Operation of Amorphous TiO₂-Protected Photocathodes Described with the Maxwell Equivalent Circuit

The use of amorphous TiO₂ (a-TiO₂), deposited by atomic layer deposition, is a common strategy to protect semiconductors from degradation when used in water-splitting photoelectrochemical (PEC) cells. Electrochemical impedance spectroscopy (EIS) is a suitable technique to study these PEC cells because it is capable of deconvoluting multiple processes occurring during operation, therefore providing information about mechanisms leading to the overall device performance. When biased under hydrogen evolution conditions, EIS shows that two simultaneous processes occur in a-TiO₂-protected photocathodes, which introduces an ambiguity in choosing the correct equivalent circuit to describe the operating device. In this report, a model p-Si|a-TiO₂|Pt photocathode system is used to show that the Maxwell circuit best describes the operating mechanism, as opposed to the more commonly used Voight and nested circuits. This indicates that, under hydrogen evolution conditions, both faradaic and nonfaradaic processes are occurring. Whereas the faradaic process corresponds to the hydrogen evolution reaction itself, the nonfaradaic process is traced to the p-Si|a-TiO₂ interface.