Tire Waste Derived Turbostratic Carbon as an Electrode for a

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Tire Waste Derived Turbostratic Carbon as Electrode for Vanadium Redox Flow Battery Rudra Kumar, Thiruvelu Bhuvana, and Ashutosh Sharma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00113 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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ACS Sustainable Chemistry & Engineering

Tire Waste Derived Turbostratic Carbon as Electrode for Vanadium Redox Flow Battery Rudra Kumar‡#, Thiruvelu Bhuvana#†* and Ashutosh Sharma‡* ‡

Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208016, India

† Department of Mechanical Engineering, Indian Institute of Technology, Kanpur 208016, India *Email: [email protected], [email protected] #

contributed equally

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Abstract: For the first time, we report the synthesis of turbostratic carbon derived from tire waste with high surface area and its utilization as an electrode material in vanadium redox flow batteries (VRFBs). The tire waste carbon was initially subjected to acid demineralization followed by KOH activation wherein the carbon to KOH ratio was varied (1:1, 1:2 and 1:5) and their electrochemical performance towards VO2+/VO2+, V3+/V4+ and V2+/V3+ redox reactions were investigated. The turbostratic nature of carbon derived from tire waste was confirmed by highresolution transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. BET measurements revealed the surface area was as high as 875 m2.g-1 for 1:5 KOH activated sample. The electrochemical performance of pretreated carbon (TW) and turbostratic carbon (1:1, 1:2 and 1:5) were compared by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge methods. Our results indicated that 1:5 KOH activated electrode exhibited highest surface area and performed much better than the other ratios in terms of overall electrochemical performance including high peak current, less peak potential difference, low polarization potential, small charge transfer resistance, high charge-discharge capacity, high columbic efficiency and high energy efficiency. Further, a full cell was fabricated and its electrochemical performance was tested. The results indicated impressive electrochemical performance with the columbic efficiency as 87% at 10 mA.cm-2 along with stable cycling behavior up to 200 cycles. Thus, signifying the tire-waste derived turbostratic carbon offers great promise as high-performance electrodes for VRFB applications. Keywords: Tire waste carbon, Turbostratic carbon, Acid De-mineralization, KOH Activation, Vanadium Redox Flow Battery (VRFB)


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Introduction Zeal for low-cost high–performance electrodes for energy storage devices has led to an investigation of many precursors for generation of different forms of carbon as an electrode material. Activated carbon produced from agricultural based products such as cellulose, sugarcane bagasse, sucrose, corn and others and recycled waste products such newspaper, wood, and tires have attracted enormous attention.1 As tires are made up of primarily rubber and are non-biodegradable, used tires are mostly disposed in landfills, which affect our ecosystem. To address this issue various applications are being implemented such as ground tires are made use for highway construction, groundcover, roofs, mats, etc. or as solid fuels especially in cement kilns. However, these applications do not appear as sustainable solutions and therefore proper recycling of used tires has become a critical issue. With high carbon content up to 75% in the tires, production of activated carbon can be rewarding as it can be used for various applications ranging from pollutant adsorbent to electrodes for energy devices such as supercapacitor2 and sodium or lithium ions batteries.3, 4 On the other hand, electrodes for flow batteries are in demand as flow batteries are simple, efficient and have long durability. In the flow batteries, it is the electrolyte which undergoes redox reaction and the electrode surface do not undergoes any physical or chemical change; it primarily role is to provide electrochemically active sites for the redox reaction to generate power. Conventionally, graphite felt, carbon felt, carbon cloth, carbon paper, etc. have been used as electrode material due to their large surface area and corrosion resistance in the acidic solution.5, 6 Thus, the surge for efficient electrode with excellent electrical conductivity, more electroactive surface area and excellent electrocatalytic activity in acidic conditions has led to the investigation of new materials for electrode.

6, 7

One of the most common approach is to



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chemically treat the electrode surface in order to improve active reaction sites. 8-12 For example, KOH-activated carbon-cloth and carbon paper electrode resulted in creating nanopores and improving the surface area by more than six to ten times and resulting energy efficiency more than 80 %.8-11 The electrochemical activation of graphite felt increased functional groups on the surface and 4-5% higher energy efficiency than the cell using pristine graphite felt has been reported.11 Introducing metal/metal oxide nanoparticles onto the electrode surface resulted in improved electrocatalytic property.

13-15

A layer of carbon material with the high surface area

such as carbon nanotubes16, reduced graphene oxide17-20, nanoparticles21 and nanofibers22 onto the electrode surface has demonstrated improved results. Further, in a study related to biomass-derived carbon, Ulaganathan et al.

23

reported that

mesoporous carbon derived from coconut shell resulted in V3+/V4+ redox couple along with conventional V4+/V5+ redox couple to improve electrochemical performance of VRFBs. To our knowledge, neither tired waste derived carbon nor turbostratic carbon has ever reported as an electrode material for VRFBs. The advantages of using tire waste derived turbostratic carbon is primarily its cost-effective synthesis as compared to other sustainable precursors such as wood, sucrose, coconut shell, etc. the cost of activated carbon derived from recycled tire waste is much cheaper and equal percentage of carbon yields23 and varieties of applications such as electrode materials for energy storage devices like supercapacitor and batteries, catalysis, heavy metal removal, used as adsorbents, etc. Also, we believe its electrochemical performance is comparable to commercially available electrode materials such as carbon paper, graphite felt, etc. its electrochemical performance is comparable to commercially available electrode materials such as carbon paper, graphite felt, etc. Therefore it is worthwhile to recycle the tire waste and produce electrode for vanadium redox flow battery as a high value product. To validate our 4 ACS Paragon Plus Environment

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hypothesis on the potential use of tire waste derived turbostratic carbon as an electrode for VRFBs, a high-performance turbostratic carbon has been synthesized. The surface and morphological characteristics of the tire waste carbon were modified by treatment with strong acids such as HCl and HF and further by activation with KOH. We hypothesized that the tire waste derived well-developed porous carbon structure after acid treatment and KOH activation may result in an enhanced electrochemical performance. Thus synthesized turbostratic carbon with the high surface area was tested as an electrode material for VRFBs and electrochemical performance of these electrodes in terms of galvanostatic charge−discharge capacity, cycling stability, coulombic and energy efficiency were studied. A full cell was fabricated and its electrochemical performance is also discussed. Experimental Materials and Method Tire waste carbon powder was obtained from a local tire industry. Hydrogen fluoride (HF), hydrochloric acid (HCl), sulphuric acid (H2SO4) and potassium hydroxide (KOH) were obtained from Fisher Scientific. Vanadyl sulfate (VOSO4) and N-methyl-2-pyrrolidone (NMP) was purchased from Loba Chemi. Polyvinylidene fluoride (PVDF) was purchased from Sigma Aldrich. Conducting carbon black (Super-P) was purchased from MTI Corporation. Carbon paper (CP) and Nafion 117 membrane were purchased from Fuel store. Acid Pretreatment: The tire waste carbon powder (unTW) was pretreated with the mixture of 1M HCl and 1M HF in DI water for 24 hours. This pretreatment removed the metallic impurities and sulfur present in the carbon powder. After the pretreatment, the sample (TW) was washed



with water and ethanol several times till the residual acid present was completely removed. Finally, TW was dried at 120°C in a vacuum oven for overnight. Activation of Carbon: A known amount of TW was mixed with a different weight ratio of KOH (1:1, 1:2 and 1:5 w/w carbon: KOH) in water and kept for 12 hours. After the penetration of KOH, the sample was dried in an oven at 150°C for 12 hours to evaporate the water. The dried samples were heated in the furnace at 5 °C min-1 at 900°C for 2 hours in a nitrogen environment with the flow rate 150 sccm. After cooling to room temperature, the obtained product was washed several times with water and ethanol to completely remove KOH and finally vacuum dried at 100°C for overnight and the samples were named as 1:1, 1:2 and 1:5 depending on their mixing ratio. Scheme 1 details the synthesis of turbostratic carbon from the tire waste.

Scheme 1: Schematic of synthesis of turbostratic carbon derived from tire waste General Characterization: The morphology of the turbostratic carbon was observed in fieldemission scanning electron microscope (FESEM, SUPRA VP40, Germany) and transmission


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electron microscope (TEM, Technai G2). The specific surface area and the pore size distribution were measured by N2 adsorption at 77 K using an Autosorb-1C instrument (model AS1-C, Quantachrome Instruments, Boynton Beach, FL). 40-50 mg of samples were degassed at 150°C for 8 hours in a vacuum environment and the total pore volume was measured by the amount of N2 adsorbed at a relative pressure close to unity (0.9994). The BET surface area was measured by the multipoint BET method in the pressure range between 0.05 and 0.35. The pore size distribution (PSD) was determined using the desorption isotherm. The micropore and mesopore volumes were calculated using density functional theory and the Barrett−Joyner−Halenda (BJH) method, respectively. X-ray diffraction (XRD) was performed (PANanalytical, Dresden, Germany with Cu Kα radiation (λ= 1.5406 Å) with 2θ in the range of 10 to 90° and scan speed of 2° min-1. Contact angle measurement was performed using Drop Shape Analyzer (Krüss DSA25, GmbH - Germany). Raman spectroscopy (WITec alpha300R, Germany) was performed in the frequency range of 400−3000 cm−1 with a 514 nm laser source. XPS measurements were carried out with PHI 5000 versa probe II, FEI Incorporation with Mg-Kα X-rays (hν = 1253.6 eV) in the range of 0-1000 eV. Fabrication of electrode: A slurry was prepared by mixing the turbostratic carbon with the polyvinylidene fluoride (PVDF) binder and conducting carbon black (Super-P) in a mass ratio of 8:1:1 and dispersed in the N-methyl-2-pyrrolidone (NMP) solvent. The slurry was then coated onto CP and dried at 120 °C in a vacuum and further used as electrodes. The mass loading of the electrode was about 1 mg cm-2. Electrodes with different compositions namely 1:1, 1:2, 1:5 and TW were prepared by coating the slurry of respective composition and a bare CP was also tested for comparative study.



Fabrication of electrochemical cell: A static cell was fabricated by placing 1:5 KOH activated electrode in each half-cell separated by Nafion 117 membrane. The active area of each electrode was 4 cm2. The copper plates were used as current collectors. Finally, the assembled cell was subjected to the electrochemical testing. The electrolyte solutions for both positive electrode and the negative electrode were 0.1 M VOSO4 in 2 M H2SO4, and the volume of the positive electrolyte was doubled to avoid over-charging the positive electrolyte. Galvanostatic cycling was performed between the charge and discharge voltage of 1.6 and 0.6 V respectively at 10 mA. cm-2. Three different single cells have been tested to check the reproducibility. From the chargedischarge plot, the coulombic, voltage and energy efficiencies were estimated for the full cell. Electrochemical study: Electrochemical tests were performed on a three-electrode system with Pt rod and Ag/AgCl (with a saturated solution of KCl as a counter and reference electrode respectively. The electrolyte employed was 0.1 M VOSO4 in 2M H2SO4. Cyclic voltammetry (CV) was performed in the range of -0.8 to 1.6 V with scan rate varying between 2 to 100 mVs-1. The galvanostatic charge−discharge (GCD) experiment was performed with a specific current from 40 to 100 mA.cm-2. Electrochemical impedance (EIS) spectra were acquired from 30 kHz to 0.01 Hz with an open circuit at AC amplitude of 10 mV. Results and discussion Characterization The morphology of the tire waste derived carbon before (unTW), after pretreatment (TW) and followed by KOH activation is observed under FESEM and the results are shown in Figure 1 indicating a significant difference in morphology after activation. The morphology of unTW shows interconnected polydispersed carbon nanoparticles of the size range 40-100 nm (Figure 1a). After HCl and HF pretreatment, TW sample shows the carbon nanoparticles are still 8 ACS Paragon Plus Environment

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interconnected (Figure 1b). However, after different KOH activation of TW, the average particle size is ~50 nm for 1:1, 1:2 and 1:5 KOH activated samples as seen in Figure S1 (Supporting Information) and Figure 1c but the surface appeared more porous after KOH activation as seen in Figure 1c in comparison to Figure 1a-b, and this is ideal as this will result in an increase in the active surface area during electrochemical studies. The morphology of these samples is also characterized by TEM (Figure 1d-e). For TW sample (Figure 1d), carbon nanoparticles with a size of 40-50 nm are observed and selected area electron diffraction patterned (SAED) in the Figure 1d inset, suggest amorphous nature of carbon. Similar sized nanoparticles are observed for 1:5 KOH activated sample (Figure 1e) but the interconnected carbon nanoparticles create a mesoporous gap, which formed an adequate space for the penetration of electrolytes. The SAED in the Figure 1e inset shows a few bright ring patterns corresponding d-spacing of 0.39 nm.24 The increased d-spacing as compared to graphite (d-spacing 0.34 nm) confirmed turbostratic carbon.25 Similarly, HRTEM of 1:5 KOH activated sample showed in Figure 1f, demonstrated the clear lattice fringes with the inter planner d-spacing of 0.39 nm supporting turbostratic nature of carbon.26, 27



Figure 1: (a-c) FESEM of unTW, TW and 1:5 KOH activated samples. TEM images of (d) TW and (e) 1:5 KOH activated sample with their SAED patterns. (f) HRTEM of 1:5 KOH activated sample. To further confirm the turbostratic form of carbon, XRD and Raman spectroscopy are performed and the porosity of the samples is measured by BET and the results are discussed in Figure 2. Diffractograms are recorded for 2θ values of 10-90° as shown in Figure 2a. The XRD of unTW showed a broad diffraction peak at 2θ angle of 23°, which confirmed the (002) lattice plane of the carbon along with other peaks due to the Si impurity present in tire waste carbon. 28 After the acid pre-treatment, the impurity peaks are not present in TW sample as HF acid completely removed the Si impurities. With 1:5 KOH activation, only two peaks are found at 22.75° and 43.32° confirming the (002) and (001) diffraction planes of carbon. The d-spacing of 1:5 KOH activated sample is calculated to be 0.39 nm, which matched well with the HRTEM lattice fringes (Figure 1e-f). The increased d-spacing obtained from XRD peak further confirmed the 10 ACS Paragon Plus Environment

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formation of turbostratic carbon.24 Figure 2b displayed Raman spectra recorded for unTW and 1:5 samples. The D band corresponded to the disordered nature of the carbon or amorphous carbon, while the G band corresponded to the ordered carbon structure or graphitic carbon.29 The ratio between ID/IG indicated the nature of carbon, high values indicated highly defective form whereas low values supported the presence of graphitic carbon. For unTW sample, the band at 1344 and 1580 cm-1, combined with second-order 2D band at 2751 cm-1 confirmed the amorphous carbon along with few bands at lower frequency range observed due to impurities, such as, bands at 350 and 425 cm-1 corresponded to ZnO30, 286 and 352 cm-1 attributed to ZnS31 and 670 and 190 cm-1 are associated with iron oxide. 32 SiO2 impurity is confirmed by a distinct band at 115 cm-1 and a broad band at around 460 cm-1.33 The impurity bands are not present in the case of TW and 1:5 KOH activated sample. For 1:5 KOH activated sample, first order D and G band along with little subdued second order 2D band is observed indicating the formation of turbostratic carbon.24 The ID/IG for 1:5 KOH activated sample is 1.06 showing more than the ratio observed for unTW, further supporting the formation of turbostratic carbon. In order to confirm the increased surface area due to KOH activation, nitrogen adsorptiondesorption is carried out at 77 K for all the samples. Figure 2c and Figure S2 (see Supporting Information) show the BET surface area measurements performed on these samples. The shape of the BET isotherm is type-II, which confirm the pore size in the range of micropores to macropores.24 The lower pressure region shows the micropores with pore size distribution, which is also confirmed by the pore size distribution curve (Figure 2d). The average pore diameter of 1:5 KOH activated sample is found to be ~3.5 nm. The slope obtained at medium relative pressure along with hysteresis loop confirmed the mesopore formation by KOH activation. At higher relative pressure ~1.0, a sharp increase in the slope is observed that corresponded to 11 ACS Paragon Plus Environment


macroporous carbon structure. For unTW, the surface area is observed to be quite low ~ 40 m2g1

. With acid pretreatment, the surface area increased to 145 m2g-1 for TW due to the formation of

mesopores. The specific surface area increased with the increase of C/KOH ratio, as more nanopores are generated with the reaction of KOH with carbon. For 1:1 KOH activated sample, from Figure S2 (Supporting Information), the surface area is measured as 227 m2g-1 and the pore volume of 0.066 cm3g-1 is observed which is lowest among the three KOH activated samples and the highest values are observed for 1:5 sample, with specific surface area of 875 m2g-1 and pore volume of 0.201 cm3g-1. Table 1 lists the details of the measured surface area for tire waste derived carbon activated with KOH under different conditions. It is clearly illustrated from the table that surface area is the best achieved so far with the tire waste derived carbon from this process and the yield of 30% is also higher than most of the reported values.34 Further, these values are comparable to or even larger than those reported in the H3PO4, steam, and CO2 activation. 2, 35-37 Further FTIR spectra of KOH activated carbon samples were studied and found that some functional groups being present. The absorption peak identified at 3423 cm-1 corresponded to the O–H stretching vibrations, which is possibly due to the adsorption of water molecule from the atmosphere. Further, the absorption peaks identified in between 1760–1665 cm-1, 1760–1690 cm-1, 1600–1585 cm-1, 1500–1400 cm-1 and 1320–1000 cm-1 corresponded to C=O stretching of carbonyls, C=O stretching in carboxylic acids, C–C stretching in aromatic ring present in carbon and C–O stretching of carboxylic acids groups, respectively (see Figure S3, Supporting Information). This functional groups play a crucial role in determining the efficiency of the electrode by accelerating electron transfer reaction.38


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Figure 2: (a) XRD, (b) Raman spectra, (c) BET isotherms and (d) Pore size distribution of unTW, TW and 1:5 KOH activated samples. Table 1: Comparison of the surface area obtained for tire waste carbon activated with KOH under different conditions. Activation conditions C:KOH/ Temp (°C)/ Heating time (h) 1:4 700 0.0 1:1 850 1.5 1:6 800 1.0 1:4 900 0.0 1:3 900 3.0 1:4 800 1.0 1:4 800 1.0 1:4 650 2.0 1:4 800 0.75 1:5 900 2.0

Surface area (m2g-1) 474 820 814 582 621 402 574 558 359 875


Reference 39 40 41 42 43 44 45 46 47

This work


As maximum surface area and pore volume are observed for 1:5 KOH activated sample, its composition is analyzed using XPS and the results are shown in Figure 3. The survey scan (Figure 3a) indicates the presence of carbon and oxygen elements. The XPS spectrum of C 1s (Figure 3b) is deconvoluted into total six peaks. The peak at 284.5 eV is assigned to sp2 C–C, and the other peaks are attributed to sp3 C–C at 285.6 eV, C–OH hydroxyl groups at 286.3 eV, C–O–C ether groups (287.4 eV), –C=O carbonyl or quinone groups at 288.5 eV and O–C=O carboxyl groups at 289.9 eV.

48, 49

From the C 1s signal, the content of sp2 and sp3 hybridised

carbon is estimated by their signal ratio. For amorphous carbon, the ratio is 1:2 and for turbostratic and crystalline carbon the ratio is 2:1. The content of sp2 and sp3 hybridized carbon is estimated to be 2.45, which further confirmed the high graphitic domain of carbon which is turbostratic in nature.24 Similarly, the O 1s spectrum (Figure 3c) is deconvoluted into three peaks with the binding energies of about 530.9, 532.3 and 533.7 eV corresponding to carbonyl/quinone groups, ether and carboxyl group, respectively.50

Figure 3: XPS spectra of 1:5 KOH activated sample (a) Survey scan and deconvoluted spectra of (b) C 1s and (c) O 1s


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The wettability is important criteria which signify the electrolyte penetration and spreading in the pores of the electrode, therefore, contact angle measurements are performed on these electrodes (Figure 4). The results of contact angle measurement of CP and 1:5 KOH activated electrode are shown in Figure 4a and 4b respectively along with contact angles for other electrodes. The contact angle of CP is 138.2° whereas for 1:5 KOH activated sample is 62.1°. The values of rest of the electrodes lie in this range as seen from the table in Figure 4. The lower value of contact angle indicated the wettability of the TW carbon is improved greatly by KOH activation, which causing better penetration and spreading of electrolyte and thereby improving the electrochemical performance.

Figure 4: Contact angle measurement on (a) CP and (b) 1:5 KOH activated sample and table listing contact angles for various electrodes.

Further, the electrical conductivity of all tire derived carbon electrodes is measured using a fourpoint probe method at room temperature and the corresponding current-voltage (I−V) curves are showed in Figure S4 (see Supporting Information). Prior to the conductivity measurement, the tire derived carbon was formed into pellet. The four probe electrical conductivity was calculated 15 ACS Paragon Plus Environment


by the equations as σ = 1/ρ

- (i), and ρ = 4.52 × (V/I) × ∆t - (ii), where, σ represents the

electrical conductivity (S.m−1), ρ, V, I, and ∆t represents the resistivity (Ω .m), measured voltage (V), current (A), and thickness (m) of the sample, respectively.50, 51 The electrical conductivity measurement revealed that the conductivity of 1:5 KOH activated sample is 1089.9 S/m, highest compared to all other tire waste derived carbon. Electrochemical studies After detailed analysis of the morphology and chemical composition of the turbostratic carbon, electrochemical studies are performed. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) is performed to understand the electrochemical performance (Figure 5) of TW, 1:1, 1:2 and 1:5 samples towards the vanadium redox reaction and since the active material is coated on carbon paper (CP), bare CP is also subjected to similar tests. Figure 5a display the CV recorded for different samples at 10 mV s-1 in the voltage range from -0.8 to 1.6 V in 0.1 M VOSO4 and 2.0 M H2SO4 solution. For all the electrode samples, three pairs of redox peak appeared indicating the electrochemical reaction of VO2+/VO2+, V3+/V4+ and V2+/V3+ system. A closer look at CV curves reveal the anodic peak for CP electrode appear at 1.2 V whereas for tire waste based samples it is much lower. For 1:1, 1:2 and 1:5 KOH activated electrodes the anodic peak appears at 1.04, 1.06 and 1.07 V respectively. Further, anodic peaks appear around 0.12 V and -0.45 V corresponding to V3+/V4+ and V2+/V3+ redox systems. The lower anodic peak value implies a lower charge voltage of VRFB and a higher energy storage efficiency which is ideal.52 But the large current response for 1:5 KOH activated electrode in comparison to 1:2 and 1:1 and TW electrodes is mainly correlated to the larger surface area and better electrical conductivity (see Supporting Information Figure S4). A very low current response for TW electrode is attributed to its low surface area, pore volume, and poor electrical conductivity as supported by 16 ACS Paragon Plus Environment

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SEM, BET and electrical conductivity measurements (Figure 1, 2 and S4). For bare CP electrode, the current response is better than 1:1 KOH activated sample, which can be attributed to the better electrical conductivity. The reversibility of the redox reaction is another important parameter, which can be estimated by the ratio of the redox peak current (Ipa/Ipc)53 and is calculated for VO2+/VO2+redox peaks. The Ipa/Ipc value closer to unity indicates a higher degree of electrochemical reversibility with a lower activation barrier. Further, the smaller value of ∆Ep indicates a better electrochemical activity towards VO2+/VO2+ redox reaction. The values of redox peak current and other electrochemical properties VO2+/VO2+ redox system for the electrodes are listed in Table S1 (see Supporting Information). The ∆Ep value of 1:5, 1:2 and 1:1 KOH activated electrodes were calculated to be 0.36, 0.39 and 0.38 V, respectively. The low ∆Ep value of 1:5 KOH activated electrode confirmed better electrochemical activity towards VO2+/VO2+ redox reaction as compared to other electrodes. For 1:5 KOH activated electrode, Ipa/Ipc is 1.2 which is closer to unity indicating good reversibility in turn suggesting the faster electron transfer process and leading to improved energy transfer efficiency.54 Based on the XPS study, carbon to oxygen ratio (C/O) of the electrode is compared (data not shown). The calculated C/O ratio from the XPS survey scan for TW, 1:1, 1:2 and 1:5 KOH activated electrode and the values are found to be 25.3, 17.5, 13.2 and 12.7. This ratio indicates, lower the ratio more functional groups are present and thereby causing reduction in overpotential, leading to increase in energy and columbic efficiency of the VFRB.54 Further, carbon oxygen bonding based of XPS studies (data not provided) in TW, 1:1 and 1:5 KOH activated electrodes are calculated to be 33.62, 54.6, and 59.4 %, respectively, which also a good agreement with VFRB performance.55 Electrochemical impedance spectroscopy (EIS) at the peak potential is performed to investigate the electron transfer/charge transfer resistance (Rct) and to evaluate the electrode activity towards 17 ACS Paragon Plus Environment


VO2+/VO2+. Figure 5b show the Nyquist plot of all electrodes, in the high-frequency region, a semicircle is observed which correspond to Rct at the electrolyte/electrode interface. It is clearly seen that Rct is greatly reduced from TW > 1:1 > 1:2 > 1.5 electrodes, which are consistent with the CV anodic peak currents (Ipa). For 1:5 KOH activated electrode, the semicircle diameter is the smallest as compared to the other electrodes, which confirmed the small charge transfer resistance and hence implying fastest electron transfer process for better electrochemical performance. Additionally, the ohmic resistance (Rs) for 1:5 KOH activated electrode show the lowest value compared to other electrodes. This is attributed to the low solution resistance which is mainly due to high electrical conductivity and large surface area of the material. Further, CVs are recorded at different scan rates ranging from 2 to 100 mV s-1 in 0.1 M VOSO4 and 2.0 M H2SO4 solution for 1:5 KOH activated electrode and results are shown in Figure 5c. With increasing the scan rate, the peak potential separation changed slightly suggesting good reversibility on this electrode and further the peak current increased confirming that better charge/ion transport at higher scan rates. A linear relationship between the anodic and cathodic peak current values and the square root of the scan rate was observed (Figure 5d) indicating the redox process is mainly controlled by diffusion from the electrolyte to the electrode surface.


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Figure 5: (a) CV and (b) Nyquist plots at peak potential for CP, TW, 1:1, 1:2 and 1:5 KOH activated electrodes. (c) CV for different scan rate and (d) a plot of anodic and cathodic peak current vs. the square root of scan rate for 1:5 KOH activated electrode. The voltage range was – 0.8 to 1.6 V in 0.1 M VOSO4 and 2.0 M H2SO4 solution.

To understand the role of surface area of the electrode in VRFBs, galvanostatic charge-discharge was performed in a three-electrode cell. Figure 6a showed the discharge plot of 1:5 KOH activated electrode for VRFB application at various current densities (40-100 mA.cm-2) in 0.1 M VOSO4 and 2.0 M H2SO4 solution in the voltage range -0.4 to 1.6 V. For comparison, the discharge curves for CP, TW 1:1 and 1:2 KOH activated electrodes are shown in Figure S5 (see 19 ACS Paragon Plus Environment


Supporting Information). For all the electrodes, the discharging time increased with a decrease in current density. The discharge time of 1:5 KOH activated electrode is higher than other electrodes at all current densities, which is primarily due to its higher surface area and pore volume. The discharge profile of CP electrode is higher than TW electrode, corroborated the good electron transport in CP while poor electron transport in TW electrode. Also, the TW electrode coated with binder decreases the surface area and electrical conductivity which impeded the penetration of ions/electrons into the pores. The discharge voltage plateau of CP at 0.7 V for 40 mA.cm-2 current density, which signified the higher polarisation behavior compared to 1:5 KOH activated electrode. Figure 6b showed the columbic efficiency of all electrodes at various current densities starting from 40 to 100 mA.cm-2 with 20 mA.cm-2 with increment for 25 cycles. The columbic efficiency of 1:5 KOH activated electrode decreased from 87% to 83% when the current density increased from 40 to 100 mA.cm-2 and regained once again when the current density was decreased to 40 mA.cm-2. Further, the columbic efficiency of all electrodes is decreased with increase in current densities, implying that decrease in the amount of charge production and the charge transfer for higher current density leading to lower columbic efficiency. The superior columbic efficiency of 1:5 KOH activated electrode than other electrodes confirmed the good electrochemical performance in VRFBs. Here we also note that the columbic efficiency of CP electrode is higher than TW and 1:1 KOH activated electrodes due to higher electrical conductivity. In some studies, wherein corn derived carbon the at 150 mA cm-2, the coulombic efficiency is 75%56, 57 and close to 85% energy efficiency for bio-mass derived mesoporous carbon.56 Thus we believe that our results are quite encouraging and this work will lead to better utilization of waste tires which are otherwise environmental concern.


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Figure 6: (a) Discharge plot for different current densities (40-100 mA.cm-2) for 1:5 KOH activated electrode and (b) Columbic efficiency for different electrodes at different current densities.

The stability of 1:5 KOH activated electrode was evaluated toward the VO2+/VO2+ and V3+/V4+ redox reactions by a continuous charge-discharge between -0.4 and 1.6 V and the results are shown in Figure S6 (see Supporting Information). Figure S6a showed the charge-discharge curve of 1:5 KOH activated electrode for initial 10 cycles at 40 mA.cm-2 current density. Further, the discharge plateau of 1:5 KOH activated electrode is at a lower potential at 0.8 V while the charge plateau is higher at 1.1 V under the same current density confirming the lower polarisation resistance of the electrode during the charge-discharge process. Further, another discharge plateau at 0.3 V, while the charge plateau around 0.05 V suggests the participation of V3+/V4+ redox. The presence of V3+/V4+ redox system indicates improved electrochemical performance as this system will certainly supply the additional electrons for the electrochemical reaction. Figure S6b showed the coulombic efficiency and energy efficiency of 1:5 KOH activated electrode for 200 cycles. The constant coulombic efficiency and energy efficiency of around 87% and 85% respectively confirmed that the stable electrode performance for VRFBs of



1:5 KOH activated electrode with its voltage efficiency (VE) as 96%. This is an encouraging result indicating the transport behavior of the electrode in solution does not degrade even after 200 cycles. Based on these results, a static full cell was fabricated and tested. The electrochemical performance of the full cell is summarized in Figure 7. CV recorded at different scan rate in the voltage range from -0.8 to 1.6 V is shown in Figure 7a. Redox peaks appear indicating the electrochemical reaction of VO2+/VO2+, V3+/V4+ and V2+/V3+ system similar to half-cell study (see Figure 5a) and the current increased as the scan rate is increased. Further, Figure 7b showed the discharge plot of full cell at various current densities (40-100 mA.cm-2). The discharge time increased as the current density is decreased but the discharge time was much higher than that of half-cell study. Further the discharge plot for lower current density (10 mA.cm-2) in the potential range of 1.6 to 0.6 V is provided in Figure S7 (see Supporting Information). Figure 7c show the GCD curve for initial 10 cycles and Figure 7d showed the coulombic efficiency and energy efficiency for 200 cycles of full cell at 10 mA.cm-2 current density. The constant coulombic efficiency and energy efficiency of around 87% and 84% respectively confirmed that the stable electrode performance for VRFBs of 1:5 KOH activated electrode in full cell configuration. Further, its electrochemical performance is better than commercially available electrode materials such as pristine carbon paper (EE is 73% at 60 mA.cm-2)23 , pristine carbon felt (EE 77% at 50 mA.cm-2)52 and pristine graphite felt (EE is 72% at 60 mA.cm-2.58 The chemical composition and elemental electronic state of the 1:5 KOH activated electrode was analyzed by XPS analysis, before and after 200 continuous chargedischarge cycles in 0.1 M VOSO4 and 2.0 M H2SO4 solution (Figure S8-S9, see Supporting Information). The chemical composition of the electrode has not changed in the presence of the 22 ACS Paragon Plus Environment

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electrolyte and its electrochemical performance has also not degraded after 200 cycles, thus indicating that the synthesized turbostratic carbon is a robust material for VRFB applications.

Figure 7: (a) CV for different scan rate, (b) Discharge plot for different current densities (40-100 mA.cm-2), (c) Galvanic charge-discharge plot at 10 mA.cm-2 current density and (d) coulombic efficiency and energy efficiency for 200 cycles for the full cell.

Conclusions In summary, we successfully demonstrated synthesis of a high surface area turbostratic carbon from tire waste and its utilization as an efficient electrode material for VRFBs for the first time. 23 ACS Paragon Plus Environment


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The KOH activation was carried out in three different C:KOH ratios namely 1:1, 1:2 and 1:5. As compared to lower surface area activated electrodes (1:1 and 1:2), 1:5 KOH activated electrode showed high surface area of 875 m2.g-1. During electrochemical studies 1:5 KOH activated electrode showed higher peak current and lower peak potential difference and lesser polarisation behavior. The high surface area of turbostratic carbon exhibit improved electrochemical performance along with better cycle retention. The improved electrochemical activity of 1:5 KOH activated electrode, such as, high columbic efficiency can be attributed to its higher surface area, larger pore volume, and good electrical conductivity which can accommodate a large number of vanadium ions along with good redox behavior and better electron transport in comparison to 1:1 and 1:2 KOH activated electrode.

Electrochemical studies on full cell

demonstrated coulombic efficiency and energy efficiency of around 87% and 84% respectively at a current density of 10 mA.cm-2 is quite encouraging. Given this material is synthesized from waste and its performance is comparable to commercially available electrode material, this work opens up new avenues for the utilization of turbostratic carbon from tire waste for all VRFB applications.

Acknowledgement: T. B. thanks the Department of Science & Technology (DST), Government of India, for Start-up Research

Grant

(Young

scientist)

(SB/FT/CS-060/2012)

and

INSPIRE

Fellowship

(DST/INSPIRE/04/2014/002251, IFA13/MS-36) for the financial support. This work was supported by Centre of Nanosciences, Indian Institute of Technology, Kanpur. Supporting Information


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TOC Synoposis: Turbostratic carbon derived from tire waste, a sustainable precursor, is demonstrated as an excellent electrode material in VRFBs.


Tire Waste Derived Turbostratic Carbon as an Electrode for a

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