Water splitting to produce solar hydrogen using silicon thin film
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 tandem-type 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-) solution can generate electricity via the reverse reaction of HI decomposition, enabling a solar energy cycle.
Hydrogenated microcrystalline silicon (µc-Si:H) thin films promise new solar-cell materials. Their advantages include minimal use of semiconductor resources, large-area fabrication using low-cost methods, and no photodegradation of solar cell characteristics. We used µc-Si:H thin-film photoelectrodes in the photodecomposition of HI (see Figure 2).6 (Neither a p-type semiconductor layer nor a transparent conducting layer, which is necessary to fabricate solid-state solar cells, is necessary for PEC solar cells.) Platinum nanoparticles were deposited onto the µc-Si:H thin film by immersing the electrode in a plating solution for 2min to improve the electrode's stability and catalytic activity.
For the photoelectrochemical decomposition of HI, a two-compartment cell was used. The platinum-particle-deposited µc-Si:H photoelectrode was immersed in a mixed solution of HI and I2 in the anode compartment. A platinum counterelectrode was immersed in a HBr solution in the cathode compartment. In short-circuit conditions under simulated solar illumination (AM1.5G, 100mWcm-2), we obtained a photocurrent of 6.8mAcm-2, the solution color on the µc-Si:H surface darkened, and gas evolution occurred on the platinum cathode surface. These results clearly show that the PEC solar cell equipped with a platinum-particle-deposited µc-Si:H film electrode can decompose HI into H2 and I2 (I3-) with no external bias. The Gibbs energy change for the HI decomposition in the present solutions was 0.34eV. Thus, the efficiency of solar to chemical conversion via the photoelectrochemical decomposition of HI is calculated at 2.3%.
In conclusion, µc-Si:H thin film used as a prospective low-cost semiconductor material can generate hydrogen and iodine (I3-) through the photoelectrochemical decomposition of HI in an aqueous solution with no external bias. Furthermore, platinum-particle-deposited µc-Si:H film electrodes can be effective as a narrow-band-gap photoelectrode in a multi-photoelectrode or tandem-type composite photoelectrode system for solar water splitting.
Shinji Yae is an associate professor at the University of Hyogo and a member of the Solar Water Splitting with a Composite Semiconductor Electrode research project established by the Japan Science and Technology Agency. He was a research associate at Osaka University from 1990 to 1998 and an assistant professor at Himeji Institute of Technology (now the University of Hyogo) from 1998 to 2002.
