Oxygen-rich hierarchical porous graphene as an excellent electrode

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Oxygen-rich hierarchical porous graphene as an excellent electrode for supercapacitors, aqueous Al-ion battery and capacitive deionization Mohanapriya K, and Neetu Jha ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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ACS Sustainable Chemistry & Engineering

Oxygen-rich hierarchical porous graphene as an excellent electrode for supercapacitors, aqueous Al-ion battery and capacitive deionization Mohanapriya K. and Neetu Jha* Department of Physics, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga East, Mumbai, India – 400019.

*Corresponding author: Email: [email protected] Tel: +91-22-33612663 Fax: +91-22-33612020

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ACS Sustainable Chemistry & Engineering 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

Abstract In this study, we have described the synthesis of oxygen-rich hierarchical porous graphene (O-PG) using focused solar radiation on nitric acid treated graphene oxide film. This nitric acid treatment for graphene oxide helps in generating micropores onto the graphene sheets and introducing oxygen-containing functional groups when solar light is focused. This unique hierarchical porous structure along with high surface area and oxygenated functional groups makes O-PG as suitable electrode in energy storage and also for capacitive deionization. The as-prepared O-PG exhibits high specific capacitance of 354 F g-1 along with the energy storage of 110.6 Wh kg-1 at 1 A g-1 current density. O-PG also delivers high specific capacity of 90 mAh g-1 as high performance cathode in aqueous Al-ion battery with an exceptionally high cyclic stability upto 10000 cycles of charging-discharging processes at high current density of 0.5 A g-1. In addition, the electrosorption studies of salt ions were done using capacitive deionization technique. The extraordinary salt removal capacity of 21.1 mg g-1 has been obtained for 500 mg L-1 NaCl solution at voltage of 1.4 V. This material not only removes NaCl from salty water, but also removes other major salts such as MgCl2 and Ca(SO4)2. Hence, this work demonstrates a simple way of utilizing solar radiation to prepare high performance O-PG and also its applicability as an excellent electrode in high energy supercapacitors, aqueous Al-ion battery and capacitive deionization.

Keywords: Supercapacitor; aqueous Al-ion battery; capacitive deionization; oxygen-rich porous graphene; energy storage


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Introduction In recent years, socio-economic changes have led to significant increase in demand for grid storage, electric hybrid vehicles and portable electronics. This has a positive impact on global research interest in the area of electrochemical energy-storage devices, such as supercapacitors and rechargeable batteries. Devices with high power and energy densities along with long lifetime are present requirement of the society

1–5.

Supercapacitors or

electrochemical capacitors (EC) have shown substantial interest because of its high power density, good rate capability and excellent cycling stability. However, its application in various electronic devices is limited owing to low energy density 6. Last decade witnessed the emergence of various electrode materials ranging from activated carbon, carbon nanotubes and graphene7–11 for electrical double layer supercapacitors to materials like transition metal oxides, its hydroxides and electrically conducting polymers for pseudocapacitors

12–17.

Actually, electrode material with large surface area, good ionic mobility, high electrical conductivity and excellent electrochemical stability can lead to better energy storage performance. Over a decade, one of the carbonaceous materials such as porous activated carbon has been used as electrodes in commercial EC. This offers large surface area, good electrical conductivity, high power, excellent cycle life, low cost and availability6. But, this electrode material failed in charge storage performance (80-120 F g-1) due to larger pore size which reduced the ionic transport rate. Basically, along with high surface area, the pore size of electrode materials should match with the electrolyte ion size for better accessibility. Aluminium, being the most abundant metal in the Earth’s crust, owing to 3-electrons redox property of aluminium (Al), rechargeable Al-based batteries in principle offers costeffective, high specific capacity and safe alternative energy storage device, which would lead to a significant advancements in energy storage technology18. However, the perception about Al-ion storage is not new and almost for three decades it failed to compete with other energy



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storage devices such as, Li-ion and Li polymer batteries, due to the limitations such as electrode material decompose, low voltage profile (0.55V), and unsatisfactory life cycle (< 100 cycles) with fast decay in capacity of 26–85% over 100 cycles19–22. Therefore, the desirable electrode material for Al-ion storage should possess exceptionally high operating voltage profiles, superior stabilities over more number of cycles and excellent rate capabilities. Another major problem which society deals with is water scarcity. With increasing population growth and industrial demands, water is becoming one of our most valuable and scarce commodities. Of the earth’s water capacity, 97.5% is currently available as salt water and remaining 2.5% as fresh water. It is estimated that only 0.3% considered as naturally available renewable source of fresh water and the rest is inaccessible due to the fact that it is frozen in ice caps and glaciers23. There are quite a few technologies today for removing salt from saline water. Reverse osmosis and multi-stage flash are the current leading desalination technologies. But, these processes works at high pressure, are energy intensive and possess large capital cost.

Due to these restrictions, there is an urgent need for a competitive

technology that can operate in smaller, portable units with low pressures.

Capacitive

deionization (CDI) is an upcoming technique that possesses potential to fill that requirement. The salt ions can be adsorbed onto the surface of the positive and negative electrodes by flowing salty water with an applied voltage thereby removing the ions from salty water and the electrodes can be regenerated. While there is significant potential for the application of energy storage and capacitive deionization technologies, their operating performance is still lagging behind the increasingly harsh requirements of the industry. One key challenge is the identification of ideal electrode materials that satisfy the requirements such as high charge storage and better deionization properties. Carbon materials such as activated carbon24,25, carbon aerogel26, ordered


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mesoporous carbon27, nanoporous carbon28, carbon nanotubes29,30 and graphene31,32 have been investigated extensively in many energy storage devices and capacitive deionization. Among these, graphene has received enormous attention in the last decade due to its theoretical specific surface area (2600 m2 g-1) is high, exceptional electrical conductivity of 106 Scm-1 at room temperature, broad electrochemical window, high mechanical strength and good chemical stability32. These exceptional properties of 2D structure of graphene make it a promising candidate for widespread applications in nanoelectronics33,34, sensors35–37, catalysis38,39,

gas

separation/storage40,

fuel

cells,

supercapacitors11,41,

capacitive

deionization42,43 and batteries44–46. However, strong van dar Waal interaction between graphene sheets leads to π – π stacking, which reduces the experimentally obtainable surface area significantly below the theoretical value. This surface area reduction imposes huge mass transport limitation on the diffusion of electrolytic ions, which ultimately leads to deteriorated electrode performance

10,47,48.

To overcome these limitations, increasing effort

has been put to insert active materials like “spacers” in-between graphene sheets. However, the inserted pseudocapacitive materials (metal hydroxides, metal oxides and conducting polymers) undergo moderate change in its structure during the repeated reduction-oxidation reactions during charging and discharging processes, which leads to the poor cycling stability of device. Alternative approach is the engineering of graphene and other 2D structures like highly curved10, corrugated49, crumpled graphene balls50, porous graphene51,52, carbon nanomesh53, open porous 3D turbostratic graphene54, 3D intercalated graphene sheet-sphere microstructure55, sandwich-like n-doped graphene56 and 2D boron carbon nitride nanosheets57. These structures would enhance the accessible surface area to facilitate the electrolytic ions transportation, favouring the double layer formation. Among all modified graphene structures, graphene with certain amount of interconnected micropores and mesopores is anticipated to provide transport channel to the electrolyte ions and hence



reducing mass transfer limitation. In addition to the structure, the chemical nature of electrode material is an equally important factor which affects the performances of the electrode material. The oxygen-based functional groups attached with the graphene sheets can improve the wettability, hence increasing the charge storage and salt adsorption capacity. However, among the conventional approaches available for pore formation and functionalization are normally coupled with high energy consumption and complex process. So, graphene-based electrodes with high SSA and reduced restacking issues, with suitable pore size distribution and surface functionalities are anticipated to outperform other carbon electrodes in the energy storage devices and capacitive deionization. Here, we report the development of simple technique for the preparation of oxygenrich hierarchical porous graphene (O-PG). When the naturally available solar radiation was focused onto the nitric acid treated graphene oxide film, nitric acid molecules which were adsorbed on the graphene oxide sheets generates pores on its surface. During this process, along with pores oxygen functional groups were also attached at pore sites and edge planes of graphene sheets. When studied as supercapacitor electrode, the O-PG framework shows attractive energy storage performances in both, aqueous (1M H2SO4) and ionic liquid (EMIMBF4) electrolytes. It exhibits high specific capacitance, excellent energy and power densities, along with long cycle life. The performance of adsorption and desorption of Alions onto O-PG has also been evaluated for electrochemical Al-ion storage application where it shows high capacity and outstanding cyclic stability. Using the similar working principle, the removal of salt ions from salt solution was done using capacitive deionization technique. So, this material can be considered as one of the best electrode material for energy storage and also in capacitive deionization applications.


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Experimental Section Synthesis of O-PG At first, graphene oxide sheets were synthesized using Simplified Hummers method58. To make GO suspension, 0.3 g of GO sheets were dispersed in ethanol (150 ml) and then sonicated for 2 hours in bath sonicator. Then, conc. HNO3 (6 ml) was added into GO suspension and again continued the sonication for ~1 hour. After that, the mixture of acid and GO dispersion was left as such overnight without any stirring. Later, the acid-treated GO sheets (a-GO) were allowed to settle down. Finally, the supernatant was discarded and settled a-GO was dried at atmospheric conditions. The dried a-GO film was peeled off carefully and reduced by focused solar radiation with the help of convex lens. In detail, the solar radiation was focused using 90 mm diameter of convex lens on a-GO (2cm x 2cm) material. The reduction process of GO initiates rapidly due to rapid climb in temperature by focused solar irradiation, which provides the required energy for the exfoliation as explained earlier59. This process also simultaneously leads to the formation of pores, wherever nitric acid molecules were attached on a-GO. These processes have taken approximately two seconds to finish the reduction and as well as pore formation. The measured power of focused solar radiation was approximately 5.4 W and the temperature of the focused spot reached up to 204 C. The as-synthesized O-PG was used as it is for further analysis.

Characterization techniques The morphology of O-PG sample was studied using high resolution transmission electron microscopy (HRTEM) using JEOL JEM. For HRTEM analysis, as synthesized PG sample (about 0.5mg) was dispersed in 2 mL of ethanol using bath sonicator for 30 mins. After sonication, a drop of this suspension was added on carbon coated 200 mesh Cu grid using capillary tube and evaporated the ethanol using IR lamp.

The surface area was

measured using nitrogen adsorption and desorption isotherms with BET sorptometer (BET



201A). Raman spectroscopy (Horiba HR 800 model) was performed to understand the scattering centre of porous structure analyzed in the range of 1000 - 3000 cm-1 at room temperature (303K) with laser excitation wavelength of 514.5 nm, spot size with 1μm and incident power of ~10mW. XRD pattern was obtained on powder X-ray diffractometer (Bruker D8 Advance) using Cu Kα radiation with the wavelength of 1.54 Å. The X-ray photoelectron spectroscopic (XPS) measurements were done using PHI 5000 Versaprobe II model.

Atomic force microscopy (AFM) images and height profiles were analyzed to

understand the number of layers present in the material using Digital Instrument from Veeco. Dynamic contact angle was measured using DIGIDROP DS model from GBX. FTIR spectra were recorded for GO and O-PG by Fourier transform infrared spectroscopy with Perkin Elmer Spectrum BX model.

Thermo gravimetric analysis (Perkin Elmer analyzer) was

performed in air atmosphere from RT to 800 C with the rate of heating is about 5 C/min. The reduction extent of GO was examined by CHNSO analysis (Thermo Finnigan, FLASH EA 1112). The intensity of solar radiation was measured using Radiation indicator (Dynalab AN 2104) and digital thermometer (DTM-100) of K1-type thermocouple was used to measure the temperature of focused solar radiation.

Electrochemical measurements Electrochemical performance of O-PG in high energy density supercapacitors: Supercapacitor cell was fabricated using symmetric two electrode coin cell. The suspension (3mg/ml) of O-PG sample prepared by mixing the sample with solvent (ethanol) using bath sonicator. This suspension was spin coated on graphite sheet and dried overnight at ambient conditions. The difference in weight of graphite sheet before and after loading the electrode material is considered as the mass of active material to be analyzed. The total mass loaded on two graphite sheets was maintained 2.5 mg. The glass fibre filter paper (whatman) was wetted with electrolyte solution and placed in-between two electrodes and the


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supercapacitor cell was assembled in CR2032 type coin cell. To perform the electrochemical measurements

1M

H2SO4

aqueous

solution

and

3-ethyl

1-methylimidazolium

tetrafluoroborate (EMIMBF4) ionic liquid were used as aqueous and ionic liquid electrolytes. All electrochemical tests were carried out with electrochemical setup (µAUTOLAB Type III). The cyclic voltammetry and charging/discharging measurements were performed in the potential range 0 - 1 V for aqueous and 0 - 3 V for ionic liquid electrolytes, respectively. The specific capacitance (Cs) values of the sample was calculated using the equation (1) given below, 𝐶𝑠 = 4

(𝐼 × ∆𝑡)

(1)

(𝑚 × ∆𝑉)

Energy density (E) and power density (P) were calculated using the following equations (2) and (3), 𝐸=

1 𝐶(𝛥𝑉)² 2

(2)

𝑃=

𝐸 𝛥𝑡

(3)

Where I is the constant discharge current (A), ∆𝑡 is the total discharge time (s), m is the weight of the active material on both electrodes (g) and ∆𝑉 is the active voltage profile (excluding voltage drop) (V).

Electrochemical performance of O-PG as cathode for aqueous Al-ion battery: Electrochemical properties of O-PG as cathode in aqueous Al-ion battery was examined using three-electrode method, where platinum rod and Ag/AgCl were used as counter and reference electrode, respectively60. To this study, the working electrode was prepared by the following method. First, O-PG suspension was prepared by dispersing it in ethanol using bath sonicator for 30 mins. Then, O-PG suspension was spin coated on graphite sheet (1 mg loaded on graphite sheet of 1 cm2). The aqueous 1M AlCl3 electrolyte solution



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was prepared by adding AlCl3 in deionized water. All the electrochemical studies were performed in a Metrohm (µAUTOLAB Type III) instrument.

Salt removal performance of O-PG using capacitive deionization (CDI) technique: The O-PG electrode material was mixed with ethanol (solvent) under sonication for 1 hour without adding any binder and the obtained slurry was brush-coated on to the two graphite sheet substrates. These electrodes kept for drying in oven at 60 ̊C for 1 hour to ensure the complete evaporation of the solvent.

The loaded electroactive material was

weighed exactly 0.05 g on each graphite sheet. The CDI experimental setup was fabricated for the batch-mode continuous flow system. During each experiment, the salt solution was continuously pumped by a peristaltic pump into CDI cell and the effluent returned to the feed tank. To conduct the salt removal study, aqueous NaCl solutions were prepared with the initial concentration of 300, 400 and 500 mg L-1. The total volume of the NaCl solution was maintained as 75 ml and the temperature of 29°C during the experiment. The relationship between concentration and conductivity was obtained according to a calibration curve made prior to the experiment. The voltage was applied from 0.8 to 1.4 V between the electrodes with an increment of 0.2 V in the CDI cell. The change in conductivity was recorded at different time intervals. The salt adsorption capacity (SAC) was calculated based on the following equation, SAC =

(Co - 𝐶𝑒) ∗ 𝑉

(4)

𝑚

Where SAC (mg g-1) is the salt adsorption capacity, Co and Ce are the initial and final concentrations (mg L-1), V is the volume of NaCl solution used (L) and m is the total mass of both electrodes (g).


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The salt adsorption rate (SAR) of the electrode material was calculated according to the following equation: SAR =

𝑆𝐴𝐶

(5)

𝑡

Where SAR (mg g-1 min-1) is the salt adsorption rate, SAC is the salt adsorption capacity and t is the salt adsorption time.

Results and Discussion Morphology and structure of O-PG

(a)

(b)



Fig. 1. (a) Digital images of (i) GO sheets in HNO3 solution, (ii) a-GO film and (iii) O-PG. (b) Schematic illustration of fabrication of O-PG. The electrolyte ions (red spheres) in (iii) can diffuse throughout the O-PG framework rapidly through the in-plane and cross-plane paths. The synthesis method for the formation of oxygen-rich hierarchical O-PG framework is explained with the help of schematic in Figure 1. It has been reported that nitric acid reacts actively with carbon hexagons under the specific conditions to produce defects in CNTs, cut CNTs and produce carbon dots61–63. We employed a mild condition and less sonication time to obtain a better control over defect generation process. Therefore, nitric acid has been utilized in the present case as pore generator to form PG framework. First, conc. nitric acid was added into GO suspension to attach and intercalate the acid molecules in-between the GO sheets. Then, the GO sheets with attached nitric acid solution was poured into petridish and dried in air atmosphere at ambient conditions. Acid incorporated GO film (a-GO) was obtained after peeling the dried film off from peteridish. The a-GO film was reduced into PG under focused solar radiation in less than two seconds, which was associated with a cracking sound and was evident by the change in colour from yellowish brown to black. In this process, the attached nitric acid molecules on the surface of GO reacts with its adjacent carbon atoms at the imperfection sites and existing edges of GO to form pores at the temperature of 204 °C (focused spot temperature of solar light using 90 mm diameter of convex lens)59. Meanwhile, the reduction of GO was also taking place i.e., removal of the oxygen-containing functional groups (-COOH and –OH) from the surface of GO sheets. During the reduction process, outgoing gases such as CO2 and H2O molecules could initiate an extra force for further expansion of the graphene sheets and helping in formation of unique porous frameworks64. Along with the pore formation, solar radiation also reduces GO sheets into O-PG with ~3 layers. The number of layers was obtained using height profile technique in AFM analysis as shown in Figure S1.


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The surface morphology of O-PG was observed using HRTEM technique as shown in Figure 2. Similar to the source (GO) structure, O-PG shown in Figure 2a also has a sheet-like structure but with the observable pores in its sheets. The control experiment shows that the nitric acid etching is the key step to obtain O-PG. Otherwise, our previously reported SRGO (solar reduced graphene oxide) material without pores like morphology will be obtained as shown in Figure S2. The HRTEM image obtained at higher resolution confirms the abundant micropores, created on the surface of graphene sheets as shown in Figure 2b. This is due to the nitric acid molecules attached onto the GO sheets, which reacts with the neighbouring carbon atoms and form carbon vacancies. These vacancies forms pore and act as transport channel for the electrolyte ions to move through the graphene sheets in the cross-plane direction and assist ion access to the whole electrode volume, which is limited in case of nonporous graphene electrode.

Fig. 2. HRTEM images of O-PG. Nitrogen adsorption-desorption isotherm were also analyzed to study surface area and porous nature of O-PG sample. As shown in Figure 3a, O-PG exhibits type IV isotherm characteristics with a distinctive H3 type hysteresis loop in the relative pressure (P/P0) range from 0.5 to 1. It signifies the existence of mesopores and macropores, which was caused by accumulation of sheets in the frameworks during reduction process65–67.

However, the

adsorption volume shows sharp rise at very low relative pressure (< 0.04), which suggests the



existence of microspores as also confirmed by HRTEM image. The BET specific surface area of 341 m2g-1 and pore volume of 0.51 cc g-1 was obtained based on Brunauer-Emmett-Teller (BET) analysis. Figure 3b confirms the BJH pore size distribution of O-PG sample below 12.7 nm. The maximum numbers of pores are in the size range from 0.9 nm to 2.1 nm, which indicates the presence of micropores and mesopores. High surface area along with hierarchical pore structure composition is anticipated to be advantageous for the electrochemical energy storage and capacitive deionization applications.

Fig. 3. (a) Nitrogen adsorption/desorption isotherm, (b) Pore size distribution vs Pore volume, (c) XRD patterns of GO and O-PG and (d) Raman spectra of GO and O-PG. To get further insight of the structure of O-PG, Raman spectroscopy and X-ray diffraction (XRD) analysis were studied in detail. Raman spectra obtained for GO and O-PG samples shows two peaks at 1592 and 1345 cm-1 for G band and D band respectively. These


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peaks are the signature peaks for graphitic carbon nanomaterials, as shown in Figure 3c. The G band corresponds to first-order Raman scattering of the E2g phonon at the Brillouin zone centre of sp2 carbon atoms. The appearance of D band is due to the sp3 contribution in the sp2 planar structure from defects and disorders. Generally, the relative intensity ratio (ID/IG) indicates the amount of disorder or defects (structure quality) in the carbon structure. The ID/IG ratio (1.21) obtained for O-PG confirms the existence of defects in the in the form of pores on the graphene structure. This attributes to the existence of more disordered and defective carbons at the pore edges of graphene sheet in O-PG. The crystalline nature of GO and O-PG was analyzed using XRD and the patterns are shown in Figure 3d. GO showed a characteristic peak at 2θ = 9.6 (002), confirming the oxidation of graphite. It is also indicating by the increment in interplanar distance from 0.34 nm (natural graphite) to 0.93 nm for GO. This enhancement in distance is due to the oxygenated functional groups have attached in between the graphitic layers during oxidation of graphite. After the focussed solar irradiation treatment of a-GO, the broad diffraction peak at 2θ of 23.4° can be assigned to (002) plane of a graphitic-type lattice. A susceptible diffraction at 42.6° belongs to (100) of graphitic-kind carbon structure, signifying the amorphous nature of O-PG caused by defects within the form of pores and presence of oxygen functional groups on the sheets as revealed by Raman spectra.



Fig. 4. (a) Survey scan, (b) C1s spectra, (c) O1s spectra and (d) area percentages obtained from deconvoluted O1s spectra of GO and O-PG. Further, XPS analysis was used to investigate the detailed information about surface functionalities of O-PG. Figure 4a represents the survey scan of GO and O-PG, this spectra shows two main characteristic peaks at 282.6 and 530.4 eV attributed to C1s and O1s, respectively. The decreased intensity of O1s in O-PG suggests that removal of oxygencontaining functional groups was taking place during solar reduction of a-GO. This was further confirmed by calculating the C/O atomic ratio using XPS survey spectra and this ratio was increased from 2.7 (73.1 at. % of C and 26.9 at. % of O) for GO to 4.6 (82.3 at. % of C and 17.7 at. % of O) for O-PG after solar reduction process. Figure 4b shows the deconvoluted C1s spectra of GO and O-PG. The C1s spectra showed three peaks centred at 284.8, 287 and 288.7 eV, which corresponds to C-C (sp2), C-O (epoxide/hydroxyl) and C=O


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(carbonyl), respectively68. After reduction process, the area percentage of sp2 C-C peak was increased from 55.6 % to 70 % and the oxygen-containing peaks were decreased from 44.4 % to 30 %, respectively. This increment in C content and reduction in O contents suggests that reduction of GO was taken place. But still, O-PG contained more oxygen-containing functional groups than our previously reported SRGO (C/O ratio was 6.1). To understand the bonding configurations of O atoms in O-PG were investigated by deconvolution of O1s spectra and the results are shown in Figure 4c and 4d. The O1s peak of GO was deconvoluted into two peaks located at 531.2 eV and 532.7 eV attributed to C=O (2.2 area %) and C-O (97.8 area %), respectively. After nitric acid treatment and solar reduction, the C-O peak was completely removed and O1s peak of O-PG still showed two peaks located at 531.3 eV and 533.3 eV attributed to C=O (39.3 area %) and O-H (60.7 area %), respectively. This means that O-PG has obtained with increased C=O and new O-H peaks than GO after nitric acid treatment. These newly increased oxygen-containing functional groups in O-PG not only improve the electrochemical performance and also the wettability of the surface. To further confirm the reduction extent of GO, elemental analysis was done. The exact percentage of C, O and H elements are given in Table S1. Further the confirmation of oxygen-containing groups was extended to contact angle measurement for O-PG. Contact angle of 72.4° was obtained for a water droplet on O-PG as shown in Figure S3. This angle clearly confirms that O-PG has oxygen groups at the pores and edges of graphene planes. The obtained results are in strong agreement with the XPS analysis.

Oxygen-rich hierarchical porous graphene (O-PG) as electrodes in energy storage Supercapacitor performance of O-PG in 1M H2SO4 electrolyte: To study the energy storage properties of O-PG, electrochemical measurements have been studied using a symmetric two-electrode system as shown in Figure S6. O-PG is



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expected to possess better electrochemical properties, owing to the availability of higher accessible surface area and presence of cross-plane diffusion paths for electrolyte ions leading to its improved mass transport. At first, cyclic voltammetry (CV) studies were carried out at various scan rates (50 – 200 mV/s) using 1M H2SO4 as electrolyte (Figure 5a). The CV curves demonstrate semi rectangular-like identical representation attributes of capacitive behaviours with relation to zero current line that indicates excellent electrochemical capacitive performances. Figure 5b shows the galvanostatic charge/discharge (GCD) characteristics of O-PG electrodes at different current density ranging from 1 - 20 A g-1.

Nearly linear and symmetric GCD profiles were obtained, confirming excellent

electrochemical property like reversibility. The specific capacitance values at various current densities are calculated utilizing the equation 1 and appeared in Figure 5c. The O-PG presents high specific capacitance of 354 F g-1 at low current density (1 A g-1), that stays upto 194 F g1

at higher current density of 20 A g-1. This excellent performance showing the fast ion

transport qualities likely because of the well interconnected porous structure of O-PG framework. It is important to note that the as-prepared O-PG sample shows less SSA than the commercially available activated carbon (> 2000 m2 g-1), but higher specific capacitance was accomplished. This great charge storage property of O-PG should be credited by the joined commitment from electrode surface wettability caused by oxygenated functional groups and its distinctive porous framework. The higher concentration of C=O (carbonyl) groups on OPG added to a slightly pesudocapacitance owing to the following reaction69: > 𝐶 = 𝑂 + 𝐻 + + 𝑒 - ⇄ > 𝐶𝐻 - 𝑂

(6)

Meanwhile, the availability of the micropores and mesopores in O-PG framework is favourable in utilizing the active SSA and contributes majority of double layer capacitance. The O-PG possesses micropores with the size of 0.9 nm, which can be accessible by aqueous


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electrolyte ions. Therefore the pore size plays an equally crucial role in deciding the charge storage property of electrode material. Thus engineering the pores for greater accessibility of solvated ions is very important. On the other hand, comparable amount of mesopores give successful pathways for ion transportation inside the micropores with a low ion penetration distance and in addition decreased inner resistance. Moreover, the plenitude of hydrophilic oxygen containing functional groups in the hierarchical O-PG contributes pseudocapacitance and also essentially enhances the hydrophilic property of the electrode, which is useful for charge storage behaviour in aqueous electrolytes. We have further studied the kinetics of ion transport using electrochemical impedance spectroscopy (EIS) with the frequency ranging from 100 kHz to 10 mHz. The point where the plot intersecting the real axis (x-axis) in Nyquist plot (Figure 5d) corresponds to equivalent series resistance (ESR) and the value was found to be 0.42Ω. This indicates resistance experienced due to the electrical conductivity of the electrode and great electrical contact at the electrode – current collector interface. Furthermore, in the low frequency region, this electrode exhibits a nearly vertical line, indicating a good capacitive behaviour. The total cell resistance (Rcell) can be obtained from extrapolating the vertical portion (low frequency region) to the real axis. It is important to note that the lower Rcell (1.1 Ω) mainly originates from its lower charge transfer resistance (Rct). Rct was obtained from diameter of the semicircle at the high to medium frequency region and the obtained Rct value is 0.22 Ω. Subtraction of Rct and ESR from Rcell gives the Warburg resistance (WR) and the obtained WR was 0.42 Ω. The smaller Rct and WR demonstrate more proficient charge transfer and ion penetration that prompt a great rate performance70.



Fig. 5. Electrochemical performance of O-PG in 1M H2SO4 electrolyte. (a) CV curves at different scan rates ranging from 50 – 200 mV s-1, (b) galvanostatic charge/discharge (GCD) curves at the different current density ranging from 1 – 20 A g-1, (c) specific capacitance values calculated at different current density, (d) Nyquist plot, (e) Bode plot, (f) Ragone plot (Energy density vs. Power density) and (g) cyclic stability test.


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The Bode plot, shown in Figure 5e, provides the valuable insight into the rate capability of O-PG. The knee frequency (fo) at a phase angle of -45o results the point where the capacitive and resistive behaviours are equal. The reciprocal of this knee frequency represents the time constant (τ0), which indicates the minimum time required for the electrode to charge and discharge while maintaining good capacitive behaviour. This O-PG electrode exhibit fo of 1.66 Hz which relates to τo (1/fo) of 0.6s. This is significantly lower than that of commercially available AC-based electrochemical capacitors (10 s). The fast frequency response of hierarchical O-PG further suggests, significantly enhanced rate of mass transport into the electrodes. Figure 5f presents the Ragone plot, demonstrating the connection between the energy density (E) and power density (P). The E and P values were calculated from the discharge curves as indicated by the equations (2) and (3).

This electrode material has obtained

excellent energy density of 12.3 Wh kg-1 and the power density of 15075 W kg-1. The energy density held over greater than 50% of its initial value when the consistent current density expanded from 1 to 20 A g-1. This magnificent rate performance of O-PG is because of the oxygenated functional groups and hierarchical porous framework of graphene sheets. Cycle life is an important parameter of supercapacitors in practical applications. The cycle stability of the O-PG was evaluated by GCD process at a current density of 10 A g-1 upto 25000 cycles in 1M H2SO4 electrolyte. As shown in Figure 5g, the supercapacitor fabricated with O-PG electrode retains 93.6 % of its initial capacitance even after 25000 cycles. The improved wettability due to oxygen-containing functional groups and porous structure of the O-PG electrode employed an excellent cycle life.



Supercapacitor performance of O-PG in ionic liquid electrolyte: It is well known that the energy stored in the supercapacitor is specifically corresponding to the square of its working voltage as appeared in equation 2. The energy density can be expanded further by expanding the working voltage window using nonaqueous electrolyte i.e. ionic liquid. By using the EMIMBF4 ionic liquid, active voltage of the device could be increased from 1V to 4V71. Hence, we further test the electrochemical performance of O-PG in EMIMBF4 ionic liquid in the wide voltage window of 0-3 V as shown in Figure 6a. The obtained CV curves are in quasi-rectangular shape due to oxygen functional groups at various scan rates ranging from 50 to 200 mV s-1. GCD curves of O-PG showed almost symmetric triangular in shape with little voltage drop at the discharge curve at the current density of 1 A g-1 as shown in Figure 6b. Figure 6c shows the obtained specific capacitance values from GCD curves at various constant current densities for O-PG. With the increasing current density from 1 to 10 A g-1, the specific capacitance value of 354 F g-1 was retained upto 193 F g-1 (54.5%). This can be assumed that the decrease in ionic penetration into the graphene framework is due to increase in current density. Though, the EMIM+ cation (0.79 nm) and BF4- anion (0.48 nm) sizes in ionic liquid matches with the size of micropores formed in O-PG72,73. Hence, O-PG exhibits good electrochemical performance in ionic liquid electrolyte as well because of its unique hierarchical porous structure which helps in efficient diffusion of electrolyte ions through cross-plane direction in addition with in-plane diffusion. The electrochemical properties were further studied in detail by EIS with frequency ranging from 100 kHz to 0.01 Hz. The obtained Nyquist plot and the fitted circuit is given in inset as shown in Figure 6d. The parameters, total cell resistance (Rcell), equivalent series resistance (ESR), charge transfer resistance (RCT), and Warburg resistance (WR), obtained from fitting the curve to the equivalent circuit are presented in the table in inset. The obtained


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values are 3.42 Ω, 2.1 Ω, 0.38 Ω and 0.94 Ω for Rcell, ESR, RCT and WR, respectively. At the low frequency region the O-PG electrode exhibits nearly vertical line indicating high specific capacitance and fast charge transfer.

Fig. 6. Electrochemical performance of O-PG in EMIMBF4 ionic liquid electrolyte. (a) CV curves at different scan rates ranging from 50 – 250 mV s-1, (b) galvanostatic charge/discharge (GCD) curves at the different current density ranging from 1 – 10 A g-1, (c) specific capacitance values calculated at different current density, (d) Nyquist plot, (e) Ragone plot (Energy density vs. Power density) and (f) cycle stability.



Ragone plot gives the connection between energy density (E) vs. power density (P) which is calculated using equation 2 and 3. The obtained values of energy and power densities are shown in Figure 6e. The highest energy density of 110.6 Wh Kg-1 has been obtained at the power density of 1563 W Kg-1, which was achieved at 1 A g-1 current density for O-PG. When the current density increases, the energy density gets reduced as expected due to limited accessibility of pores by the electrolyte at high current density. Xu et. al. reported the usage of 3D holey graphene frameworks for supercapacitor electrodes and reported high energy density of 127 Wh kg-1, where the electrode material was synthesized using H2O2 reduction followed by hydrothermal reduction. Same group reported holey graphene hydrogel and reported energy density of 116 Wh g-1. In the present study we focused on the usage of environmental safe and abundantly available solar energy for the pore formation. The state-of-the-art supercapacitor performances using porous graphene based materials synthesized by different processes are summarized in Table S2 for comparison. Long cycling life is a key requirement for energy storage device in practical applications. The cycle stability of the O-PG was investigated by employing the GCD test for 12000 cycles. As shown in Figure 6f, it is important to note that the supercapacitor fabricated with O-PG electrode retains 91.5% of its underlying specific capacitance even after 12000 cycles. This is attributed to the improved wettability of the electrode along with high stability of the basic graphene structure, implying its admirable reversibility in the long cycling.

Al-ion storage performance of PG as cathode material using 1M AlCl3 aqueous solution Owing to the natural availability and low cost of Al, Al-ion batteries have recently become one of the main candidates for the upcoming energy storage devices. One of the most important components of batteries is cathode because its main job is intercalation and


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deintercalation of electrolyte ions while charging and discharging. Recently, various cathode materials such as VO2, V2O5, anatase TiO2, 3D graphite foam, copper hexacyanoferrate and Prussian blue analogues have been studied for Al-ion batteries. However, none of them shows satisfactory performance for practical applications due to the limitations in terms of capacity and life cycle. It is anticipated that O-PG performs better for the charging and discharging of Al-ions owing to its unique pore structure. Here we demonstrate its Al-ion storage performances in 1M AlCl3 aqueous electrolyte solution using three-electrode system. Al-ion storage of O-PG as cathode was first evaluated by CV at the scan rates between 1 – 200 mV s-1 in the voltage profile range -0.5 to 0.5 V. CV curves (Figure 7a) of the O-PG sample shows broad rectangular-like curves indicating storage mechanism is double-layer capacitance behaviour. In addition, a pair of broad bumps also appears due to redox reactions caused by oxygen-containing functional groups. Charging/discharging measurements were done in the current density ranging from 0.5 A g-1 to 5 A g-1 and the charge-discharge profile is shown in Figure 7b and 7c. This O-PG electrode exhibits high capacity of 90 mAh g-1 at the constant current of 0.5 A g-1 and retains its initial specific capacity up to 70.8 % (63.75 mAh g-1) even at the high current of 5 A g-1. It is very important to note that the Al3+ ion has a smaller radius (53.5 pm) than Li+ ion (76 pm) and Mg2+ ion (72 pm), indicating the ease of using Al3+ ion in insertion/extraction chemistry74.

This porous structure of electrode

increases the diffusivity of electrolyte ions and high amount of oxygen groups imparts high wettability. The discharge profile of the O-PG indicates slanting voltage profiles with high voltage hysteresis, which exhibits that aluminium storage is mostly because of Al-ion adsorption on O-PG layers as officially seen by CV. Besides, the potential profile changes continuously with no plateau amid charge/discharge process, which shows that the electrochemical response at O-PG cathode depends on quick surface responses and this marvel resembles the electrochemical capacitors. The same mechanism was observed by Yoo



et al. for the accommodation of Li-ions in new class of carbonaceous materials such as graphene nanosheets (GNS), GNS+CNTs and GNS+C60 are different from graphite75. It is because metallic ions are electrochemically adsorbed on both sides of single-layer GNS like “falling cards”76.

Thus aluminium inclusion/extraction processes in O-PG cathode are

undifferentiated from those of Al-ion storage45. The electrode also shows an outstanding cyclic performance of charging/discharging for 10000 cycles and hence demonstrates excellent capacity retention of 92.6% as shown in Figure 7d. The state-of-the-art electrochemical performance of Al-ion batteries using various cathode materials are tabulated in Table S3. To the best of our knowledge, this is the best electrochemical performance which we got with O-PG electrode.

Fig. 7. Al-ion storage performance of O-PG in 1M AlCl3 aqueous electrolyte solution. (a) CV curves at different scan rates ranging from 1-200 mV s-1 in a voltage range of -0.5 to +0.5 V. (b) Charge-discharge profile at different current densities ranges from 0.5 to 5 A g-1. (c) Specific capacity vs current density and (d) cycle stability at a current density of 5 A g-1.


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Oxygen-rich porous graphene as electrodes in capacitive deionization Electrochemical properties: Capacitive deionization (CDI) is an electrosorption process to remove the salt ions from salty water. This CDI technique works on similar operating principle as supercapacitors. To examine the electrochemical properties, the CV was used to investigate the electrosorption capacity of the O-PG electrode, since specific capacitance is a fundamental component for assessing the salt removal capacity. CV curves were measured in 1M NaCl electrolyte solution at the scan rates 30-200 mVs -1 in the potential range of 0 – 1 V. Figure 8a shows that the CV curves are in typical rectangular shape, suggesting that O-PG electrode shows EDLC behaviour due to coulombic interactions rather than faradaic capacitance. Indeed, the rectangular shape of CV curves maintained even at higher scan rate demonstrates quick ion movement of salt ions on to PG electrodes. The GCD curves were obtained at the current density of 1 – 10 A g-1 as shown in Figure 8b. It demonstrates typical symmetric shapes in accordance with the CV results. The specific capacitance values were calculated using equation 1 from GCD curves at different current densities as shown in Figure 8c. This higher specific capacitance of O-PG sample is attributed to its wettability caused by oxygenated functional groups and unique porous framework. Electrochemical impedance spectroscopy (EIS) was used to look at the internal resistivity of O-PG electrode. The Nyquist plot of O-PG electrode in 1M NaCl aqueous solution is shown in Figure 8d. This plot shows a linear feature at low-frequency range and a semi-circle at high-frequency. In the lowfrequency area, the inclined line is taken from the standard EDLC. The point intersecting the real axis is related to equivalent series resistance (RESR) of the electrode which could be the result of ionic resistance of salt water, the intrinsic resistance of electrodes and also the contact resistance at the interface of active electrode material-current collector. The obtained RESR value is 1.45Ω, this low value indicates efficient and fast salt ions penetration into



hierarchical porous structure with abundant micropores. Moreover, the excellent electrochemical properties of O-PG electrode are additionally accentuated by their predominant cyclic stability. The stability of the electrode was assessed by performing GCD for 500 cycles and the result is shown in Figure 8e. The capacitive performance was retained by 99.3 % even at 500th cycle at the current density of 10 Ag-1. This confirms the aspiring applicability of O-PG electrodes for the removal of salt ions from salty water.


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Fig. 8. Electrochemical studies of O-PG using 1M NaCl as electrolyte. (a) CV at different scan rates ranging from 30-200 mV s-1. (b) GCD profile at various current density ranging from 1-10 A g-1. (c) Specific capacitance vs Current density, (d) Nyquist plot and (e) cycle stability test of upto 500 cycles.

Capacitive deionization performance: The schematic principle of CDI is represented in Figure 9a. The working principle of CDI depends on external electrostatic field applied between the electrodes in order to drive charged ions toward oppositely charged electrodes. The key parameters that can affect the salt adsorption capacities of NaCl onto the O-PG are applied voltage and solution concentration.

The CDI performances were studied systematically by varying these

parameters. First, we investigated the effect of applied voltage (0.8 – 1.4 V) on the CDI using NaCl concentration of 500 mg L-1, flow rate of 25 ml min-1 and 75 ml of solution volume. When the voltage difference is applied across the O-PG electrodes, the ions move towards electrode surface from the solution and get adsorbed on it. The conductivity of the salt solution decreases quickly in the initial couple of minutes as displayed in Figure 9b. The availability of electrodes surface for the ions to get adsorbed gradually decreases after 30 minutes during adsorption process, implying that the electrodes are close to saturation. It is clear that the rate of adsorption in the first stage is faster when a higher voltage (1.4V) is applied. The ragone plot shifts upward and right when increasing the voltage from 0.8 V to 1.4 V, indicating higher salt adsorption capacity (SAC) and salt adsorption rate (SAR) as shown in Figure 9c. The SAC and SAR values were calculated using the equation 4 and 5. When the applied voltage increases from 0.8 V, the solution conductivity clearly decreases with time, demonstrating that the higher voltage prompts a higher salt adsorption capacity due to enhanced Coulombic interaction. It should be noted that the internal resistance of the electrodes mainly comes from the current collector and if any binder used during the fabrication of electrodes which may consume certain amount of applied voltage during the CDI process. In this work, the resistance obtained was only from current collector because



binder was not used for the fabrication of electrodes. Figure 9d shows the salt adsorption capacity at different applied voltages. Clearly, the salt adsorption capacity of O-PG electrode increased with the applied voltage. When the voltage increased from 0.8 to 1.4 V, the salt adsorption capacity also increased from 7.8 to 21.1 mg g-1. Next, we explored the impact of salt concentration (300-500 mg L-1 of NaCl) on the CDI process. As shown in Figure 9e, a higher salt concentration moved the plot toward the upper, right locale, showing higher SAC and SAR. The better capacity and rate are primarily a direct result of compaction of the double layer and therefore the resultant ascent in capacitance at higher salt concentration. The calculated salt adsorption capacity of O-PG (equation 4) at various introductory NaCl concentrations is displayed in Figure 9f. At the point when the solution concentration increased from 300 to 500 mg L-1, the salt adsorption capacity also raised from 13.5 to 21.1 mg g-1. Hence, a higher salt concentration is favourable in light of the fact that fast deionization can be accomplished contrasted with a lower salt concentration. The salt desorption (regeneration) performance of the O-PG electrode is an important process for CDI, as rapid salt desorption rate is preferentially required for regeneration of the electrodes in practical applications. As shown in Figure 9g, the adsorption-desorption performance were conducted with a initial concentration of 500 mg L-1 at 1.4 V applied voltage. Once, voltage was applied, the NaCl solution conductivity began to decrease gradually which indicates the adsorption of salt ions. Then, the change in solution conductivity gradually became smaller until the electrode reached the saturation point. Subsequently, the CDI cell was reverse polarized, the conductivity returned to initial value due to desorption of ions. Therefore, O-PG electrode showed excellent adsorption-desorption performance in NaCl solution due to its unique porous structure.


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1.01

(a)

(b)

Conductivity (mS cm-1)

1.00 0.99 0.98 0.97 0.96 0.8V 1.0V 1.2V 1.4V

0.95 0.94 0 25

0.8V 1V 1.2V 1.4V

1

10

20

30

40

50

60

Time (min)

(d)

21.1

20

SAC (mg g-1)

SAR (mg g-1min-1)

(c)

0.1

14.5

15 10.7

10

7.8

5

1

10

0 0.6

100

SAC (mg g )

0.8

1.0

10

(e)

25 -1

300 mg L 400 mg L-1 500 mg L-1

SAC (mg g-1)

0.1

1.4

(f)

1.6

21.1

20

1

1.2

Voltage (V)

-1

SAR (mg g-1min-1)

15

17.6 13.5

10 5

0.01

1

10

0

100

300

-1

1.01

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SAC (mg g )

Conductivity (mS cm-1)

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


Concentration (mg L )

(g)

500 mg L-1 at 1.4 V

1.00 0.99 0.98 0.97 0.96 0.95 0.94 0

50

100

150

200

250

Time (min)

Fig. 9. (a) , (b) Conductivity of NaCl Vs time plot, (c) CDI Ragone plot and (d) salt removal capacity for different concentrations at 1.4 V and flow rate of 25 ml min-1, (e) CDI Ragone plot, (f) salt removal capacity for different voltages at 500 mg L-1 NaCl concentration at flow rate of 25 ml min-1 and (g) Regeneration performance of O-PG electrode.



Removal of various cations: Salt adsorption capacity of different salt solutions such as NaCl, MgCl2 and Ca (SO4)2 were also studied using O-PG electrode. The initial conductivity of all salt solutions were maintained at 0.6 ms cm-1, flow rate of 25 ml min-1, applied voltage of 1.4V and volume of 75 ml under atmospheric conditions. For electrosorption process, higher valence ions with smaller hydrated radius can be more efficiently removed than smaller valence ions due to stronger electrostatic force. Therefore, the predicted salt adsorption capacity for these salt solutions is Ca2+ (4.12 Å) > Mg2+ (4.28 Å) > Na+ (3.58 Å). Figure 10a shows the plots of conductivity with time for different salt solutions. The salt adsorption capacity was calculated for O-PG electrodes using equation 4.

Fig. 10. Effect of different ion species removal studies using CDI technique. (a) Conductivity with time plot at the applied voltage and initial concentration was 1.4 V and ~0.6 ms cm-1 and (b) salt removal capacity of NaCl, MgCl2 and Ca (SO4)2, respectively. The acquired salt adsorption capacities of NaCl, MgCl2 and Ca(SO4)2 are 13.5 mg g-1, 16.6 mg g-1 and 18.6 mg g-1, respectively as shown in Figure 10b. It clearly indicates that when voltage (1.4 V) is applied between the O-PG electrodes, highly charged multivalent ions from salt solution is strongly attracted. For same valence ions (Ca2+ and Mg2+), the one with smaller hydrated radius (Ca2+) would be more adequately removed. As per the above


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observation, for the hierarchical O-PG based CDI, fundamentally enhanced deionization performance can be attributed to the high specific surface area, improved electrode wettability caused by oxygenated functional groups on the graphene sheets and unique porous framework. Naked at al. 77 suggested that only microporous based electrodes are not the best choice for electrosorption process because of their slow kinetics. The electrosorption of ions can be improved by the unique framework with mesopores dominated pore structure as it improves the kinetics of electrosorption process. The O-PG electrode has the combination of both mesopores and micropores, which is mandatory for the electrosorption process. From this, we can presume that the high specific surface area, surface wettability and the conjunction of mesopores and micropores, provides high electrosorption locales for the EDL development and are basic requirement for salt adsorption capacity.

Conclusions In summary, oxygen rich hierarchical porous graphene electrode material has been successfully prepared by focussing solar radiation on the nitric acid intercalated graphene oxide. When tested as supercapacitor electrode, this material shows excellent energy storage performance, where specific capacitance of 354 F g-1 was obtained. Energy density of this supercapacitor achieved is 110.6 Wh kg-1, which is exceptionally high compared to other porous carbon graphene based supercpacitor devices. In addition, electrochemical Al-ion storage performance of this unique porous graphene framework as cathode material in 1M AlCl3 aqueous solution were also studied and specific capacity of 90 mAh g-1 has been obtained at 0.5 A g-1 current density. Along with good charge storage property this novel material also shows high electrosorption capacity. The O-PG electrode shows the salt adsorption capacity as high as 21.1 mg g-1 in 500 mg L-1 NaCl solution and excellent cycle



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stability over 500 cycles. Our outcomes exhibit that the O-PG is a promising material for next- generation high performance energy storage devices and capacitive deionization.

Associated content Supporting information AFM images, HRTEM image, CHNSO analysis, Dynamic contact angle measurement, FTIR spectra, TGA Analysis, Schematic illustration of supercapacitor, Coin cell image, LED light image and Comparison of literature of different samples for supercapacitors, Al-ion battery and capacitive deionization.

Acknowledgements The authors acknowledge University Grant Commission- SAP, UGC-Networking Resource Center,

Department

of

Science

and

Technology

(SR/NM/NS-1110/2012)

BRNS(2013/20/34/1/BRNS) Government of India for the financial support.


and

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Table of Content Oxygen-rich porous graphene was prepared by focusing the solar radiation on acid treated graphene oxide film as electrode for supercapacitors, aqueous Al-ion battery and capacitive deionization.


Oxygen-rich hierarchical porous graphene as an excellent electrode

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