Development of triazine-based covalent organic frameworks for enhanced electrochemical energy storage applications

Abstract

Herein, two distinct porous covalent organic frameworks (COFs) were developed by polycondensation of a heteroatom-rich rigid triazine-based triamine linker, namely, 5,5′,5′′-(1,3,5-triazine-2,4,6-triyl)tris(pyridin-2-amine) (TPA), with two structurally flexible aldehyde linkers. Two dissimilar aldehyde linkers, 4,4′,4′′-((1,3,5-triazine-2,4,6-triyl)tris(oxy))tribenzaldehyde (TPT-CHO) and 2,4,6-tris-(2-methoxy-4-formyl-phenoxy)-1,3,5-triazine (TMPT-CHO), were chosen on the basis of the impact of incorporating methoxy groups and varying heteroatom contents within the monomers, leading to the synthesis of two diverse COF materials named TPTTPA-COF and TMPTTPA-COF. The incorporation of methoxy (–OCH₃) functional groups in TMPTTPA-COF was aimed at enhancing redox activity, while the higher surface area of TPTTPA-COF (207.71 m² g⁻¹ vs. 104.85 m² g⁻¹ for TMPTTPA-COF) was expected to facilitate superior charge storage. Electrochemical investigations in a three-electrode setup demonstrated that TPTTPA-COF exhibited a specific capacitance of 277.5 F g⁻¹ at 5 mV s⁻¹ and 347 F g⁻¹ at 0.5 A g⁻¹, primarily governed by an electric double-layer capacitance (EDLC) mechanism. In contrast, TMPTTPA-COF displayed superior capacitance of 382 F g⁻¹ at 5 mV s⁻¹ and 383 F g⁻¹ at 1 A g⁻¹ due to additional pseudocapacitive contributions from the methoxy (–OCH₃) groups. Computational analysis revealed a lower bandgap for TMPTTPA-COF compared to TPTTPA-COF, which correlates with improved electronic conductivity and charge transfer kinetics. Cycling stability studies demonstrated excellent capacitance retention, with TPTTPA-COF retaining 90% and TMPTTPA-COF retaining 91% of their initial capacitance after 10 000 cycles, alongside coulombic efficiencies of 94% and 95%, respectively.