Ultrasonic Spray Coating for High-Efficiency Catalyst Layers: Fundamentals, Droplet Dynamics and Lab Process Framework

Preparing a catalyst layer is not simply a matter of placing catalyst ink on a membrane or porous transport substrate. The deposition process influences thickness, pore structure, ionomer distribution, electrical contact and the accessibility of catalytic sites. Ultrasonic Spray Coating for High-Efficiency Catalyst Layers is attractive to researchers because atomization and deposition can be adjusted independently to support thin, spatially controlled coatings while limiting the need for high-pressure atomizing gas. The method is useful for laboratory development of membrane electrode assemblies (MEAs), polymer electrolyte membrane fuel cells (PEMFCs), and water-electrolysis electrodes.

1. The physical basis of ultrasonic atomization

An ultrasonic nozzle uses a vibrating surface, commonly driven by a piezoelectric transducer, to transfer high-frequency mechanical energy into a liquid film. Surface waves develop on the liquid and, when the operating conditions are suitable, droplets detach from the surface. The resulting spray is governed by the liquid properties and the nozzle system, including vibration frequency, amplitude, liquid feed rate, surface tension, viscosity and the geometry of the atomizing tip. It is therefore misleading to treat frequency as the only control variable. Two inks processed at the same nominal frequency can atomize differently because their rheology and solvent composition are not the same.

Droplet formation and distribution

Ultrasonic atomization is often associated with a relatively fine and controllable droplet population. Actual droplet-size distribution depends on the formulation and operating window, and should be measured or otherwise verified for the specific ink. A broad distribution can produce nonuniform wetting: larger droplets may create local wet spots or coalescence, while very fine droplets can be more susceptible to solvent evaporation and airflow-driven displacement. The objective is not to obtain the smallest possible droplets, but to produce a stable distribution that transports catalyst solids consistently and wets the selected substrate without damaging it.

2. From droplets to a functional catalyst layer

After atomization, droplets travel through the nozzle-to-substrate gap under the influence of their initial momentum, carrier-gas flow, gravity, evaporation and ambient conditions. On impact, the liquid spreads, partially evaporates and leaves catalyst particles, ionomer and other ink constituents behind. Repeated passes build the layer. The deposited structure is consequently determined by more than the atomizer: stand-off distance, traverse speed, overlap, substrate temperature, pass count, drying interval and ink stability all contribute.

Deposition kinetics can be considered at three connected scales. At the droplet scale, wetting and evaporation determine the footprint left by each impact. At the track scale, adjacent spray paths overlap to form a continuous band. At the electrode scale, multiple bands and passes establish the target loading and thickness. Defects may arise at any scale: isolated droplets can leave discontinuities, track overlap can create ridges, and excessive wet build-up can trap solvent or cause redistribution during drying.

3. A practical laboratory workflow

Substrate preparation and baseline checks

Begin by identifying the substrate type, surface condition, dimensions and sensitivity to heat or solvent. Membranes, gas-diffusion layers, porous transport layers and release substrates have different wetting and mechanical requirements. Record the substrate lot, conditioning history and initial mass where gravimetric measurement is appropriate. Secure the substrate flat without stretching it. A reproducible fixture and masking strategy are essential because edge effects and movement can be mistaken for atomization problems.

Ink preparation and conditioning

Prepare catalyst ink using a documented sequence for solvent addition, ionomer incorporation, mixing and dispersion. Record solids content, catalyst and ionomer masses, solvent identity, mixing time, temperature and any sonication or milling step. Avoid assuming that a visually uniform ink is fully dispersed. Check for sedimentation, agglomerates and viscosity drift over the expected coating period. If the formulation changes, re-evaluate atomization and deposition rather than transferring old settings without verification.

Establishing the spray window

Use a controlled design of experiments rather than changing many settings at once. Relevant factors include nozzle frequency or drive setting, liquid feed rate, carrier-gas flow where used, nozzle-to-substrate distance, traverse speed, path spacing, substrate temperature and number of passes. Start with a simple witness substrate to assess the spray footprint and track overlap. Then coat a representative coupon and determine dry loading and thickness. Keep the same measurement method and conditioning protocol across trials so that differences are attributable to the process rather than the measurement.

4. Parameter interactions researchers should understand

Travel speed and liquid feed rate jointly determine the amount of wet ink delivered per unit area. Slower travel or higher feed generally increases local wet deposition, but the resulting dry layer also depends on solids content and transfer losses. Path spacing controls overlap: excessive spacing can leave gaps, while very tight spacing can create ridges or local over-wetting. Substrate temperature changes evaporation and spreading, but excessive heat may affect a polymer membrane or cause premature skin formation. These variables should be optimized together within substrate and formulation limits.

Ink rheology affects both atomization and post-impact behavior. Higher viscosity can resist film breakup and alter droplet formation; very low viscosity may encourage spreading, migration or runoff. Surface tension and solvent volatility influence wetting and drying. The useful operating range is therefore formulation-specific. A process recipe should report measured or controlled properties rather than relying on labels such as “low viscosity” or “fast drying.”

5. Drying, post-treatment and quality control

Drying must remove solvent without causing cracking, delamination, membrane distortion or undesirable ionomer migration. Depending on the chemistry and substrate, staged drying may be preferable to an aggressive single thermal step. Temperature, airflow, time and humidity should be documented. Any hot pressing, annealing or conditioning step should be treated as a separate operation with its own controls. A good-looking surface is not proof of a functional catalyst layer; evaluate loading, thickness distribution, adhesion, morphology and electrochemical response.

At minimum, a laboratory record should include ink batch identity, substrate identity, nozzle and operating settings, coating path, environmental conditions, dry loading, thickness measurement locations, drying history and observed defects. For MEA work, correlate these records with cell assembly and test conditions. This creates a traceable process–structure–performance relationship and helps distinguish coating variation from membrane, catalyst or test-fixture variation.

6. Common troubleshooting logic

Streaks may indicate unstable feed, inconsistent traverse motion, poor wetting or path overlap problems. Pinholes can arise from contamination, trapped air, substrate defects or incomplete coverage. Coffee-ring-like deposits can reflect evaporation-driven particle migration. Cracking may indicate excessive wet thickness, rapid drying, poor binder distribution or a mismatch between coating and substrate mechanics. Change one controllable factor at a time and preserve a reference coupon. When a defect appears intermittently, investigate ink aging and nozzle cleanliness before redesigning the entire recipe.

7. Building a transferable process framework

A robust laboratory method is a controlled sequence: characterize the ink, verify the spray footprint, establish a deposition window, coat replicate coupons, dry under defined conditions, measure physical properties and validate electrochemical behavior. Report both nominal settings and the rationale for selecting them. Researchers should distinguish equipment capability from a validated process result: the same spray platform may support different materials, but each formulation and substrate combination requires its own qualification.

For an overview of equipment approaches, see Cheersonic’s ultrasonic coating equipment solutions and its ultrasonic atomizing coating information.

Conclusion

Ultrasonic spraying offers a controllable route for depositing catalyst inks, but high-quality layers result from coordinated control of atomization, transport, wetting, drying and characterization. A documented laboratory framework allows researchers to move beyond trial-and-error and develop repeatable coatings for meaningful MEA comparisons. 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.