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Robust, Environmentally Benign Synthesis of Nanoporous Graphene Sheets from Bio-waste for Ultrafast Supercapacitor Application Katchala Nanaji, Upadhyayula Venkata Varadaraju, Tata Narasinga Rao, and Srinivasan Anandan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05419 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018
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ACS Sustainable Chemistry & Engineering
Robust, Environmentally Benign Synthesis of Nanoporous Graphene Sheets from Bio-waste for Ultrafast Supercapacitor Application Katchala Nanaji, a,b Varadaraju Upadhyayula,b Tata Narasinga Rao,a Srinivasan Anandan *,a a Centre
for Nano Materials, International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad-500005, Telangana, India.
6 7 8
b Department
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Corresponding author:
[email protected] of Chemistry, Indian Institute of Technology Madras, Chennai-600036, Tamil Nadu, India.
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ABSTRACT
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In this study, we adopted a simple method to synthesize a graphene like structured nanoporous
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carbon using jute stick as carbon precursor and studied the electrochemical properties for
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supercapacitors. The synthesized nanoporous carbon is composed of graphene sheet like network
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and amorphous carbon and the ratio between these two components is tuned by the activation
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temperature. As the activation temperature is increased, the amorphous carbon is converted into
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a stable graphene like network with high specific surface area of 2396 m2/g, with a graphene
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sheet like morphology and a highly ordered graphitic sp2 carbon. For supercapacitor application,
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the nanoporous carbon is studied in aqueous as well as organic electrolytes and the material
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shows excellent electrochemical performance in both the cases. It exhibited a high specific
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capacitance of 282 F/g and shows excellent rate capability with almost 70% capacitance
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retention at high current rates. Furthermore, the assembled symmetric supercapacitor displays a
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remarkable energy density of 20.6 W h kg-1 at a high power density of 33,600 W kg-1 and the
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benchmark studies revealed that the nanoporous carbon developed in the present study is better
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than the commercially available supercapacitive carbon (YP-50 F). A cylindrical supercapacitor
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device of capacitance 20 F with 2.7 V was fabricated using the nanoporous carbon electrode and
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tested for running practical devices. The excellent electrochemical performance of the electrode
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material can be attributed to the high electrical conductivity of the ordered graphene network
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coupled with high specific surface area and optimum pore size distribution of nanoporous
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carbon. These results demonstrate a facile, low-cost, eco-friendly design of materials for energy
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storage applications.
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KEYWORDS
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jute stick, nanoporous carbon, graphene sheets, electrode material, supercapacitor.
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INTRODUCTION
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In recent years, energy storage devices such as supercapacitors (SC) and Li-ion batteries (LIB)
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have attracted worldwide interest due to their critical role in replacing the conventional fuels in
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the transportation sector and also owing to their promising electro-chemical characteristics like
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long cycle life, high energy density, high power density and low toxicity.1 SCs, widely used in
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automobile industry and in consumer electronics due to its high power density, and long cycle
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(>106 cycles) life.2 SCs bridge the gap between conventional dielectric capacitors and primary or
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secondary Li-ion batteries in terms of their energy and power densities. Based on charge storage
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mechanisms, SCs are classified into two types namely (i) based on electrostatic charge separation
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in the Helmholtz double layer at the electrode-electrolyte interface giving rise to electric double
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layer capacitance (EDLC), and (ii) based on reversible faradaic redox reactions on the electrode
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surface, called as pseudocapacitance.3,4 In SCs, carbon material is an important electrode
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component and plays a pivotal role in the electrochemical performance. So, the development of
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carbon materials by a facile process using cost-effective carbon precursor is pivotal and
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challenging.
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Various carbon materials, such as activated carbon5, mesoporous carbon6, graphene7,8, 2D
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hierarchical porous carbon9, ZIF-8 derived carbons10 and MOF derived carbons3,11 have been
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synthesized and generally adopted as electro-active materials for SCs. Among them, activated
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carbon has been commercially utilized as an electrode material in conventional supercapacitors
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owing to its high specific surface area and also the charge storage predominately takes place at
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the micropores.5 However, the specific capacitance and power density characteristics of the
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activated carbons are not up to the mark due to their intrinsic limitations such as inappropriate
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and blocked microporous structure due to small size of micropores (pore size < 2nm), poor
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electrical conductivity, presence of more micropores and less of mesopores, low graphitic nature
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of carbon etc., that restricts the mass transfer and diffusion of electrolyte ions into the pores of
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electrode at higher current rates.12 For example, though graphitic carbon such as CNTs have high
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electrical conductivity and suitable porous structure for better ionic diffusion and charge
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propagation, the less specific surface area limits the capacitive properties of CNT in EDLCs. In
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general, graphitization improves the electrical conductivity of carbon materials significantly, but 2 ACS Paragon Plus Environment
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simultaneously reduces the specific surface area of carbon materials drastically.13 Thus, it is
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understood that in order to enhance the electrochemical properties of the carbon-based electrode
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materials in EDLCs, it is important to maintain the balance between specific surface area and
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graphitization degree of carbon materials. Currently, porous graphitic carbon electrode material
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is considered as the next-generation supercapacitor material owing to its excellent physico-
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chemical characteristics including the large specific surface area, porous structure, high pore
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volume and good electrical conductivity, which facilitates electrolyte ion penetration coupled
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with
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charging/discharging process.14,15 Various approaches including soft and hard templating
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strategy16, chemical vapor deposition7, and chemical exfoliation14 were extensively studied to
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synthesize porous graphitic carbon electrode materials. However, the practical application of
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porous graphitic carbon electrode materials in the market is limited due to the high cost of raw
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materials involved, environmental safety issues arising from synthesis and difficulty in up-
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scaling. Hence, it is a great challenge to prepare porous graphitic carbon electrode materials by a
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facile process using cost effective carbon precursor, suitable for EDLC applications. Therefore,
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alternative to wet chemical methods, a lot of research is focused to produce porous graphitic
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carbon electrode materials using green, recyclable bio-waste or its derivatives, which is critical
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for sustainable development and environmental protection.
low
diffusion
(ion/electron)
resistance
and
short
diffusion
length
during
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Recently, bio-mass derived carbons are finding their path as promising electrode materials not
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only in energy storage applications (Figure S1) but also used in catalysis17, adsorption18 and
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separation19 studies because of the promising characteristics like large specific surface area,
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larger pore volume, hierarchical porous structure, high graphitic content, high degree of surface
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reactivity etc. Various bio-wastes such as seaweeds20,21, rice husk22,23, dead leaves24, human
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hair25, pollen26, willow catkin27,28, egg whites and egg shell membranes29,30, corn31,32, hemps12
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have been used as carbon precursors and converted into porous carbon material through
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pyrolysis and activation. The biggest advantage of carbon derived bio-mass method over
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chemical methods is that the synthesis procedure of the former is cost effective & eco-friendly
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and more importantly the bio-wastes are renewable.
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In the present study, we have chosen Jute stick as bio-waste to synthesize porous graphene
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sheets like carbon materials. Jute, Corchorus olitorius is an ancient plant belonging to Malvacea
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family and it is broadly cultivated in tropical Asian countries (India, Bangladesh, China), Egypt,
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Brazil etc. The Corchorus olitorius produces jute fiber which is of commercial value and is 3 ACS Paragon Plus Environment
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widely used for manufacturing commercial items such as gunny bags, hessian for carpet backing,
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decorative fabrics, etc.33 On the other hand, jute stick, which is obtained after the extraction of
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fiber from jute plant is often used for domestic purposes such as firewood, thatching roofs,
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temporary fencing, etc., has less commercial importance unlike jute fiber and was used for
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applications such as catalysis, separation and adsorption studies.17–19 The extraction of every
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tonne of jute fiber from the jute plant, produces about 2.5 tonnes of jute sticks that goes as a solid
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waste.33 Interestingly, as per statistics, 4 million tonnes per annum of jute stick dumped as a solid
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waste in India, is often discarded or thrown away into the soil, which undergoes degradation
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without any utilization. Since the chemical composition of jute stick constitutes carbon sources
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like α-cellulose, lignin and hemi cellulose33, it would be advantageous if it is converted into
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useful carbon material which is suitable for energy storage application like EDLCs. Very few
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studies have been reported on the conversion of jute fiber into carbon material and its application
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for energy related applications.34 Nevertheless, the potential application of porous carbon with
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graphene sheet like morphology (from jute stick) as electrode is not realized till now for
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supercapacitor application. Hence, in the present study, jute sticks are used as a carbon precursor
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to synthesize porous carbon that could drastically result in waste minimization and remarkable
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cost saving.
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In this study, we adopted a simple method to synthesize a nanoporous carbon from jute stick
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waste, characterized with high specific surface area coupled with graphene sheet network and
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highly ordered graphitic carbon. The resulting nanoporous carbon under optimized condition
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delivers a high specific capacitance of 282 F/g at 0.5 A/g current density and exhibits excellent
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capacitance retention of 70% at high current rates, attributing to the high degree of graphitization
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and high specific surface area coupled with narrow pore size distribution which plays a vital role
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for improved performance of nanoporous carbon material reported in the present study.
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RESULTS AND DISCUSSION
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Material characterization.
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The synthesis of jute stick derived nanoporous carbon from jute stick waste is illustrated in
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Scheme 1. The ground jute stick powder after pre-carbonization is activated with KOH at
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different temperatures to obtain activated carbon. The microstructure of the nanoporous carbons
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synthesized at different carbonization temperatures (800, 900, 1000 oC) derived from jute stick 4 ACS Paragon Plus Environment
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was characterized by FE-SEM and the images are shown in Fig. 1. For comparison, the
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microstructures of the as prepared jute stick carbon without activation and also the cross section
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of a carbonized jute stick was also included in Fig. 1. The microstructure of carbonized stick
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(Fig. 1A) shows uniformly arranged pores, size varying from 10-20 microns. The non-activated
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carbon (Fig. 1B & 1C) did not show any specific morphological features and shows rough flat
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surface. Interestingly, after activation, the flat surface morphology changes into a porous scaffold
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with a 3D connected channels. With increasing activation temperature from 800 oC to 1000 oC,
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the channel walls become flat nanosheets that are oriented at a certain direction that were stacked
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together, to form a squashed 3 dimensional porous structure (Fig. 1D, 1F and 1H). Further, with
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the rise in activation temperature, the surface of the channel walls all over the frame work
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became very rough and shows the presence of sheet like structure with high porosity. However,
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some of the wall surfaces of porous carbon frame work reassembled with small carbon flakes.
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The high magnification images (Fig. 1E, 1G and 1I) exhibits the surfaces of nano flakes and the
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walls of 3D porous frame work are full of open and interconnected pores, attributes to the strong
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reaction between carbon frame work and KOH with increasing the activation temperature.
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To further investigate the crystalline and porous nature of carbon material synthesized at
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different activation temperatures, TEM images (Fig. 2) were also taken for nanoporous carbon.
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As seen in Fig. 2, with increasing activation temperature from 800 oC to 1000 oC, the
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nanoporous carbon materials show distinctively different morphologies. The non-activated
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sample does not show any porous structure (Fig. 2A, 2B) and the sample activated at 800 oC
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shows porous morphology but with more of amorphous carbon content (Fig. 2C, 2D). In
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contrast, the nanoporous carbon activated at 900 oC & 1000 oC, show a crumpled graphene
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sheet like microstructure as seen in images taken at low (Fig. 2E, 2G) and high (Fig. 2F, 2H)
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magnification. The nanosheet exhibits the typical morphology of a crystalline carbon. The HR-
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TEM images of JC 900 and JC 1000, show the lattice fringes of crystalline carbon layers with the
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measured inter planar distance of 0.361 nm in JC 900 material and 0.350 nm in JC 1000 material.
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The inter planar distance in JC 900 and JC 1000 is close to the d-values of graphene (002)
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typically corresponding to the interlayer distance of graphene sheets (0.34 nm)35, confirming the
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formation of good quality graphene like structured carbon. From the HR-TEM images, it is
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confirmed that JC 1000 exhibits a graphene-like layer structure with random orientation and
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number of graphene layers varies from 3 to 25. Further, the selected area electron diffraction
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(SAED) pattern (inset in Fig. 2H) reveals the arrangement of carbon atoms in a hexagonal 5 ACS Paragon Plus Environment
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lattice, indicating the formation of graphene layers. The presence of graphene like structure of JC
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1000 is further confirmed by the electron energy loss spectroscopic (EELS) analysis (Fig. 2I).
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The EELS spectrum of C K-edge shows two features for the near edge fine structure showing a
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narrow peak at around 285 eV (Transition from 1s to π* states - characteristic for the sp2
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hybridized carbon) and a sequence of peaks, ranging from 292 eV (Transitions from the 1 s to
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the unoccupied σ*).7 The shape and the intensity of the peaks suggest the presence of highly
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ordered sp2 carbon in JC 1000. The crystalline graphitic carbon is advantageous for
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supercapacitor application as it helps in improving the electronic conductivity of the ionic
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species and there by enhances the electrochemical performance at higher current rates. Above
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results conclude that the connected thin walls present in JC 900 (Fig. 2E) and JC 1000 (Fig. 2G)
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are graphene layers. To further confirm the above conclusion, the nanoporous carbons
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synthesized by the activation at different temperatures were also measured by XRD & Raman
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analysis and the results are shown in Figure 3. XRD patterns of nanoporous carbons prepared at
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various activation temperatures (JC 800, JC 900, and JC 1000) and non-activated JC are shown
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in Fig. 3A (a-d). The XRD pattern displays two characteristic diffraction peaks at 2 theta values
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of 25.8o and 43.3o, which are similar to 2 theta values of (002) and (101) reflections of graphitic
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carbon (JCPDS card no. 41 -1487). Interestingly, the (002) interlayer diffraction peak of JC 1000
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is much intense and narrow compared with materials (JC 800, JC 900 and JC-NA), implying the
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high crystalline nature of JC 1000. The peak of (002) and (101) reflections of JC 900 is slightly
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narrower but not as intense and narrow like JC 1000, indicating the existence of limited degree
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of graphitization in JC 900.36 On the other hand, the peaks of (002) and (101) reflections of JC
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800 and JC-NA are broad, indicating the presence of amorphous structure in the carbon. The
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graphitic characteristics of nanoporous carbons are further confirmed by Raman analysis and the
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spectra of JC carbons are shown in Fig. 3B (b-d). The Raman spectrum of carbon material
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synthesized without any KOH activation is also included in Figure 3B (a) for comparison with
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activated samples. All JC materials with/without KOH activation show peaks at ~1350 cm-1 and
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1581 cm-1, showing the presence of ordered graphitic carbon (sp2) and disordered carbon (sp3)
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respectively.37 The D band intensity is lesser than the G band intensity for KOH activated JC
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materials (JC 800, JC 900, and JC 1000) [Fig. 3B (b-d)] whereas the intensity of the D band and
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G band is nearly equal for JC-NA material [Fig. 3B (a)], indicating the presence of more sp2
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hybridized carbon in the KOH activated JC materials than JC material without KOH activation.
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Though all JC materials showed the presence of D and G bands, the quality of carbon is 6 ACS Paragon Plus Environment
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monitored by calculating the intensity ratio of D and G bands, i.e. (ID/IG) which is used to
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evaluate the ordered and disordered nature of carbon materials quantitatively.16 It is reported that
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carbon with less ID/IG ratio leads to formation of more graphitic (sp2) structured carbon than
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disordered (sp3) carbon in which former leads to improved electronic conductivity of electrode
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materials than the latter, which is advantageous for energy storage applications.38 The ID/IG ratios
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calculated for JC 800, JC 900, JC 1000 and JC-NA are 0.998, 0.557, 0.180 and 1.034
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respectively, implying that the ID/IG ratio decreases with increase in the activation temperature
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from 800 to 1000 oC. Particularly, when the temperature is increased to 1000 oC, the ID/IG ratio
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drastically decreases to 0.180, indicating the presence of high concentration of ordered sp2
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carbon in JC1000 material. Further, the characteristic 2D peak at 2700 cm-1, indicates the
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presence of graphene layers.39 It is to be noted that carbon materials namely JC 900 and JC1000
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exhibit 2D peak in the Raman spectrum, whereas carbon without activation and JC 800 does not
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show any 2D peak, revealing that chemical activation at high temperature induces graphitic
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properties in the carbon. Since the electronic conductivity of graphitic sp2 carbon is higher than
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disordered amorphous carbon38, it is expected that JC 900 and JC1000 may exhibit better
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electrochemical performance than JC 800 and JC without KOH activation. The enhanced degree
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of graphitization with the increase in activation temperature indicates that the process not only
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results in sheet like morphology, but also enhances the degree of graphitized carbon in the
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carbon synthesized from Jute stick based bio-waste.
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Nitrogen adsorption desorption isotherms are used to examine various textural parameters like
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specific surface area, pore volume and pore size distribution of nanoporous carbons (Fig. 4A &
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4B). As illustrated in Fig. 4A, all the nanoporous carbon samples exhibits type IV adsorption
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isotherms with H2 type hysteresis loop in the relative pressure region of 0.4–0.9 P/Po, which
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indicates the presence of mesopores.15 In addition, a steep increase of nitrogen uptake is
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observed for JC 1000 at low relative pressure (
