Electrolyzers and Fuel cells are opposite sides of the same coin. Fuel cells take hydrogen in and put electricity out, while electrolyzers take electricity in and produce hydrogen. Both of them have a similar membrane assembly and require a gas diffusion layer, but the gas diffusion layers used for them have different requirements.

The market is definitely calling for electrolyzers. Biogas plants for example, use the electrolyzers that generate the H2 to convert Carbon Dioxide into Methane and water. They are also used for Hydrogen fuel, for electromobility applications and power grid stabilization. Finally, remote hydrogen production, not necessarily for energy storage, but for the production of hydrogen is really driving the use of electrolyzers.

Proton Exchange Membrane (PEM) Electrolyzers

PEM electrolyzers use a Proton Exchange Membrane which use a solid polymer electrolyte.

When current is applied on the cell stack, the water splits in hydrogen and oxygen and the hydrogen protons pass through the membrane to form H2 gas on the cathode side.

The polymer electrolyte membrane (PEM)-based electrolyzer is very popular, and many modern electrolyzers are built with PEM technology. The PEM electrolyzer uses the same type of electrolyte as a PEM fuel cell. Check Fuel Cell Page The electrolyte is a thin, solid ion-conducting membrane, which is used instead of the aqueous solution. Hydrogen produced by this type of electrolyzer is highly pure. (Link)

What is the operating principle of a PEM electrolyzer?

A PEM (Proton Exchange Membrane) electrolyzer is a device that uses an acidic polymer membrane as an electrolyte to split water molecules into hydrogen and oxygen through a process called electrolysis. The chemical principle behind a PEM electrolyzer is based on the principles of electrochemistry.

In electrochemistry, chemical reactions are driven by the transfer of electrons from one substance to another. When an electrical current is applied to an electrolyte solution, it causes the electrolyte solution to undergo a process called electrolysis. During this process, the electrical current causes the water molecules to be split into their constituent atoms of hydrogen and oxygen.

In a PEM electrolyzer, the process is made possible by the presence of the acidic polymer membrane, which acts as a proton exchange medium. The membrane is sandwiched between two electrodes, an anode and a cathode, which are both coated with catalysts to facilitate the reaction.

• When an electrical current is applied, water molecules at the anode are oxidized to form oxygen gas, positively charged hydrogen ions (H+), and electrons:  2H2O → O2 + 4H+ + 4e-
• The hydrogen ions are attracted to the cathode, where they pass through the polymer membrane, while the electrons flow through an external circuit to the cathode. At the cathode, the hydrogen ions combine with electrons and are reduced to form hydrogen gas:  4H+ + 4e- → 2H2
• Overall, the reaction can be expressed as:  2H2O → 2H2 + O2

The acidic polymer membrane plays a crucial role in the process by allowing the positively charged hydrogen ions to pass through it while preventing the negatively charged oxygen ions from passing through. This selective permeability is what separates the two half-cells and creates the necessary potential difference for the reaction to occur.

The catalysts on the electrodes also play an important role in the process by increasing the rate of the reaction. Typically, platinum groum metals are used as the catalyst on the electrodes in a PEM electrolyzer.

In summary, a PEM electrolyzer works by using an acidic polymer membrane as an electrolyte to facilitate the transfer of protons (H+) between the electrodes and the water molecules, resulting in the production of hydrogen and oxygen gas.

Carbon Plates VS Sintered titanium for PEM electrolyzers

Carbon papers can be used as Gas Diffusion Layers (GDLs) in PEM electrolyzers. GDLs are essential components of PEM electrolyzers that facilitate the transport of reactants (hydrogen and oxygen gas) to and from the electrodes, while also providing structural support and electron conduction.

Carbon paper is a popular material for GDLs in PEM electrolyzers because of its high porosity, good electrical conductivity, and chemical stability under the harsh operating conditions of the electrolyzer. The carbon fibers in the paper create a network of interconnected channels that allow the gas to flow through the electrode, while also providing a large surface area for the electrochemical reaction to occur.

However, it is important to note that the selection of the appropriate GDL material for a specific application depends on several factors, including the operating conditions (such as temperature, pressure, and humidity), the desired performance metrics (such as efficiency and durability), and the overall system design. Therefore, a thorough evaluation of the performance characteristics of different GDL materials should be conducted before selecting the appropriate GDL for a specific PEM electrolyzer application.

Compared to sintered titanium there's a thickness advantage for the carbon sheets. Sintered titanium can also be made thicker, but there's a liner relationshipt to the cost of the material. Carbon plates though do now have a linear cost relation when you go to thicker substrates. On the cathode side, there's also the long term reliability concern where compressed hydrogen makes sintered titanium malleable. Carbon panels do not exhibit the same effect.

These sheets can also be compressed quite significantly. 20% compressibility is not uncommon, depending on the grade, something that also creates a nice seal that aids with avoiding leakage. Sintered titanium on the other hand is not compressible at all. Pore size on Sintered titanium can be around 20-100um with a 30-50% porosity whereas carbon panels have a random pore distribution with an average pore size of 20um and a porosity of 70-75%. To add to that, Microporous layers can be applied directly on the carbon panel while in the case of sintered titanium a standalong MPL layer is required. Finally, the cost of carbon papers is significantly lower than the one of Sintered titanium, which can be twice as expensive in lower volumes.

If you take a look at the surface of these carbon panels, it actually looks quite smooth. When we are talking about the surface area we are not talking about what the bare eye can see. If you can zoom in you can see that is actually a quite complex mesh of fibers that have been graphitized. And this complex mesh gives quite the surface area around the individual carbon fibers which allows for a high surface area tied to these GDLs.

How can you reduce the cost of PEM electrolyzers?

1. The first thing you can do is increase the active area. A large active area decreases the cost per kg of H2 produced. Linqcell carbon panels are typically 40 x 40cm but larger sheets can be produced if there is a significant business case.
2. High-pressure PEMS. If the GDLs are compressible than we eliminate a compression step that allows customers to buy less expensive compressors and reduce their overall costs. Linqcell papers withstand a significant amount of pressure and can be compressed up to 15-20% of their initial thickness, depending on the grade.
3. Using Carbon panels instead of Sintered titanium reduces the material cost.
4. Carbon panels have a very long useful life, usually outcompeting the ion exchange membranes themselves. so this is an extra cost saving step that graphitized carbon panels can help us with.

When we are looking specifically on how we can reduce the cost of the panels themselves, there are three areas that we're focusing on.

One area is to optimize the furnace runs. Furnace runs up to 2000°C are very expensive. With the rising cost of energy this is one the most important aspects of carbon panel production since this is almost one third of the cost. If we can optimize those and make sure that ovens are always full and we are taking full advantage of the space and consumed energy, we will be able to reduce the cost of the panels. This means that low volume run are very expensive since we are consuming the same energy either with 1 sheet or with 10000 sheets. Optimization is key. That's why there is a linear relationship between cost and the number of units produced.

Another one is machining costs. If we can work together to be able to accept a greater tolerance, then the machining cost which represents almost a third of the final cost can be reduced.

Finally, it comes down to materials and size. If you are asking for die-cut circles there is going to be a long of materials waste and machining cost. Custom molds can be made but the volumes and commitment needs to be there, in order to justify such an investment.

Applications of Carbon Panels

GDLs can be used in a variety of Technical Engineering Applications such as

• PEM Electrolyzers
• Fuel Cells
• Heat Management
• Redox Flow Batteries
• Gas Diffusion Layers
• Gas Diffusion Media
• Porous Graphitized Carbon Sheets
• Fuel Cell Powered City Buses

Benefits:

• Lower Cost compared to Titanium Mesh / Sintered Ti for Cathode (H2 Side) Electrode Thicker Plates (Up to 5mm)
• Higher Density (up to 1 g/cc)
• Higher Bending Stiffness vs Stacking of Thin Papers
• High Elastic Spring Compression Constant (up to 30% compression with full recovery)
• Good Machinability
• Hybrid Constructions Possible
• Long Term Performance / Corrosion Resistance
• Demonstrated: Commercial / Military Applications
• Laboratory/ Prototype and Commercial Scale

Figure 7. Clockwise from left: PEM Electrolyzers, PEM Electrolyzer stack, Electrolyzers for military and self-containing applications (in this case, the O2 is produced to provide breathable air), China Bus program used Spectracarb carbon paper for fuel cells.

Using Carbon Paper & Panels as Cathode-side Current Collectors