A Novel Method for Humidity-Dependent Through-Plane Impedance Measurement for Proton Conducting Polymer Membranes

Abstract

In this study, we introduce a through-plane electrochemical measurement cell for proton conducting polymer membranes (PEM) with the ability to vary temperature and humidity. Model Nafion and 3M membranes, as well as anisotropic composite membranes, were used to compare through plane and in plane conductivity. Electrochemical impedance spectroscopy (EIS) was applied to evaluate the proton conductivity of bare proton exchange membranes. In the Nyquist plots, all membranes showed a straight line with an angle of 60–70 degrees to the Z’-axis. Equivalent circuit modeling and linear extrapolation of the impedance data were compared to extract the membrane resistance. System and cell parameters such as high frequency inductance, contact resistance and pressure, interfacial capacitance were observed and instrumentally minimized. Material-related effects, such as swelling of the membranes and indentation of the platinum mesh electrodes were examined thoroughly to receive a reliable through-plane conductivity. The received data for model Nafion and 3M membranes were in accordance with literature values for in-plane and through-plane conductivity of membrane electrode assemblies. Anisotropic composite membranes underlined the importance of a sophisticated measurement technique that is able to separate the in-plane and through-plane effects in polymer electrolytes.

Generalized Distribution of Relaxation Times Analysis for the Characterization of Impedance Spectra

“…In general, impedance corresponds to the transfer function of current and voltage in frequency domain Z( jω) = U( jω) I( jω) =û (ω) sin(ωt) i(ω) sin(ωt + ϕ(ω)) = Z( jω) e jωϕ (1) The input or excitation signal could be current (galvanostatic mode) or voltage (potentiostatic mode). Electrochemical impedance spectroscopy (EIS) was applied to a variety of electrochemical systems covering, e.g., sensors [1], fuel cells [2,3], and batteries [4][5][6][7][8], as well as for material characterization that looks at transport and charge transfer processes, interface phenomena, conductivity, and diffusion [9][10][11][12]. Furthermore, impedance spectroscopy using current and voltage as excitation and system response is applied to photovoltaic cells [13,14] and under the label of bioelectrical impedance analysis (BIA) for clinical studies, tissue characterization [15], and for meat quality control [16,17].…”

A Review of Recent Developments and Advanced Applications of High-Temperature Polymer Electrolyte Membranes for PEM Fuel Cells

“…To measure the proton conductivity in liquid water or at relative humidity lower than 100%, the cell is placed in a distilled deionized water or a humid chamber with controlled humidity (thermos-controlled humid chamber), respectively. In-plane conductivity can be calculated as follows [214][215][216][217]:…”

“…To determine the resistance, the high-frequency intercept of the impedance with the real axis should be used. The following equation can be used for the through-plane conductivity calculations [214][215][216]:…”

Effect of Increased Ionic Liquid Uptake via Thermal Annealing on Mechanical Properties of Polyimide-Poly(ethylene glycol) Segmented Block Copolymer Membranes

“…To measure resistance from a Nyquist plot using the electrochemical impedance spectroscopy (EIS) method, there are mainly three alternatives [91][92][93][94][95][96][97]: (1) linear extrapolation of the measured spectra data to the Z'-axis of the Nyquist plot, where the Z' value at the cross-section is taken as the membrane resistance, (2) reading Z' real corresponding to the lowest value of Z" imaginary at high frequency from the impedance spectra obtained from alternating current (AC) impedance, where the semicircle starts, read is taken as the membrane resistance, or (3) applying equivalent circuit fitting.…”