Efficient Electrochemical Reduction Of Nitrate To Ammonia Via Cu-based Electrodes And Its Mechanism

Many fields of production and life in human being involve nitrate(NO3-)wastewater discharge problems,such as nuclear industry and chemical industry,etc.which is harmful to human health.Therefore,it is necessary to develop green and sustainable technologies for the treatment of nitrate pollutants.Among the existing new technologies,electrochemical reduction of nitrate to ammonia(ERNA)process has received more attention,since it can efficiently,safely and efficiently convert nitrate into ammonia,solving the problem of environmental pollution,and recycle ammonia of economic production.However,to achieve efficient ERNA performance,it is a key challenge to develop electrode materials with robust reactivity.Herein,a series of Co-based and Cu-based electrodes were developed in this study,including Co3O4/CF(cobalt(II,III)oxide/carbon felt)electrode,R-Cu2O/Cu/CF(reduced-copper(I)oxide/copper/carbon felt)electrode,Co3O4/Cu(cobalt(II,III)oxide /copper)electrode,Co O/Cu foam(cobalt(II)oxide /copper foam)electrode.This study proposed methods about how to modify electrodes to increase ERNA reactivity,and it was proved that the ERNA performance of double active site electrode was higher than the single active site electrode,and it was further found that increasing the amount of H*(active hydrogen),enhancing the adsorption of NO3-,and optimizing the adsorption strength of the intermediates(such as: nitrite(NO2-)and nitric oxide(NO),etc.),which were beneficial for improving ERNA activity of electrodes.In addition,constructing a CO2 absorption system can stabilize the p H value of the solution via adsorbing CO2 from waste gas.Even though current density was high,CO2-assisted absorption system can help the electrode maintain relatively efficient ERNA reactivity,and also synthesize ammonium bicarbonate(NH4HCO3)of high-economic value product,providing a technical reference for the recyclable and comprehensive treatment of nitrate wastewater and CO2 waste gas.The main research results and conclusions are as follows:First,a Co3O4/CF electrode was prepared by a combination of electrodeposition and calcination.This electrode exhibited a high rate(1.18×10-4 s-1 cm-2)in removing NO3-.And,the corresponding ammonia nitrogen yield and selectivity also reached 0.12 mg cm-2 h-1 and 82.1%,respectively.For mechanistic studies,the synergistic effect of Co(III)and Co(II)in ERNA was elucidated by density functional theory(DFT)calculations and electron spin resonance analysis(ESR): Co(III)is good at adsorbing NO3-,and Co(II)is beneficial to generate H* to reduce NO3-.Based on this synergy,it was found that Co3O4/CF electrode with an electrode-surficial Co(II)/Co(III)ratio of 1.3 had the best activity.Therefore,it was suggested that regulating the ratio of metal valence states on the surface of electrode is an effective method to enhance ERNA activity of metal oxide electrodes.Secondly,a R-Cu2O/Cu/CF electrode with Cu2O/Cu interface was prepared by pulse electrodeposition and electroreduction,and its FE value reached 84.3% with ammonia yield rate of 2.17 mg cm-2 h-1(589-1-1catμg mg h)and selectivity of 94.4%,respectively.And it was found that the key factor of affecting the ERNA activity on RCu2O/Cu/CF electrode was the migration rate of NO2-molecules on the electrode surface and electron transfer by electrochemical tests.DFT calculations showed that the Cu2O/Cu interface decreased the adsorption energy of NO2-from-2.02 e V of pure copper to-1.59 e V,improving the adsorption energy of NO2-to increase migration rate of NO2-molecules on the electrode surface.At the same time,Cu2O/Cu interface can also make the d-band center(εd)of Cu move up(-2.25 e V vs.-2.48 e V(pure Cu))close to the lowest unoccupied molecular orbital of NO3-,increasing the electron transfer.Thus,it is proved that changing the composition of the oxide layer on the metal surface is an effective strategy to enhance ERNA activity of the electrode.In addition,through constructing a dual-active-site Co3O4/Cu electrode,the scaling relations in single-active-site electrode(the excessively strong adsorption of reactants leads to the difficulty of product desorption)was broke.DFT calculations showed that building of dual active sites of Co3O4 and Cu on the electrode can achieve strong adsorption of nitrate(-2.91 e V)with low desorption energy barrier(0.13 e V)ofammonia,overcoming the impediment of the scaling relations.Co3O4/Cu electrodes were fabricated by sequential electrodeposition,calcination,and electroreduction,achieving a high FE of 94.6% with ammonia yield rate of 0.67 mg cm-2 h-1(684-1-1catμg mg h)and selectivity of 92.2%.The selectivity is significantly higher than that of single active site Co3O4 electrode(FE of 32.3%)and Cu electrode(FE of 50.9%).And it was also found that the ERNA happened on the Co3O4/Cu electrode mainly relied on the NO3-adsorption on Cu and the reduction of Co3O4 by producing H*.Furthermore,in-situ fourier transform infrared Spectrometer(in-situ FTIR)and differential electrochemical mass spectrometry(DEMS)jointly demonstrated that the indirect pathway(dominant of H*)was the main reaction pathway.Therefore,it can be considered that the construction of dual active sites on electrodes is a reliable method to enhance ERNA reactivity of electrodes.Then,methods for optimizing dual active site electrodes were investigated by preparing Co O/Cu foam electrodes.By X-ray photoelectron spectroscopy(XPS)analysis,it was found that Cu0 of Cu foam shifted to higher binding energy by 0.19 e V after loading Co O,while the peak position of Co(II)of Co O was shifted to lower binding energy by 0.17,thus proving the existence of an interfacial electric field on the Co O/Cu foam electrode to facilitate electron transfer.Co O/Cu foam electrode finally achieved an ammonia yield of 4.3 mg cm-2 h-1with FE of 96.7% and selectivity of 97.3% under the help of interfacial electric field.In particular,XPS characterization and DFT calculations found that interfacial electric field could cause Cu on the Co O/Cu foam electrode to lose part of its electrons and become positively charged,which was beneficial for the adsorption of NO3-and enhanced electron transfer.However,Co O obtains extra electrons due to interfacial electric field,which can fill the anti-bonding orbital during ERNA,decreasing the adsorption of NO of intermediate,resulting in improving the selectivity of ammonia.In addition,DEMS,in-situ FTIR and DFT calculations also indicated that ERNA happened on the electrode was an indirect pathway.Therefore,it was demonstrated that controlling the interfacial electric field is an effective method to enhance ERNA reactivity of dual-active-site electrodes.Finally,a CO2 absorption system device was constructed to assist in enhancing ERNA reactivity of the electrode.Through the electrochemical activity test,it was found that ERNA activity of Cu foam and Co O/Cu foam electrodes were significantly improved under the assistance of adsorption of CO2.And,the performance of Co O/Cu foam electrode was better.Under the current density of 100 m A cm-2,Co O/Cu foam electrode obtained the yield rate of ammonium bicarbonate(NH4HCO3)of 28.4 mg cm-2 h-1(ammonia yield rate of 6.47 mg cm-2 h-1),the selectivity of ammonia of 96.6%,and the energy consumption of 4.74 k Wh kg-1 NH4HCO3,respectively.Secondly,using DFT calculation,in-situ FTIR and DEMS to study mechanism,it was found that the absorption of CO2 can not only neutralized OH-generated by reducing NO3-,but also formed bicarbonate ion(HCO3-)to provide protons to the ERNA and enhance the adsorption of NO3-,boosting the ERNA reactivity of the electrode.Therefore,the construction of a CO2 absorption system can improve the ERNA activity of the electrode and generate high-valued NH4HCO3.