Exemplos de comportamento complexo podem ser encontrados em vários sistemas na natureza, como dinâmica de populações, formação de padrões em pelos de animais e a auto regulação do metabolismo. Em geral, sistemas químicos e eletroquímicos apresentam também esse tipo de comportamento complexo e são comumente escolhidos como sistemas-modelo para estudar esses aspectos, uma vez que as variáveis são facilmente controláveis e as medidas são precisas. Apesar dos esforços realizados nas últimas duas décadas, existem ainda lacunas a serem preenchidas entre o mecanismo de reação e a teoria que explica as oscilações. Visando contribuir para o assunto, o presente trabalho trata da eletro-oxidação de etileno glicol sobre platina em meio alcalino em regime potenciostático e galvanostático. Utilizando Espectroscopia de Infravermelho por Transformada de Fourier in situ acoplada com técnicas eletroquímicas, é observado um decréscimo na razão entre a produção de carbonato/oxalato quando o raio dos cátions alcalinos é diminuído na seguinte ordem: Li+ < Na+ < K+. A origem de tal tendência pode ser dada em termos das interações não-covalentes entre os cátions e as espécies oxigenadas adsorvidas na superfície. Os aspectos não-lineares da reação são estudados aumentando a resistência entre o eletrodo de trabalho e o potenciostato. Oscilações com frequências tão altas quanto 40 Hz são observadas, sendo que estas se tornam mais lentas e complexas com o aumento da resistência, mimetizando o comportamento em controle galvanostático. O pH mostrou-se fundamental para o aparecimento do comportamento oscilatório, observado somente nos valores de pH 0, 2 e 14. Baixas correntes de oxidação e baixas frequências de oscilação em meio ácido são contrastadas com altas correntes e frequências em meio alcalino. Além disso, o efeito da temperatura é testado tanto em regime voltamétrico quanto nas séries temporais oscilatórias em pH ácido e alcalino. Em meio ácido, a energia de ativação decresce à medida que o potencial torna-se próximo ao potencial de pico e esta heterogeneidade está, provavelmente, conectada à diferentes rotas reacionais. A energia de ativação calculada através da frequência remete a valores intermediários aos encontrados em regime voltamétrico. Apesar de valores comuns encontrados para os termos de ativação sob regime voltamétrico em meio alcalino, as frequências de oscilação se mostraram quase invariantes com a temperatura, indicando que o sistema apresenta a chamada compensação de temperatura. / Examples of complex behavior can be found in several systems in nature such as population dynamics, animal coat patterns formation and metabolism self-assembly. In general, chemical and electrochemical systems also display this complex behavior and are commonly chosen as workhorses to study these aspects, once the variables are easily controlled and the measurements are precise. Despite the efforts made in this area in the last two decades, there are still gaps between the reaction mechanism and the theory underlying the oscillations. To shed some light on the non-linear aspects of alcohol electro-oxidation, the present work deals with the study the ethylene glycol electro-oxidation reaction in platinum and alkaline media from both gavanostatic and potenciostático control. By means of in situ Fourier Infrared Spectroscopy coupled with electrochemical techniques, it is found a decrease in the ratio of carbonate/oxalate production when the alkaline cations size decreases in the following order: Li+ < Na+ < K+. The origin of this tendency can be rationalized in terms of non-covalent interaction between cations and adsorbed oxygenated species. These interactions are proportional to the cation size and it influences directly the number of available free sites for alcohol adsorption. The non-linear aspects of the reaction are also studied increasing the total resistance between the working electrode and the potenciostat. It is observed oscillation frequencies as high as 40 Hz. Those non-linearities turned slower and more complex as the resistance increases, mimicking the behavior observed under galvanostatic control. The pH is a decisive parameter to the oscillatory behavior, observed only at pH 2, 4 and 14. Low oxidation currents under potential control followed by low frequency oscillations observed in acid media are in contrast to the high current and frequency in alkaline. Moreover the temperature effect is tested in both cyclic voltammetry profile and oscillatory time series in acid and alkaline media. In acid media, the activation energy decrease as the potential turns closer to the peak potential region, this heterogeneity is probably caused by different reactions. The activation energy calculated by oscillation frequency remits to intermediates values in relation to that found under voltammetry control. Although ordinary values for the activation factors are found under linear voltammetry control mode at pH 14, the oscillations frequencies are almost independent of the temperature, indicating that the system shows the, so called, temperature compensation.

Os problemas ambientais originados pela produção e consumo das tradicionais fontes de energia pressionam a sociedade pelo desenvolvimento e utilização de fontes de energia limpas e renováveis. Dentre as novas tecnologias, células a combustível de membrana de troca protônica (PEMFC) apresentam-se como uma alternativa viável, aliando elevadas taxas de conversão energética a níveis mínimos de poluentes gerados. No entanto, a utilização plena desses dispositivos depende de fatores como desempenho, estabilidade e custo dos mesmos. Um dos principais elementos que afeta o desempenho de uma PEMFC é a eficiência com o qual o oxigênio é reduzido no cátodo. Assim, diversos estudos visando a obtenção de eletrocatalisadores à base de Pt que apresentem bom desempenho frente à reação de redução de oxigênio (RRO) e elevada estabilidade têm sido desenvolvidos, recebendo destaque os catalisadores Pt-Y. Neste trabalho catalisadores Pt-Y, com diferentes proporções entre os metais, suportados em carbono de alta área superficial foram sintetizados através de uma modificação no método do ácido fórmico. Os materiais foram caracterizados através das técnicas de EDX, XRD, TEM e XPS, e avaliados frente à RRO através de medidas em meia célula, empregando-se a técnica de eletrodo de disco rotatório, e em célula unitária, como cátodo de PEMFC. O catalisador Pt-Y/C 7:3 foi o que apresentou melhor desempenho dentre os materiais bimetálicos. Através dos testes de envelhecimento acelerado (TEA) constatou-se que, além da degradação das nanopartículas de platina, o ítrio passa por dissolução. / The environmental impacts generated by the production and consumption of traditional energy sources leads society to develop clean and renewable energy sources. Among the new technology, proton exchange membrane fuel cells (PEMFC) appear as viable alternative, allying high energy conversion rates to minimum levels of pollutants generated. However, the full utilization of these devices depends on factors such as its performance, stability and cost. One of the major elements that affect the PEMFC performance is the cathode performance. Thus, several studies aiming at obtaining Pt based electrocatalysts with good performance for oxygen reduction reaction (ORR) and high stability have been developed, receiving attention the Pt-Y catalysts. In this work Pt-Y catalysts, supported on high surface area carbon, were synthesized through a modification on formic acid method. The materials were characterized using EDX, XDR, TEM and XPS, and evaluated towards ORR through measurements in half cell, using rotating disk electrode technique, and in unit cell, as cathode of PEMFC. The catalyst Pt-Y/C 7:3 had the best performance among the bimetallic materials. Through accelerated aging tests (AGT), in addition to platinum nanoparticles degradation, yttrium dissolution was also observed.

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.

Progress in nanotechnology has promoted an increasing interest in the rational design of the emerging hierarchical aerogels, which represents a second stage of the NC-based aerogel research. By fine-tuning the surface properties of the backbones, metallic hierarchical aerogels are able to address the growing demands of advanced electrocatalysts. In this dissertation, three types of metallic hierarchical aerogels were designed by introducing different nanostructures (i.e. hollow, porous/dendritic and core-shell) and alloy effects (with noble or transition metals) into the aerogels. Thus, as a proof-of-concept for fuel cells, advanced electrocatalytic performances have been achieved on the resulting metallic hierarchical aerogels towards both anode (oxidation of ethanol) and cathode (reduction of oxygen) reactions. First, alloyed PdxNi hollow nanospheres with controlled composition and shell thickness were utilized as building blocks for the design of hierarchical aerogels. The combination of transition-metal doping, hollow interior, as well as the 3D aerogel structure make the resulting aerogels promising electrocatalysts for ethanol oxidation with a mass activity up to 5.6-fold higher than that of the Pd/C. Second, continuously shape-engineering of the building blocks (ranging from hollow shells to dendritic shapes) was achieved by the synthesis of a series of multimetallic Ni-PdxPty hierarchical aerogels. By optimization of the nanoscale morphology and the chemical composition, the Ni-Pd60Pt40 aerogel exhibits remarkable electrocatalytic activity for oxidation of ethanol. Moreover, the particle growth mechanism underlying the galvanic replacement was revealed in terms of nanowelding of the nanoparticulate reaction intermediates based on experimental and theoretical results. Third, a universal approach was demonstrated for core-shell structuring of metallic aerogels by coating of an ultrathin Pt shell on a composition-tunable Pd-based alloyed core. Their activities for oxygen reduction exhibit a volcano-type relationship as a function of the lattice parameter of the core substrate. Largely improved Pt utilization efficiency was accomplished based on the core-shell motifs, as the mass activity reaches 5.25 A mg-1Pt which are 18.7 times higher than those of Pt/C. Different from the conventional aerogels with nanowire-like backbones, those hierarchical aerogels are generally comprised of at least two levels of architectures, i.e. an interconnected porous structure on the macroscale and a specially designed configuration at local backbones at the nanoscale. This combination “locks in” the inherent properties of the NCs, so that the beneficial genes obtained by nano-engineering are retained in the resulting monolithic hierarchical aerogels. These results expand the exploitation approach of the electrocatalytic properties of aerogels into morphology control of their NBBs and are of great importance for the future development of aerogels for many other electrochemical reactions.

A sustainable solution for meeting the energy demands at our planet is by utilizing wind-, solar-, wave-, thermal-, biomass- and hydroelectric power. These renewable and CO2 emission-free energy sources are highly variable in terms of spatial and temporal availability over the Earth, introducing the need for an appropriate method of storing and carrying energy. Hydrogen has gained significant attention as an energy storage- and carrier media because of the high energy density that is exploited within the ‘power-to-gas’ process chain. A robust way of producing sustainable hydrogen is via electrochemical water splitting. In this work the search for new heterogeneous catalyst materials with the aim of increasing energy efficiency in water splitting has involved methods of both electrochemical water splitting and chemical water oxidation. Some 21 compounds including metal- oxides, oxofluorides, oxochlorides, hydroxide and metals have been evaluated as catalysts. Two of these were synthesized directly onto conductive backbones by hydrothermal methods. Dedicated electrochemical cells were constructed for appropriate analysis of reactions, with one cell simulating an upscale unit accounting for realistic large scale applications; in this cell gaseous products are quantified by use of mass spectrometry. Parameters such as real time faradaic efficiency, production of H2 and O2 in relation to power input or overpotentials, Tafel slopes, exchange current density and electrochemical active surface area as well as turnover numbers and turnover frequencies have been evaluated. Solubility, possible side reactions, the role of the oxidation state of catalytically active elements and the nature of the outermost active surface layer of the catalyst are discussed. It was concluded that metal oxides are less efficient than metal based catalysts, both in terms of energy efficiency and in terms of electrode preparation methods intended for long time operation. The most efficient material was Ni-Fe hydroxide electrodeposited onto Ni metal foam as conductive backbone. Among the other catalysts, Co3Sb4O6F6 was of particular interest because the compound incorporate a metalloid (Sb) and redox inert F and yet show pronounced catalytic performance. In addition, performance of materials in water splitting catalysis has been discussed on the basis of results from electron microscopy, solubility experiments and X-ray diffraction data.

Nickel-based catalysts in aqueous alkaline media are low-cost electrode materials for electrolytic hydrogen generation, a renewable method of producing fuel and industrial feedstock. However, further work is necessary to develop inexpensive electro-catalyst materials with high activity and long-term stability. This thesis employs spectroscopic and electrochemical methods to directly address specific research problems for the development of improved materials and devices with commercial or industrial value. The first chapter reviews the applications of nickel electrodes; the structures of nickel, nickel hydroxides, and nickel hydrides; and techniques for measuring the electrochemically active surface area (AECSA) of nickel. In the second chapter, electrochemically precipitated nickel hydroxide materials are fully characterized by Raman spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). This work unifies and simplifies the large body of literature on the topic by considering two fundamental phases, α- and β-Ni(OH)2, and various types and extents of structural disorder. The third chapter examines and demonstrates the potential applications of in situ Raman spectroscopy by monitoring the spontaneous ageing of α-Ni(OH)2 to β-Ni(OH)2 in pure water at room temperature. The fourth chapter considers the longstanding problem of electrode deactivation, the gradual decrease in nickel electro-catalyst activity during prolonged hydrogen production. Voltammetric and XRD evidence demonstrates that hydrogen atoms can incorporate into the electrode material and cause structural disorder or the formation of α-NiHx and β-NiHx at the surface. The voltammetric formation of NiOx, α-Ni(OH)2, β-Ni(OH)2, and β-NiOOH surface species are examined by electrochemical and XPS measurements. The fifth chapter of this thesis presents a new method to measure the AECSA by adsorption of oxalate to the (001) surface of the surface Ni(OH)2, as evidenced by voltammetric and attenuated total reflectance (ATR) FT-IR spectroscopy measurements. The adsorbed oxalate limits the surface hydroxide to a single layer. The surface NiOOH/Ni(OH)2 reduction peak during the reverse scan may be used to accurately and precisely measure the AECSA. The error of this method is estimated at < 10 %.

abstract: Chemical modification of (semi)conducting surfaces with soft-material coatings containing electrocatalysts provides a strategy for developing integrated constructs that capture, convert, and store solar energy as fuels. However, a lack of effective strategies for interfacing electrocatalysts with solid-state materials, and an incomplete understanding of performance limiting factors, inhibit further development. In this work, chemical modification of a nanostructured transparent conductive oxide, and the III-V semiconductor, gallium phosphide, is achieved by applying a thin-film polymer coating containing appropriate functional groups to direct, template, and assemble molecular cobalt catalysts for activating fuel-forming reactions. The heterogeneous-homogeneous conducting assemblies enable comparisons of the structural and electrochemical properties of these materials with their homogeneous electrocatalytic counterparts. For these hybrid constructs, rational design of the local soft-material environment yields a nearly one-volt span in the redox chemistry of the cobalt metal centers. Further, assessment of the interplay between light absorption, charge transfer, and catalytic activity in studies involving molecular-catalyst-modified semiconductors affords models to describe the rates of photoelectrosynthetic fuel production as a function of the steady-state concentration of catalysts present in their activated form. These models provide a conceptual framework for extracting kinetic and thermodynamic benchmarking parameters. Finally, investigation of molecular ‘proton wires’ inspired by the Tyrosine Z-Histidine 190 redox pair in Photosystem II, provides insight into fundamental principles governing proton-coupled electron transfer, a process essential to all fuel-forming reactions relevant to solar fuel generation. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2020

In the context of the future hydrogen economy, effective production of hydrogen (H2) from readily available and sustainable resources is of crucial importance. Hydrogen generation via water splitting by solar energy or electricity has attracted great attention in recent years. In comparison with photocatalytic water-splitting directly using solar light, which is ideal but the relevant technologies are not yet mature, electrolysis of water with catalyst is more practical at the current stage. The Pt-group noble metals are the most effective electrocatalysts for hydrogen evolution reaction (HER) from water, but their high costs limit their applications. Due to the earth-abundance and low price, MoS2 is expected to be a good alternative of the Pt-group metals for HER. Plenty of researches have been conducted for improving the HER activities of MoS2 by optimizing its synthesis method. However, it remains challenging to prepare MoS2 catalysts with high and controllable activity, and more investigations are still needed to better understand the structure-performance correlation in this system. In this thesis, we report a new strategy for fabricating MoS2 eletrocatalysts which gives rise to much improved HER performance and allows us to tune the electrocatalytic activity by varying the preparation conditions. Specifically, we sulfurized molybdenum oxide on the surface of a Ti foil electrode via a facile chemical vapor deposition (CVD) method, and directly used the electrode for HER testing. Depending on the CVD temperature, the MoO2-MoS2 nanocomposites show different HER activities. Under the optimal synthesis condition (400ºC), the resulting catalyst exhibited excellent HER activity: an onset potential (overpotential) of 0.095 V versus RHE and the Tafel slope of 40 mv/dec. Such a performance exceeds those of most reported MoS2 based HER electrocatalysts. We demonstrated that the CVD temperature has significant influence on the catalysts in crystallinity degree, particle size and dispersion, morphology, and density of the edge sites etc., and these factors in turn determine the HER activity.

No description available.

Le SmI₂ en tant que réactif de transfert monoélectronique a été largement utilisé en chimie organique depuis les premiers travaux de Kagan. Cependant, la quantité stœchiométrique ou en excès de SmI₂ et d'additifs toxique tels que HMPA sont utilisés normalement pour améliorer la réactivité. De plus, à cause de sa sensibilité à l'oxygène, le stockage de la solution de SmI₂ dans le THF est difficile. Récemment, nous avons développé une nouvelle méthode électrocatalytique basée sur la régénération électrochimique de Sm²⁺. Par rapport à la réactivité du SmI₂ classique, notre approche utilise une quantité catalytique de Sm. Premièrement, pour la réduction de nitroarènes, la réaction a sélectivement fourni les composés aromatiques azoïques et les anilines en fonction du solvant choisi. Notamment, c'est la première fois que la réaction Sm²⁺ se produit dans le méthanol dans le cas des anilines. Deuxièmement, dans le cas de la réduction des sulfoxydes par SmI₂, en général, l'HMPA était nécessaire comme additif. Dans notre procédé électrocatalytique, les sulfoxydes ont été transformés en sulfures avec une chimiosélectivité élevée et des excellents rendements toujours à température ambiante sans besoin ni de HMPA ni d’atmosphère protectrice. Enfin, les dérivés d'isoindolinone sont des séries de produits importants en chimie organique, la réduction des phtalimides est l'approche la plus pratique pour les obtenir. Avec les alcools, l'alcoxylation réductrice de phtalimides a eu lieu pour la première fois avec le Sm²⁺ électrocatalytique dans nos conditions. Et si on ajoute d'autres sources de protons, ce procédé a fourni les ω-hydroxylactames et isoindolinones correspondants avec des rendements élevés. / The SmI₂ as a single electron transfer reagent has been widely used in organic chemistry since the pioneering works by Kagan. However, the stoichiometric or excess amount of SmI₂ and harmful additives such as HMPA are used normally to enhance the reactivity, moreover, due to the oxygen sensitive, the storage of SmI₂ solution is difficult.Recently, we have developed a new electrocatalytic method based on the electrochemical regeneration of Sm²⁺. Compared to the classic SmI₂ reaction, our process occurred with a catalytic amount of Sm. In the reduction of nitroarenes, it selectively afforded the azo aromatic compounds and anilines depending on different solvents system. Notably, it’s the first time that the Sm²⁺ reaction occurred in the methanol. Normally, the HMPA was the additive in the reduction of sulfoxides by SmI₂. Under our electrocatalytic process, the sulfoxides were converted into sulfides in high chemoselectivity and yield at room temperature without HMPA and protecting atmosphere.The isoindolinone derivatives are series of important products in organic chemistry, the reduction of phthalimides is the most convenient approach to provide them. With alcohols, the unprecedented Sm²⁺ electrocatalyzed reductive alkoxylation of phthalimides was established. Moreover, adding other proton sources, this process afforded the corresponding ω-hydroxylactams and isoindolinones.