Electrochemical capacitance of a carbon quantum dots–polypyrrole/titania nanotube hybrid

A carbon quantum dots modified polypyrrole/titania (CQDs–PPy/TiO2) nanotube hybrid was designed as a supercapacitor electrode material for energy storage. CQDs–PPy/TiO2 was prepared by incorporating CQDs-hybridized PPy into a well-aligned titania nanotube array. CQDs–PPy/TiO2 exhibited a highly-ordered heterogeneous coaxial nanotube structure. A CQDs hybridized modification could improve the electrical conductivity of PPy. The charge transfer resistance decreased from 22.4 mΩ cm−2 to 9.3 mΩ cm−2 and the ohmic resistance decreased from 0.817 to 0.154 Ω cm−2 when PPy/TiO2 was converted into the CQDs–PPy/TiO2 nanotube hybrid. The specific capacitance was accordingly enhanced from 482 F g−1 (or 161 mF cm−2) for PPy/TiO2 to 849 F g−1 (or 212 mF cm−2) for CQDs–PPy/TiO2 at a current density of 0.5 A g−1. The capacitance retention was slightly increased from 78.5% to 89.3% after 2000 cycles at a high current density of 20 A g−1. The effective incorporation of CQDs into PPy could simultaneously increase the electrochemical capacitance and cycle stability of PPy, leading to a superior electrochemical performance. A flexible solid-state supercapacitor based on the CQDs–PPy nanohybrid exhibited the stable capacitive performance in both planar and bent states. CQDs-hybridized PPy presented promising applications as a supercapacitor electrode material for energy storage.