(La0.75Sr0.25)(Cr0.5Mn0.5)O3/YSZ composite anodes for methane oxidation reaction in solid oxide fuel cells

The synthesis and performance of (La_0.75Sr_0.25)(Cr_0.5Mn_0.5)O₃/Y₂O₃–ZrO₂ (LSCM/YSZ) composites are investigated as alternative anodes for the direct utilization of methane (i.e., natural gas) in solid oxide fuel cells. Addition of YSZ phase greatly improves the adhesion and reduces the electrode polarization resistance of the LSCM/YSZ composite anodes. LSCM/YSZ composite anodes show reasonably good performance for the methane oxidation reaction in wet CH₄ and the best electrode performance was achieved for the composite with LSCM contents of 50–60 wt.% with polarization resistances of 2–3 Ω cm² in 97% CH₄/3% H₂O at 850 °C. The electrode impedance for the methane oxidation in wet CH₄ on the LSCM/YSZ composite anodes was characterized by three separable arcs and the electrode behavior could be explained based on the ALS model for the reaction on the MIEC electrode. The results indicate that electrocatalytic activity of the LSCM/YSZ composite anodes for the methane oxidation is likely limited by the oxygen vacancy diffusion in the substituted lanthanum chromite-based materials.     

Composite polymeric electrolytes based on PMMA-LiCF3SO3-SiO2

Preliminary results on the mechanical, optical and electrical properties of composite gel electrolytes (CGEs) with fumed silica (SiO₂) as a filler added to gel polymeric electrolyte (GPE) based on PMMA, LiCF₃SO₃ and PC are presented in this paper. Added fumed silica is seen to enhance the mechanical properties of the GPE without changing the conductivity significantly. The high ionic conductivity (×10⁻³ S/cm), high transmission in the visible region and nominal variance of conductivity and viscosity over a wide temperature window show that these CGEs are potential electrolytes for electrochromic windows (ECWs).     

Heterogeneous ceramics formed by grain boundary engineering

Two examples were selected to emphasize the potential of grain boundary engineering in the performance design of heterogeneous ceramics. Gadolinium-doped ceria-based powders were co-fired with additions of silica, and silica and lanthanum oxide, to test the silica scavenging role of lanthanum. The formation of one ionic conducting secondary phase, instead of an insulating phase, was attempted. The structural, microstructural, and electrical characterization of these samples confirmed the formation of one apatite-type lanthanum silicate-based phase and a significant enhancement of the grain boundary conductivity of these materials. One second approach addressed the formation of one mixed conductor, with electronically conductive grain boundaries, surrounding the grains of one lanthanum gallate-based electrolyte (core-shell type microstructure). Fe-doped grain boundaries were formed by selective Fe-diffusion (thermally assisted) from lanthanum ferrite screen printed layers. Combined microstructural and electrical characterization showed that the adopted solution was also effective.     

Oxygen ion conductors for fuel cells and membranes: selected developments

Recent developments on single-phase and composite materials with oxygen ionic conductivity are briefly reviewed. Attention is focused on electrolytes with apatite-like structure, yttria-stabilized zirconia (YSZ) ceramics containing silica and apatite admixtures, and composite mixed conductors where chemical interaction of the components is large or severely constrained. The results show, in particular, a key role of phase interaction and microstructural design on materials performance. The ideas behind the adopted procedures can be extended to other systems and combinations of properties based on thermodynamic equilibria information.     

Performance of ferrite fillers on electrical behavior of polymer nanocomposite electrolyte

Dispersal of nanofillers in polymer electrolytes have shown to improve the ionic properties of Polyethylene oxide (PEO)-based polymer electrolytes in recent times. The effects of different nanoferrite fillers (i.e., Al–Zn ferrite, Mg–Zn ferrite, and Zn ferrite) on the electrical transport properties have been studied here on the composite polymer electrolyte system. The interaction of salt/filler with electrolyte has been investigated by XRD studies. SEM image and infrared spectral studies give an indication of nanocomposite formation. In conductivity studies, all electrolyte systems are seen to follow universal power law. Composition dependence (with ferrite filler) gives the maximum conductivity in [93PEO–7NH₄SCN]: X ferrite (where X?=?2% in Al–Zn ferrite, 1% Mg–Zn ferrite, and 1% Zn ferrite) system.     

Characterization of doped La0.7A0.3Cr0.5Mn0.5O3 − δ (A = Ca,Sr, Ba) electrodes for solid oxide fuel cells

Doped lanthanum manganese chromite based perovskite, La_0.7A_0.3Cr_0.5Mn_0.5O_3 ? δ (LACM, A = Ca, Sr, Ba), on yttria-stabilized zirconia (YSZ) electrolyte is investigated as potential electrode materials for solid oxide fuel cells (SOFCs). The electrical conductivity and electrochemical activity of LACM depend on the A-site dopant. The best electrochemical activity is obtained on the La_0.7Ca_0.3Cr_0.5Mn_0.5O_3 ? δ/YSZ (LCCM/YSZ) composite electrodes. The conductivity of LCCM is 29.9 S cm^? 1 at 800 °C in air, and the electrode polarization resistance (R_E) of the LCCM/YSZ composite cathode for the O₂ reduction reaction is 0.5 Ω cm² at 900 °C. The effect of Gd-doped ceria (GDC) impregnation on the LCCM cathode polarization resistances is also studied. GDC impregnation significantly enhances the electrochemical activity of the LCCM cathode. In the case of the 6.02 mg cm^? 2 GDC-impregnated LCCM cathode, R_E is 0.4 Ω cm² at 800 °C, ~ 60 times smaller than 24.4 Ω cm² measured on a LCCM cathode without the GDC impregnation. Finally the electrochemical activities of the doped lanthanum manganese chromites for the H₂ oxidation reaction are also investigated.     

Spray pyrolysis of electrolyte interlayers for vacuum plasma-sprayed SOFC

The effects of gadolinia-doped ceria (CGO, Ce_0.8Gd_0.2O_1.9−x) and yttria-doped zirconia (8YSZ, Zr_0.92Y_0.08O_2−x) interlayers prepared by spray pyrolysis between vacuum plasma-sprayed 8YSZ electrolytes (8YSZ–VPS) and screen-printed (La_0.8Sr_0.2)_0.98MnO₃ cathodes (LSM) on the power output of solid oxide fuel cells (SOFC) are investigated. Amorphous thin films are deposited and then converted to nanocrystalline electrolyte–cathode interlayers during the first heat-up cycle of a SOFC to the operating temperature. CGO thin films between the YSZ plasma-sprayed electrolyte and the LSM cathode increased the power output by more than 20% compared to cells without interlayers, whereas YSZ films degraded the power output of cells. It is assumed that CGO improves the charge transfer at the electrolyte–cathode interface and that the CGO layer prevents the formation of undesirable insulation of La-zirconate at the interface 8YSZ/LSM.     

Characterization of CaTi0.9Fe0.1O3/La0.98Mg0.02NbO4 composite

A composite of CaTi_0.9Fe_0.1O₃ and electrolyte material, i.e. magnesium doped La_0.98Mg_0.02NbO₄ was prepared and studied. The phase content and the sample microstructure was examined by an X-ray diffraction method and scanning electron microscopy. EDS measurements were done both for composite samples and the diffusion couple. The electrical properties were studied by four terminal DC method. The high-temperature interaction between the two components of the composite has been observed. It has been suggested that lanthanum diffused into the perovskite phase and substituted for calcium whereas calcium and niobium formed the Ca₂Nb₂O₇ pyrochlore phase. At 1500°C very large crystallites of the pyrochlore were observed. Regardless of strong interaction between the composite components, its total conductivity was weakly dependent on the sintering temperature.     

Behaviour of a dense In2O3/ YSZ electrode in oxygen containing atmospheres

A study of electrical and electrochemical properties of a dense In₂O₃ electrode in contact with a single crystal YSZ electrolyte was carried out by d.c. and a.c. methods. As a result, it was found that dense In₂O₃ electrodes have high electrical conductivity but very low electrochemical activity. In a vicinity of the equilibrium potential and under the anodic polarisation, the rate of Faraday reaction at the In₂O₃ electrodes was as low as to consider the electrode a blocking one. The blocking properties of the In₂O₃ electrodes were used to measure the hole conductivity of the YSZ electrolyte in the temperature range between 795 to 1163 K and oxygen partial pressure from 1 to 10⁵ Pa. A comparison with the literature data confirmed that the dense In₂O₃ electrode blockes the ionic transfer through the YSZ. A set of experiments indicated that the oxygen exchange between the indium oxide surface presented to the oxygen containing gaseous phase and this phase is very poor. A route of the electrode process at O₂, In₂O₃ / YSZ electrode was proposed a limiting stage of which is the discharge of the oxygen ions to the atomic oxygen adsorbed on the electrode surface: $$O_0 ^x \left( {In_2 O_3 } \right)_s = V_0 ^{ \bullet \bullet } \left( {In_2 O_3 } \right)_s + O_{ad} \left( {In_2 O_3 } \right)_s + 2e'\left( {In_2 O_3 } \right).$$ The polarisation resistance decreases when platinum or the praseodymium oxide is deposited on the surface of the In₂O₃ electrodes. The cathodic polarisation also increases the electrochemical activity of the electrodes. Both the establishment of the steady state of the electrode under polarisation and the recovery of the equilibrium state by the electrode are very long processes, which are probably related to the diffusion mechanism by which the stoichiometry of the indium oxide is changed.     

Composite polymeric electrolytes based on PMMA-LiCF3SO3-SiO2

Preliminary results on composite gel electrolytes (CGEs) with fumed silica (SiO₂) as filler added to gel polymeric electrolyte (GPE) based on PMMA, LiCF₃SO₃ and PC are presented in this paper. Added fumed silica is seen to enhance the mechanical properties of the GPE without changing the σ significantly. The high ionic conductivity (×10⁻³ S/cm), high transmission in the visible region and nominal variance of σ and ν over a wide temperature window makes these CGEs potential electrolytes for electrochromic windows (ECWs). Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Effect of sintering temperature on the elemental diffusion and electrical conductivity of SrTiO<Subscript>3</Subscript>/YSZ composite ceramic

The 50 vol% SrTiO₃/yttria-stabilized zirconia (YSZ) composite ceramic was prepared through powder sintering route in 1400～1500 °C. Only the cubic YSZ and SrTiO₃ phases are detected in all the sintered ceramics, and the typical XRD peak positions of both phases have varied dramatically. The grain sizes and relative densities of all specimens increase evidently with the sintering temperature. The width of the SrTiO₃/YSZ interfacial region increases from 100.4 to 468.8 nm as the sintering temperature rises from 1400 to 1500 °C. The total electrical conductivities of the sample sintered at 1500 °C are remarkably higher than those at 1400 and 1450 °C, while the ion transference numbers drop from 0.837 to 0.731 with sintering temperature from 1400 to 1500 °C. The variations in the electrical properties above can be interpreted based on the effects of sintering temperature on the elemental diffusions during the sintering process.     

High performance PEO-based polymer electrolytes and their application in rechargeable lithium polymer batteries

Solvent-free, lithium-ion-conducting, composite polymer electrolytes have been prepared by a double dispersion of an anion trapping compound, i.e., calyx(6)pyrrole, CP and a ceramic filler, i.e., super acid zirconia, S-ZrO₂ in a poly(ethylene oxide)-lithium bis(oxalate) borate, PEO–LiBOB matrix. The characterization, based on differential thermal analysis and electrochemical analysis, showed that while the addition of the S-ZrO₂ has scarce influence on the transport properties of the composite electrolyte, the unique combination of the anion-trapping compound, CP, with the large anion lithium salt, LiBOB, greatly enhances the value of the lithium transference number without depressing the overall ionic conductivity. These unique properties make polymer electrolytes, such as PEO₂₀LiBOB(CP)_0.125, of practical interest, as in fact confirmed by tests carried out on lithium battery prototypes.     

Physico-chemical investigations on selected gallium doped lanthanum chromites

Investigations on lanthanum gallium chromium mixed oxides of the compositions La_1.0Ga_0.2Cr_0.8O_3−d and La_0.9Ga_0.2Cr_0.8O_3-d are presented regarding their structure, redox stability, conductivity and catalytic activity for the oxidation of propene. The mixed oxide has perovskite-type structure, high redox stability, an electronic p-type conductivity and low catalytic activity. If the perovskite-type compound has a deficiency of lanthanum, the electronic conductivity, surface area and catalytic activity are significantly higher. The catalytic activity is likely comparable to that of gold. Similar to gold electrodes of solid electrolytes, oxygen electrodes formed with gallium doped lanthanum chromite show a relatively high sensitivity to hydrocarbons such as propene at temperatures of about 700 °C. The mixed oxide is possibly suitable as electrode material for exhaust gas sensors using oxide-ion conducting solid electrolytes. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, September 13–20, 1998.     

Highly dispersed anodes for solid oxide fuel cells using NiO/YSZ/BZY triple-phase composite powders prepared by spray pyrolysis

NiO/Y₂O₃-stabilized ZrO₂ (YSZ)/Y-doped BaZrO₃ (BZY) triple-phase composite powders were prepared by spray pyrolysis and evaluated for Ni/YSZ/BZY cermet anodes, which are considered effective for dry CH₄ operation in solid oxide fuel cells. The structure of the particles in these powders was fine crystal fragments, and the individual material phases were clearly separated and highly dispersed within the particles. The Ni/YSZ/BZY cermet anodes fabricated with these composite powders maintained a fine electrode microstructure equivalent to that in a simple Ni/YSZ cermet anode manufactured using a composite powder prepared by spray pyrolysis. Furthermore, the addition of BZY improved the anode performance in humidified H₂ and dry CH₄ operation.     

Iron oxide nanoparticles hosted in silica aerogels

The investigated hybrid materials consist of non-agglomerated iron oxide particles hosted in silica aerogels. The composite material can be produced as a monolith, in any shape, and with different dilutions of the iron oxide phase. Two sol–gel chemistry routes have been followed: a solution of Fe(NO₃)₃·9H₂O has been added either to the silica gel or to the initial sol; in the latter, the iron salt provided the water required for the gel polymerisation. To obtain monolithic aerogels, the gels were dried by hypercritical solvent evacuation. On the other hand, some gels were dried by slow and controlled evaporation of the solvent, resulting in xerogels. Several heat treatments have been performed and the iron oxide particle phase, growth mechanism and crystallinity have been analysed. The composite materials were characterised by means of X-ray diffraction, M?ssbauer spectrometry and superconducting quantum interference device magnetometry. It was observed that the particle sizes (in the nanometre range) and the thermal stability of the iron oxide phases strongly depend on the preparation method that determines the microstructure of the host material. Consequently, the magnetic properties of the nanoparticles can be controlled via synthesis conditions, matrix properties and thermal treatments. Received: 5 March 2001 / Accepted: 16 June 2001 / Published online: 30 August 2001     

Specifics of the Electrical Properties of Composite Solid Oxide Membranes Based on SrTi<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>O<Subscript>3–δ</Subscript>

The electrical properties of dual-phase fluorite-pervoskite oxide systems based on strontium titanate- ferrite (SrTi_0.5Fe_0.5O_3–δ) are studied. We find that the oxygen ionic and ambipolar conductivities of strontium titanate-ferrite can be considerably improved by introducing the fluorite phase Ce_0.8(Sm_0.8Sr_0.2)_0.2O_2–δ. This is advantageous considering the prospects of applying these types of composite materials in different electrochemical devices, e.g., as membrane materials in electrochemical converters for the production of hydrogen and syngas and anode materials in solid oxide fuel cells.     

Effect of ZrO2 on the conductivity of PVC-LiAsF6-DBP polymer electrolytes

The preparation and characterization of composite polymer electrolytes PVC-LiAsF₆-DBP for different concentrations of ZrO₂ have been investigated. X-ray and FTIR studies indicate complex formation between the polymer and salt and that the complex remains mainly in the amorphous phase. The electrical conductivity values measured by a.c impedance spectroscopy are found to depend upon the ZrO₂ concentration. The temperature dependence of the conductivity of the polymer films obeys the VTF relation. The conductivity values are presented and the results are discussed.     

Electrical properties of thin yttria-stabilized zirconia overlayers produced by atomic layer deposition for solid oxide fuel cell applications

Thin films of yttria-stabilized zirconia (YSZ) electrolyte were prepared by atomic layer deposition at 300 °C for solid oxide fuel cell (SOFC) applications. YSZ samples of 300-1000 nm thickness were deposited onto La_0.8Sr_0.2MnO₃ (LSM) cathodes. A microstructural study was performed on these samples and their electrical properties were characterised between 100 and 390 °C by impedance spectroscopy. A remarkable feature is that the as-deposited layers were already crystalline without any annealing treatment. Their resistance decreased when reducing the layer thickness; nevertheless, their conductivity and activation energy were significantly lower than those reported in the literature for bulk YSZ.     

Microstructure and performance of novel Ni anode for hollow fibre solid oxide fuel cells

Nickel anodes were deposited on hollow fibre yttria-stabilised zirconia (YSZ) electrolyte substrates for use in solid oxide fuel cells (SOFCs). The hollow fibres are characterised by porous external and internal surfaces supported by a central gas-tight layer (300 μm total wall thickness and 1.6 mm external diameter). The YSZ hollow fibres were prepared by a phase inversion technique followed by high temperature sintering in the range 1200 to 1400 °C. Ni anodes were deposited on the internal surface by electroless plating involving an initial catalyst deposition step with PdCl₂ followed by Ni plating (with a NiSO₄, NaH₂PO₂ and sodium succinate based solution at 70 °C). Fabrication and nickel deposition parameters (nature of solvents, air gap, temperature, electroless bath composition) and heat treatments in oxidising/reducing environments were investigated in order to improve anode and electrolyte microstructure and fuel cell performance. A parallel study of the effect of YSZ sintering temperature, which influences electrolyte porosity, on electrolyte/anode microstructure was performed on mainly dense discs (2.3 mm thick and 15 mm diameter). Complete cells were tested with both disc and hollow fibre design after a La_0.2Sr_0.8Co_0.8Fe_0.2O_3?δ (LSCF) cathode was deposited by slurry coating and co-fired at 1200 °C. Anodes prepared by Ni electroless plating on YSZ electrolytes (discs and hollow fibres) sintered at lower temperature (1000–1200 °C) benefited from a greater Ni penetration compared to electrolytes sintered at 1400 °C. Further increases in anode porosity and performance were achieved by anode oxidation in air at 1200–1400 °C, followed by reduction in H₂ at 800 °C.     

CeO<Subscript>2</Subscript> and Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> modified Ni-YSZ anode for solid oxide fuel cell

Ni sintering at high temperature (～ 800 °C) operation drastically degrades the performance of Ni-yttria-stabilized zirconia (YSZ) anode in solid oxide fuel cell (SOFC). Mixed ionic and electronic conductive oxides such as CeO₂ and Nb₂O₅ enhance the dispersion of Ni, CeO₂ enhances the redox behavior and promotes charge transfer reactions, and Nb₂O₅ increases the triple phase boundary. In the present work, anode-supported SOFC is fabricated and tested in H₂ fuel at 800 °C. YSZ and lanthanum strontium manganite (LSM)-YSZ are used as the electrolyte and composite cathode with NiO-YSZ, CeO₂-NiO-YSZ, and Nb₂O₅-NiO-YSZ as an anode. The peak power density obtained for the cell with 10% CeO₂–30% NiO-YSZ anode at the 5 and 25 h of operation is 330 and 290 mW cm^?2 which is higher than that for 40% NiO-YSZ anode (275 mW cm^?2 at 5 h). The peak power density obtained for the cell with 10% Nb₂O₅–30% NiO-YSZ anode at the 5 and 25 h of operation is 301 and 285 mW cm^?2 which is higher than that for 40% NiO-YSZ anode (275 mW cm^?2 at 5 h). Physical characterization has been carried to study morphology, elemental analysis, particle size, and phase formation of the fabricated anode before and after cell operation to correlate the cell performance.