Self-Supporting Microporous Layer for Polymer Electrolyte Fuel Cells Using Thin Carbon Mesh Gas Diffusion Layer
Authors: Yoshikawa Makoto , Noda Zhiyun , Yasutake Masahiro , Kitahara Tatsumi , Lyth Stephen Matthew , Matsuda Junko , Nishihara Masamichi , Sasaki Kazunari 
Published: December 2024

Introduction

Research and development of polymer electrolyte fuel cells (PEFC) have been conducted extensively to improve power density. The gas diffusion layer (GDL) and microporous layer (MPL) used in PEFCs are much thicker than the electrocatalyst layer and electrolyte membrane. Down-thinning GDL and MPL to 50 μm or less is desirable to reduce gas diffusion resistance and to increase the power density per volume of the cell stack [1]. However, while both the electrocatalyst layer and electrolyte membrane have been thinned down to about 10 μm, the GDL with MPL is still thick at about 150 to 200 μm. Previous studies have considered the possibilities and technical issues in using porous metals as GDLs, which can make GDL thinner with high mechanical strength [2]. In addition, a composite MPL with both hydrophobic and hydrophilic channels was developed and its effect on oxygen transport resistance was investigated [3]. Based on these findings, the aim of this study is to develop a self-supporting thin MPL/GDL using carbon mesh as a GDL, which is relatively stable and thinner than carbon paper, and commercially available. In addition, contact resistance is reduced by printing thinner MPL, for improving power density.

Experimental

In this study, the MPL/GDL was fabricated by printing MPL on a carbon mesh (50 µm thick, Cosmotech, Japan) as a GDL using a screen-printing method. Cells were fabricated by using these materials. Current-voltage characteristics, microstructural observation, and gas diffusion resistance measurements were conducted. The MPLs were composed of carbon black, carbon nanotubes (CNT), polytetrafluoroethylene (PTFE), and solvent (a mixture of polyethylene glycol 600, and pure water). After mixing, the first MPL “layer” was printed on a dense PTFE sheet using a 30-µm-thick screen and dried at 150°C for 30 min. Afterwards, this first MPL “layer” on the PTFE sheet was transferred to the porous GDL by pressing the MPL/PTFE sheet and the porous GDL together. The second MPL “layer” was then directly screen-printed by using a 10-µm-thick screen and then heat-treated at 300°C for 30 min to prepare the MPL/GDL. A standard Pt/C electrocatalyst (TEC10E50E, Tanaka Kikinzoku, Japan) was used for preparing membrane-electrode assemblies (MEAs). As the electrochemical characterization in this study, current-voltage characteristics were evaluated using an electrochemical impedance analyzer (SAS SP-240, Bio-Logic Science Instruments, France) and various overvoltages were separated.

Results and discussion

Figure 1 shows the current-voltage characteristics of the cells using various MPL/GDLs. The thickness of MPL/GDLs fabricated was ranging from 80 to 100 µm. For comparison, the characteristics of a cell using a standard, commercially-available MPL/GDL (22BB, SGL Carbon, Germany) are also shown. These results show that the use of MPL significantly improves cell performance even with thin GDLs. The cells with carbon mesh but without MPL exhibited particularly high concentration overvoltage and low I-V characteristics compared to the cells with an MPL. Without MPL, the contact resistance between the carbon mesh and the electrocatalyst layer was high due in part to the limited contact area, and water flooding may occur at high current densities. In contrast, when the MPLs with highly conductive materials were printed, cell performance was improved, and comparable results to the standard MPL/GDL were obtained. The presence of the highly conductive MPL improves conductivity and current collection, resulting in lower ohmic overvoltage. The hydrophobicity of the PTFE-containing MPL suppresses an increase in concentration overvoltage. In addition, the addition of CNTs may increase the porosity at a nanometer level and improve the drainage of generated water (Figure 2). These suggest that the self-supporting thin MPL/GDL has a potential to improve the power density per cell stack volume.

Acknowledgements

This paper is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). An educational part for young scientists was supported by the Japan Science and Technology Agency (JST) as a part of Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE), Grant Number JPMJAP2307.

References

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Figure 1