Polymer electrolyte fuel cells (PEFCs) are one of the most promising power sources for vehicles. A catalyst layer of PEFCs is a porous layer prepared from the catalyst ink consisting of ionomer solutions, carbon black supporting Pt nanoparticles, and dispersing solvents. Drying of a thin film from the catalyst ink on a substrate often causes the formation of cracks. Since the cracks will affect the performance and durability of PEFCs, the crack formation should be controlled and restricted. Therefore a comprehensive understanding of the factors that control the crack formation is highly required. Cracking has been investigated on model colloidal suspensions, and it is well known that the critical crack thickness (CCT) below which films do not crack during drying depends on the fracture resistance of the films. In this work, we investigated whether the CCT exists in the catalyst layers and is useful as an index value for the fracture resistance of the layers or not. The catalyst layers with thickness gradient were prepared from the two catalyst inks including ethanol or 1-propanol as a dispersing medium (E-ink and P-ink). The thickness dependence of the lengths of cracks formed on the layers was investigated by optical microscope observation. The dispersion stability of their inks was investigated in terms of the amount of absorbed ionomer on the carbon particles and rheology. The catalyst layers prepared from the P-ink (P-CLs) had no cracks below the thickness of 2.3 µm and showed a proportional increase in the length and number of the cracks above 2.3 µm. That is, the CCT of the P-CLs was 2.3 µm. On the other hand, the CCT of the catalyst layers prepared from the E-ink (E-CLs) was 5.8 µm, and the cracks of the E-CLs were less than these of the P-CLs. The amount of absorbed ionomer on the carbon particles in the E-ink was higher than that in the P-ink. In addition, the E-ink had lower viscosity and better dispersion stability than the P-ink. Thus, the E-CLs are supposed to have the higher fracture resistance than the P-CLs by increasing the adhesion strengths between particle pairs with ionomer as the binder. As a result, the E-CLs have the higher CCT and less cracks than the P-CLs. In conclusion, the fracture resistance of catalyst layers can be evaluated by measuring the CCT.

Colloid chemical approach to cracking formation in fuel cell catalyst layers

255th National Meeting (2018)

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