Third-Body Effects of Surfactants Adsorbed on Pt Electrocatalysts in Proton Exchange Membrane Fuel Cells

Abstract

In the high-temperature PEMFCs that utilize H3PO4-doped polybenzimidazole (PBI) membranes, Pt poisoning by H3PO4 is an important technical issue, while higher ORR kinetics are expected at elevated operating temperatures (~ 180oC).1 In this study, the surface of Pt nanoparticles was modified by surfactant adsorption, in order to enhance ORR activity and decrease H3PO4 poisoning. Previously, the Conway group reported so-called the “third-body (or ensemble) effect”, which refers to the blocking of poisonous spectator species on the electrocatalyst.2 When commercial Pt/C catalysts were modified by oleylamine (OA), ORR activity could be significantly enhanced, due to the variation in the electronic structure of the Pt nanoparticles.3 The ORR activity of the OA-adsorbed Pt/C was decreased by 29% when H3PO4 was added to the electrolyte solution, whereas the activity decay was much larger for untreated Pt/C (53%). This can be understood as a result of the third-body effect by the adsorbed oleylamine molecules. In addition, a direct synthesis procedure was studied to produce OA-adsorbed Pt/C.4 The Pt/C nanoparticle electrocatalysts were prepared by conventional colloidal-reduction method, but the surfactant removal step by heat-treatment or acid treatment was eliminated. The morphology and crystal structure were analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The surface modification by surfactant was confirmed by near-edge X-ray absorption fine structure (NEXAFS) and cyclic voltammetry (CV). The synthesized Pt/C electrocatalysts were characterized by half-cell test, and the expected third-body effect of adsorbed surfactants could be confirmed. A membrane electrode assembly (MEA) was fabricated with synthesized Pt/C and a homemade H3PO4-doped para-polybenzimidazole (p-PBI) membrane, and its single cell performance was evaluated at 160oC. As the MEA compression during cell assembly is an important factor, catalyst layer thickness were similarly controlled. In the low-current density region, the cell voltage of the MEA with synthesized OA-adsorbed Pt/C was higher than that with untreated commercial Pt/C. From extrapolation in Tafel plots, the kinetic current densities were measured to confirm the third-body effect for Pt nanoparticles supported on carbon substrate. Cell voltage was stable over 150 h, indicating that the adsorbed surfactants are durable under high-temperature conditions.