A Combined Theoretical and Experimental Investigation of the Transport Properties of Water in a Perfluorosulfonic Acid Proton Exchange Membrane Doped with the Heteropoly Acids, H3PW12O40or H4SiW12O40

The 3M 825EW perfluorosulfonic acid (PFSA) ionomer was doped with the heteropoly acids (HPAs), H3PW12O40(HPW) and H4SiW12O40(HSiW). Dynamic vapor sorption measurements at 95% RH showed a decrease in water content as a result of HPA doping from λ = 8.05 for the undoped 825EW 3M ionmer to λ = 4.40 for a 5% HSiW doped film. FTIR measurement revealed strong interactions between the HPAs, ionomer, and H2O. Irrespective of hydration level, it was found that the PFSA films showed tortuous proton diffusion behavior. At maximum hydration and 25 °C, the self-diffusion coefficient of water was found to decrease upon addition of HPA from 4.97 to 2.19 (×10–6cm2/s) for the undoped 825EW 3M ionomer and 1% HPW, respectively, in excellent agreement with computation. The model at low HPA loadings revealed the decreased diffusion coefficient was due to the water preferentially residing near the HPA as opposed to the SO3–groups. The addition of HPA generally improved overall conductivity due to additional formation of hydrogen-bonding networks between the HPA particles. At high RH, it was observed that the proton conductivity increased while the water diffusion coefficient decreased as HPA was incorporated. In addition, a mismatch between the conductivity values and those calculated from the Nernst–Einstein equation using the self-diffusion coefficient of water in the system indicated an enhancement in Grotthuss hopping mechanism upon addition of HPA.