Fabrication Of Novel Cobalt Cathode Combine Performance And Mechanism Of Electrocatalytic Nitrate Reduction

Nitrate（NO₃^-）ubiquitous in water is considered as a potentially harmful contaminant which can be converted to nitrite anions（NO2-）in the human body,causing serious health problems such as the liver damage,methemoglobinemia and blue baby syndrome.In this regard,the removal technologies of NO₃^-are imperious demands in populated regions.The electrochemical reduction process for denitrification has attracted increasing attention owing to the advantages of environmental compatibility,reagent free,high efficiency and ambient operating conditions.However,it is believed that the advancement of electrochemically reductive NO₃^-treatment has been limited by the development bottleneck of electrode materials.This article aimed to develop green,sustainable and economical materials for emerging electrochemical remediation of nitrate（NO₃^-）contaminant.To this end,we prepared a novel cathode,Co₃O₄-TiO₂/Ti,by combining sol-gel and calcination methods,which seems to be promising for remarkably facilitate the electrocatalytic reduction of NO₃^-.The effects of electrode preparation factors（calcination temperature and coating times）and operation parameters（p H,current density and chloride ions concentration）on NO₃^-reduction were also investigated.In addition,based on the characterization of the prepared catalysts using XRD,SEM,HRTEM,EDS elemental mapping and XPS,we tried to relate the results to the catalytic activity.Furthermore,electrochemical analyses,radical scavenging experiments and density functional theory calculations（such as adsorption configuration and differential charge density distribution of NO₃^-）were utilized to inspect the possible NO₃^-reduction mechanisms.For the first time,the direct nitrate reduction mechanism mediated by the Co2+–Co3+–Co2+ redox cycle in Co₃O₄ was proposed,and the defect that its atomic hydrogen H* adsorption capacity was weak and resulted in a low proportion of indirect nitrate reduction（18%）was found.Aiming at the defects of intrinsic catalytic activity of the above cathode,we further prepared P-Co₃O₄/NF cathode by hydrothermal method and low temperature phosphating method.The P-doped cathode exhibited a full range of reduction activity enhancement,the nitrate removal rate and reduction rate are increased to 2.64 and 8.45 times,the normalized kinetic constant increased to 2.65 times,the TN removal efficiency and nitrogen selectivity increased to 2 and 5 times.This increase is attributed to the reduction of the activation energy required for the generation and adsorption of atomic hydrogen H* by P doping,which in turn increases the contribution of indirect nitrate reduction（44%）.In the end,the treatment of actual desulfurization wastewater was performed.NO₃^--N was reduced below 10 mg L-1 within 80 min and approximately 92% TN removal efficiency was obtained within 120 min with an energy consumption of 0.157 k Wh gN-1.Furthermore,the 10-cycle electrocatalytic reduction experiment of NO₃^--N shows an acceptable decrease of activity at the P2.1-Co₃O₄/NF cathode with a 91% removal efficiency obtained after ten degradation experiments,confirming the satisfactory long-term durability under the strong potential conditional.Generally,this study provided a new paradigm for designing high performance,great stability and low cost practical non-metallic catalyst for NO₃^-reduction,which can meet the scientific and societal demands.