Characterization of infiltrated (La0.75Sr0.25)0.95MnO3 as oxygen electrode for solid oxide electrolysis cells

Porous strontium doped lanthanum manganite (LSM)-yttria-stabilized zirconia (YSZ) composite has been made by an impregnation method as oxygen electrodes for solid oxide electrolysis cells. X-ray diffraction and SEM results showed that LSM powders with well-crystallized perovskite phase uniformly distributed in the porous YSZ matrix. Impedance spectra and voltage-current density curves were measured as a function of absolute humidity at different temperatures to characterize the cell performance. The LSM infiltrated cell has an area specific resistance (ASR) of 0.20 Ω cm² at 900 °C at open circuit voltage with 50% absolute humidity (AH), which is relatively lower than that of the cell with LSM-YSZ oxygen electrode made by a conventionally mixing method. Electrolysis cell with LSM infiltrated oxygen electrode has demonstrated stable performance under electrolysis operation with 0.33 A/cm² and 50 vol.% AH at 800 °C.     

La0.75Sr0.25Cr0.5Mn0.5O3 as hydrogen electrode for solid oxide electrolysis cells

La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ (LSCM) has been applied as hydrogen electrode (cathode) material in solid oxide electrolysis cells operating with different steam concentrations (20, 40, 60 and 80 vol.% absolute humidity (AH)) using 40 sccm H₂ carrier gas at 800, 850 and 900 °C, respectively. Impedance spectra and voltage-current curves were measured as a function of cell electrolysis current density and steam concentration to characterize the cell performance. The cell resistance decreased with the increase in electrolysis current density while increased with the increase in steam concentration under the same electrolysis current density. At 1.6 V applied electrolysis voltage, the maximum consumed current density increased from 431 mA cm⁻² for 20 vol.% AH to 593 mA cm⁻² for 80 vol.% AH at 850 °C. Polarization and impedance spectra experiments revealed that LSCM-YSZ hydrogen electrode played a major role in the electrolysis reaction.     

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.     

Enhanced performance of solid oxide fuel cells with Ni/CeO2 modified La0.75Sr0.25Cr0.5Mn0.5O3−δ anodes

The optimization of electrodes for solid oxide fuel cells (SOFCs) has been achieved via a wet impregnation method. Pure La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCrM) anodes are modified using Ni(NO₃)₂ and/or Ce(NO₃)₃/(Sm,Ce)(NO₃)_x solution. Several yttria-stabilized zirconia (YSZ) electrolyte-supported fuel cells are tested to clarify the contribution of Ni and/or CeO₂ to the cell performance. For the cell using pure-LSCrM anodes, the maximum power density (P_max) at 850 °C is 198 mW cm⁻² when dry H₂ and air are used as the fuel and oxidant, respectively. When H₂ is changed to CH₄, the value of P_max is 32 mW cm⁻². After 8.9 wt.% Ni and 5.8 wt.% CeO₂ are introduced into the LSCrM anode, the cell exhibits increased values of P_max 432, 681, 948 and 1135 mW cm⁻² at 700, 750, 800 and 850 °C, respectively, with dry H₂ as fuel and air as oxidant. When O₂ at 50 mL min⁻¹ is used as the oxidant, the value of P_max increases to 1450 mW cm⁻² at 850 °C. When dry CH₄ is used as fuel and air as oxidant, the values of P_max reach 95, 197, 421 and 645 mW cm⁻² at 750, 800, 850 and 900 °C, respectively. The introduction of Ni greatly improves the performance of the LSCrM anode but does not cause any carbon deposit.     

La0.4Ce0.6O1.8–La0.8Sr0.2MnO3–8 mol% yttria-stabilized zirconia composite cathode for anode-supported solid oxide fuel cells

The composite cathodes of La_0.4Ce_0.6O_1.8 (LDC)–La_0.8Sr_0.2MnO₃ (LSM)–8 mol% yttria-stabilized zirconia (YSZ) with different LDC contents were investigated for anode-supported solid oxide fuel cells with thin film YSZ electrolyte. The oxygen temperature-programmed desorption profiles of the LDC–LSM–YSZ composites indicate that the addition of LDC increases surface oxygen vacancies. The cell performance was improved largely after the addition of LDC, and the best cell performance was achieved on the cells with the composite cathodes containing 10 wt% or 15 wt% LDC. The electrode polarization resistance was reduced significantly after the addition of LDC. At 800 °C and 650 °C, the polarization resistances of the cell with a 10 wt% LDC composite cathode are 70% and 40% of those of the cell with a LSM–YSZ composite cathode, respectively. The impedance spectra show that the processes associated with the dissociative adsorption of oxygen and diffusion of oxygen intermediates and/or oxygen ions on LSM surface and transfer of oxygen species at triple phase boundaries are accelerated after the addition of LDC.     

Synthesis and performance of Sr1.5LaxMnO4 as cathode materials for intermediate temperature solid oxide fuel cell

A-site non-stoichiometric materials Sr_1.5La_xMnO₄ (x = 0.35, 0.40, 0.45) are prepared via solid state reaction. The structure of these materials is determined to be tetragonal. Both the lattice volume and the thermal expansion coefficient reduce with the decrease of lanthanum content. On the contrary, the conductivity increases and the maximum value of 13.9 S cm⁻¹ is found for Sr_1.5La_0.35MnO₄ at 750 °C in air. AC impedance spectroscopy and DC polarization measurements are used to study the electrode performance. The optimum composition of Sr_1.5La_0.35MnO₄ results in 0.25 Ω cm² area specific resistance (ASR) at 750 °C in air. The oxygen partial pressure measurement indicates that the charge transfer process is the rate-limiting step of the electrode reactions.     

Unique porous thick Sm0.5Sr0.5CoO3 solid oxide fuel cell cathode films prepared by spray pyrolysis

Ultrasonic spray pyrolysis assisted by an electrostatic field was used to deposit thick Sm_0.5Sr_0.5CoO₃ (SSC) films (>40 μm) as solid oxide fuel cell (SOFC) cathodes with a unique porous columnar structure. The high porosity and great thickness provided many active sites for reduction reaction. The space between columns, as well as the large pores (∼100 nm) inside the columns allowed gas molecules to diffuse quickly to the reaction sites; thus, very low interfacial resistance values (0.20 and 0.035 Ω cm² at 600 and 700 °C, respectively) were obtained. Moreover, the high deposition rate, ease of operation in open air and low cost make the ultrasonic spray pyrolysis assisted by an electrostatic field a particularly useful method for preparation of films ideal for SOFC operation.     

Improved high temperature performance of La0.8Sr0.2MnO3 cathode by addition of CeO2

Ceria is proposed as an additive for La_0.8Sr_0.2MnO₃ (LSM) cathodes in order to increase both their thermal stability and electrochemical properties after co-sintering with an yttria-stabilized zirconia (YSZ) electrolyte at 1350 °C. Results show that LSM without CeO₂ addition is unstable at 1350 °C, whereas the thermal stability of LSM is drastically improved after addition of CeO₂. In addition, results show a correlation between CeO₂ addition and the maximum power density obtained in 300 μm thick electrolyte-supported single cells in which the anode and modified cathode have been co-sintered at 1350 °C. Single cells with cathodes not containing CeO₂ produce only 7 mW cm⁻² at 800 °C, whereas the power density increases to 117 mW cm⁻² for a CeO₂ addition of 12 mol%. Preliminary results suggest that CeO₂ could increase the power density by at least two mechanisms: (1) incorporation of cerium into the LSM crystal structure, and (2) by modification or reduction of La₂Zr₂O₇ formation at high temperature. This approach permits the highest LSM-YSZ co-sintering temperature so far reported, providing power densities of hundreds of mW cm⁻² without the need for a buffer layer between the LSM cathode and YSZ electrolyte. Therefore, this method simplifies the co-sintering of SOFC cells at high temperature and improves their electrochemical performance.     

Development and operation of alternative oxygen electrode materials for hydrogen production by high temperature steam electrolysis

High temperature steam electrolysis (HTSE) is one of the most promising ways for hydrogen mass production. To make this technology suitable from an economical point of view, each component of the system has to be optimized, from the balance of plant to the single solid oxide electrolysis cell. At this level, the optimization of the oxygen electrode is of particular interest since it contributes to a large extent to the cell polarization resistance. The present paper is focused on alternative oxygen electrode materials with improved performances compared to the usual ones mainly based on perovskite structure. Two nickelates, with compositions La₂NiO_4+δ and Nd₂NiO_4+δ are investigated and evaluated in HTSE operation at the button cell level. The performances of the Ln₂NiO_4+δ - containing cells (Ln = La, Nd) is improved compared to a cell containing the classical Sr-doped LaMnO₃ (LSM) perovskite oxygen electrode showing that nickelates are promising candidates for HTSE oxygen electrodes, especially for operation below 800 °C. Indeed, current densities determined at 1.3 V are 1.1 times larger for the La₂NiO_4+δ - containing cell and 1.6 times larger for the Nd₂NiO_4+δ one compared to the LSM - containing cell at 850 °C, whereas at 750 °C they are 1.8 and 4.4 times larger, respectively. Thanks to the use of a reference electrode, by coupling impedance spectroscopy and polarization measurements, the overpotential of each working electrode is deconvoluted from the complete cell voltage under HTSE operating conditions.     

Evaluation of Ni80Cr20/(La0.75Sr0.25)0.95MnO3 dual layer coating on SUS 430 stainless steel used as metallic interconnect for solid oxide fuel cells

Ni₈₀Cr₂₀/(La_0.75Sr_0.25)_0.95MnO₃ dual-layer coating is deposited on SUS 430 alloy by plasma spray for solid oxide fuel cell (SOFC) interconnect application. The phase structure, area specific resistance (ASR), and morphology of the coating are studied. A two-cell stack is also assembled and tested to evaluate coating performance in an actual SOFC stack. The NiCr/LSM coating adheres well to the SUS 430 alloy after oxidation in air at 800 °C for 2800 h. The ASR and its increasing rate of coated alloy are 25 mΩ cm² and 0.0017 mΩ cm²/h, respectively. In an actual stack test, the maximum output power density of the stack repeating unit increases from 0.32 W cm⁻² to 0.45 W cm⁻² because of the application of NiCr/LSM coating. The degradation rate of the stack repeating unit with no coating is 4.4%/100 h at a current density of 0.36 A cm⁻², whereas the stack repeating unit with NiCr/LSM coating exhibits no degradation. Ni₈₀Cr₂₀/(La_0.75Sr_0.25)_0.95MnO₃ dual-layer coating can remarkably improve the thermal stability and electrical performance of metallic interconnects for SOFCs.     

Plasma spray synthesis of La10(SiO4)6O3 as a new electrolyte for intermediate temperature solid oxide fuel cells

The apatite-type lanthanum silicate films were successfully synthesized by modified atmosphere plasma spraying using lanthanum oxide and silicon oxide mixed powders and precalcined hypereutectic powders in the size range 1–3 μm and 5–8 μm, respectively, as starting feedstock materials. The films differed not only in microstructural scale, but also in the characteristic of the degree of film densification. A detail describing the evolution of microstructure has been discussed. A considerable improvement in densification of the La₁₀(SiO₄)₆O₃ electrolyte films has been observed.     

Surface and interface behaviors of (La0.8Sr0.2)xMnO3 air electrode for solid oxide cells

In solid oxide cell operation, the stoichiometry of the air electrode is an important factor for its interaction with electrolyte and interconnect and long-term cell performance. In this study, tri-layer samples of yttria stabilized zirconia (YSZ)/(La_0.8Sr_0.2)_xMnO₃ (LSM)/AISI 441 stainless steel are made and thermally treated in dry air atmosphere at 800 °C for 500 h. The air electrode composition is varied by changing the x value in (La_0.8Sr_0.2)_xMnO₃ from 0.95 to 1.05 (LSM95, LSM100, and LSM105). The LSM composition segregation, YSZ/LSM/AISI 441 interfacial interaction, and the reaction of volatile chromium species with the LSM surface are characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Surface segregation of Sr and La are detected for all the LSM samples. Cr deposition is found across the LSM surface. For the LSM95 sample, Sr-containing compound leads to a high Cr content at the YSZ/LSM interface. For the LSM105 sample, on the other hand, the enrichment of La at the YSZ/LSM interface hinders the Cr deposition, leading to a very low Cr content. The mechanisms of LSM elemental surface segregation and Cr deposition are discussed.     

Performance and structural stability of Gd0.2Ce0.8O1.9 infiltrated La0.8Sr0.2MnO3 nano-structured oxygen electrodes of solid oxide electrolysis cells

Effect of Gd_0.2Ce_0.8O_1.9 (GDC) infiltration on the performance and stability of La_0.8Sr_0.2MnO₃ (LSM) oxygen electrodes on Y₂O₃-stabilized ZrO₂ (YSZ) electrolyte has been studied in detail under solid oxide electrolysis cell (SOEC) operating conditions at 800 °C. The incorporation of GDC nanoparticles significantly enhances the electrocatalytic activity for oxygen oxidation reaction on LSM electrodes. Electrode polarization resistance of pristine LSM electrode is 8.2 Ω cm² at 800 °C and decreases to 0.39 and 0.09 Ω cm² after the infiltration of 0.5 and 1.5 mg cm⁻² GDC, respectively. The stability of LSM oxygen electrodes under the SOEC operating conditions is also significantly increased by the GDC infiltration. A 2.0 mg cm⁻² GDC infiltrated LSM electrode shows an excellent stability under the anodic current passage at 500 mA cm⁻² and 800 °C for 100 h. The infiltrated GDC nanoparticles effectively shift the reaction sites from the LSM electrode/YSZ electrolyte interface to the LSM grains/GDC nanoparticle interface in the bulk of the electrode, effectively mitigating the delamination at the LSM/YSZ interface. The results demonstrate that the GDC infiltration is an effective approach to enhance the structural integrity and thus to achieve the high activity and excellent stability of LSM-based oxygen electrode under the SOEC operating conditions.     

Low-temperature solid oxide fuel cells with La1−xSrxMnO3 as the cathodes

La_1−xSr_xMnO₃ (LSM) has been widely developed as the cathode material for high-temperature solid oxide fuel cells (SOFCs) due to its chemical and mechanical compatibilities with the electrolyte materials. However, its application to low-temperature SOFCs is limited since its electrochemical activity decreases substantially when the temperature is reduced. In this work, low-temperature SOFCs based on LSM cathodes are developed by coating nanoscale samaria-doped ceria (SDC) onto the porous electrodes to significantly increase the electrode activity of both cathodes and anodes. A peak power density of 0.46 W cm⁻² and area specific interfacial polarization resistance of 0.36 Ω cm² are achieved at 600 °C for single cells consisting of Ni-SDC anodes, LSM cathodes, and SDC electrolytes. The cell performances are comparable with those obtained with cobalt-based cathodes such as Sm_0.5Sr_0.5CoO₃, and therefore encouraging in the development of low-temperature SOFCs with high reliability and durability.     

Characterization of atmospheric plasma-sprayed La0.8Sr0.2Ga0.8Mg0.2O3 electrolyte

La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) deposit in free standing planar shape was prepared by atmospheric plasma spraying (APS) to examine the coating microstructure and electrical conductivity to aim at applying APS LSGM to solid oxide fuel cells (SOFCs). The electrical conductivity of the plasma-sprayed LSGM coating was investigated. The coating microstructure was characterized by X-ray diffraction and scanning electron microscopy. The result showed that a fraction amorphous phase was present in the as-sprayed LSGM deposit, which starts to recrystallize at the temperature of 785 °C. The electrical conductivities of the LSGM with recrystallization treatment are 0.04 and 0.09 S cm⁻¹ at 1000 °C at the directions perpendicular and parallel to the coating surfaces, respectively. The electrical conductivity at perpendicular direction is about one-tenth that of sintered bulk at 1000 °C. This result is due to the lamellar structure feature with the limited interface bonding which dominates the electrical conductivity of APS coatings. The activation energy for ion conduction within APS-deposited LSGM deposit depends on temperature range. The change of activation energy indicates that the ion transportation dominant changes with temperature.     

Potential oscillation of methane oxidation reaction on (La0.75Sr0.25)(Cr0.5Mn0.5)O3 electrodes of solid oxide fuel cells

Oscillation of open circuit potential (OCP) and potential is observed for the methane oxidation reaction on (La_0.75Sr_0.25)(Cr_0.5Mn_0.5)O₃ (LSCM) and LSCM/YSZ composite electrodes of solid oxide fuel cells (SOFCs) in weakly humidified methane (i.e., 97%CH₄/3%H₂O). In dry methane (i.e., 100%CH₄), the potential oscillation is reduced significantly. The oscillation behaviour of OCP is also found to be strongly related to the temperature, the microstructure of the composite electrode and the fuel composition. The results indicate that the potential oscillation is thermally activated and is most likely associated with the adsorbed oxygen species on the electrode surface.     

A model for the delamination kinetics of La0.8Sr0.2MnO3 oxygen electrodes of solid oxide electrolysis cells

A theoretical model is developed to simulate the delamination kinetics of La_0.8Sr_0.2MnO₃ (LSM) electrode from YSZ electrolyte in solid oxide electrolysis cells (SOECs). The delamination is caused by the total stress including the internal oxygen pressure in LSM near the electrode/electrolyte interface, and the tensile stress by the oxygen migration from the YSZ electrolyte to LSM lattice. Weibull theory is used to determine the survival probability of electrode/electrolyte interface under the total stress. The relaxation time corresponding to the time for oxygen diffusion from the interface to the microcracks in La_0.8Sr_0.2MnO₃ links the survival probability with polarization time, thus the survival interface area can be predicted with varying anodic polarization time. The model is validated with experimental data. The effects of applied anodic current and operating temperature are discussed. The present model provides a starting point to study more complex cases, such as composite oxygen electrodes.     

Characteristic of (La0.8Sr0.2)0.98MnO3 coating on Crofer22APU used as metallic interconnects for solid oxide fuel cell

This study reports the high temperature oxidation kinetics, area specific resistance (ASR), and interfacial microstructure of metallic interconnects coated by (La_0.8Sr_0.2)_0.98MnO₃ (LSM) in air atmosphere at 800 °C. An efficient LSM conductive layer was fabricated on metallic interconnects for solid oxide fuel cells (SOFCs) by using a wet spray coating method. The optimum conditions for slurries used in the wet spray coating were determined by the measurement of slurry viscosity and coated surface morphology. The surface roughnesses of the substrates were increased through sandblast treatment. The adhesive strength of the interface between the coated layer and the metal substrate increased with increased surface roughness of the metallic interconnects. The electrical conductivities of the coated substrates were measured by using a DC two-point and four-wire method under air atmosphere at 800 °C. Of note, the Crofer22APU treated at 1100 °C in N₂ with 10 vol.% H₂ showed long-term stability and a lower ASR value than other samples(heat-treated at 800 °C and 900 °C). After an 8000-h oxidation experiment the coated Crofer22APU substrate, the ASR showed a low value of 23 mΩ cm². The thickness of the coated conductive oxide layer was about 10-20 μm. These results show that a coated oxide layer prevents the formation and the growth of scale (Cr₂O₃ and (Mn, Cr, Fe)₃O₄ layer) and enhances the long-term stability and electrical performance of metallic interconnects for SOFCs.     

Durable high-performance Sm0.5Sr0.5CoO3–Sm0.2Ce0.8O1.9 core-shell type composite cathodes for low temperature solid oxide fuel cells

Sm_0.5Sr_0.5CoO₃ (SSC)-Sm_0.2Ce_0.8O_1.9 (SDC) core-shell composite cathodes are synthesized via a polymerizable complex method, and the durability of a cell incorporating the cathodes is examined. Nanocrystalline SSC powders have been coated onto the surfaces of SDC cores to enable the formation of a rigid backbone structure, over which the catalyst phase is effectively dispersed. A symmetrical SSC-SDC |SDC| SSC-SDC half-cell exhibits a polarization resistance of 0.098 Ω cm² at 650 °C. The durability and microstructure of the cathode are investigated by electrochemical impedance spectroscopy and thermo-cycle tests at temperatures in the range of 100 °C-650 °C. After 30 cycles, the polarization resistance is found to increase by 9.04 × 10⁻² Ω cm², a 3.56% rise with respect to the initial resistance. Coarsening of the SSC catalyst phase has been prevented with the use of core-shell type powders, as confirmed by a nearly constant low frequency polarization resistance and a microstructural analysis. The performance of a unit cell comprised of the core-shell type cathode exhibits 1.07 W cm⁻² at 600 °C and 0.62 W cm⁻² at 550 °C.