Hydrophobic Gas-Diffusion Media for Polymer-Electrolyte Fuel Cells by Direct Fluorination

Abstract

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. A proton-exchange-membrane (PEM) fuel cell depends on proper water management to obtain high power density and energy efficiency. During operation, liquid water is generated in the fuel cell at the cathode due to oxygen reduction, and may also exist at the anode due to transport through the membrane or condensation. When it is not properly and adequately removed, this liquid-water accumulation leads to poor fuel-cell performance by blocking the gas pores in the catalyst and gas-diffusion media (GDM) and forming an additional transport barrier over the reactive area. To address this problem, GDM, such as non-woven carbon papers or woven carbon cloths, used in PEM fuel cell electrodes are treated with polytetrafluoroethylene (PTFE) or tetrafluoroethylene-hexafluoropropylene (FEP) to wet-proof the substrate surface. Wet-proofed surface in the GDM repels liquid water and provides pathways for gaseous reactant transport.1-4 Furthermore, when bilayer GDM that have a dense and hydrophobic microporous layer (MPL) supported by a macroporous layer are used in a PEM fuel cell, it is believed that lower liquid-water-saturation levels in the cathode are achieved because the dense and highly hydrophobic MPL 1) reduces the liquid-water saturation level at the catalyst layer/MPL interface thus enabling gas access from the GDM to the catalyst layer, 2) reduces the amount and locations of liquid water that can be transported from the catalyst layer into the macroporous support layer, and 3) forces a significant amount of liquid water in the cathode across the membrane back to the anode, thereby helping to make the anode side of the membrane more hydrated. [5][6] In addition to...

Understanding Impacts of Catalyst-Layer Thickness on Fuel-Cell Performance via Mathematical Modeling

“…GDL hydrophobization can possibly reduce the porosity and permeability of a GDL, as reported in the literature. [81][82][83][84] In addition, compression and landchannel effects can have similar effects on transport parameters. 37,[85][86][87][88][89][90][91][92] Locations of GDLs under land are more compressed, thus having lower porosity and hence lower permeability.…”

The effects of the composition of microporous layers on the permeability of gas diffusion layers used in polymer electrolyte fuel cells

“…To efficiently perform the above functions, the GDL is usually wet-proofed [3][4][5][6][7]. Further, it is normally coated with the so-called micro-porous layer (MPL) on the side facing the catalyst layer in order to enhance the electrical contact with the catalyst layer and properly handle the liquid water emerging from the cathode catalyst layer [1][2][9][10].…”

Multiwall Carbon Nanotube-Based Microporous Layers for Polymer Electrolyte Membrane Fuel Cells

“…GDLs were prepared with homogenous hydrophobicity distributions in the absence of pore structure modifications. Nguyen et al 10 introduced the direct fluorination of the GDL for providing homogenous hydrophobicity. Less liquid water content was observed in these novel GDLs compared to the conventional GDL (SGL 24 BC) when visualized with soft X-rays.…”