Double-trap kinetic equation for the oxygen reduction reaction on Pt(111) in acidic media.

We derived an intrinsic kinetic equation for the four-electron oxygen reduction reaction (ORR) in acidic media using free energies of activation and adsorption as the kinetic parameters. Our kinetic model consists of four essential elementary reactions: a dissociative adsorption (DA) and a reductive adsorption (RA), which yield two reaction intermediates, O and OH; a reductive transition (RT) from O to OH; and a reductive desorption (RD) of OH. Analytic expressions were found for the O and OH adsorption isotherms by solving the steady-state rate equations. For the ORR on Pt(111) in 0.1 M HClO4 solution, we analyzed the measured polarization curves, thereby deducing activation free energies that are consistent with the values from theoretical calculations. The reductive adsorption (DeltaG*0RA=0.46 eV) is not the rate-determining step (RDS) for the ORR on Pt because dissociative adsorption (DeltaG*0DA=0.26 eV) offers a more favorable pathway at high potentials. It, however, generates strongly adsorbed O. The high activation barriers for the O to OH transition (DeltaG*0RT=0.50 eV) and OH desorption (DeltaG*0RD=0.45 eV) cause a large potential loss for the desorption-limited ORR. As the OH coverage increases to a constant value with decreasing potential, the Tafel slope increases to the value determined by a symmetric electron-transfer coefficient. We discuss the role of adsorption isotherm in kinetic analysis and, via activity-and-barrier plots, illustrate why the RDS may vary with reaction conditions or may not exist. Recognizing such features of electrocatalytic reactions can facilitate reaching the long-standing goal of quantitative descriptions and predictions of electrocatalysts' activities.