Popcorn-Derived Porous Carbon Flakes with an Ultrahigh Specific

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Popcorn-Derived Porous Carbon Flakes with an Ultrahigh Specific Surface Area for Superior Performance Supercapacitors Jianhua Hou, Kun Jiang, Rui Wei, Muhammad Tahir, Xiaoge Wu, Ming Shen, Xiaozhi Wang, and Chuanbao Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07746 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 19, 2017

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ACS Applied Materials & Interfaces

Popcorn-Derived Porous Carbon Flakes with an Ultrahigh Specific Surface Area for Superior Performance Supercapacitors Jianhua Hou, *,† Kun Jiang, ‡ Rui Wei, † Muhammad Tahir, §Xiaoge Wu, *,† Ming Shen, ‡ Xiaozhi Wang, † and Chuanbao Cao*,§ †

Jiangsu Key Laboratory of Environmental Material and Engineering, School of Environmental Science and Engineering, Yangzhou University, Yangzhou, 225000, P.R. China ‡

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225000, P.R. China §

Research Centre of Materials Science, Beijing Institute of Technology, Beijing 100081, P.R. China * E-mail: [email protected]; [email protected]; [email protected]. KEYWORDS: carbon materials; microwave; flake-like; sub-nanopores; supercapacitors.

ABSTRACT

Popcorn-derived porous carbon flakes (PCFs) have been successfully fabricated from the biomass of maize. Utilizing the “puffing effect”, the nubby maize grain turned into materials with interconnected honeycomb-like porous structure composed of carbon flakes. The following chemical activation method enabled the as-prepared products to possess optimized porous structure for electrochemical energy storage devices, such as multi-layer flake-like structures, ultrahigh specific surface area (SBET: 3301 m2 g-1), a high 1

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content of micropores (microporous surface area of 95%, especially the optimized subnanopores with the size of 0.69 nm) that can increase the specific capacitance. The asobtained sample displayed excellent specific capacitance of 286 F g-1 @ 90 A g-1 for supercapacitors. Moreover, the unique porous structure demonstrated an ideal way to improves the volumetric energy density performance. A high energy density of 103 Wh kg-1 or 53 Wh L-1 has been obtained in case of ionic liquid electrolyte, which is the highest among reported biomass-derived carbon materials and will satisfy the urgent requirements of primary power source for electric vehicles. This work may prove a fast, green and large-scale synthesis route by using the large nubby granular materials to synthesize applicable porous carbons in energy storage devices.

INTRODUTION

Electric double-layer capacitors (EDLCs)/supercapacitors have gained much attention in energy storage application on account of their ultrahigh specific power density, good stability and long cycle life.1,2 However, they still suffer from low energy densities2,3 (less than 8 Wh kg-1) as compare to commercial lithium-ion batteries (~180 Wh kg-1),4,5 which has significantly limited their further application as primary power sources. Several nanostructured carbon-based materials, such as porous carbons,6 graphene,7-9 carbon nanotubes,10,11 carbon fibers,12 carbon aerogels,13 and 2


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mesoporous carbon,14,15 have been extensively used as an electrode materials in supercapacitors. Pre-existing studies have demonstrated that specific surface area (SSA), surface microstructures and chemical components of carbon materials are essential to the high electrochemical performance of EDLCs.12-14 In particular, the controllable size and geometry of the nanopores2,16-18 have become the focus of attention as the nanoporous structures strongly influence the power density and energy density. Till now, abundant efforts have been made to optimum the microstructures, including the application of high-temperature chlorination of carbide materials,18 various template methods14,15 and KOH activation.19-21 In fact, the mesoporous carbon templating and KOH activation methods are recognized as a well-accepted method to develop large SSA with highly porous structures, that serve as ion highways for achieving high power and gravimetric capacitance.19-22 However, the creation of large amount of pores can cause the collapse of their microstructures and may bring high pore volume. When SSA values are more than 2400 m2 g-1, pore volumes are usually in excess of 1.9 cm3 g-1.2, 22-24 The resulting high porosity leads to the declining volumetric capacitance and energy density.25-28 In addition, template methods and high-temperature chlorination in most cases are time-consuming with complicated fabrication routes. Recently, it is revealed that with different types of micropores, especially sub-nanopores2,29-31 (< 1nm), the desolvation of the ions leads towards the significantly enhancement in the energy 3



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density of supercapacitors through “micropore effect”.29-33 However, the slow ionshell desolvation and ion movement in the sub-nanopores lead to a moderate rate performance.34 Hence, for the long term, it is a great challenge to synthesize renewable porous carbon materials with ultra-large SSA and appropriate microstructures to achieve high volumetric energy densities supercapacitors by avoiding the decreasing of rate performance via a practical fabrication method.

In this work, through the “bombing” process, maize turn into crispy and delicious popcorn, whose production mechanism is the “puffing effect” by the thermal effect.35 With the liquid gasifies, induced by external energy supply, material’s internal pressure increases rapidly and rice transforms into popcorn after the pressure is released. Interestingly, popcorn’s volume increased 20 times than granular maize, mainly arising from the honeycomb flake-like structure instantaneously from the bombing process (Figure 1). The “puffing effect” via microwave pulses (10 min) applied to the preparation of micro-nano hierarchical flake-like carbon from nubby granular materials. The “puffing effect” is a pure physical chemistry effect which is rapid and eco-friendly. Therefore, in this report, popcorn-derived porous carbon flakes for electrode materials for supercapacitors have been obtained from crop maize through the combination of the physical chemistry (puffing effect) and the alkali activation. The as-prepared materials have ultrahigh SSA but relatively lower pore volume induced by numerous micropores 4


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(especially ion-accessible sub-nanopores). The SSA value of 3301 m2 g-1 (microporous surface area of 95%, especially the optimized size of the subnanopores of 0.69 nm) and layered flake-like structures has the optimum capacitance performance of 348 F g-1 at the current density of 0.2 A g-1 in 6 M KOH electrolyte, and 286 F g-1 at high current density of 90 A g-1. Notably, the energy density reached to 103 Wh kg-1 (53 Wh L-1) in ionic liquid electrolyte, the highest among the ever-reported carbon materials derived from biomass. Thus, we present a new synthesis strategy that utilizes readily available, renewable and cheap raw materials (maize) to prepare optimal electrode materials for highperformance supercapacitors through a rapid, green and industrialized method.

Scheme 1. Illustration of synthesis process for the PCFs. The maize turned into popcorn by 2 min microwave pulses at 900 W, followed by the pre-carbonization (microwave 8 min)` and then activated with KOH (weight KOH: weight carbon =3:1).

RESULTS AND DISCUSSIONS

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Figure 1 (a-b) SEM images of popcorn (c) SEM images of PCF (d) SEM images of PCF900 (e-h) TEM (HR-TEM) images of PCF-900 (i) HAADF-STEM image and the corresponding EDX elemental mapping images of PCF-900. Pretreatment of granular maize turned into popcorn though the “puffing effect” by 2 minutes’ microwave pulses at 900 W.35, 36 Followed further microwave and then activate with KOH (Scheme 1). During the bombing process, obtained popcorn show inerratic honeycomb-like structures (pore size: approximately 10 µm) composed of thin flakes with 400~900 nm thickness (Figure 1a, b). The original skeleton structure is not changed during the process of pre-carbonization (microwave 8 min) (Figure 1c). Figure S1 indicates hierarchical porous structure and the rough surface of popcorn flakes is formed,

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beneficial to the penetration with KOH in the following activation process. As a result, KOH etching leads to a micro-structure, i.e. thinner laminar structure with multi-layers (Figure 1d-f). Besides, Figure 1g evidently reveals a high content of nanopores emerged on the flake and lateral distances is around 1.0 nm (Figure 1h). The HAADF-STEM image (Figure 1i and Figure S2) further demonstrates the layered flake-like structures in final products, consistent with Figure 1e. The elemental mapping shows that nitrogen atoms are homogeneously dispersed in the carbon matrix. Elemental mapping along with XPS were conducted to exactly demonstrate the content of elements and showed similar results (Table S1 and Figure S3). With the increase activation temperature, the nitrogen and oxygen content decrease, while the carbon content is enhanced. For PCF-850 and PCF-900, carbon content is 90.56 wt.% and 92.51 wt.% by XPS, which is closer to YP17D (93.43 wt.%). However, O, N and S contents in PCF-900 still present 6.41, 1.08 and 0.3 wt.%, respectively. Heteroatoms can enhance wettability of carbon based materials through proton donation and hence increases the pseudocapacitance of carbon electrode materials.14,37-39

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Figure 2 (a) XRD patterns (b) Raman spectrum (c) Nitrogen adsorption/desorption isotherms (d) pore size distribution curve of PCF-X (inset: cumulative pore volume). Table 1 Texture Properties of PCF-X and commercial activated carbon (YP-17D). SBETa

SDFTb

SDFTb

SDFTb

SDFTb

(m2/g)

(m2/g)

S

Popcorn-Derived Porous Carbon Flakes with an Ultrahigh Specific

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