Abstract
Direct Methanol Fuel Cells (DMFCs) are considered a great alternative to lithium-ion batteries for powering the next generation of mobile devices. However, methanol crossover in DMFCs is a serious drawback to the cell’s performance. Methanol crossover and its effects can be mitigated by (1) developing alternative membranes, (2) improving the electro-oxidation process in the catalyst layer and improving the structure of the catalyst and gas diffusion layers, and (3) flow field and membrane electrode assembly design optimization which could be achieved by studying current density distribution. The last two points are the focus of this dissertation. The effects of cathode catalyst layer (CCL) thickness on the detrimental effect of methanol crossover in a DMFC under various operating conditions are studied. Three membrane electrode assemblies (MEAs) with different CCL thicknesses but identical catalyst loading and identical anode catalyst layer are used. The results show that, when a thicker CCL, approximately twice the thickness of the base case, is used, the fuel cell performance increases significantly. The increase in performance with a thicker CCL is attributed to the oxidation of the crossed-over methanol in part of the catalyst layer and leaving the rest of the catalyst layer free from methanol contamination, leading to mitigations of the effects of mixed potentials. The results of electrochemical impedance spectroscopy (EIS) shows that the charge transfer resistance for the fuel cell with twice the thickness of CCL is 30% lower compared to that for the base case, indicating that the active catalyst area available for oxygen reduction reaction (ORR) is indeed greater. A much thicker CCL, about five times of that for the base case, is also used and the cell performance is also higher than that for the base case. The experimental results show that there exists an optimum cathode catalyst layer thickness and the thickness of cathode catalyst layer has a significant effect on DMFC performance. A two-dimensional, single phase, multi-component model is developed to study the effects of cathode catalyst layer thickness on DMFC’s performance. The simulations consider the effects of mixed potentials as well as the distribution of methanol concentration in the cathode catalyst layer which is normally neglected by most DMFC models. COMSOL Multiphysics V4.3, a finite element analysis solver and simulation software, is employed to solve the fully coupled set of equations for electrochemical kinetics, continuity, momentum and species equations. There is a good agreement between the modeling results and the experimental data. The model shows that neglecting methanol contamination in the CCL overpredicts the true performance of the cell. It is believed that the separate measurement of current density under the land and the channel in DMFCs can explain some of the coupled parameters that affect the cell performance. Therefore, a novel technique is used to (a) separately measure the current density under the channel and under the land to determine where the current density is higher, and (b) separately measure methanol crossover rate under the channel and under the land to determine where the permeation rate is higher. In this method, the anode side of the cell is partially catalyzed depending on the area of interest (land, channel or full) whereas the cathode side always has a full size catalyst layer. Experimental results show that current density under the land is higher than under the channel, because the land has higher ECA and higher methanol concentration and they outweigh the negative effects of mixed potentials. The results also show that methanol crossover rate under the land is higher than under the channel. The same technique of individually catalyzing the land and channel is used to study the effects of under land convection on DMFC performance. This is achieved by comparing the performance of an MEA with catalyzed land only in a parallel flow field plate with another in a serpentine plate. It is found that at lower methanol concentrations, the under land convection negatively affects the cell performance due to higher methanol crossover rate. However, it enhances the performance of the cell when higher methanol concentrations are used, although the permeation rate is still greater. This enhancement comes from having an abundance of reactants in the catalyst layer which compensates for the adverse effects of mixed potentials.