Abstract
Proper understanding of the major limitations of current catalysts for oxygen reduction reaction (ORR) is essential for further advancement. Herein by studying representative Pt and non-Pt ORR catalysts with a wide range of redox potential (E-redox) via combined electrochemical, theoretical, and in situ spectroscopic methods, we demonstrate that the role of the site-blocking effect in limiting the ORR. varies drastically depending on the E-redox of active sites; and the intrinsic activity of active sites with low E-redox have been markedly underestimated owing to the overlook of this effect. Accordingly, we establish a general asymmetric volcano trend in the ORR activity: the ORR. of the catalysts on the overly high E-redox side of the volcano is limited by the intrinsic activity; whereas the ORR of the catalysts on the low E-redox side is limited by either the site-blocking effect and/or intrinsic activity depending on the E-redox.