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International audience; Palladium nanoparticles are grown on TiO 2 nanotubes by atomic layer deposition (ALD) and the resulting three dimensional nanostructured catalysts are studied for ethanol electrooxidation in alkaline media. The morphology, the crystal structure and the chemical composition of the Pd particles are fully characterized using scanning and transmission electron microscopies, x-ray diffraction and x-ray photoelectron spectroscopy. The characterization revealed that the deposition proceeds onto the entire surface of the TiO 2 nanotubes leading to the formation of well-defined and highly dispersed Pd nanoparticles. The electrooxidation of ethanol 2 on Pd clusters deposited on TiO 2 nanotubes show not only a direct correlation between the catalytic activity and the particle size but also a steep increase of the response due to the enhancement of the metal-support interaction when the crystal structure of the TiO 2 nanotubes is modified by annealing at 450°C in air.

Este trabalho teve como objetivo a combinação de ânodos e eletrólitos otimizados, para a formação de células a combustível de etanol direto (DEFC), operantes em elevadas temperaturas (130 ºC). Como materiais de ânodo, foram produzidos eletrocatalisadores baseados em PtSn/C, com diversas razões atômicas Pt:Sn, preparados pelo método do poliol modificado, essa metodologia possibilita a produção de eletrocatalisadores auto-organizados com estreita distribuição de tamanhos de partículas e elevado grau de liga. Os eletrocatalisadores foram caracterizados por DRX e stripping de CO. Os resultados mostraram que esses materiais apresentaram elevado grau de liga e Eonset de oxidação de CO em potenciais menores do que os materiais comerciais. Como eletrólito, foram sintetizados híbridos Nafion-SiO2 com a incorporação do óxido diretamente nos agregados iônicos de diversos tipos de membranas Nafion. Os parâmetros de síntese, tais como o solvente em meio solgel, a espessura da membrana, e a concentração do precursor de sílica foram avaliados em termos do percentual de sílica incorporada e da estabilidade mecânica do híbrido. Por fim, ânodos e eletrólitos otimizados foram avaliados em DEFCs nas temperaturas de 80 e 130 ºC. Os resultados mostraram um significativo incremento no desempenho de polarização (122 mW cm-2), resultado da aceleração na taxa de oxidação de etanol devido ao material de ânodo otimizado e do aumento de temperatura de operação, uma vez que o uso de eletrólitos híbridos possibilita o aumento da temperatura sem perdas de condutividade. Nesse sentido, a combinação de eletrodos e eletrólitos otimizados é uma alternativa promissora para o desenvolvimento de tais dispositivos. This work has as objective to evaluate anodes and electrolytes in direct ethanol fuel cells (DEFC) operating at high temperature (130 ºC). As anode materials, electrocatalysts based on PtSn/C were prepared by Modified Polyol Method with various Pt:Sn atomic ratios. Such methodology promotes selforganized electrocatalysts production with narrow particle size distribution and high alloying degree. The eletrocatalysts were characterized by XRD, and CO stripping. The results showed that these materials presented high alloying degree and Eonset CO oxidation at lower potential as commercial materials. As electrolyte, Nafion-SiO2 hybrids were synthesized by sol-gel reaction, by the incorporation of oxide directly into the ionic aggregates of various kinds of Nafion membranes. The synthesis parameter, such sol-gel solvent, membrane thickness and silicon precursor concentration were studied in terms of silica incorporation degree and hybrid mechanical stability. Finally, the optimized anodes and electrolytes were evaluated in DEFC operating at 80 130 ºC temperature range. The results showed a significant improvement of the DEFC performance (122 mW cm-2), resulted from the acceleration of ethanol oxidation reaction rate due to anode material optimization and high temperature operation once the use of hybrids possibilities the increase of temperature without a significant conductivity loses. In this sense, the combination of optimized electrodes and electrolytes are a promising alternative for the development of these devices.

A high-surface-area PtSn electrocatalyst supported on carbon was prepared by using a formic acid reduction method. The catalyst was characterized by X-ray fluorescence, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The electrochemical properties of the electrocatalyst were investigated by using polarization measurements, cyclic voltammetry, potentiostatic durability tests, and electrochemical impedance spectroscopy. An endurance test of 220 h at 0.4 V with recirculation of ethanol was carried out. Under continuous operation in a direct ethanol fuel cell, the electrocatalytic system shows an initial decay, but suitable stability is observed after about 100 h.

No presente trabalho, o desempenho elétrico dos eletrocatalisadores anódicos Pt/C, Pt3Sn/C(liga), Pt(SnO2)/C, Pt3(SnO2)/C e Pt9(SnO2)/C para as reações de eletro-oxidação de etanol, acetaldeído e ácido acético foi investigado. Testes em célula unitária mostraram que a adição de Sn seja na forma de liga Pt-Sn ou na forma de óxido (SnO2) coexistente com a platina metálica aumenta consideravelmente a resposta elétrica gerada pela célula. A melhora no desempenho elétrico dos catalisadores a base de PtSn é resultado da capacidade em oxidar o acetaldeído, majoritariamente produzido pelo Pt/C, em ácido acético. Pt3(SnO2)/C exibiu a melhor resposta elétrica tanto para o etanol quanto para acetaldeído como combustíveis, alcançando valores de densidade de potência máxima de 127 e 58 mW cm-2, respectivamente. Misturas entre os combustíveis mostraram que o acetaldeído é um composto que leva a uma rápida desativação dos catalisadores, enquanto que o ácido acético, embora não seja oxidado a CO2 nas condições de operação, não afeta o desempenho elétrico da célula. In the present work, the electrical performance of the anodic electrocatalysts Pt/C, Pt3Sn/C(alloy), Pt(SnO2)/C, Pt3(SnO2)/C and Pt9(SnO2)/C towards ethanol, acetaldehyde and acetic acid electro-oxidation reactions was investigated. Single cell tests showed that the Sn addiction whether in Pt-Sn alloyed form or in oxide (SnO2) form coexisting with metallic platinum increases considerably the electrical response generated by the cell. The electrical performance improvement of PtSn-basis catalysts is a result of the ability to oxidize acetaldehyde, mostly produced by Pt/C, in acetic acid. Pt3(SnO2)/C exhibited the highest electrical performance both for ethanol and acetaldehyde as fuels, reaching maximum power density values of 127 and 58 mW cm-2, respectively. Mixtures between fuels showed that acetaldehyde is a compound which leads a rapid deactivation of the catalysts, while acetic acid, although not oxidized to CO2 under operating conditions, does not affect the cell electrical performance.

Made available in DSpace on 2014-10-09T12:35:40Z (GMT). No. of bitstreams: 0 Made available in DSpace on 2014-10-09T14:03:55Z (GMT). No. of bitstreams: 0 Os eletrocatalisadores PtSn / C-ATO com Pt e Sn com diferentes rela????es at??micas (90:10, 70:30 e 50:50) foram preparados em uma ??nica etapa pelo processo de redu????o por ??lcool usando H2PtCl6.6H2O e SnCl2.2H2O como fontes de metais e etileno glicol como solvente e agente redutor e de uma mistura f??sica de carbono Vulcan XC72 (85% em peso) e ATO(Sb2O5.SnO2) (15% em peso) como o suporte (C-ATO). Os materiais obtidos foram caracterizados por difra????o de raios X (DRX) e microscopia eletr??nica de transmiss??o (MET). A atividade catal??tica para oxida????o eletroqu??mica do etanol em meio ??cido foi investigada por voltametria c??clica e cronoamperometria em c??lulas unit??rias de combust??vel de etanol direto (DEFC). As an??lises de DRX revelou que Pt (FCC), SnO2, carbono e fases ATO coexistem nos materiais obtidos. Os estudos eletroqu??micos mostraram que os eletrocatalisadores PtSn / C-ATO s??o mais ativos para oxida????o eletroqu??mica do etanol em compara????o ao eletrocatalisador de PtSn / C . As experi??ncias a 100 ?? C em c??lulas a combust??vel unit??rias (DEFC) mostrou que a densidade de potencia da c??lula usando PtSn / C-ATO (90:10) foi de aproximadamente 100% mais elevado do que o obtido utilizando PtSn / C (50:50). Os experimentos de infra vermelho FTIR in-situ indicaram que a adi????o de ATO no suporte para eletrocatalisadores PtSn / C favorece a forma????o do ??cido ac??tico como produto, enquanto para PtSn / C o acetalde??do foi o principal produto formado. Tese (Doutoramento) IPEN/T Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Células a combustível de óxido sólido suportadas no eletrólito de zircônia estabilizada com ítria (YSZ) foram fabricadas usando a técnica do recobrimento por rotação (spin-coating) para deposição de catodos de manganita de lantânio dopada com estrôncio (LSM) e anodos compósitos de níquel e YSZ (Ni-YSZ). Parâmetros microestruturais dos eletrodos, tais como espessura, tamanho médio de partículas e temperatura de sinterização foram otimizados, visando reduzir a resistência de polarização da célula e melhorar o seu desempenho. Estes estudos serviram de base para a fabricação de células com camada catalítica para uso com etanol direto. Sobre o anodo Ni-YSZ da célula foi depositada uma camada catalítica de céria dopada com gadolínia (CGO) com 0,1% em peso de irídio (Ir-CGO). A camada catalítica visa reformar o etanol antes do seu contato com o anodo Ni-YSZ, evitando o depósito de carbono na superfície do Ni que inviabiliza o uso de combustíveis primários contendo carbono nestas células a combustível. Inicialmente, a célula a combustível foi testada com etanol e as melhores condições de operação foram determinadas. Em seguida, as células unitárias foram testadas com etanol sem adição de água por períodos de tempo de até 390 horas. As células a combustível a etanol direto com camada catalítica operam no modo de reforma interna gradual, apresentando boa estabilidade e densidades de corrente similares às obtidas na operação com hidrogênio. Após a operação das células a combustível a etanol direto, análises de microscopia eletrônica de varredura mostraram que não houve formação significativa de depósitos de carbono na superfície do Ni, indicando que a camada catalítica de Ir-CGO foi efetiva para operação com o etanol. Testes de células a combustível a etanol direto sem a camada catalítica revelaram uma rápida degradação nas horas iniciais de operação com formação de grandes quantidades de depósitos de carbono identificados visualmente. Considerando-se a operação estável com etanol a seco por tempos relativamente longos de operação, os resultados alcançados representam um avanço significativo e apontam para o desenvolvimento de células a combustível a etanol direto usando-se os componentes tradicionais com a adição de uma camada catalítica. Yttria-stabilized zirconia (YSZ) electrolyte supported solid oxide fuel cells were fabricated with spin-coated strontium-doped lanthanum manganite (LSM) cathodes and Ni-YSZ cermet anodes. The microstructural parameters of the electrodes such as thickness, average particle size, and sintering temperature were optimized to decrease the polarization resistance of the single cells and to improve their electrochemical performance. These preliminar studies provided the basis for the fabrication of single fuel cells with a catalytic layer of gadolinia-doped ceria (CGO) and 0.1 wt% iridium (Ir-CGO) deposited onto the anode. The catalytic layer aims at the stable operation with dry (direct) ethanol; it avoids the contact of the alcohol with the anode, preventing the anode degradation by carbon deposition. Initially, the single cells were tested with ethanol and optimized operating parameters were determined. Then, the single cells were operated with anhydrous ethanol for periods of time up to 390 hours. The single cells with catalytic layer operate by the gradual internal reforming of ethanol, with good stability and delivering similar electric current densities as the ones measured using hydrogen as fuel. After single cell operation on direct ethanol, scanning electron microscopy analyses identified no significant carbon deposition on the surface of Ni, indicating that the Ir-CGO catalytic layer was effective for the reforming of ethanol. Such results were compared to the ones of standard single cells operating on dry ethanol, which showed a fast degradation and the formation of large amounts of carbon deposits. Considering the rather stable performance of single cells running on dry ethanol for relatively long times, such results represent a significant advance towards the development of direct ethanol solid oxide fuel cells using the standard components and a catalytic layer.

To find a more durable anode with high performance for direct ethanol fuel cells (DEFCs), the present study investigates a series of quaternary electrocatalysts, Pt 30 Ru 30 Ir 40− x Sn x /C (wt.%), for the ethanol electro-oxidation reaction (EOR). The carbon-supported Pt 30 Ru 30 Ir 40− x Sn x /C electrocatalysts were prepared by a known impregnation-reduction (borohydride) method. The microstructure and chemical composition were determined by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). The activity of the electrocatalysts for EOR was compared to commercial Pt 67 Ru 33 /C (HISPEC5000) using linear sweep voltammetry (LSV) based on similar Pt loading. The results of this study show that electrocatalyst composition with 10 and 20% Ir (wt.%) exhibit higher electrocatalytic activity than the commercial PtRu electrocatalyst. The single fuel cell testing at 90 °C comparing Pt 30 Ru 30 Ir 40− x Sn x /C to commercial Pt 67 Ru 33 /C and Pt 83 Sn 17 /C anodes showed an enhancement of Pt activity (normalized to Pt loading) in the following order: Pt 30 Ru 30 Ir 10 Sn 30 > Pt 30 Ru 30 Sn 40 ≥ Pt 30 Ru 30 Ir 40 ≥ Pt 83 Sn 17 > Pt 67 Ru 33 . After a long-term performance test, the activity changed to the following order: Pt 30 Ru 30 Ir 10 Sn 30 > Pt 30 Ru 30 Ir 40 > Pt 30 Ru 30 Sn 40 > Pt 83 Sn 17 > Pt 67 Ru 33 . Pt 30 Ru 30 Ir 10 Sn 30 /C exhibited both a higher performance with a specific power density of 29 mW mg Pt −1 without O 2 backpressure at the cathode and an excellent long-term stability in a DEFC operating at 90 °C.

Three cathode catalysts (60% Pt/C, 30% Pt/C and 60% Pt–Fe/C), with a particle size of about 2–3 nm, were prepared to investigate the effect of ethanol cross-over on cathode surfaces. All samples were studied in terms of structure and morphology by using X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. Their electrocatalytic behavior in terms of oxygen reduction reaction (ORR) was investigated and compared using a rotating disk electrode (RDE). The tolerance of cathode catalysts in the presence of ethanol was evaluated. The Pt–Fe/C catalyst showed both higher ORR activity and tolerance to ethanol cross-over than Pt/C catalysts. Moreover, the more promising catalysts were tested in 5 cm2 DEFC single cells at 60 and 80 °C. An improvement in single cell performance was observed in the presence of the Pt–Fe catalyst, due to an enhancement in the oxygen reduction kinetics. The maximum power density was 53 mW cm−2 at 2 bar rel. cathode pressure and 80 °C., Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas

Three Pt-based anode catalysts supported on Vulcan XC-72R (VC) were prepared by using a modified polyol process. These materials were characterized and tested by X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF) and Transmission Electron Microscopy (TEM). XRD and TEM analysis indicated that especially the ternary anode catalysts consisted of uniform nanosized particles with sharp distribution. The Pt lattice parameter was smaller, in the ternary PtSnIr catalyst whereas it increased with the addition of Sn and Rh, in the corresponding binary and ternary catalysts. Cyclic voltammetry (CV) measurements showed that Sn, Ir and Rh may act as promoter of Pt enhancing ethanol electro-oxidation activity. It was found that the direct ethanol fuel cell (DEFC) performances were significantly improved with these modified anode catalysts. This effect on the DEFC performance is attributed to the so-called bi-tri-functional mechanism and to the electronic interaction between Pt and additives. The performance increased significantly with the temperature. However, it was also possible to observe some decay with time for all catalysts due to the formation of surface poisons, probably consisting in CO-like species. At 60 °C, the PtSnIr catalyst showed the best performance, as a result of a proper morphology and promoting effect, Facultad de Ciencias Exactas, Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas