H2 Production Efficiency in PEM Water Electrolysis Cells – Evaluating the Effect of the Electrode-Electrolyte Interface on O2 and H2 Crossover
Authors: Freiberg Anna T.S. , Thiele Simon 
Published: December 2024

PEM water electrolyzers (PEMWEs) are a promising candidate to store fluctuating renewable energy due to their wide current density operating regime. This flexibility is a significant advantage compared to alkaline water electrolysis. However, this flexibility in current density is also a challenge to fully understand local and interfacial effects at different operating conditions minding the different scales that have to be considered in PEMWEs – from the nanometer-sized ionomer thin film to millimeter-sized channel structures for water supply and gas removal.
Gas crossover is a big challenge in PEMWEs. It does not just limit the operating regime due to possible formation of explosive gas mixtures on the anode side. 1 It also contributes significantly to the overall efficiency of the system by causing some hydrogen that was produced by the electrical energy input to not reach the cathode exhaust. Crossover hydrogen is lost to the anode exhaust and crossover oxygen chemically recombines with hydrogen, which is therewith also lost when considering the overall hydrogen production efficiency. 2

Compared to PEM fuel cells, gas crossover characteristics in PEMWEs under operation are diverging more from steady-state permeation data. 1, 3 This can be caused by operation dependent two-phase flow characteristics of liquid water and gaseous product. While the anode is actively flushed with liquid water as reactant supply, the cathode of a PEMWE also experiences two-phase operation; either due to being flushed with water additionally or caused by the significant electro-osmotic drag at high current densities leading to water droplet formation in the cathode catalyst layer. The exact environment of the electrode-electrolyte interface will dictate the crossover behavior at different operating points but it is not well understood.
In this study we shed some light on the effect of the interfacial two-phase characteristics on the gas crossover behavior at different operating conditions. Hydrogen and oxygen steady-state permeation over PEMWE membranes is studied using a mass spectrometer at different single- and two-phase flow conditions that can be decoupled from the electrochemical gas production and electro-osmotic drag effects due to active hydrogen and oxygen gas supply. By a thorough parameter variation we are able to distinguish solubility, diffusivity and fugacity effects on the overall permeability of the active gases. We show that common pure gas-phase measurements lead to gas permeabilities that show a severalfold difference compared to the results from the two-phase measurements.
By comparing such steady-state permeation with gas crossover characteristics at different operating points a better understanding of the electrode-electrolyte interface composition during PEMWE full cell operation can be achieved. While the exact interfacial characteristics depend on the catalyst layer and transport media structure, this experimental data shall serve as a starting point for revisiting the tuning of structural parameters in order to achieve PEMWEs with improved hydrogen production efficiencies. Such dynamic effects will be important in large-scale cells where the two-phase characteristics can change along the cell, additionally.

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