Graphitized Carbon Panels and Papers for GDL

Graphitized Carbon Fiber Paper and Panel Manufacturing

Graphitized carbon fiber papers and plates begin as a thin fiber mat or veil, created through a wet-laying process similar to traditional papermaking. The mat is then impregnated with a thermosetting resin, which serves as a binder, bonding the fibers and providing strength and the desired thickness. Depending on the required thickness, multiple layers of the veil are bonded together and undergo thermal pressing. The material is then carbonized, converting the resin into carbon. Finally, the carbonized product undergoes graphitization at temperatures ≥ 1600 °C.

For products requiring additional microporous layers (MPL) and PTFE coating, the graphitized part is fed into a coating machine, where it is coated with the MPL or PTFE dispersion. After coating, the material is dried in an oven to evaporate the solvent.

Hydrophobization (PTFE Treatment) in Graphitized Carbon Fiber Papers and Panels

Graphitized carbon paper and plate after manufacturing is somewhat hydrophobic. Water contact angle of graphitized carbon papers that are untreated are usually around 100 °. For some applications, like in fuel cells, more hydrophobic character is needed as water management is critical for fuel cell stacks. For this reason, a hydrophobic, polymer coating is applied to the graphitized carbon fiber product. 

Comparison of the water contact angle on graphitized carbon paper before and after hydrophobic PTFE treatment.

The untreated carbon paper (left) has a contact angle of 114°, indicating moderate hydrophobicity. After PTFE treatment (right), the contact angle increases to 139°, enhancing water repellency. This improvement helps prevent water accumulation in fuel cells and electrochemical applications, optimizing gas diffusion and overall performance.

Microporous Layers in Graphitized Carbon Fiber Papers and Panels

The polymer coating on carbon paper and panels makes them more hydrophobic, which helps with water management. However, this comes at a cost. It reduces both porosity and electrical conductivity:

❌ The polymer coating fills some of the pores in the carbon paper or panel, which reduces the overall porosity.

❌ At the same time, most polymer coatings are electrically insulating. When the coating covers the carbon fibers, it disrupts the conductive pathways, increasing electrical resistance and lowering the overall conductivity of the material.

To balance this, a thin microporous layer (MPL) is added on top of the hydrophobic coating. The MPL is made of carbon powder mixed with a polymer and has very small pores, typically around 100 nm in size (ranging from 50 nm to a few micrometers). Because of its small pores and hydrophobic nature, the MPL prevents liquid water from clogging the material, which is especially useful in fuel cell stacks. Additionally, the carbon in the MPL improves electrical conductivity and lowers contact resistance between the gas diffusion layer (GDL) and the catalyst layer. This helps manage water buildup without negatively affecting the GDL’s conductivity.

 

LINQCELL Graphitized Carbon Fiber Papers with MPL and PTFE Treatment

Note: The standard width of a roll of graphitized carbon fiber paper with MPL and PTFE coating is 40 cm.

Is it possible to coat thick (thickness > 340 um) with MPL and PTFE coating?

Yes, it is possible, though more challenging and cost-intensive compared to thinner materials. While roll-to-roll processing is typically more efficient for thinner sheets (<340 µm), we have successfully coated thicker materials as well. Although less commonly requested, CAPLINQ has experience with this and continues to optimize the process for improved efficiency.

What does Graphitization do?

Graphitization is a heat treatment of carbon-based materials typically above 1000 °C under controlled conditions. It is similar to carbonization, but carbonization occurs at lower temperatures, between 600 °C and 1000 °C. During carbonization, volatile materials like water, gases, and light organic compounds are removed from carbon fiber papers and panels, resulting in a carbon-rich material. After carbonization, the carbon fiber paper generally retains its original structure and properties.

The story is different during graphitization. In addition to being carbon-rich, the material undergoes structural changes as the high temperature provides the energy needed for the carbon atoms to rearrange into a more ordered, crystalline structure similar to that of graphite. This transformation enhances the material properties, such as electrical conductivity, mechanical properties, and thermal stability.

Structural Changes after Graphitization

✅ Increased Crystallinity
Carbon atoms form a hexagonal structure.

✅ Graphite-like Structure
Parallel stacking of hexagonal carbon rings, resembling graphite.

✅ Higher Carbon Purity
Loss of N, H, and other volatiles

✅ Defect Reduction
Fewer disordered or amorphous regions as sp² bonding increases.

What happens at increasing graphitization temperatures?

Our LINQCELLTM graphitized carbon fiber papers and panels have been graphitized at 1600 and 2000 °C. As a general rule of thumb, higher graphitization temperatures yield higher degree of graphitization. In other words, carbon-based material becomes more graphite-like with increasing graphitization temperature. From this, materials graphitized at higher temperatures are expected to exhibit more (improved) graphite-like properties. That is, at increasing graphitization temperatures, higher Young’s modulus (lower compressibility), higher electrical and thermal conductivities, and better chemical stabilities are to be expected.

Cost Drivers for LINQCELL Graphitized Carbon Fiber Papers and Panels

Graphitization enhances the electrical, thermal, mechanical, and electrochemical properties of carbon fiber products, but it comes with a hefty price tag. Furnaces running at 2000 °C for hours are as expensive and energy-intensive as it sounds. Optimizing carbon paper production schedules can procure both financial and environmental benefits.

Strategies to Improve Production Costs of Graphitized Carbon Fiber Papers

Optimize Furnace Runs

Furnace runs are expensive, contributing to over 50% of operating costs. Running full furnace loads (instead of small batches) minimizes cost per sheet.

Reduce Machining Costs

Using maximum sheet sizes directly from the furnace can cut costs by up to 25%. Relaxing dimensional tolerances reduces material waste and improves yield.

Optimize Material Usage & Sizing

Avoid cutting odd shapes; sheets are rectangular by default. Designing around furnace dimensions (50 cm× 50 cm) maximizes efficiency. Minimizing unnecessary cutting lowers costs and reduces environmental impact.

Testing Methods for Graphitized Carbon Fiber Papers and Panels

Thickness Measurement

Thickness of Graphitized Carbon Fiber Paper and Panels/Plates refers to the physical measurement of the material's depth or height, typically measured in millimeters (mm) or micrometers (µm). This thickness is important in different applications, especially in electrochemical devices like fuel cells, water electrolyzers, and other energy storage systems.

• Graphitized Carbon Fiber Paper: It typically ranges from 100 µm to 500 µm, depending on the specific use case. These thin papers are often used as gas diffusion layers (GDLs) in fuel cells. The thickness of the paper affects the transport properties, such as gas diffusion and conductivity.

• Graphitized Carbon Fiber Panels/Plates: These can vary more widely, often between 1 mm to several millimeters in thickness, depending on their intended use, structural requirements, and the desired mechanical properties. In water electrolyzers, thicker panels may be used to handle higher mechanical stresses or improve thermal conductivity.

  Read more about why thin graphitized carbon fiber papers are used in fuel cells →

Thickness Gauge: Mitutoyo Absolute Digimatic Indicator ID-S 543
Test Standard: ASTM D645 (Standard Test Method for Thickness of Paper and Paperboard)
Weight Load: 50 kPa according to ASTM D645 but can be chosen according to specifications.

Note: The thickness of our products is reported at 50 kPa, per the standard, unless otherwise specified.

Parallelism

Parallelism quantifies the degree at which two or more surfaces on a plate or sheet maintain a consistent distance or are equidistant from each other along a specific reference plane or axis. It is typically expressed as the difference between the thickness of the highest and lowest points on an object or between two surfaces.

Parallelism Measurement Equipment

• Precision steel base (600 × 600 mm2) and testing parts
• Electronic meter
• Statistical software, 4-pin signal output box, and foot switch
• Working table (maximum load = 1000 kg)

Parallelism Measurement Procedure

1. Fix the sample on the flat plate.
2. Measure straight by moving either the sample or the height gauges. The number of measurement points will vary depending on the sample dimensions. For example, a 54 × 54 cm2 carbon sheet has 16 measurement points.
3. Parallelism is the difference between the highest and lowest measurements. For example, if the thickest point on a plate measures 2.307 µm and the thinnest point measures 2.302 µm, the parallelism is within 0.005 µm. The plate is reported to be parallel within 0.005 µm.

Electrical Resistance Measurement and Resistivity Calculation

The electrical resistivity of graphitized carbon fiber paper, plates, or panels refers to the material's inherent resistance to the flow of electric current. It is a measure of how strongly the material resists the passage of an electric current and is typically expressed in ohm·centimeters (Ω·cm). It is calculated from the resistance measurement of the material. In the case of graphitized carbon, the resistivity depends on factors like the degree of graphitization, fiber alignment, and material density.

Test Standard: ASTM C611-98

Pressure: 200 psi (14.1 kgf/cm2)

Electrical Resistivity Measurement Procedure

1. Prepare a circular sample with a diameter of 5cm.
2. Set up the TSURUGA 3566 battery internal resistance AC tester according to the manufacturer's instructions.
3. Calibrate the tester to ensure accurate resistance measurements.
4. Place the prepared circular testing sample in the heat press.
5. Apply pressure to the sample using the hydraulic cylinder (area = 9.7 cm2).
6. Using the calibrated TSURUGA 3566 AC tester, measure the resistance (R) of the sample under the specified test conditions.
7. Calculate the resistivity of the sample.

Voltage Loss in Graphitized Carbon Fiber Paper, Plates, or Panels

Voltage loss refers to the reduction in electrical potential as the current flows through a material. In the case of graphitized carbon fiber paper, plates, or panels, it is associated with the material's electrical resistivity and resistance to the flow of electric current. The lower the resistivity and resistance, the lower the voltage loss, which means less energy is wasted as heat. For graphitized carbon materials, a lower voltage loss improves performance.

Pressure: 20 N/cm2
Constant current: 500 mA/cm2

Voltage Loss Measurement Procedure

1. Ensure that the gold-plated copper stamp and the gold-plated base plate are clean and free from contaminants that could affect electrical contact.
2. Set up the voltage measurement instrument (digital multimeter) and connect it to the electrical contact points.
3. Position the sample between the gold-plated copper stamp and the gold-plated base plate.
4. Apply the pressure and constant current.
5. Measure the voltage drop across the contact points using the voltage measurement instrument.

Graphitized Carbon Fiber Papers as Gas Diffusion Layers in Fuel Cells

Thin (thickness < 1 mm) graphitized carbon fiber papers effectively function as both the anode and cathode gas diffusion layers (GDLs) in fuel cells. Gas diffusion layers (GDLs) are key components in fuel cells and electrolyzers, performing several key functions. They ensure efficient diffusion of feed gasses to the catalyst layer, which is essential for continuous electrochemical reactions and electricity generation. The porous structure of GDLs optimizes gas permeability, while their conductive properties facilitate electron transport.

In proton exchange membrane (PEM) fuel cells , the anode GDL enables the diffusion of H2 gas to the catalyst layer, where it undergoes the hydrogen oxidation reaction (HOR), producing H+ ions and electrons. These electrons travel through the external circuit to the cathode, passing through the GDL. At the same time, the cathode GDL allows the diffusion of O2 gas, which reacts with the H+ ions that permeate through the membrane, forming liquid H2O. From this, GDLs not only facilitate gas diffusion but also aid electron transport and assist in water removal to prevent stack swelling.

Functions of Graphitized Carbon Fiber Papers in Fuel Cells

• Hydrogen and Oxygen Delivery: Facilitates the transport of input hydrogen and oxygen gas to the catalyst layer for the electrochemical reaction.
• Electron Conduction: Serves as an electrically conductive pathway, efficiently transferring electrons from the anode to the external circuit.
• Water Management: Assists in removing produced water while maintaining a balance to prevent membrane dehydration or flooding.
• Heat Dissipation: Aids in thermal management by transferring excess heat to other components, such as the bipolar plate.

How Graphitized Carbon Papers Meet the Requirements for Gas Diffusion Layers in PEM Fuel Cells

LINQCELLTM Graphitized Carbon Fiber Paper Performance in Fuel Cells

Representative polarization curves for some LINQCELLTM Graphitized Carbon Fiber Paper Products

Parameter	Value
Reaction Area	300 cm²
Fuel Supply Method	Constant Flow
Relative Humidity	100%
Test Temperature	60 °C
Anode Flow Rate	1.5 splm (H2)
Cathode Flow Rate	2.5 splm (Air)
Operating Pressure	10 psi
Clamping Pressure	10 psi

Parameter	Value
Active Area	25 cm2
Anode/Cathode Stoichiometric Ratio	2.5/1.5
Anode Humidification Temperature	65 °C
Cathode Humidification Temperature	36 °C
Cell Temperature	60 °C
Torque	25 kgf⋅cm

Graphitized Carbon Fiber Panels as Cathode Porous Transport Layers in Water Electrolyzers

Porous transport layers (PTLs) or liquid/gas diffusion layers are important components in water electrolyzer stacks. s shown in the figure below, PTLs facilitate the delivery of liquid reactants from the flow plate channels to the catalyst layer (①) while also allowing for the efficient removal of gaseous products, oxygen at the anode and hydrogen at the cathode (②). This process creates a countercurrent two-phase flow through the PTL. If the product gasses are not effectively removed from the system, H2 and O2 can clog the pores of the PTL. This blockage not only decreases the availability of liquid reactants for the reaction but also leads to membrane dehydration, which negatively impacts its ionic conductivity. 

Gas diffusion layers vs. Porous Transport Layers? Gas diffusion layers and porous transport layers are similar terms but with distinct contexts. PTLs is a broader term, commonly used in water electrolyzers, to describe materials that facilitate reactant and product transport. GDLs, on the other hand, are commonly used fuel cells and are designed to manage gas diffusion and electron conduction. While both terms involve mass transport, GDLs are specialized for fuel cells, while PTLs have a more general application.

As mentioned above, graphitized carbon fiber papers are used at the anode and cathode of fuel cells. Is it the same for graphitized carbon fiber panels or plates in water electrolyzers? No. The placement of the PTL is important because the anode and cathode sides of water electrolyzers operate under different conditions and environments. Each side experiences distinct chemical reactions that do not only affect the performance but also the stability of the PTL.

At which side of water electrolyzers are graphitized carbon fiber panels used?

As mentioned above, graphitized carbon fiber papers are used at the anode and cathode of fuel cells. Is it the same for graphitized carbon fiber panels or plates in water electrolyzers?

The placement of the PTL is important because the anode and cathode sides of water electrolyzers operate under different conditions and environments. Each side experiences distinct chemical reactions that do not only affect the performance but also the stability of the PTL.

By convention, the cathode is the site where reduction reactions occur, and in water electrolyzers, this is where the hydrogen evolution reaction (HER) takes place, generating H₂ gas. As a result, the cathode side operates under a reducing environment, which minimizes the likelihood of oxidation reactions. Materials on the cathode side are less prone to corrosion because they are not exposed to harsh oxidative conditions. Since oxidation is not a major concern, carbon materials are considered as the benchmark PTL for the cathode side. 

Key Features and Benefits of LINQCELLTM Graphitized Carbon Fiber Panels as Cathode PTLs of Water Electrolyzers

Customizable Compressibility of LINQCELLTM Graphitized Carbon Fiber Panels for Water Electrolyzers

CAPLINQ can fine-tune compressibility to achieve the perfect balance of electrical conductivity, gas permeability, and mechanical strength.

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