Molecular Design Strategy of π-Conjugated Polymers for Efficient Visible-Light-Driven Photoelectrocatalytic O2 Reduction to H2O2 Production.

Toward sustainable hydrogen peroxide (H2O2) production, photo (electro) catalytic oxygen (O2) reduction/H2O2 production has attracted increasing attention. Recently, we have found that a thin film of the π-conjugated polymer, poly (1, 4-bis (2-thienyl) benzene) (PBTB), exhibits exceptionally high (photo) electrocatalytic activity for O2 reduction/H2O2 production. To achieve higher photoelectrocatalytic activity and efficient visible-light-driven photoelectrocatalytic H2O2 production, we investigated the molecular design related to the highest occupied molecular orbital (HOMO) energy level (EHOMO) of these polymers. We designed and synthesized poly (1, 4-bis (2-thienyl) naphthalene) (PBTN), in which replacing the phenyl unit of PBTB with a naphthalene unit-a stronger electron-withdrawing group-and increasing the polymer chain twist angle selectively deepened EHOMO relative to PBTB. The degree of EHOMO deepening quantitatively affected the onset potential of PBTN. Under visible-light irradiation and 0 V vs. Ag/AgCl, the PBTN thin film achieved a high O2 reduction/H2O2 production rate (1. 11 × 103 mmol H 2 O 2 (mmol) ₇_₂ O₂ /gphotoelectrocatalyst), 1. 47 times higher than that of PBTB, with excellent Coulombic efficiency (99%) and selectivity (99%). The onset potential of PBTN for visible-light-assisted O2 reduction enabled a photocatalytic H2O2 production setup. Upon visible-light irradiation, this setup achieved a high photocatalytic O2 reduction/H2O2 production rate of 128 mmol H 2 O 2 (mmol) ₇_₂ O₂ /gphotocathode. These results clearly demonstrate the tunability of the photoelectrocatalytic activity of π-conjugated polymers through EHOMO-related molecular design.