Peri-Xanthenoxanthene-Based Covalent Organic Frameworks for High-Performance Aqueous Zn-Ion Hybrid Supercapacitors

Aqueous zinc-ion hybrid supercapacitors (Zn-HSCs) are promising devices for sustainable and efficient energy storage. However, they suffer from a limited energy density compared to lithium-ion batteries. This limitation can be overcome by developing novel electrode materials, with covalent organic frameworks (COFs) standing out as a particularly intriguing option. Herein, peri-xanthenoxanthene (PXX) has been integrated for the first time into a COF scaffold to take advantage of its straightforward synthesis, chemical stability, π-conjugated backbone, and heteroatom content endowing reversible redox reactions at low potentials. Two novel hexagonal COFs have been designed and synthesized by tethering of a PXX-diamine unit having a C₂ symmetry with two distinct tris-aldehydes acting as C₃-symmetric cornerstones, i.e., triformyl benzene (TFB) and triformylphloroglucinol (Tp), ultimately yielding COF PXX(PhNH₂)₂-TFB and COF PXX(PhNH₂)₂-Tp, respectively. As cathodes in Zn-HSCs, COF PXX(PhNH₂)₂-Tp exhibits a remarkable specific capacitance, energy, and power densities (237 F g^-1, 106.6 Wh kg^-1, and 3.0 kW kg^-1, respectively), surpassing those of COF PXX(PhNH₂)₂-TFB (109 F g^-1, 49.1 Wh kg^-1, and 0.67 kW kg^-1). Importantly, both COFs display outstanding long-term stability, over 5000 charge/discharge cycles, with capacitance retention >92%. These findings underscore the potential of PXX-based COFs as high-performance cathode materials for HSCs, thereby offering a promising new avenue for energy storage technologies.