A TWO-PHASE FLOW STUDY ON CARBON DIOXIDE BUBBLES EVOLUTION IN DIRECT-METHANOL FUEL CELLS WITH DEGASSING CHANNELS

Low-temperature fuel cells have the potential to be used in portable and automotive applications, and the direct-methanol fuel cell is a good candidate for such applications as the fuel is in the liquid phase, making it easy to store and handle. Active direct-methanol fuel cells (DMFCs) operate on a liquid methanol solution as a fuel that is pumped to the anode flow channels. Gaseous carbon dioxide can accumulate in the anode channels and block the diffusion layer (DL), limiting the transport of reactants through the DL to the functional layer. This causes a drop in the rate of reaction and therefore limits the maximum current density achievable by the cell. A degassing channel concept is investigated in this study to collect Carbon dioxide bubbles and keep them away from the main channel. A 3D two-phase flow model is developed and validated to investigate bubble development and actuation to the degassing channel. Different wettability conditions for the degassing channel are investigated, and the resulting bubble development pattern is analyzed for the expected effect on mass transport of reactants to the DL. Results indicate that the use of a degassing channel significantly improves the transport of reactants in the main channel, which in turn improves the cell's performance. Furthermore, using a degassing channel with a slightly hydrophobic treatment achieved around 37% faster bubble actuation rate from the main channel, compared to a degassing channel with plain wettability. However, this treatment created longer bubble slugs that travel about 25% slower than in the plain case. In summary, using this configuration will improve the mass transport of reactants to the diffusion layer, at the expense that bubbles will be extracted more slowly from the flow field overall. These findings create the opportunity to improve the performance of DMFCs by the improvement of mass transport of reactants on the anode side.