Structural Modulation Of ZIF-8-based Nitrogen-doped Porous Carbon And Its Electrocatalytic Performance For Oxygen Reduction

Fuel cells are a new type of power generation device with great research and development prospects.However,the slow kinetic reaction rate of the oxygen reduction reaction(ORR)involved in the cathode limits its development.Designing and synthesizing a metal-free nitrogen-doped carbon ORR catalyst with high activity and readily available raw materials is a scientific problem that needs to be solved urgently.ZIF-8 has become a potential metal-free nitrogen-doped ORR electrocatalyst precursor due to its high specific surface area,high-temperature volatile metal Zn,and rich nitrogen content.However,the activity of the nitrogen-doped carbon directly derived from ZIF-8 is limited by the microporous structure and poor conductivity.For these scientific questions,this research aims to improve the pore structure and increase the number of active sites to modify ZIF-8,thereby improving the ORR performance of metal-free nitrogen-doped porous carbon(NPC)electro catalysts.The specific research content is as follows:(1)Preparation of NPC-1 electrocatalyst based on the regulation of the hydrolysis process of ZIF-8.The newly prepared ZIF-8 was added to the glucose(Glu)solution,hydrothermally heated for 4 hours to obtain Glu/ZIF-8,and then carbonized at high temperature to obtain NPC-1.The study found that the size of Glu/ZIF-8 is about half that of ZIF-8.This is because the H+produced by the hydrolysis of Glu protonates the imidazole matrix,which etches the surface of ZIF-8,thereby reducing the size of Glu/ZIF8.Compared with the carbon nitrogen(CN)derived from ZIF-8,NPC-1 has a higher content of mesopores and macropores,a higher degree of graphitization,and a higher content of pyridine and graphitic nitrogen.The onset potential(Eo)of NPC-1 is 0.99 V,the half-wave potential(E1/2)is 0.88 V,and the limiting current(JL)is 5.43 mA cm-2,which is much higher than CN and close to commercial Pt/C.NPC-1 also has good resistance to methanol and stability.It shows that the introduction of Glu can improve the ORR activity by changing the structure of NPC-1.The synthesis method of this work is simple,the preparation process is relatively environmentally friendly,and it provides a new idea for the preparation of high-performance metal-free catalysts.(2)Zn-MOF-74/ZIF-8 was prepared based on the idea of coordination competition,and carbonization to synthesize NPC-2 electrocatalyst.The competitive ligand 2,5dihydroxyterephthalic acid(DHTP)was added to ZIF-8 to obtain composite Zn-MOF74/ZIF-8.Studies have shown that the composite material has both the crystalline structure of Zn-MOF-74 and ZIF-8,indicating that there is a coordination competition between DHTP and the ligand 2-methylimidazole of ZIF-8,which can realize ZIF-8 partial conversion to Zn-MOF-74.Compared with the carbonized products of Zn-MOF74 and ZIF-8,NPC-2 has higher mesoporous macropore content,larger pore size,and higher pyridine/graphite nitrogen content.These characteristics are all conducive to improving the ORR activity of NPC-2.The activity of NPC-2(1.00 V,0.87 4.94 mA cm-2),is close to commercial Pt/C and much higher than ZIF-8 derived CN and Zn-MOF74 derived porous carbon(PC),NPC-2 also has better resistance to methanol and durability.The reason why NPC-2 has better ORR activity is mainly because it has both the microporous structure of CN and the mesoporous structure of PC.This work provides a new research idea for the preparation of composite types of MOFs,and also has enlightening ideas for the study of metal-free ORR catalysts.