1. Introduction

Various carbon materials are currently used in PEFCs.^1,2 In particular, relatively expensive porous carbon materials such as carbon fiber cloth and carbon paper with a thickness of around 0.2 mm are commonly used as the gas diffusion layers (GDLs), which ensure gas flow, electron transport, and water management in fuel cells.^3,4 However, electrical conductivity of these carbon materials could be lower than that of metallic materials. Another technical issue is the reduction of GDL thickness to achieve higher power density per cell stack volume, which could however lead to a decrease in mechanical strength of GDLs.⁵ Here in this study, we evaluate and compare several types of metallic materials to apply as GDLs. Using such alternative metallic GDLs, current-voltage (I-V) characteristics were measured and overvoltages were separated. The purpose of this study is to examine possibilities of using metallic GDLs for PEFCs, by varying materials, thickness, porosity, and preparation conditions.

2. Experimental

Ti fiber sheets and stainless steel mesh sheets were used as GDL substrates, and Sn coating was made on the sheets. Then, current-voltage characteristics of the cells using these GDLs were evaluated. Five types of porous metallic sheets were used as GDLs: (i) Ti fiber sheet with a porosity of 70% and thickness of 100 μm, (ii) Ti fiber sheet with Sn plating layer (Sn/Ti sheet), (iii) Ti fiber sheet with Pt nanoparticles and Sn plating layer (Sn/Pt/Ti sheet), (iv) SUS316 mesh with a porosity of 60% and thickness of 32 μm, and (v) SUS316 mesh with porous Sn layer. For all the evaluations, the electrocatalyst layers were prepared by dispersing Pt/C (Pt 46.5%, TEC10E50E, Tanaka Kikinzoku Kogyo Corp., Japan), 99.5% ethanol, pure water, and 5% Nafion solution using an ultrasonic homogenizer. The dispersions were printed on the electrolyte membrane (Nafion 212) using a spray printer (C-3 J, Nordson). The electrode area was 1.0 cm² (1.0 cm × 1.0 cm), and the Pt loading on both electrodes was 0.3 mg-Pt cm^-2. The metallic GDL was used for the cathode, and a carbon paper (EC-TPI-060T) was used as the standard GDL for the anode. In the electrochemical characterization of this study, electrochemical impedance spectroscopy (EIS, 1255WB, Solatron) was applied to measure the current-voltage characteristics and to separate overvoltages.

3. Results and discussion

I-V characteristics of the cells with the MEAs (i) to (v) are shown in Figure 1. Decrease in cell performance was mainly caused by an increase in ohmic losses. Comparing the MEA (i) and the MEA (iv), Ti fiber sheet showed higher electrical conductivity than stainless steel mesh sheet. Comparing the MEA (i) and the MEA (ii), it was confirmed that the Sn plating on the Ti fiber contributed to a reduction of ohmic loss and concentration overvoltage. Furthermore, the current-voltage curve of the MEA (iii) indicates higher conductivity than that of the MEA (ii), suggesting that the Pt coating promoted the Sn plating or acted as catalysts on the electrocatalyst layer side during the operation of the cell. When the SUS316 mesh sheet was used for the MEA (iv), the concentration overvoltage tended to fluctuate in each measurement. For the MEA (v), a decrease in cell performance was observed. These could be caused by, e.g., degradation of stainless steel due to strong acidity, and contact resistance between the Sn porous layer and the mesh sheet. Further study is in progress to improve I-V characteristics of MEAs by optimizing materials and structure of metallic GDLs.

Acknowledgement

Financial support from New Energy and Industrial Technology Development Organization (NEDO) is gratefully acknowledged (Contract No. 20001214-0).

References

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