Spatial distribution and dynamics of proton conductivity in fuel cell membranes: potential and limitations of electrochemical atomic force microscopy measurements

Abstract

The proton conductivity of a Nafion 112 membrane is measured with a high spatial resolution using electrochemical atomic force microscopy. Image analysis reveals an inhomogeneous conductivity distribution which is attributed to the limited connectivity of hydrophilic domains. This information relates to the micro-morphology which is due to phase separation of the hydrophobic polymer backbone and the hydrophilic pendant groups. The direct images relate to a different length scale and are complementary to the x-ray diffraction investigations which provide only average information. Furthermore, the measured current values reveal an interesting correlation with the size of the conductive areas. A bimodal conductivity distribution suggests that there are different mechanisms which contribute to the proton current in Nafion. Additionally, time dependence in local conductivity is found and interpreted in terms of redistribution of water in the membrane. A statistical analysis of the current distribution is performed and compared with theoretical simulations. Evidence is found for the existence of a critical current density. On a timescale of seconds the response of the conductive network is probed by applying voltage steps to the atomic force microscope tip.

Atomic Force Microscopy on Cross Sections of Fuel Cell Membranes, Electrodes, and Membrane Electrode Assemblies

“…With advanced tapping mode techniques, the differentiation between different materials, including different phases, allows a direct imaging of nanostructure under various temperature and humidity conditions. Using conductive AFM, analysis of the structures under nonequilibrium conditions as current flow has been conducted [8], [23], [29], [30], [31], [32]. The disadvantage of this approach is the restriction of the measurement to surface or subsurface properties and the dependence of the obtained resolution on the tip size and shape.…”

Proton Conductive Areas on Sulfonated Poly(Arylene Ketone) Multiblock Copolymer Electrolyte Membrane Studied by Current-Sensing Atomic Force Microscopy

“…[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] This current-sensing AFM (CS-AFM) technique has been used on Nafion [21][22][23][24][25][26][27][28][29][30][31][32]37 as well as HC-based membranes. [34][35][36] Several groups reported that the distribution of the proton conductive domains was correlated to the hydration level of the membranes.…”

Conductive AFM Study to Differentiate between the Surface Ionic Conductivity of Nafion and Electrospun Membranes

“…The ionic selectivity has been shown to drastically change with the relative hydration of the PEM (1). When the PEM is not hydrated, the transport of protons across the membrane effectively goes to zero due to the incomplete disassociation of the ionic groups (-SO 3 -H + ) within the membrane and on the surface (2). When a membrane is too hydrated, the polymer swells causing the ionic pathways through the membrane to open up and allow larger chemical species to diffuse across the membrane.…”

“…The increased number of ionic pathways will raise the overall efficiency of the PEM fuel cell, while prevention of membrane swelling will enable increased chemical selectivity through the membrane. The potential and limitations of AFM have been extensively studied by Aleksandrova (2). Ionescu-Zanetti explored the local charge transfer properties of polymer blends using multimodal AFM (3).…”