Fabrication of hierarchical porous hollow carbon spheres with few-layer graphene framework and high electrochemical activity for supercapacitor

Accepted Manuscript Short Communication Fabrication of Hierarchical Porous Hollow Carbon Spheres with Few-layer Graphene Framework and High Electrochemical Activity for Supercapacitor Jing Chen, Min Hong, Jiafu Chen, Tianzhao Hu, Qun Xu PII: DOI: Reference:

S0169-4332(18)30395-7 https://doi.org/10.1016/j.apsusc.2018.02.052 APSUSC 38510

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

9 September 2017 5 February 2018 5 February 2018

Please cite this article as: J. Chen, M. Hong, J. Chen, T. Hu, Q. Xu, Fabrication of Hierarchical Porous Hollow Carbon Spheres with Few-layer Graphene Framework and High Electrochemical Activity for Supercapacitor, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.02.052

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Fabrication of Hierarchical Porous Hollow Carbon Spheres with Few-layer Graphene Framework and High Electrochemical Activity for Supercapacitor Jing Chen,‡ Min Hong,‡ Jiafu Chen,* Tianzhao Hu, and Qun Xu* School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, P.R. China, 450001

Abstract

Porous amorphous carbons with large number of defects and dangling bonds indicate great potential application in energy storage due to high specific surface area and strong adsorption properties, but poor conductivity and pore connection limit their practical application. Here fewlayer graphene framework with high electrical conductivity is embedded and meanwhile hierarchical porous structure is constructed in amorphous hollow carbon spheres (HCSs) by catalysis of Fe clusters of angstrom scale, which are loaded in the interior of crosslinked polystyrene via a novel method. These unique HCSs effectively integrate the inherent properties from two-dimensional sp2-hybridized carbon, porous amorphous carbon, hierarchical pore structure and thin shell, leading to high specific capacitance up to 561 F g -1 at a current density

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of 0.5 A g-1 as an electrode of supercapacitor with excellent recyclability, which is much higher than those of other reported porous carbon materials up to present.

Keywords: hierarchical porous structure, hollow carbon spheres, few-layer graphene, Fe cluster, fabrication 1. Introduction Highly porous carbon materials have great potential applications in supercapacitors, lithium ion batteries and fuel cells due to their high specific surface area (SSA), high electronic conductivity, excellent chemical and electrochemical stability [1-6]. The performance of these energy storage devices fundamentally depends on the capability of porous carbon to interact with electrolyte ions and to transport electrons [7]. High conductivity for electron transport, high SSA for effective ion adsorption/desorption and appreciate pore architecture for rapid access of ions are necessary to achieve excellent performance for supercapacitors [8-12]. As a matter of fact, graphitic carbon made from random stacking two-dimensional (2D) graphene sheets has high electrical conductivity due to highly sp2 hybridization of carbon atoms, but Van der Waals force results in unavoidable restacking and aggregation of graphene, dramatically decreasing its SSA and pore connectivity [7,13]. Highly porous amorphous carbon can provide high SSA and strong adsorption ability for electrolyte ions due to large number of defects and dangling bonds but suffer from poor pore connectivity and electrical conductivity due to low sp 2 hybridization of carbon atoms [14,15]. Therefore, the integration of 2D graphene sheets, porous amorphous carbon and hierarchical porous structure is crucial to improve performance for supercapacitors. Porous hollow carbon spheres (HCSs) as a zero-dimensional porous amorphous carbon possess high SSA and a minimized ion transfer/diffusion resistance but poor electrical conductivity and

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pore connection [16,17]. Our interest is to prepare porous HCSs with high electrical conductivity and good pore connection. If few-layer graphene (FLG) sheets with high electrical conductivity were embedded in HCS shells as framework, the electrical conductivity of HCSs would be greatly improved. Considering retention of original pores, FLG could be constructed in HCSs only by low-temperature catalytic graphitization of amorphous carbon using transition metal catalysts [18-23]. Uniform distribution of nano-sized catalysts in amorphous carbon tends to formation of graphene sheets, meanwhile producing hierarchical pores [24,25]. However, it is difficult for nano-sized catalysts to load in the interior of amorphous carbon due to small size and poor connectivity of micropores. Therefore, it is crucial that a simple, efficient method is developed to uniformly load catalyst of angstrom scale in the interior of carbon precursor for construction of FLG framework (FLGF), remaining a great challenge. In this communication, we describe a novel method to load Fe clusters of angstrom scale in the interior of carbon precursor, and successfully construct FLGF and hierarchical porous structure in HCSs by Fe cluster-catalyzed phase transformation. These unique HCSs effectively integrate the inherent properties from 2D sp2-hybridized carbon, porous amorphous carbon, hierarchical pore architecture and thin shells, leading to high specific capacitance (Cs) up to 561 F g-1 at 0.5 A g-1 as an electrode of supercapacitor with excellent recyclability. This is much higher than those of other reported porous carbon, porous graphene platelets or carbon nanosheets, N-doped porous HCSs or nanofibers, carbon foam with multiscale pore network, etc. [26-31]. 2. Experimental section 2.1. Chemicals

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Analytically pure (AP) tetrachloromethane (CCl 4) was purchased from Luoyang Chemical Reagent Factory. AP anhydrous aluminum trichloride (AlCl3) and 1,2-dichloroethane (Cl(CH2)2Cl) were supplied by Tianjin Damao Chemical Reagent Factory. Chemically pure anhydrous iron trichloride (FeCl3) was supplied by Sinopharm Chemical Reagent Co., Ltd. Dilute hydrochloric acid (10 wt%) was prepared from concentrated hydrochloric acid (37 wt%). 2.2. Construction of Pores in the Shell of Hollow Poly(styrene-co-divinylbenzene) Capsules Hollow poly (styrene-co-divinylbenzene) capsules (HPCs) as precursor were firstly synthesized [29]. The HPCs with divinylbenzene (DVB) dosage of 20 wt% and 30 wt% were respectively labeled as HPC20 and HPC30. The pores were constructed in HPC shell by hypercrosslinking reaction employing FeCl3 as catalyst and Cl(CH2)2Cl as crosslinker and solvent, respectively. In a typical process, 1.80 g of anhydrous FeCl3 was added into Cl(CH2)2Cl (30 mL) in a three-neck flask under vigorous stirring of 160 rpm at room temperature. 20 min later, 0.30 g of HPCs was put into the flask. 40 min later, the flask was put into a water bath of 40 °C, and this moment was regarded as the start time of reaction. After the reaction had lasted for 16 h, a large number of pores were produced in HPC shell and filled with FeCl3 solution. These hypercrosslinked HPCs were marked as HHPC-D, where D represented Cl(CH2)2Cl. 2.3. Precipitation in Situ of FeCl3 in Pores in HHPC-D The HHPC-D with pores filled with FeCl3 solution was collected by centrifugation. FeCl3 homogeneously precipitated in the pores after toluene was added into HHPC-D. The obtained product was dried at 45 °C. The FeCl3-loaded HHPC-D from HPC20 and HPC30 were respectively labeled as HHPC20-D-Fe and HHPC30-D-Fe.

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2.4. Precipitation in Situ of FeCl3 in Pores of HHPCs with Higher Crosslinking Density The precursor (0.3 g) with higher crosslinking density was obtained by crosslinking reaction of HPCs. The crosslinking reaction used CCl4 (30 mL) as crosslinker and solvent, and AlCl3 (1.8 g) as catalyst. After the reaction, the product was collected by centrifugation and washed by acetone, dilute hydrochloric acid and deionized water, and finally was dried at 45 °C. The products obtained from HPC20 and HPC30 were respectively denoted as HHPC20-T and HHPC30-T, where T represented CCl4. Construction of pores in HHPC-T precursor and precipitation in situ of FeCl3 in pores were same to HPC precursor. The final FeCl3-loaded products from HHPC20-T and HHPC30-T were respectively marked as HHPC20-T-D-Fe and HHPC30-T-D-Fe. 2.5. Pyrolysis of FeCl3-loaded HHPCs Pyrolysis of FeCl3-loaded HHPCs was carried out in a tube furnace under nitrogen atmosphere. The temperature was elevated to 700 °C at a heating rate of 2 °C min -1 and then held at 700 °C for 2 h. After the furnace was naturally cooled to room temperature, the black product was collected and soaked in dilute hydrochloric acid for 24 h to remove Fe-containing salt. The HCSs obtained from HHPC20-D-Fe, HHPC30-D-Fe, HHPC20-T-D-Fe and HHPC30-T-D-Fe were labeled as HCS20-D, HCS30-D, HCS20-T-D and HCS30-T-D, respectively. 2.6. Characterization The morphology and structure of the products were observed by a field-emission scanning electron microscope (FE-SEM, Quanta 200) and a transmission electron microscope (TEM, FEI Tecnai G2 20, 200 KV). X-ray diffraction (XRD) patterns of the samples were tested on a Y-

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2000 X-ray diffractiometer with Cu Kα radiation (λ = 0.15406 nm, 40 kV, 40 mA). Raman spectra were performed on a Renishaw

microscope system RM 2000.

Nitrogen

adsorption/desorption measurements were analyzed using a Micromeritics ASAP 2020 instrument at 77 K. Pore size distribution (PSD) curves were obtained by density function theory (DFT) methods. At a relative pressure of P/P 0 = 0.99, the total pore volume was calculated. The capacitance performance of the samples was evaluated in 2.0 M KOH aqueous solution at ambient temperature by a CHI 660D electrochemical workstation with a standard three-electrode system using a glassy carbon electrode (Φ = 5 mm) as the working electrode, Pt foil as the counter electrode, and Ag/AgCl electrode as the reference electrode. The inks were prepared by ultrasonically dispersing 1 mg of HCSs in ethanol (1 mL) and naphthol (5 μL). The uniformly dispersed solution (20 μL) was dropped on the electrode and dried at room temperature. Cyclic voltammetry (CV) curves were obtained at different scan rates and voltage range of -0.5 ~ 0 V. The chronopotentionmetry (CP) curves were obtained at various current densities within the same voltage range. Electrochemical impedance spectroscopy (EIS) was measured within 10 MHz to 100 kHz frequency range at the open circuit voltage of the AC amplitude of 5 mV. 3. Results and discussion The synthetic procedure of hierarchical porous HCSs with FLGF using HPCs as precursor is schematically shown in Fig. 1. Firstly, a large number of micropores are introduced into the HPC shell by hypercrosslinking reaction using FeCl3 as catalyst and Cl(CH2)2Cl as crosslinker and solvent, and filled with FeCl3 solution. Secondly, FeCl3 is in situ precipitated and uniformly disperses in the micropores of HHPCs by anti-solvent effect caused by toluene. Thirdly, after pyrolysis of FeCl3-loaded HHPCs, FLG and meso- or macropores appear in HCSs by catalysis of Fe clusters from reduction of FeCl3. Finally, hierarchical porous HCSs with FLGF are obtained

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after removing Fe cluster. This provides a route to load metallic catalyst of angstrom scale in carbon precursors and further construct FLGF by catalysis of metallic cluster. HPC20 precursor (about 310 nm) seriously deforms due to low strength of thin shells (Fig. 2a). After the hypercrosslinking reaction, a great quantity of micropores are produced in the shells of HHPC20-D (320 nm) [33,34], and are full of FeCl3 solution. It is important that FeCl3 in situ precipitates and homogeneously loads in the pores of HHPC20-D after addition of toluene (Fig. S1). And the FeCl3 in the outer pores covers the surface of HHPC20-D (Fig. 2b). The FeCl3- loaded HHPC20-D is pyrolyzed into HCSs containing metallic Fe (JCPDS, 01-87-0721) (Fig. 2c, d) which derives from reduction of FeCl3. Fe and C mapping images indicate that metallic Fe evenly dispersed in HCSs (Fig. 2e, f). HRTEM image of HCS20-D-Fe clearly shows that metallic Fe exists as clusters of 0.5-0.6 nm and uniformly distributes in graphitized carbon matrix (Fig. 2g). Formation of graphitic carbon should be ascribed to catalysis of Fe clusters. This further confirms that FeCl3 has been successfully loaded in the micropores of HHPC20-D. Discrete, uniform HCSs (about 250 nm) were obtained after chemical etching of Fe (Fig. 2h, i). Open hole on HCSs and sharp contrast between pale center and dark edge indicate nature of hollow structure. Clear and bright ED rings demonstrate high crystallinity of HCSs (Fig. 2i), which is confirmed by distinct lattice fringes of (002) planes (Fig. S1f). Fig. 2j illustrates that graphitic carbon emerges as stacks of graphene layers which are highly conductive and interweave in porous amorphous carbon [32,35,36]. After selectively removing porous amorphous carbon in HCSs by etching of H2O2 for 48 h, the remained graphitic product shows FLG sheets (Fig. 2k and S1g). XRD pattern further confirms high crystallinity of FLG sheets (Fig. S1g). Therefore, electrical conductivity of porous HCSs with FLGF should be sharply improved.

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FeCl3 loads in micropores which are produced by crosslinking of swelled polymer chains of HPCs [33]. However, FeCl3 cannot get into the crosslinked section of HPCs for the crosslinked polymer chains cannot be easily swelled. Therefore, the crosslinking density of HPCs directly influences loading and distribution of FeCl3 in HHPCs, determining graphitization and FLGF structure of HCSs. The HPC30 with higher crosslinking density (about 340 nm) is used as precursor to prepare porous HCSs with FLGF (about 290 nm) by catalysis of Fe clusters (Fig. S2a-d). High crystallinity of HCS30-D is confirmed by clear and bright ED rings and perfect lattice fringes (Fig. S2d, e). FLG sheets interweave in porous shells of HCS30-D (Fig. S2f), and their size is slight smaller than that of HCS20-D, which should be attributed to lower loading of FeCl3 in HHPC30 caused by higher crosslinking density of HPC30. The crosslinking density of precursor is drastically improved by crosslinking reaction (Fig. 3a). Highly crosslinked HHPC20-T (about 320 nm) is employed as precursor to synthesize porous HCSs with FLGF (HCS20-T-D, about 245 nm) by catalysis of Fe clusters (Fig. 3b-d). Compared with HCS20-D, the ED ring brightness of HCS20-T-D is weaker and obscurer, and further the (100) ring disappears (Fig. 3d). The lattice fringes of (002) planes are intermittent (Fig. 3e). Similar results appear for HCS30-T-D (Fig. S3). It is suggested that higher crosslinking density of precursor results in lower loading of Fe cluster and lower graphitization of HCSs. The graphitization of HCSs is investigated by Raman spectroscopy and XRD. The D-band at 1350 cm-1 is related to the vibrations of carbon atoms with dangling bonds in plane terminations of the disordered graphite, and the G-band around 1580 cm-1 is ascribed to the vibration of sp2hybridized carbon atoms in a two-dimensional hexagonal lattice (Fig. 2l). The low ID/IG ratios of HCS20-D, HCS30-D, HCS20-T-D and HCS30-T-D are respectively 0.45, 0.50, 0.53 and 0.56, and show characteristic of a graphite lattice with perfect two-dimensional order in FLG. The

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graphitization degree of HCSs decreases with increase of crosslinking density of precursor, which confirms the TEM results. The 2D-band at 2700 cm-1 as a second order peak of D-band is sensitive to the number of graphene layers. The I2D/IG ratios of HCS20-D, HCS30-D, HCS20-TD and HCS30-T-D are respectively 0.42, 0.41, 0.40 and 0.40, well validating FLG structure. XRD patterns of the HCS powder (Fig. S6d) demonstrate two broad, weak diffraction peaks at 26.4° and 43.5° corresponding respectively to the (002) and (100) planes of graphitic carbon, further confirming that disordered FLG exist in amorphous HCSs. The porosity of HCSs is measured by nitrogen adsorption/desorption at -196 ℃ . Nitrogen adsorption/desorption isotherms belong to hysteresis curve of type IV (Fig. 4a). The steep increase in nitrogen uptake below P/P0 of 0.02 and the uptake at P/P0 from 0.02 to 0.2 indicate the presence of micro- and mesopores. The significant increase in adsorption quantity at P/P 0> 0.95 is probably attributed to macropores. Significant hysteresis loops in the desorption process of large relative pressure ranges exhibit a wide PSD, which is further proved by t-plots (Fig. S5a). PSD curves also show coexistence of the micro-, meso- and macropores (Fig. 4b). The micropores derive from the precursor micropores produced by hypercrosslinking reaction. Mesoand macropores are produced by volume shrinkage due to local catalysis graphitization and interstitial spaces among HCSs. It is suggested that the HCSs have hierarchical porous structure with good pore connectivity. The textural parameters of the HCSs are summarized in Table 1. With the increase of crosslinking density of precursors in the order of HPC20, HPC30, HPC20-T and HPC30-T, the SSA, Smicro, Vt and Vmicro decreased. HCS20-D has the largest Vt of 0.7 cm3/g, the highest SSA of 805 m2/g and Smicro of 562 m2/g. These should be ascribed to the highest loading of FeCl3 in HPC20 due to its lowest crosslinking density.

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Thanks to high SSA, good pore connectivity and high conductivity, these HCSs are considered as good supercapacitor electrode material. Their capacitance performance is estimated by a three-electrode system (Fig. 5a-c, 6, S6a-c, and S7). All the CV curves of the HCS20-D present a quasi-rectangular shape at different scan rates even at 200 mV s -1 (Fig. 5a), which indicates a typical characteristic of double-layer capacitance and a small equivalent series resistance (ESR) with rapid ion response [34,37,38]. The CP curves show almost symmetrical triangle at 0.5 A g -1, and remain symmetric triangles without distinct voltage drop (IR) related to resistance even at 5.0 A g-1 (Fig. 5b), exhibiting the fast transmission of ions in pores [39]. Similar capacitance performance can be obtained for HCS20-T-D, HCS30-D and HCS30-T-D (Fig. S6 and S7). The impedance of the HCS electrode materials is studied by the Nyquist plots shown in Fig. 6. Unconspicuous semicircle in the high-frequency region also indicates minimum ESR, facilitating ion diffusion into pore channels [40]. A straight line in the low-frequency region of demonstrates ideal capacitive performance [41]. The slope of straight lines increases with decrease of the diffusion resistance of ions, which is related to the rate capability of HCS electrode material. The diffusion resistance of ions increases in the order of HCS30-D, HCS30T-D, HCS20-T-D and HCS20-D, agreeing with the rate capability of the HCS30-D, HCS30-T-D, HCS20-T-D and HCS20-D electrode materials, respectively (Fig. 5c). The correlation between Cs and scan rates for HCSs is presented in Fig. 5c. HCS20-D exhibits the highest Cs of 554 F g-1 at 5 mV s-1, and a high capacitance retention of 64 % at 200 mV s -1 implies a good rate capability. The approximate effective radii of K+ and OH- in aqueous solution are respectively 0.3 nm and 0.35 nm. The micropores of < 2 nm is a bit larger than the electrolyte ions which can enter and move slowly in the micropores, providing effective adsorption sites of K+ and OH-. The HCS20-D electrode shows a relatively high Cs at small scan

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rate because of relatively large DHK of 0.77 nm and high Smicro (Table 1). With increase of scan rate, the electrolyte ions can’t reach adsorption sites in a short time due to transmission resistance, leading to a sharply decrease of Cs at small scan rate below 10 mV s -1 due to the presence of micropores. The mesopores of ˃ 2 nm provide a large channel for rapid diffusion of electrolyte ions, and the electrolyte ions can get to adsorption sites of micropores in the walls of mesopores in time at high scan rate. Hence the presence of mesopores results in a slow decrease of Cs at scan rate from 10 to 200 mV s-1. In comparison, HCS20-D manifests supercapacitor performance greatly superior to HCS30-D, HCS20-T-D and HCS30-T-D (Fig. S6a-c and S7), which is attributed to the largest Smicro, Vmicro and Vmeso (Table 1) that provide dominant adsorption sites, the highest Vt with good pore connection, and the highest crystallinity of FLGF with high electrical conductivity (Fig. 2j-l). The rate capability of the HCS20-D electrode is lower than those of the HCS30-D, HCS20-T-D and HCS30-T-D electrodes, which might be ascribed to their differential pore connection. The Cs (561 F g-1 at 0.5 A g-1) of HCS20-D from CP curve (Fig. S6) is much higher than those of other reported so far porous or mesoporous carbon [26,42], porous graphene platelets or carbon nanosheets [27,28], N-doped porous HCSs or nanofibers [29,30], porous carbon microcapsules [43], carbon foam with multiscale pore network [31] etc. (Fig. 5d, Table S1), exhibiting significance of the synergistic effect of microporous carbon, hierarchical porous structure and FLGF. Furthermore, the HCS20-D shows the excellent chargedischarge recyclability of supercapacitor (Fig. 5e). The HCS20-D electrode undergoes nine charge-discharge cycles at different scan rate. After 54 cycles, the scan rate reverses back to 5 mV s-1 and the capacitance retention maintains around 99.3%, indicating the structure of HCS keeps stable after large scan rates. Considering practical levels of mass loading, the mass loading of the HCS20-D electrode is increased to 1.16 mg. The capacitance performance is evaluated in

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an aqueous 2.0 M KOH solution by using nickel foam as the working electrode, Hg/HgO electrode as a reference electrode, and Pt foil as the counter electrode (Fig. S8). In general, higher mass-loaded electrodes demand higher ion and electron current across a longer charge transport distance to maintain the same gravimetric capacity and current density [44]. In despite of high mass loading, the HCS20-D electrode also exhibits the ideal capacitive behavior including high Cs (392 F g-1 at 5 mV s-1), minimum ESR and resistance, good rate capability and excellent charge and discharge recyclability (Fig. S8), which should be contributed to interconnected FLG network and hierarchical porous structure of HCSs. It is obvious that hierarchical porous HCSs with FLGF have great potential application in energy storage devices. 4. Conclusions In summary, we have developed a novel method to load Fe cluster of angstrom scale in carbon precursor, and successfully prepared hierarchical porous HCSs with FLGF by Fe clustercatalyzed phase transformation. FLG sheets form in shells of HCSs due to catalysis of evenly distributed Fe clusters of 0.5-0.6 nm, which is influenced by crosslinked density of HPC precursor. The micropores in HCSs are from the precursor micropores produced by hypercrosslinking reaction. Meso- and macropores are produced by volume shrinkage due to local catalysis graphitization and interstitial spaces among HCSs. These unique HCSs effectively integrate the inherent properties from two-dimensional sp2-hybridized carbon, porous amorphous carbon and hierarchical pore architecture, leading to high Cs up to 561 F g -1 at 0.5 A g-1 as an electrode of supercapacitor with excellent charge-discharge recyclability, which is much higher than those of other reported so far porous or mesoporous carbon, porous graphene platelets or carbon nanosheets, N-doped porous HCSs or nanofibers, porous carbon microcapsules, and

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carbon foam with multiscale pore network, etc. This method to load Fe clusters of angstromscale and Fe cluster-catalyzed phase transformation provides an efficient route to obtain hierarchical porous HCSs with FLGF which have great potential application in energy storage devices. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 51202223). Appendix A. Supplementary data Supplementary data related to this article can be found at References [1] Q. Lv, K. Li, C. Liu, J. Ge, W. Xing, TiO 2 inserted Carbon Materials with Fine-Tuned Pore Structure as Effective Model Supports for Electrocatalysts of Fuel Cells, Carbon 98 (2016) 126-137. [2] F. Cheng, H. Xu, W. Xu, P. Zhou, J. Martin, K. P. Loh, Controlled Growth of 1D MoSe2 Nanoribbons with Spatially Modulated Edge States, Nano Lett. 17 (2) (2017) 1116-1120. [3] A. H. Lu, W. C. Li, G. P. Hao, B. Spliethoff, H. J. Bongard, B. B. Schaack, et al., Easy Synthesis of Hollow Polymer, Carbon, and Graphitized Microspheres, Angew. Chem. 49 (9) (2010) 1615-1618. [4] R. J. White, K. Tauer, M. Antonietti, M. M. Titirici, Functional Hollow Carbon Nanospheres by Latex Templating, J. Am. Chem. Soc. 132 (49) (2010) 17360-17363. [5] X. Sun, Y. Li, Colloidal Carbon Spheres and Their Core/Shell Structures with Noble-Metal

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Figure captions

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Fig. 1 Schematic illustration of the preparation procedure for hierarchical porous HCSs with FLGF. Fig. 2 SEM images of (a) HPC20 and (b) HHPC20-D-Fe. (c) SEM image, (d) Fe mapping and (e) C mapping of HCS20-D-Fe, showing that Fe element disperses homogenously in the HCS. (f) XRD pattern of HCS-D-Fe, indicating metallic Fe. (g) High-resolution TEM (HRTEM) image of HCS20-D-Fe, demonstrating that Fe clusters of about 0.5-0.6 nm exist uniformly in graphitized carbon area. (h) SEM, (i) TEM image and electron diffraction (ED) pattern of HCS20-D. (j) HRTEM of HCS20-D, exhibiting stacks of graphene layers. (k) TEM images of sheet-like material from HCS20-D etched by H2O2 for 48 h (inset: the enlarged image of red line region, revealing that sheet-like material consisting of 2 countable, stacked graphene layers of extended lateral dimension is FLG). (l) Raman spectra of HCSs. Fig. 3 SEM images of (a) HHPC20-T, (b) HHPC20-T-D-Fe and (c) HCS20-T-D. (d-f) TEM images and ED pattern of the HCS20-T-D. Fig. 4 (a) Nitrogen adsorption/desorption isotherms of the HCSs. (b) PSD of the obtained HCSs derived from the DFT method. Fig. 5 (a) CV curves of HCS20-D electrode at different scan rates. (b) CP curves of HCS20-D electrode at various current densities. (c) The correlation of specific capacitance with scan rates for HCSs. (d) Comparison of specific capacitance of HCS20-D and other carbon-based material recently reported, S1 (mesoporous carbon) [42], S2 (porous carbon) [26], S3 (porous carbon microcapsules) [43], S4 (N-doped porous HCSs) [29], S5 (porous carbon nanosheets) [28], S6 (carbon foam with multiscale pore network) [31], S7 (porous graphene platelets) [27]. (e) The recyclability showed by the capacitance retention of the HCS20-D at different scan rates compared to scan rate of 5 mV s-1.

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Fig. 6 Nyquist plots of (a) HCS20-D, (b) HCS20-T-D, (c) HCS30-D, and (d) HCS30-T-D electrode materials (inset: the enlarged view of Nyquist plots in the high-frequency region).

Fig. 1 Schematic illustration of the preparation procedure forhierarchical porous HCSs with FLGF.

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Fig. 2 SEM images of (a) HPC20 and (b) HHPC20-D-Fe. (c) SEM image, (d) Fe mapping and (e) C mapping of HCS20-D-Fe, showing that Fe element disperses homogenously in the HCS. (f) XRD pattern of HCS-D-Fe, indicating metallic Fe. (g) High-resolution TEM (HRTEM) image of HCS20-D-Fe, demonstrating that Fe clusters of about 0.5-0.6 nm exist uniformly in graphitized carbon area. (h) SEM, (i) TEM image and electron diffraction (ED) pattern of HCS20-D. (j) HRTEM of HCS20-D, exhibiting stacks of graphene layers. (k) TEM images of sheet-like

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material from HCS20-D etched by H2O2 for 48 h (inset: the enlarged image of red line region, revealing that sheet-like material consisting of 2 countable, stacked graphene layers of extended lateral dimension is FLG). (l) Raman spectra of HCSs.

Fig. 3 SEM images of (a) HHPC20-T, (b) HHPC20-T-D-Fe and (c) HCS20-T-D. (d-f) TEM images and ED pattern of the HCS20-T-D.

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Fig. 4 (a) Nitrogen adsorption/desorption isotherms of the HCSs. (b) PSD of the obtained HCSs derived from the DFT method.

Fig. 5 (a) CV curves of HCS20-D electrode at different scan rates. (b) CP curves of HCS20-D electrode at various current densities. (c) The correlation of specific capacitance with scan rates for HCSs. (d) Comparison of specific capacitance of HCS20-D and other carbon-based material

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recently reported, S1 (mesoporous carbon) [42], S2 (porous carbon) [26], S3 (porous carbon microcapsules) [43], S4 (N-doped porous HCSs) [29], S5 (porous carbon nanosheets) [28], S6 (carbon foam with multiscale pore network) [31], S7 (porous graphene platelets) [27]. (e) The recyclability showed by the capacitance retention of the HCS20-D at different scan rates compared to scan rate of 5 mV s-1.

Fig. 6 Nyquist plots of (a) HCS20-D, (b) HCS20-T-D, (c) HCS30-D, and (d) HCS30-T-D electrode materials (inset: the enlarged view of Nyquist plots in the high-frequency region).

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Table 1 Textural parameters of the HCSs Samples

SBETa(m2/g)

Smicrob(m2/g)

Vtc,d(cm3/g)

Vmicrod(cm3/g)

DHKe(nm)

HCS20-D

805

562

0.70

0.26

0.77

HCS30-D

652

514

0.45

0.24

0.71

HCS20-T-D

621

490

0.48

0.23

0.73

HCS30-T-D

536

371

0.34

0.17

0.80

a

Specific surface area (SSA) calculated by the Brunauer-Emmett-Teller (BET) method at P/P0

from 0.01 to 0.2 (Fig. S4b-f). bMicropore surface calculated by the t-Plot method. cTotal pore volume. dPore volume obtained by the DFT method. ePore diameter obtained by the HK method.

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Highlights 

Fe cluster is loaded in precursor by in situ constructing pores and precipitation.



FLGF is constructed in HCSs by Fe cluster-catalyzed phase transformation.



Hierarchical porous structure, high SSA and FLGF coexist in HCSs.



This unique HCSs show high specific capacitance up to 561 Fg-1 at 0.5 Ag-1.



HCSs indicate excellent charge-discharge recyclability of supercapacitor.

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Fabrication of Hierarchical Porous Hollow Carbon Spheres with Few-layer Graphene Framework and High Electrochemical Activity for Supercapacitor Jing Chen,‡ Min Hong,‡Jiafu Chen,* Tianzhao Hu, and Qun Xu* School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, P.R. China, 450001

We describe a route to load Fe cluster of angstrom scale and successfully construct FLGF in shells of hierarchical porous HCSs by Fe cluster catalysis. This unique HCSs show high specific capacitance of 561 F g-1 at 0.5 A g-1 as an electrode of supercapacitor with excellent recyclability, which is much higher than those of other reported porous carbon materials. [*] Corresponding authors, E-mail: [email protected]; [email protected] [?] These authors contribute equally to this work.

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