Citrus-Peel-Derived, Nanoporous Carbon Nanosheets Containing

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Citrus peel-derived, nanoporous carbon nanosheets containing redox-active heteroatoms for sodium-ion storage Na Rae Kim, Young Soo Yun, Min Yeong Song, Sung Ju Hong, Minjee Kang, Cecilia Leal, Yung Woo Park, and Hyoung-Joon Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10657 • Publication Date (Web): 12 Jan 2016 Downloaded from http://pubs.acs.org on January 13, 2016

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ACS Applied Materials & Interfaces

Citrus peel-derived, nanoporous carbon nanosheets containing redox-active heteroatoms for sodium-ion storage Na Rae Kim,1,† Young Soo Yun,1,† Min Yeong Song,1 Sung Ju Hong,2 Minjee Kang,3 Cecilia Leal,3 Yung Woo Park,2 and Hyoung-Joon Jin1,*

1

Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea

2

Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea

3

Department

of

Materials

Science

and

Engineering,

University

of

Illinois

at

Urbana−Champaign, Urbana, Illinois 61801, United States

†

These authors contributed equally to this work.

KEYWORDS: carbon nanosheet · porous carbon · pyrolysis · electrode · supercapacitor · sodium-ion battery

ACS Paragon Plus Environment

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ABSTRACT

Advanced design of nanostructured functional carbon materials for use in sustainable energy storage systems suffers from complex fabrication procedures and the use of special methods and/or expensive precursors, limiting their practical applications. In this study, nanoporous carbon nanosheets (NP-CNSs) containing numerous redox-active heteroatoms (C/O and C/N ratios of 5.5 and 34.3, respectively) were fabricated from citrus peels by simply heating the peels in the presence of potassium ions. The NP-CNSs had a 2D-like morphology with a high aspect ratio of >100, high specific surface area of 1,167 m2 g-1, and a large amount of nanopores between 1-5 nm. The NP-CNSs also had an electrical conductivity of 2.6 × 101 S cm-1, which is approximately 50 times higher than that of reduced graphene oxide. These unique material properties resulted in superior electrochemical performance with a high specific capacity of 140 mAh g-1 in the cathodic potential range. In addition, symmetric full-cell devices based on the NP-CNSs showed excellent cyclic performance over 100,000 repetitive cycles.


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INTRODUCTION

Nanostructured carbon-based materials (NCMs) have attracted much attention in energy storage fields due to their physicochemically unique properties including a high specific surface area and high aspect ratio (surface to volume ratio).1-9 In addition, nanoscale effects of these materials such as nanoionics10 and nanoelectronics11 enables storage of large amounts of charge and fast ion/electron transfer kinetics.12,13 The electrochemical performance of NCMs can be further boosted by introducing numerous nanopores4-7 and redox active heteroatoms5-9 as well as improving their electrical transport properties.14,15 Accordingly, many different types of NCMs have been designed from various organic precursors by carbonization.5-8,16-22 Among these NCMs, carbohydrate-based materials are some of the most common precursors because they are cheap, sustainable, and eco-friendly.4,7,17-24 Hollow carbon nanospheres have been typically fabricated from cellulose-based molecules (mono- or di-saccharides) via a template method.17-19 Carbon nanofibers and carbon nanosheets have been prepared from regenerated cellulose-based materials by electrospinning and solvent casting, respectively.20-22 However, these methods are not easy to perform and require complex processes and/or several steps, limiting their practical application in sustainable energy storage devices. Meanwhile, advances are being made in topdown methods using cellulose-based biomass.7,23,24 In particular, NCMs fabricated from waste resulting from the production of cereals24-28 and fruits29-32 have great potential as sustainable power sources for energy storage devices because the source material is cheap, abundant, ubiquitous, and environmentally friendly. Citrus fruits, of which the annual production accounts for about 15% of total fruit production (629,272,000 tons),33 are some of the most harvested fruits worldwide. Citrus peels,


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which account for half of the total weight of citrus fruits, are generally discarded and some compounds such as phenols emitted from the waste can lead to soil pollution.34 Therefore, disposal of the waste from citrus fruits is an important environmental issue. A possible strategy to reuse citrus peels is to make carbonaceous electrode materials out of them29,30 because citrus peels are mainly composed of cellulose, hemicellulose, lignin, and pectin, which are known to be good carbon precursors. Nevertheless, few studies have reported carbonization and activation of citrus peels for use as charge storage electrode materials. Furthermore, no previous studies have reported the nanostructured design of citrus peels. In this study, we fabricated nanoporous carbon nanosheets containing numerous redoxactive heteroatoms (NP-CNSs) from citrus peels by simple pyrolysis in the presence of potassium ions. The material characteristics and electrochemical properties of the NP-CNSs were investigated and NP-CNS-based full-cell devices using Na-ion charge carriers for energy storage were prepared. This study not only presents a unique strategy for designing sustainable, nanostructured functional materials from waste but also describes how these materials can be used to synthesize high performance energy storage devices.


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EXPERIMENTAL SECTION Preparation of NP-CNSs: Citrus peels were obtained from Jeju in Korea. The peels were washed with distilled water several times to remove adhered dirt and then chopped into small pieces. The small pieces of the peels (100 g) were soaked in a 30 wt% KOH (95%, Samchun Pure Chemical Co., Ltd., Korea) aqueous solution for 30 min and then dried at 80°C for 36 h. The product materials were pyrolyzed at 800°C for 2 h under a nitrogen atmosphere in a tubular furnace where a heating rate of 5°C min-1 was applied. The resulting product was washed using distilled water and ethanol several times and then stored at 30°C. Control samples, which are referred to as citrus peel-derived carbons (CPCs), were prepared by the same procedure without soaking in a KOH aqueous solution.

Characterization: The morphology of the samples was examined by field-emission scanning electron microscopy (FE-SEM, S-4300, Hitachi, Japan) and field-emission transmission electron microscopy (FE-TEM, JEM2100F, JEOL, Japan). Topographical images of the samples were obtained using an atomic force microscope (Cypher, Oxford Instruments AFM Inc.) with a tapping mode cantilever. Raman spectra were recorded using a continuous-wave linearly polarized laser (514.5 nm wavelength, 2.41 eV, 16 mW power). The laser beam was focused by a 100× objective lens, resulting in a spot with a diameter of ~1 µm. The acquisition time and number of circulations to collect each spectrum were 10 s and 3, respectively. The chemical composition of the samples was examined by X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA, USA) with monochromatic Al Kα radiation (hν = 1,486.6 eV). The porous properties of the samples were analyzed using nitrogen adsorption and desorption isotherms obtained using a surface area and porosimetry analyzer (ASAP 2020, Micromeritics, USA) at -196°C. The


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micropore surface area and pore volume were obtained using the t-plot theory. To measure the electrical transport properties, the NP-CNSs were deposited onto a 285 nm thick SiO2/highly pdoped Si wafer. The electrode was fabricated by conventional electron beam lithography (15 keV acceleration voltage). Cr/Au (3/100 nm) was deposited using an electron gun evaporation system under high vacuum ( 100. These unique material properties resulted in high electrochemical performance as a cathode material for sodium ion storage. A specific capacity of 140 mAh g-1 at 0.1 A g-1, good rate performance at different current rates ranging from 0.1 to 20 A g-1, and stable cycles over 2,000 repetitive discharge/charge cycles were achieved. In addition, symmetric full-cell devices based on NPCNSs using a sodium-ion charge carrier showed a high specific capacitance of 110 F g-1, good power characteristics, and excellent cyclic performance over 100,000 cycles.


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AUTHOR INFORMATION Corresponding Author *H.-J. Jin, E-mail address: [email protected] ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2012M1A2A2671806) and Basic Science Research Program funded by the Ministry of Education (NRF-2013R1A1A2A10008534). This work was also supported by Industrial Strategic Technology Development Program, (Project No. 10050477, Development of separator with low thermal shrinkage and electrolyte with high ionic conductivity for Na-ion batteries) funded By the Ministry of Trade, Industry & Energy (MI, Korea). S. J. H. and Y. W. P. acknowledge the support by the Swedish-Korean Basic Research Cooperative Program (2014R1A2A1A12067266) of the NRF, Korea.

ASSOCIATED CONTENT Supporting Information. Additional information relating to the SAXS data of NP-CNSs, electrochemical performances of CPCs, Nyquist plot of NP-CNSs and CPCs, and morphologies of citrus peels, CPCs, and NP-CNSs fabricated with different heating rates and KOH contents is included in the Supporting Information. “This material is available free of charge via the Internet at http://pubs.acs.org.”


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Figure 1. Morphological characteristics of NP-CNSs. (a, b) FE-TEM images of the NP-CNSs at different magnifications. (c) AFM image of the NP-CNSs where the inset shows an optical image. (d, e) FE-SEM images of the NP-CNSs at different magnifications.


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Figure 2. Materials characteristics of the NP-CNSs. (a) Raman spectrum and (b) XPS spectra of the NP-CNSs. (c) Temperature dependent current-voltage (I-V) characteristics where the inset shows an optical image of the two electrodes used for characterization. (d) The conductivity curve of the NP-CNSs where the inset shows an AFM image of the NP-CNS sample used for this test. The scale bar is 2 µm. (e) Nitrogen adsorption and desorption isotherm curves and (f) pore size distribution plot of the NP-CNSs.


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Figure 3. Electrochemical properties of the NP-CNSs in a 1 M NaClO4 electrolyte dissolved in an EC:PC (1:1 v/v) solution in the potential range from 1.2 to 4.2 V vs. Na/Na+. (a) Cyclovoltammograms of the NP-CNSs obtained at a scan rate of 5 mV s-1 in different potential ranges. (b) Galvanostatic discharge/charge profiles and (c) rate capabilities of the NP-CNSs at different current rates ranging from 0.1 to 20 A g-1. (d) Cyclic performance of the NP-CNSs over 2,000 repetitive cycles at a current density of 1 A g-1.


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Figure 4. Electrochemical performance of NP-CNS pseudocapacitors in 1 M NaClO4 electrolyte dissolved in an EC:PC (1:1 v/v) solution in the potential range from 1.2 to 4.2 V vs. Na/Na+. (a) Cyclovoltammograms of NP-CNS pseudocapacitors obtained at scan rates of 5, 10, and 20 mV s1

. (b) Galvanostatic charge/discharge profiles of NP-CNS pseudocapacitors obtained at current

rates of 100, 200, and 400 mA g-1. (c) Rate capabilities of NP-CNS pseudocapacitors measured at different current rates ranging from 0.1 to 10 A g-1. (d) Cyclic performance of NP-CNS pseudocapacitors over 100,000 repetitive cycles at a current density of 5 A g-1 where the inset shows galvanostatic charge/discharge profiles of (left, blue line) the initial 10 cycles and (right, red line) the last 10 cycles.


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Citrus-Peel-Derived, Nanoporous Carbon Nanosheets Containing

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