Operation of PEMFC Anodes in Dead-Ended Vs. Flow-through Modes

Abstract

Low system complexity and high hydrogen utilization are important attributes for effective proton exchange membrane fuel cells (PEMFCs). Operating a PEMFC with a dead-ended anode (DEA) mode configuration is a simple and effective way to solve these two challenges1, 2. In comparison with the hydrogen flow-through and recirculation mode, in DEA mode the hydrogen supplied to the anode compartment is controlled by pressure regulation instead of a mass flow controller, where a pressure regulator is installed at the anode inlet and a normally closed solenoid valve blocks the outlet. Over time, accumulated water and nitrogen diffused from the cathode are removed by forced gas convection through intermittent opening of the solenoid valve.The anode and cathode gas diffusion media (GDM) play a role in PEMFC water management and have a significant impact on PEMFC performance. We compare an asymmetric GDM pairing with Freudenberg GDM (H24C3 at anode, H23C2 at cathode) to a symmetric GDM pairing frequently used in the open literature containing SGL 29BC at both the anode and cathode, to highlight the impact of the GDM water management on fuel cell operation in flow-through mode and DEA mode. We have previously shown in open-cathode fuel cells that an asymmetric GDM pairing featuring higher porosity in the anode GDM than the cathode significantly improves hydration and power production3, 4.The results at 25% relative humidity (RH) are shown in Figure 1. Consistent with our prior results5, in regular flow-through mode, Figure 1A shows the PEMFC with an asymmetric Freudenberg GDM pairing has significantly higher current densities vs. the PEMFC with symmetric SGL 29BC GDM at an operating cell voltage ≤ 0.60 V. The advantage of using asymmetric GDM observed in flow-through mode is maintained when the PEMFCs are operated in DEA mode (i.e. open symbols). There is only a marginal decrease in cell voltages during operation in DEA mode compared to flow-through mode probably due to the use of dry hydrogen on the anode side during testing in DEA mode.Figure 1B shows that the cell voltage decay is significantly different for a PEMFC containing the symmetric SGL 29BC GDM compared one with asymmetric Freudenberg GDM when tested in DEA mode configuration. The time between purge events is much greater for the asymmetric Freudenberg GDM pairing. The anode is purged with 99.999% H2 when the cell voltage drops by 100 mV. The PEMFC tested with the asymmetric Freudenberg GDM pairing dwells longer at steady-state prior to voltage decay, and its periods of voltage decay are more gradual. The PEMFC containing the symmetric SGL 29BC pairing has a mean purge interval of about 2min 33s and a total of fourteen purge events are necessary over 80 minutes operation, compared to a mean purge interval of about 18min 29s and a total of four purge events are necessary for the PEMFC containing the asymmetric Freudenberg pairing.A longer purge duration is desirable in a fuel cell system because it increases H2 utilization and reduces valve wear. The results show that the asymmetric GDM pairing improves water management in PEMFCs and improves both power density and system level performance.Acknowledgements:The authors are grateful to the Office of Naval Research for support of this research. References I.-S. Han, J. Jeong and H. K. Shin, Int. J. Hydrogen Energy, 38, 11996 (2013).K. Nikiforow, H. Karimäki, T. M. Keränen and J. Ihonen, J. Power Sources, 238, 336 (2013).R. W. Atkinson, M. W. Hazard, J. A. Rodgers, R. O. Stroman and B. D. Gould, J. Electrochem. Soc., 166, F926 (2019).R. W. Atkinson, J. A. Rodgers, M. W. Hazard, R. O. Stroman and B. D. Gould, J. Electrochem. Soc., 165, F1002 (2018).R. W. Atkinson, Y. Garsany, B. D. Gould, K. E. Swider-Lyons and I. V. Zenyuk, ACS Appl. Energy Mater., 1, 191 (2018) Figure 1. (A) Comparison of I-V polarization curves measured for a PEMFC containing a typical symmetric SGL 29BC GDM pairing on the anode and cathode side and a PEMFC containing an asymmetric Freudenberg GDM pairing . Polarization curves are measured in DEA mode and flow-through mode at 25 % RHinlet cathode. In all cases the cell temperature is 65 °C, in H2|air, with air supplied to the cathode a stoichiometric ratio of 2 at atmospheric pressure. During DEA mode, dry H2 is supplied to the anode at 2 psi. (B) Time evolution of the cell voltage obtained at a current density of 1200 mA cm-2 for an H2 inlet pressure of 2 psi and 25 % RHinlet cathode. Figure 1