A direct methanol fuel cell (DMFC) is an electrochemical energy-conversion device that converts methanol directly to electric energy without the use of reformers. Because of its simplicity and high energy density, the DMFC has been viewed as one of the most promising candidates for powering portable electronic devices that are typically powered by primary or secondary batteries. The electrochemical reactions occurring in the DMFC are intrinsically coupled with a series of complex liquid-gas two-phase mass transport processes in both the anode and the cathode. A better understanding of these mass transport processes in the DMFC is essential for the improvement of cell performance and for the optimal design of the DMFC system.
This thesis work is focused on the mass transport phenomena of methanol and water in the DMFC. Both experimental and theoretical investigations have been carried out. The thesis starts with reporting on an analysis of the mass transport of methanol at the anode of the DMFC. An analytical expression to quantify the overall mass transport coefficient from the anode flow field to the catalyst layer was obtained for the first time. This expression explicitly gives the relationship between the overall mass transport coefficient and the limiting current density, which means that the very complex mass transport coefficient can now be obtained by measuring the limiting current density. This expression was then used to guide experimental investigations. One of the most significant findings is that the overall mass transfer coefficient was nearly independent of current density, but was strongly affected by the methanol flow rate.
The effect of the anode backing layer on the mass transport of methanol and cell performance was then studied. We found that a too thin anode backing layer resulted in lower cell voltages in the entire current density region, whereas a too thick backing layer led to a lower limiting current density. The reduced cell performance as a result of thinning the backing layer may be attributed to the increased under-rib mass transport polarization due to the weaker under-rib convection in a thinner backing layer. The experimental results also showed that the use of a PTFE-treated backing layer resulted in a lower limiting current density, due primarily to the increased through-plane methanol transport resistance as a result of the PTFE treatment.
To enhance the mass transport of methanol in both the in-plane and through-plane directions at the anode, a new flow field, termed convection-enhanced serpentine flow field (CESFF), was invented. This new flow field can enhance the in-plane mass transport, resulting in more uniform distributions of reactants over the fuel cell electrodes. The most significant advantage of this flow field that it enables the fuel cell to discharge stably at low flow rates of reactants (which means smaller pumping work). As a result, the use of the new flow field can lead to a higher overall fuel cell efficiency.
The mass transport of water through the membrane-electrode assembly (MEA) used for DMFCs has also been studied in this thesis work. A measurement method that enables in-situ quantification of the water-crossover flux through the membrane was developed. With this method, we investigated each of the water transport mechanisms, and the effects of the MEA design and operating parameters on water loss and cell performance. The experimental data showed that diffusion dominated the total water-crossover flux at low current densities due to the high water concentration difference across the membrane. With the increase in current density, the water flux by diffusion decreased, but the flux by back convection increased. The corresponding net water-transport coefficient was also found to decrease with current density. The effects of both the PTFE loading in the cathode backing layer (BL) as well as in the micro-porous layer (MPL) and the carbon loading in the MPL on both the water transport and cell performance were also studied.
The thesis also presents a one-dimensional, isothermal two-phase mass transport model for water transport through the MEA. The liquid (methanol-water solution) and gas (gas CO
2, methanol vapor and water vapor) two-phase mass transport in the porous anode and cathode was formulated based on classical multiphase flow theory in porous media. In the anode and cathode catalyst layers, the simultaneous three-phase (liquid and vapor in pores as well as dissolved phase in the electrolyte) water transport was considered and the phase exchange of water was modeled with finite-rate interfacial exchanges between different phases. This model enables further investigations of the respective effect of three water transport mechanisms, such as diffusion, electro-osmotic drag, and convection, through the membrane for the DMFC. With this model, the effect of the MEA design, particularly the cathode micro-porous layer, on water crossover and cell performance was investigated.
Keywords: Direct Methanol Fuel Cell; Cell Performance; Diffusion layer; Flow field; Mass transport; Methanol Concentration; Micro-porous layer; Non-uniformity; Oxygen transport; Two-phase flow; Two-phase mass transport model; Water crossover; Water flooding; Water management; Wettability.
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