Carbon Fiber Paper for Fuel Cells and Gas Diffusion Layer (GDL)

Carbon fiber paper is a paper-like composite material made from chopped carbon fibers as raw material, with natural or synthetic pulp as the matrix, and supplemented with binders and fillers through a papermaking process. Carbon paper is the main substrate of the gas diffusion layer in fuel cells. The process of turning carbon fibers into carbon paper is one of the core challenges in gas diffusion layer production, requiring the material to meet multiple performance requirements, including controllable porosity, excellent thermal and electrical conductivity, sufficient mechanical strength, strong hydrophobicity, and high corrosion resistance.

1. Production Process

Carbon paper production processes are divided into wet and dry processes. The wet process is a mature technology, used by manufacturers such as Toray and SGL Carbon. Carbon paper prepared by the wet process has good uniformity and a dense structure, and is the method commonly used for high-performance carbon paper in fuel cells abroad.

The dry process for preparing carbon paper is a rapidly developing technology in recent years. This method uses air as a medium, employing an airflow web-forming process to process the paper into a base paper, followed by subsequent processes such as sizing, drying, and carbonization. The key feature of carbon paper is its high carbon fiber content and high product strength.

1. Dry Forming Process for Preparing Carbon Fiber Paper

Dry forming is a rapidly developing papermaking technology in recent years. First, short-cut carbon fibers are opened and dispersed into uniform single fibers. Then, using air as a medium, a web is quickly formed to create the base paper. This is followed by bonding with an adhesive, hot pressing, curing, carbonization, and graphitization to produce carbon fiber paper.

The short-cut carbon fibers used in the dry forming process are typically 40-50 mm in length. The resulting carbon fiber paper has a high carbon fiber content, high product strength, and high conductivity.

However, due to the longer fiber length, the fibers are difficult to disperse, easily entangled and knotted. The overlap between carbon fibers also enhances the "bridging" effect, forming a large-pore structure, resulting in relatively poor uniformity and a loose structure in the carbon fiber paper.

2. Wet Forming Process for Carbon Fiber Paper

Currently, the most researched and widely applied preparation process for high-performance carbon fiber paper for PEMFCs, both domestically and internationally, is the wet forming process. This process uses water as a medium, uniformly dispersing short-cut carbon fibers (3-20 mm) of varying lengths in water.

The carbon fiber base paper is then rapidly vacuum-filtered on a filter screen using a paper machine. Subsequent processes include resin impregnation, hot-press curing, and carbonization/graphitization to produce the final product.

Compared to dry forming, wet forming produces carbon fiber paper with excellent density and uniformity, making it highly suitable for processing into PEMFC-specific carbon fiber paper.

Furthermore, the performance indicators of carbon fiber paper produced domestically and internationally are generally referenced to the "TGPH" series of PEMFC-specific carbon fiber paper manufactured by Toray Industries, Inc. of Japan.

2. Technical Challenges

Continuous production of carbon paper faces numerous engineering challenges. Ensuring continuous production and improving product quality uniformity and stability are key concerns during mass production.

Currently, large-scale mass production of carbon paper is difficult in China, primarily due to the complex processes and equipment involved in carbon fiber graphitization and post-processing.

1. Process Coordination in Roll-to-Roll Continuous Production

Some carbon paper manufacturers can only produce sheet-like carbon paper products, with intermittent production processes that make it difficult to guarantee product quality uniformity.

Roll-to-roll production is an important method for continuous carbon paper production, which can be divided into dispersion forming, resin impregnation and hot pressing, and carbonization and graphitization.

In the dispersion forming stage, the uniform dispersion of carbon fibers and the web forming are important factors affecting continuous carbon paper production.

Since the wet carbon paper blank has no adhesive force, it is prone to tearing when subjected to inconsistent stress. Therefore, different stages such as fiber dispersion, web forming, carbon paper blank speed, and carbon paper winding need to be coordinated and synchronized to ensure continuous carbon paper forming.

During the resin impregnation and hot pressing stage, coordinated operation including resin impregnation quantity control, carbon paper drying, continuous hot pressing, and carbon paper winding is crucial.

During the high-temperature carbonization or graphitization stage, the coordination between the carbon paper's travel speed and the residence time within different temperature ranges in the high-temperature furnace is key to continuous operation.

2. Product Quality Uniformity Control

Poor batch quality uniformity is one of the main factors restricting the current lack of domestic substitution for carbon paper.

Inconsistent thickness leads to deviations in bulk density during carbon fiber stack assembly, affecting water distribution and permeability of the electrodes.

Inconsistent resistivity degrades the conductive network structure of the diffusion layer, impacting current density uniformity.

Inconsistent mechanical properties result in variations in tensile and flexural strength, potentially leading to electrode breakage during stacking.

The poor quality uniformity issue in carbon paper mass production is essentially due to the non-uniform and unstable carbon network structure after carbonization of the carbon fiber-resin composite structure.

3. Engineering Breakthroughs for Key Core Equipment

Forming equipment is the core technology for carbon paper manufacturers. Further research is needed to meet requirements for forming single-layer high-basis-weight carbon paper for specific applications.

Resin impregnation equipment is also crucial in continuous carbon paper production. A key challenge lies in quantitatively impregnating the resin to ensure a uniform and even resin load.

Continuous hot-pressing and curing becomes more difficult in mass production. Therefore, programmed step-type flatbed hot presses and double-belt hot presses have gained attention.

Foreign double-belt hot presses offer higher control precision, while domestic equipment still lags behind.

Finally, continuous carbonization and graphitization equipment presents the greatest challenge. By precisely controlling the heating process, carbon paper can achieve ideal performance indicators.

In continuous carbonization and graphitization, the air curtain protection system is crucial. By installing gas seals or air curtain devices at both ends of the furnace, it isolates outside air, reduces oxygen erosion, and maintains a low-oxygen environment conducive to uniform carbonization and graphitization.