Evaluation of a sol–gel process for the synthesis of La1−xSrxMnO3+δ cathodic multilayers for solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrical energy conversion devices with high efficiency and low pollution. In order to increase performances of SOFCs at intermediate temperature (700–800 °C) and to decrease materials cost, an alternative sol–gel synthesis method has been investigated to deposit La1−xSrxMnO3+δ (LSMx) as cathode thin films. Polycrystalline LSMx thin films were prepared by dip-coating using a polymeric solution. Lanthanum, strontium and manganese nitrates were used as raw materials. The viscosity of the solution was adjusted and the solution was deposited on polycrystalline ZrO2–8% Y2O3 ceramics. Prior to experiments, the substrate surface was eroded until a roughness of 20 nm and then cleaned with ethanol and dried. Film thicknesses were adjusted with the number of layers. Porosity and grain size of monolayers or multilayers were evaluated. Typical thickness of monolayer is 250 nm. A key parameter in the multilayer process was the intermediate calcination temperature (400, 700 or 1000 °C) of each further layer deposition. A correlation between this intermediate temperature and morphology, thickness and porosity was found; porosity is ranging from 3 to 40% and thickness can reach 1 micron for multilayers. Concerning electrochemical performances, the best results were obtained for LSM0.4 multilayers with an intermediate calcination temperature (called Ti) of 400 °C

Synthesis by sol–gel route of oxyapatite powders for dense ceramics : applications as electrolytes for solid oxide fuel cells

Solid oxide fuel cells have considerable interest in recent years, because of their high efficiency and environmentally friendly nature. Such systems required oxygen-conducting electrolytes and now the most common electrolyte is yttria stabilized zirconia (YSZ). This compound exhibits high oxide ion conductivity at elevated temperatures (850–1000 °C). However, this high working temperature causes problems in terms of materials selection and lifetime. One solution is to develop new oxide ions conductors exhibiting high oxide ion conductivity at intermediary temperatures (700–800 °C). Recent work has identified Ln10−xSi6O26±z (Ln = rare earths) as a good fast oxide ion conductor. Undoped and doped Ln10−xB6O26±z (B = Si or Ge) oxides are currently prepared by solid-state methods. In that work, we propose a sol–gel process to synthesize powders of La9.33Si6O26 type-silicated apatites. The main advantage is to decrease the crystallization temperature in ,comparison to the conventional methods, allowing the synthesis of reactive powders with nanometric particles size. These oxides are synthesized using silicon alkoxide and lanthanum nitride as precursors. In the litterature, no study refers to the synthesis of mixed oxides with silicon alcoxides. However, there are several studies on sol–gel synthesis of glasses with this precursor. In this study, several processing parameters have been investigated (the hydrolysis ratio, the concentration of metallic precursors in the sol and the role of organic compounds) in order to synthesize pure phases after the decomposition of the sols. Pure powders of La9.33Si6O26 type-silicated apatites are obtained at 800 °C. These powders were used to prepare ceramics. Several processing parameters as morphology of powders (agglomeration, particle sizes) and, heating profiles have been studied on the densification. Dense ceramics (90–95%) have been prepared at temperatures around 1400 °C. The used of sol–gel powders allow the decrease of the sintering temperature of about 200 °C

High power density electrodes for Carbon supercapacitor applications

This paper presents results obtained with 4 cm2 Carbon/Carbon supercapacitors cells in organic electrolyte. In the first approach, a surface treatment for Al current collector foil via the sol–gel route has been used in order to decrease the Al/active material interface resistance. Performances obtained with this original process are: a low equivalent series resistance (ESR) of 0.5 Ω cm2 and a specific capacitance of 95 F g−1 of activated carbon. Then, supercapacitors assembled with treated Al foil and active material containing activated carbon/carbon nanotubes (CNTs) with different compositions have been studied. Galvanostatic cycling measurements show that when CNTs content increases, both ESR and specific capacitance are decreased. Fifteen percent appears to be a good compromise between stored energy and delivered power with an ESR of 0.4 Ω cm2 and a specific capacitance of 93 F g−1 of carbonaceous active material. Finally, cells frequency behaviour has been characterized by Electrochemical Impedance Spectroscopy. The relaxation time constant of cells decreases when the CNTs content increases. For 15% of CNTs, the time constant is about 30% lower as compared to a cell using pure activated carbon-based electrodes leading to a higher delivered power

Lanthanum ferromanganites thin films by sol–gel process. Influence of the organic/inorganic R ratio on the microstructural properties

Strontium-substituted lanthanum ferromanganites, La0.8Sr0.2Mn1−yFeyO3+δ (y=0, 0.2, 0.5, 0.8, 1), LSMF2Y (Y=10y) for solid oxide fuel cell (SOFC) cathode applications have been synthetized by a polymeric sol–gel route and deposited on YSZ substrates by a dip-coating process. The influence of the sol synthesis parameters (metal composition, organic and metal salt concentrations) on the thin film microstructure has been investigated. In this study, it has been shown that the organic/inorganic ratio, called R, appears as a key parameter to control the microstructural properties of final coatings

Microcontact printing process for the patterned growth of individual CNTs

We report an original approach to pattern a substrate with isolated carbon nanotubes. Through the improvement of the microcontact printing technique by the use of a new composite stamp, we were able to produce on flat substrates micrometric features of a catalyst suitable for the localised growth of single-walled carbon nanotubes by catalytic chemical vapour deposition. This catalyst material is for the first time prepared via an original sol–gel process. The growth of straight carbon nanotubes between the patterns was observed and a method to promote the controlled growth of such isolated nanoobjects is thus conceivable

Thick films of YSZ electrolytes by dip-coating process

Yttria stabilized zirconia (YSZ, 8% Y2O3) thick films were coated on porous Ni-YSZ substrates using the dip-coating process and a suspension with a new formulation. The suspension was obtained by addition of a polymeric matrix in a stable suspension of a commercial YSZ (Tosoh) powders dispersed in an azeotropic MEK-EtOH mixture. The green layers were densified after an optimization of the suspension composition. YSZ Tosoh particles encapsulated by a zirconium alkoxide sol and added with colloidal alkoxide precursor are used to load the suspension. The in situ growth of these colloids increases significantly the layers density after an appropriated heat treatment. The obtained films are continuous, homogeneous and 20 μm thick. Different microstructures are obtained depending on the synthesis parameters of the suspension

CO oxidation over nonstoichiometric nickel manganite spinels

Nonstoichiometric nickel–manganese spinel oxides, NixMn3−x ▭3δ/4O4+δ (1≥x≥0), have been synthesized by calcination in air of mixed oxalates at 623 K. These materials are shown to be highly reactive for CO oxidation, some conversion being observed at room temperature for the most active solid (x=1.0). The interaction of CO and O2 with these oxides has been studied by in situ IR spectroscopy under steady-state and transient reaction conditions. A detailed mechanism is proposed wherein CO reacts with coordinatively unsaturated cations to give carbonyl complexes which in turn react with surface oxygen activated on anionic vacancies. Adsorbed and gaseous CO2 also undergo much slower side reactions with lattice oxygen or surface hydroxide groups to give more stable hydrogen carbonate and carbonates species, which lead to catalyst deactivation. Marked effects of pretreatment are explained on the basis of the observed kinetics and the proposed mechanis

X-ray photoelectron spectroscopic study of non-stoichiometric nickel and nickel-copper spinel manganites

The surface of non-stoichiometric nickel and nickel-copper spinel manganites has been investigated by X-ray Photoelectron Spectroscopy (XPS). The oxidation states of the nickel, copper and manganese cations present on the surface of the samples were determined from the analysis of the M 2p(3/2) core levels (M=Ni, Cu, Mn). In particular both Cu2+ and Cu+ were evidenced in the determined from the analysis of the M 2p(3/2) core levels (M=Ni, Cu, Mn). In particular, both Cu structure whereas only bivalent nickel was observed. The partial substitution of manganese by copper led to a chemical shift towards lower binding energy in the Ni 2p(3/2) region, which was explained by the displacement of some Ni2+ cations from tetrahedral to octahedral sites of the spinel structure. Finally, the surface atomic ratios Ni/Mn for nickel manganites, Ni/(Mn+Cu) and Cu/(Mn+Ni) for nickel-copper manganites, determined from XPS data, were compared to the ratios corresponding to the bulk composition. This study shows in all cases a nickel enrichment at the surface which is not affected by the copper content of the oxide. On the contrary, the ratio Cu/(Mn+Ni) was found to be lower than the corresponding bulk value

“Chimie douce”: A land of opportunities for the designed construction of functional inorganic and hybrid organic-inorganic nanomaterials

Abstract“Chimie douce” based strategies allow, through the deep knowledge of materials chemistry and processing, the birth of the molecular engineering of nanomaterials. This feature article will highlight some of the main research accomplishments we have performed during the last years. We describe successively the design and properties of: sol–gel derived hybrids, Nano Building Blocks (NBBs) based hybrid materials, nanostructured porous materials proceeds as thin films and ultra-thin films, aerosol processed mesoporous powders and finally hierarchically structured materials. The importance of the control of the hybrid interfaces via the use of modern tools as DOSY NMR, SAXS, WAXS, Ellipsometry that are very useful to evaluate in situ the hybrid interfaces and the self-assembly processes is emphasized. Some examples of the optical, photocatalytic, electrochemical and mechanical properties of the resulting inorganic or hybrid nanomaterials are also presented

Composition and porosity graded La2−xNiO4+δ (x≥0) interlayers for SOFC: Control of the microstructure via a sol–gel process

We have developed composition and porosity graded La2−xNiO4+δ (x≥0) cathode interlayers for low-temperature solid oxide fuel cell that exhibit good adhesion with the electrolyte, controlled porosity and grain size and good electrochemical behaviour. La2−xNiO4+δ (x≥0) monolayers are elaborated from a derived sol–gel method using nitrate salts, acetylacetone and hexamethylenetetramine in acetic acid. As a function of the organic concentration and the molar ratio of lanthanum to nickel, these layers present platelets or spherical shape grains with a size distribution ranging from 50 to 200 nm, as verified by SEM-FEG. On the basis of this processing protocol, we prepared porosity and composition graded lanthanum nickelates interlayers with effective control of the pore distribution, the nanocrystalline phase, the thickness and the subsequent electrochemical properties