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Biomass is receiving considerable attention because of its significant advantages as a sustainable and renewable material. Willow catkins, which have a single-walled microtubular structure are used as both a template and a precursor for synthesizing porous graphitic carbon microtubes (PGCMT) induced by the simultaneous activation–graphitization of K 4 Fe(CN) 6 . In addition to providing low-resistant pathways and short ion diffusion channels, as-obtained PGCMT with tubular structure also serves as an ideal platform for anchoring MnO 2 . The PGCMT/MnO 2 composite electrode obtained by MnO 2 electrodeposition expressed excellent electrochemical performance, including a significantly enhanced specific capacitance (550.8 F g −1 for the mass of MnO 2 at a current density of 2 A g −1 ), a high capacitance retention of 61.8% even at a high current density of 50 A g −1 , and an excellent cycling stability of 89.6% capability retention after 5000 cycles. These findings offer a simple and environmentally friendly strategy for preparing advanced energy materials by utilizing the unique structure of biomass waste from nature. [ABSTRACT FROM AUTHOR]

In this paper, we prepared three different kinds of hollow activated carbon fibers (HACFs) from willow catkins (WCs), phenolic- and pitch-based hollow fibers, respectively. The morphology, pore structure, surface chemical composition and electrochemical properties of these hollow fibers were studied in parallel. Due to its high-hollow, cost-effective as well as eco-friendly nature, HACFs derived from WCs can be served as excellent electrode materials for electrochemical energy storage devices. Electrochemical measurements illustrate that the WCs derived HACFs exhibit not only high specific capacitance of 333 F g −1 at 0.1 A g −1 but also considerable rate capability with a retention of 62.7% (209 F g −1 at 10 A g −1 ). Symmetric supercapacitor devices that using WCs derived HACFs as electrodes deliver a maximum energy density of ∼8.8 Wh kg −1 at power density of 50 W kg −1 and good cycling performance with 95.5% retention over 3000 cycles at 5 A g −1 in 6 M KOH aqueous electrolytes. [ABSTRACT FROM AUTHOR]

A novel bio-inspired hollow activated carbon microtubes (HACMs) was prepared from low-cost, renewable biomass willow catkins by low temperature pre-carbonization combined with KOH chemical activation and used as supercapacitor electrode materials. The as-prepared HACMs exhibit hollow fiber-like structure with thin wall thickness of less than 1 µm, high microporosity surface area (997 m 2 g −1 ) and concentrated pore size distribution (1–2 nm). The electrochemical measurement results demonstrate that the HACMs electrode materials have high gravimetric capacitance of 306 F g −1 superior to most of the previously reported activated carbons and outstanding volumetric capacitance of 303 F cm −3 . [ABSTRACT FROM AUTHOR]

Unusual sheets-like primary activated carbon particles interconnected into three-dimensional micrometer-level large pores were prepared from a novel biomass named willow catkins (WCs) by KOH chemical activation process and used as electrode materials for supercapacitors. The pore structures, surface area and chemical properties could be facilely adjusted by changing the activation temperature. When the activation temperature increased from 600 to 800 °C, the specific surface area of the porous carbon product increased remarkably while the contents of nitrogen and oxygen co-doped decreased, which significantly affected the electrochemical properties of the porous carbon-based supercapacitors. The activated carbons from 600 °C activation possesses despite moderate specific surface area (645 m 2 g −1 ), concentrated pore size distribution of 0.77 nm, but high nitrogen (2.51 wt.%) and oxygen (13.28 wt.%) contents, high graphitization degree as well as good electrical conductivity. The supercapacitors with the carbon electrode reached maximal specific capacitances of 340 F g −1 and high specific surface capacitance of 52.7 μF cm −2 at the current density of 0.1 A g −1 , good rate capability (231 F g −1 at 10 A g −1 ) and good cycling stability (92% capacitance retention over 3000 cycles). The favorable capacitive performances make the waste biomass WCs act as a new resource of carbonaceous materials for high performance supercapacitors. [ABSTRACT FROM AUTHOR]