Fluorine-free gas diffusion layers via covalent hydrocarbon grafting

Covalent grafting of linear hydrocarbon chains onto carbon surfaces can provide a controllable route to hydrophobic interfaces, in which chain length governs surface energetics and liquid repellency while improving coating robustness relative to non-covalent hydrophobization approaches. This fluorine-free strategy is therefore hypothesized to provide a fundamentally relevant route for re-engineering porous transport materials, where fluoropolymer-based wetting control remains the prevailing paradigm in electrochemical energy systems.

A chemical grafting route using an 18‑carbon alkyl chain was evaluated, while electrochemical grafting was systematically applied to unbranched hydrocarbon chains containing 10, 14, 16, and 18 carbon atoms. The resulting interfaces were analyzed by microscopy and spectroscopy to verify coating formation, and by contact-angle and electrochemical measurements to relate molecular structure to wettability, surface energy, and transport-relevant behavior.

Within the electrografted series, the grafted layers establish a clear chain length-dependent evolution of interfacial properties. Increasing alkyl chain length progressively lowers the solid surface energy and enhances liquid repellency, demonstrating that hydrophobicity can be tuned through molecular design of the grafted interface. The electrografted C18 surface reaches a surface energy of 22.3 mJ m^-2, close to that of polytetrafluoroethylene (20.4 mJ m^-2), despite being fluorine-free. These results show that covalent hydrocarbon grafting offers a robust platform for engineering low-energy carbon interfaces and establish molecular chain length as a key parameter governing the macroscopic wetting behavior of porous carbon materials.