La pile à combustible directe à borohydrures (DBFC en anglais), qui est une sous-catégorie des piles à combustible alcalines, bénéficie des avantages de son combustible, le borohydrure de sodium (NaBH4), qui confère à ce système des caractéristiques thermodynamiques et énergétiques très intéressantes. Cependant, la réaction d’électrooxydation de NaBH4 (BOR en anglais) est très complexe et reste à ce jour encore peu étudiée et mal comprise sur la majorité des électrocatalyseurs (la plupart étant sous forme de nanoparticules métalliques supportées sur des noirs de carbone). De plus, de récentes études ont montré l’agressivité du milieu alcalin sur la durabilité des électrocatalyseurs conventionnels, révélant une grande perte de surface catalytique active, due principalement à un détachement des nanoparticules du support carboné. Dans ce contexte, ces travaux de thèse se sont orientés vers trois axes d’étude : (i) l’étude de la BOR sur des électrocatalyseurs à base de palladium dans des conditions proches des conditions réelles de fonctionnement de la DBFC ; (ii) l’étude de l’impact de la structure de l’anode sur les performances globales de la DBFC, et (iii) l’étude du mécanisme de dégradation d’électrocatalyseurs à base de métaux nobles dans un environnement alcalin. Les expérimentations ont été réalisées en étroite collaboration avec le U.S. Naval Research Laboratory (Washington, USA).Les résultats obtenus ont montré qu’une grande concentration en NaBH4 entraine un ralentissement de la cinétique de la réaction, due en partie à un fort empoisonnement de la surface catalytique. Par ailleurs, des marqueurs d’activité pour la BOR ont été proposés. Ensuite, l’utilisation d’électrodes à gradient de catalyseurs s’est avérée être une solution prometteuse pour mieux valoriser l’hydrogène produit via des réactions secondaires à la BOR. Enfin, l’utilisation de la spectroscopie infrarouge à transformée de Fourier couplée à de la microscopie électronique en transmission à localisation identique a permis de détecter la formation de carbonates au cours d’un test de vieillissement accéléré d’électrocatalyseurs à base de métaux nobles en milieu alcalin. Ce mécanisme explique, en partie, le détachement des nanoparticules observé au cours du test. / The direct borohydride fuel cell (DBFC), a subclass of alkaline fuel cells, benefits from the advantages of its fuel, sodium borohydride (NaBH4), which exhibits very interesting thermodynamic and energetic characteristics. However, the NaBH4 electrooxidation reaction (BOR) is very complex; to date it remains poorly studied and understood on many electrocatalysts (most of them are in the form of metal nanoparticles supported on carbon black). In addition, recent studies reported the aggressiveness of the alkaline medium on the durability of conventional carbon-supported electrocatalysts, revealing a large loss of the active catalytic surface, mainly due to the detachment of nanoparticles from the carbon support. In this context, this thesis focused on three main areas of study: (i) the study of the BOR on palladium-based electrocatalysts in conditions close to the real operating conditions of the DBFC; (ii) the study of the impact of the anode structure on the overall performance of the DBFC, and (iii) the study of the degradation mechanism of noble metal electrocatalysts in alkaline environment. The experiments were carried out in close collaboration with the U.S. Naval Research Laboratory (Washington, USA).The results obtained showed that a high concentration of NaBH4 leads to a decrease of the reaction kinetics, due in part to poisoning of the catalytic surface. In addition, activity markers for the BOR have been proposed. Then, the use of catalysts-gradient electrodes proved to be a promising solution to better valorize the hydrogen produced via side reactions of the BOR. Finally, the use of Fourier transform infrared spectroscopy coupled with identical-location transmission electron microscopy enabled to detect the formation of carbonates during the accelerated stress test of carbon-supported noble metal electrocatalysts in alkaline medium, explaining, in part, the detachment of nanoparticles observed during the test.

This work examines various light metal borohydrides, particularly those formed from group II metals, with the aim of understanding their fundamental physical properties and improving their hydrogen storage ability. The structure of a new phase (γ) of Mg(BH₄)₂ is reported and the decomposition is fully characterized in a combination of diffraction and thermogravimetric studies. The bulk properties of γ-Mg(BH₄)₂ are compared to those of an SiO₂ isostructure and probed by various neutron scattering techniques. Negative thermal expansion is observed at low temperatures and the material absorbs up to 1.5 moles of hydrogen gas to form one of the most gravimetrically hydrogen-dense materials ever reported. The structural evolution of Ca(BH₄)₂ under different synthetic conditions and external influences (e.g. temperature) is studied up until the material decomposes. The effects of various additives on group II metal borohydrides are also examined and the influence of each is justified by observing subtle structural changes in the mixed system via in situ synchrotron X-ray powder diffraction and ¹¹B NMR measurements.

Apesar de toda a atenção dada à eletro-oxidação do íon boroidreto, pela possibilidade de seu uso em células a combustível, pouco se tem discutido a respeito das instabilidades cinéticas associadas a esse processo. A maioria das pesquisas tem como foco o estudo da cinética dos processos na região de baixos sobrepotenciais, numa região relativamente próxima ao estado de equilíbrio termodinâmico. Em uma região afastada do equilíbrio, diferentes mecanismos regem o sistema e desta forma diferentes informações podem ser obtidas. O objetivo da presente dissertação consiste na investigação sistemática da cinética complexa associada aos processos de adsorção, oxidação e interação do íon boroidreto em superfícies policristalinas de platina. As investigações foram realizadas em condições afastadas do equilíbrio utilizando-se técnicas eletroquímicas convencionais, incluindo: voltametria cíclica, cronoamperometria, experimentos quase estacionários sob controle potenciostático utilizando diferentes valores de resistência total e transientes de circuito aberto. Dentre os resultados obtidos pode-se destacar: a alta velocidade na redução dos óxidos de platina em transientes de circuito aberto (i.e. 0,5 s), sendo esta mais alta do que para álcoois ou hidrogênio; dois processos auto-catalíticos no perfil de potencial de circuito aberto, que podem ser relacionadas com uma biestabilidade do sistema; as oscilações de corrente obtidas em experimentos quase estacionários utilizando diferentes valores de resistência externa; e as oscilações de baixa frequência obtidas na ausência de resistência externa presentes em uma ampla faixa de potencial. Estes resultados apresentam aspectos tecnológicos, como a relação direta entre o baixo tempo de redução de óxido e a perda de desempenho de células a combustível por conta de difusão de combustível, e fundamentais, como a emergência de oscilações e a caracterização do sistema como N-NDR, que podem contribuir para um maior entendimento sobre o uso deste combustível. Esses são resultados ainda não explorados na literatura e o presente trabalho propõe o preenchimento desta lacuna. / Despite all attention given to the electro-oxidation of the ion borohydride, for its applicability in fuel cells, few discussions have been made on the kinetical instabilities possibly present on this process. Most researches focuses on the kinetical study of these processes at a low overpotential range, relatively close to the thermodynamic equilibrium. Under far from thermodynamic condition, different mechanisms rule the system and different information can be obtained. The present dissertation aims at a systematic investigation of the complex kinetic associated to the adsorption, oxidation and interaction processes of the borohydride ion over polycrystalline platinum surface. The investigations were realized under far from thermodynamic equilibrium conditions using conventional electrochemical techniques, including: cyclic voltammetry, chronoamperometry, quasi-steady experiments under potentiostac control using different values of external resistance and open circuit transients. Among the results it is possible to highlight: the fast reduction of platinum oxides in open circuit transients (i.e. 0.5s), which is faster than when alcohol or hydrogen is used; two auto-catalytic processes in the open-circuit profile, which can be related to a bistability of the system; current oscillations in quasi-steady experiments using different values of external resistances; and the low frequency oscillations obtained without external resistance under a large potential range. These results show technological aspects, such as the direct relationship of the short time for oxide reduction and the performance loss of a fuel cell by crossover, and fundamental, such as the emergency of oscillations and the characterization of the system as a N-NDR, which can contribute to a better understanding of this fuel. These are yet not explored results in literature and the present research propose filling the gap.

Este trabalho teve por objetivo a pesquisa e o desenvolvimento de catalisadores formados por ligas formadoras de hidreto metálico (LaNi4,7Sn0,2Cu0,1, LaNi4,78Al0,22, LaNi4,78Mn0,22 e LaNi4,7Sn0,2Co0,1), com e sem depósitos de metais nobres (Pt, Au, Pd), para uso no ânodo de células a combustível alcalinas. Buscou-se o entendimento do mecanismo da reação de oxidação de boroidreto de sódio em meio alcalino nestes diferentes materiais em diferentes configurações de eletrodos e em diversas concentrações de boroidreto. As ligas foram preparadas pelo método de fusão em forno a arco e os metais nobres foram depositados por troca galvânica. A caracterização físico-composicional foi feita por energia dispersiva de raios X (EDS), análise elementar, difração de raios X (DRX) e microscopia eletrônica de transmissão (MET). Já a caracterização eletroquímica foi feita por voltametria cíclica, curvas de capacidade de descarga, curvas de polarização e espectrometria de massas eletroquímica online (OLEMS). Os resultados mostraram que a quantidade de material nobre presente nas ligas foi da ordem de 0,1 a 0,3 % e suas composições e organizações estruturais foram investigadas por EDS, DRX e MET. Os resultados eletroquímicos mostraram que o estado de hidretação da liga base é um fator importante na definição da cinética da reação de oxidação de boroidreto e que a extensão da formação de hidreto é dependente da velocidade do processo de hidrólise dos íons boroidreto, da capacidade da liga em estocar hidrogênio e da cinética de difusão dos átomos de hidrogênio atômico no interior das ligas. A atividade para a reação direta de oxidação de boroidreto foi baixa para as ligas que não continham em suas composições os metais nobres, entretanto a taxa de hidrólise para estes foi menor e a capacidade de armazenamento de hidrogênio foi maior. Os resultados também mostraram, para todas as diferentes composições, que as ligas podem ser carregadas com hidrogênio atômico tanto eletroquimicamente quanto pela exposição à solução de boroidreto. Foi observado que houve contínua hidretação de todas as ligas durante ensaios cronoamperométricos na presença de boroidreto. Esses fenômenos indicaram que a liga LaNi4.7Sn0.2Cu0.1 sem a deposição de metais nobres é a mais indicada para uma futura aplicação em um sistema DBFC/MHB (célula a combustível de boroidreto direto/bateria de hidreto metálico). / This work provides insights into the processes involved in the borohydride oxidation reaction (BOR) and hydrogen oxidation reaction in alkaline media on metal hydride alloys formed by LaNi4.7Sn0.2Cu0.1 and LaNi4.78Al0.22 with and without deposited Pt, Pd and Au, LaNi4,78Mn0,22 and LaNi4,7Sn0,2Co0,1. Measurements of BET, TEM, in situ XRD, DRIFTS, RRDE (gold ring) and OLEMS were made with the aim to characterize all the materials by different methodologies. The present investigation showed that the state of hydriding of LaNi4.7Sn0.2Cu0.1 and LaNi4.78Al0.22 is important in defining the kinetics and outcome of the BOR at these materials. The extent of hydride formation has been shown to depend on the rate of the BH4- hydrolysis, the hydrogen storage capacity of the alloy, and kinetics of H atom diffusion inside the alloy. The activity for the direct BOR is low in both bare metal hydride alloys, but the rate of the BH4- hydrolysis and the hydrogen storage capacity are higher, while the rate of H diffusion is slower in the bare LaNi4.78Al0.22. Results have shown that both the bare and all the noble metal modified alloys can be hydrided either electrochemically or by exposure to BH4- solutions, although for the LaNi4.78Al0.22-based materials the extent of this phenomenon is smaller. A continuous hydriding of all LaNi4.7Sn0.2Cu0.1-based and the bare LaNi4.78Al0.22 alloys are observed in the chronoamperometric BH4- oxidation measurements at low current densities. Addition of Pt on both alloys resulted in an increase of the BH4- hydrolysis, but the H2 formed is rapidly oxidized, confirming the initial predictions for this noble metal. In addition, the rates of the alloy hydriding/de-hydriding were not significantly affected by the presence of Pt, but this was not the case for Pd and Au, for which there was a drastic reduction of the rate of these processes. In the case of gold some increase of the BH4- hydrolysis is observed, although its presence does not change significantly the performances of the bare alloys. It was possible to confirm the formation of BH3OH- for all the samples by RRDE measurements, however for LaNi4.7Sn0.2Cu0.1 the formation of this product was lower compared to the other samples. Results showed that the first step of the electrode process is the hydriding of alloy by the hydrogen formed in the BH4- hydrolysis. The in situ XRD results have shown that the method of charging of the alloy, electrochemical or chemical, leads to different phase predominance and different site occupancies, but both methods lead to almost the same discharge capacity. In the electrolysis process, the α-β phase transition is predominant while in the chemical charging by the exposure to borohydride solution the α-β phase transition is more important. The results have also shown that the alloys with and without Pt lead essentially to the same phenomena, either with respect to the alloy structure and the electrochemical characteristics. All these phenomena point to the bare LaNi4.7Sn0.2Cu0.1 as a more adequate alloy system for applications in DBFC/MHB batteries.

Este trabalho teve por objetivo a pesquisa e o desenvolvimento de catalisadores formados por ligas formadoras de hidreto metálico (LaNi4,7Sn0,2Cu0,1, LaNi4,78Al0,22, LaNi4,78Mn0,22 e LaNi4,7Sn0,2Co0,1), com e sem depósitos de metais nobres (Pt, Au, Pd), para uso no ânodo de células a combustível alcalinas. Buscou-se o entendimento do mecanismo da reação de oxidação de boroidreto de sódio em meio alcalino nestes diferentes materiais em diferentes configurações de eletrodos e em diversas concentrações de boroidreto. As ligas foram preparadas pelo método de fusão em forno a arco e os metais nobres foram depositados por troca galvânica. A caracterização físico-composicional foi feita por energia dispersiva de raios X (EDS), análise elementar, difração de raios X (DRX) e microscopia eletrônica de transmissão (MET). Já a caracterização eletroquímica foi feita por voltametria cíclica, curvas de capacidade de descarga, curvas de polarização e espectrometria de massas eletroquímica online (OLEMS). Os resultados mostraram que a quantidade de material nobre presente nas ligas foi da ordem de 0,1 a 0,3 % e suas composições e organizações estruturais foram investigadas por EDS, DRX e MET. Os resultados eletroquímicos mostraram que o estado de hidretação da liga base é um fator importante na definição da cinética da reação de oxidação de boroidreto e que a extensão da formação de hidreto é dependente da velocidade do processo de hidrólise dos íons boroidreto, da capacidade da liga em estocar hidrogênio e da cinética de difusão dos átomos de hidrogênio atômico no interior das ligas. A atividade para a reação direta de oxidação de boroidreto foi baixa para as ligas que não continham em suas composições os metais nobres, entretanto a taxa de hidrólise para estes foi menor e a capacidade de armazenamento de hidrogênio foi maior. Os resultados também mostraram, para todas as diferentes composições, que as ligas podem ser carregadas com hidrogênio atômico tanto eletroquimicamente quanto pela exposição à solução de boroidreto. Foi observado que houve contínua hidretação de todas as ligas durante ensaios cronoamperométricos na presença de boroidreto. Esses fenômenos indicaram que a liga LaNi4.7Sn0.2Cu0.1 sem a deposição de metais nobres é a mais indicada para uma futura aplicação em um sistema DBFC/MHB (célula a combustível de boroidreto direto/bateria de hidreto metálico). / This work provides insights into the processes involved in the borohydride oxidation reaction (BOR) and hydrogen oxidation reaction in alkaline media on metal hydride alloys formed by LaNi4.7Sn0.2Cu0.1 and LaNi4.78Al0.22 with and without deposited Pt, Pd and Au, LaNi4,78Mn0,22 and LaNi4,7Sn0,2Co0,1. Measurements of BET, TEM, in situ XRD, DRIFTS, RRDE (gold ring) and OLEMS were made with the aim to characterize all the materials by different methodologies. The present investigation showed that the state of hydriding of LaNi4.7Sn0.2Cu0.1 and LaNi4.78Al0.22 is important in defining the kinetics and outcome of the BOR at these materials. The extent of hydride formation has been shown to depend on the rate of the BH4- hydrolysis, the hydrogen storage capacity of the alloy, and kinetics of H atom diffusion inside the alloy. The activity for the direct BOR is low in both bare metal hydride alloys, but the rate of the BH4- hydrolysis and the hydrogen storage capacity are higher, while the rate of H diffusion is slower in the bare LaNi4.78Al0.22. Results have shown that both the bare and all the noble metal modified alloys can be hydrided either electrochemically or by exposure to BH4- solutions, although for the LaNi4.78Al0.22-based materials the extent of this phenomenon is smaller. A continuous hydriding of all LaNi4.7Sn0.2Cu0.1-based and the bare LaNi4.78Al0.22 alloys are observed in the chronoamperometric BH4- oxidation measurements at low current densities. Addition of Pt on both alloys resulted in an increase of the BH4- hydrolysis, but the H2 formed is rapidly oxidized, confirming the initial predictions for this noble metal. In addition, the rates of the alloy hydriding/de-hydriding were not significantly affected by the presence of Pt, but this was not the case for Pd and Au, for which there was a drastic reduction of the rate of these processes. In the case of gold some increase of the BH4- hydrolysis is observed, although its presence does not change significantly the performances of the bare alloys. It was possible to confirm the formation of BH3OH- for all the samples by RRDE measurements, however for LaNi4.7Sn0.2Cu0.1 the formation of this product was lower compared to the other samples. Results showed that the first step of the electrode process is the hydriding of alloy by the hydrogen formed in the BH4- hydrolysis. The in situ XRD results have shown that the method of charging of the alloy, electrochemical or chemical, leads to different phase predominance and different site occupancies, but both methods lead to almost the same discharge capacity. In the electrolysis process, the α-β phase transition is predominant while in the chemical charging by the exposure to borohydride solution the α-β phase transition is more important. The results have also shown that the alloys with and without Pt lead essentially to the same phenomena, either with respect to the alloy structure and the electrochemical characteristics. All these phenomena point to the bare LaNi4.7Sn0.2Cu0.1 as a more adequate alloy system for applications in DBFC/MHB batteries.

Recent studies have shown that ruthenium(III) acetylacetonate is acting as homogeneous catalyst in the hydrolysis of sodium borohydride. Although trimethlyphosphite is found to be a poison for the catalytic hydrolysis of sodium borohydride, a longer observation of the reaction in the presence of ruthenium(III) acetylacetonate and 2 equivalent trimethylphosphite shows an unexpected enhancement in the catalytic activity after an induction period. The same rate enhancement is observed when 2 equivalent triphenylphosphine is added into the reaction solution. Addition of 1 equivalent 1,2-bis(diphenylphosphino)ethane, dppe, into the solution shows similarly a rate enhancement in the hydrolysis of sodium borohydride catalyzed by ruthenium(III) acetylacetonate. The effect of 1,2-bis(diphenylphosphino)ethane on the catalytic activity of ruthenium(III) acetylacetonate in the hydrolysis of sodium borohydride was studied by varying mole ratio of dppe / Ru(acac)3, ruthenium concentration, substrate concentration and temperature. The highest enhancement in the rate of hydrolysis was obtained when 1 equivalent dppe was used and therefore, this mole ratio of dppe / Ru(acac)3 was used in the further studies. The rate of the reaction was found to be first order in catalyst concentration and zero order in substrate concentration. From the evaluation of rate constant versus temperature data, the activation parameters for the hydrolysis of sodium borohydride catalyzed by ruthenium(III) acetylacetonate plus 1 equivalent dppe were found to be Ea= 59 &plusmn / 2 kJ/mol, &amp / #8710 / H&amp / #8800 / = 60 &plusmn / 1 kJ.mol-1 and &amp / #8710 / S&amp / #8800 / = -50 &plusmn / 3 J.(mol.K)-1. A series of control experiments were performed to characterize the active catalyst. However, the only useful information could be obtained by comparison of the UV-vis electronic absorption spectra taken from the solution during the catalytic reaction, is that, ruthenium(III) is reduced to ruthenium(II) in the course of reaction. It was concluded that a ruthenium(II) species is formed as a transient and may be the active catalyst in the reaction. After the reaction, the only ruthenium species isolated from the solution was the ruthenium(III) acetylacetonate.

No description available.

Antisense oligodeoxynucleotides (asODNs) show strong binding and high selectivity and can be constructed to recognize specific cellular targets such as gene regulated mRNA. Radiolabelled asODNs have the potential to image gene expression through mRNA targeting and could be a valuable tool in the early assessment of outcome to cancer treatment. We have explored the potential of in vivo imaging of p21 gene expression, using fluorine-18 labelled asODNs ([18F]asODNs) and in vitro techniques, recognizing the relationship between the expression of this gene and resistance of cancer cells to radiation therapy. Radiolabelling of fully phosphorothioated, 20-mer ODNs was performed using the [18F]-labelled prosthetic group, 4-N-[18F]fluorobenzyl-2-bromoacetamide ([18F]FBBA). [18F]FBBA was first synthesized in an automated synthesis unit, resulting in a modest radiochemical yield. Methods to improve the yield were investigated using a metal catalyst-assisted borohydride exchange resin. Alkylation of [18F]FBBA to ODN resulted in radiochemical yields of 40%. Cellular uptake and retention studies were performed in human carcinoma cells expressing p21+/+ (HCT116) and the p21 knock-out cell line, 80S4, using both [18F]-labelled antisense and random sequence ODNs. Nonradioactive FBBA-labelled ODNs were used to evaluate the antisense effectiveness and distribution of the FBBA-modified ODNs. In vitro studies demonstrated that FBBA did not interfere with the antisense effect of ODNs against p21 mRNA; however, the probes required a transfection agent to observe an antisense effect. Cell fractionation studies with [18F]ODNs revealed increasing accumulation of liposome-transfected [18F]asODN in the cytoplasm of HCT116 cells over time. A biocompatible spermine-grafted block copolymer (SP) was subsequently evaluated as a potential vector to improve the delivery of [18F]asODN into cells. SP was shown to direct [F]-labelled ODNs to the cytoplasm, whereas naked [F]ODNs remained sequestered in vesicles, and liposome-transfected [F]ODNs localized mostly in the nucleus. Selective uptake and retention of [18F]asODN was observed in p21+/+ cells only when the probe was transfected with SP. Based on these studies, it can be concluded that [18F]asODNs have the potential to image gene expression, however the focus may need to be directed to find an appropriate vector which can rapidly deliver [18F]-labelled asODNs to the target tissue in vivo.

Made available in DSpace on 2016-08-19T12:56:47Z (GMT). No. of bitstreams: 1 Dissertacao LEANDRO LIMA CARVALHO.pdf: 1286583 bytes, checksum: 3198a5ae4efaeaf29e291b70322cab13 (MD5) Previous issue date: 2014-04-04 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Some substances sound considered promising as a fuel to power electrochemical energy converters. These substances sound oxidized at the anode, while pure or oxygen from air is reduced at the cathode in galvanic cells without production of unhealthy pollutants, generating current cleanly and efficiently. The electrocatalysis of methanol, ethanol, ethylene glycol, glycerol, sodium borohydride and formaldehyde are topics of great interest, justified by the demand for new systems more efficient converters and cleaner energy. In this work Pd, Pd₇₅Ni₂₅, Pd₆₆Ni₃₄ and Pd₅₀Ni₅₀ nanoparticles supported on carbon Vulcan XC - 72 were prepared by the method of reduction by sodium borohydride in alkaline medium. The catalysts were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The results showed that nanoparticles of Pd and PdxNiy, which were uniformly dispersed on carbon, had a diameter of 2-7 nm. Studies of cyclic voltammetry (CV ) and Linear sweep voltammetry (LSV) oxidation of methanol , ethanol , ethylene glycol, glycerol and sodium borohydride showed that the addition of Ni in the catalyst nanoparticles changes the mechanism of the reactions of Pd / C and the electrocatalytic activity increases in the order Pd₇₅Ni₂₅ / C < Pd₆₆Ni₃₄ / C < Pd₅₀Ni₅₀ / C in alkaline medium . So electrocatalysts produced with Ni nanoparticles become promising candidates for the direct alcohol cells (DAFCs) cells and direct borohydride (DBFCs) fuel fuel. / Algumas substâncias são consideradas promissoras como combustível para alimentar conversores eletroquímicos de energia. Estas substâncias são oxidadas no ânodo, enquanto oxigênio puro ou proveniente do ar é reduzido no cátodo, em células galvânicas, sem produção de poluentes nocivos à saúde, gerando corrente de forma limpa e eficiente. As eletrocatálises de metanol, etanol, etilenoglicol, glicerol, boroidreto e formaldeído são temas de grande interesse, justificado pela procura de novos sistemas conversores de energia mais eficientes e menos poluentes. No presente trabalho, nanopartículas de Pd, Pd₇₅Ni₂₅, Pd₆₆Ni₃₄ e Pd₅₀Ni₅₀ suportadas sobre carbono Vulcan XC-72, foram preparadas pelo método de redução por boroidreto de sódio em meio alcalino. Os catalisadores foram caracterizados por difração de raios-X (DRX), fluorescência de raios-X (FRX), voltametria cíclica (VC) e varredura linear (VL). Os resultados mostraram que as nanopartículas de Pd e PdxNiy, que foram uniformemente dispersas sobre carbono, apresentaram de 2-7 nm de diâmetro. Os estudos de voltametria cíclica (VC) e voltametria linear (VL) das oxidações de metanol, etanol, etilenoglicol, glicerol e boroidreto de sódio mostraram que a adição de nanopartículas de Ni nos catalisadores, altera o mecanismo das reações sobre Pd/C e a atividade eletrocatalítica aumenta na ordem Pd₇₅Ni₂₅/C < Pd₆₆Ni₃₄ /C < Pd₅₀Ni₅₀/C em meio alcalino. Portanto, eletrocatalisadores produzidos com nanopartículas de Ni tornam-se candidatos promissores para células a combustível de álcool direto (CCADs) e células a combustível de boroidreto direto (CCBDs).

Apesar de toda a atenção dada à eletro-oxidação do íon boroidreto, pela possibilidade de seu uso em células a combustível, pouco se tem discutido a respeito das instabilidades cinéticas associadas a esse processo. A maioria das pesquisas tem como foco o estudo da cinética dos processos na região de baixos sobrepotenciais, numa região relativamente próxima ao estado de equilíbrio termodinâmico. Em uma região afastada do equilíbrio, diferentes mecanismos regem o sistema e desta forma diferentes informações podem ser obtidas. O objetivo da presente dissertação consiste na investigação sistemática da cinética complexa associada aos processos de adsorção, oxidação e interação do íon boroidreto em superfícies policristalinas de platina. As investigações foram realizadas em condições afastadas do equilíbrio utilizando-se técnicas eletroquímicas convencionais, incluindo: voltametria cíclica, cronoamperometria, experimentos quase estacionários sob controle potenciostático utilizando diferentes valores de resistência total e transientes de circuito aberto. Dentre os resultados obtidos pode-se destacar: a alta velocidade na redução dos óxidos de platina em transientes de circuito aberto (i.e. 0,5 s), sendo esta mais alta do que para álcoois ou hidrogênio; dois processos auto-catalíticos no perfil de potencial de circuito aberto, que podem ser relacionadas com uma biestabilidade do sistema; as oscilações de corrente obtidas em experimentos quase estacionários utilizando diferentes valores de resistência externa; e as oscilações de baixa frequência obtidas na ausência de resistência externa presentes em uma ampla faixa de potencial. Estes resultados apresentam aspectos tecnológicos, como a relação direta entre o baixo tempo de redução de óxido e a perda de desempenho de células a combustível por conta de difusão de combustível, e fundamentais, como a emergência de oscilações e a caracterização do sistema como N-NDR, que podem contribuir para um maior entendimento sobre o uso deste combustível. Esses são resultados ainda não explorados na literatura e o presente trabalho propõe o preenchimento desta lacuna. / Despite all attention given to the electro-oxidation of the ion borohydride, for its applicability in fuel cells, few discussions have been made on the kinetical instabilities possibly present on this process. Most researches focuses on the kinetical study of these processes at a low overpotential range, relatively close to the thermodynamic equilibrium. Under far from thermodynamic condition, different mechanisms rule the system and different information can be obtained. The present dissertation aims at a systematic investigation of the complex kinetic associated to the adsorption, oxidation and interaction processes of the borohydride ion over polycrystalline platinum surface. The investigations were realized under far from thermodynamic equilibrium conditions using conventional electrochemical techniques, including: cyclic voltammetry, chronoamperometry, quasi-steady experiments under potentiostac control using different values of external resistance and open circuit transients. Among the results it is possible to highlight: the fast reduction of platinum oxides in open circuit transients (i.e. 0.5s), which is faster than when alcohol or hydrogen is used; two auto-catalytic processes in the open-circuit profile, which can be related to a bistability of the system; current oscillations in quasi-steady experiments using different values of external resistances; and the low frequency oscillations obtained without external resistance under a large potential range. These results show technological aspects, such as the direct relationship of the short time for oxide reduction and the performance loss of a fuel cell by crossover, and fundamental, such as the emergency of oscillations and the characterization of the system as a N-NDR, which can contribute to a better understanding of this fuel. These are yet not explored results in literature and the present research propose filling the gap.