Effect of through-Plane Polytetrafluoroethylene Distribution in a Gas Diffusion Layer

During operation of a proton exchange membrane fuel cell (PEMFC), the oxygen reduction reaction rate at the cathode is the limiting kinetic step and determines the maximum operating current density. This rate strongly depends on the transport rate of oxygen from the flow channel to the catalyst sites. Although the electrolyte membrane needs liquid water to retain proton conductivity, the oxygen gas transport can be greatly hindered when liquid water accumulates either in the catalyst layer (CL) or the gas diffusion layer (GDL) of the cathode. Consequently, for optimal water management and performance in PEMFCs, water retention/expulsion properties must be carefully balanced.

We previously investigated the relationship between the treatment process of polytetrafluoroethylene (PTFE) and the resulting PTFE distribution in through-plane direction of carbon papers [1]. The dependence of PTFE distribution on the PTFE drying conditions was examined using SEM-EDS analysis. The EDS image maps revealed that the PTFE distribution strongly depended on the drying condition; PTFE-drying under atmospheric pressure created highly heterogeneous PTFE distributions in the through-plane direction, with high concentrations near both the upper and lower surfaces, whereas PTFE-drying under vacuum pressure created a relatively uniform PTFE distribution.

In this study, in order to investigate the effect of PTFE though-plane distribution on the water discharge characteristic, limiting current measurements were performed under various humidity conditions using a small-scale PEMFC, which has the electrode area of 1 cm2. Based on the limiting current measurement, we could evaluate the gas transport resistance through the carbon paper GDL [2,3]. Because the small-scale cell was applied, we could minimize the effect of the distribution of current density and humidification, that is, the effect of local flooding across the flow channel depending on the change of humidification.

References

1. H. Ito et al., J. Power Sources , 248, 822 (2014).

2. D. R. Baker et al., J. Electrochem. Soc., 156, B991 (2009).

3. N. Nonoyama et al., J. Electrochem. Soc., 158, B416 (2011).