Fabrication and nanostructure control of super-hierarchical carbon materials from heterogeneous bottlebrushes

Super-hierarchical carbons with a unique carbonaceous hybrid nanotube-interconnected porous network were fabricated by utilizing well-defined carbon nanotube@polystyrene bottlebrushes as building blocks.


Synthesis of CNT-2-Br.
In a typical synthesis, pristine CNT-2 (5.7 g) was added to a mixture of 65% HNO 3 (174 ml) and H 2 O (21 ml). After ultrasonication for 30 min, the mixture was stirred for 24 h at 120 °C under reflux. The product was collected by filtrating, washed with water for several times and dried under vacuum 90 o C overnight, leading to formation of the carboxyl groups functionalized CNT-2 (CNT-2-COOH). Subsequently, CNT-2-COOH was suspended in 60 ml of SOCl 2 and stirred at 70 o C for 24 h, to give the carbonyl chloride groups functionalized CNT-2 (CNT-2-COCl). After removing the excess SOCl 2 by vacuum, 120 mL of anhydrous glycol was added to CNT-2-COCl and stirred at 120 o C for 48 h. The solid was filtered off and washed efficiently with anhydrous tetrahydrofuran (THF). After drying under vacuum overnight, the hydroxyl groups functionalized CNT-2 (CNT-2-OH) was obtained. Then, CNT-2-OH (2.8 g), CHCl 3 (70 mL), 4-dimethylaminopyridine (0.2 g) and triethylamine (3.0 mL) were placed in a flask immersed in an ice/water bath. The flask was sealed and flushed with N 2 . A solution of 2-bromo-2-methylpropionyl bromide (1.44 mL) dissolved in anhydrous CHCl 3 (15 mL) was added dropwise and the flask was maintained at 0 o C for 3 h and then at room temperature for 48 h. The product was filtered off under vacuum, thoroughly washed with CHCl 3 and dried in a vacuum oven overnight, leading to formation of the Br-modified CNT-2 (CNT-2-Br). was bubbled with N 2 for another 1 h to remove air completely. After that, the flask was sealed and put into a water bath of 90 °C. The reaction was stopped by opening the flask and exposing the catalyst to air after different times. The mixture was separated by centrifuging.

Synthesis of CNT@PS
The transparent green solution was passed through a column of neutral alumina and then precipitated into a large excess of methanol, to give the free PS for measuring the molecular weight by GPC. Meanwhile, the black solid was purified by repeated redispersing in THF and centrifuging for several times, until no precipitation could be collected when the liquid was added into an excess of methanol. After drying, CNT@PS bottlebrushes were obtained. The resulting CNT@PS bottlebrushes were denoted as CNT@PS x , where the x indicates their DP of PS side chains. Among them, CNT@PS 160 , CNT@PS 450 and CNT@PS 1100 were fabraicated from CNT-1-Br, while CNT@PS 850 was fabraicated from CNT-2-Br.

Synthesis of SHCs.
Typically, 5.00 g of anhydrous AlCl 3 and 50 mL of CCl 4 were mixed and then heated at 75 °C for 0.5 h with magnetic stirring in a three-neck flask with a condenser.
Then, CNT@PS bottlebrushes were well dispersed into 50 mL of CCl 4 and were subsequently transferred to the above mixture, followed by heating at 75 °C for 28 h with magnetic stirring.
One hundred milliliters of 1 mol/L HCl was added slowly to the above mixture and then was heated at 75 °C for 1 h with magnetic stirring. The product was filtered off, washed with acetone, 1 mol/L HCl, and pure water, followed by drying at 80 °C overnight. After that, the resulting CNT@xPS was carbonized at 900 °C for 3 h in N 2 flow with a heating rate of 5 °C/min, leading to formation of SHCs. The resulting SHCs were denoted as SHC-y, where the y indicates the DP of PS side chains of their precusors (i.e., CNT@PS bottlebrushes). For comparison, a carbon control sample, i.e., HPC, was synthesized from PS 450 instead of CNT@PS 450 . Its preparation procedure was exactly the same as that of the SHC-450 except that the free PS 450 was employed as the precursor. Besides, in order to adjust the nanostructures of SHCs, various targeted SHCs were synthesized by carbonizing the CNT@xPS 850 at 900 °C for the desired hours with different heating rates in a furnace under a N 2 flow. The applied carbonization times were varied from 1 to 25 h with heating rate of 2~10 °C/min.

Characterization
Stuctual characterization. The microstructure of the samples was investigated by a JSM-6330F scanning electron microscope (SEM) and a FEI Tecnai G2 Spirit transmission electron microscope (TEM). About 100 nanotubes in a SEM image were picked at random, and then a statistical analysis of the diameter distribution was carried out. XRD patterns were recorded on a D-MAX 2200 VPC diffractometer using Cu K radiation (40 kV, 26 mA). Raman measurement was carried out with inVia-Reflex Renishaw Raman system. Macromolecular weight was analyzed with a Waters Breeze gel permeation chromatography (GPC). The thermogravimetric analysis (TGA) was performed under flowing N 2 condition at a heating rate of 20 o C/min. X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ESCALAB250 instrument. N 2 adsorption measurements were carried out using a Micromeritics ASAP 2020 analyzer at 77K. The BET surface area (S BET ) was analyzed by Brunauer-Emmett-Teller (BET) theory. The micropore surface area (S mic ) was determined by t-plot method, and then the external surface area (S ext ) was obtained by subtracting the S mic 5 from the S BET . The pore size distribution was analyzed by original density functional theory (DFT) combined with non-negative regularization and medium smoothing. The total pore volume (V t ) was calculated according to the amount adsorbed at a relative pressure P/P 0 of about 0.99.

Fabrication and measurements of supercapacitors.
The electrodes in the form of round sheet were obtained by pressing a mixture film of 92 wt.% carbon sample and 8 wt.% poly(tetrafluorethylene). Nickel foam and aluminum grid were used as the current collector in the aqueous electrolyte supercapacitor cell and organic electrolyte supercapacitor cell, respectively. 6 mol/L KOH aqueous solution was used as the electrolyte. The electrochemical measurements were characterized with the assembled coin-type supercapacitor. Cyclic voltammetry (CV) was carried out using an IM6ex electrochemical workstation.
Galvanostatic charge-discharge behavior was characterized by BT2000 (ARBIN Instruments).  The weight loss of CNT-1, CNT-1-OH and CNT-1-Br below 550 °C is 1.7%, 5.4% and 8.1%, respectively. According to these results, the density of Br atom on the surfaces of the resulting CNT-1-Br is measured to be 0.186 mmol/g.           Generally, an ideal nanostructure of supercapacitive electrodes can offer fast ion transport pathways and thus the electrical double layer can be re-organized quickly at the switching potentials, exhibiting a rectangle-shaped CV curve. 1 Thus, the ion diffusion behaviors within a nanoporous carbon structure can be estimated by the rectangle degree. The higher the rectangle degree, the better the ion diffusion behavior. At low sweep rates, the electrolyte ions have enough time to move into the nanopores for the EDL formation; therefore, the CV curves present a good rectangular shape for both SHC-450 and HPC at sweep rate of 5 mV/s. But with increasing the sweep rate to 100 mV/s, the rectangle degree of HPC decreases.
Comparatively, SHC-450 has much better rectangle-shaped CV curves at high sweep rates such as 100 mV/s as compared to HPC, demonstrating that the ion diffusion rate within the SHC-450 is faster than that within the HPC.