Study On The Capacitance Behavior Of Manganese Dioxide And Its Composites

With the increasing demand for energy of modern society, the research of new type charge storage elements allows of no delay. Supercapacitors, a kind of unique energy storage device filling the gap between conventional dielectric capacitors and batteries, have broad application prospects in many fields. Thereinto, electrode material is vital to charge storage performance. Manganese dioxide (MnO2) with excellent capacitance performance receives much concern in the study of electrode materials. However, nano MnO2 particles are prone to aggregate, resulting in low utilization ratio and energy density. So, combining MnO2 with other kind of electrode materials, such as conducting polymers, metal oxides and carbon materials, has become a research hotspot.In this paper, MnO2 and its binary composites PoPD/MnO2 and MnO2/CeO2 was prepared by liquid co-precipitation, one-step chemical oxidation method and hydrothermal synthesis, respectively. In order to improve electrode materials energy density, surfactant sodium dodecyl sulfate (SDS) and polyethylene oxide-polypropylene oxide-polyethylene oxide (P123), the structure directing agents, were employed to control MnO2 and PoPD/MnO2 morphology, paricle size and dispersity respectively, thus to increase the capacitance performance. A variety of characteration methods were used to confirm the optimal addition amount and to varify the possible mechanism on capacitance behavior from surfantants. Then, coupling the individual advantage, the reversible redox properties from rare earth oxide CeO2 and excellent cycle stability and capacitance retention from MnO2, we got binary composite metal oxide electrode materials MnO2/CeO2 with preferable capacitance performance. We systematically investigated the effect of CeO2 content on the capacitance performance. In addition, the material structure, morphology and specific surface area are measured by X-ray absorption near-edge spectroscopy (XANES), infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray spectroscopy (EDS), N2 adsorption and so on. The electrochemical properties such as capacitance performance, rate capability, power characteristics, cycle stability, impedance of MnO2 and its composite electrode materials were also tested by cyclic voltammetry (CV), constant current charge and discharge (GCD) and AC impedance (EIS). The main conclusions are as follows:(1)With the assistance of SDS, MnO2 was prepared by liquid co-precipitation. The influence law and mechanism of SDS addition amount on the MnO2 microstructure and cpacitance behavior of MnO2 electrode were studied. The results show that the obtained MnO2 are all typical amorphous α-MnO2 with poor crystallinity. Meanwhile, SDS addition amount has significant influence on MnO2 morphology, dispersity and particle size. It is worthwhile noting that the morphology of MnO2 gradually undergoes a changeable process:fibrous, pine needle-like, cotton-like, round bubble-like, flocculent and nervous tissue-like as SDS increases. In terms of materials capacitance performance, SDS addition amount 0.2 g (0.017 mol·L-1) is found to be the optimal effect value, and the as-prepared Mn-0.2 obtains the highest specific capacitance (Csp) 154.5 F·g-1 at current density of 500 mA·g-1. Compared to the sample Mn-0 synthesized without SDS, the Csp increases by about 50%, which can be attributed to its largest Brunauer-Emmett-Teller (BET) specific surface area (SBET) of 255.9 m2·g-1, best particle dispersity and smallest particle size 50-80 nm approximately. Meanwhile, the rate capability and cycle stability of Mn-0.2 also improves obviously, and the equivalent series resistance (Rs) decreases obviously, only 0.120Ω.(2) Composite material PoPD/MnO2 was prepared by one-step chemical oxidation method in the presence of P123. The original idea is to combine the bulk phase reaction mechanism of PoPD with surface reaction mechanism of MnO2, making use of individual advantages to improve composite performance. It can be comfirmed that the nonionic surfactant P123 can regulate the morphology, structure and dispersity of PoPD/MnO2, improving.SBET and Csp.The synthetic series PoPD/MnO2 are all amorphous structure with weak crystallinity. As P123 increases, PoPD/MnO2 initially presents as large irregular blocky aggregates, gradually evolves into near-spherical nanoparticles (50-80) nm, and eventually changes to the coexistence state of both. Sample PoPD/MnO2-0.8 obtained under the optimum P123 dosage 0.8 g (0.0035 mol·L-1) has the largest SBET 123.2 m2·g-1 and Csp 292.4 F·g-1, much higher than that of PoPD/MnO2-0 without P123 (47.2 m2·g-1] and 62.1 F·g-1). Meanwhile, the rate capability of PoPD/MnO2-0.8 is the best for its highest Smeso 156.9 m2·g-1. Afer 500 GCD cycles, the Csp of PoPD/MnO2-0.8 almost decreases no longer, showing good cycle stability. In addition, the Rs, electrons transfer resistance (Rct) and diffusion impedance (Zw) of samples prepared with suitable amount of P123 all decreased to some extent.(3) Hierarchical porous nanocomposites MnO2/CeO2 were prepared by hydrothermal synthesis. Results indicate that MnO2/CeO2 have been composited successfully with good crystallinity, α-MnO2 and fluorite CeO2 respectively. CeO2 particles uniformly load on MnO2 nanorods surface and the size significantly reduced. Meanwhile, the agglomeration was relieved obviously for loose structure and uniform arrangement of MnO2/CeO2. Meaningfully, the SBET of is larger than the original single component, MnO2 or CeO2. And the MnO2/CeO2 are typical hierarchical mesoporous materials, much different from MnO2 and CeO2. Thereinto, the SBET and Csp of MnO2/CeO2-1 are as high as 73.4 m2·g-1 and 274.3 F·g-1 respectively, much higher than that of pure MnO2 (20.6 m2·g-1 and 190.2 F·g-1) and CeO2 (63.6 m2·g-1 and 52.8 F·g-1). After 1000 GCD cycles, the Csp no longer decreases and remains 93.9%. In addition, the Rct, Rct and Zw of MnO2/CeO2-1 all reduce. In general MnO2/CeO2-1 is a good supercapacitor electrode material.