3D mesoporous reduced graphene oxide with remarkable supercapacitive performance

Accepted Manuscript 3D mesoporous reduced graphene oxide with remarkable supercapacitive performance Plawan Kumar Jha, Kriti Gupta, Anil Krishna Debnath, Shammi Rana, Rajendrakumar Sharma, Nirmalya Ballav PII:

S0008-6223(19)30306-9

DOI:

https://doi.org/10.1016/j.carbon.2019.03.082

Reference:

CARBON 14073

To appear in:

Carbon

Received Date: 5 March 2019 Revised Date:

24 March 2019

Accepted Date: 25 March 2019

Please cite this article as: P.K. Jha, K. Gupta, A.K. Debnath, S. Rana, R. Sharma, N. Ballav, 3D mesoporous reduced graphene oxide with remarkable supercapacitive performance, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.03.082. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

AC C

EP

TE D

M AN U

SC

Graphical Abstract

RI PT

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

3D Mesoporous Reduced Graphene Oxide with Remarkable Supercapacitive Performance

RI PT

Plawan Kumar Jha,a Kriti Gupta,a Anil Krishna Debnath,b Shammi Rana,a Rajendrakumar Sharma,c and Nirmalya Ballav*a,d a

Department of Chemistry, Indian Institute of Science Education and Research (IISER), Dr.

b

SC

Homi Bhabha Road, Pune, Maharashtra – 411 008, India.

Thin Film Devices Section, Technical Physics Division, Bhabha Atomic Research Centre

c

M AN U

(BARC), Trombay, Mumbai, Maharashtra – 400 085, India.

SPEL Technologies Private Limited, Ramtekadi Industrial Area, Pune – 411013, India

d

Centre for Energy Science, Indian Institute of Science Education and Research (IISER), Dr.

Homi Bhabha Road, Pune, Maharashtra – 411 008, India.

AC C

EP

TE D

*Corresponding author. E-mail: [email protected]

1

ACCEPTED MANUSCRIPT

Abstract Chemical reduction of graphene oxide (GO) to reduced graphene oxide (rGO) is an important process in view of the development of graphene-based supercapacitors on industrial level. We report an in situ chemical reduction of GO by copper(I) salt (CuCl) and isolation of

RI PT

semiconducting rGO material with three-dimensional (3D) mesoporous structure. Fabricated all solid-state supercapacitors of our rGO exhibited specific capfacitance and energy density values as high as 310 F/g at a current density of 1 A/g and 10 Wh/kg, respectively in an eco-friendly aqueous gel polymer electrolyte system. Furthermore, increasing the mass loading of rGO

SC

boosted the areal capacitance to a record value of about 580 mF/cm2 at 1 mA/cm2 current density. More than 80% capacitance was retained beyond 100,000 continued charge-discharge

M AN U

(CD) cycles. Also, sustainability of our rGO supercapacitor over switching current densities in the CD cycles was excellent resembling the rate performance in battery-like energy storing devices. The use of organic electrolyte boosted the energy density of rGO to very high level of ~22 Wh/kg.

TE D

1. Introduction

In the recent years of high energy demand and electric mobility, electrochemical capacitors are evolving spectacularly to store and supply of energy in an efficient manner.[1-8] Enormous variations of active materials for electrodes such as graphene, carbon nanotubes, metal oxides,

EP

and conducting polymers along with variable electrolytes are presently witnessing the push of energy density values of supercapacitors towards batteries without losing the power density values.[1, 5, 9-20] Amongst carbon-based materials graphene – a two-dimensional allotrope of

AC C

carbon element – can in principle deliver electrical double layer capacitance (EDLC) of ~550 F/g.[21-24] In fact, recent developments aid into the betterment of performances energy storage devices through viable engineering of graphene-based materials.[9, 21, 23, 25, 26] However, due to difficulties in large-scale synthesis of pristine graphene by bottom-up approach, top-down approach is largely accepted whereby graphite (Gr) is first oxidised to graphene oxide (GO) and subsequently reduced to reduced graphene oxide (rGO) resembling physicochemical properties close to pristine graphene.[27-29]

One critical aspect in the development of rGO-based

supercapacitors is the choice of reducing agent in the reduction of GO to rGO – conventional

2

ACCEPTED MANUSCRIPT

(hydrazine and borohydride) or unconventional (metal/acids and ascorbic acid) – which could drastically alter the extent of π-conjugation as well as degree of oxygen functionality in the material vis-à-vis its physicochemical properties affecting electrochemical performance.[28-33] Another crucial parameter is the presence of three-dimensional (3D) hierarchical porous

RI PT

structure in rGO which could serve as ion-buffering reservoirs and facilitate ion transport across channels favouring the electrochemical kinetics.[21, 34, 35] Note that two-dimensional (2D) layered structure of rGO derived from chemical reduction of GO frequently leads to stacking of the sheets in course of various electrochemical processes and detriments the overall

SC

supercapacitor performance.[21] To address such important issues either production of graphene nano-mesh by multi-step chemical synthesis involving rGO-metal ion complex or generation of

M AN U

3D hierarchical porous rGO by freeze drying technique would be desirable for attaining highperformance in supercapacitor applications.[21, 34-36]

Herein, we have employed Cu(I) chloride (CuCl) for the in situ reduction of GO cum complexation followed by removal of Cu(II) ions by HCl to produce rGO (Figure S1, S3). Such an elegant chemical reduction produced electrically conductive rGO with 3D mesoporous structure without the need of freeze-drying process and/or involvement of cumbersome multi-

TE D

step chemical synthesis. We have fabricated all solid-state supercapacitors of rGO employing eco-friendly aqueous H2SO4-polyvinylalcohol (PVA) gel polymer electrolyte. Supercapacitor performance including specific gravimetric (CG) and areal capacitance (CA) values, energy density (Ed) and power density (Pd) values, and sustainability over prolonged charge-discharge

EP

(CD) cycles even at switching current densities are remarkable and overall setting up record in

AC C

the domain of all-solid-state supercapacitors of rGO-only materials (c.f. Table S1). 2. Experimental Section

2.1 Chemicals. Graphite flake (+100 mesh), copper chloride (CuCl), and Nafion® were purchased from Sigma-Aldrich. Sulfuric acid (H2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), Hydrochloric acid (HCl), acetone, methanol (MeOH), N-methyl-2pyrrolidone (NMP), were purchased from RANKEM (India). Poly(vinyl alcohol) (PVA, n=17001800) was purchased from Loba Chemie. Millipore water (MQ, ~18 Ω) was used wherever needed.

3

ACCEPTED MANUSCRIPT

2.2 Synthesis of GO and rGO. Graphite oxide was synthesized from graphite (Gr) via modified Hummer’s method and exfoliated in the water to obtain graphene oxide (GO). In short, at low temperature (on ice bath) 6 gm Gr was mixed with 120 ml H2SO4 and stirred for an hour. After one hour 18 gm crushed KMnO4 was added in portion for half an hour. Mixture was stirred at

RI PT

~40 °C for ~1 hr and then for next 24 hr at room temperature (RT) (Note: after few hours mixture solidifies) followed by careful and slow addition of 240 ml MQ water. Mixture was further stirred at ~85 °C for next 3 hr and finally brought to RT. Finally, 720+60 ml H2O+H2O2 solution was added and stirred for 15 min. The precipitate was washed with MQ water, 5%

SC

aqueous HCl, acetone and dried in vacuum oven at 70 °C for 24 hr.

GO was centrifuged at 1000 rpm and subsequently removed the multilayers. About 0.9 gm

M AN U

CuCl was dispersed in 150 ml MQ water and mixed with 150 ml GO (~1 mg/ml) in a 1 L round bottom flask and stirred at 94 °C for 24 hr. A light green precipitates then normally filtered and washed with water/methanol and dried under vacuum at 150 °C for ~10 hr. About 250 mg greylight green powder was then mixed in 50 ml 0.5 M HCl and stirred for 48 hr at RT, followed by washing with water/acetone and dried under vacuum at 70 °C for 12 hr.

TE D

2.3 Material Characterization. Microscopy techniques such as FESEM, and TEM were used to observe morphology. Spectroscopy techniques included Raman spectroscopy (λmax = 488 nm laser), fourier transformed infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD), and X-ray photoelectron spectroscopy (XPS) used to elucidate the degree of oxidation, reduction and

EP

chemical entities in the samples. Thermogravimetric analysis (TGA), four probe I-V characteristic, and N2 adsorption-desorption isotherm at 77K were recorded to speculate the

AC C

stability, conductivity, and BET surface area of the materials respectively. XPS measurements were carried out using Mg-Kα (1253.6 eV) source and DESA-150 electron analyzer (Staib Instruments, Germany). The binding-energy scale was calibrated to Au-4f7/2 line of 84.0 eV. The analyzer was operated at 40 eV pass energy. Pressure in the chamber during analysis was ~ 5 x 10-9 Torr. All Electrochemical characterizations were performed on PARSTAT MC Galvanostat/Potentiostat, PMC-2000.

4

ACCEPTED MANUSCRIPT

2.4 10 wt% PVA-H2SO4 Gel Preparation. About 2 gm PVA + 20 ml MQ water stirred at ~80 °C until clear solution. About 1 ml concentrated H2SO4 was added into the clear solution of PVA and further stirred for 1 hr at RT

RI PT

2.5 20 wt% H2SO4 in PVA-H2SO4 Gel Preparation. About 2 gm PVA + 20 ml MQ water stirred at ~80 °C until clear solution. About 2 ml concentrated H2SO4 was added into the clear solution of PVA and further stirred for 1 hr at RT

2.6 Fabrication of All-Solid-State Supercapacitors. Electrode Preparation: A thick ink of rGO

SC

was made by mixing 95% active material and 5% binder (nafion) in NMP solvent and drop casted on 1x1 cm2 area of grafoil sheets followed by drying at 95 °C for 12-15 hr in order to

M AN U

allow the complete gel formation along with evaporation of physisorbed residual water. About 1x2 cm2 extra area of grafoil was left in order to use it for current collector during the measurement. In order to fabricate all solid-state supercapacitors of rGO, above made electrodes were dipped and soaked in 10 and 20 wt% H2SO4 in PVA-H2SO4 gel polymer electrolytes, air dried for 12 hr at RT and ambient pressure. Two similar electrodes were sandwiched using Celgard 3501 membrane separator. Further, to avoid leakage/water evaporation, if any, we have

TE D

sealed the active area of electrode by Teflon tape. Active material loading on each electrode was ~1 mg/cm2 and 3 mg/cm2. The electrochemical measurements of rGO all solid-state supercapacitors were performed at RT and ambient pressure. The potential window used for

EP

aqueous gel polymer electrolyte (GPE) is 1 V.

For organic electrolyte, 1 M TEABF4 was prepared in acetonitrile (AN) and electrochemical measurements were performed in two electrodes liquid-state system applying the potential

AC C

window of 2 V at RT and ambient pressure.

3. Results and Discussion

Combined field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analyses revealed 3D hierarchical porous structure with large channels in our rGO material (Figure 1a-d and S2-4). Such morphology is usually observed in the case of freezedried rGO gel leading to high-capacitance characteristics.[21, 37, 38] In our approach, reduction

5

ACCEPTED MANUSCRIPT

of GO to rGO by Cu(I) benefited to generate the large cavities all over rGO and avoided the stacking of sheets in the material which generally happens due to strong π-π interactions between the 2D sheets of rGO. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDXS) data showed negligible amount (almost absent) of Cu in rGO (Figure S5).

RI PT

N2 adsorption-desorption isotherms were recorded at 77 K where BET surface area of rGO was estimated to be around 362 m2/g (Figure 1e). The hysteresis loop in the graph indicates mesoporous behaviour usually observed for porous graphene-based materials.[39] The average pore diameter of our rGO was found to be ~8.7 nm (Figure 1e, inset). Such high-surface area and

SC

pore diameter in rGO are indeed desirable for high-performance supercapacitor applications. The DC electrical transport measurements performed on pressed films of rGO by conventional four-

M AN U

probe method showed conductivity value of >1.5 S/cm which is comparable to the rGOs derived chemically by conventional reducing agents (Figure 1f and S6).[30] Overall, our method of GO reduction do have potential to generate highly porous and conducting rGO which is otherwise rarely observed in the domain of chemically derived rGO. In-detail characterization of various materials viz. GO and rGO including experimental procedures and fabrications of all-solid-state

AC C

EP

TE D

supercapacitors are presented in Supporting Information (Figure S7-10).

Figure 1. (a-b) FESEM and (c-d) TEM images of rGO. (e) N2 adsorption-desorption isotherms of rGO at 77 K and (f) DC current-voltage (I-V) characteristics of rGO.

6

ACCEPTED MANUSCRIPT

To configure all-solid-state supercapacitors, rGO ink was drop-casted on grafoil sheet and two similar electrodes were sandwiched together using Celgard 3501 separator and 10 wt% PVAH2SO4 GPE (Figure 2a). The devices were subjected to thorough electrochemical investigations by cyclic voltammetry (CV), galvanic charge-discharge (CD), and electrochemical impedance

RI PT

spectroscopy (EIS) techniques. Subsequently, information on EDLC characteristics, specific capacitance (CS), equivalent series resistance (RS), energy density (Ed), power density (Pd), and cycle stability were extracted (Figure 2, 3, 4, and S8-10). Partial rectangular CV curves depict electrical double layer (EDL) formation between electrode and electrolyte which is generally

SC

observed for graphene-based materials (Figure 2b and S8a).[40] Consistent appearance of quasirectangular CV curves at low as well as high scan rate (varied from 10 to 500 mV/s) indicates

M AN U

facile percolation of ions through the channels in the material (Figure S8a). The CD plots recorded upon varying the current density from 1 to 10 A/g corroborated the CV plots (Figure 2c and S8b). The triangular CD curves at lower current density with negligible IR drop (V = IR, I = current, R = resistance) suggested fast percolation of electrolytes into the channels (Figure 2c and S8b). At higher current density a very small IR drop (~0.1 V) was observed which could be due to the lower resistance of the ions during the percolation process as was also visible in the

TE D

EIS data (Figure 2d and S8b). In the Nyquist plot (120 kHz -10 mHz), the curve edge at high frequency showed equivalent series resistance (RS) of the device to be as low as ~1.22 Ω (Figure 2d, inset) which is possibly due to the facile mobility of ions inside the pores of the rGO.[28, 40, 41] Nyquist plot at the low frequency region is nearly perpendicular to the x-axis indicating the

EP

EDL characteristics and complementing both CV and CD data (Figure 2d).[28, 41] The capacitance values extracted from the CD measurements are plotted against current density (J)

AC C

and the maximum specific capacitance (CS) value was found to be ~240 F/g at a current density of 1 A/g (Figure 2e). Also, at high current density of 10 A/g the capacitance value was found to be ~90 F/g. The time constant was calculated from Bode phase plot and was found to be ~1 sec (Ƭ=1/ƒₒ, ƒₒ=1 Hz at ~45° phase angle) which is in fact very low as far as rGO-based materials are concerned (Figure S8c). The time independent capacitance of rGO obtained after employing the following formula, C = k1 + k2v-1/2 (where, C = capacitance, k1 = rate independent component attributing for EDLC, k2v-1/2 represents long-T capacitance data) was observed to be ~112 F/g (extrapolating the curve to discharge time1/2 = 0) which is noteworthy (Figure 2f).[42, 43] In our system, Ed and Pd values from the CD measurements were estimated to be ~8 Wh/kg and ~245

7

ACCEPTED MANUSCRIPT

W/kg, respectively, at a current density of 1 A/g (Figure 3a). Further increase in the current density to 10 A/g boosted the Pd value beyond 2.2 kW/kg maintaining the Ed value at ~2.5 Wh/kg (Figure 3a). Such values of Ed and Pd involving our rGO material were noted to be

M AN U

SC

RI PT

comparable or higher than other chemically synthesised rGOs (Table S1).

TE D

Figure 2. (a) Schematic presentation of all solid-state supercapacitor. (b-d) CV, CD, and Nyquist plots, (e) CS versus current density (J) plots and (f) CS versus (discharge time)1/2 plots, of rGO all solid-state supercapacitors in 10 and 20 wt% H2SO4 in PVA-H2SO4 GPE.

EP

In view of massive energy consumption, supercapacitors needs to undergo multiple chargedischarge cycles, hence long term cycling stability of active material is very important. In order

AC C

to test the durability of our rGO material, 100,000 continued CD cycles were performed at a high current density of 20 A/g. Remarkably, after 100,000 cycles the capacitance retention of rGO was found to be more than 80% thereby strongly validating the high-stability of the active material (Figure 3b and S9). In energy storage devices like supercapacitors and batteries, Ed and Pd values along with durability decide the commercial viability.[44] It is noticeable that the Ed of symmetric supercapacitor can be enhanced either by increasing the capacitance value or by expanding the voltage window by applying non-aqueous electrolyte in the system.[40] Recent reports on rGO based supercapacitors strongly suggest to avoid the use of often flammable organic electrolytes in the view of eco-friendly operation.[9] Thus, a rational way to boost the Ed

8

ACCEPTED MANUSCRIPT

value of rGO-based symmetric two electrodes all-solid-state supercapacitors in pure aqueous GPE (without any additives) would be to increase the capacitance value itself. Here, we have employed a novel approach of fabricating all-solid-state supercapacitor of rGO in 20 wt% H2SO4 in PVA-H2SO4 GPE and compared the electrochemical performance with those in 10 wt%

RI PT

H2SO4 in PVA-H2SO4 GPE – keeping the other parameters identical. The EDL formation between electrode and electrolyte was realised by appearance of partial rectangular CV and triangular CD curves (Figure 2b, c and S8d, e). In CV, the current density significantly increased from 10 to 20 wt% system which indicates the higher charge accumulation in the material as a

SC

result of the increase in the concentrations of H+ and SO42- in the electrolyte (Figure 2b). The discharge time was also increased from 10 to 20 wt% system in CD data complementing the CV

M AN U

data (Figure 2c). From Nyquist plot, RS value of ~1.29 Ω suggests that the higher concentration of electrolytic ions did not affect the internal resistance of the device (Figure 2d, inset). Imperceptible semicircle in high frequency region indicates the consequence of higher concentration of H+ and SO42- ions in 20 wt% system – the concentration of ions per unit volume probably reducing the charge-transfer resistance and accelerating the percolation of ions inside the channels of rGO (Figure 2d, inset). Moreover, at low frequency region, real Z values in 20

TE D

wt% H2SO4 in PVA-H2SO4 GPE are lower than that of 10 wt% PVA-H2SO4 indicating the superior capacitive behaviour of our rGO in the former system (Figure 2d). The capacitance value of rGO in 20 wt% H2SO4 in PVA-H2SO4 was remarkably increased (1.3 times) as compared to that in 10 wt% PVA-H2SO4 (Figure 2e). Both at low and high current densities of 1

EP

and 10 A/g, the capacitance values were calculated to be ~310 F/g and ~165 F/g, respectively, in the 20 wt% H2SO4 in PVA-H2SO4 system (Figure 2e). We have also estimated the time

AC C

independent capacitance value of ~185 F/g which is higher compared to that in 10 wt% PVAH2SO4 GPE (Figure 2f).

9

ACCEPTED MANUSCRIPT

Figure 3. (a) Ed and Pd, (b) Durability cycles at a current density of 20 A/g, of rGO all solid-state supercapacitors in 10 and 20 wt% H2SO4 in PVA-H2SO4 GPE and (c) Durability cycles at different current densities (switching of J consecutively after 10000 cycles, in 20 wt% H2SO4 in

RI PT

PVA-H2SO4 GPE. Finally, the Ed value in 20 wt% H2SO4 in PVA-H2SO4 is found to be ~10.5 Wh/kg which is increased by a factor of 1.3 as compared to the value calculated for 10 wt% PVA-H2SO4 – keeping the Pd value unaffected (Figure 3a). To examine the durability of our rGO all-solid-state

SC

supercapacitor at relatively higher concentrations of H2SO4 in PVA-H2SO4, we have carried out 100,000 continued CD cycles at a current density of 20 A/g. Noticeably, even after 100k cycles, more than 80% capacitance retention was observed which indicates that such a dramatic

M AN U

modulation in the concentration of GPE did not affect the stability of our rGO at all and in fact, facilitated the energy storage capacity (Figure 3b). In both 10 as well as 20 wt%, coulombic efficiency was 100% (Figure S9). From Bode phase plot time constant was calculated to be around 4 sec (Figure S8f). The sustainability of any supercapacitor depends on the response over switching current input. Our rGO supercapacitor was subjected to 30k CD cycles at different current densities. After first set of 10k cycles at 15 A/g, capacitance retention was >80% which

TE D

was continued for next 10k cycles at 20 A/g. Interestingly, when current density was switched back to 15 A/g for 10k cycles, capacitance value came to its initial value (100% retention), overall reflecting that a sudden variation of current density could not alter the capacitive

AC C

EP

performance of our rGO material (Figure 3c).

10

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Figure 4. (a) Specific capacitance (CA and CG) versus current density plots, (b) Specific Ed vs Pd plots, (c) Durability cycles at a current density of 15 A/g, and (d) Durability cycles at different current densities (switching of J consecutively after 5000, of rGO (rGO mass loading = 3

TE D

mg/cm2) all solid-state supercapacitors in 20 wt% H2SO4 in PVA-H2SO4 GPE. Mass loading of the active material on electrodes plays a pivotal role in the supercapacitor performance as it can primarily change the capacitance value. We have increased the mass loading of rGO from 1 to 3 mg/cm2 and fabricated all solid-state supercapacitors in 20 wt%

EP

H2SO4 in PVA-H2SO4 GPE. The partial rectangular CV and triangular CD curves at low scan rate and current density, respectively, and the Nyquist plot consistently show the formation of

AC C

EDL between electrode and electrolyte (Figure S10). CS (areal capacitance (CA) and gravimetric capacitance (CG)) were calculated from the CD curves. Remarkably, at higher mass loading of rGO, the CA value was found to be around 579 mF/cm2 at a current density of 1 mA/cm2 while the CG value was estimated to be around 193 F/g at a current density of 0.33 A/g (Figure 4a) which is as per our knowledge could be the highest value in the domain of all solid-state supercapacitors of rGO- only materials in aqueous PVA-H2SO4 GPE (Table S1). The maximum areal Ed (39.7 µWh/cm2) and Pd (12.4 mW/cm2) and gravimetric Ed (6.6 Wh/kg) and Pd (2.1 kW/kg) of rGO were realised to be very impressive (Figure 4b). The rate independent capacitance of rGO was noted to be (CA) ~350 mF/cm2 and (CG) ~120 F/g (Figure S10d). The

11

ACCEPTED MANUSCRIPT

device was subjected for 50,000 CD cycles at a current density of 15 A/g, and the retention of CS was observed to be close to ~100% (Figure 4c). After 50,000 CD cycles, current density was switched to 30 mA/cm2 (10 A/g) for next 5,000 cycles and again switched back to 45 mA/cm2 (15 A/g) for next 5,000 cycles, and process was continued. More than 85% retention of CS and

RI PT

100% coulombic efficiency after complete CD cycles at J = 30→45→30→45→30→45→30 mA/cm2 for each 5,000 cycles (total 35,000 cycles) is extraordinary resembling the high rate performance feature of battery (Figure 4d). Overall, such supercapacitor performances of our rGO material presented in this work appear unprecedented in the domain of 2D as well as 3D

SC

rGO-only materials which we attribute to the quality of rGO which could be inculcated upon reduction of GO with Cu(I) salt (Table S1). Specifically, XPS and FTIR data revealed that –OH

M AN U

group is almost absent in our rGO which is strikingly distinctive from other reported rGOs derived by chemical reduction (Figure S5). Additionally, upon reduction of GO by Cu(I) the concomitant formation of Cu(II) in the reaction media could heal the rGO material via intramolecular cross dehydrogenative (ICDC) coupling mechanism likewise we observed in the case of GO reduction by Fe(II).[28] Such preliminary investigations on the mechanistic insights

AC C

EP

TE D

motivate an in-depth fundamental study in the future.

Figure. 5. (a) CD plot, (b) Cs versus current density (J), (c) Ragon plot, and (d) Durability cycles at 10 A/g, of rGO all solid-state supercapacitor in 1 M TEABF4/AN electrolytic solution.

12

ACCEPTED MANUSCRIPT

In order to increase the potential window (2V) thus boost the energy density of rGO, we have used an organic electrolyte (tetraethylammonium tetrafluoroborate in acetonitrile solution (TEABF4/AN)). The CV and CD plot characteristics indicating the EDL formation (Figure 5a and S11a, b). The maximum specific capacitance calculated from CD curve found to be as high

RI PT

as 165 F/g at a current density of 1 A/g (Figure 5b). Also, the capacitance value was more than 50% beyond 10 A/g current density, hence indicates the stability of the material over various current densities. From Ragone plot the maximum Ed and Pd was found to be 22 Wh/kg and 4.2 kW/kg, respectively which is remarkable (Figure 5c). Finally, even after 5000 continuous CD

SC

cycles more than 90% capacitance retention was realised (Figure 5d).

M AN U

4. Conclusions

In conclusions, high-quality rGO material with an optimal balancing of surface area (~360 m2/g), electrical conductivity (>1.5 S/cm), and morphology (3D mesoporous) was produced upon wet-chemical reduction of GO by CuCl. All solid-state supercapacitor devices of rGO as such (without any post-synthetic modification) in aqueous H2SO4- PVA GPE system were successfully fabricated. An increase in the concentration of H2SO4 (from 10 to 20 wt%) in

TE D

H2SO4-PVA remarkably boosted the respective specific capacitance (CS) and energy density values beyond 300 F/g and 10 Wh/kg at a current density of 1 A/g without any loss in maximum power density value (2.3 kW/kg) as well as cycling stability (>80% retention of CS value beyond 100,000 charge-discharge (CD) cycles). Also, increasing the mass loading of rGO from 1 mg to

EP

3 mg enhanced the areal capacitance to a record value of about 580 mF/cm2 at 1 mA/cm2 current density. Sustainability of all-solid-state supercapacitors at switching current densities over

AC C

thousands of CD cycles was noteworthy. We have also boosted the energy density of our rGO material by using organic electrolyte. Our elegantly simple approach of producing rGO using transition metal salts could lead to the development of sustainable and high-performance graphene-based supercapacitors fulfilling industrial demands. Conflicts of interest There are no conflicts to declare. Acknowledgment

13

ACCEPTED MANUSCRIPT

Financial supports from MHRD-FAST (India, Project – CORESUM), DST-Nano Mission (India, Project No. SR/NM/TP-13/2016(G)), SERB (EMR/2016/001404), and IISER Pune are thankfully acknowledged. P.K.J thanks SERB and Infosys. P.K.J., S.R., and K.G., thanks IISER

RI PT

Pune for fellowship. Appendix A: Supplementary Information.

References

SC

[1] Z. Lin, E. Goikolea, A. Balducci, K. Naoi, P.L. Taberna, M. Salanne, G. Yushin, P. Simon, Materials for supercapacitors: when Li-ion battery power is not enough, Mater. Today 21(4)

M AN U

(2018) 419-436.

[2] Y. Wang, Y. Song, Y. Xia, Electrochemical capacitors: mechanism, materials, systems, characterization and applications, Chem. Soc. Rev. 45(21) (2016) 5925-5950. [3] A. González, E. Goikolea, J.A. Barrena, R. Mysyk, Review on supercapacitors: technologies and materials, Renew. Sustain. Energy. Rev. 58 (2016) 1189-1206.

[4] A. Vlad, N. Singh, C. Galande, P.M. Ajayan, Design considerations for unconventional

TE D

electrochemical energy storage architectures, Adv. Energy Mater. 5(19) (2015) 1402115. [5] B.C. Kim, J.-Y. Hong, G.G. Wallace, H.S. Park, Recent progress in flexible electrochemical capacitors: electrode materials, device configuration, and functions, Adv. Energy Mater. 5(22) (2015) 1500959.

EP

[6] P. Simon, Y. Gogotsi, B. Dunn, Where do batteries end and supercapacitors begin?, Science 343(6176) (2014) 1210-1211.

AC C

[7] S. Chu, A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature 488(7411) (2012) 294-303.

[8] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum, New York, USA, 1999. [9] J.Y. Hwang, M.F. El-Kady, M. Li, C.-W. Lin, M. Kowal, X. Han, R.B. Kaner, Boosting the capacitance and voltage of aqueous supercapacitors via redox charge contribution from both electrode and electrolyte, Nano Today 15 (2017) 15-25.

14

ACCEPTED MANUSCRIPT

[10] J. Yang, E. Zhang, X. Li, Y. Yu, J. Qu, Z.Z. Yu, Direct reduction of graphene oxide by Ni foam as a high-capacitance supercapacitor electrode, ACS Appl. Mater. Interfaces 8(3) (2016) 2297-305. [11] Q. Wang, J. Yan, Z. Fan, Carbon materials for high volumetric performance

729-762.

RI PT

supercapacitors: design, progress, challenges and opportunities, Energy Environ. Sci. 9(3) (2016)

[12] E. Mourad, L. Coustan, P. Lannelongue, D. Zigah, A. Mehdi, A. Vioux, S.A. Freunberger, F. Favier, O. Fontaine, Biredox ionic liquids with solid-like redox density in the liquid state for

SC

high-energy supercapacitors, Nat. Mater. 16 (2016) 446-453.

[13] M.R. Lukatskaya, B. Dunn, Y. Gogotsi, Multidimensional materials and device

M AN U

architectures for future hybrid energy storage, Nat. Commun. 7 (2016) 12647. [14] W.K. Chee, H.N. Lim, Z. Zainal, N.M. Huang, I. Harrison, Y. Andou, Flexible graphenebased supercapacitors: a review, J. Phys. Chem. C 120(8) (2016) 4153-4172. [15] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, A review of electrolyte materials and compositions for electrochemical supercapacitors, Chem. Soc. Rev. 44(21) (2015) 74847539.

TE D

[16] Z. Yu, L. Tetard, L. Zhai, J. Thomas, Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions, Energy Environ. Sci. 8(3) (2015) 702-730. [17] Z. Niu, L. Liu, L. Zhang, W. Zhou, X. Chen, S. Xie, Programmable nanocarbon-based architectures for flexible supercapacitors, Adv. Energy Mater. 5(23) (2015) 1500677.

EP

[18] K. Chen, S. Song, F. Liu, D. Xue, Structural design of graphene for use in electrochemical energy storage devices, Chem. Soc. Rev. 44(17) (2015) 6230-57.

AC C

[19] F. Bonaccorso, L. Colombo, G. Yu, M. Stoller, V. Tozzini, A.C. Ferrari, R.S. Ruoff, V. Pellegrini, Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage, Science 347(6217) (2015) 1246501. [20] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Adv. Mater. 22(8) (2010) E28-62.

[21] Y. Shao, M.F. El‐Kady, C.W. Lin, G. Zhu, K.L. Marsh, J.Y. Hwang, Q. Zhang, Y. Li, H. Wang, R.B. Kaner, 3D freeze‐casting of cellular graphene films for ultrahigh‐power‐density supercapacitors, Adv. Mater. 28(31) (2016) 6719-6726.

15

ACCEPTED MANUSCRIPT

[22] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306(5696) (2004) 666-669. [23] M.F. El-Kady, Y. Shao, R.B. Kaner, Graphene for batteries, supercapacitors and beyond,

RI PT

Nat. Rev. Mater. 1(7) (2016) 16033.

[24] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6(3) (2007) 183-191.

[25] K. Yuan, Y. Xu, J. Uihlein, G. Brunklaus, L. Shi, R. Heiderhoff, M. Que, M. Forster, T. Chassé, T. Pichler, T. Riedl, Y. Chen, U. Scherf, Straightforward generation of pillared,

SC

microporous graphene frameworks for use in supercapacitors, Adv. Mater. 27(42) (2015) 67146721.

M AN U

[26] M.F. El-Kady, M. Ihns, M. Li, J.Y. Hwang, M.F. Mousavi, L. Chaney, A.T. Lech, R.B. Kaner, Engineering three-dimensional hybrid supercapacitors and microsupercapacitors for highperformance integrated energy storage, Proc. Natl. Acad. Sci. U S A 112(14) (2015) 4233-8. [27] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc 80(6) (1958) 1339-1339.

[28] P.K. Jha, S.K. Singh, V. Kumar, S. Rana, S. Kurungot, N. Ballav, High-level

TE D

supercapacitive performance of chemically reduced graphene oxide, Chem 3(5) (2017) 846-860. [29] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, carbon 45(7) (2007) 1558-1565.

EP

[30] C.K. Chua, M. Pumera, Chemical reduction of graphene oxide: a synthetic chemistry viewpoint, Chem. Soc. Rev. 43(1) (2014) 291-312.

AC C

[31] C.K. Chua, A. Ambrosi, M. Pumera, Graphene oxide reduction by standard industrial reducing agent: thiourea dioxide, J. Mater. Chem. 22(22) (2012) 11054-11061. [32] A. Kumar, M. Khandelwal, A novel synthesis of ultra thin graphene sheets for energy storage applications using malonic acid as a reducing agent, J. Mater. Chem. A 2(47) (2014) 20345-20357.

[33] S. Pei, J. Zhao, J. Du, W. Ren, H.-M. Cheng, Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids, Carbon 48(15) (2010) 44664474.

16

ACCEPTED MANUSCRIPT

[34] H. Jiang, P.S. Lee, C. Li, 3D carbon based nanostructures for advanced supercapacitors, Energy Environ. Sci. 6(1) (2013) 41-53. [35] X.-N. Tang, C.-Z. Liu, X.-R. Chen, Y.-Q. Deng, X.-H. Chen, J.-J. Shao, Q.-H. Yang,

supercapacitor electrodes, Carbon 146 (2019) 147-154.

RI PT

Graphene aerogel derived by purification-free graphite oxide for high performance

[36] H.-K. Kim, S.-M. Bak, S.W. Lee, M.-S. Kim, B. Park, S.C. Lee, Y.J. Choi, S.C. Jun, J.T. Han, K.-W. Nam, K.Y. Chung, J. Wang, J. Zhou, X.-Q. Yang, K.C. Roh, K.-B. Kim, Scalable

applications, Energy Environ. Sci. 9(4) (2016) 1270-1281.

SC

fabrication of micron-scale graphene nanomeshes for high-performance supercapacitor

[37] Y. Wang, J. Chen, J. Cao, Y. Liu, Y. Zhou, J.-H. Ouyang, D. Jia, Graphene/carbon black

269-277.

M AN U

hybrid film for flexible and high rate performance supercapacitor, J. Power Sources 271 (2014)

[38] S. Chen, W. Ma, H. Xiang, Y. Cheng, S. Yang, W. Weng, M. Zhu, Conductive, tough, hydrophilic poly(vinyl alcohol)/graphene hybrid fibers for wearable supercapacitors, J. Power Sources 319 (2016) 271-280.

[39] J.C. Groen, L.A.A. Peffer, J. Pérez-Ramı́rez, Pore size determination in modified micro- and

TE D

mesoporous materials. Pitfalls and limitations in gas adsorption data analysis, Micropor. Mesopor. Mat. 60(1–3) (2003) 1-17.

[40] Y. Meng, Y. Zhao, C. Hu, H. Cheng, Y. Hu, Z. Zhang, G. Shi, L. Qu, All‐graphene core‐ sheath microfibers for all‐solid‐state, stretchable fibriform supercapacitors and wearable

EP

electronic textiles, Adv. Mater. 25(16) (2013) 2326-2331. [41] E. Casero, A.M. Parra-Alfambra, M.D. Petit-Domínguez, F. Pariente, E. Lorenzo, C.

AC C

Alonso, Differentiation between graphene oxide and reduced graphene by electrochemical impedance spectroscopy (EIS), Electrochem. Commun. 20 (2012) 63-66. [42] V. Augustyn, J. Come, M.A. Lowe, J.W. Kim, P.-L. Taberna, S.H. Tolbert, H.D. Abruña, P. Simon, B. Dunn, High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance, Nat. Mater. 12(6) (2013) 518-522. [43] Q. Chen, Y. Meng, C. Hu, Y. Zhao, H. Shao, N. Chen, L. Qu, MnO2-modified hierarchical graphene fiber electrochemical supercapacitor, J. Power Sources 247 (2014) 32-39. [44] Z.S. Wu, K. Parvez, X. Feng, K. Müllen, Graphene-based in-plane micro-supercapacitors with high power and energy densities, Nat. Commun. 4 (2013) 2487.

17