Influence of the Gas Diffusion Layer Compression on the Oxygen Mass Transport in PEM Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFC) are believed to play an important role in the growing field of electric mobility. A key component is the gas diffusion layer (GDL), which has the task to supply reactant gases to the electrodes, to efficiently remove product water, and to provide good electrical and thermal contact.¹ However, at high current densities, the formation of liquid water in the GDL can substantially reduce oxygen transport and thus fuel cell performance.²

GDL compression is an important parameter in the construction of a fuel cell. While high contact pressure reduces the contact resistance,³ high compression also results in a decrease in porosity, which leads to a lower effective diffusivity and higher mass transport resistance at dry conditions.^4,5 Thus, there exists an optimum compression, taking into account the electrical and mass transport losses.⁶ However the influence of GDL compression on water condensation and mass transport resistance at wet conditions is still unclear.

In this study we measure mass transport limiting currents for a commercial GDL (SGL Sigracet 25 BC) with a microporous layer (MPL), using a differential-flow cell at a variety of GDL compressions. By diluting the oxygen flow on the cathode, we could extract the mass transport resistance at both dry and wet conditions, following the analysis by Caulk and Baker.² Figure 1 shows polarization curves in the mass transport limited region below a cell voltage of 0.3 V for two GDL compressions corresponding to 7% and 20% of the initial thickness and three dry mole fractions of oxygen. At a low dry mole fraction of 1%, the limiting current densities of the two compressions are almost superimposed. However, at a high dry mole fraction of 24%, i.e., when large amounts of water are formed, the compression of 20% shows a much higher limiting current density compared to the less compressed material.

In our study we will show how the mass transport resistance is affected by GDL compression at dry and wet conditions. Furthermore, we will show that the behavior at low compressions is consistent with the formation of a film of liquid water between the catalyst layer and the MPL, generating a barrier for the diffusion of oxygen to the electrode.

Literature
1.  J. M. Morgan and R. Datta, Journal of Power Sources 2014, 251, 269-278.
2.  D. A. Caulk and D. R. Baker, Journal of the Electrochemical Society 2010, 157, B1237-B1244.
3.  M. F. Mathias, J. Roth, J. Fleming and W. Lehnert in Handbook of Fuel Cells, Vol. 3, Eds.: W. Vielstich, H. A. Gasteiger and A. Lamm, John Wiley and Sons, Ltd, 2010.
4.  D. R. Baker, D. A. Caulk, K. C. Neyerlin and M. W. Murphy, Journal of the Electrochemical Society 2009, 156, B991-B1003.
5.  P. K. Das, X. G. Li and Z. S. Liu, Applied Energy 2010, 87, 2785-2796.
6.  J. Ge, A. Higier and H. Liu, Journal of Power Sources 2006, 159, 922-927.

Acknowledgements
This work has been supported by the German Federal Ministry of Economy and Technology (BMWi) under agreement number 03ET6015E (“Optigaa 2” project).

Figure 1: Polarization curves in the mass transport limited region below E_cell = 0.3 V for dry oxygen mole fractions (x_O2,dry) of 1%, 8% and 24 % and for GDL compressions of 7% and 20% of the initial thickness. Measurements are performed in a differential-flow cell at T_cell = 50 °C, p_in,abs = 200 kPa and RH_An/Ca = 77%. Dotted boxes indicate the extracted limiting current density i_lim.