Preparation via flocculation of reduced graphene oxide/cationic polyacrylamide composites and their electrochemical properties

Abstract

Composites of reduced graphene oxide (RGO) and cationic polyacrylamide (CPAM) are prepared through a facile method. The effect of CPAM content on the specific capacitance of the composite is investigated. Compared with the pure RGO, the composite shows a relatively high capacitance, due to the fact that CPAM improves the wettability and surface area of RGO.

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Supercapacitors have become promising energy storage devices for portable electronic devices and electric vehicles because of their high power capabilities and power densities, ultra-long cycling life, wide thermal operating ranges, and low maintenance costs.^1,2 Graphene, a new kind of carbon material with a unique two-dimensional layer atomic structure, has been proven to possess excellent mechanical, electronic and catalytic properties.³ A high conductivity and an in-plane surface area make it a potential electrode material for use in supercapacitors.⁴

Graphene oxide (GO) is often considered as a precursor to low-cost and scalable production of graphene based materials. It bears a large number of hydroxyl, epoxide and carbonyl groups,^5,6 which can be eliminated by chemical reduction or thermal annealing. However, irreversible agglomeration while removing oxygen containing groups is the main problem. So far, many efforts have been made to prepare functionalized or dispersed graphene, either by chemical reduction of GO in the presence of stabilizers, such as amphiphilic polymers⁷ or polypyrrole microspheres,⁸ or by the reduction of functional GO with organic ionic surfactants⁹ or isocyanates.¹⁰

Cationic polyacrylamide (CPAM), a common polymer electrolyte, is widely used in wastewater treatment¹¹ and paper-making.¹² It can be used as a flocculant¹³ or a dispersing agent¹⁴ owing to the active amide groups in the polymer chain. GO sheets with negatively charged oxygen groups can intercalate with CPAM chains via electrostatic interactions. Yu et al.¹⁵ reported a simple and highly effective method for the preparation of a conductive graphene-based thin film by self-assembly of graphene oxide on a CPAM layer, followed by chemical reduction. The RGO/CPAM composite film exhibited a sheet resistance of 3.1 kΩ sq⁻¹. Furthermore, CPAM can be used as a flocculant for GO, owing to the co-action of charge neutralization and polymer bridging. Preparation of a RGO/CPAM composite through the process of flocculation in an aqueous solution has never been reported.

In the present work, we report a simple route for the preparation of a RGO/CPAM composite (Scheme 1). A certain amount of CPAM solution was added to the completely exfoliated GO aqueous solution, and the mixture was stirred with glass rods for several minutes. It was observed that the GO sheets were flocculated due to the use of CPAM (see Fig. S1†). The resulting brown GO/CPAM flocks were subsequently reduced by sodium borohydride, which is an environmentally friendly chemical in comparison with hydrazine.¹⁶ The entire reaction finished in just 2 h and the black soft RGO/CPAM composites were easily obtained by centrifugation. This reduced graphene oxide, which was flocculated with CPAM, can be obtained in gram quantities quickly, and exhibits good dispersibility in aqueous solvents. CPAM intercalation between graphene sheets can prevent the aggregation of graphene sheets during reduction. Meanwhile, the presence of hydrophilic CPAM in graphene materials may enhance their surface wettabilities and improve their specific capacitances. Finally, RGO/CPAM composites with different weight ratios (GO : CPAM), 10 : 1 and 10 : 2, were fabricated for the purpose of studying the effect of CPAM addition on the electrochemical performances of the products. The obtained composites were named as RGO/CPAM1 and RGO/CPAM2, respectively.

Scheme 1 The preparation route for the RGO/CPAM composite.

To investigate the morphology of the prepared RGO and the RGO/CPAM composite, SEM and TEM images were obtained. The SEM images (see Fig. S2†) show that the surface of RGO is smooth, which indicates that the RGO sheets are stacked together. Meanwhile, the CPAM-intercalated RGO nanosheets are well dispersed in pieces, which suggests that the graphene sheets were stabilized by CPAM during the reduction treatment. More details can be observed from the TEM images (Fig. 1). The RGO sheets show typical wrinkles and crumple-like structures (Fig. 1a and c). The inset of Fig. 1c shows a selected area of the electron diffraction pattern (SAED) of RGO, showing a clear diffraction spot that is indexed to a hexagonal graphite crystal structure. Compared with RGO, some dark flakes are seen in the images of the RGO/CPAM2 composite (Fig. 1b and d), due to parts of the CPAM chains being mutually entangled, aggregated and randomly dispersed between the RGO sheets. This indicates that CPAM was attached to the RGO sheets.¹⁷ The SAED pattern of CPAM shows the crystalline nature of the polymer (inset of Fig. 1d).¹⁸

Fig. 1 TEM images of RGO (a and c) and the RGO/CPAM2 composite (b and d) under different magnifications (0.5 μm and 200 nm scale bar).

The FTIR spectra of GO, RGO, the RGO/CPAM2 composite and pure CPAM were taken and the results are shown in Fig. 2. The spectrum of GO illustrates the presence of C=O (ν_C=O at 1719 cm⁻¹), benzenoid C=C (ν_C=C at 1647 cm⁻¹), C–O–C (ν_C–O–C at 1501 cm⁻¹), and O–H (ν_H–O at 3404 cm⁻¹).^19,20 For pure CPAM, C=O stretching of the –CO–NH₂ group, –NH₂ bending vibrations and C–N stretching vibrations were observed at 1694, 1638 and 1280 cm⁻¹ respectively. Compared with GO, the spectra of RGO and the RGO/CPAM2 composite have no peaks at 1719 and 1501 cm⁻¹, which indicates the successful removal of the oxygen-containing groups from the sheets. The image on the right shows the spectra of RGO and the RGO/CPAM2 composite in the range of 1500 to 400 cm⁻¹. Compared with RGO, new peaks at 1367 and 939 cm⁻¹ are observed for the RGO/CPAM2 composite, which is attributed to the C–N stretching vibrations. This proves that CPAM had intercalated with the RGO sheets.

Fig. 2 FTIR spectra of CPAM (a), RGO (b), the RGO/CPAM2 composite (c) and GO (d).

The structures of RGO, CPAM and the RGO/CPAM2 composite were further studied using their Raman spectra as shown in Fig. 3. It can be seen that both RGO and the RGO/CPAM2 composite show double peaks at about 1320 and 1590 cm⁻¹, while CPAM does not. The peak at ∼1320 cm⁻¹ (D band) is attributed to the vibrations of sp³ carbon atoms in defects and disorders, and the peak at ∼1590 cm⁻¹ (G band) corresponds to the vibration of sp² carbon atoms in the 2D hexagonal lattice. Compared with RGO, the D/G ratio of the RGO/CPAM2 composite is slightly increased from 2.6 to 2.77. This illustrates that the RGO/CPAM2 composite has a lower percentage of sp² domains but they are larger in size.²¹ This can be explained by considering that CPAM is still connected to the RGO sheets via polymer bridging after the removal of oxygen groups, which leads to the larger size of the graphitic domains.

Fig. 3 Raman spectra of RGO (a), the RGO/CPAM2 composite (b) and CPAM (c).

XRD patterns were also collected to study the structure of RGO and the RGO/CPAM2 composite (see Fig. S3†). The interlayer distance of the RGO/CPAM2 composite is 0.39 nm, larger than that of RGO (0.35 nm). This indicates that CPAM is intercalated between RGO sheets after the removal of oxygen-containing groups in the plane. However, the increase of the interlayer distance is minimal, which is due to the polymer bridging of CPAM between the RGO sheets. This is consistent with the results of the Raman spectrum.

The electrochemical performances of the RGO/CPAM composites with different weight ratios were tested. The as-obtained samples were fabricated into electrodes and characterized by cyclic voltammetry (CV) and galvanostatic charge–discharge measurements (Fig. 4). Typical CV responses of RGO, RGO/CPAM1 and RGO/CPAM2 at 5 mV s⁻¹ are shown in Fig. 4a. The specific capacitance (C_S) is proportionate to the average area of the CV curve.²² The average specific capacitances from the CV curves were calculated according to the following equation:

where m is the mass of the active electrode material, v is the scan rate, V_f and V_i are the integration potential limits of the voltammetric curve, and I(V) is the voltammetric current.²³ The specific capacitances of RGO, RGO/CPAM1 and RGO/CPAM2 are shown in Table 1. RGO possesses quite a low specific capacitance, which is similar to that of chemically reduced graphene oxide (GR) previously reported.²⁴ It is noteworthy that the specific capacitance of the composite is dramatically enhanced. The capacitance of RGO/CPAM2 is nearly 7 times larger than that of RGO, which suggests that CPAM intercalation between the RGO sheets plays an important role in preventing the aggregation of graphene sheets. Furthermore, the presence of hydrophilic CPAM leads to the enhancement of wettability and a considerable increase in the capacitance of the composite.

Fig. 4 The cyclic voltammograms at a scan rate of 5 mV s⁻¹ (a) and the galvanostatic charge–discharge curves at 0.1 A g⁻¹ (b) of RGO, RGO/CPAM1 and RGO/CPAM2 in 6 M KOH solution.

Table 1 The specific capacitances of all samples

Sample	RGO	RGO/CPAM1	RGO/CPAM2
CSCVs (F g^−1)	10.18	39.34	103.52
CSgalvanostatic (F g^−1)	11.73	56.18	167.6

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Fig. 4b shows galvanostatic charge–discharge measurements of RGO, RGO/CPAM1 and RGO/CPAM2 at 0.1 A g⁻¹. The values of C_S were calculated based on data obtained from the discharge curves according to the following equation:

C_S = IΔt/(m × ΔV) (2)

where I is the constant discharge current (A), m is the mass of the sample (g), ΔV is the potential charge during the discharge process and Δt is the discharge time.⁹ The capacitance results of all the samples are also given in Table 1, and are consistent with the conclusions drawn from the CVs. It can be observed that the charge and discharge curves of all the samples show approximately linear properties, which demonstrates their electric double layer capacitance characteristics.

In addition, the relationships between the C_S values and the current densities measured in a three-electrode system are presented in Fig. 5a. A small decrease (about 33%) in the C_S value of RGO/CPAM2 is observed as the current density changes from 0.1 to 1.0 A g⁻¹ (Fig. 5b). The highest specific capacitance of 167.6 F g⁻¹ obtained at 0.1 A g⁻¹ is similar to the value of 135 F g⁻¹, which was obtained from reduced-graphene oxide using ultra-thin graphene membranes.²⁵

Fig. 5 The galvanostatic charge–discharge curves of the RGO/CPAM2 composite with different current densities (a); the specific capacitance of RGO/CPAM2 at different current densities (b).

In summary, we have reported a novel RGO/CPAM composite fabricated on a gram scale through a facile method, which includes flocculation of GO using CPAM, followed by reduction with sodium borohydride. The CPAM was reported to be attached to the graphene sheets after the removal of the oxygen-containing groups. In comparison with pure RGO, the specific capacitance of the RGO/CPAM composite has been improved vastly. This can be attributed to the fact that intercalated-CPAM effectively prevents aggregation of the RGO sheets and enhances the wettability of the composite electrode material. According to the present results, this original method for preparation of a RGO/CPAM composite is expected to offer a great opportunity for the application of graphene based composite materials in supercapacitor electrodes.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and supplementary characterisation data. See DOI: 10.1039/c6ra17371c

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