<p>The particles of the Pt metal were well dispersed on carbon nanotubes when EG was used and in isopropanol poor dispersion was observed and no further investigation was done on them. The platinum wt% on the supports observed from EDS was 21.8, 19.10 and 16.74wt% for Pt/EMWCNT, Pt/LPGCNT and Pt/ commercial CNT respectively. Pt/LPGMWCNT showed high electro-catalytic activity of 2.48 mA and active surface area of 76 m2/g, toward oxygen reduction, observed from cyclic voltammogram in iv sulfuric acid. Pt/LPGMWCNT also showed better tolerance toward methanol, however it was not highly active towards methanol, and hence the methanol oxidation peak current observed between 0.75 and 08 potential was the smallest. In this study a wide range of instruments was used to characterize the properties and behavior of Platinum nanoparticles on multi-wall carbon nanotubes. To add to the already mentioned, Scanning electrochemical microscopy (SEM), proton induced x-ray emission (PIXE), scanning electrochemical microscopy (SECM) and Brunauer-Emmett Tellar (BET) were also used.</p>

<p>The particles of the Pt metal were well dispersed on carbon nanotubes when EG was used and in isopropanol poor dispersion was observed and no further investigation was done on them. The platinum wt% on the supports observed from EDS was 21.8, 19.10 and 16.74wt% for Pt/EMWCNT, Pt/LPGCNT and Pt/ commercial CNT respectively. Pt/LPGMWCNT showed high electro-catalytic activity of 2.48 mA and active surface area of 76 m2/g, toward oxygen reduction, observed from cyclic voltammogram in iv sulfuric acid. Pt/LPGMWCNT also showed better tolerance toward methanol, however it was not highly active towards methanol, and hence the methanol oxidation peak current observed between 0.75 and 08 potential was the smallest. In this study a wide range of instruments was used to characterize the properties and behavior of Platinum nanoparticles on multi-wall carbon nanotubes. To add to the already mentioned, Scanning electrochemical microscopy (SEM), proton induced x-ray emission (PIXE), scanning electrochemical microscopy (SECM) and Brunauer-Emmett Tellar (BET) were also used.</p>

Magister Scientiae - MSc / The particles of the Pt metal were well dispersed on carbon nanotubes when EG was used and in isopropanol poor dispersion was observed and no further investigation was done on them. The platinum wt% on the supports observed from EDS was 21.8, 19.10 and 16.74wt% for Pt/EMWCNT, Pt/LPGCNT and Pt/ commercial CNT respectively. Pt/LPGMWCNT showed high electro-catalytic activity of 2.48 mA and active surface area of 76 m2/g, toward oxygen reduction, observed from cyclic voltammogram in iv sulfuric acid. Pt/LPGMWCNT also showed better tolerance toward methanol, however it was not highly active towards methanol, and hence the methanol oxidation peak current observed between 0.75 and 08 potential was the smallest. In this study a wide range of instruments was used to characterize the properties and behavior of Platinum nanoparticles on multi-wall carbon nanotubes. To add to the already mentioned, Scanning electrochemical microscopy (SEM), proton induced x-ray emission (PIXE), scanning electrochemical microscopy (SECM) and Brunauer-Emmett Tellar (BET) were also used. / South Africa

Low temperature fuel cells like proton exchange membrane fuel cells (PEMFC) are expected to play a crucial role in the future hydrogen economy, especially for transportation applications. These electrochemical devices offer significantly higher efficiency compared to conventional heat engines. However, use of exotic and expensive platinum as the electrocatalyst poses serious problems for commercial viability. In this regard, there is an urgent need to develop low-platinum or non-platinum electrocatalysts with electrocatalytic activity for the oxygen reduction reaction (ORR) superior or comparable to that of platinum. This dissertation first investigates non-platinum, palladium-based alloy electrocatalysts for ORR. Particularly, Pd-M (M = Mo and W) alloys are synthesized by a novel thermal decomposition of organo-metallic precursors. The carbon-supported Pd-M (M = Mo, W) electrocatalyts are then heat treated up to 900 oC in H2 atmosphere and investigated for their phase behavior. Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements reveal that the alloying of Pd with Mo or W significantly enhances the catalytic activity for ORR as well as the stability (durability) of the electrocatalysts. Additionally, both the alloy systems exhibit high tolerance to methanol, which is particularly advantageous for direct methanol fuel cells (DMFC). The dissertation then focuses on one-pot synthesis of carbon-supported multi-metallic Pt-Pd-Co nanoalloys by a rapid microwave-assisted solvothermal (MW-ST) method. The multi-metallic alloy compositions synthesized by the MW-ST method show much higher catalytic activity for ORR compared to their counterparts synthesized by the conventional borohydride reduction method. Additionally, a series of Pt encapsulated Pd-Co nanoparticle electrocatalysts are synthesized by the MW-ST method and characterized to understand their phase behavior, surface composition, and electrocatalytic activity for ORR. Finally, the dissertation focuses on carbon-supported binary Pt@Cu and ternary PtxPd1-x@Cu “core-shell” nanoparticles synthesized by a novel galvanic displacement of Cu by Pt4+ and Pd2+ at ambient conditions. Structural characterizations suggest that the Pt@Cu nanoparticles have a Pt-Cu alloy layer sandwiched between a copper core and a Pt shell. The electrochemical data clearly point to an enhancement in the activity for ORR for the Pt@Cu “core-shell” nanoparticle electrocatalysts compared to the commercial Pt electrocatalyst, both on per unit mass of Pt and per unit active surface area basis. The increase in activity for ORR is ascribed to electronic modification of the outer Pt shell by the Pt-Cu alloy core. However, incorporation of Pd to obtain PtxPd1-x@Cu deteriorates the activity for ORR. / text

Amelioration of the building blocks is a plausible approach to graft aerogels with distinguished properties while preserving the aerogel superiority. However, the incorporation of designated properties into metallic aerogels, especially catalytically beneficial morphologies and transition metal doping, still remains a challenge. Here, we report on the first case of an aerogel electrocatalyst composed entirely of alloyed PdNi hollow nanospheres (HNSs) with controllable chemical composition and shell thickness. The synergy of the transition metal doping, combined with the hollow building blocks and the three dimensional network structure make the PdNi HNS aerogels promising electrocatalysts towards ethanol oxidation, among which the Pd83Ni17 HNS aerogel shows a 5.6-fold enhanced mass activity compared to commercial Pd/C. This work expands the exploitation approach of electrocatalytic properties of aerogels into morphology and composition control of its building blocks.

Eletrocatalisadores de Pt-Sn suportados em carbono de elevada área superficial foram preparados por redução química de precursores metálicos em solução usando como agente redutor o ácido fórmico 0,5 M, 0,1 M e 2 M e caracterizados em termos de estrutura, morfologia e propriedades superficiais para aplicação em células a combustível a etanol direto (DEFC). Os catalisadores foram caracterizados por fluorescência de raios X (XRF), difração de raios X (DRX), espectroscopia de fotoelétrons de raios X (XPS), microscopia eletrônica de transmissão (TEM) e análise termogravimétrica (TGA). Os testes eletroquímicos dos catalisadores foram realizados com o uso de medidas de polarização, voltametria cíclica e linear, testes de estabilidade a potencial constante, e espectroscopia de impedância eletroquímica. A melhor composição atômica superficial, em termos de desempenho eletroquimico foi Pt3Sn1/C. Sobrepotenciais ânodicos são necessários de modo a gerar espécies oxidadas na superfície de platina, desta maneira a reação é favorecida. Em termos de produtos de oxidação, o acetaldeído e o ácido acético foram os principais produtos detectados por HPLC, com um baixo teor de CO2. A seletividade para o ácido acético com respeito ao acetaldeído aumentou com o teor de Sn e diminuiu com a menor concentração do agente redutor utilizado na preparação do catalisador. Estudos de estabilidade com base no catalisador Pt3Sn1/C foram realizados e o efeito de diferentes procedimentos de liga/desconexão da célula investigados. Testes de 250h a 0.4V incluindo tres ciclos de liga/desconexão a 50h, 150h e 200h são apresentados. Na sequência foram analisados os materiais com testes de estabilidade contínuo de 220 h em 0,4 V com recirculação de etanol. Pode-se dizer que no caso de um procedimento contínuo o sitema é menos afetado devido a menor perda em relação a corrosão do material. Análise morfológica e estrutural pós-testes forneceram informações complementares no que diz respeito ao diagnóstico eletroquímico in-situ e sobre o mecanismo de degradação. O aspecto mais crítico em relação às perdas irrecuperáveis aparece estar relacionado com a corrosão de Sn. Com base nos resultados obtidos neste trabalho, o protocolo de liga/desconexão foi o que mais afetou o desempenho geral da célula. / Pt-Sn electrocatalysts supported on carbon having high surface area were prepared by chemical reduction of metal precursors in solution using 0.5 M, 0.1 M and 2 M formic acid. These electrocatalysts used as anode materials for direct ethanol fuel cells (DEFC), have been physico-chemically characterized in terms of structure, morphology and surface properties. The catalysts were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM) e Thermogravimetric Analysis (TGA). The electrochemical properties of such electrocatalysts were investigated by using polarization measurement, cyclic and linear voltammetry, potentiostatic lifetime tests and electrochemical impedance spectroscopy (EIS). The best composition, in terms of electrochemical performance was the Pt3Sn1/C sample. High anode overpotentials are required in order to generate oxidized species on the platinum surface, which favor the reaction is favored. Essentially, acetaldehyde and acetic acid were detected as the main reaction products, whereas, a low content of CO2 was detected. The selectivity towards acetic acid with respect to acetaldehyde increased with the increase of the Sn content and decreased by decreasing the concentration of the reducing agent used in the catalyst preparation. Stability tests of a DEFC based on a Pt3Sn1/C catalyst was studied and the effect of different cell shut-down procedures investigated. Tests of 250 h at 0.4 V cell potential which include three intermediate shut-down/start-up cycles at 50, 150 and 200 h were was analyzed. Stability tests of 220 h at 0.4 V with recirculation of ethanol were carried out. In the case of a continuous procedure the system is less affected due to the lower loss compared material. Morphological analysis of structural and post-tests provide additional information regarding the diagnosis and in-situ electrochemical effects on the mechanism of degradation. The most critical aspect in respect of irrecoverable losses appears to be related to the corrosion of Sn. Based on the results obtained in this study, the protocol of start/stop was the one that most affects the overall performance of the cell.

Proton exchange membrane fuel cells (PEMFC) are attractive power sources as they offer high conversion efficiencies with low or no pollution. However, several challenges, especially the sluggish oxygen reduction reaction (ORR) and the high cost of Pt catalysts, impede their commercialization. With an aim to search for more active, less expensive, and more stable ORR catalysts than Pt, this dissertation focuses on the development of non-platinum or low-platinum Pd-based nanostructured electrocatalysts and a fundamental understanding of their structure-property-performance relationships. Carbon-supported Pd–Ni nanoalloy electrocatalysts with different Pd/Ni atomic ratios have been synthesized by a modified polyol reduction method, followed by heat treatment in a reducing atmosphere at 500–900 oC. The Pd–Ni sample with a Pd:Ni atomic ratio of 4:1 after heat treatment at 500 °C exhibits the highest electrochemical surface area and catalytic activity. The enhanced activity of Pd80Ni20 compared to that of Pd is attributed to Pd enrichment on the surface and the consequent lattice-strain effects. To improve the catalytic activity and long-term durability of the Pd–Ni catalysts, Pd–Pt–Ni nanoalloys have been synthesized by the same method and evaluated in PEMFC. The Pt-based mass activity of the Pd–Pt–Ni catalysts exceeds that of commercial Pt by a factor of 2, and its long-term durability is comparable to commercial Pt within the testing duration of 180 h. Both the favorable and detrimental effects of Pd and Ni dissolution on the performance of the membrane-electrode assembly (MEA) have been investigated by compositional analysis by transmission electron microscopy (TEM) of the MEAs before and after the fuel cell test. The MEAs of the Pd–Pt–Ni catalyst have then been characterized in-situ by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) to better understand the performance changes during cell operation. The surface state change from Pd-enrichment to Pt-enrichment and the consequent decrease in the charge transfer resistance during cell operation is believed to contribute to the activity enhancement. To further improve the MEA performance and durability, the as-synthesized Pd–Pt–Ni catalysts have been pre-leached in acid and Pd–Pt alloy catalysts have been synthesized to alleviate contamination from dissolved metal ions. Compared to the pristine Pd–Pt–Ni catalyst, the preleached catalyst shows improved performance and the Pd–Pt catalyst exhibits similar performance in the entire current density range. Finally, the catalytic activities for ORR obtained from the rotating disk electrode (RDE) and PEMFC single-cell measurements of all the catalysts are compared. The improvement in the activities of the Pd-Pt-based catalysts compared to that of Pt measured by the RDE experiments is much lower than that obtained in single cell test. In other words, RDE tests underestimate the value of the Pd-Pt-based electrocatalysts for real fuel cell applications. Also, based on the RDE data, the Pd–Pt–Cu catalyst exhibits the highest catalytic activity among all the Pd–Pt–M (M = Fe, Ni, Cu) catalysts studied. / text

Eletrocatalisadores de Pt-Sn suportados em carbono de elevada área superficial foram preparados por redução química de precursores metálicos em solução usando como agente redutor o ácido fórmico 0,5 M, 0,1 M e 2 M e caracterizados em termos de estrutura, morfologia e propriedades superficiais para aplicação em células a combustível a etanol direto (DEFC). Os catalisadores foram caracterizados por fluorescência de raios X (XRF), difração de raios X (DRX), espectroscopia de fotoelétrons de raios X (XPS), microscopia eletrônica de transmissão (TEM) e análise termogravimétrica (TGA). Os testes eletroquímicos dos catalisadores foram realizados com o uso de medidas de polarização, voltametria cíclica e linear, testes de estabilidade a potencial constante, e espectroscopia de impedância eletroquímica. A melhor composição atômica superficial, em termos de desempenho eletroquimico foi Pt3Sn1/C. Sobrepotenciais ânodicos são necessários de modo a gerar espécies oxidadas na superfície de platina, desta maneira a reação é favorecida. Em termos de produtos de oxidação, o acetaldeído e o ácido acético foram os principais produtos detectados por HPLC, com um baixo teor de CO2. A seletividade para o ácido acético com respeito ao acetaldeído aumentou com o teor de Sn e diminuiu com a menor concentração do agente redutor utilizado na preparação do catalisador. Estudos de estabilidade com base no catalisador Pt3Sn1/C foram realizados e o efeito de diferentes procedimentos de liga/desconexão da célula investigados. Testes de 250h a 0.4V incluindo tres ciclos de liga/desconexão a 50h, 150h e 200h são apresentados. Na sequência foram analisados os materiais com testes de estabilidade contínuo de 220 h em 0,4 V com recirculação de etanol. Pode-se dizer que no caso de um procedimento contínuo o sitema é menos afetado devido a menor perda em relação a corrosão do material. Análise morfológica e estrutural pós-testes forneceram informações complementares no que diz respeito ao diagnóstico eletroquímico in-situ e sobre o mecanismo de degradação. O aspecto mais crítico em relação às perdas irrecuperáveis aparece estar relacionado com a corrosão de Sn. Com base nos resultados obtidos neste trabalho, o protocolo de liga/desconexão foi o que mais afetou o desempenho geral da célula. / Pt-Sn electrocatalysts supported on carbon having high surface area were prepared by chemical reduction of metal precursors in solution using 0.5 M, 0.1 M and 2 M formic acid. These electrocatalysts used as anode materials for direct ethanol fuel cells (DEFC), have been physico-chemically characterized in terms of structure, morphology and surface properties. The catalysts were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM) e Thermogravimetric Analysis (TGA). The electrochemical properties of such electrocatalysts were investigated by using polarization measurement, cyclic and linear voltammetry, potentiostatic lifetime tests and electrochemical impedance spectroscopy (EIS). The best composition, in terms of electrochemical performance was the Pt3Sn1/C sample. High anode overpotentials are required in order to generate oxidized species on the platinum surface, which favor the reaction is favored. Essentially, acetaldehyde and acetic acid were detected as the main reaction products, whereas, a low content of CO2 was detected. The selectivity towards acetic acid with respect to acetaldehyde increased with the increase of the Sn content and decreased by decreasing the concentration of the reducing agent used in the catalyst preparation. Stability tests of a DEFC based on a Pt3Sn1/C catalyst was studied and the effect of different cell shut-down procedures investigated. Tests of 250 h at 0.4 V cell potential which include three intermediate shut-down/start-up cycles at 50, 150 and 200 h were was analyzed. Stability tests of 220 h at 0.4 V with recirculation of ethanol were carried out. In the case of a continuous procedure the system is less affected due to the lower loss compared material. Morphological analysis of structural and post-tests provide additional information regarding the diagnosis and in-situ electrochemical effects on the mechanism of degradation. The most critical aspect in respect of irrecoverable losses appears to be related to the corrosion of Sn. Based on the results obtained in this study, the protocol of start/stop was the one that most affects the overall performance of the cell.

Developing electrocatalysts with low cost, high activity, and good durability is urgently demanded for the wide commercialization of fuel cells. By taking advantage of nanostructure engineering, we fabricated PtAg nanotubular aerogels (NTAGs) with high electrocatalytic activity and good durability via a simple galvanic replacement reaction between the in situ spontaneous gelated Ag hydrogel and the Pt precursor. The PtAg NTAGs have hierarchical porous network features with primary networks and pores from the interconnected nanotubes of the aerogel and secondary networks and pores from the inter-connected thin nanowires on the nanotube surface, and show very high porosities and large specific surface areas. Due to the unique structure, the PtAg NTAGs exhibit greatly enhanced electrocatalytic activity towards formic acid oxidation, reaching 19 times higher metal based mass current density as compared to the commercial Pt black. Furthermore, the PtAg NTAGs show outstanding structural stability and electrochemical durability during the electrocatalysis. Noble metal based NTAGs are promising candidates for applications in electrocatalysis not only for fuel cells, but also for other energy related systems.

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%.