Towards Design of Fuel-Cell Catalyst Layers

Clean polymer-electrolyte fuel cells (PEFCs) are a viable alternative to combustion engines, especially for heavy-duty vehicles and long-term energy storage. The key to performance of a fuel cell is two separate PEFC catalyst layers (CLs) where hydrogen oxidation and oxygen reduction reactions occur, respectively. Catalyst layers are prepared from colloidal suspensions (inks) composed of platinum-activated carbon particles, a perfluorinated-sulfonic-acid polymer (PSFA) (e.g., Nafionâ„¢), and an aqueous/alcohol solvent that is subsequently dried into paper-thin sheets. The current design of CLs is Edisonian.

Towards a rational design of CLs, we describe a new understanding of their colloidal- chemistry underpinnings. Focus here is on the dissolved PFSA polymer that serves both as a stabilizing agent and later, when dried, an H₃O⁺ ion-conducting binder for the electron-conducting Pt-activated carbon particles. Nafionâ„¢ polymer in water/alcohol mixtures exists not as dissolved molecules but rather as aggregates. How these aggregates form or their roles are not understood. We propose that DLVO type interaction potentials control PFSA aggregation. However, the electrostatic interaction energies are not classical because PFSA is a strong acid releasing H₃O⁺ ions to neutralize the resulting anionic PFSA aggregates. No indifferent electrolyte is present, only surface-dissociated hydrogen ions neutralize the PFSA aggregates. Thus, classical DLVO electrostatic potential energies are not applicable. We derive new single-counterion interaction potential energies that, in contrast to classical potentials strongly depend particle size and on suspension volume fraction. The pivotal idea is that single-ion diffuse double layers emanating from a Nafion particle always encounter the ionic coronas of surrounding aggregates

We then utilize irreversible Smoluchowski perikinetics to predict Nafion aggregation kinetics. As particles grow, the electrostatic barriers in the stability ratio become so large that growth self-quenches, as depicted in Figure 1. Thus, Nafion particles size distributions remain static to further growth even though no reversible peptization occurs. Comparison of theory is made first to solution pH measurements with the hypothesis that aggregates bury internal charge groups with only the externally exposed charges governing the interaction potentials. Second, we compare theory qualitatively to a measured size distribution for Aquivionâ„¢, a slightly different PFSA, in an aqueous electrolyte solution. For both comparisons, the proposed theory is in excellent agreement.