Preparation and characterization of hierarchical porous carbons derived from solid leather waste for supercapacitor applications

Accepted Manuscript Title: Preparation and characterization of hierarchical porous carbons derived from solid leather waste for supercapacitor applications Author: Niketha Konikkara L. John Kennedy J. Judith Vijaya PII: DOI: Reference:

S0304-3894(16)30592-1 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.06.037 HAZMAT 17826

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

19-3-2016 6-6-2016 19-6-2016

Please cite this article as: Niketha Konikkara, L.John Kennedy, J.Judith Vijaya, Preparation and characterization of hierarchical porous carbons derived from solid leather waste for supercapacitor applications, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.06.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and characterization of hierarchical porous carbons derived from solid leather waste for supercapacitor applications Niketha Konikkaraa, L. John Kennedya*, J. Judith Vijayab a

Materials Division, School of Advanced Sciences, Vellore Institute of Technology (VIT)

University, Chennai Campus, Chennai - 600 127, India b

Catalysis and Nanomaterials Research Laboratory , Department of Chemistry, Loyola College

(Autonomous) Chennai - 600 034, India

* Corresponding author Tel.: +91 4439931326; fax: +91 44 39932555. E-mail address: [email protected] (L.J. Kennedy).

1   

GRAPHICAL ABSTRACT

 

Highlights

 Solid leather waste was used as a precursor for preparing HPCs – waste to energy storage.  The textural, structural and morphological properties show the hierarchical porous nature  Porous carbon with surface area 716 m2/g and pore volume 0.4030 cm3/g has been produced

2   

 HPCs based supercapacitor electrodes are fabricated with three electrode system in 1M KCl  Specific capacitance of 1960 F/g is achieved at scan rate of 1 mV/s in 1M KCl.

Abstract Utilization of crust leather waste (CLW) as precursors for the preparation of hierarchical porous carbons (HPC) were investigated. HPCs were prepared from CLW by pre-carbonization followed by chemical activation using KOH at relatively high temperatures. Textural properties of HPC’s showed an extent of micro-and mesoporosity with maximum BET surface area of 716 m2/g. Inducements of graphitic planes in leather waste derived carbons were observed from X-ray diffraction and HR-TEM analysis. Microstructure, thermal behavior and surface functional groups were identified using FT-Raman, thermo gravimetric analysis and FT-IR techniques. HPCs were evaluated for electrochemical properties by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) by three electrode system. CLC9 sample showed a maximum capacitance of 1960 F/g in 1M KCl electrolyte. Results achieved from rectangular curves of CV, GCD symmetric curves and Nyquist plots show that the leather waste carbon is suitable to fabricate supercapacitors as it possess high specific capacitance and electrochemical cycle stability. The present study proposes an effective method for solid waste management in leather industry by the way of converting toxic leather waste to new graphitic porous carbonaceous materials as a potential candidate for energy storage devices.

3   

Keywords: Porous carbon, Activated carbon, Leather waste, Supercapacitors, KCl, Electrochemical behavior

1. Introduction Leather making is an old conventional process that serve up social needs, employs many skilled/unskilled persons while being one of the uppermost contributors of global market. Global production of leather is approximately around 4.8 billion sqft. It is projected that globally about 6.5 million tons of hides and skins are processed annually and 3.5 million tons of various chemicals are used in leather processing [1]. The leather industries discharge large amount of liquid effluents and solid wastes as a result of significant part of the chemicals used in the leather processing. The solid wastes include fleshings, wet blue splits, trimmings, shavings, buffing dust and etc [2]. The solid wastes from tanneries may have significant Cr(III) content. Even though Cr (III) is viewed as not toxic, possible oxidation of Cr(III) to Cr(VI), due to acid rains or incineration, threats the environment since Cr(VI) is a more toxic species. Leather tanning process uses Cr(III) with, among other tanning agents, leading to a product called wet blue leather, stable and inert due to a poly nuclear chromium–collagen complex, which is formed in the tanning process. Wet blue leather is one of the most used processed leathers, and therefore the tanning industries produce a large amount of solid waste containing chromium [2-4] The conventional disposal methods such as land-filling and incineration cannot be considered a solution to the disposal problem of tanned leather wastes in eco-friendly

4   

manner. In literature, there are few studies on the treatment of tanned leather wastes mainly, the extraction of chromium from wastes to re-use and isolation of protein fractions [5, 6]. Few investigations were done in this area for converting chromium and vegetable tanned leather shaving wastes to porous carbons by physical and chemical activation methods [5, 7]. Porous carbons which are prepared from leather wastes were mainly used for removal of toxic materials [3], organic dyes [4] etc. Although carbon prepared from leather waste has been used for adsorption studies, yet no reports are available for using leather waste carbon for supercapacitor applications. Supercapacitors also known as electric double layer capacitors (EDLC) or ultracapacitors are energy storage devices, exhibit a promising set of features such as high power density fast rates of charge-discharge, reliable cycling life and safe operation [8-13]. Due to these features supercapacitors are used in electric hybrid vehicles, digital communication devices, digital cameras, mobile phones, electrical tools, pulse laser techniques and uninterruptible power supplies [13].  According to the energy storage mechanism, supercapacitors are classified based on non-faradic and faradic process. In nonfaradic process, the electrolyte ions are accumulated at the electrode/electrolyte interface forming the double, while in faradic process; pseudocapacitance emerges due to redox reactions from electroactive species, such as polymeric materials, metal oxides and certain functional groups [14-16]. Nevertheless, hybrid supercapacitors, combine these two charge storage mechanisms (faradaic and non-faradaic), bringing about enhanced device characteristics. The energy storage capacity of a supercapacitor is controlled by the choice and structure of the electrode materials. In these materials porous carbons have attracted broad interest as an electrode material for energy storage and conversion because of its low 5   

cost, high specific surface area, excellent electrical conductivity and chemical stability, environmental friendliness, and long cycling life [17]. To overcome the limit of the iontransport kinetics in porous carbons, many efforts have been devoted to the design and fabrication of carbon-based electrode materials with a hierarchical porous structure. Hierarchical porous carbons (HPCs) are the materials with interconnected, three-dimensional, porous architecture with micro-, meso-, and macropores. HPCs offer a highly electrochemically active surface area, short diffusion distance, and high mass transfer rate [18]. They combine the advantages of organized multimodal porosity and are especially desired as electrodes for energy storage/conversion devices such as supercapacitors.  The micropores enhances the electrical double layer formation, the mesopores provides iontransport pathways with low resistance, and the macropores serve as ion-buffering reservoirs to reduce the diffusion distance [19]. As a result, carbon with hierarchical porous channels has been considered as one of the most promising electrode materials for EDLCs in the present days. Hence the present work is focused on the preparation and characterization of hierarchical porous carbons (HPC) from crust leather waste (CLW) that are thrown as solid wastes in dump yards. The main aim is to develop an electrode material from CLW for supercapacitor and to evaluate its electrochemical properties. The potential of converting hazardous leather waste to energy storage device concept are explored. 2. Materials and methods 2.1. Materials

6   

Crust leather wastes (CLW) were obtained from Sri Chamundi leathers, Kolappakkam, Chennai, Tamilnadu, India for the preparation of HPCs after through washing and drying. All chemicals used in the experiment were analytical of grade. Nafion solution was obtained from Sigma – Aldrich. Nickel foam was purchased from Winfay Group Company Limited, Shanghai, China. 2.2. Preparation of hierarchical porous carbons Hierarchical porous carbon (HPC) was prepared in the laboratory using the dried crust leather wastes (CLW) as the precursor material by two-stage process: precarbonisation and chemical activation. In the precarbonisation process, CLW was heated to 400 °C at the rate of 5 °C min−1 and soaked at the same temperature for about 4 h followed by cooling to room temperature at the same rate. This is labeled as precarbonized crust leather carbon (CLCP). In the chemical activation process, 30 g of the precarbonized carbon was agitated with 120 g of aqueous solution of KOH (3.56 M). The ratio of KOH to precarbonized carbon was optimized and fixed to 4. KOH and CLCP were thoroughly mixed at 50 °C for 18 h. After mixing, CLCP slurry was dried under vacuum at 110 °C for 1 h. The pyrolysis temperature was decided from the TGA recorded for CLW (Fig.9). From the plot it is observed that the major weight loss happened before 500oC and gradual weight loss due to the slow expulsion of light bound volatiles started taking place from 600oC till about 800oC creating porosities. However from 800 to 900 oC the weight loss is almost constant indicating the formation of a stable carbon structure. Hence in order to find the effect of porosity the pyrolysis temperature was varied from 600 to 900 oC. The resulting samples were then activated in a furnace at four different temperatures 600, 700, 800 and 900 °C, at a heating rate of 5 °C min−1 using a microprocessor controlled programmer and maintained at

7   

the same temperature for 2 h, before cooling to obtain hierarchical porous carbon (HPC). After cooling, the hierarchical porous carbons were washed several times with hot water, until the pH became neutral. The washed samples were dried at 110 °C for 1 hour to get the final product. The samples prepared at temperatures 600, 700, 800 and 900 °C were labeled as CLC6, CLC7, CLC8 and CLC9 respectively.

2.3. Material characterizations N2 adsorption – desorption isotherms of the porous carbons were measured using an automatic adsorption instrument (Micromeritics - ASAP 2020 unit) for the determination of BET surface area and total pore volume. Prior to the measurement the samples were degassed at 200 o

C for 12h. The nitrogen adsorption – desorption data were recorded at liquid nitrogen

temperature, 77K. The surface areas were calculated using BET equation [20]. The pore size distribution was determined using HK and BJH method. X-ray diffraction experiments were performed with Bruker, Model-D8 difractometer for 2θ values ranging from 10 to 80o using CuKα radiation at a wavelength of λ = 1.5406 Å. Surface morphology were observed using Hitachi S - 4800 high resolution scanning electron microscope. High resolution transmission electron microscope JEOL 3010 instrument was employed for viewing the microstructure of the porous carbons. Surface functional groups of porous carbon were analyzed by the Thermo scientific Nicolet iS10 infrared spectrometer. FT-Raman spectra were recorded using BRUKER RFS – 27 model interferometer. The spectra were recorded in the region 4000 - 50 cm−1 with Nd: YAG laser operating at 100 mW power continuously with 1064 nm excitation. Thermo gravimetric analysis of sample were measured by Exstar TG/DTA 7200 instrument from SII Nanotech instruments, under N2 atmosphere at a heating rate of 5 oC / min. The ultimate analyses 8   

such as carbon, hydrogen, nitrogen (CHN) contents were performed with Elementar Vario EL III CHN analyzer. The proximate analysis of the hierarchical porous carbon samples were calculated according to the standard procedure given in ASTM 3173-3175 standards. The yield of the porous carbon was calculated according to Eq.(1) [21].

Yield %



x 100

(1)

where, W0 is the original mass of the precursor on dry basis and W2, the mass of the carbon after activation, washing and drying. 2.4. Electrochemical measurements All the electrochemical characterizations like cyclic voltammetry (CV), galvanostatic

charge–discharge

(GCD)

measurements

and

electrochemical

impedance

spectroscopy (EIS) were carried out using a PARSTAT 4000 electrochemical impedance analyzer in three electrode system. Electrodes were prepared by mixing 95 wt. % porous carbons and 5 wt. % nafion to form a paste followed by drying at 70 oC in an oven for 2 h. The typical mass of the electrode material was approximately 0.5 mg. The carbon coated on the nickel foam, Ag/AgCl and platinum wire was used as the working, reference and counter electrode, respectively. 1M KCl served as the electrolyte. 3. Results and discussion 3.1.1. Nitrogen adsorption - desorption isotherms Fig.1 represents nitrogen adsorption/ desorption isotherms at 77K for CLCP, CLC6, CLC7, CLC8 and CLC9 samples respectively. All the samples except CLCP show a sharp 9   

increase in N2 adsorption at very low relative pressures indicating the micropore filling. IUPAC has classified the isotherms based on the shapes obtained at different relative pressures as type I to type VI isotherm curves. Each type signifies the porous or non-porous behavior. Type I isotherms are given by microporous solids having relatively small external surfaces, the limiting uptake being governed by the accessible micropore volume rather than by the internal surface area [22]. The isotherms of CLCP are of type I as the adsorption and desorption branches remain horizontal over the entire range of relative pressures indicating the microporous behavior .Conversely, for CLC6, CLC7, CLC8 and CLC9 an appearance of round knee at relative pressure (0.05 < P/Po < 0.15) show the gradual filling of wider micropores and approximate location of monolayer formation. As the relative pressure increases CLC6, CLC7, CLC8 and CLC9 is observed with a mild slope indicating multilayer formation. Type IV isotherm have hysteresis loop, which is associated with capillary condensation taking place in mesopores, and the limiting uptake over a range of high P/Po. Thus CLCP exhibits micropore structure while other samples show a mixed type I and type IV isotherms micropore and mesopore structure as defined by IUPAC classification [23, 24]. The uptake of nitrogen increased with increase in activation temperature up to 800 oC and relatively decreased at 900 oC. The highest value of N2 adsorption, 261 cm3/g achieved for CLC8 may be due to the creation of slit or cylindrical pores or pores of any kind. Marginal decrease in the nitrogen uptake for CLC9 (258 cm3/g) shall be due to pore contraction and shrinkage effects that can happen at higher temperatures.

10   

3.1.2. Surface area and pore volume CLCP had a low surface area of 3 m2/g and this indicates during precarbonisation process, the raw material was converted to carbon with the release of volatile components. The BET surface area of CLC6, CLC7 and CLC8 increased with increase in the activation temperature from 600 to 800 oC and thereafter decreased for CLC9. The BET surface area values for CLC6, CLC7, CLC8 and CLC9 are 628, 657, 716 and 613 m2/g respectively. The value of nitrogen adsorption capacities of the samples are in good agreement with that of the BET surface area. The increase in surface area for the samples can be ascribed to the liberation of definite volatile and organic components from the carbon matrix leading to pore evolution as a result of KOH impregnation and activation. The decrease in the surface area for CLC9 could be due to factors like contraction, coalescence and heat shrinkage of pores probably due to higher thermal profile (900 oC) employed [25]. The HPCs prepared at 600 to 900 oC possessed 57 – 93 % of micropore surface area whereas 7 - 43 % of mesopore surface area. The micropore surface area (Smic) was obtained by subtracting the mesopore surface area (Smeso) from the total BET surface area. As seen in Table.1 the micropore surface area increased from 584 m2/g to 608 m2/g for carbons activated at 600 to 800 oC, and thereafter decreased to 350 m2/g at 900 oC. The decrease in the micropore surface area at 900°C is due to extensive thermal activation which led to micropore widening to mesopores and macropores. As a result, surface area due to mesopore and macropores will increase. The mesopore surface area moderately increased from 44 m2/g to 108 m2/g from 600 to 800 oC and abruptly increased to 263 m2/g at 900 oC activation.

11   

Total pore volumes were estimated from nitrogen adsorption at a relative pressure of 0.99. Micropore volumes were determined from the t-plot method. The mesopore volume was estimated by subtracting the micropore volume from the total pore volume. It is observed that the pore volume slightly increases from 0.3150 cm3/g (600 oC) to 0.4030 cm3/g (800 oC) and reaches almost saturation at high temperature due to limited expulsion of volatile components are limited leading to the formation of stable carbon structure. Although CLC6, CLC7, CLC8 has higher micropore volume content than mesopore volume, the situation reverses with CLC9.

The

observed reversal with higher mesopore volume (0.2321 cm3/g) and lesser micropore volume (0.1672 cm3/g) in CLC9 could be due to the pore widening effect dominating the pore opening effects. 3.1.3. Pore size distribution The pore size distribution (PSD) for the HPCs obtained from Horvath-Kawazoe (H-K) model for micropores and Barrette-Joyner and Halenda (BJH) model for mesopore are shown in Fig. 2(a) & 2(b) respectively. During pyrolysis the crust leather waste is first transformed into aromatic structures without formation of any liquid phase intermediates. During this process many bonds in the aromatic structures are left incomplete and dangling, leading to creation of molecular level internal pore structure and carbonaceous solid development. The thermal profile employed would also favour the random ordering of imperfect aromatic sheets in the porous carbon resulting in incompletely saturated valencies and unpaired electrons. With increase in temperature this would lead to the formation of ordered and disordered carbons. The ordered carbon forms the stable graphitic structure while the disordered carbon forms turbostratic carbon with volumetric shrinkage. Micro pores and mesopores forms in the regions where the

12   

impingement of graphene sheets and turbostratic crystallites prevent further volumetric shrinkage. A pore begins as a structural defect formed when local decomposition of disordered carbon can no longer accommodate uniform crystallite growth. In addition the carbonization process may also expel some bound volatiles from deep inside the carbonaceous solid, escaping through the surface and forms cracks in the stable structure and hence become a source of porosity (micro-, meso- and macro pores) of different dimensions leading to hierarchical porous carbon structures. The average pore size of the hierarchical porous carbon samples increased with increase in activation temperatures. The pore creation, pore shrinkage and pore enlargement depends on the chemical activants and thermal profile employed. For CLC6, CLC7, CLC8 and CLC9, from Fig.2a the micropore PSD centered at 0.51 nm, 0.53 nm, 0.54 nm 0.55 nm and from Fig.2b the mesopore PSD centered at 4.18 nm, 4.11 nm, 4.14 nm 3.96 nm respectively. The average pore diameter obtained for CLC6, CLC7, CLC8 and CLC9 were 2.011 nm, 2.369 nm, 2.251 nm and 2.607 nm respectively. The increase in pore size for CLC7 in comparison to CLC6 shall be due to pore widening effect dominating the pore drilling effect. But the reduction in pore size for CLC8 in comparison to CLC7 may be due to the creation of additional micropores due to expulsion of bound volatiles at 800oC leading to pore drilling effect. Again for CLC9, the pore widening takes place due to saturation in removal of bound volatiles is reached. Thus simultaneous pore drilling and pore widening effects are observed. 3.2. X-Ray diffraction studies X-ray diffraction (XRD) patterns of the leather waste carbons are shown in the Fig.3. CLCP showed no diffraction peaks representing amorphous carbon and lacks the formation of

13   

stable crystalline structures. CLC6, CLC7 and CLC8 showed broad peaks at 2θ values of 26 o, 44o and 64o corresponding to (0 0 2), (1 0 0/1 0 1) and (2 2 1) planes of the graphitic structure and matches with the JCPDS card number 75 – 2078 [26]. However CLC9 show sharp and distinct peaks, at 2θ values of 26o in addition to the broad peaks at 44o and 64oindicating the activation temperature at 900 degrees favors the growth of the crystallites in the (0 0 2) plane direction. The peak at 64o may also be attributed to the presence of trace amount of Cr2O3 [27] present in all the carbon samples (JCPDS No: 82-1484) which is evident from the EDAX analysis. 3.3. Surface morphology Surface morphologies of CLCP and HPCs are shown in Fig.4. CLCP show that the surface is fibrous in nature. The collagen fibers that were originally present in the raw leather waste got converted into fibrous carbonaceous material. No significant pores are observed in CLCP. The precarbonisation temperature was insufficient enough to remove the volatile components and for the creation of well-defined porosity. In contrast CLC6, CLC7, CLC8 and CLC9 are witnessed with well-established pores on their surface due to removal of non-carbonaceous materials and extraction of other mineral components by the combined effect of the activation agents and the temperature employed. Burrow like pores of deep, narrow and broad dimensions are seen in the image illustrate that volatile components that were slowly expelled from interior of the carbon structures. EDAX analysis were carried out to determine the quantitative elemental composition of the prepared carbons (Fig.5). All the carbons showed the presence of chromium as it was added during the leather tanning process for the sake of bringing hardness to leather. HR-TEM images were taken on CLCP and CLC9 are shown in the Fig. 6(a).

14   

Additionally, selected area electron diffraction (SAED) pattern for CLC9 (Fig.6b) show the interlayer distance 0.3522 nm corresponding to (0 0 2) plane of graphite. 3.4. FT-IR analysis The functional groups present in the carbon materials would lead to the increase in the surface hydrophilic nature and wettability. In the FT-IR analysis band at 1600 cm-1 indicates the presence of water adsorbed during in the carbon samples. From Fig.7 it can be clearly seen that decrease in the intensity of band at 1600 cm-1 when the activation temperature is increased. Also the increase in the intensity of oxygen containing functional groups such as bands in the region of 1200 - 1000 cm−1 represented as –C–O– (e.g., hydroxyl or ether) and broad band at around 3400 cm-1 can be assigned to the –OH (stretching vibration of hydroxyl group) and other hydrophilic functional groups will increase the wettability and will lead to increase in specific capacitance as expected [28, 29]. The bands observed at 2840 and 2942 cm-1 are attributed to the symmetric and asymmetric stretching vibrations of -CH2- groups. Similarly the band at 2350 cm-1 represents C≡C stretching vibrations. The band at 1380 cm-1 is attributed to the bending vibrations of -CH2 groups. Finally the band at 660 cm-1 represents C-O-H twists and at 768 cm-1 is related to C-H out of plane vibrations in substituted aromatic rings. [30] 3.5. FT- Raman studies The FT-Raman spectra of CLC6, CLC7, CLC8 and CLC9 in Fig.8 shows D and G peaks at 1317 cm-1 and 1590 cm-1. The D peak is assigned to edge planes and disordered structures and is referred to as the disorder band or the defect band in carbon materials. The G peak is assigned to bond stretching of all pairs of sp2 atoms in both rings and chains of carbon materials [31]. The intensity of D band is slightly higher than the intensity of G band in all the hierarchical porous carbon samples [32] indicating the presence of certain degree of disorderness. Fig.8 also show 15   

the presence of a broad band around 2700 cm-1, known as the graphitic overtone or second harmonic of D band frequency and represented as G’. The G’ is not intense than the G band in any of the porous carbons and confirms the stacking of sp2 bonded planar graphene sheets as in graphite [33]. The intensity ratio of the G band to the D band (IG/ID) for CLC6, CLC 7, CLC 8 and CLC 9 are 0.63, 0.68, 0.75 and 0.86 respectively. Thus it is found that the increase in activation temperature of the carbon samples leads to increase in the degree of graphitization which in turn can have a contained relationship with the specific capacitance values. 3.6. TGA studies Fig.9. shows the thermo gravimetric analysis (TGA) curves of CLW, CLCP and HPCs.

It can

be seen that CLW, CLCP have similar TG curves with 3 stages: (i) dehydration stage, room temperature to 100 oC due to elimination of moisture (ii) acute weight loss stage 100 - 450 oC due to elimination of volatile compounds (iii) gradual weight loss 450 - 800 oC due to formation of stable carbon structures. For CLC6 to CLC8 at around 100 oC, moisture content present in the samples are eliminated and till 700 oC there is no sudden weight loss. However the curve gradually slopes down which may be due to elimination of tightly bounded volatile compounds that remained still even after activation. But CLC9 shows one step weight loss at 100 oC due to adsorbed moisture and at higher temperatures a stable curve indicating no possibility of volatile compounds to escape in the sample as stable porous carbon structure is formed. The limited moisture content for CLC9 may be due to low surface area and reduction in pore volume compared to other HPCs and very in particular, the reduction in micropore volume for CLC9 might have led to lower condensation or absorption of water vapor or moisture. 3.7. Chemical characteristics and production yield

16   

Proximate and ultimate analysis of the hierarchical porous carbon samples were done to find out the composition and yield of the leather waste carbon and the results are shown in the Table 2. Lower yield obtained at a higher temperature is caused by a larger release of volatile compounds and gasification of carbons. 3.8. Electrochemical characterization Cyclic voltammetry (CV) measurements were conducted to test the electrochemical performances of the crust leather carbons using 1M KCl. Fig.10 shows the cyclic voltammograms of CLW carbon based supercapacitors scanned from 1 to 500 mV/s. CV measurements are based on the principle of electric double-layer that is formed at the interface between porous carbon electrode and electrolyte. The CV curves exhibit excellent rate performance without any redox peak in the chosen voltage range. The specific capacitance of carbon electrode can be calculated from the cyclic voltammograms according to the following equation [34]

Csp =

(1)

where, Csp is the specific capacitance (F/g), is a current response in accordance with the sweep voltage ( A),

is the potential scan rate (V/s), V-Vo is the potential window (V) and m is the

mass of the electrode in grams. It is observed that CLCP presented a rectangular voltammogram shape indicating capacitive behavior of the crust leather carbons. For CLCP with surface area of about 3 m2/g, specific capacitance value of 70 F/g is observed at a scan rate of 0.001V/s (Table.3). This may be

17   

accredited to the presence of ultra small micropores which could not be explored by N2 adsorption [35]. HPCs show clearly superior performance over CLCP as electrode material and this can be evident from the Csp values of these carbons (Table.3). This high specific capacitance is attributed to the vastly developed surface area with a suitable pore size which leads to exceptionally high capability for charge accumulation in electric double layer of such carbons. Although CLC8 possessed the high BET surface area, CLC9 gives the highest capacitance of 1960 F/g at the scan rate of 1 mV/s (Table.3). This could be due to the increase in average pore size for CLC9. The increase in the specific capacitance values with increase in activation temperature increases the degree of graphitization and in turn reflects high edge plane fraction to that of basal plane in the graphitic structure evolution in HPCs. This would lead to an increase in HCP electrical conductivity and provide improved ion diffusion to happen quickly at the edge of the crystallites and contribute to enhance electrochemical performance and making HPCs ideal for EDLC application. [36, 37]. Furthermore, for CLC6, CLC7, CLC8 and CLC9 the voltammograms are gradually tilted but still maintains a rectangular like shape even at higher scan rates (0.5V/s) as shown in Fig.10. There are no appearances of redox peaks at any of the scan rate sweeps. Hence the tilting could not be assigned to the oxygen functional groups, but shall be due to the lower content of graphitization and considerable amount of micropore fractions resulting to electrode and electrolyte resistances. For all the carbon samples it is observed that specific capacitance (Csp) increases with a decrease in scan rate which is due to the fact that at a low scanning rate, there is adequate time for ions to diffuse into the inner pores of the electrodes, which is essential for the formation of electric double layers. Hence, more ions are adsorbed on the surface of the electrode, which is associated with a better capacitive behavior. Besides, the ohmic resistance for

18   

ions transportation in the pores is low at the low scanning rate, which do not hinders the formation of double layers. The charge discharge profiles of all the samples exhibit almost symmetrical triangular curves, which indicate good capacitive behavior, electrochemical stability, reversibility and cycle life [38]. The Galvanostatic charge/discharge curves of CLC9 at 2 mA are shown in the inset of Fig.11. A sudden potential drop observed in the beginning of the curve shall be attributed to the ohmic resistance in the electrode systems. The specific capacitances determined from charge discharge are calculated by Eq. (2)

Csp =

∆ ∆

(2)

where Csp is the specific capacitance, ‘I’ is the current,’t’ is the time in seconds,’V’ is the voltage and ‘m’ is the mass of the sample [39]. The obtained values of Csp are almost in good agreement with the values obtained from CV curves (Table.4). Fig.11 also shows the variation of specific capacitance with cycle number for CLC9 that shows stability of capacitance over a cycle of 500 revealing the good life time of the electrodes. Electrochemical impedance spectroscopy (EIS) of the porous carbon electrode in 1M KCl was conducted within the frequency range of 10 mHz to 1 KHz with 10 mV AC voltage. From the Nyquist plot in (Fig.12) semicircle at high frequency region should be related to the electronic resistance which includes the solution resistance of the electrolyte, charge transfer resistance of the electrode and equivalent series resistance of the electrode. Slope of the line close to 45 o at middle frequency ascribed to the diffusion of electrolyte ions in the electrode pores, while the inclined line at low frequency corresponds to the capacitive behavior [39]. To

19   

get a further insight into the charge transfer mechanism, the impedance spectra were fitted into an equivalent circuit (Fig.12 - inset) comprising of solution resistance (ohmic resistance of the electrolyte and internal resistance of the electrode materials, Rs) in series with the parallel combination of double layer capacitance (Cdl), interfacial charge-transfer resistance (Rct) and Warburg impedance (W). The factor Rs for all the samples were obtained in the range of 0.46 - 1.59 Ω which is attributed to resistance of KCl electrolyte. Here, Rct for CLCP carbons is 193 Ω and for all other porous carbon samples it’s below 10 Ω, a lower value of Rct leads to the shortening of the ion diffusion path which reflects the higher charge discharge performance. The third factor Cdl, provides information of the electrode capacitance behavior. Capacitance of the electrodes can be evaluated using the low frequency data of the Nyquist plot and it is given by Eq. (3)

Cdl

=

(3)

where ‘f ‘is the frequency and ‘Zim’ is the imaginary part of the impedance which is obtained from the y axis of the Nyquist plot [39]. A good correlation has been found in all the techniques even though values given by impedance spectroscopy are lower and that is typical for this method since alternating current penetrates into the pore with hindrance [39] (Table.4). Warburg impedance values are obtained in the range of 0.05 - 6.07 Ω corresponding to the electrolyte ion diffusion into the porous network of the of the carbon electrode material. Warburg impedance has small contribution towards the total impedance indicating good access for the electrolyte ions into the porous carbon material even at higher frequencies. 4. Conclusion

20   

This work investigates the preparation of hierarchical porous carbons (HPC) from crust leather waste by two stage method. The HPCs possessed BET surface area in the range 628 to 716 m2/g. The average pore size distribution was in the range 2.011 to 2.607 nm. Deep burrow like pores are exposed from the SEM images. XRD and HR-TEM results confirm the growth of graphitization HPCs corresponding to JCPDS card number 75 - 2078. The presence of surface functional groups in the CLW carbon samples were identified from the FT-IR spectra. Raman studies confirmed the graphitic nature of porous carbons by the appearance of D and G peaks. CV analysis gives good future prospects for leather waste porous carbons as supercapacitor electrode materials by yielding significantly high specific capacitance value of 1960 F/g for CLC9 in 1 M KCl. Lower the voltage scan rate, higher was the specific capacitance values. Charge/discharge profiles confirms the electrochemical stability, reversibility and excellent cycle life for HPCs. From the values of Rs, Rct, ESR and Cdl obtained from EIS studies suggest that hierarchical porous carbons developed from leather solid waste could be an excellent electrode material for supercapacitors. The present study carried out thus provides a proposal of “Waste to Energy” and can be applicable in the leather industries for solid waste management. Acknowledgments The first author sincerely thanks the VIT University Chennai management for providing financial assistance through Teaching Research Associateship. The authors are highly thank the authorities of Sri Chamundi Leathers, Kolappakkam, Chennai, for leather waste samples.

   

21   

References [1] J. Kanagaraj, T. Senthilvelan, R. C. Panda, S. Kavitha, Eco-friendly waste management strategies for greener environment towards sustainable development in leather industry: a comprehensive review, Journal of Cleaner Production, 89 (2015) 1 - 17. [2] J. Kanagaraj, K. C. Velappan, N. K. C. Babu , S. Sadulla, Solid waste generation in the leather industry and its utilization for cleaner environment- A Review, Journal of Scientific and Industrial Research, 65 (2006) 541 - 548. [3] I. C. Kantarli, J. Yanik, Activated carbon from leather shaving wastes and its application in removal of toxic materials, Journal of Hazardous Materials, 179 (2010) 348 - 356. [4] L. C. A. Oliveira, C. V. Z. Coura, I. R. Guimaraes, M. Goncalves, Removal of organic dyes using Cr containing activated carbon prepared from leather waste , Journal of Hazardous Materials , 192 (2011) 1094 - 1099. [5] A. Cassano, E. Drioli, R. Molinari, C. Bertolutti, Quality improvement of recycled chromium in the tanning operation by membrane processes, Desalination, 108 (1996) 193 - 203. [6] D. Petruzzelli, R. Passino, G. Tiravanti, Ion Exchange Process for Chromium Removal and Recovery from Tannery Wastes, Industrial and Engineering Chemistry Research, 34 (8), (1995), 2612 - 2617. [7] I. C. Kantarli, M. Yuksel, M. Saglam, J. Yanik, Conversion of leather wastes to useful products, Resources Conservation and Recycling, 49 (4) (2007) 436 - 448. [8] S. Bose, T. Kuila, A. K. Mishra, R. Rajasekar, N. H. Kim and J. H. Lee, Carbon-based nanostructured materials and their composites as supercapacitor electrodes, Journal of Materials Chemistry, 22 (2012) 767 - 784.

22   

[9] W. Ricketts and C. Ton, Self-discharge of carbon-based supercapacitors with organic electrolytes, Journal of Power Sources, 89 (2000) 64 - 69. [10] M. Jayalakshmi K. Balasubramanian, Simple Capacitors to Supercapacitors - An Overview, International Journal of Electrochemical Science, 3 (2008) 1196 - 1217. [11] Elmouwahidi, Z. Z. Benabithe, F. C. Martin and C. M. Castilla, Activated carbons from KOH-activation of argan (Argania spinosa) seed shells as supercapacitor electrodes, Bioresource Technology, 111 (2012) 185 -190. [12] D. Kalpanaa, S. H. Cho, S. B. Lee, Y. S. Lee, R. Misra , N. G. Renganathan, Recycled waste paper - A new source of raw material for electric double-layer capacitors, Journal of Power Sources, 190 (2009) 587- 591. [13] E. Frackowiak, F. Beguin, Carbon materials for the electrochemical storage of energy in capacitors, Carbon, 39 (6) (2001) 937 - 950. [14] H. Yang, M. Yoshio, K. Isono, R. Kuramoto, Electrochemical and Solid State Letters 5 (6) (2002) 141 - 144. [15] A. Nishino, Journal of Power Sources 60 (1996) 137 - 147. [16] M. Endo, Y. J. Kim, K. Osawa, K. Ishii, T. Inoue, T. Nomura, N. Miyashita, M. S. Dresselhaus, Electrochemical and Solid State Letters 6 (2) (2003) 23 – 26 [17] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Materials, 7 (2008) 845 - 854. [18] S. Dutta, A. Bhaumik , K.C.W. Wu , Hierarchically porous carbon derived from polymers and biomass: effect of interconnected pores on energy applications, Energy & Environmental Science , 7 (2014) 3574-3592.

23   

[19] F. Xu, R. Cai, Q. Zeng, C. Zou, D. Wu, F. Li, X. Lu, Y. Lianga, R. Fu, Fast ion transport and high capacitance of polystyrene-based hierarchical porous carbon electrode material for supercapacitors, Journal of Materials Chemistry, 21 (2011) 1970–1976. [20] S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, 2nd Edition Academic Press, London, (1982). [21] Y. Diao, W. P. Walawender, Activated carbons prepared from phosphoric acid activation of grain sorghum, Bioresource Technology, 81 (1) (2002) 45 - 52. [22] K. S. W. Sing , D. H. Everett , R. A. W. Haul , L. Moscou R. A. Pierotti, J. Rouquerol , T. Siemieniewska, Reporting physisorption data for gas/solid systems with Special Reference to the Determination of Surface Area and Porosity, Pure and Applied Chemistry 57 (1985) 603 - 619. [23] Y. Sudaryanto, S. B. Hartono, W. Irawaty, H. Hindarso, S. Ismadji, High surface area activated carbon prepared from cassava peel by chemical activation, Bioresource Technology, 97 (2006) 734 - 739. [24] P.B. Balbuena, K. E. Gubbins, Theoretical Interpretation of Adsorption Behavior of Simple Fluids in Slit Pores, Langmuir 9 (1993) 1801-1814). [25] J. Hayashi, A. Kazehaya, K. Muroyama, A.P. Watkinson, Preparation of activated carbon from lignin by chemical activation, Carbon, 38 (2000) 1873 - 1878. [26] Y. Wang, S. Gao, X. Zang, J. Li, J. Ma, Graphene-based solid-phase extraction combined with flame atomic absorption spectrometry for a sensitive determination of trace amounts of lead in environmental water and vegetable samples, Analytica Chimica Acta 716 (2012) 112 -118.

24   

[27] F. Ma, S. Chen, H. Zhou, Y. Li, W. Lu, Revealing the ameliorating effect of chromium oxide on a carbon nanotube catalyst in propane oxidative dehydrogenation, RSC Advances, 4 (2014) 40776 - 40781. [28] W. Tan, A. L. Ahmad, B. H. Hameed, Preparation of activated carbon from coconut husk: Optimization study on removal of 2, 4, 6-trichlorophenol using response surface methodology, Journal of Hazardous Materials, 153 (2008) 709 - 717. [29] L. Z Fan, T.T Chen, W. L Song, X. Li, S. Zhang , High nitrogen-containing cotton derived 3D porous carbon frameworks for high-performance supercapacitors. Scientific Reports. 5(2015) 15388 -15399. [30] Y. Arima and H. Iwata, Effect of wettability and surface functional groups on protein adsorption and cell adhesion using well-defined mixed self-assembled monolayer’s, Biomaterials 28 (2007) 3074 - 3082. [31] V.S. Channu, R. Bobb, R. Holze, Graphite and graphene oxide electrodes for lithium ion batteries, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 436 (2013) 245 - 251. [32] G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu, J. Yao, Facile synthesis and characterization of graphene nanosheets, The Journal of physical chemistry C, 112 (2008) 8192 - 8195. [33] A.C. Ferrari, D. M. Basko, Raman spectroscopy as a versatile tool for studying the properties of graphene, Nature Nanotechnology, 8 (2013) 235 – 246. [34] Y. Tang, T. Chen, S. Yu, Y. Qiao, S. Mu, S. Zhang, Y. Zhao, L. Hou , W. Huang , F. Gao , A highly electronic conductive cobalt nickel sulphide dendrite / quasispherical

25   

nanocomposite for a supercapacitor electrode with ultrahigh areal specific capacitance, Journal of Power Sources, 295 (2015) 314 - 322. [35] M. Abioye, F.N. Ani, Recent development in the production of activated carbon electrodes from agricultural waste biomass for supercapacitors: A review, Renewable and Sustainable Energy Reviews, 52 (2015) 1282 - 1293. [36] Misnon, N. K.M. Zain, R. A. Aziz, B. Vidyadharan, R. Jose , Electrochemical properties of carbon from oil palm kernel shell for high performance supercapacitors , Electrochimica Acta , 174 ( 2015) 78–86. [37] A.G. Pandolfo , A.F. Hollenkamp, Carbon properties and their role in supercapacitors , Journal of Power Sources 157 (2006) 11 - 27. [38] P. Karthika, N. Rajalakshmi, K. S. Dhathathreyan, Functionalized Exfoliated Graphene Oxide as Supercapacitor Electrodes, Soft Nanoscience Letters, 2 (2012) 59 - 66. [39] Jurewicz, S. Delpeux, V. Bretagne, F. Beguin, E.

Frackowiak, Supercapacitors from

nanotubes / polypyrrole composites, Chemical Physical letters, 347 (2001) 36 - 40.

26 

Figure

Fig.1. Nitrogen adsorption/desorption isotherms at 77 K of CLW carbon samples.

33

Fig.2 (a). Micropore distribution of HPC carbons using H-K method.

28   

Fig.2 (b). Mesopore distribution of HPC carbons using BJH method.

29   

Fig. 3. XRD patterns for CLW carbon samples.

30   

Fig.4. Scanning Electron Micrographs CLW carbon samples. 31   

32   

Fig.5. Energy dispersive analysis studies for the precarbonized and HPC samples, inset - EDAX analysis data for precarbonized and HPC samples. .

Fig.6(a). HR-TEM image of precarbonized carbon (CLCP) and HPC sample (CLC9).

33   

Fig.6(b). Selected area electron diffraction (SAED) pattern of CLC9 carbon sample.

34   

Fig.7. FT-IR spectra for CLW carbon samples.

35   

`

Fig.8. FT-Raman spectra for precarbonized and HPC samples. .

36   

Fig.9. Thermo gravimetric analysis (TGA) curves of crust leather waste, CLCP and HPCs.

37   

Fig.10. Cyclic voltammetry studies of precarbonized and HPCs in 1M KCl at different scan rates (0.001V/s, 0.0025V/s, 0.005 V/s, 0.01 V/s, 0.025 V/s, 0.05 V/s, 0.1 V/s, 0.25 V/s, and 0.5 V/s).

38   

Fig.11. Specific capacitance Vs cycle number and inset - Charge discharge curves of CLC9 at 20A/g

39   

Fig.12. Nyquist plots for precarbonized and HPCs (inset - high frequency region HPCs) and equivalent circuit for EDLC in 3 electrode system.

40   

Table.1. Sample identification, average pore diameter, surface area for precarbonized and HPCs

VMicro/ V Total

VMeso/ VTotal

%

%

0.0164

2.38

97.6

0.026

0.2347

0.0803

74.5

25.5

2.011

0.3893

0.2411

0.1482

61.93

38.07

2.369

108

0.4030

0.2642

0.1388

65.56

34.44

2.251

263

0.3993

0.1672

0.2321

41.87

58.13

2.607

S a Sample BET (m2/g)

Smicb (m2/g)

Smesoc (cm3/g)

VTotald (cm3/g)

VMicroe (cm3/g)

VMesof (cm3/g)

CLCP

3

1

2

0.0168

0.0004

CLC6

628

584

44

0.3150

CLC7

657

594

63

CLC8

716

608

CLC9

613

350

a

BET surface area.

b

Micropore surface area.

c

mesopore surface area.

d

Total pore volume.

e

Micropore volume.

f

Mesopore Volume.

g

Average pore diameter.

41   

D pg nm

Table.2. Ultimate analysis, proximate analysis and yield calculation for precarbonized and HPC samples. .

Ultimate Analysis Sample Code

Sl. No

1

Proximate Analysis

Carbon %

Hydrogen Nitrogen % %

Volatile Fixed Oxygen* Ash% Moisture% content carbon % % %

Yield %

49.02

4.01

10.60

36.37

20.9

7.5

19.6

52

40.0

2

CLCP CLC6

35.89

2.95

3.90

57.26

21

20.4

20.2

38.4

27.1

3

CLC7

30.10

4.65

3.61

61.64

21.1

21.4

24

33.5

23.9

4

CLC8

29.40

3.97

2.04

64.59

24.5

22.7

25.6

27.2

20.0

5

CLC9

23.55

3.08

1.16

72.21

29.6

11.8

35.8

22.8

17.7

*by difference

42   

Table.3. Specific capacitance values for precarbonized and HPC samples at different scan rates. .

Specific Capacitance (F/g) SCAN RATE(V/s)

CLCP

CLC6

CLC7

CLC8

CLC9

0.001

70

333

850

1183

1960

0.0025 0.005

49 25

236 133

350 208

540 341

1633 1223

0.01

15

62

127

243

985

0.025

7

24

67

162

879

0.05

4

16

62

131

514

0.1

3

10

56

75

195

0.25 0.5

1 0.65

5 3

32 22

41 38

135 42

43   

Table.4. Comparison of specific capacitance values obtained from cyclic voltammetry (CV) at a scan rate of 0.001V/s and galvanostatic charge discharge methods (GCD) at a constant current of 2mA and electrochemical impedance spectroscopy (EIS) for precarbonized and HPC samples.

SAMPLE CLCP CLC6 CLC7 CLC8 CLC9

Csp (CV) F/g 70 333 850 1183 1960

Csp (GCD) F/g 59 304 813 1130 1935

Csp (EIS) F/g 52 212 605 934 1650

Table.5. Parameters for precarbonized and hierarchical porous carbon samples obtained from electrochemical impedance spectroscopy measurements.

Sample

Rs (Ω)

Rct(Ω)

ESR (Ω)

W(Ω)

CLCP

1.30

193

191.7

0.051

CLC6

1.21

9.65

8.44

0.043

CLC7

1.59

5.89

4.30

0.305

CLC8

0.87

4.06

3.19

0.867

CLC9

0.46

2.13

1.67

6.070

 

44