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Targeted synthesis of unique nickel sulfides (NiS, NiS2) microarchitectures and the applications for the enhanced water splitting system Pan Luo, Huijuan Zhang, Li Liu, Yan Zhang, Ju Deng, Chaohe Xu, Ning Hu, and Yu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13984 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 23, 2016

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ACS Applied Materials & Interfaces

Targeted Synthesis of Unique Nickel Sulfides (NiS, NiS2) Micro-Architectures and the Applications for the Enhanced Water Splitting System Pan Luoa, Huijuan Zhanga, Li Liua, Yan Zhanga, Ju Denga, Chaohe Xub, Ning Hub,Yu Wanga* a The State Key Laboratory of Mechanical Transmissions and School of Chemistry and Chemical Engineering, bCollege of Aerospace Engineering, Chongqing University, Chongqing, China, 400044. E-mail: [email protected]; Abstract Water splitting is one of ideal technologies to meet the ever increasing demands of energy. Many materials have aroused great attention in this field. The family of nickel-based sulfide is one of examples that possess interesting property in water splitting fields. In this paper, a controllable and simple strategy to synthesize nickel sulfides was proposed. Firstly, we fabricated NiS2 hollow microspheres via a hydrothermal process. After a precise heat control in specific atmosphere, NiS porous hollow microspheres were prepared. NiS2 was applied in hydrogen evolution reaction (HER) and shows a marvelous performance both in acid medium (an overpotential of 174 mV to achieve a current density of 10 mA/cm2 and the Tafel slope is only 63 mV/dec) and alkaline medium (an overpotential of 148 mV to afford a current density of 10 mA/cm2 and the Tafel slope is 79 mV/dec). NiS was used in oxygen evolution reaction (OER) showing a low overpotential of 320 mV to deliver a current density of 10 mA/cm2, which is meritorious. These results enlighten us to make an efficient water splitting system, including NiS2 as HER catalyst in cathode and NiS as OER catalyst in anode. The system shows high-activity and good stabilization. Specifically, it displays a stable current density of 10 mA/cm2 with the applying voltage of 1.58 V, which is a considerable electrolyzer for water splitting. Keywords: Nickel sulfides, hollow microspheres, overall synthesized strategy, hydrogen evolution reaction, oxygen evolution reaction, water splitting system Introduction Energy crisis is a hot topic in the current society, which encourages scientists to pour their efforts into energy field. Among the numerous energy techniques (lithium ion battery, wind energy source, etc. ), water splitting is seen as the extremely important one that maybe replaces fossil fuels as renewable and clean energy, since the water resource is earth-abundant and the technology of water splitting becomes mature day by day. As we all know, the electrolysis of water, which includes cathodic HER and anodic OER, remains limited by the high-cost of noble metal materials. Actually, Pt-based and Ru-based materials are typical electrocatalysts applied in HER and OER respectively, demonstrating high activities for water splitting. In recent years, much effort has been devoted to the research of the HER/OER for the water electrolysis1. Transition-metal oxides, metal sulfides and phosphides have been widely reported as alternative candidates for noble metal2. For instance, Cu2S3, MoS24, CoP5, etc., have been intensively studies as HER and OER catalysts. However, there remains much space to explore in this field. Metal sulfide is one class that has unlimited potential in this domain. Among the family of metal sulfides, nickel sulfide is an appealing class which demonstrates low overpotential and decent stability in HER. As previously reported, nickel sulfides have various phases, such as NiS, NiS2, Ni3S2, Ni7S6, and Ni9S86. Wherein, NiS and NiS2 have been

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widely studied for supercapacitor7, lithium ion battery8, dye-sensitized solar cells9-10, hydrodesulfurization catalyst11. NiS2 has two main phases, especially, the cubic phase shows a better catalytic performance than triclinic phase in HER12. Compared with NiS2, NiS is more suitable for OER13. When water splitting is applied in practice, the challenges are that almost all HER work better in acidic medium and most of OER perform well in neutral or alkaline medium. Therefore, we need to coordinate the working medium of water electrolysis. From the current studies, NiS2 can efficiently work as HER catalyst in basic solution14, which inspires us to select NiS2 as HER catalyst in basic solution for the part of water splitting. Much work concentrates on the research of single electrode or bifunctional electrocatalyst. The water splitting is an integral work, so it is not enough to study single electrode in practical water splitting. Besides, the bifunctional electroatalyst may just show a high performance in either anode or cathode15. Accordingly, some different electrocatalysts are assembled for full water splitting, but it is complicated to synthesize the catalyst. There are few reports about designing a water splitting system containing nickel sulfides. Hence, a high-efficient and low-cost water splitting system was put forward in this paper. More importantly, NiS2 and NiS were obtained in an overall strategy and they are rationally considered as cathode and anode for water splitting, respectively. Moreover, the NiS2 and NiS show hollow microsphere morphology, displaying some interesting properties in electrochemical performance. In this system, NiS2 was used as HER catalyst in cathode, while NiS was served as OER catalyst in anode. According to our experience, the morphology and structure of materials have a major impact on electrochemical performance. Hence, we put much attention on the morphology and structure of materials16. Herein, we oriented synthesis of unique nickel sulfide with a facile way. NiS2 hollow microspheres were successfully synthesized by hydrothermal method. In the ensuing section, NiS2 hollow microspheres were annealed under mixed atmosphere. Finally, NiS porous hollow microspheres were obtained. NiS2 and NiS demonstrate a higher activity with such microstructure when applied in water splitting. It maybe stems from its special microstructure which endows a large specific surface area that could provide more reactive sites. At the same time, mesoporous and hollow structures would facilitate diffusion of electrolyte and accelerate the reaction. It is no doubt that this work would take a guide meaning for other applications, such as, photocatalysis and supercapacitor17. Results and discussion As shown in figure 1, NiS2 hollow microspheres were directly prepared by hydrothermal method. And then, NiS2 was placed into tube furnace and calcined under hydrogen and argon mixed atmosphere. After that, the NiS2 was reduced to NiS by H2 and mesoporous structure was generated. Finally, NiS porous hollow microspheres were synthesized. The X-ray diffraction (XRD) patterns of NiS and NiS2 are presented in figure 2. It is clearly shown in figure 2a, all the peaks match with the standard card (JCPDS No.86-2280) of NiS, which indicates that it is pure NiS. When it comes to NiS2, the XRD pattern of the NiS2 hollow microspheres (fig. 2b) can be indexed to cubic NiS2 (JCPDS No.88-1709) implying that NiS2 is highly purity materials. Moreover, the narrow half-peak width suggests the well crystallization of products. To learn the optical properties of the NiS2 and NiS, the Raman spectrum are investigated in room temperature. The Raman spectrum of NiS is shown in figure S1a, and there are five bands, locating at 142.7, 244.6, 293.4, 347.2 and 370.4 cm-1, which are in agreement with previous studies18-19. From the


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Raman spectrum of NiS2 (figure S1b), three peaks are appeared at 269.8, 475.1 and 970.3 cm-1, attributing to Eg, Ag and 2Ag phonons of NiS2, respectively20-21.

Figure 1.Schematic illustration of the synthesis process of NiS2 hollow microspheres and NiS porous hollow microspheres.

Figure 2. The X-ray diffraction patterns of NiS (a) and NiS2 (b). The morphological details of samples are investigated by Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Figure 3a is low-magnification FESEM image of NiS2 hollow microspheres, which shows that NiS2 microspheres are prepared in a large scale. More specifically, the diameter of NiS2 hollow microspheres is about 3 µm (fig. 3b) and the surface of microspheres is rough. The hollow structure could be observed clearly in figure 3c and the thickness of the wall is approximate to 200 nm. The lattice fringes of NiS2 were clearly displayed in High-resolution TEM image (fig. 3d). The lattice distance is 0.286 nm which corresponds to the (2 0 0) crystal plane of NiS2. At the same time, a great number of NiS porous hollow microspheres are demonstrated in the figure 4a, which means NiS porous hollow microspheres have been successfully fabricated. The surface texture of NiS is illustrated in figure 4b, where a continuous ligament-pore structure could be detected easily. What’s more, as shown in figure 4c, the hollow structure inherits from NiS2 perfectly. These pores and channels (insert in fig.4b) existing on the surface of NiS are crucial for the high-efficient catalytic performance. They provide the connection between inner and surface of NiS, which is considered to be the essential ingredients for the diffusion of electrolyte. The



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HRTEM image of NiS shows interlaced lattice fringes (fig. 4d). The lattice distances of 0.295 and 0.476 nm corresponds to the (1 0 1) and (1 1 0) planes of the NiS, respectively. As shown in figure 5, TEM images of NiS2 hollow microspheres (fig. 5a) and NiS porous hollow microsphere (fig. 5b) are presented, which disclose the hollow structure clearly. Besides, the energy-dispersive spectrum (EDS) shows that the atom ratio of Ni:S is about 1.2 in figure S2a while the value is close to 0.5 in figure S2b, which further verifies that the samples are NiS and NiS2 respectively.

Figure 3. The different magnification FESEM images of NiS2 hollow microspheres (a)-(c) and a HRTEM image of NiS2(d).


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Figure 4. The different magnification FESEM images of NiS porous hollow microspheres (a)-(c) and a HRTEM image of NiS(d).

Figure 5. The TEM images of NiS2 (a) and NiS (b). Brunaner Emmet Teller (BET) analysis is introduced. Figure S3b shows a relatively high specific surface of NiS2 (20.8 m2/g) and the pore-size distribution mainly centered at 20 nm (inset



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of fig. S3b). As expected, NiS owns a higher specific area of 52.3 m2/g (fig. S3a) and the pore-size distribution concentrated on 20 nm too (inset fig. S3a), which is favorable for providing numerous adsorption sites and facilitating the penetration of the electrolyte22. The pore volume of NiS is as high as 0.218 cm3/g, which supports the SEM results that the surface of NiS is porous texture. Moreover, the adsorption-desorption isotherms of NiS can be classified as IV type, which further proves that the obtained NiS is mesoporous material23. The HER electrocatalytic performance for NiS2 was measured both in acid and alkaline medium (0.5 M H2SO4 and 1.0 M KOH) with three-electrode system. For comparison, the performance of commercial Pt/C (20 wt %) was carried out in the same set. The linear sweep voltammetry (LSV) curves are obtained (figure S4a), when the samples work in acidic electrolyte. NiS2 requires overpotentials of 89 mV, 174 mV and 283 mV to afford current densities of 1, 10 and 50 mA/cm2, respectively. As anticipated, Pt/C shows much higher HER activity in acidic medium. From the LSV curves, the Pt/C exhibits a negligible onset potential (~11 mV), more specifically, it needs an overpotentials of only 49 mV and 125 mV to reach 10 mA/cm2 and 50 mA/cm2, respectively. The corresponding Tafel plots were presented in figure S4b. The Tafel slope of Pt/C (30 mV/dec) is in agreement with previous studies. While the NiS2 yields a Tafel slope of 63 mV/dec. A comparison with others materials (Table S1) shows that it has not bad HER performance in acidic electrolytes. However, the HER performance was actually tested in alkaline medium when we assembled NiS2 as cathode for practical water splitting system, so we would pay more attention to the catalytic performance in strong alkaline. As shown in figure. 6a, remarkably, NiS2 maintains a considerable performance, which needs an overpotentials of 50 mV, 148 mV and 243 mV to afford current densities of 1, 10 and 50 mA/cm2, respectively. It is worth mentioning that there are few materials demonstrating a better HER property in alkaline solution than acidic solution. In contrast, the Pt/C shows a poor performance in alkaline medium, the overpotentials of Pt/C required to approach current densities at 1, 20, 50 mA/cm2 are 23 mV, 96 mV and 242 mV, respectively. The Tafel slopes (fig. 6b) of NiS2 (82 mV/dec) is almost the same as Pt/C (79 mV/dec). Furthermore, the increasing current density of NiS2 even surpasses Pt/C at the potential below 245 mV. The stability of NiS2 was examined. The polarization curve shows slight change both in acid and alkaline solution, after continuous cyclic voltammetry (CV) for 2000 cycles. The overpotential increases about 6 mV (in acid solution, fig. S4c) and 5 mV (in alkaline solution, Fig. 6c) at the current density of 50 mA/cm2 after 2000 cycles, respectively. The Chronopotentiometric curve was also presented, which suggests a slight increase of ~10 mV at a continuous current density (50 mA/cm2) after 10 hour in acid medium (fig. S4d). And it also shows a negligible change of overpotential at the same current density in alkaline solution (fig. 6d). Particularly, the measurement of double-layer capacitance (Cdl) was carried out to value the electrochemical active surface area24. The Cdl performed by CV test in non-faradaic potential region at different scan rates. Cyclic voltammograms of NiS2 in the voltage windows of -0.1~0.1 V (vs.RHE) are demonstrated (fig. S5a and S5c). It is obvious that the Cdl value in basic solution (6.18 mF/cm2, fig. S5d) is larger than that in acid solution (5.12 mF/cm2, fig. S5b). A table of comparing the HER performance with that of other non-noble metal catalysts in basic electrolytes (Table S2) is presented, which well proves its good performance in basic electrolytes. All the results indicate that NiS2 has a considerable HER performance in alkaline solution compared with most non-noble metal catalysts.


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Figure 6. (a) LSV curves of NiS2 and 20% commercial Pt/C at a scan rate of 5 mV for HER. (b) Tafel plot of NiS2 and Pt/C derived from LSV curve. (c) Stability test for the NiS2 with initial LSV polarization curve and after 2000 cycles. (d) Chronopotentiometric curve of NiS2 at constant current density of 50 mA/cm2. All the measurements were performed in 1.0 M KOH. The OER electrocatalytic performance for NiS was studied with three-electrode system. From the LSV curves of NiS, a small anodic peak (~1.4 V) can be observed (fig. 7a), which points to the Ⅱ Ⅲ oxidation of Ni to Ni .14 Noticeably, the onset potential is as low as 1.42 V that is superior to commercial IrO2 (~1.47 V vs RHE). In addition, the NiS affords a current density of 10, 20 mA/cm2 at an overpotential of 320 mV 390 mV respectively. What’s more, the Tafel slope of NiS is 59 mV/dec (fig. 7b), suggesting the high activity of NiS for OER. To test the stability of NiS for OER, CV test was carried out. There is negligible change of polarization curves after continuous CV scanning for 2000 cycles (fig. 7c). The Cdl value of NiS is 1.49 mF/cm2 (fig. S6b), implying that it has a high active surface area. Chronopotentiometry measurement was tested to study the stability of NiS. Figure. 7d shows the potential increased slightly (~6 mV) at a continuous current density of 10 mA/cm2 after 10 h. From the comparison with others reported (Table S3), it suggests that NiS shows an excellent OER performance. To learn conductivity of the samples, electrochemical impedance spectroscopy (EIS) was performed. As shown in figure S7, the NiS2 shows almost the same charge transfer resistance both in acidic and alkaline electrolyte. Compared with NiS2 hollow microsphere, NiS porous hollow microspheres exhibits smaller charge transfer resistance, implying it has higher electron transfer efficiency25.



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Figure 7. (a) Polarization curve of NiS porous hollow microspheres at a sacn rate of 2 mV. (b) Tafel plot of NiS for OER. (c) Stability measurement of NiS for 2000 CV cycles. (d) Chronopotentiometric curve of NiS at constant current density of 10 mA/cm2. All the measurements were performed in 1.0 M KOH. The above discussion about NiS2 applied in HER and NiS performed in OER has enlightened us to design an overall water splitting system rationally. The water electrolysis performance of this system (NiS2 used as cathode and NiS served as anode) was carried out in a two electrode configuration in 1.0 M KOH. This system which only needs a cell voltage of 1.58 V to deliver 10 mA/cm2, exhibits a remarkable water splitting performance (fig. 8a). What’s more, the cell voltage only increases to 1.69 V to afford 20 mA/cm2. Compared with the previous studies on Ni-based bifunctional catalysts applied in water splitting (NiSe/NF ~1.63 V26, Ni0.85Se ~1.7 V27, Ni(OH)2/NF ~1.82 V28, Ni0.9Fe0.1 ~1.64 V29, NiCo2O4 ~1.65 V30), this system have a better performance in full water splitting. The stability performance was also measured in 1.0 M KOH for 12 hours, this water splitting system demonstrates a slight degradation of applied voltage (~10mV) at the current density of 10 mA/cm2 (fig. 8b). Furthermore, i-t curve (fig. 8c) was recorded at the potential of 1.6 V, indicating a superior stability performance of this system.


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Figure 8. (a) Cyclic voltammograms of water splitting for NiS (anode) || NiS2 (cathode) system with initial CV curve and after 1000 cycles. (b) Chronopotentiometric curve for NiS (anode) || NiS2 (cathode) system at constant current density of 10 mA/cm2. (c) Time-dependent current density curve for NiS (anode) || NiS2 (cathode) system with a static potential of 1.6 V. All the measurements were performed in 1.0 M KOH. In order to investigate the surface state of samples before and after the catalysis, X-ray photoelectron spectroscopy (XPS) surveys were introduced in the present work. The XPS spectra of NiS and NiS2 before the catalytic reaction are shown in Figure 9. The Ni 2p spectrum of NiS (fig. 9a) can be resolved into four peaks. Two peaks are located at 873.6 eV (Ni 2p1/2) and 855.9 eV (Ni 2p3/2)31, indicating the binding energy of Ni2+. Another two peaks (879.8ev and 861.8 eV) are the satellite peaks of NiO, which implies the existence of surface oxidation on NiS32. The Ni 2p spectrum (fig. 9c) of NiS2 also can be divided into four peaks. Accordingly, the peaks at 873.7 eV and 855.8 eV, corresponding to Ni 2p1/2 and Ni 2p3/2 features respectively. The peak at 879.5 eV and 860.9 eV are the satellite peaks of NiO. It is generally accepted that the Ni 2p3/2 is mainly excited by metal character rather sulfur atoms, thus the Ni 2p spectra of NiS and NiS2 are similar. Two distinct peaks are observed in the S 2p (Fig. 9b) spectrum of NiS. The peaks at 162.8 eV and 161.6 eV associated with S 2p1/2 and S 2p3/2 respectively, are consistent with S 2p of NiS in previous studies33. In figure 9d, two strong peaks at 163.8 eV and 162.6 eV are attributed to S 2p1/2 and S 2p3/2 of NiS234.



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Figure 9. XPS survey spectra of the prepared samples before catalytic reaction, (a) Ni 2p and (b) S 2p of NiS. (c) Ni 2p and (d) S 2p of NiS2. The XPS survey after catalytic reaction is also introduced. In the XPS spectra of NiS, two peaks around 873.1 eV and 855.3 eV (fig. 10a) are attributed to the Ni 2p1/2 and Ni 2p3/2 of Ni2+ species respectively. It is notable that the satellite peaks (879.4 eV and 860.6 eV) of NiO become relatively stronger, which implies that the surface oxides may get thicker after OER cycles. Moreover, all the peaks have a slight shift, the production of O2 maybe make some effect on it, and the situation of S 2p is the same, two peaks (162.6 eV and 161.4 eV) are assigned to S 2p1/2 and S 2p3/2 of NiS respectively (fig. 10b)20. When it comes to NiS2, the peaks located at 873.6 eV and 855.7 eV are belong to Ni 2p1/2 and Ni 2p3/2 (fig. 10c), while another two peaks (879.7eV and 861.7 eV) are the satellite peaks. The peak of Ni 2p is merely located at the same position as that of NiS2 before reaction. The S 2p spectrum (fig. 10d) can be deconvoluted into four peaks. The presence of two main peaks (163.5 eV and 162.8 eV) is ascribed to (S2)2- 2p1/2 and (S2)2- 2p3/2, while the other peaks (162.2 eV and 161.5 eV) are corresponds to S2- 2p1/2 and S2- 2p3/2. It means that the production of H2 may partly change the localized surface chemical states. However, the XRD patterns of NiS and NiS2 after reaction are also presented (fig. S8), suggesting that there was not phase change of our samples after catalytic reaction. The SEM images of samples that after the catalytic reaction are shown in figure S9. We couldn’t find the obvious change of morphology, which means the samples can well maintain the microstructure during the water splitting. All the results suggest that the samples possess good stability. These merits are well linked with the unique structure of the samples.


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Figure 10. XPS survey spectra of the samples after catalytic reaction, (a) Ni 2p and (b) S 2p of NiS. (c) Ni 2p and (d) S 2p of NiS2. The formation mechanism of unique hollow structure of NiS2 attracts our great interest. However, two theories, Ostwald ripening35 and Kirkendall Effect36, have been widely known as the formation mechanism of core-shell structure. We proposed that the Ostwald ripening can make hollow microspheres NiS2 possibly11. Ostwald ripening plays an ingredient part in the formation mechanism. A most common understanding of Ostwald ripening is that the smaller crystals have a higher solubility than lager crystals which would make larger crystals bigger while smaller ones come to dissolve37. This phenomenon is due to the smaller crystals have a higher surface energy and it is easier to dissolve. The solid NiS2 microspheres are consisted of many inner crystallites, which tends to dissolve and reorganization. With a long time reaction, some interior voids would form and the microsphere would grow bigger, thus the hollow structure can be obtained. Time-dependent experiments are conducted to investigate the structural evolution process of NiS2 hollow microspheres (fig. S10). The figure S10 clearly shows the formation process of NiS2 hollow microsphere, which grow up step by step and become hollow spheres gradually. Especially, the NiS2 solid spheres are observed at the react time of 7 h while it becoming hollow spheres at the react time of 8 h. These evidences well supported our hypothesis. The formation mechanism of NiS porous hollow microspheres is well explained via the reduction reaction between NiS2 and H2. However, we suppose that some tiny channels may be formed with the growing of NiS2 hollow micrpshperes. During the reaction process, H2 would react with these delicate defects preferentially. Finally, the unique mesoporous structure was prepared. Conclusions



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In summary, NiS2 hollow microspheres and NiS porous hollow microspheres were prepared through a whole strategy with a facile process. The samples with unique structure have a great prospect in different fields. Herein, we elaborately design a water splitting system, using NiS as anode and NiS2 as cathode, which shows an impressive performance of water electrolysis in alkaline medium. It only needs a cell voltage of 1.58 V to afford a current density of 10 mA/cm2, which is better than commercial water electrolyzer (1.8~2.0 V). It is believed that the intriguing microstructure of samples and the ingenious water splitting system have a great contribution to the high performance in water electrolysis.


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Experimental Materials All reagents were analytical grade, and used without further purification. Nickel nitrate (Ni(NO3)2, Aldrich,99.9%), sodium thiosulfate (Na2S2O3, Aldrich), Ethylene glycol (Fisher chemical, 99.99%), sulfuric acid (H2SO4, Aldrich), potassium hydroxide (KOH, Aldrich, 99.9%), pure ethanol (Fisher Chemical, 99.99%), oxalic acid (Aldrich, 99.9%). Preparation of NiS2 hollow microspheres NiS2 hollow microspheres were obtained with the hydrothermal method. 0.2 M Ni(NO3)2 solution (10 ml) was added into the 0.2 M Na2S2O3 aqueous solution (10 ml) to form a mixed solution. In the ensuing section, Ethylene glycol (10 ml) and oxalic acid (3 ml) were slowly added. After strong stirring for 20 min, the mixed solution was transferred into Teflon-lined stainless-steel autoclave (50 ml). And then, it would be heated in an electric oven at 180oC for 12 h. After the reaction, it needed to cool down to room temperature by natural. Finally, the product was washed with deionized water and pure ethanol respectively, and dried in vacuum oven at 60oC overnight. Preparation of NiS porous hollow microspheres The as-prepared samples, was placed into tube furnace and calcined in a mixed-atmosphere (Ar and H2 with a half and half ratio) from room temperature to 250oC with a ramp rate of 3oC min-1. And then, the temperature rise to 310oC with heating rate of 1oC min-1 and keep the temperature for 30 min. After cooling down slowly, NiS porous hollow microspheres were obtained. Characterization The crystalline phase of the synthesized materials was identified by X-ray diffraction (XRD, Bruker D8 Advance). The morphology and structure of the samples were investigated by transmission electron microscopy (TEM, philips, tecnai, F30, 200 kV) and field emission scanning electron microscopy (FESEM, JEOL, JSM-7800F, 5 kV) which equipped with an Oxford INCA x-sight energy dispersive X-ray spectrometer (EDS). X-ray photoelectron spectroscopy (XPS) that equipped with an Al Kα X-ray source (1486.6 eV) was applied to analyze the valence state of products. Brunauer-emmett-teller (BET, Quantachrome Autosorb-6B) was also employed. Raman spectrum was measured by an Ar ion laser at 514 nm excitation wavelength (LabRam HR Evolution Raman microscope). Electrochemical measurements All electrochemical measurements were performed by electrochemical workstation (CHI 760D, shanghai chenhua Instrument) in a standard three-electrode systems at the room temperature. A platinum foil and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively, while our samples were regarded as working electrode. It is worthwhile to mention that NiS and NiS2 (5 mg active materials) were dispersed in a mixed solution (490 µl absolute ethanol and 10 µl Nafion solution) and then 5 µl ink was loaded on glassy carbon electrode as working electrode and a loading amount of 0.7±0.2 mg/cm2. All measured potentials were calibrated to RHE following the equation: E (RHE) =E (SCE) +0.059pH+0.244. Linear sweep or cyclic voltammetry of samples was tested in 1 M KOH electrolyte. For water splitting



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was performed in a two-electrode system. In this system, NiS2 was used as HER catalyst in cathode, while NiS was served as OER catalyst in anode. The potential scan range was from 1.0 to 2.0 V with a scan rate of 2 mV/s. Electrochemical impedance spectroscopy(EIS) measurements were employed at 0.65 V (vs. SCE) with frequency from 100 KHz to 0.01 Hz and an AC voltage of 5 mV. Supporting Information Raman spectra of NiS and NiS2 (figure S1), The EDS spectra of NiS and NiS2 (figure S2), the N2-adsorption-desorption isotherm curve and the corresponding pore-size distribution of NiS and NiS2 (figure S3), HER performance of NiS2 in acid medium (figure S4), the double layer capacitance (Cdl) of NiS2 (acid and alkaline medium, figure S5) and NiS (figure S6), Nyquist plots of NiS2 and NiS (figure S7), The XRD patterns (figure S8) and SEM (figure S9) of samples after catalytic reaction, The FESEM images of the NiS2 (synthesized in different conditions , figure S10), Comparison of NiS2 hollow microspheres HER performance with other reported non-noble metal electrocatalyst in acidic electrolytes (Table S1) and alkaline electrolytes (Table S2), Comparison of OER performance of porous hollow microspheres NiS with other reported non-noble metal electrocatalysts in basic electrolytes (Table S3)


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Acknowledgements This work was financially supported by the Thousand Young Talents program of the Chinese Central Government (Grant no. 0220002102003), National Natural Science Foundation of China (NSFC, Grant no. 21373280 and 21403019), Beijing National Laboratory for Molecular Sciences (BNLMS), the fundamental Research Funds for the Central Universities (106112015CDJZR285519) and Hundred Talents Program at Chongqing University (Grant no.0903005203205).



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