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