GUILHERME LUIS CORDEIRO

GUILHERME LUIS CORDEIRO

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Y-TZP reinforced with reduced graphene oxide
2018 - MANARAO, D.S.; CORDEIRO, G.L.; TERTULIANO, A.J.O.; MACHADO, I.F.; LAZAR, D.R.R.; USSUI, V.; CESAR, P.F.
Purpose/aim: To develop a processing method for yttrium stabilized zirconia pollycrystal (YTZP) reinforced with reduced graphene oxide (rGO) and to verify the effect of rGO concentration on hardness and fracture toughness of the material. Materials and methods: The composite production included several steps: (a) synthesis of Y-TZP powder by coprecipitation route, (b) synthesis of graphene oxide from chemical exfoliation of graphite (modified Hummer’s method) followed by reduction with ascorbic acid, (c) sonication of reduced graphene oxide in Y-TZP suspension followed by drying (d) uniaxial pressing in metal device with diameter of 5mm and (e) sintering in a conventional tubular furnace (Argon/4%hydrogen atmosphere) or spark plasma sintering (SPS). The concentration of rGO in Y-TZP was fixed between 0.01 and 2.0 wt%. Sintered samples were characterized by X-ray diffraction, scanning electron microscopy, density measurements, and Vickers method for hardness and fracture toughness determination (indentation fracture). Data were analyzed by ANOVA and Tukey’s test with global significance level of 5%. Results: Results (Table 1) showed that the procedure stablished for dispersion of rGO in the Y-TZP resulted in good physical homogeneity of rGO and Y-TZP. Regarding the sintering procedure, it was observed that conventional sintering in a controlled atmosphere was not effective for ceramic densification due to microcrack formation at the ceramic surface. For conventional sintering, the hardness obtained for the rGO concentration of 2% was significantly lower than those obtained for all other concentrations, however, for this processing method, fracture toughness was not affected by rGO concentration. For SPS, both fracture toughness and hardness were affected by rGO concentration, with the lowest hardness mean value measured for the concentration of 0.05% and the lowest fracture toughness value measured for specimens with addition of 0.01% of rGO. Conclusions: The production of the composite Y-TZP/rGO was proved possible, and sintering via spark plasma resulted in higher mechanical properties of the composite material compared to conventional sintering. rGO concentration affected the hardness of the composite for both processing methods (conventional and SPS), however fracture toughness was only affected by rGO concentration for specimens processed via SPS.
Defect engineering in reduced graphene oxide toward advanced energy conversion
2018 - CORDEIRO, GUILHERME L.
Defect engineering in reduced graphene oxide (rGO) for a smart design of fuel-cell supports has become an effective approach to improve the restricted two-dimensional (2D) mass and charge transfer and to boost the alcohol oxidation reaction. The present minireview describes recent trends across prominent characteristics of tailored reduced graphene oxides, which include but are not restricted to, engineered three-dimensional (3D) nanostructures for better mass transport, tuned electron/hole conduction for easier electrical transport, and hybridized surfaces for high electrocatalytic activity. Special focus fixes upon the experimental progress on defect engineering, from three-dimensional structure assembly to surface metal complexation and heteroatom doping to size-controlled defect formation. Given their crucial impact on reduced graphene oxide properties, controlled methods for synthesis, and processing offer considerable promise toward next-generation carbon nanomaterials for electrocatalysis.
Evaluation of tin-doped indium oxide synthesized by Pechini polymeric precursor route as eletrocatalyst support for ethanol electrooxidation
2018 - LAZAR, D.R.R.; CAMARGO, E.F. de; CORDEIRO, G.L.; USSUI, V.; OLIVEIRA NETO, A.
The generation of electric energy with low environmental impact and efficiency is a motivation for the development of fuel cells systems which traditionally are fed by gaseous hydrogen. The interest for the use of methanol and ethanol has also been considered in order to reduce the infrastructure required to store and supply the fuel. However, the low alcohol electrooxidation kinetics is a problem. In the case of polymeric membrane fuel cell, platinum is the state-of-the art electrocatalysts used to solve this drawback. Carbon black is always employed as platinum support, but problems related to low corrosion resistance and poisoning by absorption of CO species reduces the fuel cell performance with time. In order to improve electrocatalysts efficiency, some metal oxides have been proposed as candidate for oxidation resistant catalysts support. Inspired by the good electrical conductivity of tin doped indium oxide (ITO) for some applications such as liquid crystal displays, ITO has been considered as platinum support. In the present work, ITO powders (10wt% SnO2 – 90wt% In2O3) were synthesized by the Pechini method. Produced powders were characterized by TG/DTA analysis, X-ray diffraction, energy-dispersive X-ray spectroscopy and scanning electron microscopy. Pt supported on ITO was prepared by borohydride reduction method. Pt deposited on commercial ITO and on carbon black (Vulcan) were also prepared for comparison purposes. Electrochemical behavior of ethanol electrooxidation reaction (EOR) was performed using cyclic voltammetry and chronoamperometry techniques. Results showed that ITO powders prepared with the molar ratio citric acid/metals 1:1, followed by calcination at 600oC, are porous and soft aggregate composed by nanoparticles with cubic structure and established nominal composition. This material allowed an electrochemical behavior similar to commercial ITO due to its particle size and suitable agglomerate porosity for ethanol electrooxidation reaction.
Síntese e processamento de óxido de grafeno reduzido: abordagens no desenvolvimento de eletrocatalisadores suportados para oxidação de etanol
2018 - CORDEIRO, GUILHERME L.
A adoção de políticas de incentivo ao desenvolvimento de novas tecnologias de geração de energia tem sido consenso entre especialistas de gestão ambiental. Nesta perspectiva, sistemas de conversão como células a combustível de baixa temperatura de operação (60120 °C) constituem alternativas propícias para fornecimento de energia com emissão reduzida de poluentes. Em adição, o etanol representa uma opção atrativa como combustível devido às vantagens relacionadas à característica renovável e à consolidação da indústria de bioetanol em países como Brasil e Estados Unidos. Contudo, a oxidação do etanol resulta em perdas de eficiência por causa da dificuldade de clivagem da ligação carbono-carbono. Tradicionalmente, platina tem sido usada como catalisador em decorrência das elevadas energia de adsorção e densidade de corrente de troca. Entretanto, a melhora na cinética de reação requer maior utilização catalítica. Neste contexto, um processo de síntese de óxido de grafeno reduzido foi desenvolvido para estabilizar e suportar a fase metálica, aperfeiçoando a área eletroquimicamente ativa. Uma metodologia convencional de síntese química e processamento de óxido de grafeno, a partir de grafite, foi aprimorada visando à fabricação de materiais com características desejáveis aos processos eletrocatalíticos. As estratégias foram baseadas em princípios combinados para introdução controlada de defeitos, redução no estado de aglomeração com auxílio de funcionalização direcionada com surfactante brometo de cetiltrimetilamônio (CTAB) e manufatura de dispersões com aumentada estabilidade coloidal. A nova nanoestrutura forneceu elevada densidade de sítios ativos, que incorreu em valores de corrente mássica aumentados em torno de 2,5 e 5,4 vezes frente à platina suportada em negro de fumo e grafeno aglomerado, respectivamente.
Improved Pt/CeO2 electrocatalysts for ethanol electro-oxidation
2018 - CORDEIRO, GUILHERME L.; CAMARGO, ELAINE F. de; SANTOS, MONIQUE C.L.; PEREIRA, CONRADO V.; USSUI, VALTER; LIMA, NELSON B. de; NETO, ALMIR O.; LAZAR, DOLORES R.R.
A simple and low-cost approach for the preparation of wide-gap (Eg = 3.43 eV), fine cerium dioxide crystals (CeO2, d = 8.1 nm), was developed in order to support Pt nanoparticles as electrocatalysts for ethanol oxidation. Cerium oxide powders were prepared by precipitation from cerium chloride solution in ammonia medium and the influence of calcination temperature was evaluated. Platinum nanoparticles were incorporated by a wet chemical reduction of chloroplatinic acid with sodium borohydrate. Compared to a state-of-the-art Pt/C, the nanocomposite exhibited a much higher mass activity (2.5×) and significant anti-poisoning ability. The promotional effect of the CeO2 nanocrystals was discovered to be affected by heat treatment conditions. Compared to oven-dried samples, calcination allows to (i) an enhanced metal–support interaction between CeO2 and Pt and (ii) an enhancement of electron transportation to catalyst owing to the shrinkage of CeO2 bandgap, thereby promoting the oxidation of the strongly adsorbed CxHy. Therefore, the Pt/CeO2 nanocomposite appears to be a promising electrocatalyst for advanced energy conversion.
Engineering graphene surface toward design of aggregation-resistant catalyst supports for advanced energy conversion
2017 - CORDEIRO, GUILHERME L.; CAMARGO, ELAINE F. de; USSUI, VALTER; LIMA, NELSON B. de; NETO, ALMIR O.; LAZAR, DOLORES R.R.
Graphene has been considered an emerging fuel cell catalyst support due to its excellent chemical and electrical properties. The evaluation of the unit activity on each catalytic site (intrinsic) of low-loading supported metal nanoparticles (NPs), however, is often hampered by face-to-face aggregation of graphene sheets. Herein, we demonstrate the critical role of the expansion between sheets in a pivotal electrocatalytic process for green energy conversion through ethanol oxidation in acid medium. In order to reduce mass-transport resistances and incomplete utilization of the supported NPs, a one-step design strategy is proposed for tuning a desired physicochemical property of graphene: surface area. This step is based on the principle that the apparent activity is governed by the extrinsic activity, i . e ., the number of exposed active sites for a particular mass loading. Our design principle is achieved by a two-stage method involving a chemical delamination process of graphite (1) with an in situ surfactant functionalization/intercalation–reduction approach (2). As a result, not only an interlayer expansion was attained but also a short-ranged layered structure was assembled. This structural reorganization substantially affected the ethanol oxidation reaction (EOR) over platinum NPs. The unique nanoarchitecture provided a high density of EOR active sites, which incurred in a specific current value increased by about 2.5 and 5.4 times compared to platinum supported on state-of-the-art carbon black and restacked graphene, respectively.
Synthesis of cerium oxide nanopowders for improving catalysts electroactivity in direct ethanol fuel cells
2017 - CAMARGO, ELAINE F. de; CORDEIRO, GUILHERME L.; SANTOS, MONIQUE C.L.; PEREIRA, CONRADO de V.; USSUI, VALTER; LIMA, NELSON B. de; NETO, ALMIR O.; LAZAR, DOLORES R.R.
The search for alternative direct ethanol fuel cell catalyst supports to improve platinum electrocatalysis requires the development of novel nanostructured conducting materials. In order to boost energy conversion efficiencies, high surface-area supports with enhanced oxophilicity have been addressed. This procedure might aid breaking the C–C bond and the poisoning of platinum by strongly adsorbed species coming from the dissociative adsorption of ethanol. In the present work, synthetic cerium oxide nanopowders were compared to state-of-the-art carbon black supports for the ethanol oxidation reaction (EOR). Cerium oxides were prepared via a simple chemical precipitation method using ammonium hydroxide as a precipitant agent and the effect of thermal treatment of the synthesized nanopowders was investigated. Microstructural analyses revealed an increase in crystallite size from 8.1 to 25.7 nm after heating from 400 to 800 °C, respectively. Supported catalysts containing 20 wt.% Pt were prepared by a sodium borohydride impregnation–reduction method, in aqueous solution, at room temperature. Electrochemical measurements of the EOR showed highest electrocatalytic activity over samples calcined at 400 °C as a result of a combinantion between an appropriate nano-scaled structure and oxygen transport in cerium oxide.
Effect of Sn loading on the characteristics of Pt electrocatalysts supported on reduced graphene oxide for application as direct ethanol fuel cell anode
2017 - CORDEIRO, GUILHERME L.; USSUI, VALTER; MESSIAS, NILDEMAR A.; PIASENTIN, RICARDO M.; LIMA, NELSON B. de; NETO, ALMIR O.; LAZAR, DOLORES R.R.
The effect of Sn loading (5–30 mol %) on Pt catalysts supported on reduced graphene oxide was investigated for ethanol electro-oxidation in acidic medium. Reduced graphene oxide was synthesized via graphite oxidation–exfoliation process in liquid phase and chemical reduction. Pt and PtxSny catalysts were deposited on reduced graphene oxide by a NaBH4 impregnation–reduction method. The adopted procedures allowed the synthesis of graphene-like nanosheets where single Sn-doped Pt nanoparticles were impregnated. Pt lattice parameter and micro-strain increased with Sn addition, confirming the formation of a solid solution. Concerning ethanol electro-oxidation, Pt was more active when supported on reduced graphene oxide whereas the introduction of Sn enhanced the catalyst activity, leading to lower ethanol oxidation potentials and higher current densities.
Avaliação da influência do agente redutor nas características de filmes de óxido de grafeno
2016 - SOUZA, R.R. de; CORDEIRO, G.L.; USSUI, V.; YOSHITO, W.K.; LIMA, N.B.; NETO, A.O.; LAZAR, D.R.R.
O desenvolvimento de metodologias de síntese química eficientes e de baixo custo tem sido considerado na preparação de óxido de grafeno reduzido para aplicações em diversos campos tecnológicos incluindo as áreas de microeletrônica, biomédica e de sistemas de conversão de energia. Considerando esta abordagem, o método comumente empregado para obtenção de óxido de grafeno, inicialmente desenvolvido por Hummers e Offeman, caracteriza-se por um processo em duas etapas envolvendo a oxidação do grafite e a esfoliação do óxido correspondente por tratamento ultrassônico e centrifugação. A conversão do óxido de grafeno para óxido de grafeno reduzido, por sua vez, tem sido alcançada por meio de reações químicas utilizando uma ampla variedade de agentes redutores. Neste estudo, avaliou-se o efeito de diferentes substâncias, tais como, ácido ascórbico, bissulfito de sódio, borohidreto de sódio e sulfito de sódio na redução de filmes de óxido de grafeno. Os materiais, após lavagem e secagem, foram caracterizados por difração de raios X, espectroscopia no infravermelho e Raman, e microscopia eletrônica de transmissão. Os resultados indicaram que todos os reagentes são promissores para a redução química do óxido de grafeno, com destaque para o bissulfito e o sulfito de sódio pela melhor capacidade de promover a desoxigenação do óxido de grafeno. Dentre estes, o método de redução empregando sulfito de sódio possibilitou a obtenção de filmes com a menor concentração de defeitos estruturais.
Efeito do condicionamento de pós de óxido de grafeno reduzido na atividade de nanoplatina para eletro-oxidação de etanol
2016 - CORDEIRO, G.L.; SOUZA, R.R. de; YOSHITO, W.K.; LIMA, N.B.; USSUI, V.; NETO, A.O.; LAZAR, D.R.R.
O recente interesse em filmes de óxido de grafeno reduzido tem motivado o desenvolvimento de processos para obtenção de nanopós visando aplicações em células a combustível poliméricas. Neste estudo, verificou-se o efeito de técnicas de condicionamento nas características da superfície de pós de óxido de grafeno reduzido, os quais foram obtidos por processos de oxidação-esfoliação do grafite e redução química. As técnicas de destilação azeotrópica e tratamento hidrotérmico foram avaliadas na produção dos pós. Os pós obtidos foram caracterizados por espectroscopia de infravermelho, difração de raios X e microscopia eletrônica de varredura (MEV-FEG). Catalisadores de platina suportados nos nanopós sem e com os referidos tratamentos foram preparados e testados em reação anódica de oxidação de etanol. Conforme os resultados de voltametria cíclica e cronoamperometria, todos os catalisadores suportados nos pós de óxido de grafeno reduzido apresentaram maior atividade em relação ao material convencional de platina/carbono. Resultados promissores foram obtidos para os nanocompósitos submetidos aos condicionamentos, em termos de potencial de início da oxidação e corrente gerada.