Introduction

In recent years, renewable energy has become an urgent issue on consideration of alleviating global warming and lessening our reliance on fossil fuels [1, 2]. Exploitation of abundant and renewable energy sources has attracted much attention [3, 4]. Unlike fossil fuels, hydrogen is considered as a clean carrier for energy storing and transporting [5, 6]. It is regarded as the most promising candidate for replacing fossil fuels in energy devices because of its numerous advantages, such as satisfactory recyclability, free pollution, and high efficiency when consumed [7,8,9]. However, hydrogen does not exist abundantly on earth, and we have to prepare it before use. Therefore, it is of great practical significance to develop highly efficient hydrogen production technology.

Among a great deal of techniques for hydrogen generation (steam methane reforming, coal gasification, chlor-alkali electrolyzers and water-alkali electrolyzers, etc.), electrocatalytic water splitting offers an attractive avenue to convert electricity harvested from water into high-purity hydrogen without any pollution and emission of carbon dioxide; therefore, it has been regarded as a clean energy technology enabling a hydrogen economy in the future [10,11,12]. Water splitting consists of two half reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) [13,14,15]. The slow kinetics and transfer of multiple electrons in water splitting can result in a considerable electrochemical overpotential, which is energy consumptive. Hence, it is desirable to develop highly efficient electrocatalysts with low overpotential toward water electrolysis to accelerate the HER rate and to thus improve the energy conversion efficiency.

Nowadays, platinum-based catalysts are considered to be the state-of-the-art electrocatalysts for HER owing to their low overpotential and high electroactivity in acidic media [16, 17]. However, the extreme scarcity, high cost, and limited durability severely restrict their widespread utilization in hydrogen production through water splitting on a global scale [18,19,20]. Therefore, researchers have been exploring earth abundant, stable, and efficient hydrogen evolution electrocatalysts with great enthusiasm to alternate commercially available Pt-based electrocatalysts in the past few years, and transition metals (Fe, Co, and Ni) and their compounds (carbides, nitrides, and phosphides) are considered as promising substitutes for platinum-based electrocatalysts [21,22,23,24]. Among these materials, transition metal chalcogenides account for the largest proportion [25, 26].

Compared with other transition metal sulfides, cobalt disulfides (CoS2) are more attractive in energy storage and conversion for the facile preparation method and favorable thermal stability [27,28,29]. It was reported that CoS2 exhibited better overall performances than FeS2 and NiS2 in HER due to its intrinsically metallic features and disulfide-terminated edges as active sites [30]. Despite the advantages above, the electronic conductivity and acidic durability of CoS2 still need to be enhanced in consideration of electrocatalytic performances and energy consumption.

In principle, the electrocatalytic performances of electrocatalysts can be enhanced through the following two approaches. One is to tailor the size of electrocatalyst particles into nanoscale to increase the specific surface area [31]. The other is to incorporate electrocatalysts with large-surface substrates, such as carbon materials, to modulate the electronic structure on the surface of electrocatalysts, which is beneficial to enhance the conductivity [32, 33].

As one kind of carbon materials, carbon nanotubes (CNTs) attract researchers’ attention for the applications in electrochemical energy storage and transformation devices due to the structural integrity, large surface area, compact arrangement, and favorable mesoporosity [34, 35]. CNTs are mainly divided into two categories: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). MWCNTs have advantages of large specific surface area, good conductivity, and structural flexibility, and thus, MWCNTs are used to improve the electronic conductivity and stability of HER electrocatalysts in the long-term operation [36, 37]. Based on the considerations above, CoS2/MWCNT nanocomposite is likely to be an efficient and stable electrocatalyst for HER.

Herein, CoS2/MWCNT nanocomposite was successfully synthesized through a one-step hydrothermal method with conductive matrix of acid-treated MWCNTs and assistant of cetyltrimethyl ammonium bromide (CTAB), working as surfactant and soft template. The introduction of MWCNTs alleviated the aggregation of as-prepared CoS2, and furthermore, it enhanced the conductivity and electrocatalytic activity of CoS2. The results showed that CoS2/MWCNT nanocomposite exhibited excellent electrocatalytic activity and favorable stability for HER in acidic medium.

Materials and methods

Reagents and materials

CoCl2·6H2O, CTAB, HNO3, and C2H5OH (> 99.95 wt.%) were analytical grade and purchased from Chengdu Kelong Chemical Reagent Factory (Chengdu, China). CH4N2S and H2SO4 were analytical grade and purchased from Aldrich Chemical Reagent Co., Ltd., (Shanghai, China). Nafion solution (5 wt.%) was supplied by Jinan Henghua Chemical Reagent Factory (Jinan, China). The doubly distilled water used throughout the whole experiment was obtained from a Millipore system.

Commercial MWCNTs (outer diameter × inner diameter × length 20~40 nm × 10~15 nm × 5~15 μm, purity > 95%), purchased from Chengdu Institute of Organic Chemistry, Chinese Academy of Science (Chengdu, China), were treated with the mixed solution of concentrated H2SO4 and HNO3 (volume ratio of 3:1) for 2 h with ultrasonication to remove the impurities and endow the surface with hydrophilic carboxylic acid groups. After the reaction, the obtained sample was filtered off, washed with distilled water, and dried at 80 °C under vacuum for 24 h.

Preparation of CoS2/MWCNT nanocomposite

Shown in Scheme 1 was schematic illustration of the preparation of CoS2/MWCNT nanocomposite based on hydrothermal method. In a typical synthesis, 0.15 g acid-treated MWCNTs was firstly added into 50 mL deionized water, and then, the mixture was stirred ultrasonically for 1 h at room temperature, resulting in a homogeneous suspension. Subsequently, 0.36 g CTAB as surfactant along with soft template and 1.04 g CoCl2·6H2O as cobalt source were added into the above suspension, followed by mechanical stirring for 30 min. Afterwards, 1.55 g thiourea was added as sulfur precursor and reductant. Then, pH was adjusted to 6.5 by dropping 0.10 M HCl solution, and the suspension was diluted to 80 mL, followed by ultrasonication for 1 h. The final suspension was transferred into a 100 mL Teflon-lined autoclave and then heated at 180 °C for 18 h. As the autoclave cooled, the precipitate was collected, washed with distilled water and absolute ethanol thoroughly, and further dried at 60 °C for 24 h in air. The obtained substance was denoted as CoS2/MWCNT nanocomposite.

Scheme 1
scheme 1

Schematic illustration of synthetic procedure of CoS2/MWCNT nanocomposite

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For comparison, pure CoS2 was synthesized according to the similar procedure above without addition of MWCNTs.

Preparation of modified electrodes

Prior to modification, glassy carbon electrode (GCE, Ф = 3 mm) was polished with 500 and 50 nm aluminum oxide powders to a mirror-like appearance, respectively, and then washed successively with ethanol and doubly distilled water for several times. Subsequently, the cleaned GCE was gently blown under a nitrogen stream.

The fabrication procedure of working electrodes was as follows. Five milligram as-prepared CoS2/MWCNT nanocomposite and 30 μL 5 wt.% of Nafion solution were dispersed in 1.0 mL solution composed of water and ethanol with a volume ratio of 1:1, followed by ultrasonication for 30 min. Subsequently, 5.0 μL as-prepared dispersion was dropped onto the surface of a polished GCE and naturally dried in air at room temperature to form uniform films. And the CoS2/MWCNT nanocomposite-modified GCE (CoS2/MWCNTs/GCE) with a mass loading of 0.35 mg cm−2 was obtained.

For comparison, MWCNT-modified GCE (MWCNTs/GCE) and CoS2-modified GCE (CoS2/GCE) were fabricated according to the similar process above.

Characterization of as-prepared materials

The morphologies of as-prepared materials were investigated with scanning electron microscope (SEM) images acquired from Ultra 55 microscope (Carl Zeiss AG, Germany). The crystalline structures of as-prepared materials were characterized by X-ray diffraction (XRD) (X’ Pert PRO, Netherlands) with Cu Kα radiation (λ = 0.154060 nm) and recorded in 2θ range from 10° to 80° at a speed of 2° min−1. Fourier transform infrared spectra (FTIR) of as-prepared materials were obtained with a Fourier transform infrared spectrometer (Nicolet 5700, USA) in the wavenumber range of 4000~1000 cm−1 with KBr pellet. Raman spectra of as-prepared materials were characterized by InVia (Renishaw Instrument Co., UK) in the wavenumber range of 2500~100 cm−1.

Electrochemical measurements

All electrochemical measurements were conducted in 0.50 M H2SO4 solution with a three-electrode test system comprising the platinum electrode as counter electrode and as-prepared material-modified GCE as working electrode referred to saturated calomel electrode (SCE). The electrolyte was purged with high-purity nitrogen (99.999%) before electrochemical measurements. Electrochemical impedance spectroscopy (EIS) measurement was carried out with a PARSTAT 2273 electrochemical workstation (Princeton Applied Research, USA). Tafel plot, polarization curves, and cyclic voltammetry (CV) curves were obtained with a CHI 760C electrochemical workstation (CH Instruments, China). All the reported potentials were calibrated to the reversible hydrogen electrode (RHE) scale at 298 K on the basis of Nernst equation as follows:

$$ {\mathrm{E}}_{\mathrm{RHE}}={\mathrm{E}}_{\mathrm{SCE}}+0.059\ \mathrm{pH}+0.242\ \mathrm{V} $$
(1)

Results and discussion

Physical characterizations of as-prepared materials

FTIR spectra analysis

Shown in Fig. 1 were FTIR spectra of raw MWCNTs and acid-treated MWCNTs. The weak bands located at 2915 and 2835 cm−1 in curve a corresponded to -CH stretching vibration. The bands at 1384 and 1113 cm−1 were ascribed to stretching vibration of C-C and C-O, respectively. The strong bands observed at 3435 cm−1 and the weak one at 1632 cm−1 were attributed to the stretching vibration and bending vibration, respectively, arisen from trace amounts of water [38, 39]. Compared with curve a, FTIR spectrum of acid-treated MWCNTs (curve b) exhibited an additional band at 1710 cm−1, corresponding to stretching vibration of C=O of -COOH group [40]. This additional band indicated hydrophilic -COOH group on the wall and port of MWCNTs, which further enhanced the dispersity of MWCNTs.

Fig. 1
figure 1

FTIR spectra of raw MWCNTs (a) and acid-treated MWCNTs (b)

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XRD analysis

For confirming the as-prepared CoS2 and CoS2/MWCNTs, XRD patterns are shown in Fig. 2. As for pure CoS2 (curve a), the diffraction peaks at 27.9°, 32.4°, 36.3°, 40°, 46.4°, 55.1°, 60.4°, and 63.04° corresponded to the lattice planes (111), (200), (210), (211), (220), (311), (230), and (321) of cattierite CoS2 (JCPDS No. 41-1471), respectively [41]. Compared with pure CoS2, the diffraction angle and intensity of CoS2/MWCNTs (curve b) did not change obviously except that two additional peaks appeared at 26.1° and 44.5°, which corresponded to the lattice planes (002) and (100) of hexagonal graphite-like structure of MWCNTs, respectively, implying the successful combination of CoS2 and MWCNTs [42].

Fig. 2
figure 2

XRD patterns of CoS2 (a) and CoS2/MWCNTs (b)

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Raman spectra analysis

Raman spectra in Fig. 3 showed the structural information of MWCNTs and CoS2/MWCNTs. As seen from the Raman spectrum of MWCNTs (curve a), there were characteristic D band and G band at 1360 and 1589 cm−1, respectively. The D band arose from sp3 hybridization of carbon and the G band was associated with sp2-bonded graphite-like carbon atoms. Shown in curve b, the peaks at 190, 471, and 672 cm−1 signified the successful fabrication of CoS2, which was consistent with the results of XRD analysis [41]. The co-existence of peaks at 1360 and 1589 cm−1 of MWCNTs and peaks at 190, 471, and 672 cm−1 of CoS2 verified the fabrication of CoS2/MWCNT composite, which was in well agreement with the results obtained from XRD analysis. Furthermore, it was obvious that the ratio of ID/IG of CoS2/MWCNTs was larger than that of pure MWCNTs, indicating that the structure of MWCNTs was destroyed during the hydrothermal process, and thus, more defect sites were exposed [43].

Fig. 3
figure 3

Raman spectra of MWCNTs (a) and CoS2/MWCNTs (b)

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SEM analysis

Shown in Fig. 4 were SEM images of CoS2, MWCNTs, and CoS2/MWCNTs. As can be seen from Fig. 4a, CoS2 was sphere-like particles with a diameter of 0.7 μm and it stacked loosely, which indicated that CoS2 particles aggregated together during hydrothermal process. It was worth noting that MWCNTs constructed a conductive network, providing a great deal of attachment sites for CoS2. During the hydrothermal process, CoS2 adhered onto the surface of MWCNTs and combined well with MWCNTs (Fig. 4c). Notably, the aggregation of CoS2 was relived, and the diameter of CoS2 decreased with the introduction of MWCNTs, leading to a much larger specific surface area and thus more exposed electroactive sites.

Fig. 4
figure 4

SEM images of CoS2 (a), MWCNTs (b), and CoS2/MWCNTs (c)

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Electrochemical performances of as-prepared modified electrodes

In order to investigate the influence of MWCNTs on the electrocatalytic performances of CoS2 for HER, polarization curves of MWCNTs/GCE, CoS2/GCE, and CoS2/MWCNTs/GCE were tested in 0.50 M H2SO4 solution. As shown in Fig. 5, it was clearly observed that the potential of CoS2/MWCNTs/GCE was more positive than that of CoS2/GCE at the same current density. The potentials of CoS2/GCE and CoS2/MWCNTs/GCE at the current density of 10 mA cm−2 were − 290 and −257 mV, respectively, demonstrating higher electrocatalytic activity of CoS2/MWCNTs/GCE. When current density reached 29 mA cm−2, the required potential of CoS2/MWCNTs/GCE (− 303 mV) shifted positively about 100 mV compared with that of pure CoS2/GCE (− 401 mV). In addition, it was obvious that the current density of CoS2/MWCNTs/GCE (73 mA cm−2) was higher than that of CoS2/GCE (29 mA cm−2) and MWCNTs/GCE (3 mA cm−2) at the potential of − 400 mV, indicating better conductivity of CoS2/MWCNTs/GCE. The favorable electrocatalytic performances of CoS2/MWCNTs/GCE were attributed to the introduction of MWCNTs, which significantly alleviated the aggregation of CoS2 and increased the effective surface area, thus facilitated the diffusion of electrolytes and electrons to the electroactive electrocatalysts, leading to more exposed catalytic active sites of CoS2 and increased electronic conductivity.

Fig. 5
figure 5

Polarization curves of MWCNTs/GCE (a), CoS2/GCE (b), and CoS2/MWCNTs/GCE (c) in 0.50 M H2SO4 solution. Scan rate 2 mV s−1

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To gain further insights into the HER kinetics, Tafel plots of as-prepared electrodes were investigated (Fig. 6). The linear regions of Tafel plots fitted the Tafel equation as follows.

$$ \eta =a+\mathrm{b}\;\log\;j $$
(2)

where a was the constant, b the Tafel slope (mV dec−1), and j the current density (mA cm−2). The CoS2/GCE and MWCNTs/GCE exhibited Tafel slopes of 88 and 253 mV dec−1, respectively, while the slope of CoS2/MWCNTs/GCE was 83 mV dec−1, demonstrating faster HER kinetics of CoS2/MWCNTs/GCE.

Fig. 6
figure 6

Tafel plots of MWCNTs/GCE (a), CoS2/GCE (b), and CoS2/MWCNTs/GCE (c) in 0.50 M H2SO4 solution. Scan rate 1 mV s−1

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To gain a direct comparison, electrocatalytic performances toward HER of CoS2-based electrodes reported in literatures are listed in Table 1. It was clearly observed that the electrocatalytic activity of CTAB-assisted synthesized CoS2/MWCNT-based electrode in this work was higher or comparable with those in reported literatures, showing that as-prepared CoS2/MWCNT-based electrode exhibited excellent electrocatalytic performances.

Table 1 Comparison of HER electrocatalytic performance of CoS2-based electrodes
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EIS measurement was carried out to obtain kinetic parameters of HER at the electrode/electrolyte interface with AC perturbation of 5 mV in the frequency range from 105 to 10−2 Hz. Shown in Fig. 7 were Nyquist plots of as-prepared modified electrodes, and the insert was an equivalent circuit for fitting the impedance data of CoS2/MWCNTs/GCE, where Rs was the resistance at electrode/electrolyte interface, Rct the charge transfer resistance, Zw the Warburg resistance, and CPE the constant phase element.

Fig. 7
figure 7

Nyquist plots of MWCNTs/GCE (a), CoS2/GCE (b), and CoS2/MWCNTs/GCE (c) in 0.50 M H2SO4 solution

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It was obviously observed that all the Nyquist plots consisted of two regions: a semicircle at high frequencies and a linear part at low frequencies. Rct for MWCNTs/GCE, CoS2/GCE, and CoS2/MWCNTs/GCE were 154, 700, and 300 Ω, respectively, indicating faster HER kinetics of CoS2/MWCNTs/GCE than that of CoS2/GCE. The lower Rct of CoS2/MWCNTs/GCE originated from the excellent conductivity of MWCNTs.

Besides the electrocatalytic activity, stability was another significant criterion to evaluate an advanced electrocatalyst. To investigate the long-term cycling stability of as-prepared CoS2/MWCNTs/GCE in acidic environment, polarization curves were recorded after performing continuous cyclic voltammetry scans between − 0.6 and + 0.1 V at 50 mV s−1 for 1000 cycles and shown in Fig. 8. No obvious differences on onset potential or current density were observed between the initial plot and the last one, indicating that the CoS2/MWCNTs/GCE exhibited excellent long-term stability for HER.

Fig. 8
figure 8

Polarization curves of CoS2/MWCNTs/GCE before and after 1000 CV scans in 0.50 M H2SO4 solution. Scan rate 2 mV s−1

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Conclusion

In this work, a novel CoS2/MWCNT nanocomposite for HER was designed and constructed hydrothermally in the presence of CTAB. Compared with CoS2/GCE and MWCNTs/GCE, CoS2/MWCNTs/GCE required much lower potential (− 257 mV) to reach the current density of 10 mA cm−2. Meanwhile, CoS2/MWCNTs/GCE exhibited low Tafel slope (83 mV dec−1), low charge transfer resistance (300 Ω), and long-term stability. The outstanding electrocatalytic activity of as-prepared CoS2/MWCNT nanocomposite was attributed to the highly exposed sulfur edges of CoS2 and excellent electrical conductivity of MWCNTs. CoS2/MWCNT nanocomposite was a promising candidate for highly efficient electrocatalyst for practical hydrogen evolution through water splitting under acidic conditions.