Auther: Jiayou Tao 2013-8-29

A solid-state flexible supercapacitor (SC) based on organic-inorganic composite structure-polypyrrole (PPy)-MnO2 nanoflakes-carbon fiber (CF) hybrid structure was fabricated through an “in situ growth for conductive wrapping” by Prof. Yihua Gao’s group in the Center for Nanoscale Characterization & Devices (CNCD) of WNLO. The conductive organic material of PPy greatly improved the electrochemical performance of the device. With a high specific capacitance of 69.3 F cm-3 at a discharge current density of 0.1 A cm-3 and an energy density of 6.16×10-3 Wh cm-3 at a power density of 0.04 W cm-3, the device can drive a commercial liquid crystal display (LCD) after being charged. The organic-inorganic composite active materials have enormous potential in energy management and the “in situ growth for conductive wrapping” method might be generalized to open up new strategies for designing next-generation energy storage devices.  Paper link:

http://60.10.59.134/nature/production/SREP-13-01914-fd7cf6e6-30c7-4dbb-ba13-bb5d112093d2.pdf

 

Owing to the rapid development of portable personal electronics, flexible electronics has attracted intense interests. Much effort has been dedicated in varied fields and great progress has been made to fabricate flexible devices, such as artificial electronic skin, roll-up displays and distributed sensors. In order to realize fully flexible devices, all of these electronics require flexible, lightweight and high efficient energy storage units. Conventional energy storage devices, such as batteries, have limitations such as inflexible, relative low power and long charging time.Supercapacitors(SCs), also known as electrochemical capacitors, in which electrical energy is mainly stored by fast and reversible redox reactions or phase changes on the surface or subsurface of electrodes, exhibit a promising set of features such as high power density, fast rates of charge-discharge, good cycle stability and safe operation. In comparison with the SCs using liquid electrolyte, all solid-state SCs have certain advantages such as good flexibility, high safety and lightweight, which are in demand of flexible and portable devices.

MnO2, compared to the other transition metal oxides, is the most thoroughly investigated for pseudocapacitorson the basis of its high theoretical specific capacitance of 1370 F g-1, relatively low cost and environmentally benign nature.Being limited by its poor electrical conductivity, the theoretical specific capacitance of MnO2 has rarely been achieved in experiment.The capacitance and conductivity of the hybrid structure based on MnO2can be improved more by conductive polymers according to the investigations, such as PPy, polyaniline and polythiophene, whichhave high electrical conductivity and high specific capacitance,andare easy to be polymerized and hold particular promise for potential large-scale energy storage systems. Even so, their improvement is not significant on the capacitanceof MnO2 and the reason is not clear. On the other hand, to meet the need of the flexibility of SCs, the flexibility of the current collector of SCs should be taken into account.CFis such a kind of materials with high flexibility as a current collector,and otheradvantages, such as high knittability, good conductivity. Therefore, CFs were applied in previous researches of SCs.

Herein, Mr. Jiayou Tao, Dr. Nishuang Liu and Prof. Yihua Gao’sothergroupmembers developed an “in situ growth for conductive wrapping” method to rationally design an all solid-state SC based on PPy-MnO2 nanoflakes-CF composites, which exhibited high flexible, high electrochemical performances. CF acted as the current collector, and MnO2 nanoflakes were deposited through a electrodeposition process. A thin layer of PPy wrapped around MnO2 nanoflakes uniformly by “in situ growth” method which was different from the “dip-coating” process,not only provided an additional electron transport path besides CFs underneath MnO2 nanoflakes but also actively participates in the charge storage processof electric double layer capacitance or pseudo-capacitance.In addition, PPy can prevent MnO2 nanoflakes from corrosion in acidic electrolyte, which ensure the full release of the electrochemical performances of MnO2 nanoflakes in the whole device. Electrochemical and mechanical measurements indicated that the as-fabricated device showed a high specific capacitance of 69.3 Fcm-3 at a discharge current density of 0.1 A cm-3with a total capacitance of ~ 0.35 mF, which weremuchhigherthan thosevaluesreported in literatures, and a high energy density of 6.16×10-3 Whcm-3 at a power density of 0.04 W cm-3. Furthermore, when it was rolled up, the electrochemical performance of the SC only had a slightfluctuation of about 0.24%.This work will significantly facilitate the developmentsof wearable electronics.

This work was supported by the National Basic Research Program (2011CB933300) of China, the National Natural Science Foundation of China (11204093, 11074082), and the Fundamental Research Funds for the Central Universities (HUST: 2012QN114, 2013TS033).

(a) Two PPy-MnO2-CFs were assembled on a piece of preservative film to form a PPy-MnO2-CF SC. (b) A SEM image of a MnO2-CF shown that the region I without PPy coating was eroded by H3PO4 solution. (c) Optical images of the bended SCs. (d) Optical image of thethree SCs in series drove a LCD.