If you’ve ever felt lost in a sea of jargon while working with energy storage devices like fuel cells, water electrolyzers, and batteries, you are not alone. Terms like “paper,” “cloth,” and “felt” can seem interchangeable, but they actually refer to different types of carbon substrates. In this blog article, we’ll clarify these terms and examine how each carbon substrate’s unique properties affect the performance of your energy storage devices.
Difference according to the Manufacturing Process
The main difference between these three carbon substrates is really in how they are made.
Carbon cloth has a woven structure. Like carbon paper and felt, the fiber precursor used in carbon cloth is usually pitch or polyacrylonitrile (PAN). Instead of being formed into a mat, the precursor is spun into continuous fibers, which are then bundled and twisted into yarns. These yarns are woven or knitted together, producing carbon cloth.
In contrast, carbon paper and felt are both non-woven. Carbon paper starts out as a thin fiber mat or veil, which is produced through a wet-laying process similar to traditional papermaking. Carbon felt, on the other hand, is made by bonding the fibers together through felting, thermal bonding, or mechanical entanglement.
To simplify it: carbon cloth is made by weaving continuous fibers, while carbon paper and felt are made by bonding non-woven, chopped fibers.
▼Comparison of Woven Carbon Cloth and Non-woven Carbon Paper Base Materials
Woven carbon cloth is produced by interlacing carbon fibers in a controlled weaving process, resulting in a highly durable and flexible material. In contrast, nonwoven carbon paper is manufactured through a papermaking process, where randomly oriented carbon fibers are bonded together, forming a lightweight and porous structure.
Now, how do we distinguish carbon paper from carbon felt, considering that they are both non-woven? As discussed in our previous blog, How are carbon papers made?, carbon paper is made by first forming a very thin carbon fiber mat. This mat is then impregnated with a thermosetting resin, which acts as a binder, bonding the fibers together and giving the paper its strength and desired thickness.
In contrast, carbon felt does not go through this resin impregnation process. Instead, its thickness and structure are achieved through felting or mechanical entanglement of the fibers.
Difference according to Material Properties
Different manufacturing processes yield different material structures and consequently, different material properties. So, even though all three substrates are made of carbon, their properties will vary, specifically their structural properties. Interestingly, even carbon paper and carbon felt will have different microstructures despite being both non-woven. The reason? Carbon paper undergoes resin impregnation, while carbon felt does not. “Small” process changes, big consequences!
Differences in Pore Characteristics
Carbon cloths are highly porous (porosity = 60–80%) due to their woven structure. Interestingly, this structure also results in a wide variation in pore characteristics, both in terms of pore geometry and size distribution. The pores in carbon cloth are produced from spaces between overlapping yarn sections, gaps between fibers within the yarn, and voids on each fiber surface. As a result, you get a broad range of pore sizes, from 2 to 100 µm. Larger pores are typically found between yarn sections and due to fiber separation, while smaller pores are located on the fiber surfaces. Additional treatments, such as activation (oxidation) and etching, can further refine the pore structure of carbon cloth. These processes create even smaller pores on the fiber surfaces, with sizes ranging from 10 to 60 nm.
Unlike the pore size distribution in carbon cloths, carbon felt has a narrower range of pore sizes. In carbon felt, pores are mainly formed by voids between interconnected carbon fibers, and these primary pores can be as large as 100 µm. Similar to carbon cloths, carbon felts can undergo additional treatments to create small secondary pores on the fiber surfaces, which are at the nanometer scale. However, untreated carbon felts typically do not have pores smaller than 10 µm, with the modal pore diameter usually ranging between 30 and 40 µm. This structure gives carbon felts an exceptionally high porosity. In fact, among the three carbon substrates, carbon felts have the highest porosity (>90%).
Like carbon felt, carbon papers also have a more uniform pore size distribution compared to carbon cloths. However, carbon papers have smaller pores (10 to 30 µm) than those of carbon felts. This is due to the cured resin, which reduces the size of the gaps between interconnected carbon fibers. In some instances and applications, carbon papers are also treated with a microporous layer made of carbon powder and a hydrophobic agent (often polytetrafluoroethylene), which introduces additional secondary micropores in the nanometer range. Despite this, carbon papers are still highly porous, but their porosity is the lowest among the three materials, with a value <60%.
Differences in Compressibility
The compressibility of a fibrous substrate is defined as the relationship between its thickness (t) and the applied pressure (p): t = f(p). In simpler terms, compressibility measures how much a material’s volume changes, observed as changes in thickness, in response to pressure. For carbon substrates, factors like fiber properties (bending modulus, fiber size), fiber packing density, and porosity all influence compressibility. The differences in the compressibilities of carbon cloths, papers, and felt can be analyzed using the differences in their porosity.
As discussed in the section above, the porosities of the three carbon fiber substrates follow the order: carbon paper (>60%) < carbon cloth (60–80%) < carbon felt (>90%). So, how are porosity and compressibility related? Porosity is the volume fraction in a material occupied by voids or empty spaces. Materials with lower porosity, like carbon paper, have fewer voids compared to those with higher porosity, such as carbon cloth and carbon felt. This means carbon paper has a higher volume fraction occupied by a solid material (fiber and cured resin). Generally, solids are less compressible than gases, which fill the voids in highly porous materials like carbon cloth and carbon felt. Therefore, considering their porosity, the compressibility of these carbon substrates follows this order: carbon paper < carbon cloth < carbon felt.
Here’s a quick reference guide to the key differences between carbon cloth, carbon paper, and carbon felt. This cheat sheet highlights their distinct properties to help you choose the right material for your energy storage applications.
Graphitized Carbon Fiber Paper, Cloth, and Felt Available at CAPLINQ
At CAPLINQ, we understand how the intricate structure-property relationships of materials can affect your projects. Whether you’re using carbon cloth, paper, or felt, we know the right choice can make a big difference in performance, efficiency, and durability.
From high-porosity carbon felts to the more rigid carbon papers and plates, we’ve got a range of products to fit various needs. Our detailed product selector guides can help you choose the best material for your specific application by highlighting key features and benefits.
| Product | Thickness [mm] | Density [g/cm3] | Basis weight [g/m2] | TP Resistivity [mΩ⋅cm] | Voltage Loss [mV] |
| GDP 180 | 0.18 | – | 50 | – | – |
| GDP 210 | 0.21 | – | 51 | – | 14 |
| GDP 240 | 0.24 | – | 90 | – | – |
| GDP 340 | 0.34 | – | 90 | – | – |
| GDL 1500 | 1.5 | 0.60 | 858 | 90.6 | 24.3 |
| GDL 1500B | 1.5 | 0.60 | 670 | 140 | 39 |
| GDL 1850 | 1.85 | 0.85 | 1562 | 70.5 | 25.5 |
| GDL 2200 | 2.2 | 0.6 | 1550 | 110 | 35 |
| GDL 2900 | 2.9 | 0.60 | 1734 | 87.7 | 27.6 |
| Product | Thickness [µm] | Basic Weight [g/m²] | Specific Surface Area [m²/g] | Benzene Adsorption [wt. %] | Iodine Adsorption [mg/g] | Electrical Resistivity [Ω⋅cm] |
| LINQCELL CF350 | 350 | 135 | 2500 | 65 | 2050 | 2 |
| LINQCELL CF400-MP | 400 | 135 | 2500 | 65 | 2050 | 2 |
If you have questions or need help with your projects, feel free to reach out. We’re here to provide the best material solutions for your energy storage needs.
The blog image features SEM images of carbon cloth, paper, and felt, adapted from Y. Popat, D. Trudgeon, C. Zhang, F. C. Walsh, P. Connor, X. Li, ChemPlusChem 2022, 87, e202100441.
