Optimization of a Conductive Polymer Layer on MOF-Derived Carbon Materials for High-Performance Supercapacitor Electrodes

In this study, we report the preparation of a highly conductive metal–organic framework-derived carbon nanocomposite material with excellent electrochemical activity through a simple surface coating of carbonized-ZIF-8 (C-ZIF-8) with poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS). X-ray diffraction results indicate the successful transformation of crystalline ZIF-8 to amorphous carbon (C-ZIF-8) during carbonization at 850 °C. As-prepared C-ZIF-8 and C-ZIF-8/PEDOT–PSS nanocomposites with different weight ratios of 2:1, 8:1, and 14:1 (C-ZIF-8:PEDOT–PSS) were found to have specific surface areas of 920, 450, 740, and 810 m2 g–1, respectively. Importantly, the increase in the ratio of PEDOT–PSS (to 2:1) reveals that the PEDOT–PSS seems to block the micro- and mesopores of C-ZIF-8 due to the high density of the PEDOT–PSS coating. In cyclic voltammetry results, the specific capacitances of C-ZIF-8 and C-ZIF-8/PEDOT–PSS (14:1, 8:1, 2:1) at a scan rate of 5 mV S–1 were 44.1, 114.2, 149.4, and 110.3 F g–1, respectively. The C-ZIF-8/PEDOT–PSS nanocomposite exhibited a 4-fold increase in specific capacitance (77 F g–1) compared to that of pristine C-ZIF-8 (18.4 F g–1) at a current density of 0.5 A g–1 by galvanostatic charge–discharge. The retention of specific capacitance is maintained over 86% subsequent to 1100 cycles at 2 A g–1 by cyclic stability tests. Furthermore, the specific capacitances of C-ZIF-8/PEDOT–PSS coated on a bendable carbon cloth in its initial state and after the bending cycle test were 208.2 and 186.8 F g–1, respectively. This indicates that it maintained approximately 90% of its specific capacitance after 1000 bending cycles. Our experiments demonstrate that the C-ZIF-8/PEDOT–PSS nanocomposite exhibits a continuous conductive pathway and electrochemical activity due to PEDOT–PSS along with high porosity, a high specific surface area, abundant electroactive sites, and faster ion diffusion kinetics due to C-ZIF-8.