CoreâShell Structure of NiSe2 Nanoparticles@Nitrogen-Doped

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Core-shell structure of NiSe nanoparticles@nitrogen-doped graphene for hydrogen evolution reaction in both acidic and alkaline media Wenxin Li, Bo Yu, Yang Hu, Xinqiang Wang, Dongxu Yang, and Yuanfu Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06195 • Publication Date (Web): 13 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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ACS Sustainable Chemistry & Engineering

Core-shell structure of NiSe2 nanoparticles@nitrogen-doped graphene for hydrogen evolution reaction in both acidic and alkaline media Wenxin Li, Bo Yu*, Yang Hu, Xinqiang Wang, Dongxu Yang*, and Yuanfu Chen*

School of Electronic Science and Engineering, and State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, No.4, Section 2, North Jianshe Road, Chengdu 610054, PR China

*Corresponding authors. E-mails: [email protected] (B. Yu), [email protected] (D.X. Yang), [email protected] (Y.F. Chen).

Address: State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, No.4, Section 2, North Jianshe Road, Chengdu 610054, PR China 1 ACS Paragon Plus Environment

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ABSTRACT It is extremely significant to develop nonprecious electrocatalysts with high efficiency and remarkable stability in both acidic and alkaline media for hydrogen evolution reaction (HER). Herein, for the first time, we synthesize a core-shell structure of NiSe2@nitrogen-doped graphene (NiSe2@NG), which is constructed by NiSe2 nanoparticles encapsulated in ultrathin N-doped graphene shells derived from Ni-based metal-organic framework. The optimized NiSe2@NG hybrid exhibits outstanding electrocatalytic performance. In acidic (or alkaline) media, the hybrid has a low onset potential of -163 (or -171) mV vs. RHE, a small overpotential of 201 (or 248) mV vs. RHE at -10 mA cm-2, and particularly a low Tafel slope of 36.1 (or 74.2) mV dec-1. Furthermore, the hybrid also delivers outstanding cycling and current-time stability in acidic and alkaline electrolytes. The outstanding catalytic performance of the hybrid is attributed to its unique core-shell architecture, which not only significantly improves the conductivity and creates numerous active sites to enhance the electrocatalytic activity, but also guarantees the chemical and structural stability of the NiSe2 core thus improving the stability of electrocatalyst. This work provides a novel perspective to design and synthesize high-efficient nonprecious electrocatalysts for HER in both acidic and alkaline solutions.

Keywords: Hydrogen evolution reaction; Core-shell structure; NiSe2@nitrogen-doped graphene; Metal-organic framework; Outstanding electrocatalytic performance

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INTRODUCTION In order to solve energy depletion and related environmental issues, hydrogen energy has attracted great attention as a clean, eco-friendly and renewable energy.1-4 An efficient method for generating hydrogen is hydrolysis through hydrogen evolution reaction (HER) with the help of electrocatalysts.5-7 At present, platinum (Pt)-based catalysts are the most efficient noble-metal-based catalysts for HER, but their expensive price and scarcity greatly hinder their practical applications.8-10 Therefore, developing low-cost, highly efficient and earth-abundant HER catalysts to replace noble metals has been especially urgent and necessary.11-14 In recent years, because of their high activities, low cost and earth-abundance, transition-metal dichalcogenides (TMDs) have been considered as ideal replacements of noble metals for HER.15-17 However, most of TMDs (like MoS2,18 WS2,19 ReS2,20 MoSe2,21 WSe2,22 ReSe214) confirmed by former experimental data and theoretical calculation are still limited to the poor intrinsic HER capability compared with noblemetal materials. Fortunately, NiSe2 has relatively good catalytic performance for HER both in acidic and alkaline solutions among the TMDs.23-26 But NiSe2 in usual forms such as aggregated particles or bulk generally has no distinct advantages in catalytic applications because of its few electrocatalytic active sites, small specific surface areas and low conductivity. Meanwhile the electrocatalytic activity of bare NiSe2 is not enough stable under strong acidic and alkaline electrolytes or high overpotential. To address such issues, several strategies have been presented. The first strategy is to increase the number of catalytic sites by designing nanostructure or porous structure 3 ACS Paragon Plus Environment


for the catalysts. For instance, Zhang et al. reported MoSe2/NiSe2 nanowires which exhibited good HER activity in acidic medium due to its unique nanostructure.27 Zhou et al. synthesized a porous NiSe2/Ni catalyst with a low Tafel slope in acidic medium because of its porous structure.25 The second strategy is to introduce conductive carbonaceous materials to increase the overall conductivity of the catalysts. For example, Song et al. synthesized NiSe2 embedded in carbon nanowires, which delivered excellent electrocatalytic performance in acidic solution by using carbonaceous materials to improve the conductivity.26 However, compared with Pt catalyst, it is still challengeable to further improve the HER activity and long-term stability of NiSe2-based catalysts in acidic and alkaline electrolytes. Designing a rational nanoarchitecture and utilize the synergistic effects might be an effective solution. Recently, Deng et al. reported the enhanced catalytic activity of CoNi nanoparticles encapsulated in thin graphene layers.28 Xu et al. reported that nickel nanoparticles encapsulated in N-doped graphene showed significant enhancement in its catalytic activity.29 It suggests that the catalytic activity of metallic nanoparticles can be remarkably improved after encapsulated by graphene layers. In addition, heteroatom-doped graphene can also effectively promote catalysts’ electrochemical performance. For instance, the nitrogen dopant can increase the electron density in graphene layers, which can improve the HER activity.28 However, to our best knowledge, such synergistic strategies have not been taken in transition-metal-based selenides. Metal-organic framework (MOF) as a type of porous crystalline coordination 4 ACS Paragon Plus Environment

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polymers, has caught much attention due to its unique structure, large specific surface and good conductivity. Using MOF as precursor has been proved to be an effective method to fabricate various functional materials controllably by annealing in restrained atmosphere.30, 31 In order to significantly improve the catalytic activity and the stability of NiSe2-based electrocatalysts, particularly in both acid and alkaline media, herein we rationally design and simply synthesize a core-shell structure of NiSe2@N-doped graphene (NiSe2@NG), constructed by NiSe2 nanoparticles encapsulated in ultrathin N-doped graphene shells derived from Ni-based metal-organic framework (Ni-MOF). Due to the synergistic effects of unique core-shell nanoarchitecture, enhancement of reactive sites and overall conductivity, the NiSe2@NG delivers excellent catalytic activity and stability.

the synthesis route of NiSe2@NG-140.

EXPERIMENTAL SECTION Synthesis of Ni-MOF. Ni-MOF was fabricated by a solvothermal reaction method. First, 0.62 g of Ni(NO3)2•6H2O was dissolved into 50 mL of N,N-dimethylformamide (DMF). Then we added 0.355 g of terephthalic acid and 0.112 g of triethylene diamine



hexahydrate (ted) to the above solution and stirred it for 30 min until a uniformly dispersed cyan-color solution was obtained. The resultant solution was removed into a 100 mL Teflon-lined autoclave and heated at 140 °C for 24 h. The obtained cyan precipitates were washed with DMF and ethanol for three times and then dried in a vacuum oven at 60 °C overnight. For comparison, the synthesis temperatures of NiMOF were changed at 100 °C, 120 °C, 160 °C and 180 °C, respectively, and the other synthetic steps were unchanged. Synthesis of a core-shell structure of NiSe2@nitrogen-doped graphene (NiSe2@NG). The as-prepared Ni-MOF was annealed at 600 °C for 2 h under an argon flow to obtain Ni@NG powder. Next, Ni@NG and selenium powder were mixed at a mass ratio of 1:2. Finally, the obtained mixture was annealed at 300 °C under a flow of argon for 3 h to receive NiSe2@NG products. According to the different synthesis temperatures of the Ni-MOF precursors, the NiSe2@NG samples were designated as NiSe2@NG-100, NiSe2@NG-120, NiSe2@NG-140, NiSe2@NG-160 and NiSe2@NG180, respectively. Normal NiSe2 sample was synthesized by annealing the NiCl2 at 600 °C for 2 h and then selenization at 300 °C for 3 h under a flow of argon. Materials characterization. The X-ray diffraction patterns of Ni-MOF, Ni@NG and NiSe2@NG were characterized by a powder X-ray diffraction (Rigaku D/MAX-rA diffractometer). The XPS spectrums of the as-prepared samples were obtained by Xray photoelectron spectroscopy using Al Kα radiation (Kratos XSAM 800). The Raman spectrum of NiSe2@NG was recorded by a Raman microscope (532nm, Renishaw). The specific surface area of samples was examined by the Brunauer-Emmett-Teller 6 ACS Paragon Plus Environment

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technique (ASAP 2020). The structures and morphologies of Ni-MOF, Ni@NG and NiSe2@NG were observed by transmission electron microscope (TEM, Tecnai F20) and scanning electron microscope (SEM, JSM-7000F). Measurements of electrocatalytic properties. The HER performance in acidic and alkaline electrolytes of NiSe2@NG was tested with a standard three electrode setup at an electrochemical station (CHI660D). The counter electrode was a graphite rod (or a Pt wire) and the reference electrode was a saturated calomel electrode (SCE) (or a standard Hg/HgO electrode) in 0.5 M H2SO4 (or 1 M KOH). The measured potential vs SEC (or Hg/HgO) was calibrated to the RHE scale by the Nernst equation: ERHE = ESCE + 0.251 V (or ERHE = EHg/HgO + 0.931 V). Detailed RHE calibration information is shown in Figure S1. To make the testing electrode, NiSe2@NG sample (4 mg) was added to the co-solvent of 250 μL of ethanol and 750 μL of water, and ultrasonicated for 20 min to acquire a uniform suspension. Next, 60 μL of Nafion was transferred into the above suspension and then the resulting mixed solution was ultrasonicated for 10 minutes. Finally, 5 μL of the dispersing liquid was dropped on a glassy carbon electrode (GCE) by using a 2-10 μL micropipette and the as-prepared electrode was placed in a 60 °C drying oven for several minutes. Before any measurements, in order to activate the catalysts, all working electrodes need cycle for 20 cycles between -0.349 and +0.251 (or -0.469 and 0.131) V vs. RHE in acidic (or in alkaline) electrolytes. The onset potential is defined as a potential with a current density of 1 mA/cm2.6 EIS was tested at a bias potential of -0.229 V vs. RHE with frequency range of 0.1-100000 Hz under an AC voltage of 5 mV both in 0.5 M H2SO4 and 1 M KOH. The durability of 7 ACS Paragon Plus Environment


NiSe2@NG was recorded by continuous cyclic voltammetry (CV) and current density time dependent (IT) tests. Time dependence of current density was measured under constant potential of -0.229 (or -0.309) vs. RHE in acidic electrolyte (or alkaline electrolyte) and CV method was obtained at 100 mV s-1 scan rate for 1500 cycles.

RESULTS AND DISCUSSION The synthetic procedures of NiSe2@NG-140 are shown in Figure 1. Ni-MOF-140 was synthesized by using Ni(NO3)2•6H2O, Terephthalic acid and Triethylene diamine hexahydrate (ted) in DMF solution under 140 °C for 24 h. Then through the annealing step, Ni-MOF-140 was transformed into Ni@NG-140, because Ni cation was reduced into Ni-metal nanoparticles and organic components were pyrolyzed into N-doped graphene layers. Finally, after in-situ selenization, Ni@NG-140 was converted into NiSe2@NG-140.

Ni@NG-140 and NiSe2@NG140.

the enlargement

cm-1) of NiSe2@NG-140. 8 ACS Paragon Plus Environment

region from 100 cm-1 to 400

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In order to characterize the crystalline structure and material composition of Ni-MOF, Ni@NG and NiSe2@NG, we used X-ray diffraction (XRD) to test above samples. Figure 2a shows the diffraction patterns of Ni-MOF-140, Ni@NG-140 and NiSe2@NG-140. The characteristic peaks of Ni@NG-140 at 44.2°, 52.1° and 76.5° correspond to the (111), (200) and (220) crystalline phases of the Ni nanoparticle (PDF#87-0712).26 The diffraction peaks of NiSe2@NG-140 at 29.8°, 33.4°, 36.8°, 42.7°, 50.6°, 53.1°, 55.4°, 57.7°,62.1°, 72.5° and 74.5° are assigned to the (200), (210), (211), (220), (311), (222), (023), (321), (400), (421) and (332) crystalline phases of NiSe2 (PDF#88-1711).30 However, since the content of N-doped graphene is much less than that of NiSe2 in NiSe2@NG samples, the characteristic peak of C cannot be clearly observed in the XRD image.31 The XRD patterns of all the other NiSe2@NG samples are shown in Figure S2. The crystal sizes of NiSe2@NG-100, NiSe2@NG-120, NiSe2@NG-140, NiSe2@NG-160, NiSe2@NG-180 have been evaluated as ~81.3 nm, ~49.2 nm, ~35.6 nm, ~54.6 nm and ~63.9 nm by using Scherrer Formula from XRD data in JADE software, respectively. The Raman spectrum of NiSe2@NG-140 is shown in Figure 2b. It can be found that the Raman peaks of the sample at 1589 and 1347 cm1

correspond to the G band and D band of graphene layers, respectively, implying the

presence of C in the composite.32, 33 In the enlargement area of the Ag peak at 205 cm1

, there are four Raman peaks which are at 144, 167, 210 and 233 cm-1, respectively.

This result is consistent with the NiSe2 reported previously.25, 27



NiSe2@NG-140 The X-ray photoelectron spectroscopy (XPS) was employed to characterize the chemical states of the elements of NiSe2@NG-140. As shown in Figure S3, the survey XPS spectrum of NiSe2@NG-140 shows the signals of Se, Ni, C and N elements with the peak of O owing to oxidation of the sample. Figure 3a, b exhibit the XPS spectrums of Se 3d and Ni 2p peaks. From Figure 3a, the Se 3d3/2 (55.61 eV) and Se 3d5/2 (54.75 eV) peaks prove the existence of Se-Ni bonds and the peak at 59.24 eV is assigned to the surface oxidation of Se.16, 32, 34, 35 The peaks of Ni 2p3/2 and Ni 2p1/2 shown in Figure 3b appear at 856.15 and 874.05 eV respectively, which are both resolved into three 10 ACS Paragon Plus Environment

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bands. The main peaks at 854.12 eV of Ni 2p3/2 (S1) and 870.56 eV of Ni 2p1/2 (S1’) originate from Ni2+. The peaks at 855.95 eV (S2) and 873.90 eV (S2’) are Ni3+ in the oxidation state and the peaks at 860.30 eV (S3) and 879.30 eV (S3’) are from the oxidation phase of Ni2+.23, 27 In Figure 3c, the peak of C 1s is divided into three peaks, which are positioned at 284.55, 285.25 and 286.60 eV corresponding to C-C, C-O and C-N bonds, respectively.36-38 In Figure 3d, the XPS spectrum of N 1s is fitted by three



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peaks which correspond to pyridinic N (398.75eV), pyrrolic N (400.80eV) and graphitic N (402.60eV).29, 39 The Brunauer-Emmet-Teller (BET) is used to confirm the specific surface area of NiSe2@NGs. As shown in Figure S4, compared with other NiSe2@NGs, NiSe2@NG-140 displays the largest specific surface area (22.22 m2 g-1). This result indicates that the proper synthesis temperature of precursors could improve the specific surface area of NiSe2@NG, thus promoting the access of electrolytes and enhancing the number of active sites. The SEM images of Ni-MOF-140, Ni@NG-140 and NiSe2@NG-140 are shown in Figure 4a-f. In Figure 4a, the morphology of Ni-MOF-140 is cuboid and the average length of the cuboid is up to 3 μm. Enlarging the cuboid surface, a hierarchical structure composed of nanosheets is shown in Figure 4b. Figure 4c, d show the SEM images of the Ni@NG-140, exhibiting that a large number of spherical nanoparticles with the diameter of 10-20 nm are distributed in the porous and unconsolidated sheet structure. In Figure 4e, f, the sheet structure becomes more porous and hollow and the diameter of spherical nanoparticles of NiSe2@NG-140 has increased to 30-40 nm after in-situ selenization. Furthermore, we used TEM and high-resolution TEM (HRTEM) to further confirm the mircrostructure of NiSe2@NG-140. In Figure 4g, the TEM image identifies NiSe2 nanoparticle cores encapsulated into thin N-doped graphene layers. Figure 4h shows the HRTEM image of NiSe2@NG-140, in where interplanar spacings of 0.24 and 0.27 nm are respectively assigned to (211) and (210) lattice of NiSe2.30,

40

In

addition, it can be observed that the NiSe2 nanoparticle is surrounded by N-doped graphene shells with a distance between lattice planes of 0.34 nm.41-43 The results are 12 ACS Paragon Plus Environment

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good proof that the NiSe2 nanoparticle is encapsulated in N-doped graphene shells. The HER performance of NiSe2@NG-140 (the catalyst loading: 0.267 mg cm-2) was investigated at 5 mV s-1 scan rate in 0.5 M H2SO4 and 1 M KOH solutions. For



comparison, GCE, 20 wt % Pt/C catalyst, NiSe2@NG-100, NiSe2@NG-120, NiSe2@NG-160 and NiSe2@NG-180 were also evaluated. As shown in Figure 5a, linear sweep voltammetry (LSV) curves of the above samples in acidic electrolyte are exhibited and the current density is normalized by GC electrode geometric surface area. Pt/C catalyst shows excellent performance. Compared with other NiSe2@NGs, NiSe2@NG-140 shows the best HER activity with onset potential of -163 mV vs. RHE. In the meantime, Figure 5b exhibits the LSV curves of the above samples in alkaline electrolyte, in where NiSe2@NG-140’s onset potential (-171 mV vs. RHE) is lower than those of other control samples. Furthermore, Figure 5c (or Figure 5d) shows that NiSe2@NG-140 need an overpotential of 201 (or 248) mV vs. RHE to achieve -10 mA cm-2 in acidic (or alkaline) solution, which is the lowest among above samples. Generally, the electrocatalytic kinetics of HER is tested by the Tafel slope. The linear part of the Tafel points is fitted with the Tafel equation (𝜂 = 𝑏𝑙𝑜𝑔 (𝑗) + 𝑎, where j and b is current density and the Tafel slope, respectively).44, 45 From Figure 5e, f, Pt/C catalyst shows small Tafel slopes in acidic (29.7 mV dec-1) and alkaline (35.3mV dec1

) electrolytes. The Tafel slope of NiSe2@NG-140 is 36.1 (or 74.2) mV dec-1 in acidic

(or alkaline) solutions, which is smaller than those of other NiSe2@NGs and most recently reported catalysts (Table S1). A smaller Tafel slope implies more positive reaction kinetics in HER process. Meanwhile, compared with normal NiSe2, NiSe2@NG-140 obviously delivers lower onset potentials and Tafel slopes in both acidic and alkaline electrolytes (Figure S5). These results directly demonstrate that NiSe2@NG-140 has better electrocatalytic performance in both acidic and alkaline 14 ACS Paragon Plus Environment

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electrolytes. Commonly, there are two main HER theories that are widely adopted by the academic community. They both have two step reaction and their first step was same: Volmer reaction. This reaction is about electrochemical mechanism of hydrogen proton adsorption (when in acidic solution: H3 O+ + e− → Hads + H2 O, or in alkaline solution: H2 O + e− → Hads + OH − ). And the second step reaction is either Heyrovsky reaction (when in acidic solution: H3 O+ + Hads + e− → H2 ↑ +H2 O, or in alkaline solution: Hads + H2 O + e− → H2 ↑ +OH − ) or Tafel reaction (both in acidic and alkaline solutions: Hads + Hads → H2 ↑).32 These two reactions are related to electrochemistry

The estimation of double-layer capacitance (Cdl) for NiSe2@NGs Nyquist plots of NiSe2@NGs at -0.229 V vs. RHE in

. (inset: the equivalent circuit) 15 ACS Paragon Plus Environment


desorption and chemistry desorption, respectively. In term of previous treatises, different step reactions determine different reaction rates which are reflected by the Tafel slope, so the Tafel slopes of 30, 40 and 120 mV dec-1 correspond to Tafel, Heyrovsky and Volmer step, respectively.32, 46 The Tafel slopes of NiSe2@NG-140 are 36.1 and 74.2 mV dec-1 in acidic and alkaline electrolytes, which clearly reveals that the Volmer-Tafel and Volmer-Heyrovsky combination mechanisms are the rate determining steps of HER in acidic and alkaline solutions, respectively. Generally, electrochemical surface area (ECSA) of the electrocatalysts was evaluated by using the double-layer capacitance (Cdl) parameter.25 The cyclic voltammograms of NiSe2@NGs are shown in Figure S8. These data were tested in the potential region from 0.621 to 0.721 V vs. RHE in 0.5 M H2SO4 and 0.381 to 0.481 V vs. RHE in 1 M KOH. The data in Figure 6a, b are obtained from the difference of positive and negative current density (△ 𝑗 = 𝑗𝑎 − 𝑗𝑐 at 0.671 V vs. RHE in 0.5 M H2SO and 0.431 V vs. RHE in 1 M KOH) plotted against the different scan rates (20-200 mV s-1). The Cdl values in Figure 6a, b equal to the linear slopes. The Cdl values of NiSe2@NG-140 in acidic and alkaline electrolytes are 6.57 and 5.79 mF cm-2, respectively, which are the biggest among those of as-prepared samples. Therefore, NiSe2@NG-140 has the largest electrochemical surface area, resulting in its best HER performance. The electrochemical impedance spectroscopy (EIS) measurement is an effective way to reveal the electrode kinetics in the HER.47 Figure 6c, d show the Nyquist plots of asprepared samples in acidic and alkaline media, respectively. The charge-transfer resistance (Rct) of NiSe2@NG-140 in acidic solution are 44.39 Ω, which is much 16 ACS Paragon Plus Environment

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smaller than those of other NiSe2@NGs (204.2 Ω of NiSe2@NG-100, 96.69 Ω of NiSe2@NG-120, 103.7 Ω of NiSe2@NG-160 and 148.5 Ω of NiSe2@NG-180). Furthermore, as shown in Figure 6d, the Rct of NiSe2@NG-140 (77.08 Ω) in alkaline solution is also much lower than those of other samples (342.7 Ω of NiSe2@NG-100, 126.5 Ω of NiSe2@NG-120, 149.9 Ω of NiSe2@NG-160 and 300.4 Ω of NiSe2@NG180). The above data indicate that, under acidic and alkaline electrolytes, NiSe2@NG140 has faster transfer rate of electrons in the HER process, which means it has better HER performance. Stability is a very important indicator in electrocatalyst’s practical application. In

Durability test of NiSe2@NG-140 after 1500 CV cycles in Chronoamperometric curve 0.229 V vs. RHE in acidic electrolyte and (d) -0.309 vs. RHE in alkaline electrolyte. 17 ACS Paragon Plus Environment

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Figure 7a, b, NiSe2@NG-140’s polarization curves at 100 mV s-1 scan rate in acidic and alkaline solutions almost coincide with the initial curves after 1500 CV cycles, which

makes

clear

that

this

catalyst

has

excellent

cycling

stability.

Chronoamperometric curve at -0.229 (or -0.309) vs. RHE under acidic electrolyte (or alkaline electrolyte) is shown in Figure 7c (or Figure 7d), which demonstrates that NiSe2@NG-140 electrocatalyst can maintain an excellent electrocatalytic activity for at least 18 h in acidic electrolyte (or alkaline electrolyte). In order to confirm the stability of NiSe2@NG-140 for HER process in both acidic and alkaline media, the morphologies and crystalline structure of NiSe2@NG-140 after 1500 CV cycles were characterized by SEM and XRD, respectively. As displayed in Figure S6, there is nearly no obvious change in morphology after long-term test. Meanwhile, the XRD pattern of NiSe2@NG-140 after cyclic test (Figure S7) is almost the same as before the test (Figure 2a). These results confirm the good stability of NiSe2@NG-140 during the long-term electrochemical cycling process.

NiSe2@NG and its catalytic process in HER. 18 ACS Paragon Plus Environment

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The catalytic process of NiSe2@NG is schematically explained in Figure 8. When electrons pass through thin N-doped graphene layers to the surface of NiSe2 cores, the water molecules at the active sites of NiSe2 nanoparticles are reduced and then hydrogen is released. The excellent HER properties and good durability of NiSe2@NG can be owed to the unique core-shell structure constructed by coating the NiSe2 nanopaticles with ultrathin N-doped graphene shells, which not only observably enhance the conductivity and number of active sites to strengthen the electrocatalytic activity, but also improve the structural stability of NiSe2 cores thus improving the longterm stability of the electrocatalyst.

CONCLUSIONS To sum up, for purpose of improving the electrocatalytic activity and the durability of NiSe2-based catalysts, we have presented a simple MOF-annealing and then in-situ selenization strategy to synthesize a core-shell structure of NiSe2@nitrogen-doped graphene (NiSe2@NG). Meanwhile, we affirm that the synthesis temperature of NiMOF precursor has a huge impact on the HER Performance of NiSe2@NGs and corresponding optimization of the electrocatalytic performance can be achieved. Compared with other NiSe2@NGs, NiSe2@NG-140 shows the best electrocatalytic properties and excellent durability for HER in acidic and alkaline solutions. NiSe2@NG-140 has a minor onset potential of -163 (or -171) mV vs. RHE, a low Tafel slope of 36.1 (or 74.2) mV dec-1 and outstanding cycling stability in acidic (or alkaline) electrolyte. The facile synthesis, low-cost, as well as excellent HER properties and 19 ACS Paragon Plus Environment


long-time stability of the NiSe2@NG catalyst make it an appropriate replacement for noble-metal-based electrocatalysts for HER.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. RHE calibration; XRD pattern and nitrogen adsorption/desorption isotherms of all NiSe2@NGs; XPS spectrum; LSV curve; Tafel plots; SEM image; cyclic voltammograms of the as-prepared samples in both acidic and alkaline media; Table S1. (PDF)

AUTHOR INFORMATION Corresponding Authors *

Y.F. Chen. E-mail: [email protected].

*

B. Yu. E-mail: [email protected].

*

D.X. Yang. E-mail: [email protected].

ORCID Wenxin Li: 0000-0002-7061-6754 Bo Yu: 0000-0003-3438-0761 Yang Hu: 0000-0003-2731-335X Yuanfu Chen: 0000-0002-6561-1684 Notes 20 ACS Paragon Plus Environment

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Grant Nos. 21773024 and 51372033), and National High Technology Research and Development Program of China (Grant No. 2015AA034202).

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Core-shell structure of NiSe2@NG delivers excellent HER performance and stability in both acidic and alkaline media 316x138mm (150 x 150 DPI)

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CoreâShell Structure of NiSe2 Nanoparticles@Nitrogen-Doped

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