Ag₃O₄ grafted NiO nanosheets for high performance supercapacitors

Abstract

Ag₃O₄ grafted NiO nanosheets were successfully synthesized via hydrothermal route. The morphological and structural analysis was performed using scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies. The grafting of Ag₃O₄ on NiO nanosheets enhanced the conductivity of the nanosheets and resulted in a maximum specific capacitance of 1504 F g⁻¹ for 5 wt% of Ag on NiO (AGN2) at a scan rate of 3 mV s⁻¹. The porous nature of AGN2 provides larger paths for ions, which significantly increases the intercalation of ions and utilization rate of electrode materials. Galvanostatic charge discharge (GCD) analysis reveals a specific capacitance of 1524 F g⁻¹ for AGN2 at a current density of 1 A g⁻¹ with excellent cyclic stability (12% loss after 1500 cycles). An asymmetric supercapacitor device (AGN2//RGO) achieved a specific capacitance of 274 F g⁻¹ with an energy density of 37 Wh kg⁻¹ and the power density of 499 W kg⁻¹ at a discharge current of 0.2 A g⁻¹.

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1. Introduction

Over the past decades, the energy demand has tremendously increased due to increase in population, vehicles and energy based appliances, which results in a huge utilization of fossil fuels, CO₂ emissions and environmental issues. To overcome such issues, an advanced energy conversion, as well as storage device is needed. Recent interest has focused towards electrochemical capacitors (ECs, also called supercapacitors (SC)), as a type of efficient energy storage device, which fills the gap between batteries and conventional capacitors. Supercapacitors exhibit a long cycle life (up to 10⁴ cycles),^1–3 excellent charge–discharge capability and environment friendly nature. These qualities make them an ideal choice for a wide range of potential applications like electric vehicles, short-term power sources for mobile electronic devices and other renewable energy storage systems.^4–9

Supercapacitors are classified as: pseudocapacitors and EDLC's (Electric Double Layer Capacitors – carbon based capacitors). A pseudocapacitive electrode material undergoes Faradaic redox reactions, which delivers higher energy density compared to carbon based electrode materials (non-faradaic reactions). Among the transition metal oxides (RuO₂, NiO, Co₃O₄ and MnO₂),¹⁰ nickel oxide (NiO) is one of the most important electrode material owing to its low cost, abundance, and good electrochemical capacitance (2584 F g⁻¹ within 0.5 V).¹¹ However, a low electrical conductivity and crystal shrinkage/expansion during the cycling process retards their motion towards practical applications. Both the problems should be addressed to produce commercially useful supercapacitors. To overcome this barrier two strategies may be adopted (i) form mixed transition metal oxides (NiO–TiO₂, NiCo₂O₄, NiCo₂O₄@MnO₂, Ni_x Co_1−xO, Ni–Cu hydroxides, NiO–CdS),^12–17 and carbon based (CNT, graphene)^18,19 composites and (ii) a selective conductive coating on the electrode material. Cheng et al.²⁰ reported that the incorporation of Ag on Co₃O₄ enhances the conductivity and results in a higher specific capacitance. MnO₂–Au nanoporous electrode delivers an excellent rate performance.²¹ Au decorated NiO was reported to exhibit an excellent electrochemical performance.²² Silver oxide is an interesting material due to its multiple oxidation states (Ag⁺, Ag²⁺, Ag³⁺)²³ with good electrical conductivity. Among the oxides of silver, Ag₃O₄ is an interesting candidate due to its mixed valance states, i.e., silver oxide (Ag(III) and Ag(II)) with a composition of Ag₂O₃ and AgO, and electrochemical redox reaction leads to the formation of metallic silver (Ag₃O₄ → AgO → Ag₂O → Ag).²⁴ This property inspired us to think about grafting nickel oxide with silver oxide to increase conductivity, for the first time using a hydrothermal method. The loading level of silver on NiO was varied and its effects on the structural, morphological and electrochemical properties of nickel oxide are reported and discussed in this paper.

2. Experimental section

2.1 Synthesis of Ag₃O₄ grafted NiO

Analytical grade nickel nitrate, silver nitrate, sodium lauryl sulfate (SDS) and urea were purchased from Sigma Aldrich and used without further purification. In a typical synthesis, Ni(NO₃)₂·6H₂O and Ag(NO₃)₂·6H₂O were taken as the precursors. Initially, 0.1081 g of SDS was dissolved in 40 mL of double distilled water and stirred for 30 min at room temperature. Subsequently, 0.4523 g of Ni(NO₃)₂·6H₂O was dissolved in 10 mL of double distilled water and 0.0104 g (corresponds to 2.5 wt% of Ag) of Ag(NO₃)₂·6H₂O was separately dissolved in 10 mL of double distilled water. The precursor solutions were slowly added to the SDS solution under constant stirring. Finally, 0.1081 g of urea was dissolved in 15 mL of double distilled water and the same was added to the solution containing SDS and the precursors. The resultant solution was allowed to stir for 3 h at room temperature. The as-obtained solution was transferred into 100 mL Teflon lined stainless steel autoclave and it was maintained at 160 °C for 24 h. After cooling to room temperature, a solid green color precipitate was obtained. It was repeatedly washed with ethanol and double distilled water. Ag₃O₄ grafted NiO was obtained by annealing the green powder at 300 °C for 2 h in an atmosphere of air. The silver level was varied from 2.5 to 10 wt% in steps of 2.5 wt% to prepare the Ag₃O₄ grafted NiO and the samples were named AGN1, AGN2, AGN3 and AGN4, respectively.

2.2 Material characterization

The morphology of the samples was analyzed by scanning electron microscopy with an energy dispersive X-ray spectrometer (SEM, JSM-6390-JEOL) and a scanning transmission electron microscope (STEM, FE-QUANTA). The crystalline nature of the sample was identified using X-ray diffraction analysis using a PANAlytical X'PERT-PRO X-ray diffractometer with CuKα radiation. An automated adsorption apparatus (Micromeritics ASAP 2020 V3.00 H) was used to analyze the textural characteristics of the sample using physisorption at 77 K. Conductivity measurements were made by using a two probe HIOKI 3532-50LCR Hi Tester (pellet method).

2.3 Fabrication of the electrodes and electrochemical measurements

Initially, the current collector (Ni foam) was repeatedly washed with acetone/double distilled water to ensure that the surface was clean. The electrodes used for evaluating the electrochemical properties of the synthesized Ag₃O₄ grafted NiO were prepared by mixing the electroactive material (85%), activated carbon (10%) and poly(tetrafluoroethylene) (PTFE 5%) with a few drops of ethanol to form a homogenous slurry. The slurry was coated onto the current collector (1 cm² of nickel foam) and was dried at 80 °C for 8 h. The electrochemical properties of the samples were investigated under a three-electrode cell configuration with Ag₃O₄ grafted NiO as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode with a 2 M KOH solution as the electrolyte. The Cyclic Voltammetry (CV), galvanostatic charge-discharge (GCD) and Electrochemical Impedance Spectroscopy (EIS) measurements (including equivalent circuit fitting) were carried out using an Electrochemical Workstation (CHI 660D, USA). Cyclic voltammograms were recorded in the potential range of 0 to 0.55 V and the specific capacitance was calculated from the average current developed in the CV plot. GCD measurements were performed at different current densities within the potential range of −0.15 to 0.4 V. Impedance studies on the electrode materials were conducted between 0.01 Hz to 100 kHz at an amplitude of 5 mV.

2.4 Fabrication of the asymmetric supercapaitor

A two electrode system was fabricated by coating AGN2 (1 mg) and RGO (reduced graphene oxide, 1 mg) on Ni foams of area 1 sq cm, to serve as the anode and cathode, respectively. A polypropylene separator (Celgard-2400) was used as the separator to make a sandwich type device with 6 M KOH solution as the electrolyte. RGO was prepared by a modified Hummers method.²⁵

3. Results and discussions

3.1 Structural and morphological investigations

The purity, phase and crystal structure of the samples were examined by XRD analysis. Fig. 1 shows the XRD patterns of the Ag₃O₄ grafted NiO. The diffraction peaks located at 43.2° and 62.9° correspond to the (200) and (220) planes of cubic structured NiO (JCPDS file no. 78-0643), which is in very good agreement with the reported results.^26–30 The other peaks at 34.37°, 37.82°, 38.62°, 65.02°, 75.82° and 78.17° could be indexed to the (012) (112) (022) (113) (222) (143) crystal planes of monoclinic structured Ag₃O₄ (JCPDS file no. 84-1261). The NiO diffraction peaks become sharper and intense with the increase in the Ag level, which confirms the enhancement in the crystalline nature of NiO and results in an increase in the conductivity of the material (Fig. S1†).

Fig. 1 XRD patterns of the Ag₃O₄ grafted NiO electrodes.

Fig. 2 shows the SEM and STEM images of the Ag₃O₄ grafted NiO. The concentration of Ag alters the morphology of the NiO. Initially NiO nanosheets are formed due to the anionic surfactant.³¹ AGN2 shows a flower like structure with good cross linked nanosheets. The increase in Ag content leads to the formation of an ordered structure up to 5 wt% and further increase in Ag content results in the collapse of the flowerlike structure and the formation of individual nanosheets. The grafting of Ag₃O₄ on the NiO system is clearly shown in Fig. 2e. The presence of Ag in the NiO system is confirmed by the energy dispersive X-ray analysis (Fig. S2†).

Fig. 2 SEM images of (a) AGN1, (b) AGN2, (c) AGN3, (d) AGN4 and (e) STEM image of AGN2.

3.2 Electrochemical behavior

Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance analysis were performed to evaluate the electrochemical performance of the Ag₃O₄ grafted NiO electrodes. The cyclic voltammograms were recorded in a voltage range from 0 to 0.55 V at a wide range of scan rates ranging from 3 to 20 mV s⁻¹. Typical CV curves of the Ag₃O₄ grafted NiO are shown in Fig. 3a–d. The shape of the CV curves clearly reveals the pseudocapacitive behavior of the electrodes, which is totally different from EDLCs.

Fig. 3 (a–d) Cyclic voltammograms of the Ag₃O₄ grafted electrodes at different scan rates. (e) Effect of the scan rate on specific capacitance.

The current response is found to be proportionally increased with the increasing scan rates, which reveals that the rate of the electronic and ionic transport are not limited by higher scan rates.³² The specific capacitance (C_s) was estimated using the relation,

where I (A) is the average current, m (g) is the mass of the active material, and ν (V) is the voltage sweep rate. The C_s (F g⁻¹) values are 1130, 1504, 1059 and 946 F g⁻¹ at 3 mV s⁻¹ and 315, 408, 272 and 165 F g⁻¹ at 50 mV s⁻¹ for AGN1, AGN2, AGN3 and AGN4, respectively. The cyclic voltammograms at higher scan rates are shown in Fig. S3.† AGN2 shows a maximum enhanced specific capacitance when compared to pure NiO (500 F g⁻¹ at 3 mV s⁻¹). Fig. S4† shows the cyclic voltammograms and specific capacitance of pure NiO at different scan rates. NiO/MnO₂ core–shell nanocomposites were reported to exhibit a specific capacitance of 528 F g⁻¹ at 1 mV s⁻¹.³³ Chen et al.³⁴ reported a specific capacitance of 677 F g⁻¹ at a scan rate of 1 mV s⁻¹ for the flower-like porous NiCo₂O₄ nanostructures. The improvement in the specific capacitance of AGN2 is mainly due to the porous flower like structure, which enhanced the transportation of ions. The crystalline nature of Ag₃O₄ increases with the Ag level (from XRD). The compact crystalline layers of Ag₃O₄ on NiO may restrict the ions with access to NiO and result in a lower capacitance. The nitrogen adsorption–desorption isotherm of AGN2 is shown in Fig. S5.† The Ag₃O₄ grafted NiO shows a type IV isotherm and a narrow hysteresis loop at low relative pressures, thus indicating the existence of mesopores.³⁵ The porous nature of the electrode leads to the creation of plenty of space for the movement of ions. The specific surface area of AGN2 is 131 m² g⁻¹, which is higher than the values reported in the literature.^27,36,37 The pores are centered at 9.3, 11.8, 15.9, 22.8, 26.8 nm. The pore size distribution of the sample is shown as an inset in Fig. S5.†

The effect of scan rate on the specific capacitance is shown in Fig. 3e. At lower scan rates, ions have enough time to reach the inner electrode sites and this results in a higher specific capacitance while at higher scan rates the ions are not able to access the inner regions.³⁸

To quantify the specific capacitance and rate capability of the Ag₃O₄ grafted NiO electrodes, GCD measurements were carried out. The nonlinearity in the discharge curves (Fig. 4) shows the pseudocapacitive behavior of the electrodes. The specific capacitance (F g⁻¹) was obtained using the following equation.

where I (A) is the discharge current, Δt (s) is the discharge time, m (g) is the mass of the active material, and Δv (V) is the potential difference during the discharge. The samples AGN1, AGN2, AGN3 and AGN4 exhibit the specific capacitances of 1362, 1524, 1085, 815 F g⁻¹ at 1 A g⁻¹, respectively. It should be noted that these values are relatively consistent with the specific capacitance calculated from the CV curves. In general, the specific capacitance decreases with the increase in discharge current density (Fig. S6†). The efficiency of the utilization of the active material decreases at higher current densities due to an increase in the internal diffusion resistance within the pseudoactive material and results in a lower specific capacitance.³⁹ Lei et al.⁴⁰ reported a 3D flower shaped NiCo₃O₄ microspheres synthesized via the hydrothermal route to yield a specific capacitance of 1006 F g⁻¹ at a current density of 1 A g⁻¹. Huang et al.⁴¹ reported a specific capacitance of 280 F g⁻¹ in the case of hierarchical NiO nanoflake coated CuO flower nanostructures at a current density of 1 A g⁻¹.

Fig. 4 Discharge curves of the Ag₃O₄ grafted NiO electrodes at different current densities.

Energy and power density are the two crucial factors to evaluate the commercial application of electrochemical supercapacitors. A good electrochemical supercapacitor is expected to provide a high energy density or high specific capacitance at high charge–discharge rates (current densities). The Ragone plot (power density vs. energy density) of the Ag₃O₄ grafted NiO electrodes are shown in Fig. 5a. The energy and power densities of the samples were calculated from the charge–discharge curves at different current densities using following equations:

where C is the specific capacitance at a particular current density, Δv is the potential window, and t is the discharge time. The energy density (Wh kg⁻¹) of the electrode materials AGN1, AGN2, AGN3 and AGN4 are 57, 64, 45 and 34 Wh kg⁻¹ at the current density of 1 A g⁻¹, respectively. The sample AGN2 exhibits a maximum power density (W kg⁻¹) of 823 W kg⁻¹. The results obtained in the present work are superior to the results reported earlier by other workers.^42–44

Fig. 5 (a) Ragone plot; (b) capacitance retention with continuous charge–discharge cycles of the AGN2 sample at 5 A g⁻¹ (c) impedance plot of the AGN1, AGN2, AGN3 and AGN4 samples at 0.4 V; inset shows the low frequency region of AGN4 (d) equivalent fitting circuit.

The cycle life is another important factor to evaluate the performance of supercapacitors. The percentage retention of the capacitance of AGN2 is shown in Fig. 5b. The specific capacitance does not show any appreciable change during the first 200 cycles, whereas it gradually increases up to 400 cycles, indicating the activation of the electrode material. Beyond 400 cycles, the specific capacitance remains unaffected up to 1400 cycles and finally it shows 88% capacity retention at 1500 cycles. It is worth mentioning that the cyclic stability of the Ag₃O₄ grafted NiO (5 wt% of Ag) is better compared to other reported results.^45–47 The flower like structure may provide the easiest intercalation/deintercalation paths for ions without much change in the structure for prolonged cycles, and thus results in a higher cyclic stability.

To elucidate the electrical conductivity and ion transfer ability of the electrodes, electrochemical impedance analysis was performed. Fig. 5c shows the Nyquist plot of the Ag₃O₄ grafted NiO electrode materials (AGN1, AGN2, AGN3 and AGN4) recorded in a frequency range of 0.01 Hz to 100 kHz at the potential of 0.4 V. The equivalent fitting circuit is shown in Fig. 5d, which contains the solution resistance R_s, charge transfer resistance R_ct, double layer capacitance (C_dl), pseudocapacitive capacitance (C_L) and Warburg impedance (W). The faradaic pseudocapacitance C_L is charged via a faradaic resistance R_ct, and C_L is leaked by a desorption faradaic resistance R_L. The impedance spectrum is composed of a semicircle at the high frequency region followed by one linear component (Warburg) at the low frequency region, which shows the frequency dependence of ion diffusion in the electrolyte to the electrode interface. The charge transfer resistances (R_ct) of the electrode materials AGN1, AGN2, AGN3 and AGN4 are 1.39, 1.31, 0.96, and 0.37 Ω, respectively. Liu et al.⁴⁸ reported three dimensional tubular arrays of MnO₂–NiO nanoflakes with an R_ct of 18.7 Ω. Co_xNi_1−xO nanorods synthesized via a bio inspired method shows an R_ct of 2.29 Ω.⁴⁹ Lee et al.⁵⁰ reported a charge transfer resistance of 5 Ω for NiO/CNT nanocomposites prepared via the precipitation method. Compared to other reports, the electrodes prepared in the present work exhibit lower R_ct values and this probably increased the conductivity of the electrodes due to excellent electrolyte accessibility.⁵¹ The increase in the Ag level decreases the R_ct value and this is in good agreement with the DC conductivity studies, but the formation of compact crystallites and change in morphology may play a vital role in deciding the electrochemical performance of the electrodes.

In the view of potential applications, asymmetric supercapacitors were fabricated to evaluate the capacitive features of the Ag₃O₄ grafted NiO. The working electrodes were fabricated as discussed earlier in the Experimental section. Fig. 6a shows the schematic illustration of the fabrication of the asymmetric supercapacitor, which consists of an anode (AGN2), polypropylene separator (Celgard-2400), cathode (reduced graphene oxide-RGO) and electrolyte (6 M KOH). The electrochemical performance of RGO is shown in Fig. S7.† The CV and GCD analysis were carried out to explore the electrochemical performance of the two electrode system. The potential window of 1 V (0–1 V) was fixed for both the analysis. The CV curves (Fig. 6b) show excellent redox peaks i.e., capacitance arises mainly due to the peudocapacitive behavior of the material prepared in the present work. The specific capacitance estimated from the CV are 186, 140, 84, 55, 44 and 37 F g⁻¹ at a scan rate of 2, 3, 5, 10, 15 and 20 mV s⁻¹, respectively. The GCD analysis exhibits the specific capacitance of 274, 135, 80 and 30 F g⁻¹ (Fig. 6c) at 0.2, 0.5, 1 and 2 A g⁻¹, respectively, with a maximum energy density of 38 Wh kg⁻¹ at the power density of 499 W kg⁻¹. Fig. S8† shows the Ragone plot of the asymmetric supercapacitor.

Fig. 6 (a) Schematic representation of the asymmetric supercapacitor (b) CV plot of the asymmetric supercapacitor at different scan rates (c) discharge curves of the asymmetric supercapacitor at different current densities.

4. Conclusion

A simple and facile hydrothermal approach was used to fabricate Ag₃O₄ grafted NiO nanosheets. Electrochemical measurements reveal the pseudocapacitive nature of the Ag₃O₄ grafted NiO with the maximum specific capacitance of 1524 F g⁻¹ (AGN2) at a current density of 1 A g⁻¹. The enhanced electrochemical performance is attributed to the Ag₃O₄ grafting on NiO and the formation of flower like cross linked nanosheets with a mesoporous structure. 88% capacity retention was observed even after 1500 continuous charge–discharge cycles at a current density of 5 A g⁻¹. An asymmetric supercapacitor (AGN2//RGO) was constructed and it exhibits the maximum specific capacitance of 274 F g⁻¹ at a discharge current of 0.2 A g⁻¹ with the power density of 499 W kg⁻¹ and these results make the Ag₃O₄ grafted NiO (AGN2) an optimistic potential nominee for electrochemical energy storage.

Acknowledgements

One of the authors (K.K.P.) thanks The Department of Science and Technology-SERC, New Delhi, India, for providing the financial support under Young Scientist scheme (No. SR/FTP/PS-030/2011) and Nanotechnology Research Centre, SRM University, Kattankulathur, India, for providing XRD and STEM facility.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ta04586f

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