Structural Factors of Platinum-Supported Carbon Influencing Ionomer Adsorption in Fuel Cell Catalyst Inks

High Resolution Image Download MS PowerPoint Slide In catalyst inks for polymer electrolyte fuel cells, the adsorption of ionomers onto platinum-supported carbon (Pt/C) critically influences the formation of Pt/C agglomerate structures, which, in turn, affect both the processability and performance of the catalyst layer. However, the Pt/C properties that determine ionomer adsorption are still not well understood. Here, we investigate the ionomer adsorption ratio (Γ), defined as the ratio of the adsorbed ionomer to the total ionomer in the catalyst ink, using various carbon materials with different key Pt/C properties. These properties include the accessible specific surface area ( A acc, defined as the BET specific surface area excluding micro- and mesopores and Pt-covered regions), the radius ( R agg ) of carbon aggregates composed of primary particles, the surface density of acid functional groups (ρ acid ) and platinum weight ratio. By analyzing the correlation between these properties and Γ, we derived a universal equation that describes the relationship between the adsorption ratio and these properties. The ionomer adsorption ratio Γ increases proportionally with the accessible specific surface area ( A acc ) of the support carbon. In contrast, Γ decreases inversely with an increasing radius R agg of the carbon aggregates. Additionally, Γ depends on ρ acid and the platinum weight ratio. When ρ acid is below a certain threshold (ρ acid max ), Γ remains nearly constant regardless of ρ acid . However, when ρ acid exceeds ρ acid max, Γ sharply decreases with increasing ρ acid . The threshold ρ acid max decreases exponentially with an increasing platinum weight ratio. The overall relationship can be expressed as Γ = c 1 A acc /[ R agg (1 + c 2 exp{− c 3 [1 – (ρ acid /ρ acid max )]})], where c 1, c 2, and c 3 are universal constants. This equation offers a valuable guideline for designing the Pt/C agglomerate structure in catalyst inks and the resulting catalyst layers after drying.