Investigation on Effect of PTFE Treatment on GDL Micro-structure by High-resolution X-ray CT

Abstract

To remove liquid water from a gas diffusion layer (GDL) effectively and minimize an increase of transport resistance, hydrophobic treatment with a polytetrafluoroethylene (PTFE) is widely used. Because the GDL micro-structure has strong impact on transport properties, detailed understanding of fiber and porous network and PTFE distribution is strongly requested. To investigate correlation between the GDL micro-structure, PTFE treatment, and transport properties, micro-structure of the GDL was visualized by high-resolution X-ray computed tomography (X-ray CT). Newly designed sample holder allowed to regulate compression pressure. Visualization results with spatial resolution of 1.0 um made it possible to distinguish carbon fibers, binder material, PTFE, and voids (air), and non-uniform distribution of PTFE was observed. Because non-uniform porosity and contact angle distribution in the direction of thickness change the liquid water morphology within the GDL, understanding and controlling the GDL micro-structure is important and manufacturing process of GDL should be scientifically developed.

X-ray Tomographic Analysis of Porosity Distributions in Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells

“…As it can be seen, the area percentage porosity in both IP directions fluctuates marginally and randomly, and matches well with the bulk porosity value, whereas there is great variation in the TP direction porosity. These results follow the trends that have been reported in other studies using mCT [29,30], and may be in part from the preferred gathering of PTFE in the lower porosity regions of the untreated GDL, and/or in part from the drying process of the PTFE in the GDL [7,21,29,30].…”

“…Using mCT, Markotter et al [19,20] have shown vivid visualizations of the water transport paths in the GDL with an in-situ PEMFC setup after ceasing the reactant flow. mCT has also been used to show the effects of the PTFE content in the GDL [21], to study the effect of compression on the GDL morphology [22,23], to identify and segment the MPL within the GDL [24], and to visualize the entire membrane electrode assembly (MEA) [25]. Kim et al [26] have used mCT to study the porosity variation of paper-based and felt-based GDLs under freeze-thaw cycles and have also provided an extensive review on the evaluation of water management using mCT [27].…”

Pore-Scale Investigation of the Effect of Micro-Porous Layer on Water Transport in Proton Exchange Membrane Fuel Cell

“…iv. The PTFE is added into the carbon structure and preferentially generated on the fiber connection part 6 until the PTFE content is reached.…”

“…For this purpose, the state-of-the-art PTL is usually manufactured as a bi-layered structure with both gas diffusion layer (GDL) and micro-porous layer (MPL). 5 The GDL is typically made of micrometer-scale carbon paper with some hydrophobic substance, such as polytetrafluoroethylene (PTFE), 6 and the MPL is constructed at the nanometer-scale with carbon powder glued with hydrophobic substance 7 and added to enhance the electrical conductivity, support the catalyst layer (CL) and balance the water content in the membrane. Many experiments have proved that the MPL can improve the fuel cell performance at high current density, which is attributed to better water management.…”

A method for measuring relative in-plane diffusivity of thin and partially saturated porous media: An application to fuel cell gas diffusion layers

“…These additives tend to form web-like structures spanning the IP direction as shown by SEM images [4], [42]. Also, depending on the drying method the distribution of PTFE can be concentrated at the surfaces leaving the bulk of the material free of PTFE [43], [44], [45]. Modelling of the single-phase transport properties of GDLs with binder and PTFE structures has been conducted by El Hannach et al [46].…”