Electrochemical studies on Sr doped LaMnO<Subscript>3</Subscript> and LaCoO<Subscript>3</Subscript> layers synthesised in a low-pressure plasma reactor equipped with a convergent nozzle

The aim of this work was to synthesise and deposit a perovskite (La_1-x Sr _x MnO₃ or La_1-x Sr _x CoO₃) cathode on an SOFC electrolyte (8% Yttria Stabilised Zirconia – YSZ). The recombination of atomic oxygen on Sr doped LaMnO₃ was performed in order to study the properties of perovskite for the adsorption of oxygen. The electrical resistance of the layers was measured as a function of the chemical composition. Impedance measurements on different samples were performed in order to analyse the electrochemical response of the cathode – electrolyte stack as a function of temperature and nature of the atmosphere. Moreover, the Faradaic impedance representing the electrochemical processes at the cathode – electrolyte interface was calculated from the global impedance in various conditions.     

Oxygen permeability and structural stability of La<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>Co<Subscript>0.2</Subscript>Fe<Subscript>0.8</Subscript>O<Subscript>3−<Emphasis Type="Italic">δ</Emphasis></Subscript> membrane

La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ oxides were synthesized by citrate method and hydrothermal method. The oxides prepared by citrate method are perovskite type structure, while the oxides by hydrothermal method have a small amount of secondary phase in the powder. Pyrex glass seal and Ag melting seal provided reliable gas-tight sealing of disk type dense membrane in the range of operation temperature, but commercial ceramic binder could not be removed from the support tube without damage to the tube or membrane. Though the degree of gas tightness increases in the order of glass>Ag>ceramic binder, in the case of glass seal, the undesired spreading of glass leads to an interfacial reaction between it and the membrane and reduction of effective permeation area. The oxygen flux of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ membrane increases with increasing temperature and decreasing thickness, and the oxygen permeation flux through 1.0 mm membrane exposed to flowing air (P _h =0.21 atm) and helium (P₁=0.037 atm) is ca. 0.33 ml/cm²·min at 950 °C. X-ray diffraction analysis for the membrane after permeation test over 160 h revealed that La₂O₃ and unknown compound were formed on the surface of membrane. The segregation compounds of surface elements formed on both surfaces of membrane irrespective of spreading of glass sealing material. This paper was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Electrochemical performance of La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Cr<Subscript>0.9</Subscript>M<Subscript>0.1</Subscript>O<Subscript>3</Subscript> perovskites as SOFC anodes in CO/CO<Subscript>2</Subscript> mixtures

The performance of La_0.75Sr_0.25Cr_0.9M_0.1O₃ (M = Mn, Fe, Co, and Ni) perovskitic materials as anodes was studied for a CO-fueled solid oxide fuel cell. The electrocatalytic performance and the tolerance to carbon deposition were investigated, while electrochemical characterization was carried out via AC impedance spectroscopy and cyclic voltammetry. The La_0.75Sr_0.25Cr_0.9Fe_0.1O₃ perovskite showed the best anode performance at temperatures above 900 °C; while at temperatures below 900 °C, the best performance was achieved with the La_0.75Sr_0.25Cr_0.9Co_0.1O₃ material. AC impedance spectroscopy was used for a semi-quantitative analysis of the LSC-M_0.1 anodes performance in view of total cell and charge transfer resistance. All anode materials exhibit high electronic conductivity and presumably do not substantially contribute to the overall cell resistance and concomitant ohmic losses.     

Investigation of the conditions for synthesis of Ce<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>2</Subscript> dense coatings

The conditions for preparation of Ce_0.9Y_0.1O₂ (CYO) oxide coatings on La_0.8Sr_0.2MnO₃ (LSM) ceramic substrates by screen printing were investigated. The CYO compound was synthesized by the pyrolysis of polymer-salt composites with the aim of producing submicron powders with a uniform size distribution. Transmission electron microscopy of the microstructure of the CYO compound synthesized with ethylene glycol revealed that the synthesis product consists of ultrafine crystalline particles with an average size of 5–15 nm. The use of CYO nanopowders made it possible to prepare rather dense single-layer coatings on LSM substrates. It was demonstrated that annealing of the coatings at high temperatures leads to the recrystallization and coarsening of particles.     

Electrochemical performance of La0.8Sr0.2MnO3 oxygen electrode promoted by Ruddlesden-Popper structured La2NiO4

The effects of introducing La₂NiO₄ nanocatalyst on the electrochemical performance of La_0.8Sr_0.2MnO₃ are investigated under solid oxide electrolysis cell and fuel cell modes, as well as open circuit voltage. Extracted data from impedance spectroscopy are interpreted with the analysis of distribution of relaxation times. La₂NiO₄ infiltration effectively reduces the activation energy of the oxygen reactions from 1.35 to 0.99 eV. It also changes the rate controlling process of the overall reaction. Polarization behavior of La₂NiO₄-infiltrated La_0.8Sr_0.2MnO₃ electrode shows superior performance under electrolysis mode compared to the fuel cell mode. Drastic increase in the size of low frequency arc during anodic current passage in the non-infiltrated La_0.8Sr_0.2MnO₃ electrode is hampered by infiltration of La₂NiO₄ nanocatalyst. By applying anodic current on infiltrated La_0.8Sr_0.2MnO₃, no displacement is observed in the position of high frequency peaks in the distribution of relaxation time graphs and only a small increase in height occurs for the low frequency arc. Additionally, La₂NiO₄-infiltrated electrode impressively decreases overpotential by 74% compared to the non-infiltrated one under electrolysis mode at 800°C.     

Electrochemical Properties of La0.5Sr0.5Co0.8M0.2O3–δ (M=Mn,Fe, Ni,Cu) Perovskite Cathodes for IT‐SOFCs

The electrochemical properties of La_0.5Sr_0.5Co_0.8M_0.2O_3–δ (M=Mn, Fe, Ni, Cu) cathodes are investigated with chemical bulk diffusion coefficients (D_chem) and polarization resistances. The electrochemical performance of long‐term testing for La_0.5Sr_0.5Co_0.8Cu_0.2O_3–δ cathode was carried out to investigate its electrochemical stability. In this work, an anode‐supported single cell with a thick‐film SDC electrolyte (30 μm), a Ni‐SDC cermet anode (1 mm), and a La_0.5Sr_0.5Co_0.8Cu_0.2O_3–δ cathode (10 μm) reaches a maximum peak power density of 983 mW/cm² at 700°C. Obviously, Cu substitution for B‐site of La_0.5Sr_0.5CoO_3–δ cathode reduced thermal expansion coefficient (TEC) value and enhanced oxygen bulk diffusion and electrochemical properties. La_0.5Sr_0.5Co_0.8Cu_0.2O_3–δ is a promising cathode material for intermediate temperature solid oxide fuel cells (IT‐SOFC).     

Influences of A- or B-site substitution on the activity of LaMnO<Subscript>3</Subscript> perovskite-type catalyst in oxidation of diesel particle

LaMnO₃ was partially substituted at A- or B-site by Sol-Gel method and characterized by XRD, SEM and BET. Perovskite oxides were formed in all substitutions. The catalytic activities of substituted catalysts on carbon black oxidation were measured by Temperature Programming Oxidation (TPO). Experimental results showed that all substitutions increased the catalytic activity of LaMnO₃, and La_0.8Cs_0.2MnO₃ showed the highest catalytic activity. Under tight contact, the activity enhancement of different substitutions decreased in the order Cs>K>V>Ce>Co>Cu>Fe. Dynamic analysis showed that partial substitutions increased the pre-exponential factor and the catalytic activity by increasing the oxygen vacancy on the catalyst surface. The active components on the surfaces of La_0.8Ce_0.2MnO₃ and LaMn_0.8V_0.2O₃ included CeO₂ and LaVO₄, which changed the apparent activities and dynamic parameters of these two catalysts. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Study of electrochemical properties and the charge/discharge mechanism for Li<Subscript>4</Subscript>Mn<Subscript>5</Subscript>O<Subscript>12</Subscript>/MnO<Subscript>2</Subscript>-AC hybrid supercapacitor

Spinel Li₄Mn₅O₁₂ was prepared by a sol–gel method. The manganese oxide and activated carbon composite (MnO₂-AC) were prepared by a method in which KMnO₄ was reduced by activated carbon (AC). The products were characterized by XRD and FTIR. The hybrid supercapacitor was fabricated with Li₄Mn₅O₁₂ and MnO₂-AC, which were used as materials of the two electrodes. The pseudocapacitance performance of the Li₄Mn₅O₁₂/MnO₂-AC hybrid supercapacitor was studied in various aqueous electrolytes. Electrochemical properties of the Li₄Mn₅O₁₂/MnO₂-AC hybrid supercapacitor were studied by using cyclic voltammetry, electrochemical impedance measurement, and galvanostatic charge/discharge tests. The results show that the hybrid supercapacitor has electrochemical capacitance performance. The charge/discharge test showed that the specific capacitance of 51.3 F g⁻¹ was obtained within potential range of 0–1.3 V at a charge/discharge current density of 100 mA g⁻¹ in 1 mol L⁻¹ Li₂SO₄ solution. The charge/discharge mechanism of Li₄Mn₅O₁₂ and MnO₂-AC was discussed.     

Moisture Effect on La0.8Sr0.2MnO3 and La0.6Sr0.4Co0.2Fe0.8O3 Cathode Behaviors in Solid Oxide Fuel Cells

Moisture effect on cathode behaviors is a major issue for solid oxide fuel cells servicing under severe high temperature environments. This work studies the effect of dry air and moist air on La_0.8Sr_0.2MnO₃ (LSM821) and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF6428) cathodes at 800 °C by investigating the interfacial reaction and degradation through an AISI 441 interconnect/LSM821 (LSCF6428) electrode/yttria‐stabilized zirconia (YSZ) electrolyte tri‐layer structure. Under the same processing condition, the grain size of the LSCF6428 cathode is smaller than that of the LSM821 cathode. Ohmic resistance and polarization resistance of the cathodes are analyzed by deconvoluting the electrochemical impedance spectroscopy (EIS) results. The LSCF6428 cathode has much smaller resistance than the LSM821 cathode. Moisture produces a larger effect on the ohmic resistance and polarization resistance of the LSM821 cathode than on those of the LSCF6428 cathode. More chromium diffuses from the interconnect to the cathode for both LSM821 and LSCF6428 electrodes thermally treated in moist air. Based on the structure, elemental distribution, and EIS analysis, the interaction mechanisms between the electrodes and the AISI 441 alloy interconnect are proposed.     

Oxidative reforming of methane on structured Ni-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/cordierite catalysts

The catalytic properties of Ni/Al₂O₃ composites supported on ceramic cordierite honeycomb monoliths in oxidative methane reforming are reported. The prereduced catalyst has been tested in a flow reactor using reaction mixtures of the following compositions: in methane oxidation, 2–6% CH₄, 2–9% O₂, Ar; in carbon dioxide and oxidative carbon dioxide reforming of methane, 2–6% CH₄, 6–12% CO₂, and 0–4% O₂, and Ar. Physicochemical studies include the monitoring of the formation and oxidation of carbon, the strength of the Ni-O bond, and the phase composition of the catalyst. The structured Ni-Al₂O₃ catalysts are much more productive in the carbon dioxide reforming of methane than conventional granular catalysts. The catalysts performance is made more stable by regulating the acid-base properties of their surface via the introduction of alkali metal (Na, K) oxides to retard the coking of the surface. Rare-earth metal oxides with a low redox potential (La₂O₃, CeO₂) enhance the activity and stability of Ni-Al₂O₃/cordierite catalysts in the deep and partial oxidation and carbon dioxide reforming of methane. The carbon dioxide reforming of methane on the (NiO + La₂O₃ + Al₂O₃)/cordierite catalyst can be intensified by adding oxygen to the gas feed. This reduces the temperature necessary to reach a high methane conversion and does not exert any significant effect on the selectivity with respect to H₂.     

Preparation and electrochemical performance of Li-rich layered cathode material,Li[Ni<Subscript>0.2</Subscript>Li<Subscript>0.2</Subscript>Mn<Subscript>0.6</Subscript>]O<Subscript>2</Subscript>, for lithium-ion batteries

The Li-rich layered cathode material, Li[Ni_0.2Li_0.2Mn_0.6]O₂, was synthesized via a “mixed oxalate” method, and its structural and electrochemical properties were compared with the same material synthesized by the sol–gel method. X-ray diffraction (XRD) shows that the synthesized powders have a layered O₃–LiCoO₂-type structure with the R-3m symmetry. X-ray photoelectron spectroscopy (XPS) indicates that in the above material, Ni and Mn exist in the oxidation states of +2 and +4, respectively. The layered material exhibits an excellent electrochemical performance. Its discharge capacity increases gradually from the initial value of 228 mA hg⁻¹ to a stable capacity of over 260 mA hg⁻¹ after the 10th cycle. It delivers a larger capacity of 258 mA hg⁻¹ at the 30th cycle. The dQ/dV curves suggest that the increasing capacity results from the redox-reaction of Mn⁴⁺/Mn³⁺.     

Validation of BaIn0.3Ti0.7O2.85 as SOFC Electrolyte with Nd2NiO4, LSM and LSCF as Cathodes

Due to its high ionic conductivity level, BaIn_0.3Ti_0.7O_2.85 (BIT07) is a potential electrolyte material for SOFC. In order to validate the use of this material, its chemical, mechanical and electromechanical compatibilities with three classical cathode materials (La_0.7Sr_0.3MnO_{3 – δ} (LSM), La_0.58Sr_0.4Co_0.2Fe_0.8O_{3 – δ} (LSCF) and Nd₂NiO_{4 + δ}) have been tested. The chemical compatibility has been demonstrated by XRD experiments, which show that BIT07 does not react with the cathode materials below 1,000 °C. The mechanical compatibility has been investigated by combining measurements of the thermal expansion coefficients of the cathode materials from X‐ray thermo diffraction and SEM observations. Symmetrical cells cathode/electrolyte/cathode have been studied by electrochemical impedance spectroscopy in order to quantify the quality of the cathode/electrolyte interface and to study the long‐term stability of the cell. The main conclusion of the study is that LSCF presents the best compromise between an acceptable mechanical compatibility and very good electrical properties with BIT07.     

Performance Evaluation of Solid Oxide Fuel Cells with Thin Film Electrolyte Fabricated by Binder‐Assisted Slurry Casting

A gas‐tight yttria‐stabilized zirconia (YSZ) electrolyte film was fabricated on porous NiO–YSZ anode substrates by a binder‐assisted slurry casting technique. The scanning electron microscope (SEM) results showed that the YSZ film was relatively dense with a thickness of 10 μm. La_0.8Sr_0.2MnO₃ (LSM)–YSZ was applied to cathode using a screen‐print technique and the single fuel cells were tested in a temperature range from 600 to 800 °C. An open circuit voltage (OCV) of over 1.0 V was observed. The maximum power densities at 600, 700, and 800 °C were 0.13, 0.44, and 1.1 W cm^–2, respectively.     

Mayenite supported perovskite monoliths for catalytic combustion of methyl methacrylate

To improve their thermal stability, La_0.8Sr_0.2MnO₃ cordierite monoliths are washcoated with mayenite, which is a novel Al-based material with the crystal structure of 12MO·7Al₂O₃ (M = Ca, Sr). The monoliths are characterized by means of nitrogen adsorption/desorption, scanning electron microscopy, and X-ray diffraction. Catalytic performances of the monoliths are tested for methyl methacrylate combustion. The results show that mayenite obviously improves both the physicchemical properties and the catalytic performance of the monoliths. Because mayenite improves the dispersity of La_0.8Sr_0.2MnO₃ and also prevents the interaction between La_0.8Sr_0.2MnO₃ and cordierite or γ-Al₂O₃, both crystal structure and surface morphology of La_0.8Sr_0.2MnO₃ phase can thereby be stable on the mayenite surface even at high temperature up to 1050 °C. Under the given reaction conditions, La_0.8Sr_0.2MnO₃ monolith washcoated with 12SrO·7Al₂O₃ shows the best catalytic activity for methyl methacrylate combustion among all the tested monoliths.     

Electrochemical promotion of CO<Subscript>2</Subscript> hydrogenation on Rh/YSZ electrodes

The electrochemical promotion of the CO₂ hydrogenation reaction on porous Rh catalyst–electrodes deposited on Y₂O₃-stabilized-ZrO₂ (or YSZ), an O²⁻ conductor, was investigated under atmospheric total pressure and at temperatures 346–477 °C, combined with kinetic measurements in the temperature range 328–391 °C. Under these conditions CO₂ was transformed to CH₄ and CO. The CH₄ formation rate increased by up to 2.7 times with increasing Rh catalyst potential (electrophobic behavior) while the CO formation rate was increased by up to 1.7 times with decreasing catalyst potential (electrophilic behavior). The observed rate changes were non-faradaic, exceeding the corresponding pumping rate of oxygen ions by up to approximately 210 and 125 times for the CH₄ and CO formation reactions, respectively. The observed electrochemical promotion behavior is attributed to the induced, with increasing catalyst potential, preferential formation on the Rh surface of electron donor hydrogenated carbonylic species leading to formation of CH₄ and to the decreasing coverage of more electron acceptor carbonylic species resulting in CO formation.     

Synthesis of LSM–YSZ–GDC dual composite SOFC cathodes for high-performance power-generation systems

This article investigates a method in further improvement of a (La_0.8,Sr_0.2)MnO₃ (LSM)-Yttria-stabilized zirconia (YSZ) dual composite cathode by adding material with high ionic conductivity such as gadolinia-doped ceria (GDC). A nano-porous composite cathode containing LSM, YSZ, and GDC was prepared by a two-step polymerizable complex (PC) method which minimizes the formation of YSZ–GDC solid solution. The structure of the resulting LSM/GDC–YSZ dual composite cathode was such that the LSM and GDC phases were present on the YSZ core particles without formation of the La₂Zr₂O₇, SrZrO₃, and GDC–YSZ solid solution. At 800 °C, the electrode polarization resistance of the LSM/GDC–YSZ dual composite cathode decreased to 0.266 Ω cm², compared with 0.385 Ω cm² for the LSM/YSZ–YSZ dual composite cathode. In addition, the Ni–YSZ anode-supported single cell using a LSM/GDC–YSZ dual composite cathode with H₂ as the fuel achieved a maximum power density of 0.65 W cm⁻² at 800 °C.     

Hydrothermal synthesis of MnO<Subscript>2</Subscript>/CNT nanocomposite with a CNT core/porous MnO<Subscript>2</Subscript> sheath hierarchy architecture for supercapacitors

MnO₂/carbon nanotube [CNT] nanocomposites with a CNT core/porous MnO₂ sheath hierarchy architecture are synthesized by a simple hydrothermal treatment. X-ray diffraction and Raman spectroscopy analyses reveal that birnessite-type MnO₂ is produced through the hydrothermal synthesis. Morphological characterization reveals that three-dimensional hierarchy architecture is built with a highly porous layer consisting of interconnected MnO₂ nanoflakes uniformly coated on the CNT surface. The nanocomposite with a composition of 72 wt.% (K_0.2MnO₂·0.33 H₂O)/28 wt.% CNT has a large specific surface area of 237.8 m²/g. Electrochemical properties of the CNT, the pure MnO₂, and the MnO₂/CNT nanocomposite electrodes are investigated by cyclic voltammetry and electrochemical impedance spectroscopy measurements. The MnO₂/CNT nanocomposite electrode exhibits much larger specific capacitance compared with both the CNT electrode and the pure MnO₂ electrode and significantly improves rate capability compared to the pure MnO₂ electrode. The superior supercapacitive performance of the MnO₂/CNT nancomposite electrode is due to its high specific surface area and unique hierarchy architecture which facilitate fast electron and ion transport.     

Development of high-capacity primary alkaline manganese dioxide/zinc cells consisting of Bi-doping of MnO<Subscript>2</Subscript>

The feasibility of using Bi-doped manganese dioxide as the cathode in a primary alkaline zinc cell is discussed. A Bi-doped MnO₂ cathode material made by the CMD process is examined with respect not only to its discharge in conditioning cycles but also with achievement of practically high voltage on discharge. With suitable composition of the Bi/MnO₂ composite, remarkably high C-rates for discharge and recharge are achieved with little polarization. The Bi–Mn–O compounds with possible structures are also discussed.     

Electrochemical characteristics of La0.6Sr0.4Co1−yFeyO3 (y=0.2–1.0) fiber cathodes

Electrospun La_xSr_1−xCo_1−yFe_yO₃ (LSCF) fibers with y=0.2 – 1.0 have been investigated as the cathode of intermediate solid oxide fuel cells (IT-SOFC). The electrochemical performances of LSCF (y=0.2–1.0) fibers were studied by impedance spectroscopy in symmetrical cells containing gadolinium doped ceria (CGO) electrolyte and LSCF electrode infiltrated with CGO. Impedance measurements showed that the impedance spectra have two or three semicircles, depending on the measurement temperature. The LSCF electrodes with higher cobalt content exhibit lower polarization resistance (R_p) and the La_0.6Sr_0.4Co_0.8Fe_0.2O₃ electrode displayed the lowest polarization resistance between 500 and 900 °C, classifying this composite cathode as a promising material for intermediate temperature SOFC based on CGO electrolyte.     

Improved electrochemical performance of Bi doped La0.8Sr0.2FeO3-δ nanofiber cathode for IT-SOFCs via electrospinning

In this study, perovskite La_0.8-xBi_xSr_0.2FeO_3-δ (LBSF, x = 0.0–0.5) nanofibers with great crystallinity were prepared by electrospinning method and used as cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). The symmetric cells of nanofiber-based LBSF electrode on Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte show excellent electrochemical performance. The La_0.4Bi_0.4Sr_0.2FeO_3-δ (LBSF4) cathode has the best performance with a polarization resistance (R_P) of 0.126 Ω cm² at 650 °C. The anode-supported single cell with LBSF4 as the cathode film and Ni-SDC as the anode has a maximum power density of 448 mW cm^-2 at 650 °C using wet H₂ as the fuel. In addition, the LBSF4 cathode with fibrous structure exhibits outstanding electrochemical behavior. The catalytic activity of the cathode was improved due to the incorporation of the Bi element, indicating that LBSF4 is promising as a cathode material in the field of IT-SOFCs.