(33c) Gas and Water Management in a Dead-Ended Operating PEM Fuel Cell

            A dead-ended operating Proton Exchange Membrane (PEM) Fuel Cell is favorable for high efficiency of reactant gas utilization and self-humidification and permits the simplification of components in a balance of plant design.  However, special gas and water management is required for a dead-ended operating PEM fuel cell to ensure stable and reliable performance. 

For conventional PEM fuel cells, the gas and water management is usually addressed by applying a forced flow-through-gas to create a significant pressure difference between the inlet and outlet of each channel that provides uniform gas distribution through the cell and strong gas flow to remove product water.  It usually requires very narrow flow channels 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 dead-ended operating PEM fuel cell where pure H₂ and O₂ are applied, it creates a very different scenario of operation.  When pure O₂ is used, flow-through-gas operation is not applicable.  A rapid circulation of pure O₂ is also not desirable because of the potential fire hazards associated with fast moving mechanical components in a pure oxygen atmosphere.  ElectroChem, Inc. has developed an innovative PEM Fuel Cell with an open channel structure which provides a mechanism of gas and water management for the dead-ended operating PEM fuel cell. 

I.          Demonstration of stable operation at extremely low excess oxygen flow

  

            The fuel cell demonstrated a 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 1.  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. 

Figure 1.   Effect of oxygen excess flow rates on the cell performance

 

II.        No effects of change in orientation on cell performance

 

            The fuel cell operated under four different orientations, 0º, 90º, 180º, and 270º with 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 2, there is no significant effect on product water removal when the cell is at 180º orientation. These test results strongly implied ?against gravity? product water removal capability of the cell. 

Figure 2.  Effect of orientation on the cell performance

Cell temperature: 75oC Cell pressure: 30 psig O2 excess flow rate: 2 cc/min H2 excess flow rate: 2 cc/min

 

III.       Effect of impurity in the reactant gas on cell performance

 

            In a dead-ended operating PEM fuel cell, accumulation of inert gas can reduce the partial pressure of the reactant gas and affect the cell performance.  To investigate the impurity effect, a ultra-high-purity oxygen (99.996% O₂) is used to compare with a high-purity oxygen (estimate 99.5% O₂).  As shown in Figure 3. a dramatic drop in the cell performance was caused by the accumulation of inert gas while operating at a dead-ended condition.

Figure 3.  The effect of inert gas on a dead-ended operating PEM fuel cell

Cell temperature: 75oC Cell pressure: 30 psig Current density: 400 mA/cm2

            The open channel cell demonstrated a different gas and water management mechanism which permitted the PEM fuel cell to operate at almost no excess oxygen and hydrogen flow condition and transport product water against gravity.  However, the purity of the reactant gas will determine whether the cell can be operated at near dead-ended or complete dead-ended conditions. 

Acknowledgments

This work was funded by the National Aeronautics and Space Administration under Contract NNJ06JD71C.  Helpful discussions with the NASA teams are gratefully acknowledged.