Fe-based Oxides Research Articles

Synergistic Interplay between Fe-based Perovskite Oxides and Co in Electrolyte for Efficient Oxygen Evolution Reaction.

Perovskite oxides have been extensively investigated as active electrocatalysts for the oxygen evolution reaction (OER) in alkaline solution. While the OER activity of some perovskite oxides is positively influenced by Fe ions in the electrolyte, the impact of other transition metal ions in the electrolyte remains unclear. In this study, we compared the influence of Co ions intentionally added to the electrolyte on the OER activities of two Fe-based perovskite oxides (Ba0.5Sr0.5FeO3-δ and LaFeO3). While the OER activity ofBa0.5Sr0.5FeO3-δis significantly enhanced by adding Co ions to the electrolyte, LaFeO3 showed little difference in the OER behavior between the Co-free and Co-containing electrolytes. In the case of Ba0.5Sr0.5FeO3-δ, an amorphous layer was formed, and the Co ions from the electrolyte were incorporated on the surface as a result of OER cycling. On the other hand, Co ions were also detected on the surface ofLaFeO3, but its crystalline structure remains unchanged during the OER. Our study suggests that synergistic interplay between the perovskite oxides undergoing a structural transformation at the surface and transition metal ions in the electrolyte can improve the OER activity.

Preparation and Properties of Fe-Based Double Perovskite Oxide as Cathode Material for Intermediate-Temperature Solid Oxide Fuel Cell

Double perovskite oxides with mixed ionic and electronic conductors (MIECs) have been widely investigated as cathode materials for solid oxide fuel cells (SOFCs). Classical Fe-based double perovskites, due to their inherent low electronic and oxygen ionic conductivity, usually exhibit poor electrocatalytic activity. The existence of various valence states of B-site ions modifies the material’s catalytic activity, indicating the possibility of the partial substitution of Fe by higher-valence ions. LaBaFe2−xMoxO5+δ (x = 0, 0.03, 0.05, 0.07, 0.1, LBFMx) is used as intermediate-temperature solid oxide fuel cell (IT-SOFC) cathode materials. At a doping concentration above 0.1, the Mo substitution enhanced the cell volume, and the lattice expansion caused the formation of the impurity phase, BaMoO4. Compared with the parent material, Mo doping can regulate the oxygen vacancy concentration and accelerate the oxygen reduction reaction process to improve the electrochemical performance, as well as having a suitable coefficient of thermal expansion and excellent electrode stability. LaBaFe1.9Mo0.1O5+δ is a promising cathode material for IT-SOFC, which shows an excellent electrochemical performance, with this being demonstrated by having the lowest polarization resistance value of 0.017 Ω·cm2 at 800 °C, and the peak power density (PPD) of anode-supported single-cell LBFM0.1|CGO|NiO+CGO reaching 599 mW·cm−2.

An active and Cr-tolerant high-entropy La0.7Sr0.3FeO3-δ based cathode for solid oxide fuel cells

The development of high active and stable cathode materials is one of the main challenges of intermediate temperature solid oxide fuel cells (SOFCs). Herein, a high-entropy Fe-based perovskite oxide La0.7Sr0.3Fe0.3Ni0.3Co0.3Mo0.1O3-δ (LSFNCM) is synthesized by self-propagation combustion based on La0.7Sr0.3FeO3-δ (LSF) and the electrochemical properties are systematically tested. The symmetric half-cell with the LSFNCM cathode has a high catalytic activity with a polarization impedance of 0.12 Ω·cm2 at 800 °C, and also shows outstanding CO2 resistance and chromium resistance owing to the entropy stabilization strategy. After 50h of treatment in a 10% CO2 environment, the polarization impedance of the LSF electrode increases by 8.89 Ω·cm2, while that of the LSFNCM electrode only increases by 1.81 Ω·cm2. Additionally, after being treated for 24h in a chromium vapor environment, the polarization impedance of the LSF electrode increases by 5.95 Ω·cm2, whereas that of the LSFNCM electrode only increases by 0.44 Ω·cm2. Moreover, the single cell with LSFNCM cathode exhibits the maximum power density of 932.82 mW/cm2 and the polarization impedance of 0.34 Ω·cm2 at 800 °C with humidify hydrogen as fuel. Our results indicate that high-entropy perovskite oxide LSFNCM is an active and Cr-tolerant promising cathode for SOFCs.

Insights into the role of A/B-site substitution in chemical looping gasification of cotton stalk for enhanced syngas production over La-Co-O based perovskite oxygen carriers

Biomass chemical looping gasification (BCLG) is an emerging technology for efficient and clean utilization of cotton stalk (CS) to produce high-quality syngas. Among various oxygen carriers, perovskite oxides are holding an ever-increasing position in BCLG due to their unique structural properties and compositional flexibilities. However, research on perovskite-type oxygen carriers mostly focused on Fe-based oxides, and there is little in-depth investigation of Co-based perovskite and the role of A/B site substitution in the BCLG process. Herein, the LaCoO3 perovskite is selected as the basic oxygen carrier, and Sr, Fe are further doped on the A/B-site to form LaCo1-xFexO3 (x = 0, 0.2, 0.4, 0.6, 0.8, 1) and La1-ySryCoO3 (y = 0, 0.2, 0.4, 0.6, 0.8) series. Effects of perovskite type, gasification temperature, steam volume fraction and oxygen carrier mass fraction of the BCLG performance are investigated. Results indicate that La0.6Sr0.4CoO3 and LaCo0.2Fe0.8O3 exhibit enhanced syngas production with the maximum of 1.304 m3/kg and 1.188 m3/kg, respectively, and outstanding cyclic stability at optimal reaction conditions. Further characterizations including H2-TPR, XPS and EPR analysis reveal that Sr substitution facilitate the formation of oxygen vacancies and adsorbed oxygen species, while Fe doping leads to the increasing concentration of oxygen vacancies and surface lattice oxygen species. Combined with the experimental and characterization results, it is deduced that the oxygen vacancies which promote the adsorption of reactants and accelerate the migration of bulk lattice oxygen, play the key role in the enhanced BCLG performance.

Fe-Based High-Entropy Perovskite Oxide: A Strategy to Suppress Sr Segregation and Performance Evaluation as an Electrode Material for SOFCs

Fe-Based High-Entropy Perovskite Oxide: A Strategy to Suppress Sr Segregation and Performance Evaluation as an Electrode Material for SOFCs

One-step rapid prototyping of recyclable ultrathin CNF/MWCNT/Fe-based spinel oxide asymmetric interdigital nanopaper electrodes for high performance flexible supercapacitors

Flexible asymmetric supercapacitors (FASCs) are potential energy storage devices for wearable electronics, considering that these have a wide operating voltage window, high specific capacity, and high energy density. However, achieving high energy density through the optimization of structure and electrode remains a challenge. Herein, two electrode inks with exceptional homogeneity nature and stability are prepared by using in-situ grown Fe-based spinel oxide (MFe2O4 = Fe3O4/MnFe2O4) on multi-walled carbon nanotubes (MWCNT) combined with eco-friendly and renewable cellulose nanofiber (CNF). Meanwhile, ultrathin flexible asymmetric interdigital nanopaper electrodes (NFT@MnFe and NFT@Fe) with customized patterns are fabricated through a one-step mask-assisted vacuum filtration strategy by using the obtained electrode inks. The planar interdigital FASC device based on the NFT@MnFe cathode and the NFT@Fe anode has excellent area specific capacity (309.8 mF cm−2), energy density (110.2 μW h cm−2), and power density (0.47 mW cm−2). The FASC maintain over 95 % of the original capacity after 5000 charge/discharge cycles. Noteworthy, a stacked interdigital FASC can be achieved with an energy density of up to 273.8 μW h cm−2 at 0.47 mW cm−2 power density, combining the structural benefits of planar interdigital and sandwich-type devices.

A New High-Valent Fe Redox Couple in Intercalation Electrodes

The overwhelming majority of layered oxide cathode materials are based on the redox activity of expensive and scarce transition metals like Co and Ni. While the Fe-redox based material LiFePO4 has become increasingly attractive due to its low cost, its low gravimetric energy density and relatively low operating voltage limit its use cases. A high-voltage Fe-based layered oxide cathode would provide high energy density utilizing the redox-activity of the most abundant transition metal in Earth’s crust, but such a material that provides stable and reversible electrochemical performance remains elusive. Indeed, high-voltage Fe-redox in oxides is plagued by cation disorder and oxygen dimerization, which leads to capacity fading and large voltage hysteresis (> 1 V).[1]In contrast, the layered oxide cathode Li4FeSbO6 (equivalently, Li[Li1/3Fe1/3Sb1/3]O2, abbreviated LFSO) exhibits <0.2 V of hysteresis, and a discharge plateau at 4.2 V versus Li/Li+ with two-electron capacity.[2] Both the exceptional electrochemical reversibility of this material and its phase separating nature upon (de)intercalation raise questions about the origin of these properties that are unusual for layered Fe oxide cathodes. In this work, we will discuss our efforts to probe the electronic structure of LFSO with Fe L–edge x–ray absorption (XAS) and 57Fe Mössbauer spectroscopy, as well as O K–edge resonant inelastic x–ray scattering spectroscopy (RIXS). The combination of these element-specific and valence-state sensitive techniques, in conjunction with charge-transfer multiplet and density functional theory calculations, allows us to reveal the electronic structure changes at Fe and O as a function of state of charge for the first time. Correlating these electronic structure changes with synchrotron diffraction measurements, we reveal a unique redox compensation mechanism that readily rationalizes the exceptional electrochemical reversibility, unlocking a new direction for high-voltage, Fe-based redox systems.

Ab Initio Investigation of Oxygen Ion Diffusion in the Layered Perovskite System YSr2Cu2FeO7+δ (0 &lt; δ &lt; 1)

Extensive research on transition metal perovskite oxides as electrodes in solid oxide cells (SOC) has highlighted the potential ability of Fe-based perovskite oxides to catalyze oxygen reduction/evolution reactions (ORR/OER). The layered perovskite-type system YSr2Cu2FeO7+δ has been reported to possess attractive electrocatalytic properties. This work applies density functional theory (DFT) calculations to investigate oxygen ion diffusion in the YSr2Cu2FeO7+δ system. For δ = 0.5, it is found that in the most stable configuration, the oxygen vacancies in the FeO1+δ plane are arranged to form Fe ions in tetrahedral, square pyramid, and octahedral coordination. Ab initio molecular dynamics (AIMD) simulations for YSr2Cu2FeO7.5 (δ = 0.5) yield an oxygen ion diffusion coefficient of 1.28 × 10−7 cm2/s at 500 °C (Ea = 0.37 eV). Complementary results for YSr2Cu2FeO7.2 (δ = 0.2) and YSr2Cu2FeO7.75 (δ = 0.75) indicate that the oxygen diffusion occurs in the FeO1+δ plane, and depends on the oxygen vacancies distribution around the Fe centers.

Improved electrochemical performance of high-entropy La0.8Sr0.2FeO3-based IT-SOFC cathode

It is widely accepted that solid oxide fuel cells (SOFCs) have the potential to be the most efficient and cost-effective system for the direct conversion of various fuels into electricity with low environmental pollution and high efficiency. Herein, we demonstrate a novel Fe-based high-entropy perovskite oxide as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The high-entropy perovskite La0.2Pr0.2Sm0.2Nd0.2Sr0.2FeO3-δ (HE-LSF), composed of La, Pr, Nd, Sm, and Sr is prepared by incorporating five components into the A-site of La0.8Sr0.2FeO3-δ (LSF). Doping of Pr, Nd, Sm, and Sr into the A-site significantly enhances the redox stability of LSF and restrains the surface Sr-segregation in both oxidizing and reducing conditions. The electrode polarization resistance of the anode-supported single cell with HE-LSF cathode is significantly low at intermediate temperature. Meanwhile, at 800 °C, the power density is 1029.2 mW/cm2, and the polarization resistance (Rp) is 0.101 Ω cm2, while at 700 °C, the power density is 581.52 mW/cm2 with an Rp of 0.21 Ω cm2 in humidified H2. Further, the HE-LSF cathode presents good and long-term stability at a constant voltage of 0.7 V for 100 h at 700 °C. The excellent electrochemical performance and stability results exhibit that Fe-based high-entropy perovskite oxide is a new promising candidate and cathode material for SOFCs at intermediate temperatures.

Exploring uncommon Fe-oxides in non-metallic inclusions in ultra-high-strength steel

Investigating non-metallic inclusions within ultra-high-strength-steel via conventional methods is a known: however, the challenge is to obtain chemical information of such inclusions at the sub-micrometer level. In this context, probing Fe-based oxide in inclusions is a vital aspect for guiding steel’ performance. The vibrational properties of sub micrometer size Fe-based oxides were investigated by Raman mapping along with chemometric analysis with the aim of probing their chemical composition. Highly contrasted Raman spectra were recorded from several inclusions embedded at different spatial locations. The observed spectral features were identified as specific markers of hematite (α-Fe2O3) and magnetite (Fe3O4). Principal Component Analysis was used to confirm the presence of these markers and potentially revealing additional patterns. Their unambiguous assignment has been inferred by comparing our experimental findings with the literature data recorded either in single crystals of iron oxides or oxyhydroxides. Micro-Raman spectroscopy is proven to be a reliable, cost-effective, and non-invasive tool for the unambiguous identification of subsurface regions of steel.

Enhanced hydrogen production of Fe-based spinel by relay catalysis of Fe0 and Fe-Ov-Cr species in chemical looping H2O splitting

Chemical looping H2O splitting is considered as a promising approach to produce hydrogen thanks to its lower energy consumption and carbon footprint compared with traditional steam reforming. Fe-based oxides have been paid much attention but suffer from inferior H2 production rate and stability due to insufficient activation for H2O. Herein, it was found that the introduction of Cr into MgFe spinel oxides (MgFexCr2-xO4, MFCO) could greatly increase the performance of H2O splitting driven by CH4 reduction with the highest peak H2 production rate of about 4.65 mmol min−1 g−1, H2 productivity of about 2.88 mmol g−1 and good stability during multiple redox cycles for MFCO-46 (the atomic ratio of Fe and Cr of 2:3), which exceeded most of the state-of-the-art Fe-based oxides. This originated from the Cr doping promoting Fe0 nanoparticles exsolution by activating CH4 and Fe-Ov-Cr species formation which relay catalyzed H2O splitting and was favorable for the fast recovery of lattice oxygen converted.

Ultrathin High-Entropy Fe-Based Spinel Oxide Nanosheets with Metalloid Band Structures for Efficient Nitrate Reduction toward Ammonia.

Spinel oxides with tunable chemical compositions have emerged as versatile electrocatalysts, however their performance is greatly limited by small surface area and low electron conductivity. Here, ultrathin high-entropy Fe-based spinel oxides nanosheets are rationally designed (i.e., (Co0.2Ni0.2Zn0.2Mg0.2Cu0.2)Fe2O4; denotes A5Fe2O4) in thickness of ≈4.3nm with large surface area and highly exposed active sites via a modified sol-gel method. Theoretic and experimental results confirm that the bandgap of A5Fe2O4 nanosheets is significantly smaller than that of ordinary Fe-based spinel oxides, realizing the transformation of binary spinel oxide from semiconductors to metalloids. As a result, such A5Fe2O4 nanosheets manifest excellent performance for the nitrate reduction reaction (NO3 -RR) to ammonia (NH3), with a NH3 yield rate of ≈2.1mmol h-1 cm-2 at -0.5V versus Reversible hydrogen electrode, outperforming other spinel-based electrocatalysts. Systematic mechanism investigations reveal that the NO3 -RR is mainly occurred on Fe sites, and introducing high-entropy compositions in tetrahedral sites regulates the adsorption strength of N and O-related intermediates on Fe for boosting the NO3 -RR. The above findings offer a high-entropy platform to regulate the bandgap and enhance the electrocatalytic performance of spinel oxides.

Construction of three-dimensional porous network Fe-rGO aerogels with monocrystal magnetic Fe3O4@C core-shell structure nanospheres for enhanced microwave absorption

Single dielectric loss material has defects such as strong dielectric properties and impedance mismatch. Hence, developing electromagnetic wave (EMW) absorption materials with both outstanding EMW absorption and favourable impedance matching performances is still a great challenge. A large number of literature have reported that combining dielectric materials with magnetic materials to enhance microwave absorption. However, there is little literature on the effect of crystal structure on the absorbing properties of materials. Owing to the virtues of light weight, low density and thin thickness of aerogel, which has been widely utilized in EMW absorption materials. Herein, Fe-based metal organic framework-reduced graphene oxide (Fe-rGO) aerogel was synthesized via simple hydrothermal method and freeze-drying. Besides, Polyvinyl Pyrrolidone (PVP) was used as a surfactant to endow Fe3O4 nanospheres with uniform size and smooth morphology. Moreover, Fe3O4 nanospheres with different crystal phase (monocrystal and polycrystal) structure were obtained under different carbonization temperatures (600 and 800 °C). Through comparison, the monocrystal Fe3O4 nanospheres show better microwave absorption and impedance matching performance than that of polycrystal Fe3O4 nanospheres. In addition, the electromagnetic parameters can be adjusted by regulating the mass ratio of Fe-based metal organic framework (Fe-MOF) to GO. As a consequence, the prepared 600Fe-rGO 2:1 aerogel achieves the minimum reflection loss (RLmin) of −64.89 dB and a broad effective absorption bandwidth (EAB) of 6.96 GHz with a low filling content of 5 wt%, showing excellent EMW absorption and impedance matching property. In short, the structural design of single crystal magnetic Fe3O4 nanospheres provides inspiration and guidance for future research of enhanced microwave absorption materials.

Preparation of particle electrodes for landfill leachate treatment by doping Cu2+-modified needle coke with Fe-based oxides extracted from iron tailings

Landfill leachate is a typically difficult-to-degrade organic wastewater that, if not treated properly will cause serious harm to human health and the ecological environment. Three-dimensional electro-Fenton (3DEF) technology has received widespread attention because of its high mass transfer efficiency and current utilisation, and no secondary pollution. In this study, the Fe/Cu-loaded needle coke particle electrode (Fe/Cu-NCPE) was prepared, using which a 3DEF system was constructed with DSA as the anode and Ni/ACF as the cathode to treat mature landfill leachate. The optimal preparation conditions of the Fe/Cu-NCPE were obtained as Cu2+-modified needle coke: Fe-based oxides: binder = 32:8:15, particle size of 5 ± 0.5 mm, and Cu2+ loading of 3 %. The spectral analysis shows that the Fe/Cu-NCPE prepared in this study is a kind of particle electrode with superior performance and good stability. The optimum conditions for treating landfill leachate at an applied voltage of 9 V, an initial pH of 2, and a Fe/Cu-NCPE dosage of 25 g/L resulted in the removal of COD, colour, TN, and NH3-N of 94.2 %, 99 %, 90.3 %, and 93.8 %, respectively. In the 3DEF system for landfill leachate treatment, H2O2 is generated at the cathode and polarized Fe/Cu-NCPE surface, which is activated by loaded Fe2+ and Cu+ in a Fenton(-like) reaction to produce •OH; Meanwhile active chlorine is generated by anodic electrocatalytic oxidation of Cl-. They attack humic acid and other organics in the wastewater as well as nitrogenous reductive substances to degrade them.

Exploiting the effect of PEO parameters on the surface of AISI 1020 low-carbon steel treated in a TaOH-rich electrolyte

This study used PEO treatment and a TaOH-rich electrolyte to coat AISI 1020 low-carbon steel to enhance its anti-corrosion and wear properties, targeting its usage as a medical device. The effect of time, duty cycle, frequency, and Ta(OH)5 concentration on the chemical and phase composition, topography, wettability, and roughness of the obtained coatings were addressed. The results indicated that the Fe-based oxide coatings had a rough and super-hydrophilic surface. The phase composition of the coatings was formed mainly of Ta2O5, hematite (Fe2O3), and a minor amount of rust (Fe(OH)2), with the chemical analyses also indicating the presence of some absorbed molecules. Once the best PEO parameters were established (200 V, 5 min, 60 %, 1000 Hz, and 40 g/L Ta(OH)5), the previous mechanical, corrosion, and wear tests evidenced the positive effect of the chemical species deposited in the surface when compared to the substrate. In this way, PEO treatment applied on AISI 1020 low-carbon steel in a TaOH-rich electrolyte can be an adequate alternative to produce low-cost materials for medical devices, especially as a potential substitute for stainless steel and titanium-based alloys in surgical instruments.

Oxygen grain-boundary diffusion in (La,Sr)FeO3−δ perovskite-oxides probed by molecular-dynamics simulations

Faster grain-boundary diffusion of oxygen has been observed experimentally in polycrystalline samples of Fe-based perovskite oxides at low temperatures, but this behaviour is at present not well understood.

Iron Phosphates as Highly Efficient Precatalysts for Oxygen Evolution Reaction and Their in Situ Electrochemical Conversion

Current energy issues have prompted demand for the innovations in constructing a clean and sustainable energy platforms. One of examples for energy carrier to actualize a zero carbon society is hydrogen, which is a promising carbon-free fuel with an excellent weight energy density. Hydrogen production through alkaline water electrolysis using renewable energy resources (light and wind)-derived electricity is an attractive ecofriendly and cost-effective technology because cheap nonprecious electrocatalysts can be employed in alkaline media. However, oxygen evolution reaction (OER) at the anode side in the water electrolysis is basically slow and prevents the widespread applications of water electrolysis. Thus, highly efficient and cheap electrocatalysts for OER are highly desirable.Iron (Fe)-based compounds have recently received much attention as OER electrocatalysts because Fe is a quite earth-abundant metallic element that is a very cheap and non-toxic. Therefore, Fe is a promising candidate as electrocatalyst for OER. Due to the insufficient OER activity on simple Fe oxides, previous studies have attempted to enhance the activity by incorporating Fe and other metallic elements.[1,2] Recently, we established a novel comprehensive structural descriptor for the OER on Fe-based simple and multimetal oxides;[3] i.e., shorter minimum Fe–O bond length in the crystalline structures led to higher OER activity regardless of elemental composition, Fe–O coordination number, and crystal category. Thus, this descriptor enabled us to quickly develop excellent OER catalyst, Ba0.65Ca0.35Fe12O19, comprising extremely short Fe–O bond lengths.[4] Furthermore, we found that post-spinel CaFe2O4 catalyzes OER via "multi-iron-site mechanism" involving a reaction intermediate with O–O direct formation on multiple Fe atoms, and CaFe2O4 consequently displayed outstanding OER activity despite the long Fe–O bond length.[5] However, we speculate that OER activity of Fe-based oxides cannot be further enhanced based on crystal structures, and it necessitates to employ another type of compounds.In this study, we report the OER performances of Fe-based phosphates in alkaline media. We selected three kinds of Fe-based phosphates with distinct crystal structures: trigonal FePO4, trigonal Fe3O3(PO4), and monoclinic Fe4(P2O7)3. The electrochemical measurements and post-characterizations identified the Fe-based phosphates as efficient precatalysts that can be electrochemically converted into highly active Fe-based compounds. Notably, the OER performances of the electrochemically converted Fe-based compounds overcame those of the above-mentioned most active Fe-based multimetal oxides and any previously reported crystalline OER electrocatalysts. Therefore, the Fe-based phosphates are promising candidates as OER precatalysts, which can be in situ converted into excellent OER electrocatalysts, and have further advantages of low price, environmental friendliness, and easy production.[6] The part of this paper is based on results obtained from a project, JPNP14021, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). This paper was funded in part by the JSPS KAKENHI Grant-in-Aid (18H01786), the JST PRESTO (JPMJPR15S3) and CREST (JPMJCR16P3) programs, the Tokuyama Science Foundation, and the “Creation of Life Innovative Materials for Interdisciplinary and International Researcher Development” program of the MEXT. The authors thank Dr. Takeshi Aihara in Tokyo Institute of Technology for fruitful discussions.

Development of Novel High Capacity Fluoride Ion Battery Cathode Utilizing Anion Redox of Sulfur

All-solid-state fluoride-ion batteries (FIBs), which are based on the shuttling of fluoride ions, have been considered as one of the promising alternates for next-generation energy storage devices because of high theoretical energy density and safety. Benefiting from the multi-covalent fluorination process, metal/metal fluorides (M/MFx) have been firstly utilized as electrode materials for FIBs with high theoretical energy densities.[1] However, M/MFx systems suffer from exceptionable volume expansion upon fluorination process due to the close-packed metal atoms, which would cause blockage of the F- diffusion pathway, resulting in devastating damage to the electrode-electrolyte interfaces. To solve these problems, cathode materials that utilize topotactic fluoride ion intercalation reactions, similar to the electrode materials applied in lithium-ion secondary batteries, are being developed.[2] Cathode materials that utilize topotactic intercalation of fluoride ions are often composed of heavier elements than the cathodes of lithium-ion batteries that use the same type of reaction mechanism, and effective anion redox is required if capacity is to be increased beyond that of lithium-ion secondary batteries.In this research, we investigated Sr2F2Fe2OS2 (SFFOS) compound with a layered structure as a novel cathode material for all-solid-state FIBs, and evaluated its electrochemical properties and elucidated the charge compensation mechanism using anion redox of sulfur.SFFOS was synthesized by a solid-state reaction under vacuum environment using SrF2, SrO, Fe, and S.[3] SFFOS/La0.9Ba0.1F2.9/vapor grown carbon fiber (VGCF) was used as the composite cathode, La0.9Ba0.1F2.9 as the electrolyte and Pb/PbF2/ La0.9Ba0.1F2.9/VGCF as the composite anode to construct the electrochemical cell. Charge-discharge measurements were carried out in the cut-off voltage range of -1.5 to 1.5 V at 140°C. Ex-situ X-ray Diffraction (XRD), X-ray absorption spectroscopy (XAS) measurements of Fe K-edge, S K-edge, O K-edge and F K-edge were carried out under different state of charge during the 1st and the 2nd cycle.SFFOS showed reversible intercalation/de-intercalation of fluoride ions and a capacity of ~6.1 fluoride ions per unit cell (403 mAh g-1). The charge compensation mechanism was revealed for SFFOS by XAS measurements, where S redox contributed to the whole voltage range from -1.5 V to 1.5 V vs. Pb/PbF2, and Fe+2/+3 redox contributed from the middle SOC. The S-S dimers were formed upon charging, which was similar to the trapped O2 molecules observed in lithium-excess metal oxide during charge process[4]. Not only can the F- anions insert around Fe to form Fe-F bonds, but the excess F- anions can also occupy the Sr-S interstitial layers in the original lattice. Compared to the Fe-based layered oxide Sr3Fe2O5, the Fe-based oxysulfide SFFOS showed a lower average voltage but a higher specific capacity. We believe that this study could provide a new understanding of sulfur-based charge compensation and electrochemical fluorination reactions. Reference s : [1] D. Zhang, K. Yamamoto, Y. Uchimoto et al., J. Mater. Chem. A, 2021, 9, 406–412.[2] Y. Wang, K. Yamamoto, Y. Uchimoto, et al., Chem. Mater., 2022, 34, 609–616.[3] Kabbour H, Janod E, Corraze B, et al ., J. Am. Chem. Soc., 2008, 130(26): 8261-8270.[4] R. A. House, P. G. Bruce, et al., Nat. Energy 2020, 5, 777–785. Figure 1

Tuning the ORR catalytic activity of LaFeO3-δ-based perovskite cathode for solid oxide fuel cells by doping with alkaline-earth metal elements

Co-based perovskite cathodes have demonstrated impressive catalytic activity for oxygen reduction reaction (ORR) at intermediate temperatures. However, these cathodes face the challenges such as high cost and thermal expansion coefficient (TEC) mismatch. On the other hand, Fe-based perovskite oxides like LaFeO3 exhibit more stable properties and lower TEC, but their ORR activity is not as remarkable as that of Co-based cathodes. In this study, La3+ at the A-site of LaFeO3 was substituted by aliovalent alkaline-earth metal cations. The research findings reveal that the doping of alkaline-earth metal ions induces the formation of structural defects and alters the valence state of Fe. This leads to an increase in the conductivity and oxygen vacancy concentration, which are critical for ORR kinetics in both H–SOFC and O–SOFC. Among the various dopants, Sr doped LaFeO3 (LSrF) exhibits the most outstanding ORR catalytic activity. This can be attributed to its highest total conductivity, O2− conductivity, oxygen surface exchange coefficient, and oxygen bulk diffusion coefficient. At 700 °C, single cell with LSrF cathode achieves peak power densities of 0.781 and 0.894 W cm−2, for O2--conducting solid oxide fuel cell (SOFC) and H+-conducting SOFC, respectively. These performances are essentially twice that of single cell with LaFeO3 cathode. The findings of this study provide valuable insights into doping strategies for Fe-based perovskite cathodes and contribute to the advancement of IT-SOFC cathode research.

P2-type Na0.67Fe0.5Mn0.35Co0.15O2 cathode modified by an active coating layer of K2Na(Co(NO2)6) for sodium-ion batteries with improved cycling stability

P2-type Fe-based oxides cathodes (Na0.67Fe0.5Mn0.35Co0.15O2, NFMC) have the disadvantage of a high air sensitivity and are prone to P2-O2 phase transition during charging to high voltage. In this work, an active coating layer of K2Na(Co(NO2)6) (KNC) is successfully coated on the surface of NFMC materials. The ex-situ X-ray diffraction patterns and the microstructure analysis of the KNC@NFMC show that no phase change occurs during charging/discharging in the range of 1.5–4.2 V. The electrochemical tests demonstrate that long-time stability of the active K2Na(Co(NO2)6) coating layer enhances the reversible reaction of Mn3+/Mn4+ and effectively suppresses the increase of charge transfer impedance during cycles. Compared to Na0.67Fe0.5Mn0.35Co0.15O2 oxide, the modified material has superior capacity retention of 89.7% for 200 cycles at 1 C (1 C=180 mAh/g) with a discharge specific capacity of 94.6 mAh/g.