In a previous blog article, we explored the differences between carbon felt, cloth, and paper, focusing on the effect of manufacturing processes on the structure–property of porous carbon materials. Here’s a quick cheat sheet:
Now, the question is: How do these differences impact the suitability of carbon felt, cloth, and paper for various applications?
Porous Carbon Materials as Porous Transport and Gas Diffusion Layers in Electrolyzers and Fuel Cells, and as Electrodes in Flow Batteries
Regardless of their differences in porosity and compressibility, carbon cloth, paper, and felt all share common traits like high chemical and thermal stability, mechanical strength, large surface area, and high permeability. These qualities make them ideal for applications such as gas diffusion layers (GDLs) in fuel cells, porous transport layers (PTLs) in water electrolyzers, and electrodes in redox flow batteries.
In water electrolyzers and fuel cells, porous carbon substrates serve as porous transport layers (a more general term) or gas diffusion layers. These layers connect the bipolar plate to the catalyst layer, helping to transport reactants to the reaction sites while also providing pathways for electron transport. The open structure of GDLs allows for effective removal of product water, which prevents excess water buildup and flooding—especially important in fuel cells.
In redox flow batteries, porous carbon substrates like carbon (graphite) felt act as electrodes. They perform similar functions to the porous transport layers or gas diffusion layers in fuel cells and water electrolyzers, but without a catalyst layer in flow batteries. This means reactions take place directly on the electrode surface, which has important implications for the properties of materials that are ideal for flow battery electrodes.
Typically, carbon cloths and papers are the go-to materials for fuel cells and water electrolyzers. They’ve also been used in redox flow batteries, like vanadium redox flow systems, but in that market, carbon (graphite) felts are recognized as the benchmark electrode.
So, why is that? Why do carbon papers and cloths work well in fuel cells and water electrolyzers and can even perform in batteries? Yet, carbon felts, though ideal for batteries, aren’t recommended for use in fuel cells or water electrolyzers?
The answer lies in the differences in the (structural) properties of these materials and how they interact with the operating conditions of these systems.
Let me break this down for you.
Why do carbon papers and cloths work well in fuel cells and water electrolyzers and can even perform in batteries?
It is well-established that carbon papers and cloths work very well in fuel cells and water electrolyzers. So, I’ll go ahead and address why they also perform kind of well in batteries.
Redox flow batteries, water electrolyzers, and fuel cells operate on different mechanisms. In redox flow batteries, energy is stored and released through reversible redox reactions in liquid electrolytes that circulate through porous electrodes. Electron transfer between the electrolyte and the external circuit occurs during both charge and discharge cycles. What makes redox flow batteries unique is that their power density and energy capacity can be scaled independently since the capacity is determined by the external electrolyte.
Scaling the Energy Capacity of Conventional Static and Redox Flow Batteries
For this reason, porous electrodes for redox flow batteries need to have high surface areas to maximize the interaction of the electrolyte and increase battery efficiency. Needless to say, they need to be porous to allow the liquid electrolyte to permeate through them effectively. Carbon paper, cloth, and felt all meet these material requirements, but felts are better than papers and cloths when it comes to porosity. Additionally, electrodes in vanadium redox flow batteries are often quite thick, with thicknesses ranging from 1 to 3 mm, and sometimes up to 10 mm. In this regard, carbon felts beat paper and cloth. While thicker papers, now referred to as plates or laminates (such as LINQCELL GDL2900), can be produced, it is generally easier to manufacture felts at these thicknesses.
Why are carbon (graphite) felts, though ideal for batteries, aren’t recommended for use in fuel cells or water electrolyzers?
The high compressibility and porosity of carbon (graphite) felts make them ideal for certain applications, like redox flow batteries, but less suitable for water electrolyzers and fuel cells.
Water electrolyzers and fuel cells operate at high pressures: electrolyzers typically run at 1 to 30 bar, with some advanced systems reaching up to 100 bar. In these high-pressure environments, the high compressibility of graphite felt can cause excessive deformation. This deformation can disrupt the crucial contact between the catalyst layer and the current collector or flow field, leading to reduced electrical conductivity and inefficient operation.
Additionally, the high porosity of graphite felt can negatively impact flow distribution. In fuel cells, it can lead to uneven gas distribution across the catalyst layer, causing some areas to react inefficiently. In electrolyzers, excessive porosity can result in poor water management, leading to flooding and membrane issues, which affects the overall efficiency of the reaction process. If you’re curious about how porosity impacts the performance of GDLs in water electrolyzers and fuel cells, take a look at our article, “Understanding Porosity in Gas Diffusion Layers for Fuel Cells and Electrolyzers“.
Choosing Your Carbon Substrate: Materials Available at CAPLINQ
At CAPLINQ, we offer a range of carbon materials, including cloth, paper, and felt, ensuring that our customers have access to the right substrates for their specific electrochemical applications. Our expertise in these materials allows us to guide clients in selecting the most suitable options based on their unique requirements. Whether it’s for optimizing performance in fuel cells, enhancing efficiency in water electrolyzers, or maximizing energy storage in flow batteries, we understand how to leverage the properties of each material for the best results. If you have questions or need assistance in choosing the right carbon substrate for your project, don’t hesitate to contact us.
LINQCELLᵀᴹ Carbon Solutions for Redox Flow Batteries, Fuel Cells, and Water Electrolyzers
