Photoelectrochemical Reduction Products of Carbon Dioxide at Metal Coated p-GaP Photocathodes in Aqueous Electrolytes

In order to increase the current efficiency of the photoelectrochemical reduction of CO2 in aqueous electrolytes, metals such as Pb and Zn, which have a higher overpotential for the hydrogen evolution reaction, were coated on a p-GaP photocathode by sputtering and electroplating. Au was also coated by sputtering. Then, the photoelectrolytic experiments were carried out in a gas-tight cell with a cyclic gas-flow system. The products in a gas phase were determined as well as the products in the electrolyte. Accordingly, CO in addition to HCOOH as a reduction product of CO2 was found to be produced as well as substantial amount of H2 as a by-product. However, other products such as HCHO and CH3OH were not found at all. In the case of the Pb coated p-GaP electrode, the current (faradaic) efficiency, η, for HCOOH formation was increased up to 48.2%, although in the case of bare p-GaP it was only 4.8% at maximum. On the other hand, the η for CO formation at the Zn-coated p-GaP electrode went up to 16.5%. While, in the case of an Au-coated p-GaP, the η for the CO2 reduction was not so changed as for the bare p-GaP photocathode.