A Reação de Redução de Oxigênio (RRO) foi estudada em eletrocatalisadores constituídos por platina e paládio suportados em carbono em eletrólito de H2SO4. A caracterização estrutural foi realizada por DRX (difratometria de raios X) e a composição dos catalisadores foi determinada por EDX (espectroscopia de energia dispersiva de raios X). As propriedades eletrônicas foram investigadas por XAS (X-ray absorption spectroscopy) in situ, na região de XANES (X-ray absorption near edge structure). As atividades dos catalisadores foram comparadas por curvas de Tafel corrigidas por transporte de massa. Os materiais formados por Pt-Pd suportados em carbono mostraram maior atividade específica para a RRO quando comparados com os catalisadores constituídos somente por platina suportada em carbono. A maior atividade foi relacionada a uma menor força de adsorção de espécies oxigenadas nesses catalisadores. / Oxygen reduction reaction (ORR) on platinum-palladium electrocatalysts supported on carbon was studied in H2SO4 electrolyte. The structure of the catalysts was characterized by XRD (X-ray diffractometry) and the composition was determined by EDX (Energy dispersive X-ray spectroscopy). The electronic properties were investigated by in situ XAS (X-ray absorption spectroscopy) in the XANES (X-ray absorption near edge structure) region. Mass-transport corrected Tafel plots were used to compare the electrocatalytic activity of the materials. The Pt-Pd catalysts have shown higher specific electrocatalytic activity than Pt only catalysts. The higher activity of Pt-Pd catalysts was attributed to a lower adsorption energy of oxygenated species in these catalysts.

A cinética da reação de redução de oxigênio (RRO) foi estudada em eletrólito alcalino em diferentes eletrocatalisadores. As atividades eletrocatalíticas medidas experimentalmente em metais puros foram correlacionadas com propriedades eletrônicas do eletrocatalisador, como o centro em energia da banda d (εd). A RRO também foi investigada em Pt e Ag modificados pela formação de liga com outros metais de transição. Por fim, a RRO foi conduzida em diferentes óxidos de manganês sintetizados em laboratório, com o objetivo de se determinar a fase mais ativa e o mecanismo da reação nos diferentes óxidos. A atividade eletrocatalítica frente à RRO dos metais puros ou das monocamadas de platina suportadas em diferentes substratos metálicos monocristalinos apresentou uma dependência tipo “vulcão" em função da energia do centro da banda d do metal eletrocatalisador. Estes resultados indicam que tanto a quebra da ligação O-O como a hidrogenação dos intermediários reacionais têm que ser facilitados, de forma que a cinética das duas reações, as quais são aceleradas por propriedades antagônicas, seja otimizada. O ganho de atividade para a RRO observado para as ligas de Pt em relação à Pt pura foi associado à menor reatividade da Pt na ligas, o que leva à uma menor força da adsorção Pt-O- e, consequentemente, maior cinética de eletroredução dos intermediários oxigenados. A maior atividade das ligas de Ag comparada com a Ag pura foi atribuída à uma mais forte adsorção Ag-O-, o que resulta em maior cinética da quebra da ligação O-O. Os resultados para os diferentes óxidos de Mn mostraram que a ativação para a RRO é maior para os materiais com alto conteúdo de MnO2. A atividade eletrocatalítica dos óxidos de manganês foi associada com um mecanismo acoplado envolvendo uma mudança do estado de oxidação de Mn (IV) para Mn (III), com a transferência de elétrons do Mn (III) para o O2 adsorvido [], em processo via 2 elétrons, com subsequente reação de desproporcionamente do intermediário HO2-, recirculando O2 no sistema, tendendo à um mecanísmo global via 4 elétrons por molécula de O2. / The oxygen reduction reaction (ORR) was studied on electrodes formed by Pt monolayers deposited on different metallic substrates, and on carbon-supported electrocatalysts composed by PtM/C (M = V, Cr e Co) and AgPt/C alloys, and on different Mn oxides (Mn3O4/C, Mn2O3/C and MnO2/C) in alkaline electrolyte. The experimentally measured electrocatalytic activities of the different metal catalysts and of the Pt monolayers were plotted against the metal d-band center values (_d). In all cases, the electronic features of the metal electrocatalysts were used for understanding the catalytic activities, and trying to establish the electronic/ORR kinetics relationship. The XANES results for the PtM/C alloys at high electrode potentials have shown lower vacancy of the Pt 5d band compared to pure Pt/C, indicating lower Pt reactivity for adsorbates. The enhanced catalytic activity of Pt in the alloys was attributed to a faster electroreduction of oxygenated intermediates. For the AgPt/C alloys, the XANES results have shown a emptying of the Ag orbitals due to a charge transfer to Pt, and the increased activity of the Ag atoms was ascribed to an electronic effect induced by the presence of Pt, increasing the Ag-O adsorption strength For the manganese oxide materials, the XANES results indicated a chance of the Mn oxidation state as a function of the electrode potential, and higher electrocatalytic activity was observed for MnO2/C. This was explained based on the activation for the ORR, which is higher for the material with higher MnO2 contents and the occurrence of a mediation processes involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen.

This research exploits mild sonication in a thin layer electrochemical cell to enhance rates of reaction in systems under voltammetric perturbation. Sound waves propagate through a thin layer of condensed fluid to provide energy to the electrode solution interface in the form of pressure and temperature. The sonic energy provided in three dimensions can be exploited to enhance rates of heterogeneous electron transfer as the energy is harnessed at the two dimensional electrode interface. Enhanced rates of heterogeneous electron transfer are of interest both for fundamental reasons and for exploitation in electrochemical energy systems. The initial pilot studies were directed at demonstrating the impact of acoustic energy on heterogenous electron transfer. Redox couples with different electron transfer rates were evaluated. Whereas compounds with reversible electron transfer kinetics demonstrated little improvement, redox couples such as ferric ion (Fe3+) with slow electron transfer kinetics exhibited an increase in the standard heterogeneous electron transfer rate constant, k0 with an increase in acoustic energy. The reduction of oxygen is a complex four proton, four electron process that is of technological importance. Slow kinetics of the oxygen reduction is a primary loss of efficiency in electrochemical power sources. Much like the ferric ion, oxygen kinetic rats improve. Preliminary studies in the oxidation of methanol demonstrated a sonocatalyic effect in methanol electrolysis that is of particular interest for the development of liquid based fuel cells. Sonication can both clean and destroy surface materials. The cleaning power inherent in sonication improves electrocatalysis and removes deposits and oxides from the electrode surface.

Rapid industrial globalization and technological development and energy consumption across the globe has significantly increased in response to mounting energy needs. The necessity for alternative and sustainable energy conversion devices has become apparent with the growth of energy utilization. In recent years, many research efforts have been made in the development of low-cost, efficient, environmentally friendly energy conversion devices. One type of energy conversion device, polymer electrolyte membrane fuel cells (PEMFCs), uses hydrogen oxidation at the anode and oxygen reduction at the cathode, with a solid-state proton conducting membrane between to generate energy with water as a by-product. PEMFCs use Nafion®, a sulfonated fluoropolymer-copolymer for proton transport; however, temperature restraints and the need for hydration limits the efficacy of this polymer. Moreover, the kinetics of oxygen reduction (ORR) are significantly slower at the cathode than the anode. Platinum is currently the industry standard, but these materials have limited resources, are expensive, and can be sensitive to carbon monoxide poisoning. Platinum is also the preferred catalyst for hydrogen evolution reactions (HER)—critical electrochemical reactions at the cathode for water splitting applications for the generation of hydrogen. Metal-Organic Frameworks (MOFs) have been explored for proton conductivity and as electrode catalysts. The tunability of metal ions and organic linkers both in situ and post-synthesis allows for the targeted design of specific surface areas and topologies while fine tuning selective functionality. Furthermore, due to morphology retention upon pyrolysis, MOFs are good platforms for logical design both pre- and post- carbonization. Taking advantage of the amendable design, along with tunable porosity and growth in controlled dimensions, this work explores the modification of a zinc based MOF as a possible candidate for proton conduction, as well modification of zinc, cobalt, and iron based MOFs for ORR catalysis. Post-synthetic modification was employed as a technique to oxidize the imidazolate ligand to include carboxylic acid functionality of a zinc based MOF. Proton conductivity generally arises from the mobility of the charge carriers present (i.e. carboxylates and phosphates). The incorporation of Brønsted acidity by way of free carboxylates is often challenging, as these are generally sites of coordination in the framework. Herein, we report the successful augmentation of Brønsted acidity with retention of framework crystallinity in a robust MOF. Additionally, the effects of metal content and carbonization temperature of MOFs were explored for ORR and HER. Cobalt and iron were doped either pre- or post-synthesis and carbonized in an inert atmosphere at various temperatures to generate MOF-derived carbons with catalytically active centers without the need for additional support. Carbons with parent MOFs containing moderate amounts of cobalt doping in a bimetallic Co/Zn MOF, or carbons that contained no zinc in the parent material, showed excellent electrocatalytic performance for ORR when carbonized at temperatures just at or above the boiling point of zinc. Zinc based MOFs were doped with various amounts of iron post-synthesis and prior to carbonization in an inert atmosphere. The formation of iron nanoflakes and nanorods on the surface of these carbons generated from the pyrolysis of these iron doped MOFs yielded high surface areas and outstanding electrochemical performance for ORR in both acidic and alkaline media. Likewise, excellent HER catalysis was exhibited by the MOF-derived carbon matrix with the highest iron loading pre-carbonization and more disperse nanorods. Not only does the amenability of MOFs make them a good platform for the direct inclusion of essential electrochemically active moieties, but it also allows for more targeted, nuanced, and rational design of materials needed to enhance proton conduction and electrochemical performance, particularly in cases on non-precious metal electrocatalysts where mechanisms are often not well-understood.

Pt-based alloy surfaces are used to catalyze the electrochemical oxygen reduction reaction (ORR), where molecular oxygen is converted into water on fuel cell electrodes. In this work, we address challenges due to the cost of high Pt loadings in the cathode electrocatalyst, as well as those arising from catalyst durability. We aim to develop an increased understanding of the factors that determine ORR activity together with stability against surface segregation and dissolution of Pt-based alloys. We firstly focus on the problem of determining surface atomic distribution resulting from surface segregation phenomena. We use first-principles density functional theory (DFT) calculations on PtCo and Pt3Co overall compositions, as well as adsorption of water and atomic oxygen on PtCo(111) and Pt-skin structures. The bonding between water and surfaces of PtCo and Pt-skin monolayers are investigated in terms of orbital population. Also, on both surfaces, the surface reconstruction effect due to high oxygen coverage and water co-adsorption is investigated. Although the PtCo structures show good activity, a large dissolution of Co atoms tends to occur in acid medium. To tackle this problem, we examine core-shell structures which showed improved stability and activity compared to Pt(111), in particular, one consisting of a surface Pt-skin monolayer over an IrCo or Ir3Co core, with or without a Pd interlayer between the Pt surface and the Ir-Co core. DFT analysis of surface segregation, surface stability against dissolution, surface Pourbaix diagrams, and reaction mechanisms provide useful predictions on catalyst durability, onset potential for water oxidation, surface atomic distribution, coverage of oxygenated species, and activity. The roles of the Pd interlayer in the core-shell structures that influence higher ORR activity are clarified. Furthermore, the stability and activity enhancement of new shell-anchor-core structures of Pt/Fe-C/core, Pt/Co-C/core and Pt/Ni-C/core are demonstrated with core materials of Ir, Pd3Co, Ir3Co, IrCo and IrNi. Based on the analysis, Pt/Fe-C/Ir, Pt/Co-C/Ir, Pt/Ni-C/Ir, Pt/Co-C/Pd3Co, Pt/Fe-C/Pd3Co, Pt/Co- C/Ir3Co, Pt/Fe-C/Ir3Co, Pt/Co-C/IrCo, Pt/Co-C/IrNi, and Pt/Fe-C/IrNi structures show promise in terms of both improved durability and relatively high ORR activity.

Controlling catalyst morphology and composition are required to make meaningful structure-activity/stability relationships for the design of future catalysts. Herein, we have employed strategies of presynthesis and infusion or electroless deposition to achieve exquisite control over catalyst composite morphology. The oxygen reduction (ORR) and the oxygen evolution reactions (OER) were chosen as model systems, as their slow kinetics is a major limiting factor preventing the commercialization of fuel cells and rechargeable metal air batteries. In acid, bimetallic (Pt-Cu, Pd-Pt) and monometallic (Pt) catalysts were presynthesized in the presence of capping ligands. Well alloyed Pt-Cu nanoparticles (3-5 nm) adsorbed on graphitic mesoporous carbon (GMC) displayed an ORR activity >4x that of commercial Pt. For both presynthesized Pt and Pt-Cu nanocrystals on GMC, no activity loss was also observed during degradation cycling due to strong metal-support interactions and the oxidation resistance of graphitic carbon. Similar strong metal-support interactions were achieved on non-graphitic carbon for Pd3Pt2 (<4 nm) nanoparticles due to disorder in the metal surface This led to enhanced mass activity 1.8x versus pure Pt, as well as improved stability. For basic electrolytes, we developed an electroless co-deposition scheme to deposit Ag (3 nm) next to MnOx nanodomains on carbon. We achieved a mass activity for Ag-MnOx/VC, 3x beyond the linear combination of pure component activities due to ensemble effects, where Ag and MnOx domains catalyze different ORR steps, and ligand effects from the unique electronic interaction at the Ag-MnOx interface. Activity synergy was also shown for Ag-Pd alloys (~5 nm), achieving up to 5x activity on a Pd basis, resulting from the unique alloy surface of single Pd atoms surrounded by Ag. Lastly, we combined arrested growth of amorphous nanoparticles with thin film freezing to create a high surface area, pure phase perovskite aggregate of nanoparticles after calcination. Sintering was mitigated during the high temperature calcination required to form the perovskite crystals. The high surface areas and phase purity led to OER mass activities ~2.5x higher than the benchmark IrO2 catalyst. / text

Solid oxide fuel cells (SOFCs) are electrochemical devices that can convert a variety of fuels directly into electricity. Their commercialization requires efficient operation of its components. The sluggish kinetics for the oxygen reduction reaction (ORR) at the SOFC cathode contributes to the loss in the fuel cell efficiency. In this work, the ORR was investigated for the strontium-doped lanthanum manganite cathode (LSM) and yttria-stabilized zirconia electrolyte (YSZ) system. A combined mathematical modelling and experimental framework was developed to estimate, for the first time, the kinetics of the elementary processes of the ORR for porous LSM cathodes. The kinetics of each process was then analyzed to identify the contribution to the cathode resistance. The steady state and impedance response for polarized and unpolarized LSM cathodes was collected over a temperature range between 750C and 850C and two different oxygen partial pressure (pO2) ranges: (i) between 0.0001atm and 0.001atm, where LSM is considered to be stoichiometric with respect to oxygen and (ii) between 0.01atm and 0.21atm, where LSM is considered to be superstoichiometric with respect to oxygen. A mathematical model was developed to analyze both the steady state and impedance data. Two pathways for the ORR were considered: one where oxygen is transported in the gas phase and one where oxygen is transported along the surface of the LSM cathode. Rate constants, transport coefficients and their respective activation energies were obtained for the adsorption/desorption, surface diffusion and charge transfer processes. The experimental results indicated different polarization behavior between low and high pO2. It is hypothesized that the concentration of cation vacancies on the LSM surface changes with both pO2 and extent of polarization and that cation vacancies on the LSM surface can promote the ORR. Modelling results at low pO2 suggested that the adsorption reaction was slow and that thermodynamic limitations resulting in low equilibrium oxygen surface coverage can play an important role at both low and high polarizations. Modelling in high pO2 was complicated by the nature of the LSM surface in these conditions and suggests an electrochemical reaction at the gas/LSM interface and the transport of charged adsorbed oxygen atoms. / Thesis (Ph.D, Chemical Engineering) -- Queen's University, 2009-12-31 11:53:23.535

A cinética da reação de redução de oxigênio (RRO) foi estudada em eletrólito alcalino em diferentes eletrocatalisadores. As atividades eletrocatalíticas medidas experimentalmente em metais puros foram correlacionadas com propriedades eletrônicas do eletrocatalisador, como o centro em energia da banda d (&#949;d). A RRO também foi investigada em Pt e Ag modificados pela formação de liga com outros metais de transição. Por fim, a RRO foi conduzida em diferentes óxidos de manganês sintetizados em laboratório, com o objetivo de se determinar a fase mais ativa e o mecanismo da reação nos diferentes óxidos. A atividade eletrocatalítica frente à RRO dos metais puros ou das monocamadas de platina suportadas em diferentes substratos metálicos monocristalinos apresentou uma dependência tipo “vulcão” em função da energia do centro da banda d do metal eletrocatalisador. Estes resultados indicam que tanto a quebra da ligação O-O como a hidrogenação dos intermediários reacionais têm que ser facilitados, de forma que a cinética das duas reações, as quais são aceleradas por propriedades antagônicas, seja otimizada. O ganho de atividade para a RRO observado para as ligas de Pt em relação à Pt pura foi associado à menor reatividade da Pt na ligas, o que leva à uma menor força da adsorção Pt-O- e, consequentemente, maior cinética de eletroredução dos intermediários oxigenados. A maior atividade das ligas de Ag comparada com a Ag pura foi atribuída à uma mais forte adsorção Ag-O-, o que resulta em maior cinética da quebra da ligação O-O. Os resultados para os diferentes óxidos de Mn mostraram que a ativação para a RRO é maior para os materiais com alto conteúdo de MnO2. A atividade eletrocatalítica dos óxidos de manganês foi associada com um mecanismo acoplado envolvendo uma mudança do estado de oxidação de Mn (IV) para Mn (III), com a transferência de elétrons do Mn (III) para o O2 adsorvido [], em processo via 2 elétrons, com subsequente reação de desproporcionamente do intermediário HO2-, recirculando O2 no sistema, tendendo à um mecanísmo global via 4 elétrons por molécula de O2. / The oxygen reduction reaction (ORR) was studied on electrodes formed by Pt monolayers deposited on different metallic substrates, and on carbon-supported electrocatalysts composed by PtM/C (M = V, Cr e Co) and AgPt/C alloys, and on different Mn oxides (Mn3O4/C, Mn2O3/C and MnO2/C) in alkaline electrolyte. The experimentally measured electrocatalytic activities of the different metal catalysts and of the Pt monolayers were plotted against the metal d-band center values (_d). In all cases, the electronic features of the metal electrocatalysts were used for understanding the catalytic activities, and trying to establish the electronic/ORR kinetics relationship. The XANES results for the PtM/C alloys at high electrode potentials have shown lower vacancy of the Pt 5d band compared to pure Pt/C, indicating lower Pt reactivity for adsorbates. The enhanced catalytic activity of Pt in the alloys was attributed to a faster electroreduction of oxygenated intermediates. For the AgPt/C alloys, the XANES results have shown a emptying of the Ag orbitals due to a charge transfer to Pt, and the increased activity of the Ag atoms was ascribed to an electronic effect induced by the presence of Pt, increasing the Ag-O adsorption strength For the manganese oxide materials, the XANES results indicated a chance of the Mn oxidation state as a function of the electrode potential, and higher electrocatalytic activity was observed for MnO2/C. This was explained based on the activation for the ORR, which is higher for the material with higher MnO2 contents and the occurrence of a mediation processes involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen.

A Reação de Redução de Oxigênio (RRO) foi estudada em eletrocatalisadores constituídos por platina e paládio suportados em carbono em eletrólito de H2SO4. A caracterização estrutural foi realizada por DRX (difratometria de raios X) e a composição dos catalisadores foi determinada por EDX (espectroscopia de energia dispersiva de raios X). As propriedades eletrônicas foram investigadas por XAS (X-ray absorption spectroscopy) in situ, na região de XANES (X-ray absorption near edge structure). As atividades dos catalisadores foram comparadas por curvas de Tafel corrigidas por transporte de massa. Os materiais formados por Pt-Pd suportados em carbono mostraram maior atividade específica para a RRO quando comparados com os catalisadores constituídos somente por platina suportada em carbono. A maior atividade foi relacionada a uma menor força de adsorção de espécies oxigenadas nesses catalisadores. / Oxygen reduction reaction (ORR) on platinum-palladium electrocatalysts supported on carbon was studied in H2SO4 electrolyte. The structure of the catalysts was characterized by XRD (X-ray diffractometry) and the composition was determined by EDX (Energy dispersive X-ray spectroscopy). The electronic properties were investigated by in situ XAS (X-ray absorption spectroscopy) in the XANES (X-ray absorption near edge structure) region. Mass-transport corrected Tafel plots were used to compare the electrocatalytic activity of the materials. The Pt-Pd catalysts have shown higher specific electrocatalytic activity than Pt only catalysts. The higher activity of Pt-Pd catalysts was attributed to a lower adsorption energy of oxygenated species in these catalysts.

Ce manuscrit présente le développement de nouveaux complexes de cuivre conçus comme catalyseurs. Ceux-ci ont été développés en s’inspirant des sites actifs des enzymes à cuivre comme les oxydases à cuivre. Trois complexes binucleaires de cuivre ont été synthétisés et caractérisés, avec une étude approfondie de leur comportement redox pour évaluer leur utilisation potentielle comme catalyseurs de la réduction de l’oxygène. Ces études, couplées avec des analyses spectroscopiques ont démontré l’influence de l’environnement de coordination sur la stabilité des complexes. En fait, quand les centres de Cu(II) sont coordonnés par de ligands trop faibles, ils ont tendance à se réduire spontanément en solution, produisant des mélanges de différentes espèces impossibles à purifier. D’autre part, quand les centre Cu(II) sont stabilisés dans des structures plus rigides le complexe montre des transferts d’électrons plus lents, une caractéristique peu désirable dans le développement de catalyseurs de la réduction de l’oxygène. Pour ces raisons, des ligands plus sophistiqués ont été synthétisés avec le but d’obtenir des complexes trinucléaires de cuivre capables de catalyser efficacement la réaction de réduction de l’oxygène. Un des complexes binucléaires développés a été testé comme catalyseur de transfert de nitrene, en particulier dans la synthèse d’aziridines. Le complexe montre une bonne performance catalytique à la fois sous les formes bis-Cu(II) et bis-Cu(I). L’activité catalytique a été analysée aussi du point de vue mécanistique et des études préliminaires suggèrent la présence de différents mécanismes d’interaction entre les donneurs de nitrene et les deux formes du complexe. / This manuscript presents the development of new copper complexes for catalytic purposes. The design of the complexes takes inspiration from the active sites of multicopper oxidases. Three binuclear copper complexes have been synthesized and characterized, with a focus on their electrochemical behaviour and their potential use as catalysts for oxygen reduction. Spectroscopic and electrochemical studies revealed the importance of the coordination environment on the stability of the complexes. Cu(II) centers coordinated by not strong enough ligands are spontaneously reduced in solution, giving rise to mixtures of different complexes. At the same time more rigid structures capable of stabilising the two copper centers give rise to slow electron transfers, an undesirable characteristic in electrocatalysts. For these reasons more complex ligands have been synthesized, with the aim of obtaining trinuclear copper complexes with good properties as catalysts for dioxygen reduction. One of the binuclear complexes have been tested as oxidation catalyst in nitrene transfer reactions. The complex shows good performance in aziridination catalysis, both in its bis-Cu(II) and bis-Cu(I) forms. The catalytic activity have been analysed also from the mechanistic point of view. Preliminary results suggest different mechanisms of interaction of the reduced and oxidized forms of the complex with the nitrene donors.