Product Regulation Of CO₂ Electroreduction On N-doped Carbon Nanotubes Encapsulating Transition Metal Electrocatalysts

Electrocatalytic reduction of CO₂ to produce high-value fuels and chemicals using renewable energy under mild conditions is an important frontier of energy chemistry,which has broad application prospects.It is of great significance to develop the low-cost and high-efficiency electrocatalyst.Carbon-based electrocatalysts have attracted much attention due to their excellent conductivity,stability and high specific surface area.In this paper,the N-doped carbon nanotubes encapsulating Ni-based alloy electrocatalysts were prepared through co-pyrolysis of cheap C,N source and transition metal salts at high temperature.The syngas with adjustable CO/H₂ ratio and ethylene could be obtained from electroreduction of CO₂ by adjusting the composition of encapsulated Ni alloy and surface active sites.In order to gain syngas with tunable CO/H₂ ratio from electroreduction of CO₂,a series of N-doped carbon nanotubes encapsulating Ni alloy nanoparticles with different compositions（NCNT@M_xNi,M=Fe,Co）were successfully fabricated through co-pyrolysis of melamine and metal precursors.The NCNT@M_xNi samples exhibited bamboo-like nanotube structure with large specific areas and high graphitization degree.The results indicated that the reduction products were CO and H₂,and the syngas with a wide range of CO/H₂ ratios from 0.5:1 to 3.4:1 has been achieved on NCNT@M_xNi.More importantly,the stable CO/H₂ ratio（2:1 and 1.5:1）syngas over a potential window from-0.8 V to-1.2 V（versus reversible hydrogen electrode）could be realized on NCNT@Co₁Ni and NCNT@Co₂Ni respectively,which meets the requirement for syngas fermentation.This feature is relatively essential when using the renewable wind-or solar-electricity energy to drive the reduction of CO₂.In addition,NCNT@Ni was used as a typical catalyst to investigate the activity of encapsulated metal nanoparticles.On the one side,the Faraday efficiency of CO decreased from 89%to 65%after removing coated Ni particles by concentrated acid heat treatment,which indicated that the encapsulated Ni nanoparticles played an important role in boosting CO formation from electroreduction of CO₂.On the other side,SCN^-was added to the electrolyte to poison the M-N_x metal sites on the catalyst surface.The results showed that the Faraday efficiency of CO remained unchanged,indicating the M-N_x metal sites did not contribute to the activity.CO₂ reduction products were regulated by loading Cu on the surface of NCNT@Ni and NCNT@Co_xNi.The effects of preparation methods and support materials on the activity and products distribution of CO₂ electroreduction were investigated.The results indicated the hydrogen evolution reaction was inhibited,meanwhile,formic acid and ethylene were produced by introducing surface active site Cu on the carbon nanotube surface.The Cu/NCNT@Ni＿EG prepared by the liquid phase reduction with ethylene glycol as reducing agent exhibited the best performance.The Cu nanoparticles of Cu/NCNT@Ni＿EG were uniform,and the electrochemical active surface area of Cu/NCNT@Ni＿EG was the largest.The Faraday efficiency of formic acid was 16%and the Faraday efficiency of ethylene was 14%at-1.5 V（versus reversible hydrogen electrode）.