Carbon Dioxide Bubbles Removal by Capillary Actuation in the Anode Channel of Direct Methanol Fuel Cells

Abstract

On the anode side of a direct-methanol fuel cell, carbon dioxide bubbles are generated as a result of the methanol oxidation reaction. The accumulation of such bubbles prevents methanol from reaching the diffusion layer (DL). Hence, a reduction in the reaction rate occurs, which limits the maximum current density of the cell. To keep carbon dioxide bubbles away from the diffusion layer surface, a new design of the anode flow channel besides wall surface treatment is developed. Such a design can introduce capillary actuation, which forces the carbon dioxide bubbles to move away from the diffusion layer due to capillary forces. This can be achieved by using a trapezoidal shape of the flow channel, as well as the combined effect of hydrophilic and hydrophobic surface treatments on the diffusion layer and top wall, respectively. To identify the optimal design of the anode flow channel, a three-dimensional, two-phase flow model is developed. The model is numerically simulated, and the results are validated with available measurements. Results indicated that treating the diffusion layer with a hydrophilic layer increases the area in direct contact with liquid methanol. Besides, the hydrophobic top channel wall makes it easier for the carbon dioxide bubbles to attach and spread out on the top surface. However, super-hydrophobic treatment of the top wall should be avoided, as it can cause difficulty in bubble extraction from the channel. The current findings create a promising opportunity to improve the performance of direct-methanol fuel cells.