Immense microporous carbon@hydroquinone metamorphosed from

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Immense microporous carbon@hydroquinone metamorphosed from nonporous carbon as a supercapacitor with remarkable energy density and cyclic stability Chanderpratap Singh, and Amit Paul ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01239 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 29, 2018

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ACS Sustainable Chemistry & Engineering

Immense microporous carbon@hydroquinone metamorphosed from nonporous carbon as a supercapacitor with remarkable energy density and cyclic stability Chanderpratap Singh, and Amit Paul* Department of Chemistry, Indian Institute of Science Education and Research (IISER) Bhopal, MP, 462066, India Email: [email protected] Keywords: Ultramicroporous Carbon, Microporous Carbon, Supercapacitor, Energy Density, Electrochemical Impedance Spectroscopy.

ABSTRACT

We report transformation of a cost effective nonporous carbon (NC) to an immense microporous carbon (IMC) employing a simple chemical activation route at 750 °C. N2 adsorption/desorption experiments revealed a remarkable increase in BET surface area (80 to 3030 m2 g-1) for successor nanomaterial (IMC) in comparison to precursor nanomaterial (NC) presumably due to enhanced accessibility of reaction surface area on carbon material for oxidants to react. In consequence, 250 times specific capacitance enhancement (2.5 to 605 F g-1 at 0.5 A g-1 current density) was observed in 2 M H2SO4 using three electrodes configuration. Further, a massive specific capacitance of 1177 F g-1 with a remarkable energy density of 163 Wh kg-1 has been achieved by addition of hydroquinone in electrolyte with IMC (IMC@H2Q) employing two electrodes

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configuration. Notably, a simple electrode potential dependent chemical reversibility for hydroquinone redox chemistry in the long term cyclic experiment (95% capacitance retention after 5,000 cycles) has been demonstrated wherein a strong electric field helped to avoid agglomeration of hydroquinone molecules inside the nanomaterial while hydrogen bonds formation in IMC@H2Q prevented chemical decomposition of benzoquinone and thus also provided efficient routes for electron/proton transport eluding annihilation of charge carriers.

INTRODUCTION

Porous carbons ranging from activated carbons (AC)1 to one-dimensional carbon nanotubes2 to two-dimensional graphenes3-5 have generated immense interest due to their scientific/ technological applications such as supercapacitor, baterries, H2 storage etc and popularity of these materials were further enhanced due to their economical large scale synthesis.6 Among these applications, in an electrical double layer supercapacitor, energy storage occurs due to fast physical adsorption/desorption of electrolyte ions which results in rapid charging/discharging.4,7 Intense research has shown microporous materials (pore size 500 °C).6,11 Among these oxidants, KOH was found to be the most effective which etches the carbon nanostructure to enhance the porosity and hence surface areas of these materials were further elevated.6 In consequence, supercapacitor


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performances of devices based on ACs were improved as well.6 Significant attention was also provided towards generation of AC materials by activating metal-organic frameworks, polymers etc at elevated temperatures.12,13 In order to boost the charge storage capacity further, incorporation of pseudocapacitive materials such as metal oxides and electrochemically redox active materials have been thought to be an alternative pathway which stores extra charge by virtue of surface redox reactions.4,14-18 Electrochemically active organic molecules such as quinones, anthraquinones etc have been utilized as redox additives in supporting electrolyte or for the preparation of redox responsive composite electrodes due to their scalability, cost effectiveness, and benignity.16,19-21 Among various organic redox molecules, hydroquinone (H2Q) is the tiniest redox probe which can inject two electrons within a very narrow potential window utilizing proton-coupled electron transfer (PCET)(2H+/2e-).22,23 Moreover, due to (2H+/2e-) PCET, it also avoids generation of highly reactive radical species during oxidation and can easily penetrate through narrow micropores compared to other redox molecules. Recent reports suggest that H2Q based covalent-organic frameworks (COFs) can achieve superior performance as a supercapacitor compared to anthraquinone based COF presumably due to reduced pore diameter of H2Q based COF.24,25 In spite of all these advantages, research indicated chemical reversibility of redox chemistry of H2Q is still not adequate enough for supercapacitor application since oxidized species of H2Q, i.e. benzoquinone (Q) decomposes rapidly which results in significant drop in specific capacitances within few cycles.26 Previous effort suggested physisorbed H2Q on AC may provide desired chemical stability because of hydrogen bonds formation between H2Q and oxide sites present on AC.23 However, 18% drop in specific capacitance was still observed after 1000 cycles23 which was presumably due to minor



aggregation between H2Q molecules.27 Attempts have been made to overcome the issue of H2Q decomposition by positioning it on several carbon substrates but the challenge remained open.20

Herein, we have conceptualized a novel yet a very simple hypothesis to improve the surface area and porosity of carbon materials. We hypothesized that instead of activating a micro/mesoporous carbon (MMC), activation of a nonporous carbon (NC) material employing the most effective oxidant (KOH) may result in remarkable improvement of porosity/surface area, since effective interactions between KOH and carbon materials can enhance significantly due to improvement of surface area on carbon material wherein KOH can bombard (Scheme 1a). Furthermore, this hypothesis may also generate immense microporous carbon (IMC) by controlling the reaction condition which is a preferable requirement for superior performance of a supercapacitor as discussed earlier (Scheme 1).8,9


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Scheme 1. Metamorphosis of carbon during chemical activation by KOH. (a) Precursor nonporous carbon (NC) to successor immense microporous carbon (IMC). (b) Precursor micro/mesoporous to successor micro/mesoporous carbon (MMC). In the next few sections, we discuss the synthesis, characterization and electrochemical performance of immense microporous carbon (IMC) metamorphosed from a nonporous carbon (NC) by a simple chemical activation route using KOH at 750 °C.28 In order to verify our hypothesis, experiments were also performed with two additional micro/mesoporous carbon materials (MMC-1A and MMC-2A) prepared via identical chemical activation method but the precursor materials also consisted of micropores and mesopores (MMC-1 and MMC-2) (Scheme 1b). Moreover, with addition of a redox additive (H2Q) in supporting electrolyte solution, specific capacitance has been improved significantly and by application of a higher



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electrode potential, aggregation of H2Q molecules were avoided which resulted remarkable performance in long term cyclic experiments. EXPERIMENTAL SECTION Chemicals. Nonporous carbon (4-8 mesh (NC), micro/mesoporous carbon 8-20 mesh (MMC-1) and 100 mesh

(MMC-2)),

concentrated

sulfuric

acid,

potassium

hydroxide,

hydroquinone,

tetrabutylammonium hexafluorophosphate, N-methyl-2-pyrrolidone (NMP), poly(vinylidene fluoride) (PVDF) and potassium bromide were purchased from Sigma-Aldrich. Platinum (Pt) foil and Pt wire were purchased from Alfa Aesar. Standard calomel electrode (SCE) was purchased from CH Instruments, TX, USA. Hydrogen peroxide (50%) and potassium permanganate were purchased from Rankem, India and Fischer Scientific respectively. Acetonitrile (spectroscopy grade) and acetone were purchased from Spectrochem, India respectively. Mili-Q water (> 18 MΩ.cm) was used during all the synthesis. KOH activation of carbon materials. Activation of nonporous carbon (NC), micro/mesoporous carbons (MMC-1, and MMC-2) were performed by using a chemical process wherein KOH has been used as an oxidant.28 First, 1 gm of NC was mixed with powder KOH, wherein weight ratio of NC:KOH were varied from 1:2 to 1:7. Samples were transferred in an alumina crucible and inserted into the temperature controlled tube furnace. The samples were carbonized under continuous N2 flow in a horizontal cylindrical furnace and heated to an elevated temperature of 750 °C with a ramping rate of 5 °C/min. After that, samples were kept at 750 °C for 1 h. Then, samples were allowed to cool naturally under N2 atmosphere. Thereafter, samples were washed several times using 5 M HCl solution, followed by distilled water. HCl was used to remove mineral contents and water eliminated chlorine ions.


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Carbon materials prepared from NC were abbreviated as immense microporous carbon (IMC), since N2 adsorption/desorption studies revealed that these materials were heavily microporous in nature (vide infra). Samples were named as IMC-2, IMC-3 and IMC-7 wherein ratio of NC:KOH were 1:2, 1:3 and 1:7 respectively. Sample prepared from NC:KOH=1:5 provided the best results (vide infra) and detailed investigation was performed on this sample. This sample has been named as IMC. Same synthesis condition was used to prepare two more samples from MMC-1 and MMC-2 and they were named as MMC-1A and MMC-2A respectively. Literature reports suggest following reactions occur during activation of carbon by KOH.11,29 6KOH+C→ 2K+3H2+2K2CO3 K2CO3→ K2O+CO2 CO2+C→ 2CO K2CO3+2C→ 2K+3CO Characterization. Powder X-ray diffraction (PXRD) patterns were recorded by Bruker AXS D8 Advance with Cu Kα radiation (1.54 Å) with a step size of 0.02° in a 2θ range of 0-80°. For morphological studies, dried samples were spread over carbon tape and gold coated for 120 s, and scanning electron microscopy (SEM) experiments were performed using Carl ZEISS (ultraplus) FE-SEM at 20 kV. Transmission electron microscopy (TEM, FEI TALOS 200S) experiments were performed under an accelerating voltage of 200 kV. TEM samples were prepared by drop casting the suspension of the sample (0.5 mg in 15 mL isopropanol) onto a carbon-coated copper grid and solvent was evaporated under ambient conditions overnight. Raman spectroscopy experiments were performed by Lab RAM HR 800 (HORIBA) with excitation wavelength of 632.8 nm. N2 adsorption/desorption experiments were performed on Quantachrome Autosorb QUA211011



equipment for surface area measurements. Samples were degassed under high vacuum at 100 °C for 20 h. X-ray photoelectron spectroscopy (XPS) experiments were performed on vacuum dried powder sample with drop size of (1.5 mm radius) on sample holder by using PHI 5000 Versa ProbII, FEI Inc. with scan time one hour per element for core level scan (Energy band of 20 eV, with Pass setting of 23.5 eV, 0.025 eV Step, 100 ms time per step, 5 cycles). Electrochemical measurements. Electrochemical experiments were carried out using CH Instruments, Austin, TX bipotentiostat (Model CHI 760D), potentiostat (Model CHI 620E) and BioLogic instrument model (SP300). For three electrodes configuration supercapacitor studies, saturated calomel electrode (SCE), Pt wire, and activated carbon coated Pt foil were used as reference, counter, and working electrodes respectively. For fabrication purpose, first Pt foils were cleaned in acetone by sonication for half an hour and dried in air. After that, 1 mL NMP, 2 mg acetylene black, 2 mg PVDF binder, and 10 mg of AC were added. The solution mixture was stirred for 6 h to enhance the homogeneity of the solution. 100 μl of aliquot was drop casted and spread on an electrode covering 1 cm2 area and dried at 100 °C for 14-18 h in a hot air oven. The mass of material deposited on each electrode was 1.2 mg/cm2. Cyclic voltammetry (CV) experiments were performed in a potential range of -0.15 to 0.75 V versus SCE at various scan rates (1, 2, 5, 10, 20, 50 and 100 mV s-1) in 2 M H2SO4. Galvanostatic charge/discharge experiments were performed in a potential range of 0.15 to 0.75 V versus SCE at different current densities (0.5, 0.7, 0.8, 1, 2, 5 and 10 A g-1) in 2 M H2SO4 solution. Electrochemical impedance spectroscopy (EIS) experiments have been collected at a potential of -0.15 V bias over frequency range of 0.01-105 Hz with an amplitude of 0.01 V. Using eq. 1, specific capacitance values were calculated from CVs, wherein Csp, I, E, ν and m represent specific capacitance, current, potential window, scan rate, and mass of deposited


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material respectively. The numerator of this equation was calculated by integrating the area of CV. Specific capacitances from galvanostatic charge/discharge curves were calculated using eq. 2, wherein t is the time taken to complete charge/discharge process.

C sp =

∫ I .dE 2 mE υ

C sp =

I .t 2 mE

(1) (2)

For two electrodes configuration studies, both Pt electrodes were used as current collectors and Whatman No.1 filter paper was used as a separator. The calculations of specific capacitance values were made according to eq. 3 and 4, wherein m is the weight of material on each electrode, and a factor of 2 was applied since both electrodes were connected in series. Other terms have similar meaning as described above.

C sp =

C sp =

2 ∫ I .dE

2 mE υ

2 . I .t 2 mE

(3)

(4)

Supercapacitor experiments were also performed in two electrodes configuration with redox additive (H2Q) in electrolyte and mass of hydroquinone was also included in specific capacitance calculation (See supporting information for details). Eq. 5 was used for the calculation of specific capacitances wherein non-linear galvanostatic charge/discharge curves30 were observed. Herein, im, E are current densities (A g-1) and potential respectively. The numerator of this equation was calculated by integrating the area of charge/discharge curves. Ef and Ei are final and initial potentials respectively.



C sp =

2 .i m . ∫ Edt

Ef

E2

(5)

Ei Energy densities (E.D.) and power densities (P.D.) were calculated by using eq. 6 and 7, respectively, wherein Csp, E, and t are specific capacitance, potential window, and discharging time, respectively.

E .D =

1 C sp E 2 (6) 2

P .D =

E .D t

(7)

RESULTS AND DISCUSSION Probe for microporosity generation. The synthesis of IMC was optimized by varying the ratio of NC and KOH. Brunauer-EmmettTeller (BET) N2 adsorption/desorption experiments and electrochemical results indicated that nanomaterials prepared from 1:5 ratio of NC and KOH provided best successor nanomaterial (IMC) (Figure S1 and related discussion in supporting information (SI)). Reaction time for KOH activation at 750 °C were varied from 1 to 3 h and 1 h activation provided best performance (Figure S2 and related discussion in the SI). These conditions were chosen as the default synthesis methodology for other carbon materials as well.


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Figure 1. Characterizations of NC and IMC. (a) PXRD patterns, (b) N2 adsorption/desorption isotherms. (c) Cumulative pore volume and (inset) pore size distribution (calculated using slit/cylinder NLDFT model). (d) TEM image of IMC. (e) and (f) are HR-TEM images of IMC and NC respectively. Powder X-ray diffraction (PXRD) patterns have been shown in Figure 1a for NC and IMC which showed complete decay of (002) peak at 2θ maxima of 23.5° for IMC while peak was



visible for NC (Figure 1a). Disappearance of (002) peak of IMC was presumably due to significant decrease of graphitic layered structure, i.e. predominant presence of single layer sheets.31 Dahn et al. provided an empirical parameter (R) which is defined as the ratio of the length of (002) Bragg peak to the background, wherein a lower R value (~1) signifies larger single layer content, while larger R value (>1) signifies higher degree of graphitization (Inset of Figure 1a).32 R values were found to be 1.47 and ~1 for NC and IMC respectively (Inset Figure 1a) which quantitatively supports the same conclusion. N2 adsorption/desorption experiments exhibited type-I isotherms according to IUPAC classification with no apparent hysteresis for NC and IMC (Figure 1b). A remarkable 38 times BET surface area enhancement (from 80 to 3030 m2 g-1) was observed for IMC in comparison to NC. Non localized density functional theory (NLDFT) pore size analysis revealed IMC was consist of ultramicropores (0.56 nm) and micropores (1.15 nm) while precursor NC was nonporous (Figure 1c inset). Furthermore, cumulative pore volume of IMC rapidly increased in micropore region and reached a plateau having a value of 1.5 cm3 g-1 in mesopore region (Figure 1c, Table 1). In contrary, BET analysis of porous precursors (MMC-1 and MMC-2) and successor nanomaterials (MMC-1A and MMC-2A) didn’t show significant enhancement of BET surface area by embracing same KOH activation route (Figure 2 and Table 1). Moreover, MMC-1A and MMC-2A were also consist of mesopores since precursor nanomaterials also had mesopores (Figure 2b and 2d) which does not play significant role towards enhancement of charge storage capacity.33 A markedly increased scattering in the low angle region of PXRD also suggest presence of high density of pores in IMC (Figure 1a).34 It is important to emphasize that BET surface area of IMC was towards higher end (Table S2, SI) and nearly 8 times higher than that of hydrothermally synthesized porous carbon with the assistance of sacrificial template.35 Furthermore, in past, synthesis of AC


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having high surface area and uniform pore size were attempted by several approaches such as template technique,35,36 chemical,11 and physical6 activations. However, among these methods, chemical activation process has been perceived as a better methodology due to lower activation temperature, higher yield, less activation time etc.6 Nevertheless, ACs having well defined narrow micropores only having very high BET surface area and high pore volume remained elusive.6 Herein, as a proof-of-concept study, we have demonstrated that employing KOH activation and nonporous carbon (NC), synthesis of carbon material having predominantly micropores (IMC) only has been achieved while activation of MMC yielded minor improvement in BET surface areas besides having wide pore size distribution as well. Transmission electron microscopy (TEM) image of IMC exhibited that the structure was disordered and isotropic, consist of curled single carbon layers with no obvious graphitization (Figure 1d).37 High resolution TEM images of IMC revealed presence of porous network (Figure 1e), while NC didn’t display porous network (Figure 1f). SEM images did not display significant changes before and after activation (Figure S3, SI). However, carbon material textures were constituent of small intestine kind of particles (marked in red dotted circle, Figure S3, SI). NC and IMC were consisting of greater number intestine particle in comparison of MMC-1 and MMC-1A (Figure S3, SI). Besides, MMC-2 and MMC-2A also showed agglomeration of particles (Figure S3, SI). Impact of choice of carbon precursor on surface area and capacitive performance. N2 adsorption/desorption isotherms for MMC-1, MMC-2, MMC-1A and MMC-2A, exhibited mixed type-I and type-IV isotherm profile with hysteresis loop of H4 type (according to IUPAC classification) in the range of (0.45-0.9) for P/Po (Figures 2a, c). It is important to note that H4



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types of loops are also often associated with narrow slit pore (micropore region). Micropore volume (calculated by applying Dubinin-Radushkevich (DR) analysis) and total pore volumes (calculated from NLDFT method) are tabulated in Table 1. BET surface areas (SBET) were calculated in the relative pressure range of 0.05-0.2. Furthermore, in order to eliminate the discrepancies in the surface area measurement for microporous carbons, surface areas of ACs were calculated using NLDFT method and have been compared in Table 1 with the BET surface areas.38,39 Table 1. BET results of ACs before and after KOH activation. Sample

BET surface area (m2 g-1)

IMC

Total Pore volume (cm3 g-1)

Micropore pore volume (cm3 g-1)

3030

NLDFT surface area (m2 g-1) 2953

1.5

1.42

MMC-1A

860

1070

0.59

0.47

MMC-2A

3090

2882

2.3

1.54

MMC-1

740

970

0.57

0.17

MMC-2

1212

1404

1.01

0.77


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1.5

Adsorption Desorption 400 2 -1 MMC-1A, (SBET= 860 m g ) (a)

Differential Pore Volume 3 -1 -1 dv (cm g nm )

Volume/cm g

3 -1

500

300 200

2 -1

MMC-1, (SBET= 740 m g )

100

3 -1

Volume/cm g

0.2

0.4

0.6

0.8

1.0

0.5 ~1.2 nm ~5.3 nm

P/Po

2

1500

-1

MMC-2A, (SBET= 3090 m g )

1000 500 MMC-2, (SBET= 1212 m2 g-1) 0.2

0.4

0.6

0.8

P/Po

4000

(e)

2

-1

BET Surface area (m g )

0.0

2

1.2

Adsorption Desorption

(c)

1

1.0 Differential Pore Volume 3 -1 -1 dv (cm g nm )

0.0

0

MMC-1A MMC-1

~0.58 nm

0.0

0

2000

(b)

1.0

3 4 Pore size (nm)

~0.56 nm (d) 1.0 ~1.3 nm

6

MMC-2A MMC-2

0.8 0.6 ~2.56 nm

0.4

~3.54 nm ~5.24 nm

0.2 0.0

1

2

3 4 5 Pore size (nm)

Surface area Specific Capacitance

250 200

2000

150 100

1000

7

50

-1

MMC-1A MMC-2 MMC-2A

6

300

3000

MMC-1

5

Specific Capacitance (F g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) N2 Adsorption/desorption isotherms of MMC-1 and MMC-1A. (b) Pore size distribution (calculated using slit/cylinder NLDFT model) of MMC-1, and MMC-1A. (c) N2 Adsorption/desorption isotherms of MMC-2 and MMC-2A. (d) Pore size distribution (calculated using slit/cylinder NLDFT model) of MMC-2, and MMC-2A. (e) Comparison plots of BET surface area and specific capacitance (calculated at 0.5 A g-1 current density) for two different porous carbons before activation (MMC-1, and MMC-2) and after KOH activation (MMC-1A, MMC-2A).



BET results comparison between precursor MMC-1 and successor MMC-1A revealed surface area enhancement was nominal (740 to 860 m2 g-1), however improvement of specific capacitance was 3 fold (82 to 238 F g-1) (Figure 2e). Significant enhancement of ultramicropore (0.58 nm) (Figure 2b) distribution presumably lead to 3 fold improvement in the capacitance value (Figure 2e). Comparison between MMC-2 and MMC-2A revealed 2 fold increase in the specific capacitance values (128 to 251 F g-1) although BET surface area improvement was 2.5 times (1212 to 3090 m2 g-1) (Figure 2e). Figure 2d revealed ultramicropore distribution (0.68 nm) didn’t improve significantly in MMC-2A compared to MMC-2 (Figure 2d) which presumably was the reason for minor specific capacitance enhancement. In contrast, comparison between NC and IMC revealed a 250 times specific capacitance enhancement and highest specific capacitance for IMC was 605 F g-1 (vide infra). The superior supercapacitor performance of IMC compared to MMC-1A and MMC-2A was due to significantly higher intensity of ultamicropores (0.68 nm) in IMC than that of MMC-1A and MMC-2A (Figures 1c (inset), 2b, 2d), since pore volume of MMC-2A was higher than that of IMC while BET surface area and micropore volume were similar (Table 1). These results clearly establish the necessity of narrow micropores/ultramicropores for superior energy storage compared to high BET surface area with miscellaneous larger pore distributions. X-ray photoelectron spectroscopy (XPS) and Raman characterization of carbon nanomaterials. X-ray photoelectron spectroscopy (XPS) revealed presence of oxygen functionalities with C/O (weight %) ratio of 8.1, 5.3 and 4.0 for IMC, MMC-1A and MMC-2A respectively and further suggested carbidic to graphitic like structural transition during activation (Figure S4 and related discussion, SI). C 1s spectrum of NC revealed peaks for presence of carbidic carbon40 (carbon


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with partially negative charge surface carbonaceous Cn- species) and C-OH (hydroxyl)5 at 282.3, 283.0 and 286.0 eV (Figure S4a, SI). XPS of IMC revealed carbidic carbons were transformed to graphitic carbon during activation (Figure S4b, SI).41 Peaks corresponding to C=C, C-C/C-H, and C=O (ketone) were observed at 283.9, 284.6, and 288.3, respectively (Figure S4b, SI). MMC-1A revealed peaks for presence of carbidic carbon and C=O (ketone) 283.2, 283.7 and 287.3 eV (Figure S4c, SI). MMC-2A showed peaks for the presence of carbidic carbon and COH (hydroxyl) at 282.4, 283.1 and 286.6 eV (Figure S4d, SI). Raman spectra for different ACs have been shown in Figure S5 and they exhibited D (correspond to disorder) and G (correspond to in-plane vibration of C atoms) bands at 1330 and 1590 cm-1, respectively. The intensity ratios of these two bands (ID/IG) are measure of degree of disorder.42 Interestingly, between NC and IMC, G band positions were shifted from 1575 to 1590 cm-1 which is close to 1600 cm-1 observed for nanocrystalline graphite. This shift in G band position was due to the evolution of D band which is superimposed with G band and often indistinguishable from the G band, as suggested previously.43 On the other hand, the positions of D and G bands remain unaltered for samples MMC-1 and MMC-2 before and after activation (Figure S5, SI). Electrochemical performances of carbon nanomaterials. Inspired by BET and other characterization results, AC materials were tested for supercapacitor application using three and two electrodes configurations in 2 M H2SO4. In three electrodes configuration, IMC exhibited nearly rectangular cyclic voltammograms (CVs) indicating rapid charge storage (Figure 3a). A redox peak was observed around 0.4 V vs. standard calomel electrode (SCE) which was presumably due to oxygen functionalities present on carbon texture (Figure 3a).4,5,23 Galvanostatic charge/discharge experiments revealed symmetric curves for charging/discharging (Figure 3b) and specific capacitances calculated by two electrochemical



techniques were in agreement with each other (Table S3, SI). Highest specific capacitance observed for IMC was 605 F g-1 at a current density of 0.5 A g-1 which was nearly 250 folds higher than that of precursor NC (2.5 F g-1) (Figure 3c and Table S3, SI). This remarkable specific capacitance enhancement of IMC in comparison to NC was predominantly due to presence of ultramicropores (

Immense microporous carbon@hydroquinone metamorphosed from

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