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.