Fuel cells are electrochemical devices that rely on the transport of reactants (oxygen and hydrogen) and products (water and heat). These transport processes are coupled with electrochemistry and further complicated by phase change, porous media (gas diffusion electrodes) and a complex geometry. This thesis presents a three dimensional, non-isothermal computational model of a proton exchange membrane fuel cell (PEMFC). The model was developed to improve fundamental understanding of transport phenomena in PEMFCs and to investigate the impact of various operation parameters on performance. The model, which was implemented into a Computational Fluid Dynamics code, accounts for all major transport phenomena, including: water and proton transport through the membrane; electrochemical reaction; transport of electrons; transport and phase change of water in the gas diffusion electrodes; temperature variation; diffusion of multi-component gas mixtures in the electrodes; pressure gradients; multi-component convective heat and mass transport in the gas flow channels. Simulations employing the single-phase version of the model are performed for a straight channel section of a complete cell including the anode and cathode flow channels. Base case simulations are presented and analyzed with a focus on the physical insight, and fundamental understanding afforded by the availability of detailed distributions of reactant concentrations, current densities, temperature and water fluxes. The results are consistent with available experimental observations and show that significant temperature gradients exist within the cell, with temperature differences of several degrees Kelvin within the membrane-electrode-assembly. The three-dimensional nature of the transport processes is particularly pronounced under the collector plates land area, and has a major impact on the current distribution and predicted limiting current density. A parametric study with the single-phase computational model is also presented to investigate the effect of various operating, geometric and material parameters, including temperature, pressure, stoichiometric flow ratio, porosity and thickness of the gas diffusion layers, and the ratio between the channel with and the land area. The two-phase version of the computational model is used for a domain including a cooling channel adjacent to the cell. Simulations are performed over a range of current densities. The analysis reveals a complex interplay between several competing phase change mechanisms in the gas diffusion electrodes. Results show that the liquid water saturation is below 0.1 inside both anode and cathode gas diffusion layers. For the anode side, saturation increases with increasing current density, whereas at the cathode side saturation reaches a maximum at an intermediate current density (≈ 1.1Amp/cm2) and decreases thereafter. The simulation show that a variety of flow regimes for liquid water and vapour are present at different locations in the cell, and these depend further on current density. The PEMFC model presented in this thesis has a number of novel features that enhance the physical realism of the simulations and provide insight, particularly in heat and water management. The model should serve as a good foundation for future development of a computationally based design and optimization method. / Graduate

This research focuses on new rapid prototype development techniques for Proton Exchange Membrane (PEM) fuel cell gas delivery plates. The study addresses several key issues in the design, analysis and manufacturing of fuel cell plates. The approach combines theoretical modeling, experimental study, physical plate making process, and virtual prototyping to form a new scheme for the rapid prototype development of fuel cell plates. The research extends the newly introduced screen-printing layer deposition manufacturing technique to complete the entire cycle of rapid plate development. A number of key issues on plate materials and the layer deposition process are addressed. The study has identified the cause of the problem with the poster-ink based screen-print ink material, explored various alternative composite ink materials, and narrowed down to the promising “conductive polymer ∼ epoxy ∼ graphite power” composite. A new, concurrent approach for developing new composite materials through various experiments and material tests has been introduced, and demonstrated through the development of one particular ink composite with promising results. In this research, the method for virtual prototyping fuel cell gas delivery plate using advanced CAD/CAE commercial software is introduced. The method allows a “virtual prototype” of the fuel cell plate to be constructed and the performance of the plate to be evaluated through various analyses as “virtual prototype tests.” These include the prediction of fuel cell performance through the CFD calculation on the average oxygen concentration; as well as the assessments of the maximum stress and undesirable temperature variation on the printed fuel cell plate through finite element analysis. Design optimization is conducted using the virtual prototypes to improve the design of the flow field and the plate. Three disciplinary models are simulated and their results are subject to disciplinary optimizations. Global design optimization is carried out using multiple objective optimization, combining the functional performance measures from three disciplinary models. This multi-disciplinary optimization integrates performance considerations of PEM fuel cell plate, and provides guidelines to the plate development. The research contributes to a new approach for the rapid development of fuel cell plates with a great potential to be applied to other mechanical parts. The study also extends the methodology of computational design and rapid prototyping. / Graduate

Proton exchange membrane (PEM) fuel cells are at the forefront among different types of fuel cells and are likely to be important power sources in the near future. PEM is a key component of the PEM fuel cells. The objective of this research is to investigate the fundamental aspects of PEM in terms of thermodynamics and proton transport in the membrane, so that the new proton conducting materials may be developed based on the detailed understanding. Since the proton conductivity increases dramatically with the amount of water in PEM, it is important to maintain a high humidification during the fuel cell operation. Therefore, the water uptake characteristics of the membrane are very important in developing fuel cell systems. Thermodynamic models are developed to describe sorption in proton-exchange membranes (PEMs), which can predict the complete isotherm as well as provide a plausible explanation for the long unresolved phenomenon termed Schroeder¡¯s paradox, namely the difference between the amounts sorbed from a liquid solvent versus from its saturated vapor. The sorption isotherm is a result of equilibrium established in the polymer-solvent system when the swelling pressure due to the uptake of solvent is balanced by the surface and elastic deformation pressures that restrain further stretching of the polymer network. The transport of protons in PEMs is intriguing. It requires knowledge of the PEM structure, water sorption thermodynamics in PEM, proton distribution in PEM, interactions between the protons and PEM, and proton transport in aqueous solution. Even proton conduction in water is anomalous that has received considerable attention for over a century because of its paramount importance in chemical, biological, and electrochemical systems. A pore transport model is proposed to describe proton diffusion at various hydration levels within Nafion¢ÃƒÂ§ by incorporating structural effect upon water uptake and various proton transport mechanisms, namely proton hopping on pore surface, Grotthuss diffusion in pore bulk, and ordinary mass diffusion of hydronium ions. A comprehensive random walk basis that relates the molecular details of proton transfer to the continuum diffusion coefficients has been applied to provide the transport details in the molecular scale within the pores of PEM. The proton conductivity in contact with water vapor is accurately predicted as a function of relative humidity without any fitted parameters. This theoretical model is quite insightful and provides design variables for developing high proton conducting PEMs. The proton transport model has been extended to the nanocomposite membranes being designed for higher temperature operation which are prepared via modification of polymer (host membrane) by the incorporation of inorganics such as SiO2 and ZrO2. The operation of fuel cells at high temperature provides many advantages, especially for CO poisoning. A proton transport model is proposed to describe proton diffusion in nanocomposite Nafion¢ÃƒÂ§/(ZrO2/SO42-) membranes. This model adequately accounts for the acidity, surface acid density, particle size, and the amount of loading of the inorganics. The higher proton conductivity of the composite membrane compared with that of Nafion is observed experimentally and also predicted by the model. Finally, some applications of PEM fuel cells are considered including direct methanol fuel cells, palladium barrier anode, and water electrolysis in regenerative fuel cells.

Thesis (Ph. D.)--Michigan State University. Dept. of Chemistry, 2006. / Title from PDF t.p. (viewed on June 19, 2009) Includes bibliographical references (p. 148-154). Also issued in print.

Thesis (Ph. D.)--Worcester Polytechnic Institute. / Keywords: kinetic modeling; electrocatalysis; CO tolerance; PEM fuel cells. Includes bibliographical references.

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: Thermodynamics; Fuel Cell; Proton-exchange membranes; Proton Transport. Includes bibliographical references.

No description available.

No description available.

The major challenge of the research was to characterize and develop concepts for establishing structure/property relationships between the functionality of the polymer backbone, the states of water and the membrane transport properties. Most of the hydrocarbon based random copolymers reported in the literature show reduced proton conductivity at low water content. This was attributed to the formation of an isolated morphology. Over the last few years our group has synthesized thermally stable multiblock copolymers with varying chemical structures and compositions. Block copolymers consist of two or more incompatible polymers (i.e. blocks) that are chemically conjoined in the same chain. The transport properties of the multiblock copolymers showed a strong dependence on the morphology in contrast to the random copolymers. Irrespective of the nature of the backbone, the transport properties scaled with the block lengths of the copolymers. An increase in block length for a given series of block copolymer was associated with improved proton conduction, particularly under partially hydrated conditions compared to the random copolymers. The structure-property relationship of the proton conductivity and self-diffusion coefficient of water was obtained as a function of the volume fraction of water for all the random and block copolymers. At a given volume fraction, the block copolymers displayed both higher self-diffusion coefficients of water and proton conductivities relative to the random copolymers. This improvement in transport properties indicates the presence of desired and favorable morphology for the blocks. For DMFC applications, the block copolymers also showed low methanol permeability and high selectivity. The states of water in the copolymers were characterized using DSC and NMR relaxation techniques. At similar ionic contents, the free water concentration increased with increasing block lengths. The distribution of the states of water in the copolymers correlates to transport properties. This knowledge, coupled with the state of water experiments, transport measurements, and chemical structure of the copolymers provided a fundamental picture of how the chemical nature of a phase separated copolymer influences its transport properties. The experimental procedure involved impedance spectroscopy, DSC, TGA, FTIR, DMA, pulse gradient stimulated echo (PGSE) NMR, NMR relaxation experiments and various electrochemical fuel cell performance experiments. / Ph. D.

High performance engineering materials, poly(arylene ether)s, having very good mechanical properties, excellent oxidative and hydrolytic stability are promising candidates for alternative materials used in the field of Proton Exchange Membrane Fuel Cells (PEMFCs) and Reverse Osmosis (RO) applications. In particular, wholly aromatic sulfonated poly(arylene ether sulfone)s are of considerable interest in the field of PEMFCs and RO, due to their affordability, high Tg, and the ease of sulfonation. Proton exchange membrane fuels cells (PEMFCs) are one of the primary alternate source of energy. A Proton exchange membrane (PEM) is one of the key component in a PEMFC and it needs to have good proton conductivity under partially humidified conditions. One of the strategies to increase proton conductivity under partially RH conditions is to synthesize hydrophobic-hydrophilic multiblock copolymers with high Ion exchange capacity (IEC) values to ensure sufficient ion channel size. In this thesis two multiblock systems were synthesized incorporating trisulfonated hydrophilic oligomers and were characterized in the first two chapters of the thesis. The first multiblock system incorporated a non-fluorinated biphenol-based hydrophobic block. The second study was focused on synthesizing a fluorinated benzonitrile-based hydrophobic block. A fluorinated monomer was incorporated with the aim to improve phase separation which might lead to increased performance under partially humidified conditions. The third study featured synthesis and characterization of a novel hydroquinone-based random copolymer system precursor, which after post-sulfonation, shall form mono-sulfonated polysulfone materials with potential applications in reverse osmosis. The ratio of the amount of hydroquinone incorporated in the copolymer were varied during the synthesis of the precursor to facilitate control over the post-sulfonation process. The simple and low cost process of post-sulfonating the random copolymer enables the precursor to be a promising material to be used in the reverse osmosis application. / Master of Science