Optimizing Slot-Die Coating for Fuel Cells: New Research Improves Catalyst Ink and Electrode Performance
A recent study titled "The effect of catalyst ink formulations and slot-die coating parameters on PEMFC GDE fabrication" offers new insights into the fabrication of gas diffusion electrodes (GDEs) for fuel cells using slot-die coating. The research focuses on how ink composition and coating parameters impact the quality, consistency, and performance of electrodes in low-temperature polymer electrolyte membrane fuel cells (PEMFCs).
With the global push toward zero-emission energy, fuel cells are becoming an attractive solution for powering vehicles and drones. But for this technology to reach mass-market adoption, the production methods must become more scalable, efficient and cost-effective. This study helps close that gap by exploring how slot-die coating, a method commonly used in high-throughput manufacturing, can be optimized for fuel cell component fabrication.
Key Highlights from the Study
Inks with 75 percent water and 25 percent 1-propanol performed best in terms of viscosity and coating quality.
Shear-thinning inks led to better coating performance than inks that exhibited Newtonian (constant viscosity) behavior.
Catalyst inks with higher ionomer-to-carbon ratios had lower viscosity, which improved flow and coatability.
High platinum content increased ink viscosity, strengthening inter-particle interactions.
Slot-die coating windows were established to determine defect-free operating conditions for various ink formulations.
Cracks in dried coatings increased with thickness, but higher ionomer content helped reduce visible crack density.
MEA performance improved when an ionomer overlayer was applied and hot-pressing was used, especially at higher current densities.
One of the key advantages of R2R processing for MEAs is its scalability. By eliminating the need for batch processing, R2R enables continuous production with high throughput, reducing manufacturing costs and improving overall efficiency.
Why This Study Matters for Fuel Cell Manufacturing
The membrane electrode assembly (MEA) is the core of a PEM fuel cell, where electrochemical reactions convert hydrogen into electricity. Within the MEA, gas diffusion electrodes (GDEs) play a crucial role in facilitating this process. Traditionally, applying the catalyst layer onto these electrodes involves complex and slow processes.
Slot-die coating provides a promising alternative. This technique allows continuous deposition of materials on substrates and is already widely used in industries such as batteries and solar cells. When applied to fuel cells, it can streamline production, reduce waste and improve consistency.
However, successful slot-die coating requires precise control of several variables, including catalyst ink viscosity, solvent ratios, coating speed and ink flow rates. This study investigates those variables to identify optimal conditions for producing high-quality GDEs.
Understanding Catalyst Ink Behavior for Slot-Die Coating
Rheological Testing and Ink Formulations
The study prepared several catalyst inks using different combinations of platinum loading, ionomer content and solvent ratios. These inks were tested for viscosity changes at various shear rates, a key factor in determining their suitability for slot-die coating.
Three main ink types were compared:
CatInk01: 75/25 water to 1-propanol, I/C ratio of 0.9. Showed strong shear-thinning behavior and good coatability.
CatInk02: 90/10 water to 1-propanol, I/C ratio of 0.9. Behaved more like water and had poor coating properties.
CatInk03: 75/25 water to 1-propanol, I/C ratio of 1.8. Lower viscosity than CatInk01 and easier to coat, but less shear-thinning.
Ink rheology was modeled using the Power Law and Carreau-Yasuda equations. These models helped explain how ink viscosity responded to different shear rates, which simulates what happens during the slot-die coating process.
Wettability and Contact Angle Testing
Contact angle measurements showed how well each ink spread on the GDL surface. A lower angle means better spreading and adhesion. CatInk01 had a contact angle of just 22 degrees, indicating excellent wettability. CatInk02 exceeded 120 degrees, confirming its poor performance in coating tests.
The Laboratory Roll-to-Roll Coater is the ideal coating machine for scaling up to roll-to-roll processing of battery electrode materials or for testing and optimizing battery electrode slurries.
Testing Slot-Die Coating Windows for Electrode Fabrication
The study tested different combinations of coating speed and ink dispense rate to define "coating windows" — safe operating ranges where coatings could be applied without defects like ribbing, rivulets or voids.
Both CatInk01 and CatInk03 produced reliable coatings within a reasonable window. CatInk02 failed to produce usable coatings due to its poor wettability and unstable flow behavior.
One key finding was that defect-free coatings occurred when shear rates exceeded the point where ink viscosity had stabilized. This reinforces the importance of controlling both ink formulation and slot-die process parameters.
Cracking and Catalyst Layer Uniformity
Even with defect-free coatings, some cracking was observed in dried catalyst layers. Thicker layers showed more extensive cracking, while higher ionomer content reduced the number and size of visible cracks. Optical microscopy revealed differences in surface morphology depending on ink type and layer thickness.
The presence of fine agglomerates in higher I/C inks suggested better dispersion and reduced crack propagation. However, drying conditions and solvent choice still need to be optimized to minimize these defects in a production setting.
MEA Performance and Electrochemical Testing
The coated electrodes were assembled into membrane electrode assemblies (MEAs) and tested under standard fuel cell conditions. The performance was measured by analyzing current density, power output and electrochemical surface area (ECSA).
Effects of Ionomer Content
MEA with I/C ratio 1.8 slightly outperformed I/C ratio 0.9.
Higher I/C may have led to better bonding between the catalyst layer and membrane.
Finer cracks and improved catalyst layer contact likely contributed to lower resistance.
Impact of Ionomer Overlayer and Hot-Pressing
Adding a thin ionomer overlayer improved high-current-density performance by reducing interfacial resistance.
Hot-pressing provided additional improvements, especially in mass transport regions, though effects were moderate.
Combining IO and hot-pressing led to the highest overall performance.
The study confirmed that optimized post-processing steps can significantly enhance MEA function, even when initial ink and coating parameters are favorable.
Double-sided battery coating on a Laboratory Roll-to-Roll Coater.
Conclusions: Improving Slot-Die Coating for Fuel Cell Scalability
This study demonstrates that slot-die coating is a viable method for high-throughput production of gas diffusion electrodes in PEM fuel cells. By understanding how catalyst ink formulation affects coating behavior, manufacturers can produce more consistent, defect-free layers that perform well in real-world fuel cell applications.
Fine-tuning solvent ratios, ionomer content and platinum loading allows for better control of ink viscosity and wettability. These factors, in turn, influence coating quality and electrode durability. Combined with smart post-processing techniques like ionomer overlayers and hot-pressing, slot-die coating can be a powerful tool in the future of clean energy manufacturing.
Authors
Iosif Vazirgiantzikis
Cecil Felix (Corresponding Author)
Mphoma Matseke
Olivia Barron
Mpfunzeni Raphulu
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