Potentialities of the sol-gel route to develop cathode and electrolyte thick layers Application to SOFC systems

In this work, we report the potential of sol–gel process to prepare cathode and electrolyte thin and thick layers on anodic NiO-YSZ supports which were also made from powders prepared by sol–gel route. YSZ and La2 − xNiO4 + δ, La4Ni3O10 were synthesized as electrolyte and cathode materials for SOFC applications. For electrolyte shaping, yttria stabilized zirconia (YSZ, 8% Y2O3) thick films were cast onto porous NiO-YSZ composite substrates by a dip-coating process using a new suspension formulation. Part of the YSZ precursor colloidal sol was added in the suspension to ensure both homogeneity and adhesion of the electrolyte on the anodic substrate after thermal treatment at 1400 °C for 2 h. By precisely controlling the synthesis parameters, dense and gas-tight layers with thicknesses in the range of 10–20 μm have been obtained. Gas-tightness was confirmed by He permeation measurements. Concerning cathode processing, a duplex microstructured cathode consisting of both La2 − xNiO4 + δ ultra-thin films (few nanometers) and La2 − xNiO4 + δ and/or La4Ni3O10 thick layers (few micrometers) was prepared on YSZ substrates by the dip-coating process, with the thickness being dependent on the nature of the dip-coated solution (polymeric sol or adequate suspension). The derived cathode microstructure, related to the number/thickness of layers and type of architecture, was correlated to the good cell electrochemical performances. Concerning cathode processing, a duplex microstructured cathode consisting of both La2 ? xNiO4 + ? ultra-thin films (few nanometers) and La2 ? xNiO4 + ? and/or La4Ni3O10 thick layers (few micrometers) was prepared on YSZ substrates by the dip-coating process, with the thickness being dependent on the nature of the dip-coated solution (polymeric sol or adequate suspension). The derived cathode microstructure, related to the number/thickness of layers and type of architecture, was correlated to the good cell electrochemical performances

Development of lanthanum nickelate as a cathode for use in intermediate temperature solid oxide fuel cells

The performance of lanthanum nickelate, La2NiO4+δ (LNO), as a cathode in IT-SOFCs with the electrolyte cerium gadolinium oxide, Ce0.9Gd0.1O2−δ (CGO), has been investigated by AC impedance spectroscopy of symmetrical cells. A significant reduction in the area specific resistance (ASR) has been achieved with a layered cathode structure consisting of a thin compact LNO layer between the dense electrolyte and porous electrode. This decrease in ASR is believed to be a result of contact at the electrolyte/cathode boundary enhancing the oxygen ion transfer to the electrolyte. An ASR of 1.0 Ω cm2 at 700 °C was measured in a symmetrical cell with this layered structure, compared to an ASR of 7.4 Ω cm2 in a cell without the compact layer. In addition, further improvements were observed by enhancing the cell current collection and it is anticipated that a symmetrical cell consisting of a layered structure with adequate current collection would lower these ASR values further

Laser induced densification of cerium gadolinium oxide: application to single-chamber solid oxide fuel cells

International audienceIn single-chamber solid oxide fuel cells (SC-SOFC), anode and cathode are placed in a gas chamber where they are exposed to a fuel/air mixture. Similarly to conventional dual-chamber SOFC, the anode and the cathode are separated by an electrolyte. However, as in the SC-SOFC configuration the electrolyte does not play tightness role between compartments, this one can be a porous layer. Nevertheless, it is necessary to have a diffusion barrier to prevent the transportation of hydrogen produced locally at the anode to the cathode that reduces fuel cell performances. This study aims to obtain directly a diffusion barrier through the surface densification of the electrolyte Ce0.9Gd0.1O1.95 (CGO) by a laser treatment. KrF excimer laser and Yb fiber laser irradiations were used at different fluences and number of pulses to modify the density of the electrolyte coating. Microstructural characterizations confirmed the modifications on the surface of the electrolyte for appropriate experimental conditions showing either grain growth or densified but cracked surfaces. Gas permeation and electrical conductivities of the modified electrolyte were evaluated. Finally SC-SOFC performances were improved for the cells presenting grain growth at the electrolyte surface

A novel approach of a fully inkjet printed SnO₂-based gas sensor on a flexible foil

International audienceIn recent years, printed and flexible gas sensors have quickly emerged as an innovative area of great interest because of their lightness and low cost. These flexible sensors can be easily integrated into autonomous systems for many applications such as smart food packaging and premature disease detection. In this paper, a novel approach was applied to manufacture a fully inkjet-printed gas sensor on a flexible polymeric foil. Platinum heater and gold electrodes were printed on the top side of the substrate, separated by a thin insulating layer of printed polyimide. An aqueous sol-gel process was adopted to synthesize nanosized SnO2-based sol that guaranty a crystallization at 350 °C, which is entirely consistent with the polyimide foil. Then, the sol was transformed into a stable ink and inkjet printed over the gold electrodes. The printability of different inks was optimized to ensure flawless ejection of droplets, and the complex physico-chemical interactions between the inks and different interfaces were controlled to get well-defined patterns with high resolution. Finally, electrical measurements of the printed sensor were performed to characterize the response and the sensitivity to different concentrations of ethanol, ammonia and carbon monoxide gases, at working temperature of 300 °C, in dry and wet air

A new route to prepare anodic coatings on dense and porous metallic supports for SOFC application

Metallic cell supports have been developed for the new generation of fuel cells. Sol–gel process has been used to prepare anodic coatings on these supports at moderate thermal treatment temperature, in order to keep a good support mechanical behavior and limit metallic corrosion. Indeed, we take advantage of the numerous reaction routes that sol–gel method can offer to first synthesize NiO–YSZ (yttria-stabilized zirconia) homogeneous composites, and then to process films of different thicknesses on metallic supports by dipcoating. In this work, the metallic supports could be either dense or porous. To begin with, duplex microstructured anodes were prepared from both thin and thick layers, directly deposited on dense metallic supports. The interfacial anodic layer, around 100 nm thick, improves adhesion and accommodates stresses between metallic interconnect and active thick anode. Moreover, by dipping the substrate into an optimized slurry containing sol–gel composite powders, films of a few microns thick have been obtained and constituted the active anodic part. A heat treatment at only 800 °C leads to a coherent anodic duplex stacking which is continuous, homogeneous and adherent. Subsequently, thick anodic films have also been deposited on two different porous supports, with both dip-coating process and slurries routes. These thick anodic coatings were characterized after thermal treatment at 800 °C

Elaboration of metallic compacts with high porosity for mechanical supports of SOFC

The development of third generation Solid Oxide Fuel Cells (SOFC) with metallic mechanical supports presents several advantages over that of ceramic stacks by offering a lower cost and longer lifetime of the stacks. As a consequence, it is necessary to prepare metallic porous compacts that remain stable at the operating temperature of the SOFC (700–800 C) under reductive atmosphere. This paper presents an innovative process to elaborate iron, nickel and cobalt porous compacts. The process is based on the thermal decomposition of metal oxalate precursors with controlled morphology into metallic powders with coralline shape. Uniaxial compaction of such powders (without binder addition to the powders) under low uniaxial pressures (rising from 20 to 100 MPa) gave rise to green compacts with high porosity and good mechanical properties. After annealing at 800 C under H2 atmosphere, the compacts still present interconnected porosity high enough to allow sufficient gas flow to feed a SOFC single cell in hydrogen: the porosity rises from 25 to 50% for iron compacts, from 20 to 50% for cobalt compacts, and is higher than 40% for nickel compacts. Results from physicochemical characterization (XRD, SEM, gas permeation, Hg porosimetry) corroborated the process for SOFC application

Development of a sensitive and selective mixed-potential ammonia sensor for automotive exhausts

Session: A5 Oxide Based Sensors and ActuatorsInternational audienceOne of the most effective technologies in decreasing large-scale NOx emission produced by diesel engine vehicles is Urea-SCR (selective catalytic reduction) system. In order to prevent inducing excessive ammonia to the environment, an NH3 sensor is required at the exit of this system. In this study, highly selective ammonia sensors were developed to detect ammonia emissions from automotive exhaust.The sensors were fabricated with 8-YSZ electrolyte, a platinum reference electrode and a working electrode of Au-V2O5 (mass ratio: 85:15), screen-printed on an alumina supports. A platinum resistor was printed at the backside of the support to control the sensor temperature. The measured sensor response (ΔV) is the potential difference between reference and working electrodes. Figure 1 shows the responses of two identical sensors to 100 ppm CO, NO2, NO and 20 ppm of NH3 at four different temperatures. It can be seen that the sensors respond to all gases at lower temperatures while by increasing temperature to 600 °C the selectivity to NH3 is greatly improved. The selectivity of sensors was also confirmed by testing other possible interfering gases and no responses were observed for 20ppm of H2 and 100ppm of a hydrocarbon mixture. The stability of such sensors was studied at 550 °C and 600 °C. Since sensors showed no long term stability at 600 °C (electrode degradation), but remain stable results at 550 °C, investigations were made to decrease the selective working temperature while maintaining selectivity. After testing different mass percentages of V2O5 in working electrode, we observed that by increasing this value to 50%, the working temperature of selective ammonia sensors could be decreased to 550 °C with stable responses. Further investigations will be performed in order to gain deeper insight in sensing mechanism of V2O5 based working electrodes, which governs the sensor’s performance

Anode supported single chamber solid oxide fuel cells operating in exhaust gases of thermal engine

International audienceThis project deals with the development and the electrochemical characterization of anode supported single chamber SOFC in a simulated environment of thermal engine exhaust gas. In the present work, a gas mixture representative of exhaust conditions is selected. It is composed of hydrocarbons (HC: propane and propene), oxygen, carbon monoxide, carbon dioxide, hydrogen and water. Only oxygen content is varied leading to different gas mixtures characterized by three ratios R = HC/O2. Concerning the cell components, a cermet made of nickel and an electrolyte material, Ce0.9Gd0.1O1.95 (CGO) is used as anode and two cathode materials, La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and Pr2NiO4+δ (PNO), are evaluated. The prepared cells are investigated in the various gas mixtures for temperatures ranging from 450 °C to 600 °C. Ni-CGO/CGO/LSCF-CGO cell has delivered a maximum power density of 15 mW cm−2 at 500 °C with R = HC/O2 = 0.21, while lower power densities are obtained for the other ratios, R = 0.44 and R = 0.67. Afterwards, LSCF and PNO cathode materials are compared and LSCF is found to deliver the highest power densities. Finally, by improving the electrolyte microstructure, some cells presenting a maximum power density of 25 mW cm−2 at 550 °C are produced. Moreover, up to 17% of initial HC are eliminated in the gas mixture

Tunable architecture for flexible and highly conductive graphene-polymer composites

International audiencePrinted electronics, particularly on flexible and textile substrates, raised a strong interest during the past decades. This work presents a good candidate for conductive inks based on a graphene/polymer nanocomposite material that gathers three main benefits that are 1 - neither clogging nor flocculation, 2 - spontaneous film formation around room temperature, 3 - high conductivity. Nanosized Multilayered Graphene (NMG) is produced through a solvent-free procedure, using a grinding process in water. These NMG suspensions are used to elaborate conductive composite materials through physical blending with emulsifier-free latex. The nanocomposite microstructure exhibits a well-defined cellular architecture that highlights the formation of continuous paths of fillers throughout the material. The conductivity behavior of the nanocomposite material was efficiently described using a percolation model: the conductivity can be tuned by changing the NMG content and the latex size. A low percolation threshold (0.1 vol%) was obtained and the electrical conductivity reached 217 S m−1 for 6 vol% NMG. Efficient film forming occurs at room temperature leading to continuous and deformable materials, which is adequate for printing on flexible and textile substrates. The applicability in electronics is demonstrated by the use of the nanocomposite material in replacement of copper wires in a LED setup