Hierarchically Porous Electrocatalyst with Vertically Aligned Defect

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Hierarchically Porous Electrocatalyst with Vertically Aligned Defect-rich CoMoS Nanosheets for HER in Alkaline Medium Zexing Wu, Junpo Guo, Jie Wang, Rong Liu, Weiping Xiao, Cuijuan Xuan, Kedong Xia, and Deli Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15244 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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ACS Applied Materials & Interfaces

Hierarchically Porous Electrocatalyst with Vertically Aligned Defect-rich CoMoS Nanosheets for HER in Alkaline Medium Zexing Wu, Junpo Guo, Jie Wang, Rong Liu, Weiping Xiao, Cuijuan Xuan, Kedong Xia, Deli Wang*

Key Laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P.R. China.

E-mail: [email protected].

KEYWORDS hydrogen

evolution

reaction,

hierarchically

porous,

vertically

aligned,

electrocatalysts, defect-rich nanosheets

ABSTRACT

Effective electrocatalysts for hydrogen evolution reaction (HER) in alkaline electrolytes can be developed via a simple solvothermal process. In this work, firstly, the prepared CoMoS nanomaterials through solvothermal treatment have a porous, defect-rich, and vertically aligned nanostructure, which is beneficial for HER in an

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alkaline medium. Secondly, the electron transfer from Co to MoS2 that reduces the unoccupied d-orbitals of Mo, can also enhance the HER kinetics in an alkaline medium. This has been demonstrated via comparison of catalytic performances of CoMoS, CoS, and MoS2. Thirdly, the solvothermal treatment time evidently impacts the electrocatalytic activity. As a result, after 24 h of solvothermal treatment, the prepared CoMoS nanomaterials exhibit lowest onset potential (42 mV) and overpotential (98 mV) for delivering a current density of 10 mA cm-2 in 1 M KOH solution. Thus, this study provides a simple method to prepare efficient electrocatalysts for HER in alkaline medium.

1. Introduction Hydrogen has been regarded as promising alternative energy source to traditional sources in the future, due to the fact that it is an eco-friendly and sustainable energy carrier1. Compared with traditional technologies, electrochemical hydrogen evolution reaction (HER) is considered as the most promising energy craft to generate hydrogen gas and has consequently attracted extensive attention in recent years2-3. During the process of electrocatalytics, the electrocatalyst acts as a significant role in reducing the overpotential, while improving the catalytic efficiency for HER4-6. At present, platinum group metals possess the outstanding catalytic activity, which is capable of splitting water efficiently at potentials close to the thermodynamic value7-8. Nevertheless, the high cost and scarcity limit its large-scale application9. Thus, it is of


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paramount importance to explore earth abundant, cost-effective, highly-efficient, and stable alternatives to replace platinum group metals for HER10-12.

Due to its elemental abundance, high activity, and specific two-dimensional layered structure, molybdenum sulfide (MoS2) has been widely investigated as HER electrocatalyst to replace Pt-group metals in acid medium13-14. However, both experimental results and theoretical calculations demonstrated that the active sites of MoS2 are located at the exposed edges rather than basal planes15-16. Thus, tremendous efforts have been devoted to develop various structured MoS2 nanomaterials with more abundant exposed edges sites, including few-layered MoS217, defect-rich mono-layered MoS218, perpendicular structured MoS2 nanosheets19, and porous-rich MoS2 nanorod

20-21

. However, the catalytic activity is still unsatisfactory in alkaline

medium despite the performance has been evidently improved in acid medium. It has been reported that, the combination of 3d-transition metals and molybdenum can improve the catalytic performance of hydridesulfurization (HDS), the reaction mechanism of which is similar to HER22, due to the charge transfer and morphology change after introduction of a foreign metal elemental into MoS223-24. Furthermore, the combination of Co and Mo possesses outstanding catalytic activity for HER compared with Fe and Ni elementals23.

Here, a hierarchical porous, defect-rich, and vertically aligned CoMoS electrocatalyst was developed via a simple solvothermal avenue (Scheme 1), which possesses outstanding catalytic activity for HER in alkaline medium. The porous



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structure is beneficial for the contact between the prepared catalyst and electrolytes25. Furthermore, the prepared CoMoS nanosheets possess abundant defects, which can act as catalytic sites because the catalytic process of MoS2 takes place at the basal edges5, 26. The coexistence of Co and Mo can evidently enhance the catalytic activity compared with CoS and MoS2 because the electron transfer from cobalt to molybdenum enhances the HER kinetics more efficiently27. This study demonstrates a simple process to prepare efficient electrocatalysts for HER in alkaline medium.

Scheme 1 Schematic illustration of CoMoS nanomaterial synthesis.

2. Experimental Methods

Sample preparation

The CoMoS nanosheets were prepared via solvothermal method similar with previously reported study28. In a typical protocol, 1.646 g Co(NO3)2.6H2O and 2 g (NH4)6Mo7O24.4H2O were dissolved in 20 mL distilled water and 1 g sulfur powder was dispersed in the above solvent by ultra-sonication. After 30 min, 16 mL hydrazine monohydrate was added into the mixture solution and the obtained 4 ACS Paragon Plus Environment

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homogeneous solution was transferred into the 50 mL Teflon-lined steel autoclave, where it was heated to 180 ºC for variable time which were named as CoMoS-12 h, CoMoS-24 h, CoMoS-36 h. After that, the solvothermal products were washed with distilled water, hydrochloric acid, and ethanol several times and then freeze dried. The obtained product was heated to 300 ºC for 2 h with a heating rate of 5 ºC min-1 under N2 atmosphere. For comparison, MoS2, and CoS were synthesized using identical process in absence of Co(NO3)2.6H2O and (NH4)6Mo7O24.4H2O, respectively. Physical Characterization Powder X-ray diffraction (XRD) patterns of synthesized catalysts were conducted, using an X'Pert PRO diffractometer and morphologies were measured via scanning electron microscopy (SEM, Sirion200). Transmission electron microscopy (TEM) images of various samples were obtained via a JSM-2100 transmission electron microscopy (JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurements were carried out on an AXIS-ULTRA DLD-600W instrument. ICP-AES was measured by IRIS Advantage (Thermo Elemental Co. USA). Raman spectra were obtained by a LabRam HR800 spectrometer.

Electrochemical measurements Electrochemical measurements were carried out in a typical three-electrode setup in N2 saturated 1 M KOH electrolyte. Glassy carbon substrate (with 5 mm diameter), a reverse hydrogen electrode, and a graphite rod served as working electrode, counter electrode, and reference electrode, respectively. Typically, 5 mg of catalysts was 5 ACS Paragon Plus Environment


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added in 1 mL Nafion/isopropanol mixture solution and then sonicated to form uniform ink. Then, 33 µL of the catalysts suspension were pipetted onto a glassy carbon electrode and dried at room temperature (loading ca. 0.84 mg cm-2). Linear sweep voltammograms (LSV) were carried out with a scanning rate of 5 mV s-1 in the potential region from 0.2 to -0.5 V vs. RHE in an N2 saturated electrolyte. The stability experiments were recorded by repeating the potential scan from 0.2 to -0.3 V for 5000 cycles. The double-layer capacitances were performed through cyclic voltammograms with different scan rates (20, 40, and 60 mV s-1 etc.) in the range of 0.2-0.4 V. Electrochemical impedance (EIS) experiments were performed in the frequency range of 100 KHz - 0.1 Hz at -0.1 V with a perturbation of 5 mV.

3. Results and Discussion

The XRD results of the prepared electrocatalysts are illustrated in Figure 1a. MoS2 nanomaterial was successfully prepared according to the standard card (JCPDS NO. 37-1492). No byproducts were found in the XRD pattern of CoS (JCPDS NO.65-8977), demonstrating the purity of the efficient process to synthesize CoS. It can be seen that CoMoS is composed of both CoS and MoS2 phases in the corresponding XRD pattern, indicating the coexistence of cobalt and molybdenum elements in the prepared catalysts. As shown in Figure S1, MoS2 and CoS are formed after 12 h solvothermal treatment, but crystalline quality is weak compared with CoMoS-24 h and CoMoS-36 h which maybe affect the catalytic performance for HER. The crystalline quality of CoMoS-36 h is similar to CoMoS-24 h, thus the


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microstructure and electro-conductivity are possible the main hinders for HER. The morphologies of the prepared catalysts were first characterized by scanning electron microscopy (SEM). As shown in Figure 1b, the prepared CoS aggregated evidently which can’t provide sufficient active sites for HER. For MoS2 (Figure 1c), numerous nanosheets folded together and resembled the morphology of crumpled bulk balls, which reduced the exposed edges for HER because the catalytic active sites of MoS2 locate at the basal edges29-30. Compared with CoS and MoS2 catalysts, CoMoS possesses a hierarchically porous vertically aligned structure, composed of small nanosheets (Figure 1d). The porous structure is beneficial for the interfacial contacts between the electrocatalyst and the electrolyte31, which is expected to effectively enhance the catalytic activity32. It is also can be seen from STEM image (Figure S2) that the vertically aligned morphologies of prepared CoMoS. The average thickness of CoMoS nanosheets is about 10 nm, which is half as thin as MoS2 (20 nm).



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Figure 1 (a) XRD patterns of CoS, MoS2, and CoMoS. SEM images of CoS (b), MoS2 (c), and CoMoS (d).

Apparently, the combination of Co and Mo can form different morphologies compared with the sole existence of Co or Mo. Perhaps, the reason is that the elemental of Co restrains MoS2 crystal growth along the direction of the basal plane, hence lowering the probability to form aggregations and coalescences along the nanosheets33. The elemental distribution of the CoMoS catalyst was analyzed via field emission SEM (FE-SEM) (Figure S3a-d). Apparently, the content of elemental Co is 8 ACS Paragon Plus Environment

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lower compared to the other two elements, however, uniformly distributed in the catalysts.

Transmission electron microscopy (TEM) was utilized to better research the morphologies of prepared catalysts. Both MoS2 and CoS are composed of nanosheets, however, they are stacked and severely wrinkled (Figure 2a, b). In contrast, CoMoS possesses smaller nanosheets (Figure 2c, d). Furthermore, CoMoS catalysts exhibit interconnected porous structure (red circle), which is beneficial for the electrocatalytic performance for HER. The porosity of the synthesized CoS, MoS2, and CoMoS were demonstrated via nitrogen adsorption-desorption measurements (Figure S4). It can be seen that CoMoS exhibits relative larger surface area (18.6 cm2 g-1) than CoS (10.4 cm2 g-1) and MoS2 (13.2 cm2 g-1), which is in accordance with the observed morphologies. Furthermore, CoMoS possesses the most abundant mesoporous structure, facilitating the flow of electrolyte and gas release during the catalytic process34. The crystalline quality of CoMoS was evidently weakened relative to MoS2, and substantial defects can be observed (see the yellow circles in Figure 2e), enhancing the reactive sites for HER. Due to the Co2+ might influence the reaction of (NH4)6Mo7O24.4H2O with sulfur powder and the Co atoms substitute Mo in the MoS2 matrix toward one side and bond with only four S atoms, leaving other S atoms unsaturated, all the above reasons will induce the formation of sulfur vacancies and thus formed defect sites. Except TEM measurement, Raman measurement can also be used to investigate the defects of the prepared catalysts. As shown in Figure S5, two distinct peaks of A1g and E12g are detected for both MoS2 and CoMoS, except CoS, 9 ACS Paragon Plus Environment


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which are characteristic Raman shifts of hexagonal MoS2. Compared with MoS2, the relatively weaker intensity and larger peak width of E12g of CoMoS indicates the crystal structure intensity of MoS2 in CoMoS is relative weak and in-layer disorder or defects between Mo and S atoms35-36 which is in accordance with the TEM images. Furthermore, the lower Raman shifts of both A1g and E12g peaks in CoMoS demonstrating the formation of sulfur vacancies which is also benefit for HER37. As discussed above, the TEM and Raman measurements can be verified the rich defects of the prepared CoMoS. Furthermore, the interlayer distance was significantly increased to 0.83 nm compared to 0.65 nm of MoS2, which can act as catalytic sites for HER38. The increased interlayer distance maybe due to the coexistence of CoS and MoS2 in CoMoS, due to the interaction between CoS and MoS2. The formation of CoS can also enlarge the interlayer distance which can act as a role like carbon matrix39. As shown in corresponding mapping images in Figure 2 f-i, the elements of cobalt, molybdenum, and sulfur were uniformly distributed across the catalysts, which is in accordance with the SEM results. The accurate chemical formula of CoMoS is CoMo1.9S5.3 by EDX measurement (Figure S6), determined by which is similar with ICP-AES results CoMo1.8S4.9. The catalysts were named CoMoS for convenience.


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Figure 2 TEM images of CoS (a), MoS2 (b) and CoMoS (c, d, e). Elemental mapping of Co (f), Mo (g), S (h), and composites (i).

The electrocatalytic performance of the catalysts prepared at different solvothermal time were evaluated in a typical three-electrode system in N2- saturated 1 M KOH electrolyte, utilizing a rotating disk electrode (RDE). An effective electrocatalyst for HER should possess a small onset potential and a low overpotential for delivering a specific current density. As shown in Figure 3a, CoMoS-24 h presents the smallest onset potential of 42 mV to deliver a current density of 1 mA cm-2 relative to CoMoS-12 h (144 mV) and CoMoS-36 h (58 mV). For delivering a current 11 ACS Paragon Plus Environment


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density of 10 mA cm-2, a significant parameter to evaluate the catalytic activity of prepared catalysts40, only 98 mV was needed for CoMoS-24 h, which was 40 mV and 122 mV lower than CoMoS-12 h and CoMoS-36 h, respectively. These values were superior to the most reported electrocatalysts (Table S1). Furthermore, the current density of CoMoS-24 h is the highest among the whole potential region. As mentioned above, CoMoS-24 h presents the lowest onset potential, overpotential, and highest current density, demonstrating that the solvothermal time can significantly impact catalytic activity.

The Tafel slope was obtained through the Tafel equation η = b log j + a (j is the current density and b is the slope) to investigate the reaction mechanism and the inherent properties of the catalysts for HER41. Apparently, CoMoS-24 h possesses the smallest Tafel slope with 82 mV dec-1 compared with CoMoS-12 h (106 mV dec-1) and CoMoS-36 h (115 mV dec-1) (Figure 3b), demonstrating that the kinetics of the water splitting were effectively facilitated on the prepared CoMoS-24 h electrode27. Moreover, a lower Tafel slope of CoMoS-24 h indicates a lower overpotential to deliver a specific current density relative to both other catalysts42. Exchange current density (j0) is also a kinetic parameter to evaluate the intrinsic catalytic performance for HER of the prepared materials43 and the value can be obtained by extrapolating the Tafel plots to an overpotential of 0 V (Figure 3c). An excellent electrocatalyst with outstanding catalytic activities for HER should possess high exchange current density. It is evident that CoMoS-24 h exhibits the highest exchange current density (j0) relative to CoMoS-12 h and CoMoS-36 h, indicating the fastest electron transfer 12 ACS Paragon Plus Environment

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rate. Electrochemical impedance spectroscopy (EIS) was conducted to further investigate the electrocatalytic activity of electrocatalysts for HER (Figure 3d). The semicircle in the high frequency range contributes to the charge-transfer resistance (Rct). In general, the value of Rct varies inversely to the electrocatalytic reaction rate. Apparently, CoMoS-24 h exhibits the lowest Rct among all three catalysts, demonstrating the fastest electron transfer rate during HER, which can be contributed to the excellent catalytic activity. Electrochemical double layer capacitance (EDLC), a significant parameter to evaluate the contact area between catalyst and electrolyte, was calculated via cyclic voltammetry plots in the range of 0.2 - 0.4 V, in which no faradic reactions occurred with various scan rates from 20 mV s-1 to 180 mV s-1 (Figure S7 a, b, and c). The value of EDLC was obtained by plotting △j/2 at 300 mV against the scan rates (Figure S7d). Apparently, CoMoS-24 h possesses the highest EDLC value (102 mF cm-2) compared to CoMoS-12 h (84 mF cm-2) and CoMoS-36 h (55 mF cm-2), thus CoMoS-24 h exhibits the largest contact area with the electrolyte. The SEM images of different solvothermal times also demonstrate that CoMoS-24 h possesses a more porous structure compared to other composites, which severely aggregate, resulting in lower catalytic sites for HER (Figure S8).



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Figure 3 (a) Polarization curves of CoMoS prepared at different solvothermal time in 1 M KOH electrolytes at a scan rate of 5 mV s-1. (b) Tafel slopes of obtained CoMoS with 12, 24, and 36 h solvothermal time. Exchange current densities of prepared catalysts, calculated via extrapolation methods (c) and Nyquist plots (d). To investigate the advantages of cobalt and molybdenum elements coexistence in the prepared CoMoS, CoS and MoS2 were prepared, using an identical synthetic process (see experimental section). For comparison, Pt/C was also measured in 1 M KOH purged with N2, which presents excellent HER electrocatalytic performance with an extraordinarily low overpotential, onset potential, and high current density, in accordance with the reported value40. As shown in Figure 4a, the onset potential is about 135 mV and 82 mV higher on CoS and MoS2 than that on CoMoS. For 14 ACS Paragon Plus Environment

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delivering a current density of 10 mA cm-2, CoMoS exhibits an overpotential of 206 mV and 123 mV lower than CoS and MoS2 (Figure 4a). Furthermore, CoMoS exhibits the largest current density in the whole potential range relative to other composites. The electrochemical testing results indicate that the coexistence of cobalt and molybdenum can effectively enhance the catalytic performance. Correspondingly, Tafel slopes in Figure 4b indicates that CoMoS exhibits much lower Tafel slope (82 mV dec-1) than CoS (136 mV dec-1) and MoS2 (103 mV dec-1), although the value is higher than that on Pt/C (53 mV dec-1). The smaller Tafel slope on CoMoS indicates the fastest reaction kinetics for HER. Exchange current density (j0) was also calculated to investigate the intrinsic activity of the catalysts (Figure S9a). Compared to CoS and MoS2, CoMoS exhibits the highest exchange current density, demonstrating optimal catalytic activity.

Electrochemical impendence spectroscopy (EIS) was measured at the overpotential of 100 mV to investigate the kinetics of the catalysts for HER (Figure 4c). It can be seen that Pt/C presents the lowest resistance relative to the prepared catalysts which is benefit for the hydrogen evolution reaction (HER). Apparently, CoMoS exhibits the lowest resistance relative to CoS and MoS2, which is beneficial for electron transfer and catalytic activity enhancing. As shown in Figure 4d, the EDLC value of CoMoS was 102 mF cm-2, which is 73 mF cm-2 and 86 mF cm-2 higher than MoS2 and CoS nanomaterials, respectively which were calculated from CVs in Figure S 9b-d. The larger EDLC on CoMoS is beneficial for enhancing the catalytic activity for HER, due to a large contact area between catalyst and electrolyte. 15 ACS Paragon Plus Environment


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Figure 4 (a) Polarization curves of CoMoS, CoS, and MoS2 in 1 M KOH electrolyte at a scan rate of 5 mV s-1 and corresponding Tafel slopes (b). Nyquist plots (c) and linear fitting of the capacitance currents of CoMoS, CoS, and MoS2 vs. scan rates (d).

To illustrate the mechanism of enhanced electrocatalytic performance for HER with the prepared composite catalysts, X-ray photoelectron spectroscopy (XPS) was performed. Due to the electronegativity difference between cobalt (1.88) and molybdenum (2.16), the electron transfer from cobalt to molybdenum can effectively reduce the unoccupied d-orbitals of molybdenum44. As shown in Figure 5a, the Co 2p spectra in both CoS and CoMoS are composed of the Co2+ oxidation state45, demonstrating the existence of CoS in both electrocatalysts. Apparently, the Co 2p 16 ACS Paragon Plus Environment

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peaks in the CoMoS shift to a higher binding energy compared with CoS, indicating a higher Co valence in the prepared CoMoS. Furthermore, the loss of the satellite peak for Co2+ also demonstrates the higher Co valence in CoMoS due to electron transfer from Co to Mo. For Mo 3d in Figure 5b, the characteristic peaks of Mo 3d in CoMoS shift to lower binding energy relative to MoS2, demonstrating lower Mo valence in CoMoS due to electron transfer from Co. According to these insights, the coupling of Co and Mo might enable the transfer of electrons from cobalt to MoS2, resulting in higher cobalt valence and lower molybdenum valence for CoMoS nanomaterials, which activates HER and thus, accounts for the excellent HER catalytic activity. As discussed above, the outstanding catalytic activity of CoMoS compared with CoS and MoS2 can be attributed to several factors: The larger surface area and porous structure of CoMoS relative to CoS and MoS2 facilitate the contact between the catalysts and electrolyte31, 39; the abundant defects in the layer of CoMoS is beneficial for the HER catalytic process because the catalytic active site locates at the basal edges46-47; the electron transfer from Co to MoS2 can lower the kinetic energy barrier of the initial water dissociation and the adsorption of OH-, which is the main HER kinetics for the catalytic process in alkaline media27, 48; the size of CoMoS was evidently reduced compared to CoS and MoS2, which can provide more catalytic active sites for HER33, 49

.



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Figure 5 XPS high-resolution spectra of Co 2p for CoS and CoMoS (a). High resolution Mo 3d XPS spectra for MoS2 and CoMoS (b). Durability is an important parameter to evaluate the electrocatalyst for the industrial productions of HER. To this end, the electrochemical stability of CoMoS was performed via 5000 potential cycling measurements in the potential range of -0.3 V and 0.2 V at a scan rate of 100 mV s-1 (Figure 6a). The results revealed that the current decay on CoMoS catalyst is slight, demonstrating the excellent stability. Furthermore, the morphology of CoMoS, such as the layer structure and defect sites, remains intact after 5000 potential cycles (Figure 6b). The prepared catalyst aggregated after cycling but the phase kept well (Figure 6c, d), indicating that the HER performance of CoMoS degraded after cycling due to the aggregation of the catalyst. The long term stability was measured at a specific overpotential of 98 mV for delivering a current density of 10 mA cm-2. As shown in Figure S10, the current density degraded slightly after measured at the overpotential of 98 mV for 12 h in basic medium which is in accordance with the CV scanning measurement.


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Figure 6 (a) Durability test for CoMoS catalyst via CV scanning for 5000 cycles in 1 M KOH. TEM (b) and SEM (c) images of CoMoS after 5000 potential cycles in 1 M KOH. (d) XRD pattern of CoMoS after 5000 potential cycles in 1 M KOH.

4. Conclusions

In summary, porous and defect-rich CoMoS was successfully prepared via a simple solvothermal process. The CoMoS exhibits excellent electrocatalytic performance for HER in alkaline electrolyte with onset potential of 42 mV and overpotential of 98 mV for delivering a current density of 10 mA cm-2. Merits of the outstanding CoMoS catalytic abilities are its porous structure and rich defects. Furthermore, the electron transfer from Co to MoS2 can reduce the kinetic energy barrier of the initial water dissociation and the adsorption of OH- during the HER process. Thus, this study



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provides a simple avenue to prepare outstanding catalysts for HER in an alkaline medium.

ASSOCIATED CONTENT

Supporting Information.

Detailed additional SEM images, electrochemical performance. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation (21573083), the Program

for

New

Century

Excellent

Talents

in

Universities

of

China

(NCET-13-0237), the Doctoral Fund of Ministry of Education of China (20130142120039), 1000 Young Talent (to Deli Wang), and initiatory financial support from Huazhong University of Science and Technology (HUST). The authors thank the Analytical and Testing Center of HUST for allowing use its facilities.


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TOC


Hierarchically Porous Electrocatalyst with Vertically Aligned Defect

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