Construction Of Cu-based Composite Electrode And Its Performance For Electrochemical Nitrate Reduction In Water

Water is the source of life and the essential material for human existence.With the development of industrialization and the wide application of fertilizer in agriculture,the contamination of surface and ground water by nitrate(NO₃^-)has become a global issue.Electrochemical denitrification has been the most important technology for controlling NO₃^-pollution due to its high efficiency,simple operation,and broad application prospect.However,the main produc tion of electrochemical denitrification includes nitrate(NO₂^-),ammonium(NH₄⁺),and nitrogen(N₂).Among them,NO₂^-and NH₄⁺are the toxic and harmful substance.In addition,the control requirements for total inoriganic nitrogen(NO₂^-+NO₃^-+NH₄⁺)in pollutant discharge in China are becoming more and more stricts.Therefore,the directional conversion of NO ₃^-into N₂to achieve complete removal of total inoriganic notrogen from water is the key to popularize the practical application of electrochemical denitrification.The electrochemical NO₃^-reduction is really related to the physicochemical and electrochemical properties of electrode.Although the emerging Cu-based electrode has attracted intensive attentions in electrochemical NO ₃^-reduction,the issues such as restricted activity,stability,and selectivity are still unresolved.To solve the above problems,in this work,a series of copper matrix composite electrodes were prepared and then applied to electrochemical NO₃^-reduction.Firstly,the traditional palladium-copper nanoparticle(PdCuNPs)electrode for electrocatalytic NO₃^-reduction has the problems of poor electrocatalytic activity and low energy utilization.Compared with PdCuNPs,PdCunanowires(PdCuNWs)obtained through wet reduction had smaller particle size and better stability.The PdCuNWs was immobilized on the nickel foam(NF)surface by a binder to obtain the PdCuNWs/NF composite electrode,which was used as cathode for electrochemical reduction of NO₃^-.The effects of PdCuratio,cathodic potential,initial NO₃^-concentration,and initial solution pH on electrochemical denitrification were also investigated.When the initial NO₃^-concentration was 50 mg N/L,the resulting Pd₆Cu₁ NWs/NF electrode showed excellent performance for NO₃^-reduction(85.5%)with high N₂ selectivity(94.3%)at-0.8 V vs.AgAgCl.The cyclic voltammetry(CV)and electrochemical impedance spectra(EIS)demonstrated that electrochemical NO ₃^-reduction was achieved through electron transfer between NO₃^-and Cu⁰ and simultaneously NO₂^-and Cu₂O were generated.The PdNWs not only served as the electron mediator to facilitate the electron-transfer for NO₃^-reduction,but also acted as the stable electrocatalyst to produce atomic H^*toward conversion of NO₂^-into N₂ and transformation of Cu₂O into Cu⁰.The cyclic experiments also demonstrated the reusability and stability of composite electrode.Meanwhile,the PdCuNWs/NF composite electrode processed desirable electrochemical performance on the removal of NO ₃^-from different water sources,further demonstrating its practical application potentials.Secondly,in view of the high cost of the precious metal,the use of binder reduced the catalytic activity and stability of catalysts,and the low selectivity of N ₂ for Cu electrode,a self-supported composite electrode(Cu₃P/CF)was first prepared by direct growth of copper phosphide on copper foam and then applied to electrochemical NO ₃^-reduction.The resulting Cu₃P/CF electrode showed enhanced electrochemical performance for NO₃^-reduction(84.3%)with high N₂ selectivity(98.01%)under the initial conditions of 1500 mg L^-1 Cl^-and 50 mg N L^-1 NO₃^-.The cyclic voltammetry(CV)and electrochemical impedance spectra(EIS)demonstrated that electrochemical NO₃^-reduction was achieved through electron transfer between NO ₃^-and Cu⁰originated from CF.The in-situ grown Cu₃P served as the bifunctional catalyst,the electron mediator or bridge to facilitate the electron-transfer for NO₃^-reduction and the stable catalyst to produce atomic H^*toward NO₂^-conversion.Meanwhile,the Cu₃P/CF remained its electrocatalytic activity even after eight cyclic experiments.In addition,a 2-stage treatment strategy,pre-oxidation by Ir-Ru/Ti anode and post-reduction by Cu₃P/CF cathode,was designed for effective electrochemical chemical oxygen demand(COD,91.4%)and total nitrogen(TN,80.3%)removal from real wastewater.Finally,in terms of the dissolution Cu²⁺and phosphate using Cu₃P/CF electrode,a binder-free composite electrode(CuFeNPs@N-C/NF)was prepared by sol-gel,dip-coating,and high-temperature carbonization reduction method and then then applied to electrochemical NO₃^-reduction.The effects of CuFeratios,cathodic potential,initial NO₃^-concentration,initial solution pH,and initial Cl^-concentration on electrochemical denitrification were also studied.Although the lower cathodic potential made for NO₃^-reduction,the corresponding energy consumption was enhanced.The efficient electrocatalytic NO₃^-reduction by CuFeNPs@N-C/NF could be realized under different initial solution pH and NO ₃^-concentration,while lower initial pH and NO₃^-concentration were in favor of solution reduction.The addition of Cl^-was beneficial to the directional conversion of NO ₃^-into N₂.The NO₃^-removal efficiency and N₂ selectivity were 74.2%and 93.9%under-1.4 V vs.Ag/AgCl in the presence of 1000 mg/L Cl^-as the initial NO₃^-concentration was 50 mg N/L.Moreover,the reaction mechanism of electrochemical NO ₃^-reduction by CuFeNPs@N-C/NF cathode was proposed.Besides,the obtained CuFeNPs@N-C/NF electrode remained its electrocatalytic activity even after 15^th cyclic tests.The PdCuNWs/NF,CuFeNPs@N-C/NF,and Cu₃P/CF composite electrode were first fabricated and then applied to electrochemical NO₃^-reduction.The study 1)solved the presented problems of traditional electrode such as high cost,lower N ₂ selectivity,and poor stability;2)realized the complete removal of total inorganic nitrogen from water;3)investigated the effects of impact factors on the electrochemical NO ₃^-reduction;4)proposed the reaction mechanism;and 5)explored the practical application prospect of this technology.This work might provide the basic theoretical support for the practical application of electrochemical denitrification and has practical guiding significance for the following researches.