Self-Supported NiSe2 Nanowire Arrays on Carbon Fiber Paper as

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Self-supported NiSe nanowire arrays on carbon fiber paper as efficient and stable electrode for hydrogen evolution reaction Yajie Guo, Dong Guo, Feng Ye, Ke Wang, Zhongqi Shi, Xianjue Chen, and Chuan Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02164 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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ACS Sustainable Chemistry & Engineering

Self-supported NiSe2 nanowire arrays on carbon fiber paper as efficient and stable electrode for hydrogen evolution reaction Yajie Guo†,∥, Dong Guo†, Feng Ye†, Ke Wang‡,§,*, Zhongqi Shi§,*, Xianjue Chen∥, Chuan Zhao∥,* †

School of Materials Science and Engineering, Chang’an University, Middle-section

of Nan'er Huan Road, Xi'an, ShaanXi Province, 710064, P. R. China; ‡

Institute for Materials and Processes, School of Engineering, the University of

Edinburgh, Sanderson Building, Robert Stevenson Road, The King's Buildings, Edinburgh, EH9 3FB, UK; §

State Key Laboratory for Mechanical Behavior of Materials, School of Materials

Science and Engineering, Xi’an Jiaotong University, 28, Xianning West Road, Xi'an, Shaanxi Province, 710049, P.R. China; ∥School

of Chemistry, the University of New South Wales, Dalton Building, UNSW,

Kensington, Sydney, NSW, 2052, Australia.

Corresponding Authors *

E-mail: [email protected] (K. Wang); [email protected] (Z. Shi);

[email protected] (C. Zhao)

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ABSTRACT: NiSe2 nanowire (NW) arrays grown on flexible carbon fiber paper (CFP) were synthesized, for the first time, through a novel thermal selenization method in vacuum instead of flowing inert gas. The ultrathin NiSe2 NWs uniformly cover the CFP, contributing to the large surface area and fast charge transportation. Desirably, this binder-free and hierarchical electrode shows a favorable activity for hydrogen evolution reaction (HER). It only requires an overpotential of 172 mV to afford the current density of 100 mA cm−2 in 0.5 M H2SO4, corresponding to a low Tafel slope of 32.44 mV dec-1. After 1000 continuous cycles, the HER activity of NiSe2 NWs is well maintained. Moreover, the current density remains unchanged in prolonged electrolysis, indicating an excellent durability of NiSe2 NWs. The novel preparation method described in this study could inspire future work on the synthesis of transition metal dichalcogenides with high purity and ultrasmall nanostructure towards improved electrocatalytic activity.

KEYWORDS: transition metal dichalcogenide; electrocatalyst; nanowires; hydrogen evolution reaction

INTRODUCTION Hydrogen is believed to be a promising substitute for fossil fuels to meet the future energy demand due to its renewability, eco-friendly characters and high energy density.1 Among the various hydrogen generation methods, electrochemical water splitting is preferable for scalable and sustainable hydrogen production owing to its 2 ACS Paragon Plus Environment

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inherent advantages such as accessible reactants, stable output, and highly pure products.2 However, the hydrogen evolution reaction (HER) is an uphill reaction, making electrocatalysts indispensable to lower the overpotential in the HER.3 So far, Pt group noble metals show the best performance for HER, but large-scale application is hindered due to their scarcity and high costs.4 Therefore, developing earth-abundant and cost-effective electrocatalysts is highly desired. Recently, the pyrite-type transition metal dichalcogenides (MX2, where typically M= Fe, Co, or Ni and X= S or Se) have emerged to be competitive HER electrocatalysts due to their reasonable price, high activity and stability in both acidic and alkaline solutions.5-15 Among these, nickel diselenide (NiSe2) has a high intrinsic conductivity, making it appealing as noble-metal-free electrocatalyst to achieve high HER performance.16-29 Kong et al. prepared NiSe2 nanoparticle film on glassy carbon using a thermal selenization method, and the overpotential to drive the current density of 10 mA cm-2 was 170 mV in 0.5 M H2SO4.16 Zhou et al. synthesized NiSe2 nanoparticle film through direct selenization of Ni foam, and the overpotential to drive the current density of 10 mA cm-2 was remarkably lowered.20 Besides, by utilizing a similar thermal selenization method, Wang et al fabricated Se-enriched NiSe2 nanosheets (NSs) on carbon fiber paper (CFP) which exhibited significantly enhanced electrocatalytic performance.22 Additionally, Tang et al showed that the growth of NiSe2 nanowalls via hydrothermal anion exchange on carbon cloth (CC) could effectively improve the HER activity.23 However, it is still an ongoing 3 ACS Paragon Plus Environment


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challenge to further boost the HER performance of NiSe2 catalysts to the same level of Pt-based catalysts. Generally, the effective strategies to optimize the catalytic activity of electrocatalysts include: (i) increasing the electrochemically accessible area by nanostructure design; modification;34,

35

30-33

(ii) improving the intrinsic activity by component

(iii) enhancing the electrical conductivity by combining the

catalysts with highly conductive substrates.36, including nanoparticles (NPs),16,

17, 21

37

Nanostructures of NiSe2 catalysts

nanowalls,23 nanosheets22 and coral-like

network28 have been synthesized to enhance the catalytic activity. Ultrathin nanowire architecture generally has large accessible area, fast electron transport along the axial direction, efficient mass transfer in-between the nanowires and fast release of gas bubbles, which are favorable features for enhancing the HER performance.11,

38

However, to the best of our knowledge, the synthesis of NiSe2 nanowires or the use of them as HER electrocatalysts have not been reported yet due to the challenge in maintaining an ultrathin structure in the selenization process. The widely used thermal reaction methods conducted in flowing inert atmosphere16, 22, 39-43 fails to synthesize such an ultrathin selenide structure owing to the high temperature required to generate Se vapour. The high temperature can promote the robust Oswald ripening of the ultrathin selenides , which results in the degradation of the nanostructures.44 Herein, we present a novel approach to synthesize self-supported NiSe2 nanowire arrays on CFP (NiSe2 NWs/CFP) by the conversion of Ni2(CO3)(OH)2 precursor into 4 ACS Paragon Plus Environment

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NiSe2 in vacuum, which ensures a lower sublimation temperature of Se and conversion. As a consequence, the as-prepared integrated electrode shows well inherited ultrathin nanostructure and remarkable electrocatalytic activity for HER. It only needs a low overpotential of 172 mV to deliver the current density of 100 mA cm–2 with a ultralow Tafel slope of 32.44 mV dec-1 in H2SO4, which outperforms conventional NiSe2 nanostructures such as NSs and NPs. With the successful preparation of NiSe2 ultrasmall NW, this new vacuum selenization method shows promise for future works on morphology-controllable syntheses of other low melting point transition metal chalcogenides and further improving the performance of NiSe2 towards HER.

EXPERIMENTAL SECTION Synthesis of Ni2(CO3)(OH)2 NWs/CFP. Ni(NO3)2·6H2O (0.6 g, Aladdin) and CO(NH2)2 (0.35 g, Aladdin) were dissolved into distilled water (80 mL). After stirring, the homogenous solution was transferred into a Teflon-lined stainless steel autoclave (100 mL). A CFP substrate (60 mm ×10 mm, Toray TGP-H-060) cleaned by ethanol, 10% H2SO4 (Sinopharm group) and distilled water, successively, was then placed in and against the inner wall of the autoclave. The autoclave was sealed and heated to 100 °C for 15 h. After cooling, the precursor was rinsed under sonication in water and ethanol, respectively, and finally dried in air at 80

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o

C. For comparison,


Ni2(CO3)(OH)2 NSs/CFP was also synthesized according to procedure reported in previous work.45 Thermal selenization of Ni2(CO3)(OH)2 NWs/CFP. A modified thermal selenization method was used for the conversion of the precursor NWs into NiSe2 NWs, as shown in Figure 1. Firstly, the Ni2(CO3)(OH)2 NWs/CFP and selenium powders (Aladdin) were placed into a porcelain crucible, separately. Afterwards, the uncovered crucible was placed into a tube furnace which was then purged with Ar three times to remove oxygen. The sample was heated at a ramping rate of 10 oC min-1 to 315 oC and maintained at this temperature for 0.5 h. We note that during the heating and the subsequent cooling, a low vacuum (1.3 Pa) instead of flowing inert gas was maintained in the quartz tube to ensure the formation of NWs. By contrast, conducting the thermal selenization in flowing Ar (99.999%) gas at 350 °C (the minimum required temperature for successful selenization) for 0.5 h yields NiSe2 NPs/CFP. In the same way, the NiSe2 NSs/CFP was converted from Ni2(CO3)(OH)2 NSs/CFP in flowing Ar (99.999%) gas at 350 °C for 0.5 h. Characterizations. The crystal structures and valence state information of the as-synthesized samples as well as the as-tested samples were identified by X-ray diffractometer (XRD, Cu Kα, D8 Advance, Bruker) and X-ray photoelectron spectroscopy (XPS, PHI-5400, Physical Electronics, Inc.), respectively. The morphology and the composition of the products was characterized by field emission scanning electron microscopy (FESEM, S4800, Hitachi) equipped with energy 6 ACS Paragon Plus Environment

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dispersive X-ray spectrometer (EDS, INCA 250, Oxford Instruments) and high-resolution transmission electron microscope (HRTEM, JEM-F200, JEOL). The loadings of catalysts were determined by a high-precision electronic balance (0.01 mg, BT 25 s, Sartorius). Electrochemical tests. The HER performance of NiSe2 NWs/CFP in N2-saturated 0.5 M H2SO4 solution was measured on an electrochemical workstation (CHI 660D, CH Instruments, Inc.) at room temperature by using a typical three-electrode configuration. The samples (20×10 mm2) were used as the working electrode with a graphite rod as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Before testing, electrochemically inert silicon rubber was used to define an active geometric area of 10×10 mm2. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 100 kHz to 0.01 Hz at the overpotential of 150 mV with the amplitude of 10 mV. Linear sweeping voltammetry (LSV) was recorded from 0.2 to -0.3 V (vs RHE) at a scan rate of 5 mV s-1. The double-layer capacitances (Cdl) was determined by cyclic voltammetry (CV) between 0.1 and 0.2 V (vs RHE) at the scan rates of 5, 10, 20, 50, 100 and 200 mV s-1. All potentials reported here are calculated versus the reversible hydrogen electrode (RHE) according to the following Equation (1):

Evs RHE = Evs SCE + 0.244 + 0.059pH

(1)

For comparison, the activity of bare CFP, commercial Pt/C catalyst (20 wt.% Pt), NiSe2 NPs/CFP and NiSe2 NSs/CFP was also determined under the same conditions. 7 ACS Paragon Plus Environment


Continuous cyclic voltammetry test for the NiSe2 NWs/CFP was conducted for 1000 cycles at a scanning rate of 100 mV s-1. A long-term stability test was also taken by chronoamperometry (CA) under a fixed overpotential (148 mV) with an initial current density of ~13.5 mA cm-2 for 40 h.

Figure 1. Schematic diagram of synthesizing NiSe2 NW arrays on CFP.

RESULTS AND DISCUSSION As illustrated in Figure 1, a two-step procedure has been used to fabricate NiSe2 NWs. In the first step, Ni2(CO3)(OH)2 NWs as the precursor were synthesized by a facile hydrothermal route, and the XRD pattern of the precursor is shown in Figure S1 (Supporting information). As observed, the characteristic peaks match with the standard PDF card of Ni2(CO3)(OH)2 (JCPDS 35-0501), verifying the formation of the precursor. During the second step, the precursor was subjected to thermal 8 ACS Paragon Plus Environment

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selenization in vacuum. Figure 2a shows the XRD pattern of the product. The two peaks at 26.38o and 54.54o are from CFP (JCPDS No. 41-1487). Noticeably, the diffraction peaks at 29.95o, 33.58o, 36.89o, 50.74o and 57.81o can be indexed to the (200), (210), (211), (311), and (321) planes of cubic-phase NiSe2 (JCPDS No. 88-1711) respectively, suggesting the successful conversion of Ni2(CO3)(OH)2 into NiSe2 by this modified process. XPS survey spectrum further confirmed the presence of Ni and Se elements in the samples (Figure 2b). The signals of C and O elements can also be detected because of carbon contamination during the tests and the surface oxidation of the products. The main peaks exhibits at 853.7 eV and 871.2 eV assigned to Ni2+ 2p3/2, 2p1/2 peaks in NiSe2 (Figure 2c), which are in agreement with previous report. 46 The main peak shoulders centred at 855.6 eV and 873.7 eV can be assigned to Ni2+-O which is due to the surface oxidation. As shown in Figure S2, the pure NiO as a reference sample shows the unique high binding energy shoulder along with the main peaks having a well-known doubly peaked structure, which originates from the nonlocal screening effects.47-49 On the contrary, the multiplets corresponding to different electronic configurations of NiSe2 with pyrite structure show smaller discrepancy in binding energy and the single main peak feature is achieved.50 Based on the characteristics of the Se 3d spectra (Figure 2d), the product is mainly composed of NiSe2 with a slight degree of surface oxidation. The third peaks for both Ni 2p3/2 and 2p1/2 correspond to



the satellite peaks (identified as Sat.), which are derived from the electron rearrangement after X-ray excitation.

Figure 2. Characterization of the as-prepared NiSe2 NWs/CFP by XRD and XPS. (a) XRD pattern. (b) full range XPS spectrum. XPS spectra of (c) Ni 2p and (d) Se 3d regions.

SEM images of the as-prepared precursor are shown in Figure S3a and b. As illustrated, the carbon fibers were uniformly covered with dense and ultrathin Ni2(CO3)(OH)2 NWs which are almost vertical to the substrate surface. TEM images indicate that the average length of these 1D NWs is ~100 nm and each NW consists of necklace-like linked particles, 3~5 nm in diameter (Figure S3c and d). Figure 3 shows


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the morphology of the product after thermal selenization. As can be seen from the SEM images (Figure 3a), the Ni2(CO3)(OH)2 NWs morphology remains almost intact after conversion except that the surface of NiSe2 NWs are relatively coarser and rougher (Inset in Figure 3a). The EDS analysis shown in Figure S4 indicates the atomic ratio of Ni:Se is close to the stoichiometric value of 1:2. Accordingly, the TEM images reveal that the average length of the NiSe2 NWs remains ~100 nm, whereas the average diameter of the NWs is almost three times larger than that of the precursor (Figure 3b). This is likely due to the combination of a bundle of Ni2(CO3)(OH)2 NWs into single NiSe2 NW in the thermal selenization treatment. HRTEM images in Figure 3c verifies lattice fringes with d-spacings of 0.21 nm and 0.27 nm corresponding to the (220) and (210) planes of NiSe2, respectively. The selected area electron diffraction (SAED) pattern indicates that the NiSe2 NWs are polycrystalline rather than single crystalline (Figure 3d). EDS element mapping in scanning TEM mode of a single NiSe2 nanowire confirms the uniform distribution of Ni and Se elements and low O contents (Figure 3e). By contrast, the selenization conducted at 315 oC in flowing Ar gas can only yield partially converted selenides (as shown in Figure S5). Moreover, increasing the temperature or decreasing the heating rate resulted in the collapse of the NW structure and aggregated nanoparticles (Figure S6). For comparison, the morphology of NiSe2 NPs/CFP prepared by the optimal process in flowing Ar is shown in Figure S7. Similar to the NPs synthesis, the NiSe2 NSs/CFP was also synthesized in the flowing Ar with the NS precursor, and their 11 ACS Paragon Plus Environment


morphologies are also presented in Figure S8. Notably, the crystalline size of NSs (~20 nm) is significantly larger than that of NWs.

Figure 3. (a) Low- and high-magnification (inset) SEM images of NiSe2 NWs/CFP. (b) TEM and (c) HRTEM images of NiSe2 NWs. (d) SAED pattern and (e) EDS elemental mapping.

The HER performance of NiSe2 NWs/CFP was examined at room temperature in 0.5 M H2SO4 with a typical three-electrode configuration. The HER activity of NiSe2 NPs/CFP and NiSe2 NSs/CFP (with the same loading of 1.23 mg of NiSe2 NWs), bare CFP, as well as commercial Pt/C (20 wt % Pt) were also tested for comparison. Figure 4a shows their polarization curves after iR-corrections. It can be seen that the Pt/C


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catalyst shows the highest HER activity, featuring low overpotentials of 38 mV and 46 mV for driving the current density of 10 mA cm−2 and 100 mA cm−2, respectively. In contrast, the bare CFP shows no HER activity. Apparently, NiSe2 NWs/CFP exhibits considerably enhanced HER performance compared to NiSe2 NPs/CFP and NiSe2 NSs/CFP. In specific, the NiSe2 NWs/CFP requires overpotentials of 142 mV and 172 mV to maintain the current density of 10 mA cm−2 and 100 mA cm−2, respectively, which are much lower than those of NiSe2 NSs/CFP (159 mV and 200 mV) and NiSe2 NPs/CFP (185 mV and 246 mV). Moreover, the onset overpotential of NiSe2 NWs/CFP (100 mV) is also lower than that of NiSe2 NSs/CFP (124 mV) and NiSe2 NPs/CFP (145 mV). The corresponding Tafel slopes of the three catalysts were also determined (Figure 4b), where that of NiSe2 NWs/CFP is only 32.44 mV dec-1, which is very close to that of Pt/C catalyst (30.70 mV dec-1). In contrast, NiSe2 NSs/CFP and NiSe2 NPs/CFP exhibit higher Tafel slopes of 39.29 mV dec-1 and 46.07 mV dec-1, respectively. It is widely accepted that the HER via water splitting is a classic two-electron reaction during which the hydrogen adsorption is the first step via the Volmer reaction:

H3O+ + e- + M → Hads +H2O

(2)

and followed by hydrogen desorption derived from two competing processes51, 52:

Hads + H3O+ + e- → H2 + H2O (Heyrovsky reaction) or 13 ACS Paragon Plus Environment

(3)


Hads +Hads → H2 (Tafel reaction)

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(4)

At 25 oC, if the Tafel slope is about 120 mV dec-1, it suggests the Volmer reaction is the rate determining step (RDS) for the HER. The Heyrovsky reaction or Tafel reaction is believed to be the RDS if the Tafel slopes are 39 or 29 mV dec-1, respectively.53-55 In our case, the Tafel slope of NiSe2 NWs is close to 29 mV dec-1, suggesting the Tafel reaction (electrochemical desorption step) is the RDS for HER and the electrocatalysis proceeds mainly via the Volmer-Tafel mechanism. By contrast, the Tafel slopes of NiSe2 NSs and NPs are close to 39 mV dec-1, indicating that the Volmer–Heyrovsky route is the dominant process, during which the RDS is the Heyrovsky reaction. The performance NiSe2 NWs is also very competitive among the state-of-the-art noble metal free electrocatalysts. As summarized in Table S1, the NiSe2 NWs outperform NiSe2 NPs,17,

19, 25, 27

NiSe2/Ni,24 NiSe2 nanowalls/CC,23

coral-like NiSe228 and NiSe2/MoSe2 composite NWs46 reported in previous works, and also exceed many pyrite-type transition metal dichalcogenides such as CoSe2 NWs,44 CoS2 NWs 11 as well as NiS2 NWs.56


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Figure 4. (a) Polarization curves (iR-corrected) of NiSe2 NWs/CFP and other catalysts; (b) the corresponding Tafel slopes; (c) double-layer capacitances (Cdl) of the three electrodes derived from the cyclic voltammograms (CVs); (d) Electrochemical impedance spectroscopies (EIS) of NiSe2 NWs/CFP, NiSe2 NSs/CFP and NiSe2 NPs/CFP, respectively.

As a representative parameter to evaluate the electrochemically surface area (ECSA) at the solid-liquid interface, the double-layer capacitances (Cdl) of NiSe2 NWs/CFP and other samples were determined by cyclic voltammograms (CVs), as shown in Figure 4c. The original CVs curves under various scan rates are shown in Figure S9. The CVs of the NiSe2 NWs, NSs and NPs were determined in the range from 0.1 to 15 ACS Paragon Plus Environment


0.2 V (vs RHE) under various scan rates (5-200 mV s-1). The current density differences (∆j= (janodic − jcathodic) at 0.15 V (vs RHE) are then plotted against the various scan rates. The electrochemical double-layer capacitance (Cdl) values are equal to half of the linear slopes in Figure 4c. It can be observed that the Cdl of NiSe2 NSs/CFP electrode is 19.45 mF cm-2, which is much higher than that of NiSe2 NPs/CFP (5.95 mF cm-2). While the Cdl of NiSe2 NSs/CFP electrode is obviously smaller than that of NiSe2 NWs/CFP electrode (28.15 mF cm-2). The differences in Cdl strongly suggest that larger ECSA of NiSe2 NWs significantly contributes to the enhanced HER performance. Aside from the ECSA, electrochemical impedance spectroscopy (EIS) also provides insights to the high performance of NiSe2 NWs/CFP. As shown in Figure 4d, the EIS of NiSe2 NWs/CFP, NiSe2 NPs/CFP and NiSe2 NSs/CFP was recorded at the same overpotentials and fitted into hemispheres by using a simple equivalent Randles circuit model (Insert in Figure 4d). As expected, the series resistances (Rs) of the three electrodes are remarkably low (~2.0 Ω), demonstrating the contact between the catalysts and the carbon fiber is reasonably reliable. The charge transfer resistance (Rct) of NiSe2 NWs/CFP is 2.21 Ω, which is apparently lower than that of NiSe2 NSs/CFP (3.32 Ω) and NiSe2 NPs/CFP (5.96 Ω), respectively. It is widely recognized that smaller Rct indicates faster charge transfer, implying superior kinetics for HER.51 Collectively, the significantly improved activity of NiSe2 NWs for HER can be attributed to the following aspects: (i) the dense and ultrathin NWs have higher specific surface area, offering a large number of active 16 ACS Paragon Plus Environment

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sites on the surface;52, 57 (ii) the NW architecture facilitates the electron transport along the axial direction and mass transfer between NWs;58,

59

(iii) the porous

nanostructure as well as the sharp tips of NiSe2 NW, promoting the detaching of the H2 bubbles.11,

60

To further illustrate the effects of morphology on the intrinsic

catalytic activity of NiSe2-based catalysts, TOFs (Turnover frequency) per active site were estimated according to the method reported in previous work.45 The calculated TOFs per site for NiSe2 NWs, NSs and NPs are 0.557 H2 s-1, 0.173 H2 s-1 and 0.142 H2 s-1, respectively, at the overpotential of 172 mV (Supporting information). The three or four times larger TOF indicates a much higher intrinsic activity of NiSe2 NWs/CFP compared with the counterparts NiSe2 NSs/CFP or NiSe2 NPs/CFP, which can be attributed to the smaller crystal size of NiSe2 NWs. Such smaller crystals contains more unsaturated surface atoms, grain boundaries and defects,61 which contribute to a lower energy barrier and fast kinetics.62 Also, The mass activity values of different samples were calculated from the electrocatalyst loading and the measured current density j (mA cm2) at η=50 mV, 75 mV, 100 mV, 125 mV and 150 mV.63, 64 As can be seen in Figure S10, the mass activity of NiSe2 NWs (12.20 A g-1) is almost 2.2 times greater than that of NiSe2 NSs (5.53 A g-1) and 4.4 times greater than that of NiSe2 NPs (2.76 A g−1) at the overpotential of 0.15 V, demonstating that such an activity improvement is independent of the loading of catalyst. The H2 production measured by gas chromatography (GC) shows near 100% Faradic efficiency (FE)



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within the measurement error (Supporting information), suggesting the current density is directly used for hydrogen generation. The performance of NiSe2 NWs/CFP subjected to a continuous CV testing of more than 1000 cycles (Figure 5a) was examined. There is only a slight difference between the polarization curves of the electrode before and after the test, indicating the excellent

electrochemical

stability

of

NiSe2

NWs/CFP.

Furthermore,

the

chronoamperometry was also employed to assess the practical operation of the catalyst (Figure 5b). At a fixed overpotential of 148 mV, the initial current density appears to be unchanged at 13.5 mA cm-2 over 40 h. SEM images of the NiSe2 NWs after the long-term tests verify the NW morphology was unchanged (Figure S12a and b). The corresponding XRD pattern reveals the stability of the NiSe2 phase after the long-term electrolysis (Figure S12c). Besides, according to the XPS spectra, the after-test NiSe2 NWs/CFP electrode contains Ni, Se and O elements centering at the binding energies similar to those in the as-prepared sample (Figure S12d and e). EDS analysis demonstrates the elemental ratio of Ni:Se is still close to the stoichiometry of 1:2 (Figure S12f). The stability of morphology, phase structure and composition after long-term electrolysis were confirmed by TEM analysis (Figure S13). The exceptional durability of the NiSe2 NWs for HER in strong acidic solution suggests its potential long lifetime in practical application.


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Figure 5. (a) Polarization curves (iR-corrected) of NiSe2 NWs/CFP before and after 1000 cycles. (b) Current density vs time (j−t) curve of NiSe2 NWs/CFP over 40 h.

CONCLUSION

NiSe2 NWs were successfully synthesized on CFP through an improved low temperature thermal selenization method during which the conversion of the precursor NWs was conducted in vacuum instead of a flowing inert gas atmosphere. This hierarchical NiSe2 nanowire arrays directly grown on CFP exhibits remarkable activity for HER in acidic solution. The integrated electrode only requires a small overpotential of 172 mV to deliver the current density of 100 mA cm-2, corresponding to a Tafel slope as low as 32.44 mV dec-1. Additionally, NiSe2 NWs/CFP displays an impressive stability in the strong acidic solution. The NiSe2 NW presents a superior performance than its counterpart NiSe2 NS and NP, which proves the success of the morphology optimization in further improving the HER performance. The method introduced herein provides a novel approach for the design and synthesis of



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hierarchical transition chalcogenide nanomaterials as the high performance and cost-effective hydrogen generation electrocatalysts.

ASSOCIATED CONTENT

Supporting Information. XRD pattern; SEM images; TEM images; EDS; CVs; XPS; TOF calculation; Faradaic efficiency calculation; Tables S1.

AUTHOR INFORMATION

Corresponding Authors *

E-mail: [email protected] (K. Wang)

*

E-mail: [email protected] (Z. Shi)

*

E-mail: [email protected] (C. Zhao)

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors acknowledge financial support by National Natural Science Foundation of China (51401034), the Key Research and Development Plan in Shaanxi Province of

China

(2017GY-033),

Shaanxi

Postdoctoral

Science

Foundation

(2017BSHTDZZ01), the China Scholarship Council (CSC, No.201706565012), the 20 ACS Paragon Plus Environment

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Special Fund for Basic Scientific Research of Central Colleges, Chang’an University (310831172001), and Australian Research Council (DP160103107, FT170100224).

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Table of contents

Synopsis:

The preparation method described here could inspire future researches on the synthesis of transition metal dichalcogenides as cost-effective, high performance and long-term electrocatalysts for sustainable production of hydrogen as green alternative fuels.


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Self-Supported NiSe2 Nanowire Arrays on Carbon Fiber Paper as

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