Introduction

With the increasingly serious consumption of fossil fuels and environmental pollution issue, the searching for clean and sustainable energy sources is imperative1,2. Hydrogen as a clean renewable energy is a potential alternative to traditional fossil fuels3,4. Electrochemical splitting water into hydrogen (2H2O → 2H2 + O2) is a promising pathway for sustainable hydrogen energy production, where the oxygen evolution reaction (OER) is the bottleneck owing to the sluggish four electron transfer process (4OH− → O2 + 2H2O + 4e−), which is accompanied with the breakage of O-H bond and the combination of O-O bond5,6. Great efforts had been expended to exploring high efficiency and stable oxygen evolution electrocatalysts7,8. Currently, the most active OER electrocatalysts are oxides of noble metals (e.g., IrO2 and RuO2), while the concerns related to its high cost and scarcity are big obstacles in the real application9,10,11.

Hence, cobalt based compounds have been exploited as candidate electrocatalysts owing to their abundance in nature, unique d-orbital electronic structure and low Gibbs adsorption energy. These compounds have shown high catalytic activity close to RuO2 towards water splitting12,13. Several families of cobalt based electrocatalysts, especially cobalt oxides, nitrides, hydroxides, sulfides and phosphides have shown attractive electrocatalytic activity towards water splitting14,15,16,17,18,19. Additionally, a lot of researches about identifying the high-efficient electrocatalytic mechanism of cobalt based electrocatalysts has been carried out through experiments, operando techniques and theoretical calculations20,21,22. Among them, cobalt sulfides, inspired by hydrogenase, have been suggested as promising electrocatalysts for overall water splitting23,24. Although the high efficient electrocatalytic activity of cobalt based electrocatalysts has been demonstrated, it is still far from satisfactory comparing with commercial electrocatalysts (e.g., RuO2, IrO2). The intrinsic low electron transport efficiency and limited number of catalytic active sites of cobalt based electrocatalysts are the main obstacles for further improving the electrocatalytic performance of water splitting25,26,27. Hence, extensive efforts have been dedicated to improve OER electrocatalytic activity of cobalt sulfides. The common methods include morphology control, phase engineering, doping and hybridization. For example, investigators constructed two-dimensional nanosheets, core-shell nanostructure, nickel-iron disulfide and oxhydroxide heterostructure, cubic cobalt sulfide with layered molybdenum disulfide hybridization towards enhancing the electrocatalytic of water splitting10,28,29,30,31.

It has been demonstrated that reduced graphene oxide (rGO) was an ideal matrix with uniformly grown cobalt sulfides to accelerate electron transmission and buffer volume changes during oxygen evolution process32,33. In-situ growth nanosheets on rGO had superiority of inhibiting the aggregation of nanosheets and fully exposing surface area to short the electron transport distance. At the same time, this proposal solves the problem that cobalt sulfides as electrocatalysts suffer from dissolution, losing and agglomeration after electrochemical reaction process34,35. For example, Li et al. reported the in-situ growth of transition metal dichalcogenides (TMDs) nanocages encapsulated by rGO, which greatly improved the ion/electron transport along the interfaces and efficiently mitigated volume dilation during LIBs delithiation34. In addition, the heteroatoms doped rGO with numerous of defective pockets as support coordinating with metal ions has its advantage of high electron transport efficiency. Fischer et al. designed nitrogen doped rGO/Ni7S6 and enabled the practicability of active nickel sites and synergetic catalysis with nitrogen doped rGO in electrochemical water splitting reaction36. Other works through heteroatoms doped rGO was devoted to maximizing the electron transmission ability of electrocatalysts with high efficient activity and durability37,38,39,40. Our approach of improving the OER activity of cobalt sulfides with enhanced the electron transfer and exposed electrocatalytic active sites is designing configuration. Hierarchical structures constructed by ultrathin two-dimensional nanosheets in-situ growth on heteroatoms doped rGO is in favour of exposing more active sites, and also shortening the distance of charge diffusion. Therefore, in-situ growth of ultrathin nanosheets on N, S doped rGO is a prospective way to promote oxygen evolution performance.

Herein, we integrated a new way of constructing Co9S8/N,S-rGO hierarchical structures through polyol refluxing, sulfurization and calcination process. This Co9S8/N,S-rGO hierarchical architectures which Co9S8 nanosheets with thickness of 3∼4 nm vertically and densely grew on N,S-rGO nanosheets were favoured for exposing surface actives sites and facilitating electron transport, which exhibited high-efficient oxygen evolution electrocatalytic performance and robust durability for electrolytic water in alkaline media.

Results and Discussion

Co9S8/N,S-rGO hierarchical structures were prepared via a two-step method as illustrated in Fig. 1. Typically, during the refluxing process, GO was reduced to rGO by ethylene glycol. Oxygenic groups (such as hydroxyl or carboxyl) in the GO can absorb and form bonds with Co2+ 39,40. The Co2+ on the surface of rGO reacted with hexamethylenetetramine under the alkaline condition, forming sandwiching Co(OH)2/rGO. The SEM and TEM images of Co(OH)2/rGO (Fig. S1), revealed that ultrathin Co(OH)2 nanoflakes were uniformly on the surface of two-dimensional rGO, demonstrating the formation of sandwiched structures. XRD patterns, Elemental mappings and Raman spectroscopy indicated that the successful growth of Co(OH)2/rGO sandwiched structure (Figs S2–S4). During the sulfuration procedure, TAA slowly released S2−ions, converting Co(OH)2/rGO to Co9S8/rGO41. To increase the crystallinity of Co9S8, Co9S8/rGO was further annealed at 350 °C for 2 h under Ar flow. After the annealing process, N, S atoms in-situ doped in rGO and Co9S8/N,S-rGO hierarchical structure formed.

Figure 1
figure 1

Schematic illustration of synthetic routes for Co9S8/N,S-rGO hierarchical structures.

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In Fig. 2a, presented the XRD pattern of Co9S8/N,S-rGO, in which diffraction peaks were readily indexed to cubic phase Co9S8 (JCPDS no. 65–1765). To further investigate the role of interaction between Co9S8 and N,S-rGO, N,S-rGO nanosheets (Fig. S5) and Co9S8 nanospheres (Fig. S6) were also synthesized via the same synthetic route with the exception that the graphene oxides or Co(Ac)2 precursors were not involved in the synthesis, respectively. The broad peak at 24° revealed the representative carbon peaks in N,S-rGO. For Co9S8 sphere, all the diffraction peaks were the same as Co9S8/N,S-rGO and corresponded to cubic Co9S8 (JCPDS no. 65–1765, Fig. 2a). In Fig. 2b, Raman peaks at 459, 503 and 662 cm−1 were assigned to Co9S8 and the observed D bands (1349 cm−1), G bands (1591 cm−1) were assigned to N,S-rGO, respectively33,42,43,44,45,46.

Figure 2
figure 2

(a) XRD patterns and (b) Raman spectra of Co9S8, N,S-rGO and Co9S8/N,S-rGO.

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The morphological information of Co9S8/N,S-rGO was gained from SEM images in Fig. 3a,b, where the sandwiched structure was maintained after sulfuration. In Fig. 3c, the ultrathin Co9S8 nanosheets aligned vertically and grew densely on the surface of N,S-rGO. TEM images displayed the internal structure of Co9S8/N,S-rGO in Fig. 3d and e, in which the surface of N,S-rGO was evenly and vertically aligned by ultrathin Co9S8 nanosheets with diameters of ∼3–4 nm. SAED pattern taken on Co9S8/N,S-rGO was displayed in Fig. 3f inset, in which all diffraction rings were corresponded to (311), (222), (511) and (440) planes of cubic Co9S8. These results illustrated a polycrystalline nature of Co9S8/N,S-rGO, which is in agreement with the XRD results in Fig. 2a. The element mapping revealed the co-existence and uniform dispersion of C, N, O, S and Co (Fig. S7).

Figure 3
figure 3

Structural characterization of Co9S8/N,S-rGO hierarchical structures obtained after sulfuration and calcination process. (a–c) Low and high magnification SEM images; (d) TEM images and (e,f) high magnification TEM images show the diameters of ultrathin Co9S8 nanosheets vertically aligned on the surface of N,S-rGO (inset is SAED pattern of cubic phase Co9S8).

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To analyse the chemical states of different elements in Co9S8/N,S-rGO, X-ray photoelectron spectrometry (XPS) measurement was performed. As shown in the full spectrum in Fig. 4a, C, O, N, S and Co were probed, inferring the co-existence of these elements. In Fig. 4b, the spectra of C 1 s were deconvoluted into four major functional groups, corresponding to the non-oxygenated C-C bond (284.8 eV), C-O bond (288.6 eV), C-S (285.4 eV) and C-N bond (286.4 eV) in rGO40. In Fig. 4c, four peaks in N 1 s spectrum were respectively assigned to pyridinic N (398.8 eV), Co-Nx (399.7 eV), pyrrolic N (400.5 eV) and graphitic N (401.3 eV), indicating that N was successfully doped into rGO29,47. The S 2p XPS spectra (Fig. 4d) were attributed to the typical S 2p3/2 (161.7 eV) and S 2p1/2 (162.6 eV) of cobalt-sulfide bonds. The covalent S-C bond peak was located at 163.8 eV48. The peak at 167.7 eV was corresponded to S-Ox, in which contributed to the partial oxidization of sulfur in ambient atmosphere48. The XPS survey spectra indicated the successful doping of N and S in rGO and surface atom contents was calculated to be 6.19 and 14.45 at%, respectively. The doublet peaks of Co 2p3/2 and 2p1/2 with binding energies centered at 778.0, 792.9 eV and 780.7, 796.7 eV were ascribed to Co3+ and Co2+ state (Fig. 4e)49,50. All the above spectra confirmed the formation of Co9S8/N,S-rGO. The specific surface area of the as-prepared Co9S8/N,S-rGO was obtained from Brunauer-Emmett-Teller (BET) measurement (Fig. 4f). The Co9S8/N,S-rGO afforded a high BET specific surface area of 77.2 m2·g−1. This highly open architecture with ultrathin Co9S8 nanosheets vertically aligned on N,S-rGO should be helpful for fully accessing electrolyte and exposing catalytic active sites during electrochemical reaction51,52. To further investigate the structure advantages in Co9S8/N,S-rGO, Co9S8 nps/N,S-rGO were prepared by same steps except the addition of hexamethylenetetramine, as shown in Figs S8, 9. In Co9S8 nps/N,S-rGO, cubic Co9S8 nanoparticles were uniformly distributed on the N,S-rGO.

Figure 4
figure 4

(a) High-resolution XPS survey spectra; (b) C1s; (c) N 1s; (d) S 2p; (e) Co 2p and (f) N2-adsorption-desorption isotherm and pore size distribution plots (insert) of Co9S8/N,S-rGO.

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The OER electrocatalytic activity of Co9S8/N,S-rGO was performed in alkaline electrolyte (1 M KOH) in a typical three electrode configuration. All data was corrected with iR compensation. In the experiment, RuO2 nanopowders were used as benchmark catalyst. OER performance of Co9S8, N,S-rGO and Co9S8 nps/N,S-rGO were also performed (Fig. 5). The overpotential to achieve a current density of 10 mA cm−2 is an important factor to evaluate the oxygen evolution activity of electrocatalysts. According to the linear sweep voltammograms (LSV) curves displayed in Fig. 5a, the overpotential at current density of 10 mA cm−2 for Co9S8/N,S-rGO was only 266 mV, which is much lower than that of Co9S8 (297 mV) and Co9S8 nps/N,S-rGO (292 mV), and closed to that of benchmarking RuO2 (232 mV). The bare N,S-rGO possessed an overpotential of more than 462 mV even at 2 mA cm−2, suggesting that in Co9S8/N,S-rGO, Co9S8 is the main contributor for OER electrocatalytic active sites. Moreover, Co9S8/N,S-rGO with ultrathin Co9S8 nanosheets vertical growth on N,S-rGO can provide large exposed catalytic active sites, probably accounting for higher activity in comparsion to Co9S8 nps/N,S-rGO. Such an overpotential is much lower than other cobalt compounds in alkaline media towards OER, such as hierarchical cobalt sulphide53, O-CoS2-MoS2 heteronanosheet54, CoO/N-Graphene55, etc. Tafel slopes were calculated to analyze the OER kinetics of all electrocatalysts. The high OER activity of Co9S8/N,S-rGO was confirmed by the small Tafel slope (75.5 mV dec−1) in Fig. 5b, which was much lower than those of Co9S8 nps/N,S-rGO (90 mV dec−1), Co9S8 (117 mV dec−1) and N,S-rGO (110 mV dec−1). The different Tafel slope values of electrocatalysts signified the different electrochemical oxidation pathways and rate-determining steps. The N,S-rGO with the largest Tafel slope value demonstrated the intrinsically inferior oxygen evolution property. Compared with Co9S8 and Co9S8 nps/N,S-rGO, Tafel slope of Co9S8/N,S-rGO showed distinct acceleration of oxygen evolution kinetics arising from the interaction between Co9S8 and N,S-rGO. The small Tafel slope values demonstrated that the high OER kinetics and low hydroxyl free radical absorption free energy of Co9S8/N,S-rGO during oxygen evolution. Figure 5c displayed the comparison of Tafel slope and potential data of current density at 10 mA cm−2. The potential of Co9S8/N,S-rGO was 1.496 V vs. RHE, smaller than Co9S8 nps/N,S-rGO, Co9S8 and N,S-rGO. To further explore the catalytic activities of Co9S8/N,S-rGO, the mass activity and specific activity were also calculated (Table 1). At overpotential of 0.27 V, the mass activity and specific activity of Co9S8/N,S-rGO were 27.03 A g−1 and 0.035 mA cm−2, respectively. These excellent performance mainly attributed to the hierarchical architecture with ultrathin Co9S8 nanosheets vertically grown on N,S-rGO which can increase the number of catalytic active sites by exposing large effective surface area and facilitate fast electron transport. The synergetic catalysis of ultrathin Co9S8 nanosheets and N,S-rGO greatly facilitated the intrinsic catalytic activity of Co9S8 and boosted the sluggish reaction kinetics of oxygen evolution56.

Figure 5
figure 5

(a) IR-corrected polarization curves; (b) Corresponding Tafel slopes; (c) Summary of Tafel slope and overpotential at current density of 10 mA cm−2 for Co9S8/N,S-rGO, Co9S8, N,S-rGO, Co9S8 nps/N,S-rGO and RuO2 in 1 M KOH and (d) Electrochemical impedance spectra with overpotential of 274.7 mV for Co9S8/N,S-rGO, Co9S8, and N,S-rGO.

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Table 1 OER activity of Co9S8/N,S-rGO, Co9S8 nps/N,S-rGO and Co9S8.
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EIS measurements were undertaken in the frequency range from 100 kHz to 0.01 Hz (Fig. 5d). The charge transfer resistance (Rct) was corresponded to the depressed semicircle in high-frequency region. The Rct of Co9S8/N,S-rGO was much lower than Co9S8 at overpotential of 274.7 mV. The Nyquist spectrum of N,S-rGO was composed by a semicircle at high frequencies and a linear part at low frequencies, revealing a lower charge transfer resistance and favorable of mass transport. These suggested that Co9S8/N,S-rGO electrode possess much faster charge transfer process and high electron transmission efficiency during the electrochemical reaction. The significantly decreased charge transfer resistance of Co9S8/N,S-rGO was attributed to the structures advantages and synergetic effect of ultrathin Co9S8 nanosheets vertical growth on N,S-rGO, which facilitate the charge and electron transfer process.

For comprehending the significant difference in OER catalytic performance between Co9S8 and N,S-rGO, electrochemical double layer capacitances (Cdl) was applied to calculate the electrochemical surface area (ECSA) of electrocatalysts (Figs S10, 11). The Cdl of Co9S8/N,S-rGO (90 mF cm−2) was much larger than that of Co9S8 (55 mF cm−2), Co9S8 nps/N,S-rGO (10.9 mF cm−2) and N,S-rGO, signifying Co9S8/N,S-rGO with more exposing active sites in electrochemical process. The large ECSA for Co9S8/N,S-rGO was owed to the advantages of unique hierarchical structure with ultrathin Co9S8 nanosheets vertical growth on N,S/rGO and large exposed catalytic active sites.

The rotating ring-disk electrode (RRDE) technique was used to confirm the products formed on electrocatalysts surface during the OER process. As shown in Fig. 6, when the potential of Pt ring electrode is 1.20 V, a microamp-scale oxidation current is recorded (red line), implying negligible production of hydrogen peroxide in the system. Instead, an apparent oxygen reduction reaction (ORR) current was obtained when the potential on Pt ring was at 0.60 V, which verified that the final products were O2 during OER process.

Figure 6
figure 6

RRDE technique detection of OER products with Pt ring at different potentials for Co9S8/N,S-rGO in oxygen evolution process.

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Electrochemical durability of catalysts is important for practical water splitting applications. Hence, the durability for Co9S8/N,S-rGO was further evaluated. As shown in Fig. 7a, a continuous potential cycling test (between 1.0–1.5 V vs. RHE) showed a negligible shift in the polarization curves after 1000 cycles, depicting high durability of Co9S8/N,S-rGO in long term OER under alkaline conditions. The stability of Co9S8/N,S-rGO was investigated by the chronopotentiometry measurements. As depicted in Fig. 7b, the overpotential at current density of 10 mA cm−2 was no obvious decay after running for 20 h, indicating the excellent and stable oxygen evolution ability of Co9S8/N,S-rGO in long term OER under strong alkaline condition. At higher overpotential of 350 mV, the current density remains stable at ~30 mA cm−2 for oxygen evolution over 20 h.

Figure 7
figure 7

(a) Polarization curves of Co9S8/N,S-rGO before and after 1000 cycles stability test (between 1.0–1.5 V vs. RHE); (b) Chronopotentiometry curves of Co9S8/N,S-rGO and commercial RuO2 at a constant current density of 10 and 30 mA cm−2.

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Excitingly, besides the high OER electrocatalytic activity, Co9S8/N,S-rGO also revealed superior catalytic activity towards HER in 1.0 M KOH. As demonstrated in Fig. 8a, a current density of 10 mA cm−2 can be achieved at a small overpotential of 332.4 mV, which was much lower than other materials. In comparison with bare N,S-rGO, Co9S8 (368.2 mV) and Co9S8 nps/N,S-rGO (334.2 mV), distinctly enhanced activity is observed on Co9S8/N,S-rGO. The Tafel slope of Co9S8/N,S-rGO was 131.4 mV dec−1, hence the Heyrovsky reaction (Hads + H+  + e− → H2) is the rate determining step (Fig. 8b)57. The N,S-rGO with the highest Tafel slope value demonstrated the intrinsically poor hydrogen evolution property. The Tafel slope value of Co9S8 nps/N,S-rGO may be restricted by the low exposed electrochemical active surface area. In addition, the almost overlapping LSV curves in Fig. 8c indicated that Co9S8/N,S-rGO can retain its electrocatalytic activity after 1000 cycles. These results were further verified by the chronoamperometry test that the potential has no obvious increase after 10 h (Fig. 8d), implying the high durability of commercial Pt/C and Co9S8/N,S-rGO towards HER. By contrast, Co9S8/N,S-rGO basically retained the initial potential towards HER even after 10 h.

Figure 8
figure 8

(a) Polarization curves; (b) Corresponding Tafel slopes of Co9S8/N,S-rGO, Co9S8, N,S-rGO and Co9S8 nps/N,S-rGO for HER in 1 M KOH; (c) Polarization curves before and after 1000 cycles stability test of Co9S8/N,S-rGO; (d) Chronopotentiometry curves of Co9S8/N,S-rGO and commercial Pt/C with constant current density of 10 mA cm−2 in 1 M KOH.

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Conclusions

In summary, Co9S8/N,S-rGO was fabricated by a facile and cost-effective way as promising substitutes for noble metal OER electrocatalysts. The unique Co9S8/N,S-rGO hierarchical structure with ultrathin Co9S8 nanosheets vertically grown on the surface of N and S co-doped reduced graphene oxide has superiority of large exposed electrocatalytic active sites and efficient electron transfer efficiency towards enhancing OER kinetics. This work demonstrates that Co9S8/N,S-rGO has highly efficient and durable electrocatalytic performance for stable oxygen evolution and moderates hydrogen evolution activity in alkaline condition. This can provide scientific guidance for future research work and understanding the role of electrocatalysts in the electrochemical process.

Methods

Materials

Graphene oxide nanosheets (Nanjing XFNANO Materials Tech), Nafion (Sigma-Aldrich), Cobalt acetate, hexamethylenetetramine (C6H12N4), thioacetamide (TAA) and all others reagents were purchased from Sinopharm.

Materials synthesis

Synthesis of Co(OH)2/rGO sandwiched structures: 10 mg of graphene oxide, 1 mmol of Co(Ac)2 · 4H2O and 1 mmol of hexamethylenetetramine were dispersed in 30 mL ethylene glycol and sonicated for 2 h, respectively. The as-formed supensions were added into a flask and the mixture was stirred at 180 °C for 1 h. After centrifugation and lyophilisation, Co(OH)2/rGO sandwiched structures were obtained.

Synthesis of Co9S8/N,S-rGO hierarchical structures: Co(OH)2/rGO sandwiched structures and 1 mmol of thioacetamide were added into 30 mL ethyl alcohol absolute solutions. The mixture was transferred into a Teflon-lined stainless-steel autoclave and heated up at 120 °C for 12 h. The product was washed and centrifugated, followed by carbonization at 350 °C for 2 h under Ar atmosphere.

The fabrications of Co9S8, N,S-rGO or Co9S8 nps/N,S-rGO were similar with that of Co9S8/N,S-rGO but without adding graphene oxide, cobalt acetate or hexamethylenetetramine, respectively.

Materials characterization

The morphology and crystal structure characterization were performed on a field emission scanning electron microscope (FE-SEM, FEI Quanta 200F), Transmission electron microscopy (TEM, JEOL JEM-2100) and X-Ray Diffraction (XRD, Rigaku D/Max-γB diffractometer). The surface chemical compositions were analysed by X-ray photoelectron spectroscopy (XPS, VG Kα Probe, Thermo Fisher Scientific). Nitrogen adsorption–desorption measurements were conducted by using a Micromeritics ASAP 2020 system and calculated using BET method.

Electrochemical measurements

Electrochemical performance of catalysts was evaluated on Wavedrive 20 potentiostat (PINE). A three-electrode configuration was constituted with reference electrode (Ag/AgCl), working electrode (rotating disk electrode (RDE)) and counter electrode (Pt wire). 21 μL of catalyst ink were casted onto glassy carbon (GC) electrode as the working electrode. The overpotentials (η), Tafel slope, mass activity (A g−1) and specific activity (mA m−2) were calculated on the basis of the following formula:

$$\eta ={E}_{{\rm{Ag}}/{\rm{AgC}}l}+0.196{\rm{V}}+0.059{\rm{pH}}-1.23{\rm{V}}$$
(1)
$$\eta =b\,\mathrm{log}\,j+a$$
(2)
$${\rm{Mass}}\,{\rm{activity}}=J/m$$
(3)
$${\rm{Specific}}\,{\rm{activity}}=J/(10{S}_{{\rm{BET}}}m)$$
(4)

where j is the current density (mA cm−2); b is the Tafel slope (mV dec−1); J (mA cm−2) denotes the current density at η = 0.27 V; m denotes the catalyst loading (0.37 mg cm−2); SBET (m2 g−1) is the BET surface area. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 0.01 Hz to 100 kHz with constant potential of 1.5 V vs. RHE.