Fuel-Cell Hydrophobic Materials

In the field of renewable energy, Proton Exchange Membrane Fuel Cells (PEMFC) are the devices which mostly have benefited of the unique characteristics of fluorinated materials. Many fluorinated sulphonic membranes have been developed for PEMFC and, in addition, fluorinated polymers are present in many other parts of the cell, such as the catalytic layer (CL) and the gas diffusion layer (GDL). The main function of the GDL is to promote the distribution of the gaseous reagents from the bipolar plates to the catalyst layer. However, GDL has other important functions: it allows the electric contact for conduction of electrons, it works as mechanical support for the catalytic layer and the membrane, it helps to remove the combustion heat, and it plays a key role in the water management. In fact, the overall fuel cell performances can be improved by a correct water management; on the contrary performances can be dramatically limited by the accumulation of liquid water inside the cell. Therefore, a correct water management is needed to reach power and current densities required for commercial applications. Thus, the conductive carbonaceous materials composing the GDL are usually coupled with a hydrophobic fluorinated material, usually polytetrafluoroethylene (PTFE), in order to obtain a performing water management. Fluorinated materials are suitable for this purpose because one of their core characteristics is the very low surface tension that means low water polymeric surface interaction and therefore a very high hydrophobicity. Actually, carbon-based materials are the reference class of materials used for GDL, because they have high gas permeability, good stability in acid environment and good electric conductivity.

In our laboratoty, we developed an innovative methodology to confer a stable hydrophobicity to carbonaceous materials is the by the chemical linkage of perfluoropolyether (PFPE) chains through the chemical treatment with PFPE peroxides. PFPE peroxides are industrially available perfluorinated polymers whose structures are characterized by the alternation of fluorinated carbon-based units and oxygen units. A thermal treatment at 150-200C can link PFPE chains to carbonaceous materials by a radical pathway . PFPE chains have the typical properties of perfluorinated polymers, like chemical stability, thermal stability and high hydrophobicity, and also some peculiar characteristic such as liquid physical form and high gas permeability. These modified carbonaceous materials are super hydrophobic and show surprisingly high conductivity compare to carbons fluorinated with F2. Carbon based materials commonly used in PEMFC are carbon cloth (CC) and carbon black (CB). The functionalization of such as substrates by means of PFPE peroxide lead to high conductive and superhydrophobic composite materials. Test on introduction of PFPE functionalized materials in PEMFCs resulted to be a great improvement in water management and consequently in feeding gases diffusion compared to commonly used materials. Indeed PFPE-functionalized GDLs allow to reach higher currents as well as power density. The introduction of PFPE-functionalized carbonaceous materials improved the overall PEMFC performances by focusing on the requirements for their upcoming widespread commercial application.

Polarization and power density curves of standard GDLs (functionalized by PTFE) and PFPE functionalized GDL