Triazatrinaphthylene-based metal-free polycyclic aromatic frameworks for electrochemical nitrate reduction to ammonia

Area of Science:

• Electrochemistry
• Materials Science
• Sustainable Chemistry

Background:

• Electrochemical nitrate reduction (NO3RR) converts waste nitrate to ammonia, a valuable chemical.
• Efficient, earth-abundant catalysts are crucial for the multielectron NO3RR process.
• Noble metal catalysts are effective but scarce, driving research into alternatives.

Purpose of the Study:

• To synthesize and evaluate novel metal-free catalysts for the electrochemical nitrate reduction reaction.
• To explore the potential of porous, crystalline covalent organic frameworks (COFs) for NO3RR.
• To understand the catalytic mechanism and active sites using computational methods.

Main Methods:

• Synthesis of six covalent organic frameworks (COFs) using a triamine derivative (TNP-NH2) based on a nitrogen-doped planar carbon structure.
• Electrochemical characterization including ammonia yield rate and faradaic efficiency measurements.
• Long-term chronoamperometry for durability assessment.
• Density Functional Theory (DFT) calculations to identify active sites and reaction energetics.

Main Results:

• All synthesized COFs demonstrated good to moderate ammonia yields.
• Achieved a maximum ammonia yield rate of 2282 µg h⁻¹ mgcat⁻¹.
• Attained a highest faradaic efficiency of 82% at -1.0 V vs. RHE.
• DFT calculations identified specific active sites and stabilizing intermediates on the COF backbone.

Conclusions:

• The synthesized nitrogen-doped COFs are effective metal-free catalysts for the electrochemical nitrate reduction reaction.
• The catalysts exhibit structural durability and efficient conversion of nitrate to ammonia.
• Computational studies provide insights into the catalytic mechanism and active site identification.