Biomass-derived CaO in situ catalyzing approach towards hierarchical porous graphene nanosheets for high-rate performance supercapacitors

Highlights: • Hierarchical porous graphene is fabricated by direct pyrolysis of lotus leaves. • The inherent Ca-containing species in biomass transfers into CaO by CaCO₃. • The imbedded CaO serves as in situ template and catalyst to form 3D graphene. • High rate capability of 85.3% from 1 to 20 A/g and recyclability of 98.2% cycles at 20 A/g are gained. -- Abstract: Hierarchical porous graphene is first fabricated from lotus leaves by naturally CaO in situ self-catalyzing strategy without the utilization of any chemical or physical agent. The intrinsic Ca-containing species in biomass first converses into CaCO₃ and then decomposes into CaO during the pyrolysis process, and the imbedded CaO serves as in situ template and catalyst to generate hierarchical structure and graphitic structure. The resultant graphene exhibits 3D structural hierarchy with micro-sized in-plane vacancies, meso-sized channels and macro-sized cavities, contributing to rapid charge transfer, short ion diffusion pathway and robust framework. It has been demonstrated as a high-rate performance supercapacitor electrode (without the addition of any conductive agent) with outstanding rate capability (85.3% retention rate from 1 to 20 A/g) and high scan rate of 3000 mV/s. No obvious deterioration (98.2% capacitance retention) was observed after 10,000 charge-discharge cycles at 20 A/g. The first synthesis of nanoarchitecture graphene via one-step pyrolysis method by taking advantage of the unique natural features of biomass opened up new angle for the green and facile preparation of 3D graphene for energy storage.