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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 Biomass carbon-based supercapacitors is efficiently constructed through coupling multi-step carbonization and mild-activation strategy from soybean straw. [Display omitted] • Biomass-based hierarchical porous carbon is prepared from soybean straw. • Multi-step carbonization and mild-activation improve the electrochemical performance. • Soybean straw waste can be used to produce highly efficient supercapacitors. [ABSTRACT FROM AUTHOR]

Constructing biomass hierarchical porous carbon composite electrodes with ultra-high specific surface area, reasonable distribution of pores, and fast redox atoms for super-capacitors (SCs), is still a constant pursuit goal of current researchers. Herein, biochar containing heteroatoms with micro-meso-macro porous were successfully prepared from soybean straw as the precursor. Remarkably, an exclusive synthesis strategy, via hydrothermal reaction, alkali activation, and acid etching dual chemical treatments at specific carbonization temperature, was applied for controlling pore structure in this work, affording three dimensions (3D) hierarchical porous and a certain disordered graphitized structure of carbon material: SSP-900. SSP-900 exhibits the property of ultra-high specific surface area and reasonable distribution (S BET =1283 m2 g−1, S micro /S BET = 65%, Pore Volume= 0.73 cm3 g−1). Furthermore, it also displays the key merits of electrode materials candidate for SCs, including the superb specific capacity of 357 F g−1 at 0.5 A g−1 in the three-electrode system. Additionally, single electrode of SSP-900//SSP-900 demonstrates a splendid specific capacitance of 320 F g−1 in current density of 0.15 A g−1, the high energy density (11.2 Wh kg−1) and power density (37.8 W kg−1), a great rate capability retention (42%), and outstanding cycling stability up to 10,000 times (only 10% loss of capacitance) in two-electrode system. Its splendid electrochemical performances originate from the synergistic contribution of the special pore structure and built-in redox atom (N and O). As such SSP-900 may be seen as a promising SCs electrode material. This research provides an effective method for controlling the pore structure of biochar to be of great application in the SCs field. • Soybean straw waste is used to prepare 3-D hierarchical porous carbon electrode material. • Carbon materials show micro-meso-macroporous and disordered graphitized structure. • Its performance has been improved by combined alkali activation with acid etching. [ABSTRACT FROM AUTHOR]