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[Gd(H4nmp)(H2O)2]Cl·2H2O (1) converts into [Gd2(H3nmp)2]·xH2O (2) (x = 1 to 4) with a notable increase in proton conductivity., [Gd(H4nmp)(H2O)2]Cl·2H2O (1) converts into [Gd2(H3nmp)2]·xH2O (2) (x = 1 to 4) with a notable increase in proton conductivity. 1 is a charged layered material counter balanced by chloride ions, with proton conductivity values of 1.23 × 10–5 S cm–1 at 98% relative humidity (RH) and 40 °C. At 98% RH and 94 °C the observed conductivity is 0.51 S cm–1, being to date one of the highest values ever reported for a proton-conducting coordination polymer. This increase is observed during a structural transformation into 2 that occurs at high temperature and RH. While this remarkable conductivity is observed only after transformation and by maintaining high humidity conditions, as-synthesized 2 also shows a conductivity value of 3.79 × 10–2 S cm–1 at 94 °C and 98% RH, still ranked as one of the highest reported values. Moreover, it is shown that the key factor for high proton conduction is the unusual dynamic structural transformation with water insertion and release of chloride ions.

Hydrogen is a clean energy carrier and fuel cells (FCs) are an efficient conversion technology, and thus have become mainstream research topics that could contribute to sustainable and clean energy supply in modern society. Among the various hydrogen FC types that convert chemical energy into electrical energy via redox reactions, polymer electrolyte fuel cells (PEFCs) are the most studied technology due to their vast range of applications both mobile and stationary. The use of bio-based materials as substrates for the manufacture of the key constituent components of FCs, to minimize the impact of their production, although in its infancy, is growing exponentially as part of the general concern for sustainability. In fact, natural polymers such as cellulose (i.e. the most abundant biopolymer), and in particular its nanoscale forms, viz. cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs) and bacterial nanocellulose (BNC), are appropriate materials to engineer the key components of PEFCs, namely the ion-exchange membrane and the catalyst layer (anode and cathode). Despite the intense R&D activity, little has been reviewed about the potential application of these three forms of cellulose with at least one dimension in the nanoscale in the field of FCs. Hence, the overarching goal of this review is to concentrate on the most recent efforts in the domain of bio-based PEFC components, namely the ion-exchange membrane and the electrodes, by targeting the use of nanocellulose as a renewable and easily foldable or stackable nanomaterial. The application of these nanocellulose-based components in microbial fuel cells (MFCs) is also covered.

Porous robust materials are typically the primary selection of several industrial processes. Many of these compounds are, however, not robust enough to be used as multifunctional materials. This is typically the case of Metal-Organic Frameworks (MOFs) which rarely combine several different excellent functionalities into the same material. In this report we describe the simple acid-base postsynthetic modification of isotypical porous rare-earth-phosphonate MOFs into a truly multifunctional system, maintaining the original porosity features: [Ln(H3pptd)]·xSolvent [where Ln3+ = Y3+ (1) and (Y0.95Eu0.05)3+ (1_Eu)] are converted into [K3Ln(pptd)]·zSolvent [where Ln3+ = Y3+ (1K) and (Y0.95Eu0.05)3+ (1K_Eu)] by immersing the powder of 1 and 1_Eu into an ethanolic solution of KOH for 48 h. The K+-exchanged Eu3+-based material exhibits a considerable boost in CO2 adsorption, capable of being reused for several consecutive cycles. It can further separate C2H2 from CO2 from a complex ternary gas mixture composed of CH4, CO2, and C2H2. This high adsorption selectivity is, additionally, observed for other gaseous mixtures, such as C3H6 and C3H8, with all these results being supported by detailed theoretical calculations. The incorporation of K+ ions notably increases the electrical conductivity by 4 orders of magnitude in high relative humidity conditions. The conductivity is assumed to be predominantly protonic in nature, rendering this material as one of the best conducting MOFs reported to date. published

This paper describes a novel method to produce poly(lactic acid) (PLA) stereocomplex (SC) crystallites directly from high molecular weight PLLA by incorporation of small quantities of choline dihydrogen phosphate ([Ch][DHP]), at room temperature. Combined results of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) confirm the formation of the PLA SC crystals. 1H NMR spectroscopy indicates a modification of the short range structural environment of choline in the blend with PLA in good agreement with the major long-range structural changes suggested by XRD, both suggesting a direct role of the choline in the PLA stereocomplexation. The electrical conductivity of these membranes measured using impedance spectroscopy is in the 6 × 10−7–4 × 10−6 S cm−1 range, which increases with increasing choline content and with increasing relative humidity (RH). The dependence on RH is weaker than for pure PLA, thus suggesting that the SC enhances the hydrophobic character of PLA, in agreement with contact angle measurements. Atomic force microscopy (AFM) and piezoresponse force microscopy (PFM) reveal a new phase with piezoelectric properties at the microsphere interfaces, with d33 corresponding to ∼60% of d33 of a periodically poled lithium niobate reference, which is directly related to the formation of SC crystalline domains. This first report of intrinsic piezoelectricity in PLA stereocomplexes opens the possibility for the development of new types of bio-based piezoactive polymers.

A positively charged lamellar coordination polymer based on a flexible triphosphonic acid linker is reported. [Gd(H(4)nmp)(H2O)(2)]Cl center dot 2H(2)O (1) [H(6)nmp=nitrilotris(methylene-phosphonic acid)] was obtained by a one-pot approach by using water as a green solvent and by forcing the inclusion of additional acid sites by employing HCl in the synthesis. Compound 1 acts as a versatile heterogeneous acid catalyst with outstanding activity in organic reactions such as alcoholysis of styrene oxide, acetalization of benzaldehyde and cyclohexanaldehyde and ketalization of cyclohexanone. For all reaction systems, very high conversions were reached (92-97%) in only 15-30 min under mild conditions (35 degrees C, atmospheric pressure). The coordination polymer exhibits a protonic conductivity of 1.23 x 10(-5) Scm(-1) at 98% relative humidity and 40 degrees C.

Lightweight, three-dimensional (3D) cellular structures of graphene/silicon carbide (SiC) showing very low densities (

This work reports the synthesis of nanostructured Ce1-xNixO2-δ (x = 0.05, 0.1, 0.15 and 0.2) oxides prepared by cation complexation route and with the main objective of studying their redox properties using a combination of electron microscopy, synchrotron radiation X-ray diffraction (SR-XRD) and X-ray absorption near-edge spectroscopy (XANES). The Ce1-xNixO2-δ series of nanopowders maintain the cubic crystal structure (Fm3m space group) of pure ceria, with an average crystallite size of 5-7 nm indicated by XRD patterns and confirmed by transmission electron microscopy. In situ SR-XRD and XANES carried out under reducing (5% H2/He; 5% CO/He) and oxidizing (21%O2/N2) atmospheres at temperatures up to 500 °C show a Ni solubility limit close to 15 at.% in air at room temperature, decreasing to about 10 at.% after exposure to 5% H2/He atmosphere at 500 °C. At room temperature in air, the effect of Ni on the lattice parameter of Ce1-xNixO2-δ is negligible, whereas a marked expansion of the lattice is observed at 500 °C in reducing conditions. This is shown by XANES to be correlated with the reduction of up to 25% of Ce4+ cations to the much larger Ce3+, possibly accompanied by the formation of oxygen vacancies. The redox ability of the Ce4+/Ce3+ couple in nanocrystalline Ni-doped ceria is greatly enhanced in comparison to pure ceria or achieved by using other dopants (e.g. Gd, Tb or Pr), where it is limited to less than 5% of Ce cations. Fil: Fuentes, Rodolfo Oscar. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Acuña, Leandro Marcelo. Ministerio de Defensa. Instituto de Investigaciones Científicas y Técnicas para la Defensa; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Albornoz, Cecilia Andrea. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina Fil: Leyva, A. G.. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina Fil: Sousa, N.. University of Aveiro; Portugal Fil: Figueiredo, F. M.. University of Aveiro; Portugal

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.

Silicon nitride (Si3N4) processed with up to 25 vol.% of graphene nanoplatelets (GNPs) gives conductive composites with the highest electrical conductivity (40 Scm−1) reported for these ceramics with added conductive particles. During compaction and pressure-assisted densification of the composites in the spark plasma sintering (SPS), a preferred orientation of GNPs occurs. Consequently, the electrical conductivity measured along the direction perpendicular to the SPS pressing axis is more than one order of magnitude higher than the one measured along the parallel direction. Percolation in the composites is observed for 7–9 vol.% of GNPs, depending on the measuring direction, perpendicular or parallel to the pressing axis. Different conduction mechanisms are apparent for the two orthogonal orientations. Charge transport along the direction defined by the graphene ab-plane (perpendicular direction) may be explained by a two dimensional variable range hopping mechanism, whereas conduction in the parallel direction shows a more complex behavior, with a metallic-type transition (dσ/dT

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.

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.

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

By unzipping multi-walled carbon nanotubes (MWCNTs) it is possible to obtain graphene nanoribbons (GNRs) that could then be used as fillers in ceramic composites. Here we report the fabrication of silicon nitride (Si3N4) ceramics with different contents of GNRs by spark plasma sintering. The GNR fillers confer electrical conductivity to the Si3N4 composites, following a semiconducting-like behavior at relatively low volume filler concentrations (0.04). In addition, a toughening effect, produced by GNRs bridging the cracks was observed. GNRs appear to be an efficient alternative to graphene-based composites, useful in the fabrication of novel multifunctional ceramic composites. (C) 2015 Elsevier Ltd. All rights reserved.

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.

Solid oxide-alkaline carbonate (Li2CO3 : Na2CO3, 1 : 1 molar ratio) composite electrolytes were prepared using different solid oxide matrices (TiO2, HfO2, Yb2O3, Y2O3, Dy2O3, Gd2O3 and La2O3), to cover a wide range of ceramic chemical characteristics. The chemical and microstructural stability of these oxides with the mixed carbonates were studied by powder X-ray diffraction, scanning electron microscopy, infrared and laser Raman spectroscopic techniques after reacting them at 690 degrees C for 1 h in air. The electrical performance of selected composites was evaluated using impedance spectroscopy, in air. Amongst the oxides hereby tested, TiO2 is found to be the most unstable in contact with the molten carbonates whereas Yb2O3 is quite stable. The corresponding composites have ionic conductivities (3.3 x 10(-1) S cm(-1) at 580 degrees C, in air) close to those reported for state-of-the-art ceria-based composite electrolytes. A draft equivalent circuit model underlines the transport in the carbonate phase and across the carbonate/oxide interfaces as the dominant contributions to the total conductivity of these composites. Yb2O3 + Li2CO3 : Na2CO3 composites show chemical stability at operating temperatures in the order of 690 degrees C, standing as a potential candidate for intermediate temperature applications.

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.

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.

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.

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

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.

Oxygen permeation and faradaic efficiency measurements of perovskite solid solutions Sr0.97Ti1 − x − yFexMgyO3 − δ (x = 0.20–0.40; y = 0–0.10) at 973–1223 K showed that the oxygen transport at membrane thicknesses below 2 mm is limited by both bulk ionic conductivity and the surface exchange kinetics. Incorporation of either iron or magnesium into the B sublattice of strontium titanate results in greater p-type electronic and oxygen ionic conductivities. For Sr0.97(Ti,Fe)O3 − δ solid solutions, the role of the surface exchange as the permeation-determining factor decreases with reducing temperature. In contrast, the limiting effect of the interphase exchange on oxygen transport through Sr0.97Ti0.70Fe0.20Mg0.10O3 − δ membranes is observed to be significant within the studied temperature range, suggesting that doping with magnesium leads to higher ionic conductivity and lower surface exchange rates in comparison with Sr0.97Ti0.60Fe0.40O3 − δ perovskite which exhibit similar permeation fluxes. The ion transference numbers of the solid solutions in air, estimated from the oxygen permeation and faradaic efficiency results, do not exceed 0.14. TGA/DTA results demonstrated the stability of the perovskite phases in CO2-containing atmospheres at temperatures above 770 K. The effect of the surface exchange limitations on the faradaic efficiency results is analysed.

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.

A simple experimental setup, consisting of a closed chamber with two electrochemically active walls (the material being studied and one electrochemical pump), was designed to measure the electrochemical permeability of mixed conductors. The basic relations between material properties and measurable parameters are revised. Experiments showed that the quality of sealing is a critical issue to be taken into consideration. Careful analysis of the cell steady-state and transient behaviors (e.g. recovery from reducing to oxidizing conditions) is shown to be quite effective in evaluating the role of physical leaks in the overall experiment.

Thermodynamic calculations on high temperature reactions between an YSZ electrolyte and LaCoO 3 cathode materials identify the limiting conditions of temperature and oxygen partial pressure ranges required to prevent formation of resistive reaction products. The time dependence of the fraction reacted was evaluated under isothermal conditions and as a function of temperature. Theoretical models for both diffusion or phase boundary controlled reactions fail to fit the experimental data. This failure is probably related to loss of contact at the LaCoO 3 | La 2 Zr 2 O 7 interface, when significant oxygen pressures are built up inside compact reacting mixtures of YSZ and LaCoO 3 .

Graphene/silicon nitride (Si3N4) composites with high fraction of few layered graphene are synthesized by an in situ reduction of graphene oxide (GO) during spark plasma sintering (SPS) of the GO/Si3N4 composites. The adequate intermixing of the GO layers and the ceramic powders is achieved in alcohol under sonication followed by blade mixing. The reduction of GO occurs together with the composite densification in SPS, thus avoiding the implementation of additional reduction steps. The materials are studied by X-ray photoelectron and micro-Raman spectroscopy, revealing a high level of recovery of graphene-like domains. The SPS graphene/Si3N4 composites exhibit relatively large electrical conductivity values caused by the presence of reduced graphene oxide (similar to 1 S cm(-1) for similar to 4 vol.%, and similar to 7 S cm-1 for 7 vol.% of reduced-GO). This single-step process also prevents the formation of highly curved graphene sheets during the thermal treatment as the sheets are homogeneously embedded in the ceramic matrix. The uniform distribution of the reduced GO sheets in the composites also produces a noticeable grain refinement of the silicon nitride matrix. (C) 2013 Elsevier Ltd. All rights reserved.

A study of the influence of the substitution of Al for Ga in the ceramic processing and electrical properties of La0.95Sr0.05Ga0.90–x Al x Mg0.10O3–δ (0 ≤ x ≤ 0.3) solid electrolytes is presented. The materials retained orthorhombic symmetry over the entire substitution range, whereas a deviation from Vegard’s law for x > 0.20 suggested a maximum Al solubility of x = 0.20. Scanning electron microscopy analysis of ceramic samples revealed that grain growth was inhibited for x ≥ 0.2. This microstructural change was related to an apparent deterioration of mechanical properties, as suggested by room-temperature Vickers hardness measurements. Impedance spectroscopy revealed a significant degradation of the grain-boundary electrical properties for x ≥ 0.20, whereas the bulk conductivity was enhanced for 0.10 ≤ x ≤ 0.15. Oxygen-permeability measurements confirmed that the studied materials remain essentially pure ionic conductors. An ionic conductivity maximum of 0.047 S/cm at 700 °C was obtained for x = 0.10. The effect of aluminium in the grain-bulk ionic conductivity is discussed in terms of defect cluster models and assuming fast oxygen diffusion along domain walls.

Gadolinia-doped ceria electrolyte Ce 0.8 Gd 0.2 O 2− δ (CGO) and perovskite-type mixed conductor La 0.8 Sr 0.2 Fe 0.8 Co 0.2 O 3− δ (LSFC), having compatible thermal expansion coefficients (TECs), were combined in dual-phase ceramic membranes for oxygen separation. Oxygen permeability of both LSFC and composite LSFC/CGO membranes at 970–1220 K was found to be limited by the bulk ambipolar conductivity. LSFC exhibits a relatively low ionic conductivity and high activation energy for ionic transport (∼200 kJ/mol) in comparison with doped ceria. As a result, oxygen permeation through LSFC/CGO composite membranes, containing similar volume fractions of the phases, is determined by the ionic transport in CGO. The permeation fluxes through LSFC/CGO and La 0.7 Sr 0.3 MnO 3− δ /Ce 0.8 Gd 0.2 O 2− δ (LSM/CGO) composites have comparable values. An increase in the p-type electronic conductivity of ceria in oxidizing conditions, which can be achieved by co-doping with variable-valence metal cations, such as Pr, leads to a greater permeability. The oxygen ionic conductivity of the composites consisting of CGO and perovskite oxides depends strongly of processing conditions, decreasing with interdiffusion of the phase components, particularly lanthanum and strontium cations from the perovskite into the CGO phase.