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Tracking pseudocapacitive contribution to superior energy storage of MnS nanoparticles grown on carbon textile Muhammad Sufyan Javed, Xiangyu Han, Chenguo Hu, Meijuan Zhou, Zhiwei Huang, Xingfu Tang, and Xiao Gu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07924 • Publication Date (Web): 25 Aug 2016 Downloaded from http://pubs.acs.org on August 27, 2016

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ACS Applied Materials & Interfaces

Tracking Pseudocapacitive Contribution to Superior Energy Storage of MnS Nanoparticles Grown on Carbon Textile Muhammad Sufyan Javed†, ‡, Xiangyu Han†, Chenguo Hu†*, Meijuan Zhou§, Zhiwei Huang§, Xingfu Tang§, Xiao Gu†* †

Department of Applied Physics, Chongqing University, Chongqing 400044, P. R. China

‡

Department of Physics, COMSATS Institute of Information Technology Lahore 54000, Pakistan

§

Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, P. R. China ∗

Corresponding author. Tel: +86 23 65670880; Fax: +86 23 65678362

E-mail address: [email protected] (CG Hu); [email protected] (X Gu)

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Abstract: Transition metal chalcogenides have emerged as a new class of electrode materials for energy storage devices with superior electrochemical performance. We have directly synthesized manganese sulfide nanoparticles on carbon textile substrate, and used them as electrodes to fabricate flexible all-solid-state supercapacitors. By voltammetry analysis, we have studied the electrochemical properties of MnS-CT composites, which reveal that the Faradic diffusion controlled process dominates at low scan rates (82.85 % at 5 mV s-1) and even at high scan rates (39 % at 20 mV s-1). The MnS-CT electrode shows high capacitance of 710.6 F g-1 in LiCl aqueous electrolyte and the surface redox reactions on MnS nanoparticles are found to be responsible for the high pseudocapacity, and which is further analyzed by XRD and HRTEM. Furthermore, MnS-CT supercapacitor exhibits excellent pseudocapacitive performance (465 Fg-1 at 5 mV s-1), excellent stability, light weight (0.83 g as a whole device), and high flexibility. The device has also achieved high energy density and high power density (52 Wh kg-1 at 308 W kg-1 and 1233 W kg-1 with 28 Wh kg-1, respectively). In practice, three charged supercapacitors in series can power four red light emitting diodes (LED’s) (2.0 V, 15 mA) for 2 minutes. All of the evidence shows that MnS nanoparticles combined with carbon textile is a promising electrode material for pseudocapacitors. Key

words:

Carbon

textile,

nanoparticles,

Li+

intercalation,

pseudocapacitance

2


Faradic

process,

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1.

Introduction

The performance of the electrochemical devices highly depends on the properties of the electrode materials. It is well known that, the electrode materials with proper nanostructures and high surface area would enhance the rate capability, cycling stability and energy density of the energy storage devices.1-2 There are various kinds of nanostructured electrode materials for supercapacitors, including carbon3, graphene4, conducting polymers5, transition metal oxide (TMO)6 and transition metal chalcogenides (TMCs).7 By implementing pseudocapacitance into traditional double-layer capacitors, electrochemical energy of storage devices has been greatly improved in the last decade. TMO’s are considered as promising electrode materials for pseudocapacitors owing to their fast redox reactions and high specific capacitance. For example, MnO2 have been extensively investigated as positive electrode material for supercapacitors due to its high theoretical capacitance, environmental friendliness and low cost.8 However, the electrical conductivity of pristine MnO2 is rather poor which leads to low specific capacitance and low rate capability.9 The electrical conductivity and charge storage capability of MnO2 could greatly be enhanced by making composites with high conductive materials such as graphene, carbon and conducting polymers or incorporation with other metals,9-10 and sulfides.11 As an emerging new class of electrode materials with superior electrochemical activity, transition metal chalcogenides have attracted much attention. TMCs have high electrical conductivity, thermal and mechanical stability than those of their corresponding metal oxides12, which make them promising electrode materials. In addition, for practical prospective the TMCs are more attractive because they possess significantly low volumetric expansion during charging/discharging as compared with TMOs. This quality improves the stability and life time of the supercapacitor.13 Manganese sulfide (MnS) is p-type

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semiconductor with wide band energy (3.7 eV) and it exists in three types of distinct phases, green stable (α-phase) with rock salt structure, zinc-blende (β-phase) and wurtzite with (γphase). Among all the phases of MnS, α-phase is the most stable one.14-15 Manganese sulfide is used in various energy related applications, such as optoelectronic devices,16 solar cells17 and Li+ and Na+ batteries.18-19 However, little attention has been paid to the investigation of the detailed electrochemical properties of MnS as electrode for supercapacitors. Besides, only a few reports are found for supercapacitor electrodes based on composite with reduced graphene. For example, Li et. al. reported the hydrothermal synthesis of γ-MnS/rGO composite for supercapacitor electrode and it achieved 802.5 F g-1 at 5 A g-1 in KOH aqueous electrolyte with good stability till 2000 cycles.15 Tang et.al. reported the synthesis of MnS nanocrystals and further used as electrode materials for supercapacitors with 704.5 F g-1 at 1 mV s-1 in aqueous electrolyte and it retained 62.6 % capacitance after 5000 cycles.11 These materials are in powder form and need to be mixed with conductive additives and polymer binders, which directly affect the electrochemical performance and stability of the supercapacitor. More effective approach to enhance the electrical conductivity and transport kinetics would be growing active materials on conductive substrates (nickel foam, copper foam Ti foil and carbon textiles) with direct mechanical and electrical contact.20-21. The directly growing architecture allows easier ion diffusion, decreases internal resistance and enhances electrochemical activity. Additionally, flexibility of the devices could fulfill requirements of roll-up and flexible displays, artificial electronic skins and flexible photovoltaic cells.22 Therefore, many efforts have been devoted to develop high efficient energy/power devices along with light weight and flexibility.23 These investigations are also limited in aqueous and ionic-liquid-based electrolytes, which may create leakage and require special safety management. To our best

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knowledge, no all-solid-state flexible supercapacitor based on MnS nanoparticles was demonstrated before. In the present work, we apply facile hydrothermal route to synthesize MnS nanoparticles on conductive flexible carbon textile (CT) substrate. MnS nanoparticles have strong contact with CT forming stable MnS-CT composite and could be directly used as binder free electrodes for supercapacitors. Furthermore, an all-solid-state supercapacitor was fabricated using PVA-LiCl as solid electrolyte by composing two MnS-CT flexible electrodes and a separator. The fabricated supercapacitor device exhibits outstanding electrochemical performance, achieving 465 F g-1 at a scan rate 5 m Vs-1 with 92.47 % retention after 10,000 charging-discharging cycles with outstanding coulombic efficiency. High energy density of 52.03 Wh kg-1 at power density of 307.5 Wh kg-1 is achieved. Hence, the MnS nanoparticles grown on flexible conductive carbon textile offer great potential as an electrode for high energy storage device. 2.

Experimental

2.1.

Synthesis of MnS-nanoparticles assembled carbon textile

Carbon textiles used as template and the MnS nanoparticles were grown on CTs by low temperature hydrothermal method. Typically, commercially available CTs were first cut and washed with acetone, deionized water and ethanol for 15 minutes each, respectively. After drying, the cleansed CTs were dipped into 0.3 M aqueous solution of manganese chloride (MnCl2.4H2O) for 10 hours and then heated in air atmosphere at 350 oC for 30 minutes to form thin seed layer on the surface of CTs. In a typical synthesis of MnS nanoparticles, the reaction solution containing the mixture of 2.5 mmol manganese acetate [(MnCH3COO)2] and 0.2 g of polyvinylpyrrolidone (PVP-55000) in 36 mL deionized water were stirred until become a homogenous milky solution, then 1.0 mmol of elemental sulfur powder (S) was 5



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added to the solution. The reaction solution was transferred into 50 mL capacity Teflon-lined stainless steel autoclave and the treated CTs were immersed and stood vertically on the wall of the autoclave. After that, the autoclave was placed into an electric oven (preheated 180 o

C) for 12 h. After the hydrothermal reaction, the autoclave was cool down naturally to

normal temperature. The samples were cleansed with deionized water and absolute ethanol. The MnS-CTs were first dried in oven at 60 oC for 24 h and then annealed at 350 oC for 2 h in air atmosphere. Digital photographs of pristine CT and annelid MnS-CTs are demonstrated in Fig. S1A-B, respectively. The schematic illustration of the synthesis process of MnS-CTs along with short post annealing process is shown in Figure 1. 2.2.

Preparation of working electrode and solid state supercapacitor

The MnS-CT working electrode of MnS-CT for three electrodes system was fabricated as follows: first, the MnS-CT was cut into 2.5 × 1.5 cm2. The MnS mass on the surface of CT substrate is about 1.25 mg cm-2, carefully calculated by the weight difference between pristine and sintered MnS-CTs. The solid state symmetric supercapacitors were fabricated by combining two MnS-CT electrodes with dimensions (4.5 × 1.2 cm2) face-to-face separated by a filter paper (Whatman 8 µm). LiCl-PVA solid electrolyte was prepared by mixing the 12.6 g LiCl and 6 g polyvinyl alcohol (PVA) in 60 mL deionized water at constant temperature of 85 oC under stirring. After solidification at room temperature for one day the solid state supercapacitor device is ready for electrochemical measurements. 3.

Results and discussions

3.1.

Structural and morphological analysis

Self-supported MnS nanoparticles were fabricated on CT substrates by simple hydrothermal process at 180 oC and then followed by a subsequent annealing treatment in air to remove the

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residual organics and improve the crystalline nature of the prepared MnS nanoparticles. The X-ray diffraction (XRD) pattern of the MnS nanoparticles without carbon textile is presented in Figure 2A. All diffraction peaks could be indexed to the cubic phase α-MnS with standard JCPDS No: 72-1534; space group Fm-3m (225), with lattice parameters a = 5.22 Å. No peaks from impurity or from other phases of MnS are detected, indicating the high purity and complete formation of α-MnS in the final product. The sharp and intense peaks suggest that the product possess good crystallinity. The particle size of MnS is found to be 46.63 nm, calculated using Debye Scherer’s formula (Calculation details in supporting information). The crystal structure of the cubic phase α-MnS is shown in Figure 2B. After annealing the MnS-CTs, the black color of pristine CT turned to the dark gray color, indicating that the CT is uniformly coated with MnS nanoparticles with high yields as can be seen in Figure. S1 A-B, respectively. The morphology of the pristine CT and MnS-CT are shown in Figure 3. SEM image taken from thread of pristine CT and is presented in Figure 3A, demonstrating a compact bundle of fibers with 2D rough texture network, which is excellent for direct growth of nanomaterials. In comparison with the SEM image of pristine CT, we can see the successful growth of MnS nanoparticles on the surface on CT after hydrothermal synthesis, as shown in Figure 3B. The entire surface of CT fibers was covered with high density MnS nanoparticles with average diameter of 100 nm (Fig. S2A), forming a porous structure. Low and high magnified SEM images of the as-synthesized MnS nanoparticles on single CT fiber are shown in Figure 3C and 3D, respectively. No obvious changes or collapse are observed in the morphology of the MnS nanoparticles during the heat treatment, as shown in Fig. S2B. HRTEM images of MnS is shown in Figure 3E, where crystalline of cubic MnS is clearly observed. Figure 3F shows the interplanar spacing of 0.259 nm, which agrees with XRD lattice plane of (200).

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The porosity and specific surface area of MnS nanoparticles were investigated by BrunauerEmmett-Teller (BET) nitrogen adsorption/desorption measurements. The surface area of MnS nanoparticles is 41.08 m2 g-1, which is higher than that of other TMCs used for supercapacitor’s electrodes, such as ZnS nanospheres (28.5 m2 g-1),23 Cu7S4 nanowires (34.23 m2 g-1),24 MoS2 nanospheres (35 m2 g-1),25 Co3S4 hollow nanospheres (14 m2 g-1)26 and β-NiS (30.40 m2 g-1).27 The absorption-desorption curves belong to a typical type IV isotherm with a distinct hysteresis loop indicating the presence of mesoporous features and is presents in Fig. S3A. The pore size distribution is calculated using Barrett-Joyner-Halenda (BJH) method, which further confirms the existence of the mesoporous structure. The pore distribution is relatively narrow and centered at 21 nm (Fig. S3B), illustrates high specific surface area and suitable mesopores for efficient transmission of electrolyte ions,23, 28 which offers rich electroactive sites and shortened ion diffusion paths. High specific surface area is beneficial to enhancing the exposed active available sites for high Faradic redox reactions on the surface/interior portion of the electrode materials. 3.2.

Charge storage analysis

The cyclic voltammetry (CV) is a powerful tool to analyze the electrochemical kinetics of electrode materials. The amount of charges stored in a supercapacitor electrode is the combination of both diffusions controlled Faradic redox reactions and capacitive process. To investigate the kinetics, we first carried out CV of MnS-CT in three electrode configuration in 3 M LiCl aqueous electrolyte. Figure 4A displays the CV curves of MnS-CT electrode at scan rate of 1 to 100 m Vs-1 in potential window of 0.0 to 0.8 V. The redox peaks come from the Mn+2/Mn+3, which is attributed to the electrochemical pseudocapacitive nature of the MnS nanoparticles. The specific capacitance (Cs) of a single MnS-CT electrode in aqueous electrolyte is calculated from CV curves8,

23-25

(Calculation details in supporting

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information), which indicates the highest capacitance of 710.6 and 243.4 F g-1 at scan rate of 1 m Vs-1 and 100 m Vs-1, respectively (Fig. S4). All CV curves have the same shape with small shifting of anodic peaks to higher potentials with increasing scan rates, demonstrating that the capacitive charge storage process increases at higher scan rates.29 The Li+ ion reactions kinetics could be examined by plotting the log(i) versus log(ν) according to the power’s law, and the scan rate dependence of the current can be expressed as29-32

i (V ) = aν

b

(1)

where, i, ν and (a, b) are the current response in CV curves (A g-1), scan rate (V s-1), and adjustable parameters, respectively. The values of a and b can be found from the slope of log(ν)-log(i) plot at a fixed potential 1.0 V. The b-value has two well defined conditions; the b = 0.5 indicates the total diffusion controlled Faradaic reactions process at the surface/inner of electrode and b = 1.0 indicates the total capacitive process. The log(ν)-log(i) plot for MnS-CT electrode in the rage of 1 to 100 mV s-1, as shown in Figure 4B. The b-value is 0.649 for an anodic peak demonstrating that the current partially comes from both Li+ Faradaic reactions and capacitive process.31-33 However, the diffusion controlled Faradaic process dominates the charge storage rather than capacitive process in MnS-CT electrode as b-value is closer to 0.5. The total stored charges could be known at a certain scan rate by separating the specific contributions from diffusion controlled and capacitive process at particular voltage according to the following equation.30-33 i(V ) = k1ν + k 2 ν

Or

i(V )

ν = k1 ν + k 2

(2)

where, k1 and k2 are constants and the terms k1ν , k 2 ν corresponds to the contributions from the capacitive and diffusion controlled Faradaic process, respectively. The values of k1 9



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and k2 can be determined from the slope and y-intercept of ν versus i(V )

ν plot as

shown in Figure S5. It is clear that most charge storage is attributed to the diffusion controlled Faradaic process. A comparison of both kinds of storage at a scan rate of 5 m Vs-1 is shown in Figure 4C, which indicates that 82.85 % capacitance is contributed to Faradaic process and the remaining 17.15 % is from capacitive process. The contribution ratios between these two different storage processes at different scan rates are also quantified, which indicates that the capacitive property increases gradually with increase in scan rates and reaches at maximum value of 40 % at scan rate of 20 mV s-1, as shown in Figure 4D. Two possible redox reactions are proposed for electrochemical process during the charging and discharging of MnS-CT electrode according to literatures.11, 18, 33 MnS + 2 Li + + xe − ↔ Li 2 MnS

(3)

Li 2 MnS + 2e − ↔ Mn + Li 2 S

(4)

During the discharging process the Li ions interact with the MnS which leads to the formation of complex Li2MnS, besides, Mn and Li2S are produced. The charge/discharge mechanism is further analyzed using XRD patterns and HRTEM, the typical XRD patterns of the MnS-electrode at charged state is presented in Figure 4E. During the discharging process the small diffraction peaks belonging to the elemental Mn and Li2S are observed (Figure 4F), which is further confirmed by HRTEM and is shown in Figure 4G (insets “x” and “y” shows the high resolution of selected part from Fig.4G). Where the interplanar spacing of 0.321 and 0.451 nm are in good agreement with the lattice plane of (111) for Li2S (JCPDS: 77-2145) and (110) for elemental Mn (JCPDS: 89-4857), and are elaborated in Figure 4H-I, respectively. 3.3.

Solid-state supercapacitor characteristics 10


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To explore the potential applications of MnS-CT as energy storage device, the symmetric supercapacitor is analyzed by CV and GCD measurements. Figure 5A-B, illustrates the typical CV curves of MnS-CT based supercapacitor at various scan rates in potential windows of -0.8 to 0.8 V and 0.0-1.0 V, respectively. It should be noted that the CV curves are different from typical electrochemical double layer capacitive behavior, which is described by rectangular shaped CV curves.3-4 All CV curves display strong redox peaks, indicating the pseudocapacitive nature of MnS-CT electrodes. As the scan rate increases, the anodic and cathodic current peaks significantly shift towards lower potentials and this is attributed to the increase in capacitive process, which well agrees with power law analysis.3132

The higher current response of MnS-CT also demonstrates the outstanding charge storage

capability and fast ion transportation with rapid I-V response. The comparison between CV curves of pristine CT and MnS-CT based supercapacitors is also depicted in Fig. S6, demonstrating the negligible contribution in capacitance from CT substrate. The specific capacitance (Cs) of the supercapacitor is calculated by integrating the area under the CV curves according to the Eqn.18-23-25 (Calculation details in supporting information) and is depicted in Figure 5C. The specific capacitance of 465, 390, 310, 250 and 150 F g-1 are achieved at 5, 15, 13, 50 and 150 m Vs-1 respectively, which is much higher than those of the previously reported values for solid-state SCs based on MnO2 and TMC’s such as Cu-doped porous δ-MnO2 microsphere (300 F g-1 @ 5 m Vs-1),8 γ-MnS nanocrystals (110.4 F g-1 @ 1.0 mA),14 cupper sulfide nanowires (400 F g-1 @ 10 m Vs-1),24 and MoS2 nanospheres (368 F g1

@ 5 m Vs-1).25 Furthermore, the MnS-CT supercapacitor exhibits excellent rate capability

by attaining the high capacitance of 250 and 178 F g-1 at 50 and 100 m Vs-1, respectively. Therefore, the enhanced capacitance and rate capability are mainly attributed to the wellordered pore distribution and high specific surface area.

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Galvanometric charging/discharging (GCD) measurement is conducted to further evaluate the pseudocapacitive properties of MnS-CT supercapacitor at current density from 0.1 to 0.7 mA cm-2 in potential window of 0.0 to 0.8 V and the results are revealed in Figure 5D. Evidently, all GCD curves are non-linear and present the pseudocapacitive characteristics with outstanding coulombic efficiency (152.5 % at 0.1 mAcm-2), indicating excellent reversibility of redox reactions.14-34 The long discharging time indicates the excellent storage feature with outstanding coulombic efficiency. No obvious iR-drop at the beginning of discharge curve indicates the low internal ohmic resistance and excellent capacitive behavior. Figure 5E presents the charging-discharging profiles recorded at a constant current density of 0.5 mA cm-2 at various potential windows, which follows the same tracing path for charging up to 1.25 V for single supercapacitor, demonstrating that the potential window can be easily extended up to high potentials. To further explain the origin of the high electrochemical activity, electrochemical impedance spectroscopy (EIS) is performed and the corresponding Nyquist plot is depicted in Figure 5F. The EiS spectrum is analyzed using ZSimWin software based on the equivalent electrical circuit (inset Fig. 5F). The EiS spectrum has a semicircle in high frequency range and a quasi-vertical line in the low frequency range. The quasi-vertical makes an angle of 60o with real axis, which is larger than a typical Werburg angle (45o)35 indicating better electrolyte diffusion controlled process on the surface of electrodes, which well agrees with above power law analysis. The diameter of the semicircle in high frequency region represents the electron transfer resistance (Rct ~ 3.53 Ω) and at the intersection point on real axis represents the equivalent series resistance of electrochemical system (Rs ~ 4.25 Ω).23 These results reveals that a fast charge transfer process and the low internal resistance could be achieved due to the direct contact of MnS nanoparticles with substrate without any binder and pressing. 12


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Flexibility is one of the important properties for the wearable electronic device applications. The CV and GCD curves at normal and bent conditions (0o, 90o and 180o) are presented in Figure 6A-B respectively, illustrating that there is no obvious change at normal and bent conditions even at bending of 180o. The whole weight and thickness of MnS-CT supercapacitor are 0.83 g and~1.12 mm, respectively. The capacitor can be bended in any angle without damaging its physical appearance, as shown in Figure 6C. These results demonstrate that the MnS-CT supercapacitor has light weight and possess highly flexibility. The charging/discharging processes for consecutive 10,000 cycles are used to characterize the lifetime of MnS-CT supercapacitor at current density of 0.8 mA cm-2 in 0.0 to 0.8 V for consecutive 10,000 cycles (Fig. S8). The GCD curves of first and last five cycles and the capacitance retention versus number of cycles are presented in Figure 7A-B, respectively. MnS-CT supercapacitor has maintained 92.47 % of its initial capacitance after 10,000 cycles as shown in Figure 7B demonstrating the excellent stability of MnS nanoparticles as supercapacitor’s electrode material. EIS before and after cycling process is performed to characterize the stability of MnS-CT supercapacitor. The charge transfer resistance (Rct) before and after 10,000 cycles is evaluated to be 3.53 and 3.89 Ω, respectively and the equivalent series resistance (Rs) is calculated to be 4.25 and 5.11 Ω, respectively as shown in Fig. S7B. Furthermore, the morphology and crystal structure is also analyzed after the cycling process. The SEM image of MnS-nanoparticles suggests that the basic particle-like morphology is preserved without structural deformation after the cycling process and MnSnanoparticles still have good adhesion with CT substrate (Fig. S8A). However, some agglomerations are also observed, as shown in circled part in Fig. S8B. XRD patterns of MnS-nanoparticles before and after the cycling are shown in Fig. S8C, in which all diffraction peaks could be still indexed as α-MnS, demonstrate the excellent stability of αMnS after long term cycling process. Energy and power densities are key parameters to the 13



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evaluation of the overall performance of the supercapacitor. The energy versus power densities (Ragone plots) of MnS-CT supercapacitor is shown in Figure 7C. The high energy density of 52.03 Wh kg-1 at power density of 307.5 W kg-1 and 28.23 Wh kg-1 even at high power density of 1233 W kg-1 is achieved. These values are much higher than those of MnO2,8 ZnS,23 CuS,24 MoS225 and carbon/graphene based symmetrical supercapacitors.36-38 The high electrochemical performance of the MnS-CT supercapacitor might be ascribed to the following features: (1) The MnS-nanoparticles directly grown on the CT and have an excellent electrical conductivity due to their good mechanical adhesion. (2) The open spaces between nanoparticles allows easy diffusion paths for Li+ ions to every active site of electrodes. (3) MnS-CT can easily accommodate the strain released during the surface reactions and prohibit the structural collapse, which results in an excellent performance even after continuous long term running. For practical demonstrations, three fabricated supercapacitors based on MnS-CT connected in series can easily charge up to 2.5 V and corresponding GCD curves at current density of 5 mA cm-2 are shown in Figure 7D. Three charged supercapacitors can power 4 red-color LED’s (2.0 V, 15 mA) for 2 min. The digital photograph of lighted LED’s is shown in Figure 7E, representing the ability of the MnS-CT supercapacitor as a superior energy storage device. 4. Conclusions

In summary, we have directly synthesized MnS nanoparticles on the carbon textile via simple hydrothermal process and further used it as additive/binder free electrochemical electrode, which exhibits high capacitance of 710.6 F g-1 at scan rate of 1 mV s-1 in aqueous electrolyte. The charge storage process in MnS-CT electrode is quantitatively analyzed in detail and the results demonstrates that the total capacitance is the combination of capacitive

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charge and diffusion controlled Faradic redox process. Faradic process dominates at lower scan rates (82.85 % at 5 mV s-1) while capacitive charge process dominates at higher scan rates (61 % at 20 mV s-1). These results clearly indicate that the MnS nanoparticles can effectively store charges on the surface. Furthermore, an all-solid-state supercapacitor is fabricated with PVA-LiCl solid electrolyte. The MnS-CT supercapacitor exhibits outstanding electrochemical performance by achieving the 465 F g-1 at scan rate 5 m Vs-1 with 92.47 % retention after 10,000 charging-discharging cycles, while the Faradic process dominates. High energy density of 52.03 Wh kg-1 at power density of 307.5 Wh kg-1 is achieved along with outstanding coulombic efficiency of 152.5 % at 0.1 mAcm-2. The supercapacitor is highly flexible and three charged supercapacitors in sereis can light four LED’s (2V, 15 mA) for 2 min. The MnS nanoparticles combined flexible conductive carbon textile offers great potential as electrode for high energy storage supercapacitors.

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Acknowledgments

This work is supported by NSFC (51572040, 51402112), Chongqing University Postgraduates’ Innovation Project (CYB15044), National High Technology Research and Development Program of China (2015AA034801), the Fundamental Research Funds for the Central Universities (CQDXWL-2014-001), NSFCQ (cstc2015jcyjA20020), The Science and Technology Research Project of Chongqing Municipal Education Commission of China (KJ1400607).

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References

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Figure 1: Schematic demonstration of the synthesis process of MnS nanoparticles via hydrothermal process on the surface of carbon textile with short post annealing process.

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Figure 2. (A) The XRD pattern of as-synthesized MnS nanoparticles and (B) Crystal structure of MnS, where purple balls indicate the Mn atoms and yellow balls indicate the sulfur atoms.

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Figure 3. SEM and TEM images of the MnS nanoparticles grown on carbon textile at different magnifications. (A) SEM image of bare carbon textile, (B) MnS nanoparticles grown on carbon textile, (C) MnS nanoparticles grown on single fiber of carbon textile, (D) high resolution SEM image selected part from (C) presents the average size of particle is 100 nm, (E) High resolution TEM image of MnS nanoparticles and (F) magnified image of selected part form (E) indicating the lattice spacing of 0.259 nm which is well agreement with X-ray diffraction peak corresponding to (200). 22


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Figure 4. Kinetic analysis of the electrochemical behavior of MnS-CT electrode and structural characterization by in-situ XRD and HRTEM. (A) Cyclic voltammetric from 1.0 to 100 mV s-1 in potential window of 0.0 to 0.8 V, (B) Determination of b-value from the anodic peak currents using power law, which shows that the b-value is 0.649 (inset Fig. B), (C) Typical separation of capacitive and Faradic diffusion controlled charge storage process at scan rate of 5 mV s-1, (D) Contribution ratio of capacitive and Faradic diffusion controlled charge storage, (E-F) The comparison between XRD patterns of fresh, charged and discharged states of MnS electrode, (G) HRTEM at discharged state and (H-I) HRTEM of selected part from Fig. G, x and y, respectively.

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Figure 5. Electrochemical performances of the MnS-CT solid-state supercapacitor. (A) CV curves at different scan rates ranging from 5 to 150 m Vs-1 in potential window of -0.8 to 0.8 V, (B) CV curves at different scan rates ranging from 25 to 100 m Vs-1 in potential window of 0.0 to 1.0 V, (C) Specific capacitance as a function of scan rate, (D) Charging-discharging curves taken at different current densities ranging from 0.1 to 0.7 mA cm-2 in potential window of 0.0 to 0.8 V, (E) Charging-discharging curves taken at constant current density of 0.5 mA cm-2 at different potential windows from 0.0 to 1.25 V for single supercapacitor and (F) Nyquist plot of impedance from 0.01 to 100 kHz, inset is the corresponding equivalent electrical circuit.

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Figure 6. Flexible performance of MnS-CT solid-state supercapacitor under different bending conditions, demonstrating that the bent device has no effect on its performance as seen in (A) CV curves at constant scan rate of 50 mV s-1, (B) Charging-discharging curves taken at flat and bent position at 180 degrees at constant current density of 0.6 mA cm-2 and (C) digital photographs of solid state supercapacitor bent in different angles showing the flexibility of the device.

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Figure 7. Long term cycling performance and practical demonstration of the fabricated MnSCT solid state supercapacitors (A) Charging-discharging curves of first five cycles and last five cycles after 10,000 cycles at constant current density of 0.8 mA cm-2, (B) Cyclic performance at constant current density of 0.8 mA cm-2, cycle number verses capacitance retention, (C) Ragone plots (Energy density vs. Power density) of the supercapacitor, (D) Charging-discharging curves of three supercapacitors connected in series which could be charged up to 2.5 V at current density of 5 mA cm-2, demonstrating the stable performance at high voltage and (E) Three charged supercapacitors connected in series could light the 4 redcolor LEDs (2 V, 15 mA) for 2 min.

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Graphical Abstract

The surface redox reactions on MnS nanoparticles are found to be a dominant part in energy storage process at low scan rate (82.85 % @ 5 mV s-1). The MnS-CT supercapacitor is achieved capacitance retention of 92.47 % after consecutive 10,000 charging/discharging cycles although the Faradic diffusion controlled process is proceeds.

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Highlights

1. The MnS-nanoparticles are directly grown on the conductive carbon textile (MnSCT) with large specific surface area. 2. Pseudocapacitive contribution to the energy storage of MnS-CT is investigated. 3. The charge storage process dominates by Faradic diffusion controlled process at low scan rate. 4. The MnS-CT based solid state supercapacitor shows the excellent electrochemical performance.

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Tracking Pseudocapacitive Contribution to ... - ACS Publications

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