Investigation of the through-plane impedance technique for evaluation of anisotropy of proton conducting polymer membranes

Two-probe electrochemical cells were designed for proton conductivity evaluation in the X, Y and Z directions using electrochemical impedance spectroscopy (EIS). Nafion® 112, 115, 1135, 117 and 211 membranes were used to examine the conductivity in the three given directions, and to determine the influence of cell configuration and probe geometry on the accuracy and reproducibility of the measurement. Emphasis is placed on obtaining an understanding of the real system and adequate transfer of the physical processes into the form of the equivalent circuit for the extraction of reliable conductivity data. In order to verify that the method can determine anisotropic conductivity, novel fluorous block copolymers with various anisotropic phase segregated morphologies were evaluated. All polymer electrolyte membranes examined exhibited a Nyquist plot response in the form of a straight line with an angle of approximately 70°–80° to the Z′-axis. Interfacial capacitance, membrane bulk resistance and membrane bulk capacitance were considered as contributing parameters to the impedance of the system; high frequency inductance and contact resistance were instrumentally minimized. A marginally anisotropic conductivity (σ=/σ⊥ = 1.0–1.4) was found in all Nafion® samples. Fluorous block copolymers with disordered morphology exhibited higher conductivity in the through-plane direction by a factor of 1.4. For the polymers with perforated lamellae and lamellae morphologies, in-plane conductivities were larger by factors of 2.4 and 5.5, respectively.