Aili, David1; Savinell, Robert F4; Jensen, Jens Oluf1; Cleemann, Lars Nilausen1; Bjerrum, Niels J.1; Li, Qingfeng1
1 Department of Energy Conversion and Storage, Technical University of Denmark2 Proton conductors, Department of Energy Conversion and Storage, Technical University of Denmark3 Case Western Reserve University4 Case Western Reserve University
Highly conductive phosphoric-acid-doped poly(perfluorosulfonic acid) membranes have long been known to malfunction in fuel cells. This is investigated and found to be due to failure of the anode, in which a limiting current is observed in the very low current-density range. It is proposed that the strongly acidic sulfonic acid groups protonate the phosphoric acid under anhydrous conditions, forming excess proton defects that are involved in proton conduction by means of the vehicle mechanism. The slow back-diffusion of phosphoric acid molecules as proton carriers thus limits the long-range conductivity of the membranes during fuel cell operation. The hypothesis is experimentally verified using a specially designed halfcell test.