Diffusion layer hydrophobic treatment

Ji and Kumar [107] presented an invention in which, after treatment with PTFE, the cathode diffusion layers are coated with a silicon solution in order to enhance the hydrophobic properties of the DFs. This silicon does not have to be coated over the whole surface of the DL, but could be coated in just certain areas, depending on the design of the cell, the location of fhe cell wifhin a stack, and the desired hydrophobic properties. It was demonstrated how a DL with the silicon coating improved the performance of a single fuel cell when operating at high relative humilities [107]. 

H. Nakajima, T. Konomi, and T. Kitahara. Direct water balance analysis on a polymer electrolyte fuel cell (PEFC) Effects of hydrophobic treatment and microporous layer addition to the gas diffusion layer of a PEFC on its performance during a simulated start-up operation. Journal of Power Sources 171 (2007) 457-463. 

The gas diffusion layers, one next to the anode and the other next to the cathode, are usually made of a porous carbon paper or carbon cloth, typically 100 pm to 300 pm thick. Fig. 14 shows a porous GDL made of carbon paper, which is partially covered by catalyst layer. The porous nature of the backing layer ensures effective diffusion of feed and product components to and from the electrode on the MEA. The correct balance of hydrophobicity in the backing material, obtained by PTFE treatment, allows the appropriate amount of water vapor to reach the MEA, keeping the membrane humidified while allowing the liquid water produced at the cathode to leave the cell. The permeability of oxygen in the GDL affects the limiting current density of ORR, and thus the performance of PEMFC.[ l... 

Hydrophobic treatment (HT) and micro-porous layer (MPL) in addition to gas diffusion layer (GDL) effect on water balance... 

The XPS results indicated that there were about 3-5 at. % sulfur and 27-47 at. % oxygen incorporated onto the sulfur dioxide plasma treated LDPE substrate surfaces (Table 1). The sulfur atomic concentration reached a maximum at about 50 A from the sample surface (0 = 30°) right after the plasma treatment (Figure 3). (The uncertainty of the XPS multiplex scan for atomic concentration analysis is believed to be 0.5-1.0 at. %.) The sulfur-containing species diffused into the bulk of the polymer (> 100 A) as shown from the XPS data collected eleven days after the plasma reaction. This phenomenon is due to the mobility of the polymer surface. s After the sulfur dioxide plasma modification, the hydrophilic sulfonyl groups on the LDPE backbone diffuse away from the polymer surface toward the bulk of the material so that a lower surface energy can be attained. Because the air/LDPE interface has a low surface tension, thermodynamic equilibrium favors a hydrophobic surface. As a result, the sulfur atomic concentrations in the top 100 A of the substrates decreased with time as the sulfonyl groups diffused away from this surface layer. 

The cost of Gas Diffusion Media (MPL + GDL) [13] is quoted to be aroimd 10- 15 per m, and is likely to be primarily determined by processing costs. Both carbon/graphite paper as woven stmctures are being used as substrate, which is made hydrophobic by, for example, a PTFE coating. The microporous layer mostly consists of carbon powder mixed with a PTFE emulsion, which is cured by a heat treatment. Clear directions for cost reduction have not been found. As the gas diffusion media play a critical role in the performance of the PEMFC, especially determining the maximum power output makes it worthwhile to focus on the GDM performance, rather than on the cost per m. ... 

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