Ultrasonic Spray Coating for High-Efficiency Catalyst Layers: Key Ink Formulation Rules for PEMFC CCM Fabrication

In catalyst-coated membrane (CCM) fabrication for polymer electrolyte membrane fuel cells, the ink is a functional materials system rather than a simple suspension. It must carry catalyst particles, distribute ionomer, wet the membrane or transfer substrate, atomize reliably and form a porous layer after drying. Ultrasonic Spray Coating for High-Efficiency Catalyst Layers can provide fine deposition control, but the coating process cannot compensate for an unstable or poorly designed ink. Formulation and spray parameters must be developed together.

1. Define the formulation around the electrode function

A PEMFC catalyst layer requires interconnected pathways for electrons, protons and reactant gases, while also managing product water. Catalyst particles provide electrochemical active sites; ionomer supplies proton-conducting contact and contributes to particle binding; pore space supports gas transport and water removal. The formulation therefore involves trade-offs. Too little ionomer can reduce protonic connectivity and weaken the layer. Too much can cover catalyst surfaces, narrow pores or increase transport resistance. The optimum is not universal: it depends on catalyst support, particle size, membrane chemistry, target loading, operating humidity and electrode architecture.

Catalyst-to-ionomer ratio

Express the ionomer-to-carbon or ionomer-to-catalyst ratio clearly and state the basis used. For supported platinum catalysts, the carbon support mass is often relevant to the ionomer distribution, so a ratio based only on total catalyst powder can obscure differences between powders with different metal fractions. Record the ionomer equivalent weight and dispersion concentration where available. A useful screening study brackets a formulation range and compares layer integrity, electrochemical surface area (ECSA), polarization behavior and transport response rather than selecting a ratio from appearance alone.

2. Solvent systems: dispersion, wetting and drying

Binary solvent systems are commonly explored because one solvent can help disperse or solvate ionomer while another adjusts volatility, surface tension and substrate wetting. The right balance depends on the ionomer product, membrane chemistry and catalyst surface. A solvent combination that works well on a transfer film may behave differently on a hydrated membrane. Researchers should document solvent identity, purity, mixing order and composition, and should consult material supplier compatibility information before coating.

Solvent volatility controls the time available for droplets to spread and coalesce. Very rapid evaporation may cause dry spray, poor inter-track fusion or particle-rich deposits. Slow evaporation can produce excessive wetting, migration, long drying cycles or membrane exposure to solvent. Surface tension influences the footprint and continuity of deposited tracks. Rather than treating one property in isolation, assess the solvent system through spray stability, contact behavior, drying morphology and final electrode function.

3. Solids content and rheology

Solids content affects the amount of dry material delivered by a given wet volume. Increasing solids can reduce solvent burden and change the number of passes needed, but it also raises viscosity and can increase agglomeration risk. A dilute ink may atomize readily yet require many passes and extended drying. A concentrated ink may build loading efficiently but become difficult to atomize or distribute uniformly. Establish a workable range by measuring solids content gravimetrically and checking viscosity under a defined temperature and shear protocol.

Rheology is process-specific

Viscosity measured at one shear rate may not describe the conditions inside a feed line, at the nozzle or during droplet formation. If the ink is shear-thinning, report the measurement range and temperature. Record any time-dependent behavior, such as thixotropic recovery, that could change between mixing and coating. A batch that thickens during a long run can cause loading drift even when the machine settings remain constant. Keep a sample before and after coating to identify such changes.

4. Dispersion stability and agglomerate management

Large agglomerates can obstruct fluid paths, disturb atomization and create local defects or catalyst-rich islands. Dispersion procedures may include controlled stirring, bath sonication, probe sonication or other suitable methods, but energy input must be selected carefully. Excessive processing can heat the ink, alter ionomer structure or introduce contamination from processing tools. A reproducible protocol should state equipment type, energy or power setting where meaningful, duration, temperature control and sequence of additions.

Evaluate stability over the actual coating window. Visual inspection can identify gross sedimentation, but it does not quantify particle-size distribution or prove that the ink remains unchanged. Where available, use particle-size analysis, microscopy, rheology and a simple redispersion assessment. Define an acceptable hold time and remixing procedure. If the formulation requires continuous agitation, make that part of the documented process rather than an informal operator practice.

5. How ink properties influence catalyst-layer structure

Droplet wetting, evaporation and particle movement shape the dry layer. Solvent-rich droplets can spread and merge; as solvent leaves, capillary forces and local concentration gradients influence particle packing. Ionomer can migrate during drying, creating differences between the membrane-facing and gas-facing regions. These effects influence pore size distribution, connectivity and catalyst accessibility. A formulation that gives a smooth top surface may still contain buried cracks, dense regions or nonuniform ionomer distribution.

Porosity and ionomer coverage affect mass transport resistance. Excessive ionomer can impede gas diffusion and water removal, while insufficient ionomer can interrupt proton transport to active sites. ECSA is a useful indicator of accessible catalyst surface, but it should be interpreted alongside loading, catalyst utilization and cell-level behavior. A higher ECSA measurement does not by itself guarantee lower polarization losses under every operating condition.

6. A practical formulation screening matrix

Start with a baseline formulation supported by material supplier guidance or prior laboratory experience. Select a limited number of variables: ionomer ratio, solvent composition and solids content are common first factors. Hold catalyst lot, substrate, nozzle, traverse path and drying conditions constant. Prepare replicate coupons and record wet ink properties immediately before coating. Measure dry loading, thickness, surface morphology and adhesion, then assemble MEAs using a controlled protocol for electrochemical testing.

Interpreting ECSA and polarization data

Use consistent conditioning and test protocols when comparing ECSA. Report the method, potential range, scan conditions and normalization basis. For H₂/air polarization, control temperature, pressure, humidity, stoichiometry and break-in procedure. If a formulation improves low-current performance but worsens high-current behavior, investigate protonic resistance, oxygen transport, flooding and catalyst-layer thickness rather than attributing the result to a single ink variable. Replicates and uncertainty estimates are essential for distinguishing real trends from assembly variation.

7. Formulation records and scale-up readiness

A useful ink batch sheet includes supplier and lot information, catalyst composition, ionomer type and concentration, solvent proportions, total solids, addition order, mixing energy, temperature, viscosity method, storage conditions and age at coating. Link each batch to its spray recipe and coupon results. This traceability makes it possible to compare batches and identify whether performance drift originates in raw materials, dispersion, coating or MEA assembly.

Cheersonic provides information on ultrasonic coating equipment solutions and ultrasonic atomizing coating. Equipment selection should follow the ink’s verified rheology, substrate geometry and research objectives, not the other way around.

Conclusion

PEMFC CCM ink design is a coupled optimization of catalyst utilization, proton conduction, gas transport, water management and sprayability. By measuring formulation properties and linking them to layer morphology and electrochemical response, laboratories can develop recipes that are reproducible and transferable. Qualified manufacturers, R&D teams and production engineers are invited to contact Cheersonic to discuss their coating project and arrange a sample coating test. Email: market2@cheersonic.com.