Efficient construction of a carbon-based symmetric supercapacitor from soybean straw by coupling multi-stage carbonization and mild activation

Abstract

Hierarchical porous carbon materials were successfully prepared and controlled through a multi-step carbonization and mild-activation route using soybean straw from agricultural and forestry waste as raw material. Developing the unique hierarchical structure of soybean straw as a template during pre-carbonization process, is beneficial for regulation of micro-mesoporous structure at mild-activation route. The specific surface area of the obtained sample is controlled by adjusting the pyrolysis temperature, and reaches 2266.19 m2 g−1 in the presence of a small amount of alkaline activator (mass ratio of soybean straw carbon: KOH=1:2). Moreover, the abundant porosity and specific chemical structure of nitrogen and oxygen in soybean straw-based carbon materials are favorable for an better electrochemical behavior of carbon-based supercapacitor devices. In the three-electrode system, the optimal sample (SSC-700) exhibits the capacitance as high as 380.5 F g−1 at a current density of 0.5 A g−1, and capacitance retention 73.97% after 10000 charge and discharge cycles, and produces a high energy density of 13.2 W h kg−1 at a power density of 52.03 W kg−1. In the two-electrode system, the SSC-700-based symmetric supercapacitor exhibits a high energy density of 8.95 W h kg−1 at a power density of 25 W kg−1 and is able to maintain 5 W h kg−1 at 2500 W kg−1. Furthermore, carbon-based symmetrical supercapacitors also show good cycle capacity, with a capacity loss rate of 0.5% in 5000 cycles. This work shows that soybean straw-based carbon materials have great application potential in high-performance energy storage devices