High-Performance Symmetrical Supercapacitor with a Combination of a ZIF-67/rGO Composite Electrode and a Redox Additive Electrolyte.

The synthesis of a highly porous composite of ZIF-67 and reduced graphene oxide (rGO) using a simple stirring approach is reported. The composite has been investigated as an electrode to be assembled in a supercapacitor. In the presence of an optimized redox additive electrolyte (RAE), that is, 0.2 M K3[Fe(CN)6] in 1 M Na2SO4, the ZIF-67/rGO composite electrode has combined the properties of improved conductivity, high specific surface area, and low resistance. The proposed composite electrode in the three-electrode system shows an ultrahigh specific capacitance of 1453 F g-1 at a current density of 4.5 A g-1 within a potential window of -0.1 to 0.5 V. Further, the ZIF-67/rGO composite electrode was used to fabricate a symmetrical supercapacitor whose operation in the presence of the RAE has delivered high values of specific capacitance (326 F g-1 at a current density of 3 A g-1) and energy density (25.5 W h kg-1 at a power density of 2.7 kW kg-1). The device could retain about 88% of its initial specific capacitance after 1000 repeated charge-discharge cycles. The practical usefulness of the device was also verified by combining two symmetrical supercapacitors in series and then lighting a white light-emitting diode (illumination for 3 min). This study, for the first time, reports the application of a ZIF-based composite (ZIF-67/rGO) in the presence of an RAE to design an efficient supercapacitor electrode. This proposed design is also scalable to a flexible symmetric device delivering high values of specific capacitance and energy density.

Introduction

The ever-growing environmental concerns originating with the present energy demands have motivated the researchers to develop alternative technologies for energy conversion and storage. Lithium (Li)-based rechargeable batteries and supercapacitors have received significant attention in the above context. Li batteries demonstrate high energy density but do not necessarily possess sufficient cycle life.[1] On the other hand, supercapacitors are considered as a good alternative in terms of high power delivery and long cycle life but they usually deliver moderate levels of energy density.[1−3] Therefore, the development of next generation supercapacitors demands for the realization of both high energy and high power density features. Such supercapacitors can be considered useful for various electronic applications, such as hybrid electric vehicles, portable power supplies, pulsed electronic devices, and so forth.[2,4−6]

The last decade has witnessed an enormous expansion in the applications of metal–organic frameworks (MOFs) in areas such as gas storage, catalysis, biosensors, solar cells, and so forth.[7,8] MOFs are characterized with unique features of high specific surface area, hierarchical pore size distribution, structural tailorability, and high chemical and thermal stabilities.[8,9] Recently, various MOFs have also been advocated as potential electrodes in supercapacitors on virtue of their highly porous structures.[8] Nonetheless, most of the pristine MOFs bear poor electrical conductivity. Consequently, MOF-based electrodes do not support efficient transfer of charge to the current collector. This limitation of MOFs can be overcome by forming their composites with other conductive materials such as conducting polymers, graphene, and so forth. Because of the presence of MOFs, such electrically conductive composites possess a high surface area characteristic which is an advantageous factor over a competitive electrode material, such as bare graphene, carbon nanotubes, and other forms of carbon.

Salunkhe et al. have reported the use of a composite of zeolitic imidazolate framework-8 (ZIF-8) with polyaniline (PANI) as a supercapacitor electrode (electrolyte = 1 M H2SO4).[10] The electrode demonstrated a specific capacitance value of 236 F g–1 at a current density of 1 A g–1. The application of a composite electrode of UIO-66 with reduced graphene oxide (rGO) in the presence of 6 M KOH as an aqueous electrolyte has been demonstrated to yield a moderate value of specific capacitance, (302 F g–1).[11] However, this value of specific capacitance was achievable only at a low current density, for example, 0.15 A g–1. The functioning of a Mn–MOF-derived Mn3O4/graphene composite electrode has been proposed wherein the supercapacitor was able to yield a specific capacitance of 456 F/g (1 A g–1).[12] Xu et al. reported a ZIF-67/polypyrrole nanotube composite-based electrode to attain a specific capacitance of 597.6 F g–1 at a current density of 0.5 A g–1 (electrolyte = 1 M Na2SO4).[13] Several other MOF-based electrodes have also been proposed to design supercapacitors with considerable levels of specific capacitance.[14−20] As the literature survey has revealed, most of the MOF composite-based supercapacitor studies have been carried out in the presence of aqueous electrolytes. The assembly and complete device performance of those systems have not been reported.

The characteristics of an electrode material significantly influence the output parameters such as specific capacitance, cyclic stability, and energy and power densities of a supercapacitor. Simultaneously, the electrolytes also play a big role to decide the overall performance (e.g., energy density and cyclic stability) of a supercapacitor.[21−23] Apart from the commonly used aqueous and organic solutions, some redox active species such as K3[Fe(CN)6], hydroquinone, and potassium iodide can be listed as alternative electrolytes.[8,23−25] They can also be mixed with common aqueous electrolytes (e.g., H2SO4, Na2SO4, and KOH).[26] In some studies, the use of redox additive electrolytes (RAEs) has been suggested to enhance the electrochemical performance of supercapacitors.[8,27−29] Most of these studies have been carried out on graphene and metal oxide/hydroxide electrodes-based supercapacitors.[26,30,31] As per our knowledge, no previous data are available to assess the performance of MOF or MOF composite-based supercapacitors in the presence of RAEs.

The present work reports a simple, stirring-based, environment-friendly, and acid-free synthesis of a highly porous ZIF-67/rGO composite. ZIF-67 is a large surface area material (e.g., 947 m2 g–1) that, when mixed with rGO, has yielded the formation of a highly porous and electrically conductive hybrid composite. The ZIF-67/rGO composite introduces improved electron transport and electrolyte ion diffusion characteristics as compared to the bare ZIF-67.[32] In this work, we have investigated the application of the ZIF-67/rGO composite as a supercapacitor electrode material in the presence of an optimized RAE solution, that is, 0.2 M K3[Fe(CN)6] in 1 M Na2SO4. The use of the RAE is explored in view of its apparent advantages as discussed in the preceding paragraph. Our studies have revealed that the ZIF-67/rGO-redox system could deliver high values of specific capacitance and energy density, supplemented with an excellent cyclic stability. The studies have been extended to assemble a symmetrical supercapacitor device using two identical ZIF-67/rGO electrodes. The symmetrical supercapacitor has also delivered enhanced values of specific capacitance and energy density compared to the earlier reported MOF-based devices. Further, the device (series of two symmetrical supercapacitors) has been demonstrated to energize a white light-emitting diode (LED). As per the authors’ knowledge, this is the first report to underline the excellent supercapacitor performance of a ZIF-composite electrode/RAE system.

Results and Discussion

MOFs (including ZIFs) are known for their high specific surface area and hierarchical pore size distribution characteristics. The hybrid structure of ZIF-67 with rGO should offer the synergistic benefits of both components. The polyhedral-shaped crystals of ZIF-67 decorate over the 2D rGO sheets and the resulting composite attains properties of high surface area, porosity, capacitance, and enhanced charge transfer. The detailed studies, as elaborated in the following paragraphs, outline various characteristics of the ZIF-67/rGO composite along with its potential use as an efficient supercapacitor electrode material.

The structure information and the crystal phase of the samples were studied by X-ray diffraction (XRD) analysis of a reference (ZIF-67) and the synthesized composite (ZIF-67/rGO) (Figure ). Both the samples of ZIF-67 and ZIF-67/rGO have shown characteristic diffraction peaks of the MOF. This observation is indicative of a successful integration of ZIF-67 in the synthesized composite.[33−35] The notable peaks in the XRD pattern of ZIF-67/rGO can be listed as: 7.2° (011), 10.4° (002), 12.7° (112), 14.7° (022), 16.4° (013), 18° (222), 22.1° (114), 24.5° (233), 25.6° (002), 26.5° (134), 29.6° (044), 31.3° (244), 32.5° (235), and 43.1° (100).[33−35] In particular, a low intensity 2θ peak at 25.6° (002) is an indicator of the presence of rGO (CCDC no. #671073).[36] It may also be noted here that rGO is unlikely to reflect many diffraction peaks due to its mono- to few-layered structure.[37] Nonetheless, the XRD analysis has provided useful evidence about successful formation of the desired nanocomposite of ZIF-67/rGO.[33]

Figure 1

XRD patterns of ZIF-67/rGO and ZIF-67 samples.

ZIF-67 is a zeolitic type of MOF having a general formula of Co(2-MeIM)2. The morphological characterization of the samples (ZIF-67 and ZIF-67/rGO) has been performed with field emission scanning electron microscopy (FESEM) analysis (Figure a,b). Figure a shows the morphology of ZIF-67, which indicates a 3D polyhedral shape of the ZIF-67 particles with an average size of 500 nm. The formation of the ZIF-67/rGO composite was evidenced with the apparent uniform growth of the MOF crystals over the sheet-like surface of rGO. TEM analysis of the sample also supports the formation of the composite between ZIF-67 and rGO in which a crystal of ZIF-67 resides over the rGO sheet [Figures c,d and S1 show a high-resolution transmission electron microscopy (HRTEM) image]. The energy-dispersive X-ray spectrometry (EDXS) analysis-based elemental composition of the ZIF-67/rGO composite has been presented in Table , which confirms the presence of cobalt (from ZIF-67) along with carbon, oxygen, and nitrogen contents.

Figure 2

FESEM images of (a) ZIF-67, (b) ZIF-67/rGO. (c,d) TEM images of the ZIF-67/rGO composite.

Table 1

EDXS Analysis of the ZIF-67/rGO Composite

element	wt %
carbon (C)	53.30
oxygen (O)	20.98
nitrogen (N)	10.75
cobalt (Co)	14.97

As the morphological and microstructural analyses with FESEM and TEM suggest, the proposed approach to synthesize the ZIF-67/rGO composite has resulted in the formation of the desired composite wherein the structural features of both the components could be preserved. This was important to ensure efficient charge storage through enhanced transport of electrolyte ions inside the electrode material.[8] The introduction of additional porosity owing to the presence of ZIF-67 should facilitate the electrode material with good supercapacitor performance.

Raman spectroscopy has been used to study the vibrational modes of samples under study (Figure a). These studies were carried out over a spectral region of 200–1400 cm–1 under room temperature (RT) conditions. The ZIF-67/rGO sample was characterized with two prominent peaks at 1349 and 1579 cm–1, related with D and G bands, respectively, of rGO nanosheets.[38] It is well known that the D band correlates with the stretching of C–C bonds and is common to all sp2 hybridized carbon lattices. The D band is an also an indicator of the disorder- and defect-induced breathing mode of sp2 rings.[38] The intensity of the D band was larger than the G band indicating the presence of defects in the rGO fraction which might have been introduced during the chemical reduction step. The appearance of some other peaks at 262, 424, 521, and 681 cm–1 can be associated with the presence of ZIF-67.[35,39]

Figure 3

(a) Raman and (b) FTIR spectra of ZIF-67 and ZIF-67/rGO composites.

Fourier transform infrared (FTIR) spectra of ZIF-67 and ZIF-67/rGO are shown in Figure b. Both the samples showed a common band around 3450 cm–1 to signify the surface O–H stretching vibrations of the C–OH group.[34] The sample of rGO showed a main band at 1662 cm–1 corresponding to epoxide which helped us to verify the reduction of GO into rGO. A relatively less intense band at 1662 cm–1 was due to the skeleton vibration of rGO. No major signal related to (nonreduced) graphene oxide (GO) [e.g., at 1220 cm–1 (C–O), 1621 cm–1 (C=O), 3400 cm–1 (O–H), 1415 cm–1 (C–O–H), 1732 cm–1 (C=O), and 1040 cm–1 (C–OC)] was observed to confirm the presence of graphene as rGO. The bands related with ZIF-67 appeared at 759, 996, 1142, 1304, 1428, and 1580 cm–1.[33,40] The bands in the region of 600–1502 cm–1 corresponded to the bending and stretching modes of ZIF-67, while a specific signal at 1580 cm–1 could be associated with the stretching modes of C=C and C=N. Another band at 3135 cm–1 could be ascribed to the stretching mode of C–H from the aliphatic chain in the linker (2-methylimidazole). The presence of characteristic signals from both rGO and ZIF-67 provided evidence of successful formation of the ZIF-67/rGO composite.

The N2 adsorption–desorption isotherms of ZIF-67 and ZIF-67/rGO composites (performed at 77 K) are shown in Figure . The results for rGO are summarized in Figure S2. Both the samples of ZIF-67 and ZIF-67/rGO showed typical type-I surface area profiles with microporous characteristics in majority (Figure a). The adsorption isotherms showed a steep rise at low relative pressure and then quickly attained a balance to suggest dominant microporous characteristic. The values of specific surface area for ZIF-67, ZIF-67/rGO, and rGO were estimated to be 947, 571, and 58.2 m2 g–1, respectively. A significantly large specific surface area of ZIF-67 can be linked with the presence of micropores.[41]Figure b shows the Barrett–Joyner–Halenda pore size distribution plots for the samples of ZIF-67 and ZIF-67/rGO. The diameter of most of the pores was found to lie within the range of 1–20 nm. The values of mean centered porosities for ZIF-67 and ZIF-67/rGO were assessed as 1.26 and 1.73 nm, respectively, which verified their microporous characteristics.

Figure 4

(a) N2 adsorption–desorption isotherms for ZIF-67 and ZIF-67/rGO and (b) Pore size distribution plot for ZIF-67 and ZIF-67/rGO.

In literature, significant focus has been laid on the development of new materials to achieve improved electrochemical performance of supercapacitor devices. Some recent studies have highlighted the fact that the selection of an appropriate electrolyte can also play a vital role.[8] For instance, the introduction of redox additives in parent aqueous or organic electrolytes has been suggested to produce enhanced supercapacitor performance.[8,31] The application of such RAEs facilitates redox reactions on the surface of the active electrode material, thereby leading to enhancement in the pseudocapacitance of the system.[42] In the present study, for the first time, we have investigated the performance of a ZIF-67-based composite electrode in conjunction with a RAE. For this, an aqueous electrolyte of 1 M Na2SO4 was mixed with a redox active species, that is, potassium ferricyanide, in varying concentrations from 0.05 to 0.5 M. Amongst various tested combinations, highest ionic conductance was realized from a composition of 0.3 M K3[Fe(CN)6] in 1 M Na2SO4 (Figure S3). The addition of 0–0.3 M K3[Fe(CN)6] in 1 M Na2SO4 was found to boost the ionic conductance of the resulting electrolyte with a maximum conductance for 0.3 M K3[Fe(CN)6].

Based on some initial tests, the desirable charging–discharging conditions of the ZIF-67-redox or ZIF-67/rGO-redox supercapacitor system were satisfactorily met with the application of 0.2 M K3[Fe(CN)6] in 1 M Na2SO4 as the optimized RAE composition. The use of K3[Fe(CN)6] in concentrations larger than 0.2 M did not allow complete discharging of the supercapacitor system even at higher current densities. Therefore, 0.2 M K3[Fe(CN)6] in 1 M Na2SO4 was selected as the optimized composition of the RAE for all other electrochemical studies. For a thorough evaluation and comparison, in all studies, we have investigated the supercapacitor performance of the ZIF-67 or ZIF-67/rGO electrode in the presence of both 1 M Na2SO4 (reference electrolyte) and 0.2 M K3[Fe(CN)6] in 1 M Na2SO4 (RAE).

The ZIF-67 or ZIF-67/rGO composite electrode was first characterized with electrochemical impedance spectroscopy (EIS) to probe its potential as an electrode for supercapacitors. Nyquist plots have been used to determine the values of series resistance (Rs), charge transfer resistance (Rct), electric double layer capacitance (EDLC), and Warburg resistance (W or Rw) (Figure ). The electrode characteristics were found better in the presence of RAE because of a faster rate of electron transport and electrochemical reactions. ZIF-67 has a relatively less conductivity in its native form. After the incorporation of rGO, the ZIF-67/rGO electrode was characterized with lower values of Rs and Rct (Rs = 3.9 Ω and Rct = 1.1 Ω) than the pristine ZIF-67 electrode (Rs = 4.7 Ω and Rct = 1.8 Ω). Apparently, the ZIF-67/rGO electrode was characterized with a lower charge transfer resistance in the redox electrolyte. This can be attributed to the formation of the [Fe(CN)6]3–/[Fe(CN)6]4– redox couple that enhanced the ionic concentration (and conductivity) of the system and allowed faster transport of more number of electrolyte ions toward the surface of the active electrode material. As a result, we observed a decrease in both the solution and charge transfer resistances.[30,43]

Figure 5

Nyquist plots for ZIF-67 and ZIF-67/rGO electrodes in the presence of the RAE (inset shows corresponding equivalent circuit).

Cyclic voltammetry (CV) and galvanostatic charge discharge (GCD) measurements have been performed to evaluate the specific capacitance, energy density, and power density of the fabricated electrodes. Figure a shows the cyclic voltammograms recorded during the functioning of ZIF-67/rGO electrodes in the optimized composition of the RAE (0.2 M K3[Fe(CN)6] in 1 M Na2SO4) at various scan rates (100, 50, 20, 10, and 5 mV s–1) within a potential range of −0.1 to +0.5 V. Comparative voltammograms with 1 M Na2SO4 aqueous electrolyte solution are shown in Figure S4a. The ZIF-67/rGO electrode has shown distinct anodic and cathodic peaks because of the Faradaic nature of the ZIF-67 component. CV data were used to estimate the values of specific capacitances (Cs, in F g–1) according to the expression given in eq S1. Clearly, lower scan rates should yield better values of specific capacitance because of the availability of sufficient time for the electrolyte ions to undergo adsorption/desorption at the electrode interface.[8,44,45] (Figures a and S4a). The above comparative study also confirmed that the ZIF-67/rGO electrode would yield much higher current values in the presence of RAE. In comparison to the simple aqueous electrolyte, the use of RAE was clearly beneficial to obtain a much improved value of specific capacitance. For instance, for a fixed scan rate of 5 mV s–1, ZIF-67/rGO yielded a specific capacitance of 562 F g–1 with the use of the RAE compared to a value of 46.5 F g–1 obtained with the simple 1 M Na2SO4 aqueous electrolyte.

Figure 6

(a) CV curves for the ZIF-67/rGO electrode at different scan rates, and (b) comparative CV curves for ZIF-67/rGO and ZIF-67 electrodes (scan rate = 5 mV s–1). Electrolyte = 0.2 M K3[Fe(CN)6] in 1 M Na2SO4.

A similar positive effect with the use of the RAE was also realizable with the use of the bare ZIF-67 electrode as well (Figure b). The bare ZIF-67 system yielded a specific capacitance value of 510 F g–1. Therefore, the abovementioned investigations have clearly highlighted that the selection of RAE with the MOF/graphene composite electrode should be more effective to achieve high values of specific capacitance and energy density. The formation of the Fe(CN)63–/Fe(CN)64– redox couple happened to be the major factor contributing toward enhanced pseudocapacitance and energy density.[23,30] The addition of the redox active K3[Fe(CN)6] species in the parent 1 M Na2SO4 medium ensured extra density of charge carriers (electrons). Both ZIF-67 and the ZIF-67/rGO electrodes showed distinct anodic and cathodic peaks when they interacted with the redox electrolyte (Figure b). This can be attributed to the surface redox reactions that took place in the presence of the redox electrolyte. Cobalt ions (of ZIF-67) undergo different oxidation states via Faradaic reactions for the system to deliver pseudocapacitive behavior.[46] Subsequently, overall conductivity of the composite electrode improved as a result of faster electrochemical reactions.[30] The abovementioned combined effects synergized to facilitate a remarkable electrochemical performance of the proposed ZIF-67/rGO-redox supercapacitor.

The contribution of rGO in the ZIF-67/rGO composite was also evident in delivering a better specific capacitance (Figure b). The mixing of ZIF-67 with rGO introduced conductive pathways to the electrode resulting in an improved electronic transport between the composite electrode and its associated current collector.

Along with CV, GCD analysis of a supercapacitor system also allows us to assess various important electrochemical parameters, for example, specific capacitance and energy and power densities. The results of GCD investigations for the ZIF-67/rGO electrode using both redox additive and 1 M Na2SO4 electrolytes at various values of current density are presented in Figures a and S4b. The ZIF-67/rGO electrode delivered better electrochemical activity at lower current densities in both the cases of redox additive and simple aqueous electrolytes. The GCD profiles exhibited nonlinear characteristic with a small voltage drop to suggest a low internal resistance of the electrode.[30] Further, increase in the current density caused a slower discharging of the system. A relatively fast (thus incomplete) diffusion of electrolyte ions into the pores of the electrode material at high current densities can be associated with the abovementioned observation.[8] GCD data were used to calculate the specific capacitance (Cs) of the electrodes according to the mathematical expression given in eq S2 of the Supporting Information.[47] A depiction of the specific capacitance values as a function of current density is made in Figure b, which is indicative of the fact that a low current density facilitates better specific capacitance. This property is attributed to the fact that at lower current densities, electrolyte ions have extra time to access the pores of the active electrode surface which in turn allows more efficient electronic and ionic transport.[2,6]

Figure 7

Electrochemical performance using optimized RAE (a) GCD curves for the ZIF-67/rGO electrode at different current densities, (b) values of specific capacitance from the ZIF-67/rGO electrode at varying current densities, (c) comparative GCD curves for ZIF-67/rGO and ZIF-67 electrodes, and (d) capacity retention versus cycle number plot for the ZIF-67/rGO electrode.

Figure c shows comparative GCD curves for the ZIF-67/rGO and ZIF-67 electrodes using the optimized RAE at a constant current density of 4.5 A/g. The results again clearly verified the improved performance of the ZIF-67/rGO composite electrode as compared to the ZIF-67 electrode. Nonlinear GCD patterns have been observed because of the occurrence of quasireversible Faradaic reactions. Because of these Faradaic reactions, the charge/voltage ratio does not remain constant and varies with time. Therefore, the values of specific capacitances in the abovementioned systems have been calculated by integrating the discharging current area (eq S2).[47] The values of specific capacitance with ZIF-67/rGO and ZIF-67 electrodes (at a current density of 4.5 A g–1) have been estimated as 1453 and 1014 F g–1, respectively. A higher specific capacitance by ZIF-67/rGO is explained by the incorporation of rGO nanosheets which facilitated faster electron transport and more efficient diffusion of ions into the redox sites of the composite material.

Further, the ZIF-67/rGO electrode has also shown a good long-term cyclic stability as it retained almost 90% of its capacity after 1000 continuous charge–discharge cycles (Figure d). The abovementioned value of cycle stability suggests an extraordinary performance of the proposed ZIF-67/rGO composite electrode. The abovementioned feature can be attributed to the combination of EDLC and Faradaic capacitance achieved by the double layer charging process. A comparison of the performance of the ZIF-67/rGO electrode proposed in this study with earlier reported MOF-based supercapacitor electrodes is given in Table , which clearly indicates that the ZIF-67/rGO-redox system is superior in terms of all three critical performance parameters, that is, specific capacitance, energy density, and power density.

Table 2

Comparison of Supercapacitor Performance Parameters of the ZIF-67/rGO-Redox System with Previously Reported MOF Composite Electrodes

s. no.	electrode material	electrolyte	scan rate/current density	specific capacitance (F g–1)	capacitance retention % (wrt cyclic tability)	refs
1	Co MOF-derived Co3O4 nano-/microsuperstructures	6 M KOH	1 A g–1	208	97	(40)
2	ZIF-8/PANI	1 M H2SO4	1 A g–1	236	86	(10)
3	UIO-66/rGO	6 M KOH	0.15 A g–1	302	94	(11)
4	ZIF-67/PANI	3 M KCl	10 mV s–1	371	80	(48)
5	Cu MOF/rGO	PVA–Na2SO4	1 A g–1	385	98.5	(49)
6	Mn MOF-derived Mn3O4/graphene	1 M Na2SO4	1 A g–1	456	98.1	(12)
7	ZIF-67/polypyrrole nanotubes	1 M Na2SO4	0.5 A g–1	597.6	90.7	(13)
8	Mo MOF-derived MoO3/rGO	PVA–H2SO4	1 A g–1	617	87.5	(50)
9	Ni-MOF-5/rGO	1 M KOH	1 mV s–1	758	∼100	(51)
10	Ni MOF-derived nanoparticles/graphene	1 M H2SO4	1 A g–1	886	84	(52)
11	MOF-derived LDH/GO films	6 M KOH	1 A g–1	904.3	90.3	(53)
12	ZIF-67/rGO composite	0.2 M K3[Fe(CN)6] + 1 M Na2SO4	4.5 A g–1	1453	90.5 (1000 cycles)	this work

Amongst the two components of the ZIF-67/rGO composite, ZIF-67 stores charge via pseudocapacitive behavior, whereas rGO accumulates charge via the EDLC mechanism. Because of this synergy, the use of the ZIF-67/rGO-redox system has shown promising performance in terms of specific capacitance and other critical parameters as well. After assessing the different electrode characteristics, we have assembled a symmetrical supercapacitor using two equally weighed ZIF-67/rGO electrodes. About 1 mg cm–2 of the active material was deposited over each electrode. A filter paper (0.22 μ pore size) was used as the separator. The two ZIF-67/rGO electrodes were then sandwiched with the separator paper after soaking all of them in the redox electrolyte solution. The separator paper prevented the device from short circuit while allowing transport of ions across the two electrodes. The device was then tested in a two-electrode symmetrical cell configuration.

Figure shows the results of the EIS studies (Nyquist plots) carried out for the ZIF-67/rGO//ZIF-67/rGO-redox supercapacitor device. The high- and low-frequency regions were analyzed to estimate the values of Rs and Rct as 22.9 and 8.3 Ω, respectively. A semicircle shape in the high-frequency region confirmed pseudocapacitive characteristic of the device, facilitated by the application of RAE. A low value of Rct (along with a linear behavior) in the low-frequency region was indicative of the fast charge transfer mechanism during the charging or discharging of the developed symmetrical supercapacitor device.[49]

Figure 8

Nyquist plot for the ZIF-67/rGO//ZIF-67/rGO symmetric supercapacitor device (inset shows the equivalent circuit).

The two-electrode symmetrical supercapacitor device, that is, “ZIF-67/rGO//ZIF-67/rGO” was first studied for its working potential window. The related CV curves, recorded at a scan rate of 100 mV/s, for different potential windows (from 0 to +1.5 V) are shown in Figure a. A potential range of 0–1.5 V was found suitable to obtain maximum current from the device; hence, those conditions were maintained in all further studies carried out on the device. CV curves (investigated at different scan rates, e.g., 5–50 mV s–1) followed identical patterns and maintained their shape (Figure b). However, the best current values were achievable with a scan rate of 5 mV s–1.

Figure 9

Electrochemical performance of the ZIF-67/rGO//ZIF-67/rGO symmetric supercapacitor device in 0.2 M K3[Fe(CN)6] + 1 M Na2SO4 RAE. (a) CV curves for different potential windows, (b) CV curves at different scan rates, (c) GCD curves at different current densities, and (d) variation of specific capacitance as a function of current density.

Figure c shows the GCD curves for the symmetrical supercapacitor device at various current densities (3–10 A g–1). A longest discharging time was found for a current density of 3 A g–1. The device parameters (specific capacitance, energy density, and power density) have been calculated according to eqs S3–S5″. The maximum specific capacitance calculated from the GCD curves was 326 F g–1 at a current density of 3 A g–1, which is an excellent value for a full symmetrical supercapacitor cell. The variation of specific capacitance as a function of current density is also shown in Figure d. Here also better values of specific capacitance were observed at lower current densities. The estimated values of energy and power densities at different current densities are indicated in Figure a. The Ragone plot has been used to evaluate the “ZIF-67/rGO//ZIF-67/rGO” symmetrical supercapacitor for its maximum energy density, which was found to be 25.5 W h kg–1 (at a power density of 2.7 kW kg–1). This significant high value of energy density reflects an excellent performance of the proposed device. The favorable material characteristics of the ZIF-67/rGO electrode along with enhanced redox behavior from the chosen RAE might be accounted for an enhanced performance of the present supercapacitor design.

Figure 10

(a) Ragone plot, (b) capacity retention of the “ZIF-67/rGO//ZIF-67/rGO” symmetrical supercapacitor device during successive 1000 charge–discharge cycles.

The cyclic stability of the symmetrical supercapacitor device was also assessed (Figure b). The device retained 88.8% of its initial specific capacity after 1000 continuous charge–discharge cycles. Therefore, the ZIF-67/rGO-redox system could be projected as a practically viable supercapacitor design with better performance than most of the previously reported MOF-based supercapacitors.

Two symmetrical supercapacitors were joined in a series configuration and then tested to power a white LED (2.5 V, 1.8 mA, 45 mW). The joining of two symmetrical supercapacitors helped to increase the overall potential of the device. The white LED remained illuminated for at least 3 min. This practical demonstration of the supercapacitor’s efficiency is a vital indicator of potentiality of the ZIF-67/rGO composite to explore this composite material in future energy storage devices.

Conclusions

The present research has demonstrated the application of a ZIF-67/rGO composite as an efficient supercapacitor electrode material. This supercapacitor design has been found to work much more efficiently in the presence of a RAE [0.2 M K3(Fe(CN)6) + 1 M Na2SO4] than the simple aqueous electrolyte. The ZIF-67/rGO composite was prepared from aqueous medium by a simple stirring-based one-pot method. The resulting ZIF-67/rGO composite was characterized with a large specific surface area and good electrochemical performance. The porosity of the material allowed it to behave as an ion reservoir with short diffusion pathways, thereby enabling rapid transport of ions. The synergistic enhancements in the composite’s properties facilitated its application as a supercapacitor electrode to attain high specific capacitance (∼1453 F g–1 at a current density of 4.5 A g–1) with an excellent cyclic stability (∼90.5% even after performing 1000 cycles of charging–discharging). We have also demonstrated the assembly of a symmetrical supercapacitor device using two ZIF-67/rGO electrodes. The operation of the symmetrical supercapacitor in the presence of RAE delivered a high energy density of 25.5 W h kg–1 at a power density of 2.7 kW kg–1. The device also showed a fair level of cyclic stability (∼88.8%) even after 1000 charge–discharge cycles. A series of two symmetrical devices could illuminate a white LED for at least 3 min. To conclude, it can be outlined that a combination of high surface to volume ratio, desirable porous properties, and high electrical conductivity make the ZIF-67/rGO composite as an attractive material to be explored in high efficiency supercapacitor electrodes. The use of RAE is useful for further enhancing the electrochemical performance of the MOF-based supercapacitor electrodes.

Materials and Methods

Materials

All different chemicals/materials used in this study were of analytical or high purity grade and used without any further purification. Graphite powder and cobalt nitrate hexahydrate (Co(NO3)2·6H2O) were purchased from Sigma-Aldrich, India. Potassium permanganate (KMnO4), hydrochloric acid (HCl), sulfuric acid (H2SO4), hydrogen peroxide (H2O2), potassium ferricyanide (K3[Fe(CN)6]), and sodium sulfate (Na2SO4) were purchased from Merck, India. Polyvinylidene fluoride (PVDF), 2-methylimidazole, and L-ascorbic acid were purchased from Himedia, India.

Synthesis of GO and rGO

GO was synthesized using graphite powder as the starting material (modified Hummers method).[38] Briefly, graphite powder was mixed with KMnO4 in a weight ratio of 1:6 (i.e., 1.5 g graphite + 9 g KMnO4). A mixture of H2SO4 (180 mL) and H3PO4 (20 mL) was added to the abovementioned contents causing an exothermic reaction that raised the temperature of the reaction mixture to about 35–40 °C. The temperature was raised further (by heating) to about 50 °C after which the contents were stirred well for another 2 h. Next, the reaction mixture was cooled to the RT (25 ± 2 °C) and then placed in an ice bath before adding 1.5 mL of 30% H2O2. It was followed by another round of stirring (30 min). Subsequently, the reaction mixture was centrifuged (8000 rpm, 15 min) to remove impurities such as amorphous carbon and acid content. For further purification, the recovered product was washed by centrifugation in the presence of deionized water, 30% HCl, and ethanol in that order. The above synthesized and purified GO was dried at 80 °C. The product was manually crushed to form a fine powder before preserving it at RT conditions.[38]

For some comparative studies, rGO was synthesized using ascorbic acid as the reducing agent. Briefly, 1 mg mL–1 aqueous dispersion of GO was mixed with 500 mg of L-ascorbic acid, and the resulting mixture was left to stir for 1 h maintaining a solution temperature of 95 °C. After the reaction, the settled black powder was recovered by centrifugation at 8000 rpm (15 min). The sample was washed multiple times with deionized water and then left to dry in an oven (80 °C, 12 h).

Synthesis of ZIF-67/rGO Composite

Cobalt nitrate hexahydrate (450 mg) and GO (45 mg) were dispersed separately in 3 and 20 mL of deionized water, respectively. The GO sample was sonicated for 2 h to get a clear dispersion, followed by the addition of 5.5 g of 2-methylimidazole (linker). The metal ion solution was then added to the mixture of GO + linker, and the resulting reaction contents were left to stir for 6 h at RT. After the completion of the reaction, the composite of ZIF-67/rGO was recovered by centrifugation. The product was washed with deionized water (twice) and methanol (once) before drying it under vacuum conditions (80 °C, 12 h).

Material Characterization and Equipment

XRD patterns were collected using an X-ray diffractometer ( Bruker, D8 ADVANCE; scan speed = 4° min–1, Cu Kα radiation wavelength = 1.54060 Å). Raman spectra were recorded on a Renishaw (inVia) system by exciting the sample with a 532 nm laser source. Fourier transform infrared (FTIR) spectroscopy measurements were carried out by a FTIR spectrometer (Nicolet iS10; scan rate of 2.5 cm s–1). Brunauer–Emmett–Teller surface area measurements were performed on a BELSORP-max system from Microtrac. The ionic conductivity of different concentrations of K3[Fe(CN)6] in 1 M Na2SO4 has been evaluated using a portable handheld ionic conductivity meter (model no. Lovibond sense-direct CON-200). Morphology/microstructural analysis were investigated with field emission scanning electron microscopy (FESEM, Hitachi S4300; applied voltage = 5–7 kV) and TEM (Technai G20; accelerating voltage = 200 KV). The electrochemical measurements were performed with the PGSTAT 302N instrument (Autolab).

Electrochemical Testing

Different electrochemical measurements, including CV, GCD, and EIS, were carried out with a three-electrode system which consisted of a platinum wire (Pt) as counter, Ag/AgCl as reference, and the active material over the grafoil substrate as working electrodes.

The working electrode was prepared by mixing the active material (ZIF-67/rGO composite) with carbon black and polyvinylidene fluoride (PVDF) in a w/w ratio of 8:1:1. A slurry of this mixture was formed in the copresence of N-methyl pyrrolidone. The slurry was then uniformly coated over a carbon paper (grafoil). Thus, the obtained electrode paper was left to dry in an oven (80 °C) for 12 h. As determined by the weight analysis, the active mass ratio of the coated material over the electrode was 2.58 mg cm–2 for redox electrolyte measurements in three-electrode measurements.

The electrochemical studies in three-electrode systems were carried out exploring two types of electrolytes, namely, 0.2 M K3[Fe(CN)6] in 1 M Na2SO4 (RAE) and 1 M Na2SO4 (aqueous electrolyte). CV curves were collected at various scan rates (100, 50, 20, 10, and 5 mV s–1) over a potential range of −0.1 to 0.5 V. GCD measurements were also performed at various current densities (e.g., 8.5, 7, 6, 5.5, and 4.5 A/g) setting the cutoff potential in the range of −0.1 to 0.5 V (vs the Ag/AgCl reference electrode). EIS measurements were performed over a frequency range of 0.1 Hz to 100 kHz maintaining an alternating current amplitude of 10 mV.