Influence of GDL Coated with MPL Containing CNTs on PEFC Performance Under Low and High Humidity Conditions

A hydrophobic microporous layer (MPL) coated on a gas diffusion layer (GDL) has been commonly used to improve the water management characteristics and thereby enhance the performance 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]. 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 [2, 3]. In the present study, a new GDL coated with an MPL containing hydrophilic carbon nanotubes (CNTs) was developed to achieve further enhancement of the PEFC performance under both low and high humidity conditions.

The GDL used at the anode was a commercial carbon paper without an MPL (SGL24BA). MPL coated GDLs were used at the cathode. The MPL coated GDL without the CNTs consisted of a substrate (SGL24BA) coated with an MPL composed of 80 mass% carbon black and polytetrafluoroethylene (PTFE). The MPL coated GDL with the CNTs consisted of a substrate coated with an MPL made from 4 mass% CNTs (NITTA CORPORATION), 76 mass% carbon black and PTFE. The contact angle of the MPL without the CNTs was 132° [1]. The contact angle of the CNT surface was 30°, which exhibited relatively high hydrophilicity. However, because the MPL pore surfaces consist of a combination of the CNTs, carbon black and PTFE, the contact angle of the MPL with the CNTs was 111°.

PEFC performance tests were conducted under both low and high humidity conditions. The cell temperature was set at 75 °C. The hydrogen and air utilizations were set to 70 and 60%, respectively. During tests under low humidity conditions, the relative humidity (RH) of the gas supplied to the cathode was set to 0%, while that supplied to the anode was maintained at 60% RH. During tests under high humidity conditions, the RH of the gases supplied to both the anode and cathode was set to 100%.

In the case of the MPL without the CNTs, decreasing the maximum pore diameter of the MPL from 40 to 5 μm enhanced the ability to prevent dehydration of the MEA, which improved the PEFC performance under low humidity conditions. Under high humidity conditions, decreasing the maximum pore diameter to 20 μm was effective to reduce flooding and also enhance the PEFC performance. However, when the pore diameter became too small, such as at 5 μm, the discharge of excess water through the MPL was inhibited, which promoted flooding, so that no significant enhancement in the PEFC performance was possible. The appropriate pore diameter for a conventional MPL is different under low and high humidity conditions. An MPL coated GDL designed to prevent dehydration of the MEA under low humidity conditions is inferior at reducing flooding under high humidity conditions.

In the case of the MPL with the CNTs, decreasing the maximum pore diameter from 40 to 5 μm was effective to enhance the ability to prevent dehydration of the MEA. The performance obtained with the MPL containing the CNTs was higher than that observed when using an MPL without the CNTs. Since the hydrophobicity of the MPL with the CNTs was reduced, its ability to retain humidity in the catalyst layer was improved. This resulted in higher PEFC performance compared with that for an MPL without the CNTs. Under high humidity conditions, decreasing the pore diameter to 5 μm enhanced the PEFC performance. For an MPL with the CNTs, the less hydrophobic pores promoted the transport of excess water from the catalyst layer through the MPL to the substrate. As a result, it was possible to decrease the maximum pore diameter to 5 μm without lowering the PEFC performance under high humidity conditions. These results demonstrate that the MPL with the CNTs is effective to achieve further enhancement of the PEFC performance under both low and high humidity conditions compared to that for a conventional 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, M. Morishita, Journal of Power Sources 234 (2013) 129-138.

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