Nanoscale Characterization and Scanning Probe Microscopy
Studying the kinetic scaling behavior of nano-structured systems can be quite complicated. In fuel cell systems, for example, it has long been believed that the oxygen reduction reaction (ORR) can only occur at confined spatial regions, called "triple phase boundaries" (TPB's) where the electrolyte, gas, and electrically connected catalyst particles contact. However, the reaction-zone structure of a state of the art fuel cell is complex, consisting of a porous, heterogeneous mixture of conductive carbon powders and platinum particles, often mixed with a solid polymer electrolyte binder. Due to this complexity, the true amount of TPB in a fuel cell is difficult, if not impossible, to determine. Thus, the relationship between catalyst microstructure (ie, TPB geometry) and fuel cell performance is still unclear.
Figure 1. A simplified schematic diagram of the electrode/electolyte interface in a fuel cell, illustrating the triple phase boundary reaction zones where the catalytically active electrode particles, electrolyte phase, and gas pores intersect.
In order to begin answering some of these questions, our laboratory employs novel micro and nano-scale electrochemical characterization techniques to study electrochemical phenomena at the sub-micron length scale.
For example, we employ well-defined platinum micro-electrode geometries to directly examine the intrinsic catalytic performance and TPB properties of Nafion/Pt interfaces. By constructing reproducible, geometrically simple, well-defined electrocatalyst structures of various sizes, a relationship between electrocatalyst geometry and electrochemical behavior is clearly delineated. These studies have provided perhaps the most direct experimental validation to date of the TPB theory.
Figure 2. Electron micrograph of a series of Pt-microelectrodes directly patterned via FIB onto the surface of a Nafion membrane "half-cell" MEA. (b) Higher magnification electron micrograph of a portion of an FIB patterned array of 1micron radii Pt microelectrodes. The electrochemical properties of these FIB patterned Pt particles are probed using a micro-probe station.
Extending our characterization ability to the nano-scale, we have developed a technique called AFM impedance imaging. AFM impedance imaging allows highly localized measurements of electrochemical properties to be acquired across sample surfaces. We have used this technique to study the electrochemical behavior of fuel cells and electrocatalysts. However, characterizing and understanding nano-structures is a challenge that extends far beyond the fuel cell realm. Many other devices, such as solar cells, sensors, and thermoelectric converters also benefit from nano-structured materials. The parallels between these systems and fuel cells make them highly amenable to the same type of nanometer scale visualization and measurement techniques, offering rich opportunities for further research.
Figure 3. Bulk vs. AFM impedance measurement. (a) General concept of a two-electrode impedance measurement. An impedance measurement system acquires an ac impedance spectrum from a sample of interest sandwiched between two bulk electrodes. (b) In an AFM impedance measurement, impedance is measured between a local probe (the SPM tip) and a bulk electrode, enabling the acquisition of spatially resolved electrochemical information at the sub-micron scale.
This work is supported by the U.S.Army Research Office under grant number: W911NF-07-1-0258 and Capital Equipment number: W911NF-08-1-0292.