Reaction Pathways toward Sustainable Photosynthesis of Hydrogen Peroxide by Polymer Photocatalysts

Abstract

Harnessing solar energy to generate hydrogen peroxide (H 2 O 2 ) from H 2 O and O 2 via artificial photosynthesis is an attractive route, as this approach only uses sunlight as the energy input. Organic polymers have emerged as a promising class of materials for solar-driven H 2 O 2 production, owing to their virtually unlimited molecular building blocks and rich bondforming reactions. This distinctive feature leads to the existence of different reaction pathways characterized by different electron transfer numbers. For the overall photosynthesis of H 2 O 2 , the O 2 reduction reaction and the H 2 O oxidation reaction must occur concurrently. Thus, in-depth insights into these reaction pathways are crucial for solar-driven H 2 O 2 production, with the eventual aim of steering these pathways to optimize efficiency. In this perspective, we primarily focus on the state-of-the-art progress in developing polymer photocatalysts for the overall photosynthesis of H 2 O 2 via coupling different O 2 reduction and H 2 O oxidation reactions. We also present key challenges and opportunities in developing polymer photocatalysts for H 2 O 2 production in the future. Organic polymers offer an ample molecular-level design space. They have now found extensive applications in solar-driven photochemical reactions. Therefore, this perspective serves as a guideline for designing polymer photocatalysts toward sustainable photosynthesis of H 2 O 2 and has significant implications for the future development of polymer materials in the broad area of solar-to-chemical energy conversion research.

Conjugated Organic Polymers with Anthraquinone Redox Centers for Efficient Photocatalytic Hydrogen Peroxide Production from Water and Oxygen under Visible Light Irradiation without Any Additives

“…Conjugated organic polymers (COPs) with semiconductor properties have attracted extensive interest as a complement to the inorganic counterpart due to their high surface area, tunable photoelectric responsiveness, and extended π-conjugated structure. − The synthesis mode modules provide rich options for photocatalytic applications to regulate porosity, electronic/band structures, and interface properties at the molecular level. , The unique π-conjugated structure endows them with rather improved visible light harvesting and expedited photo-excited charge carrier separation/migration due to large delocalization, presenting exciting potential in artificial photosynthesis . With constant efforts, a variety of COP-based photocatalytic systems with outstanding structures and properties have been developed rapidly, which can promote efficient O 2 reduction to generate H 2 O 2 . However, the photocatalytic efficiency is relatively low when inorganic sacrificial agents (ethyl alcohol, ascorbic acid, and triethanolamine) are used. ,, Several strategies have been employed to enhance photocatalytic activity such as creating donor–acceptor dyads, doping heteroatoms, forming heterojunctions, and so forth, ,, but the lack of specific redox centers is still the main reason for low photocatalytic H 2 O 2 generation efficiency from H 2 O and O 2 in net water without any additives.…”

“…33 With constant efforts, a variety of COP-based photocatalytic systems with outstanding structures and properties have been developed rapidly, which can promote efficient O 2 reduction to generate H 2 O 2 . 34 However, the photocatalytic efficiency is relatively low when inorganic sacrificial agents (ethyl alcohol, ascorbic acid, and triethanolamine) are used. 27,33,35 Several strategies have been employed to enhance photocatalytic activity such as creating donor− acceptor dyads, doping heteroatoms, forming heterojunctions, and so forth, 26,36,37 38 although the conventional anthraquinone method is criticized due to the involvement of substantial energy input, severe environmental contamination, and significant safety issues.…”

Extended Conjugation Tuning Carbon Nitride for Non‐sacrificial H2O2Photosynthesis and Hypoxic Tumor Therapy**

“…Owing to the strengthened delocalization of π‐electrons, C 5 N 2 showed unusually low conduction (>0 V vs. NHE) and valence band position, promoting realistic selectivity in thermodynamics and high activity in kinetics for overall H 2 O 2 production through synergistic 2 e − ORR and 2/4 e − WOR. As a result, C 5 N 2 rivals the state‐of‐the‐art photocatalysts without sacrificial reagents and co‐catalysts for H 2 O 2 production, [2d, 8, 11a, 12a, 14] exhibiting by far the highest activity of metal‐free polymers under hypoxic conditions to our knowledge. C 5 N 2 was successfully applied to hypoxic tumor PDT/CDT for the first time, demonstrating competitive performance.…”

“…In principle, for non‐sacrificial H 2 O 2 photosynthesis, both highly selective 2 e − O 2 reduction reaction (ORR) and 2 e − /4 e − water oxidation reaction (WOR) are needed [Eq. (1)–(3)] [8] . Nonetheless, due to the mismatch between the band position of photocatalysts and the standard electrode potential of the above reactions along with the overpotential, side reactions, such as hydrogen evolution reaction (HER) and 4 e − ORR [Eq.…”

“…(1)-(3)]. [8] Nonetheless, due to the mismatch between the band position of photocatalysts and the standard electrode potential of the above reactions along with the overpotential, side reactions, such as hydrogen evolution reaction (HER) and 4 e À ORR [Eq. ( 4) and (5)], would also occur substantially.…”