Effect of Carbon Structure and Ink Composition on 3M Ionomer Adsorption

Abstract

Conventional catalyst layers (CLs) in Proton Exchange Membrane Fuel Cells (PEMFCs) involve an ionomeric component to provide a proton-conducting pathway to the catalyst sites. Such CLs are commonly formed by a mixture of the components, also known as catalyst ink, which comprise the carbon-supported catalyst (e.g. Pt/C) and perfluorosulfonic acid (PFSA) ionomer, dispersed in an aqueous solvent. The resulting CL structure is influenced by the dispersion of the components in solution which, in turn, depend on the dispersion medium.Different CL fabrication techniques, however, require dispersion media with specific characteristics. Fabrication techniques such as hand-painting, screen-printing or roll-to-roll coating require a viscous ink with a high solid content (>5 wt. %) and high-boiling-point additives.(1) In contrast, the spray-coating technique requires a lower solid content (<2 wt. %) and a volatile medium, typically using a water or alcohol solution. The early work of Aldebert et al. found a significant influence of the nature of the solvent on the morphology of Nafion in solution.(2) The Small-Angle Neutron Scattering (SANS) spectra of Nafion solutions in water and ethanol evidenced significant differences in the polymer morphology.Here, the interactions of standard carbon support structures and carbon-supported Pt materials with 3M ionomer were studied with adsorption isotherm in aqueous solution based on 19F NMR and ICP-OES. We provide an analysis of the adsorption considering the results of applying several physical characterization methods to explore CL structure and interactions with 3M 825 EW ionomer.Figure 1 shows the adsorption behavior of the carbon samples: Vulcan XC-72 (V72), Black Pearls 2000 (BP), acetylene black (AB), and Ketjenblack (KB). A Langmuir approach was used to obtained information on the adsorption behavior of the carbon samples. These materials exhibit degrees of graphitization as analyzed through Raman spectroscopy. The intensity ratio of the D band over the graphitic band intensities (ID/IG) was used to evaluate the degree of disorder of the carbon samples.The highest equilibrium constant and area-specific maximum surface coverage correspond to the BP and V72 samples, respectively. These samples exhibited intermediate values of the ID/IG ratio. In contrast, both the AB and KJB samples, which exhibited the lowest and highest ID/IG ratios, respectively, showed lower and values. This trend suggests that the carbon preferentially adsorbs on the amorphous/graphitic boundaries present in the turbostratic structure of carbon. These sites of the microcrystalline carbon domains areFurthermore, the effect of the solvent structure on the 3M ionomer distribution can also be explored through this methodology. The dissolution of the 3M ionomer in a series of solvents (isopropanol, ethylene glycol, methanol, glycerin, propylene glycol) can provide information on the impact of the solvent nature on the adsorption and coverage of the ionomer on the various substrates.