Direct alcohol fuel cells (DAFC) are attracting considerable interest to meet a variety of energy needs as they offer higher efficiency with less pollution compared to other conventional energy-conversion devices. However, the sluggish alcohol oxidation reaction kinetics and durability problems of the conventional Pt-Ru anode catalyst hamper the commercialization of the DAFC systems. With an aim to overcome these problems, there have been intensive efforts to alloy Pt-Ru with other metals. Although such strategies have led to some enhancement in activity, the durability problem caused by the instability of Ru could still not be alleviated. In this regard, this dissertation focuses on the development of non-Ru electrocatalysts with high activity and durability for DAFC applications. First, Ru-free, Pt-based bimetallic electrocatalysts for methanol oxidation reaction (MOR) were studied. Particularly, Pt-Sn and Pt-CeO₂ catalysts were synthesized, respectively, by a polyol method and a one-step reverse microemulsion (RME) method. The prepared samples are investigated for phase and morphological evaluations by various material-characterization techniques. Cyclic voltammetry and accelerated durability tests revealed that these alternative catalysts have much higher stability with a catalytic activity for MOR comparable to that of Pt-Ru. In the case of Pt-CeO₂, an improved particle morphology is obtained by the RME synthesis, and the advantage of the RME method is reflected by a higher catalytic activity in comparison to that of Pt-CeO₂ synthesized by the conventional synthesis method. It has been known that Pt-Sn is better than Pt-Ru for ethanol oxidation reaction (EOR), and the direct ethanol fuel cells (DEFC) employing Pt-Sn as the anode catalyst have better durability than the DMFC system employing a Pt-Ru anode catalyst. Therefore, this dissertation then focused on the enhancement of the catalytic activity for EOR by incorporating a third metal M to the Pt-Sn catalyst. Following the synthesis and characterization of the Pt-Sn-M (M = Mo and Pd) alloys, the effect of M on the enhanced catalytic activity of Pt-Sn-M is presented. The activity enhancement of the above catalysts is based on the promoting effect of the second or third elements added to Pt. However, in the final chapter of this dissertation, the activity enhancement of Pt nanoparticle itself through the formation of low energy surfaces is investigated. Carbon-supported Pt nanoparticles are synthesized in mixed water-ethylene glycol solvent, and the positive effect of the mixed solvent on both the morphology and surface structure of the Pt nanoparticles for COad oxidation is discussed. / text

Este trabalho mostra estudos das propriedades eletroquímicas do eletrodo de diamante dopado com boro (DDB) e descreve a utilização de eletrodos de DDB, modificados direta ou indiretamente pelo método Sol-gel, para a oxidação de metanol e de etanol em meio ácido. Do estudo das propriedades superficiais do diamante concluiu-se que a polarização catódica é bastante apropriada para um bom desempenho do material é que não introduz diferenças estruturais detectáveis no filme de DDB, indicando que a melhora na resposta eletroquímica é devida somente a mudanças superficiais. Entretanto, polarizações catódicas consecutivas e extensivas por longos períodos de tempo produzem uma erosão da superfície do eletrodo. Adicionalmente, estudos realizados usando voltametria cíclica e espectroscopia fotoeletrônica de raios X, mostraram que a superfície de eletrodos de DDB pré-tratados catodicamente tem um comportamento eletroquímico dinâmico, apresentando uma perda da reversibilidade para o sistema Fe(CN)63-/4- em função do tempo de exposição ao ar. Isto é devido, possivelmente, à perda de hidrogênio superficial decorrente da oxidação da superfície do eletrodo pelo oxigênio do ar. Este comportamento dinâmico da superfície do DDB é inversamente dependente ao teor de boro do eletrodo. Foi observado também que a quantidade de sítios ativos disponíveis na superfície do eletrodo é diretamente proporcional à dopagem do mesmo e deve ser conseqüência da quantidade de boro superficial. Desses estudos concluiu-se que todos os eletrodos de DDB com distintas dopagens usados nesta tese (300, 800, 2000 e 8000 ppm de B) apresentam uma superfície eletroquímica heterogênea (sítios ricos em boro que apresentariam uma alta condutividade e sítios de diamante com terminação hidrogênio com menor condutividade), quando polarizados catodicamente e parcialmente bloqueada (diamante com terminações oxigênio) quando polarizados anodicamente, formando arranjos de microeletrodos (provavelmente sítios ricos em boro). As superfícies do diamante foram modificadas diretamente com Pt, Pt-RuO2 e Pt-RuO2-RhO2 pelo método Sol-gel. Estes depósitos de catalisadores apresentavam tamanhos nanométricos e mostraram um bom contato elétrico com a superfície do diamante e elevada pureza, assim como, uma composição atômica bem controlada e uma distribuição homogênea na superfície do DDB. Os estudos da oxidação eletroquímica de metanol e de etanol sugerem fortemente que o envenenamento da superfície do eletrodo é grandemente inibido sobre o catalisador ternário Pt-RuO2-RhO2/DDB quando comparado com os eletrodos Pt-RuO2/DDB e Pt/DDB. Adicionalmente, esse eletrodo mostra uma excelente atividade catalítica para a oxidação de etanol, provavelmente devido à habilidade que o ródio tem para quebrar a ligação C-C somada ao mecanismo bifuncional que acontece no catalisador contendo Pt e Ru. Em seguida, realizaram-se modificações indiretas do DDB pela síntese de compósitos de pó de carbono modificados com metais e óxidos metálicos e posterior fixação usando uma solução de Nafion. A oxidação dos álcoois foi estudada por voltametria cíclica e curvas de polarização em estado estacionário (diagramas de Tafel). Os sistemas investigados incluíram misturas metálicas binárias, ternárias e quaternárias contendo Pt, Ru, Ir, Rh, PbOx, TaOx e MoOx num total de 16 sistemas diferentes. Assim, os melhores catalisadores para a oxidação de metanol foram: Pt-Ru-TaOx-PbOx > Pt-Ru-TaOx-MoOx ≈ Pt-Ru-MoOx-Ir enquanto que, para a oxidação de etanol foram Pt-Ru-TaOx-PbOx e Pt-Ru-PbOx-Rh. Consequentemente, catalisadores do tipo Pt-Ru-TaOx-M são promissores para futuras aplicações práticas. Finalmente, estudos preliminares realizados usando pó de DDB modificado diretamente com Pt-RuOx pelo método Sol-gel mostraram que este compósito apresenta uma excelente atividade catalítica para oxidação de metanol, que foi maior do que a observada sobre um catalisador comercial considerado como o estado da arte (Pt-Ru/C da E-TEK) nas mesmas condições experimentais. Desta forma, foi mostrada a possibilidade do uso de pó de DDB como suporte para eletrocatalisadores para aplicações em células a combustível que funcionem diretamente com álcoois. / The electrochemical properties of boron-doped diamond (BDD) surfaces as well as studies of the oxidation of methanol and ethanol in acid media on BDD surfaces modified with catalysts by the Sol-gel method are presented here. Studies of the surface properties of BDD electrodes revealed that a cathodic polarization (-3,0 V vs. HESS, 30 min) is necessary for a good performance of the system while no important bulk structural differences are introduced in the film indicating that the enhanced electrochemical responses brought on by the cathodic pre-treatment is only due to superficial changes. Meanwhile, repeated and extensive cathodic polarizations led to severe erosion of the electrode surface. On the other hand, studies performed using cyclic voltammetry and X-ray photoelectron spectroscopy have shown that, after a cathodic pre-treatment, the BDD electrode surface presents a dynamic behaviour that results in a loss of the reversibility towards the Fe(CN)6 4−/3− redox couple as a function of the time exposed to atmospheric conditions. This dynamic behaviour must be associated to a loss of superficial hydrogen due to oxidation by the air and is inversely proportional to the BDD doping level suggesting that the boron content has a stabilizing effect on the H-terminated surface. It was also observed that the amount of electrochemical active sites of the BDD electrode has a direct dependence with the doping level (maybe due to the amount of superficial boron). From those studies, it was concluded that all BDD electrodes used in this work having different doping levels (300, 800, 2000 and 8000 ppm of boron) posses, after cathodic polarization, electrochemical heterogeneous surfaces (i.e. boron-rich sites with high conductivity and H-terminated diamond sites with lower conductivity) while after anodic polarization the surface is partially blocked by O-terminated diamond sites, resulting in a behaviour that resembles microelectrode arrays (probably formed by the few boron-rich sites). The BDD surface was also modified directly with Pt, Pt-RuO2 and Pt-RuO2-RhO2 using the Sol-gel method. These catalysts deposits have nanometric sizes, good electrical contact with the diamond surface and high purity. They also show a well controlled atomic composition and a homogeneous distribution on the BDD surface. Methanol and ethanol electrochemical oxidation studies on those modified electrodes suggested that poisoning of the surface by undesired intermediates (most xvi probably CO) is highly inhibited on the ternary catalyst Pt-RuO2-RhO2/BDD when compared with the Pt-RuO2/BDD and Pt/BDD responses. Additionally, the observed catalytic activity for ethanol oxidation was excellent, probably due to the rhodium ability to break the C-C bond which was added to the bifunctional mechanism operating in Pt-Ru catalysts. In the sequence, indirect modifications of the BDD surface were carried out by the production of carbon powder composites modified with metals and metallic oxides by the Sol-gel method and their subsequent anchoring to the BDD using a Nafionsolution. The alcohols oxidation on these electrodes was studied by cyclic voltammetry and steady-state polarization curves (Tafel plots). The investigated systems included binary, ternary and quaternary catalysts containing Pt, Ru, Ir, Rh, PbOx, TaOx and MoOx in a total of 16 different systems The higher catalytic activity towards methanol oxidation was observed in the sequence: Pt-Ru-TaOx-PbOx > Pt-Ru-TaOx-MoOx _ Pt-Ru-MoOx-Ir while for ethanol oxidation, the catalysts activity sequence was: Pt-Ru-TaOx-PbOx _ Pt-Ru-PbOx-Rh > Pt- Ru-Rh. Consequently, it was concluded that catalysts of the type Pt-Ru-TaOx-M are very promising systems to be used in future practical applications. Finally, preliminary studies carried out using BDD powder directly modified with Pt-RuOx by the Sol-gel method showed that this material has an excellent catalytic activity towards methanol oxidation reaction, being higher than that observed on a state of the art commercial catalyst (Pt-Ru/C from E-TEK) under the same experimental conditions. Thus, the feasibility of the use of BDD powder as catalysts support in direct-alcohol fuel cell systems was clearly established.

Este trabalho mostra estudos das propriedades eletroquímicas do eletrodo de diamante dopado com boro (DDB) e descreve a utilização de eletrodos de DDB, modificados direta ou indiretamente pelo método Sol-gel, para a oxidação de metanol e de etanol em meio ácido. Do estudo das propriedades superficiais do diamante concluiu-se que a polarização catódica é bastante apropriada para um bom desempenho do material é que não introduz diferenças estruturais detectáveis no filme de DDB, indicando que a melhora na resposta eletroquímica é devida somente a mudanças superficiais. Entretanto, polarizações catódicas consecutivas e extensivas por longos períodos de tempo produzem uma erosão da superfície do eletrodo. Adicionalmente, estudos realizados usando voltametria cíclica e espectroscopia fotoeletrônica de raios X, mostraram que a superfície de eletrodos de DDB pré-tratados catodicamente tem um comportamento eletroquímico dinâmico, apresentando uma perda da reversibilidade para o sistema Fe(CN)63-/4- em função do tempo de exposição ao ar. Isto é devido, possivelmente, à perda de hidrogênio superficial decorrente da oxidação da superfície do eletrodo pelo oxigênio do ar. Este comportamento dinâmico da superfície do DDB é inversamente dependente ao teor de boro do eletrodo. Foi observado também que a quantidade de sítios ativos disponíveis na superfície do eletrodo é diretamente proporcional à dopagem do mesmo e deve ser conseqüência da quantidade de boro superficial. Desses estudos concluiu-se que todos os eletrodos de DDB com distintas dopagens usados nesta tese (300, 800, 2000 e 8000 ppm de B) apresentam uma superfície eletroquímica heterogênea (sítios ricos em boro que apresentariam uma alta condutividade e sítios de diamante com terminação hidrogênio com menor condutividade), quando polarizados catodicamente e parcialmente bloqueada (diamante com terminações oxigênio) quando polarizados anodicamente, formando arranjos de microeletrodos (provavelmente sítios ricos em boro). As superfícies do diamante foram modificadas diretamente com Pt, Pt-RuO2 e Pt-RuO2-RhO2 pelo método Sol-gel. Estes depósitos de catalisadores apresentavam tamanhos nanométricos e mostraram um bom contato elétrico com a superfície do diamante e elevada pureza, assim como, uma composição atômica bem controlada e uma distribuição homogênea na superfície do DDB. Os estudos da oxidação eletroquímica de metanol e de etanol sugerem fortemente que o envenenamento da superfície do eletrodo é grandemente inibido sobre o catalisador ternário Pt-RuO2-RhO2/DDB quando comparado com os eletrodos Pt-RuO2/DDB e Pt/DDB. Adicionalmente, esse eletrodo mostra uma excelente atividade catalítica para a oxidação de etanol, provavelmente devido à habilidade que o ródio tem para quebrar a ligação C-C somada ao mecanismo bifuncional que acontece no catalisador contendo Pt e Ru. Em seguida, realizaram-se modificações indiretas do DDB pela síntese de compósitos de pó de carbono modificados com metais e óxidos metálicos e posterior fixação usando uma solução de Nafion. A oxidação dos álcoois foi estudada por voltametria cíclica e curvas de polarização em estado estacionário (diagramas de Tafel). Os sistemas investigados incluíram misturas metálicas binárias, ternárias e quaternárias contendo Pt, Ru, Ir, Rh, PbOx, TaOx e MoOx num total de 16 sistemas diferentes. Assim, os melhores catalisadores para a oxidação de metanol foram: Pt-Ru-TaOx-PbOx > Pt-Ru-TaOx-MoOx ≈ Pt-Ru-MoOx-Ir enquanto que, para a oxidação de etanol foram Pt-Ru-TaOx-PbOx e Pt-Ru-PbOx-Rh. Consequentemente, catalisadores do tipo Pt-Ru-TaOx-M são promissores para futuras aplicações práticas. Finalmente, estudos preliminares realizados usando pó de DDB modificado diretamente com Pt-RuOx pelo método Sol-gel mostraram que este compósito apresenta uma excelente atividade catalítica para oxidação de metanol, que foi maior do que a observada sobre um catalisador comercial considerado como o estado da arte (Pt-Ru/C da E-TEK) nas mesmas condições experimentais. Desta forma, foi mostrada a possibilidade do uso de pó de DDB como suporte para eletrocatalisadores para aplicações em células a combustível que funcionem diretamente com álcoois. / The electrochemical properties of boron-doped diamond (BDD) surfaces as well as studies of the oxidation of methanol and ethanol in acid media on BDD surfaces modified with catalysts by the Sol-gel method are presented here. Studies of the surface properties of BDD electrodes revealed that a cathodic polarization (-3,0 V vs. HESS, 30 min) is necessary for a good performance of the system while no important bulk structural differences are introduced in the film indicating that the enhanced electrochemical responses brought on by the cathodic pre-treatment is only due to superficial changes. Meanwhile, repeated and extensive cathodic polarizations led to severe erosion of the electrode surface. On the other hand, studies performed using cyclic voltammetry and X-ray photoelectron spectroscopy have shown that, after a cathodic pre-treatment, the BDD electrode surface presents a dynamic behaviour that results in a loss of the reversibility towards the Fe(CN)6 4−/3− redox couple as a function of the time exposed to atmospheric conditions. This dynamic behaviour must be associated to a loss of superficial hydrogen due to oxidation by the air and is inversely proportional to the BDD doping level suggesting that the boron content has a stabilizing effect on the H-terminated surface. It was also observed that the amount of electrochemical active sites of the BDD electrode has a direct dependence with the doping level (maybe due to the amount of superficial boron). From those studies, it was concluded that all BDD electrodes used in this work having different doping levels (300, 800, 2000 and 8000 ppm of boron) posses, after cathodic polarization, electrochemical heterogeneous surfaces (i.e. boron-rich sites with high conductivity and H-terminated diamond sites with lower conductivity) while after anodic polarization the surface is partially blocked by O-terminated diamond sites, resulting in a behaviour that resembles microelectrode arrays (probably formed by the few boron-rich sites). The BDD surface was also modified directly with Pt, Pt-RuO2 and Pt-RuO2-RhO2 using the Sol-gel method. These catalysts deposits have nanometric sizes, good electrical contact with the diamond surface and high purity. They also show a well controlled atomic composition and a homogeneous distribution on the BDD surface. Methanol and ethanol electrochemical oxidation studies on those modified electrodes suggested that poisoning of the surface by undesired intermediates (most xvi probably CO) is highly inhibited on the ternary catalyst Pt-RuO2-RhO2/BDD when compared with the Pt-RuO2/BDD and Pt/BDD responses. Additionally, the observed catalytic activity for ethanol oxidation was excellent, probably due to the rhodium ability to break the C-C bond which was added to the bifunctional mechanism operating in Pt-Ru catalysts. In the sequence, indirect modifications of the BDD surface were carried out by the production of carbon powder composites modified with metals and metallic oxides by the Sol-gel method and their subsequent anchoring to the BDD using a Nafionsolution. The alcohols oxidation on these electrodes was studied by cyclic voltammetry and steady-state polarization curves (Tafel plots). The investigated systems included binary, ternary and quaternary catalysts containing Pt, Ru, Ir, Rh, PbOx, TaOx and MoOx in a total of 16 different systems The higher catalytic activity towards methanol oxidation was observed in the sequence: Pt-Ru-TaOx-PbOx > Pt-Ru-TaOx-MoOx _ Pt-Ru-MoOx-Ir while for ethanol oxidation, the catalysts activity sequence was: Pt-Ru-TaOx-PbOx _ Pt-Ru-PbOx-Rh > Pt- Ru-Rh. Consequently, it was concluded that catalysts of the type Pt-Ru-TaOx-M are very promising systems to be used in future practical applications. Finally, preliminary studies carried out using BDD powder directly modified with Pt-RuOx by the Sol-gel method showed that this material has an excellent catalytic activity towards methanol oxidation reaction, being higher than that observed on a state of the art commercial catalyst (Pt-Ru/C from E-TEK) under the same experimental conditions. Thus, the feasibility of the use of BDD powder as catalysts support in direct-alcohol fuel cell systems was clearly established.

Electrocatalysis plays a vital role in the transition from fossil fuel to renewable energy infrastructure. Bimetallic systems can provide enhanced electrocatalytic activity and/or selectivity due to their altered electronic and/or crystal structures. These two effects are the main parameters responsible for the enhancement of the catalytic properties of multi-metallic systems. In practice, they are often interrelated and can be difficult to distinguish from one another due to random distribution and segregation of the elements in substitutional alloys. With well-defined crystal and electronic structures, intermetallic compounds provide excellent platform materials for a knowledge-based approach aiming for the evaluation and optimization of structural and/or electronic effects in heterogeneous (electro) catalysis. The present PhD thesis focuses on the investigation of the correlations between electronic, geometric and electrocatalytic properties of anode materials in the methanol oxidation reaction (MOR). This is achieved by substitution of indium (three valence electrons) with tin (four valence electrons) in the isostructural series In1-xSnxPd2, which allows for a systematic variation of the total number of electrons per unit cell with a minor variation of the cell parameters. Geometric effects were evaluated by substitution of indium with gallium in the isostructural Ga1-xInxPd2 series, which allows for a systematic variation of the cell parameters (interatomic distances) with the same number of valence electrons per unit cell. By substitution of gallium with tin in the Ga1-xSnxPd2 series, both effects are combined and addressed simultaneously. Single-phase samples of the isostructural series In1-xSnxPd2, Ga1-xInxPd2 and Ga1-xSnxPd2 (0 ≤ x ≤ 1), were synthesized and characterized by metallography, powder X-ray diffraction, and electron microscopy to establish the phase composition and to determine the variation of the lattice parameters with composition. The MOR current densities show a distinct change in slop as the fraction of tin increases in the In1-xSnxPd2 series with a minimum at x = 0.8 which is attributed to the alteration of the electronic properties of the materials. For the GaxIn1-xPd2 series, the MOR current densities show a maximum at x = 0.5 which is attributed to the alteration of the structural properties of the materials. The Ga1-xSnxPd2 series shows two maxima at x = 0.15 and 0.93. The high activity at x = 0.15 and 0.93 is attributed to a synergy of simultaneous alteration of electronic and geometric influences and the catalytic properties. The results contribute to the knowledge-based development of catalytic materials with direct experimental evidence of fine-tuning of electronic and/or geometric influences using isostructural intermetallic compounds as platform materials. This provides a basis of model catalysts for further studies to advance fundamental, as well as applied research in catalysis for the development of a green, sustainable future for the new generations.