An investigational method of improving the performance of a fuel cell that contains a polymer-electrolyte membrane (PEM) is based on the concept of roughening the surface of the PEM, prior to deposition of a thin layer of catalyst, in order to increase the PEM/catalyst interfacial area and thereby increase the degree of utilization of the catalyst. The roughening is done by means of laser ablation under carefully controlled conditions. Next, the roughened membrane surface is coated with the thin layer of catalyst (which is typically platinum), then sandwiched between two electrode/catalyst structures to form a membrane/electrode assembly.

Figure 1. This Scanning Electron Micrograph shows portions of a PEM before and after roughening by laser ablation.
The feasibility of the roughening technique was demonstrated in experiments in which proton-conducting membranes made of a perfluorosulfonic acid-based hydrophilic, proton-conducting polymer were ablated by use of femtosecond laser pulses. It was found that when proper combinations of the pulse intensity, pulse-repetition rate, and number of repetitions was chosen, the initially flat, smooth membrane surfaces became roughened to such an extent as to be converted to networks of nodules interconnected by filaments (see Figure 1).

Figure 2. These EIS Data were acquired in measurements at frequencies from 100 kHz down to 10 mHz on 0.4-mm-thick smooth and roughened PEMs that had lateral dimensions of 3 by 3 mm, were 0.4 mm thick, and were coated with Pt on both faces. The smaller imaginary components of impedance of the roughened specimens are attributed to greater capacitances, which, in turn, are attributed to greater surface areas. Roughened specimens 1 and 2 were subjected to different laser-ablation conditions.
In further experiments, electrochemical impedance spectroscopy (EIS) was performed on a pristine (smooth) membrane and on two laser-roughened membranes after the membranes were coated with platinum on both sides. Some preliminary EIS data were interpreted as showing that notwithstanding the potential for laser-induced damage, the bulk conductivities of the membranes were not diminished in the roughening process. Other preliminary EIS data (see Figure 2) were interpreted as signifying that the surface areas of the laser-roughened membranes were significantly greater than those of the smooth membrane. Moreover, elemental analyses showed that the sulfur-containing molecular groups necessary for proton conduction remained intact, even near the laser-roughened surfaces. These preliminary results can be taken as indications that laser-roughened PEMs should function well in fuel cells and, in particular, should exhibit current and power densities greater than those attainable by use of smooth membranes.

This work was done by Jay Whitacre of Caltech and Steve Yalisove of the University of Michigan for NASA’s Jet Propulsion Laboratory.


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Laser Ablation Increases PEM/Catalyst Interfacial Area

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This article first appeared in the March, 2009 issue of NASA Tech Briefs Magazine.

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