Building hybrid structure of monolayered S-depleted Mo–S nanocrystals and 3D graphene towards promising aqueous supercapacitor applications

The supercapacitors (SC) is a crucial energy storage technique, which attracted significant interests because of their high power density, long cycle life, and fast charge/discharge features [1, 2]. It is known that the electrode material of SC should possess high electrical conductivity and high accessible active surface area. In this regard, two-dimensional (2D) molybdenum disulfide (MoS₂) showed great potentials owing to its high carrier mobility, high thermal/chemical stability, high surface to volume ratio and fast 2D capacitive behaviors [3–8]. The short mass transport distance and fast electron transport of the 2D surface make it comparable to carbon nanotubes and metal oxides in delivering either the fast energy storage or the high capacitance. However, practical applications of the most stable 1H MoS₂ are still limited by their inert basal planar. In the stacked 2D 1H MoS₂, the poor electron hopping efficiency further reduces their energy storage performance [9].

To improve the surface activity of the 2D 1H MoS₂, several strategies have been proposed, including the formation of crystal boundary and heterostructure and the chemical doping on the surface [7, 10]. The former strategy still cannot meet practical demands because of the limited number of active sites, while the later one required complex and expensive techniques to finely control the inert 2D surface. Recent investigations suggested that the inert basal plane of 2D MoS₂ can be activated by forming the metastable 1T and 1T' phase [10–13]. Although stable and excellent electrochemical applications of these metastable phases were demonstrated on electrochemical hydrogen evolution reactions, the unstable nature of the metastable phase should be well resolved, in particular when it is used in anode electrochemical reactions and for long-term storages [10].

Tackling these challenges may need significant efforts. Nevertheless, recent developments on 1H MoS₂ nanocrystals (NCs) may provide an alternative solution to improving their surface activity, and sustaining the chemical stability in the meantime [14, 15]. Unlike traditional 2D MoS₂ sheets, monolayered 1H MoS₂ NCs have much enhanced metallicity and edge ratios [14, 15]. By incorporating S vacancies, the density of state (DOS) of NCs at near the Fermi level can be further enhanced, providing the increased binding energy to electrolyte ions [7, 10, 14, 15]. Besides, the S vacancy is usually accompanied with the structural re-arrangement, which could improve the thermodynamic stability of the material [16]. All these clues implied that S-depleted 1H Mo–S NCs may have much better energy storage performance compared to traditional 2D 1H monolayers. Nevertheless, demonstrations of such S-depleted Mo–S NCs in SCs are rarely reported due to likely technical challenges in electrode fabrications. Specially, Mo–S NCs are typically stably dispersed in solvents such as ethanol and water. The layer stacking and agglomeration of NCs are inevitable when they are collected as the powder form for electrochemical applications. These NCs also cannot be well deposited on the 3D metal frameworks like those demonstrated elsewhere [17, 18]. Direct combination of the NCs and other materials with techniques such as microwave-assisted synthesis may also destroy the 2D surface to reduce the activity [19–24]. Therefore, it is challenge but meaningful to investigate the activation of these NCs and explore a simplified approach for preparing the electrode materials to overcome above challenges.

In this study, we examined the electrochemical properties of monolayered MoS₂ NCs. By introducing S vacancies, we were able to activate the inert basal plane, which effectively doubled the specific capacitance. To solve the stacking problem of 2D phases, a relatively stable hybrid structure of 2D NCs and 3D expanded graphene (EG) was designed and fabricated by electrostatic incorporation (Scheme 1). In this structure, the EG provided interconnected skeleton, rich porous structure and large accessible surface area for an ideal electrochemical process. As a result, the promising energy storage of 1H Mo–S NCs was effectively developed.

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2.1. Fabrication of the EG

2.2. Fabrication of MoS₂ and MoS_1.5 NCs

2.3. Fabrication of the MoS_1.5 NCs/EG

2.4. Characterizations

2.5. Electrochemical measurements

Upon the S-depletion, Mo–S NCs changed to be dark brown from faint yellow (figure 1(a) inset). According to the XPS survey (figure S1), the S/Mo ratios of NCs with and without S-depletion were confirmed as 1:2 and 1:1.5 respectively. Shown as figures 1(a), (b) are the AFM and TEM images of MoS_1.5 NCs. The majority of (>99%) MoS_1.5 NCs are monolayered with the AFM thickness of around 1 nm and lateral size of ∼12 nm (figure 1(a)). MoS₂ NCs should have similar lateral size and thickness to MoS_1.5 NCs. High-resolution TEM (HRTEM) suggested the highly defected lattice of the MoS_1.5 NCs (figure 1(c)) which was fully different to the MoS₂ NCs with ordered lattices (figure 1(d)). Given the electrical positivity of S-depleted Mo–S NCs and the electronegativity of EG, the electrostatic adsorption composite of these two components is highly likely to be achievable (will be further confirmed with Raman and XPS characterizations). On the basis of the above fabrications, different amounts of MoS_1.5 NCs were loaded onto the EG via a simple stirring adsorption reaction to fabricate MoS_1.5 NCs/EG-1, MoS_1.5 NCs/EG-2 and MoS_1.5 NCs/EG-3. Fabrications and basic characterizations of this EG were fully described in our previous report and will not be further extended here [25]. In principle, single-layer NCs should have no any x-ray diffraction peaks unless they are stacked to each other. XRD patterns of different samples suggested that the stacking of NCs (diffraction peak at ∼14.7°) happened in the case of MoS_1.5 NCs/EG-2 and MoS_1.5 NCs/EG-3, while the NCs in MoS_1.5 NCs/EG-1 should be highly dispersed (figure 2(a)). The MoS_1.5 NCs/EG-2 was then examined with the SEM as an example. In this sample, the highly curved and folded graphene structure was evident, where the Mo and S elements were uniformly dispersed on the graphene sheet (figure 2(b)). MoS_1.5 NCs/EG-2 exhibited a substantial Brunauer–Emmett–Teller (BET) surface area of 282 m² g⁻¹, along with an abundant porous structure ranging from 2.5 to 8 nm, thereby providing abundant anchoring sites for MoS_1.5 NCs (figure S3).

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Raman and XPS analyses further confirmed the successful hybrid combination of NCs and graphene. In the Raman spectrum, D and G modes of graphene in MoS_1.5 NCs/EG-2 were recognized at ∼1350 and 1593 cm⁻¹, respectively (figure S2). This G band position was blue-shifted from 1589 cm⁻¹ of EG under identical measurement conditions (figure S2) because of the chemical interaction between Mo–S NCs and graphene sheets. Besides, Raman responses of the oxidized Mo phases were also found in MoS_1.5 NCs/EG-2 at 658 (Mo–O–Mo), 818 (Mo=O) and 991 cm⁻¹ (Mo–O–Mo) [33], which again demonstrated that the Mo–S NCs were chemical interacted with the graphene through the oxygen atom. The Mo 3d and S 2p XPS of MoS_1.5 NCs and MoS_1.5 NCs/EG-2 were compared in figures 2(c), (d). The MoS_1.5 NCs showed Mo 3d3/2 (232.1 eV), Mo3d5/2 (229.0 eV) and S2s (226.3 eV) peaks corresponding to the 1H structure [10, 22, 23]. The defect associated Mo 3d3/2 and Mo 3d5/2 were found at 233.9 and 230.8 eV, respectively [34, 35]. Such defect peaks were weakened upon the combination of NCs with graphene, while the Mo⁵⁺ states in MoS_1.5 NCs were transformed into a new strong Mo⁶⁺ oxidation state in MoS_1.5 NCs/EG-2 [36, 37]. The S2p spectra of MoS_1.5 NCs and MoS_1.5 NCs/EG-2 both contained S 2p3/2(162.3 eV), S 2p1/2(163.5 eV) and S defect peaks (164.7 eV) at the same position, indicating that the S state was almost the same after the loading of NCs onto the EG. All above characterizations suggested that NCs have been successfully loaded on the graphene layer of EG through chemical interactions, which is crucial in sustaining a stable electrochemical process.

In electrochemical processes, the stacking of 1H MoS₂ severely affected the current response duo to the low hopping efficiency between layers, and the capacitance measurement of 1H Mo–S NCs was highly difficult [38, 39]. To tackle this challenge, the pure NCs were loaded on the CP to alleviate the layer stacking. The capacitance of the working electrode against the loading ratio of NCs was then plotted. In principle, the capacitance and loading ratio of NCs should present a linear function if NCs are highly dispersed on the CP. Otherwise, the measured capacitance should be degenerated upon the NCs stacking. To eliminate the error introduced by the CP capacitance, the capacitance background of pure CP was measured in 1.0 M KCl, 1.0 M NaCl and 1.0 M LiCl electrolytes (figures S4 and S5) initially. Different amounts of Mo–S NCs were drop casted onto the same CP for CV tests (figures S4 and S5) to obtain the capacitance of Mo–S NCs. According to the concentration of the suspension (MoS₂ and MoS_1.5 NCs were 0.76 mg ml⁻¹ and 0.69 mg ml⁻¹, respectively, see experimental section for the determination of the concentration), the mass of NCs loaded on the CP was calculated, and thus the relationship between the mass and capacitance of NCs can be exploited. When a small amount of Mo–S NCs (<7 μg) was loaded on the CP, the capacitance value of MoS_1.5 and MoS₂ NCs in all three electrolytes (1.0 M KCl, 1.0 M NaCl and 1.0 M LiCl) were both increased linearly with the loading ratio, while the mass specific capacitance remained unchanged (figures 3(a)–(c)). Since NCs gradually fell on the CP surface and became saturated, re-stacking of NCs was tended to form, thereby reduced electrical conductivity and hindered ion transport. Although the electrode capacitance was increased by adding more NCs on the CP, the value of the electrode capacitance against the mass of NCs was actually decreased (the labeled yellow zone in figures 3(a)–(c)). A further increase in NCs loading significantly reduced the electrode capacitance (labeled as blue zone in figures 3(a)–(c)). In this case, NCs should have been severely stacked to each other and the electrical resistance changed to be dominant to decide the capacitance. Therefore, the real specific capacitance of the NCs can only be measured with a low mass loading on the CP (e.g. 5 μg). On the basis of above measurements, the real specific capacitance of both MoS_1.5 and MoS₂ NCs were summarized in figure 3(d). MoS₂ NCs have the specific capacitance of 88, 84 and 85 F g⁻¹ at 5 mV s⁻¹ in 1.0 M LiCl, 1.0 M NaCl and 1.0 M KCl, respectively. By contrast, the specific capacitance of MoS_1.5 NCs were more than doubled compared to MoS₂ NCs, which were 178, 206 and 215 F g⁻¹ at 5 mV s⁻¹ in above three electrolytes, respectively. Evidently, the S vacancy improved the capacitance of the 1H Mo–S NCs significantly.

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The serious degradation of capacitor performance and the mass limitation of NCs encouraged us to fabricate a 3D structure combined with 2D NCs to alleviate the stacking problem. Although XRD patterns showed that there was still a slight stacking of NCs in MoS_1.5 NCs/EG composites (figure 2(a)), the loading capacity of NCs was increased by an order of magnitude through simple stirring adsorption reaction. The difference caused by mass loading in electrochemical impedance and reaction kinetics of EG, MoS_1.5 NCs, and MoS_1.5 NCs/EG-2 electrodes were investigated by EIS. As shown in figure 4(a), MoS_1.5 NCs exhibited the highest R_ct and R_s values due to the high charge transfer resistance. By contrast, EIS curves of EG and MoS_1.5 NCs/EG-2 were more vertical at low frequencies, indicating near ideal supercapacitor behavior [40, 41]. The semicircle of MoS_1.5 NCs/EG-2 at middle-high frequency region was also not obvious, and suggested the low R_ct. Therefore, the hybrid structure of NCs and EG should have enhanced the conductivity and diffusion kinetics of electrolyte ions, giving promising energy storage applications.

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The energy storage performance of MoS_1.5 NCs/EG-2 was further evaluated in 1.0 M LiCl electrolyte with three-electrode technique, and compared to other MoS_1.5 NCs/EG samples and EG. As shown in figures 4(b)–(d), the CV curves of all MoS_1.5 NCs/EG samples at different scanning rates have a pair of redox peaks at both ends of the potential, corresponding to the anodic peak at −0.4 V and the cathodic peak at −0.8 V. The formation of reduction and oxidation peaks may be associated to the formation of Li_y MoS_x and the oxidation of Mo atoms into MoS_x , respectively. Both the reduction and oxidation peaks of MoS_1.5 NCs/EG composites shifted toward both ends of the potential with the increase of scanning rates, while typical CV curves of electric double layer capacitors appeared on the EG (figures 4(b), S6), indicating that NCs had contributed to the electrode capacitance. Figure 5(a) compares specific capacitance graphene and MoS_1.5 NCs/EG composites electrodes at different scanning rates. Among all composite samples, the MoS_1.5 NCs/EG-2 showed the highest specific capacitance up to 620 F g⁻¹ at the scan rate of 1 mV s⁻¹, which can be attributed to the relative large loading of dispersed NCs on EG. Further increasing the loading of MoS_1.5 NCs led to the re-stacking and agglomeration of NCs, which reduced the capacitance of electrode materials. All above information suggested that the proper combination of MoS_1.5 and EG can readily solve the unfavorable stacking issue of NCs.

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With MoS_1.5 NCs/EG-2, the symmetric coin cells (SCC) were further assembled to evaluate their potential applications in supercapacitor devices. In this case, the 1.0 M Na₂SO₄ electrolyte was instead used to avoid the polarization-like signal. Figure 5(b) shows the CV curve of the SCC at scan rates from 5 to 60 mV s⁻¹ in 1.0 M Na₂SO₄ electrolyte. CV curves were quasi-rectangular without obvious redox peaks, and showed little change with increasing scan rate, indicating that the electric double layer energy storage was the main contributions of SCC [42, 43]. Figure 5(c) shows GCD curves of the charge and discharge voltages of the SCC in the range of 0–1 V. As the current density increased, the curve was also remained as a symmetrical triangle. The battery capacity peaked at 0.1 A g⁻¹ as 314 F g⁻¹ and remained at 62.9% even after increasing the current density to 5 A g⁻¹, indicating good rate performance of the device. The energy density of the device reaches 8.8 Wh kg⁻¹ at the power density of 250 W kg⁻¹. It can still reach 6.7 Wh kg⁻¹ even at the high power density of 2500 W kg⁻¹. The energy density of MoS_1.5 NCs/EG SSC (6.7 Wh kg⁻¹) is higher compared to the MoS₂ based SSC device in aqueous electrolytes such as MoS₂/RGO/MoS₂@Mo (6.22 Wh kg⁻¹) [44], 1T-MoS₂ (6 Wh kg⁻¹) [45], MoS₂/CB-C (5.2 Wh kg⁻¹) [46] and 2H-MoS₂ (4.5 Wh kg⁻¹) [47]. Besides, the SCC device also has good stability under 0.5 A g⁻¹, by showing ∼90.5% capacitance retention after 5000 charge/discharge cycles (figure 5(d)), which demonstrated well the robust structure of the hybrid MoS_1.5 NCs and EG composite. However, the capacity degeneration was still not good enough compared to the pure carbon materials, which may induced by the decomposition of O-functionalized groups on the EG, the destroy of the chemical interaction between NCs and graphene, and the degeneration of the Mo–S NCs as we demonstrated elsewhere [8]. To improve the stability of the device, more techniques may need to be investigated and introduced in future (e.g. the use of solvothermal reaction to improve the chemical binding between materials).

In 2D materials, current research is focusing on the metastable phase structures and amorphous structures [10–13, 48–51]. Dangling states of these materials are highly improved from the most stable 2D phases, giving high surface chemical activity. The challenge here is how to stabilize these metastable structures when it is used for electrochemical processes [7, 10]. In this regard, the chemical doping on the surface of metastable materials may be considered, which was proved by recent reports, where the single metal atoms were introduced onto the 2D surface [52]. However, compared to the 1H phase, fabrication of the metastable counterparts are not well—developed [7, 10]. In industry, the 1H phase is more suited to be fabricated in large scale and stored for a long period. The little problem on the most stable 1H phase here is the accurate measurement on the electrochemical capacity, which was solved by using our method demonstrated above (e.g. drop-casted NCs onto the CP and plot the linear function of the capacitance against mass loading). As for the surface activity, the investigation here demonstrated that it can be feasibly activated via the formation of S vacancy. According to our previous investigations, the 2D size reduction of the 1H Mo–S gives the improved DOS at near the Fermi level, resulting multi-layer chemical adsorptions and fast 2D capacitive feature [8]. Investigations also suggested that the structure of 1H Mo–S with high ratio of S vacancies be stabilized with the structural re-organization [53]. All these features imply that the stable 1H MoS materials can be well used in energy storage devices, although more works are need to be introduced towards industrial implementations.

Monolayered MoS_1.5 NCs were fabricated by 'K-intercalated unzipping' technique, followed by the surface S depletion with cation exchange resin. The surface S vacancy activated the chemical activity on the basal plane, which doubled the specific capacitance (∼200 F g⁻¹ in 1.0 M LiCl, 1.0 M NaCl and 1.0 M KCl electrolytes) compared to the pure monolayered MoS₂ NCs. For the real electrode application, MoS_1.5 NCs was chemically bound on the surface of EG. This hybrid structure overcame challenges of the 2D stacking of NCs and offered high exposure of active sites, and therefore efficiently improved the capacitance (620 F g⁻¹ in 1.0 M LiCl, measured at 1 mV s⁻¹) performance compared to pure MoS_1.5 NCs (178 F g⁻¹) and EG (225 F g⁻¹). With this hybrid electrode, a 310 F g⁻¹ of the two-electrode device was also demonstrated at 0.1 A g⁻¹ in 1.0 M Na₂SO₄, giving the maximum energy density of 6.7 Wh kg⁻¹ at the power density of 2500 W kg⁻¹ and high capacitance retention (∼90.2%) after 5000 charge/discharge cycles. Results discovered above are exciting, which confirmed that 1H MoS₂ materials can be highly promising electrode materials for the application in SCs. Further improvement on the materials can be expected by combining other protocols such as chemical doping and construction of heterojunction structure of MoS₂, improving the chemical interactions between materials and further prevention of the re-stacking issue of 2D layers.

This work was supported by 'Independent Innovation Projects of the Hubei Longzhong Laboratory' (2022ZZ-03). Liangxu Lin is also supported by the National Natural Science Foundation of China (52172228).

All data that support the findings of this study are included within the article (and any supplementary files).

This work was supervised by L Lin and D Liu. L Zhong, Y Pi, Y Gao, Y He and L Wang performed experiments, collected data and drafted the manuscript under supervisions. All authors discussed the results and commented on the manuscript.

There is no competing interest involved in this work.