Phosphoric acid-assisted synthesis of layered MoS₂/graphene hybrids with electrolyte-dependent supercapacitive behaviors

Abstract

A reformative graphene-supported MoS₂ hybrid has been synthesized by a phosphoric acid (H₃PO₄)-assisted hydrothermal process. The effect of H₃PO₄ on the growth, morphology, structure, and composition of the MoS₂/graphene hybrid has been explored by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The results indicate that H₃PO₄ can control the process of both the reduction of graphene oxide and the crystallization of MoS₂. The electrochemical performance suggests that involvement of H₃PO₄ in the reaction bestows the hybrid with electrolyte-dependent capacitive behaviors in which contribution from electric double-layer capacitance (EDLC) and pseudocapacitance can be distinguished in acidic and alkaline electrolytes, respectively. While the quasi-EDLC behavior of the hybrid dominates in an alkaline electrolyte (258 F g⁻¹ at 2 A g⁻¹), the pseudocapacitance of the hybrid in an acidic electrolyte can be significantly enhanced due to oxygen-containing groups and Mo active in the total capacitance value (351 F g⁻¹ at 2 A g⁻¹). Moreover, a superior cycling performance and quick frequency response of the hybrid reinforces its potential for supercapacitor applications.

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1. Introduction

Supercapacitors (SCs) have been regarded as a high performance electrochemical energy storage device having many advantages over batteries and traditional capacitors, featuring higher power density than batteries and better energy-storage ability than capacitors.^1,2 From the viewpoint of charge storage mechanism, SCs can be divided into electric double layer capacitors (EDLC) and pseudocapacitors. The most representative electrode material for commercial SCs is activated carbon (AC) which works based on the electrostatic attraction of ions at the electrode/electrolyte interface (EDLC one) whereas it has limited capacitance (around 100–200 F g⁻¹, depending on the electrolytic medium). Besides the capacitance derived from electrostatic forces, a great enhancement in capacitance values can be obtained provided that quick faradic reactions take place at the electrode/electrolyte interface.³ To achieve this, many composites have now been designed to increase the effective total capacitance value of the electrode and cycling performance as well.⁴

Recently, two-dimensional (2D) materials and their hybrids have attracted numerous attention in energy conversion and storage applications due to their fascinating physicochemical properties and structural flexibility for heterostructure design,⁵ among which the hybrid structures of graphene and MoS₂ were actively explored particularly for lithium-ion batteries^6–9 and SCs applications.^10,11 Most recently, Dryfe¹² demonstrated a 2D MoS₂–graphene composite electrode (prepared by mechanical mixing) with excellent durability owing to the ion intercalation/deintercalation (pseudocapacitance contribution). Nevertheless, the most reported capacitance value of MoS₂/graphene hybrids still remains low (200–250 F g⁻¹) and the assembled supercapacitors cannot contend with the energy-storage ability of batteries. Rao¹³ ever reported a high capacitance of 416 F g⁻¹ (at 5 mV s⁻¹) based on MoS₂/graphene hybrid electrode, however, this hybrid only exhibited a mediocre capacitance value (249 F g⁻¹ at 0.3 A g⁻¹) from galvanostatic charge–discharge tests. As reported in the literature,^14–16 this capacitive decay behavior at high current could be ascribed to the limited ion transfer channel and poor intrinsic conductivity of the electrode materials. In addition, an unexploited (or partially exploited) capacitance derived from electrochemical active pairs (such as quinine/hydroquinone pair resided in the graphene edges and varied valence state Mo sites) remained to be another factor^17,18 constraining the further development of MoS₂/graphene hybrid-based electrode materials, which may be caused by unfavorable electrolytes or synthetic factors. To solve the problems mentioned above, novel techniques are expected by fully utilizing the active sites of MoS₂/graphene hybrids in producing high performance electrochemical capacitors.

The H₃PO₄-activation technique in promoting the supercapacitor performance of N-doped graphene nanosheets has recently been reported,¹⁹ and the improved effect is attributed to the increase of the pore volume and the beneficial chemical modification during the annealing process. Apart from that, some surface-controlled crystallization techniques (via the introduction of inorganic acids or pH adjustment) have been addressed in terms of controlling the growth of specific inorganic nanostructures.^6,20–22

In this work, the role of H₃PO₄ will be addressed in terms of controlling the hydrothermal growth of the MoS₂/graphene hybrids, in which the introduced H₃PO₄ could be a buffering agent rendering homogeneous formation of well-dispersed MoS₂ nanosheets on the graphene surface. The capacitance value of the hybrid electrode material is measured to be a high level of 351 F g⁻¹ (from 164 F g⁻¹) and 258 F g⁻¹ (from 172 F g⁻¹) at 2 A g⁻¹ in 1 M H₂SO₄ and 6 M KOH electrolyte, respectively. The capacitive contribution from EDLC or pseudocapacitance is to be investigated based on the electrochemical data derived from both acidic and alkaline electrolytes.

2. Experimental section

2.1 Preparation of pure MoS₂ and MoS₂/graphene hybrids

Pure MoS₂ were prepared using the typical hydrothermal technique. Firstly, 0.18 g Na₂MoO₄·2H₂O and 0.36 g L-cysteine were mixed in 65 ml H₂O, and to get the homogeneous solution via ultrasonic treatment for 0.5 h. Then the solution was poured into Teflon reactor and sealed closely with stainless steel holder. Subsequently, the system was moved to the heating oven and reacted at 200 °C for 24 h, followed by natural cooling to room temperature. The resulting black solution was washed with filtered water for several times, together with a filtering process, and the target solid was obtained until a neutral upper solution was indicated. Finally, the MoS₂ sample was dried at 80 °C for 12 h.

Self-made²³ graphene oxide (GO, 1 mg ml⁻¹) solution was used in the following experiments. 1 ml 0.1 wt% CTAB solution was dripped into 40 ml GO solution, and 60 mg Na₂MoO₄·2H₂O crystal was introduced to get a mixture A. Next, 120 mg L-cysteine was added to 20 ml pre-prepared phosphoric acid solution (1.66 ml 85% H₃PO₄ was included) to get solution B. With the help of ultrasonic treatment, A and B solution were blended to get a homogeneous faint blue solution. The pH value of this solution was estimated to be 2.3 by pH meter. Again, the mixture was transferred to the autoclave and heated at 200 °C for 24 h (the pH value of the resulting solution was measured to be 2.5). Then the sample was washed and purified with water combined with vacuum filtration operations until pH value of upper solution approached 7, and the obtained filter cake was dried at 80 °C for 12 h. Finally the solid was moved to furnace and annealed at 500 °C at a mixed atmosphere of Ar/H₂ (9 : 1) gas. The unannealed and annealed sample were marked as P-MG and P-MG(T), respectively.

Processes for the preparation of MG and annealed MG(T) is the same as that of P-MG and P-MG(T), except for the removing of H₃PO₄ (the pronounced phosphoric acid solution was replaced by pure water). But the pH value of the resulting solution after the hydrothermal reaction increased to 11 (from initial pH = 3.2).

2.2 Characterizations

X-ray diffraction (XRD) patterns of the samples were recorded by a Bruker D8 ADVANCE X-ray diffractometer with Cu Kα radiation (λ = 0.154187 nm). High resolution transmission electron microscopy (HRTEM) images were obtained by a Tecnai G2 F20 S-TWIN field-emission TEM operating at 200 kV. Morphology analysis was performed with scanning electron microscope (SEM). The elemental composition of the samples was analyzed by X-ray photoelectron spectroscopy (XPS). Raman spectra were collected on a Jobin Yvon Labor Raman HR-800 spectrometer with an argon ion laser (λ = 514 nm) in ambient atmosphere.

2.3 Electrochemical measurements in a three-electrode system

A mixture containing 80 wt% active materials (3 mg), 10 wt% carbon black, and 10 wt% polytetrafuoroethylene (PTFE) was well mixed in N,N-dimethylformamide (DMF) until they formed a slurry with the proper viscosity, and then the slurry was uniformly laid on a piece of Ni foam about 1 cm² that was used as a current collector and then dried at 80 °C for 2 h. The Ni foam coated with the composite was pressed for 1 min under 8.0 MPa and dried at 120 °C for another 12 h. A Pt electrode was used as the counter electrode, Hg/HgO electrode (0.165 V vs. SCE) filled with 1 M KOH aqueous solution and Ag/AgCl electrode (0.29 V vs. SCE) was used as the counter reference electrode for 6 M KOH and 1 M H₂SO₄ electrolyte, respectively. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were measured on a CHI760E electrochemical workstation in a three-electrode system. The specific capacitance is calculated according to equation C = It/V, where I is the mass normalized current (A g⁻¹), t is the discharge time (t), and V is the voltage (V). The reported specific capacitance values were all normalized to the weight of sample. And the voltage value was all converted to vs. SCE.

3. Results and discussion

3.1 Morphology and microstructure

The structure and morphology of pristine MoS₂ and MoS₂/graphene hybrids were observed by scanning electron microscopy (SEM). The pristine MoS₂ exhibits conglomerate grainy morphology (Fig. 1a) with diameter of several hundred nanometers and the inset clearly shows the coarse surface due to the formation of sheet-like subunits.²⁴ When the graphene was introduced, the morphology of MoS₂ changed greatly. MoS₂ crystal particles with slight aggregation and graphene lamella are randomly presented in MG(T) (Fig. 1b). While P-MG(T) hybrid shows the cross-linked lamellar framework homogeneously decorated with MoS₂ flakes (Fig. 1c). Meanwhile, the grainy morphology detected in MG(T) almost vanishes here, suggesting that MoS₂ is well embedded into the flexible rGO nanosheets (see the inset of Fig. 1c) in the presence of H₃PO₄. The similar morphology of MoS₂/C hybrid was also previously reported.^6,25 The formation of the uniform and well separated MoS₂/graphene sheets was ascribed to the slow growth of MoS₂ crystal during the pH-buffered (H₃PO₄) hydrothermal process.²² Transmission electron microscope (TEM) pictures of MG(T) and P-MG(T) are given in Fig. S1c† and 1d, in which the MG(T) nanohybrids with 3D assembled structure (Fig. S1c†) are yielded in the absence of H₃PO₄ and folded MoS₂ edges corresponding to the black lines can be clearly discerned. For the P-MG(T) hybrid, graphene is evenly covered by few-layer MoS₂ nanosheets, revealing a co-planar growth habit of MoS₂ crystals on the graphene surface for the latter case. In addition, these clearly separated graphene nanosheets (Fig. S1†) provide sufficient contact area for electrolyte ions when they are applied as electrodes for energy storage. HRTEM picture in Fig. 1e shows an uniform layered morphology of the hybrids, in which parallel lines with d = 0.62 nm spacing can be assigned to the curled edges of MoS₂ nanosheets, confirming the homogeneous and 2D nature of the as-prepared hybrids. Lattice fringe with the spacing distance of 0.20 nm can be indexed as (101) crystal planes of graphene.

Fig. 1 The SEM pictures of (a) bare MoS₂, (b) MG(T), and (c) P-MG(T). The inset picture gives the detailed morphology of the marked area; (d) TEM picture of P-MG(T); (e) HRTEM picture gives the local lattice of P-MG(T).

3.2 XRD and Raman

The peaks at 14.2°, 29.0°, 33.2°, 40.1°, 45.2°, 59.4°, and 60.2° can be indexed to (002), (004), (100), (103), (006), (110), and (008) diffraction planes of pure hexagonal MoS₂ (JCPDS, 37-1492), respectively. After the combination with graphene nansheets, only two depressed peaks ((002) and (100) crystal facets) can be observed, demonstrating that the pre-existing graphene nanosheets could change the growth behaviors of MoS₂ crystals.⁷ Typically the shift of (002) peak toward a lower degree for MG(T) and P-MG(T) implies the few-layer structural feature of MoS₂ within two hybrids.²⁶ A relative stronger diffraction signal is observed for P-MG(T) compared to that of MG(T) due to the buffering effect of H₃PO₄, and this crystalline protecting behavior will be detailedly discussed later. The inset in the Fig. 2a shows the XRD pattern of P-MG before annealing, the peak (marked by red cycle) arises from the (002) plane signal of the few-layer graphene sheets, apparently, it is severely suppressed in P-MG(T) due to the improvement of the crystal quality of MoS₂ component after heating treatment.

Fig. 2 The (a) XRD patterns and (b) Raman spectrum of bare MoS₂, MG(T), and P-MG(T). The inset picture gives the XRD pattern of P-MG.

Raman measurement was performed on three samples and the results were displayed in Fig. 2b. The bare MoS₂ exhibits two representative peaks at around 376 cm⁻¹ and 404 cm⁻¹, which can be attributed to the in-plane vibrations of S–Mo–S (E¹_2g mode) and out-of-plane vibrations of S atoms along the c-axis (A_1g mode),^6,7 respectively. These two characteristic signals for MoS₂ are suppressed greatly for MG(T) hybrid, while they remain clear for P-MG(T) hybrid, confirming promotion effect of phosphoric acid for the crystallinity of the MoS₂ phase. The spectrum collected in the range of 1000–2000 cm⁻¹ is shown in the Fig. 2b, and the D- and G-band peaks of graphene centred at ∼1358 cm⁻¹ and ∼1586 cm⁻¹, can be observed for the two hybrids, respectively. G-band is caused by the vibrations of sp² carbon in the carbon lattice and herein can be regarded as a proof for the presence of high crystalline graphite. Generally, the D-band derives from the defects resided in the basal plane and edge of carbon-based materials, and the ratio of I_D/I_G is used to measure the defect density of the sample. One can easily discern that the I_D/I_G value of P-MG(T) (0.84) is smaller than that of MG(T) (1.05), which is significantly lower than that of rGO prepared using other technique,^27–30 indicating a high restoration degree of sp² carbon matrix for P-MG(T) after the hydrothermal reduction and the annealing treatment.

3.3 XPS

XPS is an important tool to investigate the chemical composition and bonding state of sample. In Table S1,† C, O, P, Mo, and S atomic percentage of each sample is listed. All samples have a S/Mo value of 2.03–2.08 which is slightly higher than that of the pristine MoS₂ crystal, which may indicate the existence of high valence state Mo(Mo⁵⁺/Mo⁶⁺). It can be seen that all four hybrid samples contain relatively high oxygen (9.5–16.2 at%) species which is remained from the pristine graphene oxide and can play a vital role in the electrochemical applications, depending on the bonding types.³¹ And the deoxygenization behavior due to annealing can be clearly observed as well.

To scrutinize the bonding configuration of the C, S, Mo and P species, fitted peaks are presented. The high-resolution C 1s spectrum (Fig. 3b and S2b†) of MG(T), P-MG, and P-MG(T) all contain following three sub-peaks: C–C (284.5 eV), O–C–O&C–OH (286.2 eV) and C=O&O–C=O (288.5 eV), a higher value of C=O&O–C=O can be seen for P-MG and P-MG(T). In addition, the extra peak at ∼283.1 eV can be attributed to conjugated C=C.³² These confirm that the introduced H₃PO₄ buffer can protect the C=O and O–C=O groups, which can be easily reduced in an alkaline aqueous condition,²⁸ simultaneously promoting the recovery of graphitic aromatic sp² structure. During the thermal treatment (Fig. S2a†), these electrochemistry active groups³¹ generally on the graphene edge remained due to good thermal stability, and only internal oxygenic groups were evidently eliminated. The high-resolution Mo 3d spectra of the P-MG(T) is shown in Fig. 3c which can be divided into five peaks, and the one centered at 226.3 eV corresponds to S 2s of MoS₂. The two intense Mo 3d_5/2 (229.3 eV) and Mo 3d_3/2 (232.4 eV) components are characteristic peaks of MoS₂, while the peaks centered at 230.2 and 235.9 eV confirm the presence of Mo⁵⁺ and Mo–O(3d_5/2),³³ wherein the Mo–O bond was proposed to connect the graphene and MoS₂ layers according to the previous study.^15,23 Likewise, S species are determined from the high-resolution XPS S 2p spectrum (Fig. 3d). The main doublet located at binding energies of 162.0 and 163.1 eV corresponds to the S 2p_3/2 and S 2p_1/2 lines of MoS₂.³⁴ Meanwhile, the high-energy component at 169.2 eV can be assigned to S⁴⁺ species in sulfate groups (SO₃²⁻),³³ and these groups could locate at the edges of MoS₂ layers, which are non-trival to be eliminated due to the strong covalent link. Finally, we need to mention that the dominated chemical state of P atoms in our sample is P=O bond, and these exterior P-containing groups normally introduced at a relative low temperature (<500 °C) are proven to be favorable for enhancing the cation exchange ability of carbon-based materials.^35,36

Fig. 3 The (a) XPS survey spectra and (b) C 1s spectra of MG(T) and P-MG(T); (c) Mo 3d, (d) S 2p, and (e) P 2p spectra of P-MG(T).

3.4 Formation mechanism

The spatially interconnected graphene layers framework interspersed with refined MoS₂ nanosheets was successfully fabricated by a moderate hydrothermal reaction plus post-annealing technique (Fig. 4), within which the role of H₃PO₄ could be addressed as follows:

Fig. 4 Schematic illustration of the formation process of P-MG(T) hybrid.

It has been stated that the growth habit of crystals can be modulated with the aid of specific organic proton acid.^6,20,21 Similarly, the addition of H₃PO₄ in our experiment could act as a pH-regulator via consuming the ammonia released from L-cysteine during the hydrothermal reaction. It is well known that increasing pH value can give rise to negative charge on the GO surface³⁷ (corresponding to the case of MG(T)), hence a decreased absorption amount of MoO₄⁻ on the GO surface, and the stacking of MoS₂ crystals were finally formed due to the lack of adequate adsorption sites (Fig. 1b). In contrast, keeping pH value at 2–3 (corresponding to the case of P-MG(T), see Experimental section) could promote the formation of well-distributed gauzy MoS₂ onto the graphene surface (Fig. 1c). Moreover, the difference in chemical composition between MG(T) and P-MG(T) (see XPS results) may be explained by the pH-dependent surface properties of GO³⁷ and the ammonia-related reducing ability of the system on GO, such as the conservation of carboxyl groups and the higher oxygen value for P-MG(T) product.³⁸ Finally, a small fraction of phosphorus-containing groups which survived via dehydration reactions during thermal treatment may act as an efficient linker that connects the edge of graphene layers to form an interconnected structure.²¹

4. Electrochemical performance

The electrochemical performance of the pure MoS₂ and MoS₂/graphene hybrids were evaluated by using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques in a three-electrode system. Herein, two different electrolytes (1 M H₂SO₄ and 6 M KOH, also frequently-used ones in the graphene-based electrode materials) were employed elaborately in consideration of the electrolyte-dependent electrochemical properties^10,39 of our samples.

4.1 Capacitive properties in 6 M KOH electrolyte

All CV curves measured in alkaline condition exhibit a quasi-rectangular shape (see Fig. 5a and b), attributing to the typical double-layer response. An increase in capacitive performance of P-MG(T) is intuitively observed from the surround CV area, which indicates that the H₃PO₄-involved treatment can trigger the EDLC-related properties of the electrode material, such as the wettability of the electrode surface and its electronic conduction ability. For the quantitative study of this improvement, GCD curves were recorded (the obtained symmetrical charge–discharge lines are also the characteristic of EDLC behavior), from which the specific capacitance of the electrode can be obtained using the equation: C_sp = It/ΔEm. Where C_sp, I, t, ΔE, and m are specific capacitance (F g⁻¹), constant current (A), discharge time (s), potential window (V) and mass of the active material (g), respectively. From the calculated results as shown in Fig. 5d, one can find that there is a significant increase in C_sp value after the H₃PO₄ treatment (for instance, from 172 F g⁻¹ to 258 F g⁻¹ in 2 A g⁻¹), and a negligible C_sp value of pure MoS₂ is observed. This result can be correlated to suitable oxygen content, low defect density and the interconnected 3D architecture of P-MG(T) hybrid, which is similar to those of high-class graphene-based EDLC electrode materials.¹

Fig. 5 Electrochemical performance recorded in 6 M KOH: (a) CV curves of P-MG(T) at a scan rate of 10–50 mV s⁻¹; (b) CV curve comparison among pure MoS₂, MG(T), and P-MG(T) samples; (c) GCD curves of P-MG(T) at a normalized current density of 2–10 A g⁻¹; (d) calculated C_sp value for three samples from GCD curves.

4.2 Capacitive properties in 1 M H₂SO₄ electrolyte

Fig. 6a and b shows CVs of the three samples. Contrary to the situation in the alkaline electrolyte, several obvious humps are obtained in pure MoS₂ and P-MG(T), which implies the presence of electrochemical active sites on the samples. At a scan rate of 50 mV s⁻¹, P-MG(T) shows a rectangle-like trend with three broad peaks, indicating the synergistic effect of EDLC and pseudocapacitance. The peak pair emerged at around 0.7–0.8 V of pure MoS₂ in Fig. 6a can account for the sole hump of cathodic process of P-MG(T). It was reported that redox reaction of Mo⁴⁺ ↔ Mo⁵⁺ and electrochemical adsorption of H⁺ onto the MoS₂ surface can contribute to the pseudocapacitance of MoS₂ nanosheets,^10,15,40 hence, the same perspective can be claimed in our case considering the existence of Mo⁵⁺ in the sample (Fig. 3c) and the H₂SO₄ electrolyte employed here. And the peak pair observed at ∼0.6 V of anodic scan and ∼0.55 V of cathodic scan correspond to faradic reactions of oxygen-containing groups (benzoquinone/hydroquinone, supported by XPS results) according to the previous reports^41,42 (0.5–0.8 V vs. SCE). The relative low anodic signal (at ∼0.8 V) from the electrochemical reactions of MoS₂ may be overwhelmed by the oxygen-related peak in P-MG(T) sample. With increasing the scan rate (Fig. 6b), the peak pair coming from the oxygenic groups was separated gradually from each other, attributing to the accompanying polarization process. The density of MoS₂-related cathodic peak decreases highly at the high scan rate, which indicates that this pseudocapacitance behavior is restricted by its rate-determining steps (for example, the diffusion of H⁺ ions⁴³), while oxygen-related peak current enhances with increasing scan rates, demonstrating a quick kinetics of this redox reaction. The specific capacitance obtained from CVs reaches a high level of 357 F g⁻¹ at the scan rate of 10 mV s⁻¹ for P-MG(T) (see Fig. S3†), which is almost two times higher than that of MG(T). The subsequent tiny drop in capacitance value at high scan rates indicate a quick ion-diffusion response and a accessible surface of P-MG(T) electrode material.

Fig. 6 Electrochemical performance recorded in 1 M H₂SO₄: (a) CV curves of P-MG(T) at a scan rate of 10–50 mV s⁻¹; (b) CV curve comparison among pure MoS₂, MG(T), and P-MG(T) samples; (c) GCD curves of P-MG(T) at a normalized current density of 2–10 A g⁻¹; (d) calculated C_sp value for three samples from GCD curves.

The discharge curves of P-MG(T) are presented in Fig. 6c. The pronounced mutations correspond to the humps appeared in CVs, confirming the occurrence of redox reactions during its charge/discharge process. The inset gives the discharge curve of P-MG(T) at 10 A g⁻¹, in which the distortions can be identified and the persistence of electrochemical sites resided in P-MG(T) at a relative high current load can be stated. As illustrated in Fig. 6d and Table S1,† the highest C_sp value obtained from GCD is observed for P-MG(T) (351 F g⁻¹ at 2 A g⁻¹). It can be seen that the ability for charge accumulation diminishes with the increase in current load, but P-MG(T) sample is still able to supply a capacitance of 173 F g⁻¹ at an extremely high current density of 20 A g⁻¹. Significantly, over two-fold C_sp value was achieved after the treatment with H₃PO₄, benefiting from the improvements in both chemical composition (pseudocapacitance) and architectural construction (accessible surface).

4.3 Cycling performance

The cycling performance is illustrated in Fig. 7a, after 1000 cycles at 4 A g⁻¹, and it can be indicated that 89.5%, 83.7%, and 75% of the initial capacitance is held for P-MG(T) (in 1 M H₂SO₄), P-MG(T) (in 6 M KOH), and MG(T) (in 1 M H₂SO₄), respectively. Commonly, a significant drop in the first 100 cycles is observed for the three curves, probably due to the degenerated redox pairs or degradation of electrode in the ability to accommodate ions. The highest retention of P-MG(T) (in 1 M H₂SO₄) suggests good cycling stability of electrochemical sites resided in the hybrid (comparative study on the electrochemical performance of MoS₂/graphene is given in Table S2†). The cycling performance of pure MoS₂ is given in Fig. S3b and a† clear retention improvement after the incorporation of graphene can be manifested.

Fig. 7 (a) Cycling performance for MG(T) and P-MG(T); (b) Nyquist plots of MoS₂, MG(T), and P-MG(T); (c) bode plots of the imaginary/real capacitance as a function of frequency (log f) for P-MG(T).

4.4 EIS performance

The Nyquist plots of three samples in 1 M H₂SO₄ all show excellent electrochemical behaviors with very small impedance at the high frequency range and nearly perfect electrochemical response in the low frequency range (except for pure MoS₂), indicating a capacitive behavior of P-MG(T) and MG(T). The equivalent series resistance (ESR, coming from the resistance of electrolyte and/or contact resistance) and charge-transfer resistance of all samples are very low (∼0.75 Ω for ESR) here, favoring the quick redox reactions. Significantly, there exists little difference in impedance behaviors when different electrolytes (1 M H₂SO₄ and 6 M KOH) are employed, demonstrates that the enhanced capacitance value of P-MG(T) in H₂SO₄ is indeed owing to the specific electrochemical sites.

An alternative approach to describe the supercapacitors is to construct the relationship between capacitance (C) and frequency (f). It is known that

It can also be written under its complex from

Z(ω) = Z′(ω) + jZ′′(ω) (2)

It is possible to define

C(ω) = C′(ω) + jC′′(ω) (3)

leading towhere C′(ω) and C′′(ω) is the real and imaginary part of the capacitance C(ω), respectively.

Fig. 7c presents the relationship of C′(ω) vs. frequency (log f) and C′′(ω) vs. frequency (log f) using the impedance data of P-MG(T) (in 1 M H₂SO₄). It can be seen that when the frequency decreases, C′ sharply increases, then tends to be less frequency-dependent, which reveals characteristics of the electrode kinetics. Most importantly, the relaxation time constant τ₀ indexed as the supercapacitor factor of merit by Miller,^44,45 can be calculated to be ∼1.8 s from the peak frequency (f₀) of C′′(ω) vs. frequency (log f) plots as τ₀ = 1/f₀, which is comparable to that of carbon-based electrode materials,^46–48 suggesting that P-MG(T) hybrid is a favored candidate for the supercapacitor applications.

4.5 Electrolyte-dependent capacitive performance

Based on the above results, the electrolyte-dependent capacitive performance of P-MG(T) may be explored. Indeed, two distinct contributions to the enhanced capacitance value of P-MG(T) hybrid (compared with MG(T)) can be claimed here: EDLC and pseudocapacitance, which has been testified in an alkaline and acidic electrolyte, respectively. To distinguish these two parts quantitatively, the dependence of voltammetric charge (q) on the scan rate of CV (v) for P-MG(T) in two electrolytes was estimated according to the studies by Trasatti⁴⁹ and its further evolution by Hu^39,50. It was stated that the voltammetric charge, q_dl, coming from electric double-layer (which is related to the “outer” and more accessible surface) can be given from the extrapolation of q to v = ∞ from the fitted q vs. v^−1/2 plot (Fig. 8a); the total voltammetric charge, q_t, is given from the extrapolation of q to v = 0 from the fitted 1/q vs. v^1/2 plot (Fig. 8b). Accordingly, the charge coming from the pseudocapacitance, q_p, due to redox reaction,⁵¹ can be obtained from the difference between q_t and q_dl. From the equation: C = q/ΔU (ΔU = 0.8 V in our case), the total capacitance, EDLC, and pseudocapacitance can be derived by the substitution of q_t, q_dl, and q_p, respectively, and corresponding value is listed in Table 1. As shown in Fig. 7, the fitted lines are harmonious with the raw date, which confirms the pronounced relationship between q and v. The obtained total capacitance (C_t) of P-MG(T) is 390 F g⁻¹ in acidic electrolyte, wherein 247 F g⁻¹ of which derives from EDLC and the other 143 F g⁻¹ (36.7%) comes from pseudocapacitance, attributing to benzoquinone/hydroquinone pair and faradaic charging of MoS₂ nanosheets. Relatively, almost pure EDLC behavior is assumed for P-MG(T) in the alkaline electrolyte (C_t, C_dl, and C_p is 278, 262, and 16 F g⁻¹, respectively), which is consistent with the CVs results. In addition, the P-MG(T) shows closed C_dl value because of its similar impedance behavior for two kinds of electrolytes, which is supported by EIS analysis.

Fig. 8 (a) Dependence of 1/q on v^1/2 and (b) dependence of q on v^−1/2 for P-MG(T) in KOH and H₂SO₄ electrolytes.

Table 1 Collection of total capacitance (C_t), EDLC (C_dl), and pseudocapacitance (C_p) for P-MG(T) from the plots of 1/q vs. v^1/2 and q vs. v^−1/2

Electrolyte	Ct (F g^−1)	Cdl (F g^−1)	Cp (F g^−1)	Cp/Ct (%)
1 M H2SO4	390	247	143	36.7
6 M KOH	278	262	16	5.7

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5. Conclusion

Layered MoS₂/graphene hybrids have been successfully prepared via a H₃PO₄-assisted hydrothermal reaction coupled with a subsequent annealing treatment. The introduction of H₃PO₄ gives rise to the hybrid with certain crystal structure and morphology which enhances electrochemical performance. The different electrochemical behavior could be attributed to the EDLC and pseudocapacitance effect in alkaline and acidic electrolyte, respectively. The strategy presented in this work can also be extended to the synthesis of other graphene-based inorganic hybrids.

Acknowledgements

This project is supported by National Basic Research Program of China (973 Program, Grant No. 2014CB660815), the National Natural Science Foundation of China (Grant No. 41202022, 21303129, 51372063), the Fundamental Research Funds for National University (CUG150413, 130403, 1410491B03) China University of Geosciences (Wuhan).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17109e

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