Precisely Engineering Asymmetric Atomic CoN₄ by Electron Donating and Extracting for Oxygen Reduction Reaction

Area of Science:

• Electrochemistry
• Materials Science
• Catalysis

Background:

• Understanding the relationship between catalyst structure and activity is crucial for developing efficient oxygen reduction reaction (ORR) catalysts.
• Precisely defined active sites in nonpyrolytic catalysts offer a pathway to investigate these structure-activity relationships.
• Cobalt-nitrogen-4 (CoN4) centers are promising active sites for ORR catalysis.

Purpose of the Study:

• To synthesize model electrocatalysts with well-defined CoN4 centers and controllable coordination environments.
• To investigate the impact of ligand substituent electronic properties on ORR catalytic activity.
• To establish a correlation between the electron density of CoN4 centers and their ORR performance.

Main Methods:

• Sequential chelation of cobalt salts and substituted 1,10-phenanthroline ligands onto covalent triazine frameworks (CTFs).
• Synthesis of a series of catalysts (BCTF-Co-phen(X), where X = OH, CH3, H, Br, Cl) to systematically vary electronic properties.
• Electrochemical evaluation of ORR activity, including half-wave potential and turnover frequency (TOF) measurements in an alkaline environment.

Main Results:

• Catalyst activity was directly correlated with the electron-donating ability of the substituent groups on the phenanthroline ligands.
• The BCTF-Co-phen(OH) catalyst, featuring an electron-donating hydroxyl group, exhibited the highest ORR catalytic activity.
• This optimized catalyst achieved a half-wave potential of 0.80 V vs. RHE and a TOF of 47.4 × 10^-3 Hz at 0.80 V vs. RHE.

Conclusions:

• Modulating the electronic properties of ligands provides an effective strategy to tune the activity of CoN4-based ORR catalysts.
• The electron-donating hydroxyl group significantly enhances the ORR performance of the CoN4 active site.
• These findings offer valuable insights for the rational design of advanced electrocatalysts for energy conversion applications.