Characterization of Water Management in PEM Fuel Cells with Microporous Layer Using Electrochemical Impedance Spectroscopy

Transcription

1 Characterization of Water Management in PEM Fuel Cells with Microporous Layer Using Electrochemical Impedance Spectroscopy Dzmity Malevich, Ela Halliop, Kunal Karan, Brant A Peppley and Jon Pharoah

2 Water management in PEM fuel cell Flooded e e e e Hydrogen Water Oxygen water content e e membrane catalyst porous transport layer Dry

3 Microporous Layer (MPL) Porous Transport Layer (PTL) (Gas Diffusion Layer) Catalyst Coated Membrane (CCM) Proton Exchange Membrane (PEM) 200μm 200μm 1μm porous carbon backing microporous layer catalyst layer 2

4 Effect of MPL on PEMFC performance 1.2 Positive effects of MPL: E cell / V PEMFC with MPL PEMFC without MPL without MPL with MPL i / A/cm 2 reduces cathode flooding U. Pasaogullari, C.-Y. Wang, Electrochim. Acta 49 (2004) 4359 L. R. Jordan, A. K. Shukla, T. Behrsing, N. R. Avery, B. C. Muddle, M.Forsyth, J. Power Sources 86 (2000) 250 improves catalyst utilization Z. Qi, A. Kaufman, J. Power Sources, 109 (2002) 38 Reduces variability in cell performance Z. Qi, A. Kaufman, J. Power Sources, 109 (2002) 38 Polarization curves for PEMFCs without MPL and with MPL on anode and cathode sides. Error bars represent standard deviation within batch of identically built cells. D. Malevich, E. Halliop, B. Peppley, J. Pharoah, K. Karan, J. Electrochem. Soc., 156, B216 (2009) 3

5 Theories explaining MPL effect membrane cathode catalyst layer MPL Oxygen Hydrophobic characteristic of the MPL forces water from cathode towards FFP reducing flooding Porous transport layer (aka GDL) Water Proposed in: A. Z. Weber and J. Newman, J. Electrochem. Soc., 152, A677 (2005) 4

6 Hydrogen Theories explaining MPL effect anode catalyst layer membrane cathode catalyst layer MPL Oxygen water drag with proton water back diffusion MPL facilitates water back diffusion providing better humidification of membrane and catalyst layers Water Proposed in: G. Lin and T. V. Nguyen, J. Electrochem. Soc., 152, A1942 (2005). 5

7 Theories explaining MPL effect membrane cathode catalyst layer MPL water Oxygen water MPL makes open pathways for gaseous oxygen to be transported to catalyst layer water Proposed in: Porous transport layer (aka GDL) Water J. T. Gostick, M. A. Ioannidis, M. W. Fowler, M. D. Pritzker, Electrochem. Comm. 11, 576 (2009) 6

8 Approaches to examine proposed mechanisms Flow visualization Neutron radiographs of water distribution in flow field channels effect of PTL (J.P. Owejan, T.A. Trabold, D.L. Jacobson, M. Arif, S.G. Kandlikar, Int. J. Hydrogen Energy, 32, 4489 (2007) 7

9 Approaches to examine proposed mechanisms Electrochemical Impedance Spectroscopy (EIS) Setup 1 or Setup 2 V impedance analyser I=V*R s -1 resistance R s EIS enabled potentiostat or potentistat impedance analyser electronic load High-current impedance Low-current impedance PEM fuel cell 8

10 EIS response of PEM fuel cell Z'' / Ω cm Hz 12 Hz 3 Hz Hz 1 Hz 0.1 Hz Z' / Ω cm 2 Highfrequency intercept Highfrequency arc Mid-frequency arc Lowfrequency arc 9

11 Mid-frequency arc: charge transfer resistance effect of current density on EIS response -Z'' / Ω cm ma cm ma cm ma cm ma cm -2 R(i=500) R(i=210) Z' / Ω cm 2 D. Malevich, E. Halliop, B. Peppley, J. Pharoah, K. Karan, J. Electrochem. Soc., 156, B216 (2009) 10

12 Low-frequency arc: mass-transport resistance effect of cathode gas on EIS response Hz 12 Hz 3 Hz Z'' / Ω cm Hz H 2 /(20%O 2 He) cell 1 Hz 0.1 Hz H 2 /Air cell Z' / Ω cm 2 D. Malevich, E. Halliop, B. Peppley, J. Pharoah, K. Karan, J. Electrochem. Soc., 156, B216 (2009) 11

13 High-frequency arc: membrane resistance -0.3 Z'' / Ω cm khz 12 Hz 24 Hz 2 Hz 1.5 khz H H 2 /O 2 cell 2 /H 2 cell H 2 /Air cell 0.1Hz E cell -E cell(j=0) / mv H2 / H2 R=0.1 Ω cm 2 H2 / Air i / ma cm -2 E cell -E cell(j=0) / mv Z' / Ω cm 2 D. Malevich, E. Halliop, B. Peppley, J. Pharoah, K. Karan, ECS Transactions 156, 1763 (2008) Diameter of high-frequency arc found to be consistent with the resistance calculated from the known geometry and conductivity of the Nafion membrane J. M. Le Canut, R. Latham, W. Mérida, D. A. Harrington, J. Power Sources, 192, 457 (2009) 12

14 EIS response of PEM fuel cell: arc assignment E / V polarization curve total cell resistance (R cell ) R cell Hz 12 Hz Z'' / Ω cm Hz i = 0.21 A cm Hz Hz Z' / Ω cm 2 In-series resistance (FFP, endplates, cables etc.) Membrane resistance Cathode chargetransfer resistance Cathode masstransport resistance 13

15 Effect of MPL on PEMFC EIS response Z'' (Ω cm 2 ) Hz 600 Hz Hz with MPL 3 Hz 1 Hz 0.1 Hz without MPL Z' (Ω cm 2 ) Impedance diagrams for PEMFC with (2,3) and without (1) MPL fed with H 2 /Air. Current density 0.21 A cm

16 Effect of MPL: influence of current density without MPL 0.7 A cm -2 with MPL 0.3 A cm -2 15

17 Equivalent circuit fitting 0.2 -Z'' / Ω cm Randles circuit: Z' / Ω cm 2 R o CPE m CPE dl W R m R ct Z w 16

18 Effect of current density on cathode charge-transfer and Warburg resistances R / Ω cm R ct without MPL R w with MPL R w R ct i / ma cm -2 R / Ω cm R ct R w 5 R ct R w i / ma cm -2 17

19 Time constants for oxygen transport process Experimentally determined: Theoretically predicted: 18

20 MPL effect models Model A: MPL promotes water back diffusion Model B: MPL forces water to FFP observed Expected effects: Increased amount of water in cathode catalyst layer Reduced membrane resistance Reduced water content in cathode PTL Expected effects: Reduced amount of water in cathode catalyst layer Increased water content in cathode PTL not observed 19