Improvement,of,the,Selectivity,for,Hydrogen,Peroxide,Production,via,the,Synergy,of,TiO2,and,Graphene①

SUN Yun-Long HUANG Jun-Heng

a (CAS Key Laboratory of Design and Assembly of Functional Nano-structures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter,Chinese Academy of Sciences, Fuzhou 350002, China)

b (Fuzhou University, Fuzhou 350108, China)

ABSTRACT To replace the four-electron transferred pathway of oxygen reduction reaction (ORR) by twoelectron transferred pathway of ORR (2e- ORR) is desirable for the production of hydrogen peroxide with addedvalue. The development of electrocatalysts with high selectivity toward 2e- ORR is of great interest but it is still a challenge. Here, we synthesized the graphene-supported titanium dioxide nanocomposite as the 2e- ORR catalysts by a combinative process of hydrothermal methods and calcination. Due to the synergistic effect between the graphene with high conductivity and the titanium dioxide with defect sites, the composite TiO2/graphene exhibits the improved selectivity (up to 90%) for oxygen converting into hydrogen peroxide.

Keywords: oxygen reduction reaction, hydrogen peroxide, titanium dioxide, graphene, electrochemistry;

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 industries 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 H2O2production 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 costeffectiveness.

As a rule, electrochemical ORR in alkaline electrolyte generally needs to execute one of the following two reactions:the 4e-process to convert O2to H2O (Eq. (1)) or the 2e-process for the reduction of O2into HO2-(Eq. (2)).

E0is 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 HO2-(generate H2O2in acidic media or over-protonated anions, or HO2-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 considered 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 performance 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 H2O2from O2reduction, 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 metalsupport 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. 1 Synthesis of the materials

We synthesize TiO2/RGO composite nanomaterials through a combination of hydrothermal methods and calcination. 0.4 mmol TiCl4was 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. TiO2is 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-1in 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 AlKαsource),and XPS data were obtained by correcting the binding energies of C1sto 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 2.5µL 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 H2O2selectivity (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,IDis the disk electrode current andIRis the ring electrode current, andNis the ring electrode collection efficiency, which is determined using the RRDE electrode and usually has a value of 0.35 in our test.

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

We herein report the fabrication of TiO2supported 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 TiO2on the surface of reduction graphene oxide. Moreover, for the preparation of a series of contrasted samples, the products of TiO2were formed in a similar method under the absence of GO, and RGO was obtained without adding TiCl4.

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

Fig. 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

X-ray diffraction (XRD) (Fig. 2) is carried out to identify the structural features of TiO2/RGO, TiO2and RGO. The TiO2and 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).

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 TiO2nanomaterials 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.

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, TiO2and 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 ofsp2-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.

Fig. 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

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 electrocatalysts. TiO2/ RGO exhibits a higher onset potential of 0.72 VvsRHE at -0.1 mA/cm2and the bigger disk current density(Jdisk) and ring current (Iring) than those of TiO2and 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 electrochemically 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].

Fig. 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

Table 1. Electrocatalytic Performance of Different Catalyst Materials

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 H2O2electrosynthesis. 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.