English

• 1. INTRODUCTION

  The increasing energy demand and the increasing environmental pollution problem caused by fossil fuels have attracted great research attention, which is an urgent need for the design and development of sustainable energy storage and conversion equipment. Hydrogen peroxide (H₂O₂)^[1-6], as a valuable chemical, is a potential energy carrier and an environmentally friendly oxidant for various chemical indus-tries and environmental rectification, including the paper and pulp, the textile, the electronic industries, the waste treatment, the chemical oxidation and others. At present, the process of industrialization scale synthesis of hydrogen peroxide is very mature, which is a kind of high energy consumption of anthraquinone uninterrupted continuous oxidation/reduction process, produces high concentration organic wastewater and causes environmental pollution, so we urgently need a distributed approach to the production of hydrogen peroxide in a timely manner. An attractive and selective approach to H₂O₂ production through the two-electron oxygen reduction reaction (ORR)^[7-12] in a fuel cell setup has attracted much attention on account of the advantages afforded by such electrochemical processes, including low energy efficiency and cost-effectiveness.

  As a rule, electrochemical ORR in alkaline electrolyte generally needs to execute one of the following two reactions: the 4e^– process to convert O₂ to H₂O (Eq. (1)) or the 2e^– process for the reduction of O₂ into HO₂⁻ (Eq. (2)).

  $ \text{O}_{2} + 2\text{H}_{2}\text{O} + 4e^{−} → 4\text{OH}^{−} $

  $ E^{0} = 1.23 \text{V versus RHE} $

  (1)

  $ \text{O}_{2} + \text{H}_{2}\text{O} + 2e^{−} → \text{HO}_{{2}^{-}} + \text{OH}^{−} $

  $ E^{0} = 0.76 \text{V versus RHE} $

  (2)

  E⁰ is the standard equilibrium potential, calculated by the free energy of this reaction, and RHE is the reversible hydrogen electrode. The reaction shown in Eq. (1) is very important for fuel cell, and the chemical energy in the gas fuel is converted into electrical energy through an electrochemical reaction, whereas the environment-friendly and versatile HO_2^- (generate H₂O₂ in acidic media or over-protonated anions, or HO_2^- in alkaline media) form in the reaction shown as in Eq. (2).

  It should be pointed out that the synthesis of hydrogen peroxide depends on how to find an efficient catalyst to produce hydrogen peroxide in a two-electron transfer way, instead of breaking the O–O bond^[13-17] to generate water in a four-electron reaction. At present, a daunting challenge is to find a suitable catalyst to depress the competing reaction of four electrons to produce water. To predict and screen the candidates, a volcano relationship diagram with the *OOH bond energy as a descriptor was regarded as a powerful tool^[18]. With this method, some precious metal alloy materials have been consi-dered as the suitable catalysts for deterring the 4-electron ORR process. Nevertheless, there remains a great space for mining non-noble metal electrocatalysts to improve catalytic perfor-mance for cutting down the cost.

  The focus of our research is to find a high effective and selective catalyst in Eq. (2). A variety of materials have been probed as the feasible electrocatalysts for the production of H₂O₂ from O₂ reduction, including pure metals, metal alloys, carbon materials and metal oxides. At present, graphene-based material, having a large specific surface area and high electrical conductivity, is widely used as catalysts on the variously electrocatalytic reactions. For example, Yang group^[19] from the University of California used the graphene oxide of thermal reduction as a non-noble metal ORR catalyst to synthesize hydrogen peroxide. In addition, Yu group^[20] from the University of Science and Technology of China has synthesized oxygen-deficient titanium dioxide materials as an effective 2-electron ORR^{[21, 22]} electrocatalyst, but the low conductivity of titanium dioxide limits the further enhancement of the activity and selectivity.

  Strong metal-support interaction (SMSI) of supported metal catalysts is an important notion to depict the effect of metal-support interaction about structures and catalytic performances of supported metal oxide particles^[23-27]. The strong metal interacts with the support will change the electronic structure of electrocatalyst, which further gives the opportunity to enhance the activity and selectivity of electrocatalytic production of hydrogen peroxide.

  Based on this background, we employ a strong interaction strategy between titanium dioxide nanosheets and multilayer reduced graphene nanosheets to improve the activity and selectivity of 2e^- ORR in alkaline medium. Thanks to the synergistic effect between titanium dioxide and reduced graphene oxide^[28-36], the composite electrocatalyst can effectively inhibit the decomposition of hydrogen peroxide and hinder the 4e^- ORR to produce H₂O, and the selectivity of hydrogen peroxide also reaches up to 90%.

  2. EXPERIMENTAL

  2.1 Synthesis of the materials

  We synthesize TiO₂/RGO composite nanomaterials through a combination of hydrothermal methods and calcination. 0.4 mmol TiCl₄ was added to a mixture of 16 mL ethanol and 16 mL ethylene glycol. 1 mL (12 mg/mL) of (graphene oxide) GO solution and 1 mL of deionized water were added, followed by ultrasonic and stirring for 30 minutes. Finally, the solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave and followed by hydrothermal reaction at 130 ℃ for 24 hours. After reaction, the products were centrifugally washed with water and ethanol and then freeze-dried overnight. The dried sample is placed in a tube furnace filled with 10% hydrogen and argon mixture and calcined at 600 ℃ for 2 hours. TiO₂ is prepared by a similar procedure without the addition of GO, and RGO is prepared by a similar procedure without adding TiO₂.

  2.2 Characterization

  X-ray powder diffraction patterns were collected on an XRD instrument (Miniflex6000, Rigaku) using a CuKα (λ = 1.5417 Å) radiation source, which was operated at a scan rate of 3 °⋅min^–1 in the 2θ range from 10° to 80°. TEM and HRTEM characterizations were operated on a FEI F20 microscope with an acceleration voltage of 200 kV. XPS analysis was conducted by ESCALAB 250Xi XPS (spectrometer with AlKα source), and XPS data were obtained by correcting the binding energies of C1s to 284.6 eV.

  2.3 Electrochemical measurements

  The electrochemical measurements of catalyst were studied by using a CHI 770E electrochemistry workstation with a three-electrode system. ORR electrochemical tests were carried out in 0.1 M KOH electrolyte. The glassy carbon electrode supporting with the catalyst material was used as the working electrode, graphite rod and Hg/HgO electrodes were used as counter and reference electrodes. Typically, 2 mg catalyst and 10 μL Nafion solution (10 wt%) were dissolved in 1 mL mixture of ethanol and water, which was supersonic for one hour to form a uniform catalyst ink. Then, working electrode was prepared by coating the glassy carbon electrode with 2.5 μL catalyst ink. The linear sweep voltammetry (LSV) and cyclic voltammetry measurements (CV) of ORR were conducted at scan rates of 5 and 50 mV/s, respectively. The H₂O₂ selectivity (H₂O₂%) and the electron transfer number (n) were calculated by equations (3) and (4). Cyclic voltammetry was performed in the Faraday region of 1.0~1.1 V to obtain the electrochemically active area, and the scan was increased from 50 to 90 mV/s. The IR-corrected LSV with resistance compensation of 70% was obtained. All potentials were converted to a reversible hydrogen electrode. In the following equations, I_D is the disk electrode current and I_R is the ring electrode current, and N is the ring electrode collection efficiency, which is determined using the RRDE electrode and usually has a value of 0.35 in our test.

  $ {\mathrm{H}}_{2}{\mathrm{O}}_{2}\mathrm{\%}=200\times \frac{{I}_{\mathrm{R}}/N}{{{I}_{\mathrm{D}}+I}_{\mathrm{R}}/N} $

  (3)

  $ n=4\times \frac{{I}_{\mathrm{D}}}{{{I}_{\mathrm{D}}+I}_{\mathrm{R}}/N} $

  (4)

  3. RESULTS AND DISCUSSION

  We herein report the fabrication of TiO₂ supported on RGO by the hydrothermal and calcination method. As described in Fig. 1, the desired TiO₂/RGO material was prepared by using the mixture solution of metal chloride, alcohol and GO as precursors, and the subsequent hydrothermal reaction of mixture solution results in bottom-up self-assembly of the crystallization of TiO₂ on the surface of reduction graphene oxide. Moreover, for the preparation of a series of contrasted samples, the products of TiO₂ were formed in a similar method under the absence of GO, and RGO was obtained without adding TiCl₄.

  Figure 1

  

  Figure 1.  Schematic illustration of the preparation of TiO₂/RGO

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  X-ray diffraction (XRD) (Fig. 2) is carried out to identify the structural features of TiO₂/RGO, TiO₂ and RGO. The TiO₂ and TiO₂/RGO samples mainly present the sharply characteristic peaks of (101), (004), (200), (211), (204), (200) and (224) faces, which are consistent with the standard XRD pattern of anatase TiO₂ (PDF# 72-1764).

  Figure 2

  

  Figure 2.  X-ray diffraction patterns of TiO₂, RGO and TiO₂/RGO

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  The morphology and size of the as-synthesized materials were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images of TiO₂, RGO and TiO₂/RGO (Fig. 3a) show that the calcination will cause serious agglomeration for pure titanium dioxide, while the addition of GO can inhibit the agglomeration of TiO₂. In addition, the reduced graphene exhibits an obvious layered structure. The TEM images of TiO₂/RGO (Fig. 3b) show that the TiO₂ nanomaterials were loaded on RGO, and the layer spacing of titanium dioxide was determined to be 0.372 and 0.338 nm, corresponding to (200) and (204) crystal faces, respectively. As can be seen from Fig. 3c, the STEM-HAADF and corresponding elemental mapping Ti, O and C elements are uniformly distributed across the TiO₂/RGO composites. Graphene-based materials with good electrical conductivity can improve the electrical conductivity of the composite material, so the 2-dimensional hybrid structure has the potential to improve the electrochemical performance of TiO₂/RGO.

  Figure 3

  

  Figure 3.  (a) SEM images of TiO₂, TiO₂/RGO, RGO; (b) TEM images of TiO₂, TiO₂/RGO, RGO; (c) HAADF-STEM image of TiO₂/RGO and the corresponding STEM-EDX elemental mapping images of the as-prepared TiO₂/RGO sample for oxygen element, carbon element and titanium element

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  The valence state information and composition were ensured by X-ray photoelectron spectroscopy (XPS). The elementary composition of oxygen (O), carbon (C), and titanium (Ti) was firstly identified by the typical wide-scan XPS survey spectrum for TiO₂/RGO, TiO₂ and RGO samples in Fig. 4. Fig. 4b displays the high resolution X-ray photoelectron spectroscopy (XPS) of C atoms in TiO₂/RGO and RGO respectively, which consists of sp²-hybridized graphitic carbon (284.6 eV), C–OH (285.5 eV), C–O (286.5 eV) and C=O (289.1 eV), indicating a similar electronic structure for TiO₂/RGO and RGO. Fig. 4c shows the electronic structure of O atoms in TiO₂/RGO consisting of Ti–O (530.6 eV), C–O (532.5 eV) and O–H (533.5 eV). Fig. 4d reveals a small amount of Ti³⁺ atoms (458.2 eV) in TiO₂/RGO, indicating the formation of oxygen defect after annealing under hydrogen and argon mixture gas.

  Figure 4

  

  Figure 4.  (a) XPS spectra in the survey region of TiO₂/RGO, (b) XPS spectra in the C 1s region of TiO₂/RGO, (c) XPS spectra in the O 1s region of TiO₂/RGO, (d) XPS spectra in the Ti 2p region of TiO₂/RGO

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  The electrocatalytic activity of electrochemical oxygen reduction reaction (ORR) was measured by Rotating Ring Disk electrode (RRDE) system. Fig. 5a firstly confirms the ORR activity for TiO₂, RGO and TiO₂/RGO by the obvious oxygen reduction peak appearing at 0.6, 0.66 and 0.58 V, respectively. The linear scan voltammetry (LSV) curve (Fig. 5b) shows different ORR performance for three electro-catalysts. TiO₂/RGO exhibits a higher onset potential of 0.72 V vs RHE at –0.1 mA/cm² and the bigger disk current density (J_disk) and ring current (I_ring) than those of TiO₂ and RGO catalysts. Through the cyclic voltammetry test in the non-Faraday region, the specific of these three materials are presented in Fig. 5c. TiO₂/RGO shows the largest electro-chemically active area among these catalysts. The hydrogen peroxide selectivity of TiO₂/RGO is up to 90% over a wide range of potential at 0.2~0.6 V (Fig. 5d). Accordingly, the electron transfer number (n) less than 2.2 further confirms an almost complete 2-electron pathway. Therefore, these results prove that the adverse O–O bond cracking on 4-electron ORR to produce water can be effectively inhibited by the synergistic effect between the titanium dioxide with the oxygen defect and graphene with high conductivity, as well as the large specific surface of TiO₂/RGO. According to literature research, it is found that TiO₂/RGO has the best electrocatalytic performance in 0.1M KOH, as shown in Table 1^[37-41].

  Figure 5

  

  Figure 5.  (a) Cyclic voltammetry curves of TiO₂, RGO and TiO₂/RGO in 0.1 MKOH, 50 mV/s (b) Linear scan voltammetry curves, in 0.1 M KOH, rotation rate: 1600 rpm, scan rate: 5 mV/s, (c) Electrochemical active area normalization, (d) Selectivity of hydrogen peroxide produced by oxygen reduction

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  Table 1

  

  Table 1.  Electrocatalytic Performance of Different Catalyst Materials

  DownLoad: CSV

  Catalyst	Electrolyte	Selectivity	Reference
  Co–N–C	0.1M KOH	60% (at 0.5 V)	37
  Co1–NG(O)	0.1M KOH	80% (at 0.5 V)	38
  Carbon	0.1M KOH	60% (at 0.5 V)	39
  Ni3B	0.1M KOH	85% (at 0.5 V)	40
  MesoC	0.1M KOH	70% (at 0.5 V)	41
  TiO2/RGO	0.1M KOH	90% (at 0.5 V)	This work

  4. CONCLUSION

  In summary, we synthesized TiO₂/RGO, 2-dimensional nanomaterials with abundant oxygen defect and high specific surface by a combination of hydrothermal and calcination methods. The as prepared TiO₂/RGO catalysts exhibit good activity and the selectivity of more than 90% for H₂O₂ electrosynthesis. The enhanced ORR performance of TiO₂/RGO may be attributed to the strong interaction between titanium dioxide and reduced graphene oxide. This work has given a novel approach for the reasonable design of 2-electron ORR electrocatalysts, and make an important development of TiO₂-based materials in the application of sustainable energy storage and conversion technology.

  
  
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• 

  Figure 1  Schematic illustration of the preparation of TiO₂/RGO

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  Figure 2  X-ray diffraction patterns of TiO₂, RGO and TiO₂/RGO

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  Figure 3  (a) SEM images of TiO₂, TiO₂/RGO, RGO; (b) TEM images of TiO₂, TiO₂/RGO, RGO; (c) HAADF-STEM image of TiO₂/RGO and the corresponding STEM-EDX elemental mapping images of the as-prepared TiO₂/RGO sample for oxygen element, carbon element and titanium element

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  Figure 4  (a) XPS spectra in the survey region of TiO₂/RGO, (b) XPS spectra in the C 1s region of TiO₂/RGO, (c) XPS spectra in the O 1s region of TiO₂/RGO, (d) XPS spectra in the Ti 2p region of TiO₂/RGO

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  Figure 5  (a) Cyclic voltammetry curves of TiO₂, RGO and TiO₂/RGO in 0.1 MKOH, 50 mV/s (b) Linear scan voltammetry curves, in 0.1 M KOH, rotation rate: 1600 rpm, scan rate: 5 mV/s, (c) Electrochemical active area normalization, (d) Selectivity of hydrogen peroxide produced by oxygen reduction

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  Table 1.  Electrocatalytic Performance of Different Catalyst Materials

  Catalyst	Electrolyte	Selectivity	Reference
  Co–N–C	0.1M KOH	60% (at 0.5 V)	37
  Co1–NG(O)	0.1M KOH	80% (at 0.5 V)	38
  Carbon	0.1M KOH	60% (at 0.5 V)	39
  Ni3B	0.1M KOH	85% (at 0.5 V)	40
  MesoC	0.1M KOH	70% (at 0.5 V)	41
  TiO2/RGO	0.1M KOH	90% (at 0.5 V)	This work

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