Revealing the structural evolution in catalyst layer of fuel cell during drying process via multistage ink analysis

The controllable design of the catalyst layer (CL) structure provides a feasible way to achieve the commercialization of proton exchange membrane fuel cells (PEMFC). However, the structural transition process from catalyst ink to the CL remains to be elucidated. This study investigates the nanostructure evolution of agglomerates and free ionomers within catalyst inks during the drying process. A novel in-situ monitoring method is applied to obtain the real-time compositional variation of the catalyst inks and calculate the variation of catalyst ink properties. On the basis, six catalyst inks at different drying time-nodes are prepared and their characteristics are analyzed by ink rheology, agglomerate particle size and zeta potential tests, free ionomer quantification and conformational analysis. The results indicate that the agglomerate structure of the ionomer-covered catalyst particles mainly undergoes two forms of variation, including the adsorption of free ionomers and the secondary aggregation of the agglomerates. The free ionomers tend to adsorb to the agglomerate surface, and they also gradually entangle and aggregate to form secondary aggregates of the ionomers. Ultimately, a complete and detailed structural evolution model is proposed, which provides insights into the CL structural transition during the drying process. • A novel in-situ monitoring method is presented to obtain ink composition variation. • A designed thermogravimetric analysis is applied to quantify free ionomers. • The agglomerate structural evolution model of catalyst ink is proposed. • Adsorption of free ionomer and secondary aggregation of agglomerate is confirmed. • Free ionomer gradually entangle and form secondary ionomer cluster during drying.