Anodic flow field

Through the use of a transparent fuel cell, Spernjak et al. [87] were able to visualize the anode FF plate (and DL without MPL) while operating the fuel cell with a cathode that had MPL on the DL. It was observed that liquid water was present in the anode flow field only when an MPL on the cathode side was used. Again, this is an indication that the cathode side creates a pressure barrier that pushes the water toward the anode. These observations agree with the ones presented mathematically by Weber and Newman [148]. Although they did not do any experimental work, their two-phase fuel cell model concluded that the MPL acts as a valve that pushes water away from the DL toward the anode though the membrane. 

H. Yang, T. S. Zhao, and Q. Ye. In situ visualization study of CO2 gas bubble behavior in DMFC anode flow fields. Journal of Power Sources 139 (2005) 79-90. 

The fuel is fed into the anode flow field, moves through the diffusion medium, and reacts electro-... 

Figure 1 shows the schematic of a PEM fuel cell where there is an H2/02 front dividing the cell into two portions one is H2-rich and generates electric power as a power source, and the other is H2-ffee and becomes a load driven by the power source. The existence of the H2/02 front results from two scenarios (a) start-stop events frequently seen in automotive application and (b) local H2 starvation caused by local blockage of the H2 fuel supply, e.g., part of anode flow-field is filled with liquid water. As shown in Fig. 1,... 

Figure 1. The schematic of a PEMFC having the H2/O2 front in the anode and major electrochemical reactions considered in the analysis. The H2/O2 front divides the fuel cell into the power source and the load. In the load portion, anode flow-field (FF) is occupied by air (or O2+N2) in the case of start-stop, while it is filled with liquid water in the case of local H2 starvation.

The two-dimensional, coupled kinetic and transport model can also be used to simulate start-stop processes. Figure 14 plots cathode potential and carbon corrosion current distribution at three instants when the H2/02 front passes through 10, 50, and 90% of anode flow path during the start process. As H2 displaces air in the anode flow-field, the size of the power source increases and the load size decreases accordingly. The balanced current density becomes larger, causing higher carbon corrosion current density. 

Figure 14. Calculated cathode potential and carbon corrosion current distributions when H2/02 front passes through 10%, 50%, and 90% of anode flow-field during a cell start from air/air state (80 0 C, 101 kPaabs, 66% RHin). The cell has a catalyst loading of 0.4 mgpt/cm2 using a 50%wt Pt/Vulcan catalyst in both anode and cathode electrodes.

Figure 15. Predicted accumulated carbon loss distribution along anode flow-field over a complete start-stop cycle for a controlled start-stop experiment as shown above the plot at 80 °C, 101 kPaabs, 66% RHjn, and residence time of 1.5 s based on anode void volume (including flow-field and diffusion medium). The model predicts nearly symmetric carbon loss at anode inlet and outlet because the stop process essentially mirrors the start process by switching H2 and air periodically at anode inlet.

Figure 19. Predicted carbon loss distribution along anode flow-field channel over a complete H2/air-front start—stop cycle using the pseudo-capacitance model in comparison with one-dimensional, normalized mass activity from Fig. 17. The pseudo-capacitance value used in the model is obtained from AC-impedance measurements as described in references (42, 43).

Figure 20. The impact of a faster H2/air-front moving through die anode flow-field. 100% relative carbon loss is defined as the localized damage prediction when no pseudo-capacitance is considered in die model. Significantly less carbon corrosion is expected at the anode inlet region as the speed of H2/air-front increases but much less benefit at the anode outlet region.

Subcell Approach Stumper et al.135 presented the subcell approach to measure localized currents and localized electrochemical activity in a fuel cell. In this method a number of subcells were situated in different locations along the cell s active area and each subcell was electrically isolated from each other and from the main cell. Separate load banks controlled each subcell. Figure 8 shows the subcells in both the cathode and anode flow field plates (the MEA also had such subcells). The current-voltage characteristics for the... 

Finally the possibility to utilize electrolysers to elevate hydrogen pressure and recirculate it inside anode flow fields in H2FCS power sources has been recently considered [10]. This electro-chemical approach for hydrogen pumping could be effected with an external additional smaller PEMFC component, connected to the stack devoted to electric power generation, or alternatively using in electrolysis mode a group of cells of the main PEMFC stack. 

In another experiment the depth of the anode flow field was varied between 25 and 40 pm. Here the differences of the F//curve is within the measurement error. Cells with lower channel depth show higher power fluetuations during long-term tests. 

Wong CW, Zhao TS, Ye Q, Liu JG (2006) Experimental investigations of the anode flow field of a micro direct methanol fuel cell. J Power Sources 155 291-296... 

Fig. 8.3 Anode flow field with bubbles (Source [16] reproduced with permission of Elsevier)...

No matter what heating method is used, a thermal couple needs to be placed within the flow-field plates (not the end plates) it is better to place the thermal couple as close as possible to the GDM in order to estimate the temperature of the MEA more accurately. If the purpose is to evaluate the temperature distribution across the entire MEA, multiple thermal couples that are placed at different positions will be needed. The thermal couples can be placed in the anode flow-field plate, the cathode flow-field plate, or both. 

The fundamental start/stop mechanism was first reported by Reiser et al. [ 11], and occurs when one part of the anode flow-field is partially filled with hydrogen and another part is filled with air, a situation which occurs during the start-up of a fuel cell (hydrogen displacing air in the anode flow-field) or during shutdown (air... 

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