Angstrom-scale vanadium carbide rods as Pt electrocatalyst support for efficient methanol oxidation reaction

Abstract

Angstrom-scale vanadium carbide rods combined with carbon (denoted as C–V₈C₇ (rods)) are synthesized through an ion-exchange route. The angstrom-scale C–V₈C₇ (rods) show better promotion effect on Pt than the nanoscale C–V₈C₇ (particles) towards methanol oxidation reaction (MOR). Furthermore, Pt particle loading on C–V₈C₇ (rods) (denoted as Pt/C–V₈C₇ (rods)) show much higher MOR activity and stability than commercial Pt/C electrocatalyst. The present method is imagined to be adopted to easily synthesize other angstrom-scale materials.

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1. Introduction

Transition metal carbides are efficient as promoters to improve the performances of noble metal based electrocatalysts due to synergistic effects.^1–4 By introducing carbides, the use level of noble metals can be greatly reduced to achieve the same or higher activity.^5–12 The literatures indicate that the interaction between carbide and noble metal increases with the decrease of carbide particle size.^13–15 The larger carbide particles have lower specific surface area, resulting in poor dispersion of loaded noble metal nanoparticles. It is imagined that the mixing of carbide and noble metal at the atomic level would lead to full interaction.

The carbides that have been most extensively studied as electrocatalyst promoter are tungsten carbide (WC)^16–23 and molybdenum carbides (MoC and Mo₂C).^24–28 The recent study indicated that vanadium carbide is more efficient²⁹ in promoting the activity of platinum (Pt) than WC and Mo₂C, which might make vanadium carbide a new research focus as catalyst promoter. However, the traditional synthesis methods of vanadium carbide (and also other carbides) lead to large particle sizes that are tens or hundreds of nanometers in diameter.^30,31 Recently, in situ exchange routes to vanadium carbide with the diameter down to 2–10 nm (ref. 32–35) have been reported, great promotion effects of V₈C₇ on activities of Pt or Pd have been found.

Herein, we report a synthesis of angstrom-scale V₈C₇ with rod-shape. To the best of our knowledge, this is the first attempt to prepare such small V₈C₇ material. It is also found that Pt being loaded on the synthesized angstrom-scale V₈C₇ rods show excellent activity and stability towards MOR in acidic media.

2. Experimental

2.1 Synthesis of C–V₈C₇ composites

Typically, polyacrylic weak-base anion-exchange resin (10 g, D201×1 resin, Hebi Power Resin Factory, China) was impregnated in 100 mL sodium orthovanadate (SOV, Na₃VO₄·12H₂O, A.R., Shanghai Ekear Biological Technology Co., Ltd., China) for 6 h, then the solid was separated and dried at 80 °C and heated at 800 °C for 1 h. After cooled down to room temperature, the sample was ground into powder to get the C–V₈C₇ composite. The samples with SOV concentrations of 0.02 and 0.002 mol L⁻¹ were denoted as C–V₈C₇ (particles) and C–V₈C₇ (rods) respectively.

2.2 Preparation of electrocatalysts

Pt particles were loaded on the C–V₈C₇ to form Pt/C–V₈C₇ electrocatalyst. Typically, C–V₈C₇ (60 mg) was added into a mixture of 20 mL glycol (A.R., Tianjin Fuyu Fine Chemicals Co., Ltd, China) and chloroplatinic acid (H₂PtCl₆, containing 40 mg Pt, A. R., Sinopharm Chemical Reagent Co., Ltd) and dispersed to form a uniform ink in ultrasonic bath for 30 min. The pH of the mixture was adjusted to 10 by adding 1 mol L⁻¹ NaOH/glycol solution. The sample was then put into a microwave oven (900 W) for heating at a 10 s on and 10 s off procedure for 12 times.³⁶ Afterwards, the mixture was washed with deionized water and dried in vacuum at 50 °C for 24 h to get the Pt/C–V₈C₇ electrocatalyst. The Pt particles being loaded on C–V₈C₇ (particles) and C–V₈C₇ (rods) were denoted as Pt/C–V₈C₇ (particles) and Pt/C–V₈C₇ (rods), respectively. The Pt contents in the electrocatalysts were 40 wt% stoichiometricly.

2.3 Preparation of electrodes

Pt/C–V₈C₇ (5 mg) or commercial Pt/C (4 mg, 47.6 wt% Pt, TKK, Japan) were dispersed in mixture of 0.05 mL 5 wt% Nafion suspension (DuPont, USA) and 1.95 mL ethanol in ultrasonic bath to form the electrocatalyst ink. The ink (0.005 mL) was deposited on surface of a glass carbon electrode (0.25 cm²) and dried at room temperature. The total Pt loadings were 0.02 mg cm⁻².

2.4 Electrochemical characterization

All electrochemical measurements were performed in a three-electrode cell on a potentiostat at 30 °C. A platinum foil (1.0 cm²) and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. All chemicals were of analytical grade and used as received.

2.5 Physical characterization

The morphologies of the synthesized materials were characterized by transmission electron microscopy (TEM, JOEP JEM-2010, JEOL Ltd.) operating at 200 kV. The structures of the samples were determined on an X-ray diffractometer (XRD, D/Max-IIIA, RigakuCo., Japan, CuK1, λ = 1.54056 Å radiation).

3. Results and discussion

Fig. 1a is the TEM image of the C–V₈C₇ (rods), which shows the rods with the width of around 1 nm and length of several nanometers that are uniformly dispersed on carbon matrix. Fig. 1b is the magnified TEM image of C–V₈C₇ (rods), inset shows a rod with the width of 1.5 nm, length of 5.6 nm and lattice of V₈C₇ (222) facet. Fig. 1c displays the corresponding width distribution of the rods. From randomly selected 100 rods, the average width of the rods was calculated as 0.8 nm, being angstrom scale. Fig. 1d is the TEM image of C–V₈C₇ (particles), which shows obvious agglomerations, due to the high concentration of SOV in the preparation step.

Fig. 1 (a) TEM and (b) magnified TEM images of C–V₈C₇ (rods); (c) rods' width distribution of V₈C₇ (rods); (d) TEM image of C–V₈C₇ (particles). Inset of (b) is the TEM image of a V₈C₇ rod.

Fig. 2 shows the XRD patterns of C–V₈C₇ (particles) and C–V₈C₇ (rods) (solid lines). The diffraction peaks at 2θ of 37.4°, 43.4°, 63.0° and 75.6° correspond to (222), (400), (440) and (622) facets of V₈C₇ crystal respectively, confirming that the rods and particles in Fig. 1 are V₈C₇. The peak intensity of the V₈C₇ weakened with the decreasing concentration of SOV, which related to the V₈C₇ particle size.

Fig. 2 XRD patterns of C–V₈C₇ (rods), C–V₈C₇ (particles), Pt/C–V₈C₇ (rods) and Pt/C–V₈C₇ (particles).

The formation of V₈C₇ rods should be due to long-chain structure of the D201×1 resin. A proper concentration of SOV would lead to discontinuous chain of V₈C₇, i.e. V₈C₇ rods. However, a high concentration of SOV would lead to conglomerated chain, i.e. V₈C₇ particles.

The XRD patterns of Pt/C–V₈C₇ (rods) and Pt/C–V₈C₇ (particles) are also shown in Fig. 2 (dotted lines). The peaks at 39.8°, 46.2° and 67.5° correspond to the (111), (200) and (220) facets of Pt crystal respectively. And the V₈C₇ peaks and the Pt peaks are overlapped with each other.

Fig. 3 shows the TEM images of Pt/C–V₈C₇ (rods) and Pt/C–V₈C₇ (particles). Due to that the C–V₈C₇ composites have different V₈C₇ content (leading to different density), Pt particles on C–V₈C₇ (rods) are sparse (see Fig. 3a), and on C–V₈C₇ (particles) seem to be abundant (see Fig. 3b). Inset of Fig. 3a shows the EDS pattern of Pt/C–V₈C₇ (rods) with C, Cu, V, Pt and Cr elements (Cu and Cr come from sample bracket). And Fig. 3c shows the V₈C₇ (400) and Pt (111) lattices in Pt/C–V₈C₇ (rods). They all prove the coexistence of V₈C₇ and Pt.

Fig. 3 TEM images of (a) Pt/C–V₈C₇ (rods) and (b) Pt/C–V₈C₇ (particles); (c) HRTEM image of Pt/C–V₈C₇ (rods). Inset of (a) is the corresponding EDS pattern.

The MOR performances of the Pt/C–V₈C₇ electrocatalysts were tested and shown in Fig. 4. Fig. 4a shows the cyclic voltammograms of MOR on Pt/C–V₈C₇ (rods), Pt/C–V₈C₇ (particles) and commercial Pt/C electrocatalysts. It can be seen that the onset potential of these electrocatalysts is in the following order: Pt/C–V₈C₇ (rods) (+0.25 V) < Pt/C–V₈C₇ (particles) (+0.27 V) < commercial Pt/C (+0.39 V). The mass current densities are in the following order: Pt/C–V₈C₇ (rods) (1605 mA mg_Pt⁻¹) > Pt/C–V₈C₇ (particles) (1377 mA mg_Pt⁻¹) > commercial Pt/C (775 mA mg_Pt⁻¹). And the corresponding mass current density ratio is 2.07 : 1.78 : 1. Significantly, the onset potential for methanol oxidation on Pt/C–V₈C₇ (rods) was negatively shifted for 140 mV compared with that on Pt/C. This is a great improvement since a direct methanol fuel cell gives only less than 0.5 V output at reasonable current density, leading to an expected 28% improvement in electric efficiency. Fig. 4b shows the electrochemical active surface areas (EASAs) of the electrodes. The EASA is in the following order: Pt/C–V₈C₇ (rods) (62.0 m² g⁻¹) > Pt/C–V₈C₇ (particles) (50.1 m² g⁻¹) > commercial Pt/C (44.7 m² g⁻¹). And the corresponding EASA ratio is 1.39 : 1.12 : 1. It can be seen that the mass current density ratio (2.07 : 1.78 : 1) is larger than the EASA ratio (1.39 : 1.12 : 1), that is to say, the EASA is not the sole factor to determine the catalytic performances. By comparing the ingredients of Pt/C–V₈C₇ and Pt/C, it can be deduced that the other factor to increase the catalytic activity of Pt/C–V₈C₇ should be the promotion effect of V₈C₇ on Pt. As to the higher activity of Pt/C–V₈C₇ (rods) than that of Pt/C–V₈C₇ (particles), it is related to the smaller size of V₈C₇ rods, which have higher specific surface area or more sites to interact with the Pt particles. Significantly, it is very energy-efficient that the Pt/C–V₈C₇ (rods) has only one-tenth V₈C₇ content that of the Pt/C–V₈C₇ (particles). More significantly, the enhancement of the current densities at lower potentials was much higher as shown in Fig. 4c, showing that the ratio of the current density on Pt/C–V₈C₇ (rods) over the current density on Pt/C exceeds 9 at 0.4 V.

Fig. 4 The cyclic voltammograms on Pt/C–V₈C₇ (rods), Pt/C–V₈C₇ (particles) and Pt/C electrocatalysts in (a) 0.5 mol L⁻¹ H₂SO₄/1.0 mol L⁻¹ methanol solution and (b) 0.5 mol L⁻¹ H₂SO₄ solution, with the scan rate of 50 mV s⁻¹ at 30 °C; (c) the plot of the ratio of the current density on Pt/C–V₈C₇ (rods) to that on Pt/C.

The stabilities of the Pt/C–V₈C₇ (rods) and Pt/C electrodes for MOR are shown in Fig. 5. The shadows in Fig. 5 are the cycling difference between the 1^st cycle and the 2000^th cycle. It is clear that the activity of the commercial Pt/C reduced 27.7% from 775 mA mg_Pt⁻¹ to 560 mA mg_Pt⁻¹ for MOR by comparing the peak current density. However, the activity of the Pt/C–V₈C₇ (rods) reduced 9.4% from 1605 mA mg_Pt⁻¹ to 1470 mA mg_Pt⁻¹. The results indicate that the Pt/C–V₈C₇ (rods) electrocatalyst is more stable than the commercial Pt/C.

Fig. 5 The cyclic voltammograms on Pt/C–V₈C₇ (rods) and Pt/C electrodes in (a) 0.5 mol L⁻¹ H₂SO₄/1.0 mol L⁻¹ methanol solution and (b) 0.5 mol L⁻¹ H₂SO₄ solution, with the scan rate of 50 mV s⁻¹ at 30 °C. The shadows show the difference between the 1^st cycle and the 2000^th cycle.

Literature reported that electron transfer (synergistic effect) exists between carbides and the loaded noble metals, which not only improves the oxidation of methanol and the intermediate poisons due to easy electron-transfer,¹ but also increases the stability due to increased linkage strength between carbides and noble metals.²⁴ The synergistic effect may account for the promotion effect of V₈C₇ on the excellent activity and stability of the Pt/C–V₈C₇ (rods) electrocatalyst.

4. Conclusions

Angstrom-scale V₈C₇ rods have been successfully synthesized through an ion-exchange route. The angstrom-scale C–V₈C₇ (rods) show higher promotion effect than nanoscale C–V₈C₇ (particles) on the performance of Pt electrocatalysts, although the former has only one-tenth V₈C₇ content that of the latter. Moreover, the Pt/C–V₈C₇ (rods) electrocatalyst shows much higher activity and stability than the commercial Pt/C for MOR.

Acknowledgements

This work was funded by National Natural Science Foundations of China (21306067), China Postdoctoral Science Foundation (2014T70481), Natural Science Foundation of Jiangsu (BK20130490, BK20140531), Industry High Technology Foundation of Jiangsu (BE2013090) and Zhenjiang Industry Supporting Plan (GY2013023).

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