High performance BaFe1−xBixO3−δ as cobalt-free cathodes for intermediate temperature solid oxide fuel cells

Bi₂O₃ doped BaFeO_3?δ on the B-site as a cobalt free perovskite cathode for intermediate temperature solid oxide fuel cells is evaluated. The BaFe_1?xBi_xO_3?δ (BFBx) powders are synthesized by solid state reaction. It is found that Bi₂O₃ doping stabilizes the BaFeO₃ cubic phase. The new cathode is compatible with Gd_0.1Ce_0.9O_1.95 even calcined at 1000 °C for 10 h. The electronic conductivity shows a transformation from semiconductor to metal conductor, and achieves its maximum value of 28.1 S cm^?1 for BFB10 at 800 °C. The δ is as high as 0.408 for BFB10 determined by iodometric titration. This leads to the free volume in crystal lattice of BFB10 21.60% higher than that of BaNb_0.05Fe_0.95O_3?δ. The area specific resistance is only 0.133 Ω cm² for BFB10 at 750 °C and the average TEC is 26.697 × 10^?6 K^?1 measured from room temperature to 800 °C. The peak power density of Ni-YSZ|YSZ|GDC|BFB10 cell is 646.28 mW cm^?2 at 750 °C, higher than that of single cell using LSCF as cathode. These show that BFBx perovskite oxides with cubic phase are promising cathodes for intermediate temperature solid oxide fuel cells.     

Modified La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes with the infiltration of Er0.4Bi1.6O3 for intermediate-temperature solid oxide fuel cells

Aiming to lower the activation energy and expedite the oxygen reduction reaction (ORR) process of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) cathodes for application in intermediate-temperature solid oxide fuel cells (IT-SOFCs), Er_0.4Bi_1.6O₃ (ESB) modified LSCF was prepared by infiltrating using organic solvents. The infiltration of ESB dramatically reduces the polarization resistances of LSCF cathodes (from 0.27 to 0.11 Ω cm² at 700 °C, from 0.58 to 0.25 Ω cm² at 650 °C), and lowers their activation energy (from 100.28 to 97.15 kJ mol^?1). Also, ESB makes the rate-limiting step of LSCF cathodes at high frequency change from the charge transfer process on the cathode to the adsorption and diffusion of oxygen on cathode surface. The single cell with ESB infiltrated LSCF cathodes shows a peak power density of 469 mW cm^?2 at 700 °C using humid hydrogen and air as fuels and oxidants, respectively, as well as a good short-term stability for 50 h.     

Effect of humidity on La0.4Sr0.6Co0.2Fe0.7Nb0.1O3−δ cathode of solid oxide fuel cells

Solid oxide fuel cells cathode often suffers from degradation caused by water vapor in air. Here, we report a cathode material, La_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O_3−δ (LSCFN), and evaluate its humidity tolerance by the characterization of the materials in wet air with different water vapor concentration at different temperature. The X-ray diffraction analysis indicates that the crystal structure of LSCFN is relatively stable in wet air with no observable impurity. However, a crystalline contraction is observed. Exposure of wet air to LSCFN causes the decrease of electrical conductivity and increase of polarization resistance because H₂O might occupy the active sites for oxygen reduction reaction. For long-term operation, higher H₂O concentration in air accelerates the degradation of LSCFN cathode.     

Investigating the electrochemical performance of Nd1-xSrxCo0.8Fe0.2O3−δ (0 ≤ x ≤ 0.85) as cathodes for intermediate temperature solid oxide fuel cells

Perovskite-type structure oxides with the nominal chemical composition Nd_1-xSr_xCo_0.8Fe_0.2O_3−δ (0 ≤ x ≤ 0.85) (NSCF) were synthesized by solid-state method to investigate the effect of Sr-doping on the crystal structure and electrochemical performance in the intermediate temperature range of 600 °C–750 °C. The electrical conductivity of the sintered NSCF pellets was found to be in the range of 300–1000 S cm⁻¹. All the NSCF compositions showed a transition from semiconducting to metallic behavior with an increase in temperature. NSCF showed reactivity with the La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte. The electrochemical performance was tested by preparing the symmetrical cells with the configuration 70 wt% NSCF +30 wt% LSGM using LSGM electrolyte in the temperature range of 650–800 °C. AC-impedance results showed a decrease in polarization resistance (Rp) of the cathode with increase in Sr-doping due to increase in electrical conductivity. Among the samples studied, composite electrode of Nd_0.3Sr_0.7Co_0.8Fe_0.2O_3−δ – LSGM showed the lowest area specific resistance (ASR) of 0.1 Ω cm² at 750 °C in air. It was chosen to investigate the effect of pO₂ on the electrochemical performance of the cathode to determine the rate determining step (RDS) in oxygen reduction reaction (ORR). Dissociation of molecular oxygen into oxygen atoms seems to be the RDS in the ORR.     

In-situ strategy to suppress chromium poisoning on La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes of solid oxide fuel cells

The surface segregation of strontium in the La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) electrode interacts with volatile contaminants such as chromium in the solid oxide fuel cell (SOFC) interconnect, causing deterioration in cell performance. A simple in-situ reaction strategy has been exploited to synergistically improve oxygen reduction reaction (ORR) activity in air and anti-chromium stability of LSCF electrode via infiltration and calcination of nickel nitrate and ferrite nitrate (NF) precursor on the LSCF backbone. The chemical compatibility, electrochemical performance, interfacial element distribution and stability in chromium-containing atmosphere of the as-prepared hybrid electrodes were systematically investigated. At a calcination temperature of 1100 °C, Sr(Co,Ni)O_3-δ layer was formed owing to Co diffusion and Sr precipitation from LSCF and the reaction with Ni atoms at the surface of LSCF. This will promote anti-chromium ability for the hybrid LSCF@NF cathode material. After the symmetrical cells were operated at 750 °C for 400 h under Cr contamination, the polarization resistance of LSCF@NF was only half of that of blank LSCF electrode with much less Cr species. This strategy via in-situ reaction may be extended to other high temperature energy conversion systems such as anti-sulfur and anti-carbon deposition of SOFC anodes and CO₂ resistance of cathodes.     

Synthesis and electrochemical characterization of La0.6Sr0.4Co0.2Fe0.8O3–δ / Ce0.9Gd0.1O1.95 co-electrospun nanofiber cathodes for intermediate-temperature solid oxide fuel cells

Synthesis and electrochemical characterization of composite cathodes, formed from a mixture of La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ (LSCF) and Ce_0.9Gd_0.1O_1.95 (GDC) nanofibers, is reported. The electrodes are obtained by simultaneous electrospinning of the two precursor solutions, using apparatus equipped with two spinnerets working in parallel. Results of electrochemical testing carried out through electrochemical impedance spectroscopy (EIS) are presented and discussed. The results suggest that the electrochemical reaction takes place in an electrode region close to the electrode/current collector interface and that the oxygen ions then flow along the ionic conducting path of the GDC fibers. At 650 °C, the polarization resistance is R_p = 5.6 Ω cm^?2, in line with literature values reported for other IT-SOFC cathodes.     

A novel Nb and Cu co-doped SrCoO3-δ cathode for intermediate temperature solid oxide fuel cells

The extensive explorations of potential cathode materials are prominently critical for the rapid development of high performance solid oxide fuel cells (SOFCs). Herein, we develop a novel Nb and Cu co-doped SrCoO_3-δ (SCNC) cathode base on solid state reaction, which exhibits decent compatibility with gadolinium doped cerium oxide (GDC) electrolyte. The SCNC is successfully stabilized with cubic structure at room temperature when incorporating of small amount of high valence Nb⁵⁺. Meanwhile, the oxygen vacancy concentration of SCNC is efficiently improved with the addition of Cu. The Nb and Cu co-doping also substantially promotes the electronic conductivity, achieving 550 S cm⁻¹ for the optical doped SrCo_0.85Nb_0.05Cu_0.10O_3-δ (SCNC10) at 400 °C. In addition, the polarization of SCNC is remarkably reduced, reaching as low as 0.021 Ω cm² for SCNC10 at 700 °C. The activation energy for reaction is also significantly lowered to 0.78 eV. The reaction order m is deduced to be about 0.30, implying that the rate determination step for SCNC10 is the charge transfer reaction. The peak power density of the single cell reaches 780 mW cm⁻² at 800 °C. All these outstanding performances demonstrate that SCNC is a promising cathode for SOFCs when operating at intermediate temperature (IT).     

Effects of PdO modification on the performance of La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes for solid oxide fuel cells: A first principle study

Effects of palladium (Pd) impregnation on the performance of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) cathodes are investigated with density functional theory plus U (DFT + U) and experimental methods. In-situ high temperature X-ray diffractometer results show that the impregnated Pd species exist at states of palladium oxide (PdO) at 700 °C. The measured electrochemical impedance spectroscopy at 700 °C indicates PdO modification promotes the catalytic activity of LSCF cathodes. The modification structure of PdO on LSCF surfaces and effects of PdO modification on the performance of LSCF cathodes are investigated with DFT + U methods. The results show that B-8 with PdO molecule modification by a parallel posture on LSCF surface is the most stable structure. O₂ prefers to be adsorbed on AO-terminated surfaces rather than that on BO₂-terminated ones. The oxygen surface adsorption activity of LSCF surface is improved by PdO modification. The calculated partial densities of states (PDOS) and Fermi level of O₂ adsorption on LSCF surfaces imply that the charge transfer is easier with PdO modification than that without PdO modification because PdO acts as a metal-like modification. The PdO modification on LSCF surface leads to a better oxygen surface adsorption activity of LSCF cathodes.     

Novel,cobalt-free,and highly active Sr2Fe1.5Mo0.5−xSnxO6−δ cathode materials for intermediate temperature solid oxide fuel cells

One of the technical hurdles to commercialization of intermediate temperature solid oxide fuel cells (IT-SOFCs) is the requirement of highly efficient cathode materials. Herein, we report the evaluation of Sr₂Fe_1.5Mo_0.5?xSn_xO_6?δ (x = 0, 0.1, 0.3 and 0.5, abbreviated as SFM, SFMS1, SFMS3, and SFS) oxides as cobalt-free cathode materials of IT-SOFCs. XPS analysis demonstrates the presence of variable valences among Fe, Mo and Sn elements, suggesting a small polaron hopping mechanism for electronic conduction. First principle calculations reveal that SFMS3 provides the lowest average formation energy of oxygen vacancy (E_vacO*) among these perovskites. The relatively low area specific resistances are obtained with SFMS3 electrode based on La_0.8Sr_0.2Ga_0.8Mg_0.2O_3?δ electrolytes, indicating its high activity for oxygen reduction reaction. Power density of the single cell using SFMS3 cathode as high as 618 mW cm^?2 at 800 °C is achieved, and operation lasts for 200 h without obvious degradation. The encouraging results promise SFMS3 as an alternative cathode material for IT-SOFCs.     

Crystal structure and functional properties of Nd1.6Ca0.4Ni1-yCuyO4+δ as prospective cathode materials for intermediate temperature solid oxide fuel cells

Complex oxides Nd_1.6Ca_0.4Ni_1-yCu_yO_4+δ (y = 0.0–0.4) have been prepared by a pyrolysis of glycerol-nitrate compositions. According to the X-ray diffraction analysis, the materials are single-phase up to y = 0.3 and crystallize in an orthorhombic structure (Bmab) at room temperature. High-temperature studies assert that they all undergo a phase transition from orthorhombic to tetragonal (I4/mmm) structure in a range of 300–400 °C. With Cu doping, the over-stoichiometric oxygen content δ decreases from 0.07 (y = 0.0) down to 0.00 (y = 0.3). The studies on the compact samples reveal the maximum value of total conductivity (165 S cm^?1 at 420 °C) and the minimum value of the linear coefficient of thermal expansion (11.9·10^?6 K^?1 in a range of 400–1000 °C in air) at y = 0.2. Chemical compatibility of the Nd_1.6Cа_0.4Ni_1-yCu_yO_4+δ (y = 0.0, 0.2) oxides with oxygen- and proton conducting electrolytes (Ce_0.9Gd_0.1O_1.95, Ce_0.8Sm_0.2O_1.9 and BaCe_0.5Zr_0.3Y_0.1Yb_0.1O_3-δ) up to a temperature of 1100 °C is demonstrated.     

Investigation of La0.6Sr0.4Co1-xNixO3-δ (x=0, 0.2, 0.4, 0.6, 0.8) catalysts on solid oxide fuel cells anode for biogas dry reforming

The introduction of catalyst on anode of solid oxide fuel cell (SOFC) has been an effective way to alleviate the carbon deposition when utilizing biogas as the fuel. A series of La_0.6Sr_0.4Co_1-xNi_xO_3-δ (x = 0, 0.2, 0.4, 0.6, 0.8) oxides are synthesized by sol-gel method and used as catalysts precursors for biogas dry reforming. The phase structure of La_0.6Sr_0.4Co_1-xNi_xO_3-δ oxides before and after reduction are characterized by X-ray diffraction (XRD). The texture properties, carbon deposition, CH₄ and CO₂ conversion rate of La_0.6Sr_0.4Co_1-xNi_xO_3-δ catalysts are evaluated and compared. The peak power density of 739 mW cm^?2 is obtained by a commercial SOFC with La_0.6Sr_0.4Co_0.4Ni_0.6O_3-δ catalyst at 850 °C when using a mixture of CH₄: CO₂ = 2:1 as fuel. This shows a great improvement from the cell without catalyst for internal dry reforming, which is attributed to the formation of NiCo alloy active species after reduction in H₂ atmosphere. The results indicate the benefits of inhibiting the carbon deposition on Ni-based anode through introducing the La_0.6Sr_0.4Co_0.4Ni_0.6O_3-δ catalyst precursor. Additionally, the dry reforming technology will also help to convert part of the exhaust heat into chemical energy and improve the efficiency of SOFC system with biogas fuel.     

Investigation of structural,electrical and electrochemical properties of La0.6Sr0.4Fe0.8Mn0.2O3-δ as an intermediate temperature solid oxide fuel cell cathode

La_0.6Sr_0.4Fe_0.8Mn_0.2O₃ (LSFM) compound is synthesized by sol-gel method and evaluated as a cathode material for the intermediate temperature solid oxide fuel cell (IT-SOFC). X-ray diffraction (XRD) indicates that the LSFM has a rhombohedral structure with R-3c space group symmetry. The XRD patterns reveal very small amount of impurity phase in the LSFM and Y₂O₃-stabilized ZrO₂ (YSZ) mixture powders sintered at 600, 700, 800 and 850 °C for a week. The maximum electrical conductivity of LSFM is about 35.35 S cm⁻¹ at 783 °C in the air. The oxygen chemical diffusion coefficients, D_Chem, are increased from 1.39 × 10⁻⁶ up to 1.44 × 10⁻⁵ cm² s⁻¹. Besides, the oxygen surface exchange coefficients, k_Chem, are obtained to lie between 2.9 × 10⁻³ and 1.86 × 10⁻² cm s⁻¹ in a temperature range of 600–800 °C. The area-specific resistances (ASRs) of the LSFM symmetrical cell are 7.53, 1.53, 1.13, 0.46 and 0.31 Ω cm² at 600, 650, 700, 750 and 800 °C respectively, and related activation energy, E_a, is about 1.23 eV.     

Co-free La0.6Sr0.4Fe0.9Nb0.1O3-δ symmetric electrode for hydrogen and carbon monoxide solid oxide fuel cell

Co-free La_0.6Sr_0.4FeO_3-δ (LSFNb₀) and La_0.6Sr_0.4Fe_0.9Nb_0.1O_3-δ (LSFNb_0.1) perovskite oxides were prepared by a standard solid-state reaction method. The structural stability and electrochemical performance of La_0.6Sr_0.4Fe_0.9Nb_0.1O_3-δ as both cathode and anode were studied. Nb dopant in LSFNb₀ significantly enhances the structural and chemical stability in anode condition. At 800 °C, the polarization resistances (Rp) of LSFNb_0.1 symmetric electrode based on YSZ electrolyte are 0.5 and 0.05 Ω cm² in H₂ and air, respectively. The peak power densities of LSFNb_0.1 based on LSGM electrolyte-supported SSOFCs are 934 and 707 mW cm⁻² at 850 °C in H₂ (3% H₂O) and dry CO, respectively. Moreover, the symmetric cell exhibits reasonable stability in both H₂ and CO fuel, suggesting that La_0.6Sr_0.4Fe_0.9Nb_0.1O_3-δ may be a potential symmetric electrode material for hydrogen and carbon monoxide SOFCs.     

Validating the efficiency of γ-Al2O3/La0.6Sr0.4Co0.2Fe0.8O3-δ double-layer electrolyte for low temperature solid oxide fuel cell

Perovskite La_0.6Sr_0.4Co_0.2Fe_0.8O_3+δ (LSCF) as a promising cathode material possessed overwhelming electronic conduction along with certain ionic conductivity. Its strong electron conduction capability hinder the application of pure-phase LSCF as electrolyte in semiconductor membrane fuel cell (SMFC). In order to constrain the electron transport and take advantage of the decent ion conduction of LSCF, a thin layer of γ-Al₂O₃ with insulating property was added as an electron barrier layer and combine with LSCF to form a two-layer structure electrolyte. Through adjusting the weight ratio of LSCF/γ-Al₂O₃ to optimize the thickness of double layers, an open circuit voltage of 0.98 V and a maximum power density of 690 mW/cm² was received at 550 °C. At the same time, SEM, EIS and other characterization technology had proven that the LSCF/γ-Al₂O₃ bi-layer electrolyte can work efficiently at low temperature. The advantage of this work is the application of double-layer (γ-Al₂O₃/LSCF) structure electrolyte to instead of mixed material electrolyte in low-temperature solid oxide fuel cells. Structural innovation and the using of insulating materials provided clues for the further development of SMFC.     

Electrochemical properties and durability of in-situ composite cathodes with SmBa0.5Sr0.5Co2O5+δ for metal supported solid oxide fuel cells

The electrochemical properties and long-term performance of an in-situ composite cathode comprised of SmBa_0.5Sr_0.5Co₂O_5+δ (SBSCO) and Ce_0.9Gd_0.1O_2?δ (CGO91) are investigated for metal supported solid oxide fuel cell (MS-SOFC) application.The Area Specific Resistance (ASR) of an in-situ composite cathode comprised of 50 wt% of SBSCO and 50 wt% of CGO91 (SBSCO:50) is 0.031 Ω cm² in the first stage of measurement at 700 °C; this value of ASR increases to 0.138 Ω cm² after 1000 h. The ASR of SBSCO:50 (in-situ sample at 750 °C) is 0.014 Ω cm² at the initial stage of measurement; the increase of ASR after 1000 h at 750 °C is only 0.067 Ω cm². These results suggest that the optimum temperature for in-situ firing of an SBSCO:50 cathode sample of MS-SOFC is higher than 700 °C, ideally around 750 °C.     

The investigation of Ag & LaCo0.6Ni0.4O3−δ composites as cathode contact material for intermediate temperature solid oxide fuel cells

The high interface resistance between cathodes and interconnects is a major cause for performance degradation of solid oxide fuel cells (SOFCs). Ag particles were mixed to LaCo_0.6Ni_0.4O_3?δ (LCN) matrix which prevented the silver densification and demonstrated porosity microstructure. The composites with different Ag content were evaluated as cathode contact materials with SUS430 alloy as interconnects. The area specific resistance (ASR) of SUS430/10%Ag & LCN/SUS430 showed the optimal performance in which the ASR was 73 mΩ cm² after 50 h at 750 °C and showed stable property in 10 thermal cycles from 200 °C to 750 °C. The excellent performance of 10%Ag & LCN is attributed to the high conductivity of silver, the stable microstructure of LCN and its good interface adhesion with the interconnect alloy. With 10%Ag & LCN as cathode contact materials, the power density of a single cell reached 0.623 W/cm² at 750 °C and the average degradation is lower than 1% in 3 thermal cycles.     

Highly carbon– and sulfur–tolerant Sr2TiMoO6−δ double perovskite anode for solid oxide fuel cells

Ni-based cermets are most commonly used anode materials for solid-oxide fuel cells (SOFCs), but poor stability operating on hydrocarbon fuels seriously hampers their commercialization due to carbon deposition and sulfur poisoning. Here, we report a carbon– and sulfur–tolerant double perovskite anode Sr₂TiMoO_6−δ (STMO) combining the characteristics of two simple perovskites of SrTiO₃ and SrMoO₃. The STMO anode exhibits excellent thermal and chemical compatibility with La_0.9Sr_0.1Ga_0.8Mg_0.2O_3–δ (LSGM) and Ce_0.8Sm_0.2O_1.9 (SDC) electrolytes in 5% H₂/Ar. The single cell with STMO anode demonstrates good stability and excellent coking resistance and sulfur tolerance in H₂S-containing syngas during a 60-h period. The maximum power density (P_max) values of a LSGM-electrolyte-supported single cell with STMO anode are 505 and 275 mW cm⁻²at 850 °C in H₂ and H₂S-containing syngas, respectively. The electrochemical performance is further improved by impregnation of Pd nanoparticles, where the P_max values achieve 1009 and 586 mW cm⁻² at 850 °C under the same conditions, respectively, showing great potential as an anode material for SOFCs operating on H₂S-containing syngas. Our study provides a strategy to develop versatile double perovskite materials by combining the relevant characteristics of two separate perovskites.     

Synthesis and characterization of cobalt-free SrFe0·8Ti0·2O3-δ cathode powders synthesized through combustion method for solid oxide fuel cells

Cobalt-free SrFe_0·8Ti_0·2O_3-δ cathode powders were synthesized through the combustion method. Results of thermogravimetric and Fourier transform infrared analyses suggested that a perovskite oxide started to form at temperatures above 1100 °C. X-ray diffraction and Rietveld refinement analyses confirmed that the single-phase cubic structure (Pm-3m) of the SrFe_0·8Ti_0·2O_3-δ cathode was produced after calcination at 1300 °C. The average sizes of the particles were 1.0827 and 1.4438 μm as revealed by field emission scanning electron microscopy and dynamic light scattering analysis, respectively. In addition, energy dispersive X-ray analysis coupled with mapping revealed the homogeneous distribution of elements in the cathode. The thermal expansion coefficient of the SrFe_0·8Ti_0·2O_3-δ cathode is 16.20 × 10⁻⁶ K⁻¹. For the electrochemical behavior, the area specific resistance of cathode (0.60–13.57 Ω cm²) was obtained at 600–800 °C, and the activation energy was 121.77 kJ mol⁻¹. This work confirmed the potential of a SrFe_0·8Ti_0·2O_3-δ cathode in the intermediate temperature solid oxide fuel cell.     

Enhancing catalysis activity of La0.6Sr0.4Co0.8Fe0.2O3-δ cathode for solid oxide fuel cell by a facile and efficient impregnation process

The development of high performance electrocatalysts to promote oxygen reduction reaction (ORR) and to prolong the durability of cathodes of solid oxide fuel cell is essential at intermediate or low temperatures. Here, we report a facile and efficient spray impregnation strategy in enhancing catalytic activity of La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) due to the introduction of a multitude of homogeneous nanoparticles. With a highly active surface abundance in B-site cations, the modified LSCF cathode manifests an area-specific resistance (ASR) of ∼0.140 Ω cm², only a fifth of that for a pristine LSCF cathode (∼0.764 Ω cm²) at 600 °C, and anode-supported fuel cells with the decorated LSCF cathode show markedly improved peak power densities (∼0.94 W cm⁻² at 700 °C). Furthermore, the ORR kinetics of the modified LSCF cathode can be further enhanced by impregnating Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O solution again. X-ray photoelectron spectroscopy analysis indicates that the homogeneous nanoparticles alter the distribution of Sr_surface and O_surface. It is found that ‘Co’ decoration can effectively alleviate the surface aggregation of Sr and ‘Co’ and ‘Ni’ decoration play a pivotal role in the reactivation of electrode surface.