A novel anode-supported stable BaCe0.7Ta0.1Y0.2O3− δ membrane prepared by all-solid-state process for solid oxide fuel cells

BaCe_0.7Ta_0.1Y_0.2O_{3− δ} (BCTY) and BaCe_0.8Y_0.2O_{3− δ} (BCY) were synthesized by solid-state reaction method at 1,300 °C for 20 h. After being exposed in 3% CO₂ + 3% H₂O + 94% N₂ at 700 °C for 20 h, the BCTY exhibited adequate chemical stability against carbonations while BCY decomposed into BaCO₃ and CeO₂. The BCTY showed the similar thermal expansion behavior to BCY from room temperature to 1,000 °C in air. The BCTY displayed a conductivity of 0.007 S/cm at 700 °C in humid hydrogen, lower than that of BCY (0.009 S/cm). A fuel cell with 10-μm thick BCTY membrane prepared through an all-solid-state process exhibited 1.004 V for OCV, 330 mW/cm² for maximum output at 700 °C, respectively. Short-term test shows that the fuel cell performance does not degrade after 20 h.     

Hydrogen separation from syngas using high-temperature proton conductors

Hydrogen pumps using high-temperature proton conductors have been examined as a candidate means of hydrogen separation from syngas. Durability to CO₂ is a main concern for the alkali-earth-containing perovskites and was tested for a few typical compositions. Ce-excluded and cerate–zirconate solid solution electrolytes, SrZr_0.9Y_0.1O_3-α, and BaCe_0.6Zr_0.3Nd_0.1O_3-α were stable in CO₂-containing atmospheres, whereas they showed poor overpotential characteristics when platinum electrodes were used. It is demonstrated that the hydrogen pumping properties can be much improved for the case of SrZr_0.9Y_0.1O_3-α by the use of palladium anode and SrCe_0.95Yb_0.05O_3-α interlayer for the cathode.     

H2/Pt/Ce0.9Gd0.1O1.95/Pt/O2 fuel cell operated in the intermediate temperature range 500 – 700°C

Ce_0.9Gd_0.1O_1.95 (GCO), is one of the potential candidate electrolytes for intermediate temperature Solid Oxide Fuel Cells (ITSOFC). GCO has high oxide ion conductivity in the intermediate temperature range (500 – 700 °C) compared to other Ce_1−yGd_yO_2-2/y compositions and the Gd³⁺ ion is the most appropriate dopant material compared to other rare earth materials such as Sm³⁺, Y³⁺, Zr³⁺, etc. Our results show that the fuel cell H₂/Pt/Ce_0.9Gd_0.1O_1.95/Pt/O₂ operated in the temperature range 500 – 700°C gives the maximum power densities 0.0049 W/cm² at 500 °C and 0.0126 W/cm² at 650 °C for cell voltages 0.6275 V and 0.6278 V, respectively, where the electrolyte was kept in 5% H₂ (+ Argon) for 12 hours before use in the fuel cell. Maximum power densities are 0.0038 W/cm² at 500 °C and 0.0270 W/cm² at 650 °C for cell voltages 0.5986 and 0.5913 V, respectively, where the electrolyte was kept in 2 % O₂ (+ Argon) for 12 hours before use in the fuel cell. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Structural,thermal and electrical properties of Bi and Y co-doped barium zirconium cerates

Bismuth- and yttrium-co-doped barium cerates were successfully synthesised by solid-state reactions followed by sintering between 1,400 and 1,500 °C for 1 to 6 h allowing densification above 98 % to be obtained. All samples were found to retain their original orthorhombic structure after treatment in either oxidising or reducing atmospheres (dry and wet). Mechanical strength was affected by structure upon reduction due in part to strains and stresses induced by bismuth ionic size variations. Conductivity values as high as 0.055 S/cm were obtained for sample BaCe_0.6Zr_0.1Y_0.1Bi_0.2O_3?δ and of 0.0094 S/cm for the Zr-free compound BaCe_0.7Y_0.2Bi_0.1O_3?δ at 700 °C in air. In all the investigated materials, sample BaCe_0.6Zr_0.1Y_0.1Bi_0.2O_3?δ exhibits the highest conductivity in both air and wet 5 % H₂/Ar with good mechanical strength. BaCe_0.6Zr_0.1Y_0.1Bi_0.2O_3?δ is a promising mixed H⁺/e^? conductor, a potential component of composite anode for solid oxide fuel cells.     

Electrical conductivity and chemical stability of perovskite-type BaCe0.8-xTixY0.2O3-δ

Here we report the synthesis, chemical stability, and electrical conductivity of Ti-doped perovskite-type BaCe_0.8-x Ti _x Y_0.2O_3-δ (x = 0.05, 0.1, 0.2, and 0.3; BCTY). Samples were synthesized by conventional solid state (ceramic) reaction from corresponding metal salts and oxides at elevated temperature of 1,300–1,500 °C in air. The powder X-ray diffraction confirmed the formation of a simple cubic perovskite-type structure with a lattice constant of a = 4.374(1), 4.377(1), and 4.332(1) ? for x = 0.05, 0.1, and 0.2 members of BCTY, respectively. Like BaCe_0.8Y_0.2O_3-δ (BCY), Ti substituted BCTY was found to be chemically not stable in 100% CO₂ and form BaCO₃ at elevated temperature. The bulk electrical conductivity of BCTY decreased with increasing Ti content and the x = 0.05 member exhibited the highest conductivity of 2.3 × 10⁻³ S cm⁻¹ at 650 °C in air, while a slight increase in the conductivity, especially at low temperatures (below 600 °C), was observed in humidified atmospheres.     

Performance of palladium electrode for electrochemical hydrogen pump using strontium-zirconate-based proton conductors

An electrode design with no use of three-phase boundary was investigated using palladium electrode. The hydrogen evolution rate of the palladium electrode cell using SrZr_0.9Y_0.1O_3 − α electrolyte followed Faraday’s law up to 180 mA cm⁻², and the anode and cathode overpotentials were significantly lower than those of a platinum electrode cell, suggesting that the palladium electrode is effective to improve the performance of the hydrogen-pumping cell using SrZrO₃-based electrolyte. The rate-determining step (RDS) for electrode reaction was also investigated by changing the electrode morphology and hydrogen partial pressure, and it was suggested that the RDS of the anode is a reaction at electrode/electrolyte interface.     

Structural and electrical properties of pure and H<Subscript>2</Subscript>SO<Subscript>4</Subscript> doped (PVA)<Subscript>0.7</Subscript>(NaI)<Subscript>0.3</Subscript> solid polymer electrolyte

Solid polymer electrolytes (SPE) based on poly-(vinyl alcohol) (PVA)_0.7 and sodium iodide (NaI)_0.3 complexed with sulfuric acid (SA) at different concentrations were prepared using solution casting technique. The structural properties of these electrolyte films were examined by X-ray diffraction (XRD) studies. The XRD data revealed that sulfuric acid disrupt the semi-crystalline nature of (PVA)_0.7(NaI)_0.3 and convert it into an amorphous phase. The proton conductivity and impedance of the electrolyte were studied with changing sulfuric acid concentration from 0 to 5.1 mol/liter (M). The highest conductivity of (PVA)_0.7(NaI)_0.3 matrix at room temperature was 10⁻⁵ S cm⁻¹ and this increased to 10⁻³ S cm⁻¹ with doping by 5.1 M sulfuric acid. The electrical conductivity (σ) and dielectric permittivity (ε′) of the solid polymer electrolyte in frequency range (500 Hz–1 MHz) and temperature range (300–400) K were carried out. The electrolyte with the highest electrical conductivity was used in the fabrication of a sodium battery with the configuration Na/SPE/MnO₂. The fabricated cells give open circuit voltage of 3.34 V and have an internal resistance of 4.5 kΩ.     

A study on ac and dc conductivity characteristics of Y-doped BaCeO3 modified by Ar+ ion beam

The effects of surface modification on electrical characteristics in bulk, grain boundary and interface (electrolyte/electrode) of BaCe_0.9Y_0.1O_3-δ were investigated. The surface modification was performed by means of two processes: specimen was firstly irradiated by 10 keV Ar⁺ ion with dose of 1 × 10¹⁸ ions/cm² and then exposed to air. The modified surface was investigated by elastic recoil detection analysis (ERDA) for quantitative analysis of hydrogen concentration on the surface and alternating current (AC) and direct current (DC) conductivity measurements, respectively. The ERDA results showed that hydrogen concentration and reaction rate on the modified surface increased. The increase of hydrogen concentration was explained in terms of the increase of proton due to interaction between oxygen vacancy formed by modification and H₂O. In AC and DC electrical conductivity measurements, it concluded that the proton and electronic carrier generated on the surface by modification attributed to the increase of bulk, grain boundary and interface conductivity.     

Preparation of proton conducting BaCe0.8Gd0.2O3 thin films

Thin films of BaCe_0.8Gd_0.2O₃ were prepared by solid state reaction of two screen-printed layers over porous substrates. The first layer consists of the oxygen ion conductor Ce_0.8Gd_0.2O₂ with a fluorite structure, whereas the top layer consists of BaCO₃. After decomposition of the carbonate, BaO reacts with Ce_0.8Gd_0.2O₂ forming the perovskite oxide BaCe_0.8Gd_0.2O_3−δ with protonic conductivity. The in-situ reaction and densification on the porous substrates results in gastight thin layers of 10 to 50 μm and allows overcoming the problems due to the poor sinterability of the proton conductor. Two different porous substrates prepared by warm-pressing were studied as membrane supports, i.e., (i) porous composite NiO–Zr_0.85Y_0.15O₂, commonly employed as solid oxide fuel cell anode and (ii) porous Ce_0.8Gd_0.2O₂ oxide. The structural properties of the layer, compositional gradients and occurring phases are described, as well as water uptake, gastightness (He leaking rate) and emf measurement. Protonic conducting membranes are particularly suited not only for hydrogen separation combined with reforming and water–gas-shift converters but also as a protonic fuel cell electrolyte.     

Pulsed laser deposition of high temperature protonic films

Pulsed laser deposition has been used to fabricate nanostructured BaCe_0.85Y_0.15O_3−δ films. Protonic conduction of the fabricated BaCe_0.85Y_0.15O_3−δ films was compared to the sintered BaCe_0.85Y_0.15O_3−δ. Sintered samples and laser targets were prepared by sintering BaCe_0.85Y_0.15O_3−δ powders derived by solid state synthesis. Films 1 to 8 μm thick were deposited by KrF excimer laser on porous Al₂O₃ substrates. Thin films were fabricated at deposition temperatures of 700 to 950 °C at O₂ pressures up to 200 mTorr using laser pulse energy densities of 1.4–3 J/cm². Fabricated films were characterized by X-ray diffraction, electron microscopy and electrical impedance spectroscopy. Single phase BaCe_0.85Y_0.15O_3−δ films with a columnar growth morphology are observed with preferred crystal growth along the [100] or [001] direction. Results indicate [100] growth dependence upon laser pulse energy. Electrical conductivity of bulk samples produced by solid state sintering and thin film samples were measured over a temperature range of 100 to 900 °C. Electrical conduction behavior was dependent upon film deposition temperature. Maximum conductivity occurs at deposition temperature of 900 °C; the electrical conductivity exceeds the sintered specimen. All other deposited films exhibit a lower electrical conductivity than the sintered specimen. Activation energy for electrical conduction showed dependence upon deposition temperature, it varied from 115 to 54 kJ. Film microstructure was attributed to the difference in electrical conductivity of the BaCe_0.85Y_0.15O_3−δ films.     

Electrical conductivity of BaCe0.9Nd0.1O3−α in H2+H2O+Ar gas mixture

Electroconductivity of BaCe_0.9Nd_0.1O_3−α was studied as a function of the composition of the H₂+H₂O+Ar mixture and temperature in the interval from 873 to 1173 K. It was shown that the electroconductivity was independent of P_H2 (0.97 to 0.10 atm) and P_O2 (10⁻²¹ to 10⁻²⁶ atm), but depended on P_H2O (0.08 to 0.005 atm). A mathematical processing of the P_H2O dependencies of the electroconductivity, which was performed in terms of a classical model of defect formation in high-temperature proton-conducting solid electrolytes, yielded equilibrium constants of the reaction of water dissolution in BaCe_0.9Nd_0.1O_3−α and mobilities of protons and oxygen ions. The temperature dependencies of these quantities were used to determine the mobility activation energies of protons (E_a=34±7 kJ/mole) and oxygen ions (E_a=72±8 kJ/mole), and also the enthalpy (ΔH=−150±25 kJ/mole) and the entropy (ΔS=153±26 kJ/mole·K) of the reaction of water dissolution in BaCe_0.9Nd_0.1O_3−α.     

The development of multivalent ion conductors and their application for chemical sensors

New types of multivalent ion conducting solid electrolytes have been extensively developed and their applications for chemical sensors were investigated. Among the trivalent ion conductors, the highest ion conductivity was realized with the (Al_xZr_1−x)_4/(4−x)Nb(PO₄)₃ solid electrolyte and the value reaches the region between yttria stabilized zirconia (YSZ) and calcia stabilized zirconia (CSZ) of the representative oxide anion conductors. The above described Al³⁺ ion conducting (Al_xZr_1−x)_4/(4−x)Nb(PO₄)₃ solid electrolyte was combined with YSZ, with accompanying the Y₂O₃-KNO₂ solid solution as an auxiliary electrode for nitrogen monoxide (NO) gas sensing. The sensor response was rapid and a reproducible output was continuously observed obeying the Nernst theoretical relationship in a typical NO gas content region in exhaust gases. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Application of ionic liquid doped solid polymer electrolyte

The electrical, structural, and photoelectrochemical properties of the polymer electrolyte PEO:NaI/I₂ doped with an ionic liquid 1-ethyl 3-methylimidazolium dicyanamide (EMImDCN) have been reported. Incorporation of the ionic liquid (IL) increases the membrane homogeneity, decreased surface roughness, and enhances the short current (J _sc). Additionally, the doping of IL provides more charge carriers which enhances overall ionic conductivity (σ). The optimized σ was found at 40 wt.% IL composition. The fabricated DSSC using this new solid electrolyte showed 1.43% efficiency at 100 mW cm⁻².     

Reactive sputtering deposition and electrochemical characterisation of thin solid electrolyte membranes for solid oxide fuel cells

Solid oxide fuel cells directly convert the chemical energy of a fuel into electricity. To enhance the efficiency of the fuel cells, the thickness of the gastight solid electrolyte membranes should be as thin as possible. Y₂O₃-stabilised ZrO₂ (YSZ) electrolyte films were prepared by reactive sputtering deposition using Zr/Y targets in Ar/O₂ atmospheres. The films were 5 – 8 μm thin and were deposited onto anode substrates made of a NiO/YSZ composite. After deposition of a cathode with the composition La_0.65Sr_0.35MnO₃ the electrochemical properties of such a fuel cell were tested under operating conditions at temperatures between 600 °C and 850 °C. Current-voltage curves were recorded and impedance measurements were performed to calculate apparent activation energies from the fitted resistance data. The conductivity of the YSZ films varied between 4.6·10⁻⁶ S/cm and 2.2·10⁻⁵ S/cm at 400 °C and the fuel cell gave a reasonable power density of 0.4 W/cm² at 0.7 V and 790 °C using H₂ with 3 % H₂O as fuel gas. The gas compositions were varied to distinguish the electrochemical processes of the anode and cathode in the impedance spectra. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Synthesis of Yb-doped Ba(Ce,Zr)O3 ceramic powders by sol–gel method

This study was aimed to synthesize a ceramic electrolyte of BaCe_0.76Zr_0.19Yb_0.05O_2.975 using acetate and chloride precursors. The transparent sol was stable for more than 10 months but only 3 months for the gel. The dried gel was characterized using thermogravimetric analysis/differential scanning calorimetry, particle analyzer, X-ray diffraction (XRD), and scanning electron microscopy/dispersive X-ray (SEM/EDX) analysis. Thermal behavior analysis showed three steps of weight losses. All the processes were exothermic reactions as shown by derivative thermogravimetric analysis signal. Two groups of particle size were observed in the particle size distribution for the sample in the form of sol and powders ranging from 100 to 1,100 nm. A high purity single-phase sample with orthorhombic structure was identified by XRD. The theoretical density estimated from the unit cell parameters was 6.40 g cm⁻³. SEM of the powdered sample showed homogeneous distribution of particles and the grains size were in the range of 0.7–1.3 μm. EDX data revealed that the residue of small amount of chloride (≈0.25%) was still present in the final product of the compound.     

New χ (3)-nonlinear-laser manifestations in tetragonal LuVO4 crystal: more than sesqui-octave Raman-induced Stokes and anti-Stokes comb generation and cascaded self-frequency “tripling”

We report the experimental investigations of nonlinear-laser effects in LuVO₄ vanadate under one-micron picosecond Nd³⁺:Y₃Al₅O₁₂ pumping. In this tetragonal host-crystal for Ln³⁺ lasants for the first time we excited ultra-broad, more than one and half octave (13500 cm⁻¹) Raman induced Stokes and anti-Stokes generation combs and observed multi-step cascaded parametric χ ⁽³⁾-lasing in UV spectral region. All generation lines were identified and attributed to SRS-promoting modes of the crystal (ω _SRS1≈900 cm⁻¹ and ω _SRS2≈113 cm⁻¹). We classified this vanadate as a promising material for self-Raman laser converters.     

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.     

Structural and electrical characterisation of strontium zirconate proton conductor co-sputter deposited coatings

SrZr_1−x Y _x O₃ coatings were co-sputtered from metallic Zr–Y (84–16 at.%) and Sr targets in the presence of a reactive argon–oxygen gas mixture. The structural and chemical features of the film have been assessed by X-ray diffraction and scanning electron microscopy. The electrical properties have been investigated for different substrates by Complex Impedance Spectroscopy as a function of crystalline state, temperature and atmosphere. The as-deposited coatings are amorphous and crystallise after annealing at 673 K for 2 h under air. The stabilisation of the perovskite structure is a function of the nominal composition. The films are dense and present a good adhesion on different substrates. Crystallisation and mechanical stresses are detected by alternating current (AC) impedance spectroscopy. Significant ionic conductivity in the 473–823 K temperature range is evidenced in air. Two different conduction regimes in the presence of steam are attributed to a modification of the charge carrier nature. In spite of low conductivity values (σ ~10⁻⁶ S.cm⁻¹ at 881 K), the activation energies are in agreement with that of Y-doped strontium zirconate ceramics (~0.7 eV in air).     

Structure and impedance spectroscopy of Pr1? x Sr x Fe0.8Co0.2O3? δ ( x =0.1, 0.2, 0.3) thin films grown by laser ablation

Polycrystalline samples of Pr_1−x Sr _x Fe_0.8Co_0.2 O_3−δ (x=0.1, 0.2, 0.3) (PSFC) were prepared by the combustion synthesis route at 1200°C. The structure of the polycrystalline powders was analysed with X-ray powder diffraction data. The X-ray diffraction (XRD) patterns were indexed as the orthoferrite similar to that of PrFeO₃ having a single-phase orthorhombic perovskite structure (Pbnm). Pr_1−x Sr _x Fe_0.8Co_0.2O_3−δ (x=0.1, 0.2, 0.3) films have been deposited on yttria-stabilized zirconia (YSZ) single-crystal substrates at 700°C by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The structure of the films was analysed by XRD, scanning electron microscopy (SEM) and atomic force microscopy (AFM). All films are polycrystalline with a marked texture and present pyramidal grains in the surface with different size distributions. Electrochemical impedance spectroscopy (EIS) measurements of PSFC/YSZ single crystal/PSFC test cells were conducted. The Pr_0.7Sr_0.3Fe_0.8Co_0.2O_3−δ film at 850°C presents a lower area specific resistance (ASR) value, 1.65 Ω cm², followed by the Pr_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (2.29 Ω cm² at 850°C) and the Pr_0.9Sr_0.1Fe_0.8Co_0.2O_3−δ films (5.45 Ω cm² at 850°C).     

Conductivity studies of starch-based polymer electrolytes

A proton-conducting polymer electrolyte based on starch and ammonium nitrate (NH₄NO₃) has been prepared through solution casting method. Ionic conductivity for the system was conducted over a wide range of frequency between 50 Hz and 1 MHz and at temperatures between 303 K and 373 K. Impedance analysis shows that sample with 25 wt.% NH₄NO₃ has a smaller bulk resistance (R _b) compared to that of the pure sample. The amount of NH₄NO₃ was found to influence the proton conduction; the highest obtainable room temperature conductivity was 2.83 × 10⁻⁵ S cm⁻¹, while at 100 °C, the conductivity in found to be 2.09 × 10⁻⁴ S cm⁻¹. The dielectric analysis demonstrates a non-Debye behavior. Transport parameters of the samples were calculated using the Rice and Roth model and thus shows that the increase in conductivity is due to the increase in the number of mobile ions.