# Bimetallic CoMoO4 Nanosheets on Freestanding Nanofiber as Wearable Supercapacitors with Long-Term Stability

Authors: Ashwin Khadka; Edmund Samuel; Bhavana Joshi; Yong Il Kim; Ali Aldalbahi; Mohamed El-Newehy; Hae-Seok Lee; Sam S. Yoon
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Currently, lightweight wearable energy storage devices are in great demand owing to their use in wearable electronics and energy-efficient electric vehicles. Freestanding carbon nanofibers replace the need for metal substrates while providing a rapid electrical network owing to their excellent electrical properties. Bimetallic oxides with multivalent oxidation states facilitate the rapid transfer of electrolytic ions owing to efficient Faradaic reactions, thereby enhancing the overall energy storage capability. In this study, CoOx@CNF was derived from ZIF-67 (zeolitic-imidazolate framework) @PAN-2MI fibers that were stabilized in air at 280°C and then annealed in argon at 900°C. Subsequently, Co was seeded on the annealed CoOx@CNF and subjected to a hydrothermal process in sodium molybdate dihydrate solution to grow CoMoO4 nanosheets, eventually forming bimetallic CoMoO4@CNF. The concentration of sodium molybdate solution was varied to determine the optimal growth conditions for CoMoO4 nanosheets. The energy density of the optimal bimetallic CoMoO4@CNF sample was 166.5 μWh cm-2 at a power density of 200 μW cm-2; this represented a nearly twofold increase compared to that of the single metallic CoOx@CNF. Powering humidity sensors using only one CoMoO4@CNF supercapacitor was demonstrated. The optimal sample remained stable during long-term galvanostatic charge and discharge cycles (Ncyc=30,000) and retained 100% of its specific capacitance.


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## Body

## 1. Introduction

Supercapacitors are energy storage devices that are used in various fields of application such as healthcare devices, remote sensors, electric vehicles, and wearable electronics owing to their excellent power density, fast charging capability, and long lifetime [1–3]. The use of aqueous electrolytes in supercapacitors ensures their commercial viability and environmental safety [4]. The electrostatic charge accumulation of carbon materials and the capacitive Faradaic reaction of pseudocapacitive materials are the basic energy storage mechanisms of supercapacitors [5–7].Pandey et al. demonstrated that lightweight energy storage devices can significantly improve the energy consumption efficiency of an electric vehicle [8]. Therefore, the fabrication of electrodes without a metallic substrate is of great interest to the energy storage community. Electrospinning is a facile technique for meeting this demand [9–11]. Electrospun carbon nanofibers (CNFs) with superior electrical properties can be used as freestanding and lightweight supercapacitors without the use of metallic substrates [12]. Furthermore, CNFs have a high surface-to-volume ratio that promotes interfacial electrochemical activity between the active material and electrolytes [13, 14]. In addition, the physical properties of the CNFs can be easily tuned by controlling the electrospinning operating conditions. Moreover, CNFs can be decorated with functional materials through dip-coating and/or hydrothermal processes to confer significantly favorable effects on their energy storage capability [15–17].Carbon-based supercapacitors have low weight, high stability, and high surface-to-volume ratio; however, their energy density is limited by the intrinsic nature of the electric double layer (EDL) process [18]. Therefore, pseudocapacitive materials need to be supplemented to form EDL-pseudocapacitive hybrid composites comprising both carbon and pseudocapacitive materials. Such composites are capable of storing considerably large numbers of charges while facilitating rapid Faradaic reactions on pseudocapacitor surfaces [19–21]. The pathway of electrolytic ions would be lengthened if pseudocapacitive materials were embedded inside the CNF. In such a case, ion access to the pseudocapacitive material would be hindered, thereby slowing down Faradaic reactions. To avoid this issue, pseudocapacitive materials can be grown on the fiber surface to considerably reduce the ionic pathway and thus promote Faradaic reactions. Such decoration at the fiber surface, in particular with a 3D dodecahedral metal-organic framework, would significantly increase the electrochemical activity, leading to an excellent supercapacitor performance [22].The binder-free approach is often used to reduce the overall weight of supercapacitors by removing the binders [23–25]. Freestanding binder-free CNFs decorated with ZIF-67-derived dodecahedral CoOx have been used in supercapacitors [26, 27]. In other studies, the only decoration material used was cobalt oxide from ZIF-67, and the use of bimetallic materials would have resulted in superior supercapacitor performance [28, 29]. Bimetallic oxides with multivalent oxidation states facilitate the rapid transfer of electrolytic ions owing to the enhanced electron mobility between the two metals and efficient Faradaic reactions [23, 24, 30, 31]. For example, the addition of Mo with high oxidation states (i.e., Mo2+, Mo4+, and Mo6+) to Co (i.e., Co2+ and Co3+) enhances the Faradaic reactions at the electrochemically active sites. As a result, bimetallic oxides together with carbon are capable of storing a large number of electrolytic ions, thereby increasing the overall supercapacitor performance [32]. In addition, Co and Mo are abundant and inexpensive and thus have great commercial viability [33–35].Veerasubramani et al. reported a capacitance of 400 mF cm–2 using CoMoO4 and activated C for an asymmetric supercapacitor with an operating voltage of 1.5 V and 2 M KOH and 0.02 M K3Fe (CN)6 as the electrolyte [36]. CoMoO4 nanoplates were grown on a nickel foam substrate using a hydrothermal process. Galvanostatic charging and discharging with Ncyc=2000 were demonstrated in their study. Ahmad et al. [28] reported a capacitance of 1125 mF cm–2 at a scan rate of 2 mV s–1 for an rGO/CoMoO4 composite and demonstrated galvanostatic charging and discharging of Ncyc=1500 cycles. The rGO/CoMoO4 composites were prepared by a hydrothermal process, and their slurry was deposited on a carbon cloth [28]. Li et al. introduced a CoMoO4 hollow microplate array on Ni foam using an ion exchange process [37]. They reported a capacitance of 902 mF cm–2 at a current density of 2 mA cm–2. Further, capacitance retention of 95% was reported for Ncyc=5000 cycles. However, till date, bimetallic CoMoO4 has never been grown onto CNFs and used as a freestanding and binder-free supercapacitor, and its flexibility or wearability has never been studied.Herein, we demonstrate the fabrication of freestanding, binder-free, lightweight, and highly flexible CNFs decorated with bimetallic CoMoO4 nanosheets for use in supercapacitors. Electrospun PAN-2MI fibers were impregnated with ZIF-67, stabilized in air at 280°C, and then annealed in argon at 900°C; Co was additionally seeded on the annealed Co-decorated CNF and subjected to a hydrothermal process inside sodium molybdate dihydrate to grow CoMoO4 nanosheets, eventually forming CoMoO4@CNF. The concentration of the molybdate solution was varied to determine the optimal fabrication conditions. The flexibility of the fabricated supercapacitor was tested in various bending modes. Long-term galvanostatic charging and discharging cycles at a high current density were also presented. The powering of LEDs and humidity and temperature sensors was demonstrated in a severe bending mode.


## 2. Experimental Procedure

### 2.1. Chemicals

Polyacrylonitrile (PAN;Mw=150 kDa; Sigma Aldrich, USA), 2-methylimidazole (2MI; 99%; Sigma Aldrich, USA), and N,N-dimethyl formamide (DMF; anhydrous 99.8%; Sigma Aldrich, USA) were used for electrospinning PAN-2MI fibers. Cobalt (II) nitrate hexahydrate (CoNt; ≥98%; Sigma Aldrich, USA), 2-methylimidazole (2MI; 99%; Sigma Aldrich, USA), and HPLC-grade methanol (Honeywell, USA) were used for the synthesis of ZIF-67. Polyvinylpyrrolidone (PVP; Mw=40 kDa; Sigma Aldrich, USA) was used to control the particle size during ZIF-67 formation. Sodium molybdate dihydrate (SMD; ≥99%; Sigma Aldrich, USA), cobalt (II) nitrate hexahydrate, and sodium sulfate (Sigma Aldrich, USA) were used for the synthesis of CoMoO4 nanosheets.


### 2.2. Synthesis of CoMoO4 on CNF

#### 2.2.1. ZIF-67 on PAN-2MI Nanofibers

PAN-2MI fibers were prepared by electrospinning a solution of PAN (10 wt.%) and 2MI (1 wt.%) mixed in DMF (80°C for 24 h) over a circular drum by applying a voltage of 10 kV for 5 h at 30°C and 30% humidity (Figure1). ZIF-67 was deposited by dip-coating 6×3 cm electrospun PAN-2MI fibers in a solution of CoNt (12.5 mM), 2MI (50 mM), and 180 mL methanol for 1.5 h, and PVP (0.6 g) was added to tune the size of ZIF-67. The fibers were then washed with methanol, dried for 12 h at room temperature, and dried for 12 h at 70°C.Figure 1
Schematic of decoration of CoMoO4 nanosheets onto CNF, and SEM images corresponding to various stages of fibers with a scale bar of 2 μm. Symmetric supercapacitor cell assembly and its wearable application.


#### 2.2.2. CoOx@CNF

Porous CoOx@CNF was prepared by consecutive carbonization of ZIF67@PAN-2MI fibers at 900°C for 1 h in argon atmosphere after stabilization at 280°C in air for 30 min (Figure 1).


#### 2.2.3. CoMoO4 Nanosheets on CoOx@CNF

CoOx@CNF electrodes were used to deposit CoMoO4 sheets. First, CoOx@CNF was soaked in 10 mM CoNt solution for 1 h and dried at 70°C for 1 h to activate; it assisted in the growth of nanosheets. A 100 mL solution of CoNt (5 mM) was prepared with the addition of SMD (5, 10, and 20 mM separately), and sodium sulfate (10 mM) was added to slow down the deposition process. The solution was stirred for 1 h. CoMoO4 nanosheets (Figure 1) were grown over the dried CoOx@CNF by placing them in a Teflon-lined stainless steel autoclave with hydrothermal solution prepared above for 6 h at 120°C. When the autoclave was cooled, the sheets loaded with CNF were taken out, washed with DI water, and dried at 70°C for 12 h to obtain three different electrode samples with 5, 10, and 20 mM SMD with bare CoOx@CNF being a 0 mM sample. The samples prepared with different concentrations of SMD (5, 10, and 20 mM) are hereafter referred as CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF, respectively.


### 2.3. Characterization Techniques

Transmission electron microscopy (TEM) with electron-dispersive X-ray spectroscopy (JEOL-2100F, JEOL) was used to analyze the morphology and perform elemental mapping of CoMo10mMO4@CNF. The morphology of all samples was analyzed by scanning electron microscopy (SEM; S-5000, Hitachi Ltd., Japan). The crystal structure of CoOx@CNF and CoMo10mMO4@CNF was studied by X-ray diffraction (XRD) analysis (SmartLab, Rigaku) over the 2θ range of 8°–80° at a scan rate of 0.5°/min using Cu Kα radiation. Raman spectroscopic analysis (LabRam ARAMIS IR2, HORIBA JOBIN YVON) was performed using a 532 nm excitation laser to examine the molecular vibrational modes of CoOx@CNF and CoMo10mMO4@CNF. Finally, the chemical states of the samples were determined by X-ray photoelectron spectroscopy (XPS; X-TOOL, ULVAC-PHI).


### 2.4. Electrochemical Characterization

Electrochemical measurements were performed on symmetric supercapacitors (SSCs) with two typical cell assemblies—coin cells and pouch cells. To make a coin cell, two circular electrodes with a 14 mm diameter were prepared by punching freestanding electrodes of each case (0, 5, 10, and 20 mM of SMD). Celgard 3501 (Celgard, Chungbuk, South Korea) polymer film was punched to 16 mm diameter size for use as a separator between two electrodes, and a few drops of 6 M KOH aqueous electrolyte was added. The coin cell was assembled within a CR2032 cell casing to prepare a symmetric supercapacitor for each case. Impedance analysis was performed using electrochemical impedance spectroscopy (EIS) of the coin cell at room temperature using a potentiostat (VersaSTAT-3, Princeton Applied Research, USA) over a frequency range of 100 kHz to 0.1 Hz. Cyclic voltammetry (CV) analysis was performed at various scan rates using a potentiostat (VersaSTAT-3, Princeton Applied Research, USA). Galvanostatic charge–discharge (GCD) measurements were performed using a WBCS3000 battery cycler (WonATech, Seoul, South Korea) at 25°C to examine the electrochemical performance of the prepared cells.The pouch cell for the best-case electrode (10 mM) was cut into dimensions of3cm×1.5cm. A Celgard 3501 separator was sandwiched between two electrodes to electrically insulate them, and an appropriate amount of 6 M KOH electrolyte was added. A nickel foil was cut and connected to each electrode using silver paste to serve as a current collector. The symmetric supercapacitor device thus fabricated was then sealed with Scotch tape. The CV test of the pouch cell was carried out under various bending angles and bending cycles. The flexibility and wearability of the fabricated supercapacitor for practical applications were tested by powering a humidity and temperature sensor, as shown in Figure 1.The areal capacitance of each electrode,Cs (mF cm–2), was calculated by substituting the discharge time obtained from GCD in [38, 39]
(1)Cs=4IaΔtΔV.Here,I (A), a (cm–2), ΔV (V), and Δt (s) are the applied constant current, total area of electrodes used, potential window, and discharge time, respectively, during the GCD measurements. The areal energy density (E, Wh cm–2) and power density (P, W·cm–2) were, respectively, obtained using [40]
(2)E=12×3600CsΔV2,(3)P=3600ΔtE,where Δt (s) and ΔV (V) are the discharge time and potential window, respectively, obtained from the GCD measurements.


## 2.1. Chemicals

Polyacrylonitrile (PAN;Mw=150 kDa; Sigma Aldrich, USA), 2-methylimidazole (2MI; 99%; Sigma Aldrich, USA), and N,N-dimethyl formamide (DMF; anhydrous 99.8%; Sigma Aldrich, USA) were used for electrospinning PAN-2MI fibers. Cobalt (II) nitrate hexahydrate (CoNt; ≥98%; Sigma Aldrich, USA), 2-methylimidazole (2MI; 99%; Sigma Aldrich, USA), and HPLC-grade methanol (Honeywell, USA) were used for the synthesis of ZIF-67. Polyvinylpyrrolidone (PVP; Mw=40 kDa; Sigma Aldrich, USA) was used to control the particle size during ZIF-67 formation. Sodium molybdate dihydrate (SMD; ≥99%; Sigma Aldrich, USA), cobalt (II) nitrate hexahydrate, and sodium sulfate (Sigma Aldrich, USA) were used for the synthesis of CoMoO4 nanosheets.


## 2.2. Synthesis of CoMoO4 on CNF

### 2.2.1. ZIF-67 on PAN-2MI Nanofibers

PAN-2MI fibers were prepared by electrospinning a solution of PAN (10 wt.%) and 2MI (1 wt.%) mixed in DMF (80°C for 24 h) over a circular drum by applying a voltage of 10 kV for 5 h at 30°C and 30% humidity (Figure1). ZIF-67 was deposited by dip-coating 6×3 cm electrospun PAN-2MI fibers in a solution of CoNt (12.5 mM), 2MI (50 mM), and 180 mL methanol for 1.5 h, and PVP (0.6 g) was added to tune the size of ZIF-67. The fibers were then washed with methanol, dried for 12 h at room temperature, and dried for 12 h at 70°C.Figure 1
Schematic of decoration of CoMoO4 nanosheets onto CNF, and SEM images corresponding to various stages of fibers with a scale bar of 2 μm. Symmetric supercapacitor cell assembly and its wearable application.


### 2.2.2. CoOx@CNF

Porous CoOx@CNF was prepared by consecutive carbonization of ZIF67@PAN-2MI fibers at 900°C for 1 h in argon atmosphere after stabilization at 280°C in air for 30 min (Figure 1).


### 2.2.3. CoMoO4 Nanosheets on CoOx@CNF

CoOx@CNF electrodes were used to deposit CoMoO4 sheets. First, CoOx@CNF was soaked in 10 mM CoNt solution for 1 h and dried at 70°C for 1 h to activate; it assisted in the growth of nanosheets. A 100 mL solution of CoNt (5 mM) was prepared with the addition of SMD (5, 10, and 20 mM separately), and sodium sulfate (10 mM) was added to slow down the deposition process. The solution was stirred for 1 h. CoMoO4 nanosheets (Figure 1) were grown over the dried CoOx@CNF by placing them in a Teflon-lined stainless steel autoclave with hydrothermal solution prepared above for 6 h at 120°C. When the autoclave was cooled, the sheets loaded with CNF were taken out, washed with DI water, and dried at 70°C for 12 h to obtain three different electrode samples with 5, 10, and 20 mM SMD with bare CoOx@CNF being a 0 mM sample. The samples prepared with different concentrations of SMD (5, 10, and 20 mM) are hereafter referred as CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF, respectively.


## 2.2.1. ZIF-67 on PAN-2MI Nanofibers

PAN-2MI fibers were prepared by electrospinning a solution of PAN (10 wt.%) and 2MI (1 wt.%) mixed in DMF (80°C for 24 h) over a circular drum by applying a voltage of 10 kV for 5 h at 30°C and 30% humidity (Figure1). ZIF-67 was deposited by dip-coating 6×3 cm electrospun PAN-2MI fibers in a solution of CoNt (12.5 mM), 2MI (50 mM), and 180 mL methanol for 1.5 h, and PVP (0.6 g) was added to tune the size of ZIF-67. The fibers were then washed with methanol, dried for 12 h at room temperature, and dried for 12 h at 70°C.Figure 1
Schematic of decoration of CoMoO4 nanosheets onto CNF, and SEM images corresponding to various stages of fibers with a scale bar of 2 μm. Symmetric supercapacitor cell assembly and its wearable application.


## 2.2.2. CoOx@CNF

Porous CoOx@CNF was prepared by consecutive carbonization of ZIF67@PAN-2MI fibers at 900°C for 1 h in argon atmosphere after stabilization at 280°C in air for 30 min (Figure 1).


## 2.2.3. CoMoO4 Nanosheets on CoOx@CNF

CoOx@CNF electrodes were used to deposit CoMoO4 sheets. First, CoOx@CNF was soaked in 10 mM CoNt solution for 1 h and dried at 70°C for 1 h to activate; it assisted in the growth of nanosheets. A 100 mL solution of CoNt (5 mM) was prepared with the addition of SMD (5, 10, and 20 mM separately), and sodium sulfate (10 mM) was added to slow down the deposition process. The solution was stirred for 1 h. CoMoO4 nanosheets (Figure 1) were grown over the dried CoOx@CNF by placing them in a Teflon-lined stainless steel autoclave with hydrothermal solution prepared above for 6 h at 120°C. When the autoclave was cooled, the sheets loaded with CNF were taken out, washed with DI water, and dried at 70°C for 12 h to obtain three different electrode samples with 5, 10, and 20 mM SMD with bare CoOx@CNF being a 0 mM sample. The samples prepared with different concentrations of SMD (5, 10, and 20 mM) are hereafter referred as CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF, respectively.


## 2.3. Characterization Techniques

Transmission electron microscopy (TEM) with electron-dispersive X-ray spectroscopy (JEOL-2100F, JEOL) was used to analyze the morphology and perform elemental mapping of CoMo10mMO4@CNF. The morphology of all samples was analyzed by scanning electron microscopy (SEM; S-5000, Hitachi Ltd., Japan). The crystal structure of CoOx@CNF and CoMo10mMO4@CNF was studied by X-ray diffraction (XRD) analysis (SmartLab, Rigaku) over the 2θ range of 8°–80° at a scan rate of 0.5°/min using Cu Kα radiation. Raman spectroscopic analysis (LabRam ARAMIS IR2, HORIBA JOBIN YVON) was performed using a 532 nm excitation laser to examine the molecular vibrational modes of CoOx@CNF and CoMo10mMO4@CNF. Finally, the chemical states of the samples were determined by X-ray photoelectron spectroscopy (XPS; X-TOOL, ULVAC-PHI).


## 2.4. Electrochemical Characterization

Electrochemical measurements were performed on symmetric supercapacitors (SSCs) with two typical cell assemblies—coin cells and pouch cells. To make a coin cell, two circular electrodes with a 14 mm diameter were prepared by punching freestanding electrodes of each case (0, 5, 10, and 20 mM of SMD). Celgard 3501 (Celgard, Chungbuk, South Korea) polymer film was punched to 16 mm diameter size for use as a separator between two electrodes, and a few drops of 6 M KOH aqueous electrolyte was added. The coin cell was assembled within a CR2032 cell casing to prepare a symmetric supercapacitor for each case. Impedance analysis was performed using electrochemical impedance spectroscopy (EIS) of the coin cell at room temperature using a potentiostat (VersaSTAT-3, Princeton Applied Research, USA) over a frequency range of 100 kHz to 0.1 Hz. Cyclic voltammetry (CV) analysis was performed at various scan rates using a potentiostat (VersaSTAT-3, Princeton Applied Research, USA). Galvanostatic charge–discharge (GCD) measurements were performed using a WBCS3000 battery cycler (WonATech, Seoul, South Korea) at 25°C to examine the electrochemical performance of the prepared cells.The pouch cell for the best-case electrode (10 mM) was cut into dimensions of3cm×1.5cm. A Celgard 3501 separator was sandwiched between two electrodes to electrically insulate them, and an appropriate amount of 6 M KOH electrolyte was added. A nickel foil was cut and connected to each electrode using silver paste to serve as a current collector. The symmetric supercapacitor device thus fabricated was then sealed with Scotch tape. The CV test of the pouch cell was carried out under various bending angles and bending cycles. The flexibility and wearability of the fabricated supercapacitor for practical applications were tested by powering a humidity and temperature sensor, as shown in Figure 1.The areal capacitance of each electrode,Cs (mF cm–2), was calculated by substituting the discharge time obtained from GCD in [38, 39]
(1)Cs=4IaΔtΔV.Here,I (A), a (cm–2), ΔV (V), and Δt (s) are the applied constant current, total area of electrodes used, potential window, and discharge time, respectively, during the GCD measurements. The areal energy density (E, Wh cm–2) and power density (P, W·cm–2) were, respectively, obtained using [40]
(2)E=12×3600CsΔV2,(3)P=3600ΔtE,where Δt (s) and ΔV (V) are the discharge time and potential window, respectively, obtained from the GCD measurements.


## 3. Results and Discussion

### 3.1. Materials Characterization and Analysis

The crystallinities of CoOx@CNF and CoMo10mMO4@CNF (10 mM Mo) were confirmed using XRD. Figure 2(a) shows the XRD spectra of CoOx@CNF and CoMoO4@CNF. Markers indicate the peaks associated with C, Co, and CoMoO4 in Figure 2(a). The C peaks for CoOx@CNF and CoMo10mMO4@CNF were observed at 24.1° and were attributed to the (002) plane with d spacing of 0.36 nm. The higher value of d compared to that of graphite (0.34 nm, JCPDS 75-1621) is due to the presence of disordered C-C (sp3), N, O, and H [41]. The deconvolution of the broad peak associated with C for CoOx@CNF and CoMoO4 @CNF provided information about disordered and graphitic C, as shown in Figure S1A and S1B. The amounts of disordered C were 62.6% and 45.3% in CoOx@CNF and CoMoO4@CNF samples, respectively. The CoOx@CNF and CoMoO4@CNF samples showed 37.4% and 39.0% deconvolution of graphitic C, respectively [42]. Such a combination of graphitic and disordered C in CNF will be beneficial for faster electron transport and charge storage, respectively [43–45]. The PAN fiber decorated with carbonized ZIF-67 exhibited face-centered cubic (FCC) cobalt oxide (Co3O4) peaks corresponding to the (111), (400), and (220) planes at 19°, 44°, and 75.9° (JCPDS 76-1802), respectively [26]. The synthesis of CoMoO4 nanosheets was confirmed by the presence of peaks at 9.0°, 18.6°, 27.8°, 33.5°, and 58.9° for the CoMo10mMO4@CNF samples [46]. These peaks are confirmed from JCPDS cards 14-0587 and 25-1434, corresponding to the α and β phases of CoMoO4, respectively [47].Figure 2
(a) XRD and (b) RAMAN spectrum of annealed CoOx@CNF and CoMo10mMO4@CNF samples.
(a)(b)Raman spectroscopy was performed to study the vibrational bands of the compounds present in the samples. The results indicate the presence of vibrational modes associated with the space group C2/m of CoMoO4 and Fd3m of cobalt oxide (Figure 2(b)). The broad peak centered at 351 cm-1 was due to the combination of the symmetric and asymmetric torsional modes of O-Mo-O, and the peaks at 812, 871, and 933 cm–1 were assigned to the symmetric and asymmetric stretching modes of O-Mo-O [19, 48]. The peaks designated by A1g, Eg, and three F2g mode vibrations linked with the Fd3m space group of the cobalt oxide structure are present in each spectrum. In case of CoOx@CNF, the cobalt oxide is derived from ZIF-67 through annealing. However, the presence of cobalt oxide in CoMoO4@CNF could be due to an excess amount of Co seeding and ZIF-67, as shown in Figure 2(b) [49, 50]. The Raman spectrum of CoOx@CNF suggests the existence of cobalt oxide; this is consistent with the XRD results. In addition, A1g of CoMo10mMO4@CNF shows a red shift, as is clear in Figure 2(b); it is attributed to the influence of the symmetric stretching of the Co-Mo-O bond [48]. The higher wavenumber region shows C-related D and G bands that can be attributed to disordered and graphitic C structures of the CNF, respectively, at approximately 1350 and 1590 cm–1, as depicted in Figure S1C [51–53]. The ID/IG ratios were calculated to be 1.22 and 1.29 for CoOx@CNF and CoMo10mMO4@CNF, respectively. The minor change in the ID/IG ratio of CoOx@CNF and CoMoO4@CNF indicates that the ultrathin CoMoO4 nanosheet protected the CNF.Figure3(a) shows the survey spectra of the CoOx@CNF and CoMo10mMO4@CNF samples. The survey spectra confirmed the presence of C, O, and Co in the CoOx@CNF sample and C, O, Co, and Mo in the CoMo10mMO4@CNF sample. The chemical states of Co, C, and Mo are presented in Figures 3(b)–3(d) (CoMo10mMO4@CNF sample), and the corresponding elemental core spectra of O is shown in Figure S2A. A detailed analysis of the chemical states of the elements was carried out by deconvoluting the core spectra using the XPSpeak41 software. The main peaks of Co 2p3/2/Co 2p1/2 were observed at 778.7/794 and 780.3/796 eV with a spin-orbit splitting of 15.3 and 15.7 eV for CoOx@CNF and CoMo10mMO4@CNF samples, respectively. After deconvolution, the Co 2p core spectrum of CoMo10mMO4@CNF (Figure 3(b)) shows doublet splitting of Co 2p3/2 and 2p1/2 with peaks at 782.2 and 798.9 eV corresponding to the +2 oxidation state and those at 780.1 and 796.4 eV corresponding to the +3 oxidation state, respectively. In addition, Co 2p exhibits satellite peaks at 785.9 and 802.8 eV, confirming the +2 oxidation state of Co in the CoMo10mMO4@CNF sample [36, 54].Figure 3
(a) XPS survey spectrum of CoOx@CNF and CoMo10mMO4@CNF. Deconvoluted narrow XPS spectra of (b) Co 2p, (c) C 1s, and (d) Mo 3d for CoMo10mMO4@CNF.
(a)(b)(c)(d)For the CoOx@CNF sample (Figure S2B), the peak at 778/794 eV in the deconvoluted Co 2p3/2 and 2p1/2 peaks clearly indicates the presence of metallic Co, which is consistent with the XRD results. For the CoMo10mMO4@CNF sample, no metallic Co was observed (see Figure 3(b)). Furthermore, a spin orbit doublet at 781.6 and 798.4 eV corresponds to the +2 oxidation state of Co. Notably, the satellite peaks present at 785.9 and 802.8 in the Co 2p core spectrum of the CoOx@CNF sample have relatively lower intensities (Figure S2B).The 3d5/2 peak of Mo on deconvolution shows peaks at 231.1 and 232.1 eV, corresponding to the Mo5+ and Mo6+ oxidation states, respectively. Similarly, the deconvoluted peaks of 3d3/2 at 234.2 and 235.5 eV correspond to the Mo5+ and Mo6+ oxidation states, respectively, as shown in Figure 3(d) [55, 56]. Upon deconvolution, the C 1s peaks for both samples (CoMo10mMO4@CNF and CoOx@CNF) showed three significant peaks related to sp3, sp2, and C=O at 284, 285, and 288 eV, respectively (Figure 3(c) and S2D) [57]. In addition, the O 1s peak of O can be deconvoluted into three peaks, namely, C-O, C=O, and metal oxides related to Co and Mo, as shown in Figure S2A and S2C [26].The SEM images in Figure4(a) show the morphology of dodecahedral CoOx decorated on the CNF. The nanosheets of cobalt molybdate oxide were grown onto CoOx@CNF through a hydrothermal process (Figures 4(b)–4(d)). The cobalt molybdate oxide nanosheet was engineered for optimal morphology by varying the Mo salt concentration up to 20 mM. The low concentration of Mo salt (mM) in the precursor solution was not sufficient for the growth of the nanosheets to decorate the CNF. The undergrown nanosheets were easily visible in the high-magnification SEM image (Figure 4(b)). The low-magnification SEM images indicate several undergrown CoMoO4 nanosheets (Figure 4(b)). Thus, the SMD concentration (10 mM) was increased, and the decoration of the entire CoOx@CNF surface by nanosheets was observed in the low- and high-magnification SEM images, as shown in Figure 4(c). The undergrowth of the CoMoO4 nanosheets was due to the less available molybdate anion for 5 mM SMD. Increasing the SMD concentration to 10 mM provided a larger number of molybdate anions, resulting in the excellent decoration of CoMoO4 nanosheets on the CNF. The nanosheet decoration over the CNF could be helpful for enhancing the electron transfer rate owing to their ultrathin cross-sectional width. Furthermore, when the SMD was increased to 20 mM in the hydrothermal precursor solution, excessive growth of CoMoO4 nanosheets occurred on CNF and in voids between CNFs (see Figure 4(d)). The SEM images at various magnifications (Figure 4(d)) clearly indicate that the width of the CoMoO4 nanosheet-decorated CNF increased significantly. The CoMoO4 nanosheets results suggest that the electrochemical kinetics of the pseudocapacitive material can be improved to provide an electrochemical characteristic signature similar to that of EDLC [58, 59].Figure 4
SEM images of (a) CoOx@CNF, (b) CoMo5mMO4@CNF, (c) CoMo10mMO4@CNF, and (d) CoMo20mMO4@CNF samples from high to low magnification moving from left to right.
(a)(b)(c)(d)Elemental and crystallographic analyses were carried out using TEM (see Figure5) for the optimal sample CoMo10mMO4@CNF. Figure 5(a) shows a high-magnification TEM image indicating the growth of CoMoO4 on the CNF. The SAED pattern presented in Figure 5(b) confirms the presence of the (002), (111), and (3¯52) crystallographic planes associated with C, Co, and CoMoO4, respectively. The HR-TEM result in Figure 5(c) shows lattice fringes marked by red, yellow, and green squares that are, respectively, well resolved in Figures 5(d)–5(f) with d spacing values of 0.36, 0.21, and 0.16 nm corresponding to graphitic C (002), FCC Co (111), and CoMoO4 (3¯52). The Gatan software was used to observe the HR-TEM images with better clarity, as presented in Figures 5(d)–5(f). The SAED pattern and HR-TEM analysis were consistent with the XRD analysis presented in Figure 2(a). The elemental mapping in Figures 5(g)–5(j) reflects the presence of C, Co, Mo, and O attributed to CoMo10mMO4@CNF; the line profiles of the elements are shown in Figure S3.Figure 5
(a) TEM image of a cross-sectional view of CoMo10mMO4@CNF. (b) SAED pattern and (c–f) HR-TEM results. (g) Elemental mapping of C, Co, Mo, and O.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)


### 3.2. Electrochemical Measurement and Analysis

CV is a widely used electrochemical performance characterization technique to obtain qualitative information on electrodes for the capacitive potential window of supercapacitors. The capacitive and possible resistive characteristic features of the CV curves can be studied by varying the scan rate and current response. Figure6(a) shows the CV curves of CoOx@CNF at varying scan rates. The CV curves are almost rectangular, indicating excellent EDLC capacitive contribution from the CNF and rapid capacitive Faradic reaction at the surface of the ZIF-67-derived CoOx. The CV curves exhibited excellent capacitive characteristics for a potential window range of 0–1 V and scan rates of 10–100 mV s–1. The quasi-rectangular CV curve for 100 mV s–1 could be attributed to the excellent electron transfer from ZIF-67-derived CoOx to CNF and the shortening of the distance of the electrolytic ion migration from the solution to the surface of CoOx and CNF.Figure 6
CV curves of (a) CoOx@CNF and (b) CoMo10mMO4@CNF SSC with varying scan rates. (c) EDL- and Faradaic-mechanism-based current contribution. Galvanostatic charge–discharge of (d) CoOx@CNF and (e) CoMo10mMO4@CNF with varying current densities. (f) Capacitance retention and Coulombic efficiency for 30,000 cycles. Inset shows the first and last three cycles. (g) Capacitance, (h) Ragone plot, and (i) Nyquist plot (inset showing closer view), of various samples.
(a)(b)(c)(d)(e)(f)(g)(h)(i)To enhance the electrochemical performance of CoOx@CNF, Mo was introduced owing to its multiple oxidation states and reversible redox activity. The CV curve of CoMo10mMO4@CNF in Figure 6(b) exhibits a higher current response than that of CoOx@CNF. However, the elliptical, and not the quasi-rectangular, shape of the CV curve indicates the slightly resistive characteristic of the CoMo10mMO4 nanosheets. Dense CoMo10mMO4 nanosheets increase the electron transfer path toward the CNF. The CV curves of the CoMo5mMO4@CNF-and CoMo20mMO4@CNF-based SSCs are presented in Figure S4A and S4B, respectively. All CV curves for each case exhibited no signs of redox peaks and a consistent current increase response corresponding to an increase in the scan rate. Thus, the CV curves indicate rapid Faradaic reactions for the storage of large charges [60]. Both morphologies (dodecahedral and nanosheets) are well suited for rapid Faradaic reactions owing to the enhanced electrode/electrolyte interfacial area. The CV curves of all cases reveal that the CoMo10mMO4@CNF-based SSC has a superior current response in the potential window of 0 to 1 V using a 6 M KOH aqueous electrolyte. The well-grown nanosheets and availability of a large number of electrochemically active sites in CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF improved the performance of these electrodes. Nevertheless, the presence of Mo in the composite electrode is clearly beneficial for boosting the electrochemical performance and storing more energy. The 10 mM concentration of Mo in the precursor solution for hydrothermal treatment is optimal for achieving the pertinent growth of CoMo10mMO4 nanosheets (cf. SEM images in Figure 4(c)) onto the CoOx@CNF and achieving a superior electrochemical performance [61, 62].When both pseudocapacitive and C materials are present in a supercapacitor, the total capacitance is attributed to the EDL capacitance and Faradaic capacitance. Therefore, the capacitance contribution of CoMo10mMO4@CNF from the EDL and Faradaic processes was studied (see Figure 6(c)). The Faradaic process is severe at low scan rates. Therefore, CV was performed with lower scan rates (0.5–2.5 mV s–1), as shown in Figure S5A. The peak current values of the CV curves at low scan rates are shown in Figure S5B. The plot (Figure S5B) reveals a slight nonlinear relation between the peak current values and square root of the scan rates, indicating the possibility of limited diffusion of electrolytic ions in the CoMo10mMO4 nanosheets and the dominant rapid Faradaic reaction at the surface of the nanosheets. Furthermore, the slow scan rate CV curves were used to plot a graph of logvvs. logI, as shown in Figure S5C; it is plotted using the current and scan rate relations:
(4)I=avb,where I (mA) and v (mV s-1) are the peak current and scan rate, respectively, and a and b are constants. As the logarithmic transformation of Equation (4) is analogous to the line equation, b corresponds to the slope of the fitted line of the logvvs. logI graph, which was obtained as 0.83. The expression 0.5<b<1 indicates that the electrodes have pseudocapacitive characteristics owing to the fast Faradaic reaction at the nanosheet surface. This study further extended the investigation of the EDL capacitance and Faradaic capacitance contribution by using [63]
(5)I=k1v1/2+k2v,where I and v denote the peak current and scan rate, respectively. Constants k1 and k2 are proportionality constants with the k1v1/2 pseudocapacitive current resulting from the Faradaic reaction and capacitive current due to EDL formation. The values of the constants were calculated from the linear fitting of Equation (5) using the observed data. In Figure 6(c), the current contribution in percentage from the EDL and Faradaic charge storage mechanism is estimated using the slow scan rate CV curves of symmetric supercapacitor cells using CoMo10mMO4@CNF freestanding electrodes. The current due to EDL increased from 41% to 61% when the CV scan rate increased from 0.5 to 2.5 mV s–1, respectively (see Figure 6(c)). In contrast, the current contribution due to diffusion was reduced; it could be attributed to a more significant amount of the electrolytic ion forming monolayers at the surface of the CoMo10mMO4 nanosheets. Therefore, the rapidly electrosorbed monolayer could enhance the EDL-like characteristics; subsequently, the CV curves exhibit a higher EDLC current at a higher scan rate.GCD data were acquired to calculate the capacitance and consequently determine the energy and power densities of the supercapacitor cells. Figures6(d) and 6(e) show the results for Co/CNF- and CoMo10mMO4@CNF-based symmetric supercapacitors, respectively. The GCD data showed triangular charging and discharging characteristics with almost >97% Coulombic efficiency. Similar characteristics were exhibited by the CoMo5mMO4@CNF and CoMo20mMO4@CNF electrode-based supercapacitor cells. All four cases exhibit excellent reversible characteristics at all current densities, as shown in Figure S6. The electrochemical performance (CV and GCD) of the EDLC/pseudocapacitive material composite (i.e., CoOx@CNF and CoMoO4@CNF) and the absence of sharp redox peaks confirm their capacitive characteristics. It should be noted that the pseudocapacitive material should exhibit EDLC-like electrochemical characteristics [58, 59]. For example, a Co-based pseudocapacitive material (i.e., Co3O4-PANI@ZIF-8NPC) did not exhibit any sharp redox peaks in the CV curves [50]. Similarly, a Ni-based pseudocapacitive materials (i.e., (Ni-Fe)-P-C@HCNFs) exhibited quasirectangular CV curves without any sharp redox peaks [9]. Both of these Co- and Ni-based materials were tested using three-electrode and asymmetric supercapacitor measurements.Although each electrode comprises carbon and a pseudocapacitive material, the capacitive characteristics were maintained in the CV and galvanostatic measurements. The pseudocapacitive material is known to store charges by the Faradaic process with the electrosorption or electrodesorption of a monolayer of reactive electrolyte species. As a result, the Faradaic reaction is much faster in supercapacitors/pseudocapacitors, and it yields a quasirectangular shape for CV curves and triangular GCD profiles for symmetric supercapacitors in all cases. Therefore, the pseudocapacitive materials exhibit characteristics similar to EDLC characteristics but unlike battery redox characteristics [58].A long-term stability test with 30,000 cycles for a CoMo10mMO4@CNF electrode-based supercapacitor was evaluated at a current density of 5 mA cm-2 in the potential window of 0–1 V. The excellent electrochemical stability of CoMo10mMO4@CNF is seen from the long-term stability results shown in Figure 6(f). The capacitance retention after 30,000 GCD cycles was determined to be almost 100%. The Coulombic efficiency of the galvanostatic measurements during long-term cycling is presented in Figure 6(f); it appears to be ~100% during all cycles. The low initial Coulombic efficiency could be attributed to lower pore openings during the first charging and discharging cycle. The inset in Figure 6(f) shows the first and last three GCD cycles; these overlap, indicating that no capacitance loss occurred. The excellent electrochemical stability could be attributed to the optimal growth of bimetallic (CoMo10mMO4) nanosheets and CNF. Owing to their morphology, the nanosheets quickly transfer electrons from particle-to-particle and particle-to-CNF. The highly annealed CNF exhibited excellent conductivity and thus was able to receive electrons for densely packed nanosheets of CoMo10mMO4.Figure6(g) shows the capacitance plot for various samples at current densities of 0.1–5 mA cm–2. The higher charging and discharging times for the CoMo10mMO4@CNF case resulted in higher capacitance values compared with those in the other cases (see Figure 6(g)). The maximum capacitance of 1199 mF cm–2 at 0.1 mA cm–2 within a potential window of 0–1 V is considerably better than the values reported in numerous studies (Table 1). Moreover, the CoMo10mMO4@CNF case exhibited a high current rate capability of 72% capacitance retention when the current density increased from 0.1 to 5 mA cm–2. The capacitance values estimated using Equation (1) at a current density of 0.1 mA cm–2 for CoOx@CNF, CoMo5mMO4@CNF, and CoMo20mMO4@CNF are 649, 789, and 872 mF cm–2, respectively. CoOx@CNF exhibited a moderate electrochemical performance compared to the other cases owing to the absence of Mo. With the introduction of Mo (CoMo5mMO4/CNF), the charge storage capability of the electrode increased, as shown in Figure 6(g). However, the morphology observed using SEM clearly indicates inadequate growth of the CoMO4 nanosheets (see Figure 4(b)). In contrast to CoMo5mMO4/CNF, the excess growth of CoMO4 nanosheets for CoMo20mMO4/CNF resulted in a longer electron transfer path from the surface of the electrode to the CNF (interior), resulting in inferior electrochemical performance compared to that of CoMo10mMO4/CNF. The direct growth of CoMoO4 nanosheets on CNF was random, as seen in Figure S7A–7C; it could have caused the relatively poor electrochemical performance compared to that of CoMo10mMO4/CNF (Figure S7D–7F).Table 1
Comparison of prepared CoMoO4@CNF supercapacitor with other Mo- and Co-based supercapacitors.
Electrode materialSubstrateCapacitance (mF cm-2)Current density (mA cm-2)ElectrolyteP.W. (ΔV) (V)Retention (%)Number of cyclesRef.MoS2/Co3O4Glassy carbon2210.140.5 M Na2SO40.987.010000[29]VN@C/CNTF//MNCO/CNTFCNT fibers233.71.0KOH/PVA1.690.23500[3]CoMoO4Nickel foam4002.5K3Fe(CN)61.5100.02000[36]MoS2/SWNT/CNFFreestanding4450.075PVA/H2SO4 gel0.891.010000[5]rGO/CoMoO4Carbon cloth11252 (mV/s)K3Fe(CN)60.898.01500[28]Co3O4@NiMoO4Nickel foam230013 M KOH0.5581.04000[2]CoMoO4@CNFFreestanding110616 M KOH1.5100.030000PresentThe energy and power densities of the CoOx@CNF, CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF electrodes are shown in Figure 6(h). The Ragone plots of the various samples show identical characteristics to those of the capacitance plot. For the CoMo10mMO4@CNF electrode-based symmetric supercapacitor cell, a maximum energy density of 166.5 μWh cm–2 was obtained at 200 μW cm–2, and a density of 119.4 μWh cm–2 was obtained at 10,000 μW cm–2. Furthermore, 0, 5, and 20 mM samples, respectively, had energy densities of 90.2, 109.6, and 121.2 μWh cm–2 at a power density of 200 μW cm–2 and 64.4, 77.2, and 92.8 μWh cm–2 at a power density of 10,000 μW cm–2. This extraordinary energy-to-power ratio verifies that CoMoO4@CNF is a potential material for energy storage in practical devices.EIS is useful for determining the charge-transfer capability of the samples. The Nyquist data for CoOx@CNF, CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF are presented in Figure 6(i). The Nyquist plots were fitted using the Randles equivalent circuit using the Zsimp software, as shown in Figure S8. The solution resistance (Rs) and charge-transfer resistance (Rct) were obtained by appropriately fitting each Nyquist plot. For CoOx@CNF, CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF, the Rs values were 2.7, 2.7, 2.2, and 2.3 Ω, respectively, and Rct values were 0.8, 0.7, 1.2, and 3.8 Ω, respectively. The relatively larger Rct value for the CoMo20mMO4@CNF composite can be attributed to the thicker CoMo20mMO4 decorated CNF.GCD is also useful for determining the operating potential window of the supercapacitor by slowly increasing the potential window until the triangular shape of the charging and discharging plot is maintained [38]. As indicated by Stoller et al., two-electrode testing gives a closer matching to that of packaged supercapacitors than three-electrode testing [39]. Therefore, a pouch-like symmetric supercapacitor was assembled using two electrodes of CoMoO4@CNF to determine the maximum possible operating potential window. The electrode in a pouch-like symmetric supercapacitor using CoMoO4@CNF can store charges in the form of a double layer through EDLC or a capacitive Faradaic reaction at the surface, which is beneficial for providing a wide potential window, as depicted in Figure 7(a). The pseudocapacitive material, as in this case, is a CoMoO4 nanosheet capable of the rapid adsorption and desorption of electrolytic ions. Simultaneously, a characteristic of CNF is to accumulate charges electrostatically. Thus, the layer of opposite charge on each electrode develops a potential, resulting in a supercapacitor with a wide potential window. This is because the layer of opposite charge at the surface of two electrodes in a symmetric supercapacitor acts as an individual capacitor having an equal amount of developed potential [27]. By using this specific feature of a symmetric supercapacitor, the GCD plot was determined by increasing the potential window from 1 V to 1.5 V. However, the GCD profile was distorted when the voltage window was further increased; as a result, the charging and discharging times were not triangular in shape. Therefore, we limited the voltage window measurement to 1.5 V. The increase in the potential window with stable electrochemical performance can be attributed to the presence of CoMoO4 nanosheets for a CoMoO4@CNF symmetric supercapacitor. In addition, the presence of CNF significantly provided excellent electrical conductivity and EDLC (see Figure 7(a)) to achieve stable electrochemical performance with capacitive characteristics even in the wide potential window range of 0–1.5 V. The triangular shapes of the charging and discharging times were consistent for each potential window, as shown in Figure 7(b). The overlapping of the charging times indicates the excellent stability and reversible Faradaic reaction capability of the electrodes. This is consistent with the excellent electrochemical performance stability for 30,000 galvanostatic charging and discharging cycles as shown in Figure 6(f).Figure 7
(a) Schematic representation of charge storage phenomena at C and pseudocapacitive material. (b) GCD and (c) Ragone plot for various potential windows. CV curve of CoMo10mMO4@CNF flexible pouch-like symmetric supercapacitor cell with varying (d) scan rates, (e) bending angles (inset is a snapshot with 10 mm bending of electrode), and (f) number of bending cycles. (g) LED and humidity gauge with thermometer device under (h) flat and (i) bending conditions are powered by the pouch-like supercapacitor.
(a)(b)(c)(d)(e)(f)(g)(h)(i)The electrode capacitance calculated using the discharge time from the GCD plot shown in Figure7(b) increased from 1054 to 1106 mF cm–2 at a current density of 1 mA cm-2 for CoMoO4@CNF. An increase in the potential window significantly increased the energy density, as shown in Figure 7(c). The energy density of CoMoO4@CNF increases from 146.3 to 345.5 μWh cm–2 for the corresponding power density of 2 to 3 mW cm–2, as shown in Figure 7(c). As the energy density is dependent on time as well as the square of the potential window, the increase in energy density is 236%. On the contrary, the power density is proportional only to the potential window, and it increases by 150% upon increasing the potential window from 1 to 1.5 V, as shown in Figure 7(c).A freestanding CoMo10mMO4/CNF pouch cell was fabricated and its flexibility was demonstrated. Initially, the CV curve of the pouch cell at various scan rates was acquired within a potential window of 0–1 V. The CV curves of the pouch cell presented in Figure 7(d) exhibit similar symmetric coin cell features (cf. Figure 6(b)) at all corresponding scan rates. Further, to investigate the bendability of the freestanding CoMo10mMO4-based supercapacitor, the CV curve of a pouch cell at 100 mV s–1 under bending angles of 0°, 45°, and 90° was evaluated. The CV curves with bending angles of 45° and 90° exhibited slight deviations from the CV curves without bending (see Figure 7(e)). The inset in Figure 7(e) is a snapshot of a pouch cell with a 10 mm bending radius.Furthermore, a cyclic bending test with an 8 mm bending radius was performed for 500 cycles using a cyclic bending machine, as shown in Figure7(f) and its inset. The supercapacitor pouch cell shows a very small distortion from the original curve without any significant difference, owing to its good mechanical strength during bending. The practical ability of the pouch cells to operate various devices was also tested, as shown in Figures 7(g)–7(i). The LED was powered by two pouch cells connected in series. A single pouch cell was sufficient to operate the humidity gauge with a thermometer, as shown in Figure 7(h). The humidity gauge operated even under the bent condition of a pouch-like supercapacitor cell (see Figure 7(i)). The excellent electrochemical performance of CoMo10mMO4 under severe conditions such as high current density, long-term cycling, and bending conditions suggests that CoMoO4 nanosheet-decorated CNF is an extraordinary capacitive material for use in consumer electronic devices and a step further for wearable electronic devices.


## 3.1. Materials Characterization and Analysis

The crystallinities of CoOx@CNF and CoMo10mMO4@CNF (10 mM Mo) were confirmed using XRD. Figure 2(a) shows the XRD spectra of CoOx@CNF and CoMoO4@CNF. Markers indicate the peaks associated with C, Co, and CoMoO4 in Figure 2(a). The C peaks for CoOx@CNF and CoMo10mMO4@CNF were observed at 24.1° and were attributed to the (002) plane with d spacing of 0.36 nm. The higher value of d compared to that of graphite (0.34 nm, JCPDS 75-1621) is due to the presence of disordered C-C (sp3), N, O, and H [41]. The deconvolution of the broad peak associated with C for CoOx@CNF and CoMoO4 @CNF provided information about disordered and graphitic C, as shown in Figure S1A and S1B. The amounts of disordered C were 62.6% and 45.3% in CoOx@CNF and CoMoO4@CNF samples, respectively. The CoOx@CNF and CoMoO4@CNF samples showed 37.4% and 39.0% deconvolution of graphitic C, respectively [42]. Such a combination of graphitic and disordered C in CNF will be beneficial for faster electron transport and charge storage, respectively [43–45]. The PAN fiber decorated with carbonized ZIF-67 exhibited face-centered cubic (FCC) cobalt oxide (Co3O4) peaks corresponding to the (111), (400), and (220) planes at 19°, 44°, and 75.9° (JCPDS 76-1802), respectively [26]. The synthesis of CoMoO4 nanosheets was confirmed by the presence of peaks at 9.0°, 18.6°, 27.8°, 33.5°, and 58.9° for the CoMo10mMO4@CNF samples [46]. These peaks are confirmed from JCPDS cards 14-0587 and 25-1434, corresponding to the α and β phases of CoMoO4, respectively [47].Figure 2
(a) XRD and (b) RAMAN spectrum of annealed CoOx@CNF and CoMo10mMO4@CNF samples.
(a)(b)Raman spectroscopy was performed to study the vibrational bands of the compounds present in the samples. The results indicate the presence of vibrational modes associated with the space group C2/m of CoMoO4 and Fd3m of cobalt oxide (Figure 2(b)). The broad peak centered at 351 cm-1 was due to the combination of the symmetric and asymmetric torsional modes of O-Mo-O, and the peaks at 812, 871, and 933 cm–1 were assigned to the symmetric and asymmetric stretching modes of O-Mo-O [19, 48]. The peaks designated by A1g, Eg, and three F2g mode vibrations linked with the Fd3m space group of the cobalt oxide structure are present in each spectrum. In case of CoOx@CNF, the cobalt oxide is derived from ZIF-67 through annealing. However, the presence of cobalt oxide in CoMoO4@CNF could be due to an excess amount of Co seeding and ZIF-67, as shown in Figure 2(b) [49, 50]. The Raman spectrum of CoOx@CNF suggests the existence of cobalt oxide; this is consistent with the XRD results. In addition, A1g of CoMo10mMO4@CNF shows a red shift, as is clear in Figure 2(b); it is attributed to the influence of the symmetric stretching of the Co-Mo-O bond [48]. The higher wavenumber region shows C-related D and G bands that can be attributed to disordered and graphitic C structures of the CNF, respectively, at approximately 1350 and 1590 cm–1, as depicted in Figure S1C [51–53]. The ID/IG ratios were calculated to be 1.22 and 1.29 for CoOx@CNF and CoMo10mMO4@CNF, respectively. The minor change in the ID/IG ratio of CoOx@CNF and CoMoO4@CNF indicates that the ultrathin CoMoO4 nanosheet protected the CNF.Figure3(a) shows the survey spectra of the CoOx@CNF and CoMo10mMO4@CNF samples. The survey spectra confirmed the presence of C, O, and Co in the CoOx@CNF sample and C, O, Co, and Mo in the CoMo10mMO4@CNF sample. The chemical states of Co, C, and Mo are presented in Figures 3(b)–3(d) (CoMo10mMO4@CNF sample), and the corresponding elemental core spectra of O is shown in Figure S2A. A detailed analysis of the chemical states of the elements was carried out by deconvoluting the core spectra using the XPSpeak41 software. The main peaks of Co 2p3/2/Co 2p1/2 were observed at 778.7/794 and 780.3/796 eV with a spin-orbit splitting of 15.3 and 15.7 eV for CoOx@CNF and CoMo10mMO4@CNF samples, respectively. After deconvolution, the Co 2p core spectrum of CoMo10mMO4@CNF (Figure 3(b)) shows doublet splitting of Co 2p3/2 and 2p1/2 with peaks at 782.2 and 798.9 eV corresponding to the +2 oxidation state and those at 780.1 and 796.4 eV corresponding to the +3 oxidation state, respectively. In addition, Co 2p exhibits satellite peaks at 785.9 and 802.8 eV, confirming the +2 oxidation state of Co in the CoMo10mMO4@CNF sample [36, 54].Figure 3
(a) XPS survey spectrum of CoOx@CNF and CoMo10mMO4@CNF. Deconvoluted narrow XPS spectra of (b) Co 2p, (c) C 1s, and (d) Mo 3d for CoMo10mMO4@CNF.
(a)(b)(c)(d)For the CoOx@CNF sample (Figure S2B), the peak at 778/794 eV in the deconvoluted Co 2p3/2 and 2p1/2 peaks clearly indicates the presence of metallic Co, which is consistent with the XRD results. For the CoMo10mMO4@CNF sample, no metallic Co was observed (see Figure 3(b)). Furthermore, a spin orbit doublet at 781.6 and 798.4 eV corresponds to the +2 oxidation state of Co. Notably, the satellite peaks present at 785.9 and 802.8 in the Co 2p core spectrum of the CoOx@CNF sample have relatively lower intensities (Figure S2B).The 3d5/2 peak of Mo on deconvolution shows peaks at 231.1 and 232.1 eV, corresponding to the Mo5+ and Mo6+ oxidation states, respectively. Similarly, the deconvoluted peaks of 3d3/2 at 234.2 and 235.5 eV correspond to the Mo5+ and Mo6+ oxidation states, respectively, as shown in Figure 3(d) [55, 56]. Upon deconvolution, the C 1s peaks for both samples (CoMo10mMO4@CNF and CoOx@CNF) showed three significant peaks related to sp3, sp2, and C=O at 284, 285, and 288 eV, respectively (Figure 3(c) and S2D) [57]. In addition, the O 1s peak of O can be deconvoluted into three peaks, namely, C-O, C=O, and metal oxides related to Co and Mo, as shown in Figure S2A and S2C [26].The SEM images in Figure4(a) show the morphology of dodecahedral CoOx decorated on the CNF. The nanosheets of cobalt molybdate oxide were grown onto CoOx@CNF through a hydrothermal process (Figures 4(b)–4(d)). The cobalt molybdate oxide nanosheet was engineered for optimal morphology by varying the Mo salt concentration up to 20 mM. The low concentration of Mo salt (mM) in the precursor solution was not sufficient for the growth of the nanosheets to decorate the CNF. The undergrown nanosheets were easily visible in the high-magnification SEM image (Figure 4(b)). The low-magnification SEM images indicate several undergrown CoMoO4 nanosheets (Figure 4(b)). Thus, the SMD concentration (10 mM) was increased, and the decoration of the entire CoOx@CNF surface by nanosheets was observed in the low- and high-magnification SEM images, as shown in Figure 4(c). The undergrowth of the CoMoO4 nanosheets was due to the less available molybdate anion for 5 mM SMD. Increasing the SMD concentration to 10 mM provided a larger number of molybdate anions, resulting in the excellent decoration of CoMoO4 nanosheets on the CNF. The nanosheet decoration over the CNF could be helpful for enhancing the electron transfer rate owing to their ultrathin cross-sectional width. Furthermore, when the SMD was increased to 20 mM in the hydrothermal precursor solution, excessive growth of CoMoO4 nanosheets occurred on CNF and in voids between CNFs (see Figure 4(d)). The SEM images at various magnifications (Figure 4(d)) clearly indicate that the width of the CoMoO4 nanosheet-decorated CNF increased significantly. The CoMoO4 nanosheets results suggest that the electrochemical kinetics of the pseudocapacitive material can be improved to provide an electrochemical characteristic signature similar to that of EDLC [58, 59].Figure 4
SEM images of (a) CoOx@CNF, (b) CoMo5mMO4@CNF, (c) CoMo10mMO4@CNF, and (d) CoMo20mMO4@CNF samples from high to low magnification moving from left to right.
(a)(b)(c)(d)Elemental and crystallographic analyses were carried out using TEM (see Figure5) for the optimal sample CoMo10mMO4@CNF. Figure 5(a) shows a high-magnification TEM image indicating the growth of CoMoO4 on the CNF. The SAED pattern presented in Figure 5(b) confirms the presence of the (002), (111), and (3¯52) crystallographic planes associated with C, Co, and CoMoO4, respectively. The HR-TEM result in Figure 5(c) shows lattice fringes marked by red, yellow, and green squares that are, respectively, well resolved in Figures 5(d)–5(f) with d spacing values of 0.36, 0.21, and 0.16 nm corresponding to graphitic C (002), FCC Co (111), and CoMoO4 (3¯52). The Gatan software was used to observe the HR-TEM images with better clarity, as presented in Figures 5(d)–5(f). The SAED pattern and HR-TEM analysis were consistent with the XRD analysis presented in Figure 2(a). The elemental mapping in Figures 5(g)–5(j) reflects the presence of C, Co, Mo, and O attributed to CoMo10mMO4@CNF; the line profiles of the elements are shown in Figure S3.Figure 5
(a) TEM image of a cross-sectional view of CoMo10mMO4@CNF. (b) SAED pattern and (c–f) HR-TEM results. (g) Elemental mapping of C, Co, Mo, and O.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)


## 3.2. Electrochemical Measurement and Analysis

CV is a widely used electrochemical performance characterization technique to obtain qualitative information on electrodes for the capacitive potential window of supercapacitors. The capacitive and possible resistive characteristic features of the CV curves can be studied by varying the scan rate and current response. Figure6(a) shows the CV curves of CoOx@CNF at varying scan rates. The CV curves are almost rectangular, indicating excellent EDLC capacitive contribution from the CNF and rapid capacitive Faradic reaction at the surface of the ZIF-67-derived CoOx. The CV curves exhibited excellent capacitive characteristics for a potential window range of 0–1 V and scan rates of 10–100 mV s–1. The quasi-rectangular CV curve for 100 mV s–1 could be attributed to the excellent electron transfer from ZIF-67-derived CoOx to CNF and the shortening of the distance of the electrolytic ion migration from the solution to the surface of CoOx and CNF.Figure 6
CV curves of (a) CoOx@CNF and (b) CoMo10mMO4@CNF SSC with varying scan rates. (c) EDL- and Faradaic-mechanism-based current contribution. Galvanostatic charge–discharge of (d) CoOx@CNF and (e) CoMo10mMO4@CNF with varying current densities. (f) Capacitance retention and Coulombic efficiency for 30,000 cycles. Inset shows the first and last three cycles. (g) Capacitance, (h) Ragone plot, and (i) Nyquist plot (inset showing closer view), of various samples.
(a)(b)(c)(d)(e)(f)(g)(h)(i)To enhance the electrochemical performance of CoOx@CNF, Mo was introduced owing to its multiple oxidation states and reversible redox activity. The CV curve of CoMo10mMO4@CNF in Figure 6(b) exhibits a higher current response than that of CoOx@CNF. However, the elliptical, and not the quasi-rectangular, shape of the CV curve indicates the slightly resistive characteristic of the CoMo10mMO4 nanosheets. Dense CoMo10mMO4 nanosheets increase the electron transfer path toward the CNF. The CV curves of the CoMo5mMO4@CNF-and CoMo20mMO4@CNF-based SSCs are presented in Figure S4A and S4B, respectively. All CV curves for each case exhibited no signs of redox peaks and a consistent current increase response corresponding to an increase in the scan rate. Thus, the CV curves indicate rapid Faradaic reactions for the storage of large charges [60]. Both morphologies (dodecahedral and nanosheets) are well suited for rapid Faradaic reactions owing to the enhanced electrode/electrolyte interfacial area. The CV curves of all cases reveal that the CoMo10mMO4@CNF-based SSC has a superior current response in the potential window of 0 to 1 V using a 6 M KOH aqueous electrolyte. The well-grown nanosheets and availability of a large number of electrochemically active sites in CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF improved the performance of these electrodes. Nevertheless, the presence of Mo in the composite electrode is clearly beneficial for boosting the electrochemical performance and storing more energy. The 10 mM concentration of Mo in the precursor solution for hydrothermal treatment is optimal for achieving the pertinent growth of CoMo10mMO4 nanosheets (cf. SEM images in Figure 4(c)) onto the CoOx@CNF and achieving a superior electrochemical performance [61, 62].When both pseudocapacitive and C materials are present in a supercapacitor, the total capacitance is attributed to the EDL capacitance and Faradaic capacitance. Therefore, the capacitance contribution of CoMo10mMO4@CNF from the EDL and Faradaic processes was studied (see Figure 6(c)). The Faradaic process is severe at low scan rates. Therefore, CV was performed with lower scan rates (0.5–2.5 mV s–1), as shown in Figure S5A. The peak current values of the CV curves at low scan rates are shown in Figure S5B. The plot (Figure S5B) reveals a slight nonlinear relation between the peak current values and square root of the scan rates, indicating the possibility of limited diffusion of electrolytic ions in the CoMo10mMO4 nanosheets and the dominant rapid Faradaic reaction at the surface of the nanosheets. Furthermore, the slow scan rate CV curves were used to plot a graph of logvvs. logI, as shown in Figure S5C; it is plotted using the current and scan rate relations:
(4)I=avb,where I (mA) and v (mV s-1) are the peak current and scan rate, respectively, and a and b are constants. As the logarithmic transformation of Equation (4) is analogous to the line equation, b corresponds to the slope of the fitted line of the logvvs. logI graph, which was obtained as 0.83. The expression 0.5<b<1 indicates that the electrodes have pseudocapacitive characteristics owing to the fast Faradaic reaction at the nanosheet surface. This study further extended the investigation of the EDL capacitance and Faradaic capacitance contribution by using [63]
(5)I=k1v1/2+k2v,where I and v denote the peak current and scan rate, respectively. Constants k1 and k2 are proportionality constants with the k1v1/2 pseudocapacitive current resulting from the Faradaic reaction and capacitive current due to EDL formation. The values of the constants were calculated from the linear fitting of Equation (5) using the observed data. In Figure 6(c), the current contribution in percentage from the EDL and Faradaic charge storage mechanism is estimated using the slow scan rate CV curves of symmetric supercapacitor cells using CoMo10mMO4@CNF freestanding electrodes. The current due to EDL increased from 41% to 61% when the CV scan rate increased from 0.5 to 2.5 mV s–1, respectively (see Figure 6(c)). In contrast, the current contribution due to diffusion was reduced; it could be attributed to a more significant amount of the electrolytic ion forming monolayers at the surface of the CoMo10mMO4 nanosheets. Therefore, the rapidly electrosorbed monolayer could enhance the EDL-like characteristics; subsequently, the CV curves exhibit a higher EDLC current at a higher scan rate.GCD data were acquired to calculate the capacitance and consequently determine the energy and power densities of the supercapacitor cells. Figures6(d) and 6(e) show the results for Co/CNF- and CoMo10mMO4@CNF-based symmetric supercapacitors, respectively. The GCD data showed triangular charging and discharging characteristics with almost >97% Coulombic efficiency. Similar characteristics were exhibited by the CoMo5mMO4@CNF and CoMo20mMO4@CNF electrode-based supercapacitor cells. All four cases exhibit excellent reversible characteristics at all current densities, as shown in Figure S6. The electrochemical performance (CV and GCD) of the EDLC/pseudocapacitive material composite (i.e., CoOx@CNF and CoMoO4@CNF) and the absence of sharp redox peaks confirm their capacitive characteristics. It should be noted that the pseudocapacitive material should exhibit EDLC-like electrochemical characteristics [58, 59]. For example, a Co-based pseudocapacitive material (i.e., Co3O4-PANI@ZIF-8NPC) did not exhibit any sharp redox peaks in the CV curves [50]. Similarly, a Ni-based pseudocapacitive materials (i.e., (Ni-Fe)-P-C@HCNFs) exhibited quasirectangular CV curves without any sharp redox peaks [9]. Both of these Co- and Ni-based materials were tested using three-electrode and asymmetric supercapacitor measurements.Although each electrode comprises carbon and a pseudocapacitive material, the capacitive characteristics were maintained in the CV and galvanostatic measurements. The pseudocapacitive material is known to store charges by the Faradaic process with the electrosorption or electrodesorption of a monolayer of reactive electrolyte species. As a result, the Faradaic reaction is much faster in supercapacitors/pseudocapacitors, and it yields a quasirectangular shape for CV curves and triangular GCD profiles for symmetric supercapacitors in all cases. Therefore, the pseudocapacitive materials exhibit characteristics similar to EDLC characteristics but unlike battery redox characteristics [58].A long-term stability test with 30,000 cycles for a CoMo10mMO4@CNF electrode-based supercapacitor was evaluated at a current density of 5 mA cm-2 in the potential window of 0–1 V. The excellent electrochemical stability of CoMo10mMO4@CNF is seen from the long-term stability results shown in Figure 6(f). The capacitance retention after 30,000 GCD cycles was determined to be almost 100%. The Coulombic efficiency of the galvanostatic measurements during long-term cycling is presented in Figure 6(f); it appears to be ~100% during all cycles. The low initial Coulombic efficiency could be attributed to lower pore openings during the first charging and discharging cycle. The inset in Figure 6(f) shows the first and last three GCD cycles; these overlap, indicating that no capacitance loss occurred. The excellent electrochemical stability could be attributed to the optimal growth of bimetallic (CoMo10mMO4) nanosheets and CNF. Owing to their morphology, the nanosheets quickly transfer electrons from particle-to-particle and particle-to-CNF. The highly annealed CNF exhibited excellent conductivity and thus was able to receive electrons for densely packed nanosheets of CoMo10mMO4.Figure6(g) shows the capacitance plot for various samples at current densities of 0.1–5 mA cm–2. The higher charging and discharging times for the CoMo10mMO4@CNF case resulted in higher capacitance values compared with those in the other cases (see Figure 6(g)). The maximum capacitance of 1199 mF cm–2 at 0.1 mA cm–2 within a potential window of 0–1 V is considerably better than the values reported in numerous studies (Table 1). Moreover, the CoMo10mMO4@CNF case exhibited a high current rate capability of 72% capacitance retention when the current density increased from 0.1 to 5 mA cm–2. The capacitance values estimated using Equation (1) at a current density of 0.1 mA cm–2 for CoOx@CNF, CoMo5mMO4@CNF, and CoMo20mMO4@CNF are 649, 789, and 872 mF cm–2, respectively. CoOx@CNF exhibited a moderate electrochemical performance compared to the other cases owing to the absence of Mo. With the introduction of Mo (CoMo5mMO4/CNF), the charge storage capability of the electrode increased, as shown in Figure 6(g). However, the morphology observed using SEM clearly indicates inadequate growth of the CoMO4 nanosheets (see Figure 4(b)). In contrast to CoMo5mMO4/CNF, the excess growth of CoMO4 nanosheets for CoMo20mMO4/CNF resulted in a longer electron transfer path from the surface of the electrode to the CNF (interior), resulting in inferior electrochemical performance compared to that of CoMo10mMO4/CNF. The direct growth of CoMoO4 nanosheets on CNF was random, as seen in Figure S7A–7C; it could have caused the relatively poor electrochemical performance compared to that of CoMo10mMO4/CNF (Figure S7D–7F).Table 1
Comparison of prepared CoMoO4@CNF supercapacitor with other Mo- and Co-based supercapacitors.
Electrode materialSubstrateCapacitance (mF cm-2)Current density (mA cm-2)ElectrolyteP.W. (ΔV) (V)Retention (%)Number of cyclesRef.MoS2/Co3O4Glassy carbon2210.140.5 M Na2SO40.987.010000[29]VN@C/CNTF//MNCO/CNTFCNT fibers233.71.0KOH/PVA1.690.23500[3]CoMoO4Nickel foam4002.5K3Fe(CN)61.5100.02000[36]MoS2/SWNT/CNFFreestanding4450.075PVA/H2SO4 gel0.891.010000[5]rGO/CoMoO4Carbon cloth11252 (mV/s)K3Fe(CN)60.898.01500[28]Co3O4@NiMoO4Nickel foam230013 M KOH0.5581.04000[2]CoMoO4@CNFFreestanding110616 M KOH1.5100.030000PresentThe energy and power densities of the CoOx@CNF, CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF electrodes are shown in Figure 6(h). The Ragone plots of the various samples show identical characteristics to those of the capacitance plot. For the CoMo10mMO4@CNF electrode-based symmetric supercapacitor cell, a maximum energy density of 166.5 μWh cm–2 was obtained at 200 μW cm–2, and a density of 119.4 μWh cm–2 was obtained at 10,000 μW cm–2. Furthermore, 0, 5, and 20 mM samples, respectively, had energy densities of 90.2, 109.6, and 121.2 μWh cm–2 at a power density of 200 μW cm–2 and 64.4, 77.2, and 92.8 μWh cm–2 at a power density of 10,000 μW cm–2. This extraordinary energy-to-power ratio verifies that CoMoO4@CNF is a potential material for energy storage in practical devices.EIS is useful for determining the charge-transfer capability of the samples. The Nyquist data for CoOx@CNF, CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF are presented in Figure 6(i). The Nyquist plots were fitted using the Randles equivalent circuit using the Zsimp software, as shown in Figure S8. The solution resistance (Rs) and charge-transfer resistance (Rct) were obtained by appropriately fitting each Nyquist plot. For CoOx@CNF, CoMo5mMO4@CNF, CoMo10mMO4@CNF, and CoMo20mMO4@CNF, the Rs values were 2.7, 2.7, 2.2, and 2.3 Ω, respectively, and Rct values were 0.8, 0.7, 1.2, and 3.8 Ω, respectively. The relatively larger Rct value for the CoMo20mMO4@CNF composite can be attributed to the thicker CoMo20mMO4 decorated CNF.GCD is also useful for determining the operating potential window of the supercapacitor by slowly increasing the potential window until the triangular shape of the charging and discharging plot is maintained [38]. As indicated by Stoller et al., two-electrode testing gives a closer matching to that of packaged supercapacitors than three-electrode testing [39]. Therefore, a pouch-like symmetric supercapacitor was assembled using two electrodes of CoMoO4@CNF to determine the maximum possible operating potential window. The electrode in a pouch-like symmetric supercapacitor using CoMoO4@CNF can store charges in the form of a double layer through EDLC or a capacitive Faradaic reaction at the surface, which is beneficial for providing a wide potential window, as depicted in Figure 7(a). The pseudocapacitive material, as in this case, is a CoMoO4 nanosheet capable of the rapid adsorption and desorption of electrolytic ions. Simultaneously, a characteristic of CNF is to accumulate charges electrostatically. Thus, the layer of opposite charge on each electrode develops a potential, resulting in a supercapacitor with a wide potential window. This is because the layer of opposite charge at the surface of two electrodes in a symmetric supercapacitor acts as an individual capacitor having an equal amount of developed potential [27]. By using this specific feature of a symmetric supercapacitor, the GCD plot was determined by increasing the potential window from 1 V to 1.5 V. However, the GCD profile was distorted when the voltage window was further increased; as a result, the charging and discharging times were not triangular in shape. Therefore, we limited the voltage window measurement to 1.5 V. The increase in the potential window with stable electrochemical performance can be attributed to the presence of CoMoO4 nanosheets for a CoMoO4@CNF symmetric supercapacitor. In addition, the presence of CNF significantly provided excellent electrical conductivity and EDLC (see Figure 7(a)) to achieve stable electrochemical performance with capacitive characteristics even in the wide potential window range of 0–1.5 V. The triangular shapes of the charging and discharging times were consistent for each potential window, as shown in Figure 7(b). The overlapping of the charging times indicates the excellent stability and reversible Faradaic reaction capability of the electrodes. This is consistent with the excellent electrochemical performance stability for 30,000 galvanostatic charging and discharging cycles as shown in Figure 6(f).Figure 7
(a) Schematic representation of charge storage phenomena at C and pseudocapacitive material. (b) GCD and (c) Ragone plot for various potential windows. CV curve of CoMo10mMO4@CNF flexible pouch-like symmetric supercapacitor cell with varying (d) scan rates, (e) bending angles (inset is a snapshot with 10 mm bending of electrode), and (f) number of bending cycles. (g) LED and humidity gauge with thermometer device under (h) flat and (i) bending conditions are powered by the pouch-like supercapacitor.
(a)(b)(c)(d)(e)(f)(g)(h)(i)The electrode capacitance calculated using the discharge time from the GCD plot shown in Figure7(b) increased from 1054 to 1106 mF cm–2 at a current density of 1 mA cm-2 for CoMoO4@CNF. An increase in the potential window significantly increased the energy density, as shown in Figure 7(c). The energy density of CoMoO4@CNF increases from 146.3 to 345.5 μWh cm–2 for the corresponding power density of 2 to 3 mW cm–2, as shown in Figure 7(c). As the energy density is dependent on time as well as the square of the potential window, the increase in energy density is 236%. On the contrary, the power density is proportional only to the potential window, and it increases by 150% upon increasing the potential window from 1 to 1.5 V, as shown in Figure 7(c).A freestanding CoMo10mMO4/CNF pouch cell was fabricated and its flexibility was demonstrated. Initially, the CV curve of the pouch cell at various scan rates was acquired within a potential window of 0–1 V. The CV curves of the pouch cell presented in Figure 7(d) exhibit similar symmetric coin cell features (cf. Figure 6(b)) at all corresponding scan rates. Further, to investigate the bendability of the freestanding CoMo10mMO4-based supercapacitor, the CV curve of a pouch cell at 100 mV s–1 under bending angles of 0°, 45°, and 90° was evaluated. The CV curves with bending angles of 45° and 90° exhibited slight deviations from the CV curves without bending (see Figure 7(e)). The inset in Figure 7(e) is a snapshot of a pouch cell with a 10 mm bending radius.Furthermore, a cyclic bending test with an 8 mm bending radius was performed for 500 cycles using a cyclic bending machine, as shown in Figure7(f) and its inset. The supercapacitor pouch cell shows a very small distortion from the original curve without any significant difference, owing to its good mechanical strength during bending. The practical ability of the pouch cells to operate various devices was also tested, as shown in Figures 7(g)–7(i). The LED was powered by two pouch cells connected in series. A single pouch cell was sufficient to operate the humidity gauge with a thermometer, as shown in Figure 7(h). The humidity gauge operated even under the bent condition of a pouch-like supercapacitor cell (see Figure 7(i)). The excellent electrochemical performance of CoMo10mMO4 under severe conditions such as high current density, long-term cycling, and bending conditions suggests that CoMoO4 nanosheet-decorated CNF is an extraordinary capacitive material for use in consumer electronic devices and a step further for wearable electronic devices.


## 4. Conclusion

An electrospun PAN-2MI nanofiber mat was dip-coated and decorated with ZIF-67. The dip-coated sample was stabilized in air at 280°C and then annealed in argon at 900°C to produce CoOx-decorated flexible CNFs. The sample then underwent a hydrothermal process inside a sodium molybdate dehydrate solution to produce CoMoO4 nanosheets on the CNFs. The optimal concentration of 10 mM molybdate solution yielded the highest capacitance of 1199 mF cm–2 at a current rate of 0.1 mA cm–2. The capacitive characteristics of the fabricated supercapacitors were maintained for a wide range of potential window (0–1.5 V), increasing their energy storage capability. The capacitance retention of the optimal sample was 100% after long-term GCD cycles (Ncyc=30,000), thereby demonstrating its electrochemical stability. The supercapacitor could power an LED light and humidity/temperature sensor under severe bending, thereby demonstrating its mechanical durability.



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*Source: 2910207-2023-03-03.xml*