4.4: Direct Methanol Fuel Cells
4.4 Direct Methanol Fuel Cells
In comparison with PEM fuel cells, the technical and economical advantages of direct methanol fuel cells (DMFCs) include (1) elimination of the fuel reforming unit in the power plant and (2) easy storage of the methanol liquid than the hydrogen gas , , , , , . The component materials of DMFCs are similar to those of PEM fuel cells with the main difference being the adoption of platinum-ruthenium as the anode catalyst. An aqueous methanol solution of low molarity, introduced in the fuel channel, diffuses through the backing layer to the anode catalyst layer. The global methanol oxidation reaction at the platinum-ruthenium catalyst sites generates carbon dioxide, protons, and electrons, as shown in Table 4.1. Air is fed to the cathode side flow channel, and the oxygen species react with the protons transported through the polymer membrane and the electrons from the external circuit to form water.
The flow field at the anode and cathode sides of DMFCs involves gas-liquid two-phase flow with different characteristics. A distinctive flow pattern in DMFCs is caused by the formation of CO 2 bubbles on the anode side , , . Carbon dioxide bubbles resulting from the anode electrochemical reaction nucleate at certain locations and grow with time to form discrete gas slugs in the flow channel. Small bubbles tend to cover the surface of the backing layer by strong surface tension, and thus decrease the effective mass...