Structural Engineering Of Coal-derived Carbon-based Electrocatalysts For CO₂ Reduction

The conversion of CO2 into high value-added chemicals by electrochemical technology can not only realize the resource utilization of CO2,but also help alleviate the greenhouse effect and energy shortage.However,the molecule structure of CO2 is extremely stable,resulting in slow kinetics of CO2 reduction reaction(CO2RR).Meanwhile,the low solubility and slow diffusion rate of CO2 in water electrolyte also limit the improvement of electrocatalytic performance.Therefore,the development of a new type of catalyst with high catalytic activity and mass transfer rate is the key to realize the practical application of CO2RR.In this paper,a series of new carbon-based electrocatalysts with controllable structure,abundant active sites and excellent conductivity were prepared from carbobitumen by using template induction,ammonia etching,solvothermal and defect engineering strategies.The main contents are as follows:Preparation of nitrogen-doped porous carbon nanosheets.To address the problem of poor conductivity of porous carbon-based catalysts.The wrinkled carbon nanosheets with rich surface nitrogen defects and well-developed pore structure was prepared by precarbonization and high temperature ammonia treatment,with coal tar pitch as carbon precursor,and sodium chloride particles as templates.Nitrogen defects with high electrocatalytic activity were formed on the surface of the material by ammonia etching.Large specific surface area and rich micro/mesoporous structure were favorable for electrolyte infiltration and mass transfer.The graphene-like nanosheet structure enhances the contact between the electrolyte and active sites,shortens the mass transfer path.The surface wrinkles can prevent the stacking of the carbon nanosheets,increasing the exposure of active sites.Meanwhile,the continuously distributed high graphitization region in the carbon matrix forms electron transport channels and facilitates electrochemical reaction.When the material was used as an electrocatalyst for CO2RR,the initial reaction overpotential was only 0.19 V.At the overpotential of 0.49 V,the Faradaic efficiency of CO can reach 84%.The current density can be maintained at 1.5 mA cm-2 after 8 h of electrolysis,and the Faradaic efficiency of CO can be maintained at over 80%.Construction of oxygen vacancy-enriched SnO2/carbon foam monolithic catalyst.Aiming at the problems of low current density and low yield of the formate in the traditional tin powder catalyst,the asphaltene-based carbon foams were prepared by template replication with asphaltene extracted from coal residue as carbon source and polyurethane foam as structure-oriented template.After that,the monolithic carbon foam supported oxygen vacancy-enriched tin oxides nanosheets(Vo-SnO2/CF)were prepared by hydrothermal and Ar plasma engraving.Asphaltene-based carbon foams have excellent conductivity and rich macroporous structure,are ideal carriers for electrocatalysts.The oxygen vacancy-enriched tin oxides nanosheets grow vertically on the surface of the carbon foams,enhancing the contact between the SnO2 nanosheets and electrolyte.The oxygen vacancies enhanced the interaction between the catalysts and the reactants and intermediates,and enhanced the catalytic activity of formate production.When the material was used as an electrocatalyst for CO2RR,the Faradaic efficiency of formate reached 86%at the potential of-1.0 V vs.RHE,and the partial current density of formate reached 30 mA cm-2.At this potential,the Faradaic efficiency of formate could remain above 80%after 8 hours of electrolysis.The maximum yield of formate reached 432.8 μmol h-1 cm-2.Creation of nitrogen-doped tubular carbon foam gas diffusion electrodes.To address the issue of the syngas cannot be generated stably in a wide potential window,which caused by the small three-phase interface between the conventional planar electrodes,electrolyte and CO2,as well as the limited mass transfer.Nitrogen-doped tubular carbon foam materials with dual functions of gas diffusion electrode and self-supporting electrocatalyst were successfully prepared by high temperature ammonia etching and mechanical processing with asphaltene-based carbon foam as precursor.The tortuously complex pore structure of carbon foam combined with the tubular structure makes the CO2 gas form forced convection inside the electrode,thus increasing the three-phase contact interface,reducing the CO2 diffusion distance and increasing CO2 concentration.Which breaking the mass transfer limit,and making the electrocatalytic reaction proceed smoothly within the ultra-wide potential window of-0.5～-1.3 V vs.RHE.By changing the etching conditions of ammonia,the surface chemical properties of carbon foam were regulated,and the CO/H2 ratio in syngas was controlled within the range of 1:0.5-3.The catalyst worked continuously for 8 h,and the ratio of syngas did not change obviously,which showed excellent stability.