Ph.D Thesis


Ph.D StudentHen Dotan
Subjectalpha-Fe2O3 Photoelectrodes for Solar-Induced Water
Splitting
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Rothschild Avner


Abstract

Solar-induced water photoelectrolysis can provide a clean and renewable method to store solar power in the form of hydrogen. Hematite (α-Fe2O3) photoanodes are well-suited for this purpose, but they also have some drawbacks, most notably massive charge carrier recombination. To tackle these drawbacks, creative designs are required to overcome the trade-off between light harvesting and charge collection. Much progress has been achieved by producing mesoscopic hematite photoanodes with co-catalysts at the surface. Most of the progress has been achieved by trial and error, without detailed understanding of the fundamental limitations. This thesis aims to rectify this shortcoming.

The thesis provides a rational approach to identify the key losses and understand the underlying physics and electrochemistry in order to design efficient hematite photoanodes. In order to identify the bottleneck of hematite photoanodes, a new electrochemical analysis method was developed. Using a holes scavenger (H2O2), bulk and surface recombination process were quantified. It was found that mesoscopic hematite photoanodes suffer from severe bulk recombination and a new approach is needed to overcome this loss. An innovative photoanode design was developed, using resonant light trapping in ultrathin hematite films in order to enhance the light harvesting efficiency of 20-30 nm thick films that are thin enough for photogenerated minority carriers to reach the surface before recombination takes place. This design achieved a new record of water photo-oxidation current density with hematite photoanodes.

Detailed physical modeling of the underlying optics and charge transport process enabled to predict optimal light trapping structures. On top of that, the kinetics of the water photo-oxidation reaction was modeled by combining semiconductor physics and chemical reaction kinetics. The combined model predicts how material properties and operation conditions influence the water photo-oxidation current, thereby providing an important tool to study hematite photoanodes and optimize their performance. A new analysis method was developed in order to quantify the photoanode intrinsic solar to chemical power conversion efficiency and identify the optimal operation conditions. This method is essential for rational optimization of hematite photoanodes and tandem cells for solar energy conversion and storage.

This thesis demonstrates that the resonance between experimental and theoretical work provides comprehensive understating of the operation mechanism of hematite photoanodes, leading to innovative design of efficient photoanodes. This is an important progress in the field of photoelectrolysis and I hope that it will advance the field from lab experiments into a new technology for solar energy conversion and storage.