In Situ Synthesis of a Polyaniline/ Fe-Ni Codoped Co3O4 Composite for the Electrode Material of Supercapacitors with Improved Cyclic Stability.

Conductive polymers have become a remarkable candidate for electrode materials of supercapacitors. Polyaniline (PANI) is the most promising contender for supercapacitors because of its easy method of synthesis, low cost, and higher choice in the improvement of energy storage applications. The main issue in the use of PANI in supercapacitors is its lower stability. In this work, PANI@Fe-Ni codoped Co3O4 (PANI@FNCO) nanocomposite has been prepared by in situ addition of 10 wt % FNCO as fillers in the PANI matrix. The nanocomposites were then characterized via scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry to observe the morphology, crystal structure, functional groups, and thermal stability of samples, respectively. SEM results showed that FNCO was fairly dispersed in the PANI matrix, while XRD results showed a broad peak for nanocomposites because of the semicrystalline nature of polymers. The electrochemical properties of the samples were analyzed via cyclic voltammetry, galvanostatic charge and discharge, and electrochemical impedance spectroscopy. PANI@FNCO nanowires are found to overcome the shortcomings in electrochemical energy storage devices by exhibiting a higher value of specific capacitance of 1171 F g-1 and energy density of 144 W h kg-1 at a current density of 1 A g-1. Moreover, the FNCO nanowires also showed a cyclic charge/discharge stability of 84% for 2000 cycles.

Introduction

The demand for the development of new materials for energy storage devices is derived because of the rise in demand for reliable and efficient sources of energy. Electrochemical energy storage devices such as electrochemical batteries and electrochemical capacitors represent a new class of energy storage devices.[1−4] The electrochemical capacitor which is also known as the supercapacitor has gained remarkable attention because of its energy storage mechanism, fast charging/discharging, high power density, long life of up to a few million cycles, and high energy density as compared to dielectric capacitors. However, there is room to improve the energy density of supercapacitors to use them in portable electronics, that is, mobile phones, laptops, and so forth, to replace the batteries, which contain more energy density. Supercapacitors are further classified into two types based on their working mechanisms: electrochemical double-layer capacitor (EDLC) and pseudocapacitors.[5,6] EDLCs work by charge accumulation on the interface of the electrode material and electrolyte. The materials with a high surface area are required to sustain more charges such as carbon-based materials, activated carbon, and graphene, and so forth. While, pseudocapacitors work because of the reversible redox reactions of the electrode material within the electrolyte. The transition-metal oxides, Co3O4, NiO, Fe3O4, and so forth, metal sulfides FeS and MnS, and conductive polymers such as polyaniline (PANI), polythiophene, polypyrrole, and their derivatives exhibit good redox reaction which makes these materials compatible for pseudocapacitors.[7−14] Transition-metal oxides have been extensively studied in the development of the electrode material of supercapacitors. Co3O4 has been the center of interest for researchers in the development of the electrode material. The main drawback in the development of Co3O4 is its high resistance and low ionic conductivity. Many efforts have been devoted to the improvement of performance and cyclic stability of Co3O4 to use in the electrode material of supercapacitors. A facile method to improve the performance of the transition-metal oxides is the insertion of different metal cations in the crystal structure of the transition-metal oxides or making its composite with conductive polymers.[14,15] PANI has gained great attention because of its remarkable advantage over other pseudocapacitive materials such as ease method synthesis, good ionic conductivity, high theoretical capacitance, wide operating potential window, flexibility in modification, and cost-effectiveness. Because of these remarkable attributes, PANI has been used in supercapacitors intrinsically and with composites of other electrode materials.[16] While, PANI has few drawbacks such as its mass loss with heat, low practical capacitance, and low cyclic stability of charging and discharging. It exhibits many unreactive sites after exposure with electrolyte and causes structural change, which reduces its reactive sites for redox reactions and results in poor super capacitive performances. Many works have been done to improve the thermal, chemical, and electrical properties of PANI by making its composites with, carbon nanotubes, metals nanoparticle, and metal oxides.[17−20] These composites have been extensively used in electrochemical sensors, supercapacitor electrodes, batteries, electromagnetic interference shielding, and light-emitting diodes.

Here, in this work, first, we hydrothermally synthesized Fe and Ni metals codoped with Co3O4 (FNCO) to improve the performance of Co3O4. The insertion of Fe and Ni in Co3O4 reduces the charge transfer resistance and makes it suitable for supercapacitors. Then, we synthesized PANI@FNCO by in situ polymerization of aniline on an optimized amount of FNCO by a simple, scalable, and inexpensive chemical oxidation method to improve its cyclic stability (Figure ). In the composite of PANI@FNCO, PANI provides good ionic conductivity and exhibits good redox reaction for the pseudocapacitive performance of the supercapacitors, while FNCO provides structural and cyclic stability. Collectively in the PANI@FNCO composite, PANI wrapped on FNCO nanowires makes electrolytes easily accessible to the pores of the composite and provides a continuous path for the transport of electrons and ions to improve the performance of the electrode material of supercapacitors. This composite has shown a tremendous performance up to 1197 F g–1 with a better cyclic stability of 84% capacitance retention after 2000 cycles on a wide operating potential window of 1.4 V. While pristine PANI exhibited a specific capacitance of 834 F g–1 with 69% cyclic stability over 2000 cycles.

Figure 1

Formation of PANI and PANI@FNCO.

Results and Discussion

The crystal structure of the composite was studied by X-ray diffraction (XRD). The XRD spectra for FNCO revealed that all the peaks matched with JCPDS card number 01-080-1542 for the crystal structure of Co3O4. There was no additional peak detected in the XRD pattern for FNCO, which revealed the successful replacement of Co cations with Fe and Ni. The XRD pattern for PANI consisted of broad peaks, which revealed the semicrystalline structure for PANI. The XRD diffraction peaks of PANI were at 2θ of 14.75, 20.57, and 25.52° corresponding to the planes of (0 1 1), (0 2 0), and (2 0 0) of PANI, respectively.[21] The XRD spectra for PANI@FNCO revealed the existence of both amorphous and crystal phases. The broad peaks were due to the existence of PANI, and the crystallinity was due to the metal oxide peaks, as shown in Figure . Scanning electron microscopy (SEM) was carried out to study the morphological analysis of the prepared sample. The SEM image of FNCO showed the formation of a ball-type structure of FNCO consisting of nanowires, as shown in Figure a. The SEM image of pure PANI revealed the formation of mesh-type nanofibers, as shown in Figure b. This type of morphology of PANI enhances the surface area of PANI, thus providing more interfacial contact area with the electrolyte. The SEM images reveal that these wires were arranged in regular order. The SEM image for PANI@FNCO showed the formation of the mesoporous-type structure of nanofibers shown in Figure c. Fourier transform infrared (FTIR) spectroscopy was carried out to study the presence of functional groups and composite formation. Figure shows the FTIR spectra of PANI, FNCO, and PANI@FNCO. In spectra of FNCO, two characteristic peaks at 549 and 647 cm–1 correspond to the Co–O bond of tetrahedral Co2+ and octahedral Co3+.[22,23] A peak at 831 cm–1 corresponds to CO32–, which is during the hydrothermal reaction of urea, and a peak at 1420 cm–1 exhibits the residue contents of NO32–, which remained unreactive during the reaction.[24−26] A belly-shaped peak at 3437 cm–1 is due to the presence of the O–H group from an ambient environment. In the case of PANI, two characteristic peaks at 1635 and 1440 cm–1 correspond to C=C stretching of the quinonoid ring and benzenoid ring, respectively. A broad peak at 3431 cm–1 corresponds to N–H stretching vibration of an amino group of PANI. Two strong peaks at 2849 and 2916 cm–1 are assigned to C–H stretching. A characteristic peak at 1290 cm–1 is assigned to the secondary C–N stretching amine group. A characteristic peak at 1126 cm–1 corresponds to the C–H bending of the quinonoid ring and benzenoid ring.[27−31] The presence of these groups endorsed the successful formation of PANI. In the case of PANI@FNCO, the absorption peaks of PANI are observed, while the characteristic peaks of FNCO were not observed, which revealed the full coating of the PANI chain of its Co3O4 spinal structure.[29,32−34] Differential scanning calorimetry (DSC) thermogram of PANI shows endothermic peaks at 25–78 °C and the exothermic peak at 78–444 °C. The first endothermic peak is mostly due to the removal of water content and the second endothermic peak at 444 °C is due to the cross-linking/oxidation reaction. Whereas, a small change in temperature caused the degradation of the polymer chain. In comparison to the thermal properties of PANI, the DSC thermogram of PANI@FNCO is also plotted, which exhibits the endothermic peak at 25–85 °C and the exothermic peak at 85–576 °C. The first endothermic peak is mostly due to the removal of water content and the second endothermic peak at 576 °C is due to the cross-linking/oxidation reaction.[35] Whereas, a small change in temperature caused the degradation of the polymer backbone in the PANI@FNCO composite. The glass transition temperature Tg of PANI and PANI@FNCO can be calculated from the DSC thermogram, as shown in Figure a. It is well known that glass transition temperature appears as a sudden change in the slope or with an endothermic dip at a lower temperature.[36] It can be seen from Figure a that the endothermic dip at 78 °C is Tg for PANI and a deflection at 444 °C is the Tm of PANI In comparison the Tg value of PANI@FNCO is observed at 85 °C and Tm at 576 °C. The increase in Tg and Tm of the PANI composite with the transition metal oxide is due to the restriction in chain mobility. Tg of the polymer composite depends on the free volume movement and the interaction between the polymer chain and metal oxide particles. However, an increase in Tg and Tm of the PANI@FNCO composite is due to the strong interaction between FNCO and the polymer chain. Thermogravimetric analysis (TGA) was carried out to examine the thermal stability of PANI and PANI@FNCO. Figure b shows the TGA thermogram of PANI and PANI@FNCO from room temperature 25 to 1000 °C. It can be seen from Figure b that the weight loss consisted of three different steps with increasing temperature. The first weight loss was from 25 to 127 °C for PANI and 25–119 °C for PANI@FNCO. This weight loss was due to the evaporation of volatile impurities, that is, surface-adsorbed or interlayer water molecules. The second weight-loss temperature region was 119–343 °C for PANI and 127–362 °C for PANI@FNCO. It was observed that weight loss in this region was taken gradually, and it was assigned to deprotonation of PANI through the dopant of HCl and removal of other oligomers and unreacted monomers in the specimens.[35−37] The third part was due to the degradation of PANI in the crystalline FNCO. It was seen that the PANI was completely degraded up to 657 °C, while 68% of PANI@FNCO was degraded up to 544 °C. PANI@FNCO showed that 32% of the total mass remained. It was observed that the degradation rate of pure PANI is quite higher than the composite of PANI made with FNCO. This shows that the thermal stability of PANI was improved by making its composite with FNCO. It was due to the strong interaction found between the polymer chains and nanoparticles.

Figure 2

XRD pattern of FNCO, PANI, and PANI@FNCO.

Figure 3

SEM image of (a) FNCO, (b) PANI, and (c) PANI@FNCO.

Figure 4

FTIR spectra of PANI, FNCO, and PANI@FNCO.

Figure 5

(a) DSC thermogram of PANI and PANI@FNCO and (b) TGA thermogram of PANI and PANI@FNCO.

The electrochemical performance of PANI and PANI@FNCO as the electrode material for supercapacitors was studied by several techniques [cyclic voltammetry (CV), galvanostatic charge and discharge (GCD), and electrochemical impedance spectroscopy (EIS)].[38]Figure a shows the cyclic voltammogram for PANI and PANI@FNCO at a scan rate of 10 mV s–1. Cyclic voltammogram for pristine PANI@FNCO and PANI at different scan rates is shown in Figure b,c, respectively. The nonrectangular shape of cyclic voltammograms endorsed the pseudocapacitive nature of the material. The cyclic voltammogram for PANI and PANI@FNCO revealed the presence of two redox peaks for their working. It was seen that the current passing increased for the PANI@FNCO composite as compared to the pristine PANI. In the case of PANI@FNCO, the redox peaks became sharper and the potential difference between both peaks was reduced. Besides, it was noted that the area under the curve for PANI@FNCO increased, which proposed the enrichment of capacitance in comparison to PANI. It can be revealed in the careful analysis that the main capacitance from PANI and its properties were improved by the addition of FNCO. This improvement relies on the synergetic effect of Fe and Ni doping in Co3O4, further improvement in operating potential window was caused by making its composite with PANI.[16] The CV curves exhibit a rise in current and area with increasing scan rate; this endorsed the pseudocapacitive nature. The specific capacitance calculated by CV for PANI was 1073—, 942—, 717—, 623—, and 462 F g–1 at scan rate of 10, 20, 40, 50, and 100 mV s–1. GCD was performed to further study the capacitive nature of electrode materials. Figure d shows the GCD curve of PANI and PANI@FNCO at a current density of 1 A g–1. The discharging time for PANI@FNCO was higher than PANI, which endorsed the better performance of PANI@FNCO. The GCD curve for PANI@FNCO and PANI at different current densities is shown in Figure e,f, respectively. The shape of GCD curves is nonlinear, which endorsed the pseudocapacitive nature of materials. The discharging time was reduced by increasing the current density and resulted in decreased specific capacitance. This phenomenon was observed because of the provision of less time for reaching of the electrolyte to the electrode material’s active sites and resulted in limited charge storage. The specific capacitances for PANI were 834, 733, 622, and 481 F g–1 at the current densities of 1, 2, 5, and 10 A g–1, respectively. The discharging time for PANI@FNCO was higher than the pristine PANI. This was because of the synergetic effect of PANI and FNCO. The specific capacitances for PANI@FNCO were 1171, 959, 750, and 603 F g–1 at the current density of 1, 2, 5, and 10 A g–1, respectively. Figure a shows the comparison of specific capacitance for PANI and PANI@FNCO calculated by GCD at different current densities. Furthermore, energy density and power density were also calculated for prepared electrode materials. These parameters have important significance for energy storage devices to use in practical applications. Figure b shows the Ragone plot for PANI and PANI@FNCO. PANI@FNCO exhibited an energy density of 144 W h kg–1 corresponding to a power density of 470 W kg–1, which is higher in comparison to PANI.

Figure 6

(a) Comparison of cyclic voltammogram at a scan rate of 10 mV s–1 between PANI and PANI@FNCO, cyclic voltammogram of (b) PANI@FNCO and (c) PANI at different scan rates, (d) comparison of GCD curve at a current density of 1 A g–1 between PANI and PANI@FNCO, and GCD curve of (e) PANI@FNCO and (f) PANI at different current density.

Figure 7

(a) Histogram between specific capacitance and current densities, (b) Ragone plot for PANI and PANI@FNCO, (c) Nyquist plot for PANI and PANI@FNCO, and the inset is the equivalent circuit used for z-fitting, and (d) cyclic life stability of PANI and PANI@FNCO.

EIS was carried out within the frequency range from 1 MHz to 100 mHz to study the capacitive behavior and charge transfer mechanism involved in the performance of PANI and PANI@FNCO between the electrolyte and electrode.[39,40]Figure c shows the z-fitted Nyquist plot using the equivalent circuit shown in the inset to Figure c for pristine PANI and PANI@FNCO. The fitted Nyquist plot consisted of two sections. One is a linear line at a lower frequency, which revealed the capacitive nature of the material and a diagonal line at intermediate frequencies, which exhibits the internal resistances of the system, that is, the electrolyte and electrode material. For ideal supercapacitors, this line should be vertical. While, the third region was a semicircle at higher frequencies, which exhibits the charge transfer resistance between the electrode and electrolyte, which depends on the diffusion of the ions in the electrolyte to the electrode interface.[41,42] The radius of the semicircle is directly proportional to the value of charge transfer resistance.[43] The charge transfer resistance of the PANI and PANI@FNCO was 696 and 120 Ω. The small amount of the charge transfer resistance affirmed the fast charge transfer. The reduction in charge transfer resistance for PANI@FNCO was due to the formation of mesoporous structure affirmed by BET results, as shown in Figure S1 of Supporting Information, which endorsed the creation of an easily accessible path to allow penetration of the electrolyte into the pores of the composite and high surface area, which allows maximum contact between the electrode and electrolyte, and the strong interaction between the PANI and FNCO, which makes an easy and continuous path for the transportation of electron and ions. EIS was further carried out to study the diffusion coefficient of H+ ions (DH) by using the low-frequency region of the Nyquist plot by using the mathematical equation as follows[40]Here, R is the ideal gas constant, T is absolute temperature, A is the area of the electrode, n is the number of electrons, F is Faraday’s constant, C is the molar concentration of electrolyte, and σ is the Warburg coefficient. The value of σ was calculated by evaluating the slope of the curve between the Re and ω–1/2 as reported in the literature. The value of DH for PANI@FNCO and PANI was 1.1 × 10–8 and 1.11 × 10–11, respectively. This revealed the higher value of the diffusion coefficient for PANI@FNCO, which caused improvement in the electrochemical properties of the electrode.

A key parameter for an electrode material to be used in practical application is its charge stability. We carried out the GCD test for 2000 cycles at a current density of 25 A g–1 to visualize the stability of PANI@FNCO and PANI to sustain a charge for a large number of cycles, as shown in Figure d. This showed a Coulombic efficiency close to 100% for all cycles. It was observed that PANI@FNCO and PANI exhibit 84 and 69% capacitance retention after 2000 GCD cycles, respectively. This loss in specific capacitance can be caused because of the removal of the material from the electrode or blocking the pores to diminish the electrode/electrolyte interface with the passage of charging/discharging cycles.

Conclusions

PANI@FNCO was successfully synthesized by the in situ polymerization of aniline on FNCO. The microstructural analysis of PANI@FNCO showed mesoporosity. The thermal stability of PANI@FNCO was improved by the thermal stability of FNCO nanofibers. The mesoporous structure of PANI@FNCO allowed maximum interfacial contact between the electrolyte and electrode to store more charges. The specific capacitance and charge stability of PANI@FNCO were improved by the synergetic effect of the PANI chain and FNCO nanofibers.

Experimental Section

Synthesis of FNCO

Metal oxide was synthesized by two steps of the hydrothermal method. In the first step, 0.2 mmol nickel nitrate, 0.2 mmol iron nitrate, 0.4 mmol cobalt nitrate, 3 g of urea, and 0.5 g of ammonium fluoride were dissolved in 40 mL of deionized (DI) water and transferred to a 50 mL Teflon-lined autoclave tube. The autoclave was heated at 135 °C for 5 h in an electric oven. After completing the reaction, the synthesized powder was vacuum-filtered and washed with DI water and ethanol at least 3 times to remove all the byproducts and impurities. After filtration, the powder was dried at 80 °C for 6 h. In the second step of synthesis, the dried powder was annealed at 350 °C for 2 h with a ramp rate of 3 °C in a muffle furnace. After annealing, the powder was collected and stored in a vacuum desiccator for further use and characterizations.

Synthesis of the PANI@FNCO Composite

Initially synthesized FNCO (130 mg) was mixed homogeneously in 40 mL of DI water through 80 min of ultrasonication. Then, the blend of aniline 1.2 and 3.9 mL of HCl was formed in 17 mL of DI water by 5 min of sonication. This formed blend was added in the suspension of metal oxides dropwise, and the mixture was stirred magnetically for 1 h to make the solution homogeneous. After one hour of stirring, the solution was placed in an ice bath to achieve a temperature 0–3 °C. Subsequently, 0.1 M ammonium persulfate was dissolved in 13 mL of DI water. This solution was added in a prior mixture of metal oxide and aniline as a reactive agent to start the process of polymerization. After the addition of the ammonium persulfate solution, stirring was further performed for 5 h while keeping the temperature below 4 °C. These experimental conditions and the amount of the materials were optimized after running several experiments. The pristine PANI was synthesized using the same procedure without the addition of FNCO. A schematic for the formation of PANI and PANI@FNCO is shown in Figure .

Characterizations

The phase-structural analysis of the synthesized material was studied by powder-based XRD spectroscopy. The microstructural study was performed by SEM. FTIR was carried out to study the functional groups attached to the material. The thermal stability of the material was studied by TGA and DSC analysis. All the electrochemical analyses were carried out on a potentiostat (VSP-300; Bio-Logic Science Instruments, France) using the three-electrode system in 1 M H2SO4 electrolyte. The active material (PANI and PANI@FNCO) on glassy carbon acted as the working electrode, while Pt wire was used as a counter electrode, and Ag/AgCl was used as a reference electrode. The capacitive performance of the material was examined by electrochemical techniques such as CV, GCD, and EIS. The specific capacitance of the material was calculated by CV using this formula

The specific capacitance of the material was calculated by GCD using this formula[44]

The energy density of the material was calculated by GCD using this formula

The power density of the material was calculated by GCD using this formula