2 Experimental

2.1 Preparation of gas diffusion electrode

(1) Pretreatment of stainless steel mesh:
Firstly, the stainless steel mesh (40 mesh, Runshi metal products factory) is cut into length × width =5cm×5cm specification, and soaked in 5% of the mass of PTFE emulsion (PTFE) for 5-10s. Then we take it out and put it in a oven at 105℃ to dry. Repeating the above operation several times, until the weight of stainless steel mesh is doubled, and dry for 1h.
(2) Preparation of carbon black, pore-forming agent and binder paste:
According to the dry weight ratio, we weigh the carbon materials(Vulcan XC-72R, AR, Cabot, USA; Graphite powder, AR, Tianjin chemical reagent factory), binder (60wt % of PTFE, Japan Daikin company) and pore-forming agent (NH4HCO3, AR, Tianjin wind ship chemical technology co., LTD.). Then, we mix them evenly, and add an appropriate amount of ethanol (AR, characters). Ultrasonic shocks for 20min and disperses them evenly, and volatilizes the ethanol at 40℃ to a thick paste, which is carbon black, pore-forming agent and binder paste.
The typical formula is carbon material and PTFE mass ratio of 2:1, at this case, electrode abbreviation is Gr/PTFE0.5 or CB/PTFE0.5, Gr: CB: PTFE= 1:1:1, electrode abbreviation is Gr0.5CB0.5 /PTFE0.5. Where: Gr and CB are graphite powder and carbon black (Vulcan XC-72R) powder.
(3) Preparation and molding of gas diffusion electrode
The paste is evenly coated on the hydrophobic stainless steel mesh and pressed at 6MPa pressure for 20min to obtain the initial gas diffusion electrode. The initial gas diffusion electrode is placed in a muffle furnace and calcined at 330℃ for 1h. After cooling, the electrode is taken out and soaked in deionized water for 2h.

2.2 Characterization and analytical methods

(1) Nitrogen adsorption and desorption test (BET)
The pore size distribution and specific surface area of electrode materials are measured by ASAP2460 gas adsorption analyzer.
(2) Raman spectral representation
Conducted in the SENTERRA Raman Scope (BRUKER) tester, the excitation source wavelength is 532nm and the power is about 2mW.
(3) Fourier-transform infrared spectroscopy (FTIR)
The vibration absorption characteristics of chemical bonds or functional groups in molecules are used to test the molecular structure and functional groups of electrode materials by FTIR.
(4) Scanning electron microscopy (SEM)
SEM is used to observe the structure, material distribution and microscopic pore distribution of the electrode surface.
(5) Electrochemical Tests
The performance of selective oxygen reduction is assessed in a standard three-electrode system controlled by a potentiostat (CHI 760E) with a RDE setup. A glassy carbon (GC) disc (RRDE-3A; Pine Instrument Company; 0.0707cm2 of disc area) is employed as the working electrode. The Pt wire and saturated Ag/AgCl electrode are used as the counter and reference electrode, respectively. All the reported potentials are referred to as the reversible hydrogen electrode (RHE) potential.
The catalyst ink is prepared by dispersing 15mg of carbon materials catalyst in 800μL of isopropyl alcohol, 800μL of deionized water and 14μL of 5wt% Nafion solution, followed by a sonication of 30min. After that, 9.6μL of the catalyst ink is deposited on the GC part of the RDE electrode and dried to form a uniform thin film at 95℃ keeping 12h. An O2-saturated 0.5 M Na2SO4 (pH=1.0) aqueous solution is used as an electrolyte for measurement. The working electrode is rotating at 1600rpm.
Linear sweep voltammetry (LSV) is carried out to compare the electrochemical behavior during H2O2production, and are performed by the electrochemical workstation at a scan rate of 5mV•s-1 in the three-electrode cell system. The prepared cathode is used as working electrode. The Pt wire and saturated Ag/AgCl electrode are used as the counter and reference electrode, respectively. All the reported potentials are referred to as the reversible hydrogen electrode (RHE) potential. The LSV curves of the working electrodes are obtained in the potential range of 0.0–0.9V in O2-saturated Na2SO4solution (pH=1.0), ambient temperature.

2.3 Experimental methods and devices

Gas diffusion electrode is assembled in the removable H-type experiment device, the typical conditions of the preparation of hydrogen peroxide are the cathode chamber volume 7 cm3, the thickness of 1cm, anode platinum electrode, Proton Exchange Membrane (PEM), electrolyte, 0.5 mol•L–1Na2SO4 solution ( pH=1.0), current 1.05A , O2 flow rate 30ml•min– 1, circulating pump flow 20ml•min- 1, reaction, every 10min sampling, the determination of H2O2 concentration.

2.4 Analysis and calculation methods

(1) Determination of H2O2 concentration
The concentration of H2O2 is determined by colorimetry, using 0.1 mol•L–1Ti(SO4)2 as chromogenic agent. The ultraviolet spectrophotometer detects that its maximum absorption wavelength is at 407nm. Diluted with 30wt% H2O2 into a series of concentration gradients of 1-60mg•L-1, under 407nm absorption wavelength, the standard curve of absorbance (A) and H2O2 concentration (C) is made, which meets the following requirements: A = 0.023C. concentration of experimental sample can be calculated according to the standard curve of absorbance-H2O2 concentration.
(2)The current efficiency of the H2O2electrosynthesis was calculated using Eq. (1)
\(FE=\frac{\text{nFCV}}{\text{It}}\times 100\%\) (1)
Where n is the number of transferred electrons in the reduction of O2 to H2O2 (n=2),F is the Faraday constant (96485C•mol−1),C is the concentration of H2O2 (mol•L–1),V is the volume of electrolyte (L), I is the current intensity (A), and t is the electrocatalysis time (s).
(3) Calculation of H2O2 generation rate
The apparent generation rate of H2O2 is the difference between the net generation rate of H2O2 and the decomposition rate. We usually use the net generation rate of H2O2 to describe the electrochemical activity of the electrode. According to the relationship between the concentration of H2O2 (C ) and the reaction time (t ), the apparent formation rate of H2O2 is r = dC / dt . After reasonable derivation and calculation, the influence of H2O2 decomposition is eliminated, and when r is extrapolated to t=0 min, the net generation rate of H2O2 (rt=0 ) is obtained to reflect the electrochemical activity of the electrode.