>Magister Scientiae - MSc / Polymer electrolyte membrane fuel cells (PEMFC) are in the forefront of energy production and have drawn a great deal of attention in both fundamental and application in recent years. It is a promising energy system used in commercialized electric vehicles presenting with the following advantages: low-temperature operation, high power density (40%–60%), nearly zero pollutants compared to conventional internal combustion gasoline vehicles, simple structure, and so on. There are, however, two major obstacles which obstruct PMFCs pathway to commercialization— durability and cost. Recent advances in PEMFC systems showed the most common fuel cell catalysts to be Platinum (Pt) (or platinum alloys) supported by high surface carbon in both the cathode and anode. However, carbon is very susceptible to corrosion and results in lower durability of Pt supported catalysts.

Oxygen reduction reaction (ORR) has considerable potential for the pro-duction of electricity, issues with water splitting and many other applications in the energy sector. But in order to increase the efficiency of the reaction an electrocatalyst needs to be introduced. In today’s industrial devices precious and costly metals such as platinum (Pt) are used as catalysts. Other more abundant and cheaper alternatives, for example cobalt oxides, are therefore being investigated. In this thesis, pure cobalt (Co) thin films were synthesised to investigate if thin films can be used for the catalysis of ORR. This was successfully carried out by electrochemically modifying the thin films and grow catalytically ac-tive hexagonal cobalt oxide nanoparticles.   Multicomponent system CoCrFeNi is an emerging alloy system with high research interest for its high corrosion resistance suitable for harsh environments in which the applications for ORR are found. In this thesis, CoCrFexNi were synthesised as thin films. The corrosion resistance of the films was investigated in addition to their catalytic activity. The effect of Fe content on these properties was also studied. The presence of Fe was crucial for the electrochemical activation of films and catalytic activity towards ORR.  In summary, this thesis shows that cobalt based thin films can be used as catalysts combined with corrosion resistance for ORR applications.

No description available.

No description available.

No description available.

No description available.

No description available.

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.