Transient hydrogen crossover in dynamically operated PEM water electrolysis cells - A model-based analysis

Hydrogen crossover in polymer electrolyte membrane electrolysis cells is important concerning faradaic efficiency, flammability hazards, and degradation phenomena. In recent years, steady-state H2-in-O2 measurements have demonstrated that the hydrogen crossover increases with current density, due to mass transport limitations in the cathode catalyst layer. However, hydrogen crossover during dynamic operation has not been investigated yet. Therefore, this study investigates the hydrogen crossover with a dynamic macroscopic 1-D through-plane model of a polymer electrolyte membrane electrolysis cell. The model focuses on the detailed description of the dynamics of the reactions and mass transport of hydrogen in the membrane electrode assembly. Simulated down steps in current density, lead to transient overshoots in the H2-in-O2 content at the anode side. The membrane acts as short-term mass storage for the dissolved hydrogen, and mass transport lags the instant response of the current density. Under specific conditions with high cathode mass transport limitations, the lower explosion limit of H2-in-O2 can be transiently exceeded. This work provides for the first-time insights into transient hydrogen crossover phenomena and is a further step into dynamic model-based analysis of polymer electrolyte water electrolysis cells.