Tomographic Analysis of Polymer Electrolyte Fuel Cell Catalyst Layers: Methods, Validity and Challenges

The catalyst layer (CLs) are the central part of Polymer Electrolyte Fuel Cells. Consisting of catalyst particles, ionomer, carbon black plus numerous gases and liquid water during operation, the CL contains not less than five different phases with a plethora of mutual physical interactions. Further, all these phases have characteristic length scales in the nanometer range. Due to its nanoscale, multi-phase nature the CL bears complex physical phenomena that are very challenging to describe and understand.

The precise morphology of all phases within a CL plays a key role for reactant transport and thus  performance: Platinum catalyst particles with poor connection to ion conducting, electron conducting or gas transporting phases cannot contribute to the fuel cell reaction. Also, the precise shape of the pathways of the reaction species determines their effective transport properties. Wetting or non-wetting pore wall areas influence water precipitation and therefore the generation of liquid water networks. Such networks both improve ionic transport and adulterate gas transport. 

Facing the tremendous influence morphology has on PEMFC performance, cost and durability, tools to image morphology are an obvious necessity. While there is a variety of imaging methods that allow three-dimensional reconstructions, only few are able to resolve the nanostructure of CLs: x-ray tomography (Xt), focused ion beam / scanning electron microscopy tomography (FIB-SEMt) and transmission electron microscopy tomography (TEMt) [1].

Both Xt and FIB-SEMt have been proven to enable differentiating porous and solid phase within PEMFC CLs [2,3]. TEMt allows imaging with resolutions below 1 nm and can be directly used to image Platinum particles [4]. To image all important phases multiple tomographic techniques must be used. In this talk we give an overview on the latest tomographic analysis techniques for PEMFC CL reconstruction with a particular emphasis on FIB-SEMt, multi-scale imaging approaches and validity of the existing reconstruction methods.

References

[1]   G. Möbus, B.J. Inkson, Materials Today 10 (2007) 18–25.

[2]   C. Ziegler, S. Thiele, R. Zengerle, J. Power Sources 196 (2011) 2094–2097.

[3]   W.K. Epting, J. Gelb, S. Litster, Advanced Functional Materials (2012).

[4]   H. Uchida, J.M. Song, S. Suzuki, E. Nakazawa, N. Baba, M. Watanabe, The Journal of Physical Chemistry B 110 (2006) 13319–13321.

[5]   S. Thiele, T. Fürstenhaupt, D. Banham, T. Hutzenlaub, V. Birss, C. Ziegler, R. Zengerle, J. Power Sources 228 (2013) 185–192.