Preparation Of ZIF-8 Based Multi-stage Pore Carbon Material By ZnO Template Method And Its Electrochemical Properties

Due to its high specific surface area,modifiable surface,adjustable pore structure and high nitrogen content,ZIF-8 is often used as a precursor to prepare MOFs-derived porous carbon materials and applied in the field of supercapacitors.However,the porous carbon material prepared by direct carbonization of ZIF-8 precursor still has the problem of single pore structure.They are mainly micropores but lack mesopores,which limits the transport rate of electrolyte ions,thereby hindering the improvement of the capacitive properties of carbon materials.In this paper,The surface of the ZnO template was actively treated with nitrogen-containing PVP(polyvinylpyrrolidone)as a surfactant.After being configured as a ZnO suspension,Zn(NO3)2 methanol solution and 2-methylimidazole methanol solution were added successively to to grow ZIF-8 with ZnO as the crystal core,and ZnO@ZIF-8 material with core-shell structure was synthesized.Then,the ZnO@ZIF-8 precursor was pre-oxidized,and a multi-stage pore(microporous-mesoporous)ZIF-8-based carbon material with good capacitance performance was prepared by "one-step" carbonization.The materials were characterized by morphology,phase structure,thermal properties,elemental composition,specific surface area and pore structure.The electrochemical performance of the prepared porous carbon electrode material was studied using an electrochemical workstation.The specific research content is as follows.(1)ZnO was used as the template and ZIF-8 was used as the precursor to synthesize ZnO@ZIF-8.In order to study the effect of mass ratio of PVP and ZnO and the titration rate of 2-methylimidazole methanol solution on the morphology of ZnO@ZIF-8.The synthesis process of ZnO@ZIF-8 was also optimized.The results showed that when the mass ratio of PVP to ZnO was 1:1 and the titration rate was 0.6ml/min,the crystal size of ZnO@ZIF-8 was uniform and polyhedral morphology,and ZnO template wasn’t observed on the surface.(2)The ZnO@ZIF-8 was carbonized at different temperatures to prepare ZIF-8-based porous carbon materials.The effects of carbonization temperature on the phase structure,specific surface and pore structure of ZIF-8-based porous carbon materials were investigated.The results showed that when the carbonization temperature was 950°C,the ZnO template could be removed smoothly,and the multi-porous carbon material with microporous-mesoporous structure could be prepared.(3)ZnO@ZIF-8 was pre-oxidized and carbonized to form ZIF-8-based porous carbon material.The effect of pre-oxidation process of ZnO@ZIF-8 on the nitrogen content and capacitance of porous carbon materials was studied.The results showed that the thermal stability of nitrogen-containing PVP was improved by pre-oxidation of ZnO@ZIF-8 precursor,so that the nitrogen of PVP itself remain in the porous carbon material during carbonization,which improved the nitrogen content of carbon material.The amount of nitrogen content reached 8.49 wt%,which improved the capacitance performance of the carbon material.When the current density was 0.1 A/g,the specific capacitance value was 241.1 F/g,and the specific capacitance retention rate was 55.1%.(4)By adjusting the concentration of ZnO suspension,ZnO@ZIF-8 materials with different ZnO doping amounts were synthesized and carbonized to prepare multi-stage pore carbon materials.The effects of ZnO suspension concentration on nitrogen content,specific surface area,pore structure and electrochemical properties of porous carbon materials were investigated.The results showed that with the increase of ZnO suspension concentration,the specific surface area of porous carbon material decreased gradually,the mesopores increased first and then decreased,and the nitrogen content increased.when the concentration of ZnO suspension is 6mg/ml,the mesopores were the most,and the mesoporous specific surface area was 454.1 m2/g.The prepared porous carbon material had the best electrochemical performance.When the current density was 0.1A/g,the specific capacitance value reached 275.3F/g,while at the current density of 10A/g,the specific capacitance value is still 166.4F/g,and the specific capacitance retention rate was 60.4%.After charging and discharging 10000 times at a current density of 5 A/g,the specific capacitance retention rate reached 96.77%.