Controlled Synthesis Of Graphene-Based Hybrids For Their Supercapacitor Application

Sustainable energy production, storage, and consumption are a major challenge in today’s society. At present, the key aim is not only to build the new-type energy sources with renewability and sustainability, but more important to efficiently store and release energy in order to meet the practical application, such as electric vehicles, portable electronic products, energy storage systems for solar and wind power, and so on. The supercapacitors are ideal electrochemical energy storage devices, bridging the gap between the traditional capacitor and the battery with the high power density, excellent cycle stability, good reliability, and relatively low production costs, and have received considerable attention. Graphene, a single layer of two-dimensional carbon atoms, holds the potential promise in the energy storage applications due to its extraordinary electrical conductivity, high surface area, and good chemical stability. In this thesis, main efforts focus on the controlled synthesis of graphene-based hybrids with high performances and low costs and their supercapacitor application. Several high-performance graphene/inorganic compounds hybrid electrodes were constructed and designed using graphene as conductive network and mechanical support. Their formation mechanism and the effects of their microstructure on the electrochemical properties were explored, which is expected to promote the development of new high-performance electrode materials. The main research contents and progresses are as follows:（1） Graphene/self-assembled α-Fe₂O₃ mesocrystals nanohybrids have been prepared and have high conductivity, a large specific surface area, and a unique mesocrystal porous structure. Their growth process and formation mechanism were investigated, and the self-assembled forming mechanism is presented. The α-Fe₂O₃ mesocrystals on graphene surface were formed by self-assembly of Fe OOH nanorods as the primary building blocks on graphene and concomitant phase transition from Fe OOH to α-Fe₂O₃. The charge/discharge curve shows an exceptional specific capacitance of the as-prepared graphene/α-Fe₂O₃ mesocrystals nanohybrid, which is 306.9 F g-1 at 3 Ag^-1 in 1 M Na₂SO₄ aqueous electrolyte. Even at 10 Ag^-1, the nanohybrids still yields a high specific capacitance（98.2 F g-1） due to the enhanced ion and charge transport, exhibiting improvrd rate capability. The graphene/α-Fe₂O₃ mesocrystal nanohybrid electrode also shows excellent cyclic performance, which is superior to reported graphene/α-Fe₂O₃ hybrid electrode, suggesting they are highly stable as an electrode for supercapacitor.（2） Although porous α-Fe₂O₃ on graphene has been synthesized, its pore sizes and crystallinity are uncontrollable. To improve its electrochemical properties, graphene/heating-rate-induced porous α-Fe₂O₃ composites, in which the porous α-Fe₂O₃ has well-controlled pore sizes and crystallinity, have been synthesized by a simple hydrothermal method combined with a annealing route. The pore size and crystallinity of the porous α-Fe₂O₃ in the hybrids can be controlled by varying the heating rate. Among them, the composites obtained under a slow heating rate of 1 °C min-1（S-PIGCs） exhibit an ultrahigh specific capacitance of 343.7 F g-1 at a current density of 3 Ag^-1 due to the optimally architecture, narrow pore size distribution, proper crystallite size, and enhanced electrical conductivity. Even at a current density as high as 10 Ag^-1, the S-PIGCs still retain a capacitance of 182.1 F g-1, showing good rate capability. Furthermore, the S-PIGCs electrode shows excellent cycling stability（95.8 % capacitance retention after 50,000 cycles） and high Columbic efficiency（98.6 %）.（3） Graphene is considered as an excellent substrate for pseudocapacitive materials due to its large surface area, high conductivity, great chemical stability, and high mechanical flexibility. So far, the as-synthesized graphene nanosheets exhibit low conductivity and intrinsic capacitance owing to the abundant oxygen-containing groups on their surface. Nitrogen doping in graphene can effectively increase its conductivity and surface active sites. Based on the advantage resulted from the nitrogen doping in graphene, several nanohybrids consisting of nitrogen-doped graphene sheets and manganese-containing oxides are successfully prepared by a facile one-step strategy.Firstly, nitrogen-doped graphene/ultrathin Mn O2 sheet hybrids（NGMCs） were synthesized through an onestep hydrothermal route at low temperature（120 °C）, in which the urea acts as a nitrogen source. During the hydrothermal process, ultrathin Mn O2 sheets were tightly anchored on graphene sheets, while nitrogen atoms were doped in graphene. Nitrogen doping in graphene not only increases the conductivity of NGMCs, but also prevents the aggregation of Mn O2 sheets on graphene surfaces. Because of the optimal combination between highly conductive nitrogen-doped graphene sheets and two-dimensional Mn O2 sheets, NGMCs electrode exhibited enhanced capacitive performances relative to those of undoped graphene/ultrathin Mn O2 sheets composites（GMCs）. As the current density increased from 0.2 to 2 Ag^-1, the capacitance of NGMCs still retained ⁷4.9 %, which was considerablely higher than that of GMCs（27 %）. Moreover, over 94.2 % of the original capacitance was maintained after 2000 cycles, indicating a good cycle stability of NGMCs electrode materials.Secondly, crumpled nitrogen-doped graphene/ultrafine Mn₃O₄ nanohybrids（CNGMNs） were prepared through a one-step hydrothermal approach, in which the aniline acts as a nitrogen source, and their electrochemical properties for supercapacitor were measured. During the hydrothermal treatment, ultrafine Mn₃O₄ nanoparticles anchoring on NG and the nitrogen doping in graphene were achieved concomitantly under the assistance of aniline. For as-prepared CNGMNs, the nitrogen doping and the optimal combination between crumpled nitrogen-doped graphene sheets and ultrafine Mn₃O₄ nanoparticles can substantially increase their conductivity and surface area, improve the electrochemical utilization of Mn₃O₄, and thus effectively improve their electrochemical properties. The specific capacitance of this hybrid was 205.5 Fg^-1 at 1 Ag^-1, nearly five times that of the pure Mn₃O₄ counterpart. Even at a high discharge current density of 10 Ag^-1, it still delivers a comparable rate capacitance of 110 Fg^-1. Additionally, good cycling stability（⁹8.7 % capacitance retention after 2000 cycles） was also obtained.Finally, nitrogen-doped graphene/Mn OOH hybrid nanowires（MNGHNs） were developded with the aid of formamide. Then the as-prepared MNGHNs were mixed with AGO suspension. The flexible freestanding MNGHNs/AGO sandwich film was fabricated by vacuum filtration using the mixture obtained. The as-fabricated MNGHNs/AGO electrodes satisfy all of the kinetic requirements for an ideal electrode material, such as numerously porous channels for the access of electrolyte, enhanced electrical conductivity for fast electron transport, high content（70 wt %） of MNGHNs for high capacitance and energy density of the entire electrode, and stable nanostructures and mechanical robustness for excellent cycle life. Based on aforementioned many advantages, the flexible freestanding MNGHNs/AGO sandwich film shows excellent electrochemical performance. The film achieves an excellent areal capacitance of 173.2 m F cm^-2 at current densities of 1 m A cm^-2. Finally, the film as all-solid-state SCs electrode also exhibits a volumetric capacitance as high as 26.3 F cm^-3 at 0.1 A cm^-3, a maximum energy density of up to 2.34 m Wh cm^-3 at the power density of 0.04 W cm^-3, and an outstanding cycling stability of retaining 91.5 % of the initial capacity even after 200,000 cycles at 5 A cm^-3.