polyaniline nanocomposite: A superior electrode material for asymmetric supercapacitor device

Journal Pre-proof Adsorbed Cr(VI) based activated carbon/polyaniline nanocomposite: A superior electrode material for asymmetric supercapacitor device Sourav Acharya, Sumanta Sahoo, Sonalika Sonal, Joong Hee Lee, Brijesh K. Mishra, G.C. Nayak PII:

S1359-8368(19)33813-2

DOI:

https://doi.org/10.1016/j.compositesb.2020.107913

Reference:

JCOMB 107913

To appear in:

Composites Part B

Received Date: 2 August 2019 Revised Date:

23 February 2020

Accepted Date: 23 February 2020

Please cite this article as: Acharya S, Sahoo S, Sonal S, Lee JH, Mishra BK, Nayak GC, Adsorbed Cr(VI) based activated carbon/polyaniline nanocomposite: A superior electrode material for asymmetric supercapacitor device, Composites Part B (2020), doi: https://doi.org/10.1016/ j.compositesb.2020.107913. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.

CREDIT AUTHOR STATEMENT

Sourav Acharya: Conceptualization, Methodology, Writing - Original Draft, Investigation, Sumanta Sahoo: Writing - Original Draft, Formal analysis. Sonalika Sonal: Writing - Original Draft, Investigation Joong Hee Lee: Review & Editing, Resources. Brijesh K. Mishra: Supervision, Visualization, Writing - Review & Editing G. C. Nayak: Supervision, Visualization, Writing - Review & Editing.

Graphical Abstract A cost-effective approach to mitigate two of the major problems of toxicity of mine water and energy storage have been proposed by fabrication of asymmetric supercapacitors through the adsorption of Cr(VI) from mine water with modified filter-derived activated carbon. The incorporation of Cr(VI) and subsequent conversion to Cr(III) enhanced the performance of the electrode towards energy storage. This approach will facilitate both water purification and development of efficient electrodes for energy storage.

Adsorbed Cr(VI) based Activated Carbon/Polyaniline Nanocomposite: A superior electrode material for Asymmetric Supercapacitor Device Sourav Acharyaa, Sumanta Sahooa, Sonalika Sonalb, Joong Hee Leec,d, Brijesh K. Mishra*,b, G. C. Nayak,*,a a

Department of Chemistry, Indian Institute of Technology (ISM), Dhanbad-826004 Jharkhand, India

b

Department of Environmental Science and Engineering, Indian Institute of Technology (ISM), Dhanbad-826004 Jharkhand, India c

Advanced Materials Institute of BIN Convergence Technology (BK21 Plus Global) and Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea d

Carbon Composite Research Centre, Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonju, Jeonbuk, 54896, Republic of Korea * E-mail Id of Corresponding authors: [email protected], [email protected]

Abstract: Combating heavy metal pollution and energy crisis together, while maintaining the green chemistry standard is a great challenge. Among heavy metals, the toxicity of hexavalent chromium compounds is well known one which can be adsorbed from water and can be used as an electrode for energy storage. In this work, a nanocomposite of exhausted activated carbon (extracted from commercial water filter) and polyaniline was used as an adsorbent for toxic Cr(VI) adsorption from mine water, and a high-performance asymmetric energy storage device was developed from it. Firstly, the adsorbent was explored for the proficient removal of toxic Cr(VI) from water. The adsorption data was found to be well fitted with the Langmuir isotherm, and the pseudo-second order kinetics suggested the monolayer chemisorbed nature. In the next step, the adsorbed material was investigated as the cathode material for supercapacitors. The nanocomposite based on Cr(VI)-adsorbed material exhibited a high specific capacitance of 219.6 F/g at 1 A/g current density and moderate cycling stability of 77% after 5000 charges/discharge cycles. An asymmetric device developed using this hybrid material achieved an elevated energy density of 60.8 Wh/kg in organic electrolyte. The device was used to light up a strip of 43 red LEDs connected in parallel.

1

Keywords: Nanocomposite; Heavy metal Pollution; Adsorption; Cr(VI); activated carbon; asymmetric supercapacitor.

1. Introduction: Heavy metals are those which have a relatively high density or high relative atomic weight. These are required by our body as micronutrients in definite concentration. Rapid industrialization and uncontrolled mining process have led to the release of these metals in higher concentrations in surface water. Heavy metal poisoning, one of the most burning problems of this century, occurs due to higher levels, i.e., in toxic amounts, of accumulation of heavy metals in various soft tissues of the body due to their non-biodegradable nature [1]. Chromium is one of those heavy metals which have now become one of the top priority contaminants according to Superfund priority list of hazardous substances [2]. The increased use of chromium is due to the rapid industrialization where it is explicitly used in the alloy and steel, tannery, paint, electroplating, and glass manufacturing industries [3]. Chromium mainly exits in three oxidation states (0), (III), and (VI). The dominant form of chromium in water is Cr(VI). Hexavalent chromium is dangerous for our body as it affects the physiological processes of the body and also has adverse effects on the lungs, liver, and kidneys leading to cancer [4-5]. The primary source of Cr(VI) in the surface water is untreated industrial effluents from the above-mentioned industries and mine water from chromite mines [5-6]. Depending upon the pH, Cr(VI) in waters is mainly present in the form of dichromate (Cr2O72-), chromate (CrO42-), and bichromate (HCrO4-) [7]. Cr(VI) concentrations of as high as 270 mg/L have been reported in industrial effluents and mine water in some parts of the world whereas the tolerance limit is 0.1 mg/L and 0.05 mg/L for discharge in inland surface waters and potable water respectively [8]. Thus, removing Cr(VI) from the water before releasing is very important. Many methodologies and technologies have been developed for the removal of Cr(VI) from water. Some of them include reduction and precipitation, ion exchange resins, electrochemical treatments, use of membrane technology, adsorption technologies, and various biological treatments [9-14]. Among all, adsorption treatment is the most widely used technique due to its versatility and economic viability. Among the adsorbents, activated carbon (AC) produced from cheaper, and readily available sources have served well as lowcost adsorbents [15-18]. Activated carbons developed from various sources have a high surface area, and with modifications can yield very high chromate removal efficiency from mine water [19]. They are an integral part of water filter systems currently deployed all over the world. Tap water supplied in houses across the world contains a variety of dissolved 2

metal ions. Thus a water-filter derived activated carbon would contain a number of adsorbed metal ions, which is beneficial for it to be used as an active material for energy storage. But for this filter derived activated carbon, the porous nature decreases due to the accumulation of adsorbed metal ions, thus blocking the active sites for adsorption. This, along with slow adsorption kinetics, makes it unsuitable for adsorption of metal ions. To get rid of this problem and to increase the contribution of the material towards energy storage electrode material, this activated carbon is further modified with a coating of polyaniline to get active sites for adsorption and also increase the conductivity of the material. Polyaniline (PANI) is one of the conducting polymer materials which have found a new use in the field of removal of chromate from mine water. Polyaniline synthesized in many forms have found uses as electrode material in energy storage, the adsorbent in metal ion removal, in electronics for making PCB boards etc [20-22]. Many of its composites are being developed which can efficiently remove Cr(VI) from water [22-23]. In this study, the nitrogen centers act as the main active sites where an ion-exchange type mechanism works to remove the chromium as shown in scheme 2.

Scheme 1. Synthesis of polyaniline by APS The adsorption technologies though very efficient and cost-effective have the problem of regeneration and waste disposal once lifetimes of the adsorbents are finished. Thus this results in increased maintenance problems which result in untreated water being disposed into water bodies. In recent years many works have come up regarding the use of chromium and its compounds as a supercapacitor electrode material. Both oxides and mixed oxides of chromium have shown to give a high specific capacitance for supercapacitor applications, while chromium nitride has also been used for supercapacitor applications [24-26]. Different oxidation states of chromium have also been used for redox flow batteries due to their true interchangeable nature [27]. The presence of adsorbed chromium on the polyaniline adds more pseudocapacitive nature to the material thus improving its charge storage nature. As shown in scheme 2 the hexavalent form of Cr is reduced to its trivalent form simultaneously 3

after adsorption and forms a chelate type complex with the nitrogen centers of PANI [28]. The trivalent form during the charging process gets converted into the divalent form and vice versa during the discharge process thus setting up a redox couple which along with PANI gives the pseudocapacitive nature to the material. The idea behind the current work is to give a value-added product from the toxic waste that is being dispersed into water bodies. This will both ensure better deployment of the adsorption system thus ensuring that heavy metal pollution decreases and also getting a useable product from it. With this view we have tried to combine the concepts of adsorption technologies and supercapacitors together to get a more sustainable and green way of both removing toxic chromium from mine water and its utilization for the development of energy storage devices.

Scheme 2. Adsorption and reduction mechanism of Cr(VI)

2. Materials and Methods: 2.1. Materials: The activated carbon was obtained from a commercial water filter system, used for 6 months. Polyaniline (PANI) used to modify the activated carbon was synthesized from aniline (99.5%, AR grade) as monomer and ammonium persulphate (APS) (98% purity) as initiator. Both were obtained from SRL PVT. Ltd. pH of the solutions was maintained by HCl (Merck, 37%) and 0.1 M NaOH (Fisher scientific, 97%). Commercial activated carbon was purchased from AKSHAR EXIM CO. PVT. LTD, India and activated charcoal were obtained from Merck (Particle size ~10-15 µm). For adsorption study a 1000 mg/L stock solution of potassium dichromate salt (Merck, 99.9%) was prepared. All the stock solutions were prepared using Millipore water. DMSO(99.9 %)used was purchased from Rankem PVT LTD and TEABF4 (99%) was purchased from Alfa Aesar.

2.2. Characterizations used: The characterizations of the materials were done by FTIR (Perkin Elmer, Spectrum Two), solid-state UV-Vis spectrophotometer (Agilent Cary 5000), FESEM (55, Carl Zeiss, Germany), SEM-EDX Mapping (Hitachi, S-3400N), TGA (NETZSCH STA 449F3), BET (

4

Quantachrome Nova Win ) and ED-XRF(Rigaku ED-XRF model NEX DE analyzer) analysis. The batch adsorption studies were performed in a thermostatic orbital shaker, Rivotek, India. The electrochemical analysis was done in Biologic SP 300. Samples for FESEM and SEM-EDX were dried overnight while for BET analysis the samples were degassed at 60oC for 7 hours.

2.3. Synthesis of the adsorbents: 2.3.1. Preparation of Filter derived Activated Carbon: Activated carbon in the granular form was extracted from a waste commercial water filter. This activated carbon was ground down into smaller particles using a kitchen grinder. The ground down particles was first sieved through a 100 mesh and then the resultant particles were sieved through a 150 mesh. Thus particles in size range of 100-149 µm were obtained. This particular size range was chosen for the study because it can both maintain the properties of activated carbon like high porosity and surface area and can also be very easily removed from the water after adsorption. The activated carbon so obtained was washed with water to remove the clay deposited from the water treatment process and then dried at 110oC overnight in a hot air oven. The activated carbon hence obtained was labeled as C.

2.3.2. Synthesis of Cl doped-Polyaniline (PANI): Polyaniline (PANI) was synthesized by in-situ polymerization of aniline by using APS as initiator as shown in scheme 1[29]. At first 1 mL of aniline was added to 50 mL of 0.1 M HCl solution and was stirred for about 15 mins. Then 2.54 g of ammonium persulphate (APS) dissolved in 50 mL of distilled water was added dropwise to the above solution, maintained at 0oC with continuous stirring. The mixture turned to deep green color upon the addition of APS. To complete the polymerization process the mixture was kept in the refrigerator for 7 hours. Solid PANI was then obtained by filtration in Whatman 42 filter paper and then washed with distilled water and ethanol several times. The product so obtained was dried at 60oC overnight and labeled as PANI.

2.3.3. Synthesis of Cl-doped PANI/Activated Carbon composite: The composite was prepared by the same method as shown in section 2.3.2. Firstly, 2.5 g of C was sonicated for 30 mins and then stirred for in 50 ml of 0.1 M HCl to get a uniform suspension. 1 mL of aniline was added to it and stirred to get a homogenous mixture. Then 2.54 g of APS dissolved in 50 mL of water was added dropwise to the mixture at 0oC. The addition of APS turned the solution to deep green color. The polymerization was completed by keeping the mixture in the refrigerator for 7 hours. The solid product was 5

filtered and washed with water and ethanol several times followed by drying at 60oC overnight. The obtained product was labeled as CP.

2.4. Batch adsorption studies: 2.4.1. Kinetic and equilibrium study: The adsorption studies were performed in a temperature-controlled thermostatic orbital shaker at 120 rpm at different temperatures. 50 ml of different Cr(VI) were prepared by diluting the stock solution. The pH of the solutions was varied from 2-10 by the addition of either 0.1 M HCl or 0.1 M NaOH. The adsorbent was kept constant to 0.1 g for all the adsorption studies except for dose optimization. Absorbents were separated from the solutions by filtration and the residual Cr(VI) for each solution was determined by UV-Vis spectrophotometer. The removal efficiency of Cr(VI) was calculated according to Eq. (1).



% =

X 100

(1)

Where Co and Ce are the initial and equilibrium concentrations of Cr(VI) in mg/L respectively. The equilibrium adsorption capacity qe in mg/g was calculated according to Eq. (2).

=

(2)

Where m is the mass of the adsorbent in g and V is the volume of solution used for adsorption in L. Adsorption isotherm for the adsorption process was investigated at three temperatures of 25ºC, 35ºC and 45ºC and fitted to isotherm models. For this study the concentration was varied from 20 to 240 mg/L at each temperature with the above-mentioned conditions at the natural pH of the solution. The maximum adsorption capacity at each concentration was calculated using Eq. (2). Also the thermodynamic parameters like ∆H°, ∆G°, ∆S° were evaluated from the equilibrium data. To assess the rate of adsorption of Cr(VI) on the adsorbent, the kinetic study of the adsorption was performed. For this study, 14 No. of 50 mL solution of 100 mg/L each taken in a 150 ml stoppered conical flask were prepared and shaken in an orbital shaker at each temperature (25ºC, 35ºC and 45ºC). Then the solutions were removed one by one at predetermined time intervals and were filtered and analyzed for residual Cr(VI) concentration. For further characterizations of the adsorbent, CP, after adsorption was saturated with Cr(VI). For this, 3 g of CP was taken in a 1 L closed round bottom flask containing 500 mL 6

of 1000 mg/L solution of dichromate and stirred for 4 hours at 45oC. Then the solution was filtered and the solid obtained was washed with water and labeled as CP-Cr.

2.4.2. Analysis of residual Cr(VI): The concentration of the residual Cr(VI) in the solution was calculated by UV analysis. Then from the data the amount of Cr(VI) adsorbed was calculated by subtracting the final conc. from initial.

2.5. Electrochemical Measurements: The electrochemical properties of the samples were measured by cyclic voltammetry (CV),

galvanostatic

charging-discharging

(GCD),

and

electrochemical

impedance

spectroscopy (EIS) using a conventional three-electrode system in 1 M TEABF4/DMSO as the electrolyte, at room temperature. The three-electrode set up was constituted of platinum foil as the counter electrode, Ag/AgCl electrode as reference electrode and sample-coated graphite rod as the working electrode. For preparing the working electrodes, the samples were mixed with carbon black (for better electrical conductivity) and PVDF (binder) in the ratio 80:15:5 and dispersed into NMP (solvent) followed by drop-casting on the flat top of the rod and dried overnight. For the asymmetric cell fabrication, the cathode was prepared by coating the electrode materials (active material:PVDF:carbon black = 80:5:15 in NMP) on stainless steel mesh (100 mesh) and anode was prepared by coating carbon black (Carbon black:PVDF = 90:10 in NMP) in another piece of stainless steel mesh. Whatman 42 filter paper and 1 M TEABF4 in DMSO was used as separator and electrolyte, respectively. The cell thus assembled was pressed between two wooden plates by a G-clamp and the electrochemical analyses were carried out. For device testing similar cells were set up and were tested by glowing 43 red LEDs connected in parallel and by running a 2 V DC mini fan. The specific capacitance (Fg-1) was calculated from the GCD analysis according to Eq.(3):

=

×∆

(3)

×

Where I is the charging and discharging current, ∆t is the discharge time in seconds, m is the mass of the sample loaded and V is the potential window. The energy density (Whkg-1) and power density (Wkg-1) were calculated according to Eq. (4) and Eq. (5) respectively.

= "=

!

.! ×#$ ∆

7

(4) (5)

3. Results and Discussion: 3.1. Structural Characterizations: FTIR analysis, shown in figure 1(a), was used to find the interactions among the materials. The C showed the characteristic peaks for activated carbons at 1400 cm-1 for bending of terminal C-CH3 and at 1653 cm-1 for C=C stretching [30-31]. PANI shows characteristic peaks at 1564 and 1481 cm-1 for C=C stretching of the quinoid ring and benzenoid ring, respectively while the peaks at 1293 and 1105 cm-1 are for the C-N stretching of the benzenoid ring and quinoid ring, respectively [22]. The above peaks are all present in the composite CP depicting the successful polymerization of aniline in the presence of C. The 1400 cm-1 peak of C is present in both the composites which show the presence of C in the prepared composites. Along with that the peaks of polyaniline at 1564, 1481, 1293 and 1105 cm-1 were shifted to 1577, 1495, 1300, and 1122 cm-1 for CP due to the interactions with the filter-derived carbon. The further shifting of peaks in the case of CP-Cr can be attributed to the formation of interaction between N of PANI and Cr due to the adsorption of chromate (HCrO4-) on the electropositive nitrogen followed by reduction to Cr(III), thus changing the electronic nature of N in PANI. This in turn changes the bonding properties of N. Solid-state UV-Vis analysis was carried out to study possible electronic transitions among the components. Figure 1(b) shows the solid-state UV-Vis spectra of the prepared materials recorded in the 200 to 800 nm range. The spectrum of CP showed broadband around 300 nm (due to the ᴨ-ᴨ* transition of the benzenoid ring of PANI), 370 nm (due to polaronic transition of PANI) and 680 nm (due to the bipolaronic transitions). However, in the case of CP-Cr the intensity of 370 nm peak has diminished and the peak at 680 nm shifted to 530 nm which can be assigned to the transitions of the quinoid rings of PANI. The diminishing of the peak at 370 nm and the shifting of the peak from 680 to 530 nm suggest the deprotonation of PANI indicating the reduction of adsorbed Cr(VI) to Cr(III). A similar observation was reported for the protonation/deprotonation of PANI in chitosan-graft-PANI composite in a previous study [32]. It can be concluded that the reduction was made by the lost protons (H+) of PANI on converting from benzenoid to quinoid ring. Therefore, the successful polymerization of aniline in the presence of C and the reduction of Cr(VI) in CPCr has been confirmed by FTIR and UV-Vis spectroscopy. However, to confirm the adsorption of Cr(VI), morphological analysis was done for the prepared materials.

8

Figure 1. (a) FTIR spectra and (b)Solid-state UV-Vis spectra of C, CP and CP-Cr.

3.2. Morphological Characterizations: To explore the surface morphology and coating FESEM analysis of adsorbent C, CP and CP-Cr were carried out and the micrographs were presented in figure 2. FESEM image of C revealed a highly porous structure (figure 2(a)) which is a common feature associated with activated carbons. Coating of polyaniline over C (in adsorbent CP) can be confirmed from figure 2(b) which shows the rougher surface as compared to C. A closer look of the surface showed fiber like PANI deposition on the carbon surface (inset of figure 2(c)). This fiber-like morphology could be beneficial for the chromate adsorption during mine water treatment. Form figure 2(b and c) it can be concluded that the coating of thread-like PANI on has been achieved during the synthesis process. Further, Figure 2(d) shows the morphology of PANI coated C after chromate adsorption which confirmed the stability of the structure during adsorption. Elemental mapping with EDX was carried out to confirm the adsorption of chromate in CP. Figure 2 (f-k) shows the elemental mapping of C, O, N, Cr, Al, and Si, respectively. The presence of Si and Al in the sample is due to the adsorption of clay particles on the activated carbon during the water filtration process. Figure 2(e) represents the corresponding SEM image of the mapped area. Figure 2 (l) and (m) represent the EDX plot of CP, before and after chromate adsorption where the presence of Cr can be confirmed after adsorption which indicates successful adsorption. In addition elemental mapping of Cr (figure 2(i)) shows the uniform adsorption of Cr on the CP. This adsorption could have resulted from the ion exchange type adsorption mechanism between Cl- and chromate ions.

9

Figure 2. FESEM images of (a) C (b) CP (c) Magnified image of CP (d) CP-Cr(e) SEM image of CP-Cr,(f-k) Elemental mapping of C, O, N, Cr, Al, Si in CP-Cr and EDX spectra of (l) CP (m) CP-Cr.

10

3.3. TGA analysis: To explore the thermal stability and decomposition process, TGA analysis was carried out. All the analysis were carried out in oxygen atmosphere in the temperature range 28oC to 800oC and shown in figure 3(a). All the materials showed an initial weight loss at around 100oC which can be attributed to the evaporation of surface adsorbed water molecules. However, an abnormal weight gain in the temperature range of 100oC to 450oC can be attributed to the formation of metal oxide of the different metals present in the filter derived activated carbon C. To confirm the composition of C, ED-XRF study was carried out and presented in supporting information (figure S1 and table S2shows qualitative and quantitative data of composition of filter derived activated carbon). As can be seen from XRF analysis the activated carbon contains a number of metal ions sourced from the water filtration process. These metal ions converted to metal oxides by reacting with atmospheric oxygen and hence resulted in weight gain in TGA. Followed by this weight gain, C showed significant weight loss at around 500oC,due to the decomposition of carbon to CO and CO2 which escaped as volatile components [34]. The TGA and DTG plots of CP and CP-Cr, presented in figure 3(a and b)showed the characteristic two-step degradation of polyaniline. Both of them showed a little mass gain after the evaporation of surface adsorbed water. The first decomposition around 400oC can be attributed to the loss of dopant HCl from the PANI matrix [34]. This is followed by significant decomposition around 500oC to 600oC related to the combined degradation of PANI and carbon [35]. Both CP and CP-Cr exhibit lesser residual weight %, at 800oC, than C due to decomposition of PANI. Among CP and CP-Cr, CP-Cr possessed higher residual weight which can be ascribed to the adsorbed chromium during the adsorption process. This further proves that the adsorption of chromate has taken place.

3.4. N2 adsorption-desorption study: Surface area, pore size and pore size distribution plays a critical role in the adsorption process. Hence, to find out the surface area and pore geometry BET nitrogen adsorptiondesorption study was carried out and presented in figure 3 (c and d). Table 1 shows the pore size data calculated by the BJH method. All the samples show a type IV like adsorption isotherm and are mesoporous in nature [36]. From the table, it is evident that surface area of CP reduced as compared to C which can be due to deposition of polyaniline which has blocked some of the pores of C. However, an increase in surface area was observed for CP-Cr which can be ascribed to the additional contribution from adsorbed chromium. We believe

11

adsorption and coordination of chromium might have altered the chain orientation of polyaniline which might have to open up some of the pores blocked by polyaniline coating. This resulted in increased surface area and average pore volume for CP-Cr.

Figure 3. (a) TGA plots (b) DTG plots (c) BET surface area and (d) Pore diameter distribution of C, CP and CP-Cr. Table 1. Surface area and pore size distribution data

Sample

Surface area (m²/g)

Avg. Pore Diameter (nm)

Avg. Pore Volume (cc/g)

C

196

3.9

2.2

CP

74

3.3

0.99

CP-Cr

124

3.3

1.05

3.5. Batch adsorption studies: 3.5.1. Effect of initial pH: To study the effect of pH on the adsorption of chromate on CP 9, 50 mL solutions of 80 mg/L each were prepared and their pH was maintained from 2-10.0.1 g of adsorbent CP 12

was added to each solution and shaken in an orbital shaker at 120 rpm for 2 hours. The pH variation data represented in figure 4(a) shows that a wide pH range of 4-8 is suitable for the adsorption of chromate. The data is well correlated to the Pzc study of CP represented in figure S3 which shows that above pH=2.5 the surface is positively charged hence facilitating Cr(VI) adsorption. This can be attributed to the presence of Cr(VI) in the form of CrO42-, HCrO4- and Cr2O72- depending upon the pH of the medium and the mechanism of adsorption followed[7]. Adsorption of chromate over CP follows an ion-exchange type mechanism (shown in scheme 2) where Cl- attached to electropositive nitrogen is exchanged with the chromate species present in the solution and is subsequently reduced to Cr(III) and stabilized in the PANI framework [37]. This mechanism is indicated in the UV-Vis analysis and EDX analysis of CP and CP-Cr samples which shows the deprotonation of nitrogen centers and the exchange of Cl- ions with the chromate ions. However at higher pH due to the deprotonation of the nitrogen centers, the primary electrostatic attraction decreases leading to a decrease in adsorption. Thus for the rest of the study the batch adsorption experiments were carried out at the natural pH of the solutions.

3.5.2. Dose optimization: The effect of various doses of the adsorbent is shown in figure 4(b). The doses taken were 0.01, 0.05, 0.1, 0.15 and 0.2 g respectively and the initial concentration of the solution was 80 mg/L. The solutions in their natural pH were shaken in an orbital shaker for 2 hours at 120 rpm. The percentage removal obtained were 48.3%, 73.7%, 99.8%, 97.4% and 90.7% for 0.01, 0.05, 0.1, 0.15 and 0.2 g respectively. The increase of chromate adsorption with the increase in dose is due to the availability of more active sites for adsorption while for the larger amount of adsorbent dose overcrowding of particles diminishes adsorbate adsorbent interactions due to the attainment of equilibrium [38]. Thus a dose of 0.1 g (2 g/L) was obtained as the optimized dose amount and is used for the rest of the study.

3.5.3 Kinetics of absorption: The study of the kinetics of the adsorption process gives us an insight into the mechanisms of adsorption. Kinetics of an adsorption process is controlled by three steps, transport of solute from bulk to the surrounding of adsorbent, surface adsorption of the solute and intra-particle diffusion [39]. The adsorption showed fast kinetics as shown in figure 4(c) with a saturation time of 10 to 20 mins. This is quite high as compared to other adsorbents and can be attributed to the rapid exchange of Cl- ions and the chromate ions in the solution.

13

The adsorption data collected at different time intervals were fitted to the pseudofirst-order model and pseudo-second-order model respectively. A simple pseudo first order Eq. is represented in Eq. (6).

%&'

= %&'

−

−

)

(6)

!.# #

Where qe and qt are the mass adsorbed of Cr(VI) in mg/g at equilibrium time and at any time t respectively, and k1 (min -1) is the rate constant of 1st order reaction. k1 is calculated from the slope of the straight line obtained in the plot of log (qe-qt) vs t. The data did not fit well with this model. The Eq. for the pseudo-second order adsorption at equilibrium is shown in Eq. (7).

= )

!

!

+

(7)

where k2 is the pseudo-second order rate constant (g/mg min) and qe and qt are the mass adsorbed at equilibrium and at any time t respectively. qe and k2 can be calculated from the slope and intercept of the graph between t/qt vs. t respectively. The adsorption data fitted better to the pseudo-second order model. The fit of the second-order kinetic model is shown in figure 4(d) and the corresponding calculations are shown in table (2). The correlation coefficient of fitting for the 25ºC, 35ºC and 45ºC sets are 0.98, 0.98 and 0.97 showing the excellent fitting of the data to the pseudo-second order model. The values of k2 and qe are shown in table (2). Thus it is indicated that the rate of the adsorption depends on two factors, the concentration of the active sites (the electropositive nitrogen centers) and the concentration of the adsorbate (Cr(VI) concentration). This is in accordance with our proposed mechanism that Cr(VI) adsorbed is reduced to Cr(III) at the nitrogen center and thus the adsorption follows a pseudo-second order model. Table 2: Kinetic parameters for the removal of Cr(VI) by CP Temperature (oC)

Pseudo-Second order

Pseudo-First order R2

qe

k2

(mg/g)

(g/mg min)

25

38.02

0.0065

35

42.14

45

47.4

R2

qe

K1

(mg/g)

(min-1)

0.98

11.1

0.00276

0.46

0.0036

0.98

17.03

0.00392

0.52

0.0021

0.97

32.1

0.00322

0.7

14

3.5.4 Adsorption isotherms: To find out the isotherm followed by the adsorption, initial concentration of the solutions were varied from 20 mg/L to 240 mg/L at25ºC, 35ºC and 45ºC and the data obtained were fitted to Langmuir isotherm and Freundlich isotherm. The linear form of Langmuir isotherm is represented in Eq. (8) as:

=

+ ,-

+

+

(8)

Where Ce is the equilibrium concentration in mg/L, qe is the mass adsorbed in mg/g at equilibrium, qmax is the maximum mass adsorbed in mg/g that the adsorbent can absorb at that temperature. KL (L/mg) is the Langmuir constant. The qmax and the KL can be calculated from the slope and intercept of the graph between Ce/qe vs. Ce which is a straight line. The linear form of Freundlich isotherm is expressed in Eq. (9) as:

=

,. +

(9)

Where KF is the Freundlich adsorption coefficient and n is the nonlinearity of the isotherm associated with the sorption intensity. The plot of ln qe vs. ln Ce gives a straight line and the values of KF and n can be calculated from the intercept and slope respectively. The correlation coefficient for the fit of the adsorption data to the Langmuir model shown in figure 4(e) was 0.99, 0.98 and 0.99 for 25ºC, 35ºC and 45ºC indicating a better fit to the model. The values of KL and qmax as calculated from the graph are reported in table 3. KL represents the extent of adsorption of Cr(VI) over the adsorbent and also talks about the feasibility of the adsorption process. The maximum adsorption capacity of CP for the adsorption of Cr(VI) was calculated from the Langmuir isotherm to be 64.5 mg/g at a temperature of 45ºC. The fit of the data to the Langmuir isotherm points to the fact that the adsorption is monolayer in nature. This complements our proposed mechanism of adsorption that there is one to one interaction between the adsorbed Cr(VI) and the electropositive nitrogen center. This monolayer formation points to the fact that the physical adsorption is restricted for the adsorbate-adsorbent pair. This increases the binding of the Cr(VI) to the PANI chain due to minimized desorption (as all obtained) thus increasing the stability of the cathode of the asymmetric device fabricated at a later part of the study.

15

Table 3. Isotherm parameters for the removal of Cr(VI) by CP Temperature o

( C)

Langmuir

Freundlich R2

nF

KF

R2

44.8

0.99

4.56

16.716

0.48

0.221

50.26

0.98

5

19.1

0.54

0.112

64.516

0.99

2.8

28.03

0.48

KL

qmax

(L/mg)

(mg/g)

25

0.3038

35 45

Thermodynamic parameters: To better understand the nature of the sorption behavior and thermodynamic feasibility of the adsorption, the thermodynamic parameters like entropy (∆S), enthalpy (∆H) and Gibbs free energy change (∆G) for the chromate adsorption were calculated at different temperature, which also showed whether the adsorption of Cr(VI) over CP is endothermic or exothermic. ∆G at each temperature was calculated according to Eq. (10):

∆/ = − 0

,

(10)

where R is the universal gas constant with value 8.314 J/(mol K), T is the temperature and Kc is the equilibrium constant which is calculated as in Eq. (11):

, =

(11)

Kc, ∆S and ∆H are related to each other by Eq. (12) as:

%1 , =

∆2

−

∆3 0

(12)

Thus by plotting a graph between Kc and 1/T we can calculate the values of ∆S and ∆H from the intercept and slope respectively. The graph of ln Kc vs. 1/T for the adsorption is shown in figure 4(f) with the corresponding values of ∆G, ∆H and ∆S shown in table 4. The positive ∆H value of the adsorption indicates towards its endothermic nature. The chromate ions in water are well solvated and thus some extra energy is needed to overcome this force to create the adsorbate-adsorbent interactions. Also the adsorption of Cr(VI) and its conversion to Cr(III) (presence of which is indicated from solid-state UV-Vis spectra and latter from the CV analysis) by reduction needs some extra energy. This all adds up to give a positive enthalpy for the process. The positive ∆S can be attributed to the release of Cl- from PANI at the adsorbent-solution interface [22]. The negative ∆G value shows the feasibility of the adsorption process. On increasing the temperature ∆G becomes more negative which can be attributed to the fact that at a higher temperature the Cr(VI)-CP interaction increases and also 16

the reduction of Cr(VI) to Cr(III) is increased. Thus the adsorption of Cr(VI) on CP is spontaneous and endothermic in nature. Table 4. Thermodynamic parameters for the removal of Cr(VI) by CP Temperature (K)

∆Go (kJ/mol)

25

-1.145

35

-2.527

45

-5.847

∆Ho (kJ/mol)

∆So (JK/mol)

68.590

233.5

Figure 4. (a) Effect of initial pH of solution on adsorption of Cr(VI) (initial concentration 80 mg/L, vol of solution 50 mL, amount of CP 0.1 g, temperature 25oC. )(b) Effect of adsorbent dose on adsorption of Cr(VI) (initial concentration 80 mg/L, vol of solution 50 mL, temperature 25oC.) (c) Effect of contact time on adsorption of Cr(VI) (initial concentration 140 mg/L, vol of solution 50 mL) (d) Fit of kinetic adsorption data at 25ºC, 35ºC and 45ºC using pseudo-second-order model (e) Fit of adsorption data to Langmuir isotherm at 25ºC, 35ºC and 45ºC (f) Plot to calculate the thermodynamic parameters.

3.5.5. Mine water adsorption study: To see the practical ability of the adsorbent CP towards the removal of Cr(VI) from mine water in the presence of other ions, a time variation adsorption study of Cr(VI) from 17

mine water collected from a chromite mine wash was carried out. The concentration of Cr(VI) in the water was 1.28 mg/L. For the study we used the previously optimized conditions of 0.06 g (2 g/L) adsorbent dose, the temperature of 45ºC and natural pH of the solution and used a volume of 30 mL. Time was varied from 0 to 60 mins as concluded that the time of attainment of equilibrium is 10 to 20 mins and collections were taken at regular intervals. The data is summarized in figure 5 shows that a higher time of around 30 mins was required to reach the equilibrium due to the competition from the other existing ions. The maximum percentage of removal obtained was 77% after 60 min.

Figure 5. Adsorption of Cr(VI) by CP from mine water.

3.6. Electrochemical analysis: To evaluate the electrochemical properties of the materials, cyclic voltammetry (CV), galvanostatic charging-discharging (GCD), and electrochemical impedance analysis (EIS) were performed in the conventional 3-electrode system within the voltage range of 0 to 1.2 V using TEABF4/DMSO as the electrolyte. The electrochemical performance of any materials depends on a few factors like electrolytes, working potential limit etc. Therefore, we have chosen the electrolyte and potential range on the basis of the following factors. Herein, the conventional aqueous electrolyte like KOH has not been used due to the presence of PANI, which generally degrades in basic pH (>10). Additionally, KCl has not also been used since the electrode material does not exhibit pseudocapacitive behavior, which is the prime requirement of this study. The voltage range was selected based on the thermodynamic degradation of water (1.23 V), which may be associated with DMSO during analysis. To further investigate the practical ability of CP-Cr, its electrochemical properties were tested in a 2-electrode asymmetric cell system with carbon black as a negative electrode and sample as a positive electrode. The full cell set up was constructed by using stainless steel (SS) mesh as a current collector, TEABF4/DMSO as the electrolyte, and the commercial

18

filter paper as a separator. Moreover, the practical utilization of this asymmetric cell was done by glowing a series of LEDs and operating a 2 V Mini fan.

3.6.1. 3-electrode Study: The cyclic voltammetric analysis of the electrode materials is represented in Figure 6. Figure 6(a) shows the cyclic voltammetry comparison of the different materials in the voltage range 0 to 1.2 V at 2 mV/sec scan rate. Sample C exhibits a rectangular-shaped CV curve indicating the typical EDLC behavior activated carbon [40]. However, the coating of polyaniline over C adds pseudocapacitive nature to it as evident from the redox peak in the CV curve of CP at around 0.4 V, which corresponds to the variable oxidation states of nitrogen in polyaniline. As compared to C, the minute decrease in the area of CV curve of CP is caused by the decrease in surface area as evident from the BET analysis. However, it has been observed that, despite having a much lower surface area, there is a minute decrement in the area of the CV curves of CP, as compared to C. Herein, it can be concluded that the additional capacitance contribution is caused by the pseudocapacitive PANI. On the other hand, there is an increase in the CV curve area for CP-Cr than the other two electrode materials. This increase is due to the facile electron transfer, which is caused by the increased interaction between the Cr(III), obtained from the reduction of adsorbed Cr(VI) with the iron present in C (presence confirmed from ED-XRF analysis as shown in figure S1) due to the setting up of iron/chromium redox system. This redox system is a well-established part of the Fe(III)/Cr(III) redox flow cell [27]. The increase in the CV area of CP-Cr as compared to CP can also be ascribed to the increase in EDLC contribution from the increase in surface area from CP to CP-Cr (as evident from Bet surface area analysis). To find out the Cr(III)/Cr(II) redox couple, CV analysis was performed at 20 mV/sec within the voltage range 0 to -0.8 V and the corresponding CV curves are shown in Figure 6(c). The CV curve of CP-Cr showed a peak around -0.4 V, corresponding to the Cr(III)/Cr(II) redox couple, which confirms the efficient Cr(III) adsorption. Figure 6(b) shows the CV scans of CP-Cr in different scan rates ranging from 2 to 50 mV/sec. The general trend of increased CV curve area with increasing scan rate is clearly noticeable, which occurs due to the slower diffusion kinetics at a lower scan, caused by the

formation of a double-layer far away from the electrode surface

generating a lower flux and hence lower current density response [29]. To further establish the use of CP-Cr as a positive electrode material, the CV of CP-Cr was performed in TEABF4/Acetonitrile electrolyte at 5 mV/sec represented in figure S3. The plot clearly shows a peak around 1.2V which corresponds to the Cr(VI)/Cr(III) couple, which is the active redox

19

reaction during the use of the material as a positive electrode. The peak around 0.9 V in the plot is due to the redox behavior of polyaniline. To study the practical applicability of the samples towards energy storage, GCD analysis of the samples was performed in a potential range of 0 to 1.2 V. Figure 6(d) shows the comparison of the electrode materials at 1 A/g scan rate . From the discharge time of the materials, their specific capacitance was calculated. CP-Cr exhibited a maximum capacitance of 219.6 F/g, which is found to much higher than both CP (57.6 F/g) and C (33.9 F/g). The superior discharge time for CP-Cr can be attributed to the slow redox reactions that are occurring between the Cr(III) and Fe. Moreover, the coating of conductive PANI facilitates the movement of electrons from the adsorbed Cr and Fe to the current collector. The charge/discharge profiles of CP-Cr at different current densities of 2, 3, 4, 5, and 10 A/g indicates typical electrochemical behavior (Figure 6(e)). Herein, the gradual decrease of discharge time with increasing the current density is attributed to the lesser availability of time for the occurrence of redox reactions. Most importantly, even at a high scan rate of 10 A/g, the electrode exhibited a moderate capacitance of 52.4 F/g. The capacitive contribution of adsorbed Cr is evident from the increase in capacitance value of CP-Cr, as compared to CP. The result of GCD analysis at different current densities is summarised in figure 6(f).

Figure 6. (a) CV comparison of the different materials at 2 mV/sec (b) CV of CP-Cr at different scan rates (c) CV of composites in 0 to -0.8 V at 20 mV/secs (d) GCD plots of the

20

different materials at 1 A/g (e) GCD plot of CP-Cr in different current densities, (f) Bar plot showing specific capacitance variation with current density. To study the electron charge storage and flow mechanisms in the materials, the EIS study was carried out at open circuit potential in the range 1 to 100000 Hz and represented in Figure 7(a). The semicircle type area in the high-frequency region of the Nyquist plot is represented as RCT (charge transfer resistance), which reflects the ease of electron transfer within the electrode and at the electrolyte-electrode surface due to the various reactions occurring in the electrode. The straight line obtained in the low-frequency region is mainly due to the capacitive behavior of the materials. The part in front of the semicircle indicates the combination of contact resistance and solution resistance (denoted by RS). The Nyquist plots of the materials, shown in Figure 7(a) show their corresponding fitted curves to the circuit, shown in the inset of figure 7(a). Moreover, the corresponding fitted data are represented in table S5. The RCT value of CP-Cr is found to be minimum among the three, indicating the trouble-free flow of electrons between Cr(III)/Fe system and PANI (which also contributes due to its variable oxidation states). The contribution of PANI in the case of CP also lowers its RCT than C which does have only EDLC behavior. The addition of conducting PANI improved the internal resistance of the electrodes, which is confirmed by the lowering of RS value for CP and CP-Cr in comparison with C. On the other hand, the straight line at low-frequency region is found to be much steeper toward y-axis in case of C, indicating higher EDLC behavior than the other two. A similar trend has been observed in the case of the Bode plot (Figure 7(b)). This particular phenomenon is also well-supported by the n value, which is found to be higher than the others. The Warburg impedance (W) designates the ease of movement of electrolytes inside the pores of the material. The low W value of C can be correlated to the larger pore diameter and surface area. However, in the case of CP and CP-Cr, the high value of W can be explained by the closing up of pores of the activated carbon through PANI coating. However, CP-Cr exhibits slightly higher W value as compared to CP, which can be associated with the additional blocking of pores though Cr adsorption. Therefore, the occurrence of different electron transfers within the electrode material and at the electrode-electrolyte surface has been verified by EIS analysis. To further evaluate the stability of the materials, GCD analysis was carried out at 10 A/g for 5000 cycles (Figure 7(c)). Herein, C retained almost 100% of its initial capacitance indicating no structural degradation or deformation during cycling. On the other hand, CP and CP-Cr exhibited 81% and 77% capacitance retention, respectively. The decrease in cycling stability in the case of the composites was caused by the structural deformation due to 21

the volumetric swelling and shrinkage of PANI. For further investigation, we compared the CV curve of CP-Cr before and after 5000 cycles at the scan rate of 10 mV/sec (inset of Figure 7(c)). The reduction of the area of the CV curve from the 1st cycle to the 5000th cycle can be attributed to the material loss of the electrode during the cyclic stability analysis. To analyze whether Cr(VI) has been leached out from the electrode surface and present in the solution, UV-Vis analysis of the electrolyte was performed after cycle test and compared to a 50 mg/L solution of dichromate (Figure 7(d)). From the spectra, it is clear that the peaks obtained at 270 nm and 370 nm corresponds to polyaniline, whereas the peaks at 250 and 350 nm attributes to dichromate, respectively [41]. Therefore, we conclude that during continuous charging/discharging, a little amount of polyaniline has been detached from the electrode surface, accounting for the material loss of the electrode. The increment of the capacitance during the initial cycles (up to 500 cycles) can be attributed to the formation of new Cr(III) centers by gradual reduction of Cr(VI) and the activation of the electrode material. However, once this conversion was over, the degradation of electrode material took on the dominant form and hence the capacitance decreased.

Figure 7. (a) Nyquist plot of the materials (b) Bode plot of the materials (c) % Retention of specific capacitance of the materials at 10 A/g (inset showing CV analysis of CP-Cr at 10 22

mV/sec scan rate for 1st (black) and 5000th (red) cycle) (d) UV-Vis spectrum of electrolyte after cyclic GCD analysis of CP-Cr.

3.6.2. 2-electrode study: The practical utility of CP-Cr was further analyzed by fabricating an asymmetric supercapacitor cell through coating the 2 cm × 2 cm area of both the cathode and anode, as mentioned in section 2.5. In order to check the feasibility, the electrochemical performance of the CP-Cr device was also compared with the devices of chromium-adsorbed commercially available activated carbon (PANI-newAC-Cr) and chromium-adsorbed commercially available activated charcoal (Char-PANI-Cr). Devices for these two materials were fabricated by following a similar procedure as that of CP-Cr. From the CV analysis, the area under CV curves (at the scan rate of 10 mV/sec) is found to be similar for CP-Cr and Char-PANI-Cr, whereas for PANI-newAC-Cr it is slightly more, which can be attributed to the higher effective surface area of commercial AC (Figure S6(a)). The shape of the GCD plots deviates from the ideal rectangular shape indicating the pseudocapacitive behavior (Figure S6(b)). The specific capacitance for the devices was calculated from the GCD analysis according to Eq. (14)., while energy density and power density were calculated according to Eq. (4). And Eq. (5). respectively and is represented in table S7 [41].

=

!× ×∆ ×

(13)

Where I is the charging and discharging current, ∆t is the discharge time, m is the mass of the sample loaded and V is the potential window. From the comparative study, it is clear that the newly developed device exhibits capacitance value in the range of the other commercial-AC based devices. Moreover, the device also exhibits a high energy density of 60.8 Wh/kg, which is better than PANI-NewAC-Cr (51.7 F/g) and comparable to CharPANI-Cr (62.1 Wh/kg) (Table S7). The CV analysis of the CP-Cr asymmetric device at a constant scan rate of 50 mV/sec at different potential ranges indicates that the device can be operated up to a high working potential of 3V (Figure 8(a)). On the other hand, the CV analysis of CP-Cr cells at different scan rates exhibits the conventional pseudocapacitive nature within the wide potential limit (Figure 8(b)). Moreover, the shape of the CV curve remained intact at an elevated scan rate of 200 mV/s, indicating excellent electrochemical stability of the device. The optimization of the working potential of the device was further confirmed by GCD analysis at a fixed current density of 1 A/g (Figure 8(c)). Herein, it’s clear that the nature of the GCD profiles has not

23

been altered with increasing the potential range, which represents the accurate optimization of the working potential. On the other hand, the GCD profiles of the device at different current densities are represented in Figure 8(d). The device exhibited a maximum capacitance of 48.7 F/g. Even though the capacitance of the device was moderate, it achieved a high energy density of 60.8 Wh/kg due to its broad working potential. Even at a high current density of 5 A/g, the device still delivered an energy density of 6.8 Wh/kg (Table S9). Most importantly, energy density of the device is significantly higher than previously reported devices

like

PANI-CNT//PANI-CNT

(6.16

Wh/kg),

Vanadium

Pentoxide

nanofibres//Polyaniline nanofibres (26.7 Wh/kg), Activated Carbon Fibre-PANI//AC (20 Wh/kg), Graphene-wrapped-PANI-Nanofibres//AC (19.5 Wh/kg) and PANI//Graphene (4.86 Wh/kg) and is comparable to PANI-CuCo2O4//AC(76 Wh/kg) [20,42-46]. as shown in the Ragone plot (Figure 8(e)). For reference, the pictorial representation of the assembled device is shown in Figure 8(f). From the device study, we can conclude that the asymmetric device demonstrated promising electrochemical performance.

Figure 8. (a) CV analysis of CP-Cr asymmetric cell at different voltage range at 50 mV/sec scan rate (b) CV analysis of CP-Cr asymmetric cell at different scan rate (c) GCD analysis of CP-Cr asymmetric cell at different voltage window at 1 A/g current density. (d) GCD analysis of CP-Cr asymmetric cell at different current densities (e) Ragone plot of the asymmetric device (f) Device assembly representation. 3.6.3. Asymmetric cell fabrication and testing using CP-Cr as cathode: To check the practical exploitation of the device, we tested the single device (4×4 cm) to glow red LEDs (1.7 V) and operate a mini fan (2 V). Further, to enhance the performance, 24

we connected three of such devices in series. The charging setups of the single-cell and 3 cell mode is represented in figure S12 and figure S13. From Figure 9, it is clearly visible that the fabricated device is able to glow the strip of red LEDs with the logo “IIT ISM”. In fact, after charging for 30 secs, the device lighted up the LEDs for the duration of 90 secs (Supporting video 1). But, the device could not run the mini fan. However, as shown in figure S14 the mini fan was operated for 10 secs when we combined three devices in series connection (Supporting video 2). Most importantly, the series-connected devices were able to light up the LEDs for the duration of 9 mins 30 secs (570 secs), after the charging time of 30 secs.

Figure 9. Single-cell mode discharging. The enhanced electrochemical performance of CP-Cr material has been attributed to the following factors – 1. The incorporation of pseudocapacitive Cr through adsorption enhanced the capacitive properties of CP-Cr. 2. The high surface area of the adsorbed material played an essential role in enhancing the capacitance. 3. A porous network of the hybrid is very feasible for the effortless passageway of ions and electrons. 4. The synergistic effect of each component also enhanced electrochemical performance, which has been qualitatively analyzed through different characterization techniques. We expect that the current work will open an innovative door for the utilization of waste materials in the area of energy storage. The enhancement of the electrochemical performance of any supercapacitor electrodes by adsorbing the pseudocapacitive element is a novel approach. Moreover, this unique combination of pseudocapacitive and EDLC materials 25

is highly feasible for future energy transport. Lastly, we believe that this “waste to wealth” approach has a significant impact on supercapacitor technology.

4. Conclusion: The formation of the adsorbent CP was confirmed through different characterization techniques including FTIR, FESEM, TGA and BET analysis. Moreover, the adsorption of toxic Cr(VI) and its reduction to Cr(III) on the adsorbent was confirmed from SEM-EDX, solid-state UV-vis and TGA analysis. Finally, the mechanism of Cr(VI) adsorption and stabilization have been established. The electrochemical analysis confirmed the enhancement of electrochemical properties of CP after Cr(VI) adsorption. The enhanced electrochemical performance was caused by the additional pseudocapacitive contribution of Chromium. Here, it is crucial to declare that the facile synthesis, the ready availability of the constituting materials, and the multiple problem-solving abilities made this hybrid material suitable for practical applications in everyday life. Overall, our approach of fabricating efficient energy storage devices based on the adsorption of toxic and harmful Cr(VI) is an instant solution to two major problems of this century - heavy metal pollution and energy crisis.

Acknowledgments The authors are grateful to the Indian Institute of Technology (ISM) Dhanbad for providing research facilities ,working labs and fellowship. The authors are also thankful to SAIF, Punjab and Sprint testing solutions, Maharashtra for providing analytical and testing data. The authors are also thankful to Ms. Pupulata Saren, Ms. Shrabani De, Mr. Chandan Kumar Maity and Mr. Salim Hassan Siddiqui for their help form time to time.

Associated Contents Supporting Information attached Supporting video 1 Supporting video 2

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Highlights •

Cr (VI) removal from mine water by modified activated carbon-based nanocomposite.

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Fabrication of asymmetric supercapacitor device by “Waste to Wealth” approach.

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Waste management and Development of Efficient Energy storage devices.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: