Solution plasma synthesis process of tungsten carbide on N-doped carbon nanocomposite with enhanced catalytic ORR activity and durability

Abstract

In this study, the enhancement of ORR activity and durability by an N-doped carbon nanocomposite on tungsten carbide (WC) nanoparticles was reported. The nanocomposite of tungsten carbide on two different carbon matrices, pure carbon matrix (WC/C) and N-doped carbon matrix (WC/N–C), was at first prepared by a simple discharge process in the mixture of benzene/dodecane and pyrrole/dodecane. The nanoparticles of tungsten carbide were formed via the sputtering effect of tungsten electrodes during discharge. The results of TEM and XRD demonstrated that tungsten carbide nanoparticles with a mean size of 6 nm were evenly dispersed on both carbon matrices. The results of cyclic voltammetry measurements showed that both obtained metal/carbon matrices promoted a significant oxygen reduction reaction (ORR) in alkaline solution. The ORR potential of tungsten carbide/carbon matrix and nitrogen-doped carbon were −0.29 V and −0.36 V, respectively. The enhancement of ORR activity in WC/N–C was attributed to the combined catalytic effects of WC and N in the carbon matrix. Although the ORR activity of WC/N–C was still incomparable with commercial Pt/C, the durability of the catalyst was significant higher than that of Pt/C in a methanol environment. The catalyst did not exhibit an evident change of initial current after 4000 s. Therefore, the inexpensive N-doped WC/C nanocomposite might be a promising and highly durable catalytic material for cathodes in fuel cell applications.

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1. Introduction

Tungsten carbide (WC) is widely used in various industrial applications due to its outstanding properties such as high melting point, good chemical stability, and superior hardness and wear resistance. Normally, WC is utilized as a cutting tool owing to its good mechanical properties. Recent studies showed that, WC particles could be applied as catalysts in various chemical reactions such as hydrogen evolution, oxygen reduction, and methanol oxidation reactions.^1–4 In particular, WC has high resistance of carbon monoxide which offers a long-term durability.^6,7 However, the catalytic activity is strongly depending on the dispersion of catalysts. Generally, a uniform dispersion and high surface area of support material are essential for improving its catalytic potential.

Recently other methods have been investigated to increase the catalytic activity without applying metal catalyst. It was found that N-doped carbon material such as carbon nanotubes and graphene indicate higher catalytic activity.^8–10 The motivation of preparing N-doped carbon matrix is due to the electron accepting ability of nitrogen dopants. According to quantum mechanics calculations, the nitrogen dopants created positively charges on neighboring carbon atom in the matrix.^11,12 Accordingly, for improving catalytic potential and durability of cathode, the combination of WC with N-doped carbon matrix is considered as a potential electrode material of fuel cell application.

Meanwhile, different methods were applied for synthesizing WC/carbon (WC/C) particles, including vapor deposition (chemical vapor deposition and physical vapor deposition), pyrolysis, solution synthesis, as well as microwave heating.^5,13–16 However, these techniques are complicated and generally require multiple step process, high temperature, long processing time, and chemical agent. It is a challenge to find out a simple method to synthesize WC/C nanocomposite with good dispersion. Solution plasma is defined as one type of non-equilibrium and cold plasma in liquid. Solution plasma process (SPP) has been recently used for nanoparticle synthesis, organic-compound decomposition, and carbon related materials synthesis. These processes are achieved due to the role of activated species, in particular, free radicals in the plasma.^17–19 SPP has been applied for carbon particles synthesis^19,20 directly from benzene and metal nanoparticles as a catalyst by electrode sputtering process of the working electrode within short processing time at atmospheric pressure. In addition, synthesized metal nanoparticles can be uniformly dispersing on carbon particles because the synthesis and loading processes were occurring at the same time.

In this work, the application of SPP was further studied with the discharge in the new combination of organic solvent. A new, simple and effective method to synthesize WC/C and WC/N–C in the form of nanocomposite using one-step process by SPP was reported. Since SP is still an emerging technology, the chemical reaction and mechanism occurred during discharge were firstly discussed for better understanding. Then, the physical and chemical characteristics of the obtained WC/C and WC/N–C were investigated. Currently, WC has been utilized as the catalyst for oxygen reduction reaction (ORR) with platinum or other novel metals. In this study, nitrogen was used as an alternative material to improve the ORR activity in WC/C. Due to its crystal structure, WC/C as catalyst in the electrochemical reaction, especially ORR was evaluated. Moreover, the enhancement on catalytic activity by the present of nitrogen within the carbon matrix was discussed in detail. The durability of WC/N–C was compared to that of commercial Pt/C in methanol environment.

2. Experimental

2.1 Synthesis of WC/C and WC/N–C nanocomposite by solution plasma process

The schematic of SPP is demonstrated in Fig. 1. For WC/C catalyst, tungsten electrode and a mixed solution of benzene (C₆H₆) (kando Chemical) and dodecane (Kanto Chemical) in the ratio of (8 : 2) were applied, respectively, as the precursors of WC and carbon nanoparticles. In order to dope nitrogen atom into the carbon matrix, pyrrole (C₄H₅N), was chosen as the precursor instead. Thus, for WC/N–C, the nitrogen–carbon matrix was synthesized under SP in the mixed solution of pyrrole (Tokyo chemical industry) and dodecane (Kanto Chemical) in the ratio of 8 : 2. Since the main precursors, benzene (C₆H₆) and pyrrole (C₄H₅N), exhibit low electrical conductivity in nature, it is an undesirable aspect for a stable discharge conditions in SP. Dodacane was introduced to the solution in order to enhance the stability of discharge during the process.

Fig. 1 Schematic of solution plasma process setup.

A bipolar pulsed power generator (Kurita, Japan) was applied to generate the discharge. The pulse width and frequency of the power supply were fixed at 2 μs and 15 kHz, respectively. An optical fiber/PC computer-based optical spectrometer (Ocean Optics Inst. Co. 200–800 nm) recorded the time dependent optical emission spectrum through the beaker. Chemical reaction with radicals in the solution was carried out by gas chromatography-mass spectrometry (GC-MS, JMS-Q1050GC, JEOL).

As for the preparation of catalysts, the obtained suspension was filtered and dried in a vacuum oven at 60 °C. It was then heat-treated at 900 °C for 30 minutes under nitrogen atmosphere. Heat treatment was required to increasing the conductivity of carbon matrix and the conversion from metastable carbide (cubic WC_1−X) to carbon rich carbide (hexagonal WC).

2.2 Characterization

Field emission scanning electron microscopy (FESEM, JEOL-JSM7500FA at 10 kV) and transmission electron microscopy (TEM, JEOL-JEM2500SE at 200 kV) observations were conducted for study of the microstructure, shape and size of the synthesized WC/C nanocomposite. X-ray diffraction patterns (XRD) were collected by Rigaku Smart lab to examine the crystal structure of the particles. The XRD analysis was carried out using Cu Kα (1.54 Å) as a target over a scan range 2θ of 15–90° with 0.02° step size and 3° min⁻¹ scan speed. The chemical composition of hydrogen, nitrogen, and carbon was measured by elemental analysis (YANACO MT-6). Fourier transform infrared spectrometry (FTIR) measurements were carried out on IR Prestige-21 spectrometer (Shimadzu).

2.3 Electrode preparation and electrochemical characterization

The electrochemical properties of the WC/C and WC/N–C were evaluated. The electrochemical properties of the catalyst were measured by cycle voltammetry (CV) and linear sweep voltammetry (LSV) using a three-electrode cell (Hokuto Denko Inc. HZ5000) in 0.1 M KOH (Kanto chemical) electrolyte and O₂ gas was purged in the solution during measurement. The glassy carbon (GC) disk (3 mm in diameter), platinum, and Ag/AgCl (saturated KCl) electrode were used as working, counter, and reference electrodes, respectively. The sample on GC electrode was prepared by ultrasonicating the mixture of 5 mg WC/C powder, 0.5 ml ethanol, and 50 μl Nafion solution (Aldrich, 5 wt% Nafion) until the homogeneous suspension was formed. The obtained suspension was then spread on GC electrode and dried at room temperature prior to measurement. In this study, the electrochemical durability and methanol tolerance of the catalyst were measured by chronoamperometric responses in 0.1 M KOH electrolyte with and without methanol addition. It was evaluated at −0.4 V for 4000 s and 20 000 s, respectively.

3. Results and discussion

3.1 Solution plasma process

The morphology of solution plasma process and its fundamental has been published in previous paper by the co-authors.^19–21 Thus in this study, we focused on the specific properties of SP process in benzene/dodacane and pyrrole/dodacane. The optical emission spectrum in Fig. 2(a) and (b), demonstrate the active species presented during the plasma discharge in benzene/dodacane and pyrrole/dodacane, respectively.

Fig. 2 Optical emission spectrum obtained from liquid discharge zone in (a) benzene and (b) pyrrole as the carbon precursor, respectively.

The major active species in benzene/dodacane were C₂, H and W radicals. The formation of carbon particles was highly dependent on C₂ radicals^22–24 while the generation of WC were initiated by the reaction between W and C₂ radicals. In the case of pyrrole/dodacane, weak intensities of N₂ and CN were also detected besides C₂, H and W. The formation of N-doped carbon matrix might be dependent on the combination of CN and C₂ radiacls.^25,26 It is assume that these radicals generation can be derived from repetitive dissociation and recombination.²⁷ At the beginning of the discharge, the solutions were firstly gasified and excited. The resonance of their ring structure was being destroyed and became an open structure. Consequently, the radicals can be generated continuous by the plasma occurred in the solution. Carbon nanoparticle was synthesized by dissociation and recombination of benzene/dodacane and pyrrole/dodacane solutions during discharge processes. In order to have a better understanding of the formation mechanisms carbon and carbon–nitrogen matrices, the intermediate formed in the solution were examined by GC-MS according to the discharge time at 3, 40, 60 and 180 seconds. The intermediates were identified and their chemical formulas were listed in the ESI (S1 and S2†). The results showed that benzene and pyrrole were gasified and excited at the initial stage, and the intermediate products were generated by continuous reactions with radicals and solution. Further decomposition and recombination in the discharge resulted in gradual growth of intermediates and formed into more complicated network. The proposed synthesis mechanisms of WC/C and WC/N–C are shown respectively, in Fig. 3(a) and (b).

Fig. 3 Synthesis mechanism of (a) carbon nanoparticles and with (b) metal nanoparticles.

3.2 Characterizations of WC/carbon and WC/N-doped carbon

The microstructure of nanocomposites was revealed by the electron micrographs in Fig. 4. Fig. 4(a) and (b) show the FESEM images of aggregation nanoparticles in WC/C and WC/N–C. The particle size of aggregated carbon nanoparticles was estimated to be 27.04 ± 0.50 nm from TEM images, the inset of Fig. 4(a) and (b). The high magnification TEM in Fig. 4(c) and (d) showed that the spherical WC nanoparticles were uniformly dispersed in the carbon layer with the mean size of 6.87 ± 0.24 and 5.43 ± 0.17 nm in carbon and N-doped carbon matrix, respectively.

Fig. 4 Electron micrographs, (a) FESEM and TEM (inset) and (c) high magnification TEM of WC/C, (b) FESEM and TEM (inset) and (d) high magnification TEM of WC/N–C.

Fig. 5 displays the XRD patterns of the nanocomposites synthesized at different carbon precursors. The peaks of untreated samples (Fig. 5(a)) were detected at 36.70, 42.47, 61.85, 74.05, and 78.04° which identified as (111), (200), (220), (311), and (222) planes of WC_1−X. After heat treatment (Fig. 5(b)), the peak intensity of WC_1−X was reduced and the additional peaks were observed at 31.50, 35.54, 48.25, 64.02, and 75.13°, which is corresponding to (001), (100), (101), (110), and (111) planes of WC.^4,5,10 The small peak at 38.5° was also observed. It was suggested to be W (110) reflection and we believed that these particles were originated from occasional electrode erosion by locally concentrated sputtering on the surface of electrode. However, Based on the intensity of W (110) reflection, the amount of tungsten was relatively low and randomly dispersed. The crystallite size of WC was also calculated from the obtained results using Scherrer equation.

where D is mean size of the ordered crystallite, K is a dimensionless shape factor. Since the samples were highly ordered spherical particles, a typical value of 0.9 is suitable for K. λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM) in radians and θ is the Bragg angle.

Fig. 5 XRD patterns of WC/C nanocomposites, (a) before and (b) after heat treatment.

As shown in the ESI Table S3,† As-prepared WC nanoparticles were smaller than 5 nm, and the size increased to 5–10 nm after heat-treatment. These results agree with TEM results. The results indicated that the crystalline phase was transformed from WC_1−X to WC by heat treatment.

Among several phases of WC including tungsten rich carbide (hexagonal W₂C), metastable carbide (cubic WC_1−X), and carbon rich carbide (hexagonal WC), WC phase is well-known for the highest chemical stability.²⁸ Thus, the results suggested that WC/C nanocomposites with heat treatment might be better for electrocatalysis. The elemental analysis and FTIR measurement were investigated for further understanding the elemental composition and chemical structure of WC/C before and after heat treatment. Fig. 6 shows the elemental composition and FTIR spectra of WC/C. The elemental content in Fig. 6(a) found that the percentages of hydrogen in WC/C and WC/N–C before heat treatment were 3.65 and 4.76 wt%, respectively, and the hydrogen content decreased after heat treatment. The result indicated that the structure of carbon in WC/C was changed to highly crystalline structure. In WC/N–C, the content of nitrogen was 12.28 and 3.91 wt% before and after heat treatment. FTIR spectra in Fig. 6(b) obviously observe the absorbance peaks at 1400–1500 (C–C stretching) and 1640–1670 cm⁻¹ (C=C stretching) which identified to the carbon structure. In addition, the peak at 1640–1680 cm⁻¹, it was not only originated from C=C bond but might also generated by C=N bond at 1650–1690 cm⁻¹. The doped N-atoms into the carbon network, which shows a mixed mode of C=N and C=C, which was slightly shifted wavenumbers as compared to the C=C bond in WC/C.²⁹ As a result, the C=C bond with adjacent C=N bond in the matrix lead to a stronger signal compared to that of WC/C. The peaks at 2100–2200 and 1200–1400 cm⁻¹ corresponding to C≡N and C–N stretching were additionally detected with WC/N–C due to the incorporated nitrogen via the reaction of nitrogen-containing precursor.

Fig. 6 Elemental analysis (a) of as-prepared and heat-treated WC/C nanocomposites and (b) FTIR spectra of heat-treated WC/C.

3.3 Catalytic activity of WC/C and WC/N–C

Fig. 7 shows the cyclic voltammetry (CV) of WC/C and WC/N–C in O₂ and N₂-saturated. The oxygen reduction potential of WC/C was presented (black and blue lines) in the range of −0.29 to −0.40 V in the O₂-saturated, but not N₂-saturated. The heat-treated WC/C nanocomposites significantly improved not only ORR activity but also the output current density. This improvement was consistent with phase transformation of WC/C after heat treatment from XRD result. Additionally, the ORR potential of WC/N–C after heat treatment was −0.29 V, which was higher than that of WC/C at −0.34 V. It is noticeable that the existence of nitrogen in enhanced the ORR activity. Theoretically, the carbon materials have the possibilities of catalysts for ORR due to their unique electrical properties, in particular, flowing π electrons. According to quantum mechanics calculations, the nitrogen dopants as the electron acceptor created net positively charges on neighbouring carbon atom in the matrix.^11,12 Therefore, activated carbon atom can reduce oxygen molecules and the catalytic activity is improved. Thus, improved catalytic activity of WC/N–C might be originated from the synergetic effect between N-doped to WC nanoparticles. We further performed rotating disk electrode (RDE) voltammetry. As shown in Fig. 7(c), WC/N–C indicated higher current density and on-set potential than that of WC/C. However, in comparison of the commercial Pt-supported carbon (Pt/C) catalyst (Fig. 7(c)), the ORR activity of WC/N–C was still incompatible to that of Pt.

Fig. 7 Cyclic voltammetry of as-prepared (inset) and heat-treated catalysts in N₂ and O₂-saturated 0.1 M KOH at a scan rate of 20 mV s⁻¹ (a) WC/C and (b) WC/N–C. (c) Linear sweep voltammetry at a rotation rate of 1600 rpm and a scan rate of 10 mV s⁻¹.

Beside of current density, long-term durability of the catalyst is another important issue in direct methanol fuel cell (DMFC). Methanol crossover is occurring at the cathode to leak the methanol from the anode. During the methanol oxidation, CO can be adsorbing on the active site. Consequentially, it is depleting the efficiency of the catalyst. The electrocatalytic stability of the WC/N–C and Pt/C were measured at −0.4 V with addition of methanol. As shown in Fig. 8(a), the observed current for both electrodes have quickly reached its stable values. WC/N–C does not show evident change of initial currents after 4000 s. On the other hand, an obvious decrease in current was observed for the commercial Pt/C catalyst. The electrocataytic durability between WC/N–C and Pt/C were also compared. As shown in Fig. 8(b), relative current for both electrodes decreased over a period of time. The current of WC/N–C decreased about 13% of initial currents while the commercial Pt/C dropped about 53% after 20 000 s. From the results discussed above, we can conclude that WC/N–C nanocomposites presented a higher durability compared to commercial Pt/C catalyst.

Fig. 8 The current–time (i–t) chronoamperometric responses of WC/N–C and commercial Pt/C catalysts in O₂-saturated 0.1 M KOH at −0.4 V (a) with methanol and (b) without methanol addition.

4. Conclusions

WC/C nanocomposites were first synthesized by SPP. During the SPP process, active species including C₂, H and W radicals were observed in beneze/dodecane, where C₂, H, W and CN were detected in pyrrole/dodecane. It was confirmed that the N-doped carbon was successfully formed by SP. In TEM results, WC nanoparticles with mean size about 6 nm were dispersed evenly in carbon matrix with an average size of 27 nm. XRD results demonstrated that phase transformation occurred from WC_1−X to hexagonal WC after heat treatment at 900 °C. The ORR activity of WC nanocomposites increased from −0.36 V to −0.29 V with the present of Nitrogen within the nanostructure matrix. Although the catalytic activity of WC/N–C was lower than that of the commercial Pt/C, the WC/N–C indicated significant higher methanol tolerance and durability compared to the noble metal catalyst.

Notes and references

1. L. H. Bennett, J. R. Cuthill, A. J. McAlister, N. E. Erickson and R. E. Watson, Science, 1974, 184, 563–565.
2. R. Ganesan and J. S. Lee, Angew. Chem., Int. Ed., 2005, 44, 6557–6560.
3. P. N. Ross Jr and P. Stonehart, J. Catal., 1977, 48, 42–59.
4. H. Meng and P. K. Shen, J. Phys. Chem. B, 2005, 109, 22705–22709.
5. W. Liu, Y. Sondeda, M. Kodama, J. Yamashita and H. Hatori, Carbon, 2007, 45, 2759–2767.
6. V. Nikolova, I. Nikolova, P. Andreev, V. Najdenov and T. Vitanov, J. Appl. Electrochem., 2000, 30, 705–710.
7. K. Lee, A. Ishihara, S. Mitsushima, N. Kamiya and K. I. Ota, Electrochim. Acta, 2004, 49, 3479–3485.
8. K. Gong, F. Du, Z. Xia, M. Durstock and L. Dai, Science, 2009, 323, 760–764.
9. S. Wang, D. Yu and L. Dai, J. Am. Chem. Soc., 2011, 133, 5182–5185.
10. S. Chen, J. Bi, Y. Zhao, L. Yang, C. Zhang, Y. Ma, Q. Wu, X. Wang and Z. Hu, Adv. Mater, 2012, 24, 5593–5597.
11. X. L. Li, X. R. Wang, L. Zhang, S. W. Lee and H. J. Dai, Science, 2008, 319, 1229–1232.
12. Y. Shao, S. Zhang, M. H. Engelhard, G. Li, G. Shao, Y. Wang, J. Liu, I. A. Aksay and Y. Lin, J. Mater. Chem., 2010, 20, 7491–7496.
13. I. J. Luxmoore, I. M. Ross, A. G. Cullis, P. W. Fry, J. Orr, P. D. Buckle and J. H. Jefferson, Thin Solid Films, 2007, 515, 6791–6797.
14. I. Tallo, T. Thomberg, K. Kontturi, A. Janes and E. Lust, Carbon, 2011, 49, 4427–4433.
15. J. L. Lu, Z. H. Li, S. P. Jiang, P. K. Shen and L. Li, J. Power Sources, 2012, 202, 56–62.
16. Z. Yan, G. He, M. Cai, H. Meng and P. K. Shen, J. Power Sources, 2013, 242, 817–823.
17. P. Pootawang, N. Saito and O. Takai, Mater. Lett., 2011, 65, 1037–1040.
18. P. Pootawang and S. Y. Lee, Mater. Lett., 2012, 80, 1–4.
19. J. Kang, O. L. Li and N. Saito, Carbon, 2013, 60, 292–298.
20. O. L. Li, J. Kang, K. Urashima and N. Saito, Int. J. Environ. Sci. Technol., 2013, 7, 31.
21. O. Takai, Pure Appl. Chem., 2008, 80, 2003–2011.
22. A. N. Goyette, J. E. Lawler, L. W. Anderson, D. M. Gruen, T. G. McCauley and D. Zhou, Plasma Sources Sci. Technol., 1998, 7, 149–153.
23. J. Khachan, B. W. James and A. Marfoure, Appl. Phys. Lett., 2000, 77, 2973–2975.
24. P. Gopinath and J. Gore, Combust. Flame, 2007, 151, 542–550.
25. E. G. Wanga, Z. G. Guo, J. Ma, M. M. Zhou, Y. K. Pu, S. Liu, G. Y. Zhang and D. Y. Zhong, Carbon, 2003, 41, 1827–1831.
26. J. M. Stillahn and E. R. Fisher, J. Phys. Chem. C, 2009, 113, 1963–1971.
27. T. Yamamoto, Moon Planets, 1981, 24, 453–463.
28. M. B. Zellner and J. G. Chen, Catal. Today, 2005, 99, 299–307.
29. J. H. Kaufman, S. Metin and D. D. Saperstein, Phys. Rev. B: Condens. Matter Mater. Phys., 1989, 39, 13053–13060.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02380c

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