Letter www.acsami.org
Rational Design of Efficient Electrocatalysts for Hydrogen Evolution Reaction: Single Layers of WS2 Nanoplates Anchored to Hollow Nitrogen-Doped Carbon Nanofibers Sunmoon Yu,†,‡ Jaehoon Kim,†,§ Ki Ro Yoon,‡ Ji-Won Jung,‡ Jihun Oh,*,§ and Il-Doo Kim*,‡ ‡
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea § Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *
ABSTRACT: To exploit the benefits of nanostructuring for enhanced hydrogen evolution reaction (HER), we employed coaxial electrospinning to synthesize single-layered WS2 nanoplates anchored to hollow nitrogen-doped carbon nanofibers (WS2@HNCNFs) as efficient electrocatalysts. For comparison, bulk WS2 powder and single layers of WS2 embedded in nitrogen-doped carbon nanofibers (WS2@ NCNFs) were synthesized and electrochemically tested. The distinctive design of the WS2@HNCNFs enables remarkable electrochemical performances showing a low overpotential with reduced charge transfer resistance, a small Tafel slope, and excellent durability. The experimental results highlight the importance of nanostructure engineering in electrocatalysts for enhanced HER. KEYWORDS: single layer, tungsten disulfide, hydrogen evolution reaction, electrocatalyst, hollow carbon nanofiber
H
recent years has been shifted toward earth-abundant, low-cost HER electrocatalysts with superb catalytic activities comparable to PGMs. Such materials include transition metal dichalcogenides (TMDs) as molybdenum disulfide (MoS2) and tungsten disulfide (WS2). Several TMDs possess a layered structure where each layer is coupled by weak van der Waals forces while intralayer bonds are predominantly covalent.6 The layered structure gives rise to anisotropic chemical, electronic, and electrochemical characteristics.7−9 For example, electrical conductivity along a basal plane is much more rapid than that perpendicular to the planes. Furthermore, dimension reduction from three-dimensional to two-dimensional TMDs leads to the emergence of extraordinary physicochemical properties that are different from those of their bulk material counterparts.7 With regard to electrocatalytic activity, the hydrogen adsorption energies on edge sites of the TMDs are close to zero, which indicates that, according to the Sabatier principle, the edges are optimal electrocatalytic sites for the HER.10−12 In other words, only the edges of the TMDs are electrocatalytically active for the HER whereas their basal planes remain inert.10 As a result, the practical use of bulk
ydrogen has been regarded as an ideal energy carrier, which can potentially replace fossil fuels, by virtue of its highest gravimetric energy density without carbon dioxide (CO2) emission.1,2 However, unfortunately, hydrogen does not exist in the form of a free molecule even though it is the most abundant element on the earth.3 Hence, sustainable production of H2 with excellent efficiency is of great importance for largescale production. Electrochemical water splitting can be a promising environment-friendly way to produce H2 compared to a conventional steam reforming process of hydrocarbons.4 Nevertheless, its efficiency has to be much improved from a practical perspective. The hydrogen evolution reaction (HER) refers to the cathodic half reaction of water splitting, where 2H+ + e− → H2 in acidic electrolytes.2,4 To drive the reaction, an activation energy barrier has to be overcome, which is an extra energy in addition to thermodynamically required energy.2 For efficient H2 production to be accomplished, electrocatalysts are employed to lower the activation energy, or overpotential in electrochemistry, consequently facilitating the reaction. Among heterogeneous HER electrocatalysts, platinum-group metals (PGMs) show the best electrocatalytic performances in the HER.2,5 However, Pt, for example, is one of the precious elements and highly expensive. It is worth noting that costs of electrocatalysts are more relevant to practical limitations on the large-scale implementation of HER electrocatalysts than their thermodynamic efficiencies.5 In this context, much attention in © XXXX American Chemical Society
Received: October 6, 2015 Accepted: December 14, 2015
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DOI: 10.1021/acsami.5b09447 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. Schematic illustration of (a) coaxial electrospinning, (b) as-spun core−sheath SAN@WS2 precursor/PAN nanofiber composite, which undergoes a subsequent thermal treatment, resulting in (c) a core−sheath nanofiber structure where single layers of WS2 nanoplates are uniformly anchored to hollow N-doped carbon nanofibers.
(between 2 and 50 nm in diameter) can be formed directly by thermal decomposition of the sacrificial SAN phase.19 For comparison purposes, WS2@NCNFs was manufactured via single-spinneret electrospinning of PAN solution with the same WS2 precursor well-dispersed in the solution, and the subsequent two-step thermal treatment. On the other hand, bulk WS2 powder was also synthesized via the thermal treatment of the WS2 precursor but without any electrospinning process. The carbon contents of the WS2@HNCNFs and the WS2@NCNFs, measured by elemental analysis (EA), were 34.5 and 32.5 wt %, respectively. This was further confirmed by thermogravimetric analysis (TGA) in Figure S1. The weight loss of 5.5 wt % around 300 °C in the TGA curve corresponds to the oxidation of WS2 nanoplates into WO3 while that of 33 wt % above 400 °C represent the decomposition of the HNCNFs. Scanning electron microscopy (SEM) images of as-spun core−sheath SAN@WS2 precursor/PAN nanofiber composite and as-spun WS2 precursor/PAN nanofiber are shown in Figure S2. After the thermal treatment, WS2@HNCNFs was obtained and its morphology is shown in Figure 2a. The inner diameter of the core−sheath structure was ca. 220 nm with the wall thickness of ca. 40 nm. Meanwhile, the WS2@NCNFs has a diameter of ca. 200 nm (Figure S3a). The hollow structure of the WS2@HNCNFs was clearly observed from transmission electron microscopy (TEM) images of the WS2@HNCNFs in Figure 2b and c. Furthermore, we confirmed that single layers of WS2 nanoplates with the lateral dimension of ca. 5 nm are uniformly anchored to HNCNFs as shown in high-resolution TEM (HRTEM) images (Figure 2d and e). Similarly, singlelayered WS2 nanoplates are homogeneously embedded in NCNFs as shown in Figure S3b and c. This growth behavior can be attributed to the confined growth of WS2 within carbon matrix, which can provide nucleation sites but simultaneously restrict the stacking of WS2 nanoplates.20,21 However, in the case of the WS2@HNCNFs, micropores and mesopores were observed in the vicinity of single layered WS2 nanoplates. On the contrary, single layers of WS2 nanoplates in the WS2@ NCNFs were densely packed without obvious voids. As a result, single-layered WS2 nanoplates in the WS2@HNCNFs were more exposed as shown in Figure 2e. The high angle annular dark field scanning transmission electron microscopy (HAADFSTEM) image with energy-dispersive X-ray spectroscopy (EDS) line-scan profile superimposed in Figure 2f explicitly reveals that WS2 nanoplates and doped N atoms are
TMDs to catalyze the HER has been neglected due to their poor electrocatalytic activities stemming from their limited number of exposed edge sites in the bulk forms.13 In an effort to climb up “volcano plot” for the HER, which correlate exchange current density with hydrogen adsorption energy reflecting the Sabatier principle, two viable approaches have been proposed in general: (i) nanostructure engineering, and (ii) electronic structure engineering.3,4,14 The nanostructure engineering approach intends to significantly increase the number of electrocatalytically active sites which are electrically accessible as well for the HER.15−17 On the other hand, the latter approach involves the modification of intrinsic chemical properties such as electronic band structure to be optimized for efficient HER. Herein, we report a rationally designed electrocatalyst fabricated via coaxial electrospinning for enhanced HER; single-layered WS2 nanoplates are uniformly anchored to hollow nitrogen-doped carbon nanofibers (WS2@HNCNFs), fully exploiting the benefits of nanostructuring. The schematic illustration in Figure 1 shows the synthetic procedure of WS2@ HNCNFs via coaxial electrospinning of poly(styrene-acrylonitrile) (SAN) solution for the core and poly(acrylonitrile) (PAN) solution for the sheath in which WS2 precursor is homogeneously dispersed (Figure 1a). SAN was chosen as a polymeric sacrificial component owing to its immiscibility with PAN solution, providing robust support for the sheath structure.30 Subsequently, a two-step thermal treatment was conducted in a reducing atmosphere (H2/N2, 5% v/v) (Figure 1b and c). The first step at 400 °C was intended to thermally decompose (NH4)2WS4 resulting in single layers of WS2 nanoplates as expressed in reaction 1 and reaction 2, and to stabilize PAN while thermal decomposition of SAN occurs. The ensuing second step at 700 °C was conducted for the carbonization of the HNCNFs. (NH4)2 WS4 → 2NH3 ↑ +H 2S ↑ +WS3
(1)
WS3 → WS2 + S↑
(2)
As expressed in reaction 1, ammonia (NH3) gas is generated, and this allows for in situ N-doping of the HCNFs.18 Also, the reducing gas (H2/N2, 5% v/v) used in the thermal treatment can further participate in the in situ N-doping. Hydrogen sulfide (H2S) and sulfur gases are also emitted during the thermal treatment. The gases penetrating the carbon matrix can create micropores (less than 2 nm in diameter) while mesopores B
DOI: 10.1021/acsami.5b09447 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 3. (a) XRD patterns of WS2@HNCNFs, WS2@NCNFs, and WS2 powder. (b) Raman spectra of WS2@HNCNFs, HCNFs, and WS2 powder. In-plane vibrational mode (E12g) and out-of-plane vibrational mode (A1g) are illustrated in the schematic diagram where green and yellow balls represent W and S, respectively. XPS spectra of WS2@HNCNFs: (c) W 4f and 5p3/2, and (d) S 2p. (e) Nitrogen adsorption/desorption isotherms and (f) pore size distribution curves of WS2@HNCNFs and WS2@NCNFs.
Figure 2. (a) SEM image of WS2@HNCNFs (scale bar in the inset corresponds to 200 nm). (b, c) TEM images and (d, e) HRTEM images of WS2@HNCNFs. Dashed white ellipses indicate pores in the carbon matrix. A white ellipse with a white allow in (e) points at a WS2 single layer. (f) STEM image of WS2@HNCNFs on which EDS linescan profile of W, S, N, and C is superimposed.
nanoplates have poor crystallinity. This defect-rich nature of single-layered WS2 nanoplates in the WS2@HNCNFs and the WS2@NCNFs can be beneficial in terms of electrocatalytic activity. This is because the existence of plentiful defects inevitably creates numerous edge sites with lower coordination number, promoting its electrocatalytic activity for the HER.22 The Raman spectra of the WS2@HNCNFs, HCNFs, and WS2 powder using 514 nm excitation are shown in Figure 3b. The peaks for the in-plane vibrational mode (E12g) and the outof-plane vibrational mode (A1g) of the WS2@HNCNFs were located at 353.1 and 417.1 cm−1, respectively. Thus, the difference between two peak locations was 64 cm−1, and the intensity of E12g was much stronger than that of A1g. It has been demonstrated that the difference between the two peak locations tends to increase, and the intensity ratio of the two peaks decreases as the number of WS2 layers increases.23 Correspondingly, the peaks for E12g and A1g of the WS2 powder were identified at 353.1 and 421.5 cm−1, respectively; hence, the difference in location between two peaks was widened to 68.5 cm−1, and their peak intensities became comparable. The D-band associated with defects and the G-band related to graphitic crystallites in the carbon matrix for the both of the WS2@HNCNFs and the HCNFs were found at 1357 and 1603 cm−1, respectively. The anchored WS2 nanoplates may possibly induce more defects and disorder in the hollow N-doped carbon nanofibers. However, the intensity ratio of the D-band
homogeneously distributed in the hollow carbon matrix. Note that the shape of EDS line-scan profile of the WS2@HNCNFs is different from that of EDS line-scan profile of the WS2@ NCNFs (Figure S 3d) due to the hollow architecture of the WS2@HNCNFs. On the other hand, WS2 micropowder synthesized without electrospinning was composed of multilayered (5−15 layers) WS2 with the longer lateral dimension of over 15 nm whose SEM and TEM images are shown in Figure S4. The X-ray diffraction (XRD) patterns of WS2@HNCNFs, WS2@NCNFs, and WS2 powder are shown in Figure 3a. The diffraction peaks of the WS2 powder well correspond to 2HWS2 (P63/mmc space group, JCPDS no. 08−0237). However, the (002) peak is absent in the XRD patterns of the WS2@ HNCNFs and the WS2@NCNFs, which implies that the number of the WS2 layers are less than five.20 The selected area electron diffraction (SAED) patterns of the WS2@HNCNFs and the WS2 powder in Figure S5 agree well with the XRD data. The SAED pattern of the WS2@HNCNFs does not have the (002) plane ring pattern, and diffraction ring patterns are relatively vague whereas that of the WS2 powder has much more obvious diffraction rings including the (002) plane. The blurry ring pattern signifies that the single-layered WS2 C
DOI: 10.1021/acsami.5b09447 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. (a) Linear sweep voltammetry (after iR-compensation) of various electrocatalysts as indicated. (b) Corresponding Tafel plots for the WS2based electrocatalysts and a commercial Pt/C electrocatalyst. (c) EIS spectra of WS2-based electrocatalysts, collected at the potential of −0.3 V (vs RHE). Inset shows an equivalent circuit model used for EIS. Rct is the charge transfer resistance, Rs is the resistance of the electrolyte, and CPE is the constant phase element. (d) The chronoamperometry of WS2@HNCNFs at −0.3 V (vs RHE). Inset shows polarization curves before and after 8 h. Digital images show the evolution of H2 from the WS2@HNCNFs as time passes. The scale bars in the digital images indicate 5 mm.
in Figure 3e. The surface area of the WS2@HNCNFs and the WS2@NCNFs determined by Brunauer−Emmett−Teller (BET) analysis were 34.6 and 6.7 m2 g−1, respectively. The much enhanced surface area is beneficial for facilitating the HER in terms of the increase in the number of active sites.1 The isotherm of the WS2@HNCNFs falls into type IV indicating a highly porous nanostructure according to the IUPAC classification.25 However, the isotherm of the WS2@ NCNFs belongs to type I, which suggests that its structure possesses mainly micropores. Corresponding pore size distribution curves are plotted in Figure 3f using Barrett− Joyner−Halenda (BJH) method, which supports the analysis of the isotherm curves. In the case of the WS2@NCNFs, only small amount of byproduct gases including NH3 are generated from WS2 precursor during the thermal treatment. As these gases are penetrating the carbon matrix, they can create micropores, and this explains a small peak less than 2 nm in its pore size distribution curve. On the other hand, sacrificial SAN in the core of the WS2@HNCNFs is thermally decomposed, and plays a critical role in developing largely mesopores.19,26 The sharp peak around 5 nm and the broad peak in a range of 10−100 nm in the pore size distribution curve can be interpreted as pores formed by abrupt gases evolved from the sacrificial component and subsequently penetrating the sheath. Figure 4a compares the electrocatalytic HER activities of various WS2-based and nanofiber-based electrocatalysts in 0.5 M H2SO4. As shown in Figure 4a, the WS2@HNCNFs exhibit enhanced HER catalytic performance, compared to the WS2 powder or the WS2@NCNFs. For example, the onset potential
to the G-band (ID/IG) was approximately 0.97 for the both of them, which suggests that WS2 nanoplates trivially affect the quality of the carbon matrix. The Raman spectra of WS2@ NCNFs and CNFs are also shown in Figure S6. The Raman spectra of the WS2@NCNFs is analogous to that of the WS2@ HNCNFs, whereas that of the CNFs is similar to that of the HCNFs. The X-ray photoelectron spectroscopy (XPS) survey spectra confirm the presence of W, S, C, and N elements in the WS2@ HNCNFs (Figure S7a). The W XPS spectra in Figure 3c display W 4f and 5p3/2 peaks at 32.8, 35, and 38.3 eV. Oxidized WS2 peaks were hardly identified. The two S 2p peaks were located at 162.6 and 163.8 eV as shown in Figure 3d. Moreover, the C 1s peaks in Figure S7b can be deconvoluted into C−C, C−N, CO, and CO-O at 284.5, 285.5, 289.1, and 292.3 eV, respectively. Meanwhile, the N 1s peaks in Figure S7c can be assigned to pyridinic N (N-6), pyrrolic N (N-5), quaternary N (N-Q), and pyridinic-N oxide (N−O), identified at 398.5, 400.1, 400.9, and 403.3 eV, respectively, and these four forms of N are schematically illustrated in Figure S7d. In particular, pyridinic N (N-6) and quaternary N (N-Q) are sp2 hybridized in the carbon matrix; thus, they can contribute to improving the electrical conductivity and accordingly enhanced electron transport.24 Even though electron transport is a separate process from the charge transfer in the course of the HER, it has a great impact on the overall electrocatalytic activity of a catalyst.3,14 The N 2 adsorption−desorption isotherms of WS 2 @ HNCNFs and WS2@NCNFs performed at 77 K are displayed D
DOI: 10.1021/acsami.5b09447 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces for HER of WS2@HNCNFs is −185 mV (vs RHE) while those of the WS2@NCNFs and the WS2 powder are −240 and −330 mV (vs RHE), respectively. In addition, the WS2@HNCNFs exhibits reduced overpotential about 90 mV at −10 mA cm−2, compared to the WS2@NCNFs. The improved catalytic activity is in part attributed to increased catalytic active sites (i.e., edge sites) of single-layered WS2 in the WS2@HNCNFs; the surface area of the WS2@HNCNFs (34.6 m2 g−1) is approximately five times larger than that of the WS2@NCNFs (6.7 m2 g−1) (see Figure 3e). Note that, as shown in Figure 4b, Tafel slope of the WS2@HNCNFs is 60 mV dec−1, whereas those of the WS2@ NCNFs and the WS2 powder are 110 and 160 mV dec−1, respectively. To investigate the electrode kinetics, we conducted electrochemical impedance spectroscopy (EIS) measurement for WS2based electrocatalysts at the applied potential of −300 mV (vs RHE), as shown in Figure 4c. The small series resistance (Rs) of ∼13 Ω was observed for all the WS2-based electrocatalysts in the Nyquist plot (Figure 4c). The charge transfer resistance (Rct) is then obtained by subtracting the Rs of ∼13 Ω from the second x-intercept of the semicircle. Accordingly, the WS2@ HNCNFs exhibit the smallest Rct of 24 Ω. In addition, the Rct value of the WS2@NCNFs (50 Ω) is also significantly lower than that of the WS2 powder (∼1700 Ω). These remarkably reduced Rct values of the WS2-based nanofibers can be attributed to the use of highly conductive supporting material. Such a carbon matrix can serve as an effective electrical bridge to minimize ohmic losses between the electrode and the catalyst (i.e., WS2), supporting facile electron transport to the electrolyte. Meanwhile, the small Rct differences between the WS2@HNCNFs (24 Ω) and the WS2@NCNFs (50 Ω) are due to the increased catalytically active sites from large surface area achieved by hollow structured NCNFs. Finally, the stability of the WS2@HNCNFs was investigated using chronoamperometry at the constant voltage of −300 mV (vs RHE) for 8 h. As shown in Figure 4d, the WS2@HNCNFs exhibit stable HER current density for 8 h. Note that the significantly diminished current density of the WS2@HNCNFs during the chronoamperometry is believed to originate from the evolved H2 attached to the electrode as seen in digital images in Figure 4d; the attached H2 bubbles reduce the active electrode area. The excellent durability of the WS2@HNCNFs is also confirmed from the identical polarization curves of the WS2@HNCNFs before and after 8 h of operation. The polarization curve after 8 h of operation is measured after removing H2 bubbles on the electrode. In summary, we first demonstrated that single-layered WS2 nanoplates anchored to hollow N-doped carbon nanofibers is an efficient electrocatalyst for the hydrogen evolution reaction. The complementary coupling of single-layered WS2 nanoplates with HNCNFs enabled extraordinary electrocatalytic activity compensating for insulating characteristic of WS2. The WS2@ HNCNFs exhibit relatively low overpotential of 280 mV (vs RHE) to reach −10 mA cm−2 as well as a small Tafel slope of 60 mV dec−1, compared to the WS2@NCNFs and the WS2 powder. Moreover, the WS2@HNCNFs shows excellent durability even after 8 h-operation. The remarkable electrocatalytic performance of the WS2@ HNCNFs can be attributed to its unique nanostructure: (i) edge-rich, defect-abundant single layers of WS2 nanoplates provide numerous active sites; (ii) electrical resistance between interlayers can be eliminated due to single-layered characteristic; (iii) N-doping into the carbon matrix promotes facile
electron transport; (iv) enhanced surface area implies increased number of active sites; (v) gas evolution from sacrificial component can create pores, and consequently expose more active sites. The experimental results clearly illuminate the importance of nanostructure engineering in electrocatalysts for efficient HER.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09447. Experimental section; TGA and DTG curves of WS2@ HNCNFs; SEM images of as-spun products; SEM, TEM, and STEM images of WS2@NCNFs; SEM and TEM images of WS2 powder; SAED patterns of WS2@ HNCNFs and WS2@NCNFs; Raman spectra of WS2@ NCNFs and CNFs; XPS spectra of WS2@HNCNFs with schematic illustration of N atoms in four forms (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions †
S.Y. and J.K. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a grant from the Korea CCS R&D Center (KCRC) funded by the Korean government (Ministry of Science, ICT & Future Planning) (NRF2014M1A8A1049303).
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REFERENCES
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