Although the absolute H2 crossover was higher for STz25 than for Zirfon, the specific permeability was similar, which indicates that the difference is mainly a consequence of the difference in thicknesses. However, it is also known that the reported H2 crossover of Zirfon is largely varying in the scientific literature, which may be due to batch-to-batch variations, differences in experimental protocols, and influenced by the degree of zero-gap for the specific cell. Figure