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LaMnO3/RGO/PANI Ternary nanocomposite for Supercapacitor Electrode Application and Their Outstanding Performance in All-Solid-State Asymmetrical Device Design P. Muhammed Shafi, V Ganesh, and Arumugam Chandra Bose ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00459 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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ACS Applied Energy Materials

LaMnO3/RGO/PANI Ternary nanocomposite for Supercapacitor Electrode Application and Their Outstanding Performance in AllSolid-State Asymmetrical Device Design P. Muhammed Shafia, V. Ganeshb and A.Chandra Bosea*

(a)

Nanomaterials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli 620015

(b)

Electrodics and Electrocatalysis (EEC) Division, CSIR-Central Electrochemical Research Institute (CSIR-CECRI), Karaikudi - 630 003, India *Corresponding Author: [email protected]

Keywords: Ternary Nanocomposite; LaMnO3/RGO/PANI; Efficient Electrode Material; Asymmetric Supercapacitor; All Solid-State Device. Abstract The rapid growth of energy demand and lack of sustainable energy conversion/ storage devices have made the supercapacitor as an inevitable substitute to current storage systems. However, the major limitation to the cutting edge lies in their relatively low energy density compared to batteries. Here, we report LaMnO3/RGO/PANI ternary composite fabricated via in situ polymerisation route as an efficient electrode material with enhanced energy density. The all-solid-state designs of both LaMnO3/RGO/PANI║LaMnO3/RGO/PANI symmetric supercapacitor and LaMnO3/RGO/PANI║RGO asymmetric supercapacitor have been highlighted in this work.

The asymmetric supercapacitor device exhibits a superior

electrochemical performance (Sc = 111 Fg-1 at 2.5 Ag-1 and > 50 % retention at 20 Ag-1) than symmetric supercapacitor device although both these devices have an appreciable

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electrochemical behaviour. The asymmetric supercapacitor device delivers a maximum energy density of 50 Wh kg-1 at a power density of 2.25 kW kg-1. The device still delivers an energy density of 25 Wh kg-1 at a power density as high as 18 kW kg-1. It is an outstanding result among LaMnO3 based supercapacitors. In addition, the asymmetric supercapacitor device executes an excellent retention of 117 % even after 100k cycles. This extraordinary performance of asymmetric supercapacitor device favours their practical application to the current demand. INTRODUCTION Supercapacitors have achieved a great attention among energy storage devices due to their ability to bridge the gap between power density and energy density. High energy batteries (deliver energy consistently for prolonged period) and high power capacitors (quick delivery and charging) are equally important in energy storage/conversion field. Hence obtaining high energy density at large power density is a great task among researchers. Coming to the mechanism behind the electrochemical charge storage on supercapacitors, there are electric double layer capacitance (EDLC) and pseudocapacitance depending upon the materials used as electrodes in the device. Former one is due to the accumulation/polarization of charges on electrode surface and is more or less equivalent to conventional capacitors (physical storage). Usually carbon or carbon derived materials exhibit this kind of charge storage behaviour. The latter one is purely faradic, as it happens through fast redox reaction over electrode material (chemical storage). Metal oxides and conducting polymers are the usual candidates for this type of storage mechanism. Supercapacitors usually have superior EDLC mechanism and can deliver large power density where as batteries usually store energy through faradic process and hence it has large energy density. Thus to obtain large energy density with high power density, batteries and supercapacitors are jointly used in many applications including hybrid vehicles. It is quite difficult to acquire both energy and power densities in a single unit. 2 ACS Paragon Plus Environment

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However, researchers are making considerable effort to develop such storage units by developing suitable psuedocapacitive materials and fabrication techniques. It is quit logical that supercapacitors with pseudocapacitive electrodes can meet this requirement to a particular extent. Thus a number of studies and investigations have been employed to develop such electrochemical storage devices over the last couple of decades [1-7]. Among the pseudocapacitive materials used for supercapacitor electrode application, transition metal oxides have attracted considerable attention as electrode materials for supercapacitors and for other electrochemical storage devices owing to their following merits: variable oxidation state, good chemical and electrochemical stability, ease of preparation and convenience in handling. Hydrated RuO2 prepared by sol-gel process at low temperature has a specific capacitance as high as 720 F/g due to solid state pseudo Faradic reaction [8]. But their unaffordable cost and toxic nature conquered commercializing them to practical supercapacitors. While searching for an alternate to RuO2, manganese based metal oxides were found to be an excellent substitute owing to their favourable electrochemical properties, natural abundance, low cost, rich redox activity, eco-friendliness and large theoretical capacitance [9-15]. Their charge staorage mechanism is more or less similar to that of hydrous RuO2, and hence it can meet the particular requirements for energy storage applications. Moreover, ternary metal oxides with two different metal cations are found to be suitable for advanced electrode materials owing to their multiple oxidation states and improved electronic conductivity [16-18]. Similarly, perovskite oxides with ABO3 structure have achieved tremendous attraction to replace metal oxides as electrode material in electrochemical energy storage devices. The availability of A-site and B-site, where A site is usually occupid by lanthanides and B site by transition metal provide enough sites for cationic intercalation [19]. Further, their properties strongly vary, depending on the A and B ion’s nature and oxidation states. In majority of cases, ion A (rare-earth) is catalytically 3 ACS Paragon Plus Environment


inactive and merely provides thermal stability, while ion B (transition metal) is the active component [20]. Among the LnMnO3 materials (Ln = Lanthanides), lanthanum manganite (LaMnO3) shows high thermal stability and oxygen mobility [20, 21]. The energy conversion process of LaMnO3 perovskite oxide materials in metal-air battaries and solid oxide fuel-cell (SOFC) electrode are well established due to their structural stability and oxygen mobility. Further, due to the ability of cationic substitution at both A and B sites with ions of varying oxidation states, cation deficient defective nature and potential to accommodate mobile oxygen ions, LaMnO3 has an excellent catalytic property for oxygen reduction reaction (ORR) application [22,23]. All these properties of LaMnO3 made them the best candidate for SOFC electrode, metal-air battery electrode and ORR catalyst could and researchers found that all these properties could be exploited for electrochemical storage mechanisms also [2329]. Mefford et al recently reported the oxygen vacancy-mediated redox pseudocapacitance property of LaMnO3 nanomaterials by preparing both oxygen excess and oxygen deficit LaMnO3 [23]. They found an enhanced specific capacitance for oxygen deficit LaMnO3 electrode which is due to the oxygen mediated ionic intercalation. The role of oxygen vacancy in pseudocapacitance property of the LaMnO3 material is well explained in our previous report [30]. Further due to its inherent property like oxygen vacancies and ability of cationic substitution on both A and B sites with ions of variable oxidation states, it can be introduced to both anionic and cationic charge storage devices [23,31-33]. However, poor electrical conductivity and ease of aggregation hinder the smooth ionic intercalations and as a consequence, it could not achieve the expected capacitance value. Hence, some significant research have been done by various groups to conquer these drawbacks of LaMnO3 [2428,30,34,35]. Graphene has achieved considerable attention due to its extra-large theoretical surface area (2630 m2g-1), excellent electrical conductivity and extended mechanical flexibility and 4 ACS Paragon Plus Environment

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structural stability. Hence, tremendous efforts have been made by researchers to achieve high energy supercapacitors using different forms of graphene [36-43]. It has been used largely as a negative electrode in asymmetric supercapacitors [6,44-46]. The extended surface area with mechanical flexibility brought graphene as a promising candidate of large EDLC. Hence compositing with graphene not only provide large surface area and electrical conductivity but also provide extended mechanical and structural stability to the composite electrode over thousands of cycles. However, the specific capacitance value of graphene derivatives is limited due to its purely double layer behaviour. Conducting polymers are another class of electrode materials which possess pseudocapacitive property. Hence it has large capacitance value besides its conductivity and mechanical flexibility. Polyaniline has drawn considerable attention among other conducting polymers owing to their low cost, environmental benignity, ease of preparation, relatively high conductivity and is well known to increase the specific capacitance of active materials [47]. Hence, combining different class materials together to form composite electrode is considered as a promising approach to fabricate electrode materials. Consequently, such composites exhibit enhanced surface area, appreciable mechanical stability, flexibility, induced porosity, reduced agglomeration, enhanced electron and proton conduction, improved access of active sites and extra pseudocapacitance [5,7,47,48]. Herein we report the fabrication of LaMnO3-reduced graphene oxide-polyaniline (LaMnO3/RGO/PANI) ternary nanocomposite as an efficient electrode for supercapacitor application. The RGO support and in situ PANI polymerisation are carried out systematically to develop LaMnO3/RGO/PANI composite electrode. It is found that the as-prepared composite exhibits a far better electrochemical performance compared to individual materials. Moreover, to explore the favourable electrochemical property of the as-prepared electrode

material

to

practical

applications,

LaMnO3/RGO/PANI║LaMnO3/RGO/PANI,

symmetric


we

assembled

supercapacitor

(SSC)

both and


LaMnO3/RGO/PANI║RGO, asymmetric supercapacitor (ASC) devices and the performances were compared. Although both these devices exhibited better electrochemical performances, the ASC device showed higher specific capacitance and capacitive retention (111 Fg-1 at 1 Ag-1 and 55.5 Fg-1 at 20 Ag-1). Also we could elevate the cell voltage up to 1.8 V by constructing ASC device and hence we achieved outstanding energy and power densities (energy density of 50 Whkg-1 at a power density of 2.25 kWkg-1 and the device was still found to deliver an energy density of 25 Whkg-1 at high power density as high as18 kW kg-1). The RGO support and PANI coating again provided excellent structural stability and good electrical conductivity, which is directly reflected on the device cyclic stability (117 % retention after 100k cycles). Hence from these findings, we propose the fabricated LaMnO3/RGO/PANI electrode as a promising candidate to the future supercapacitor electrode application. EXPERIMENTAL SECTION Preparation of RGO RGO was prepared by reducing the graphite oxide (GO) which was prepared by modified Hummers method under chemical reduction. The procedure is as follows: 125 mg of GO powder was dissolved in 100 mL double distilled (DD) water under 2h of sonication. The solution containing RB flask was then transferred to an oil bath with a condenser setup under vigorous stirring at 100 ˚C. After 30 minutes of stirring, 1 mL of hydrazine hydrate was added to the solution and the stirring and condensation processes were continued for 48 h at 100 ˚C. Finally, the dark powder was filtered and separated by centrifugation drying process. Preparation of RGO/ LaMnO3


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RGO/LaMnO3 composite was prepared by insitu precipitation method. 100 mg as-prepared RGO powder was dissolved in 100 mL DD water and kept for sonication for 30 minutes. 0.1M Lanthanum nitrate (LaNO3.6H2O) and manganese sulphate (MnSO4.H2O) were separately dissolved in 20 mL of DD water and poured to the RGO solution. After 30 minutes of vigorous stirring, 0.6 M KOH solution was added and the stirring was continued for 3 h. Finally, the precipitate was washed, dried and calcinated at 750 ˚C for 12 h. Preparation of RGO/ LaMnO3/PANI composite The ternary composite was prepared by in situ polymerization with the prepared RGO LaMnO3 suspension as follows. In typical synthesis procedure, 100 mg RGO LaMnO3 composite was suspended in 1 M HCl solution (75 mL DD water and 25 mL HCl 35%) by continuous stirring. Then 0.23 mL purified aniline was added to the above solution and the resultant solution kept in ice bath maintaining the temperature around 0-5 ˚C. Then a precooled solution of 15 mL (0.68 g) of ammonium peroxodisulfate (APS) was added drop wise to the above solution and the ice bath was continued for 5 h with vigorous stirring. Finally the solution was washed several times with water and ethanol and dried at 40 ˚C. Material Characterization The crystalline structure of the prepared samples was analysed using Ultima III Rigaku X-ray diffractometer with Cu Kα1 radiation of wavelength 1.0546 Å. The oxidation states and chemical compositions are confirmed by X-ray photoelectron spectroscopy (Kratos Axis ULTRA X-ray Photoelectron Spectrometer incorporating a 165 mm hemispherical electron energy analyser). Morphological investigations of the prepared materials are done by FESEM imaging (Carl ZEISS NEON 40 FESEM, USA) and HRTEM imaging (JEOL-JEM 2100, USA, with LaB6 source).



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Electrochemical properties of prepared samples are investigated by using BioLogic SP-150 instrument with 3 electrode as well as 2 electrode symmetric and asymmetric cell systems. Cyclic voltammetry (CV), Galvanostatic charge-discharge technique (GCD) and electrochemical impedance spectroscopy (EIS) studies are carried out in 3 M KOH aqueous electrolyte solution. The working electrodes were fabricated by mixing the active material, carbon black, and polyvinylidene fluoride (PVDF) in a weight ratio 8:1:1 and dispersed in dimethylformamide (DMF) solution by using ultrasonicator in order to get homogeneous slurry. A known weight of the resulting slurry was coated on Ni foam substrate (1 cm ×1 cm) of thickness 1 mm yielding an active material loading of 0.5 mg cm-2 and it is allowed to dry at room temperature. The foam is then manually pressed to become a thickness of 0.2 mm. The electrochemical behaviour of the materials was characterized using a KCl saturated Ag/AgCl reference electrode and a platinum wire as a counter electrode along with prepared working electrode in three electrode system. Cyclic voltammetry is carried out for different scan rates, 2 mV/s, 5 mV/s, 10 mV/s, 20 mV/s, 30 mV/s, 50 mV/s, 75 mV/s, 100 mV/s, and 150 mV/s within a potential range of 0 to 0.5 V. Galvanostatic charge/discharge studies are performed to analyze the charge/discharge time of the LaMnO3 and its composites for different current densities (1 A/g, 2 A/g, 3A/g, 4 A/g, 5A/g, 7.5 A/g, 10 A/g, and 15 A/g) in a potential window of 0 to 0.5 V. Further electrochemical impedance spectroscopy studies are done in a frequency range of 1 mHz to 100 kHz with a perturbation potential of 5 mV. For two electrode device fabrication, PVA-KOH gel electrolyte has been prepared through a modified method and is sandwiched in between the electrodes [49,50]. In a symmetrical supercapacitor

(SSC),

both

positive

and

negative

electrodes

are

acted

by

LaMnO3/RGO/PANI composite whereas for asymmetric supercapacitor (ASC) device, LaMnO3/RGO/PANI acted as positive electrode and RGO acted as negative electrode. All


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electrochemical analyses were repeated for a potential range of -0.7 to +0.7 V for SSC and 0 to 1.8 V for ASC. RESULTS AND DISCUSSION The crystalline formation of LaMnO3 and reduction of GO to RGO are confirmed from the XRD pattern of each sample as shown in figure 1. All diffraction peaks of LaMnO3 can be indexed to rhombohedral LaMnO3 perovskite (JCPDS: 898775). The broad peak observed at 24.5˚ for RGO corresponds to (002) plane and it reveals the reduction of GO to RGO. The XRD pattern of LaMnO3/RGO/PANI composite exhibits intense peaks corresponding to LaMnO3 planes whereas there are two broad peaks around 16˚ and 25.6˚ corresponding to (011) and (200) planes of PANI, indicated that the LaMnO3 and PANI coexist in the prepared LaMnO3/RGO/PANI ternary composite material. The information about chemical composition via valance state was studied by XPS. The presence of each component of LaMnO3/RGO/PANI ternary composite is confirmed from the survey spectrum (figure 2a). XPS analysis is a surface specific technique usually employed with a sampling depth of 10 nm [47]. Hence this technique always provides the chemical composition information with high accuracy at the surface of the sample and least accuracy for bulk sample. Here the XPS spectra of LaMnO3/RGO/PANI exhibit distinct peaks corresponding to N1s, C1s and O1s whereas peaks corresponding to the A site and B site ions were diminished considerably. This reveals the uniform coating of PANI and the incorporation of RGO with LaMnO3 composite material. The chemical bond state of carbon in the composite material can be further determined by deconvoluting the high resolution C1s spectrum into four peaks (figure 2b). The peaks at 284.4 eV and 284.6 eV are attributed to the sp2 hybridized carbon C=C originating from RGO and PANI respectively [51]. Peaks at 286.4 eV and 288.5 eV are exerted from the C-C and C=O bond respectively [51,52]. The



area difference between C-C and C=O confirms the incorporation of RGO in a considerable amount. The high resolution O1s spectrum (figure 2c) exhibits two distinct peaks at 529.4 eV and 531.2 eV, which are corresponds to crystal lattice oxygen (OL) and surface absorbed oxygen (OH) respectively [53]. This OL indicates the Mn-O-Mn bond of MnO6 and OH reveals the possible oxygen vacancies [54]. Figure 3 displays the TEM and HRTEM images of LaMnO3, LaMnO3/RGO binary composite and LaMnO3/RGO/PANI ternary composite with SAED pattern. Figure 3a represents the TEM images of pure LaMnO3 nanoparticles with a particle size range 20 – 30 nm. Figure 3b shows the LaMnO3/RGO composite and it is well observed that the LaMnO3 nanoparticles are anchored over the RGO sheet. The LaMnO3/RGO/PANI composite shows well interconnected structure (figure 3c-d). The presence of PANI and RGO is confirmed with HRTEM images as shown in figure 3e. The observed lattice fringes in SAED pattern reveal that it is corresponds to rhombohedral LaMnO3 (JCPDS: 89-8775). Prior to asymmetric cell fabrication, the electrochemical analysis was performed using a three electrode configuration and the results of LaMnO3/RGO/PANI ternary composite were compared with bare LaMnO3, LaMnO3/PANI and LaMnO3/RGO electrodes. Typical cyclic voltammogram corresponding to LaMnO3, LaMnO3/PANI, LaMnO3/RGO, LaMnO3/RGO/PANI electrodes recorded at a sweep rate of 10 mVs-1and bare Nickel foam substrate at 20 mVs-1 in a potential window of 0–0.5 V are shown in figure 4a. The CV corresponding to bare Ni foam substrate does not own much current response with the potential scan, which clearly points out the inconsiderable contribution of the substrate to the capacitance of prepared materials. The CV corresponding to LaMnO3 and its composites exhibit a pair of redox peaks, indicating the superior faradic contribution as pseudocapacitance which results to an enhanced specific capacitance of the material. However, the broad peaks on both positive as well as negative sweep provide a larger loop 10 ACS Paragon Plus Environment

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area, implying the presence of EDLC too. Since larger loop area represents the larger specific capacitance, the composite electrode, LaMnO3/RGO/PANI has higher specific capacitance compared to other electrodes. However, the electrode, RGO/LaMnO3 exhibits a larger enhancement in loop area compared to pure LaMnO3, which clearly points to the enhanced specific capacitance of LaMnO3/RGO composite electrode. The enhancement in specific capacitance of LaMnO3/RGO is attributed to the increased conductivity acquired by compositing with RGO. It is well known that the removal of oxygen containing functional group can enhance the electrical conductivity of the material considerably [47,55]. In the case of LaMnO3/RGO/PANI ternary composite electrode, the enhanced capacitance is ascribed to the synergetic effect between LaMnO3, RGO and PANI. The double layer capacitance from RGO and pseudocapacitance from LaMnO3 and PANI collectively contribute to an enhanced total specific capacitance of the composite electrode. Here apart from RGO, the polymer PANI has excellent conductive property and they together enhance the conductivity of the composite material. Thus the highly conductive PANI and the bridge function of RGO for the electronic hopping between PANI chains together contribute to an enhanced electrical conductivity which results in an enhanced specific capacitance [47,56]. Rate capability is an important factor which has direct impact on practical application as it affects the power delivery nature of the material. In order to understand the detailed capacitive property and rate capability of LaMnO3/RGO/PANI electrode, the cyclic voltammogram was recorded at different scan rates (figure 4b). The pseudocapacitive nature of the CV curve still maintained as the scan rate increases from 5 to 100 mV/s, indicating the excellent retention on capacitive performance of the electrode. In addition, the current response and scan rate exhibit a linear relation reflecting the faster and easier ionic transport through electrode- electrolyte interface. It is quite common that the effective interaction of electrolyte ions with active sites of electrode material will decrease with increased scan rate as it won’t get sufficient time to



intercalate completely. Consequently, the separation between anodic and cathodic peaks will increase greatly. However, here the separation is less and the redox behaviour is retained even at higher scan rates, revealing the negligible polarisation and excellent rate capability of the electrode [47,57]. Figure 4c represents the Galvanostatic charge-discharge (GCD) curves corresponding to each electrode at a current density of 1 Ag-1. Interestingly, the charging part is symmetric with the corresponding discharging counter parts, which reveal the appreciable coulombic efficiency. The observed curvature for charging as well as discharging exactly matches with the peak potentials of CV curve, which revealed that the redox reactions happened at that particular potential range. The discharging time of the electrodes increases in the order of LaMnO3 < LaMnO3/PANI < LaMnO3/RGO < LaMnO3/RGO/PANI, which points out the drastic enhancement of specific capacitance by compositing with RGO and PANI, since discharging time is directly proportional to the specific capacitance of the material. The specific capacitance values of each electrode can be estimated using the following formula [58].

S

c

=

I∆ t m ∆ V

(1)

where ‘I’ (A) is the applied constant current in Ampere, ‘∆t’ (s) is the discharge time in seconds, ‘m’(g) is the mass of active material in gram and ‘∆V’ (V) is the applied potential window of charge-discharge curve in volt. The calculated Sc values for LaMnO3, LaMnO3/PANI, LaMnO3/RGO and LaMnO3/RGO/PANI electrodes are 324 Fg-1, 390 Fg-1 416 Fg-1and 802 Fg-1 respectively. The drastic increment in capacitance value of LaMnO3/RGO/PANI composite from pure LaMnO3 and LaMnO3/RGO binary composite is achieved due to the synergetic effect between LaMnO3, RGO, PANI of the ternary composite. Additionally, the incorporation of RGO and PANI provides conductive substrate


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and binding to LaMnO3 nanoparticles, which could effectively enhance the ionic and electronic transport through the electrode electrolyte interface resulting in an enhanced specific capacitance and rate capability. The rapid charging is another key significant factor of supercapacitor, which is highly demanded in practical application. Hence, the LaMnO3/RGO/PANI electrode is further subjected to GCD at different current densities (1 to 15 Ag-1) as shown in figure 4d. The shape of GCD curve is maintained even at higher current density, which implies the possibility of rapid charging with reversible redox reaction. Also the charge time and discharge time are equal irrespective of the applied current density, revealing the retention of columbic efficiency at various charging rate. The IR drop is observed to be minimum at lower current density and increasing with current density. However, LaMnO3/RGO/PANI possess negligible IR drop at current density of 1 Ag-1. The specific capacitance of LaMnO3/RGO/PANI electrode at various current densities is calculated and is presented in figure 4e. It is obvious to observe the reduction in specific capacitance value at higher current densities as the electrolyte ions have insufficient time to intercalate/deintercalate at respective sites of electrode materials. However, more than 66 % of initial specific capacitance (802 Fg-1 at 1 Ag-1) is retained at a current density of 10 Ag-1 and it exhibits above 55 % even for 15 Ag-1. This outstanding rate capability is achieved from the structural stability through RGO support and the enhanced electrical and ionic conductivity acquired by compositing with RGO as well as PANI, which will reduce the diffusion path and may ease the charge/ion transport through electrode electrolyte interface. The electrical conductivity and charge/ion transport kinetics are well investigated by EIS technique employed in a frequency range of 100 mHz to 100 kHz at a bias voltage of 5 mV. Typical Nyquist plots corresponding to each electrode are displayed in figure 4f. Basically the Nyquist plot consists of three parts. First part is at higher frequency region, the x-intercept of starting curve represents the effective series resistance (ESR). This is the sum of solution



resistance, intrinsic resistance of the active material and contact resistance of the active material to the current collector substrate. Here all three electrodes exhibit very less ESR value (≤ 1). The second part, the semi-circle at mid frequency region indicates the charge transfer resistance (Rct) at the electrode-electrolyte interface and is equal to the diameter of the semi-circle. It is clear from the figure that the Rct value is reduced for LaMnO3/RGO composite and further reduced for LaMnO3/RGO/PANI ternary composite compared to pure LaMnO3 electrode. It is ascribed to the enhanced electrical conductivity acquired through compositing with RGO and PANI. The effect is reflected in CV and GCD curves of the respective electrode and is explained in the previous session. The third part is the vertical spike at low frequency region, representing the diffusion resistance or Warburg resistance (Zw) to the ionic transport at electrode-electrolyte interface. For an ideal capacitor this vertical line will be exactly parallel to the imaginary axis of EIS plot (≈ 90˚). Here the LaMnO3/RGO/PANI electrode exhibits a nearly vertical line, indicating a better capacitive behaviour compared to pure LaMnO3, LaMnO3/PANI and LaMnO3/RGO binary composite electrodes. For accuracy, we fitted each EIS curve with an equivalent circuit (figure 4f inset) and the fitted values of each component are tabulated in table 1. To explore the practical applications of the LaMnO3/RGO/PANI ternary composite, we fabricated all solid state supercapacitor with both symmetric and asymmetric device configurations by sandwiching PVA-KOH gel electrolyte in between the positive and negative electrodes. This PVA-KOH gel acts as both electrolyte and separator at the same time. For LaMnO3/RGO/PANI║LaMnO3/RGO/PANI SSC device, LaMnO3/RGO/PANI is used as both positive and negative electrode with equal mass loading whereas in LaMnO3/RGO/PANI║RGO ASC device configuration, LaMnO3/RGO/PANI is used as positive electrode and RGO as the negative electrode. The mass loading ratio of active material on positive (m+) and negative electrode (m-) is confirmed by the equation, m+/m- = 14 ACS Paragon Plus Environment

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C-∆V-/C+∆V+ [6,58,59]. Here C-, C+, ∆V- and ∆V+ represent the specific capacitance of RGO electrode, LaMnO3/RGO/PANI electrode, potential window of RGO electrode and potential window of LaMnO3/RGO/PANI electrode respectively. The capacitance value of RGO (C-) and LaMnO3/RGO/PANI (C+) electrode are calculated from the GCD curve recorded in their respective potential window. In order to fix the stable working potential window of SSC, a series of CV have been recorded with varying potential window as shown in figure 5a. It is observed that the area bounded by CV curve increases as potential window increase up to 1.4 V, while there is no further enhancement in area observed on further increasing the potential window. Thus from the figure it is clear that the SSC will give a maximum Sc value at a potential window of ∆V = 1.4 V. Hence it is fixed for further studies of SSC device. The stable working potential window of the ASC device is confirmed as 1.8 V by recording the cyclic voltammogram of both positive and negative electrodes at possible potential range as shown in figure 5b. Hence the capacitive property and electrochemical performances of the SSC and ASC devices were investigated by employing the CV and GCD techniques at the optimised potential windows. Typical cyclic voltammogram recorded for SSC at different scan rates (5 – 150 mVs-1) is presented in figure 5c. It is well clear that the CV curve reflects both faradic and double layer capacitance property of the SSC device. However as scan rate increases, the redox behaviour gets declined, which reveals that only double layer capacitance maintain the rate capability. Thus there is a considerable drop in specific capacitance of the SSC at higher scan rates. However, in the case of ASC device, faradic capacitance is dominant over double layer capacitance (figure 5d). Besides, the CV curve exhibited a negligible attenuation on shape and redox behaviour even at higher scan rates, which conveyed the outstanding rate capability of the ASC device. The charging and discharging process of SSC and ASC devices are further evaluated by employing the GCD technique for various current densities (2.5 – 20 Ag-1) at respective potential windows. Figure 5e shows the



charge discharge performances of the SSC device at different current densities. The faradic behaviour of the GCD profile with plateau region corresponding to the redox potential observed in CV diagram, revealed the excellent pseudo capacitive behaviour of the fabricated SSC device. The plateau potential is observed to be increasing on charging and decreasing on discharging as current density increases, pointing out the discrepancy in rapid charge transport. This is quite similar to that of observed from CV for higher scan rates. Figure 5f depicts the GCD profile at different applied current densities. Here also the faradic nature is reflected and the plateau region well matches with the redox potential observed on CV diagram. Unlike SSC, ASC device does not exhibit any potential shift on plateau region either for charging or discharging, which shows the ability of ASC device to perform at higher current densities. The specific capacitance values of both SSC and ASC devices at different current densities were estimated using the equation 1 and is presented in figure 6a, where m denotes the total mass of active material coated on positive and negative electrodes. The calculated specific capacitance values of SSC and ASC at a current density of 2.5 Ag-1 are 89 Fg-1 and 111 Fg-1 respectively. The rate capability of these devices can be easily examined from figure 6a and it confirms the result obtained from CV diagram and GCD profiles of different charging/discharging rate. The SSC and ASC devices still exhibit a specific capacitance of 31.5 Fg-1 and 55.5 Fg-1 at current density as a large as 20 Ag-1and it is about 35 % and 50 % retention of their capacitance at 2.5 Ag-1 respectively. This superior retention of ASC over SSC is attributed to the excellent rate capability of both faradic and non-faradic capacitance of ASC device which is observed from the shape of CV and GCD curves at various charge-discharge rates. The rate capability of ASC device is further evaluated by subjecting the device to CV of high scan rates, up to 10 V/s. The recorded CV curves exhibit redox behaviour despite the scan rate as shown in figure 6b. The enhancement of area bounded by the CV curve with distinct redox peaks on increasing the scan rate proves


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the existence of both EDLC and pseudocapacitance even at higher scan rates. This outstanding retention, even at 10 V/s is clearly evident to the excellent rate capability of the ASC device. Hence from these studies it is well clear that the ASC device can perform more reliably for high power and energy demands. Energy density and power density are the two critical parameters of any energy storage device which gauge the device performance in practical life. The energy density, E (Wh/kg) and power density, P (W/kg) of both SSC and ASC devices were estimated from equation (2) and (3) [60].

E =

P

0 . 5 Sc ( ∆ V ) 2

(2)

3 .6

=

E

(3)

∆ t

The estimated E and P values corresponding to both SSC and ASC are presented as Ragone plot as shown in figure 6c. From equation 2, it is clear that the energy density of a device is directly proportional to specific capacitance value and square of operating potential window. Hence, by assembling an asymmetric device, we could extend the potential window up to 1.8 V whereas for symmetric device it is 1.4 V, which is reflected on the Ragone plot. Since both Sc values and ∆V are superior for ASC device, the ASC device exhibits a drastic enhancement on E and P values. The ASC device could deliver a maximum energy density of 50 Wh kg-1 at a power density of 2.25 kW kg-1. Moreover, even at high power density as high as 18 kW kg-1, the device still delivers an energy density of 25 Wh kg-1.These results are compatible among reported asymmetric devices and superior over LaMnO3 based supercapacitors [1-6,23,31-33,53,61,62]. The energy and power densities are compared with some recent reports in Ragone plot (figure 6c). The cycle life of an energy storage device is a crucial factor which distinguishes the supercapacitor from other storage devices apart from power density. The cyclic stability of the ASC device is investigated by employing the GCD 17 ACS Paragon Plus Environment


techniques between 0 to 1.8 V at an applied current density of 20 Ag-1 for 100k cycles. Typical capacitive retention plot is presented in figure 6d. It is quite interesting that the ASC device does not attenuate the capacitance as cycling proceeds, rather it exhibits an enhanced specific capacitance after 100 k cycles. Initially, the specific capacitance is observed to be increasing drastically up to 1000 cycles, which is ascribed to the preconditioning of electrodes by continuous charging and discharging [63]. However, after this initial cycling (electrode activation), the device still manifests a slight increase as cycling proceeds. Hence the LaMnO3/RGO/PANI║RGO ASC device exhibits 117% of retention on its initial capacitance even after 100k cycles. This result again demonstrates the practical application of LaMnO3/RGO/PANI║RGO ASC device. CONCLUSION LaMnO3/RGO/PANI ternary composite material has been fabricated by a two step process including in situ PANI polymerisation. The formation of composite has been confirmed chemically and physically from XRD, XPS and TEM analyses. As-prepared ternary composite exhibits an outstanding and superior electrochemical performance over individual materials. The incorporation of RGO and PANI to the LaMnO3 nanoparticles has improved its structural stability, electrical conductivity and electrochemical performances. The LaMnO3/RGO/PANI ternary composite has exhibited specific capacitance of 802 Fg-1 at 1 Ag-1 in three electrode configuration. In order to confirm the practical application of this ternary composite electrode, all solid state LaMnO3/RGO/PANI║LaMnO3/RGO/PANI symmetric supercapacitor as well as LaMnO3/RGO/PANI║RGO asymmetric supercapacitor devices are assembled. It is found that the ASC device can perform over an extended potential range of 1.8 V and exhibit a specific capacitance of 111 Fg-1 at a current density of 2.5Ag-1. Moreover, the Sc value has a retention of 50 % even at a current density as high as 20 Ag-1. Also, the ASC device has significantly enhanced energy density and power density 18 ACS Paragon Plus Environment

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values as compared to that of SSC device. The ASC device could deliver a maximum energy density of 50 Wh kg-1 at a power density of 2.25 kW kg-1 and the device still delivers an energy density of 25 Wh kg-1 at power density as high as 18 kW kg-1. It is an outstanding result among LaMnO3 based supercapacitors. In addition, the ASC device executes an excellent retention of 117 % even after 100k cycles. These findings on LaMnO3/RGO/PANI ternary composite suggest that this composite can be used as an efficient positive electrode material for supercapacitor devices. ACKNOWLEDGEMENT P.M.S. is grateful to MHRD, Government of India for funding the SRF grant to undertake research work. A.C.B. acknowledge DST-Serb project (EMR/2016/002115).

Figure 1. XRD patterns of pure LaMnO3, RGO, LaMnO3/RGO composite and LaMnO3/RGO/PANI composite. 19 ACS Paragon Plus Environment


Page 20 of 33

(a)

(b)

(c)

Figure 2. XPS spectrum of LaMnO3/RGO/PANI composite: (a) survey, (b) and (c) are high resolution spectrum of C1s and O1s states.


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(c)

(b)

(a)

RGO

LaMnO3

(128) (214) (024) (104)

RGO LaMnO3

(104)

5 1/nm

RGO

(024)

(e) (d)

PANI

10 nm

2 nm

(f)

Figure 3. TEM images of pure LaMnO3 (a), LaMnO3/RGO (b), LaMnO3/RGO/PANI (c and d) and HRTEM images of LaMnO3/RGO/PANI (e and f).



Page 22 of 33

(a)

(b)

(d)

(c)

(e)

(f)

Cdl Rs

Q Rct

Figure 4. (a) CV curve recorded at a scan rate of 10 mV/s for all samples and at 20 mV/s for Ni foam. (b) CV curve of LaMnO3/RGO/PANI composite at different scan rates, (c) Chargedischarge curve recorded for all samples at a current density of 1mA/g. (d) GCD curve of LaMnO3/RGO/PANI composite at various current densities (e) capacitive retention at higher current densities and (f) EIS plot for all electrodes. 22 ACS Paragon Plus Environment

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(b)

(a)

(d)

(c)

(f)

(e)

Figure 5. (a) Cyclic voltammetry of SSC device with different working potential (b) CV of LaMnO3 and RGO electrodes at 10 mV/s in three electrode system, CV recorded for SSC (c) and ASC (d) at different scan rates, GCD measurement for SSC (e) and ASC (f) at different current densities.



Page 24 of 33

(b)

(a)

(c)

(d)

Figure 6. (a) Specific capacitance as a function of current density for both SSC and ASC devices. (b) CV recorded at extra-higher scan rates for ASC device, (c) Ragone plot derived from GCD techniques for both SSC and ASC devices and (d) cyclic stability of ASC device.


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Table 1. The fitted values of each component in equivalent circuit for different electrodes.

Electrodes

Rs (Ω)

Rct (Ω)

Cdl (µF)

Q (Fs(a-1))

a

LaMnO3

0.75

13.24

19

0.00429

0.502

LaMnO3/RGO

0.426

7.79

339

0.0481

0.571

LaMnO3/PANI

0.435

5.57

426

0.036

0.611

LaMnO3/RGO /PANI

1.124

3.75

1.44

0.108

0.784

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electrode

and

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TOC


PANI Ternary ... - ACS Publications

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