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
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