Spinel Co₃O₄ On N-doped Porous Carbon Derived From ZnCo ZIF Particles For Highly Selective Reduction Of CO₂ To Formic Acid

One of the major objectives of energy and environmental science is the construction of a carbon-neutral energy utilization system,which requires more sufficient approach to recycle the waste CO₂.Electrochemical reduction of CO₂?ERC?has gained worldwide interests due to its huge potential benefits from both environmental and economic point of view.The reaction mechanism and products vary depending on electrode/catalyst materials and reaction medium.Considering such a complex situation,the dedicated design of catalysts becomes the key challenge,which includes:high efficiency and selectivity at low over-potential;high stability;low cost and earth-abundant composition.Herein,we have developed a simple and highly scalable procedure for the synthesis of a special nanocomposite that comprises cubic spinel Co₃O₄ NPs and CNCo-n?N-doped porous carbon material derived from Zn/Co bimetal ZIFs,n indicates the molar ratio of Zn/Co?.This nanocomposite was optimized with the aim of combining the high catalytic activity of Co₃O₄ together with the excellent electronic properties of CNCo-n.Eventually,this approach has produced a hybrid material that has shown a very high activity toward the reduction of CO₂ to formic acid with excellent selectivity at lower overpotential.First,we studied the ERC activity of spinel Co₃O₄ on porous carbon derived from pyrolysis of ZIF-8,which was synthesized by a one-pot method.XRD,FTIR,SEM,and TGA were used to characterize the structure and properties of the ZIF-8 crystals.Spinel Co₃O₄ was loaded by hydrothermal method on the surface of N-doped porous carbon formed by carbonization of ZIF-8 at 900??Co₃O₄@C-N?.At the same time,commercial graphite was used as a contrast?Co₃O₄@C?.Electrochemical research showed that the onset potential and peak potential of formic acid for Co₃O₄@C were-0.925 V vs SCE and-1.03 V vs SCE,respectively,which were positively shifted to-0.894 V vs SCE and-0.958 V vs SCE for Co₃O₄@C,respectively.Moreover,the peak current density reached 3.62 mA·cm^-2,indicating that N-doped porous carbon can effectively promote the ERC activity of spinel Co₃O₄.In addition,the ECSA of Co₃O₄@C-N was 1.18 cm²·mg^-1,which is higher than that of Co₃O₄@C,explaining that the catalytic better performance of Co₃O₄@C-N.Co₃O₄@C-N has the highest formate acid efficiency of 53%,which is much higher than that of Co₃O₄@C.Secondly,we synthesized different different N-doped porous carbon materials CNCo-n by pyrolyzation of bimetallic ZIF materials with different Zn/Co ratios,and the same content of spinel Co₃O₄ was loaded to synthesize Co₃O₄@CNCo-n composite catalyst.Electrochemical experiments showed that onset potential of Co₃O₄@CNCo-1was the lowest,but the peak current density was only 6.227 mA·cm^-2.For Co₃O₄@CNCo-20,although the initial potential is about 50 mV higher than that of Co₃O₄@CNCo-1,the current density increased by 50%.The electrochemically active area indicates that Co₃O₄@CNCo-20 has the highest specific surface area for electrochemical activity,which was considered to be dominate factor for catalyst activity.Finally,CNCo-20 was selected as the carrier to study the effect of Co₃O₄ loading on catalytic performance.Co₃O₄-n@CNCo?n denotes the mass percentage of Co?were characterized by Raman,SEM,XPS,etc.The experimental results show that the peak current density of Co₃O₄-20@CNCo is 9.228 mA·cm^-2,which is twice that of Co₃O₄-10@CNCo(4.512 mA·cm^-2).At the same time,Co₃O₄-20@CNCo has the highest mass activity,reaching 39.20 mA·mg^-1_Co3O4.In addition,Co₃O₄-20@CNCo has the highest Faradaic efficiency of formic acid and reaches the maximum faradaic efficiency of 84%at-1.0 V vs SCE with the catalyst current density remained stable for8 hours at least.