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Hydrophilic cobalt sulfide nanosheets as a bifunctional catalyst for oxygen and hydrogen evolution in electrolysis of alkaline aqueous solution
Journal of Colloid and Interface Science xxx (2017) xxx–xxx
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Hydrophilic cobalt sulfide nanosheets as a bifunctional catalyst for oxygen and hydrogen evolution in electrolysis of alkaline aqueous solution Mingchao Zhu a, Zhongyi Zhang a,⇑, Hu Zhang b, Hui Zhang a, Xiaodong Zhang a, Lixue Zhang a, Shicai Wang a a b
College of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071, PR China Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 17 June 2017 Revised 16 September 2017 Accepted 21 September 2017 Available online xxxx Keywords: Water splitting Bifunctional catalyst Hydrophilic nanosheet Hydrogen evolution Oxygen evolution
a b s t r a c t Hydrophilic medium and precursors were used to synthesize a hydrophilic electro-catalyst for overall water splitting. The cobalt sulfide (Co3S4) catalyst exhibits a layered nanosheet structure with a hydrophilic surface, which can facilitate the diffusion of aqueous substrates into the electrode pores and towards the active sites. The Co3S4 catalyst shows excellent bifunctional catalytic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline solution. The assembled water electrolyzer based on Co3S4 exhibits better performance and stability than that of Pt/C-RuO2 catalyst. Thereforce the hydrophilic Co3S4 is a highly promising bifunctional catalyst for the overall water splitting reaction. Ó 2017 Published by Elsevier Inc.
1. Introduction Hydrogen is widely considered a clean, alternative energy resource to fossil fuel [1–4]. Water splitting provides a promising ⇑ Corresponding author. E-mail address: [email protected] (Z. Zhang).
and reliable method to generate hydrogen from renewable energy sources, such as solar, wind and hydropower. In the water-splitting process, the use of high-performance electrodes is critical for effective minimization of energy consumption [5–10]. The activation energy and overpotential at the electrodes have been shown to be significantly reduced by using Electro-catalysts, resulting in faster reaction rates and lower operating voltages. Hence, high-
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performance electro-catalysts are highly desirable for improving the efficiency of the water-splitting process [11–14]. Currently, IrO2 and RuO2 catalysts show the highest efficiency for the oxygen evolution reaction (OER), while, Pt is the best performing catalyst for the hydrogen evolution reaction (HER) [15]. However, these noble metal catalysts are expensive and are not compatible with each other in the same electrolyte system [16– 18]. Therefore, the development of bifunctional metal catalysts with excellent catalytic activities for both HER and OER can simplify the system and lower the cost [1,19–21]. However, only a few such catalysts have been realized so far. Among them, cobalt-based catalysts (e.g., Co2B [22], CoP [23], CoO [24] and NiCo2S4 nanowires [18]), constitute an important class that has attracted great attentions in recent years. In particular, cobalt sulfide (Co3S4) is known to show excellent catalytic activity for both HER in acidic electrolytes [20] and OER in alkaline electrolytes [25]. Compared with phosphate cobalt [26], the synthesis of cobalt sulfide does not produce toxic phosphine gas. However, Co3S4 has not been reported to show bifunctional catalytic activity for both OER and HER in the same electrolyte system. In the OER and HER process, catalysts with hydrophilic sites often show higher reactivity by maximizing the contact between the catalyst and electrolyte. However, most synthetic processes of cobalt sulfides involve the use of hydrophobic long alkyl chain precursors or solvents, such as Oleyl amine and 1-Dodecanethiol [20]. These strategies inevitably lead to the long hydrophobic chains covering the surface of catalyst and consequently preventing the aqueous substrates (H2O, OH or H+) from contact with the catalytic active sites. To provide a hydrophilic catalytic surface, we used hydrophilic solvent and precursors to synthesize Co3S4. The as-prepared Co3S4 exhibits a layered nanosheet structure with good water dispersibility. This 2D nanostructure is conductive and supports an enlarged catalytic interface that improves charge transfer efficiency. More importantly, the hydrophilic Co3S4 (Co3S4-L) showed excellent bifunctional catalytic activity for both OER and HER, as well as good stability in alkaline solution in a water electrolyzer. 2. Experimental 2.1. Catalyst synthesis 2.1.1. 1Synthesis of hydrophilic Co3S4 (Co3S4 -L) 3 mmol C4H6CoO44H2O was dissolved in Triethylene glycol (30 mL) in a two-neck flask. After all the solid was dissolved, 8 mmol C3H8O2S was added to the mixture. Then it was heated to 220 °C under argon atmosphere. The reaction was kept at that temperature for about 3 h under the magnetic stirring. Then, the generated black solution was cooled to room temperature and then 10 mL of ethanol was added. The formed precipitate was centrifuged (at 5000 rpm, for 5 min), washed with ethanol for several times, and vacuum-dried at 60 °C for 4 h to provide the purified product. 2.1.2. Synthesis of hydrophobic Co3S4 (Co3S4-B) 3 mmol C4H6CoO44H2O and 30 mL Oleyl amine were put in a two-neck flask and stirred magnetically under argon atmosphere. After all the solid was dissolved, 8 mmol CH4N2S was added to the reaction mixture, then it was heated to 220 °C and kept at this temperature for 3 h. After cooling to room temperature, a black precipitate was obtained by adding 30 mL methanol to the solution and separated by centrifugation (at 5000 rpm, for 5 min). Then the black precipitate was washed with a mixture of methanol and methylbenzene (volume ratio is 5:1) for three times. Finally, Co3S4-B were obtained by drying in vacuum at 60 °C for 4 h.
2.2. Characterization Scanning electron microscope (SEM) images, Transmission electron microscopy (TEM) and energy dispersive spectra (EDS) were acquired using a Hitachi SU-8010, Hitachi HT-7700 fieldemission electron microscope. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurement were carried out using Bruker D8 Advance X-ray diffractometer and PHI Quantera X-ray Photoelectrometer Spectrometer, respectively. Contact angles were tested by Dataphysics OCA15 Pro. Infrared absorption spectrum was acquired using a Thermo Nicolet IR spectrophotometer model (IR 460). 2.3. Assembly of Water-splitting electrolyzer A mixture of the prepared catalysts, 1-Methyl-2-pyrrolidone and PVDF (Polyvinylidene Fluoride) (5%) brushed as an ink onto the carbon cloth (1 cm 1 cm). After drying, the sample was vacuum dried at 60 °C for 1 h. 2.4. Electrochemical measurements Electrochemical measurements were performed on CHI630e electrochemistry workstation. The working electrodes consisted of 5 mm glassy carbon stick, with surface area of 0.197 cm2. Samples were prepared for analysis by dispersing 3 mg of catalyst in 50 lL Nafion solution with 950 lL of H2O. 27.5 lL of the catalyst ink was deposited onto the glassy carbon electrodes (0.42 mg cm 2). All measurements were performed in 1M KOH solution, at the scan rate of 5 mV/s with 95% iR-compensation (iRcorrected), using a three-electrode cell that contained an Ag/AgCl3 M KCl (with a double junction) as the reference electrode and a platinum wire (isolated in a proton exchange membrane sealed tube) as the counter electrode. The potential could be easily converted to the one versus Reversible Hydrogen Electrode (RHE) by using the Nernst equation: (ERHE = EAg/AgCl + 0.059 pH + E0Ag/AgCl). Additionally, the catalytic stability test was carried out using graphite plate electrode as the counter electrode. Before analysis, the electrolyte solution was purged with O2 for at least 15 min. A constant stream of bobbing O2 was maintained for all further electrochemical experiments. All measurements were performed at room temperature. 3. Results and discussion In order to achieve a hydrophilic surface, hydrophilic solvent and precursors was selected to synthesize the target catalyst (Co3S4-L). Our previous research has showed that 1-Thioglycerol is a highly reactive sulfur precursor for the synthesis of metal sulfides [27]. Therefore, we chose 1-Thioglycerol as the sulfur precursor. Triethylene glycol was used as the solvent because of its high boiling point and hydrophilic nature. Catalyst with hydrophobic surface (Co3S4-B) was also synthesized and studied to demonstrate the critical role of hydrophilic surface in the high bifunctional catalytic performance of Co3S4-L. 3.1. Structure and morphology analysis X-ray diffraction (XRD) was used to characterize the crystal structure of the cobalt sulfides synthesized following our protocol. Fig. 1a shows the XRD patterns of the Co3S4-L. All of the diffraction peaks matched well with the structure of Co3S4 (JCPDS#42-1448), with the diffraction peaks at 2H = 31.4°, 38.1° and 55.0° attributed to (3 1 1), (4 0 0), and (4 4 0) crystal faces, respectively. Co3S4-B showed similar XRD patterns, suggesting that the catalysts synthe-
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and Co3+ [31]. The sulfur 2p spectrum showed two peaks at 161 and 162.1 eV that correspond to S 2p3/2 and S 2p1/2 of Co3S4 respectively (Fig. 2d). The broad peak observed at 167.8 eV can be assigned to oxidized sulfur species which is likely due to exposure to air [20]. A scanning electron microscopy (SEM) image of Co3S4-L, showed stacked 2D nanosheets (Fig. 3a). The transmission electron microscopy (TEM) image revealed that the Co3S4-L exhibits a layered structure, with the size of single nanosheet about 200 nm in length and width (Fig. 3b). Three High-Resolution TEM (HRTEM) images focused on the edges of the nanosheets, showed that the thickness of the lamellar structure is approximately 3–4 nm (Fig. S1). HRTEM image showed two well-resolved lattice fringes with interplanar spacing of 0.23 and 0.16 nm, corresponding to (4 0 0) and (4 4 0) planes of Co3S4, respectively (Fig. 3c). The polycrystalline selected-area electron diffraction (SAED) pattern is indexed with rings to (3 1 1), (4 0 0), (5 1 1) and (4 4 0) (Fig. 3d), which is consistent with the results of XRD patterns of Co3S4 (JCPDS#42-1448). The elemental mapping images (Fig. 3e–g and Fig. S2) confirm that the Co and S elements are uniformly distributed, without any elemental segregation. Fig. 1. XRD patterns of (a) Co3S4-L, (b) Co3S4-B and (c) Standard card (JCPDS#421448) of Co3S4.
sized via hydrophilic (Co3S4-L) or hydrophobic (Co3S4-B) routes have the same crystal structure as Co3S4 (JCPDS#42-1448). X-ray photoelectron spectroscopy (XPS) was applied to probe the surface chemical structure of Co3S4-L. As shown in the carbon 1 s spectrum (Fig. 2a), C-C (284 eV) and C–O (285.2 eV) [28] are attributed to the organic residues on the surface of catalyst .Those organic residues can also be found in oxygen 1s spectrum, such as CAO (531.5 eV) and CAOH (532 eV) (Fig. 2b) [29]. The cobalt 2p spectrum showed two major peaks at 778.5 and 794.3 eV with two shakeup satellites respectively (Fig. 2c), which correspond to Co 2p3/2 and Co 2p1/2 [30]. The energy difference between Co 2p3/2 and Co 2p1/2 is 15.9 eV, suggesting the coexistence of Co2+
3.2. Study on Interfacial hydrophilicity These 2D layered Co3S4 (for Co3S4-L), synthesized from hydrophilic nonionic solvents and precursors, have good water dispersibility. As shown in Fig. 4g, the dispersion of Co3S4-L (with the different concentration of 10, 20, 30, 50 ppm) in water was homogenous and there was no precipitation after standing for 2 h. In comparison, the hydrophobic Co3S4-B cannot be dispersed well in water and precipitation at the bottom of glass bottles was observed (Fig. 4h). The high dispersibility of Co3S4-L can be attributed to suggest its hydrophilic surface. In order to further investigate the state of hydrophilic groups on the surface of Co3S4-L, we obtained the infrared reflection absorption spectrum (IRAS) of Co3S4-L as well as that of the two organic reactants (Fig. S3). It is clearly shows that the hydrophilic organic residues are bonded
Fig. 2. High-resolution XPS of Co3S4-L and (a) is C 1s spectrum, (b) is O 1s spectrum, (c) is Co 2p spectrum and (d) is S 2p spectrum.
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Fig. 3. (a) SEM and (b) TEM image of Co3S4-L. (c) HRTEM image and (d) SAED pattern of Co3S4-L. (e) TEM image and EDS mapping for (f) Cobalt and (g) Sulfur elements of Co3S4-L.
on the surface to provide the catalyst high hydrophilicity (see supporting information). The presence of nonionic groups not only makes a hydrophilic catalytic surface, but also avoids the repulsion between the surface charge and substrate ions (such as OH or H+). In contrast, the IRAS of Co3S4-B (in Fig. S4) revealed that the surface of Co3S4-B is covered with long alkyl chains, which lead to its hydrophobic surface. To evaluate the wettability for the electrode coated with Co3S4L, dynamic contact angles were measured for Co3S4-L loading on carbon cloth. Fig. 4a–e show the contact angles at different wetting time for the electrode of Co3S4-L. A plot of the contact angle versus wetting time. Shows that the contact angles of Co3S4-L decreased rapidly after the start time and can be reduced to less than 5°
within 2 s (Fig. 4f). This result indicates that the electrode surface of Co3S4-L was able to achieve super hydrophilic state rapidly. While, as a comparison, the contact angles have been maintained at larger values (>90°) for the electrode of Co3S4-B and blank carbon cloth (Fig. S5), consistent with their hydrophobic nature.
3.3. Electrochemical HER and OER performance To study the catalytic properties of the catalysts in HER and OER, linear sweep voltammetry (LSV) was carried out in 1M KOH solution. The commercial 10% Pt/C catalyst and RuO2 were examined as a comparison.
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Fig. 4. (a)–(e) Contact angle measurements of water drop on Co3S4–L electrode as a function of time. (f) The detailed data curve of the contact angle versus time. (g) Photos of the dispersions for Co3S4-L and (h) for Co3S4-B, with different concentration of 10, 20, 30, 50 ppm, after standing for 2 h.
In HER, the onset potential of Co3S4-L is more positive and the current density is much larger than that of Co3S4-B (Fig. 5a). The current density of Co3S4-L achieves 10 mA/cm2 at 0.27 V (vs. RHE), with the overpotential only about 0.15 V larger than that of Pt/C. The linear parts of the Tafel plots revealed the Tafel slopes of 34.5, 124.5, and 219.7 mV/decade for Pt/C, Co3S4-L and Co3S4B, respectively (Fig. 5b). These results demonstrate that Co3S4-L have much higher HER catalytic activity than Co3S4-B. In OER, at a given potential Co3S4-L showed the largest current density among all the electrocatalysts tested (Fig. 5c). Co3S4-L reached a current density of 10 mA/cm2 at 1.59 V (vs. RHE) which is much lower than Co3S4-B. The overpotential of Co3S4-L is 0.31 V, which is only about 0.05 V larger than that of RuO2. The Tafel slopes of Co3S4-L was calculated to be 84.7 mV/decade (Fig. 5d). The small Tafel slope [32] indicates the outstanding intrinsic OER activity of Co3S4-L compared with other catalysts. These results suggest that the catalytic activity of Co3S4-L for OER was even better than that of RuO2 in alkaline solution. The above analysis demonstrated that Co3S4-L has outstanding bifunctional catalytic activity for both OER and HER in alkaline solution. It should be pointed out that, Co3S4-L showed much better bifunctional performance than Co3S4-B under the same test condition, which provides strong evidence that the hydrophilic surface is very important to prompt bifunctional activities. Besides catalytic activities, durability is another important aspect in evaluating a electrocatalyst. Chronoamperometry (i-t testing) of HER and OER were measured to investigate the longterm electrochemical stability. The catalyst was loaded on carbon cloth and a high purity graphite plate was used as the counter electrode. As shown in Fig. 5e, Co3S4-L electrode exhibited a more stable HER performance than Co3S4-B. The Co3S4-L electrode also showed a more stable OER performance than Co3S4-B (Fig. 5f). After 5 h, both the HER and the OER current retention rate of Co3S4-L electrode are still above 90%.
We also measured the HER and OER performance of Co3S4-L in different pH (0.5 M H2SO4, 0.1M KOH, 1M KOH) aqueous solutions with different catalyst dosage (0.42 mg/cm2, 0.84 mg/cm2, 1.26 mg/cm2) (Fig. S6). It can be seen that the rate of HER and OER slightly increased with the increasing of catalyst loading.
3.4. Performance of assembled Water-splitting electrolyzer Based on the aforementioned results, we anticipated that the Co3S4-L could act as a bifunctional electrocatalyst for overall water splitting. Hence, a two-electrode configuration was employed to build a water-splitting electrolyzer with Co3S4-L as the catalyst on both the cathode and anode. As a comparison, a watersplitting cell based on commercial 10% Pt/C catalyst (as cathode) and RuO2 (as anode) was also examined. Fig. 6a shows the polarization curves of the electrolysis test. When the current density of Co3S4-L cell reached 10 mA/cm2 (at 1.63 V), gas evolution was observed at both electrodes. The current onset is 1.45 V for Co3S4-L, which means the corresponding water-splitting electrolyzer can produce H2 and O2 at a very small voltage [33,34]. The current density of Co3S4-L cell can reach 10 mA/cm2 when the voltage runs up to 1.63 V. Subsequently, continuous electrolysis was tested with the applied voltage of 1.63 V for Co3S4-L cell. As shown in Fig. 6b, after 10 h, the water-splitting cell based on Co3S4-L still performed well, with only about 5% of current decay. However, the current density of Pt/C-RuO2 and Co3S4-B cell systems degraded dramatically in the meantime. The detailed performances of several reported bifunctional electrocatalysts are compared with that of Co3S4-L (in supporting information). Table S1 lists the overpotentials (gHER, gOER and goverall is potential) at the current density of 10 mA cm 2 for HER, OER and water electrolyzer, respectively. Compared to the
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Fig. 5. (a) HER polarization curves and (b) the corresponding Tafel plots of Co3S4-L Co3S4-B, and Pt/C tested in 1M KOH with the scan rate of 5 mV/s 95% iR-compensation. (c) OER polarization curves and (d) the corresponding Tafel plots of Co3S4-L Co3S4-B, and RuO2 tested in 1M KOH with the scan rate of 5 mV/s and 95% iR-compensation. (e) Timedependent current density curve of HER tested at the same starting current density for Co3S4-L and Co3S4-B. (f) Time-dependent current density curve of OER tested at the same starting current density for Co3S4-L and Co3S4-B.
existing bifunctional catalysts, the hydrophilic Co3S4-L is superior in overall water splitting.
4. Conclusion In conclusion, a water dispersible, hydrophilic cobalt sulfide (Co3S4-L) was synthesized using hydrophilic solvent and precursors in a practical manner. The synthesized Co3S4-L exhibited 2 D nanosheet morphology with hydrophilic catalytic surface, which facilitates the diffusion of aqueous substrates into the pore of electrodes approach the active sites. Co3S4-L exhibited excellent bifunctional catalytic activities for both OER and HER in alkaline
solution. The current density of HER reached 10 mA/cm2 at 0.27 V (vs.RHE), with an overpotential only about 0.15 V larger than that of Pt/C. The current density of OER reached 10 mA/cm2 at 1.59 V (vs. RHE), with an overpotential only about 0.05 V larger than that of RuO2. The onset potential and goverall (the potential that provides a current density of 10 mA/cm2) of the watersplitting electrolyzer based on Co3S4-L are 1.45 V and 1.63 V, respectively. In addition, the assembled electrolyzer of Co3S4-L also exhibits better stability than that of Pt/C-RuO2 system. These results suggest that Co3S4-L is superior to reported base metal bifunctional catalysts in term of both activity and durability. This work represents the first example of using Co3S4 as catalyst for
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Fig. 6. (a) Polarization curves and (b) Chronoamperometry curves of the assembled water splitting cell based on Co3S4-L (red line), Co3S4-B (blue line) and Pt/CRuO2 (black line) with the scan rate of 5 mV/s in 1M KOH solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
both OER and HER in one electrolyte system. The researches in this paper demonstrate conclusively that the surface hydrophilicity is very important for exploring bifunctional catalyst of overall water splitting. Acknowledgements This work was supported by Natural Science Foundation of Shandong Province (Grant No. ZR2015BM002), Key Research and Development Plan of Shandong Province (Grant No. 2017GGX40119), the Program of the Qingdao Key Lab of Solar Energy Utilization and Energy Storage Technology (Grant No. QDKLSE201602), the Research Program of Qingdao Municipal Science and Technology Commission (Grant No. 16-5-1-44-jch). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2017.09.076. References [1] T. Liu, D. Liu, F. Qu, D. Wang, L. Zhang, R. Ge, S. Hao, Y. Ma, G. Du, M. Arisiri, L. Chen, X. Sun, Adv. Energy Mater. 7 (2017) 1700020. [2] X. Niu, Q. Tang, B. He, P. Yang, Electrochim. Acta 208 (2016) 180–187. [3] C. Tang, R. Zhang, W. Lu, L. He, X. Jiang, M. Asiri, X. Sun, Adv. Mater. 29 (2017) 1602441. [4] T. Pandiarajan, S. Ravichandran, L.J. Berchmans, RSC Adv. 4 (2014) 64364– 64370. [5] T. Wang, X. Wang, Y. Liu, J. Zheng, X. Li, Nano Energy 22 (2016) 111–119. [6] M.R. Gao, J.X. Liang, Y.R. Zheng, Y.F. Xu, J. Jiang, Q. Gao, J. Li, S.H. Yu, Nat. Commun. 6 (2015) 5982. [7] L. Chen, X. Dong, Y. Wang, Y. Xia, Nat. Commun. 7 (2016) 11741. [8] W. Liu, E. Hu, H. Jiang, Y. Xiang, Z. Weng, M. Li, Q. Fan, X. Yu, E.I. Altman, H. Wang, Nat. Commun. 7 (2016) 10771.
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Regular Article
Hydrophilic cobalt sulfide nanosheets as a bifunctional catalyst for oxygen and hydrogen evolution in electrolysis of alkaline aqueous solution Mingchao Zhu a, Zhongyi Zhang a,⇑, Hu Zhang b, Hui Zhang a, Xiaodong Zhang a, Lixue Zhang a, Shicai Wang a a b
College of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071, PR China Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 17 June 2017 Revised 16 September 2017 Accepted 21 September 2017 Available online xxxx Keywords: Water splitting Bifunctional catalyst Hydrophilic nanosheet Hydrogen evolution Oxygen evolution
a b s t r a c t Hydrophilic medium and precursors were used to synthesize a hydrophilic electro-catalyst for overall water splitting. The cobalt sulfide (Co3S4) catalyst exhibits a layered nanosheet structure with a hydrophilic surface, which can facilitate the diffusion of aqueous substrates into the electrode pores and towards the active sites. The Co3S4 catalyst shows excellent bifunctional catalytic activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline solution. The assembled water electrolyzer based on Co3S4 exhibits better performance and stability than that of Pt/C-RuO2 catalyst. Thereforce the hydrophilic Co3S4 is a highly promising bifunctional catalyst for the overall water splitting reaction. Ó 2017 Published by Elsevier Inc.
1. Introduction Hydrogen is widely considered a clean, alternative energy resource to fossil fuel [1–4]. Water splitting provides a promising ⇑ Corresponding author. E-mail address: [email protected] (Z. Zhang).
and reliable method to generate hydrogen from renewable energy sources, such as solar, wind and hydropower. In the water-splitting process, the use of high-performance electrodes is critical for effective minimization of energy consumption [5–10]. The activation energy and overpotential at the electrodes have been shown to be significantly reduced by using Electro-catalysts, resulting in faster reaction rates and lower operating voltages. Hence, high-
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performance electro-catalysts are highly desirable for improving the efficiency of the water-splitting process [11–14]. Currently, IrO2 and RuO2 catalysts show the highest efficiency for the oxygen evolution reaction (OER), while, Pt is the best performing catalyst for the hydrogen evolution reaction (HER) [15]. However, these noble metal catalysts are expensive and are not compatible with each other in the same electrolyte system [16– 18]. Therefore, the development of bifunctional metal catalysts with excellent catalytic activities for both HER and OER can simplify the system and lower the cost [1,19–21]. However, only a few such catalysts have been realized so far. Among them, cobalt-based catalysts (e.g., Co2B [22], CoP [23], CoO [24] and NiCo2S4 nanowires [18]), constitute an important class that has attracted great attentions in recent years. In particular, cobalt sulfide (Co3S4) is known to show excellent catalytic activity for both HER in acidic electrolytes [20] and OER in alkaline electrolytes [25]. Compared with phosphate cobalt [26], the synthesis of cobalt sulfide does not produce toxic phosphine gas. However, Co3S4 has not been reported to show bifunctional catalytic activity for both OER and HER in the same electrolyte system. In the OER and HER process, catalysts with hydrophilic sites often show higher reactivity by maximizing the contact between the catalyst and electrolyte. However, most synthetic processes of cobalt sulfides involve the use of hydrophobic long alkyl chain precursors or solvents, such as Oleyl amine and 1-Dodecanethiol [20]. These strategies inevitably lead to the long hydrophobic chains covering the surface of catalyst and consequently preventing the aqueous substrates (H2O, OH or H+) from contact with the catalytic active sites. To provide a hydrophilic catalytic surface, we used hydrophilic solvent and precursors to synthesize Co3S4. The as-prepared Co3S4 exhibits a layered nanosheet structure with good water dispersibility. This 2D nanostructure is conductive and supports an enlarged catalytic interface that improves charge transfer efficiency. More importantly, the hydrophilic Co3S4 (Co3S4-L) showed excellent bifunctional catalytic activity for both OER and HER, as well as good stability in alkaline solution in a water electrolyzer. 2. Experimental 2.1. Catalyst synthesis 2.1.1. 1Synthesis of hydrophilic Co3S4 (Co3S4 -L) 3 mmol C4H6CoO44H2O was dissolved in Triethylene glycol (30 mL) in a two-neck flask. After all the solid was dissolved, 8 mmol C3H8O2S was added to the mixture. Then it was heated to 220 °C under argon atmosphere. The reaction was kept at that temperature for about 3 h under the magnetic stirring. Then, the generated black solution was cooled to room temperature and then 10 mL of ethanol was added. The formed precipitate was centrifuged (at 5000 rpm, for 5 min), washed with ethanol for several times, and vacuum-dried at 60 °C for 4 h to provide the purified product. 2.1.2. Synthesis of hydrophobic Co3S4 (Co3S4-B) 3 mmol C4H6CoO44H2O and 30 mL Oleyl amine were put in a two-neck flask and stirred magnetically under argon atmosphere. After all the solid was dissolved, 8 mmol CH4N2S was added to the reaction mixture, then it was heated to 220 °C and kept at this temperature for 3 h. After cooling to room temperature, a black precipitate was obtained by adding 30 mL methanol to the solution and separated by centrifugation (at 5000 rpm, for 5 min). Then the black precipitate was washed with a mixture of methanol and methylbenzene (volume ratio is 5:1) for three times. Finally, Co3S4-B were obtained by drying in vacuum at 60 °C for 4 h.
2.2. Characterization Scanning electron microscope (SEM) images, Transmission electron microscopy (TEM) and energy dispersive spectra (EDS) were acquired using a Hitachi SU-8010, Hitachi HT-7700 fieldemission electron microscope. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurement were carried out using Bruker D8 Advance X-ray diffractometer and PHI Quantera X-ray Photoelectrometer Spectrometer, respectively. Contact angles were tested by Dataphysics OCA15 Pro. Infrared absorption spectrum was acquired using a Thermo Nicolet IR spectrophotometer model (IR 460). 2.3. Assembly of Water-splitting electrolyzer A mixture of the prepared catalysts, 1-Methyl-2-pyrrolidone and PVDF (Polyvinylidene Fluoride) (5%) brushed as an ink onto the carbon cloth (1 cm 1 cm). After drying, the sample was vacuum dried at 60 °C for 1 h. 2.4. Electrochemical measurements Electrochemical measurements were performed on CHI630e electrochemistry workstation. The working electrodes consisted of 5 mm glassy carbon stick, with surface area of 0.197 cm2. Samples were prepared for analysis by dispersing 3 mg of catalyst in 50 lL Nafion solution with 950 lL of H2O. 27.5 lL of the catalyst ink was deposited onto the glassy carbon electrodes (0.42 mg cm 2). All measurements were performed in 1M KOH solution, at the scan rate of 5 mV/s with 95% iR-compensation (iRcorrected), using a three-electrode cell that contained an Ag/AgCl3 M KCl (with a double junction) as the reference electrode and a platinum wire (isolated in a proton exchange membrane sealed tube) as the counter electrode. The potential could be easily converted to the one versus Reversible Hydrogen Electrode (RHE) by using the Nernst equation: (ERHE = EAg/AgCl + 0.059 pH + E0Ag/AgCl). Additionally, the catalytic stability test was carried out using graphite plate electrode as the counter electrode. Before analysis, the electrolyte solution was purged with O2 for at least 15 min. A constant stream of bobbing O2 was maintained for all further electrochemical experiments. All measurements were performed at room temperature. 3. Results and discussion In order to achieve a hydrophilic surface, hydrophilic solvent and precursors was selected to synthesize the target catalyst (Co3S4-L). Our previous research has showed that 1-Thioglycerol is a highly reactive sulfur precursor for the synthesis of metal sulfides [27]. Therefore, we chose 1-Thioglycerol as the sulfur precursor. Triethylene glycol was used as the solvent because of its high boiling point and hydrophilic nature. Catalyst with hydrophobic surface (Co3S4-B) was also synthesized and studied to demonstrate the critical role of hydrophilic surface in the high bifunctional catalytic performance of Co3S4-L. 3.1. Structure and morphology analysis X-ray diffraction (XRD) was used to characterize the crystal structure of the cobalt sulfides synthesized following our protocol. Fig. 1a shows the XRD patterns of the Co3S4-L. All of the diffraction peaks matched well with the structure of Co3S4 (JCPDS#42-1448), with the diffraction peaks at 2H = 31.4°, 38.1° and 55.0° attributed to (3 1 1), (4 0 0), and (4 4 0) crystal faces, respectively. Co3S4-B showed similar XRD patterns, suggesting that the catalysts synthe-
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and Co3+ [31]. The sulfur 2p spectrum showed two peaks at 161 and 162.1 eV that correspond to S 2p3/2 and S 2p1/2 of Co3S4 respectively (Fig. 2d). The broad peak observed at 167.8 eV can be assigned to oxidized sulfur species which is likely due to exposure to air [20]. A scanning electron microscopy (SEM) image of Co3S4-L, showed stacked 2D nanosheets (Fig. 3a). The transmission electron microscopy (TEM) image revealed that the Co3S4-L exhibits a layered structure, with the size of single nanosheet about 200 nm in length and width (Fig. 3b). Three High-Resolution TEM (HRTEM) images focused on the edges of the nanosheets, showed that the thickness of the lamellar structure is approximately 3–4 nm (Fig. S1). HRTEM image showed two well-resolved lattice fringes with interplanar spacing of 0.23 and 0.16 nm, corresponding to (4 0 0) and (4 4 0) planes of Co3S4, respectively (Fig. 3c). The polycrystalline selected-area electron diffraction (SAED) pattern is indexed with rings to (3 1 1), (4 0 0), (5 1 1) and (4 4 0) (Fig. 3d), which is consistent with the results of XRD patterns of Co3S4 (JCPDS#42-1448). The elemental mapping images (Fig. 3e–g and Fig. S2) confirm that the Co and S elements are uniformly distributed, without any elemental segregation. Fig. 1. XRD patterns of (a) Co3S4-L, (b) Co3S4-B and (c) Standard card (JCPDS#421448) of Co3S4.
sized via hydrophilic (Co3S4-L) or hydrophobic (Co3S4-B) routes have the same crystal structure as Co3S4 (JCPDS#42-1448). X-ray photoelectron spectroscopy (XPS) was applied to probe the surface chemical structure of Co3S4-L. As shown in the carbon 1 s spectrum (Fig. 2a), C-C (284 eV) and C–O (285.2 eV) [28] are attributed to the organic residues on the surface of catalyst .Those organic residues can also be found in oxygen 1s spectrum, such as CAO (531.5 eV) and CAOH (532 eV) (Fig. 2b) [29]. The cobalt 2p spectrum showed two major peaks at 778.5 and 794.3 eV with two shakeup satellites respectively (Fig. 2c), which correspond to Co 2p3/2 and Co 2p1/2 [30]. The energy difference between Co 2p3/2 and Co 2p1/2 is 15.9 eV, suggesting the coexistence of Co2+
3.2. Study on Interfacial hydrophilicity These 2D layered Co3S4 (for Co3S4-L), synthesized from hydrophilic nonionic solvents and precursors, have good water dispersibility. As shown in Fig. 4g, the dispersion of Co3S4-L (with the different concentration of 10, 20, 30, 50 ppm) in water was homogenous and there was no precipitation after standing for 2 h. In comparison, the hydrophobic Co3S4-B cannot be dispersed well in water and precipitation at the bottom of glass bottles was observed (Fig. 4h). The high dispersibility of Co3S4-L can be attributed to suggest its hydrophilic surface. In order to further investigate the state of hydrophilic groups on the surface of Co3S4-L, we obtained the infrared reflection absorption spectrum (IRAS) of Co3S4-L as well as that of the two organic reactants (Fig. S3). It is clearly shows that the hydrophilic organic residues are bonded
Fig. 2. High-resolution XPS of Co3S4-L and (a) is C 1s spectrum, (b) is O 1s spectrum, (c) is Co 2p spectrum and (d) is S 2p spectrum.
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Fig. 3. (a) SEM and (b) TEM image of Co3S4-L. (c) HRTEM image and (d) SAED pattern of Co3S4-L. (e) TEM image and EDS mapping for (f) Cobalt and (g) Sulfur elements of Co3S4-L.
on the surface to provide the catalyst high hydrophilicity (see supporting information). The presence of nonionic groups not only makes a hydrophilic catalytic surface, but also avoids the repulsion between the surface charge and substrate ions (such as OH or H+). In contrast, the IRAS of Co3S4-B (in Fig. S4) revealed that the surface of Co3S4-B is covered with long alkyl chains, which lead to its hydrophobic surface. To evaluate the wettability for the electrode coated with Co3S4L, dynamic contact angles were measured for Co3S4-L loading on carbon cloth. Fig. 4a–e show the contact angles at different wetting time for the electrode of Co3S4-L. A plot of the contact angle versus wetting time. Shows that the contact angles of Co3S4-L decreased rapidly after the start time and can be reduced to less than 5°
within 2 s (Fig. 4f). This result indicates that the electrode surface of Co3S4-L was able to achieve super hydrophilic state rapidly. While, as a comparison, the contact angles have been maintained at larger values (>90°) for the electrode of Co3S4-B and blank carbon cloth (Fig. S5), consistent with their hydrophobic nature.
3.3. Electrochemical HER and OER performance To study the catalytic properties of the catalysts in HER and OER, linear sweep voltammetry (LSV) was carried out in 1M KOH solution. The commercial 10% Pt/C catalyst and RuO2 were examined as a comparison.
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Fig. 4. (a)–(e) Contact angle measurements of water drop on Co3S4–L electrode as a function of time. (f) The detailed data curve of the contact angle versus time. (g) Photos of the dispersions for Co3S4-L and (h) for Co3S4-B, with different concentration of 10, 20, 30, 50 ppm, after standing for 2 h.
In HER, the onset potential of Co3S4-L is more positive and the current density is much larger than that of Co3S4-B (Fig. 5a). The current density of Co3S4-L achieves 10 mA/cm2 at 0.27 V (vs. RHE), with the overpotential only about 0.15 V larger than that of Pt/C. The linear parts of the Tafel plots revealed the Tafel slopes of 34.5, 124.5, and 219.7 mV/decade for Pt/C, Co3S4-L and Co3S4B, respectively (Fig. 5b). These results demonstrate that Co3S4-L have much higher HER catalytic activity than Co3S4-B. In OER, at a given potential Co3S4-L showed the largest current density among all the electrocatalysts tested (Fig. 5c). Co3S4-L reached a current density of 10 mA/cm2 at 1.59 V (vs. RHE) which is much lower than Co3S4-B. The overpotential of Co3S4-L is 0.31 V, which is only about 0.05 V larger than that of RuO2. The Tafel slopes of Co3S4-L was calculated to be 84.7 mV/decade (Fig. 5d). The small Tafel slope [32] indicates the outstanding intrinsic OER activity of Co3S4-L compared with other catalysts. These results suggest that the catalytic activity of Co3S4-L for OER was even better than that of RuO2 in alkaline solution. The above analysis demonstrated that Co3S4-L has outstanding bifunctional catalytic activity for both OER and HER in alkaline solution. It should be pointed out that, Co3S4-L showed much better bifunctional performance than Co3S4-B under the same test condition, which provides strong evidence that the hydrophilic surface is very important to prompt bifunctional activities. Besides catalytic activities, durability is another important aspect in evaluating a electrocatalyst. Chronoamperometry (i-t testing) of HER and OER were measured to investigate the longterm electrochemical stability. The catalyst was loaded on carbon cloth and a high purity graphite plate was used as the counter electrode. As shown in Fig. 5e, Co3S4-L electrode exhibited a more stable HER performance than Co3S4-B. The Co3S4-L electrode also showed a more stable OER performance than Co3S4-B (Fig. 5f). After 5 h, both the HER and the OER current retention rate of Co3S4-L electrode are still above 90%.
We also measured the HER and OER performance of Co3S4-L in different pH (0.5 M H2SO4, 0.1M KOH, 1M KOH) aqueous solutions with different catalyst dosage (0.42 mg/cm2, 0.84 mg/cm2, 1.26 mg/cm2) (Fig. S6). It can be seen that the rate of HER and OER slightly increased with the increasing of catalyst loading.
3.4. Performance of assembled Water-splitting electrolyzer Based on the aforementioned results, we anticipated that the Co3S4-L could act as a bifunctional electrocatalyst for overall water splitting. Hence, a two-electrode configuration was employed to build a water-splitting electrolyzer with Co3S4-L as the catalyst on both the cathode and anode. As a comparison, a watersplitting cell based on commercial 10% Pt/C catalyst (as cathode) and RuO2 (as anode) was also examined. Fig. 6a shows the polarization curves of the electrolysis test. When the current density of Co3S4-L cell reached 10 mA/cm2 (at 1.63 V), gas evolution was observed at both electrodes. The current onset is 1.45 V for Co3S4-L, which means the corresponding water-splitting electrolyzer can produce H2 and O2 at a very small voltage [33,34]. The current density of Co3S4-L cell can reach 10 mA/cm2 when the voltage runs up to 1.63 V. Subsequently, continuous electrolysis was tested with the applied voltage of 1.63 V for Co3S4-L cell. As shown in Fig. 6b, after 10 h, the water-splitting cell based on Co3S4-L still performed well, with only about 5% of current decay. However, the current density of Pt/C-RuO2 and Co3S4-B cell systems degraded dramatically in the meantime. The detailed performances of several reported bifunctional electrocatalysts are compared with that of Co3S4-L (in supporting information). Table S1 lists the overpotentials (gHER, gOER and goverall is potential) at the current density of 10 mA cm 2 for HER, OER and water electrolyzer, respectively. Compared to the
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Fig. 5. (a) HER polarization curves and (b) the corresponding Tafel plots of Co3S4-L Co3S4-B, and Pt/C tested in 1M KOH with the scan rate of 5 mV/s 95% iR-compensation. (c) OER polarization curves and (d) the corresponding Tafel plots of Co3S4-L Co3S4-B, and RuO2 tested in 1M KOH with the scan rate of 5 mV/s and 95% iR-compensation. (e) Timedependent current density curve of HER tested at the same starting current density for Co3S4-L and Co3S4-B. (f) Time-dependent current density curve of OER tested at the same starting current density for Co3S4-L and Co3S4-B.
existing bifunctional catalysts, the hydrophilic Co3S4-L is superior in overall water splitting.
4. Conclusion In conclusion, a water dispersible, hydrophilic cobalt sulfide (Co3S4-L) was synthesized using hydrophilic solvent and precursors in a practical manner. The synthesized Co3S4-L exhibited 2 D nanosheet morphology with hydrophilic catalytic surface, which facilitates the diffusion of aqueous substrates into the pore of electrodes approach the active sites. Co3S4-L exhibited excellent bifunctional catalytic activities for both OER and HER in alkaline
solution. The current density of HER reached 10 mA/cm2 at 0.27 V (vs.RHE), with an overpotential only about 0.15 V larger than that of Pt/C. The current density of OER reached 10 mA/cm2 at 1.59 V (vs. RHE), with an overpotential only about 0.05 V larger than that of RuO2. The onset potential and goverall (the potential that provides a current density of 10 mA/cm2) of the watersplitting electrolyzer based on Co3S4-L are 1.45 V and 1.63 V, respectively. In addition, the assembled electrolyzer of Co3S4-L also exhibits better stability than that of Pt/C-RuO2 system. These results suggest that Co3S4-L is superior to reported base metal bifunctional catalysts in term of both activity and durability. This work represents the first example of using Co3S4 as catalyst for
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Fig. 6. (a) Polarization curves and (b) Chronoamperometry curves of the assembled water splitting cell based on Co3S4-L (red line), Co3S4-B (blue line) and Pt/CRuO2 (black line) with the scan rate of 5 mV/s in 1M KOH solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
both OER and HER in one electrolyte system. The researches in this paper demonstrate conclusively that the surface hydrophilicity is very important for exploring bifunctional catalyst of overall water splitting. Acknowledgements This work was supported by Natural Science Foundation of Shandong Province (Grant No. ZR2015BM002), Key Research and Development Plan of Shandong Province (Grant No. 2017GGX40119), the Program of the Qingdao Key Lab of Solar Energy Utilization and Energy Storage Technology (Grant No. QDKLSE201602), the Research Program of Qingdao Municipal Science and Technology Commission (Grant No. 16-5-1-44-jch). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2017.09.076. References [1] T. Liu, D. Liu, F. Qu, D. Wang, L. Zhang, R. Ge, S. Hao, Y. Ma, G. Du, M. Arisiri, L. Chen, X. Sun, Adv. Energy Mater. 7 (2017) 1700020. [2] X. Niu, Q. Tang, B. He, P. Yang, Electrochim. Acta 208 (2016) 180–187. [3] C. Tang, R. Zhang, W. Lu, L. He, X. Jiang, M. Asiri, X. Sun, Adv. Mater. 29 (2017) 1602441. [4] T. Pandiarajan, S. Ravichandran, L.J. Berchmans, RSC Adv. 4 (2014) 64364– 64370. [5] T. Wang, X. Wang, Y. Liu, J. Zheng, X. Li, Nano Energy 22 (2016) 111–119. [6] M.R. Gao, J.X. Liang, Y.R. Zheng, Y.F. Xu, J. Jiang, Q. Gao, J. Li, S.H. Yu, Nat. Commun. 6 (2015) 5982. [7] L. Chen, X. Dong, Y. Wang, Y. Xia, Nat. Commun. 7 (2016) 11741. [8] W. Liu, E. Hu, H. Jiang, Y. Xiang, Z. Weng, M. Li, Q. Fan, X. Yu, E.I. Altman, H. Wang, Nat. Commun. 7 (2016) 10771.
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