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Sagar Mothkuri MnO2 paper

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Sagar Mothkuri MnO2 paper
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  Synthesis of MnO 2  nano-flakes for high performance supercapacitorapplication Sagar Mothkuri, S. Chakrabarti ⇑ , Honey Gupta, Balaji Padya, T.N. Rao, P.K. Jain Center for Carbon Materials, International Advanced Research Centre for Powder Metallurgy and New Materials, Balapur P.O., Hyderabad 500001, India a r t i c l e i n f o  Article history: Received 14 March 2019Received in revised form 29 March 2019Accepted 31 March 2019Available online xxxx Keywords: MnO 2  nanoflakesHydrothermal techniqueSupercapacitorPerformance parametersPseudo capacitance a b s t r a c t Among various energy storage devices, pseudo capacitive supercapacitors have gained much attentiondue to its unique features of high-power density and fair enough energy density. This work is aimedto fabricate high power density and energy density supercapacitor by using MnO 2  nano-flakes as elec-trodematerial.MnO 2 nano-flakeshavebeensynthesizedusinghydrothermaltechniqueandincorporatedin supercapacitor structure to study the performance. The electrochemical performance of MnO 2  nano-flakeelectrode was investigated andobtained aspecificcapacitance of 145F.g  1 at 5mV.s  1 withenergydensity of 20.16Wh.kg  1 and power density of 363.03W.kg  1 .   2019 Elsevier Ltd.Peer-review under responsibility of the scientific committee of the International Conference on NanoScience & Engineering Application (ICONSEA-2018) Centre for Nano Science and Technology, ICONSEA-2018. 1. Introduction The concern towards the deteriorating climatic conditionsaround the world has put forward the thought for replacement of existing fossil fuel-based energy sources with alternative energysources. The conventional form of energy resources is depletingfast, hence demands have generated towards the development of alternative energy sources. This has brought forth the need for anextensive research on renewable energy sources like solar (sun),wind (turbine), hydel (water), tidal (waves), biomass (biowaste)etc.Duetosomeseasonalavailabilityissueoftheseenergysources,storing of such energy became vital.Energystoragedevicesareessentialtotapandstoretherenew-able source of energy. For the development of energy storagedevices, mostly carbon related materials such as activated carbon,carbon nanotubes, graphene, and other materials are used widelyowing to their excellent chemical stability, thermal stability, andenvironmental friendliness [1–4]. Commercially used activatedcarbon has the limitation in terms of meeting high energy densityand high conductivity. Hence, the research for high energy densitymaterials such as metal oxides, nitrides, sulfides, and other con-ducting polymer are widely taken up to solve the issues of energydensity.Althoughthesepseudo-materialsareknownfortheirpoorconductivity, it is overlooked for their other features such as highsurface area and multiple oxidation states during electrochemicalreactions. RuO x  is the most explored material in the beginningfor its high energy density [5]. But owing to the fact that it isexpensive, other pseudo-materials have also been explored [6–8].Manganese oxide (MnO 2 ) is promising among the many pseudo-materials with theoretically higher specific capacitance (1370F.g  1 ) [9]. It is superior for its tunnel like crystal structure whichcan be filled with cations such as K + , Li + , Na + etc. [6,7]. This tunnellike structure of MnO 2  results in the low density of the material,high permeation, and complete utilization of the material, thusleading to the utilization of the higher surface area [10,11]. Otherkey benefits of MnO 2  include its fast ion and electron transport,low cost, high electrochemical activity/stability in alkali/neutralmedia, environmentally benign, and abundant in nature [12–14].The carbon based electrode materials used for supercapacitorapplications do not have high specific capacitance values. Forexample, high surface area carbon materials such as activated car-bon (50–125F.g  1 ), carbon aerogel (5–80F.g  1 ), carbon nanofiber(50–100F.g  1 ) and carbon nanotube (12–120F.g  1 ) [15] still fallbehind MnO 2  in terms of specific capacitance. Among the mostcommonly used pseudo materials, MnO 2  leads the race owing toits unique tunnel-like crystal structure that can host cations of the electrolyte in its layered structure. Owing to the pseudo-capacitive nature of MnO 2  it has high energy density besides high https://doi.org/10.1016/j.matpr.2019.03.2362214-7853/   2019 Elsevier Ltd.Peer-review under responsibility of the scientific committee of the International Conference on Nano Science & Engineering Application (ICONSEA-2018) Centre for NanoScience and Technology, ICONSEA-2018. ⇑ Corresponding author. E-mail address:  supriya.c@arci.res.in (S. Chakrabarti).Materials Today: Proceedings xxx (xxxx) xxx Contents lists available at ScienceDirect Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr Pleasecitethisarticleas:S.Mothkuri,S.Chakrabarti,H.Guptaetal.,SynthesisofMnO 2 nano-flakesforhighperformancesupercapacitorapplication,Mate-rials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.236  specific capacitance. Nanostructured MnO 2  has become the mostsuitable electrode material for supercapacitor due to its enlargedworking potential window in natural aqueous electrolytes. Otheradvantages of MnO 2  electrode material are lowcost, natural abun-dance, large theoretical capacity, and low toxicity. To increase thespecific capacitance of MnO 2  based electrodes, researchers haveexplored nanostructured MnO 2  based electrodes with porousstructures, large pore volume and high specific surface areathrough novel morphologies like nanospheres, nanowires/-nanorods, thin films, and nanotubes. The studies have indicatedthat MnO 2 -based nanomaterials with controllable particle size,morphology, crystallinity, highspecific surfaceareaand goodelec-tricalconductivityarecriticalforimprovingcapacitance,ratecapa-bility and cycling stability. Another effective approach to improvethe performance is the synthesis of MnO 2 -based composites byusing highly conductive materials.Among several routes of synthesis, hydrothermal technique isselected because of its non-complex nature and possibility of large-scale production at lower cost. This technique is also advan-tageous in terms of no solvent, simple-setup, and lowtemperaturesynthesis [16,17]. In this work, the optimized hydrothermal syn-thesis of MnO 2  with flake like morphology and the fabrication of super capacitor device is reported. The detailed crystal structural,morphologicalandcompositional analysis of synthesizedflakelikeMnO 2  has been studied and complete electrochemical study hasbeen done to understand its performance as supercapacitor. 2. Experimental section All the chemicals used in this work are of analytical grade andused without further purification. MnO 2  was prepared viahydrothermal technique. KMnO 4  was dissolved in deionized waterand stirred vigorously until a homogeneous solution was formed.During stirring, a small amount (0.5ml) of conc. H 2 SO 4  was addedand continued stirring for another 15min. The KMnO 4  solutionwas then transferred into a stainless-steel teflon lined autoclaveand heat treated at 160  C for 4h. Finally, a light brownish colourprecipitate was collected, washed, filtered and dried at 90  C for16h to obtain MnO 2  with flake like morphology. 3. Characterization techniques Thecharacterizationtoolsandtheirparametersutilizedforana-lyzing MnO 2  are discussed below.The crystal structure of MnO 2  was analyzed using D8 Advance,Bruker X-ray powder diffraction analysis. The crystal structurestudies was done using Cu K a 1  radiation of wavelength0.15406nm.ThemorphologyofMnO 2 wasobservedusingZEISSGeminiSEM500 apparatus with an EDAX attachment which was used for theelemental analysis. The sample was observed both at lower andhigher magnifications as discussed in the Section 4. The samplewas submitted in the form of fine powder and was analyzed at5kV voltage.The electrochemical analysis was done using BioLogic ScienceInstruments. Inthis,threemaintechniqueswereusedfortheanal-ysis vis-a-vis: cyclic voltammetry (CV), galvanostatic charge dis-charge (GCD), and electrochemical impedance spectroscopy (EIS).Forthisanalysis,theMnO 2 powderalongwithtwoothermaterials,carbonblack(conductingcarbon) and PVDF(binder) in the ratioof 8:1:1 was ground into a fine powder with few drops of N-Methyl-2-pyrollidone (NMP) in order to make slurry. The slurry was latercoated on to a 1cm diameter circular graphite foil and then vac-uum dried at 90  C for overnight. The electrodes were arrangedinto a two-electrode system using a Swagelok cell as shown inthe graphical abstract. The cell was prepared by sandwiching theWhatman separator wetted with 3M KOH electrolyte in betweenthe two electrodes. Finally, the analytical formulae employed inevaluating key performance parameters are given below [5,15]. From cyclic voltammetry Specific capacitance: C s  ¼ R   IdvV s  D V  m units  :  F : g  1 ð 1 Þ where R   IdvistheareaundertheCVcurve, V s  isthescanrate,  D Visthe potential window, and m is the total mass of active material inthe electrochemical cell. Energy density: E d  ¼ 12  C s  D V 2  10003600 units  :  Wh : kg  1 ð 2 Þ whereC s  isthespecificcapacitanceobtainedfromEq.(1),and D Visthe potential applied. Power density: P d  ¼ 12  C s  D V  V s  units  :  W : kg  1 ð 3 Þ where C s  is the specific capacitanceobtained fromEq. (1),  D Vis thepotential window, and V s  is the scan rate. From Galvanostatic charge discharge Specific capacitance: C s  ¼ 4  I  D t D V  m units  :  F : g  1 ð 4 Þ where I is the cathodic current applied,  D t is the discharging time, D V is the discharging voltage, and m is the mass of the total activematerial in the electrochemical cell.The energy density is calculated with a similar equation asgiven in Eq. (2), except that C s  is the value obtained from Eq. (4). Power density: P d  ¼ E d  3600 D t units  :  W : kg  1 ð 5 Þ where E d  is the energy density obtained from the above discussionand  D t is the discharging time. 4. Result and discussion 4.1. X-ray diffraction The crystal structure was determined from X-ray diffractionanditwasfoundthatMnO 2  wascrystallineinnatureandbelongedto monoclinic (birnessite) crystal system with diffraction peaks at12.4   (001), 25   (002), 36.6   (201)/(110), and 65.8   (310)/(020) as shown in the Fig. 1. The diffraction data matched with JCPDS 80–1098 according to which the crystal structure consistsof layers with K + cations and water molecules in the interlayerspacings.Thisisinagreementwithotheralreadypublishedreports[13]. 4.2. Morphology The morphology of the sample was studied using FESEM. Fromthe Fig. 2(a) and 2(b), it can be seen that the MnO 2  has severalflakes in its structure which is different from the established mor-phologies of MnO 2  like nanorods, urchin-like, spherical etc. Thethicknessofflakesisfoundtobebetween25and35nm,estimatedusing ImageJ software. This flake like morphology is advantageousas it allows easy interaction of the maximum surface area of thematerial with electrolyte, thus enhancing the capacitive perfor-mance of the material. The elemental analysis was performed 2  S. Mothkuri et al./Materials Today: Proceedings xxx (xxxx) xxx Pleasecitethisarticleas:S.Mothkuri,S.Chakrabarti,H.Guptaetal.,SynthesisofMnO 2 nano-flakesforhighperformancesupercapacitorapplication,Mate-rials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.236  using EDAX and shown in Fig. 2(c) which indicates few traces of potassium confirming the birnessite like nature besides followingthe stoichiometric ratio of MnO 2  as shown in the atomic percent-age in Fig. 2(d). 4.3. Electrochemical analysis Theelectrochemicalanalysisof thematerialwasdoneinatwo-electrode system. Three main techniques were used for the analy-sis namely: cyclic voltammetry, galvanostatic charge discharge,and electrochemical impedance spectroscopy. Fig. 3(a) and 3(b) show the results based on cyclic voltammetry. The analysis wasdone using a potential window of 1V with different scan rates. Itwas observed that MnO 2  had undergone redox reaction showingredox peaks at 0.42V and 0.30V in the forward scan and reversescan respectively in Fig. 3(a). The loop under the CV-curveincreasedwiththescanrate.Withincreasingscanrate,thecurrentdrawn increases with the voltage thus resulting in an increase inthe area of the loop. But the specific capacitance decreases withthe increase of scan rate because the interaction between elec-trolyte and electrode is time-limited; hence the utilization of thesurfaceareaofthematerialisalsolimitedorreducedwithincreas-ing scan rate. This results in the decrease of capacitance value asthe capacitance is directly related with the active surface area of the material. A specific capacitance of 145.21F.g  1 was obtainedat a scan rate of 5mV.s  1 and the value decreased with increasingscan rate. This inverse relationship between scan rate and specificcapacitance is shown in the Fig. 3(b).A similar observation was made from the galvanostatic chargedischarge analysis. The analysis was done at different current den-sities based on the mass of the active material used in making of the electrodes of the cell. As the current density increased, thecharge-discharge time decreased owing to the fact that at highercurrent density the up-rise of the output voltage happened muchfaster, which resulted in the faster charging-discharging times asshown in the Fig. 3(c). A specific capacitance of 118.45F.g  1 wasfound at a current density of 0.25 A.g  1 . Fig. 3(d) shows the rela-tionship between specific capacitance and current density whichfollowed a similar decreasing trend as seen in cyclic voltammetry.Inthiselectrochemicalanalysis,theinteractionbetweenMnO 2  andaqueous electrolyte (in this case KOH) can be described as givenbelow: MnO 2 þ K þ þ e  $ MnOOK Fig. 1.  XRD pattern of MnO 2 .   (d) Element Atomic (%)O k   62.56K  k 3.63Mn k  33.81 Fig. 2.  FESEM micrographs of MnO 2  at (a) lower magnification and (b) higher magnification (c) EDAX of MnO 2  and its corresponding (d) element percentage. S. Mothkuri et al./Materials Today: Proceedings xxx (xxxx) xxx  3 Pleasecitethisarticleas:S.Mothkuri,S.Chakrabarti,H.Guptaetal.,SynthesisofMnO 2 nano-flakesforhighperformancesupercapacitorapplication,Mate-rials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.236  The charge storage in the material happened because of eitherby the surface limited non-faradaic reaction (i.e. adsorption) orby the intercalation of ions of the electrolyte into the interlayerspacing of the material [17].All the performance parameters calculated from both cyclicvoltammetry and galvanostatic charge discharge are summarizedin the Table 1.Furtheranalysisofthematerialwasdoneusingelectrochemicalimpedance spectroscopy. The analysis was done using a sinusoidalvoltage of V rms  of 5mV in the frequency range of 10kHz to 0.1Hz.Fig. 4presents theNyquist plot of MnO 2 . This plot givesthe detailsof internal resistance and charge transfer resistance developed intheelectrochemical cell. It isseenfromtheinsetof  Fig. 4, at higherfrequencies, the curve makes an intercept with the real axis. Theintercept gives equivalent series resistance (ESR) resulting fromthe contribution of electrolyte resistance, electrode (material)ingredients, and diffusion resistance which was found to be0.535 O  and 1.17 O  before and after 1000 cycles. It was alsoobserved that the impedance curve had developed charge transferresistance after running 1000 number of cycles termed as CTR which was evaluated from the radius of the semicircle developedat higher frequencies and was found to be 3.32 O .Fig. 5(a) and 5(b) show key performance parameters of this supercapacitor. Fig. 5(a) is a plot between energy density andpower density of MnO 2 , called Ragone plot. The supercapacitorshowed an energy density of 20.16 Wh.kg  1 and correspondingpower density of 363.03W.kg  1 at a scan rate of 5mV.s  1 . Theenergy density decreases with scan rate (or current density)because it is directly proportional to the specific capacitance. Andpower density increases with scan rate as it offers less resistance Fig. 3.  Electrochemical analysis of MnO 2  (a) cyclic voltammetry at different scan rates (b) plot of specific capacitance against scan rate from cyclic voltammetry (c)galvanostatic charge discharge at different current densities and (d) plot of specific capacitance against current density from galvanostatic charge discharge.  Table 1 Performance parameters from cyclic voltammetry and galvanostatic charge discharge. Cyclic voltammetry Galvanostatic charge dischargeScan rate(mV.s  1 )Specific capacitance(F.g  1 )Energy density(Wh.kg  1 )Power density(W.kg  1 )Current density(A.g  1 )Specific capacitance(F.g  1 )Energy density(Wh.kg  1 )Power density(W.kg  1 )5 145.21 20.16 363.03 0.25 118.45 16.45 25010 102.0 13.86 499.03 0.5 95.42 13.25 50020 86.27 11.72 844.18 1 78.46 10.89 100050 43.59 5.92 1066.27 2 62.88 8.73 2000100 48.51 4.66 1679.52 5 38.3 5.31 5000200 34.33 3.71 2675.86 10 23.6 3.27 10,0004  S. Mothkuri et al./Materials Today: Proceedings xxx (xxxx) xxx Pleasecitethisarticleas:S.Mothkuri,S.Chakrabarti,H.Guptaetal.,SynthesisofMnO 2 nano-flakesforhighperformancesupercapacitorapplication,Mate-rials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.236  at higher scan rates as the interaction between electrolyte andelectrodeisverylimited.Thecellwastestedbyrunning1000num-ber of charge-discharge cycles and the performance of the electro-chemical cell is shown in the Fig. 5(b) which gives the details of capacitance retention for 1000 cycles. In the insets of  Fig. 5(b),the first 10 cycles and intermediate cycles i.e. 550–560 are shown.It was observed that the cell had retained a capacitance of 55.52%fromits initial value. The poor capacitance retention can be attrib-utedtothepoorconductivityofMnO 2 .Fromtheinsetsitisevidentthat the charge–discharge profiles are retained even after 1000number of cycles. 5. Conclusion MnO 2  nanoflakes with birnessite monoclinic microstructurewas synthesized using hydrothermal techniques. It exhibitedexcellent specific capacitance value of 145.21F.g  1 at a scan rateof 5mV.s  1 from cyclic voltammetry with corresponding energydensity of 20.16Wh.kg  1 and power density of 363.03W.kg  1 .The ESR value was evaluated from electrochemical impedanceanalysis and was found to be 1.17 O  after running 1000 number Fig. 4.  Nyquist plot of MnO 2  before and after cycles; the inset shows the plot athigher frequencies. Fig. 5.  (a) Ragone plot (Energy density vs Power density) (b) Cycle performance (inset shows the cycles of first 10 cycles and 550–560 cycles). S. Mothkuri et al./Materials Today: Proceedings xxx (xxxx) xxx  5 Pleasecitethisarticleas:S.Mothkuri,S.Chakrabarti,H.Guptaetal.,SynthesisofMnO 2 nano-flakesforhighperformancesupercapacitorapplication,Mate-rials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.236

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