Role of pz band in hybrid carbon-bimetallic subnanocluster PtM (M = 3d, 4d, 5d block metals) catalysts to boost electrochemical oxygen reduction reaction

The utilization of carbon-encapsulated Pt or Pt-alloy subnanocluster catalysts for proton exchange membrane fuel cells is a promising strategy to further reduce Pt loadings, enhancing catalytic activity and stability. However, such subnanocluster catalysts with carbon encapsulation remain prospective nanomaterials since they have been rarely explored to date. Here, using spin-polarized density functional theory (DFT) calculation, the carbon-encapsulated Pt and Pt-alloy catalysts (Ptn@C and Pt3M3@C) featuring subnanoclusters are developed. Unlike the dissociative oxygen reduction occurring on a variety of metals, The Pt6@C offered facile four-electron oxygen reduction reaction (ORR) pathway via H2O2 decomposition with low kinetic barrier (0.11 eV) at unique active site (carbon surface), and exhibited improved ORR activity with higher onset potential of 0.60 V over against Pt(111) catalyst (0.52 V). To reduce Pt loading and tune catalytic activity of Pt6@C, the binary Pt3M3 alloy subnanoclusters (M = 3d, 4d and 5d block metals) were introduced. Using activity descriptor (*OOH adsorption energy), the screening of Pt3M3@C candidates was conducted. It suggested new Pt3Co3 (0.62 V), Pt3Rh3 (0.60 V), Pt3Ta3 (0.65 V), Pt3Re3 (0.61 V) alloy subnanoclusters possessing even or better ORR activity relative to Pt6@C. The achievement of high ORR performance was also unveiled through an effective charge transfer from the metal subananocluster to the carbon shell. This leads to the down-shift of pz band center of the carbon sites and in turn the formation of bonding orbital between *OOH and carbon at deeper energy level, which consequently strengthens *OOH adsorption and decreases the overpotential. Our study can provide valuable insight into developing the hybrid metal-carbon catalysts with highly reduced Pt loadings for the efficient ORR as well as other electrocatalysis applications.