Perforated Anode Gas Diffusion Layer for Direct Methanol Fuel Cells

Abstract

Direct methanol fuel cells (DMFCs) are very promising electrochemical power sources. They are very suitable for portable electronics and transportation vehicles and considered as a potential alternative to lithium-ion batteries. Methanol is easily stored and transported and can be supplied in liquid form to a low temperature/pressure operating fuel cell. Direct operation on liquid fuels is attractive because of the exceptionally high energy density and convenience of liquid fuels [1]. However, DMFC’s suffer from two major obstacles, namely, the sluggish reaction at the anode and the methanol cross over to the cathode. The sluggish reaction at the anode is due to many individual steps required for methanol electrooxidation. The formation of CO2 as a product and its removability also contributes to the sluggishness of the reaction. CO2 bubbles, that can block reaction sites from reactants, must be transported from catalyst layer to flow channels and out of the cell as fast as possible. Most research of DMFCs is primarily focused on two aspects: 1. developing a new catalyst or reduce the use of precious metals. 2. Developing a more methanol resistant membranes to reduce the methanol cross over to the cathode that leads to useless mixed potentials at the cathode. If the anode structure is well designed in a way that prevents large amounts of methanol to transport through the membrane, the cell efficiency will be improved. The design and optimization of the anode electrode is very crucial in the development of DMFC’s. In this work, anode gas diffusion layer is modified by laser perforation with 500μm diameter holes along the channels. The perforation creates hydrophilic regions around the holes due to the loss of the hydrophobic PTFE material after being exposed to the hot laser beam. Therefore, the perforation not only creates pathways for reactants and products to transported but also change the wettability of the gas diffusion layer. The change in wettability and the pathways created by the laser reduce kinetic, ohmic and concentration polarization compared to the gas diffusion layer without perforation. Figure1 shows polarization curves comparison between a perforated anode gas diffusion layer and a non-perforated one (virgin GDL). High frequency resistance measurements and EIS (electrochemical impedance spectroscopy) of the anode and cathode of the virgin gas diffusion layer and the new perforated gas diffusion layer have also been studied. The results show that the difference in performance is attributed to the changed structure of the anode gas diffusion layer, when taking EIS of the anode at relatively low current density, the charge transport resistance arc (magnitude) of the perforated anode gas diffusion layer is much less than the virgin gas diffusion layer. To confirm that the results are solely due to the anode structure, the cathode impedance at low current density was also obtained and showed no difference in the charge transport resistance between the two tested gas diffusion layers. It was also noticed that the high frequency resistance (HFR) changed when increasing the current density; the perforated GDL maintain lower HFR for tested regions of the current density. This finding could be explained by the ability of better hydration of the membrane that perforated pathways create. Figure 1