Domingo Pérez-Coll

Surface proton conductivity of dense nanocrystalline YSZ

Ionic transport in nanocrystalline solid oxides is of considerable current interest. Several studies have reported room-temperature proton conductivity in nanoscaled 8 mol% Y2O3-doped zirconia (YSZ), although the location of the transport species is not clear. In this study, nanocrystalline YSZ with a grain size of ∼50 nm was prepared by spark-plasma sintering of nanoscaled commercial powder to a density of >97% of the theoretical value. Impedance spectroscopy was employed to analyze the electrical behavior in the temperature range 25–600 °C in H2O- and D2O-wetted atmospheres (air, O2 and 10% H2:90% N2). Transport in wet conditions below 50 °C is limited to the sample surface and occurs via proton hopping (Grotthus mechanism), as demonstrated by a conductive H+/D+ isotope effect. The impedance in these conditions is dominated by a single arc which can be modelled with parallel paths for proton transport on the lateral sample surface and a capacitance with values of the order of those of the grain interior. Surface proton transport is considerable, exhibiting a resistance at 26 °C in wet atmospheres comparable to that obtained at ∼300 °C. In contrast, transport by volumetric processes (grain, grain boundary or nanopores) was demonstrated to be insignificant by experiments involving conductivity measurements with a coated lateral surface, with electrode configurations of different areas, and emf measurements in a water-vapour concentration cell.

Synthesis, structures and electrical transport properties of the La2−xSrxNiTiO6−δ (0 ≤ x ≤ 0.5) perovskite series

Perovskites (ABO3) with mixed transition-metal cations are of particular interest for high-temperature electrochemical and electrocatalytic devices. In this regard, the effects of Sr doping on the structural and electrical properties of the double perovskite La2NiTiO6 have been studied. High-resolution neutron powder diffraction (NPD) indicates that the symmetry of the La2−xSrxNiTiO6−δ series changes from monoclinic (space group, P21/n) for low Sr contents, associated with B-site cation ordering, to orthorhombic (Pnma) for x > 0.1 with increasing B-site disorder. The substitution of Sr for La is primarily charge compensated by the creation of oxygen vacancies, as indicated by NPD and analysis of the Ni oxidation state by chemical titration thermal analysis and magnetic measurements. The system exhibits p-type electronic conductivity for oxygen partial pressures (pO2) > 10−18 atm, increasing with increasing Sr content (σ = 1.18 S cm−1 at 900 °C for x = 0.5 in air). A p–n transition is observed in the range 10−18 ≤ pO2 ≤ 10−19 atm. Sr-doped materials are less conductive in the n-type regime than the Sr-free parent phase; however, n-type conductivity increases with greater x in the range 0.1–0.5, most probably due to increasing reducibility associated with a higher degree of B-site disorder and defect association.

Methodology for the study of mixed transport properties of a Zn-doped SrZr0.9Y0.1O3−δ electrolyte under reducing conditions

The mixed ionic–electronic transport properties of the protonic ceramic electrolyser material SrZr0.9Y0.1O3−δ, with the addition of 4 mol% ZnO as a sintering additive, are analysed under reducing conditions. The study is performed by means of an active-load modification of the classical electromotive-force method to account for the non-negligible effect of the electrodes on the obtained electrical-transport numbers. The methodology is developed in detail in order to link the electrochemical criteria to simulated equivalent circuits. The observed electromotive force of the system is considerably affected by the introduction of the polarisation resistance of the electrodes in the corresponding analysis, resulting in a high deviation between the present results and those obtained by a classical analysis without attending to electrode effects. Under wet reducing conditions (pH2 ≈ 0.05 atm, pH2O ≈ 3 × 10−3–10−2 atm), the oxide-ionic transport number is negligible in the range of 600–900 °C, whereas pure protonic conductivity is observed for temperatures ≤700 °C and pH2O ≥ 5.6 × 10−3 atm. For higher temperatures and/or lower pH2O, mixed protonic–electronic conduction is exhibited. The electronic contribution under reducing conditions is consistent with n-type electronic behaviour.

Exploring the mixed transport properties of sulfur(VI)-doped Ba2In2O5 for intermediate-temperature electrochemical applications

A stabilised orthorhombic perovskite Ba2In1.8S0.2O5+δ was synthesised by a solid state reaction at low-temperature. The mixed (electronic–protonic–oxide-ionic) conducting properties of this composition were investigated in detail for potential interest in a wide range of membrane applications including protonic ceramic fuel cells. The electrochemical analyses performed include impedance spectroscopy under wet/dry conditions of N2 and O2 and modified electromotive force measurements in wet/dry mixtures of N2/O2. Under dry oxidising conditions, the sample possesses mixed electronic–oxide-ionic contributions to the electrical transport in the whole range of temperatures, associated with the equilibrium between oxygen vacancies and holes. In wet atmospheres, protonic species arise from the hydration reaction. Protons represent the dominant charge carriers for temperatures lower than 550 °C, while oxide-ions and holes are the major mobile species for temperatures higher than 700 °C. Appreciable mixed contributions to the electrical transport from protons, oxide ions and holes are confirmed between 550 and 700 °C. The experimental data obtained from modified electromotive force measurements and impedance spectroscopy fit well with the expected results according to the proposed defect chemistry model.

Electrical properties of nanometric CGO-thin films prepared by electron-beam physical vapour deposition

Thin-film oxide-ion-conducting electrolytes are of considerable interest for the development of micro-solid oxide fuel cells (μ-SOFCs). In this study, nanocrystalline CGO thin films (with thickness ranging from 0.3 to 1.35 μm) were deposited from a Ce0.9Gd0.1O2−δ target by electron-beam physical vapour deposition onto a selection of substrates (Si, glass, fused silica and Al2O3) which were either maintained at room temperature or at 700 °C during deposition. Films have a columnar microstructure with (111) texture in the surface plane. The crystallinity of the films deposited at a substrate temperature of 700 °C was found to be much superior to those deposited at RT and subsequently heated. Impedance spectroscopy revealed that the in-plane resistivity of the films in air at 900 °C is greater than that of bulk CGO by around two orders of magnitude, with this difference increasing with decreasing temperature. The poor electrical properties of the nanocrystalline films may be considerably affected by the expected higher concentration of strains in comparison with the micrometric sample. The composition of the thin films was also affected by a de-doping process. The in-plane charge transport in films is limited by grain boundaries, mainly due to both a greater grain-boundary volume and higher specific grain-boundary resistivity. Conductivity measurements as a function of oxygen partial pressure reveal a considerably narrower electrolytic domain for the thin films than that of bulk CGO.

Graphene Covered Alumina Nanofibers as Toughening Agent in Alumina Ceramics

Graphene is a promising component for next-generation high-performance structural and multifunctional composite materials. Graphene deposited onto nanofibers of high aspect ratio can serve as reinforcement agent for improving ceramic fracture toughness and electroconductivity. It was found that quality and quantity of graphene sheets on the fiber surface essentially depends on the pyrolysis of carbon source conditions such as gas flow, duration, temperature and the composition of the gas mixture. The alumina/graphene composites of 10 and 15 wt% of nanofibers covered by graphene were produced by spark plasma sintering (SPS) at 1380 °C. Both composites show improvement in mechanical and electrical properties as compared to the monolithic alumina. The main advantage of the graphene growth on the fibers surface is a lack of complicated step of constituents mixing. Graphene platelets are believed to act as toughening agents prevailing crack propagation under loading.

Hybrid Graphene/Alumina Nanofibers for Electrodonductive Zirconia

Ceramic materials have become of high industrial importance in some applications as their properties outperform ones of metallic components. However, use of ceramics is limited due to the difficulties in shaping. Electrically conductive ceramics can be machined by Electro-Discharge Machining (EDM) irrespective of its hardness or strength. In this study, yttria stabilized zirconia (YTZP) conductive composite was produced by incorporation of the cost-effective graphene coated alumina nanofibers (ANFC) into the matrix. Almost fully dense YTZP/5 vol.% ANFC nanocomposite was obtained by spark plasma sintering (SPS) at 1250 °C with uniaxial pressure of 40 MPa. Scanning electron microscopy observation of the microstructures showed that ANFCs were homogeneously dispersed in the matrix. Addition of ANFC resulted in slightly decreased mechanical properties, but the electrical resistivity of the composite dropped 9 orders of magnitude compared to monolithic zirconia, exhibiting 1.4 Ω∙m, satisfying the required condition for the EDM.

Co-precipitate precursor-based synthesis of new interstitial niobium molybdenum nitrides

A simple method of preparation of interstitial niobium molybdenum nitride solid solutions in the series Nb1−xMoxNy (xxa0=xa00.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) has been developed. It is based on direct ammonolysis of precursors resulting from co-precipitation of aqueous solutions of the appropriate common metal salts. A study of the effect of method conditions on the outcome of the procedure is presented. Compounds in this series were prepared as single phases by nitridation at 1,173xa0K followed by rapid cooling of the samples. Similarly to the individual nitrides NbN and Mo2N, all the Nb1−xMoxNy compounds in this series have the rock-salt crystal structure in which the metal atoms are in an fcc arrangement with N atoms occupying octahedral interstitial positions. The materials were characterized by X-ray powder diffraction, elemental analysis, scanning electron microscopy, and thermogravimetry under oxygen flow.

Crystal structure and compositional effects on the electrical and electrochemical properties of GdBaCo2−xMnxO5+δ (0 ≤ x ≤ 2) oxides for use as air electrodes in solid oxide fuel cells

The effects of the substitution of Co with Mn in the crystal structure, oxygen content, thermal stability and expansion and electrical properties of GdBaCo2−xMnxO5+δ (0 ≤ x ≤ 2) oxides are reported. Composites of GdBaCo2−xMnxO5+δ–Ce0.9Gd0.1O2−δ (70u2006:u200630 wt%) are used as cathode materials and their electrochemical behaviour is presented. Layered-type ordering of Ba and Gd cations in the perovskite structure occurs in the whole system when the materials are prepared in argon but only for compositions in the range corresponding to x < 1.4 when the materials are prepared in air. The oxygen content increases with increasing the Mn content, causing thermal stability to improve and thermal expansion to decrease. However, lowering of the dc conductivity and an increase of electrode polarization resistances are observed by Mn substitution for Co. Cation ordering of the Gd and Ba atoms seems to affect the electrochemical properties of the materials.

Influence of the synthesis conditions on the crystal structure and properties of GdBaCo2−xFexO5+δ oxides as air-electrodes for intermediate temperature solid oxide fuel cells

The influence of the synthesis conditions on the crystal structure and properties of the oxides of the system GdBaCo2−xFexO5+δ is reported in this article. The materials are prepared by a ceramic method either in air or in an argon atmosphere. Crystal structure characterization is carried out by a combination of powder X-ray diffraction (PXRD) with advanced methods of high resolution transmission electron microscopy (HRTEM), scanning-transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). Thermal expansion coefficients of the oxides decrease with increasing substitution of Co by Fe. Impedance spectroscopy analysis of symmetrical cells using these materials as electrodes reveals area-specific polarization resistance (ASR) values low enough for the use of the oxides as air electrodes in intermediate temperature solid oxide fuel cells (IT-SOFCs).