Construction Of Photoelectro/Piezoelelctric Catalytic Systems With The Mo-containing Electrodes For Recalcitrant Organics Removal And Simultaneous Hydrogen Evolution

Photo-electro-chemical cells（PEC）,photocatalytic fuel cells（PFC）and piezoelectric catalysis（PZC）can recover energy during wastewater treatment,which have attracted much attention in recent years.The electrodes with high catalytic activity and stability are the keys to realize the effective utilization of the above technologies.Molybdenum（Mo）-containing catalytic electrodes in the forms of ferrous molybdate（Fe Mo O₄）and molybdenum disulfide（MoS₂）have the advantages of easy availability of raw materials,simple preparation,low synthesis costs and environmental friendliness.However,in existing studies,Fe Mo O₄ was often reported as an anode material in electrocatalysis（EC）.Its application form and application field need to be further expanded.For MoS₂,the catalytic activity was still limited by its composition,crystal structure and surface morphology.Accurate regulation and optimization based at the micro level is needed.In addition,the existing PZC systems are usually driven by ultrasonic vibration or stirring,which results in high energy consumption and low catalytic efficiency.To solve the above problems,four kinds of Mo-containing cathodes were designed and fabricated:（1）Fe Mo O₄/graphene oxide（GO）loaded on carbon fiber cloth（CFC）,termed as Fe Mo O₄/GO@CFC;（2）iron（Fe）doped MoS₂/reduced graphene oxide（r GO）loaded on CFC,termed as Fe-MoS₂/r GO@CFC;（3）silver（Ag）deposited MoS₂ loaded on carbon fiber felt（CF）,termed as Ag-MoS₂@CF;（4）Ag-MoS₂/r GO loaded on CF,termed as Ag-MoS₂/r GO@CF.The catalytic cathodes were separately used to construct the systems including active chlorine-PEC,dual photoelectrode-PFC,persulfate（PS）-PZC and PZC-EC,which were applied in the removal of recalcitrant organics and hydrogen evolution.Mechanisms of the systems were proposed.Specific research contents and conclusions are as follows:（1）GO was introduced into Fe Mo O₄ to regulate the composition and structure of the electrode,accelerating the surface electron migration and improving the separation efficiency of free carriers.With 20 m L GO(2.83 g·L^-1),the Fe Mo O₄/GO-20@CFC was used as the cathode to construct an active chlorine-PEC system.Berberine（BBR）was used as the target pollutant to investigate the photoelectrocatalytic performance and stability of the system.The removal and mineralization rates of BBR(10 mg·L^-1)reached 98.6%and 74.9%（in 30 min）,respectively.The reaction kinetic constant of the system was 2.14×10^-1 min^-1（first order）,which was 1.48 times higher than that of the system with Fe Mo O₄/GO-0@CFC cathode.During the BBR removal,the hydrogen evolution rate on the Fe Mo O₄/GO-20@CFC was 2.34μmol·cm^-2·min^-1（namely heat of 161 J）,and the energy consumption of the system was 432 W·h·m^-3,thus the energy recovery rate was 25.9%.The mechanism investigation revealed the leading role of active chlorine besides singlet oxygen.（2）In order to reduce the energy consumption of the PEC and improve the system performance in pollutants removal and simultaneous hydrogen evolution under illumination,MoS₂ was used to substitute Fe Mo O₄ in electrode optimization.Fe₃O₄ and r GO were introduced to modify MoS₂ in activating its inert base surface,to expose more active site,and improve electron migration rate.By using the Fe-MoS₂/r GO@CFC as the cathode,a dual photoelectrode-PFC was constructed with the anode of Zinc ferrite/Ag⁰/silver vanadate loaded CF.The results showed that the system improved the separating efficiency of photo-generated carriers,realizing electricity generation and hydrogen evolution when removing organic pollutants:the removal rate of methyl orange(MO,10 mg·L^-1)reached 97.2%（in 120 min）,the corresponding reaction kinetic constant was 1.20×10^-1 mg·L^-1·min^-1（0 order）,the voltage output was～400 m V,and the hydrogen evolution rate was 3.66×10^-6μmol·cm^-2·min^-1.The energy consumption of PFC was 1.93 k W·h·m^-3,and the electric energy recovered was 1.15J.The presence of organic pollutants inhibited the hydrogen evolution.The mechanism investigation illustrated the main active species of holes and hydroxyl radicals in pollutants removal.（3）To modify the MoS₂ in piezoelectric property and optimize the driving mode of PZC,Ag⁰ was deposited on MoS₂ to regulate the compositing ratio of 1T phase and 2H phase.The modified Ag-MoS₂@CF was used to construct a PZC system,and gas agitation was applied to drive the system.PS was added to enhance the removal efficiency of pollutants.The results showed that the removal rate and mineralization rate of rhodamine B(RhB,100 mg·L^-1)reached 92.5%and 75.1%（in 15 min）,respectively.Under the intermittent driving mode of gas agitation for 5 min and stop for 1 min,the reaction kinetic constant for RhB removal was 6.60×10^-2 min^-1（first order）,which was 1.41 times higher than that of continuous driving mode.The PS-PZC system can realize simultaneous hydrogen evolution during pollutant removal,and it consumed energy of 2.54 k W·h·m^-3.The mechanism investigation illustrated the main role of singlet oxygen and sulfate radicals in organic pollutant degradation.（4）Taking advantages of the piezoelectric and electrocatalytic properties of Ag-MoS₂@CF,a PZC-EC system was constructed to enhanced the MO removal efficiency and the hydrogen evolution rate of PZC.r GO was introduced into Ag-MoS₂@CF to improve the stability of the cathode and the electron transfer property between the cathode and the conductive substrate.The results showed that the system with Ag-MoS₂/r GO@CF cathode removed 95.9%of MO(100 mg·L^-1)in 60 min with the reaction kinetic constant of 3.50×10^-2 min^-1（first order）,which was 1.17 times higher than that of Ag-MoS₂@CF.The hydrogen evolution rate was 8.75×10^-1μmol·cm^-2·min^-1（namely heat of 300 J）,and the energy consumption was 13.0 k W·h·m^-3.The mechanism investigation illustrated that the superoxide anion radicals and holes both functioned in MO removal together with the direct EC oxidation of the platinum anode.