Indirect and Direct Observation of Ionomer Colloidal Systems with Applications to Fuel-Cell Catalyst Layers
Authors: Hatzell Kelsey B. , Kusoglu Ahmet , Dudenas Peter , Kariuki Nancy , Myers Deborah J , Weber Adam Z 
Published: February 2020

Manufacturing methods for fuel cell catalyst layers are critically important for achieving high material utilization and performance. Typically, electrode systems are cast from an ‘ink’ or ‘colloidal’ state, where the solid material is suspended in a solution containing a solvent and a polymer. For polymer-electrolyte fuel cells, a perfluorinated sulfonic-acid (PFSA) ionomer, typically Nafion, is used in the ink and catalyst layer, where its roles are to act as a binding agent and conduit for proton conduction to the reaction site.1 Recently, it has been suggested that the ionomer film that surrounds the platinum reaction site may contribute to increased resistances that limit performance at low Pt loadings.1,2 Thus, it is of great interest to understand the factors controlling the film formation in catalyst layers to optimize material properties and cell performance. The nature of the dispersion and casting process could have significant influence on the morphology and properties of the dispersion-cast membranes.3-5 To isolate these effects, the morphology of PFSA dispersed in various solvents (glycerol, water/2-propanol, and NMP) was reported previously using small angle neutron scattering (SANS), where catalyst layers from the different solvents exhibit different performance.6 The understanding of local‘ink’ structures as a function of active material (polymer and solid) volume fraction (Φ) and across solvent space and length scales can aid in understanding bulk properties and the impact of ink formulation in cast systems. In this talk, a direct observation of the colloidal suspension of the ionomer in different solvents will be described using confocal laser scanning microscopy (CLSM). CLSM has long been utilized as a tool for characterizing the structure of biphasic colloidal gels. We combine dynamic light scattering, ultra-small angle x-ray scattering, and confocal microscopy to understand PFSA size and structure in dilute (1,3, and 5 wt%) ionomer solutions. The structural formation of nafion membranes are observed in-situ with small angle x-ray scattering. Finally, we contrive a model ‘ink’ composed of attractive and repulsive silica spheres and PFSA suspended in different solvents (water, NMP, and isopropyl-alcohol) and use CLSM to gain a greater understanding about solid|PFSA interactions in model inks.

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