Experimental investigations of the anode flow fields of micro direct methanol fuel cells

Low temperature direct methanol fuel cells (DMFCs) convert liquid methanol (MeOH) and water into electrical energy and are a promising technology for portable devices. Compared with other fuel cell systems, the liquid-feed DMFC is relatively simple and could be easily miniaturized since it does not need a fuel reformer, complicated humidification, or thermal management system. Furthermore, methanol has a high energy density in comparison with lithium polymer and lithium ion polymer batteries. Therefore, the micro DMFC has been projected to be one of the most promising candidates for powering next-generation portable consumer electronics devices, which demand small, lightweight power sources with high power density and energy capacity. This thesis presents a systematic investigation of the effect of the anode flow field design on the performance of an in-house fabricated micro direct methanol fuel cell (μDMFC) with an active area 1.0 cm x 1.0 cm. Single serpentine and parallel flow fields consisting of microchannels were fabricated and tested. The experimental results indicated that the serpentine flow field exhibited significantly higher cell voltages than did the parallel flow field, particularly at high current densities. The study of the effect of channel width of the serpentine flow field suggested that there exists an optimal channel width for the same channel depth and the same open ratio when the same methanol flow rate is supplied; either wider or narrower channels will lead to a reduction in the cell performance. The in-situ visualization study revealed that the flow field was blocked periodically by elongated gas slugs due to the increased capillary force in micro channels. This transient capillary blocking caused CO2 bubbles to be evolved in the flow field and removed from the cell periodically. It was further found that with a reduction in channel size both gas slugs and the residence time of gas slugs in the flow field became longer. As a result, the effective mass transfer area of methanol solution on the diffusion layer became smaller, causing the cell performance to decline. On the other hand, for the same fuel feed rate, a smaller flow channel leads to a higher mass transfer coefficient. The competition between the favorable effect of the increased mass transfer coefficient and the adverse effect of the reduced mass effective mass transfer area results in an optimal channel size that gives the best cell performance. The study of channel depth of the serpentine flow field also suggested that there exists an optimal channel depth for the same channel width and the same open ratio when the same methanol flow rate is supplied; either shallower or deeper channels will lead to a reduction in the cell performance. It can be explained as the similar mechanism as the effect of channel width in the serpentine flow field. The experiments also indicated that for a given DMFC hardware module that is operating with a given methanol concentration and at a given temperature, there exists a critical methanol solution flow rate, above which an increase in flow rate will not lead to a further improvement in the cell performance. Finally, it was demonstrated that performance of the μDMFC is insensitive to the cell orientations, which is different from the DMFCs with larger flow channels reported in the literature