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

Abstract

The electrochemical nitrate reduction reaction (NO3RR) offers a sustainable route to convert waste NO3− into valuable ammonia (NH3) using renewable electricity. However, the multielectron nature of the NO3RR necessitates the use of efficient, earth-abundant catalysts. While noble metals show high activity, their scarcity motivates the exploration of carbon-based alternatives. Here, we synthesized a triamine derivative based on the triazatrinaphthylene unit (TNP-NH2), a nitrogen-doped planar carbon structure, for the synthesis of six highly porous, crystalline covalent organic frameworks (COFs) as metal-free NO3RR catalysts. All the COFs exhibited good to moderate ammonia yields, achieving the highest yield rate of 2282 µg h−1 mgcat−1 and the highest faradaic efficiency of 82% at −1.0 V vs. RHE. Long-term chronoamperometry confirmed the structural durability of the COF during catalysis. DFT calculations further identified the specific active sites and reaction energetics on the COF backbone responsible for stabilizing key intermediates, consistent with the observed catalytic performance.