Nanomaterials in Advanced Batteries and Supercapacitors by Kenneth I. Ozoemena & Shaowei Chen

Author:Kenneth I. Ozoemena & Shaowei Chen
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

8.4.2 Chemical Reduction Method

Because there are some oxygen-containing functional groups (hydroxyl, carboxyl, and epoxy groups) on GO sheets, the multifunctional reduction reagent such as hydrazine, NaHSO3, Na2S, L-ascorbic acid (vitamin C), HI, hydroquinone, sodium ascorbate, hypophosphorous acid–iodine, and ethylene diamine (EDA) can combine with these oxygen-containing functional groups, which not only reduce the GO into graphene but also can impel graphene layer cross-link together by chemical bonding. Zhang et al. fabricated graphene aerogels with hierarchically porous structure by only heating GO with vitamin C followed by supercritical CO2 drying or freeze-drying [112]. Vitamin C served as a reducing agent for the reduction of GO, and the as-prepared foam showed lightweight (12–96 mg cm−3), large specific surface area (512 m2 g−1), and large pore volume (2.48 cm3 g−1). The graphene hydrogel (GH) was mechanically strong and electrically conductive (102 S m−1), could support at least 14,000 times its own weight and showed a specific capacitance of 128 F g−1 at a current density of 50 mA g−1. Subsequently, in order to obtain higher conductivity of graphene hydrogel, Shi and coworkers further used hydrazine or hydroiodic acid (HI) for reduction of the graphene hydrogels [113]. The chemical reduction of graphene hydrogels was carried out by immersing it into an aqueous solution of HI (55 %) or hydrazine monohydrate (50 %) in a sealed cuvette. The reduction was conducted for 3 or 8 h at 100 °C for HI and 95 °C for hydrazine, respectively. The chemically reduced graphene hydrogels possess high conductivities of 1.3–3.2 S m−1 and specific surface areas in the range from 780 to 950 m2 g−1. Moreover, the further treatment of graphene hydrogel with reducing agent has less impact on the microscopic pore structure of graphene hydrogel. The supercapacitor based on the HZ-reduced graphene hydrogel exhibited a high specific capacitance of 220 F g−1 at 1 A g−1 in 5 M KOH aqueous electrolyte, and this capacitance can be maintained for 74 % as the discharging current density was increased up to 100 A g−1. Furthermore, the capacitor exhibited a high power density of 30 kW kg−1 and energy density of 5.7 W h kg−1 at 100 A g−1. This is possibly attributed to the improved conductivity of reduced hydrogels accelerating its charge transfer during the discharge processes at high current densities. Moreover, Hur and coworkers demonstrated that a graphene foam was fabricated by the cross-linking reaction between GO and ethylenediamine (EDA) and then submerged into hydrazine at 90 °C for 2 days followed by freeze-drying [114]. The as-prepared graphene foam showed high electrical conductivity of 1351 S m−1, high SSA of 745 m2 g−1, high break strength of 10.3 MPa, and high specific capacitance of 232 F g−1 in the supercapacitor application.

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