Trace amounts of carbon monoxide, typically as low as 10 ppm CO, have a deleterious effect on the activation overpotential losses in proton exchange membrane (PEM) fuel cells. This is because CO preferentially adsorbs on the Pt electrocatalyst at the anode at typical PEM fuel cell operating temperatures, thereby preventing the absorption and ionisation of hydrogen. The inability of current preferential oxidation steps to completely remove CO from hydrogen-rich gas streams has stimulated research into CO tolerant anodes. As opposed to other CO oxidation catalysts, metal oxide supported gold catalysts have been shown to be active for the afore mentioned reaction at low temperatures, making it ideal for the 80°C operating temperatures of PEM fuel cells. The objective of this study was to investigate the viability of incorporating titanium dioxide supported gold (Au/TiO2) catalysts inside a PEM fuel cell system to remove CO to levels low enough to prevent poisoning of the Pt-containing anode. Two distinct methods were investigated. In the first method, the incorporation of the said Au/TiO2 catalyst inside the membrane electrode assembly (MEA) of a PEM fuel cell for the selective/preferential oxidation of carbon monoxide to carbon dioxide in hydrogen-rich gas fuels, facilitated by the injection of an air bleed stream, was investigated. It was important for this study to simulate typical fuel cell operating conditions in an external CO oxidation test rig. Factors such as gold loading, oxygen concentration, temperature, pressure, membrane electrode assembly constituents, water formation, and selectivity in hydrogen-rich gas streams, were investigated. The Au/TiO2 catalysts were prepared via deposition-precipitation, a preparation procedure proven to yield nano-sized gold particles, suggested in literature as being crucial for activity on the metal oxide support. The most active catalysts were incorporated into the MEA and its performance tested in a single cell PEM fuel cell. The catalysts proved to yield exceptional activity for all test conditions inside the CO oxidation test rig. However, no significant improvement in CO tolerance was observed when these catalysts were incorporated into the MEA. It was concluded that the thin bilayer configuration resulted in mass transfer and contact time limitations between the catalysts and the simulated reformate gas mixture. Other factors highlighted as possible causes of deactivation included the deleterious effect of the acidic environment in the fuel cell, the formation of liquid water on the catalyst’s surface, and the adverse effect of the organic MEA constituents during the MEA production procedure. The second method investigated was the incorporation of the Au/TiO2 catalyst in an isolated catalyst chamber in the hydrogen feed line to the fuel cell, between the CO contaminated hydrogen gas cylinder and the anode humidifier. Test work in a CO oxidation test rig indicated that with this configuration, the Au/TiO2 catalysts were able to remove CO from concentrations of 2000 ppm to less that 1.3 ppm at a space velocity (SV) of 850 000 ml.gcat -1.h-1 while introducing a 2 per cent air bleed stream. Incorporation of this Au/TiO2 preferential oxidation system into a Johnson Matthey single cell PEM fuel cell test station prevented any measurable CO poisoning when 100 and/or 1000 ppm CO, 2 per cent air in hydrogen was introduced to a 0.39 mg Pt.cm-2 Pt/C anode. These results were superior compared to other state of the art CO tolerance technologies. An economic viability study indicated that the former can be achieved at a cost of gold equal to 0.8 per cent of the USDoE target cost of $45/kW. This concept might allow fuel cells to operate on less pure hydrogen-rich gas, e.g. from H2 that would be stored in a fuel tank/cylinder but that would have some CO contamination and would essentially be dry. The use of less pure H2 should allow a cost incentive to the end user in that less pure H2 can be produced at a significantly lower cost.

O material (pó) de nanofase de Ce0,9W0,1O2 foi sintetizado por coprecipitação de oxalatos de cério (IV) e cátions de tungstênio (IV). A redução da platina (2%) foi feita pelo método da redução por álcool, utilizando uma solução de ácido hexacloroplatínico (H2PtCl6.6H2O) como fonte do metal, óxido de cério dopado com tungstênio (Ce0,9W0,1O2) utilizado como suporte e, uma solução de etilenoglicol/água (75/25, v/v) como solvente e agente redutor. Os materiais 2%Pt-Ce0,9W0,1O2 foram misturados em Pt/C E-TEK 20%, utilizando-se processo de mistura física para produzir os eletrocatalisadores de 20%Pt-(2%Pt-Ce0,9W0,1O2)/C. Os eletrocatalisadores obtidos foram caracterizados por espectroscopia de energia dispersiva de raios X (EDX) acoplado à microscopia eletrônica de varredura (MEV), análises de difração de raios X (DRX), e microscopia eletrônica de transmissão (MET). O conjunto eletrodos-membrana (MEAs) foram preparados para o ânodo com cargas iguais a 0,401, 0,364, 0,328 mg Pt cm-2 de eletrocatalisadores 20%Pt-(2%Pt-Ce0,9W0,1O2)/C produzidos. No cátodo foi usada uma carga de 0,4 mg Pt cm-2 de eletrocatalisador Pt/C ETEK. A polarização anódica foi realizada para oxidação de H2/CO (100 ppm de CO). A tolerância ao CO foi estudada utilizando o processo eletroquímico (stripping de CO e medidas de curvas de polarização). Os resultados obtidos mostraram que a oxidação de CO adsorvido a CO2 na superfície de platina ocorre em potenciais menos positivos mostrando tolerância ao CO adsorvido nestes eletrocatalisadores (20%Pt-(2%Pt-Ce0,9W0,1O2)/C (50:50, 60:40 e 70:30)) a uma temperatura de 85 ºC e com pressão absoluta de 2 bar para ânodo e cátodo, comparado com Pt/C E-TEK 20%. / The nanophase material (powder) of Ce0,9W0,1O2 was synthesized via coprecipitation of oxalates of cerium (IV) and tungsten cations. The reduction of platinum (2%) was made by the method of alcohol reduction, using an acid solution hexachloroplatinic (H2PtCl6.6H2O) as metal source, cerium oxide doped with tungsten (Ce0,9W0,1O2) used as support and the solution of ethylene glycol/water (75/75, v/v) as solvent and reducing agent. The 2%Pt-Ce0,9W0,1O2 materials were mixed in Pt/C E-TEK 20% using physical mixing process to produce the 20%Pt-(2%Pt-Ce0,9W0,1O2)/C electrocatalyst. The materials were characterized by energy dispersive X-ray spectroscopy (EDX) coupled to scanning electron microscopy (SEM), X-ray difratometry analysis (XRD) and transmission electronic microscopy (TEM). The membrane electrodes assembly (MEAs) were prepared with loads equal to 0.401, 0.364, 0.328 mg Pt cm-2 for 20%Pt(2%Pt-Ce0,9W0,1O2)/C electrocatalysts produced. In the cathode a load of 0.4 mg Pt cm-2 of commercial Pt/C ETEK electrocatalyst was used. The anodic polarization was carried out for oxidation of the mixture H2/CO(100 ppm CO). The CO tolerance was studied using electrochemical process (CO stripping and measurements of polarization curves). The results showed that the oxidation of CO adsorbed to CO2 on the surface of platinum occur at less positive potentials showing tolerance to CO adsorbed on these 20%Pt-(2%Pt-Ce0,9W0,1O2)/C (50:50, 60:40 and 70:30) electrocatalysts at a temperature of 85 ºC and absolute pressure of 2 bar for anode and cathode, compared with Pt/C E-TEK 20%.

Fuel cells are expected to play an important role in future power generation. However, significant technical challenges remain and the commercial breakthrough of fuel cells is hindered by the high price of fuel cell components. As is well known, the fuel cells do not provide the robust source characteristics required to effectively follow the load during significant load steps and they have limited overload-handling capability. Further, the performance of the fuel cell is significantly degraded when the CO (Carbon Monoxide) is contained in the hydrogen fuel. In this thesis several new approaches to improve the performance of PEM based fuel cell power systems are discussed. In the first section an impedance model of the Proton Exchange Membrane Fuel Cell Stack (PEMFCS) is first proposed. This equivalent circuit model of the fuel cell stack is derived by a frequency response analysis (FRA) technique to evaluate the effects of the ripple current generated by the power-conditioning unit. Experimental results are presented to show the effects of the ripple currents. In the second section, a fuel cell powered UPS (Uninterruptible Power Supply) system is proposed. In this approach, two PEM Fuel Cell modules along with suitable DC/DC and DC/AC power electronic converter modules are employed. A Supercapacitor module is also employed to compensate for instantaneous power fluctuations including overload and to overcome the slow dynamics of the fuel processor such as reformers. A complete design example for a 1-kVA system is presented. In the third section, an advanced power converter topology is proposed to significantly improve the CO tolerance on PEM based fuel cell power systems. An additional two-stage dc-dc converter with a supercapacitor module is connected to the fuel cell to draw a low frequency (0.5Hz) pulsating current of the specific amplitude (20-30[A]) from the fuel cell stack. CO on the catalyst surface can be electro-oxidized by using this technique, and thereby the CO tolerance of the system can be significantly improved. Simulation and experimental results show the validity and feasibility of the proposed scheme.

O material (pó) de nanofase de Ce0,9W0,1O2 foi sintetizado por coprecipitação de oxalatos de cério (IV) e cátions de tungstênio (IV). A redução da platina (2%) foi feita pelo método da redução por álcool, utilizando uma solução de ácido hexacloroplatínico (H2PtCl6.6H2O) como fonte do metal, óxido de cério dopado com tungstênio (Ce0,9W0,1O2) utilizado como suporte e, uma solução de etilenoglicol/água (75/25, v/v) como solvente e agente redutor. Os materiais 2%Pt-Ce0,9W0,1O2 foram misturados em Pt/C E-TEK 20%, utilizando-se processo de mistura física para produzir os eletrocatalisadores de 20%Pt-(2%Pt-Ce0,9W0,1O2)/C. Os eletrocatalisadores obtidos foram caracterizados por espectroscopia de energia dispersiva de raios X (EDX) acoplado à microscopia eletrônica de varredura (MEV), análises de difração de raios X (DRX), e microscopia eletrônica de transmissão (MET). O conjunto eletrodos-membrana (MEAs) foram preparados para o ânodo com cargas iguais a 0,401, 0,364, 0,328 mg Pt cm-2 de eletrocatalisadores 20%Pt-(2%Pt-Ce0,9W0,1O2)/C produzidos. No cátodo foi usada uma carga de 0,4 mg Pt cm-2 de eletrocatalisador Pt/C ETEK. A polarização anódica foi realizada para oxidação de H2/CO (100 ppm de CO). A tolerância ao CO foi estudada utilizando o processo eletroquímico (stripping de CO e medidas de curvas de polarização). Os resultados obtidos mostraram que a oxidação de CO adsorvido a CO2 na superfície de platina ocorre em potenciais menos positivos mostrando tolerância ao CO adsorvido nestes eletrocatalisadores (20%Pt-(2%Pt-Ce0,9W0,1O2)/C (50:50, 60:40 e 70:30)) a uma temperatura de 85 ºC e com pressão absoluta de 2 bar para ânodo e cátodo, comparado com Pt/C E-TEK 20%. / The nanophase material (powder) of Ce0,9W0,1O2 was synthesized via coprecipitation of oxalates of cerium (IV) and tungsten cations. The reduction of platinum (2%) was made by the method of alcohol reduction, using an acid solution hexachloroplatinic (H2PtCl6.6H2O) as metal source, cerium oxide doped with tungsten (Ce0,9W0,1O2) used as support and the solution of ethylene glycol/water (75/75, v/v) as solvent and reducing agent. The 2%Pt-Ce0,9W0,1O2 materials were mixed in Pt/C E-TEK 20% using physical mixing process to produce the 20%Pt-(2%Pt-Ce0,9W0,1O2)/C electrocatalyst. The materials were characterized by energy dispersive X-ray spectroscopy (EDX) coupled to scanning electron microscopy (SEM), X-ray difratometry analysis (XRD) and transmission electronic microscopy (TEM). The membrane electrodes assembly (MEAs) were prepared with loads equal to 0.401, 0.364, 0.328 mg Pt cm-2 for 20%Pt(2%Pt-Ce0,9W0,1O2)/C electrocatalysts produced. In the cathode a load of 0.4 mg Pt cm-2 of commercial Pt/C ETEK electrocatalyst was used. The anodic polarization was carried out for oxidation of the mixture H2/CO(100 ppm CO). The CO tolerance was studied using electrochemical process (CO stripping and measurements of polarization curves). The results showed that the oxidation of CO adsorbed to CO2 on the surface of platinum occur at less positive potentials showing tolerance to CO adsorbed on these 20%Pt-(2%Pt-Ce0,9W0,1O2)/C (50:50, 60:40 and 70:30) electrocatalysts at a temperature of 85 ºC and absolute pressure of 2 bar for anode and cathode, compared with Pt/C E-TEK 20%.

Antipredator defenses are ubiquitous in aquatic ecosystems. In the widely studied Chaoborus-Daphnia predator-prey system, Daphnia elicit a variety of phenotypically plastic responses to Chaoborus including: morphological, life history, and behavioral responses. While these inducible defenses benefit the prey, metal contaminants have been shown to interfere with chemosensory functions, thereby inhibiting antipredator defenses and decreasing survivorship. However, in lakes with a history of metal contamination, such as Kelly Lake in Sudbury, Ontario, there is evidence to suggest that Daphnia may have adapted to high, ambient copper concentrations. Using seven distinct Daphnia clones that were hatched from resting eggs from Kelly Lake, we examined morphological and life history traits when clones were exposed to either a nominal concentration of copper, kairomone, or a combination of both. As expected, clones displayed a variety of inducible responses in both kairomone-control and kairomone-copper treatments, which was attributed to genetic variability. Expected trade-offs in life history traits were not always observed, suggesting that inducible traits may be coupled. Furthermore, in contradiction to life history theory, one clone exhibited both increased somatic growth and increased reproductive output, indicating that clones likely adopted adaptive strategies to stressors rather than elicitng trade-offs in traditional traits. Our results indicate that environmentally relevant copper concentrations do not inhibit the induction of antipredator defenses in Daphnia from Kelly Lake, and we conclude that Kelly Lake Daphnia have developed an adaptive tolerance to copper. Adaptation to copper contamination may have implications for resilience in natural Kelly Lake populations. / Thesis (Master, Biology) -- Queen's University, 2012-04-03 19:33:59.137

Este trabalho consiste na síntese, caracterização físico-química e eletroquímica de catalisadores de Pt, bimetálicos de PtRu e trimetálicos de PtRuMo suportados em grafeno (GF) e grafeno funcionalizado com amônia (GFN), comparados com seus homólogos comerciais suportados em carbono Vulcan E-Tek (C-Etek), avaliando o desempenho da reação de oxidação de hidrogênio (ROH) na presença de CO (ROH/CO) para ânodos em células a combustível tipo PEM. A finalidade desta tese consistiu em entender as propriedades físicas e eletroquímicas do suporte de grafeno e grafeno funcionalizado, o seu papel na atividade catalítica, com o objetivo de melhorar o desempenho, estabilidade e durabilidade dos catalisadores na ROH e ROH/CO, com o intuito de obter catalisadores mais homogêneos, eficientes e duráveis comparados com seus homólogos suportados em carbono Vulcan. Na síntese e avaliação dos catalisadores de Pt, PtRu e PtRuMo, a eficiência no ânodo foi determinada de acordo com as variáveis a saber: o método de síntese, efeito do segundo metal e o tipo de suporte como C-Etek, GF e GFN. As propriedades físicas de cada catalisador, como sua estrutura, tamanho médio de cristalito, composição mássica e atômica dos metais e seus estados de oxidação foram determinadas mediante diferentes técnicas de caracterização, como difração de raios X (XDR), análise termogravimétrica (TGA), energia dispersiva de raios X (EDX), espectroscopia de fotoelétrons excitados por raios X (XPS), microscopia eletrônica de transmissão (TEM) e espectroscopia de absorção de raios X (XAS). Testes de desempenho e estabilidade foram realizados em célula unitária mediante curvas de polarização, as quais determinaram para cada catalisador sua eficiência na oxidação de H2, com e sem contaminação com CO, no ânodo em condições reais de operação. Os catalisadores também foram avaliados em medidas de meia célula por voltametria cíclica e strinpping de CO, assim como testes de envelhecimento acelerado (TEA) para determinar a estabilidade dos catalisadores após de longos tempos de operação (5000 ciclos Vs. ERH), acompanhando a mudança na área eletroativa dos mesmos. Os catalisadores de Pt-Ru e Pt-Ru-Mo suportados sobre GNF apresentaram melhor desempenho na ROH/CO, assim como a melhor estabilidade após 5000 ciclos. / This work consists in the synthesis of Pt, bimetallic PtRu and trimetallic PtRuMo catalysts supported on graphene (GF) and ammonia functionalized graphene (GFN), being determined their physical and electrochemical properties to be compared with their commercial catalysts Pt/C-Etek and PtRu/C-Etek. The synthetized catalysts were tested and compared the performance for the hydrogen oxidation reaction (ROH) in the presence of CO for anodes in PEM-type fuel cells. The objective consisted in understanding the effect of the Pt-M1-M2 alloy and the role of different type of graphene support in the catalytic activity of ROH/CO, with the aim of improving the performance, stability and durability of the catalyst. The performance of the catalysts of Pt, PtRu and PtRuMo was determined according to the variables: synthesis method, second metal effect and support effect (C-Etek, GF and GFN). The physical properties of each catalyst such as mean particle size, element content, oxidation states and Pt band changes were determined by different characterization techniques, such as X-ray diffraction (XDR), thermogravimetric analysis (TGA), dispersive energy of X-ray (EDX), X-ray excited photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS). Tests of performance and stability in unit cell were carried out by polarization curves and cyclic voltammetries, which determined the efficiency and tolerance of ROH/CO reaction in the anode, under real operating conditions. The catalysts were also evaluated in half cell measurements by cyclic voltammetry and CO stripping, as well as accelerated aging tests (TEA) to determine the stability of the catalysts after long operating times (5000 cycles Vs. ERH), following the change in their electroactive area. Pt-Ru and Pt-Ru-Mo catalysts supported on GNF showed bests ROH/CO performance, as well as better stability after 5000 cycles.

Elevada tolerância ao CO e alta estabilidade são necessário para o eletrocatalisador anódico de células a combustível de membrana protônica (PEMFC) para o melhoramento do desempenho do sistema operando com combustível reformado. Neste trabalho eletrocatalisadores suportados em carbono e carbetos (carbetos de molibdênio e tungstênio) foram estudados para a reação de oxidação de hidrogênio (ROH), a tolerância ao CO e a estabilidade no ânodo de PEMFC. Os materiais investigados incluem: PtMo/C, PtMo/C tratado-termicamente, Pt/Mo2C/C, PtMo/Mo2C/C, PtW/C, Pt/WC/C e Pt/C. As diferenças na morfologia dos eletrocatalisadores foram caracterizadas por redução com temperatura programada (TPR), difração de raios-x (XRD), microscopia eletrônica de transmissão (MET), energia dispersiva de raios-x (EDX), espectroscopia de absorção de raios-x (XAS), microscopia eletrônica de varredura (MEV) e espectroscopia de dispersão de comprimento de onda (WDS). As características e as atividades eletroquímicas dos electrocatalisadores foram avaliadas para ROH e tolerância ao CO por medidas de curvas de polarização em célula unitária e voltametria cíclica, sob a forma de elétrodos de difusão de gás. Espectrometria de massa (EMS) \"on-line\" e experimentos de stripping de CO foram realizados para avaliar o mecanismo de tolerância ao CO dos eletrocatalisadores. Voltametria cíclica conduzida até 5000 ciclos voltamétricos foi realizada para avaliar a estabilidade dos eletrocatalisadores. Tanto o eletrocatalisador de PtMo suportado em carbono e tratado a 600ºC, com tamanho médio de cristalito de 16,7 nm, como o eletrocatalisador de Pt suportado em carbeto de molibdênio mostraram atividade mais elevada para a oxidação de hidrogênio na presença de 100 ppm de CO e uma estabilidade melhorada, em comparação com os catalisadores de PtMo suportado em carbono e PtMo suportado em carbeto de molibdênio. Semelhantamente, melhor tolerância ao CO e maior estabilidade foram mostradas por Pt suportado em carbeto de tungstênio, como comparado com electrocatalisador de PtW suportado em carbono. Os resultados de voltametria cíclica e WDS mostraram que ocorre uma dissolução parcial de Mo e W e a sua migração/difusão do ânodo para o cátodo durante o período de ciclagem, o que é a maior causa das perdas em desempenho destes eletrocatalisadores. Os resultados mostraram que a atividade catalítica e a estabilidade podem ser melhoradas com o tratamento térmico, a despeito de um aumento do tamanho das partículas do catalisador, ou pela formação de carbetos nos suportes dos eletrocatalisadores. / Enhanced CO tolerance and stability of proton exchange membrane fuel cell (PEMFC) anode electrocatalysts is necessary for the improvement in the performance of PEM fuel cell system operating with reformate fuel. In this work carbon and carbides (molybdenum and tungsten carbides) supported electrocatalysts have been studied for the activity of hydrogen oxidation reaction (HOR), CO tolerance and stability in the anode of proton exchange membrane fuel cell. The materials investigated include: PtMo/C, PtMo/C heat-treated, Pt/Mo2C/C, PtMo/Mo2C/C, PtW/C, Pt/WC/C and Pt/C. Differences in electrocatalysts morphology were characterized by temperature programmed reduction (TPR), x-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), x-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM) and wavelength dispersive spectroscopy (WDS). The electrochemical characteristics and activity of electrocatalysts were evaluated for the HOR and CO tolerance by single cell polarization measurements and cyclic voltammetry in the form of gas diffusion electrodes. Online mass spectrometry (OLMS) and CO stripping experiments were performed to evaluate the CO tolerance mechanism of the electrocatalysts. Cyclic voltammetry up to 5000 potential cycles was conducted to evaluate the electrocatalyst stability. It was observed that the carbon supported PtMo heat-treated at 600 °C with average crystallite size of 16.7 nm and Pt supported on Mo2C/C showed an enhanced stability and a good hydrogen electrooxidation activity in the presence of 100 ppm CO, as compared to as prepared carbon supported PtMo and molybdenum carbide supported PtMo electrocatalysts. Similarly, a better CO tolerance and improved stability was shown by tungsten carbide supported Pt electrocatalyst, as compared to carbon supported PtW electrocatalyst. Cyclic voltammetry and wavelength dispersive spectroscopy results showed that a partial dissolution of Mo and W and their migration/duffusion from the anode toward cathode take place during the cycling process, which is the major cause of performance losses of these electrocatalysts. On the basis of polarization measurements and cyclic voltammograms of both anodes and cathodes, it was concluded that the stability of these electrocatalysts may be improved either by heat-treatment or by the formation of carbides in the catalyst supports

A busca por fontes alternativas de energia é uma tendência mundial e, neste contexto, as células a combustível alimentadas com hidrogênio obtido pela reforma de biocombustíveis constitui uma das alternativas mais promissoras. Entretanto, o desempenho das células a combustível de membrana de troca protônica (PEMFC) com ânodos baseados em Pt é drasticamente reduzido quando se utiliza hidrogênio contaminado por CO, o qual é produzido no processo de reforma. Neste trabalho, a eletrocatálise da tolerância ao CO e a estabilidade de Pt/C, PtRu/C, PtFe/C, PtMo/C e PtW/C como eletrocatlisadores anódicos de célula a combustível PEM foram investigadas através de curvas de polarização e medidas on line de espectrometria de massas (EMS), análises de voltametria cíclica, difração de raios X (DRX) e absorção de raios X (XAS). Para todos os eletrocatalisadores bimetálicos, os quais apresentaram alta tolerância ao CO, os resultado de EMS mostraram que a produção de CO2 inicia-se a menores sobrepotenciais em relação ao eletrodo de hidrogênio quando comparado a Pt/C, confirmando a ocorrência do conhecido mecanismo bifuncional. Por outro lado, os resultados de XANES indicam um aumento de vacância da banda 5d da Pt para todos os catalisadores bimetálicos, particularmente para PtFe/C, o que leva a um enfraquecimento da ligação Pt-CO e conseqüente aumento da tolerância ao CO (efeito eletrônico). Para PtMo/C e PtRu/C alimentado com H2/CO, a formação de CO2 é observada mesmo quando a célula opera em circuito aberto, confirmando alguma eliminação de CO por um processo químico, muito provavelmente uma reação de deslocamento gás-água. Uma deterioração do desempenho de célula a combustível foi observada em uma função do tempo de operação. As causas desta degradação durante a operação a longo prazo fazem parte de um processo complexo que envolve diversos mecanismos paralelos, tais como: perda ou redistribuição do eletrocatalisador, corrosão do suporte de carbono e degradação do eletrólito (Nafion®). / The search for alternative sources of energy is a global trend, and in this context, the fuel cell supplied with hydrogen obtained by biofuels reforming is one of the most promising alternative. However, the performance of proton exchange membrane fuel cells (PEMFC) with Pt-based anode is drastically lowered when using CO-contain hydrogen, as that produced by reform. In this work, the electrocatalysis of CO tolerance and the stability of Pt/C, PtRu/C, PtFe/C, PtMo/C, and PtW/C electrocatalysts at a PEM fuel cell anode has been investigated using single cell polarization and on line electrochemical mass spectrometry (EMS) measurements, and cyclic voltammetry, X-ray diffraction (XRD), and X-ray absorption near edge structure (XANES) analyses of the electrocatalysts. For all bimetallic electrocatalysts, which presented higher CO tolerance, EMS results have shown that the production of CO2 starts at lower hydrogen electrode overpotentials as compared to Pt/C, confirming the occurrence of the so-called bifunctional mechanism. On the other hand, XANES results indicate an increase in the Pt 5d-band vacancies for the bimetallic catalysts, particularly for PtFe/C, this leading to a weakening of the Pt-CO bond, helping to increase the CO tolerance (the so-called electronic effect). For PtMo/C and PtRu/C supplied with H2/CO, the formation of CO2 is observed even when the cell is at open circuit, confirming some elimination of CO by a chemical process, most probably the water gas shift reaction. A decay of the fuel cell performance was observed as a function of the operation time. The causes of degradation during long-term operation were found to be a complex process that involves several parallel mechanisms, including: electrocatalyst loss or redistribution, carbon corrosion, and electrolyte (Nafion®) degradation.

"The need for an efficient, non-polluting power source for vehicles in urban environments has resulted in increased attention to the option of fuel cell powered vehicles of high efficiency and low emissions. Of various fuel cell systems considered, the proton exchange membrane (PEM) fuel cell technology seems to be the most suitable one for the terrestrial transportation applications. This is thanks to its low temperature of operation (hence, fast cold start), and a combination of high power density and high energy conversion efficiency. Besides automobile and stationary applications (distributed power for homes, office buildings, and as back-up for critical applications such as hospitals and credit card centers), future consumer electronics also demands compact long-lasting sources of power, and fuel cell is a promising candidate in these applications. The goal of a cost effective and high performance fuel cell has resulted in very active multidisciplinary research. Although significant progress has been made on PEM fuel cells over the last twenty years, further progress in fuel cell research is still needed before the commercially viable fuel cell utilization in transportation, potable and stationary applications. A chief goal among others is the design of PEM fuel cells that can operate with impure hydrogen containing traces of CO, which has been the objective of this research. Standard Pt and PtRu anode catalyst has been studied systematically under practical fuel cell conditions, in an attempt to understand the mechanism and kinetics of H2/CO electrooxidation on these noble metal catalysts. In the study of Pt as anode catalyst, it was found that the fuel cell performance was strongly affected by the anode flow rate and cathode oxygen pressure. A CO electrooxidation kinetic model was developed taking into account the CO inventory in the anode, which can successfully simulate the experimental results. It was found that there is finite CO electrooxidation even on Pt anode with H2/CO as anode feed. Thus, anode overpotential and outlet CO concentration is a function of anode inlet flow rate at a constant current density. The on-line monitoring of CO concentration in PEM fuel cell anode exit has proved that the ~{!0~}ligand mechanism~{!1~} and ~{!0~}bifunctional mechanism~{!1~} coexist as the CO tolerance mechanisms for PtRu anode catalyst. For PtRu anode catalyst, sustained potential oscillations were observed when the fuel cell was operated at constant current density with H2/CO as anode feed. Temperature was found to be the key bifurcation parameter besides current density and the anode flow rate for the onset of potential oscillations. The anode kinetic model was extended further to unsteady state which can reasonably reproduce and adequately explain the oscillatory phenomenon. The potential oscillations are due to the coupling of anode electrooxidation of H2 and CO on PtRu alloy surface, on which OHad can be formed more facile, preferably on top of Ru atoms at lower overpotentials. One parameter bifurcation and local linear stability analysis have shown that the bifurcation experienced during the variation of fuel cell temperature is a Hopf bifurcation, which leads to stable potential oscillations when the fuel cell is set at constant current density. It was further found that a PEM fuel cell operated in an autonomous oscillatory state produces higher time-averaged cell voltage and power density as compared to the stable steady-state operation, which may be useful for developing an operational strategy for improved management of power output in PEM fuel cells with the presence of CO in anode feed. Finally, an Electrochemical Preferential Oxidation (ECPrOx) process is proposed to replace the conventional PrOx for cleaning CO from reformate gas, which can selectively oxidized CO electrochemically while generating supplemental electrical power without wasting hydrogen."

A busca por fontes alternativas de energia é uma tendência mundial e, neste contexto, as células a combustível alimentadas com hidrogênio obtido pela reforma de biocombustíveis constitui uma das alternativas mais promissoras. Entretanto, o desempenho das células a combustível de membrana de troca protônica (PEMFC) com ânodos baseados em Pt é drasticamente reduzido quando se utiliza hidrogênio contaminado por CO, o qual é produzido no processo de reforma. Neste trabalho, a eletrocatálise da tolerância ao CO e a estabilidade de Pt/C, PtRu/C, PtFe/C, PtMo/C e PtW/C como eletrocatlisadores anódicos de célula a combustível PEM foram investigadas através de curvas de polarização e medidas on line de espectrometria de massas (EMS), análises de voltametria cíclica, difração de raios X (DRX) e absorção de raios X (XAS). Para todos os eletrocatalisadores bimetálicos, os quais apresentaram alta tolerância ao CO, os resultado de EMS mostraram que a produção de CO2 inicia-se a menores sobrepotenciais em relação ao eletrodo de hidrogênio quando comparado a Pt/C, confirmando a ocorrência do conhecido mecanismo bifuncional. Por outro lado, os resultados de XANES indicam um aumento de vacância da banda 5d da Pt para todos os catalisadores bimetálicos, particularmente para PtFe/C, o que leva a um enfraquecimento da ligação Pt-CO e conseqüente aumento da tolerância ao CO (efeito eletrônico). Para PtMo/C e PtRu/C alimentado com H2/CO, a formação de CO2 é observada mesmo quando a célula opera em circuito aberto, confirmando alguma eliminação de CO por um processo químico, muito provavelmente uma reação de deslocamento gás-água. Uma deterioração do desempenho de célula a combustível foi observada em uma função do tempo de operação. As causas desta degradação durante a operação a longo prazo fazem parte de um processo complexo que envolve diversos mecanismos paralelos, tais como: perda ou redistribuição do eletrocatalisador, corrosão do suporte de carbono e degradação do eletrólito (Nafion®). / The search for alternative sources of energy is a global trend, and in this context, the fuel cell supplied with hydrogen obtained by biofuels reforming is one of the most promising alternative. However, the performance of proton exchange membrane fuel cells (PEMFC) with Pt-based anode is drastically lowered when using CO-contain hydrogen, as that produced by reform. In this work, the electrocatalysis of CO tolerance and the stability of Pt/C, PtRu/C, PtFe/C, PtMo/C, and PtW/C electrocatalysts at a PEM fuel cell anode has been investigated using single cell polarization and on line electrochemical mass spectrometry (EMS) measurements, and cyclic voltammetry, X-ray diffraction (XRD), and X-ray absorption near edge structure (XANES) analyses of the electrocatalysts. For all bimetallic electrocatalysts, which presented higher CO tolerance, EMS results have shown that the production of CO2 starts at lower hydrogen electrode overpotentials as compared to Pt/C, confirming the occurrence of the so-called bifunctional mechanism. On the other hand, XANES results indicate an increase in the Pt 5d-band vacancies for the bimetallic catalysts, particularly for PtFe/C, this leading to a weakening of the Pt-CO bond, helping to increase the CO tolerance (the so-called electronic effect). For PtMo/C and PtRu/C supplied with H2/CO, the formation of CO2 is observed even when the cell is at open circuit, confirming some elimination of CO by a chemical process, most probably the water gas shift reaction. A decay of the fuel cell performance was observed as a function of the operation time. The causes of degradation during long-term operation were found to be a complex process that involves several parallel mechanisms, including: electrocatalyst loss or redistribution, carbon corrosion, and electrolyte (Nafion®) degradation.