Cation intercalated one-dimensional manganese hydroxide nanorods and hierarchical mesoporous activated carbon nanosheets with ultrahigh capacitance retention asymmetric supercapacitors

Journal Pre-proofs Cation Intercalated One-Dimensional Manganese Hydroxide nanorods and Hierarchical Mesoporous Activated Carbon Nanosheets with Ultrahigh Capacitance Retention Asymmetric Supercapacitors Aravindha Raja Selvaraj, Hee-Je Kim, Karuppanan Senthil, Kandasamy Prabakar PII: DOI: Reference:

S0021-9797(20)30132-6 https://doi.org/10.1016/j.jcis.2020.01.117 YJCIS 25990

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

4 October 2019 21 January 2020 28 January 2020

Please cite this article as: A. Raja Selvaraj, H-J. Kim, K. Senthil, K. Prabakar, Cation Intercalated OneDimensional Manganese Hydroxide nanorods and Hierarchical Mesoporous Activated Carbon Nanosheets with Ultrahigh Capacitance Retention Asymmetric Supercapacitors, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.01.117

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Cation Intercalated One-Dimensional Manganese Hydroxide nanorods and Hierarchical Mesoporous Activated Carbon Nanosheets with Ultrahigh Capacitance Retention Asymmetric Supercapacitors Aravindha Raja Selvaraj a, Hee-Je Kima, , Karuppanan Senthilb, Kandasamy Prabakara, * aDepartment

of Electrical Engineering, Pusan National University, 2 Busandaehak-ro 63beon-gil,

Geumjeong-gu, Busan-46241, Republic of Korea. bDepartment

of Physics, Bannari Amman Institute of Technology, Sathyamangalam 638 401,

Tamil Nadu, India. ABSTRACT: We have reported the electrochemical performance of K+ ion doped Mn(OH)4 and MnO2 nanorods as a positive electrode and a highly porous activated carbon nanosheet (AC) made from Prosopis Juliflora as negative electrode asymmetric supercapacitor (ASC) with high rate capability and capacity retention. The cation K+ doped Mn(OH)4 and MnO2 nanorods with large tunnel sizes allow the electrolyte to penetrate through a well-defined pathway and hence benefits from the intercalation pseudocapacitance and surface redox reactions. As a result, they exhibit good electrochemical performance in neutral aqueous electrolytes. More specifically, the K+Mn(OH)4 nanorods exhibit higher capacitance values than K+-MnO2 nanorods due to the homogenous distribution of 1D nanorods and optimum amount of O-H bonds. The fabricated K+Mn(OH)4 symmetric electrochemical Pseudocapacitor shows very high energy density of 10.11 Wh/kg and high-power density of 51.04 W/kg over the range of 1.0 V in aqueous electrolyte. The energy density of AC || K+-Mn(OH)4 ASC is improved significantly compared to those of symmetric supercapacitors. The fabricated ASC exhibits a wide working voltage window (1.6

1

V), high power (143.37 W/kg) and energy densities (41.38 Wh/kg) at 0.2 A g−1, and excellent cycling behavior with 107.3% capacitance retention after 6000 cycles at 2 A g−1 indicating the promising practical applications in electrochemical supercapacitors. KEYWORDS: Manganese oxide 1D nanorods, Cation intercalation, symmetrical Supercapacitors, Asymmetric capacitors, Pseudocapacitance. INTRODUCTION Supercapacitors (SCs) with short charging time, high power density, and moderately high energy density have attracted great attention all over the world as a promising energy storage devices due to the growing demand for portable electronic devices and hybrid electrical vehicles [1–4]. The energy density of existing commercialized SC is still low and cannot satisfy the fastgrowing power demands of the future electronic devices [5,6]. Hence, research works have been focused on asymmetric supercapacitors (ASC) to improve their energy density and make them comparable to Li ion batteries [6,7]. In ASC, the electric double layer capacitor (EDLC) carbon negative electrodes are paired with pseudocapacitive or battery type positive electrodes [6,8]. The mass (charge) balances should be considered for improving the operating potential window of the ASC, since both the electrodes essentially work in the similar electrolyte. So, both the positive and negative electrode materials play key roles in developing high performance ASC [7,9]. Pseudocapacitors (PCs) are the family of electrochemical capacitors (ECs) having fast reversible redox reactions, but their charge storage mechanism is more like electrolytic capacitors such as fast charging/discharging over the complete potential window [5,10]. PC

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stores electrical energy Faradaically by electron transfer between electrode and electrolyte interface. The electrode material can store and release the charges either by redox pseudocapacitance or intercalation pseudocapacitance coupled with reversible redox reactions of cations at or near the electrode/electrolyte interface [2,6]. Manganese oxides (MnOx) are one of the classical metal oxide PCs electrodes because of their high theoretical capacitance (1380 Fg-1), wide operational window (0 – 1V), natural abundance, and low cost [11–14]. The wide oxidation diversity (Mn2+, Mn3+,Mn4+) and structures including MnOOH [15], MnO2 [12], Mn2O3 [16], and Mn3O4 [17] existing in different crystal phases provide fast single electron transfer during oxidation and reduction process and hence enhances the performance of PC [13–15]. MnOx crystal structures can be divided into three categories based on different [MnO6] links such as tunnel shaped α-MnO2 [18], β-MnO2 [19], γ-MnO2 [13]), spinel type 3D λ-MnO2 [20] and layered δ-MnO2 [21]. It is generally accepted that crystallographic phase, the textural and morphological effects of the electrodes influence the electrochemical activity [11–13]. Hence, large varieties of one-dimensional (1D) size, shape and surface dependent MnOx nanoarchitectures have been explored for the short and linear transportation of electrons or cations between the electrode/electrolyte interface to increase the electrochemical stability and reversibility [22–24]. However, due to the manganese oxide’s (MnOx) low intrinsic electrical conductivity (10-5 to 10-6 S cm -1) and poor structural stability, their ion/electron transport efficiency and cycling stability is reduced [23]. In order to alleviate those drawbacks, considerable attention is given to boost the conductivity of MnOx by introducing large cations (K+, Na+, H+ and Li+) into the tunnel or 1D layered MnOx nanostructures during the synthesis process [25–28]. It improves not only the intercalation PCs but also maintain the tunnel or layer structures during electrochemical reactions 3

[27,29]. Hence, in the present investigation, an efficient simple one-pot hydrothermal approach is used without any physical template and surfactant to synthesis cation intercalated Mn(OH)4 and MnO2 nanorods by reducing KMnO4 and aniline in aqueous medium and at various temperatures and time. KMnO4 is an effective precursor to introduce K+ ions into the tunnel or layered MnOX nanostructures with different crystallographic forms and functionalities [29,30]. Hydrothermal method is a practical and efficient technique which directly influence the surface morphology [31–34] and crystalline phases of the MnOX nanomaterials [12,13,35]. Moreover, hydrothermally prepared MnOX nanomaterials possess high specific surface area, optimum H-O-H bonds on oxide lattice and high crystallinity which effectively enhance the redox reactions [31,32,36,37]. The electrochemical properties of ACs materials are improved by tailoring the pore size (volume), specific surface area (SSA), and defective sites on the carbon edges to ensure the reduced ion-diffusion length with enhanced rate capability [38,39] For this reason, developing a functionalized hierarchical porous carbon nanostructure is necessary for high-performance ASC. Conventionally, ACs are prepared from non-renewable fossil fuels and derivatives and hence, it was decided to synthesis environmental friendly and renewable ACs from biomass material at low cost [40,41]. Therefore, in this work, it is aimed to explore the electrochemical properties of symmetric Mn(OH)4 nanorods and ASC using AC as the negative electrode and Mn(OH)4 material as the positive electrode in a 3 M aqueous Na2SO4 electrolyte to improve the energy density by extending the potential window. 2. EXPERIMENTAL: 2.1 Chemicals and Reagents All the analytical grade reagents such as potassium permanganate (KMnO4), aniline (C6H5NH2), sodium sulphate (Na2SO4), potassium hydroxide (KOH), 1-methyl-2-pyrrolidinone 4

(NMP), acetyleneblack and polyvinylidene difluoride (PVdF) were purchased from SigmaAldrich and used without further purification. 2.2 Material Synthesis The Mn(OH)4 and MnO2 synthesis was carried by a hydrothermal reaction using potassium permanganate (KMnO4) and organic monomer aniline (C6H5NH2) at various temperatures and different times without addition of any extra surfactant or template. 12.6 g of KMnO4 was dissolved in 80 mL of deionized water, and then 2 mL of colourless aniline solution was slowly dropped into the reaction mixture. After stirring for 10 min, the solution was loaded into a stainless-steel autoclave and held at various optimized temperatures and times. The asprepared products were filtered, washed with double deionized water, and dried at 80°C for 12 hours. The synthesized samples were named as, A1 (6 hours, 80°C,), A2 (8 hours,120°C), A3 (16 hours,160 °C) and A4 (24 hours,160 °C). 2.3. Preparation of Activated Carbon: The AC was prepared from the Prosopis juliflora wood by KOH activation. The wood was washed with double deionized water and then pre-heated at 200oC for 24 hours to remove moisture and by-products. Then, 20g of pre-heated sample was mixed with KOH (weight ratio 1:1). The dried wood was carbonized from room temperature to 800oC at a heating rate of 2°C min−1 for 4 hours under N2 atmosphere in a tubular furnace. The carbonized product was washed several times with HCl solution and double deionized water and then dried at 100oC for 2h. 2.4. Material characterization: The crystal structure of the material was studied by powder X-ray diffraction (XRD; Shimadzu XRD-6000, Cu Kα, λ =1.5406 Å) at a scanning rate of 2ᴏ min-1. Fourier Transform 5

Infrared Spectroscopy (FTIR) was recorded in the range from 400 to 4000 cm−1 with a resolution of 2 cm−1. The surface area, pore size, and pore distribution of the ACs were determined using Brunauer− Emmett−Teller (BET) method. The structure and morphology of AC and MnOx were investigated by field emission scanning electron microscope (FESEM Zeiss SUPRA 25) and high-resolution transmission electron microscope (HRTEM, JEOL TEM, 2100F FEG) techniques. The energy dispersive X-ray analysis (EDX) was used to analyse the composition of elements presents in the synthesized materials. XPS measurements were performed with a Thermo Scientific Escalab 250 system using monochromatic Al Ka (1486.6 eV) radiation. Raman spectra were obtained using XperRam 200 (Nano Base, Korea). 2.5 Electrochemical measurements: Electrochemical experiments were carried in both three and two electrode systems. The working electrodes were fabricated by mixing the as-prepared MnOx powders or AC (85 wt.%), acetylene black (10 wt. %) and polytetrafluorene ethylene (PTFE, 5 wt.%) and grounded in a mortar. A small amount of 1-Methyl-N-pyrodilline was added to the mixture to produce a homogeneous paste. The paste was coated (4 mg) on the pre-treated Ni foam current collectors (1 cm x 1 cm) and dried at 100°C for 2 hours. All electrochemical measurements including cyclic voltammetry (CV), galvanostatic charge discharge (GCD), and cycling performance were conducted on an electrochemical workstation (Biologic SP150). Three-electrode electrochemical measurements were carried in a conventional three electrode cell consisting of Pt wire and saturated calomel electrode used as the counter and reference electrodes, respectively. Finally, we assembled both the symmetric and ASC using the Mn(OH)4 as the positive electrode and AC as the negative electrode in 3M Na2SO4 aqueous solution. The detailed electrochemical calculations are given in the supporting information. 6

3. Results and discussion 3.1 Positive electrode materials: 3.1 XRD and FTIR analysis: The crystalline phase of MnOx nanorods measured by XRD is shown in Figure 1. The as synthesized nanostructure (A1) have a single phase of Mn(OH)4 and tetragonal crystal structure (JCPDS:00-015-0604) with a predominant orientation along the (211) crystal plane. Sample A2 showed a phase transition from Mn(OH)4 to MnO2 (JCPDS 01- 072-1982) with a predominant preferential orientation along the (110) crystal plane. The formation of the pure MnO2 phase occurred after an extended hydrothermal reaction time of 16 hours at 160°C (A3) in which all diffraction peaks can be exclusively indexed to tetragonal MnO2 (JCPDS 01-072-1982) with lattice constants of a = 9.8150 Å, b = 9.8150 Å and c = 2.68470 Å. The crystallinity was increased with increasing the hydrothermal dwell time of 24 h at 160°C as shown in Figure 1 (A4). No other characteristic peaks were observed, demonstrating high crystallinity of the sample. The XRD results reveal that the hydrothermal temperature and time has a

significant

influence on crystal structure of the MnOX nanorods. The interaction between Mn and O-H bond in the Mn(OH)4 and MnO2 nanorods studied by the FTIR spectrum is shown in (Figure S1, Supporting information). The absorption peaks observed at low-frequency region around 600700 cm-1 due to stretching bonds of Mn-O and O-Mn-O of Mn(OH)4 and MnO2, confirmed the presence of the MnO2 in the sample [33,35]. In addition, the broad band around 3400 to 3500 cm-1 resulting from O-H stretching vibration of H-O-H and the broad band around 1500-1600 cm-1 shows the bending vibration of water molecules existing in the MnOx nanostructures [13,18,42,43]. The existence of intramolecular bound lattice water (H-O-H) is beneficial for the electroactive species intercalation/deintercalation in the MnOx nanostructures [13,18,43] 7

3.2 Morphological study: The growth morphologies of the MnOX nanorods analysed by FESEM are shown in Figure 2. All four samples (both Mn(OH)4 and MnO2 nanorods) show 1D non-uniform nanorods of sizes varying from 100 nm to 600 nm. The nanorods aggregated continuously until it forms denser MnO2 nanorods from 8 to 16 hours and temperature from 120°C to 160°C (Figure 2a-c). Further increasing the hydrothermal reaction time from 16 to 24 hours, these non-uniform nanorods and nanoclusters were completely converted into compactly connected nanorods with uniform diameters and lengths as shown in Fig 2d. The MnOX nanostructures were further analysed by HRTEM and EDX analysis. The HRTEM images shown in Figure 2(e-h)) confirms the formation of 1D nanorods as observed in FESEM. The lengths of the nanorods are apparently different with the processing temperature and time. At the initial stage of the reaction, mixture of Mn(OH)4 1D nanorods (length: 434 nm, diameter: 52 nm) and some irregular nanostructures were observed (Figure 2e). While increasing the hydrothermal temperature and time, MnOx nanostructures aggregated to form irregular 1D nanorods (Figure 2f) at different directions. The average diameter of the individual nanorods was about 20 nm with a length of about 300 nm. The diameter of the nanorods decreased to 12 nm and the length of the nanorod increased to 600 nm at 16 h, 160°C as shown in Figure 2g. As the reaction time increased to 24h (figure 2h), a perfectly smooth 1D MnO2 nanorods (diameter: 5-10 nm, length 50-100 nm) with clear edges were formed. Growth evolution of 1D manganese oxide Nanorods: The formation of nanorods proceeds via three main factors such as initial nuclei stage, crystal growth process and evolution of morphology. The initial nucleation would have been initiated by forming tiny crystalline nuclei of Mn(OH)4 and MnO2 (Mn4+) from precursor 8

KMnO4 in which the K+ ions might act not only as an initiator but also as a growth directing agent [18,24,30,44]. As the reaction goes forward, the aggregated crystal sites start growing at a faster rates driven by Ostwald ripening process [24,31,32]. The Mn(OH)4 and MnO2 nanoclusters merged to form rod like shaped nanostructures at higher temperature. Since hydrothermal medium is thermodynamically stable, the formation of highly crystalline nanorods is much easier. At higher hydrothermal temperature (>160°C), more MnO2 nanoparticles aggregated to form nanorods and so the nanorods grow longer. Composition of the MnOx nanorods analysed by EDX is given in Table TS1 (Supporting information). All the samples show the existence of manganese, oxygen and potassium ions. 3.3 XPS Analysis: XPS provides the chemical state and composition of the samples at the surface. Figure 3a shows the XPS spectra of K 2p peaks around 296.4 to 297.9 eV (K 2p1/2) and 292.9 to 294.1 eV (K 2p 3/2) [24,45]. This result evidenced the presence of K+ ions in Mn(OH)4 and MnO2 nanorods are denoted as K+-Mn(OH)4 and K+-MnO2. The atomic percentages of K+ ions were decreased with increasing the hydrothermal time and temperature due to the conversion of KMnO4 (Mn7+) into manganese hydroxide/oxide nanorods and is given in table TS2 (Supporting information). The fitted XPS spectra of Mn 2p are shown in the Figure 3b. The high-resolution Mn 2p peaks were fitted with three constraints such as equal full width and half maximum for the same oxidation states, the peak area ratio of 2p3/2:2p1/2 is 2:1 and spin orbit splitting of 11.2 eV. The two strong peaks at 641.1 eV and 642.18 eV of (Mn2p3/2) in Mn(OH)4 nanorods (Figure A1) can be attributed to MnOOH (Mn3+) and Mn(OH)4 (Mn4+), respectively. [46,47] where as the binding energies found at 641.2 and 642.59 eV in A2 show a mixed oxidation states between Mn3+and Mn4+ [46–48]. It was found that increasing the temperature to 160°C and the growth time to 16 9

hours (Figure A3), the Mn2p3/2 show Mn3+ at 641.4 eV and another shoulder found at 642.8 eV was assigned to Mn4+ [45,46]. Further increasing the growth time to 24 hours, the Mn 2p3/2 spectra showed a major shift towards higher binding energies of 642.7 and 644.5 eV corresponding to

Mn4+ and

Mn7+, respectively [45–47]. It clearly demonstrates that the

oxidation state of Mn was increased with temperature and time. 3.4 Negative electrode materials Figure 4a shows the XRD pattern of AC with two broad peaks at 25 and 43°, corresponding to (002) and (100) diffraction planes respectively, revealing the existence of graphitic carbon. Raman spectrum (inset in Figure 4a) shows the G band (1584 cm −1) due to the graphitic layers of AC while D band (≈1350 cm

−1)

corresponds to defective carbon [49,50].

Generally, the relative peak intensities of D and G bands (ID/IG) reveals the defective nature of carbon. The ID/IG value of 0.91 indicates that the as synthesized AC have high degree of graphitization as well as with carbon defects [51,52]. The high resolution C 1s spectrum was deconvoluted into four peaks, corresponds to the C-C bonds (284.5 eV), C–O bonds (285.7 eV), C=O bonds (286.7 eV) and O–C=O bonds (288.5 eV) [51]. The O 1s spectrum was fitted with three peaks located at 531.3 eV, 532.4 eV, and 533.7 eV, which are attributed to C=O-N, C-O–C and C–O, respectively [49]. Figure 4d shows the porosities of the AC analyzed by N2 sorption isotherm and is identified as type IV according to the IUPAC classification and mesoporous nature was confirmed by the pore size distribution (inset Figure 4d) along with little macropores [50,53]. The BET surface area, average pore volume and average pore size were found to be 1632 m2 g-1, 1.81 cm3g-1 and 4.45 nm respectively. FESEM (Figure 4e) image clearly shows that the carbon is chemically activated and structurally opened to have rough unsmooth sheet like surface structure due to the KOH chemical activation process. HRTEM (Figure 4f) images 10

expose the amorphous porous carbon in the center with some partially aligned graphitic layers on the edges of the nanosheets. 3.5 Electrochemical Studies The initial electrochemical performance of potassium doped K+-Mn(OH)4 and K+-MnO2 nanorods was evaluated by a conventional three-electrode system in an aqueous 3 M Na2SO4 electrolyte solution, and the results are shown in Figure 5. Figure 5a shows the comparison of CV curves in the potential range of 0 to 1 V and at a scan rate of 10 mV/S. All CV curves (both K+-Mn(OH)4 and K+-MnO2 nanorods) show nearly rectangular shape, displays ideal pseudo capacitive behaviour [13,14,29,36]. The area of the CV curve is higher for K+-Mn(OH)4 nanorods (A1) due to the existence more H-O-H bonds in the nanorod, which can improve the high ionic conductivity and reversible polarity due to high dipole moment [13,23,37]. Figure 5b shows the CV curves of K+-Mn(OH)4 nanorods at different scan rates from 2 to 200 mVs–1. The electrode exhibits high specific capacitance and ideal rectangular behaviour at lower scan rates, since the cations could access outer surface area as well as diffuse from electrolyte into electrode active material. At higher scan rates, specific capacitance decreases and the rectangular shape is deviated to quasi rectangular due to the limited access of electrolyte cations on the electrode surface [13,37,54]. However, the rectangular shape of the CV curve is maintained even at high scan rates (up to 100 mVs-1) indicating the excellent electrochemical reversibility and high-rate performance [16,44]. The GCD curves of K+-Mn(OH)4 and K+-MnO2 nanorods (Figure 5C) at a current density of 0.2 Ag-1 shows triangular pattern confirming the linear charge/discharge slopes with negligible IR drop, revealing the pseudocapacitance nature of the electrode [37,44,55]. The discharging time for the K+-Mn(OH)4 nanorods is much larger than that of other K+-MnO2 nanorods, demonstrating their higher charge retention capacity agrees with their CV curves 11

[13,23,36,44]. The specific capacitances of the all electrodes were determined from GCD @ 0.2 Ag-1 current density over the identical potential window of CV. The specific capacitance values of A1, A2, A3 and A4 electrodes were 319, 248, 281 and 309 Fg−1, respectively (See Figure S2 Supporting Information). The origin of the electrode rate kinetics was understood from the electrochemical impedance spectroscopy (EIS) carried out at frequency from 10 kHz to 0.01 Hz with an alternating current amplitude of 10 mV. Figure 5d, shows the Nyquist plot for the A1, A2, A3 and A4 electrodes. All the EIS spectra exhibited a semicircle in the high-frequency range and a quasi-vertical line in the low-frequency range. The high frequency intercepts on the real axis is series resistance (Rs) and the semicircle stands for the charge transfer resistance at the interface (RCT) between electrode and electrolyte [31,36,37]. All the electrodes show considerably low Rs < 6 Ω and hence rapid ion transport occurs within the active electrode material and current collector because of higher conductivity between the electrode material and electrolyte [29,56,57].

Among the electrodes, small RCT of 2.11 Ω was observed for A1

indicating the good ionic and electronic conductive nature of the electrode. All the electrodes exhibit vertical line at low frequency region and makes an angle of 60° to 80° > 45° (Warburg angle) with the real axis indicating better electrolyte diffusion-controlled process on the surface of nanorods, which further confirms the typical capacitive behaviour [28,29,37,56]. These electrochemical results demonstrate that both K+-Mn(OH)4 and K+-MnO2 nanorods have enhanced pseudocapacitive nature without much reduction in specific capacitance.

The

physiochemical and electrochemical results show that K+-Mn(OH)4 nanorods have lowest RCT due to the polar H-O-H bonds and the high concentration of K+ ions that can provide the efficient transport of electrons and ions, leading to the high electrochemical capacity of the electrode material [13,18,37,58].

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To evaluate the electrochemical performance of the AC (negative electrode material), the CV, GCD and EIS were carried out by a three-electrode system in Na2SO4 aqueous electrolyte. Figure 5e displays the CV curves of AC at 5 mVS-1 exhibits a near rectangular shape (without any redox peaks) demonstrating ideal EDLC behaviour [49,52]. The CV curves at different scan rates (Figure 5f) exhibited a quasi-rectangular shape at higher scan rates. It is suggested that AC electrode has excellent rate capability due to the synergetic effect of high specific surface area and the porous carbon nanosheets generate a rapid diffusion pathway and fast ion transportation [38,51]. The GCD curves (Figure 5g) performed at various current densities (0.2 to 2 Ag− 1) show symmetric triangular shape curves without IR drop. The specific capacitance of the AC was found to be 198 Fg-1 at a current density of 0.2 Ag-1. EIS (Figure 5h) curve comprised a semicircle in the high frequency region and a straight line in the low frequency region. The AC electrode exhibited low intrinsic resistance (Rs=0.43Ω) and smaller charge transfer resistance (Rct), indicating high conductivity and good electron transfer rates. In low frequency region, almost a vertical straight line (Warburg angle) with a slope close to 90° is related to the ion diffusion/transport process between electrode/electrolyte[38,50]. 3.6. Electrochemical performance of symmetric supercapacitor device Symmetric devices were made to check the viability of commercial applications for the best performing K+-Mn(OH)4 nanorods. Figure S3 a and b (supporting information) displays the CV curves at 5 mVs-1 and at different scan rates up to 200 mV s-1 respectively. Noticeably, at low scan rate (5 mV s-1), the shape of curve exhibits perfect rectangular and pseudo-constant rate over the entire voltammetric cycle. However, The CV profiles deviates from rectangular shape with increasing the scan rate and displays perfect PC behaviour[18,56]. Figure S3 c and d (supporting information) shows the GCD curves and the Nyquist plot respectively. The GCD 13

curves exhibit the nearly linear charge/discharge profile at current densities from 0.2 A g−1 to 2 Ag-1 and without any IR drop, suggesting excellent electrochemical capacitive characteristics and reversible Faradaic reaction between Na+ and 1D K+-Mn(OH)4[18,59]. The Nyquist plot of K+Mn(OH)4 symmetric electrode having a semicircle in the high-frequency region followed by a line at the low-frequency region. The linear region of the plot exhibits an angle between 45° and 90° relative to the real axis, indicating that the electrode process is not perfectly pseudocapacitive in nature but has diffusion nature [36,56,57]. 3.7 Electrochemical performance of asymmetric AC||K+-Mn(OH)4 supercapacitor ASC has an advantage of wide operating voltage and high energy density. Hence, we assembled an ASC using the K+-Mn(OH)4 nanorods as the positive electrode and AC as the negative electrode with a cell voltage extending up to 1.6 V in an aqueous electrolyte. The AC electrode was measured in the potential window of −1.0 – 0.0 V, while the Mn(OH)4 electrode was scanned in the range of 0.0–1.0 V at a scan rate of 5 mV s−1 as shown in Figure 6a. Both CV curves exhibited excellent reversibility over the potential range. Figure 6b shows the symmetrical CV curve of the ASC electrode at a scan rate of 5 mV s−1 indicating the excellent capacitive behaviour [57,59,60]. Figure 6c shows the CV curves of the ASC device at different scan rates and is observed a nearly semi rectangular shape CV curves even at a high scan rate of 100 mV/s, demonstrating that the ASC exhibits fast electron transfer and excellent capacitive performance. Figure 6d shows a good linear variation and symmetrical charging/discharging curve at various current densities, clearly confirms the admirable electrochemical reversibility [23,59,60]. We calculated and compared the specific capacitances of the symmetric and ASC from the GCD curve (Figure.6e). The specific capacitance of ASC device was found to be 399 Fg-1 at a current density of 0.4 Ag-1 which is almost double than that of symmetric SC (216 Fg-1). 14

Figure 6f displays the Ragone plot of symmetric Mn(OH)4 and asymmetric AC||Mn(OH)4 supercapacitor calculated from GCD curves. The symmetric device exhibited an energy density of 10.11 Wh Kg-1at a power density of 51.04 W kg -1 whereas ASC device delivered high energy density of 41.38 Wh Kg-1 at power density of 143.3W kg-1. The ASC device showed high energy and power densities than the symmetric device due to the improved operating potential window of ASC device. The long-term cycling stability of both the symmetric and ASC device was tested by constant GCD method between 0 and 1.0 V at an applied current density of 2 A g−1 for 6000 cycles to check the suitability of its practical applications. Figure 7a and b shows the capacitance retention of asymmetric and symmetric device with an operating window of 1.6 V for 6000 cycles. Both the asymmetric and symmetric devices exhibited capacitance retention of 107.1 and 103.3 %, respectively due to the electrode activation by continuous charging and discharging indicating a prominent improvement in cyclic stability [63]. The ASC shows (inset of Figure 7a) enhanced discharge time after 6000 cycles and the remarkable enhancement of the performance of AC||K+Mn(OH)4 can be attributed to the synergetic effects between the pseudocapacitance Mn(OH)4 nanorods and the EDLC of AC [61,62]. The EIS studies shown in Figure 7 c and d for the asymmetric and symmetric devices exhibited a slight decrease in Rs and Rct for the 6000 cycled samples due to improved Warburg impedance (Zw) values. This confirms the higher electrical conductivity and enhanced electrochemical activity of the devices. The superior capacitance retention of both symmetric K+-Mn(OH)4 and AC||K+-Mn(OH)4 ASC increased over 100 % is due to the improved surface wetting (electrolyte penetration into the tunnel structures) at the electrode/electrolyte interface over the time period, called electro activation process [62,64]. During the electro activation process, the electrolyte ion intercalated at the electrode surface, 15

thereby improving the electrochemically exposed surface area and hence enhanced capacitance was observed. Moreover, it was observed that the conductivity (Figure 7 C and D) has been increased over the extended cyclic process due to the electro activation process[62,65].

Conclusion A facile one pot hydrothermal synthesis has been developed for the synthesis of manganese oxyhydroxide and manganese oxide nanorods by reducing potassium permanganate. We have concluded from the physiochemical and electrochemical studies that the doped cation (K+) act as stabilizer for the tunnels, growth indicator for 1D nanorods and enhance the electrochemical activity of K+-MnOx nanorods. The assembled AC||Mn(OH)4 nanorods shows an outstanding cyclic stability between 0 and 1.6 V and displays a high energy density of 34.68 Wh Kg-1 at a power density of 113.77 W Kg-1. The ability of the both asymmetric and symmetric supercapacitor electrode preserved ~100% capacitive performance (6,000 cycles) reflects its excellent long‐term stability and proved the efficiency of cation (K+) doped into the Mn(OH)4 tunnel. Thus, this work provides a simple way to synthesize supercapacitor electrodes to overcome the drawbacks of short cycle-life for both symmetric and asymmetric supercapacitors made of manganese hydroxide nanorods.

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ASSOCIATED CONTENT: Supporting Information: The Figures S1, S2, and S3 shows the FTIR, Specific capacitance comparison and Symmetric Supercapacitor device of MnOx nanorods as stated in paper (PDF). This information is available via the Internet at free of charge.

 AUTHOR INFORMATION: Email: [email protected] ACKNOWLEDGEMENTS This research was supported by Basic Research Laboratory through the National Research Foundations of Korea funded by the Ministry of Science, ICT and Future Planning (NRF2015R1A4A1041584).

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FIGURE 1:

Figure. 1. XRD patterns of the MnOX nanorods (A1) 80 °C for 6 h, (A2) 120 °C for 8h, (A3) 160 °C for 16 h and (A4) 160°C for 24 h.

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FIGURE 2:

Figure 2: FESEM and HRTEM images of MnOX nanorods prepared at various hydrothermal temperatures (a and e) 80 °C for 6 h (b and f) 120 °C for 8h (c and g) 160 °C for 16 h (d and h) and 160°C for 24 h.

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FIGURE 3:

Figure 3 (a) XPS core level K 2P Spectra of MnOX nanorods and (b) Core level XPS spectra of Mn in manganese oxide Nanorods (A1) 80 °C for 6 h (A2) 120 °C for 8h (A3) 160 °C for 16 h (A4) and 160°C for 24 h.

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FIGURE 4:

Fig. 4. (a) XRD pattern and inset Raman Spectra, XPS spectra of (b) C 1s, (c) O1s, (d) N2 adsorption-desorption isotherm and BJH pore-size distribution curve (inset), (e) SEM image and (f) HRTEM image of Activated carbon nanosheets.

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FIGURE 5:

Figure 5: (a) Cyclic Voltammograms of MnOX nanorods @ 10mVs-1 (b) Mn(OH)4 nanorods at various scan rates (c) GCD @0.2 Ag-1 (d) Nyquist plot (Inset: High frequency region) and (e) CV curves of AC at 5 mVs-1 , (f) CV curves at different scan rates and (g) GCD curves at various current densities of the AC nanosheets (h) E.I.S Spectra of AC

22

FIGURE 6:

Figure 6: (a)CV of asymmetric of Mn(OH)4 nanorods and AC electrodes at 5 mVs-1, (b) CV curves of at 5 mVs-1 between 0 and 1.6 V , (c) CV curves at different potential ranges (d) GCD curves at different current densities .(e) Specific Capacitance Vs Current density and (f) Ragone plot for symmetric Mn(OH)4 and Asymmetric AC//Mn(OH)4 supercapacitor

23

FIGURE 7:

Figure 7: (a) cyclic Stability of (AC)//Mn(OH)4 asymmetric supercapacitor electrode at 2 A g-1(Inset: 1st and 6000th GCD Curves) (b) cyclic stability of symmetric of Mn(OH)4 electrode at 2 Ag-1(Inset:1st and 6000th Cycle GCD Curves) (c) and (d) Electrochemical impedance spectra of asymmetric and symmetric supercapacitors before and after cyclic stability

24

Scheme 1. Schematic Design of the Fabrication Process of an ASC Based on Activated carbon as the Negative electrode and K+-Mn(OH)4 as the positive electrode.

25

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Credit Author Statement

Aravindha Raja Selvaraj : Conceptualization, Formal analysis, Writing - Original Draft Hee-Je Kim: Resources Karuppanan Senthil: Writing - Review & Editing Kandasamy Prabakar :Project administration, Supervision

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.

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

36

Graphical abstract

37

HIGHLIGHTS  Pre-intercalated K+ Manganese hydroxy Mn(OH)4 and oxide (MnO2) nanorods synthesized by one step hydrothermal method.  Activated carbon nanosheets (A.C.N) with high specific surface area are made from Prosopis Juliflora biomas.

 Preinserted K+ ions maintain the structural stability during charge discharge process as well as improve the Pseudocapacitive performance.  Both the asymmetric and symmetric electrochemical capacitors retain 100% retention even after 6,000 cycles.

38