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manganese oxide composite for supercapacitor
Journal of Energy Storage 28 (2020) 101219
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Ultrahigh specific energy of layer by layer polypyrrole/graphene oxide/ multi-walled carbon nanotube| polypyrrole/manganese oxide composite for supercapacitor
T
Shalini Kulandaivalua, Muhammad Naim Mohd Azaharia, Nur Hawa Nabilah Azmana, ⁎ Yusran Sulaimana,b, a b
Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Selangor43400 Malaysia Functional Devices Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, Serdang, Selangor 43400 Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: Layer-by-layer Specific capacitance Specific energy Conducting polymer Carbon material Metal oxide
A layer-by-layer (LBL) composite of polypyrrole/graphene oxide/multi-walled carbon nanotube| polypyrrole/ manganese oxide (PGM|PMnO2) was fabricated potentiostatically. The PGM|PMnO2 composite unveiled outstanding supercapacitive performance with a maximum specific capacitance of 755.99 F g−1 and remarkably retain 110% of its initial capacitance for 5000 cycles compared to PGM (49%) and PMnO2 (32%). The specific energy of as high as 66 Wh kg−1 and specific power of 726 W kg−1 were achieved on account of the synergistic effect between each material. The LBL composite also manifested the highest electronic conductivity with the lowest ESR value (40.01 Ω) compared to PGM (47.78 Ω) and PMnO2 (43.02 Ω). Thus, PGM|PMnO2 LBL composite is a promising electrode material for supercapacitors.
1. Introduction
introduced with PPy to overcome the mechanical instability issue of PPy [10,11]. In spite of this, fabricating an advanced electrode on the basis of PPy-based hybrid materials is still a challenge to overcome certain issues such as achieving high specific energy with great cycle life and developing environment-friendly electrodes. Adding to that, the challenges in producing high capacitive electrode materials also rely on fabrication approaches. Most of the approaches require sophisticated procedures and methods. Whereas, layer-by-layer (LBL) assembly is an interesting technique to effectively fabricate nanostructured films using any various materials due to its simplicity of preparation. A few studies have been reported using this technique to fabricate electrode materials for supercapacitors. For instance, Lee, Yun, Park, Sharma, Song and Kim [12] have successfully prepared LBL assembled of polyaniline (PANi) with GO nanosheets and the composite displayed an outstanding specific capacitance of 375.2 F g−1 at 0.5 A g−1 and able to retain 90.7% of its original capacitance over 500 cycles. Besides that, De la Fuente Salas, Sudhakar and Selvakumar [13] have prepared PPy/GO multilayer film electrode by depositing a different number of PPy layers on GO and the specific capacitance obtained was 332 F g−1 which is higher compared to a single layer of PPy deposited on GO (215 F g−1). Apart from that, LBL multilayers comprising of ZnO nanoparticles complexed with polyallylamine hydrochloride (PAH) and MWCNTs had also been prepared
Supercapacitors are one of the potential contenders for electrochemical energy storage devices owing to its impressive properties such as long life cycle stability, high specific power, and fast charge/discharge rates [1,2]. However, the low specific energy in comparison to batteries is the major bottleneck that obstructing their efficient use. In this instance, electrode materials are holding the key in determining the electrochemical performance and charge storage ability of supercapacitors [3]. To date, carbonaceous materials, transition metal oxides, and conducting polymers are the most widely explored electrode materials. In that context, the polypyrrole (PPy) is a conducting polymer known for its high conductivity, high charge density, and rapid reversible doping and dedoping ability [2]. In contrast, manganese oxide (MnO2) is a renowned transition metal oxide with a high theoretical specific capacitance of ~1110 F g−1 [4,5] and a well-explored pseudocapacitive material. PPy is often coupled with MnO2 and the remarkable outcome of their combination revealed that the polypyrrole/ manganese oxide (PMnO2) composite is suitable for supercapacitor [6–9]. However, the stability of PMnO2 still needs to be improved with further efforts. Similarly, carbonaceous materials like multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO) are also usually ⁎
Corresponding author. E-mail address: [email protected] (Y. Sulaiman).
https://doi.org/10.1016/j.est.2020.101219 Received 18 October 2019; Received in revised form 15 January 2020; Accepted 15 January 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 28 (2020) 101219
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ITO deposited with composites. Both cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements were conducted at potential ranging from 0 to 1.0 V at various scan rates (5 200 mV s−1) and current densities (1.5 - 3.5 A g−1), respectively. Electrochemical impedance spectroscopy (EIS) analysis were recorded from 0.01 Hz to 100 kHz at 5 mV with anopen circuit potential.
and the composite able to retain 96% of its original capacitance even after 1000 cycling cycles [14]. The composite also successfully recorded a superior areal capacitance of 1000 F cm−2. Herein, we demonstrate the preparation of a novel LBL assembled PPy/GO/MWCNT|PPy/MnO2 (PGM|PMnO2) composite for supercapacitor. The as-fabricated LBL composite with good electrical conductivity exhibited high specific energy and enhanced specific capacitance. In addition, this binder-free electrode with reduced interfacial resistance has shown great potential to be used in supercapacitor applications.
2.5. Calculations
2. Experimental
The specific capacitance, Csp (F g−1) of the composites was calculated from the CV and GCD according to the following Eq. (1) and (2), respectively:
2.1. Materials/Chemicals
Csp =
MWCNTs (10 ± 1 nm outer diameter, 4.5 ± 0.5 nm inner diameter and 3–6 µm length; 98%) were acquired from Sigma Aldrich and functionalized prior to use with a mixture of concentrated nitric acid (65%, Fisher Scientific) and concentrated sulphuric acid (96%, Fisher Scientific) and in a ratio of 1:3. The mixture was left overnight before filtering and washing with deionized water. The obtained pre-treated MWCNTs were completely dried at 60 °C for overnight. GO, sodium sulfate (Na2SO4; 99%) and manganese sulfate monohydrate (MnSO4⋅H2O) were obtained from Graphenea, Merck and Sigma Aldrich, respectively. The monomer, pyrrole (Py; 97%) acquired from Merck was purified prior to use. Acetone (99.5%) and ethanol (95%) and acetone (99.5%) were supplied from Friendemann Schmidt and J. Kollin Chemicals, respectively. Indium tin oxide (ITO) glass was acquired from Xin Yan Technology Limited. Millipore Milli-Q deionized water (18.2 MΩ.cm at 25 °C) was used throughout the experiments.
Csp =
∫ I dV s×m×V
(1)
i × Δt m × ΔV
(2)
Where, ∫ I dV is integrated area under the CV curve, V is potential window applied (V), s is scan rate (V s−1), i is applied current (V), Δt is discharging time (s), m is mass of the electrode material (g), and ΔV is voltages differences. While, the specific energy, E (Wh kg−1) and specific power, P (W kg−1) of the composites were derived from the discharging part of GCD curves using the following equations:
E= P=
ΔV 2 × Csp (3)
2 ΔV × i 2m
(4) −1
where, Csp is the specific capacitance (F g ), ΔV is the voltage window (V), t is discharging time (s), and m is the average mass of electrode (g).
2.2. Preparation of layer-by-layer assembled PGM with PMnO2 The preparation of electrode materials was carried out using the electrochemical deposition method using a three-electrode system, in which ITO, platinum wire and silver/silver chloride as working, counter and reference electrodes. Prior to use, ITO was cleaned ultrasonically with ethanol, acetone and DI sequentially for 10 min. Firstly, a layer of PGM was electrodeposited on the ITO from a solution consisting of 100 mM Py, 0.3 mg/ml functionalized MWCNTs and 1 mg/ml GO. The first layer was left dried for a few minutes, the second layer of PMnO2 was electrodeposited on the first layer from a solution containing 100 mM Py and 0.1 M manganese sulfate monohydrate in order to form PGM|PMnO2 LBL composite. Each layer was electrodeposited potentiostatically at 0.8 V for 10 min. The average mass loading of the active material was 0.1 mg/cm2.
3. Results and discussion 3.1. Fourier transform infrared spectroscopy (FTIR) FTIR spectra (Fig. 1) were recorded to confirm the chemical structure of as-fabricated PGM, PMnO2 and LBL composites by identifying the functional groups. In the FTIR spectra, the bands at 3224 and 3331 cm−1 are ascribed to the stretching vibrations of OeH in GO and MWCNTs. The main characteristic peaks of PPy in all spectra are noticed in the region between 1700 and 800 cm−1. The band around
2.3. Materials characterization The fabricated composites were analyzed with a Fourier transform infrared (FTIR) spectrophotometer (Shimadzu) at a wavenumber of 400 to 4000 cm−1 to characterize the functional groups present in the samples. Raman analysis (Witec Brand, Alpha 300R) was carried out at an excitation wavelength of 532 nm between 500 and 3500 cm−1 to determine the molecular vibration structure of the sample. The Shimadzu X-ray diffractometer (XRD) was used to analyze the crystallinity of the sample with Cu Kα radiation (λ=1.54 Å). A field emission scanning electron microscope (FESEM, JEOL JSM-7600F) was used to study the surface morphology of the samples. 2.4. Electrochemical measurements All the electrochemical analyses were executed in a two-electrode configuration using Autolab M101 potentiostat/galvanostat equipped with NOVA software. In a two-electrode setup, a filter paper immersed in 1 M Na2SO4 was served as a separator in between the two pieces of
Fig. 1. FTIR spectra of PGM, PMnO2 and PGM|PMnO2 LBL composite. 2
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Fig. 2. Raman spectra of PGM, PMnO2 and PGM|PMnO2 LBL composite. Fig. 3. XRD patterns of (a) PMnO2, (b) PGM and (c) PGM|PMnO2 LBL composite.
1529 cm−1 is attributed to the C=C backbone stretching of PPy. The peak around 1124 and 1010 cm−1 can be assigned to CeN stretching from the PPy skeleton [15]. Whereas, the peak around 872 cm−1 is related to the NeH wagging of secondary amine indicating the polymerization of pyrrole [2]. Notably, the distinctive peaks of MWCNTs and GO also present in this region, which situated around 1660 cm−1 (C=O stretching of carboxylic acid and carbonyl), 1453 cm−1 (CeO of phenol group) and 1282 cm−1 (CeOH) [16]. The presence of MnO2 in the PMnO2 spectrum is indicated at lower wavenumber ranging from 800 to 550 cm−1, which is corresponded to the vibration modes of MnO [17]. It is observed that peaks of PPy, GO, MWCNTs and MnO2 that present in PGM and PMnO2 spectra are also present in LBL composite spectrum with the wavenumbers slightly shifted implying that the LBL composite was successfully prepared.
overlapped with PPy diffraction peak. Whereas, a distinct peak at 2θ = 10.2° (002) in the PGM reveals the characteristic peak of GO which is also noticed in the LBL composite [23]. Apart from the peaks belong to ITO, the other well-defined peaks are indicative of the highly crystalline nature of composite. Moreover, all the peaks in LBL composite are consistent with PMnO2 and PGM indicating the LBL approach does not affect the crystallinity of composite. 3.4. Field emission scanning electron microscope (FESEM) Fig. 4 displays the FESEM micrographs of PGM, PMnO2 and LBL composite. The PGM composite exhibits a compact and dense morphology with the crumpled surface (Fig. 4a). The crumples or wrinkles observed in the composite indicate the presence of GO, wherein the PPy is grown uniformly with GO sheets. It can be seen that the MWCNTs are randomly distributed in the PPy/GO (PG) matrix without any entangles (as pointed by the arrows in Fig. 4a) and also prevents the aggregations of GO in the composite. Additionally, the presence of MWCNTs in the matrix can effectively enlarge the contact area and increase the conductive pathway. Whereas, FESEM micrograph of PMnO2 in Fig. 4b shows homogenous tiny granular morphology, implying the embedment of MnO2 with PPy particles. The LBL composite has a similar granular morphology of PMnO2 (Fig. 4c), however, the granular morphology of LBL composite appears bigger and rougher. Such morphology offers a better diffusion of electrolyte and subsequently will enhance the pseudocapacitive and electrical double layer capacitive contribution of the composite. From the cross-sectional image of the LBL composite in Fig. 4d, it can be seen clearly there are two different morphologies at the top and the bottom layer of the LBL composite. Both layers are responsible for improving the conductivity and thus enhancing the Csp of the LBL composite. The presence of Mn can be confirmed from the elemental mapping image shown in Fig. 4e as Mn is observed homogenously distributed on the LBL composite.
3.2. Raman spectroscopy Raman measurements were executed to validate the formation of the composites. The Raman spectra of PGM, PMnO2 and LBL composite are displayed in Fig. 2. The peak at 927 cm−1 corresponds to the bipolaron ring deformation of PPy. Meanwhile, the peak at 1053 cm−1 is ascribed to the CeH in-plane vibrations [18]. In addition, the peak at 1584 cm−1 is designated to the C=C stretching mode of PPy [19]. The peaks at 1330 and 1382 cm−1 in the PMnO2 and LBL composite spectra are attributed to the CeN stretching mode in PPy [20]. While, the intense peaks at 1340 cm−1 (D-band) and 1584 cm−1 (G-band) [15] indicate the presence of GO and MWCNT in the PGM composite. The presence of MnO2 in the PMnO2 spectrum is identified at 617 and 678 cm−1 which are corresponded to stretching vibrations of Mn-O in the basal plane of MnO2 [17]. It is observed that all the peaks of GO, MWCNTs and MnO2 are present in the spectrum of LBL composite, indicating that the LBL composite was successfully prepared. 3.3. X-ray diffractometry (XRD) The phase structure and crystallinity of the LBL composite was verified by XRD and presented in Fig. 3. The spectrum of PMnO2 clearly shows intense peaks at 17.4° (200), 20.5° (220), 27.4° (310), 37.9° (211), 44.1° (301), 49.8° (411) and 60.6° (521) which are indexed to the tetragonal α-MnO2 crystalline structure (JCPDS. No 44–0141) [21]. A broad peak between 20° to 30° specifies the presence of amorphous PPy (002) in PGM and LBL composite spectra [22]. No obvious diffraction peak of MWCNTs at ~25° is observed in the PGM spectrum since it is
3.5. Cyclic voltammetry (CV) The electrochemical properties of the electrodeposited composites were determined in 1.0 M KCl as shown in Fig. 5a. The electrochemical performance was firstly determined by CV (Fig. 5b), where PMnO2 and LBL composite show a quasi-rectangular shape, indicating the Csp is influenced by both electrical double layer capacitance and 3
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Fig. 4. FESEM images (a) PGM composite (b) PMnO2 composite (c) PGM|PMnO2 LBL composite (d) cross-section of PGM|PMnO2 LBL composite and (e) elemental mapping of Mn for PGM|PMnO2.
integrating the CV curves are shown in Fig. 5d. At the lowest scan rate (5 mV s − 1), the Csp of 755.99 F g−1 is obtained for LBL composite, which is much higher than PMnO2 (389.13 F g−1) and PGM (88.36 F g−1). With the increase in scan rates, the Csp values are dropped gradually implying the limited time period for the counter ions to diffuse and absorb into the materials [17]. Obviously, the formation of layered nanostructured electrode material is able to improve the Csp because of the additional active sites provided by the layers (PGM and PMnO2). It also expected that the movement of ions inside the bulk
pseudocapacitance. Apparently, the PGM composite has a deformed rectangular shaped curve, manifesting a combination of pseudocapacitive and electrical double layer capacitive behavior. Undoubtedly, the LBL composite exhibits a larger CV loop, signifying higher Csp compared to the single layers. Additionally, the CV curves of LBL composite at higher scan rates show leaf-like shape (Fig. 5c), indicating the slower rate of insertion/de-insertion and diffusion of ions compared to the movement of electrons in the composite [24]. Meanwhile, the Csp values of the composites at various scan rates, which were calculated by
4
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Fig. 5. (a) The diagram of two-electrode system, (b) CV curves of PGM, PMnO2, and PGM|PMnO2 LBL composite at a scan rate of 5 mV/s (c) CV curves of PGM|PMnO2 and (d) Specific capacitance of PGM, PMnO2, and PGM|PMnO2 LBL composite at scan rates of 5 to 200 mV s−1.
electrode [33]. Overall, the GCD results are in good agreement with CV results. A comparison of Csp of PGM, PMnO2 and LBL composite at different current densities was studied and shown in Fig. 6c. The highest Csp of the LBL composite is calculated to be 512 F g−1 at 1.5 A g−1, which is about 3.78- and 12.5-fold higher than PMnO2 and PGM, respectively. The synergistic combination of both layers contributing to the excellent Csp in LBL composite. According to the Ragone plot in Fig. 6d, the highest E of 66 Wh kg−1 at P of 726 W kg−1 is recorded for LBL composite at 1.5 A g−1, indicating superior energy storage capability. These values are better and even considerably comparable than those reported studies such as MnO2/carbon nanotube (13.3 Wh kg−1, 2100 W kg−1) [34], PEDOT/GO|PEDOT/NCC (10 Wh kg−1, 2276 W kg−1) [35], GO/PPy/silver (25.5 Wh kg−1, 3994 W kg−1) [36] and GO/induced porous PPy/PET-non woven (37.4 Wh kg−1, 175 W kg−1) [37].
electrode is greatly benefited from the interconnected of MWCNTs in the PG matrix, in which each constituent in PGM is bonded through interfacial interactions (π-π stacking interaction and hydrogen bonding). While, the high pseudocapacitance properties of MnO2 and PPy also lead to an appreciable increase in Csp. The Csp of LBL composite is much greater compared to previously reported PPy-based work electrode, namely PPy/carbon nanotube/cotton (201.99 F g−1) [25], graphene/PPy nanosheets (318. 6 F g−1) [26], MnO2/PPy/rGO (404 F g−1) [22], rGO/MnO2/PPy (682 F g−1) [27], carbon nanotube/PPy/ MnO2 (281 F g−1) [28] and PPy-decorated MnO2/rGO/MWCNT (295.83 F g−1) [29].
3.6. Galvanostatic charge-discharge (GCD) The electrochemical performance of as-prepared electrodes was further assessed by GCD. The GCD curve of PGM at 1.5 A g−1 is in a perfect symmetrical triangular shape as shown in Fig. 6a indicating good reversibility and suggesting the main contribution of electrical double layer capacitance in the composite [30]. In contrast, PMnO2 and LBL composites have distorted triangular shaped GCD curves, manifesting pseudocapacitive properties in the composite [30]. The discharging curve of LBL composite is found to exhibit the longest discharging time compared to single layers, which can be related to its high Csp, which is also demonstrating good charge storage behavior in the LBL composite [31]. The LBL composite at various current densities shows good triangular characteristics as depicted in Fig. 6b, signifying good charge-discharge efficiency and electrochemical reversibility [32]. Furthermore, at the initial part of the discharging curve of the LBL composite, the negligible IR drop indicates the good conductivity of the
3.7. Electrochemical impedance spectroscopy (EIS) Further investigation on the charge transfer process and interface resistance of the electrodes was carried out through EIS. Briefly, the Nyquist plots of PGM, PMnO2 and LBL composite have a straight line and a semicircle at the low and high-frequency region, respectively (Fig. 7a). The semicircle signifies the charge transfer resistance (Rct) that caused by the faradaic reaction at the electrode/electrolyte interface, whereas the straight line which is a Warburg line denotes the ion diffusion within the electrodes and characteristic of capacitive behavior [38,39]. The Rct for PGM|PMnO2 LBL composite is the largest (74.08 Ω) compared to PGM (2.14 Ω) and PMnO2 (46.30 Ω), implying it has large electron transfer resistance with poor charge transport behavior [40] 5
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Fig. 6. GCD curves of (a) PGM, PMnO2, and PGM|PMnO2 composite at a current density of 1.5 Ag−1 (b) PGM|PMnO2 composite at current densities of 1.5, 2.0, 2.5, 3.0, 3.5 A g−1 (c) Specific capacitance of PGM, PMnO2, and PGM|PMnO2 LBL composite at current densities of 1.5, 2.0, 2.5, 3.0, 3.5 Ag−1 and (d) Ragone plot of PGM|PMnO2 LBL composite.
due to combined resistance of both layers. Meanwhile, equivalent series resistance (ESR) is derived from the x-intercept of the Nyquist plots that is associated to the interfacial contact resistance between the current collector and electrode, the intrinsic resistance of electrode and electrolyte solution resistance [41,42]. The ESR for LBL composite (40.01 Ω) is lower compared to PGM (47.78 Ω) and PMnO2 (43.02 Ω) single layer composite which indicates the highest electronic conductivity of the LBL composite [38]. Moreover, these results indicate that the electrons are efficiently moving in the layers resulting in reduced resistance. Among the three electrodes, the Warburg line for LBL composite appears the shortest, manifesting shorter ion diffusion path [43]. Moreover, for a high-performance supercapacitor, a shorter ion diffusion path is an important criterion. The Nyquist plots were fitted with an equivalent circuit (Fig. 7b) comprises of ESR, Rct, constant phase element (CPE) which indicates inhomogenity of electrode's surface [44], and the Warburg element (W) which implies the diffusion of the electrolyte.
several fluctuations in retention during the whole process. The increment of Csp at the beginning and ending of cycles may be related to the self-activation process [45]. Moreover, in this study the LBL composite shows the best stability performance than that of previously reported PPy-based electrode composites, such as ternary nickel cobaltite/PPy nanowire (89.2% over 5000 cycles) [46], carbon nanofiber/MnO2/PPy (82.5% over 2000 cycles) [17] and molybdenum disulfide/PPy (85% over 4000 cycles) [47]. Swelling and shrinking of PPy during the intercalation and de-intercalation process is responsible for the mechanical degradation of most PPy-based composites. However, in the current study, such behavior is not observed due to a strong synergistic effect between the materials in the composite. Furthermore, it indicates that the formation of layers slows down the deterioration of the structure of composite. Adding to this, the CV curves of LBL composite at different cycles (Fig. 7d) show the curves still maintain quasi-rectangular shapes without any distortion, concluding that LBL composite is a promising material as an electrode for supercapacitor.
3.8. Stability
4. Conclusion
Prolonged stability is an imperative factor as any other measurements in a supercapacitor application to ensure their good performance during long-term usage. Therefore, cycling stability measurement of LBL composite was performed over 5000 cycles at 100 mV s−1 (Fig. 7cd) and compared the performance with its corresponding single layers. Impressively, the LBL composite exhibited outstanding electrochemical stability with Csp retention of 110% of its original capacitance after 5000 cycles compared to PGM (49%) and PMnO2 (32%) aside from
A composite consisting of PGM and PMnO2 was successfully prepared potentiostatically via the LBL approach. The presence of PPy, GO, MWCNT and MnO2 in the LBL composite were proven by Raman spectroscopy and FTIR. The cross-sectional view of the LBL composite reveals the granular particles of PMnO2 were densely attached to PGM. The synergistic effect between the layers in the LBL composite has shown improvement in the supercapacitor performance with the maximum specific capacitance of 755.99 F g−1, a high specific energy 6
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Fig. 7. (a) Nyquist plots (0.01 Hz to 100 kHz), (b) electrical equivalent circuit (c) Capacitance retention for PGM, PMnO2 and PGM|PMnO2 LBL at 100 mV s−1 after 5000 cycles and (d) CV curves of PGM|PMnO2 LBL at 100 mV s−1 at different cycles.
(66 Wh kg−1) and specific power (726 W kg−1) at 1.5 A g−1. In addition, the LBL composite possesses extraordinary cycling stability (110% capacitance retention) and low ESR value (40.01 Ω). This indicates that PGM|PMnO2 LBL composite is a reassuring future generation contender for the supercapacitor application.
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CRediT authorship contribution statement Shalini Kulandaivalu: Investigation, Data curation, Formal analysis, Writing - original draft. Muhammad Naim Mohd Azahari: Investigation, Data curation, Formal analysis. Nur Hawa Nabilah Azman: Visualization, Writing - original draft. Yusran Sulaiman: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgment The authors would like to appreciate funding from Universiti Putra Malaysia under the research grant of GP-IPS/2017/9580500. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2020.101219. 7
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[13] I.M. De la Fuente Salas, Y.N. Sudhakar, M. Selvakumar, High performance of symmetrical supercapacitor based on multilayer films of graphene oxide/polypyrrole electrodes, Appl. Surf. Sci. 296 (2014) 195–203. [14] V.O. Fávero, D.A. Oliveira, J.L. Lutkenhaus, J.R. Siqueira, Layer-by-layer nanostructured supercapacitor electrodes consisting of ZnO nanoparticles and multiwalled carbon nanotubes, J. Mater Sci. 53 (2018) 6719–6728. [15] S.N.J. Syed Zainol Abidin, N.H.N. Azman, S. Kulandaivalu, Y. Sulaiman, Poly(3,4ethylenedioxythiophene) doped with carbon materials for high-performance supercapacitor: a comparison study, < 2017 (2017) 13. [16] M.A. Atieh, O.Y. Bakather, B. Al-Tawbini, A.A. Bukhari, F.A. Abuilaiwi, M.B. Fettouhi, Effect of carboxylic functional group functionalized on carbon nanotubes surface on the removal of lead from water, Bioinorg. Chem. Appl. 2010 (2011). [17] M.A.A. Mohd Abdah, N. Mohammed Modawe Aldris Edris, S. Kulandaivalu, N. Abdul Rahman, Y. Sulaiman, Supercapacitor with superior electrochemical properties derived from symmetrical manganese oxide-carbon fiber coated with polypyrrole, Int. J. Hydrogen Energy 43 (2018) 17328–17337. [18] P.K. Kalambate, R.A. Dar, S.P. Karna, A.K. Srivastava, High performance supercapacitor based on graphene-silver nanoparticles-polypyrrole nanocomposite coated on glassy carbon electrode, J. Power Sources 276 (2015) 262–270. [19] W.-.H. Khoh, J.-.D. Hong, Solid-state asymmetric supercapacitor based on manganese dioxide/reduced-graphene oxide and polypyrrole/reduced-graphene oxide in a gel electrolyte, Colloids and Surfaces A 456 (2014) 26–34. [20] B. Zhang, Y. Xu, Y. Zheng, L. Dai, M. Zhang, J. Yang, Y. Chen, X. Chen, J. Zhou, A facile synthesis of polypyrrole/carbon nanotube composites with ultrathin, uniform and thickness-tunable polypyrrole shells, Nanoscale Res. Lett. 6 (2011) 431. [21] P.-.Y. Kuang, M.-.H. Liang, W.-.Y. Kong, Z.-.Q. Liu, Y.-.P. Guo, H.-.J. Wang, N. Li, Y.-.Z. Su, S. Chen, Anion-assisted one-pot synthesis of 1D magnetic α- and β-MnO2 nanostructures for recyclable water treatment application, New J.Chem. 39 (2015) 2497–2505. [22] G. Han, Y. Liu, E. Kan, J. Tang, L. Zhang, H. Wang, W. Tang, Sandwich-structured MnO2/polypyrrole/reduced graphene oxide hybrid composites for high-performance supercapacitors, RSC Adv. 4 (2014) 9898. [23] M. Deng, X. Yang, M. Silke, W. Qiu, M. Xu, G. Borghs, H. Chen, Electrochemical deposition of polypyrrole/graphene oxide composite on microelectrodes towards tuning the electrochemical properties of neural probes, Sens. Actuators B 158 (2011) 176–184. [24] R.K. Sharma, A.C. Rastogi, S.B. Desu, Manganese oxide embedded polypyrrole nanocomposites for electrochemical supercapacitor, Electrochim. Acta 53 (2008) 7690–7695. [25] S. Huang, P. Chen, W. Lin, S. Lyu, G. Chen, X. Yin, W. Chen, Electrodeposition of polypyrrole on carbon nanotube-coated cotton fabrics for all-solid flexible supercapacitor electrodes, RSC Adv. 6 (2016) 13359–13364. [26] C. Xu, J. Sun, L. Gao, Synthesis of novel hierarchical graphene/polypyrrole nanosheet composites and their superior electrochemical performance, J. Mater. Chem. 21 (2011) 11253–11258. [27] H. Zhou, Z. Yan, X. Yang, J. Lv, L. Kang, Z.-.H. Liu, RGO/MnO2/polypyrrole ternary film electrode for supercapacitor, Mater. Chem. Phys. 177 (2016) 40–47. [28] S.R. Sivakkumar, J.M. Ko, D.Y. Kim, B.C. Kim, G.G. Wallace, Performance evaluation of CNT/polypyrrole/MnO2 composite electrodes for electrochemical capacitors, Electrochim. Acta 52 (2007) 7377–7385. [29] S.A. El-Khodary, I.S. Yahia, H.Y. Zahran, M. Ibrahim, Preparation of polypyrroledecorated MnO2/reduced graphene oxide in the presence of multi-walled carbon nanotubes composite for high performance asymmetric supercapacitors, Physica B 556 (2019) 66–74. [30] N.P.S. Chauhan, M. Mozafari, N.S. Chundawat, K. Meghwal, R. Ameta, S.C. Ameta, High-performance supercapacitors based on polyaniline–graphene nanocomposites:
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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Ultrahigh specific energy of layer by layer polypyrrole/graphene oxide/ multi-walled carbon nanotube| polypyrrole/manganese oxide composite for supercapacitor
T
Shalini Kulandaivalua, Muhammad Naim Mohd Azaharia, Nur Hawa Nabilah Azmana, ⁎ Yusran Sulaimana,b, a b
Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, UPM Serdang, Selangor43400 Malaysia Functional Devices Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, Serdang, Selangor 43400 Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: Layer-by-layer Specific capacitance Specific energy Conducting polymer Carbon material Metal oxide
A layer-by-layer (LBL) composite of polypyrrole/graphene oxide/multi-walled carbon nanotube| polypyrrole/ manganese oxide (PGM|PMnO2) was fabricated potentiostatically. The PGM|PMnO2 composite unveiled outstanding supercapacitive performance with a maximum specific capacitance of 755.99 F g−1 and remarkably retain 110% of its initial capacitance for 5000 cycles compared to PGM (49%) and PMnO2 (32%). The specific energy of as high as 66 Wh kg−1 and specific power of 726 W kg−1 were achieved on account of the synergistic effect between each material. The LBL composite also manifested the highest electronic conductivity with the lowest ESR value (40.01 Ω) compared to PGM (47.78 Ω) and PMnO2 (43.02 Ω). Thus, PGM|PMnO2 LBL composite is a promising electrode material for supercapacitors.
1. Introduction
introduced with PPy to overcome the mechanical instability issue of PPy [10,11]. In spite of this, fabricating an advanced electrode on the basis of PPy-based hybrid materials is still a challenge to overcome certain issues such as achieving high specific energy with great cycle life and developing environment-friendly electrodes. Adding to that, the challenges in producing high capacitive electrode materials also rely on fabrication approaches. Most of the approaches require sophisticated procedures and methods. Whereas, layer-by-layer (LBL) assembly is an interesting technique to effectively fabricate nanostructured films using any various materials due to its simplicity of preparation. A few studies have been reported using this technique to fabricate electrode materials for supercapacitors. For instance, Lee, Yun, Park, Sharma, Song and Kim [12] have successfully prepared LBL assembled of polyaniline (PANi) with GO nanosheets and the composite displayed an outstanding specific capacitance of 375.2 F g−1 at 0.5 A g−1 and able to retain 90.7% of its original capacitance over 500 cycles. Besides that, De la Fuente Salas, Sudhakar and Selvakumar [13] have prepared PPy/GO multilayer film electrode by depositing a different number of PPy layers on GO and the specific capacitance obtained was 332 F g−1 which is higher compared to a single layer of PPy deposited on GO (215 F g−1). Apart from that, LBL multilayers comprising of ZnO nanoparticles complexed with polyallylamine hydrochloride (PAH) and MWCNTs had also been prepared
Supercapacitors are one of the potential contenders for electrochemical energy storage devices owing to its impressive properties such as long life cycle stability, high specific power, and fast charge/discharge rates [1,2]. However, the low specific energy in comparison to batteries is the major bottleneck that obstructing their efficient use. In this instance, electrode materials are holding the key in determining the electrochemical performance and charge storage ability of supercapacitors [3]. To date, carbonaceous materials, transition metal oxides, and conducting polymers are the most widely explored electrode materials. In that context, the polypyrrole (PPy) is a conducting polymer known for its high conductivity, high charge density, and rapid reversible doping and dedoping ability [2]. In contrast, manganese oxide (MnO2) is a renowned transition metal oxide with a high theoretical specific capacitance of ~1110 F g−1 [4,5] and a well-explored pseudocapacitive material. PPy is often coupled with MnO2 and the remarkable outcome of their combination revealed that the polypyrrole/ manganese oxide (PMnO2) composite is suitable for supercapacitor [6–9]. However, the stability of PMnO2 still needs to be improved with further efforts. Similarly, carbonaceous materials like multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO) are also usually ⁎
Corresponding author. E-mail address: [email protected] (Y. Sulaiman).
https://doi.org/10.1016/j.est.2020.101219 Received 18 October 2019; Received in revised form 15 January 2020; Accepted 15 January 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
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ITO deposited with composites. Both cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements were conducted at potential ranging from 0 to 1.0 V at various scan rates (5 200 mV s−1) and current densities (1.5 - 3.5 A g−1), respectively. Electrochemical impedance spectroscopy (EIS) analysis were recorded from 0.01 Hz to 100 kHz at 5 mV with anopen circuit potential.
and the composite able to retain 96% of its original capacitance even after 1000 cycling cycles [14]. The composite also successfully recorded a superior areal capacitance of 1000 F cm−2. Herein, we demonstrate the preparation of a novel LBL assembled PPy/GO/MWCNT|PPy/MnO2 (PGM|PMnO2) composite for supercapacitor. The as-fabricated LBL composite with good electrical conductivity exhibited high specific energy and enhanced specific capacitance. In addition, this binder-free electrode with reduced interfacial resistance has shown great potential to be used in supercapacitor applications.
2.5. Calculations
2. Experimental
The specific capacitance, Csp (F g−1) of the composites was calculated from the CV and GCD according to the following Eq. (1) and (2), respectively:
2.1. Materials/Chemicals
Csp =
MWCNTs (10 ± 1 nm outer diameter, 4.5 ± 0.5 nm inner diameter and 3–6 µm length; 98%) were acquired from Sigma Aldrich and functionalized prior to use with a mixture of concentrated nitric acid (65%, Fisher Scientific) and concentrated sulphuric acid (96%, Fisher Scientific) and in a ratio of 1:3. The mixture was left overnight before filtering and washing with deionized water. The obtained pre-treated MWCNTs were completely dried at 60 °C for overnight. GO, sodium sulfate (Na2SO4; 99%) and manganese sulfate monohydrate (MnSO4⋅H2O) were obtained from Graphenea, Merck and Sigma Aldrich, respectively. The monomer, pyrrole (Py; 97%) acquired from Merck was purified prior to use. Acetone (99.5%) and ethanol (95%) and acetone (99.5%) were supplied from Friendemann Schmidt and J. Kollin Chemicals, respectively. Indium tin oxide (ITO) glass was acquired from Xin Yan Technology Limited. Millipore Milli-Q deionized water (18.2 MΩ.cm at 25 °C) was used throughout the experiments.
Csp =
∫ I dV s×m×V
(1)
i × Δt m × ΔV
(2)
Where, ∫ I dV is integrated area under the CV curve, V is potential window applied (V), s is scan rate (V s−1), i is applied current (V), Δt is discharging time (s), m is mass of the electrode material (g), and ΔV is voltages differences. While, the specific energy, E (Wh kg−1) and specific power, P (W kg−1) of the composites were derived from the discharging part of GCD curves using the following equations:
E= P=
ΔV 2 × Csp (3)
2 ΔV × i 2m
(4) −1
where, Csp is the specific capacitance (F g ), ΔV is the voltage window (V), t is discharging time (s), and m is the average mass of electrode (g).
2.2. Preparation of layer-by-layer assembled PGM with PMnO2 The preparation of electrode materials was carried out using the electrochemical deposition method using a three-electrode system, in which ITO, platinum wire and silver/silver chloride as working, counter and reference electrodes. Prior to use, ITO was cleaned ultrasonically with ethanol, acetone and DI sequentially for 10 min. Firstly, a layer of PGM was electrodeposited on the ITO from a solution consisting of 100 mM Py, 0.3 mg/ml functionalized MWCNTs and 1 mg/ml GO. The first layer was left dried for a few minutes, the second layer of PMnO2 was electrodeposited on the first layer from a solution containing 100 mM Py and 0.1 M manganese sulfate monohydrate in order to form PGM|PMnO2 LBL composite. Each layer was electrodeposited potentiostatically at 0.8 V for 10 min. The average mass loading of the active material was 0.1 mg/cm2.
3. Results and discussion 3.1. Fourier transform infrared spectroscopy (FTIR) FTIR spectra (Fig. 1) were recorded to confirm the chemical structure of as-fabricated PGM, PMnO2 and LBL composites by identifying the functional groups. In the FTIR spectra, the bands at 3224 and 3331 cm−1 are ascribed to the stretching vibrations of OeH in GO and MWCNTs. The main characteristic peaks of PPy in all spectra are noticed in the region between 1700 and 800 cm−1. The band around
2.3. Materials characterization The fabricated composites were analyzed with a Fourier transform infrared (FTIR) spectrophotometer (Shimadzu) at a wavenumber of 400 to 4000 cm−1 to characterize the functional groups present in the samples. Raman analysis (Witec Brand, Alpha 300R) was carried out at an excitation wavelength of 532 nm between 500 and 3500 cm−1 to determine the molecular vibration structure of the sample. The Shimadzu X-ray diffractometer (XRD) was used to analyze the crystallinity of the sample with Cu Kα radiation (λ=1.54 Å). A field emission scanning electron microscope (FESEM, JEOL JSM-7600F) was used to study the surface morphology of the samples. 2.4. Electrochemical measurements All the electrochemical analyses were executed in a two-electrode configuration using Autolab M101 potentiostat/galvanostat equipped with NOVA software. In a two-electrode setup, a filter paper immersed in 1 M Na2SO4 was served as a separator in between the two pieces of
Fig. 1. FTIR spectra of PGM, PMnO2 and PGM|PMnO2 LBL composite. 2
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Fig. 2. Raman spectra of PGM, PMnO2 and PGM|PMnO2 LBL composite. Fig. 3. XRD patterns of (a) PMnO2, (b) PGM and (c) PGM|PMnO2 LBL composite.
1529 cm−1 is attributed to the C=C backbone stretching of PPy. The peak around 1124 and 1010 cm−1 can be assigned to CeN stretching from the PPy skeleton [15]. Whereas, the peak around 872 cm−1 is related to the NeH wagging of secondary amine indicating the polymerization of pyrrole [2]. Notably, the distinctive peaks of MWCNTs and GO also present in this region, which situated around 1660 cm−1 (C=O stretching of carboxylic acid and carbonyl), 1453 cm−1 (CeO of phenol group) and 1282 cm−1 (CeOH) [16]. The presence of MnO2 in the PMnO2 spectrum is indicated at lower wavenumber ranging from 800 to 550 cm−1, which is corresponded to the vibration modes of MnO [17]. It is observed that peaks of PPy, GO, MWCNTs and MnO2 that present in PGM and PMnO2 spectra are also present in LBL composite spectrum with the wavenumbers slightly shifted implying that the LBL composite was successfully prepared.
overlapped with PPy diffraction peak. Whereas, a distinct peak at 2θ = 10.2° (002) in the PGM reveals the characteristic peak of GO which is also noticed in the LBL composite [23]. Apart from the peaks belong to ITO, the other well-defined peaks are indicative of the highly crystalline nature of composite. Moreover, all the peaks in LBL composite are consistent with PMnO2 and PGM indicating the LBL approach does not affect the crystallinity of composite. 3.4. Field emission scanning electron microscope (FESEM) Fig. 4 displays the FESEM micrographs of PGM, PMnO2 and LBL composite. The PGM composite exhibits a compact and dense morphology with the crumpled surface (Fig. 4a). The crumples or wrinkles observed in the composite indicate the presence of GO, wherein the PPy is grown uniformly with GO sheets. It can be seen that the MWCNTs are randomly distributed in the PPy/GO (PG) matrix without any entangles (as pointed by the arrows in Fig. 4a) and also prevents the aggregations of GO in the composite. Additionally, the presence of MWCNTs in the matrix can effectively enlarge the contact area and increase the conductive pathway. Whereas, FESEM micrograph of PMnO2 in Fig. 4b shows homogenous tiny granular morphology, implying the embedment of MnO2 with PPy particles. The LBL composite has a similar granular morphology of PMnO2 (Fig. 4c), however, the granular morphology of LBL composite appears bigger and rougher. Such morphology offers a better diffusion of electrolyte and subsequently will enhance the pseudocapacitive and electrical double layer capacitive contribution of the composite. From the cross-sectional image of the LBL composite in Fig. 4d, it can be seen clearly there are two different morphologies at the top and the bottom layer of the LBL composite. Both layers are responsible for improving the conductivity and thus enhancing the Csp of the LBL composite. The presence of Mn can be confirmed from the elemental mapping image shown in Fig. 4e as Mn is observed homogenously distributed on the LBL composite.
3.2. Raman spectroscopy Raman measurements were executed to validate the formation of the composites. The Raman spectra of PGM, PMnO2 and LBL composite are displayed in Fig. 2. The peak at 927 cm−1 corresponds to the bipolaron ring deformation of PPy. Meanwhile, the peak at 1053 cm−1 is ascribed to the CeH in-plane vibrations [18]. In addition, the peak at 1584 cm−1 is designated to the C=C stretching mode of PPy [19]. The peaks at 1330 and 1382 cm−1 in the PMnO2 and LBL composite spectra are attributed to the CeN stretching mode in PPy [20]. While, the intense peaks at 1340 cm−1 (D-band) and 1584 cm−1 (G-band) [15] indicate the presence of GO and MWCNT in the PGM composite. The presence of MnO2 in the PMnO2 spectrum is identified at 617 and 678 cm−1 which are corresponded to stretching vibrations of Mn-O in the basal plane of MnO2 [17]. It is observed that all the peaks of GO, MWCNTs and MnO2 are present in the spectrum of LBL composite, indicating that the LBL composite was successfully prepared. 3.3. X-ray diffractometry (XRD) The phase structure and crystallinity of the LBL composite was verified by XRD and presented in Fig. 3. The spectrum of PMnO2 clearly shows intense peaks at 17.4° (200), 20.5° (220), 27.4° (310), 37.9° (211), 44.1° (301), 49.8° (411) and 60.6° (521) which are indexed to the tetragonal α-MnO2 crystalline structure (JCPDS. No 44–0141) [21]. A broad peak between 20° to 30° specifies the presence of amorphous PPy (002) in PGM and LBL composite spectra [22]. No obvious diffraction peak of MWCNTs at ~25° is observed in the PGM spectrum since it is
3.5. Cyclic voltammetry (CV) The electrochemical properties of the electrodeposited composites were determined in 1.0 M KCl as shown in Fig. 5a. The electrochemical performance was firstly determined by CV (Fig. 5b), where PMnO2 and LBL composite show a quasi-rectangular shape, indicating the Csp is influenced by both electrical double layer capacitance and 3
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Fig. 4. FESEM images (a) PGM composite (b) PMnO2 composite (c) PGM|PMnO2 LBL composite (d) cross-section of PGM|PMnO2 LBL composite and (e) elemental mapping of Mn for PGM|PMnO2.
integrating the CV curves are shown in Fig. 5d. At the lowest scan rate (5 mV s − 1), the Csp of 755.99 F g−1 is obtained for LBL composite, which is much higher than PMnO2 (389.13 F g−1) and PGM (88.36 F g−1). With the increase in scan rates, the Csp values are dropped gradually implying the limited time period for the counter ions to diffuse and absorb into the materials [17]. Obviously, the formation of layered nanostructured electrode material is able to improve the Csp because of the additional active sites provided by the layers (PGM and PMnO2). It also expected that the movement of ions inside the bulk
pseudocapacitance. Apparently, the PGM composite has a deformed rectangular shaped curve, manifesting a combination of pseudocapacitive and electrical double layer capacitive behavior. Undoubtedly, the LBL composite exhibits a larger CV loop, signifying higher Csp compared to the single layers. Additionally, the CV curves of LBL composite at higher scan rates show leaf-like shape (Fig. 5c), indicating the slower rate of insertion/de-insertion and diffusion of ions compared to the movement of electrons in the composite [24]. Meanwhile, the Csp values of the composites at various scan rates, which were calculated by
4
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Fig. 5. (a) The diagram of two-electrode system, (b) CV curves of PGM, PMnO2, and PGM|PMnO2 LBL composite at a scan rate of 5 mV/s (c) CV curves of PGM|PMnO2 and (d) Specific capacitance of PGM, PMnO2, and PGM|PMnO2 LBL composite at scan rates of 5 to 200 mV s−1.
electrode [33]. Overall, the GCD results are in good agreement with CV results. A comparison of Csp of PGM, PMnO2 and LBL composite at different current densities was studied and shown in Fig. 6c. The highest Csp of the LBL composite is calculated to be 512 F g−1 at 1.5 A g−1, which is about 3.78- and 12.5-fold higher than PMnO2 and PGM, respectively. The synergistic combination of both layers contributing to the excellent Csp in LBL composite. According to the Ragone plot in Fig. 6d, the highest E of 66 Wh kg−1 at P of 726 W kg−1 is recorded for LBL composite at 1.5 A g−1, indicating superior energy storage capability. These values are better and even considerably comparable than those reported studies such as MnO2/carbon nanotube (13.3 Wh kg−1, 2100 W kg−1) [34], PEDOT/GO|PEDOT/NCC (10 Wh kg−1, 2276 W kg−1) [35], GO/PPy/silver (25.5 Wh kg−1, 3994 W kg−1) [36] and GO/induced porous PPy/PET-non woven (37.4 Wh kg−1, 175 W kg−1) [37].
electrode is greatly benefited from the interconnected of MWCNTs in the PG matrix, in which each constituent in PGM is bonded through interfacial interactions (π-π stacking interaction and hydrogen bonding). While, the high pseudocapacitance properties of MnO2 and PPy also lead to an appreciable increase in Csp. The Csp of LBL composite is much greater compared to previously reported PPy-based work electrode, namely PPy/carbon nanotube/cotton (201.99 F g−1) [25], graphene/PPy nanosheets (318. 6 F g−1) [26], MnO2/PPy/rGO (404 F g−1) [22], rGO/MnO2/PPy (682 F g−1) [27], carbon nanotube/PPy/ MnO2 (281 F g−1) [28] and PPy-decorated MnO2/rGO/MWCNT (295.83 F g−1) [29].
3.6. Galvanostatic charge-discharge (GCD) The electrochemical performance of as-prepared electrodes was further assessed by GCD. The GCD curve of PGM at 1.5 A g−1 is in a perfect symmetrical triangular shape as shown in Fig. 6a indicating good reversibility and suggesting the main contribution of electrical double layer capacitance in the composite [30]. In contrast, PMnO2 and LBL composites have distorted triangular shaped GCD curves, manifesting pseudocapacitive properties in the composite [30]. The discharging curve of LBL composite is found to exhibit the longest discharging time compared to single layers, which can be related to its high Csp, which is also demonstrating good charge storage behavior in the LBL composite [31]. The LBL composite at various current densities shows good triangular characteristics as depicted in Fig. 6b, signifying good charge-discharge efficiency and electrochemical reversibility [32]. Furthermore, at the initial part of the discharging curve of the LBL composite, the negligible IR drop indicates the good conductivity of the
3.7. Electrochemical impedance spectroscopy (EIS) Further investigation on the charge transfer process and interface resistance of the electrodes was carried out through EIS. Briefly, the Nyquist plots of PGM, PMnO2 and LBL composite have a straight line and a semicircle at the low and high-frequency region, respectively (Fig. 7a). The semicircle signifies the charge transfer resistance (Rct) that caused by the faradaic reaction at the electrode/electrolyte interface, whereas the straight line which is a Warburg line denotes the ion diffusion within the electrodes and characteristic of capacitive behavior [38,39]. The Rct for PGM|PMnO2 LBL composite is the largest (74.08 Ω) compared to PGM (2.14 Ω) and PMnO2 (46.30 Ω), implying it has large electron transfer resistance with poor charge transport behavior [40] 5
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Fig. 6. GCD curves of (a) PGM, PMnO2, and PGM|PMnO2 composite at a current density of 1.5 Ag−1 (b) PGM|PMnO2 composite at current densities of 1.5, 2.0, 2.5, 3.0, 3.5 A g−1 (c) Specific capacitance of PGM, PMnO2, and PGM|PMnO2 LBL composite at current densities of 1.5, 2.0, 2.5, 3.0, 3.5 Ag−1 and (d) Ragone plot of PGM|PMnO2 LBL composite.
due to combined resistance of both layers. Meanwhile, equivalent series resistance (ESR) is derived from the x-intercept of the Nyquist plots that is associated to the interfacial contact resistance between the current collector and electrode, the intrinsic resistance of electrode and electrolyte solution resistance [41,42]. The ESR for LBL composite (40.01 Ω) is lower compared to PGM (47.78 Ω) and PMnO2 (43.02 Ω) single layer composite which indicates the highest electronic conductivity of the LBL composite [38]. Moreover, these results indicate that the electrons are efficiently moving in the layers resulting in reduced resistance. Among the three electrodes, the Warburg line for LBL composite appears the shortest, manifesting shorter ion diffusion path [43]. Moreover, for a high-performance supercapacitor, a shorter ion diffusion path is an important criterion. The Nyquist plots were fitted with an equivalent circuit (Fig. 7b) comprises of ESR, Rct, constant phase element (CPE) which indicates inhomogenity of electrode's surface [44], and the Warburg element (W) which implies the diffusion of the electrolyte.
several fluctuations in retention during the whole process. The increment of Csp at the beginning and ending of cycles may be related to the self-activation process [45]. Moreover, in this study the LBL composite shows the best stability performance than that of previously reported PPy-based electrode composites, such as ternary nickel cobaltite/PPy nanowire (89.2% over 5000 cycles) [46], carbon nanofiber/MnO2/PPy (82.5% over 2000 cycles) [17] and molybdenum disulfide/PPy (85% over 4000 cycles) [47]. Swelling and shrinking of PPy during the intercalation and de-intercalation process is responsible for the mechanical degradation of most PPy-based composites. However, in the current study, such behavior is not observed due to a strong synergistic effect between the materials in the composite. Furthermore, it indicates that the formation of layers slows down the deterioration of the structure of composite. Adding to this, the CV curves of LBL composite at different cycles (Fig. 7d) show the curves still maintain quasi-rectangular shapes without any distortion, concluding that LBL composite is a promising material as an electrode for supercapacitor.
3.8. Stability
4. Conclusion
Prolonged stability is an imperative factor as any other measurements in a supercapacitor application to ensure their good performance during long-term usage. Therefore, cycling stability measurement of LBL composite was performed over 5000 cycles at 100 mV s−1 (Fig. 7cd) and compared the performance with its corresponding single layers. Impressively, the LBL composite exhibited outstanding electrochemical stability with Csp retention of 110% of its original capacitance after 5000 cycles compared to PGM (49%) and PMnO2 (32%) aside from
A composite consisting of PGM and PMnO2 was successfully prepared potentiostatically via the LBL approach. The presence of PPy, GO, MWCNT and MnO2 in the LBL composite were proven by Raman spectroscopy and FTIR. The cross-sectional view of the LBL composite reveals the granular particles of PMnO2 were densely attached to PGM. The synergistic effect between the layers in the LBL composite has shown improvement in the supercapacitor performance with the maximum specific capacitance of 755.99 F g−1, a high specific energy 6
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Fig. 7. (a) Nyquist plots (0.01 Hz to 100 kHz), (b) electrical equivalent circuit (c) Capacitance retention for PGM, PMnO2 and PGM|PMnO2 LBL at 100 mV s−1 after 5000 cycles and (d) CV curves of PGM|PMnO2 LBL at 100 mV s−1 at different cycles.
(66 Wh kg−1) and specific power (726 W kg−1) at 1.5 A g−1. In addition, the LBL composite possesses extraordinary cycling stability (110% capacitance retention) and low ESR value (40.01 Ω). This indicates that PGM|PMnO2 LBL composite is a reassuring future generation contender for the supercapacitor application.
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CRediT authorship contribution statement Shalini Kulandaivalu: Investigation, Data curation, Formal analysis, Writing - original draft. Muhammad Naim Mohd Azahari: Investigation, Data curation, Formal analysis. Nur Hawa Nabilah Azman: Visualization, Writing - original draft. Yusran Sulaiman: Conceptualization, Writing - review & editing, Supervision, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgment The authors would like to appreciate funding from Universiti Putra Malaysia under the research grant of GP-IPS/2017/9580500. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2020.101219. 7
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