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High Performance Supercapacitors from Novel Metal Doped Ceria Decorated Aminated Graphene Rudra Kumar, Aman Agrawal, Rajaram K Nagarale, and Ashutosh Sharma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09062 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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The Journal of Physical Chemistry

High Performance Supercapacitors from Novel Metal Doped Ceria Decorated Aminated Graphene Rudra Kumar, Aman Agrawal, Rajaram K. Nagarale*†, Ashutosh Sharma* Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208016, U.P., India Author Information Corresponding author E-mail: [email protected] , [email protected] Telephone Number: +91 512 259 7026 †: Present Address: Central Salt and Marine Chemicals Research Institute, Gijubhai Badheka Marg, Bhavnagar364002, Gujarat, India. E-mail: [email protected] Telephone Number: +91 278 256 7760

Abstract Herewith we are reporting half-cell study of ceria electrode material made by uniform doping of silver and platinum nanoparticles for high performance supercapacitor applications. The hydrothermally synthesized electrode material had 5-7 nm size doped ceria particles decorated on graphene sheet. The electrodes were prepared by dip coating. The measured specific capacitance for neat Gr-CeO2 electrode was found to be 208 Fg-1 at 1 Ag-1 current density. Doping with silver and platinum, far increase in specific capacitance was observed. Silver doped gave 1017 Fg-1 specific capacitance at 1Ag-1 current density, a fivefold increase. While platinum doping gave ten tenfold increase in specific capacitance i.e. 1987 Fg-1 at 1 Ag-1 current density. The maximum energy and power density obtained was 41 kWkg-1 and 400 Whkg-1, respectively at 1 Ag-1 current density for Gr-CeO2-Pt electrode. The significant increase in specific capacitance was argued on the basis of enhancement in electrical

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conductivity and improvement in the electron transfer between electrodes and electrolyte. The stability check of the electrodes were performed by charging/discharging cycles at 5 Ag-1 current density in 6 M KOH solution. In a 1000 cycle study, exceptional high specific capacitance was found in the initial few charging/discharging cycles. The constant capacitance of about 600 Fg-1and 206 Fg-1 at 5 Ag-1 current density for Gr-CeO2-Pt and GrCeO2-Ag, respectively were found after 500 cycles, Suggesting ceria doped materials will be an excellent candidate for new generation supercapacitor applications. Keywords: Silver; Platinum; CeO2; Graphene; Nanocomposite; Supercapacitor Introduction Supercapacitors (SCs) are excellent means for rechargeable storage of energy, where they can meet the high energy and power requirements, with rapid charge and discharge rates and long life, for practical applications such as electric vehicles, laptops, cell phones, flashlights, and memory cards.1-4 However they have the disadvantage of a low energy density compared to rechargeable Li-ion batteries and fuel cells. A Supercapacitor can be classified as an electrochemical double layer capacitor and pseudo capacitor. The former stores charge by generating an electric double layer, while the latter does so by a Faradic redox reaction at the electrode/electrolyte interface.5-7 Electrode materials with a high active surface area and electrical conductivity can achieve an enhanced charge density and a better SC performance.8 However, most of the electrode materials used have low surface area9 and low electrical conductivity.10-12 Graphene, a widely used carbon material, has high electrical conductivity, high surface area (2630 m2 g−1), high electronic carrier mobility13, good electrochemical stability, and is easy to synthesize.14-16 Neat graphene has a specific capacitance of 200 F g−1. A large increase in the specific capacitance has been reported for graphene foam and graphene composites with


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metal oxides.17-22 Further Chen et al. showed the 5-fold increase in specific capacitance of graphene paper in K3Fe(CN)6 redox active electrolyte.23 Liu et al. reported the improvement of specific capacitance value of graphene up to 279 Fg-1 by tuning the C/O ratio.24 Rai et al. investigated the supercapacitor performance of electrodes composed of the aluminum-doped lanthanum ferrite. Doping with aluminum increases the specific capacitance from 200 to 260 F g−1.25 The electrochemical capacitance of polypyrrole–lanthanum strontium manganite oxide La0.8Sr0.2MnO3 (PPy-LSMO) nanocomposites was investigated by Chellalchamy et al., who measured a specific capacitance of 530 F g−1 for this material at the scan rate of 2 mV s−1.26 Swati et al. studied La2Te3 thin films as asymmetric supercapacitors, and reported a specific capacitance of 469 F g−1 at a 2 mV s−1 scan rate.19 Padmanathan et al. investigated NiO/CeO2 as an electrode material for supercapacitor applications, finding a specific capacitance of 305 F g−1.27 The utility of CeO2 nanoparticles with a fast electron transfer ability and a large specific surface area has been reported with a specific capacitance of 208 F g−1.28 Wang et al. reported a specific capacitance of 208 F g−1 at a current density of 1 A g−1 for three-dimensional graphene ceria composite electrodes. Similarly, Chen et al.29 reported the specific capacitance of 89 Fg-1 for 3D graphene aerogel cerium oxide composite. This value was remarkably low compared to those of novel and transition metal oxides.28 Maiti et al. reported a high pseudo capacitance in metal organic framework derived cerium oxide and K4Fe(CN)6-doped KOH electrolyte.30 The exceptionally high value (1204 F g−1 at 0.2 A g−1) was due to a reversible surface redox reaction between cerium oxide and the Fe(CN)64−/Fe(CN)63− redox couple. Ceria has been used in many applications aside from capacitors, such as chemical and biosensors31, 32, electrolytes for solid oxide fuel cell33, and solar cells34 owing to its optical and catalytic properties18, 35, photocatalytic activity36, high oxygen storage capacity37,

38

, coating39, and electronic transport properties.40 The redox



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behavior of the ceria has been studied in detail and it is used as a reference electrode in glucose sensors.41 Herewith, we demonstrate for the first time the effect of various metal/metal oxide-doped ceria materials for supercapacitor applications. The materials were synthesized by a hydrothermal method in a single step, and electrochemically characterized. Electrodes were prepared by dip coating, and the performance of the half-cell was evaluated in 6 M KOH solution.

Experimental Section 2.1 Materials Natural graphite flakes (45 µm) were purchased from Alfa Aeser. Sulfuric acid, hydrochloric acid, potassium permanganate, sodium nitrate, hydrogen peroxide (30% v/v), cerium nitrate hexahydrate, silver nitrate, liquor ammonia (25% w/v), isopropanol, and potassium hydroxide were purchased from Fisher Scientific. Nafion solution (5 wt.% in isopropanol) and chloroplatinic acid were received from Sigma Aldrich. Nickel foam was purchased from MTI Corporation. All chemicals and reagents were analytical grade and used as received. 2.2 Experimental Procedure 2.2.1 Synthesis of CeO2, RGO, Gr-CeO2, Gr-Ag, Gr-Pt, Gr-CeO2-Ag, and Gr-CeO2-Pt Composites: Graphitic oxide was synthesized as we have reported previously.42 Graphene decorated with ceria was synthesized as follows. In brief, 150 mg of graphene oxide was dispersed in 30 mL of deionized water by ultrasonication for 2 h. A 434 mg sample of cerium nitrate hexahydrate was added to the graphitic oxide suspension, and stirred vigorously. 1 ml of 25 wt % liquor ammonia was added to the resulting homogeneous solution, which was transferred to a 40 mL Teflon-lined hydrothermal reactor. The reactor was kept in an oil bath


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at 180°C for 30 min with constant stirring. After completion of the reaction, the reactor was cooled to room temperature, and the desired product was recovered by centrifugation, washed with distilled water and isopropanol several times, and vacuum dried overnight. Pure CeO2 and reduced graphene oxide (RGO) was synthesized by the same method without the addition of graphene oxide and cerium salt, respectively. Gr-Ag was also synthesized by same method by addition of silver nitrate in to GO solution without addition of cerium salt. Gr-Pt composite was synthesized hydrothermally in 1:1 volume ratio of ethylene glycol and water mixture with chloroplatinic acid and graphene oxide. For the synthesis of Gr-CeO2-Ag and Gr-CeO2-Pt composite, the same procedure was used with the addition of 10 wt.% silver nitrate and chloroplatinic acid, respectively, with respect to cerium nitrate hexahydrate.

Characterization 3.1 General Characterization The presence of ceria and doped material in Gr-CeO2, Gr-CeO2-Ag, and Gr-CeO2-Pt composites was confirmed by their crystalline structures analyzed by X-ray diffraction (XRD) (PAN Analytic Germany) using Cu Kα radiation (λ = 1.5406 Å) from 5° to 80° at a scanning speed of 2° min−1. The presence of functional groups in all composites was confirmed by FTIR (Tensor 27, Bruker, Germany) in the 400 - 4000 cm−1 frequency range. Raman spectroscopy (Model: Alpha, Make: WITec, Germany) was performed in the 400– 3000 cm−1 frequency range with a 514 nm laser source to confirm the graphitic and amorphous nature of carbon. It also provides valuable information on the characteristic stretching vibrations of cerium-oxygen and novel metal/metal oxygen bonds. Morphological



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characterizations were performed by field emission scanning electron microscopy (FESEM, ZEISS Supra 40VP, Germany) operated at 10 kV, and transmission electron microscopy (TEM) (TECNAI-G2) operated at 200 kV. The presence of doped metal/metal oxide in the composite electrodes was confirmed by energy dispersive X-ray spectroscopy (EDX) with elemental mapping measured by Oxford elemental system combined with FESEM. The selected-area electron diffraction (SAED) patterns of Ag, Pt, and CeO2 nanoparticles embedded on the graphene sheets were analyzed by TEM. The thermal stability of the developed materials was analyzed by thermogravimetric analysis (TGA, TA Instruments 2960) at a heating rate of 10°C min−1 from room temperature to 800°C in a nitrogen environment.

Nitrogen

adsorption/desorption

experiments

were

performed

by

Brunauer−Emmett−Teller (BET) analysis using a surface area analyzer (Quantachrome). Xray photoelectron spectroscopy (XPS) of all samples was performed (PHI 5000 Versa Probe II, ULVAC-PHI, Inc.) using a scanning XPS microprobe in the 0 -1000 eV range. 3.2 Electrode Preparation All electrodes were fabricated on the nickel foam substrate. The nickel foam was first cleaned to remove the oxide layer by ultrasonication in aqueous 1 M HCl solution for 5 min, followed by washing with methanol and water. The washed foam was air dried and stored in a desiccator. Coating of electroactive material was performed by dip coating in carbon paste prepared by homogeneous dispersion of 200 mg of aminated graphene decorated with metal/metal oxide-doped ceria in 20 mL of isopropanol and 1 mL of Nafion solution and dried in an oven at 100 °C for 1 h. The dried electrodes were used for all experiments in this study. 3.3 Electrochemical Measurement


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The electrochemical tests were performed with a potentiostat/galvanostat (Autolab, 302N Metrohm). A three-electrode system, with the electroactive material-coated nickel foam as the working electrode, platinum wire as the counter electrode, and Ag/AgCl (3 M KCl) as the reference electrode, was used for cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) analyses. A 6 M KOH aqueous solution was used as the electrolyte. The specific capacitance, C (F g−1), was calculated from the galvanostatic charge–discharge curve by using the relation: C = I ∆t / m ∆V

(1)

where I is the current applied (A), ∆t is the discharge time (s), m is the mass of active material deposited (g), and ∆V is the potential window (V). Energy density, E (W h kg−1), and power density, P (W kg−1), were calculated from equations (2) and (3), respectively: E = ½ C V2

(2)

P = E / ∆t

(3)

Results and Discussion

Figure 1: Schematic of the synthesis of novel metal/metal oxide doped ceria decorated aminated graphene 4.1 General Characterization



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Metal/metal oxide-doped ceria-decorated graphene was synthesized hydrothermally at 180°C in a single step, as shown in Figure 1. It is an alkaline reduction of graphitic oxide in the presence of ammonia with simultaneous formation of graphene and metal/metal oxide-doped ceria from their corresponding hydroxides. The formation of ceria and different functional groups on graphene was analyzed by FTIR. Figure 2(a) Shows the FTIR spectra of GO, GrCeO2, Gr-CeO2-Ag, and Gr-CeO2-Pt composites. The peaks observed at 3432 cm−1 and 1398 cm−1 were assigned to O–H bending modes, while the C=O and C–OH stretching modes of the –COOH group were observed at 1735 cm−1 and 1040 cm−1 respectively. An epoxy absorption band was observed at 1232 cm−1. The presence of a vibrational band at 1625 cm−1 was assigned to the bending mode of water molecules adsorbed on GO sheets.42 The CeO2GO material produced a characteristic peak at 462 cm−1 corresponding to the Ce–O vibration.43 In the case of Gr-CeO2-Ag and Gr-CeO2-Pt composites, a decrease in the –OH stretch absorption intensity of the oxygen-containing functional groups was observed.44, 45 Further support for the formation of GO, Gr-CeO2, and Ag and Pt-doped Gr-CeO2 was provided by UV-Visible spectroscopy. Figure S1 shows the UV-Visible spectrum in the 200–800 nm wavelength range for all samples examined. Absorption peaks for GO were observed at 232 nm, corresponding to the π–π* transition of C=C, and 300 nm, for the n–π* transition of C=O.42 The peak at 3300-3500 cm-1 corresponds to the stretching mode of N-H bonds present in amine group, while the peak designates at 1580 cm-1 is in plane N-H stretching. The red shift in the spectrum of Gr-CeO2 confirms the reduction of GO with simultaneous formation of CeO2.46 The presence of absorption at 320 nm confirms the formation of CeO2. The absorption at 258 nm corresponds to platinum complex formation, while the peak at 270 nm corresponds to the reduction of the graphene sheet.47 Similarly, the peak appearing at 437 nm confirms the presence of silver nanoparticle decorations on the graphene sheet.48


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The doping of the ceria with metal/metal oxide was confirmed with wide angle XRD. Figure 2(b) shows the X-ray diffraction patterns of the GO, RGO, Gr-CeO2, Gr-CeO2-Ag, and GrCeO2-Pt composites. The GO peak was observed at an angle of 11.20° for the (001) plane with a d-spacing of 0.83 nm. This is due to the formation of the oxygen containing functional group on the surface of the graphene sheets. After the reduction of GO to RGO, the peak was observed at 25.6° for the (002) plane with d spacing of 0.34 nm. The broad peak observed in the RGO pattern indicates that the graphene sheets are loosely stacked, behavior that is different to that of pure graphite. Furthermore, diffraction peaks observed at 2θ angles of 28.7, 33.2, 47.7, and 56.6° may be assigned to the (111), (200), (220), and (311) planes, corresponding to the formation of the face-centered CeO2 crystal structure (JCPDS 340394).49 Similarly, the peaks observed at 2θ angles of 38.1, 44.3, 64.4, and 77.4° correspond to the (111), (200), (220), and (311) planes of silver.(ref) Moreover, the peaks found at angles of 39.9, 46.4, and 67.7° correspond to the (111), (200), and (220) planes, confirming the formation of platinum nanoparticles in the Gr-CeO2-Pt composites (JCPDS 04-0802).50



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Figure 2: Spectroscopic characterizations of GO, Gr-CeO2, Gr-CeO2-Ag and Gr-CeO2-Pt (a) FTIR spectra recorded in KBr (b) wide angle powder XRD was carried out at 2°/min scan rate with 1.54 Å wavelength of Cu-Kα (c) Raman spectrum recorded in the frequency range of 400-3000 cm-1 with 514 nm laser source and (d) TGA analysis was performed in nitrogen environment with a heating rate of 10 °C/min.

Raman spectroscopy is a standard technique for determining the ordered and disordered nature of carbon based materials. Figure 2(c) shows the Raman spectra of GO, Gr-CeO2, GrCeO2-Ag, and Gr-CeO2-Pt. Peaks were observed at 1352 cm−1 and 1590 cm−1 in the GO spectrum, which confirm the D and G bands, respectively. The D band describes the disordered nature of graphitic carbon and the vibrational mode of the k-point phonons of A1g symmetry51, while the G band corresponds to the in-plane bond stretching of sp2 hybridized carbon structure, and indicates the graphitic nature of carbon (E2g phonon vibration).52 However, the ratio of the peak intensities of the D and G bands (Id/Ig) describes the disordered nature and structural defects of graphitic carbon. The Id/Ig ratios of the Gr-CeO2,


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Gr-CeO2-Ag, and Gr-CeO2-Pt composites are 1.15, 1.17, and 1.2, respectively. The increase in Id/Ig ratio in the composites confirms that CeO2, Ag, and Pt nanoparticles are embedded into the graphene sheets, creating more disorder and defects. Furthermore, there was small shift in the G band of the Gr-CeO2-Ag and Gr-CeO2-Pt composites, which confirms the reduction of graphene.47, 53 The peak observed at ~463 cm−1 is related to the vibrational mode of Ce–O8 in CeO2/GO composites.49 However, the small peaks at 260, 450, and 550 cm−1 correspond to silver and CeO2 nanoparticles, and the 550 and 695 cm−1 peaks to platinum complex formation.47 The quantitative estimation of the compositions of CeO2, CeO2-Ag, and CeO2-Pt in the composites was performed by TGA analysis. Figure 2 (d) shows the TGA curves for GO, Gr-CeO2, Gr-CeO2-Ag, and Gr-CeO2-Pt composites. From the figure, the maximum weight loss was observed for GO (68%), followed by Gr-CeO2 (27%), Gr-CeO2-Ag (25%), and GrCeO2-Pt (21%). From the weight differences, the calculated abundance of ceria is 42%, and of silver is 2%, which represents a silver doping of 4.8 wt.% with respect to ceria.54,

55

Similarly, the total amount of platinum present was 5 wt.%, corresponding to an 11.9 wt.% doping with respect to ceria.



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Figure3: High Resolution XPS spectra of (a) Ce 3d (b) Ag 3d and (c) Pt 4f Figure 3 shows the X-ray photoelectron spectra for GO, Gr-CeO2, Gr-CeO2-Ag, and GrCeO2-Pt in the 0–1000eV range for confirmation of the presence of novel metals in their native and/or oxide forms. The full survey scans of Gr-CeO2-Ag and Gr-CeO2-Pt composites are shown in Figure S2 (a-b). The deconvoluted spectra (Figure S2(c-e)) show peaks at 286 400 eV and 532 eV corresponding to C 1s, N 1s and O 1s electrons in GO.56 The high resolution deconvoluted XPS spectrum (Figure 3a) of Gr-CeO2 shows peaks at 882.5, 888.6, 898.5, and 900.8 eV corresponding to Ce 3d5/2 transitions. The peaks at 907.7 and 916.7 eV are assigned as Ce 3d3/2 of CeO240, whereas peaks at 885.2 and 905.4 eV were assigned to the 3d electrons of the Ce3+ oxidation state responsible for oxygen vacancies in CeO2 crystal. The deconvoluted XPS spectrum of Gr-CeO2-Ag (Figure 3b) shows peaks at 368.2 and 374.2 eV, corresponding to Ag 3d5/2 and Ag 3d3/2, respectively, for native silver; i.e. Ag0.57 Similarly,


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peaks are evident in the deconvoluted spectrum of the Pt 4f region of Gr-CeO2-Pt composites (Figure 3c). Peaks observed at 71.2 eV and 74.6 eV correspond to Pt0, whereas those at 72.8, 76.1, and 75.0 eV are assigned to the Pt2+ oxidation state, and the 78.2 eV peak to the Pt4+ oxidation state.58 This suggests that platinum doped into ceria adopts both the native and the oxide form. The surface areas of the Gr-CeO2, Gr-CeO2-Ag, and Gr-CeO2-Pt composites were measured by a multi-point BET surface area analyzer. Figure S3 shows the nitrogen adsorption/desorption isotherms. Surface areas of 46, 80, and 245 m2 g−1 and total pore volumes of 0.081, 0.15, and 0.16 cm3 g−1 were found for the Gr-CeO2, Gr-CeO2-Ag, and GrCeO2-Pt composites, respectively. The observed increases in surface area and pore volume clearly indicate the stumpy agglomeration of ceria particles with doping. The change in morphology also enabled better intercalation of ceria particles between graphene sheets. The surface morphology and microstructure of the as-synthesized GO, Gr-CeO2, Gr-CeO2Ag, and Gr-CeO2-Pt composites were analyzed by FESEM. Figures 4(a) and (b) show the SEM images of GO and Gr-CeO2 composite. Crumpled sheets were observed for GO (Figure 4(a)), whereas crumpled sheets uniformly seeded with 5–7-nm CeO2 nanoparticles were observed for Gr-CeO2 (Figure 4(b)). A similar particle size was distribution was observed for the platinum-doped material (Figure 4(d)). However, different behavior was observed in the silver-doped material, where aggregation of particles to a size of ~100 nm was observed (Figure 4(c)). The surface morphology of CeO2, Gr-Ag and Gr-Pt were shown in Figure S4 (a-c). Silver nanoparticles were uniformly distributed on the surface of graphene sheet. Platinum nanoparticles were not clearly visible in SEM image due to very small size. But EDX spectra confirms its formation. Figure S5(a-c)) shows the EDX spectra for CeO2, GrAg and Gr-Pt composites. EDX with elemental mapping for Gr-CeO2-Ag and Gr-CeO2-Pt materials (Figure S6 and S7) confirms the uniform distribution of carbon, oxygen, cerium,



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silver, and platinum, suggesting that the composite is homogeneous in nature with nanometer sized separation between the ceria phase and silver and/or platinum/platinum oxide.

Figure 4: (a) SEM image of GO, (b) Gr- CeO2, (c) Gr-CeO2-Ag, (d) Gr-CeO2-Pt composite

Figure 5 shows the TEM images of as-synthesized GO, Gr-CeO2, Gr-CeO2-Ag, and GrCeO2-Pt composites. From these images, the even distribution of CeO2 nanoparticles across surface of the graphene sheets is clearly visible (Figure 5(b)), suggesting that the hydrothermal method is a simple and efficient way of synthesizing 5–7-nm ceria nanoparticles. Figure 5(a) shows the TEM image of pure graphene oxide. The aggregation of silver was clearly visible for Gr-CeO2-Ag composites (Figure 5(c)), however, no change in particle size was observed for Gr-CeO2-Pt (Figure 5(d)). The insets in Figure 5(b-d) show the selected area electron diffraction (SAED) of the Gr-CeO2, Gr-CeO2-Ag, and Gr-CeO2-Pt composites, respectively. The observed bright rings suggest that the nanoparticles have a polycrystalline character.46 Figure S8 shows the TEM image of CeO2, Gr-Ag and Gr-Pt


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composites. 5-10 nm size of CeO2 nanoparticles is clearly visible in Figure S8(a). TEM image of silver and platinum decorated graphene sheets are shown in Figure S8(b) and S8(c), respectively. The decoration of 2-3 nm platinum nanoparticles on graphene sheet is shown in Figure S8(d).

Figure 5: TEM image of Graphene, Gr-CeO2, Gr-CeO2-Ag and Gr-CeO2-Pt composite. Inset

in figure 5(b-d) shows the corresponding SAED pattern.

4.2 Electrochemical Characterization



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4.2.1 Cyclic Voltammetry The electrochemical performance of CeO2, RGO, Gr-Ag, Gr-Pt, Gr-CeO2, Gr-CeO2-Ag, and Gr-CeO2-Pt was analyzed by CV in a 6 M KOH electrolyte. All CV curves were measured in the potential window of −0.2–0.45 V (Ag/AgCl). Figures 6(a-c) show the cyclic voltammograms of Gr-CeO2, Gr-CeO2-Ag, and Gr-CeO2-Pt composite electrodes at scan rates of 5 to 100 mV s−1. The cyclic voltammograms of neat CeO2. RGO, Gr-Ag and Gr-Pt composites are shown in Figure S9 (a-d). The observed cyclic voltammograms were symmetrical with anodic and cathodic peak potentials respectively at 0.360 and 0.190 V for Gr-CeO2, 0.281 and 0.167 V for Gr-CeO2-Ag, and 0.360 and 0.190 V for Gr-CeO2-Pt electrodes. A difference of about 15 ± 2 mV between the anodic and cathodic peak potentials was observed at slow scan rate. This difference increased with increasing scan rate due to the resistance of the electrode.59 In comparison to neat nickel foam (Figure 6d; inset), the anodic and cathodic peak potentials are shifted by roughly 16, 65, and 16 mV for Gr-CeO2, GrCeO2-Ag, and Gr-CeO2-Pt, respectively. The neat ceria nanoparticles have a standard oxidation/reduction potential (E°) of 50 mV vs Ag/AgCl, while E° for Ag/Ag2O is 120 mV (alkaline electrolyte) vs Ag/AgCl. However, an exceptionally high increase in the anodic and cathodic peak currents was observed for electroactive ceria composite material after coating on nickel foam. The increases of 3.5, 20, and 23 mA for Gr-CeO2, Gr-CeO2-Ag and GrCeO2-Pt, respectively, suggest amplification of current by ceria nanocomposites. The plot of the square root of scan rate against anodic and cathodic peak current (Figure 6(a-c); inset) gives a straight line, indicating that the observed electrochemical behavior is a surface confined phenomena. The area swept in CV curve for neat Ni foam is very low in comparison to that of electrodes made by coating with electroactive ceria nanocomposite materials. It confirms the negligible contribution to the specific capacitance by the nickel foam. In the present study, the highest specific capacitance of about 1980 F g−1 was observed for the Gr-


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CeO2-Pt composites. This is due to the increase in electrical conductivity of these composites. Incorporation of platinum also prevents the aggregation of ceria particles and graphene sheets, which increases the total surface area, and the corresponding specific capacitance.60

Figure 6: Cyclic Voltammetry of (a) Gr-CeO2 (b) Gr-CeO2-Ag (c) Gr-CeO2-Pt composite (d) comparison of CV curve at the scan rate of 50mV/s. Inset shows the anodic and cathodic current vs. square root of scan rate for corresponding electrodes. Ag/AgCl as reference and Pt wire as counter electrode was used.

4.2.2 Galvanostatic Analysis Figures 7(a-c) show the galvanostatic charge/discharge (GCD) curves of the Gr-CeO2, GrCeO2-Ag, and Gr-CeO2-Pt composite electrodes in the 0–0.4 V potential window. Whereas



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Figure S10 (a-d) shows the GCD curve of pure CeO2, RGO, Gr-Ag and Gr-Pt composites. The specific capacitance variation with respect to current density is shown in Figure S11 (ad). The value of specific capacitance calculated by the galvanostatic charge-discharge (GCD) method for neat CeO2, at current densities of 0.5, 1, 2 and 5 A g−1 was found to be 95, 87, 78 and 55 Fg-1, respectively, whereas for RGO was 158, 137, 105 and 57 Fg-1 respectively. The specific capacitance value obtained for RGO is higher than colloidal graphene (56 Fg-1) synthesized by electrochemical exfoliation of graphite61 and slightly less than the folded structure graphene paper (172 Fg-1 at 1 Ag-1 current density).62The composite of graphene oxide and silver, the specific capacitance increased to 418, 348, 275 and 212 Fg-1, at 0.5, 1, 2 and 5 Ag-1, respectively. With addition of platinum to graphitic oxide, the observed specific capacitance was 986, 955, 830 and 512 Fg-1 at the current density of 0.5, 1, 2 and 5 Ag-1, respectively. The values of specific capacitance for Gr-CeO2 at current densities of 0.5, 1, and 2 A g−1 are 216, 208, and 110 F g−1, respectively. These values are consistent with values reported in the literature.28 With doping of Ag and Pt, a substantial increase in capacitance was observed. The values calculated for the Ag-doped composite are 1012, 964, 844, and 562 F g−1 at 0.5, 1, 2, and 5 A g−1, respectively, For the Gr-CeO2-Pt composite, specific capacitance values of 1980, 1978, 1740, and 1462 F g−1 at 0.5, 1, 2, and 5 A g−1, respectively, are calculated. This specific capacitance obtained for Gr-CeO2-Pt is higher than Gr-Pt composite and also reported values for Pt/carbon63, Ag/Gr/MnO264, Ag/CuO65 and MnO2/Pt/NPG66 composites. The large increase in specific capacitance caused by doping with Ag and Pt is due to the increase in the electrical conductivity and surface area. There is also a major contribution from the decrease in charge transfer resistance between the electrode surface and electrolyte molecules.60, 65, 67 Figure 7(d) shows the comparative study of charge/discharge behavior for each of the composite electrodes at a current density of 2 A g−1. The discharge time for Gr-CeO2-Pt is longer than those of the Gr-CeO2-Ag and Gr-CeO2


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composite electrodes. This is attributed to the higher specific capacitance of the Gr-CeO2-Pt electrode.

Figure 7: Galvanostatic charge-discharge (GCD) of (a) Gr-CeO2 (b) Gr-CeO2-Ag (c) GrCeO2-Pt composite (d) comparison of GCD curve at the 2A/g current density

Figure 8(a) shows specific capacitance plotted against current density for each composite electrode. The specific capacitance decreases with increasing current density owing to reduced penetration of electrolyte into the pores of electrode materials. The stability of the electrodes was studied by considering charge/discharge cycles at a current density of 5 A g−1, as shown in Figure 8(b). The graph is plotted as specific capacitance against the number of cycles for the Gr-CeO2-Ag and Gr-CeO2-Pt composite electrodes. In the first 100 cycles, a steep decrease in capacitance can be observed for both the materials. However, for the next



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900 cycles, the capacitance decreases at a slow rate as compared to the initial 100 cycles, with the initial value of 562 F g−1 for the Gr-CeO2-Ag electrode falling to 150 F g−1, and a decrease from 1462 to 537 F g−1 for Gr-CeO2-Pt electrode. This decay in specific capacitance during cycling was mainly due to the mechanical expansion/contraction of electrode material by insertion/exertion of electrolyte ions. This expansion/contraction also leads to aggregation of nanoparticle. This in turn, leads to a decrease in the redox activity of the ceria redox couple, which mainly resides at the interface between the nanoparticle and the electrolyte. The process also leads to dissolution of loosely attached electroactive material into the electrolyte, and hence adds to the decrease in capacitance.68, 69 After the initial decay, the specific capacitance remains constant between 400 and 1000 cycles, suggesting a good longterm stability of the materials.70, 71


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Figure 8: (a) Specific Capacitance as a function of current density (b) Cycling performance of Gr-CeO2-Ag and Gr-CeO2-Pt composites at 5A/g current density. (c) EIS of Gr-CeO2, GrCeO2-Ag and Gr-CeO2-pt composites (d) Energy density Vs Power density

4.2.3 Electrochemical Impedance Spectroscopy EIS was performed over the 0.01 Hz–100 kHz frequency range with an amplitude of 10mV. Figure 8(c) shows the Nyquist plots for each electrode. The series resistance, Rs, of Gr-CeO2, Gr-CeO2-Ag, and Gr-CeO2-Pt is 0.89, 0.67, and 0.56 ohms, respectively. The higher Rs value for the Gr-CeO2 based composite electrodes gives lesser conductivity. Figure 8 (d) shows energy density vs. power density plots for each composite electrode. The energy density of the Gr-CeO2-Pt composite electrode was 47.8, 44, 38.6, and 32.5 kWh kg−1 at current densities of 0.5, 1, 2, and 5 A g−1, respectively, while the energy density values of was 22.5, 21.4, 18.7, and 12.5 kWh kg−1 were found for the Gr-CeO2-Ag composite electrode at 0.5, 1, 2, and 5 A g−1, respectively. These results demonstrate that the high specific capacitance and energy density of the Gr-CeO2-Pt and Gr-CeO2-Ag composites promise better electrode performance for practical applications. The two-dimensional mesoporous character of the graphene sheet decorated with CeO2 and novel metal composites provide a high surface area nanostructure. These composite electrode materials deliver efficient ion and electron transport, giving rise to faster kinetics and resulting in high charge/discharge capacities even at high current densities. Conclusion We have shown that graphene composites the silver and platinum nanoparticles decorated CeO2 were synthesized by the hydrothermal method. Silver or platinum nanoparticles, along with CeO2 nanoparticles, are uniformly deposited on the graphene sheets. The intercalation of nanoparticles between the graphene sheets not only prevents restacking, but also improves



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the electrical conductivity and surface area of the composite. In analysis of the electrochemical performance of Gr-CeO2, Gr-CeO2-Ag, and Gr-CeO2-Pt composites, specific capacitance values of 208, 1017, and 1987 F g−1, respectively, were measured at a current density of 1 A g−1. After 1000 charge/discharge cycles, the Gr-CeO2-Pt and Gr-CeO2-Ag composites maintain a specific capacitance of 540 F g−1and 150 F g−1, respectively, at a current density of 5 A g−1. Energy densities of 44 and 21.4 kWh kg−1 were obtained at 1 A g−1 with a power density of 200 W kg−1 for the Gr-CeO2-Pt and Gr-CeO2-Ag composite electrodes respectively. The results clearly demonstrate that novel metal doped ceria decorated aminated graphene is a promising material for high performance supercapacitor applications. Acknowledgment This research was supported by the Department of Science &Technology (DST), Government of India and to the Centre of Nano sciences at IIT Kanpur. RKN Thanks the Department of Science & Technology (DST), Government of India, for Ramanujan Fellowship (SR/S2/RJN18/2011) award and financial support, (grant Number SR/S3/CE/034/2013). Supporting Information UV-Vis spectra, EDX with elemental mapping, BET characterization of the Gr-CeO2-Ag and Gr-CeO2-Pt composite, CV, GCD, and specific capacitance values of RGO, CeO2, Gr-Ag and Gr-Pt composites. This material is available free of charge via the Internet. References 1. Armand, M.; Tarascon, J. M., Building Better Batteries. Nature 2008, 451, 652-657. 2. Wang, G.; Zhang, L.; Zhang, J., A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. 3. Miller, J. R.; Simon, P., Electrochemical Capacitors for Energy Management. Science 2008, 321, 651-652.


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