High-temperature variant of oxygen-rich covalent triazine frameworks as multifunctional electrocatalysts

Abstract

Metal-free electrocatalysis provides a sustainable alternative to critical metal (platinum/ruthenium/iridium)-based catalysts by lowering cost, resource dependence, and environmental impact while offering high stability, poison resistance, and tuneable active sites. Herein, we report two oxygen-rich covalent triazine frameworks (CTFs), ht-Oxy-CTF750 and ht-Oxy-CTF850, synthesized from 2,5-dimethoxyterephthalonitrile via ZnCl₂-mediated nitrile trimerization under ionothermal conditions at 750 and 850 °C – temperatures well beyond those conventionally employed (≤600 °C). These conditions exploit the dynamic polymerization and simultaneous framework rearrangement intrinsic to CTF chemistry, wherein the methoxy groups function as thermally labile self-templates and oxygen dopants, ultimately leading to a pronounced enhancement in multifunctional electrocatalytic activities. Comprehensive structural and spectroscopic analysis supported by DFT calculations reveals that ht-Oxy-CTF850 possesses a higher proportion of catalytically active pyridinic-N and graphitic-N species (85.8%), enriched with an optimum ratio of carbonyl/methoxy functionalities, and an increased defect density (I_D/I_G = 1.33), which collectively modulate the framework's electronic properties, optimize charge distribution, facilitate analyte (O₂/H₂/OH⁻) adsorption/activation, and enhance hydrophilicity and mass transport pathways within the framework. As a result, ht-Oxy-CTF850 delivers outstanding ORR performance with a half-wave potential of 0.88 V vs. RHE along with excellent HER and comparable OER performance with overpotentials of 81.5 mV and 389 mV at 10 mA cm⁻², respectively. When assembled as a cathode to fabricate a rechargeable zinc–air battery, ht-Oxy-CTF850 delivers a high discharge specific capacity of 792 mA h g⁻¹, superior rate capability across 5–100 mA cm⁻², and exceptional cycling stability over 340 h, rivalling commercial Pt/C. These results highlight synergistic heteroatom chemistry, defect engineering, and oxygen functionalization as an effective strategy for advanced metal-free electrocatalysts for sustainable energy storage and generation.