Efficient Preparation,Growth Mechanism,and Functionazition Of Polypropylene-based Graphene Sheets

Graphene exhibits broad application prospects in optoelectronic devices,sensors,high-efficiency radiators,flexible wearable devices,and energy storage due to its unique two-dimensional structure and extraordinary properties.With the development of preparation technology and fundamental research on application,graphene has been widely explored in functional composite,new energy,and textile fields.However,the high price of graphene seriously hinders its application in these fields.Although the chemical vapor decomposition（CVD）method using a gas hydrocarbon as the carbon source is the mainstream technology for graphene synthesis,the extensive application is limited by its low preparation efficiency and the time-consuming post-processing process.Therefore,it still remains a key challenge for science and industry to achieve low-cost preparation of graphene,which is of great significance to the mass production and industrial application of graphene.In this paper,the one-pot pyrolysis method was successfully developed for preparing graphene using polypropylene（PP）as the carbon source and diatomite（DM）as the catalyst.The growth mechanism of graphene during the thermal degradation of PP was studied.Then,the nitrogen-doped graphene microsphere（NG）was synthesized by co-sintering technology using melamine（MEL）as the solid nitrogen source.In addition,the application of as-prepared graphene sheet（microsphere）in fuel cell microporous layer（MPL）and PP-based composite was explored.The major contents and results are summarized as follows:（1）Low-cost preparation technology of graphene was studied via the one-pot pyrolysis method using PP as the solid carbon source and DM as the catalyst.The effect of pyrolysis atmosphere,pyrolysis temperature,holding time,DM content,and size on the yield and structure of graphene was studied.The results demonstrated that the addition of H₂ promoted the growth of graphene on the DM surface under the same condition of DM content and pyrolysis process.When Ar/H₂=175/25 sccm,the graphene yield was 14.6%,which was 20.67%higher than that when only Ar（200 sccm）was injected.The graphene yield increased from 14.6%to 20.6%with the median particle size(D₅₀)of DM decreasing from 22μm to 0.41μm,indicating that the decrease in DM particle size facilitates the growth of graphene.In addition to the above factors,the graphene yield greatly depended on the DM content in DM/PP composite.When the pyrolysis temperature was 1000°C and holding time was 100 min,the yield increased with the increase of DM content in the range of 15～30 wt.%DM content.Graphene yield reached the highest（25.86%）when the DM content was 30 wt.%.Meanwhile,the pyrolysis temperature and holding time had little effect on the graphene yield.With the same condition as other process parameters,the key factor affecting the graphene structure was the pyrolysis temperature.The pyrolysis temperature was less than 850°C,no graphene was formed while the temperature increased to 1000°C,graphene with a high degree of graphitization was obtained.The I_D/I_G reached the lowest（0.68）,and the I_2D/I_G reached the highest（0.39）.When the temperature was further increased,the layer number and defect of graphene increased.In addition,the DM content and holding time had little effect on the graphene structure.After experiments under different conditions,the process parameters for the efficient preparation of high-quality graphene were established as Ar/H₂=175/25 sccm,1000℃,100 min,and 20 wt.%DM with D₅₀=0.41μm,respectively.The graphene yield was20.6%,and the obtained graphene was the few-layer graphene sheet.（2）The growth mechanism of polypropylene-based graphene was investigated.The effect of DM on the pyrolysis behavior of PP was studied using TG-FTIR and GC-MS techniques.The results showed that the thermal decomposition temperature of PP with 20 wt.%DM was 39℃lower than that of pure PP,indicating DM has a catalytic effect on the thermal degradation of PP.Further analysis by TG-FTIR and GC-MS showed that the addition of DM could change the cracking pathway of PP（moreβscission occurred）and promote the formation of olefins.The total yield of main olefin pyrolysis products from DM/PP was increased by 32%compared to that of pure PP.FTIR analysis further proved the interaction between DM and radical fragments of pyrolysis products,and the Si-O-C bond was formed during the heat treatment around400～500°C.The formation of the infusible cross-linked structure could facilitate the growth of graphene during a high-temperature graphitization process.The growth mechanism of graphene can be summarized as the following four steps:（i）the catalytic effect of DM on thermal degradation of PP resulting in promoting the formation of olefins;（ii）the interaction between radical fragments of olefins and DM;（iii）the formation of the infusible cross-linked structure at higher temperatures via radical fragments of olefins;（iv）the growth of graphene during the graphitization process when pyrolysis temperature is over 850°C.（3）Nitrogen-doped modified polypropylene-based graphene microsphere was synthesized using the spherical DM catalyst prepared by ball milling and MEL as the solid nitrogen source via co-sintering technology.The effect of MEL content and pyrolysis temperature on the yield,structure,and nitrogen doping configuration of NG was investigated.The results showed that the addition of MEL was beneficial to increasing the NG yield under the same pyrolysis process,which could be attributed to the carbon atoms decomposed from MEL participating in the growth of graphene.Under the condition of the same MEL content,the pyrolysis temperature had little effect on the yield.The structure of NG greatly depended on the MEL content and pyrolysis temperature.Under the condition of the same pyrolysis temperature,the I_D/I_G of NG increased when the MEL content increased,which implied the point defects in the sp²carbon network increased.Meanwhile,the I_2D/I_G decreased with the increase of MEL content,indicating the number of layers of NG increased.When the MEL content was constant,I_D/I_G increased and I_2D/I_G decreased as the pyrolysis temperature increased（900～1000℃）.The interconversion between nitrogen doping configurations of NG could be induced by the increase of MEL content and pyrolysis temperature.Through experiments under different conditions,when the MEL content was 20 wt.%and the pyrolysis temperature was 1000℃,the highest NG yield（25.89%）was obtained while the I_D/I_G ratio reached the lowest（0.97）,and the I_2D/I_G ratio reached the highest（0.71）.The total nitrogen doping content of NG was 2.53 at.%,and the content of graphitic N（42.55%）and pyridine N（34.17%）both reached the highest.The incorporation of nitrogen atoms could improve the electrical conductivity of NG,compared to that of non-nitrogen-doped graphene.（4）Application of nitrogen-doped graphene microspheres with a hollow structure in the MPL of fuel cells was studied.The MPL-NG for the proton exchange membrane fuel cell（PEMFC）was constructed using NG as the material of MPL.The effect of the hollow structure of NG on the water transport behavior of MPL-NG and the performance of the as-prepared membrane electrode was studied.The results showed that the uniform and flat MPL was successfully fabricated.The single-cell performance test showed that the maximum power density of the cell using MPL-NG as the cathode was 478.72 m W/cm²,which was improved by 23.9%compared with that of MPL-CB prepared from the commercial carbon black（CB）.The polarization curve demonstrated that the concentration polarization was greatly affected by NG,indicating that the improvement of battery performance was closely related to the discharge of water from the cathode.The water transport behavior test showed that MPL-NG had lower breakthrough pressure and higher initial water flow rate than that of MPL-CB,which further confirmed the volume buffering effect of NG played a key role in the discharge of the water generated by the cathode in the reaction process.（5）Graphene-coated diatomite/polypropylene（GDM/PP）composites were prepared by melt blending using graphene-coated diatomite（GDM）synthesized from the spherical DM((D₅₀=0.41μm)as the raw material.The effect of GDM on the fluidity of PP and on the structure and properties of GDM/PP were investigated.The steady-state rheological test showed that when a small amount（0.5～2 wt.%）of GDM was added,the complex viscosity of GDM/PP increased with the increase of GDM content.When GDM content reached 5 wt.%,the complex viscosity decreased significantly,which was lower than neat PP,indicating the addition of sufficient GDM could have a“slip effect”.The dynamic rheological test showed the“terminal effect”of the homogeneous systems between G’,G”andωof GDM/PP,which implied the interaction between GDM filler and PP matrix was reduced due to the coating of graphene.When the GDM content was less than 5 wt.%,the tensile strength of GDM/PP increased with the increase of GDM content.However,when the GDM content increased to 5 wt.%,the tensile strength and elongation at break decreased,respectively,which could be attributed to the pore defects caused by the“slip effect”of GDM during the molding process.In addition,the volume resistivity of GDM/PP decreased with the increase of GDM content.When the GDM content was 5 wt.%,the volume resistivity of GDM/PP decreased to 1.19ⅹ10⁹Ω·cm,which was seven orders of magnitude lower than that of pure PP(1.16ⅹ10¹⁶Ω·cm).