Although the change in protection layer has no discernable effect on device performance (Figure ), a significant difference is observed in the Nyquist plots negative of the onset potential. Figure compares these Nyquist plots, which were obtained under 0.1 sun illumination at -0.55 V vs RHE. At this potential, all devices are operating in depletion and show a saturated photocurrent. Clearly, for the two alternate protection layers only one semicircle is visible and therefore the Randles circuit can be used unambiguously to extract resistances and capacitances for these devices during the HER. This also indicates that the low frequency process observed in a-TiO2 protected devices originates from the presence of a-TiO2 itself.