Effect of anode channel shape and wettability on CO2 bubble evolution in direct methanol fuel cells

Abstract

Active direct-methanol fuel cells operate on a liquid supply of reactants to the anode flow channels. Gaseous carbon dioxide is produced during operation forming large bubbles on the top side of diffusion layer, limiting the transport of reactants to the functional layer. This causes a significant drop in the rate of reaction and therefore limits the maximum current density. To collect CO2 bubbles away from the diffusion layer, a new design is proposed. It includes a degassing channel placed at the top of the main trapezoidal anode channel. The wettability of the degassing channel and the dihedral angle of the anode channel are investigated. To assess the effect of these parameters, a three-dimensional, two-phase flow model is developed and numerically simulated. Results show that adding the degassing channel is advantageous in terms of bubble collection. A trapezoidal main channel achieves a significantly higher rate of bubble actuation compared to a rectangular channel. In addition, using a dihedral angle of 20° causes a decrease in the pumping pressure, which reduces pumping losses. Moreover, a contact angle of 100° for the degassing channel provides the best compromise in terms of actuation rate, extraction rate out of the channel, and pressure drop along the channel. However, degassing channels can yield up to three times longer bubbles, which are around 75% slower. These findings create the opportunity to improve the performance of direct-methanol fuel cells by enhancing/optimizing the mass transport of reactants on the anode side.