Why Do Weak-Binding M-N-C Single-Atom Catalysts Possess Anomalously High Oxygen Reduction Activity?

Area of Science:

• Materials Science
• Electrochemistry
• Catalysis

Background:

• Single-atom catalysts (SACs) with metal-nitrogen-carbon (M-N-C) structures are promising for oxygen reduction reactions (ORR).
• Classical understanding favors moderate metal-support binding (e.g., Fe/Co-N-C) based on the Sabatier principle.
• Weakly binding M-N-C catalysts (e.g., Ni/Cu-N-C) exhibit high ORR activity, necessitating new mechanistic insights.

Purpose of the Study:

• To investigate the underlying mechanism of high ORR activity in weakly binding M-N-C single-atom catalysts.
• To challenge and expand the classical understanding of catalyst-intermediate binding strength in ORR.
• To propose and verify a novel reaction pathway for weak-binding SACs.

Main Methods:

• Integrated a pH-field coupled microkinetic model with experimental electron state analyses.
• Utilized synchrotron spectra analysis to probe catalyst electronic structure and bonding.
• Performed detailed theoretical calculations to analyze adsorption scaling relations, electric field effects, and solvation.

Main Results:

• Identified oxygen adsorption at the metal-nitrogen bridge site as a key step in the ORR pathway for weak-binding SACs.
• Demonstrated that this novel step alters adsorption scaling relations, electric field responses, and solvation effects.
• Observed increased electron density on N antibonding π orbitals and confirmed N-O bond formation in weak-binding M-N-C catalysts via synchrotron spectra.

Conclusions:

• Redefined the understanding of catalytic mechanisms for weak-binding M-N-C single-atom catalysts.
• The novel metal-nitrogen bridge site adsorption significantly impacts the kinetic barrier and pH-dependent performance.
• Opened new avenues for designing and optimizing SACs for clean energy applications, particularly in ORR.