Philosophiae Doctor - PhD / In this study, the OMCVD method is demonstrated as a powerful, fast, economic and environmental friendly method to produce a set of PGMelectrocatalysts with different supports, metal content and metal alloys in one step and without the multiple processing stages of impregnation, washing, drying, calcinationsand activation. / South Africa

To achieve sustainable production of H2 fuel through water splitting, low-cost electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are required to replace Pt and IrO2 catalysts. Here, for the first time, we present the interface engineering of novel MoS2/Ni3S2 heterostructures, in which abundant interfaces are formed. For OER, such MoS2/Ni3S2 heterostructures show an extremely low overpotential of ~218 mV at 10 mA cm-2, which is superior to that of the state-of-the-art OER electrocatalysts. Using MoS2/Ni3S2 heterostructures as bifunctional electrocatalysts, an alkali electrolyser delivers a current density of 10 mA cm-2 at a very low cell voltage of ~1.56 V. In combination with density function theory (DFT) calculations, this study demonstrates that the constructed interfaces synergistically favor the chemisorption of hydrogen and oxygencontaining intermediates, thus accelerating the overall electrochemical water splitting.

No description available.

>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.

No description available.

No / Here we report a hierarchical composite structure composed of few-layers molybdenum disulfide nanosheets supported by vertical graphene on conductive carbon cloth (MDNS/VG/CC) for high-performance electrochemical hydrogen evolution reaction (HER). In the fabrication, 3D vertical graphene is first prepared on carbon cloth by a micro-wave plasma enhanced chemical vapor deposition (MPCVD) and then few-layers MoS2 nanosheets are in-situ synthesized on the surface of the vertical graphene through a simple hydrothermal reaction. This integrated catalyst exhibits an excellent HER electrocatalytic activity including an onset potential of 50 mV, an overpotential at 10 mA cm(-2) (eta(10)) of 78 mV, a Tafel slop of 53 mV dec(-1), and an excellent cycling stability in acid solution. The excellent catalytic performance can be ascribed to the abundant active edges provided by the vertical MoS2 nanosheets, as well as the effective electron transport route provided by the graphene arrays on the conductive substrate. Moreover, the vertical graphene offers robust anchor sites for MoS2 nanosheets and appropriate intervals for electrolyte infiltration. This not only benefits hydrogen convection and release but also avoids the damaging or restacking of catalyst in electrochemical processes. / This work was financially supported by the National Natural Science Foundation of China (Grant nos. 61176007, 51372213, and 51402343).

A tecnologia de células a combustível associada à crescente exigência de baixo impacto ambiental tornou-se bastante promissora no cenário mundial de energia. As células a combustível são, em princípio, dispositivos que convertem energia química diretamente em energia elétrica e térmica, possuindo, entretanto, uma operação contínua, graças à alimentação constante de um combustível. Particularmente, o negro de fumo Vulcan XC72 é usualmente empregado como suporte dos eletrocatalisadores, e alguns fatores como uma superfície acessível e área superficial suficientemente grande para uma máxima dispersão dos cristalitos dos eletrocatalisadores, além de tamanho dos poros, distribuição dos poros adequada e a presença de grupos funcionais na superfície do negro de fumo são considerados fundamentais para o desenvolvimento de materiais inovadores. Entretanto, o material denominado Vulcan XC72 ainda revela condições insuficientes para este fim. Este estudo consiste na preparação e caracterização físico-química de carbono funcionalizado por peróxido de hidrogênio e com cadeias poliméricas do tipo poliestireno sulfonado condutoras de prótons, visando sua posterior utilização como suporte de eletrocatalisadores para células a combustível tipo PEMFC e DMFC. Após a funcionalização do carbono, obteve-se uma melhora da dispersibilidade do negro de fumo em solução aquosa, efeito este benéfico para a preparação dos eletrocatalisadores. Observou-se também que os grupos funcionais e as cadeias poliméricas funcionaram como estabilizadores do crescimento dos cristalitos produzindo catalisadores mais homogêneos e com menor diâmetro médio dos cristalitos; e especialmente, no caso da funcionalização com cadeias poliméricas, obteve-se uma diminuição da queda ôhmica do sistema, referente à melhoria da transferência protônica. / The fuel cell technology associated with the growing exigency of low environmental impact energy became prosperous in the world energy scenery. The fuel cell is basically a device that converts directly the chemical energy of a fuel into electrical and thermal energy with a continuous operation by the constant feed of a fuel. Especially, the carbon black Vulcan XC72 is usually employed as an electrocatalyst support, and some factors as an accessible and high surface area in order to get maximum particles dispersion, pore size, adequate pore distribution and the presence of functional groups in the carbon black surface are considered fundamental characteristics for an innovative materials development. However, the Vulcan XC72 still reveals insufficient conditions for these purposes. This study consists in the preparation and in the physical chemical characterization of functionalized carbon black by hydrogen peroxide and by polymeric chains with proton conduction properties, and its posterior utilization as electrocatalyst support for PEMFC and DMFC application. After the carbon functionalization, an improvement in the carbon black dispersibility in water media was observed, a beneficial effect for electrocatalyst preparation. It was also observed, that the functional groups and the polymeric chains worked as stabilizers in the particle growing, producing much more homogeneous electrocatalysts, exhibiting smaller average particle size. Especially, in the case of polymeric chains functionalization, a decrease in the ohmic drop was observed for this system, attributed to an improvement in the proton transference.

La thèse présentée traite principalement de l'influence de la composition et de l’état de surface d’électrodes à base de Ni sur la cinétique et le mécanisme des réactions d'oxydation/dégagement de l'hydrogène (HOR/HER) en milieu alcalin. En combinant les résultats de mesures électrochimiques avec une modélisation microcinétique, il a pu être montré que l'activité spécifique du Ni pour l’HOR/HER augmente jusqu'à 10 fois en présence à la fois d’oxydes de Ni et de Ni métallique à la surface de l'électrode. En outre, l'influence de l'addition d’un second métal aux électrocatalyseurs à base Ni sur leurs activités pour l’HOR/HER a été étudiée dans le cas des systèmes NiMo/C et NiCu/C. Dans les deux cas, une augmentation de l'activité spécifique a été observée par rapport à l'échantillon Ni/C de référence et a été attribuée à une diminution de l'énergie d'adsorption de l'hydrogène adsorbé sur Ni, espèce intermédiaire de l’HOR/HER. / The present thesis is mainly focused on the influence of the surface state of Ni electrodes on the kinetics and the mechanism of the hydrogen oxidation/evolution reactions (HOR/HER) in alkaline medium. By combining the results of electrochemical measurements with microkinetic modeling, it was shown that specific activity of Ni in the HOR/HER increases up to 10 times if along with metallic Ni, Ni oxide species are present on the electrode surface. In addition, the effect of the addition of a second metal to Ni electrocatalysts on their activity in the HOR/HER was investigated for NiMo/C and NiCu/C systems. In both cases an enhancement of specific activity was observed in comparison with the reference Ni/C sample, which was assigned to a decrease of the adsorption energy of the hydrogen intermediate on Ni participating in the HOR/HER.

Στην παρούσα διδακτορική διατριβή έγινε η μελέτη της φύσης των επιφανειακών ειδών άνθρακα που δημιουργούνται στην επιφάνεια κεραμομεταλλικών υλικών βασισμένα στο Ni-YSZ κατά την θερμοδυναμική ισορροπία της διασπαστικής ρόφησης του μεθανίου. Κύριος στόχος της μελέτης ήταν να διερευνηθεί η επίδραση της παρουσίας ενός δεύτερου μετάλλου στο Ni-YSZ, όσον αφορά τον σχηματισμό και/ή την αναστολή του ανενεργού άνθρακα. Η επικάλυψη της επιφάνειας του Ni με μικρή ποσότητα Au (≤1%at σε σχέση με το Ni) επηρέασε σημαντικά την κινητική της διασπαστικής ρόφησης του CH4 και την ενέργεια δεσμού των ροφημένων ειδών CHx στην επιφάνεια του Ni. Ο σχηματισμός γραφιτικού άνθρακα παρεμποδίστηκε, ενώ η επιφανειακή δραστικότητα για την υδρογόνωση των CHx προς CH4 ήταν χαμηλότερη από ότι στον Ni-YSZ. Αυτό έχει ως αποτέλεσμα την υψηλότερη σταθερότητα και την επιμήκυνση του χρόνου ζωής των CHx πάνω στην επιφάνεια του NiAu/YSZ πριν την διάσπαση τους προς επιφανειακά καρβιδικά είδη. Με βάση τις πειραματικές μετρήσεις, προτάθηκε κινητικό μοντέλο για την περιγραφή της μερικής οξείδωσης του μεθανίου για την παραγωγή αερίου σύνθεσης. Οι αντιδράσεις οξείδωσης περιλαμβάνουν την οξείδωση των ειδών CHx προς CHxO και την επακόλουθη διασπασή τους σε υψηλές θερμοκρασίες (700 Κ) προς CO και H2, ενώ τα καρβιδικά είδη οξειδώνονται εκλεκτικά προς CO2 από χαμηλότερες θερμοκρασίες (500 Κ). Από αυτή την άποψη, η εκλεκτικότητα ως προς το αέριο σύνθεσης εξαρτάται από δύο παράγοντες: (i) τον χρόνο ζωής και την επιφανειακή συγκέντρωση των ειδών CHx, και (ii) τη θερμοκρασία διάσπασης των ειδών CHxO. Ως γενικό συμπέρασμα, ο ηλεκτροκαταλύτης Ni(1%at Au)/YSZ αναμένεται να έχει υψηλή αντοχή στην εναπόθεση άνθρακα όπως και εκλεκτικότητα για την μερική οξείδωση του CH4 προς αέριο σύνθεσης. / In order to investigate the possibility of the direct introduction of CH4 into a SOFC, the dissociative adsorption of CH4 and the nature of the various carbon adspecies on Ni-based cermets were studied in the present thesis by means of thermodynamic equilibrium measurements and temperature programming methods. The aim of this study was mainly to elucidate the effect of a second metal addition on Ni-YSZ, regarding the formation and/or inhibition of inactive carbon. The decoration of the Ni surface with very small quantity of Au (≤1at% with respect to Ni) affected significantly both the kinetics of CH4 dissociative adsorption and the binding strength of the adsorbed CHx species on the Ni surface. The formation of graphitic carbon was significantly inhibited, while the surface reactivity towards the hydrogenation of CHx species into CH4 was lower than on the unmodified Ni/YSZ surface. This resulted in the higher stability and elongation of the CHx species life time on the NiAu/YSZ surface prior to their decomposition into surface carbidic species. Based on the experimental results and discussion, a kinetic model is proposed for the description of the partial oxidation of methane towards the production of synthesis gas. The oxidation reactions involve both the oxidation of CHx species into CHxO and its subsequent decomposition at elevated temperatures (700 K) into CO and H2, while carbidic species are selectively oxidized into CO2 at temperatures as low as 500 K. In this respect, the selectivity towards synthesis gas depends on two factors: (i) the lifetime and surface concentration of CHx species, and (ii) the decomposition temperature of the CHxO species. As a conclusion, the Ni(1%at Au with respect to Ni)/YSZ catalyst is expected to be highly carbon tolerant and selective catalyst/electrode for the CH4 partial oxidation reaction for the production of synthesis gas.

Made available in DSpace on 2014-10-09T12:26:36Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:06:22Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP