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copper cobaltite for high performance asymmetric supercapacitor application
Accepted Manuscript Title: Binary composite of polyaniline/copper cobaltite for high performance asymmetric supercapacitor application Author: Fatin Saiha Omar Arshid Numan Navaneethan Duraisamy Mohammad Mukhlis Ramly K. Ramesh S. Ramesh PII: DOI: Reference:
S0013-4686(17)30006-3 http://dx.doi.org/doi:10.1016/j.electacta.2017.01.006 EA 28673
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
8-3-2016 10-8-2016 1-1-2017
Please cite this article as: Fatin Saiha Omar, Arshid Numan, Navaneethan Duraisamy, Mohammad Mukhlis Ramly, Ramesh K., Ramesh S., Binary composite of polyaniline/copper cobaltite for high performance asymmetric supercapacitor application, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.01.006
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Binary composite of polyaniline/copper cobaltite for high performance asymmetric supercapacitor application
Fatin Saiha Omar1δ, Arshid Numan1, Navaneethan Duraisamy2#, Mohammad Mukhlis Ramly1, K. Ramesh1, S. Ramesh1*
Center for Ionics University of Malaya, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia 2
Department of Chemistry, Periyar University, Salem-636011, Tamilnadu, India
Corresponding authors *
E-mail: [email protected], Tel: +603-7967 4391, Fax: +603-7967 4146
#
E-mail: [email protected]
δ
E-mail: [email protected]
1
Abstract This article presents the effect of polyaniline (PANI) embedded copper cobaltite (CuCo2O4) as an electrode material for high performance supercapacitor application. The composite of PANI-CuCo2O4 was prepared via blending process. The formation of PANICuCo2O4 composite was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analysis. The surface morphologies showed that the spinel structure of CuCo 2O4 (average particle size of 30 nm) was well distributed on PANI matrix, suggest the effective intercalation of CuCo2O4 with PANI matrix. The electrochemical properties of CuCo2O4, PANI and PANICuCo2O4 composite were investigated using cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS) in 1 M of KOH as an aqueous electrolyte. The PANI-CuCo2O4 composite showed the improved specific capacitance of 403 C/g than that of pure CuCo2O4 and PANI.The enhanced electrochemical performance was obtained due to the augmentation of redox active sites and synergetic effect between PANI and CuCo2O4 nanoparticles. Additionally, the fabricated (activated carbon (AC)/PANI-CuCo2O4) asymmetric supercapacitor device can be cycled reversibly at a cell voltage of 1.5 V, which exhibited excellent electrochemical performances with an energy density of 76 Wh/kg and a power density of 599 W/kg. It also presented a superior life cycle with 94% capacitance retention after 3000 cycles.
Keywords: Cobaltite;Polyaniline;Composites;Electrode materials;Supercapacitor
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1.
Introduction Supercapacitors are receiving extensive attention due to their efficient capability of storing
and discharging energy than that of other primary energy sources like fuel cells and batteries. In recent years, several works have been developed in order to get the high energy density, long cycling life and low internal resistance based energy system [1,2]. However, still, we have some limitation to develop the practical applications due to the lack of appropriate electrode materials with optimal electrochemical properties. Generally, there are two different ways to store the energies such as capacitive (electric double layer capacitors (EDLCs)) and pseudocapacitive (pseudocapacitor) nature. In EDLCs, the capacitance arises from the adsorption of electrolyte ions on the electrode/electrolyte interface without involve faradaic reaction leading to high power density. While for pseudocapacitors, the capacitance arises from the reversible faradaic reactions at the surface of electrode material as well as the insertion of cations from the electrolyte [3–5]. Ruthenium oxide (RuO2) has been recognized as the best pseudocapacitive material due to its high specific capacitance, good electrical conductivity and reversible chargedischarge properties [6,7]. However, there are some limitations to use RuO2 as an electrode material in a commercial scale such as expensive and high toxicity. In recent years, it has been demonstrated that spinel structure of transition metal oxides is an effective strategy to improve the performance of single-component metal oxides such as nickel cobaltite (NiCo2O4), zinc cobaltite (ZnCo2O4), cobalt ferrite (CoFe2O4), manganese cobaltite (MnCo2O4) and etc [8–11]. The presence of two metal oxides within the one molecule exhibited better physico-chemical properties such as good electrical conductivity, high stability and improved redox sites with numerous oxidation states than that of single metal oxides [12]. Among these spinel metal oxides, CuCo2O4 has been realized as a good electrode material for 3
supercapacitor application. CuCo2O4 spinel has unique structure where divalent Cu ions occupy the tetrahedral site and the trivalent Co ions occupy the octahedral site in the cubic spinel structure. In addition, CuCo2O4 offers several advantages including abundant resources, low cost and environmental friendliness [13]. However, the capacitance and operation voltage are still the challenging task to improve the supercapacitor performance. Polyaniline (PANI) is one of the most interesting and widely used conductive polymers in various applications such as batteries [14], sensors [15] and supercapacitor [16]. PANI has several advantages in terms of good electrical conductivity, high electrochemical activity, chemical stability, excellent redox properties, low cost and flexibility. In general, PANI exhibiting three different structural forms such as fully reduced state (leucoemeraldine), fully oxidized state (pernigraniline) and half-oxidized state (emeraldine base)
[17]. Herein, the
emeraldine form is the most conductive part and also is the key factor to enhance the electrochemical activity. The present work is investigating the electrochemical performance of CuCo2O4 intercalated PANI matrix. The improved electrochemical properties in spinel structured CuCo2O4 are offered by the incorporation of PANI, which lead to restrict the aggregation of CuCo2O4 with improved electrochemical surface area. To the best of our knowledge, no attention has been paid to the comparison between the pure CuCo2O4 and the PANI-CuCo2O4 composite especially for supercapacitor application. In addition, the performance of PANI-CuCo2O4 in asymmetric supercapacitor is also studied.
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2. Materials and Method 2.1. Materials Copper acetate dihydrate, Cu(O₂CCH₃)₂.2H2O and cobalt acetate tetrahydrate Co(C₂H₃O₂)₂.4H2O were purchased from Friendemann Schmidt, Malaysia. Urea, ammonia (28% purity), aniline,
ammonium
peroxydisulfate (APS), hydrochloric acid
(HCl),
polyvinylidene fluoride (PVdF), acetylene black and activated carbon (AC) were received from Sigma-Aldrich, Malaysia. Ethanol (95% purity) was purchased from J. Kollin Chemical, Malaysia. 2.2. Synthesis of polyaniline (PANI) PANI was synthesized by chemical oxidation polymerization method. 0.25 M aniline was added into 20 ml of 1M aqueous HCl. The solution was stirred at room temperature for the formation of anilinium ions. Next, the freshly prepared APS solution (0.2 M APS dissolved into 30 ml of 1M HCl) was added dropwise into the above solution and allowed for continuous stirring until a green precipitate observed. Then the precipitate was washed with acetone and deionized water for several times and dried at 60 ºC for 1 hour. Finally, the observed green color was the evidence of the polyaniline formation in its conductive form of emeraldine salt (ES). 2.3. Synthesis of CuCo2O4 nanoparticles CuCo2O4 nanoparticles were synthesized by a simple hydrothermal treatment. Cu(O₂CCH₃)₂.2H2O (15 mmol), Co(C₂H₃O₂)₂.4H2O (15 mmol), urea (20 mmol) and 3.4 mL of ammonia were added into the solution of water-ethanol mixture and stirred for 19 hours. Next, the mixture was transferred into a 80 mL capacity of Teflon-lined stainless steel autoclave, which was heated and maintained at 180 ºC for 6 hours. After that the obtained sample was 5
washed with deionized water for several times to remove the redundant materials and then dried at 60 ºC. 2.4. Synthesis of PANI-CuCo2O4 nanoparticles PANI-CuCo2O4 nanocomposite was prepared by blending of PANI with CuCo2O4 (weight ratio of PANI:CuCo2O4= 4:1) in a pestle mortar to achieve a homogeneous mixture of PANI with CuCo2O4 composite. The entire preparation process was schematically illustrated in Fig. 1. 2.5. Characterization of the samples The crystalline phases of the samples were determined via X-ray diffraction (XRD; D5000, Siemens), using copper Kα radiation (λ=1.5418 Å) at a scan rate of 0.02º s-1. The morphological studies of synthesized pure CuCo2O4 nanoparticles, pure PANI and PANICuCo2O4 composite were carried out using JEOL JSM-7600F, field emission scanning electron microscopy (FESEM). Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet ISIO Smart ITR) analysis used to study the presence of functional groups and purity of samples, which was in the region from 400 to 2500 cm-1 at a resolution of 1 cm-1. 2.6. Electrochemical studies PANI-CuCo2O4 electrode was fabricated by mixing 75 wt% of the active material, 15 wt% of acetylene black and 10 wt% of PVdF in NMP (solvent) to achieve homogeneous slurry. Then, the cleaned nickel foam (1x1 cm2 area) was coated with the slurry and dried in an oven at 90 ºC for 14 hours. The mass of coated active material (PANI-CuCo2O4) was approximately ~3.75 mg. A standard three-electrode cell system was used to examine the electrochemical measurements. The PANI-CuCo2O4 coated Ni foam was used as the working electrode, Ag/AgCl 6
and platinum wire were used as the reference and counter electrodes, respectively. The measurements were conducted in a 1 M KOH at room temperature. Cyclic voltammetry (CV), galvanostatic charge-discharge test (GCD) and electrochemical impedance spectroscopy (EIS) with a frequency range of 0.01 Hz to 100 kHz were conducted using Gamry Instrument. For the comparison study, the same experiment was repeated using pure CuCo2O4 as an electrode material. 3. Results and discussion 3.1. X-ray diffraction (XRD) The structural crystallinity and phase purity of synthesized materials were investigated by X-ray diffraction (XRD) analysis. The XRD of pure CuCo2O4, PANI and PANI-CuCo2O4 was represented in Fig. 2. The diffraction peaks were observed at 2θ values of 19.07º, 31.36º, 36.96º, 38.96º, 45.07º, 56.03º, 59.59 º and 65.70º correspond to the (111), (220), (311), (222), (400), (422), (511) and (440) lattice planes respectively. This pattern were clearly indexed as a cubic CuCo2O4 with a spinel structure (JCPDS card no. 001-1155) and a space group of Fd3m [18]. No other additional peaks were detected suggesting the high purity of CuCo2O4. PANI showed the partial crystalline state with the diffraction peaks at 2θ = 14.67º, 20.72º and 25.59º attributed to the (011), (020) and (200) lattice planes of PANI chains in its emeraldine salt form respectively [19]. Moreveor, a weaker and broader diffraction peaks were observed in PANI-CuCo2O4 composite,which was due to the intercalation CuCo2O4 with PANI matrix.
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3.2. Fourier transform infrared spectroscopy (FTIR) The purity of prepared samples was further characterized by FTIR analysis. Fig. 3 represented the FTIR spectra of CuCo2O4, PANI and PANI-CuCo2O4 composite. CuCo2O4 revealed two strong absorption bands at 554 and 657 cm-1 correspond to the stretching vibration of Co3+ ̶ O2- in the tetrahedral complexes and Cu2+ ̶ O2- in the octahedral complexes. The peaks at ~832, 970, 1037, 1340 and 1540 cm-1 were assigned to the carbonate anion (derived from slow hydrolysis of urea during hydrothermal process) [20]. In case of PANI, the peaks around 1475 cm-1 and 1556 cm-1 correspond to –C=C stretching vibrations of benzenoid and quinonoid structures respectviely. The observed band at 1287 cm-1 was assigned to the C–N stretching of secondary aromatic amine. The bands at 785 cm-1and 1099 cm-1 were ascribed to C–H bending vibration of aromatic ring [21]. All characteristic peaks of PANI and CuCo2O4 can be observed in PANI-CuCo2O4 composite, which was confirmed the intercalation of CuCo2O4 with PANI matrix. 3.3. Field emission scanning electron microscopy (FESEM) The surface morphologies of pure PANI, CuCo2O4 and PANI-CuCo2O4 were investigated by FESEM analysis. Fig. 4a and 4b showed the branched geometrical surface morphology of pristine PANI structure with 100 nm in width, while Fig. 4c and 4d displayed the irregular shaped morphology of CuCo2O4nanoparticles with an average particle size of 33 nm. Fig. 4e and 4f represented the nanostructured CuCo2O4 decorated PANI matrix. High magnification of FESEM image (Fig. 4f) exhibited well embedded CuCo2O4 nanoparticles on PANI matrix, which was confirmed that the blending method able to form the effective interfacial contact between PANI and CuCo2O4. The incorporation of PANI with CuCo2O4 lead to control the aggregation of CuCo2O4 nanoparticles in PANI-CuCo2O4 composite. This kind of structure may increases the 8
effective liquid-solid interfacial area and allow the rapid electrolyte ions transportation during charge/discharge process [22].
3.4 Electrochemical characterizations 3.4.1. Cyclic voltammetry (CV) CV test was used to investigate the occurrence of faradaic or non-faradaic reactions in the electrode. The CuCo2O4, PANI and PANI-CuCo2O4 electrodes were measured for the comparison of capacitance difference in pure CuCo2O4 and after the introduction of PANI. The CV tests were performed in a potential range of 0.0‒0.5 V (vs. Ag/AgCl) using 1 M KOH electrolyte. Fig. 5a-5c exhibited the the CV curves of the pure CuCo2O4, PANI and PANICuCo2O4 electrodes with various scan rates ranging from 1 mV/s to 30 mV/s. The CV curves of CuCo2O4 exhibited well-defined redox peaks, which is a characteristic of faradaic behavior of battery-type electrode. It is noteworthy to mention that often transition metal oxide-based battery-type materials are misled as pseudocapacitive materials due to the similar charge storage mechanism [23]. The redox peaks correspond to the reversible reaction of Co4+/Co3+ and Cu2+/Cu+ transitions associated with hydroxyl ion (OH¯) from the KOH electrolyte. The redox reactions in alkaline medium are based on the equation (1), (2) and (3) [24]: CuCo2O4 + H2O + e¯ ↔ 2CoOOH + Cu(OH)¯
(1)
CoOOH + OH¯ ↔ CoO2 + H2O + e¯
(2)
CuOH + OH¯ ↔ Cu(OH)2 + e¯
(3)
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CV curves of PANI and PANI-CuCo2O4 electrodes also exhibit redox peaks contributed from the faradaic reactons. Furthermore, it was well known that the charge stored with respect to the area under the CV curves. It can be found that the integrated area of PANI-CuCo2O4 was larger than that of pure CuCo2O4 and PANI, indicating that the maximum capacity provided by PANI-CuCo2O4 [25]. In addition, while increasing the scan rate, peak currents are found to increase, and the anodic and cathodic peaks shifted towards higher and lower potential, respectively. This was due to the strengthened electric polarization and the possible kinetic irreversibility of electrolyte ions at the electrode surface during the redox reaction at high scan rates [26]. Notice that the shape of the CV curves was not significantly influenced by increasing scan rates, implying fine reversibility of redox reaction and good power property [27]. The materials exhibiting redox peaks in CV should not be interpret as pseudocapacitor as the capacitance of the electrode keep on changing over the whole potential window. Therefore, the electrochemical performance of CuCo2O4, PANI and PANI-CuCo2O4 was measured in terms of specific capacity instead of specific capacitance [28]. The specific capacity of the electrodes was calculated based on the CV curves using equation (4). 1
Vf
𝑄S = νm ∫Vi I × VdV
(4)
where 𝑄S is the specific capacity of the samples (C/g), ν is the scan rate (V/s), m is the mass of the active material (g) and the integral term is equal to the area under the CV curve. The calculated specific capacity values for pure CuCo2O4, PANI and PANI-CuCo2O4 were found to be 97, 281 and 312 C/g, respectively at a scan rate of 1 mV/s.
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3.4.2. Charge-discharge study and electrochemical impedance spectroscopy The GCD is a reliable method to evaluate the electrochemical capacitance of materials under controlled conditions. Fig. 6a-6c manifest the galvanostatic discharge curves of CuCo2O4, PANI and PANI-CuCo2O4 with a potential range of 0‒0.38 V. Herein, the electrode materials showed the nonlinear discharge curves confirmed the faradaic behavior of battery-type electrode. Apparently, the discharge time of PANI-CuCo2O4 was longer than that of CuCo2O4 and PANI, implying better electrochemical capability of the PANI-CuCo2O4 [29]. The specific capacity of the electrodes was calculated from the galvanostatic discharge curves using equation (5); 𝑄𝑠 =
I ×∆t m
(5)
where 𝑄𝑠 is the specific capacity of the electrode (C/g), I is the current (A), m is the mass loading of the active material on electrode (g) and ∆𝑡 is the discharge time (s). The specific capacity values of PANI-CuCo2O4 were found to be 403, 378, 303 and 218 C/g at current density of 0.4, 0.5, 1 and 2 A/g, respectively (Fig. 6d). On the other hand, the specific capacity values for CuCo2O4 were found to be 206, 118, 69 and 66 C/g, while PANI delivered specific capacity of 240, 232, 189 and 163 C/g at current density of 0.4, 0.5, 1 and 2 A/g respectively, which are far lower than specific capacity values of PANI-CuCo2O4 electrode. The maximum specific capacity of PANI-CuCo2O4 was due to the effective ion or charge transfer mechanism between PANI and CuCo2O4. The disparity in the capacity between CuCo2O4, PANI and PANI-CuCo2O4 composite could be interpreted by the following reasons; (i) the large surface area of PANI can act as a spacer to prevent the aggregation of CuCo2O4 nanoparticles, providing more redox active sites for the accessibility of OH¯ ions. While, in pure CuCo2O4 nanoparticles, closely packed particles lower the number of redox active sites for rapid OH¯accessibility (ii) the contribution from the
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conductivity of PANI itself in the nanocomposite are partially involved during the redox reaction (iii) the synergetic effect formed between conductive PANI and CuCo2O4 nanoparticles contributed to the enhancement of specific capacitance [30,31]. EIS was employed to investigate the conductivity and interfacial charge transfer process at the electrode/electrolyte interface. Fig. 7 showed the EIS plot (Nyquist) plot between Z’ and – Z” recorded between 0.01 Hz-100 kHz frequency region. Inset of the Fig. 7 showed the enlarge portion of the high frequency region. It can be observed that the low equivalent series resistance (ESR) values (0.65 Ω) for PANI-CuCo2O4 than other electrodes manifesting the low internal resistance of the electrode. The diameter of the semicircle observed at low frequency region estimates the faradaic charge transfer resistance (Rct) which was related to the electrode surface area and electrical conductivity of the electrode material. In this respect, CuCo2O4 illustrated larger semicircle showed the higher resistance involved during charge transfer at the electrode material/electrolyte interface. There was no obvious semicircle observed at PANI-CuCo2O4, suggesting a small charge transfer resistance attributed to the improved electrical conductivity of the PANI-CuCo2O4. Straight line at low frequency region (Warburg impedance) originated from OH- diffusion from the electrolyte to the electrode surface. PANI-CuCo2O4 exhibited more vertical line parallel to the imaginary axis than CuCo2O4 and PANI, which was supported high capacitive behavior of PANI-CuCo2O4 [32]. Furthermore, the line for PANI-CuCo2O4 was shorter than its counterparts indicated the shorter ion diffusion path through the electrode [33]. Therefore, the incorporation of PANI with CuCo2O4 nanoparticles exhibited better conductivity with rapid electron shuttling play a prominent role in energy storage application.
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3.4.3 Asymmetric supercapacitor assembly and electrochemical characterization Besides specific capacitance, extend in operation voltage window was one of the factors that would enhance the energy density of real capacitor according to the equation of the stored energy: E=0.5 CV2. Thus, asymmetric supercapacitor was fabricated using PANI-CuCo2O4 as the battery-type positive electrode and activated carbon (AC) as a capacitive negative electrode as shown in Fig 8a. Prior to the fabrication of the supercapacitor, CVs of AC and PANICuCo2O4 were recorded in three electrode cell to decide the maximum working potential of the supercapacitor. Fig. 8b shows the CV curves of both AC (at scan rate of 10 mV/s) and PANICuCo2O4 (at scan rate of 5 mV/s) respectively. It was noted that the stable potential window of the fabricated supercapacitor can be extended to 1.5 V, above 1.5 V the curve was seriously deviated from rectangularity (Fig. 8c).The asymmetric supercapacitor device exhibited quasi rectangular shape with broad redox peak [34]. Fig. 8d showed the CV curves of supercapacitor device at the different scan rates. The shape of the CV curves was well maintained, which was suggesting the good rate capability of the device [35]. The GDC curves at different current densities were shown in Fig 9a. The charging and discharging curves were almost symmetric, suggesting the high reversible electrochemistry of the fabricated device [36]. The specific capacity, Q, from the charge-discharge curve was calculated according to the following equation; Q=
2×I×∆t m
(6)
Where I is the discharge current and ∆t is the discharge time after IR drop. The specific capacity of AC//PANI-CuCo2O4 was 367.5 C/g at a current density of 0.4 A/g. Furthermore, the
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fabricated device showed good rate performance with 60% of specific capacity retained when the current density increased from 0.4 A/g to 2 A/g (Fig. 9b). The Ragone plot of the device describing the relationship between energy density and power density as shown in Fig. 9c. The energy density (E) and power density (P) were calculated using the following equations; Q∆V
E (Wh/kg)=2×3.6 P (W/kg) =
E ×3600 Δt d
(7)
(8)
Where Q is specific capacity (C/g), ∆𝑉 is the potential window and td is the discharge time, respectively.The fabricated device stores the maximum energy density of 76 Wh/kg and delivers power density of 599 W/kg. The energy density decreased from 76 to 46 Wh/kg, while the power density increased from 599 to 3011 W/kg at current density of 0.4 – 2 A/g. These values are higher than recently reported work in literature such as CuCo2O4@MnO2//AG [37], NixCo3-xO4//AC [38], PCF@ZnCo2O4//PCF [39] and PANI//MoO3 [40]. Since the long-term cyclic stability was another crucial parameter for supercapacitor electrode materials, the fabricated asymmetric PANI-CuCo2O4 was subjected to 3000 continuous cycles of chargingdischarging at a current density of 0.8 A/g. Fig 9d displayed the capacitance retention of the fabricated asymmetric supercapacitor as a function of the cycle numbers. Noticeably, the slight increase of capacity percentage up to the maximum 113% before 200 cycles due to the full activation process of the device. Then the capacity retention slightly decayed to 94% after 3000 cycles which was due to the swelling and shrinkage of PANI during the long-term chargedischarge process [41]. This result suggesting that the superior electrochemical stability of the asymmetric supercapacitor device. 14
Conclusion The spinel structure of CuCo2O4 nanoparticles synthesized via a facile hydrothermal method which was authenticated by XRD and FTIR analysis. Upon introduction of PANI to CuCo2O4 nanoparticles, the conductivity was enhanced due to the effective intercalation of conducting polymer with CuCo2O4. As compared with pure CuCo2O4 and PANI, the PANICuCo2O4 composite exhibited enhanced specific capacitance from cyclic voltammetry and galvanostatic charge-discharge analyses. This improvement was mainly due to the presence of of PANI which was promoting the effective charge transport mechanism. Meanwhile, the fabricated asymmetric supercapacitor delivered a high energy density (76 Wh/kg) at high power density (595 W/kg), which was attributed to the combination of PANI-CuCo2O4 with AC. In addition, the device showed superior long-term stability, which was retained 94% of initial capacitance after 3000 cycles. Acknowledgment This work was supported by the High Impact Research Grant (H-21001-F000046), Fundamental Research Grant Scheme (FP012-2015A) from Ministry of Education, Malaysia and Postgraduate Research Grant (PG034-2015A). One of the author Dr. Navaneethan Duraisamy acknowledges UGC-Dr. D.S. Kothari Postdoctoral Fellowship (Ref no: No.F.4-2/2006 (BSR)/EN/15-16/0031).
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References [1]
A. Ramadoss, S.J. Kim, Vertically aligned TiO2 nanorod arrays for electrochemical supercapacitor, J. Alloys Compd. 561 (2013) 262–267.
[2]
R.R. Salunkhe, K. Jang, H. Yu, S. Yu, T. Ganesh, S.H. Han, et al., Chemical synthesis and electrochemical analysis of nickel cobaltite nanostructures for supercapacitor applications, J. Alloys Compd. 509 (2011) 6677–6682.
[3]
T. Brousse, B. Daniel, To Be or Not To Be Pseudocapacitive?, J. Electrochem. Soc. 162 (2015) 5185–5189.
[4]
D. Guo, L. Lai, A. Cao, H. Liu, S. Dou, J. Ma, Nanoarrays: design, preparation and supercapacitor applications, RSC Adv. 5 (2015) 55856–55869.
[5]
C. Ramirez-Castro, O. Crosnier, L. Athouel, R. Retoux, D. Belanger, T. Brousse, Electrochemical Performance of Carbon/MnO2 Nanocomposites Prepared via Molecular Bridging as Supercapacitor Electrode Materials, J. Electrochem. Soc. 162 (2015) A5179– A5184.
[6]
M.G. Jeong, K. Zhuo, S. Cherevko, W.J. Kim, C.H. Chung, Facile preparation of threedimensional porous hydrous ruthenium oxide electrode for supercapacitors, J. Power Sources. 244 (2013) 806–811.
[7]
D.P. Dubal, G.S. Gund, R. Holze, H.S. Jadhav, C.D. Lokhande, C.J. Park, Solution-based binder-free synthetic approach of RuO2 thin films for all solid state supercapacitors, Electrochim. Acta. 103 (2013) 103–109.
[8]
Q. Wang, X. Wang, J. Xu, X. Ouyang, X. Hou, D. Chen, et al., Flexible coaxial-type fi ber supercapacitor based on NiCo2O4 nanosheets electrodes, Nano Energy. 8 (2014) 44–51.
[9]
Q. Wang, J. Du, Y. Zhu, J. Yang, J. Chen, C. Wang, et al., Facile fabrication and supercapacitive properties of mesoporous zinc cobaltite microspheres, J. Power Sources. 284 (2015) 138–145.
[10] L. Lv, Q. Xu, R. Ding, L. Qi, H. Wang, Chemical synthesis of mesoporous CoFe2O4 nanoparticles as promising bifunctional electrode materials for supercapacitors, Mater. Lett. 111 (2013) 35–38. [11] L. Li, Y.Q. Zhang, X.Y. Liu, S.J. Shi, X.Y. Zhao, H. Zhang, et al., One-dimension MnCo2O4 nanowire arrays for electrochemical energy storage, Electrochim. Acta. 116 (2014) 467–474. [12] Z. Wu, Y. Zhu, X. Ji, NiCo2O4-based materials for electrochemical supercapacitors, J. Mater. Chem. A. 2 (2014) 14759. [13] J. Cheng, H. Yan, Y. Lu, K. Qiu, X. Hou, J. Xu, et al., Mesoporous CuCo2O4 nanograss as multi-functional electrodes for supercapacitors and electro-catalysts, J. Mater. Chem. A. 3 (2015) 1–8. [14] A.. MacDiarmid, L.. Yang, W.. Huang, B.. Humphrey, Polyaniline: Electrochemistry and application to rechargeable batteries, Synth. Met. 18 (1987) 393–398. [15] A. Mirmohseni, A. Oladegaragoze, Detection and determination of CrVI in solution using polyaniline modified quartz crystal electrode, J. Appl. Polym. Sci. 85 (2002) 2772–2780. 16
[16] K.S. Ryu, K.M. Kim, N.-G. Park, Y.J. Park, S.H. Chang, Symmetric redox supercapacitor with conducting polyaniline electrodes, J. Power Sources. 103 (2002) 305–309. [17] M. Khairy, M.E. Gouda, Electrical and optical properties of nickel ferrite/polyaniline nanocomposite, J. Adv. Res. 6 (2014) 555–562. [18] A. Pendashteh, M.S. Rahmanifar, R.B. Kaner, M.F. Mousavi, Facile synthesis of nanostructured CuCo2O4 as a novel electrode material for high-rate supercapacitors, Chem. Commun. 50 (2014) 1972. [19] J. Epstein, X-ray Structure of Polyaniline, Macromolecules. 24 (1991) 779–789. [20] B.M. Abu-Zied, S. a. Soliman, S.E. Abdellah, Enhanced direct N2O decomposition over CuxCo1−xCo2O4 (x=0.0≤x≤1.0) spinel-oxide catalysts, J. Ind. Eng. Chem. 21 (2014) 814– 821. [21] C. Li, J. Wang, Y. Wen, Y. Ning, X. Yuan, M. Li, et al., Polyaniline/CeO2 Nanofiber Composite Membrane as a Promoter of Pt for Formic Acid Electro-Oxidation, ECS Electrochem. Lett. 2 (2012) H1–H4. [22] Q. Wang, F. Qu, Towards three-dimensional hierarchical ZnO nanofiber@Ni(OH)2 nanoflake core–shell heterostructures for high-performance asymmetric supercapacitors, J. Mater. Chem. A Mater. Energy Sustain. 3 (2015) 18413–18421. [23] R. Kumar, P. Rai, A. Sharma, 3D urchin-shaped Ni3(VO4)2 hollow nanospheres for highperformance asymmetric supercapacitor applications, J. Mater. Chem. A. 4 (2016) 9822– 9831. [24] S. Vijayakumar, S. Lee, K. Ryu, Hierarchical CuCo2O4 nanobelts as a supercapacitor electrode with high areal and specific capacitance, Electrochim. Acta. 182 (2015) 979– 986. [25] K. Wang, C. Zhao, S. Min, X. Qian, Electrochimica Acta Facile Synthesis of Cu2O/RGO/Ni(OH )2 Nanocomposite and its Double Synergistic Effect on Supercapacitor Performance, Electrochim. Acta. 165 (2015) 314–322. [26] E. Umeshbabu, G. Rajeshkhanna, P. Justin, G.R. Rao, Synthesis of mesoporous NiCo2O4 –rGO by a solvothermal method for charge storage applications, RSC Adv. 5 (2015) 66657–66666. [27] H.Y. Wu, H.W. Wang, Electrochemical synthesis of nickel oxide nanoparticulate films on nickel foils for high-performance electrode materials of supercapacitors, Int. J. Electrochem. Sci. 7 (2012) 4405–4417. [28] P. Simon, Y. Gogotsi, B. Dunn, Where Do Batteries End and Supercapacitors Begin?, Science (80-. ). 343 (2014) 1210–1211. [29] L.-L. Zhang, H.-H. Li, C.-Y. Fan, K. Wang, X.-L. Wu, H.-Z. Sun, et al., A vertical and cross-linked Ni(OH)2 network on cellulose-fiber covered with graphene as a binder-free electrode for advanced asymmetric supercapacitors, J. Mater. Chem. A. 3 (2015) 19077– 19084. [30] L. Chen, Z. Song, G. Liu, J. Qiu, C. Yu, J. Qin, et al., Synthesis and electrochemical performance of polyaniline–MnO2 nanowire composites for supercapacitors, J. Phys. Chem. Solids. 74 (2013) 360–365. 17
[31] Jaidev, R.I. Jafri, A.K. Mishra, S. Ramaprabhu, Polyaniline–MnO2 nanotube hybrid nanocomposite as supercapacitor electrode material in acidic electrolyte, J. Mater. Chem. 21 (2011) 17601. [32] V. Sahu, S. Goel, R.K. Sharma, G. Singh, Zinc oxide nanoring embedded lacey graphene nanoribbons in symmetric/asymmetric electrochemical capacitive energy storage, Nanoscale. 7 (2015) 20642–51. [33] S.K. Balasingam, J.S. Lee, Y. Jun, Few-layered MoSe2 nanosheets as an advanced electrode material for supercapacitors, Dalt. Trans. 44 (2015) 15491–15498. [34] L. Zhang, K.N. Hui, K. San Hui, H. Lee, High-performance hybrid supercapacitor with 3D hierarchical porous flower-like layered double hydroxide grown on nickel foam as binder-free electrode, J. Power Sources. 318 (2016) 76–85. [35] Q. Wang, D. Chen, D. Zhang, Electrospun porous CuCo2O4 nanowire network electrode for asymmetric supercapacitors, RSC Adv. 5 (2015) 96448–96454. [36] Z. Li, Z. Zhou, G. Yun, K. Shi, X. Lv, B. Yang, High-performance solid-state supercapacitors based on graphene-ZnO hybrid nanocomposites, Nanoscale Res. Lett. 8 (2013) 1–9. [37] M. Kuang, X.Y. Liu, F. Dong, Y.X. Zhang, Tunable design of layered CuCo2O4 nanosheets@MnO2 nanoflakes core–shell arrays on Ni foam for high-performance supercapacitors, J. Mater. Chem. A Mater. Energy Sustain. 3 (2015) 21528–21536. [38] X. Wang, C. Yan, A. Sumboja, P.S. Lee, High performance porous nickel cobalt oxide nanowires for asymmetric supercapacitor, Nano Energy. 3 (2014) 119–126. [39] H. Niu, X. Yang, H. Jiang, D. Zhou, X. Li, T. Zhang, et al., Hierarchical core–shell heterostructure of porous carbon nanofiber@ZnCo2O4 nanoneedle arrays: advanced binder-free electrodes for all-solid-state supercapacitors, J. Mater. Chem. A. 3 (2015) 24082–24094. [40] H. Peng, G. Ma, J. Mu, K. Sun, Z. Lei, Low-cost and high energy density asymmetric supercapacitors based on polyaniline nanotubes and MoO3 nanobelts, J. Mater. Chem. A. 2 (2014) 10384–10388. [41] G. Xiong, C. Meng, R.G. Reifenberger, P.P. Irazoqui, T.S. Fisher, Graphitic petal electrodes for all-solid-state flexible supercapacitors, Adv. Energy Mater. 4 (2014) 897– 905.
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Figure captions Fig. 1. Schematic diagram of PANI-CuCo2O4 composite preparation Fig. 2. XRD pattern of CuCo2O4, PANI and PANI-CuCo2O4 Fig. 3. FTIR spectra of CuCo2O4, PANI, PANI-CuCo2O4 Fig. 4. FESEM image: (a) PANI at low magnification(b) PANI at high magnification (c) CuCo2O4 at low magnification (d) CuCo2O4 at high magnification (e) PANI-CuCo2O4 at low magnification (f) PANI-CuCo2O4 at high magnification Fig. 5. Cyclic voltammogram of (a) CuCo2O4 (b) PANI and (c) PANI-CuCo2O4 at different scan rates. Fig. 6. Galvanostatic discharge curves of (a) CuCo2O4 (b) PANI and (c) PANI-CuCo2O4 at different current densities (c) Specific capacity with respect to current densities. Fig. 7. Nyquist plots of CuCo2O4, PANI and PANI-CuCo2O4. Inset shows the enlarged EIS of CuCo2O4, PANI and PANI-CuCo2O4 Fig. 8. (a) Schematic illustration of the as-sembled asymmetric supercapacitor (b) Comparative CV curves of PANI-CuCo2O4 as a positive electrode (at 1 mV/s) and activated carbon (at 10 mV/s) as a negative electrode performed in a three-electrode cell in 1 M KOH electrolyte (b) CV curves of AC//PANI-CuCo2O4 supercapacitor measured at different potential windows at a scan rate of 20 mV/s (d) CV curves of AC//PANI-CuCo2O4 supercapacitor measured at different scan rates. Fig. 9. (a) Galvanostatic charge/discharge curves of AC//PANI-CuCo2O4 supercapacitor at different current densities (b) Specific capacitance of the asymmetric supercapacitor at different current densities (c) Ragone plot related to energy and power densities of AC//PANI-CuCo2O4 supercapacitor (d) Cycling stability of AC//PANI-CuCo2O4 supercapacitor
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S0013-4686(17)30006-3 http://dx.doi.org/doi:10.1016/j.electacta.2017.01.006 EA 28673
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
8-3-2016 10-8-2016 1-1-2017
Please cite this article as: Fatin Saiha Omar, Arshid Numan, Navaneethan Duraisamy, Mohammad Mukhlis Ramly, Ramesh K., Ramesh S., Binary composite of polyaniline/copper cobaltite for high performance asymmetric supercapacitor application, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.01.006
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Binary composite of polyaniline/copper cobaltite for high performance asymmetric supercapacitor application
Fatin Saiha Omar1δ, Arshid Numan1, Navaneethan Duraisamy2#, Mohammad Mukhlis Ramly1, K. Ramesh1, S. Ramesh1*
Center for Ionics University of Malaya, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia 2
Department of Chemistry, Periyar University, Salem-636011, Tamilnadu, India
Corresponding authors *
E-mail: [email protected], Tel: +603-7967 4391, Fax: +603-7967 4146
#
E-mail: [email protected]
δ
E-mail: [email protected]
1
Abstract This article presents the effect of polyaniline (PANI) embedded copper cobaltite (CuCo2O4) as an electrode material for high performance supercapacitor application. The composite of PANI-CuCo2O4 was prepared via blending process. The formation of PANICuCo2O4 composite was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analysis. The surface morphologies showed that the spinel structure of CuCo 2O4 (average particle size of 30 nm) was well distributed on PANI matrix, suggest the effective intercalation of CuCo2O4 with PANI matrix. The electrochemical properties of CuCo2O4, PANI and PANICuCo2O4 composite were investigated using cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS) in 1 M of KOH as an aqueous electrolyte. The PANI-CuCo2O4 composite showed the improved specific capacitance of 403 C/g than that of pure CuCo2O4 and PANI.The enhanced electrochemical performance was obtained due to the augmentation of redox active sites and synergetic effect between PANI and CuCo2O4 nanoparticles. Additionally, the fabricated (activated carbon (AC)/PANI-CuCo2O4) asymmetric supercapacitor device can be cycled reversibly at a cell voltage of 1.5 V, which exhibited excellent electrochemical performances with an energy density of 76 Wh/kg and a power density of 599 W/kg. It also presented a superior life cycle with 94% capacitance retention after 3000 cycles.
Keywords: Cobaltite;Polyaniline;Composites;Electrode materials;Supercapacitor
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1.
Introduction Supercapacitors are receiving extensive attention due to their efficient capability of storing
and discharging energy than that of other primary energy sources like fuel cells and batteries. In recent years, several works have been developed in order to get the high energy density, long cycling life and low internal resistance based energy system [1,2]. However, still, we have some limitation to develop the practical applications due to the lack of appropriate electrode materials with optimal electrochemical properties. Generally, there are two different ways to store the energies such as capacitive (electric double layer capacitors (EDLCs)) and pseudocapacitive (pseudocapacitor) nature. In EDLCs, the capacitance arises from the adsorption of electrolyte ions on the electrode/electrolyte interface without involve faradaic reaction leading to high power density. While for pseudocapacitors, the capacitance arises from the reversible faradaic reactions at the surface of electrode material as well as the insertion of cations from the electrolyte [3–5]. Ruthenium oxide (RuO2) has been recognized as the best pseudocapacitive material due to its high specific capacitance, good electrical conductivity and reversible chargedischarge properties [6,7]. However, there are some limitations to use RuO2 as an electrode material in a commercial scale such as expensive and high toxicity. In recent years, it has been demonstrated that spinel structure of transition metal oxides is an effective strategy to improve the performance of single-component metal oxides such as nickel cobaltite (NiCo2O4), zinc cobaltite (ZnCo2O4), cobalt ferrite (CoFe2O4), manganese cobaltite (MnCo2O4) and etc [8–11]. The presence of two metal oxides within the one molecule exhibited better physico-chemical properties such as good electrical conductivity, high stability and improved redox sites with numerous oxidation states than that of single metal oxides [12]. Among these spinel metal oxides, CuCo2O4 has been realized as a good electrode material for 3
supercapacitor application. CuCo2O4 spinel has unique structure where divalent Cu ions occupy the tetrahedral site and the trivalent Co ions occupy the octahedral site in the cubic spinel structure. In addition, CuCo2O4 offers several advantages including abundant resources, low cost and environmental friendliness [13]. However, the capacitance and operation voltage are still the challenging task to improve the supercapacitor performance. Polyaniline (PANI) is one of the most interesting and widely used conductive polymers in various applications such as batteries [14], sensors [15] and supercapacitor [16]. PANI has several advantages in terms of good electrical conductivity, high electrochemical activity, chemical stability, excellent redox properties, low cost and flexibility. In general, PANI exhibiting three different structural forms such as fully reduced state (leucoemeraldine), fully oxidized state (pernigraniline) and half-oxidized state (emeraldine base)
[17]. Herein, the
emeraldine form is the most conductive part and also is the key factor to enhance the electrochemical activity. The present work is investigating the electrochemical performance of CuCo2O4 intercalated PANI matrix. The improved electrochemical properties in spinel structured CuCo2O4 are offered by the incorporation of PANI, which lead to restrict the aggregation of CuCo2O4 with improved electrochemical surface area. To the best of our knowledge, no attention has been paid to the comparison between the pure CuCo2O4 and the PANI-CuCo2O4 composite especially for supercapacitor application. In addition, the performance of PANI-CuCo2O4 in asymmetric supercapacitor is also studied.
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2. Materials and Method 2.1. Materials Copper acetate dihydrate, Cu(O₂CCH₃)₂.2H2O and cobalt acetate tetrahydrate Co(C₂H₃O₂)₂.4H2O were purchased from Friendemann Schmidt, Malaysia. Urea, ammonia (28% purity), aniline,
ammonium
peroxydisulfate (APS), hydrochloric acid
(HCl),
polyvinylidene fluoride (PVdF), acetylene black and activated carbon (AC) were received from Sigma-Aldrich, Malaysia. Ethanol (95% purity) was purchased from J. Kollin Chemical, Malaysia. 2.2. Synthesis of polyaniline (PANI) PANI was synthesized by chemical oxidation polymerization method. 0.25 M aniline was added into 20 ml of 1M aqueous HCl. The solution was stirred at room temperature for the formation of anilinium ions. Next, the freshly prepared APS solution (0.2 M APS dissolved into 30 ml of 1M HCl) was added dropwise into the above solution and allowed for continuous stirring until a green precipitate observed. Then the precipitate was washed with acetone and deionized water for several times and dried at 60 ºC for 1 hour. Finally, the observed green color was the evidence of the polyaniline formation in its conductive form of emeraldine salt (ES). 2.3. Synthesis of CuCo2O4 nanoparticles CuCo2O4 nanoparticles were synthesized by a simple hydrothermal treatment. Cu(O₂CCH₃)₂.2H2O (15 mmol), Co(C₂H₃O₂)₂.4H2O (15 mmol), urea (20 mmol) and 3.4 mL of ammonia were added into the solution of water-ethanol mixture and stirred for 19 hours. Next, the mixture was transferred into a 80 mL capacity of Teflon-lined stainless steel autoclave, which was heated and maintained at 180 ºC for 6 hours. After that the obtained sample was 5
washed with deionized water for several times to remove the redundant materials and then dried at 60 ºC. 2.4. Synthesis of PANI-CuCo2O4 nanoparticles PANI-CuCo2O4 nanocomposite was prepared by blending of PANI with CuCo2O4 (weight ratio of PANI:CuCo2O4= 4:1) in a pestle mortar to achieve a homogeneous mixture of PANI with CuCo2O4 composite. The entire preparation process was schematically illustrated in Fig. 1. 2.5. Characterization of the samples The crystalline phases of the samples were determined via X-ray diffraction (XRD; D5000, Siemens), using copper Kα radiation (λ=1.5418 Å) at a scan rate of 0.02º s-1. The morphological studies of synthesized pure CuCo2O4 nanoparticles, pure PANI and PANICuCo2O4 composite were carried out using JEOL JSM-7600F, field emission scanning electron microscopy (FESEM). Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet ISIO Smart ITR) analysis used to study the presence of functional groups and purity of samples, which was in the region from 400 to 2500 cm-1 at a resolution of 1 cm-1. 2.6. Electrochemical studies PANI-CuCo2O4 electrode was fabricated by mixing 75 wt% of the active material, 15 wt% of acetylene black and 10 wt% of PVdF in NMP (solvent) to achieve homogeneous slurry. Then, the cleaned nickel foam (1x1 cm2 area) was coated with the slurry and dried in an oven at 90 ºC for 14 hours. The mass of coated active material (PANI-CuCo2O4) was approximately ~3.75 mg. A standard three-electrode cell system was used to examine the electrochemical measurements. The PANI-CuCo2O4 coated Ni foam was used as the working electrode, Ag/AgCl 6
and platinum wire were used as the reference and counter electrodes, respectively. The measurements were conducted in a 1 M KOH at room temperature. Cyclic voltammetry (CV), galvanostatic charge-discharge test (GCD) and electrochemical impedance spectroscopy (EIS) with a frequency range of 0.01 Hz to 100 kHz were conducted using Gamry Instrument. For the comparison study, the same experiment was repeated using pure CuCo2O4 as an electrode material. 3. Results and discussion 3.1. X-ray diffraction (XRD) The structural crystallinity and phase purity of synthesized materials were investigated by X-ray diffraction (XRD) analysis. The XRD of pure CuCo2O4, PANI and PANI-CuCo2O4 was represented in Fig. 2. The diffraction peaks were observed at 2θ values of 19.07º, 31.36º, 36.96º, 38.96º, 45.07º, 56.03º, 59.59 º and 65.70º correspond to the (111), (220), (311), (222), (400), (422), (511) and (440) lattice planes respectively. This pattern were clearly indexed as a cubic CuCo2O4 with a spinel structure (JCPDS card no. 001-1155) and a space group of Fd3m [18]. No other additional peaks were detected suggesting the high purity of CuCo2O4. PANI showed the partial crystalline state with the diffraction peaks at 2θ = 14.67º, 20.72º and 25.59º attributed to the (011), (020) and (200) lattice planes of PANI chains in its emeraldine salt form respectively [19]. Moreveor, a weaker and broader diffraction peaks were observed in PANI-CuCo2O4 composite,which was due to the intercalation CuCo2O4 with PANI matrix.
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3.2. Fourier transform infrared spectroscopy (FTIR) The purity of prepared samples was further characterized by FTIR analysis. Fig. 3 represented the FTIR spectra of CuCo2O4, PANI and PANI-CuCo2O4 composite. CuCo2O4 revealed two strong absorption bands at 554 and 657 cm-1 correspond to the stretching vibration of Co3+ ̶ O2- in the tetrahedral complexes and Cu2+ ̶ O2- in the octahedral complexes. The peaks at ~832, 970, 1037, 1340 and 1540 cm-1 were assigned to the carbonate anion (derived from slow hydrolysis of urea during hydrothermal process) [20]. In case of PANI, the peaks around 1475 cm-1 and 1556 cm-1 correspond to –C=C stretching vibrations of benzenoid and quinonoid structures respectviely. The observed band at 1287 cm-1 was assigned to the C–N stretching of secondary aromatic amine. The bands at 785 cm-1and 1099 cm-1 were ascribed to C–H bending vibration of aromatic ring [21]. All characteristic peaks of PANI and CuCo2O4 can be observed in PANI-CuCo2O4 composite, which was confirmed the intercalation of CuCo2O4 with PANI matrix. 3.3. Field emission scanning electron microscopy (FESEM) The surface morphologies of pure PANI, CuCo2O4 and PANI-CuCo2O4 were investigated by FESEM analysis. Fig. 4a and 4b showed the branched geometrical surface morphology of pristine PANI structure with 100 nm in width, while Fig. 4c and 4d displayed the irregular shaped morphology of CuCo2O4nanoparticles with an average particle size of 33 nm. Fig. 4e and 4f represented the nanostructured CuCo2O4 decorated PANI matrix. High magnification of FESEM image (Fig. 4f) exhibited well embedded CuCo2O4 nanoparticles on PANI matrix, which was confirmed that the blending method able to form the effective interfacial contact between PANI and CuCo2O4. The incorporation of PANI with CuCo2O4 lead to control the aggregation of CuCo2O4 nanoparticles in PANI-CuCo2O4 composite. This kind of structure may increases the 8
effective liquid-solid interfacial area and allow the rapid electrolyte ions transportation during charge/discharge process [22].
3.4 Electrochemical characterizations 3.4.1. Cyclic voltammetry (CV) CV test was used to investigate the occurrence of faradaic or non-faradaic reactions in the electrode. The CuCo2O4, PANI and PANI-CuCo2O4 electrodes were measured for the comparison of capacitance difference in pure CuCo2O4 and after the introduction of PANI. The CV tests were performed in a potential range of 0.0‒0.5 V (vs. Ag/AgCl) using 1 M KOH electrolyte. Fig. 5a-5c exhibited the the CV curves of the pure CuCo2O4, PANI and PANICuCo2O4 electrodes with various scan rates ranging from 1 mV/s to 30 mV/s. The CV curves of CuCo2O4 exhibited well-defined redox peaks, which is a characteristic of faradaic behavior of battery-type electrode. It is noteworthy to mention that often transition metal oxide-based battery-type materials are misled as pseudocapacitive materials due to the similar charge storage mechanism [23]. The redox peaks correspond to the reversible reaction of Co4+/Co3+ and Cu2+/Cu+ transitions associated with hydroxyl ion (OH¯) from the KOH electrolyte. The redox reactions in alkaline medium are based on the equation (1), (2) and (3) [24]: CuCo2O4 + H2O + e¯ ↔ 2CoOOH + Cu(OH)¯
(1)
CoOOH + OH¯ ↔ CoO2 + H2O + e¯
(2)
CuOH + OH¯ ↔ Cu(OH)2 + e¯
(3)
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CV curves of PANI and PANI-CuCo2O4 electrodes also exhibit redox peaks contributed from the faradaic reactons. Furthermore, it was well known that the charge stored with respect to the area under the CV curves. It can be found that the integrated area of PANI-CuCo2O4 was larger than that of pure CuCo2O4 and PANI, indicating that the maximum capacity provided by PANI-CuCo2O4 [25]. In addition, while increasing the scan rate, peak currents are found to increase, and the anodic and cathodic peaks shifted towards higher and lower potential, respectively. This was due to the strengthened electric polarization and the possible kinetic irreversibility of electrolyte ions at the electrode surface during the redox reaction at high scan rates [26]. Notice that the shape of the CV curves was not significantly influenced by increasing scan rates, implying fine reversibility of redox reaction and good power property [27]. The materials exhibiting redox peaks in CV should not be interpret as pseudocapacitor as the capacitance of the electrode keep on changing over the whole potential window. Therefore, the electrochemical performance of CuCo2O4, PANI and PANI-CuCo2O4 was measured in terms of specific capacity instead of specific capacitance [28]. The specific capacity of the electrodes was calculated based on the CV curves using equation (4). 1
Vf
𝑄S = νm ∫Vi I × VdV
(4)
where 𝑄S is the specific capacity of the samples (C/g), ν is the scan rate (V/s), m is the mass of the active material (g) and the integral term is equal to the area under the CV curve. The calculated specific capacity values for pure CuCo2O4, PANI and PANI-CuCo2O4 were found to be 97, 281 and 312 C/g, respectively at a scan rate of 1 mV/s.
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3.4.2. Charge-discharge study and electrochemical impedance spectroscopy The GCD is a reliable method to evaluate the electrochemical capacitance of materials under controlled conditions. Fig. 6a-6c manifest the galvanostatic discharge curves of CuCo2O4, PANI and PANI-CuCo2O4 with a potential range of 0‒0.38 V. Herein, the electrode materials showed the nonlinear discharge curves confirmed the faradaic behavior of battery-type electrode. Apparently, the discharge time of PANI-CuCo2O4 was longer than that of CuCo2O4 and PANI, implying better electrochemical capability of the PANI-CuCo2O4 [29]. The specific capacity of the electrodes was calculated from the galvanostatic discharge curves using equation (5); 𝑄𝑠 =
I ×∆t m
(5)
where 𝑄𝑠 is the specific capacity of the electrode (C/g), I is the current (A), m is the mass loading of the active material on electrode (g) and ∆𝑡 is the discharge time (s). The specific capacity values of PANI-CuCo2O4 were found to be 403, 378, 303 and 218 C/g at current density of 0.4, 0.5, 1 and 2 A/g, respectively (Fig. 6d). On the other hand, the specific capacity values for CuCo2O4 were found to be 206, 118, 69 and 66 C/g, while PANI delivered specific capacity of 240, 232, 189 and 163 C/g at current density of 0.4, 0.5, 1 and 2 A/g respectively, which are far lower than specific capacity values of PANI-CuCo2O4 electrode. The maximum specific capacity of PANI-CuCo2O4 was due to the effective ion or charge transfer mechanism between PANI and CuCo2O4. The disparity in the capacity between CuCo2O4, PANI and PANI-CuCo2O4 composite could be interpreted by the following reasons; (i) the large surface area of PANI can act as a spacer to prevent the aggregation of CuCo2O4 nanoparticles, providing more redox active sites for the accessibility of OH¯ ions. While, in pure CuCo2O4 nanoparticles, closely packed particles lower the number of redox active sites for rapid OH¯accessibility (ii) the contribution from the
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conductivity of PANI itself in the nanocomposite are partially involved during the redox reaction (iii) the synergetic effect formed between conductive PANI and CuCo2O4 nanoparticles contributed to the enhancement of specific capacitance [30,31]. EIS was employed to investigate the conductivity and interfacial charge transfer process at the electrode/electrolyte interface. Fig. 7 showed the EIS plot (Nyquist) plot between Z’ and – Z” recorded between 0.01 Hz-100 kHz frequency region. Inset of the Fig. 7 showed the enlarge portion of the high frequency region. It can be observed that the low equivalent series resistance (ESR) values (0.65 Ω) for PANI-CuCo2O4 than other electrodes manifesting the low internal resistance of the electrode. The diameter of the semicircle observed at low frequency region estimates the faradaic charge transfer resistance (Rct) which was related to the electrode surface area and electrical conductivity of the electrode material. In this respect, CuCo2O4 illustrated larger semicircle showed the higher resistance involved during charge transfer at the electrode material/electrolyte interface. There was no obvious semicircle observed at PANI-CuCo2O4, suggesting a small charge transfer resistance attributed to the improved electrical conductivity of the PANI-CuCo2O4. Straight line at low frequency region (Warburg impedance) originated from OH- diffusion from the electrolyte to the electrode surface. PANI-CuCo2O4 exhibited more vertical line parallel to the imaginary axis than CuCo2O4 and PANI, which was supported high capacitive behavior of PANI-CuCo2O4 [32]. Furthermore, the line for PANI-CuCo2O4 was shorter than its counterparts indicated the shorter ion diffusion path through the electrode [33]. Therefore, the incorporation of PANI with CuCo2O4 nanoparticles exhibited better conductivity with rapid electron shuttling play a prominent role in energy storage application.
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3.4.3 Asymmetric supercapacitor assembly and electrochemical characterization Besides specific capacitance, extend in operation voltage window was one of the factors that would enhance the energy density of real capacitor according to the equation of the stored energy: E=0.5 CV2. Thus, asymmetric supercapacitor was fabricated using PANI-CuCo2O4 as the battery-type positive electrode and activated carbon (AC) as a capacitive negative electrode as shown in Fig 8a. Prior to the fabrication of the supercapacitor, CVs of AC and PANICuCo2O4 were recorded in three electrode cell to decide the maximum working potential of the supercapacitor. Fig. 8b shows the CV curves of both AC (at scan rate of 10 mV/s) and PANICuCo2O4 (at scan rate of 5 mV/s) respectively. It was noted that the stable potential window of the fabricated supercapacitor can be extended to 1.5 V, above 1.5 V the curve was seriously deviated from rectangularity (Fig. 8c).The asymmetric supercapacitor device exhibited quasi rectangular shape with broad redox peak [34]. Fig. 8d showed the CV curves of supercapacitor device at the different scan rates. The shape of the CV curves was well maintained, which was suggesting the good rate capability of the device [35]. The GDC curves at different current densities were shown in Fig 9a. The charging and discharging curves were almost symmetric, suggesting the high reversible electrochemistry of the fabricated device [36]. The specific capacity, Q, from the charge-discharge curve was calculated according to the following equation; Q=
2×I×∆t m
(6)
Where I is the discharge current and ∆t is the discharge time after IR drop. The specific capacity of AC//PANI-CuCo2O4 was 367.5 C/g at a current density of 0.4 A/g. Furthermore, the
13
fabricated device showed good rate performance with 60% of specific capacity retained when the current density increased from 0.4 A/g to 2 A/g (Fig. 9b). The Ragone plot of the device describing the relationship between energy density and power density as shown in Fig. 9c. The energy density (E) and power density (P) were calculated using the following equations; Q∆V
E (Wh/kg)=2×3.6 P (W/kg) =
E ×3600 Δt d
(7)
(8)
Where Q is specific capacity (C/g), ∆𝑉 is the potential window and td is the discharge time, respectively.The fabricated device stores the maximum energy density of 76 Wh/kg and delivers power density of 599 W/kg. The energy density decreased from 76 to 46 Wh/kg, while the power density increased from 599 to 3011 W/kg at current density of 0.4 – 2 A/g. These values are higher than recently reported work in literature such as CuCo2O4@MnO2//AG [37], NixCo3-xO4//AC [38], PCF@ZnCo2O4//PCF [39] and PANI//MoO3 [40]. Since the long-term cyclic stability was another crucial parameter for supercapacitor electrode materials, the fabricated asymmetric PANI-CuCo2O4 was subjected to 3000 continuous cycles of chargingdischarging at a current density of 0.8 A/g. Fig 9d displayed the capacitance retention of the fabricated asymmetric supercapacitor as a function of the cycle numbers. Noticeably, the slight increase of capacity percentage up to the maximum 113% before 200 cycles due to the full activation process of the device. Then the capacity retention slightly decayed to 94% after 3000 cycles which was due to the swelling and shrinkage of PANI during the long-term chargedischarge process [41]. This result suggesting that the superior electrochemical stability of the asymmetric supercapacitor device. 14
Conclusion The spinel structure of CuCo2O4 nanoparticles synthesized via a facile hydrothermal method which was authenticated by XRD and FTIR analysis. Upon introduction of PANI to CuCo2O4 nanoparticles, the conductivity was enhanced due to the effective intercalation of conducting polymer with CuCo2O4. As compared with pure CuCo2O4 and PANI, the PANICuCo2O4 composite exhibited enhanced specific capacitance from cyclic voltammetry and galvanostatic charge-discharge analyses. This improvement was mainly due to the presence of of PANI which was promoting the effective charge transport mechanism. Meanwhile, the fabricated asymmetric supercapacitor delivered a high energy density (76 Wh/kg) at high power density (595 W/kg), which was attributed to the combination of PANI-CuCo2O4 with AC. In addition, the device showed superior long-term stability, which was retained 94% of initial capacitance after 3000 cycles. Acknowledgment This work was supported by the High Impact Research Grant (H-21001-F000046), Fundamental Research Grant Scheme (FP012-2015A) from Ministry of Education, Malaysia and Postgraduate Research Grant (PG034-2015A). One of the author Dr. Navaneethan Duraisamy acknowledges UGC-Dr. D.S. Kothari Postdoctoral Fellowship (Ref no: No.F.4-2/2006 (BSR)/EN/15-16/0031).
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References [1]
A. Ramadoss, S.J. Kim, Vertically aligned TiO2 nanorod arrays for electrochemical supercapacitor, J. Alloys Compd. 561 (2013) 262–267.
[2]
R.R. Salunkhe, K. Jang, H. Yu, S. Yu, T. Ganesh, S.H. Han, et al., Chemical synthesis and electrochemical analysis of nickel cobaltite nanostructures for supercapacitor applications, J. Alloys Compd. 509 (2011) 6677–6682.
[3]
T. Brousse, B. Daniel, To Be or Not To Be Pseudocapacitive?, J. Electrochem. Soc. 162 (2015) 5185–5189.
[4]
D. Guo, L. Lai, A. Cao, H. Liu, S. Dou, J. Ma, Nanoarrays: design, preparation and supercapacitor applications, RSC Adv. 5 (2015) 55856–55869.
[5]
C. Ramirez-Castro, O. Crosnier, L. Athouel, R. Retoux, D. Belanger, T. Brousse, Electrochemical Performance of Carbon/MnO2 Nanocomposites Prepared via Molecular Bridging as Supercapacitor Electrode Materials, J. Electrochem. Soc. 162 (2015) A5179– A5184.
[6]
M.G. Jeong, K. Zhuo, S. Cherevko, W.J. Kim, C.H. Chung, Facile preparation of threedimensional porous hydrous ruthenium oxide electrode for supercapacitors, J. Power Sources. 244 (2013) 806–811.
[7]
D.P. Dubal, G.S. Gund, R. Holze, H.S. Jadhav, C.D. Lokhande, C.J. Park, Solution-based binder-free synthetic approach of RuO2 thin films for all solid state supercapacitors, Electrochim. Acta. 103 (2013) 103–109.
[8]
Q. Wang, X. Wang, J. Xu, X. Ouyang, X. Hou, D. Chen, et al., Flexible coaxial-type fi ber supercapacitor based on NiCo2O4 nanosheets electrodes, Nano Energy. 8 (2014) 44–51.
[9]
Q. Wang, J. Du, Y. Zhu, J. Yang, J. Chen, C. Wang, et al., Facile fabrication and supercapacitive properties of mesoporous zinc cobaltite microspheres, J. Power Sources. 284 (2015) 138–145.
[10] L. Lv, Q. Xu, R. Ding, L. Qi, H. Wang, Chemical synthesis of mesoporous CoFe2O4 nanoparticles as promising bifunctional electrode materials for supercapacitors, Mater. Lett. 111 (2013) 35–38. [11] L. Li, Y.Q. Zhang, X.Y. Liu, S.J. Shi, X.Y. Zhao, H. Zhang, et al., One-dimension MnCo2O4 nanowire arrays for electrochemical energy storage, Electrochim. Acta. 116 (2014) 467–474. [12] Z. Wu, Y. Zhu, X. Ji, NiCo2O4-based materials for electrochemical supercapacitors, J. Mater. Chem. A. 2 (2014) 14759. [13] J. Cheng, H. Yan, Y. Lu, K. Qiu, X. Hou, J. Xu, et al., Mesoporous CuCo2O4 nanograss as multi-functional electrodes for supercapacitors and electro-catalysts, J. Mater. Chem. A. 3 (2015) 1–8. [14] A.. MacDiarmid, L.. Yang, W.. Huang, B.. Humphrey, Polyaniline: Electrochemistry and application to rechargeable batteries, Synth. Met. 18 (1987) 393–398. [15] A. Mirmohseni, A. Oladegaragoze, Detection and determination of CrVI in solution using polyaniline modified quartz crystal electrode, J. Appl. Polym. Sci. 85 (2002) 2772–2780. 16
[16] K.S. Ryu, K.M. Kim, N.-G. Park, Y.J. Park, S.H. Chang, Symmetric redox supercapacitor with conducting polyaniline electrodes, J. Power Sources. 103 (2002) 305–309. [17] M. Khairy, M.E. Gouda, Electrical and optical properties of nickel ferrite/polyaniline nanocomposite, J. Adv. Res. 6 (2014) 555–562. [18] A. Pendashteh, M.S. Rahmanifar, R.B. Kaner, M.F. Mousavi, Facile synthesis of nanostructured CuCo2O4 as a novel electrode material for high-rate supercapacitors, Chem. Commun. 50 (2014) 1972. [19] J. Epstein, X-ray Structure of Polyaniline, Macromolecules. 24 (1991) 779–789. [20] B.M. Abu-Zied, S. a. Soliman, S.E. Abdellah, Enhanced direct N2O decomposition over CuxCo1−xCo2O4 (x=0.0≤x≤1.0) spinel-oxide catalysts, J. Ind. Eng. Chem. 21 (2014) 814– 821. [21] C. Li, J. Wang, Y. Wen, Y. Ning, X. Yuan, M. Li, et al., Polyaniline/CeO2 Nanofiber Composite Membrane as a Promoter of Pt for Formic Acid Electro-Oxidation, ECS Electrochem. Lett. 2 (2012) H1–H4. [22] Q. Wang, F. Qu, Towards three-dimensional hierarchical ZnO nanofiber@Ni(OH)2 nanoflake core–shell heterostructures for high-performance asymmetric supercapacitors, J. Mater. Chem. A Mater. Energy Sustain. 3 (2015) 18413–18421. [23] R. Kumar, P. Rai, A. Sharma, 3D urchin-shaped Ni3(VO4)2 hollow nanospheres for highperformance asymmetric supercapacitor applications, J. Mater. Chem. A. 4 (2016) 9822– 9831. [24] S. Vijayakumar, S. Lee, K. Ryu, Hierarchical CuCo2O4 nanobelts as a supercapacitor electrode with high areal and specific capacitance, Electrochim. Acta. 182 (2015) 979– 986. [25] K. Wang, C. Zhao, S. Min, X. Qian, Electrochimica Acta Facile Synthesis of Cu2O/RGO/Ni(OH )2 Nanocomposite and its Double Synergistic Effect on Supercapacitor Performance, Electrochim. Acta. 165 (2015) 314–322. [26] E. Umeshbabu, G. Rajeshkhanna, P. Justin, G.R. Rao, Synthesis of mesoporous NiCo2O4 –rGO by a solvothermal method for charge storage applications, RSC Adv. 5 (2015) 66657–66666. [27] H.Y. Wu, H.W. Wang, Electrochemical synthesis of nickel oxide nanoparticulate films on nickel foils for high-performance electrode materials of supercapacitors, Int. J. Electrochem. Sci. 7 (2012) 4405–4417. [28] P. Simon, Y. Gogotsi, B. Dunn, Where Do Batteries End and Supercapacitors Begin?, Science (80-. ). 343 (2014) 1210–1211. [29] L.-L. Zhang, H.-H. Li, C.-Y. Fan, K. Wang, X.-L. Wu, H.-Z. Sun, et al., A vertical and cross-linked Ni(OH)2 network on cellulose-fiber covered with graphene as a binder-free electrode for advanced asymmetric supercapacitors, J. Mater. Chem. A. 3 (2015) 19077– 19084. [30] L. Chen, Z. Song, G. Liu, J. Qiu, C. Yu, J. Qin, et al., Synthesis and electrochemical performance of polyaniline–MnO2 nanowire composites for supercapacitors, J. Phys. Chem. Solids. 74 (2013) 360–365. 17
[31] Jaidev, R.I. Jafri, A.K. Mishra, S. Ramaprabhu, Polyaniline–MnO2 nanotube hybrid nanocomposite as supercapacitor electrode material in acidic electrolyte, J. Mater. Chem. 21 (2011) 17601. [32] V. Sahu, S. Goel, R.K. Sharma, G. Singh, Zinc oxide nanoring embedded lacey graphene nanoribbons in symmetric/asymmetric electrochemical capacitive energy storage, Nanoscale. 7 (2015) 20642–51. [33] S.K. Balasingam, J.S. Lee, Y. Jun, Few-layered MoSe2 nanosheets as an advanced electrode material for supercapacitors, Dalt. Trans. 44 (2015) 15491–15498. [34] L. Zhang, K.N. Hui, K. San Hui, H. Lee, High-performance hybrid supercapacitor with 3D hierarchical porous flower-like layered double hydroxide grown on nickel foam as binder-free electrode, J. Power Sources. 318 (2016) 76–85. [35] Q. Wang, D. Chen, D. Zhang, Electrospun porous CuCo2O4 nanowire network electrode for asymmetric supercapacitors, RSC Adv. 5 (2015) 96448–96454. [36] Z. Li, Z. Zhou, G. Yun, K. Shi, X. Lv, B. Yang, High-performance solid-state supercapacitors based on graphene-ZnO hybrid nanocomposites, Nanoscale Res. Lett. 8 (2013) 1–9. [37] M. Kuang, X.Y. Liu, F. Dong, Y.X. Zhang, Tunable design of layered CuCo2O4 nanosheets@MnO2 nanoflakes core–shell arrays on Ni foam for high-performance supercapacitors, J. Mater. Chem. A Mater. Energy Sustain. 3 (2015) 21528–21536. [38] X. Wang, C. Yan, A. Sumboja, P.S. Lee, High performance porous nickel cobalt oxide nanowires for asymmetric supercapacitor, Nano Energy. 3 (2014) 119–126. [39] H. Niu, X. Yang, H. Jiang, D. Zhou, X. Li, T. Zhang, et al., Hierarchical core–shell heterostructure of porous carbon nanofiber@ZnCo2O4 nanoneedle arrays: advanced binder-free electrodes for all-solid-state supercapacitors, J. Mater. Chem. A. 3 (2015) 24082–24094. [40] H. Peng, G. Ma, J. Mu, K. Sun, Z. Lei, Low-cost and high energy density asymmetric supercapacitors based on polyaniline nanotubes and MoO3 nanobelts, J. Mater. Chem. A. 2 (2014) 10384–10388. [41] G. Xiong, C. Meng, R.G. Reifenberger, P.P. Irazoqui, T.S. Fisher, Graphitic petal electrodes for all-solid-state flexible supercapacitors, Adv. Energy Mater. 4 (2014) 897– 905.
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Figure captions Fig. 1. Schematic diagram of PANI-CuCo2O4 composite preparation Fig. 2. XRD pattern of CuCo2O4, PANI and PANI-CuCo2O4 Fig. 3. FTIR spectra of CuCo2O4, PANI, PANI-CuCo2O4 Fig. 4. FESEM image: (a) PANI at low magnification(b) PANI at high magnification (c) CuCo2O4 at low magnification (d) CuCo2O4 at high magnification (e) PANI-CuCo2O4 at low magnification (f) PANI-CuCo2O4 at high magnification Fig. 5. Cyclic voltammogram of (a) CuCo2O4 (b) PANI and (c) PANI-CuCo2O4 at different scan rates. Fig. 6. Galvanostatic discharge curves of (a) CuCo2O4 (b) PANI and (c) PANI-CuCo2O4 at different current densities (c) Specific capacity with respect to current densities. Fig. 7. Nyquist plots of CuCo2O4, PANI and PANI-CuCo2O4. Inset shows the enlarged EIS of CuCo2O4, PANI and PANI-CuCo2O4 Fig. 8. (a) Schematic illustration of the as-sembled asymmetric supercapacitor (b) Comparative CV curves of PANI-CuCo2O4 as a positive electrode (at 1 mV/s) and activated carbon (at 10 mV/s) as a negative electrode performed in a three-electrode cell in 1 M KOH electrolyte (b) CV curves of AC//PANI-CuCo2O4 supercapacitor measured at different potential windows at a scan rate of 20 mV/s (d) CV curves of AC//PANI-CuCo2O4 supercapacitor measured at different scan rates. Fig. 9. (a) Galvanostatic charge/discharge curves of AC//PANI-CuCo2O4 supercapacitor at different current densities (b) Specific capacitance of the asymmetric supercapacitor at different current densities (c) Ragone plot related to energy and power densities of AC//PANI-CuCo2O4 supercapacitor (d) Cycling stability of AC//PANI-CuCo2O4 supercapacitor
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Figures
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