Improving Photoelectrochemical Hydrogen Generation from Water Splitting using Ultrathin Overlayers on Hematite Photo-anodes

Abstract

Solar energy is the energy source for the future, because it is renewable and environmentally safe. An application to convert the solar energy into a useful energy form is photoelectrochemical (PEC) water splitting, in which the solar energy is converted to chemical energy in hydrogen gas. Hematite (-Fe2O3) is one of the most promising semiconductors that are used in PEC cells, mainly because of its band gap of 1.9 − 2.2 eV, its chemical stability, and that it is an abundant material. Hematite is used as a photo-anode in the PEC cell, but its performance is limited due to for instance poor oxidation kinetics at the hematite/electrolyte interface. In this work, three different ultrathin overlayers (FeOOH, NiOOH, NiFeOx) are deposited on hematite in order to improve the performance of the PEC cell by reducing recombination of photogenerated electron−hole pairs and facilitate hole transport at the hematite/overlayer/electrolyte interface. With these overlayers, the photocurrent density of hematite is increased and the onset potential is lowered. The hematite electrode with the FeOOH overlayer shows the best performance, where the photocurrent density is increased from 0.087 mA/cm2 for bare hematite to 0.275 mA/cm2 at 1.23 VRHE under 1 sun illumination, and the onset potential is lowered by 220 mV compared to bare hematite. This improved performance is a result of the improved oxidation kinetics, which for instance can be attributed to an increased surface charge transfer efficiency.