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Polyaniline-cobalt hydroxide hybrid nanostructures and their supercapacitor studies
Materials Chemistry and Physics xxx (2016) 1e11
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Polyaniline-cobalt hydroxide hybrid nanostructures and their supercapacitor studies Janardhan H. Shendkar a, b, Manohar Zate b, Kailas Tehare b, Vijaykumar V. Jadhav b, 1, Rajaram S. Mane b, c, d, *, Mu Naushad c, Je Moon Yun d, **, Kwang Ho Kim d, *** a
S. S. J. E S. Arts, Comm. and Sci. College, Gangakhed, Dist. Parbhani, India School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded, India Department of Chemistry, College of Science, Bld-5, King Saud University, Riyadh, Saudi Arabia d Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, San 30 Jangjeon-dong, Geumjung-gu, Busan, 609-735, South Korea b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Adverse supercapacitive performance in hybrid electrode than pristine is explored. Change in structure and morphology of hybrid electrode compared with separate once has been attempted initially. Change in supercapacitve performance is explained on the basis of inner and outer charge contributions. Obtained results are corroborated using Nyquist spectra.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2015 Received in revised form 4 May 2016 Accepted 31 May 2016 Available online xxx
We attempt to demonstrate comprehensive analysis of experimentally obtained adverse supercapacitive performance of cobalt hydroxide (Co(OH)2)-polyaniline (PANI) hybrid nanocomposites (HNs), prepared potentiostatically via electrochemical deposition method, compared to phase pure individual nanostructures. Morphologically amorphous Co(OH)2, PANI and HNs are entirely different from one another. The electrochemical properties of the Co(OH)2, PANI and HNs electrodes have been investigated by cyclic voltammograms, galvanostatic charge-discharge and electrochemical impedance spectroscopy measurements. The specific capacitances of PANI, HNs and Co(OH)2 are found to be 3.06F/g, 215F/g and 868F/ g, respectively, at a sweep rate of 10 mV/s in 1.0 M NaOH electrolyte, whereas the stabilities and voltammetric charges, assigned to these electrodes, are 47%, 60% & 55% (after 1000 cycles) and 17.22 mC/ cm2, 836.39 mC/cm2 and 1128.73 mC/cm2, respectively. Obtained adverse (inferior) supercapacitive performance values of HNs electrodes have been demonstrated on occurrence of inner and outer charges concept. Our work demonstrates plausible causes for observed smaller supercapacitor performance in
Keywords: Electrochemical supercapacitor Electrochemical deposition Polyaniline Nanofibers Nanoplatelets
* Corresponding author. School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded, India. ** Corresponding author. *** Corresponding author.1 E-mail addresses: [email protected] (R.S. Mane), yunjemoon@gmail. com (J.M. Yun), [email protected] (K.H. Kim). 1 Present address: Shivaji Mahavidyalaya, Udgir, Dist. Latur, Maharashtra, India. http://dx.doi.org/10.1016/j.matchemphys.2016.05.070 0254-0584/© 2016 Elsevier B.V. All rights reserved.
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hybrid/composite nanostructured electrodes (not a common practice) and useful to researchers working in composite/hybrid symmetric and asymmetric supercapacitor fields. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Batteries, fuel cells and electrochemical supercapacitors (ESs) are electrochemical energy conversion and storage devices, wherein batteries exhibit lower power and higher energy density and traditional capacitors demonstrate higher power and lower energy density [1]. Therefore, ESs with higher power as compared to batteries, higher energy density than the traditional capacitors, and function as a bridge between batteries and traditional capacitors [2], have attracted considerable attention in portable, reliable and environmentally friendly commercial electrochemical devices [3]. Based on energy storage mechanism, ES are of two categories viz. electrical double layer capacitors (EDLCs) and pseudocapacitors. In EDLCs, the electrostatic charge accumulation occurs at the interface between the surface of electrode and the electrolyte i.e. electrosorption of electrolyte ions on materials like porous carbon. EDLCs provide relatively higher power and excellent cyclability i.e. durability because of the involvement of nondegradative processes between the electrode and the electrolyte [4]. The performance of EDLCs is determined by the available electrode surface area and the finite charge separation between the electrode and the electrolyte [5]. Cathode materials include metal oxides/hydroxides, polymers or hybrid nano-composites. However, in a pseudocapacitors, the fast and reversible Faradaic reactions taking place near the surface confirm their energy storage capability [6]. Typically, the specific capacitance (SC) of a pseudocapacitor electrode is better than that of an electrode operating as an EDLC [7]. Pseudocapacitors provide higher energy densities but disadvantageously suffer from the durability test and are generally preferred in applications where high capacitance is desired. Polyaniline (PANI) is one the most employed conducting polymers from last four decades due to its low-cost, availability of simple synthesis methods, excellent environmental stability, and competitive redox reversibility by both charge transfer doping and protonation [8e10]. Backbone of pi-conjugated chains in PANI is responsible for an electrical conduction where the conjugation is due to overlap of the pi (p) electrons of the carbon atoms and their wave function to form a sequence of alternating double and single bonds, resulting in unpaired electrons delocalized along the backbone chain. The delocalization of pi-electrons over the backbone is with low ionization potentials and high electron affinity [11]. The oxidation state of the polymer and its degree of protonation are essentially important during electrical conduction process of PANI [12]. Emeraldine state of the polyaniline, consists of equal proportion of amines (eNHe) and imines (¼N-), is highly conducting state among all other existing states [13] whose base is an insulator, but on protonation it changes to the conducting form (emeraldine salt). It is reported that conductivity of emeraldine is due to the formation of polarons and bipolarons at the amines and imines in high acidic medium [14e16]. Similarly, the Faradaic supercapacitive behaviour of the PANI electrode is due to redox reversible electrochemical dopingededoping of the protons, where PANI can be charged positively or negatively with ion insertion in the polymer matrix and the injected charge contributes to the total capacitance. In the redox reaction of the PANI, oxidation is considered as the de-protonation and reduction is protonation. The protonation and de-protonation processes are
taking place at amine and imine nitrogen groups, respectively. Therefore, protonic (acidic) medium is required for electrochemical behaviour of the PANI. Therefore, at solution pH 4, the emeraldine salt cannot form and thereby loses its electrical conductivity and also electrochemical activity [17e20]. The extension of electrochemical activity of PANI to high H p medium is a quite challenging task. One has to introduce either acidic groups into the PANI ring chains or impede the deprotonation. For example in self-doped ring sulfonated polyaniline [21], the self doped m-aminobenzoic acid and m-aminobenzenesulphonic acid [22], the electropolymerized of poly(aniline) in the presence of poly(vinylsulfonate) counter ions [23], poly(styrenesulfonate) [24], poly-(acrylic acid) [25], camphorsulfonic acid [26], poly-(aniline boronic acid) [27], naphthalenesulfonic acid [28] and poly(2-acrylamido-2-methyl-1propanesulfonic acid) PAAMPSA [29], the redox activity of PANI has been improved (in neutral and alkaline aqueous solutions). However, its chemical as well as mechanical stabilities are unsatisfactory. By developing hybrid nanocomposites (HNs) of the PANI with other electrochemically active materials, one can improve the supercapacitance performance by the effect of size confinement and additional functionalities [4]. The HNs of the PANI and polypyrrole electrodes have confirmed an enhanced supercapacitive performance in neutral medium with poor stability [30]. The HNs of the PANI with other electrode materials are generally obtained by either mixing physically or by strong chemical bonding, to withstand electrode in alkaline medium. The HN of PANI-with activated carbon [31], carbon nanotubes [32], ordered mesopours carbon [33] and carbonization of the PANI nanowires [34], three-dimensional graphene [35], MnO2 [36,37], CoMoO4$0.75H2O [38], WO3 [39], V2O5 [40] and graphene/Fe2O3 [41] etc., have demonstrated a strong synergistic effect in literature due to which the electrochemical performance in neutral and/or alkaline medium has been considerably boosted. In this work, we turned our attention to HN film electrodes based on PANI and cobalt hydroxide [Co(OH)2], which were fabricated by using electro-polymerization of aniline onto a stainlesssteel (SS) substrate and electrochemical deposition of Co(OH)2 on PANI HNs processes. The electrochemical activities of the Co(OH)2 film onto PANI i.e. HNs were investigated and compared with that of the pristine PANI and Co(OH)2 film electrodes. Pristine PANI and Co(OH)2 electrodes were also used separately for their supercapacitive measurements. To the best of our knowledge, there is no record in the literature of studying the electro-activity of the HN electrodes composed with PANI and Co(OH)2 nanostructures in alkaline medium. The focus of the present work was to study electroactivities of pristine and HN electrodes and their comparison with detailed scientific understanding using inner & outer charge, cycling stability, charge-discharge and electrochemical impedance measurement studies. 2. Experiment section 2.1. Chemicals All the chemicals were analytical reagent grade of Merck. Aniline monomer (99% pure), sulfuric acid (H2SO4), cobalt nitrate
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Fig. 1. XRD patters of; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrodes.
hexahydrated (99% pure), sodium hydroxide, acetone & distilled water were used without further purification.
2.2. Preparation of PANI electrode Polymerization of aniline was conducted in a one compartment glass cell in atmospheric air, at room temperature (27 C) using three-electrode potentiostatical approach, controlled by computer. In three-electrode potentiostat, platinum plate of area 1.5 cm 1.5 cm, Ag/AgCl electrode & SS of area 1 cm 4 cm were used as a counter, reference and working electrodes, respectively. The pieces of SS substrate were cleaned ultrasonically in acetone
3
solution for 10 min, and then dried by air. The aniline monomer solutions of 0.5 M and 0.5 M solution of sulfuric acid were prepared separately in distilled water. The 0.5 M, 20 ml solution of aniline was stirred for 5 min at room temperature to produce well mixed homogeneous solution due to low solubility of aniline monomer in water and 0.5 M solution of sulfuric acid was poured drop-by-drop slowly to avoid chemical polymerization by instant addition of sulfuric acid and to adjust the H p of solution <2. The low H p value of the electrodepositing solution not only allows to produce nano-fibrous morphology but also to deposit more mass of the PANI on the conducting substrate, in vicinity [42]. The solution was further stirred for 5 min. This 20 ml solution of aniline þ sulfuric acid was poured in glass cell and the platinum electrode, Ag/AgCl electrode and SS electrode were immersed 1 cm apart and 1 cm deep in solution parallel to each other. PANI films were electrodeposited at a fixed potential of þ0.75 V on 1 cm2 surface area of SS substrate vs. Ag/AgCl electrode for 5 min. The dried film of PANI was dipped in distilled water for 2 min and dried for further use. Pristine PANI electrode was preferred as A electrode. 2.3. Preparation of PANI þ Co(OH)2 HN and Co(OH)2 electrodes A 0.05 M aqueous solution of cobalt nitrate was prepared and electrodepozation (cathodization) of Co(OH)2 was carried out in the one compartment glass cell at room temperature using threeelectrode system (in the present case D electrode). The Co(OH)2 film electrodes were electrodeposited (cathodized) at fixed potential of 1.0 V on previously deposited PANI film (deposited on SS) vs. Ag/AgCl electrode for 10, & 20 min time intervals (B and C electrodes). These HN electrodes were dried and immersed in distilled water for 2 min to remove impurities, if there is any. These electrodes were again dried in atmospheric air at room temperature before final use.
Fig. 2. EDX mapping on; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrode surfaces.
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Fig. 3. (a) Low (1.00kx) & (b) high magnification (100kx) FESEM surface images of; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrode surfaces.
2.4. Characterization techniques The morphologies and elemental analyses of all the electrodes were investigated by a Hitachi S-4800 field-emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray analysis (EDX). X-ray diffraction (XRD) measurements were carried out by using a (Ultima IV, Rigaku 2500) diffractometer with a Cu Ka radiation in the 2q range of 10e80 . The electrochemical deposition and the cyclic voltammograms (CVs) of the pristine and HN film electrodes were carried out through a WonAtech (WPG 100 Potentionstat/Galvanostat Workstation). The galvanostatic charging-discharging and electrochemical impedance spectroscopy (EIS) studies were obtained by IVIUMSTAT. All the electrochemical
properties were studied in 1.0 M NaOH electrolyte solution. 3. Results and discussion 3.1. Reaction kinetics The PANI film was developed from an electrochemical oxidation of the 0.5 M aniline monomer and 0.5 M sulfuric acid as dopant in a single glass cell with a constant voltage of 0.75 V for 5 min. The polymerization of the aniline inherently formed bluish film through electrochemical oxidation process. The Co(OH)2 films were cathodically deposited on PANI and SS electrode using 0.05 M Co(NO3)2 solution by reduction of the NO3‾ ions as [43].
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is in amorphous nature. The Co(OH)2 is also showing amorphous nature. In the present work, the conventional method of structure determination i.e. XRD pattern cannot support for phase purity confirmation and for prediction of elements present in pristine and HNs due to their amorphous nature. From EDX spectrums [Fig. 2] of the energy versus relative number of kilo counts of the detected X-rays qualitatively, carbon, C, peak at 0.24 keV, nitrogen, N, peak at 0.35 keV, oxygen, O, peak at 0.52 keV are confirmed. The C and N peaks are accounted for the PANI. The oxygen ‘O’ peak in EDX of A electrode is obtained which is due to water absorbed content in the PANI film, at the time when it is immersed in the distilled water. The PANI film is only dried at room temperature in atmosphere, which fails to take off the water content. In the EDX analyses of B and C samples, an additional peak of cobalt from Co(OH)2 is evidenced. The EDX spectrum of D is of phase pure Co(OH)2. The quantitative determination of the elements obtained from EDX mapping on 2 mm2 area, in terms of weight percentage [Wt.%] and atomic percentage [At.%] is presented as an inset of each EDX spectrum [Fig. 2]. With increasing the cathodization time, the atomic percentage proportion of cobalt is more in C than B, but is less than that of in Co(OH)2 electrode i.e. D. In B, C and pristine D electrode materials the ratio of oxygen to cobalt is approximately equal to 2, supporting for the formation of Co(OH)2 phase. 3.3. Surface morphology change
Fig. 4. (a) CV curves of (B) electrode at 10 mV/s scan rate in 0.5 M different electrolytes, and (b) CV curves at 10 mV/s for; (A) PANI (0.6 V to 0.5 V), (B) 10 min HN (0.1 V to 0.5 V), (C) 20 min HN (0.2 V to 0.5 V), and (D) Co(OH)2 (0.2 to 0.5 V) electrodes.
NO 3 þ H2 O þ 2e /NO2 þ2OH
(1)
Co2þ þ2OH /CoðOHÞ2
(2)
The OH‾ ions formed at cathode surface due to reduction of NO3‾ ions combine with Co2þ ions, resulting in the formation of Co(OH)2 film on SS substrate and on the PANI cathode electrode. 3.2. Structural elucidation and compositional analysis The XRD spectra of the pristine PANI and Co(OH)2 and (B) and (C) HN electrodes are presented in Fig. 1. Intensity peaks (marked as *) in the XRD patterns belong to SS. Thus, PANI electrode is amorphous in nature showing absence of long rang order in the back bone of benzenoid and/or quinonoid structural units. The crystalline structure is formed only when PANI is in its polaron and/or bipolaron form due to protonation of PANI by protonic acid. But in the present study, it is immersed in distilled water to remove the dopant acid used at the time of polymerization. HNs get reduced at the time of (cathodization at 1.0 V) electrodeposition of the Co(OH)2, hence PANI is converted into leucoemeraldine base, which
The FESEM images of all electrodes are shown in Fig. 3. The FESEM of pristine PANI electrode A confirms nanofibers-type appearance with an average 80 (±10) nm fiber diameter. It is inferred that the nanofiber network is highly porous (with mass loading of 2.8 mg). The FESEM of B and C are of HN type wherein Co(OH)2 was deposited on PANI electrode for 10 and 20 min times at a constant voltage of 1.0 V. With increasing the cathodization time, the amount of electrodeposited Co(OH)2 mass (for B 3.2 mg ¼ 2.8 mg þ 0.4 mg and for C 3.9 mg ¼ 2.8 mg þ 1.1 mg) is increased on the PANI electrode which is also confirmed from EDX spectrum (Fig. 2). In the B FESEM image, the mass deposition of Co(OH)2 is less, few nano-fibrous are inter-connected. The increase in the deposited mass of Co(OH)2 covers more nanofibers and finally at few locations nanoplates Co(OH)2 are grown on PANI, as seen in C part of Fig. 3. The pristine Co(OH)2 (2.5 mg) has 2D interlocked-type plate morphology. The nanoplates of Co(OH)2 formed on the PANI electrode i.e. HN have larger widths than that of obtained in the pristine Co(OH)2 case. The surfaces of the nanoplates of the Co(OH)2 in HN are smother than that of pristine Co(OH)2. 3.4. Electrochemical measurements The CVs of B electrode are shown in Fig 4a at the scan rate of 10 mV/s in 0.5 M concentrated different electrolyte, on averaging anodic and cathodic current values, we found that NaOH demonstrates better performance than other electrolytes used. The inset of Fig 4a represents enlarged view of CV curves in electrolytes which otherwise are unclear. The cyclic CVs of the PANI, Co(OH)2 and NH electrodes are presented in Fig. 4b. All CVs were scanned in 1.0 M NaOH electrolyte solution with a potential window of 0.6 V to 0.5 V for A, and 0.2 V to 0.5 V for B, C & D vs. Ag/AgCl electrode at a scan rate of 10 mV/s. However, for pristine PANI electrode i.e. A, prominent redox peak has not been detected; indicating the contribution in supercapacitance is due to electrostatic double layer capacitance origin. While presence of couple of redox peaks in the CV curves of HN and pure Co(OH)2 electrodes is attributed to the surface faradaic reaction at low potential as [44];
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Fig. 5. Effect of scan rate on CV curves of; (A) PANI (0.6 V to 0.5 V), (B) 10 min HN (0.1 V to 0.5 V), (C) 20 min HN (0.2 V to 0.5 V), and (D) Co(OH)2 (0.2 to 0.5 V) electrodes in 1.0 M NaOH electrolyte.
Fig. 6. (a) Plots of potential vs. scan rate, (b) plots of current density vs. scan rate for; (A) PANI (0.6 V to 0.5 V), (B) 10 min HN (0.1 V to 0.5 V), (C) 20 min HN (0.2 V to 0.5 V), and (D) Co(OH)2 (0.2 to 0.5 V) electrodes in 1.0 M NaOH electrolyte.
CoðOHÞ2 þOH 4CoOOH þ H2 O þ e
(3)
While the faradaic reaction at higher potential can be presented as;
CoOOH þ OH 4CoO2 þH2 O þ e
(4)
This indicates that the measured pseudocapacitance is mainly originated by redox mechanism. The strong anodic peak at positive potential is due to the oxidation of Co(OH)2 to CoOOH and the strong cathodic peak is for the reverse process. The weak anodic peak at more positive potential is due to the oxidation of CoOOH to CoO2, and the weak cathodic peak is for the reverse process. The shape of CV of HNs is more rectangular than the pristine Co(OH)2 as well as better stability (discussed later) due to PANI material composition, suggesting an increase of active sites due to high
Table 1 Scan rate effect on SC values of A, B, C and D electrodes. Scan rate(mV/s)
Specific capacitance (F/g)
Electrodes
A
B
C
D
10 20 30 40 50 60 70 80 90 100 % of retention in SC % of decrease in SC
3.06 1.60 1.22 0.86 0.68 0.64 0.58 0.48 0.43 0.39 13% 87%
124 94 82 73 67 62 58 57 54 52 42% 58%
215 162 141 126 115 107 101 96 92 88 41% 59%
868 685 563 499 450 415 387 364 344 327 38% 62%
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distribution of active sites for the redox reaction (4) in HNs. The highest geometric specific capacitance (SC, F/g) values obtained for A, B, C and D electrodes are 3.06, 124, 215, and 868 F/g, respectively, which were calculated by using relation,
I
C¼ m
Fig. 7. SC vs. scan rate plots of; (A) PANI (0.6 V to 0.5 V), (B) 10 min HN (0.1 V to 0.5 V), (C) 20 min HN (0.2 V to 0.5 V), and (D) Co(OH)2 (0.2 to 0.5 V) electrodes.
(5)
dV dt
where, C is SC of the electrode (F/g), I is the average of anodic and cathodic currents (mA), m is the mass of the active electrode materials (mg) and dV/dt is the scan rate (mV/s). Additional masses of A, B, C and D products on SS substrate are 2.8 mg, 3.2 mg, 3.9 mg, and 2.5 mg, respectively. Moreover, Fig. 5 (AeD) depicts the CV spectra of PANI, HN and Co(OH)2 electrodes obtained for different scan rates i.e. from 10 to 100 mV/s. With increasing scan rate the area of the CV increases by acquiring approximately similar shape change in CV. This means that increased scan rate i.e. potential per second is able to increase the value of current across all the electrodes either by electric double layer charge formation or by redox reaction. It is found that the cathodic (reduction) peaks are shifted to more negative potential side, while anodic (oxidation) peaks are shifted to more positive potential at higher scan rate (Fig. 6a). The peak current density value in all electrodes is increased with scan rate. The
Fig. 8. Plots of; (a) of 1/q* vs. v1/2 with magnified image for B, C, D electrodes (b), and (c) q* vs. 1/v1/2 for A, B, C, & D electrodes.
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Table 2 Total charge qt*, outer charge qo*, inner charge qi*, ratio of inner-to-total and outerto- total charge contributions of A, B, C and D electrodes. Electrodes
qt*(mC/cm2)
qo*(mC/cm2)
qi*(mC/cm2)
qi*/qt*
qo*/qt*
A B C D
17.22 244.98 836.39 1128.73
2.78 100.71 345.52 570.18
14.44 144.27 491.16 558.55
0.8385 0.5889 0.5870 0.4948
0.1614 0.4111 0.4130 0.5052
nearly linear dependence of the current density on the scan rate reveals the reversible stability and fast response to oxidation/ reduction, as these current changes are supporting for diffusion limited process rather than surface limited (Fig. 6b) [45]. Further, regarding the electrochemical capacitance trends with increasing current density, the capacitance of PANI, HN and Co(OH)2 electrodes rapidly decline as the current density is increased [see Table 1 & Fig. 7 for details]. This declination in SC with scan rate can also be understood from outer charge and inner charge contributions to SC value of electrode as with increasing scan rate the outer charge contribution is increased (Fig. 8b) and inner charge contribution is decreased. This means inner pores inaccessibility by electrolyte ions with increasing scan rate is also responsible for decrease in SC value for all electrodes. Noteworthy, it is to be noted that the SC values of HN electrodes are smaller than pristine Co(OH)2, so an attempt has been made to investigation possible reasons for the same. Fig. 8 depicts the dependence of voltammetric charge (q*) on the scan rate of CV to identify the effect of the ion-diffusion resistance of electrochemically derived pristine and HN electrodes. Since, the voltammetric
Fig. 10. Plots of SC values vs. number of cycles of; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrodes.
charge is widely recognized as an index measuring the electrochemically active sites coupled with the ion-exchange between the electrode materials and the aqueous electrolyte. The outer electrochemical surface estimated from the outer charge (qo*) corresponds to the region touching the electrolyte directly; the inner electrochemical surface, estimated from inner charge (qi*),
Fig. 9. CV stability curves of; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrodes in 1.0 M NaOH electrolyte.
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J.H. Shendkar et al. / Materials Chemistry and Physics xxx (2016) 1e11 Table 3 The relation between SC values and cycle number for; [A] PANI, [B] 10 min HN, [C] 20 min HN, and [D] Co(OH)2 electrodes. Cycle no.
Specific capacitance (F/g)
Electrodes
A
B
C
D
1 200 400 600 800 1000 % of retention in SC % of decrease in SC
0.86 0.55 0.51 0.49 0.47 0.46 47% 53%
73 48 43 41 40 40 55% 45%
126 104 93 87 81 75 60% 40%
499 390 355 330 308 274 55% 45%
indicates the regions of pores, grain boundaries, voids, crevices, and cracks etc. The total charge (qt*) is estimated from the extrapolation of q* to v ¼ 0, from the plot of 1/q* versus v1/2 (Fig. 8a), that is, the charge involved to the whole inner and outer active sites. Fig. 8b is the enlarged view of the total charge contributions for B, C, and D electrodes, as in Fig. 8a variations are not clearly distinguished. The amount of outer charge (qo*) is estimated from the extrapolation of q* to v ¼ ∞, from the plot of q* versus1/v1/2 (Fig. 8c). Then, one can easily determine the charge contribution due to the inner sites (qi*), from the difference between qt* and qo* [46e50]. The estimated voltammetric charges and their ratios for electrodes under study are presented in Table 2. The ratios of qi/qt and qo/qt indicate the shares of inner and outer active sites, respectively. From the values represented in Table 2, the outer active sites contribution for D electrode is slightly higher than the inner active sites to the total voltammetric charge, implying less porosity. In A, B and C electrodes, the contributions of the inner active sites to the total voltammetric charge are higher than the outer active sites, implying the presence of an excess porosity. Hence, electrodes A, B and C are expected to be more porous than only Co(OH)2 i.e. D, showing the intercalation of cations/anions (Naþ/OH‾) in the electrode materials which also governs the charge storage mechanism process. The cycling stabilities of the pristine and HN electrodes are depicted in Fig. 9 in 1 M NaOH electrolyte at a scan rate of 40 mV/s in the potential range of 0.2 V to 0.5 V for over 1000 cycles (except for the electrode A the potential range of 0.6 V to 0.5 V is selected). The SC values are decreased for all electrodes slowly with increasing cycle number and remained to 47%, 55%, 60%
9
& 55% after 1000 cycles of the their initial values [Fig. 10 and Table 3]. In case of electrode A, the decrease of SC value is due to the degradation by intercalation/deintercalation of cations or anions [51]. The fading of SC is due to slight shifting of the oxidation peak of Co(OH)2 to CoOOH towards CoOOH to CoO2 peak, while the reduction CoO2 to CoOOH peak is due to shifting of CoOOH to Co(OH)2 peak. The reduction peak CoOOH to Co(OH)2 is disappeared for electrodes B & C after 600 cycles, indicating the slow oxidation of Co2þ [Co(OH)2] to Co3þ [CoOOH]. In case of electrode D, the oxidation current density is higher than the reduction current density which is due to a slow oxidation of Co2þ to Co3þ [52,53]. Galvanostatic charge-discharge curves [Fig. 11] of A, B, C & D electrodes were investigated for a current of 1 mA/cm2 in the potential range from 0 to 0.5 V. The entire charge-discharge curves are symmetric, showing the characteristic shape of pseudocapacitance. The charge-discharge curve [inset of Fig. 11)] is an enlarged view of electrode A, which is not symmetric. The EIS spectra were carried out in the frequency range of 0.01 Hze1.5 MHz with ac amplitude of 5 mV. The Nyquist plots between the real (ohmic) part of impedance Z´ (KU) along the Xaxis and the imaginary (usually with capacitive part) part of impedance Z00 (KU) along the Y-axis were measured and are presented in Fig. 12a. The frequency range from 1.5 MHz to 0.01 Hz was divided in three parts on account of the different functions of the supercapacitor electrode [51,54]. First, at high frequency (1.5 MHze60.28 kHz) observed half semicircle is concern with the internal resistance of the electrode which is due to ionic resistance of the electrolyte. The intrinsic resistance of the active material and the contact resistance at the active material/current collector interface can be obtained from Fig. 12b. Second, the high to middle frequency [60.28 kHz to 1 Hz] gives the pseudocharge (faradaiccharge) transfer resistance, which is associated with the porous structure of the active material electrode. The faradaic-charge transfer resistance of electrode/electrolyte interface for electrodes are in the increasing order for A, B, D & C (8.36U, 8.51U, 9.94U & 12.7U) electrodes at 60.28 kHz frequency. With the beginning of high to medium frequency region, a sloppy lines of phase angle between 45 and 90 for all electrodes shows more efficient ions diffusion process more in electrodes B & C and than that of electrodes A & D. And third, the medium to low frequency (1 Hze0.01 Hz) region is the characteristic feature for pure capacitive behaviour. In this frequency region second semicircle is observed, arising either due to a potential-dependent redox reaction or an over potential deposition process, as a result of a potential dependence of the coverage by an adsorbed intermediate [55]. 4. Conclusion
Fig. 11. Charging-discharging curves at 1 mA in 1.0 M NaOH electrolyte of; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrodes.
Experimentally obtained adverse supercapacitive performance results of cobalt hydroxide (Co(OH)2)-polyaniline hybrid nanocomposite electrodes, synthesized potentiostatically via electrochemical deposition method from an alkaline medium, are explained on the basis of outer and inner charge contributions concept. Structural study has supported an amorphous character of pristine as well as composite electrodes but there is significant change in surface appearance where on addition of Co(OH)2, nanofibrous morphology of PANI has been changed to mixed type, which in fact, is in platelets form (for pure phase). Due to loss of electrical conductivity in alkaline medium PANI electrode has demonstrated inferior performance than HNs and Co(OH)2 (stable in alkaline medium) electrodes. The respective specific capacitance values of these electrodes are, 3.06F/g, 215F/g and 868F/g at a sweep rate of 10 mV/s in 1.0 M NaOH electrolyte. Due to loss of electrical conductivity and electrochemical activity values of both inner charge
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Fig. 12. Nyquist plots of; (a) PANI (A), 10 min HN (B), 20 min HN (C), and Co(OH)2 (D) electrodes in 1.0 M NaOH electrolyte. Fig. (b) and (c) are enlarged views of A, B, C & D plots at high frequencies and of A & B at low and high frequencies, respectively.
and outer charge contributions of PANI are fewer compared to HNs and Co(OH)2 electrodes. Over potential resistance values of PANI dominant electrodes are smaller than PANI scare electrodes. Acknowledgments The authors acknowledge the King Saud University, Deanship of Scientific Research, College of Science Research Center for the support. This research was supported by Global Frontier Program, through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013M3A6B1078874). References [1] M. Winter, R. Brodd, J. Chem. Rev. 104 (2004) 4245. [2] C. Largeot, C. Portet, J. Chmiola, P. Taberna, Y. Gogotsi, P. Simon, J. Am. Chem. Soc. 130 (2008) 2730. [3] L.L. Zhang, X.S. Zhao, Chem. Soc. Rev. 38 (2009) 2520. [4] P. Simon, Y. Gogotsi, Nat. Mater 7 (2008) 845. [5] W. Chen, R.B. Rakhi, L. Hu, X. Xie, Y. Cui, H.N. Alshareef, Nano Lett. 11 (2011) 5165. [6] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. van Schalkwijk, Nat. Mater 4 (2005) 366. [7] G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J.R. McDonough, X. Cui, Y. Cui, Z. Bao, Nano Lett. 11 (2011) 2905. [8] A. Buker, J. Power Sources 91 (2000) 37. [9] D. Li, J.X. Huang, R.B. Kaner, Acc. Chem. Res. 42 (2009) 135. [10] S. Bhadra, D. Khastgir, N.K. Singha, J.H. Lee, Prog. Polym. Sci. 34 (2009) 783. [11] J.D. Stenger-Smith, Progr. Polym. Sci. 23 (1998) 57. [12] E.M. Genisis, A. Boyle, M. Lapkowski, C. Tsintavis, Synth. Met. 36 (1990) 139. [13] J.C. Chiang, A.G. MacDiarmid, Synth. Met. 13 (1986) 193. [14] A.G. Macdiarmid, J.C. Chiang, A.H. Richter, Synth. Met. 18 (1987) 285. [15] A. Ray, G.E. Asturias, D.L. Kershner, A.F. Richter, A.G. Macdiarmid, Synth. Met. 29 (1989) E141.
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Contents lists available at ScienceDirect
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Polyaniline-cobalt hydroxide hybrid nanostructures and their supercapacitor studies Janardhan H. Shendkar a, b, Manohar Zate b, Kailas Tehare b, Vijaykumar V. Jadhav b, 1, Rajaram S. Mane b, c, d, *, Mu Naushad c, Je Moon Yun d, **, Kwang Ho Kim d, *** a
S. S. J. E S. Arts, Comm. and Sci. College, Gangakhed, Dist. Parbhani, India School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded, India Department of Chemistry, College of Science, Bld-5, King Saud University, Riyadh, Saudi Arabia d Global Frontier R&D Center for Hybrid Interface Materials, Pusan National University, San 30 Jangjeon-dong, Geumjung-gu, Busan, 609-735, South Korea b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Adverse supercapacitive performance in hybrid electrode than pristine is explored. Change in structure and morphology of hybrid electrode compared with separate once has been attempted initially. Change in supercapacitve performance is explained on the basis of inner and outer charge contributions. Obtained results are corroborated using Nyquist spectra.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 December 2015 Received in revised form 4 May 2016 Accepted 31 May 2016 Available online xxx
We attempt to demonstrate comprehensive analysis of experimentally obtained adverse supercapacitive performance of cobalt hydroxide (Co(OH)2)-polyaniline (PANI) hybrid nanocomposites (HNs), prepared potentiostatically via electrochemical deposition method, compared to phase pure individual nanostructures. Morphologically amorphous Co(OH)2, PANI and HNs are entirely different from one another. The electrochemical properties of the Co(OH)2, PANI and HNs electrodes have been investigated by cyclic voltammograms, galvanostatic charge-discharge and electrochemical impedance spectroscopy measurements. The specific capacitances of PANI, HNs and Co(OH)2 are found to be 3.06F/g, 215F/g and 868F/ g, respectively, at a sweep rate of 10 mV/s in 1.0 M NaOH electrolyte, whereas the stabilities and voltammetric charges, assigned to these electrodes, are 47%, 60% & 55% (after 1000 cycles) and 17.22 mC/ cm2, 836.39 mC/cm2 and 1128.73 mC/cm2, respectively. Obtained adverse (inferior) supercapacitive performance values of HNs electrodes have been demonstrated on occurrence of inner and outer charges concept. Our work demonstrates plausible causes for observed smaller supercapacitor performance in
Keywords: Electrochemical supercapacitor Electrochemical deposition Polyaniline Nanofibers Nanoplatelets
* Corresponding author. School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded, India. ** Corresponding author. *** Corresponding author.1 E-mail addresses: [email protected] (R.S. Mane), yunjemoon@gmail. com (J.M. Yun), [email protected] (K.H. Kim). 1 Present address: Shivaji Mahavidyalaya, Udgir, Dist. Latur, Maharashtra, India. http://dx.doi.org/10.1016/j.matchemphys.2016.05.070 0254-0584/© 2016 Elsevier B.V. All rights reserved.
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hybrid/composite nanostructured electrodes (not a common practice) and useful to researchers working in composite/hybrid symmetric and asymmetric supercapacitor fields. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Batteries, fuel cells and electrochemical supercapacitors (ESs) are electrochemical energy conversion and storage devices, wherein batteries exhibit lower power and higher energy density and traditional capacitors demonstrate higher power and lower energy density [1]. Therefore, ESs with higher power as compared to batteries, higher energy density than the traditional capacitors, and function as a bridge between batteries and traditional capacitors [2], have attracted considerable attention in portable, reliable and environmentally friendly commercial electrochemical devices [3]. Based on energy storage mechanism, ES are of two categories viz. electrical double layer capacitors (EDLCs) and pseudocapacitors. In EDLCs, the electrostatic charge accumulation occurs at the interface between the surface of electrode and the electrolyte i.e. electrosorption of electrolyte ions on materials like porous carbon. EDLCs provide relatively higher power and excellent cyclability i.e. durability because of the involvement of nondegradative processes between the electrode and the electrolyte [4]. The performance of EDLCs is determined by the available electrode surface area and the finite charge separation between the electrode and the electrolyte [5]. Cathode materials include metal oxides/hydroxides, polymers or hybrid nano-composites. However, in a pseudocapacitors, the fast and reversible Faradaic reactions taking place near the surface confirm their energy storage capability [6]. Typically, the specific capacitance (SC) of a pseudocapacitor electrode is better than that of an electrode operating as an EDLC [7]. Pseudocapacitors provide higher energy densities but disadvantageously suffer from the durability test and are generally preferred in applications where high capacitance is desired. Polyaniline (PANI) is one the most employed conducting polymers from last four decades due to its low-cost, availability of simple synthesis methods, excellent environmental stability, and competitive redox reversibility by both charge transfer doping and protonation [8e10]. Backbone of pi-conjugated chains in PANI is responsible for an electrical conduction where the conjugation is due to overlap of the pi (p) electrons of the carbon atoms and their wave function to form a sequence of alternating double and single bonds, resulting in unpaired electrons delocalized along the backbone chain. The delocalization of pi-electrons over the backbone is with low ionization potentials and high electron affinity [11]. The oxidation state of the polymer and its degree of protonation are essentially important during electrical conduction process of PANI [12]. Emeraldine state of the polyaniline, consists of equal proportion of amines (eNHe) and imines (¼N-), is highly conducting state among all other existing states [13] whose base is an insulator, but on protonation it changes to the conducting form (emeraldine salt). It is reported that conductivity of emeraldine is due to the formation of polarons and bipolarons at the amines and imines in high acidic medium [14e16]. Similarly, the Faradaic supercapacitive behaviour of the PANI electrode is due to redox reversible electrochemical dopingededoping of the protons, where PANI can be charged positively or negatively with ion insertion in the polymer matrix and the injected charge contributes to the total capacitance. In the redox reaction of the PANI, oxidation is considered as the de-protonation and reduction is protonation. The protonation and de-protonation processes are
taking place at amine and imine nitrogen groups, respectively. Therefore, protonic (acidic) medium is required for electrochemical behaviour of the PANI. Therefore, at solution pH 4, the emeraldine salt cannot form and thereby loses its electrical conductivity and also electrochemical activity [17e20]. The extension of electrochemical activity of PANI to high H p medium is a quite challenging task. One has to introduce either acidic groups into the PANI ring chains or impede the deprotonation. For example in self-doped ring sulfonated polyaniline [21], the self doped m-aminobenzoic acid and m-aminobenzenesulphonic acid [22], the electropolymerized of poly(aniline) in the presence of poly(vinylsulfonate) counter ions [23], poly(styrenesulfonate) [24], poly-(acrylic acid) [25], camphorsulfonic acid [26], poly-(aniline boronic acid) [27], naphthalenesulfonic acid [28] and poly(2-acrylamido-2-methyl-1propanesulfonic acid) PAAMPSA [29], the redox activity of PANI has been improved (in neutral and alkaline aqueous solutions). However, its chemical as well as mechanical stabilities are unsatisfactory. By developing hybrid nanocomposites (HNs) of the PANI with other electrochemically active materials, one can improve the supercapacitance performance by the effect of size confinement and additional functionalities [4]. The HNs of the PANI and polypyrrole electrodes have confirmed an enhanced supercapacitive performance in neutral medium with poor stability [30]. The HNs of the PANI with other electrode materials are generally obtained by either mixing physically or by strong chemical bonding, to withstand electrode in alkaline medium. The HN of PANI-with activated carbon [31], carbon nanotubes [32], ordered mesopours carbon [33] and carbonization of the PANI nanowires [34], three-dimensional graphene [35], MnO2 [36,37], CoMoO4$0.75H2O [38], WO3 [39], V2O5 [40] and graphene/Fe2O3 [41] etc., have demonstrated a strong synergistic effect in literature due to which the electrochemical performance in neutral and/or alkaline medium has been considerably boosted. In this work, we turned our attention to HN film electrodes based on PANI and cobalt hydroxide [Co(OH)2], which were fabricated by using electro-polymerization of aniline onto a stainlesssteel (SS) substrate and electrochemical deposition of Co(OH)2 on PANI HNs processes. The electrochemical activities of the Co(OH)2 film onto PANI i.e. HNs were investigated and compared with that of the pristine PANI and Co(OH)2 film electrodes. Pristine PANI and Co(OH)2 electrodes were also used separately for their supercapacitive measurements. To the best of our knowledge, there is no record in the literature of studying the electro-activity of the HN electrodes composed with PANI and Co(OH)2 nanostructures in alkaline medium. The focus of the present work was to study electroactivities of pristine and HN electrodes and their comparison with detailed scientific understanding using inner & outer charge, cycling stability, charge-discharge and electrochemical impedance measurement studies. 2. Experiment section 2.1. Chemicals All the chemicals were analytical reagent grade of Merck. Aniline monomer (99% pure), sulfuric acid (H2SO4), cobalt nitrate
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Fig. 1. XRD patters of; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrodes.
hexahydrated (99% pure), sodium hydroxide, acetone & distilled water were used without further purification.
2.2. Preparation of PANI electrode Polymerization of aniline was conducted in a one compartment glass cell in atmospheric air, at room temperature (27 C) using three-electrode potentiostatical approach, controlled by computer. In three-electrode potentiostat, platinum plate of area 1.5 cm 1.5 cm, Ag/AgCl electrode & SS of area 1 cm 4 cm were used as a counter, reference and working electrodes, respectively. The pieces of SS substrate were cleaned ultrasonically in acetone
3
solution for 10 min, and then dried by air. The aniline monomer solutions of 0.5 M and 0.5 M solution of sulfuric acid were prepared separately in distilled water. The 0.5 M, 20 ml solution of aniline was stirred for 5 min at room temperature to produce well mixed homogeneous solution due to low solubility of aniline monomer in water and 0.5 M solution of sulfuric acid was poured drop-by-drop slowly to avoid chemical polymerization by instant addition of sulfuric acid and to adjust the H p of solution <2. The low H p value of the electrodepositing solution not only allows to produce nano-fibrous morphology but also to deposit more mass of the PANI on the conducting substrate, in vicinity [42]. The solution was further stirred for 5 min. This 20 ml solution of aniline þ sulfuric acid was poured in glass cell and the platinum electrode, Ag/AgCl electrode and SS electrode were immersed 1 cm apart and 1 cm deep in solution parallel to each other. PANI films were electrodeposited at a fixed potential of þ0.75 V on 1 cm2 surface area of SS substrate vs. Ag/AgCl electrode for 5 min. The dried film of PANI was dipped in distilled water for 2 min and dried for further use. Pristine PANI electrode was preferred as A electrode. 2.3. Preparation of PANI þ Co(OH)2 HN and Co(OH)2 electrodes A 0.05 M aqueous solution of cobalt nitrate was prepared and electrodepozation (cathodization) of Co(OH)2 was carried out in the one compartment glass cell at room temperature using threeelectrode system (in the present case D electrode). The Co(OH)2 film electrodes were electrodeposited (cathodized) at fixed potential of 1.0 V on previously deposited PANI film (deposited on SS) vs. Ag/AgCl electrode for 10, & 20 min time intervals (B and C electrodes). These HN electrodes were dried and immersed in distilled water for 2 min to remove impurities, if there is any. These electrodes were again dried in atmospheric air at room temperature before final use.
Fig. 2. EDX mapping on; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrode surfaces.
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Fig. 3. (a) Low (1.00kx) & (b) high magnification (100kx) FESEM surface images of; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrode surfaces.
2.4. Characterization techniques The morphologies and elemental analyses of all the electrodes were investigated by a Hitachi S-4800 field-emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray analysis (EDX). X-ray diffraction (XRD) measurements were carried out by using a (Ultima IV, Rigaku 2500) diffractometer with a Cu Ka radiation in the 2q range of 10e80 . The electrochemical deposition and the cyclic voltammograms (CVs) of the pristine and HN film electrodes were carried out through a WonAtech (WPG 100 Potentionstat/Galvanostat Workstation). The galvanostatic charging-discharging and electrochemical impedance spectroscopy (EIS) studies were obtained by IVIUMSTAT. All the electrochemical
properties were studied in 1.0 M NaOH electrolyte solution. 3. Results and discussion 3.1. Reaction kinetics The PANI film was developed from an electrochemical oxidation of the 0.5 M aniline monomer and 0.5 M sulfuric acid as dopant in a single glass cell with a constant voltage of 0.75 V for 5 min. The polymerization of the aniline inherently formed bluish film through electrochemical oxidation process. The Co(OH)2 films were cathodically deposited on PANI and SS electrode using 0.05 M Co(NO3)2 solution by reduction of the NO3‾ ions as [43].
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is in amorphous nature. The Co(OH)2 is also showing amorphous nature. In the present work, the conventional method of structure determination i.e. XRD pattern cannot support for phase purity confirmation and for prediction of elements present in pristine and HNs due to their amorphous nature. From EDX spectrums [Fig. 2] of the energy versus relative number of kilo counts of the detected X-rays qualitatively, carbon, C, peak at 0.24 keV, nitrogen, N, peak at 0.35 keV, oxygen, O, peak at 0.52 keV are confirmed. The C and N peaks are accounted for the PANI. The oxygen ‘O’ peak in EDX of A electrode is obtained which is due to water absorbed content in the PANI film, at the time when it is immersed in the distilled water. The PANI film is only dried at room temperature in atmosphere, which fails to take off the water content. In the EDX analyses of B and C samples, an additional peak of cobalt from Co(OH)2 is evidenced. The EDX spectrum of D is of phase pure Co(OH)2. The quantitative determination of the elements obtained from EDX mapping on 2 mm2 area, in terms of weight percentage [Wt.%] and atomic percentage [At.%] is presented as an inset of each EDX spectrum [Fig. 2]. With increasing the cathodization time, the atomic percentage proportion of cobalt is more in C than B, but is less than that of in Co(OH)2 electrode i.e. D. In B, C and pristine D electrode materials the ratio of oxygen to cobalt is approximately equal to 2, supporting for the formation of Co(OH)2 phase. 3.3. Surface morphology change
Fig. 4. (a) CV curves of (B) electrode at 10 mV/s scan rate in 0.5 M different electrolytes, and (b) CV curves at 10 mV/s for; (A) PANI (0.6 V to 0.5 V), (B) 10 min HN (0.1 V to 0.5 V), (C) 20 min HN (0.2 V to 0.5 V), and (D) Co(OH)2 (0.2 to 0.5 V) electrodes.
NO 3 þ H2 O þ 2e /NO2 þ2OH
(1)
Co2þ þ2OH /CoðOHÞ2
(2)
The OH‾ ions formed at cathode surface due to reduction of NO3‾ ions combine with Co2þ ions, resulting in the formation of Co(OH)2 film on SS substrate and on the PANI cathode electrode. 3.2. Structural elucidation and compositional analysis The XRD spectra of the pristine PANI and Co(OH)2 and (B) and (C) HN electrodes are presented in Fig. 1. Intensity peaks (marked as *) in the XRD patterns belong to SS. Thus, PANI electrode is amorphous in nature showing absence of long rang order in the back bone of benzenoid and/or quinonoid structural units. The crystalline structure is formed only when PANI is in its polaron and/or bipolaron form due to protonation of PANI by protonic acid. But in the present study, it is immersed in distilled water to remove the dopant acid used at the time of polymerization. HNs get reduced at the time of (cathodization at 1.0 V) electrodeposition of the Co(OH)2, hence PANI is converted into leucoemeraldine base, which
The FESEM images of all electrodes are shown in Fig. 3. The FESEM of pristine PANI electrode A confirms nanofibers-type appearance with an average 80 (±10) nm fiber diameter. It is inferred that the nanofiber network is highly porous (with mass loading of 2.8 mg). The FESEM of B and C are of HN type wherein Co(OH)2 was deposited on PANI electrode for 10 and 20 min times at a constant voltage of 1.0 V. With increasing the cathodization time, the amount of electrodeposited Co(OH)2 mass (for B 3.2 mg ¼ 2.8 mg þ 0.4 mg and for C 3.9 mg ¼ 2.8 mg þ 1.1 mg) is increased on the PANI electrode which is also confirmed from EDX spectrum (Fig. 2). In the B FESEM image, the mass deposition of Co(OH)2 is less, few nano-fibrous are inter-connected. The increase in the deposited mass of Co(OH)2 covers more nanofibers and finally at few locations nanoplates Co(OH)2 are grown on PANI, as seen in C part of Fig. 3. The pristine Co(OH)2 (2.5 mg) has 2D interlocked-type plate morphology. The nanoplates of Co(OH)2 formed on the PANI electrode i.e. HN have larger widths than that of obtained in the pristine Co(OH)2 case. The surfaces of the nanoplates of the Co(OH)2 in HN are smother than that of pristine Co(OH)2. 3.4. Electrochemical measurements The CVs of B electrode are shown in Fig 4a at the scan rate of 10 mV/s in 0.5 M concentrated different electrolyte, on averaging anodic and cathodic current values, we found that NaOH demonstrates better performance than other electrolytes used. The inset of Fig 4a represents enlarged view of CV curves in electrolytes which otherwise are unclear. The cyclic CVs of the PANI, Co(OH)2 and NH electrodes are presented in Fig. 4b. All CVs were scanned in 1.0 M NaOH electrolyte solution with a potential window of 0.6 V to 0.5 V for A, and 0.2 V to 0.5 V for B, C & D vs. Ag/AgCl electrode at a scan rate of 10 mV/s. However, for pristine PANI electrode i.e. A, prominent redox peak has not been detected; indicating the contribution in supercapacitance is due to electrostatic double layer capacitance origin. While presence of couple of redox peaks in the CV curves of HN and pure Co(OH)2 electrodes is attributed to the surface faradaic reaction at low potential as [44];
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J.H. Shendkar et al. / Materials Chemistry and Physics xxx (2016) 1e11
Fig. 5. Effect of scan rate on CV curves of; (A) PANI (0.6 V to 0.5 V), (B) 10 min HN (0.1 V to 0.5 V), (C) 20 min HN (0.2 V to 0.5 V), and (D) Co(OH)2 (0.2 to 0.5 V) electrodes in 1.0 M NaOH electrolyte.
Fig. 6. (a) Plots of potential vs. scan rate, (b) plots of current density vs. scan rate for; (A) PANI (0.6 V to 0.5 V), (B) 10 min HN (0.1 V to 0.5 V), (C) 20 min HN (0.2 V to 0.5 V), and (D) Co(OH)2 (0.2 to 0.5 V) electrodes in 1.0 M NaOH electrolyte.
CoðOHÞ2 þOH 4CoOOH þ H2 O þ e
(3)
While the faradaic reaction at higher potential can be presented as;
CoOOH þ OH 4CoO2 þH2 O þ e
(4)
This indicates that the measured pseudocapacitance is mainly originated by redox mechanism. The strong anodic peak at positive potential is due to the oxidation of Co(OH)2 to CoOOH and the strong cathodic peak is for the reverse process. The weak anodic peak at more positive potential is due to the oxidation of CoOOH to CoO2, and the weak cathodic peak is for the reverse process. The shape of CV of HNs is more rectangular than the pristine Co(OH)2 as well as better stability (discussed later) due to PANI material composition, suggesting an increase of active sites due to high
Table 1 Scan rate effect on SC values of A, B, C and D electrodes. Scan rate(mV/s)
Specific capacitance (F/g)
Electrodes
A
B
C
D
10 20 30 40 50 60 70 80 90 100 % of retention in SC % of decrease in SC
3.06 1.60 1.22 0.86 0.68 0.64 0.58 0.48 0.43 0.39 13% 87%
124 94 82 73 67 62 58 57 54 52 42% 58%
215 162 141 126 115 107 101 96 92 88 41% 59%
868 685 563 499 450 415 387 364 344 327 38% 62%
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J.H. Shendkar et al. / Materials Chemistry and Physics xxx (2016) 1e11
7
distribution of active sites for the redox reaction (4) in HNs. The highest geometric specific capacitance (SC, F/g) values obtained for A, B, C and D electrodes are 3.06, 124, 215, and 868 F/g, respectively, which were calculated by using relation,
I
C¼ m
Fig. 7. SC vs. scan rate plots of; (A) PANI (0.6 V to 0.5 V), (B) 10 min HN (0.1 V to 0.5 V), (C) 20 min HN (0.2 V to 0.5 V), and (D) Co(OH)2 (0.2 to 0.5 V) electrodes.
(5)
dV dt
where, C is SC of the electrode (F/g), I is the average of anodic and cathodic currents (mA), m is the mass of the active electrode materials (mg) and dV/dt is the scan rate (mV/s). Additional masses of A, B, C and D products on SS substrate are 2.8 mg, 3.2 mg, 3.9 mg, and 2.5 mg, respectively. Moreover, Fig. 5 (AeD) depicts the CV spectra of PANI, HN and Co(OH)2 electrodes obtained for different scan rates i.e. from 10 to 100 mV/s. With increasing scan rate the area of the CV increases by acquiring approximately similar shape change in CV. This means that increased scan rate i.e. potential per second is able to increase the value of current across all the electrodes either by electric double layer charge formation or by redox reaction. It is found that the cathodic (reduction) peaks are shifted to more negative potential side, while anodic (oxidation) peaks are shifted to more positive potential at higher scan rate (Fig. 6a). The peak current density value in all electrodes is increased with scan rate. The
Fig. 8. Plots of; (a) of 1/q* vs. v1/2 with magnified image for B, C, D electrodes (b), and (c) q* vs. 1/v1/2 for A, B, C, & D electrodes.
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J.H. Shendkar et al. / Materials Chemistry and Physics xxx (2016) 1e11
Table 2 Total charge qt*, outer charge qo*, inner charge qi*, ratio of inner-to-total and outerto- total charge contributions of A, B, C and D electrodes. Electrodes
qt*(mC/cm2)
qo*(mC/cm2)
qi*(mC/cm2)
qi*/qt*
qo*/qt*
A B C D
17.22 244.98 836.39 1128.73
2.78 100.71 345.52 570.18
14.44 144.27 491.16 558.55
0.8385 0.5889 0.5870 0.4948
0.1614 0.4111 0.4130 0.5052
nearly linear dependence of the current density on the scan rate reveals the reversible stability and fast response to oxidation/ reduction, as these current changes are supporting for diffusion limited process rather than surface limited (Fig. 6b) [45]. Further, regarding the electrochemical capacitance trends with increasing current density, the capacitance of PANI, HN and Co(OH)2 electrodes rapidly decline as the current density is increased [see Table 1 & Fig. 7 for details]. This declination in SC with scan rate can also be understood from outer charge and inner charge contributions to SC value of electrode as with increasing scan rate the outer charge contribution is increased (Fig. 8b) and inner charge contribution is decreased. This means inner pores inaccessibility by electrolyte ions with increasing scan rate is also responsible for decrease in SC value for all electrodes. Noteworthy, it is to be noted that the SC values of HN electrodes are smaller than pristine Co(OH)2, so an attempt has been made to investigation possible reasons for the same. Fig. 8 depicts the dependence of voltammetric charge (q*) on the scan rate of CV to identify the effect of the ion-diffusion resistance of electrochemically derived pristine and HN electrodes. Since, the voltammetric
Fig. 10. Plots of SC values vs. number of cycles of; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrodes.
charge is widely recognized as an index measuring the electrochemically active sites coupled with the ion-exchange between the electrode materials and the aqueous electrolyte. The outer electrochemical surface estimated from the outer charge (qo*) corresponds to the region touching the electrolyte directly; the inner electrochemical surface, estimated from inner charge (qi*),
Fig. 9. CV stability curves of; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrodes in 1.0 M NaOH electrolyte.
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J.H. Shendkar et al. / Materials Chemistry and Physics xxx (2016) 1e11 Table 3 The relation between SC values and cycle number for; [A] PANI, [B] 10 min HN, [C] 20 min HN, and [D] Co(OH)2 electrodes. Cycle no.
Specific capacitance (F/g)
Electrodes
A
B
C
D
1 200 400 600 800 1000 % of retention in SC % of decrease in SC
0.86 0.55 0.51 0.49 0.47 0.46 47% 53%
73 48 43 41 40 40 55% 45%
126 104 93 87 81 75 60% 40%
499 390 355 330 308 274 55% 45%
indicates the regions of pores, grain boundaries, voids, crevices, and cracks etc. The total charge (qt*) is estimated from the extrapolation of q* to v ¼ 0, from the plot of 1/q* versus v1/2 (Fig. 8a), that is, the charge involved to the whole inner and outer active sites. Fig. 8b is the enlarged view of the total charge contributions for B, C, and D electrodes, as in Fig. 8a variations are not clearly distinguished. The amount of outer charge (qo*) is estimated from the extrapolation of q* to v ¼ ∞, from the plot of q* versus1/v1/2 (Fig. 8c). Then, one can easily determine the charge contribution due to the inner sites (qi*), from the difference between qt* and qo* [46e50]. The estimated voltammetric charges and their ratios for electrodes under study are presented in Table 2. The ratios of qi/qt and qo/qt indicate the shares of inner and outer active sites, respectively. From the values represented in Table 2, the outer active sites contribution for D electrode is slightly higher than the inner active sites to the total voltammetric charge, implying less porosity. In A, B and C electrodes, the contributions of the inner active sites to the total voltammetric charge are higher than the outer active sites, implying the presence of an excess porosity. Hence, electrodes A, B and C are expected to be more porous than only Co(OH)2 i.e. D, showing the intercalation of cations/anions (Naþ/OH‾) in the electrode materials which also governs the charge storage mechanism process. The cycling stabilities of the pristine and HN electrodes are depicted in Fig. 9 in 1 M NaOH electrolyte at a scan rate of 40 mV/s in the potential range of 0.2 V to 0.5 V for over 1000 cycles (except for the electrode A the potential range of 0.6 V to 0.5 V is selected). The SC values are decreased for all electrodes slowly with increasing cycle number and remained to 47%, 55%, 60%
9
& 55% after 1000 cycles of the their initial values [Fig. 10 and Table 3]. In case of electrode A, the decrease of SC value is due to the degradation by intercalation/deintercalation of cations or anions [51]. The fading of SC is due to slight shifting of the oxidation peak of Co(OH)2 to CoOOH towards CoOOH to CoO2 peak, while the reduction CoO2 to CoOOH peak is due to shifting of CoOOH to Co(OH)2 peak. The reduction peak CoOOH to Co(OH)2 is disappeared for electrodes B & C after 600 cycles, indicating the slow oxidation of Co2þ [Co(OH)2] to Co3þ [CoOOH]. In case of electrode D, the oxidation current density is higher than the reduction current density which is due to a slow oxidation of Co2þ to Co3þ [52,53]. Galvanostatic charge-discharge curves [Fig. 11] of A, B, C & D electrodes were investigated for a current of 1 mA/cm2 in the potential range from 0 to 0.5 V. The entire charge-discharge curves are symmetric, showing the characteristic shape of pseudocapacitance. The charge-discharge curve [inset of Fig. 11)] is an enlarged view of electrode A, which is not symmetric. The EIS spectra were carried out in the frequency range of 0.01 Hze1.5 MHz with ac amplitude of 5 mV. The Nyquist plots between the real (ohmic) part of impedance Z´ (KU) along the Xaxis and the imaginary (usually with capacitive part) part of impedance Z00 (KU) along the Y-axis were measured and are presented in Fig. 12a. The frequency range from 1.5 MHz to 0.01 Hz was divided in three parts on account of the different functions of the supercapacitor electrode [51,54]. First, at high frequency (1.5 MHze60.28 kHz) observed half semicircle is concern with the internal resistance of the electrode which is due to ionic resistance of the electrolyte. The intrinsic resistance of the active material and the contact resistance at the active material/current collector interface can be obtained from Fig. 12b. Second, the high to middle frequency [60.28 kHz to 1 Hz] gives the pseudocharge (faradaiccharge) transfer resistance, which is associated with the porous structure of the active material electrode. The faradaic-charge transfer resistance of electrode/electrolyte interface for electrodes are in the increasing order for A, B, D & C (8.36U, 8.51U, 9.94U & 12.7U) electrodes at 60.28 kHz frequency. With the beginning of high to medium frequency region, a sloppy lines of phase angle between 45 and 90 for all electrodes shows more efficient ions diffusion process more in electrodes B & C and than that of electrodes A & D. And third, the medium to low frequency (1 Hze0.01 Hz) region is the characteristic feature for pure capacitive behaviour. In this frequency region second semicircle is observed, arising either due to a potential-dependent redox reaction or an over potential deposition process, as a result of a potential dependence of the coverage by an adsorbed intermediate [55]. 4. Conclusion
Fig. 11. Charging-discharging curves at 1 mA in 1.0 M NaOH electrolyte of; (A) PANI, (B) 10 min HN, (C) 20 min HN, and (D) Co(OH)2 electrodes.
Experimentally obtained adverse supercapacitive performance results of cobalt hydroxide (Co(OH)2)-polyaniline hybrid nanocomposite electrodes, synthesized potentiostatically via electrochemical deposition method from an alkaline medium, are explained on the basis of outer and inner charge contributions concept. Structural study has supported an amorphous character of pristine as well as composite electrodes but there is significant change in surface appearance where on addition of Co(OH)2, nanofibrous morphology of PANI has been changed to mixed type, which in fact, is in platelets form (for pure phase). Due to loss of electrical conductivity in alkaline medium PANI electrode has demonstrated inferior performance than HNs and Co(OH)2 (stable in alkaline medium) electrodes. The respective specific capacitance values of these electrodes are, 3.06F/g, 215F/g and 868F/g at a sweep rate of 10 mV/s in 1.0 M NaOH electrolyte. Due to loss of electrical conductivity and electrochemical activity values of both inner charge
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Fig. 12. Nyquist plots of; (a) PANI (A), 10 min HN (B), 20 min HN (C), and Co(OH)2 (D) electrodes in 1.0 M NaOH electrolyte. Fig. (b) and (c) are enlarged views of A, B, C & D plots at high frequencies and of A & B at low and high frequencies, respectively.
and outer charge contributions of PANI are fewer compared to HNs and Co(OH)2 electrodes. Over potential resistance values of PANI dominant electrodes are smaller than PANI scare electrodes. Acknowledgments The authors acknowledge the King Saud University, Deanship of Scientific Research, College of Science Research Center for the support. This research was supported by Global Frontier Program, through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013M3A6B1078874). References [1] M. Winter, R. Brodd, J. Chem. Rev. 104 (2004) 4245. [2] C. Largeot, C. Portet, J. Chmiola, P. Taberna, Y. Gogotsi, P. Simon, J. Am. Chem. Soc. 130 (2008) 2730. [3] L.L. Zhang, X.S. Zhao, Chem. Soc. Rev. 38 (2009) 2520. [4] P. Simon, Y. Gogotsi, Nat. Mater 7 (2008) 845. [5] W. Chen, R.B. Rakhi, L. Hu, X. Xie, Y. Cui, H.N. Alshareef, Nano Lett. 11 (2011) 5165. [6] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. van Schalkwijk, Nat. Mater 4 (2005) 366. [7] G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J.R. McDonough, X. Cui, Y. Cui, Z. Bao, Nano Lett. 11 (2011) 2905. [8] A. Buker, J. Power Sources 91 (2000) 37. [9] D. Li, J.X. Huang, R.B. Kaner, Acc. Chem. Res. 42 (2009) 135. [10] S. Bhadra, D. Khastgir, N.K. Singha, J.H. Lee, Prog. Polym. Sci. 34 (2009) 783. [11] J.D. Stenger-Smith, Progr. Polym. Sci. 23 (1998) 57. [12] E.M. Genisis, A. Boyle, M. Lapkowski, C. Tsintavis, Synth. Met. 36 (1990) 139. [13] J.C. Chiang, A.G. MacDiarmid, Synth. Met. 13 (1986) 193. [14] A.G. Macdiarmid, J.C. Chiang, A.H. Richter, Synth. Met. 18 (1987) 285. [15] A. Ray, G.E. Asturias, D.L. Kershner, A.F. Richter, A.G. Macdiarmid, Synth. Met. 29 (1989) E141.
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