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

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 (~ 180^oC).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 160^oC. 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.

References

1. R. Gisbert, G. Garcı´a and M. T. M. Koper, Electrochim. Acta 55 (2010) 55 7961.

2. H. Angerstein-Kozlowska, B. MacDougall and B. E. Conway, J. Electrochem. Soc. 120 (1973) 756.

3. Y.-H. Chung, D. Y. Chung, N. Jung, Y.-E. Sung, J. Phys. Chem. Lett. 4 (2013) 1304.

4. Y.-H. Chung, S. J. Kim, D. Y. Chung, H. Y. Park, Y.-E. Sung, S. J. Yoo, J. H. Jang 51 (2015) 2968.