Cerium species are added to membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells as radical quenchers to suppress the degradation caused by radical species derived from the reaction between impurity and hydrogen peroxide formed during fuel cell operation [1]. During long-term operation, the radical quencher of the dissolved trivalent cerium ions moves toward the cathode electrolyte interface, which negatively affects the durability of the fuel cell. To clarify the transport phenomena of radical quenchers, composition mapping analysis of disassembled cells after degradation test has been mainly performed. However, to estimate the quantitative value such as mobility and diffusion coefficient, it is necessary to directly observe the transport phenomena of cerium species under operating conditions of fuel cells. In the previous report, cerium ion transport with in-plane direction has analyzed using X-ray fluorescence mapping [2]. Compared with the in-plane analysis, operando analysis of cerium distribution with through-plane in MEA is challenge due to the quite thin thickness of electrolyte membrane. In this study, we have developed a technique for cerium ion mapping with through-plane MEA under fuel cell operation using high-energy micro-beam X-ray fluorescence spectroscopy. This technique is applied to analyze the transport phenomena of radical quenchers with through-plane MEA in 2nd-generation MIRAI fuel cells.

The fluorescence X-ray measurements were conducted at BL37XU of SPring-8. Monochromatized X-ray of 45 keV was focused to less than 200 nm by Kirkpatrick-Baez mirror. The X-rays were incident vertically from the membrane thickness direction, and the X-ray fluorescence spectra were measured using a Ge detector. MEAs used in the 2nd-generation MIRAI were cut and sandwiched placed in custom made carbon separators and end-plates. The effective electrode area is 1.0 cm². Ce-Ka fluorescence X-ray was monitored under fuel cell operation at 60°C by supplying humidified hydrogen and oxygen gases.

In the open circuit condition, cerium is mainly detected in anode MPL and cathode MPL, while there is low concentration of cerium in PEM. This is consistent with the report of the elemental analysis for 1st-generation MIRAI [3]. During fuel cell operation at 1.0 A cm^-2, the part of concentrated concentration shifts toward to cathode side. The enriched cerium in anode MPL is slightly shaved from the anode catalyst layer. We obtained the time-resolved cerium concentration profile, which can be used to estimate the apparent diffusion coefficient of cerium.

Acknowledgment

This work is based on results obtained from the project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. MEAs were provided by Toyota Motor Corporation.

References

[1] E. Endoh, N. Onoda, Y. Kaneko, Y. Hasegawa, S. Uchiike, Y. Takagi, T. Take, ECS Electrochem. Lett., 2 F73-F75 (2013).

[2] A.M. Baker, S.K. Babu, R. Mukundan, S.G. Advani, A.K. Prasad, D. Spernjak, R.L. Borup, J. Electrochem. Soc., 164 F1272-F1278 (2017).

[3] R.L. Borup, K.L. More, D.J. Myers, FC-PAD: Fuel Cell Performance and Durability Consortium Update to USCAR Analysis of Toyota Mirai Components Provided by USCAR, https://www.osti.gov/servlets/purl/1440417.