Nitrogen-doped Carbon Coated Metal Nanoparticles For Electrocatalytic Reduction Of CO₂

Carbon dioxide(CO₂) from the use of fossil fuels such as coal,natural gas and oil has led to a gradual increase in global temperatures and worsening of the climate.Therefore,it is urgent to solve the CO₂ problem.Electrocatalytic CO₂ reduction(ECR)has a high application prospect due to its advantages of simple operation and environmental friendliness.At present,the focus of ECR research is to prepare electrocatalysts with high catalytic activity,selectivity and stability.ECR is a multi-electron transfer and multi-proton reaction,so variety of products may be obtained.Among many products,the syngas composed of CO and H₂ can be directly used as industrial raw materials to produce high added value products.However,the difficulty of ECR synthesis gas is how to efficiently prepare syngas with controllable H₂/CO ratio under high current density conditions.In this work,rGO supported nitrogen-doped carbon-coated nickel particle composite catalyst(Ni@N-C/rGO)was prepared using Ni-2-methylimidazole complex as precursor and GO as carrier.Ni@N-C is the active site for ECR to prepare CO,and rGO is the active site for catalytic hydrogen evolution reaction(HER)to prepare H₂.The catalytic performance is regulated by regulating the content of elements in the catalyst,and then the synthesis gas product with controllable hydrogen to carbon ratio is obtained.XRD,XPS,SEM and TEM were used to study the crystal phase structure,composition,element valence state and morphology of the prepared composite catalyst in detail.The active center controllable Ni@N-C/rGO catalyst was successfully prepared.Under the condition of high current density(>15 m A·cm^-2),the H₂/CO of the synthesized gas product can be accurately regulated in the range of9/1?1/9.The CO Faraday efficiency(FE_(CO)%)of the catalyst 40Ni@N-C/rGO can reach 90%and the current density can reach 25 m A·cm^-2 at-0.97 V(vs.RHE).At the same time,the current density of 40Ni@N-C/rGO catalyst was not significantly reduced after 15 h constant voltage test.These results indicate that the pyrolysis Ni-imidazole complex can be used to prepare ECR catalysts with excellent performance.On the basis of the above work,we further studied the influence of different imidazole ligands on the structure and catalytic performance of the prepared catalyst.Firstly,the Ni-imidazole complex was prepared with Ni²⁺by imidazole(IM),2-methylimidazole(M-IM),2-ethylimidazole(E-IM),2-propylimidazole(P-IM)and2-phenylimidazole(PH-IM),respectively.The Ni@N-C composites with different carbon contents were obtained by direct pyrolysis.The crystal structure,chemical composition,valence state and morphology of different Ni@N-C materials were studied by different characterization methods,and their catalytic performance was also studied.The results show that as the carbon chain of the substituent in the ligand increases,the carbon content in the obtained derivative material gradually increases,and the catalytic performance gradually decreases.The optimal ECR properties of Ni@IM composites were obtained when the molar ratio of Ni/imidazole was 1:6 and the pyrolysis temperature was 800?.The FE_(CO)%reached 92%and the current density was 8.2 m A?cm^-2 at-0.97 V(vs.RHE),and the current density of Ni@IM-700remains at 14.2 m A·cm^-2 after a 30 h long-term durability test,and the FE_(CO)% remains basically unchanged.