Engineering Thin MoS2 Nanosheets on TiN Nanorods: Advanced

Subscriber access provided by UNIV OF NEWCASTLE

Letter 2

Engineering Thin MoS Nanosheets on TiN Nanorods: Advanced Electrochemical Capacitor Electrode and Hydrogen Evolution Electrocatalyst Minghao Yu, Shaobin Zhao, Haobin Feng, Le Hu, Xiyue Zhang, Yinxiang Zeng, Yexiang Tong, and Xihong Lu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00602 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Energy Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

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

ACS Energy Letters

Engineering Thin MoS2 Nanosheets on TiN Nanorods: Advanced Electrochemical Capacitor Electrode and Hydrogen Evolution Electrocatalyst Minghao Yu,†,‡ Shaobin Zhao,‡ Haobin Feng,‡ Le Hu,‡ Xiyue Zhang,‡ Yinxiang Zeng,‡ Yexiang Tong‡ and Xihong Lu*,†,‡ †

School of Applied Physics and Materials, Wuyi University, Jiangmen, Guangdong 529020,

People’s Republic of China ‡

MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of

Environment and Energy Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACS Paragon Plus Environment

1

ACS Energy Letters

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

Page 2 of 29

Abstract: The poor intrinsic conductivity of MoS2 presents a huge barrier for the exploitation of its versatile properties, especially as an electrochemical capacitors (ECs) electrode and hydrogen evolution reaction (HER) catalyst. Toward this challenge, TiN nanorods coated by randomly oriented MoS2 nanosheets (TMS) are engineered as state-of-the-art electrodes for ECs and HER. In the light of the synergistic effects, TMS electrodes show favorable performance as both binder-free ECs electrode and HER catalyst. Importantly, the optimal TMS achieves an areal capacitance of 662.2 mF cm-2 at 1 mA cm-2 with superior rate capability and ultra-long cycling stability. As catalyst for HER in 0.5 M H2SO4, it shows an overpotential of 146 mV at 10 mA cm-2, favorable Tafel slope and good electrocatalytic stability. All the results highlight the favorable integration of TiN and MoS2, and provide clear insight correlating the hybrid structure and the corresponding electrochemical performance.

TOC GRAPHICS


2

Page 3 of 29

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

ACS Energy Letters

The critical shortage of fossil energy and global warming issue prompt an urgent development of renewable energies

1-3

. To address the geographically, seasonally, and temporally unbalanced

distribution of renewable sources, renewable energies are generally converted into electrical energy and stored. Electrochemical capacitors (ECs) provide a viable solution for electrical energy storage, which own plenty of desirable properties over batteries, like fast charge/discharge ability, excellent power density and long cycling stability 4-6. Nevertheless, ECs still suffer from the insufficient energy density, severely restricting their opportunity to be applied in most of modern electronics. As such, it is extremely attractive to fabricate highcapacitance electrodes in a low cost, facile and easy way. On the other hand, electrochemical water splitting technology that efficiently converts electricity into chemical energy stored in high-purity H2, also offers an effective solution for energy storage and transport

7-9

. Even though

platinum based catalysts are considered as the most effective HER electrocatalysts, the high price and scarcity rule them out as scale-up deployment. Great deals of efforts have been devoted toward exploring cost-effective and earth-abundant replacements for platinum based catalysts. However, further breakthrough in improving the sluggish kinetics of nonprecious catalysts still remains a vital challenge to date. MoS2, a layered transition metal dichalcogenide, is made up of covalently bonded S–Mo–S monolayers via van der Waals interactions

10-12

. Such fascinating layered properties provide

MoS2 a great possibility in versatile energy related applications, particular in energy storage and HER. The wide and weak van der Waals gaps are favorable for the fast intercalation of guest species (like H+, Li+, and Na+), endowing MoS2 as promising alternatives for Li ion batteries13-14, Na ion batteries15-16 and ECs11-12. Meanwhile, the edge sites of MoS2 can accelerate the hydrogen binding process and present high catalytic activity for HER

10, 17

. However, the performances of


3

ACS Energy Letters

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

Page 4 of 29

MoS2 still fall short of expectations, which are plagued with two main challenges: semiconducting nature with poor conductivity and limited exposure of their edge sites. These render MoS2 slow kinetics and unexploited active centers for both ion intercalation and catalytic reaction. Thus, it is highly desirable to design well-defined MoS2 nanostructures with efficient charge transport pathways and abundant exposure of edge sites. Herein, we engineered MoS2 nanosheets on TiN nanorods, which offer a smart binder-free protocol for high-performance ECs electrodes and HER catalysts. Randomly oriented MoS2 nanosheets were coated onto TiN nanorods, which vertically aline on carbon cloth in a homogeneous manner, exhibiting a widely open structure with abundant edge sites exposed. Featuring highly conductive interior and sufficient active centers to accommodate fast ion intercalation and catalytic reaction, the hybrid electrode shows a superb activity as flexible and binder-free EC electrode and HER electrocatalyst. The designed hierarchical structure will pave a novel way for the construction of state-of-the-art ECs electrodes and HER catalysts.


4

Page 5 of 29

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

ACS Energy Letters

Figure 1. (a, b) SEM, (c-e) TEM, (f) high angle annular dark-field STEM images and corresponding elemental mapping of TMS sample. Vertically aligned TiN nanorods on carbon cloth were firstly obtained through growing TiO2 nanorods using a seed-assisted hydrothermal method, followed by annealing under NH3 atmosphere. The scanning electron microscopy (SEM) images display numerous TiN nanorods with diameters of 180-250 nm (Figure S1). Figure S2a shows the TEM image of TiN nanorods with rough surfaces. The lengths of these nanorods are about 0.8-1.5 μm. The ring-shaped selected area electron diffraction (SAED) patterns shown in Figure S2b indicate the polycrystalline nature of TiN. All the SAED patterns are in good agreement with osbornite TiN, no pattern related to TiO2 is observed, implying the complete conversion form TiO2 to TiN.


5

ACS Energy Letters

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

Page 6 of 29

Subsequently, thin MoS2 nanosheets were coated on the surface of TiN nanorods through a second hydrothermal process. As revealed from the SEM images (Figure 1a and 1b), the hybrid TiN@MoS2 (defined as TMS) preserves the 1D morphology well. In comparison with the smooth surface of TiN nanorods, TMS nanorods show a much rough surface with obvious ravine. The transmission electron microscopy (TEM) images (Figure 1c and 1d) of TMS nanorods present the typical core-shell configuration constructed by the even coverage of randomly oriented MoS2 on the entire TiN skeleton. These spatially dispersed MoS2 nanosheets form a widely open micro-forest, which is particularly beneficial for electrode/electrolyte interaction. Moreover, the high-resolution TEM image of MoS2 edge view (Figure 1e) identifies the interlayer distance is 0.64 nm, which corresponds to the (002) plane of MoS2 (JCPDS No. 371492). The MoS2 nanosheets display a very small thickness with only 4-6 layers. Figure 1f further collects the elemental distribution of the hybrid TMS nanorods. The well-defined coreshell structure was unambiguously verified with that Ti and N elements distribute on the core area, while Mo and S elements are showed on the shell part. For comparison, MoS2 nanosheets directly synthesized on carbon cloth were also obtained. The SEM images of pristine MoS 2 (Figure S3) show numerous nanosheets stacked into large aggregated nanoparticles, implying an insufficient exposure of surface sites.


6

Page 7 of 29

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

ACS Energy Letters

Figure 2. (a) XRD patterns of TiN, MoS2 and TMS samples. Core-level (b) Mo 3d – S 2s and (c) S2p XPS spectra of TMS sample. (d) Core-level Ti 2p spectra of TiN and TMS samples. X-ray diffraction (XRD) and X-ray photo-electron spectroscopy (XPS) measurements were carried out to gain more detailed insights into the compositions. Figure 2a compares the XRD pattern of TiN nanorods on carbon cloth, MoS2 nanosheets on carbon cloth and hybrid TMS samples. The major reflections of TiN nanorods on carbon cloth and MoS2 nanosheets on carbon cloth are well indexed to osbornite phase (JCPDS No. 38-1420) and molybdenite 2H phase (JCPDS No. 37-1492), respectively. Meanwhile, the diffraction peaks of TMS perfectly match with the overlay pattern of TiN nanorods on carbon cloth and MoS2 nanosheets on carbon cloth.


7

ACS Energy Letters

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

Page 8 of 29

It is noticeable that the (002) peak of 2H MoS2 presents a broad feature, which is indicative of the nanoscale sized and random arranged MoS2 sheets. The survey XPS spectrum of TMS indicates the presence of Ti, N, Mo, S, and O elements (Figure S4). Elemental O owes to the formation of the oxide passivation layer on TiN when exposed to the air18. The +4 oxidation state of Mo is evidenced by the high-resolution Mo 3d spectrum (Figure 2b), which consists of Mo 3d5/2 (228.9 eV) and Mo 3d3/2 (232.0 eV) peaks

12, 19

. Likewise, the S 2p spectrum (Figure 2c)

depicts a doublet shape with S 2p3/2 peak located at 161.7 eV and S 2p1/2 peak located at 162.9 eV, confirming the -2 oxidation state of sulfur in MoS219. It is also revealed that the Mo/S ratio is 1:2.1, in good agreement with the stoichiometric ratio in MoS2. In addition, the Ti 2p spectra were detected to assess the surface composition of TiN. For pristine TiN nanorods on carbon cloth, the broad Ti 2p3/2 peak can be de-convoluted into three dependent peaks at 458.6 eV, 455.8 eV, and 457.2 eV associated with TiO2 (Ti-O), TiN (Ti-N), and intermediate state (Ti-NO), which corroborate the formation of oxide passivation layer18. By contrast, the Ti 2p3/2 peak of TMS sample shows a much lower intensity due to the coverage of MoS2 nanosheets and exhibits a sharp peak corresponding to Ti4+. According to the previous reports on metal-oxide host material18, these bonds play a crucial role on connecting MoS2 and TiN by chemical interactions, which also are beneficial for the efficient charge transfer between MoS2 and TiN and promote the synergistic effect between them. It should be pointed out that the XPS analysis only provides the composition information of several nanometers in depth. The interior parts of both TiN nanorods on carbon cloth and the TMS core remain nitride nature, which is confirmed by the aforementioned XRD and EDX mapping results.


8

Page 9 of 29

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

ACS Energy Letters

Figure 3. (a) Imaginary capacitance vs. the logarithm of frequency and (b) areal capacitances of all electrodes at various scan rates. (c) GCD curves with different current densities of TMS6 electrode. (d) Areal capacitance as a function of the current density for TiN, MoS 2, TMS6 and recently reported ECs anodes. (e) Cycling performance obtained for TMS6 electrode.


9

ACS Energy Letters

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

Page 10 of 29

To probe the superiority of the as-fabricated TMS electrode in energy storage, the capacitive properties were evaluated. The hydrothermal reaction time was controlled from 1 to 8 hours to optimize the performance (denoted as TMS1, TMS2, TMS4, TMS6, and TMS8), while pristine TiN and MoS2 electrodes were tested for comparison. Electrochemical impedance spectroscopy (EIS) was firstly used to examine the kinetics of ion and charge transport in the electrodes. As revealed by the Nyquist plots (Figure S5), the TiN and all TMS electrodes present almost vertical lines at low frequency region, the slope of which are remarkably higher than that of MoS 2 electrode, suggesting TiN promoted an ideal capacitive feature. The internal resistances (Rs) of all electrodes were obtained by fitting with an equivalent circuit. As expected, TiN electrode possesses the lowest Rs of 3.52 Ω, whereas MoS2 shows the highest Rs of 6.10 Ω, demonstrating the superior conductivity of TiN to that of MoS2. For better understanding the frequency behavior of all electrodes, we further assessed the capacitance models of all electrodes by distinguishing the real (C'(ω)) and imaginary (C''(ω)) part of the whole capacitance (C(ω))20-21. The real capacitances of all electrodes depict downward trend along with the increase of frequency, indicating the transition from resistive behavior at high frequency to capacitive behavior at low frequency (Figure S5b). Meanwhile, C''(ω) curves with peak shapes were observed, the maximum values of which are about equal to the half of the total capacitances of electrodes (Figure 3a). Clearly, for TMS electrodes, this value is directly proportional to the mass loading of MoS2 (Figure S6a), implying MoS2 occupies the dominant position as active materials for charge storage. However, the maximum C''(ω) apparently shifts toward a lower frequency along with the increasing mass loading of MoS2. The relaxation time τ0 can be determined according to τ0 = 1/f0, which reflects the time required to efficiently deliver or store energy. As observed, TiN electrode displays the smallest τ0 of 0.15 s, while pristine MoS2


10

Page 11 of 29

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

ACS Energy Letters

electrode owns the largest one of more than 25 s. All these facts evidence the significant power contribution of TiN and the main capacitance contribution of MoS2. Furthermore, a series of rate-dependent CV curves of all electrodes were obtained to provide an overall evaluation of the electrochemical performance (Figure S6b-f, S7). Figure 3b depicts the calculated areal capacitances of all the electrodes. As an optimal electrode, TMS6 electrode exhibits an excellent areal capacitance of 719 mF cm-2 at 10 mV s-1, and a remarkable rate capability with capacitance retention of 58.5 % when the scan rate reaches 100 mV s-1. Figure S8 compares the gravimetric capacitances of MoS2 and the capacitance retention in MoS2 electrode and TMS6 electrode. It is worth noting that the loading masses of MoS2 in MoS2 electrode and TMS6 electrode are close, and the capacitive contribution of TiN has been excluded. As revealed, the gravimetric capacitances of MoS2 in TMS6 are about three times higher than those of MoS2 electrode. Moreover, TMS6 exhibits much higher retentions than MoS2 electrode does, implying the better rate capability of TMS6. We next turn to use the galvanostatic charge/discharge (GCD) measurement to examine the capacitive behavior of TMS6 electrode. All GCD curves (Figure 3c) present triangular traces with almost no obvious IR drop, suggesting the purely capacitive and small resistive behavior. The corresponding areal capacitances were calculated as shown in Figure 3d. The maximum areal capacitance of TMS6 electrode reached 662.2 mF cm-2 at 1 mA cm-2. This areal capacitance is 4.5-time that of pristine MoS2 electrode and 4.9-time that of TiN electrode tested at the same current density, giving clear evidence of the synergistic effect of MoS 2-TiN composite. Excluding the capacitive contribution of TiN, the gravimetric capacitance of MoS 2 in TMS6 is estimated to be 278.5 F g-1 at 1 mA cm-2 (Figure S9), which outperforms not only that of pristine MoS2 electrode (67.4 F g-1), but also those of latest reported 1T phase MoS2 (105 F g-1


11

ACS Energy Letters

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

at 5 mV s-1 in 0.5 M Li2SO4)12, MoS2 monolayers (140 F g-1 at 10 mV s-1 in 1 M KCl)

Page 12 of 29

11

,

flowerlike MoS2 nanosheets (200 F g-1 at 2 A g-1 in 6 M KOH)22, 3D tubular MoS2 (235 F g-1 at 0.5 A g-1 in 1 M H2SO4)23, MoS2/graphene (243 F g-1 at 1 A g-1 in 1 M Na2SO4)24, and MoS2/RGO (265 F g-1 at 10 mV s-1 in 1 M HClO4)25. More importantly, the data shown in Figure 3d indicates the areal capacitances of TMS6 electrode are significantly superior to the results from recently reported anodes, such like Ppy@WO326,WO3-x/MoO3-x27, Fe2O328-29, WON30, TiN31, and VN32. Finally, the cycling stability of TMS6 electrodes was investigated by performing 10000 charge/discharge cycles each at 50 mV s-1, 100 mV s-1, 200 mV s-1, 100 mV s1

, and 50 mV s-1(Figure 3e). After 50000 cycles, the electrode showed negligible degradation in

capacitance, implying its ultra-stable electrochemical performance. The TEM image of TMS6 after cycling test has been detected (Figure S10). It can be observed the electrode well remained its original core-shell structure, implying its superior mechanical and structural stability. The interlayer distance increased to 0.69 nm, which can be attributed to the continually insertion/desertion of Li ions.


12

Page 13 of 29

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

ACS Energy Letters

Figure 4. (a) Polarization curves, (b) onset potential and potential at 10 mA cm-2, and (c) Tafel plots of all catalysts in 0.5 M H2SO4. (d) Time dependence of catalytic current density during electrolysis for TMS6 at -180 mV. The inset of (d) compares the polarization curves of TMS6 before and after long-term electrolysis. We further sought to assess the catalytic activities of our TMS for HER in 0.5 M H 2SO4. The polarization curves after iR correction (Figure 4a and S11) show the TiN electrode possessed negligible catalytic activity with sluggish current response to the applied potential. Figure 4b summarizes the onset potentials and the potentials producing a current density of 10 mA cm-2. Remarkably, all TMS electrodes showed better HER catalytic activity with considerably smaller onset potentials (-119 mV ~ -176 mV) and the potentials at 10 mA cm-2 (-146 mV ~ -195 mV) in


13

ACS Energy Letters

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

Page 14 of 29

comparison with pristine MoS2 electrode (-200 mV and -252 mV). This indicates the superior HER catalytic activity originating from the effective integration of TiN and MoS2. The catalytic activity of TMS electrodes follows the order of TMS6 > TMS4 > TMS2 > TMS8 > TMS1. Figure S12 compares the SEM images of TMS1, TMS2, TMS4 and TMS8. Taking the SEM image of TMS6 (Figure 1b) into consideration, it is clearly revealed the coverage of MoS2 nanosheets on TiN nanorods turn to abundant along with the increasing of synthesis time. The increasingly rich edge exposure explains the increasing performance from TMS1 to TMS6. On the other hand, when the synthesis time reached 8 h, the large MoS2 loading led to the formation of bulky aggregates (Figure S12d) and poor intrinsic conductivity, which, in turn, restrict the contact between electrolyte ions and active sites. Furthermore, Tafel plots were obtained to provide more insight into their catalytic behaviors. As shown in Figure 4c, Tafel slopes of all electrodes, ranging from 44.8 to 65.6 mV dec-1, clearly reveals the Volmer-Heyrovsky mechanism of the HER, where electrochemical desorption determines the reaction rate. That means two steps are mainly involved for HER. A proton firstly combines with an electron to form an adsorbed H atom on TMS surface, and then the adsorbed H atom receives an electron from TMS surface and reacts with a hydrated proton from the electrolyte to form the H2 molecule. The lowest Tafel slope of TMS6 electrode (44.8 mV dec-1) further corroborates the optimal HER activity as it suggests the largest increased HER rate with a decrease in potential, whereas the pristine MoS2 depicts the highest slope. Moreover, the HER catalytic activity in acidic electrolyte of our TMS6 electrode is apparently superior to those of recently reported HER catalysts as listed in Table S1. In order to certify the applications of TMS6 electrode over the long term, the practical operation of TMS6 electrode is performed by electrolysis at fixed potential of -180 mV. The exceptional stability is corroborated by the fact that the current density


14

Page 15 of 29

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

ACS Energy Letters

remains at about 65 mA cm-2 with negligible degradation (Figure 4d and the polarization curve after this durability test overlaps almost exactly the initial one (the inset in Figure 4d).

Figure 5. (a) Polarization curves of TMS6 under flat, bent and bent with two divided parts conditions in 0.5 M H2SO4. (b) Polarization curves of TMS6 in 0.5 M H2SO4, 0.5 M phosphate buffer solution, and 1 M KOH. (c) Schematic illustration showing the structure of TMS. The viability of using TMS6 catalytic electrode in complex situations was also evaluated. As shown in the insets of Figure 5a, the HER catalytic activity of TMS6 in 0.5 M H2SO4 was tested under flat, bent and bent with two divided parts conditions. As expected, the high flexibility of TMS6 catalytic electrode was witnessed by the almost unchanged polarization curves (Figure 5a), implying our TMS6 electrode could endure complicated operation environment. In addition, TMS6 also presents decent catalytic performance for HER in neutral and basic electrolyte. As


15

ACS Energy Letters

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

Page 16 of 29

shown in Figure 5b, the polarization curves performed in 1 M KOH achieved an onset potential of -190 mV vs. RHE and a potential of -238 mV vs. RHE at the current density of 10 mA cm-2. This result is much better than recently reported non-noble metal HER catalysts in basic electrolyte, such like MoN33, amorphous MoSx34, Co-S35, Co/N-CNTs36, and Ni37. Meanwhile, the catalytic activity of TMS6 in a benign 0.5 M phosphate buffer (pH 7) depicts slower kinetics and lower electrocatalytic activity than the ones in acidic and basic medias, which is in agreement with already reported result36, 38. However, our TMS6 achieved an onset potential of 200 mV vs. RHE and a potential of -440 mV vs. RHE at 10 mA cm-2, implying much better catalytic performance than recently reported Co/N-CNTs36 and Co@Co-oxo/hydroxo phosphate39. Additionally, the electrocatalytic stability of TMS6 electrode in basic and neutral media was verified by the stable current density during long-term potentiostatic hydrogen evolution reaction (Figure S13). For better understanding the merits of our designed TMS hierarchical hybrid, the structural schematic of TMS (Figure 5c) offers clear insight correlating the favorable hybrid structure and the corresponding superior capacitive and HER performance. First, the MoS2 nanosheets are electronically connected to highly conductive TiN nanorods, ensuring good transport kinetics for electrons. Second, the TiN nanorods as the support for MoS 2 provides a large surface area, allowing more active sites exposed for electrolyte access. Third, the widely open configuration and randomly oriented MoS2 thin nanosheets facilitate the ion diffusion. Fourth, the novel flexible configuration of our samples holds great promise as the flexible electrode for both ECs and fuel cells. In summary, we have successfully engineered vertically aligned TiN nanorods electronically connected with randomly aligned MoS2, which shows outstanding features as both supercapacitor anode and HER catalyst. Originating from the widely open core-shell


16

Page 17 of 29

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

ACS Energy Letters

configuration, the as-obtained TMS electrode provides efficient electron transfer channels, short ion diffusion pathways, and abundant exposed active sites. Significantly, the optimal TMS6 electrode achieved an outstanding areal capacitance of 662.2 mF cm-2 at 1 mA cm-2 with high rate capability and ultra-long electrochemical stability. In addition, our TMS6 also reached an apparent enhancement in electrocatalytic activity for HER, exhibiting a low overpotential of 119 mV producing 10 mA cm-2, favorable Tafel slope of 44.8 mV dec-1 and long electrocatalytic stability. The robust catalytic activity of ourTMS6 electrode was also verified under bending conditions and in all-pH media. All these findings offer a new way to further excavate the potential use of MoS2 electrodes in ECs and HER.


17

ACS Energy Letters

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

Page 18 of 29

EXPERIMENTAL SECTION Details are provided in the Supporting information.

ASSOCIATED CONTENT Supporting Information. Experimental section, SEM images, TEM images, XPS, and extra electrochemical experiment results.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the financial support of this work received by the Guangdong Natural Science Funds for Distinguished Young Scholar (2014A030306048), Pearl River S&T Nova Program of Guangzhou (201610010080), Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (2015TQ01C205), Natural Science Foundation of China (21403306),and

Training

Program

of

Scientific

and

Technological

Innovation

for

Undergraduates (pdjh2016a0006 and pdjh2016a0007).


18

Page 19 of 29

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

ACS Energy Letters

REFERENCES (1) Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X. A Metal–Organic Framework-Derived Bifunctional Oxygen Electrocatalyst. Nat. Energy 2016, 1, 15006. (2) Chen, R.; Qu, W.; Guo, X.; Li, L.; Wu, F. The Pursuit of Solid-State Electrolytes for Lithium Batteries: from Comprehensive Insight to Emerging Horizons. Mater. Horiz. 2016, 3, 487-516. (3) Zhang, C.; Huang, Y.; Tang, S.; Deng, M.; Du, Y. High-Energy All-Solid-State Symmetric Supercapacitor Based on Ni3S2 Mesoporous Nanosheet-Decorated ThreeDimensional Reduced Graphene Oxide. ACS Energy Lett. 2017, 2, 759-768. (4) Yu, M.; Cheng, X.; Zeng, Y.; Wang, Z.; Tong, Y.; Lu, X.; Yang, S. Dual-Doped Molybdenum Trioxide Nanowires: A Bifunctional Anode for Fiber-Shaped Asymmetric Supercapacitors and Microbial Fuel Cells. Angew. Chem. Int. Ed. 2016, 55, 6762-6766. (5) Zhu, C.; Yang, P.; Chao, D.; Wang, X.; Zhang, X.; Chen, S.; Tay, B. K.; Huang, H.; Zhang, H.; Mai, W.; et al. All Metal Nitrides Solid-State Asymmetric Supercapacitors. Adv. Mater. 2015, 27, 4566-4571. (6) Xia, X.; Zhang, Y.; Chao, D.; Xiong, Q.; Fan, Z.; Tong, X.; Tu, J.; Zhang, H.; Fan, H. J. Tubular TiC Fibre Nanostructures as Supercapacitor Electrode Materials with Stable Cycling Life and Wide-Temperature Performance. Energy Environ. Sci. 2015, 8, 15591568. (7) Long, X.; Li, G.; Wang, Z.; Zhu, H.; Zhang, T.; Xiao, S.; Guo, W.; Yang, S. Metallic Iron-Nickel Sulfide Ultrathin Nanosheets As a Highly Active Electrocatalyst for


19

ACS Energy Letters

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

Page 20 of 29

Hydrogen Evolution Reaction in Acidic Media. J. Am. Chem. Soc. 2015, 137, 1190011903. (8) Liang, K.; Yan, Y.; Guo, L.; Marcus, K.; Li, Z.; Zhou, L.; Li, Y.; Ye, R.; Orlovskaya, N.; Sohn, Y. H.; et al. Strained W(SexS1–x)2 Nanoporous Films for Highly Efficient Hydrogen Evolution. ACS Energy Lett. 2017, 1315-1320. (9) Qu, C.; Zhao, B.; Jiao, Y.; Chen, D.; Dai, S.; Deglee, B. M.; Chen, Y.; Walton, K. S.; Zou, R.; Liu, M. Functionalized Bimetallic Hydroxides Derived from Metal–Organic Frameworks for High-Performance Hybrid Supercapacitor with Exceptional Cycling Stability. ACS Energy Lett. 2017, 1263-1269. (10) Yu, X. Y.; Hu, H.; Wang, Y.; Chen, H.; Lou, X. W. Ultrathin MoS 2 Nanosheets Supported on N-doped Carbon Nanoboxes with Enhanced Lithium Storage and Electrocatalytic Properties. Angew. Chem. Int. Ed. 2015, 54, 7395-7398. (11) Tang, H.; Wang, J.; Yin, H.; Zhao, H.; Wang, D.; Tang, Z. Growth of Polypyrrole Ultrathin Films on MoS2 Monolayers as High-Performance Supercapacitor Electrodes. Adv. Mater. 2015, 27, 1117-1123. (12) Acerce, M.; Voiry, D.; Chhowalla, M. Metallic 1T Phase MoS2 Nanosheets as Supercapacitor Electrode Materials. Nat. Nanotechnol. 2015, 10, 313-318. (13) Chen, Y. M.; Yu, X. Y.; Li, Z.; Paik, U.; Lou, X. W. Hierarchical MoS 2 Tubular Structures Internally Wired by Carbon Nanotubes as a Highly Stable Anode Material for Lithium-Ion Batteries. Sci. Adv. 2016, 2, e1600021.


20

Page 21 of 29

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

ACS Energy Letters

(14) Jiang, H.; Ren, D.; Wang, H.; Hu, Y.; Guo, S.; Yuan, H.; Hu, P.; Zhang, L.; Li, C. 2D Monolayer MoS2-Carbon Interoverlapped Superstructure: Engineering Ideal Atomic Interface for Lithium Ion Storage. Adv. Mater. 2015, 27, 3687-3695. (15) Liu, Y.; He, X.; Hanlon, D.; Harvey, A.; Coleman, J. N.; Li, Y. Liquid Phase Exfoliated MoS2 Nanosheets Percolated with Carbon Nanotubes for High Volumetric/Areal Capacity Sodium-Ion Batteries. ACS Nano 2016, 10, 8821-8828. (16) Xie, X.; Makaryan, T.; Zhao, M.; Van Aken, K. L.; Gogotsi, Y.; Wang, G. MoS2 Nanosheets Vertically Aligned on Carbon Paper: A Freestanding Electrode for Highly Reversible Sodium-Ion Batteries. Adv. Energy Mater. 2016, 6, 1502161. (17) Liu, P.; Zhu, J.; Zhang, J.; Xi, P.; Tao, K.; Gao, D.; Xue, D. P Dopants Triggered New Basal Plane Active Sites and Enlarged Interlayer Spacing in MoS2 Nanosheets toward Electrocatalytic Hydrogen Evolution. ACS Energy Lett. 2017, 2, 745-752. (18) Cui, Z.; Zu, C.; Zhou, W.; Manthiram, A.; Goodenough, J. B. Mesoporous Titanium Nitride-Enabled Highly Stable Lithium-Sulfur Batteries. Adv. Mater. 2016, 28, 69266931. (19) Cook, J. B.; Kim, H. S.; Lin, T. C.; Lai, C. H.; Dunn, B.; Tolbert, S. H. Pseudocapacitive Charge Storage in Thick Composite MoS2 Nanocrystal-Based Electrodes. Adv. Energy Mater. 2016, 7, 1601283. (20) Taberna, P. L.; Simon, P.; Fauvarque, J. F. Electrochemical Characteristics and Impedance Spectroscopy Studies of Carbon-Carbon Supercapacitors. J. Electrochem. Soc. 2003, 150, A292.


21

ACS Energy Letters

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

Page 22 of 29

(21) Gao, P. C.; Tsai, W. Y.; Daffos, B.; Taberna, P. L.; Pérez, C. R.; Gogotsi, Y.; Simon, P.; Favier, F. Graphene-Like Carbide Derived Carbon for High-Power Supercapacitors. Nano Energy 2015, 12, 197-206. (22) Hao, C.; Wen, F.; Xiang, J.; Wang, L.; Hou, H.; Su, Z.; Hu, W.; Liu, Z. Controlled Incorporation of Ni(OH)2 Nanoplates Into Flowerlike MoS2 Nanosheets for Flexible AllSolid-State Supercapacitors. Adv. Funct. Mater. 2014, 24, 6700-6707. (23) Ren, L.; Zhang, G.; Yan, Z.; Kang, L.; Xu, H.; Shi, F.; Lei, Z.; Liu, Z. H. ThreeDimensional Tubular MoS2/PANI Hybrid Electrode for High Rate Performance Supercapacitor. ACS Appl. Mater. Interfaces 2015, 7, 28294-28302. (24) Huang, K. J.; Wang, L.; Liu, Y. J.; Liu, Y. M.; Wang, H. B.; Gan, T.; Wang, L. L. Layered MoS2–Graphene Composites for Supercapacitor Applications with Enhanced Capacitive Performance. Int. J. Hydrogen energy 2013, 38, 14027-14034. (25) Da Silveira Firmiano, E. G.; Rabelo, A. C.; Dalmaschio, C. J.; Pinheiro, A. N.; Pereira, E. C.; Schreiner, W. H.; Leite, E. R. Supercapacitor Electrodes Obtained by Directly Bonding 2D MoS2 on Reduced Graphene Oxide. Adv. Energy Mater. 2014, 4, 1301380. (26) Wang, F.; Zhan, X.; Cheng, Z.; Wang, Z.; Wang, Q.; Xu, K.; Safdar, M.; He, J. Tungsten Oxide@Polypyrrole Core-Shell Nanowire Arrays as Novel Negative Electrodes for Asymmetric Supercapacitors. Small 2015, 11, 749-755. (27) Xiao, X.; Ding, T.; Yuan, L.; Shen, Y.; Zhong, Q.; Zhang, X.; Cao, Y.; Hu, B.; Zhai, T.; Gong, L.; et al. WO3−x/MoO3−x Core/Shell Nanowires on Carbon Fabric as an Anode for All-Solid-State Asymmetric Supercapacitors. Adv. Energy Mater. 2012, 2, 1328-1332.


22

Page 23 of 29

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

ACS Energy Letters

(28) Lu, X.; Zeng, Y.; Yu, M.; Zhai, T.; Liang, C.; Xie, S.; Balogun, M. S.; Tong, Y. Oxygen-Deficient Hematite Nanorods as High-Performance and Novel Negative Electrodes for Flexible Asymmetric Supercapacitors. Adv. Mater. 2014, 26, 3148-3155. (29) Yang, P.; Ding, Y.; Lin, Z.; Chen, Z.; Li, Y.; Qiang, P.; Ebrahimi, M.; Mai, W.; Wong, C. P.; Wang, Z. L. Low-Cost High-Performance Solid-State Asymmetric Supercapacitors Based on MnO2 nanowires and Fe2O3 nanotubes. Nano Lett. 2014, 14, 731-736. (30) Yu, M.; Han, Y.; Cheng, X.; Hu, L.; Zeng, Y.; Chen, M.; Cheng, F.; Lu, X.; Tong, Y. Holey Tungsten Oxynitride Nanowires: Novel Anodes Efficiently Integrate Microbial Chemical Energy Conversion and Electrochemical Energy Storage. Adv. Mater. 2015, 27, 3085-3091. (31) Lu, X.; Wang, G.; Zhai, T.; Yu, M.; Xie, S.; Ling, Y.; Liang, C.; Tong, Y.; Li, Y. Stabilized TiN Nanowire Arrays for High-Performance and Flexible Supercapacitors. Nano Lett. 2012, 12, 5376-5381. (32) Lu, X.; Yu, M.; Zhai, T.; Wang, G.; Xie, S.; Liu, T.; Liang, C.; Tong, Y.; Li, Y. High Energy Density Asymmetric Quasi-Solid-State Supercapacitor Based on Porous Vanadium Nitride Nanowire Anode. Nano Lett. 2013, 13, 2628-2633. (33) Ma, L.; Ting, L. R. L.; Molinari, V.; Giordano, C.; Yeo, B. S. Efficient Hydrogen Evolution Reaction Catalyzed by Molybdenum Carbide and Molybdenum Nitride Nanocatalysts Synthesized via the Urea Glass Route. J. Mater. Chem. A 2015, 3, 83618368.


23

ACS Energy Letters

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

Page 24 of 29

(34) Merki, D.; Fierro, S.; Vrubel, H.; Hu, X. Amorphous Molybdenum Sulfide Films as Catalysts for Electrochemical Hydrogen Production in Water. Chem. Sci. 2011, 2, 12621267. (35) Sun, Y.; Liu, C.; Grauer, D. C.; Yano, J.; Long, J. R.; Yang, P.; Chang, C. J. Electrodeposited Cobalt-Sulfide Catalyst for Electrochemical and Photoelectrochemical Hydrogen Generation from Water. J. Am. Chem. Soc. 2013, 135, 17699-17702. (36) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmekova, E.; Asefa, T. Cobalt-Embedded Nitrogen-Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values. Angew. Chem. Int. Ed. 2014, 53, 4372-4376. (37) McKone, J. R.; Sadtler, B. F.; Werlang, C. A.; Lewis, N. S.; Gray, H. B. Ni–Mo Nanopowders for Efficient Electrochemical Hydrogen Evolution. ACS Catalysis 2013, 3, 166-169. (38) Wang, J.; Zhong, H. X.; Wang, Z. L.; Meng, F. L.; Zhang, X. B. Integrated ThreeDimensional Carbon Paper/Carbon Tubes/Cobalt-Sulfide Sheets as an Efficient Electrode for Overall Water Splitting. ACS Nano 2016, 10, 2342-2348. (39) Cobo, S.; Heidkamp, J.; Jacques, P. A.; Fize, J.; Fourmond, V.; Guetaz, L.; Jousselme, B.; Ivanova, V.; Dau, H.; Palacin, S.; et al. A Janus Cobalt-Based Catalytic Material for Electro-Splitting of Water. Nat. Mater. 2012, 11, 802-807.


24

Page 25 of 29

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

ACS Energy Letters

Figure 1. (a, b) SEM, (c-e) TEM, (f) high angle annular dark-field STEM images and corresponding elemental mapping of TMS sample. 201x141mm (300 x 300 DPI)


ACS Energy Letters

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

Figure 2. (a) XRD patterns of TiN, MoS2 and TMS samples. Core-level (b) Mo 3d – S 2s and (c) S2p XPS spectra of TMS sample. (d) Core-level Ti 2p spectra of TiN and TMS samples. 189x153mm (300 x 300 DPI)


Page 26 of 29

Page 27 of 29

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

ACS Energy Letters

Figure 3. (a) Imaginary capacitance vs. the logarithm of frequency and (b) areal capacitances of all electrodes at various scan rates. (c) GCD curves with different current densities of TMS6 electrode. (d) Areal capacitance as a function of the current density for TiN, MoS2, TMS6 and recently reported ECs anodes. (e) Cycling performance obtained for TMS6 electrode. 198x199mm (300 x 300 DPI)


ACS Energy Letters

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

Figure 4. (a) Polarization curves, (b) onset potential and potential at 10 mA cm-2, and (c) Tafel plots of all catalysts in 0.5 M H2SO4. (d) Time dependence of catalytic current density during electrolysis for TMS6 at 180 mV. The inset of (d) compares the polarization curves of TMS6 before and after long-term electrolysis. 206x154mm (300 x 300 DPI)


Page 28 of 29

Page 29 of 29

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

ACS Energy Letters

Figure 5. (a) Polarization curves of TMS6 under flat, bent and bent with two divided parts conditions in 0.5 M H2SO4. (b) Polarization curves of TMS6 in 0.5 M H2SO4, 0.5 M phosphate buffer solution, and 1 M KOH. (c) Schematic illustration showing the structure of TMS. 204x132mm (300 x 300 DPI)


Engineering Thin MoS2 Nanosheets on TiN Nanorods: Advanced

Recommend Documents