Tailoring of Electrocatalyst Inks for Performance Enhancement in Proton Exchange Membrane Fuel Cells

One of the main cost and durability controlling components of proton exchange membrane fuel cells (PEMFCs) is the cathode catalyst layer where the oxygen reduction takes place [1]. While the underlying electrochemical mechanisms are currently intensely explored on the fundamental level [2], [3], the ink formulation, which is the most critical step in PEMFC processing is still optimized empirically. As a result, poor formulations with less stable materials show low performance on a lab-scale, hampering their commercialization. Typical inks for catalyst layers consist of Pt nanoparticles supported on carbon-based materials (active material for conductivity, electron transport), and perfluorinated sulfonic acid ionomer (binder for proton transport and mechanical strength) dispersed in suitable solvents. Weakly dispersed inks result in a poor ionomer distribution, form large agglomerates, negatively affect ink rheology due to low colloidal stability, and limit catalyst utilization due to heat and mass transport limitations. This, in turn, significantly reduces the fuel cell performance [4]. For an effective and scalable electrode fabrication, it is necessary to unravel the underlying complex interactions between the multiple nanoparticle species and the solvent mixture forming the catalyst ink. All the techniques that were employed so far to characterize the catalyst inks require modifications to the original ink composition, such as dilution or drying. This makes the assessment of such complex systems challenging and laborious, and sometimes falsifies the results [5], [6]. In this study, we experimentally examined the colloidal stability of inks from their settling behavior at their original concentration used in PEMFC manufacturing. Analytical centrifugation (AC) was used to systematically investigate the effects of processing (i.e. energy input during dispersion) on the stability of inks with identical composition. In addition to this, we developed visual maps to elucidate agglomeration behavior of processed inks which is a critical phenomenon for scalable manufacturing of fuel cell electrodes. Finally, this information was linked to the catalyst particle morphology and structure, handling properties and the final electrode performance to close the design chain from process to structure to property. In combination with electron microscopy and rheological investigations, tailored catalyst inks were developed for economical and durable formation of fuel cell cathodes. References: [1] M. Thoma, W. Lin, E. Hoffmann,MM. Sattes, D. Segets, C. Damm, and W. Peukert, “Simple and Reliable Method for Studying the Adsorption Behavior of Aquivion Ionomers on Carbon Black Surfaces,” Langmuir, vol. 34, pp. 12324–12334, 2018. [2] Y. Bultel, K. Wiezell, F. Jaouen, P. Ozil, and G. Lindbergh, “Investigation of mass transport in gas diffusion layer at the air cathode of a PEMFC,” Electrochim. Acta, vol. 51, pp. 474–488, 2005. [3] J. St-Pierre, Y. Zhai, and J. Ge, “Relationships between PEMFC cathode kinetic losses and contaminants’ dipole moment and adsorption energy on pt,” J. Electrochem. Soc., vol. 163, pp. F247–F254, 2016. [4] M. Bredol, A. Szydło, I. Radev, W. Philippi, R. Bartholomäus, V. Peinecke, and A. Heinzel, “How the colloid chemistry of precursor electrocatalyst dispersions is related to the polymer electrolyte membrane fuel cell performance,” J. Power Sources, vol. 402, pp. 15–23, 2018. [5] H. Mizukawa and M. Kawaguchi, “Effects of perfluorosulfonic acid adsorption on the stability of carbon black suspensions,” Langmuir, vol. 25, pp. 11984–11987, 2009. [6] B. G. Pollet, “The use of ultrasound for the fabrication of fuel cell materials,” Int. J. Hydrogen Energy, vol. 35, pp. 11986–12004, 2010.