Fuel cell bipolar plate materials and preparation-Ⅱ

Graphite Bipolar Plate

Graphite is currently the mainstream application of bipolar plates in China due to its high electrical conductivity, strong chemical and thermal stability, and corrosion resistance. Graphite is a porous and brittle material with low strength and high brittleness. It cannot meet the air tightness requirements of bipolar plates. It needs to be repeatedly impregnated and carbonized to make non-porous graphite plates.

Non-porous graphite plates are generally prepared by graphitizing carbon powder/graphite powder and graphitized resin under high temperature (2500°C) conditions. This process requires strict heating procedures, so the production cycle is long and the cost is high. In addition, new pores may appear due to the evaporation of impurities after graphitization, resulting in a porosity of 20% to 30% on the surface of the graphite plate. The existence of pores has the risk of causing PEMFC leakage, thereby reducing the concentration of reaction gases and thus reducing the performance of the stack. Therefore, the graphite plate needs to be impregnated to reduce its porosity and improve its surface quality. Currently, the most widely used is after repeated Impregnated non-porous graphite sheets.

Machined graphite plate

Most domestic manufacturers use artificial graphite mechanical processing to achieve this.The process steps mainly include:

(1) Raw material preparation: Mix coke and asphalt at a temperature of 1 000 to 1 300 ℃ and coke to form carbon. Then the carbon material is impregnated with asphalt, baked, and then heated in an electric furnace at 2 500 to 3 000 ℃ to graphitized.

(2) Slicing: Roughly slice according to the size of the bipolar plate;

(3) Impregnation: After the graphite slices are impregnated with resin, the purpose of impregnation is to fill the pores on the surface and inside of the graphite with synthetic resin. It is usually impregnated for 24 hours, and then heat treated to solidify the resin. Depending on the impregnation resin, Divided into phenolic impregnated graphite and furfural alcohol impregnated graphite;

(4) Grinding: Since the size after slicing is relatively rough, and the surface finish of the impregnated graphite plate is also poor, coarse grinding, medium grinding and fine grinding are required;

(5) Engraving: Engraving is a key step in bipolar plate processing. The dimensional tolerance of the bipolar plate and the quality of the flow field depend on the accuracy of the engraving machine.

Due to the low strength and brittleness of graphite plates, this processing method is not suitable for forming ultra-thin bipolar plates (less than 1.5 mm). This method may cause excessive friction between the tool and graphite, resulting in poor dimensional accuracy and surface quality of the bipolar plate. Lei  found that during the machining process, graphite is crushed into small particles and fine dust, and the tool is subjected to high impact and compressive stress at the tip. The processing of the flow channel also increases the total cost. Therefore, this method has gradually become popular abroad. Being eliminated, most foreign manufacturers adopt injection molding or compression molding production methods.

Injection molded graphite plate

Injection molding graphite plate is to feed a certain proportion of graphite and resin mixture from the hopper of the injection molding machine into the barrel. The heated and melted mixture is injected into the closed mold through the nozzle under pressure. After cooling and shaping, the product is demoulded. . In order to improve the conductivity of the bipolar plate, some metal powder can be added to the mixture, while carbon fiber or ceramic fiber can be added to improve the mechanical strength. However, injection molding also has many disadvantages, such as long binder removal (up to 7 days), cracking in thick sections, dimensional limitations, and defects. Further graphitization can improve the properties of the sheet, but this will significantly increase the cost and, therefore, the method is not suitable for large-scale production.

Molded graphite sheet

In order to solve the problems of high processing costs and large-scale production, researchers began to use the molding process to manufacture bipolar plates. The process flow is: first prepare the mixed material of graphite powder and resin, then pre-process the mixed material and the mold, use the melting temperature of the polymer and a certain pressure to make the powder flow in the mold and fill the entire cavity, and then obtained the bipolar plate after  demoulding solidify. If the adhesive is a thermosetting plastic, it usually only takes a few minutes to solidify and demold; if the adhesive is a thermoplastic, the mold needs to be cooled to a temperature below the melting point of the adhesive before demoulding.

Judging from the current actual operation of hydrogen fuel cell vehicles in China, commercial vehicles such as passenger cars, logistics vehicles, sanitation trucks, and forklifts have become the main models in the market. The high-frequency use scenarios of commercial vehicles determine their requirements for the durability of parts. higher. Graphite bipolar plates are more suitable for the current demonstration operation of fuel cell commercial vehicles due to their long durability.

Metal bipolar plate

Compared with graphite bipolar plates, metal bipolar plates have good electrical conductivity, thermal conductivity, machinability, fewer manufacturing processes, can produce ultra-thin bipolar plates, and the mass production process is mature, which can greatly reduce the quantity. production costs have attracted much attention. So far, stainless steel, aluminum alloy, titanium alloy, nickel alloy, copper alloy and metal matrix composite materials have been used in bipolar plate manufacturing.