Towards a Consistent Interpretation of Electro‐osmotic Drag in Polymer Electrolyte Membranes

In many experimental studies of polymer electrolyte membranes (PEMs), the electro‐osmotic drag coefficient is a property of primary interest. When protons are the only type of mobile ion present in the system, the coefficient is typically defined as the ratio between the molar water flux and the molar proton flux across the PEM. While most, if not all, experimental results suggest a value of order one, experimentalists have yet to arrive at a consensus regarding its exact value for a specific PEM at specific operating conditions. One reason for the variance in reported results could lie in the misinterpretation of the electro‐osmotic drag. This contribution uses physical models to clarify what a consistent definition and measurement of electro‐osmotic drag coefficients entails, thereby providing guidance for experimental studies of these membranes.