Hongxiu Du

Electrochemical capacitance of polypyrrole–titanium nitride and polypyrrole–titania nanotube hybrids

Both polypyrrole–titanium nitride (PPy–TiN) and polypyrrole–titania (PPy–TiO2) nanotube hybrids have been prepared by incorporating electroactive polypyrrole into well-aligned titanium nitride and titania nanotube arrays through a normal pulse voltammetry deposition process. Microstructure characterization shows that the polypyrroles have been fully coated on the titanium nitride and titania nanotube arrays to form coaxial heterogenous nanohybrids. The galvanostatic charge–discharge measurements indicate that the PPy–TiN and PPy–TiO2 nanotube hybrids have specific capacitances of 1265 and 382 F g−1 at a current density of 0.6 A g−1. Both nanotube hybrids have similar cyclability, exhibiting stable capacitances of 459 and 72 F g−1 after 2000 cycles at a high current density of 15 A g−1. The highly conductive titanium nitride substrate can promote the electrochemical capacitance of polypyrrole more significantly, as compared to the titania semiconductor, contributing to a higher supercapacitance performance of PPy–TiN. This indicates that PPy–TiN nanotube hybrids can be more suitable to act as supercapacitor electrode materials.

Preparation and electrochemical capacitance of graphene/titanium nitride nanotube array

Two kinds of graphene composite electrode materials were synthesized on a titanium nitride nanotube array (TiN NTA) and a nickel foam (NF) substrate by a simple adsorption–reduction process, forming a graphene/titanium nitride (G/TiN) NTA and graphene/nickel foam (G/NF). Both the chemical reduction method and the thermal reduction method were used to convert graphene oxide into graphene, forming a G/TiN NTA and thermal reduction graphene/titanium nitride (TRG/TiN) NTA. The morphology and microstructure of the G/TiN NTA, G/NF and TRG/TiN NTA were characterized by scanning electron microscopy, Raman spectra and X-ray diffraction analysis. The electrochemical performance of G/TiN NTA, G/NF and TRG/TiN NTA were investigated by electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge–discharge measurements. The specific capacitances of G/NF and G/TiN NTA were 198.7 and 333.7 F g−1, respectively, at a current density of 1 A g−1 with respect to the mass of graphene. TiN NTA was more suitable as a substrate material for the graphene composite electrode and G/TiN NTA achieved a higher capacitance performance. Additionally, the specific capacitance of G/TiN NTA and TRG/TiN NTA was 230.7 and 197.9 F g−1, respectively, at a current density of 4 A g−1. This indicated that the chemical reduction method was effective for producing graphene composite electrodes with a higher capacitive performance. An all-solid-state flexible supercapacitor was also constructed using two symmetric G/TiN NTA electrodes and a gel electrolyte of polyvinyl alcohol–potassium hydroxide–potassium iodide, achieving an energy density of 34.2 W h kg−1 and a power density of 11.3 kW kg−1.

Preparation of carbon-coated lithium iron phosphate/titanium nitride for a lithium-ion supercapacitor

Carbon-coated lithium iron phosphate (C-LiFePO4) supported on a titanium nitride (TiN) substrate was designed as the electrode material for a lithium-ion supercapacitor for an energy storage application. C-LiFePO4 nanoparticles were prepared via a hydrothermal synthesis and carbonization treatment process. TiN nanowires were prepared using an anodization oxidation and nitridization process. A C-LiFePO4/TiN nanowire network was synthesized by loading C-LiFePO4 nanoparticles onto TiN nanowires through a chemical bath deposition method. The surface morphology and microstructure of C-LiFePO4/TiN were characterized using scanning electron microscopy, X-ray diffraction and Raman spectrum analysis. The lithium-ion insertion–extraction behavior of the C-LiFePO4/TiN in a Li2SO4 aqueous electrolyte was investigated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge measurements. C-LiFePO4/TiN exhibited a high capacitance of 972 F g−1 at the current density of 1.0 A g−1, presenting a capacity improvement of 210% when compared with 314 F g−1 for LiFePO4/TiN. The C-LiFePO4/TiN nanowire network also exhibited good cycle stability and high rate capability, presenting a promising application of the lithium-ion supercapacitor.

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.