The role of nano-hydroxyapatite bearing zwitterion within carboxylated chitosan hydrogel electrolyte in improving supercapacitor performance

In this study, we intend studying the role of nano-hydroxyapatite bearing zwitterion within carboxylated chitosan hydrogel electrolyte in improving supercapacitor performance. First, we carry out the ring-opening addition of (3-Aminopropyl) triethoxysilane with 1,4-Butane sultone to obtain zwitterionic silane (ZSi). Then, ZSi is grafted onto nano-hydroxyapatite (nHA) by condensation process to obtain nano-hydroxyapatite grafted zwitterionic silane (nHA-ZSi). The successful synthesis of nHA-ZSi is confirmed by Fourier transform infrared spectroscopy and ¹³C Nuclear magnetic resonance. Differential scanning calorimetry shows that the incorporation of nHA-ZSi reduces the crystallization of the polymer matrix due to filler effect. Afterwards, nHA-ZSi with 5%, 10%, 20%, 30% are mixed with carboxylated chitosan (CCS) to obtain composite films (CCS/nHA-ZSi). The stress-strain curves of CCS/nHA-ZSi are characterized by tensile testing. Ionic conductivities of hydrogel electrolytes are estimated by soaking CCS/nHA-ZSi in 1 M Na₂SO₄ via electrochemical impedance. Furthermore, CCS/nHA-ZSi hydrogel electrolytes are assembled with symmetric carbon electrode for electrochemical tests, including cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance. The specific capacitance increases with the addition of nHA-ZSi; reaches the maximum at 20%; and then decreases due to aggregation. After 5000 cycles, CCS/20% nHA-ZSi supercapacitor has 98% capacitance retention at 6 mA/cm². This study demonstrates that nano-hydroxyapatite bearing zwitterion is a beneficial additive for both mechanical strength and ionic conductivity in hydrogel electrolytes.