Study On Preparation Of Graphene-Based Iron Oxide Three Dimensional Composites For Energy Storage

Graphene, the2D lattices of sp2carbon atoms, has spurred great research interest as electrode materials in LIBs owing to the following features:First, the high intrinsic surface area and outstanding electrical conductivity of graphene provide an ideal platform for the storage and transportation of lithium ions and electrons. Second, graphene derivates with different oxygen containing groups such as graphene oxide (GO) and reduced graphene oxide (RGO) can serve as appealing2D substrates for the anisotropic growth of various metal or metal oxide nanoparticles as electrode mateials. The aggregation of these NPs during the cycling can be effectively suppressed by the graphene matrices. Recently, graphene and graphene-based composites has spurred great research interest as electrode materials.Rechargeable lithium ion batteries (LIBs) have become one of the most promising types of battery technology for electrochemical energy storage due to the high energy density, low maintenance, and relatively low self-discharge. However, in the classical commercial LIBs, graphitic carbon is the most popular anode material, which only delivers a theoretical specific capacity of372mAh g-1. To develop new electrode materials with high energy densities has been an important way to satisfy the ever-growing demand for high performance LIBs. Nanostructured metal oxides (MOs) are regarded as potential anode materials for LIBs because of their various advantages, such as high reversible capacity, fast power capability, good safety, and long cycle life. Among them, Fe2O3has attracted much attention, owing to its appealing features, including high theoretical specific capacity (1005mAh g-1), natural abundance, low-cost, and environmental friendliness. Nevertheless, because of the low conductivity and strong agglomeration during the charge and discharge processes, Fe2O3particles suffer from rapid capacity fading caused by volume changes and subsequent particle pulverization. To overcome these obstacles, conducting carbon matrices have been introduced to absorb the volume changes and to improve the structural stability of the electrodes.Meanwhile, Fe2O3, like other transition metal oxides, is a candidate for electrode materials for pseudo-supercapacitors owing to its high theoretical capacitance, low cost and low toxicity. However, few reports have concentrated on Fe2O3used as an electrode material in supercapacitors, due to its relatively low conductivity. To circumvent these problems, a conductive phase, e.g. conducting polymer, or carbon matrix may be added to improve the performance of the electrode materials. Graphene, has been considered as the one of the most appealing carbon matrices for MOs particles because of its outstanding charge carrier mobility, mechanical robustness, and thermal and chemical stability. Polyaniline (PANI), the earliest discovery of a conductive polymer, has become the most widely used for the electrode material in supercapacitors, due to its multiple redox state and environmental friendliness. Moreover, nanostructures of PANI are easily to fabricate. However, due to the volume change occurs in the repeated charge-discharge process, PANI is prone to decomposition. To this end, the introduction of the flexible graphene sheet may accommodate the volume expand, and the high conductivity of graphene can further reduce the resistance of the electrode material.Furthermore, it was revealed that the assembly of2D graphene sheets into3D architectures can provide resultant graphene-based composites with strong mechanical strengths, and fast mass and electron transport kinetics due to the combination of the3D interconnected framework and the intriguing properties of graphene.In this article, GO was used as substrate to prepare graphene-based MOs or conducting polymer3D composites, and the electrochemical properties of the composites were evaluated. The details are as follows:1. GO was synthesized from natural graphite flakes using a modified Hummers method. Exfoliation was carried out by ultrasonicating the GO dispersion under ambient conditions for further characterization. A graphene aerogel (GA)-supported Fe2O3particles with three-dimensional (3D) architecture (Fe2O3/GAs) was fabricated via a combined hydrothermal self-assembly and freeze-drying process. Uniform Fe2O3particles and supporting graphene networks were simultaneously synthesized through a hydrothermal procedure using FeCl3and2D graphene oxide (GO) as the precursors. Compared with the Fe2O3particles supported on2D graphene sheets (Fe2O3/GNs), Fe2O3/GAs demonstrate3D interconnected macroporous architecture with a uniform deposition of Fe2O3particles, which provides highly conductive networks with enhanced surface areas and short diffusion path lengths for lithium ion transport. As a result, Fe2O3/GAs exhibit outstanding reversible capacity and excellent rate performance (995mAh g-1after50cycles at a charge-discharge rate of100mA g-1and reversible discharging capacity372mAh g-1at5000mA g-1), when applied as the anode material for lithium storage.2. The PANI/Fe2O3/GAs ternary composite was fabricated by an in situ polymerization of aniline on the2D FeO(OH)/GO sheet. GO was reduced via a hydrothermal self-assembly process. The resultant composites were subject to different characterization, and it turned out that the conducting PANI was uniformly deposited on each sheet of the as-obtained composite. And such a nanostructure of ternary composite, with the well-dispersed Fe2O3particles anchored firmly on the3D graphene network and protected by PANI. When employed as the electrode materials for supercapacitors, PANI/Fe2O3/GAs exhibits a high specific capacitance of318F g-1at a current density of1A g-1, which is much higher than pure Fe2O3(4F g-1) and Fe2O3/GAs (50F g-1).