Pyrite-Type CoS2 Nanoparticles Supported on Nitrogen-Doped Graphene for Enhanced Water Splitting

It is extremely meaningful to develop cheap, highly efficient, and stable bifunctional electrocatalysts for both hydrogen and oxygen evolution reactions (HER and OER) to promote large-scale application of water splitting technology. Herein, we reported the preparation of CoS₂ nanoparticles supported on nitrogen-doped graphene (CoS₂@N-GN) by one-step hydrothermal method and the enhanced electrochemical efficacy for catalyzing hydrogen and oxygen in water electrolysis. The CoS₂@N-GN composites are composed of nitrogen-doped graphene and CoS₂ nanocrystals with the average size of 73.5 nm. Benefitting from the improved electronic transfer and synergistic effect, the as-prepared CoS₂@N-GN exhibits remarkable OER and HER performance in 1.0 M KOH, with overpotentials of 243 mV for OER and 204 mV for HER at 10 mA cm⁻², and the corresponding Tafel slopes of 51.8 and 108 mV dec⁻¹, respectively. Otherwise, the CoS₂@N-GN hybrid also presents superior long-term catalytic durability. Moreover, an alkaline water splitting device assembled by CoS₂@N-GN as both anode and cathode can achieve a low cell voltage of 1.53 V at 60 °C with a high faraday efficiency of 100% for overall water splitting. The tremendously enhanced electrochemical behaviors arise from favorable factors including small sized, homogenously dispersed novel CoS₂ nanocrystals and coupling interaction with the underlying conductive nitrogen-doped graphene, which would provide insight into the rational design of transition metal chalcogenides for highly efficient and durable hydrogen and oxygen-involved electrocatalysis.

Introduction

The large demand of clean and sustainable energy stimulated intensive research on the development of efficient and robust electrochemical energy conversion systems such as water splitting that can produce hydrogen and oxygen (Wang J. et al., 2016; Jia et al., 2017; Li H. et al., 2017; Zheng et al., 2017). It consists of two half reactions: the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) (Dong et al., 2015; Menezes et al., 2015; Sun et al., 2017; Han et al., 2018). However, slow reaction dynamics and high overpotential limit the wide application of water electrolyzer (Cheng et al., 2017a; Ma et al., 2017; Huang et al., 2018). High-performance electrocatalysts are urgently needed to reduce the overpotential and improve the energy efficiency (Fang et al., 2017; Yin et al., 2017; Shit et al., 2018). Currently, Pt-based materials and noble metal oxides (RuO₂, IrO₂) are the best catalysts for HER and OER (Han et al., 2014; Lee et al., 2015; Wang et al., 2015), respectively. However, high price and limited reserves restrict the wide application of these noble metal-based materials (Cheng et al., 2017b; Su et al., 2017). Moreover, these catalysts are still faced the problems of inferior long-term stability and unsatisfied bifunctional activity. Therefore, it is of great significance to design and develop high-abundant, high active and stable precious metal-free electrocatalysts (Forgie et al., 2010; Mccrory et al., 2015).

Recently, transition metal sulfides, especially cobalt sulfide, have attracted much research attention because of their earth-abundance, environmentally green, and significant chemical/electrochemical stability (Wang et al., 2009; Zheng et al., 2015; Han et al., 2017; Liu et al., 2017; Wu et al., 2017). However, the limited active sites and intrinsically low conductivity of traditional cobalt sulfide materials hinder their activity enhancement for catalyzing HER and OER (Liu et al., 2013). Therefore, the combination of metal sulfides and conductive materials is supposed to increase the electrical conductivity and meanwhile synergistically promoting the catalytic activity (Zhou et al., 2019). For example, Zn–Co-mixed sulfide nanostructure on carbon fiber paper has been synthesized for efficient rechargeable zinc–air batteries and water electrolysis (Wu et al., 2017); Co₉S₈ nanoparticles were anchored on nitrogen and sulfur dual-doped carbon nanosheets for bifunctional oxygen evolution and reduction reactions (Gulzar et al., 2017), CoS_x/Ni₃S₂ heterostructure supported on nickel foam was developed as an efficient catalytic electrode for accelerating HER and OER (Wang et al., 2018). Previous investigations demonstrated that, among various cobalt sulfides (i.e., Co₃S₄, Co₉S₈, CoS₂, etc.), the pyrite-type CoS₂ exhibited the superior intrinsic performance owning to the abundant cobalt active sites and more proton-acceptor centers as a results of the unique crystal structure and S-rich nature (Kumar et al., 2018). Accordingly, it appears to be a smart strategy to design CoS₂ nanostructure/conductive supported hybrid material to further optimize the HER and OER capability to replace the precious metal-based catalysts, thereby promoting the large-scale implementation of overall water splitting technologies (Wang J. et al., 2016; Xia et al., 2017).

In this work, we prepared the composite material of CoS₂ uniformly supported on nitrogen-doped graphene (CoS₂@N-GN) by one-step hydrothermal method and its application as bifunctional electrocatalytic catalyst for OER/HER and overall water-splitting in alkaline media. The average size of CoS₂ is about 73.5 nm, homogeneously dispersed on the N-doped reduced graphene oxide nanosheet. Electrochemical tests reveal that the CoS₂@N-GN hybrid exhibits superior catalytic performance than bare CoS₂ and physically mixture sample (CoS₂/N-GN), delivering low overpotentials of 243 and 204 mV at current density of 10 mA cm⁻² for OER and HER, respectively. The in-situ grown composite electrode also presents remarkable long-term catalytic durability with negligible activity decay after 12 h period. Furthermore, the alkaline electrolyzer using CoS₂@N-GN loaded on carbon fiber paper as both anode and cathode electrodes can achieve 10 mA cm⁻² at a low cell voltage of 1.53 V at 60°C with a faradaic efficiency of 100% for overall water splitting. Our research will provide insights on the development of low cost and efficient hybrid electrocatalysts for next-generation energy conversion devices.

Experimental Section

Regents and Materials

Nitrogen doped graphene was purchased from Aladdin. Cobalt (II) acetate tetrahydrate (Co(AC)₂·4H₂O, 99.5%) and Potassium hydroxide (KOH, 99.99%) were purchased from Beijing Chemicals (Beijing, China). Carbon disulfide (CS₂, 99%) and ethylenediamine (EN, 99%) were obtained from Tianjin Yuanli Chemical Co. Ltd. (Tianjin, China). The carbon paper (CP) was purchased from Phychemi Company Limited and used as the substrate of active substance. The deionized water (18.2 MΩ·cm⁻¹) was obtained via Millipore, an ultrapure water system. High purity nitrogen (Air Product, purity 99.995%) gas was used to deaerate the 1 M KOH solution. All the reagents were of analytical grade and used as received without further purification.

Materials Synthesis

The CoS₂@N-GN hybrid was synthesized by one step hydrothermal method. Typically, 30 mg of nitrogen doped graphene was put into a beaker with 100 mL and then 35 mL of deionized water was added followed by 30 min of ultrasonic agitation. Then, 0.4 mmol of Co(CH₃COO)₂·4H₂O was added into the above mixed solution, followed by magnetic stirring under 800 rounds per minute (rpm) for 10 min. Then 0.16 mL of EN was added into the solution with magnetic stirring for 20 min. Afterwards, 0.16 mL of CS₂ was dropped into the solution to form a uniform mixture after magnetic stirring for another 20 min. The above aqueous solution was then transferred into a 50 mL Teflon-lined stainless autoclave and then maintained at 200°C for 9 h. After cooling naturally to room temperature, the product was separated by centrifugal force, washed by distilled water for three times, and then lyophilized. The preparation of pure CoS₂ was the same as the above route without adding 30 mg nitrogen doped graphene. The synthesis of physical mixture of CoS₂ and N-doped graphene was achieved by mechanically mixed two components at mass ratio of 1:1.

Materials Characterization

The phase purity of the as-prepared samples was characterized by X-ray diffraction (XRD, Bruker/D8 Advanced with Cu Kα radiation) at a scanning rate of 2° min⁻¹. The morphological micro-nanostructures were characterized by field-emission scanning electron microscopy (SEM, Hitachi S4800, 30 kV) and transmission electron microscopy (TEM, JEOL JEM-2100F, 200 kV) equipped with an energy-dispersive spectrometer (EDS). The Brunauer-Emmett-Teller (BET) specific surface area was determined by N₂ adsorption/desorption isotherms at 77 K using the AutosorbiQ instrument (Quantachrome U.S.) with a 6 h outgas at 80°C. X-ray photoelectron spectroscopy (XPS) was conducted by using a Perkin Elmer PHI 1600 ECSA system.

Electrocatalytic Measurements

The electrocatalytic properties of the synthesized catalysts were tested on an IviumStat workstation using a three-electrode configuration. All the electrochemical data was performed in 1.0 M KOH electrolyte, which was saturated with high-purity N₂ (Air Product, purity 99.995 %) for OER and HER for at least 30 min before each test and maintained under the corresponding atmosphere during the whole experiment. In addition, a saturated calomel electrode (SCE), and a platinum foil electrode were employed as reference electrode, and counter electrode. The linear sweeping voltammetry (LSV) of ORR and OER were scanning at a same scan rate of 5 mV s⁻¹. The HER and OER potentials were corrected to the reversible hydrogen electrode (RHE) on the basis of following equation:

$\begin{array}{rc}E(vs.RHE)=E(vs.SCE)+0.059\times pH+0.241V& (1)\end{array}$

The electrochemical data of HER and OER was measured from −0.8 to −1.6 V vs. SCE, and 0.2 to 1.0 V vs. SCE, respectively. Electrochemical impedance spectra was carried out in a frequency range of from 100 to100 mHz at a potential of 0.6 V vs. RHE. The obtained HER and OER linear sweeping voltammetry (LSV) data was treated with iR-compensation according to the equation: E_c = E_m - I_m × R_s, where E_c, E_m, I_m, and R_s stand for compensated voltage, measured voltage, measured voltage and electrolyte resistance, respectively. The mass loading of synthesized electrocatalysts was about 1.5 mg cm⁻². The RuO₂ and Pt/C electrodes were also prepared for comparison with the same mass loading.

Results and Discussion

The synthetic procedure of the composite materials follows a facile, one-step, and hydrothermal method. As shown in Figure 1a, the power X-ray diffraction (XRD) pattern of CoS₂@N-GN can be assigned to cubic CoS₂ (JCPDS no. 41-1471), suggesting the successful formation of pyrite CoS₂ phase in the hybrid. The morphological structures were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM) techniques. SEM image in Figure 1b revealed that CoS₂ nanoparticles were uniformly anchored on nitrogen-doped graphene nanosheets. The average crystallize size is around 73.5 nm (Figure S1), which coincides with the Scherrer analysis based on the XRD pattern. The homogeneous distribution of CoS₂ nanocrystals on graphene is further confirmed by the TEM imaging (Figure 1c). Moreover, in the high-resolution TEM (HRTEM) of inset in Figure 1c, an observed lattice spacing of 0.28 nm matches the (200) lattice plane of CoS₂ (Ma et al., 2018), further proving the XRD analysis. Otherwise, the elemental mapping reveal the homogeneous dispersion of Co, S, C and N in the composite (Figure 1d), indicating the uniform distribution of CoS₂ nanoparticles on the N-doping graphene substance. These observations collectively demonstrate the successful synthesis of CoS₂@N-GN hybrid through stepwise controlled strategies. The Brunner–Emmet–Teller (BET) surface area of CoS₂@N-GN hybrid was characterized to be 79.8 m² g⁻¹ (Figure S2). Thus, the CoS₂@N-GN shows a significantly high BET area, which can provide more electroactive sites and facilitate electrolyte infiltration during the catalytic process (Li et al., 2018). The content of CoS₂ in the hybrid is determined to be around 51 wt% (Figure S3) by thermal gravimetric analysis (TGA) (Liang et al., 2012).

FIGURE 1

Figure 1. (a) XRD, (b) SEM, (c) TEM image, and (d) elemental mapping of CoS₂@N-GN hybrid. Inset in (a) shows the crystal structure of pyrite-type CoS₂. Inset in (c) shows the HRTEM image.

The chemical valence states of N, Co, O and S in CoS₂@N-GN were characterized by X-ray photoelectron spectroscopy (XPS, Figure 2). The XPS survey spectrum in Figure 2A shows the existence of C, N, Co, S, as well as O. The determined stoichiometric ratio is consistent with the result from energy-dispersive spectroscopy (Figure S4). The existence of O is due to exposure to air (Zhang et al., 2017). Additionally, the high-resolution N 1s spectrum (Figure 2B) can be deconvoluted into three sub-peaks located at 400.30 eV (pyridinic N), 402.20 eV (pyrrolic N), and 403.38 eV (graphitic N) (Fu et al., 2016; Li G. et al., 2016; Wang Z. et al., 2016). It is well known that doped N into carbon framework can potentially increase the active sites and thereby benefit the electrochemical catalytic activity enhancement (Ma et al., 2014; Li et al., 2018). The Co 2p spectra (Figure 2C) can be de-convoluted into six species, including pairs of fitting peaks for Co²⁺ and Co³⁺, and their shakeup satellites (Xiao et al., 2013; Ma et al., 2016; Zhu et al., 2016). The Co 2p_3/2 peaks at 778.9 and 782.4 eV can be assigned to Co atoms in CoS₂@N-GN and surface oxidized cobalt species coordinated with oxide or hydroxyl groups, respectively (Li C. et al., 2017). The main peak centered at 798.9 eV corresponds to spin–orbit characteristic peak of Co 2p_1/2 in CoS₂ compounds. For the high-resolution S 2p spectrum (Figure 2D), a weak doublet situated at 163.3 eV corresponds to S 2p. The peak located at 163.7 eV is assigned to the S 2p_1/2 of S²⁻ ions that matched with metal ions (Du et al., 2015). The peak at 169.7 eV indicates the presence of a oxygen-sulfur (O-S) bond in the CoS₂@N-GN compound, which may be ascribed to the surface oxidation, as confirmed by previous observations (Sivanantham et al., 2016).

FIGURE 2

Figure 2. (A) XPS survey, (B) N 1s, (c) Co 2p, and (D) S 2p XPS spectra of the CoS₂@N-GN hybrid.

The electrocatalytic OER performance of synthesized CoS₂@N-GN, together with CoS₂/N-GN (the mechanical mixtures of pure CoS₂ and nitrogen-doped graphene) and pure CoS₂, was assessed by a three-electrode configuration in 1.0 M KOH. As shown in the polarization curves (Figure 3A and Table 1), CoS₂@N-GN hybrid displays an overpotential of 204 mV to achieve an OER current density of 10 mA cm⁻², which is much lower than those of CoS₂/N-GN (278 mV) and pure CoS₂ (297 mV), highlighting the superior OER activity of in-situ fabricated CoS₂@N-GN. The fitted Tafel value of the CoS₂@N-GN catalyst is 51.8 mV dec⁻¹, which is lower than those of CoS₂/N-GN (71.6 mV dec⁻¹) and pure CoS₂ (81.4 mV dec⁻¹) (Figure 3B), indicating more favorable OER kinetics over the CoS₂@N-GN surface. Otherwise, the corresponding charge transfer resistance (R_ct) values of CoS₂@N-GN, CoS₂/N-GN and pure CoS₂ are fitted to be 1.85, 3.2, 4.02 Ω, respectively (Figure 3C), suggesting that the efficient charge transfer contributes the superior activity of CoS₂@N-GN electrode (Li P. et al., 2016). The presented OER performance parameters here are among the non-noble metal-based OER electrocatalysts reported in literatures (Table S1). In addition to good catalytic activity, long-term stability is also a critical factor in evaluating its practical performance. As we can see, the OER polarization curve of CoS₂@N-GN after 1000 OER cycles at a scanning rate of 200 mV s⁻¹ almost overlays the initial one (Figure 3D), evidencing that the activity is considerably maintained after the long-term continuous cycles. This is also corroborated by the chronopotentiometric response (Figure 3E), in which that the overpotential does not increase after 12 h period at an anodic current density of 10 mA cm⁻². The SEM image of CoS₂@N-GN further shows that the morphology is substantially unchanged after the 500 and 1000 OER cycles (Figure S5 and Figure 3f), signaling the remarkable structure durability of the composite catalyst. This may be mainly ascribed to the intrinsic crystal stability of CoS₂ and firm attachment between the in situ grown sulfide nanostructures and the graphene support. The possible reaction mechanism of Co-based materials for catalyzing OER in alkaline media follows the sequence (Chen et al., 2015; Ma et al., 2018):

$\begin{array}{rc}\text{C}\text{o}+2\text{O}{\text{H}}^{-}\to \text{C}\text{o}{(\text{O}\text{H})}_2+2{\text{e}}^{-}& (2)\end{array}$

$\begin{array}{rc}3\text{C}\text{o}{(\text{O}\text{H})}_2+2\text{O}{\text{H}}^{-}\to \text{C}{\text{o}}_3{\text{O}}_4+4{\text{H}}_2\text{O}& (3)\end{array}$

$\begin{array}{rc}\text{C}{\text{o}}_3{\text{O}}_4+{\text{H}}_2\text{O}+\text{O}{\text{H}}^{-}\to 3\text{C}\text{o}\text{O}\text{O}\text{H}+{\text{e}}^{-}& (4)\end{array}$

$\begin{array}{rc}\text{C}\text{o}\text{O}\text{O}\text{H}+\text{O}{\text{H}}^{-}\to \text{C}\text{o}{\text{O}}_2+{\text{H}}_2\text{O}+{\text{e}}^{-}& (5)\end{array}$

$\begin{array}{rc}\text{O}\text{v}\text{e}\text{r}\text{a}\text{l}\text{l}:4\text{O}{\text{H}}^{-}\to 2{\text{H}}_2\text{O}+{\text{O}}_2+4{\text{e}}^{-}& (6)\end{array}$

As shown in Figure S6a, after 1000 OER cycles, it can be clearly seen that the proportion of Co³⁺ is significantly increased. Meanwhile, the proportion of S-O is also significantly increased (Figure S6b), suggesting the considerable oxidation, consistent with previous observations (Han et al., 2018; Ma et al., 2018).

FIGURE 3

Figure 3. (A) OER LSVs of CoS₂@N-GN, CoS₂/N-GN, pure CoS₂ in 1.0 M KOH at a scanning rate of 5 mV s⁻¹. (B) Corresponding Tafel curves. (C) Nyquist plots of EIS at the potential of 1.56 V. (D) Polarization curves of CoS₂@N-GN before and after 1000 cycles. (E) Chronopotentiometric response of CoS₂@N-GN at a constant current density of 10 mA cm⁻². (F) SEM image of the CoS₂@N-GN catalyst after OER for 1000 cycles.

TABLE 1

Table 1. Summary of the electrochemical activities of CoS₂@N-GN, CoS₂/N-GN and pure CoS₂ electrodes.

In the context of developing bifunctional catalysts for overall water oxidation, the HER performance of synthesized samples was tested in the same electrolyte. All the linear sweeping voltammetry (LSVs) measurements were collected at a scan rate of 5 mV s⁻¹ while the electrolyte is saturated with N₂ during the experiment. From the HER LSVs in Figure 4A, it is clear that the pure CoS₂ shows weak electrocatalytic HER activity. The in-situ fabricated CoS₂@N-GN achieves a current density of 10 mA cm⁻² at an overpotential of 204 mV, which is much lower than that of physical mixture of CoS₂/N-GN (278 mV) and pure CoS₂ (297 mV).

FIGURE 4

Figure 4. (A) HER LSV curves of CoS₂@N-GN, CoS₂/N-GN, pure CoS₂ in 1.0 M KOH at a scanning rate of 5 mV s⁻¹. (B) Corresponding Tafel slopes. (C) HER polarization curves of CoS₂@N-GN before and after 1,000 cycles. (D) Chronopotentiometric response of CoS₂@N-GN at a constant current density of 10 mA cm⁻².

The corresponding Tafel slopes of N-GP@CoS₂, N-GP/CoS₂, and pure CoS₂ are 108.5, 139.1, 144.7 mV dec⁻¹, respectively, which is comparable to those of transition metal sulfide catalysts reported recently (Figure 4B) (Liang et al., 2016; Sivanantham et al., 2016). In particular, N-GP@CoS₂ shows the lowest Tafel slope of 108.5 mV dec⁻¹ among three synthesized catalysts. The disparity in Tafel slopes further corroborates the advantage of the synergistic effect between N-GN and CoS₂. The double-layer capacitance (C_dl) values are determined to be 76.7, 20.1 and 13.6 mF cm⁻² for CoS₂@N-GN, CoS₂/N-GN, and pure CoS₂ (Figure S7), demonstrated that the CoS₂@N-GN could provide larger electrochemical active area for electrolyte soakage and water adsorption, which is another favorable factor in enhancing the electrocatalytic capability. The HER typically occurs through two reactions in alkaline solution (Li H. et al., 2017; Ma et al., 2018) Volmer reaction (H₂O + e⁻ + M → M-H${}_{\text{ad}}^{*}$ + OH⁻, where H${}_{\text{ad}}^{*}$ is a reactive intermediate and M is the catalytically active site) and Heyrovsky reaction (H₂O + M-H${}_{\text{ad}}^{*}$ + e⁻ → M + H₂ + OH⁻). The key HER performance parameters presented here are among the best non-noble metal HER electrocatalysts reported in literatures (Table S2). Moreover, after 1000 CV cycles, the polarization curve of CoS₂@N-GN is remarkably maintained in comparison with the initial state (Figure 4C). The morphology of CoS₂@N-GN electrode was still essentially preserved (Figure S8), further confirming the structural durability, which is mainly due to the intrinsic stability of CoS₂ and the strong chemical interaction between two components. In a continuous polarization period of 12 h (Figure 4D), the HER current retention of CoS₂@N-GN again confirms its remarkable long-term stability for catalyzing the HER in alkaline media.

As shown in Figure 5A, to further investigate its practical application for overall water-splitting, CoS₂@N-GN electrode was applied as both anode and cathode to assemble an electrochemical device, which presents low cell voltages of 1.70 V at 25°C and 1.53 V at 60°C to achieve 10 mA cm⁻² in 1 M KOH. The generated oxygen and hydrogen bubbles on both anode and cathode surface can be obviously seen at 10 mA cm⁻² (Figure 5B). In addition, the stability of the electrolyzer device was tested with sustained polarization for 12 h at 1.90 V. The activity degradation of CoS₂@N-GN (8.1%) is even lower than that of the combined catalyst of precious Pt/C + RuO₂ (17.6%, Figure 5C), further demonstrating the considerable durability of the CoS₂@N-GN electrode in a practical water splitting device. As displayed in Figure 5D, the amounts of collected H₂ and O₂ match well with the calculated values at a constant current density of 10 mA cm⁻², which shows advanced concept of efficient energy conversion from electric energy to chemical fuel gas. The relationship between time and gas quantity is calculated according to the following formula:

$\begin{array}{ll}\frac{I*\mathrm{\text{t}}}{4*e}=\frac{V*N_A}{22.4}& (7)\end{array}$

I, t, e, V and N_A are current (mA), time (s), 1.6 × 10⁻¹⁹ C, volume (L) and avogadro constant (6.02 × 10²³).

FIGURE 5

Figure 5. (A) Overall water splitting performance of N-GP@CoS₂ at different temperatures. (B) The photograph of gas evolution at 10 mA cm⁻². (C) Chronoamperometric response of two overall water splitting systems at 1.9 V. (D) The volumes of H₂ and O₂ experimentally measured and theoretically calculated vs. time at 10 mA cm⁻².

Conclusion

In summary, a novel CoS₂@N-GN hybrid was successfully prepared through a facile and one-step hydrothermal strategy. Compared with CoS₂/N-GN and pure CoS₂ catalyst, CoS₂@N-GN hybrid exhibits remarkable overall electrocatalytic activity toward OER and HER in alkaline electrolyte as well as the enhanced long-term stability. Moreover, the CoS₂@N-GN enables the assembled water splitting device with low cell voltage, high efficiency and prolonged operational life. The remarkable electrochemical properties are attributed to the high intrinsic activity of CoS₂, efficient electron transfer provided by N-doped graphene and the synergetic coupling interaction between two components. This work establishes the low cost and earth-abundant metal sulfide-based composite catalyst as a promising and high performance functional electrode for promoting large-scale water electrolyzer technologies.

Author Contributions

WZ conducted the experiments and helped write the manuscript. XM helped with operating the experiments and data analysis. CZ and YD interpreted the results. XH, TM, and WH supervised the research. All authors read and approved the final manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (51602216, U1601216), Tianjin Natural Science Foundation (17JCQNJC02100), and Shen-zhen Science and Technology Foundation (JCYJ20170307145703486).

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2018.00569/full#supplementary-material

References

Chen, P. Z., Xu, K., Fang, Z. W., Tong, Y., Wu, J. C., Lu, X. L., et al. (2015). Metallic Co₄N porous nanowire arrays activated by surface oxidation as electrocatalysts for the oxygen evolution reaction. Angew. Chem. Int. Ed. 54,14710–14714. doi: 10.1002/anie.201506480

PubMed Abstract | CrossRef Full Text | Google Scholar

Cheng, H., Li, M. L., Su, C. Y., Li, N., and Liu, Z. Q. (2017a). Cu-Co bimetallic oxide quantum dot decorated nitrogen-doped carbon nanotubes: a high-efficiency bifunctional oxygen electrode for zn-air batteries. Adv. Funct. Mater. 27:1701833. doi: 10.1002/adfm.201701833

CrossRef Full Text | Google Scholar

Cheng, H., Su, C.-Y., Tan, Z.-Y., Tai, S.-Z., and Liu, Z.-Q. (2017b). Interacting ZnCo₂O₄ and Au nanodots on carbon nanotubes as highly efficient water oxidation electrocatalyst. J. Power Sources 357, 1–10. doi: 10.1016/j.jpowsour.2017.04.091

CrossRef Full Text | Google Scholar

Dong, U. L., Park, M. G., Park, H. W., Min, H. S., Ismayilov, V., Ahmed, R., et al. (2015). Highly active Co-doped LaMnO₃ perovskite oxide and N-doped carbon nanotube hybrid bi-functional catalyst for rechargeable zinc–air batteries. Electrochem. Commun. 60, 38–41. doi: 10.1016/j.elecom.2015.08.001

CrossRef Full Text | Google Scholar

Du, Y., Zhu, X., Zhou, X., Hu, L., Dai, Z., and Bao, J. (2015). Co₃S₄ porous nanosheets embedded in graphene sheets as high-performance anode materials for lithium and sodium storage. J. Mater. Chem. A 3, 6787–6791. doi: 10.1039/c5ta00621j

CrossRef Full Text | Google Scholar

Fang, L., Li, W., Guan, Y., Feng, Y., Zhang, H., Wang, S., et al. (2017). Tuning Unique Peapod Like Co(S_xSe_1−−x)₂ nanoparticles for efficient overall water splitting. Adv. Funct. Mater. 27:1701008. doi: 10.1002/adfm.201701008

CrossRef Full Text | Google Scholar

Forgie, R., Bugosh, G., Neyerlin, K. C., Liu, Z., and Strasser, P. (2010). Bimetallic Ru electrocatalysts for the OER and electrolytic water splitting in acidic media. Electrochem. Solid ST 13, 36–39. doi: 10.1149/1.3290735

CrossRef Full Text | Google Scholar

Fu, J., Hassan, F. M., Li, J., Lee, D. U., Ghannoum, A. R., Lui, G., et al. (2016). Flexible rechargeable zinc-air batteries through morphological emulation of human hair array. Adv. Mater. 28, 6421–6428. doi: 10.1002/adma.201600762

CrossRef Full Text | Google Scholar

Gulzar, A., Xu, J., Yang, P., He, F., and Xu, L. (2017). Upconversion processes: versatile biological applications and biosafety. Nanoscale 9, 12248–12282. doi: 10.1039/c7nr01836c

PubMed Abstract | CrossRef Full Text | Google Scholar

Han, X., Cheng, F., Zhang, T., Yang, J., Hu, Y., and Chen, J. (2014). Hydrogenated uniform Pt Clusters supported on porous CaMnO₃ as a bifunctional electrocatalyst for enhanced oxygen reduction and evolution. Adv. Mater. 26, 2047–2051. doi: 10.1002/adma.201304867

CrossRef Full Text | Google Scholar

Han, X., Wu, X., Deng, Y., Liu, J., Lu, J., Zhong, C., et al. (2018). Ultrafine Pt nanoparticle-decorated pyrite-Type CoS₂ nanosheet arrays coated on carbon cloth as a bifunctional electrode for overall water splitting. Adv. Energy Mater. 8:1800935. doi: 10.1002/aenm.201800935

CrossRef Full Text | Google Scholar

Han, X., Wu, X., Zhong, C., Deng, Y., Zhao, N., and Hu, W. (2017). NiCo₂S₄ nanocrystals anchored on nitrogen-doped carbon nanotubes as a highly efficient bifunctional electrocatalyst for rechargeable zinc-air batteries. Nano Energy 31, 541–550. doi: 10.1016/j.nanoen.2016.12.008

CrossRef Full Text | Google Scholar

Huang, C., Ouyang, T., Zou, Y., Li, N., and Liu, Z.-Q. (2018). Ultrathin NiCo₂P_x nanosheets strongly coupled with CNTs as efficient and robust electrocatalysts for overall water splitting. J. Mater. Chem. A. 6, 7420–7427. doi: 10.1039/C7TA11364A

CrossRef Full Text | Google Scholar

Jia, Y., Zhang, L., Gao, G., Chen, H., Wang, B., Zhou, J., et al. (2017). A Heterostructure coupling of exfoliated Ni–Fe hydroxide nanosheet and defective graphene as a bifunctional electrocatalyst for overall water splitting. Adv. Mater. 29:1700017. doi: 10.1002/adma.201700017

PubMed Abstract | CrossRef Full Text | Google Scholar

Kumar, N., Su, W., Veselý, M., Weckhuysen, B. M., Pollard, A. J., and Wain, A. J. (2018). Nanoscale chemical imaging of solid-liquid interfaces using tip-enhanced Raman spectroscopy. Nanoscale 10, 1815–1824. doi: 10.1039/c7nr08257f

PubMed Abstract | CrossRef Full Text | Google Scholar

Lee, Y., Jin, S., May, K. J., Perry, E. E., and Yang, S. H. (2015). Synthesis and activities of rutile IrO₂ and RuO₂ nanoparticles for oxygen evolution in acid and alkaline solutions. J. Phys. Chem. Lett. 3, 399–404. doi: 10.1021/jz2016507

PubMed Abstract | CrossRef Full Text | Google Scholar

Li, C., Du, Y., Wang, D., Yin, S., Tu, W., Chen, Z., et al. (2017). Unique P-Co-N surface bonding states constructed on g-C₃N₄ nanosheets for drastically enhanced photocatalytic activity of H₂ evolution. Adv. Funct. Mater. 27:1604328. doi: 10.1002/adfm.201604328

CrossRef Full Text | Google Scholar

Li, G., Wang, X., Fu, J., Li, J., Park, M. G., Zhang, Y., et al. (2016). Pomegranate-inspired design of highly active and durable bifunctional electrocatalysts for rechargeable metal-air batteries. Angew. Chem. Int. Ed. 55, 4977–4982. doi: 10.1002/anie.201600750

PubMed Abstract | CrossRef Full Text | Google Scholar

Li, H., Wen, P., Li, Q., Dun, C., Xing, J., Lu, C., et al. (2017). Earth-abundant iron diboride (FeB₂) nanoparticles as highly active bifunctional electrocatalysts for overall water splitting. Adv. Energy Mater. 7:1700513. doi: 10.1002/aenm.201700513

CrossRef Full Text | Google Scholar

Li, P., Yang, Z., Shen, J., Nie, H., Cai, Q., Li, L., et al. (2016). Subnanometer molybdenum sulfide on carbon nanotubes as a highly active and stable electrocatalyst for hydrogen evolution reaction. ACS Appl. Mater. Interfaces 8, 3543–3550. doi: 10.1021/acsami.5b08816

PubMed Abstract | CrossRef Full Text | Google Scholar

Li, Y., Zhong, C., Liu, J., Zeng, X., Qu, S., Han, X., et al. (2018). Atomically thin mesoporous Co₃O₄ layers strongly coupled with N-rGO nanosheets as high-performance bifunctional catalysts for 1D knittable zinc-air batteries. Adv. Mater. 30:1703657. doi: 10.1002/adma.201703657

CrossRef Full Text | Google Scholar

Liang, Y., Liu, Q., Luo, Y., Sun, X., He, Y., and Asiri, A. M. (2016). Zn_0.76Co_0.24S/CoS₂ nanowires array for efficient electrochemical splitting of water. Electrochim. Acta 190, 360–364. doi: 10.1016/j.electacta.2015.12.153

CrossRef Full Text | Google Scholar

Liang, Y., Wang, H., Zhou, J., Li, Y., Wang, J., Regier, T., et al. (2012). Covalent hybrid of spinel manganese-cobalt oxide and graphene as advanced oxygen reduction electrocatalysts. J. Am. Chem. Soc. 134, 3517–3523. doi: 10.1021/ja210924t

PubMed Abstract | CrossRef Full Text | Google Scholar

Liu, J., Wang, J., Zhang, B., Ruan, Y., Lv, L., Ji, X., et al. (2017). Hierarchical NiCo2S4@NiFe LDH Heterostructures Supported on Nickel Foam for Enhanced Overall-Water-Splitting Activity. ACS Appl. Mater. Interfaces 9, 15364–15372. doi: 10.1021/acsami.7b00019

PubMed Abstract | CrossRef Full Text | Google Scholar

Liu, Q., Jin, J., and Zhang, J. (2013). NiCo₂S₄@graphene as a bifunctional electrocatalyst for oxygen reduction and evolution reactions. ACS Appl. Mater. Interfaces 5, 5002–5008. doi: 10.1021/am4007897

PubMed Abstract | CrossRef Full Text | Google Scholar

Ma, F. X., Yu, L., Xu, C. Y., and Lou, X. W. (2016). Self-supported formation of hierarchical NiCo₂O₄ tetragonal microtubes with enhanced electrochemical properties. Energy Environ. Sci. 9, 862–866. doi: 10.1039/c5ee03772g

CrossRef Full Text | Google Scholar

Ma, T. Y., Dai, S., Jaroniec, M., and Qiao, S. Z. (2014). Graphitic carbon nitride nanosheet–carbon nanotube three-dimensional porous composites as high-performance oxygen evolution electrocatalysts. Angew. Chem. Int. Ed. 53, 7281–7285. doi: 10.1002/anie.201403946

PubMed Abstract | CrossRef Full Text | Google Scholar

Ma, X., Wang, J., Liu, D., Kong, R. M., Hao, S., Du, G., et al. (2017). Hydrazine-assisted electrolytic hydrogen production: CoS₂ nanoarray as a superior bifunctional electrocatalyst. New J. Chem. 41, 4754–4757. doi: 10.1039/C7NJ00326A

CrossRef Full Text | Google Scholar

Ma, X., Zhang, W., Deng, Y., Zhong, C., Hu, W., and Han, X. (2018). Phase and composition controlled synthesis of cobalt sulfide hollow nanospheres for electrocatalytic water splitting. Nanoscale 10, 4816–4824. doi: 10.1039/c7nr09424h

PubMed Abstract | CrossRef Full Text | Google Scholar

Mccrory, C. C. L., Jung, S., Ferrer, I. M., Chatman, S. M., Peters, J. C., and Jaramillo, T. F. (2015). Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J. Am. Chem. Soc. 137, 4347–4357. doi: 10.1021/ja510442p

PubMed Abstract | CrossRef Full Text | Google Scholar

Menezes, P. W., Indra, A., Littlewood, P., Schwarze, M., Göbel, C., Schomäcker, R., et al. (2015). Nanostructured manganese oxides as highly active water oxidation catalysts: a boost from manganese precursor chemistry. Chemsuschem 7, 2202–2211. doi: 10.1002/cssc.201402169

PubMed Abstract | CrossRef Full Text | Google Scholar

Shit, S., Chhetri, S., Jang, W., Murmu, N. C., Koo, H., Samanta, P., et al. (2018). Cobalt sulfide/nickel sulfide heterostructure directly grown on nickel foam: an efficient and durable electrocatalyst for overall water splitting application. ACS Appl. Mater. Interfaces 10, 27712–27722. doi: 10.1021/acsami.8b04223

PubMed Abstract | CrossRef Full Text | Google Scholar

Sivanantham, A., Ganesan, P., and Shanmugam, S. (2016). Hierarchical NiCo₂S₄ nanowire arrays supported on ni foam: an efficient and durable bifunctional electrocatalyst for oxygen and hydrogen evolution reactions. Adv. Funct. Mater. 26, 4661–4672. doi: 10.1002/adfm.201600566

CrossRef Full Text | Google Scholar

Su, C. Y., Cheng, H., Li, W., Liu, Z. Q., Li, N., Hou, Z., et al. (2017). Atomic modulation of FeCo–nitrogen–carbon bifunctional oxygen electrodes for rechargeable and flexible all soli-state zinc-air battery. Adv. Energy Mater. 7:1602420. doi: 10.1002/aenm.201602420

CrossRef Full Text | Google Scholar

Sun, Y., Hang, L., Shen, Q., Zhang, T., Li, H., Zhang, X., et al. (2017). Mo doped Ni₂P nanowire arrays: an efficient electrocatalyst for the hydrogen evolution reaction with enhanced activity at all pH values. Nanoscale 9, 16674–16679. doi: 10.1039/c7nr03515b

PubMed Abstract | CrossRef Full Text | Google Scholar

Wang, H., Lee, H. W., Deng, Y., Lu, Z., Hsu, P. C., Liu, Y., et al. (2015). Bifunctional non-noble metal oxide nanoparticle electrocatalysts through lithium-induced conversion for overall water splitting. Nat. Commun. 6, 7261–7269. doi: 10.1038/ncomms8261

PubMed Abstract | CrossRef Full Text | Google Scholar

Wang, J., Zhong, H., Wang, Z., Meng, F., and Zhang, X. (2016). Integrated three-dimensional carbon paper/carbon tubes/cobalt-sulfide sheets as an efficient electrode for overall water splitting. ACS Nano 10, 2342–2348. doi: 10.1021/acsnano.5b07126

PubMed Abstract | CrossRef Full Text | Google Scholar

Wang, K., Liu, X. K., Chen, X. H., Yu, D. G., Yang, Y. Y., and Liu, P. (2018). Electrospun hydrophilic janus nanocomposites for the rapid onset of therapeutic action of helicid. ACS Appl. Mater. Interfaces 10, 2859–2867. doi: 10.1021/acsami.7b17663

PubMed Abstract | CrossRef Full Text | Google Scholar

Wang, X. Z., Liu, S. S., Sun, Y., Wu, J. Y., Zhou, Y. L., and Zhang, J. H. (2009). Beta-cypermethrin impairs reproductive function in male mice by inducing oxidative stress. Theriogenology 72, 599–611. doi: 10.1016/j.theriogenology.2009.04.016

PubMed Abstract | CrossRef Full Text | Google Scholar

Wang, Z., Li, B., Ge, X., Goh, F. W. T., Zhang, X., Du, G., et al. (2016). Co@Co₃O₄@PPD Core@bishell nanoparticle-based composite as an efficient electrocatalyst for oxygen reduction reaction. Small 12, 2580–2587. doi: 10.1002/smll.201503694

PubMed Abstract | CrossRef Full Text | Google Scholar

Wu, X., Han, X., Ma, X., Zhang, W., Deng, Y., Zhong, C., et al. (2017). Morphology-controllable synthesis of Zn-Co-mixed sulfide nanostructures on carbon fiber paper toward efficient rechargeable Zinc-air batteries and water electrolysis. ACS Appl. Mater. Interfaces 9, 12574–12583. doi: 10.1021/acsami.6b16602

PubMed Abstract | CrossRef Full Text | Google Scholar

Xia, C., Liang, H., Zhu, J., Schwingenschlögl, U., and Alshareef, H. N. (2017). Active edge sites engineering in nickel cobalt selenide solid solutions for highly efficient hydrogen evolution. Adv. Energy Mater. 7:1602089. doi: 10.1002/aenm.201602089

CrossRef Full Text | Google Scholar

Xiao, J., Kuang, Q., Yang, S., Xiao, F., Wang, S., and Guo, L. (2013). Surface structure dependent electrocatalytic activity of Co₃O₄anchored on graphene sheets toward oxygen reduction reaction. Sci. Rep. 3, 2300–2308. doi: 10.1038/srep02300

PubMed Abstract | CrossRef Full Text | Google Scholar

Yin, J., Li, Y., Lv, F., Lu, M., Sun, K., Wang, W., et al. (2017). Oxygen vacancies dominated NiS₂/CoS₂ interface porous nanowires for portable Zn–air batteries driven water splitting devices. Adv. Mater. 29:1704681. doi: 10.1002/adma.201704681

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhang, G., Wang, P., Lu, W. T., Wang, C. Y., Li, Y. K., Ding, C., et al. (2017). Co nanoparticles/Co, N, S tri-doped graphene templated from in-situ-formed Co, S Co-doped g-C₃N₄ as an active bifunctional electrocatalyst for overall water splitting. ACS Appl. Mater. Interfaces 9, 28566–28576. doi: 10.1021/acsami.7b08138

PubMed Abstract | CrossRef Full Text | Google Scholar

Zheng, G., Peng, Z., Jia, D., Al-Enizi, A. M., and Elzatahry, A. A. (2015). From water oxidation to reduction: Homologous Ni-Co based nanowires as complementary water splitting electrocatalysts. Adv. Energy Mater. 5:1402031. doi: 10.1002/aenm.201402031

CrossRef Full Text | Google Scholar

Zheng, M., Ding, Y., Yu, L., Du, X., and Zhao, Y. (2017). In Situ grown pristine cobalt sulfide as bifunctional photocatalyst for hydrogen and oxygen evolution. Adv. Funct. Mater. 27:1605846. doi: 10.1002/adfm.201605846

CrossRef Full Text | Google Scholar

Zhou, S., Zhou, Q.-X., Su, H., Wang, Y., Dong, Z., Dai, X., et al. (2019). Hybrid of Fe₃C@N, S co-doped carbon nanotubes coated porous carbon derived from metal organic frameworks as an efficient catalyst towards oxygen reduction. J. Colloid Inter. Sci. 533, 311–318. doi: 10.1016/j.jcis.2018.06.091

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhu, Y. P., Ma, T. Y., Jaroniec, M., and Qiao, S. Z. (2016). Self-templating synthesis of hollow Co₃O₄ microtube arrays for highly efficient water electrolysis. Angew. Chem. Int. Ed. 56, 1324–1328. doi: 10.1002/anie.201610413

CrossRef Full Text | Google Scholar