Zn-HHTP Metal Organic Framework Nanoflowers for High-Performance Flexible Electrochemical Capacitors

In this work Zn-HHTP nanoflower structured metal organic framework (MOF) is prepared by controlling the deprotonation during solvothermal reaction for the development of next-generation energy storage devices. Here, the Zn-HHTP synthesised at a 10:84 NaOH-to-Zn weight ratio (ZH3) exhibited a flower-like morphology with excellent crystallinity and wettability (35° contact angle), thereby enhancing the electrochemical performance of flexible energy storage. The three-electrode measurement analysis shows that the prepared ZH3 materials exhibit a specific capacitance of 18 F.g ⁻1 and an energy density of 2.5 W.h.kg ⁻1 at 1 mV.s ⁻1 . Using ZH3 as the active material, a symmetrical flexible electrochemical capacitor (EC) is developed, and its performance is compared separately with salt-in-water (SIW) and water-in-salt (WIS) electrolytes. The ZH3-WIS device outperformed the ZH3-SIW system by suppressing water decomposition and extending the operating voltage beyond the 1.23 V limit of conventional SIW electrolytes, thereby significantly enhancing energy density. The ZH3-WIS EC achieved an operating voltage of 2.0 V, a capacitance of 5.9 F.g ⁻1 , and an energy density of 3.28 W.h.kg ⁻1 , with 72.69% capacity retention after 6,000 cycles. The device also demonstrated excellent mechanical flexibility under different bending angles. Practical applicability was validated through successful integration into low-power electronic systems. These results highlight the potential of fabricated ECs to advance safe, high-voltage aqueous systems, establishing a flexible energy storage platform suitable for wearable electronics.