Tuning the Composition and Structure of Amorphous Molybdenum Sulfide/Carbon Black Nanocomposites by Radiation Technique for Highly Efficient Hydrogen Evolution

Amorphous molybdenum sulfide/carbon black (MoSx/C) nanocomposites are synthesized by a facile one-step γ-ray radiation induced reduction process. Amorphous MoSx shows better intrinsic activity than crystalline MoS2. And the composition and amorphous structure of MoSx could be expediently tuned by absorbed dose for excellent catalytic activity. Meanwhile, the addition of carbon black leads to a significant decrease of charge transfer resistance and increase of active sites of MoSx/C composite. Consequently, MoSx/C nanocomposite shows Pt-like catalytic activity towards hydrogen evolution reaction (HER), which requires an onset over potential of 40 mV and over potential of 76 mV to achieve a current density of 10 mA cm−2, and the corresponding Tafel slope is 48 mV decade−1. After 6000 CV cycles, the catalytic activity of MoSx/C shows no obvious decrease. However, when platinum (Pt) foil is used as counter electrode, MoSx/C composite show better catalytic activity abnormally after long-term cycling tests. The dissolution of Pt was observed in HER and the Pt dissolution mechanism is elucidated by further analyzing the surface composition of after-cycling electrodes, which offers highly valuable guidelines for using Pt electrode in HER.

Hydrogen, because of its advantages of environmental friendliness and high energy density, has been considered as an ideal energy candidate for the sustainable development 1,2 . However, now most of the hydrogen is produced by steam-reforming reaction and chlor-alkali industry. Electrochemical production of hydrogen from the splitting of water by the hydrogen evolution reaction (HER) has long been considered as a highly promising way to produce hydrogen on a large scale 3 . Pt-group metals are considered as the most effective HER catalysts. However, they are not suitable for large-scale application due to their high cost and low abundance, so novel catalysts with low cost, earth abundance, high catalytic activity and stability in strong acids are needed for HER 4,5 . Recently, transition-metal-based materials, such as transition metal nitrides 6 , transition metal oxides and hydroxides 7 , transition-metal dichalcogenides 8 have been widespread reported as new competent electrocatalysts for water splitting. For example, molybdenum sulfide nanoparticles, including crystalline molybdenum disulfide (MoS 2 ) 9,10 and amorphous molybdenum sulfide (MoS x ) [11][12][13] , have become a promising alternative for their potential to meet these demands.
Crystalline MoS 2 has a typical transition-metal dichalcogenide (MX 2 ) layered sandwich structure. The edge planes of crystalline MoS 2 are active sites for HER, whereas their basal planes are chemically inert 14 . Benefitting from widely research, three universal strategies have been suggested to improve the HER activity of crystalline MoS 2 -based catalysts 15 : (1) increasing the density of active sites; (2) enhancing the intrinsic catalytic activity; (3)

Results and Discussion
Considering the interaction of γ ray with EG can produce solvated electrons 21 , and solvated electrons can reduce MoS 4 2− to MoS x nanoparticles, and in the presence of CB, the as-prepared MoS x nanoparticles will load on the sheets of CB to produce MoS x /C composite. Firstly, the influence of preparation conditions on the composition and structure of MoS x /C composites are investigated and the results are listed in Table 1.
As shown in Table 1, ICP-AES and XPS analysis show a consistent trend that the S/Mo ratio decreases gradually with the increasing of absorbed dose. Moreover, the S/Mo ratio is between 2.5 and 2.2, indicating the structure of MoS x is neither typically crystalline MoS 2 nor amorphous polymeric MoS 3 . And at the same absorbed dose, the S/Mo ratio of MoS x /C-2 is larger than that of MoS x . In the preparing process of MoS x /C-2, part of radiation energy from γ-rays was absorbed by CB, the energy absorbed by EG was reduced, and this will reduce the numbers of solvated electrons for reduction of (NH 4 ) 2 MoS 4 .
To further explore the composition and chemical states of both Mo and S in MoS x /C nanocomposites, XPS data was carefully analyzed. Pure MoS 2 powder was used as a reference material. Figure 1a and b show the Mo 3d and S 2p spectrum of pure MoS 2 powder, respectively. As shown in Fig. 1a, the doublet peaks located at 232.1 eV and 228.9 eV correspond to characteristic peaks of Mo 4+ 3d 3/2 and 3d 5/2 of MoS 2 , respectively. A S 2 s peak is observed at about 226.0 eV. The doublet peaks located at higher binding energy may be caused by the slight oxidation of MoS 2 . And in Fig. 1b ; b S lower represents S atoms of unsaturated S 2− and terminal S 2 2− . and 2p 1/2 of MoS 2 , respectively. However, the chemical states of MoS x synthesized by γ-ray radiation are much more complicated. All the radiation synthesized MoS x materials show similar XPS spectrum. Figure 1c and d show the typical Mo 3d and S 2p spectrum of MoS x /C-2, respectively. As shown in Fig. 1c, besides the doublet peaks at lower binding energy (232.5 eV and 229.3 eV) which belong to Mo 4+ of MoS 2 , the doublet peaks at higher binding energy of 235.9 eV and 232.7 eV can be attributed to unreduced Mo 6+ , This result indicates the structure of MoS x /C is different with that of hydrothermal synthesized MoS 2 /CB in our previous work 24 . And in Fig. 1d, the S 2p spectrum shows no obvious spin-orbit splitting, indicating the bonding states of S atoms in MoS x /C-2 are complex. Herein, our result was analyzed according to an analysis process proposed by Hu and coworkers 35 . The energy difference between S 2p 3/2 and 2p 1/2 was set as 1.18 eV for data fitting. The doublet peaks at binding energy of 163.5 eV and 162.3 eV can be attributed to the unsaturated S 2− and terminal S 2 2− 2p 1/2 and 2p 3/2 of MoS 2 , respectively. The doublet peaks at higher binding energy of 165.0 eV and 163.8 eV can be assigned to apical S 2− and/or bridging S 2 2− 2p 1/2 and 2p 3/2 of MoS x , respectively. According to Yeo and co-workers' research 18 , S atoms with higher binding energy should be the catalytic active sites. The ratios of different types of Mo and S atoms are calculated and listed in Table 1. As shown in Table 1, the ratio of Mo 4+ /Mo 6+ increases with the increasing of absorbed dose. This is due to that the Mo 6+ ions were reduced by the solvated electrons and higher absorbed dose would produce more solvated electrons. This result is in consistent with the trend of S/Mo atomic ratio. With the increasing percentages of low valence Mo 4+ ions, the S/Mo ratio decreases gradually and becomes closer to that of MoS 2 , which means the reduction degree of MoS 4 2− increases with the increasing of absorbed dose. Meanwhile, the ratio of S higher /S lower shows an opposite tendency with the increasing of absorbed dose.
Powder XRD was used to investigate the structure of as-prepared MoS x /C nanocomposites. Figure S1 shows the XRD patterns of CB, MoS x and MoS x /C composites. No obvious diffraction peak was found in the XRD spectrum of MoS x , which indicates the amorphous structure of MoS x produced by γ-ray radiation. All the MoS x /C samples show similar XRD patterns, and no other diffraction peak appears except the (002) and (100) graphitic reflection plane of CB, which indicates that the structure of MoS x maintains amorphous and does not change significantly with the increase of dose.
TEM amnalysis further demonstrates the morphology and structure of MoS x /C nanocomposites. Figure 2a shows the TEM image of CB. The marked inter planar d-spacing of 0.34 nm corresponds to the (002) lattice plane of graphitic CB, which agrees well with the XRD result. However, as shown in Fig. 2b, no ordered structure is observed, indicating the formation of amorphous MoS x . Figure 2c shows the image of MoS x /C-2, CB and amorphous MoS x can be observed easily, indicating MoS x retains amorphous structure in the nanocomposite. Annealing process is always used to rearrange the atoms and adjust the structure of materials. An annealing process was used to treat MoS x /C-2 composite. MoS x /C-2 was heated to 350 °C, and maintained at this temperature for 12 hours in the atmosphere of N 2 . As shown in Fig. 2d, after the annealing process, typical two-dimension nanosheets and the layers of MoS 2 can be observed. This result indicates that annealing process leads to the formation of crystalline MoS 2 in the nanocomposite. The marked inter planar d-spacing of 0.66 nm corresponds to the (002) lattice plane of MoS 2 , and this spacing is larger than the layer-to-layer spacing of 0.61 nm in bulk MoS 2 , indicating a significant lattice expansion 36 . Moreover, twisted and discontinuous crystal fringes can be observed, indicating the low crystallinity even after the annealing process.
Based on the above analysis, the possible reactions during the process of synthesis are as follows: To characterize the HER performance of the as-synthesized materials, MoS x , all MoS x /C nanocomposites and commercial 20% Pt/C were tested in a standard three-electrode system. Figure 3 shows the catalytic activity of different samples. As shown in Fig. 3a, the commercial 20% Pt/C electrocatalyst exhibits the lowest onset over potential of 10 mV and an over potential of 25 mV to achieve a current density of 10 mA cm −2 . This result is in accordance with previous report 37 . CB is inert to catalyze HER, and amorphous MoS x shows poor HER activity. In contrast, MoS x /C-2 exhibits high HER activity, which requires an onset over potential of 40 mV and an over potential of 76 mV to achieve 10 mA cm −2 . This performance is better than most reported MoS x -based catalysts (Table S1) and even close to that of Pt/C catalyst. Furthermore, the current density of MoS x /C-2 increases sharply with the increasing of over potential. However, after annealing treatment, the catalytic activity of annealed MoS x /C-2 is much lower than initial MoS x /C-2, which requires an onset over potential of 150 mV and an overpotential of 206 mV to achieve 10 mA cm −2 . Compared with MoS x and annealed MoS x /C-2, the highly efficient catalytic activity of MoS x /C-2 is attributed to the addition of CB and the composition and amorphous structure of MoS x .
The mechanism of HER in acidic medium can be summarized to three elementary reactions 37 : Firstly, the Volmer reaction occurs, (Equation 3). After the Volmer reaction, hydrogen may be generated by two different reactions: one is the Heyrovsky reaction, (Equation 4), the other is the Tafel reaction, (Equation 5). So, for an integrated HER process, Volmer-Heyrovsky mechanism or Volmer-Tafel mechanism should be involved.
Lg (6) The relationship between over potential and current density is shown in Equation 6 (Tafel formula), where η is the over potential, j is the current density, a is a Tafel constant, and b is Tafel slope. Tafel slope is always used to reveal the inherent reaction processes of HER, because it is determined by the rate-limiting step of HER. If the rate-limiting step is Volmer, Heyrovesy or Tafel reaction, the corresponding Tafel slope is about 120, 40, 30 mVdecade −1 , respectively. Therefore, the polarization curves were presented in Tafel plots to explore the detailed mechanism of HER. As shown in Fig. 3b, the Tafel slope of CB is 118 mV decade −1 , indicating the rate-limiting step is Volmer reaction. The 20% Pt/C exhibits a value of 33 mV decade −1 , indicating the Volmer-Tafel mechanism of HER, and this result is consistent with previous research results 4,14 . MoS x /C-2 exhibits a small Tafel slope of 48 mV decade −1 , which corresponds to the Volmer-Heyrovesy mechanism. Whatever, the annealed MoS x /C-2 exhibits a much higher Tafel slope of 68 mV decade −1 . Thus, in the application of MoS x /C for HER, the crystalline structure of MoS x is not favor for the catalytic performance.
Since absorbed dose can influence the ratio of S/Mo and the ratios of different types of Mo and S atoms, the catalytic activity of different MoS x /C materials were also investigated. As shown in Fig. 3c, for MoS x /C-1, MoS x /C-2 and MoS x /C-3, the onset over potential is 99 mV, 40 mV and 84 mV, respectively, and the over potential of 10 mA cm −2 is 126 mV, 76 mV and 124 mV, respectively. MoS x /C-2 shows the best catalytic activity among all the MoS x /C nanocomposites, and this result is different from Yeo's report 18 , which reports that catalysts containing higher percentages of S active sites have better catalytic activity. Nevertheless, in our case, MoS x /C-1 has the highest percentages of S atoms with higher binding energy, but the catalytic activity of MoS x /C -2 is much better than MoS x /C -1, and MoS x /C-3 shows a similar catalytic activity with MoS x /C-1. As shown in Table 1, the variation of reduction degree (the ratio of Mo 4+ /Mo 6+ ) and percentages of S atoms with higher binding energy (S higher / S lower ) is in contrast with the absorbed dose. Therefore, the composition and structure of MoS x /C composites and their catalytic activity can be tuned by absorbed dose. And for the catalytic activity of MoS x /C composites, 100 kGy is an optimal absorbed dose. The Tafel slopes are 55, 48, 69 mV decade −1 for MoS x /C-1, MoS x /C-2 and MoS x /C-3, respectively, indicating the catalytic mechanism of all the MoS x /C materials is Volmer-Heyrovesy mechanism.
In order to further investigate the mechanism of the excellent performance of MoS x /C nanocomposite in HER, electrochemical impedance spectroscopy (EIS) was performed to study the electrode kinetics. As shown in Fig. 4a, the EIS spectra of MoS x and MoS x /C-2 were dominated by a single capacitive semicircle at medium frequency range, suggesting the catalytic reaction was limited by the charge transfer steps. For MoS x , the charge transfer resistance (R CT ) is about 120 Ω, which is obtained by fitting the electrochemical impedance data. And for MoS x /C-2, due to the addition of highly conductive CB, the R CT decreases to about 2 Ω. R CT is related to the electrocatalytic kinetics at the catalyst/electrolyte interface, and a lower value corresponds to a faster electron transfer, so the significant decrease of R CT indicates a fast electron transfer and consequently facile HER kinetics at the catalyst/electrolyte interface. These experimental results identify that the strategy of combination MoS x with CB is highly effective to enhance the HER activity, because the presence of CB will lead to rapid electron transfer from the catalyst to the electrode.
Intrinsic activity is another significant factor to assess the property of a catalyst. Exchange current density (j 0 ) and per-site intrinsic catalytic activity reflect the inherent catalytic properties of catalysts. j 0 is obtained by applying extrapolation method to the Tafel plots. As shown in Table 2, for MoS x /C catalysts, the j 0 is 72, 223 and 103 μA cm −2 for MoS x /C-1, MoS x /C-2 and MoS x /C-3, respectively. The j 0 of all the MoS x /C catalysts are about 20 times larger than the reported crystalline MoS 2 38 , and the j 0 of annealed crystalline MoS x /C-2 is only about one-ninth of the initial MoS x /C-2, which suggests that the amorphous structure is better than the crystalline structure. With a modest reduction degree and active sites, MoS x /C-2 shows the highest intrinsic activity. The j 0 of MoS x is also much smaller than those of MoS x /C catalysts, this can be attributed to the addition of CB. The per-site intrinsic catalytic activity of as-prepared catalysts are further assessed by using the turnover frequency (TOF). TOF represents the number of hydrogen molecules produced per second per active site. We followed an estimation process proposed by Jaramillo and coworkers 20 Table 2. HER parameters of various samples. a j 0 is obtained from Tafel curves by using extrapolation methods; b TOFs are calculated at the over potential of 150 mV.
The capacitance of the catalyst was estimated with cyclic voltammetry performed at various scan rates in a potential window of 0.05-0.25 V. The representative cyclic voltammograms of MoS x /C-2 are shown in Figure S2. The pseudo-rectangular shapes indicate there is no obvious Faradaic current in this potential window. The current density readings at 0.15 V were extracted from the cyclic voltammograms. As shown in Fig. 4b, current density is proportional to the scan rate, indicating a pure non-Faradaic response. The capacitance of the catalyst is half of the slopes. As shown in Table 2, the specific capacitance (C dl ) of MoS x is only 0.8 mF cm −2 while all the MoS x /C materials show much larger value of about 11 mF cm −2 . The enhancement of C dl identifies that the addition of CB can reduce the aggregation of the formed MoS x nanoparticles. Roughness factor was then obtained by using the reported value of 60 μF cm −2 for an atomic flat MoS 2 catalyst. As shown in Table 2, the roughness factor of MoS x is 14, and for MoS x /C nanocomposites, the roughness factors are about 13 times larger than the value of MoS x . Larger roughness factor indicates more effective active sites, which is beneficial for the HER. Table 2 shows the TOF of MoS x and all the MoS x /C nanocomposites at the overpotential of 150 mV. MoS x /C-2 shows the highest TOF value of 1.4 H 2 s −1 per active site, which means it has the highest per-site intrinsic catalytic activity, and this result is consistent with previous analysis.
Based on the above analysis, the excellent performance of MoS x /C-2 in HER can be attributed to the following two reasons: (1) optimal absorbed dose tunes the composition and structure of MoS x ; (2) the addition of CB leads to a significant decrease of charge transfer resistance and increase of active sites.
Long-time stability is another significant factor to evaluate a catalyst. Long-term cycling test of representative MoS x /C-2 catalyst was measured by CV test. As shown in Fig. 5a, when graphite rod is used as counter electrode, the catalytic activity of MoS x /C-2 shows no obvious decrease after 6000 CV cycles, indicating the excellent catalytic stability of MoS x /C catalysts during HER process.
However, when Pt foil is used as counter electrode, the catalytic activity of after-cycling MoS x /C-2 material is even better than the initial material and 20% Pt/C. As shown in Fig. 5b, after 6000 CV cycles, the after-cycling MoS x /C-2 requires an onset over potential of nearly 0 mV and an over potential of only 9 mV to achieve 10 mA cm −2 . This performance is even much better than commercial 20% Pt/C catalyst, which is recognized as the best catalyst for HER. Furthermore, the Tafel slope of after-cycling MoS x /C-2 material reduces to 33 mV decade −1 , which is similar to the 32 mV decade −1 of Pt/C ( Figure S3). This result indicates that the catalytic mechanism of after-cycling MoS x /C-2 proceeds a Volmer-Tafel mechanism. Moreover, the change of catalytic mechanism indicates that there may be a change of structure and composition in MoS x /C-2 during the CV test.
In order to explain the enhancement of catalytic activity of MoS x /C-2, we investigated the structure and composition change of MoS x /C-2 during the CV test. XPS and TEM analysis were performed on after-cycling MoS x /C-2. Figure 6a shows the XPS spectrum of MoS x /C-2 before and after 6000 CV cycles. Compared with initial MoS x /C-2, F and Pt elements appear in the after-cycling MoS x /C-2. F can be attributed to Nafion membrane we used during the preparation of working electrode, and Pt should come from the Pt counter electrode. Figure 6b shows Pt 4 f spectrum of after-cycling MoS x /C-2. The doublet peaks locate at 71.4 eV and 74.7 eV are assigned to metallic Pt 0 and the doublet peaks locate at higher binding energy (72.4 eV and 75.7 eV) are related to Pt 2+ 39 . Figure 6c shows the Mo 3d spectrum of after-cycling MoS x /C-2. Compared with initial MoS x /C-2, the peaks corresponding to Mo 6+ disappeared. The doublet peaks at 229.8 eV and 233.0 eV are assigned to the Mo 4+ ion in MoS 3 , and the doublet peaks at higher binding energy (230.5 eV and 233.7 eV) can be attributed to a Mo ion in molybdenum oxysulfides 35 . This result indicates that high valence Mo ions are reduced during the HER. But due to the addition of Nafion membrane which contains S as well, it is difficult to calculate the S/Mo ratio of after-cycling MoS x . Figure 6d shows the S 2p spectrum of after-cycling MoS x /C-2, the doublet peaks assigned to apical S 2− and/or bridging S 2 2− reduced significantly compared with initial MoS x /C-2, and the signal at about 169 eV is attributed to the sulfonic group of Nafion membrane. XPS analysis verifies that the composition and chemical state of MoS x /C-2 indeed changed during the CV test. At the same time, Pt counter electrode dissolved during the HER test for some reasons.
Generally, Pt is regarded as a chemical inert and stable material, so it is always used as counter electrode in an electrochemical test system. However, Pt dissolution from the electrode have been found in polymer electrolyte  40,41 . Nevertheless, the potential range applied in PEFCs and operation temperature are higher than those in HER, so there are few reports about Pt dissolution in HER. In 2015, Dong and co-workers 42 observed Pt dissolution during HER and they speculated the Pt dissolution mechanism. But till now, the detailed mechanism of Pt dissolution during HER is still not very clear, and Pt electrode is still widely used as counter electrode during HER. In this work, the dissolution mechanism of the Pt counter electrode was studied further. Figure S4 shows the photographs of Pt counter before and after CV tests. The surface of Pt foil is covered by golden-colored material after long-time CV test. XPS analysis was then carried out to study the composition and chemical state of the golden-colored material. As shown in Fig. 6e, C, S, O and Pt elements can be observed. S is assigned to adsorbed SO 4 2− . The chemical states of Pt were then carefully evaluated by fitting the XPS spectrum of Pt 4 f. As shown in Fig. 6f, the doublet peaks at 72.5 eV and 75.8 eV are assigned to Pt 2+ and the doublet peaks at higher binding energy (75.1 eV and 78.4 eV) are related to Pt 4+ 43 . No signal of metallic Pt 0 was observed, indicating the chemical states of Pt on the surface of counter electrode are Pt 2+ and Pt 4+ . So the possible dissolution mechanism is as follows: the oxidizing species formed in the anode reaction can react with Pt foil to generate Pt 2+ and Pt 4+ , then the oxidized Pt species (Pt 2+ and Pt 4+ ) dissolve in the solution and then migrate to the cathode and are reduced to Pt 2+ and metallic Pt 0 at last. There are few reports about the dissolution of Pt during the HER process 42,44,45 in recent two years, our experimental results confirmed the previous speculation. Figures S5, S6 and S7a show the TEM and HRTEM images of after-cycling MoS x /C-2. As shown in Figure S5, when graphite rod is used as counter electrode, no obvious difference was observed even after 6000 CV cycles. However, the Pt aggregations containing many Pt nanoparticles with a diameter less than 5 nm were observed when Pt foil is used as counter electrode ( Figure S6). Figure S7a shows the HRTEM image of Pt nanoparticles, the marked interplanar d-spacing of 0.25 nm corresponds to the (110) lattice plane of Pt nanoparticles, and corresponding EDS spectrum ( Figure S7b) confirms the formation of Pt nanoparticles.
The influence of deposited Pt content on the electrode is evaluated as well. Figure S8a shows the first 50 CV cycles of MoS x /C-2. With the increasing of cycles, the onset over potential decreases gradually while the current density at the same over potential increases, indicating Pt loading on working electrode increases with the CV tests. Figure S8b shows the polarization curves of MoS x /C-2 after different CV cycles. When CV cycles are less than 300, the catalytic activity of MoS x /C-2 increases gradually. However, further increase in CV cycles makes no significant difference between 300 cycles and 6000 cycles. ICP-MS analysis shows Pt loadings of MoS x /C-2 after 300 cycles and 6000 cycles are 1.5 μg cm −2 and 35 μg cm −2 , respectively. The Pt loading of MoS x /C-2 after 300 cycles is much smaller than that after 6000 cycles, but their catalytic activity shows no obvious difference. Higher Pt loading will aggravate the aggregation of Pt nanoparticles, and thereby reduce the utilization efficiency of Pt. The mass activity of MoS x /C-2 after 300 cycles is 3.1 × 10 5 A g −1 (Pt) at the over potential of 90 mV, which is about 400 times larger than that of 20% Pt/C. Therefore, the improved catalytic performance of after-cycling MoS x /C-2 towards HER should be attributed to the deposition of Pt nanoparticles on the working electrode. It is suggested that Pt dissolution should be emphasized for evaluation of the catalyst towards HER when Pt is applied as counter electrode.

Conclusions
In summary, amorphous MoS x /C composites are synthesized successfully by a facile one-step γ-ray radiation reduction process for the first time. The resultant MoS x /C shows excellent catalytic activity and cycle stability towards HER, which requires an over potential of 76 mV to achieve a current density of 10 mA cm −2 , and the corresponding Tafel slope is 48 mV decade −1 . Amorphous structure of MoS x with suitable reduction degree and active sites and presence of CB support play important roles in the catalytic performance in HER. In addition, the dissolution of Pt was observed during the long-term cycling tests when Pt is used as counter electrode. And the dissolution mechanism is further elucidated by analyzing the surface composition of after-cycling electrode, which is highly valuable for using Pt electrode towards HER.

Methods
Materials. Ammonium tetrathiomolybdate ((NH 4 ) 2 MoS 4 , 99.95%) was purchased from Acros. Cobot Vulcan XC-72 CB and commercial 20% Pt/C catalyst were purchased from Macklin. Pure molybdenum disulfide (MoS 2 ) powder was purchased from Alfa Aesar. Nitric acid (HNO 3 , AR), ethylene glycol (EG, AR) and sulfuric acid (H 2 SO 4 , AR) were purchased from Beijing Chemical Works. Nitrogen gas (99.999%) was purchased from Beijing Haikeyuanchang Practical Gas Co., Ltd. All materials were used as received without further purification. Synthesis of MoS x /C nanocomposites. MoS x /C composites were synthesized by a simply one-step radiation induced reduction process. Typically, 20 mg CB and 40 mg (NH 4 ) 2 MoS 4 were added to 20 mL EG and sonicated for 20 min. Then the mixed solution was saturated with high purity nitrogen gas before the sealing treatment. After that, the suspension was exposed to γ radiation using a 60 Co source for different doses with a dose rate of 300 Gy min −1 at room temperature. After irradiation, the precipitates were separated from the solutions by filtration and washed with pure water and ethanol for several times, and then dried at 40 °C under vacuum. For comparison, the solution of (NH 4 ) 2 MoS 4 without CB was treated according to the above procedure under the same experimental conditions to prepare MoS x .
Composition and Structure Characterization of MoS x /C nanocomposites. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were detected by a Prodigy ICP from Teledyne Leeman Labs. X-ray photoelectron spectroscopy (XPS) measurements were performed on an Axis Ultra X-ray photoelectron spectrometer from Kratos Analytical with an exciting source of Al Kα = 1486.7 eV. The binding energies obtained in the XPS spectral analysis were corrected for specimen charging by referencing C 1 s to 284.8 eV, and Powder X-Ray Diffraction (XRD) was performed on a Philips X'Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were carried out on a FEI TECNAI F20 field emission electron microscope at an acceleration voltage of 200 kV. The inductively coupled plasma mass spectrum (ICP-MS) was detected by an ELEMENTAL XR ICP-MS from Thermo Fisher.
Preparation of Working Electrodes. The carbon paper working electrode was prepared as follows: carbon paper was cut into strips with a width of 5 mm. Then 50 μL catalyst ink was loaded onto the carbon paper strip (area ~0.3 cm 2 , loading ~0.667 mg cm −2 ) and then dried under an infrared lamp.
Electrochemical Measurements. All the electrochemical tests were performed in a three-electrode system. The details are consistent with the tests we demonstrated in previous work 27 except that the counter electrode is a platinum foil (~1 cm 2 ) or graphite rod (diameter = 5 mm). Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) tests were performed on a CHI 760e electrochemical station. Electrochemical impedance spectroscopy (EIS) was performed on an Autolab PGSTAT302N electrochemical station. All the potential was transferred to reversible hydrogen electrode (RHE) potential by the equation E(RHE) = E(SCE) + 0.260 V. All the polar curves were iR corrected, where R is ohmic resistance obtained by the EIS. Pt dissolution was observed when Pt foil was used as counter electrode. The Pt doped working electrode was then characterized and measured with the same methods.