Effect of Graphitization: Mechanical, Chemical & Electrical

How does graphitization affect the structure and properties of carbon fiber papers?

The manufacturing process of our LINQCELLTM carbon-based gas diffusion (or porous transport) layers involves three major steps: (1) paper wet laying, (2) carbonization, and (3) graphitization.

Manufacturing process of carbon-based gas diffusion layers

During the wet paper laying step, the carbon fibers produced from carbon sources, commonly polyacrylonitrile (PAN), are impregnated with carbonizable resins, which serve as binders for the individual fiber strands. Thin layers of impregnated carbon fibers are formed using a paper making machine. Once the desired thickness or number of stacked layers is reached, the material is compressed and heat-rolled to fuse the carbon fibers and solidify the binder resin. Then, curing through carbonization is performed, followed by graphitization.

Carbonization vs. Graphitization

Carbonization and graphitization are both heat treatments. How do these two processes differ from each other? Carbonization and graphitization are processes that transform organic (carbon-based) materials; however, they yield different forms of carbon with different properties. Carbonization is typically performed at 600 to 1200 °C under inert conditions (i. e., in the absence of air nor oxygen). Carbonization removes volatile materials, such as water, gases, and some light organic compounds, from the carbon fiber papers, thereby producing a carbon-rich material. After carbonization, the carbon fiber paper presumably retains its initial structure and properties.

On the other hand, graphitization is performed at higher temperatures than those employed in carbonization. Graphitization is the heat treatment of carbon-based materials at temperatures typically above 1000 °C under controlled conditions. As you would have expected, such high-temperature heating also changes the composition of the carbon fiber paper. Similar to carbonization, graphitization produces carbon-rich materials, which have an even higher carbon concentration than those that have only undergone carbonization.

In fact, as shown in Figure 1, the carbon content of PAN-derived carbon fibers increased from 93.85% to 99.87% as the graphitization temperature was increased from 1300 to 2700 °C. Aside from that, graphitized carbon-based materials underwent dehydrogenation and denitrogenation. Case in point, hydrogen and nitrogen concentrations were both lower than 0.05% after graphitization at 2700 °C.

Elemental Composition of PAN-derived Carbon Fibers at different Graphitization Temperatures

These compositional changes observed during graphitization are often accompanied by structural changes. Please stay with me as we dig deeper into these changes.

Structural changes due to graphitization

Amorphous, Crystalline, and Graphitic Structure

What do I exactly mean with “structural change” or at least, with the word “structure?” Structure describes how atoms are arranged in a material. To identify the structural changes, we have to look into the arrangement of carbon atoms before and after graphitization.

Initially, carbon fibers are amorphous materials, meaning, their atomic arrangements do not exhibit long-range order. To put it simply, the carbon fibers are composed of randomly-oriented carbon atoms at first. During the initial stages of heat treatment, carbon–carbon bonds are broken down, and volatile components like hydrogen, nitrogen, and others evaporate, leaving behind a carbon-rich structure. Eventually, prolonged heating at such high temperatures gives the carbon atoms sufficient energy for them to rearrange themselves and achieve a more-ordered structure. During graphitization, carbon atoms tend to form hexagonal rings and then stack up into parallel layers, forming a structure that is similar to that of graphite. Simply put, graphitization makes the carbon atoms in the carbon fiber paper assume a more crystalline structure, resembling that of graphite. However, graphitization does not transform the entire carbon network structure into graphite. For this reason, the degree of graphitization is used to quantify the extent at which the carbon structure is transformed into layers of hexagonally bonded atoms. More formally, DOG is a measure of the sp3-to-sp2 hybridization transition in the carbon material. In general, high DOG implies that there are more graphite-like structures in the material. As such, the material possibly possesses a higher degree of crystallinity (i. e., higher degree of structural order).

Changes in Carbon Structure during Graphitization

Our LINQCELLTM graphitized carbon-based gas diffusion layers have been graphitized at 1600 and 2000 °C. Now, how does graphitization temperature affect the resulting structure of the graphitized carbon fiber papers in terms of the degrees of crystallinity and graphitization? Raman spectroscopy and X-ray diffraction measurements demonstrate that higher graphitization temperatures yield higher graphitic ordering, meaning carbon-based materials subjected to higher temperatures exhibit higher degrees of crystallinity and graphitization. Higher graphitization temperatures provide the carbon atoms more energy, making them more mobile and facilitating the formation of the preferred graphitic structure.

Property changes due to graphitization

Imagine atoms as LEGO blocks forming your buildings. Arranging LEGO blocks in different ways creates different structures, which in turn affect how strong, flexible, and stable your creation is. This briefly explains why probing structural changes in carbon fibers after graphitization is important. Property is a function of structure. Ergo, identifying structural changes helps us identify material property changes after a treatment.

Since graphitization transforms the originally amorphous structure of the carbon fiber paper into a graphitic one, we can expect that the properties of the graphitized carbon fiber papers will become similar to those of graphite. Graphite has been used in a wide array of applications, particularly in electrochemical energy storage devices as electrode materials or substrates for supercapacitors, batteries, fuel cells, and water electrolyzers, owing to its excellent mechanical properties, good chemical stability, and high thermal and electrical conductivity. As such, the following property changes are expected after graphitization.

Mechanical property changes

Graphitization is known to increase a material’s Young’s modulus. Young’s modulus (also known as the modulus of elasticity) is a determinant of a material’s stiffness or resistance to (recoverable) deformation under an applied load. High Young’s modulus means that the material is stiff and is unlikely to deform under an applied force. Increasing the Young’s modulus is beneficial for graphitized carbon papers used as electrode materials in electrochemical energy storage devices (like fuel cells and water electrolyzers). At the core of these devices is the membrane electrode assembly, which is fabricated by applying high compressive forces. Since graphitized carbon fiber papers exhibit improved (higher) Young’s modulus, they can withstand high compressive stresses without undergoing excessive deformation. This is really crucial in maintaining its porosity and desirable electrochemical properties in general. CAPLINQ’s LINQCELL GDL1500B (improved version of GDL1500) is a graphitized carbon plate that has been well used as gas diffusion layer or porous transport layer of fuel cells and electrolyzers. Aside from its excellent electrical properties, GDL1500B also exhibits optimum compressibility. From its original thickness of 1.5 mm, GDL1500B can be compressed to approximately 1.3 mm at 2 MPa. This facilitates seamless contact with the catalyst layer and thereby enhances the overall performance of the device.

Electrical and thermal conductivity changes

Graphitization increases the material’s electrical conductivity. As discussed above, graphitization transforms the structure of amorphous graphitizing carbon-based materials into layers of hexagonal carbon rings. The carbon atoms in each ring are covalently bonded, whereas the layers or stacks are kept together by a weak van der Waals force. This structural re-arrangement forms electrical pathways at which electrons can move through the material, which considerably minimizes the resistance and improves the electrical conductivity of the graphitized carbon fiber papers.

On a similar note, graphitization increases the material’s thermal conductivity. Heat can travel faster in the graphitic structure because the carbon–carbon bonds form a system that allows the vibrations caused by the temperature differences to propagate rapidly in between the layers of the stacked structure.

Chemical stability

Chemical stability refers to the ability of a material to retain its properties and structure under exposure to different chemical environments. Graphitization increases the resistance to oxidation, reduces the reactivity, and stabilizes the thermal behavior of carbon fiber papers. These have positive implications in the performance of graphitized carbon fiber papers over significant periods of usage or application.

TLDR: Graphitization improves the mechanical (i.e., increases the Young’s modulus), electrical and heat transport capabilities (i.e., increases the electrical and thermal conductivities), and chemical stability of carbon-based materials.

Now, what happens to these material properties with increasing graphitization temperature?

Advantages of higher graphitization temperature

As a general rule of thumb, higher graphitization temperatures yield higher degree of graphitization. In other words, carbon-based material become 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.

Given these considerable structural and property improvements after graphitization, you may find our graphitized carbon fiber papers fit for your applications, may it be in fuel cells, water electrolyzers, supercapacitors, and batteries. Contact us, and our application engineers will assist you in selecting the most suitable products tailored to your application.

About Rose Anne Acedera

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