Microemulsion-synthesized Pt/Ru/Sn nanoparticles on BDD for alcohol electrooxidation

Ternary Pt-based nanoparticles (nominal composition Pt80Ru10Sn10 ) were synthesized via the microemulsion route and deposited onto boron-doped diamond (BDD) electrode. The particle size measured by transmission electron microscopy was in the 2–5nm diam range. X-ray photoelectron spectroscopy revealed a slightly different surface composition from that of the bulk and showed no chemical shift of the Pt4f7∕2 line in ternary Pt∕Ru∕Sn nanoparticles. For both methanol and ethanol electro-oxidation, the ternary catalyst exhibited lower onset potentials than either pure Pt or the corresponding bimetallic (Pt∕Ru and Pt∕Sn ) catalysts. The ternary Pt∕Ru∕Sn catalyst was more efficient for methanol oxidation than for complete oxidation of ethanol, because ethanol electro-oxidation was stopped at acetaldehyde and/or acetic acid formation, the ternary catalyst being unable to activate the C-C bond scission.

Self and induced stabilized nanoparticles for electrocatalysis

Mériadec Limat

Since few decades the search of new catalysts formulations for fuel cell applications has motivated numerous projects especially due to the future lack of fossil fuel, environmental and economical reasons. Consequently, Direct Alcohol Fuel Cell (DAFC) or Direct Formic Acid Fuel Cell domains have received great attention. In spite of the promising results reported recently, these types of fuel cell still exhibit some problems. In fact, platinum, bulk or nanoparticles, undergoes surface poisoning involved by intermediates (as acetaldehyde, acetic acid, carbon monoxide) which are strongly bonded to the surface. These poisoning species block the active sites at the surface electrode and drastically decrease the catalytic activity and also the life-time of the cell. Probable solutions are suggested essentially in Pt-based alloys catalysts or nanocatalysts in order to avoid this phenomenon. Therefore, multicomponent catalysts deposited at carbon electrodes (Boron-Doped Diamond and Highly Oriented Pyrolytic Graphite) has been investigated during this thesis. These two substrates have been chosen due to their outstanding properties. In the first part of this study, self stabilized nanoparticles at BDD substrate have been studied. Ex-situ ternary Pt-Ru-Sn microemulsion-synthesized nanoparticles deposited at BDD substrate were investigated towards both methanol and ethanol electrooxidation. Such type of composite electrode seemed to possess a greater efficiency towards methanol oxidation than observed for ethanol, leading essentially for the latter to C2 oxidation products. The inability of this catalyst to activate the C-C bond scission can explain this observation. However, the major enhancement was observed for the methanol electrooxidation. Recent studies have shown that gold nanoparticles exhibited surprising catalytic activity, very different from the bulk, making them a privileged candidate to replace or be added to platinum. Gold nanoparticles prepared with a twofold technique (sputtering followed by a heat treatment in air) have been characterized with both outer and inner-sphere redox reactions. It has been observed that the resulting electrochemical behavior of such type of composite electrode was similar to a gold microelectrodes array, due to the high difference between the two material electrochemical rate constants. In a morphological point of view, the processes of nucleation and growth engaged in the stabilized and well-dispersed gold nanoparticles formation have especially attracted a great attention. At BDD surface, the gold nanoparticles were located at specific sites (grain boundaries, defects, impurities,…). This phenomenon has motivated further investigations at HOPG substrate, known to possess a quite smooth and perfect surface. In the second part of this thesis, induced stabilized nanoparticles at HOPG surface were studied. In a first time, HOPG was morphologically and electrochemically characterized. The surface state has appeared to be a key parameter, essentially with the presence of the two surface species, the edge plane and the basal plane. High modifications in the electrochemical behavior of HOPG electrode were observed after an anodic pretreatment in acidic media, due to surface concentration increase of the edge plane species, which are highly reactive. Based on these results a tailoring of the HOPG has been performed in order to create nanocontainers for a further gold deposition. HOPG surface has been bombarded by gold clusters and then oxidized (etching step) under well-defined experimental conditions, leading in a tailored pattern of nanopits with high reliability and reproducibility. Gold deposition was performed by evaporation. The gold nanoparticles were located in the created nanocontainers and were found to be stable under cyclic voltammetric measurements in acidic media. A new synthesis method for a bimetallic or alloy system formed by Au and Pt nanoparticles stabilized at tailored HOPG surface was developed. Pt electrodeposited at Au/HOPG electrode surface have been observed to preferentially deposit on Au. In fact, gold nanoparticles were used as nucleation sites for the platinum particles. The resulting Pt-Au/HOPG electrodes were then heated at 400 °C in vacuum conditions to improve interactions between both metals. The appearance of an alloy was observed and characterized by a negative shift in the peak potential of Pt oxides reduction. Thanks to these results, Pt-Au/HOPG composite electrodes were applied for electrocatalytic oxidation. At a surface composition of P55-Au45 alloy, Pt-Au/HOPG after the heat treatment has demonstrated a higher electrocatalytic activity than before the heat treatment towards formic acid electrooxidation. It seemed that electronic and geometrical effect was responsible of such enhancement. Finally, bismuth adsorption at both P55-Au45/HOPG and polycrystalline platinum was investigated towards formic acid electrooxidation. The effect of Bi adatoms has been essentially ascribed to geometrical restrictions for CO to adsorb at platinum active sites. Drastic enhancement in current density at a fixed potential and dramatic decrease of the onset potential of formic acid oxidation were observed. Thus, it seemed that avoiding the CO poisoning can be the most judicious development for new electrocatalysts in fuel cell applications.

EPFL2009

Selective solar absorber coatings on receiver tubes for CSP - energy-efficient production process by sol-gel dip-coating and subsequent induction heating

Yannik Antonetti, Thomas Gascou, Aïcha Hessler-Wyser, Martin Joly, Martin Python, Jean-Louis Scartezzini, Andreas Schueler

The energy efficiency of production processes for components of solar energy systems is an important issue. Other factors, which are important for the production of black selective solar coatings include production speed, cycle time and homogeneity of the coating, as well as the minimization of the quantity of the needed chemical precursors. In this paper a new energy-efficient low-cost production process is presented for production of optically selective coatings for solar thermal absorbers. The used method to produce such coatings is sol-gel dip-coating for which all the solutions have been synthetized at the Solar Energy and Building Physics Laboratory of EPFL. The used precursors are tetraethyl orthosilicate, manganese acetate tetrahydrate, copper chloride dihydrate and cobalt chloride hexahydrate. Solutions were obtained by dissolving these precursors in a solution based on a mixture of absolute ethanol, nitric acid and demineralized water. The layers deposited on sheetlike substrates were annealed in a benchtop furnace. For 2 meter long austenitic stainless steel tubes, a novel, fast and energy-efficient process based on induction heating was developed for the thermal annealing. An induction coil passes along the tube. An alternating current flowing in the coil induces a current in the tube. By resistive heating, the temperature of both the metallic tube wall and the deposited film increases. The homogeneity of the temperature distribution of the tube after a single passage of the coil was monitored by infrared imaging. The optical and morphological properties of the Cu-Co-Mn-Si-0 based triple layer have been characterized by spectrophotometry, transmission electron microscopy (TEM), time-of-flight secondary ion mass spectroscopy (ToFSIMS) and X-ray photoelectron spectroscopy (XPS). After optimization of the multilayer design, a solar absorptance of 0.95 and a thermal emissivity of 0.12 at 100 C have been achieved. The intermediate Cu-Co-Mn-Si-0 layer was analyzed by transmission electron microscopy. The likewise obtained images show an agglomeration of crystalline grains with 5-20 nm in diameter. Therefore, we can consider that the Cu-Co-Mn-Si-0 phase is nanocrystalline. In order to roughly estimate the corrosion resistance of the coating in an acidic environment, a simple corrosion test in harsh conditions confirmed the durability of the novel sol-gel coating. Moreover, the excellent stability at elevated temperatures in ambient air makes the coating an interesting candidate for solar applications involving concentrated solar radiation, such as the generation of solar electricity (concentrated solar power), industrial process heating and solar cooling. For that reason, prototype coatings consisting of stacks of three individual layers were deposited on 2 meter long stainless steel tubes. (C) 2014 Published by Elsevier Ltd.

Elsevier Science Bv2014

Multi-objective optimization of a sugarcane biorefinery for integrated ethanol and methanol production

Priscilla Caliandro, François Maréchal, Sandro Onorati, Emanuela Peduzzi, Adriano Viana Ensinas

The present study evaluates a sugarcane biorefinery producing ethanol through juice fermentation and methanol via gasification of sugarcane lignocellulosic residues and liquid fuel synthesis. Two technologies of gasification named entrained flow and circulating fluidized bed are compared. Flowsheet modeling and thermo-economic analysis methods are applied, followed by a multi-objective optimization based on a genetic algorithm. The optimum Ethanol Methanol biorefinery design options are compared with other previously studied sugarcane biorefineries. The results show that the biorefinery's energy efficiency increases significantly with the integration of a methanol production plant in a conventional ethanol distillery. The configuration using an entrained flow gasifier presents lower conversion efficiency than the one using a circulating fluidized bed gasifier. However, for the entrained flow gasifier configuration, the size of the methanol production process could be bigger since more heat is available for the ethanol process favouring the integration with the ethanol plant. Higher energy efficiency due to increase of methanol production leads to a higher total investment for both gasification technologies. The cost analysis shows that the calculated methanol production cost is 30% higher than its current market price. Environmental incentives for biofuels could change this scenario but are not in the scope of this study. (c) 2015 Elsevier Ltd. All rights reserved.