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

Abstract

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 (SO₃⁻) 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/Ų) and carbon (− 0.0608 eV/Ų) 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 H₃PO₄ 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 (${\rho }_{\mathrm{P}}{g}_{\mathrm{Pt}-\mathrm{P}}\left(r\right)$) increased with increasing temperature because of the greater mobility and miscibility of H₃PO₄ at 433.15 K than at 298.15 K. The coordination numbers (CNs) of Pt–P(H₃PO₄) gradually decreased with increasing ratio of the Nafion binders until the Nafion binder ratio reached 50%, indicating that the adsorption of H₃PO₄ onto the Pt surface decreased because of the high adsorption energy of SO₃⁻ groups with Pt. However, the CNs of Pt–P(H₃PO₄) gradually increased when the Nafion binder ratio was greater than 50% because excess Nafion binder agglomerated with itself via its SO₃⁻ groups. Surface coverage analysis showed that the carbon surface coverage by H₃PO₄ 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 H₃PO₄ 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 H₃PO₄ 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 H₃PO₄ contact with the Pt.