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

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 = c₁A_acc/[R_agg(1 + c₂ exp{-c₃[1 - (ρ_acid/ρ_acid^max)]})], where c₁, c₂, and c₃ 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.