Albert Tarancón

Layered perovskites as promising cathodes for intermediate temperature solid oxide fuel cells

The suitability of GdBaCo2O5+δ as a cathode material for intermediate temperature solid oxide fuel cells has been evaluated. The 18O/16O isotope exchange depth profile (IEDP) method has been used to obtain the oxygen surface exchange and oxygen tracer diffusion coefficients yielding optimum values for applicability in fuel cells (k* = 2.8 × 10−7 cm s−1 and D* = 4.8 × 10−10 cm2 s−1 at 575 °C) especially in terms of low activation energies (EAk = 0.81(4) and EAD = 0.60(4) eV). The same material has been characterized electrically as a part of a symmetrical electrochemical system (GdBaCo2O5+δ/Ce0.9Gd0.1O2−x/GdBaCo2O5+δ), by means of impedance spectroscopy measurements, corroborating an excellent performance in the classical intermediate temperature range for solid oxide fuel cells (500–700 °C). An area specific resistance (electrode–electrolyte interface) of 0.25 Ω cm2 at 625 °C was achieved for a cell processing temperature of 975 °C. Finally, layered perovskites are presented as a promising new family of materials for cathode use in solid oxide fuel cells at low temperatures.

Oxygen ion diffusion in cation ordered/disordered GdBaCo2O5+δ

We use molecular dynamics in conjunction with an established set of Born model potentials to examine the oxygen ion diffusion mechanism in the double perovskite GdBaCo2O5+δ. We predict that the mechanism of oxygen diffusion is highly anisotropic diffusion occurring only in the Gd–O and adjacent Co–O layers. For GdBaCo2O5.5 the activation energy of oxygen diffusion is 0.5 eV. We investigate the effect that cation disorder of the Gd–Ba sublattice has upon the diffusivity, the anisotropy and the diffusion mechanism in GdBaCo2O5+δ, which is a model system for double perovskites and other layered compounds. Cation disorder results in a reduction of the oxygen diffusivity and the appearance of diffusion along the c-axis of the material. Oxygen diffusion becomes effectively isotropic when the cation sublattice is disordered.

Fabrication and electrical characterization of circuits based on individual tin oxide nanowires

Two-xa0and four-probe electrical measurements on individual tin oxide (SnO(2)) nanowires were performed to evaluate their conductivity and contact resistance. Electrical contacts between the nanowires and the microelectrodes were achieved with the help of an electron-xa0and ion-beam-assisted direct-write nanolithography process. High contact resistance values and the nonlinear current-bias (I-V) characteristics of some of these devices observed in two-probe measurements can be explained by the existence of back-to-back Schottky barriers arising from the platinum-nanowire contacts. The nanoscale devices described herein were characterized using impedance spectroscopy, enabling the development of an equivalent circuit. The proposed methodology of nanocontacting and measurements can be easily applied to other nanowires and nanometre-sized materials.

Synthesis of nanocrystalline materials for SOFC applications by acrylamide polymerisation

AbstractUltrafine powders with applicability in solid oxide fuel cells (SOFCs) were prepared by a novel method based on a polyacrylamide gel-combustion process: Zr 0.84 Y 0.16 O 1.92 (8YSZ), Ce 0.8 Gd 0.2 O 1.9 (CGO), La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 2.85 (LSGM), La 2 Mo 2 O 9 ,La 0.8 Sr 0.2 CoO 3 d (LSC) and La 0.8 Sr 0.2 FeO 3 d (LSF). Synthesized powders present desirable characteristics for powder consolidation and sintering, includingnanometric crystal size (10–40 nm), narrow size distribution and the possibility of aggregate disagglomeration via soft ball milling. Aclassical screen-printing method is presented as a novel thin dense layer deposition technique. First results on deposition of quasi-full-densitythin films of 8YSZ (around 5 mm thick) were obtained at a sintering temperature of 1300 8C with sintering times of 10 h in air.# 2003 Elsevier Science B.V. All rights reserved. Keywords: SOFC; Synthesis; Acrylamide; Methacrylamide; Screen-printing; Dense electrolyte 1. IntroductionSolidionicandmixedionic-electronicconductors(MIECs)have received special attention in recent years in relation totheir applicability in solid oxide fuel cells (SOFCs) [1,2] aselectrolyte and electrode materials, respectively. Fluorite (i.e.Zr

Portable microsensors based on individual SnO2 nanowires

Individual SnO(2) nanowires were integrated in suspended micromembrane-based bottom-up devices. Electrical contacts between the nanowires and the electrodes were achieved with the help of electron- and ion-beam-assisted direct-write nanolithography processes. The stability of these nanomaterials was evaluated as function of time and applied current, showing that stable and reliable devices were obtained. Furthermore, the possibility of modulating their temperature using the integrated microheater placed in the membrane was also demonstrated, enabling these devices to be used in gas sensing procedures. We present a methodology and general strategy for the fabrication and characterization of portable and reliable nanowire-based devices.

Water vapor detection with individual tin oxide nanowires

Individual tin oxide nanowires (NWs), contacted to platinum electrodes using focused ion beam assisted nanolithography, were used for detecting water vapor (1500-32u2009000xa0ppm) in different gaseous environments. Responses obtained in synthetic air (SA) and nitrogen atmospheres suggested differences in the sensing mechanism, which were related to changes in surface density of the adsorbed oxygen species in the two cases. A model describing the different behaviors has been proposed together with comparative evaluation of NW responses against sensors based on bulk tin oxide. The results obtained on tenxa0individual devices (tested >6 times) revealed the interfering effect of water in the detection of carbon monoxide and illustrated the intrinsic potential of nanowire-based devices as humidity sensors. Investigations were made on sensitivity, recovery time and device stability as well as surface-humidity interactions. This is the first step towards fundamental understanding of single-crystalline one-dimensional (1D) tin oxide nanostructures for sensor applications, which could lead to integration in real devices.

Grain-boundary resistivity versus grain size distribution in three-dimensional polycrystals

We have studied the accuracy of the bricklayer model in the evaluation of the grain-boundary resistivity associated to a real three-dimensional (3D) polycrystal. An impedance network electrically equivalent to the 3D structure has been solved by direct calculation. In order to examine the progressive evolution from the bricklayer model to a completely disordered system, the ideal polycrystal was modified in a controlled way, according to a Voronoi diagram approach. From this, the grain-boundary resistivity of a polycrystal, as deduced from the 3D bricklayer model, is lower than the real value. The upper limit of the error made is around 15% in relation to the real one.

Power Response of a Planar Thermoelectric Microgenerator Based on Silicon Nanowires at Different Convection Regimes

Abstract A thermoelectric microgenerator based on multiple silicon nanowire arrays is fabricated and its performance evaluated for different convection regimes. Mature silicon microfabrication technology is used to fabricate the device structure. As a post-process, a bottom-up approach is used to grow silicon nanowires by a VLS-CVD mechanism. The thermal design of the microgenerator features a thermally isolated silicon platform which is connected to the bulk silicon rim through several arrays of silicon nanowires. Simulations are carried out to evaluate the need of an external heat sink to improve the thermal gradient seen by the nanowires and the power output of the microgenerator. Results show a significant improvement with a heat sink raising the thermal gradient from 3u2006K to approximately 100u2006K when the external temperature gradient is 300u2006K. Experimental measurements with different convection regimes also show a radical improvement on the power output comparing natural convection and two different forced convection regimes. The first forced convection regime is a broad airflow from a commercial CPU fan placed on top of the device, while the second (air jet forced convection) uses a syringe to focus the airflow from the compressed air line to the platform. The maximum output power for a natural convection regime is 2.2u2006nW for a hotplate temperature of 200u2006°C, while the air jet forced convection regime generates up to 700u2006nW, which correspond to 35u2006µW/cm2 considering a device footprint of 2u2006mm2.

Micro solid oxide fuel cells: a new generation of micro-power sources for portable applications

Portable electronic devices are already an indispensable part of our daily life; and their increasing number and demand for higher performance is becoming a challenge for the research community. In particular, a major concern is the way to efficiently power these energy-demanding devices, assuring long grid independency with high efficiency, sustainability and cheap production. In this context, technologies beyond Li-ion are receiving increasing attention, among which the development of micro solid oxide fuel cells (μSOFC) stands out. In particular, μSOFC provides a high energy density, high efficiency and opens the possibility to the use of different fuels, such as hydrocarbons. Yet, its high operating temperature has typically hindered its application as miniaturized portable device. Recent advances have however set a completely new range of lower operating temperatures, i.e. 350-450°C, as compared to the typical <900°C needed for classical bulk SOFC systems. In this work, a comprehensive review of the status of the technology is presented. The main achievements, as well as the most important challenges still pending are discussed, regarding (i.) the cell design and microfabrication, and (ii.) the integration of functional electrolyte and electrode materials. To conclude, the different strategies foreseen for a wide deployment of the technology as new portable power source are underlined.

Fabrication of bottom-up gas sensors based on individual SnO 2 nanowires and suspended microhotplates

In this work, bottom-up devices based on individual monocrystalline SnO2 nanowires (NWs) were fabricated using FIB nanolithography on top of suspended microhotplates, with integrated heater and interdigitated microelectrodes. The electrical characterisation of such devices in the presence of different gases show that devices with improved gas sensing properties can be fabricated.