Structure, Properties, and Degradation of Nanothin Ionomer Films in Fuel Cell Catalytic Layers

The structure of electrodes for proton-exchange membrane fuel cells and electrolysis has a major impact on performance and degradation. Ionomer films covering Pt/C agglomerates in fuel cell electrodes have a thickness in the range of 4 to 15 nm. Such nanothin ionomer films differ significantly in structure and conductivity from bulk ionomer. To access their thickness-structure-conductivity correlations, self-assembled model layers deposited on platinum-covered Si were investigated by material-sensitive and conductive AFM at conditions close to operation. The dependence of transient current on ionomer film height measured directly at samples with a varying thickness has two distinct changes of slope which indicates three distinct length scales (< 4-5 nm, similar to 4-8 nm, > 8 nm) over which structure and subsequently the conductivity of nanothin films varies.