Coupled Simulation of Chemical Membrane Degradation and Oxygen Crossover in PEM Water Electrolysis

Abstract

The membrane of polymer electrolyte membrane water electrolysis (PEMWE), as one of the lifetime limiting components, does not only have an impact on performance, but also plays an important role for safety issues over time. Membrane thinning is a key phenomenon caused by chemical membrane degradation and directly affects the conductivity and gas crossover. Therefore, a chemical membrane degradation model based on Fenton equations [1,2] coupled with a gas crossover model [3] is presented in this work. The presence of Oxygen at cathode side is considered by oxygen crossover, which is strongly enforced by supersaturation effects. Specifically, this cathode-side oxygen is conducive to hydrogen peroxide formation. In interaction with metal ions, e.g. iron, from the feed water, which partially migrate through the membrane, hydroxyl radicals can be formed. These attack the polyfluorinated sulfonicacid (PFSA) membrane at different points, releasing sulfon acid head groups (SO3H-) and fluride ions (F-), which can be measured as sulfur release rates (SRR) and fluride release rates (FRR) in the effluent water, respectively. The degradation model provides concentration profiles for the relevant species involved in the process, which, e.g., influence the thickness and conductivity of the membrane via structure-function relations. In particular, the changed thickness again influences the crossover effect, which itself represents an input for the degradation model. Through this coupling, both models can be updated at specific times. Implications of the model-based analysis on the mechanistic understanding of chemical membrane degradation by radical attack in PEMWE are discussed. Furthermore, the model results can be verified and calibrated by experimental data in combination with adapted accelerated stress tests (AST) and also encourage their enhancement.