Surfactant-assisted Preparation And Electrochemical Properties Of MOFs-derived Materials

The energy crisis and environmental issues have gradually become the top challenge facing the development of human societies.The demand for clean energy has gradually stimulated further research on efficient,cheap,and environmentally friendly conversion and storage devices.As one of the most promising candidates for new energy sources technology,fuel cells have aroused great interest.However,due to the slow kinetics of the cathode oxygen reduction reaction and the high cost of the only commercially available Pt/C catalyst,widespread application still faces serious challenge.Non-noble metal catalysts with excellent catalytic capabilities,low prices and excellent stability,have become a research hotspot recently.Derivatives of metal organic frameworks（MOFs）,especially zeolite imidazole frameworks（ZIF）,are used as precursors for the preparation of carbon-based non-noble metal catalytic materials due to adjustable structure,product stability,and low cost.Surfactants,as structure-directing agents,not only can adjust the morphology,increase the porous structure and conductivity,but also can improve the catalytic activity.In this paper,surface-active materials are used as additives to prepare cobalt-nitrogen-carbon doped porous catalysts,and the effects of different factors on catalyst performance are studied in detail.Major findings and conclusions are as follows:（1）Different Zn/Co ratios bimetallic MOF materials with even cubic box morphology,stable structure,uniform element distribution and variety of micropore-mesoporous structures were synthesized by adding the Hexadecyl trimethyl ammonium Bromide（CTAB）as surfactants.A very thin carbon interface structure forms on the surface of the ZIF structure after the decomposition of the CTAB,which can increase the conductivity of the final product.The material has the best catalytic ability both under acidic and alkaline conditions when the mole ratio of Co/Zn is 4:96.Under alkaline conditions,the half-wave potential of oxygen reduction reaction（ORR）text E1/2=0.88 V,which is better than commercial Pt/C catalysts.Under acidic conditions,the half-wave potential（0.77 V）is slightly lower than Pt/C catalysts.The one reason for the excellent performance is that Co at this ratio can effectively disperse and properly expose a large number of catalytic sites to facilitate the reaction with oxygen species.The other is that the existence of the hierarchical micro-/meso-pores formed due to the volatilization of the Zn and the carbonization decomposition of the CTAB,which can avail the rapid transmission of substances.Based on the calculation of the oxygen reduction reaction kinetics,under acidic and alkaline conditions,the electron revolutions of the ORR reaction of NC-4Co96Zn is approximately 4,which is similar to the commercial Pt/C catalyst,and has better stability under 0.1M KOH.It is also more resistant to methanol.Through A.C.impedance test,the charge transfer resistance of the synthetic sample is lower than that of other synthetic materials.（2）Another catalytic material was prepared by the assist of the bimetallic ZIFs and different amounts of polyvinyl pyrrolidone（PVP）surface active materials.The prepared composite material processes tiny nano-microspheres.After the carbonization of PVP and ZIF composite materials at high temperature,a lot of vacancies and defects are introduced for the material with the decomposition of PVP.In addition,the specific surface area of the carbonized material is increased while maintaining the original crystal form.Further,the catalytic activity enhanced with the increasing of the boundary unsaturated carbon content due to the mesoporous structure generated after the volatilization of Zn and the decomposition of PVP,which facilitates the transmission of oxygen.Of all samples,the one with PVP content of 45 mg has the best catalytic ability,the ORR test half-wave potential is 0.89 V,the OER test is in 0.1 M KOH,and the potential is 1.72 V at 10 m A cm^-2,which is better than commercial Pt/C catalyst,making it to be a promising dual-function catalyst for fuel cells.By the LSV testing at different speeds and the K-L equation calculating,the electron transfer path of the P-CNCo-3 materials is closer to the"direct four-electron path".Through the i-t measurement,this material have excellent durability and resistance methanol capacity.