Facile Synthesis of N-Doped WS 2 Nanosheets as an E ﬃ cient and Stable Electrocatalyst for Hydrogen Evolution Reaction in Acidic Media

: Transition metal chalcogenides have been widely studied as a promising electrocatalyst for the hydrogen evolution reaction (HER) in acidic conditions. Among various transition metal chalcogenides, tungsten disulﬁde (WS 2 ) is a distinguishable candidate due to abundant active sites and good electrical properties. Herein, we report a facile and selective synthetic method to synthesize WS 2 with an intriguing two-dimensional nanostructure by using cysteine (C 3 H 7 NO 2 S) as a chemical agent. In addition, nitrogen can be incorporated during chemical synthesis from cysteine, which may be helpful for enhancing the HER. The electrocatalytic activity of N-doped WS 2 exhibits a promising HER in acidic conditions, which are not only higher than W 18 O 49 nanowires and hex-WO 3 nanowires, but also comparable to the benchmark Pt / C. Moreover, excellent electrocatalytic stability is also demonstrated for acidic HER during long-term tests, thus highlighting its potential use of practical applications as an electrolyzer. catalyst. In addition, excellent electrocatalytic stability was demonstrated for the WS 2 (cysteine 1:2) using a long-term stability test. These results reveal that our facile chemical synthetic method is powerful for the design of high-performance electrocatalysts with controlled phase and morphology.


Introduction
Hydrogen is a promising future energy resource, which has the potential to replace fossil fuel stocks [1,2]. The electrochemical electrolyzer has become a crucial energy conversion system to produce hydrogen in a cost-effective and environmentally friendly way [3][4][5]. Compared to the conventional alkaline electrolyzer, the proton exchange membrane (PEM) electrolyzer has attracted great attention from researchers due to a much higher energy conversion efficiency. With this in mind, electrocatalysts for the hydrogen evolution reaction (HER) in acidic media becomes of paramount importance for efficient electrochemical water splitting using a PEM electrolyzer [6]. Noble metals such as Pt or Pd are currently marked as the best HER electrocatalysts in acidic media [7]. However, the scarcity and high cost of novel metals block their practical use. As such, low-cost electrocatalysts with a high performance need to be developed in order to replace the benchmark novel-metal-based electrocatalysts for the HER.
Tungsten-based compounds (i.e., chalcogenides or oxides) are reported as promising materials for acidic HER [8][9][10]. A multivalent W in the oxides or chalcogenides can give rise to a high probability of electrochemical reactions with electrochemical robustness in harsh acidic conditions. Especially, tungsten chalcogenides such as WS x can exhibit promising electrocatalytic activity due to abundant active sites where both the metal and chalcogenide atoms are electrocatalytically active [11][12][13][14][15][16]. In addition, N dopants, possessing a high affinity to transition metals, can further enhance interfacial charge transfer in WS x . The incorporated N atoms can modulate the local electronic structure of transition metal atoms. Thus, water dissociation can be much more favorable in N-doped WS x compared to the pristine WS x , resulting in an enhanced HER [15].
In this context, here we report a facile synthetic method for N-doped WS 2 as a high-performance HER electrocatalyst in acidic media. Our chemical method could selectively synthesize WS x or WO x by simply changing chemical agents. In addition, an intriguing two-dimensional nanosheet structure was obtained for WS 2 with a successful incorporation of N via a one-step hydrothermal reaction using cysteine (C 3 H 7 NO 2 S) as a chemical agent. The best sample showed a low overpotential of 130 mV for generating a current density of 10 mA cm −2 with excellent catalytic stability up to 12 h in 0.5 M H 2 SO 4 . Figure 1 shows X-ray diffraction (XRD) patterns of W 18 O 49 , WO 3 , WS 2 (cysteine 1:2, 0.01 M), and WS 2 (cysteine 1:4, 0.02 M). XRD results reveal that phase-pure W 18 O 49 (JCPDS: 01-084-1516), WO 3 (JCPDS: 033-1387), and WS 2 (JCPDS: 008-0237) were successfully synthesized using our method. No secondary phases or impurities were presented for all samples. Notably, selective synthesis for each phase could be achieved by only adding thiourea or cysteine to the starting WCl 6 solution. This highlights the practical potential of our synthetic method for preparing tungsten-based compounds.

Results and Discussion
Catalysts 2020, 10, x FOR PEER REVIEW 2 of 9 interfacial charge transfer in WSx. The incorporated N atoms can modulate the local electronic structure of transition metal atoms. Thus, water dissociation can be much more favorable in N-doped WSx compared to the pristine WSx, resulting in an enhanced HER [15]. In this context, here we report a facile synthetic method for N-doped WS2 as a high-performance HER electrocatalyst in acidic media. Our chemical method could selectively synthesize WSx or WOx by simply changing chemical agents. In addition, an intriguing two-dimensional nanosheet structure was obtained for WS2 with a successful incorporation of N via a one-step hydrothermal reaction using cysteine (C3H7NO2S) as a chemical agent. The best sample showed a low overpotential of 130 mV for generating a current density of 10 mA cm −2 with excellent catalytic stability up to 12 h in 0.5 M H2SO4. Figure 1 shows X-ray diffraction (XRD) patterns of W18O49, WO3, WS2 (cysteine 1:2, 0.01 M), and WS2 (cysteine 1:4, 0.02 M). XRD results reveal that phase-pure W18O49 (JCPDS: 01-084-1516), WO3 (JCPDS: 033-1387), and WS2 (JCPDS: 008-0237) were successfully synthesized using our method. No secondary phases or impurities were presented for all samples. Notably, selective synthesis for each phase could be achieved by only adding thiourea or cysteine to the starting WCl6 solution. This highlights the practical potential of our synthetic method for preparing tungsten-based compounds. SEM images of W18O49 are shown in Figure 2. A homogeneous nanowire morphology was observed for W18O49. A thickness on the order of a few tens of nanometers was observed for the W18O49, and no significant aggregation of nanowires occurred during the solvothermal synthesis ( Figure 2b). After the addition of thiourea (CH4N2S) during the wet-chemical synthesis, a pristine nanowire morphology was well retained while the crystalline phase was changed from W18O49 to hexagonal WO3 ( Figure 3). With respect to the morphology, the nanowire thickness was slightly increased to about a few hundred nanometers for WO3 compared to that of W18O49 (Figure 3b). Substantial changes in morphology occurred when cysteine (C3H7NO2S) instead of thiourea (CH4N2S) was added to the WCl6 solution during wet-chemical synthesis ( Figure 4). An intriguing nanoflower  Figure 2. A homogeneous nanowire morphology was observed for W 18 O 49 . A thickness on the order of a few tens of nanometers was observed for the W 18 O 49 , and no significant aggregation of nanowires occurred during the solvothermal synthesis ( Figure 2b). After the addition of thiourea (CH 4 N 2 S) during the wet-chemical synthesis, a pristine nanowire morphology was well retained while the crystalline phase was changed from W 18 O 49 to hexagonal WO 3 (Figure 3). With respect to the morphology, the nanowire thickness was slightly increased to about a few hundred nanometers for WO 3 compared to that of W 18 O 49 (Figure 3b). Substantial changes in morphology occurred when cysteine (C 3 H 7 NO 2 S) instead of thiourea (CH 4 N 2 S) was added to the WCl 6 solution during wet-chemical synthesis ( Figure 4). An intriguing nanoflower morphology was observed on the surface of the WS 2 nanoparticles (Figure 4b). The WS 2 (cysteine 1:2) nanoparticles showed a spherical shape with a diameter of about 300~500 nm. Interestingly, two-dimensional (2D) nanosheets with a thickness of a few nanometers were homogeneously grown on the surface of the WS 2 (cysteine 1:2) nanoparticle. For the WS 2 (cysteine 1:4) sample, a nearly identical morphology with WS 2 (cysteine 1:2) was observed ( Figure 5), except that the particle size was slightly increased to about 500~700 nm. These results indicate that cysteine (C 3 H 7 NO 2 S) in the starting WCl 6 solution can facilitate the formation of a tungsten sulfide phase (i.e., WS 2 ) in a 2D nanosheet structure during the hydrothermal reaction, whereas thiourea (CH 4 N 2 S) promotes the one-dimensional (1D) growth of tungsten oxide phases such as WO 3 or W 18 O 49 . Hence, a selective synthesis with phase and morphology control for tungsten-based nanomaterials can be realized via our simple and facile synthetic method. Notably, a well-defined 2D nanoflower morphology of WS 2 (cysteine 1:2) may be of importance for enhancing electrocatalytic activity due to the high surface area for electrochemical reactions.

Results and Discussion
Catalysts 2020, 10, x FOR PEER REVIEW 3 of 9 morphology was observed on the surface of the WS2 nanoparticles ( Figure 4b). The WS2 (cysteine 1:2) nanoparticles showed a spherical shape with a diameter of about 300~500 nm. Interestingly, twodimensional (2D) nanosheets with a thickness of a few nanometers were homogeneously grown on the surface of the WS2 (cysteine 1:2) nanoparticle. For the WS2 (cysteine 1:4) sample, a nearly identical morphology with WS2 (cysteine 1:2) was observed ( Figure 5), except that the particle size was slightly increased to about 500~700 nm. These results indicate that cysteine (C3H7NO2S) in the starting WCl6 solution can facilitate the formation of a tungsten sulfide phase (i.e., WS2) in a 2D nanosheet structure during the hydrothermal reaction, whereas thiourea (CH4N2S) promotes the one-dimensional (1D) growth of tungsten oxide phases such as WO3 or W18O49. Hence, a selective synthesis with phase and morphology control for tungsten-based nanomaterials can be realized via our simple and facile synthetic method. Notably, a well-defined 2D nanoflower morphology of WS2 (cysteine 1:2) may be of importance for enhancing electrocatalytic activity due to the high surface area for electrochemical reactions.   Catalysts 2020, 10, x FOR PEER REVIEW 3 of 9 morphology was observed on the surface of the WS2 nanoparticles ( Figure 4b). The WS2 (cysteine 1:2) nanoparticles showed a spherical shape with a diameter of about 300~500 nm. Interestingly, twodimensional (2D) nanosheets with a thickness of a few nanometers were homogeneously grown on the surface of the WS2 (cysteine 1:2) nanoparticle. For the WS2 (cysteine 1:4) sample, a nearly identical morphology with WS2 (cysteine 1:2) was observed ( Figure 5), except that the particle size was slightly increased to about 500~700 nm. These results indicate that cysteine (C3H7NO2S) in the starting WCl6 solution can facilitate the formation of a tungsten sulfide phase (i.e., WS2) in a 2D nanosheet structure during the hydrothermal reaction, whereas thiourea (CH4N2S) promotes the one-dimensional (1D) growth of tungsten oxide phases such as WO3 or W18O49. Hence, a selective synthesis with phase and morphology control for tungsten-based nanomaterials can be realized via our simple and facile synthetic method. Notably, a well-defined 2D nanoflower morphology of WS2 (cysteine 1:2) may be of importance for enhancing electrocatalytic activity due to the high surface area for electrochemical reactions.       X-ray photoelectron spectroscopy (XPS) was performed on the WS2 (cysteine 1:2) sample to study chemical and electronic structures of the surface (Figure 6). High-resolution XPS scans were measured on W 4f, S 2p, O 1s, and N 1s. The presence of W-S bonds in WS2 (cysteine 1:2) was confirmed because W 4f3/2 and W 4f1/2 peaks were observed at 32.18 and 34.38 eV, respectively ( Figure  6a) [14]. Two doublets were observed for W 4f3/2 and W 4f1/2 peaks, with the corresponding satellite peaks at 33.18 and 35.08 eV indicating the existence of W 4+ in the hexagonal WS2 phase. In the S 2p spectra, the XPS peaks for W-S were observed at ~161.88 eV (Figure 6b). The XPS peaks in the S 2p spectra near 163.18 and 164.68 eV are associated with S-O and S-OH bonds, respectively, which can be formed during solvothermal reaction using ethanol as a solvent [14]. The high-resolution XPS spectra of the O 1s core level is shown in Figure 6c. The prominent peak at binding energies of 531.78 eV can be ascribed to O-W 6+ or O-W 5+ bonds, respectively, which may be presented in the surface (oxy)hydroxide layers [6,17]. Notably, the high-resolution N 1s XPS spectra showed the presence of N in the WS2 (cysteine 1:2) sample, which could have originated from cysteine (C3H7NO2S) during the synthesis. The amount of nitrogen in the sample was determined as 2.41 at% by analyzing the relative peak area in the XPS spectrum. In addition, the deconvoluted N 1s peak revealed that the incorporated nitrogen is primarily present as quaternary nitrogen (~401.48 eV) with an additional sp2 hybridized nitrogen component (~399.8 eV). Given that pyridinic nitrogen can act as an active site for electrocatalytic reactions or promote charge transfer between the adsorbents and the catalyst surface, improved catalytic performance can be expected for the WS2 (cysteine 1:2) sample owing to the pyridinic nitrogen [18][19][20][21][22]. X-ray photoelectron spectroscopy (XPS) was performed on the WS 2 (cysteine 1:2) sample to study chemical and electronic structures of the surface (Figure 6). High-resolution XPS scans were measured on W 4f, S 2p, O 1s, and N 1s. The presence of W-S bonds in WS 2 (cysteine 1:2) was confirmed because W 4f 3/2 and W 4f 1/2 peaks were observed at 32.18 and 34.38 eV, respectively (Figure 6a) [14]. Two doublets were observed for W 4f 3/2 and W 4f 1/2 peaks, with the corresponding satellite peaks at 33.18 and 35.08 eV indicating the existence of W 4+ in the hexagonal WS 2 phase. In the S 2p spectra, the XPS peaks for W-S were observed at~161.88 eV (Figure 6b). The XPS peaks in the S 2p spectra near 163.18 and 164.68 eV are associated with S-O and S-OH bonds, respectively, which can be formed during solvothermal reaction using ethanol as a solvent [14]. The high-resolution XPS spectra of the O 1s core level is shown in Figure 6c. The prominent peak at binding energies of 531.78 eV can be ascribed to O-W 6+ or O-W 5+ bonds, respectively, which may be presented in the surface (oxy)hydroxide layers [6,17]. Notably, the high-resolution N 1s XPS spectra showed the presence of N in the WS 2 (cysteine 1:2) sample, which could have originated from cysteine (C 3 H 7 NO 2 S) during the synthesis. The amount of nitrogen in the sample was determined as 2.41 at% by analyzing the relative peak area in the XPS spectrum. In addition, the deconvoluted N 1s peak revealed that the incorporated nitrogen is primarily present as quaternary nitrogen (~401.48 eV) with an additional sp2 hybridized nitrogen component (~399.8 eV). Given that pyridinic nitrogen can act as an active site for electrocatalytic reactions or promote charge transfer between the adsorbents and the catalyst surface, improved catalytic performance can be expected for the WS 2 (cysteine 1:2) sample owing to the pyridinic nitrogen [18][19][20][21][22].
The electrocatalytic HER performance of W 18 O 49 , WO 3 , WS 2 (cysteine 1:2), and WS 2 (cysteine 1:4) was tested in 0.5 M H 2 SO 4 solution using a three-electrode measurement system with a rotating disk electrode (RDE, glassy carbon) and a loading amount of 0.28 mg cm −2 . For comparison, the bare glassy carbon electrode (GCE) and the benchmark Pt/C (20 wt %) catalyst were also tested. Linear sweep voltammetry (LSV) was performed at a scan rate of 2 mV s −1 while the RDE was continuously rotated at 1600 rpm to remove any bubbles generated during the measurements. All the measured potentials were iR-compensated, which were then referenced to a reversible hydrogen electrode (RHE). Figure 7a shows LSV polarization curves for the measured samples. An overpotential to deliver −10 mA cm −2 (η 10 ) is widely used as a figure of merit to compare catalytic activity for the HER. Among the tested samples, the WS 2 (cysteine 1:2) sample exhibited the lowest η 10 (130 mV), indicating its high electrocatalytic activity for the HER in acidic media, which is comparable to that of the benchmark Pt/C sample (80 mV). The η 10 values of W 18 O 49 , WO 3 , and WS 2 (cysteine 1:4) were determined as 233, 181, and 143 mV, respectively. Figure 7b shows the calculated Tafel slope for the tested samples. Notably, the Tafel slope of WS 2 (cysteine 1:2) (45 mV dec −1 ) exhibited a much lower value compared to the control samples, except for Pt/C (31 mV dec −1 ). This reveals that the WS 2 phase with a 2D nanosheet morphology is advantageous to obtain a superior electrocatalytic activity for the acidic HER compared to the W 18 O 49 or WO 3 phases with a 1D nanowire morphology. Two major factors could be responsible for the superior electrocatalytic HER performance of the WS 2 (cysteine 1:4) and Catalysts 2020, 10, 1238 5 of 9 (cysteine 1:2): (i) The higher electrical conductivity of the tungsten sulfide phase compared to the tungsten oxide phase can enhance charge transfer during the HER. (ii) A well-defined 2D nanosheet morphology can secure a high electrochemically active surface area for the HER. In addition, the higher catalytic activity of the WS 2 (cysteine 1:2) in comparison to the WS 2 (cysteine 1:4) can be attributed to the smaller particle size, leading to a higher surface area. The electrocatalytic stability of the WS 2 (cysteine 1:2) was tested using a chronoamperometric (CA) study. Additionally, the HER performance of WS 2 (cysteine 1:2) is compared with other recently reported catalysts based on transition metal chalcogenides in Table 1. The WS 2 (cysteine 1:2) showed outstanding HER performance in terms of low overpotential and the Tafel slope, highlighting its potential as a promising HER catalyst. Figure 7c presents CA data for the WS 2 (cysteine 1:2) sample for 10 h under the applied voltage of 0.14 V RHE in 0.5 M H 2 SO 4 . Notably, no noticeable current decrease was observed during the CA test for 10 h. Moreover, the LSV polarization curve was also measured for the WS 2 (cysteine 1:2) sample after a continuous CA test for 10 h (Figure 7d). Almost identical LSV curves were obtained before and after the CA test. These results highlight the excellent electrocatalytic stability of the WS 2 (cysteine 1:2) sample for the HER in acidic conditions. These results demonstrate that the WS 2 (cysteine 1:2) can be a promising electrocatalyst with high activity and stability for the acidic HER. In addition, our facile synthetic method to simultaneously control materials' phases and nanostructures is of great significance for designing new functional catalytic nanomaterials. The electrocatalytic HER performance of W18O49, WO3, WS2 (cysteine 1:2), and WS2 (cysteine 1:4) was tested in 0.5 M H2SO4 solution using a three-electrode measurement system with a rotating disk electrode (RDE, glassy carbon) and a loading amount of 0.28 mg cm −2 . For comparison, the bare glassy carbon electrode (GCE) and the benchmark Pt/C (20 wt %) catalyst were also tested. Linear sweep voltammetry (LSV) was performed at a scan rate of 2 mV s −1 while the RDE was continuously rotated at 1600 rpm to remove any bubbles generated during the measurements. All the measured potentials were iR-compensated, which were then referenced to a reversible hydrogen electrode (RHE). Figure  7a shows LSV polarization curves for the measured samples. An overpotential to deliver −10 mA cm −2  Catalysts 2020, 10, x FOR PEER REVIEW 6 of 9 morphology can secure a high electrochemically active surface area for the HER. In addition, the higher catalytic activity of the WS2 (cysteine 1:2) in comparison to the WS2 (cysteine 1:4) can be attributed to the smaller particle size, leading to a higher surface area. The electrocatalytic stability of the WS2 (cysteine 1:2) was tested using a chronoamperometric (CA) study. Additionally, the HER performance of WS2 (cysteine 1:2) is compared with other recently reported catalysts based on transition metal chalcogenides in Table 1. The WS2 (cysteine 1:2) showed outstanding HER performance in terms of low overpotential and the Tafel slope, highlighting its potential as a promising HER catalyst. Figure 7c presents CA data for the WS2 (cysteine 1:2) sample for 10 h under the applied voltage of 0.14 VRHE in 0.5 M H2SO4. Notably, no noticeable current decrease was observed during the CA test for 10 h. Moreover, the LSV polarization curve was also measured for the WS2 (cysteine 1:2) sample after a continuous CA test for 10 h (Figure 7d). Almost identical LSV curves were obtained before and after the CA test. These results highlight the excellent electrocatalytic stability of the WS2 (cysteine 1:2) sample for the HER in acidic conditions. These results demonstrate that the WS2 (cysteine 1:2) can be a promising electrocatalyst with high activity and stability for the acidic HER. In addition, our facile synthetic method to simultaneously control materials' phases and nanostructures is of great significance for designing new functional catalytic nanomaterials.

Synthesis of W 18 O 49 Nanowires
The material was synthesized using a solvothermal technique. In a typical synthesis, 0.005 M of WCl 6 (Sigma-Aldrich, St. Louis, MO, USA) was dissolved in 80 mL ethanol (Sigma-Aldrich, St. Louis, MO, USA) at room temperature and stirred for 30 min at a rotation speed of 400 rpm. The above solution was transferred to a 100 mL autoclave chamber. Then, the Teflon-linked stainless steel autoclave was heated at 180 • C for 24 h in a vacuum oven. The product was collected by centrifuging and washing several times using ethanol and distilled water. Finally, the sample was dried at 60 • C overnight in a hot air oven.

Synthesis of Hex-WO 3 Nanowires
During the synthesis of W 18 O 49 , 0.01 M of thiourea (CH 4 N 2 S, Sigma-Aldrich, St. Louis, MO, USA) was added to the WCl 6 -ethanol solution and stirred for 30 min with a rotation speed of 400 rpm at room temperature. Then the Teflon-lined stainless-steel autoclave was heated at 180 • C for 24 h in a vacuum oven. The blue-colored precipitate powder was collected by centrifuging and washing several times using ethanol and distilled water. Finally, the sample was dried at 60 • C overnight in a hot air oven.

Synthesis of Hex-WS 2 Nanosheets
During the synthesis of hex-WO 3 , 0.01 M and 0.02 M of cysteine (C 3 H 7 NO 2 S, Sigma-Aldrich, St. Louis, MO, USA) was used by replacing thiourea (CH 4 N 2 S), keeping rest of the parameters fixed. The black-colored precipitate powder was collected by centrifuging and washing several times using ethanol and distilled water. Finally, the sample was dried at 60 • C overnight in a hot air oven.

Structural and Chemical Characterizations
An X-ray diffractometer (XRD, D/MAX RINT-2000, Rigaku, Tokyo, Japan) was employed to investigate the crystal structures of the samples using Cu Kα X-rays (1.54 Å) at 40 kV and 40 mA. Field-emission scanning electron microscopy (FE-SEM, JSM-7600F, Jeol, Tokyo, Japan) was used to study the morphology of the samples. The electronic structure and valence states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha + , Thermo Fisher Scientific, Waltham, MA, USA) with 100 and 20 eV for the XPS survey and high-resolution scans, respectively.

Electrochemical Characterization
The electrochemical properties of the samples were studied using a standard three-electrode system (Autolab PGSTAT, Metrohm Autolab, Utrecht, Netherlands). A rotating disk electrode (RDE, Meetrohm Autolab, Utrecht, Netherlands) configuration was used for all electrochemical tests. A 0.5 M H 2 SO 4 electrolyte was used for the electrochemical tests. A quantity of 5 mg of the catalyst powder was mixed well in a solution of 1 mL of water and ethanol with a volume ratio of 3:1. A Nafion ® solution (5 wt %, 30 µL, Sigma-Aldrich, St. Louis, MO, USA) was added to secure a rigid bind between catalyst powder and the electrode surface. Five microliters of as-prepared catalyst ink was drop-coated on the glassy carbon (GC) RDE electrode with a diameter of 3 mm. Then, the electrode was dried overnight at room temperature. The RDE electrode as the working electrode, Ag/AgCl (3.0 M KCl, Meetrohm Autolab, Utrecht, Netherlands) as the reference electrode, and graphite rod as the counter electrode were used for all electrochemical tests. Voltage calibration was performed to change the measured working potential to the reversible hydrogen electrode (RHE). A scan rate of 2 mV s −1 was used for the linear sweep voltammetry.

Conclusions
W 18 O 49 , WO 3 , WS 2 (cysteine 1:2), and WS 2 (cysteine 1:4) were synthesized via a one-step facile solvothermal method. The phase-selective synthesis was achieved by simply changing starting precursors such as cysteine (C 3 H 7 NO 2 S) or thiourea (CH 4 N 2 S). XRD and SEM results confirm that cysteine (C 3 H 7 NO 2 S) facilitates the formation of tungsten sulfide phases in a 2D nanosheet morphology, while thiourea (CH 4 N 2 S) promotes the formation of tungsten oxide phases in a 1D nanowire structure. The electrocatalytic activity of the samples was tested for the acidic HER, and the WS 2 (cysteine 1:2) showed the best catalytic performance among the tested samples, comparable to that of the benchmark Pt/C catalyst. In addition, excellent electrocatalytic stability was demonstrated for the WS 2 (cysteine 1:2) using a long-term stability test. These results reveal that our facile chemical synthetic method is powerful for the design of high-performance electrocatalysts with controlled phase and morphology.