(Invited) Direct Observations of Liquid Water Formation at Nano- and Micro-Scale in Platinum Group Metal-Free Electrodes By Operando X-Ray Computed Tomography
Authors: Zenyuk Iryna V. 
Published: February 2020

Platinum group metal (PGM)-free catalyst materials are promising alternatives to Pt-based catalysts for use in polymer electrolyte fuel cells (PEFCs) providing for reduced cost and potentially broadening technology footprint because of sources availability. Optimizing the morphology of these PGM-free catalyst layers for enhanced transport properties is critical for reaching high power-density targets. Unfortunately, the current understandings of both morphology and transport processes are very limited due to the novelty of the materials and the challenges in designing operando characterization techniques. In order to bridge these gaps in understanding, we used operando synchrotron X-ray computed tomography to visualize water transport in operating PEFCs.
In this study, we present a non-destructive observation of operando morphology and water transport processes for PGM-free PEFCs. Our previous studies highlighted some of the morphological features of PGM-free electrodes on nano and micro-scale (1, 2). Ex-situ nano-CT data complements operando micro-CT observation by providing water distribution in smaller macropores (< 1 µm). Figure 1 shows grey-scale cross-section four stitched FOVs along one of the channels for OCV and 50 % RH (Figure 1a) and 40 mAcm-2 at 100 % RH (Figure 1b) operando conditions. The anode, cathode layers, as well, as membrane are clearly seen from this cross-section view. No liquid water was observed at 50 % RH condition, whereas for 100 % RH and under applied current density large water droplets are seen in the cathode channel and within the cathode PGM-free layer. Figure 1b and c show zoom-in locations within the MEA. Tree-like agglomerates were observed within the cathode PGM-free GDE that were compressed against 50 µm membrane. Under applied current density of 40 mAcm-2 liquid water was clearly observed within both large and small voids of the GDE and membrane swelling to 72 µm was also measured. From Figure 1d and b membrane expands into the GDE voids upon humidification resulting in several locations in loss of contact with the anode GDE.
In this presentation we demonstrate that (1) gas-diffusion electrodes (GDEs) form highly non-uniform interfaces with the membrane and under constant current operation, liquid water formed in electrochemical reactions fills the larger macropores within the PGM-free electrode and at the electrode-membrane interface; (2) from nano-CT data ionomer swelling in macropores is 16 % going from 50 % RH to 100 % RH but no notable structural changes occur within CL at nano-scale; (3) water is removed into the cathode channel and is also observed under location of the land-area (which is cooler but also has larger voids); and (4) water preferentially fills larger voids, that are lower capillary pressure locations, due to hydrophobic nature of the catalyst.

Figure 1. Along-the-channel cross-section tomographs for temperature 60oC a) 50 % RH OCV and b) 100 % RH 40 mAcm-2, where c and d) are corresponding zoom-in locations from a and b. Locations for liquid water are marked, as well as membrane thicknesses.
References

A. Serov, A. D. Shum, X. Xiao, V. De Andrade, K. Artyushkova, I. V. Zenyuk and P. Atanassov, Applied Catalysis B: Environmental (2017).

S. Kabir, K. Lemire, K. Artyushkova, A. Roy, M. Odgaard, D. Schlueter, A. Oshchepkov, A. Bonnefont, E. Savinova, D. C. Sabarirajan, P. Mandal, E. J. Crumlin, I. Zenyuk, P. Atanassov and A. Serov, Journal of Materials Chemistry A (2017).

Figure 1