For the oxygen reduction reaction (ORR) in acidic media, proton is a key component in the hydrogenation of O2, O, and OH. Modeling a proton requires the explicit account of its solvation and dynamic nature in the interfacial solution region. We employed ab initio molecular dynamics method to study such reactions on Pt(111), a model problem in electro-catalysis. Our results show that the branching ratio for the two hydrogenation channels of O atoms adsorbed on Pt(111) shifts dramatically with the electrode potential. This kinetic factor underlies the electrochemical observations peculiar to ORR on Pt(111), and provides an explanation for the long standing puzzle of its high onset overpotential.