Fuel cells are complex systems whose performance and degradation depend on a number of factors spreading over multiple scales. At the lowest spatial scale are nano-sized Pt-group or Pt-group-metal-free (PGM-free) catalysts, various types of catalyst supports, proton-conducting nanometer-thin ionomer film, and gas-transporting nano-pores, all contained within the electrodes of a membrane electrode assembly (MEA). Their properties, composition and spatial distribution largely determine fuel cell performance and affect degradation trends. At the next scale are electrodes, membrane and gas diffusion layer, dictating mass, electrical and thermal transport at the micron-scale. Finally, millimeters-thick flow field plates and larger system components affect the gas transport, water and overall system management. Understanding how all of these factors affect fuel cell performance and degradation is of crucial importance, and it is only possible by applying highly correlated efforts in testing, structural characterization and mathematical modeling. The focus of this talk is to present the latest advancements in the structural characterization approaches for fuel cells at multiple scales.

In the recent years, a whole suite of novel 2D and 3D characterization approaches have been developed to provide information ranging from nano-sized catalysts used in the electrodes to macro-level flow field plates. This talk will give an overview of a number of advanced characterization techniques used for fuel cell analysis, starting from some more typically used, such as scanning electron microscopy (SEM), to highly sophisticated techniques such as 3D electron tomography. Techniques such as 2D and 3D high-resolution transmission electron microscopy (HR-TEM), 2D and 3D scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS), 3D focused-ion beam SEM (FIB-SEM), electron energy loss spectroscopy (EELS), scanning transmission X-ray microscopy (STXM), atomic force microscopy (AFM), and micro- and nano-computed tomography (CT), as well as developed in-situ techniques will be discussed. Challenges in characterization, ranging from sample preparation, ionomer and water imaging, image processing and quantification will be addressed.

Finally, the author’s work on advanced characterization of fuel cells will be reported. Multi-scale approach in 3D imaging and property modeling, utilizing electron tomography, FIB-SEM and micro-CT, will be presented with encountered challenges [1]. Advanced 2D STEM characterization with quantification and its application for analysis of fuel cell degradation will be discussed [2]. Lastly, a collaborative work on in-situ water imaging using X-ray synchrotron imaging and revealed effect on membrane changes during operation at high current densities will be shown [3]. The talk will close by discussing the challenges and plan forward for continuing development of fuel cell characterization approaches.

1. Jankovic, S. Zhang, A. Putz, M. S. Saha, D. Susac, Multi-scale imaging and transport modeling for fuel cell electrodes, (invited) Journal of Materials Research, 34 (4) (2019).
2. Kneer, J. Jankovic, D. Susac, A. Putz, N. Wagner, M. Sabharwal, M. Secanell, Correlation of changes in electrochemical and structural parameters due to voltage cycling induced degradation in PEM fuel cells, Journal of The Electrochemical Society, 165 (6) F3241-F3250 (2018).
3. N. Ge, R. Banerjee, D. Muirhead, J. Lee, H. Liu, P. Shrestha, A.K.C. Wong, J. Jankovic, M. Tam, D. Susac, J. Stumper, A. Bazylak, Membrane dehydration with increasing current density at high inlet gas relative humidity in polymer electrolyte membrane fuel cells, Journal of Power Sources, 422 (2019) 163-174.