Nanostructured Hematite Photoanodes for Enhanced Photoelectrochemical (PEC) Water Splitting

Abstract

Hydrogen is very promising fuel that may become one of the important energy carrier to meet our energy demands instead of fossil fuels to a near future. It can easily convert into electricity using fuel cells or it can directly use in internal combustion engines and turbines. Hydrogen is an environmentally clean source of energy and there is no need to use a carbon source. However, since hydrogen is not naturally occurred to nature, it is necessary to develop clean, sustainable, and economic way for hydrogen production to replace the huge usage of fossil fuel. Photoelectrochemical (PEC) water splitting is a very attractive route for solar hydrogen production. The ultimate goal in the field of PEC water splitting is to make an efficient and stable photoelectrode which is made at low cost. In PEC water splitting process, water oxidation part is more difficult than reduction process because oxidation requires four electrons transfer whereas reduction needs two electrons transfer. Therefore, overall efficiency for water splitting is totally up to the efficiency of water oxidation. To overcome current PEC efficiency limits and step forward to the practical use, development of highly efficient and durable photoanode is the most urgent challenge. The aim of this study is the development of hematite photoanode and their modification for high efficient and durable photoelectrochemical (PEC) water splitting. Hematite is almost the only oxide semiconductor material that can be put to the practical use due to the narrow band gap and earth abundance element. This study is organized into five parts. The first chapter describes the brief motivation for solar hydrogen production, basic PEC process, and general background of iron oxide for photoanode in PEC system. From the second chapter, each development technique of hematite photoanodes and their modification by various techniques was covered. In second chapter, a simple dual modification of Sn(IV) doping and hydrogen treatment was demonstrated on hematite. In third chapter, porous hematite films doped with Sn(IV) and coated with an ultrathin (~2 nm thick) Nb2O5 passivation layer were synthesized, and the photoelectrochemical (PEC) water oxidation performance and durability of the hematite were examined in detail. In fourth chapter, ultrahigh-efficiency photoelectrochemical water oxidation using modified hematite nanorod arrays is included. The hematite nanorod arrays are synthesized using chemical bath deposition and further modified by hydrogen treatment, loading of a ~3.5-nm-thick TiO2 overlayer, and deposition of a cobalt phosphate (CoPi) catalyst. In the final chapter, three-dimensional (3D) SnO2 nanohelix (NH) structures coated with Fe2O3 layer is presented to show a remarkable improvement on Fe2O3 layer in photoelectrochemical (PEC) performance.