Photoelectrochemical Systems for Hydrogen Production.

Abstract

The goal of this research is to advance the understanding and development of efficient and stable photoelectrochemical cells for renewable hydrogen production. To this end, three main strategies were investigated for improving the photoanode performance: producing the semiconducting oxide (i.e. titanium dioxide, TiO2) in the form of long nanotube arrays, incorporating gold nanoparticles onto the surface, and combining this photocatalyst with a solar cell. Highly ordered TiO2 nanotube (TiNT) arrays were fabricated using an anodization process. By varying the anodization conditions, TiNTs with different dimensions were fabricated. Increasing the nanotube length resulted in increased photocurrents up to lengths that exceeded the diffusion length of electrons in TiO2 (~20 µm). Gold nanoparticles with average diameter ranging from 3-12 nm were deposited onto selected TiNTs using a modified deposition precipitation method. The pH of the solution used during the Au loading is the crucial parameter determining the gold particle size and metal loading. Furthermore, small gold nanoparticles (less than 5 nm) significantly improved the electrocatalytic properties of TiO2 by adding active sites for water oxidation. Studies relating Au particle size and hydrogen rate per active Au species suggested that for Au particles bigger than 5 nm the most active sites were located on the surface of the metal, and for Au particles smaller than 5 nm the most active sites seemed to be at the perimeter in contact with the oxide support. Efficiencies for PEC cells were calculated and the Au/TiNT photoelectrodes shown efficiencies in excess of 1.2 %, which are one order of magnitude higher than the efficiencies reported for TiO2 powder photoelectrodes. In addition, this efficiency is about 100% higher than the efficiencies reported in the literature for photoanodes made similar nanotube arrays. The novel Au/TiNT photocatalyst was combined with Si solar cells in a hybrid arrangement. In this tandem cell the photocatalyst film and the solar cell were connected in series (adding the voltage produced by each component) and gave a conversion efficiency of 1.6 %.