integrated ni p s nanosheets array as superior electrocatalysts for hydrogen generation

Li, Integrated Ni-P-S nanosheets array as superior electrocatalysts for hydrogen generation, Green Energy & Environment 2017, doi: 10.1016/... Here, we present the synthesis of integrate

Integrated Ni-P-S nanosheets array as superior electrocatalysts for hydrogen

To appear in: Green Energy and Environment

Received Date: 23 November 2016

Revised Date: 22 December 2016

Accepted Date: 27 December 2016

Please cite this article as: H Zhang, H Jiang, Y Hu, H Jiang, C Li, Integrated Ni-P-S nanosheets array

as superior electrocatalysts for hydrogen generation, Green Energy & Environment (2017), doi: 10.1016/

Haoxuan Zhang, Haibo Jiang, Yanjie Hu, Hao Jiang*, Chunzhong Li*

Key Laboratory for Ultrafine Materials of Ministry of Education & School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China

Email: jianghao@ecust.edu.cn (Prof H Jiang) and czli@ecust.edu.cn (Prof C Z Li)

Abstract

Searching for efficient and robust non-noble electrocatalysts for hydrogen generation is extremely desirable for future green energy systems Here, we present the synthesis of integrated Ni-P-S nanosheets array including Ni2P and NiS on nickel foam by a simple simultaneous phosphorization and sulfurization strategy The resultant sample with optimal composition exhibits superior electrocatalytic performance for hydrogen evolution reaction (HER) in a wide pH range In alkaline media, it can generate current densities of 10, 20 and 100 mA cm-2 at low overpotentials of only -101.9, -142.0 and -207.8 mV with robust durability It still exhibits high electrocatalytic activities even in acid or neutral media Such superior electrocatalytic performances can be mainly attributed to the synergistic enhancement of the hybrid Ni-P-S nanosheets array with integration microstructure The kind of catalyst gives a new insight on achieving efficient and robust hydrogen generation

Keywords: Nanosheets array, nickel phosphide, nickel sulfide, overpotential, hydrogen

generation

is severely hindered by the scarcity and expensiveness In this case, it is entirely preferable to develop efficient and low-cost earth-abundant electrocatalysts, such as

transition metal chalcogenides and oxides, etc [5-9] It is noted that nickel phosphide

(Ni2P) is a representative hydrodesulfurization (HDS) catalyst, which is also widely served as an excellent HER catalyst in acid media because of their similar catalytic reaction mechanism [10-12] Nevertheless, in most reported works, the pure Ni2P catalyst shows limited exposure of active sites in alkaline media, resulting in unsatisfactory electrocatalytic activity [13-15] Although hybridizing Ni2P with other compounds has been considered as an effective strategy to improve it, it is still a challenge to achieve efficient and robust Ni2P-based electrocatalysts in acid or neutral

or alkaline media [16, 17]

It is reported that metal chalcogenides possess a high activity in alkaline media because of their layered structure with rich edge active sites For example, Feng et al [18] developed an effective approach to improve the sluggish HER kinetics of the MoS2

electrocatalysts by engineering the edge sites, which accelerates water dissociation with remarkably enhanced hydrogen generation in alkaline Xie et al [19] reported a CoSe2phase-transformation engineering to realize an enhanced electrocatalytic activity due to the

ideal water adsorption energy of c-CoSe2 Recently, it is also found that the synergistic effects

of different chalcogenides can further improve the electrocatalytic activity with rapid charge

transfer, e.g MoS2-coated CoSe2 [20], MoS2/Ni3S2 heterostructures and Cu7S4/MoS2 ultrasmall nanohybrids [21, 22] Therefore, combining the advantages of metal chalcogenides in alkaline, the rational hybridization with Ni2P will be a promising strategy

to achieve highly efficient HER performance in a wide pH range

Herein, we demonstrate a hybrid Ni-P-S nanosheets array with integration structure on Ni foam by a simultaneous phosphorization and sulfurization treatment, which exhibits superior electrocatalytic performance for hydrogen generation in a wide pH range

(Figure 1) As we predicted, the optimized Ni-P-S nanosheets array generates cathodic

current densities of 10, 20 and 100 mA cm-2 at low overpotentials of only -101.9, -142.0 and -207.8 mV with robust durability for 16 h in alkaline media Meanwhile, the nanosheets array can also possess great catalytic activities under both acid and neutral conditions Such excellent performances primarily benefit from the unique structure of the hybrid Ni-P-S nanosheets array with synergistic enhancement interaction The present catalyst design idea gives a new pathway for achieving highly efficient and stable hydrogen generation

2 Experimental section

2.1 Synthesis of Ni-P-S nanosheets array

Prior to further dealing, a piece of Ni foam was soaked in 3 M HCl for 20 min to remove oxide layer on the surface, then rinsed with de-ionized water and absolute ethanol for three times, and finally dried at 45 °C for 1 h In a typical synthesis, a

ceramic boat loaded with the above Ni foam (0.5*1 cm2) was placed in the downside

of a tube furnace, and meanwhile another boat loaded with 1.0 g of S powder was below it Subsequently, 1.5 g of NaH2PO2· H2O was placed 10 cm away from Ni foam

in the upside The mass ratio of NaH2PO2· H2O and S is 6 to 4 After that, the tube furnace purged with Ar (99.999%) at a flow rate of 50 SCCM was heated to 305 °C at

2 °C min-1 for 2 h After naturally cooled down to room temperature, the as-obtained sample was alternatively washed by de-ionized water and absolute ethanol for several times

2.2 Characterization

The structure and morphology of as-obtained product was characterized by X-ray powder diffraction (XRD; Rigaku D/Max 2550, Cu Kα radiation) at a scan rate of 1° min-1, scanning electron microscopy (FESEM, Hitachi, S-4800, 15 kV), transmission electron microscopy (TEM; JEOL, JEM-2100F) with an X-ray energy dispersive spectrometer (EDS) at an accelerating voltage of 200 kV, and X-ray photoelectron spectra (XPS; Thermal Scientific, EscaLab 250Xi) The sample was directly conducted

by X-ray diffraction and scanning electron microscopy, and was dispersed in absolute ethanol for 10-min ultrasound bath before transmission electron microscopy, and was grinded to powder for X-ray photoelectron spectra

2.3 Electrochemical Measurements

Electrochemical measurements were conducted in a three-electrode system controlled by a CHI 660E electrochemical workstation with saturated Ag/AgCl and graphite electrode as reference electrode and counter electrode, respectively All potentials measured were calibrated to reversible hydrogen electrode (RHE) by the following equation:

at least 30 min, and kept gas saturation during whole experiment All measurements were performed without activation process at ambient temperature Liner sweep voltammetry was performed at 1 mV s-1 iR drop was compensated at 90% through the positive feedback according to the impedance (R) result tested in HER-condition potential range Chronopotentiometry was carried out under same conditions without

iR compensation The resulting sample was directly evaluated without any treatment And as comparisons, pristine Ni foam, Pt/C (20% Pt) catalysts supported on Ni foam (loading mass = 0.3 mg cm-2) as well as Pt sheet (10*10*0.1 mm) were also measured for HER performance

3 Results and discussion

Figure 2a-2b show the digital photographs of Ni foam and the hybrid Ni-P-S nanosheets

array with a color change from gray to black, implying the generation of products Figure 2c

is the X-ray diffraction (XRD) patterns of Ni-P-S nanosheets array and Ni Foam, indicating the combined formation of hexagonal-type Ni2P (JCPDS No 65-3544) and millerite-type NiS (JCPDS No 12-0041) after a simultaneous phosphorization and sulfurization treatment The energy dispersive spectrum (EDS) further confirms the existence of Ni, P and S elements with

a molar ratio of 3.4 : 1 : 3.5 (Figure S1) From scanning electron microscopy (SEM) images

of as-obtained products in Figure 2d-2e, it is observed that the three-dimensional (3D)

framework has been well-maintained with Ni-P-S nanosheets array surface, which are interconnected each other with an average thickness of ~ 50 nm

The detailed microstructure of Ni-P-S nanosheets array was further investigated by

transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) Figure 3a is a

representative TEM image of single Ni-P-S nanosheet, displaying rich porous arrangement on

whole structure This feature will provide more electroactive area with short electron transfer

pathway by efficient electrolyte infiltration [23] The corresponding HRTEM image (Figure

3b) gives the lattice spacing of 0.16, 0.29 and 0.25 nm, fitting well with the (211) plane of

Ni2P as well as (101) and (021) planes of NiS Moreover, the elemental distribution of a

typical Ni-P-S nanosheet is examined by TEM-EDS mapping (Figure 3c-3f), which validates

the uniform distribution of Ni, P and S elements over the whole nanosheet Therefore, we successfully obtained the integrated Ni-P-S nanosheets array

The morphologies of such hybrid Ni-P-S nanosheets array can be controlled just by changing the reactant ratio of NaH2PO2· H2O and S, e.g nanorods array (Figure S2a) and

dendritic nanosheets array (Figure S2b) Before we measure the HER performance,

the electrochemical active surface area (ECSA) of the three samples has been firstly evaluated by testing the charge of double layer, which shows the areal capacitance of 60.1, 52.3 and 22.7 mF cm-2 for the Ni-P-S nanosheets array, the nanorods array and the

dendritic nanosheets array, respectively (Figure S3) Obviously, the integrated Ni-P-S

nanosheets array possesses a highest active area, suggesting a high HER performance To

further verify it, we evaluate HER activities of the three samples, as shown in Figure S4,

which is in good agreement with the ECSA results Therefore, we further evaluate the detailed HER performance of the integrated Ni-P-S nanosheets array in the following discussion

It is well-known that the HER performance is closely related to the wettability of the

catalysts As illustrated in Figure 4a-4b, we found that the surface of Ni foam is very

hydrophobic with the initial contact angle of 130.1 ° enduring for 3 minutes while the Ni-P-S nanosheets array has excellent hydrophilicity where droplet is entirely absorbed simply within 0.05 s The improved wettability helps the infiltration between electrocatalyst and electrolyte as well as efficient bubble detachment during HER process [24] The linear

polarization curve of the Ni-P-S nanosheets array was evaluated in 1 M KOH at a scan rate of

1 mV s-1 with 90% iR-compensation, as shown in Figure 4c For comparison, we also test the

commercial Pt/C (20%), Pt sheet and Ni foam It can be found that the Ni-P-S nanosheets array shows a superior HER activity with current densities of 10, 20, 100 mA cm-2 at small

overpotentials of -101.8, -142.0 and -207.8 mV The inset in Figure 4c shows abundant H2

bubbles on the surface of catalyst, which has been detailed recorded in Video S1 This performance outperforms most non-precious electrocatalysts in the literature (Table S1), and

even is much closer to Pt sheet at high overpotentials The Ni-P-S nanosheets array also shows a low Tafel slop of 73.5 mV dec-1 (Figure 4d), which is in the range of 40-120 mV

dec-1, indicating the catalytic process complies with Volmer–Heyrovsky model [25] The impressive HER performance was further supported using electrochemical impedance spectroscopy (EIS) measurement at an applied potential (-0.2 V vs RHE) As shown in

Figure 4e, the Ni-P-S nanosheets array exhibits a remarkably reduced charge-transfer

resistance (Rct) with a small semicircle, which is comparable to the commercial Pt/C catalyst and much smaller than the pristine Ni foam This result reveals facile kinetics of our sample in alkaline media with an enhancing catalytic activity The durability of catalysts is also very

important for their practical applications In Figure 4f, our Ni-P-S nanosheets array shows a

slight potential shift of less than 30 mV at 10 and 50 mA cm-2 Meanwhile, the microstructure

of the catalyst can be well-maintained even through 16 h continuous chronopotentiometry

measurement (Figure S5), verifying its robust durability in alkaline media

To deeply probe the composition change before and after HER, X-ray photoelectron spectrum (XPS) was conducted to investigate the surface characterization of the Ni-P-S

nanosheets array The XPS survey spectrum is provided in Figure S6, exhibiting the presence

of Ni, P, S elements Figure 5 further gives the high-resolution XPS spectra of each element before and after HER measurement For the pristine sample, Ni 2p region exhibits two sharp

result in Figure S1 The phenomenon manifests that the Ni hydroxides dominate the surface

of catalysis after HER, illustrating the generation of the intermediate Ni hydroxides active sites

It is well-known that an efficient HER catalyst should work well in different pH electrolytes because of the inevitable proton concentration change in a typical electrocatalytic process We also measure the activity and durability of our as-synthesized Ni-P-S nanosheets

array both in acid and neutral conditions Figure 6a shows the corresponding linear

polarization curves at a scan rate of 1 mV s-1 with 90% iR-compensation In 0.5 M H2SO4, it exhibits current densities of 10, 20 and 100 mA cm-2 at overpotentials of -185.0, -209.0 and -254.8 mV, respectively And the overpotentials of -394.6, -495.0 and -546.6 mV are required to attain current densities of 10, 20 and 100 mA cm-2 in 1 M PBS Furthermore, the stability of our sample in varied pH range is also evaluated, revealing a negligible degradation after 16 h long-term test at 50 mA cm-2 (Figure 6b)

spectra (Figure 5b) of the ternary Ni-P-S nanosheets array, which strengthens the

surface adsorption of electrolyte Besides, the introduction of S-included compound

and element (Figure 5c) will promote the production of metal hydroxides, and hence

leading to a high catalytic activity (b) The well-defined and interconnected nanosheets array creates rich porous structure with a big and valid surface, resulting in an

improved ECSA (Figure S3) and reduced charge-transfer resistance, which is

favourable to expose more electrocatalytic active sites and boost HER reaction kinetics (c) Our Ni-P-S nanosheets array also exhibits excellent wettability, which not only reinforces the intimate contact between catalyst and electrolyte, but also accelerates bubble separation from the catalyst surface In addition, the simultaneous phosphorization and sulfurization strategy on Ni foam enables it very stable and the materials design concept can

be further extended to exploit other efficient and stable HER electrocatalysts

4 Conclusions

In summary, we demonstrate the synthesis of the hybrid Ni-P-S nanosheets array with integrated microstructure on nickel foam through a simultaneous phosphorization and sulfurization process Such an integrated structure is highly vital for boosting HER performance in a wide pH range due to its strong synergistic interactions between Ni2P and NiS nanosheets as well as the improved kinetics and hydrophilic interface As a

-142.0 and -207.8 mV The current (e.g 10 and 50 mA cm-2) can be well-maintainefor

16 h without obvious change Moreover, Even in acid (pH = 0) and neutral media (pH

= 7), they also exhibit stable and excellent catalytic activities The present catalyst design idea gives a new pathway for achieving highly efficient and stable hydrogen generation

Conflict of interests

There is no conflict of interest

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21522602,

51672082, 91534202), the International Science and Technology Cooperation Program of China (2015DFA51220), the Research Project of Chinese Ministry of Education (113026A), the Program for Shanghai Youth Top-notch Talent, and the Fundamental Research Funds for the Central Universities

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