A reação de redução de oxigênio (RRO) foi estudada em eletrodos lisos de platina e prata recobertos com filmes de quitosana e em eletrodos porosos de Pt/C, Ag/C, MnyOx/C e TiO2/C na ausência e na presença de filmes à base de quitosana, em eletrólito alcalino. Nestes estudos foram empregadas curvas de polarização em estado estacionário, obtidas em eletrodos de disco rotatório (EDR) e em células a combustível alcalina (CCA) unitárias, através das quais foram construídos os correspondentes diagramas de curvas de Tafel e os gráficos de Levich. Os estudos realizados em EDRs permitiram estabelecer-se uma boa compreensão da atividade catalítica e dos mecanismos envolvidos na RRO. Foi observada uma diminuição da atividade eletrocatalítica dos eletrodos metálicos (Pt e Ag lisos e porosos) quando na presença dos filmes de quitosana, havendo também uma mudança no mecanismo da reação. Assim, nos experimentos de EDR verificou-se que em altas densidades de corrente a etapa determinante da velocidade é a adsorção do oxigênio, em contraste com os eletrodos sem o filme para os quais a etapa determinante é a difusão do oxigênio. Nestes casos não foi possível calcular o número de elétr. Já, nos casos dos eletrodos de MnyOx/C e TiO2/C não foi observada nenhuma diminuição significativa na atividade eletrocatalítica destes materiais na presença dos filmes de quitosana, assim como no mecanismo da reação na presença dos filmes (inclusive no número de elétrons envolvidos na reação), apesar de se notar um pequeno aumento nos valores dos coeficientes de Tafel. Foram também realizados testes de alguns dos materiais como cátodos em células a combustível alcalinas unitárias alimentadas com hidrogênio/oxigênio, tendo sido verificado que o desempenho é promissor, apesar de ser inferior ao de células com eletrólito de membrana ácidas. / The oxygen reduction reaction was studied on smooth platinum and silver electrodes recovered with chitosan films and on Pt/C, Ag/C, MnyOx/C e TiO2/C porous electrodes in the presence or absence of chitosan-based films in alkaline electrolyte. These studies were carried out using steady state polarization measurement, obtained in a rotating disc electrode (RDE) and in alkaline fuel cells (AFC) (single cell), from which the corresponding Tafel curves and Levich plots were built. The studies carried out using the RDE had allowed establishing a good understanding of the catalytic activity and of the involved RRO mechanisms. A reduction in the electrocatalytic activity of the metallic electrodes and a change in the reaction mechanism (Pt and Ag smooth and porous) were caused by the presence of the chitosan films Thus, the RDE experiments showed that at high current densities the rate determining step of the ORR is the oxygen adsorption, in contrast to the electrodes without the film for which the rate determining step is the oxygen diffusion. In these cases it was not possible to calculate the number of electrons involved in the reaction. In the cases of the MnyOx/C and TiO2/C electrodes no significant change in the electrocatalytic activity, as well as in the reaction mechanism were caused due to the presence of the films (also in the number of electrons involved in the reaction), although a small increase in the Tafel coefficient values was noted. Also, tests of some of the materials as cathodes in AFC single cells with hydrogen/oxygen were carried out. These results showed that the performance is promising, although inferior to those of acid membrane electrolyte fuel cells.

O envelhecimento dos eletrocatalisadores utilizados em cátodos de células a combustível de eletrólito polimérico (PEMFCs) é um dos principais fatores que restringem sua aplicação como conversores de energia em larga escala. Esse trabalho visa contribuir para o aprimoramento da estabilidade de nanopartículas de platina (NPs de Pt) por meio da modificação do suporte catalítico aos quais encontram-se impregnadas. Desse modo, foram realizadas sínteses de suportes catalíticos baseados em óxidos de molibdênio (MoO3 e MoO2) ancorados em carbono Vulcan® XC72-R, sendo os materiais produzidos caracterizados física, estrutural e eletroquimicamente antes e após a impregnação de NPs de Pt. Para investigar a estabilidade dos eletrocatalisadores, foi realizado um teste de degradação eletroquímico acelerado, o qual consistiu em aplicar os ciclos de potenciais entre 0,6 e 1,0 V vs. ERH por curto período de tempo. Os resultados mostraram que os métodos de síntese utilizados foram satisfatórios, levando a formação dos catalisadores com as proporções bem próximas das requeridas. O catalisador Pt/MoO3-C apresentou a maior atividade específica frente a reação de redução de oxigênio (RRO), atribuída a efeitos sinérgicos metal/suporte. Quando investigada a estabilidade dos materiais frente ao teste de degradação eletroquímico acelerado, observou-se que, a princípio, nenhum dos óxidos de molibdênio diminui a extensão da degradação da platina. Analisando-se as atividades específicas frente à RRO para cada catalisador antes e após o envelhecimento eletroquímico, foi observado que Pt/MoO2-C apresentou-se como o material mais estável dentre os demais. / The aging of Pt based electrocatalysts used in the polymer electrolyte fuel cell (PEMFC) cathodes is one of the main issues that restrict its wide application as energy converters. This work aims to contribute to the improvement of the stability of platinum nanoparticles (Pt NPs) by modification of the catalyst support at which they are impregnated. Thus, syntheses of catalyst supports based on molybdenum oxide (MoO3 and MoO2) anchored on Vulcan® XC72-R carbon were carried out and the produced materials were characterized physically, structurally and electrochemically prior and after impregnation of the Pt NPs. To investigate their stability, an electrochemical accelerated degradation test was performed, which consisted of applying a large number of short duration potential cycling steps between 0.6 and 1.0 V vs. RHE. The results showed that the synthetic methods used here were satisfactory, leading to the formation of catalysts with compositions near to those expected. The Pt/MoO3-C catalyst showed the highest specific activity toward the oxygen reduction reaction (ORR), and this was attributed to metal/support synergistic effects. When the stability against electrochemical accelerated degradation test of the materials was investigated, it was observed that, in principle, none of the molybdenum oxides really decreases the extent of platinum degradation. However, comparing the specific activities towards the ORR for each catalyst, before and after electrochemical aging, it is concluded that Pt/MoO2-C is the most stable material among all others.

To investigate conditions of silver electroless deposition on silicon (100), optical microscope, atomic force microscope (AFM) and attenuated total reflection infrared spectroscopy (ATR-FTIR) spectroscopy were used. Twenty second dipping in 0.8mM AgNO3/4.9% solution coats a silicon (100) wafer with a thin film of silver nanoparticles very well. According to AFM results, the diameter of silver particles is from 50 to 100nm. After deposition, arithmetic average of absolute values roughness (Ra) increased from ~0.7nm to ~1.2nm and the root mean square roughness (Rq) is from ~0.8nm to ~1.5nm. SCN- ions were applied to detect the existence of silver on silicon surface by ATR-FTIR spectroscopy and IR spectra demonstrate SCN- is a good adsorbent for silver metal. Patina is the general name of copper basic salts which forms green-blue film on the surface of ancient bronze architectures. Patina formation has been found on the surface of platinum or ruthenium after several scans of cyclic voltammetry in 2mM CuSO4/0.1M K2SO4, pH5 solution. Evidence implies that oxygen reduction reaction (ORR) triggers the patina formation. ORR is an important step of fuel cell process and only few sorts of noble metals like platinum can be worked as the catalyst of ORR. Mechanisms of patination involving ORR were investigated by cyclic voltammetry, optical microscope, AFM, rotating disk electrode and other experimental methods: the occurrence of ORR cause the increase of local pH on electrode, and Cu2+ ions prefer to form Cu2O by reduction. Patina forms while Cu2O is oxidizing back to Cu2+.

Les électrocatalyseurs nanostructurés à base de fer, de carbone et d’azote (Fe/C/N) sont de bonnes alternatives au platine dans les piles à combustible acide. Les Fe/C/N sont synthétisés par pyrolyse laser et sont obtenus à partir de deux précurseurs de fer (FeOOH ou Fe(acac)3), de la pyridine, (avec ou sans méthilimidazole) en présence de NH3. La variation de la fraction volumique de NH3 (RNH3) est étudiée sur une large gamme. Les caractéristiques physico-chimiques des matériaux évoluent de façon monotone avec la variation de RNH3. Les analyses par XPS permettent d’identifier une phase assimilable à du nitrure de fer pour les fortes valeurs de RNH3. Les matériaux les plus actifs en éléctrochimie sont ceux contenant une phase de nitrure de fer. Ceci suggère que la présence de nitrure de fer, permet la formation de sites actifs pour la réduction de O2..L’utilisation de toluène (mélangé à de la pyridine ou à du méthylimidazole) en présence ou non de Fe(acac)3 permet d’obtenir de nouveaux matériaux Fe/C/N ou C/N. Les matériaux C/N moins actifs que ceux contenant du fer, indiquent la présence de sites actifs à base de fer. Des recuits sous Ar ou sous NH3 à 1100°C améliorent l’activité des matériaux. Elle est due à l’élimination de sites azotés inactifs et à la transformation de sites pyridiniques en sites graphitiques. Le recuit sous NH3, plus efficace que celui sous Ar, induit une augmentation de la surface spécifique (Sspé max > 1100 m²/g). Pour un même matériau recuit sous Ar ou sous NH3 la sélectivité de la réduction de O2 (n) atteint 3,70 e- contre 3,93 e- respectivement. Le plus actif d’entre eux atteint un Edép > 950mV/ENH. / Nanostructured Iron-nitrogen-carbon (Fe/N/C) electrocatalysts is a good substitute for platinum in acidic fuel cells. Laser pyrolysis synthesis allows to obtain iron nanocomposites (Fe/N/C). The reaction involves two iron precursors, FeOOH and Fe(acac)3, combined with pyridine (and possibly methylimidazole) in the presence of NH3. The effect of large range variations of NH3’s volume fraction (RNH3) in the reactions is studied. Physicochemical properties of the materials increase monotonically with RNH3. XPS analysis shows that an iron nitride phase appears when RNH3 increase, and electrochemical analysis shows that materials with this iron nitride phase are the most active ones. These results suggest that iron nitride presence triggers the formation of active sites for the oxygen reduction reaction (ORR).Finally, we synthetize new Fe/N/C, or N/C materials using toluene (mixed with pyridine or methylimidazole) in the presence or absence of Fe(acac)3. The obtained Fe/N/C materials are more active than the N/C materials which indicates that iron plays a role in the presence of active sites for the ORR. Annealing under Ar or NH3 at a temperature of 1100°C increases the activity of all the materials. This improvement is due to the suppression of inactive nitrogen sites, and the transformation of some pyridinic sites to graphitic sites. Annealing under NH3 is more effective under Ar beacuse of the increase of the specific surface area (Sspe max > 1100 m2/g). For the same material annealed under Ar versus NH3, the selectivity (n) of the ORR goes from 3.70 up to 3.93 e- respectively. The most active one reaches Edep > 950 mV/ENH.

abstract: The fuel cell is a promising device that converts the chemical energy directly into the electrical energy without combustion process. However, the slow reaction rate of the oxygen reduction reaction (ORR) necessitates the development of cathode catalysts for low-temperature fuel cells. After a thorough literature review in Chapter 1, the thesis is divided into three parts as given below in Chapters 2-4. Chapter 2 describes the study on the Pt and Pt-Me (Me: Co, Ni) alloy nanoparticles supported on the pyrolyzed zeolitic imidazolate framework (ZIF) towards ORR. The Co-ZIF and NiCo-ZIF were synthesized by the solvothermal method and then mixed with Pt precursor. After pyrolysis and acid leaching, the PtCo/NC and PtNiCo/NC were evaluated in proton exchange membrane fuel cells (PEMFC). The peak power density exhibited > 10% and 15% for PtCo/NC and PtNiCo/NC, respectively, compared to that with commercial Pt/C catalyst under identical test conditions. Chapter 3 is the investigation of the oxygen vacancy (OV) effect in a-MnO2 as a cathode catalyst for alkaline membrane fuel cells (AMFC). The a-MnO2 nanorods were synthesized by hydrothermal method and heated at 300, 400 and 500 ℃ in the air to introduce the OV. The 400 ℃ treated material showed the best ORR performance among all other samples due to more OV in pure a-MnO2 phase. The optimized AMFC electrode showed ~ 45 mW.cm-2, which was slightly lower than that with commercial Pt/C (~60 mW.cm-2). Chapter 4 is the density functional theory (DFT) study of the protonation effect and active sites towards ORR on a-MnO2 (211) plane. The theoretically optimized oxygen adsorption and hydroxyl ion desorption energies were ~ 1.55-1.95 eV and ~ 0.98-1.45 eV, respectively, by Nørskov et al.’s calculations. All the configurations showed oxygen adsorption and hydroxyl ion desorption energies were ranging from 0.27 to 1.76 eV and 1.59 to 15.0 eV, respectively. The site which was close to two Mn ions showed the best oxygen adsorption and hydroxyl ion desorption energies improvement with the surface protonation. Based on the results given in Chapters 1-4, the major findings are summarized in Chapter 5. / Dissertation/Thesis / Doctoral Dissertation Systems Engineering 2019

No description available.

No description available.

We are facing an energy crisis because of the limitation of the fossil fuel and the pollution caused by burning it. Clean energy technologies, such as fuel cells and metal-air batteries, are studied extensively because of this high efficiency and less pollution. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential in the process of energy storage and conversion, and noble metals (e.g. Pt) are needed to catalyze the critical chemical reactions in these devices. Functionalized carbon nanomaterials such as heteroatom-doped and molecule-adsorbed graphene can be used as metal-free catalysts to replace the expensive and scarce platinum-based catalysts for the energy storage and conversion. Traditionally, experimental studies on the catalytic performance of carbon nanomaterials have been conducted extensively, however, there is a lack of computational studies to guide the experiments for rapid search for the best catalysts. In addition, theoretical mechanism and the rational design principle towards ORR and OER also need to be fully understood. In this dissertation, density functional theory calculations are performed to calculate the thermodynamic and electrochemical properties of heteroatom-doped graphene and molecule-adsorbed graphene for ORR and OER. Gibb's free energy, overpotential, charge transfer and edge effect are evaluated. The charge transfer analysis show the positive charges on the graphene surface caused by the heteroatom, hetero-edges and the adsorbed organic molecules play an essential role in improving the electrochemical properties of the carbon nanomaterials. Based on the calculations, design principles are introduced to rationally design and predict the electrochemical properties of doped graphene and molecule-adsorbed graphene as metal-free catalysts for ORR and OER. An intrinsic descriptor is discovered for the first time, which can be used as a materials parameter for rational design of the metal-free catalysts with carbon nanomaterials for energy storage and conversion. The success of the design principle provides a better understanding of the mechanism behind ORR and OER and a screening approach for the best catalyst for energy storage and conversion.

No description available.

No description available.