Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode

Journal of Colloid and Interface Science xxx (xxxx) xxx

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Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode Yunping Wu, Tianyi Ding, Rui Zhai, Sa Jiao, Wei Wei ⇑ Department of Chemistry, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Materials, Xi’an Key Laboratory of Sustainable Energy Material Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, PR China

g r a p h i c a l a b s t r a c t Assembly of cellulose macromolecules engineers surface properties of high quality graphene for efficient aqueous processing and material synthesis.

a r t i c l e

i n f o

Article history: Received 17 September 2019 Revised 8 November 2019 Accepted 11 November 2019 Available online xxxx Keywords: High quality graphene Cellulose Molecular grafting Aqueous dispersion Supercapacitor

a b s t r a c t High quality graphene (HQG) offers unconventional properties and is desirable for a variety of applications. However, facile solution processing (especially in water) and chemical bonding of functional components with the aim of achieving high-yield, green, and controllable synthesis of advanced graphene materials are of great concern. Herein, the surface chemistry of HQG is effectively tailored using a hydrophobic-driven assembly of cellulose macromolecules (CM) with various functionalities. In contrast to bulk or nanocellulose modifiers, surface engineering of HQG with densely carboxyl grafted CM renders stable aqueous graphene colloids via electrostatic repulsion. It also enables the use of efficient, low-cost, aqueous-phase synthetic techniques to create new HQG-based materials and devices. Highly exposed and reactive carboxyl and hydroxyl groups lead to in situ formation of evenly distributed Co3O4 nanoparticles on HQG sheets (HQG-COOH-Co3O4). We further demonstrate the potential application of twodimensional HQG-COOH-Co3O4 heterostructures as supercapacitor electrodes with high power and energy density. Ó 2019 Elsevier Inc. All rights reserved.

⇑ Corresponding Author. E-mail address: [email protected] (W. Wei). https://doi.org/10.1016/j.jcis.2019.11.043 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

Please cite this article as: Y. Wu, T. Ding, R. Zhai et al., Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.043

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1. Introduction Graphene is a two-dimensional (2D) carbon nanomaterial that is one-atom thick and has superior mechanical, electronic, and optical properties. Graphene has been the focus of material synthesis and applications in a variety of fields, including batteries, supercapacitors, nanoelectronics, sensors, and electrocatalyst [1–3]. However, similar to other nanomaterials, pristine graphene sheets tend to aggregate into multiple layers or even restack into graphite structures because of strong van der Waals forces [4,5]. This phenomenon diminishes the intriguing properties that are normally associated with individual sheets, and this is one major obstacle for exploring advanced materials and devices based on graphene [6]. Therefore, obtaining stable dispersion with respect to efficient solution processing of graphene is of great concern. In particular, dispersion of graphene in water is crucial for advancing many practical technology applications. However, because of their hydrophobic nature, stable dispersion of graphene sheets in water has been generally considered to be a primary challenge. In addition, pristine graphene is composed of sp2-conjugated carbon with perfect atomic lattice structures. Thus, the intrinsic absence of functional groups on graphene sheets impedes their chemical interactions and reactions with various foreign components. To overcome these drawbacks, significant efforts have been undertaken to fabricate graphene materials using graphene oxide (GO) or reduced GO (rGO) as the building blocks [7,8]. The presence of oxygen-containing groups (hydroxyl, epoxy and carboxyl) in GO offers facile solution processing and the possibility of further covalent and/or noncovalent functionalization with various organic and inorganic compounds [9,10]. It is noteworthy that the production of GO and rGO involves aggressive oxidation/ reduction processes, and this results in abundant structural defects and produces a large amount of toxic liquid wastes. Consequently, the quality and properties of either GO and rGO remain substantially inferior to that of pristine graphene. Very recently, we and other groups demonstrated efficient production techniques of HQG which has low defect density and provides enormous opportunities for fabricating advanced graphene materials on a large scale [11]. It is known that HQG can only been stably dispersed in a small number of solvents, such as N-methylpyrrolidone (NMP) and N,Ndimethylformamide (DMF). As a result, the limited solvents and low concentratiobn of graphene sheets considerably limit solution processing of HQG [12]. Previous work reported on the use of a variety of stabilizers (e.g., polymers and surfactants) to achieve aqueous dispersion of HQG for proposed applications, such as sodium dodecyl sulfate, tetradecyltrimethylammonium bromide, and 1-pyrenebutyrate [13,14]. However, most of the synthetic stabilizers are expensive, and the interfacial contact between the stabilizer and graphene is poor and unstable. Moreover, the stabilizers commonly are not able to render functionalities, and this hinders the use of HQG in facile and controllable manners. Because of the diverse molecular structures and properties, combining graphene with various natural/biological components is a promising way to engineer the surface properties of HQG [15–17]. In particular, cellulose is the most abundant and renewable biopolymer in nature [16,18]. The unique amphiphilic nature of cellulose macromolecules (CM) facilitates interfacial assembly with hydrophobic HQG and simultaneously improves the processability and functionalities (e.g., hydroxyl groups) of HQG. However, the highly crystalline structure that is associated with strongly hydrogenbonded interactions makes them difficult to dissolve and process continuously [19]. Although a few works have demonstrated that cellulose is capable of dispersing graphene [20], the direct use of either bulk or nano cellulose materials is difficult to achieve

efficient aqueous processability and render sufficient functionalities on HQG surface. In this study, we demonstrate an efficient and controlled biomolecular assembly strategy for improving the processing and reaction abilities of HQG in an aqueous environment. Homogeneous modification of cellulose in ionic liquid (IL) leads to various functionalities of CM that have tunable grafting degrees. The backbone anhydroglucose units (AGU) of CM spontaneously adsorb onto the surface of HQG via hydrophobic and van der Waals forces, and the innate or grafted functional groups (i.e., AOH, ACOOH, and ANH2) serve as highly exposed and reactive sites for stable dispersing and coupling with foreign species. Remarkably, an assembly of carboxyl-grafted cellulose and HQG (HQG-COOH) affords stable aqueous dispersions and also facilitates homogeneous nucleation and bonding of active metal compounds (e.g., Co3O4) onto HQG. The as-obtained 2D heterostructured materials (HQG-COOHCo3O4) has a high specific capacitance of 1268 F g
1 at a current density of 1 A g
1, excellent rate capability, and pronounced cycling stability with ~100% capacity retention after 6000 cycles. Further, an asymmetric supercapacitor with a high energy density of 48.0 Wh kg
1 at a power density of 775 W kg
1 has been achieved using HQG-COOH-Co3O4 as the positive electrode and activated carbon (AC) as the negative electrode, and this indicates the great potential for using this material as an advanced electrode material toward energy storage applications. 2. Experimental section 2.1. Materials Natural graphite powder, microcrystalline cellulose, and succinic anhydride (SA) were purchased from Aladdin Reagent Co. (Shanghai). 1-Butyl-3-methylimidazolium chloride ([Bmim]Cl), potassium hydroxide (KOH), hydrochloric acid (HCl), ammonium hydroxide (NH3H2O), and urea were supplied by Shanghai Chengjie Chemical Co. All of the solvents (including NMP, DMF, and ethanol) were analytically pure and purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. All of the regents were used without further purification. 2.2. Preparation of HQG HQG was prepared via a liquid-phase exfoliation method. In a typical procedure, natural graphite powder (3 g) was dispersed in NMP solvent (250 mL) in a 300 mL beaker. The mixture was then sonicated for 3 h at room temperature with a 5 min break for every 10 min of operation. The resulting suspension was maintained for 3 days to precipitate unexfoliated graphite flakes. The upper dispersion was then filtered and dried for characterizations and material synthesis. 2.3. Molecular grafting of CM To reduce the crystalline texture, microcrystalline cellulose was first hydrolyzed using sulfuric acid to obtain nanocrystalline cellulose. Typically, 2 g of microcrystalline cellulose was slowly added into 17 mL of H2SO4 solution (64 wt%) under constant magnetic stirring at 45 °C and stirred for 45 min. Distilled water (170 mL) was then poured into the mixture to terminate the hydrolysis reaction. The as-obtained solution was centrifugated at 10000 rpm for 10 min to obtain the hydrolysis products. The solution was then neutralized via dialysis and freeze-dried to yield nanocrystalline cellulose powder. To achieve homogeneous and molecular functionalization of cellulose, 20 mg of the as-obtained nanocrystalline

Please cite this article as: Y. Wu, T. Ding, R. Zhai et al., Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.043

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cellulose was added to 200 mg of [Bmim]Cl and stirred at 80 °C for a few minutes to ensure complete dissolution of the cellulose powder. A certain amount of SA was then mixed with the cellulose/IL solution and stirred for 30 min to obtain carboxyl-grafted cellulose macromolecules (CM-COOH). Furthermore, anime-grafted cellulose macromolecules (CM-NH2) were prepared via the continuous addition of urea and reaction for another 10 min under the same conditions. To tune the grafting degree of carboxyl groups, different dosages of SA were added and reacted under the same conditions. Specifically, the molar ratio of the AGU of cellulose to SA were 1:2, 1:4, 1:6, 1:8, 1:10 and 1:12.

30 to 800 °C. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an XPS system (ESCALAB Xi+, ThermoScientific, America). IR spectra analysis was carried out using attenuated total reflectance (ATR) with a Fourier transform IR spectrometer (FTIRPE-2000, the United States). The average zeta potential values of the samples were monitored to measure the surface charge of HQG-COOH samples in water using a Zeta analyzer (Nano-ZSE, Malvern, Britain). The Brunauer-Emmett-Teller (BET) surface area of the samples were determined by using a surface area & pore size analyzer (ASAP 2020 Plus HD88, China). 2.7. Electrochemical measurements

2.4. Assembly of HQG-COOH The as-exfoliated HQG (5 mg) was dispersed in 20 mL of DMF and mixed with the CM-COOH/IL solutions. After mild stirring at ambient temperature for 4 h, the CM-COOH/HQG assembly (HQG-COOH) was obtained via filtering and washing. The assembly was then redispersed in water for further use. 2.5. Synthesis of HQG-COOH-Co3O4 hybrids The synthetic procedure for HQG-COOH-Co3O4 hybrids was as follows: 60 mg Co(NO3)2 was dissolved in aqueous HQG-COOH dispersions, and 0.25 mL of NH3H2O (30 wt% in water) was added. The resulting solution was kept at 80 °C under stirring for 20 h and then transferred and sealed in a 50 mL Teflon-lined stainless steel autoclave for hydrothermal reaction at 150 °C for 3 h. After the solution was cooled to room temperature, the collected materials were washed and dried at 60 °C to produce HQG-COOHCo3O4. For comparison, HQG-Co3O4 hybrids were prepared via the same synthetic procedures using pristine HQG (without the CM-COOH assembly) as the building blocks. 2.6. Characterizations The grafting degree (GD) of CM-COOH was evaluated via back titration [21]. A known weight of CM-COOH was dissolved in 50 mL of KOH (0.1 M) under stirring at 60 °C for 4 h. The excess KOH was back-titrated with a HCl titration solution (0.1 M) using phenolphthalein as an indicator. Each back titration was repeated three times and the results were averaged. GD was calculated according to the following equations:

W SA ¼

98ðC 0 V 0
CV Þ 2000W

The electrochemical capacitor performances of HQG-derived Co3O4 hybrids were measured in 2 mol L
1 KOH aqueous solution using a typical three-electrode system. A platinum (Pt) foil and a saturated calomel electrode (SCE) reference electrode were used as the counter and reference electrodes, respectively. To fabricate a working electrode, HQG-COOH-Co3O4 (or HQG-Co3O4) powder, carbon black (Super-P), and polymer binder (polyvinylidene fluoride, PVDF) were mixed in a mass ratio of 70:20:10 in NMP and stirred to obtain a homogeneous black slurry. The slurry was then coated on 1 cm  1 cm Ni foam and transferred into a vacuum oven where it was dried at 80 °C for 10 h. The mass loading of active materials was 1 mg cm
2. Cyclic voltammetry (CV) curves were recorded from 0 to 0.5 V (vs. SCE) using an electrochemical workstation (CHI660E, Shanghai Chenhua Machinery Co., Ltd., China). The galvanostatic charge/discharge tests were conducted over the potential range of 0–0.45 V (vs. SCE), and electrochemical impedance spectra (EIS) were measured over the frequency range of 0.01–100 kHz. The specific capacitance (SC) was calculated using the equation [22]:

SC ¼

I  Dt m  DV

where I (A g
1) is the discharge current density, Dt (s) is the discharge time, m is the mass load of the active material, and DV (V) is the discharge voltage range. HQG-COOH-Co3O4 and AC were used as the positive and negative electrodes, respectively, to construct asymmetric supercapacitors (ASC). The ASC was used in 2 M KOH aqueous solution with one piece of filter paper as a separator. The mass loading ratio of the active materials (HQG-COOH-Co3O4: AC) was estimated to be 0.3. The energy density (E) and power density (P) of the ASC devices based on both electrodes were calculated using the following equations:

C  DV 2 2  3:6

3:6  E Dt

162W SA DS ¼ 98ð1
W SA Þ

E¼

where 162 g mol
1 is the molar weight of an anhydroglucose unit (AGU), 98 g mol
1 is the net increase in the weight of an AGU for each succinoyl unit that is grafted, and W is the weight of the sample that was analyzed. Co (M) and Vo (mL) are respectively the the concentration and volume of KOH, and C (M) and V (mL) are respectively the concentration and consumptive volume of the HCl titration solution. The crystal structures of the samples were examined using an X-ray diffractometer (D8 ADVANCE, Bruker, Germany). Raman spectra were recorded on a Raman spectrometer with a 532 nm laser (HR800, Horiba, Japan). Field emission scanning electron microscopy (FESEM, JEOL JSM 7500F, Japan) and Transmission electron microscopy (TEM, JEM F200, JEOL, Japan) were employed to observe the microstructure of HQG and HQG-derived materials. Thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses were carried out on a TGA instrument (TGA-2, Mettler Toledo) at a heating rate of 10 °C min
1 under an air flow from

where C (F g
1) is the specific capacity, DV (V) is voltage range, and Dt (s) is the discharge time.

and P ¼

3. Results and discussion The procedure for continuous aqueous processing of HQG is illustrated in Scheme 1. HQG is first prepared according to a modified procedure via liquid-phase exfoliation of natural graphite flakes in NMP. Surface properties of HQG are carefully engineered via interfacial hydrophobic assembly of CM to obtain aqueous processable HQG. It has previously been reported that cellulose can be used to modify carbon materials taking advantage of the abundant hydroxyl groups. However, chemical grafting only occurs on the surface of bulk or nanocellulose fibers, resulting in extremely low modification efficiency [23]. Herein, IL is used as a green solvent and homogenous functionalization medium to achieve molecular grafting of various functional groups with tunable grafting degrees.

Please cite this article as: Y. Wu, T. Ding, R. Zhai et al., Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.043

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Scheme 1. Schematic diagram of procedures for aqueous dispersion and synthesis of HQG materials.

The hydrophobicity-driven assembly is then trigged when the modified CM in IL is diluted with HQG dispersions in DMF under stirring. The formation of aqueous processable HQG with functional groups on the surface enables in situ growth of Co3O4 nanoparticles (NPs) via a facile hydrothermal treatment, and this is important for the development of HQG-based energy materials.

Typically, esterification grafting of COOH groups is achieved by fully dissolving cellulose and SA modifier in [Bmim]Cl followed by moderate heating. FTIR analysis is used to confirm the grafting of CM. Compared with the FTIR spectrum of pristine cellulose (Fig. S1a in the Supplementary Information), the FTIR spectrum of the grafted material shows a characteristic peak at 1723 cm
1 (Fig. 1a), which is ascribed to the stretching vibration of carbonyl

Fig. 1. (a) ATR-FTIR spectra and (b) GD of CM-COOH prepared with different molar ratios of cellulose AGU to SA modifier; (c) Photographs illustration on aqueous stability and (d) corresponding zeta potential values of the HQG and HQG-COOH-X samples.

Please cite this article as: Y. Wu, T. Ding, R. Zhai et al., Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.043

Y. Wu et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

in the carboxyl and ester groups [24], and this indicates the successful succinoylation of CM (denoted as CM-COOH). The relationship between the initial molar ratios of cellulose (calculated based on AGU) to SA and the grafting degree (GD) of CM-COOH was also investigated. The FTIR spectra of all of the controlled samples (Fig. 1a) have enhanced peak intensity at 1723 cm
1 with an increase in the SA modifier dosage. Moreover, the GD of CM-COOH was quantified via back titration (see details in the Experimental Section). Notably, the amount of SA modifier exerts significant influence on GD (Fig. 1b). Specifically, with an increase in the molar ratio from 1:2 to 1:12, the GD of CM-COOH significantly increases from 0.23 to 1.30, and this verifies that SA is highly efficient for modifying cellulose at a molecular level. It is worth noting that the as-prepared CM-COOH can be used to graft new functional groups, such as amino groups (ANH2), via coupling of the carboxyl groups with the urea modifier (as depicted in Fig. S1a), and this gives rise to aminografted cellulose macromolecules (CM-NH2). The FTIR spectrum of CM-NH2 (Fig. S1a) has a peak corresponding to NAH bending at 1625 cm
1 and peaks corresponding to ester carbonyl and amide carbonyl vibrations at 1713 and 1665 cm
1 [25,26], respectively. These observations reveal that amino groups were molecularly grafted onto CM-COOH. CM with a high GD of COOH possesses a large amount of hydrophilic sites. Therefore, the assembly of CM-COOH and HQG provides sufficient interfacial hydrophilicity and improves the aqueous processability of HQG. Photographs of the aqueous dispersion of pristine HQG and HQG-COOH that have different amounts of the CM-COOH assembly (denoted as HQG-COOH-X, where X represents the mass ratio of HQG to CM-COOH) are shown in Fig. 1c. It is clear that both the HQG-COOH-0.5 and HQG-COOH-1 samples form stable aqueous dispersions and remain stable after standing for 48 h, whereas pristine HQG tends to agglomerate quickly and precipitate completely under the same conditions. This comparison indicates that the assembly of CM-COOH provides the aqueous stability of HQG. The observed Tyndall effect (Fig. S1b) of the HQG-COOH dispersion under laser irradiation confirms that stable HQG-COOH colloids formed [27]. It is interesting to note that the HQG-COOH-4 and HQG-COOH-8 samples precipitated upon standing rather than forming stable dispersions, and this can be attributed to the large loading weight and increased van der Waals interactions of the HQG-COOH assemblies. The feasibility of forming stable aqueous HQG dispersions using the CM-COOH assembly is further elucidated via zeta potential measurements (Fig. 1d). It is well known that a zeta potential value that is more negative than
30 mV indicates a sufficient repulsive force to ensure the stability of dispersions [28]. The zeta potential of pristine HQG is as high as
0.282 mV (close to 0), and this reveals that there is a lack of surface charges and a tendency of the material to agglomerate. In contrast, the zeta potential of HQG-COOH-0.5 decreases to
31.7 mV and reaches
38.3 mV for HQG-COOH-1, and this is apparently a result of the ionization of carboxylic groups that are bonded to the HQG surface. Further increasing the CM-COOH amount for HQG-COOH-4 and HQGCOOH-8 leads to positively shifted zeta potential values (
27.2 and
19.0 mV, respectively), and this is mainly attributed to the reduced interfaces and mutual repulsions. This is consistent with the precipitation phenomena, as observed in Fig. 1c. Therefore, an optimal assembly of CM-COOH can form aqueous HQG colloids via electrostatic repulsions. The microstructures of HQG before and after assembly of CMCOOH were examined using FESEM and TEM. As seen in Figs. S2a and S3a, as-exfoliated HQG exhibits multilayered structures. The sheet edges tend to fold and corrugate slightly, and this is related to the thin and flexible features of graphene. After assembly of CM-COOH, the morphology of the resulting HQG-COOH-1 is similar to that of pristine HQG (Figs. S2b and S3b), and there are no obvi-

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ous CM-COOH agglomerates detected on the graphene surface. This implies that the coating of the CM-COOH macromolecules is uniform. The EDS (energy dispersive spectrum) elemental mapping of C and O indicate that CM-COOH is evenly distributed on HQG surface (Fig. S2d). It is worth noting that further increasing the amount of CM-COOH for HQG-COOH-8 results in a cloud-like wrapping structure around the HQG surface (Fig. S2c and S3c), and this is attributed to the agglomeration of a significant amount of the regenerated CM-COOH Fig. S2b. FTIR analysis verifies the surface functionalization of HQG by CM-COOH (Fig. 2a). In the FTIR spectrum of pristine HQG, the absorption band at 1575 cm
1 is assigned to the skeletal vibration of graphene sheets [29]. No obvious oxygen-containing groups are detected, and this indicates the high-quality character of the asexfoliated graphene. For HQG-COOH, the absorbance peaks detected at 3435, 2900, 1635, 1387, 1167, and 1021 cm
1 are associated with native cellulose and correspond to the AOH stretching vibration, aliphatic CAH stretching vibrations, HAOAH bending of the absorbed water, OAH deformation, CAO antisymmetric stretching vibration in ester, and CAOAC pyranose ring skeletal vibration, respectively [30,31]. The absorption peak at 1723 cm
1 provides the succinoylation evidence of CM, and this is consistent with the observation from the FTIR spectrum of CM-COOH (Fig. 1a) [32]. Fig. 2b presents the Raman spectra of HQG and the HQG-COOH assembly. The characteristic peaks at 1350, 1580, and 2715 cm
1 respectively correspond to the D, G, and 2D bands of HQG [33]. The intensity ratio of D and G bands (ID/IG) in HQG is calculated to be 0.13, and this suggests that there is a very low defect density in the HQG sheets. In contrast to HQG, HQG-COOH exhibits two characteristic peaks at 2890 and 1100 cm
1, and these are attributed to the symmetric stretching vibrations of ACH2 and to asymmetric stretching of CAOAC in the glycosidic link in cellulose, respectively [34]. The chemical compositions of HQG and HQGCOOH were probed using XPS (Fig. 2c). HQG exhibits approximately 2.3 at.% oxygen content, which is considerably less than the oxygen content in rGO (~15–35%) or graphene obtained via anodic electrochemical exfoliation (~5–10%). The oxygen content of HQG-COOH increases to 17.5 at.% because of the interfacial assembled CM-COOH species. The high-resolution C1s spectra of HQG (Fig. 2d) can be further deconvoluted into two signals with binding energies at 284.6 and 285.6 eV, which correspond to C@C and CAO, respectively. The intensity of the CAO peak is very low, and this indicates the high purity of as-exfoliated HQG. In contrast, the C1s XPS spectrum of HQG-COOH reveals that there is a significant amount of oxygen and that there are three bonding configurations (CAO (285.6 eV), C@O (287.1 eV), and OAC@O (288.2 eV)) [35]. The increased intensity of the CAO peak and the appearance of peaks associated with C@O and OAC@O configurations confirm the successful assembly of HQG and CM-COOH. The CM-COOH assembly lead to the formation of stable HQG dispersions and also promotes interfacial chemical interactions that enable the development of advanced functional materials in aqueous conditions. As a typical example, we used HQG-COOH as a substrate with cobalt nitrate as the metal precursor to fabricate HQG-based heterostructures using continuous coordination and hydrothermal approaches. HQG-COOH has an abundance sites for surface adsorption and coordination with Co2+ or [Co(NH3)6]2+. Subsequent reflux and hydrothermal processes transfer the metal precursors into cobalt hydroxide and finally to Co3O4 on both sides of HQG [36]. The morphology of the as-obtained HQG-COOH-Co3O4 was characterized using FESEM and TEM. As shown in Fig. S4a, the HQG-COOH-Co3O4 exhibits a curled morphology as a result of ultrathin feature of the HQG substrates. Owing to very small particle size (see below), the anchored Co3O4 NPs cannot be observed based on the SEM image. Nevertheless, the EDS mapping images

Please cite this article as: Y. Wu, T. Ding, R. Zhai et al., Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.043

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Fig. 2. (a) FTIR, (b) Raman, (c) overview XPS, and (d) C1s XPS spectral comparisons of HQG and HQG-COOH.

of HQG-COOH-Co3O4 in Fig. S4b show that the C, O and Co elements are homogeneously distributed throughout the HQGCOOH sheets, revealing uniform growth of Co3O4 NPs onto HQG surface. TEM observation in Figs. 3a and S5a demonstrate that a large number of Co3O4 NPs are homogeneously distributed and tightly anchored on the HQG-COOH surface. From highresolution TEM (HR-TEM) of HQG-COOH-Co3O4 in Fig. 3b, it is determined that the anchored Co3O4 NPs have an average diameter of 10.7 nm (Fig. S5c, size distribution), and the interplanar lattice spacing of 0.29 nm corresponds to the (2 2 0) planes of crystalline Co3O4 [37]. For comparison, pristine HQG sheets without the CMCOOH assembly were used for in situ growth of Co3O4 NPs. Most of the Co3O4 NPs were randomly distributed around the HQG sheets instead of being uniformly coated (Fig. S5b). This validates that the surface-engineered HQG has a sufficient amount of exposed binding sites (i.e., carboxylic groups) to interact with Co2+ precursors and to direct nucleation and growth. Moreover, the corresponding size distribution of Co3O4 (Fig. S5d) indicates that HQG-Co3O4 has a larger particle size (22.3 nm) than that of HQG-COOH-Co3O4. This may be interpreted to mean that there is a high concentration of Co2+ in free solutions because of a lack of surface bonding, and this is favorable for the formation of large NPs upon the addition of ammonia. Further, the specific surface area of the HQG-Co3O4 and HQG-COOH-Co3O4 hybrids was determined by BET analysis (Fig. S6). The nitrogen adsorption/desorption isotherms of both samples exhibit the type IV isotherms pattern with a H3 hysteresis loop according to IUPAC classification[38], which is related to the aggregate apertures from the graphene sheets. The HQG-COOH-Co3O4 reveals a BET surface area of 356.1 m2 g
1, which is significantly higher than that of HQG-Co3O4 (39.6 m2 g
1). Thereby, the surface area of the HQG hybrids can be

increased by interfacial bonding of small Co3O4 NPs and the formation of 2D heterostructures, which may lead to promising application of the HQG-COOH-Co3O4 as high performance supercapacitor electrodes. The crystallographic phases of HQG-COOH-Co3O4 hybrids were investigated using XRD (Fig. 3c). The diffraction peaks at 31.3°, 36.8°, 44.8°, 59.4°, and 65.2° are perfectly indexed to the (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) of Co3O4 (JCPDS No. 43-1003), respectively [39]. The crystalline peak at 26.2° of the asexfoliated HQG can be indexed to the (0 0 2) plane of hexagonal crystalline graphite (JCPDS No. 41-1487) [40,41], and this is consistent with the rather limited ordering from the limited few-layers HQG. Furthermore, TG analysis was carried out to quantify the loading amount of Co3O4 NPs (Fig. 3d), and it can be seen that the maximum rate of lose weight of hybrids is 216 °C because the surface of the assembled CM-COOH decomposes. Subsequently, the hybrids dramatically lose weight because of the burning of HQG around 605 °C. Thus, the content of Co3O4 is estimated to be about 55 wt% in the hybrids. Cyclic voltammetry (CV) was performed at different scanning rates (10–100 mV s
1) to evaluate the electrochemical behavior of HQG-COOH-Co3O4 and HQG-Co3O4 electrodes for supercapacitors. As seen in Figs. 4a and S7a, both of the hybrid electrodes exhibit faradic pseudocapacitance features with an obvious redox peak, which corresponds to the redox reactions of Co3O4 during the cathodic and anodic sweeps. The two anodic peaks at 0.24 and 0.30 V correspond to the conversions of Co3O4/CoOOH and CoOOH/CoO2, respectively. However, only one cathodic peak at 0.16 V is observed during the reverse sweep, and this probably corresponds to the simultaneous reduction of the CoOOH product into Co3O4 [42]. With an increased scan rate, the anodic peaks shift to

Please cite this article as: Y. Wu, T. Ding, R. Zhai et al., Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.043

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Fig. 3. (a) TEM images of HQG-COOH-Co3O4 hybrids and (b) HRTEM image focusing on a single Co3O4 NP; (c) XRD pattern and (d) TG/DTG curves of the HQG-COOH-Co3O4 hybrids.

higher potential, whereas the cathodic peaks shift to lower potential, and these shifts are a result of the limited ion diffusion to satisfy electronic neutralization during the redox reactions. Within the potential scan range, the electrochemical reactions correspond to the Co3O4/CoOOH/CoO2 conversions in the presence of OH
anions, as follows [43,44]:

Co3 O4 + OH
+ H2 O $ 3CoOOH + e
CoOOH + OH
$CoO2 + H2 O + e
Fig. 4b shows a comparison of the CV curves of both hybrid electrodes at a scanning rate of 10 mV s
1 in the potential window of 0–0.5 V. The larger integral area of the CV curves for HQG-COOHCo3O4 indicates that charge storage capability of HQG-COOHCo3O4 is superior to that of HQG-Co3O4. The galvanostatic charge-discharge curves of both electrodes are also presented (Figs. 4c and S7b). Remarkably, the curves of the HQG-COOHCo3O4 and HQG-Co3O4 electrodes are nearly symmetric at different current densities from 1 to 20 A g
1, and this confirms the excellent reversibility of the hybrids. The presence of discharge/charge plateaus contribute to pseudocapacitance from the redox reactions of Co3O4, and this observation is in good agreement with the redox couples in the CV curves. Notably, the HQG-COOH-Co3O4 electrode displays a longer discharge time than the HQG-Co3O4 electrode at various current densities, and this indicates that the HQG-COOH derived hybrids have enhanced charge-storage ability. These comparative results can be ascribed to the excellent interfacial contact between smaller Co3O4 NPs and HQG substrates in HQG-COOHCo3O4 hybrids compared to that in HQG-Co3O4. Therefore, surface engineering of HQG is potentially useful for creating a heterostructured architecture and significantly improving the electrochemical activity of the bonded pseudocapacitive Co3O4 materials [45]. EIS measurements were recorded to measure the ion and electron transport properties and to understand the electrochemical

behavior of the electrodes. Nyquist plots of both hybrid electrodes and an equivalent circuit are shown in Fig. 4d. The equivalent circuit is comprised of the internal resistance Rs, charge transfer resistance Rct, diffusion resistance of electrolyte ions (Warburg) Wd, double layer capacitance element Cdl, and faradaic capacitance element CF [46]. Using the ZSimpWin software (Princeton Applied Research, Oak Ridge, TN, USA), the evaluated Rs value of the HQG-COOH-Co3O4 (1.08 O) is slightly higher than that of HQGCo3O4 (1.05 O). This can be attributed to surface coating of cellulose functionalities on HQG for the HQG-COOH-Co3O4. The Rct values for HQG-Co3O4 and HQG-COOH-Co3O4 are 0.78 and 0.64 O, respectively. The reduced Rct of HQG-COOH-Co3O4 in comparison to HQG-Co3O4 electrode can be attributed to the formation of 2D heterostructures and uniform distribution of highly acitive Co3O4 NPs. In the low frequency region, the steeper slope of the Warburg-type line based on HQG-COOH-Co3O4 electrode indicates lower ion diffusion resistance than that of HQG-Co3O4 electrode. This result can be ascribed to higher specific surface area of HQG-COOH-Co3O4 than that of HQG-Co3O4 electrode. This effectively improves the accessible surface area of Co3O4 and consequently reduces the ion diffusion distance from the aqueous electrolyte. These comparisons clearly demonstrate that the HQG-COOH-Co3O4 electrode possesses fast kinetics, leading to enhanced electrochemical properties. Furthermore, the specific pseudocapacitances of the HQGCOOH-Co3O4 and HQG-Co3O4 electrodes at different current densities were calculated and plotted (Figs. 4e and S7c). The HQGCOOH-Co3O4 hybrids deliver high capacitances of 1268.3, 1210.9, 1178.8, 1126.6, 1027.9, 1025.6 and 853.8 F g
1 at current densities of 1, 2, 3, 5, 8, 10 and 20 A g
1, respectively, and these observations confirm that the hybrids have good capacitive capability (low capacitance degradation rate) over an extensive range of operating current densities. For example, 80.9% of the initial capacity of the HQG-COOH-Co3O4 hybrids (at 1 A g
1) was readily obtained when

Please cite this article as: Y. Wu, T. Ding, R. Zhai et al., Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.043

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Fig. 4. (a) CV curves of the HQG-COOH-Co3O4 hybrids at various scan rates and (b) CV curves of the HQG-COOH-Co3O4 and HQG-Co3O4 hybrids at a scan rate of 10 mV s
1; (c) galvanostatic charge-discharge curves of the HQG-COOH-Co3O4 hybrids at various current densities; (d) Nyquist plots of the HQG-COOH-Co3O4 and HQG-Co3O4 hybrids with the equivalent circuit; (e) specific capacitance of the HQG-COOH-Co3O4 electrodes calculated from discharge curves as a function of current densities; and (f) cycling stability of the HQG-COOH-Co3O4 hybrids.

the current density was increased to 10 A g
1, indicating that the hybrids had a high rate performance. In contrast, the specific capacitances of the HQG-Co3O4 electrode at the same current densities are only 477.7, 468.4, 461.7, 447.9, 420.6, 403.8 and 341.8 F g
1, respectively. This capacitance performance of HQG-COOHCo3O4 is among the best reported values for Co3O4-based supercapacitors that have various carbon matrixes (Table S1). Impressively, the HQG-COOH-Co3O4 hybrids exhibit excellent cycling stability with a reversible capacitance of 1117 F g
1 and a capacity retention of ~100% even after 6000 cycles (Fig. 4f). This is in sharp contrast to the cycling stability and capacity retention of conventional carbon-based hybrid supercapacitors (Table S1). The above results clearly demonstrate the considerably improved capacitance performance and cycling stability of the Co3O4 hybrid electrode in supercapacitor applications, and this can be attributed to the use of HQG as a unique substrate and to rational surface engineering to form 2D heterostructures. The excellent electrical conductivity and structural integrity of HQG offer efficient on-plane electron

transport pathways, and the assembled CM functional groups contribute to homogeneous growth and stable interfacial adhesion between Co3O4 NPs and HQG. Thus, the long-term rapid charge accumulation and delivery capability of the whole heterostructured electrode are greatly enhanced. To demonstrate the potential application of the HQG-COOHCo3O4 electrode, an ASC was assembled using HQG-COOH-Co3O4 as the positive electrode and AC as the negative electrode in 2 M KOH electrolyte. Fig. 5a shows the potential windows of the HQG-COOH-Co3O4 electrode and AC electrode at a scan rate of 10 mV s
1. Thereby, it is anticipated that the maximum operation potential of the ASC can reach 1.6 V. Fig. 5b shows the CV curves of the assembled HQG-COOH-Co3O4//AC ASC at different scan rates. There are no obvious changes to the CV curves even at a scan rate as high as 100 mV s
1, and this suggests that the ASC has good electrochemical activity and reversibility. The galvanostatic charge-discharge tests of the ASC were performed at various current densities. As seen in Fig. 5c, all of the curves have a nearly

Please cite this article as: Y. Wu, T. Ding, R. Zhai et al., Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.043

Y. Wu et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx

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Fig. 5. (a) CV curves of the HQG-COOH-Co3O4 and the AC electrode performed in a three-electrode cell at a scan rate of 10 mV s
1; (b) CV curves of the HQG-COOH-Co3O4//AC ASC at different scan rates ranging; (c) Galvanostatic charge-discharge curves of the HQG-COOH-Co3O4//AC ASC at different current densities from 1 to 10 A g
1. (d) Ragone plot of the ASC device.

triangular shape, and this indicates that the ASC has good capacitive behavior. The specific capacitance based on the total mass of active materials reaches 143.7 F g
1 at a current density of 1 A g
1 and remains 83.9 F g
1 at a high current density of 10 A g
1, revealing that the ASC has a high rate capability. A Ragone plot based on both electrodes shows that the energy density versus power density is an important indicator of the ASC for practical applications. As depicted in Fig. 5d, the HQG-COOHCo3O4//AC ASC delivers a high energy density of 48.0 Wh kg
1 at a power density of 775 W kg
1, and even retains an energy density of 28.0 Wh kg
1 at a power density of 7750 W kg
1. Remarkably, these values for the ASC that was developed in this work are much higher than those of recent reports on cobalt oxides ASC devices, such as Co3O4/NHCS//AC (34.5 Wh kg
1 at 753 W kg
1) [47], CoOx/C-10//carbon foam (32 Wh kg
1 at 650 W kg
1) [48], CO/ CNT/Gr//CO/Gr (27.9 Wh kg
1 at 3100 W kg
1) [49], CNMnL-m// AC (38.4 Wh kg
1 at 800 W kg
1) [50], Co3O4-G > N-PEGm//GCA (34.4 Wh kg
1 at 400 W kg
1) [51] and Co3O4@NiO UNAs//AC (30.2 Wh kg
1 at 201.5 W kg
1) [52]. Besides, the energy density and power density of HQG-COOH-Co3O4//AC ASC are superior to other metal-derived ASC devices, such as MgAl2O4/rGO//AC (16.2 Wh kg
1 at 4000 W kg
1) [53] and Co-Al LDH/rGO-3//AC (44.6 Wh kg
1 at 799.6 W kg
1) [54]. The enhanced capacitive performance of HQG-COOH-Co3O4 electrode can be attributed to the combination of wide operating voltage of assembled ASC device and the high specific capacitance of the HQG-COOH-Co3O4 hybrids with unique heterostructure.

tunable grafting types and degrees along with the hydrophobicity-driven assembly enable the surface properties of HQG to be engineered in controllable ways. Carboxyl-grafted CM (CM-COOH) promotes the aqueous processability of HQG and also offers a substantial amount of exposed reactive sites for efficient coupling of functional components. The as-prepared HQG-COOHCo3O4 exhibits pronounced electrochemical performances (including high reversible capacitance, good rate capability, and long-term cycling stability) when it is used as an electrode in supercapacitors. These performances are superior to those of the aqueous processable GO-based counterparts that are used as supercapacitor electrodes. Such an interfacial assembly strategy provides a facile and eco-friendly approach to large scale development of new 2D heterostructural materials that are based on HQG and used for a wide range of applications. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51602245), the Natural Science Foundation of Shaanxi Province (No. 2019JQ-362), the Fundamental Research Funds for the Central Universities (2016qngz07), and the ‘‘Young Talent Support Plan” of Xi’an Jiaotong University (HX1J002).

4. Conclusion Appendix A. Supplementary material In summary, aqueous processing and functionalization of HQG were achieved via the interfacial assembly of cellulose macromolecules (CM) and HQG. Homogeneous modification of CM with

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.11.043.

Please cite this article as: Y. Wu, T. Ding, R. Zhai et al., Efficient aqueous processing and utilization of high-quality graphene for high performance supercapacitor electrode, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.043

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