Fig.3 Device and performance test for generating H2O2.(a) Schematic diagram of h-type device generating H2O2, specification: length × width × height =7cm×5cm×5cm, electrolytic liquid volume 20ml; (b) At the electrode Gr0.5CB0.5/PTFE0.5, the net generation rate of H2O2 changes with the throughput of O2 under the conditions of current 150mA•cm-2, 0.5 mol•L–1Na2SO4 solution and pH=1.0. (c)The net generation rate and Faraday current efficiency of H2O2 changed with the current density of electrodes with different compositions under the condition of O2 flow rate of 30ml•min-1, 0.5 mol•L–1 Na2SO4solution, pH=1.0; (d) At the electrode Gr0.5CB0.5/PTFE0.5, the net generation rate of H2O2 varies with the pH of the electrolyte under the conditions of O2flow rate of 30ml•min-1, current 150mA•cm-2 and 0.5 mol•L–1Na2SO4 solution; (e) Electrode Gr0.5CB0.5/PTFE0.5, under the condition of O2 flow rate of 30ml•min-1, 0.5 mol•L–1Na2SO4 solution, pH=1.0, the cumulative concentration of H2O2 changes with reaction time at different current density; (f) Electrode Gr0.5CB0.5/PTFE0.5 the Faraday current efficiency of generating H2O2 with different current density changes with reaction time under the condition of O2flow rate of 30ml•min-1, 0.5 mol•L–1 Na2SO4solution, pH=1.0.
Fig.3a shows the schematic diagram of the hydrogen peroxide generating unit. Oxygen passes through the gas diffusion electrode integrating the diffusion layer and the reaction layer, and the excess oxygen is discharged from the hydrogen peroxide solution in the cathode chamber. Fig.3b shows the relationship between the hydrogen peroxide generation rate and the oxygen flow rate at the current density of 150mA•cm-2. It can be seen that when the O2 flow rate is no less than 30ml•min-1, the influence of O2throughput on the generation of H2O2 can be excluded. Unless otherwise stated, the oxygen flow rate in the paper is 30ml•min-1. The investigation results of some influencing factors such as the type of pore-making agent, partial pressure of oxygen, and addition ratio of carbon materials are shown in Fig.S3.
It can be seen from Fig.3c and Fig.S4c that no matter what kind of electrode, the hydrogen peroxide generation rate increases as the current density increases, but the increasing slope becomes smaller and smaller. After the current density exceeds 200mA•cm-2, it gradually becomes stable. The maximum hydrogen peroxide generation rate is about 2.5mmol•cm-2•h-1, and the corresponding Faraday efficiency also decreases gradually. Comparing different electrodes, it can be found that the hydrogen peroxide generation rate increases with the increase of CB content. When the mass ratio of CB/Gr≥1, the electrode performance does not increase significantly. Compared with Gr0.5CB0.5/PTFE0.5 and CB/PTFE0.5, the pore volumes are significantly different, but the electrochemical active surface area is similar. We believe that the production rate of hydrogen peroxide depends on the electrochemical active surface area. Using carbon materials such as carbon black, which are much cheaper than graphene and carbon nanotubes, and simply adding PTEF, gas diffusion electrodes with high hydrogen peroxide rate can be obtained compared with other carbon materials.
As pH goes from 0 to 2, the formation rate of H2O2 increases linearly, but when pH=2 to 4, the reaction rate is basically independent of pH. With different pH range, the corresponding speed control steps of hydrogen peroxide generation may be different.
Fig.3e shows that with the increase of the current density, the generation rate of H2O2 increases, and the maximum cumulative concentration can be reached in a shorter time. At the current density of 50mA•cm-2 and O2 flow rate of 30ml•min-1, after 780min, the maximum cumulative concentration of H2O2 reaches 117g•L-1, i.e. 3.44mol•L-1, or about 11.8% of the mass fraction. The maximum cumulative concentration is related to the decomposition of hydrogen peroxide. When the decomposition rate is increased to equal to the rate of hydrogen peroxide formation, the hydrogen peroxide concentration will remain constant for a long time if there is no mechanical damage to the electrode.
Fig.3f is the average Faraday efficiency of hydrogen peroxide concentration accumulated to a certain point. It can be seen that the electrodes are activated at the beginning. The Faraday current efficiency decreases rapidly with the increase of hydrogen peroxide concentration, reflecting that the decomposition of hydrogen peroxide into water will affect the overall electrode efficiency. When the hydrogen peroxide concentration is no longer rising, the instantaneous Faraday efficiency drops to zero. In addition, too high current density will reduce the service life of the electrode, and the surface of the electrode where the micro-pore structure is damaged has hydrogen generation, which will also reduce the Faraday efficiency of hydrogen peroxide generation.

3.4 Simulating the polarization curve under real reaction conditions based on Bulter-Volmer equation