A Novel Rolling Air-breathing Cathode For The Electro-synthesis Of Hydrogen Peroxide In Electrochemical Advanced Oxidation Processes:Fabrication,optimization,and Application

Recently,advanced oxidation processes（AOPs）based on the catalysis of hydrogen peroxide（H₂O₂）to generate free·OH radical has gradually attracted people’s attention.In-situ generating-utilizing H₂O₂ is regarded as a novel kind of method of pollution treatment.Producing H₂O₂ through two-electron oxygen reduction reaction（ORR）in the electrochemical system and then build a quick pollutants removal process gradually attracted people’s attention.This process is named as electrochemical advanced oxidation process（EAOP）.Numerous carbonaceous cathodes were developed due to the high conductivity,low price,good stability properties of carbon materials.Based on our rolling method,a sandwich-like air breathing cathode（ABC）consisting of a catalytic layer（CL）,a metal collector and a gas diffusion layer（GDL）was prepared.Firstly,the prepared electrodes were placed in the electrochemical system and the bio-electrochemical system respectively for H₂O₂ synthesis tests.The results showed the performance of the ABC prepared by the graphite and CB mass ratio of 5:1 was significantly improved.The characterization results showed after effective mixing of graphite and CB,CB particles were embedded between the graphite sheets,which increased the specific surface area and exposed active sites,and significantly improved the performance of the electrode.The H₂O₂ yield was 11.9 mg·L^-1·h^-1·cm^-2(50 mg·h^-1)at-1.4 V（vs.Ag/Ag Cl）with 12.3 m A·cm^-2 and the current efficiency reached 92%.The novel ABC uses oxygen in the air to diffuse freely into the CL for ORR.There is no need for aeration and pressurization device,which simplifies the synthesis system and has more application potential.Afterwards,the effect of liquid-gas-solid three-phase interfaces on ORR and its mechanism in rolling ABC were studied.Two kinds of air cathode based on the same CL were prepared:ABC and air aeration cathode（AAC）.H₂O₂ synthesis electrolytic experiments and electrochemical tests were conducted on both ABC and AAC.The response current,H₂O₂ production,O₂ and H₂ content in electrolyte were in-situ monitored.The oxygen supply pathway of the electrode and the side reactions that lead to the decrease of current efficiency of the electrode under different conditions were determined.The hydrogen evolution reaction of the air aerated cathode is the main reaction when the current density surpass 2.5 m A·cm^-2,and the current efficiency of H₂O₂ production is reduced to less than 10%.CLs preferred to use oxygen from the free diffusion of air,and used very little dissolved oxygen from the aeration.It was proven that the mass transfer and diffusion of dissolved oxygen in water limits the application of the electrode to oxygen.The current efficiency of the ABC also decreased with the current density increasing to 35 m A·cm^-2.The side reaction under high current was the H₂O₂further reduction to H₂O.A large amount of H₂O₂ generated at the electrode interface can be pushed away from the reaction interface with the help of a pressurized directional airflow to reduce side reactions and thus improve the current efficiency.At35 m A·cm^-2,the yield of H₂O₂ is 461±11 mg·L^-1·h^-1(138.3 mg·h^-1),and the current efficiency is 89±2%.The stability of the electrode is as important as its catalytic activity.The performance attenuation of ABCs in three EAOP systems for a long-term operation and the attenuation factors causing the performance decay were studied.Three EAOPs,including EF,UV/H₂O₂ and photo-electro-Fenton（PEF）,were established to realize fast degradation of 100 mg·L^-1 phenol solution.10 control groups were set up for long-term operation.The results showed that the performance of the ABC with only H₂O₂production decreased only by 17.8%after 200 h operation,while the H₂O₂ yield of the electrode with different EAOP systems decreased by 43-85%.Through various physical and chemical characterization analysis,four attenuation factors of alkali salting precipitation,·OH radical erosion,iron compound deposition and organic pollution were summarized.And the mechanisms were illustrated.A solution to prolong the operating life of the electrode was proposed.In order to achieve the removal of algal cells and algal toxins in algal bloom water,a switchable three-electrode system was built based on our ABC.The electro-coagulation（EC）and electro-Fenton（EF）functions were coupled into a reactor through circuit switching.And the coagulant（Fe（OH）₃）,catalyst（Fe ion）as well as oxidant（H₂O₂）were all generated in-situ in our system.The orthogonal experiments were carried out for aeration,initial p H,reaction time and other factors during the EC process.Through optimization,it was found that the effective removal of algal cells（91%）with low energy consumption(0.28±0.04 k Wh·m^-3)could be achieved under the conditions of 100 m A,initial p H 8 and running time of 40 min with aeration.The remaining algal toxins in water can be degraded by following EF process.Through the control group,it was found that single EC process could not effectively remove algal toxins in the water（15%at most）.As contrast,single EF process could conduct the non-selective mineralization of all organic matter in the algal bloom water and realized satisfied removal of cyanobacterial cells and toxin,while the energy consumption(5.6±0.02k Wh·m^-3)was 11 times of the coupling process(0.5±0.04 k Wh·m^-3).Therefore,the electro-coagulation-Fenton coupling process can achieve high efficiency and low energy consumption for algal bloom treatment.In order to further promote the application of ABC,the structure of ABC was upgraded and improved.In order to solve the problem of water flooding in long-term operation,the rolling GDL was replaced by nano-waterproof coating.The electrode was successfully amplified from 7 cm² to 240 cm²,and the moulding of the electrode was changed.The modularized electrode system was prepared and tested for EF degradation of antiviral lamivudine in a 2 L pre-pilot system.The degradation intermediates of lamivudine were analyzed and the degradation path was predicted.Finally,the electricity cost and preparation cost of hydrogen peroxide synthesis by ABC are calculated.The application prospect and optimization direction of the electrode are evaluated.