By Wenzhen Ma,Wuhan National Laboratory for Optoelectronics (WNLO), HUST

A novel cable-type flexible supercapacitor with excellent performance is fabricated utilizing three-dimensional Polypyrrole(PPy)-MnO2-CNT-cotton thread multi-grade nanostructure based electrodes by Prof. Yihua Gao’s group in the Center for Nanoscale Characterization & Devices (CNCD) of WNLO. The group members Dr. Nishuang Liu, Mr. Wenzhen Ma, Mr. Jiayou Tao and others worked hard for 7 months on it. The multiple supercapacitors with a high areal capacitance 1.49 F cm-2 at a scan rate of 1 mVs-1 connected in series and in parallel can successfully drive a LED segment display. Such high performance is attributed to the cumulative effect of conducting single-walled carbon nanotubes on cotton thread, active mesoporous flower-like MnO2 nanoplates, and PPy conductive wrapping layer improving the conductivity and acting as pseudocapacitance material simultaneously (Adv. Mater., DOI: 10.1002/adma.2013001311,IF:14.829).


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With many novel features, such as flexibility, lightweight, and foldability, wearable electroniccomponentshave rapidly been developed overthe last decade and have already revealedmany smart designs and applications previously inconceivable. In order to achieve wearable displays,embedded vital signs monitoring devices, and portable power devices, a lightweight and wearable power storage device is necessary.Asupercapacitor is such a very attractive device, owing to fast charge-discharge rate, high power density, and reliable cycling life. Recently, cable-type devices, a new concept ofdevice architecture, which can maximize the mechanical flexibility and provide the breakthrough necessary in wearable electronics, have been developed and applied in the field of energy conversion and storage. This kind of cable-type architecture can remove traditionalrestriction and achieve a subversive technology that could open up a path for design innovation. Meanwhile, cable-type electronic devices also show perfect bending properties that could meet the requirements of wearable electronic devices. Therefore, it must be meaningful to develop new cable-type supercapacitors with high performance.

Generally, in order to fabricate cable-type device, a linear shaped electrode material is necessary. Cotton thread is a flexible and porous material composed of multiple individual weaving cotton fibrils, which are made ofmultiple cotton microfibrils bundled together.The poly-D glucose chains based microfibrils have a strong adsorption capacity for water and other polar solvents.Single walled carbon nanotubes (SWCNTs) have strong van der Waals interactions with this kind of poly-D glucose chains based microfibrils. Therefore, SWCNTs can be coated on the surface of a cotton threadvia its simpleimmersion in aSWCNT solution.Thiscan make cotton thread highly conductive without affectingits shape. Suchhierarchical network createscomplicated surface morphology, high porosity and high conductivity, which meet the requirementsfor an ideal supercapacitor platform. The porous structure permits high mass loading of active materials which could further increase the energy storagecapability. Especially, macroscopic linear shape and excellentmechanical flexibility of such material are particularly valuable for cable-type devices.

In this work, Prof. Yihua Gao’s groupreporteda novel high-performance cable-type supercapacitor based on three-dimensional Polypyrrole(PPy)-MnO2-CNT-cotton thread multi-grade nanostructures preparedvia a facile and reproduciblethree-step process. At first, theycoated SWCNTs on porous cotton threads. The SWCNT coating makes these threads (with a diameter of about 0.3 mm) highly conductive with a line resistance of less than 20 Ωcm-1. And then, MnO2 nanostructures and PPy film were simply grown on SWCNT coated cotton threads via electrochemical deposition process. It is well known that MnO2 is one of the most promising pseudocapactive materials for supercapacitors because of its high theoretical specific capacitance (1370 Fg-1), environmental friendliness, low-cost and natural abundance. However, unfortunately, the poor electrical and ionic conductivity of MnO2 usually depresses its capacitor performance. Meanwhile, PPy is a common conductive polymer with high conductivity and specific capacitance, which makes it suitable for conductive wrapping ona supercapacitor electrode. Therefore, theyin-situ deposited a thin film of PPy on the surface of MnO2 nanostructures and SWCNTs to improve the electrical conductivity and electrochemical performance of the entire electrode system. The three-dimensional PPy-MnO2-CNT-cotton thread multi-grade nanostructures show outstanding mechanical and electrochemical properties. When functionalized cotton threads are used as electrodes and pristine cotton textile are used as separators, newcable-type supercapacitors become available, which 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)    Schematic diagram of a cable-type supercapacitor (the inset is a photograph of a twisting cable-type supercapacitor).

(b)    Cyclic voltammograms of the cable-type supercapacitor from CNT, MnO2-CNT and PPy-MnO2-CNT.

(c)    Photograph of a LED segment display driven by cable-type supercapacitors.