Electrode materials and reaction mechanisms in solid oxide fuel cells: a brief review

Following previous surveys of the solid electrolyte ceramics and electrode reaction mechanisms in solid oxide fuel cells, this review is focused on the comparative analysis of electrochemical performance, thermal expansion, oxygen ionic and electronic transport, and durability-determining factors in the major groups of electrode materials. The properties of mixed-conducting oxide phases with perovskite-related and fluorite structures, ceramic–metal and oxide composites, and catalytically active additives are briefly discussed, with emphasis on the approaches and findings reported during the last 10–15 years. The performance of conventional and alternative electrode materials in the cells with ZrO₂-, CeO₂-, LaGaO₃-, and La₁₀Si₆O₂₇-based electrolytes is appraised in the context of potential optimization strategies. Particular attention is centered on the cathode and anode compositions providing maximum electrochemical activity and stability and on the critical aspects relevant for electrode microstructure engineering.     

Study of transition metal oxide doped LaGaO3 as electrode materials for LSGM-based solid oxide fuel cells

Transition metal oxide doped lanthanum gallates, La_0.9Sr_0.1Ga_0.8M_0.2O₃ (where M=Co, Mn, Cr, Fe, or V), are studied as mixed ionic-electronic conductors (MIECs) for electrode applications. The electrochemical properties of these materials in air and in H₂ are characterized using impedance spectroscopy, open cell voltage measurement, and gas permeation measurement. Three single cells based on La_0.9Sr_0.1Ga_0.8 Mg_0.2O₃ (LSGM) electrolyte (1.13 to 1.65 mm thick) but with different electrode materials are studied under identical conditions to characterize the effectiveness of the lanthanum gallate-based MIECs for electrode applications. At 800 °C, a single cell using La_0.9Sr_0.1- Ga_0.8Co_0.2O₃ as the cathode and La_0.9Sr_0.1Ga_0.8Mn_0.2O₃ as the anode shows a maximum power density of 88 mW/cm², which is better than that of a cell using Pt as both electrodes (20 mW/cm²) and that of a cell using La_0.6Sr_0.4CoO₃ (LSC) as the cathode and CeO₂-Ni as the anode (61 mW/cm²) under identical conditions. The performance of LSGM-based fuel cells with MIEC electrodes may be further improved by reducing the electrolyte thickness and by optimizing the microstructures of the electrodes through processing. Received: 9 January 1998 / Accepted: 1 May 1998     

Electrode materials and reaction mechanisms in solid oxide fuel cells: a brief review. III. Recent trends and selected methodological aspects

Continuing previous reviews on mixed-conducting electrodes for intermediate-temperature solid oxide fuel cells (IT SOFCs), this work presents a short overview of novel cathode and anode materials, their electrochemical performance in contact with oxygen anion- and proton-conducting solid electrolytes, and specific features determining possible applications. Priority was given mainly to recent research reports published during the last 2–5 years. Particular emphasis is focused on the relevant methodological aspects, potential limitations and drawbacks, and factors affecting electrode polarization and durability. Typical ranges of the polarization resistances, overpotentials, power densities in the cells with various current collectors, and the electrode materials total conductivity and thermal expansion are compared. The electrode compositions appraised in single-chamber and micro-SOFCs, hydrocarbon- and carbon-fueled cells, high-temperature electrolyzers, and other solid-electrolyte appliances are briefly covered in light of their similarity to the common SOFC materials discussed in the previous parts.     

Cathode materials for solid oxide fuel cells: a review

The composition and microstructure of cathode materials has a large impact on the performance of solid oxide fuel cells (SOFCs). Rational design of materials composition through controlled oxygen nonstoichiometry and defect aspects can enhance the ionic and electronic conductivities as well as the catalytic properties for oxygen reduction in the cathode. Cell performance can be further improved through microstructure optimization to extend the triple-phase boundaries. A major degradation mechanism in SOFCs is poisoning of the cathode by chromium species when chromium-containing alloys are used as the interconnect material. This article reviews recent developments in SOFC cathodes with a principal emphasis on the choice of materials. In addition, the reaction mechanism of oxygen reduction is also addressed. The development of Cr-tolerant cathodes for intermediate temperature solid oxide fuel cells, and a possible mechanism of Cr deposition at cathodes are briefly reviewed as well. Finally, this review will be concluded with some perspectives on the future of research directions in this area.     

X-ray absorption spectroscopic studies on solid oxide fuel cells and proton-conducting ceramic fuel cells

X-ray absorption spectroscopy (XAS) is one of the best techniques to obtain the information on the electronic and local structures of materials. In the last few decades, XAS becomes a common analytical technique for the investigation of solid oxide fuel cells and proton-conducting ceramic fuel cells. In particular, operando and/or advanced XAS measurements can be recently available with the increased accessibility of synchrotron radiation. In this article, recent trends of solid oxide fuel cell and proton-conducting ceramic fuel cell researches using XAS are overviewed.     

Effect of pretreatments on the anode structure of solid oxide fuel cells

An important objective in the development of solid oxide fuel cell (SOFC) is to produce thin stabilized zirconia electrolytes that are supported upon the nickel–zirconia composite anode. Although this will reduce some of the problems associated with SOFCs by permitting lower temperature operation, this design may encounter problems during start- up. The first step in a start-up involves the reduction of nickel oxide in the anode to metallic nickel and increase of three-phase boundary will be beneficial for further reaction. In this study, two pretreatment methods are investigated for their effects on the performances of SOFC. Performances of the SOFCs are influenced by the pretreatment conditions, which included exposure of the cells to dilute H₂/O₂ either under open-circuit or closed-circuit conditions before their performance studies. By carrying out the methods, the pretreatment using the closed circuit is found to attain much higher performances effectively and efficiently. Accompanying with SEM and element analysis, increase of three-phase boundary is considered to give rise to changes in the anode microstructure, leading to activation of the anode. Mechanisms of NiO in anode reducing to Ni and porous structure via different pretreatments and their effects on the anode microstructure are proposed.     

Separation of cationic and anionic conductivity constituents in solid polymer electrolytes comprising a copolymer of acrylonitrile and butadiene (40:60) and lithium hexafluoroarsenate

A technique for separating cationic and anionic constituents of the ionic conductivity of amorphous solid polymer electrolytes in the region of low salt concentrations is proposed. The technique employs model equations for separately describing transport of cations and anions that differently interact with the polymer matrix. It is shown that the temperature dependences of the anionic conductivity obey the Vogel-Tamman-Fulcher equation and those of the cationic conductivity, the Miyamoto-Shibayama equation. By separating the overall ionic conductivity into constituents, cationic transport numbers are evaluated in broad ranges of salt concentrations and temperatures in the system comprising a copolymer of acrylonitrile and butadiene (40:60) and lithium hexafluoroarsenate. At nearly ambient temperatures the anionic constituent is predominant. Two different cation transport mechanisms are established at elevated temperatures.     

Neutron powder diffraction as a characterization tool of solid oxide fuel cell materials

Neutron diffraction is a powerful tool for the characterization of materials and, particularly, oxides. Oxide materials find applications in solid oxide fuel cells (SOFCs) as solid electrolytes as well as anode and cathode materials. As a structural probe, neutrons are specially suitable for the crystallographic study of oxides, given the comparable scattering factors of O and other heavier elements, allowing its precise localization in the crystal structure. Many problems can be addressed by neutrons, related to the octahedral tilting in perovskites, phase transitions, order–disorder phenomena, presence of anionic vacancies, etc. Neutrons make possible an accurate determination of the thermal factors and provide a visualization of the diffusion paths in ionic conductors. Neutrons allow the localization of light atoms such as hydrogen, and make possible the distinction between neighbouring elements, typically Fe and Mn. In this work we will describe some recent applications of this technique in the field of solid electrolytes and electrode materials, including some examples from our group.     

通过溅射与退火制备的用于固体氧化物燃料电池的氧化钆掺杂氧化铈电解质隔层

采用溅射或溅射与退火相结合的方法制备了一系列氧化钆掺杂的氧化铈（GDC）隔层,并考察了其对固体氧化燃料电池性能的影响. 结果表明,200 ℃下溅射获得了立方结构氧化钆掺杂的氧化铈均匀薄膜,在900-1100 ℃范围内的退火处理使得GDC薄膜致密,从而有效阻止了氧化钇掺杂的氧化锆电解质与阴极材料之间的反应,大幅度提高了电池的电化学性能.     

LaNi0.6Fe0.4O3 as a cathode contact material for solid oxide fuel cells

In solid oxide fuel cells (SOFCs) the interconnects electrically link air and fuel electrodes on either side to produce a practical electrical power output. The long-term stability of intermediate temperature (650–800 °C) SOFC operation strongly depends on the composition of the ferritic steel interconnection material and the steel/ceramic interface. During high-temperature operation the Cr-containing ferritic steel forms an oxide scale at its surface, thereby causing high ohmic electrical contact resistance when connected to the surface of an electronically conducting ceramic cathode material. In the long run, the vaporization of Cr species from these oxide scales also affects the cathode activity, eventually leading to cell deterioration. One way of overcoming the problem is to incorporate another electronically conducting ceramic compliant layer, commonly known as the contact layer, between the cathode and metallic interconnect. In this contribution, LaNi_0.6Fe_0.4O₃ was tested as a cathode contact material. Its performance at 800 °C in the form of a ~50 μm thick film applied on two ferritic steel compositions was examined. After 600 h of testing, contact resistances of 60 and 160 mΩ cm² were obtained. The different values are explained by the variation in steel composition.     

High temperature thermo-photocatalysis driven carbon removal in direct biogas fueled solid oxide fuel cells

Solid oxide fuel cells (SOFCs) can directly convert renewable biogas into electricity with high efficiency at high temperature, however the long-term stability of SOFCs is significantly affected by the carbon deposition on the anode during cell operation. Herein, we report a novel carbon removal approach by high temperature infrared light driven photocatalytic oxidation. Upon the comparison of electrochemical performance of Ni-YSZ anode and TiO₂ modified Ni-YSZ anode in the state-of-the-art single cell (Ni-YSZ/YSZ/LSCM), the modified anodes exhibit markedly improved peak powder density with simulated biogas fuel (70% CH₄+ 30% CO₂) at 850 °C with less coking after 40 h operation. The high activity and carbon deposition resistance of the modified anode is possibly attributed to the in situ generated hydroxyl radical from the reduced TiO_x powder under high temperature infrared light excitation, which is supported by detailed analysis of microstructural information of anodes and the powder-based thermo-photocatalytic experiments.     

Effect of polarization on the electrode behavior and microstructure of (La,Sr)MnO<Subscript>3</Subscript> electrodes of solid oxide fuel cells

The electrode behavior and microstructure of freshly prepared (La_0.8Sr_0.2)_0.9MnO₃ (LSM) electrodes were investigated under various polarization conditions. The original, large agglomerates in freshly prepared LSM electrodes were broken down into sphere-shaped grains when exposed to cathodic or anodic current passage of 200 mA cm^–2 at 800 °C in air for 3 h. Microstructural changes under cathodic polarization could be related to the pronounced diffusion and migration of oxygen vacancies and Mn ions on the LSM surface and lattice expansion, while lattice shrinkage under oxidation conditions most likely contributes to the structural changes under anodic polarization. Such morphological changes were irreversible and were found to be beneficial to the performance of freshly prepared LSM electrodes. Freshly prepared LSM electrodes behaved very differently with respect to the cathodic and anodic current passage treatment.     

Anodically electrodeposited Co+Ni mixed oxide electrode: preparation and electrocatalytic activity for oxygen evolution in alkaline media

Co+Ni mixed oxides on Ni substrate were prepared through anodic electrodeposition from Co(NO₃)₂ and Ni(NO₃)₂ aqueous solutions with five different Co²⁺/Ni²⁺ ratios beside only Co²⁺. By the electrochemical measurements, the optimum performance in electrocatalytic activity for oxygen evolution reaction in alkaline media was obtained on the Co+Ni mixed oxide deposited from the solution containing Co²⁺/Ni²⁺ ratio of 1:1. The mixed oxide is corresponding to about 68 at% Co contents with spinel-type NiCo₂O₄ phase and porosity surface structure. The electrochemical kinetic parameters including exchange current density, Tafel slopes, reaction order with respect to [OH⁻] and standard electrochemical enthalpy of activation were analyzed also. A possible mechanism involving the formation of a physisorbed hydrogen peroxide intermediate in a slow electrochemical step was presented, which accounts for the values of the experimental results.     

Design of materials for solid oxide fuel cells,permselective membranes,and catalysts for biofuel transformation into syngas and hydrogen based on fundamental studies of their real structure,transport properties,and surface reactivity

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Oxygen ionic and electronic transport in apatite-type La10−x(Si,Al)6O26±δ

The main factor governing the oxygen ionic conductivity in apatite-type La_10−xSi_6−yAl_yO_{27−3x/2−y/2} (x=0-0.33; y=0.5-1.5) is the concentration of mobile interstitials determined by the total oxygen content. The ion transference numbers, measured by modified faradaic efficiency technique, vary in the range 0.9949-0.9997 in air and increase on reducing oxygen partial pressure due to decreasing p-type electronic conduction. The activation energies for ionic and hole transport are (56-67)±3 kJ/mol and (57-100)±8 kJ/mol, respectively. Increasing oxygen content leads to higher hole conduction in oxidizing atmospheres and promotes minor oxygen losses from the lattice when the oxygen pressure decreases, although the overall level of ionic conductivity is almost constant in the p(O₂) range from 50 kPa down to 10⁻¹⁶ Pa. Under reducing conditions at temperatures above 1100 K, silicon oxide volatilization from the surface layers of apatite ceramics results in a moderate decrease of the conductivity with time. This suggests that the operation of electrochemical cells with silicate-based solid electrolytes should be limited to the intermediate-temperature range, such as 800-1000 K, where the ionic transport in most-conductive apatite phases containing 26.50-26.75 oxygen atoms per unit formula is higher than that in stabilized zirconia. The average thermal expansion coefficients of apatite ceramics, calculated from dilatometric data in air, are (8.7-10.8)×10⁻⁶ K⁻¹ at 300-1300 K.     

Development of an O-ring from NR/EPDM filled silica/CB hybrid filler for use in a solid oxide fuel cell testing system

This research presents the effects of oxygen pressure and ambient temperatures on the crack behavior of O-rings from a semi-EV of NR/EPDM rubber with silica/CB filler, exposed to the inlet flow and outflow oxygen pressure in a Solid Oxide Fuel Cell (SOFC) environment. Blends of NR/EPDM were prepared with various ratios of silica/CB filler at 00/60, 10/50, 20/40, 30/30, 40/20, 50/10, and 60/00 phr. The fabricated O-ring complied with the standard for O-rings (TIS 2728-2559), with a minimum hardness of 65–75 Shore A, minimum tensile strength of 9 MPa, minimum elongation at break of 200%, and a minimum 100% modulus of 2.7 MPa. The mechanical properties of the compounds were tested, and the appropriate compound was chosen to make the O-rings to test in SOFC. The crack morphology of the fabricated O-rings was investigated and compared with a commercial O-ring after testing in the SOFC. As a result, the compound with silica/CB of 40:20 ratio provided the optimum mechanical properties, and passed the criteria standard of TIS 2728-2559. The mechanical properties of the prepared and commercial O-rings were similar (P-value of commercial with 60/00 = 0.273, 50/10 = 0.273, 40/20 = 0.144, 30/30 = 0.465, 20/40 = 0.465, 10/50 = 1.000 and 00/60 = 0.273; all > 0.05) and and both could still be continued to be used in SOFC despite some inner cracks after 24 h. The price of the prepared O-ring is cheaper than the commercial O-rings due to the low price of NR used in its formulation. Therefore, a prepared O-ring can be used in a SOFC, or other applications due to their mechanical properties and their reasonable price.     

Biogas as a fuel for solid oxide fuel cells and synthesis gas production: effects of ceria-doping and hydrogen sulfide on the performance of nickel-based anode materials

Numerous investigations have been carried out into the conversion of biogas into synthesis gas (a mixture of H(2) + CO) over Ni/YSZ anode cermet catalysts. Biogas is a variable mixture of gases consisting predominantly of methane and carbon dioxide (usually in a 2 : 1 ratio, but variable with source), with other constituents including sulfur-containing gases such as hydrogen sulfide, which can cause sulfur poisoning of nickel catalysts. The effect of temperature on carbon deposition and sulfur poisoning of 90 : 10 mol% Ni/YSZ under biogas conversion conditions has been investigated by carrying out a series of catalytic reactions of methane-rich (2 : 1) CH(4)/CO(2) mixtures in the absence and presence of H(2)S over the temperature range 750-1000 °C. The effect of ceria-doping on carbon dioxide reforming, carbon deposition and sulfur tolerance has also been investigated by carrying out a similar series of reactions over ceria-doped Ni/YSZ. Ceria was doped at 5 mol% of the nickel content to give an anode catalyst composition of 85.5 : 4.5 : 10 mol% Ni/CeO(2)/YSZ. Reactions were followed using quadrupolar mass spectrometry (QMS) and the amount of carbon deposition was analysed by subjecting the reacted catalyst samples to a post-reaction temperature programmed oxidation (TPO). On undoped Ni/YSZ, carbon deposition occurred predominantly through thermal decomposition of methane. Ceria-doping significantly suppressed methane decomposition and at high temperatures simultaneously promoted the reverse Boudouard reaction, significantly lowering carbon deposition. Sulfur poisoning of Ni/YSZ occurred in two phases, the first of which caused the most activity loss and was accelerated on increasing the reaction temperature, while the second phase had greater stability and became more favourable with increasing reaction temperature. Adding H(2)S significantly inhibited methane decomposition, resulting in much less carbon deposition. Ceria-doping significantly increased the sulfur tolerance of Ni/YSZ, however, in the presence of H(2)S ceria did not promote the reverse Boudouard reaction and at high temperatures carbon deposition was greater over ceria-doped Ni/YSZ. In order to further study the effects of ceria-doping, a solid oxide fuel cell (SOFC) was constructed with a ceria-doped anode cermet and its electrical performance on simulated biogas compared to hydrogen was tested. This fuel cell was subsequently ran for 1000 h on simulated biogas with no degradation in its overall electrical performance.     

Melt crystallization and segmental dynamics of poly(ethylene oxide) confined in a solid electrolyte composite

The isothermal melt crystallization and the corresponding segmental dynamics, of a high molecular weight poly(ethylene oxide) (PEO) confined by Li₇La₃Zr₂O₁₂ (LLZO) particles in solid electrolyte composites, were monitored by differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS), respectively. Our results show that the overall crystallinity is positively correlated with the surface area of LLZO particles. The primary and secondary crystallization processes are identified by a modified Avrami equation, while two dynamic modes, the α relaxation and α′ relaxation, were in the DRS measurements. The results reveal an unambiguous correlation between the primary crystallization and the α relaxation, while a correlation between the second crystallization and the α′ relaxation concurrently exist in the electrolyte composites. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 466–477     

Identifying the O2 diffusion and reduction mechanisms on CeO2 electrolyte in solid oxide fuel cells: A DFT + U study

The interactions and reduction mechanisms of O₂ molecule on the fully oxidized and reduced CeO₂ surface were studied using periodic density functional theory calculations implementing on‐site Coulomb interactions (DFT + U) consideration. The adsorbed O₂ species on the oxidized CeO₂ surface were characterized by physisorption. Their adsorption energies and vibrational frequencies are within ?0.05 to 0.02 eV and 1530–1552 cm^?1, respectively. For the reduced CeO₂ surface, the adsorption of O₂ on Ce⁴⁺, one‐electron defects (Ce³⁺ on the CeO₂ surface) and two‐electron defects (neutral oxygen vacancy) can alter geometrical parameters and results in the formation of surface physisorbed O₂, O₂^a? (0 < a < 1), superoxide (O₂^?), and peroxide (O₂^2?) species. Their corresponding adsorption energies are ?0.01 to ?0.09, ?0.20 to ?0.37, ?1.34 and ?1.86 eV, respectively. The predicted vibrational frequencies of the peroxide, superoxide, O₂^a? (0 < a < 1) and physisorbed species are 897, 1234, 1323–1389, and 1462–1545 cm^?1, respectively, which are in good agreement with experimental data. Potential energy profiles for the O₂ reduction on the oxidized and reduced CeO₂ (111) surface were constructed using the nudged elastic band method. Our calculations show that the reduced surface is energetically more favorable than the unreduced surface for oxygen reduction. In addition, we have studied the oxygen ion diffusion process on the surface and in bulk ceria. The small barrier for the oxygen ion diffusion through the subsurface and bulk implies that ceria‐based oxides are high ionic conductivity at relatively low temperatures which can be suitable for IT‐SOFC electrolyte materials. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Development of a voltage-applicable heating specimen holder for in situ observations of solid oxide fuel cells with an environmental transmission electron microscope

A voltage-applicable heating specimen holder is developed to observe solid oxide fuel cells (SOFCs) in an environmental transmission electron microscope. An SOFC specimen can be heated and has a voltage applied in an oxygen gas atmosphere using the holder. Oxygen ion migration and redox reactions in the specimen could also be realized. The heating unit, which consists of nickel–chromium alloy mounted on the tip of the developed specimen holder, can heat the specimen up to 1200 K in an oxygen gas atmosphere. A specimen preparation method for the SOFC structure is also established using a focused ion beam technique. The holder has high stability for high-resolution imaging and electron energy loss spectroscopy under the in situ condition. The mechanical and electrical stabilities are estimated from high-resolution images and electron energy loss spectra of the heated and voltage-applied specimen in an oxygen gas atmosphere. The developed holder is a powerful tool to reveal the microstructural and electronic structural changes that occur by electrochemical reactions at the interfaces of an SOFC.