Electrochimica Acta 54 (2009) 7173–7179 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Factors influencing MnO 2 /multi-walled carbon nanotubes composite’s electrochemical performance as supercapacitor electrode Rongrong Jiang, Tao Huang, Yang Tang, Jiali Liu, Leigang Xue, Jihua Zhuang ∗ , Aishui Yu ∗ Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200433, PR China article info Article history: Received 22 April 2009 Received in revised form 19 June 2009 Accepted 14 July 2009 Available online 21 July 2009 Keywords: Manganese dioxide Supercapacitor MWCNTs Conductivity Energy storage abstract Poor crystallined -MnO 2 grown on multi-walled carbon nanotubes (MWCNTs) by reducing KMnO 4 in ethanol are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Brunauer–Emmett–Telle (BET) surface area measurement, which indicate that MWCNTs are wrapped up by poor crystalline MnO 2 and BET areas of the composites maintain the same level of 200 m 2 g -1 as the content of MWCNTs in the range of 0–30%. The electrochemical performances of the MnO 2 /MWCNTs composites as electrode materials for supercapacitor are evaluated by cyclic voltammetry (CV) and gal- vanostatic charge–discharge measurement in 1 M Na 2 SO 4 solution. At a scan rate of 5 mV s -1 , rectangular shapes could only be observed for the composites with higher MWCNTs contents. The effect of additional conductive agent KS6 on the electrochemical behavior of the composites is also studied. With a fixed carbon content of 25% (MWCNTs included), MnO 2 with 20% MWCNTs and 5% KS6 has the highest specific capacitance, excellent cyclability and best rate capability, which gives the specific capacitance of 179 F g -1 at a scan rate of 5 mV s -1 , and remains 114.6 F g -1 at 100 mV s -1 . © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Electrochemical supercapacitors (ECs) are intermediate devices between conventional batteries and dielectric capacitors and what make the supercapacitors becoming focus of the worldwide researchers is its safety, short charging time, electrochemical stabil- ity and not to mention its high power density [1–3]. According to the charge storage mechanism, ECs are categorized as electrochemical double layer capacitors (EDLCs) and pseudocapacitors. The former is based on charge separation at the electrode/solution interface, whereas the latter is based on Faradaic redox reactions occurring within the active electrode materials. Various materials are investigated for the electrodes of ECs, including carboneous materials [4,5], conducting polymers [6,7] and transition-metal oxides [8–10]. MnO 2 has become promising alternative candidate as the electrode material of pseudocapacitors, because of its abundance, low cost and environmental friendliness. To date, MnO 2 has been synthesized via various methods, such as sol–gel, hydrothermal, reduction reactions and co-precipitation [11–16]. The values of specific capacitance reported for manganese oxide were between 100 and 250 F g -1 , which are far from the the- oretical value of 1000 F g -1 . The intrinsic problem with pure MnO 2 as a supercapacitor electrode is that only a very thin surface layer takes part in ∗ Corresponding authors. Tel.: +86 21 55664259; fax: +86 21 65642403. E-mail address: asyu@fudan.edu.cn (A. Yu). the charge–storage mechanism while the underlying MnO 2 mate- rial cannot participate the reaction because of its poor electronic conductivity. MWCNTs possess unique structural and electronic properties, such as a good conductivity and a high surface area to weight ratio as well as the ability to form a three-dimensional con- ducting matrix. For the aim of enhancement in the electrochemical capacitor applications, the metal oxide/MWCNTs composites have been widely investigated [17–19]. The advantages for this composite, combining the high conduc- tivity of MWCNTs and high capacitance of MnO 2 , are creating a large surface area by MWCNTs matrix and enhancing material utiliza- tion. Consequently, an increase in both specific power and energy of electrode can be achieved. The ratio of MnO 2 to MWCNTs in the composite plays a critical role in the electrochemical perfor- mance of electrode [20]. With more MWCNTs in the composite, the electrode performs ideal characterization of supercapacitor but a comparatively low specific capacitance, whereas less MWCNTs, the electrode risks high ohmic resistance. Moreover, direct growth of MnO 2 particles on the MWCNTs could also lead to low conductivity of the composite. Among various strategies to improve the electrochemical per- formance of MnO 2 /MWCNTs composite via the effective utilization of conductivity of carbon and surface area of MnO 2 , uniform disper- sion of nanosized MnO 2 particles on MWCNTs matrix is preferred. Both electrochemical and chemical routes were applied to synthe- size MnO 2 /MWCNTs composite with MnO 2 uniformly dispersed on MWCNTs [20–24]. In this paper, a simple approach for precip- itation of nanoscale manganese oxide particles on MWCNTs was 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.07.041