Structure in Catalyst Layers Redefined

Higher Pt loading causes a transition of the Pt/C surface from predominantly hydrophobic to predominantly hydrophilic. The change in surface wettability of Pt/C triggers a structural inversion in the ionomer film. This inversion of the orientation of the ionomer film has implications for catalyst utilization, density distribution and conductivity of protons, electrocatalytic activity, and water balance in catalyst layers. 

The structural picture of ionomer in catalyst layers, unraveled in this section, suggests that extrapolation of bulk membrane properties in terms of water uptake, water binding, and proton transport skews specific properties of ionomer in CLs and is not generally feasible for the purpose of CL modeling. One needs to adapt mechanisms of water and proton transport to the thin-film ionomer morphology, where (i) proton transport is dominated by surface properties of ionomer and (ii) electrocatalytic properties are determined by the interfacial thin-film structure formed by Pt/C surface, ionomer film, and a thin intermediate water layer. 

Since ionomer is unlikely to penetrate into micropores, the major portion of Pt particles will not be in contact with ionomer, but with water in pores. Therefore, connectivity of ionomer phases should not be a critical requirement for proton transport. It is merely the continuity of water channels that is required. Proton diffusion through water in pores, between disconnected ionomer domains, may contribute significantly to the proton conductivity. 

Finally, to understand properties of various types of ionomers in CLs (Astill, 2008 Astill et al., 2009 Holdcroft, 2014), one has to study, using techniques proposed in this section, their self-organized morphologies in the presence of Pt/C, their tendency to form a thin, adhesive, and well-connected film at aggregated Pt/C particles, and the arrangement of sidechains in terms of their density and orientation relative to the Pt/C surface. 

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Xc 0.12 (Eikerling, 2006 Eikerling and Komyshev, 1998 Eikerling et al 2004, 2007a). It should be noted that this parameterization does not take into account the specific thin-film morphology of ionomer in CLs discussed in the section lonomer Structure in Catalyst Layers Redefined. ... 

As discussed in the section Ionomer Structure in Catalyst Layers Redefined in Chapter 3, a theory of composition-dependent effective properties that incorporates recent insights into stmcture formation in CCLs is yet to be developed. At present, the relations presented in the section Effective Catalyst Layer Properties from Percolation Theory in Chapter 3 do not account for agglomerate formation and skin-type morphology of the ionomer film at the agglomerate surface. Qualitative trends predicted by the simple structure-based catalyst layer theory should be correct, as confirmed by the results discussed in this section. 

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