Bifunctional Oxygen Electrocatalysis through Chemical Bonding … · 2019. 5. 16. · Bifunctional...

Full p

aper

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 9) 1602217wileyonlinelibrary.com

Bifunctional Oxygen Electrocatalysis through Chemical Bonding of Transition Metal Chalcogenides on Conductive Carbons

Anand P. Tiwari, Doyoung Kim, Yongshin Kim, and Hyoyoung Lee*

A. P. Tiwari, D. Kim, Y. Kim, Prof. H. LeeCentre for Integrated Nanostructure Physics (CINAP)Institute for Basic Science (IBS)Suwon 16419, Republic of KoreaE-mail: [email protected]. P. Tiwari, Y. Kim, Prof. H. LeeDepartment of ChemistrySungkyunkwan University (SKKU)Suwon 16419, Republic of KoreaD. Kim, Prof. H. LeeDepartment of Energy ScienceSungkyunkwan University (SKKU)Suwon 16419, Republic of Korea

DOI: 10.1002/aenm.201602217

performance and commercialization of fuel cells and metal–air batteries.[9,10] Furthermore, searching for bifunctional electrocatalysts for both ORR and OER is critically important for the development of electrochemical devices, especially for rechargeable metal–air batteries. Platinum (Pt) and Pt-based materials are the most efficient ORR catalysts, while ruthenium oxide (RuO2) and iridium oxide (IrO2) are considered as the best electrocatalysts for OER.[11,12] However, the use of these noble metal materials is limited because of their high cost and scarcity. To make mat-ters worse, the efficiency of Pt or RuO2/IrO2 for ORR and OER simultaneously is very poor; this is the case because Pt is only active for ORR (but inactive for OER) while RuO2/IrO2 is only active for OER (but inactive for ORR).[13] Therefore, exten-sive research efforts have been taken to

develop non-noble metal and efficient bifunctional electrocata-lysts toward both ORR and OER.

In this respect, many kinds of bifunctional electrocatalysts have been investigated, such as alloys of Pt and Ir/Ru catalysts, toward ORR and OER.[14–16] However, the high cost of noble metals limits their large-scale applications in electrochemical energy devices. However, earth-abundant conductive carbon materials and their derivatives, such as graphene and carbon nanotubes (CNTs), can replace noble metal alloys for practical applications in electrochemical energy devices. Recently, het-eroatom B- and N-doped carbon materials showed the pos-sibility of simultaneous OER and ORR activity.[17–27] However, obtaining OER activity on heteroatom-doped carbon materials is still challenging because of the low catalytic activity of water splitting; additionally, although the ORR activity can match commercial Pt/C catalysts, it suffers from poor stability.[28,29] In order to address this challenge, composites of transition metals, including cobalt oxide/cobalt sulphide and carbon materials, have been proposed to actively catalyze OER with low overpo-tentials.[30–32] Moreover, a metal-doped transition metal oxide on graphene has also been shown to be a good electrocatalyst toward OER.[33–36] However, developing carbon-based electrocat-alysts toward ORR and OER is still challenging due to the insta-bility of transition metals and the nonintimate contact between the conductive carbonaceous materials and the doping/deco-rated active material.[37]

Improving the electrochemical performance of both the oxygen reduc-tion reaction (ORR) and oxygen evolution reaction (OER) has been of great interest in emerging renewable energy technologies. This study reports an advanced bifunctional hybrid electrocatalyst for both ORR and OER, which is composed of tungsten disulphide (WS2) and carbon nanotube (CNT) con-nected via tungsten carbide (WC) bonding. WS2 sheets on the surface of CNTs provide catalytic active sites for electrocatalytic activity while the CNTs act as conduction channels and provide a large surface area. Moreover, the newly formed WC crystalline structure provides an easy path for electron transfer by spin coupling and helps to solve stability issues to enable excel-lent electrocatalytic activity. In addition, it is found that four to five layers of WS2 sheets on the surface of CNTs produce excellent catalytic activity toward both ORR and OER, which is comparable to noble metals (Pt, RuO2, etc.). These findings show the many advantages enabled by designing highly active, durable, and cost-effective ORR and OER electrocatalysts.

1. Introduction

Highly efficient renewable energy systems (e.g., fuel cells, metal–air batteries, and water splitting devices) are extremely desirable to fulfill the needs of and further the utilization of sustainable energies.[1–4] The electrochemical oxygen reduc-tion reaction (ORR) and oxygen evolution reaction (OER) have been considered as two of the most important processes in a wide range of renewable energy technologies.[5–8] The develop-ment of highly active electrocatalysts for both ORR and OER has received a great amount of attention from researchers because the slow kinetics of these two reactions restricts the

Adv. Energy Mater. 2017, 1602217

www.advenergymat.dewww.advancedsciencenews.com

http://doi.wiley.com/10.1002/aenm.201602217

Full

paper

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1602217 (2 of 9) wileyonlinelibrary.com

In order to obtain nonmetal-doped compounds, layered tran-sition metal dichalcogenides (TMDs), such as MoS2, have been extensively studied in attempts to obtain efficient electrochem-ical hydrogen and oxygen evolutions.[38–40] However, van der Waals interactions between nanosheets of TMD materials block active edge sites for efficient catalytic activity.[38] In addition, the conductivity of TMD materials as an electrocatalyst is also very limited.[41] In this regard, hybrid structures of TMD sheets (e.g., MoS2 sheets) and conductive carbon materials (e.g., modified graphene heterostructures) have been investigated as advanced electrocatalysts for ORR.[42–45] However, the nonintimate inter-actions between TMD and the carbon materials restrict the stability and cause homogeneity issues for electrochemical ORR.[46–48] In addition, the hybrid structure of TMD materials and carbon materials for bifunctional electrocatalysts toward both ORR and OER has yet to be investigated.

Herein, we report a new concept for the development of a hybrid structure of tungsten disulphide (WS2) and CNTs, which are interconnected by the formation of tungsten carbide (WC) bonding, for bifunctional catalytic activity toward both ORR and OER. We propose that the stacking of WS2 nanosheets on con-ductive CNT surfaces may provide a low-resistance connection for electron transfer from the substrate to active sites, forming an advanced catalyst for electrochemically bifunctional ORR and OER activity. In addition, the formation of the WC crystal-line structure is an effective way to change the charge density and spin density of carbon atoms in the CNTs to enhance the electrocatalytic activity as well as to solve the nonintimate inter-action issues between CNTs and WS2 to obtain a stable hybrid structure. The synergistic effect of the WS2 sheets and the sur-face engineering of CNTs (by the formation of new WC bonds) provide a highly efficient bifunctional electrocatalyst for both ORR and OER. For the synthesis of a hybrid hierarchical struc-ture, we carefully design a simple step-by-step method to grow WS2 sheets on CNTs by taking advantage of the WC crystalline structure.

2. Results and Discussions

2.1. Characterization of the Morphology and Structure

As designed, sulphur is coated on the CNTs, and the resulting sulphur-coated CNTs are reacted with the W source (WCl4) (Scheme 1). In addition, we also performed a control experi-ment with different amounts of WS2 sheets on the sur-face of the CNTs to determine the most suitable material for

bifunctional ORR and OER activity. By changing the amount of the W source, we synthesized three different composites with 0.06, 0.125, and 0.25 mmol of WS2; these are referred to as CSW1, CSW2, and CSW3, respectively, and contain different numbers of sheets on the surface of the CNTs. The hybrid-structured WS2–CNTs composites, which are interconnected via WC bonding, are first confirmed by X-ray diffraction (XRD). The XRD patterns of sulphur- and WS2-coated CNTs samples, including S-CNTs, CSW1, CSW2, and, CSW3, are shown in Figure 1a. First, the XRD patterns in Figure 1a are indexed as hexagonal 2H-WS2 (JCPDS 84-1398). The single strong (002) reflection at 2θ = 14.2° corresponds to a d-spacing of 0.615 nm, which indicates the well-stacked layered structure of WS2 for all three compositions (CSW1, CSW2, and CSW3). Moreover, the diffraction peaks of the CSW1, CSW2, and CSW3 samples at 2θ = 33.5°, 39.5°, and 59.1° correspond to the (101), (103), and (110) planes of WS2, respectively. Furthermore, the WC crystal-line structure for samples CSW1, CSW2, and CSW3 is indexed as a hexagonal structure with space group P-6m2 (JCPDS 84-1398). The single strong (100) reflection at 2θ = 35.6° reveals the well-stacked structure of WC with a d-spacing of 0.25 nm. Moreover, the diffraction peaks at 31.4° and 48.3° correspond to the (101) and (001) planes of WC, respectively. In addition, the absence of sulphur peaks in the CSW1, CSW2, and CSW3 com-posites indicates that no sulphur impurities are present in the composites. Therefore, we can conclude from the XRD patterns of the three composites that the WS2 sheets are grown on the CNTs and connected via WC bonding.

To investigate the microstructure of the WS2–CNT hybrid structure (interconnected by WC bonds), transmission elec-tron microscopy (TEM) experiments were performed for the different samples, as shown in Figure 1b–f. It can be clearly seen that all of the as-synthesized samples contain well-stacked WS2 sheets with a 0.61 nm interlayer distance on the surface of the CNTs. It was also found that increasing the amount of the W source increases the thickness and number of WS2 sheets. Figure 1d shows that the CSW1 sample has two to three layers of WS2 sheets on the CNT surfaces. Moreover, with increasing the W source in CSW2 and CSW3 leads to the formation of four to five layers and eight to ten layers of WS2 on the surface of CNTs, respectively (shown in Figure 1b,e). In addition, elec-tron energy-dispersive X-ray spectroscopy maps were obtained for the CSW2 sample, as shown in Figure 1c (corresponding dark-field image shown in Figure S4a, Supporting Informa-tion). The atomic percentage of C, W, and S are 90.33%, 3.24%, and 6.43%, respectively, which are well matched with the stoichiometric ratio for WS2 and agrees with our X-ray


www.advenergymat.de www.advancedsciencenews.com

Scheme 1. Schematic illustration of the synthesis of WS2 sheets on the surface of CNTs with interconnection of WC bonding.

Full p

aper


photoemission spectroscopy (XPS) results in Figure 2a–d. In addition, to confirm the formation of WC, we have performed high-resolution TEM for CSW2 sample and result is shown in Figure 1f. It is revealed that the after WS2 sheets the WC layers are formed with the interlayer spacing 0.30 nm on the CNT surfaces, which is well agree with XRD and XPS results. However, it is confirmed from high resolution transmission electron microscope (HRTEM) of CSW2 that the thickness of WC is 3–4 nm. Moreover, the scanning electron microscopy (SEM) image of the CSW2 sample reveals that WS2 sheets are

homogeneously attached to the CNT surfaces (Figure S4b, Sup-porting Information).

To determine the chemical compositions of the hybrid-structured WS2–CNTs (interconnected via WC bonding), XPS was performed. The XPS spectra of the as-synthesized CSW2 composite are shown in Figure 2a–d. First, a peak at 243.9 eV is observed; this indicates the W 4d5/2 state of tungsten and confirms the formation of bonds between tungsten and carbon (WC bond) on the surface of the CNTs (Figure 2a). Further-more, the peaks at 32.8 and 35.2 eV indicate the W 4f7/2 and



Figure 1. a) XRD patterns of as synthesized samples. b) HRTEM images of the CSW2. c) EDS mapping of the CSW2 sample. d) HRTEM images of the CSW1 sample. e) HRTEM images of the CSW3 sample. f) HRTEM image of CSW2 sample for showing the formation of WC bond.

Figure 2. a) Full scan XPS spectrum of CSW2 sample. b–d) Individual XPS spectrums for different elements of CSW2 sample. e) Raman spectra of as synthesized samples.

Full

paper


W 4f5/2 states of W, respectively, and the peaks at 161.8 and 162.9 eV correspond to the S 2p3/2 and S 2p1/2 states of S, respectively, confirming the formation of WS2 on the surface of CNTs (formed via WC bonds; Figure 2b,c). Moreover, the peaks at 284.1 and 285.7 eV confirm the C1s state of carbon, providing information about the CNT structure (Figure 2d). In addition, there are no additional peaks present in the XPS spectrum, confirming that the as-synthesized samples do not have any impurities or additional phases. All three composi-tions (CSW1, CSW2, and CSW3) show similar chemical com-positions, indicating that WS2 is formed on the surface of CNTs (via WC bonding) to successfully synthesize stoichiometric compounds (Figure S1 and Table S1, Supporting Information).

To confirm the formation of WS2 sheets on the CNT sur-faces, the possible vibration modes of the as-synthesized com-posites have been confirmed by Raman spectroscopy (using an excitation wavelength of 514 nm). The Raman spectra of the different composites are shown in Figure 2e. It can be seen that the Raman spectra of the CSW1, CSW2, and CSW3 samples have the E2g and A1g peaks of WS2 in-plane and out-of-plane vibrations, confirming that the WS2 sheets are well stacked on the surface of the CNTs. Moreover, it can be observed that the intensity ratio of the E2g and A1g peaks gradually increases as the amount of WS2 sheets on the surface of the CNTs increases. This may be due to the increased number of WS2 layers (sum-marized in Table S2, Supporting Information). However, the D and G bands of CNTs can be seen in all of the as-synthesized composites. It was also found that the intensity ratio between the D and G peaks increases after the formation of WS2 sheets on the CNT surfaces. Enhancement in the intensity ratio of D and G band comes from defects on the CNT surface caused

by the formation of WC bonds on the CNT surface. An increase in the intensity ratio between the D and G peaks is also observed when comparing the bare CNTs with the S-coated CNTs (shown in Figure S2, Supporting Information). It has been observed that all three composites have in-plane and out-of-plane vibrational modes that are well matched to those of WS2 and CNT vibration modes, which reveals that we success-fully obtained WS2 with a layered structure on the surface of CNTs (interconnected via WC bonding).

2.2. Electrocatalytic Performance for ORR

To evaluate the catalytic activity of different samples toward ORR, linear sweep voltammetry (LSV) is carried out in an O2-sat-urated 0.1 m KOH electrolyte on a rotating disk electrode (RDE) at a scan rate of 10 mV s−1 with a rotation rate of 1600 rpm. LSV data for the different sample electrodes are shown in Figure 3a. It can be seen that by changing the number of WS2 sheets, the CSW2 sample shows the best catalytic activity toward ORR with an onset potential −11 mV; this value is very close to that of a Pt/C electrode (−1 mV). Alternatively, the CSW1 and CSW3 samples have onset potentials of −88 and −240 mV, respectively. It is worth noting that, as the voltage is increased, the CSW1 sample carries a higher current density than the CSW2 and CSW3 samples due to the increased amount of CNTs in com-parison to WS2. The decrease in the onset potential that occurs when the amount of WS2 sheets is increased from CSW1 (two to three layers of WS2) to CSW2 (four to five layers of WS2) can be explained by the fact that the smaller amount of WS2 is not as effective at transferring electrons from the CNT surface



Figure 3. Electrocatalytic performance for ORR. a) Linear sweep voltammetry of the different as synthesized samples. b) RDE experiment of the CSW2 sample. c) K–L plot for the CSW2 sample. d) Comparison of n values and diffusion current density for the different as synthesized samples. e) Cyclic voltammetry in O2 and N2 saturated electrolyte for CSW2 sample. f) Long-term stability test (chronoamperometry at −0.60 V) for the CSW2 sample.

Full p

aper


to WS2, which impacts the catalytic activity toward ORR. How-ever, by increasing the amount of WS2 sheets from four to five layers (CSW2) to eight to 10 layers (CSW3), the onset poten-tial is increased and the current density begins to decrease because the large number of WS2 layers on the CNT surfaces hinders the path electrons and decreases the catalytic activity toward ORR. The improvement in the ORR activity can also be confirmed by making comparisons with bare CNTs (onset potential: −261 mV) and WS2 (onset potential: −283 mV). These results further indicate the important role of both CNTs (as the conducting agent) and WS2 sheets (as the electron-transferring agent), which both contribute to the synergetic effect. Moreover, half wave potentials E1/2 of the as synthesized CSW1, CSW2, and CSW3 are measured and summarized in Table 1. In addi-tion, we have performed the controlled electrochemical cata-lytic activity of WC on CNT, S on CNT (CNT-S), and physically mixed WS2–CNT samples toward ORR and compare the results with CSW2 sample and WS2 (shown in Figure S5a, Supporting Information). It is revealed that the hybrid structure of only WC on CNT, CNT-S, and physically attached WS2–CNT samples are not effective electrocatalyst as WS2 sheets on the surface of CNT with interconnection of WC toward ORR.

To estimate the catalytic activity of an electrocatalyst toward ORR performance, the electron transfer number (n) and H2O2 production yield are very important indicators. Thus, LSV experiments are conducted at different rotation speeds with an RDE system for all of the as-synthesized materials. The Koutecky–Levich (K–L) plot of j−1 versus ω−1/2, at a potential of −0.40 V, is also plotted. LSV data at different rotation speeds for CSW2 are shown in Figure 3b, and the corresponding K–L plot is plotted and shown in Figure 3c. To calculate the electron-transfer number (n), the Koutecky–Levich equation is used

1 1 1 1 1

L K12 Kj j j

Bjω

= + = +

(1)

η=

−0.62 0 0

23 1/6B nFC D

(2)

K 0j nF kC= (3)

Details of these calculations and the interpretation of these equations are described in the Supporting Information. Briefly, j is the measured current density, jK and jL are the kinetic- and diffusion-limiting current densities, respectively, ω is the angular velocity of the disk, and n is the overall number of elec-trons transferred in the oxygen reduction. By using the K–L plot, we calculated the electron- transfer number for the dif-ferent materials. The calculated n value of the CSW2 electrode,

as shown in Figure 3d, is 3.86; this value is very close to a com-mercially available Pt/C electrode material (3.99). Moreover, the calculated n values of the CSW1 and CSW3 electrodes are 3.71 and 3.45, respectively. Furthermore, the calculated jK value for the CSW2 electrode is 6.92 mA cm−2 at −0.40 V, which is much higher than that of Pt/C (5.87 mA cm−2 at −0.40 V). In fact, it is higher than any of the previously reported carbon and hybrid-structured materials.[5–15] The jK and corresponding n values for the different electrodes are summarized in Figure 3d. It is clear that the ORR data of the WS2–CNTs (interconnected via WC bonding) follows an efficient one-step, four-electron pathway over the entire potential range with a low H2O2 pro-duction yield (H2O2 production yield of different samples are summarized in Table 1).

To assess the ORR activity mechanism, cyclic voltammo-grams (CVs) are measured in N2- and O2-saturated 0.1 m KOH solutions for the CSW2 electrode at a scan rate of 10 mV s−1, as shown in Figure 3e. It can be clearly seen that the CSW2 electrocatalyst shows a substantial oxygen reduction process in the O2-saturated electrolyte, but not in the N2-purged solution. It has been proposed that O2 reduction in an alkaline solution follows the associative rather than the dissociative mecha-nism.[24] It is assumed that, upon the introduction of WC bonds at the WS2–CNT interface, O2 adsorption becomes energeti-cally favorable. However, the spin polarization of W-bonded C atoms near the active WS2 sheets plays a key role in enhancing the binding of O2.[49,50] Furthermore, the weak OH bonding (relative to strong O bonding) leads to low protonation, which can increase barriers for the two rate-determining steps during the formation of H2O2. Therefore, the presence of WS2 sheets on the CNT surfaces further suggests that WC bonding plays a key role in enhancing the ORR performance of WS2–CNT composites.

The long-term stability of the CSW2 is analyzed by chrono-amperometry measurement (j ∼ t) with a high catalyst loading of 2 mg cm−2 on a rotating glassy carbon disk electrode at a con-stant potential of −0.60 V versus Ag/AgCl for 15 h. As shown in Figure 3f, the current densities CSW2 sample constant over the initial 1 h, and then increased over 15 h of continuous opera-tion. Notably, after 15 h of catalytic activity, structural charac-terizations such as XRD, TEM, and SEM of CSW2 indicate no change in the structure of WS2–CNT hybrid structure intercon-nected with WC bond (shown in Figure S6, Supporting Infor-mation). Moreover, chemical characterization (XPS) reveals that no change in chemical states of the CSW2 sample and the homogeneous elemental distribution are maintained (Figure S7, Supporting Information), indicating the robustness of the materials. In addition, LSV has been measured after stability test and illustrating that there is little shift in onset potential for the CSW2 sample shown in inset of Figure 3f.

2.3. Electrocatalytic Performance for OER

Similar to the ORR measurements, our investigation into the OER performance of the WS2–CNTs electrocatalysts (intercon-nected via WC bonding) is also carried out by dropping the catalyst slurry onto a RDE with a mass loading of 0.2 mg cm−2 in the O2-saturated 0.1 m KOH with a rotation rate of 1600 rpm.



Table 1. Electrocatalytic behavior of different samples toward ORR.

Sample Half-wave potentials (E1/2) Electron transfer number (ne) H2O2 yield

CSW1 −0.44 V 3.71 14.5%

CSW2 −0.17 V 3.86 7%

CSW3 −0.44 V 3.45 27.5%

Full

paper


LSV curves for the different electrodes are shown in Figure 4a. It can be seen that the CSW2 electrode shows the lowest onset potential (≈0.70 V) with the highest catalytic current among all of the as-synthesized materials. Moreover, the onset poten-tials for the CSW1 and CSW3 electrodes are ≈0.73 and ≈0.76 V, respectively. In addition, the onset potentials required at a cur-rent density 10 mA cm−2 is always regarded as the benchmark for estimating the activity of OER catalysts. The onset potentials of all of the electrocatalysts at 11 mA cm−2 were measured and their results are summarized in Table 2. The onset potential of the CSW2 electrode at 10 mA cm−2 is 0.77 V, which is the best value among the as-synthesized materials. This is very close to value of conventional RuO2 electrodes (0.61 V). This indicates that the WS2 sheets on the CNT surfaces (interconnected via WC bonds) are also effective toward OER because the WS2 sheets provide catalytic-active sites and the CNTs improve the electrical conductivity. Moreover, it is clear that a similar trend

also occurs for the OER activity in the terms of the amount of WS2 on the CNT surfaces. Furthermore, to investigate the OER kinetics of the as-synthesized WS2–CNT composites, Tafel plots (Figure 4b) are fitted according to the polarization curves. The CSW2 electrode shows the smallest Tafel slope (62 mV per dec) among the as-synthesized and reported carbon hybrid mate-rials. This explains the intrinsic reason why the CSW2 electrode exhibits excellent OER activity. The Tafel slopes of the as-syn-thesized electrocatalysts are summarized in Table 2. Based on these results, the WS2 sheets on the CNT surfaces intercon-nected with WC exert high OER activity, which is better than nonmetallic 2D carbon materials and heterostructure of TMD and carbon materials (Table S3, Supporting Information).[43] In addition, we have performed the controlled experiment with WC on CNT, S on CNT (CNT-S), and physically mixed WS2–CNT samples and found that the WS2 sheets on the surface of CNT with the interconnection of WC showed much better performance than only WC–CNT, CNT-S, and physically mixed WS2–CNT samples hybrid structure (Figure S5b, Supporting Information).

To evaluate the long-term catalytic stability for OER, chrono-amperometry measurements (j ∼ t) are conducted at a constant potential of 1.2 V for the CSW2 electrode, as shown in Figure 4c. It can be seen that after 15 h of continuous electrolysis, the cur-rent density remains constant at the CSW2 electrode, thereby confirming that the material possesses good stability in alka-line conditions. However, severe reduction in current density over the initial hour might be caused by access of electrolyte to surface of the CSW2 sample.[51] In addition, the CSW2 electro-catalyst shows a similar onset potential after 15 h of continuous



Figure 4. Electrocatalytic performance for OER. a) Linear sweep voltammetry of the different as synthesized samples. b) Tafel plots of the different as synthesized samples. c) Chronoamperometry for the CSW2 sample. d) Electrochemical impedance spectroscopy (EIS) of different samples.

Table 2. Electrocatalytic behavior of different samples towards OER.

Samples Onset potentials at 11 [mA cm−2]

Tafel slopes

RuO2 0.62 V 47 m V per dec

CNT 1.08 V 162 m V per dec

WS2 1.20 V 305 m V per dec

CSW1 0.82 V 74 m V per dec



Full p

aper


electrolysis; the initial onset potential is only slightly shifted to a higher potential, which confirms the robustness of the cata-lytic behavior of this material toward OER activity (the inset of Figure 4c). In addition, the chemical and structural characteri-zations after stability test show that there is no change in struc-ture and chemical state of the material (shown in Figures S6 and S7, Supporting Information). As a result, the WS2–CNT hybrid structure (interconnected with WC bonding) shows high stability and excellent activity towards OER, even in an alkaline medium.

Electrochemical impedance spectroscopy (EIS) is performed to confirm changes of the surface property of the WS2–CNT hybrid structure interconnected with WC. The Nyquist plot of the impedance spectrum of different samples is shown in Figure 4d. The plot for the samples has a semicircular domain which diameter corresponds to electron transfer resistance (Rct). The CSW2 sample has Rct of 40.34 Ω, which is lowest among reported WS2 hybrid structure.[52] Moreover, the Rct values for CSW1 and CSW3 samples are 62 and 77.85 Ω, respectively. The results indicate that WS2 on the CNT surface interconnected with WC provides lower resistance connection for electrons transfer for electrocatlytic activity towards ORR and OER.

2.4. Interpretation of the Active Sites for Oxygen Electrolysis

The active sites for ORR and OER of as synthesized WS2–CNT hybrid structure interconnected with WC are explained in Figure 5. First, for the ORR, oxygen molecules are partly bonded with tungsten to generate radical O* and H+ ions are connected with sulfur atoms via hydrodesulfurization. More-over, the metallic behavior of WC helps the formation of radical oxygen via bonding with W and hydrodesulfurization by fast transferring the electrons from WS2 to CNT and enhances the electroactivity toward ORR.[53] For the OER, OH− radicals are interconnected with sulfur atoms via hydrodesulfurization and enhanced by the metallic WC via fast charge transfer. So we can conclude that the S atoms of the WS2 are acting as the active

sites for electrocatlysis for both ORR and OER via hydrodesul-furization and the interconnected metallic WC between W and C is enhancing the hydrodesulfurization by fast charge transfer. Moreover, the tungsten atoms are effective for oxygen reduction for ORR and enhanced by the charge transfer via WC bonding with CNT.

3. Conclusions

In summary, we have synthesized a new hybrid structure con-taining WS2 and CNTs that is interconnected with WC bonding. The new hybrid structure is an efficient bifunctional electrocat-alyst that shows excellent electrocatalytic performance for both ORR and OER. WS2 sheets on CNT surfaces provide active sites for electrocatalytic activity while the CNTs provide conducting channels and a large surface area. In addition, the chemical bonds between W and C help to increase the stability of the hybrid structure and provide efficient pathways for transferring electrons by spin polarization, which leads to high activities for both ORR and OER. Furthermore, by controlling the number of WS2 sheets on the CNT surfaces, we determined that the sample with four to five layers of WS2 (CSW2) shows excellent ORR activity, close to commercial Pt/C catalyst; this sample also showed simultaneously great OER activity close to the best-known OER catalyst (RuO2). Our new concept, using the dual-functioned heterostructure of a transition metal chalcogenide and conductive carbon material interconnected via intimate chemical bonding, provides a novel and efficient approach to prepare hybrid, bifunctional, metal-free electrocatalysts for both ORR and OER.

4. Experimental SectionMaterial Synthesis: First, 200 mg of sulphur was dissolved in

50 mL of a CS2 solvent. Then, 200 mg of multiwalled carbon nano tubes (MWCNTs) was mixed using a mortar to produce a sulphur-coated



Figure 5. Interpretation of the active sites for oxygen electrolysis.

Full

paper

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1602217 (8 of 9) wileyonlinelibrary.com Adv. Energy Mater. 2017, 1602217


CNT composite (S-CNT). The resulting mixture was dried at 40 °C to evaporate the CS2 solvent. For preparation of the WS2 sheets on CNT surfaces, 0.125 mmol of tungsten chloride (WCl4) was dissolved in ethanol and the resulting solution was grinded with the S-CNT mixture with a mortar. The WCl4-S–CNT mixture was dried under vacuum at 60 °C. To form the WS2 sheets and WC on the surface of CNTs, the resulting mixture was treated with microwaves in an inert atmosphere at 700 W for 30 min to allow reaction with the incorporated sulphur and WCl4. Here, three different composites were synthesized with different amounts of WCl4: 0.06 mmol WCl4 (CSW1), 0.125 mmol WCl4 (CSW2), and 0.25 mmol WCl4 (CSW3).

Chemical and Microstructural Characterizations: The morphology, microstructure, and chemical composition of the as-synthesized samples were investigated using a field-emission scanning electron microscope (JEOL JMM-740F), a transmission electron microscope (TEM) (JEOL JEM-2100F), a powder X-ray diffractometer (XRD) (Rigaku Ultima IV), Raman spectroscopy, and X-ray photospectroscopy (ESCA 2000, VG Microtech).

Electrochemical Characterizations: All electrode fabrication was carried out using the same method with a consistent cell configuration. First, 4 mg of the active material was suspended in a solution containing 660 µL of DI H2O, 220 µL of ethanol, and 80 µL of a Nafion 117 solution (5%). This solution was then ultrasonicated for 2 h. 3 µL of the prepared solution was then dropped on a rotating disk electrode (d = 3 mm). After air drying overnight, the electrodes were further dried in a vacuum for 1 h. Electrochemical analyses were performed with a CHI660c electrochemical workstation with a rotating disk electrode system (RRDE-A3, CHI). The electrochemical performance was determined with a three-electrode system. The working electrode was the active material/glassy carbon, the reference electrode was Ag/AgCl in 3 m KCl, and the counter electrode was a Pt mesh. A 0.1 m KOH solution was used as an electrolyte. All of the measurements were carried out after activating the electrode by 20 cycles of CV with a scan rate of 50 mV s−1 in an N2-saturated electrolyte. For the RDE measurements, the working electrode was rotated between 400 and 2025 rpm under a continuous flow of O2.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was supported by the Institute for Basic Science (IBS-R011-D1).

Received: October 6, 2016Revised: January 17, 2017

Published online:

[1] A. S. Arico, P. Bruce, B. Scrosati, J. M. Tarascon, W. Schalkwijk, Nat. Mater. 2005, 4, 366.

[2] Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, Chem. Soc. Rev. 2015, 44, 2060.

[3] Y. Hou, Z. Wen, S. Cui, S. Ci, S. Mao, J. Chen, Adv. Funct. Mater. 2015, 25, 872.

[4] Z. L. Wang, D. Xu, H. X. Zhong, J. Wang, F. L. Meng, X. B. Zhang, Sci. Adv. 2015, 1, e1400035.

[5] W. T. Hong, M. Risch, K. A. Stoerzinger, A. Grimaud, J. Suntivich, Y. Shao-Horn, Energy Environ. Sci. 2015, 8, 1404.

[6] H. X. Zhong, J. Wang, Y. W. Zhang, W. Xu, W. Xing, D. Xu, Y. F. Zhang, X. B. Zhang, Angew. Chem., Int. Ed. 2014, 53, 14235.

[7] G. L. Tian, M. Q. Zhao, D. Yu, X. Y. Kong, J. Q. Huang, Q. Zhang, F. Wei, Small 2014, 10, 2251.

[8] F. Y. Song, Y. Ding, B. C. Ma, C. M. Wang, Q. Wang, X. Q. Du, S. Fu, J. Song, Energy Environ. Sci. 2013, 6, 1170.

[9] C. Jin, F. Lu, X. Cao, Z. Yang, R. Yang, J. Mater. Chem. A 2013, 1, 12170.

[10] C. Xu, M. Lu, Y. Zhan, J. Y. Lee, RSC Adv. 20144, 25089.[11] E. Antolini, ACS Catal. 2014, 4, 1426.[12] J. Wang, K. Li, H. X. Zhong, D. Xu, Z. L. Wang, Z. Jiang, Z. J. Wu,

X. B. Zhang, Angew. Chem., Int. Ed. 2015, 54, 10530.[13] J. Liang, Y. Jiao, M. Jaroniec, S. Z. Qiao, Angew. Chem., Int. Ed.

2012, 51, 11496.[14] B. C. H. Steele, A. Heinzel, Nature 2001, 414, 345.[15] C. C. L. McCrory, S. Jung, J. C. Peters, T. F. Jaramillo, J. Am. Chem.

Soc. 2013, 135, 16977.[16] F. D. Kong, S. Zhang, G. P. Yin, N. Zhang, Z. B. Wang, C. Y. Du,

Electrochem. Commun. 2012, 14, 63.[17] K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Science 2009, 323, 760.[18] D. S. Yang, D. Bhattacharjya, S. Inamdar, J. Park, J. S. Yu, J. Am.

Chem. Soc. 2012, 134, 16127.[19] S. A. Wohlgemuth, R. J. White, M. G. Willinger, M. M. Titirici,

M. Antonietti, Green Chem. 2012, 14, 1515.[20] Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, Energy Environ. Sci.

2011, 4, 3167.[21] Y. Zhao, L. Yang, S. Chen, X. Wang, Y. Ma, Q. Wu, Y. Jiang, W. Qian,

Z. Hu, J. Am. Chem. Soc. 2013, 135, 1201.[22] Z. Lin, G. Waller, Y. Liu, M. Liu, C. P. Wong, Adv. Energy Mater. 2012,

2, 884.[23] Y. Zhao, R. Nakamura, K. Kamiya, S. Nakanishi, K. Hashimoto,

Nat. Commun. 2013, 4, 2390.[24] K. Parvez, S. Yang, Y. Hernandez, A. Winter, A. Turchanin, X. Feng,

M. Klaus, ACS Nano 2012, 6, 9541.[25] R. M. Yadav, J. Wu, R. Kochandra, L. Ma, C. Tiwary, L. Ge, G. Ye,

R. Vajtai, J. Lou, P. M. Ajayan, ACS Appl. Mater. Interfaces 2015, 7, 11991.

[26] J. Wu, L. Ma, R. M. Yadav, Y. Yang, X. Zhang, R. Vajtai, J. Lou, P. M. Ajayan, ACS Appl. Mater. Interfaces 2015, 7, 14763.

[27] Y. Gong, H. Fei, X. Zou, W. Zhou, S. Yang, G. Ye, Z. Liu, Z. Peng, J. Lou, R. Vajtai, B. I. Yakobson, J. M. Tour, P. M. Ajayan, Chem. Mater. 2015, 27, 1181.

[28] G. S. Hutchings, Y. Zhang, J. Li, B. T. Yonemoto, X. Zhou, K. Zhu, F. Jiao, J. Am. Chem. Soc. 2015, 137, 4223.

[29] Z. Han, Z. Junfeng, Y. Ruoping, D. Mingliang, W. Qingfa, G. Guohua, W. Jiong, W. Guangming, Z. Ming, L. Bo, Adv. Mater. 2015, 27, 4752.

[30] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Nat. Mater. 2011, 10, 780.

[31] G. Wu, N. H. Mack, W. Gao, S. Ma, R. Zhong, J. Han, J. K. Baldwin, P. Zelenay, ACS Nano 2012, 6, 9764.

[32] H. Zhong, J. Wang, F. Meng, X. Zhang, Angew. Chem. Int. Ed. 2016, 55, 9937.

[33] Q. Liu, J. Jin, J. Zhang, ACS Appl. Mater. Interfaces 2013, 5, 5002.[34] F. Meng, Z. Wang, H. Zhong, J. Wang, J. Yan, X. Zhang, Adv. Mater.

2016, 28, 7948.[35] W. Jiang, L. Gu, L. Li, Y. Zhang, X. Zhang, L. Zhang, J. Wang, J. Hu,

Z. Wei, L. Wan, J. Am. Chem. Soc. 2016, 138, 3570.[36] U. L. Dong, B. J. Kim, Z. Chen, J. Mater. Chem. A 2013, 1, 4754.[37] Y. Gong, S. Yang, Z. Liu, L. Ma, R. Vajtai, P. M. Ajayan, Adv. Mater.

2013, 25, 3979.[38] T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch,

I. Chorkendorff, Science 2007, 317, 100.[39] Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong, H. Dai, J. Am. Chem. Soc.

2011, 133, 7296.[40] M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li, S. Jin,

J. Am. Chem. Soc. 2013, 135, 10274.

Full p

aper

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (9 of 9) 1602217wileyonlinelibrary.comAdv. Energy Mater. 2017, 1602217


[41] A. B. Laursen, S. Kegnaes, S. Dahl, I. Chorkendorff, Energy Environ. Sci. 2012, 5, 5577.

[42] L. Tao, X. Duan, C. Wang, X. Duan, S. Wang, Chem. Commun. 2015, 51, 7470.

[43] S. T. Dou, J. Li Huo, S. Wang, L. Dai, Energy Environ. Sci. 2016, 9, 1320.

[44] H. Zhong, K. Li1, Q. Zhang, J. Wang, F. Meng, Z. Wu, J. Yan, X. Zhang, NPG Asia Mater. 2016, 8, e308.

[45] K. Zhao, W. Gu, L. Zhao, C. Zhang, W. Peng, Y. Xian, Electrochim. Acta 2015, 169, 142.

[46] H. Yoo, A. P. Tiwari, J. Lee, D. Kim, J. H. Park, H. Lee, Nanoscale 2015, 7, 3404.

[47] X. Song, Y. Zheng, Y. Zhao, H. Yin, Mater. Lett. 2006, 60, 2346.[48] L. Ma, W. Chen, Z. Xu, J. Xia, X. Li, Nanotechnology 2006, 17, 571.[49] X. Li, J. Zhang, R. Wang, H. Huang, C. Xie, Z. Li, J. Li, C. Niu, Nano

Lett. 2015, 15, 5268.[50] J. Lin, O. Pitkänen, J. Mäklin, R. Puskas, A. Kukovecz,

A. Dombovari, G. Toth, K. Kordas, J. Mater. Chem. A 2015, 3, 14609.

[51] A. P. Tiwari, D. Kim, Y. Kim, O. Prakash, H. Lee, Nano Energy 2016, 28, 366.

[52] D. Chu, C. Zhang, P. Yang, Y. Du, C. Lu, Catalysts 2016, 6, 136.[53] E. R. Castellón, A. Jiménez-López, D. Eliche-Quesada, Fuel 2008, 87,

1195.

Bifunctional Oxygen Electrocatalysis through Chemical Bonding … · 2019. 5. 16. · Bifunctional...

Documents

Transcript of Bifunctional Oxygen Electrocatalysis through Chemical Bonding … · 2019. 5. 16. · Bifunctional...