Managing the Pore Morphologies of Microporous Layers for Polymer Electrolyte Fuel Cells with a Solvent-Free Coating Technique

Abstract

We report a solvent-free coating process for microporous layers (MPLs) in polymer electrolyte fuel cells to eliminate solvents, dispersing agents, and thickening agents of a conventional wet-coating process. In this environmentally friendly process, the MPLs were fabricated by depositing composite powder consisting of carbon and poly­(tetrafluoroethylene) directly onto a fibrous carbon substrate using an electrostatic screen printer. This enables the fabrication of sufficiently sized MPLs for the cells of first-generation Toyota MIRAI fuel-cell vehicles. Microscopic observation of the surface and the cross sections of dry-coated MPLs revealed smooth defect-free surfaces. Static water contact angle measurements revealed the hydrophobicity of dry-coated MPLs to be equivalent to that of wet-coated MPLs. Moreover, the pore morphologies of dry-coated MPLs could be managed by controlling the particle arrangement by electrostatic screen printing. The particle and pore morphologies of dry-coated MPLs were characterized by mercury intrusion porosimetry and synchrotron X-ray computed nanotomography. From electrochemical characterizations for dry-coated MPLs, a porosity-graded MPL exhibits excellent flooding tolerance under operating conditions below the dew point, corresponding to 120% relative humidity. A multilayer obtained with the dry-coating process requires a single annealing process and has the necessary gradient features, thus potentially reducing the environmental impact of manufacturing next-generation fuel-cell vehicles.

Effects of the Surface Structure on the Water Transport Behavior in PEMFC Carbon Fiber Papers

“…Due to the presence of PTFE, the pore size of PTFE-CFP is slightly smaller but the pore-size distribution still conforms to the normal distribution. The morphology of the MPL is shown in Figure e, and the CFP surface is covered by powdery layers, resulting in tiny pore structures . The micropores are situated among the carbon black aggregates and agglomerations, having a size between 0.09 and 0.35 μm.…”

“…PTFE was introduced into the CFP by dipping, so it could ensure that the pores inside PTFE-CFP and MPL-CFP were also hydrophobic. It is well known that both surface energy and surface morphology play important roles in contact angle. , Both PTFE-CFP and MPL have been subjected to hydrophobic treatment, so their surface energy is low. In addition, the MPL is composed of carbon particles and has a small pore size (inset in Figure e); this microstructure limits the water infiltration, so the MPL is more hydrophobic …”

“…The morphology of the MPL is shown in Figure 1e, and the CFP surface is covered by powdery layers, resulting in tiny pore structures. 19 The micropores are situated among the carbon black aggregates and agglomerations, 20 having a size between 0.09 and 0.35 μm. The pore-size distribution of the MPL is relatively uniform, so as to better contact with the CL.…”

Observation of water droplets in microporous layers for polymer electrolyte fuel cells by X-ray computed nano-tomography

“…X-ray nano-CT measurements for visualization of liquid water in the MPL pores were then performed. The MPL specimen was a simulated MPL fabricated in a glass capillary (Yoshimune et al, 2021). Nanosize carbon black particles (Vulcan XC-72R, Cabot Corp.) and PTFE particles (KTL-500F, Kitamura, Japan) in powder form were used.…”

Determination of the Porosity and its Heterogeneity of Fuel Cell Microporous Layers by X-Ray Tomographic Microscopy

“…A delicate balance between maintaining an ionconductive polymer electrolyte and preventing water flooding in the porous structures is necessary for operating PEFCs durably and efficiently. To enhance PEFC performance, the application of microporous layers (MPLs) on the gas diffusion layers (GDLs) and its advanced structural engineering are a common practice [2,3].…”