La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes infiltrated with samarium-doped cerium oxide for solid oxide fuel cells

Porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathodes are coated with a thin film of Sm_0.2Ce_0.8O_1.95−δ (SDC) using a one-step infiltration process. Examination of the microstructures reveals that small SDC particles are formed on the surface of LSCF grains with a relatively narrow size distribution. Impedance analysis indicates that the SDC infiltration has dramatically reduced the polarization of LSCF cathode, reaching interfacial resistances of 0.074 and 0.44 Ω cm² at 750 °C and 650 °C, respectively, which are about half of those for LSCF cathode without infiltration of SDC. The activation energies of the SDC infiltrated LSCF cathodes are in the range of 1.42-1.55 eV, slightly lower than those for a blank LSCF cathode. The SDC infiltrated LSCF cathodes have also shown improved stability under typical SOFC operating conditions, suggesting that SDC infiltration improves not only power output but also performance stability and operational life.     

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.     

Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and La0.6Ba0.4Co0.2Fe0.8O3−δ (LBCF) cathodes prepared by combined citrate-EDTA method for IT-SOFCs

The potential candidates for IT-SOFCs cathode materials, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and La_0.6Ba_0.4Co_0.2Fe_0.8O_3−δ (LBCF), were synthesized by the combined citrate-EDTA method. The BSCF and LSCF aqueous precursors solutions were prepared from Sr(NO₃)₂, Ba(NO₃)₂, La(NO₃)₃·6H₂O, Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, citric acid and EDTA-NH₃. BSCF precursor solutions with different pH values were dried at 130 °C and subsequently calcined at various temperatures. Symmetrical electrochemical cells consisting of porous BSCF or LBCF electrodes and a GDC electrolyte were fabricated by the screen-printing technique, and the cathode performance of the interfaces between the porous electrode (BSCF or LBCF) and GDC electrolyte was investigated at intermediate temperatures (500–700 °C) using AC impedance spectroscopy. The pH value of the precursor solution did not affect the phase evolution behavior of the BSCF powder. On the other hand, it appears that a low pH value results in the calcined BSCF powder having a more porous microstructure. The cathode performances of the BSCF and LBCF electrodes were sensitive to the powder preparation conditions. The BSCF electrode prepared from the precursor solution with a pH value of 8 showed low polarization resistance, and its area specific resistances (ASR) were 1.1, 0.15 and 0.035 Ω cm² at 500, 600 and 700 °C, respectively. On the other hand, the cathode polarization resistances of the LBCF electrode were slightly higher than those of the BSCF electrode.     

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.     

Electrospinning La0.8Sr0.2Co0.2Fe0.8O3−δ tubes impregnated with Ce0.8Gd0.2O1.9 nanoparticles for an intermediate temperature solid oxide fuel cell cathode

In order to reduce the polarization resistance of the cathode, we have developed one-dimensional (1D) nanostructured La_0.8Sr_0.2Co_0.2Fe_0.8O_3−δ (LSCF) tubes/Ce_0.8Gd_0.2O_1.9 (GDC) nanoparticles composite cathodes for solid oxide fuel cell. Uniform LSCF/PVP composite nanofibers have been firstly synthesized by a single-nozzle electrospinning technique, followed by firing at 800 °C for 2 h to form one-dimensional LSCF tubes. Subsequently, the GDC phases were introduced into tube structured LSCF scaffold pre-sintered on a GDC pellet by a multi-impregnation process. Electrochemical Impedance spectra reveal that nanostructured LSCF tubes/GDC nanoparticles composite cathodes have a better electrochemical performance, achieving area-specific resistances of 4.70, 1.12, 0.27 and 0.07 Ω cm² at 500, 550, 600 and 650 °C for the composite of GDC and LSCF in a weight ratio of 0.52:1. The low ASR values are mainly related to its optimal microstructure with larger triple-phase boundaries and higher porosity. These results suggest that LSCF tube/GDC nanoparticle composite can be an alternative cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC).     

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.     

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.     

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.     

Performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes under solid oxide fuel cells operation conditions

The performance stability and degradation mechanism of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathodes and LSCF impregnated Gd_0.1Ce_0.9O_2−δ (LSCF-GDC) cathodes are investigated under solid oxide fuel cell operation conditions. LSCF and LSCF-GDC cathodes show initially performance improvement but degrade under cathodic polarization treatment at 750 °C for 120 h. The results confirm the grain growth and agglomeration of LSCF and in particular GDC-LSCF cathodes as well as the formation of SrCoO_x particles on the surface of LSCF under cathodic polarization conditions. The direct observation of SrCoO_x formation has been made possible on the surface of dense LSCF electrode plate on GDC electrolyte. The formation of SrCoO_x is most likely due to the interaction between the segregated Sr and Co from LSCF lattice under polarization conditions. The formation of SrCoO_x would contribute to the deterioration of the electrocatalytic activity of the LSCF-based electrodes for the O₂ reduction in addition to the agglomeration and microstructure coarsening.     

A La0.8Sr0.2MnO3/La0.6Sr0.4Co0.2Fe0.8O3−δ core–shell structured cathode by a rapid sintering process for solid oxide fuel cells

A La_0.8Sr_0.2MnO₃ (LSM)/La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) core–shell structured composite cathode of solid oxide fuel cells (SOFCs) has been fabricated by wet infiltration followed by a rapid sintering (RS) process. The RS is carried out by placing LSCF infiltrated LSM electrodes directly into a preheated furnace at 800 °C for 10 min and cooling down very quickly. The heating and cooling step takes about 20 s, substantially shorter than 10 h in the case of conventional sintering (CS) process. The results indicate the formation of a continuous and almost non-porous LSCF thin film on the LSM scaffold, forming a LSCF/LSM core–shell structure. Such RS-formed infiltrated LSCF–LSM cathodes show an electrode polarization resistance of 2.1 Ω cm² at 700 °C, substantially smaller than 88.2 Ω cm² of pristine LSM electrode. The core–shell structured LSCF–LSM electrodes also show good operating stability at 700 °C and 600 °C over 24–40 h.     

Investigation of Ba1−xSrxCo0.8Fe0.2O3−δ as cathodes for low-temperature solid oxide fuel cells both in the absence and presence of CO2

Ba_1−xSr_xCo_0.8Fe_0.2O_3−δ (BSCF)(0 ≤ x ≤ 1) composite oxides were prepared and tested as cathodes for low-temperature solid oxide fuel cells (SOFCs) both in the absence and presence of CO₂. It is found that the BSCF cathodes in the whole range of strontium doping levels show promising performance at 500–600 °C in the absence of CO₂, among which the SrCo_0.8Fe_0.2O_3−δ (SCF) cathode gives the highest power density while BaCo_0.8Fe_0.2O_3−δ (BCF) cathode shows the lowest performance. The impedance analysis reveals that both the ohmic resistance and polarization resistance of the fuel cell increases when the strontium content decreases. It is believed that the microstructure and electrical conductivity simultaneously affect the process of oxygen reduction. The presence of CO₂ deteriorates the BSCF performance by adsorbing on the cathode surface and thus obstructing the oxygen surface exchange reaction. The CO₂ exerts a more intense influence on BSCF with higher barium content.     

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.     

Palladium and ceria infiltrated La0.8Sr0.2Co0.5Fe0.5O3−δ cathodes of solid oxide fuel cells

La_0.8Sr_0.2Co_0.5Fe_0.5O_3−δ (LSCF) cathodes infiltrated with electrocatalytically active Pd and (Gd,Ce)O₂ (GDC) nanoparticles are investigated as high performance cathodes for the O₂ reduction reaction in intermediate temperature solid oxide fuel cells (IT-SOFCs). Incorporation of nano-sized Pd and GDC particles significantly reduces the electrode area specific resistance (ASR) as compared to the pure LSCF cathode; ASR is 0.1 Ω cm² for the reaction on a LSCF cathode infiltrated with 1.2 mg cm⁻² Pd and 0.06 Ω cm² on a LSCF cathode infiltrated with 1.5 mg cm⁻² GDC at 750 °C, which are all significantly smaller than 0.22 Ω cm² obtained for the reaction on a conventional LSCF cathode. The activation energy of GDC- and Pd-impregnated LSCF cathodes is 157 and 176 kJ mol⁻¹, respectively. The GDC-infiltrated LSCF cathode has a lower activation energy and higher electrocatalytic activity for the O₂ reduction reaction, showing promising potential for applications in IT-SOFCs.     

Enhancing oxygen reduction activity of perovskite cathode decorated with core@shell nano catalysts

Highly active catalysts towards oxygen reduction reaction (ORR) with good stability are critical for reduced temperature solid oxide fuel cells (SOFCs). Nano Ag@M_0.2Ce_0.8O_2-δ (M = La, Sm, Pr) catalyst with core@shell structure is in-situ synthesized via a facile hydrothermal method on porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) scaffold as SOFC cathode. The diameter of the silver core is ∼60 nm with a uniform outer shell ∼20 nm in thickness. A dramatic decrease (∼80% at 550 °C) on the interfacial polarization resistance was observed for this catalyst-decorated cathode compared to the pristine one. The durability test demonstrated a relatively stable operation for Ag@Pr_0.2Ce_0.8O_2-δ decorated LSCF cathode. The excellent electrochemical performance of this hybrid electrode with convenient fabrication process presents a new strategy for rational design of SOFC cathode with excellent ORR activity and stability.     

Investigation of single SOEC with BSCF anode and SDC barrier layer

In this paper, Sm_0.2Ce_0.8O_1.9 (SDC) is used as a barrier interlayer between Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) anode and 8YSZ electrolyte to avoid solid state interaction of solid oxide electrolysis cells (SOEC) for high temperature application. The crystal structure and surface morphologies of BSCF and SDC powders were characterized, respectively. BSCF-SDC/YSZ/SDC-BSCF symmetric cells and BSCF-SDC/YSZ/Ni-YSZ single button cells were prepared and the related electrochemical performances were tested at 850 °C. The results showed that ASR data of BSCF-SDC/YSZ is 0.42 Ω cm² at 850 °C. The hydrogen production rate of the single SOEC using BSCF/SDC anode can be up to 177.4 mL cm⁻² h⁻¹, also the cell exhibits excellent stability, which indicates that it could be a potential candidate for the future application of SOEC technology.     

Performance stability of impregnated La0.6Sr0.4Co0.2Fe0.8O3−δ–Y2O3 stabilized ZrO2 cathodes of intermediate temperature solid oxide fuel cells

The stability of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ impregnated Y₂O₃ stabilized ZrO₂ (LSCF–YSZ) cathodes was investigated under the condition of open circuit or current polarization at 750 °C in air. The electrochemical measurement and the microstructure characteristic show that the flattening of LSCF particles has great contribution to the increase of resistance of LSCF–YSZ cathodes after 500 h heat treatment at 750 °C. Microstructure coarsening and the damage of well-connected porous structure are main reasons of the performance degradation for LSCF–YSZ cathodes testing at 200 mA cm⁻² and 750 °C in air. Higher current density of 500 mA cm⁻² applying on cathodes accelerates degradation processes. X-ray photoelectron spectroscopy (XPS) shows that Sr concentration on the cathode surface decreases after current polarization, which plays a main role in performance activation processes observed at the beginning stage. The enhancement of cobalt activity in LSCF lattice by current polarization increases the conductivity and decreases the stability of LSCF–YSZ cathodes.     

Ni–YSZ-supported tubular solid oxide fuel cells with GDC interlayer between YSZ electrolyte and LSCF cathode

Highly sinterable gadolinia doped ceria (GDC) powders are prepared by carbonate coprecipitation and applied to the GDC interlayer in Ni–YSZ (yttria stabilized zirconia)-supported tubular solid oxide fuel cell in order to prevent the reaction between YSZ electrolyte and LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ) cathode materials. The formation of highly resistive phase at the YSZ/LSCF interface was effectively blocked by the low-temperature densification of GDC interlayer using carbonate-derived active GDC powders and the suppression of Sr diffusion toward YSZ electrolyte via GDC interlayer by tuning the heat-treatment temperature for cathode fabrication. The power density of the cell with the configuration of Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF was as high as 906 mW cm⁻², which was 2.0 times higher than that (455 mW cm⁻²) of the cell with the configuration of Ni–YSZ/YSZ/LSM(La_0.8Sr_0.2MnO_3−δ)–YSZ/LSM at 750 °C.     

Long term behaviors of La0.8Sr0.2MnO3 and La0.6Sr0.4Co0.2Fe0.8O3 as cathodes for solid oxide fuel cells

This work studies the electrochemical performance and stability of La_0.8Sr_0.2MnO₃ (LSM) and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathodes in a AISI441 interconnect/cathode/YSZ electrolyte half-cell configuration at 800 °C for 500 h. Ohmic resistance and polarization resistance of the cathodes are analyzed by deconvoluting the electrochemical impedance spectroscopy (EIS) results. The LSM cathode has much higher resistance than the LSCF electrode even though the respective cathode resistance either decreases or stays stable over the long term thermal treatment. During the 500 h thermal treatment, dramatic elemental distribution changes influence the electrochemical behaviors of the cathodes. Chromium diffusion from the interconnect into the LSM electrode at triple phase boundaries (TPBs) leads to segregation of Sr away from La and Mn. For the LSCF cathode, Sr and Co segregation is dominant. The fundamental processes at the TPBs are proposed. Overall, LSCF is a much preferred cathode material because of its much smaller resistance for the 500 h thermal treatment time.     

Preparation of La0.6Sr0.4Co0.2Fe0.8O3 nanoceramic cathode powders for solid oxide fuel cell (SOFC) application

The La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) nanoceramic powders were prepared by sol–gel process using nitrate based chemicals for SOFC applications since these powders are considered as more promising cathode materials for SOFC. The citric acid was used as a chelating agent and ethylene glycol as a dispersant. The powders were calcined at 650 °C/6 h, 900 °C/3 h and 1150 °C/2 h in air using Thermolyne 47900 furnace. These powders were characterized by employing SEM/EDS, XRD, porosimetry and TGA/DTA techniques.     

LSCF–SDC core–shell high-performance durable composite cathode

Core–shell type La_0.6Sr_0.4Co_0.2Fe_0.8O_3−d (LSCF)–Sm_0.2Ce_0.8O_2−d (SDC) powders are synthesized to achieve a high-performance durable cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The SDC core size is controlled so that all core particles are surrounded by the LSCF particles with no unattached spots. Such a core–shell composite cathode develops an ideal microstructure with improved phase contiguity, homogeneity, and maximized triple-phase boundary density. The cathode that involves an SDC core of 500 nm exhibits the lowest interfacial polarization resistance (0.265 Ω cm² at 650 °C), as well as long-term stability during both thermo-cyclic and electrochemically accelerated tests.