Theoretical Analysis of Relationship Between Porous Electrode Structure and Mass Transfer Performance for PEFCs By Direct Measurement and Simulation

In order to reduce the cost of polymer electrolyte fuel cell system, it is important to increase current density. However, the limiting current density cannot be expanded because it depends on the oxygen transfer performance from gas channels to electrode catalyst surfaces. Therefore, it is important to reduce the oxygen diffusion resistance of the porous electrode components, which are composed of gas diffusion layer (GDL), micro-porous layer (MPL) and catalyst layer (CL). In this study, the oxygen diffusion resistances of these porous media were directly measured and calculated by three-dimensional porous structure reconstructed by X-ray CT and FIB-SEM, and the relationship between heterogeneous structure and the mass transport performance was examined. Figure 1 shows the example of reconstructed image. The porous structure of GDL was obtained by X-ray CT under compression. The porous structures of MPL and CL were obtained by FIB-SEM with different carbon-PTFE weight ratio and different ionomer-carbon weight ratio, respectively. And the validity of 3D structure, which was obtained after image processing, has been confirmed by pore size distribution and porosity. Effective oxygen diffusion coefficient of each porous electrode was experimentally measured by O₂-N₂ inter-diffusion resistance method ^[1]. In addition, the effect of Knudsen diffusion can be separated from overall diffusion resistance by the dependency of full pressure. Furthermore, the effective diffusion coefficient with or without Knudsen effect could be obtained by tortuosity and pore size calculated in porous structure shown in Fig.1 ^[2]. Figure 2 shows the relationship between porosity and relative diffusion coefficient of each component and model structures (vertical orientation, sphere packing, fibrous structure). In the case of GDL and MPL, experimental values are almost equal to calculation results from reconstructed structure. In the case of CL, these experimental and calculated results are almost same. However, although the value of MPL agreed with that of model structure of particle packing, the relative diffusion coefficients of MPL and CL, which has the same carbon black particles, were different from each other. It is conceivable that the difference is caused by the effect of ionomer distribution. In order to compare the effect of ionomer coating distribution, simulated catalyst layer was numerically made by packing carbon black aggregate and modeling ionomer heterogeneous coating^[3], and it was confirmed that the ionomer coating distribution affected the oxygen diffusion performance in catalyst layer.

Acknowledgement

This work is a part of the project, Strategic Development of PEFC Technologies for Practical Application, by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

References

[1] K.Yokoyama et al., FC-DIC Transactions145-150 (2013)

[2] G.Inoue et al., J. Power Sources 175, 145-158 (2008)

[3] G.Inoue et al., 218th ECS Meeting #817 B7 (2010)