Noninvasive Measurement of Conductivity Distribution in Polymer Electrolyte Fuel Cells (PEFCs)

Abstract

Water management in Polymer Electrolyte Fuel Cells (PEFCs) is a crucial topic. Indeed, a homogeneous distribution of the water in the membrane plane is important as it dictates its conductivity. Furthermore, local dry outs or flooding event can damage the cell. Existing measurement methods that are able to provide spatially resolved information on the current density, conductivity or water content distribution are usually invasive and therefore difficult to use in actual applications. A new method, based on the learnings of the development of Electrical Impedance Tomography (EIT) for PEFCs [Schuller and Eller 2020 ECS Trans. 98 3], is used to determine 1D humidity profiles between the gas inlet and outlet in a 6-cell commercial stack. Local AC current injections and voltage measurements are performed using electrodes positioned all around the fuel cell. Each measurement characterizes a defined area of the membrane as the current crosses it very locally. In a first step, the method is calibrated using nitrogen flow at different well-defined humidity levels. Using the calibration data the local boundary voltages can be converted into local humidity conditions when the stack is operated. In Figure 1a the reconstructed conductivity distributions of the stack being operated at four different current densities with inlet relative humidity (RH) of 60 % at the anode and 30 % at the cathode in counter flow configuration is shown. The y-axis represents the relative membrane conductivity value where 1 corresponds to the reference measurement with the highest inlet RH (90%). The conductivity increases with increasing current densities and there tends to be a higher conductivity towards the cathode outlet due to water drag through the membrane. Thanks to the fast multi-channel data acquisition setup, the method can be also used to study the dynamic behavior of a cell. Figure 1b shows the time resolved reconstruction of the conductivity distribution when flushing the cell with nitrogen (anode and cathode at 40% RH) after the stack is operated at 0.8 A/cm2. The method is capable to resolve the quick decrease in membrane conductivity at both inlets and to follow the much slower drying process of the membrane in the center of the cell. Figure 1