Article pubs.acs.org/cm
Oriented and Interlinked Porous Carbon Nanosheets with an Extraordinary Capacitive Performance Xiaoyu Zheng,†,‡,§ Wei Lv,†,⊥ Ying Tao,‡ Jiaojing Shao,‡ Chen Zhang,‡,§ Donghai Liu,‡,§ Jiayan Luo,‡,§ Da-Wei Wang,*,# and Quan-Hong Yang*,‡,§,⊥ ‡
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China § Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300072, China ⊥ Engineering Laboratory for Functionalized Carbon Materials and Shenzhen Key Laboratory for Graphene-based Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China # School of Chemical Engineering, UNSW Australia (The University of New South Wales), Sydney, NSW 2052, Australia S Supporting Information *
ABSTRACT: Highly porous carbon nanosheets that are oriented and interlinked are prepared by a one-step KOH activation of polymerized glucose spheres (pGSs) that are hydrothermally derived from glucose at 180 °C yet without carbonization. This is totally unexpected because a spherical microporous carbon is produced by a normally employed twostep process that includes a precarbonization and a successive KOH activation. In our one-step activation, the melt of potassium species directed the formation of oriented carbon nanosheets; the oxygen constituents in the pGSs are critical for the morphological evolution from sphere to sheet. The carbon nanosheets have a large surface area (2633 m2 g−1) and a high pore volume (1.86 cm3 g−1). The oriented structure and hierarchical porosity impose advantages for mass transfer and ion accommodation and give rise to an ultrahigh rate performance (184 F g−1 at 100 A g−1) of the carbon nanosheets in supercapacitors in comparison with the case of microporous carbon spheres (56 F g−1 at 100 A g−1). polymerization method.19,20 The nanoporous carbon coatings not only partially prevent aggregation but also increase the porosity of the hybrid nanosheets. This kind of 2D hybrid material combines the flexibility of graphene and the high porosity of nanoporous carbons. These diverse approaches have significantly increased the electrochemically active surface area of graphene-based electrodes. Very recently, highly porous carbon nanosheets made from biomass precursors have attracted increasing attention in regards to their intrinsically high porosity.21−24 Macroscopically assembled carbon nanosheets were fabricated using a sugarblowing technique, which demonstrated a very high power performance as supercapacitor electrodes.25 Aggregated carbon nanosheets were prepared by the chemical activation of a wide range of bioprecursors, such as celtuce leaves, coconut or peanut shells.26,27 Porous carbon sheets synthesized by simultaneous activation-graphitization showed a specific surface area and a total pore volume of 1874 m2 g−1 and 1.21 cm3 g−1, respectively.28 It exhibited a high specific capacitance of 268 F
1. INTRODUCTION Two-dimensional (2D) sheetlike materials have emerged as new-generation functional materials for electrochemical energy storage.1−3 The exposed active sites on the 2D surfaces and the unique electronic properties of graphene and other inorganic 2D materials give them extraordinary performance in supercapacitors and batteries.4−7 The high porosity and surface area of these 2D materials, however, is far less than their theoretical value, which limits the practical performance. Graphene, as a typical 2D material, has a theoretical specific surface area (SSA) of 2630 m2 g−1, but this is impossible to achieve because of the inevitable restacking and agglomeration.8−11 To date, there are a few promising methods, such as chemical activation, to increase the accessible surface area of graphene-derived materials.12,13 A monolithic hydrogel consisting of graphene sheets has also exhibited a greatly increased SSA (1016 m2 g−1).14,15 Li’s group reported a unique liquid-mediated condense graphene hydrogel with an extremely compact capacitance.16 Our group used evaporation induced assembly to achieve a highly dense (1.58 g cm−3) yet porous (370 m2 g−1) graphene monolith.17 An alternative strategy is to fabricate graphene-porous carbon hybrid nanosheets by carbonization of a polymer in the presence of graphene18 or an in situ © XXXX American Chemical Society
Received: October 18, 2014 Revised: November 1, 2014
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dx.doi.org/10.1021/cm503845q | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
Figure 1. Scheme of transformation of polymerized glucose spheres (pGSs) into the carbon nanosheets (Route 1) and pGS-HT-A (Route 2). Models and SEM images of (a) polymerized glucose spheres (pGSs), (b) carbon nanosheets, (c) carbon spheres (pGS-HT), and (d) activated carbon spheres (pGS-HT-A). (b, inset) Nitrogen adsorption−desorption isotherms of activated carbon spheres and the carbon nanosheets.
g−1 in KOH and 196 F g−1 in an organic electrolyte because of its low-resistance pathways and short ion-diffusion channels.28 However, because the randomly stacked nanosheets impede electron and ion transport, it is critical to assemble these carbon nanosheets in an oriented direction to facilitate ion migration.29,30 Furthermore, aligned growth of carbon nanosheets by chemical vapor deposition cannot guarantee high porosity.31−33 Therefore, the synthesis of oriented carbon nanosheets with high porosity remains a great challenge. Here we report a sheet-like carbon consisting of highly porous, oriented and interlinked carbon nanosheets by directly pyrolyzing a mixture of KOH and polymerized glucose spheres (pGSs) at 800 °C. The pGSs were first prepared by a hydrothermal treatment of a glucose solution at 180 °C. We show a high specific surface area (2633 m2 g−1) and a large capacitance (257 F g−1) for the product. The unique morphology of the novel carbon nanosheets is clearly distinguished from the spherical, carbonized and activated pGSs samples produced from the traditional activation process. Our results show that novel carbon nanostructures can be produced by simply modifying traditional chemical activation. The oriented nanosheet structure facilitates the motion of electrolyte ions and electrochemical access to carbon surfaces, leading to a remarkably high rate performance. When the current density was increased 200 times from 0.5 to 100 A g−1,
the capacitance was preserved at 184 F g−1 with a capacitance retention ratio of 71.6%.
2. RESULTS AND DISCUSSION The SEM images shown in Figures 1a and Figure S1a in the Supporting Information demonstrate the spherical shape and uniform size (∼100 nm) of pGSs. One-step activation was used to prepare the sheetlike porous carbon with the thickness of about 120 nm from pGSs. The pGSs were first soaked in a KOH aqueous solution followed by drying. The dried powder was then subjected to thermal treatment at 800 °C for 1 h in an Ar atmosphere (Route 1 in Figure 1). For comparison, a traditional two-step activation method was also applied to the pGSs (Route 2 in Figure 1). The first step is carbonization at 800 °C (denoted pGS-HT, Figure 1c) and then is the chemical activation of pGS-HT with KOH at 800 °C for 1 h in an Ar atmosphere (denoted pGS-HT-A, Figure 1d). Both products were washed with dilute HCl to remove metal residues and then rinsed with distilled water. The sheet-like product from Route 1 consisted of oriented carbon nanosheets as shown in Figure 1b. The spaces surrounding the interlinked carbon nanosheets are macropores. TEM images (see Figure S1 in the Supporting Information) show that mesopores (5−10 nm) and micropores (2 nm) help to reduce the ion transport impedance both of which are favorable for the ion penetration to the micropores. In comparison, pGS-HT and pGS-HT-A have larger particle size (50−100 nm) and only micropores (
