Στην παρούσα διατριβή αναπτύχθηκε ένα μοντέλο βασιζόμενο σε πρώτες αρχές με σκοπό την περιγραφή και την πρόβλεψη της πρωτονιακής αγωγιμότητας των πλήρως ενυδατωμένων μεμβρανών Nafion (το κυριότερο υλικό που χρησιμοποιείται ως ηλεκτρολύτης στις κυψέλες καυσίμου τύπου PEM) και των ιδιαίτερων χαρακτηριστικών αυτής, όπως τη γραμμική εξάρτηση από το πάχος της μεμβράνης, το δυναμικό του κελιού και τη μερική πίεση του υδρογόνου. Το μοντέλο εστιάζει στη μετανάστευση των πρωτονίων που είναι συνδεδεμένα στις σουλφονομάδες και χρησιμοποιεί την κατανομή φορτίου Poisson-Boltzmann γύρω από κάθε πρωτόνιο, σε συνδυασμό με την εξίσωση Gamow, που δίνει την πιθανότητα να συμβεί το φαινόμενο σήραγγος σε παραβολικό φράγμα δυναμικού. Προτείνεται ότι το μήκος που διανύεται κατά την πραγματοποίηση του φαινομένου σήραγγος ισούται με το μήκος κύματος του πρωτονίου και ότι κάθε πρωτόνιο περιβάλλεται από το νέφος Debye-Hückel. Το μοντέλο, που δεν εμπεριέχει προσαρμόσιμες παραμέτρους, λύνεται αναλυτικά και οι προβλέψεις του είναι σε ημιποσοτική συμφωνία με το πείραμα. Στις προβλέψεις αυτές συμπεριλαμβάνεται η τάξη μεγέθους της αγωγιμότητας, η γραμμική εξάρτηση της αγωγιμότητας με το πάχος της μεμβράνης, η εκθετική εξάρτηση από το δυναμικό και η ισχυρή εξάρτηση με τη μερική πίεση του υδρογόνου. Μελετήθηκε η κινητική, οι ταλαντώσεις και η ηλεκτροχημική ενίσχυση της οξείδωσης του CO σε καταλυτικό υμένιο Pt εναποτεθειμένο σε YSZ, χρησιμοποιώντας ως βάση την απόλυτη κλίμακα του ηλεκτροδίου του οξυγόνου. Βρέθηκε ότι η ηλεκτροχημική ενίσχυση είναι μικρή (ρ<3, Λ~300) όταν οι τιμές του δυναμικού του καταλύτη, UWR, είναι ανάμεσα σε 0.2 και 0.4 V και πολύ σημαντική (ρ~9, Λ~1500) όταν το UWR υπερβαίνει την τιμή 0.4 V. Η απότομη αλλαγή που παρατηρείται κατά την μετάβαση στην έντονη ηλεκτροχημική ενίσχυση συνοδεύεται και από απότομη αλλαγή στην κινητική της αντίδρασης και στο δυναμικό του καταλύτη. Μέσω της σύγκρισης των πειραματικών αποτελεσμάτων του κεφαλαίου και ανεξάρτητων μετρήσεων του έργου εξόδου μεταβάλλοντας το δυναμικό του καταλύτη για το ίδιο σύστημα, βρέθηκε ότι η μετάβαση οφείλεται στην έντονη μετανάστευση προωθητικών ειδών, Ο2-, από τη YSZ στην επιφάνεια του καταλύτη και την συνεπαγόμενη δημιουργία μιας πυκνής αποτελεσματικής διπλοστιβάδας στη διεπιφάνεια του καταλύτη με τα αέρια αντιδρώντα. Τέλος εξετάστηκε μια τροποποιημένη κυψέλη καυσίμου τύπου PEM η οποία λειτουργεί σε συνθήκες δηλητηρίασης από CO κατά την τριοδική λειτουργία. Στο τριοδικό κελί καυσίμου τύπου PEM εκτός της ανόδου και της καθόδου εισάγεται ένα τρίτο ηλεκτρόδιο το οποίο δημιουργεί ένα επιπλέον βοηθητικό κύκλωμα το οποίο λειτουργεί με ηλεκτρολυτικά ρεύματα και επιτρέπει τη λειτουργία της κυψέλης σε δυναμικά μεταξύ της ανόδου και της καθόδου ανέφικτα κατά τη συμβατική λειτουργία. Βρέθηκε ότι είναι δυνατό να ενισχυθεί η συνολική θερμοδυναμική ενεργειακή απόδοση όταν χρησιμοποιείται ως καύσιμο μίγμα αναμόρφωσης μεθανόλης που αντιστοιχεί σε συνθήκες έντονης δηλητηρίασης από CO. / In the present work a first principles model was developed to describe and predict the protonic conductivity of fully hydrated Nafion membranes and its peculiar non-linear dependence on membrane thickness, potential and PH2. The model focuses on the surface migration of protons between adjacent sulfonate groups and utilizes the Poisson-Boltzmann charge distribution around each proton combined with the basic Gamow equation of quantum mechanics for proton tunneling, for parabolic potential barrier. It was shown that the proton tunneling distance equals the proton wavelength and that each proton surrounded by its Debye-Hückel cloud behaves as a leaking nanobattery. The model, which contains no adjustable parameters, is solved analytically and its predictions are in semiquantitative agreement with experiment, including the magnitude of the conductivity, its linear increase with membrane thickness, its exponential increase with potential and its strong dependence on partial pressure of hydrogen. Moreover it was investigated the kinetics, rate oscillations and electrochemical promotion of CO oxidation on Pt deposited on YSZ using a standard oxygen reference electrode. It was found that electropromotion is small (ρ<3) when the catalyst potential UWR, is between 0.2 and 0.4V and very pronounced (ρ~9, Λ~1500) when UWR exceeds 0.4V. This sharp transition in the electropromotion behavior is accompanied by an abrupt change in reaction kinetics and in catalyst potential. It was shown via comparison with independent catalyst potential–catalyst work function measurements that the transition corresponds to the onset of extensive O2- spillover from YSZ onto the catalyst surface, and concomitant establishment of an effective double layer at the catalyst-gas interface, which is the cause of the highly active electropromoted state Furthermore it was studied a modified PEM fuel cell running in CO poisoning conditions by the triode operation. In addition to the anode and cathode, the triode PEM fuel cell introduces a third electrode together with an auxiliary circuit which is run in the electrolytic mode and permits fuel cell operation under previously inaccessible anode-cathode potential differences. It was found that it is possible to enhance the overall thermodynamic efficiency when it is used a methanol reformate mixture as a fuel, which corresponds to intense CO poisoning conditions.

Porous, conductive metallic nanostructures are required in several fields, such as energy conversion, low-cost sensors etc. This thesis reports on the development of an electrocatalytically active and conductive membrane for use in Polymer Electrolyte Membrane Fuel Cells (PEMFCs) and fabrication of low-cost substrates for Surface Enhanced Raman Spectroscopy (SERS). One of the main challenges facing large-scale deployment of PEMFCs currently is to fabricate a catalyst layer that minimizes platinum loading, maximizes eletrocatalytically active area, and maximizes tolerance to CO in the feed stream. Modeling the kinetics of platinum catalyzed half cell reactions occurring in a PEMFC using the kinetic theory of gases and incorporating appropriate sticking coefficients provides a revealing insight that there is scope for an order of magnitude increase in maximum current density achievable from PEMFCs. To accomplish this, losses due to concentration polarization in gas diffusion layers, which occur at high current densities, need to be eliminated. A novel catalyst design, based on a porous metallic nanostructure, which aims to overcome the limitations of concentration polarization as well as minimize the amount of platinum loading in PEMFCs is proposed. Fabrication steps involving controlled in-plane fusion of self-assembled arrays of core-shell gold-platinum nanoparticles (Au@Pt) is envisioned. The key steps involved being the development of a facile synthesis route to form Au@Pt nanoparticles with tunable platinum shell thicknesses in the 5 nm size range, the formation of large-scale 2D arrays of Au@Pt nanoparticles using guided self-assembly, and optimization of an RF plasma process to promote in-plane fusion of the nanoparticles to form porous, electrocatalytically active and electrically conductive membranes. This thesis consists of seven chapters. The first chapter provides an introduction into the topic of PEMFCs, some perspective on the current status of research and development of PEMFCs, and an outline of the thesis. The second chapter provides an overview on the methods used, characterization techniques employed and protocols followed for sample preparation. The third chapter describes the modelling of a PEMFC using the Kinetic theory of gases to arrive at an estimate of the maximum feasible current density, based on the kinetics of the electrocatalytic reactions. The fourth chapter presents the development of a simple protocol for synthesizing Au@Pt nanoparticles with control over platinum shell thicknesses from the sub monolayer coverage onwards. The results of spectroscopic and microscopic characterization establish the uniformity of coating and the absence of secondary nucleation. Chapter five describes the formation of a nanoporous, electrocatalytically active membrane by self-assembly to form bilayers of 2D arrays of Au@Pt nanoparticles and subsequent fusion using an RF plasma based process. The evolution of the electrocatalytic activity and electrical conductivity as a function of the duration of RF plasma treatment is monitored for Au@Pt nanoparticles with various extent of platinum coating. Spectroscopic, microscopic, electrical and cyclic voltammetry characterization of the samples at various stages were used to understand the structural evolution with RF plasma treatment duration and discussed. Next durability studies were carried out on the nanoporous, Au@Pt bilayer nanoparticle array with an optimum composition of Pt/Au atomic ratio of 0.88 treated to 16 minutes of argon plasma exposure. After this the novel catalyst membrane design of PEM fuel cell is revisited. Two different techniques are proposed so that the thin, nanoporous, metallic catalyst membrane achieves horizontal electronic resistance equivalent to that of the conventional gas diffusion layer with catalyst layer. The first technique proposes the introduction of gold coated polymeric mesh in between the thin, nanoporous, metallic catalyst membrane and bipolar plate and discusses the advantages. Later the gold coated polymeric mesh is introduced in a conventional membrane electrode assembly and efficiency of the polarization curves probed with and without the introduction of gold coated polymeric mesh. The second technique describes the results of fabrication of a nanoporous metallic membrane using multiple layers of 2D Au@Pt nanoparticle arrays at an optimum composition of Pt/Au atomic ratio of 0.88 to reduce the horizontal electronic resistance. Preliminary studies on the permeability of water through such membranes supported on a porous polycarbonate filter membrane are also presented. In chapter six, a simple reactive inkjet printing process for fabricating SERS active silver nanostructures on paper is presented. The process adapts a simple room temperature protocol, using tannic acid as the reducing agent, developed earlier in our group to fabricate porous silver nanostructures on paper using a commercial office inkjet printer. The results of SERS characterization, spectroscopic and microscopic characterizations of the samples and the comparison of the substrate’s long-term performance with respect to a substrate fabricated using sodium borohydride as the reducing agent is discussed. Preliminary findings on attempts to fabricate a conductive silver network using RF plasma induced fusion area also presented. Chapter seven provides a summary of the results, draws conclusions and a perspective on work required to accomplish the goals of incorporating the porous metallic nanostructures into PEMFCs.

Fuel cell systems are becoming more commonplace as a power generation method and are being researched, developed, and explored for commercial use, including portable fuel cells that appear in laptops, phones, and of course, chargers. This thesis examines a model constructed on inspiration from the myFC PowerTrekk, a portable fuel cell charger, using COMSOL Multiphysics, a finite element analysis software. As an educational tool and in the form of zero-dimensional, two-dimensional, and three-dimensional models, an investigation was completed into the geometric construction, air conditions and compositions, and product materials with a best case scenario completed that summarizes the results identified. On the basis of the results of this research, it can be concluded that polyoximetylen and high-density polyethylene were considered as possible materials for the majority of the product, though a more thorough investigation is needed. Air flow of above 10 m/s, air water vapour mass fraction below 50% and initial temperature between 308K and 298K was considered in this best scenario. Suggestions on future expansions to this project are also given in the conclusion.