1 Introduction

Hydrogen peroxide (H2O2), one of the 100 most important chemical substances, is a versatile and environmentally friendly oxidizer [1], which is widely used in chemical synthesis [2], water treatment[3]-[6] and environmental treatment[7]-[8]. At present, the main industrial production method of H2O2 is anthraquinone method [9]-[12]. However, the direct catalytic method of hydrogen and oxygen[13]-[14] and the electrocatalytic method[15]-[27] to produce H2O2 have the advantages of green and environmental protection, and are feasible alternative methods for industrial production. It is safer to produce H2O2 by 2e-ORR on the surface of gas diffusion electrode (GDE). Hydrogen peroxide prepared in the field can be directly used for disinfection and sewage treatment.
The reaction mechanism of oxygen reduction under acidic conditions mainly involves two electrons and four electrons:
O2 + 2e- + 2H+ → H2O2 (0.695 Vvs. SHE) (1)
H2O2 + 2e- + 2H+ → 2H2O (1.760 Vvs. SHE) (2)
O2 + 4e- + 4H+ → 2H2O (1.229 Vvs. SHE) (3)
High H2O2 selectivity and current efficiency can be obtained by increasing the rate of reaction (1) and inhibiting the occurrence of reactions (2) and (3). Previous studies have found that platinum-based materials are excellent ORR catalysts[28]-[29], while the relatively cheap carbon oxide materials have a significant advantage in 2e-ORR activity[30]-[35]. For most carbon-based materials (such as carbon nanotubes (CNTs) [36]-[39], carbon black [40]-[42], acetylene black[43]-[44], graphene[45]-[48] and carbon fiber[49]), the 2e-overpotential of O2conversion to H2O2 is low, and the modification and regulation of carbon materials are convenient. By the introduction of nonmetal impurity atoms (such as O[50]-[52], N[53]-[57], F[58]-[59], P[60], B[61], S[62]-[64], etc.) or the introduction of metal oxides [65]-[68]of the doping modification methods for carbon materials, we can change the physical structure or electron configuration, and reach the purpose of increasing its intrinsic activity, and obtain high efficiency, high selectivity, high stability of the 2e--ORR catalysts.
At present, the means to improve the catalytic performance of carbon materials are mainly focus on the regulation of defects and active sites, but there are few reports on optimizing the interface design of carbon-based catalysts to improve gas transport and fabricate more three-phase interfaces. In the process of electrocatalysis, especially under the condition of high current density, fabricating a developed hierarchical pores network is conducive to diffusion and mass transfer. O2 diffuses to the surface of the catalytic active site through the pores, and reacts with the H+ in the electrolyte and the electrons in the catalytic layer at the gas-solid-liquid three-phase interface, promoting the improvement of H2O2 reaction rate.
In this paper, cheap conductive carbon black (Vulcan XC-72R) and graphite are used as electrode materials, by adding PTFE、pore-making agent and simple processing, then a gas diffusion electrode with diffusion layer and catalytic layer is prepared. Under the optimized ratio and processing conditions, the electrode not only has rough surface and multi-stage micro-nano scale composite structure, but also the polymer phase regulates the wetting property of interface gas and liquid. The obtained concentration of hydrogen peroxide at 11.7wt% is superior to most reported materials. Based on the theory of Bulter-Volmer polarization curve, we also set up a mass transfer model for the three-phase interface reaction under the real reaction condition, and simulate the variation law of concentration polarization and current density in the three-phase region. This study demonstrates the great potential of PTFE in the construction of high-performance three-phase interfaces using simple porous materials and provides new ideas for the design of key catalytic materials and functional interfaces in other areas of electrochemical catalysis.