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Energy, Environmental, and Catalysis Applications

Flower-like NiCo2S4 Hollow Sub-microspheres with Mesoporous Nanoshells Support Pd Nanoparticles for Enhanced Hydrogen Evolution Reaction Electrocatalysis in Both Acidic and Alkaline Conditions Guoqing Sheng, Jiahui Chen, Yunming Li, Huangqing Ye, Zhixiong Hu, Xian-Zhu Fu, Rong Sun, Weixin Huang, and Ching-Ping Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05427 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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ACS Applied Materials & Interfaces

Flower-like NiCo2S4 Hollow Sub-microspheres with Mesoporous Nanoshells Support Pd Nanoparticles for Enhanced Hydrogen Evolution Reaction Electrocatalysis in Both Acidic and Alkaline Conditions Guoqing Sheng,a,b Jiahui Chen,a Yunming Li,a Huangqing Ye,a Zhixiong Hu,a,c Xian-Zhu Fu,a,c,* Rong Sun,a,* Weixin Huang,d Ching-Ping Wonge, f a

Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055,

China b

Nano Science and Technology Institute, University of Science and Technology of China, Suzhou

215123, China c

College of Materials Science and Engineering, Shenzhen University, Shenzhen 518055, China

d

Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of

Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China e

Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong,

China f

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA

30332, United States

1

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ABSTRACT： Flower-like NiCo2S4 hollow sub-microspheres are synthesized through Cu2O templates to support Pd nanoparticles as high-efficiency catalysts for HER. The diameter and shells size of NiCo2S4 hollow sub-microspheres are about 400 nm and 16 nm, respectively. In addition, the surface of shells is constructed by petal-like nanosheets. About 3 nm Pd particles uniformly incorporate with the flower-like NiCo2S4 hollow sub-microsphere to form NiCo2S4/Pd heterostructure. The NiCo2S4/Pd catalysts exhibit significantly lower overpotential of only 87 mV and 83 mV at 10 mA/cm2 for HER in both acidic and alkaline conditions, respectively, relative to NiCo2S4 (247 mV, 226 mV) and Pd (175 mV, 385mV) catalysts. Besides, the NiCo2S4/Pd catalysts also exhibit excellent stability of HER in these two conditions. The superior HER performance of NiCo2S4/Pd might be resulted from the unique architecture of metal nanoparticles anchored on the bimetallic sulfides flower-like hollow sub-microspheres which could provide high surface area, lots of active sites, strong synergetic effect and stable structure.

KEYWORDS: hollow structure, electrocatalysts, hydrogen evolution reaction, metal chalcogenides, palladium, synergetic effect

1. Introduction The constant depletion of non-renewable energy urge us to seek green and efficient energy, such as biomass energy, nuclear energy, and hydrogen energy and so on.1-2 Hydrogen energy will become the most potential renewable and pollution-free energy in place of tradition non-renewable energy.3-4 Therefore, many researchers spared no effort to explore a convenient and effective way for producing hydrogen.5-7 Among the various reliable strategies, electrolysis of 2


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water is regarded as an attractive and green device to generate hydrogen.8-10 However, the electrocatalytic performance and stability of HER in cathode is significantly determined by catalysts, due to that it can significantly decrease the reactive energy barrier. To date, platinum (Pt) is assumed to be the highest performance catalyst for HER, but high-cost restrict its extensive applications.11-13 Therefore, exploring and developing novel low-cost and well performance catalysts is a vital task for HER. An ideal catalyst is also supposed to possess well performance in multi-pH conditions, just like the Pt catalyst.3, 14-15 Recently, many low-cost transition metal based catalysts were studied as substitutes, such as metal16, metal alloy17, oxides18-21, nitrides22, carbides23-25, sulfides26-31, selenides32-33, and borides34. Nonetheless, most of them exhibit well HER performance only in acidic or alkaline conditions, which apparently restrict their actual applications.35 Noticeably, NiCo2S4 catalysts possess excellent electrical conductivity, relatively low band-gap energy, and structural stability in both acidic and alkaline conditions.36 But the electrocatalytic activity of NiCo2S4 catalysts still need to be improved. So we were devoted to exploit inexpensive, highly performance, and highly stability NiCo2S4 catalysts for HER in these two conditions. Herein, we fabricate Pd nanoparticles modified flower-like NiCo2S4 hollow sub-microspheres with mesoporous nanoshells (NiCo2S4/Pd) as HER catalysts. The NiCo2S4 and Pd catalysts both show a certain HER electrocatalytic performance. After loading Pd nanoparticles to form NiCo2S4/Pd, the HER electrocatalytic performance shows an extensive improvement, which reveals remarkably lower overpotential of only 87 mV and 83 mV in both acidic and alkaline aqueous solutions, respectively. Besides, the NiCo2S4/Pd catalysts also demonstrate excellent stability for HER in these two conditions. 3



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2. Experimental Section 2.1 Materials Cupric acetate monohydrate (Cu(CH3COO)2•H2O), glucose (C6H12O6), polyvinylpyrrolidone (PVP, K-30), cobalt chloride hexahydrate (CoCl2•6H2O), nickel chloride hexahydrate (NiCl2•6H2O), and N,N-dimethylformamide (DMF) were bought from Sinopharm Chemical Reagent Co., Ltd. Sodium tetrachloropalladate (Na2PdCl4) and other reagents were bought from Aladdin.

2.2 Synthesis of monodisperse Cu2O sub-microspheres Monodisperse Cu2O sub-microspheres were synthesized according to the previous report.37 The successful preparation of monodisperse Cu2O sub-microspheres were strongly determined by the amount of PVP, the reaction time and temperature. Briefly, Cu(CH3COO)2·H2O (1.60 g), glucose (1.5232 g) and PVP (K-30, 0.66 g) were added in DMF solvent(120 mL) under vigorous stirring for about 1 h. Subsequently, the mixture were kept stirring for 7 min to form orange-yellow solution when the temperature was heated to 80 oC. The products were collected by centrifugation, washed several times with ethanol and water.

2.3

Synthesis

of

flower-like

Ni-Co

hydroxides

(NiCo2(OH)x)

hollow

sub-microspheres In a typical procedure, Cu2O (50 mg), NiCl2·6H2O (16.7 mg), CoCl2·6H2O (33.3 mg) and PVP (K-30, 830 mg) were added in the mixed solvent of water and ethanol (50 mL, v/v 1:1) under stirring for about 10 min, and then Na2S2O3 solution (20 mL, 1.0 M) was dripped into these mixed solvent. After reaction for 1 h, the color of solvent changed from orange-yellow to light green, demonstrating the successful formation the NiCo2(OH)x hollow sub-microspheres. Finally, the 4


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products were centrifuged, washed 3 times with water and ethanol, and finally redispersed in 30 mL deionized water.

2.4 Synthesis of flower-like NiCo2S4 hollow sub-microspheres To obtain NiCo2S4, Na2S·9H2O (0.28 g) was added into 30 mL of NiCo2(OH)x aqueous solution under ultrasonication for about 10 min. Then, the mixture was moved to Teflon-lined stainless-steel 50 mL autoclave, and then the temperature of reaction was heated to 160 oC for keeping 8 h. After the reaction, the product was collected by centrifugation, washed several times with water and ethanol.

2.5 Synthesis of flower-like NiCo2S4 /Pd hollow sub-microspheres The synthesis of NiCo2S4/Pd catalysts were carried out by simply mixing the above NiCo2S4 into a Na2PdCl4 (2.5 mM, 20mL) aqueous solution at 50 oC for 30 min. The Na2PdCl4 could react with trace amounts of the unreacted NiCo2(OH)x to form Pd nanoparticles. After that, the products were collected, washed and then dried for further tests. For comparison, Pd was prepared through that NiCo2(OH)x reacted with Na2PdCl4 to form the NiCo2(OH)x/Pd in the same conditions and then using acid etch it.

2.5 Material characterization. Scanning electron microscopy (FEI Nova Nano SEM 450) and transmission electron microscopy (FEI Tecnai G2 F30) were conducted to observe the morphologies of as-obtained products. The compositions of the samples were analyzed by Rigaku, D/max 2500 PC (XRD, Cu Kα radiation). The chemical state of near-surface elements of samples was collected by PHI-1800 X-ray photoelectron spectroscopy. Micromeritics ASAP 2020 BET apparatus was operated to obtain the suface area and pore size distribution of samples. The electrocatalytic performance tests were 5



characterized by an CHI 440C electrochemical workstation.

2.6 Electrochemical tests. For the electrochemical tests, as-synthesized catalyst (4 mg) was dissolved in 1.0 mL ethanol, and then 15 µL of catalyst solution was added on the glassy carbon electrode (GCE, diameter: 5 mm, area: 0.196 cm2) and dried at 60 oC. Subsequently, 15 µL of Nafion ethanol solution (0.1 wt%) was also added on the GCE. The HER performances were studied in a typical three-electrode system using a graphite rod and Ag/AgCl electrode as counter electrode and reference electrode, respectively. All the studies were analyzed in 0.5 M H2SO4 aqueous electrolyte or 1.0 M KOH aqueous electrolyte by LSV at 5 mV s-1. The recorded potentials were converted to E(RHE) from E(Ag/AgCl) using the equation: E(RHE) = E(Ag/AgCl) + 0.197 + 0.0592 × pH. All curves were not corrected with iR-compensation. The electrochemical active surface area (ECSA) of catalysts was analyzed by measurement of the capacitive currents in a potential range where can’t observe obvious Faradaic effect. We sweep the potential between 0 -0.1 V and 0.1-0.2 V at each of six different scan rates in 0.5 M H2SO4 and 1.0 KOH, respectively. The changes of current density (∆j = ja - jc) in the middle of potential against each scan rate are fitted to analyze the double layer capacitance (Cdl), and then calculate the ECSA from the following equation:3 ECSA = Cdl (mF cm-2) /[0.04 (mF cm-2) per cm2]. Calculation of Turnover Frequency (TOF): TOF = j×S/(2×n×F), where j is the current density at specific overpotential; S is the area of GCE (0.196 cm2); 2 refers to the two electron involved HER that generates 1 mol H2; n is the moles of active metal sites of the catalysts; F is the Faradic constant (96485 C mol-1).38 6


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3. Results and Discussion 3.1

Synthesis

and

characterization

of

flower-like

NiCo2S4/Pd

hollow

sub-microspheres Scheme 1 shows the synthesis strategy of flower-like NiCo2S4/Pd hollow sub-microspheres. Uniform monodisperse Cu2O sub-microspheres, as the templates, are prepared through reduction of Cu2+ by glucose in solution. Then flower-like Ni-Co double hydroxides hollow sub-microspheres (NiCo2(OH)x) are gradually obtained while Cu2O are etched at room temperature following “coordinating etching and precipitating” step (Scheme 1 (i)).39 Afterwards, NiCo2(OH)x are converted into NiCo2S4 by sulfuration (Scheme 1 (ii)).29 After sulfuration, there is little NiCo2(OH)x residual, due to incomplete sulfuration.8, 40-41 Finally, as-synthesized flower-like NiCo2S4 hollow sub-microspheres are dispersed in Na2PdCl4 solution, and the unreacted NiCo2(OH)x react with Na2PdCl4 to form heterostructure of Pd nanoparticles on the NiCo2S4 surface (Scheme 1 (iii)).42

Scheme 1. Schematic diagram of the synthesis of flower-like NiCo2S4 hollow sub-microspheres 7



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support Pd nanoparticles heterostructures. The synthesis steps involve (i) coordinating etching and precipitating, (ii) sulfuration, and (iii) Pd loading.

The morphologies and structures of as-synthesized materials are observed by SEM and TEM. The SEM images in Figure 1a shows the uniform monodisperse spherical Cu2O with an average size of about 400 nm, and Figure 1b reveals the rough surface of Cu2O. The NiCo2(OH)x exhibit a flower-like sub-microspheres structure which inherits the sizes and geometrics of the Cu2O templates (Figure 1c, d). The interior and architectural construction of the NiCo2(OH)x are further studied by TEM. From Figure S1, the hollow structure is clearly observed, and the petal-like nanoflakes are also revealed on the shell of the NiCo2(OH)x. After sulfuration treatment of

the

corresponding

NiCo2(OH)x

sub-microspheres,

hollow

flower-like

NiCo2S4

sub-microspheres with the shell thickness of about 16 nm are readily obtained, which inherit the similar features of the NiCo2(OH)x. But nanosheets change rougher and shorter after sulfuration (Figure 1e, f and S2a-c). From HR-TEM image (Figure S2d), the plane spacing of 0.33 nm belongs to the (220) plane of NiCo2S4. From Figure S4a, the Ni : Co : S molar ratio is 13.73 : 24.24 : 62.03. Subsequently, loading Pd on the surface of NiCo2S4 is performed through the unreacted NiCo2(OH)x react with Na2PdCl4. The morphologies and features of NiCo2S4/Pd inherit the hollow flower-like structure of NiCo2S4 (Figure 1g, h). The hollow structure and detailed parts of NiCo2S4/Pd are also further observed by TEM in Figure 2 and S3. In Figure 2a and b, the sub-microspheres possess obvious hollow cavity and petal-like nanosheets are also retained on the surface (inset), and nanoflakes change rougher and shorter which attribute to the anion exchange reaction43-45, relative to NiCo2(OH)x (Figure S2). However, the surface changes rougher, which 8


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would be beneficial to increase specific surface area and close contact with electrolyte. Figure 2c and d show the surface details of NiCo2S4/Pd, in which plane spacing of 0.271 nm and 0.225 nm match well with the (222) and (111) planes of NiCo2S4 and Pd, respectively. The anchored Pd nanoparticles are about 3 nm, which are tightly supported on the nanoshells of NiCo2S4 hollow sub-microspheres. In addition, Figure 2e shows the elemental mapping images of NiCo2S4/Pd, in which Ni, Co, S, and Pd element are uniformly distribution throughout the sub-microspheres. Besides, the elemental line-scanning profile is further analyzed in Figure 2f, proving that the hollow cavity is existence in the interior of catalyst and the Pd uniformly distributes through the sub-microspheres which is well in consistent with the elemental mapping results. And the molar ratio of Ni : Co : S : Pd is 11.29 : 21.38 : 61.37 : 5.96 (Figure S4b).

Figure 1. SEM images of (a, b) Cu2O nanospheres, (c, d) NiCo2(OH)x, (e, f) NiCo2S4, and (g, h) NiCo2S4/Pd.

9



Figure 2. (a, b) TEM images of NiCo2S4/Pd. (c, d) HR-TEM image of NiCo2S4/Pd. (e) Elemental mapping images of NiCo2S4/Pd. (f) Line-scanning profile of NiCo2S4/Pd.

The compositions of the as-obtained materials are analyzed by XRD. The Cu2O sample shows strong diffraction peaks which belong to the cubic structure of Cu2O in Figure 3a (JCPDS 05-0667). The NiCo2(OH)x do not show obvious diffraction peaks, indicating the amorphous feature (Figure 3a). After sulfuration, the peaks at 26.8°, 31.6°, 38.3°, 50.5°and 55.3° could be attributed to the (220), (311), (400), (511) and (440) planes of cubic phase of NiCo2S4 (JCPDS 20-0782). The weak and wide peaks show the poor crystallinity and small size of NiCo2S4 (Figure 3b). Subsequently, after loading Pd nanoparticles, the weak (111) diffraction peak of Pd is observed in area marked by a black box in Figure 3b, demonstrating that it may be the formation 10


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of metallic Pd nanoparticles. The results can be further confirmed by XPS.

Figure 3. XRD patterns of (a) Cu2O and NiCo2(OH)x, (b) hollow flower-like NiCo2S4 and NiCo2S4/Pd nanospheres.

To further study the surface elemental state of the as-obtained NiCo2S4 and NiCo2S4/Pd samples, XPS analysis are employed as shown in Figure 4 and Figure S5. The survey XPS spectra of the NiCo2S4 and NiCo2S4/Pd (Figure S5) reveal the existence of Ni, Co, S and Pd elements. The C (as reference) and O elements are derived from the oxidation on the surface of both NiCo2S4 and Pd, due to exposure to air.36,

46-47

The XPS spectrum (Figure 4a) of Co 2p shows two

spin-orbit doublets, attributing to Co3+ and Co2+, and two shake-up satellites (Sat.).40 The integrated area of Co3+ peaks is larger than that of Co2+ peaks, demonstrating that the majority of cobalt cation is Co3+. In regard to the Ni 2p spectrum (Figure 4b), it also contains two spin-orbit doublets and two Sat., which show the presence of Ni2+ and Ni3+.36 What’s more, the majority of nickel cation is Ni2+ from the integrated area of fitted peaks. The S 2p spectrum (Figure 4c) could be fitted with two main peaks of S 2p1/2 and 2p3/2, corresponding to typical metal-sulfur bonds, and two Sat.. The S 2p peak at about 162.5 eV is corresponding to the sulfur ion in low 11



coordination at the surface.48 As for Pd 3d spectrum (Figure 4d), it is composed of Pd 3d3/2 and 3d5/2 peaks, which represent the metallic state Pd and PdO.47 By the comparison of the relative areas, the metallic state Pd (91.7%) is absolutely main in NiCo2S4/Pd.49 Combined with XRD results, it proves that metallic Pd nanoparticles have formed on the surface of NiCo2S4. In addition, after loading Pd, the Co 2p, Ni 2p and S 2p in NiCo2S4/Pd show a 0.7 eV, 0.2 eV, and 0.4 eV negative shift compared with those in NiCo2S4, respectively, confirming the electron transfer between Co, Ni, S with Pd atoms owing to the electronegative difference and thus further changing the charge density around the corresponding atoms.50 According to the XPS analysis, it proves the existence of Co2+, Co3+, Ni2+, Ni3+, S2- ,Pd and Pd2+, which is in consistent with the NiCo2S4/Pd.48, 51

Figure 4. The XPS spectra of (a) Co 2p orbital, (b) Ni 2p orbital, (c) S 2p orbital and (d) Pd 3d orbital of NiCo2S4, NiCo2S4/Pd catalysts. 12


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The specific surface area and pore size distribution of NiCo2S4 and NiCo2S4/Pd are analyzed in Figure 5. The isothermals of these two catalysts are a typical Ⅳ type with an obvious hysteresis loop in the range of 0.5-1.0 (p/p0).52 The BET specific surface area of NiCo2S4 and NiCo2S4/Pd is calculated to be 24.39 and 25.34 m2/g, respectively (Figure 5a). BJH results show a narrow pore size distribution (2-15 nm) centered at about 4 nm in both two catalysts, indicating a typical mesoporous structure and benefiting the diffusion of active species in catalytic materials. Also, the BET and BJH results demonstrate that the procedure of loading Pd does not change the morphology and structure of NiCo2S4. Moreover, the hollow and mesoporous structures not only efficient improve the contact area of active materials and electrolyte, but also provide lots of active sites. Therefore, hollow flower-like NiCo2S4/Pd electrocatalyst would exhibit huge promising potential for HER.

Figure 5. (a) N2 adsorption and desorption isothermal, and (b) BJH pore size distribution of the flower-like NiCo2S4 and NiCo2S4/Pd hollow sub-microspheres.

13



3.2. Electrochemical tests The HER performance of as-prepared NiCo2S4/Pd electrocatalyst was studied through a typical three-electrode system tests in 0.5 M H2SO4 and 1.0 M KOH solution at room temperature, respectively. For comparison, NiCo2S4, NiCo2(OH)x, Pd catalysts and bared glass carbon are also tested. Acidic-effective catalysts have made considerable progress in the past few years, since the acidic condition is more beneficial to the generation of H2.3 To investigate the HER performance of as-prepared catalysts in acidic condition, the polarization curves are obtained by LSV at 5 mV/s in 0.5 M H2SO4 electrolyte. In Figure 6a, c, the NiCo2S4 and Pd show a relatively large overpotential (ŋ10) of 247 mV and 175 mV at the current density of 10 mA/cm2, respectively, indicating weak HER electrocatalytic performance. Nevertheless, NiCo2S4/Pd catalysts show an excellent performance with an overpotential of only 83 mV, far below those of NiCo2S4 and Pd catalysts. The results suggest the strong synergetic effect between NiCo2S4 and Pd of the NiCo2S4/Pd catalysts for HER electrolysis in acidic condition. To further reveal the electron-transfer kinetics, Tafel plots are analyzed in Figure 6b and d. The NiCo2S4/Pd catalysts possess the Tafel slope of 70 mV/dec, far smaller than those of NiCo2S4 (146 mV/dec) and Pd (89 mV/dec) and also suggests that the Volmer-Heyrovsky mechanism plays a leading role in HER.3

14


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Figure 6. HER performances characterization in 0.5 M H2SO4. (a) LSV curves of as-prepared NiCo2S4/Pd, NiCo2S4, Pd and glass carbon (GC). (b) Tafel plots of the corresponding catalysts. (c) Overpotential of as-prepared catalysts at 10 mA/cm2. (d) The corresponding Tafel slops.

The HER performance is also estimated in alkaline condition, owing to that alkaline water splitting is the most commonly used methods in the industry. However, only few catalysts possess excellent HER performance in both acidic and alkaline conditions.

53-54

In addition, the

performance in alkaline condition is often about 2-3 orders of magnitude smaller than those in acidic condition.55 Therefore, to explore the HER performance of catalysts in alkaline at the same time, all the same tests mentioned above are executed in 1.0 M KOH solution. In Figure 7a, c, the NiCo2S4/Pd catalysts show well HER electrocatalytic performance with the overpotential of 83 mV, close to that in acidic condition, far less than those of NiCo2S4 (226 mV), Pd (385 mV), and NiCo2(OH)x (459 mV). The results suggest the strong synergetic effect between NiCo2S4 and 15



Pd of the NiCo2S4/Pd catalysts for HER electrolysis also in alkaline condition. The Tafel slope of NiCo2S4/Pd is 123 mV/dec, smaller than those of NiCo2S4 (166 mV/dec), Pd (219 mV/dec), and NiCo2(OH)x (159 mV/dec), suggesting that the Volmer reaction step is probably the rate-limiting step.7

Figure 7. HER performances characterization in 1.0 M KOH. (a) LSV curves of as-prepared NiCo2S4/Pd, NiCo2S4, NiCo2(OH)x, Pd and glass carbon (GC). (b) The corresponding Tafel plots of catalysts. (c) Overpotential of as-prepared catalysts at 10 mA/cm2. (d) The corresponding Tafel slops.

To further study the durability of the NiCo2S4/Pd, we conduct the stability test for HER (Figure 8 inset), only a slight decrease is observed during continuous 10 hours at a constant 10 mA/cm2, respectively. Furthermore, the LSV curve of NiCo2S4/Pd before and after stability test for HER is also investigated. As shown in Figure 8, the performance of NiCo2S4/Pd exhibits 16


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nearly no decay and a slight loss in acidic and alkaline conditions after stability test, respectively. Figure S6 shows that the morphology and structure of NiCo2S4/Pd is basically preserved after stability tests with some disintegration in these two conditions. Such structure degradation of the materials may be a cause of the slight performance loss. These results demonstrate that flower– like NiCo2S4/Pd hollow sub-microspheres catalysts reveal not only excellent HER electrocatalytic activity but also excellent stability in both acidic and alkaline conditions.

Figure 8. HER polarization curves of NiCo2S4/Pd before and after stability test in (a) 0.5 M H2SO4 solution and (b) 1.0 M KOH solution. Chronoamperometry u-t curve of NiCo2S4/Pd at a constant 10 mA/cm2 in (a inset) 0.5 M H2SO4 solution and (b inset) 1.0 M KOH solution.

To further study the intrinsic performances of as-prepared catalysts, electrochemical impedance spectra (EIS) and the electrocatalytic active surface areas (ECSA) are analyzed. From Figure S7, the NiCo2S4/Pd catalysts show the smallest Rct value, indicating the lowest electron and charge transfer resistance and thus faster reaction rates.56 In addition, the double layer capacitances (Cdl) is measured by CV curves with no obvious Faradaic effect in 0.5 M H2SO4 and 1.0 M KOH, respectively, and then used Cdl to evaluate the ECSA.8 The NiCo2S4/Pd catalysts 17



exhibit the largest Cdl (Figure S8 and Figure S9), implying the highest ECSA (177.5 cm2 in

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0.5

M H2SO4, 120.6 cm2 in 1.0 M KOH). These results reveal that this unique structure can improve the HER electrocatalysis, due to enhancing the intrinsic performances of its active sites. The static contact angle method is used to estimate the wettability of NiCo2S4/Pd catalysts by a water droplet (50 µL) in Figure S10. Before and after coating the catalysts, contact angle has changed in substrate, due to their different surface roughness. As expected, NiCo2S4/Pd catalysts exhibit contact angle of about 9.3±1o, which is much smaller than that of glass (35.6±1 o), indicating the extensive hydrophilic nature of the NiCo2S4/Pd catalysts. The contact angle of a water droplet is affected by material surface roughness, indicating the different wettability features. 48

The results reveal that NiCo2S4/Pd catalysts possess rough surface and thus benefits to

strengthen the close contact with aqueous electrolyte, thereby enhancing the HER electrolysis performance. To better evaluate the intrinsic performances of as-prepared catalysts, turnover frequency (TOF) is analyzed, which all the metal atoms are assumed to be catalytic active.38 In the overpotential of 100 mV, the TOF of NiCo2S4/Pd is calculated to be about 7.68×10-2 s-1, which is higher than those of NiCo2S4 (1.24×10-2 s-1), and Pd (5.86×10-3 s-1) in acidic condition, demonstrating that the intrinsic performances of NiCo2S4/Pd catalysts are higher than that of NiCo2S4 and Pd single component (Table 1). In addition, in alkaline condition, NiCo2S4/Pd catalysts also show the highest TOF (6.54×10-2 s-1) than those of NiCo2S4, Pd, and NiCo2(OH)x (Table 2). Besides, similar trend of TOF can also be found at overpotential of 150, 200 mV in both acidic and alkaline conditions. These results confirm that the NiCo2S4/Pd catalysts provide higher the intrinsic HER performances and effectively promote reactions at a given time, which is 18


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consistent with the results of EIS and ECSA.

Table 1. TOF of NiCo2S4/Pd, NiCo2S4, and Pd at Overpotential of 100, 150, and 200 mV in 0.5 M H2SO4 solution. TOF s-1 [mV]

NiCo2S4/Pd

NiCo2S4

Pd

100

7.68 × 10-2

1.24 × 10-2

5.86 × 10-3

150

2.82 × 10-1

1.72 × 10-2

2.58 × 10-2

200

5.68 × 10-1

2.66 × 10-2

6.36 × 10-2

Table 2. TOF of NiCo2S4/Pd, NiCo2S4, NiCo2(OH)x, and Pd at Overpotential of 100, 150, and 200 mV in 1.0 M KOH solution. TOF s-1 [mV]

NiCo2S4/Pd

NiCo2S4

NiCo2(OH)x

Pd

100

6.54 × 10-2

1.39 × 10-2

9.62 × 10-3

2.84 × 10-3

150

1.42 × 10-1

1.92 × 10-2

1.01 × 10-2

3.30 × 10-3

200

2.70 × 10-1

3.62 × 10-2

1.03 × 10-2

3.90 × 10-2

The excellent HER electrocatalytic performances of NiCo2S4/Pd might be ascribed to the unique flower-like hollow heterostructure, which possess high exposure of active sites and well synergetic effect between NiCo2S4 and Pd. Also, the architecture of hollow sub-microshperes with flower-like mesoporous nanoshells is well beneficial to the release of the generated H2 and ions transportion during HER. In addition, loading Pd nanoparticles on the surface of NiCo2S4 might also promote electron and charge transfer to further accelerate the reaction rate and enhance the electrocatalytic performance of HER.

19



4. Conclusions The NiCo2S4/Pd catalysts are successfully synthesized through Cu2O sub-microsphere template. The Pd nanoparticles are supported on the flower-like hollow sub-microspheres with mesoporous nanoshells. Compared to the NiCo2S4 and Pd single component, the NiCo2S4/Pd catalysts show much higher electrocatalytic performance and good stability for HER in both acidic and alkaline conditions. The unique architecture of flower-like NiCo2S4 hollow sub-microspheres with mesoporous nanoshells supporting Pd nanoparticles could improve the exposure of active sites and form synergetic effect, thus enhancing the HER electrocatalytic performance.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge via the Internet at http://pubs.acs.org. TEM images of NiCo2(OH)x, NiCo2S4, and NiCo2S4/Pd. EDS analysis of NiCo2S4 and NiCo2S4/Pd. XPS survey spectra of NiCo2S4 and NiCo2S4/Pd. SEM images of NiCo2S4/Pd after stability test. Nyquist plots of NiCo2S4/Pd, NiCo2S4, and Pd. CV curves of NiCo2S4/Pd and NiCo2S4. Contact angle images of NiCo2S4/Pd on glass.

AUTHOR INFORMATION Corresponding authors *E-mail address: [email protected]; *E-mail address: [email protected].

ORCID Xian-Zhu Fu: 0000-0003-1843-8927 20


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Rong Sun: 0000-0001-9719-3563

Notes The authors declare no competing financial interest.

Acknowledgements Financial support from the National Natural Science Foundation of China (No.21203236), Guangdong Department of Science and Technology (2017A050501052), and Shenzhen research plan (JCYJ20160229195455154) is gratefully acknowledged.

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