Abstract
Hydrogen is a crucial chemical for society, both today and in the future. Today, it is used for fertilizer and steel production. In the future, it has the potential to replace fossil fuels in many transportation sectors. To ensure that production of the hydrogen itself does not involve carbon dioxide emissions, solar energy can be used to synthesize hydrogen directly from water using photoelectrochemical (PEC) cells. In this work, the hydrogen evolution reaction (HER) is investigated using amorphous TiO2 (a-TiO2) protected PEC cells. Specifically, the mechanism of charge transfer through the device is determined using electrochemical impedance spectroscopy (EIS). A model system of p-Si│a-TiO2│Pt is established for this study. It is found that two distinct processes occur within the MHz–Hz frequency range during the HER for these devices. While the fast process is attributed to the semiconductor layer through Mott-Schottky analysis, the slow frequency process is found to be due to the presence of a-TiO2. These two simultaneous processes induce an ambiguity to the mechanism of charge transfer in these devices, as multiple equivalent circuits (ECs) are mathematically valid to model the EIS response. This ambiguity is resolved by comparison to devices without the a-TiO2 protection layer. Ultimately, it is found that the Maxwell EC represents the charge transfer mechanism in a-TiO2 protected photocathode devices during the HER. Physically, this corresponds to one faradaic process through the PEC device in parallel with a short-range, non-faradaic process within the device. Possible physical mechanisms of the short-range process are investigated and discussed. These include the a-TiO2│electrochemical interface, the p-Si│a-TiO2 interface and proton intercalation in the a-TiO2 protection layer.
Service, Erin