CoMn₂O₄ Nanoparticles Decorated on 2D MoS₂ Frame: A Synergetic Energy Storage Composite Material for Practical Supercapacitor Applications

HIGHLIGHTS

• Facile synthesis of mesoporous CoMn₂O₄@MoS₂ nanocomposite.

• Content of MoS₂ has been optimized to obtain best CoMn₂O₄@MoS₂ nanocomposite.

• Best CoMn₂O₄@MoS₂ sample shows specific capacitance of 422 F g⁻¹ @ 0.5 A g⁻¹.

• Fabricated asymmetric supercapacitor exhibits maximum specific energy of 37 W hkg⁻¹.

• Such fabricated single asymmetric supercapacitor can light up a red LEDs for more than 3 min.

Abstract

In material science, the synergistic effect comes into the picture when the physical and/or chemical properties of a composite material improve noticeably in comparison to that demonstrated by its forming individual components. This work represents the facile synthesis of CoMn₂O₄ (CMO) and CoMn₂O₄@MoS₂ (CMOS) nanocomposites via a co-precipitation synthesis approach. In this study, amounts of CMO precursors were kept constant and the effect of MoS₂ addition on the electrochemical properties of the nanocomposite has been investigated. The substantial improvement in the electrochemical performance of the nanocomposite after adding MoS₂ contents with CMO can be attributed to the synergistic effect. The CMOS nanocomposite synthesized using 20% MoS₂ uniform dispersion (abbreviated as CMOS20) exhibits maximum improvement in the electrochemical properties which is ascribed to its higher specific surface area (74 m² g⁻¹) and hierarchical pore size distribution. When examined in a conventional three-electrode system with 2 M KOH aqueous electrolyte, CMOS20 nanocomposite demonstrates high specific capacitances of 422 F g⁻¹ at 0.5 A g⁻¹, good cyclability, high-rate capability and higher diffusion coefficients (1.96 × 10⁻¹⁰ cm² s⁻¹). An asymmetric supercapacitor device designed using CMOS20 nanocomposite cathode and activated carbon anode exhibit maximum specific energy of 37 W h kg⁻¹ and the maximum specific power of 5000 W kg⁻¹. This practical device can light up a red LED for more than 3 min. We believe the facile synthesis approach and promising electrochemical results assert the potential of our designed CMOS20 nanocomposite in the development of high-performance practical supercapacitor devices.

Introduction

With the continuous increase in the human population, the energy demand is increasing exponentially. Since the beginning, fossil fuels are the main resources of global primary energy recruitment, but these are not permanent and their too much consumption has become a survival issue as it increases global warming, pollution, and several health-related problems. Therefore, the scientific community all around the world is working toward the utmost utilization of renewable energy sources to produce cleaner and greener energy to fulfil the global energy demand. The main problem associated with renewable energy sources like solar and wind, is intermittency, as the sun may disappear behind clouds and not shines at night, while wind flow will not be the same all the time. The practical solution to mitigate this problem is the coupling of these renewable energy sources with some efficient energy storage devices (Li-ion batteries and supercapacitors), which can provide energy whenever needed. Recently, Supercapacitors have drawn comprehensive research attention as the sustainable and renewable energy storage device, due to its outstanding properties including ultrafast charging-discharging, excellent cycle life, higher specific power, easy handling, low cost and non-polluting nature [1], [2], [3], [4], [5], [6]. Based on energy storage mechanism, supercapacitors have been divided mainly into two categories; (1) Electrical double-layer capacitors (EDLCs) in which charge storage happens at the electrode-electrolyte interface purely via electrostatic attraction and no faradic reactions are involved, and (2) Pseudocapacitors in which charge storage occurs due to fast reversible reactions at the electrode-electrolyte interface and inside the active material near the surface up to depth L ≪ (2Dt)^1/2, where D is the diffusion coefficient for charge-compensating ions (cm² s⁻¹), and t is time (s) [7]. EDLC materials like graphene, CNTs and activated carbon exhibit high specific power but poor specific energy (3-5 W h kg⁻¹), which is 1-2 orders lower than commercial Li-ion batteries (100-275 W h kg⁻¹), restrict their potential utilization in high-end applications like hybrid electric vehicles (HEVs), wind-generated energy storage and regenerative breaking [8], [9], [10]. In this regard, transition metal oxides, hydroxides, and conducting polymers, which demonstrate high specific capacitance and hence high specific energy (E = 1/2CV²) are gaining considerable attention of energy researchers. However, due to low electronic conductivity, poor specific surface area, and the presence of some irreversible electrochemical reactions, pseudocapacitor materials generally exhibit lower specific power and cyclability than that of EDLC materials [11]. Therefore, composites of these materials with the materials which can improve their conductivity, surface area, porous structure, and electrochemical stability at lower expense are highly desirable.

In the last decade, transition metal oxides (TMOs) having variable oxidation states, which perform redox reactions, have been tested more as energy storage materials, because of their high theoretical specific capacitances. Recently, binary TMOs having spinel structures such as MCo₂O₄ (M = Ni, Co, Cu, Zn, Mn) [12], [13], [14], [15], MMn₂O₄ (M = Zn, Co, Ni) [16], [17], [18], and MFe₂O₄ (M = Ni, Co, Zn, Mn) [19,20], etc. have gained huge research interest as these exhibit enhanced electronic conductivities, higher specific capacitance, the lower activation energy for charge transfer between cations, good structural stability and more active sites for rapid reversible faradic reactions than that of unitary metal oxides.

Among these, Mn-based binary TMOs are the most promising materials for supercapacitor electrodes because of their huge abundance, low-cost, eco-friendly nature, and easy accessibility. Co-Mn oxides, specially CoMn₂O₄ which are formed by replacing the Mn-cation by Co-cation at the tetrahedral and/or octahedral position in the spinel Mn₃O₄ structure usually demonstrates a mixed spinal structure in which both the cations occupy both octahedral and tetrahedral sites and the cationic distribution is represented as$\left[A_x^{2+}{B}_{1-x}^{3+}\right]\left[A_{1-x}^{2+}{B}_{1+x}^{3+}\right]O_4$, where x is the degree of inversion and its value depends on synthesis conditions. A normal spinal structure is obtained for x = 0, when x = 1 inverse spinal structure is formed and for 0< x <1 a mixed spinal structure is obtained [21], [22], [23]. Because of the combined effect of Mn⁺³ and Co⁺² cations, CoMn₂O₄ exhibits exceptional pseudocapacitive performance as cobalt demonstrates higher oxidation potential while manganese transports more electrons and earns higher capacitance [24]. Because of this, the supercapacitor performance of CoMn₂O₄ has been investigated by many research groups. For example, L. Ren et al. developed CoMn₂O₄ microspheres-based supercapacitor electrode material with specific capacitance 188 F g⁻¹ at the specific current of 1 A g⁻¹ and 93% cycling stability after 1000 GCD cycles in 1 M Na₂SO₄ aqueous electrolyte [25]. J. Bhagwan et al. studied the electrochemical performance of CoMn₂O₄ nanofibers and observed the specific capacitance of 320 F g⁻¹ at 1 A g⁻¹ on graphite sheet in 1 M H₂SO₄ aqueous electrolyte [17]. Z. Chaozheng et al. grown CoMn₂O₄ nanosheets on Ni foam and observed the specific capacitance of 475 F g⁻¹ @ 5 A g⁻¹ and 95% capacitance retention after 500 cycles [26]. C. Zhang et al. studied the electrochemical performance of CoMn₂O₄/3DrGO composite on Ni-foam and found specific capacitance of 1028 F g⁻¹ at 1 A g⁻¹ and 89.7% capacitance retention after 5000 cycles [27]. Results show that the electrochemical performance of TMOs greatly depends on electronic conductivity, specific surface area, and pore size distribution. In this regard, various composites of CMO with the appropriate amount of materials having high specific surface area and electronic conductivity have been synthesized which due to the combined effect of both the two forming individuals demonstrate enhanced electrochemical performance [27], [28], [29]. Recently, 2D layered transition metal dichalcogenides (TMDCs), especially MoS₂, have gained much research interest as a supercapacitor electrode material because of their sheet-like layered morphology, large surface area, high electronic conductivity, and easy synthesis in the large amount [30], [31], [32], [33], [34], [35]. According to the reported data, MoS₂ deliver high specific capacitance with good cycling stability, as a consequence of both non-faradaic and faradaic reactions during charging-discharging [36], [37], [38], [39].

In the present study, we have synthesized CoMn₂O₄@MoS₂ nanocomposites via a simple approach and investigated their electrochemical performance. To the best of our knowledge, supercapacitor properties of CoMn₂O₄@MoS₂ nanocomposite have not been reported yet. The best optimized CoMn₂O₄@MoS₂ nanocomposite (CMOS20) exhibits a high specific capacitance of 422 F g⁻¹ at 0.5 A g⁻¹ and ~90% capacitance retention after 1000 successive cycles in 2 M KOH aqueous electrolyte at room temperature in a three-electrode system. Further to test the practical utility of CMOS20 as a supercapacitor electrode material, an asymmetric device has been designed with activated carbon as a negative electrode. This device exhibits maximum specific energy of 37 W h kg⁻¹ and a maximum specific power of 5000 W kg⁻¹. This single device can light up a red LED for more than 3 min, which verifies CMOS20 could be a good supercapacitor electrode material for practical applications.