Zeolite imidazole framework-67 (ZIF-67)/phosphorus-doped carbon nanofiber hybrid materials for binder-free supercapacitor electrodes

In this work, phosphoric acid is utilized as a precursor to improve electrospinnability of lignin/polyvinylpyrrolidone (lignin/PVP) solution as well as flexibility and uniformity of the resulting lignin-based carbon nanofibers. By treating with phosphoric acid, the carbon nanofibers obtained are doped with phosphorus, resulting in improved surface wettability and electrochemical properties. In the range of 0–60 wt% H3PO4, phosphorus-doped (P-doped) carbon nanofibers prepared from 40 wt% H3PO4 (PCNFs40), as electrode materials, show the highest specific capacitance of 153.9 F/g at a current density of 0.1 A/g. To enhance electrochemical performance of the P-doped carbon electrodes, we further develop zeolite imidazole framework-67 (ZIF-67)/P-doped carbon nanofiber hybrid materials (ZIF-67@PCNFs40) by simple deposition of ZIF-67 on PCNFs40 at ambient temperature. After calcination at 300–400 °C in air, ZIF-67 on ZIF-67@PCNFs40-300C and ZIF-67@PCNFs40-400C is transformed into cobalt oxide. Due to their high specific surface area (507.3 m2/g) and abundance of active surface elements, ZIF-67@PCNFs40-300C exhibit high specific capacitance (307.7 F/g at a current density of 0.1 A/g in a 2.0 M H2SO4 electrolyte) with excellent cycling stability (99.9 % capacitance retention after 5,000 cycles). The ZIF-67@PCNFs40-300C show a great potential as flexible and binder-free hybrid electrodes for high performance supercapacitor with an energy density as high as 42.7 Wh/kg at a power density of 270 W/kg.