High-Resolution Mapping of the PFSA Polymer Distribution in PEFC Electrode Layers

Perfluorosulfonic acid (PFSA) ionomer is a key component within the electrode layers of polymer electrolyte fuel cells (PEFCs) since this solid electrolyte provides an efficient proton transport path to the active catalyst sites.  Both the average loading on a 100-nm scale as well as the actual film thickness distribution on a 1-nm scale of the PFSA ionomer in the porous layer structure are critical to electrode performance so quantitative measurements of both properties are highly desired.

Scanning transmission electron microscopy (STEM) is one useful tool to map this PFSA distribution in the electrode layer [1]. However, it has long been recognized that fluorinated compounds are sensitive to electron beam radiation damage [2,3]. By maintaining an electron dose below 1x10⁴ e^-/nm² at 100-nm lateral resolution and cooling the specimen below -100ºC, the total fluorine mass loss for Nafion^® ionomer is held below 10% a/a whereby the electron beam damage threshold for a typical Pt/C electrode layer is improved by 2-3 orders-of-magnitude at this sample temperature.

 Atomic force microscopy (AFM) [4] is a complementary tool suitable to map the Nafion^® coverage on a 1-nm length scale on the electrode surface. The adhesion of the AFM tip to Nafion^® in air was measured to be very low as the fluorocarbon backbone was preferentially exposed at polymer-air interface.

 In this paper, we present ionomer mapping measurements of two common PEFC electrode constructions. Figure 1 shows Pt/C/Nafion^® layers at 8 μm dry thickness that are coated from the same electrode ink onto either a decal substrate (CCD) or directly onto the gas diffusion media. The other electrode layer comprises a PtCoMn nanostructured thin film (NSTF) electrode [5] that is overcoated with Nafion^® ionomer using a preferred adsorption scheme to a targeted 2.0 nm dry thickness.

 Figure 2 shows that a significantly different through-layer ionomer/carbon (I/C w/w) profile is measured by energy-dispersive X-ray spectroscopy (EDS) for the CCD and CCDM electrode layers that can be ascribed to partial permeation of the electrode ink solution into the porous gas diffusion media substrate. In addition, the AFM adhesion measurement in Figure 3 shows a linear increase in ionomer coverage on the NSTF electrode surface to the adsorption limit (at 2 nm dry thickness).

 ORNL Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) the Shared Research Equipment (ShaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy.