Scandia-stabilized Zirconia Electrolyte Research Articles

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

AbstractSolid 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.

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.

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.

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.

(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.

In-Situ Observation of Temperature Distribution on a Planar Type SOEC During Start-Stop Cycle Operation

Introduction It is important to establish various analytical methods for solid oxide electrolysis cells (SOECs) under practical operating conditions in order to improve their performance and to analyze their degradation phenomena. In-situ observation of temperature distribution on a planar type SOEC enables to reveal the distributions of electrochemical reactions and the cell degradation. While there have been several reports on the internal visualization of solid oxide fuel cells (SOFCs)1, 2, there have been only a few reports on that of SOECs. Since the endothermic electrolysis reaction occurs in SOECs, comparing with SOFCs, a different temperature distribution will appear. Therefore, it should be necessary to evaluate the phenomena on the planar SOECs for analyzing the temperature distribution taking gas flow and current distribution into consideration.The purpose of this study is to evaluate the effect of cell degradation caused by the start-stop cycle operation on the temperature distribution using a 5 cm × 5 cm planar cell observed by infrared camera. Experimental In this study, a planar cell was fabricated using a scandia-stabilized zirconia electrolyte (ScSZ, 200 µm thick, 5 cm × 5 cm in area) as a substrate. LSM ((La0.8Sr0.2)0.98MnO3) and LSM-ScSZ composite materials were used for the air electrode, and nickel and ScSZ cermet were used for the fuel electrode. The electrode area was 4 cm × 4 cm (16 cm2). Each electrode material was screen-printed and sintered. A platinum mesh was used as the current collector.In this test, the cell was operated at 800 °C and 50%-humidified hydrogen (200 ml min-1) was supplied to the fuel electrode. The durability test was conducted under the accelerated degradation conditions of a start-stop cycle repeated in every 1 hour with 0.2 A cm-2 for 500 cycles (1000 hours). Electrochemical characteristics were measured by connecting Pt wires, spot-welded every 1 mm on one side of both electrode surfaces, to the voltage terminals of the electrochemical measurement setup and to the current terminals of the external power supply unit.Figure 1 shows a schematic view of the experimental setup and temperature change compared with the initial condition. The temperature distribution was observed by infrared camera (FLIR SC2500 - NIR, uncertainty ±0.02 oC) from the air electrode side open to the electric furnace. The difference between the temperatures before and during current loading was derived as the distribution of temperature changes. Results and discussion First, the electrochemical characteristics of the planar type SOECs during the start-stop cycle test were obtained. The cell voltage at 0.2 A cm-2 increased from 1.19 V to 1.29 V during the test up to 500 cycles. The cell voltage was 1.27 V even after the initial 50 cycles. This suggests that SOEC electrode degradation3,4 occurred in the early stage of the start-stop cycling.The temperature changes after 50 cycles showed that each temperature change was relatively small because the cell was operated near the thermoneutral potential (around 1.29 V at 800 °C). It was confirmed that the endothermic electrolysis reaction was predominant near the fuel inlet. In addition, heat generation due to Joule heating was also appeared near the current collecting Pt wire connected to the power supply. Therefore, in SOEC operation, it is suggested that the temperature distribution is affected by the shape of current collector, gas supply condition, and electrochemical reaction distribution. The detailed analysis on the SOEC electrochemical properties and temperature distribution observed by in-situ measurements during cycling tests is also presented. Acknowledgements A part of this research was supported by New Energy and Industrial Technology Development Organization (NEDO), project No. JPNP14026. References (1) S. Shinichi and H. Iwai, J. Power Sources, 482, 229070 (2021).(2) Y. Shiratori, T. Ogura, H. Nakajima, M. Sakamoto, Y. Takahashi, Y. Wakita, T. Kitaoka, and K. Sasaki, Int. J. Hydrogen Energy, 38 (25), 10542 (2013).(3) M. Keane, M. K. Mahapatra, A. Verma, and P. Singh, Int. J. Hydrogen Energy, 37 (22), 16776 (2012).(4) M. Hubert, J. Laurencin, P. Cloetens, B. Morel, D. Montinaro, and F. Lefebvre, J. Power Sources, 397, 240 (2018). Figure 1

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.

Performance and Durability of Solid Oxide Electrolysis Cell Air Electrodes Prepared By Various Conditions

Introduction Fuel electrode materials are important for achieving higher performance and durability of Solid Oxide Fuel Cell (SOFC), Solid Oxide Electrolysis Cell (SOEC), and Reversible Solid Oxide Cells (r-SOCs). On the other hand, the air electrode also faces performance and durability issues. For example, the diffusion of Sr ions from the air electrode materials has been reported.1, 2 In SOECs, if electrolysis can be performed at a thermoneutral potential balancing heat absorption by electrolysis and heat generation by the internal resistance of the electrolysis cell, hydrogen can be generated theoretically without heat supply. The purpose of this study is to evaluate the electrochemical performance and durability of the SOECs by conducting electrolysis performance tests of LSCF-based air electrodes with different preparation conditions, and electrolysis durability tests at the thermoneutral potential. Experimental Each test was conducted using a cell with a scandia-stabilized zirconia electrolyte (ScSZ, 200 µm thick) as a substrate. For the fuel electrode, Ni-GDC co-impregnated fuel electrode was used3. A mixture of Ni(NO3)2･6H2O, Gd(NO3)3･6H2O, and Ce(NO3)3･6H2O was impregnated into the porous LST-GDC framework, a mixture of LST powder (La0.1Sr0.9TiO3) and GDC powder (Gd0.9Ce0.1O₃). LSCF, (La0.6Sr0.4)(Co0.2Fe0.8)O3, was used for the air electrodes. GDC was inserted between the LSCF layer and the electrolyte plate as a buffer layer. In fabricating air electrodes, four types of air electrodes were applied, at shown in Table 1. For the electrochemical measurements of the air electrodes, Pt reference electrode was attached to the electrolyte on the fuel electrode side, and the voltage terminals of the electrochemical measurement system were connected between the reference electrode and the air electrode to measure air electrode voltage.In the performance tests, the voltage (potential) and impedance of the air electrode were measured at operating temperatures of 800°C, 750°C, and 700°C. In the durability tests, the air electrode was maintained at thermoneutral potentials at 800°C and 700°C (1.286 V and 1.283 V, respectively), and the voltage and impedance of the air electrodes were measured before and after the durability tests to evaluate the degradation of the air electrodes. Results and discussion Figure 1 shows the air electrode voltage obtained from the performance tests at each operating temperature for the four types of air electrodes shown in Table 1. First, regarding the dependence of the electrolysis performance on operating temperature, an increase in air electrode voltage (degradation of the air electrode) was found at 800°C.The increase in air electrode voltage with current density was almost linear, but at lower operating temperatures, the increase in air electrode voltage at lower current densities becomes larger. In other words, the I-V curve was rather ohmic at 800°C, but non-ohmic at 700°C.Figure 2 shows the results of an 80-hour durability test at different temperatures. The current density of the cell corresponding to the thermoneutral potential was -0.298 Acm-2 when operated at 800°C and -0.018 Acm-2 when operated at 700°C.The degradation rate was calculated from the difference in air electrode voltage before and after the performance and durability test at these current densities: 0.172% at 800°C and 0.792% at 700℃. The degradation rate increased as the operating temperature decreased in the SOEC mode, despite the low current density. Microstructural observation of the air electrode after the durability test showed that Sr diffused to the LSCF/GDC interface similar to SOFCs mode4, and that the diffusion of Sr was accelerated at 700°C. In addition, Co diffusion was also observed. Such diffusion may occur during LSCF firing process and/or during SOEC operation. More detailed investigation is in progress. Acknowledgements This research was supported in part by NEDO (New Energy and Industrial Technology Development Organization). We thank all parties involved. References (1) V. Subotić, S. Futamura, G. F. Harrington, J. Matsuda, K. Natsukoshi, and K. Sasaki, J. Power Sources, 492, 229600 (2021).(2) S. P. Simner, M. D. Anderson, M. H. Engelhard, and J. W. Stevenson, Electrochem. Solid-State Lett., 9 A478 (2006).(3) K. Natsukoshi, K. Miyara, Y. Tachikawa, J. Matsuda, S. Taniguchi, G. F. Harrington, and K. Sasaki, ECS Trans., 03, 203 (2021).(4) S. Kanae, Y. Toyofuku, T. Kawabata, Y. Inoue, T. Daio, J. Matsuda, J,-T. Chou, Y. Shiratori, S. Taniguchi, and K. Sasaki, ECS Trans., 68, 2463 (2015). Figure 1

Atmospheric plasma spraying to fabricate metal‐supported solid oxide fuel cells with open‐channel porous metal support

AbstractMetal‐supported solid oxide fuel cells (MS‐SOFCs) have been fabricated by applying phase‐inversion tape‐casting and atmospheric plasma spraying (APS). The effect of the binder amount of the phase‐inversion slurries on the microstructure development of the 430L stainless steel metal support was investigated. The pore structures, the viscosity of the slurry, porosity and permeability of the as‐prepared metal supports are significantly influenced by the amount of the binder. NiO–scandia‐stabilized zirconia (ScSZ) anode, ScSZ electrolyte and La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathode layers were consecutively deposited on the metal support with an ideal microstructure by APS process. The effect of plasma power of the APS on the microstructure of the electrolyte and cathode was investigated. A dense electrolyte layer and a porous cathode layer were successfully obtained at 40 and 6 kW of the APS plasma power, respectively. MS‐SOFCs, with a cell configuration of 430L/Ni‐ScSZ/ScSZ/LSCF, achieved a maximum cell power density of 1079 mW cm−2 at 700°C using humidified H2 as fuel and ambient air as oxidant. The corresponding ohmic resistance and total resistance of MS‐SOFCs was 0.14 and 0.32 Ω cm2, respectively. This work demonstrates the feasibility of fabricating high‐performance MS‐SOFCs with economical and scalable techniques.

High-conductivity electrolyte with a low sintering temperature for solid oxide fuel cells

Bismuth oxide and scandia co-doped zirconia (Sc2O3)0.06(Bi2O3)x(ZrO2)0.94–x (ScSZB, x = 0, 0.01, 0.03, 0.05, 0.07, 0.1) powders are prepared via a citrate sol-gel method. Bi2O3 promotes the sintering process of scandia stabilized zirconia (ScSZ) and increases electrical conductivity of system. A high conductivity of ∼0.094 S/cm at 800 °C is achieved on 5 mol% Bi2O3 doped ScSZ (ScSZB05). X-ray Rietveld refinement and transmission electron microscope (TEM) analysis of the ScSZB05 reveal the formation of cubic phase and rhombohedral phase at room temperature. The electrolyte-supported cell constructed by the ScSZ electrolyte gives the maximum power density of 258.3 mW/cm2 at 800 °C, while the cell with ScSZB05 electrolyte shows a higher value of 387.6 mW/cm2. The performance obtained by theoretical simulation of the two electrolyte-supported cells is in good agreement with the experimental results.

On the mechanism of Mn(II)-doping in Scandia stabilized zirconia electrolytes

Cubic Scandia-stabilized zirconia (ScSZ) is an attractive electrolyte material for solid oxide cells due to its significant ionic conductivity, provided that the phase transition to its rhombohedral polymorph upon cooling is suppressed. The latter is achieved with addition of a secondary co-dopant, albeit it may be at the detriment of its ionic conductivity Here, we thoroughly investigate how MnO2 (0.5–10 mol%) as a co-dopant impacts on the sinterability, thermal expansion, crystal structure and ionic conductivity of ZrO2 doped with 10 mol% Scandia (10ScSZ), and we provide new insight on the chemistry of dissolved manganese in the fluorite lattice. Reactive sintering of 2 mol% MnO2 mixed with 10ScSZ enables to produce dense electrolyte with significant reduction of the peak sintering temperature and stabilisation of the cubic structure down to room temperature. Combined density functional theory and X-ray photoelectron spectroscopy analyses reveal that manganese predominantly enters the structure as Mn2+ during reactive sintering, with a prevalence of higher valence states at the surface and grain boundaries. The highest oxide ion conductivity is achieved for 2 mol% doped 10ScSZ (120 mScm−1 at 800 °C) and it decreases with increasing Mn concentration. For all compositions, the bulk conductivity remains independent of pO2 – corroborating a limited electronic conductivity contribution from Mn-doping. The grain boundary conductivity is found to decrease with sintering time and pO2, which is attributed to the chemistry and concentration of segregated manganese at the surface and grain boundaries, yielding depletion of oxygen vacancies in the space charge layer.

Effect of nanostructured grains in co-sputtered Ni-GDC thin-film anode on methane conversion kinetics for low temperature solid oxide fuel cells operating on nearly dry methane

Direct-methane solid oxide fuel cells (DMSOFCs) have recently attracted substantial attention due to their simplified system, reduced cost, and the direct availability of methane fuel obtained from natural gas. Among oxygen-ion conductive materials, doped-ceria such as gadolinium-doped ceria (GDC) or samarium-doped ceria can be incorporated into Ni-based anodes to reinforce their coking resistance, enlarge their electrochemical reaction area, and improve the kinetics of the internal reforming/electrochemical oxidation of methane. To reduce the range of operating temperatures of DMSOFCs while maintaining their performance, the thin film deposition technique of magnetron sputtering was adopted in this work. An Ni-GDC thin-film anode and a Pt thin-film cathode were deposited on scandia-stabilized zirconia (ScSZ) electrolyte supports. This fuel cell was tested with directly supplied methane fuel (3% H2O) at 500 °C. The results demonstrated the effects of the GDC volume fraction in the anode—which was controlled by co-sputtering power—on open circuit voltage and electrochemical performance. The co-sputtered Ni-GDC anode was able to survive through 36-h operation, although there was some performance degradation. Field-emission scanning electron microscopy results revealed no formation of filamentous carbon on the Ni catalysts, despite the fact that both X-ray photoelectron spectroscopy and Raman spectroscopy analyses detected carbon coking. The relatively high performance and resistance to carbon coking of co-sputtered thin-film anode were attributed to its intrinsic small grain size.

Effect of Microstructure of the Anode Support Tubes Towards SOFC Performance

Reversible solid-oxide fuel cells (rSOFC) attract enormous attentions because of their capability to operate in both fuel cell and electrolysis modes. rSOFC with a tubular cell design has multiple advantages over a planar design for example allowing faster startup and shutdown. Successful fabrication of tubular cells needs to overcome numerous technological challenges, including control of its mechanical stress and high interlayer electrode resistance. The scope of the present work involves fabrication and testing of anode supported rSOFC single cells using both ionic conducting electrolyte and protonic conducting electrolyte. For the ionic conducting cells, the rSOFCs constituted Ni + yttria-stabilized zirconia (YSZ) anode support tube, Ni + scandia-stabilized zirconia (ScSZ) anode functional layer, scandia-stabilized zirconia (ScSZ) electrolyte, samarium doped ceria (SDC) protective layer, La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) + SDC cathode functional layer, and LSCF cathode current collector layer, respectively. For the protonic conducting cells, BaZr0.1Ce0.7Y0.1Yb0.1O3-d (BZCYb) was used as electrolyte.In this work, sequential dip-coating coupled with co-firing technique was implemented over the thin film cell structure fabrication. The nickel based tubular anode supports were made from various methods, including extrusion, freeze-casting and dipping processes. To achieve low interlayer resistance and dense electrolyte layer, we have optimized the ceramic slurry (ink) formulation parameters. Cell performance and microstructure of the rSOFC will be discussed and compared with the available data.

Comparison of electrochemical impedance spectra for electrolyte-supported solid oxide fuel cells (SOFCs) and protonic ceramic fuel cells (PCFCs)

Protonic ceramic fuel cells (PCFCs) are expected to achieve high power generation efficiency at intermediate temperature around 400–600 °C. In the present work, the distribution of relaxation times (DRT) analysis was investigated in order to deconvolute the anode and cathode polarization resistances for PCFCs supported on yttria-doped barium cerate (BCY) electrolyte in comparison with solid oxide fuel cells (SOFCs) supported on scandia-stabilized zirconia (ScSZ) electrolyte. Four DRT peaks were detected from the impedance spectra measured at 700 °C excluding the gas diffusion process for ScSZ and BCY. The DRT peaks at 5 × 102–1 × 104 Hz and 1 × 100–2 × 102 Hz were related to the hydrogen oxidation reaction at the anode and the oxygen reduction reaction at the cathode, respectively, for both cells. The DRT peak at 2 × 101–1 × 103 Hz depended on the hydrogen concentration at the anode for ScSZ, while it was dependent on the oxygen concentration at the cathode for BCY. Compared to ScSZ, steam was produced at the opposite electrode in the case of BCY, which enhanced the cathode polarization resistance for PCFCs.

Ultra-Thin GDC Buffer Layer Fabricated By Vacuum Slurry Coating Method for High Performance Intermediate-Temperature SOFC

The Vacuum Slurry Coating (VSC) method has the advantage of being able to produce thin films with a simple device and is cost effective over other thin film coating methods. In solid oxide fuel cells (SOFCs), gadolinium doped ceria (GDC) buffer layer should be thin and dense to minimize ohmic resistance as well as prevent the undesirable reactions between scandia-stabilized zirconia (ScSZ) electrolyte and cobaltite perovskite cathodes. In this study, GDC buffer layer was coated on pre-sintered anode-supported half-cell comprising an ScSZ electrolyte layer on the NiO-YSZ anode using vacuum slurry coating process. Then GDC coated cells were sintered at 1400oC. The GDC buffer layer coated on the pre-sintered cell at 1200oC was thin and dense regardless of the immersion time. The maximum power density of the SOFC using VSC was 400 mW/cm2 (@ 700oC), which is 38 % higher than that using screen printing method (290 mW/cm2). The microstructure of the coated GDC buffer layer using the VSC method was much denser than using the screen printing method.

Advanced modification of scandia‐stabilized zirconia electrolytes for solid oxide fuel cells application—A review

SummarySolid oxide fuel cells (SOFCs) are recognized as an energy generation device with high power density and energy conversion efficiency, low noise, low greenhouse gas emissions, positive environmental effect, and high fuel consumption versatility. Nevertheless, the high operating temperature of SOFCs has restricted the development in various applications due to challenges in material compatibility and working process. To address these issues, scandia‐stabilized zirconia (ScSZ) electrolytes are commonly utilized to explore its potentials as a solid electrolyte toward lowering the operating temperature due to high conductivity properties at low‐to‐intermediate temperature conditions. Thus, this paper provides an advanced modification of ScSZ electrolytes in SOFCs toward lowering the operating temperature. This review presents the potential properties and the progress modification of ScSZ electrolyte including challenges, and advances, summary, and future perspectives in the low‐to‐intermediate operating temperature of SOFCs. This review can serve as a first reference for identifying specific parameters and prospective studies to enhance the properties of ScSZ electrolytes for the application of SOFCs. Based on the literature review, there are no previous review works that have been published on the modification of ScSZ electrolytes.

Phase stability and conductivity of rare earth co-doped nanocrystalline zirconia electrolytes for solid oxide fuel cells

Abstract Solid oxide fuel cell (SOFC) is a green energy technology that directly coverts chemical energy into electricity. Scandia stabilized zirconia (SSZ) shows the highest conductivity among zirconia based electrolytes for SOFCs. However, the stability of the cubic phase, which is the desirable phase for high conductivity, can be an issue in SSZ electrolytes due to its transformation to other low-conducting phases at higher temperatures. In the present investigation, SSZ electrolyte was co-doped with ytterbia, gadolinia and ceria with an objective of improving the high-temperature phase stability. Both the doped and co-doped compositions exhibited a single cubic phase in the as-processed condition. The phase stability at high temperature was studied by aging the sintered pellets at 900 °C for 500 h in air. X-ray diffraction and transmission electron microscopy analysis revealed formation of small amount of the low-conducting tetragonal phase in 1 mol % ytterbia and gadolinia co-doped compositions on ageing which resulted in conductivity degradation. Increasing the doping level to 2 mol% prevented the formation of the tetragonal phase. The ceria co-doped composition (1 mol%), on the other hand, was clean without any sign of the secondary phases even after high-temperature ageing. The rhombohedral ‘β’ phase formed in the binary composition (SSZ) after sintering but was absent in all the co-doped compositions. The conductivity of the co-doped samples was higher than the binary SSZ. Thus, it can be said that rare earth co-doping is an effective way of improving the phase stability and conductivity of SSZ electrolytes.

High-performance solid oxide fuel cells with fiber-based cathodes for low-temperature operation

Low-temperature operation of solid oxide fuel cells (SOFCs) results in deterioration in electrochemical performance due to sluggish oxygen reduction reaction (ORR) at the cathode. To enhance the reaction pathway for ORR, La0.8Sr0.2MnO3 (LSM) nanofibers were fabricated by electrospinning and used for low-temperature solid oxide fuel cells operated at 600–700 °C. The morphological and structural characteristics show that the electrospun LSM nanofiber has a highly crystallized perovskite structure with a uniform elemental distribution. The average diameter of the LSM nanofiber after sintering is 380 nm. A symmetric cell of nanofiber-based LSM cathode on scandia-stabilized zirconia (SSZ) electrolyte pellet exhibits much lower area specific resistances compared to commercial LSM powder-based cathode. A single cell based on the nanofiber LSM cathode on yttrium-doped barium cerate-zirconia (BCZY) electrolyte exhibits a power density of 0.35 Wcm−2 at 600 °C, which increases to 0.85 Wcm−2 at 700 °C. The cell has an area specific resistance (ASR) of 0.46 Ωcm2 at 600 °C, which decreases to 0.07 Ωcm2 at 700 °C. The results indicate that the LSM electrode fabricated by the electrospinning process produces a nanostructured porous electrode which optimizes the microstructure and significantly enhances the ORR at the cathode of SOFCs.

Fabrication of Scandia-Stabilized Zirconia Thin Films by Instant Flash Light Irradiation

In this study, scandia-stabilized zirconia (ScSZ) electrolyte thin-film layers were deposited via chemical solution deposition (CSD). We selected 10ScSZ (10% Sc 2 O 3 , 90% ZrO 2 molar ratio) as the target material, and the precursor solution was prepared by precise calculations. The 10ScSZ solution was deposited on Al2O3 substrate using a spin-coating method. Then, the substrate was sintered using two methods: flash light irradiation and thermal. The characteristics of the thin films were compared, including ionic conductivity, surface morphology, and chemical composition. Pulsed light sintering was applied in the sintering step under a variety of energy density conditions from 80 to 130 J/ cm 2 , irradiation on/off times of 10 ms and 10 ms/500 ms, number of pulses, and bottom heat from 300 to 600 °C. The ionic conductivity of the ScSZ electrolyte layers fabricated by thermal or flash light irradiation methods was tested and compared. The results show that the ScSZ electrolyte layer sintered by flash light irradiation within a few seconds of process time had similar ionic conductivity to the electrolyte layer that was thermal sintered for about 10 h including cooling process.