Gas transport properties of catalyst layers (CLs) used in proton exchange membrane fuel cells (PEMFCs) are one of the important factors which affects cell performance. The gas transport properties are strongly affected by the porous structure of the CLs. In this study, a measurement system of gas diffusion flux through the CLs by using a microfluidic device was developed. Parallel channels were made on a Si chip by microfabrication techniques. Air and nitrogen were supplied to the two parallel channels in the device, respectively. Oxygen was transferred to a nitrogen flow channel through the CL under a rib between the two channels. Oxygen partial pressure in the nitrogen flow channel was measured at the outlet of the channel by using an optical oxygen sensor. The porous structure of the measured CLs was characterized. Effective gas diffusivity and tortuosity factor of the CLs can be calculated based on the measured gas transfer and porous structure properties.

Several types of CLs were fabricated and characterized. The thickness and overall porosity of the CLs were evaluated from scanning electron microscopy and weight measurement. Pore size distribution and effective porosity were evaluated by using nitrogen physisorption measurement. Then effective diffusivity of CLs can be calculated by measuring oxygen partial pressure at the inlet and outlet of the flow channel. The CL, which has 1.0 of ionomer to carbon ratio, was evaluated. Oxygen partial pressure at the outlet of the nitrogen flow channel decreases with flow rate as shown in Figure 1. Effective gas diffusivities obtained from each flow rate showed almost constant value. Tortuosity factor was calculated from the effective diffusivity and measured structural properties and indicated around 3. This evaluation method was applied to the CLs containing a different type of carbon materials, which show different porous structure. Adding multi-walled carbon nanotube in the CL(1) instead of carbon black resulted in an increase of effective gas diffusivity, while geometrical porosity was decreased.

Acknowledgments

A part of this work was financially supported by JSPS KAKENHI Grant Number 16K18028. This study was supported by Osaka University Nanofabrication Platform [F-18-OS-0006] in Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

References

1. T. Suzuki, R. Hashizume and M. Hayase, J. Power Sources, 286, 109 (2015).