InGaN-based epitaxial films as photoelectrodes for hydrogen generation through water photoelectrolysis and CO₂ reduction to formic acid

Hydrogen gas (H₂) and formic acid (HCOOH) are simultaneously generated through direct water photoelectrolysis and CO₂ reduction, respectively. These processes are demonstrated in this study by using photoelectrochemical (PEC) cells with working electrodes composed of InGaN/GaN epitaxial wafers in CO₂-containing NaCl electrolyte. In particular, the electrolyte used in this study is the mixture of NaCl and water, which is an environment friendly aqueous solution rather than the artificial acid or alkaline solution, such as HCl or KOH. The working electrodes composed of double-sided epitaxial wafers, where Si-doped InGaN and GaN layers are separately grown on both surface sides of a sapphire substrate, exhibit higher hydrogen gas production and CO₂ reduction rates than the working electrodes consisting of single-sided epitaxial layer. The typical energy conversion efficiency of HCOOH and H₂ generated from the double-sided working electrodes without an external bias are estimated as 1.09% and 5.48%, respectively.