Oxygen reduction mechanism of NdBaCo2O5+δ cathode for intermediate-temperature solid oxide fuel cells under cathodic polarization

The oxygen reduction reaction mechanism of NdBaCo₂O_5+δ cathode for intermediate-temperature solid oxide fuel cells was investigated by the electrochemical impedance spectroscopy under cathodic polarization. The Nyquist diagrams showed the different changes with the applied cathodic voltage at three temperature ranges: at 500 and 550 °C, at 600 °C, at 650 and 700 °C, which might be related to the changes in the charge transfer and/or oxygen diffusion processes including O₂ adsorption/desorption. Besides, the diffusion process was more easily affected by the increase of applied cathodic voltages than the charge transfer process, which was ascribed to the low activation energy of the diffusion process.     

Nanoscale bismuth oxide impregnated (La,Sr)MnO3 cathodes for intermediate-temperature solid oxide fuel cells

Bismuth oxide based oxygen ion conductors are incorporated into (La,Sr)MnO₃ (LSM), the classical cathode material for solid oxide fuel cells (SOFC), to improve the cathode performance. Yttria-stabilized bismuth oxide (YSB) is taken as an example and is impregnated into a preformed porous LSM frame, forming a highly active cathode for intermediate-temperature SOFCs (IT-SOFCs) with doped ceria electrolytes. X-ray diffraction indicates that YSB is chemically compatible with LSM at intermediate temperatures below 800 °C. The impregnated YSB particles are nanosized and are deposited on the surface of the framework. Significant performance improvement is achieved by introducing nanosized YSB into the LSM electrodes. At 600 °C, the interfacial polarization resistance under open-circuit conditions for electrodes impregnated with 50% YSB is only 1.3% of the original value for a pure LSM electrode. The resistance is further reduced dramatically when current is passed through. In addition, the YSB impregnated LSM electrodes has the highest electrochemical performance among those based on LSM. Single cell with 25% of YSB impregnated LSM cathode generates maximum power density of 300 mW cm⁻² at 600 °C, indicating the promise of using LSM-based electrodes for IT-SOFC.     

Preparation and characterization of silver-modified La0.8Sr0.2MnO3 cathode powders for solid oxide fuel cells by chemical reduction method

Herein a chemical reduction method is proposed in order to modify the solid oxide fuel cells (SOFC) traditional cathode material La_0.8Sr_0.2MnO_3−x (LSM). Silver nanoparticles were prepared by the reduction of ammoniacal silver nitrate with ascorbic acid in dilute aqueous solutions containing PVP. The obtained LSM–Ag composite powders were characterized by XRD, SEM, EDX, and STEM. The results showed that the LSM–Ag composite powder possess an elaborated fine structure with a homogeneous distribution of Ag and LSM, which effectively shortens the diffusion pathway for electrons and adsorbed oxygen. The electrochemical performance of the LSM–Ag cathode with different Ag loadings was investigated. A cathode loading with 1 wt.% Ag exhibited an area specific resistance as low as 0.45 Ω cm² at 750 °C, compared to around 1.1 Ω cm² for a pure LSM electrode. Similarly an anode-supported SOFC with 1 wt.% Ag in the cathode shows a peak power density of 1199 mW cm⁻¹, higher than the value of 717 mW cm⁻¹ achieved for a similar cell with a LSM cathode. Increasing the Ag loading is shown to have an insignificant effect on improving electrocatalytic performance at 750 °C, however it can increase output power at 650 °C.     

Electrochemical reduction of CO2 in solid oxide electrolysis cells

This paper describes results on the electrochemical reduction of carbon dioxide using the same device as the typical planar nickel-YSZ cermet electrode supported solid oxide fuel cells (H₂-CO₂, Ni-YSZ|YSZ|LSCF-GDC, LSCF, air). Operation in both the fuel cell and the electrolysis mode indicates that the electrodes could work reversibly for the charge transfer processes. An electrolysis current density of ≈1 A cm⁻² is observed at 800 °C and 1.3 V for an inlet mixtures of 25% H₂-75% CO₂. Mass spectra measurement suggests that the nickel-YSZ cermet electrode is highly effective for reduction of CO₂ to CO. Analysis of the gas transport in the porous electrode and the adsorption/desorption process over the nickel surface indicates that the cathodic reactions are probably dominated by the reduction of steam to hydrogen, whereas carbon monoxide is mainly produced via the reverse water gas shift reaction.     

Failure mechanism of (La,Sr)MnO3 oxygen electrodes of solid oxide electrolysis cells

The delamination behavior of La_0.8Sr_0.2MnO₃ (LSM) oxygen electrode of solid oxide electrolysis cell (SOEC) is studied in detail under anodic current passage of 500 mA cm⁻² and 800 °C. The delamination or failure of LSM oxygen electrode is observed after the current passage treatment for 48 h, and is accompanied by the significant increase in the electrode polarization and ohmic resistances. The delaminated electrode and electrolyte interface is characterized by the formation of nanoparticles within LSM contact rings on the electrolyte surface. SEM analysis of the interface at different stages of the polarization indicates that the formation of these nanoparticles is caused by the localized disintegration of the LSM grains at the electrode/electrolyte interface. The formation of nanoparticles is most likely due to the migration or incorporation of oxygen ions from the YSZ electrolyte into the LSM grain, leading to the shrinkage of LSM lattice. The shrinkage of the LSM lattice will create local tensile strains, resulting in the microcrack and subsequent formation of nanoparticles within LSM particles at the electrode/electrolyte interface. The formation of nanoparticle clusters weakens the anode/electrolyte interface, eventually leading to the delamination and failure of the LSM oxygen electrode under high internal partial pressure of oxygen at the interface.     

Performance and durability of nanostructured (La0.85Sr0.15)0.98MnO3/yttria-stabilized zirconia cathodes for intermediate-temperature solid oxide fuel cells

In this communication we report the fabrication of nanostructured (La_0.85Sr_0.15)_0.98MnO₃ (LSM)/yttria-stabilized zirconia (YSZ) composite cathodes consisting of homogeneously distributed and connected LSM and YSZ grains approximately 100 nm large. We also investigate for the first time the role of the cathode nanostructure on the performance and the durability of intermediate-temperature solid oxide fuel cells. The cathodes were fabricated using homogenous LSM/YSZ nanocomposite particles synthesized by co-precipitation, using YSZ nanoparticles of 3 nm as seed crystals. Detailed microstructural characterization by transmission electron microscopy with energy-dispersive X-ray spectroscopy revealed that many of the LSM/YSZ junctions in the cathode faced the homogeneously connected pore channels, indicating the formation of a considerable number of triple phase boundaries. The nanostructure served to reduce cathodic polarization. As a result, these anode-supported solid oxide fuel cells showed high power densities of 0.18, 0.40, 0.70 and 0.86 W cm⁻² at 650, 700, 750 and 800 °C, respectively, under the cell voltage of 0.7 V. Furthermore, no significant performance degradation of the cathode was observed during operation at 700 °C for 1000 h under a constant current density of 0.2 A cm⁻².     

Sr2Fe4/3Mo2/3O6 as anodes for solid oxide fuel cells

Sr₂Fe_4/3Mo_2/3O₆ has been synthesized by a combustion method in air. It shows a single cubic perovskite structure after being reduced in wet H₂ at 800 °C and demonstrates a metallic conducting behavior in reducing atmospheres at mediate temperatures. Its conductivity value at 800 °C in wet H₂ (3% H₂O) is about 16 S cm⁻¹. This material exhibits remarkable electrochemical activity and stability in H₂. Without a ceria interlayer, maximum power density (P_max) of 547 mW cm⁻² is achieved at 800 °C with wet H₂ (3% H₂O) as fuel and ambient air as oxidant in the single cell with the configuration of Sr₂Fe_4/3Mo_2/3O₆|La_0.8Sr_0.2Ga_0.83Mg_0.17O₃ (LSGM)| La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF). The P_max even increases to 595 mW cm⁻² when the cell is operated at a constant current load at 800 °C for additional 15 h. This anode material also shows carbon resistance and sulfur tolerance. The P_max is about 130 mW cm⁻² in wet CH₄ (3% H₂O) and 472 mW cm⁻² in H₂ with 100 ppm H₂S. The cell performance can be effectively recovered after changing the fuel gas back to H₂.     

Optimization of La0.6Ca0.4Fe0.8Ni0.2O3-Ce0.8Sm0.2O2 composite cathodes for intermediate-temperature solid oxide fuel cells

Sample of nominal composition La_0.6Ca_0.4Fe_0.8Ni_0.2O₃ (LCFN) was prepared by liquid mix method. The structure of the polycrystalline powder was analyzed with X-ray powder diffraction data. This compound shows orthorhombic perovskite structure with a space group Pnma. In order to improve the electrochemical performance, Sm-doped ceria (SDC) powder was added to prepare the LCFN-SDC composite cathodes. Electrochemical characteristics of the composites have been investigated for possible application as cathode material for an intermediate-temperature-operating solid oxide fuel cell (IT-SOFC). The polarization resistance was studied using Sm-doped ceria (SDC). Electrochemical impedance spectroscopy measurements of LCFN-SDC/SDC/LCFN-SDC test cell were carried out. These electrochemical experiments were performed at equilibrium from 850 °C to room temperature, under both zero dc current intensity and air. The best value of area-specific resistance (ASR) was for LCFN cathode doped with 10% of SDC (LCFN-SDC9010), 0.13 Ω cm² at 850 °C. The dc four-probe measurement exhibits a total electrical conductivity over 100 S cm⁻¹ at T ≥ 600 °C for LCFN-SDC9010 composite cathode.     

Electrochemical characteristics of an La0.6Sr0.4Co0.2Fe0.8O3–La0.8Sr0.2MnO3 multi-layer composite cathode for intermediate-temperature solid oxide fuel cells

An La_0.6Sr_0.4Co_0.2Fe_0.8O₃–La_0.8Sr_0.2MnO₃ (LSCF–LSM) multi-layer composite cathode for solid oxide fuel cells (SOFCs) was prepared on an yttria-stabilized zirconia (YSZ) electrolyte by the screen-printing technique. Its cathodic polarization curves and electrochemical impedance spectra were measured and the results were compared with those for a conventional LSM/LSM–YSZ cathode. While the LSCF–LSM multi-layer composite cathode exhibited a cathodic overpotential lower than 0.13 V at 750 °C at a current density of 0.4 A cm⁻², the overpotential for the conventional LSM–YSZ cathode was about 0.2 V. The electrochemical impedance spectra revealed a better electrochemical performance of the LSCF–LSM multi-layer composite cathode than that of the conventional LSM/LSM–YSZ cathode; e.g., the polarization resistance value of the multi-layer composite cathode was 0.25 Ω cm² at 800 °C, nearly 40% lower than that of LSM/LSM–YSZ at the same temperature. In addition, an encouraging output power from an YSZ-supported cell using an LSCF–LSM multi-layer composite cathode was obtained.     

Ni-Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells with stabilized zirconia electrolytes

Ni-LnO_x cermets (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), in which LnO_x is not an oxygen ion conductor, have shown high performance as the anodes for low-temperature solid oxide fuel cells (SOFCs) with doped ceria electrolytes. In this work, Ni-Sm₂O₃ cermets are primarily investigated as the anodes for intermediate-temperature SOFCs with scandia stabilized zirconia (ScSZ) electrolytes. The electrochemical performances of the Ni-Sm₂O₃ anodes are characterized using single cells with ScSZ electrolytes and LSM-YSB composite cathodes. The Ni-Sm₂O₃ anodes exhibit relatively lower performance, compared with that reported Ni-SDC (samaria doped ceria) and Ni-YSZ (yttria stabilized zirconia) anodes, the state-of-the-art electrodes for SOFCs based on zirconia electrolytes. The relatively low performance is possibly due to the solid-state reaction between Sm₂O₃ and ScSZ in fuel cell fabrication processes. By depositing a thin interlayer between the Ni-Sm₂O₃ anode and the ScSZ electrolyte, the performance is substantially improved. Single cells with a Ni-SDC interlayer show stable open circuit voltage, generate peak power density of 410 mW cm⁻² at 700 °C, and the interfacial polarization is about 0.7 Ω cm².     

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.     

Investigation of layered Ni0.8Co0.15Al0.05LiO2 in electrode for low-temperature solid oxide fuel cells

A symmetrical cell composed of Ce_0.9Gd_0.1O_2?δ electrolyte is constructed with 0.5 mm thickness and Ni_0.8Co_0.15Al_0.05LiO₂ (NCAL)-foam Ni composite electrodes. Electrochemical performance of the cell and electrochemical impedance spectra (EIS) are measured using the three-electrode method. The maximum power densities of the cell are 93.6 and 159.7 mW cm^?2 at 500 and 550 °C, respectively. The polarization resistances of the cathode are 0.393 and 0.729 Ω cm^?2 at 550 and 500 °C, indicating that NCAL has good ORR activity. HT-XRD results for NCAL do not show phase transitions or any additional new phases at elevated temperatures, indicating that NCAL has a stable phase structure. The surface characteristics of the NCAL powders are studied by XPS and FTIR. The results reveal that Li₂CO₃ and the cation-disordered “NiO-like” phase are formed on the surface of the layered NCAL structure due to prolonged exposure to air and contain a large number of oxygen vacancies. The cation-disordered “NiO-like” phase and Li₂CO₃ composite in the melt and partial melt states in the high temperature region are considered to possess very high ionic conductivity and lower activation energy for oxygen reduction reactions.     

Measurement of oxygen chemical potential in Gd2O3-doped ceria-Y2O3-stabilized zirconia bi-layer electrolyte, anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFC) were fabricated with gadolinia-doped ceria (GDC)-yttria stabilized zirconia (YSZ), thin bi-layer electrolytes supported on Ni + YSZ anodes. The GDC and YSZ layer thicknesses were 45 μm, and ∼5 μm, respectively. Two types of cells were made; YSZ layer between anode and GDC (GDC/YSZ) and YSZ layer between cathode and GDC (YSZ/GDC). Two platinum reference electrodes were embedded within the GDC layer. Cells were tested at 650 °C with hydrogen as fuel and air as oxidant. Electric potentials between embedded reference electrodes and anode and between cathode and anode were measured at open circuit, short circuit and under load. The electric potential was nearly constant through GDC in the cathode/YSZ/GDC/anode cells. By contrast, it varied monotonically through GDC in the cathode/GDC/YSZ/anode cells. Estimates of oxygen chemical potential, μO₂, variation through GDC were made. μO₂ within the GDC layer in the cathode/GDC/YSZ/anode cell decreased as the current was increased. By contrast, μO₂ within the GDC layer in the cathode/YSZ/GDC/anode cell increased as the current was increased. The cathode/YSZ/GDC/anode cell exhibited maximum power density of ∼0.52 W cm⁻² at 650 °C while the cathode/GDC/YSZ/anode cell exhibited maximum power density of ∼0.14 W cm⁻² for the same total electrolyte thickness.     

Electrochemical characteristics of solid oxide fuel cell cathodes prepared by infiltrating (La,Sr)MnO3 nanoparticles into yttria-stabilized bismuth oxide backbones

The electrochemical characteristics of the solid oxide fuel cell (SOFC) cathodes prepared by infiltration of (La_0.85Sr_0.15)_0.9MnO_3−δ (LSM) nanoparticles into porous Y_0.5Bi_1.5O₃ (YSB) backbones are investigated in terms of overpotential, interfacial polarization resistance, and single cell performance obtained with three-electrode cell, symmetrical cell, and single cell, respectively. X-ray diffraction confirms the formation of perovskite LSM by heating the infiltrated nitrates at 800 °C. The electrical conductivity of the electrode measured using Van der Pauw method is 1.67 S cm⁻¹, which is acceptable at the typical SOFC operating temperatures. The single cell with the LSM infiltrated YSB cathode generates maximum power densities of 0.23, 0.45, 0.78, and 1.13 W cm⁻² at 600, 650, 700, and 750 °C, respectively. The oxygen reduction mechanism on the cathode is studied by analyzing the impedance spectra obtained under various temperatures and oxygen partial pressures. The impedance spectra under various cathodic current densities are also measured to study the effect of cathodic polarization on the performance of the cathode.     

New ionic diffusion strategy to fabricate proton-conducting solid oxide fuel cells based on a stable La2Ce2O7 electrolyte

A composite of NiO–BaZr_0.1Ce_0.7Y_0.2O_3−δ (NiO-BZCY) was successfully prepared by a simple one-step-combustion process and applied as an anode for solid oxide fuel cells based on stable La₂Ce₂O₇ (LCO) electrolyte. A high open circuit voltage of 1.00 V and a maximum power density of 315 mW cm⁻² were obtained with NiO-BZCY anode and LCO electrolyte when measured at 700 °C using humidified hydrogen fuel. SEM-EDX and Raman results suggested that a thin BaCeO₃-based reaction layer about 5 μm in thickness was formed at the anode/electrolyte interface for Ba cations partially migrated from anode into the electrolyte film. Impedance spectra analysis showed that the activation energy for LCO conductivity differed with the anode materials, about 52.51 kJ mol⁻¹ with NiO-BZCY anode and 95.08 kJ mol⁻¹ with NiO-LCO anode. The great difference in these activation energies might suggest that the formed BaCeO₃ reaction layer could promote the proton transferring numbers of LCO electrolyte.     

Performance of (La,Sr)MnO3 cathode based solid oxide fuel cells: Effect of bismuth oxide sintering aid in silver paste cathode current collector

Effects of a bismuth oxide (Bi₂O₃) sintering aid in the silver paste cathode current collectors on the electrochemical performance of solid oxide fuel cells with (La,Sr)MnO₃ cathode is investigated. Anode-supported single cells are prepared and applied with pure and Bi₂O₃-added silver pastes for cathode current collecting. Cell performances are evaluated using a current-voltage test and electrochemical impedance spectroscopy. The results indicate that the Bi₂O₃-added silver paste cathode current collector artificially increases the power density and lowers the polarization resistance of single cell, which may be attributed to the observation of the improved cathode current collector surface morphology and enhanced contact at the cathode-current collector interface, as well as the migration of the Bi₂O₃ and silver into the cathode from the Bi₂O₃ contained silver paste cathode current collector.     

Ce0.7Bi0.3O1.85-(La0.8Sr0.2)0.9MnO3-Y0.16Zr0.84O1.92 ternary cathodes for low temperature solid oxide fuel cells

The ternary cathodes of Ce_0.7Bi_0.3O_1.85-(La_0.8Sr_0.2)_0.9MnO₃-Y_0.16Zr_0.84O_1.92 (BDC-LSM-YSZ) are fabricated through infiltration for low temperature solid oxide fuel cells. The infiltrated BDC particles are 10–20 nm in size and cover on LSM and YSZ particles. The 10 wt% and 20 wt% BDC-LSM-YSZ samples show a large peak for the desorption of surface oxygen species and a large peak for the evolution of lattice oxygen, reflecting their good redox property. 0.1BDC-LSM-YSZ cell and 0.2BDC-LSM-YSZ cell give the power density at 0.6 V of 387.8 and 521.7 mWcm^?2 at 600 °C, which is 3.7 and 4.9 times higher than that of LSM-YSZ cell, respectively. 0.1BDC-LSM-YSZ cell and 0.2BDC-LSM-YSZ cell exhibit low ohmic resistance and low total polarization resistance. The DRT analysis reveals that charge transfer reaction and surface diffusion are greatly accelerated on the BDC-LSM-YSZ cathodes.     

The heterogeneous electrolyte of CuFeO2 nano-flakes composited with flower-shaped ZnO for advanced solid oxide fuel cells

The flower-shaped ZnO was synthesized to form composite with the delafossite structure CuFeO₂. The composite heterojunction formed for the ZnO-CuFeO₂ composite material demonstrates a profound significance for exploring novel materials in solid oxide fuel cell (SOFC) field. At 550 °C, power outputs of 300 mW cm^?2 and 468 mW cm^?2 were achieved for SOFC devices using pure ZnO and composite with CuFeO₂ as the electrolytes, respectively. The composite showed a good performance at low temperatures, for instance, it showed a power output of 148 mW cm^?2 at 430 °C. The studies on photocurrent-time curves with visible light on/off irradiation provided an evidence for electron-hole separation. The heterojunctions separate holes and electrons, preventing short-circuiting while used in the SOFC device. These results demonstrate that introducing the heterojunctions in the electrolyte is an innovative approach for advanced SOFCs.     

A novel electrolyte for intermediate solid oxide fuel cells

Scheelite-type, La_xCa_1−xMoO_4+δ electrolyte powders, are prepared by the sol-gel process. The crystal structure of the samples is determined by employing the technique of X-ray diffraction (XRD). According to XRD analysis, the continuous series of La_xCa_1−xMoO_4+δ (0 ≤ x ≤ 0.3) solid solutions have the structure of tetragonal scheelite. Their lattice parameters are greater than that of the original sample, and increase with increasing values of x in the La-substituted system. Results of sinterability and electrochemical testing reveal that the performances of La-doped calcium molybdate are superior to that of pure CaMoO₄. La_xCa_1−xMoO_4+δ ceramics demonstrate higher sinterability. The La_0.2Ca_0.8MoO_4+δ sample that achieved 96.5% of the theoretical density was obtained after being sintered at 1250 °C for 4 h. The conductivity increases with increasing lanthanum content, and a total conductivity of 7.3 × 10⁻³ S cm⁻¹ at 800 °C could be obtained in the La_0.2Ca_0.8MoO_4+δ compound sintered at 1250 °C for 4 h.     

Study on oxygen activation and methane oxidation over La0.8Sr0.2MnO3 electrode in single-chamber solid oxide fuel cells via an electrochemical approach

Single-chamber solid oxide fuel cells (SC-SOFCs), which apply fuel-oxidant (air) gas mixture as the atmosphere for both anode and cathode, are receiving many interests recently. This study aims to clarify the mechanism of oxygen reduction and methane oxidization over La_0.8Sr_0.2MnO₃ (LSM) cathode in SC-SOFCs by an electrochemical method in combination with mass spectrometry (MS). Before cathodic polarization, a large polarization resistance (R_p) for oxygen reduction reaction (ORR) was observed and methane did not cause obvious effect on ORR because of the weak adsorption of methane over LSM surface. Cathodic polarization could decrease the R_p obviously due to the in-situ creation of oxygen vacancies; methane likely adsorbed on those oxygen vacancy sites to enhance its effect on ORR. Both the anodic and cathodic polarizations significantly increased the rate of methane oxidation over LSM electrode; in particular, the pumped oxygen anion was highly active for methane oxidation.