Electrochemical performance of binder-free NiO-PANI on etched carbon cloth as active electrode material for supercapacitor

Materials and Design 153 (2018) 24–35

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Electrochemical performance of binder-free NiO-PANI on etched carbon cloth as active electrode material for supercapacitor S.A. Razali, S.R. Majid ⁎ Centre for Ionics University of Malaya, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia

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

• EC-NiP electrode has been prepared by dual step synthesis method. • Binder-free EC-NiP composite fabricated electrode exhibits good electrochemical performance of 193 Fg−1 at 0.5 Ag−1. • The electrodes show excellent reversibility with 72% capacitance after 4500 cycles at different current density.

a r t i c l e

i n f o

Article history: Received 6 April 2018 Received in revised form 25 April 2018 Accepted 27 April 2018 Available online 04 May 2018 Keywords: Hybrid supercapacitor Pseudocapacitor Conducting polymer Nickel oxide Polyaniline Carbon fiber cloth Polymer electrolyte

a b s t r a c t Binder free electrode for supercapacitor application was successfully fabricated which consists of carbon cloth nickel oxide-polyaniline (EC-NiP). The composite electrode was prepared by growing NiO on EC via hydrothermal followed by the electrodeposition of PANI. The crystallite sizes of NiO were varied from 5.73 nm to 17.81 nm over the temperature range of 200 to 500 °C. The optimized electrode, heated at 300 °C (NIP300) showed good specific capacitance of 192.31 Fg−1 with energy density of 21.63 mWhkg−1 and 4.81 Wkg−1 of power density at 0.5 Ag−1 current density in 0.5 M H2SO4 electrolyte. The symmetrical NIP300//PVA + 0.5 M H2SO4//NIP300 cell exhibit excellent reversibility where the specific capacitance retained 72% of the initial value after 4500 cycles. © 2018 Published by Elsevier Ltd.

1. Introduction Supercapacitors have been widely studied as electrical energy storage devices due to their ability to store great amount of energy [1]. ⁎ Corresponding author. E-mail address: [email protected] (S.R. Majid).

https://doi.org/10.1016/j.matdes.2018.04.074 0264-1275/© 2018 Published by Elsevier Ltd.

According to its energy storage mechanism, electrochemical capacitor can be classified as either an electrical double layer capacitor (EDLC) or pseudocapacitors [2]. Reversible ion absorption at electrode/electrolyte interface, play a role in charge storage system in EDLC while in pseudocapacitors redox active employ the fast and reversible Faradaic reaction that occurs on electrode surface which gives high specific capacitance compared to EDLC [3]. High surface area materials such as

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activated carbon (AC), carbon nanotubes (CNT), and reduced graphene oxide (RGO) are usually used in EDLC [4]. However, the presence of the micropores has minimize the coverage of electrolyte hence limits the utilization of the large surface area. Other than that, high cost in CNT and aggregation of graphene are considered in choosing electrode materials for EDLC [5]. During the past few years, hybrid supercapacitors have emerged as a promising energy storage device. This is due to the ability of the device to achieve higher energy density than that of EDLCs, by reason of the extended voltage window and the significantly enhanced capacitance. Hybrid supercapacitor involves a battery-type faradaic electrode which contributes to higher energy density and a capacitive electrode for high power density and long cycle life. The hybridization of these two electrodes can further broaden the operating voltage and upsurge the capacitance of the hybrid capacitor which leads to even higher energy density. In a typical hybrid supercapacitor, the capacitive electrode is usually employs activated carbon while the battery-type electrode utilizes lithium intercalation compounds or transition metal compounds to be used as electrode material. Nowadays, nanostructured transition metal oxide has been investigated as active electrode material in pseudocapacitor by the reason of their ability to perform fast and reversible redox reaction such as RuO2, MnO2, Co3O4 and VO [6]. The other type of material that is currently being widely explored is conductive polymer such as PANI, PPy, and PEDOT-PSS. PANI electrodes have been attracting the attention of researchers due to their unique doping/dedoping behavior, environmental stability, high conductivity and ease of synthesis [7]. Cheng et al. [8] has reported his work on the preparation of electrode for supercapacitor by having PANI being directly deposited on the substrate. The specific capacitance obtained is ~673 Fg−1 which shows a clear gap compared with Kwang et al. [9] where the prepared electrode consists of PANI mixed with PVdF that act as binder. The reported value of the electrode prepared in his work is ~130 Fg−1 which is relatively decreased due to the presence of binder. Having PANI being directly deposited onto a conductive substrate clearly shown to exhibit good specific capacitance value. The condition where no binder is being used decreases the value of resistance [10]. However, directly deposited PANI has its own flaw. The PANI will undergo rapid dissolution which contributes to low cycle rate of the electrode. Originating from doping process, the storage mechanism of conducting polymer can improve the storage ability resulting in the decrement of self-discharging caused by the high doping/de-doping rates during charge-discharge processes [11]. Owing to ion doping and dedoping effect, the PANI skeleton will be modified in which it will affect the lifespan of the device. This situation can be circumvented by having PANI to be coupled with carbon based material or metal oxide [11]. The PANI skeleton will be modified due to the ion doping and dedoping effect, PANI contract and expand in volume during charge/ discharge process. Having a relatively low rate of charge-discharge gives a significant drawback for conducting polymer. To overcome the situation, PANI can also be coupled with other material to form a hybrid. Metal oxide is one of the best choices to act as a holder for PANI to prevent the dissolution effect and also able to contribute in the conductivity of the electrode [11]. Metal oxide is chosen due to its good chemical, electrical and mechanical properties [12]. In this work, we have studied the grown NiO on CFC through hydrothermal method followed by the electrodeposition of PANI by using chronoamperometry mode. The effect of calcinations temperature after the growing process of NiO has been extensively studied which we believed able to produce different structures and morphologies of NiO that eventually influence the deposition of PANI on top of NiO. The results show that the effect between applied calcinations temperature gives significant effect on the electrochemical performance of the supercapacitor electrode [13].

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In this work, we grew nickel oxide on EC through hydrothermal method followed by the electrodeposition of PANI via chronoamperometry mode. The influence of heat treatment on the morphology of the grown metal oxide has been systematically studied at different temperatures to obtain NiO-EC samples. This procedure is significant in order for PANI to be coated on NiO homogenously. It is expected that enhancement on cycling stability of composite electrode can be achieved when NiO is incorporated with PANI-based electrode system.

2. Experimental 2.1. Materials Carbon fiber cloth with thickness of 454 μm, 170 g−2 basic weight and 0.377 gcm−3 density was purchased from ELAT. The web material was PTFE treated woven fiber of 99.5% carbon. Nickel nitrate hexahydrate (Ni(NO3)2.6H2O) and urea were analytical grade obtained from Ajax Finechem and Duschema Biochemie respectively. The aniline and sulfuric acid (H2SO4) were supplied by Sigma-Aldrich. The aniline was purified before use.

2.2. Sample preparation 2.2.1. Electro-etching of carbon fiber cloth The electro-etching of carbon fiber cloth was conducted using a PGSTAT30, which was operated at 2 V for 10 min in 1 M H2SO4 electrolyte. The pristine carbon fiber cloth (CCP) (1 × 2 cm) was used as a working electrode (WE), while a platinum wire and Ag/AgCl were used as the counter electrode (CE) and reference electrode (RE), respectively. The distance between the WE and CE was fixed at 1.5 cm [8]. The electro-etched electrode is referred to as EC.

2.2.2. Growing of nickel oxide (NiO) The nickel hydroxide (NiOH) was prepared by using hydrothermal method [14]. 4 mmol of Ni(NO3)2.6H2O was dissolved together with 2 mmol of urea in 40 mL of distilled water for 10 min. The resulted mixture was transferred into a Teflon-lined autoclave with a stainless-steel shell together with EC, maintained at 120 °C for 6 h. The product grown on EC obtained were washed with distilled water and heated at different temperatures i.e. 200, 300, 400 and 500 °C for 3 h to obtain NiO on EC (EC-Ni).

2.2.3. Electrodeposition of PANI PANI nanowires were synthesized using the chronoamperometry method on the EC-Ni substrate [8]. The deposition electrolyte consists of 0.5 M purified aniline monomer in 0.5 M H2SO4 solution. The applied potential was fixed at 1.4 V for 30 s deposition time. The electrochemical cell consists of EC-Ni substrate as the WE, platinum wire as the CE, and Ag/AgCl as the RE. Prepared electrodes were heated in a furnace at 60 °C for 10 h before being characterized. The samples designated names are as stated in Table 1. Table 1 Sample designation with respect to the heating temperature applied. Heating temperature (°C)

200 300 400 500

Designation Without PANI

With PANI

NI200 NI300 NI400 NI500

NIP200 NIP300 NIP400 NIP500

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2.3. Sample characterization The thermal stability of electrolytes was determined using a Mettler Toledo analyzer, which consists of a TGA/SDTA851e main unit in the temperature range from 30 °C to 900 °C at the heating rate of 10 °C min−1 in a nitrogen atmosphere. The X-ray diffraction (XRD) patterns of deposited samples from the top of the carbon fiber were obtained using a PANalytical Empyrean with CuKα monochromatized radiation at 40 kV and 40 mA. The XRD deconvolution was done using OriginPro 8.5 software. The Fourier Transform Infrared (FTIR) spectra were studied by using the Scientific Nicolet iSIO smart FTIR. The morphology structure of the depositions were examined by a Field Emission Scanning Electron Microscope (FESEM) using a Jeol JSM 7600 and Transmission Electron Microscopy (TEM) images were captured using a Jeol JEM 2100F. The Energy Dispersive X-ray (EDX) spectrum was collected using an Oxford Instruments apparatus for the elemental analysis of the electrode sample. The electrochemical tests of charge/discharge (CD), cyclic voltammetry (CV), and electrical impedance spectroscopy (EIS) were conducted using a potentiostat (Autolab, PGSTAT30). Platinum wire and Ag/AgCl were used as a CE and RE, respectively. Before the electrochemical tests were performed, the mass of active material was measured by an analytical semi micro-balance (XP205, 0.01 mg). The specific capacitance was calculated from discharged curves using the equation:

Cs ¼

I  Δt m  ΔV

ð1Þ

where C (F g−1) is the specific capacitance, I is the current applied, Δt is the time taken during discharge process, m stands for the mass of active material and Δv (V) is the potential difference. 3. Results and discussion Fig. 1 displays the steps involved in the fabrication of the electrode for supercapacitor. Based on the observation, it is interesting to highlight the interaction between urea and Ni2+ ions that has been involved in the growth mechanism of the precursor Ni(OH)2 synthesis. Acting as a precipitation agent, urea plays an important role in the process. Ni (NO3)2·6H2O used in this work acts as the source of Ni2+ ions besides having free water molecules being existed in the composite. Fig. 1 (a) shows the schematic diagram of the Ni(OH)2 precursor and NiO growth mechanism. The reactions between nitrogen and oxygen atom in the polar groups of one urea unit occupy the empty orbitals of the metal ion in the solution which eventually forming coordinate bonds between urea and Ni2+ ions. On parallel occasion, the Ni2+ ions also unite with OH– from the self-ionization of water simultaneously [14–17]. Afterwards, the nuclei grew larger, resulting interactions between the chains to form networks under the heat application. This process leads to the formation of nanosheets product which followed by having self-oriented and assembled by degrees towards ordered flower-like structure to reduce the surface energy [18]. These processes occurred at the surface of the EC fiber resulting the flower-like structure of Ni(OH)2 precursor to automatically grow on the EC without binder. The mechanism of PANI electrodeposition started during the redox transition of leucoemeraldine and the polaronic emeraldine of PANI

Fig. 1. (a) Schematic diagram of the NiO growth mechanism, (b) Schematic diagram for deposition mechanism of PANI onto NiCC, and (inset) CA curve of PANI electrodeposition process.

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when potential is applied in the electrodeposition electrolyte solution containing 0.5 M of aniline in H2SO4 with the NiCC acts as the WE at a specific deposition time [19,20]. Fig. 1(b) displays the schematic diagram of the electrodeposition process of PANI on NiCC as the substrate together with the CA curve obtained from the electrodeposition process where the “*” indicates the point of the phase transitions from leucomeraldine to polaronic emaraldine [19]. Thermal analyses of the prepared samples were conducted from room temperature to 800 °C, Fig. 2. The weight loss of 9 wt% before 240 °C can be due to the dehydration of absorbed water on Ni(OH)2 sample. The phase transformation of nickel hydroxide to nickel oxide can be inferred from major weight loss of 23 wt% observed at 240 °C until 343 °C [21] according to equation: NiðOHÞ2 →NiO þ H 2 O

ð2Þ

The same trend is observed in EC-Ni sample with the weight loss of 12 wt% at 343 °C. This shows that this sample is more stable compared to Ni(OH)2. Weight loss observed on sample EC_PANI at 150 °C until 341 °C of 6 wt% is due to the deprotonation of the PANI losing dopant H2SO4. PANI then undergoes degradation after 341 °C which lead to loss of 10 wt% that can be seen at the temperature from 341 °C to 800 °C. This is related to the PANI decomposition and degradation with different polymerization degree [22]. In sample, NIP300, the initial weight loss from 77 °C to 191 °C is due to the evaporation surface-absorbed moisture molecule. Whereas, the weight loss of 11 wt% from 191 °C to 368 °C is due to the loss of H2SO4 dopant and phase transformation of hydroxide to oxide from PANI and nickel respectively which occurred simultaneously. The structure composition of samples was studied by X-ray diffraction (XRD) patterns. Fig. 3 displays the XRD patterns of prepared electrodes after heat treatment at 200, 300, 400 and 500 °C. Peaks observed at 2θ = 37°, 44° and 63° in all samples can be assigned to the NiO plane of (111), (200) and (220) respectively [18,23,24]. As shown in Fig. 3(a), the peaks became sharper as the temperature increased, implying a more ordered structure was formed. The characteristic peak of PANI and EC are present at 2θ = 25°. To distinguish those peaks, deconvolution of overlapped peaks have been carried out and the resolved peaks of PANI and EC are shown in Fig. 3(b). Peak of PANI can be noticed at 2θ = 25.5° in which, the broad with low intensity peak indicating the formation of amorphous PANI [19,25]. The resolved peak observed at 2θ = 25° is corresponding to (002) plane of carbon in EC which related to amorphous carbon materials with low degree of graphitization [26–31]. Changes in intensity and the shape of these peaks have proven the formation of NiO-PANI on EC substrate.

Fig. 2. TGA spectra of Ni(OH)2, EC-Ni, EC-P and sample NIP300.

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To further investigate the structural changes of sample during heating process, evolution of the crystal size (B) at different heating temperatures was calculated using Scherrer equation, Eq. (3) and shown in Fig. 3(c). Bð2θÞ ¼

Kλ L cos θ

ð3Þ

The calculation was done based on peak 2θ = 44°. In general, the crystallite size will increase with the increasing of heating temperature [32]. The plot shows that the calculated crystallite size of each sample has strong relationship with heating temperature. The variation in size for NiO is from 5.73 nm to 17.81 nm over the temperature range 200 to 500 °C. The crystallite size of NiO increases linearly from 300 °C until the heating temperature reaches 500 °C. The smaller crystallite size provides higher surface area for ion penetration. This leads to higher specific capacitance which has good concurrence with other reports [33,34]. The element composition of the deposit obtained from EDX is shown in Fig. 3(d, e). The element of C, S, Ni, and O are found which belong to EC-Ni, Fig. 3(d). There are additional N and O in NIP300 electrode, where N is originated from PANI and O might be from the adsorption of moisture by PANI, Fig. 3(e). In order to see clearly the evidence of deposited PANI, FTIR has been investigated. FTIR spectra analyses of the electrodes are conducted to investigate the formation of NiO and PANI on EC. The spectra are shown in Fig. 4. In the spectrum of EC, the prominent bands at 2325, 2088 and 1992 cm−1 are typical C`C of alkyne group which can be associated with carbon material. In the spectrum of EC-Ni, the intensity of these three peaks is decreasing and remains at its position in all studied samples. For most of the oxide's characteristic, the transmittance bands are in the lower wavenumber fingerprint region. Hence, the bands that occurred at 656 and 717 cm−1 are assigned to the bonding between Ni and O [35]. The FTIR spectrum of EC_PANI shows that characteristic bands located at 1568, 1471 and 795 cm−1 are assigned to the C_C stretching vibration of quinoid rings, C_C stretching of benzoid rings and psubstituted ring of PANI, respectively [36,37]. The spectra of prepared samples in the region from 900 to 1220 cm−1 is shown in Fig. 4(b). The deconvoluted spectrum of NiO powder is shown in Fig. 4(c, I) where four deconvoluted bands at 1033, 1046, 1068 and 1103 cm−1 can be observed in this region. To further confirm the bands, the spectrum of NiO on EC substrate is deconvoluted, Fig. 4(c, II). Bands at 1049 and 1069 cm−1 remain at its position but have changes in intensity. Meanwhile, based on the deconvoluted spectrum of PANI on EC, (Fig. 4(c, III)) it shows four intensive bands at 962, 1023, 1113 and 1155 cm−1. Bands at 962 and 1155 cm−1 can be attributed to C\\H out of plane and C\\N stretching of second aromatic amine of PANI [38]. The spectrum of samples consist of EC, NiO and PANI heated at 200 °C (NIP200), Fig. 4(c, IV) reveals the band at 1037 cm−1 which can be attributed to the merged bands of 1049 and 1021 cm−1 originated from the C\\O stretching bond of NiO and PANI [39], respectively. This band appeared at the same wavenumbers in all prepared samples. Same observation also can be made for the band at 1068 cm−1 where the peak remains at its position as the prominent peak. Band at 1110 cm−1 from EC-Ni remains at the same position in sample NIP200 but with higher intensity and become more prominent while bands at 1134 and 1113 cm−1 from NiO on EC and PANI on EC spectra respectively merged and centered at 1126 cm−1. The band at 1110 cm−1 from NiO on EC and NIP200 spectra later combined together with the band at 1126 cm−1 in samples heated at higher temperature. While in sample NIP300, peak at 1155 cm−1 shifted to higher wavenumber, 1165 cm−1 without any changes in the remaining two samples (NIP400 and NIP500). The chemical structure of the NiCC_PANI is further explored by using Raman spectra as shown in Fig. 5. From the spectra in Fig. 5(a), three

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Fig. 3. (a) XRD diffractogram of EC, NiO, EC_P, NIP200, NIP300, NIP400, NIP500, (b) deconvoluted diffractogram on region I, and (c) crystal size with respect to the heating temperature of electrodes NIP200, NIP300, NIP400 and NIP500. EDX spectra of (d) NI300 and (e) NIP300.

peaks were observed in all spectra located at ~1345 cm−1, ~1579 cm−1 and ~2673 cm−1 which correspond to the peaks of EC is due to the formation of semiquinone radical cations, that is, the p-disubstituted benzene rings. Five small peaks located at ~494 cm−1, ~522 cm−1, ~605 cm−1 and ~810 cm−1 are observed in G band, D band and 2D band, which corresponding to the peaks of NiO. Peaks 490 cm−1 and 520 cm−1 are assigned to the first-order transverse optical (1TO) and longitudinal optical (1LO) phonon modes of NiO, respectively while peak 600 cm−1 and 810 cm−1 are attributed to the second order transverse optical (2TO) longitudinal optical (2LO) phonon modes of NiO [40–43].

One broad pocket of peaks in region I (900–2100 cm−1) is deconvoluted as displayed in Fig. 5(b) to distinguish the peaks between NiO and PANI that has been overlapped. Peaks located at ~1165 cm−1, ~1314 cm−1, 1492 cm−1 and ~1555 cm−1 are believed to be originated from NiO. Peaks at 1165 cm−1 and 1492 cm−1 are associated with twophonon scattering and the excitation of two magnons respectively while 1314 cm−1 and 1492 cm−1 are observed in D band and G band respectively [41,44,45]. The characteristic bands of PANI can be seen at 1143 and 1217 cm−1 which can be referred to C\\H bending modes. Rings of C\\C stretching mode can be captured at 1591 cm−1 while for C\\N stretching mode is observed at 1380 cm−1. The conductive

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Fig. 4. (a) FTIR spectra of NiO, EC, EC-Ni, EC_PANI and all prepared samples at wavenumber of 650–3200 cm−1 and (b) at 900–1220 cm−1, (c) deconvoluted spectra of (I) pure NiO, (II) ECNi, (III) EC_PANI, (IV) NIP200, (V) NIP300, (VI) NIP400 and (VII) NIP500.

property of PANI can be detected in the region from 1100 to 1150 cm−1 which can be indicated as the delocalization of charge on the polymer backbone [46]. The morphology of our electrode was investigated through FESEM and TEM studies (Fig. 6). Fig. 6(a1) reveals the nanoflower-like shaped NiO formed on sample NI300 by growing it directly on the EC. Same shape also can be seen from sample NI400 (Fig. 6(c1)) but in a more densely packed order as shown in Fig. 6(c2) which represents the higher magnification of the sample compared to sample NI300 (Fig. 6 (a2)) where the plates are more loosely arranged thus providing more spaces for PANI deposition. This situation is proven in Fig. 6(b1) where sample NIP300 has uniform pattern of PANI deposition. The deposited PANI is able to fill up the gap between the plates provided by the NiO that has grown beforehand. Ultimately, a more uniform shape and well-developed nanowire of PANI inevitably gives more area for electrolyte contact when electrochemical analysis took place.

Unlike the case of sample NIP400 (Fig. 6(d1)), the PANI deposited is not in proper condition. Agglomeration occurred on the surface of the electrode which might be due to the limitation of spaces provided by the NiO for PANI to attach. This leads to PANI being unable to grow well on its substrate, hence decreases the area for electrolyte contact among the active electrode material which automatically gives low specific capacitance value. A representative TEM image of the selected electrode is shown in Fig.4(g). This figure verified that deposited PANI had built up from small PANI nanowire, which is in agreement with FESEM results. From these structural studies, applied heating temperature was shown to strongly influence the morphology and porosity of the deposits. It is believed that these factors will affect the PANI electrodeposition process which influences the electrochemical performance. To evaluate the electrochemical behavior of the samples as electrode materials for supercapacitor, CV, GCD, EIS and cycle stability tests have

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Fig. 5. (a) Raman spectra of EC and all deposited PANI electrodes and (b) deconvoluted spectra at region I.

Fig. 6. FESEM images of (a1, a2) NI300, (b1, b2) NIP300, (c1,c2) NI400, (d1, d2) NIP400 and (e & f) TEM images of NIP300 at low and high magnification.

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been conducted. CV measurement of all fabricated electrodes are shown in Fig. 7(a). The CV was measured at potential interval from −0.2 to 0.7 V in 0.5 M H2SO4 electrolyte at the scan rates of 1 to 20 mVs−1. From Fig. 7(a), two pairs of redox peaks marked as A/A' and B/B′ were observed showing pseudocapacitance property. Peak A/A' is related to the transformations from leucomeraldine to emaraldine of the PANI chain, while the B/B′ peak is attributed to emaraldine-pernigraniline transformation [47]. The electrochemical performance of the electrodes were measured in terms of specific capacitance (Cs). The Cs based on CV curves were calculated using Eq. (4); Cs ¼

ʃivdv 2μmΔV

ð4Þ

where Cs represents the specific capacitance (Fg−1), I is the response current (A), v is the potential (V), μ is the scan rate (mVs−1) and m is the mass of active material of the electrode (g). The mass of active materials is in between 0.00645 g to 0.00800 g. The calculated Cs is presented in Fig. 7(b). As can be seen, the Cs value of NIP300 is the highest, 193.38 Fg−1 at scan rate of 1 mVs−1 compared to other samples. A series of CV with different scan rates from 1 to 20 mVs−1 were also conducted on sample NIP300 in order to get more information on the electrochemical behavior of the electrodes and displayed in Fig. 7(c). The CV profile shows unambiguous anodic and cathodic peaks which can be inferred to the redox activity contributed from faradaic reaction of the sample [48]. As the scan rate being increased from 1 to 20 mVs−1, the anodic and cathodic peak experience changes in their intensity and positions. The anodic peak shows increment in its current peak value and shifted to higher potential. On the other hand, the cathodic peak has experienced decrement in its current value and shifted to lower

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potential region. This observation may be due to the strengthened electric polarization and kinetic irreversibility of ions originate from electrolyte at the electrode's surface while undergoing redox reaction at higher scan rates [49]. The shape of the CV curves progressively transform into rectangular with increasing scan rate. The Cs values of electrode NIP300 measured at different scan rates are displayed in Fig. 7(d). The specific capacitance value decreases with the increment of scan rate as reported by other researcher [50]. The studied sample shows a good stability when the Cs value remains the same even after it has been tested at 1 to 10 mVs−1. However, there is a decrement of Cs value from 15 to 20 mVs−1. The Cs drops at 20 mVs−1 scan rate is probably due to the internal resistance of the electrode hindering the charge collection and restriction of H+ ion mobility into the electrode [51]. These factors suggest that there are disordered ionic intercalation in the system which affect the charge storing capability at higher scan rate [51]. The charge storage capacity of the prepared electrodes was investigated by galvanostatic charge/discharge (GCD) measurements. Fig. 8 (a) presents the GCD curves at current density of 0.5 Ag−1 in the potential windows ranging from −0.2 to 0.7 V. The non-linear charge and discharge curve is consistent with the peaks marked from the CV profile which can be corroborated to pseudocapacitance behavior contributed from the electrode. The specific capacitance is calculated using Eq. (1) and shown in Fig. 8(b). The calculated Cs highlights the effect of the heating temperature during the sample preparation. The Cs increased when the heating temperature is at 300 °C. This might be associated to the uniformity of grown PANI in which provides sufficient active site for better absorption of H+ ion. The increment of Cs can also be inferred to the small crystal size. Further increment of heating temperatures to 400 and 500 °C has caused drop in Cs due to the crystal size enlargement as mentioned before in XRD results. This enlargement will lead to PANI agglomeration due to the ineffective PANI deposition

Fig. 7. (a) CV profiles of NIP200, NIP300, NIP400 and NIP500 at scan rate of 1 mVs−1, (b) with the value of calculated specific capacitance, (c) CV curves of NIP300 at various scan rates together with (d) the graph of its specific capacitance with different scan rate applied in 0.5 M H2SO4 electrolyte.

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Fig. 8. (a) CD profiles of NIP200, NIP300, NIP400 and NIP500 at current density of 0.5 Ag−1, (b) calculated specific capacitance, (c) CD curves of NIP300 at various current densities and (d) the Nyquist plot of all samples in 0.5 M H2SO4 electrolyte.

on NiO surfaces in which reduced utilization of active site for electrolyte ion absorption/desorption [52]. Therefore, the calculated specific capacitance of sample NIP400 and NIP500 (110.37 Fg−1 and 77.81 Fg−1) are found to be lower than the Cs value of sample NIP300 (192.31 Fg−1) measured at 0.5 Ag−1 in 0.5 M H2SO4. High specific capacitance in sample NIP300 can be due to the efficient ion charge transfer provided by the ordered PANI nanorods on NiO [53]. Comparison of GCD curves at different current densities for sample NIP300 show almost similar trend in the charging and discharging profiles, Fig. 8(c). The Cs of sample NIP300 at current density of 0.5 and 1.0 Ag−1 are 192.31, and 185.62 Fg−1 respectively. With the interval of current density from 0.5 to 1.0 Ag−1, the Cs retention is 96%. High capacitance retention of 96% indicates good charge storage capacity which might due to fast charge transfer related to the increment in current densities [53]. The same observation has been reported in other works such as Xue and coworkers [54], Han and coworkers [50], as well as Ma and coworkers [53] for PANI based composite electrode. The energy and power density of NIP300 at current density of 0.5 Ag−1 are 21.63 mWhkg−1 and 4.74 Wkg−1. At current density of 1.0 Ag−1, the calculated energy density and power density are 20.83 mWhkg−1 and 2.36 Wkg−1. The EIS study of the electrodes in 0.5 M H2SO4 electrolyte is shown in Fig. 8(d). All plots exhibited similar electrochemical impedance characteristics where, the EIS is built up with high and low frequency region that consists of ordinary semicircle and oblique line in the respective frequency. In the high frequency region there is an intercept of the arc on the x-axis called the equivalent series resistance (Rs), which represents a combination of ionic resistance of electrolyte, contact resistance, and internal resistance of the material [55] while the transfer resistance (Rct) is obtained from the electrochemical reaction resistance that can be inferred from the high frequency semicircle [55]. A more than 45°

linear slope can be observed in the low frequency oblique line which can be ascribed to the diffusion and kinetics process called the Warburg impedance (W) [56]. The equivalent circuit of the fitted curve is displayed in Fig. 8(d) inset. The Rct values of sample NIP200, NIP300, NIP400 and NIP500 are 3.00 Ω, 2.07 Ω, 2.79 Ω and 2.22 Ω. This result indicates that NIP300 electrode exhibited the lowest Rct value among all tested samples which is convenient for diffusion and migration of electrons that contribute to higher electrochemical performance. The uniform deposition of PANI on the electrode also plays an important role in better accessibility of cations to the electrode matrix [48], which is parallel with the FESEM result. The achieved performances from previous reports are as stated in Table 2. From the table, sample NIP300 is likely to have good specific capacitance compared to other works ref. A symmetrical NIP300//PVA + 0.5 M H2SO4//NIP300 cell was fabricated and tested for 4500 cycles continuously under different current densities of charging and discharging process to investigate the cycling stability of the NIP300 electrode with total mass load of 0.01140 g. The cell shows good retention stability under continuously cycling at different current densities. Fig. 9(a) displayed the sequence of charge and discharge cycles. The cell was first cycled at 200 mAg−1 and then continued to 300 and 400 mAg−1. The current densities were reversed to 300 and 200 mAg−1 within 1000 cycles continuously. The specific capacitance for the first 1000 cycles at 200 mAg−1 was decreased by 27% of the initial cycle. During the next 1000 cycles at 300 mAg−1, the specific capacitance experienced a major dropped of 46% from the cycle of 1000th to 1500th. The value then gradually increased to 66% in the next 500 cycles to 66% of the initial capacitance. This is possibly due to the activation process where insufficient utilization of NiO during the early cycles occurred [59]. The values of specific capacitance for the subsequent current densities are calculated to be 76% and 82% of initial

S.A. Razali, S.R. Majid / Materials and Design 153 (2018) 24–35

33

Table 2 Electrochemical performance achieved from previous report. Electrodes

Electrolyte

Specific capacitance (Fg−1)

Current density

Cycle stability (%) (cycles)

Refs.

EC-NiO-PANI NiO nanosheets PANI-carbon Carbon with PTFE PANI-MnHCF

0.5 M H2SO4 3 M KOH 1 M H2SO4 1 M H2SO4 0.5 M H2SO4 + 0.5 M Na2SO4

193 82 160 95 730

0.5 Ag−1 0.5 Ag−1 0.5 Ag−1 0.5 Ag−1 1.0 Ag−1

72 (4500) 79 (3000) 90 (1000) 95 (1000) 85 (1000)

This work [13] [57] [57] [58]

capacitance respectively. When the current density is switched back to 200 mAg−1, the cell is able to store the charge and retained approximately 72% of the initial capacitance. This indicates that the capacitance retention is slightly attenuated after testing at higher current densities implying good electrochemical performance of NIP300 electrode. EIS measurements were plotted before and after the cycles were done as presented in Fig. 9(b) and (c). After 4500 continuous cycle of the symmetrical cell, the Nyquist curve has shifted towards higher resistance values and a broadening of the high-frequency semicircle was

observed with Rct value calculated to be 0.3 and 4.0 Ω for before and after the cycle. This can be ascribed to the larger resistance resulted from the repeated charge-discharge processes. As can be seen from Fig. 9(c), the second semicircle with resistance of 31.0 Ω appeared in the Nyquist plot after 4500 cycles. This semicircle may indicates to the formation of interface layer in which affects the charge-discharge process and leads to the decreased in value of specific capacitance retention [60]. Fig. 9(d) shows the specific capacitance as a function of frequency (0.01 Hz–10 kHz) of NIP300//PVA + 0.5 M H2SO4//NIP300 cell before

Fig. 9. (a) Retention plot for symmetric cell NIP300//PVA + 0.5 M H2SO4//NIP300 at different current densities (b) Nyquist plot of the cell before cycling process and (c) after cycling process, (d) specific capacitance versus frequency plot of the cell before and after cycling process and (e) energy and power density calculated at different current densities.

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and after cycling stability test. The specific capacitance value was gradually decreased until 100 Hz and remains constant in the sample before cycling stability test. At higher frequencies (100 Hz–10 kHz), ions in electrolyte are not able to follow the electric field oscillations. A different trend was observed for the sample after 4500 cycles where, the plateau region begins to appear at lower frequency value (0.1 Hz–10 kHz). This is probably due to the increment of resistance which is in agreement with Fig. 9(b) and (c) as the Rb value was found to be larger after the cycling stability process [61]. Fig. 9(e) shows the plot of energy and power density versus cycle number of NIP300//PVA + 0.5 M H2SO4//NIP300 cell. From figure, it can be seen that both energy and power density of the cell decreases with respect to the increment of current density applied for each 1000 cycles. After the cell undergoes 1000 cycles at current density of 200 mAg−1, it delivers energy density of 17.04 mWhkg−1 with power density of 1.33 Wkg−1. As the current density increased to 300 mAg−1, the energy density is 10.97 mWhkg−1 with power density of 0.72 Wkg−1. The energy density decreased to 5.79 mWhkg−1 at 400 mAg−1 after 3000 cycles. When the current density returned to 200 mAg−1, the cell successfully restored 13.87 mWhkg−1 energy density compared to the value of the first 1000 cycles. It can be seen that the NIP300//PVA + 0.5 M H2SO4//NIP300 cell possessed electrochemical reversibility with low resistance value. This might be due to the good synergistic effect between the NiO and PANI [53]. Besides that, the porous structure of carbon fiber cloth is expected to facilitate the diffusion of electrolyte into the electrode material not only on the surface hence provide more channels for rapid ion transport [8].

4. Conclusion In summary, we have successfully synthesized NiO incorporated with PANI embedded on carbon cloth by combining hydrothermal and electrodeposition methods. Nanorod PANI was formed by potentiostatic electrodeposition on the NiO layer as evidenced from FESEM images. Small crystal size nanoflower-like NiO was obtained after heat treatment at 300 °C as determined from XRD peak. The highest specific capacitance of sample EC-NiO-PANI (NIP300) is 192.31 Fg−1 at 0.5 Ag−1 in 0.5 M H2SO4 electrolyte with the energy and power density of 21.63 mWhkg−1 and 4.81 Wkg−1. The NIP300 electrode also possessed excellent synergistic effect of NiO-PANI as the Cs was retained at 96% measured at current densities between 0.5 Ag−1 and 1.0 Ag−1. A symmetrical NIP300//PVA + 0.5 M H2SO4//NIP300 showed good electrochemical reversibility when discharged at different current densities over 4500 cycles. The cell retained 72% of its initial capacitance after discharge current density is switched back to 200 mAg−1. This study proved that binder-free NiO-PANI on EC as active electrode material exhibited good electrochemical reversibility with high capacitance retention even after continuous 4500 cycles.

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