Layer-by-layer assembly of manganese–cobalt–nickel oxide nanosheets/graphene composite films
Graphical abstract
Introduction
Electrochemical capacitors have attracted considerable attention because of their high capacities, high power densities, and long lifetimes. They can be used as the assistant and buffering systems for primary power sources (e.g., electric vehicles, hybrid electric vehicles, and many other stop-and-go systems) and the renewable energy generating systems [1], [2], [3]. In general, energy can be stored in an electrochemical capacitor by either ion adsorption at the electrode/electrolyte interface (electrical double-layer capacitors, EDLCs) or fast and reversible Faradic reactions (pseudocapacitors) [4], [5]. Electrochemical capacitance is influenced by the electrode materials, electrolyte, and the assembled device, with the most important factor being the electrode materials [6]. Porous carbon materials, conducting polymers, and transition metal oxides are currently the best candidates for the electrochemical electrode materials [7]. Among the transition metal oxides containing metal atoms with various valence states, manganese oxide is selected to be one of the most promising pseudocapacitance electrode materials because of its high specific capacitance, environmental friendliness, and cost effectiveness [8], [9]. Nevertheless, MnO2 does not usually deliver ideal specific capacitance behavior because of its poor electrical conductivity and electrochemical dissolution during cycling [10]. So as to achieve higher electrochemical performance, an effective way to improve the capacity and cycling stability has been found, in which when Ni and Co are co-incorporated into the MnO2 host layer. As a result, a Ni–Co–Mn ternary layered cathode material with good capacitance performance and high energy density can be obtained [11], [12], [13].
However, the poor electrical conductivity and the electrochemical dissolution of MnO2-based cathode materials still need to be resolved. Therefore, carbonaceous materials with high electrical conductivities have been widely chosen as matrices for MnO2-based materials to improve their conductivities and stabilities [14], [15], [16], [17]. Compared with other carbon matrices, such as graphite, carbon black, and carbon nanotubes, graphene is emerging as one of the most appealing carbon materials because of its unique properties, including superior electrical conductivity, excellent mechanical flexibility, and high thermal and chemical stabilities [18]. However, the capacitive behavior of pure graphene is much lower than its predicted value because of its agglomeration during preparation, thus limiting its practical applications [19], [20]. Therefore, the heterogeneous nanostructured materials with multi-nanocomponents have been proposed and prepared; the individual components interact synergistically and greatly increase the supercapacitive performance [21]. Among the various hybrid approaches, the MnO2–graphene combination has shown great potential [22]. Up to now, several kinds of graphene–MnO2-based composites with good capacitance performances and energy densities have been prepared by flocculation techniques or layer-by-layer (LBL) self-assembly method. A graphene–MnO2 hybrid cathode prepared by Deng et al. had a wide voltage range of 0–1.7 V, a high energy density of 10.03 Wh kg−1, and good stability (69%) after 10,000 cycles [23]. The graphene–cobalt oxide cathode had a high specific capacitance of about 1100 F g−1 at a current density of 10 A g−1 with excellent cycling stability [24]. The Lv et al. reported a specific capacitance of 150–220 F g−1 at a current density of 100 mA g−1 [25].
The self-assembly by electrostatic attractive interactions of inorganic nanosheets with different charge properties is a promising approach to prepare electrode materials. This approach has been used to create several robust hybrid materials [26], [27]. For example, MnO2/graphene hybrid materials made with poly(diallyldimethylammonium chloride)-functionalized graphene (FRGO), (FRGO–p–MnO2), have been fabricated by dispersing negatively-charged MnO2 nanosheets on FRGO nanosheets by an electrostatic coprecipitation method [28]. The present work took advantage of a synergy between the pseudocapacitance of manganese–cobalt–nickel oxide nanosheets and the high electrical conductivity and stability of graphene nanosheets. A heterogeneous manganese–cobalt–nickel–graphene composite film was prepared by using LBL self-assembly process. Moreover, the capacitance of the obtained composite film was also investigated.
Section snippets
Materials
Crude flake graphite (carbon content: 99.9%) was purchased from Qingdao Aoke Co., China. Poly(diallyldimethylammonium chloride) (PDDA) (Mw about 300,000, 20 wt%) was purchased from Aldrich Co. Tetramethylammonium hydroxide (TMAOH, 25 wt%) was purchased from Alfa Aesar Co. Hydrazine solution (50%), ammonia solution (25%), H2O2 (30%), H2SO4 (98%), HCl (37%), Mn(Ac)2·4H2O, Co(Ac)2·4H2O, Ni(Ac)2·4H2O, K2S2O8, P2O5, and KMnO4 were analytical grades and used without further purification. Deionized
Results and discussion
The XRD patterns of the various intermediate materials are shown in Fig. 1. The precursor Li[Mn1/3Co1/3Ni1/3]O2 has a layered structure with a planar spacing of 0.472 nm; no impurity peaks are present (Fig. 1(a)) [12]. H[Mn1/3Co1/3Ni1/3]O2 still retains the layered structure after acid treatment. Protonation causes a slight shift of the diffraction peaks toward the low-angle side (0.481 nm), indicating the replacement of interlayer lithium ions with larger H3O+ ions (Fig. 1(b)) [32]. A dark
Conclusions
MnCoNiO2–FRGO composite films are prepared by LBL self-assembly and spontaneous flocculation between FRGO nanosheets bearing positive charges and negatively-charged MnCoNiO2 nanosheets. The MnCoNiO2–FRGO film electrode takes full advantage of the pseudocapacitance of the manganese–cobalt–nickel oxide, as well as the high electrical conductivity and stability of graphene. A strong synergistic effect between the MnCoNiO2 and FRGO electrode materials is found in the MnCoNiO2–FRGO composite film.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51172137), Changjiang Scholars and Innovative Research Team in University (IRT1070) and the Fundamental Research Funds for the Central Universities (GK201101003 and GK201301002).
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