A Stack Design for Portable Direct Methanol Fuel Cell Based on the Newly Developed Heterogeneous Composite Bipolar Plate

Abstract

There are several problems associated with the marketing of portable direct methanol fuel cell (DMFC). The most critical one is how to supply fuel to all cells without supplementary devices. It requires a serial connection of 12, 24 and 48 cells to acquire enough voltage to operate a cellular phone, a digital video camera and a personal computer, respectively. How can methanol be supplied to all these cells if the conventional stacking method is adopted? Supplementary devices such as pumps, fans, and complex piping system must be used and, as a result, the word “portable” no longer applies. New stacking approach must be developed before portable DMFC can be put onto the market. The idea behind the promising “banded” type stack is simple. Problems only come when one tries to implement it. What is the actual device that can collect and transport those electrons between electrodes with minimum electrical resistance and, yet, be able to survive in the acidic environment inside the cell and, more importantly, not to harm the membrane electrode assembly (MEA)? Currently, there is none with reasonable cost. The new heterogeneous composite bipolar plate developed in our laboratory is composed of rows of carbon fiber bunches and plastic main plate body. The carbon fiber bunches are perpendicular to and penetrate through the main plate body. Besides being light, better performance, and low cost, the new plate has a unique property. Since the main plate is an insulator, the rows of fiber bunches may be arranged to be electrically independent with each other. When MEA is made into banded structure and with each band corresponding to one row of fiber bunch on the plate, all bands will function as different cells and higher voltage may be acquired in a smaller volume. Thus, just two bipolar plates (“unipolar” should be called in this case) with one “banded” MEA in between can form a very simple multi-cell stack. The serial connection may be done outside the cell and, therefore, low cost metal such as copper, which has great electrical conductibility but is harmful to MEA, may be used to transport the electrons which are conveyed outside the cell through carbon fiber bunches in the first place. The fact that it has only one anode chamber and one cathode chamber eliminates the difficult problem of distributing fuel sufficiently to all cells without supplementary devices. As a matter of fact, the anode chamber may be used as a methanol storage tank to further reduce the volume. The structure of the new stack design presented in this paper is simple, durable and low cost. It holds the key to the success in marketing the portable DMFC.