Simulation of Agglomeration in Polymer Electrolyte Fuel Cell Catalyst Inks

Inoue, Gen

Polymer electrolyte fuel cells (PEFCs) are expected to use as a power source for new generation automobiles, especially heavy-duty vehicles because of their low environmental affection. To be widely spread, it is essential to increase their power output and reduce the cost of catalyst platinum. Since oxygen reduction reaction (ORR) at the cathode is the dominant reaction, there is a need to improve the oxygen transport and proton conductivity. In recent years, porous carbon has been studied and it has been found its high activity and durability. However, the catalyst layer made of porous carbon gets a lower voltage than that made of non-porous carbon in the high current density area. This output reduction results from high oxygen transport resistance, but it is unclear how the porous structure influences the performance.

So in this study, we assumed the following relation. Firstly, the difference in voltage at a high current density between carbons results in oxygen transport properties. Secondly, their properties depend on an effective oxygen diffusion coefficient, which is determined by the porous structure in the catalyst layer. And finally, the factor that attributes the catalyst layer structure is the size of the agglomerate in the catalyst ink because the catalyst layer is fabricated by the wet process. Therefore, we focused on the particle agglomeration process in catalyst ink and calculated the agglomerate behavior under various ink conditions using a numerical model. The differences in the agglomeration and its factors were also examined for carbon with different internal pores.

A three-dimensional discrete element method (DEM) was used to calculate agglomeration. Initially, aggregates, the smallest unit of carbon black, were placed randomly in the cubic region. In this simulation, aggregates were regarded as spheres. The distance of movement was determined by solving the equation of motion. For each particle, the fluid drag, Brownian motion, and particle interactions were considered. The particle-particle interaction included the forces based on DLVO theory [1] and the ionomer effect. Several studies show that ionomer adsorbed on the carbon aggregates improves their stability. So it was assumed that the ionomer effect was the repulsive force and modeled using the same equation as for the EDL force. The dielectric constant dependence of this effect was used in the function previously reported by Magnus et al. [2]. The carbon particles assumed here were non-porous (Vulcan) and porous (Ketjen), and were coated with Pt particles. The initial average diameter of the aggregates was 300 nm. Hamaker coefficient was expressed as an average of the physical properties of platinum and carbon black, which was weighted by the percentage of platinum loaded on the surface of the carbon particles. The solvent was a mixture of 1-propanol (NPA) and water, and the dielectric constant of the ink was expressed as a weighted average of the composition ratio of water and NPA. The simulation was conducted under various water/alcohol ratios and ionomer/carbon ratios (I/C).

From this model, it was confirmed that the carbon internal pores affect the size of the agglomerate in the catalyst ink.

References

[1] B. Derjaguin et al., Prog. Surf. Sci., 43 30 (1993).

[2] M. So et al., Int. J. Hydrog. Energy, 44 28984 (2019).