Engineering TM-N₂@C₁₅N₅S₃H₅-Based Covalent-Organic Frameworks for Enhanced Water-Splitting and Oxygen Reduction...

Area of Science:

• Materials Science
• Catalysis
• Computational Chemistry

Background:

• Single-atom catalysts (SACs) offer superior atom utilization and activity for energy conversion and storage.
• Bifunctional SACs, capable of catalyzing multiple reactions like hydrogen evolution (HER) and oxygen evolution/reduction (OER/ORR), present enhanced efficiency and economic value over monofunctional counterparts.

Purpose of the Study:

• To systematically evaluate the catalytic potential of transition metal (TM)-N₂@C₁₅N₅S₃H₅ monolayers for HER, OER, and ORR.
• To identify stable TM-N₂ configurations and assess their bifunctional catalytic activity and performance under varying conditions.

Main Methods:

• Density functional theory (DFT) calculations were employed to investigate the structural stability and electronic properties of TM-N₂@C₁₅N₅S₃H₅ monolayers.
• Machine learning (ML) techniques were utilized to explore structure-activity relationships and predict catalytic performance.
• Calculations included detailed analysis of reaction mechanisms, adsorption energies, and the influence of pH and applied potential.

Main Results:

• Twenty-seven transition metal atoms were found to be stable within the N₂@C₁₅N₅S₃H₅ framework, forming TM-N₂ coordination sites.
• Rh-N₂@C₁₅N₅S₃H₅ demonstrated superior HER activity compared to platinum and also catalyzed OER, acting as a bifunctional catalyst.
• Ni-N₂@C₁₅N₅S₃H₅ exhibited excellent OER and ORR activity, functioning as another bifunctional catalyst. Its performance was attributed to favorable Ni-d orbital interactions with O-p orbitals, enhancing OH* adsorption.
• Both Ni-N₂@C₁₅N₅S₃H₅ and Rh-N₂@C₁₅N₅S₃H₅ showed enhanced OER in acidic media. Ni-N₂@C₁₅N₅S₃H₅ excelled in alkaline ORR, while Rh-N₂@C₁₅N₅S₃H₅ was more effective for ORR in acidic conditions.

Conclusions:

• The TM-N₂@C₁₅N₅S₃H₅ monolayer system is a promising platform for developing highly efficient bifunctional single-atom catalysts for energy applications.
• Rh-N₂@C₁₅N₅S₃H₅ and Ni-N₂@C₁₅N₅S₃H₅ represent advanced bifunctional catalysts with tunable performance based on reaction conditions and pH.
• The integrated DFT and ML approach effectively correlates structural/atomic properties with catalytic activity, paving the way for rational catalyst design.