A microporous layer (MPL) is conventionally applied to the gas diffusion layer (GDL) in order to improve liquid water transport and to provide a smooth contact between GDL and electrode. It typically consists of carbon components (e.g., carbon black, graphite) and a hydrophobic binder (e.g., PTFE).¹ The small hydrophobic pores prevent condensed water from accumulating inside the layer or at the interface between MPL and electrode, which allows an efficient oxygen transport to the cathode.^2,3

Figure 1 shows polarization curves at standard and humid conditions for 3 different GDL materials. At standard conditions (full symbols) where little amounts of liquid water are expected inside the MEA and GDL, the fuel cell performances of the GDL substrate without MPL and with different MPLs are very similar. On the other hand, the polarization curves at humid conditions (open symbols) are very different: the substrate without MPL reveals an early voltage drop at 2.5 A cm^-2 caused by flooding of the cathode/GDL interface, while the addition of an MPL clearly improves the performance, with the MPL based on carbon fibers exhibiting the highest current densities. To characterize the oxygen transport, we determine the total oxygen transport resistance (R_T,O2) from limiting current density measurements with diluted oxygen.⁴ The bar graph in Figure 1 illustrates R_T,O2 at the two conditions. While all materials behave very similar at standard conditions, at humid conditions the application of the carbon fiber MPL decreases R_T,O2 by 50%. This shows that MPL structural differences as indicated by the SEM images significantly impact the liquid water transport.

In our study we will show to how MPL properties influence the oxygen transport for (1) various operating conditions, (2) various oxygen concentrations, and (3) different GDL substrates. With this work, we aim to illustrate under which operating conditions and for which materials (e.g., type of GDL substrate, fuel cell operating strategy) advanced MPLs help to improve the overall fuel cell performance at high current densities.

Literature

1. M. F. Mathias, J. Roth, J. Fleming and W. Lehnert, in Handbook of Fuel Cells, W. Vielstich, H. A. Gasteiger and A. Lamm (Editors), John Wiley and Sons, Ltd (2010).
2. J. T. Gostick, M. A. Ioannidis, M. W. Fowler and M. D. Pritzker, Electrochem. Commun., 11, 576 (2009).
3. J. P. Owejan, J. E. Owejan, W. B. Gu, T. A. Trabold, T. W. Tighe and M. F. Mathias, J. Electrochem. Soc., 157, B1456 (2010).
4. D. A. Caulk and D. R. Baker, J. Electrochem. Soc., 157, B1237 (2010).

Acknowledgments

The authors gratefully acknowledge the financial support by the German Federal Ministry for Economic Affairs and Energy (BMWi), Freudenberg Performance Materials SE & Co. KG, and Daimler AG under agreement number 03ET6015E (“Optigaa 2” project).

Figure 1: Polarization curves for three different GDL materials on the cathode (also see SEM images in the figure): GDL substrate (Freudenberg H1410 I4) without MPL (green symbols), with carbon black based MPL (blue symbols), and with carbon fiber based MPL (red symbols). Measurements are performed with differential gas flow rates (2 nlm H₂ and 5 nlm air for active area of 5 cm²) at standard conditions (full symbols; T_cell = 80 °C, p_abs = 170 kPa, RH = 100%) and at humid conditions (open symbols; T_cell = 50 °C, p_abs = 300 kPa, RH = 120%). The bar graph in the figure illustrates the total oxygen transport resistance (R_T,O2) at standard and humid conditions determined from the limiting current density at a dry oxygen content of 2%.