Synthesis of morphology-tunable electroactive biomass/graphene composites using metal ions for supercapacitors

Tannic acid (TA) is a natural polyphenolic biomass, which shows high electro-activity and can be considered for supercapacitor applications. However, the negligible electronic conductivity of TA hinders its direct use as an electrode. In order to achieve the electrochemical activity of TA, herein, a three-dimensional porous TA/graphene composite (TAG) is prepared by mixing TA with graphene oxide (GO) via hydrothermal assembly, and various structural composites are realized by adding metal ions into the system before hydrothermal treatment. Metal ions can chelate with TA molecules and coordinate with GO via electrostatic interactions. As a result, a uniform and well-defined three-dimensional porous network (TAG(Ni)), a regularly arranged scale-like microstructure (TAG(Cu)) and a flower-like structure (TAG(Fe)) are achieved by introducing Ni2+, Cu2+ and Fe3+, respectively. The as-prepared TAG, TAG(Ni), TAG(Cu) and TAG(Fe) electrodes exhibit a high specific capacitance of 373.6, 412.4, 460.4 and 429.4 F g(-1) at 1 A g(-1), respectively, and excellent cycling stability. The TAG, TAG(Ni), TAG(Cu) and TAG(Fe) assembled symmetric supercapacitors display a favorable energy density of 14.76, 16.76, 19.13 and 17.6 W h kg(-1) at 300 W kg(-1), respectively. The morphology-tunable TA/graphene composites with excellent electrochemical performance are promising for renewable energy storage device applications.