Graphene/manganese oxide composites for energy storage

Abstract

Due to the increasing energy requirements of the portable electronic devices, backup power sources and radio frequency identification (RFID) tags, there is a growing need of high performance energy storage devices, mainly including supercapacitors and lithium ion batteries. Particularly, a simple and large scale production of high performance electrode materials is highly sought. By now, many novel nanostructured materials, including carbon nanotubes, graphene, metal oxides, transition metal dichalcogenides (TMDs) and black phosphorus have been reported to be the promising candidates as electrodes for energy storage devices. Among them, graphene and manganese dioxide (MnO₂) have attracted much attention. On one hand, graphene performs unique electrical, thermal and mechanical properties, especially outstanding electrical conductivity (106 S·cm⁻¹) and large specific surface area (~2630 m2·g⁻¹), but suffers from a relatively low theoretical capacity (550 F·g⁻¹ for supercapacitors and 1116 mAh·g⁻¹ for Li ion batteries). On the other hand, MnO₂ exhibits a high theoretical capacity (1370 F·g⁻¹ for supercapacitors and 1232 mAh·g⁻¹ for Li ion batteries) but suffers from poor electrical conductivity (10⁻⁶-10⁻⁷ S·cm⁻¹). Hence, the MnO₂/graphene composites are expected as the promising candidates of the electrode materials with improved electrochemical performances for both rechargeable batteries and supercapacitors. In this work, the synthesis of MnO₂/graphene composites with different morphologies from zero- to three-dimensional were investigated. The electrochemical performances of as-prepared composites for lithium-ion batteries and supercapacitors were tested. MnO₂/graphene composite aerogel (MnGA) was prepared via a fast and simple wet chemical process. The as-prepared composite aerogel exhibits 3D rigid graphene networks embedded by MnO₂ nanoparticles. A maximum specific capacitance of 534 F·g⁻¹ at 1 mV·s⁻¹ is achieved by 1.33 wt% MnGA electrode due to the synergistic effect. The aerogel could be a promising candidate for large-scale production of energy storage devices. MnO₂/graphene composite ink (MnGI) was prepared via a facile synthetic path. The MnGI is formed by 2D hexagonal MnO₂ nanosheets and reduced graphene oxide (rGO) sheets. The ink could be printed on different substrates and suitable for mass production in industry. A maximum specific capacitance of 648 F·g⁻¹ at 5 mV·s-1 is achieved. All these merits allow the MnGI to be a promising candidate for high performance energy storage devices. A free standing, compact and robust paper comprised of reduced graphene oxide (rGO), multi-walled carbon nanotubes (MCNTs) and manganese dioxide nanowires (MnNWs) was prepared via a simple strategy. The combination of rGO, MCNTs and MnNWs exhibits high packing density but hierarchical porous structure, which facilitates the energy capacity, rate capability and long term stability. The rGO/MCNTs/MnNWs (GMM) paper electrode retains a specific capacitance of 48 mF·cm⁻² (28 F·cm⁻³) at a high current density of 20 mA·cm⁻² (11,765 mA·cm⁻³). A symmetric capacitor assembled by two GMM paper electrodes was investigated. The as-prepared device achieves a maximum specific energy and power densities of 7.96 mWh·cm⁻³ and 10,470 mW·cm⁻³ respectively, with a capacitance retention of 99% after 5,000 cycles at 1,176 mA·cm⁻³. All the results indicate that the free standing GMM paper electrode has a potential for the large scale production, low cost, environmental friendly and high performance capacitive energy storage devices. Furthermore, the synthesis strategy may be extendable to other composite materials for batteries, solar cells, fuel cells or other related fields. MnO₂/graphene quantum dots composite (MnGQDs) were synthesized via a low-temperature chemical reaction process. The MnO₂ quantum dots (MnQDs) were well mixed with graphene quantum dots (GQDs) to form a homogeneous quantum dots composite powder. To investigate the electrochemical performances, the as-obtained MnGQDs served as the anode materials for lithium ion batteries. An initial charge and discharge specific capacity of 662 and 1444 mAh·g⁻¹ are achieved respectively. Besides, a high coulombic efficiency of nearly 100% and good rate capability were measured. The discharge specific capacity remains at 246 mAh·g⁻¹ after 50 cycles. The results show that the MnGQDs anode could be an ideal candidate for lithium ion batteries with high efficiency and rate capability.