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PEM Fuel Cell with Dead-ended Operation Michael Pien, Marvin Warshay, Steven Lis, Radha Jalan ElectroChem, Inc., 400 West Cummings Park Woburn, Massachusetts 01801 USA ElectroChem, Inc. has demonstrated a dead-ended fuel cell operating at very low excess oxygen requirement and maintaining very stable performance. The fuel cell not only showed significantly higher voltages at high current density but also demonstrated the gravity-independent operation. The dead-ended operation significantly enhances the fuel cell power density by creating self-humidification and obtaining high efficiency of gas utilization. Introduction One of the issues with PEM fuel cell system stability is related to the formation of condensed water in the fuel cell causing cell flooding condition which results in severe operational problems, and even cell failure. For conventional H2/Air PEM power systems, the problem is usually addressed by causing the air blower to create a significant pressure difference between the inlet and outlet of each channel that is strong enough to force out any water droplets that might be formed. This usually requires creating very narrow channels and then operating with flow-through gas to create the needed flow rate and pressure. In some other cases, fuel cell operators will "burp the stack" causing a sudden release of gas in the hope of removing the water droplets. In a H2/O2 PEM fuel cell, since pure O2 and H2 are used, a flow-through operation becomes impractical in terms of the efficiency of gas utilization. While a H2/O2 fuel cell can be operated by both hydrogen and oxygen dead-ended operation, it requires a very different scenario of controlling the gas and removal of product water. The rapid circulation of pure O2 has been used in H2/O2 PEM fuel cells but is not desirable because of the potential fire hazards associated with fast moving mechanical components in a pure oxygen atmosphere. ElectroChem has demonstrated a solution that makes the flow channel into an open channel structure with wicking which automatically clears the liquid water from blocking gas passages and moves the water toward the end of the cell. Performance A. Fuel cell with open channel demonstrates higher performance than the conventional flow channel cell. As shown in Figure 1, the open channel cell produced a higher cell voltage than did the baseline cell with the conventional flow channel. The impact of the design upon cell voltage was most pronounced at the highest current densities. 1.1 1 H2, O2 30 psig, 75C 0.9 Cell Voltage, V 0.8 0.7 0.6 conventional channel 0.5 open channel 0.4 0.3 0.2 0.1 0 0 10 20 30 40 50 60 70 80 Current, A 25 H2, O2 30 psig, 75C Power output, Watt 20 15 conventional channel open channel 10 5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Current density, A.cm2 Figure 1. The advantage of the cell in producing higher voltages B. The cell demonstrates Stable Operation at Extremely Low Excess Oxygen Flow Rates—Enabling Passive Operation The second important result was the demonstration of stable operation at extremely low oxygen flow rates, i.e., at extremely low percentages of excess oxygen (over stoichiometric value). The cell required a thirtieth of the excess oxygen of that the conventional cell requires to maintain stable operation. The demonstration of the fuel cell with stable operation at extremely low excess oxygen flow rates is indicated in the test results seen in Figure 2. To reveal the effect of low oxygen excess flow rate on the cell performance, the tests were taken with two consecutive runs with a difference in the oxygen excess flow rate. 0.75 Cell voltage, V 0.74 0.73 20 sccm/[email protected] 10 sccm/[email protected] 0.72 0.71 0.7 0 50 100 150 200 250 Time, min 0.75 Cell voltage, V 0.74 0.73 10 sccm/[email protected] 5 sccm/[email protected] 0.72 0.71 0.7 0 50 100 150 200 250 300 350 Time, min 0.75 Cell voltage, V 0.74 0.73 5 sccm/[email protected] 2 sccm/[email protected] 0.72 0.71 0.7 0 50 100 150 Time, min 200 250 300 0.75 Cell voltage, V 0.74 0.73 2 sccm/[email protected] 1 sccm/[email protected] 0.72 0.71 0.7 0 50 100 150 200 250 300 Time. min Figure 2. Effect of oxygen excess flow rates on the cell performance C. The Cell demonstrates no effects of Change in Orientation on performance—Potential for “Zero g” Water Removal The cell operated under four different orientations, 0º, 90º, 180º, and 270ºwith respect to the location of the cell’s oxygen exit. At 180º, the oxygen exit is located at the top of the cell in which the production water is required to move upward against the gravity to leave the cell. As shown in Figure 3, there is no significant negative effect on product water removal when the cell is at 180º orientation. These test results indicated that “Zero gravity” product water removal is enabled for the cell. 180o Orientation 0.9 50 mA /cm 2 50 mA/cm 2 0.85 Cell voltage, V 100 mA/cm 2 100 mA /cm 2 0.8 200 mA/cm 2 200 mA/cm 2 0.75 0.7 400 mA/cm 2 0.65 0 100 200 300 400 500 Time, min Cell temperature: 75oC Cell pressure: 30 psig O2 excess flow rate: 2 cc/min H2 excess flow rate: 2 cc/min Figure 3. Effect of orientation on the cell performance 600 Conclusion The open channel results in increased exposure of the electrode to the reactant gas which supports a higher mass transport at high current densities. The reduced need for almost no excess oxygen flow and circulation results in operational benefits as well as gas utilization advantages. This design significantly simplifies the PEM fuel cell operation system while supporting high performance and high energy density Acknowledgments This work was funded by the National Aeronautics and Space Administration under Contract NNJ06JD71C.