Morphological evolution and electrochemical cycling for enhanced electrochemical activity of MnCo-layered double hydroxide

Journal Pre-proof Morphological evolution and electrochemical cycling for enhanced electrochemical activity of MnCo-layered double hydroxide Dipali S. Patil, Sachin A. Pawar, Jongwon Ryu, Jae Cheol Shin, Hyo Jin Kim PII:

S0013-4686(19)32250-9

DOI:

https://doi.org/10.1016/j.electacta.2019.135378

Reference:

EA 135378

To appear in:

Electrochimica Acta

Received Date: 3 June 2019 Revised Date:

5 November 2019

Accepted Date: 25 November 2019

Please cite this article as: D.S. Patil, S.A. Pawar, J. Ryu, J.C. Shin, H.J. Kim, Morphological evolution and electrochemical cycling for enhanced electrochemical activity of MnCo-layered double hydroxide, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135378. 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.

Date: 11/27/2019

Graphical abstract

Schematic illustrations of the stepwise growth process of MnCo hydroxide structures controlled by the hydrothermal reaction time. Nucleation and growth (I), aggregation and coalescence (II), crystal growth; at this stage the crystal consist of three different faces such as flat faces (F), stepped (or ledge) faces (S) and kinked faces (K) (III), further growth proceeds through terrace-ledge-kink growth model (IV,V and VI)

Morphological evolution and electrochemical cycling for enhanced electrochemical activity of MnCo-layered double hydroxide Dipali S. Patila, Sachin A. Pawara*, Jongwon Ryua, Jae Cheol Shina*, Hyo Jin Kimb* a

Department of Physics, Yeungnam University, Gyeongsan, Gyeongbuk 38541, South Korea. b Korea Photonics Technology Institute, Buk-gu, Gwangju 61007, South Korea. *Corresponding Author Emails: [email protected], [email protected], [email protected] Abstract

We report a stepwise morphological evolution of MnCo-layered double hydroxide (LDH) cubes with the hydrothermal reaction time. The electrochemical performance of the fabricated electrode at each step was studied in detail. The obtained MnCo-LDH exhibited maximum areal capacities of 1.58 Ccm-2 (at a scan rate of 5 mVs-1) and 1.89 Ccm-2 (at an applied current of 3 mA), good rate capability (74% at a scan rate of 100 mVs-1), and excellent energy efficiency (80% at an applied current of 4.5 mA). The electrochemical performances of all synthesized electrodes were enhanced during the electrochemical cycling, which led to morphological modifications. In addition, for practical applications, we fabricated an asymmetric cell based on the MnCo-LDH and activated carbon electrode and studied its electrochemical performance in detail.

Keywords: MnCo, Layered double hydroxide, electrochemical cycling, asymmetric cell, hydrothermal.

1

1. Introduction

Layered double hydroxides (LDHs) attract considerable attention for use in supercapacitors owing to their unique features such as good anion exchange abilities, high redox activities, easily tunable chemical compositions, high chemical stabilities, and large surface areas.1–5 Among the LDHs, MnCo-LDH, CoAl-LDH, and NiMn-LDH are very promising electrode materials for supercapacitors as a class of two-dimensional metal hydroxides.6–11 MnCo-LDH is the most desirable electrode material for energy storage applications in the LDH class, as cobalt can be introduced in the LDH to improve the conductivity of the electrode material; Co2+ can be oxidized to the conductive CoOOH during the electrochemical reactions. Similarly, the introduction of manganese helps improve the

electrochemical

activity

owing

to

its

multiple

oxidation

states.12,13

Supercapacitors outperform batteries in terms of power density and cycling stability. Therefore, supercapacitors are required in various applications such as hybrid electric vehicles and portable electronics. However, the lower energy densities of supercapacitors hinder their applications in energy storage devices. Regarding the supercapacitors, the pseudocapacitors where the energy is stored by Faradaic reactions attributed to the efficient redox reactions are superior and exhibit higher energy storage abilities than those of electrical double-layer capacitors, which are composed mainly of carbon materials.14–17 It is imperative to develop electrode materials that can match the energy densities of the batteries by architectural engineering for supercapacitors. MnCo-LDH is a promising electrode 2

material for the energy storage technology to replace the commercial carbon-based supercapacitor by delivering a high energy density. However, the LDHs exhibit lower capacities than the predicted theoretical values owing to their low conductivities and sluggish ion transfer kinetics.6 Different routes have been employed to synthesize the MnCo-LDH as an electrode in a supercapacitor such as electrodeposition, chemical bath deposition, sol–gel method, and hydrothermal reaction.18,19 The main approach is to form composite structures of LDH/transition metal oxides to achieve superior capacitances. Gao et al.18 studied a NiO-bridged MnCo-LDH for a flexible high-performance fibershaped energy storage device. An enhancement of 210% in areal capacitance compared to that of the pristine MnCo-LDH was achieved; however, they did not consider the effect of the morphological evolution on the performance. Zhou et al.20 studied a metal–organic framework template-directed MnCo2O4@Co3O4 for energy storage using MnCo-LDH nanoneedles and ZIF-67 rhombic dodecahedral, which achieved a high capacity of 1440 Ccm-2 in a battery-type supercapacitor. However, they did not carry out performance and cyclic stability analyses of the pristine MnCo-LDH. Their ternary metal oxide and its composites including MnCo2O4 and MnCo2O4@Co3O4 exhibited low cycling performances. Various studies on MnCo2O4 and its composites for energy storage have been reported. However, no extensive studies on the use of the pristine MnCo-LDH or its composites for energy storage applications have been reported.21,22 Wu et al. investigated hierarchical hollow cages of MnCo-LDH for use in supercapacitors.21 3

However, they studied the cycling stability only up to 2000 cycles and achieved a capacitance retention of ~92%. Liu et al. achieved a very high specific capacitance of 2320 Fg-1 using a composite MnCo-LDH@Ni(OH)2 core–shell heterostructure.6 In order to achieve a superior energy storage performance, it is important to better understand the energy storage mechanism attributed to the material shape and nanostructure. In addition, it is crucial to assess the supercapacitor performance through electrochemical cycling. In this regard, Chen et al. investigated the morphological evolution of MnO2 nanostructures during electrochemical cycling.23 It is worth noting that no studies on stepwise evolution of the MnCo-LDH nanostructure and its electrochemical performance after cycling have been reported. The effect of the electrochemical cycling on the pristine MnCo-LDH providing an improved supercapacitor performance was ignored. In this study, we systematically investigated the stepwise growth of nanostructured MnCo LDH and their electrochemical performances for energy storage. We prepared different MnCo LDH nanostructures with well-controlled morphologies using a simple hydrothermal method by varying the reaction time. The electrochemical performances of the MnCo-LDH nanostructures were studied in detail. Moreover, we discuss the morphological evolution during the electrochemical cycling, which enhanced the electrochemical activity. 2. Experimental Details Different nanostructures of MnCo-LDH were grown on Ni foams by a simple hydrothermal route. In a typical process, 0.01 M of manganese sulphate 4

(MnSO4), 0.01 M of cobalt nitrate (CoNO3 (6H2O)), and 0.2 M of urea were dissolved in 60 mL of distilled water under constant stirring. The obtained homogeneous solution was then transferred to an 80-mL Teflon-lined stainlesssteel autoclave. A piece of cleaned Ni foam was immersed in the solution by inclining it against the wall of the Teflon liner. The autoclave was then sealed and placed in an oven at 140 °C for 8 h. After cooling to room temperature, the Ni foam coated with the product was removed from the autoclave, thoroughly rinsed using distilled water, and kept at room temperature for air-drying. The obtained electrode is denoted as MnCo-8. The hydrothermal reaction time was varied (12, 16, 24, 32, and 40 h) while maintaining all other parameters. These electrodes are denoted as MnCo-12, MnCo -16, MnCo-24, MnCo-32, and

MnCo-40,

respectively. 2.1 Characterizations of electrodes X-ray diffraction (XRD; PANalytical) was carried out using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS; K-alpha, Thermo Scientific, UK) was performed to determine the surface chemical composition. The surface morphology of the film was analyzed by field-emission scanning electron microscopy (FE-SEM; S-4800 HITACHI, Ltd., Japan). Surface morphology analyses were carried out by high-resolution transmission electron microscopy (HRTEM; Tecnai F21, FEI Company). Electrochemical measurements, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) measurements, were performed in a 3-M 5

KOH electrolyte in a conventional three-electrode arrangement consisting of a graphite counter electrode and saturated calomel electrode as the reference electrode using a ZIVE SP5 electrochemical workstation (WonAtech). For each electrode, the area of the deposited MnCo-LDH was constant (1 cm × 1 cm). For the analysis of the device performances, an asymmetric device was fabricated using MnCo-32 as a positive electrode and activated carbon (AC) as a negative electrode. The AC electrode consist of activated carbon, carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 80:10:10. The obtained slurry was coated onto pre-cleaned Ni foam followed by drying at 75˚ C for 12 h. The device was assembled by sandwiching the soaked filter paper in a 3-M KOH between the two electrodes and sealed using a parafilm. 3. Results and discussion To study the stepwise growth of the MnCo-LDH, the hydrothermal reaction time was varied (8, 12, 16, 24, 32, and 40 h). Figs. 1A and 1B show the surface morphologies of the MnCo-LDH cubes with the increase in the reaction time. Complete formations of MnCo-LDH cubes were achieved at reaction times of 32 and 40 h, as shown in Figs. 1B (f) and (j), respectively. After 8 h, nucleation and growth of granular nanoparticles (Figs. 1A (a–d)) are observed. With the increase in the reaction time to 12 h (Figs. 1A (e–h)), the granular nanoparticles accumulated together forming the base of the cube. With the further increase in the reaction time (up to 16 h), they turned into asymmetric cubes, consisting of stairs having different sizes (500 nm to 1 µm) (Figs. 1A (i–l)). The surface of the stairs 6

comprises many small steps. With the further increase in the reaction time, the small steps on the stair surface are filled and a smooth surface with a uniform size (~500 nm) is formed (Figs. 1B (a–d)). At the reaction time of 32 h, the curvy edges of the cube are completely filled and the formation of the whole MnCo-LDH cube is observed (Fig. 1B (f)). Each face of the cube includes the steps with decreasing sizes from top (400 nm) to bottom (1.2 µm). With the further increase in the reaction time to 40 h, the steps disappeared from each face of the cube and the absolute formation of the cube with a rough surface was observed (Figs. 1B (i–l)). The results suggest that the growth process proceeds as construction of building blocks (one block on top of another block). Recently, Li et al. reported the growth process of layered stacked MnCo2O4 cubes using ethylene glycol and acetate sources of Mn and Co.24 They initially observed the formation of nanostrips, which then shrank into nanoparticles, and finally the accumulation of nanoparticles led to the formation of the layers of the stacked cubes. It is well known that the nanostructure formation strongly depends on the metal salt precursors.25 The binding affinity of the metal–ligand directly influences the dissociation rate of the metal ions, which leads to different growth processes of nanostructures.26 The basicity of (CH3COOH) is smaller than those of 2+  SO and Co2+ ions, which leads to  and NO , providing the quick release of Mn

the faster growth of MnCo2O4 cubes, skipping the intermediate growth steps. In our case, the formation of nanoparticles, accumulation of the nanoparticles forming the base of the cube, and stepwise growth of the MnCo-LDH cube are 7

 observed. Owing to the relatively high basicities of the SO  and NO (compared

to that of CH3COOH) ligands, which release Mn2+ and Co2+ ions at a relatively low rate, we can achieve the controlled and stepwise growth of MnCo-LDH cubes. To develop the preferred morphology for a particular purpose, it is important to understand the growth process of the MnCo-LDH structure. Fig. 2 shows the schematic illustrations of stepwise growth of MnCo-LDH cubes with the increase in the hydrothermal reaction time. In the first step (Fig. 2 (I)), the particles are deposited on the surface, adsorb, and then diffuse around the surface and can be bound to the surface. This leads to the nucleation and growth of granular particles, which act as seeds for the further growth. The epitaxy growth proceeds following the aggregation and coalescence of the particles (Fig. (II)).27 The next step of the crystallization is the crystal growth (Fig. 2 (III)). At this stage, the crystal consists of three different faces, flat (F), stepped (or ledge) (S), and kinked (K) faces.28,29 Therefore, after the aggregation of the particles, the MnCo hydroxide growth triggers the terrace–ledge–kink growth of crystalline surfaces. In general, the growth process of the deposition starts from the kink or ledge surface of the crystal.30,31 Likewise, in this study, the growth proceeds through the kink and ledge faces and fills up kink sites (Fig. 2 (IV)). With the further increase in the reaction time (from 24 to 32 h), deposition occurs at the ledge and flat faces, which forms the whole cube with steps at the faces (Fig. 2 (V)). Finally, at the reaction time of 40 h, the growth occurs at all ledge faces, leading to the development of the cube with a slight overgrowth (Fig. 2 (VI)). 8

To identify the elemental compositions of the synthesized materials, energydispersive X-ray spectroscopy (EDS) was carried out at an electron energy of 15 keV using FE-SEM. The ED spectra of the MnCo-LDH samples at different hydrothermal reaction times are shown in Figs. S1 (a–f). The results show the existence of Mn and Co in all samples. At the hydrothermal reaction time of 8 h, the atomic ratio of Mn:Co is 7.83:26.11, which is close to 1:3. This implies that the initial Co nanoparticles quickly deposited onto the substrate, compared to Mn, and thus the sample is Co-rich. The atomic concentration of Mn increases with the further increase in the reaction time. At the different periods of reaction, the contents of Mn and Co atoms persistently alter. Only the MnCo-32 sample is Mnrich. The elemental compositions of all synthesized samples at different reaction times are shown in Figs. S1 (a–f). The crystal structures and phase purities of the as-prepared MnCo-LDH nanostructures were characterized by XRD, as presented in Fig. 3a. The three sharp diffraction peaks observed for all samples at 44.55, 51.91, and 76.47˚ can be attributed to the Ni foam (Joint Committee on Powder Diffraction Standards (JCPDS) card no. 1-1266). The observed XRD peaks are well matched with the (003), (006), (012), and (110) reflections of the typical LDH phase.32 In addition, few small peaks are observed, attributed to the precursors used for the synthesis of the materials. XPS was carried out to analyze the valance state of the synthesized material, as shown in Figs. 3(b–d). The peaks at 642.3 and 653.7 eV are attributed to Mn 2p3/2 9

and Mn 2p1/2, respectively, suggesting the presence of Mn3+ in the sample.33 The core-level Co 2p spectrum consists of two main peaks attributed to Co 2p3/2 and Co 2p1/2 (Fig. 3c). Each of these peaks was deconvoluted into two peaks with their satellite line. The deconvolution peaks centered at binding energies of 780.4 and 795.3 eV correspond to Co3+, whereas those at binding energies of 782.4 and 797.1 eV correspond to Co2+.34 For O 1s (Fig. 3d), the peaks centered at 529.6, 531.0, and 532.1 eV are associated with Co–O, hydroxyl groups (OH-), and surface contamination, respectively.35 The interior of the MnCo-LDH cube was characterized by TEM and HRTEM. The low-magnification TEM images in Figs. 4a and 4b demonstrate the formation of MnCo-LDH cubes. The lattice fringes in Fig. 4c have an interplanar distance (d) of 0.25 nm, which could be assigned to the (110) planes of MnCo-LDH. This further confirms the formation of crystalline MnCo-LDH cubes, which is consistent with the XRD results. The Manganese, cobalt and oxygen distribution on MnCo-LDH was evaluated by quantitative EDS elemental mapping. As shown in Fig. 4(e-g) the cyan region in the image (Fig.4e) is the manganese containing regions, magenta region corresponds to cobalt (Fig.4f), whereas the green region belong to the oxygen (Fig.4g) containing portions of MnCo-LDH electrode. To evaluate the electrochemical performances of the synthesized electrodes as active materials for supercapacitors, CV and GCD measurements were carried out in a three-electrode configuration. Fig. 5a shows the CV curves of the MnCo-LDH electrodes with different reaction periods (8, 12, 16, 24, 32, and 40 h) recorded at 10

20 mVs-1. As shown in Fig. 5a, the CV curves for all electrodes exhibit a pair of well-defined redox peaks involving the faradaic charge storage mechanism of the MnCo-LDH. The GCD curves of the MnCo-LDH electrodes with various hydrothermal reaction times (8, 12, 16, 24, 32, and 40 h) are measured at an applied current of 3 mA, as shown in Fig. 5b. The redox peaks in CV and plateaus in CD curves display that the reversible reactions occur in a narrow voltage window. Hence, the concept of capacitance as the derivative of the charge with respect to the potential, stated by B.E Conway is not satisfied.36 Based on this, it is essential to discuss the electrochemical results of the MnCo-LDH in terms of specific capacity because of their battery type behavior.37, 38 Remarkably, the area under the CV curve and discharge time (GCD) of the MnCo-LDH electrode increase with the hydrothermal reaction time reaching the maximum for MnCo-32. Consequently, the MnCo-32 electrode exhibited the highest charge storage properties. CV curves recorded at different scan rates and GCD curves measured at different applied currents for all electrodes are presented in Supporting Information (Figs. S2 and S3). The changes in areal capacities with the scan rate and applied current are presented in Figs. 5c and 5d, respectively. Excellent areal capacities of 1.58 Ccm-2 (at a scan rate of 5 mVs-1) and 1.89 Ccm-2 (at an applied current of 3 mA) were achieved for the MnCo-32 electrode. The capacities of MnCo-32 was 1.17 Ccm-2, even at a high scan rate of 100 mVs-1, with a capacity retention of 74%, whereas that at an applied current of 4.5 mA was 1.56 Ccm-2, with an energy efficiency of 80%. The areal capacities of all synthesized electrodes 11

obtained at a scan rate of 5 mVs-1 and applied current of 3 mA are summarized in Table S1. To evaluate the influences of the cycling on the electrochemical performances of the synthesized electrodes, CV and GCD, of all electrodes were carried out before and after 5000 cycles. Figs. S4 and S5 show the CV and GCD curves at 5 mVs-1 and 3 mA, respectively, for all synthesized electrodes recorded before and after 5000 cycles. It is worth noting that increases in current and discharge time were observed in the CV and GCD curves for all electrodes after 5000 cycles, signifying the increases in electrochemical performances upon the cycling. Capacities increases of approximately 1.5, 12.7, 14.8, 10.8, 8.9, and 14.2% were observed for MnCo-8, MnCo-12, MnCo-16, MnCo-24, MnCo-32, and MnCo-40, respectively, after 5000 cycles at 5 mVs-1, which show the increased electrochemical performances after the cycling. Likewise, according to the GCD measurements, capacities increases of approximately 8.9, 18.6, 22.5, 17.6, 20.4, and 23.2% were observed for MnCo-8, MnCo-12, MnCo-16, MnCo-24, MnCo-32, and MnCo-40, respectively, after 5000 cycles at 3 mA. The electrochemical cycling of the synthesized electrodes provided the enhanced electrochemical performances. To characterize possible phase changes upon the cycling, XRD measurements were carried out (Fig. S6). It is worth noting that no significant difference was observed between the XRD patterns recorded before and after the cycling, demonstrating the absence of phase alteration during the cycling. To further elucidate the origin of the enhanced electrochemical 12

performances of the electrodes after the long cycling, a surface morphology analysis was carried out, as shown in Fig. 6. Remarkable differences in surface morphologies were observed for all electrodes after 5000 cycles. In the electrode grown for 8 h (Figs. 6a and 6b), the granular nanoparticles of MnCo hydroxide turned into hexagonal sheets, whereas in the electrode grown for 12 h (Figs. 6c and 6d), the accumulated nanoparticles converted into the connected network of nanofibers. In the MnCo-16 electrode (Figs. 6e and 6f), each step of the cubes separated out into single sheets. The surface morphology of MnCo-24 remained the same (Fig. 6g) after 5000 cycles, suggesting the tight bonding between two sheets. Fig. 6h shows the formation of tiny nanoparticles onto the cube surface. The complete transformation was observed for the MnCo-32 electrode (Figs. 6i and 6j); the displacements in steps are observed along with the formation of spherical nanoparticles onto the surface. With the increase in the reaction time (MnCo-40) (Fig. 6k), a slight modification occurred in the shape of the cube upon the cycling. At a higher magnification (Fig. 6l), development of voids throughout the cube surface was observed, attributed to the cycling. These morphological modifications are associated with the repeated insertion and extraction of electrolyte ions into and out of the MnCo-LDH during the cycling. The formation of hexagonal sheets and nanofiber network, split-up of sheets, and formation of particles and voids on the surfaces of the cubes facilitate the access of the ions into the electroactive material and thus lead to enhanced electrochemical performances. This is attributed to the improved surface wetting and electrochemical activation of 13

the MnCo-LDH during the intercalation and deintercalation. In addition, the EDS analysis of the MnCo-32 electrode after the 5000 cycles (Fig. S7) indicates the large content of K ions in the sample, confirming the strong interaction between the electrode and KOH electrolyte. For practical applications, the electrochemical performance of the MnCo-32 electrode was evaluated in a two-electrode system by assembling it in an asymmetric cell. As we know the asymmetric cell provides a larger operating potential window than that of symmetric cell.39 Therefore, we have fabricated asymmetric cell using superior MnCo-32 electrode as the positive electrode and activated carbon (AC) as the negative electrode. The mass of the AC as a negative electrode was calculated by following charge balance theory equation such as:



=

 

where    denotes mass loading and specific capacity of

positive (MnCo-32) electrode, respectively. Likewise,    denotes mass loading and specific capacity of negative (AC) electrode, respectively. From the charge balance, for the asymmetric supercapacitor the mass ratio between the positive and negative electrode is 0.42, in which the mass loading on positive electrode is 1.9 mg whereas negative electrode is 4.5 mg. Fig. 7 (a) represents the CV curves of the MnCo-32 electrode with potential window, 0 to 0.5V versus SCE and AC electrode obtained with potential window, -1 to 0 V versus SCE were measured in a three electrode system at a scan rate of 10 mVs-1. To determine the operating potential window the CV curves of MnCo-32//AC

14

asymmetric supercapacitor recorded with increasing potential window (from 0-1V to 0-1.8V) as shows in Fig. 7 (b) at 10 mVs-1, scan rate. Beyond, the operating potential window 0-1.6V, the polarization becomes noticeable; signifying the maximum operating potential window for the MnCo-32//AC asymmetric supercapacitor device is 0-1.6 V. The CV curves of the MnCo-32//AC asymmetric supercapacitor device recorded with scan rate from 10 to 100 mVs-1, within potential window, 0-1.6V is as shown in Fig. 7 (c). Fig. 7 (d) and (e) displays the GCD curves of MnCo-32//AC asymmetric supercapacitor device measured within different potential window (from 0-1V to 0-1.6 V at 5 mA) and at different applied currents (5, 10, 15 and 20 mA), respectively. Maximum areal capacities (specific capacitance) of 2445 mCcm-2 (238 Fg-1) and 1251 mCcm-2 (122 Fg-1) were achieved by the asymmetric cell assembled using MnCo-32 and AC electrode at 10 mVs-1 and 5 mA, respectively, in the voltage window of 0 to 1.6 V. Ragone plot related to the energy and power densities of the MnCo-32//AC asymmetric cell obtained at different currents is demonstrated in Fig. 7 (f). The MnCo-32//AC asymmetric capacitor exhibits a maximum energy density of 342.5µWhcm-2 and power density of 8 mWcm-2, at an applied current of 5 mA. 4. Conclusions

We

studied

the

growth

process,

supercapacitor

electrochemical

performances, and effect of the cycling of the hydrothermally derived MnCo-LDH electrodes with different reaction times. The growth of the MnCo-LDH involved 15

the nucleation, growth, aggregation, coalescence of nanoparticles, and further crystal growth in the terrace–ledge–kink growth mode. The electrochemical cycling led to an enhanced electrochemical performance of the resultant electrode. The evaluation of the MnCo-LDH electrode after the cycling demonstrated that the changes in morphological features led to the porosity of the material as well as effective access of the electrolytes to the surface beneficial for improved electrochemical reactions, leading to the charge storage performance enhancement. Systematic structural, morphological, and electrochemical analyses of the MnCoLDH electrode before and after the cycling were carried out. This study provides valuable insights into the growth mechanism with respect to the hydrothermal reaction time, electrochemical performance enhancement, and cycling stability of the MnCo-LDH nanostructure. Acknowledgment This study was supported by the National Research Foundation of Korea (NRF2017R1C1B2010906 and NRF-2017M1A2A2048904).

5. References (1)

Mishra, G.; Dash, B.; Pandey, S. Layered Double Hydroxides: A Brief Review from Fundamentals to Application as Evolving Biomaterials. Appl. Clay Sci. 2018, 153, 172–186. 16

(2)

Li, X.; Du, D.; Zhang, Y.; Xing, W.; Xue, Q.; Yan, Z. Layered Double Hydroxides toward High-Performance Supercapacitors. J. Mater. Chem. A 2017, 5 (30), 15460–15485.

(3)

Vialat, P.; Mousty, C.; Taviot-Gueho, C.; Renaudin, G.; Martinez, H.; Dupin, J.-C.; Elkaim, E.; Leroux, F. High-Performing Monometallic Cobalt Layered Double Hydroxide Supercapacitor with Defined Local Structure. Adv. Funct. Mater. 2014, 24 (30), 4831–4842.

(4)

Liang, T.; Xuan, H.; Xu, Y.; Gao, J.; Han, X.; Yang, J.; Han, P.; Wang, D.; Du, Y. Rational Assembly of CoAl-Layered Double Hydroxide on Reduced Graphene Oxide with Enhanced Electrochemical Performance for Energy Storage. ChemElectroChem 2018, 5 (17), 2424–2434.

(5)

Liu, X.; Zhou, A.; Pan, T.; Dou, Y.; Shao, M.; Han, J.; Wei, M. Ultrahigh-RateCapability of a Layered Double Hydroxide Supercapacitor Based on a SelfGenerated Electrolyte Reservoir. J. Mater. Chem. A 2016, 4 (21), 8421–8427.

(6)

Liu, S.; Lee, S. C.; Patil, U.; Shackery, I.; Kang, S.; Zhang, K.; Park, J. H.; Chung, K. Y.; Chan Jun, S. Hierarchical MnCo-Layered Double Hydroxides@Ni(OH) 2 Core–Shell Heterostructures as Advanced Electrodes for Supercapacitors. J. Mater. Chem. A 2017, 5 (3), 1043–1049.

(7)

Zhao, J.; Chen, J.; Xu, S.; Shao, M.; Zhang, Q.; Wei, F.; Ma, J.; Wei, M.; Evans, D. G.; Duan, X. Hierarchical NiMn Layered Double Hydroxide/Carbon Nanotubes Architecture with Superb Energy Density for Flexible Supercapacitors. Adv. Funct. Mater. 2014, 24 (20), 2938–2946. 17

(8)

Sanati, S.; Rezvani, Z. G-C3N4 Nanosheet@CoAl-Layered Double Hydroxide Composites for Electrochemical Energy Storage in Supercapacitors. Chem. Eng. J. 2019, 362, 743–757.

(9)

Jagadale, A. D.; Guan, G.; Li, X.; Du, X.; Ma, X.; Hao, X.; Abudula, A. BinderFree Electrodes of CoAl Layered Double Hydroxide on Carbon Fibers for AllSolid-State Flexible Yarn Supercapacitors. Energy Technol. 2016, 4 (8), 997–1004.

(10) Wan, H.; Liu, J.; Ruan, Y.; Lv, L.; Peng, L.; Ji, X.; Miao, L.; Jiang, J. Hierarchical Configuration of NiCo 2 S 4 Nanotube@Ni–Mn Layered Double Hydroxide Arrays/Three-Dimensional Graphene Sponge as Electrode Materials for HighCapacitance Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7 (29), 15840– 15847. (11) Guo, X. L.; Zhang, J. M.; Xu, W. N.; Hu, C. G.; Sun, L.; Zhang, Y. X. Growth of NiMn LDH Nanosheet Arrays on KCu 7 S 4 Microwires for Hybrid Supercapacitors with Enhanced Electrochemical Performance. J. Mater. Chem. A 2017, 5 (39), 20579–20587. (12) Xu, Y.; Wang, X.; An, C.; Wang, Y.; Jiao, L.; Yuan, H. Facile Synthesis Route of Porous MnCo 2 O 4 and CoMn 2 O 4 Nanowires and Their Excellent Electrochemical Properties in Supercapacitors. J. Mater. Chem. A 2014, 2 (39), 16480–16488. (13) Guo, X. L.; Liu, X. Y.; Hao, X. D.; Zhu, S. J.; Dong, F.; Wen, Z. Q.; Zhang, Y. X. Nickel-Manganese Layered Double Hydroxide Nanosheets Supported on Nickel Foam for High-Performance Supercapacitor Electrode Materials. Electrochim. Acta 18

2016, 194, 179–186. (14) Sahoo, R.; Sasmal, A. K.; Ray, C.; Dutta, S.; Pal, A.; Pal, T. Suitable Morphology Makes CoSn(OH) 6 Nanostructure a Superior Electrochemical Pseudocapacitor. ACS Appl. Mater. Interfaces 2016, 8 (28), 17987–17998. (15) Singh, A. K.; Sarkar, D.; Khan, G. G.; Mandal, K. Hydrogenated NiO Nanoblock Architecture for High Performance Pseudocapacitor. ACS Appl. Mater. Interfaces 2014, 6 (7), 4684–4692. (16) Seok, J. Y.; Lee, J.; Yang, M. Self-Generated Nanoporous Silver Framework for High-Performance Iron Oxide Pseudocapacitor Anodes. ACS Appl. Mater. Interfaces 2018, 10 (20), 17223–17231. (17) Jiang, Y.; Wang, P.; Zang, X.; Yang, Y.; Kozinda, A.; Lin, L. Uniformly Embedded Metal Oxide Nanoparticles in Vertically Aligned Carbon Nanotube Forests as Pseudocapacitor Electrodes for Enhanced Energy Storage. Nano Lett. 2013, 13 (8), 3524–3530. (18) Gao, L.; Fan, R.; Xiao, R.; Cao, K.; Li, P.; Wang, W.; Lu, Y. NiO-Bridged MnCoHydroxides for Flexible High-Performance Fiber-Shaped Energy Storage Device. Appl. Surf. Sci. 2019, 475, 1058–1064. (19) Su, D.; Tang, Z.; Xie, J.; Bian, Z.; Zhang, J.; Yang, D.; Zhang, D.; Wang, J.; Liu, Y.; Yuan, A.; et al. Co, Mn-LDH Nanoneedle Arrays Grown on Ni Foam for High Performance Supercapacitors. Appl. Surf. Sci. 2019, 469, 487–494. (20) Zhou, J.-J.; Han, X.; Tao, K.; Li, Q.; Li, Y.-L.; Chen, C.; Han, L. Shish-Kebab Type MnCo2O4@Co3O4 Nanoneedle Arrays Derived from MnCo-LDH@ZIF-67 19

for High-Performance Supercapacitors and Efficient Oxygen Evolution Reaction. Chem. Eng. J. 2018, 354, 875–884. (21) Liu, Y.-L.; Yan, C.; Wang, G.-G.; Zhang, H.-Y.; Dang, L.-Y.; Wu, B.-W.; Lin, Z.Q.; An, X.-S.; Han, J.-C. Achieving Ultrahigh Capacity with Self-Assembled Ni(OH) 2 Nanosheet-Decorated Hierarchical Flower-like MnCo 2 O 4.5 Nanoneedles as Advanced Electrodes of Battery–Supercapacitor Hybrid Devices. ACS Appl. Mater. Interfaces 2019, 11 (10), 9984–9993. (22) Wu, N.; Low, J.; Liu, T.; Yu, J.; Cao, S. Hierarchical Hollow Cages of Mn-Co Layered Double Hydroxide as Supercapacitor Electrode Materials. Appl. Surf. Sci. 2017, 413, 35–40. (23) Chen, W.; Rakhi, R. B.; Wang, Q.; Hedhili, M. N.; Alshareef, H. N. Morphological and Electrochemical Cycling Effects in MnO 2 Nanostructures by 3D Electron Tomography. Adv. Funct. Mater. 2014, 24 (21), 3130–3143. (24) Li, M.; Yang, W.; Li, J.; Feng, M.; Li, W.; Li, H.; Yu, Y. Porous Layered Stacked MnCo 2 O 4 Cubes with Enhanced Electrochemical Capacitive Performance. Nanoscale 2018, 10 (5), 2218–2225. (25) Shinde, D. V.; Ahn, D. Y.; Jadhav, V. V.; Lee, D. Y.; Shrestha, N. K.; Lee, J. K.; Lee, H. Y.; Mane, R. S.; Han, S.-H. A Coordination Chemistry Approach for Shape Controlled Synthesis of Indium Oxide Nanostructures and Their Photoelectrochemical Properties. J. Mater. Chem. A 2014, 2 (15), 5490–5498. (26) Bandgar, S. B.; Vadiyar, M. M.; Ling, Y.-C.; Chang, J.-Y.; Han, S.-H.; Ghule, A. V.; Kolekar, S. S. Metal Precursor Dependent Synthesis of NiFe 2 O 4 Thin Films 20

for High-Performance Flexible Symmetric Supercapacitor. ACS Appl. Energy Mater. 2018, 1 (2), 638–648. (27) Thanh, N. T. K.; Maclean, N.; Mahiddine, S. Mechanisms of Nucleation and Growth of Nanoparticles in Solution. Chem. Rev. 2014, 114 (15), 7610–7630. (28) Alexander E.S. Van Driessche. In Situ Observation of Protein Crystal Growth by Advanced Optical Techniques; Granada, 2007. (29) María Sancho-Tomás. Calcium Carbonate Bio-Preci Pi Tat Ion in Gelling Environments via Counter-Diffusion; Paris, 2014. (30) Kaneko, S.; Ito, T.; Kato, C.; Tanaka, S.; Yasuhara, S.; Matsuda, A.; Yoshimoto, M. Layer-by-Layer Growth of Graphene on Insulator in CO 2 -Oxidizing Environment. ACS Omega 2017, 2 (4), 1523–1528. (31) Nozawa, J.; Uda, S.; Guo, S.; Toyotama, A.; Yamanaka, J.; Okada, J.; Koizumi, H. Step Kinetics Dependent on the Kink Generation Mechanism in Colloidal Crystal Growth. Cryst. Growth Des. 2018, 18 (5), 2948–2955. (32) Shao, M.; Li, Z.; Zhang, R.; Ning, F.; Wei, M.; Evans, D. G.; Duan, X. Hierarchical Conducting Polymer@Clay Core-Shell Arrays for Flexible All-SolidState Supercapacitor Devices. Small 2015, 11 (29), 3530–3538. (33) Zhao, J.; Chen, J.; Xu, S.; Shao, M.; Zhang, Q.; Wei, F.; Ma, J.; Wei, M.; Evans, D. G.; Duan, X. Hierarchical NiMn Layered Double Hydroxide/Carbon Nanotubes Architecture with Superb Energy Density for Flexible Supercapacitors. Adv. Funct. Mater. 2014, 24 (20), 2938–2946. (34) Pawar, S. A.; Patil, D. S.; Shin, J. C. Designing a Copper- and Silver-Sulfide 21

Composite with Co 3 O 4 for High-Performance Electrochemical Supercapacitors. ChemElectroChem 2019, 6 (2), 522–534. (35) Patil, D. S.; Pawar, S. A.; Shin, J. C. Core-Shell Structure of Co3O4@CdS for High Performance Electrochemical Supercapacitor. Chem. Eng. J. 2018, 335, 693– 702. (36) B. E. Conway, Electrochemical Capacitors: Scientific Fundamentals and Technology Applications, Kluwer Academic/Plenum Publishers, New-York (1999) (37) Brousse T.; Belanger D; Long J. F. To be or not to be pseudocapacitive? J. Electrochem Soc. 2015, 162, A5185. (38) Aldama I.; Barranco V.; Centeno T. A.; Ibanez J.; Rojo J. M. Composite electrodes made from carbon cloth as supercapacitor material and manganese and cobalt oxide as battery one. J. Electrochem Soc. 2016, A758, 163. (39) Akinwolemiwa B.; Wei C.; Chen G. Z. Mechanisms and designs of asymmetrical electrochemical capacitors. Electrochim Acta 2017, 247, 344.

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Figure captions Fig. 1A Scanning electron micrographs of MnCo-8 (a-d), MnCo-12 (e-h), MnCo-16 (i-l) at different magnifications. Fig. 1B Scanning electron micrographs of MnCo-24 (a-d), MnCo-32 (e-h), MnCo-40 (i-l) at different magnifications. Fig. 2 Schematic illustrations of the stepwise growth process of MnCo hydroxide structures controlled by the hydrothermal reaction time. Nucleation and growth (I), aggregation and coalescence (II), crystal growth; at this stage the crystal consist of three different faces such as flat faces (F), stepped (or ledge) faces (S) and kinked faces (K) (III), further growth proceeds through terrace-ledge-kink growth model (IV,V and VI). Fig. 3 (a) XRD patterns of the MnCo-8, MnCo-16 and MnCo-32. High resolution spectra of MnCo hydroxide, (b) Mn 2p, (c) Co 2p, (d) O 1s. Fig. 4 TEM, HR-TEM images (a, b, c) and EDS analysis (d), EDS element mapping of MnCoLDH manganese (e), cobalt (f), Oxygen (g). Fig. 5 (a) CV curves at scan rate of 20 mVs-1, (b) Charge-discharge at an applied current 3 mA, (c) Variation of areal capacitance with respect to scan rates, (d) Variation of areal capacitance with respect to applied currents, of MnCo-8, MnCo-12, MnCo-16, MnCo-24, MnCo-32, and MnCo-40 electrodes. Fig. 6 Scanning electron micrographs of MnCo-8 (a,b), MnCo-12 (c,d), MnCo-16 (e,f), MnCo24 (g,h), MnCo-32 (i, j) and MnCo-40 (k, l) after performing 5000 cycles. Fig. 7 (a) CV curves of Activated carbon and MnCo-32 electrodes at 10 mVs-1. Electrochemical measurements of MnCo-32//AC asymmetric cell device; (b) CV and (d) GCD curves at different voltage windows at 10 mVs-1 and 5 mA, respectively. (c) CV and (e) GCD curves at different

scan rates and applied currents, respectively. (f) Ragone plot, for the asymmetric device MnCo32// AC.

Fig. 1A

Fig. 1B

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Date: 11/27/2019

Highlights


MnCo layered double hydroxide synthesis.
All hydrothermal methods have been adopted.
Effect of morphology evolution and electrochemical cycling
A high areal capacity of 1.58 Ccm-2 at 5 mV s-1 scan rate is obtained.
Efficient supercapacitor fabrication by facile and one pot hydrothermal route.

Declaration of interest Statement The authors declare no competing financial interest.

Yours sincerely, Jae Cheol Shin Associate Professor, Department of Physics, Yeungnam University, 38541, KOREA.