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/Å2) and carbon (− 0.0608 eV/Å2) 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(rright)) 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.
Highlights
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 hightemperature polymer electrolyte membrane fuel cells
To prevent excessive contact of H3PO4 on the Pt surface and carbon corrosion in the catalyst layers, the binder–Pt and binder–carbon surface adsorption energies should be greater than the H3PO4–Pt and H3PO4–carbon surface adsorption energies to prevent excess leaching of the carbon surface and Pt particles
The binding energies between the PTFE binder and the Pt surface (− 0.0118 eV/ Å2) and between the PTFE binder and the carbon surface (− 0.0085 eV/Å2) are lower than those between H3PO4 and the Pt surface and between H3PO4 and the carbon surface. These results mean that the PTFE binder does not readily prevent excessive leaching of the Pt and carbon surfaces in the presence of H 3PO4 at the HT-polymer electrolyte membrane fuel cells (PEMFCs) operating temperature
Summary
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 hightemperature polymer electrolyte membrane fuel cells. HT-PEMFCs have disadvantages of lower energy efficiency than LT-PEMFCs because of the slow kinetics of the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction, which can affect cell performance and lower durability by Pt poisoning and increment of Pt particle s ize[8,17–21]. Carbon corrosion, particle agglomeration, and acid leaching in the catalyst layers of HT-PEMFCs are substantial problems that affect cell performance and durability[20]. Increasing the polymer binder content can increase the durability by preventing H3PO4 flooding in the catalyst layers[28]. Polytetrafluoroethylene (PTFE)[29–37], polyvinylidene difluoride (PVDF)29,37, PBI29,38–41, Nafion[29] and PBI–PVDF blends[29,42,43] have been mainly used as polymer binders to protect catalyst layers, and the content and type of polymer binder can affect cell performance and durability. Further investigations of the contents and types of polymer binders are needed to improve cell performance and durability
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