The goal of this research project was to characterize the physical and electrochemical properties of a commercially available fuel cell-based breath alcohol sensor. Characteristics of the existing sensor were compared with state of the art power generating fuel cells with the goal of understanding the factors that limit performance, lifetime and cost effectiveness of the sensors. This will guide the development of the next generation of breath alcohol sensors. The average lifetime of the current sensor falls short of the industry standards. In particular, sensors operating in dry conditions experience more rapid loss of sensitivity and failure. Two primary causes of degradation were investigated in this study. Loss of proton conductivity as a result of membrane dehydration was shown to be reversible by rehydrating the membrane in humid conditions. Loss of electrochemically active surface area of Pt is irreversible and seems to be caused by a change in sensor morphology after long-term exposure to dry conditions. / UOIT

Σε αυτή την μελέτη παρασκευάστηκαν ηλεκτρόδια Pt-RuO2-TiO2 και χαρακτηρίστηκαν με περίθλαση ακτίνων Χ (X-ray diffraction - XRD), φασματοσκοπία φωτοηλεκτρονίων από ακτίνες Χ (X-ray photoelectron spectroscopy - XPS), ηλεκτροχημικές τεχνικές και πειράματα ρόφησης-οξείδωσης μονοξειδίου του άνθρακα (CO stripping). Ερευνήθηκε η μείωση της περιεκτικότητας σε Pt και RuO2 χωρίς απώλειες της ηλεκτροκαταλυτικής ενεργότητας. Το TiO2 επιλέχθηκε λόγω της χημικής του σταθερότητας και του χαμηλού κόστους. Βρέθηκε ότι περιεκτικότητα σε TiO2 μέχρι 50% οδηγεί σε αύξηση της ηλεκτροχημικά ενεργής επιφάνειας (EAS) του ηλεκτροδίου. Η ηλεκτροχημικά ενεργή επιφάνεια (EAS) του ηλεκτροδίου Pt(25%)-RuO2(25%)-TiO2(50%) ήταν μεγαλύτερη του ηλεκτροδίου Pt(50%)-RuO2(50%), ενώ για περιεκτικότητα σε TiO2 μεγαλύτερη από 65% η EAS μειώνεται δραματικά. Το παραπάνω συμπέρασμα στηρίχθηκε σε μετρήσεις του φορτίου της αναγωγικής κορυφής των κυκλικών βολταμογραφημάτων και σε πειράματα ρόφησης-οξείδωσης του CO (CO stripping). Όλα τα δείγματα χρησιμοποιήθηκαν και ως άνοδοι κατά τη διάρκεια ηλεκτροχημικής οξείδωσης μεθανόλης, αιθανόλης και γλυκερόλης. Και στις τρεις περιπτώσεις το ηλεκτρόδιο Pt(25%)-RuO2(25%)-TiO2(50%) παρουσίασε τη μεγαλύτερη ηλεκτροκαταλυτική ενεργότητα. Η παρατηρούμενη αυξημένη απόδοση των ηλεκτροδίων που παρασκευάστηκαν αποδόθηκε στην αυξημένη διασπορά της Pt και του RuO2, στο σχηματισμό μικρότερων κρυσταλλιτών Pt και RuO2 με την πρόσθεση TiO2, καθώς επίσης και σε ηλεκτρονιακές αλληλεπιδράσεις μεταξύ των μετάλλων και του TiO2. / In this study Pt-RuO2-TiO2 electrodes were prepared and characterised by X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), electrochemical techniques and CO stripping. The minimization of the Pt and RuO2 loading without electrocatalytic activity losses was explored. TiO2 was chosen due to its chemical stability and low cost. It was found that TiO2 loading up to 50% resulted in an increase of the Electrochemically Active Surface (EAS). The EAS of Pt(25%)-RuO2(25%)-TiO2(50%) was higher than that of Pt(50%)-RuO2(50%) while for TiO2 loadings higher than 50% the EAS diminished. The above conclusion has been confirmed by following the charge of the platinum reduction peak on cyclic voltammograms and by CO stripping experiments. All samples were used as anodes during electrochemical oxidation of methanol, ethanol and glycerol. In all cases the Pt(25%)-RuO2(25%)-TiO2(50%) electrode exhibited better electrocatalytic activity than the Pt(50%)-RuO2(50%) anode. The observed higher performance of this electrode has been attributed to the enhanced dispersion of Pt and RuO2 particles, the formation of smaller crystallites of Pt and RuO2 by the addition of TiO2 and the electronic interactions between metals and TiO2.

>Magister Scientiae - MSc / This study explores the use of UV-assisted reduction method to synthesise the catalysts, aiming at reducing synthesis time. The Pd and Au catalyst loading is kept at 5 wt% in order to reduce the cost associated with high loading (20 wt%) of platinum group metals. The synthesised catalysts have SnO2 incorporated in them for two purposes, one being to activate the chemical reaction by absorbing UV-light and the second one is to serve as a promoter for binary and ternary catalysts. All the synthesised electrocatalysts in this study were denoted as Au/10wt%SnO2-C, Au/15wt%SnO2-C, Au/20wt%SnO2-C, Au/40wt%SnO2-C, Au/60wt%SnO2-C, Pd/10wt%SnO2-C, Pd/15wt%SnO2-C, Pd/20wt%SnO2-C, Pd/40wt%SnO2-C, Pd/60wt%SnO2-C and PdAu/10wt%SnO2-C respectively. The UV-assisted reduction method was proved to be effective with the obtained results from TEM, SEM, XRD and electrochemical studies. TEM micrographs revealed nanoparticles of Pd, Au and SnO2 which were proved by the measured d-spacing values corresponding to the element’s structures. The measured average particle size ranged from 3.05 to 14.97 nm for the electrocatalysts. The XRD profiles confirmed the face centred cubic of Pd, Au and tetragonal structures of SnO2. These electrocatalysts showed varied activity towards the oxidation of alcohols namely, methanol, ethanol, ethylene glycol and glycerol in alkaline electrolyte The cyclic voltammetry results showed improved performance towards the oxidation of glycerol on Au-based electrocatalysts, highest current density of 22.08 mA cm-2 than on Pd-based electrocatalysts. Pd-based electrocatalysts were more active towards the oxidation of ethanol than Au-based electrocatalysts with the highest current density of 19.96 mA cm-2. The co-reduced PdAu on 10wt%SnO2-C electrocatalysts showed the lowest current density of 6.88 mA cm-2 for ethanol oxidation when compared to Pd/10wt%SnO2-C and Au/10wt%SnO2-C. Linear sweep voltammograms showed more negative onset potentials on Pd-based electrocatalysts than Au-based electrocatalysts. The more negative onset potential obtained on Pd-based electrocatalysts was observed for ethanol oxidation. These results correspond to the trend observed in literature for ethanol oxidation being more favoured on Pd-based electrocatalysts whereas the polyalcohol oxidation is more favoured on Au-based electrocatalysts. The best performing and most stable electrocatalyst among the Au-based electrocatalysts is Au/10wt%SnO2-C and Pd/10wt%SnO2-C for the Pd-based electrocatalysts.