μPIV measurements of two-phase flows of an operated direct methanol fuel cell

In direct methanol fuel cells (DMFCs), two-phase flows appear in the channels of the anode side (CO₂ bubbles in a liquid water–methanol environment) as well as of the cathode side (water droplets or films in an ambient air flow). CO₂ bubbles or water droplets may almost completely fill the cross-section of a channel. The instantaneous effect of the formation of two-phase flows on the cell performance has not been investigated in detail, yet. In the current project, the micro particle image velocimetry (μPIV) technique is used to elucidate the corresponding flow phenomena on the anode as well as on the cathode side of a DMFC and to correlate those phenomena with the performance of the cell. A single-channel DMFC with optical access at the anode and the cathode side is constructed and assembled that allows for μPIV measurements at both sides as well as a detailed time-resolved cell voltage recording. The appearance and evolution of CO₂ bubbles on the anode side is qualitatively and quantitatively investigated. The results clearly indicate that the cell power increases when the free cross-section area of the channel is decreased by huge bubbles. Methanol is forced into the porous gas diffusion layer (GDL) between the channels and the membrane is oxidized to CO_2, and hence, the fuel consumption is increased and the cell performance rises. Eventually, a bubble forms a moving slug that effectively cleans the channel from CO₂ bubbles on its way downstream. The blockage effect is eliminated; the methanol flow is not forced into the GDL anymore. The remaining amount of methanol in the GDL is oxidized. The cell power decreases until enough CO₂ is produced to eventually form bubbles again and the process starts again. On the other hand under the investigated conditions, water on the cathode side only forms liquid films on the channels walls rather than channel-filling droplets. Instantaneous changes of the cell power due to liquid water formation could not be observed. The timescales of the two-phase flow on the cathode side are significantly larger than on the anode side. However, the μPIV measurements at the cathode side demonstrate the ability of feeding gas flows in microchannels with liquid tracer particles and the ability to measure in two-phase flows in such a configuration.