Hydrophilic and Hydrophobic Microporous Layer Coated Gas Diffusion Layer for Enhancing PEFC Performance

Abstract

The gas diffusion layers (GDLs) coated with a hydrophobic microporous layer (MPL) have been commonly used to improve water management properties of polymer electrolyte fuel cells (PEFCs). An MPL coated GDL designed to prevent dehydration of the membrane electrode assembly (MEA) under low humidity conditions is generally inferior at reducing flooding under high humidity conditions. Thus, it is important to develop an MPL coated GDL to enhance the PEFC performance under both low and high humidity conditions. To overcome the defects of pure hydrophobic MPL, several plans are raised to develop a composite MPL coated GDL which possesses both hydrophobic and hydrophilic structures in the same MPL to enhance PEFC performance under both low and high humidity conditions. One method is a hydrophilic and hydrophobic double MPL coated GDL. The hydrophobic MPL with 20mass% hydrophobic polytetrafluoroethylene (PTFE) and 80mass% carbon black is coated on the carbon paper substrate as the first layer, then the hydrophilic MPL with 20mass% titanium dioxide (TiO2) and 80 mass% carbon black is coated as a relatively thin second layer. The contact angle of the hydrophobic MPL is 134°, while that of the hydrophilic MPL is 82°. A thin hydrophilic layer is effective at conserving membrane humidity under low humidity, while a hydrophobic intermediate MPL between the hydrophilic layer and the carbon paper substrate prevents the removal of water from the hydrophilic layer. This results in a significant enhancement of PEFC performance under low humidity conditions. The oxygen transport resistance is measured using the limiting current density of polarization curves to evaluate the ability of the GDL to prevent flooding under high humidity conditions. This value obtained with a pure hydrophobic MPL coated GDL is 80 sm-1, while the double MPL coated GDL is 70 sm-1. In the case of the double MPL coated GDL, excess water can be expelled from the catalyst layer, thereby enhancing PEFC performance under high humidity conditions. Except for the double MPL coated GDL, directly changing the part of the hydrophobic carbon black to hydrophilic carbon black is another method. This MPL consists of 16mass% hydrophilic carbon black with 64mass% hydrophobic carbon black and 20mass% PTFE. The prepared MPL is expected to keep the hydrophilic and hydrophobic structures in the same layer. However, the contact angle of the composite MPL coated GDL is 131°. The cell performance obtained with the composite MPL is lower than that with the hydrophobic MPL. The reason is the carbon blacks with different properties in composite MPL coated GDL are completely bonding together and cannot generate hydrophobic and hydrophilic pores separately. As a result, under high humidity conditions, most pore structures of composite MPL are occupied by liquid water, and oxygen transportation is deteriorated, which results in low performance under high humidity conditions. Another designed plan is mixing hydrophilic carbon nanotubes (CNTs) with a hydrophobic MPL. The MPL that contains 4mass% CNTs, 76mass% carbon black, and 20mass% PTFE is coated on the carbon paper. Since it is difficult for the hydrophilic CNTs and PTFE to fully bond together, the hydrophobic pores and hydrophilic pores can be generated separately. In the case of a hydrophobic MPL, decreasing the mean flow pore diameter is effective at preventing membrane dehydration, which enhances cell performance under low humidity conditions. However, when the pore diameter decreases to 2 μm, the water breakthrough pressure significantly increases. This increases the oxygen transport resistance under high humidity conditions. In the case of an MPL with hydrophilic CNTs, the ability to retain membrane humidity is improved, which enhances cell performance under low humidity conditions. It is possible to decrease the pore diameter to 2 μm without raising the water breakthrough pressure. This decreases the oxygen transport resistance to 62 sm-1 under high humidity conditions. Both decreasing the pore diameter and using hydrophilic CNTs in the MPL allow much higher cell performance under both low and high humidity conditions compared to that obtained with a hydrophobic MPL coated GDL.