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The development of efficient electrocatalysts for slow reaction of the oxygen evolution reaction (OER) is fundamental for viability of the electrochemical water splitting technologies. Here we report for the first time the synthesis of NiO/carbon hollow fibres (NiO-HF) by the Solution Blow Spinning (SBS) technique, and a study of their catalytic activity towards the OER in alkaline medium. The hollow fibres were obtained with ca. 300 nm in diameter consisting of agglomerated NiO nanoparticles with an average size of 50 nm which is close to the tubular wall thickness. The formation mechanism of the hollow structure was discussed. It was revealed that the carbon from polyenic branch of polyvinylpyrrolidone (PVP) resists the firing treatment and acts as an agglomerating agent, thus ensuring a conductive and percolating path between NiO nanoparticles along the fibres. A battery of electrochemical tests of NiO-HF supported by commercial Ni foam reveals excellent electrochemical activity for OER in 1 M KOH, in comparison with reference NiO nanoparticles (NiO-NP, diameter ca. 23 nm). NiO-HF attains an overpotential of 340 mV vs. RHE at a current density of 10 mA cm−2, which is amongst the lowest values reported in the literature for undoped NiO. Chronopotentiometry reveals stable NiO-HF electrodes over 15 h under an electrolysis current of 25 mA cm−2. Microscopic analysis shows that the fibrillar morphology is completely preserved after the electrolysis test. The remarkable performance of the NiO-HF catalyst is ascribed to the enhanced electronic conductivity resulting from the interpenetrating NiO-HF/carbon microstructure.

Ceria-based composites (50 vol% of the ceramic phase) including one mixture of Li and Na carbonates (1:1 molar ratio), were prepared using either pure ceria (modest conductor) or Gd-doped ceria (excellent oxide ion conductor) as ceramic matrix. These materials were aged for periods of up to 1000 h at 550 degrees C, under pure CO2, in air and in H-2 diluted in N-2, to test their stability and electrical performance. Impedance spectroscopy measurements performed between 300 and 600 degrees C were complemented by structural and microstructural characterization. The excellent long-term stability in CO2 drops slightly when moving to air and in a pronounced manner when moving to diluted H-2. In all cases and in all conditions the best performance is observed for pure ceria-based composites. Electrode (Au) and electrolyte degradation were found interrelated. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

7 figuras, 3 tablas.-- © 2012. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/, The paper addresses the effect of the carbon support on the microstructure and performance of Pt–Ru-based anodes for direct methanol fuel cells (DMFC), based on the study of four electrodes with a carbon black functionalized with HNO3, a mesoporous carbon (CMK-3), a physical mixture of TiO2 and carbon black and a reference carbon thermally treated in helium atmosphere (HeTT). It is shown that CMK-3 hinders the growth of the electrocatalyst nanoparticles (2.7 nm) and improves their distribution on the support surface, whereas the oxidized surfaces of HNO3 carbon and TiO2+carbon lead to larger (4–4.5 nm), agglomerated particles, and the lowest electrochemical active areas (54 and 26 m2 g−1, in contrast with 90 m2 g−1 for CMK-3), as determined from CO stripping experiments. However, HNO3 and TiO2 are characterized by the lowest CO oxidation potential (0.4 V vs. RHE), thus suggesting higher CO tolerance for the se electrodes. Tests in DMFC configuration show that the three modified electrodes have clearly better performance than the reference HeTT. The highest power density attained with electrodes supported on carbon treated with HNO3 (65 mW cm−2/300 mA cm−2 at 90 °C) and the equally interesting performance of the TiO2-based electrodes (53 mW cm−2/300 mA cm−2), is a strong indication of the positive effect of the presence of oxygenated groups on the methanol oxidation reaction. The results are interpreted in order to identify separate microstructural (electrocatalyst particle size, porosity) and compositional (oxygenated surface groups, presence of oxide phase) effects on the electrode performance., J.R.C. Salgado acknowledges the financial support of Portuguese Foundation for Science and Technology (FCT, Science 2008) which has also partly funded this work through project HyPEM (FCT/FEDER/QREN-COMPETE PTDC/CTM-CER/109843/2009). J.R.C. Salgado and M.F. Montemor acknowledge the European Institute of Innovation and Technology, and the KIC InnoEnergy NewMat project.

Propylsulfonic-acid functionalized mesoporous phenylene-silicas have potential use in catalysis and applications demanding protonic conduction such as fuel cells. Their synthesis by conventional co-condensation is time consuming and with poor control of the incorporation of sulfonic acid. Here we report a fast, microwave-assisted synthetic route to prepare these materials with controlled acid load and structural order. It is found that the microwave energy strongly improves the kinetics of the various steps of the process, including the self-assembly of precursors (stirring), consolidation of the framework (hydrothermal treatment), surfactant extraction and the oxidation of the thiol to the sulfonic acid group. The concentration of incorporated acid groups increases linearly with the self-assembly time under microwave radiation, enabling the largest improvement of all synthetic steps. However, the mesopore order decreases significantly for microwave-assisted self-assemblies in excess of 6 h, and for microwave assisted hydrothermal treatments longer than 3 h. The multivariate analysis of kinetic data is presented and used to derive kinetic parameters enabling the prediction of the acid load and the structural order of the materials as a function of the synthesis time under microwave radiation. For similar acid contents, full microwave synthesis is completed within 12 h, whereas nearly 84 h are needed with conventional heating sources. The protonic conductivity increases linearly with increasing acid load regardless of the used combination of synthetic steps, including MW or not. This is seen as a confirmation that the distribution of the acid groups in the pore surface is not affected by MW. (C) 2016 Elsevier Inc. All rights reserved.

An investigation of proton conducting periodic mesoporous benzenosilica hybrids functionalised with sulfonic or phosphonic acids obtained by a co-condensation route is presented. Protonic conductivity data obtained under different temperatures and relative humidity conditions are reported in order to assess the combined effects of composition and concentration of the functional acid group, and microstructural features such as pore size, specific surface area and meso- and molecular-scale order. It is found that the conductivity increases significantly with increasing acid load, specific surface area and relative humidity. A proton conductivity of 0.3 Sm − 1 at 100 °C and 100% relative humidity is reported for a sulfonic acid functionalised PMO which is three orders of magnitude higher than the value reported for a similar material obtained by grafting.

Sixteen different thermal profiles were used to exploit the role of thermal treatment on the structure, microstructure and electrical conductivity of yttrium- and gadolinium-doped ceria electrolytes. Samples were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy in air. The bulk and grain boundary conductivity dependence on grain size and thermal treatment deviated from usual trends. Consistent changes in the fluorite lattice parameter with thermal history, reported here for the first time, reveal a tendency for solid solution demixing. Formation of localized concentrations of dopant cations (both in bulk and grain boundaries), explains the apparently abnormal electrical performance of these materials after prolonged thermal treatments.

The present work demonstrates that additions of lanthanum oxide can counter the negative grain boundary effects of relatively high silica contents in 8 mol% yttria-stabilized zirconia (YSZ). This can be predicted by thermodynamic analysis, showing strong affinity between silica and lanthanum oxide at typical sintering temperatures. Microstructural analysis revealed the onset of La- and Si-rich precipitates, and impedance spectroscopy confirmed a positive impact on grain boundary conductivity. Gradual oversaturation of lanthana in YSZ with decreasing temperature allows one to extend these effects by heat treatment at intermediate temperatures. This procedure enhances the positive effects of additions of lanthana on the specific grain boundary conductivity, while minimizing the negative impact on bulk conductivity.

The room temperature mechanosynthesis of La 1− x Sr x Ga 1− y − z Mg y Al z O 3− δ nanopowders is successfully demonstrated for a broad compositional range ( x ≤ 0.1; y ≤ 0.2, z ≤ 0.4) by resorting to a nearly amorphous alumina precursor with enhanced reactivity. It is shown that ceramics with one single phase and free from open porosity can be obtained by sintering these nanopowders at 1350–1450 °C. Microstructural data show that the substitution of Ga by Al hinders densification and decreases the grain size of ceramics. This is explained assuming the segregation of aluminum cations to the grain boundaries as a result of the decrease of the cationic diffusion coefficients.

Multilayer ceramic and salt composite cells were prepared superimposing up to seven alternate layers of Ce0.9Gd0.1O1.95 (CGO) and mixtures of Na2CO3 (NC) and Li2CO3 (LC), with final sintering around 500 °C. Various other single component cells (CGO or NC + LC) and homogeneous composites (consisting of intimate mixtures of ceramics and salts) were prepared and used as reference. All these materials were characterized by X-ray diffraction, scanning electronic microscopy and impedance spectroscopy, in air. The layered cells electrical performance was studied both as series or parallel associations to try to inspect the specific electrical performance of the ceramic/salt interface. The multilayer cells resistance showed a close agreement with prediction based on classical series or parallel association models, and no sign of special contributions originated in the ceramic/salt interface. However, the electrode impedance appeared quite distinct for the various types of cells under analysis (single component, homogeneous and layered composites). This indeed suggests that interfaces play a role on the overall cell performance.

A study of composite 8 mol% yttria stabilized zirconia (8YSZ) and 3 mol% yttria tetragonal zirconia polycrystal (3YTZP) solid electrolytes sintered under isothermal and two-step sintering cycles is reported. The nominal phase composition is retained for composites with up to 25 wt.% 3YTZP. These composites show a combination of beneficial effects with respect to pure 8YSZ, including slight improvement in sinterability, gains in bulk and grain boundary conductivity and also enhanced fracture toughness. Impedance spectroscopy revealed an enhancement of the specific grain boundary conductivity for samples with finer grain sizes, attained by increasing the fraction of 3YTZP or by hindering grain growth under two-step sintering cycles. This effect is rationalized in terms of a decrease of the grain boundary space-charge potential. The conductivity gains decrease with increasing temperature, but even at 700 °C the total ionic conductivity of ceramics with 25 wt.% 3YTZP is still higher than that of pure 8YSZ, whereas at 900 °C there is a performance loss of less than 10%. The improved mechanical and electrical performance in the intermediate temperature range represents an important advantage of the heterostructured electrolytes for low/intermediate temperature SOFC operation.

Composite ionic conductors for fuel cells were produced by combination of one ceria-based ceramic electrolyte and various mixtures of Na and Li carbonates, using several processing routes. These materials were characterized by impedance spectroscopy, scanning electron microscopy with energy dispersive spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy with attenuated total reflectance. The chemical composition of the mixed carbonates and the processing route were influential in the development of impressive conductivity levels (close to 0.1 S/cm) at modest temperatures (below 600 °C), suggesting a complex role of chemical composition (Na/Li ratio), phase composition (solid/liquid ratio), processing route (mechanothermal history) and microstructure. Furthermore, exposure to humidity showed formation of OH groups. Such hydrogenated species are believed to increase the complexity of the global surface processes and charge transport mechanisms where several native and foreign species may play a significant role.

A method is described to assess optima two-step sintering schedules of ceramic materials that combines predictions of the sintering kinetics based on shrinkage data with Taguchi fractional experimental plans. The study is based on commercial nanopowders (∼50 nm) of the partially stabilized tetragonal zirconia (ZrO2)0.97(Y2O3)0.03, and the fully stabilized cubic phase (ZrO2)0.92(Y2O3)0.08. The controlled processing parameters are the peak temperature (Tp), the temperature and dwell time of the isothermal step (Td and td) and the cooling rate (β) between Tp and Td. Dense (ZrO2)0.97(Y2O3)0.03 ceramics were obtained with an average grain size of ∼100 nm under the conditions Tp = 1320 °C, Td = 1250 °C, td = 12 h and β = 20 K min−1. Grain growth is inhibited to a lesser extent for the (ZrO2)0.92(Y2O3)0.08 ceramics, with the lowest average grain size of 450 nm obtained for Tp = 1320 °C, Td = 1270 °C, td = 12 h and β = 20 K min−1. The predictions based on Herring's scaling law applied to non-isothermal densification data are in reasonable agreement with isothermal results obtained for the tetragonal zirconia ceramics, whereas they fail for the cubic phase, probably due to the grain growth occurring up to Tp. The total ionic conductivity values at 1000 °C are 0.052 S cm−1 for (ZrO2)0.97(Y2O3)0.03 and 0.145 S cm−1 for (ZrO2)0.92(Y2O3)0.08. These values are comparable to those reported for samples with three to four times larger grain size.

Yttrium and gadolinium-doped ceria-based electrolytes (20 at% dopant cation) with and without small Ga2O3-additions (0.5 mol%) were fired at peak temperatures of 1250 and 1300 °C, or following a two-step sintering profile including one peak temperature and subsequent dwell at 1150 °C, 10 h. All materials were characterized by scanning electron microscopy, X-ray diffraction and impedance spectroscopy in air, in the temperature range 200–800 °C. Average grain sizes in the range 150–250 nm and densifications up to about 94% were found dependent on the sintering profile and presence of Ga. The grain boundary arcs in the impedance spectra increased significantly with Ga-doping, cancelling the apparently positive role of Ga on bulk transport, evidenced mostly in the case of yttrium-doped materials.

Samaria-doped ceria-based composites with a 2:1 addition of Li and Na carbonates (or simple Na carbonate as admixture) were prepared mixing nanosized powders of the ceramic phase with the remaining constituents. Samples fired at relatively low temperatures (below 700 °C) were characterized by X-ray diffraction, scanning electron microscopy combined with energy dispersive spectroscopy and impedance spectroscopy in air. These composites showed a complex but homogeneous distribution of both phases, with one ceramic skeleton of bonded nanosized grains surrounded by the carbonate-based phase. Impedance spectroscopy data was used to confirm the impressive electrical conductivity of these materials, but also to put into evidence the complex nature of the charge transport process, clearly deviating from classical electrolytes.

The formation of perovskite nanopowders of the common proton-conducting, electrolyte material Ba(Zr{sub 1-x}Y{sub x})O{sub 3-{delta}} is demonstrated by room temperature mechanosynthesis for the compositional range x=0, 0.058 and 0.148. This is achieved with a planetary ball mill at 650 rpm in zirconia vials, starting from BaO{sub 2} with ZrO{sub 2}, (ZrO{sub 2}){sub 0.97}(Y{sub 2}O{sub 3}){sub 0.03} or (ZrO{sub 2}){sub 0.92}(Y{sub 2}O{sub 3}){sub 0.08} precursors, respectively. Powder X-ray diffraction (XRD) reveals the formation of the perovskite phase in the early stages of milling with phase purity being achieved after milling times of 240 min for composition x=0.058 whereas 420 min is necessary for composition x=0.148. In contrast, traces of ZrO{sub 2} are apparent in composition x=0 even after milling times of 420 min. The use of BaCO{sub 3} as precursor does not allow the formation of the perovskite phase for any composition. The perovskite crystallites are spherical in shape with an average size determined from XRD of ca. 30 nm in agreement with transmission electron microscopy observations. FTIR spectra demonstrate that contamination levels of BaCO{sub 3} in the mechanosynthesized powders are very low. The spherical shape and nanoscale of the crystallites allow densification levels that are highly competitive when compared tomore » BaZrO{sub 3}-based materials formed by alternative synthesis techniques documented in the literature. - Graphical abstract: The formation of perovskite nanopowders of the common proton-conducting, electrolyte, material Ba(Zr{sub 1-x}Y{sub x})O{sub 3-{delta}} is demonstrated by room temperature mechanosynthesis, starting from BaO{sub 2} with ZrO{sub 2}, (ZrO{sub 2}){sub 0.97}(Y{sub 2}O{sub 3}){sub 0.03} or (ZrO{sub 2}){sub 0.92}(Y{sub 2}O{sub 3}){sub 0.08} precursors.« less

Mixed conductors based on grain boundary Fe-doped La0.95Sr0.05Ga0.90Mg0.10O3 ? ? (LSGM) ceramics were obtained by selectively doping the grain boundaries with Fe. This was achieved using LaFeO3 ? ? layers screen-printed onto LSGM, after annealing at high temperature in air for several hours to promote Fe diffusion into LSGM. The influence of the number of impregnation cycles, temperature of impregnation and microstructure of the host LSGM was evaluated by impedance spectroscopy and oxygen permeability. The impedance spectra consist of high and low frequency semicircles, ascribed to bulk and grain boundaries. The amplitude of both contributions decreases with increasing impregnation temperature and time, suggesting the onset of electronic conduction along grain boundaries. The effect is stronger for ceramics with larger grain size. The observed trends are fully consistent with estimates of p-type electronic conductivity obtained from oxygen permeability measurements using a simplified electrochemical cell. 371C-9F16-EBDE | Eduarda Gomes N/A

The room temperature mechanosynthesis of La 1 − x Sr x Ga 1 − y Mg y O 3 − δ was demonstrated for a broad compositional range ( x = 0.05, y = 0.10; x = 0.20, y = 0.20). This was achieved in atmospheric air with a planetary ball mill at 650 rpm in nylon containers rotating at 1300 rpm in the opposite direction. The maximum milling time was 360 min so that Zr contamination was kept below 0.25 mass%, as detected by ICP-AES. Powder X-ray diffraction revealed the formation of a perovskite phase since the early milling stages that becomes dominant in the patterns collected after milling for 360 min. The average crystallite diameter determined from XRD was ca. 30 nm, in good agreement with transmission electron microscopy observations. Ceramics with 93% of the theoretical density were obtained after sintering at 1300 °C.

The electrical properties of La0.95Sr0.05Ga0.90Mg0.10O3 ? ? (LSGM) were modified by selective doping of the grain boundaries, using LaFeO3 screen-printed layers and annealing at high temperature to promote Fe diffusion into LSGM. Scanning electron microscopy (SEM) and energydispersive spectroscopy (EDS) analyses showed that iron was mainly located along the grain boundaries with the bulk grain composition almost unchanged. Impedance spectra showed a significant increase in the total conductivity for the Fe-doped samples, the effect being greater for the grain boundary contribution. The formation of a parallel pathway for electronic conduction along the grain boundaries explains these effects. Ageing of these samples at high temperature, after removal of the Fe source, showed a steady shift to the original LSGM behaviour, due to dilution of Fe throughout the samples. 371C-9F16-EBDE | Eduarda Gomes N/A

The production of La0.95Sr0.05Ga0.90Mg0.10O3-δ powders was achieved at room temperature by a mechanosynthesis route in a high energy planetary ball mill starting from a mixture of lanthanum, strontium, gallium and magnesium oxides. The milling was carried out in nylon containers, using zirconia balls and a balls:powder mass ratio of 10:1. The planetary rotation was kept constant at 650 rotations per minute (rpm), and the container at 1300 rpm, in the opposite direction. The formation of the perovskite phase was detected from the early milling stages and nearly completed after milling for 360 min, as shown by powder X-ray diffraction. Transmission electron microscopy results revealed that powders consist of agglomerates of homogeneous, crystalline particles with an average equivalent diameter of about 16-17 nm, in excellent agreement with average crystallite size estimates obtained from X-ray diffraction.

Mixed oxide-ion and electronic conductivity can be exploited in dense ceramic membranes for controlled oxygen separation as a means of producing pure oxygen or integrating with catalytic oxidation. Atomistic simulation has been used to probe the energetics of defects, dopant-vacancy association, nanoscale cluster formation, and oxide-ion transport in mixed-conducting CaTiO3. The most favorable energetics for trivalent dopant substitution on the Ti site are found for Mn3+ and Sc3+. Dopant-vacancy association is predicted for pair clusters and neutral trimers. Low binding energies are found for Sc3+ in accordance with the high oxide-ion conductivity of Sc-doped CaTiO3. The preferred location for Fe4+ is in a hexacoordinated site, which supports experimental evidence that Fe4+ promotes the termination of defect chains and increases disorder. A higher oxide-ion migration energy for a vacancy mechanism is predicted along a pathway adjacent to an Fe3+ ion rather than Fe4+ and Ti4+, consistent with the higher observed activation energies for ionic transport in reduced CaTi(Fe)O3–δ.

A study of the structure of Fe IV CaTi 0.4 Fe 0.6 O 3− δ is presented and compared to data on the Fe III counterparts. The powder XRD pattern was dominated by a simple cubic perovskite unit cell; however, some small peaks indicated an orthorhombic distortion. All peaks could be indexed using a space group analogous to the Fe III phase Ca 3 TiFe 2 O 8 . From HRTEM the strong cubic peaks are well explained by the superposition of three equivalent and mutually perpendicular orthorhombic unit cells. TEM analysis further revealed a microdomain structure consisting of disordered intergrowths of CaTiO 3 - and Ca 3 TiFe 2 O 8 -like phases. Mossbauer spectra show that ca. 4% of the Fe cations are in the 4+ oxidation state. Results suggest that the Fe 4+ cations are associated with octahedral coordination and hence are associated with the CaTiO 3 -like regions, transition regions between the CaTiO 3 - and Ca 3 TiFe 2 O 8 intergrown phases and the domain boundaries. Structural models for the intergrowths are proposed based on HRTEM image simulations.

CaCO3, TiO2 and Fe2O3 were mixed in the appropriate stoichiometric quantities and calcined at 1100∘C for 10 h. These powder mixtures were uniaxially pressed and sintered at temperatures ranging from 1350 to 1500_∘C for 2 h in order to obtain dense disk-shaped samples with nominal CaTi1 − xFe x O3−δ (x = 0.05, 0.15, 0.20, 0.40 and 0.60) compositions. Dilatometry and in situ high temperature powder X-ray diffraction analysis showed a good agreement on the thermal expansion behaviour of these materials between room temperature and 1000_∘C. The estimated linear thermal expansion coefficient is close to 13× 10− 6 K− 1 and is little affected by composition. No evidence for surface carbonation was detected in the infrared spectra collected on samples previously annealed in CO2 atmospheres. The oxygen permeability measured at temperatures ranging from 750 to 1000_∘C goes through a sharp maximum for x = 0.20. This result is interpreted by structural differences related to change from disordered to ordered oxygen vacancies. The overall performance of CaTi0.80Fe0.20O3−δ is compared to other mixed conducting materials.

An impedance spectroscopy study of sodium cation-conducting ceramics, including layered compounds Na0.8Ni0.4Ti0.6O2 ,N a 0.8Fe0.8Ti0.2O2 ,N a 0.8Ni0.6Sb0.4O2 (structural type O3), Na0.68Ni0.34Ti0.66O2 (P2 type), and NASICON-type Na3Si2Zr1.88Y0.12PO11.94 and Na3.2Si2.2Zr1.88Y0.12P0.8O11.94, showed that their transport properties are essentially independent of partial water vapor pressure at temperatures above 420 K. In the low-temperature range, increasing vapor partial pressure from approximately 0 (dry air) up to 0.46 atm. leads to a reversible in- crease in the conductivity. The sensitivity of studied materials to air humidity is strongly affected by the ceramic microstructure, being favored by larger boundary area and porosity. Maximum stability in wet atmospheres was found for NASICON ceramics, which also exhibit the highest cationic conduction. The average thermal expansion coefficients at 300-1173 K are in the range (13.7-16.0) × 10 −6 K −1 for the layered materials and (5.9-6.5) × 10 −6 K −1 for NASICON-type ceramics.

Cermet anodes of Ni-CaZr 0.95 Y 0.05 O 2.975 (Ni-CZY) and Ni-SrZr 0.95 Y 0.05 O 2..975 (Ni-SZY) have been synthesised by combustion followed by sintering and reduction to Ni metal. The anodes were co-pressed and co-sintered on green YSZ electrolyte to produce anode/electrolyte/anode assemblies with planar electrodes of thickness, ca. 150 μm. The anode microstructures are composed of a uniform and homogeneous distribution of submicron Ni metal and ceramic phases. Preliminary impedance spectroscopy results on symmetrical cells indicate that the polarization resistance is composed of two or more pH 2 O-sensitive contributions and is dependent on the composition of the proton-conducting ceramic.

Polycrystalline Ca 0.97 Ti 1− x Fe x O 3− δ samples with x =0, 0.01, 0.035, 0.07 and 0.15 were prepared by a conventional ceramic route and characterised by impedance spectroscopy at different temperatures in air. Spectra obtained for samples with low iron contents ( x

Cermet anode material with a proton-conducting ceramic phase, Ni–SrCe 0.9 Yb 0.1 O 3− δ , was synthesised by combustion from mixtures of molten nitrates and urea followed by sintering and reduction. Co-pressing the anode combustion powder on “green” SrCe 0.9 Yb 0.1 O 3− δ electrolyte and co-firing produced symmetrical anode/electrolyte/anode assemblies with planar electrodes of thickness ca. 100 μm. The addition of Co(NO 3 ) 2 ·6H 2 O to the electrolyte as sintering aid lowered the sintering temperature to 1250 °C, thereby allowing high anode porosity while retaining good anode/electrolyte adherence. The anode microstructure is composed of a uniform distribution of submicron Ni and perovskite particles. Analysis of the symmetrical assemblies by a.c. impedance spectroscopy indicates that the electrode polarisation resistances are composed of at least two contributions. Stability issues concerning the Ni–SrCe 0.9 Yb 0.1 O 3− δ anodes in reducing atmospheres are discussed.

For the first time, Fe 4+ was observed and analysed in a systematic manner in CaTi 1− x Fe x O 3− δ ( x =0.05, 0.20, 0.40 and 0.60) perovskites and its presence related with the ionic conductivity of these materials. These oxides were prepared by solid state reaction under oxidizing conditions. Slowly cooled CaTi 1− x Fe x O 3− δ ( x =0.05, 0.20 and 0.40) oxides crystallise with the same orthorhombic symmetry of CaTiO 3 . Some of the weaker peaks tend to disappear with increasing x and are no longer present for x =0.60, which can be indexed on a cubic unit cell. Mossbauer spectroscopy revealed the coexistence of Fe 4+ and Fe 3+ in all compositions, with prevalence of the Fe 3+ species. The relative amounts of Fe 3+ coordinated by 6, 5 and 4 oxygen ions were estimated. Oxygen stoichiometry changes, determined by solid electrolyte potentiometry–coulometry, were found to be in good agreement with the number of oxygen vacancies per unit formula based on estimates of the relative amounts of Fe 4+ and Fe 3+ obtained from the Mossbauer spectra. The ionic conductivity follows a typical Arrhenius behaviour with a sharp maximum at x =0.20. This behaviour is explained from combined Mossbauer spectroscopy and coulometric titration data, based on the existence of ordered and disordered oxygen vacancies associated with tetracoordinated and pentacoordinated Fe 3+ ions, respectively. Differences between the ionic conductivity of ceramics prepared under reducing and oxidising conditions suggest that Fe 4+ cations may stabilise disordered structures at low temperatures, thus enhancing the transport properties.

Substitution of 2% gadolinium in the fluorite-type lattice of Ce 0.80 Gd 0.20 O 2− δ with praseodymium cations was found to increase the p-type electronic conductivity by 2.5–4 times, whereas the activation energy for the electron-hole transport decreases from 145 to 125 kJ/mol. The polarization resistance of porous Pt electrodes in contact with Ce 0.80 Gd 0.18 Pr 0.02 O 2− δ solid electrolyte, determined by impedance spectroscopy at 723–1073 K in air, is 0.5–3 times lower than that for Ce 0.80 Gd 0.20 O 2− δ ceramics. This behavior, indicating an active role of the surface of the ceria-based electrolytes in electrochemical processes, was attributed to the larger p-type conductivity as well as to the high electrocatalytic activity of praseodymium cations. Further decrease in polarization resistance can be achieved by incorporation of praseodymium oxide into the Pt metal electrode matrix. Microstructural studies of Ce 0.80 Gd 0.18 Pr 0.02 O 2− δ ceramics showed that Pr doping increases the tendency for segregation of rare-earth dopants at the grain boundaries; the segregated Pr-enriched phase may have a positive effect on electrochemical activity.

The properties of directionally solidified eutectic (DSE) zirconia-mullite composites are presented. These materials combine two oxide ion conductors in eutectic microstructures stable over broad temperature (570-1415 degrees C) and PO2 (10(-20) to 10(5) Pa) ranges, which are pertinent for their application as high temperature (>1200 degrees C) solid electrolytes, e.g. in Nernstian sensors. X-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy results reveal a composite structure comprising eutectic crystals of mullite and zirconia, and an intergranular amorphous phase rich in Al-Y-Si-O. The amorphous phase is crystallised upon annealing at 1400 degrees C, and the resulting composites have a composition close to the nominal eutectic consisting of 79 vol.% mullite and 21 vol.% zirconia. The electrical conductivity of these materials is rationalized in terms of percolation, fraction and properties of each phase, attaining values in excess of 0.01 S/cm at 1370 degrees C and displaying Arrhenius behaviour with activation energy of 70 kJ/mol. The broad electrolytic domain of these solid electrolytes is demonstrated by conductivity measurements carried out from 600 degrees C to 1370 degrees C in O2, air, Ar and 10%H-2 + 90%N-2 atmospheres. (C) 2013 Elsevier B.V. All rights reserved.

The synthesis and characterization of directionally solidified Y-doped zirconia-barium zirconate eutectic materials by the laser floating zone technique (LFZ) is presented in the search for novel mixed ionic conductors by protons and oxide ions. Emphasis is placed on the control of the LFZ conditions that promote the solubilization of Y in both eutectic phases, and how the dopant influences the ionic conductivity. Fast growth under constitutional supercooling conditions leads to the development of ZrO2-dendrites above the solidification interface, which are dipped into a finer-interpenetrated network of ZrO2 and BaZrO3. Yttrium-doping of the ZrO2 fluorite-type lattice is favored by the high thermodynamic stability of BaZrO3. However, even low Y-substitution levels in BaZrO3 lead to dominating protonic conduction in the composite under humidified atmospheres below 500 degrees C. At higher temperatures, the proton incorporation is unfavorable, and the total conductivity of the composite is determined by the hole conduction in the Ba(Zr,Y)O-3, and/or oxide-ion conductivity through the (Zr,Y)O-2. (C) 2013 Elsevier B.V. All rights reserved.

The processing and characterization of composite membranes casted from commercial Nafion (R) suspension with incorporation of benzene-bridged periodic mesoporous organo-silicas functionalised with sulfonic acid groups (S-Ph-PMO) are presented in the search for improved protonic conductivity under low humidity conditions. It is shown that the incorporation of 10 wt%S-Ph-PMO fillers into Nafion (R) improves the proton conductivity by approximately 70%, with the composite membranes reaching 0.19 S . cm(-1) in 98% relative humidity (RH), or 0.05 S . cm(-1) in 60% RH, at 94 degrees C. The filler effect on the conductivity increases at low RH, representing an enhancement of similar to 250% and similar to 425% with respect to the conductivity of Nafion (R) at 40% RH and 20% RH, respectively. The conductivity improvement is also higher at low temperature due to the higher activation energy of Nafion (R). Such effects are interpreted by the high proton mobility in the stable mesoporous channels of the fillers. (C) 2013 Elsevier B.V. All rights reserved.

Composite electrolytes based on Gd-doped ceria (from 50 to 95 vol%) and one eutectic mixture of sodium and lithium carbonates were prepared by co-firing, in order to identify the compositional range where both oxide and carbonate-ion transport are balanced, as required for high performance CO2 separation membranes. Considering the adequacy of this technique, the effects of composition and microstructure on oxide-ion transport were inspected by impedance spectroscopy. The specific features of low temperature impedance spectra allowed access to data on oxide-ion transport based on a hereby suggested equivalent circuit where the individual characteristics of both constituent phases are considered, as well as the variable nature and relevance of interfaces. Combined analysis of all information indicates that the range of interesting compositions for balanced dual ionic conduction corresponds to a ceramic phase content around 85 yo1%, for the present microstructural characteristics and a target temperature below 550 degrees C. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Benzene-bridged periodic mesoporous organosilicas functionalised with sulfonic acid (SPh-PMO) are explored as fillers to improve the protonic conductivity and the viscoelastic properties of Nafion (R). Homogeneous membranes with 5, 10 and 20 wt.% submicrometric SPh-PMO particles (roughly corresponding to 11, 20 and 36 vol.%) were obtained by control of the casting suspensions. The three composite membranes have acid loads and water uptake values similar to Nafion (R). The storage modulus of the 20 wt.% S-Ph-PMO composite (0.2 GPa at 100 degrees C and 0.05 GPa at 140 degrees C) is 2.5-15 times higher than for pure Nafion (R) (respectively 0.08 and 0.005 GPa), denoting a positive effect of the fillers on the mechanical resistance of the membranes, also observed for lower filler fractions. The protonic conductivity of the composite membranes at 20% relative humidity (RH) and 40 degrees C is up to 1.5 orders of magnitude higher than for Nafion (R). The magnitude of the effect decreases with increasing humidity, with the best composite attaining 0.03 S cm(-1) at 120 degrees C/40% RH, 3 times more than Nafion (R). All membranes have similar behaviour at 98% RH, showing a maximum of 0.2 S cm(-1) at 94 degrees C, with the composites still showing slightly better performance. The results are discussed in terms of the effect of the fillers on reducing the internal swelling pressure and the activation energy for proton migration. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Gd-doped ceria (CGO) nanopowders were mixed with up to 1 mol% of Zn (as nitrate) to exploit the role of this dopant on sintering and electrical performance. All samples were prepared by two-step sintering schedules to try to combine the advantages of both effects on low-temperature densification. Structural and microstructural analyses were complemented with impedance spectroscopy (wide range of temperatures and oxygen partial pressures) and ion-blocking measurements. The combination of all data suggests that Zn is preferentially located in the grain boundary region, playing a significant role on sintering. Zn additions (I mol%) allowed for a 100 degrees C lower sintering temperature with respect to pure CGO. A marginal effect on the oxide-ion conductivity and a small enhancement (less than 20%) in n-type electronic conductivity was found between Zn-free and 1 mol% Zn-doped CGO. The removal of Zn from CGO after sintering is discussed. (C) 2013 Elsevier B.V. All rights reserved.

CaFexTi1-xO3-y (0.05≤x≤0.60) and SrFexTi1-xO3-y (0.20≤x≤0.60) perovskites were prepared by solid state reaction and subjected to different heat treatments in different atmospheres. Mossbauer spectroscopy of the CaFexTi1-xO3-y could detect Fe4+ in all the samples, even in those heated at 1000 °C in Ar atmosphere for 10 hours. Fe3+ coordinated by six, five and four anions were also identified and the estimated hyperfine parameters were consistent with those previously reported for the CaFexTi1-xO3-x/2 oxides where all the Fe cations were present as Fe3+. The Fe4+/Fe3+ ratios estimated from Mossbauer data are in agreement with coulometric titration data. The absence of tetracoordinated Fe3+ in SrFexTi1-xO3-y suggests that in contrast to CaFexTi1-xO3-y no ordering of anion vacancies takes place even for x = 0.60 and explains the differences in the Fe concentration dependence of the electric transport properties of these materials.

NASICON-type compounds with the nominal formula, Na 3 Si 2 Zr 1.88 Y 0.12 PO 12 , were prepared by a typical ceramic route with different microstructures. The samples were fired in the temperature range 1190–1235°C with sintering periods between 2 and 80 h. Results showed a significant influence of the processing conditions on the microstructure, affecting both grain and grain boundaries. Electrical conductivity was mainly controlled by the grain boundary contribution, which is strongly dependent on the grain size and density of grain boundaries. A maximum conductivity value of about 2.7×10 −3 S cm −1 at room temperature was obtained with samples sintered at 1220°C for 40 h.

In this work, the stability of NASICON in contact with water was studied by impedance spectroscopy, structural and microstructural analyses, using both dense and powder samples. Impedance spectroscopy data showed a significant decrease in ionic conductivity of dense pellets when exposed to water for more than 300 h. While both grain and grain boundary resistivities increased, the major contribution and changes in the overall impedance spectra were due to changes in the grain boundary arc. Based on complementary microstructural analysis, small grains of a second phase were found covering large grains of NASICON. The composition of these small grains is believed to be similar to that of hydronium NASICON, identified by XRD after the reaction of NASICON powder with water.

Increasing iron concentration in perovskite-type solid solutions Sr0.97Ti1−xFexO3−δ(x=0.2–0.8) was found to decrease the unit cell volume and to increase the partial ionic and electronic conductivities as well as the thermal expansion. The oxygen permeation through Sr0.97(Ti,Fe)O3−δ membranes is limited by both bulk ionic conduction and surface exchange rates. Prolonged stabilization under oxygen chemical potential gradients suggests formation of ordered microdomains in the perovskite lattice. The activation energy for the ionic conductivity, evaluated from oxygen permeation and Faradaic efficiency data, is nearly independent of iron content, varying in the range 97–104 kJ/mol. Thermal expansion coefficients of Sr0.97Ti1−xFex O3−δ (x=0.4–0.8) vary from (11.7–13.8)×10−6 K−1 at temperatures of 300–720 K to (16.6–27.0)×10−6 K−1 at higher temperatures. In comparison with the Sr(Ti, Co)O3−δ system, strontium titanates ferrites possess somewhat poorer transport properties, but lower thermal expansion.

Homogeneous fine grained La 2 (Zr,Y) 2 O 7 /LaCoO 3− δ -based composite materials were obtained from simple mixture of the components and solid state reaction between LaCoO 3− δ and yttria-stabilized zirconia. Thermal expansion data can be fitted to simple mixture rules to predict the composite behavior from the properties of the individual components. The dependence of total and ionic conductivity on the volume fraction of the conductive phase can be described by the General Effective Media equation. The resistance to thermal cycling was evaluated and found satisfactory

LaCoO3−δ/La2(Zr,Y)2O7-based composites were designed in view of three main properties: electrical conductivity, thermal expansion and resistance to thermal cycling. Composition and processing conditions were investigated on the basis of an experimental plan according to the Taguchi method. The phase distribution was estimated from image analysis and related to electrical and thermo-mechanical behavior. Results indicate that an homogeneous and fine grained phase distribution is essential in order to obtain materials with the desired thermal expansion, electrical properties and thermomechanical properties.

Measurements of oxygen permeation through dense LaCoO 3− δ membranes prepared by different routes (standard ceramic or from organic precursors) showed a considerable role of processing conditions and microstructure on permeation fluxes. The higher permeability observed in ceramics produced by the ceramic route was attributed to a lower grain-boundary resistance to oxygen transport. Experimental data were also compared with results of numerical modeling of oxygen transport in LaCoO 3− δ , based on isotopic diffusion data in single crystals. Ionic conduction in ceramics with smaller grain size is lower than in single crystals, suggesting a significant grain boundary resistance. In contrast, oxygen permeability of LaCoO 3− δ prepared by the standard ceramic synthesis route, involving higher sintering temperatures is higher than expected. This suggests easy diffusion along grain boundaries. The influence of the ceramic microstructure on total electrical conductivity and thermal expansion of lanthanum cobaltite ceramics was found negligible.

Composite cathodes were prepared by high temperature reaction and screen printed onto a ceria-gadolinia electrolyte. Different depositions and/or firing conditions were used to change the electrode microstructures. A combination of steady state cathodic polarisation measurements and impedance spectroscopy was used to evaluate the performance of these cathodes and to investigate the effects of the most representative microstructural features. Both types of results suggest different mechanisms for relatively low and high cathodic polarisation. The results obtained for the best composite cathodes are comparable to results obtained for La 1− x Sr x Co 1− y Fe y O 3− δ , and are much better than those obtained for La 1− x Sr x MnO 3− δ .

Oxygen-ion conductivity of the perovskite-type solid solutions (La,Sr)Ga1−zM2O3−δ (M=Ti, Cr, Fe, Co; z=0–0.20), LaGa1−y−zMgyMzO3−δ (M=Cr, Fe, Co; y=0.10–0.20, z=0.35–0.60) and LaGa1−zNizO3−δ (z=0.20–0.50) was studied using the techniques of oxygen permeation, Faradaic efficiency, ion-blocking electrode and the e.m.f. of oxygen concentration cells. Oxygen-ion transference numbers vary from 2×10−6 to 0.98 throughout the series and p-type electronic conductivity increases with increasing transition metal content. Substitution of Ga with higher valence cations (Ti, Cr) decreases ionic conductivity whereas small amounts of Fe or Co (∼5%) increase ionic conductivity. For higher transition metal contents, lower levels of oxygen-ion conductivity and an increase in the activation energy, EA, for ionic transport, from 60 (5%-doped) to 230 kJ/mol (>40%-doped) are observed. In heavily doped phases, EA tends to decrease with temperature and, above 1170 K, values are similar to the undoped phase suggesting that an order–disorder transition takes place. Factors affecting the observed ionic conductivity trends are discussed.

Oxygen ionic conduction in the perovskite-type Sr 0.97 Ti 0.60 Fe 0.40 O 3− δ was studied using oxygen permeability, Faradaic efficiency and total electrical conductivity measurements at 973–1223 K. The ion transference numbers of the strontium titanate–ferrite in air vary from 0.005 to 0.08, decreasing with decreasing temperature. The electron–hole conductivity of the oxide is relatively low but exceeds the ionic conductivity. The activation energy for the electronic conductivity is 35±3 kJ/mol at 470–890 K and drops at higher temperatures. Studying the oxygen permeation through dense Sr 0.97 Ti 0.60 Fe 0.40 O 3− δ ceramic membranes as a function of membrane thickness showed that at temperatures above 1170 K the permeation fluxes are limited by both bulk ionic conductivity and surface exchange rates. Depositing of porous layers of the same material or a mixture of platinum and praseodymium oxide onto the membrane feed-side surface leads to a significant increase in the oxygen permeability. Decreasing temperature results in increasing role of the bulk ionic transport in Sr 0.97 Ti 0.60 Fe 0.40 O 3− δ as the permeation-determining factor. The oxygen permeation fluxes at 1073 K are limited predominantly by the oxygen ionic conductivity of the ceramics. The thermal expansion coefficients of the ceramic material in air were calculated from dilatometric data to be 11.7×10 −6 K −1 in the temperature range 300–720 K and 16.6×10 −6 K −1 at 720–1070 K.

The transport properties of Sr0.97(Ti,Fe)O3−y materials were reevaluated by a simple electrochemical permeability technique, comprising an electrochemical oxygen pump and a potentiometric oxygen sensor to measure the driving force. The rate of transport through single layer dense samples was significantly lower than through bilayered dense∣porous samples with the porous layer in contact with the oxidizing atmosphere (air); this suggests that the overall resistance to oxygen transfer across the mixed conducting material includes terms related to surface exchange, and mixed (electronic+oxide ion) conduction. The dependence of permeability current on the driving-force also tends to a limiting current, which was attributed to concentration overpotentials due to limitations of transport in dilute gas phases.

Chemically stable Sr0.97(Ti, Fe)O3−δ materials were prepared by a conventional solid state reaction procedure. Their transport properties were evaluated by electrochemical permeability experiments. Our results confirm that Fe additions yield SrTiO3 based materials with relatively good electronic and oxygen ion conductivities. However, the permeability experiments could not be fitted by a model behaviour derived on assuming that the ionic conductivity is independent of Po2, and a simple power law for the p-type contribution, σp=σp1(Po2)1/4.

LaCoO3−δ/La2(Zr,Y)2O7-based composites were designed with thermal expansion coefficients below 15×10−6°C−1 and dc conductivities higher than 100 S cm−1 at 800°C. Resistance to thermal cycling was evaluated by the deterioration of electrical properties and was found to be satisfactory. Percolation of the highly conducting phase is retained for fractions of LaCoO3−δ as low as about 30% in volume. Thin films were screen-printed on gadolinia-doped ceria (GCO) substrates for electrochemical characterisation. Electrode thickness and porosity were varied by changing the concentration of solids and the sintering temperature. Results suggest some dependence on cathodic polarisation on the electrode surface area in contact with the gas atmosphere. The best composite electrode has shown better electrode performance than LaMnO3-based materials and also has a good resistance to thermal cycling.

Dense disks of La1−xSrxCoO3−δ materials were prepared and characterized by electrochemical permeability experiments. Our results confirm that these materials possess relatively high ionic conductivity, and this is greatly enhanced by Sr additions. The ionic conductivity of these materials at temperatures of about 1000°C may exceed the conductivity of the most widely used electrolyte materials. However, the permeability experiments could not be fitted by a relatively simple model behaviour derived from the expected defect chemistry, in which the electrode overpotentials are neglected. Significant overpotential contributions are thus expected.