Synthesis And Application Of Nitrogen-Carbon Catalysts In Electrochemistry Reduction CO₂

The renewable-energy-powered electrochemical carbon dioxide reduction（CO₂RR）provides a feasible way to reduce atmospheric carbon dioxide concentrations and produce valuable carbon-based fuels to achieve carbon cycling.At present,various metal-based electrocatalysts,metal-free electrocatalysts and molecular electrocatalysts have made great progress in electrochemical reduction CO₂ into gaseous（CO,CH₄,etc.）and liquid products（formic acid,methanol,etc.）.Although a large number of catalysts have been studied,they suffer from some problems such as high overpotential,poor selectivity,high cost and poor durability.Therefore,it is highly compelling to design efficient,durable and economical CO₂RR electrocatalysts for selective carbon dioxide conversion.In this study,oxygen-rich Zn-MOF-74 was used as precursor,and melamine sponge（MS）was used as nitrogen source.The unique absorption property of melamine sponge was used to provide the supporting scaffold for the growth of Zn-MOF-74.A series of N-C catalysts were prepared by adjusting the content of Zn NO₃and carbonization temperature.The structure,morphology,lattice,elemental composition,valence state and specific surface area of N-C catalysts were characterized and the electrochemical performance of CO₂RR was tested.The main research contents and results are as follows:（1）The ligand content was fixed and the activated MS was used as the supporter to prepare different Zn-MOF-74/MS with different contents of Zn NO₃.Zn-MOF-74,Zn-MOF-74/MS and the activated MS were firstly were pre-carbonized at 600°C for 2h under the protection of argon,and then continued to carbonized at 900°C for 2 h.The resulting products were denoted as C-600-900,N-C-600-900 and MS-600-900,respectively.The experimental results show that C-600-900 has no CO₂RR catalytic activity.N-C-600-900 contains high concentration of active pyridine nitrogen and graphite nitrogen due to the introduction of MS,which exhibits excellent CO₂RR activity.MS-600-900 showed weak catalytic activity of CO₂RR.It was found that different contents of Zn²⁺affect the morphology of N-C catalyst and make the catalyst have different catalytic activity of CO₂RR.The experimental results show that NC-600-900-2（mass of Zn NO₃·6H₂O is 0.45 g）has the smaller charge transfer resistance（3.087Ω）and the best catalytic performance of CO₂RR.At-0.6 V（vs.RHE）,the FE_COreaches 50.4%（5.4 times of MS-600-900）,the current density is 2.46 m A cm^-2（20.5times of MS-600-900）.The results showed that the pores produced by the release of gaseous molecules（Zn,H₂O,CO₂）in the carbonization process of Zn-MOF-74 can promote the improvement of CO₂RR performance.（2）Using un-activated melamine sponge（UA-MS）as carrier,the Zn-MOF-74/UA-MS precursor was prepared,and UA-NC-600-900 was obtained by carbonization.The results show that the surface of MS becomes rough after activation,and the surface roughing MS is conducive to the uniform distribution of N-C nanorods.The FE_CO of NC-600-900-2 is 4.5 times of UA-NC-600-900 at-0.6V（vs.RHE）,and the current density is 2.7 times that of UA-NC-600-900.（3）In addition,the effect of different carbonization temperatures on the performance of N-C catalyst on CO₂RR was investigated.It was found that the pre-carbonization process at 600℃followed by pyrolysis at 900℃is beneficial to increase the pyridine nitrogen content and specific surface area of N-C catalyst,and also promoted CO₂RR performance.