Multiscale simulation approach to investigate the binder distribution in catalyst layers of high-temperature polymer electrolyte membrane fuel cells.

A multiscale approach involving both density functional theory (DFT) and molecular dynamics (MD) simulations was used to deduce an appropriate binder for Pt/C in the catalyst layers of high-temperature polymer electrolyte membrane fuel cells. The DFT calculations showed that the sulfonic acid (SO) group has higher adsorption energy than the other functional groups of the binders, as indicated by its normalized adsorption area on Pt (- 0.1078 eV/Å) and carbon (- 0.0608 eV/Å) surfaces. Consequently, MD simulations were performed with Nafion binders as well as polytetrafluoroethylene (PTFE) binders at binder contents ranging from 14.2 to 25.0 wt% on a Pt/C model with HPO at room temperature (298.15 K) and operating temperature (433.15 K). The pair correlation function analysis showed that the intensity of phosphorus atoms in phosphoric acid around Pt ([Formula: see text]) increased with increasing temperature because of the greater mobility and miscibility of HPO at 433.15 K than at 298.15 K. The coordination numbers (CNs) of Pt-P(HPO) gradually decreased with increasing ratio of the Nafion binders until the Nafion binder ratio reached 50%, indicating that the adsorption of HPO onto the Pt surface decreased because of the high adsorption energy of SO groups with Pt. However, the CNs of Pt-P(HPO) gradually increased when the Nafion binder ratio was greater than 50% because excess Nafion binder agglomerated with itself via its SO groups. Surface coverage analysis showed that the carbon surface coverage by HPO decreased as the overall binder content was increased to 20.0 wt% at both 298.15 and 433.15 K. The Pt surface coverage by HPO at 433.15 K reached its lowest value when the PTFE and Nafion binders were present in equal ratios and at an overall binder content of 25.0 wt%. At the Pt (lower part) surface covered by HPO at 433.15 K, an overall binder content of at least 20.0 wt% and equal proportions of PTFE and Nafion binder are needed to minimize HPO contact with the Pt.