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Enhanced electrocatalytic activity of WO3@NPRGO composite in a hydrogen evolution reaction
Applied Surface Science 463 (2019) 275–282
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Full Length Article
Enhanced electrocatalytic activity of WO3@NPRGO composite in a hydrogen evolution reaction Guojing Hu, Jing Li, Ping Liu, Xingqun Zhu, Xuefeng Li, Rai Nauman Ali, Bin Xiang
T
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Department of Materials Science & Engineering, CAS Key Lab of Materials for Energy Conversion, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
A R T I C LE I N FO
A B S T R A C T
Keywords: WO3@NPRGO composite Electrocatalysis Hydrogen evolution reaction
Various non-noble metal-based electrocatalysts have been studied to replace platinum for hydrogen production. The formation of a composite through adding a conductive polymer could be an efficient way to further enhance the performance of those catalysts further but applying conductive polymers in electrochemical catalysis has been hardly ever reported. In this research, we synthesized an extraordinary WO3@NPRGO composite using phosphotungstic acid-polypyrrole/reduced graphene oxide (PW12-PPy/RGO) as a precursor. The oxidation polymerization of pyrrole (Py) monomers prevents the PW12 and RGO from aggregating and WO3 nanoparticles were homogeneously imbedded in the N, P-codoped RGO nanosheets, which not only exposed massive active sites, but also resulted in faster charge transfer. Electrochemical characterizations reveal that the composite exhibits greatly enhanced electrocatalytic activity and has impressive long-term stability for the hydrogen evolution reaction. Our works opens a new path for the design and synthesis of novel nanostructure electrocatalysts.
1. Introduction With energy consumption growing and environmental contamination becoming aggravating, it is urgent to explore possible sources of sustainable and clean energy. Amongst all the energy sources, hydrogen is an environmentally friendly, renewable, and high efficient energy resources to diminish the use of fossil fuels [1–3]. It is reported that the electrocatalytic hydrogen evolution reaction (HER) is the most practicable and effective approach to produce hydrogen, so it is critical to obtain high-efficiency eletrocatalysts for HER. At present, Pt-based metals are most efficient catalytic materials, although their limited storage and high cost greatly hinder their usage in practical HER. Therefore, great efforts have been made to develop other high-performance electrocatalysts as substitute catalyst for HER [4–6]. Recently, numerous works about transition-metal compounds for HER, such as transitional metal carbides [7–9], transitional metal oxides [10–12] and transitional metal dichalcogenides (TMDs) [13–17] have been reported because they have outstanding performance for HER in acidic conditions. In particular, W-based compounds have attracted great interests in the eletrocatalytic fields because of the Pt-like catalytic performance [18–20]. As an important n-type semiconductor, tungsten oxide (WO3) has attracted enormous attention in the applications of secondary batteries, photocatalysts, and electrocatalysts in ⁎
water splitting for hydrogen production. For example, Lee’s group successfully synthesized WO3 with different morphologies such as nanorods and nanoplates by the hydrothermal process. They found the nanostructured m-WO3 performed with much higher electrocatalytic activity for HER than commercial bulk m-WO3 [21]. Luo et al. reported that the enhanced electrocatalytic activity of WO3 for HER was obtained by the incorporation of tantalum ions (Ta5+) into the lattice of WO3 [22]. However, WO3 is a semiconductor and has poor electron transport ability, which can greatly affect its electrocatalytic performance [23–25]. It is known that dispersing W-based compounds on conductive supports, such as reduced graphene oxide nanosheets (RGO NSs), carbon nanotubes (CNTs), and carbon nanofibers can greatly improve the dispersion of particles and the conductivity of the composite, which can greatly enhance the HER activity [11,20,26–29]. Nevertheless, WO3 nanoparticles (NP) tend to aggregate during hightemperature carbonization and RGO NSs usually aggregate because of their strong π-stacking, which greatly restrict their application. It is reported that conductive polymer monomers such as pyrrole (Py) form frameworks during oxidation polymerization, which could provide a good way to prevent the RGO NSs from re-stacking and the WO3 NPs from aggregation [30–33]. Herein, we synthesize WO3 NPs imbedded in N, P-codoped RGO NS (WO3@NPRGO) composite via a green “one pot” redox reaction and
Corresponding author. E-mail address: [email protected] (B. Xiang).
https://doi.org/10.1016/j.apsusc.2018.08.227 Received 5 June 2018; Received in revised form 15 August 2018; Accepted 26 August 2018 Available online 27 August 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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2.3. Synthesis of WO3@NPRGO
carbonization, obtaining a material which exhibits impressive electrochemical performance. We carried out a series of characterization methods to analyze the structure and chemical composition of the WO3@NPRGO composite, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), energy dispersive spectrometer element mapping (EDS mapping), X-ray diffraction (XRD), Raman spectra, and X-ray photoelectron spectroscopy (XPS). The as-prepared material’s effect on HER performance was investigated using an electrochemical work station.
A quartz boat with 40 mg of black PW12–PPy/RGO hybrids was put into the middle of a tubular furnace. Subsequently, it was heated to 800 °C at a rate of 5 °C/min through a 50 sccm Ar gas flow and kept at that temperature for 5 h for carbonization. At last, the tubular furnace was naturally cooled down to room temperature. The samples were put in 0.5 M H2SO4 and kept at 80 °C for 24 h under vigorous stirring. Acid etching removed the unstable and inactive species. De-ionized water was used to wash the etched samples to reach pH = 7. The final product was dried in a vacuum drying oven at 60 °C for 10 h to obtain the WO3@NPRGO composite. The pure WO3 was synthesized with the same method. A contrast experiment of the catalyst composed of the physical mixture of WO3 and NPRGO was carried out. First, 40 mg of graphene oxide powder was loaded into a quartz boat, which was put in the downstream part of a tubular furnace. Sodium hypochlorite and urea were taken as phosphorus and nitrogen sources, respectively, and these were put in the upstream part of the tubular furnace. Subsequently, the furnace was heated up to 800 °C at a rate of 5 °C/min under a 50 sccm Ar/H2 gas flow and kept at that temperature for 1 h to obtain NPRGO. After 10 mg NPRGO and 50 mg pure WO3 were put into a mortar, the mixture was ground and mixed mechanically to gain a physical mixture of WO3 and NPRGO (WO3/NPRGO).
2. Experimental 2.1. Preparation of GO solution Concentrated 10 mg ml−1 GO aqueous solution (The Sixth Element (Changzhou) Materials Technology Co., Ltd) was diluted into 1 mg ml−1 GO aqueous solution by adding DI-water and using ultrasonic processing for about 30 min. 2.2. Synthesis of PW12-PPy/RGO hybrids Firstly, 0.86 g H3PW12O40ּ⋅xH2O (PW12) was dispersed in 150 ml deionized water to form 2 mg ml−1 PW12 solution by stirring and 2 ml of Py monomer solution was dissolved into 10 ml DI-water. Then around 12.5 ml of 1 mg ml−1 GO suspension as well as 2 mg ml−1 PW12 solution were poured into a round-bottom flask, after which they were mixed uniformly by strong ultrasound. The next step was slowly adding Py monomer aqueous solution into the above mixed solution and magnetic stirring for 10 min. Then the flask was transferred into a 60 °C oil bath where it was kept under continuous magnetic stirring for 26 h. The black precipitate was generated and separated by vacuum filtration, washed with DI-water three times and alcohol two times, and dried in a vacuum drying oven at 60 °C for 10 h. Finally, the black PW12–PPy/RGO hybrids were obtained.
2.4. Characterization The morphologies of the samples were characterized by SEM (JSM6700F, JEOL). Results for TEM, HRTEM, SAED, and EDS mapping were recorded by JEOL (JEM-2010). The XRD pattern was examined by MXPAHF (Mac Science Co. Ltd., Japan). The Raman spectra were taken on a Renishaw InVia confocal microscope-based Raman spectrometer. The XPS was obtained using an ESCALab 250 (Thermo-VG Scientific). The BET result was obtained using a TriStar II 3020 V1.03.
Fig. 1. SEM and TEM characterization of the WO3@NPRGO composite. (a) SEM image of the WO3@NPRGO composite. (b) Low magnification TEM image of the WO3@NPRGO composite. (c) HRTEM image of the WO3@NPRGO composite. (d) SAED pattern of the WO3@NPRGO composite. 276
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2.5. Electrocatalytic measurements
3. Results and discussion
A CHI 660E electrochemical work station with a three-electrode system was used to test the electrochemical performance in 0.5 M sulfuric acid. A glassy carbon electrode (GCE) covered with as-prepared sample, an Ag/AgCl electrode, and a carbon rod served as the working, reference, and counter electrode, respectively. 5 mg WO3@NPRGO composite was dispersed in 1 ml mixed solution (Vethanol/ VNafion = 960/40). A homogeneous ink was formed by sonification for more than 30 min, and 5 μL of this solution was used to cover GCE which was then dried for about 10 h. Other GCEs coated with the WO3/ NPRGO, pure WO3 and 20%Pt/C were prepared as contrast samples in the same way. Linear sweep voltammetry (LSV) analysis was carried out from 0 to −1 V at 0.005 V s−1. The CV curves at −0.2 to −0.3 V at different scan rates (2, 4, 6, 8, 10 mV/s) were swept. Finally, the electrode was calibrated by a reversible hydrogen electrode (RHE). Electrochemical impedance spectroscopy (EIS) testing was performed at 0.8 V in the frequency from 106 Hz to 0.1 Hz.
Firstly, a PW12-PPy/RGO hybrid was synthesized by oxidation-reduction reaction and was carbonized at 800 °C for 5 h under Ar flow. Then, the unstable and inactive species were removed via the acid etching method and washed to reach pH = 7 to gain the WO3@NPRGO composite. The SEM image of the WO3@NPRGO composite in Fig. 1a shows sheet-like structures of RGO. Because of intercalation of PW12 and polymerization of Py, the RGO NSs present wrinkled edges and rough surfaces. From the TEM image of the WO3@NPRGO composite (Fig. 1b), it can be seen that a great quantity of WO3 NPs uniformly cover the RGO NSs, and it can be calculated that the diameter of the WO3 NPs ranges from 1 nm to 4 nm. The HRTEM image in Fig. 1c reveals the clear lattice fringes of the WO3. The lattice fringe spacing is 0.263 nm and 0.231 nm, and those values are consistent with the (2 0 0) and (2 2 0) planes, respectively. The SAED pattern displays individual spots associated with concentric rings, indicating that the WO3@ NPRGO is composed of numerous WO3 nanocrystals. Compared with the initial morphology (Fig. S1a), the TEM image of the WO3@NPRGO
Fig. 2. (a) HRTEM images of the WO3@NPRGO composite and (b-f) EDS elemental mapping of C, W, O, P and N of the WO3@NPRGO composite. 277
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located at 286.3 eV and 289.0 eV are attributed to CeO and OeC]O, respectively [8,19,32], which indicates that the RGO still has a small proportion of oxygen-containing functional groups. In addition, the XPS spectrum of C 1s and O1s of the GO were characterized as shown in Fig. S3. It can be seen that the RGO in the WO3@NPRGO composite was mostly reduced and only rare oxygen-containing functional groups remained. The N 1s spectrum (Fig. 4e) exhibits two kinds of N, which can be assigned to graphinic (401.3 eV) and pyfidinic (398.6 eV) [32,34,35]. From Fig. 4f, it can be seen that the P 2p peaks are divided into three peaks situated at 134.0 eV, 132.3 eV, and 130.7 eV, which can be assigned to P-O, P-C, and P-W, respectively [35,37]. These results indicate that N and P were doped into the RGO NSs, which would enhance the conducivity of the RGO NSs. The N2 adsorption-desorption plot (Fig. S4a) showed that the WO3@ NPRGO composite had a high surface area of 64 m2/g, a value which is 16 times larger than that of pure WO3 (4 m2/g). From the pore size distribution (Fig. S4b, c), it can be seen that the WO3@NPRGO composite takes on a mesoporous structure and the pore sizes of the composite mainly range from 15 to 20 nm. However, the corresponding pore size of pure WO3 was more than 50 nm, which indicates a macroporous structure. Overall, the larger surface area and abundant porous structures provide many active sites and efficiently facilitate charge transfer. To assess the electrocatalytic activities of the WO3@NPRGO composite, 0.5 M H2SO4 was used as an electrolyte and the test was carried out in a three-electrode system with the modified GCE, graphite rod and Ag/AgCl used as the working, counter, and reference electrodes, respectively. For comparison, WO3/NPRGO, pure WO3, and 20%Pt/C were examined in the same way. All the electrochemical measurements, without the iR compensation, are shown in Fig. 5. Usually, as a contrast, the potential when the current density reaches 10 mA cm−2 is defined as overpotential [3,36,38]. The Pt/C displays excellent activity with an overpotential of only 86 mV. However, it can be seen that the WO3/ NPRGO needs 589 mV and the pure WO3 needs 660 mV to reach 10 mA cm−2, overpotentials which are much larger than the WO3@ NPRGO composite (225 mV) (Table S1). Also, the Tafel slope and charge-transfer resistance of the WO3/NPRGO is 87 mV dec−1 and 60.64 Ω, values which are both much higher than those of the WO3@ NPRGO composite. Such a result suggests that physical mixture of WO3 and NPRGO cannot make the WO3 NPs uniformly attach to RGO NSs, so the catalytic performance cannot be improved. However, in the WO3@ NPRGO composite, WO3 NPs with small size were well dispersed in the RGO NSs, providing exposure to a large number of highly active sites, so it greatly improves the catalytic activity for HER. Apart from the overpotential, the exchange current density (j0) and the Tafel slope (b) are two other important parameters. In general, the Tafel slope (b) which is fitted by Tafel plots (η = b log j + a), can be used to evaluate the HER mechanism. The exchange current density (j0) reflects the intrinsic catalytic activity of a material. In theory the preeminent catalysts should possess very smaller b and larger j0
after 1000 cycles was also obtained (Fig. S1b). That image showed that the graphene still retained its original morphology, which means that it provides a stable skeleton. Also, a great quantity of WO3 NPs was still uniformly distributed on the RGO NSs, which confirms that the WO3@ NPRGO has a great long-term stability in HER. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.08.227. The scanning TEM (STEM) and EDS mapping images were taken to analyze the element distribution of C, W, O, P and N. From the Fig. 2, it can be seen that the C, W, O, P, and N are homogeneously scattered on the surface of the WO3@NPRGO composite, which implies there is a strong interaction between the RGO and WO3. The WO3 NPs are uniformly scattered around the RGO NSs, which can prevent the WO3 NPs agglomeration and overgrowth. Therefore, the small WO3 NPs provide high exposure of substantial available active sites. Furthermore, the intercalation of PW12 NPs and the polymerization of Py can impede the re-stacking of RGO, which can favor a large specific surface area. The synergetic effect of WO3 and RGO plays an important role in enhancing the performance of electrocatalytic hydrogen production. Moreover, it can be seen that the atomic ratio of W: O is about 1:3 according to the EDS spectrum in Fig. S2, which corresponds to the atomic ratio of the WO3. These test results prove the successful synthesis of the WO3@ NPRGO composite. The XRD pattern and Raman spectra analyzed to reveal the crystal structure and chemical composition of the WO3@NPRGO composite. From the XRD patterns (Fig. 3a), it can be seen that an inconspicuous broad peak at ∼25° is ascribed to amorphous carbon [7,32]. The other indexed peaks of the WO3@NPRGO composite correspond well to the WO3 (PDF#84-0886) without any impurity peaks. Next, the degree of graphitization of the WO3@NPRGO composite was analyzed by Raman spectra (Fig. 3b). Two representative peaks of D and G band of graphene appear at about 1350 cm−1 and 1580 cm−1 [19,27]. Normally, the ratio of ID/IG (I represents the peak intensity) can be used to judge the degree of graphitization. From the Raman spectra in Fig. 3b, it was calculated that the ratio of ID/IG in GO powder is 0.84, while that of the WO3@NPRGO composite is 1.11. The larger ratio value reveals that the GO was reduced to RGO and the defects in the graphene increased. XPS spectrum analysis was carried out to determine the valence states and elemental composition of the WO3@NPRGO composite. Fig. 4a shows the full XPS spectrum of the WO3@NPRGO composite, which indicates that W, O, C, N, and P elements exist in the WO3@ NPRGO composite. The XPS spectrum of W 4f (Fig. 4b) has two weak peaks, which are consistent with the W-P and W-C bonds located at 32.4 eV and 34.0 eV, respectively. Two strong peaks located at 36.0 eV and 38.1 eV represent the W 4f7/2 and W 4f5/2 peaks of WO3 [34,35]. From the O 1s spectra (Fig. 4c), the strong peak is divided into two peaks corresponding to the WeO and C]O bonds, which are located at 531.2 and 532.8 eV, respectively [10,23]. The C 1s core level peaks can be separated into three peaks (Fig. 4d). The CeC bond of the graphitic carbon located at 284.8 eV has the strongest intensity peak. The peaks
Fig. 3. (a) The XRD pattern of the WO3@NPRGO composite. (b) Raman spectra of the WO3@NPRGO composite and GO powder. 278
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Fig. 4. (a) XPS survey spectrum of the WO3@NPRGO composite. The high resolution of the (b) W 4f, (c) O 1s, (d) C 1s, (e) N 1s and (f) P 2p XPS spectra of the WO3@ NPRGO composite.
reacts with MHads to form H2(g). Reaction (3) is the recombination step, where two MHads combine to release H2(g). Electrocatalysis for HER typically goes through either reaction (1) and (2) (Volmer–Heyrovsky mechanism) or (1) and (3) (Volmer–Tafel mechanism), and these can happen simultaneously [3,32,35,39]. In our work, the value of the Tafel slope of the WO3@NPRGO composite is 87 mV dec−1, which reveals that this reaction follows the Volmer-Heyrovsky mechanism. The Tafel plots can be extrapolated to calculate the exchange current density (j0). The WO3@NPRGO composite exhibits a j0 of 0.34 mA cm−2, which is larger than that of WO3/NPRGO (0.054 mA cm−2) and nearly two hundred times larger than that of pure WO3 (1.28 × 10−3 mA cm−2) (Table S1). These results demonstrate that the WO3@NPRGO composite has a favorable HER performance. The double-layer capacitance was measured to reflect the electrochemical surface area (ECSA). The CV curves of WO3@NPRGO (Fig. 5c), WO3/NPRGO (Fig. S5a) and pure WO3 (Fig. S5b) at −0.1–0 V vs. RHE at different scan rates (2, 4, 6, 8, 10 mV/s) were tested. Then the differences in current density at −0.05 V vs. RHE plotted against different scan rates were fitted to estimate the electrochemical double-
[35,38,39]. From Fig. 5b, the Tafel slope of the WO3@NPRGO composite is 87 mV dec−1, a value which is much smaller than the 119 mV dec−1 value for pure WO3 and 169 mV dec−1 for WO3/NPRGO. According to the classic theory on the mechanism of hydrogen evolution, the Tafel slope is related with the rate-limiting step of electrocatalysis. The HER procedure in the acid electrolyte can experience three basic steps as follows: H3O+ + e− + M → MHads + H2O −
(1)
MHads + H2O + e → H2 + H2O↑
(2)
2MHads → 2 M + H2↑
(3)
+
The M and MHads is used as the catalyst and the adsorbed H intermediate. (1), (2), and (3) represent the three steps, which are called the Volmer step (120 mV dec−1), Heyrovsky step (40 mV dec−1) and Tafel step (30 mV dec−1), respectively. Reaction (1) is the discharge step, where an active site of the catalyst surface absorbs a free proton, and this is a rate limiting step. Reaction (2) is the electrochemical desorption step which is also rate limiting. During this step, a free proton 279
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Fig. 5. HER activity characterization. (a) Polarization curves of WO3@NPRGO WO3/NPRGO, pure WO3 and 20% Pt/C. (b) Tafel plots of WO3@NPRGO, WO3/ NPRGO, pure WO3 and 20% Pt/C. (c) The CV curves of WO3@NPRGO at different scan rates. (d) The plots show the extraction of the double-layer capacitance of WO3@NPRGO, WO3/NPRGO, as well as pure WO3. (e) Polarization curves of WO3@NPRGO at initial stage and after 1000 CV cycles. (f) Electrochemical impedance spectra (EIS) of pure WO3 and WO3@NPRGO over the frequency ranging from 1000 kHz to 0.1 Hz at the −0.8 V. The inset image showing equivalent circuit.
the equivalent electric circuit, the Rs, Rct, and CPE are the uncompensated solution resistance, charge-transfer resistance, and constant phase element, respectively. Through the fitting result, we can obtain that the solution resistances of WO3@NPRGO, WO3/NPRGO and pure WO3 are 7.15 Ω, 7.30 Ω, and 6.98 Ω, respectively. The WO3@ NPRGO has the smallest charge-transfer resistance (23.72 Ω) followed by that of WO3/NPRGO (40.23 Ω) and pure WO3 (60.64 Ω), which indicates the robust contact between WO3 NPs and RGO NSs greatly reduces the resistance during the charge transfer. The WO3@NPRGO composite was synthesized using a green and economical method. During the synthesis process, the PW12s were dispersed into the PPy framework because of the polymerization of Py. Simultaneously, RGO NSs were separated by the intercalation of PW12s and the polymerization of Py. Preparing RGO-supported WO3 nanoparticle (NP) with PW12-PPy/RGO as a precursor can not only prevent WO3 NPs and RGO NSs from aggregation during high-temperature carbonization but also provide nitrogen (N) and phosphorus (P) sources to form N, P-codoped RGO (NPRGO) [32]. The results show that the WO3@NPRGO composite has strongly enhanced eletrocatalytic activity for HER, which can be attributed to the following reasons: (1) The
layer capacitance (Cdl), which can be used to evaluate the ECSA. From Fig. 5d, it was calculated that the Cdl of WO3@NPRGO, WO3/NPRGO, and pure WO3 were 27.22 mF cm−2, 18.87 mF cm−2, and 0.89 mF cm−2, respectively. These figures indicate the WO3@NPRGO has a higher electrochemical surface area, which corresponds to the outstanding catalytic activity of WO3@NPRGO composite. Besides the catalytic activity, stability is equally important for HER catalysts [3,32,40]. To evaluate the durability of the WO3@NPRGO composite, continuous CV characterizations were performed for 1000 cycles between −1 V and 0 V at 50 mV s−1 in the acid system. Fig. 5e shows that the 1000-cycle polarization curve for the WO3@NPRGO composite remains almost the same as the initial one, indicating strong long-term stability. This is because the RGO NSs provide a substantial support, which can protect the structure from bubble corrosion during the cycling process. The electrochemical impedance spectroscopy (EIS) of the WO3@ NPRGO, WO3/NPRGO and pure WO3 was further tested to investigate the electrochemical activity. The Nyquist plots are over the frequency ranging from 106 Hz to 0.1 Hz at o.8 V as shown in Fig. 5f. The EIS was fitted by Zview, and the equivalent electric circuit was determined. In 280
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intercalation of PW12 and polymerization of Py can prevent RGO NSs from aggregating, which provides a large surface area. (2) WO3 NPs with small size were well dispersed in the RGO NSs, which provides the exposure to many active sites, so it greatly improves the catalytic activity for the HER. (3) The N, P-doped RGO has excellent electrical conductivity, accelerating the charge transfer speed and the reaction rate. (4) The robust contact between WO3 NPs and RGO NSs greatly reduces the resistance during charge transfer. To summarize, the WO3 NPs and the RGO NSs have an extraordinary synergistic catalytic effect, so the WO3@NPRGO composite exhibits outstanding catalytic activity for HER.
[8]
[9]
[10]
[11]
4. Conclusions
[12]
In summary, we developed a new pathway to synthesize WO3@ NPRGO composite by adding Py monomer to form a conductive polymer framework to prevent the aggregation of WO3 NPs and RGO NSs from aggregation. The synthesized WO3@NPRGO has a larger surface area without aggregation and offers more exposed active sites by homogeneously imbedding the NPRGO NSs. Moreover, the electron transport ability has been improved by a factor of NPRGO compared to pure WO3. The WO3@NPRGO composite presents a remarkably enhanced electrocatalytic activity for the HER with a much smaller Tafel slope compared with pure WO3, and it also has long term stability. Our works opens a new approach to developing high-performance catalysts and an approach which can also be explored to synthesize other promising energy materials.
[13]
[14]
[15]
[16]
[17]
[18]
Conflict of interest [19]
The authors declare that they have no conflict of interest. Supporting information
[20]
The TEM image of the WO3@NPRGO composite at initial stage and after 1000 cycles, the EDS spectrum of the WO3@NPRGO composite, the high resolution of the C1s and O1s XPS spectra of the GO powder, the N2 adsorption-desorption plot and the pore size distribution plot, the CV curves of WO3/NPRGO and pure WO3 at different scan rates, a comparison table of parameters of different HER catalysts.
[21]
[22]
[23]
Acknowledgements
[24]
This work was supported by the joint fund of the National Natural Science Foundation Committee of China Academy of Engineering Physics (NSAF) (U1630108) and the National Key Research and Development Program of China (2017YFA0402902). This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.
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Full Length Article
Enhanced electrocatalytic activity of WO3@NPRGO composite in a hydrogen evolution reaction Guojing Hu, Jing Li, Ping Liu, Xingqun Zhu, Xuefeng Li, Rai Nauman Ali, Bin Xiang
T
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Department of Materials Science & Engineering, CAS Key Lab of Materials for Energy Conversion, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
A R T I C LE I N FO
A B S T R A C T
Keywords: WO3@NPRGO composite Electrocatalysis Hydrogen evolution reaction
Various non-noble metal-based electrocatalysts have been studied to replace platinum for hydrogen production. The formation of a composite through adding a conductive polymer could be an efficient way to further enhance the performance of those catalysts further but applying conductive polymers in electrochemical catalysis has been hardly ever reported. In this research, we synthesized an extraordinary WO3@NPRGO composite using phosphotungstic acid-polypyrrole/reduced graphene oxide (PW12-PPy/RGO) as a precursor. The oxidation polymerization of pyrrole (Py) monomers prevents the PW12 and RGO from aggregating and WO3 nanoparticles were homogeneously imbedded in the N, P-codoped RGO nanosheets, which not only exposed massive active sites, but also resulted in faster charge transfer. Electrochemical characterizations reveal that the composite exhibits greatly enhanced electrocatalytic activity and has impressive long-term stability for the hydrogen evolution reaction. Our works opens a new path for the design and synthesis of novel nanostructure electrocatalysts.
1. Introduction With energy consumption growing and environmental contamination becoming aggravating, it is urgent to explore possible sources of sustainable and clean energy. Amongst all the energy sources, hydrogen is an environmentally friendly, renewable, and high efficient energy resources to diminish the use of fossil fuels [1–3]. It is reported that the electrocatalytic hydrogen evolution reaction (HER) is the most practicable and effective approach to produce hydrogen, so it is critical to obtain high-efficiency eletrocatalysts for HER. At present, Pt-based metals are most efficient catalytic materials, although their limited storage and high cost greatly hinder their usage in practical HER. Therefore, great efforts have been made to develop other high-performance electrocatalysts as substitute catalyst for HER [4–6]. Recently, numerous works about transition-metal compounds for HER, such as transitional metal carbides [7–9], transitional metal oxides [10–12] and transitional metal dichalcogenides (TMDs) [13–17] have been reported because they have outstanding performance for HER in acidic conditions. In particular, W-based compounds have attracted great interests in the eletrocatalytic fields because of the Pt-like catalytic performance [18–20]. As an important n-type semiconductor, tungsten oxide (WO3) has attracted enormous attention in the applications of secondary batteries, photocatalysts, and electrocatalysts in ⁎
water splitting for hydrogen production. For example, Lee’s group successfully synthesized WO3 with different morphologies such as nanorods and nanoplates by the hydrothermal process. They found the nanostructured m-WO3 performed with much higher electrocatalytic activity for HER than commercial bulk m-WO3 [21]. Luo et al. reported that the enhanced electrocatalytic activity of WO3 for HER was obtained by the incorporation of tantalum ions (Ta5+) into the lattice of WO3 [22]. However, WO3 is a semiconductor and has poor electron transport ability, which can greatly affect its electrocatalytic performance [23–25]. It is known that dispersing W-based compounds on conductive supports, such as reduced graphene oxide nanosheets (RGO NSs), carbon nanotubes (CNTs), and carbon nanofibers can greatly improve the dispersion of particles and the conductivity of the composite, which can greatly enhance the HER activity [11,20,26–29]. Nevertheless, WO3 nanoparticles (NP) tend to aggregate during hightemperature carbonization and RGO NSs usually aggregate because of their strong π-stacking, which greatly restrict their application. It is reported that conductive polymer monomers such as pyrrole (Py) form frameworks during oxidation polymerization, which could provide a good way to prevent the RGO NSs from re-stacking and the WO3 NPs from aggregation [30–33]. Herein, we synthesize WO3 NPs imbedded in N, P-codoped RGO NS (WO3@NPRGO) composite via a green “one pot” redox reaction and
Corresponding author. E-mail address: [email protected] (B. Xiang).
https://doi.org/10.1016/j.apsusc.2018.08.227 Received 5 June 2018; Received in revised form 15 August 2018; Accepted 26 August 2018 Available online 27 August 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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2.3. Synthesis of WO3@NPRGO
carbonization, obtaining a material which exhibits impressive electrochemical performance. We carried out a series of characterization methods to analyze the structure and chemical composition of the WO3@NPRGO composite, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), energy dispersive spectrometer element mapping (EDS mapping), X-ray diffraction (XRD), Raman spectra, and X-ray photoelectron spectroscopy (XPS). The as-prepared material’s effect on HER performance was investigated using an electrochemical work station.
A quartz boat with 40 mg of black PW12–PPy/RGO hybrids was put into the middle of a tubular furnace. Subsequently, it was heated to 800 °C at a rate of 5 °C/min through a 50 sccm Ar gas flow and kept at that temperature for 5 h for carbonization. At last, the tubular furnace was naturally cooled down to room temperature. The samples were put in 0.5 M H2SO4 and kept at 80 °C for 24 h under vigorous stirring. Acid etching removed the unstable and inactive species. De-ionized water was used to wash the etched samples to reach pH = 7. The final product was dried in a vacuum drying oven at 60 °C for 10 h to obtain the WO3@NPRGO composite. The pure WO3 was synthesized with the same method. A contrast experiment of the catalyst composed of the physical mixture of WO3 and NPRGO was carried out. First, 40 mg of graphene oxide powder was loaded into a quartz boat, which was put in the downstream part of a tubular furnace. Sodium hypochlorite and urea were taken as phosphorus and nitrogen sources, respectively, and these were put in the upstream part of the tubular furnace. Subsequently, the furnace was heated up to 800 °C at a rate of 5 °C/min under a 50 sccm Ar/H2 gas flow and kept at that temperature for 1 h to obtain NPRGO. After 10 mg NPRGO and 50 mg pure WO3 were put into a mortar, the mixture was ground and mixed mechanically to gain a physical mixture of WO3 and NPRGO (WO3/NPRGO).
2. Experimental 2.1. Preparation of GO solution Concentrated 10 mg ml−1 GO aqueous solution (The Sixth Element (Changzhou) Materials Technology Co., Ltd) was diluted into 1 mg ml−1 GO aqueous solution by adding DI-water and using ultrasonic processing for about 30 min. 2.2. Synthesis of PW12-PPy/RGO hybrids Firstly, 0.86 g H3PW12O40ּ⋅xH2O (PW12) was dispersed in 150 ml deionized water to form 2 mg ml−1 PW12 solution by stirring and 2 ml of Py monomer solution was dissolved into 10 ml DI-water. Then around 12.5 ml of 1 mg ml−1 GO suspension as well as 2 mg ml−1 PW12 solution were poured into a round-bottom flask, after which they were mixed uniformly by strong ultrasound. The next step was slowly adding Py monomer aqueous solution into the above mixed solution and magnetic stirring for 10 min. Then the flask was transferred into a 60 °C oil bath where it was kept under continuous magnetic stirring for 26 h. The black precipitate was generated and separated by vacuum filtration, washed with DI-water three times and alcohol two times, and dried in a vacuum drying oven at 60 °C for 10 h. Finally, the black PW12–PPy/RGO hybrids were obtained.
2.4. Characterization The morphologies of the samples were characterized by SEM (JSM6700F, JEOL). Results for TEM, HRTEM, SAED, and EDS mapping were recorded by JEOL (JEM-2010). The XRD pattern was examined by MXPAHF (Mac Science Co. Ltd., Japan). The Raman spectra were taken on a Renishaw InVia confocal microscope-based Raman spectrometer. The XPS was obtained using an ESCALab 250 (Thermo-VG Scientific). The BET result was obtained using a TriStar II 3020 V1.03.
Fig. 1. SEM and TEM characterization of the WO3@NPRGO composite. (a) SEM image of the WO3@NPRGO composite. (b) Low magnification TEM image of the WO3@NPRGO composite. (c) HRTEM image of the WO3@NPRGO composite. (d) SAED pattern of the WO3@NPRGO composite. 276
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2.5. Electrocatalytic measurements
3. Results and discussion
A CHI 660E electrochemical work station with a three-electrode system was used to test the electrochemical performance in 0.5 M sulfuric acid. A glassy carbon electrode (GCE) covered with as-prepared sample, an Ag/AgCl electrode, and a carbon rod served as the working, reference, and counter electrode, respectively. 5 mg WO3@NPRGO composite was dispersed in 1 ml mixed solution (Vethanol/ VNafion = 960/40). A homogeneous ink was formed by sonification for more than 30 min, and 5 μL of this solution was used to cover GCE which was then dried for about 10 h. Other GCEs coated with the WO3/ NPRGO, pure WO3 and 20%Pt/C were prepared as contrast samples in the same way. Linear sweep voltammetry (LSV) analysis was carried out from 0 to −1 V at 0.005 V s−1. The CV curves at −0.2 to −0.3 V at different scan rates (2, 4, 6, 8, 10 mV/s) were swept. Finally, the electrode was calibrated by a reversible hydrogen electrode (RHE). Electrochemical impedance spectroscopy (EIS) testing was performed at 0.8 V in the frequency from 106 Hz to 0.1 Hz.
Firstly, a PW12-PPy/RGO hybrid was synthesized by oxidation-reduction reaction and was carbonized at 800 °C for 5 h under Ar flow. Then, the unstable and inactive species were removed via the acid etching method and washed to reach pH = 7 to gain the WO3@NPRGO composite. The SEM image of the WO3@NPRGO composite in Fig. 1a shows sheet-like structures of RGO. Because of intercalation of PW12 and polymerization of Py, the RGO NSs present wrinkled edges and rough surfaces. From the TEM image of the WO3@NPRGO composite (Fig. 1b), it can be seen that a great quantity of WO3 NPs uniformly cover the RGO NSs, and it can be calculated that the diameter of the WO3 NPs ranges from 1 nm to 4 nm. The HRTEM image in Fig. 1c reveals the clear lattice fringes of the WO3. The lattice fringe spacing is 0.263 nm and 0.231 nm, and those values are consistent with the (2 0 0) and (2 2 0) planes, respectively. The SAED pattern displays individual spots associated with concentric rings, indicating that the WO3@ NPRGO is composed of numerous WO3 nanocrystals. Compared with the initial morphology (Fig. S1a), the TEM image of the WO3@NPRGO
Fig. 2. (a) HRTEM images of the WO3@NPRGO composite and (b-f) EDS elemental mapping of C, W, O, P and N of the WO3@NPRGO composite. 277
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located at 286.3 eV and 289.0 eV are attributed to CeO and OeC]O, respectively [8,19,32], which indicates that the RGO still has a small proportion of oxygen-containing functional groups. In addition, the XPS spectrum of C 1s and O1s of the GO were characterized as shown in Fig. S3. It can be seen that the RGO in the WO3@NPRGO composite was mostly reduced and only rare oxygen-containing functional groups remained. The N 1s spectrum (Fig. 4e) exhibits two kinds of N, which can be assigned to graphinic (401.3 eV) and pyfidinic (398.6 eV) [32,34,35]. From Fig. 4f, it can be seen that the P 2p peaks are divided into three peaks situated at 134.0 eV, 132.3 eV, and 130.7 eV, which can be assigned to P-O, P-C, and P-W, respectively [35,37]. These results indicate that N and P were doped into the RGO NSs, which would enhance the conducivity of the RGO NSs. The N2 adsorption-desorption plot (Fig. S4a) showed that the WO3@ NPRGO composite had a high surface area of 64 m2/g, a value which is 16 times larger than that of pure WO3 (4 m2/g). From the pore size distribution (Fig. S4b, c), it can be seen that the WO3@NPRGO composite takes on a mesoporous structure and the pore sizes of the composite mainly range from 15 to 20 nm. However, the corresponding pore size of pure WO3 was more than 50 nm, which indicates a macroporous structure. Overall, the larger surface area and abundant porous structures provide many active sites and efficiently facilitate charge transfer. To assess the electrocatalytic activities of the WO3@NPRGO composite, 0.5 M H2SO4 was used as an electrolyte and the test was carried out in a three-electrode system with the modified GCE, graphite rod and Ag/AgCl used as the working, counter, and reference electrodes, respectively. For comparison, WO3/NPRGO, pure WO3, and 20%Pt/C were examined in the same way. All the electrochemical measurements, without the iR compensation, are shown in Fig. 5. Usually, as a contrast, the potential when the current density reaches 10 mA cm−2 is defined as overpotential [3,36,38]. The Pt/C displays excellent activity with an overpotential of only 86 mV. However, it can be seen that the WO3/ NPRGO needs 589 mV and the pure WO3 needs 660 mV to reach 10 mA cm−2, overpotentials which are much larger than the WO3@ NPRGO composite (225 mV) (Table S1). Also, the Tafel slope and charge-transfer resistance of the WO3/NPRGO is 87 mV dec−1 and 60.64 Ω, values which are both much higher than those of the WO3@ NPRGO composite. Such a result suggests that physical mixture of WO3 and NPRGO cannot make the WO3 NPs uniformly attach to RGO NSs, so the catalytic performance cannot be improved. However, in the WO3@ NPRGO composite, WO3 NPs with small size were well dispersed in the RGO NSs, providing exposure to a large number of highly active sites, so it greatly improves the catalytic activity for HER. Apart from the overpotential, the exchange current density (j0) and the Tafel slope (b) are two other important parameters. In general, the Tafel slope (b) which is fitted by Tafel plots (η = b log j + a), can be used to evaluate the HER mechanism. The exchange current density (j0) reflects the intrinsic catalytic activity of a material. In theory the preeminent catalysts should possess very smaller b and larger j0
after 1000 cycles was also obtained (Fig. S1b). That image showed that the graphene still retained its original morphology, which means that it provides a stable skeleton. Also, a great quantity of WO3 NPs was still uniformly distributed on the RGO NSs, which confirms that the WO3@ NPRGO has a great long-term stability in HER. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.08.227. The scanning TEM (STEM) and EDS mapping images were taken to analyze the element distribution of C, W, O, P and N. From the Fig. 2, it can be seen that the C, W, O, P, and N are homogeneously scattered on the surface of the WO3@NPRGO composite, which implies there is a strong interaction between the RGO and WO3. The WO3 NPs are uniformly scattered around the RGO NSs, which can prevent the WO3 NPs agglomeration and overgrowth. Therefore, the small WO3 NPs provide high exposure of substantial available active sites. Furthermore, the intercalation of PW12 NPs and the polymerization of Py can impede the re-stacking of RGO, which can favor a large specific surface area. The synergetic effect of WO3 and RGO plays an important role in enhancing the performance of electrocatalytic hydrogen production. Moreover, it can be seen that the atomic ratio of W: O is about 1:3 according to the EDS spectrum in Fig. S2, which corresponds to the atomic ratio of the WO3. These test results prove the successful synthesis of the WO3@ NPRGO composite. The XRD pattern and Raman spectra analyzed to reveal the crystal structure and chemical composition of the WO3@NPRGO composite. From the XRD patterns (Fig. 3a), it can be seen that an inconspicuous broad peak at ∼25° is ascribed to amorphous carbon [7,32]. The other indexed peaks of the WO3@NPRGO composite correspond well to the WO3 (PDF#84-0886) without any impurity peaks. Next, the degree of graphitization of the WO3@NPRGO composite was analyzed by Raman spectra (Fig. 3b). Two representative peaks of D and G band of graphene appear at about 1350 cm−1 and 1580 cm−1 [19,27]. Normally, the ratio of ID/IG (I represents the peak intensity) can be used to judge the degree of graphitization. From the Raman spectra in Fig. 3b, it was calculated that the ratio of ID/IG in GO powder is 0.84, while that of the WO3@NPRGO composite is 1.11. The larger ratio value reveals that the GO was reduced to RGO and the defects in the graphene increased. XPS spectrum analysis was carried out to determine the valence states and elemental composition of the WO3@NPRGO composite. Fig. 4a shows the full XPS spectrum of the WO3@NPRGO composite, which indicates that W, O, C, N, and P elements exist in the WO3@ NPRGO composite. The XPS spectrum of W 4f (Fig. 4b) has two weak peaks, which are consistent with the W-P and W-C bonds located at 32.4 eV and 34.0 eV, respectively. Two strong peaks located at 36.0 eV and 38.1 eV represent the W 4f7/2 and W 4f5/2 peaks of WO3 [34,35]. From the O 1s spectra (Fig. 4c), the strong peak is divided into two peaks corresponding to the WeO and C]O bonds, which are located at 531.2 and 532.8 eV, respectively [10,23]. The C 1s core level peaks can be separated into three peaks (Fig. 4d). The CeC bond of the graphitic carbon located at 284.8 eV has the strongest intensity peak. The peaks
Fig. 3. (a) The XRD pattern of the WO3@NPRGO composite. (b) Raman spectra of the WO3@NPRGO composite and GO powder. 278
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Fig. 4. (a) XPS survey spectrum of the WO3@NPRGO composite. The high resolution of the (b) W 4f, (c) O 1s, (d) C 1s, (e) N 1s and (f) P 2p XPS spectra of the WO3@ NPRGO composite.
reacts with MHads to form H2(g). Reaction (3) is the recombination step, where two MHads combine to release H2(g). Electrocatalysis for HER typically goes through either reaction (1) and (2) (Volmer–Heyrovsky mechanism) or (1) and (3) (Volmer–Tafel mechanism), and these can happen simultaneously [3,32,35,39]. In our work, the value of the Tafel slope of the WO3@NPRGO composite is 87 mV dec−1, which reveals that this reaction follows the Volmer-Heyrovsky mechanism. The Tafel plots can be extrapolated to calculate the exchange current density (j0). The WO3@NPRGO composite exhibits a j0 of 0.34 mA cm−2, which is larger than that of WO3/NPRGO (0.054 mA cm−2) and nearly two hundred times larger than that of pure WO3 (1.28 × 10−3 mA cm−2) (Table S1). These results demonstrate that the WO3@NPRGO composite has a favorable HER performance. The double-layer capacitance was measured to reflect the electrochemical surface area (ECSA). The CV curves of WO3@NPRGO (Fig. 5c), WO3/NPRGO (Fig. S5a) and pure WO3 (Fig. S5b) at −0.1–0 V vs. RHE at different scan rates (2, 4, 6, 8, 10 mV/s) were tested. Then the differences in current density at −0.05 V vs. RHE plotted against different scan rates were fitted to estimate the electrochemical double-
[35,38,39]. From Fig. 5b, the Tafel slope of the WO3@NPRGO composite is 87 mV dec−1, a value which is much smaller than the 119 mV dec−1 value for pure WO3 and 169 mV dec−1 for WO3/NPRGO. According to the classic theory on the mechanism of hydrogen evolution, the Tafel slope is related with the rate-limiting step of electrocatalysis. The HER procedure in the acid electrolyte can experience three basic steps as follows: H3O+ + e− + M → MHads + H2O −
(1)
MHads + H2O + e → H2 + H2O↑
(2)
2MHads → 2 M + H2↑
(3)
+
The M and MHads is used as the catalyst and the adsorbed H intermediate. (1), (2), and (3) represent the three steps, which are called the Volmer step (120 mV dec−1), Heyrovsky step (40 mV dec−1) and Tafel step (30 mV dec−1), respectively. Reaction (1) is the discharge step, where an active site of the catalyst surface absorbs a free proton, and this is a rate limiting step. Reaction (2) is the electrochemical desorption step which is also rate limiting. During this step, a free proton 279
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Fig. 5. HER activity characterization. (a) Polarization curves of WO3@NPRGO WO3/NPRGO, pure WO3 and 20% Pt/C. (b) Tafel plots of WO3@NPRGO, WO3/ NPRGO, pure WO3 and 20% Pt/C. (c) The CV curves of WO3@NPRGO at different scan rates. (d) The plots show the extraction of the double-layer capacitance of WO3@NPRGO, WO3/NPRGO, as well as pure WO3. (e) Polarization curves of WO3@NPRGO at initial stage and after 1000 CV cycles. (f) Electrochemical impedance spectra (EIS) of pure WO3 and WO3@NPRGO over the frequency ranging from 1000 kHz to 0.1 Hz at the −0.8 V. The inset image showing equivalent circuit.
the equivalent electric circuit, the Rs, Rct, and CPE are the uncompensated solution resistance, charge-transfer resistance, and constant phase element, respectively. Through the fitting result, we can obtain that the solution resistances of WO3@NPRGO, WO3/NPRGO and pure WO3 are 7.15 Ω, 7.30 Ω, and 6.98 Ω, respectively. The WO3@ NPRGO has the smallest charge-transfer resistance (23.72 Ω) followed by that of WO3/NPRGO (40.23 Ω) and pure WO3 (60.64 Ω), which indicates the robust contact between WO3 NPs and RGO NSs greatly reduces the resistance during the charge transfer. The WO3@NPRGO composite was synthesized using a green and economical method. During the synthesis process, the PW12s were dispersed into the PPy framework because of the polymerization of Py. Simultaneously, RGO NSs were separated by the intercalation of PW12s and the polymerization of Py. Preparing RGO-supported WO3 nanoparticle (NP) with PW12-PPy/RGO as a precursor can not only prevent WO3 NPs and RGO NSs from aggregation during high-temperature carbonization but also provide nitrogen (N) and phosphorus (P) sources to form N, P-codoped RGO (NPRGO) [32]. The results show that the WO3@NPRGO composite has strongly enhanced eletrocatalytic activity for HER, which can be attributed to the following reasons: (1) The
layer capacitance (Cdl), which can be used to evaluate the ECSA. From Fig. 5d, it was calculated that the Cdl of WO3@NPRGO, WO3/NPRGO, and pure WO3 were 27.22 mF cm−2, 18.87 mF cm−2, and 0.89 mF cm−2, respectively. These figures indicate the WO3@NPRGO has a higher electrochemical surface area, which corresponds to the outstanding catalytic activity of WO3@NPRGO composite. Besides the catalytic activity, stability is equally important for HER catalysts [3,32,40]. To evaluate the durability of the WO3@NPRGO composite, continuous CV characterizations were performed for 1000 cycles between −1 V and 0 V at 50 mV s−1 in the acid system. Fig. 5e shows that the 1000-cycle polarization curve for the WO3@NPRGO composite remains almost the same as the initial one, indicating strong long-term stability. This is because the RGO NSs provide a substantial support, which can protect the structure from bubble corrosion during the cycling process. The electrochemical impedance spectroscopy (EIS) of the WO3@ NPRGO, WO3/NPRGO and pure WO3 was further tested to investigate the electrochemical activity. The Nyquist plots are over the frequency ranging from 106 Hz to 0.1 Hz at o.8 V as shown in Fig. 5f. The EIS was fitted by Zview, and the equivalent electric circuit was determined. In 280
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intercalation of PW12 and polymerization of Py can prevent RGO NSs from aggregating, which provides a large surface area. (2) WO3 NPs with small size were well dispersed in the RGO NSs, which provides the exposure to many active sites, so it greatly improves the catalytic activity for the HER. (3) The N, P-doped RGO has excellent electrical conductivity, accelerating the charge transfer speed and the reaction rate. (4) The robust contact between WO3 NPs and RGO NSs greatly reduces the resistance during charge transfer. To summarize, the WO3 NPs and the RGO NSs have an extraordinary synergistic catalytic effect, so the WO3@NPRGO composite exhibits outstanding catalytic activity for HER.
[8]
[9]
[10]
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4. Conclusions
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In summary, we developed a new pathway to synthesize WO3@ NPRGO composite by adding Py monomer to form a conductive polymer framework to prevent the aggregation of WO3 NPs and RGO NSs from aggregation. The synthesized WO3@NPRGO has a larger surface area without aggregation and offers more exposed active sites by homogeneously imbedding the NPRGO NSs. Moreover, the electron transport ability has been improved by a factor of NPRGO compared to pure WO3. The WO3@NPRGO composite presents a remarkably enhanced electrocatalytic activity for the HER with a much smaller Tafel slope compared with pure WO3, and it also has long term stability. Our works opens a new approach to developing high-performance catalysts and an approach which can also be explored to synthesize other promising energy materials.
[13]
[14]
[15]
[16]
[17]
[18]
Conflict of interest [19]
The authors declare that they have no conflict of interest. Supporting information
[20]
The TEM image of the WO3@NPRGO composite at initial stage and after 1000 cycles, the EDS spectrum of the WO3@NPRGO composite, the high resolution of the C1s and O1s XPS spectra of the GO powder, the N2 adsorption-desorption plot and the pore size distribution plot, the CV curves of WO3/NPRGO and pure WO3 at different scan rates, a comparison table of parameters of different HER catalysts.
[21]
[22]
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Acknowledgements
[24]
This work was supported by the joint fund of the National Natural Science Foundation Committee of China Academy of Engineering Physics (NSAF) (U1630108) and the National Key Research and Development Program of China (2017YFA0402902). This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.
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[27]
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