Facile synthesis of ternary graphene nanocomposites with doped metal oxide and conductive polymers as electrode materials for high performance supercapacitors

Supercapacitors (SCs) due to their high energy density, fast charge storage and energy transfer, long charge discharge curves and low costs are very attractive for designing new generation of energy storage devices. In this work we present a simple and scalable synthetic approach to engineer ternary composite as electrode material based on combination of graphene with doped metal oxides (iron oxide) and conductive polymer (polypyrrole) with aims to achieve supercapacitors with very high gravimetric and areal capacitances. In the first step a binary composite with graphene mixed with doped iron oxide (rGO/MeFe2O4) (Me = Mn, Ni) was synthesized using new single step process with NaOH acting as a coprecipitation and GO reducing agent. This rGO/MnFe2O4 composite electrode showed gravimetric capacitance of 147 Fg−1 and areal capacitance of 232 mFcm−2 at scan rate of 5 mVs−1. In the final step a conductive polypyrrole was included to prepare a ternary composite graphene/metal doped iron oxide/polypyrrole (rGO/MnFe2O4/Ppy) electrode. Ternary composite electrode showed significantly improved gravimetric capacitance and areal capacitance of 232 Fg−1 and 395 mFcm−2 respectively indicating synergistic impact of Ppy additives. The method is promising to fabricate advanced electrode materials for high performing supercapacitors combining graphene, doped iron oxide and conductive polymers.

their composite structure and using different charging mechanisms and possible synergistic effect between each of their components is recognized to be ideal solution to design and improve the performance of supercapacitors 9 .
Graphene has emerged as an ideal material for EDLCs due to its unique properties like high electrical conductivity, low density, high specific surface area (2670 m 2 g −1 ), chemical stability, mechanical strength and tailoring chemical functionalities 10,11 . Initial studies showed gravimetric capacitance of synthesised graphene in aqueous and organic electrolytes to be 135 Fg −1 and 99 Fg −1 respectively 12 . Wang et al., prepared graphene by using gas based hydrazine reduction of graphene oxide (GO) and measured its gravimetric capacitance as 205 Fg −1 13 . Many other studies showed different results for gravimetric capacitance of graphene from 59 Fg −1 at scan rate of 2 mVs −1 14 , 117 Fg −1 at scan rate of 100 mVs −1 and 169.3 Fg −1 at 10 mVs −1 depending on the type of graphene, its purity and electrolyte 15 . Metal ferrites having variable redox states have been extensively explored as suitable electrode materials for supercapacitors applications 16 showing an outstanding gravimetric capacitance 576. 6 Fg −1 at current density of 1 Ag −1 measured by three electrodes system 20 .
In recent years, conducting polymers in pseudocapacitors are heavily explored due to their high specific capacitance obtained through reversible redox reaction. Polypyrrole is one of conducting polymers that showed excellent high conductivity and high environmental and mechanical stability 21,22 . Recently composites of polymers and nanofillers such as carbon based materials have been successfully used as electrodes to improve performance of supercapacitors using high synergistic effect. Biswas et al., synthesized graphene/polypyrrole composite material displaying gravimetric capacitance of 165 Fg −1 at current density of 1 Ag −1 measured by two electrodes system while using 1 M NaCl aqueous solution as electrolyte 23 . Parl et al., used graphite/polypyrrole composite for supercapacitor electrodes showing gravimetric capacitance of 400 Fg −1 measured by three electrodes system 24 .
In order to gain advanced supercapacitors performances, the concept of the three-components or ternary system by combining these three components has been proposed. Chee et al., synthesized ternary polypyrrole/ graphene oxide/zinc oxide supercapacitor electrodes and measured its gravimetric capacitance in two electrodes system to be 94.6 Fg −1 at 1 Ag −1 from charge/discharge (CD) curves 25 . Lim et al., reported ternary polypyrrole/ graphene/nano manganese oxide composite, the gravimetric capacitance of synthesized composite was 320.6 Fg −1 at 1 mVs −1 which was much higher than that of polypyrrole/graphene delivering gravimetric capacitance of 255.1 Fg −1 and neat polypyrrole with gravimetric capacitance of 118.4 Fg −1 26 . Xiong et al., used three electrodes system to measure gravimetric capacitance of ternary cobalt ferrite/graphene/polyaniline nanocomposites, which showed gravimetric capacitance of 1133.3 Fg −1 at scan rate of 1 mVs −1 27 . These studies clearly indicate that design of multi-component composite electrodes for supercapacitors is a beneficial and promising approach that is able to significantly improve the performance of supercapacitors.
Inspired with these studies in the present research work, we present the synthesis and performances of ternary composite electrodes for supercapacitor applications that are specifically engineered by combination of graphene, mixed doped metal oxide and conductive polymers and their unique properties and synergistic effects. To demonstrate this concept we selected graphene (rGO), metals doped iron oxide (MeFe 2 O 4 ) and conductive polypyrrole (Ppy) polymer as model components for proposed ternary system (rGO/MeFe 2 O 4 /Ppy) that is schematically presented in Fig. 1. The aims of this work were to explore the electrochemical performance of this ternary composite material, evaluate influence of each component and their synergetic effects and demonstrate its capability to be used for designing high performing supercapacitors. For metal doping of iron oxide two metals Mn and Ni were selected as a model doping elements because of their excellent redox behaviour. These metals can contribute more efficiently than pure iron oxide in increasing gravimetric capacitance having high electronic conductivity and electrochemical performance, low cost and environmental friendly nature 28 . Finally Ppy was selected as common and highly conductive polymer having high specific capacitance of 136 Fg −1 measured by three electrodes system 29 . In addition to confirm the performance of proposed ternary electrodes system one of specific objective of this work was to develop new, simplified, environmentally friendly and scalable method to synthesize these composite materials. For that purpose we introduced one step process using NaOH to make binary composite (rGO/MeFe 2 O 4 ) instead of undergoing conventional two steps process using reduction by hydrazine hydrate to form reduced graphene oxide (rGO). In the following and final step ternary graphene/ www.nature.com/scientificreports www.nature.com/scientificreports/ metal doped iron oxide/polypyrrole (rGO/MeFe 2 O 4 /Ppy) nanocomposite was synthesized by common oxidative polymerization of pyrrole. The extensive characterization for both binary and ternary composites with mixed metals (Mn and Ni) as dopant of iron oxide was performed in order to reveal synergetic impact of each component in the system on supercapacitor performances.  www.nature.com/scientificreports www.nature.com/scientificreports/ The TGA characterizations carried out to study thermal stability of GO, rGO/MeFe 2 O 4 and rGO/MeFe 2 O 4 / Ppy nanocomposites is presented in Fig. 3c and S2D. TG curves of GO and rGO/MnFe 2 O 4 in Fig. 3c shows that a slight weight loss below 200 °C was observed for GO and rGO/MnFe 2 O 4 and it is due to water loss. For GO maximum weight is lost below 300 °C. This weight loss is due to break down of oxygen functional groups in GO. However it is apparent from TG curve of rGO/MnFe 2 O 4 that it is thermally more stable than GO. It is due to conversion of GO into rGO, which is due to removal of oxygen functional groups during reduction of GO to rGO 41

Results and Discussion
Figures 4b and S3B show the CV curves of rGO/MnFe 2 O 4 and rGO/NiFe 2 O 4 respectively at different scan rates of 10-100 mVs −1 . It is clear from the figures that current density of peak enhances with corresponding scan rate. It depicts its fairly good ion response and good EDL capacitance behaviour. High current density with increasing scan rate depicts that electrode material shows more conductivity, less internal resistance and good rate capability of electrode material in used electrolyte i.e., 1 M H 2 SO 4 as the scan rate increases 44 . Shape of CV curves remains same at different scan rates of 10-100 mVs −1 which shows that kinetics of EDL formation is quite fast and it is also indicative of fast Faradic reaction in electrodes 45 . Gravimetric capacitance of rGO/MnFe 2 O 4 electrode was 147 Fg −1 at scan rate of 5 mVs −1 . At the same scan rate, areal capacitance of rGO/MnFe 2 O 4 electrode calculated from its CV curves was 250 mFcm −2 while gravimetric and areal capacitance of rGO/NiFe 2 O 4 electrode was found to be 48 Fg −1 and 82 mFcm −2 , respectively.
CD curves of rGO/MnFe 2 O 4 at different current densities of 1 Ag −1 , 2 Ag −1 , 4 Ag −1 and 6 Ag −1 are shown in Fig. 4c. CD curves of synthesized rGO/MnFe 2 O 4 based electrodes are not symmetrical, which shows pseudocapacitive behaviour of electrode material. CD curves at different current densities are of almost same shape, showing that electrode material has ideal capacitive behaviour. Similar behaviour was observed in CD curves of rGO/NiFe 2 O 4 as shown in Fig. S3C. Figure 4d represents gravimetric and areal capacitances of rGO/MnFe 2 O 4 at different scan rates i.e., 10-2000 mVs −1 . It is evident that with increase in scan rate both gravimetric capacitance and areal capacitance decrease. It is attributed to insufficient time available for ions to diffuse and adsorb in the small pores within large particles. In addition, at high scan rate, when supercapacitor delivers high current, a noticeable voltage loss (ΔV) is originated 46 . Owing to this large current density and symmetrical behaviour of CV curves in both anodic and cathodic directions it is proposed to possess best capacitive performance among all synthesized electrodes and it is suitable supercapacitor electrode material 47 . Gravimetric capacitance of rGO/MnFe 2 O 4 /Ppy electrode was 232 Fg −1 at scan rate of 5 mVs −1 while its areal capacitance was 395 mFcm −2 . It is obvious that both gravimetric and areal capacitance of rGO/MnFe 2 O 4 /Ppy was 1.57 times greater than that of rGO/MnFe 2 O 4 due to more synergistic effect between the composite components . CV curves of both rGO/MnFe 2 O 4 /Ppy and rGO/NiFe 2 O 4 /Ppy are rectangular even at high scan rate of 100 mVs −1 showing that this electrode material has higher electrochemical performance 48 . High current density with increasing scan rate depicts that electrode material shows more conductivity, less internal resistance and good rate capability of electrode material in used electrolyte i.e., 1 M H 2 SO 4 as the scan rate increases 44 . Similar trend was observed in rGO/NiFe 2 O 4 /Ppy electrode shown in Fig. S4B.  Electrochemical Impedance spectroscopy (EIS) has been performed by two electrodes system in 1 M H 2 SO 4 at the excited potential of 5 mV between frequency range of 0.01-100 kHz in the form of Nyquist plot shown in Fig. 6. Nyquist plot reveals that rGO/MnFe 2 O 4 and rGO/MnFe 2 O 4 /Ppy show very small semicircles at high frequency region, which reveal to charge transfer resistance and solution resistance while a straight line in high frequency region reveals to Warburg resistance. rGO/MnFe 2 O 4 /Ppy shows small equivalent series resistance of 0.81 Ω while rGO/MnFe 2 O 4 shows greater resistance of 0.86 Ω. This equivalent series resistance (ESR) was calculated from first x-intercept and slope of Nyquist plot 52 . However, in low frequency region rGO/MnFe 2 O 4 /Ppy has more slope as compared to slope of rGO/MnFe 2 O 4 explaining that former has better capacitance than later that is consistent with results of CV and CD. Nyquist plots for rGO/NiFe 2 O 4 and ternary rGO/NiFe 2 O 4 /Ppy nanocomposites are shown in Fig. S5. Figure 7 summarizes results of gravimetric and areal capacitance of all synthesized binary rGO/MeFe 2 O 4 and ternary rGO/MeFe 2 O 4 /Ppy nanocomposites. Table 1 shows comparison of the electrochemical performance of synthesized ternary rGO/MeFe 2 O 4 /Ppy electrodes with electrochemical performances of previously reported electrodes. Comparison of our results with previous literature findings shows that ternary rGO/MnFe 2 O 4 /Ppy electrodes give better electrochemical performance compared with previous reports. We propose that possible reason of this improvement is the incorporation of MnFe 2 O 4 particle as third component which improves electrochemical performance due to its redox behaviour. While gravimetric capacitance of rGO/NiFe 2 O 4 /Ppy is slightly less than that of G/Ppy showing that NiFe 2 O 4 contributes less gravimetric capacitance than MnFe 2 O 4 . Gravimetric capacitance of rGO/NiFe 2 O 4 /Ppy, rGO/MnFe 2 O 4 and rGO/MnFe 2 O 4 /Ppy is better than previously reported Ppy/GO/ZnO composite electrode which may be due to reasons that rGO is more conducting than GO and MeFe 2 O 4 is better candidate than ZnO to enhance electrochemical performance. CNT/Ppy/MnO 2 electrodes deliver more gravimetric capacitance than our synthesized electrodes because in synthesis of this composite hydrous MnO 2 was used to disperse it effectively in polymer matrix. Better gravimetric capacitance of previously reported PANI-Graphene-CNT electrodes as compared to our synthesized binary and ternary composites can be explained due to presence of all three highly conducting components, each of which may effectively participate in improving the electrochemical performance of resulting composite. Moreover, electrolyte used in that supercapacitor is different than used by us and their value was calculate at low current density i.e., 0.5 Ag −1 . Usually at low current density gravimetric capacitance is high which decreases accordingly with increasing current density. Areal capacitance of previously reported PEDOT-NiFe 2 O 4 electrode is almost equal to that of binary rGO/MnFe 2 O 4 electrode. However it is less than our synthesized rGO/MeFe 2 O 4 /Ppy electrodes. It is due to presence of conducting Ppy in rGO/NiFe 2 O 4 / Ppy. Areal capacitance of our binary rGO/MnFe 2 O 4 and ternary rGO/MeFe 2 O 4 /Ppy is better than previously reported PEDOT-GO/CNTs and Ppy-GO/CNTs which is due to more conductive rGO than GO used in above

Materials and Methods
Materials. Graphite  , Ethanol and acetone were used. All chemicals were pure and of analytical grade and were used without further purification.

One step synthesis of binary graphene/metal doped iron oxide nanoparticles (rGO/MeFe 2 o 4 ).
Graphene oxide (GO) was synthesized by a modified Hummer's method 53 . Binary Graphene/manganese ferrite (rGO/MnFe 2 O 4 ) was prepared by in-situ reduction coprecipitation method described previously in literature 54  /Ppy was analysed by field emission scanning electron microscopy (FESEM, Quanta 450, FEI, USA). Nanocomposites were further investigated by using X-ray diffraction (600 Miniflex, Rigaco, Japan). Fourier Transform Infrared (FTIR) spectroscopy (Nicolet 6700 Thermo Fisher) in transmittance mode and range 400-4000 cm −1 was used to identify the functional groups of synthesized nanocomposites. Thermal stability was measured by using a thermal gravimetric analyser (TGA, Q500, TA Instruments, USA) under air where the samples were heated up to 900 °C at a heating rate of 10 °C min −1 at RT. Electrochemical characterizations. All electrochemical measurements like cyclic voltammetry (CV), galvanostatic charge/discharge (CD) and electrochemical impedance spectroscopy (EIS) were carried out in 1 M H 2 SO 4 using two electrodes system. Gravimetric and areal capacitances of electrodes were calculated from cyclic voltammetry curves by using equations 3-4, respectively.
is the integrated area for CV curve, s is the scan rate, V is 2 × (Vmax-Vmin) the potential window, m is the mass of single electrode and a is the foot-print device area. Specific power and specific energy of electrodes were calculated from cyclic voltammetry by using equations 5-6, respectively.
Electrochemical performance of synthesized nanocomposites was characterized by using CHI760C Electrochemical workstation. Electrode material was fabricated by mixing active material i.e., nanocomposites, carbon black and binder (80:10:10) in 10 mL of ethanol. Polytetrafluoroethylene (60% wt in H 2 O) was used as binder. Mixture was ultrasonicated till completely dispersed, filtered by using filter paper and dried overnight at RT. Supercapacitor was made by using two electrodes of same material with active area of 1 cm 2 . Piece of filter paper dipped in 1 M H 2 SO 4 was used as separator between two electrodes. Gold electrodes were used as current collectors. Cyclic voltammetry (CV) testing was carried out between 0-1 V at scan rates from 5-2000 mVs −1 . Charging/discharging (CD) measurements were carried out in voltage window between 0-1 Vat 1-20 Ag −1 . EIS measurements were done from 0.01 Hz to 100 KHz at an open circuit potential with an AC voltage amplitude of 5 mV. All measurements were carried out at RT. ethical approval and informed consent. The methods used in this work were carried out in accordance with the relevant guidelines and and regulations.

Conclusions
In summary a simple, scalable and environmentally sustainable method for preparation ternary composite electrodes for supercapacitors applications consisting of rGO, mixed metal doped Iron oxide and conductive Ppy is presented. New method using NaOH with double roles to replace hydrazine for GO reduction and assist the formation of MeFe 2 O 4 is demonstrated for the first time. This process allows simple one step and scalable synthesis of graphene/metal doped iron oxide (rGO/MeFe 2 O 4 ) (Me = Mn, Ni) not possible before. Electrochemical characterizations showed binary rGO/MnFe 2 O 4 composite electrode delivers specific capacitance of 147 Fg −1 and areal capacitance of 232 mFcm −2 in two electrodes system at scan rate of 5 mVs −1 . Compared with previous results this is the highest value among all synthesized rGO/MeFe 2 O 4 electrodes reported in literature. Further significant improvement in performance was observed in ternary composite system after introduction of Ppy showing the capacitances of were increased to 250 Fg −1 and 395 mFcm −2 for rGO/MnFe 2 O 4 /Ppy electrode. Compared with previous results this is the highest value among all synthesized binary rGO/MeFe 2 O 4 and ternary rGO/MnFe 2 O 4 / Ppy electrodes reported in literature. These results confirmed that using this simple synthetic strategy it is possible to prepare synergistic composites electrode materials by combination of graphene metal doped iron oxide conductive polymers and promising strategy for designing and tailoring properties of high performing supercapacitors.