Adsorption Behavior of Perfluorosulfonic Acid Ionomers on a Pt(111) Surface Observed By Atomic Force Microscopy

Abstract

Understanding of the adsorbed structure of ionomers on the Pt electrocatalyst and carbon support surfaces are very important to improve the cell performance of polymer electrolyte membrane fuel cells (PEMFCs). The effect of the solvent composition and temperature on the adsorbed structure of ionomers have been studied by using various techniques, such as 19F nuclear magnetic resonance (19F NMR) spectroscopy, cryogenic scanning electron microscopy (cryo-SEM), dynamic light scattering (DLS), small angle X-ray scattering (SAXS), small angle neutron scattering (SANS), and electron spin resonance (ESR) techniques. However, it has not been fully clarified yet, due to the complexity of the system composed of a variety of ingredients including the Pt electrocatalyst, carbon support, ionomers, solvent and other contaminants. Thus, we have investigated the adsorbed structure of ionomers on the Pt(111) surface as a model of an electrocatalyst by using atomic force microscopy (AFM) at various solvent compositions. A droplet of aqueous solutions of Nafion was placed on Pt (111) surfaces mounted on the AFM sample holder. AFM measurements were performed with Acoustic AC mode AFM using Agilent 5500 AFM microscope (Agilent Technologies, USA) with a silicon nitride cantilever. The images were recorded over a period of ~30 minutes with a scanning area of 1 µm × 1 µm. In a 2 wt% aqueous solution of Nafion, relatively small aggregates of Nafion ionomers with a height of ~2-3 nm were observed at the Pt(111) surface. Amount of adsorption increased with time and, eventually, the surface was entirely covered by randomly oriented aggregates with a thickness of ~4 nm. In a 0.2 wt% aqueous solution, aggregates larger than those observed in the 2 wt% aqueous solution were imaged, suggesting that larger aggregates were formed in the lower concentration solution. Such a concentration-dependent adsorption behavior is further discussed on the basis of solvent composition and conformation of Nafion ionomers in liquid phase. References Masuda, H. Naohara, S. Takakusagi, P. R. Singh, K. Uosaki: Chemistry Letters, 38, 884 (2009).Masuda, S. Faridah, P. Singh, H. Naohara, K. Uosaki, Journal of Physical Chemistry C, 117, 15704 (2013).Masuda, K. Ikeda, K. Uosaki, Langmuir, 29, 2420 (2013).