Facile template-free synthesis of 3D hierarchical ravine-like interconnected MnCo2S4 nanosheet arrays for hybrid energy storage device

Journal Pre-proof Facile template-free synthesis of 3D hierarchical ravine-like interconnected MnCo2S4 nanosheet arrays for hybrid energy storage device Laleh Abbasi, Majid Arvand, Seyyed Ebrahim Moosavifard PII:

S0008-6223(20)30113-5

DOI:

https://doi.org/10.1016/j.carbon.2020.01.094

Reference:

CARBON 15029

To appear in:

Carbon

Received Date: 23 October 2019 Revised Date:

8 January 2020

Accepted Date: 25 January 2020

Please cite this article as: L. Abbasi, M. Arvand, S.E. Moosavifard, Facile template-free synthesis of 3D hierarchical ravine-like interconnected MnCo2S4 nanosheet arrays for hybrid energy storage device, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2020.01.094. 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. © 2020 Published by Elsevier Ltd.

CRediT author statement Laleh Abbasi: Data curation, Writing, Original draft preparation Majid Arvand: Supervision, Methodology, Validation, Reviewing and Editing Seyyed Ebrahim Moosavifard: Software, Methodology, Validation

Graphical Abstract A template-free method has been developed to engineer 3D ravine-like interconnected MnCo2S4 nanosheet arrays as a positive electrode material for asymmetric electrochemical capacitors.

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Facile template-free synthesis of 3D hierarchical ravine-like interconnected MnCo2S4 nanosheet arrays for hybrid energy storage device

Laleh Abbasia, Majid Arvanda,*, Seyyed Ebrahim Moosavifardb,c a

Electroanalytical Chemistry Laboratory, Faculty of Chemistry, University of Guilan, Namjoo

Street, P.O. Box: 1914, Rasht, Iran b

Department of Advanced Medical Sciences & Technologies, School of Medicine, Jahrom

University of Medical Sciences (JUMS), Jahrom, Iran c

Research Center for Noncommunicable Diseases, School of Medicine, Jahrom University of

Medical Sciences (JUMS), Jahrom, Iran

*Corresponding author. E-mail address: [email protected] (M. Arvand)

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Abstract Engineering of nanostructured electrodes for enhancing their electrochemical performance is a critical issue to further development in energy storage systems. In the present study, we have developed a facile template-free method to engineer 3D hierarchical ravine-like electrode based on MnCo2S4 nanosheet arrays as an efficient material for high-performance electrochemical capacitors. The physico-chemical characteristics of ravine-like structure of MnCo2S4 nanosheets are investigated by different techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS). The as-prepared MnCo2S4 electrode exhibits an ultrahigh specific capacity of 834 C g−1 (231 mAh g−1) at the current density of 1 A g−1, excellent rate capability and good cycle performance. Thiospinel nature of the MnCo2S4 electrode and its ravine-like nanosheet structure with effective spatial confinement for the electrolyte ions and charge transportation are responsible for this remarkable performance. Furthermore, the assembled MnCo2S4//AC asymmetric device shows the maximum energy density of 57 W h kg−1 and the highest power density of 20.8 kW kg−1.

Keywords: Ravine-like; Nanosheets; MnCo2S4; Asymmetric electrochemical capacitors

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1. Introduction Diminishing fossil fuel supplies and the severe environmental impact of greenhouse gas emission into the atmosphere have motivated intense research on renewable and clean energy resources [1,2]. Moreover, with the ever-growing power demand throughout the world, there is a need to develop energy storage devices possessing high energy and high output power [3,4]. In this regards, supercapacitors (i.e., electrochemical capacitors) have enticed considerable attention owing to their high power capability with extended lifespan over batteries and rapid rates for contemporary applications such as backup/auxiliary power supplies of electric devices and smart grids [5,6]. According to charge storage mechanisms, electrochemical capacitors are classified into electrical double layer capacitors (EDLC) and pseudocapacitors [7]. The former stores charges at the interface of electrode/electrolyte by means of charge separation while, pseudocapacitors store charges based on Faradaic reaction at the nanoscale gap of the surface [8]. In the case of electrode materials, pseudocapacitors such as transition metal oxides/hydroxides and conducting polymer exhibit higher energy density and specific capacitance than EDLC (carbonaceous materials) owing to their redox reactions [9]. Nevertheless, these pseudocapacitive materials show unpleasant low conductivity and poor electrochemical stability, which can obstruct their practical application in energy storage devices [10]. Accordingly, it has been recognized that ternary transition metal sulfides are good candidates of pseudocapacitive materials with improved electrochemical performance since, in comparison to metal oxides, they exhibit lower band-gap energy and higher electric conductivity [11]. Besides, ternary transition metal sulfides possess a variety of oxidation state, which provides richer redox reaction and results in higher specific capacitance compared to mono-metal sulfides [12]. Additionally, by the replacement of oxygen with sulfur, a more flexible structure

3

can be created owing to the lower electronegativity of sulfur compared to oxygen. This prevents the change of structural integrity through the elongation between layers and provides a short transport pathway in the electrode structure for ions and electrons [13]. Because of these reasons, much research approaches have been poured into the design and fabrication of ternary transition metal sulfides such as NiCo2S4 [14], ZnCo2S4 [15], CuCo2S4 [16], FeCo2S4 [17]. Among the most potential mixed metal sulphospinels, MnCo2S4 electrodes have been considered as the fascinating electrodes owning to their higher capacitance/capacity and higher electrical conductivity than those of MnSX [18] and CoSX electrodes [19]. Until now, various morphologies of Mn-Co sulfide, such as MnCo2S4 nanorods [20], MxCo3-xS4 hollow tubular structures [15] and MnCo2S4 core-shell [21] have been reported for electrochemical capacitor application. Most of these research works paid attention to achieve high specific gravimetric capacitance resulted from constructing thin electrodes. Inactive material buried under the material's surface usually results from increasing the mass loading which leads for more thickness of the material and accordingly lower overall efficiencies [22]. However, for practical applications, the high areal capacitance of electrodes should be considered as an important criterion, which needs higher mass-loading combined with high utilization of electrochemically active material (i.e. specific gravimetric capacitance). Herein, we engineered 3D hierarchical ravine-like nanosheet arrays with a large amount of electroactive sites and easy diffusion pathway for ions and electrons to simultaneously achieve the high mass-loading along with high utilize exploiting of the material. On the other side, for maximizing the electrochemical performance of a electrochemical capacitor, one needs to design nanostructured electrode with a large amount of electroactive sites and abundant transport paths for electrolyte ions and electrons [23]. Among various nanostructures, nanoporous nanosheets directly grown on conductive substrates have been

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regarded as a favorable morphology for energy storage in the view of their high specific area and thin wall thickness, which can achieve more electroactive sites and easy diffusion pathways for electric charge carriers during the electrochemical reactions [3,24,25]. Direct growth of nanostructured arrays on conductive substrates increases in conductivity of electrode through the direct connection of nanosheets to the substrate and eliminates the need of adding conducting additives or binders [24]. Hence, it will be greatly significant to develop a simple approach to synthesis materials with these features. As per our knowledge the direct growth of MnCo2S4 nanosheet arrays on conductive substrates has rarely been reported; hence, for the first time, we present a simple template-free approach to engineer 3D hierarchical ravine-like MnCo2S4 nanoporous structures grown on conductive Ni foam as an efficient binder-free electrode for high-performance electrochemical capacitors. These electrodes revealed ultrahigh gravimetric (834 C g−1 at 1 A g−1) and areal (3.34 C cm−2 at 4 mA cm−2) capacity showing superior cycling life (capacity retention of 94% after 5000 cycles at current density of 20 A g−1). Furthermore, the resultant MnCo2S4//AC asymmetric electrochemical capacitor device delivered a high energy density of 57 W h kg−1 (1049.2 W kg−1 power density) with a good cycling life (93.7% capacity retention after 5000 cycles), which can be favorable in the development of energy storage system. Finally, for illustrating the capability of the fabricated device as a power source, we employed it in a practical application for powering several light-emitting diodes (LEDs).

2. Experimental 2.1 Synthesis of 3D hierarchical ravine-like MnCo2S4 nanosheets In this study, all the used reagents were of analytical grade and used as received. Before the synthesis, the Ni foam was thoroughly cleaned with 3 mol L–1 HCl solution for 20 min to remove

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the possible oxide layer on its surface. In a typical synthesis, a 60 mL mix solution (40 mL H2O and 20 mL of ethanol) of 33 mmol L–1 Co(NO3)2·6H2O, 16 mmol L–1 MnSO4·H2O and 100 mmol L–1 hexamethylenetetramine was transferred to a capped bottle (80 mL in capacity). Then, the cleaned Ni foam was immersed into the prepared mixture to obtain Mn-Co precursor followed by heating the bottle in an oven at 90 °C for 10 h. After the bottle naturally cooled down to room temperature, the Ni foam loaded with precursor was taken out and rinsed with ethanol and DI water several times and dried at 60 °C. Then, in order to obtain MnCo2S4, the precursor-loaded Ni foam was transferred into a 100 mL Teflon-lined stainless-steel autoclave, in which contained 75 mL of a 25 mmol L–1 Na2S solution. The autoclave was kept at 120 °C for 8 h. Finally, after the reaction, the electrode was rinsed with DI water and then dried under vacuum at 60 °C for 5 h. As a control sample, the Mn-Co precursor was annealed for 2 h at 320 °C with a ramping rate of 2 °C min−1 to obtain crystallized nanoporous MnCo2O4 nanosheet arrays. The average mass loading of the Mn-Co precursor, MnCo2O4 and MnCo2S4 was approximately 2.8, 2.5 and 4 mg cm−2, respectively.

2.2 Characterization The surface morphology of the samples was investigated by field-emission scanning electron microscope (FESEM, Mira3 XMU from TESCAN Company) and transmission electron microscope (TEM, JEOL, JEM-2100F, 200 Kv). X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi using an Mg X-ray resource) was used to analyze the chemical bonding status of the samples. The crystal structure of the synthesized electrodes was determined by X-ray diffraction (XRD, Rigaku, CuKα radiation, λ=0.15418 nm). Nitrogen

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adsorption/desorption isotherms and Barret-Joyner-Halenda (BJH) model were used to calculate the specific surface area and pore size distribution of the samples.

2.3 Electrochemical evaluation The electrochemical measurements were performed in an aqueous solution of 3 mol L–1 KOH using an Autolab PGSTAT 30 electrochemical analyzer in a three-electrode system, where the synthesized electrode, a saturated calomel electrode (SCE) and platinum plate were served as the working, counter and reference electrodes, respectively. In order to evaluate all electrochemical performances, cyclic voltammetry (CV), galvanostatic charge-discharge (CD) and electrochemical impedance spectroscopy (EIS) at ambient temperature were used. The specific capacity (Cs) of the as-synthesized electrodes was obtained from CD measurements according to the following equation: =

× ∆

(1)

Where I is the discharge current (A), ∆t is the discharge duration (s), m is the loaded mass of active material (g).

2.4 Asymmetric electrochemical capacitor fabrication An asymmetric electrochemical capacitor (AEC) was assembled by taking the MnCo2S4 and activated carbon (AC) as the positive and negative electrodes, respectively. A piece of cellulose filter paper as the separator was placed between two electrodes. Each electrode had a geometric surface area of 1 cm2 and immersed into an aqueous solution of 3 mol L–1 KOH as the electrolyte. For electrochemical measurements, the negative electrode (AC) was made by mixing AC, carbon black and polytetrafluoroethylene (PTFE) with a final composition of 85:10:5. The 7

obtained AC slurry was pasted onto Ni foam and then dried at 100 °C overnight. For balancing the charge between the two electrodes in asymmetric device, the mass ratio of positive (

) electrodes were adjusted based on the well-known charge balance equation

negative ( =

(

). The total mass of the active materials on both electrodes was about 23 mg cm–2

= 4 mg,

(

) to

= 19 mg). The voltage window was from -0.1–0.5 V vs. SCE for the MnCo2S4

cathode, -1.0–0.0 V vs. SCE for the activated carbon anode electrode, and thus 0.0–1.6 V for the asymmetric MnCo2S4//AC cell. The specific energy (Es) and the corresponding specific capacitance (Cs) and energetic efficiency (

) were obtained from the galvanostatic

charge/discharge (GC/GD) curves according to the following equations [26]: = =

,



=

/

2

,

"#,% "#,&

() 3.6

(5) is the discharge energy,

Where current,

(3)

(4)

. ,-

=

. ! (2)

is the potential at each time point,

time of discharge,

,

is the charge energy,

1,2

is the initial time of discharge,

is the specific energy,

energetic efficiency.

3. Results and discussion 8

is the constant 1,3

is the final

is the specific capacitance and

is the

3.1 Synthesis and structural characterization A facile template-free two-step method has been designed to synthesize 3D hierarchical ravine-like interconnected MnCo2S4 nanoporous structures on Ni foam substrate (Fig. 1). At the first step, hexamethylenetetramine as the capping agent was used to engineer uniform Mn-Co precursor nanosheet arrays on the substrate. At the second step, the as-prepared Mn-Co precursors were transformed into the ravine-like MnCo2S4 nanosheets in hydrothermal treatment through anion exchange reactions in pseudo-Kirkendall process [17]. Anion exchange is an advanced synthetic method in which beneficial ions are introduced to template nanomaterials [27]. During sulfide processing, sulfide ions release from Na2S and can react with metal ions to form a ravine-like surface layer of Mn-Co sulfide. Then, in the next step of reaction, sulfur ions diffuse through the interface. Since this process is isotropic, the overall morphology of the resultant Mn-Co sulfide remains unchanged.

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Fig. 1. The Schematic illustration showing the procedure to grow MnCo2O4 and MnCo2S4 ravine-like nanosheet structure.

The investigation of morphological features, component and crystalline nature of the assynthesized electrodes, were performed by FESEM, TEM, XPS and XRD analyzers. Fig. 2 displays the FESEM images of the Mn–Co precursor (Fig. 2(a–b)), Mn–Co oxide (Fig. 2(c–d)) and Mn-Co sulfide (Fig. 2(e–f)) nanosheet arrays on Ni foam. As shown, the Mn-Co precursor nanosheet arrays are vertically grown and intertwined to form a 3D hierarchical ravine-like structure. After calcination treatment at a temperature of 320 °C, the Mn-Co oxide retains the wall-like feature of the Mn-Co precursor and becomes thinner. As seen in Fig. 2d, the average thickness and length of the Mn-Co oxide nanosheets are about 20–40 nm and more than 1 µm, 10

respectively. After sulfurization (Fig. 2(e–f)), it can be found that the array structure of the MnCo sulfide electrode is successfully maintained but the nanosheets have become rougher, resulting in formation of ravine-like structure [3]. Such ravine-like morphologies are promising for many applications of energy storage owing to their substantial contact area between electrode material and electrolyte ions, nanoscale wall thickness and electrolyte ion trapping [4,28].

Fig. 2. FESEM images of (a,b) Mn-Co precursors, (c,d) MnCo2O4 and MnCo2S4 (e,f) at low and high magnification.

To evaluate the crystalline phase purity, the prepared samples were subjected to XRD analysis (Fig. 3a). Apart from three strong peaks (labelled with asterisk) arising from the Ni foam, other peaks located at 18.67º, 30.77º, 36.19º, 58.35º and 64.63º can be well indexed as the

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(111), (220), (311), (511) and (440) crystal plans of the spinel MnCo2O4 (JCPDS card No. 0011130).In the XRD pattern of the sulfide electrode, the peaks located at 31.3º, 37.9º, 50.1º and 55.3º correspond to the respective (311), (400), (511) and (440) crystal planes of the MnCo2S4. Furthermore, the diffraction peaks show a similar pattern to the record JCPDS No. 73-1703 (cubic Co3S4) which reveal that the replacement of Co by Mn ions does not change the crystal structure [21,29]. The composition and chemical valence states of the as-prepared MnCo2S4 electrode can be gained using XPS measurements. The corresponding high-resolution XPS spectral data are illustrated in Fig. 3(b–e). The Co 2p spectrum is deconvoluted into two main peaks (2p3/2 and 2p1/2) located at 781.48 and 796.98 eV, respectively, with a spin-orbit splitting of 15.5 eV suggesting the presence of both Co2+ and Co3+ species, as depicted in Fig. 3b [30]. The appearance of Mn 2p3/2 (642.43 eV) and Mn 2p1/2 (654.28 eV) in the Mn 2p spectrum (Fig. 3c) can be related to Mn2+. Moreover, two main peaks at 83.08 and 89.18 eV with the splitting energy of about 6.05 eV in the Mn 3s spectrum (Fig. 3d) confirm the oxidation state of Mn2+ [20]. Two peaks corresponding to 161.68 and 163.28 eV in the S 2p spectrum (Fig. 3e) can be ascribed to the metal-sulfur bonds (Mn-S and Co-S) and sulfur ions in low coordinated surface, respectively [20,31]. The peak observed at 168.38 eV can be also related to sulfur adsorbed to surface with higher oxidation state such as sulfates [32,33]. In order to figure out the pore-size distribution and specific surface area of the prepared samples, the nitrogen adsorption/desorption experiments were investigated. Fig. 3f elucidates that both samples have the typical IV isotherms accompanied with H3-type hysteresis loops (p/p0> 0.4) demonstrating the attendance of nanoporous structure. The nanoporous structure of the samples was further confirmed by the BJH method (inset of Fig. 3f). The MnCo2S4 sample owns a BET specific surface area of 116 m2 g−1, which is larger than MnCo2O4 (94 m2 g−1). The

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improved BET surface area of the MnCo2S4 sample can be attributed to the growth of ravine-like structures after sulfidation step, which provide larger surface compared with the smooth surface of MnCo2O4 nanosheets. Such a high surface area and nanoporous structures provide short ion diffusion paths and enhance electron transfer during the electrochemical process [34].

Fig. 3. (a) XRD patterns of MnCo2O4 and MnCo2S4 nanosheet arrays on Ni foam. (b-e) Highresolution XPS spectra and peak fitting of Co 2p, Mn 2p, Mn 3s, and S 2p, respectively. (f) N2 adsorption-desorption isotherms and the corresponding pore size distribution curves (inset) of the MnCo2S4 and MnCo2O4 samples.

In order to shed light upon the interior structure of the as-synthesized samples, TEM and HRTEM analyses were conducted. Fig. 4a,d shows the wide TEM images of the MnCo2S4 and MnCo2O4 electrodes, respectively. As seen, the nanosheets have a planar size of several micrometers, which is in line with the FESEM images. The darker surface of the MnCo2S4 nanosheet can be related to its ravine-like surface. In agreement with the BJH result, abundant 13

nanopores can be seen on the surface of the MnCo2S4 and MnCo2O4 nanosheets (Fig. 4b,e). Different lattice fringes with separation of 0.28 and 0.23 nm (outlined in yellow in Fig. 4b) can be assigned to (311), (400) crystallographic planes of MnCo2S4 respectively.

Fig. 4. (a,b) TEM images and (c) HRTEM image of MnCo2S4, (d–f) TEM images of MnCo2O4 at different magnifications.

3.2 Electrochemical evaluation in three-electrode system The electrochemical measurements of the prepared binder-free electrodes in a threeelectrode system containing 3 mol L–1 KOH as the electrolyte were systematically conducted. The comparative CV curves of the MnCo2S4 and MnCo2O4 electrodes at a sweep rate of 5 mV s−1 are depicted in Fig. 5a. It is obvious that the MnCo2S4 electrode possesses the largest surrounded area illustrating a significant increase in its specific capacitance. In addition, the

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straight line derived from Ni foam confirms that the capacitance contribution of the substrate is insignificant. Fig. 5b presents the typical CV curves at various sweep rates (5 to 100 mV s−1) for the MnCo2S4 electrode. Each CV curve consists of a pair of well-defined redox peak, attributing to the interaction between sulphospinel materials and electrolyte ion, which can be defined by the following equations [35]: MnCo2S4 + OH− ↔ MnS4-2XOH + 2CoSxOH + 2e−

(5)

CoSxOH + OH− ↔ CoSxO + H2O + e−

(6)

CoSxO + K+ ↔ CoSxO.K + e−

(7)

MnS4-2XOH + OH− ↔ MnS4-2XO + H2O + e−

(8)

MnS4-2XO + K+ ↔ MnS4-2XO.K + e−

(9)

Meanwhile, with the increase of sweep rate, the shape of CV curves remains approximately unchanged suggesting the excellent rate capability and fast redox reactions at the interface of electrode/electrolyte [36]. Fig. 5c,d provides the constant–current charge–discharge curves (CD) of the MnCo2S4 electrode in a potential window of 0 to 0.4 V (vs. SCE) at different current densities. The symmetrical shape of the CD curves with the two defined voltage plateaus verify that the Faradaic reactions are highly reversible [37], which well agrees with CV curves. According to the discharge time, the specific gravimetric and areal capacity values for MnCo2S4 and MnCo2O4 electrodes are illustrated in Fig. 5e. The MnCo2S4 electrode delivered an ultrahigh specific capacity of 834 C g−1 (231 mAh g−1) at the current density of 1 A g−1 and the corresponding areal capacity of 3.34 C cm−2 at 4 mA cm−2 which is higher than that of the MnCo2O4 electrode. It should be noted that the specific capacity of the MnCo2S4 electrode maintains about 53% of its initial value with increase in current density from 1 A g−1 (4 mA cm−2) to 50 A g−1 (200 mA cm−2), which demonstrates its superior rate capability. More

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strikingly, the specific capacitance of ravine-like MnCo2S4 nanosheet arrays is much more than the values reported previously, as shown in Table 1.

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Table 1 Comparison of the electrochemical performance of ravine-like MnCo2S4 electrode with other electrodes reported in literatures. Material

Three-electrode system

Synthesis rout

Cycle retention

CuCo2S4 flower-like NiCo2S4@NiCo2S4 nanosheets P-doped MnCo2S4 nanoneedles Ni3S2/MWCNT nanocomposites Ni-Co sulfide nanoboxes Mn-Co sulfide nanowalls MnCo2S4 hollow tubular CuCo2O4/CuO nanocomposites NiCo2S4@CoSx

Specific capacity at current density 363 C g–1, 5 mA cm–2 2.19C cm–2, 5 mA cm–2 217.3 C g–1, 1 A g–1 360 C g–1, 3.2 A g–1 794 C g–1, 2 A g–1 775.2 C g–1, 5 A g–1 547 C g–1, 10 A g–1 – 2.13 C cm–2, 5 mA cm–2

NiCo2S4 Core-shell NiCo2S4@CoS2 nanostructures CuCo2S4/CuCo2O4 nanoflowers Ni3S2/CNTs nanocomposites

0.97 C cm–2, 1 mA cm–2 860.7 C g–1, 1 A g–1 599 C g–1, 1 A g–1 657.2 C g–1, 1 A g–1

5000, 94% 8000, 91% 5000, 85% 2000, 91.5%

CuCo2S4 nanoparticles CoMoS4//rGO nanocomposites NiCo2S4 nanoparticles NiCo2S4@Co(OH)2 nanotube

261 C g–1, 1 A g–1 396.6 C g–1, 1 A g–1 486 C g–1, 2 A g–1 –

2000, 80% – 1000, 107.9% –

GO/strontium sulfide nanorods β-Ni(OH)2 nanosheets MnCo2S4 ravine-like nanosheets

732.4C g–1, 5 mA cm–2 – 834 C g−1, 1 A g–1

2000, 91.1% 5000, 82% 1000, 80% 4000, 88.2% 4000, 81.88% 20000, 94% – 1500, 76.1%

1200, 67% – 5000, 94%

Hydrothermal Hydrothermal Hydrothermal Hydrothermal Wet-chemical method Electrodeposition Hydrothermal Microwave method Hydrothermalelectrodeposition Hydrothermal Hydrothermal Hydrothermal ElectrodepositionHydrothermal Microwave method Chemical precipitation Solvothermal Hydrothermalelectrodeposition Hydrothermal Template-free growth Hydrothermal

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Two-electrode system

Refs.

Specific capacitance at current density 93.5 F g–1, 1 mA cm–2 75 F g–1, 5 mA cm–2 58.4 F g–1, 0.5 A g–1 55.8 F g–1, 1 A g–1 – 24.9 F g–1, 1 A g–1 – -1 57 F g , 1 mA cm–2 –

Cycle retention 2000, 126.4% 5000, 81% 5000, 91.1% 5000, 90% – – – 5000, 79% –

EMax (W h kg–1) 29 24.9 20.8 19.8 – 14.33 18 –

P (W kg–1) 127 334 400 798 – 74.87 259 –

[38] [39] [40] [41] [33] [42] [15] [43] [44]

0.34 F cm–2, 1mA cm–2 – –1 90.4 F g , 1 A g–1 –

5000 77% – 10000, 73% –

22.8 – 33.2 –

160 – 800 –

[45] [46] [16] [47]

39.8 F g–1, 1 A g–1 68 F g–1, 1 A g–1 – 100.94 F g–1, 0.5 A g–1

6000, 99.5% 10000, 86% 5000, 91.7 % 5000, 70%

15 27.2 28.3 35.89

400 400 245 400

[48] [49] [50] [51]

33.12 F g–1, 1 mA cm–2 105.8 F g–1, 2 mA cm–2 160 F g–1, 1 A g–1

– 1000, 92% 5000, 93.7%

10.55 36.2 57

294.3 100.6 1049

[52] [53] This work

Moreover, the long term cycling stability, a vital need for electrochemical capacitor application, was investigated for 5000 CD measurements at a current density of 20 A g−1. As shown in Fig. 5f, the specific capacity gradually decreases from 569 C g–1 to 535 C g–1 (after 5000 CD cycles) corresponding to a loss of 6% of the initial value, which is obviously lower than that of MnCo2O4 (10.3%). This demonstrates the excellent cycling stability of the MnCo2S4 electrode. The energy efficiency as an essential part of the basic research for designing electrode materials is of vital importance for the practical development [54]. In this regard, the energy efficiency of the electrodes as a function of cycle number was evaluated. As shown in Fig. 5f, during the cycling tests, the energy efficiency of the MnCo2S4 electrode at the current density of 20 A g−1 maintains around 92%, which is higher than that of MnCo2O4 (88%).

Fig. 5. Electrochemical characterizations of the as-prepared electrodes in a three-electrode system. (a) Comparative CV profiles of Ni foam, MnCo2O4 and MnCo2S4 electrodes at a sweep rate of 5 mV s−1. (b) CV profiles at various sweep rates and (c,d) CD profiles at different current densities of MnCo2S4 electrode. (e) Rate capability, (f) Energy efficiency and specific capacity as a function of cycle number for the MnCo2S4 and MnCo2O4 electrodes. 18

In order to evaluate the electrical resistance response, EIS measurements were conducted for both MnCo2S4 and MnCo2O4 electrodes. Fig. 6A shows the Nyquist plots of the experimental impedance data over the frequency range from 0.01 to 105 Hz in 3 mol L–1 KOH aqueous solution. As seen, the negligible semicircle observed for the MnCo2S4 electrode in high frequency region suggests a much faster charge transfer for it, confirming the better electrochemical performance of the MnCo2S4 electrode. In addition, in the low frequency region, the MnCo2S4 electrode shows a more vertical line than the MnCo2O4 electrode, demonstrating lower mass transfer resistance (Warburg), further confirming the better electrochemical performance which is in accordance with the specific capacity results. Moreover, the MnCo2S4 electrode exhibits a lower internal resistance (inset of Fig. 6A, the intercept of the Nyquist curve on the real axis) than that of MnCo2O4 electrode indicating its higher electrical conductivity.

3.3 Electrochemical performance of MnCo2S4//AC asymmetric device In order to investigate the potential application of the MnCo2S4 electrode in highperformance electrochemical capacitors, an asymmetric device was constructed based on the MnCo2S4 and AC as the positive and negative electrodes respectively, with a piece of cellulose paper as the separator placed between the electrodes. Before making the device, the mass loading for both electrodes materials was balanced according to their individual electrochemical behaviors (Fig. 6B) in a separate three-electrode cell.

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A

B

Fig. 6. (A) Nyquist plots of the EIS for the prepared MnCo2S4 and MnCo2O4 electrodes. (B) CV profiles of MnCo2S4 and AC electrodes in a three-electrode system after charge balancing.

A series of CV tests were operated to figure out the optimum operating voltage window (Fig. 7a). Therefore, a maximum working voltage of 1.6 V was chosen for the MnCo2S4//AC device. This value is higher than that of conventional symmetric activated carbon electrochemical capacitors in aqueous electrolytes, which are not widely used due to their low electrochemical window limiting the potentials of water decomposition (0.8–1.0 V) [55,56]. Fig. 7b displays the typical CV curves of the device at various sweep rates (5–50 mV s−1) within the potential range of 0.0–1.6 V. The obvious redox peaks in the CV curves result from the electrochemical reactions of the MnCo2S4 electrode. As the sweep rate increased up to 100 mV s−1, no noticeable distortion appears in the CV curves, which demonstrates high rate capability and fast kinetics properties of the device. The CD curves of the device achieved at different current densities (23–230 mA cm−2) are depicted in Fig. 7c. The linear CD curves with a small internal resistance even at a high current density of 230 mA cm−2 suggest high conductivity and excellent reversibility. The values of the specific gravimetric and areal 20

capacitance at different current densities are shown in Fig. 7d. As seen, our device can deliver a striking areal capacitance of 3.68 F cm−2 at a current density of 23 mA cm−2, with a 61.2% retention after 20 times increase in current density (460 mA cm−2) indicating its superior rate capability. Furthermore, a maximum capacitance value of 160 F g−1 (3.68 F cm−2) at 1 A g−1 (23 mA cm−2) is much higher than that of previously reported works [10,12,40,45]. The good cycling lifetime of the device was presented in Fig. S1 in which, by applying a high current density of 46 mA cm−2 over 5000 cycles, the real capacitance gradually decreases from 151 to 141.5 F g−1, resulting in a capacitance loss of only 6.3%. As the more demanding tests than the conventional CD cycling, the constant float voltage test and energy efficiency analysis were also performed over 100 h and 5000 cycles respectively, in order to investigate the stability of our device. In this regard, by applying a constant float voltage of 1.6 V, two CD cycles at a current density of 2 A g−1 were conducted every 1 h. As shown in Fig. 7e, during 100 h, our device delivers remarkable stability with no loss in capacitance. In addition, it is found that the energy efficiency of the device can reach up to 96.1% without noticeable decay after 5000 cycles (Fig. 7e), indicating the excellent stability of the device over cycling tests. For the evaluation of the overall performance of our device compared to previously reported devices, the Ragone plot (energy density vs. power density) was depicted in Fig. 7f. Encouragingly, our device displays a high energy density of 57 W h kg−1, corresponding to a power density 1.0 kW kg–1. The more the power density, the lower the available energy density can be obtained. However, our device at a towering power density of 20.8 kW kg–1 can deliver an energy density of 34.2 W h kg–1, which exceeds most of the reported results of asymmetric devices in literature (see Table 1).

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Fig. 7. Electrochemical performance of MnCo2S4//AC asymmetric device. (a) CV profiles of the device over various operating potential window, (b) CV profiles at various sweep rates, (c) CD profiles at different current densities, (d) Cell capacitance vs. current density, (e) Energy efficiency and float voltage test of the device over 5000 CD cycles and 100 h, respectively. (f) Ragone plot of the device at different current densities compared with previously reported devices.

Taking into all above results, the excellent performance of the proposed redox MnCo2S4 electrode might be derived from the following attributes: (1) ravine-like nanosheet characteristics of the fabricated electrode, accompanied with enhanced contact area provide plentiful electroactive sites for redox reaction. (2) The presence of void between the interconnected ravine-like nanosheet arrays could serve as ion-buffering reservoirs and facilitate the channeling of OH– and electron transport as well [24,57,58]. (3) The Thiospinel nature of the MnCo2S4 nanosheet arrays possesses higher electrical conductivity and richer redox reaction compared to

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MnCo2O4 electrode; consequently, reduced contact resistance and better electrochemical performance can be achieved [3]. Finally, in order to demonstrate the real application of our device, positive and negative electrodes connected in series, were able to power 20 red light emitting diodes (LED, with heart arrangement, connected in parallel) for more than 6 minutes as shown in Fig. 8.

Fig. 8. (a) Schematic illustration of electron and ion pathways on nanoporous ravine-like MnCo2S4 nanosheet arrays. (b) Photos showing the real application of two as-prepared devices in series to light up 20 red LEDs assembled in parallel for more than 6 minutes.

4. Conclusions In a nutshell, a template-free method has been developed to engineer 3D ravine-like interconnected MnCo2S4 nanosheet arrays as a positive electrode material for asymmetric electrochemical capacitors. With superior electrochemical properties including ultrahigh specific 23

capacity of 834 C g–1 and areal capacity of 3.34 C cm–2 at 1 A g–1 (4 mA cm–2) as well as excellent cycling stability, leading to potential practical application in energy storage devices. Benefiting from the rational structural features, the assembled MnCo2S4//AC device delivered a maximum energy density of 57 W h kg–1 and a power density up to 20.8 kW kg–1, suggesting promising electrode material for electrochemical energy storage devices.

Acknowledgement The authors are thankful to the post-graduate office of Guilan University for the support of this work.

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Highlights •

Engineering 3D hierarchical ravine-like interconnected MnCo2S4 nanosheet arrays.

•

Developing a facile template free method to grow ravine-like nanostructures.

•

Achieving an ultrahigh specific capacity of 834 C g−1 for the MnCo2S4 electrode.

•

Delivering remarkable energy density and cycling stability from the MnCo2S4//AC asymmetric device.

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.