Nafion (PEM-FC Membrane)
The AFM impedance technique may also be applied to studies of ionic materials and/or electrochemical systems. In this section, we report localized impedance measurements on the protonated form of Nafion solid polymer electrolyte.
The figure below shows sets of fixed-frequency AFM impedance images of a Nafion electrolyte membrane. These images were acquired at 1 Hz with a 30 mV excitation signal, 0.4 V dc cathodic bias (the bias favors the oxygen reduction reaction, thus decreasing the Faradaic chargetransfer resistance and improving the signal to noise). The images compare the impedance response from a dry Nafion membrane (imaged under dry, 0% relative humidity, room temperature air) versus the same area of the same membrane when hydrated (imaged under 30% relative humidity, room temperature air). The hydration level of the Nafion electrolyte significantly influences its solvated proton (H3O+.nH2O) concentration and conductivity. This is readily apparent from the AFM impedance images, which show a dramatic change in the impedance response (note change in the impedance scale).
It is well known that Nafion is not a homogeneous material, but instead consists of hydrophilic and hydrophobic phase-segregated regions. The apparent features seen in the Nafion impedance images of Fig. 15 may correspond to hydrophilic domains in the membrane. The apparent sizes of these domains (refer to the 1 um image) are on the order of several hundred nanometers.
