This study provides a direct comparison of hydrogen crossover in PEM (Nafion 117) and alkaline water electrolysis (Zirfon) at a temperature of <mml:math display="inline"><mml:mrow><mml:msup><mml:mn>60</mml:mn><mml:mo>∘</mml:mo></mml:msup><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>60∘C applying state-of-the-art separating unit materials. To this end, occurring crossover mechanisms are described first, before experimental data of the anodic hydrogen content are shown in dependence of current density, system pressure and process management strategy. The results suggest that permeation in PEM electrolyzers is mainly governed by diffusion due to a supersaturated concentration of dissolved hydrogen within the catalyst layer, showing a share of 98} of the total permeation flux at <mml:math display="inline"><mml:mrow><mml:mn>1</mml:mn><mml:mspace width="0.16em"></mml:mspace><mml:mi mathvariant="normal">A</mml:mi><mml:mspace width="0.16em"></mml:mspace><mml:msup><mml:mi>cm</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>1Acm−2 and atmospheric pressure. Permeation in alkaline electrolyzers also exhibits a significant influence of supersaturation, but the overall crossover is mainly influenced by mixing the electrolyte cycles, which makes up a share of 90} at <mml:math display="inline"><mml:mrow><mml:mn>0.7</mml:mn><mml:mspace width="0.16em"></mml:mspace><mml:mi mathvariant="normal">A</mml:mi><mml:mspace width="0.16em"></mml:mspace><mml:msup><mml:mi>cm</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>0.7Acm−2 and <mml:math display="inline"><mml:mrow><mml:mn>1</mml:mn><mml:mspace width="0.16em"></mml:mspace><mml:mi>bar</mml:mi></mml:mrow></mml:math>1bar . Generally it becomes evident that hydrogen permeation across the separating unit is more than one order of magnitude smaller in alkaline electrolysis, which is mainly a consequence of the significantly lower hydrogen solubility in concentrated KOH electrolyte. Finally, this study concludes with an assessment of the impact of separating unit thickness and provides mitigation strategies to reduce hydrogen crossover.

An important challenge for polymer electrolyte membrane (PEM) water electrolysis is to reduce the permeation of the produced gases. This crossover affects the cell efficiency and causes safety issues. The crossover increases with current density, most probably due to mass transfer resistances. This work aims to investigate the influence of the cathode ionomer content on hydrogen crossover. Therefore, the ionomer content was varied between 10 and 40 wt% to clearly influence the mass transfer resistances. The best performance and lowest crossover was obtained for 10 wt% ionomer. However, within the observed ionomer range the mass transfer resistances increase with ionomer content that cause increases in hydrogen crossover and cell voltage. Both can be entirely explained by the same quantity of supersaturated dissolved hydrogen concentrations. These supersaturated concentrations cause higher cathode half-cell potentials, which explain the cell voltage increase and lead to higher concentration gradients across the membrane, which enhance the crossover. These findings highlight the importance of mass transfer resistances within catalyst layers in terms of crossover and performance. They constitute an important step in the clarification of the complex interplay between mass transport and voltage losses, enabling the development of novel electrode architectures for PEM water electrolyzers.

Polymer electrolyte membrane (PEM) water electrolysis is a key technology for sustainable hydrogen based energy supply. Gas permeation through the PEM leads to hydrogen in oxygen at the anode side posing a safety hazard and therefore restricting the operation window of PEM water electrolysis, especially when operating under pressure. In this work the hydrogen in oxygen content at the anode is significantly reduced when a recombination interlayer is integrated into the membrane electrode assemblies (MEAs) compared to reference MEAs without interlayer. The recombination interlayer with a platinum loading of 0.02 mg cm^{− 2} is sprayed between two membranes that are coated with anode and cathode catalysts on the outside. The permeating H₂ and O₂ forms water at the recombination interlayer, leading to higher gas purity and resolving safety issues. In case of the MEAs with interlayer also a constant current hold at 1 A cm^{− 2} for 245 h revealed only a slight increase of the hydrogen in oxygen content (below 140 · 10^{− 6} vol.% h^{− 1}) whereas for the reference MEAs without interlayer a stronger increase was observed (above 1250 · 10^{− 6} vol.% h^{− 1}). Furthermore, the long-term experiments showed no increased degradation rates compared to the reference MEAs.

The present analysis shows the local distribution of current density and EIS measurements along a 50 cm single-channel proton exchange membrane water electrolysis (PEMWE) cell. Measurements for operating modes with one sufficiently high and one insufficiently low stoichiometric water ratio (λ) were carried out to observe effects on the current density distribution. Furthermore, global and local EIS measurements were performed to distinguish between the cell voltage loss differences in the two cases. The mass transport losses η_mtx and the Ohmic voltage losses η_Ω show a strong increase, when the stoichiometric water ratio falls below a level of λ ≈ 5. The reduction of the inlet water flux of the anode reduces both the proton conductivity of the ionomer within the catalyst layer and the membrane, increasing transport and Ohmic resistances, respectively. The local analysis has shown that the level of membrane and catalyst hydration under low stoichiometric conditions can be distributed highly non-homogeneous in along-the-channel direction, with the most pronounced dehydration toward the end of the channel.