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

Accepted Manuscript Integrated Ni-P-S nanosheets array as superior electrocatalysts for hydrogen generation Haoxuan Zhang, Haibo Jiang, Yanjie Hu, Prof Hao Jiang, Prof Chunzhong Li PII: S2468-0257(16)30103-0 DOI: 10.1016/j.gee.2016.12.004 Reference: GEE 44 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/ j.gee.2016.12.004 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain ACCEPTED MANUSCRIPT Integrated Ni-P-S nanosheets array as superior electrocatalysts for hydrogen generation Haoxuan Zhang, Haibo Jiang, Yanjie Hu, Hao Jiang*, Chunzhong Li * RI PT 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 SC Email: jianghao@ecust.edu.cn (Prof H Jiang) and czli@ecust.edu.cn (Prof C Z Li) M AN U 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 TE D 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 EP and 100 mA cm-2 at low overpotentials of only -101.9, -142.0 and -207.8 mV with AC C 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 ACCEPTED MANUSCRIPT Introduction Electrical-driven hydrogen generation has attracted tremendous attention to solve energy crisis and environmental pollution, which requires efficient electrocatalyts for RI PT hydrogen evolution reaction (HER) to decrease overpotential [1-4] Although platinum (Pt) is identified as the state-of-the-art HER catalyst to date, its large-scale application SC 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 M AN U 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 TE D catalyst shows limited exposure of active sites in alkaline media, resulting in unsatisfactory electrocatalytic activity [13-15] Although hybridizing Ni2P with other EP 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 AC C 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 CoSe2 phase-transformation engineering to realize an enhanced electrocatalytic activity due to the ACCEPTED MANUSCRIPT 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 RI PT 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 SC Herein, we demonstrate a hybrid Ni-P-S nanosheets array with integration M AN U 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, TE D -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 EP structure of the hybrid Ni-P-S nanosheets array with synergistic enhancement AC C interaction The present catalyst design idea gives a new pathway for achieving highly efficient and stable hydrogen generation Experimental section 2.1 Synthesis of Ni-P-S nanosheets array Prior to further dealing, a piece of Ni foam was soaked in M HCl for 20 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 h In a typical synthesis, a ACCEPTED MANUSCRIPT 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 to After that, the tube RI PT furnace purged with Ar (99.999%) at a flow rate of 50 SCCM was heated to 305 °C at °C min-1 for h After naturally cooled down to room temperature, the as-obtained sample was alternatively washed by de-ionized water and absolute ethanol for several SC times M AN U 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 TE D 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 EP 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 AC C 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: ACCEPTED MANUSCRIPT E (RHE) = E (Ag/AgCl) + 0.1976 V+ 0.0591 pH Before each set of measurement, the electrochemical cell was purged with N2 for at least 30 min, and kept gas saturation during whole experiment All measurements were performed without activation process at ambient temperature Liner sweep RI PT voltammetry was performed at 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 SC 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 for HER performance Results and discussion M AN U (loading mass = 0.3 mg cm-2) as well as Pt sheet (10*10*0.1 mm) were also measured TE D 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 EP 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 AC C energy dispersive spectrum (EDS) further confirms the existence of Ni, P and S elements with a molar ratio of 3.4 : : 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 ACCEPTED MANUSCRIPT 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 RI PT 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 SC 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 M AN U 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 TE D 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, EP 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 AC C 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 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 ACCEPTED MANUSCRIPT polarization curve of the Ni-P-S nanosheets array was evaluated in M KOH at a scan rate of 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 RI PT 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 SC 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 M AN U 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 TE D 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 EP 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 AC C 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 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 ACCEPTED MANUSCRIPT peaks at 853.4 and 856.3 eV and a broad satellite peak at 861.3 eV, which is regularly ascribed to the Ni-P/S and Ni-OH bonds [26-28] P 2p region shows a peak at 130.6 eV linking to Ni-P compounds and another peak at 134.4 eV corresponding to P-O components [29, 30] Apparently, the binding energy of Ni-P is higher than that of pure NiP compound, RI PT implying the strong effect of sulfur incorporation by raising charge transfer [31] And for S species, the peaks at 161.8 and 162.8 eV are commonly reported for NiS materials and the weak peak at 168.8 eV belongs to S-O bond [32, 33] In addition, we also observe the SC elemental S (163.8 eV), which will help the conversion of metal compounds into metal hydroxides, leading to an enhanced catalytic activity [34, 35] In O 1s region, the main peak at M AN U 531.8 eV is regarded as Ni hydroxides while another peak at 533.0 eV is from residual water [36] After HER, the significant compositional difference can be observed, in which the total amount of P and S is decreased with more OH- generation, being in accordance with the EDS result in Figure S1 The phenomenon manifests that the Ni hydroxides dominate the surface sites TE D of catalysis after HER, illustrating the generation of the intermediate Ni hydroxides active It is well-known that an efficient HER catalyst should work well in different pH EP 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 AC C array both in acid and neutral conditions Figure 6a shows the corresponding linear polarization curves at a scan rate of 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 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) ACCEPTED MANUSCRIPT Based on the aforementioned results, the as-obtained Ni-P-S nanosheets array possesses a high electrocatalytic activity with robust durability in a wide pH range Such impressive HER performances can be extremely attributed to the hybrid Ni-P-S nanosheets array (a) The reciprocal composition and integrated structure of Ni2P and RI PT NiS exceptionally generate strong synergistic interaction with abundant actives sites Detailed speaking, a remarkable binding energy shift can be observed in the XPS spectra (Figure 5b) of the ternary Ni-P-S nanosheets array, which strengthens the SC surface adsorption of electrolyte Besides, the introduction of S-included compound and element (Figure 5c) will promote the production of metal hydroxides, and hence M AN U 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 TE D (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 EP sulfurization strategy on Ni foam enables it very stable and the materials design concept can AC C be further extended to exploit other efficient and stable HER electrocatalysts 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 ACCEPTED MANUSCRIPT References [1] T F Jaramillo, K P Jorgensen, J Bonde, J H Nielsen, S Horch, I Chorkendorff, Science 317 (2007) 100-102 [2] M A Lukowski, A S Daniel, F Meng, A Forticaux, L Li, S Jin, J Am Chem RI PT Soc 135 (2013) 10274-10277 X X Zou, Y Zhang, Chem Soc Rev 44 (2015) 5148-5180 [4] P C K Vesborg, B Seger, I Chorkendorff, J Phys Chem Lett (2015) 951-957 [5] W F Chen, C H Wang, K Sasaki, N Marinkovic, W Xu, J T Muckerman, Y SC [3] M AN U Zhu, R R Adzic, Energy Environ Sci (2013) 943-951 [6] X J Fan, H Q Zhou, X Guo, ACS Nano 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(1979) 567-569 ACCEPTED MANUSCRIPT [28] M C Biesinger, B P Payne, L W Lau, A Gerson, R S C Smart, Surf Interface Anal 41 (2009) 324-332 [29] K Ichimura, M Sano, Synth Met 45 (1991) 203-211 RI PT [30] B Chowdari, K Tan, W Chia, Solid state ionics 53 (1992) 1172-1178 [31] G Curro, V Grasso, F Neri, L Silipigni, II Nuovo Cimento D 17 (1995) 37-52 [32] T Yokoyama, A Imanishi, S Terada, H Namba, Y Kitajima, T Ohta, Surf Sci SC 334 (1995) 88-94 M AN U [33] H Peisert, T Chassé, P Streubel, A Meisel, R Szargan, J Electron Spectrosc Relat Phenom 68 (1994) 321-328 [34] Y M Shul'ga, V Rubtsov, V Vasilets, A Lobach, N Spitsyna, E Yagubskii, Synth Met 70 (1995) 1381-1382 TE D [35] H Vrubel, D Merki, X Hu, Energy Environ Sci (2012) 6136 [36] T Sun, L Xu, Y Yan, A A Zakhidov, R H Baughman, J Chen, ACS Catal AC C EP (2016) 1446-1450 ACCEPTED MANUSCRIPT Figure captions Figure Schematic illustration for the hybrid Ni-P-S nanosheets array Figure Digital photographs of (a) pristine Ni Foam and (b) hybrid Ni-P-S nanosheets array, (c) XRD spectrum, (d) low- and (e) high-magnification SEM images of the Ni-P-S nanosheets RI PT array Figure (a) TEM, (b) HRTEM images and (c-f) TEM-EDS mapping of a representative hybrid Ni-P-S nanosheet Figure Wetting-ability test of (a) Ni foam and (b) hybrid Ni-P-S nanosheets array; (c) liner nanosheets array, Pt sheet, Pt/C (20%) and SC polarization curves, (d) Tafel plots, (e) Nyquist plots applied at -0.2 V vs RHE of Ni-P-S Ni foam, respectively, and (f) M AN U Chronopotentiometry measurement of Ni-P-S nanosheets array Figure XPS spectra of the Ni-P-S nanosheets array with peaks-fitting results before and after HER measurement: (a) Ni 2p region, (b) P 2p region, (c) S 2p region and (d) O 1s region Figure Electrochemical activities of the hybrid Ni-P-S nanosheets array for HER in 0.5 M TE D H2SO4 (pH = 0) and M PBS (pH = 7): (a) liner polarization curves, (b) AC C EP Chronopotentiometry curves ACCEPTED MANUSCRIPT Appendix A Supplementary data Supplementary data related to this article can be found at http://www.elsevier.com/ M AN U SC RI PT Part I: Figures AC C EP TE D Figure S1 EDS spectra of integrated Ni-P-S nanosheets array before and after HER Figure S2 The morphologies of integrated Ni-P-S nanosheets array with different reactant ratios of NaH2PO2· H2O and S (a) nanorods array, (b) dendritic nanosheets array M AN U SC RI PT ACCEPTED MANUSCRIPT Figure S3 Cyclic voltammetry curves of (a) nanorods array, (b) dendritic nanosheets array and (c) nanosheets array measured in non-faradaic potential of 0.04-0.16 V vs RHE at multiple scan rates (d) Electrochemical active surface area (ECAS) determined by the AC C EP TE D capacitive currents at 0.1 V vs RHE Figure S4 Liner polarization curves for varied morphologies RI PT ACCEPTED MANUSCRIPT EP TE D M AN U SC Figure S5 SEM image of Ni-P-S nanosheets array after 16 h chronopotentiometry measurement AC C Figure S6 XPS survey spectrum of integrated Ni-P-S nanosheets array SC RI PT ACCEPTED MANUSCRIPT AC C EP TE D M AN U Figure S7 Liner polarization curves of Ni-P-S nanosheets array without iR compensation in wide pH range Figure S8 XRD spectra of the Ni-P-S nanosheets array before and after HER ACCEPTED MANUSCRIPT Part II: Table Table S1 The HER activity of various nonprecious catalysts in 1.0 M KOH Measurement J (mA cm-2) η (mV) Tafel slope (mVdec-1) Ref Ni2P nanoparticle Glassy carbon 20 250 100 13 80 >200 100 250 10 110 100 100 MoS2-Ni3S2 Ni3Se2 nanoforest CoP nanowire NiS microsphere This work Ni foam 14 85.4 15 83 21 >250 79 24 >500 129 S1 83 S2 20 158 10 20 100 101.9 142.0 207.8 TE D References 50 SC CF@Ni-P Ni foam (3.5 mg cm-2) Carbon fiber (25.8 mg cm-2) Ni foam (9.7 mg cm-2) Ni foam (8.87 mg cm-2) Carbon cloth (0.92 mg cm-2) Ni foam (43 mg cm-2) M AN U Ni2P nanosheet RI PT Electroatalyst 73.5 [S1] J Tian, Q Liu, A M Asiri, X Sun, J Am Chem Soc 136 (2014) 7587-7590 (2016) 1486-1489 EP [S2] W Zhu, X Yue, W Zhang, S Yu, Y Zhang, J Wang, J Wang, Chem Commun 52 AC C Part III: Video S1 AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT ... its strong synergistic interactions between Ni2 P and NiS nanosheets as well as the improved kinetics and hydrophilic interface As a ACCEPTED MANUSCRIPT result, the as- obtained products present... energy systems Here, we present the synthesis of integrated Ni- P- S nanosheets array including Ni2 P and NiS on nickel foam by a simple TE D simultaneous phosphorization and sulfurization strategy... curves, (d) Tafel plots, (e) Nyquist plots applied at -0.2 V vs RHE of Ni- P- S Ni foam, respectively, and (f) M AN U Chronopotentiometry measurement of Ni- P- S nanosheets array Figure XPS spectra