Hydrophobic Gas-Diffusion Media for Polymer-Electrolyte Fuel Cells by Direct Fluorination
A polymer-electrolyte fuel cell depends on proper water management to obtain high performance. During operation, liquid water is generated in the cell. When it is not properly and adequately removed, accumulation leads to poor fuel-cell performance by reducing and blocking the gas pores in the catalyst and gas-diffusion media. To address this problem, gas-diffusion media are often coated with a wet-proofing agent. This approach results in reduced pore size and volume resulting in lower transport properties, as well as inducing durability and performance issues due to the inherent non-uniformity. To overcome these issues, an alternative wet-proofing process called direct fluorination was developed. In this approach, fluorine gas reacts with carbon to create a more uniform, durable, and consistent wet-proof surface without affecting the morphology of the media. The fluorinated media showed capillary pressure properties that are more suitable for fuel-cell application. Fuel cells with fluorinated materials in the cathodes showed better performance, lower ohmic resistance, and lower liquid water amount in the cathode. These advantages are attributed to having a better wet-proofed fluorinated media at the cathode that forces water back to the anode, thereby keeping the membrane more hydrated and reducing the amount of water in and transported out of the cathode.