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    Topic review

    MoS2 Energy Applications

    Subjects: Electrochemistry
    View times: 10
    Submitted by: Amine El Moutaouakil
    (This entry belongs to Entry Collection "Environmental Sciences ")

    Definition

    MoS2 is one of the transition metal dichalcogenides (TMDs) that has gained a high reputation in recent years due to its distinct chemical, electronic, mechanical, magnetic, and optical properties. Its unique properties enabled its use in different applications such as sensing applications, high-efficiency field effect transistors, and energy and medical (curing) applications. MoS2 exists in different crystalline structures, such as hexagonal (H), tetrahedral (T), or rhombohedral (R). It naturally exists as 2H MoS2, and its most popular structures are the semiconducting 2H and 3R phases and the 1T metallic phase, where 2H is more stable but less conductive than 1T. Metallic MoS2 has a higher conductivity (105 times) than semiconducting 2H MoS2 and high catalytic activity.

    1. Structure and Properties

    MoS2 layers are formed by covalent bonds between sulfur and molybdenum S-Mo-S, as a layer of Mo sandwiched between two layers of sulfur. The layers are connected together through weak van der Waal forces [1]. MoS2 exists in many phases, where its characteristics and properties differ according to its phase. The 1T phase is an octahedral structure, while 2H and 3R are trigonal prismatic structures [2]. The 3R phase showed better catalytic activity in hydrogen evolution reactions than the 2H and 1T phases [3]; however, not much work has been conducted on the 3R MoS2 phase. Monolayer 2H-MoS2 is semiconducting, with a direct bandgap of ~1.8 eV [4][5]. 2H MoS2 exists in nature and is stable under ambient temperature. Metallic MoS2 is a metastable structure that does not exist in nature and is synthesized from the 2H phase or formed by controlled transitions, e.g., using an electron beam [6], ion intercalation [7], or laser irradiation [8][9]. It has superconductivity and high catalytic activity [10] that render it promising for energy applications. Although the metallic phase of MoS2 has challenges with stability and synthesis, research is directed towards it because of its high conductivity, which renders it promising for energy storage applications, such as its use in supercapacitors [11][12] and batteries [13][14][15].
    The 1T MoS2 phase is metastable and coexists with other phases such as 1T’, 1T’’, and 1T’’’ (Figure 1). The phases are easily transformed to the 2H phase by annealing at nearly 70 °C [16]. The 1T’ phase is a superconductor, while 1T’’’ can be either a superconductor or an insulator depending on the synthesizing technique [16]. Generally, the 1T metastable phases have superconductivity and catalytic activity in hydrogen evolution reactions, which directed energy studies to these metastable phases. However, their electronic and magnetic properties and their device applications have not been studied extensively due to their metastability. A quantum spin Hall effect is expected from the 1T’ polytype [17]. The 1T metallic phase was proposed to decrease the contact resistance in ultrathin MoS2 transistors [7][18]. The 1T phase is laid over the 2H semiconducting phase (which is known for its high resistance (0.7–10 kΩ μm)) to decrease the contact resistance to 200–300 Ω μm at zero gate bias.
    Figure 1. MoS2 different crystal structures. (a) The top and side views of monolayer MoS2 for H and T phases. (b) Different lattice structures of MoS2 metallic phases 1T’, 1T’’, and 1T’’’. Adapted from [16]. American Physical Society 2018.

    2. Energy Applications

    2.1. Energy Storage Applications

    2.1.1. Lithium-Ion Batteries (LIB)

    Lithium-ion batteries (LIB) have a high capacity and are recycable. Most portable devices have LIB; however, their capacity is still too low to be used in some electrical vehicles. Metallic MoS2 can serve as an anode in LIB due to its high conductivity, specific capacity, and large surface area which enable better intercalation of the incoming ions and enhance the battery’s stability and rate performance [19]. The chemical composition of metallic MoS2 (1T MoS2) was investigated using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and X-ray diffraction (XRD), and it was found to have binding energies of 228.8 and 231.9 eV corresponding to the 3d5/2 and 3d3/2 components, respectively, for Mo–S bonding. The S 2p components have binding energies of 161.5 eV and 162.5 eV, corresponding to S 2P3/2 and 2P1/2, respectively [14][20]. The bonding of both Mo and S is nearly 1 eV less than that of 2H MoS2.
    The research work in this area is based on two directions, namely, whether to enhance the stability of metallic MoS2-based LIB or to enhance the conductivity of 2H MoS2 to serve as an anode in LIB, since it has better stability. In a trial to suppress the high intrinsic electric conductivity of metallic MoS2, it was alligned over graphene with a relatively large separation distance of 0.98 nm between them. The first cycle showed a high capacity of ≈1700 mA h g−1 at a current density of 70 mA g−1 and an initial coulombic efficiency of 70%. The battery had a high capacity of 666 mA g−1 at a high current density of 3500 mA g−1, with a reversible capacity of ≈1700 mA g−1 at a low current density of 70 mA g−1 [21]. MoS2 was mounted on carbon fiber cloth to obtain a high reversible specific capacity of 1789 mA h g−1 at 0.1 A g−1 and a retained capacity of 853 mA h g−1 after 140 cycles at 1 A g−1 [22]. A composite of 1T-MoS2 and conductive molybdate (NiMoO4) was used to obtain a coulombic efficiency of 99.5%, and it had stability after 750 cycle [23]. A pure metallic MoS2 structure was developed in [20] to avoid stability problems in material stacking. The battery had a specific capacity of ≈935 mA h g−1 for 200 cycles at 5 A g−1 that can be increased to 1150 mA h g−1. It had a high rate performance at the current density range from 0.2 to 20 A g−1 and a reversible capacity of 589 mA g−1Table 1 summarizes some MoS2-based LIB showing the used structure and phase of MoS2 and its specifications.
    Table 1. Energy storage applications of different MoS2 structures.
    Battery Type MoS2 Phase Structure Capacity References
    Lithium-ion 1T (Metallic) Nanotube-like MoS2 over graphene Discharge capacity = 666 mA h g−1 at
    current density = 3500 mA g−1
    [21]
    Lithium-ion 1T (Metallic) MoS2 over carbon cloth Reversible specific capacity = 1789 mA h g−1 at 0.1 Ag−1
    Retained capacity = 853 mA h g−1 after 140 cycles at 1 Ag−1
    [22]
    Lithium-ion 1T (Metallic) 1T MoS2 + (NiMoO4) Charged mass capacity = 940.1 mA h g−1
    Discharged mass capacity = 941.6 mA h g−1
    [23]
    Lithium-ion 1T (Metallic) Pure MoS2 Specific capacity ≈ 935 mA h g−1 for 200 cycles at 5 A g−1
    can be increased to 1150 mA h g−1
    [20]
    Sodium-ion 1T (Metallic) MoS2-graphene-MoS2 Capacity of 175 mA h g−1 at a high current density of 2 A g−1
    Reverse capacity of ≈313 mA h g−1 at low current density of 50 mA g−1.
    Stabilizes at current density = 313 mA h g−1 after 200 cycles
    [14]
    Sodium-ion 2H and 1T MoS2 Dual phase of 2H and 1T MoS2 Capacity = 300 mA h g−1 after
    200 cycles, and
    coulombic efficiency = 99%
    [24]
    Sodium-ion 2H phase transfers to 1T through chemical reactions MoS2 and amorphous carbon (C) Capacity = 563.5 mA h g−1 at 0.2 A g−1
    Coulombic efficiency = 86.6%
    Cyclic stability = 484.9 mA h g−1 at 2 A g−1
    [25]
    Supercapacitor 2D MoS2 Spraying MoS2 nanosheets on Si/SiO2 Area capacitance = 8 mF cm−2, and
    volumetric capacitance = 178 F cm−3
    [26]
    Supercapacitor Nanoflower-like MoS2 structure 3D-graphene/MoS2 nanohybrid Dimensions 23.6 × 22.4 × 0.6 mm3
    Specific capacitance (Csp) = 58 F g−1, energy density of 24.59 W h Kg−1, and power density of 8.8 W Kg−1 with operating window of 2.7 V (−1.5 to +1.2 V)
    [27]
    Supercapacitor Brush-like arrangement MoS2 MoS2 nanowires over Ni foam The high mass loading of MoS2 (30 mg cm−2) retains 92% of maximum capacitance after 9000 charge–discharge cycles at 5 A g−1 [28]
    Supercapacitor MoS2 QSs Exfoliated MoS2 QSs lateral size (5–10 nm) Capacitance = 162 F g−1
    Energy density = 14.4 W h kg−1
    [29]
    Hybrid
    Supercapacitor
    N-3DG and
    3D-IEMoS2@G
    Prepared using solvothermal process Energy density = 140 W h kg−1 at 630 W kg−1, and 43 W h kg−1 at power density of 103 kW kg−1
    Lifecycle over 10,000
    [30]

    2.1.2. Sodium-Ion Batteries (NIB)

    Since NIB are less efficient than LIB, there is not much research work about the role of MoS2 in Na-ion batteries; however, an early theoretical study in [31] showed that monolayer MoS2 can have a higher Na adsorption when compared to bulk MoS2. It is perfect as an anode electrode in Na-ion batteries, with a theoretical capacity of 335 mA h g−1. The monolayer maintains a lower applicable voltage of 1.0 V when compared to the bulk (1.7–2.0 V). The low mobility of Na is overcome by the monolayer structure because when the dimensions decrease, the diffusion barrier decreases from 0.7 to 0.11 eV. A graphene sandwich of MoS2, MoS2-graphene-MoS2, in [14] had a high capacity of 175 mA h g−1 at a high current density of 2 A g−1 and a reverse capacity of ≈313 mA h g−1 at a low current density of 50 mA g−1. It stabilized at a current density of 313 mA h g−1 after 200 cycles. A dual phase of 2H and 1T MoS2 was used to obtain a capacity of 300 mA h g−1 after 200 cycles and 99% coulombic efficiency. The good interlayer spacing permitted a high reversibility of Na ion intercalation [24]. MoS2 and amorphous carbon (C) microtubes (MTs) in [25] were used to improve the capacity to 563.5 mA h g−1 at 0.2 A g−1 and obtain 86.6% coulombic efficiency with cyclic stability of 484.9 mA h g−1 at 2.0 A g−1Table 1 summarizes some MoS2-based NIB showing the used structure and phase of MoS2 and its specifications.
    It is worth mentioning that, recently, multilayer intercalation of alkali metals (AM) (Li, K, Na) between bilayer graphene was possible and showed a higher storage capacity than the bulk structure [32]. A study in [33] compared the intercalation energetics of bilayer graphene and MoS2 for a number of alkali metals (Li, Na, K, Rb, Cs). The weak van der Waal forces between MoS2 layers enabled easy intercalation of Li ions without excess volume, and the Li storage capacity could reach 700 mA h g−1. The study showed that the storage capacity of MoS2 is significantly lower than graphene, but it can be increased through vertical van der Waals forces between graphene-MoS2 heterostructures where it will benefit from the light weight of graphene and the low formation energy of MoS2.

    2.1.3. Supercapacitors

    Supercapacitors are energy storage devices that have a lower energy density than batteries and a higher power density, meaning they can be used as a complementary device in electric vechiles beside batteries [19]. MoS2 is a good capacitor since it is formed of layers (sheets) that provide a large area for charge storage, where ions are inserted between layers through intercalation. The layers are exfoliated and then restacked to form electrodes with improved electrochemical features [34]. Carbon-based supercapacitors are leading the market due to their fast charge–discharge, versatile synthesis, and stability [35], but MoS2 can achieve extraordinary capacitances from 400 to 700 F cm−3 [34]. The charge storage mechanism of 1T MoS2 was investigated in [36] for an interlayer spacing ranging from 0.615 to 1.615 nm in ionic liquids. It was found that the highest volumetric and gravimetric capacitances were 118 F cm−3 and 42 F g−1, respectively, and occurred at a MoS2 interlayer spacing of 1.115 nm. A micro-supercapacitor proposed in [26], developed through spraying MoS2 nanosheets on a Si/SiO2 chip followed by laser patterning, had excellent cyclic and electrochemical performance compared to graphene-based micro-supercapacitors. It had a high area capacitance of 8 mF cm−2 and a volumetric capacitance of 178 F cm−3. The idea opens the door for portable and flexible micro-electronic devices. Some studies were directed towards the nano-MoS2 structure, where it showed a better performance in energy storage. Metallic 1T phase MoS2 nanosheets were found to efficiently intercalate ions such as H+, Li+, Na+, and K+ with capacitance values ranging from ∼400 to ∼700 F cm−3 in different aqueous electrolytes [34]. Their coulombic efficiencies were more than 95% and were stable until 5000 cycles. The MoS2 flower-shaped nanostructure was paired with 3D graphene to develop a supercapacitor prototype with dimensions of 23.6 × 22.4 × 0.6 mm3 by stacking a MoS2 nanoflower structure over 3D graphene over a graphite electrode [27]. The prototype had a high specific capacitance Csp of 58 F g−1, an energy density of 24.59 W h Kg−1, and a power density of 8.8 W Kg−1, with an operating window of nearly 2.7 V (−1.5 to +1.2 V). The study represents an inexpensive supercapacitor without the need for ionic liquid media. The nanostructures of MoS2 showed excellent supercapacitance when grown on Ni foam through the hydrothermal process [28]. It was able to maintain 92% of its maximum capacitance after 9000 charge–discharge cycles at 5 A g−1. The study confirmed that the high mass loading of MoS2 nanostructures grown over conducting substrates corresponds to superior energy storage electrodes. A recent work studying the capacitance of MoS2 quantum sheets (QSs) in [29] demonstrated that MoS2 quantum sheets have a high capacitance of 162 F g−1, which is very high if compared to typical MoS2 supercapacitors. MoS2 QSs have an energy density of 14.4 W h kg−1 and a long cycle life. In [30], a 3D interlayer-expanded MoS2/rGO nanocomposite (3D-IEMoS2@G) was synthesized and experimented as an anode in lithium-ion and sodium-ion batteries. It was then modified by pairing it with nitrogen-doped hierarchically porous 3D graphene (N-3DG) to obtain sodium and lithium hybrid supercapacitors (HSCs). The Na-HSC showed an excellent performance of 140 W h kg−1 at 630 W kg−1, and 43 W h kg−1 at an ultra-high power density of 103 kW kg−1 (charge finished within 1.5 s). It can retain its capacitance even after 10,000 cycles. Table 1 summarizes some MoS2-based supercapacitors showing the used structure and phase of MoS2 and its specifications.

    2.2. Energy Generation Applications

    2.2.1. Hydrogen Evolution Reactions (HER)

    Hydrogen was recently studied to substitute fuel as a source of energy. It is not a source of energy by itself but rather a carrier of energy. It has to be manufactured as with electricity. It has to be manufactured from coal or natural gas; however, in both cases, carbon is released, and environmental pollution occurs. It is also generated from water, which represents a better environmental solution. It is not toxic, as opposed to fuel, has a high octane number, and does not cause ozone issues [37]. MoS2 is a cheap catalyst in electrochemical HER [38] and water splitting reactions [39]. The large number of electrostatic active edges and high structural defects makes MoS2 a good catalyst. 1T MoS2 is known to be a better catalyst in HER than 2H MoS2 because of its reactive basal planes, which gains its activity from the hydrogen binding affinity at the surface S sites [40]. Studies have been conducted to enhance the catalytic activity and stability of MoS2 so that it can replace noble metals. The catalytic activity of MoS2 mainly depends on the active edges or the cell vacancies. The work in [41], based on the first-principle density functional theory (DFT), studied different possible cell vacancies of MoS2 and found that the best catalytic activity for MoS2 occurs with Mo and MoS2 cell vacancies. The efficiency of HER is enhanced when compared to platinum catalyst reactions. A later study conducted by the same researchers [39] focused on Mo defects on the inert basal plane of MoS2 and showed its better performance in HER and water splitting reactions. The active sites of MoS2 basal planes are restricted to edges and missing primitive cell vacancies. The weak van der Waal interactions between MoS2 layers result in a hydrophobic characteristic which assigns more importance to layer defects [42]. A detailed study of five types of defects in MoS2 layers was conducted [41]. The study investigated the effect of disulfur vacancy (VS2), vacancy complex of Mo and nearby tri-sulfur (VMoS3), Mo vacancy (VMo), nearby S tri-vacancy (VS3), and VMoS2, and it was found that VMo and VMoS2 can activate inert basal planes and have a role in dissociating water in HER. The Gibbs energy for hydrogen adsorption (ΔG0H)  for VMo is −0.198 eV, and for VS3, it is 0.06 eV, which is comparable to platinum, which has a value of −0.09 eV.
    The effect of strain on Mo vacancies in single-layer MoS2 was investigated in [43], where a biaxial compressive strain of 4.5%, carried out by modifying the Mo and S interaction around the vacancy, showed optimal catalytic properties, with Gibbs free energy between −0.03 and −0.04 eV at the active sites. A hybrid catalyst made by growing MoS2 over cobalt diselenide (MoS2/CoSe2) approached the commercial platinum catalyst behavior [44]. The reaction in the acidic electrolyte had a Tafel slope of 36 mV dec−1, onset potential of −11 mV, and exchange current density of 7.3 × 10−2 mA cm−1. A trial to increase the active sites of MoS2 was introduced in [45] using cracked monolayers of 1T MoS2. The monolayers were obtained through hydrothermal synthesis. 2H MoS2 was ultrasonicated with lithium which facilitated the intercalation of MoS2 layers, which were then exfoliated to obtain 1T MoS2. The resulting MoS2 had a favorable HER performance characteristic, with a low overpotential of 156 mV, at 10 mA cm−2 in an acidic medium, and a low Tafel slope of 42.7 mV dec−1Table 2 summarizes some MoS2 applications in HER.
    Table 2. Energy generation applications of different MoS2 structures and composites.
    Type of Reaction Catalyst Used Specification References
    HER (MoS2/CoSe2) Tafel slope = 36 mV dec−1
    Onset potential = −11 mV
    Exchange current density = 7.3 × 10−2 mA cm−2
    [44]
    HER 1T MoS2 Overpotential = 156 mV, at 10 mA cm−2
    Tafel slope = 42.7 mV dec−1
    [45]
    HER/OER Amorphous Ni–Co complexes hybridized with 1T MoS2 Overpotentials = 70 mV HER
    and 235 mV for OER
    at 10 mA cm−2
    Tafel slope = 38.1 to 45.7 mV dec−1
    [46]
    OER Rhombohedral MoS2 microspheres over conductive Ni Overpotential ≈ 310 mV
    Tafel slope ≈ 105 mV dec−1
    [47]
    OER MoS2 quantum dots (MSQDs) Overpotential = 280 mV
    Tafel slope = 39 mV dec−1
    [48]
    CO2 reduction Vertically aligned MoS2 nanoflakes
    (2H and 1T phases coexist)
    Overpotential = 54 mV
    Reduction current density = 130 mA cm−2 at −0.764 V
    [49]
    CO2 reduction p–n junction
    Bi2S3/MoS2 composite
    Photocatalytic CO2 reduction
    20 times higher than single
    catalysts under visible light irradiation
    [50]
    CO2 reduction 3R MoS2 nanoflower powder Synthesized using CVD
    CO production < 0.01 μmol-gcat−1 hr−1 at 25 °C
    which is negligible
    [51]

    2.2.2. Oxygen Evolution Reactions (OER)

    MoS2 acts as a catalyst in OER which is a step in water splitting. Few studies have been conducted related to the role of MoS2 in OER. 1T MoS2 with amorphous nickel–cobalt complexes was used as a catalyst in water splitting to generate hydrogen and oxygen [46]. The method represents a low-cost, easy, and stable way to perform water splitting instead of using expensive noble metal catalysts. Another hybrid nanocomposite made of MoS2 microspheres over Ni foam was proposed in [47]. The study made use of the efficient catalytic activity of MoS2 while increasing its conductivity by attaching it to the conductive Ni foam. The overpotential decreased rapidly (nearly by 290 mV) when compared with RuO2 as a catalyst. MoS2 quantum dots (MSQDs) in [48] were used as a catalyst for OER. The quantum dots were synthesized using (NH4)2MoS4 as a precursor to produce MoS2 quantum dots (MSQDs), and then activation of QDs was carried out using potential cycling under electrolyte conditions to produce the as-synthesized materials after cycling (MSQDs-AC). The catalytic current density increased as the number of potential cycles increased, and it reached its maximum after 50 potential cycles. The technique avoids using carbon that leaves carbon QDs behind which negatively interfere with the process. The resulting MSQDs-AC had the lowest Tafel slope (39 mV dec−1) when compared to other state-of-the-art catalysts such as IrO2/C, and they also had fast reaction kinetics. Table 2 summarizes some of the MoS2 applications in OER.

    The entry is from 10.3390/en14154586

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