Abstract:

MnO₂ is regarded as the most attractive electrode material for supercapacitor (SC) because of its low cost and non-toxic nature, high natural abundance, and superb theoretical specific capacitance. The tantalum capacitor electrode material MnO₂ still has a problem of poor conductivity and easy peeling during charging and discharging. In this study, a sandwich MnO₂/CNTs/MnO₂ mesoporous composite is fabricated by a facile galvanostatic electrochemical deposition approach on the surface of carbon paper pre-oxidized by nitric acid, and the middle layer of CNTs was added by a simple smear-drying method. The crystal structure, surface morphology and pore characteristics of the sandwich composite are characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen adsorption test. The prepared composite shows a sandwich structure with mesopores of about 5 nm, which could ensure the efficient diffusion of electrolyte ions. Three-dimensional carbon paper could provide abundant conductive sites for attachment of MnO₂. The synthesized α-MnO₂ has a fluffy and porous morphology, which could reduce the expansion stress of the composite effectively. The intermediate layer of carbon nanotubes (CNTs) serve as conductive media relay between the inner and outer layers of MnO₂ to further improve the conductivity of composite. The composite exhibits excellent electrochemical performance: the electrode has a reversible specific capacity of 428.8 F·g^-1 at a current density of 0.1 A·g^-1 and an outstanding specific capacitance retention of ca. 80% at 5 A·g^-1. Moreover, the electrode still has an excellent cycle stability (95% retention rate) at a current density of 1 A·g^-1 after 6000 cycles.

Key words: manganese dioxide, carbon nanotube, sandwich structure, composites, diffusion, electrochemical, supercapacitor

摘要：

MnO₂具有低成本、无毒性、高天然丰度和优异的理论比电容等优点，被认为是一种极具前景的超级电容器(SC)电极材料。赝电容电极材料MnO₂仍然存在导电性差以及充放电过程中易剥落的问题。本文利用恒电流沉积的方法在硝酸预氧化处理的碳纸表面制备了一种MnO₂/CNTs/MnO₂复合电极材料。X射线衍射(XRD)、扫描电子显微镜(SEM)和氮吸附测试证明，所制备的复合材料具有一种三明治状的夹层结构，同时富含5 nm左右的介孔，介孔结构能够保证电解液离子的高效传输。采用三维立体的碳纸能够为MnO₂提供丰富的附着位点，而电沉积法合成的α-MnO₂生长在有效的导电位点上，具有蓬松多孔的形貌，在MnO₂发生膨胀/收缩过程中，这种海绵状形貌可以有效降低材料受到的膨胀应力。中间层碳纳米管(CNTs)相互搭接于内外两层MnO₂之间，作为一种导电中继，提高了复合材料的导电性。该复合材料具有优异的电化学性能：在0.1 A·g^-1的电流密度下，能够获得428.8 F·g^-1的可逆比电容，并在5 A·g^-1的高电流密度下仍能具有80%的电容保持率。同时，电极表现出优异的循环稳定性，在1 A·g^-1循环6000次之后比电容仅衰减5%。

关键词: 二氧化锰, 碳纳米管, 三明治结构, 复合物, 扩散, 电化学, 超级电容器