Organic π‐Conjugated Polymers as Photocathode Materials for Visible‐Light‐Enhanced Hydrogen and Hydrogen Peroxide Production from Water

Abstract The photo(electro)chemical reduction of water and oxygen to produce hydrogen (water‐splitting) and hydrogen peroxide, respectively, are well developed with inorganic semiconductors. In contrast, organic π‐conjugated polymers, especially precisely synthesized ones, have been extensively studied as photoharvesting and charge‐separating materials in dry photoelectron‐conversion devices, such as organic photovoltaic cells. However, the use of conjugated polymers as photocathodes; i.e., photoelectrocalatalytic reduction‐active materials in direct contact with aqueous electrolytes, has been less explored. This review describes the fundamentals of the electrochemistry of water and oxygen reduction, as well as organic semiconductor functions for photocathode activity in water, along with figures of merit for organic photocathodes used to practically produce hydrogen and hydrogen peroxide. Recent research on conjugated polymers in this field is then surveyed after briefly referring to early trial‐and‐error investigations using conventional conjugated polymers. Basic and very simple designs of robust polythiophene films are demonstrated as examples of successful materials which act as both visible‐light harvesters and catalysts for hydrogen evolution and the production of hydrogen peroxide, and as counterparts in combination with conventional oxygen‐evolution catalysts. Perspectives for this emerging field are also briefly provided.