Water splitting to produce solar hydrogen using silicon thin film

Abstract

Solar hydrogen, which is produced by splitting water using solar energy, can be considered the perfect renewable energy source.1 Easily stored and transported, hydrogen can generate not only thermal energy but also electricity (using fuel cells) and mechanical energy (using hydrogen engines), before ultimately returning to water. Therefore, solar hydrogen could potentially form the basis of a clean, renewable energy cycle. Water splitting using photoelectrochemical (PEC) solar cells as a possible means of obtaining solar hydrogen has been attracting much attention since Fujishima and Honda’s report on TiO2 (titanium dioxide) photoelectrodes in 1972.2 PEC solar cells have several important, unique features. They are easily fabricated by immersing a semiconductor electrode and a counterelectrode into an electrolyte solution (see Figure 1). They can convert solar energy directly to storable chemical energy. And the junction of an electrolyte solution and a semiconductor can generate a high-energy barrier, thus reaching high photovoltage even with a low-cost, low-quality semiconductor.3 However, water splitting using TiO2 presents serious difficulties in hydrogen evolution. There are three solutions to these problems: using another semiconductor with an energy band gap that is wider than TiO2; using a multiphoton system equipped with multi-photoelectrodes in series or a tandemtype photoelectrode; and using an oxidation reaction other than oxygen evolution, such as oxidation of iodide ions into iodine. The Gibbs energy change for decomposition of hydrogen iodide (HI) into hydrogen (H2) and iodine (I2) (triiodide ion, I3 ) in an aqueous solution is smaller than that for water splitting. Thus, silicon photoelectrodes, which have a narrower energy band gap than TiO2, can decompose HI with no external bias, and with efficiency reaching 7.4%.4, 5 Fuel cells using H2 gas and I2 (I3 ) Figure 1. Photoelectrochemical solar cell produces hydrogen gas with no external bias.