Rate-determining step backshift effectively boosts ORR performance by excess electron transfer to the O–O antibonding orbital

Abstract

A Pt₂₃Pd₇₇ nanosheet catalyst (Pt₂₃Pd₇₇ NSs/C) was successfully synthesized with a thickness of 1.33 nm and a mass activity (MA) of 6.78 A mg_PGM⁻¹ at 0.9 V, achieving an oxygen performance 45.2 times higher than that of commercial Pt/C (0.15 A mg_Pt⁻¹). Furthermore, the Tafel slope of the Pt₂₃Pd₇₇ NSs/C catalyst is as low as 39.52 mV dec⁻¹ compared to that of Pt/C (70.55 mV dec⁻¹) indicating that the rate-determining step (RDS) shifts from *OOH cleavage to the second electron transfer. First-principles calculations show a decline in the barrier of *OOH → *O + *OH; thus, the second electron transfer succeeds the RDS. In the kinetic-controlled region, the apparent activation energy (E_a) of Pt/C does not change with the change of applied potential, and the reaction order of OH⁻ is close to 0, while the E_a of the PtPd NSs increases with the increase of applied potential, and the reaction order is negative. These results collectively demonstrate that the RDS shifts to a later stage in the mechanism. This paper provides another idea for improving the performance optimization strategy of oxygen reduction, that is, the kinetic RDS backshift.