ABSTRACT
The nature of tip-sample interaction forces in atomic force microscopy (AFM) phase imaging strongly affects the resolution of proton conducting domains mapped at the surface of Nafion membranes. Images acquired in repulsive mode overestimated the area of individual proton conducting domains by a factor of 4 (360 vs 90 nm(2)) and underestimated the number of these domains by a factor of 3 (0.9 domains per 1000 nm(2) vs 2.7 domains per 1000 nm(2)) compared to attractive mode. When the cantilever was driven above resonance or when the combination of scan parameters resulted in an AFM feedback loop that was not fully optimized, phase contrast arose not from proton conducting domains but instead from changes in topography. In attractive mode, phase contrast did not correlate with either topography or changes in topography, and the resulting images most accurately represent the fluorocarbon and aqueous domains at the surface of Nafion membranes.
ABSTRACT
The electrochemically active area of a proton exchange membrane fuel cell (PEMFC) is investigated using conductive probe atomic force microscopy (CP-AFM). A platinum-coated AFM tip is used as a nanoscale cathode in an operating PEMFC. We present results that show highly inhomogeneous distributions of conductive surface domains at several length scales. At length scales on the order of the aqueous domains of the membrane, approximately 50 nm, we observe single channel electrochemistry. I-V curves for single conducting channels are obtained, which yield insight into the nature of conductive regions across the PEM. In addition, we demonstrate a new characterization technique, phase current correlation microscopy, which gives a direct measure of the electrochemical activity for each aqueous domain. This shows that a large number ( approximately 60%) of the aqueous domains present at the surface of an operating Nafion membrane are inactive. We attribute this to a combination of limited aqueous domain connectivity and catalyst accessibility.
