Analysis of graphite bipolar plate production process

At present, the application of fuel cell system in the market is mainly commercial vehicles, of which the core component of fuel cell reactor is mainly graphite plate reactor, from the future development point of view, graphite bipolar plate will still occupy a large share of the commercial vehicle market for a long time. 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?

According to the Fuel Cell Technical Team Roadmap report released by the US Department of Energy in 2017, the requirements for the performance of fuel cell bipolar plates can be seen in the following table:

The cost, mechanical properties, air permeability, corrosion resistance, electrical conductivity and surface contact resistance of the bipolar plate have strict requirements as the core indicators of the bipolar plate, so the selection of raw materials and process processing technology of the graphite bipolar plate affect whether it can meet the needs of the final target market.

Raw material

Different manufacturers of graphite bipolar plate production process is not the same, the main materials include graphite materials, resin impregnation materials, curing materials, etc., according to different graphite plate process, raw material selection type, material, specifications and grades have a great difference.

According to different process requirements, graphite can be made into powder, coil, sheet and emulsion, but its raw materials are mainly divided into three categories:

Graphite powder: chemical reaction is very sensitive substances, in different environments inside his resistivity will change, depending on whether the graphite powder in the insulating object can ensure that the graphite powder is uninterrupted. High temperature resistance, chemical stability, plasticity and thermoplastic resistance are very good.

Expanded graphite: a loose and porous worm like material obtained from natural graphite scales by intercalation, washing, drying, and expanding at high temperature. In addition to the excellent properties of natural graphite itself, it also has the characteristics of softness, compression resilience, adsorption, radiation resistance and other properties that natural graphite does not have, and expanded graphite can instantly expand 150 to 300 times at high temperature.

Flake graphite: complete crystallization, natural crystalline graphite,its shape like phosphorus, hexagonal crystal system, layered structure, thin and good toughness, excellent physical and chemical properties, with good thermal conductivity, electrical conductivity, thermal shock resistance, corrosion resistance and so on.

The continuous production and processing of graphite bipolar plates are mainly divided into two categories, one is the sheet molding process of expanded graphite composite materials, and the other is the molding/injection molding process of graphite powder resin conforming materials.

* Source cited the analysis report of these two types of processes published by AFCC Canada in 2017:

Category 1: The expanded graphite sheet is calendered, shaped and removed, and then enters the resin impregnation, vulcanization, bonding and sealing vulcanization processes, and finally forms a bipolar plate product.

At present, the process has the following characteristics:

It can meet the requirements of the shape, surface geometry and mechanical size of the plate;

Due to the continuity of the expanded graphite material, it has high electrical and thermal conductivity characteristics;

The surface hydrophobic characteristics that meet the requirements need to be solved through collaborative treatment of process and material.

If the rolling process is adopted, the design accuracy and process accuracy should be further improved;

Closing pressure, resin ratio and post-treatment of surface contact resistance are the key factors affecting the performance of the plate.

Category 2: The mixed material of graphite powder and resin is prepared first, and then the mixed material and mold are prepressed, then moulded and vulcanized, and finally bonded and sealed to form the product.

At present, the process has the following characteristics:

1. It can well meet the requirements of the shape of the plate, the surface geometry and the mechanical size;

2. Because the mixed particles block the continuity of the graphite material, the conductivity under high current should be further improved;

3. To meet the needs of surface hydrophobic characteristics need to be solved by collaborative treatment of process and material;

4. The ratio and selection of resin, vulcanization time and post-treatment of surface contact resistance are the key factors affecting the performance of the plate.