Experimental evidence of increasing oxygen crossover with increasing current density during PEM water electrolysis

Oxygen permeation in proton exchange membrane (PEM) water electrolyzers is a critical phenomenon. Mainly, because of (i) degradation and (ii) purity of the hydrogen product gas. Additionally, but less important because of (iii) efficiency loss and (iv) safety problems. Despite these issues, oxygen permeation in PEM water electrolysis was paid less attention. This can be explained by the low oxygen crossover compared to the hydrogen crossover. In this contribution the oxygen content within the hydrogen product gas was measured for two different cathodic catalyst materials (Pt and a Pt-free catalyst) during water electrolysis in a current density range of 0.05-2 A/cm(2). In comparison to the platinum catalyst, the Pt-free catalyst leads to 3-4 times higher oxygen contents within the hydrogen product gas. This can be explained with a lower activity concerning oxygen recombination, so that less permeated oxygen is consumed and consequently, the oxygen flux within hydrogen is higher. The results of this work emphasize that the oxygen crossover increases with increasing current density, as like the hydrogen crossover does. Particularly, two effects are possible for this strong increase in oxygen permeation: supersaturation and the electro-osmotic drag. The experimental findings show that the crossover is higher as generally expected, and should receive more attention.