Ag–Ni Nanoparticle Anchored Reduced Graphene Oxide

Jun 27, 2019 - ... as Advanced Electrode Material for Supercapacitor Application .... and (Ag0.50Ni0.50)80RGO20 nanocompsoites; Figure S9: Bode plot ...
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Article Cite This: ACS Appl. Electron. Mater. 2019, 1, 1215−1224

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Ag−Ni Nanoparticle Anchored Reduced Graphene Oxide Nanocomposite as Advanced Electrode Material for Supercapacitor Application Madhurya Chandel, Priyanka Makkar, and Narendra Nath Ghosh* Nano-materials Lab, Department of Chemistry, Birla Institute of Technology and Science, Pilani K K Birla Goa Campus, Goa 403726, India Downloaded via BUFFALO STATE on July 28, 2019 at 13:47:32 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: We have reported the electrochemical properties of the nanocomposites that were synthesized by immobilizing nanoparticles of Ag and Ni on the surface of reduced graphene oxide (RGO). The hierarchical structure derived from the combination of nanoparticles of Ni, Ag, and RGO in the (AgxNi(1−x))yRGO(100−y) nanocomposite provides a synergistic effect in the enhancement of supercapacitive performance of the synthesized nanocomposite. (Ag0.50Ni0.50)90RGO10 showed a high specific capacitance value of 897 F g−1 at a current density 1 A g−1 and an energy density of 80 Wh kg−1 at 400 W kg−1 power density. This nanocomposite also exhibited ∼77% retention of the initial capacitance even after 4000 cycles. To the best of our knowledge, the electrochemical properties of the nanocomposite composed of Ni and Ag nanoparticles and RGO are reported for the first time, and the nanocomposite (Ag0.50Ni0.50)90RGO10 demonstrated its excellent supercapacitive performance. KEYWORDS: Ag−Ni reduced graphene oxide nanocomposite, supercapacitor, specific capacitance, capacity retention, cyclic stability, energy density

1. INTRODUCTION Development of highly efficient and sustainable energy storage systems has gained great importance to meet the everincreasing demands of energy in the modern world.1−3 In this context supercapacitors have attracted immense attention due to their outstanding energy storage properties, such as high power density, high energy density, long cycle life, and so on.2−6 Though compared to batteries, supercapacitors possess lower energy density but they provide higher power density (e.g., ∼10 kW kg−1) than that of a battery.4,7,8 In addition to this, supercapacitors exhibit faster charging/discharging process.4,5 Therefore, supercapacitors are showing their competence to become a potential candidate for a plethora of applications, such as portable electronics, memory backup for various consumer products (e.g., laptop, mobile phones, solid-state devices, etc.), hybridized system with batteries having a prolonged lifetime, hybrid vehicles, industrial equipment that requires a large amount of power, and so on.8,9 Supercapacitors possess a higher specific capacitance and energy density than common dielectric capacitors (for example, the values of energy density of supercapacitors and dielectric capacitors are ∼10 and ∼3 × 10−2 Wh kg−1, respectively).4,8,9 Based on the energy storage mechanism, supercapacitors are largely classified into two categories: pseudocapacitors and electric double-layer capacitors (EDLC).1,8,9 In the pseudocapacitors energy storage occurs through fast reversible faradaic redox reactions.6,7 In the EDLC © 2019 American Chemical Society

the energy storage occurs via a non-faradaic process, and the charge accumulation occurs at the interface between the electrolyte and electrode.1,3,9 Several metal oxide nanoparticles, (such as MFe2O4, Fe3O4, RuO2, MnO2, etc.) have been investigated extensively to construct pseudocapacitors.10−14 However, their supercapacitive performance often suffers from their poor conductivity.2,15−20 For example, MnO2 exhibits a specific capacitance value of ∼150 F g−1 within a potential window of 10 wt %), the RGO sheets have a greater tendency to agglomerate and hence showed a lower capacitance value.3,55 We have inspected the cycling performance of this electrode to determine its stability. The cycling performance of the (Ag0.50Ni0.50)90RGO10 electrode has been measured at a constant current density of 6 A g−1 for more than 4000 cycles (Figure 6). After the first few cycles, the equilibrium was

Figure 7. Electrochemical impedance spectra of Ag0.50Ni0.50, (Ag0.50Ni0.50)90RGO10, and RGO electrodes. The inset shows the high-frequency region of the impedance spectra and the equivalent circuit which was used for fitting the Nyquist plots. A 3 M KOH solution was used as electrolyte.

S7. The fitting results of the circuit of synthesized materials are listed in Table S2. These results revealed that when Ag and Ni nanoparticles were anchored on the RGO surface, the Rs value of the nanocomposites was decreased. For example, the Rs value (Ag0.50Ni0.50)90RGO10 was 0.639 Ω, whereas that of pure RGO was 2.15 Ω. This indicated that the internal resistance of the nanocomposites was less than that of pure RGO. (Ag0.50Ni0.50)90RGO10 also exhibited the lowest value of Rct (0.868 Ω), and this value was far less than that of pure RGO (2.86 Ω). This could be due to the following reasons: (i) the nanosize of Ag and Ni nanoparticles provides a shorter electron path length, and (ii) the well-dispersed Ag and Ni nanoparticles are in direct contact with the highly conducting RGO, which helps in the charge transfer process by minimizing the interfacial resistance.7,58,59 However, it was also observed that a further increase (beyond 10 wt %) of RGO content in the nanocomposites caused their Rct and Rs values to increase. The agglomeration of RGO sheets in the nanocomposites having higher RGO content might have caused their internal resistance to increase and limited the charge transfer process.37,59 Bode plots were also generated to understand their capacitive behavior and to determine the “knee frequency”.1,56,58 In the Bode plot (|Z| vs frequency), the electrode material having relatively lower |Z| value at the lower frequency suggests its higher conductivity.32 From Figure 8 and Figure S8, it was observed that the value of the real component of impedance (|Z|) of Ag0.50Ni0.50 was 105 Ω at the lower frequency region, and with increasing RGO wt % in the (Ag0.50Ni0.50)xRGO100−x nanocomposite the |Z| was decreased. This result also indicated that the dispersion of Ag and Ni nanoparticles on the surface of highly conducting RGO sheets caused to enhance the conductivity of the nanocomposites.43 Recently, we have reported the electronic structure and properties of Ag−Ni−graphene and the interfacial interactions between Ag, Ni, and graphene which were obtained from the density functional theory (DFT) study.45 This study clearly indicated the existence of hybridization between Ag 4d, Ag 4s, Ni 3d, and Ni 4s states in the interface between Ni and Ag and also the hybridization between the C 2p state of graphene and

Figure 6. Cyclic stability of the (Ag0.50Ni0.50)90RGO10 electrode showing the capacitance retention up to 4000 cycles. The inset shows the charge−discharge curves at a constant current density of 6 A g−1.

reached, and then a steady capacitance was observed for the subsequent cycles. The (Ag0.50Ni0.50)90RGO10 electrode exhibited specific capacitance of 607 F g−1 at 6 A g−1. The (Ag0.50Ni0.50)90RGO10 electrode demonstrated its high cycling stability by exhibiting ∼77% retention of its specific capacitance after 4000 cycles. The electrochemical impedance spectroscopic (EIS) measurements of the synthesized nanocomposites were performed to determine their charge transfer resistance (Rct) and internal resistance (Rs) and to understand their fundamental behavior for supercapacitor applications. The data which were obtained from EIS measurements were used to generate the Nyquist plot for the nanocomposites, and an equivalent circuit was constructed to fit the impedance data.32,56,57 From the Nyquist plot, the internal resistance (Rs) of the cell was estimated from the intercept of the real axis (Z′) in the high-frequency range. Rs signifies the combined effect of (i) intrinsic resistance of the electrode materials, (ii) the contact resistance between the electrode material and the current collector, and (iii) the ionic resistance of the electrolyte.7,58 The charge transfer resistance (Rct), which originates from the faradaic pseudocapacitance, was determined from the diameter of the semicircle observed at the intermediate frequency range.58,59 As a representative, the Nyquist plots of RGO, Ag0.50Ni0.50, and 1219

DOI: 10.1021/acsaelm.9b00194 ACS Appl. Electron. Mater. 2019, 1, 1215−1224

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ACS Applied Electronic Materials

resistive impedance are equal. It is well-known that the high value of knee frequency suggests the higher rate capability property of the electrode.1,56,61,62 The value of f 0 was increased from 47.1 Hz (for Ag 0.50 Ni 0.50 ) to 103.1 Hz (for (Ag0.50Ni0.50)90RGO10) and indicated the high rate capability of these nanocomposites. Moreover, the relaxation time (τ0 = f 0−1)1,56 for pure RGO was found to be much higher (0.56 s) than that of (Ag0.50Ni0.50)90RGO10 (9.69 × 10−3 s). The immobilization of Ag and Ni particles on RGO sheets in the nanocomposite creates shorter ionic transport paths and leads to a decrease in its internal resistance and an increase in electrical conductivity compared to those of pure RGO.32,56 However, the relaxation time (τ0) for the nanocomposites having more than 10 wt % RGO was found to be greater than that of (Ag0.50Ni0.50)90RGO10, indicating the higher internal resistance of (Ag0.50Ni0.50)85RGO15. This could be due to the agglomeration of RGO which resulted in higher internal resistance. From the GCD measurements, we have also observed that (Ag0.50Ni0.50)90RGO10 exhibited the highest capacitance value of 897 F g−1 whereas (Ag0.50Ni0.50)80RGO20 possessed a lower capacitance value of 540 F g−1 at 1 A g−1. The energy density (E) and power density (P) are two important indicators of the performance of the supercapacitor.2,55 We have determined the energy density and power density of the synthesized nanocomposites from GCD plots. The Ragone plots (energy density vs power density) for Ag0.50Ni0.50 and (Ag0.50Ni0.50)90RGO10 electrodes are presented in Figure 10. It was observed that Ag0.50Ni0.50 showed an

Figure 8. Bode plot (|Z| vs frequency) of the Ag0.50Ni0.50, RGO, and (Ag0.50Ni0.50)90RGO10, nanocomposites.

Ag 4d and Ni 3d at the conduction and valence band in the Ag−Ni−graphene interface. Moreover, the binding energy between graphene and Ag−Ni interface was found to be −3.32 eV, which revealed the existence of strong interfacial interaction between them. Therefore, it can be predicted that these interfacial interactions and orbital level hybridizations will help in enhancing the conductivity of Ag−Ni−RGO nanocomposites. The results obtained from EIS measurements showed the better conductivity of (Ag0.50Ni0.50)90RGO10 than that of Ag0.50Ni0.50 nanoparticles and pure RGO. This fact can be explained in light of the DFT calculation which shows the existence of orbital level hybridizations in the interface between graphene and Ni and Ag in the nanocomposites. In the Bode plot (phase angle vs frequency) the phase angle of 90° in the low frequency indicates ideal capacitive behavior of the electrode, and the phase angle of 0° suggests pure resistance behavior.1,32,56 In the present case, the phase angles at the lowest frequency were found to be increased due to the immobilization of Ag and Ni nanoparticles on the surface of RGO. The phase angle values of Ag0.50Ni0.50, (Ag0.50Ni0.50)95RGO5, and (Ag0.50Ni0.50)90RGO10 were 54°, 57°, and 73°, respectively (Figure 9 and Figure S9), indicating their pseudocapacitive behavior.32,60 At the knee frequency ( f 0), which is the frequency at which phase angle is 45°, the capacitive and the

Figure 10. Ragone plots of Ag0.50Ni0.50 and (Ag0.50Ni0.50)90RGO10 electrodes.

energy density of 34.8 Wh kg−1 at a power density of 400 W kg−1. The synthesized nanocomposites (Ag0.50Ni0.50)yRGO100−y showed the increased value of energy density in comparison with Ag 0 . 5 0 Ni 0 . 5 0 . A mong these n anocomposites (Ag0.50Ni0.50)90RGO10 showed the highest value of an energy density of 80 Wh kg−1 at a power density 400 W kg−1. The energy density was decreased from 80 to 43 Wh kg−1 when the power density was increased from 400 to 4000 W kg−1. Generally, the results obtained from the electrochemical measurements of three-electrode systems are useful to estimate the redox characteristics of the electrode materials.7,44,48 To evaluate the electrochemical performance of the electrodes for practical applications, the required measurements should be performed by constructing two-electrode systems.48,54,63 Therefore, we have assembled a symmetric two-electrode cell

Figure 9. Bode plot (phase angle vs frequency) of the Ag0.50Ni0.50, RGO, and (Ag0.50Ni0.50)90RGO10 nanocomposites. 1220

DOI: 10.1021/acsaelm.9b00194 ACS Appl. Electron. Mater. 2019, 1, 1215−1224

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Figure 11. (A) CV curves at different scan rates (10−100 mV s−1), (B) GCD curves with increasing current densities from 1 to 10 A g−1, and (C) Ragone plot of (Ag0.50Ni0.50)90RGO10 electrode in 3 M KOH in a two-electrode system.

Figure 12. Demonstration of glowing of a blue light-emitting diode (LED) (3 V) powered by three (Ag0.50Ni0.50)90RGO10 capacitors connected in series. (A−D) shows the lighting of LED at a different time intervals after connecting the LED with the (Ag0.50Ni0.50)90RGO10 capacitors.

setup where the (Ag0.50Ni0.50)90RGO10 nanocomposite was used as an active electrode material and measured its specific capacitance with the 3 M KOH electrolyte. Figure 11 shows the CV curves at different scan rates and GCD curves with varying current densities of the (Ag0.50Ni0.50)90RGO10 electrode which were measured in the two-electrode system. The specific capacitance achieved in this condition was 124 F g−1 at 1 A g−1. Although this specific capacitance is lower than the specific capacitance determined from the three-electrode

system, this trend is quite similar to the observations reported by several researchers for different systems.7,48,51,52,63,64 This is due to the fact that in the three-electrode configuration only the working electrode contains the active electrode material. The applied voltage and charge transfer across the single electrode in the three-electrode system is different than with a two-electrode cell configuration.63,64 For a three-electrode cell, the voltage potential applied to the working electrode is with respect to the reference electrode used. In a symmetrical two1221

DOI: 10.1021/acsaelm.9b00194 ACS Appl. Electron. Mater. 2019, 1, 1215−1224

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ACS Applied Electronic Materials

indicate that this nanocomposite could be considered as an excellent electrode material for a high performing supercapacitor.

electrode cell, the potential differences applied to each electrode are equal to each other and are one-half of the values of the applied voltage potential.63,64 In two-electrode system, (Ag0.50Ni0.50)90RGO10 showed the highest value of an energy density of 17 Wh kg−1 at a power density 555 W kg−1 (Figure 11C). The energy density was reduced from 17 to 3 Wh kg−1 with the increase of power density from 555 to 20000 W kg−1. To demonstrate the real application, three symmetric cells were fabricated by using (Ag0.50Ni0.50)90RGO10 as an active electrode material, and they were joined in a series. A 9 V battery was used to charge this setup. After charging for 10 min, this symmetric cell setup was connected with a blue LED bulb (3 V). This blue LED bulb was glowing for 5 min. Figure 12 illustrates the light up of the LED after connecting with the (Ag0.50Ni0.50)90RGO10 electrode. In summary, it was observed that the electrode material consisting of 50 wt % Ag and 50 wt % Ni nanoparticles showed better specific capacitance than that of pure Ni. The specific capacitance value was further increased when Ni and Ag nanoparticles were immobilized on the RGO surface (10 wt %). The presence of RGO in the nanocomposites enhances the electrical conductivity by providing a faster electron transfer path.7,11,48 In this “in situ” coprecipitation method of nanocomposite preparation, the Ag and Ni nanoparticles are formed on the RGO surface. In the nanocomposite Ag, Ni nanoparticles act as a spacer between RGO sheets. This causes the weakening of the π−π interaction between the RGO sheets, and hence the chance of agglomeration of the RGO sheets reduces.3,45 Thus, the interplanar spacing between RGO sheets increases which helps the electrolyte ions more available to the active materials. Moreover, our previous investigation on the electronic structure and electronic property of Ag−Nigraphene by DFT calculations revealed the hybridization between Ag 4d, Ni 3d, and C 2p states of graphene at the conduction and valence band.45 The presence of these hybridizations in the interface between graphene, Ni, and Ag strongly influences the electronic properties of the composite. The presence of graphene introduces several new electronic states in the composite which causes strong electronic interactions between Ag, Ni, and RGO in the nanocomposites. Because of these synergistic effects, (Ag0.50Ni0.50)90RGO10 exhibits excellent supercapacitive performance. The supercapacitive performance of (Ag0.50Ni0.50)90RGO10 is comparable and superior to some of the already reported RGO-based electrodes4,17,22,29−35,37−42,45,51,65−69 as well as commercial supercapacitors (3−9 Wh kg−1 at 3000−10000 W kg−1).7,70

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaelm.9b00194.

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4. CONCLUSION Here we have demonstrated the application of the nanocomposite that is composed of Ag and Ni nanoparticles and RGO as high performing supercapacitor material. To the best of our knowledge, the supercapacitive performance of (AgxNi(1−x))yRGO(100−y) is reported for the first time. In the (Ag0.50Ni0.50)90RGO10 nanocomposite Ag and Ni nanoparticles are immobilized on the surface of RGO. Because of the hierarchical architecture, this nanocomposite exhibits outstanding capacitive and cycling performance as electrode materials. At 1 A g−1 current density (Ag0.50Ni0.50)90RGO10 shows a maximum specific capacitance of 897 F g−1 and a high energy density of 80 Wh kg−1 at 400 W kg−1 power density. This electrode exhibits ∼77% retention of its specific capacitance up to 4000 charge−discharge cycles. These results

Table S1: comparison table of previously reported silver, nickel, and RGO based electrodes for supercapacitor application; Figure S1: room temperature wide-angle powder XRD pattern of (A) Ag 0.50 Ni 0.50 , (B) (Ag0.50Ni0.50)95RGO5, (C) (Ag0.50Ni0.50)85RGO15, and (D) (Ag0.50Ni0.50)80RGO20.; Figure S2: FESEM microgr aph of (A) Ag 0 . 5 0 Ni 0 . 5 0 , (B) RGO, (C) (Ag0.50Ni0.50)95RGO5, (D) (Ag0.50Ni0.50)85RGO15, and (E) (Ag0.50Ni0.50)80RGO20 nanocomposites; Figure S3: EDS spectra of synthesized (Ag0.50Ni0.50)90RGO10 nanocomposite; Figure S4: cyclic voltammetry curves of (Ag 0.50 Ni 0.50 ) 95 RGO 5 , (Ag 0.50 Ni 0.50 ) 85 RGO 15 , and Ag0.50Ni0.50)80RGO20 electrodes at scan rate 10 mV s−1 in 3 M KOH electrolyte; Figure S5: cyclic voltammetry curves of (A) pure Ni, (B) Ag 0.50 Ni 0.50 , (C) (Ag0.50Ni0.50)95RGO5, (D) (Ag0.50Ni0.50)85RGO15, and (E) (Ag0.50Ni0.50)80RGO20 at different scan rates (from 10 to 100 mV s−1) and (F) Randles−Sevick plot for pure Ni, Ag0.50Ni0.50, (Ag0.50Ni0.50)95RGO5, (Ag0.50Ni0.50)85RGO15, and (Ag0.50Ni0.50)80RGO20 nanocomposites in 3 M KOH; Figure S6: galvanostatic charge−discharge curves of (A) pure Ni, (B) Ag 0 . 5 0 Ni 0 . 5 0 , (C) (Ag 0 . 5 0 Ni 0 . 5 0 ) 9 5 RGO 5 , (D) (Ag0.50Ni0.50)85RGO15, and (E) (Ag0.50Ni0.50)80RGO20 at different current densities (from 1 to 10 A g−1) in 3 M KOH electrolyte; Figure S7: electrochemical impedance spectra of (Ag 0 . 5 0 Ni 0 . 5 0 ) 9 5 RGO 5 , (Ag0.50Ni0.50)85RGO15, and (Ag0.50Ni0.50)80RGO20 electrodes in 3 M KOH electrolyte, with inset showing the high-frequency region of the impedance spectra and equivalent circuit used for fitting the Nyquist plots; Figure S8: Bode plot (|Z| vs frequency) of the (Ag 0.50 Ni 0.50 ) 95 RGO 5 , (Ag 0.50 Ni 0.50 ) 85 RGO 15 , and (Ag0.50Ni0.50)80RGO20 nanocompsoites; Figure S9: Bode plot (phase angle vs frequency) of the (Ag 0.50 Ni 0.50 ) 95 RGO 5 , (Ag 0.50 Ni 0.50 ) 85 RGO 15 , and (Ag0.50Ni0.50)80RGO20 nanocomposites; Table S2: fitting results of the EIS data of all the synthesized nanocomposites (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel/fax +91 832 2580318/2557033; e-mail naren70@yahoo. com (N.N.G.). ORCID

Narendra Nath Ghosh: 0000-0002-8338-7292 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. N. N. Ghosh is thankful to the Central Sophisticated Instrumentation Facility (CSIF) of BITS Pilani K K Birla Goa campus for providing the FESEM facility. 1222

DOI: 10.1021/acsaelm.9b00194 ACS Appl. Electron. Mater. 2019, 1, 1215−1224

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