Effect Of Catalytic Graphitization By Boron On Structure And Properties Of Carbon Paper For Fuel Cell

Carbon paper is widely used as the base material of gas diffusion layer in fuel cells.In order to meet the operating conditions of fuel cells,carbon paper should have uniform porous structure,good conductivity/thermal conductivity,certain mechanical strength and high chemical stability.Among them,good conductivity is conducive to reduce the loss in the process of electron conduction,so as to improve the performance of the battery.The conductive properties of carbon paper are usually improved by increasing graphitization temperature and adding conductive carbon powder.However,higher graphitization temperature leads to lower mechanical strength,higher brittleness and higher production cost.Adding conductive carbon powder can improve the conductivity of carbon paper and maintain its mechanical strength,but it will cause the decrease of porosity and permeability.Therefore,how to prepare carbon paper with high conductivity,high strength,high permeability and low cost has become an urgent problem.Boron is the only element that can form a substitute solid solution with carbon,which can play a significant catalytic role in the graphitization of carbon materials.Therefore,in this paper,boron element with catalytic graphitization property was introduced into the preparation process of carbon paper,and the carbon paper with high graphitization and high performance were prepared at a lower temperature.（1）Effect of boronizing on impregnated paper and hot-pressed paperBoric acid（H₃BO₃）as boron source was added to phenolic resin solution and base paper was impregnated.By changing the boron concentration in the impregnating solution,three kinds of dipping paper with different boronizing amount were prepared,and subsequently the hot pressing paper containing boron was obtained by hot pressing.The composition,structure and properties of impregnated and hot-pressed paper were analyzed.The results show that the actual boron content of hot-pressed papers are 3393 ppm,8867 ppm and 14890 ppm respectively,and a new chemical cross-linking bond（B-O-C）is formed between H₃BO₃ and phenolic resin during hot pressing curing,which promotes the cross-linking of the resin.As a result,the thickness of the hot-pressed paper containing boron shrinks,the porosity and permeability decrease,but the tensile strength increases by 5.9%～22.5%.At the same time,the resistance of the non-conductive network increases by 14.2%due to the presence of boride.（2）Effect of boronizing on carbonization process of hot-pressed paperThe carbonized samples were prepared by carbonizing the hot-pressed paper.The effects of boronizing on the composition,structure and properties of the samples were studied.The results show that after carbonization at 1400℃,the actual boron content of boron containing carbon paper is 3825 ppm,12049 ppm and 12400 ppm respectively.The surface carbon structure and microcrystalline structure of the sample are not affected by boronizing.With the increase of boron content,the thickness of the sample decreases,the bulk density increases,and the porosity decreases.Appropriate boronizing has little effect on the permeability,surface resistivity and tensile strength of the samples.However,due to the excessive shrinkage of sample thickness,the permeability decreases by 22.4%,the surface resistivity decreases by 32.7%and the tensile strength increases by 7.1%under high boronizing content.（3）Effect of boronizing on the graphitization process of carbon paperGraphitized carbon paper samples were prepared by graphitization at 1800～2700℃.The effects of boronizing on the composition,structure and properties of carbon paper during graphitization were studied.The results show that boron will gradually escape from carbon paper with the increase of graphitization temperature,and the highest retention rate of boron is only 10.5%at 2700℃.Boron significantly catalyzed the graphitization of carbon paper,which reduces the graphitization transition temperature from 2400℃to 2100℃.At 2100℃,when the concentration of H₃BO₃ is 1.3 wt%,the porosity and air permeability of carbon paper are 68.4%and 331 mm/s,respectively.The surface resistivity is 5.34 mΩ·cm,which is 35.3%lower than the blank sample,and the tensile strength is 23.0 MPa,which is 13.6%higher than the blank sample at 2400℃.Therefore,the carbon paper with high strength,high conductivity,high permeability and low cost can be prepared boron catalytic graphitization.（4）Effect of boron with different chemical forms on the graphitization process of carbon paperNano boron,boron carbide and boron nitride particles were selected as boron sources,boronizing was carried out in the process of impregnation.The effects of different forms of boron on the structure and performance evolution of carbon paper in the process of graphitization were compared.The results show that all three boron compounds play a significant catalytic role in the graphitization of carbon paper.Similar to H₃BO₃,the graphitization transition temperature of carbon paper can be reduced from 2400℃to 2100℃by three different forms of boron,and the conductivity and tensile strength are significantly improved.This provides a more diversified boron source for the preparation of carbon paper with high performance and low cost.