Asymmetric Volcano Trend in Oxygen Reduction Activity of Pt and Non-Pt Catalysts: <i>In Situ</i> Identification of the Site-Blocking Effect

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 (<i>E</i>_redox) via combined electrochemical, theoretical, and <i>in situ</i> spectroscopic methods, we demonstrate that the role of the site-blocking effect in limiting the ORR varies drastically depending on the <i>E</i>_redox of active sites; and the intrinsic activity of active sites with low <i>E</i>_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 <i>E</i>_redox side of the volcano is limited by the intrinsic activity; whereas the ORR of the catalysts on the low <i>E</i>_redox side is limited by either the site-blocking effect and/or intrinsic activity depending on the <i>E</i>_redox