Hydrogen fuel cell vehicles and graphite bipolar plates

Graphite bipolar plates play a crucial role in hydrogen fuel cell vehicles. These lightweight and robust plates facilitate the efficient conversion of chemical energy stored in hydrogen into electrical energy. Designed to enhance conductivity and resist corrosion, graphite bipolar plates maximize fuel cell performance and durability. Their advanced composition, which often includes carbon and other additives, ensures superior conductivity and machinability. By providing a reliable and cost-effective solution for the transportation sector, graphite bipolar plates contribute to the widespread adoption of hydrogen fuel cell technology, reducing greenhouse gas emissions and promoting sustainable mobility.

In the future, hydrogen fuel cells, lithium batteries, and fossil fuels will share the market. Hydrogen fuel cell vehicles consist of several main components, including the battery system, drive motor system, vehicle control system, auxiliary energy storage system, and onboard hydrogen storage system. Today, we will focus on the graphite bipolar plates, which are an important component within the fuel cell stack.

The proton exchange membrane fuel cell stack is made up of individual cells that are connected in series. Each cell consists of an anode plate, a membrane electrode assembly (MEA), and a cathode plate. Typically, the operating voltage of a single cell is between 0.6V to 0.85V, and the number of cells in series can be selected according to the needs of the integrated fuel cell stack. As a crucial component of the proton exchange membrane fuel cell stack, the quality of the bipolar plates accounts for 60% to 80% of the stack's weight, 20% to 40% of its cost, and occupies almost the entire volume of the fuel cell stack.

The primary function of the bipolar plates within the fuel cell stack is to facilitate drainage and support electrical conductivity. Therefore, the bipolar plates themselves have certain performance requirements:

1. They need to have high electrical conductivity to bear mechanical strength.

2. Besides conducting electricity, they must also have good gas tightness.

3. They need to exhibit excellent corrosion resistance during use and have a similar lifespan as the vehicle itself.

4. In terms of application in automobiles, cost is a sensitive factor. Therefore, the manufacturing efficiency of the bipolar plates needs to be high to reduce production costs.

Metal bipolar plates

Currently, bipolar plates can be mainly divided into two types: metal plates and graphite plates. Graphite plates, which are commonly produced by molding, are widely used in fuel cell stacks. Different parameters of bipolar plates have their respective advantages and disadvantages, making these two types of plates both competitors and complements.

The main advantage of metal bipolar plates is their good mechanical strength and gas tightness. They can be made very thin, leading to a significant advantage in power density. However, they suffer from corrosion issues, which currently affect their lifespan.

On the other hand, graphite plates excel in conductivity, thermal conductivity, corrosion resistance, and lifespan. However, due to mechanical strength limitations, graphite plates have relatively larger thicknesses, resulting in larger overall stack volumes.

In addition, there is another type of composite bipolar plate, which can also be considered as a graphite-based bipolar plate. It consists of resin and conductive materials and is formed through molding techniques. Graphite bipolar plates strike a balance between metal bipolar plates and traditional graphite bipolar plates in terms of performance. They have high mechanical strength and toughness, contributing to a lightweight product. Additionally, they exhibit good gas tightness. However, there are currently fewer manufacturers producing them.

Currently, hydrogen fuel cell vehicles are primarily used in commercial vehicles. Commercial vehicles are less sensitive to battery size and weight compared to passenger vehicles, making graphite bipolar plates the main choice. For example, the market price for a set of graphite bipolar plates used in hydrogen fuel cell commercial vehicles is around 150 RMB, and a commercial vehicle equipped with a 75 kW fuel cell stack requires approximately 300 sets of bipolar plates, resulting in a bipolar plate cost of around 45,000 RMB and a total stack cost exceeding one million RMB. As a result, the graphite bipolar plate processing industry has a high market value.

Hydrogen-powered buses

In 2020, the application of fuel cell systems in the market mainly focused on commercial vehicles, where graphite bipolar plate stacks were dominant. Looking ahead, graphite bipolar plates will continue to hold a significant market share in the commercial vehicle sector.

Graphite is an excellent conductor of heat and electricity, possessing high electrical conductivity, chemical stability, thermal stability, corrosion resistance, and low density. These inherent characteristics make graphite an ideal material for producing bipolar plates.

However, graphite is a porous and brittle material with low strength and poor ductility. It struggles to meet the gas tightness requirements of bipolar plates. Therefore, during processing, the graphite needs to undergo repeated impregnation and carbonization treatments to create porous-free, airtight graphite bipolar plates with good gas tightness.

Thus, the manufacturing process of graphite bipolar plates demands high precision. Otherwise, the plates may have high porosity, poor gas tightness, and negatively impact the overall performance of the fuel cell stack, potentially leading to hydrogen leakage and safety hazards.

So why do graphite bipolar plates occupy such a significant market share, and how do they transform from graphite into bipolar plates? Let's explore the steps involved:

1. Material Preparation:

   - Carbonization of a mixture of coke and pitch at temperatures of 1000-1300°C to form carbon.

   - Impregnating the carbon material with resins, followed by baking, and then graphitizing them at high temperatures of 2500-3000°C.

2. Slicing:

   - Roughly cutting the graphite block into preliminary dimensions according to the bipolar plate design.

3. Impregnation:

   - Impregnating the sliced graphite with resin to fill the surface and internal pores of the graphite. Typically, the impregnation process takes 24 hours, followed by heat treatment to solidify the resin. Different types of impregnation resins can be used, such as phenolic resin or furfuryl alcohol.

4. Polishing:

   - The sliced bipolar plates have rough surfaces, and the impregnated graphite plates may lack smoothness. Therefore, polishing is required, including rough, medium, and fine grinding.

5. Engraving Processing:

   - Engraving processing is a crucial step in bipolar plate production. The dimensional tolerance and flow quality of the bipolar plate depend on the accuracy of the engraving machine.