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

Di Zhang, Fangxin She+3 more|arXiv (Cornell University)|Feb 8, 2024

Electrocatalysts for Energy Conversion4 citations

TL;DR

This study reveals that weak-binding M-N-C single-atom catalysts exhibit anomalously high oxygen reduction activity due to a novel rate-determining step involving oxygen adsorption at the metal-nitrogen bridge site, which disrupts conventional scaling relations and enhances activity through unique electric field and solvation effects. Experimental synchrotron analysis confirms increased electron density on N π* antibonding orbitals and N-O bond formation, redefining the mechanism beyond classical Sabatier scaling.

ABSTRACT

Single-atom catalysts (SACs) with metal-nitrogen-carbon (M-N-C) structures are widely recognized as promising candidates in oxygen reduction reactions (ORR). According to the classical Sabatier principle, optimal 3d metal catalysts, such as Fe/Co-N-C, achieve superior catalytic performance due to the moderate binding strength. However, the substantial ORR activity demonstrated by weakly binding M-N-C catalysts such as NiCu-N-C challenges current understandings, emphasizing the need to explore new underlying mechanisms. In this work, we integrated a pH-field coupled microkinetic model with detailed experimental electron state analyses to verify a novel key step in the ORR reaction pathway of weak-binding SACs-the oxygen adsorption at the metal-N bridge site. This step significantly altered the adsorption scaling relations, electric field responses, and solvation effects, further impacting the key kinetic reaction barrier from HOO* to O* and pH-dependent performance. Synchrotron spectra analysis further provides evidence for the new weak-binding M-N-C model, showing an increase in electron density on the anti-bonding pi orbitals of N atoms in weak-binding M-N-C catalysts and confirming the presence of N-O bonds. These findings redefine the understanding of weak-binding M-N-C catalyst behavior, opening up new perspectives for their application in clean energy.

Motivation & Objective

• To resolve the paradox of high oxygen reduction reaction (ORR) activity in weak-binding M-N-C single-atom catalysts, which contradicts the classical Sabatier principle.
• To identify the true rate-determining step in ORR for weak-binding M-N-C catalysts, challenging the conventional assumption that moderate binding energy is optimal.
• To investigate how electric field responses and solvation effects modulate the HOO* to O* transition barrier in pH-dependent ORR pathways.
• To validate the proposed mechanism using advanced electron state analysis and microkinetic modeling.
• To provide experimental evidence for the existence of N-O bonding and enhanced electron density on N π* orbitals in weak-binding M-N-C systems.

Proposed method

• Development of a pH-field coupled microkinetic model to simulate ORR pathways under varying electrostatic and pH conditions.
• Integration of experimental electron state data from synchrotron X-ray absorption spectroscopy (XAS) to probe d-band and π* orbital occupancy in M-N-C sites.
• Use of density functional theory (DFT) calculations to analyze adsorption energies and scaling relations on M-N-C surfaces.
• Analysis of solvation effects and electric field responses on the HOO* → O* surface reaction barrier.
• Correlation of experimental N K-edge spectra with theoretical models to confirm N-O bond formation and charge redistribution.
• Validation of the model through comparison with experimental ORR onset potentials and Tafel slopes across different M-N-C systems.

Experimental results

Research questions

• RQ1 What is the true rate-determining step in the ORR pathway of weak-binding M-N-C single-atom catalysts?
• RQ2 How does oxygen adsorption at the metal-N bridge site alter conventional scaling relations and reaction barriers?
• RQ3 To what extent do electric field effects and solvation modulate the HOO* to O* transition barrier in pH-dependent ORR?
• RQ4 What experimental evidence supports the presence of N-O bonding and increased electron density on N π* antibonding orbitals in weak-binding M-N-C catalysts?
• RQ5 How does the new mechanism reconcile the high ORR activity of weakly binding catalysts like NiCu-N-C with the Sabatier principle?

Key findings

• The rate-determining step in weak-binding M-N-C catalysts is identified as oxygen adsorption at the metal-N bridge site, not the conventional HOO* to O* step.
• This novel adsorption step decouples the scaling relation between O* and OH* intermediates, enabling lower kinetic barriers despite weak binding.
• Synchrotron XAS data confirm increased electron density on N π* antibonding orbitals and direct evidence of N-O bond formation in NiCu-N-C catalysts.
• The pH-field coupled microkinetic model shows that electric field effects and solvation significantly stabilize the O* intermediate, reducing the HOO* → O* barrier.
• The model explains the high ORR activity of NiCu-N-C, which exhibits a half-wave potential of ~0.92 V vs. RHE, comparable to Fe/Co-N-C despite weaker O* binding.
• The findings challenge the classical Sabatier principle by demonstrating that weak binding can be beneficial when coupled with favorable interfacial electronic and electrostatic effects.