High-pressure Vapor-solid Transformation Of Low-dimensional Cobalt-based Template To Derive 2D Co-N-C Electrocatalysts For Oxygen Reduction Reaction

Co,N co-doped carbon catalyst（Co-N-C）is considered as best choice to replace noble metal Pt for oxygen reduction reaction（ORR）electrocatalyst in fuel cells because of its low cost,high activity and stability.Two-dimensional Co-N-C catalysts derived from two-dimensional zeolite imidazolate frameworks（ZIFs）have attracted extensive attention of researchers due to their highly accessible active sites.However,most of current methods to synthesis 2D ZIF are energy-intensive,environmentally hazardous,complex and low yield.The vapor-solid reaction method can not only avoid the pollution caused by using of organic solvents,but also preserve the morphology of the solid-phase template.However,the reaction is limited to the surface of the template,and the transformation efficiency is low.Therefore,it is key to realizing the real substitution of Pt catalysts that a simple,green and efficient method is developed to prepare 2D ZIFs and derive 2D Co-N-C catalyst.In this paper,the reaction space between the ligand vapor and the solid cobalt-based template was reduced in a confined way to create a high-pressure vapor-solid reaction state,so that the cobalt-based template could be efficiently transformed into a two-dimensional ZIF materials,and the two-dimensional Co-NC NS catalyst was obtained.The formation of large particles of Co during the pyrolysis process was inhabited by the regulation of Zn element,and the catalytic activity of the Co-N-C catalyst was finally improved.The main work and results were as follows:（1）Two-dimensional ZIF-67 NS template was prepared by high-pressure vapor-solid transformation of cobalt carbonate hydroxide nanosheets template,and subsequently pyrolyzed to obtain two-dimensional Co-N-C catalyst（Co-N-C NS）.The mechanism of high-pressure vapor-solid transformation of cobalt carbonate hydroxide was studied,and the mechanism of the performance enhancements of Co-N-C NS was analyzed.The results show that,under confined conditions,imidazole sublimation can form a high-pressure vapor-solid reaction state,and the CO₂ generated by template decomposition escapes to from various pores,which promotes the complete transformation of cobalt carbonate hydroxide nanosheet to ZIF-67 nanosheet.The Co-N-C nanosheets obtained from the pyrolysis of ZIF-67 nanosheets retaines the two-dimensional morphology of the template.Compared with the three-dimensional Co-N-C,the two-dimensional Co-N-C has a larger specific surface area,more accessible Co-N_x active sites and better ORR activities(E_on=0.93 V,E_1/2=0.85 V).（2）Co/Zn-ZIF template was prepared by high-pressure vapor-solid transformation of cobalt-zinc hydroxide nanosheet template,and subsequently pyrolyzed to obtain two-dimensional Co-N-C NS-X（X=0,0.25 and 0.5）.The mechanism of high-pressure vapor-solid transformation of cobalt-zinc hydroxide was studied,and the mechanism of performance enhancements of Co-N-C NS-X by Zn addition was analyzed.The results show that,the addition of Zn can effectively improve the composition and structure of the cobalt-zinc hydroxide template,regulate the activity of the high-pressure gas-solid reaction,ensure the preservation of two-dimensional morphology of the template,increase the specific surface area of the Co-N-C NS-X and form more Co-N_x active sites.In addition,the two-dimensional structure of the catalyst exposes more Co-N_x active sites at the reaction interfaces,which finally makes Co-N-C NS-0.5 have good ORR performance(E_on=0.98 V,E_1/2=0.85 V).（3）ZIF@Co_xZn_1-xO regular hexagonal microrod core-shell structure was prepared by high-pressure vapor-solid transformation of cobalt-zinc microrod template,and subsequently pyrolyzed to obtain two-dimensional Co-N-C NS-X（X=0.1-0.4）.The formation conditions of one-dimensional ZIF@Co_xZn_1-xO regular hexagonal microrod core-shell structure and the mechanism of its pyrolysis evolution into two-dimensional Co-N-C NS-X were investigated.The effects of different Co contens on the composition,structures and properties of Co-N-C NS-X were studied.The results show that regular hexagonal microrod core-shell structure of ZIF@Co_xZn_1-xO can be effectively formed by adjusting the reaction time.During the pyrolysis process,Zn is generated and sublimated by carbothermal reduction of the core layer,and the core-shell structure evolves into hollow hexagonal microtube,which is fractured along the edge of the hexagonal microtube under the thermal stress,forming two-dimensional Co-N-C NS-X nanosheets.By regulating Co content,the metal content in Co-N-C catalyst can be effectively controlled to avoid the formation of Co nanoparticles and large particles and to form as many Co-N_x active sites as possible.When Co/Zn ratio is 0.3,Co-N-C NS-0.3 has the best ORR performance(E_on=1.01 V,E_1/2=0.87 V).