p-n Junction Silicon Electrode Coated with Noble Metal for Efficient Solar Photoelectrolysis of Hydrogen Iodide

Abstract

Abstract The photooxidation of iodide ions on n-type silicon electrodes highly doped with boron from the surface in hydrogen iodide or hydrogen iodide/iodine solutions was investigated. Two doping methods were employed: thermal diffusion and ion implantation. The electrode was remarkably stabilized by coating with a 2 to 3 nm-thick platinum layer, followed by heat-treatment at about 400 °C for ca. 10 min. The photocurrent was stable for more than 500 h at a density of the order of 10 mA cm−2. The boron-doped and Pt-coated n-Si electrode also generated a photovoltage much higher than that for a Pt-coated n-Si electrode, showing an advantage of forming the p-n junction. Photoelectrochemical cells made of a boron-doped-Pt-coated n-Si electrode, a platinized Pt counter electrode, and a commercial cation exchange membrane were found to photoelectrolyze hydrogen iodide without externally applied voltage into hydrogen and iodine with a solar-to-chemical energy conversion efficiency (φchems) under simulated AM 1 solar radiation of 8.2%, probably the highest of the φchems values observed for the cells working without external voltage.