How to manufacture the PEM fuel cell catalyst layer using flat screen screen printing?

In the field of printed electronics, screen printing technology is regarded as a reliable and suitable coating technique for high production volumes. It offers additional flexibility and enables in-plane structuring in the catalyst layer design, thereby providing possibilities for designing more complex catalyst layers in the future.

The upgrading and optimization of various manufacturing processes in fuel cell catalyst production present challenges to maintain quality requirements while increasing productivity. Different manufacturing routes can directly apply catalyst paste to membranes or gas diffusion layers, or indirectly achieve this through a transfer method using stencils.

Typically, the catalyst suspension consists of platinum particles, polymer dispersion, and solvent mixtures, adhering to carbon powder. Depending on the preferred coating technique, the solid content in the suspension can vary. After coating, the wet layer is dried, and then the catalyst layer is typically transferred using methods such as electrospinning, inkjet printing, or scraper coating.

In the process of transitioning from laboratory to industrial scale, high-throughput coating techniques need to be evaluated. Besides screen printing, offset printing, or aniline printing, the screen printing process has shown excellent reliability, uniformity of printed layers, and characteristics suitable for high production demands, which are derived from fields such as printed electronics and graphic printing.

The screen printing process can be divided into two steps. In the first step, the paste is spread on the screen, filling the screen, and then the screen is pushed downward to slightly contact the substrate. The movement of the plate drives the solder paste through the screen. At the screen, the solder paste is separated according to the break distance and the speed of the scraper.

The effect of filament breakage is not only dependent on process parameters but also on the slurry, substrate, screen surface energy, and more factors.

For example, a higher scraper speed leads to a higher deposition weight, which also depends on the viscosity of the paste. Slurries with lower viscosity can be printed at a higher scraper speed, indicating that exceeding the specified scraper speed will have a negative impact on the printing result.

Using a semi-automatic screen printing machine for flat screen printing enables process control, achieving sufficient layer thickness and the potential for high production volume, and screen printing can form a planar structure within the catalyst layer. Different printing process parameters will affect the printed layer.

Therefore, we can use a relatively economical platinum-free paste to replace the catalyst suspension and make it have similar viscosity characteristics. At the same time, the water content will also affect the stability of the screen printing process and the in-situ electrochemical performance of the fuel cell.

Experimental methods and materials

To produce platinum-free carbon paste, a mixture of Vulcan XC72R carbon black powder, polymer dispersion, and propylene glycol methyl ether is homogenized by magnetic stirring and a rotating biaxial symmetric centrifuge. Before adding the carbon powder, the moisture in the polymer dispersion has evaporated. This results in a solid content of 18% by weight in the slurry and a ratio of 0.7 between the polymer and carbon.

Functional catalyst suspension (containing platinum) is made by combining Umicore Elyst Pt50 0550 powder for the cathode side and Umicore Elyst Pt20 0390 powder for the anode side on the LSC with 20% Pt content, using the polymer type.

Flow properties

A dual-drive rotating rheometer is used for synchronous rotation of the upper and lower plates. Parallel plates with a diameter of 50 mm and a gap height of 200 μm rotate in opposite directions at a constant temperature of 20°C and have rough surfaces to reduce wall slip phenomena at low shear rates. 19, 25, 26 The paste is left to stand for 10 minutes before measurement.

During the rotational measurement process, the shear rate increases logarithmically from \dot{\gamma } = 0.1 – 3000 seconds?1 to measure the apparent viscosity \eta.19, 26 The static yield stress \tau_{y} is determined by the logarithmic growth of the shear stress during rotation. \tau = 0.1 – 1000 pascals.

Flat plate screen printing process

All coatings are applied using a semi-automatic flat plate screen printing machine and KOENEN GmbH's screen. Different process parameters such as the type of screen, the printing scraper speed and the scraper pressure are system-dependent.

Within this range of variation, a constant amount of carbon paste is applied to the corresponding screen, and then as many layers as possible are printed. Here, four layers of coating are printed simultaneously, with the screen aperture diameter being {A}_{\mathrm{CL}} = 20 cm, and the parameters remain unchanged.

Break distance a = 1.8 mm, separation distance {a}_{\mathrm{s}} = 2 mm, separation speed {v}_{\mathrm{s}} = 2 mm/s, immersion speed {v}_{\mathrm{f}} = 80 mm/s, and the printing scraper angle \alpha = 55. The contact area of the scraper on the screen can be estimated as {A}_{\mathrm{sq}} = 1 × 180 mm. Therefore, 50, 75, and 100 N of scraping force can all be converted into pressure.

To calculate the theoretical volume of the printing slurry, the cross-sectional geometry of the grid must be considered. The grid consists of open and closed parts, where the open part is filled with paste and wires. Therefore, in a grid cell, the diagonal length needs to be taken into account.

Layer thickness measurement (SEM) is favored due to its excellent cracking behavior and conductivity. Four different points were selected on each substrate for SEM cross-sectional imaging, resulting in eight photos for each process variation. Image processing tools have been developed to extract the height data of each vertical pixel column in each SEM image and calculate the average layer thickness and its deviation.

MEA manufacturing and electrochemical in situ testing Finally, functional catalyst slurry is printed on the anode and cathode to verify whether the platinum loading range is suitable for the anode and cathode sides.

The influence of water as a solvent on the screen printing process is studied. Two different screens are used to coat the anode catalyst layer to analyze the effect on Pt loading and further evaluate the performance of the fuel cell.

By hot pressing for 15 minutes at 180°C and 5 MPa pressure, the generated catalyst layer is transferred to the membrane and subjected to electrochemical testing to verify the manufacturing route and its reproducibility. Higher thermal stability and higher glass transition temperature are achieved with a heat press temperature higher than that of the commercial gas diffusion layer Freudenberg H23C9 used on both electrodes.

Flow rheological properties

The microstructure of the catalyst slurry has a significant impact on the dried catalyst layer, such as the distribution of copolymers, pore network, or Pt/C agglomeration distribution, which directly affects the electrochemical performance of the fuel cell. During the flow rheological measurement of the catalyst slurry, these microstructure features and changes are visible.

And there are usually multiple mechanisms and effects acting simultaneously. The carbon paste exhibits favorable dilution shear behavior during screen printing, which breaks the microstructure clusters in the suspension when the shear rate increases.

Therefore, the dynamic viscosity shows a significant shear dilution behavior within the amplitude range, and at higher shear rates, it eventually tends towards an infinitely large shear viscosity. During the printing process, the apparent shear rate can be estimated by the ratio of the processing speed to the grid size, which can be calculated from the number of grids and the wire diameter.

Flat plate screen printing with carbon suspension fluid

During the screen printing process, four processing steps are required, and the specific layer weights obtained can be divided into different parts during the printing process.

Firstly, the dry screen needs to be moistened with glue to ensure stability during the first and third printing processes. No further research plans are made for these layers.

In part B, a stable process runs several printing steps. Even minor changes can lead to differences in platinum loading and directly affect the electrochemical performance.

Therefore, the minimum deviation of cathode platinum loading was defined under the condition without Pt carbon paste, and the layer weight deviation can be transferred by the usual platinum content in the dried catalyst layer, resulting in a maximum allowable layer weight deviation of 0.022 milligrams per square centimeter. This maximum deviation is only applicable to the cathode layer, as the anode deviation may exist over a wider range, and this will be verified in the following electrochemical test section.

During part C, due to the low boiling point (120℃) and high vapor pressure (11.5 hPa) of PGME, the carbon paste is consumed and exposed to air, and the ratio of the volume of paste between the surface and the volume of paste between the mesh exceeds the critical value.

Therefore, the remaining paste continuously loses its solvent quality until the volume of paste on the screen is too low to fully cover the open area, which leads to the appearance of the first uneven structure and incomplete printed image.

The change in the grid causes the determination of layer weights to vary. The coarse mesh size transfers more paste volume due to its higher mesh thickness and thus higher theoretical volume, and the achieved printing quantity indicates that using a finer screen mesh and less paste volume results in a longer initial volume of paste on the screen.

In the second group, a 100 mm/s scraper speed shows the lowest specific layer weight, and the paste separation process under different scraper speeds is studied using a high-speed camera.

At the contact point between the screen and the substrate directly behind the scraper, the paste adheres simultaneously to the screen and the substrate, known as the "adhesion zone". As the screen moves away from the base, the paste separates into fine threads, known as "flow separation".

The length of each area depends on the printing process parameters, especially for lower scraper speeds, the length of the flow separation zone increases, thus forming more fine threads at lower speeds, and the increase or decrease in the layer weight of the printed layer depends on the rheology of each paste. Too low a scraper speed can lead to the overall failure of the printing process, and the wet layer thickness is extremely uneven.

Therefore, the total layer weight is higher, which may be the case in the 50 mm/s condition of this study, compared to 300 mm/s, the printing paste quality at 100 mm/s is lower, which is consistent with Reimer's publication, 20 by increasing the printing speed, the fluid static pressure within the paste increases, resulting in a higher deposition layer quality.

The layer thicknesses measured from several SEM images decrease with the increase in speed, indicating the "homogenization" of the coated surface. The SEM images of the printed layer surfaces made at different speeds, at 50 mm/s speed, show slightly larger lumps and unevenness highlights.

At higher processing shear rates, larger lumps may decompose in the carbon paste due to the instability of the printing process.

This indicates that efforts should be further made to improve the paste dispersion method before the coating process. However, at higher process speeds, more uniform layers may be beneficial for high-yield production.

When comparing the calculated grid volume to the actual paste volume, the slope verifies the correlation, and is almost parallel to the ideal guidance of the eye, meaning that the grid geometry affects the deposition volume of the paste. A large amount of paste (60-62%) is not transferred but adheres to the metal screen mesh.

Generally speaking, the solder paste separation process involves the interaction among the adhesion of the solder paste to the metal wire surface, its viscosity and internal force (cohesion), and its adhesion to the substrate.

Summary

Change the printing process parameters, such as printing speed, scraper pressure and different mesh sizes, to evaluate their effects on the specific layer weight and thickness of the PEM fuel cell catalyst layer. The adjustment of platinum loading should be accomplished by changing the mesh size, while the increase in printing speed leads to a more uniform coating thickness.

Two different water-containing and water-free catalyst pastes were printed and electrochemically tested, and analyzed through viscosity measurement. The water-free catalyst paste showed a decrease in dynamic viscosity and applied Pt loading, and performed best under dry conditions, compared to the catalyst layer made with water. 

The production of catalyst layers with constant process parameters in proportion is highly reproducible and exhibits excellent overall performance. The main challenge of the screen printing technology is the dependence of process stability on the evaporation of solvents during the printing process.

This led to an increase in platinum loading over time to test the stability of air-soluble solvents and their impact on fuel cell performance, enabling the industrial manufacture of catalyst layers through flat or rotating screen printing, aniline printing or intaglio printing.