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Electrocatalytic Activity of Small Organic Molecules at PtAu Alloy Nanoparticles for Fuel Cells and Electrochemical Biosensing Applications.

Oko, Daniel Nii (2014). Electrocatalytic Activity of Small Organic Molecules at PtAu Alloy Nanoparticles for Fuel Cells and Electrochemical Biosensing Applications. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 205 p.

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Nano science and technology have received much attention recently due to their exciting applications in the fields of clean energy technology and biomedical sensing devices. Also what makes nano science and technology more relevant in this 21st century is the increasing environmental concern and accelerated depletion of fossil fuels. This realization has spark the renewed research interests in the development of new technology using alternative energy sources, most noticeably those fuels that can be utilized by proton exchange membrane fuel cells (PEMFCs). A broad spectrum of nano materials of distinct sizes, morphologies and compositions are now available for application in clean energy and biosensing technologies. Platinum (Pt), gold (Au) and PtAu alloy nanoparticles (NPs), were applied in this work for designing cost effective novel electrocatalysts for PEMFCs and biosensing systems. These nanoparticles did not only improve the electrocatalysis of small organic molecules for fuel cell application (Methanol Fuel cells and Direct Formic acid fuel cells) but also showed enhanced performance towards the oxidation of bio-analytical molecules (ascorbic acid and dopamine) with promising values of sensitivity and selectivity. The goal of the research is to provide deeper insight into the mechanisms and structure-sensitivity of these oxidation reactions on these metal nanoparticles electrodes. . The specific objective of the thesis work was to study the oxidation of formic acid, methanol, dopamine and ascorbic acid on platinum-gold nanoparticles by employing conventional electrochemical methods. The platinum-gold alloy nanoparticles of varying composition. (PtxAu100-x alloy NPs) were supplied by our collaborators and were synthesized by pulsed laser ablation in aqueous medium via a single metal target prepared from mixtures of pure platinum and pure gold nanoparticles. Cyclic voltammetry and chronoamperometry studies of formic acid and methanol oxidation on platinum-gold alloy nanoparticles in sulphuric acid electrolyte solution were performed to determine the influence of the nanoparticles composition on the mechanism and kinetics of the oxidation reactions. As a result of the low catalytic activity of the nanoparticles towards methanol oxidation in acidic medium, we also investigated the methanol oxidation in alkaline medium. In the case of ascorbic acid and dopamine oxidation, in addition to the voltammetric studies, differential pulse voltammetry was also employed to probe the current response as an evolution of the analytes concentration changes in phosphate buffer solution (PBS) in a neutral medium. During the electrocatalytic performances of the PtAu alloy NPs towards the formic acid oxidation, the formic acid oxidation followed a dehydrogenation pathway on these as-prepared PtAu alloy NPs. However, the dehydration was the privileged mechanism on as-prepared mixtures of pure Pt and pure Au NPs. After electrochemical aging in 0.5 M sulphuric acid, the dehydrogenation pathway was the privileged reaction pathway on both PtAu alloy and Pt+Au mixture NPs. This was attributed to the structure modification of Pt and Au nanoparticles through electrochemical aging which makes the Pt+Au mixture to behavior as an alloy. It was suggested that metal dissolution and metal redeposition occur during the electrochemical aging process resulting in possible atomistic re-organization on the catalyst surface. Also, after 600 s of potentiostatic polarization, the mass activities of PtAu alloy NPs displayed a factor of 2 times larger than that of the Pt+Au mixtures with the same surface composition. Nevertheless, both types of catalysts display similar activity with respect to the total electrochemically active surface area. During methanol oxidation in alkaline environment, the bifunctional character of PtAu alloys emerged quite clearly with 50-50 bimetallic composition: the ability of Au surface atoms to provide active sites for oxygen-containing species such as OHads at low electrode potentials facilitated the oxidation of CO migrating to these sites from either Pt or Au sites. In addition, it was found that the intrinsic activity and mass activity of Pt in the PtAu NPs towards oxidation reaction of methanol reaction was 8 times and 5 times larger for Pt30Au70 NPs (Au-rich alloy) with respect to Pt NPs and Pt70Au30 NPs (Pt-rich alloy). We suggested that this enhancement was due to an additional electronic effect on the adsorption strength of CO and CO-like species on Pt-Au pair sites as against the Au sites, therefore shifting the equilibrium position of the methanol oxidation reaction (MOR) to more negative omset potential. The strong synergistic effect PtAu alloy electrode toward the electrooxidation of MOR in alkaline medium was not observed for the same composition of PtAu alloy electrodes in acidic environment. Instead, a continuous decrease in activity with increasing Au content in PtAu NPs was found in acid medium. The electrocatalytic behavior of the PtAu alloy NPs electrodes towards MOR in acidic medium were merely due to ensemble effect of neighboring atoms. This thesis also describes results of an investigation of the electrocatalytic oxidation of dopamine (DA) and ascorbic acid (AA) on AuxPt100-x alloy nanoparticles catalysts prepared by pulsed laser ablation in water. The electrochemical sensing and properties of AuPt NPs was found to be dependent on the bimetallic compositions of the NPs. Electrochemical studies have shown that the catalytic oxidation of DA and AA on AuPt NPs electrodes can afford a peak potential separation of 156 mV and 190 mV on Au and Au50Pt50 alloy NPs respectively which was large enough for the determination of DA in AA/DA mixed solution. The Au-rich alloy electrode exhibited superior electrocatalytic activity towards AA oxidation due to synergic effect between alloy Au and Pt in AuPt alloy shifting the potential to the more negative direction as compared to Au NPs. Further, while the Au and Au50Pt50 NPs favored sensing of dopamine, the Au70Pt30 NPs favored AA oxidation. The DA and AA oxidation current increases linearly with DA and AA concentration respectively for all selected electrodes. Prior to the electro-oxidation of these small organic molecules (formic acid, methanol oxidation, ascorbic acid and dopamine) on the AuPt-based alloy nanoparticles, electrochemical cycling was performed in acid, neutral and alkaline medium based on the type of molecules studied. The effect of alloying on the peak positions of the metal oxide reduction of the nanoparticles in these media was correlated. While the Pt-oxide reduction peak shifted negatively in all media with increasing Au content in the bimetallic AuPt NPs, the Au-oxide reduction peak remain fairly constant in all media with Au content in the AuPt bimetallic NPs. In general, the application of these nanoparticles could bring many advantages for the fuel cell and sensing systems investigated in this work.

Type de document: Thèse
Directeur de mémoire/thèse: Tavares, Ana
Co-directeurs de mémoire/thèse: Guay, Daniel
Informations complémentaires: Résumé avec symboles
Mots-clés libres: piles à combustible de méthanol direct; piles à combustible d'acide formique direct; biocapteurs électrochimiques; méthode voltampérométrie cyclique; méthode chronoampérométrie
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 17 mars 2015 20:31
Dernière modification: 18 mars 2016 14:04
URI: http://espace.inrs.ca/id/eprint/2626

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