2H-MoS2 with Tunable Phases and Residual S, N Co

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“Bulk” 1T/2H-MoS2 with Tunable Phases and Residual S, N co-Doped Carbon as a Highly Active and Durable Catalyst for Hydrogen Evolution Xiaobo He, Fengxiang Yin, Biaohua Chen, Guoru Li, and Huaqiang Yin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02109 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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ACS Applied Energy Materials

“Bulk” 1T/2H-MoS2 with Tunable Phases and Residual S, N co-Doped Carbon as a Highly Active and Durable Catalyst for Hydrogen Evolution Xiaobo He1, 2, Fengxiang Yin1, 2*, Biaohua Chen1,3, Guoru Li1, Huaqiang Yin4

1

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China

2

Changzhou Institute of Advanced Materials, Beijing University of Chemical Technology, Changzhou 213164, PR China

3

College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, PR China

4

Key Laboratory of Advanced Reactor Engineering and Safety, Ministry of Education, Tsinghua University, Beijing 100084, PR China

*Corresponding author Tel.: +86-519-86330253 E-mail: [email protected] (F. Yin) 1

ACS Paragon Plus Environment

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ABSTRACT: Molybdenum sulfide (MoS2) is considered as low-cost catalyst with great potential for hydrogen evolution reaction (HER). In this contribution, a promising Mo-precursor was first designed and prepared via partial reduction of commercial (NH4)6Mo7O24·4H2O by DL-tartaric acid. A simple pyrolysis method as a new “bottom-up” approach was then developed to achieve the desired HER catalysts by using the Mo-precursor. The resulting catalysts consist of multiphasic 1T/2H-MoS2 and residual S, N co-doped carbon (SNC) with oxygen functional groups. In comparison with (NH4)6Mo7O24·4H2O, Mo-precursor with high contents of Mo5+ promotes the full formation of MoS2, while its high content of carbon is more favorable to gain the residual SNC in the resulting catalysts. The further results demonstrate that the percentages of 1T-MoS2 and the contents of the residual SNC can be facilely tuned by the pyrolysis temperatures or the Mo/S feeding molar ratios. Notably, although the resulting catalysts exhibit the “bulk” and irregular morphology with low specific surface areas, the high percentages of 1T-MoS2 as the primary advantage, the highly exposed active sites mainly stemmed from disordered stacking of S-Mo-S layers, and the high contents of the SNC residues are synergistically responsible for their high electrocatalytic HER activity. The high thermal stability of 1T-MoS2 and the excellent durability and stability during HER processes is attributed to the stabilizing effects of the residual SNC. Under the optimized synthetic conditions, the achieved Mo/S(0.2)-450 has a low overpotential of ~130 mV at 10 mA cm-2, a low Tafel slope of 77 mV dec-1, a high specific activity of 17.53 A cmCat.-2, and the excellent durability and stability in 0.5 M H2SO4. This work can provide a promising Mo-precursor and a facile route to developing the highly efficient HER catalysts. 2


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KEYWORDS: Hydrogen evolution reaction; Molybdenum sulfide; Electrocatalysis; Electrocatalysts; Polymorph

3



1. INTRODUCTION With the rapid development of modern society, the increasing demands for clean energy trigger the considerable efforts to develop hydrogen energy, which benefits from its high energy density and the zero emission.1 Among the various techniques for hydrogen production, the electrochemical hydrogen evolution reaction (HER) has attracted tremendous attention.2,

3

However, the high operating overpotential during the electrochemical HER

processes still hinders the practical efficiency for hydrogen production. Thus, the active catalysts are required to minimize the overpotential of HER.4 Although Pt-based catalysts exhibit the outstanding HER electrocatalytic activity, the scarcity and high-cost limit their practical applications for the large-scale hydrogen production.5 Unsurprisingly, many recent researches have focused on the alternative catalysts based on earth-abundant elements with low cost. 6-9 Among the recently-developed earth-abundant HER catalysts, molybdenum disulfide (MoS2) has the great potential as the alternative to the Pt-based catalysts.10-13 The theoretical 12

and experimental

13

results have proven that the edge sites of hexagonal semiconducting

2H-MoS2 (the thermodynamically stable polymorph of MoS2 with trigonal prismatic coordination between the Mo center and the six surrounded S atoms), rather than the basal planes, are active toward HER.13 Although various strategies have been used to increase or expose more active edges,14, 15 the low electronic conductivity of 2H-MoS2 still hinders the further improvement of its HER activity.16 The phase engineering that transforms or directly synthesizes another polymorph, i.e., 1T-MoS2 with the octahedral coordination between the Mo center and the six surrounded S atoms, can overcome this drawback substantially, because 4


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the 1T-MoS2 is metallic. The electronic conductivity of 1T-MoS2 is approximately 107-fold higher than that of 2H-MoS2,17 which may result in the enhanced charge transfer efficiency during the electrocatalytic HER processes,18 the activation of the basal planes,19,

20

simultaneously the improved HER activity. Thus, the 1T-MoS2 has attracted much interest for further enhancing the HER activity of the MoS2-based catalysts. 21-24 A variety of chemical and/or electrochemical “top-down” routes to pure 1T-or hybrid 1T/2H-MoS2 have been developed recently,16,

18, 25

usually using bulk 2H-MoS2 as starting material followed by

various post-treatments. On the other hand, the hydro/solvo-thermal “bottom-up” approaches have been also used to prepare them in recent years,16, 26, 27 using the Mo and S molecular compounds as starting materials. The numerous researches have clearly demonstrated that 1T-MoS2 with the higher conductivity and the enhanced active sites generally exhibits the superior HER activity to its counterpart 2H-MoS2.18, 25, 28-32 Inspired by the aforementioned studies, in this contribution, a simple pyrolysis method as a new “bottom-up” route was developed to prepare the multiphasic 1T/2H-MoS2, using a solid reaction between an as-developed Mo-precursor and thiourea during the pyrolysis process. The percentage of 1T-MoS2 of the resulting catalysts can be easily tuned via the pyrolysis temperatures or the Mo/S feeding molar ratios. Under the optimal conditions, the percentage of 1T-MoS2 is high up to 80.4 %. Meanwhile, in addition to the multiphasic 1T/2H-MoS2, the residual S, N co-doped carbon (SNC) with amorphous features was also gained in the resulting catalysts, even though no additional carbon materials (such as graphene, carbon nanotubes, carbon black nanoparticles, and so on) were added as carbon sources. This is attributed to the high content 5



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of carbon in the as-developed Mo-precursor. Similar to the percentages of 1T-MoS2, the contents of the SNC residues also can be regulated via the pyrolysis temperatures or the Mo/S feeding molar ratios. As it is known, carbon-based materials are attractive in developing HER catalysts,

33-40

because both ordered carbons such as widely developed graphene

amorphous ones

41

35-37

and

can improve charge transfer during electrocatalytic HER processes, thus

enhancing HER activity. Accordingly, this work developed the multiphasic 1T/2H-MoS2 with tunable phases and residual SNC with amorphous features as the desired HER catalysts by using the promising Mo-precursor. The important roles of the Mo-precursor on developing such HER catalysts were also investigated. Surprisingly, although the resulting catalysts have the “bulk” morphological structures with very low specific surface areas, they still exhibit the outstanding HER electrocatalytic performance in acidic electrolyte, which is primarily attributed to high percentages of 1T phase, highly exposed active sites primarily originated from disordered stacking of S-Mo-S layers, and high contents of residual SNC. 2. RESULTS AND DISCUSSION Scheme 1 shows the entire preparation process of Mo/S(0.2)-450. The other HER catalysts were also synthesized via the similar preparation processes. Briefly, a light-blue Mo-precursor with the different structures from the commercial (NH4)6Mo7O24·4H2O (Figure S1 and Note S1) was first synthesized via a partial reduction of (NH4)6Mo7O24·4H2O by DL-tartaric acid. Then, the resulting HER catalysts were obtained via a simple pyrolysis of the mixture of Mo-precursor and thiourea with the Mo/S feeding molar ratio of ~0.2 at different temperatures (350, 450, 550, and 650 °C). Generally, their XRD patterns (Figure 1a) 6


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show that all of diffraction peaks are broadened, which is difficult to differentiate the specific peaks of 1T or 2H-MoS2.26 But, the peaks at ~14º could be still assigned to the (002) facet of MoS2, whatever the phases these resulting catalysts contain (2H 25 or 1T 42). Surprisingly, the diffraction peaks of (002) in Mo/S(0.2)-350 and -450 are absent. The similar thing also happens to the other catalysts with the different Mo/S feeding molar ratios (i.e., Mo/S(0.26)-450, Mo/S(0.16)-450, and Mo/S(0.16)-450), which were also prepared at 450 ºC (Figure S2). The absence of the (002) diffraction peaks in these catalysts prepared at the low pyrolysis temperatures demonstrates that they has the disturbed stacking of S-Mo-S along the c-axis. With the increase of pyrolysis temperatures, the (002) peaks of Mo/S(0.2)-550 and -650 become prominent, in comparison with Mo/S(0.2)-350 and -450, suggesting the presence of an ordered stacking of S-Mo-S layers along c-axis in them. In addition, the (002) diffraction peaks of Mo/S(0.2)-550 and -650 are shifted to the lower degrees, compared with the bulk 2H-MoS2 (2 = 14.4°, JCPDS No. 37-1492), indicating an slightly expanded interlayer spacing (0.64 Å of Mo/S(0.2)-550 and -650 vs. 0.62 Å of bulk 2H-MoS2). The Raman spectra (Figure 1b) of these catalysts prepared at different pyrolysis temperatures with the Mo/S feeding molar ratio of ~0.2 include the characteristic peaks at ~150 cm-1, ~237 cm-1, and ~336 cm-1, especially for the spectrum of Mo/S(0.2)-450. They could be attributed to the J1, J2, and J3 active mode of 1T MoS2.16,

32, 43

Those modes of

1T-MoS2 are also observed in the Raman spectra of the other catalysts prepared at 450 ºC with different Mo/S feeding molar ratios (Figure S3). The other distinct peaks at ~173 cm-1, ~283 cm-1, ~381 cm-1, and ~405 cm-1, which are attributed to the Raman active A1g-LA(M), E1g, E12g, and A1g modes of hexagonal 2H-MoS2,44 respectively, where the LA(M) (= ~232 cm-1) 7



represents the longitudinal acoustic mode at M point of the Brillouin zone of 2H-MoS2. The other controlled catalysts prepared at 450 ºC with different Mo/S feeding molar ratios also have these Raman active peaks of 2H-MoS2 (Figure S3). Notably, the J1 mode of 1T-MoS2 is too close to the second-order band of 2H-MoS2 (i.e, E12g-LA(M) = ~149 cm-1), which is difficult to be distinguished. Anyway, the Raman spectra above clearly demonstrate that all the resulting catalysts include the multiphasic 1T/2H-MoS2. In addition to the peaks attributed to those Raman active modes of the formed MoS2, both the Raman spectra of Mo/S(0.2)-350 and -450 exhibit another two peaks at ~1368 and ~1582 cm−1, corresponding to the D and G bands of the derived carbon, respectively. It is well consistent with the elemental analysis results, which demonstrate that ~12.28 wt% and ~10.84 wt% of residual carbon remains in Mo/S(0.2)-350 and Mo/S(0.2)-450, respectively, likely resulting from thiourea and the Mo-precursor containing ~7.20 wt% of C (Note S1). The similar things happen to the other catalysts with different Mo/S feeding molar ratios prepared at 450 ºC (Figure S3). Their high contents of residual carbons (~6.35 wt% for Mo/S(0.26)-450, ~14.76 wt% for Mo/S(0.16)-450, and ~18.55 wt% for Mo/S(0.13)-450) may be responsible for their distinct D and G bands in their Raman spectra. In contrast, Mo/S(0.2)-550 and -650 do not include the distinct D and G bands in their Raman spectra, which is attributed to the considerable loss of carbon at higher pyrolysis temperatures and also confirmed by the corresponding elemental analyses (very low contents of ~0.38 wt% and ~0.11 wt% of residual carbon in Mo/S(0.2)-550 and Mo/S(0.2)-650, respectively). Notably, as compared with G bands, the more distinct D bands in these Raman spectra (Figure 1b and Figure S3) also indicate that the residual carbons in resulting catalysts may contain numerous structural 8


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defects or distortions, showing amorphous features. As it is reported, carbon materials (such as carbon nanotubes, graphene and mesoporous carbon) has been widely employed to form highly efficient MoS2-based HER catalysts, which benefits from their excellent conductivity and enhanced charge transfer efficiency.45, 46 Herein, although the residual carbons are more like amorphous carbons in comparison with those highly ordered carbons (such as carbon nanotubes and graphene), Zhao et al. have found that amorphous carbons in MoS2/amorphous carbon composites can also improve charge transfer during HER processes. 41 Hence, they are expected to provide those resulting catalysts the improved charge transfer during electrocatalytic HER processes. In addition to the Raman spectra, XPS was also employed to investigate the phases of MoS2 in the resulting catalysts.16 Obviously, the Mo 3d spectra of the resulting catalysts can be deconvoluted into the Mo 3d5/2 and Mo 3d3/2 peaks (Figure 1c and S4a). Generally speaking, the binding energies of Mo 3d for 1T-MoS2 are lower than those for 2H-MoS2.16, 27, 47

Here, it is found that the binding energies of ~228.4 eV (Mo4+ 3d5/2) and ~231.5 eV (Mo4+

3d3/2) are assigned to 1T-MoS2, while the ones of ~229.2 eV (Mo4+ 3d5/2) and ~232.4 eV (Mo4+ 3d3/2) are attributed to 2H-MoS2. Similarly, the spectra of S 2p for the resulting catalysts (Figure 1d and S4b) can also be deconvoluted into the S 2p peaks for 1T-MoS2 (~161 eV for S 2p3/2 and ~162.2 eV for S 2p1/2) and 2H-MoS2 (~161.7 eV for S 2p3/2 and ~162.9 eV for S 2p1/2). In addition, the peaks with the higher binding energies in the S 2p spectra of Mo/S(0.2)-350 and -450 can be attributed to the doped S (~163.6 eV for the C-S-C bond and ~164.5 eV for the C=S bond 38) in the scaffolds of the residual carbons. The similar things are also observed in the S 2p spectra of Mo/S(0.26)-450, Mo/S(0.16)-450, and 9



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Mo/S(0.13)-450. Such observations are probably due to their high contents of the residual carbons, resulting in the visible peaks of the doped S. Based on the Mo 3d regions, the percentage of 1T-MoS2 in the resulting catalysts can be estimated as follows:16, 48 ~13.2 % for Mo/S(0.2)-350, ~80.4 % for Mo/S(0.2)-450, ~15.9 % for Mo/S(0.2)-550, ~6.8 % for Mo/S(0.2)-650, ~72.6 % for Mo/S(0.26)-450, ~45 % for Mo/S(0.16)-450, and ~17.3 % for Mo/S(0.13)-450. Accordingly, the phase compositions of the resulting catalysts can be tuned via the pyrolysis temperatures or the feeding Mo/S ratios. Among the resulting catalysts, Mo/S(0.2)-450 contains the highest percentage of 1T-MoS2. Furthermore, in comparison with the resulting catalysts with the lower contents of the residual carbons (such as Mo/S(0.2)-550 and -650), the ones with the higher contents of the residual carbons, including Mo/S(0.2)-350 and -450 (Figure 1e) as well as Mo/S(0.13)-450, Mo/S(0.16)-450 and Mo/S(0.26)-450 (Figure S4c), naturally have the more distinct N 1s peaks, which can be deconvoluted into pyrindic N (py-N, ~398.4 eV), pyrrolic N (pr-N, ~399.2 eV), and graphitic N (g-N, ~400.3 eV). Given the presence of the doped S in the residual carbons of Mo/S(0.2)-350 and -450 as well as and Mo/S(0.26)-450, Mo/S(0.16)-450, and Mo/S(0.13)-450, they include the SNC. Notably, the SNC in the resulting catalysts still contains some oxygen functional groups, such as adsorbed O (531.1-531.4 eV), C=O (532.1-532.3 eV) and C-O &C-O-C (533.0-533.2 eV),

49

as

indicated in the O 1s XPS spectra (Figure S4d and S4e, and Note S2). Figure 1f exhibits the III-type N2 sorption isotherms of the resulting catalysts prepared at 450 oC. The typical H3 hysteresis loops indicate that the slit-shaped mesopores are included in those catalysts.50 Their BET specific surface areas (SSAs) are small and listed in Figure1f. Among them, the desired Mo/S(0.2)-450 has the relative larger BET SSAs of ~8.45 m2 g-1 10


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than those of the other catalysts. The similar N2 sorption isotherms (Figure S5) are also observed in those catalysts prepared with different Mo/S feeding molar ratios. They have the smaller BET SSAs, as compared with Mo/S(0.2)-450. As revealed by the SEM image of low magnification (Figure 2a), the desired Mo/S(0.2)-450 shows the morphology of the “bulk” irregular particles with large sizes, which resembles the morphology of the Mo-precursor (Figure S1). The further observation demonstrates that some portions of the surfaces of those particles are relatively smooth (the circle-marked zones in Figure 2b and Figure S6), while some parts of surfaces are opened and rough (the square-marked zones in Figure 2b and S6). As viewed from the broken surfaces, the inner zones of those particles have the rough surfaces. Not surprisingly, such bulk morphology is consistent with its low BET SSA. The distinct layered structure is observed in Figure 2c. But, the disoriented stacking structures within the long range along c-axis are also dominant, indicating the abundant defects in it. The poor crystallinity with the disoriented stacking of S-Mo-S layers along c-axis may be responsible for the missing of related (002) peaks of the XRD pattern of Mo/S(0.2)-450 (Figure 1a). The similar disordered stacking structure along c-axis is also included in Mo/S(0.2)-350 (Figure S7a and S7b). When the pyrolysis temperatures increased to 550 and 650 oC, the stacking of S-Mo-S layers along c-axis becomes more ordered in Mo/S(0.2)-550 and -650 (Figure S7c-S7f), which have the slight expanded (002) spacing of ~0.64 nm (consistent with their XRD results in Figure 1a). Even so, there are still many defects in them. Similar to Mo/S(0.2)-450, the other catalysts prepared at 450 oC with different Mo/S feeding molar ratios (Figure S7g-S7l) also have the disordered stacking of the S-Mo-S layers, resulting in the missing of the diffraction peaks of 11



the (002) as indicated in Figure S2. Furthermore, the HRTEM image of the basal plane of Mo/S(0.2)-450 (Figure 2d) also demonstrates the co-existence of 1T and 2H phases generally in plane. The domains marked by the red squares reveal the trigonal lattices of the 1T phase with the spacing of ~0.27 nm (corresponding to the (101) plane).25 The distinct honeycomb lattices (marked by the green squares) are assigned to 2H phase, which contain the spacing of ~0.27 nm indexed to the (100) and (010) planes of 2H-MoS2.26 Notably, the sizes of 1T- and 2H-donmains are small, which agrees with the broad diffraction peaks of the XRD patterns. As discussed above, S, N co-doped residual carbons with oxygen functional groups exist in the resulting catalysts. To further investigate the existing state of residual carbons, elements mapping of Mo/S(0.2)-450 was also acquired, as shown in Figure S8. Within the acquisition zone (Figure S8a and S8b, mainly for Figure S8b), Mo (Figure S8c) and S (Figure S8d) fully covers the whole zone, which is consistent with the main component of MoS2. For C and N (Figure S8e and S8f, respectively), they scatter within the acquisition zone. The signals of O were also acquired (Figure S8g), which corresponds to the XPS results of O 1s (Figure S4d and S4e). In addition, Figure S8h shows the overlapped image of all the five elements (including Mo, S, C, N and O), while Figure S8i is the overlapped image of Figure S8b and S8h. Obviously, the scattered distribution of C (i.e., residual carbons) is distinct especially as shown in Figure S8i. The mapping result indicates that the residual carbons might exist in the form of fragments that combine with MoS2. As it is known, 1T-MoS2 is thermodynamically metastable as compared with 2H-MoS2. To investigate the stability of 1T-MoS2 in the resulting catalysts, the TG/DSC curves (Figure S9) of the mixture of Mo-precursor and thiourea with Mo/S molar ratio of 0.2 and 12


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Mo/S(0.2)-450 were acquired in N2 atmosphere from room temperature to 700 ºC at a heating rate of 5 ºC min-1. As discussed in Note S3, when residual carbons in the resulting catalysts gradually decompose as increasing temperature, a transition of the formed 1T-MoS2 to 2H-MoS2 happens, resulting in exothermic peaks that is generally higher than 600 ºC.

51

In

other words, the residual carbons have positive effects on stabilizing metastable 1T-MoS2. Actually, some efficient methods have been recently developed to stabilize 1T-MoS2, 52 such as functionalizing basal sites,

51, 53

forming nanocomposites

54

and planting single atoms in

basal planes. 55 In this work, the in-situ formed residual carbons that are combined with MoS2 still take effects on stabilizing 1T-MoS2, which is likely due to coupling effects between MoS2 and residual carbons through the oxygen functional groups in residual carbons. 41, 56 Figure 3a shows the LSV curves of the resulting catalysts prepared at different pyrolysis temperatures. Obviously, Mo/S(0.2)-450 requires the lowest overpotential (10 = ~130 mV) to deliver the current density of 10 mA cm-2 in comparison with the other catalysts (Mo/S(0.2)-350, 550 and 650) and the reference sample (Ref-Mo/S(0.2)-450), as indicated in Table 1. This typical HER activity metric (10) value is also lower than that of the other catalysts prepared with different feeding Mo/S molar ratios (i.e., Mo/S(0.26)-450, Mo/S(0.2)-450 and Mo/S(0.2)-450) as shown in Figure S10a and Table S1. Although it still needs efforts to bridge the activity gap between the prepared catalysts and Pt-group based catalysts (such as 10 = ~32 mV for 20 wt% Pt/C in Figure 3a), the 10 value of Mo/S(0.2)-450 is also comparable or lower than those recently-developed MoS2-based HER catalysts (Table S2), such as hierarchical MoS2 nanosheets (~167 mV),57 MoS2xSe2(1-x) (x = 0.54) (~219 mV),58 MoS2/graphene quantum dots (~200 mV),59 inlaid MoS2/graphene 13



Nanosheets (~110 mV),60 1T/2H MoS2 nanosheets (~220 mV),26 and vertically aligned ultrasmall monolayer MoS2 (~126 mV).61 The HER kinetics can be indicated by their Tafel slopes. The Tafel plots from the corresponding LSV curves are shown in Figure 3b. The Tafel slope of Mo/S(0.2)-450 is ~77 mV dec-1. Although this Tafel slope value is higher than that of 20 wt% Pt/C (~32 mV dec-1), it is still lower than that of Mo/S(0.2)-350, 550 and 650 and Ref-Mo/S(0.2)-450 (Table 1). It suggests that Mo/S(0.2)-450 has the more favorable HER kinetics than them, also comparable to that of the recently-reported highly efficient MoS2-based HER catalysts (Table S2). In addition, the HER kinetics of Mo/S(0.2)-450 also surpasses those of the other catalysts prepared with different feeding Mo/S molar ratios (Mo/S(0.26)-450, Mo/S(0.2)-450 and Mo/S(0.2)-450) with the higher Tafel slopes (Figure S10b and Table S1). As it is known, the rough insight into the possible HER mechanism can also be revealed by Tafel slopes.62, 63 In acidic electrolyte, the HER may involve three principle steps as rate-determining step (RDS) with their specific Tafel slopes as follows: 64 Volmer step

H+ + e- → Hads

Tafel slope: 120 mV dec-1

Heyrovsky step

Hads + H+ + e- → H2


Tafel step

Hads + Hads → H2


During a practical HER process on the active sites of catalysts, a Volmer-Heyrovsky mechanism or a Volmer-Tafel mechanism would occur.64 However, the Tafel slope could be influenced by variuos factors, such as the variety of catalysts and electrodes, catalyst preparation conditions, electrode preparation conditions and etc..65-67 Thus, it is insufficient for Tafel slopes alone to get precise insight into the HER mechanism. It should be cautious to 14


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use it. Even so, many reports tentatively proposed one HER mechanism above-mentioned (Volmer-Heyrovsky or Volmer-Tafel) on their developed catalysts by using their corresponding Tafel slopes.57, 59, 62, 68-70 Accordingly, the ~77 mV dec-1 of the Tafel slope for Mo/S(0.2)-450 suggests that the HER processes on it would follow the Volmer-Heyrovsky mechanism in acidic electrolyte.59, 68, 70 The exchange current density (j0) is widely considered as another HER activity metrics. Obviously, Mo/S(0.2)-450 has a high j0 value of ~355.6 A cm-2, which is comparable to the one of 20 wt% Pt/C (~463.1 A cm-2), but much higher than that of the other resulting catalysts (including Mo/S(0.2)-350, 550 and 650, Mo/S(0.26)-450, Mo/S(0.16)-450, and Mo/S(0.13)-450) and Ref-Mo/S(0.2)-450, as indicated in Table 1 and Table S1. Its high j0 value is also comparable or even higher than that of other MoS2-based catalysts, such as Co-doped edge-rich MoS2/nitrogenated grapheme composite (~23.6 A cm-2),66 inlaid ultrathin MoS2/grapheme nanosheets (~140 A cm-2),60 hierarchical MoS2 nanosheets (~36 A cm-2),57 defect-rich ultrathin MoS2 nanosheets (~8.91 A cm-2),10 and Pt-decorated MoS2 nanosheets (~750 A cm-2).71 Based on the j0, the specific activity of the resulting catalysts and Ref-Mo/S(0.2)-450 was obtained via normalizing the j0 values to their corresponding BET SSAs, listed in Table 1 and Table S1. Clearly, Mo/S(0.2)-450 still has the higher specific activity (~17.53 A cmCat.-2) than the others. These results of the HER activity indicate that Mo/S(0.2)-450 affords the high apparent and the intrinsic HER activity with the high kinetics, which outperforms all the other prepared catalysts (including Mo/S(0.2)-350, 550 and 650, Mo/S(0.26)-450, Mo/S(0.16)-450, and Mo/S(0.13)-450) and Ref-Mo/S(0.2)-450. To get insights into the possible origins of the enhanced HER activity of Mo/S(0.2)-450, 15



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its electrochemically active surface area (ECSA) as well as the ones of all the other resulting catalysts (including Mo/S(0.2)-350, 450, 550 and 650, Mo/S(0.26)-450, Mo/S(0.16)-450, and Mo/S(0.13)-450) and Ref-Mo/S(0.2)-450 were determined by electrochemical double-layer capacitance (Cdl) via the Eq.(1), whose detailed estimation can be referred to the experimental section. The plots of cathode and anode current variance versus scan rates are shown in Figure 3c and S10c, while the corresponding Cdl and ECSA values are listed in Table 2 and Table S3. Obviously, Mo/S(0.2)-450 has the higher Cdl (~3.03 mF) and ECSA (~50.50 cm2) values compared with all the other resulting catalysts. Furthermore, on the basis of the ECSA values and the corresponding roughness factor (RF) values (estimated via Eq. (2)), the average turnover frequency (TOF) values as intrinsic activity were also estimated by Eq. (3), as listed in Table 2 and Table S3. Distinctly, Mo/S(0.2)-450 with the high ECSA value has the higher TOF value (37.10 × 10-4 s-1 at 10 = 0 mV and 0.375 s-1 at 10 = 200 mV) than those of the other resulting catalysts and Ref-Mo/S(0.2)-450, which is also comparable to those of the reported MoS2-based catalyst, such as nC60/MoS2 on carbon fiber paper (~2.33 s-1 at 10 = 200 mV),

72

hierarchical MoS2 nanosheets (~0.41 s-1 at 10 = 150 mV),

57

(~0.25 s-1 at 10 = 200

mV) 24 and N-doped MoS2 nanosheets (~4 s-1 10 = 200 mV). 73 Thus, the TOF values above demonstrate that Mo/S(0.2)-450 has the higher intrinsic HER activity as compared with the other resulting catalysts and Ref-Mo/S(0.2)-450. According to the discussion on the general structures of the resulting catalysts above-mentioned, all of them include 1T-MoS2, while some of them also have the S-Mo-S layers with the obviously disordered stacking along c-axis, such as Mo/S(0.2)-350 and -450 (Figure 1a, 2c and S7), and the others prepared at 450 oC with different Mo/S feeding molar 16


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ratios (Mo/S(0.26)-450, Mo/S(0.16)-450, and Mo/S(0.13)-450 in Figure S2 and S7). As reported, the basal planes of 1T-MoS2 can be activated as HER active sites in addition to edge sites.19, 20 Furthermore, as discussed above, the resulting catalysts also contain more or less residual carbons with S, N dopants and oxygen functional groups. In order to get view of the effects of 1T-MoS2 and residual carbons, the relationships between them and TOF values are investigated as indicated in Figure S11. Figure S11a and S11b show the relationship between the percentages of 1T-MoS2 and TOF values for the resulting catalysts. Among the catalysts prepared at different pyrolysis temperatures, the TOFs values obviously increase in line with the increase of the percentage of 1T-MoS2 (Figure S11a). Notably, although Mo/S(0.2)-350 has more disordered stacking of S-Mo-S layers along c-axis (more favorable to expose active sites) as compared with Mo/S(0.2)-550 (Figure S7), it still has the lower TOF value due to its lower percentage of 1T-MoS2. Furthermore, although all the catalysts with different Mo/S feeding ratios obtained at 450 oC have disordered stacking of S-Mo-S layers (Figure 2 and S7), they generally have the enhanced TOF values as increasing the percentage of 1T-MoS2 (Figure S11b). Such results indicate that 1T-MoS2 with both basal planes and edges as active sites has the direct impacts on the intrinsic HER activity for the resulting catalysts, while the disordered stacking of S-Mo-S layers plays assistant roles in facilitating the exposure of active sites of MoS2 (1T-MoS2 and/or 2H-MoS2). In addition, Figure S11c and S11d show the complex relationship between the contents of residual carbons and TOF values. Not similar to what happens to 1T-MoS2, the higher contents of residual carbons may not always result in the higher TOF values. For example, Mo/S(0.2)-350 with ~12.8 wt% of residual carbons has a 17



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lower TOF value than Mo/S(0.2)-450 with ~10.84 wt% of residual carbons. Besides, as compared with Mo/S(0.2)-450, both Mo/S(0.16)-450 and Mo/S(0.13)-450 have more contents of residual carbons, but afford the lower TOF values. In other words, the residual carbons with S, N dopants and oxygen functional groups may be indirectly related with the HER intrinsic activity. On one hand, as reported, S, N dopants can activate inert carbons (especially for highly ordered carbons, such as graphene, carbon nanotubes) to be active toward HER. 35-40

However, the amorphous features of the formed residual carbons might limit the

activation effects of S, N dopants. On the other hand, although oxygen functional groups (such as ketonic C=O) can activate inert carbons (also for highly ordered ones) to be electrocatalytic active toward oxygen evolution reaction,

49

Jiao et al. have found that the

carbons with oxygen functional groups exhibit poor HER activity. 35 Accordingly, the residual carbons with such features above have indirect contributions to the HER activity. They take the indirect effects most likely through affecting charge transfer during electrocatalytic HER processes (Figure S12, see the following discussion).

41

Thus, Mo/S(0.26)-450 (as an

exception) has the slightly lower activity than Mo/S(0.16)-450 and Mo/S(0.13)-450 (Figure S11b), which is most likely due to its lower content of residual carbons (Figure S11d) and thus lower charge transfer efficiency (Figure S12d). All in all, 1T-MoS2 with both basal planes and edges as active sites may dominate the intrinsic HER activity. Under the optimal conditions, Mo/S(0.2)-450 has the optimized intrinsic and apparent HER activity. Furthermore, charge transfer efficiency is generally important to electrocatalytic HER processes. The electrochemical impedance spectra (EIS) of all the resulting catalysts were recorded. The corresponding Nyquist plots with the equivalent circuit are shown in Figure 3d 18


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and S10d. The obtained charge transfer resistance (Rct) values are listed in Table 1 and Table S3. As it is known, 1T-MoS2 has high electronic conductivity.16, 17, 31 Naturally, it can have positive contribution to charge transfer efficiency as the residual carbons do. In order to distinguish their contributions, the relationship between the percentages of 1T-MoS2 and Rct values and the one between the contents of residual carbons and Rct values is plotted in Figure S12. Generally speaking, the Rct values decrease with the increase of the percentages of 1T-MoS2, as shown in Figure S12a and S12b, indicating the enhanced charge transfer efficiencies for the resulting catalysts. Notably, Mo/S(0.2)-550 and Mo/S(0.26)-450 make the exceptions, probably due to their relatively lower contents of residual carbons, as indicated in Figure S12c and S12d, respectively. However, as shown in Figure S12c and S12d, the higher contents of the residual carbons may not always bring out the lower Rct values and thus the enhanced charge transfer efficiencies. For instances, Mo/S(0.2)-350 with the higher content of residual carbons has a higher Rct value than Mo/S(0.2)-450 with the lower content of residual carbons. In addition, as compared with Mo/S(0.2)-450, both Mo/S(0.16)-450 and Mo/S(0.13)-450 have the more contents of residual carbons, but hold the higher Rct values and thus the lower charge transfer efficiencies. Therefore, charge transfer efficiencies, like TOF values, are dominated by 1T-MoS2, while the residual carbons have assistant influences. Under the collective effects of 1T-MoS2 and residual carbons, the optimal Mo/S(0.2)-450 has the highest charge transfer efficiency with the lowest Rct value of ~2.96  among these resulting catalysts. As the results above shown, the used Mo-precursor (Figure S1 and Note S1) also play important roles in developing Mo/S(0.2)-450 with the outstanding HER activity. As the 19



reference sample, Ref-Mo/S(0.2)-450 was also prepared by using (NH4)6Mo7O24·4H2O as Mo source. As indicated in Figure S13a, the XRD pattern of Ref-Mo/S(0.2)-450 shows that it includes the remnants of (NH4)6Mo7O24·4H2O in addition to MoS2, likely due to the insufficient reaction between (NH4)6Mo7O24·4H2O and thiourea under the similar synthetic conditions for Mo/S(0.2)-450. Such insufficient reaction may also result in its much lower content of the residual carbon (~0.41 wt%) than that of Mo/S(0.2)-450 (~10.84 wt%). Although Ref-Mo/S(0.2)-450 also has the “bulk” morphology with the low BET SSA of 3.38 m2 g-1 (Figure S13b), the TEM (Figure S13c) and HRTEM (Figure S13d) images reveal that it contains the very few layered lattice fringes, indicating the poor crystallinity of MoS2 with few exposed edges in it. The XPS results (Figure S13e and S13f) suggest that almost no 1T-MoS2 in Ref-Mo/S(0.2)-450 in addition to 2H-MoS2. The high content of Mo6+ (Figure S13e) in it may correlate with the residual (NH4)6Mo7O24·4H2O and/or the oxidation of the resulting MoS2. The peaks attributed to SO42- and SO32- in its S 2p spectrum (Figure S13f) also indicates the oxidation of the resulting MoS2. In contrast, the oxidation of the MoS2 in Mo/S(0.2)-450 can be ignored as compared with Ref-Mo/S(0.2)-450 (Figure 1c and 1d). According to the discussion above, Ref-Mo/S(0.2)-450 has hardly any 1T-MoS2, the few exposed active edge sites and the low content of the residual carbon, which thus result in the smaller ECSA (~15.83 cm2), the lower TOF value (~0.035 s-1 at  =200 mV) and the larger Rct (~44.9 ) (Figure 3c and 3d, and Table 2). Unsurprisingly, Ref-Mo/S(0.2)-450 has much lower activity than Mo/S(0.2)-450 (Figure 3 and Table 1). Accordingly, it is of importance that the Mo-precursor is a better precursor than the commercial (NH4)6Mo7O24·4H2O, which ensures the formation of the 1T/2H-MoS2 with the residual carbons as highly efficient HER 20


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catalysts. Such positive effects of the Mo-precursor on the formation of the resulting catalysts are probably related with the existence of the high content of Mo5+ (the Mo5+/Mo6+ ratio of ~1.71) and the ~7.20 wt% of carbon in it, referring to Note S1. On one hand, it is easier for the lower oxidation state of Mo5+ to be further reduced to Mo4+ (favorable for the full formation of MoS2), as compared with the higher oxidation state of Mo6+. On the other hand, the high content of carbon in the Mo-precursor is more favorable for the formation of the residual carbons in the resulting catalysts, in comparison with (NH4)6Mo7O24·4H2O. But, the further investigations are required to dig out the detailed effects, which is important to develop the HER activity with the higher performance by using the Mo-precursor. As a good HER catalyst, an excellent durability is also highly required for the practical applications in addition to the excellent activity. The results of the HER activity above-discussed demonstrate that the optimized Mo/S(0.2)-450 has the outstanding activity toward HER. Here, to assess the durability of Mo/S(0.2)-450, the long-term potential cycling was performed within a potential range from 0 to -0.6 V (vs. RHE) for 4000 cycles. Figure 4a shows the LSV curves of Mo/S(0.2)-450 before and after potential cycling. Notably, its HER activity was maintained very well (only ~3 mV increased in 10). By contrast, the 10 value of 20 wt% Pt/C was increased by ~20 mV, showing the slightly reduced activity. To investigate the changes in Mo/S(0.2)-450 after CV cycling, XPS and TEM (&HRTEM) were also carried out. The XPS spectra of Mo 3d and S 2p (Figure S14a and S14b, respectively) show that the high percentage of 1T-MoS2 is still maintained in Mo/S(0.2)-450. The percentage of 1T-MoS2 (based on the Mo 3d regions) is still kept as high as ~78.9 % in Mo/S(0.2)-450 after CV cycling, which is slightly lower than that of ~80.4 % for the pristine Mo/S(0.2)-450 before 21



CV cycling. In addition, the presence of surface Mo6+ (~2.81 %) and SOx2- (SO32- + SO42- = ~6.33 %) with small amounts (Figure S14a and S14b, respectively) also indicates that Mo/S(0.2)-450 was slightly oxidized during CV cycling. The TEM (&HRTEM) images (Figure 2c and 2d vs. Figure S14c and 14d) suggest that the layered morphology with disordered stacking of S-Mo-S layers and the coexistence of 1T- and 2H-MoS2 alters slightly before and after CV cycling. In addition, to further evaluate the HER durability of Mo/S(0.2)-450, the chronopotentiometry (-t) measurement was performed at a constant current density of 10 mA cm-2 for 50 h. As shown in Figure 4b, the overpotential (i.e., 10) for Mo/S(0.2)-450 during the -t test were increased by ~27 mV. As for 20 wt% Pt/C, the 10 was increased by ~36 mV. Accordingly, Mo/S(0.2)-450 has the excellent durability and stability in addition to the high activity during the HER processes All in all, the optimized Mo/S(0.2)-450 has the outstanding HER activity may be attributed to the following advantageous facts: (i) The high percentage of 1T-MoS2 with both basal planes and edges as active sites that has dominant contributions to intrinsic activity and charge transfer efficiency; (ii) The disordered stacking of S-Mo-S layers along c-axis that facilitates the exposure of active sites; (iii) The high content of residual carbons that also provide positively assistant contributions to charge transfer efficiency. In addition, the high thermal stability of 1T-MoS2 in Mo/S(0.2)-450 and the excellent durability and stability during HER processes should be attributed to the stabilizing roles of the residual carbons via the coupling effects between MoS2 and the residual carbons. 41, 56 3. CONCLUSIONS In summary, this work first developed a promising Mo-precursor via partial reduction of 22


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commercial (NH4)6Mo7O24·4H2O by DL-tartaric acid. A simple pyrolysis method was then applied to obtain the desired HER catalysts by using the Mo-precursor, consisting of a nanocomposite of multiphasic 1T/2H-MoS2 and residual SNC with oxygen functional groups. In comparison with (NH4)6Mo7O24·4H2O, the prepared Mo-precursor is the key to achieving these desired catalysts: on one hand, the Mo-precursor with high contents of Mo5+ facilitates the full formation of MoS2; on the other hand, its high content of carbon is more favorable to gain the residual carbon in the resulting catalysts. The further results show that the percentages of 1T-MoS2 and the contents of the residual SNC in the resulting catalysts can be facilely regulated by the pyrolysis temperatures or the Mo/S feeding molar ratios. Notably, although the resulting catalysts exhibit the “bulk” and irregular morphology with low specific surface areas, the high percentages of 1T-MoS2 as the main advantage, the high exposure of active sites mainly due to the disordered stacking of S-Mo-S layers, and the high contents of the SNC residues are synergistically responsible for their high electrocatalytic HER activity. The high thermal stability of 1T-MoS2 and the excellent durability and stability during HER processes can be attributed to the stabilizing effects of the residual SNC. Under the optimal conditions, the obtained Mo/S(0.2)-450 has the outstanding HER activity and the excellent durability and stability in 0.5 M H2SO4. It is believed that this work can provide a promising Mo-precursor and an effective route to developing the highly active and durable HER catalysts. 4. EXPERIMENTAL SECTION Preparation of the Mo-precursor. 20.13 g of hexeaammonium heptamolybdate tetrahydra ((NH4)6Mo7O24·4H2O) and 4.89 g of DL-tartaric acid were dissolved in 80 mL of 23



deionized (DI) water successively under stirring. After a reflux at 80 oC for 1 h, the obtained dark blue solution was transferred into a beaker containing 200 mL of anhydrous ethanol. When naturally cooling down to room temperature, the resulting light-blue precipitate was filtered and washed with anhydrous ethanol. The Mo-precursor was finally obtained after dried at 80 °C overnight. According to the elemental analysis and the inductively coupled plasma (ICP) results, the Mo-precursor contains: C, 7.20 wt%; H, 2.66 wt%; N, 5.74 wt%; O, 15.87 wt%, and Mo, 68.53 wt%. Synthesis of the HER catalysts. In a typical synthesis, the obtained Mo-precursor and thiourea were grinded to fine powders in a mortar with the feeding mass ratio of 3:8 (i.e., Mo/S feeding molar ratio of ~0.2), and the mixture powder was annealed at a ramp rate of 5 ºC min-1 in N2 for 6 h at 350, 450, 550 and 650 ºC, respectively. The products were washed with DI water and anhydrous ethanol before dried at 80 ºC under vacuum conditions. The resulting catalysts were referred to Mo/S(0.2)-350, Mo/S(0.2)-450, Mo/S(0.2)-550 and Mo/S(0.2)-650, respectively. In addition, the other controlled catalysts with the different feeding Mo/S molar ratios (including ~0.26, ~0.16 and ~0.13) were also prepared at 450 ºC under the similar synthesis conditions, which were labeled as Mo/S(0.26)-450, Mo/S(0.16)-450 and Mo/S(0.13)-450. For comparison, (NH4)6Mo7O24·4H2O and thiourea were directly grinded to fine powders in a mortar with Mo/S feeding molar ratio of ~0.2, and the mixture powders were also annealed at 450 ºC under the similar synthesis conditions. The reference sample was labeled as Ref-Mo/S(0.2)-450. Physical characterizations. The crystal structure of the samples was analyzed by using 24


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X-ray Powder diffraction (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation (Cu K, λ = 1.5406 Å). The surface chemical states and compositions of the samples were determined by using X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo, USA). All the binding energies were calibrated to the C 1s peak at 284.6 eV. Raman spectra were recorded using a Raman microscope (Renishaw inVia, Renishaw, UK) with a 633 nm excitation laser. The morphology of the samples was observed by scanning electron microscopy (SEM, JSM-7500F, JEOL, Japan) and transmission electron microscopy (TEM, JEM-2100, JEOL, Japan). The element analysis was conducted on an elemental analyzer (Elementar, Vario EL III, Germany). The ICP results were obtained on an ICPS-7500 (Shimadzu, Japan). The N2 sorption isotherms were obtained on a TriStar II 3020 automatic analyzer (Micromeritics Instrument Corporation, USA).The corresponding specific surface areas of the samples were calculated via Brunauer-Emmett-Teller (BET) method. The thermal behaviors of precursors or samples, including thermogravimetric (TG) and differential scanning calorimetry (DSC), were analyzed on a thermal analyzer (METTLER-TOLEDO, TGA/DSC 3+, USA). Electrochemical measurements. The ink for HER working electrode was prepared as follows: 2 mg of the samples (such as the prepared catalysts, the reference sample and 20 wt% Pt/C) and 50 μL of Nafion solution (5 wt%, DuPont) were added to 1 mL of ethanol. After ultra-sonication for 30 min, a homogeneous ink was obtained. ~24 μL of the ink was dropped on a glassy carbon electrode (GCE, 0.196 cm-2, Pine Research Instrumentation). The catalyst mass loading of geometry is ~0.24 mg cm-2disk. For comparison, 20 wt% Pt/C was used as a benchmark HER catalyst with a mass loading of ~0.24 mg cm-2disk (~48 gPt 25



Page 26 of 50

cm-2disk). The electrochemical measurements of the catalysts were carried out in a three-electrode configuration in N2-saturated 0.5 M H2SO4 at room temperature, using an electrochemical workstation (CHI760E, CHI). A KCl-saturated Ag/AgCl electrode and a carbon rod were used as the reference and counter electrodes, respectively. All the final potentials were calibrated to a reversible hydrogen electrode (RHE) scale using the Nernst equation ERHE = EAg/AgCl + 0.196 V (in 0.5 M H2SO4) and corrected for iR loss. The HER linear sweep voltammetry (LSV) curves were recorded from 0.2 to -0.6 V (vs. RHE) at a sweep rate of 2 mV s-1 and at 1600 rpm. To determine ECSA and RF values, Cdl was first measured through cyclic voltammetrys (CVs) at various scan rates (10, 20, 30, 40 and 50 mV s-1) performed within a potential range from -0.25 to -0.20 V (vs. RHE) without Faradaic currents. The anodic and cathodic current differences at -0.225 V (vs. RHE) were plotted vs. scan rates, where the half of the slopes of the plots were determined as Cdl values. The corresponding ECSA values were calculated as follows: 74 ECSA = RF =

𝐶𝑑𝑙

(1)

𝐶𝑠

ESCA

(2)

Ageo

where Cs is the specific capacitance of a flat and smooth standard electrode per real surface area (cm-2), Ageo represents geometric area of the GC electrode (~0.196 cm-2). Here, as for MoS2-based electrode in 0.5 M H2SO4, the Cs of ~60 F cm-2 is adopted.75 The TOF as an essential metric for intrinsic activity for a HER catalyst was estimated via the following equation: 72, 76 26


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j𝑁𝐴

(3)

TOF = 2𝐹(𝑅𝐹)𝑛𝑓𝑙𝑎𝑡

where j is current density from LSV curves, NA is Avogadro’s constant (6.022 × 1023 mol-1), “2” is the stoichiometric number of transferred electrons per evolved H2 molecule, F is Faradaic constant (96485.3 C mol-1), RF is roughness factor which is determined by Eq. (2), nflat is the surface site number for the flat surface of crystalline MoS2 (~1.164 × 1015 cm-2 for a MoS2 unit. For sake of simplicity, it assumes that both Mo and S are active for HER, whereas S, N co-doped residual carbons with oxygen functional groups do not act as active materials with high intrinsic activity.). 75 The EIS measurements were carried out at open circuit potential with a frequency range from 106 Hz to 10 Hz. To assess the HER durability, the CVs were performed for 4000 cycles at 50 mV s-1 within the potential range from 0 to -0.6 V (vs. RHE). Before and after the CV cycling above, the LSV curves were recorded at 2 mV s-1 and at 1600 rpm. In addition to CV cycling, the chronopotentiometry (-t) was also used to evaluate the long time durability. The -t test was performed at a constant current density of 10 mA cm−2 for 40000 s. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Structural and morphology information of the Mo-precursor and the commercial (NH4)6Mo7O24·4H2O; XRD patterns, Raman spectra, XPS spectra, N2 sorption isotherms of the catalysts with different Mo/S feeding molar ratios obtained at 450 ºC; XPS spectra (O 1s) of the catalysts prepared at different pyrolysis temperatures; TEM and HRTEM images of all 27



resulting catalysts; Additional SEM, TEM and element mapping images of Mo/S(0.2)-450; TG/DSC curves of the mixture of Mo-precursor and thiourea with Mo/S molar ratio of 0.2 and Mo/S(0.2)-450; The electrocatalytic HER performance of the catalysts with different Mo/S feeding molar ratios obtained at 450 ºC; Relationship between percentages of 1T-MoS2 and TOF values for the resulting catalysts; Relationship between percentages of 1T-MoS2 and Rct values for the resulting catalysts; Structural and morphology information of Ref-Mo/S(0.2)-450; Structural and morphology information of Mo/S(0.2)-450 after CV cycling. Corresponding Author *E-mail: [email protected] (F. Yin). Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (21706010), the Natural Science Foundation of Jiangsu Province of China (BK20161200). Special thanks to the support from Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University (ACGM2016-06-02 and ACGM2016-06-03), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Key Laboratory of Advanced Reactor Engineering and Safety, Ministry of Education (ARES-2018-09). REFERENCES (1) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. 28


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39



SCHEMES, FIGURES AND LEGENDS

Scheme 1. Schematic illustration of the typical preparation process of the 1T/2H-MoS2 with the residual SNC (Mo/S(0.2)-450 as example).

40


Page 40 of 50

Page 41 of 50

(a)

(b) Mo/S(0.2)-650 A1g-LA(M) E1g

Intensity (a.u.)

Intensity (a.u.)

(002)

Mo/S(0.2)-650 Mo/S(0.2)-550

E12g A1g

Mo/S(0.2)-550 J1 (+E12g-LA(M)) J3

J2

G

D

Mo/S(0.2)-450

Mo/S(0.2)-450 Mo/S(0.2)-350

Mo/S(0.2)-350

20

30

40

(c) Mo/S(0.2)-650 Mo6+

Intensity (a.u.)

50

2-Theta (deg)

3d3/2

60

70

Mo/S(0.2)-550

2H

170

Binding energy (eV)

Mo/S(0.2)-450 pr-N

py-N

Mo/S(0.2)-350

401

400

399

C-S-C

398

168

C-S-C S=C

166

164

162

160

Binding energy (eV) Quantity adsorbed (cm3 g-1 STP)

Mo/S(0.2)-550

g-N

1T

Mo/S(0.2)-550

240 238 236 234 232 230 228 226 224

Mo/S(0.2)-650

S 2p

Mo/S(0.2)-650 2SO4

Mo/S(0.2)-350

Mo/S(0.2)-350

402

2H

Mo/S(0.2)-450

N 1s

1200 1600

(d)

1T

(e)

800

Raman shift (cm-1)

Mo 3d

3d5/2

S 2s

Mo/S(0.2)-450

200 300 400

80

Intensity (a.u.)

10

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

(f)

15

10

5

0 0.0

397

BET SSA (cm2 g-1) 3.08 Mo/S(0.2)-350 Mo/S(0.2)-450 8.45 7.91 Mo/S(0.2)-550 3.87 Mo/S(0.2)-650

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Binding energy (eV)

Figure 1. The structural features of the catalysts obtained at different pyrolysis temperatures: (a) XRD patterns; (b) Raman spectra; (c), (d) and (e) XPS spectra of Mo 3d, S 2p and N 1s, 41



respectively; (f) N2 sorption isotherms.

42


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Page 43 of 50


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 43



Figure 2. Morphological structures of Mo/S(0.2)-450: (a) SEM image of low magnification and (b) the one of high magnification from the zone in (a) marked with the purple square; (c) TEM and (d) HRTEM images, the insets are the corrected and magnified images of the inverse fast Fourier transform (FFT) images of the selected zones (1T and 2H).

44


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Page 45 of 50

0.5

(a) 20 wt% Pt/C Mo/S(0.2)-350 Mo/S(0.2)-450 Mo/S(0.2)-550 Mo/S(0.2)-650 Ref-Mo/S(0.2)-450

-5

-10 mA cm

-10

-2

0.2

114 mV dec-1 77 mV dec-1

93 mV dec-1

-0.4

-0.2

0.0

100

2.67 mF

1.15 mF

0.2 0.0

-Z" (Ohm)

0.4

0.2

0.6

)]

14 12

Mo/S(0.2)-350 Mo/S(0.2)-450 Mo/S(0.2)-550 Mo/S(0.2)-650 Ref-Mo/S(0.2)-450

60

0.4

-2 disk

10 8 6 4 2 0 8

12

16

20

Z' (Ohm)

24

40 20

0.95 mF

1.04 mF

0.0

(d)

80 3.03 mF

-0.2

log[j/(mA cm

(c)

0.6

-0.4

0.2

Potential (V vs. RHE)

Mo/S(0.2)-350 Mo/S(0.2)-450 Mo/S(0.2)-550 Mo/S(0.2)-650 Ref-Mo/S(0.2)-450

32 mV dec-1

-Z'' (Ohm)

-0.6

212 mV dec-1

163 mV dec-1

0.1

-0.1 -0.6

-20 -0.8

-0.2

0.3

20 wt% Pt/C Mo/S(0.2)-350 Mo/S(0.2)-450 Mo/S(0.2)-550 Mo/S(0.2)-650 Ref-Mo/S(0.2)-450

0.0

-15

0.8

(b)

0.4

Overpotential (V)

-2

Current density (mA cmdisk)

0

ia-ic(mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 10

20

30

40

Scan rate (mV s-1)

50

0

20

40

60

80

100

Z' (Ohm)

Figure 3. (a) The LSV curves of the resulting catalysts, the reference sample (Ref-Mo/S(0.2)-450), and 20 wt% Pt/C at 1600 rpm in 0.5 M H2SO4; (b) The corresponding Tafel plots; (c) The variance of cathodic and anodic current at -0.225 V (vs. RHE) was plotted as a function of scan rate; (d) The Nyquist plots of the prepared catalysts and the reference sample. The upper inset shows the magnified high frequency region. The lower inset is the corresponding equivalent circuit, where Rs is the solution resistance, Rct is the charge transfer resistance, CPE is a constant phase element, W is a Warburg constant element related to the diffusion.

45


Initial After 4000 cycles

0 -2

-10 mA cm

-20

Mo/S(0.2)-450

20 wt% Pt/C

-40

-60 -0.3

-0.2

-0.1

0.0

Potential (V vs. RHE)

0.1

Overpotential (V)

(a)

Current density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Overpotential (V)


0.10

Page 46 of 50

(b)

0.08 0.06

36 mV

0.04 0.02

20 wt% Pt/C

0.00 0.18 0.16 0.14

27 mV

0.12 Mo/S(0.2)-450

0.10 0

10

20

30

40

50

Time (h)

Figure 4. (a) HER LSV curves for Mo/S(0.2)-450 and 20 wt% Pt/C before and after 4000 cycles of potential sweeps; (b) Chronopotentiometry (-t) in 0.50 M H2SO4 at a constant current density of 10 mA cm-2 for 50 h.

46


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TABLES Table 1. Summary of HER activity of the prepared catalysts at different pyrolysis temperatures and the reference sample.

10 (mV)

Tafel slope (mV dec-1)

j0 (A cmdisk-2)

Specific activity (A cmCat.-2) a

Mo/S(0.2)-350

314

114

14.7

1.99

Mo/S(0.2)-450

130

77

355.6

17.53

Mo/S(0.2)-550

237

93

46.9

2.47

Mo/S(0.2)-650

417

163

12.5

1.35

Ref-Mo/S(0.2)-450

437

212

9.4

1.16

Samples

a

The specific activity was estimated via the normalization of the j0 to the corresponding BET SSAs.

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Page 48 of 50

Table 2. The Rct, Cdl, ESCA and TOF values of the prepared catalysts at different pyrolysis temperatures and the reference sample. Rct )

Cdl (mF)

ESCA (cm2)

Mo/S(0.2)-350

5.77

1.15

19.17

4.03 × 10-4

0.054

Mo/S(0.2)-450

2.96

3.03

50.50

37.10 × 10-4

0.375

Mo/S(0.2)-550

5.84

2.67

44.50

5.54 × 10-4

0.065

Mo/S(0.2)-650

9.56

1.04

17.33

3.78 × 10-4

0.048

Ref-Mo/S(0.2)-450

44.9

0.95

15.83

3.10 × 10-4

0.035

Samples

a

TOF (s-1) a

The TOF values in the left row were estimated by using j0 (i.e.,  = 0 mV), while the TOF values in the right one were estimated by using the

current density at an overpotential of 200 mV.

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TABLE OF CONTENTS (TOC)

49



Schematic illustration of the typical preparation process of the 1T/2H-MoS2 with the residual SNC (Mo/S(0.2)-450 as example). 139x77mm (300 x 300 DPI)


Page 50 of 50

2H-MoS2 with Tunable Phases and Residual S, N Co

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