Acidic and Alkaline pH Controlled Oxygen Reduction Reaction Pathway over Co-N₄C Catalyst

Area of Science:

• Electrochemistry
• Materials Science
• Catalysis

Background:

• Oxygen reduction reaction (ORR) is vital for fuel cells and metal-air batteries.
• Single-atom catalysts (SACs) with M-N4/C structures show promise for ORR.
• Understanding pH-dependent ORR mechanisms in SACs is crucial for device optimization.

Purpose of the Study:

• To synthesize and characterize a porous single-atom catalyst (pCo-N4C) for ORR.
• To investigate the mechanistic differences in ORR activity of pCo-N4C in acidic and alkaline electrolytes.
• To elucidate the role of pH on catalytic performance using computational methods.

Main Methods:

• Synthesis of a porous single-atom catalyst (pCo-N4C) with cobalt coordinated to nitrogen in a graphene framework.
• Electrochemical evaluation of pCo-N4C for ORR in 0.1 m HClO4 and 0.1 m KOH.
• Ab-initio molecular dynamics (AIMD) simulations based on density functional theory (DFT) calculations.

Main Results:

• The pCo-N4C catalyst facilitates a direct 4e- ORR process with kinetics rivaling Pt/C.
• Higher activity in acidic electrolyte attributed to porosity-induced hydrophobicity.
• Observed difference in ORR onset potentials (0.82 V in acid, 0.91 V in alkali) linked to nitrogen protonation and restricted proton-coupled electron transfer (PCET).

Conclusions:

• The synthesized pCo-N4C is an effective ORR catalyst with tunable activity based on pH.
• Protonation of coordinated nitrogen and restricted PCET significantly influence ORR performance in different electrolytes.
• The study provides insights into pH-dependent mechanisms for SACs, guiding future catalyst design for energy applications.