Localized Electrochemical Impedance Measurements on Nafion Membranes: Observation and Analysis of Spatially Diverse Proton Transport Using Atomic Force Microscopy
Authors: He Qinggang , Jiang Fangming , Wang Xiaojiang , Tsegai Habte Bereket , Zhang Shuomeng , Yang Houhua 
Published: February 2020

Nafion, the state-of-the-art-of proton exchange membrane in fuel cell, is a copolymer with a randomly tethered side-chain terminated by a pendant sulfonic acid groups adjacent to the polytetrafluoroethylene (PTFE) backbone. The PTFE backbone is hydrophobic and insulative, whereas the sulfonic acid groups are hydrophilic and proton-conductive. As a result of the intrinsic difference between the backbone and sulfonic acid groups, phase separation occurs during the formation of solid Nafion membranes from ionomer solution via a self-organizing process conducted at elevated temperature. The ionic clusters are formed in hydrophobic matrix by hydrophilic groups, interconnected by slender ionic channels which serve as the proton conductive pathways.

Furthermore, the distribution of proton-conductive pathways not only inside but on the surface of the membrane plays a significant role in improving the cell performance. The latter determines the mass transport of H2O molecules and protons through the membrane, which is the basis of good performance in fuel cells. In addition, the three-phase boundary (TPB) plays a crucial role in the performance of fuel cells, which means that the electrocatalysts (such as Pt) in fuel cells must directly contact with the conductive hydrophilic regions on the surface of PEMs to minimize the contact resistance and invalid catalysts located in the hydrophobic regions will lead to lower catalyst utilization. However, an excess of hydrophilic groups would cause flooding, and the hydrophobic phase is necessary for supporting mechanical strength and separating reagents in fuel cells. Therefore, in order to further promote fuel cell performance, detailed knowledge of phase separation with Nafion is required, and a good balance between the distributions of hydrophilic and hydrophobic groups is extremely crucial for promising ion exchange membranes.

Currently, current-sensing AFM is often used to obtain the imaging of local surface proton conductance. However, the direct current (DC) applied leads to an overall result that is affected by many factors, such as proton conductivity, mass transport and reaction kinetics. Compared to DC methods, alternate current (AC) methods such as electrochemical impedance spectroscopy (EIS) provides a powerful insight to the overall impedance response and individual processes including ionic transport can be distinguished at higher accuracy.

In our study, we constructed a home-built AFM-EIS setup and measured the localized impedance by coupling AFM with an impedance tester in a Faraday cage. In contrast to traditional average and macroscopic bulk impedance measurements, we obtained a detailed and spatially diverse distribution of EIS spectra at selected points on Nafion with AFM-EIS. By equivalent electric circuit fitting, we derived the localized pure proton transfer resistance from the EIS spectra; such results cannot be obtained by current-sensing AFM. In addition, with a customized automatic procedure, we performed proton transfer resistance imaging on the Nafion surface in an area of 2 µm*2 µm at different humidities, the results of which confirmed the widely known fact that higher water activities significantly reduce the transport resistance. However, our experimental results show a wide variation in proton transport resistance at a well-equilibrated humidity level of the membrane. Our work has led to the conclusion that the large variation arises mainly from the surface concentration of water channels due to the fact that the transport and reaction kinetics are favoured by the three-phase interface. Furthermore, to elucidate the Nafion microstructure, a numerical simulation was constructed to incorporate morphological and physical variations of the membrane, such as the heterogeneity of the hydrophilic and hydrophobic groups in the membrane, which revealed that at higher water activities, SO3H groups cluster to create a matrix of interconnected nano-water channels through which the ions are transported.

Figure 1