Impact of Microstructure of Cathode MPL on Interface Water Transport and Power Generation Characteristics of PEFC

Proper water management in cathode porous electrode of polymer electrolyte fuel cells (PEFCs) is essential for achieving better performance.  Especially at high current densities, excessive water produced by oxygen reduction reaction (ORR) on cathode side is rapidly condensed in catalyst layer (CL) and gas diffusion layer (GDL).  When the open pores in cathode CL and the interface between CL and GDL are filled with liquid water, oxygen cannot be supplied to the reaction sites.  This phenomenon, called “water flooding”, is a critical barrier for high efficiency and high power density operation.  To alleviate this issue, it is necessary to understand and control liquid water transport inside cathode porous electrode.

It is well known that microporous layer (MPL) plays a critical role in suppressing liquid water flooding in PEFCs [1,2].  MPL is a much thinner and finer porous layer, which is typically composed of carbon black (CB) particles and PTFE, and inserted between cathode CL and GDL.  Nam et al. [2] experimentally and numerically investigated the vapor condensation and liquid water breakthrough in porous layers, and showed that inserting MPL between CL and GDL reduces the size and saturation level of interfacial water droplets on CL surface.  However, there is a risk that MPL addition on cathode side increases oxygen transport resistance through porous layers.  In general, MPL is formed by coating the slurry of CB powder and PTFE onto GDL substrate and then heat-treated.  Its pore structure is complicatedly affected by various fabrication parameters such as CB loading, PTFE/CB ratio, coating and dry processes.  To control interfacial water transport between cathode CL and GDL and minimize water flooding, it is essential to optimize the fabrication parameters of MPL determining its microstructure.

In this study, we focused on CB loading as one of the major fabrication parameters of MPL, and analyzed the pore structure of MPLs with different CB loadings using SEM-EDX and mercury porosimetry.  Furthermore, the operation test and the optical visualization of liquid droplet behavior in the cathode electrode were conducted using a transparent cell, and the influence of CB loading of MPL on its microstructure, the interfacial water transport and the performance characteristics of PEFC was discussed.

In the experiment, the MPLs were coated on the hydrophobized carbon paper (Toray, TGP-H-060) by using the doctor-blade technique.  The SEM-EDX observation revealed that the average thickness of two MPLs with 1.0 and 4.0 mg/cm² CB are 33.7 and 123.6 um, respectively.  When the CB loading is increased from 1.0 to 4.0 mg/cm², the thickness of MPL is enlarged approximately 3.7 times.

Subsequently, the operation test of a PEFC was carried out at 70 ^oC under high-humidity conditions, and the effect of CB loading of MPL on the I-V characteristics was investigated.  The MPL addition on the cathode side drastically improves the limiting current density, because it suppresses the formation of liquid water film at the CL/GDL interface and reduces the concentration overpotential.  In addition, the cell voltage in the high current density range increases with an increase in CB loading of MPL.  Since the thick MPL raises the temperature near the cathode reaction sites, the rate of condensation of product water at the CL decreases in the case of 4.0 mg/cm²CB.  Therefore, the thick MPL with high CB loading beneficially reduces water flooding at the cathode CL/MPL interface and improves the cell performance.

References:
[1] A.Z. Weber and J. Newman, J. Electrochem. Soc., 152(4), A677 (2005).
[2] J.H. Nam, et al., Int. J. Heat Mass Transfer, 52, 2779 (2009).