The objective of the present study was to design and develop microfabrication process for a micro PEM (Proton Exchange Membrane) fuel cell structure, Both anode and cathode flow field plates with a cross section of 5 cm2 (22.5 mm 22.5 mm) and thickness (for a single cell) of about 700 £gm. A thickness of 30 nm Pt sputter loading deposited onto a Nafion 117 for MEA was made with both SEM and AFM characterization. The effect of different operating parameters on micro PEMFCs performances was experimentally studied for three different flow field configurations (interdigitated, mesh, and serpentine). The experiments with different cell operating temperatures, backpressures on the H2 /air flow channels as well as various combinations of these parameters have been conducted for three different flow geometries. The results are presented in the form of the polarization VI curves and PI curves under different operating conditions. A significant enhancement with three different type flow patterns for H2/air operation at 25¢J and 152 kPa on both electrodes was found. The possible transport mechanisms associated with the parametric effects were discussed. In addition, it was found that, among the three flow patterns considered, significant improvements can be reached with a specified flow geometry.

Les membranes ionomères (par exemple le Nafion®) sont utilisées en tant qu’électrolyte dans les piles à combustible à membrane échangeuse d’ions (PEMFC) dont les performances dépendent fortement de l’état d’hydratation de cette membrane. Il est donc fondamental de connaître la distribution en eau dans la membrane ainsi que dans la pile à combustible. Les coefficients d'autodiffusion ont été mesurés par Résonance Magnétique Nucléaire (RMN) employant des gradients de champ magnétique statique B0 ou des gradients de champ magnétique radiofréquence B1. Cette seconde méthode permettant de s’affranchir de l’effet des gradients internes, nous avons pu mesurer une diminution du coefficient de diffusion apparent en fonction de l'intervalle de diffusion ce que ne permet pas l’utilisation des gradients B0. L'effet du flux électro-osmotique a pu être mis en évidence dans une membrane soumise à un champ électrique. Après avoir appliqué une tension constante aux bornes de deux électrodes en platine, placées à chaque extrémité de la membrane, la migration de l'eau de l’anode vers la cathode a pu être visualisée par des techniques d’Imagerie par Résonance Magnétique (IRM). L’IRM a finalement été employée pour étudier les phénomènes de transport directement dans une PEMFC en fonctionnement. Ces expériences sont plus délicates à mettre en œuvre, et nécessitent la conception d’une PEMFC optimisée pour l’observation par IRM. Cette cellule élémentaire s'est avérée avoir des propriétés comparables à celles disponibles dans le commerce. Les résultats préliminaires montrent une accumulation progressive de l'eau près de la sortie des gaz tandis que l'admission reste sèche. / PEMFC use perfluorosulfonic acid membranes (Nafion® for example) as solid electrolyte and their performances are strongly dependent on membrane hydration. Therefore, the accurate knowledge of water distribution in the membrane and in the fuel cell is a fundamental issue. First, self diffusion coefficients have been thoroughly measured by Nuclear Magnetic Resonance (NMR) using B0 gradients and/or B1 gradients. The latter method is more suitable in the case of short relaxation times and for avoiding effects of the so-called internal gradients. Indeed, we were able to observe the decrease of the apparent diffusion coefficient as a function of the diffusion interval whereas this feature is totally absent in the data obtained by B0 gradients. Secondly, electro-osmotic flow effect has been detected in a membrane experiencing an electrical field. The setup consists of two platinum electrodes at each extremity of the membrane. We have observed by Magnetic Resonance Imaging the migration of water when a constant tension between the membranes is imposed. Finally, MRI has been used to study these phenomena into a PEMFC under operation. These experiments, carried out with a whole fuel cell, are more difficult to achieve and they require a PEMFC optimized for the MRI observation. This fuel cell proved to have performances comparable to commercially available fuel cells. Preliminary results indicate a progressive accumulation of water close to the gas outlet while the gas inlet remains dry.

In this dissertation, we present two major lines of numerical investigation based on a control-volume approach to solve coupled, nonlinear differential equations. The first model is developed to provide better understanding of the water management in PEMFC operating at less than 100ºC, under transient conditions. The model provides explanations for the observed differences between hydration and dehydration time constants during load change. When there is liquid water at the cathode catalyst layer, the time constant of the water content in the membrane is closely tied to that of liquid water saturation in the cathode catalyst layer, as the vapor is already saturated. The water content in the membrane will not reach steady state as long as the liquid water flow in the cathode catalyst layer is not at steady state. The second model is to optimize the morphological properties of HT-PEMFCs components so as to keep water generated as close as possible to the membrane to help reduce ionic resistance and thereby increase cell performance. Humidification of the feed gas at room temperature is shown to have minimal effects on the ionic resistance of the membrane used in the HT-PEMFC. Feed gases must be humidified at higher temperature to have effects on the ionic resistance. However, humidification at such higher temperatures will require complex system design and additional power consumption. It is, therefore, important to keep the water generated by the electrochemical reaction as close as possible to the membrane to hydration the membrane so as to reduce the ionic resistance and thereby increase cell performance. The use of cathode MPL helps keep the water generated close to the membrane and decreasing the MPL porosity and pore size will increase the effectiveness of the MPL in keep the water generated close to the membrane. The optimum value of the MPL porosity depends on the operating conditions of the cell. Similarly, decreasing the GDL porosity helps keep water close to the membrane and the optimum value of the GDL porosity depends on the operating conditions of the cell. / text

A numerical simulation study for an interdigitated micro-channel PEM fuel cell is presented. Hydrogen gas is supplied to the anode and air is supplied to the cathode. The fuel cell outer surfaces are maintained at a constant temperature. The SIMPLEC algorithm is employed in a control volume numerical scheme. The outflow boundary conditions are specified to all transport equations except that an outlet pressure is specified to the momentum equation. Results are compared and show good agreement with the experimental data. The effects of the mass flow rate, the outlet pressure and the cell surface temperature on the cell performance are studied. The results can provide reference for fuel cell design.

In this study, conductive particles will spray- coated onto the surface of the MPL to function as micro pillar structures that has been proved to be able to significantly increase the performance of a PEMFC. Contrast with the previously used nano imprint technology , the new method is cheap,fast, and especially suitable for the fabrication of large-area electrodes. The results show that a 30% increase in performance can be acquired by using 20um graphite particles. An increase of 57%, max power of 737mW/cm2 , may be achieved with irregular graphite flakes. The understanding of the distribution and the development of the produced water inside the cathode are essential to associate the performance increase with the microstructures. The electrode is dried gradually until a sharp increase in its impedance appeared, which indicates that the surface of the PEM begins to lose water. Then, i-v performance is measured through a cyclic test, i.e. , form a small load current to a large one and, then, buck to the small again. The performance improved with each cycle, because more and more water is produced along the test. A large performance ¡§jump¡¨ appearded at the 4th cycle only for the electrodes with the microstructure that indicates that the major reaction sites have shifted to the location of microstructures. It is concluded that, along with other evidence, a general water ¡§surface¡¨ exists and migrates form PEM towards MPL.

Abstract The process of the Membrane Electrode Assembly in the Proton Exchange Membrane Fuel Cell and the controllable variables: the pressure, the temperature, and the time of the hot pressure in the producing period would be discussed here. The experimental result of the MEA revealed that for Nafion112 and Nafion117 membranes, the conditions under the hot pressure are the same in the temperature. However, the pressure used in Nafion112 should be lower than Nafion117. In this case, the better function of the MEA can be achieved. Because Nafion112 is thinner, its water in the process of the hot pressure would be lost with extreme ease. This has a very serious impact on the function of the MEA. Therefore, to improve the MEA¡¦s function, the MEA should be boiled by water after being fabricated. The outcome of the research showed that for the purpose of improving the function of the MEA, some humidifier structure adding to the design of STACK is necessary. In order to be familiar to the related practical skills of STACK, PEMFC is brought into use for an electric bicycle in this research. Because the maximum power of STACK is only 150W, which is almost equal to the one-third power of the electric bicycles available such as the 400W of GIANT-Lafree electric bicycle. Besides, the speed of the electric bicycle is too slow when it is operated by itself, but it will be run more smoothly by the means of the assistant power. If the PEMFC electric bicycle can make more useful STACK and be redesigned, not be composed as it used to be, the use of the PEMFC electric bicycle will be steadier, more efficient, and more beneficial to the environment.

Electrode is where electricity is generated. Its quality is important to the entire battery performance. In this study, we are going to establish a stable and automatic process for making electrodes as well as required equipment. By this way, the instability in the electrode process can be improved. Electrospray technology is developed to spray the catalyst and reduce the agglomeration. It is shown that the electrode performance is 37% better than before after electrospray is adopted for producing catalyst layer. If we check the catalyst grains by AFM and TEM, we can find that the electrospray does scatter the polymers containing Nafion effectively. Under SEM, the catalyst grains are small and well proportioned on the carbon cloth. Obviously, catalysts are better utilized. All of the above can be used to explain the performance boost.

No description available.

Este trabalho apresenta o desenvolvimento de um modelo numérico computacional e respectiva metodologia para estudo e projeto de células a combustível a membrana polimérica trocadora de prótons PEM. Para a validação dos resultados experimentais, descreveu-se uma seqüência de rotinas de programação, adequadas ao ajuste dos dados obtidos em laboratório. Com relação à implementação computacional criou-se uma estratégia inovadora de acoplamento com dois modelos tridimensionais, de forma a satisfazer as exigências do modelo completo de célula a combustível, comportando suas diversas geometrias e materiais, assim como os diversos processos físicoquímicos simulados. Com a finalidade de avaliação eficaz da analogia da célula real com o modelo numérico, foram realizados estudos numéricos, comparações com valores obtidos na literatura, caracterização de variáveis por meio de experimentos laboratoriais e estimativas com base em modelos já estudados na literatura. Para a parte experimental, um protótipo de célula a combustível unitária de 144 cm2 de área geométrica foi projetado, produzido e operado em bancada com a finalidade de validação do modelo numérico computacional proposto, apresentando resultados positivos. Os resultados das simulações para as geometrias 2D e 3D propostas são apresentados em forma de curvas de polarização, destacando o modelo de camada catalítica baseado na geometria de aglomerados. Estudos paramétricos e de sensibilidade são apresentados como ilustração da variação do desempenho da célula a combustível estudada. O modelo final é robusto e útil como ferramenta de projeto e otimização de células tipo PEM em uma ampla faixa de condições de operação. / This work presents the development of a numerical computer model and methodology to study and design polymeric exchange membrane fuel cell PEM. For the validation of experimental results, a sequence of routines, appropriate to fit the data obtained in the laboratory, was described. At the computational implementation it was created a new strategy of coupling two 3-dimensional models to satisfy the requirements of the comprehensive model of the fuel cell, including its various geometries and materials, as well as the various physical and chemical processes simulated. To effective assessment of the real cell analogy with numerical model, numerical studies were carried out. Comparisons with values obtained in the literature, characterization of variables through laboratory experiments and estimates from models already tested in the literature were also performed. Regarding the experimental part, a prototype of a fuel cell unit of 144 cm2 of geometric area was designed, produced and operated at laboratory with the purpose of validating the numerical computer model proposed, with positive results. The results of simulations for the 2D and 3D geometries proposed are presented in the form of polarization curves, highlighting the catalytic layer model based on the geometry of agglomerates. Parametric and sensitivity studies are presented to illustrate the change in performance of the fuel cell studied. The final model is robust and useful as a tool for design and optimization of PEM type fuel cells in a wide range of operating conditions.

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%.