Study On Preparation And Performance Of Polyvinylidene Fluoride, Alumina Cement/Filler Composite Bipolar Plates

Bipolar plate is one of the critical components of proton exchange membrane fuel cell (PEMFC), which influences not only the performance but also the cost of the stack. At the present time, the non-porous graphite bipolar plate has been adopted extensively because of its excellent properties, however, its manufacturing technology is relatively complicated and it is expensive to mill flow channels, so its production cost is very high. Extensive efforts have been made to develop alternative materials and bipolar plates. Due to low cost, ease of manufacturing, low gas permeability , as well as high corrosion resistance, polymer/filler composite bipolar plate was considered to be one of good candidates for non-porous graphite bipolar plate. However, at the present time, little research has been done to develop new polymer/filler composite bipolar plates except for carbon-based composite bipolar plates, which has comparatively low electrical conductivity and mechanical strength. Therefore, seeking new conducive filler for the substitution of carbon and manufacturing novel low cost composite bipolar plate with high performance has been an important topic for the development of bipolar plate. In addition, humidifying through external equipment is generally adopted currently, which increase the volume of the fuel cell stack. So, it is a considerable direction to humidify the NAFION membrane through internal equipment, for example, changing the materials or structure of bipolar plate to give it humidifying function. Therefore, with the background mentioned above and in order to develop the application field of titanium silicon carbide (Ti₃SiC₂), two kinds of new composite bipolar plates are investigated in this thesis.Firstly, the polyvinylidene fluoride (PVDF)-based conductive composites have been fabricated by compression molding technique. The percolation theory is applied to explain the conduction mechanism of the composite, the influences of conductive fillers' categories and particle size of Ti₃SiC₂ on the percolation threshold and percolation transition zone are discussed, also the microstructure of the PVDF-based conductive composite was analyzed by scanning electron microscope (SEM). With PVDF as the adhesive, graphite, titanium carbide (TiC) and Ti₃SiC₂ as conductive filler respectively, the PVDF/filler composite was prepared by compression molding technique. The effect of conductive fillers' categories on the physical properties of PVDF/filler composite was investigated. The experimental results show that for a content of PVDF of 20 Wt%, the volume conductivity of PVDF/Ti₃SiC₂ composite is the highest among PVDF/Ti₃SiC₂ composite, PVDF/TiC composite and PVDF/graphite composite. The other physical properties of PVDF/Ti₃SiC₂ composite, such as flexural strength, thermal conductivity, thermal expansion coefficient and volume density, were between PVDF/TiC composite and PVDF/graphite composite. Therefore, Ti₃SiC₂ was considered a good candidate to prepare composite bipolar plate with excellent performance.PVDF/Ti₃SiC₂ composite bipolar plate was manufactured by compression molding technique using Ti₃SiC₂ as conductive filler and PVDF as the adhesive. The effects of Ti₃SiC₂ content, the particle size of Ti₃SiC₂, the particle grade of Ti₃SiC₂ and compression molding technological conditions on electrical conductivity, flexural strength, thermal conductivity, volume density and water absorption ratio of the composite were systematically investigated. The experimental results indicate that, with 80 wt% Ti₃SiC₂, the mass ratio of coarse Ti₃SiC-2 particle to fine Ti₃SiC₂ particle was 4:1 and under appropriate technological conditions, the volume conductivity and flexural strength of the self-prepared PVDF/Ti₃SiC₂ composite bipolar plate are both higher than the target value established by American Department of Energy for PEMFC bipolar plate. As a result, the PVDF/Ti₃SiC₂ composite bipolar plate appears to be a good candidate for PEMFC bipolar plate. However, its thermal and corrosion resistant properties are needed to be improved further.To further optimize the physical properties and reduce the production cost of PVDF/Ti₃SiC₂ composite bipolar plate, the effect of second phase conductive filler such as TiC or graphite on physical properties of PVDF/Ti₃SiC₂ composite was investigated.The results indicate that as TiC content increases, the volume conductivity and thermal conductivity of the composite firstly increase and then decrease, and the flexural strength of the composite increases monotonously. As graphite content increases, the volume conductivity of the composite firstly decrease and then increases, the thermal conductivity of the composite increases comparatively fastly, and the flexural strength of the composite decreases linearly.Alumina cement/filler conductive composite, which will be used as the bipolar plate of proton exchange membrane fuel cell (PEMFC), was prepared by compression molding technique at room temperature using alumina cement as adhesive and Ti₃SiC₂ combined with graphite as conductive filler. The fundamental physical properties of the composite such as the volume conductivity, the flexural strength, the corrosion current density, the volume density and water absorption ratio were measured, and morphology of these composites were investigated by scanning electron microscope (SEM),the phase composition was also investigated by X-ray diffraction technique(XRD). The results indicate that the electrical conductivity, the flexural strength and the corrosion current density of the composite bipolar plate can meet PEMFC requirements when graphite content is 40wt%, alumina cement content is 20wt% and under appropriate technological conditions. The water absorption ratio of composite bipolar plate is about 4wt% which makes it possess self- humidifying function during the operating process of fuel cell.