Studies of heteropoly acid/polyvinylidenedifluoride-hexafluoroproylene composite membranes and implication for the use of heteropoly acids as the proton conducting component in a fuel cell membrane

Complete polarization curves for a number of heteropoly acids (HPAs), H3PW12O40, alpha-H3P2W18O62, H6P2W21O71, and H6As2W21O69 as the only proton conducting component are presented for the first time. Both thin pellets of HPA and composite membranes of 1:1 (w/w) of HPA and polyvinylidenedifluoride-hexafluoropropylene (PVDF-HFP) are investigated. Although the pellets are somewhat variable, the HPA phase changes can be observed by electrochemistry and these materials show promise for solid acid fuel cell performance at >200 degrees C. The high proton conductivities reported for HPAs at RT are demonstrated in fuel cells using HPA/PVDF-HFP composites with limiting current densities as high as 1.6 A cm(-2) using dry O-2 and H-2. Moderate fuel cell activity is demonstrated for alpha-H3PW18O62 at 120 degrees C and 25%RH. Unfortunately all of the materials studied were somewhat porous and the open circuit potentials observed were somewhat low. We were also able to show that an HPA fuel cell could be shorted by reduction of the HPA to a heteropoly blue under exceptional circumstances. (C) 2007 Elsevier B.V. All rights reserved.