Hydrophobic Diffusion Media via Electrografted Organosilicons Enable Competitive Performance in Polymer Electrolyte Fuel Cells.

Abstract

Hydrophobic surface modification of porous carbon materials is critical for the performance and durability of polymer electrolyte membrane fuel cells and many other electrochemical technologies. However, conventional treatments rely on dip coating approaches using polymer dispersions containing fluorinated compounds (i.e., polytetrafluoroethylene), which pose environmental hazards and yield heterogeneous surfaces with limited control over wetting behavior and poor durability. Here, we introduce a facile method that exploits the facile electroreduction of 4-nitrobenzodiazonium tetrafluoroborate to generate aryl radicals and initiate a radical chain reaction that enables the grafting of vinyl-terminated organosilicon compounds. This method proceeds under ambient conditions and at milder potentials than traditional vinylic electrografting. We investigate three organosilicon derivativesallyltriisopropylsilane, acryloxymethyl-trimethylsilane, and monomethacryl-oxypropyl-terminated polydimethylsiloxaneas electrografted coatings to tailor surface wettability. We perform microscopic, spectroscopic, and contact angle measurements and electrochemical characterization to correlate chemical moieties with the resulting wettability and electrochemical performance in fuel cells. We find that the electrografted coatings form a covalently bonded, thin layer that significantly reduces the solid surface energy of the carbonaceous substrate, reaching values close to those of polytetrafluoroethylene. Additionally, we find a correlation between surface energy and fuel cell performance, where the less hydrophobic coatings show cell flooding under more humid conditions. Polydimethylsiloxane-based coating outperforms the commercial baseline (polytetrafluoroethylene) in operando fuel cells, which paves a promising pathway for this class of materials. Importantly, this study highlights the potential of fluorine-free alternatives to traditional fluorinated hydrophobic treatments, offering a more sustainable and environmentally friendly approach without compromising performance.