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

The electrocatalyst layer (ECL) of 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 is critically dependent on the quality of the DM. However, understanding of the influence of the quality of the DM on the hierarchical structure of 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 green catalyst ink using contrast-variation small-angle and ultra-small-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 rod-like (~1.3 nm radius and ~35.0 nm length) negatively surface charged ionomer NPs. These well characterised baseline data are then compared and contrasted with DM formulations of lower IPA content. It is observed that decreasing the alcohol content (increasing water content) increases the dielectric constant of the DM, which in turn promotes electrostatic screening and reduces Coulombic interactions. Consequently, in water rich DMs, ionomers are more dissociated (more charged) and their nanoparticles have larger dimensions, while their carbon aggregate structures shrink with thinner ionomer shells. Therefore, the changes in the interfacial structure via adjustments of the DM composition can be used to tailor the hierarchical structure of the colloidal fuel cell catalyst inks and the ECL.