Carbon Paper—A Core Material in the Hydrogen Energy Sector

As the transition of the global energy structure continues to deepen, research into new energy technologies has experienced rapid advancement. Hydrogen energy—a clean, efficient, and virtually carbon-emission-free energy solution—helps reduce reliance on fossil fuels and lower greenhouse gas emissions; consequently, it holds immense promise for both energy structural transformation and climate change mitigation. Driven by progress in hydrogen production, storage, and utilization technologies, the cost of hydrogen energy has gradually decreased while its scope of application has expanded, making it an indispensable component in the construction of a low-carbon society.

Proton Exchange Membrane Fuel Cells (PEMFCs) represent one of the core technologies for hydrogen utilization. Through electrochemical reactions, they convert hydrogen and oxygen into electrical energy, water, and heat. This entire process is highly efficient and generates virtually no pollutant emissions, making it suitable for a wide range of applications—from portable power sources to large-scale stationary power plants.The Gas Diffusion Layer (GDL) is a critical component within a PEMFC; it not only facilitates the uniform distribution of reactant gases across the electrode surface but is also responsible for expelling generated water and conducting electrons and heat. A GDL typically consists of a porous, conductive base substrate and a microporous layer. Due to its exceptional electrical conductivity, uniform pore structure, and robust mechanical properties, carbon fiber paper (hereinafter referred to as "carbon paper") has emerged as the preferred substrate material for GDLs.

Carbon fiber paper (or "carbon paper") is a paper-like composite material manufactured using a papermaking process; it is produced from chopped carbon fibers, utilizing natural or synthetic pulp as a matrix, and supplemented with binders and fillers. Carbon paper serves as the primary substrate for the gas diffusion layers in fuel cells. The process of converting carbon fibers into carbon paper constitutes one of the core technical challenges in GDL manufacturing, as the resulting material must satisfy a multitude of performance requirements: controllable porosity, excellent thermal and electrical conductivity, sufficient mechanical strength, strong hydrophobicity, and high corrosion resistance.

However, the domestic market for carbon paper remains largely monopolized by a handful of foreign companies. Domestic production relies heavily on the secondary processing of imported raw materials; consequently, both pricing and production volume are significantly constrained by raw material availability, resulting in a low cost-performance ratio. The limited domestic production capacity is insufficient to meet the surging market demand from my country's fuel cell industry, thereby—to a certain extent—hindering the independent and indigenous development of the nation's hydrogen energy and fuel cell sectors. 

01 Production Process

Regarding the production processes for carbon paper, a distinction is made between the wet method and the dry method. The wet method is considered a mature technology, and manufacturers such as Toray and SGL Carbon widely employ this approach. Carbon paper produced via the wet method exhibits excellent uniformity and a dense structure; consequently, it is the universally adopted method for manufacturing high-performance carbon paper for fuel cells internationally. The dry method for carbon paper production, conversely, is a technology that has developed rapidly in recent years. In this method, air serves as the medium; chopped carbon fibers are processed into a base sheet using an air-laid web-forming technique, followed by subsequent processing steps such as binder application, drying, and carbonization. The key characteristics of carbon paper produced via this method are its high carbon fiber content and superior product strength.

1. Dry-Laid Forming for Carbon Fiber Paper Production

Dry-laid forming is a papermaking technology that has seen rapid development in recent years. The process begins by opening and dispersing chopped carbon fibers into uniform individual strands. Using air as the medium, these fibers are then rapidly formed into a web via an air-laying technique to produce a base sheet. This base sheet subsequently undergoes binder application, hot-press curing, and carbonization/graphitization to yield the final carbon fiber paper. During the dry-laid forming process, the chopped carbon fibers typically range in length from 40 to 50 mm. The resulting carbon fiber paper features a high carbon fiber content, high product strength, and high electrical conductivity. However, due to the relatively long length of the carbon fibers, they are prone to poor dispersion—often tangling or knotting together. Furthermore, the extensive overlapping between fibers intensifies the "bridging" effect, leading to the formation of large pore structures; consequently, the resulting carbon fiber paper tends to exhibit relatively poor uniformity and a loose structure.

2. Wet-Laid Forming for Carbon Fiber Paper Production

Currently, the wet-laid forming process stands as the most extensively researched and widely applied production method—both domestically and internationally—for manufacturing high-performance carbon fiber paper intended for use in Proton Exchange Membrane Fuel Cells (PEMFCs). In this process, water serves as the medium; chopped carbon fibers of varying lengths (typically 3 to 20 mm) are uniformly dispersed within the water. A papermaking machine is then utilized to rapidly vacuum-filter this suspension onto a wire screen, thereby forming a carbon fiber base sheet. This base sheet subsequently undergoes a series of post-processing steps—including resin impregnation, hot-press curing, and carbonization/graphitization—to produce the final carbon fiber paper product. Compared to dry-forming methods, carbon fiber paper produced via wet-forming exhibits superior density and uniformity, making it highly suitable for processing into carbon fiber paper specifically designed for PEMFCs. Consequently, the subsequent discussion will focus primarily on providing a detailed overview of the wet-forming process. Furthermore, the performance metrics of carbon fiber paper produced both domestically and internationally are generally benchmarked against the "TGPH" series—a line of PEMFC-specific carbon fiber paper manufactured by Toray Industries (Japan).

02 Technical Challenges

During the continuous production of carbon paper, manufacturers face a preponderance of engineering-related technical challenges. Ensuring process continuity and enhancing the uniformity and stability of product quality are the primary areas of focus during mass production. Currently, achieving large-scale mass production of carbon paper within China remains challenging, primarily due to constraints imposed by complex processing techniques—such as carbon fiber graphitization and post-treatment—as well as issues related to manufacturing equipment.

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

Some carbon paper manufacturers are currently limited to producing only sheet-form carbon paper products; their production processes operate in a batch mode, making it difficult to guarantee consistent product quality. The roll-to-roll manufacturing process is a primary method for the continuous production of carbon paper; it can be broadly categorized into three stages: dispersion and web formation, resin impregnation and hot pressing, and carbonization and graphitization. In the dispersion and web formation stage, the uniform dispersion of carbon fibers and their subsequent formation into a web are critical factors influencing the continuity of production. The longer it takes to achieve uniform dispersion of the raw carbon fibers, the more complex the associated pretreatment equipment tends to be; conversely, the shorter the dispersion time, the more compact the equipment can be. Furthermore, since the wet carbon paper web lacks inherent bonding strength and is prone to tearing under uneven mechanical stress, various operational links—including fiber dispersion, web formation, web transport speed, and carbon paper winding—must be precisely coordinated and synchronized to ensure the continuous formation of the carbon paper web. In the resin impregnation and hot pressing stage, the coordinated operation of processes such as quantitative control of resin uptake, drying, continuous hot pressing, and winding is essential for ensuring process continuity. If the impregnation rate exceeds the hot-pressing curing rate, the carbon paper web may absorb an insufficient amount of resin, resulting in poor adhesion between the fibers and the resin matrix. Conversely, if the impregnation rate is too high, the carbon paper may become oversaturated with resin, potentially hindering the complete flow and distribution of the resin during the hot-pressing and curing phase. Finally, in the high-temperature carbonization or graphitization stage, the synchronization between the carbon paper's transport speed and its dwell time within the various temperature zones of the high-temperature furnace is the key factor for ensuring continuous operation.

2. Control of Product Quality Uniformity

Poor batch-to-batch quality uniformity is one of the primary factors currently hindering the domestic substitution of imported carbon paper. Significant variations in properties—whether between different locations within the same production batch or between different production batches—can cause substantial difficulties for downstream users. Inconsistent thickness, for instance, leads to significant deviations in volumetric density during the assembly of fuel cell stacks, thereby affecting critical electrode properties such as water distribution and gas permeability. Inconsistent electrical resistivity compromises the structural integrity of the conductive network within the gas diffusion layer, thereby undermining the uniformity of current density distribution. Furthermore, inconsistent mechanical properties result in significant variations in the tensile and flexural strengths of different carbon paper sheets, making the electrodes susceptible to damage during the stack assembly process. The issue of poor quality uniformity often encountered during the mass production of carbon paper stems fundamentally from the non-uniform and unstable carbon network structure that emerges after the carbonization of the carbon fiber-resin composite structure. The root causes likely include delayed adjustments to process parameters in response to fluctuations in raw material properties, unstable control over dispersant concentrations in the solution, and variations in the concentration of the resin-ethanol solution.

3. Engineering Breakthroughs for Key Core Equipment

Forming equipment constitutes the technological core for all carbon paper manufacturers. Currently, researchers primarily draw upon the experience gained from papermaking forming equipment to optimize and fine-tune these systems; however, whether these adjustments yield optimal forming results remains to be validated through actual production practice. Furthermore, further intensive research and development are required to address the specific challenges involved in forming single-layer, high-basis-weight carbon paper for specialized application scenarios.

Resin impregnation equipment is another critical component in the continuous production of carbon paper. Current impregnation systems largely adapt coating technologies developed in industries such as papermaking; however, the primary technical challenge in the context of carbon paper lies in achieving precise, quantitative resin impregnation—specifically, ensuring both a uniform resin loading level and a homogeneous distribution throughout the material. In laboratory settings, resin-impregnated carbon paper is typically cured via hot pressing using equipment such as flat-bed vulcanizers. However, scaling this process up to continuous mass production presents significant difficulties. Consequently, manufacturers have explored adopting the hot-roll pressing techniques utilized in the papermaking industry; yet, this approach carries the inherent risk that the line contact between the two rollers could potentially tear the carbon paper. As a result, attention has shifted toward alternative technologies—such as programmed step-wise flat-bed hot presses and double-steel-belt hot presses—with the latter offering particularly high levels of control precision.

Finally, the construction of continuous carbonization and graphitization equipment represents the most significant challenge. Such equipment enables the continuous thermal treatment of carbon paper, thereby ensuring the stability of its performance and quality. The length and temperature profiles of these continuous carbonization systems are determined based on various factors, including the carbon paper's tensile strength, tension requirements, and allowable sag. By precisely controlling the heating process, manufacturers can ensure that the carbon paper achieves its target performance specifications during the carbonization stage. In the continuous carbonization and graphitization process, the gas curtain protection system serves as a critical component for ensuring the quality of the carbon paper. By positioning gas seals or gas curtain devices at both ends of the furnace, it effectively isolates the interior from ambient air, thereby minimizing oxygen-induced corrosion of both the furnace chamber and the carbon paper. This maintains a low-oxygen working environment within the furnace, thereby facilitating the uniform carbonization and graphitization of the carbon paper.