Facile synthesis of perovskite lanthanum aluminate and its green reduced graphene oxide composite for high performance supercapacitors

Journal Pre-proof Facile synthesis of perovskite lanthanum aluminate and its green reduced graphene oxide composite for high performance supercapacitors

T.N. Vinuth Raj, Priya A. Hoskeri, H.B. Muralidhara, C.R. Manjunatha, K. Yogesh Kumar, M.S. Raghu PII:

S1572-6657(20)30013-8

DOI:

https://doi.org/10.1016/j.jelechem.2020.113830

Reference:

JEAC 113830

To appear in:

Journal of Electroanalytical Chemistry

Received date:

16 October 2019

Revised date:

6 January 2020

Accepted date:

6 January 2020

Please cite this article as: T.N. Vinuth Raj, P.A. Hoskeri, H.B. Muralidhara, et al., Facile synthesis of perovskite lanthanum aluminate and its green reduced graphene oxide composite for high performance supercapacitors, Journal of Electroanalytical Chemistry(2020), https://doi.org/10.1016/j.jelechem.2020.113830

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© 2020 Published by Elsevier.

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Facile synthesis of perovskite Lanthanum aluminate and its green reduced graphene oxide composite for high performance supercapacitors Vinuth Raj T Na, Priya A Hoskeri b, Muralidhara H Bc, Manjunatha C Rd, Yogesh Kumar Ke, Raghu M Sf* a

Department of Physics, School of Engineering and Technology, Jain University, Bangalore,562 112, India. b Department of Physics, Dayanand Sagar College of Engineering, Kumarswamy Layout, Bangalore, 560 078, India. c

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Centre for Incubation, Innovation, Research & Consultancy, Jyothy Institute of Technology, Bangalore, 560 082, India. d

Department of Chemistry, M S Ramaiah Institute of Technology, Bangalore, 560054, India.

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Department of Chemistry, School of Engineering and Technology, Jain University, Bangalore, 562 112, India. *fDepartment of Chemistry, New Horizon College of Engineering, Outer Ring Road, Bangalore, 560103, India.

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*corresponding author: Email;[email protected]

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[email protected]

Journal Pre-proof Abstract A facile simple hydrothermal synthetic route has been developed for the synthesis of perovskite lanthanum aluminate (LaAlO3) and reduced graphene oxide composite (RGO). Green tea extract here is used as reducing agent for conversion of graphene oxide to reduced graphene oxide. The synthesized materials have been characterised for its morphology using, XRD, SEM, EDAX, FTIR and BET techniques. The materials are used as electrode materials for supercapacitor applications. Electron microscopic images indicate RGO sheets are densely attached to surface of LaAlO3 with perforated cage like morphology. A high specific capacitance of 721 Fg-1 at a scan rate of 2 mVs-1 was exhibited by RGO/LaAlO3 hybrid at

the same scan rate using cyclic voltammetry (CV).

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electrodes

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structure electrode when compared to pure RGO (531 Fg-1) and pure LaAlO3 (105 Fg-1 ) By using

chronopotentiometry (CP), the RGO/LaAlO3 hybrid structure electrode out streched specific

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capacitance of 283 Fg-1 at a current density of 0.5 Ag-1 and able to retain specific capacitance (Csp) of about 157 Fg-1 even at very high current density of 20 0.5 Ag-1.RGO/LaAlO3

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composite also possesses a energy density of 57 Wh kg-1 with a maximum power density of

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569 W kg-1. RGO/LaAlO3 nanocomposite has also been fabricated for asymmetric supercapacitor device (ASD) system. From the Electrochemical Impedence Spectroscopy

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(EIS), the RGO/LaAlO3 hybrid structure electrode has shown phase angle close to -90o for frequency up to 0.01Hz, suggesting that the nanocomposite electrode material approached

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ideal capacitor behaviour.

Key words: RGO-perovskite LaAlO3 composites; Green Synthesis; Hydrothermal synthesis; Supercapacitors.

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Introduction In the Universe: curiosity and impulsive superior existence needs are the dynamic forces of foundation of all the scientific research and inventions. In the anticipation of rewarding all the ideas and comfort, we, the human beings across the globe lost our plot by destroying our environment. Despite of this, in order to enhance the ease of living and source of income, people around the world are implementing new stuffs which ultimately result in energy crisis, environmental pollution and global warming due to population explosion and

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ecosystem imbalance. Acknowledging the immense connotation of safety, eco-friendly and sustainability in energy and environment sector, scientists throughout the world are working

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on development of energy storage devices in the form of supercapacitors, lithium ion

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batteries and fuel cells.

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The supercapacitors are the type devices that stores energy electrochemically, possessing twin advantages of conventional dielectric capacitors and the rechargeable

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batteries. The properties of dielectric capacitor include high power density, long cycle life and low energy density [1,2]. In contrast, typical rechargeable batteries afford high energy

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density, yet comprise low power density and short cycle life. Supercapacitors possess blended property with merits of fast charge/discharge profile, high coulombic efficiency, little

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maintenance cost, ease of operation with safety and mainly long cycle life. The Supercapacitors offer multipurpose powering solutions to a variety of applications ranging from handy electronic devices, automobile regenerative braking systems and electric vehicles to large scale smart utility grids.

Supercapacitors are majorly of two types, on the basis of energy storage system: electrochemical double layer capacitors (EDLC) and pseudocapacitors. EDLCs store energy electrostatically, and pseudocapacitors utilize reversible superficial faradiac reactions. The carbon based materials with high surface area, like, carbon nano tubes, activated carbon, carbon aerogels and graphene are the electrode materials in EDLCs [3-5], on the other hand electrode materials in pseudocapacitors includes metal oxides, and conducting polymers like, RuO2, MnO2, Fe2O3, Zno, V2O5, polyaniline [6-11] etc. Among the perovskite materials, lanthanum

materials

like

LaMnO3,

LaMnO3@NiCo2O4,

La0.85Sr0.15MnO3,

La0.85Sr0.15MnO3@NiCo2O4 and other materials were also used as electrode materials

for

supercapacitor applications [12-18]. Though pseudocapacitor materials exhibit higher

Journal Pre-proof specific capacitance than EDLCs materials but limit their application due to instability during cycling and low power density. The present scenario on innovations of materials for supercapacitors mainly focuses on pseudocapacitor electrode materials being incorporated in material matrices of EDLCs, creating hybrid structures with the aim to attain high energy density and power density along with good stability during cycling in supercapacitors. Among the materials used in pseudocapacitors, perovskites are the family of compounds having the general formula ABO3, where A is 12 (co-ordinated) and B is 6 (co-ordinated) metals attracted tremendously due to its special electronic and magnetic

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properties. Trivalent ions like La3+, Sr3+ etc are sites at A, and where as Al, Ni, Mn, Co etc. are transition metals at B site. Lanthanum based perovskite materials are utilized as electrode

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materials for supercapacitor applications. In order to enhance the electrochemical performance of LaAlO3, it is recommended to generate composite materials by incorporation

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of good carbon based materials, which possess high surface area, good conductivity, etc.

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Recently, carbon-based materials such as carbon nanotubes (CNT), carbon aerogels, graphene, graphene oxide (GO) and reduced graphene oxide (RGO) are frequently been used

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for a variety of applications such as redox catalysts, energy storage devices i.e. supercapacitors, lithium-ion batteries [19-21]. Graphene, discovered in 2004 and it is a single

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atomic thick layer of graphite with closely packed conjugated and hexagonally connected carbon atoms, which has attracted tremendous attention because of its large specific surface

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area, high-speed electron mobility, good mechanical strength, high electric and thermal conductivity, room temperature quantum hall effect, good optical transparency, and tunable band gap [22,23].

The reduction of graphene oxide (GO) is achieved by thermal, electrochemical and chemical methods to produce reduced graphene oxide [24-26]. Among all these existing methods, chemical reduction is considered as a promising approach for bulk production as it is simple, lesser need of sophisticated equipments, cost effective etc. First graphite will undergo oxidation to form GO later followed by reduction of GO using strong reducing agents like hydrazine and sodium hydrate [27, 28]. These reducing agents are hazardous, toxic, carcinogenic non economical and show tendency of introducing the impurities to the RGO which is harmful to environment along with creating adverse effect on biological system. To overcome this, green reduction of graphene oxide to RGO is in high demand, as they are environment friendly and non toxic. The green reductants used for reduction of GO involves, glucose, vitamin C, wild carrot, bacteria etc [29-32].

Journal Pre-proof By keeping above issues in mind, authors in the present work prepared perovskite lanthanum aluminate particles embedded with green reduced graphene oxide by simple hydrothermal method, characterized for its structural and morphological properties by various techniques. The LaAlO3, RGO and RGO/LaAlO3 composite are evaluated for electrochemical performance in supercapacitors and observed superior performance in RGO/LaAlO3 composite when compared with LaAlO3, rGO particles alone. The knowledge gained out of this research work could certainly spread a valuable guidance in development of high performance supercapacitors in future.

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2. Materials and Methods 2.1. Materials

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Analytical grade chemicals were used and are carried from SD-Fine chemicals.

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Beside this purchased chemicals used were without further purification. Green tea extract, Graphite powder, Lanthanum oxide, Nitric acid, Aluminium (III) nitrate, Oxallyl di

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hydrazole, Sulphuric Acid (98 wt%), Hydrogen Peroxide (30 wt%), Potassium Permanganate,

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Sodium Nitrate (98%), and Sodium Hydroxide for the preparation of nanocomposites.

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2.2. Characterization

Bruker D2 Phaser XRD system was used to get X-ray diffraction (XRD) patterns. Scanning electron microscope (JEOL JSM 840A) was used to study the surface morphology

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(SEM) was studied by coupling with energy dispersive X-ray analyzer (EDX). BET surface area, total pore volume and average pore size were measured using ASAP 2010 Micrometrics instrument by Brunauer–Emmett–Teller (BET) method.

2.3. Preparation of Green Tea Extract Green tea leaves were collected, cleaned and shade dried completely, milled with hammer mill and passed through 1 mm mesh screen. One hundred gram of leaves was boiled with 1 litre of distilled water for 10 min at 70 ºC. The heated solution was filtered by using Whatmann No. 42 filter paper. 2.4. Preparation of LaAlO3 The procedure used to prepare LaAlO3 by the gel route and low temperature combustion method. Stoichiometric amount of La2O3 powder dissolved with the 1:1 nitrating mixture (1:1 Nitric acid and sulphuric acid) heated slowly on a sand bath up to gel formation. A

Journal Pre-proof stoichiometric amount of allumininum nitrate and oxalyl dihydrazole (both in distilled water) were added to the gel. Throughout the process, no sign of precipitation was observed. The gel was transferred in to petridish and placed in an open heater maintained at around 300 °C. Later, the gel was dehydrated and decomposed releasing abundant amounts of heat and gases. The resultant was further heated and swelled to generate foam of fine flakes and gets ignited with evolution of large quantity of gases yielding foamy powder of lanthanum aluminate (Schematically given in scheme 1). The powdered foam was then hand ground in a mortar and was calcined at a temperature 500 °C for 2h.

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2.5. Conversion of GO-RGO

Graphene oxide (GO) was prepared from natural graphite flakes by a modified Hummers

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method [33]. A suspension consisting of 0.3 mg mL-1 graphene oxide in DI water was taken

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and the reduction reaction of GO with green tea extract (GTE) was carried out in a 50 mL batch reactor-closed system. Typically, green tea extract with a weight ratio of GTE/GO=1

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was added to 30 mL of GO suspension. Subsequently, the reduction of GO was performed in the reactor for 8 h at 90 °C while the GO-GTE mixture was stirred continuously at 200 rpm

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during the reduction reaction. The suspension of the final products was filtered through a

RGO.

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nylon membrane (0.22 µm) and washed 3-5 times with DI water and collected and named as

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2.6. Preparation of RGO-LaAlO3 nanocomposite For the RGO-LaAlO3 nanocomposite preparation, 20 mg RGO was dispersed in 20 mL ethanol and sonicated for 15 min. The 0.1 g of LaAlO3 powder was added in above solution with constant stirring at room temperature for another 1 h. The above mixture was transferred to hydrothermal bomb and autoclaved at 120 °C for 5 h. Then the resultant precipitate was centrifuged with distilled water, ethanol and dried under vacuum.

2.7. Preparation of working electrodes: The working electrodes using RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 samples are prepared as follows. A slurry was prepared by mixing 85 wt% of the active material, 10 wt% of Ketjen black (EC 600JD) as conductive material and 5 wt% of polyvinylidene difluoride (PVDF) as binder dissolved in 1-methyl-2- pyrrolidinone (NMP) in an agate mortar. The prepared slurry was coated on 1 cm2 area of carbon papers. The

Journal Pre-proof fabricated electrode was vacuum dried at 80 oC for 8 h before using it as working electrode. Electrochemical characterization was carried out in a three-electrode cell with 1.0 M KOH as the electrolyte. A platinum wire and Ag/AgCl were used as the counter electrode and reference electrode, respectively. Electrochemical studies were performed by Cyclic voltammetry (CV), Chronopotentiometry (CP) and Electrochemical impedance spectroscopy (EIS) techniques using CHI-660E workstation. The electrochemical measurement of assembled asymmetric supercapacitor device ASD (two electrodes system) was fabricated by using RGO/LaAlO3 nanocomposite as the positive electrode and less expensive, commonly used activated carbon (AC) material as the

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negative electrode. Electrodes for the device were prepared by coating the slurry of the

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electroactive materials on a carbon paper of area 1 cm2. Initially a capacitor grade separator paper with thickness of 44 mm was soaked in 1.0 M KOH electrolyte solution and

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sandwiched between the two electrodes. Since the specific capacitance of the two electrode material are different, the electrode mass ratio for the optimum performance of the device

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was calculated using the following equation.

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where, mn, mp are the active masses, Cn, Cp are the specific capacitances and Vn, Vp are the operating potentials of negative and positive electrode materials respectively. The calculated

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optimum mass ratio of the two electrode materials for ASD was found to be 0.32. In designing an ASD exactly 1 mg of RGO/LaAlO3 nanocomposite and 3.1 mg of AC were coated on a carbon paper based on their specific capacitances of 612 and 64 Fg-1 respectively which was determined in a three electrode system initially by CV at a scan rate of 5 mVs-1.

3. Results and discussion

3.1. Characterization

Figure 1 illustrates the XRD patterns of the LaAlO3 and RGO/LaAlO3 powders, which reveals that the calcined powders are crystalline in nature. The obtained LaAlO3 exhibit the crystalline phase with rhombohedral shape and perovskite structure and are in good agreement with JCPDS card 31-0022.

Journal Pre-proof LaAlO3 particles obtained has preferential orientation growth towards (110) plane, and intense diffraction peak at ~26° for RGO/LaAlO3 nanocomposite is attributed to the disordered RGO sheets with no evidence of restacking of individual sheets, which is expected to be facilitated by the presence of surface anchored LaAlO3 nanoparticles. GO shows typical diffraction signature at ~10° which was not observed for the RGO/LaAlO3 nanocomposite confirming the complete reduction of graphene. The morphology of synthesized RGO/LaAlO3 nanocomposite was characterized by SEM. The Figure 2(a and b) shows the high and low resolution SEM images of samples. It clearly indicates the perforated cage like morphology formation and also indicates that RGO

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nanosheets are densely attached to the surface of the LaAlO3.

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The EDAX studies (Figure 3a and 3b) has shown that there is a close agreement with the calculated composition of nanocomposites with the loaded compositions of the precursor

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nanoparticles. The elemental composition is as shown in figure (inset). There are no characteristic peaks for any impurities. XRD and EDAX studies corroborate to the formation

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of RGO/LaAlO3 nanocomposite.

In order to know the oxidation states of the synthesized RGO/LaAlO3 nanocomposite,

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X-ray photoelectron Spectroscopic (XPS) evaluation has been conducted and given in Figure (Supplementary 1; $). Figure $1a confirms the presence of La+3 cation in the perovskite and

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exhibit two 3d5/2 binding peaks at 835.4 and 838.2 eV, for Al+3 (2p) a distinctive peak at also observed.

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binding energy 72.4 ev (figure $1b) and for O 1s from figure $1c, a peak at 530.7 eV was

The BET plots in Figure 4 revealed that there is a large enhancement in the surface area of the nanocomposites and it was found to be in the range of 250 m2 g-1. Where as pure GO was found to be 112 m2 g-1. An increase in the surface area is observed with decoration of LaAlO3 due to the high density and low surface area of LaAlO3.

Cyclic voltammetry (CV) Cyclic voltammetry studies of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrode materials were conducted by scanning a voltage sweeping rate of 2, 5, 10, 20, 40, 50, 80 and 100 mVs-1. Figure 5, presents CV curves of RGO/LaAlO3 nanocomposites electrode sample at various scan rates of 2 to 100 mVs-1 with a potential window ranging from -0.3V to 1.2V. The consistency of these nearly a rectangular shape of curves with two distinct redox peaks around 0.3V and 0.4V at various scan rates reveals the excellent

Journal Pre-proof capacitive behaviour of nanocomposite electrode. During the electrochemical reaction for negative scan, reduction of AlO3 taking place from Al+3 to Al+2 later during positive scan AlO3 retain back its +3 state in the presence of electrolyte and thereby acts as a reason for its faradaic behaviour of the nanocomposite. The oxidation and reduction peaks of CV curves shifted continuously to higher and lower potentials with increasing scan rates, resulting to the polarization effect and ohmic resistance of the electrode materials at high scanning speed. This indicates that the electrochemical reaction is quasi-reversible and the irreversibility degree becomes bigger with an increasing of potential scan rates. Figure 6 shows the comparison of CV recorded for RGO/LaAlO3 nanocomposites,

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pure RGO and pure LaAlO3 electrodes at a scan rate of 50 mVs-1. The RGO/LaAlO3

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electrode exhibited much higher current density than other pure electrodes, indicating better

calculated using the following equation (1). V

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(1)

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C sp

C 1  iV dV  W (V ) VD

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electron transportation. The specific capacitance (Csp) of the electrode materials were

Where  is the scan rate (Vs-1), W is active mass of material (g) and V is potential

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window (V). The calculated specific capacitance values of all the electrodes at various scan rates are summarized in Table 1. As the scan rate increase the specific capacitance exhibit

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decreasing trend. Figure 7 shows the calculated specific capacitance and rate performance of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes at different scan rates. The highest specific capacitance obtained for the RGO/LaAlO3 electrode was 721 Fg-1 at a scan rate of 2 mVs-1, which is much higher than the values obtained for pure RGO and pure LaAlO3 electrodes (531 and 105 Fg-1) at the same scan rate. As more oxygen ions react with electrode material, CV with larger integrated area was examined for RGO/LaAlO3 composite. The specific capacitance achieved by RGO/LaAlO3 is maximum compared to other two electrodes, because the hybrid structure leads to development of new pathways for electron transport, which inturn increases the conductivity of the composite. Because of the presence of hierarchical graphene oxide prevents the aggregation of LaAlO3 particles and the functional groups on the GO exhibited well hybridization with LaAlO3 resulting in enhanced surface area of the hybrid material has also show impact in enhancing the electrochemical performance of RGO/LaAlO3 composite. On increasing the scan rate from 2 to 100 mVs-1, the current response is enhanced, and hence indicates fast electronic and ionic transport

Journal Pre-proof during redox reaction even at high scan rate (100 mVs-1). During scanning, it is observed that, shifting of oxidation peak towards a positive potential and shifting of reduction peak towards negative potential because of the increased internal resistance of the RGO/LaAlO3 nanocomposite electrode [34]. Galvanostatic charge discharge studies (GCD): GCD curves for RGO/LaAlO3 nanocomposite electrode material between the potential window 0-1.2 V in 1M KOH aqueous electrolyte at various current densities are shown in Figure 8. The plateau region of GCD profile at lower current density (0.5 Ag-1) was well

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matches with the CV curves and confirmed EDLC/pseudocapacitive behaviour of the electrode material. That is the charging plateau region of GCD is nearly a rectangular shape

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of EDLC of RGO and linear trend during discharge cycle confirmed the pseudocapacitive

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nature of LaAlO3. Further, the initial voltage drop during discharge cycle is due to the inner resistance (IR drop), which can be reduced by finding an optimum ratio of RGO and LaAlO 3

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in the electrode. As the discharge time was found decreasing with increase in current density,

more resistive [35].

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which was justified due to the enhanced excitation carriers making the electrode materials

Figure 9 displays the comparison of GCD curves between RGO/LaAlO3

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nanocomposites, pure RGO and pure LaAlO3 electrodes. RGO/LaAlO3 nanocomposite material confirms the large discharge time than that by the individual RGO and LaAlO3

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electrodes at the same current densities, pre supposing relatively better electrical conductivity of LaAlO3 resulted by the incorporation of RGO under layer. Based on the charge/discharge curves, the specific capacitance of electrodes can be calculated using following equation and tabulated in the Table 1. Csp 

i .  V  W   t 

Where i is the current (A), (t ) is the discharge time (s), ( V ) is the potential window (V) and W is the active mass of the elctroactive material (g). Figure 10 exhibits the variation of specific capacitance as a function of various current densities for all electrode materials. The calculated specific capacitance (Csp) for the nanocomposite was found to be 283 Fg-1 at a current density of 0.5 Ag-1 and was able to retain Csp of about 157 Fg-1 even at very high current density of 20 Ag-1 with the retention

Journal Pre-proof capacity of 55.47% with its initial capacitance. Further, the sample electrode materials were tested for a large number of about 5000 charge/discharge cycles at constant current density of 20 Ag-1 and displayed in Figure 11. The RGO/LaAlO3 nanocomposite electrode material was able to retain nearly about 74% of its initial capacitance, whereas the individual pure RGO and pure LaAlO3 electrodes were able to retain only 65% and 70% with their initial capacitance respectively. The energy density (ED) and power density (PD) of the electrodes were calculated by the following equations.

E t

-p

P

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1 CV 2 2

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E

Where E is the energy density (Wh Kg-1), C is the specific capacitance (Fg-1) using CP, V is

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the potential window (V), P is the power density (W Kg-1) and t is the discharge time (s). Figure 12 exhibits the Ragone plot for all the electrode materials and Table 2 shows the

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Energy density and power densities of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes. It is obvious that the ED of RGO/LaAlO3 nanocomposites electrode can

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reach very high ED of 57 Wh kg-1 at a PD of 569 W kg-1 and ED of 31 Wh kg-1 at a PD of 22.61 kWh kg-1. Notably, these ED and PD values are very much superior to that of pure

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RGO and pure LaAlO3 electrodes. The superior supercapacitance performance of RGO/LaAlO3 electrode could bedue to their inherent oxygen vacancies. These oxygen vacancies of the perovskite materials with substantial heterogeneous microstructures on RGO sheets provide high diffusivity pathways along crystal domain boundaries and possess high surface area as well as mesoporosity, the transportation of electrolytes through their nanochannels is possibly more feasible for efficient redox reactions during faradaic charge storage process. Since oxygen vacancies are the charge carriers, the high oxygen vacancy mobility is the key factor of the high specific capacitance [36]. Electrochemical impedance spectroscopy studies: The electrochemical performance of RGO/LaAlO3 nanocomposites is further evaluated using electrochemical impedance spectroscope (EIS) at 0.3V with frequency ranging from 0.01 Hz to1 MHz. From the Figure 13, the Nyquist plots show a straight line, which is parallel to the imaginary axis indicating an ideal capacitive behaviour of RGO/LaAlO3 nanocomposite when compared to LaAlO3 and RGO electrodes. Moreover, the

Journal Pre-proof resistance of the RGO/LaAlO3 nanocomposite electrode material is very close to the intercept of the Nyquist curve on the real axis which demonstrates the low internal resistivity of the electrode and good conductivity of the electrolyte [37]. The experimental data for the sample were fitted using Zsimpwin 3.21 software is shown in Figure 13 (Inset) and the equivalent circuit consisting of double layer capacitance (Cdl), solution resistance (Rs) and charge transfer resistance (Rct) represented for RGO/LaAlO3 nanocomposite. The specific capacitance of electrodes are estimated using the equation mentioned below

Csp 

1 2fZ "

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Where f is the frequency (Hz) and Z” is the imaginary part of impedance. Figure 14 shows

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the specific capacitance values of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes obtained from impedance data. The specific capacitance of RGO/LaAlO3 electrode

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material reached as high as 272 Fg-1, which is very much superior to that of individual pure electrode materials. Figure 15 shows the Bode plots of all three electrode materials. The

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phase angle is close to -90o for frequency up to 0.01Hz, suggesting that an ideal capacitor is

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approached for the nanocomposite electrode material. The calculated relaxation time constant based on the frequency (0.297Hz) that possesses a phase angle of -45o to a point where the capacitive and resistive impedances are equal.

(ASD):

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Electrochemical performance of the assembled asymmetric supercapacitor device

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To conduct an in-depth investigation into the electrochemical performance of RGO/LaAlO3 nanocomposite an asymmetric supercapacitor is assembled and tested by the two-electrode system in 1.0 M KOH electrolyte. Figure 16 shows the CP discharge curves of the fabricated ASD at various current densities ranging from 2.5 to 50 Ag-1. Even if there is a significant IR drop, the device exhibited good capacitive response even at high current densities up to 50 Ag-1. Specific capacitance values of ASD was calculated using the same equation used in three electrode system earlier. The calculated Csp were found to be 42, 30, 26, 23, 20, 19, 19, 18, 17, 15, 15 and 14 Fg-1 at current densities of 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 25, 30, 40 and 50 Ag-1. The ED (in Wh kg-1) and PD (in W kg-1) of the ASD was calculated from CP studies using the equations used earlier three electrode system. The Ragone plot for ASD comprising of ED and PD values obtained from CP measurements are shown in figure 17. The maximum ED of the device was found to be 15 Wh kg-1. The device also exhibited a high PD of 9756 W kg-1 with a corresponding ED of 5 Wh kg-1. A significant

Journal Pre-proof performance of the AS device was observed during the cycle test where the device was cycled for 3000 continuous galvanostatic charge-discharge cycles at a high current density of 10 A g-1. It showed a very good retention capacity of 85% even at the 3000th cycle (Figure 18). The specific capacitance of RGO/LaAlO3 nanocomposite has been compared over many of the reported perovskite related material and is given in Table 3. LaMnO3 has been synthesized by solgel method and the powder was calcined at 700 oC and using cyclic voltammetry achieved the maximum specific capacitance of 73 Fg-1 at a current density of 0.5 Ag-1 using KOH as electrolyte and arrived at a trend of lower in the specific capacitance

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at high current density and obtained a specific capacitance of 40 Fg-1 at a high current density

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of 3 Ag-1 [10]. In another report a simple template-free solvothermal method has been used to synthesize perovskite LaNiO3 with hollow spherical structure and a specific capacitance of −1

has been achieved in alkaline medium at a current density of 1 Ag

−1

and the

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material exhibited a good cyclic stability [36].

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422 Fg

Four different kinds of perovskite materials LaMnO3, LaFeO3, LaCrO3, and LaNiO3 were

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prepared and investigated as anode materials for supercapacitor in LiOH medium and achieved maximum capacitance of 106.58 Fg-1 at a current density of 1 Ag-1 [37]. Kakwand et

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al, synthesized NiMnO3 nanoparticles through co-precipitation and blended with graphite and RGO. And managed to achieve the specific capacitance of 285 F g-1, 237 F g-1 and 70 F

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g-1 in potassium hydroxide medium for NiMnO3/RGO, NiMnO3/Gr and NiMnO3, respectively [38].

Electrochemical performance of LaMnO3 for has been blended with

different amount of nitrogen-doped reduced graphene oxide (NrGO) and evaluated for supercapacitor applications [39], and reported that lower amount of N-rGO (25%) exhibited the highest specific capacitance (687 Fg−1 at a scan rate of 5 mVs−1). Nitrogen-doped graphene (NDG) and Mn3O4/Fe3O4 ternary composite has been synthesized and evaluated for supercapacitor applications and obtained highest specific capacitance (158.46 Fg−1) compared to NDG/Fe3O4 (130.41 Fg−1), NDG/Mn3O4 (147.55 Fg−1), and NDG (74.35 Fg−1) in 1 M Na2SO4 medium at a scan rate of 50 mVs-1 [40]. When compared to the above reported methods, RGO/LaAlO3 composite electrode material has shown enhanced electrochemical performance in three electrode system as well as in ASDs system. The highest specific capacitance obtained are 721, 531 and 105 Fg-1 at a scan rate of 2 mVs-1 for RGO/LaAlO3, RGO and LaAlO3 electrode, respectively. Alongside good performance was also observed when fabricated for asymmetric supercapacitor device system.

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4. Conclusions In the present paper authors used simple, green approach fo0r the synthesis of RGO/LaAlO3 composite material. The RGO/LaAlO3 composite material has shown enhanced specific capacitance when compared to RGO and LaAlO3 nanoparticles. The specific capacitance of RGO/LaAlO3 composite electrode was high (721 Fg-1 at a scan rate of 2 mVs-1), which is very high compared to many of the previous methods available in the literature. The superior supercapacitance performance of RGO/LaAlO3 electrode was achieved due to the presence of inherent oxygen vacancies of the perovskite materials with considerable heterogeneous

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microstructures on RGO sheets. Thus, RGO/LaAlO3 composite material could serve as a

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potential perovskite composite to serve as a choice of material in energy storage devices.

Acknowledgement

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Authors are immensely elated and wish to express their indebtedness gratitude to the

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Management of New Horizon College of Engineering, MSRIT and Jain University for

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providing lab facilities to carry out this work.

Journal Pre-proof Figure captions Figure 1: The X-ray diffraction patterns of LaAlO3 and RGO/LaAlO3 nanocomposites. Figure 2: SEM micrographs of RGO/LaAlO3 nanocomposites. Figure 3: EDS morphology of a) LaAlO3 and b) RGO/LaAlO3 nanocomposites. Figure 4: BET of RGO/LaAlO3 nanocomposites. Figure 5: CV curves at different scan rates for RGO/LaAlO3 nanocomposites electrode.

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Figure 6: CV curves of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrode materials at scan rates of 50 mVs-1.

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Figure 7: Specific capacitance of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes as a function of scan rates.

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Figure 8: Charge/discharge curves of RGO/LaAlO3 nanocomposites at different current density.

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Figure 9: Comparison of GCD curves of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes at constant current density of 5 mA.

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Figure 10: Variation of specific capacitance as a function of various current densities for RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes. Figure 11: Plot of capacitance retention of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes at a high current density of 20 mAg-1 for 5000 cycles. Figure 12: Ragone plot of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes. Figure 13: The Nyquist plots of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes. Inset: Equivalent circuit. Figure 14: Specific capacitance values of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes obtained from impedance data. Figure 15: Bode plots of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes. Figure 16: Discharge curves of fabricated ASD using RGO/LaAlO3 nanocomposites at different current density. Figure 17: Ragone plot of fabricated ASD using RGO/LaAlO3 nanocomposite. Figure 18: Plot of capacitance retention of of fabricated ASD using RGO/LaAlO3 nanocomposite. Scheme 1: Synthesis of LaAlO3-RGO composite

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Journal Pre-proof Table 1. Specific capacitance of RGO/ LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes using CV at different scan rates and CP at different current densities.

Sl. no

Scan rate (mVs-1)

Specific capacitance (CSp) RGO/LaAlO3 nanocomposites

Pure RGO

Pure LaAlO3

Current density (Ag-1) 0.2

Pure RGO

Pure LaAlO3

-

203

43

283

178

39

1.0

240

145

34

2.5

204

123

31

5.0

197

90

31

72.5

10

181

72

31

70.5

15

166

-

-

69.4

20

157

-

-

1

2

721

535

105

2

5

612

509

91

3

10

557

456

83

4

20

517

391

78

5

40

482

354

6

50

470

7

80

449

8

100

437

l a

n r u 332 304

o J

257

Specific capacitance (CSp)

74

r P

f o

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p e 0.5

RGO/LaAlO3 nanocomposites

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Table 2: Calculated ED and PD values of RGO/LaAlO3 nanocomposites, pure RGO and pure LaAlO3 electrodes using CP data.

Sl. No.

Current density (Ag-1)

RGO/ LaAlO3 nanocomposites ED

Pure RGO

PD -1

ED

-1

PD -1

(Wkg )

-1

ED

PD -1

(Whkg )

(Wkg )

(Whkg )

(Wkg-1)

41

226

8.6

228

7.8

562

1135

6.8

1139

25

2857

6.2

2790

18

5635

6.2

5580

0.2

2

0.5

57

569

36

3

1

48

1133

29

4

2.5

41

2825

5

5

39

5674

6

10

36

11332

14

11520

6.2

11160

7

15

33

17074

-

-

-

-

8

20

31

22608

-

-

-

-

566

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1

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(Whkg )

Pure LaAlO3

Journal Pre-proof Table 3 : Comparison of the specific capacitance values of the RGO/LaAlO3 composite over reported methods. Sl. No

Material

electrolyte

Specific capacitance

Reference

2

LaMnO3

1 M KOH

73 Fg-1 @ 0.5Ag-1

[12]

3

La 1-XSrXMnO3

1 M KOH

102 Fg-1 @ 1 Ag-1

[35]

4

LaNiO3

6 M KOH

422 Fg-1 @ 1 Ag-1

[36]

5

LaMnO3

LiOH

56.78 Fg-1

6

LaFeO3

16.43 Fg

LaCro3

24.40 Fg-1

LaNiO3

106.58 Fg-1

NiMnO3/RGO

6 M KOH

l a

NiMnO3/graphite NiMnO3 7

LaMnO3-N-RGO

8

N-Graphene/Mn3O4

u o

J

N -Graphene/ Fe3O4

rn

10

LaMnO3/RGO/PANI RGO/LaAlO3 composite

e

r P

285 Fg-1 @ 1 Ag-1

[37]

[38]

237 Fg-1 @ 1 Ag-1

1 M KOH

687 Fg-1 @ 5 mVS-1

[39]

1 M Na2SO4

130.41 Fg-1 @ 50 mVS-1

[40]

147.55 Fg-1 @ 50 mVS-1 158 Fg-1 @ 50 mVS-1

N-Graphene/Mn3O4/Fe3O4 9

f o

o r p

-1

3 M KOH 1 M KOH

111 Fg-1 @ 2.5 Ag-1 -1

-1

721 Fg @ 0.2 Ag CD and @ 2 mVS-1 scan rate by CV 283 Fg-1 @ CD 0.5 Ag-1 by CP

[41] Present Work

Journal Pre-proof Author Contribution Vinuth Raj T N mainly worked on synthesis of materials in the laboratory Priya A Hoskeri Planned for the material and wrote some portions manuscript Muralidhara H B Has taken care of characterization of the materials Manjunath C R Material synthesis and manuscript preparation

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Yogesh Kumar K taken care of electrochemical measurements, ASD devise fabrication

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preparation and revision of manuscript.

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M S Raghu being a corresponding author guided for material preparation scheme, manuscript

Journal Pre-proof Declaration of interests

☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights  Hydrothermal synthesis of green reduced graphene oxide-LAAlO3 composite.  High specific capacitance over many reported methods  Eco friendly reduction of graphene oxide  High energy density supercapacitors

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