- Email: [email protected]
Directly grown Sr–Co layered double hydroxide (LDH) entangled two dimensional nanosheet film with superior performances
Journal Pre-proof Directly grown Sr–Co layered double hydroxide (LDH) entangled two dimensional nanosheet film with superior performances Deepa B. Bailmare, Kavita A. Deshmukh, P. Sivaraman, D.R. Peshwe, Bipin Kumar Gupta, S.J. Dhoble, Abhay D. Deshmukh PII:
S0013-4686(19)31934-6
DOI:
https://doi.org/10.1016/j.electacta.2019.135063
Reference:
EA 135063
To appear in:
Electrochimica Acta
Received Date: 3 April 2019 Revised Date:
20 September 2019
Accepted Date: 10 October 2019
Please cite this article as: D.B. Bailmare, K.A. Deshmukh, P. Sivaraman, D.R. Peshwe, B.K. Gupta, S.J. Dhoble, A.D. Deshmukh, Directly grown Sr–Co layered double hydroxide (LDH) entangled two dimensional nanosheet film with superior performances, Electrochimica Acta (2019), doi: https:// doi.org/10.1016/j.electacta.2019.135063. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Directly grown Sr-Co Layered double hydroxide (LDH) entangled two dimensional nanosheet film with superior performances Deepa B. Bailmare1, Kavita A. Deshmukh2, P. Sivaraman3, D. R. Peshwe2, Bipin Kumar Gupta4, S. J. Dhoble5, Abhay D. Deshmukh1* *1
Energy Materials and Devices Laboratory, Dept. of Physics, RTM Nagpur University,
Nagpur-440033, India 2
3
Dept. of MME, Visvesvaraya National Institute of Technology, Nagpur-440022, India Polymer Science and Technology Centre, Naval Materials Research Laboratory (DRDO),
Ambernath (E) 421506, India 4
5
CSIR -National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi, 110012, India Dept. of Physics, Nanomaterials Research Laboratory, RTM Nagpur University, Nagpur,
India KEYWORDS: Sr-Co LDHs, Layered double hydroxides, Supercapacitors, Two dimensional materials (2D), Energy storage.
1
ABSTRACT: Designing of electrode with electrochemically rich structure, outstanding mechanical robustness and high electrical conductivity remains a challenge. We report a design of entangled Sr-Co Layered double hydroxides (LDHs) two-dimensional structure for such electrode. The hierarchical electrode with entangled nanosheet exhibit high specific capacitance of 1415 Fg-1 at 5Ag-1 current density, excellent rate capability, high energy (12.28Whkg-1) and power density (567.788 Wkg-1) and outstanding stability (80% capacity retention over 5000 cycles). The asymmetric device cell of Sr-Co LDH/ACC gives high energy density (27.78 Whkg-1) and power density (499.96Wkg-1) with cyclic performance of 1000 cycles with 95.24% capacitance retention. Further, we provide a detailed analysis of the various electrolyte environment experienced by the electrode and their effect on electrochemical properties. We pick out three different electrolytes; LiOH, NaOH, KOH and
2
show that both selected electrolyte and metal combination for LDHs paves the way for outstanding electrochemical performance.
INTRODUCTION: Amongst two dimensional nanomaterials, the extraordinary electrochemical properties of 2D layered double hydroxides (LDHs) have ignited great interest for asymmetric supercapacitors devices. The layered double hydroxides-based materials are the multi metal clay materials that are made up of layers of metal ions surrounded by octahedral arrangement of hydroxyl groups. This class of materials rich in fast redox reaction, environment friendly nature and high electrochemical performance. In particular, great efforts have been directed towards developing various combinations of LDHs family of materials depending upon the chemistry of the compound and the atomic structure for improving the specific energy of supercapacitor. [1-3] The major focus has been given on the designing of nanostructured electrode materials or use of novel electrolytes and understating their electrochemistry to improve the total cell capacitance and widening of operating voltage. Obviously, the new LDHs materials and their combinations have to be investigated to step up the LDHs asymmetric supercapacitor performance. For example, designing of new LDHs combinations such as Zn-Co[3], CoMo[4], Ni-Co[5] Mn-Ni[6], Mn-Co[7], Co-Al [8], Ni-Fe[9], Co-Ni [10], Cu-Co[11], CoCr[12], Co-Mn [13] Sr(OH2)[14] as electrode materials has been studied in recent times. The prior work shows, mostly cobalt hydroxides have been studied extensively in case of LDHs, because of its high electrochemical activity. The cobalt content has great influence on the properties of LDH nanosheets and porous structure. Cobalt in its hydroxide form provide high conductivity with Co(OH)2 gets converted into CoOOH in electrochemical reaction and hence acts as a conductive wrapping for electro active materials which is beneficial for the fast
3
redox reaction in particular electrolyte. To improve the efficiency of electrode material it is necessary to have highly accessible open structure for making redox reaction and ion diffusion at the electrode and electrolyte interface, which exhibits high specific capacitance, high electrical conductivity and fast electron transfer reaction. Hence the effective combinations of metal oxides/hydroxides can improve the efficiency of electrode material. The presence of two different metal cations in the mixed metal hydroxides can show improvement in electrochemical performance of material due to the synergic effect of multiple metal ions. [15,16] In present work, we have chosen the combination of strontium and cobalt as the LDHs materials. Strontium is highly reactive element, therefore mostly studied in the combination with other elements to improve the properties of the bulk. It is mostly used in improving the electric and magnetic properties of ferrites and other various applications. Therefore, an attempt has been made to study the combining effect of strontium and cobalt on electrochemical properties. Due to the large number of applications of strontium and cobalt we have deposited strontium cobalt layered double hydroxide on SS mesh substrate using electrodeposition technique. Electrodeposition is easy, fast and reliable technique for depositing variable thickness and uniform deposition of electrode material on any substrate. The weight of sample coated after deposition is found to be 0.55mg/cm2 calculated by difference between weight of SS (mesh) before and after deposition which shows the excellent results with high specific capacitance of 1415Fg-1, 12.28Whkg-1energy density and 567.788 Wkg-1 power densities in 1M NaOH strong aqueous electrolyte at 5Ag-1 current density. With increasing the current density specific capacitance is also increases. By analyzing CV curve, it is clearly visible the pure pseudocapacitive behavior of prepared electrode material with
4
prominent oxidation and reduction peaks at higher and lowers potential sides respectively. This faradic nature of material shows higher specific capacitance than EDLCs. (Sr-Co LDH) shows high specific capacitance with 1M NaOH with good cycling stability of 5000 cycles with 80% capacitance retention. The two-electrode asymmetric device were demonstrated using LDHs as positive electrode and carbon cloth as negative electrode in 1M NaOH electrolyte. Specific capacitance for fabricated asymmetric device was found to be 138.88 Fg-1 at 0.2 Ag-1 of current density within the potential range of 0 to 1.2V. The two-electrode cell interestingly exhibits high energy density 27.78 Whkg-1 and power density 499.96 Wkg-1. EXPERIMENTAL SECTION: Synthesis of 3D entangled Sr-Co LDH: The electrode material were prepared by a simple one step process as follows: typically, 304 stainless steel (SS) mesh (10 mm x 20 mm x 0.1 mm, 400ppi) was successively cleaned in ultrasonic bath with acetone for 10 min and then simply rinsed with double distilled water to remove the impurities on the surface. The cleaned SS mesh was then immersed in electrochemical cell with a homogeneous solution of Sr(NO3)2.4H2O
(8mmol),
Co(NO3)2.6H2O
(16mmol),
followed
by
potentiostatic
electrodeposition for 300s at applied potential of -1.0V to grow Sr-Co LDH entangled nanosheet. All the electrodeposition processes were performed in conventional three electrode cell at room temperature comprising of the SS mesh and Ag/AgCl as working and reference electrode, and platinum foil (10 x10mm) as counter electrode respectively. The strontium and cobalt hydroxide monomers were converted to Sr-Co LDH through redox reactions. The entangled nanosheet covered substrate then washed with water and ethanol using ultrasonic cleaner to remove the surface ions and molecules, and further dried in vacuum oven at 80oC for overnight to remove solvent. The overnight heating provides a material to bind on substrate for
5
longer period of time and above 80oC drying may result in structural destruction. The mass of the entangled Sr-Co LDH nanosheet on SS mesh was determined by subtracting the weight before and after deposition. Characterization: The crystalline structure of prepared electrode material (Sr-Co LDH) was analyzed by PAN-analytical X-ray diffractometer with Cu Kα radiation with proportional detector. The morphologies were observed by Field emission scanning electron microscope (JEOL JSM 7610F FEGSEM). Electrochemical measurement: The electrochemical measurements of the entangled LDH electrodes were investigated using Metrohm AUTOLAB (128N Potentiostate) under a three-electrode cell configuration. The SS mesh supported entangled nanosheet were directly used as the working electrodes, which were soaked in 1M NaOH solution before the electrochemical test. Ag/AgCl and Platinum foil were used as reference and counter electrode, respectively. Asymmetric supercapacitor devices tested with entangled Sr-Co LDH nanosheets as positive electrode and activated carbon cloth (ACC) as negative electrode in 1M NaOH solution. The electrochemical properties of prepared electrode materials were investigated through cyclic voltammetry (CV), and Galvanostatic charge discharge process in the applied potential range -0.2 to 0.3V. The cyclic voltammetry was carried out at different scan rates from 5mV/s to 100mV/s. The electrochemical impedance spectroscopy (EIS) was performed at frequency range of 10-1Hz to 104 Hz with respect to open circuit potential (OCP) of 0.019 V. The Cs of Sr-Co LDH nanosheet was calculated from Galvanostatic charge discharge curve with the formula given as (6). I x ∆t
Cs = 2 (m x ∆V)
6
Where, Cs (Fg−1) is the specific capacitance. The Cs of asymmetric supercapacitor was calculated from Galvanostatic charge-discharge curves as follows:
Cs=
× × (×)
Where, ‘m’ is the total mass of the active material in the positive and negative electrodes. The energy and power densities of the Sr-Co LDH, ACC based electrodes, and the asymmetric supercapacitor was calculated as follows: E= 0.5×CV2, Pave= E/∆t, where, I is specific current, ∆t is the discharging time, m is the mass deposited, ∆V is the potential window, E(Whkg−1) is the energy density, V is the cell voltage excluding IR drop, Pave (W kg−1) is the average power density.
RESULT AND DISCUSSION: Structural and morphological studies: Figure1 (a) shows a schematic fabrication process for the well grown entangled Sr-Co LDH nanosheets directly grown on stainless steel mesh as a novel binder free electrode. The detailed synthesis process is given in experimental section. The entire surface of the stainless-steel mesh is uniformly covered with the Sr-Co LDHs after electrodeposition. In Figure 1(b), the magnified FESEM images validate the well distributed and vertically aligned entangled Sr-Co LDHs nanosheet with nano flakes porous morphology. After electrodeposition process, the SrCo LDHs nanosheet homogeneously anchored onto the SS surface forming the unique 3dimensional entangled structure (Figure 1(c)). The thickness of nanosheet is 20-25nm as shown in Figure 1(d). The increased thickness of the nanosheet is may be due to the amount of Cobalt added in Sr-Co LDH material. During electrodeposition process, simultaneously the instantaneous and progressive nucleation takes places which results in vertical growth of
7
nanosheet. As shown in (Supplementary Fig S1) where at the beginning of electric pulse nuclei are formed initially thereafter vertical growth occurs. The precursors having high pH value dominates the metal hydroxides formation hence our material was electrodeposited at higher pH value followed by the successive growth of the material over the current collector. The growth mechanism, morphology and the structure of the material in electrodeposition process is strictly depends upon the electrolyte concentration, pH value and other environmental conditions. The synthesized 3-dimensional porous nanosheet entangled structure provides higher electrochemical active sites results in higher specific capacitance and cycle stability of material [14]. Furthermore, (Figure 1(e)) shows the X-ray diffraction (XRD) pattern of the entangled Sr-Co LDHs material. The Sr-Co LDHs exhibit diffraction peaks for both Sr(OH)2 and Co(OH)2. [14,15] Excluding peaks originating from the SS mesh, all the noticeable peaks can be indexed to Sr(OH)2 (JCPDS No. 38-0715)18 and Co(OH)2 (JCPDS No. 42-1316), implying existence of Sr-Co LDHs structure. The Sr-Co LDH material shows less crystalline structure which further attributes to high porosity of the material. The XRD data shows the continuous peak broadening throughout the disordering process which indicates the non crystalline behavior of the material. Hence the material is related to highly disorder noncrystalline nature which lost their crystallinity due to the incorporation of highly disordered phases. Therefore, the material is trending amorphous nature and gives more active sites to the ions incorporation, which further gives high specific capacitance and cyclic stability in aqueous electrolytes.[16] The layered double hydroxides based material specially gives more porous and 3-dimensional structure and hence provides a path to improve the electrochemical performances of the electrode materials. The LDHs are nanostructured electrode materials which plays vital role in electrolytic ion transportation during charge discharge process. The
8
prevention of aggregation of LDHs nanosheets make full use of active sites and improve the electrochemical performances.
(a)
(b)
(e)
(c)
(d)
Figure 1: a) Schematic representation of the Sr-Co LDHs entangled nanosheets forming 3D interconnected network. b) Magnified FESEM image of SS stainless steel sheet. C) Entangled Sr-Co LDH nanosheet grown vertically on SS (Mesh) substrate. d) Thickness of the Sr-Co LDH nanosheet grown on SS (mesh) substrate.(e) XRD pattern of Sr-Co LDH nanosheet. To explain the electrochemical performance of electrodeposited Sr-Co LDH entangled nanosheet, we carried out three electrode electrochemical test with Ag/AgCl reference
9
electrode and platinum foil counter electrode in 1M NaOH solution. The typical CV curve (Figure 2(a)) of the obtained LDH film exhibits a pair of well-defined redox peaks within 0.2V to 0.3V at various scan rates of 5mV/s to 100mV/s. The curve show pseudocapacitive characteristics and superior reversible redox reaction of prepared electrode material. The CV curve changes accordingly with respect to scan rates. The changes in CV curve are due to the occurrence of redox intercalation between electrode and electrolyte interfaces.The cations may suffer multiple processes simultaneously during the charge storage process and hence gives the high specific capacitance. When the scan rate increases, area under the curve also increases which shows the high rate capability of the electrode material. Remarkably, the peak potential shifts only 120mV/s for 20 times increase of scan rates, suggests the Sr-Co LDH electrode possess low polarization. The high potential shift may cause due to the isolated barriers developed at the electrode and electrolyte with respect to increase in potential. Additionally, subsequent linear increase in peak current with respect to (scan rate)1/2 confirms the electrochemical redox process is diffusion controlled (Supplementary Figure S2). The comparative CV curve further indicates that Sr-Co LDH electrode possess a higher specific capacitance by utilizing the combinations of single layered Sr(OH)2and Co(OH)2 electrodes (Supplementary Figure S3 and table T1) which is estimated the synergetic effect of entangled Sr-Co double hydroxides nano sheets structure. In second part of our experimental studies, several known aqueous electrolytes were selected and considered for comparison, including 1M KOH, 1M LiOH and 1M NaOH. In order to compare the electrochemical characteristics of Sr-Co LDH in various electrolytes, their CVs are normalized to show specific current verses potential in Figure 2(b). All the CVs in Figure 2b are similar in nature with an increasing current at the upper or lower potential limits. The
10
Sr-Co LDH electrode works at less negative potential in 1M KOH than in 1M LiOH and 1M NaOH. From the selected electrolytes, NaOH is the only one that exhibits higher specific capacitance than others. The comparative tables [Supplementary Table T2] and detailed study of Sr-Co LDH electrode in various electrolytes are discussed in [Supplementary Fig S4]. Thus 1M NaOH aqueous electrolytes is more favorable for Sr-Co LDH electrode material as compare to LiOH and KOH.The rate capability of Sr-Co LDH electrode material was examined by recording galvanostatic charge discharge (GCD) process. Charge discharge curves at various current densities ranging from 5 Ag-1 to 50 Ag-1 within the applied potential window of -0.2 V to 0.3V verses (Ag/AgCl) (Figure2(c)). Sr-Co LDH was able to deliver a specific capacitance of 1415.63 Fg-1 at 5Ag-1 of current density which is much higher than the specific capacitance of Sr(OH)2 and Co(OH)2. Whereas, increase in current density by 10 folds 5 to 50Ag-1, the specific capacitance was found to be 982 Fg-1 which is 69% of initial capacitance. After cycling at higher current density of 50 Ag-1, the specific capacitance at the 5Ag-1 current density was retained which proves the Sr-Co LDH electrode has superior rate capability. The comparative specific capacitance verses current density for all the three aqueous electrolytes shown in Figure2(d), the Sr-Co LDH was tested in two electrolytes shows the specific capacitance of 1013.12 Fg-1 in 1M KOH and 843.47 Fg-1 in 1M LiOH electrolyte respectively, at 5Ag-1 of current density.
11
(a)
(c )
(b)
(d)
Fig2: (a) CV curve of Sr-Co double hydroxide electrode material at different scan rates with 1M NaOH electrolyte (b)Comparative CV curve of prepared electrode materials with different electrolytes (c) charge- discharge curve of Sr-Co double hydroxide at different current densities with 1M NaOH electrolyte (d) percent rate capability Sr-Co LDH electrode material at 1M NaOH electrolyte. The cyclic stability is another important parameter to study the cycle life of the electrode materials. In order to understand the cycling performance of Sr-Co LDH electrode, the charge discharge cycling was performed at current density of 22.5 Ag-1. After 5000 cycles, this entangled Sr-Co LDH nanosheet electrode possesses 80% (Figure 3(a)) capacitance retention, which is much higher than those of Sr(OH)2 and Co(OH)2 (Supplementary Figure S4). To analyze the effect of long cycling test on Sr-Co LDH nanosheet we compared cyclic
12
voltammetry (CV) of before and after cycling (Supplementary Fig S6). The change in CV curve clearly estimated the structural destruction of Sr-Co LDH nanosheet in 1M NaOH electrolyte. However, the Sr-Co LDH electrode exhibits an energy density of 12.28Whkg-1 and power density 567.788Wkg-1(Figure 3(b)) showed in comparative Ragone plot of Sr-Co LDH with Sr(OH)2 and Co(OH)2. The comparative performance of Co and Sr based electrodes reported in earlier literature is shown in (Supplementary Table T3). Further, Fig 3(c) shows the comparative Ragone plot of energy density and power density at different current densities with reported literature. The Sr-Co LDH nanosheet shows the good agreement with comparative results. EIS study were performed to evaluate resistive behavior of the electrode material (Supplementary Figure S7) which shows the comparative Nyquist plot of before and after cycles taken in the frequency range of 0.01 Hz to 10000 Hz with the open circuit potential of 0.019V. The charge transfer resistance Rct of the electrode material increases from 40.3Ω and 250.17Ω with (Rs 1.06Ω and 1.28 Ω), respectively, before and after 5000 cycles. This indicates that the major loss of capacitance is due to the structural destruction and increase in charge transfer resistance of the nanostructured electrode material. The comparative EIS measurement of Sr-Co-LDH, Co(OH)2 and Sr(OH)2 shown in (Supplementary Figure S3) . The Rs values of Sr-Co LDH, Sr(OH)2 and Co(OH)2 is ( 1.06 Ω, 3.09 Ω, and 4.38 Ω respectively). Rs value for Sr-Co LDH is less than Co(OH)2 and Sr(OH)2 which indicated better electronic conductivity of the material.
13
(a)
(b)
(c )
Fig3: (a) cyclic stability curve of Sr-Co double hydroxide electrode material with 1M NaOH. (b) Comparative energy density Whkg-1 vs power density Wkg-1 of various Sr-Co LDH with Sr(OH)2 and Co(OH)2. (c) comparative Ragone plot of Energy density and power density of prepared Sr-Co LDH with other materials. An excellent electrochemical performance in terms of energy density, cycle stability, rate capability and specific capacitance of Sr-Co LDH electrode mainly attributed to the following merits: i) well distributed Sr-Co LDH nanosheet on the SS mesh with entangled interconnected 3D porous architecture, enhanced porous structure, conducting behavior, enhanced electrolytic diffusion through the interconnected network which results in enhancement of rate capability. ii) The direct growth of active material on the current collector favors the easy access to electrolytes which provide fast electron transport, iii) The vertical growth of 2D nanosheet about 20-25 nm thickness reduce the dead mass of active electrode and provide more specific surface area for electrochemical reaction. iv)The entangled nanosheet 3D architecture reduces
14
the structural destructiveness during charging and discharging which was demonstrated by SrCo LDH electrode material.
(a)
(c)
(b)
(d)
Fig4 (a) CV curve of asymmetric two electrode system with 1M NaOH ((b) Charge discharge curve of asymmetric two electrode system (c)
comparative Ragone Plot
ofenergy density and power density with other reported literature (d) cycle stability plot of two electrode measurement.
15
Further, asymmetric supercapacitor was fabricated using Sr-Co-LDH as a cathode and activated carbon cloth (ACC) as an anode in 1M NaOH electrolyte solution. Before fabricating the ASC device, the electrode was dipped into the electrolyte solution for about four hours. In ASC device, the mass is the ratio of the electrode as obtained by the equation
=
.∆ .∆
Where, m+ and m- are the mass of positive and negative electrode respectively [23-27]. To evaluate the applications of Sr-Co LDH / ACC asymmetric device, we demonstrate the CV, GCD and EIS tests. Interestingly, the charge discharge curve shows the non-ideal triangular behavior due to the combinations of both non faradic and faradic reactions [2830](Fig4b). Moreover, the symmetric nature of GCD curve suggests the high reversibility and excellent columbic efficiency of electrode. The operating potential window of the ASC was optimized and it was found to be 0 to 1.2V. The capacitance of the cell is calculated by using an equation provided in experimental section. The maximum specific capacitance based on the total mass of the active material found to be 138.88 Fg-1 at current density of 0.2 Ag-1. The galvanostatic charge discharge curve and rate capability of asymmetric device at different current densities were also tested and given in Figure 4(b) and Supplementary Table T4 (55.6 Fg-1 of specific capacitance at 3Ag-1 current density). Furthermore, energy density and power density (Ragone plot) of the ASC device were calculated from the data shown in Figure 4(c). The energy density and power density (E & P) were calculated according to equation given in experimental section based on the weight of active material. The asymmetric supercapacitor device exhibits a maximum energy density of 27.78Whkg-1 at power density of 499.96 Wkg-1 and maximum power density 7200 Wkg-1 at energy density of 11.12 Whkg-1. The comparative table of supercapacitor that have been
16
reported earlier and the present asymmetric supercapacitor based on strontium is shown in Supplementary Table T6. In present study, we have also examined the areal capacitance, energy density and power density of the Sr-Co LDH/ACC asymmetric device (Supplementary Figure S8) which was found to be 666.66 mFcm-2.The total volume/areal capacitance of presented ASC device was calculated based on area of ACC and Sr-Co-LDH electrode shown in (Supplementary Table T5). Additionally, the ASC device also exhibits an excellent cycling stability of 95.24% over 1000 continuous charge discharge cycles at 3Ag-1 current density (Figure 4(d)). In order to understand the effect of cycling on the resistive behavior of ASC, EIS measurement was employed. Nyquist plot (supplementary Figure S9) clearly reveals no obvious change in Rs from 5.05 Ω and 9.10 Ω after continuous 1000 charge discharge cycles as compared to initial cycles which demonstrate the excellent stability of the asymmetric supercapacitor device.
CONCLUSION: Layered strontium cobalt double hydroxide entangled nanosheet has been grown on SS (mesh) substrate by using electrodeposition technique. The SEM image of fabricated Sr-Co double hydroxide clearly shows flakes morphology of layered nano porous structure. The unique structure of layered strontium cobalt double hydroxides exhibits high specific capacitance of 1415.63 Fg-1 at 5Ag-1 of current density with 1M NaOH strong aqueous electrolyte. The prepared electrode material runs for 5000 continuous charge discharge cycles at 22.5 Ag-1 of current density with 80% of capacitance retention. The Asymmetric devices made up of the combination of strontium cobalt double hydroxide Sr-Co LDH electrode material as one and carbon cloth as another electrode material. The two electrode system was
17
performed with 1M NaOH aqueous electrolyte. It gives maximum specific capacitance of 138.88 Fg-1 at 0.2Ag-1 of current density with good cycle stability of 1000 continuous charge discharge cycles at 3Ag-1 of current density with 95.24% of capacitance retention. Hence the prepared electrode material shows high specific capacitance and good cyclic stability. The qualities of Sr-Co double hydroxide material reveal its use for high performance supercapacitor applications.
AUTHOR INFORMATION Corresponding Author *Abhay D. Deshmukh, E-mail: [email protected], Abhay Deshmukh Orchid-id: 0000-0001-6524-2826 Author Contributions A.D.D and K.A.D proposed and supervised the project. D.B.B. designed the experiments and performed electrochemical characterization. K.A.D. and D.R.P. carried out characterization of materials. D.B.B. and A.D.D. analyzed the data and wrote the paper. S.J.D., P.S. and B.K.G.edited and corrected the manuscript. All authors discussed the results and commented on the manuscript. Funding Sources Naval Research Board (NRB), DRDO, New Delhi (NRB Sanctioned No: DRDO/NRB/4003/PG/338). ACKNOWLEDGMENT
18
ADD thanks for the financial support from Naval Research Board (NRB), DRDO New Delhi (NRB Sanctioned No: DRDO/NRB/4003/PG/338). ADD and DBB also acknowledge the RUSA (Rashtriya Uchchatar Shiksha Abhiyan), Department of Higher Education, MHRD, for Instrument grant to R.T.M. Nagpur University and EMDL Laboratory. ADD thanks to ENVIRON CARE PRODUCTS, India for providing the Activated Carbon Cloth (ACC) for this research work. ADD also thanks to Celgard, LLC, North Carolina, USA for their support to make available the material for our research. D.B.B. acknowledges CSIR for fellowship support. ABBREVIATIONS SC, Supercapacitor, LDHs, Layered double hydroxides; ACC- activated carbon cloth, CVcyclic voltammetry ASC- asymmetric supercapacitor EIS- electrochemical impedance spectroscopy, GCD- Galvanostatic charge discharge
REFERENCES: 1. Iro.Z, Subramani.C, . Dash,S.S..A Brief Review on Electrode Materials for Supercapacitor. Int. J. Electrochem. Sci.,2016. 11, 10628 – 10643.
2. Huang.L,Chen.D, Ding.Y, Feng.S, Wang.Z, and Liu.M. Nickel−Cobalt Hydroxide Nanosheets Coated on NiCo2O4 Nanowires Grown on Carbon Fiber Paper for High-Performance Pseudocapacitors.Nano Lett. 2013.13(7).3135-3139
3. Vijayakumar .S, Nagamuthu, S . Lee S-H, Ryu K-S. Porous thin layered nanosheets assembled ZnCo2O4 grown on Nifoam as an efficient electrode material for hybrid supercapacitor applications. Int J Hydrogen energy.2017.42(5).3122-3129
4. Barmi M.J, Sundaram M.M.role of polymeric surfactant in the synthesis of cobalt molybdate nanospheres for hybrid capacitor applications.RSCAdv 2016. 6(42).36152—36162.
19
5. Chen.H, Hu.L , Chen.M , Yan.Y , and Wu.L . Nickel–Cobalt Layered Double Hydroxide Nanosheets for Highperformance Supercapacitor Electrode Materials. Adv. Functional Materials.2013.24(7).932-934
6.
Tran.M.V,
Ha.A.T,
and
Le. P , Nanoflake Manganese Oxide and Nickel-Manganese Oxide Synthesized by
Electrodeposition for Electrochemical Capacitor. Journals of Nano material. 2015.10,1-12 7. Pramanik A, Maiti S, Sreemany M, Mahanty S. Carbon doped MnCo2S4 microcubes grown on Ni foam as high energy density faradaic electrode.Electrochemical Acta.2016.213,672-679 8. Zai.J , Liu Y ,Li.X , Ma.Z, Qi.R . Quan.X. 3D Hierarchical Co–Al Layered Double Hydroxides with Long-Term Stabilities and High Rate Performances in Supercapacitors.Nano micro Lett.2016.9(2),No.21
9. Wang.F, Wang.T, Sun.S, Xu.Y, Yu.R & Li.H. One-step synthesis of Nickle Ironlayered double hydroxide/reduced graphene oxide/ carbon nanofibre composites as electrode material for asymmetric supercapacitors.sci. Rep.2018 8(1)
10. Xiao.B, Zhu.W, Li.Z , ju.J , Zhu.X and Pezzotti.G .Tailoring morphology of cobalt–nickel layered double hydroxide via different surfactants for high-performance supercapacitor.Royal society.(2018).5(9),No 180867
11. Deshmukh .A.D, Urade .A.R, Nanwani A.P, Deshmukh. K.A, Peshwe D.R, Sivaraman.P , Dhoble S.J, and Gupta B.K. TwoDimensional Double Hydroxide Nanoarchitecture with High Areal and Volumetric Capacitance. ACS Omega .2018.3, 72047213
12. Kiran .S.K , Padmini .M ,Das H.T and
Elumalai.P . Performance of asymmetric supercapacitor using CoCr-
layereddouble hydroxide and reduced graphene-oxide.journal of solid state Electrochemistry.(2016).21(4).(927-938)
13. Tian.Q,
Wang.X, Huang.G, and Guo.X1.Nanostructured (Co, Mn)3O4 for High Capacitive Supercapacitor
Applications.Nano scale Research letter.(2017).12(1) No.214.
14. Kavyashree,Raut S.S, Sankapal B.R, Pandeya S.N,. Tuberose surface architecture of Sr(OH)2 film as supercapacitive electrode. Electrochimica Acta(2017).258.(34-42)
15.Rovetta.A.S, Browne. M.P , Harvey.A , Godwin.I.J , Coleman J N and Lyons M. E.G, Cobalt hydroxide nanoflakes and their application as supercapacitors and oxygen evolution catalysts. Nanotechnology(2017).28.375401.
16. Li, Q., Xu, Y., Zheng, S., Guo, X., Xue, H., & Pang, HRecent Progress in Some Amorphous Materials for Supercapacitors. Small, . (2018). 14(28),1800426.
20
17. Gong. X.F , Cheng.J.P , Ma .K.Y , Liu.F , Zhang.L , Zhang.X.B .Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance. Materals chem and Phy.(2016),173.317-324
18. Sahoo.S, Naik.K.K and Rout.C . Electrodeposition of spinel MnCo2O4 nanosheets for supercapacitor applications. Nano tech.(2015).45.No.455401 19. Li.H, Musharavati.F, Sun.J, Jaber.F, Zalnezhad.E,
Hui.K.N, and Hui .K.S.Investigation of the Electrochemical
Properties of CoAl-Layered Double Hydroxide/Ni(OH)2.J Electrocheml Soc.(2018),165,A407-A415 20. Jing.M, Hou.H, Banks.C.E, Yang.Y, Zhang.Y and Ji.X*,†. Alternating Voltage Introduced NiCo Double Hydroxide Layered Nanoflakes for an Asymmetric Supercapacitor. ACS Appl. Mater. Interfaces, 2015,7, 22741−22744.
21. Ochal, Ejeh.F.O , Madito.M.J , Momodu.D.Y , Khaleed.A.A , Olaniyan.O, Manala.N. .High performance hybrid supercapacitor device based on cobalt manganese layered double hydroxide and activated carbon derived from cork (Quercus Suber). Electrochimica Acta,2017,252,41-54
22. Zhao.C, Ren.F, Xue.X , Zheng.W , Wang.X , chans.L. A high-performance asymmetric supercapacitor based on Co(OH)2/graphene and activated carbon electrodes.J of Electroenalytical chem.2016.782,98-102 23. Rusi,and, Majid S.R .Effects of Electrodeposition Mode and Deposition Cycle on the Electrochemical Performance of MnO2-NiO Composite Electrodes for High-Energy-Density Supercapacitors.PLOS ONE.2016.11(5),No 154566.
24. Hou.L, Yuana.C, Lia.D, Yang.L, Shen.L, Zhang.F, Zhang.X. Electrochemically induced transformation of NiS nanoparticles into Ni(OH)2 in KOH aqueous solution toward electrochemical capacitors. electrochimica Acta (2011),56, 7454–7459.
25. Jokar.E, Zad,A.I & Shahrokhian.S. Synthesis and characterization of NiCo2O4 nanorods for preparation of supercapacitor electrodes. J Solid State Electrochem(2015) 19,269–274.
26.Zhou Q, Wang X, Liu Y, He Y, Gao Y, Liu J. High rate capabilities of NiCo2O4-based hierarchical superstructures for rechargeable charge storage. J Electrochem Soc 2014;161,A1922-A1926 27. Gao.H, Xiao.F, Ching C.B., Duan, H. High-Performance Asymmetric Supercapacitor Based on Graphene Hydrogel and Nanostructured MnO2,. ACS Appl. Mater. Interfaces (2012) .4, 2801–2810.
28. Cakici. M., Reddy K.R., Marroquin F.A. Advanced electrochemical energy storage supercapacitors based on the flexible carbon fiber fabric-coated with uniform coral-like MnO2 structured electrodes .Chem. Eng. J. (2017) 309,151– 158
21
29. Cheng J, Lu Y, Qiu K, Yan H, Hou X, Xu J, et al. Mesoporous ZnCo2O4 nanoflakes grown on nickel foam as electrodes for high performance supercapacitors..Phy.Chem.chem.Phy.(2015).17.17016-17022.
30. Cheng .H and Duong.H.M. Three dimensional carbon nanotube/nickel hydroxide gels for advanced supercapacitors RSC Adv., 2015, 5, 30260-30267.
22
S0013-4686(19)31934-6
DOI:
https://doi.org/10.1016/j.electacta.2019.135063
Reference:
EA 135063
To appear in:
Electrochimica Acta
Received Date: 3 April 2019 Revised Date:
20 September 2019
Accepted Date: 10 October 2019
Please cite this article as: D.B. Bailmare, K.A. Deshmukh, P. Sivaraman, D.R. Peshwe, B.K. Gupta, S.J. Dhoble, A.D. Deshmukh, Directly grown Sr–Co layered double hydroxide (LDH) entangled two dimensional nanosheet film with superior performances, Electrochimica Acta (2019), doi: https:// doi.org/10.1016/j.electacta.2019.135063. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Directly grown Sr-Co Layered double hydroxide (LDH) entangled two dimensional nanosheet film with superior performances Deepa B. Bailmare1, Kavita A. Deshmukh2, P. Sivaraman3, D. R. Peshwe2, Bipin Kumar Gupta4, S. J. Dhoble5, Abhay D. Deshmukh1* *1
Energy Materials and Devices Laboratory, Dept. of Physics, RTM Nagpur University,
Nagpur-440033, India 2
3
Dept. of MME, Visvesvaraya National Institute of Technology, Nagpur-440022, India Polymer Science and Technology Centre, Naval Materials Research Laboratory (DRDO),
Ambernath (E) 421506, India 4
5
CSIR -National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi, 110012, India Dept. of Physics, Nanomaterials Research Laboratory, RTM Nagpur University, Nagpur,
India KEYWORDS: Sr-Co LDHs, Layered double hydroxides, Supercapacitors, Two dimensional materials (2D), Energy storage.
1
ABSTRACT: Designing of electrode with electrochemically rich structure, outstanding mechanical robustness and high electrical conductivity remains a challenge. We report a design of entangled Sr-Co Layered double hydroxides (LDHs) two-dimensional structure for such electrode. The hierarchical electrode with entangled nanosheet exhibit high specific capacitance of 1415 Fg-1 at 5Ag-1 current density, excellent rate capability, high energy (12.28Whkg-1) and power density (567.788 Wkg-1) and outstanding stability (80% capacity retention over 5000 cycles). The asymmetric device cell of Sr-Co LDH/ACC gives high energy density (27.78 Whkg-1) and power density (499.96Wkg-1) with cyclic performance of 1000 cycles with 95.24% capacitance retention. Further, we provide a detailed analysis of the various electrolyte environment experienced by the electrode and their effect on electrochemical properties. We pick out three different electrolytes; LiOH, NaOH, KOH and
2
show that both selected electrolyte and metal combination for LDHs paves the way for outstanding electrochemical performance.
INTRODUCTION: Amongst two dimensional nanomaterials, the extraordinary electrochemical properties of 2D layered double hydroxides (LDHs) have ignited great interest for asymmetric supercapacitors devices. The layered double hydroxides-based materials are the multi metal clay materials that are made up of layers of metal ions surrounded by octahedral arrangement of hydroxyl groups. This class of materials rich in fast redox reaction, environment friendly nature and high electrochemical performance. In particular, great efforts have been directed towards developing various combinations of LDHs family of materials depending upon the chemistry of the compound and the atomic structure for improving the specific energy of supercapacitor. [1-3] The major focus has been given on the designing of nanostructured electrode materials or use of novel electrolytes and understating their electrochemistry to improve the total cell capacitance and widening of operating voltage. Obviously, the new LDHs materials and their combinations have to be investigated to step up the LDHs asymmetric supercapacitor performance. For example, designing of new LDHs combinations such as Zn-Co[3], CoMo[4], Ni-Co[5] Mn-Ni[6], Mn-Co[7], Co-Al [8], Ni-Fe[9], Co-Ni [10], Cu-Co[11], CoCr[12], Co-Mn [13] Sr(OH2)[14] as electrode materials has been studied in recent times. The prior work shows, mostly cobalt hydroxides have been studied extensively in case of LDHs, because of its high electrochemical activity. The cobalt content has great influence on the properties of LDH nanosheets and porous structure. Cobalt in its hydroxide form provide high conductivity with Co(OH)2 gets converted into CoOOH in electrochemical reaction and hence acts as a conductive wrapping for electro active materials which is beneficial for the fast
3
redox reaction in particular electrolyte. To improve the efficiency of electrode material it is necessary to have highly accessible open structure for making redox reaction and ion diffusion at the electrode and electrolyte interface, which exhibits high specific capacitance, high electrical conductivity and fast electron transfer reaction. Hence the effective combinations of metal oxides/hydroxides can improve the efficiency of electrode material. The presence of two different metal cations in the mixed metal hydroxides can show improvement in electrochemical performance of material due to the synergic effect of multiple metal ions. [15,16] In present work, we have chosen the combination of strontium and cobalt as the LDHs materials. Strontium is highly reactive element, therefore mostly studied in the combination with other elements to improve the properties of the bulk. It is mostly used in improving the electric and magnetic properties of ferrites and other various applications. Therefore, an attempt has been made to study the combining effect of strontium and cobalt on electrochemical properties. Due to the large number of applications of strontium and cobalt we have deposited strontium cobalt layered double hydroxide on SS mesh substrate using electrodeposition technique. Electrodeposition is easy, fast and reliable technique for depositing variable thickness and uniform deposition of electrode material on any substrate. The weight of sample coated after deposition is found to be 0.55mg/cm2 calculated by difference between weight of SS (mesh) before and after deposition which shows the excellent results with high specific capacitance of 1415Fg-1, 12.28Whkg-1energy density and 567.788 Wkg-1 power densities in 1M NaOH strong aqueous electrolyte at 5Ag-1 current density. With increasing the current density specific capacitance is also increases. By analyzing CV curve, it is clearly visible the pure pseudocapacitive behavior of prepared electrode material with
4
prominent oxidation and reduction peaks at higher and lowers potential sides respectively. This faradic nature of material shows higher specific capacitance than EDLCs. (Sr-Co LDH) shows high specific capacitance with 1M NaOH with good cycling stability of 5000 cycles with 80% capacitance retention. The two-electrode asymmetric device were demonstrated using LDHs as positive electrode and carbon cloth as negative electrode in 1M NaOH electrolyte. Specific capacitance for fabricated asymmetric device was found to be 138.88 Fg-1 at 0.2 Ag-1 of current density within the potential range of 0 to 1.2V. The two-electrode cell interestingly exhibits high energy density 27.78 Whkg-1 and power density 499.96 Wkg-1. EXPERIMENTAL SECTION: Synthesis of 3D entangled Sr-Co LDH: The electrode material were prepared by a simple one step process as follows: typically, 304 stainless steel (SS) mesh (10 mm x 20 mm x 0.1 mm, 400ppi) was successively cleaned in ultrasonic bath with acetone for 10 min and then simply rinsed with double distilled water to remove the impurities on the surface. The cleaned SS mesh was then immersed in electrochemical cell with a homogeneous solution of Sr(NO3)2.4H2O
(8mmol),
Co(NO3)2.6H2O
(16mmol),
followed
by
potentiostatic
electrodeposition for 300s at applied potential of -1.0V to grow Sr-Co LDH entangled nanosheet. All the electrodeposition processes were performed in conventional three electrode cell at room temperature comprising of the SS mesh and Ag/AgCl as working and reference electrode, and platinum foil (10 x10mm) as counter electrode respectively. The strontium and cobalt hydroxide monomers were converted to Sr-Co LDH through redox reactions. The entangled nanosheet covered substrate then washed with water and ethanol using ultrasonic cleaner to remove the surface ions and molecules, and further dried in vacuum oven at 80oC for overnight to remove solvent. The overnight heating provides a material to bind on substrate for
5
longer period of time and above 80oC drying may result in structural destruction. The mass of the entangled Sr-Co LDH nanosheet on SS mesh was determined by subtracting the weight before and after deposition. Characterization: The crystalline structure of prepared electrode material (Sr-Co LDH) was analyzed by PAN-analytical X-ray diffractometer with Cu Kα radiation with proportional detector. The morphologies were observed by Field emission scanning electron microscope (JEOL JSM 7610F FEGSEM). Electrochemical measurement: The electrochemical measurements of the entangled LDH electrodes were investigated using Metrohm AUTOLAB (128N Potentiostate) under a three-electrode cell configuration. The SS mesh supported entangled nanosheet were directly used as the working electrodes, which were soaked in 1M NaOH solution before the electrochemical test. Ag/AgCl and Platinum foil were used as reference and counter electrode, respectively. Asymmetric supercapacitor devices tested with entangled Sr-Co LDH nanosheets as positive electrode and activated carbon cloth (ACC) as negative electrode in 1M NaOH solution. The electrochemical properties of prepared electrode materials were investigated through cyclic voltammetry (CV), and Galvanostatic charge discharge process in the applied potential range -0.2 to 0.3V. The cyclic voltammetry was carried out at different scan rates from 5mV/s to 100mV/s. The electrochemical impedance spectroscopy (EIS) was performed at frequency range of 10-1Hz to 104 Hz with respect to open circuit potential (OCP) of 0.019 V. The Cs of Sr-Co LDH nanosheet was calculated from Galvanostatic charge discharge curve with the formula given as (6). I x ∆t
Cs = 2 (m x ∆V)
6
Where, Cs (Fg−1) is the specific capacitance. The Cs of asymmetric supercapacitor was calculated from Galvanostatic charge-discharge curves as follows:
Cs=
× × (×)
Where, ‘m’ is the total mass of the active material in the positive and negative electrodes. The energy and power densities of the Sr-Co LDH, ACC based electrodes, and the asymmetric supercapacitor was calculated as follows: E= 0.5×CV2, Pave= E/∆t, where, I is specific current, ∆t is the discharging time, m is the mass deposited, ∆V is the potential window, E(Whkg−1) is the energy density, V is the cell voltage excluding IR drop, Pave (W kg−1) is the average power density.
RESULT AND DISCUSSION: Structural and morphological studies: Figure1 (a) shows a schematic fabrication process for the well grown entangled Sr-Co LDH nanosheets directly grown on stainless steel mesh as a novel binder free electrode. The detailed synthesis process is given in experimental section. The entire surface of the stainless-steel mesh is uniformly covered with the Sr-Co LDHs after electrodeposition. In Figure 1(b), the magnified FESEM images validate the well distributed and vertically aligned entangled Sr-Co LDHs nanosheet with nano flakes porous morphology. After electrodeposition process, the SrCo LDHs nanosheet homogeneously anchored onto the SS surface forming the unique 3dimensional entangled structure (Figure 1(c)). The thickness of nanosheet is 20-25nm as shown in Figure 1(d). The increased thickness of the nanosheet is may be due to the amount of Cobalt added in Sr-Co LDH material. During electrodeposition process, simultaneously the instantaneous and progressive nucleation takes places which results in vertical growth of
7
nanosheet. As shown in (Supplementary Fig S1) where at the beginning of electric pulse nuclei are formed initially thereafter vertical growth occurs. The precursors having high pH value dominates the metal hydroxides formation hence our material was electrodeposited at higher pH value followed by the successive growth of the material over the current collector. The growth mechanism, morphology and the structure of the material in electrodeposition process is strictly depends upon the electrolyte concentration, pH value and other environmental conditions. The synthesized 3-dimensional porous nanosheet entangled structure provides higher electrochemical active sites results in higher specific capacitance and cycle stability of material [14]. Furthermore, (Figure 1(e)) shows the X-ray diffraction (XRD) pattern of the entangled Sr-Co LDHs material. The Sr-Co LDHs exhibit diffraction peaks for both Sr(OH)2 and Co(OH)2. [14,15] Excluding peaks originating from the SS mesh, all the noticeable peaks can be indexed to Sr(OH)2 (JCPDS No. 38-0715)18 and Co(OH)2 (JCPDS No. 42-1316), implying existence of Sr-Co LDHs structure. The Sr-Co LDH material shows less crystalline structure which further attributes to high porosity of the material. The XRD data shows the continuous peak broadening throughout the disordering process which indicates the non crystalline behavior of the material. Hence the material is related to highly disorder noncrystalline nature which lost their crystallinity due to the incorporation of highly disordered phases. Therefore, the material is trending amorphous nature and gives more active sites to the ions incorporation, which further gives high specific capacitance and cyclic stability in aqueous electrolytes.[16] The layered double hydroxides based material specially gives more porous and 3-dimensional structure and hence provides a path to improve the electrochemical performances of the electrode materials. The LDHs are nanostructured electrode materials which plays vital role in electrolytic ion transportation during charge discharge process. The
8
prevention of aggregation of LDHs nanosheets make full use of active sites and improve the electrochemical performances.
(a)
(b)
(e)
(c)
(d)
Figure 1: a) Schematic representation of the Sr-Co LDHs entangled nanosheets forming 3D interconnected network. b) Magnified FESEM image of SS stainless steel sheet. C) Entangled Sr-Co LDH nanosheet grown vertically on SS (Mesh) substrate. d) Thickness of the Sr-Co LDH nanosheet grown on SS (mesh) substrate.(e) XRD pattern of Sr-Co LDH nanosheet. To explain the electrochemical performance of electrodeposited Sr-Co LDH entangled nanosheet, we carried out three electrode electrochemical test with Ag/AgCl reference
9
electrode and platinum foil counter electrode in 1M NaOH solution. The typical CV curve (Figure 2(a)) of the obtained LDH film exhibits a pair of well-defined redox peaks within 0.2V to 0.3V at various scan rates of 5mV/s to 100mV/s. The curve show pseudocapacitive characteristics and superior reversible redox reaction of prepared electrode material. The CV curve changes accordingly with respect to scan rates. The changes in CV curve are due to the occurrence of redox intercalation between electrode and electrolyte interfaces.The cations may suffer multiple processes simultaneously during the charge storage process and hence gives the high specific capacitance. When the scan rate increases, area under the curve also increases which shows the high rate capability of the electrode material. Remarkably, the peak potential shifts only 120mV/s for 20 times increase of scan rates, suggests the Sr-Co LDH electrode possess low polarization. The high potential shift may cause due to the isolated barriers developed at the electrode and electrolyte with respect to increase in potential. Additionally, subsequent linear increase in peak current with respect to (scan rate)1/2 confirms the electrochemical redox process is diffusion controlled (Supplementary Figure S2). The comparative CV curve further indicates that Sr-Co LDH electrode possess a higher specific capacitance by utilizing the combinations of single layered Sr(OH)2and Co(OH)2 electrodes (Supplementary Figure S3 and table T1) which is estimated the synergetic effect of entangled Sr-Co double hydroxides nano sheets structure. In second part of our experimental studies, several known aqueous electrolytes were selected and considered for comparison, including 1M KOH, 1M LiOH and 1M NaOH. In order to compare the electrochemical characteristics of Sr-Co LDH in various electrolytes, their CVs are normalized to show specific current verses potential in Figure 2(b). All the CVs in Figure 2b are similar in nature with an increasing current at the upper or lower potential limits. The
10
Sr-Co LDH electrode works at less negative potential in 1M KOH than in 1M LiOH and 1M NaOH. From the selected electrolytes, NaOH is the only one that exhibits higher specific capacitance than others. The comparative tables [Supplementary Table T2] and detailed study of Sr-Co LDH electrode in various electrolytes are discussed in [Supplementary Fig S4]. Thus 1M NaOH aqueous electrolytes is more favorable for Sr-Co LDH electrode material as compare to LiOH and KOH.The rate capability of Sr-Co LDH electrode material was examined by recording galvanostatic charge discharge (GCD) process. Charge discharge curves at various current densities ranging from 5 Ag-1 to 50 Ag-1 within the applied potential window of -0.2 V to 0.3V verses (Ag/AgCl) (Figure2(c)). Sr-Co LDH was able to deliver a specific capacitance of 1415.63 Fg-1 at 5Ag-1 of current density which is much higher than the specific capacitance of Sr(OH)2 and Co(OH)2. Whereas, increase in current density by 10 folds 5 to 50Ag-1, the specific capacitance was found to be 982 Fg-1 which is 69% of initial capacitance. After cycling at higher current density of 50 Ag-1, the specific capacitance at the 5Ag-1 current density was retained which proves the Sr-Co LDH electrode has superior rate capability. The comparative specific capacitance verses current density for all the three aqueous electrolytes shown in Figure2(d), the Sr-Co LDH was tested in two electrolytes shows the specific capacitance of 1013.12 Fg-1 in 1M KOH and 843.47 Fg-1 in 1M LiOH electrolyte respectively, at 5Ag-1 of current density.
11
(a)
(c )
(b)
(d)
Fig2: (a) CV curve of Sr-Co double hydroxide electrode material at different scan rates with 1M NaOH electrolyte (b)Comparative CV curve of prepared electrode materials with different electrolytes (c) charge- discharge curve of Sr-Co double hydroxide at different current densities with 1M NaOH electrolyte (d) percent rate capability Sr-Co LDH electrode material at 1M NaOH electrolyte. The cyclic stability is another important parameter to study the cycle life of the electrode materials. In order to understand the cycling performance of Sr-Co LDH electrode, the charge discharge cycling was performed at current density of 22.5 Ag-1. After 5000 cycles, this entangled Sr-Co LDH nanosheet electrode possesses 80% (Figure 3(a)) capacitance retention, which is much higher than those of Sr(OH)2 and Co(OH)2 (Supplementary Figure S4). To analyze the effect of long cycling test on Sr-Co LDH nanosheet we compared cyclic
12
voltammetry (CV) of before and after cycling (Supplementary Fig S6). The change in CV curve clearly estimated the structural destruction of Sr-Co LDH nanosheet in 1M NaOH electrolyte. However, the Sr-Co LDH electrode exhibits an energy density of 12.28Whkg-1 and power density 567.788Wkg-1(Figure 3(b)) showed in comparative Ragone plot of Sr-Co LDH with Sr(OH)2 and Co(OH)2. The comparative performance of Co and Sr based electrodes reported in earlier literature is shown in (Supplementary Table T3). Further, Fig 3(c) shows the comparative Ragone plot of energy density and power density at different current densities with reported literature. The Sr-Co LDH nanosheet shows the good agreement with comparative results. EIS study were performed to evaluate resistive behavior of the electrode material (Supplementary Figure S7) which shows the comparative Nyquist plot of before and after cycles taken in the frequency range of 0.01 Hz to 10000 Hz with the open circuit potential of 0.019V. The charge transfer resistance Rct of the electrode material increases from 40.3Ω and 250.17Ω with (Rs 1.06Ω and 1.28 Ω), respectively, before and after 5000 cycles. This indicates that the major loss of capacitance is due to the structural destruction and increase in charge transfer resistance of the nanostructured electrode material. The comparative EIS measurement of Sr-Co-LDH, Co(OH)2 and Sr(OH)2 shown in (Supplementary Figure S3) . The Rs values of Sr-Co LDH, Sr(OH)2 and Co(OH)2 is ( 1.06 Ω, 3.09 Ω, and 4.38 Ω respectively). Rs value for Sr-Co LDH is less than Co(OH)2 and Sr(OH)2 which indicated better electronic conductivity of the material.
13
(a)
(b)
(c )
Fig3: (a) cyclic stability curve of Sr-Co double hydroxide electrode material with 1M NaOH. (b) Comparative energy density Whkg-1 vs power density Wkg-1 of various Sr-Co LDH with Sr(OH)2 and Co(OH)2. (c) comparative Ragone plot of Energy density and power density of prepared Sr-Co LDH with other materials. An excellent electrochemical performance in terms of energy density, cycle stability, rate capability and specific capacitance of Sr-Co LDH electrode mainly attributed to the following merits: i) well distributed Sr-Co LDH nanosheet on the SS mesh with entangled interconnected 3D porous architecture, enhanced porous structure, conducting behavior, enhanced electrolytic diffusion through the interconnected network which results in enhancement of rate capability. ii) The direct growth of active material on the current collector favors the easy access to electrolytes which provide fast electron transport, iii) The vertical growth of 2D nanosheet about 20-25 nm thickness reduce the dead mass of active electrode and provide more specific surface area for electrochemical reaction. iv)The entangled nanosheet 3D architecture reduces
14
the structural destructiveness during charging and discharging which was demonstrated by SrCo LDH electrode material.
(a)
(c)
(b)
(d)
Fig4 (a) CV curve of asymmetric two electrode system with 1M NaOH ((b) Charge discharge curve of asymmetric two electrode system (c)
comparative Ragone Plot
ofenergy density and power density with other reported literature (d) cycle stability plot of two electrode measurement.
15
Further, asymmetric supercapacitor was fabricated using Sr-Co-LDH as a cathode and activated carbon cloth (ACC) as an anode in 1M NaOH electrolyte solution. Before fabricating the ASC device, the electrode was dipped into the electrolyte solution for about four hours. In ASC device, the mass is the ratio of the electrode as obtained by the equation
=
.∆ .∆
Where, m+ and m- are the mass of positive and negative electrode respectively [23-27]. To evaluate the applications of Sr-Co LDH / ACC asymmetric device, we demonstrate the CV, GCD and EIS tests. Interestingly, the charge discharge curve shows the non-ideal triangular behavior due to the combinations of both non faradic and faradic reactions [2830](Fig4b). Moreover, the symmetric nature of GCD curve suggests the high reversibility and excellent columbic efficiency of electrode. The operating potential window of the ASC was optimized and it was found to be 0 to 1.2V. The capacitance of the cell is calculated by using an equation provided in experimental section. The maximum specific capacitance based on the total mass of the active material found to be 138.88 Fg-1 at current density of 0.2 Ag-1. The galvanostatic charge discharge curve and rate capability of asymmetric device at different current densities were also tested and given in Figure 4(b) and Supplementary Table T4 (55.6 Fg-1 of specific capacitance at 3Ag-1 current density). Furthermore, energy density and power density (Ragone plot) of the ASC device were calculated from the data shown in Figure 4(c). The energy density and power density (E & P) were calculated according to equation given in experimental section based on the weight of active material. The asymmetric supercapacitor device exhibits a maximum energy density of 27.78Whkg-1 at power density of 499.96 Wkg-1 and maximum power density 7200 Wkg-1 at energy density of 11.12 Whkg-1. The comparative table of supercapacitor that have been
16
reported earlier and the present asymmetric supercapacitor based on strontium is shown in Supplementary Table T6. In present study, we have also examined the areal capacitance, energy density and power density of the Sr-Co LDH/ACC asymmetric device (Supplementary Figure S8) which was found to be 666.66 mFcm-2.The total volume/areal capacitance of presented ASC device was calculated based on area of ACC and Sr-Co-LDH electrode shown in (Supplementary Table T5). Additionally, the ASC device also exhibits an excellent cycling stability of 95.24% over 1000 continuous charge discharge cycles at 3Ag-1 current density (Figure 4(d)). In order to understand the effect of cycling on the resistive behavior of ASC, EIS measurement was employed. Nyquist plot (supplementary Figure S9) clearly reveals no obvious change in Rs from 5.05 Ω and 9.10 Ω after continuous 1000 charge discharge cycles as compared to initial cycles which demonstrate the excellent stability of the asymmetric supercapacitor device.
CONCLUSION: Layered strontium cobalt double hydroxide entangled nanosheet has been grown on SS (mesh) substrate by using electrodeposition technique. The SEM image of fabricated Sr-Co double hydroxide clearly shows flakes morphology of layered nano porous structure. The unique structure of layered strontium cobalt double hydroxides exhibits high specific capacitance of 1415.63 Fg-1 at 5Ag-1 of current density with 1M NaOH strong aqueous electrolyte. The prepared electrode material runs for 5000 continuous charge discharge cycles at 22.5 Ag-1 of current density with 80% of capacitance retention. The Asymmetric devices made up of the combination of strontium cobalt double hydroxide Sr-Co LDH electrode material as one and carbon cloth as another electrode material. The two electrode system was
17
performed with 1M NaOH aqueous electrolyte. It gives maximum specific capacitance of 138.88 Fg-1 at 0.2Ag-1 of current density with good cycle stability of 1000 continuous charge discharge cycles at 3Ag-1 of current density with 95.24% of capacitance retention. Hence the prepared electrode material shows high specific capacitance and good cyclic stability. The qualities of Sr-Co double hydroxide material reveal its use for high performance supercapacitor applications.
AUTHOR INFORMATION Corresponding Author *Abhay D. Deshmukh, E-mail: [email protected], Abhay Deshmukh Orchid-id: 0000-0001-6524-2826 Author Contributions A.D.D and K.A.D proposed and supervised the project. D.B.B. designed the experiments and performed electrochemical characterization. K.A.D. and D.R.P. carried out characterization of materials. D.B.B. and A.D.D. analyzed the data and wrote the paper. S.J.D., P.S. and B.K.G.edited and corrected the manuscript. All authors discussed the results and commented on the manuscript. Funding Sources Naval Research Board (NRB), DRDO, New Delhi (NRB Sanctioned No: DRDO/NRB/4003/PG/338). ACKNOWLEDGMENT
18
ADD thanks for the financial support from Naval Research Board (NRB), DRDO New Delhi (NRB Sanctioned No: DRDO/NRB/4003/PG/338). ADD and DBB also acknowledge the RUSA (Rashtriya Uchchatar Shiksha Abhiyan), Department of Higher Education, MHRD, for Instrument grant to R.T.M. Nagpur University and EMDL Laboratory. ADD thanks to ENVIRON CARE PRODUCTS, India for providing the Activated Carbon Cloth (ACC) for this research work. ADD also thanks to Celgard, LLC, North Carolina, USA for their support to make available the material for our research. D.B.B. acknowledges CSIR for fellowship support. ABBREVIATIONS SC, Supercapacitor, LDHs, Layered double hydroxides; ACC- activated carbon cloth, CVcyclic voltammetry ASC- asymmetric supercapacitor EIS- electrochemical impedance spectroscopy, GCD- Galvanostatic charge discharge
REFERENCES: 1. Iro.Z, Subramani.C, . Dash,S.S..A Brief Review on Electrode Materials for Supercapacitor. Int. J. Electrochem. Sci.,2016. 11, 10628 – 10643.
2. Huang.L,Chen.D, Ding.Y, Feng.S, Wang.Z, and Liu.M. Nickel−Cobalt Hydroxide Nanosheets Coated on NiCo2O4 Nanowires Grown on Carbon Fiber Paper for High-Performance Pseudocapacitors.Nano Lett. 2013.13(7).3135-3139
3. Vijayakumar .S, Nagamuthu, S . Lee S-H, Ryu K-S. Porous thin layered nanosheets assembled ZnCo2O4 grown on Nifoam as an efficient electrode material for hybrid supercapacitor applications. Int J Hydrogen energy.2017.42(5).3122-3129
4. Barmi M.J, Sundaram M.M.role of polymeric surfactant in the synthesis of cobalt molybdate nanospheres for hybrid capacitor applications.RSCAdv 2016. 6(42).36152—36162.
19
5. Chen.H, Hu.L , Chen.M , Yan.Y , and Wu.L . Nickel–Cobalt Layered Double Hydroxide Nanosheets for Highperformance Supercapacitor Electrode Materials. Adv. Functional Materials.2013.24(7).932-934
6.
Tran.M.V,
Ha.A.T,
and
Le. P , Nanoflake Manganese Oxide and Nickel-Manganese Oxide Synthesized by
Electrodeposition for Electrochemical Capacitor. Journals of Nano material. 2015.10,1-12 7. Pramanik A, Maiti S, Sreemany M, Mahanty S. Carbon doped MnCo2S4 microcubes grown on Ni foam as high energy density faradaic electrode.Electrochemical Acta.2016.213,672-679 8. Zai.J , Liu Y ,Li.X , Ma.Z, Qi.R . Quan.X. 3D Hierarchical Co–Al Layered Double Hydroxides with Long-Term Stabilities and High Rate Performances in Supercapacitors.Nano micro Lett.2016.9(2),No.21
9. Wang.F, Wang.T, Sun.S, Xu.Y, Yu.R & Li.H. One-step synthesis of Nickle Ironlayered double hydroxide/reduced graphene oxide/ carbon nanofibre composites as electrode material for asymmetric supercapacitors.sci. Rep.2018 8(1)
10. Xiao.B, Zhu.W, Li.Z , ju.J , Zhu.X and Pezzotti.G .Tailoring morphology of cobalt–nickel layered double hydroxide via different surfactants for high-performance supercapacitor.Royal society.(2018).5(9),No 180867
11. Deshmukh .A.D, Urade .A.R, Nanwani A.P, Deshmukh. K.A, Peshwe D.R, Sivaraman.P , Dhoble S.J, and Gupta B.K. TwoDimensional Double Hydroxide Nanoarchitecture with High Areal and Volumetric Capacitance. ACS Omega .2018.3, 72047213
12. Kiran .S.K , Padmini .M ,Das H.T and
Elumalai.P . Performance of asymmetric supercapacitor using CoCr-
layereddouble hydroxide and reduced graphene-oxide.journal of solid state Electrochemistry.(2016).21(4).(927-938)
13. Tian.Q,
Wang.X, Huang.G, and Guo.X1.Nanostructured (Co, Mn)3O4 for High Capacitive Supercapacitor
Applications.Nano scale Research letter.(2017).12(1) No.214.
14. Kavyashree,Raut S.S, Sankapal B.R, Pandeya S.N,. Tuberose surface architecture of Sr(OH)2 film as supercapacitive electrode. Electrochimica Acta(2017).258.(34-42)
15.Rovetta.A.S, Browne. M.P , Harvey.A , Godwin.I.J , Coleman J N and Lyons M. E.G, Cobalt hydroxide nanoflakes and their application as supercapacitors and oxygen evolution catalysts. Nanotechnology(2017).28.375401.
16. Li, Q., Xu, Y., Zheng, S., Guo, X., Xue, H., & Pang, HRecent Progress in Some Amorphous Materials for Supercapacitors. Small, . (2018). 14(28),1800426.
20
17. Gong. X.F , Cheng.J.P , Ma .K.Y , Liu.F , Zhang.L , Zhang.X.B .Nanostructured nickel-cobalt sulfide grown on nickel foam directly as supercapacitor electrodes with high specific capacitance. Materals chem and Phy.(2016),173.317-324
18. Sahoo.S, Naik.K.K and Rout.C . Electrodeposition of spinel MnCo2O4 nanosheets for supercapacitor applications. Nano tech.(2015).45.No.455401 19. Li.H, Musharavati.F, Sun.J, Jaber.F, Zalnezhad.E,
Hui.K.N, and Hui .K.S.Investigation of the Electrochemical
Properties of CoAl-Layered Double Hydroxide/Ni(OH)2.J Electrocheml Soc.(2018),165,A407-A415 20. Jing.M, Hou.H, Banks.C.E, Yang.Y, Zhang.Y and Ji.X*,†. Alternating Voltage Introduced NiCo Double Hydroxide Layered Nanoflakes for an Asymmetric Supercapacitor. ACS Appl. Mater. Interfaces, 2015,7, 22741−22744.
21. Ochal, Ejeh.F.O , Madito.M.J , Momodu.D.Y , Khaleed.A.A , Olaniyan.O, Manala.N. .High performance hybrid supercapacitor device based on cobalt manganese layered double hydroxide and activated carbon derived from cork (Quercus Suber). Electrochimica Acta,2017,252,41-54
22. Zhao.C, Ren.F, Xue.X , Zheng.W , Wang.X , chans.L. A high-performance asymmetric supercapacitor based on Co(OH)2/graphene and activated carbon electrodes.J of Electroenalytical chem.2016.782,98-102 23. Rusi,and, Majid S.R .Effects of Electrodeposition Mode and Deposition Cycle on the Electrochemical Performance of MnO2-NiO Composite Electrodes for High-Energy-Density Supercapacitors.PLOS ONE.2016.11(5),No 154566.
24. Hou.L, Yuana.C, Lia.D, Yang.L, Shen.L, Zhang.F, Zhang.X. Electrochemically induced transformation of NiS nanoparticles into Ni(OH)2 in KOH aqueous solution toward electrochemical capacitors. electrochimica Acta (2011),56, 7454–7459.
25. Jokar.E, Zad,A.I & Shahrokhian.S. Synthesis and characterization of NiCo2O4 nanorods for preparation of supercapacitor electrodes. J Solid State Electrochem(2015) 19,269–274.
26.Zhou Q, Wang X, Liu Y, He Y, Gao Y, Liu J. High rate capabilities of NiCo2O4-based hierarchical superstructures for rechargeable charge storage. J Electrochem Soc 2014;161,A1922-A1926 27. Gao.H, Xiao.F, Ching C.B., Duan, H. High-Performance Asymmetric Supercapacitor Based on Graphene Hydrogel and Nanostructured MnO2,. ACS Appl. Mater. Interfaces (2012) .4, 2801–2810.
28. Cakici. M., Reddy K.R., Marroquin F.A. Advanced electrochemical energy storage supercapacitors based on the flexible carbon fiber fabric-coated with uniform coral-like MnO2 structured electrodes .Chem. Eng. J. (2017) 309,151– 158
21
29. Cheng J, Lu Y, Qiu K, Yan H, Hou X, Xu J, et al. Mesoporous ZnCo2O4 nanoflakes grown on nickel foam as electrodes for high performance supercapacitors..Phy.Chem.chem.Phy.(2015).17.17016-17022.
30. Cheng .H and Duong.H.M. Three dimensional carbon nanotube/nickel hydroxide gels for advanced supercapacitors RSC Adv., 2015, 5, 30260-30267.
22