Homogeneous sulphur-doped composites: porous carbon materials with unique hierarchical porous nanostructure for super-capacitor application

Abstract

Specific surface area, hierarchical porosity and heteroatom doping have been proved to be desirable for carbon electrode materials in advanced super-capacitors. In this study, homogeneous sulphur-doped porous carbon materials (L-HPCs) with a unique hierarchical porous nanostructure have been prepared by the combination of the two-dimensional interlayer confinement effect of a layered double hydroxide (LDH) and KOH activation method. XRD, SEM, TEM, BET, EIS, cyclic voltammetry and the constant current charge–discharge test were used to investigate the morphology, structure, surface properties and electrochemical performances of the carbon materials. The L-HPCs fabricated by this method have a large specific surface area of 2154.0 m² g⁻¹ and contain plenty of micropores and mesopores, which arise from the carbonisation of organic polymers, the presence of LDH and the catalytic effect of iron in LDH layers in the calcination process. The L-HPCs serving as an electrode material for super-capacitors exhibited a specific capacitance as high as 259 F g⁻¹ at 1 A g⁻¹ in 6 M KOH electrolyte. This excellent electrochemical performance might be due to the high surface area, high pore volume and hierarchical porous structure of the carbon materials, which provide a quick ion transfer pathway to electrolyte access into the microporous area. At the same time, the doping of heteroatoms can improve the conductivity of the electrode materials.

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Introduction

Since 1957, carbon-based materials used as super-capacitor electrode materials have been the topic of intense research.^1–3 Electrochemical double-layer capacitors (EDLC) rely on electron attachment in carbon material to achieve energy storage and conversion.^4–6 Therefore, the specific surface area, pore size distribution, conductivity and surface wettability of the carbon material become critical factors for super-capacitor materials.

One effective strategy to get high surface area, large pore volume and high specific capacitance of carbon materials is to introduce porous structures. Template techniques have been widely employed to synthesise porous carbon materials.^7–10 Introducing catalytically active metal ions (e.g. cobalt, nickel, iron) into the template or the reaction system can especially enhance the generation of porous structure in carbon mateials.^11–13 In addition, KOH activation is a well-known method to develop the pore network in carbon materials.^14–16 Other than controlling the morphology, another effective and simple way to improve the capacitive performance of carbon materials is to introduce a heteroatom in the carbon frameworks, such as N, S, P or B.^17–21 In particular, sulphur-doped porous carbon materials are expected to have a wider band gap due to the electron-withdrawing character of S. Compared with pure carbon materials, S-doped carbon materials show better electrical conductivity and surface wettability.^20,22–24 Enhanced super-capacitive behaviour of sulphur-doped carbon materials was reported recently. Zhao et al. have prepared sulphur-doped mesoporous carbon via a one-pot aqueous self-assembly synthesis strategy. And the specific capacitance of this sample was improved by 72% over that of conventional ordered mesoporous carbon in an alkaline electrolyte.²⁵ Seredych also have reported that the volumetric capacitance of sulphur-doped micro/mesoporous carbon-graphene composites reached 65 F cm⁻³, which is higher than that of pristine-ordered mesoporous carbon.²⁶ Besides, S-doping can influence the pore size and structure of the porous carbon.²⁷ Although in some cases an improvement of S-doped carbon materials was made, more research is needed. Especially, how to get homogeneous S-doped carbon materials in a very simple way without HF need to be exploited.

Herein, in this paper, homogeneous sulphur-doped porous carbon materials (L-HPCs) were easily using sodium dodecyl sulfonate (SDSO) by the two-dimensional (2D) interlayer confinement effect of a layered double hydroxide (LDH) and KOH activation. The idea of using SDSO in the first synthetic step provides a convenient way to introduce S evenly into the carbon lattices which can induce bond polarisation, lattice distortion and tailoring of charge density distribution in carbon materials; 10 wt% HCl solution instead of HF is used in the second step to remove the template in the reaction conditions and avoid hazardous gases generated. Furthermore, the presence of the LDH hosts can increase the decomposition and vaporisation temperature of the organic molecule precursors, resulting in a high yield of carbonisation, and the KOH activation method can further increase the surface area of the materials. In addition, the catalytically active iron ions in the LDH hosts can catalyse the formation of porous structures. The obtained carbon materials showed hierarchical porous structure and displayed superior electrochemical performances in aqueous electrolyte.

Experimental

Preparation of intercalated LDH precursors

First, 7.7823 g Mg(NO₃)₂·6H₂O, 1.9045 g of Al(NO₃)₃·9H₂O, 2.0469 g of Fe(NO₃)₃·9H₂O, and 2.2697 g of sodium dodecyl sulfonate (SDSO) were dissolved in hot deionised water (100 mL), which had been boiled to get rid of the carbon dioxide. After it formed a homogeneous clear solution, 23 mL of 3.5 M NaOH solution was added, dropwise. Then this system was heated at 80 °C for 8 h under dynamic flow of nitrogen. After cooling and filtration, the residue was washed and dried overnight under vacuum to afford DSO-LDH. For comparison, another precursor, DSO-LDH-w (without Fe(NO₃)₃·9H₂O in the intercalated LDH precursors), was prepared.

Preparation of S-doped porous carbon materials

The dried DSO-LDH sample was ground and calcined at 600 °C for 2 h in the N₂ flow. After cooling down, the product was dissolved in 300 mL 10 wt% HCl solution in order to remove the oxides and elemental metals. Finally, the products were centrifuged and washed until a pH of 7, and noted as L-PCs. The dried DSO-LDH-w sample was treated by the same procedure as described above. The final products were labelled as L-PCs-w. For comparison, the sample obtained by carbonising SDSO directly and acid dissolution was labelled as PC.

The mixture (the weight ratio of L-PCs and KOH is 1 : 3) was ground and calcined at 800 °C for 2 h in the N₂ flow. After cooling, the product was treated with 5 wt% HCl solution. The porous carbon material was washed and dried, and denoted as L-HPC. At the same time, another product, obtained by activating PCs, was denoted as HPC.

Characterisation

X-ray diffraction (XRD) was measured using a BRUKER D8 ADVANCE instrument with Cu-Kα radiation (λ = 0.15406 nm) operated at 40 kV and 40 mA, in the range of 1.5–70°. The morphologies of the materials were characterised by field emission scanning electron microscopy (SEM, Hitachi SU 8010) with an acceleration voltage of 15 kV, and high-resolution transmission electron microscopy (HRTEM) (JEOL JEM-2100F). The surface area and pore size distribution of the materials were calculated according to the data recorded on the AutosorbIQ2-MP apparatus. An X-ray photoelectron spectroscopy (XPS) was obtained from an ESCALAB 250 spectrometer (Thermo Fisher Scientific) with an Al Kα (1486.6 eV) X-ray source operated at 15 kV and 150 mW.

Electrochemical measurements

The electrochemical measurements were carried out using a standard three-electrode configuration. Active materials, Super P, and binder (poly-tetrafluoroethylene, PTFE) were mixed in a mass ratio of 80 : 10 : 10 by adding a small amount of water and ethanol. The mixture was rolled into carbon film, after continuously stirring, evaporated, and cut into a round film 5 × 10 mm in size (the mass loading was about 2 mg cm⁻²). Then the carbon film was pressed into nickel foam as a work electrode, and a Pt foil and Hg/HgO electrode were used as counter electrode and reference electrode, respectively. The electrolyte used in this system was 6 mol L⁻¹ KOH solution. The cyclic voltammetry (CV) and galvanostatic charge–discharge were all carried out using a CHI 660C electrochemical workstation (Shanghai CHI Instruments). The gravimetric capacitance for a single electrode C_m (F g⁻¹) was calculated from the galvanostatic charge/discharge profiles according to the equation:
where C_m (F g⁻¹) is the mass ratio of the capacitance; I (A) is the discharge current; t (s) is the discharge time; and m (g) is the mass of active materials in a single electrode.

Results and discussion

The XRD patterns of DSO-LDH are shown in Fig. 1a. The characteristic diffraction peaks (003), (006), (009), (012) and (110) of LDH were detected easily. The low angle diffraction peak (003) shifted to the position 2θ = 3.6°, corresponding to a layer distance of d₍₀₀₃₎ = 2.4 nm, and the interlayer distance was bigger than that in the nitrate intercalated hydrotalcite, suggesting that DSO entered the interlayer of LDH.²⁸ In addition, the diffraction peak (006) was located at 2θ = 7.4°, corresponding to a distance of d₍₀₀₆₎ = 1.19 nm, and the diffraction peak (009) was located at 2θ = 11.34°, corresponding to a distance of d₍₀₀₉₎ = 0.783 nm. Thus it can be seen that the d-value of the three has a very good multiple relationship, which implies intercalated LDH precursors have an ideal layered structure, which can also be seen in Fig. 1b.

Fig. 1 (a) XRD patterns, (b) SEM images of the DSO-LDH. The insertion in (a) is a small angle diffraction pattern.

Different steps were taken to obtain L-PCs, L-HPCs, PCs and HPCs. SEM and HRTEM images of four materials are shown in Fig. 2. Compared with L-PCs (Fig. 2a) with a small amount of porous structure and large differences in pore size, significant changes in pore structure and pore size took place after KOH activation, L-HPCs (Fig. 2c) presented a hierarchical porous structure with the coexistence of micropores and mesopores (Fig. 2e), so KOH etching is the most likely cause. We also observed the same phenomenon from PCs (Fig. 2b) to HPCs (Fig. 2d and f). L-PCs have a much more fluffy porous structure and more irregular pores compared with PCs, which indicated that the presence of LDH template inhibits SDSO reunion during the process of carbonisation, promoting and catalysing the formation of the porous structure, and is conducive to porous carbon material with greater surface area.¹²

Fig. 2 SEM images of (a) L-PCs, (b) PCs, (c) L-HPCs, and (d) HPCs. HRTEM images of (e) L-HPCs and (f) HPCs.

As shown in Fig. 3, two distinct diffraction peaks corresponding to (002) diffraction peak and (101) diffraction peak of graphite could be observed. The position of the (002) diffraction peak demonstrates that the lattice spacing of the graphitic layers for samples is larger than for pure graphitic carbon, along with low crystallinity, short-range order and long-range disorder. That is to say, the graphite layer lattice was softened, damaged and consumed by the KOH activation process. As a result, the amount of porous structure and the degree of disorder were increased.^29,30

Fig. 3 XRD patterns of L-HPCs and HPCs.

To further explore the microscopic pore structure of the porous carbon material, nitrogen adsorption–desorption isotherms were measured (Fig. 4). Compared with the L-PCs-w, the specific surface area and pore volume of L-PCs have significantly increased. The presence of catalytically active iron ions in LDH hosts was probably the main reason.^12,13,31 As shown in Fig. 4b, more mesopores are detected in L-PCs than L-PC-w, which tends to reduce internal resistance, improve material wettability, and enhance electrochemical properties as a super-capacitor electrode material.^32,33

Fig. 4 (a) Nitrogen adsorption/desorption isotherm and (b) pore-size distribution of all porous materials.

In addition, comparing the BET data of L-HPCs and L-PCs, it can be seen that KOH activation leads to carbons with high surface area and volume of both micropores and mesopores. Potassium steam is formed by the sublimation of potassium metal (K) at 800 °C since the boiling point of K is 762 °C. It has a strong penetration and intercalation effect, which can go deeply into the interior structure of the carbon material and further activate carbon materials.^34,35

As shown in Fig. 4b, the porous structure of HPCs mainly contains micropores and a very small amount of mesopores, while L-HPCs have a hierarchical porous structure with the coexistence of micropores and mesopores. The presence of mesopores facilitates the transport of ions to the micropores, and to some degree of electrical conductivity, which helps in attracting ions to small pores. These features appeared to be of paramount importance for charge storage following the electrical double-layer capacitance (EDLC) mechanism.²⁶ What's more, the surface areas and pore volumes of L-HPCs and HPCs are 2154.0 m² g⁻¹ and 2.309 cm³ g⁻¹, and 1458.0 m² g⁻¹and 1.093 cm³ g⁻¹, respectively, the values of which are much larger than that of other porous carbon: sulphur-doped porous carbons hybridized with graphene (SPC@G) with a BET value of 928 m² g⁻¹, S-doped graphene (SG-600) with a BET value of 771 m² g⁻¹ and S-doped 2D porous carbons (GMC-S-900) with a BET value of 642 m² g⁻¹.^36–38 These demonstrated that under the 2D interlayer confinement effect of a LDH containing a catalytic element, confined carbonisation provided an efficient route to prepare porous carbon materials.

In order to further study the characteristic structure of the samples, XPS was used to clarify the chemical state of the elements and surface chemistry of porous carbon materials. The results (see Fig. 5, left) show a strong peak within 284.3–284.8 eV in the C 1s XPS spectra, corresponding to the sp² hybridised peak of carbon atoms, which indicates that carbon elements mainly exist in graphite rings in the form of π-conjugated bond.^39,40 The S 2p peak of the samples can be split into three peaks, which are shown in Fig. 5. The former two peaks at 164.0 eV and 165.1 eV correspond to S 2p_3/2 and S 2p_1/2 of the –C–S–C– covalent bond from thiophene-S due to their spin–orbit couplings, and the peak centred at 168.9 eV corresponds to C–SO_x bonds.^36,41 The elemental contents of S in HPCs and L-HPCs are 1.05 and 1.01%, respectively. These results demonstrate that the sulphur atom has successfully entered into the carbon lattice.

Fig. 5 C 1s and S 2p XPS spectra of (a) HPCs, (b) L-HPCs.

The electrochemical performances of HPCs and L-HPCs were evaluated in 6 M KOH aqueous solution in three electrode cells. Fig. 6a shows the cyclic voltammograms (CVs) of HPCs and L-HPCs at a scan rate of 10 mV s⁻¹, which exhibits a good rectangular shape corresponding to typical capacitive behaviour; the specific capacity of L-HPCs is greater than HPCs. As shown in Fig. 6b, with the increase of scanning rate, CVs of L-HPCs maintained a good rectangular shape, indicating that L-HPCs show a good rate of performance at high rate. According to Fig. 6c and d, galvanostatic charge–discharge curves of L-HPCs and HPCs both showed good symmetry at diffident current densities. Then the specific capacitance notably decreases along with the increase of current density, mainly because of insufficient ion diffusion at high current densities.²² Owing to its large specific surface area and abundant pore structure, the electrode material has different resistances between the outer and inner pores. Thus, upon discharge, the electrode material will generate a potential difference. The IR drop (Fig. 6e), related to the equivalent series resistance of the L-HPC samples, is significantly lower than that of the HPCs.⁴² The specific capacity (Fig. 6f) calculated from the discharge curve reached 259 F g⁻¹ at a current density of 1 A g⁻¹ for L-HPCs, which is higher than HPCs (203 F g⁻¹) and other doped carbon materials that have been reported in many published studies. For example, the specific capacitance of S-doped mesoporous carbon fibres were synthesised by Ma et al. to reach up to 150 F g⁻¹ at a current density of 1 A g⁻¹.⁴³ The bamboo-like carbon nanotubes containing sulphur (BCNT-S) showed a remarkably high specific capacitance of 259 F g⁻¹ at a current density of 1 A g⁻¹.⁴⁴ And the sulphur-doped porous-reduced graphene oxide hollow nanospheres framework, as super-capacitor electrode materials, exhibited a high specific capacitance of 211 F g⁻¹ at 1 A g⁻¹.⁴⁵ What's more, at a higher current density of 10 A g⁻¹, the capacity of L-HPCs still maintains more than 57%, higher than HPCs. The reasons for L-HPCs having excellent electrochemical performance may be as follows. Micropores that facilitate ions' rapid access can provide higher surface area for the adsorption of ions, but not be conducive to the transmission of the electrolytes. According to Fig. 4, much more mesopores were produced in L-HPCs. It is good for the penetration and transfer of electrolytes, reduces the internal resistance of the material, effectively shortens the ion diffusion distance, and is conducive to the rapid formation of an electric double layer.^46,47 Thus, L-HPCs exhibit higher capacitance and are more suitable than other porous carbon material as a super-capacitor electrode material.

Fig. 6 Cyclic valtammograms (CVs) of (a) HPCs and L-HPCs at a scan rate of 10 mV s⁻¹, (b) L-HPCs at different scan rates; galvanostatic charge–discharge curves of (c) L-HPCs and (d) HPCs at diffident current densities, (e) L-HPCs at a current density of 1 A g⁻¹; (f) specific capacitance as a function of the current density.

To further identify the exact electrical conductivity of electrodes, electrochemical impedance spectroscopy (EIS) of L-HPCs and HPCs were measured in 6 M KOH aqueous solution. As shown in Fig. 7, nyquist plots of these materials are made up of a semicircle in the high-medium frequency region and a sloping line in the low frequency region. It is well known that a smaller semicircle means smaller charge transfer resistance and the steepest slope of the curve indicates faster ion diffusion and excellent conductivity.^48,49 It can be clearly seen from Fig. 7 that L-HPCs and HPCs have smaller charge transfer resistance and faster ion diffusion and show excellent conductivity, better than commercial graphite, which might be attributed to the rich pores in the samples that facilitate rapid ion transport and the introduction of S in the carbon materials.⁵⁰

Fig. 7 Electrochemical impedance spectra (EIS) of L-HPCs, HPCs and commercial graphite.

Conclusions

In summary, L-HPCs were successfully prepared by the 2D interlayer confinement effect of LDH and KOH activation. The L-HPCs show a good response speed and rate performance as super-capacitor electrode materials, which might be attributed to the hierarchical porous structure and the coexistence of micropores and mesopores, the larger specific surface area, and the doping of sulphur atoms. The specific capacitances reached 259 F g⁻¹ for L-HPCs at a current density of 1 A g⁻¹ in 6 M KOH electrolyte. This study extends the energy storage application of sulphur-doped porous carbon materials with unique hierarchical porous nanostructure by providing a facile synthetic approach to carbon-based materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21671092), the Program for Liaoning Excellent Talents in University (LNET LR2015036) and the Opening Funds of the State Key Lab of Chemical Resource Engineering.

Notes and references

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