Results and discussion

Chapter 2. Tungsten Carbide Catalysts as an Oxygen Reduction Reaction

2.3 Results and discussion

2.3.1 Synthesis and characterization of WC/CNT

WC/CNT catalyst was synthesized by thermal reaction at 900 ℃ of polyaniline, tungsten precursor and CNT mixture. This mixture was synthesized by in-situ polymerization of aniline monomer.

Polymerization was carried out for 24 hr at room temperature. This method makes uniform dispersion of tungsten precursor and CNT in the polymer matrix. I controlled the amount of CNT from 0 to 0.4 g to optimize the catalysts.

The XRD pattern for each WC/CNT catalysts are shown in Figure 2-1. The diffraction peaks at 31.51°, 35.64°, 48.29° correspond to the (001), (100) and (101) planes of hexagonal WC (JCPDS No. 00-051- 0939), while those at 36.37°, 42.89°, 62.03° correspond to WC1-x. Between them, only 0.1 and 0.2 g samples show that only pure WC phase was synthesized. The other samples show the mixed phase of WC and WC1-X.These results mean that the optimized ratio of tungsten precursor and carbon precursor is necessary to synthesize pure phase WC.[13]From the XRD results, mean particle size of WC was estimated by Scherrer equation (Table 2-1). The particle size of WC was 7-12 nm.

Table 2-2 shows the elemental analyses (C, W, N and O) results of the WC/CNT 0.2 g catalyst. The nitrogen content of this sample was 1.4 wt% and the carbon content was 61 wt%. This result shows that carbide is synthesized rather than the nitride form.

Transmission electron microscopy (TEM) images were taken to confirm support morphology (Figure 2-2). WC particles are well dispersed on the CNT and the particle size is ~11 nm which is in good agreement with the Scherrer equation results. As shown in figure 2-2 (b), the lattice spacing of 2.52 Å corresponds to the (100) facet of WC.

Figure 2-3 shows the N2 adsorption/desorption isothermal curves for WC/CNT 0.2 g catalyst. The measured surface area of the catalyst is 301.61 m²g^-1. Further analyze was carried out by Barret-Joyner- Halenda (BJH) formulas. BJH results reveals that some micropores exist on the surface of WC/CNT.

The X-ray photoelectron spectroscopy (XPS) results of C1s, W 4f and N 1s are shown in Figure 2-4.

XPS spectra are aligned to the C (sp²) peak at 284.8 eV. The C 1s spectra was fitted to the sp²C (284.8 eV), sp³C (285.5 eV), C-O -C and C-N (288.7 eV). W 4f spectrum consisted of W 4f5/2(34.2 eV) and W 4f7/2(31.9, 35.8 and 38.0 eV). 31.9 and 34.2 eV are assigned to WC. 35.8 and 38.0 eV of W 4f7/2are assigned to WO3.[14-15]The N 1s spectra were deconvoluted into three peaks, assigned to pyridinic N

N (403.4 eV, 14 at%).[16-17]From the XPS results, graphitic N (36.0 at%) shows the highest ratio.

2.3.2 Electrochemical characteristics and activity of WC/CNT

The electrocatalytic activity of the prepared WC/CNT electrocatalysts was measured by rotating ring- disk electrode (RDE) measurement. Linear sweep voltammetry (LSV) was evaluated in both acidic and alkaline condition. However, the intrinsic activity of WC catalysts is too low in acidic media. Thus, to investigate the activity difference, the as-prepared electrocatalysts were tested in O2-saturated 0.1 M KOH electrolyte at a scan rate of 10 mV s^-1and a rotation rate of 1600 rpm (Figure 2-5). WC/CNT 200 mg sample shows the best ORR activity with an onset potential of about 0.89 eV and more positive half wave potential (E1/2) than other catalysts. Thus, among these three catalysts, the WC/CNT 200 mg sample was selected as a support of platinum catalyst.

2.3.3 Synthesis and characterization of Pt/WC/CNT (XRD, TEM, XPS, ICP, Activity, stability) The Pt/WC/CNT catalysts were prepared by the polyol method. WC/CNT power was well mixed with ethylene glycol by ultrasonication for 15min. Thereafter, chloroplatinic acid (3 and 10 wt%) was added into the mixture. After constant ultrasonication for 2hr, the solution was refluxed at 120℃ for 3 hr. Finally, the powder sample was washed and dried at 70℃ in a vacuum oven.

The XRD patterns for each Pt/WC/CNT catalysts are shown in Figure 2-6. The diffraction peaks at 39.6° and 46.0° correspond to the (111) and (200) planes of cubic Pt. The pattern of 10 wt% Pt/WC/CNT shows the distinct peaks of Pt. However, in the XRD spectra of 3 wt% Pt/WC/CNT, Pt peaks are not identified due to the lower loading amount.

Inductively coupled plasma optical emission spectrometry (ICP-OES) was carried out to confirm the amount of deposited Pt on WC/CNT. From the ICP-OES results Table 2-3, the mass ratio of Pt was 2.6 and 10.5 wt% for 3 and 10 wt% Pt/WC/CNT respectively.

Figure 2-7 shows the TEM image of Pt/WC/CNT catalysts. The average particle size of 10 and 3 wt % catalysts are ~4 nm and ~2.4 nm, respectively. Energy-dispersive X-ray spectroscopy results of 3 wt%

Pt/WC/CNT are shown in Figure 2-7(c)-(d). As shown in data, Pt nanoparticles are well dispersed on the WC/CNT support. The Pt content are 9.8 and 2.5 wt% for 3wt% and 10 wt% Pt/WC/CNT, respectively.

Figure 2-8 shows the Pt 4f XPS spectra for the 10 wt% Pt/WC/CNT and 20 wt% Pt/C. XPS spectra are calibrated based on the C 1s peak at 284.8 eV as a reference value. The Pt 4f spectra shows spin- orbit splitting into a Pt 4f7/2 at 71.3 eV and a Pt 4f5/2 at 74.7 eV. The spectra of Pt/WC/CNT catalyst feature the same splitting. However, two peaks are shifted to higher binding energies by ~ 0.3 eV.

The positive shift of binding energy has two possibility of alloy formation or an increased electron density on Pt.[18]However, XRD and XPS results show no evidence of alloy formation. Therefore, binding energy shift is caused by electron transfer into Pt, which is related to the increased ORR activity and stability.

2.3.4 Electrochemical characteristics and activity of Pt/WC/CNT

Figure 2-9 (a) shows the ORR polarization curves of Pt/WC/CNT (3 and 10 wt% Pt) and Pt/C (20 wt%

Pt, Johnson-Matthey) in 0.1M HClO4at a scan rate of 10 mV s^-1and a rotation rate of 1600 rpm. The activities of the catalyst were evaluated using RDE measurement. The LSV curve demonstrates the high activities of Pt/WC/CNT catalysts in acidic solution. The half wave potential (E1/2) are 0.858 and 0.828 V for 10 and 3 wt% Pt/WC/CNT catalysts, respectively. 10 wt % sample showed the comparable activity with commercial 20 wt% Pt/C (E1/2: 0.869 V). The limiting current density of all catalysts is about 5.8 mA cm^-2. The Pt-mass activity at 0.9 V is 216.52 and 289.74 mA mgpt-1for 10 and 3 wt% Pt/WC/CNT catalysts, which are higher than that of Pt/C (215.90 mA mgpt-1). 3 wt% sample shows much higher mass activity than commercial Pt/C.

To further evaluate stability of the Pt/WC/CNT, durability was tested by 5000 cycling over the potential range of 0.6-1.0 V versus RHE at a scan rate of 200 mV s^-1. Figure 2-9 (b) shows the ORR activity after 5000 cycles stability test. After 5000 cycles test, 10 wt% Pt/WC/CNT shows almost similar ORR performance to Pt/C. As shown in figure 2-10, Pt/WC/CNT catalysts exhibit small E1/2change as 10 mV for 10 wt% catalysts and 3 wt% catalyst exhibits no difference between before and after 5000 cycles test. However, Pt/C shows the larger E1/2 difference as a 30 mV. This result indicates that Pt/WC/CNT catalysts possess excellent stability in the acidic medium. The high ORR activity and stability originates from strong electronic metal-support interactions.[19]

Figure 2-1. XRD patterns of WC/CNT (0.05, 0.1, 0.2 and 0.4 g) and WC/C.

20 25 30 35 40 45 50 55 60 65 70 75 80

In te n s it y ( a .u .)

Degree (2θ)

WC/C

WC/CNT 0.05g WC/CNT 0.1g WC/CNT 0.2g WC/CNT 0.4g

WC WC

_1-x

a b c

d e f

WC (100)

2.52 Å

Figure 2-3.The nitrogen adsorption/desorption isotherm curves and pore size distribution of WC/CNT 0.2 g.

0.0 0.2 0.4 0.6 0.8 1.0

0 200 400 600 800 1000

Volume Adsorbed / cm3 g-1

Relative Pressure /P/P₀

2 4 6 8 10 12 14 16 18 20

dV/dlog(D)

Pore size (nm)

28 30 32 34 36 38 40 42 44

In te n s it y ( a .u .)

Binding enrergy (eV)

W 4f

280 282 284 286 288 290 292 294 296

In te n si ty ( a. u .)

Binding energy (eV)

C 1s

W 4f_7/2 W 4f_5/2

W 4f_7/2

W 4f_7/2

Sp²C Sp³C

C-O-C&C-

394 396 398 400 402 404 406 408

In te n si ty ( a .u .)

Pyridinic N Graphitic N Pyrrolic N Oxdized N

398.5

400.6 401.0

403.4 N 1S

36%

14% 25%

25%

Graphitic N Pyridinic N Pyrrolic N Oxdized N

Figure 2-5. LSV curves of WC/CNT (100 and 200mg), WC/C and Pt/C in 0.1M KOH at a scan rate 10mV/s.

0.4 0.5 0.6 0.7 0.8 0.9 1.0 -6

-5 -4 -3 -2 -1 0 1

C u rr e n t d e n si ty ( m A c m

-2

)

Potential (V vs. RHE)

WC/CNT 200mg WC/CNT 100mg WC/C

Pt/C

0.1M KOH

Figure 2-6. XRD patterns of 3 and 10 wt% Pt/WC/CNT catalysts.

20 25 30 35 40 45 50 55 60 65 70 75 80 Degree (2θ)

10 wt % Pt/WC/CNT

In te n si ty ( a .u .)

3 wt % Pt/WC/CNT

WC Pt

Figure 2-7. TEM image of (a) 10 wt% and (b) 3 wt% Pt/WC/CNT. (c) HAADF-STEM and (d) elemental mapping images (C, Pt, W and N) of the 3 wt% Pt/WC/CNT.

d

a b c

d

c

Figure 2-8. XPS spectra of the Pt 4f of the 10 wt% Pt/WC/CNT and Pt/C catalysts.

66 68 70 72 74 76 78 80 82 Binding Energy (eV)

Pt/C Pt/WC/CNT

Pt 4f_7/2

Pt 4f_5/2

Δ0.3eV

Pt 4f

Figure 2-9 ORR polarization curves at 1600 rpm in O2-saturated 0.1 M HClO4 (a) before and (b) after 5000 cycles stability test.

a

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

-7 -6 -5 -4 -3 -2 -1 0 1

3 wt% Pt/WC/CNT 10 wt% Pt/WC/CNT

20 wt% Pt/C

0.1 M HClO

₄

C u rr e n t d e n s it y ( m A c m

-2

)

Potential (V vs. RHE)

b

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

-7 -6 -5 -4 -3 -2 -1 0 1

0.1 M HClO

₄

C u rr e n t d e n s it y ( m A c m

-2

)

Potential (V vs. RHE)

3 wt% Pt/WC/CNT 10 wt% Pt/WC/CNT

20 wt% Pt/C

after 5000 cycles

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -7

-6 -5 -4 -3 -2 -1 0 1

Current density (mAcm-2 )

Potential (V vs. RHE) before 5000cycles after 5000cycles

10 w t% Pt/WC/CNT

0.1 M HClO₄

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -7

-6 -5 -4 -3 -2 -1 0 1

0.1 M HClO₄ Current density (mAcm-2 )

Potential (V vs. RHE) before 5000cycles

after 5000cycles

3 wt% Pt/WC/CNT

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -7

-6 -5 -4 -3 -2 -1 0 1

Current density (mAcm-2 )

Potential (V vs. RHE) before 5000cycles after 5000cycles

20 wt% Pt/C

0.1M HClO₄ 30 mV 10 mV

Table 2-1. Tungsten carbide (WC) nanoparticle size of WC/CNT catalysts (0.05, 0.1, 0.2 and 0.4 g) calculated by the Scherrer equation

Catalyst WC particle Size

WC/CNT 0.05 g 12.3nm

WC/CNT 0.1 g 10.7nm

WC/CNT 0.2 g 12.4nm

WC/CNT 0.4 g 11.9nm

Table 2-2. Elemental analyses (C, W, N and O) of the WC/CNT 0.2 g catalyst

Sample Element [%]

C W N O

WC/CNT 0.2g 62.0 25.7 1.4 10.8

Table 2-3. ICP-OES results of the 3 and10 wt% Pt/WC/CNT catalysts

Catalyst Pt content (wt%)

3 wt% Pt/WC/CNT 2.6

10 wt% Pt/WC/CNT 10.5

2.4 Conclusion

In summary, WC/CNT nanocomposites were prepared by in-situ polymerization and pyrolysis. WC was uniformly dispersed with an average particle size of 11 nm. By the nitrogen atom in PANI precursor, nitrogen doped carbon nanotube was fabricated. Among the three types of the N species, such as pyridinic, pyrrolic and graphitic N, graphitic N showed the highest contents. The WC/CNT was further used as a support material for Pt nanoparticles. Lower concentration of Pt (3 and 10 wt%) was loaded on the WC/CNT support and they showed comparable activity with commercial 20 wt% Pt/C in acidic solution. However, the mass activity of 3 wt% Pt/WC/CNT is much higher than Pt/C. In addition, Pt/WC/CNT catalysts showed superior stability in comparison with Pt/C, over 5000 cycles cyclic voltammetry test. The high activity and stability of Pt/WC/CNT are attributed to increased electronic metal-support interaction. By using transition-metal carbide as supports, it can offer an opportunity for reduction of Pt loading and cost reduction.