Investigation of Interfacial Water Transport at the Gas Diffusion Media by Neutron Radiography

Cost reduction is the most important issue for commercialization of Fuel Cell Electric Vehicle (FCEV)., and high current density operation is one of the solutions that is currently being considered. In order to realize high current density operation, however it is necessary to reduce mass transport resistance. Carbon paper (CP) is commonly used as the gas diffusion layer (GDL), and is shown to dominate the mass transport resistance in fuel cells ^{(1, 2)}, Tabuchi et al. revealed that it is possible to achieve higher cell performance by removing carbon paper ⁽³⁾. However, mass transport phenomena, especially liquid water transport at the interface between CP, micro porous layer (MPL), and catalyst layer (CL) are yet to be fully understood. In this study, coupled cell performance evaluation, liquid water visualization by neutron radiography (NRG) method and numerical modeling based on multiphase mixture (M2) model⁽⁴⁾were performed to investigate the liquid water transport at the interfaces of components and understand its effect on cell performance.

All experiments were conducted with three types of gas diffusion media (GDMs): MPL free; CP with MPL; and CP free. CPs without PTFE treatment were utilized for all these analyses. Operating conditions are summarized in Table 1. NRG was carried out to evaluate liquid water distribution in-situ in each cell during operation. Parameters for NRG were same with T. Kotaka et al. ⁽⁵⁾ Additionally, numerical analysis with M2 model, which included sub models accounting interface effect of each component on water transport⁽⁵⁾, was conducted for thorough understanding of the effect of the interfaces on liquid water transport and cell performance.

Figure 1 shows the polarization curves of each cell. CP free cell showed much better performance rather than cells with CPs. The liquid water distributions of each cell at IR-free voltage of around 0.65 V are shown in Figure 2. In case of MPL free cell, liquid water accumulated at the gap between CL and CP, and this liquid water film was seen as a peak of water thickness toward to CL. This liquid water film caused significant increase in oxygen transport resistance and resulted in poor performance. By placing MPL at the interface, this liquid water accumulation was prevented, as MPL is easily deformed by compression and fills the gap, significantly improving the cell performance compared to MPL free case. However, there was still large amount liquid water present inside the CP, which hampers the oxygen transport. In CP free case, the effect is removed. Additionally, due to strong hydrophobicity and much finer structure of MPL, the liquid water coverage on GDM surface is small and liquid water removal is enhanced in CP free case. Thus CP free showed lower water content and better performance. These facts indicated that interfacial design is important to achieve high current density operation.

References

(1) D. R. Baker, D. A. Caulk, K. C. Neyerlin, and M. W. Murphy, J. Electrochem. Soc, 156(9), B991(2009).

(2) R. Jiang, C. K. Mittelsteadt, and C. S. Gittleman, J. Electrochem. Soc.,156(12), B1440(2009).

(3) Y. Tabuchi, T. Shiomi, Y. Fukuyama, K. Sato, et al., 222th ECS Meeting Abstract, No. 1496(2012).

(4) Y. Wang and C. Y. Wang, J. Electrochem. Soc., 154, B636 (2007).

(5) T. Kotaka, Y. Tabuchi, U. Pasaogullari, and C. Y. Wang, ECS Trans., 58(1), 1033 (2013). 

Acknowledgement

The authors acknowledge D. S. Hussy, E. Baltic and D. L. Jacobson of the NIST for technical assistance for carrying out NRG experiments and valuable discussion.