Experimental Investigation and Numerical Determination of Custom Gas Diffusion Layers to Understand Water Transports in PEMFC

Water management is an important impact to determine the polymer electrolyte membrane fuel cell (PEMFC) performance. In particular for fuel cell system operation at high humidity conditions and current densities, the proper water management requires good understanding of water transport in the different components of the cell. The gas diffusion layer (GDL) is one of the vital components of PEMFCs that has a variety of functions significant for their overall performance and durability. The GDL is usually a dual-layer carbon based porous substrate and tropically treated with polytetrafluoroethylene (PTFE) to make it hydrophobic in order to avoid liquid water blockage of its internal pores and keep them available for reactant gas transport to the adjacent catalyst layer [1]. When the gas phase is saturated with water vapor, water condensation takes place and resulting liquid water starts to fill the open pores of the GDL and covers the catalyst particles, rendering them electrochemically inactive [2]. The maximum current density is limited by oxygen concentration at the cathode catalyst surface [3]. This value represents the oxygen transport limitation through the GDL. The mass transfer through the GDL may indeed be rate limiting at high current densities when the GDL is saturated with water [4].

In this study, the different designs of GDL with the introduction of two micro porous layers were investigated on PEMFC performance through characterization of their materials and structures as shown in Fig. 1. This new design of GDLs has been modified from standard AvCarb GDLs by adding two micro porous layers. Each set has been treated with two different methods in order to provide two different value of diffusivity. Polarization curves were collected at different designs of GDL as shown in Fig. 2. Further, the internal fuel cell resistance was measured by high frequency resistance (HFR) technique at 1000 Hz. Pore size distribution (PSD) data of the different GDLs were reported and analyzed. It is well known that tortuosity (τ) and porosity (ε) are two important parameters to relate free-stream properties with the actual mass transport in GDL [5]. The relationship of tortuosity (τ) and porosity (ε) was measured as the MacMullin number.

In addition to these experiments, we present a computational fluid dynamic (CFD) analysis of the PEMFC operation with provided GDL characterization information, specifically the MacMullin number and PSD to compare with experiment data. The CFD prediction of current density distribution and gas/water transports in PEMFC will be used to understand the effect of these new designs of GDL.

Acknowledgement

This project is supported by the Department of Energy Contract Grant# DE-EE0000471

 

References

1. M.V. Williams, et al., J Electrochem Soc, 151 (2004) A1173-A1180.
2. U. Pasaogullar,i et al., J Electrochem Soc, 151 (2004) A399-A406.
3. A.A. Kulikovsky, Electrochem Commun, 4 (2002) 845-852.
4. J.T. Gostick, et al., J Power Sources, 173 (2007) 277-290.
5. M.J. Martinez et al., J Electrochem Soc, 156 (2009) B80-B85.