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Nanostructured functional metal oxide-based catalysts for direct ethanol fuel cells.

Wang, Youling (2017). Nanostructured functional metal oxide-based catalysts for direct ethanol fuel cells. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 202 p.

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Résumé

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Rising energy demands, depletion of fossil fuel reserves and environmental deteriorations, have spurred great interest in searching for energy conversion devices with high efficiency and low greenhouse gas emissions. Fuel cells powered by hydrogen (H2) or H-rich fuels (such as methanol, ethanol, ethylene glycol, etc.) have been regarded as promising alternative energy conversion devices to ease our concerns about fossil energy and the environment. Among various available fuels, ethanol becomes a more attractive fuel compared with H2 and other organic fuels. At room temperature, ethanol is in the liquid state and, unlike hydrogen, can be easily stored and transported using the current gasoline infrastructure with only slight modifications. Furthermore, it has more advantages than other liquid fuels including low toxicity, high energy density (8.0 kWh/kg), biocompatibility and wide availability from renewable resources like from sugar cane, wheat, corn, or even straw and algae. Despite these advantages, the use of ethanol in fuel cells and ultimately realizing the commercialization of direct ethanol fuel cells (DEFCs) are still being hampered by several challenges, especially the difficulties from the development of anode catalysts which are listed as follows: (i) high cost of electrocatalysts. Platinum (Pt) is the most commonly used catalysts for ethanol oxidation, but as Pt is scarce and expensive, the high loading of Pt in electrodes becomes a critical obstacle limiting the successful commercialization of DEFCs; (ii) sluggish electrochemical kinetics; (iii) low poisoning resistance to reaction intermediates (like COads); and (iv) difficulties associated with breaking the C-C bond in order to achieve the complete ethanol oxidation to CO2. Therefore, more active, selective and durable electrocatalysts have to be developed, and the preparation of multi-component (binary, ternary, or even quaternary) catalysts has been regarded as a promising strategy to meet the demands of the complicated process of ethanol oxidation. So far, a great deal of effort has been centered on modifying platinum by adding transition metals (M: Ru, Sn, Mo, Pd, Ir, etc.) to form Pt-M binary catalysts. It was found that the added metal can definitely enhance Pt activity for alcohol oxidation by the bifunctional mechanism or electronic effect between Pt and foreign metals. Alternatively, the combination of nanostructured Pt with functional metal oxides (FMO: such as CeO2, SnO2, TiO2) has recently originated a distinctive class of electrocatalysts for DEFCs applications. The interaction between Pt and metal oxides was proved to have a critical influence on the chemisorption properties and ultimate catalytic behavior. Indeed, FMOs’ role has been claimed to improve Pt nanoparticles dispersion, and/or to supply hydroxyl species at lower potentials than Pt to accomplish the oxidative removal of COads (bi-functional effect), and/or to alter the electronic structure of Pt and thus lessens the adsorption strength of COads on Pt (the electronic effect). In this project, the work centers on developing advanced binderless, nanoarchitectured catalysts layers composed of the substrate (carbon paper, CP), the catalyst support (CNTs), Pt and FMO for DEFC applications. Two dissimilar materials (Pt and FMO) could be directly deposited onto the substrate by means of pulsed laser deposition. Four different FMOs (CeO2, SnO2, MnO2, and TiO2) have been selected and incorporated respectively in our binderless catalyst layer. It is well known that interactions of Pt with FMO can radically improve the catalytic performance of the Pt-FMO composites. In order to gain insight of the role of FMO in the catalytic behavior of Pt-based composites, FMO was incorporated into catalyst layer in two manners, namely layer-by-layer architecture and co-deposition architecture. The first architecture is fabricated by depositing Pt layer onto a layer of FMO. The second architecture is the co-deposition structure of Pt and FMO obtained by ablating both targets of Pt and FMO simultaneously. Furthermore, the optimization of each type of architecture was undertaken by varying the background gas pressure in the deposition chamber. Afterward, the prepared electrodes were firstly characterized by various physicochemical techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and micro-Raman spectroscopy. The electrocatalytic performance of synthesized electrodes towards ethanol oxidation was investigated by using cyclic voltammetry (CV), linear scanning voltammetry (LSV) and chronoamperometry (CA). Based on the results obtained, the correlation between physiochemical characterization and the electrochemical performance of corresponding Pt-FMO catalysts have been discussed and established. Specifically, as for the LOL structured catalyst layer, the porosity of the top Pt layer is varied with the morphologies of the underlying FMO layer which could be tuned by varying the gas background pressure. As a result, the catalytic performance of FMO/Pt towards EOR is affected due to the variation of the microstructure of FMO layer. Similarly, the gas pressure plays an important role on the physic-chemical properties of the co-deposits of Pt and FMO. The interactions between Pt and FMO have been found to be varied with gas pressure during deposition, and thus leading to different catalytic behaviors in the electrooxidation reaction for ethanol. Comparing the FMO-based catalysts of two different architectures, co-deposited catalysts exhibited superior EOR performance with respect to catalysts with LOL structure. This is mainly related to the higher interaction degree of Pt-FMO in co-deposited structure compared with LOL structure. Based on the results of binary Pt-FMO catalysts, ternary catalysts composed of Pt, CeO2 and SnO2 were prepared in an attempt to further enhance the activity and durability with respect to that of binary catalysts. The catalytic activity of our optimized ternary catalysts under low potentials is enhanced compared with optimized Pt-CeO2 co-deposits but is still inferior to Pt-SnO2 co-deposits. Further optimization work needs to be carried out. Furthermore, cathode catalysts for oxygen reduction reactions composed of Pt and FMO catalysts have been widely investigated, owing to the outstanding proton conductivity and the promoting effect of FMO demonstrated by the spillover phenomena of FMO to Pt. As is well known, synthesis method of catalysts plays a significant role in the catalytic properties of the resulting catalysts. Therefore, it would be an interesting study to examine the PLD-synthesized Pt-FMO catalysts for oxygen reduction reaction. In this doctoral project, it was found that the TiO2 films synthesized under different gas atmosphere dramatically increased the electroactive surface area of Pt and enhanced its electroactivity towards oxygen reduction reaction as compared with bare Pt electrode. Unfortunately, I could not complete studies on all synthesized FMO-based catalysts towards ORR due to the limitation of my Ph.D. period. However, the initial work presented in this thesis could intrigue further studies on PLD synthesized FMO-based catalysts towards ORR.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Mohamedi, Mohamed
Informations complémentaires: Résumé avec symboles
Mots-clés libres: Fuel cells; anode catalysts; Direct Ethanol Fuel Cells; DEFC; ethanol oxidation; eletrochemistry
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 27 nov. 2017 21:51
Dernière modification: 27 nov. 2017 21:51
URI: http://espace.inrs.ca/id/eprint/6521

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