Evolution of the Interfacial Structure of a Catalyst Ink with the Quality of the Dispersing Solvent: A Contrast Variation Small-Angle and Ultrasmall-Angle Neutron Scattering Investigation.

Abstract

The electrocatalyst layer (ECL) of the proton-exchange membrane fuel cell (PEMFC) is commonly fabricated from colloidal catalyst ink containing carbon-supported catalyst nanoparticles (NPs), ionomer stabilizer, and dispersion medium (DM). The structure, stability, and aggregate size distribution of fuel cell catalyst ink are critically dependent on the quality of DM. However, understanding of the influence of the quality of DM on the hierarchical structure of the ECL is lacking. This work presents a systematic investigation of the effects of reducing alcohol content in isopropyl alcohol/water (IPA/H2O) binary mixtures as DM on the structural evolution of water-rich (green) catalyst ink using contrast-variation small-angle and ultrasmall-angle neutron scattering techniques. Both qualitative and quantitative information are extracted from the data to obtain information about the size, structure, and organization of the catalyst ink using different model functions fit to the experimental data. The catalyst ink prepared using 70% IPA (commonly employed in industry and extensively reported in the literature) is shown to consist of randomly distributed globular carbon aggregates (mean radius of gyration of ∼178.9 nm) stabilized by an ionomer mass fractal shell (thickness of ∼13.0 nm), which is dispersed in the matrix of rodlike (∼1.3 nm radius and ∼35.0 nm length) negatively surface-charged ionomer NPs. These well characterized baseline data are then compared and contrasted with DM formulations of lower IPA content. A sequential reduction in IPA content of DM shows a progressive increase in the ionomer NP radius and electrostatic repulsion, concomitantly with the decrease in the carbon aggregate size and ionomer shell thickness of the catalyst ink. Therefore, the changes in the interfacial structure via adjustments of the DM composition can be used as a controlling parameter to tailor the hierarchical structure of the colloidal fuel cell catalyst ink and to further optimize the performance of the ECL.