Fuels cells and Electrolyzers are two sides of the same coin. In the case of electrolysis we use electricity to create hydrogen while in the case of fuel cells we use hydrogen to create electricity. Their principle and setup is pretty similar but with a reverse structure and some crucial differences in catalyst and exchange membrane types.
Both fuel cells and electrolyzers use very similar MEA (Membrane electrode assembly) stacks with Gas diffusion layers, Electrodes (Anodes & Cathodes) and of course the exchange membrane.Graphitized Carbon plates, Carbon papers, carbon sheets, however you want to name them are crucial for a big part of these assemblies, playing a role in both the electrode and gas diffusion layer side.
Renewable energy sustainability — Courtesty of Honda Motor Co., Ltd
Gas diffusion layers (GDLs) are crucial components for proton exchange membrane fuel cells (PEMFCs) since they modulate all relevant transport processes (fuel, reaction products, electricity, heat). Gas diffusion layers act as an interface between the flow fields (structural cell parts, millimeter-sized features) and the electrocatalysts (reaction layers, nanometer-size features), directing the fuel to the active sites while removing heat and reaction products and electrically wiring the reaction layers with the current collectors.
If we want to get into carbon plate semantics there are two more “layers” that we can look into:
1. Gas Diffusion Backing Layer (GDBL)The carbon plate which is a fibrous backing material that governs the mechanical properties of the GDL (behavior upon compression, bending, and shear strength) and also impacts the thermal and electric properties.
2. Microporous Layer (MPL). Carbon plates can be treated with extra microporous layers and PTFE. This is not an essential treatment. It is an extra. The main properties of GDLs, containing GDL treated with PTFE (Teflon), can make the material hydrophobic and improve water transport. Furthermore, the GDL with MPL treatment helps to reduce the contact resistance between the GDL and catalyst layer, and limit catalyst loss to the GDL interior. MPL can also be used as a substrate to deposit catalyst particles for the manufacture of gas diffusion electrodes (GDEs).
There are multiple MPL types that can be adjusted for general fuel cells, DMFC anodes, dry condition PEMFC, low humidity FCV, High humidity FCV and High durability Vehicle Fuel cells(FCV).
Typical GDL Applications
- Proton Exchange Membrane(PEM) stacks
- Direct Methanol (DMFC) stacks
- Phosphoric Acid (PAFC) stacks
- Redox Flow Batteries
- Other electrochemical devices
Manufacturing Process of Carbon Papers
Carbon paper gas diffusion layers (prepared by the wet laying of chopped PAN-based carbon fibers) are a popular solution since they can be manufactured at high volumes (scalability) and low thickness. Steps are:
1. Chopped carbon fibers are processed to a primary carbon fiber web using a papermaking (wet-laying) technology and subsequent thermo-bonding.
2. The raw paper is then impregnated with carbonizable resins (carbonizable resins with the optional addition of carbon fillers), cured, and recarbonized/ graphitized. This procedure serves to adjust the porosity and enhance electric and thermal conductivity.
3. Different GDL backings with different filler content could be the base for the final finishing processes, such as:
- Hydrophobic treatment with PTFE
- Coating with a microporous layer (MPL)
- Sintering (thermal annealing) to bond the substrate/MPL
Simple Carbon plate process goes through the following steps
Dispersion > Resin impregnation > Molding > Carbonization > Graphitization
These thicker products have higher thermal conductivity, high durability, low carbon corrosion and are typically used for Electrolyzers and distributed power generators. They are also known as Molded graphite papers or Carbon plates.
A carbon paper with MPL treatment will traditionally go through the following manufacturing steps:
- Resin impregnation
- Low temperature carbonization
- High temperature graphitization
- Microporous Layer coating
- Resing impregnation
- Heat treatment (1600 or 2000°C)
These thinner carbon papers that usually (but not necessarily) come with an MPL coating are typically used for Fuel cells. These fuel cells are used in vehicles, forklifts, drones and residential applications. These vehicles tend to need higher power stacks with robust properties in thin assemblies with high gas permeability and porosity.
Carbon Papers as Fuel cell GDL and Electrodes
In fuel cells, these thin, porous sheets must provide high electrical and thermal conductivity and chemical/corrosion resistance, in addition to controlling the proper flow of reactant gases (hydrogen and air) and managing the water transport out of the membrane electrode assembly (MEA). This layer must also have controlled compressibility to support the external forces from the assembly, and not deform into the bi-component plate channels to restrict flow. In summary, GDL modulates all relevant transport processes: mass transport (fuel, reaction products), electrons transport, and heat transport), meanwhile, providing long-term mechanical, chemical, and thermal stability for PEM fuel cells.
Carbon papers can be used as Electrodes for the Membrane Electrode Assembly. They are used as a substrate to paint the catalyst ink that determines their Anode or Cathode nature.
Generally carbon plates can be used on the cathode side while sintered titanium is more popular and suitable for the anode side.
Anode is the part of the fuel cells where Hydrogen enters the package. It is where Hydrogen is split into ions and electrons and where the electron flow starts.
Cathode is the part of the fuel cell where Oxygen enters the package. Oxygen is being bound with the charged hydrogen ions and the excess is being removed in water form that flows out of the cell.
After all was said and done we managed to replace and fully qualify our LINQCELL Carbon plate product line after the discontinuation of Spectracarb which left many companies hanging. With full control over our supply chain and production we are now offering an excellent product portfolio that has been successfully tested and qualified but top industry players. Our GDLs range from thin, 100um carbon papers to thick 2.9mm carbon plates (or molded graphite papers). They exhibit excellent compressibility and electrical properties and have already become the product of choice for huge fuel cell manufacturers and governmental institutions.
|Properties||Unit||GDL 1500B||GDL 1850||GDL 2200||GDL 2900|
|TP resistivity||mΩ cm||140||70.5||110||87.5|
|With MPL+PTFE Treatment||NO||NO||NO||NO|
This is a top-level overview of our thicker grades. Our graphitized carbon paper product line is much more extensive and we can also discuss custom grades if you are interested in high quantities. Please contact us with your applications and product requirements and we’ll be happy to aid you with product selection.