Scandia-stabilized Zirconia Research Articles

(Invited) Solid Oxide Cell Design for High Power Density and Reliability: Applications in Power and Fuels Production

Solid oxide fuel cells (SOFC) offer a carbon-neutral solution to power generation, fuel flexibility, and robust design suitable for stationary and air transportation applications. OxEon Energy developed an air electrode supported (AES) cell design that demonstrated 63% increase in power density compared with its heritage cell design in short stack tests.OxEon Energy team has been advancing solid oxide technology for over three decades and the stacks operate fully reversibly in fuel cell and electrolysis cell modes. The SOEC/SOFC technology builds on the success of the SOEC stack installed on NASA’s Mars Perseverance Rover as part of the MOXIE (Mars Oxygen In-situ resource utilization Experiment) mission. The MOXIE stack produced propellant grade, high-purity O2 by electrolyzing Mars atmosphere CO2. MOXIE stack used an electrolyte supported design to meet the reliability requirements for a space mission.An infiltrated air electrode cell design was developed to increase fuel cell stack specific power (kW/kg). Electrolyte thickness, the most resistive element of the SOFC, was reduced from ~250 microns to 70 microns. The fabrication process results in a bi-layer of dense scandia stabilized zirconia (ScSZ) electrolyte and a porous yttria stabilized zirconia (YSZ) matrix for air electrode infiltration. Fuel electrodes are screen printed. The reduced thickness of the electrolyte offers a compromise between improved performance over electrolyte supported design, while meeting robustness required for aerospace applications.Producing infiltrated electrodes with well-adhered and homogeneous distribution of active material is a challenge that requires multiple infiltration-calcination cycles to deposit sufficient material. Key variables are precursor chemistry to reduce the number of infiltrations and prevent interaction between electrode layers; intermediate calcination temperatures; and process development using infiltration equipment that enables volume production. OxEon partnered with National Energy Technology Laboratory (NETL) for electrode infiltration process development and PNNL for button cell and short stack validation testing.SOFC fuel flexibility was demonstrated in button cells and stacks previously; performance was nearly identical among hydrogen, ammonia, and reformed natural gas. Durability testing shows similarly stable performance in each fuel.A 10-cell AES stack was tested in SOFC mode at 800 °C in H2 and NH3 fuel feeds at 0.7 V/cell, and the same stack was operated in steam electrolysis to produce H2. SOFC testing demonstrated powder density of 0.35 W/ cm2 at 0.7 V/cell in the AES stack in ammonia feed; an electrolyte supported stack with equivalent materials measured 0.20 W/cm2 at 0.7 V. Long term testing in steam electrolysis showed stable performance at 800 °C and 1.2 V/ cell.Button cell tests show the thin electrolyte cell structure is capable of an area specific resistance (ASR) of 0.25 Ω-cm2 in H2- and NH3 fuel feeds. OxEon’s continued AES cell development is focused on improving manufacturability, infiltrated electrode stability, and implementing degradation mitigation strategies developed for electrolyte supported cell structures. Improved performance with AES cell design has critical implications for future integration in OxEon’s technology suite: SOEC systems to generate H2 and syngas from renewable energy, and Fischer-Tropsch to produce synthetic fuels.This material is based on research sponsored by Air Force Research Laboratory under agreement number FA8649-22-P-0792. The views expressed are those of the authors and do not necessarily reflect the official policy or position of the Department of the Air Force, the Department of Defense, or the U.S. government. Approved for public release; distribution is unlimited. Figure 1

Prevention of morphological change for (Ba,Sr)(Co,Fe)O3 cathodes by incorporating (Ce,Gd)O2 for intermediate-temperature solid oxide fuel cells

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) generally exhibits superior cathode activity compared to La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) for intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, the chemical stability of BSCF is inferior to that of LSCF. When BSCF cathodes are sintered at a high temperature of 1050 °C, performance was compromised due to the formation of (Ba,Sr)ZrO3 between the scandia-stabilized zirconia (ScSZ) electrolyte and gadolinia-doped ceria (GDC) interlayer. The oxide ionic conductivity of (Ba,Sr)ZrO3 was low, decreasing the cathode performance. Furthermore, the BSCF grains enlarged, and cobalt oxide formed due to BSCF decomposition, decreasing the active specific surface area. However, by incorporating GDC into the BSCF cathode, the morphology remained unchanged and cobalt oxide formation was prevented, despite (Ba,Sr)ZrO3 still forming. The performance of the cell with the BSCF-GDC composite cathode surpassed that with the BSCF cathode due to decreased polarization resistance attributed to the oxygen surface exchange and diffusion processes in the cathode.

The effect of gadolinium-doped ceria interlayer on the oxygen reduction reaction in a LSCF cathode-ScSZ electrolyte supported IT-SOFCs

Solid oxide fuel cells (SOFCs) are rapidly emerging as a technology, offering the potential for carbon neutral or carbon negative generation of electricity and heat (combined heat and power) using synthetic carbohydrate fuels and hydrogen. A significant challenge associated with SOFCs is the high polarization resistance experienced at the cathode during the oxygen reduction reaction (ORR), which diminishes the cell efficiency. The kinetics of the ORR are influenced by factors such as the cathode material type, its microstructure, and the quality of the interface between the cathode and electrolyte. In our research, we have addressed this issue by modifying the interface between the state-of-the-art cathode material, Lanthanum Strontium Cobalt Ferrite—La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), and the Scandia stabilized Zirconia (ScSZ) electrolyte. This modification involved the deposition of a micron-sized film of ion-conducting gadolinium-doped ceria (Gd0.1Ce0.9O2-δ) – (GDC) as an interface layer. Our analysis involved systematic studies, including variations in cell operating temperatures and applied potentials, as well as measurements of cell performance over an extended period. We observed a significant enhancement in cell performance with the introduction of the GDC interfacial layer between the LSCF cathode and ScSZ electrolyte. Specifically, we recorded a cathode polarization resistance as low as 0.40 Ωcm2 for the modified interface, which is substantially lower compared to bare LSCF cathodes (3.04 Ωcm2) at 600 °C. This reduction in cathode resistance can primarily be attributed to the improved conditions for the oxygen reduction reaction (ORR), resulting from enhanced interfacial contact between the electrode and the electrolyte and mitigation of any zirconium interdiffusion as seen from detailed scanning electron microscopic studies.

Nanocrystalline Cubic Phase Scandium-Stabilized Zirconia Thin Films.

The cubic zirconia (ZrO2) is attractive for a broad range of applications. However, at room temperature, the cubic phase needs to be stabilized. The most studied stabilization method is the addition of the oxides of trivalent metals, such as Sc2O3. Another method is the stabilization of the cubic phase in nanostructures-nanopowders or nanocrystallites of pure zirconia. We studied the relationship between the size factor and the dopant concentration range for the formation and stabilization of the cubic phase in scandium-stabilized zirconia (ScSZ) films. The thin films of (ZrO2)1-x(Sc2O3)x, with x from 0 to 0.2, were deposited on room-temperature substrates by reactive direct current magnetron co-sputtering. The crystal structure of films with an average crystallite size of 85 Å was cubic at Sc2O3 content from 6.5 to 17.5 mol%, which is much broader than the range of 8-12 mol.% of the conventional deposition methods. The sputtering of ScSZ films on hot substrates resulted in a doubling of crystallite size and a decrease in the cubic phase range to 7.4-11 mol% of Sc2O3 content. This confirmed that the size of crystallites is one of the determining factors for expanding the concentration range for forming and stabilizing the cubic phase of ScSZ films.

3D printed electrolyte-supported solid oxide cells based on Ytterbium-doped scandia-stabilized zirconia

Solid oxide cells (SOC) are an efficient and cost-effective energy conversion technology able to operate reversibly in fuel cell and electrolysis mode. Electrolyte-supported SOC have been recently fabricated employing 3D printing to generate unique geometries with never-explored capabilities. However, the use of the state-of-the-art electrolyte based on yttria-stabilized zirconia limits the current performance of such printed devices due to a limited oxide-ion conductivity. In the last years, alternative electrolytes such as scandia-stabilized zirconia (ScSZ) became more popular to increase the performance of electrolyte-supported cells. In this work, stereolithography 3D printing of Ytterbium-doped ScSZ was developed to fabricate SOC with planar and corrugated architectures. Symmetrical and full cells with about 250 μm- thick electrolytes were fabricated and electrochemically characterized using impedance spectroscopy and galvanostatic studies. Maximum power density of 500 mW cm−2 in fuel cell mode and an injected current of 1 A cm−2 at 1.3 V in electrolysis mode, both measured at 900 °C, were obtained demonstrating the feasibility of 3D printing for the fabrication of high-performance electrolyte-supported SOC. This, together with excellent stability proved for more than 350 h of operation, opens a new scenario for using complex-shaped SOC in real applications.

Conductivity Increase As a Result of the Disappearance of Short-Range Ordering in a Scandia-Zirconia Electrolyte

Solid oxide cell electrolytes and structures have been greatly optimised to offer high performance and good durability for commercial implementation. This has highlighted an underlying issue where the slow, long-term conductivity degradation in the generally used fluorite structured oxide-ion conducting doped zirconias known to be related to reorganisation of their defect short-range structures. Among the doped zirconia systems, scandia stabilised zirconia (SSZ) gives the highest ionic conductivity due to the smallest mismatch between the dopant and host cations. However, owing to these similar and small sizes, the system is more prone to defect short-range ordering. This leads to a complex phase diagram of the Sc2O3-ZrO2 system and, depending on the composition, its ionic conductivity ages more significantly than other zirconia systems on high temperature annealing. Here, we report a magnesia and india co-doped scandia stabilised zirconia (IMSSZ) composition that offers high ionic conductivity, 0.14 at 850 ℃ and 0.4 at 1000 ℃, improved phase stability in both macro- and micro scales, and long-term conductivity stability. The conductivity after annealing the sample at 850 ℃, for different times, 0 (as-sintered), 167 and 426 hours, is correlated to the microdomain structural evolution. We show the initial of conductivity increase over time as a result of eliminating the defect short-range ordering using electron diffraction and powder neutron diffraction, resulting in a long-term stable disordered structure with improved conductivity. We will discuss here the thermodynamic driving forces for the breakdown of defect short-range ordering, and how right co-dopant sizes may play a role. Figure 1

Boosting the Stability and Performance of SOFCs: A New Approach to Oxygen Storage Capacity Using Lanthanum-Doped Ceria Interlayer Technology

Being carbon neutral is one of the world's most important objectives. More than 65 percent of the countries that emit harmful greenhouse gases have committed to achieving net-zero emissions by the middle of the century.[1] There are numerous essential ways to gather energy for worldwide missions, including fuel cells, solar power, wind power, etc. The growing marketplace of fuel cells is following the trend of green-energy demand. Solid oxide fuel cells (SOFCs) are gaining popularity in the next-generation energy industry due to their high energy conversion efficiency, fuel flexibility, and low emissions.[2]The main challenges for the commercialization of SOCs that need to address are high performance, long-term durability, and effective cost. Stabilized zirconia such as Yttria-stabilized zirconia (YSZ) and Scandia-stabilized zirconia (ScSZ) is commonly used as a self-standing electrolyte support. The inability of zirconia-based electrolytes to react with Sr-rich cathode materials like LSC, LSCF, BSCF, etc. Therefore, the electrodes and the zirconia-based electrolyte must have the best-interlayered buffer structure. Optimizing interlayer microstructures to make them more effective at high-performance ESCs is of great interest. Several Rare-earth doped ceria (RDC) materials, including (Gd, Ce)O2, (Sm, Ce)O2, and (La, Ce)O2, have been used as efficient interlayers to lessen reactivity and preserve ionic conductivity in the interface. However, the difference in the thermal expansion coefficient (TEC) between the GDC and the YSZ poses a significant problem for starting and halting SOCs. In the present work, we focus on the commercial lanthanum-doped ceria (Ce0.6La0.4O1.8 - LDC) as a buffer layer between the YSZ electrolyte and the LSCF cathode because of its higher oxygen storage capacitance. The reactivity of the YSZ/GDC composite was higher than that of the YSZ/LDC composite. And the total conductivity of the LDC/YSZ composite is higher than GDC/YSZ composite even though LDC has lower conductivity than GDC. Moreover, the mismatching of TEC was reduced by replacing the GDC with the LDC bilayer.Consequently, the maximum power density for the electrolyte-supported cell SOFC with LDC interlayer was ~0.55 W.cm−2 at 1073 K. A variety of additives (Co, Mn, and Cu) was added to the LDC buffer layer to examine the sintering properties. The addition of copper not only obtained the highest microstructural density but also improve mechanical stability under cost-effective processes.

Effects of Current Collector on Internal Visualization of Solid Oxide Cells

Introduction Visualization of spatial distribution in solid oxide fuel cells (SOFCs) operating under high temperature is difficult to use various measurement equipment and sensors due to their recommended operating temperature. Therefore, it is difficult to reveal the distributions of gas species and reactions in the cell, as well as the loading current, using only electrochemical measurement methods. Simulation technology can be used to visualize various distributions in the cell and predict cell performance. While many simulation studies have revealed various spatial distributions in the SOFCs such as current density, they have also indicated that it is essential to accurately reflect the cell shape and multiphysics to simulate the cell performance1, 2.In our previous study, simulation and experimental thermal imaging combined temperature visualization method was developed. It was suggested that the temperature distribution under actual operating conditions is affected by Joule heating due to loading current distribution depending on the current collector geometry. Therefore, in order to evaluate the influence of the collector shape on the spatial temperature distribution, the influences of Joule heating and the collector geometry on the temperature distribution are evaluated. Then, we also analyze the influence of the current collector on the SOFC performance and propose a current collection method suitable for visualizing the reaction in an SOFC. Simulation Method In this study, the calculation was performed using a three-dimensional model that simulates the actual cell geometry used in the experiments. The simulated 3-D model was based on an electrolyte-supported plate cell with Scandia-stabilized Zirconia (ScSZ, 5 cm × 5 cm in size) as the support, LSM ((La0.8Sr0.2)0.98MnO3) and LSM-ScSZ composite material for the air electrode, and Ni-ScSZ cermet for the fuel electrode, respectively. The electrode area was 4 cm × 4 cm (16 cm2), and a platinum mesh and wire were used as a current collector. A schematic diagram of one of these examples is shown in Fig. 1. COMSOL Multiphysics (Ver. 6.0) was used as the 3-D simulation software to calculate the I-V characteristics and cell temperature distribution.The cell operating conditions were set to 800 ℃ for the operating temperature, 100 ml min-1 of 3 % humidified hydrogen as the incoming fuel, and the air electrode side opened to the atmosphere. I-V characteristics and cell temperature distributions were calculated under current density loading conditions ranging from 0.01 A cm-2 to 0.15 A cm-2. Based on each result, the relationship between current density and cell temperature distribution was evaluated. Results and Discussion As a result of numerical analysis and experiments focusing on the effect of the current collector shape, it was confirmed that the influence of the current collector shape appeared significantly in the cell temperature distribution as shown in Fig. 2. In addition, it was found that the visualization of the spatial distribution may be difficult because the effect of the heat generated by the Pt wire used as the current collector appears in the results of the temperature distribution on the surface. Based on these findings, we improved the current collector shape. Although the effect of the improvement on the Joule heating at Pt wire appeared in the temperature distribution result, the temperature distribution change was confirmed due to the loading current flow pattern, which depends on the shape of the current collector. The simulation results of the further improved collector shape showed that the temperature distribution change depending on the collector shape became smaller, suggesting that this collector shape is promising as a current collecting method suitable for visualizing the cell inside. In this presentation, we report the simulation results and discussion on the changes in current density distribution, gas distribution, and I-V characteristics, in addition to the changes in temperature distribution. References (1) M. Andersson, H. Paradis, J. Yuan, and B. Sundén, Electrochimica Acta, 109, 881 (2013).(2) A. Li, C. Song, and Z. Lin, Applied Energy, 190, 1234 (2017). Figure 1

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

Introduction Reversible solid oxide cells (r-SOCs) are electrochemical energy devices that can switch reversibly between power generation as a solid oxide fuel cell (SOFC) and hydrogen production as a solid oxide electrolysis cell (SOEC). It is important to systematically understand the electrode reaction processes and degradation factors in these two modes to tailor high-performance and durable r-SOC electrodes. The dependence of polarization resistance on cell fabrication conditions and operating conditions has been individually investigated for SOFC and SOEC by impedance measurement and distribution of relaxation times (DRT) analysis [1-4], but it is necessary to evaluate r-SOC as a whole [5]. Here in this study, we aim to systematically clarify the similarities and differences in the electrode reaction processes at the fuel electrodes of both SOFC and SOEC modes of r-SOCs, with respect to fuel electrode fabrication conditions. Experimental Several types of cells were fabricated with different fuel electrode constituent materials, thicknesses, and sintering temperatures. Three types of fuel electrode materials were used: Ni-GDC cermet, a composite of Ni and mixed ionic-electronic conductor Gd0.9Ce0.1O3 (GDC); Ni-YSZ cermet, a composite of Ni and pure ionic conductor YSZ (8 mol% Y2O3 - stabilized ZrO2); and Ni-GDC co-impregnated electrode with LST-GDC as a backbone support (LST: La0.1Sr0.9TiO3). Scandia-stabilized zirconia (ScSZ) was used as the electrolyte plate, and (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF) was used as the air electrode. GDC buffer layer was prepared between the electrolyte and the air electrode to suppress the formation of interfacial insulting layers.Electrochemical impedance measurements were performed for the fuel electrode of the fabricated cell under SOFC and SOEC modes by supplying H2-H2O mixture and varying operating temperature, fuel humidification, and current density. The DRT analysis was conducted to separate various polarization resistance components of the fuel electrode with different relaxation times. The area of each DRT peak corresponds to the resistance of each electrode reaction process. In this study, activation energy was derived based on the assumption that the electrical conductance is of an Arrhenius-type thermally activated character. Its dependence on the fuel electrode fabrication conditions was investigated. Furthermore, electrode microstructure was observed by using focused-ion beam scanning electron microscopy (FIB-SEM). Results and Discussion Figure 1 shows typical DRT peaks of the Ni-GDC cermet fuel electrode in (a) SOFC and (b) SOEC modes, measured at different temperatures. In the range of 10-1 to 106 Hz where the frequency response was measured, three DRT peaks appeared, P1: 10-1 to 100 Hz, P2: 100 to 102 Hz, and P3: 102 to 103 Hz, all with a DRT peak area decreasing with increasing operating temperature. This indicates that there are at least three electrode reaction processes with different relaxation times, which are thermally active processes. Activation energies of 0.1-0.3 eV (SOFC mode) and 0.2-0.5 eV (SOEC mode) were obtained for P1, and 0.9-1.0 eV (SOFC mode) and 1.1-1.3 eV (SOEC mode) for P2. The activation energies in SOEC mode were higher than those in SOFC mode for both P1 and P2. In the presentation, the dependence of activation energies on the fuel electrode constituent materials and related electrode reaction processes will be discussed.

Low-pressure plasma sprayed dense scandia-stabilized zirconia electrolyte and its effect on SOFC performance

The plasma spraying process has been proven capable of fabricating high-performance metal-supported solid oxide fuel cells (MS-SOFCs), and the critical technology is to prepare dense MS-SOFC electrolytes. Usually, very low-pressure plasma spraying (<1000 Pa) is adopted as it can significantly improve the electrolyte density and air tightness. However, at the same time, it also has the disadvantages of low deposition efficiency and high cost. This paper explores whether it is possible to prepare dense MS-SOFC electrolytes under low-pressure conditions (1000–10000 Pa) with ideal deposition efficiency and cost. Specifically, 4000 Pa and 7000 Pa were chosen to prepare scandia-stabilized zirconia (ScSZ) electrolytes. The effects of spraying pressure and distance on the electrolyte microstructure and cell electrochemical performance were investigated. It was found that the electrolyte prepared at a pressure of 4000 Pa and a spraying distance of 325 mm had the lowest porosity of 3.28%. Meanwhile, the cell exhibited the best electrochemical performance with an open circuit voltage of 1.008 V and a maximum power density of 970 mW/cm2 at 800 °C.

Impact of Stack Temperature on Solid Oxide Electrolysis System Operation for Lunar Production of Hydrogen and Oxygen Propellants

Solid oxide electrolysis (SOEC) systems have the potential to produce hydrogen and oxygen propellant from lunar ice found in the permanently shadowed cream on the moon. Because SOEC systems electrolyze steam instead of liquid water as in more conventional alkaline and proton-exchange membrane electrolysis systems, they require lower stack voltages for thermal-neutral operation (about 1.29 V/cell vs. 1.48 V/cell for liquid-fed cells) and thus can have lower stack specific energy as defined by stack power requirements per kg of hydrogen produced. However, at a system level, power requirements in a remote lunar setting for vaporizing the steam and flowing it through the stack increase system-level power demands such that the advantages of the SOEC system are reduced due to the balance of plant loads contributing as much as 20 to 25% of the system energy at 800 deg. C. Our team at Mines collaborated with our partners at OxEon Energy to demonstrate an SOEC system with a stack operating temperature of 800 deg. C that demonstrated an overall system-level specific energy just below 50 kWh electric per kg of hydrogen. That study and a supporting model validation study provided a basis for showing the viability for high-temperature SOEC to be scaled-up for lunar production of hydrogen and oxygen propellant for propulsion. The current study builds on that previous work by exploring the impact of SOEC stack temperature on both the stack and balance of plant power requirements. This study explores the trade-offs between reduced balance of plant parasitics and increased stack mass with lower SOEC operating temperatures and explores the advantages of higher conducting mixed-ionic-electronic conducting electrolytes with electron blocking layers to increase current densities at lower temperatures.This study was performed using stack and balance of plant models that have been calibrated in the lab-scale demonstration of an SOEC system with an electrically driven steam generator, a scroll-compressor for driving the steam flow, and cathode and anode exhaust recuperators for heat recovery to the stack inlet steam flow. The system also includes H2 drying from the SOEC cathode exhaust by heat exchange with the incoming water stream. The current study looks updates the previous model by replacing the compressor with a higher pressure steam generator and an expansion valve that allows the operation of the system without the parasitic loads of a compressor. In the current study, three different electrolytes traditional yttrium-stabilized zirconia (YSZ), higher conductivity scandium-stabilized zirconia (ScSZ), and mixed ionic-electronic conducting gadolinium-doped ceria (GDC) with a YSZ blocking layer. The GDC bilayer electrolyte cells have the highest conductivity and allow for reasonable hydrogen production at operating temperatures below 700 deg. C. The results show that the lower operating temperatures enabled by the GDC bilayer electrolyte and by the ScSZ electrolytes not only reduce the system mass with smaller heat exchangers and thermal insulation, but also lower requirements for steam generation by enabling more heat recovery in the steam generator. The impact of stack operating temperatures on the feasibility of scaling up a lunar-deployed SOEC system to produce more than 1 kg/h are presented to further emphasize the feasibility of SOEC systems as part of a lunar propellant production facility.

Minimizing Coke Formation at La0.3Ca0.7Fe0.7Cr0.3O3-δ Perovskite Anodes in a Syngas Fed-SOFC

As the world moves to decarbonize the fossil fuel sector, transition technologies are needed that bridge the gap between natural gas power plants and more sustainable low-carbon energy sources. These newer technologies often still rely on fossil fuels but have improved energy conversion efficiencies and lower net carbon dioxide (CO2) outputs over conventional fossil fuel based electric power generation systems. In this work, we are exploring one such technology, namely the use of a syngas-fed solid oxide fuel cell (SOFC) to generate heat, electricity, steam, and captured CO2. Core to this technology is the mixed ion electron conductor deployed at the anode and cathode that catalyzes all of the relevant reactions, namely electrochemical oxidation of hydrogen (H2) and carbon monoxide (CO) at the anode, producing steam and CO2, and reduction of oxygen at the cathode.Carbon formation (coking) is normally a significant problem affecting SOFCs operating on carbon-based fuels, as it leads to a rapid decline in electrochemical performance by blocking catalytically active sites and pores with various carbon species, e.g., amorphous, graphitic, or nanotubular carbon.1 The formation of carbon species from syngas is known to occur through various mechanisms, with the Boudouard reaction (∆H= -172 kJ/mol) and the reduction of CO (∆H= -131 kJ/mol) being the most prominent.2 As such, temperature is a key parameter to optimize as it determines the propensity for carbon formation at equilibrium. In addition, the kinetics of carbon formation can be significantly reduced by introducing oxygen to the fuel gas stream in the form of O2, CO2, or H2O.3 The catalyst materials investigated here are mixed conducting perovskite oxides (La0.3Ca0.7Fe0.7Cr0.3O3- δ, LCFCr) that have been optimized and modified recently by our group, both in the as-prepared undoped form and after B-site doping with variable quantities of transition metals (M), e.g., Ni,4 forming nanoparticle (NP)-decorated ABO3-Mx surfaces. Our catalyst is highly active for H2 and CO oxidation, CO2 reduction, and O2 reduction, where it was demonstrated that the un-doped parent material can deliver a stable power density of 0.2 W/cm2 for several hundred hours with negligible performance degradation in 3% humidified H2.5 In more recent work, excellent resilience to carbon deposition for exsolved Fe-Ni@LCFCr up to 70:30 CO:CO2 was demonstrated.4 Herein, we show that minimal coke forms during exposure of these materials to dry syngas at 600oC, even under open circuit conditions.The catalysts were prepared using combustion synthesis and were characterized by XRD, SEM EDX, and TPO-MS in order to confirm morphology, crystal structure, and composition as a function of temperature and gas environment.4 Symmetrical electrolyte-supported SOFCs were constructed using our catalyst as both the anode and cathode. Catalyst layers of 1 cm2 were screen printed to a thickness of 25 µm on both sides of commercially available 130 µm thick samaria-doped ceria (SDC)-buffered scandia-stabilized zirconia (ScSZ) electrolyte, followed by sintering at 1100°C for 2 h,4 with porous metal current collectors used. The cells were mounted and tested in a Fiaxel SOFC test station with gas flow controlled by mass flow controllers.Preliminary electrochemistry experiments were conducted in 5:95 H2:N2, or 1:1 H2:CO (syngas) balanced by CO2 in a 1:2 ratio of fuel to oxidant into the anode chamber, and air into the cathode chamber at 600 oC, with performance evaluation carried out using CV, EIS and chronopotentiometry. The power density was found to be ca. 2x higher in dry H2 vs. in syngas, as expected, considering that H2 is a more active fuel vs. CO. Additionally, EIS exhibited ca. 2x higher resistance in the low frequency arc in syngas, which can be attributed to sluggish CO oxidation kinetics.4 Chronopotentiometry was performed for 20 h at 10 mA cm-2, showing a degradation rate of only 0.08 mV h-1, suspected to be primarily due to current collector delamination. Coking studies were also conducted on button cells at 600 oC in 1:1 H2:CO for 25 h at open circuit, comparing to a NiO standard that was painted on the electrolyte just next to the LCFCr-Ni working electrode. Imaging by SEM showed negligible carbon formation on the perovskite surface, supported by EDX analysis, compared to the extensive degree of coking observed at the standard. Further quantification was conducted by TPO-MS, also confirming minimal carbon formation. References Bengaard et al., Journal of Catalysis, 2002, 209, 365–384.Farshchi Tabrizi et al., Energy Conversion and Management, 2015, 103, 1065–1077.Sasaki et al., Journal of The Electrochemical Society, 2003, 150.Ansari et al., Journal of Materials Chemistry A, 2022, 10, 2280–2294.Addo et al., ECS Transactions, 2015, 66, 219–228.

A Study on Electrochemical Properties of Fuel-Electrode-Supported Reversible Solid Oxide Cells

Introduction Reversible solid oxide cells (r-SOCs) enable both power generation and steam electrolysis, and have attracted attention as a solid state electrochemical energy device for a decarbonized society (1). R-SOCs have the advantage of high efficiency due to high temperature operation, but there exist various technical issues remained in materials selection, start-up and shutdown, and long-term durability. Therefore, reducing operating temperature is desirable for practical applications (2,3). Electrode-supported cells, in which the electrode acts as a structural support, enable lower temperature operation by reducing the thickness of the electrolyte, which has certain electrical resistivity. Here in this study, the current-voltage characteristics and reversible cycle durability under r-SOC operating conditions are evaluated using fuel-electrode-supported cells, with the aim of developing r-SOCs capable of highly efficient fuel cell power generation and steam electrolysis. Experimental For the experiments, fuel-electrode-supported cells were fabricated using fuel electrode-supported half-cells (Japan Fine Ceramics, Japan), schematically shown in Fig. 1. The half-cell consists of scandia-stabilized zirconia (ScSZ: 10 mol%Sc2O3-1mol%CeO2-89 mol%ZrO2) or yttria-stabilized zirconia (YSZ: 8 mol%Y2O3-92 mol%ZrO2) electrolyte and a Ni-cermet fuel electrode, such as Ni-ScSZ or Ni-YSZ. (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF) was applied for the air electrode, and Gd0.1Ce0.9O2 (GDC) was inserted between the electrolyte and the air electrode to suppress interdiffusion and chemical reactions. Four types of cells were used with different combinations of the electrolyte component (YSZ or ScSZ) in the electrolyte layer or the supporting fuel electrode.In electrochemical tests, 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. R-SOC initial performance tests were conducted using an electrochemical analyzer (1255B, Solartron). Cell voltage and impedance were measured at 700-800°C at current densities ranging from -0.5 A cm-2 to +0.5 A cm-2. Positive current density means the value in SOFC mode, while negative current density means the value in SOEC mode. In r-SOC 1,000 cycle durability tests, cycles of switching between SOFC and SOEC operation were repeated 1,000 times by varying current density. The range of current density in the cycle tests was between -0.2 A cm-2 and +0.2 A cm-2. The cell impedance was measured before the cycling test and every 100 cycles. After each test, the cells were analyzed by using a focused-ion beam scanning electron microscopy (FIB-SEM) to observe and evaluate the electrode microstructure. Results and discussion Figure 2 (a) shows the r-SOC initial performance of each fuel-electrode-supported cell. The cell with the Ni-YSZ fuel electrode and the YSZ electrolyte showed better current voltage characteristics than other cells. Figure 2 (b) shows the r-SOC 1000-cycle durability of the cell with the Ni-YSZ fuel electrode and the YSZ electrolyte. The results showed a decrease in power generation and electrolysis performance with increasing the number of cycles in both SOFC and SOEC modes. Possible degradation mechanisms will be discussed. Acknowledgments This paper is based on results obtained from a project (Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration), JPNP20005, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Collaborative support by Prof. H. L. Tuller and Prof. B. Yildiz at Massachusetts Institute of Technology (MIT) is gratefully acknowledged. References (1) Venkataraman, M. Pérez-Fortes, L. Wang, Y. S. Hajimolana, C. Boigues-Muñoz, A. Agostini, S. J. Mcphail, F. Mar échal, J. V. Herle, and P. V. Aravind, J. Energy Storage, 24, 100782 (2019).(2) Subotić, S. Pofahl, V. Lawlor, N. H. Menzler, T. Thaller, and C. Hochenauer, Energy Proc., 158, 2329 (2019).(3) B. Mogensen, Current Opinion Electrochem., 21, 265 (2020). Figure 1

(Invited) Understanding the Effects of Metal Nanoparticle Exsolution from La0.3Ca0.7Fe0.7Cr0.3O3-δ Perovskites on CO2-CO Electrocatalysis

Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysis Cells (SOECs) are highly useful devices, capable of generating and storing large amounts of energy, respectively.1 They can do this by catalyzing CO oxidation at a SOFC anode and CO2 reduction (CO2RR) at a SOEC cathode, while the other electrode catalyzes oxygen reduction or evolution. The conduction of oxide ions through the electrolyte completes the circuit, making Solid Oxide Cells (SOCs) an excellent choice for clean energy production and use since there are no undesired byproducts.1 While traditional SOC electrodes are composed of oxide-conducting ceramics mixed with electronically conducting metals,1 a newer category of catalysts are Mixed Ionic Electronic Conductors (MIECs), with one example being the perovskite La0.3M0.7Fe0.7Cr0.3O3-δ (M = Sr, Ca) (LMFCr), investigated heavily by our group.2,3,4 As an MIEC, the full surface of LMFCr is electrochemically active, giving excellent activity at both the cathode and anode, including for CO2RR and CO oxidation.2,3 However, efforts are being made to further improve the CO2RR-CO oxidation kinetics and durability, with the Ca analogue having a better chemical match with standard electrolytes.4 One approach used recently to improve MIEC oxide performance is B-site doping with transition metals (TMs) while also creating an A-site deficiency, resulting in nanoparticle (NP) formation (exsolution) when the perovskite is subjected to reducing conditions.2 For instance, Fe-Ni NPs of a ~20 nm average size can be exsolved from (La0.3Ca0.7)0.95Fe0.7Cr0.25Ni0.05O3-δ(LCFCrNi), even at 600 °C in 70CO2:30CO (pO2 ~10-18 atm). Furthermore, NP features can easily be tailored by changing the reducing conditions or dopant used.2 In general, higher temperatures and lower pO2 lead to larger NPs over time, and more easily reducible metals tend to exsolve first under less harshly reducing conditions.2 Recent studies have suggested that NP formation enhances electrocatalytic activity by creating additional sites of reactivity, suggesting that strong NP-substrate interactions are important to catalysis.5 However, there is no clear understanding of the role played by exsolved NPs in catalyzing SOC reactions. To gain further insights, detailed electrochemical studies of LCFCrNi electrodes were done on 1-inch diameter cells constructed using Samarium-Doped Ceria buffered Scandia-Stabilized Zirconia electrolyte substrates. LCFCrNi was made into an ink and tape-cast on 0.5 cm2 on one side (working electrode, WE) and 1 cm2 on the other (counter electrode, CE) on each substrate. The cells were sintered at 1100 °C for 2 h, coated with Au ink, and sintered at 850 °C for 1 h. Electrochemical Impedance Spectroscopy (EIS) was performed at 600 °C with 70CO2:30CO at the WE and air at the CE, followed by exsolution in 5H2:95N2 at a higher temperature, and then EIS was repeated multiple times under the same conditions at 600 °C. Cells were imaged via Scanning Electron Microscopy (SEM) after ramping down to room temperature in N2 to determine the size and distribution of the NPs for each set of exsolution conditions.Under all conditions employed, the polarization resistance (Rp) was found to be stable before exsolution occurred, but started decreasing once the NPs formed, especially via a shrinking of the low frequency resistance. The low frequency resistance has been associated with CO oxidation,2 so the fact that it improves with exsolution is an indication that the increased electrode area serves to improve CO oxidation more than CO2RR. When exsolution was carried out at 800 °C for 25 h, Rp decreased more rapidly than after exsolution at 800 °C for 14 h. Exsolution at 800 °C for 25 h also showed a more rapid decrease in Rp compared to 700 °C exsolution for 25 h. These results argue that the initial formation of larger NPs (formed after longer time or at a higher temperature) results in a faster increase in the active surface area of the electrode at the lower 600 °C temperature. Further experiments are being carried out to better understand whether it is the growth of NPs or new NP formation after higher temperature exsolution that is the reason behind this observation. The results of operando synchrotron studies in CO2/CO environments before and after exsolving, as well as the use of different transition metal dopants, are also being investigated for this purpose.

Role of Divalent Co-Dopant as Structure Stabilizer in Scandia-Stabilized Zirconia Electrolyte for SOFC

The present investigation reports the role of divalent binary co-dopant (Ca2+) in 11 mol% scandia stabilized zirconia (11SSZ) electrolytes to resolve its severe long-term aging issue for application in solid oxide fuel cells (SOFC). Dense electrolytes were formulated via the solid-state reaction method and their crystal structure was identified by X-ray diffractometer (XRD). To examine total electrical conductivity and its stability in oxidizing and reducing atmosphere DC four-point probe measurement was used. Among all the compositions, 0.2Ca11SSZ demonstrates the highest conductivity of 0.075 S cm−1 at 800 °C, with excellent stability of 6.7%/100 h in a reducing (97 vol% H2/3 vol% H2O) atmosphere. However, the presence of 0.5 mol% calcium in 11SSZ results in more than threefold suppression of aging rate compared to undoped11SSZ i.e. 2.19%/200 h in air atmosphere at 800 °C. Additionally, the doping of divalent Ca2+ widens the electrolytic domain up to pO2 ∼ 10−26 atm at 1000 °C compared to state-of-art 8YSZ (pO2 ∼ 10−22 atm), with 0.024% linear expansion on phase transition and 172 MPa flexural strength. Convincingly, the excellent structure stability and ionic conductivity of calcium co-doped 11SSZ compared to state-of-the-art electrolytes make them potential candidates to be used as an electrolyte for SOFC application.

Evaluation of Electrolyte Materials of Gd- and Ce-Doped Scandia-Stabilized Zirconia and Yb- and Bi-Doped Gadolinium-Doped Ceria for Highly Durable Solid Oxide Fuel Cells

Evaluation of Electrolyte Materials of Gd- and Ce-Doped Scandia-Stabilized Zirconia and Yb- and Bi-Doped Gadolinium-Doped Ceria for Highly Durable Solid Oxide Fuel Cells

Optimizing LSM-LSF composite cathodes for enhanced solid oxide fuel cell performance: Material engineering and electrochemical insights

In response to pressing environmental concerns and the ever-increasing global energy demand, the pursuit of sustainable energy sources has garnered substantial momentum. A predominant focus of research within this domain revolves around the enhancement of solid oxide fuel cell performance, with particular attention directed toward the intricate oxygen reduction reaction transpiring at the cathode electrode. The present investigation embarks upon the exploration of a composite material comprising La1-xSrxMnO3-δ (LSM) and La1-xSrxFeO3-δ (LSF) as prospective cathode materials. However, the achievement of optimal composite cathodes necessitates the application of advanced materials engineering strategies. The principal aim of this comprehensive study is the development of high-performance composite cathodes, achieved through the deposition of thin film counterparts onto scandium-stabilized zirconia (ScSZ) electrolyte substrate employing the precision sputtering technique. The ensuing findings unequivocally unveil the remarkable performance exhibited by this innovative composite cathode configuration, notably manifesting its excellence at an operating temperature as modest as 780 °C. These promising outcomes imbue optimism regarding the prospective utilization of this composite material in advanced energy conversion applications, marking a significant advancement toward the realization of sustainable and efficient energy systems.

Development of Multiple Ceramic Coatings on Porous Tubular Metal Support

Micro-tubular metal supported Solid Oxide Fuel Cell (MS-SOFC) has many advantages over traditional ceramic supported SOFC, such as greater mechanical robustness, better thermal and redox cycling along with low cost of starting material. Among the multiple MS-SOFC fabrication methods, the one involving sequential dip-coating followed by co-sintering has the potential to reduce the fabrication cost further, making it attractive from a commercial production standpoint. However, the co-sintering route is challenging because it requires constrained sintering conditions, high temperatures, and inert atmosphere. Other challenges with the co-sintering route are delamination and cracking of layers, warping of cell, diffusion of Fe and Cr away from metal support, diffusion of Ni from anode, coarsening of Ni in the anode, and lack of complete densification of YSZ electrolyte even at temperatures as high as 1350 °C.In this work, we developed a new ceramic multilayer design on a porous tubular 3D-printed metal support. Scandia Stabilized Zirconia (SSZ) and, Ni and SSZ (Ni-SSZ cermet) were used as electrolyte and anode, respectively. Stainless steel 17-4 PH was identified as the metal support material due to its matching coefficient of thermal expansion with SOFC ceramic materials, low cost and corrosion resistance nature. The dip coating steps and the co-sintering process were optimized so that they could be adopted for a cost-effective fabrication of MS-SOFC. The binder-burnout step was optimized to prevent cracking of the multilayers during the co-sintering process. To avoid delamination and to impede the inter-diffusion of elements, a coating of an intermediate layer made of 17-4 PH stainless steel+Ni+YSZ was introduced between the metal support and the succeeding layers. Preliminary results suggest that the intermediate layer has a potential to prevent delamination and inter-diffusion. Introduction of the debinding step and optimized sintering profiles resulted in the formation of the uniform and dense electrolyte coating. However, certain cell warping was observed after electrolyte sintering, suggesting further improvement is still needed in the support design and ceramic processing.

Characterization and Testing of Exsolution-Based Solid Oxide Cells for Reversible Operations in CO2 Electrolysis

Reversible solid oxide cells (rSOCs) are a promising electrochemical technology able to work both as energy storage devices in solid oxide electrolysis (SOEC) mode, and as power generators in solid oxide fuel cell (SOFC) mode. rSOCs implemented for CO2 electroreduction into valuable CO-rich steams through electrolysis would provide opportunities for CO2 utilization, and hence become useful tools for the reduction of greenhouse gases emissions. High-temperature co-electrolysis of CO2/H2O mixtures has recently become of interest since high conversion and energy efficiency are thermodynamically favored in addition to the reduction in cell area-specific resistance when compared to pure CO2 electrolysis [1]. However, reversible operation with CO2-containing feeds requires developing flexible, high-performing, and long-lasting materials for the rSOCsManaging issues such as coking and low redox tolerance in the state-of-the-art (SoA) Ni-Yttria-stabilized-zirconia (Ni-YSZ) cermet fuel electrode in the rSOCs is crucial. Ni-YSZ cermet exhibits high electronic conductivity and electrochemical activity in hydrogen. Nonetheless, Ni is prone to oxidation due to operation under high steam concentrations, and rapid variations in the fuel supply. Additionally, Ni is well-known for its coke-formation tendency which becomes detrimental in CO2-rich mixtures for electrolysis [2]. Thus, ceramics with mixed ionic and electronic conductivity (MIEC) have emerged as alternative fuel electrodes thanks to the larger electrochemically active area compared to standard cermets. Among the MIEC electrodes, the SrTi0.3Fe0.7O3 (STF) perovskite has proven to give low polarization resistances especially when modification techniques such as exsolution of metal nanoparticles are employed [3]. This work focuses on the investigation of the electrocatalytic properties of exsolution-based STF electrodes operating with CO/CO2 and H2O/CO2 mixtures. The mixtures used were 50% CO2/50% CO, 25% H2O/25% CO2/25%H2/25%CO, and 45% H2O/45% CO2/10% H2. A comparison between the performance and microstructure of the studied electrode formulation is provided.Electrolyte-supported cells were manufactured via screen-printing of the electrode inks on scandia-stabilized zirconia (ScSZ) electrolytes. STF perovskite-based electrodes were used, namely: SrTi0.3Fe0.7O3 (STF), Sr0.95(Ti0.3Fe0.63Ni0.07)O3 (STF-Ni), and Sr0.95(Ti0.3Fe0.63Ru0.07)O3 (STF-Ru). The latter two formulations allowed the enhancement of the perovskite structure via exsolution of catalytic nanoparticles (Ni, Co alloyed with Fe) on the oxide surface during cell operation. Each cell configuration included a gadolinium-doped ceria (GDC) buffer layer coupled with an LSCF-GDC (La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O1.95) oxygen electrode. Baseline performances of cell mounting the Ni-YSZ cermet electrode were also evaluated for comparison with the perovskite alternatives, both in the short and long term. Electrochemical characterization was done via impedance spectroscopy, polarization experiments, and durability tests in potentiostatic mode. Chemical characterization was performed via X-ray diffraction, scanning electron microscopy, and temperature-programmed reduction analyses.Compared to Ni-YSZ, significant improvement in terms of reversibility and maximum current densities of the current-voltage (I/V) curves in both SOFC and SOEC modes is found on the STF-based cells at 750°C under CO/CO2 mixtures (Figure 1B). The initial difference in the performance of the STF cells with respect to the Ni-YSZ cermet in humidified H2 (SOFC mode, Figure 1A) was overcome by using the exsolved electrode formulations. Aging experiments up to 100 h performed in alternating operation modes from SOFC (6 h) to SOEC (6 h) indicated an initial performance degradation within the first 24 h for both the cermet and perovskite-based fuel electrodes which later stabilized in time. Compared to the undoped STF formulation, improvement in performance stability was observed for the exsolved STF-Ni and STF-Ru fuel electrodes while working under the target CO/CO2 mixture. This was attributed to the combination of the STF structure benefits (low polarization resistance towards CO/CO2 and H2O/CO2 mixtures) with the boost that metallic nanoparticles provide to the electrode’s heterogeneous catalytic reactivity and overall stability over time.

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