Exploring Polydimethylsiloxane Coating Strategies to Enhance Liquid Repellency of Carbonaceous Porous Media

Carbonaceous porous diffusion media play a critical role in electrochemical technologies sustaining multiphase (gas and liquid) flows, including low-temperature fuel cells and CO₂ electrolyzers. To resist liquid intrusion and preserve gas pathways, these materials are typically hydrophobized with polytetrafluoroethylene dispersions. However, the application of dispersion-based coatings limits uniformity and performance, while the persistence and toxicity associated with per- and polyfluoroalkyl compounds have prompted regulatory scrutiny. Here, we investigate polydimethylsiloxane (PDMS), a fluorine-free, low-surface-energy, and environmentally benign polymer, as a viable alternative. We assess four PDMS coating application strategies: (1) dip-coating, vapor deposition in (2) oxygen and (3) nitrogen atmospheres, and (4) electrografting of an amine-functionalized derivative. We perform spectroscopic, microscopic, wetting, and electrochemical double-layer measurements to correlate the surface chemical composition and morphology with the resulting wettability. Vapor deposition in an oxygen environment produces a superhydrophobic rough microstructure; however, the weak adhesion of the coating and substrate oxidation result in severe flooding when the substrate is exposed to an alkaline liquid flow. Notably, electrografting and vapor deposition in nitrogen yield thin uniform coatings with an ethanol-potassium hydroxide solution repellency in flow comparable to the polytetrafluoroethylene baseline. On the contrary, dip-coated samples feature thick unevenly distributed layers exhibiting poor liquid infiltration resistance. Under prolonged flow in the presence of 10 wt % ethanol, the desorption of the physisorbed layer vapor deposited in nitrogen led to a continuous increase in wetting, resulting in complete electrode flooding within 65 h. In contrast, the electrografted substrates showed enhanced durability under liquid flows with various compositions, retaining repellency for the same time scale comparably with the PTFE baseline, highlighting the importance of covalently attached hydrophobic coatings for long-term operation. These findings demonstrate the potential of PDMS-based coatings as sustainable fluorine-free alternatives for hydrophobizing carbonaceous porous media and provide practical guidelines for engineering the wetting behavior and coating stability.