English

• 1. INTRODUCTION

  Hydrogen peroxide (H₂O₂) is one of the 100 most important chemicals in the world^[1]. Because of its strong oxidative properties and the only by-product is water^{[2, 3]}, H₂O₂ as a green oxidant is often used in almost all chemical industry fields, such as medicine, chemical synthesis, environmental protection, paper bleaching, disinfection, water treatment and so on^{[2, 4-7]}. So far, 95% of the total H₂O₂ productions annually comes from the anthraquinone process (AO), which contains a multi-step complex process such as hydrogenation over Pd catalyst and rapid oxidation by O₂ to generate H₂O₂^{[3, 8-10]}. However, such AO process needs complicated steps and several side reactions which cause unnecessary energy consumption^{[9, 11]}. Secondly, the extremely poor stability of high-concentration H₂O₂ poses safety hazard and high transportation costs^[12]. Therefore, the development of low-cost and low-concentration H₂O₂ synthesis methods has received increasing attention^[5]. One alternative process is the direct synthesis of H₂O₂ from H₂ and O₂ with a noble metal alloy catalyst^[13-15]. However, this process is not only inefficient but also has a risk of explosion due to the hydrogen-oxygen mixture^[12]. Another alternative is photocatalytic production of hydrogen peroxide from water and dioxygen^{[16, 17]}. Nevertheless, this method also has the disadvantages of slow kinetics and low efficiency^{[1, 2]}. Recently, electrosynthesis of H₂O₂ via two-electron O₂ reduction replacing the AO process has caused more and more attention because it is a green process that is efficient, energy-saving and safe^{[2, 18]}.

  Due to its low price and abundance, transition metal oxides (such as MnO₂, Co₃O₄, Fe₃O₄, V₂O₅, Ta₂O₅ and so no) have been widely used as an alternative to precious metals in electrocatalytic oxygen reduction reaction (ORR) catalysts^[19-24]. In general, metal oxides show limited ORR performance due to poor electronic conductivity and agglomeration issues^[25].

  Ta₂O₅ is currently used as an efficient ORR catalyst due to its high oxygen reduction onset potential (∼0.95 V vs. SHE) and excellent stability under corrosive environments^[26-28]. However, the limited electronic transport of Ta₂O₅ always reduces its kinetic reaction rate^[29]. Recently, Ta₂O₅ was found to be an efficient catalyst for two electrons ORR to generate H₂O₂^{[24, 26]}. Carbon supports (such as Vulcan XC72R (V), printex L6 (P)^{[22, 30]}, RGO^[31], carbon black^[32], etc.) show good conductivity and large surface areas have been proved to be helpful to improve the electronic transport and catalytic activity of metal oxides.

  Herein, a simple and effective strategy has been proposed to improve the catalytic activity and electronic conductivity of Ta₂O₅ for H₂O₂ electrosynthesis. The polyvinylidene fluoride (PVDF) has been carbonized to high surface area F doped porous carbon and coupled with Ta₂O₅ nanospheres by a simple calcination to generate Ta₂O₅/F−C composite. The Ta₂O₅/F−C composite catalyst presents an excellent performance for H₂O₂ generation with H₂O₂ selectivity and achieves more than 80% and negligible overpotential in 0.1 M KOH alkaline solution, which is greatly enhanced compared to the counterparts of F−C and Ta₂O₅. The enhanced performance was attributed to the synergistic effect between F−C and Ta₂O₅, which increases the conductivity of Ta₂O₅ and improves the selectivity of H₂O₂.

  2. EXPERIMENTAL

  2.1 Materials

  Potassium hydroxide (98.5%) and tantalum powder (99.9%) were bought from Aladdin company. Hydrofluoric acid (40%), nitric acid (65~68%), ammonium hydroxide (25~28%), N-methylpyrrolidone (NMP, 99%), monopotassium phosphate (99.5%) isopropanol (99.7%), glucose (AR) and dipotassium phosphate (99%) were gotten from Sinopharm Chemical Reagent Co. Ltd. PVDF (HSV900) was bought from HeiFei-kejing. Nafion solution (5%) is gotten from Shanghai Hesen Electric Co. Ltd.

  2.2 Synthesis of Ta₂O₅ sphere precursor

  The Ta₂O₅ sphere precursor was prepared by a co-precipitation process in a typical previous synthesis procedure^[33]. First, 11 mL HF, 22 mL HNO₃ and 77 mL deionized water were mixed and then 1 g tantalum powder was carefully poured into the above solution. Ta powder is completely dissolved after six hours. Finally, a certain amount of ammonium hydroxide was injected to the solution and a white precipitation was generated. The white precipitates were separated, washed with deionized water for three times and then dried in an oven at 60 ℃ overnight to obtain the final products of Ta₂O₅ nanospheres.

  2.3 Synthesis of Ta₂O₅/F−C, Ta₂O₅−800 and F−C

  Ta₂O₅/F−C was prepared using one-step process. 0.1 g PVDF and 0.1 g Ta₂O₅ nanospheres were milled with a mortar for 30 minutes, then a certain amount of NMP as a binder was added during the milling process. Such viscous colloids were dried overnight in an oven at 60 ℃ and then calcinated in a tube furnace at 800 ℃ for 2 h with a heating rate of 5 ℃/min in N₂. Then, they were cooled down to room temperature naturally. The black powders were generated and marked as Ta₂O₅/F−C. As a comparison, PVDF and Ta₂O₅ nanospheres with a mass ratio of 1:2 and 2:1 were also treated with the same process. The products were marked Ta₂O₅/F−0.5C and Ta₂O₅/F−2C, respectively. 0.1 g PVDF was calcinated in a tube furnace at 800 ℃ for 2 h at a heating rate of 5 ℃/min in N₂ and marked as Ta₂O₅-800. 0.1 g Ta₂O₅ nanosphere calcinated in a tube furnace at 800 ℃ for 2 h with a heating rate of 5 ℃/min in N₂ was marked as F−C.

  2.4 Structure characterization

  The transmission electron microscopy (TEM) images were obtained by using JEOL field-emission microscope (JEOL, Tokyo, Japan) operated at 200 kV. A field emission scanning electron microscope (SEM, FESEM, JSM6700F) equipped with an energy dispersive X-ray spectroscope (EDX, Oxford INCA) is used for acquired SEM images and collecting EDX data (Random five points). X-ray diffraction (XRD) patterns were captured by Miniflex 600 X-ray diffractometer under CuKα radiation. X-ray photoelectron spectroscopy (XPS) was recorded on an ESCALAB 250Xi X-ray photoelectron spectrometer (Thermo Fisher) using monochromatized AlKα radiation (15 kV, 10 mA). The specific surface areas were measured using the Brunauer-Emmett-Teller (BET) model.

  2.5 Electrochemical measurements

  All electrochemical measurements were performed by an electrochemical workstation (Autolab, PGSTAT302N). Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) experiments were performed using a single cell on a three-electrode system including a platinum gauze as the counter electrode, a saturated Ag/AgCl electrode as a reference electrode and a rotating ring-disk electrode (RRDE, the areas of ring and desk electrodes are 0.07212 and 0.19625 cm²) as working electrode. The H₂O₂ selectivity was acquired by RRDE experiments in oxygen saturated electrolyte (two electrolytes with pH~8 (phosphate buffered saline) and pH~13 (0.1 M KOH)) at a scan rate of 10 mV/s). 6.0 mg of each catalyst was dispersed in 1.49 mL of water, 0.57 mL of isopropanol, and 0.04 mL of a Nafion solution (5%). The catalyst ink was sonicated for one hour and 16.5 µL of catalyst ink drop-casted onto the desk electrode and then dried at room temperature. As a comparison, CV and LSV experiments in nitrogen saturated electrolytes were also performed. Electrochemical impedance spectroscopy (EIS) was carried out on a CHI760E working station. Stability test was obtained by a chronoamperometry measurement using RRED under 0.5V vs. RHE with a rotation speed of 1600 rpm for 10 hours. All potentials in this work were converted to reversible hydrogen (RHE) by the following equation (1), H₂O₂ selectivity was calculated by equation (2)^[34], and electronic transfer number was calculated by equation (3)^[5].

  ${E}_{\mathrm{R}\mathrm{H}\mathrm{E}}={E}_{\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}+0.197+0.0592\times \mathrm{p}\mathrm{H}$

  (1)

  ${\mathrm{H}}_{2}{\mathrm{O}}_{2}\left(\mathrm{\%}\right)=\frac{200\times {I}_{\mathrm{r}}}{\left(N\times {I}_{\mathrm{d}}\right)+{I}_{\mathrm{r}}}$

  (2)

  $n=\frac{4\times {I}_{\mathrm{d}}}{{I}_{\mathrm{d}}+{I}_{\mathrm{r}}/N}$

  (3)

  where I_d is the disk current, I_r the ring current, and N the collection efficiency (0.249).

  3. RESULTS AND DISCUSSION

  3.1 Structure discussion

  The Ta₂O₅/F−C catalyst was successfully synthesized by calcining PVDF and Ta₂O₅ spheres in N₂ at 800 ℃. Figs. 1a and S1a show the SEM images with different magnification of the Ta₂O₅ spheres which have a smooth surface and a particle size of tens to hundreds of nanometers. For comparison, the SEM image of Ta₂O₅-800 (Ta₂O₅ spheres were calcined at 800 ℃) shown in Fig. S1b suggests that the morphology of Ta₂O₅ sphere maintained after calcination. Figs. 1b and S1c suggest that the F doped C (F−C) displayed as a block shape. Compared with the Ta₂O₅ sphere precursor, the rough surfaces of Ta₂O₅/F−C composite shown in Figs. 1c and S1d illustrate that the F doped C is coupled with Ta₂O₅ sphere. The TEM image of Ta₂O₅ spheres (Fig. 2a) further proves the Ta₂O₅ spheres are dense of several smaller nanospheres, 30~100 nm in size. In Fig. 2b, the TEM structure of F doped C indicates that F−C has a porous layered structure. The TEM image of Ta₂O₅/F−C (Fig. 2c) further suggests that the Ta₂O₅ sphere was coupled with a layer of F doped carbon. Fig. S2a~2c shows the results of five spots EDX data of Ta₂O₅/F−0.5C, Ta₂O₅/F−C and Ta₂O₅/F−2C, illustrating that the composite contains F, O, Ta, and C elements and the atomic ratios of C and Ta are 8:1, 15:1, 31:1, respectively. This corresponds to the feed ratio of Ta₂O₅ spheres and PVDF. Fig. S2d reveals the EDX data of Ta₂O₅-800 draw near to 2:5 which can further prove the presence of Ta₂O₅ phase. The EDX result of F−C (Fig. S2e) shows that the atom ratio of F element is 1.5%.

  Figure 1

  

  Figure 1.  SEM images of (a) Ta₂O₅ spheres, (b) F doped carbon and (c) Ta₂O₅/F−C

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  Figure 2

  

  Figure 2.  TEM images of (a) Ta₂O₅ spheres, (b) F doped carbon and (c) Ta₂O₅/F−C

  DownLoad: Full-Size Img PowerPoint

  The XRD (X-ray diffraction) patterns of Ta₂O₅ (Fig. S3) before calcination show two wide diffraction peaks, suggesting that Ta₂O₅ spheres synthesized at room temperature are amorphous phase^[35]. The XRD pattern of F−C (in Fig. S3) shows a wide peak of graphite carbon. The X-ray diffraction (XRD) pattern of Ta₂O₅ and Ta₂O₅/F−C (Fig. 3) mainly demonstrate the orthorhombic pattern of Ta₂O₅ (ICSD No. 25-0922).

  Figure 3

  

  Figure 3.  XRD patterns of Ta₂O₅/F−C and Ta₂O₅-800

  DownLoad: Full-Size Img PowerPoint

  The high resolution XPS spectrum for C1s is shown in Fig. 4a, which reveals that the fitting peaks at 284.5, 286.2, 288.2 and 289.8 eV are corresponding to C−C, C−O, C−F and O−C=O chemical bonds, respectively^[36]. It confirmed that F atoms are incorporated into C matrix. The fitting peaks of O 1s spectrum (Fig. 4b) at 531.4, 530.1 and 533.1 eV are corresponding to C=O, O−Ta and O−C=O bonds, which demonstrates that O atoms are combined with C and Ta atoms^{[37, 38]}. The binding energy of the F 1s peak is 688.9 eV, which corresponds to F−C bond, as shown in Fig. 4c. This further indicates that fluorine is successfully doped into carbon after PVDF carbonization^{[36, 37]}. The Ta 4f7/2 and Ta 4f5/2 peaks in Fig. 4d are at 26.8 and 28.7 eV, which indicates the presence of Ta₂O₅^[33].

  Figure 4

  

  Figure 4.  X-ray photoelectron spectra of (a) C 1s, (b) O 1s, (c) F 1s and (d) Ta 4f for Ta₂O₅/F−C, (e) Nitrogen adsorption-desorption isotherms for Ta₂O₅/F−C and (f) BJH desorption pore-size distribution of Ta₂O₅/F−C

  DownLoad: Full-Size Img PowerPoint

  The nitrogen adsorption-desorption isotherms of Ta₂O₅/F−C (Fig. 4e) demonstrate extremely rapid nitrogen absorption at very low relative pressures, indicating the typical microporosity property of Ta₂O₅/F−C^[39]. In Fig. 4f, the BJH desorption pore-size distribution suggests that the pore width distribution is 1.16 nm. The BET surface area of Ta₂O₅/F−C is 152.6 m²/g and the micropore area is 140 m²/g with a micropore volume of 0.075 cm³/g. However, Ta₂O₅-800 has no conspicuous hysteresis loop at high relative pressure compared to Ta₂O₅/F−C, as shown in Fig. S4. Its BET surface area is only 8 m²/g and the pore volume is insignificant. As a result, it can be deduced that the increased BET surface area and pore volume of Ta₂O₅/F−C mainly comes from piled pore and porous F−C. The increased BET surface area not only exposes more catalytically active sites, but also improves mass transfer. This is more conducive to the production of hydrogen peroxide^[40].

  3.2 Electrochemical discussion

  The electrochemical performance measurement of the catalysts was based on rotating ring-disk electrode (RRDE) with a rotating speed of 1600 rpm in a three-electrode system. To better evaluate the ORR activity, the ring electrode is made of Pt to oxidize H₂O₂ under a ring potential^[41]. Fig. 5a and 5b reveals the cyclic voltammetry (CV) curves and the linear sweep voltammetry (LSV) profiles of the Ta₂O₅/F−C composites in N₂ and O₂ saturated alkaline electrolytes (0.1 M KOH solution). A remarkable increase of both the ring and disk currents in the presence of O₂ respect to the N₂ background was observed. The CV curve shows a flat slope in the N₂ saturated solution and an obvious reduction peak under O₂ saturated electrolyte, which indicates the reliability of oxygen reduction performance of Ta₂O₅/F−C.

  Figure 5

  

  Figure 5.  (a) CV and (b) LSV curves of Ta₂O₅/F−C in N₂ and O₂ saturated 0.1 M KOH solution

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  Fig. 6a shows the LSV curves of Ta₂O₅/F−C, Ta₂O₅-800 and F−C in 0.1 M KOH electrolyte. Obviously, the ring current of Ta₂O₅/F−C is the largest among all the three catalysts. Although Ta₂O₅-800 has a favorable H₂O₂ selectivity, the oxygen reduction reactivity of Ta₂O₅-800 is poor. As shown in Fig. 6a and 6b, H₂O₂ selectivity of Ta₂O₅-800 can reach nearly 70% but the limiting current density is just less than 1 mA/cm² and the onset potential at 0.64 V vs. RHE (defined as the potential at which a current density of 0.1 mA/cm² is achieved). The poor activity of Ta₂O₅-800 is ascribed to the lower electronic conductivity of Ta₂O₅^[25]. As shown in Fig. 6b, the F−C has a poor H₂O₂ selectivity about 55%, which is far less than the Ta₂O₅/F−C (80.8%). The resistance measurement further confirms the enhanced conductivity of the Ta₂O₅/F−C electrode. The high frequency range of EIS (Fig. S5) illustrated charge transfer ability for Ta₂O₅/F−C and Ta₂O₅-800. EIS in high frequency region (100 kHz to 10 Hz) was fitted with an equivalent circuit diagram to get charge transfer resistance. Obviously, the charge transfer resistance of Ta₂O₅-800 is two orders of magnitude larger than Ta₂O₅/F−C, which indicates the introduction of F−C improved the conductivity of Ta₂O₅ successfully.

  Figure 6

  

  Figure 6.  (a) LSV curves of Ta₂O₅/F−C, Ta₂O₅-800 and F−C (scan rate of 10 mV/s with a rotation speed of 1600 rpm). (b) H₂O₂ selectivity of Ta₂O₅/F−C, Ta₂O₅-800 and F−C in O₂ saturated 0.1 M KOH electrolyte. (c) Tafel slopes for corresponding catalysts in O₂ saturated 0.1 M KOH electrolyte. (d) Stability test for Ta₂O₅/F−C. All plots were subtracted from the nitrogen background current and the pH of test condition is 13

  DownLoad: Full-Size Img PowerPoint

  The LSV profiles of Ta₂O₅/F−0.5C, Ta₂O₅/F−C, and Ta₂O₅/F−2C that contain different ratios of F are shown in Fig. S6a. The Ta₂O₅/F−C displayed the highest disk current and ring current among the three catalysts, demonstrating that Ta₂O₅/F−C shows an excellent activity for two-electron ORR that is better than Ta₂O₅/F−C−0.5 and Ta₂O₅/F−C−2. The Ta₂O₅/F−C exhibits a high selectivity of H₂O₂ of 80.8% at 0.49 V vs. RHE, and the selectivity is close to 80% within a wide potential range of 0.25~0.55 V. This catalyst also displays an excellent oxygen reduction activity with the onset potential of 0.78 V vs. RHE, which shows almost no overpotential. These efficient activity of Ta₂O₅/F−C surpass those of most other carbon support metal oxides in alkaline media (Table S1). The different selectivity shown in Fig. S6b can illustrate the feed ratio of C and Ta also play a nonnegligible role in the activity of such composites and the feed ratio of 1:1 is optimal. In Fig. 6a, F−C shows the largest disk current and the most positive onset potential among all catalysts, but the ring current is lower than Ta₂O₅/F−C. This unusual performance can be explained that the F−C tends to take four-electron ORR pathway to generate H₂O. The H₂O₂ selectivity of Ta₂O₅/F−C is better than that of F−C and Ta₂O₅, which demonstrates that F−C and Ta₂O₅ propose a synergistic catalytic effect. Both the ring and disk currents were also obtained by RRDE in a neutral condition (0.1 M PBS solution with PH = 8) for Ta₂O₅/F−C and F−C (Fig. S7a). In Fig. S7b, Ta₂O₅/F−C shows a considerable performance for H₂O₂ production with the selectivity of 68.5%, compared with the selectivity of 43.6% for F−C. Figs. 6c and S8 are the Tafel slopes for all the catalysts in different electrolytes. Obviously, Ta₂O₅/F−C has the lowest Tafel slope in alkaline electrolytes, so it shows the best kinetic rate than other catalysts. To evaluate its stability, a constant potential hold stability test at 0.5 V vs. RHE was performed by rotating ring-disk electrode at 1600 rpm. Fig. 6d shows that the ring and disk current density is rather stable during 10 h electrolysis for Ta₂O₅/F−C. Fig. S9 shows the H₂O₂ selectivity and electron transfer number (ETN) of the catalysts after 10 h electrolysis. The H₂O₂ selectivity kept around 80% and ETN is around 2.3 after electrolysis for 10 h.

  4. CONCLUSION

  In summary, a synergistic effect is proposed to enhance the performance of Ta₂O₅/F−C composite catalysts for efficient H₂O₂ electrosynthesis. The electrical conductivity of Ta₂O₅ was significantly increased by forming Ta₂O₅/F−C interface. The Ta₂O₅/F−C catalyst shows an excellent selectivity more than 80% and long-term stability for H₂O₂ production. The greatly enhanced performance of Ta₂O₅/F−C compared to F−C (selectivity of 59%) Ta₂O₅-800 (current density of 0.85 mA/cm²) counterparts were ascribed to the synergistic effect between F−C and Ta₂O₅. This work could provide a new way to rational design of carbon-supported metal oxide catalysts for H₂O₂ electrosynthesis.

  
  
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  Figure 1  SEM images of (a) Ta₂O₅ spheres, (b) F doped carbon and (c) Ta₂O₅/F−C

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  Figure 2  TEM images of (a) Ta₂O₅ spheres, (b) F doped carbon and (c) Ta₂O₅/F−C

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  Figure 3  XRD patterns of Ta₂O₅/F−C and Ta₂O₅-800

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  Figure 4  X-ray photoelectron spectra of (a) C 1s, (b) O 1s, (c) F 1s and (d) Ta 4f for Ta₂O₅/F−C, (e) Nitrogen adsorption-desorption isotherms for Ta₂O₅/F−C and (f) BJH desorption pore-size distribution of Ta₂O₅/F−C

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  Figure 5  (a) CV and (b) LSV curves of Ta₂O₅/F−C in N₂ and O₂ saturated 0.1 M KOH solution

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  Figure 6  (a) LSV curves of Ta₂O₅/F−C, Ta₂O₅-800 and F−C (scan rate of 10 mV/s with a rotation speed of 1600 rpm). (b) H₂O₂ selectivity of Ta₂O₅/F−C, Ta₂O₅-800 and F−C in O₂ saturated 0.1 M KOH electrolyte. (c) Tafel slopes for corresponding catalysts in O₂ saturated 0.1 M KOH electrolyte. (d) Stability test for Ta₂O₅/F−C. All plots were subtracted from the nitrogen background current and the pH of test condition is 13

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