Micro scale electrochemistry: Application to fuel cells

Fuel cells offer the tantalizing promise of cleaner electricity with less environmental impact than traditional energy conversion technologies. This is because fuel cells are direct electrochemical energy conversion devices.; Currently, fuel cell technology's greatest disadvantage is cost. Today (2004), fuel cell technology is only economically competitive onboard the Space Shuttle. Recently, fuel cell costs have declined due to technological successes from nanostructured materials. In spite of this success, greater cost reductions and other challenges remain. Furthermore, we are far from possessing a solid scientific understanding of what really goes on at the nano-scale inside fuel cells. Such understanding is critical for further progress.; This thesis pioneers several novel electrochemical techniques to study fuel cells at sub-micron length scales. A first technique employs platinum microelectrodes to examine the "triple phase boundary" (TPB) properties of polymer electrolyte fuel cells. By constructing geometrically simple, well-defined electrocatalyst structures of various sizes, a relationship between electrocatalyst geometry and electrochemical behavior is clearly delineated. This study provides perhaps the most direct experimental validation to date of the TPB theory.; Extending characterization abilities to the nano-scale, a second technique, called atomic force microscopy (AFM) impedance imaging, is developed. AFM impedance imaging allows highly localized measurements of electrochemical properties to be acquired across sample surfaces. The technique is used to qualitatively visualize sub-micron variations in the electrochemical properties of Nafion (fuel cell electrolyte) samples. The AFM impedance technique is further refined by development of a quantitative measurement methodology. This methodology is validated by AFM impedance studies of the oxygen reduction reaction (ORR) at nano-scale Platinum/Nafion contacts. Use of the quantitative AFM impedance technique provides perhaps the most direct measurement yet of ORR kinetics at nanometer length scales.; While the techniques in this thesis are employed to study fuel cells, characterizing and understanding nanostructures is a challenge extending beyond the fuel cell realm. Many other devices, such as solar cells, sensors, and thermoelectric converters also benefit from nanostructured materials. The parallels between these systems and fuel cells make them amenable to the same type of nano-scale visualization and measurement techniques, offering rich opportunities for further research.