The catalyst poisoning property of a novel ionomer, NBC4, which has two sulfonimide acid groups in its side chain ended with a perfluorobutane, instead of sulfonic acid groups as Nafion does, was theoretically and experimentally compared with that of Nafion. A density functional theory combined with a continuum electrolyte theory showed that the adsorption is weaker for sulfonimide anion than for sulfonate anion on Pt (111) surface. Electrochemical measurements using Pt (111) surfaced single crystal electrodes covered with the ionomer films showed the weaker anion adsorption and higher ORR activity by ca. 50% at 0.82 V vs the reversible hydrogen electrode for NBC4 than for Nafion. (C) 2014 The Electrochemical Society. All rights reserved.