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In this paper, to study the effect of sulfonic groups on the ORR activity of Pt and exclude the effect of polytetrafluoroethylene (PTFE) backbone of Nafion, trifluoromethanesulfonic acid (CF3SO3H) was used to simulate the SO3H-terminated side chains of Nafion. By using the CO displacement technique, it was predetermined that much more sulfonic groups are absorbed on the Pt surface in 50 mM CF3SO3H solution compared with that of 0 mM and 1 mM. The influence of sulfonic groups on the oxygen reduction reaction (ORR) activity of Pt is analyzed by using rotating Pt disk electrode (RDE) as work electrode in 0.1 M HClO4 with various CF3SO3H contents. It is found that when the content of CF3SO3H is over 5 mM, the electrochemical active area of Pt decreases with increasing the content of CF3SO3H in solution, and the oxidation peak of Pt shifts to a higher potential. Meanwhile, the kinetic current loss of Pt increases apparently along with the increase of CF3SO3H content which is 40% for 10 mM and 71.3% for 50 mM at 0.9 V, respectively. The result shows that excess sulfonic groups of Nafion adsorbed on the surface of Pt will greatly lower the ORR activity of Pt.

Effect of trifluoromethanesulfonic acid on the ORR activity of Pt in acid medium