(540f) Effect of Salt-Shielding and Additional Polycation On the Conductive and Hydration Properties of Layer-by-Layer Assembled Proton Exchange Membranes

To have an economical hydrogen fuel cell car with minimal additional parts, the development of a thin proton exchange membrane with sustained performance up to 120⁰ C and down to 50% RH is necessary.  Nafion, the industrial standard for proton exchange membranes, begins to deform at this temperature and all acid based membranes lose conductivity as a function of water loss.  It is therefore necessary to find an alternative proton exchange membrane that is stable at higher temperatures and to better control the water and conductive properties of the membrane. 

LbL assembly allows for the controlled deposition of alternating polyelectrolytes at the nanometer scale.  By changing the salt concentration during the deposition phase, both the composition and conductivity of the resulting film may be changed.  Adding and increasing the salt concentration in the anion solution creates a loopier polymer structure with more free acid groups and thus increased conductivity.  When performing the LbL assembly on Nafion, in the absence of salt, the LbL film forms on the surface of Nafion with the properties of the composite membrane a summation of the two parts.  In the presence of salt the polycation is able to penetrate into the pores and complex with the sulfonic acid groups inside the pores, reducing both the conductivity, fuel permeability, and presumably the water transport properties of Nafion.  The change in properties occurs with the first bilayer and the degree of reduction is minimally dependent on the number of bilayers but highly dependent on the concentration of salt used during the LbL process.  When a second polyanion is incorporated via the tetralayer scheme (alternating cation/anion1/cation/anion2), the resulting film exhibits a different proton conductivity behavior than the bilayer films of either polyanion.