The Beijing Winter Olympics is a good application scenario for hydrogen energy vehicles, which are used at low temperatures and have almost no carbon emissions throughout the cycle. In the future, hydrogen fuel cells, lithium batteries and fuel will be divided into three parts. Hydrogen fuel cell vehicles are mainly composed of several parts, battery system, drive motor system, vehicle control system, auxiliary energy storage system, on-board hydrogen storage system, etc. Among them, the graphite bipolar plate to be introduced today is an important component in the fuel cell stack.
The proton exchange membrane fuel cell stack is assembled from a single cell in series, wherein the single cell consists of an anode plate, an MEA (membrane electrode) and a cathode plate. Usually, the operating voltage of a single cell is 0.6~0.85V, and the number of single cells in series can be selected as required when integrating a fuel cell stack. As an important component of the proton exchange membrane fuel cell stack, the mass of the bipolar plate accounts for 60-80% of the fuel cell stack, the cost accounts for 20-40%, and almost occupies the entire volume of the entire fuel cell stack.
The role of the bipolar plate in the hydrogen fuel cell stack is mainly to drain and support conduction, so its own bipolar plate has certain performance requirements:
1. On the one hand, high conductivity is required, because it carries a certain mechanical strength;
2. In addition to current conduction, air tightness is better;
3. Another one is during use, because the bipolar plate environment needs to have a good corrosion resistance, and it needs to meet the same service life as the vehicle;
4. In addition, for the application of our automobile, there is a more sensitive cost, so the manufacturing efficiency of the electrode plate needs to be high, and its manufacturing cost needs to be reduced.
metal bipolar plate
1. At present, bipolar plates can be mainly divided into two categories, one is metal plates and the other is graphite plates. Graphite is now more commonly used in molded bipolar plates. In this piece of bipolar plates with different parameters, they have their own advantages, but there are also some disadvantages, so these two words can be said to be a competition and a complement to each other.
2. The main advantage of the metal bipolar plate is that it has good mechanical strength, good air tightness, and the plate can be made very thin, which is a great advantage in power density, but the existing problem is the problem of corrosion, which leads to There are currently some problems with the service life.
3. The graphite plate is relatively strong in terms of electrical and thermal conductivity, including corrosion resistance and service life, but at present, no matter what kind of graphite plate it is, currently due to mechanical strength, the entire double The thickness of the plate is too thick, so the volume of our stack is too large.
In addition, there is a composite bipolar plate, which is actually a graphite-based double substrate in the strict sense. Its main components include resin and conductive materials, which are formed by a molding process. Relatively speaking, graphite bipolar plates can be said to be a balance of performance between metal bipolar plates and traditional graphite bipolar plates, with relatively high mechanical strength and relatively high toughness, so in the matching design of materials, It can bring the lightness and thinness of this product, and the air tightness is better, but there are few manufacturers currently producing it. There are disadvantages of composite board, high resistance, and high initial mold cost.
At present, hydrogen fuel cell vehicles are mainly commercial vehicles. Commercial vehicles are not as sensitive to the volume and weight of batteries as passenger vehicles, and mainly use graphite bipolar plates. Taking the graphite bipolar plates used in hydrogen fuel cell commercial vehicles as an example, the current market price is about 150 yuan per group, while the number of bipolar plates required for a commercial vehicle equipped with a 75 kW stack is about 300 groups. The cost of the plate is as high as 45,000 yuan, and the cost of the entire stack is more than 100,000 yuan. It can be seen that the current output value of the graphite bipolar plate processing industry is very high.
Hydrogen energy bus
In 2020, the application of fuel cell systems in the market is mainly commercial vehicles, and the core components of fuel cell stacks are mainly graphite plate stacks. From the perspective of future development, graphite bipolar plates will still occupy commercial vehicles for a long time. larger share of the car market.
Graphite is a good conductor of heat and electricity. It has the advantages of high electrical conductivity, chemical stability, thermal stability, corrosion resistance, and low density. It has inherent and unique advantages for making bipolar plates.
Since graphite is a porous and brittle material with low strength and poor ductility (relatively brittle), it is difficult to meet the air tightness requirements of bipolar plates. Therefore, during processing, graphite needs to be repeatedly impregnated and carbonized to make it non-porous. The non-porous graphite bipolar plate with good air tightness.
Therefore, the graphite bipolar plate has high requirements on the manufacturing process during manufacturing, otherwise it is easy to make the manufactured bipolar plate have high porosity and poor air tightness, and the installation of a fuel cell stack will not only affect the performance of the stack Overall performance, it may also lead to hydrogen leakage and cause safety hazards.
So why does the graphite bipolar plate occupy such a large market share, and how does it change from graphite to bipolar plate step by step? Let's find out together:
1. Raw material preparation
At a high temperature of 1000-1300°C, coke and pitch are mixed to form carbon, and then the carbon material is impregnated with pitch, baked, and then graphitized at a high temperature of 2500-3000°C in an electric furnace.
According to the design size of the bipolar machine, preliminary rough slice processing is carried out.
After slicing, resin impregnation treatment is performed to fill the surface and internal pores of graphite, usually immersed for 24 hours, and then heat treatment is performed to cure the resin. According to the difference of the impregnating resin, it is divided into phenolic impregnated graphite and furfuryl alcohol impregnated graphite.
The size of the bipolar plate after slicing is relatively rough, and the surface finish of the impregnated graphite bipolar plate is also poor, so it is necessary to grind, which includes coarse grinding, medium grinding and fine grinding.
Engraving is a key step in the manufacture of bipolar plates. The dimensional tolerance and flow field quality of bipolar plates depend on the accuracy of the engraving machine.