Effect of Carbon Surface on Nafion Thin Film Hygro-Expansion and Thermal-Expansion

Abstract

Polymer electrolyte fuel cell (PEFC) catalyst layers consist of carbon, platinum, and ionomer. Carbon is necessary to provide a high surface area to support catalytic platinum nanoparticles and facilitate electron transport to the reaction sites whereas ionomer is required to supply protons to the reaction site. In addition, together they contribute to the structural strength of a catalyst layer. Since ionomer and carbon make up a majority of the catalyst layer volume, the ionomer in catalyst layer mostly interfaces with carbon support. Studies have shown the suppression of ionomer properties in thin film form especially below 50 nm and is also highly dependent on the substrate. For example, Key et. al measured higher water content in 19 nm thick Nafion coated on Pt than ionomer on SiO2 [1]. Recently, Orfanidi et. al, showed that the functionalization of carbon with NHx group assists in better distribution of ionomer within catalyst layer and restructure the ionomer in such a way that oxygen transport resistance across the ionomer drops [2]. Hence, from the fuel cell application perspective, it is important to study the effect of different carbon substrates on ionomer properties. There is lack of data available on fundamental properties such as hygro-expansion and thermal-expansion of nanometer thin film Nafion on carbon in general and the effect of carbon surface characteristics, in particular on these properties. This presentation will share the effect of hydrophobic carbon and functionalized hydrophilic surfaces on the hygro- and thermal-expansion of 20 and 70 nm thick Nafion films.