Research On MOF-derived Nitrogen-doped Carbon-based Materials As High-efficiency Oxygen Reduction Catalysts

Protecting the environment and conserving resources are the basic national policies of our country.We have been continuously exploring and adhering to the development model that combines economic feasibility with environmental sustainability.Direct methanol fuel cell(DMFC)has a good market prospect because of its advantages of fast startup speed,low operating temperature,small size,portability,simple structure,high energy conversion rate and environmental friendliness.However,in practical applications,the ohmic loss of the electrolyte and the lack of electrode dynamics in the DMFC lead to a significant reduction in the actual voltage and energy conversion efficiency,which hinder the development of the DMFC.It has been found that these problems can be solved by adding appropriate catalysts to the electrode side,which includes the choice of cathode catalysts.Selecting a catalyst with excellent performance can greatly affect the property and cost of the DMFC.Two ORR catalysts with comparable oxygen reduction performance to Pt/C but superior durability than Pt/C and methanol tolerance are prepared in this study.The catalysts are characterized by X-ray photoelectron spectroscopy(XPS)and scanning electron microscopy(SEM).The ORR electrochemical performance and methanol tolerance of the catalysts are tested by linear sweep voltammetry(LSV)and rotating ring disk electrode(RRDE).In this study,nitrogen-doped porous metal-free Z8-NCF-P/EG is successfully prepared as cathode ORR catalyst for DMFC under acidic conditions by simple one-pot method and high temperature carbonization,using expanded graphite(EG)as carbon carrier,ZIF-8 as nitrogen source and PVP as binder.The effect of ZIF-8 loading on the morphology and performance of the catalyst is investigated by changing the loading amount.Through a series of tests,it is concluded that Z8-NCF-P/EG-10(EG:ZIF-8:PVP=1:10:10)has the largest BET surface area,the best microscopic morphology,the most active sites,and it also has the best ORR performance,methanol tolerance and durability.EG provides the catalyst with acid resistance and high conductivity,and contributes to the uniform dispersion of ZIF-8 and prevents its stacking;ZIF-8 helps the catalyst to form a micro-mesoporous structure and provides a large number of mass transfer channels;The effective doping of N provides a large number of active sites;As an adhesive,PVP binds EG closely to ZIF-8.After carbonization,a thin carbon layer is formed to protect the integrity of ZIF-8 and prevent methanol penetration.The synergistic effect of the above features makes Z8-NCF-P/EG-10 have the best ORR performance and good methanol resistance and durability.In addition,we replaced ZIF-8 with Z8@Z67 and prepared the H-Z67-NCF-P/EG catalyst with hollow structure for the ORR reaction of DMFC under acidic conditions.It has been tested that H-Z67-NCF-P/EG-10 has good ORR performance,methanol tolerance and durability.This is related to the Co,N co-doped porous hollow structure.The presence of PVP and the high temperature pyrolysis in the inert atmosphere promote the formation of the hollow structure.Co and N doped into the carbon skeleton to form a large number of CoN_x active sites,and the cavity and mesopores provide attachment sites for the active sites,and effectively promote oxygen transport and mass transfer.The presence of Co also promotes the formation of CNTs on the catalyst surface,contributing to the conductivity and large specific surface area of the catalyst.EG can improve the conductivity and acid resistance of the catalyst and prevent methanol penetration.The synergistic effect of the above characteristics ultimately resulted in the excellent performance of H-Z67-NCF-P/EG-10 catalyst.