Ultrasonic Coating of Platinum-Based Catalysts

Application of Ultrasonic Coating Technology in the Preparation of Platinum-Based Catalysts

Ultrasonic coating is a precision coating technology based on ultrasonic energy. It involves converting a coating slurry (containing platinum-based catalyst, binder, solvent, etc.) into uniform, fine droplets, which are then precisely deposited onto the substrate surface to form a thin film. Due to its high coating uniformity, good catalyst dispersion, and high material utilization, this technology has become an important means of supporting platinum-based catalysts (such as the aforementioned lattice-distorted platinum-wrinkled nanoparticles LD-Pt WNPs) on electrode substrates (such as carbon paper, carbon cloth, and metal current collectors), and is widely applicable to the electrode preparation of hydrogen electrocatalytic devices such as fuel cells and water electrolysis for hydrogen production.

Core Principle of Ultrasonic Coating of Platinum-Based Catalysts

The core of ultrasonic coating is to utilize the cavitation and atomization effects of ultrasonic vibrations to overcome the limitations of traditional coating methods (such as drop coating and spray coating), achieving efficient and uniform loading of platinum-based catalysts. The specific process consists of three steps:

1. Slurry Preparation: Platinum-based catalysts (such as LD-Pt WNPs) are mixed with ion exchange resins (such as Nafion, a commonly used binder in proton exchange membrane fuel cells) and solvents (such as ethanol and deionized water) in a specific ratio, and stirred to form a uniformly dispersed coating slurry, ensuring that the catalyst particles do not agglomerate;

2. Ultrasonic Atomization: The slurry is delivered to the ultrasonic atomizing head through a feed pipe. The atomizing head vibrates under the action of high-frequency ultrasound (usually 20-150kHz), “breaking” the slurry into tiny droplets with a diameter of 5-50μm. The droplets are uniform in size and do not carry significant impact force;

3. Precise Deposition: Atomized catalyst droplets are directionally sprayed onto a pretreated substrate surface (such as clean carbon paper) with the assistance of an airflow (e.g., compressed air, inert gas). After rapid solvent evaporation, a thin film coating with controllable thickness (typically 1-50 μm) and uniform catalyst distribution is formed.

Core Advantages of Ultrasonic Coating in Platinum-Based Catalyst Applications

Compared to traditional coating techniques, ultrasonic coating can better leverage the catalytic performance of platinum-based catalysts, especially suitable for highly active and easily agglomerated platinum-based catalysts (such as nanoscale platinum particles with lattice distortion structures). Its core advantages are reflected in four aspects:

1. Improved Catalyst Dispersion, Exposing More Active Sites

The vibration energy of ultrasound can effectively suppress the agglomeration of platinum-based catalyst particles during the coating process. Traditional droplet coating is prone to localized catalyst accumulation due to uneven slurry flow. However, ultrasonically atomized droplets are small and uniform. After deposition, the catalyst particles are monodisperse or weakly agglomerated on the substrate surface, maximizing the exposure of platinum active sites (such as the lattice distortion region of LD-Pt WNPs) and reducing “ineffective catalyst” (the part of the agglomerated particles that cannot participate in the reaction).

2. High Coating Uniformity, Optimized Electrode Reaction Kinetics

The thickness deviation of ultrasonic coating can be controlled within ±5%, far superior to traditional spraying (±15%). A uniform platinum-based catalyst coating ensures consistent reaction site density on the electrode surface, avoiding overpotential increases caused by localized current concentration. Simultaneously, the uniform coating optimizes the contact area between the electrolyte (e.g., alkaline solution, proton exchange membrane) and the catalyst, accelerating the H⁺/OH⁻ transport rate and improving HER/HOR reaction kinetics (corresponding to the high activity advantage of LD-Pt WNPs mentioned earlier).

3. Reduced Platinum Usage, Improved Material Utilization

Platinum, as a precious metal, accounts for 30%-50% of the cost of hydrogen power devices (such as fuel cells). Ultrasonic coating offers high atomization efficiency (material utilization exceeding 85%, compared to only 50%-60% with traditional spraying), allowing for a 10%-30% reduction in platinum loading (e.g., from 0.1 mg/cm² to 0.07 mg/cm²) while maintaining electrode activity. Simultaneously, precise droplet deposition avoids catalyst waste at substrate edges, further controlling costs.

4. Protecting Catalyst Structural Integrity and Maintaining Long-Term Stability

The atomization process of ultrasonic coating is free of mechanical impact (low droplet kinetic energy), avoiding the damage to the microstructure of platinum-based catalysts caused by traditional high-pressure spraying. For example, LD-Pt WNPs, mentioned earlier, rely on lattice distortion for activity enhancement; high-pressure spraying may cause the distorted structure to collapse, while ultrasonic coating completely preserves its wrinkled morphology and lattice defects, ensuring stable catalyst activity during long-term operation (e.g., 36 hours of water electrolysis).

Key Process Parameters for Ultrasonic Coating of Platinum-Based Catalysts

The control of process parameters directly affects the coating quality and catalytic performance. Adjustments must be made based on the particle size of the platinum-based catalyst and the substrate type (e.g., carbon paper, titanium sheet). The core parameters fall into three categories:

1. Ultrasonic Parameters:

• Frequency: Typically selected between 40-80kHz—low frequencies (20-40kHz) produce larger atomized droplets, suitable for thick coatings (>10μm); high frequencies (80-150kHz) produce smaller droplets, suitable for thin coatings (<5μm), avoiding catalyst agglomeration;
• Power: Generally controlled between 10-50W—too low power can lead to incomplete atomization of the slurry, resulting in “droplet flow”; too high power may damage the nanostructure of the platinum-based catalyst (e.g., wrinkled morphology).

2. Slurry Parameters:

• Solid Content: The mass fraction of platinum-based catalyst in the slurry is typically 5%-15%. Too low a solid content increases the number of coating passes, leading to coating delamination; too high a content results in high slurry viscosity, making atomization difficult and easily clogging the atomizing head.
• Viscosity: Adjusted to 5-20 cP (centipoise) by adjusting the solvent ratio. Moderate viscosity ensures uniform droplet distribution while preventing excessive solvent evaporation that could cause coating cracking.

3. Deposition Parameters:

• Distance between Atomizing Head and Substrate: Typically 5-15 cm. Too close a distance can lead to locally excessive coating thickness; too far a distance allows droplets to diffuse easily, resulting in an uneven coating.
• Coating Speed: The substrate movement speed (e.g., conveyor belt speed) is controlled at 1-5 mm/s. Matching the speed to the atomization volume ensures uniform coating thickness in a single pass and avoids interfacial delamination from multiple coating passes.

Typical Application Scenarios of Ultrasonic Coating of Platinum-Based Catalysts

Ultrasonic coating technology has become one of the core processes for electrode preparation in hydrogen electrocatalytic devices. Typical applications include two categories:

1. Alkaline Water Electrolysis Electrode for Hydrogen Production

Platinum-based catalysts such as LD-Pt WNPs are ultrasonically coated onto titanium sheets or nickel foam substrates to form a cathode (HER) coating. A uniform catalyst coating can adapt to the high current density conditions of alkaline electrolyzers (e.g., 1000 mA/cm²), reducing electrode corrosion caused by localized hydrogen evolution and extending equipment lifespan.

2. Proton Exchange Membrane Fuel Cell (PEMFC) Electrode

A slurry of platinum-based catalysts (e.g., Pt/C, Pt-Co alloys) and Nafion binder is ultrasonically coated onto a carbon paper substrate to form an anode (HOR) or cathode (ORR, oxygen reduction reaction) coating. The coating prepared by this technology optimizes the proton (H⁺) transport path between Nafion and the catalyst, increasing the power density of the fuel cell (e.g., from 0.8 W/cm² to 1.0 W/cm²).

Technological Challenges and Future Directions

Currently, ultrasonic coating in platinum-based catalyst applications still faces two major challenges:

1. Scalability: Laboratory-level ultrasonic coating primarily uses single-nozzle equipment. Mass production requires multiple nozzles operating simultaneously, necessitating solutions to the coating adhesion problem between nozzles to avoid uneven thickness at seams.

2. Adaptability to High-Viscosity Slurries: For high-loading scenarios (e.g., platinum loading > 0.2 mg/cm²), increased slurry viscosity leads to greater atomization difficulty, requiring the development of novel ultrasonic atomizing heads (e.g., porous atomizing heads) to improve atomization efficiency.

Future directions focus on “integrated process-performance optimization”: Combining in-situ characterization techniques (e.g., in-situ XRD, SEM) to monitor the structural changes of platinum-based catalysts during coating in real time, and using AI algorithms to automatically adjust parameters such as ultrasonic power and slurry viscosity, achieving precise matching between coating process and catalytic performance, further promoting the industrialization of hydrogen fuel cell equipment.

About Cheersonic

Cheersonic is the leading developer and manufacturer of ultrasonic coating systems for applying precise, thin film coatings to protect, strengthen or smooth surfaces on parts and components for the microelectronics/electronics, alternative energy, medical and industrial markets, including specialized glass applications in construction and automotive.

Our coating solutions are environmentally-friendly, efficient and highly reliable, and enable dramatic reductions in overspray, savings in raw material, water and energy usage and provide improved process repeatability, transfer efficiency, high uniformity and reduced emissions.

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