Study On The Structure And Properties Of Carbon Paper For Gas Diffusion Layer

As the core material of fuel cells,the gas diffusion layer mainly uses carbon papers as the substrate layer.At present,carbon papers have been commercially produced abroad,while most of our research and exploitation are still at the laboratory stage.The following key problems need to be solved in the batch production of carbon papers: One is that the poor dispersion of carbon fibers results in unsatisfactory formation of raw paper,and it is difficult to obtain carbon papers with stable structure and properties.The other lies in the lack of researches on the structure and properties evolution of the matrix,carbon fibers or carbon papers during the carbonization process,thus it is unable to provide theoretical guidance for the batch production of carbon papers.In view of the above,this paper conducted the following researches:(1)The formation of carbon paper and fiber floc were quantitatively analyzed by 2D lab sensor.The effects of carbon fiber length,dispersant on paper formation and flocs distribution were investigated,and the relationship between flocs,paper formation and properties were also revealed.These results showed that there were different size flocs in raw paper,and the floc area of each size floc in raw papers decreased at first and then increased with the floc size increasing.The area of large-size flocs increased most obviously in accordance with the increase of fiber length.Under the optimal dosage of PEO,the look-through index of raw papers prepared with 2,4,6 and 8 mm carbon fiber decreased by 32.6 %,49.9 %,53.5 % and39.8 % respectively,among which PEO had a more significant dispersion effect on the large-size flocs.As the look-through index of raw paper increased,the surface roughness gradually increased,the tensile strength gradually decreased,and its mean pore diameter and maximum pore diameter increased.The in-plane resistivity of raw paper prepared with 2 mm carbon fiber increased gradually,while that of 4,6 and 8 mm basically remained unchanged.(2)The effects of different fiber length,carbonization temperatures,phenolic resin content and applied stress on the composition,structure and properties of carbon papers were investigated.The results showed that the in-plane resistivity and mechanical properties of carbon papers decreased significantly at 400-700 ?.With the increase of temperature in the range of 1000-1600 ?,the interaction between fiber and matrix gradually strengthened,leading to the occurrence of stress graphitization,and the in-plane resistivity and surface roughness of carbon papers gradually decreased,strength and toughness gradually improved.The proper increase of fiber length was beneficial to improve the strength and conductivity of carbon papers,but there were also more flocs in the carbon papers prepared with longer fiber,so the mechanical properties and electrical conductivity of the 8 mm sample were lower than that of the 4 mm sample.Furthermore,when the resin content was 300 %,the interlayer bonding of carbon papers enhanced,and then the tensile strength,bending strength and bending displacement were improved to the resin content of 60 % by 100 %,180 %,280 %respectively,while its in-plane resistivity decreased by 62 %.In addition,applying certain stress to carbon papers in carbonization stage contributed to inhibition of free shrinkage of resin carbon,restrained crack of resin carbon,improved quality yield and thereby the mechanical properties and surface roughness of carbon papers.Among them,when the applied stress was 900 Pa,the tensile strength and bending strength of carbon paper increased by 133.4 % and 57.6 % respectively compared with that without the applied stress.(3)The evolution of the structure and properties of carbon paper during graphitization was investigated.The results showed that the overall graphitization degree of carbon paper reached 83.6 % at 2400 ?.It was observed by polarized light microscope that the matrix region at the fiber-matrix interface presented anisotropy.Compared with carbon fiber,the matrix had higher graphitization degree,so the fiber length had less influence on the electrical conductivity of carbon paper.With the increase of graphitization temperature,the tensile strength and bending resistance of carbon paper first increased and then decreased,reaching a maximum value at 2100 ?.On the basis of above results,the pilot test of raw paper was carried out,and the properties of carbon paper prepared by handsheet and commercial samples were compared.It can be found that the mechanical properties and electrical conductivity of carbon paper prepared by pilot paper was better than that prepared by hand copy paper AVCARb-P75,closed to Toray-060.(4)The effects of small-diameter carbon fibers and graphite powder on the structure and properties of carbon papers were explored.The results showed that as the content of small-diameter carbon fibers was 30 %,the in-plane resistivity of carbon paper reached 12.10m?·cm,which reduced by 9.2 % compared with the blank samples.Besides,the tensile strength was 12.98 MPa and the bending strength was 33.37 MPa,both rose by 12.5 % and30.9 % respectively.When the graphite powder content was 10 %,the in-plane resistivity of carbon paper reached 9.75 m?·cm and reduced by 26.8 % compared with the blank samples.The tensile strength was 13.13 MPa and the bending strength was 32.77 MPa,with an increase of 13.9 % and 28.6 % respectively,after graphitization,the graphite powder had a more significant strengthening effect.It could be considered that adding small diameter carbon fiber or graphite powder could improve the in-plane resistivity and mechanical properties of carbon papers.