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한국센서학회 학술지영문홈페이지
[ Article ]
JOURNAL OF SENSOR SCIENCE AND TECHNOLOGY - Vol. 29, No. 4, pp.215-219
ISSN: 1225-5475 (Print) 2093-7563 (Online)
Print publication date 31 Jul 2020
Received 27 May 2020 Revised 23 Jul 2020 Accepted 29 Jul 2020
DOI: https://doi.org/10.46670/JSST.2020.29.4.215

MnCo2S4/CoS2 Electrode for Ultrahigh Areal Capacitance

Rahul B. Pujari1 ; C. D. Lokhande2 ; Dong-Weon Lee1, +
1MEMS and Nanotechnology Laboratory, School of Mechanical System Engineering, Chonnam National University, Gwangju, 61186, Republic of Korea
2Centre for Interdisciplinary Research, D.Y. Patil Education Society (Deemed to be University), Kolhapur (M.S.), 416 006, India

Correspondence to: + mems@jnu.ac.kr

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(https://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

MnCo2S4/CoS2 electrode with highly accessible electroactive sites is prepared using the hydrothermal method. The electrode exhibits an areal capacitance of 0.75 Fcm-2 at 6 mAcm-2 in 1 M KOH. The capacitance is further increased to 2.06 Fcm-2 by adding K3Fe(CN)6 and K4Fe(CN)6 (a redox couple) to KOH. This increment is associated with the redox-active properties of cobalt and manganese transition metals, as well as the ion pair of [Fe(CN)6]-3/[Fe(CN)6]-4. The capacitance retention of the MnCo2S4/CoS2 electrode is 87.5% for successive 4000 charge–discharge cycles at 10 mAcm-2 in a composite electrolyte system of KOH and ferri/ferrocyanide. The capacitance enhancement is supported by the lowest equivalent series resistance (0.62 Ωcm-2) of MnCo2S4/CoS2 in the presence of redox additive couple compared with the bare KOH electrolyte.

Keywords:

Hydrothermal, CoS2, MnCo2S4, Redox additive, Supercapacitor

1. INTRODUCTION

Supercapacitors, unlike electrostatic capacitors, are endowed with moderate energy density and high power density [1]. Energy density is directly related to capacitance, i.e., E = 0.5CV2. Out of various ways to improve the capacitance of supercapacitors, electrodes based on transition metals, e.g., MnO2 [2] and MoO3 [3], are very effective than carbon-derived materials. Recently, transition metal sulfides are intensively tested like CoS2 [4] owing to the superiority of electrically conductive nature compared with the oxide electrodes. Even bimetallic sulfide electrodes, such as NiCo2S4 [5] Zn0.76Co0.24S [6], CuCo2S4 [7], MnCo2S4 [8] and MxCo3-xS4 (M = Ni, Mn, Zn) [9], cannot improve the targeted energy storage for supercapacitor devices. It is anticipated that the composite electrode of the MnCo2S4 and CoS2 will achieve targeted capacitance and energy density of supercapacitor. The individual redox activities of MnCo2S4 and CoS2 as well as electrochemical interactions between them will enhance performance of the composite electrode.

Furthermore, the practices have been done to improve the capacitance of aqueous supercapacitors using redox additives in parent electrolytes, e.g., K3Fe(CN)6. Previously, the capacitance of the graphene paper electrode was enhanced to 475 mFcm-2 from 93 mFcm-2 (obtained from the bare Na2SO4 electrolyte) using the K3Fe(CN)6/Na2SO4 redox electrolyte system [10]. Moreover, capacitance improvement has been observed in Co-Al–layered double hydroxide [11] and Co(OH)2 [12] by pouring K3Fe(CN)6 or K4Fe(CN)6 into 1 M KOH.

However, the hydrothermal synthesis of the MnCo2S4/CoS2 electrode and its electrochemical charge storage performance are yet to be explored in a redox-active electrolyte. Therefore, we have prepared a highly accessible MnCo2S4/CoS2 electrode using the hydrothermal method, and its electrochemical charge storage is evaluated in 1 M KOH electrolyte. The capacitance increment of the MnCo2S4/CoS2 electrode is more than two times in the redox active electrolyte compared with bare KOH.

2. EXPERIMENTAL DETAILS

2.1 Synthesis of MnCo2S4/CoS2

Manganese sulfate (MnSO4), cobalt sulfate (CoSO4), thiourea (SC(NH2)2), and urea (CO(NH2)2) (Thomas Baker Pvt. Ltd., Mumbai) were used without further treatments. MnSO4 (100 mM), CoSO4 (200 mM), SC(NH2)2 (400 mM), and CO(NH2) (200 mM) were dissolved successively in 40 ml of double-distilled water (DDW) under constant magnetic stirring to produce a final solution. Then, type 304 stainless steel (SS) substrate was polished using a zero-grade silicon carbide paper and cleaned with DDW. Then, the substrate was placed and aligned with the solution in a 40-ml glass beaker. The beaker was placed in a 16-L hydrothermal SS autoclave and heated at 363 K for 5 h. After the chemical reaction, the MnCo2S4/CoS2 thin-film electrode of sky blue color was formed on the SS substrate. The electrode was removed from the autoclave and cleaned with DDW and ethanol.

2.2 Materials characterization

The growth orientation of material and crystal structure was analyzed with the Bruker X-ray diffractometer using the X-ray diffraction (XRD) technique. The chemical states of different elements present in the material were analyzed using the X-ray photoelectron spectroscopy (XPS) technique. The surface morphology of the thin film was assessed using field emission scanning electron microscopy (FE-SEM). The electrochemical charge storage of electrode was evaluated in 1 M KOH, 1 M KOH + 0.05 M K3Fe(CN)6, and 1 M KOH + 0.025 M K3Fe(CN)6 + 0.025 M K4Fe(CN)6 electrolytes. The electrochemical impedance of MnCo2S4/CoS2 was measured in the aforementioned electrolytes in the range of 0.1 Hz to 100 kHz with an AC amplitude of 10 mV and zero biased potential. Electrochemical measurements were performed using the ZIVE SP5 electrochemical workstation with three electrode cell comprising MnCo2S4/CoS2, standard calomel, and platinum plate as working, reference, and counter electrodes, respectively.

3. RESULTS AND DISCUSSIONS

3.1 Physico-chemical study

The crystallinity of material plays a significant role in the performance of electrode materials; thus, the crystal structure and growth orientation of composite material is assessed based on the XRD pattern (Fig. 1). Highly intense XRD peaks of the electrode materials suggest that they are well crystallized. The XRD peaks indicated with (200), (210), (222), and (230) are associated with the CoS2 phase of cobalt sulfide. The other peaks marked with (111), (220), and (222) should be considered for the MnCo2S4 phase of manganese cobalt sulfide [13]. Thus, the composite of the highly crystalline MnCo2S4/CoS2 material is identified using XRD.

Fig. 1.

(a) XRD pattern of MnCo2S4/CoS2.

The surface morphology of the electrode material mainly affects the electrochemical performance. The FE-SEM images of the MnCo2S4/CoS2 electrode (Fig. 2a-b) show vertical interlocked discs at micro-to nanoscale, with incorporated large pores and vertical nanoflakes present underneath the discs. Such surface morphology is highly accessible for electrolyte ions during the electrochemical performance and extremely useful for supercapacitor applications. The discs and nanoflakes are connected to their side edges, which can tolerate high-rate charging and discharging during electrochemical interactions with electrolyte ions. The porous microstructure surface material facilitates easy ion intercalation and de-intercalation into an electrode matrix, which can improve charge kinetics in the material. Such discs and 2-D flakes help reduce electron transfer with successive nanodiscs during the electrochemical performance.

Fig. 2.

(a, b) FE-SEM images of the MnCo2S4/CoS2 electrode at different resolutions. The inset shows a low-resolution image.

3.2 Electrochemical study

Cyclic voltammetry (CV) is a fundamental technique for analyzing charge storage kinetics in a material. The CV curves of the MnCo2S4/CoS2 are measured at scan rates of 5–100 mVs-1 within -0.2 to + 0.55 V/SCE potential window, initially in 1 M KOH electrolyte and then in redox active electrolytes, i.e., 0.05 M K3Fe(CN)6 + 1 M KOH and 0.025 M K3Fe(CN)6 + 0.025 M K4Fe(CN)6 + 1 M KOH. For convenience, 0.05 M K3Fe(CN)6 + 1 M KOH and 0.025 M K3Fe(CN)6 + 0.025 M K4Fe(CN)6 + 1 M KOH electrolytes are referred to as K3KOH and K3K4KOH, respectively. The CV curves of MnCo2S4/CoS2 electrode in the KOH electrolyte (Fig. 3a) show a shift in oxidation and reduction peaks toward high and low potential with increasing scan rate; this shift indicates the pseudocapacitive behavior of the electrode.

Fig. 3.

(a) CV curves at scan rates of 5–100 mVs-1. (b) GCD curves at 6–12 mAcm-2 of MnCo2S4/CoS2 in the KOH electrolyte.

Similarly, it has occurred in the case of galvanostatic charge–discharge profile of the electrode at different current densities (Fig. 3b), which supports the CV analysis. The CV curves of MnCo2S4/CoS2 (Fig. 4a) in K3KOH demonstrates an increase in cathodic and anodic current densities owing to the presence of the redox-active K3K4KOH component, and the current density further increases in case of K4Fe(CN)6 and K3K4KOH electrolytes.

Fig. 4.

CV curves of MnCo2S4/CoS2 recorded in (a) K3KOH and (b) K3K4KOH electrolytes. (c) Areal capacitances vs scan rate plot.

As compensating [Fe(CN)6]+3 and [Fe(CN)6]+4 ions are present in K3K4KOH, the symmetric CV curves of the electrode are seen for K3K4KOH than that for K3KOH electrolyte. Moreover, the CV curves (Fig. 3a) in KOH exhibit a pair of reduction and oxidation peaks, while for K3K4KOH (Fig. 4a-b), an additional pair of reduction and oxidation peaks has been observed at each scan rate owing to the charge transfer between [Fe(CN)6]3- and [Fe(CN)6]4-. The electronic exchange between KOH and MnCo2S4/CoS2 is defined by the following reaction:

MnCo2S4+3OH-↔MnS2-2XOH+2CoSXOH+3e-(1) 

For the K3KOH electrolyte, K3Fe(CN)6 molecules produce the ion pair of [Fe(CN)6]3-/[Fe(CN)6]4-, which adds pseudocapacitance, as shown in equation (2). In case of K3K4KOH, [Fe(CN)6]3- and [Fe(CN)6]4- are already present in the electrolyte in equal concentration; thus, symmetric CV curves are observed with increased current densities. Additional electron sharing between [Fe(CN)6]3- and [Fe(CN)6]4- during the charging and discharging of the electrode in K3K4KOH (rather than in K3KOH) is defined by the following reaction:

K3FeCN63-+e-↔K3FeCN64-(2) 

The areal capacitance (Ca) of the MnCo2S4/CoS2 electrode in different electrolytes are calculated using the following equation:

Ca=∫V1V2IVdVA×ν(3) 

where the numerator part is the integral area of the CV curve for each scan rate (v) and A is the area (1 cm2) of the MnCo2S4/CoS2 electrode. The v1 and v2 are the lowest negative and highest positive potentials, respectively, of the CV curves. The MnCo2S4/CoS2 electrode has exhibited the highest capacitance of 2.4 Fcm-2 at a scan rate of 5 mVs-1 in the K3K4KOH electrolyte compared with the capacitance in KOH (0.8 Fcm-2; Fig. 4c). Thus, the capacitance increment of MnCo2S4/CoS2 in K3K4KOH is attributed to pseudocapacitance of the redox activity of cobalt and manganese transition metals associated with the MnCo2S4/CoS2 electrode materials, as well as the redox activity of [Fe(CN)6]3-/[Fe(CN)6]4- present in the electrolyte solution. The decrease in the capacitance of the MnCo2S4/CoS2 electrode is observed and attributed to charge transfer and mass transfer polarization at high scan rates [3, 4].

Moreover, the charge storage of MnCo2S4/CoS2 is evaluated based on the GCD measurements in the K3KOH and K3K4KOH electrolytes (Fig. 5a-c). The GCD curves at different current densities show non-linear shapes during the charging and discharging of the electrode suggesting the pseudocapacitive contribution of the material in addition to electrochemical double layer capacitance (EDLC) [3, 4]. In the GCD curves, the discharging part shows the initial potential drop caused by the internal resistance of the electrode material and the subsequent curved portion demonstrating the redox activity of the material. Increased electrochemical reactions, as shown by equation (2), and the presence of the ferri and ferrocyanide ion pair near to the electrode surface have pushed the charging and discharging time of MnCo2S4/CoS2 to a higher scale in K3K4KOH. Therefore, the maximum areal capacitance of 2.06 Fcm-2 at a current density of 6 mAcm-2 is obtained in K3K4KOH compared with 1.08 Fcm-2 in K3KOH and 0.75 Fcm-2 in KOH (Fig. 5c). The obtained 2.06 Fcm-2 areal capacitance of the MnCo2S4/CoS2 electrode is higher than recently reported 0.475 Fcm-2 for graphene paper electrode in redox active (Composite of K3Fe(CN)6, K4Fe(CN)6 and Na2SO4) electrolyte [9]. The cyclic lifetime of the MnCo2S4/CoS2 electrode is evaluated in all three electrolytes for successive 4000 GCD cycles performed at 10 mAcm-2. Fig. 6a shows capacitive retentions of 89%, 88%, and 87.5% of MnCo2S4/CoS2 in KOH, K3KOH, and K3K4KOH, respectively, after 4000 cycles demonstrating that cycling life of electrode is not influenced with addition of K3Fe(CN)6 and K4Fe(CN)6 redox additives. Thus, exhibits stable composite system of three different electrolytes.

Fig. 5.

GCD curves of MnCo2S4 at various current densities in (d) K3KOH and (e) K3K4KOH electrolytes. (f) Variation in areal capacitance of MnCo2S4/CoS2 in different electrolytes.

Fig. 6.

(a) Capacitive retention of MnCo2S4/CoS2 in KOH, K3KOH, and K3K4KOH electrolytes for 4000 GCD cycles measured at 10 mAcm-2. (b) Nyquist curves of MnCo2S4/CoS2 in various electrolytes measured at 0.1–105 Hz; the inset shows the magnified part of the Nyquist curves in high and mid-high frequency regions.

The impedance of MnCo2S4/CoS2 was measured to analyze electrochemical reaction kinetics of the electrode in different electrolytes and calculate different resistances at the electrode–electrolyte interface. Fig. 6b shows the impedance spectra of MnCo2S4/CoS2 in different electrolytes showing initial intersection with the real axis known to equivalent series resistance (ESR), which comprises electronic resistance and ionic resistance of the electrode–electrolyte system. The electronic resistance is caused by the intrinsic resistance of electrode material as well as resistance between electrode and current collector [13]. The ionic resistance is caused by movement of electrolyte ions in the electrolyte and the pores of the electrode material. The lowest ESR of 0.62 Ω/cm-2 is obtained for MnCo2S4/CoS2 in K3K4KOH compared with that in K3KOH and KOH (0.66 and 0.73 Ω/cm-2, respectively). The lower ESR facilitates higher electronic and ionic transfer at the electrode–electrolyte interface in K3K4KOH, which assures maximum supercapacitive performance of the electrode.

4. CONCLUSIONS

The MnCo2S4/CoS2 electrode has been prepared using a single-step hydrothermal method with the surface morphology similar to interconnected vertically aligned discs. The electrode exhibited excellent electrochemical charge storage of 0.75 Fcm-2 at 6 mAcm-2 in 1 M KOH owing to the redox activity of manganese and cobalt transition metals incorporated in the electrode materials. It is increased to 2.06 Fcm-2 in K3K4KOH caused by the [Fe(CN)6]-3/[Fe(CN)6]-4 ion pair in the redox-active electrolyte. The cyclic lifetime of the MnCo2S4/CoS2 electrode exhibited capacitance retention of 87.5% in redox active electrolyte, even after 4000 GCD cycles at 10 mAcm-2, and it is not affected by the [Fe(CN)6]-3/[Fe(CN)6]-4 ion pair, which shows the stability of the K3K4KOH electrolyte. The maximum pseudocapacitive charge storage of MnCo2S4/CoS2 in K3K4KOH is supported by a decrease in ESR to 0.62 Ωcm-2 compared with 0.73 Ωcm-2 in the KOH electrolyte.

Acknowledgments

1. This study was supported by the National Research Foundation of Korea (NRF) under grant No. 2015R1A4A1041746 funded by the Korean Government (MSIP).

2. The authors are thankful to the Department of Science and Technology, Govt. of India, for financial support through the research project, Materials for Energy Storage, under sanction no. [DST/TMD/MES.2K17/04 (C&G)] dated July 17, 2018.

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Fig. 1.

Fig. 1.
(a) XRD pattern of MnCo2S4/CoS2.

Fig. 2.

Fig. 2.
(a, b) FE-SEM images of the MnCo2S4/CoS2 electrode at different resolutions. The inset shows a low-resolution image.

Fig. 3.

Fig. 3.
(a) CV curves at scan rates of 5–100 mVs-1. (b) GCD curves at 6–12 mAcm-2 of MnCo2S4/CoS2 in the KOH electrolyte.

Fig. 4.

Fig. 4.
CV curves of MnCo2S4/CoS2 recorded in (a) K3KOH and (b) K3K4KOH electrolytes. (c) Areal capacitances vs scan rate plot.

Fig. 5.

Fig. 5.
GCD curves of MnCo2S4 at various current densities in (d) K3KOH and (e) K3K4KOH electrolytes. (f) Variation in areal capacitance of MnCo2S4/CoS2 in different electrolytes.

Fig. 6.

Fig. 6.
(a) Capacitive retention of MnCo2S4/CoS2 in KOH, K3KOH, and K3K4KOH electrolytes for 4000 GCD cycles measured at 10 mAcm-2. (b) Nyquist curves of MnCo2S4/CoS2 in various electrolytes measured at 0.1–105 Hz; the inset shows the magnified part of the Nyquist curves in high and mid-high frequency regions.