on the anode side contributes to getting a stable potential within the whole working time at the selected galvanostatic conditions (500 mA cm−2). By comparing the results of these two tests, the exsolved LSCN layer showed better perfomances, because of the the B-site doped with Co instead of Mn which is responsible for an enhanced O2- transport within the crystal structures. Finally, post-reaction characterization revealed minimum/null carbonous species in both cases which suggests that the classical risks caused by the direct use of hydrocarbon fuels in commercial SOFC can be suppressed with this kind of approach which would lead us to think about the use of this type of materials in a next level or further electrochemical applications. Acknowledgements The authors acknowledge the project entitled "Direct utilisation of bio-fuels in solid oxide fuel cells for sustainable and decentralised electric power production and heat (DIRECTBIOPOWER)" Grant Agreement number: 2017FCFYHK.The authors acknowledge the Ministry of Ecological Transition of Italy (MiTE) for funding this research through the AdP "Piano Operativo di Ricerca (POR) sull'idrogeno verde Figure 1

Scenarios for Hydrogen Production from a Full-Scale Reversible Solid Oxide System with Electrolyte-Supported Stacks

Reversible solid oxide cell technology is of increasing interest and is emerging as a promising grid-flexible support system [1] with storage [2]. Among the types of cell architecture available, electrolyte-supported cells have proven to be better in terms of long-term stability and degradation of the cell, albeit sacrificing on performance as compared to anode-supported cells [3]. This makes electrolyte-supported cells better suited architecture to be used in reversible systems where higher stability is required due to the changing operating conditions in each mode. In the present work, validated cell and stack models of Nexceris electrolyte-supported cells are used to model a full-scale reversible solid oxide system designed for hydrogen production.EIS data from symmetric button cells and polarization curves from 5cm×5cm full cells are used to validate the stack model. A net 100 kWe system in fuel cell mode is modelled with electrolyte-supported stacks from Nexceris operating at a nominal temperature of 800 oC. The system is run at thermoneutral conditions in electrolysis mode and at 0.5 A/cm2 in fuel cell mode to balance between efficiency and hydrogen production. The levelized cost of hydrogen production is calculated using a price-taker model with the system pumping the produced hydrogen into a natural gas/hydrogen pipeline. The effect of various parameters, such as the capacity factor of the system, the operating pressure, grid-electricity pricing, etc. - are investigated on the levelized cost of hydrogen.[1] Guenther Glenk and Stefan Reichelstein, “Reversible Power-to-Gas systems for energy conversion and storage”, Nature Communications 13, 2010 (2022).[2] Evan Reznicek and Robert J. Braun, “Techno-economic and off-design analysis of stand-alone, distributed-scale reversible solid oxide cell energy storage systems”, Energy Conversion and Management 175, 2018.[3] Marco A. Buccheri, Anand Singh, and Josephine M. Hill, “Anode- versus electrolyte-supported Ni-YSZ/YSZ/Pt SOFCs: Effect of cell design on OCV, performance and carbon formation for the direct utilization of dry methane”, Journal of Power Sources 196, 2011. Figure 1