The Synthesis Of Three-dimensional Biomass/Graphene Hydrogels And Its Electrochemical Performance

Supercapacitors are received tremendous attention because of their high power density, fast charging/discharging processes, long cycle life, high rate capability and superior safety performance. Generally, for practical applications, it is essential to increase the energy density of supercapacitors, without sacrificing the device of power density and cycle life. The key to achieving high performance supercapacitor is to explore novel electrode materials. Electrode materials for pseudocapacitors are composed of conducting polymers and transition metal oxides and they usually exhibit high specific capacitance values, but suffer from worse rate capability and lack of stability during cycling. EDLC electrodes possess high power density, excellent cycle life, high rate capability, low internal resistance and so on, but its main disadvantages are that lower specific capacitance and energy density. In order to obtain high performance composite materials, pseudocapacitive materials combined with EDLC materials forming 3D composites have been developed to optimize the electrochemical properties from the synergistic effect between them. Biomass with low-cost, renewable, readily available and environmentally friendly properties can be served as electrode materials due to the reversible faradaic reaction of quinone/hydroquinone in biomass. However, it is difficult to directly utilize the electroactivity of this biomass, because of their insulativity. In this work, the biomass (lignosulfonate and tannic acid) have been introduced as pseudocapacitive materials. We choose the graphene serve as conductive substrate because of its excellent properties. The 3D biomass/graphene composites have been prepared to form high performance supercapacitor electrodes. The main reaserch contents are presented as follows:(1) A composite of Lig/graphene hydrogel (Lig-GH) was fabricated from a mixture of Lig and graphene oxide (GO) via convenient hydrothermal process. It was found that the hydrothermal process temperature played a significant role in forming Lig-GH. In this work, the effect of the mass ratio of Rm, (GO:Lig) on the electrochemical properties of composite hydrogels was discussed. The results disclosed that the Lig-GH prepared at a mass ratio of Rm =3:4, the temperature of 180? and the hydrothermal time of 12 h exhibited excellent electrochemical performance. The as-prepared hydrogel possessed high specific capacitance of 549.5 F g-1 at a current density of 1 A g-1 in 0.1 M HClO4 electrolyte. It also showed excellent cycling stability:83.7% capacitance retention after 1000 cycles at 20 A g-1.(2) Three-dimensional (3D) porous TA/graphene hydrogel (TAG) has been prepared by mixing TA molecule with graphene oxide (GO) via a hydrothermal assembly. The mass ratio of GO to TA and hydrothermal time on the electrochemical properties of compositess were discussed. The results disclosed that the TAG prepared at a mass ratio of Rm=3:5, the temperature of 180? and the hydrothermal time of 12 h exhibited excellent electrochemical performance. The as-prepared TAG electrode exhibit high specific capacitance of 373.6 F g-1 at 1 A g-1 and excellent cycling life with 81.8% of its initial capacitance retained after 1000 cycles at a high current density of 20 A g-1.(3) The shape controllable of TA/graphene composite hydrogels have been synthesized by introducing Ni2+, Cu2+ and Fe3+ into the mixture of TA and GO, respectively, via the hydrothermal assembly. We mainly discuss the mole ratio of TA to metal ions on the structure and electrochemical properties of composites. The uniform and well-defined 3D porous network (TAGNi), scale-like 3D microstructure (TAGCu) and flower-like structure (TAGFe) were found. The as-prepared TAGNi (mole ratio of TA:NiCl2= 0.6), TAGCu (mole ratio of TA:CuCl2= 0.5) and TAGFe (mole ratio of TA:FeCl3= 0.8) electrodes exhibit high specific capacitance of 412.4 F g-1,460.4 F g-1 and 429.4 F g-1, respectively, at 1 A g-1 and excellent cycling life with 90.8%,91.7% and 96.7%, respectively, of its initial capacitance retained after 1000 cycles at a high current density of 20 A g-1.