Chromium deposition and poisoning of cathodes of solid oxide fuel cells – A review

Intermediate temperature solid oxide fuel cells (IT-SOFCs) using chromia-forming alloy interconnect requires the development of cathode not only with high electrochemical activity but also with the high resistance or tolerance towards Cr deposition and poisoning. This is due to the fact that, at SOFC operating temperatures, volatile Cr species are generated over the chromia scale, poisoning the cathodes such as (La,Sr)MnO₃ (LSM) and (La,Sr)(Co,Fe)O₃ (LSCF) and causing a rapid degradation of the cell performance. Thus, a fundamental understanding of the interaction between the Fe–Cr alloys and SOFC cathode is essential for the development of high performance and stable SOFCs. The objective of this paper is to critically review the progress and particularly the work done in the last 10 years in this important area. The mechanism and kinetics of the Cr deposition and Cr poisoning process on the cathodes of SOFCs are discussed. Chromium deposition at SOFC cathodes is most likely dominated by the chemical reduction of high valence Cr species, facilitated by the nucleation agents on the electrode and electrolyte surface and/or at the electrode/electrolyte interface, i.e., the nucleation theory. The driving force behind the nucleation theory is the surface segregation and migration of cationic species on the surface of perovskite oxide cathodes. Overwhelming evidences indicate that the surface segregation plays a critical role in the Cr deposition. The prospect of the development in the Cr-tolerant cathodes for SOFCs is presented.     

Progress in understanding and development of Ba0.5Sr0.5Co0.8Fe0.2O3−δ-based cathodes for intermediate-temperature solid-oxide fuel cells: A review

Solid-oxide fuel cells (SOFCs) convert chemical energy directly into electric power in a highly efficient way. Lowering the operating temperature of SOFCs to around 500-800 °C is one of the main goals in current SOFC research. The associated benefits include reducing the difficulties associated with sealing and thermal degradation, allowing the use of low-cost metallic interconnectors and suppressing reactions between the cell components. However, the electrochemical activity of the cathode deteriorates dramatically with decreasing temperature for the typical La_0.8Sr_0.2MnO₃-based electrodes. The cathode becomes the limiting factor in determining the overall cell performance. Therefore, the development of new electrodes with high electrocatalytic activity for oxygen reduction becomes a critical issue for intermediate-temperature (IT)-SOFCs. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) perovskite oxide was first reported as a potential IT-SOFC cathode material in 2004 by Shao and Haile. After that, the BSCF cathode has attracted considerable attention. This paper reviews the current research activities on BSCF-based cathodes for IT-SOFCs. Emphasis will be placed on the understanding and optimization of BSCF-based materials. The issues raised by the BSCF cathode are also presented and analyzed to provide some guidelines in the search for the new generation of cathode materials for IT-SOFCs.     

Short review on cobalt-free cathodes for solid oxide fuel cells

Cobalt-containing cathodes are known for their ability to operate under high-temperature applications in solid oxide fuel cells (SOFCs). Reducing the operation temperature into intermediate temperature-to-low temperature (IT-LT) zones may lead to a mismatch in the thermal expansion coefficient between the cathodes and the developed IT-LTSOFC electrolyte materials. Hence, cathode materials are adjusted to resolve this issue. Studies on IT-LTSOFC propose cobalt-free cathodes as an alternative way to produce high electrochemical performance cells for operation within the IT-LT range. Novel cobalt-free cathode powders are developed using perovskite structured materials, such as strontium ferrite oxide, as the main components together with dopants. This paper reviews various studies on cobalt-free cathode development, including the most important parameter in determining cathode performance, namely, the polarization resistance of SOFC cathodes.     

PrBaCo2-xTaxO5+δ based composite materials as cathodes for proton-conducting solid oxide fuel cells with high CO2 resistance

PrBaCo₂O_5+δ (PBC) with high catalytic activity is identified as a prospective cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). However, its poor chemical stability hinders its application. To address this problem, a Ta-doping strategy was presented in this study. The cathode with Ta-doping PBC was applied in proton conducting SOFCs. And the influence of Ta-doping on the crystal structure, electrochemical performance, structure stability and electrical conductivity of PBC was investigated. The resistance to CO₂ of PBC at elevated temperature is significantly improved with Ta-doping. The electrochemical performance measurements indicated that a low Ta-doping concentration did not change the performance of the cells obviously, while large Ta-doping concentration could lower the fuel cell performance.     

Development of lanthanum strontium cobalt ferrite perovskite electrodes of solid oxide fuel cells – A review

There is an enormous driving force in solid oxide fuel cells (SOFCs) to reduce the operating temperatures from high temperatures (800–1000 °C) to intermediate and low temperatures (400–800 °C) in order to increase the durability, improve thermal compatibility and thermal cycle capability, and reduce the fabrication and materials costs. One of the grand challenges is the development of cathode materials for intermediate and low temperature SOFCs with high activity and stability for the O₂ reduction reaction (ORR), high structural stability as well as high tolerance toward contaminants like chromium, sulfur and boron. Lanthanum strontium cobalt ferrite (LSCF) perovskite is the most popular and representative mixed ionic and electronic conducting (MIEC) electrode material for SOFCs. LSCF-based materials are characterized by high MIEC properties, good structural stability and high electrochemical activity for ORR, and have played a unique role in the development of SOFCs technologies. However, there appears no comprehensive review on the development and understanding of this most important MIEC electrode material in SOFCs despite its unique position in SOFCs. The objective of this article is to provide a critical and comprehensive review in the structure and defect chemistry, the electrical and ionic conductivity, and relationship between the performance, intrinsic and extrinsic factors of LSCF-based electrode materials in SOFCs. The challenges, strategies and prospect of LSCF-based electrodes for intermediate and low temperature SOFCs are discussed. Finally, the development of LSCF-based electrodes for metal-supported SOFCs and solid oxide electrolysis cells (SOECs) is also briefly reviewed.     

Effects of Gd0.8Ce0.2O1.9−δ coating with different thickness on electrochemical performance and long-term stability of La0.8Sr0.2Co0.2Fe0.8O3-δ cathode in SOFCs

The commercialization of Solid oxide fuel cells (SOFCs) has always been limited by the poor catalytic activity and the severe degradation of cathode in the intermediate and low operating temperature. Here we report a Gd_0.8Ce_0.2O_1.9?δ (GDC) coated La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ (LSCF) composite cathode material, which can significantly improve the electrochemical performance and durability of LSCF cathode. The effects of different GDC coating thickness on the electrochemical performance and long-term working stability of LSCF cathode are investigated, and the optimal coating thickness is established. The polarization impedance of GDC coated LSCF (LSCF@GDC) cathode with 9 nm of GDC coating is 0.08 Ω cm² at 800 °C, which is only one quarter of that of the raw LSCF cathode, and the degradation rate of constant current polarization with 100 mA cm^?2 is only 0.42%/100 h at 700 °C, which is far less than that of the raw LSCF cathode. The X-ray photoelectron spectroscopy (XPS) results show that the degree of Sr segregation decreases with the increase of the thickness of the coated GDC layer. The potential LSCF@GDC composite material is expected to increase the operability of SOFCs and accelerate its commercialization.     

Low-temperature solid oxide fuel cells with La1−xSrxMnO3 as the cathodes

La_1−xSr_xMnO₃ (LSM) has been widely developed as the cathode material for high-temperature solid oxide fuel cells (SOFCs) due to its chemical and mechanical compatibilities with the electrolyte materials. However, its application to low-temperature SOFCs is limited since its electrochemical activity decreases substantially when the temperature is reduced. In this work, low-temperature SOFCs based on LSM cathodes are developed by coating nanoscale samaria-doped ceria (SDC) onto the porous electrodes to significantly increase the electrode activity of both cathodes and anodes. A peak power density of 0.46 W cm⁻² and area specific interfacial polarization resistance of 0.36 Ω cm² are achieved at 600 °C for single cells consisting of Ni-SDC anodes, LSM cathodes, and SDC electrolytes. The cell performances are comparable with those obtained with cobalt-based cathodes such as Sm_0.5Sr_0.5CoO₃, and therefore encouraging in the development of low-temperature SOFCs with high reliability and durability.     

A model study on correlation between microstructure-gas diffusion and Cr deposition in porous LSM/YSZ cathodes of solid oxide fuel cells

The deposition of Cr on cathode materials in solid oxide fuel cells (SOFCs) is complex and impacted by multiple factors. In this report, a mathematical model based on electrochemical Cr-poisoning mechanism is developed to investigate the correlation between gas transport and Cr deposition in porous strontium doped lanthanum manganite/yttria stabilized zirconia (LSM/YSZ) cathode. Time evolution of cathode pore size and three phase boundaries (TPBs) in different cathode regions with Cr₂O₃ deposition is analyzed. The distribution of local current density in the cathode, the lifetime, the concentration polarization, the activation polarization and area specific resistance of SOFC cathodes are subsequently assessed quantitatively. Three types of LSM/YSZ cathode structures with uniform, ascending and descending gradient pore-size distributions are compared. The results show that uniform pore distribution contributes to the best performance when all the cathodes own TPBs with the same initial length.     

Synergistically enhancing CO2-tolerance and oxygen reduction reaction activity of cobalt-free dual-phase cathode for solid oxide fuel cells

High-performance cathodes with adequate CO₂ tolerance are vital for further development of intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, there is always a trade-off between CO₂ tolerance and oxygen reduction reaction (ORR) performance for single-phase cathodes. Here, we report a cobalt-free Ba_0.6La_0.4FeO_3-δ-Ce_0.8Sm_0.2O_2-δ (BLF-SDC) dual-phase cathode with excellent ORR activity and CO₂ tolerance. Introducing ionic conductor Ce_0.8Sm_0.2O_2-δ (SDC) into the Ba_0.6La_0.4FeO_3-δ (BLF) phase can boost ORR activity due to the extended active sites and enhanced oxygen surface exchange process with a polarization resistance of 0.121 Ω cm² for the BLF-30% SDC (weight ratio, BLF-30SDC) cathode at 700 °C. The CO₂ resistance of the BLF-30SDC composite cathode outperforms BLF cathode by three times at 600 °C. This stability enhancement is owing to low CO₂ adsorption of SDC, which is confirmed from thermodynamic calculation. This work indicates that dual-phase mixed conductors can be developed as highly active and stable cathodes for IT-SOFCs.     

Electrochemical performance of nanostructured La0.6Sr0.4CoO3−δ and Sm0.5Sr0.5CoO3−δ cathodes for IT-SOFCs

The electrochemical performance of nanostructured cathodes for IT-SOFCs based on perovskite-type mixed ionic/electronic conductors (MIECs) is investigated. Different compounds (La_0.6Sr_0.4CoO_3−δ and Sm_0.5Sr_0.5CoO_3−δ) and synthesis methods (freeze-drying and citrate complexation) were evaluated. These materials exhibited excellent performance (area-specific resistance values in the range of 0.05-0.20 Ω cm² for an operating temperature of 700 °C), which improved with decreasing grain size. This performance can be attributed to the high specific surface area of these nanostructured cathodes, thus dramatically increasing the number of active sites for the oxygen reduction reaction. Under these conditions, the electrochemical properties are mainly controlled by oxide ion diffusion through the MIEC cathode, which becomes faster with decreasing grain size.     

Scientometric review of advancements in the development of high-performance cathode for low and intermediate temperature solid oxide fuel cells: Three decades in retrospect

Solid oxide fuel cells (SOFCs) have the potential to replace conventional thermal power plants due to their high efficiency and low emission. As the activation loss of the cathode usually limits the SOFC performance, the development of high-performance and durable cathode materials has received extensive attention in the past few decades. It is therefore essential to keep track of the research progress to identify significant research gaps and future directions. In this study, we retrieved the bibliometric data of 1101 cutting-edge research articles focused on cathode development for SOFCs and conducted a scientometric review. Even though significant research in cathode development for intermediate to low temperature SOFCs started in the 1990s, significant growth in the research output appeared in the year 2000 and remarkably continued till 2010 before exhibiting a sinusoidal pattern. Overall, there is a record of average decadal progress in this research area. We found that only a small percentage of countries in the world (i.e., about 29%) are involved in the research for the development of intermediate to low temperature SOFC cathodes. A highlight of core assessment criteria for cathode developments is presented with a summary of the most recent articles (i.e., including those in 2021). This paper can help early-stage researchers, journal outlets, governments, funding authorities, and investors understand the current progress in this area and how close researchers are to a breakthrough that could lead to the commercialization of this emerging technology.     

Influence of microstructure of perovskite-type oxide cathodes on electrochemical performances of proton-conducting solid oxide fuel cells operated at low temperature

The electrochemical performances of proton-conducting SOFCs with the perovskite-type oxide cathodes were investigated at low temperature of 773 K. Among the perovskite-type oxides used in the present study, La_0.7Sr_0.3FeO₃ (LSF) cathode exhibited the lowest overpotential at 773 K. The power density of the SOFC was dependent on the particle size of LSF cathode. The decrease in the particle size resulted in the decrease in overpotential. The power density of the cell with LSF cathode was also dependent on the thickness of LSF cathode; in the present condition, the LSF cathode with 13-μm thickness showed the best electrochemical performance at 773 K.     

Electrochemical performances of spinel oxides as cathodes for intermediate temperature solid oxide fuel cells

The spinel-type oxides of (Mn, Co, Cu)₃O₄ prepared via a citric–EDTA acid process were investigated as candidate cathodes of intermediate temperature solid oxide fuel cells (IT-SOFCs). (Mn, Co)₃O₄ spinel oxide shows a phase transition from tetragonal to cubic when the doping amount of cobalt element increases. Their electric conductivities increase with the cobalt content and are enough high for them used as cathodes of IT-SOFCs. A fuel cell with (Mn, Co)₃O₄ spinel cathode was successfully evaluated based on YSZ electrolyte. (Mn, Co)₃O₄ spinel cathodes show good electrochemical activities, demonstrating the feasibility of the spinel oxide being a cathode of IT-SOFC. As copper doped into (Mn, Co)₃O₄ spinel, the P_peak for Cu_0.5MnCo_1.5O₄ cathode rise to 343, 474 and 506 mW cm⁻² at 700, 750 and 800 °C, respectively. The results reveal that the spinel-type oxides are promising cathodes for IT-SOFCs, especially for Cu_0.5MnCo_1.5O₄.     

Controlling the morphology and uniformity of a catalyst-infiltrated cathode for solid oxide fuel cells by tuning wetting property

Infiltration has been widely used in surface modification of porous electrodes in solid oxide fuel cells (SOFCs). The stability and performance of a porous electrode infiltrated with a catalyst depend sensitively on the composition, morphology, and nanostructure of the catalyst. In this contribution, we report our findings on investigation into the effect of wetting property on the formation of catalyst coatings through an infiltration process. It is observed that aqueous solutions containing catalyst precursors wet SOFC electrolyte materials (e.g., yttria-stabilized zirconia or YSZ) better than cathode materials (e.g., La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ or LSCF). Controlling the wetting of catalyst precursor solutions on porous electrode backbones can dramatically improve the uniformity of the infiltrated catalyst layer on porous cathode backbone, thus enhancing the electrochemical performance of infiltrated cathodes, especially at low operating temperatures.     

Boosting electrocatalytic performance of Ruddlesden-Popper type Fe-based cathode catalysts by La and Ni co-doping for solid oxide fuel cells

Exploring advanced electrode materials with high electrochemical performance and sufficient durability is crucial to the commercialization of solid oxide fuel cells (SOFCs). Herein, a Ruddlesden-Popper Sr_2·9La_0·1Fe_1·9Ni_0·1O_7?δ (SLFN) oxide is systematically evaluated as efficient oxygen electrode material. La and Ni co-doping strategy demonstrates improved oxygen desorption ability and promoted electrochemical activity of pristine Sr₃Fe₂O_7?δ (SF) toward oxygen reduction react (ORR). Further, the ORR process of the SLFN electrode is probed by electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) technique. The button cell with the SLFN cathode delivers a peak power density of 1.01 W cm^?2 at 700 °C, along with desirable stability over a period of 60 h. This study offers a feasible strategy for developing Ruddlesden-Popper type cathode candidates for SOFCs.     

One-step synthesis of CuCo2O4-Sm0.2Ce0.8O1.9 nanofibers as high performance composite cathodes of intermediate-temperature solid oxide fuel cells

One-dimensional nanostructured CuCo₂O₄-Sm_0.2Ce_0.8O_1.9 (SDC) nanofibers are prepared by the electrospinning method and one step sintering as a cathode with low polarization resistance for intermediate temperature solid oxide fuel cells (IT-SOFC). The CuCo₂O₄-SDC nanofibers cathodes form a porous network structure and have large triple-phase boundaries. Correspondingly, the electrochemical performance of the CuCo₂O₄-SDC nanofibers composite cathodes shows significantly improve, achieving the polarization resistance of 0.061 Ω cm² and the maximum power densities of 976 mW·cm⁻² at 750 °C. Thus, these results suggest that CuCo₂O₄-SDC nanofiber could be a highly active cathode material for IT-SOFCs.     

A new solution combustion route to synthesize LiCoO2 and LiMn2O4

Commercially important, high-voltage, lithium cathodes, such as LiCoO₂ and LiMn₂O₄ have been synthesized from nitrates, following the ‘soft-chemistry’ approach using starch as the combustion-assisting component. The minimum temperature required for phase formation and the degree of crystallinity has been evaluated from thermal studies and X-ray diffraction analysis, respectively. The starch-assisted combustion (SAC) method produces mono-dispersed powders of grain size below 1.5 μm as observed from scanning electron microscopy and particle-size analysis. The electrochemical activity of the synthesized oxide powders has been examined via cyclic voltammetric and charge–discharge studies using lithium coin cells.Cyclic voltammetric data shows excellent reversibility with respect to Li⁺ and confirms the effect of crystallinity of the compounds on the electrochemical performance of the cathode materials. The electrochemical stability and performance of the cathodes over 30 cycles have been demonstrated with a capacity fade of <10% of the initial capacity. The simplicity and flexibility of this approach towards the synthesis of various other cathode materials is also discussed.     

A cathode-supported solid oxide fuel cell prepared by the phase-inversion tape casting and impregnating method

Cathode-supported Solid Oxide Fuel Cells (SOFCs) have unique advantages of stability and operating life, but the commercialization process is limited by manufacturing cost and poor electrochemical performance. In this paper, a cathode-supported SOFC with 3YSZ-LSM95| porous 8YSZ| dense 8YSZ| porous 8YSZ sandwich structure was successfully fabricated by phase-inversion tape casting and co-sintering method. The cathode support demonstrated finger shaped macropore with high porosity. The long-term stability of symmetric cells with and without impregnated LSC nanoparticals was evaluated and no obvious degregadion were observed. The peak power densities of single cell reached 464, 209, 271 and 144 mW cm^?2 at 850, 800, 750 and 700 °C respectively when Ni nano-particles as the anode catalyst and LSC nano-particles as the cathode catalyst, showing a significant improvement in electrochemical performance compared with non-LSC cell. Additionally, the distribution of relaxation times (DRT) method was empoyed to analysis the polarization process at high-resolution, for better understanding the mechanism of electrochemical reaction of cells. The results indicated the impregnated LSC particles can increase the triple phase-boundaries (TPBs) for fast oxygen reduction reaction and improve the electrochemical performance. However, the optimization of anode and cathode are needed in the future work.     

Air plasma spray processing and electrochemical characterization of SOFC composite cathodes

Air plasma spraying has been used to produce porous composite cathodes containing (La_0.8Sr_0.2)_0.98MnO_3−y (LSM) and yttria-stabilized zirconia (YSZ) for use in solid oxide fuel cells (SOFCs). Preliminary investigations focused on determining the range of plasma conditions under which each of the individual materials could be successfully deposited. A range of conditions was thereby determined that was suitable for the deposition of a composite cathode from pre-mixed LSM and YSZ powders. A number of composite cathodes were produced using different combinations of parameter values within the identified range according to a Uniform Design experimental grid. Coatings were then characterized for composition and microstructure using EDX and SEM. As a result of these tests, combinations of input parameter values were identified that are best suited to the production of coatings with microstructures appropriate for use in SOFC composite cathodes. A selection of coatings representative of the types of observed microstructures were then subjected to electrochemical testing to evaluate the performance of these cathodes. From these tests, it was found that, in general, the coatings that appeared to have the most suitable microstructures also had the highest electrochemical performances, provided that the deposition efficiency of both phases was sufficiently high.     

(Pr0.9La0.1)2(Ni0.74Cu0.21Nb0.05)O4+δ-Ce0.9Gd0.1O2−δ (GDC) as an active and CO2-tolerant nano-composite cathode for intermediate temperature solid oxide fuel cells

Nano composite of Ruddlesden-Popper electrocatalyst-oxygen ionic conductor, (Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Nb_0.05)O_4+δ (PLNCN)-Ce_0.9Gd_0.1O_2?δ (GDC), is developed as composite cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs) via a infiltration way. The electrochemical behavior of PLNCN-GDC nanostructured electrode is assessed with respect to infiltration loading and oxygen partial pressure. The optimized PLNCN loading can improve charge transfer dynamics for electrochemical oxygen reduction reaction, thus promoting cathode performance. Importantly, the encouraging results of single cell highlight high activity and good CO₂ tolerance of PLNCN-GDC nanostructured cathode.