This thesis concerns the electrochemical investigation of high-potential laccases. These multicopper oxidases are efficient electrocatalysts for the dioxygen reduction reaction. A method for stabilising laccase on a graphite electrode was established. The method involved modification of the graphite surface by diazonium coupling of a 2-anthracene molecule. A laccase ‘film’ adsorbed on this modified surface remained stable for over two months and, typically, the current density for dioxygen reduction was doubled compared to a laccase ‘film’ on an unmodified surface. Protein film voltammetry was used to investigate thermodynamic and kinetic aspects of the electrochemical behaviour of laccase. The effect of inhibitors on the magnitude of reduction current and the position of the wave (related to the overpotential for the reaction) was also studied. Fluoride, chloride and azide showed different modes of inhibition and inhibition constants ranged from micromolar for azide to millimolar for chloride. In cyclic voltammetry experiments it was only in the presence of high concentrations of the inhibitors fluoride, chloride and azide that a non-turnover signal, corresponding to a one electron transfer process, was revealed. The evidence suggested that the non-turnover signal arose from interfacial electron transfer between the electrode and the type 1 or ‘blue’ copper. Evaluation of the peak areas allowed determination of the catalytic rate constant, kcat, as 300 s–1, and the electroactive surface coverage as four pmol cm–2. The rate of interfacial electron transfer was rapid enough to not limit catalysis at high overpotentials. A spectroelectrochemical cell was designed to investigate the behaviour of the type 1 copper in the presence of inhibitors and at different pH values. The inhibitors fluoride, chloride and azide had little effect on the reduction potential of the type 1 copper, but at higher pH values the reduction potential of the type 1 copper was decreased.

A reação de redução de oxigênio (RRO) e a reação de oxidação do borohidreto (ROB) foram estudadas em eletrólito alcalino em eletrodos formados por diversos tipos de óxidos de manganês dopados com Ni (II) dispersos sobre carbonos Monarch 1000, MM225 e E350. As técnicas de caracterização físico-química foram difração de raios X (DRX), microscopia eletrônica de transmissão de alta resolução (HR-TEM) equipado com espectrômetro de energia dispersiva de raios X (EDX). Já os estudos eletroquímicos compreenderam voltametria cíclica, curvas de polarização de estado quase estacionário além das técnicas de Espectroscopia de massas diferencial on line (DEMS) e Infravermelho com transformada de Fourier in situ (FTIR). Foi observada pequena inserção dos átomos de Ni na estrutura dos MnOx. A fase correspondente a NiMnOx/C está presente na forma de aglomerados nanocristalinos ou em forma de agulhas com tamanhos da ordem de 1,5 a 6,7 nm dependendo do tipo de carbono utilizado como substrato. Manganita (MnOOH) apresentou-se como fase preponderante para óxido de manganês disperso sobre carbono Monarch 1000 enquanto que para materiais dispersos sobre carbono MM225 e E350G a fase MnO2 esta presente em maior quantidade. Estudos eletroquímicos em camada fina utilizando eletrodo disco rotatório revelaram melhores atividade para a RRO e estabilidade para MnOx dopados com níquel. A RRO procede segundo o mecanismo peróxido, seguida pela reação de desproporcionação do íon HO2- formado. A extensão da reação de desproporcionação do íon HO2- aumenta com o aumento da quantidade de Ni. Sobre eletrocatalisadores suportados em carbonos MM225 e E350 a reação de desproporcionação é mais rápida e envolve um total de 4 e- por oxigênio molecular. As curvas de polarização para RRO obtidas na presença do íon BH4- mostraram que todos os materiais são tolerantes à presença do borohidreto. Resultados de DEMS on line e FTIR in situ mostraram que óxidos de manganês dopados com Ni além de serem ativos para RRO são também ativos para a ROB, porém há uma grande influência da composição e da morfologia dos materiais uma vez que, quando fases segregadas de Ni estão presentes nas amostras, a reação compete com a hidrólise heterogênea do BH4- levando a uma diminuição da eficiência faradaica. / The oxygen reduction reaction (ORR) and the borohydride oxidation reaction (BOR) were studied in alkaline medium on Ni (II) doped MnOx catalysts supported on different carbon powder substrates. Characterizations of physico chemical properties were made by X ray diffraction (XRD), high resolution transmition electronic microscopy (HR-TEM) equipped with X ray dispersive energy spectroscopy (EDS). Electrochemical studies involved cyclic voltammetry and oxygen reduction voltammograms. Also it was used Differential Electrochemical Mass Spectrometry on line (DEMS) and Fourier Transform Infra Red Spectrometry (FTIR) in situ. A small insertion of Ni atoms in the MnOx lattice was observed, this consisting of a true doping of the manganese oxide phase. The corresponding NiMnOx phase is present in the form of needles or agglomerates, with crystallite sizes in the order of 1.5-6.7 nm. Layered manganite (MnOOH) phase has been detected for the Monarch1000 supported NiMnOx material, while different species of MnOx phases are present at the E350G and MM225 carbons. Electrochemical studies in thin porous coating active layers in the rotating ring-disk electrode setup revealed that the MnOx catalysts present better ORR kinetics and electrochemical stability upon Ni doping. The ORR follows the so-called peroxide mechanism on MnOx/C catalysts, with the occurrence of minority HO2- disproportionation reaction. The HO2- disproportionation reaction progressively increases with the Ni content in NiMnOx materials. The catalysts supported on the MM225 and E350G carbons promote faster disproportionation reaction, thus leading to an overall four-electron ORR pathway. The results towards ORR in presence of sodium borohydride showed that all materials are tolerant to the presence of BH4- ion into some extent. DEMS on line and FTIR in situ showed that NiMnOx/C are also active toward the BOR, but there is a strong influence of the nature of the electrocatalysts with respect to the morphology, composition, the nature of the carbon substrate and the Ni load. Results indicate that the electrocatalysts containing segregate Ni phases, the bohohydride oxidation occurs together with the heterogeneous hydrolysis of the BH4- ion resulting in a decrease of the faradaic efficiency.

A reação de redução de oxigênio (RRO) foi estudada em eletrocatalisadores formados por ligas de Pt-M/C (M = V, Cr, Co, Fe e Ni) em eletrólito de H2SO4, sendo estes catalisadores preparados por diversos métodos. As propriedades eletrônicas foram investigadas por XAS (X-Ray Absorption Spectroscopy) in situ, na região de XANES (X-ray absorption near edge structure) e as propriedades estruturais por DRX (difração de raio-X). As atividades eletrocatalíticas para a RRO dos diferentes eletrocatalisadores foram comparadas por curvas de Tafel corrigidas por transporte de massa. Em todos os casos, as propriedades eletrônicas dos metais eletrocatalisadores, caracterizadas pela magnitude da absorção de raio-X, foram usadas para a compreensão da atividade eletrocatalítica dos materiais, e para estabelecer a relação propriedade eletrônica/cinética da RRO. Os resultados de XANES para as ligas Pt-M/C em altos potenciais do eletrodo mostraram uma menor vacância da banda 5d da Pt comparada com Pt/C, indicando menor reatividade para adsorbatos da Pt nas ligas. As medidas eletroquímicas evidenciaram um aumento da atividade eletrocatalítica da Pt nas ligas, em comparação com Pt pura e isto foi atribuído à menor força de adsorção de espécies oxigenadas causada pelo menor valor de energia docentro da banda d (εd) ou menor reatividade da Pt. / The oxygen reduction reaction (ORR) was studied in H2SO4 electrolyte on different Pt-M/C (M = V, Cr, Co, Fe e Ni) alloys electrocatalysts, prepared by several methods. The electronic properties of the materials have been investigated by in situ XAS (X-ray absorption spectroscopy) in the XANES (X-ray absorption near edge structure) region and the structural properties by XRD (X-ray diffraction). The electrocatalytic activity for the ORR on the different catalysts was compared through mass-transport corrected Tafel plots. In all cases, the electronic properties of Pt on the metal electrocatalysts, as characterized by the magnitude of X-ray absorption, were used to understand the electrocatalytic activity and to establish a relationship between the electronic/kinetic properties. XANES results for the PtM/C alloys at high electrode potentials had shown lower Pt 5d band vacancy compared to Pt/C, indicating lower reactivity for adsorbates for the Pt alloys. The electrochemical experiments had shown enhancement of the catalytic activity in for the Pt alloys when compared with pure Pt and this was attributed to a lower adsorption strength of oxygenate species caused by the lower reactivity of Pt.

Vários trabalhos na literatura reportam a utilização de catalisadores à base de compostos macrocíclicos metálicos no estudo da reação de redução do oxigênio (RRO). Esta reação possui mecanismos complexos que ainda não foram totalmente compreendidos, sendo que os principais produtos gerados são água e peróxido de hidrogênio. A maior parte dos artigos encontrados na literatura estudam estes catalisadores com intuito de se ter como produto único ou majoritário a água, principalmente para aplicações em células a combustível. Porém, este trabalho tem como objetivo principal identificar catalisadores a base de compostos macrocíclicos metálicos, mais em específico ftalocianinas metálicas, que promovam uma geração majoritária de peróxido de hidrogênio. Em trabalhos futuros, os melhores catalisadores encontrados servirão como modificadores em eletrodos de difusão gasosa (EDG) na aplicação em sistemas de degradação de compostos orgânicos através dos processos oxidativos avançados (POA). <br/>Neste trabalho foram avaliados catalisadores contendo vários teores de ftalocianinas de manganês, ferro, cobre e cobalto incorporadas, separadamente, em uma matriz condutora de carbono Printex 6L. Para isto, foram realizadas medidas de fluorescência de raios X (XRF), medidas eletroquímicas de voltametria cíclica e linear utilizando um eletrodo disco/anel rotatório (RRDE) e medidas utilizando a técnica de microscopia eletroquímica de varredura (SECM). Os resultados de XRF obtidos indicaram que o método de incorporação das ftalocianinas metálicas foi eficaz. Os resultados da avaliação do direcionamento da RRO a partir das medidas de voltametria linear, cálculos de eficiência de corrente de H2O2, número total de elétrons trocados e gráfico de Koutecký-Levich indicaram que os catalisadores contendo ftalocianina de cobalto mostram-se promissores para a eletrogeração em grandes quantidades de H2O2, para posteriores aplicações em POA. <br/>Os resultados de SECM evidenciaram o efeito catalítico observado para o catalisador contendo 10 % de ftalocianina de cobalto, bem como revelou que a superfície do catalisador obtida indica uma distribuição não uniforme do modificador sobre a matriz Printex 6L; porém, outras medidas são necessárias para uma melhor compreensão deste fenômeno. / Several papers in the literature report the use of catalysts based on metallic macrocyclic compounds in the study of the oxygen reduction reaction (ORR). This reaction has complex mechanisms that are not yet fully understood, whereas the main products generated are water and hydrogen peroxide. Most articles in literature study these catalysts in order to have water as the major or sole product, particularly for applications in fuel cells. However, this study aims to identify catalysts containing metallic macrocyclic compounds, metallic phthalocyanines more specifically, to promote majority generation of hydrogen peroxide. In future studies, the best catalysts will serve as modifiers in gas diffusion electrodes (GDE) for application of organic compounds degradation systems, via the advanced oxidation processes (AOP). <br/>This study evaluated various contents of catalysts containing manganese, iron, copper and cobalt phthalocyanine incorporated separately into a Printex 6L carbon matrix. It was performed measurements of X-ray fluorescence (XRF), electrochemical experiments of cyclic and linear voltammetry using rotating ring/disk electrode (RRDE) and measurements using the technique of scanning electrochemical microscopy (SECM). XRF results indicated that the incorporation method of metal phthalocyanines was effective. Results of the evaluation of the ORR pathway obtained from linear voltammetry measurements, calculations of current efficiency of H2O2, total number of electrons exchanged and Koutecký-Levich plots indicated that the catalysts containing cobalt phthalocyanine proved to be promising to H2O2 electrogeneration in large quantities for further applications in AOP. <br/>The SECM results evidenced the catalytic effect observed for the catalyst containing 10% cobalt phthalocyanine and revealed its surface indicates a non-uniform distribution of the modifier on matrix Printex 6L. But other measures are necessary for a better understanding of this phenomenon.

Magister Scientiae - MSc / Fuel cells convert chemical energy from a fuel into electricity through chemical reaction with oxygen. This possesses some challenges like slow oxygen reduction reaction (ORR), overpotential, and methanol fuel cross over in a direct methanol fuel cell (DMFC). These challenges cause inefficiency and use of higher amounts of the expensive platinum catalyst.Several binary catalysts with better ORR activity have been reported. In this study we investigate the best catalyst with better ORR and MOR performances and lower over-potentials for PEMFC and DMFC applications by comparing the in-house catalysts (10%Pt/C, 20%Pt/C,30%Pt15%Ru/C, 40%Pt20%Ru/C, 30%PtCo/C, 20%Pt20%Cu/C and 20%PtSn/C) with the commercial platinum based catalysts (10%Pt/C, 20%Pt/C, 20%Pt10%Ru/C, 20%PtCo/C,20%PtCu/C and 20%PtSn/C) using the cyclic voltammetry and the rotating disk electrode to determine their oxygen reduction reaction and methanol tolerance. HRTEM and XRD techniques were used to determine their particle size, arrangement and the atomic composition. It was observed that the 20%Pt/C in-house catalyst gave the best ORR activity and higher methanol oxidation current peaks compared to others catalysts followed by 20%Pt10%Ru/C commercial catalyst. The 20%PtCo/C commercial, 30%PtCo/C in-house and 20%PtSn/C in-house catalysts were found to be the most methanol tolerant catalysts making them the best catalysts for ORR in DMFC. It was observed that the ORR activity of 20%PtCo/C commercial and 30%PtCo/C inhouse catalysts were enhanced when heat treated at 350 0C. From XRD and HRTEM studies, the particle sizes were between 2.72nm to 5.02nm with little agglomeration but after the heat treatment, the particles were nicely dispersed on the carbon support.

A complexidade da reação de redução do oxigênio (RRO) e suas perdas de potencial a fazem responsável por grande parte das perdas de eficiência nas células a combustível. Para esta reação o eletrocatalisador mais apropriado e com melhor desempenho é a Platina, um metal nobre e que torna alto o custo da tecnologia das células a combustível, aumentando as barreiras para entrar no mercado. Primeiramente o trabalho teve em vista reduzir a quantidade em massa de platina utilizada no cátodo, sendo substituída por óxidos de terras raras. Observando que os métodos mais comuns de síntese de eletrocatalisadores para a aplicação em células a combustível se realizam em enumeras etapas, este trabalho se propôs a preparar eletrocatalisadores através de etapas mais simples e que dependessem de menos etapas e tempo de preparo. Através da mistura física simples utilizando ultrassom foram preparados eletrocatalisadores de platina suportada em carbono com os óxidos das terras raras lantânio, cério e érbio, para o estudo em meia célula da RRO. O resultado do gráfico de Koutecky-Levich mostrou que entre os eletrocatalisadores preparados o Pt80Ce20/C foi o que apresentou atividade catalítica mais próxima da platina comercial BASF, sugerindo que a RRO aconteceu via 4 elétrons. Como encontrado na em alguns trabalhos da literatura, entre as terras raras aplicada no cátodo, o cério é o elemento que mais contribui para esta substituição, devido a sua capacidade de estocar e fornecer oxigênio. Esta característica é um grande atrativo para a RRO pois esta reação é primeira ordem em relação a concentração de oxigênio. O resultado mostrou que é possível diminuir a quantidade de platina mantendo atividade catalítica. / The complexity of the oxygen reduction reaction (ORR) and its potential losses make it responsible for the most part of efficiency losses at the Fuel Cells. For this reaction the electrocatalyst witch is most appropriated and shows better performance is platinum, a noble metal that elevates the cost, raising barriers for Fuel Cells technology to enter the market. First this work focuses on reducing the amount of platinum used in the cathode, by being replaced by rare earths. The most common methods of synthesis involves a large amount of steps and this work proposed to prepare the electrocatalyst through a simpler way that would not take so many steps and time to be done. Using an ultrasound mixer the electrocatalyst was prepared mixing platinum supported on carbon black and the rare earths lanthanum, cerium and erbium oxides to be applied in a half-cell study of the ORR. The Koutecky-Levich plots shows that among the electrocatalysts prepared the Pt80Ce20/C had the catalytic activity close to the commercial BASF platinum on carbon black, suggesting that the reaction was taken by the 4-electron path. As found in some works in literature, among the rare earth used to study the ORR, cerium is the one witch shows the better performance because it is able to store and provide oxygen. This feature is of great interest for the ORR because this reaction is first order to the oxygen concentration. Results show that is possible to reduce the amount of platinum maintaining the same electrocatalyst activity.

In the search for alternative energy technologies, low temperature fuel cells continue to feature as technologies with the most promise for mass commercialization. Among the low temperature fuel cells, alkaline and proton exchange membrane fuel cells are the most popular. Alkaline fuel cells have typically been used for water generation as well as auxiliary power for space shuttles. Their bulkiness however makes them undesirable for other applications, especially in automobiles, where there is a great demand for alternative technologies to internal combustion engines. Proton exchange membrane fuel cells on the other hand possess numerous qualities including their compact size, high efficiency and versatility. Their mass implementation has however been delayed, because of cost among other reasons. Most of this cost is owed to the Pt/C catalyst that accounts for about half of the price of the PEM Fuel Cell. This catalyst is used to drive the sluggish oxygen reduction reaction that occurs at the cathode of the PEM Fuel Cell. To overcome this obstacle, which is to make PEM Fuel Cell technology more affordable, reducing the amount Pt has traditionally been the approach. Another approach has been to find new ideal catalyst-support combinations that increase the intrinsic activity of the supported material. One more strategy has been to find lower cost alternative materials to Pt through synthetic and kinetic manipulations to rival or exceed the current oxygen reduction reaction activity benchmark. To this end, Palladium has garnered significant interest as a monometallic entity. Its manipulation through synthetic chemistry to achieve different morphologies - which favor select lattice planes - in turn promotes the oxygen reduction reaction to different degrees. In bimetallic or, in more recent times multimetallic frameworks, geometric and ligand effects can be used to form ideal compositions and morphologies that are synergistic for improved oxygen reduction reaction kinetics. In this dissertation, we have explored three different approaches to make contributions to the catalysis and electrocatalysis body of literature. In the first instance, we look at the influence of ligand effects through the active incorporation of a PVP capping agent on the stability of ~3nm Pt NPs. Washed (no capping agent) and unwashed (with capping agent) batches of NPs were evaluated via cyclic voltammogram analyses to evaluate differences there might be between them. It was found that the current density measurements for unwashed particle batches were higher. This increase in current density was attributed to the monodentate and bidentate ligand bonding from the PVP, which increased as a function of cycle number and plateaued when the PVP was completely decomposed. The complete decomposition of PVP during the CV experiment was estimated to occur around 200 cycles. The remaining portion of the dissertation explores the electrocatalytic properties of Palladium based NPs. The first instance, a monometallic study of Palladium cubes and dendrites was aimed at building on a recent publication on the enhanced ORR activity that was achieved with a PdPt bimetallic dendrite morphology. In our work, we sought to isolate the dendritic morphology properties of the monometallic Pd composition in order to understand what advantages could be achieved via this morphology. Pd cubes were used as a comparison, since they could be generated through the combination of a similar set of reagents simply by switching the order of addition. It was found that while there was no significant variation in the ORR activity as a function of morphology / shape, there was an interesting interaction between hydrogen and the palladium NPs in the hydrogen oxidation region that varied as a function of shape. This led to further sorption and ethylene hydrogenation studies, which suggested that, the interaction between hydrogen and Pd depended on the environment. Within the electrochemical environment, the ECSA measured, suggested that hydrogen was being reversibly absorbed into the sub-surface octahedral sites of Pd. The higher ECSA for Pd cubes corroborated with higher sorption for Pd cubes as well. However ethylene hydrogenation showed that the fringes of the Pd dendrites provided additional sites for reaction, which in turn translated to higher conversion. Furthermore, through a Koutecky-Levich analysis, it was found out that the Pd dendrites while exhibiting slightly lower activity, favored the 4-electron oxygen reduction process more than the Pd cubes. In the last part of this dissertation we explored the electrocatalytic properties of Pd-based bimetallic NPs under different morphologies including nanocages and sub-10nm alloys. With the inclusion of Ag, it was found out, through Koutecky-Levich analysis that the 4-electron process was better observed under alkaline conditions using a 0.1M NaOH(aq) electrolyte solution instead of a 0.1M HClO4 (aq) for acidic media testing. It was found that, for PdAg nanocage morphologies, where the Pd galvanically replaced the Ag to form cages, the four-electron process was suited to thinner Pd shells. Indeed the average electron numbers measured for Ag nanocubes coated with a 6nm shell was in agreement, within reason of literature values for bulk Ag. However, since the binding energy that both metals have for OH is so close, the potential for contributions to the ORR kinetics in alkaline media by Pd is a potential consideration.

Η παρούσα διατριβή αποτελεί κατά ένα μέρος μελέτη του φαινομένου της Ηλεκτροχημικής Τροποποίησης της Καταλυτικής Ενεργότητας, NEMCA βασιζόμενη τόσο στη διερεύνηση της ελεγχόμενης ιοντικής αγωγής του ζεόλιθου με την επιβολή δυναμικού με σκοπό την εφαρμογή του φαινομένου σε διεσπαρμένα συστήματα καταλυτών όσο και στην παρουσία του φαινομένου NEMCA σε υδατικά ηλεκτρολυτικά συστήματα μεταβαλλόμενου pH. Μελετήθηκε επίσης η δυνατότητα βελτίωσης της ηλεκτροχημικής ενεργότητας της καθόδου όπως επίσης και της ανόδου για τις αντιδράσεις της αναγωγής του οξυγόνου και της διάσπασης του νερού αντίστοιχα, σε κυψελίδες καυσίμου πρωτονιακής μεμβράνης χαμηλής θερμοκρασίας. Tο Κεφάλαιο 1 αποτελεί εισαγωγή στις γενικές αρχές της Ηλεκτροχημικής Προώθησης ή Μη Φαραντεϊκής Ηλεκτροχημικής Τροποποίησης της Καταλυτικής Ενεργότητας (φαινόμενο NEMCA) και περιλαμβάνει τον πλήρη κατάλογο όλων των βιβλιογραφικών αναφορών που διερευνούν και τεκμηριώνουν το φαινόμενο. Παράλληλα περιγράφονται οι πειραματικές διατάξεις που χρησιμοποιούνται στην έρευνα της ηλεκτροχημικής ενίσχυσης, αναλύονται οι παράμετροι του φαινομένου καθώς και οι κανόνες που θεμελιώθηκαν με την ταξινόμηση και ανάλυση των πειραματικών αποτελεσμάτων και αποτελούν πρακτικές αρχές ασφαλούς πρόβλεψης της συμπεριφοράς μιας αντίδρασης. Στο Κεφάλαιο 2 περιγράφονται οι βασικές ηλεκτρονιακές παράμετροι μίας στερεής επιφάνειας (μέταλλο ή ημιαγωγός) καθώς και οι θεμελιώδεις ηλεκτρικές και θερμοδυναμικές έννοιες που χαρακτηρίζουν τη διεπιφάνεια ηλεκτροδίου/ηλεκτρολύτη και γενικά τις ηλεκτροδιακές δράσεις. Στο Κεφάλαιο 3 περιγράφονται συνοπτικά οι μέθοδοι χαρακτηρισμού και ανάλυσης καθώς και οι πειραματικές διατάξεις που χρησιμοποιήθηκαν στην παρούσα διατριβή. Στο Κεφάλαιο 4 εξετάζεται ηλεκτροχημικά η ιοντική μετακίνηση Na στο σύστημα Au/NaY σε συνθήκες κενού και σε ατμόσφαιρα οξυγόνου ενώ στη συνέχεια μελετήθηκε το φαινόμενο NEMCA στο σύστημα Pt/NaY κατά την αντίδραση οξείδωσης του CO. Δομικά φαίνεται να υπάρχουν είδη Na ισχυρά δεσμευμένα στο πλέγμα του ζεόλιθου που να μη συμμετέχουν στην ιοντική αγωγιμότητα του υλικού. Αντίθετα η περίσσεια ή τα ασθενώς δεσμευμένα είδη Na δύνανται να μετακινηθούν με την επίδραση ηλεκτρικού πεδίου ώστε να δημιουργηθεί ιοντική αγωγή. Σε ατμόσφαιρα υπερυψηλού κενού και υπό την επίδραση αρνητικής υπέρτασης δεν ευνοείται η ρόφηση ειδών Na στην επιφάνεια του ηλεκτροδίου Au, κάτι που υποδηλώνει την σχεδόν μηδενική ηλεκτροκαταλυτική ενεργότητα της διεπιφάνειας Au|ζεολίθου. Αντίθετα κατά την επίδραση ατμόσφαιρας O2 και υπό αρνητική πόλωση της διεπιφάνειας Au|ζεολίθου, δημιουργείται ροφημένο Na2O στην επιφάνεια του Au. Τούτο σημαίνει ότι η παρουσία ροφημένου οξυγόνου προάγει την αντίδραση μεταφοράς φορτίου Na+ και συνεπώς αυξάνει την ηλεκτροκαταλυτική ενεργότητα στο ηλεκτρόδιο του Au. Η ηλεκτροχημική μελέτη της διεπιφάνειας Pt/NaY ζεολίθου σε κελίο δύο και τριών ηλεκτροδίων έδειξε ότι η επιβολή θετικού δυναμικού στο σύστημα Pt/NaY|Au όταν ο καταλύτης παρουσιάζει ~50% διασπορά δεν επέφερε καμμία μεταβολή στον καταλυτικό ρυθμό. Αυτό κυρίως οφείλεται στην ύπαρξη μεγάλων υπερτάσεων στην διεπιφάνεια εξαιτίας τόσο της μικρής ιοντικής και ηλεκτρονιακής αγωγιμότητας του φορέα και του ηλεκτροκαταλύτη αντίστοιχα. Η επιβολή θετικού δυναμικού στο σύστημα Pt|AuNaY|Au, Au στο οποίο ο καταλύτης έχει εναποτεθεί με την μορφή φιλμ στο φορέα οπότε και παρουσιάζει μικρή διασπορά, ενισχύει τον καταλυτικό ρυθμό της οξείδωσης του CO κατά 17%. Η ενίσχυση οφείλεται στην απομάκρυνση Na από την επιφάνεια της Pt και στην διευκόλυνση της ρόφησης των αντιδρώντων. Στο Κεφάλαιο 5 αποδεικνύεται ότι η κινητική της καταλυτικής οξείδωσης του H2 σε ηλεκτρόδιο-καταλύτη PtRu/C δύναται να επηρεαστεί δραματικά αφενός με τον έλεγχο του pH των υδατικών ηλεκτρολυτών και αφετέρου με την ηλεκτροχημική πόλωση της διεπιφάνειας ηλεκτροδίου/ηλεκτρολύτη. Η πρώτη περίπτωση ενίσχυσης της καταλυτικής ενεργότητας αποκαλείται για πρώτη φορά με τον όρο: Ηλεκτροχημική Αλληλεπίδραση Μετάλλου Φορέα (Electrochemical Metal Support Interaction, EMSI) και δύναται να θεωρηθεί ως ένα γενικό φαινόμενο αλληλεπίδρασης μετάλλου – φορέα το οποίο εμφανίζεται όταν ο φορέας παρουσιάζει ιοντική αγωγή και τεκμηριώνεται μέσω της αντίδρασης μεταφοράς φορτίου σε ισορροπία που λαμβάνει χώρα στη διεπιφάνεια καταλύτη/φορέα. Η δεύτερη περίπτωση μεταβολής της καταλυτικής ενεργότητας αφορά το ήδη αναγνωρισμένο φαινόμενο της ηλεκτροχημικής ενίσχυσης. Η μελέτη του φαινομένου NEMCA σε διάφορα υδατικά ηλεκτρολυτικά συστήματα μεταβαλλόμενου pH δείχνει την ενίσχυση του φαινομένου όταν ο καταλύτης – ηλεκτρόδιο έρχεται σε επαφή με ηλεκτρολύτες υψηλού pH όπου στη διεπιφάνεια καταλύτη/ηλεκτρολύτη υπερέχει η συγκέντρωση OH-. Συμπεραίνεται ότι με τρόπο ανάλογο όπως στα συστήματα στερεής ηλεκτροχημείας, η μη Φαρανταϊκή ηλεκτροχημική τροποποίηση της καταλυτικής ενεργότητας προάγεται με τη δημιουργία ενός στρώματος πολικών ροφημένων ειδών τα οποία παράγονται ηλεκτροχημικά, από τα υπάρχοντα OH-, στα όρια των σημείων επαφής των τριών φάσεων της εκτεθειμένης στην αέρια φάση καταλυτικής επιφάνειας. Τα Κεφάλαια 6 και 7 αποτελούν την μελέτη του ρόλου του φορέα στις ιδιότητες της Pt, της διασποράς του μετάλλου και της παρουσίας ενός δεύτερου μετάλλου κατά την αντίδραση της αναγωγής του οξυγόνου. Μελετήθηκαν ηλεκτρόδια-καταλύτες Pt διεσπαρμένοι στο οξειδικό μείγμα TiO2/WO3 με συμπαρουσία του άνθρακα. Συγκεκριμένα, παρασκευάσθηκαν τρία ηλεκτρόδια εργασίας που αποτελούνταν από καταλύτη Pt (με ποσοστά μετάλλου 10, 15 και 30%) υποστηριγμένου στον μεικτό οξειδικό φορέα TiO2/WO3 διασπαρμένο σε C. Προκειμένου σύγκρισης μελετήθηκε ο εμπορικός καταλύτης Pt/C της εταιρείας Etek ως ηλεκτρόδιο καθόδου. Τον ηλεκτρολύτη του κελίου αποτέλεσε η πολυμερική μεμβράνη Nafion 115 οπότε και η μέγιστη θερμοκρασία μελέτης της αντίδρασης της αναγωγής ήταν 80°C. Η απόδοση της αναγωγής βελτιώνεται με την αύξηση του ποσοστού της Pt σε 15% καθώς η συγκεκριμένη διεπιφάνεια ηλεκτροδίου/ηλεκτρολύτη χαρακτηρίζεται από την μειωμένη τιμή της κλίσης Tafel μικρών υπερτάσεων ίσης με b1= -0.047V/dec που απαντάται μόνο σε κελία αλκαλικών ηλεκτρολυτών και αποδίδεται στην παρουσία της αποδίδεται στην παρουσία του μεικτού οξειδίου TiO2/WO3 που δρα ως μεμβράνη ιόντων OH- τα οποία υπό συνθήκες ανοδικής πόλωσης διαχέονται και ροφούνται ηλεκτροχημικά στην μεταλλική επιφάνεια συμβάλλοντας στην διευκόλυνση της αναγωγής του οξυγόνου. Η πυκνότητα ρεύματος της αναγωγής με βάση την γεωμετρική επιφάνεια του ηλεκτροδίου υπολογίζεται σε 0.16 Α cm-2 στο δυναμικό των 0.5V στους 25°C. Η περαιτέρω αύξηση της μεταλλικής φόρτισης στον καταλύτη έχει ως αποτέλεσμα την βελτίωση της απόδοσης της αναγωγής του οξυγόνου εφόσον υπολογίζεται σε 0.2 A cm-2 στους 30°C για εξωτερική εφαρμοζόμενη τάση 0.5V. Ο καταλύτης Pt/Ebonex ως ηλεκτρόδιο διάχυσης αερίου σε κυψελίδα πολυμερικής μεμβράνης Nafion παρουσίασε σημαντικές διαφορές από την τυπική συμπεριφορά του ηλεκτροδίου της Pt στην περιοχή της αναγωγής του οξυγόνου. Η έναρξη της ηλεκτροχημικής οξειδικής ρόφησης στο ηλεκτρόδιο Pt/Ebonex προηγείται κατά 0.1V του Pt/C, ενώ η ύπαρξη των δύο καθοδικών κορυφών (I και II) στα 0.68 και 0.78V αποδίδεται στην αντιστρεπτή αναγωγή του Pt-OH και του ροφημένου οξυγόνου, αντίστοιχα. Η παρουσία του οξειδικού φορέα της Pt ως πηγή των OH, που μέσω διάχυσης έχει ως αποτέλεσμα την δημιουργία του πρωτογενούς οξειδίου Pt-OH, φαίνεται να βελτιώνει την αντίδραση της αναγωγής του οξυγόνου. Ο διμεταλλικός καταλύτης PtCo/Ebonex χαρακτηρίζεται περισσότερο ενεργός από τον καταλύτη Pt/C καθώς για τις ίδιες ηλεκτροχημικές δράσεις παρουσιάζει μεγαλύτερο ρεύμα, ενώ παράλληλα μετατοπίζει το δυναμικό της κορυφής αναγωγής κατά 0.05V σε καθοδικότερα δυναμικά όταν η έναρξη της οξειδικής κάλυψης είναι κοινή και για τα δύο ηλεκτρόδια. Αυτό σημαίνει ότι ο διμεταλλικός καταλύτης παρουσία του Ebonex δημιουργεί ασθενέστερο δεσμό με τα οξειδικά είδη και ως εκ τούτου η ρόφηση αλλά και η εκρόφηση (δηλαδή η αναγωγή των οξειδίων) είναι ηλεκτροχημικά πιο γρήγορες διαδικασίες. Η απόδοση της αντίδρασης της αναγωγής, κανονικοποιημένης ως προς την μεταλλική επιφάνεια, συμπίπτει με την ικανότητα του Pt/C γεγονός που συνδέεται με το θετικό ενδεχόμενο της μείωσης του ποσοστού της Pt στον καταλύτη. Το Κεφάλαιο 8 εστιάζεται στη συμπεριφορά των καταλυτών IrOx/C, PtVTiO2/C και RuO2-IrO2/Εbonex κατά την διάσπαση του νερού σε ηλεκτρολυτική κυψελίδα πρωτονιακής μεμβράνης Nafion. Η παρασκευή ηλεκτροδίων Ir σε υπόστρωμα άνθρακα με τη μέθοδο της καθοδικής ιονοβολής υπερέχει σε απόδοση έναντι των συμβατικών θερμικών μεθόδων παρασκευής ανόδων καθώς στους 90°C και στο δυναμικό των 1.56V η πυκνότητα ρεύματος ισούται με 1.1A cm-2 κατατάσσοντας την συγκεκριμένη άνοδο στις λειτουργικότερες για την παραγωγή O2 με γνώμονα τις βιβλιογραφικές αναφορές. Εκτός της απόδοσης, σημειώνεται η ικανή μηχανική συταθερότητα του ηλεκτροδίου αλλά και η παρουσία μικρότερων φορτίσεων καταλύτη ανά μονάδα επιφάνειας ηλεκτροδίου, παράμετροι που συνηγορούν στην προτεινόμενη χρήση τους για πρακτικές εφαρμογές. Ο διμεταλλικός καταλύτης PtV/TiO2/C με την παρουσία του βαναδίου σε υψηλές οξειδωτικές καταστάσεις (V4+ ή/και V5+), φαίνεται να διευκολύνει την ανοδική αντίδραση καθώς το δυναμικό έναρξης της παραγωγής του οξυγόνου εντοπίζεται στα ~1.25V παρουσιάζοντας μετατόπιση κατά 0.25V καθοδικότερα σε σχέση με το ηλεκτρόδιο Pt/C. / The present thesis constitutes at one part the study of phenomenon of non Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect), based on the investigation of the controlled ionic migration of a zeolite material under potential imposition and shoot for the application of NEMCA in well dispersed catalytic systems as in aqueous electrolytic systems of various pH. The second part of this work investigates the possibilities of cathode’s improvement in the case of oxygen reduction reaction as well as the anode’s for the water splitting using low temperature polymer electrolyte membrane. Chapter 1 introduces the basic concepts and terminology of Electrochemical Promotion (NEMCA effect) and includes the complete list of all bibliographic reports that investigates and corroborates the phenomenon. The experimental designs that are used in the research of NEMCA effect are described and the parameters of phenomenon are analyzed as well as the rules that were founded with the classification and analysis of experimental results and constitute practical beginnings of the behavior of any reaction. Chapter 2 describes the fundamental electronic terms of solid surfaces (metal or semiconductor) as well as the electric and thermodynamic parameters that characterize the electrode /electrolyte interface and generally an electrodic action. Chapter 3 deals with the methods of characterization and analysis as well as the experimental techniques that were used is described concisely. The scope of the first part of Chapter 4 is the migration of Na+ to/from the electrochemical interface of Au/NaY under vacuum and oxygen conditions in a controllable potensiostatic manner. In the second part of Chapter 4 the NEMCA effect is investigated on Pt/NaY under the oxidation of CO as the model reaction. It is found that two main species exist into the zeolite matrix, these that being strongly bonded in the matrix stayed unperturbed with an electric field and those that can be moved during the potential imposition resulted in the ionic conductivity. Especially, at vacuum conditions, the negative overpotential doesn’t influence the coverage of Naad on the Au surface while at oxygen atmosphere there is an enhancement by 4 times. The latter implies the significant role of oxygen that promotes the charge transfer of Na+ and consequently increases the Naad coverage on the working electrtode. The electrochemical study of the interface Pt /NaY where the metal dispersion is up to 50% showed that under positive overpotential the catalytic rate doesn’t change. However the metal dispersion decreasing provokes the catalytic rate up to 17%. The catalytic enhancement was ascribed to the depletion of Na adsorbate from Pt surface. In Chapter 5 proved that the kinetics of catalytic oxidation of H2 on PtRu/C influenced dramatically from one side with the pH variation of the aqueous electrolyte and from the other side with the electrode/electrolyte polarization. The pH effect is called for the first time with the term: Electrochemical Metal Support Interaction, EMSI and it is considered as a general phenomenon of metal-support interaction which presents when the support is an ionic conductor via the charge transfer reaction in equilibrium that takes place at the electrode/electrolyte interface. The second case of change of catalytic activity concerns the already recognized phenomenon of electrochemical promotion. NEMCA effect occurred at electrode/aqueous electrolyte interface combines with the formation of the effective double layer created at the tpb region made of . Chapters 6 and 7 deal with investigations on the influence of the support and the existence of a second metal on Pt catalysts at the oxygen reduction reaction. The studied electrodes support were the oxides of TiO2 and WO3 on carbon. The reaction is characterized by the decreased Tafel slope in the region of low overpotentials as it is found equal to -0.047V/dec which can be observed only in alkaline electrolytes and is attributed to the presence of the oxidic support that acts as membrane of OH- ions that diffused and electrochemically adsorbed on the metal surface contributing in the facilitation of reduction of oxygen. The current density of the reaction based on the geometrical area is up to 0.2 A cm -2 at 0.5V (30°C). Pt on Ebonex shows important differences from the typical behavior of investigated catalysts with the existence of two cathodic peaks at 0.68 and 0.78V (vs. RHE). The first peak is attributed to the Pt-OH reduction while the latter is being associated with the reduction of molecular oxygen. The bimetallic PtCo/Ebonex is being more active than Pt/C towards oxygen reduction reaction showing larger current densities while the reduction potential is been shifted 0.05V to the positive direction. Chapter 8 deals with investigations on the electrochemical water splitting using polymer electrolyte membrane on thin films of iridium oxide deposited by reactive magnetron sputtering. Very high performance was obtained resulted in current densities up to 1.1 A cm-2 at 1.56V and 90°C. Bimetallic PtV/TiO2/C with the presence of vanadium in high oxidative states (V4+ and/or V5+), appears to facilitate the reaction of oxygen evolution as the onset potential is shifted at ~1.25V.

Graphene has great potential for energy storage and conversion applications due to its outstanding electrical conductivity, large surface area and chemical stability. However, the pristine graphene offers unsatisfactory performance as a result of several intrinsic limitations such as aggregation and inertness. The functionalization of graphene is considered as a powerful way to modify the physical and chemical properties of graphene, and improve the material performance, which unfortunately still being preliminary and need further knowledge on controllable functionalization methods and the structure-property relationships. This thesis aims to provide in-depth understanding on these aspects. We firstly explored oxygen-functionalized graphene for supercapacitor electrodes. A mild solvothermal method was developed for graphene preparation from the reduction of graphene oxide; the solvent-dependent reduction kinetics is an interesting finding in this method that could be attributed to the solvent-graphene oxide interactions. Using the solvothermal method, oxygen-functionalized graphene with controlled density of oxygen functional groups was prepared by tuning the reduction time. The oxygen-containing groups, primarily phenols and quinones, reduce the graphene aggregation, improve the wetting properties and introduce the pseudocapacitance. Consequently, excellent supercapacitive performance was achieved. Nitrogen-doped graphene was synthesized by the pyrolysis of graphene oxide with nitrogen-containing molecules and used as an electrocatalyst for oxygen reduction reactions. We achieved the structural control of the nitrogen-doped graphene, mainly the content of graphitic nitrogen, by manipulating the pyrolysis temperature and the structure of nitrogen-containing molecules; these experiments help understand the evolution of the bonding configurations of nitrogen dopants during pyrolysis. Superior catalytic activity of the prepared nitrogen-doped graphene was found, due to the enriched content of graphitic nitrogen that is most active for the oxygen reduction reaction. Moreover, we demonstrated a facile strategy of producing superhydrophobic octadecylamine-functionalized graphite oxide films. The long hydrocarbon chain in octadecylamine reduces the surface energy of the graphene oxide film, resulting in a high water contact angle and low hysteresis. The reaction mechanism and the effect of hydrocarbon chain length were systematically investigated. In addition to the researches on graphene-based materials, some results on advanced carbon nanomaterials and polymer composites for electronic packaging will also be discussed as appendix to the thesis. These include carbon nanotube-based capacitive deionizer and gas sensor, and hexagonal boron nitride-epoxy composites for high thermal conductivity underfill.

Afin de mieux comprendre et de dépasser les limites actuelles des systèmes métal-air non-aqueux, le mécanisme de réduction de l’oxygène (ORR) a été étudié en présence de cation alcalins dans divers solvants aprotiques. Sur la base de caractérisations électrochimiques sur électrode statique et d’électrodes tournantes disque-anneau, un mécanisme unique a été proposé afin de rendre compte de l’ORR en présence de cations alcalins. De plus, les différences observées d’un solvant à l’autre ont été expliquées en termes de capacité du solvant à solvater à la fois le cation alcalin en présence et l’anion superoxyde, mais aussi à sa capacité à séparer les paires d’ions. Un modèle cinétique basé sur ce mécanisme a montré un excellent accord avec les résultats expérimentaux. / In order to better understand and overcome the current limitations of non-aqueous metal-air batteries, the oxygen reduction reaction (ORR) mechanism has been studied in presence of different alkali metal cations in several aprotic solvents. Based on electrochemical characterizations on static electrode and rotating ring-disk electrode, a unique mechanism has been proposed to account for ORR in presence of alkali metal cations. It has been further showed that the differences observed from one solvent to another could be linked to the solvent’s ability to solvate both the alkali metal cation and the superoxide anion, as well as its capability to separate ion-pairs. A kinetic model based on this mechanism has shown very good agreement with experimental results.