3D carbon/polyaniline nanostructures for energy storage

Abstract

Among various types of electrical energy storage devices, supercapacitors (high power but low energy) and batteries (high energy but low power) are extensively studied. Present work aims to develop an electrode material that combines the advantages of both supercapacitors and batteries. This was realized by rational design of the microstructure of the composite material made of aligned carbon nanotubes (ACNTs) and a pseudocapacitive component which in this study is mainly polyaniline (PANI). Other affecting factors including electrolytes, configuration of the cell and so on are temporarily not included in this study.

Initial study on the composite materials made of PANI and powder CNTs through chemical reactions has shown that the loading of PANI varied the π-π interaction between PANI and CNTs which could cause the change in the oxidation stability of the composite materials.

Deposition of PANI on ACNTs via electrochemical polymerization shows better control over the morphology and microstructure of the composite materials than via chemical polymerization. ACNTs were successfully synthesized on Ti and Al substrates in present work. PANI was firstly electrochemically polymerized on the ACNTs/Ti templates by means of cyclic voltammetry (CV). A systematic study on the composite materials containing various loadings of PANI has shown: The specific capacitance based on PANI phase reached 1100 F/g when the PANI film is thinner than 11 nm, which is approaching the limited value that PANI can provide. After 2000 cycles of charge-discharge at 2.0 A/g, the reduction in the discharge capacitance of the composites containing 63.3% and 21.4% of PANI was 7.1% and 3.3%, A principle of designing the optimal microstructure of the composite materials towards high specific power and specific energy and good cycling stability was developed: high loading and thin layer of PANI should be obtained at the same time.

This principle was realized by depositing a 9 nm of PANI on much smaller ACNTs (9 nm in diameter) which were facilely synthesized on household aluminum foil. A highly flexible symmetric supercapacitor built from the 63.8% wt. PANI/ACNTs composite material on household Al current collector exhibited a high specific capacitance of 705.8 F/g at 0.1 A/g. A small specific capacitance decay of about 5.3% from 0.1 A/g to 5.0 A/g indicated a good rate performance. The average specific energy and average specific power were found to be 18.9 Wh/kg and 220.1 W/kg at 1.0 A/g. The internal resistance of the device at 1.0 A/g was calculated to be 1.3 Ω, leading to the maximum specific power of 11.3 kW/kg. 96% of the initial capacitance was kept after 2000 cycles of charge-dishcarge at 1.0 A/g. The high specific energy of 72.4 Wh/kg and high specific power of 24.9 kW/kg were obtained with an organic electrolyte, namely, lithium hexafluorophosphate (LiPF6) at 1.0 A/g. The coin-cell showed a reduction of ~5% in the capacitance after 2000 cycles of charge-discharge from 0 to 2.5 V at 1.0 A/g.

A smarter design on the supercapacitor device was also proposed in present work aiming to enhance the performance and reduce the cost of the device.