Controlling the selectivity of solar O₂/HClO production from seawater by simple surface modification of visible-light responsible photoelectrodes

Abstract

Photoelectrochemical (PEC) production of H₂ from saltwater over a semiconductor photoelectrode under solar-light irradiation is one of the effective technologies for developing a cost-effective sustainable energy conversion process. However, because saltwater such as seawater contains Cl⁻, O₂ and HClO are produced competitively during oxidation reactions by photogenerated holes from electrolytes containing Cl⁻ during the photo-electrolysis reaction. HClO is a high value-added chemical used for bleaching, etc., however, it is also an undesirable chemical that accelerates corrosion deterioration of large-scale water splitting systems. Therefore, it is necessary to control selectivity of oxidative O₂/HClO production in electrolytes containing Cl⁻ over photoelectrodes. In this review, we summarized our recent innovations in selective O₂ or HClO production over the visible-light driven BiVO₄/WO₃ photoanodes by simple modification of metal oxides. Modifications of metal oxides such as MnO_x or CoO_x via spin-coating onto a photoelectrode could control the selectivity on the O₂/HClO production from an aqueous solution containing Cl⁻ effectively. In addition, controlling loading conditions such as the loading amount of metal oxides, and calcination temperatures after coating a metal precursor solution enabled us to prepare photoelectrodes that produce O₂ or HClO with selectivity of almost 100 % using MnO_x or CoO_x, respectively, along with maintaining their PEC performance under solar-light irradiation.