Influence of Hydrophilic and Hydrophobic Triple MPL Coated GDL on the Oxygen Transport Resistance in a PEFC under High Humidity Conditions

Gas diffusion layers (GDLs) coated with a hydrophobic microporous layer (MPL) have been commonly used to improve water management characteristics of polymer electrolyte fuel cells (PEFCs). However, the appropriate design parameters for the MPL coated GDL are different under low and high humidity conditions [1]. 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. It is highly desirable to have a PEFC that can be operated under a wide range of conditions varying from low (or no) to high humidity. The authors have reported that a hydrophilic and hydrophobic double MPL coated GDLs was more effective at enhancing the PEFC performance under low humidity conditions than a hydrophobic MPL coated GDL [2,3]. This design involves the application of a thin hydrophilic layer on top of a hydrophobic MPL. The hydrophilic layer works to conserve the water content of the MEA, while the hydrophobic MPL between the hydrophilic layer and the carbon paper substrate prevents removal of water from the hydrophilic layer. This results in a significant enhancement of the ability to prevent dehydration of the MEA.

In the present study, a hydrophilic and hydrophobic triple MPL coated GDL was developed to achieve further enhancement of the PEFC performance under high humidity conditions at a low temperature of 35°C. The active area of the MEA (GORE PRIMEA5580) was 1cm². The relative humidity (RH) of the gas supplied to the anode and cathode was varied between 50 and 200%. The oxygen transport resistance [4] was measured using the limiting current density of polarization curves to evaluate the ability of the GDL to prevent flooding under high humidity conditions. Single (hydrophobic), double and triple MPL coated GDLs were used at the cathode. The hydrophobic MPL coated GDL consisted of a carbon paper substrate (SGL24BA) coated with an MPL of 20 mass% PTFE (polytetrafluoroethylene) and carbon black. For both the double MPL and the triple MPL coated GDLs, a hydrophilic layer of 25 mass% titanium dioxide, 5 mass% silicone, and carbon black was coated on the hydrophobic MPL coated GDL. The PTFE content in the hydrophobic intermediate MPL of the double MPL coated GDL was varied between 10 and 40 mass%. For the triple MPL coated GDL, the PTFE content in the hydrophobic MPL in contact with the hydrophilic layer was set to 20 mass% and that in contact with the substrate was set to 10 mass%. The contact angle for the hydrophilic MPL was 82°, while those for the hydrophobic MPLs were greater than 120°. When the PTFE content in the hydrophobic MPL was increased from 10 to 40 mass%, the hydrophobicity was enhanced, thereby increasing the contact angle.

When the RH of the supplied gases increases from 50 to 100%, the oxygen transport resistance significantly increases due to flooding. However, the oxygen transport resistance becomes almost constant under high humidity conditions varying from 100 to 200 %RH.  A double MPL coated GDL is effective to expel excess water from the catalyst layer [2], which results in lower oxygen transport resistance under high humidity conditions, compared with that for the hydrophobic MPL coated GDL. The performance is strongly dependent on the hydrophobic intermediate MPL in the double MPL. Increasing the PTFE content from 10 to 20 mass% represents an effective means of reducing flooding, thereby lowering the oxygen transport resistance. However, when the PTFE content becomes too high, such as 40 mass%, the discharge of excess water at the catalyst layer is inhibited, which increases the oxygen transport resistance.

For the triple MPL coated GDL, the hydrophobic gradient promotes water transport from the hydrophobic MPL containing 20 mass% PFTE to the hydrophobic MPL containing 10 mass% PTFE, which causes a relatively large amount of water to be stored in the latter MPL in contact with the substrate, which is effective at expelling excess water from the catalyst layer, while maintaining the oxygen flow pathways inside the MPL from the substrate to the catalyst layer. This results in a significantly lower oxygen transport resistance than that obtained when employing the double MPL coated GDL.

References

[1] T. Kitahara, T. Konomi, H. Nakajima, Journal of Power Sources 195 (2010) 2202-2211.

[2] T. Kitahara, H. Nakajima, M. Inamoto, Journal of Power Sources 234 (2013) 129-138.

[3] T. Kitahara, H. Nakajima, M. Inamoto, Journal of Power Sources 248 (2014) 1256-1263.

[4] N. Nonoyama, S. Okazaki, A. Weber, Y. Ikogi, T. Yoshida, Journal of The Electrochemical Society 158 (2011) B416-B423.