Membrane/ionomer proton conductivity advantage

Kreuer et al. [25] investigated the membrane properties, including water sorption, transport (proton conductivity, electro-osmotic water drag and water diffusion), microstructure and viscoelasticity of the short-side-chain (SSC) perfluorosulfonic acid ionomers (PFSA, Dow 840 and Dow 1150) with different lEC-values. The data were compared to those for Nafion 117, and the implications for using such ionomers as separator materials in direct methanol and hydrogen fuel cells discussed. Tire major advantages of PFSA membranes were seen to be (i) a high proton conductivity. 

There are several advantages for the use of S-ZrOj as a catalyst support in PEMFC applications. Because of its hydrophilicity, it has been suggested that this type of fuel cell catalyst would be well suited for low-relative humidity conditions and possibly simplify fuel cell components to operate without the use of a humidifier. Due to the proton conductivity across the surface of the material, less Nafion iono-mer needs to be cast to form the TPBs. Platinum utilization increases as the S-ZrOj support acts as both the platinum and proton conductor and better gas diffusion to the catalyst site results from the decreased blockage of Nafion ionomer (Liu et al., 2006a,b). It is beheved that within porous carbon catalyst supports, platinum deposited within the pores may not have proton conductivity due to the perfluorosul-fonated ionomer unahle to penetrate into the pores. Thus, a TPB which is necessary for a catalyst active site will not be formed. Therefore, the S-ZrOj support has an additional benefit over porous carbon material supports in that by using the S-ZrOj as a support for platinum catalysts, the surface of the support can act as a proton conductor and platinum deposited anywhere on the surface of the support will provide immediate access to the electron and proton pathways thereby requiring less Nafion. Thus the use of S-ZrOj in fuel cell MEA components may potentially lower the cost of materials substantially, as the catalytic metals and membrane materials are among the most costly in a PEMFC. However, like most metallic oxides, the downside of their use stems from their relatively low electron conductivity and low surface areas that results in poor platinum dispersion. 

To benefit from these advantages, it is now becoming clear that pure ionomer membranes are not suitable, and new types of proton-conducting membranes that work at temperatures higher than 100 °C have to be developed. There are currently three principal polymer electrolyte types proposed for high temperature PEMFCs ... 

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