CoFe2O4 Hollow Spheres-Decorated Three-Dimensional rGO Sponge for Highly Efficient Electrochemical Charge Storage Devices.

The energy demand, the crisis of fossil fuels, and the increasing popularity of portable and wearable electronics in the global market have triggered the demand to develop high-performance flexible all-solid-state supercapacitors that are capable of delivering high energy at high power density as well as being safely entrenched in those electronics. Herein, we have designed a nanocomposite, 80CFhs-20rGOsp, which exhibits a high specific capacitance (C S) value of 1032 F g-1 at 3 A g-1. Utilizing this nanocomposite as the cathode and reduced graphene oxide sponge (rGOsp) as the anode, a flexible all-solid-state asymmetric device has been fabricated. In this device, poly(vinyl alcohol) (PVA) gel embedded with a mixture of 3 M KOH and 0.1 M K4[Fe(CN)6] was used as an electrolyte cum separator. The fabricated device showed the capability to deliver an energy density of 65.8 W h kg-1 at a power density of 1500 W kg-1 and retained its capability even after various physical deformations. The device also exhibited a long cycle life and retained ∼96% of its C S value after 5000 cycles. Moreover, the fabricated flexible all-solid-state device successfully illuminated light-emitting diodes, which proved its potential use in real-life supercapacitor applications. The obtained results revealed the excellent electrochemical performances of the fabricated device and rendered it a promising candidate in the energy sector.

Introduction

With the exponential increase in the world population and technological advancement, the demand for energy is increasing extensively, which is causing a rapid depletion of fossil fuels (i.e., coal, petroleum, and natural gas). Moreover, the environmental pollution occurring due to the extensive use of these fossil fuel-based energies is creating a lot of environmental awareness that is triggering the replacement of these energy sources with alternative sustainable and eco-friendly energy sources.[1,2] The natural renewable sources of energy, like solar cells, wind turbines, and hydro energy forms (waves, tides, and current), etc., are intermittent in energy production as these are dependent on the climate, seasons, or unpredictable weather conditions. In this scenario, the storage of these energies has become highly essential to deliver an uninterrupted energy supply as well as to achieve the massive global energy demand, which is increasing every year by ∼3.6%.[3] Among the electrochemical energy storage (EES) devices, batteries and supercapacitors (SCs) have emerged as global leaders in the energy sector owing to their superior characteristics. Currently, batteries (lithium-ion battery (LIB), Zn air, Na-batteries, etc.) are widely used owing to their considerable efficiency and high energy density (100–240 W h kg–1).[3] However, the processing cost and safety issues of batteries are also high due to the generation of heat and formation of dendrites at high power, which can cause very serious accidents (fire accidents and failure of Dreamliner airplanes and Tesla cars).[3,4] In contrast, supercapacitors exhibit very fast charge–discharge mechanisms (1–10 s), exceptionally long cycle life (0.5–1 million cycles), high power density, and they are eco-friendly and cheap as well as can be maintained at low cost.[3,5] All of these features make supercapacitors more favorable than batteries and widen their potential applications in light rail, beacon light, heavy-duty vehicles, storage and release of the regenerative braking energy of electric vehicles (EV), solar energy warning lights, wind power generation, charging products, current backup power supply, load-leveling systems, etc. For example, an Airbus 380 used supercapacitors in the emergency doors; supercapacitors are used to operate the public transport vehicles in China (Capabus, since 2006) as the only power source.[3,6]

Today, in this modern civilization, new technologies are emerging every day, and recently, portable electronics with foldable displays, flexible sensors, and wearable electronics have revolutionized the market of electronics.[7] The soaring popularity of these technologies is greatly accelerating the demand to develop flexible all-solid-state supercapacitor devices, and many researchers have reported various flexible devices to date.[8−11] To fulfill all of the prospects of these new technologies, the developed devices must possess the following properties: (i) Flexibility: the device must be capable of operating in various physical deformities (bent, folded, stretched, twisted, etc.). (ii) Capacity: the device should be capable of storing and delivering a large amount of energy. (iii) Durability: the device should be capable of operating for a long period of time. Lastly, (iv) the device should be light-weighted, safe to handle and compatible with human skin, etc.[9,12,13] Several supercapacitor companies (such as Nippon Chemicon (Japan), Nesscap (Korea), ELTON (Russia), CAP-XX (Australia), etc.) are developing various types of advanced supercapacitors for applications in diverse fields.[4] However, most of the commercially available supercapacitors exhibit a very low energy density (∼5–10 W h kg–1), which limits their full-blown usage in the energy sector.[3,14] So, in recent years, researchers from all over the world have been devoting enormous efforts to develop supercapacitors with high energy and power density to overcome this limitation.

To develop high-performance supercapacitor devices, the selection of suitable electrode materials is one of the critical issues. Depending upon the charge storage mechanisms, the active electrode materials of the supercapacitors can mainly be classified as electric double-layer capacitance (EDLC) materials and pseudocapacitive materials. The double-layer charge storage mechanism is the basic principle of the EDLC materials (for example, activated carbon, porous carbon, etc.). EDLCs offer tremendous power performance and cycle life.[2,4] However, their applications are restricted due to their low energy density. The charge storage mechanism in pseudocapacitive materials comprises the redox reactions at the electrode surface. Compared to EDLC materials, pseudocapacitive materials (e.g., metal oxides and hydroxides, conducting polymers, etc.) generally manifest a significantly higher capacitance and energy.[4,6] However, due to the low electrical conductivity and lethargic ion diffusion of metal oxides and poor mechanical stability of the polymers during electrochemical reactions, pseudocapacitive materials often suffer from a low power performance, short cycle life, and mechanical deformations.[2,4] Therefore, to achieve active electrode materials with the pertinent energy content, power performance, and relatively long cycle life, the design of novel materials that can exhibit the combined beneficial properties of respective EDLC and pseudocapacitive materials are the need of today’s energy storage and harvesting technology.

The performance of a supercapacitor device largely depends on its operating voltage. The stable operating potential range of the electrolyte, the standard voltage range of each electrode, and the passivation layer formed at the electrode/electrolyte junction play important roles in determining the operating voltage of the SCs.[7,15] Supercapacitors can be broadly classified as symmetric supercapacitors and asymmetric supercapacitors (ASCs). A symmetric device possesses the same active material as the positive as well as the negative electrodes with an equivalent mass loading. Both the electrodes of symmetric supercapacitors are generally built of EDLC-based materials or materials having an amalgamation of EDLC and Faradaic features. The conventional aqueous-based symmetric supercapacitors cover a narrow potential range that can be extended up to the standard decomposition voltage of water (i.e., ∼1.2 V). These factors limit the electrochemical performances of the symmetric supercapacitor devices.[7,15] To overcome the limitations of the symmetric supercapacitor devices, the development of ASC devices is an enthralling approach. ASC devices are composed of two distinctive electrode active materials, i.e., one EDLC and one pseudocapacitive, well-adjusted in the same electrolyte. ASCs can cover a wider operating voltage range (for example, ∼2.0 V in aqueous electrolytes, ∼2.7 V in the organic solvents, and ∼4 V in ionic liquids),[4,16] which causes the significant enhancement of the energy density compared to the symmetric SC devices in the aqueous electrolyte medium. The high energy density, high power density, good rate capability, high CS, and long cycle life make ASCs prominent candidates in energy harvesting and power grid applications.[2,4] Keeping these points in mind, we have developed a nanocomposite comprising CoFe2O4 hollow spheres (CFhs) and reduced graphene oxide sponge (rGOsp) to use as the active electrode materials for the fabrication of an all-solid-state high-performing flexible ASC device.

In the present study, reduced graphene oxide (rGO) has been chosen as one of the components of the nanocomposites because of its astounding electrical conductivity, mechanical strength, large surface area, good chemical stability, etc. However, two-dimensional (2-D) rGO sheets can easily be agglomerated as a result of the π–π interaction and van der Waals force of attraction among the sheets.[17,18] These factors disrupt the electrolyte ion diffusion, reduce the accessible surface area, and hence subsequently reduce its electrochemical properties. This limitation can be diminished to some extent by forming a three-dimensional (3-D) rGO sponge in which the presence of pores enhances the properties such as specific surface area, ion absorption–desorption capacity, etc.[18−20] The fluffy porous rGOsp is capable of storing more charges than that of rGO having a nanosheet structure. Moreover, the fluffiness lowers the agglomeration of the rGO sheets. The agglomeration can be further prevented by inserting nanoparticles within the porous sheets, which act as spacers as well as enhance the electrochemical properties by reducing the ion migration path.[17,21]

Here, CoFe2O4 has been chosen as another component of the nanocomposite because it possesses excellent thermal and chemical stability, high theoretical specific capacitance (CS) value (916 mA h g–1), and energy density.[22] It can be easily synthesized from highly abundant materials, which makes it economical. In the inverse spinel structure of CoFe2O4, tetrahedral and octahedral sites are occupied by Co2+ and Fe3+ ions, respectively, and the presence of the metal ions with multiple oxidation states enhances its redox activity. However, poor intrinsic electrical conductivity, prone to agglomeration during the electrochemical processes, restricts its use in high-performance SCs.[22,23] To address these limitations, two approaches have been taken here: (i) synthesis of CoFe2O4 hollow spheres, so that during the electrochemical processes, the space available inside the sphere can accommodate a significant amount of electrolyte ions which can enhance the redox reaction occurring at the electrode–electrolyte interface, and (ii) immobilization of the CoFe2O4 hollow spheres within the porous structure of rGO sponge to enhance the electron transfer process during the electrochemical processes. Several researchers have reported CoFe2O4-rGO (or graphene or porous carbon)-based nanocomposites to use as electrode materials in the supercapacitor devices (Table S2).[22,24−29] However, to the best of our knowledge, the preparation of CoFe2O4 hollow sphere-rGO sponge (CFhs-rGOsp) nanocomposites and their extensive use as electrode materials for high-performance supercapacitor devices is not yet well explored.

We have prepared nanocomposites consisting of CFhs and rGOsp to create a synergistic effect that can be generated from the beneficial features of each component due to their intimate coexistence. This nanocomposite, having enhanced electrochemical properties, was used to assemble a high-performance flexible all-solid-state asymmetric supercapacitor device. Here, we have first synthesized graphene oxide (GO) from graphite powders using the modified Hummers’ method. Then, we have created a spongelike structure of GO by applying a lyophilization (freeze-drying) technique. Finally, the GO sponge (GOsp) was converted to rGOsp by reducing it with hydrazine hydrate. Separately, we have synthesized CFhs through a hydrothermal technique. We have prepared various CFhs-rGOsp nanocomposites employing a wet-impregnation method by varying the amount of CFhs and rGOsp in them. The steps involved in the synthesis of CFhs-rGOsp nanocomposites have been depicted in Scheme . In our previous studies, we have synthesized rGO with a nanosheet-like structure. In this study, we have performed the electrochemical measurements of these rGO nanosheets and rGOsp, and the comparative studies revealed that rGOsp exhibits properties superior to that of pure rGO nanosheets. Therefore, we have designed nanocomposites by immobilizing CFhs within rGOsp, so that the nanocomposites combine the benefits of both materials. CFhs offer rich Faradaic processes and subsequently fast power delivery, whereas rGOsp offers good rate capability, better cyclic stability, and mechanical tenacity to the CFhs-rGOsp nanocomposites. From the electrochemical measurements of the synthesized nanocomposites, it was confirmed that 80CFhs-20rGOsp exhibits pre-eminent electrochemical properties, and hence, we have fabricated an all-solid-state flexible supercapacitor device using the 80CFhs-20rGOsp nanocomposite as a cathode material. The fabricated flexible device exhibited a CS value of 210.6 F g–1 at 2 A g–1, ∼96% CS retention even after ∼5000 cycles, and has the potential to deliver an appreciable energy density of 65 W h kg–1 at a power density of 1500 W kg–1. Moreover, the fabricated flexible all-solid-state asymmetric supercapacitor device is capable of operating with the same efficiency even after numerous physical deformations.

Scheme 1

Schematic Representation of the Synthetic Procedure of CFhs-rGOsp Nanocomposites

Experimental Section

Preparation of the CFhs-rGOsp Nanocomposite

The complete synthetic route of CFhs-rGOsp is elucidated in Scheme . First, graphene oxide (GO) was prepared by the modified Hummers’ method, and then it was dispersed in water and freeze-dried for 3 days to obtain GOsp, which was reduced to rGOsp by heating at a temperature of 150 °C for 3 h in the presence of hydrazine hydrate. Then, CFhs were prepared by a hydrothermal route, where a stoichiometric mixture of CoCl2·6H2O and FeCl3·6H2O in ethylene glycol was heated at 200 °C for 22 h in the presence of poly(ethylene glycol)-2000 (PEG-2000) and sodium acetate. In the final step, the synthesized CF and rGO sponge were dispersed in methanol and refluxed for 3 h to obtain the CFhs-rGOsp nanocomposite. Various nanocomposites composed of different wt% of CFhs and rGOsp were prepared by differing the amount of the materials taken at the final step. Nanocomposites with 90, 80, and 70% of CFhs and 10, 20, and 30% rGOsp, respectively, were termed 90CFhs-10rGOsp, 80CFhs-20rGOsp, and 70CFhs-30rGOsp, respectively. The complete synthetic protocols of all of the materials and their characterizations are described in detail in the Supporting Information.

Electrochemical Measurements

Three-Electrode (3-E) Measurements

The electrochemical properties of the synthesized materials were scrutinized individually from the 3-E measurements of the working electrodes fabricated by these active electrode materials. A Hg/HgO double junction electrode and a platinum wire were used as the reference and the counter electrode, respectively, and nickel foam was used as the current collector in the working electrode. Two electrolyte systems, 3 M KOH and 3 M KOH + 0.1 M K4[Fe(CN)6], were used to study the electrolyte effect on the electrochemical properties of the active electrode materials. The details of electrode preparation are mentioned in the Supporting Information. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements were conducted to obtain a stable potential window, specific capacitance value, etc., and to optimize the nanocomposite with superior properties.

Fabrication of Two-Electrode Asymmetric Supercapacitor Devices

An asymmetric two-electrode cell was assembled using the optimum composition of the CFhs-rGOsp nanocomposites (80CFhs-20rGOsp) as the cathode, rGOsp as the anode, and separated with a Whatman filter paper soaked with a 3 M KOH + 0.1 M K4[Fe(CN)6] solution. The positive-to-negative electrode mass ratio (m+/m–) was determined from the charge balance theory (q+ = q–), and the related equations are provided in the Supporting Information (eqs S1 and S2). In this case, the m+/m– value was found to be ∼0.45. The CS value, power density, and energy density of the device were calculated using eqs S3–S5. From the CV and GCD measurements, the maximum working potential of the asymmetric cell was optimized (0–1.5 V), and this working window was used for the electrochemical measurements of the flexible supercapacitor device. The fabrication of the flexible supercapacitor device is explained in detail in the Supporting Information.

Results and Discussion

Formation and Structural Characterization of CFhs, rGOsp, and CFhs-rGOsp Nanocomposites

To comprehend the formation of a spongelike structure of GO due to the freeze-drying of normal GO, retention of this spongelike morphology after conversion of GO to rGO, presence of rGOsp in the nanocomposites, and the morphology of the prepared CFhs, the microstructures of the synthesized nanocomposites were investigated by field emission scanning electron microscopy (FESEM) technique. The FESEM micrograph clearly displays that the freeze-drying of normal GO (Figure a) created the spongelike structure in GO (Figure b). Figure c confirms that the rGO formed due to the reduction of GOsp also possesses the spongy porous microstructure. For comparison, the FESEM micrograph of rGO with a nanosheet-like structure, which was obtained from the direct reduction of GO (without freeze-drying treatment), is presented in Figure d. These two micrographs distinctly highlight the difference in the microstructure of these two types of rGO. In the later sections, we will discuss how this difference in microstructure affects their electrochemical properties. Figure e shows the FESEM micrograph of pure CFhs, which depicts that small spherical CF nanoparticles (average particle size of ∼15–20 nm) are agglomerated to form hollow spheres with diameters ranging from ∼80 to 130 nm (average diameter of ∼100 nm). The broken sphere (outlined with a red circle) in the FESEM micrograph clearly shows the hollow nature of the spherical-shaped formation of the CF nanoparticles. In the FESEM micrograph of CFhs-rGOsp nanocomposite (Figure f), it is clearly visible that the CFhs are extensively immobilized on the surface as well as inside the pores of rGOsp.

Figure 1

FESEM micrographs of (a) GO, (b) freeze-dried GO (GOsp), (c) rGO sponge, (d) rGO nanosheets, (e) CF hollow spheres, and (f) 80CFhs-20rGOsp.

In the high-resolution transmission electron microscopy (HRTEM) micrograph of pure CFhs (Figure a), the light image contrast in the center indicates their hollow nature. The selected area electron diffraction (SAED) pattern of CFhs (Figure b) was indexed, which matches well with the Debye–Scherrer pattern of CoFe2O4 and also indicates its polycrystalline structure. The HRTEM micrograph of the 80CFhs-20rGOsp nanocomposite is presented in Figure c, which further confirms that CFhs are embedded on the carbon matrix of rGOsp. The lattice fringes corresponding to the (220) and (311) planes of CF nanoparticles are observed in the HRTEM micrograph of the nanocomposite (Figure d). The energy-dispersive X-ray spectroscopy (EDS) elemental mapping images (Figure e,f) showed a homogeneous spatial allocation of Co, Fe, O, and C elements in the nanocomposite. The EDS spectra of the nanocomposite (Figure S1) also manifested the presence of all of the abovementioned elements (C, O, Co, and Fe) in it.

Figure 2

(a) HRTEM micrograph. (b) SAED pattern of CFhs. (c, d) HRTEM micrographs of 80CFhs-20rGOsp, and (e, f) the EDS elemental mapping images of 80CFhs-20rGOsp.

The X-ray diffraction (XRD) patterns of CFhs, GOsp, rGOsp, and CFhs-rGOsp nanocomposites are presented in Figure a. In the XRD pattern of pure CFhs, diffraction peaks at 2θ values of 18.3, 30.1, 35.5, 37.03, 43.2, 53.6, 57.1, and 62.6° corresponding to the diffraction planes (111), (220), (311), (222), (400), (422), (511), and (440), respectively, were observed [JCPDS card no. 22-1086].[30,31] The presence of only peaks corresponding to pure CFhs indicated the formation of single-phase CFhs by the employed hydrothermal synthetic route. The XRD patterns of GO (Figure S2) and GOsp were identical to each other, which confirmed that no intrinsic structural changes occurred due to the formation of the spongelike nature. In the XRD pattern of GOsp, two diffraction peaks at 2θ values of 10.5 and 42.4° corresponding to the (001) and (101) diffraction planes were observed.[14,30] In the XRD pattern of rGOsp, the diffraction peak corresponding to the (001) plane of the GO disappeared and a new broad peak centered at a 2θ of ∼24.3° was observed that corresponds to the (002) diffraction plane, due to the reduction of functional groups containing oxygen in rGOsp.[32] The XRD pattern of rGOsp is also identical to that of rGO nanosheets (Figure S3). In the XRD patterns of all of the CFhs-rGOsp nanocomposites, the diffraction peaks that are characteristic of CFhs were observed, suggesting the presence of CFhs in the nanocomposites. Due to the low diffraction intensity and amorphous nature of rGOsp and the presence of its relatively less wt % in the nanocomposite, the diffraction peaks corresponding to rGOsp did not appear in the XRD patterns of CFhs-rGOsp nanocomposites,[33,34] but FESEM, HRTEM, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and Raman spectroscopy verified the presence of rGOsp in the nanocomposites.

Figure 3

(a) XRD patterns of (i) GO sponge, (ii) rGO sponge, (iii) 70CFhs-30rGOsp, (iv) 80CFhs-20rGOsp, (v) 90CFhs-10rGOsp, and (vi) pure CFhs. (b) Raman spectra of (i) rGO sponge, (ii) CFhs, and (iii) 80CFhs-20rGOsp. (c) TGA curves of (i) GO sponge, (ii) rGO sponge, (iii) 70CFhs-30rGOsp, (iv) 80CFhs-20rGOsp, (v) 90CFhs-10rGOsp, and (vi) pure CFhs.

FTIR spectra of GOsp, rGOsp, CFhs, and the 80CFhs-20rGOsp nanocomposite are presented in Figure S4. In the FTIR spectrum of GOsp, the peaks at 1045 cm–1 (C–O stretching), 1224 cm–1 (stretching vibrations of C–O of the epoxy group), 1364 cm–1 (stretching vibrations of C–O of the COOH group), 1620 cm–1 (due to the presence of graphitic domains), and 1716 cm–1 (C=O stretching vibrations) along with a broad peak centering at ∼3300 cm–1 corresponding to O–H stretching were observed.[35] In the FTIR spectra of rGOsp, the peaks at 1716 and 3300 cm–1 disappeared, and a notable decrease in the intensities of the peaks at 1045, 1364, and 1224 cm–1 indicated the successful reduction of the functional groups of GOsp to form rGOsp.[14,30] Pure CFhs exhibited a peak at 542 cm–1, which corresponds to the vibration of M–O (M = Co2+, Fe3+) bonds at the lattice sites.[30,31] All of the characteristic peaks of CFhs and rGOsp were observed in the FTIR spectrum of 80CFhs-20rGOsp. Raman spectra of pure CFhs, pure rGOsp, and 80CFhs-20rGOsp are displayed in Figure b. In the Raman spectrum of pure rGOsp, two Raman peaks assigned as D (1348 cm–1) and G bands (1585 cm–1) were observed, which arise due to the presence of defective carbon atoms and the in-plane stretching of symmetric sp2-hybridized carbon atoms, respectively.[6,36] Pure CFhs showed two Raman peaks at 471 and 678 cm–1, which correspond to the T2g and A1g vibrational modes of Fe3+ and Co2+, respectively, in the octahedral and tetrahedral sites of CF.[24,31] All of the characteristic bands of pure CFhs and rGOsp were observed in the Raman spectrum of 80CFhs-20rGOsp, which confirms the coexistence of both materials in the nanocomposite.

To estimate the wt% of rGOsp present in the as-prepared nanocomposites, the thermogravimetric analysis (TGA) of the as-synthesized materials was performed. The thermograms of GOsp, rGOsp, CFhs, and CFhs-rGOsp nanocomposites in the temperature range of 35–750 °C are presented in Figure c. In these thermograms, the following were observed: (i) GOsp showed a loss of ∼14% weight in the 35–150 °C temperature range due to the loss of absorbed moisture content, ∼26% weight loss was observed between 150 and 250 °C as a consequence of the removal of functional groups containing oxygen, and finally complete decomposition occurred between 300 and 600 °C due to the oxidative decomposition of residual carbon atoms of GO.[30] (ii) rGOsp exhibited an ∼5% weight loss in the temperature range of 35–200 °C resulting from the loss of surface-adsorbed moisture, and then a single step weight loss was observed between 400 and 600 °C due to the total decomposition of carbon. (iii) CFhs were thermally stable in the entire temperature range (35–750 °C). (iv) In the thermograms of 90CFhs-10rGOsp, 80CFhs-20rGOsp, and 70CFhs-30rGOsp nanocomposites, ∼10, ∼20, and ∼30% weight loss was observed, respectively, because of the complete decomposition of rGOsp present in the nanocomposites, which confirms the formation of nanocomposites with the desired wt% of CFhs and rGOsp. As CFhs were thermally stable up to 750 °C, after the decomposition of the carbonaceous mass of the nanocomposites, the residue that remained was undecomposed CFhs.

The Brunauer–Emmett–Teller (BET) surface area and porosimetry analysis of 80CFhs-20rGOsp show that the nanocomposite possesses a BET surface area and a total pore volume of ∼87 m2 g–1 and 0.204 cm3 g–1, respectively. The N2 adsorption–desorption isotherm of 80CFhs-20rGOsp (Figure S5) clearly depicts a type IV isotherm with an H3 hysteresis loop indicating the presence of a pore network consisting of meso- and macropores in it.[37,38]

The oxidation states and the elemental compositions of the CFhs-rGOsp nanocomposite were studied with the help of X-ray photoelectron spectroscopy (XPS) analysis, and the full scan XPS survey spectrum obtained is depicted in Figure S6. The presence of all of the peaks corresponding to C (285 eV), O (530 eV), Fe (712, 726 eV), and Co (782 eV) indicate their presence in the nanocomposite. The high-resolution Co 2p spectrum (Figure a) is fitted with two main peaks corresponding to the spin–orbit splitting of Co 2p1/2 (796.8 eV) and Co 2p3/2 (781.2 eV) and their two shake-up satellite peaks (803.4 and 785.9 eV), which confirms the presence of Co(II).[27,39] In the deconvoluted Fe 2p spectrum, four main peaks (711.04, 713.5, 724.3, 727.1 eV) and two shake-up satellite peaks (719.5, 733.2 eV) were observed (Figure b). The peaks at 724.3 and 711.04 eV are due to the spin–orbit splitting of Fe 2p1/2 and Fe 2p3/2 orbitals of Fe(III) present in the octahedral sites, and the peaks at 727.1 and 713.5 eV are from the Fe 2p1/2 and Fe 2p3/2 orbitals of Fe(III) in the tetrahedral sites.[27,40]Figure c depicts the high-resolution XPS spectrum of O 1s, which is fitted with two peaks assigned to the metal–oxygen (M–O) bond (530.5 eV) in the lattice sites and oxygen-containing functional groups (532.7 eV).[23,27] The high-resolution C 1s spectrum (Figure d) is comprised of four main peaks, which are assigned to C–C (284.84 eV), C–O (286.61 eV), and O–C=O bonds (287.77 eV) and one π–π* shake-up satellite peak at 291.62 eV.[16,27,41]

Figure 4

High-resolution deconvoluted XPS spectra of (a) Co 2p, (b) Fe 2p, (c) O 1s, and (d) C 1s.

XRD, FESEM, HRTEM, EDS, FTIR, Raman spectroscopy, and XPS analysis confirm the formation of CFhs, rGOsp, and the presence of both CFhs and rGOsp in the nanocomposites. The formation of CFhs-rGOsp nanocomposites and the synthetic route used to prepare these nanocomposites are demonstrated in Scheme . The presence of sodium acetate (NaAc) and poly(ethylene glycol) (PEG) plays a pivotal role in the formation of CFhs. Nucleation and crystal growth are the two major steps in the formation of nanocrystals. NaAc accelerates the nucleation rate of CF nanocrystals in the hydrothermal reaction and thus prevents the formation of multiphase CFhs. NaAc also acts as a precipitating agent and helps in the formation of the morphology of the CFhs.[14,42] The presence of PEG influences the kinetics of the crystal growth and helps in the anisotropic growth of CFhs. The addition of PEG also causes the aggregation of CF nanoparticles to form the hollow spheres and maintains the uniformity in the size of the CFhs.[24,43] In the CFhs-rGOsp nanocomposites, CFhs are dispersed and anchored on the surface as well as pores of the rGOsp, which helps to retain the fluffiness of the material by constraining the restacking of the spongy rGO sheets.

Electrochemical Characterization of the Synthesized Materials and Fabrication of ASC Devices

The electrochemical properties of the synthesized materials were studied by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) by constructing a three-electrode (3-E) cell setup. (The details are provided in the Supporting Information.) The electrochemical properties of rGOsp were determined in the operational voltage of 0 to −1 V using an aqueous solution of 3 M KOH as the electrolyte. The electrochemical properties of rGOsp were compared with those of rGO having a nanosheet morphology. For both cases, the CV profiles were found to be quasi-rectangular-shaped (Figure S7a,c), and almost linear GCD curves (Figure S7b,d) were obtained, which indicated the EDLC nature of both rGO samples. The area under the CV curve of rGOsp was greater than that of the rGO nanosheet. Moreover, rGOsp exhibited a higher current response than that of the rGO nanosheet (Figure a). CS values, determined from the GCD curves, showed that rGOsp possesses much higher CS (291 F g–1 at 1 A g–1) compared to that of rGO nanosheets (156 F g–1 at 1 A g–1). This increase in the CS value of rGOsp compared to the pure rGO nanosheets might be due to the presence of 3-D macropores in the rGOsp, which assists in the fast flow of the electrolyte ions to the entire electrode surface by providing a shorter ion transfer path and thus enhancing the rate of electrochemical processes at the electrode–electrolyte interface. From electrochemical impedance spectroscopy (EIS) measurements (Figure S7e), it was observed that the RCT (2.33 Ω) and RS values (0.68 Ω) of rGOsp are much less than those of rGO nanosheets (RCT = 3.68 Ω, RS = 1.39 Ω), which indicated the higher conductivity and better charge transfer capability of rGOsp compared to rGO nanosheets.[14,36] All of these facts demonstrate that rGOsp possesses superior electrochemical properties than that of rGO nanosheets, and thus, it was used to synthesize the nanocomposites.

Figure 5

(a) CV profiles of rGO sponge and rGO nanosheets in 3 M KOH. (b) CV profiles of CFhs in both electrolyte systems. (c) CV profiles of synthesized CFhs-rGOsp nanocomposites in 3 M KOH + 0.1 M K4[Fe(CN)6]. (d) CV curves at numerous scanning rates and (e) GCD curves at various current densities for 80CFhs-20rGOsp in 3 M KOH + 0.1 M K4[Fe(CN)6]. (f) Nyquist plots of synthesized materials; the inset shows the EIS spectra of materials in the high-frequency region and the equivalent circuit used for fitting Nyquist plots. All of the measurements were conducted in a 3-E cell set-up.

In the CV measurements of CFhs in the 3 M KOH electrolyte, a stable cyclic voltammogram was obtained in the potential range of 0–0.55 V (Figure b), and a distinct pair of redox peaks were observed at ∼0.47/0.37 V at a scanning rate of 10 mV s–1, which confirms the pseudocapacitive nature of the CFhs. The origin of this redox pair can be attributed to the following reactions of CFhs in the KOH medium (eqs –3)[22,44]The CV profiles of pure CFhs in 3 M KOH obtained at various scan rates (10–100 mV s–1) are displayed in Figure S8a, which shows that the redox peaks were shifted toward more anodic and cathodic directions with the increment of scanning rate. This increase in the separation between the anodic and cathodic peaks with the increase of scanning rate suggests that the diffusion-controlled redox processes occur in the system.[14,45] The pseudocapacitive nature of CFhs can also be confirmed from its GCD curves (Figure S8b), which shows the nonlinear charging and discharging curves. The CS value of CFhs was 202 F g–1 at a current density of 1 A g–1, which was estimated from the GCD measurements. The CS values were gradually decreased to 125 F g–1 with the increase of current densities from 1 to 10 A g–1. The diffusion of electrolyte ions into the electrode surface is less at the higher current densities in comparison to the lower current densities because of the insufficient availability of time, and this results in the decrease of CS values at relatively higher current densities.[14]

In our previous studies, it had been demonstrated that the CS value of the nanocomposites was increased with the addition of K4[Fe(CN)6] to the aqueous KOH electrolyte system, and the same effect was also reported by other researchers.[6,7,14,36,46−49] An aqueous mixture of 0.1 M K4[Fe(CN)6] and 3 M KOH was used as the electrolyte in the electrochemical measurements of CFhs, and in this electrolyte system, CFhs showed a stable cyclic voltammogram in the potential window 0–0.55 V with a pair of redox peaks at ∼0.47/0.37 V at a scan rate of 10 mV s–1 (Figure b), which confirmed that the origin of these peaks is from the same set of redox reactions (eqs –3). In the 0.1 M K4[Fe(CN)6] + 3 M KOH electrolyte system, the occlusive area of pure CFhs at 10 mV s–1 was increased drastically in comparison to that in 3 M KOH. This increase in the occlusive area indicates the enhancement in the CS value of CFhs in the 0.1 M K4[Fe(CN)6] + 3 M KOH electrolyte system. The CV profiles at various scanning rates (10–100 mV s–1) and the GCD curves at various current densities (3–15 A g–1) of the CFhs in the 0.1 M K4[Fe(CN)6] + 3 M KOH electrolyte system are shown in Figure S8c,d, respectively. The CS value of CFhs in this electrolyte system was found to be 552 F g–1 at a current density of 3 A g–1, which is considerably higher than the value obtained when the electrolyte was 3 M KOH (169 F g–1 at 3 A g–1). RS and RCT values of CFhs were determined from the EIS measurements performed in the 3 M KOH and 3 M KOH + 0.1 M K4[Fe(CN)6] electrolyte systems (Figure f). Almost no change in RS values (∼0.44 Ω) was observed in the two electrolyte systems. However, the RCT obtained in 3 M KOH (19.3 Ω) was much higher than that obtained in the 3 M KOH + 0.1 M K4[Fe(CN)6] electrolyte system (9.2 Ω). These facts indicated that the addition of 0.1 M K4[Fe(CN)6] does not affect the internal resistance of the system but enhances the charge transfer process. The drastic enhancement in the electrochemical performances of CFhs due to the addition of K4[Fe(CN)6] to the electrolyte system can be justified from the following reasons: (i) the redox couple [Fe(CN)6]4–/[Fe(CN)6]3– has a redox potential (0.2/0.37 V) that is analogous to that of CFhs; thus it provides an auxiliary electron buffer system to the electrochemical reactions occurring at the electrode–electrolyte interface and subsequently enhances the rate of electrochemical processes; (ii) CFhs has a total pore volume of ∼0.2 cm3 g–1 [(Figure S9), N2 adsorption–desorption isotherm of multiple-point BET analysis] that can easily accommodate plenty of [Fe(CN)6]4–/[Fe(CN)6]3– redox ions having a Born radius of ∼0.4 nm.[14,46−48,50] As the rate of electrochemical processes in the cell was enhanced by the aforementioned factors, the CS value of the CFhs considerably increased in the 0.1 M K4[Fe(CN)6] + 3 M KOH electrolyte system. Hence, 0.1 M K4[Fe(CN)6] + 3 M KOH was used as the electrolyte for further electrochemical studies of the synthesized electrode materials (CFhs-rGOsp nanocomposites) and fabricated ASC devices.

To obtain a high-performance active electrode material, nanocomposites were prepared by varying the wt % of rGO sponge and CFhs in it, and then the electrochemical measurements of the as-synthesized nanocomposites were performed in a three-electrode cell setup using the 3 M KOH + 0.1 M K4[Fe(CN)6] electrolyte system. The CV profiles of CFhs-rGOsp nanocomposites showed a distinct pair of redox peaks that are analogous to that of pure CFhs (Figure S10a,c). The area under the curve in the CV profiles of the nanocomposites was larger than that of the pure CFhs (Figure c), indicating the increase in the CS value due to the addition of rGOsp in the nanocomposites. The GCD measurements of the CFhs-rGOsp nanocomposites (Figure S10b,d) confirmed that due to the addition of rGOsp to the CFhs, the CS value was increased from 552 F g–1 to 853.2 F g–1 (for 90CFhs-10rGOsp), 1032 F g–1 (for 80CFhs-20rGOsp), and 768 F g–1 (for 70CFhs-30rGOsp). Among these three nanocomposites, 80CFhs-20rGOsp has shown the largest occlusive area in its CV profile (Figure c), which also confirmed its highest CS value. The CV profiles at various scanning rates and GCD at various current densities of 80CFhs-20rGOsp are shown in Figure d,e.

The RCT and RS values of CFhs and CFhs-rGOsp nanocomposites were determined from EIS measurements (Table S1), and the Nyquist plots are presented in Figure f. The RCT value of CFhs-rGOsp nanocomposites was found to be lower than that of pure CFhs. For example, the RCT of 80CFhs-20rGOsp was 0.38 Ω. However, the presence of rGOsp in the nanocomposites did not affect their RS values (∼0.40 Ω). These facts indicate that the electrochemical processes of the nanocomposites are mainly driven by the charge transfer mechanism.[51] The Nyquist plot derived from the EIS data of 80CFhs-20rGOsp recorded after ∼5000 charge–discharge cycles is presented in Figure S11. It has been observed that the RS (0.39 Ω) and RCT (0.39 Ω) values of 80CFhs-20rGOsp remain almost the same even after ∼5000 GCD cycles, which indicates the stability of the nanocomposite. The amplified electrochemical properties of the CFhs-rGOsp nanocomposites originated from the combination of the following factors: (i) a synergistic effect is established between the CFhs and rGOsp due to their intimate coexistence in the nanocomposites. (ii) The nanocomposite combines both Faradaic (from CFhs) and non-Faradaic (from rGOsp) charge storing characters. (iii) In the nanocomposites, CFhs act as spacers and prevent the spongy rGO sheets from restacking and also help to maintain their fluffiness.[14,17] (iv) The presence of rGOsp helps in the diffusion of electrolyte ions in a shorter time through its channels in the nanocomposites.[52] The plausible electron transfer and ion adsorption–desorption processes in the electrode–electrolyte interface are illustrated in Scheme .[14,53] The presence of rGOsp also provides mechanical robustness to the electrode. From the results obtained from CV, GCD, and EIS measurements, it can be concluded that 80CFhs-20rGOsp demonstrates superior electrochemical performances among the synthesized electrode materials, and thus, this nanocomposite was used as a cathode material to fabricate the ASC devices.

Scheme 2

Plausible (a) Electronic Conduction and (b, c) Ion Adsorption–Desorption by CFhs-rGOsp Nanocomposites

To assess the potential usage of the synthesized 80CFhs-20rGOsp nanocomposite in real-life supercapacitor application, a two-electrode ASC cell was constructed using rGOsp as the anode and 80CFhs-20rGOsp nanocomposite as the cathode (80CFhs-20rGOsp//rGOsp), and the electrodes were separated with a Whatman filter paper soaked in a 3 M KOH + 0.1 M K4[Fe(CN)6] solution. The details of the fabrication of the ASC cell and the equations used are given in the Supporting Information.

Figure a shows the stable voltammograms of rGOsp and 80CFhs-20rGOsp at a scanning rate of 10 mV s–1 in the potential window of −1 to 0 V and 0–0.55 V, respectively, which were obtained from the 3-E cell measurements. With the help of these CV profiles, the extended working potential window for this asymmetric cell was estimated. Figure b shows the CV profiles of 80CFhs-20rGOsp//rGOsp in different operating potentials, which indicates that the potential window for the ASC can be protracted up to a potential of 0 to 1.5 V. In the operating window beyond 1.5 V, a sudden increase in the current and an unwanted distortion in the peak was observed in the CV curve, which could be due to the evolution of oxygen from the thermodynamic breakdown of water.[7] Thus, an operational voltage of 0–1.5 V was used for further electrochemical investigations. The CV profile of the fabricated ASC (Figure c) depicts that even at a higher sweep rate of 100 mV s–1, the shape of the CV curve was well retained, indicating its good reversibility and rate capability.[7] Moreover, the GCD curves obtained at various current densities (2–12 A g–1) (Figure d) were almost symmetrical in shape, which suggests the good electrochemical reversibility and a well-balanced charge storage process in the ASC.[7,16] The CS value, calculated from the GCD measurements, was found to be 218.6 F g–1 at a current density of 1 A g–1, and at a higher current density of 12 A g–1, the fabricated ASC still retained a CS value of 128 F g–1. EIS measurements revealed that the RCT and RS values of this ASC device were 0.69 and 0.44 Ω, respectively, which are clearly displayed in the Nyquist plot (Figure e) of the device, suggesting its good ionic and electronic conductivity.[7,16] 80CFhs-20rGOsp//rGOsp ASC exhibited a high energy density of 68.3 W h kg–1 at a power density of 1500 W kg–1, and the Ragone plot, sketched from the obtained energy and power densities of the ASC, is shown in Figure f.

Figure 6

(a) CV curves of rGOsp and 80CFhs-20rGOsp in the 3-E cell. (b) CV curves at 10 mV s–1 in various operating potentials. (c) CV curves at various scan rates. (d) GCD curves at various current densities. (e) EIS curve and used equivalent circuit. (f) Ragone plot of the 80CFhs-20rGOsp//rGOsp ASC cell.

Excellent electrochemical performances of the 80CFhs-20rGOsp//rGOsp ASC cell encouraged us to construct an all-solid-state flexible supercapacitor device using the same positive and negative electrodes and to assess the potential usage of this device in flexible, portable, and wearable electronics. In this fabricated flexible ASC device, a mixture of the 3 M KOH + 0.1 M K4[Fe(CN)6] solution implanted in PVA gel was used as the separator between the cathode (80CFhs-20rGOsp) and the anode (rGOsp), as well as acted as the electrolyte in the device. The schematic representation of the fabricated all-solid-state flexible device is displayed in Figure a. The CV profiles of the fabricated device at various sweep rates (10–100 mV s–1) depicted a liver-shaped voltammogram, which is well retained even at higher scan rates (Figure b). The GCD curves of the device at various current densities (2–12 A g–1) are presented in Figure c, which shows approximately symmetrical, triangular-shaped charge–discharge curves, suggesting good electrochemical reversibility and outstanding capacitive nature of the device emanated from the coexistence of both Faradaic and non-Faradaic charge-storage mechanism.[7,16] The fabricated flexible device exhibited a CS value of 210.6 F g–1 at 2 A g–1, and the variation of the CS value and the change in the Coulombic efficiency with increasing current density is displayed in Figure S12. Figure S13 represents the Nyquist plot obtained from the EIS data of the device, which showed the impedance curve with a low intercept on the X-axis and a prominent semicircle arc having a very small diameter at the high-frequency range and a sharp slope in the inclined portion of the curve, suggesting low internal resistance, good charge-transfer process, and proper diffusion of electrolyte ions in the device.[51,54] After circuit fitting, RS and RCT values of the device were found to be 0.81 and 1.04 Ω, respectively. Apart from the exemplary electrochemical performances, the fabricated all-solid-state flexible ASC device retained ∼96% of its CS value at a current density of 5 A g–1 even after ∼5000 cycles (Figure d). The structural and morphological integrity of the electrode material after the 5000 cycles of charging and discharging was investigated by XRD and FESEM (Figure S14). The absence of any significant changes in the crystallinity or morphology proves the superb structural stability of the nanocomposite.

Figure 7

(a) Diagrammatic representation. (b) CV curves at various scan rates. (c) GCD curves at various current densities. (d) cyclic stability test of the fabricated all-solid-state flexible supercapacitor device (inset: first 15 GCD cycles).

To demonstrate the flexibility and the mechanical tenacity of the fabricated all-solid-state device, the CV and GCD measurements of the device were carried out after bending it in different shapes. Figure a,b displays the CV and GCD curves of the device at different bending angles, respectively, which are nearly identical to each other, suggesting good mechanical tenacity and high flexibility of the fabricated device even after considerable physical deformations. A Ragone plot of the device was sketched from the obtained energy and power densities and compared with a few of the reported carbon-based ASC devices, as shown in Figure c. The fabricated device has demonstrated its potentiality to deliver a high energy density of 65.8 W h kg–1 at a power density of 1500 W kg–1, which is equivalent and, in various cases, superior to that of carbon-based ASC devices found in the literature (Table S2).[8,9,14−16,24,27−29,44,55−68]Figure d portrays the optical image of the panel of 65 light-emitting diodes (LEDs) illuminated by four fabricated devices connected in series and demonstrates the potential of the all-solid-state flexible 80CFhs-20rGOs//rGOsp ASC device to be used in electronic applications.

Figure 8

(a) Cyclic voltammograms. (b) GCD curves of the fabricated device at varied bending forms. (c) Ragone plot of the device in comparison to some of the already reported carbon-based ASCs. (d) Optical image of a panel of 65 LEDs illuminated by four devices connected in series.

Conclusions

In summary, we have successfully synthesized nanocomposites in which CFhs are anchored on the rGOsp and demonstrated their outstanding electrochemical properties. The 80CFhs-20rGOsp nanocomposite exhibited its pre-eminent characteristics with a high CS value of 1032 F g–1 at 3 A g–1, owing to the crucial synergy achieved between CFhs and the rGOsp in the nanocomposite. CFhs provide a high charge–discharge rate through a fast reversible Faradaic process, whereas the 3-D macroporous structure of rGOsp enhances the electrolyte ion absorption–desorption process that subsequently enhances the electrochemical performances of the nanocomposite. We have successfully constructed a high-performance flexible all-solid-state asymmetric supercapacitor device by utilizing 80CFhs-20rGOsp as the positive and rGOsp as the negative electrode, respectively, and PVA gel embedded with the 3 M KOH + 0.1 M K4Fe[(CN)6] mixture, functioning as both the electrolyte and separator. The fabricated device exhibited a high energy density of 65.8 W h kg–1 at a power density of 1500 W kg–1 and retained ∼96% of the CS value even after ∼5000 charging–discharging cycles. The device also illustrated its good mechanical stability by retaining its capability to perform after numerous physical distortions. The obtained electrochemical performances of the device were superior to some of the already reported carbon-based devices (Table S2). The outstandingly high energy and power delivery, high rate capability, excellent cycle life, good flexibility make it an attractive candidate to use in portable and wearable electronics as well as in other energy harvesting applications.