Wetting properties of water in gas diffusion electrodes investigated with environmental scanning electron microscopy

Abstract text (incl. figure legends and references)

Proton exchange membrane fuel cells (PEMFC) are promising energetic devices for automotive applications that have been widely investigated in the last decades ¹. In order to achieve their full theoretical potential, water management at the cathode side needs to be improved to avoid mass transport losses. Although water formation in the catalyst layer comes mostly from the cathodic reaction, condensation still plays an important role, as it was proven by the effect of humidifying reactants ². Pore size of diffusion media ranges from tenths of nanometres to few microns, and pore constrictions hindering water percolation are in the range of hundreds of nanometres ³. Environmental scanning electron microscopy (ESEM) is therefore a suitable technique to investigate the condensation mechanism at the relevant scale for water saturation ^4,5.

Previous studies have correlated the wetting behaviour of microporous layers (MPL) in the ESEM with water breakthrough from diffusion media in flooding conditions ⁶, while more recent studies took advantage of water condensation to compare the wettability of nanoporous carbon scaffolds to the conventional MPL ⁷. However, many parameters related to the condensation mechanism are still poorly investigated experimentally, and the benefits of environmental electron microscopy have not been fully exploited yet.

Microporous layers and catalyst layers with different hydrophobicity have been observed in the ESEM at a pressure of 900 Pa varying the relative humidity. Temperature below 5°C were controlled by a cooling stage with a ramp of 0.1°C/min. Recordings of water breakthrough and droplet growth were acquired and methods to segment and analyse droplets were developed. Preliminary results suggest that larger pore size MPLs have slower droplet growth, which may be explained by the larger coalescence contribution in narrower pore size MPLs.

In this work, we report on the effects of the condensation mechanism parameters, such as droplet nucleation rate, spatial density of nucleation sites, droplet growth and coalescence rate of dissimilar gas diffusion layers. Further insights into water nucleation and growth could aid towards the optimization of the transport of reactants to the electrodes.

References

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