image: D-A conjugated polymers with distinct donor units (biphenyl, bithiophene, and bipyridine) were engineered to regulate the kinetics of water oxidation sites for effective H2O2 photocatalysis. Among the three polymers, the bipyridine exhibits the highest performance, achieving an attractive H2O2 production rate of 6687 μmol g-1 h-1 by promoting charge separation, accelerating water oxidation kinetics, and stabilizing superoxide radicals, thereby optimizing H2O2 synthesis.
Credit: ©Science China Press
H2O2 is a widely used green chemical with important applications in environmental remediation, disinfection, and chemical synthesis. The conventional anthraquinone process, although industrially established, requires high energy input and raises environmental concerns. As a result, producing H2O2 directly from air and water using sunlight has emerged as a promising sustainable alternative. In photocatalytic systems, H2O2 generation is governed by the coupling of two key reactions: the oxygen reduction reaction and the water oxidation reaction. While the two-electron oxygen reduction pathway favors H2O2 formation, the overall efficiency is largely limited by the sluggish kinetics of the water oxidation reaction, which involves multiple proton and electron transfer steps. This slow process is a major bottleneck for improving photocatalytic performance.
To address this challenge, a research team from Wuhan University of Technology developed a molecular engineering strategy to regulate water oxidation sites in conjugated polymer photocatalysts. The researchers designed three donor–acceptor conjugated polymers using biphenyl, bithiophene, and bipyridine as different functional units, enabling systematic tuning of water oxidation sites. Among the three materials, the polymer containing bipyridine units exhibited the best performance, achieving a high H2O2 production rate of 6687 μmol g-1 h-1 in pure water under ambient conditions and outperforming the other two polymers. This result highlights the importance of molecular-level design in optimizing photocatalytic activity.
Further studies revealed the origin of this enhanced performance. The incorporation of nitrogen-containing bipyridine units modifies the local electronic structure of the polymer, facilitating proton-coupled electron transfer and accelerating the water oxidation process. At the same time, the optimized structure improves charge separation, reducing recombination losses and allowing more charge carriers to participate in the reaction. Mechanistic studies provided deeper insights into the reaction pathways. Reactive oxygen species, including superoxide radicals and singlet oxygen, were found to play key roles in H2O2 formation. These species work cooperatively to promote the conversion process, contributing to the high efficiency observed in the optimized polymer. Advanced characterization techniques, such as Kelvin probe force microscopy and in situ infrared spectroscopy, were employed to track charge dynamics and identify key reaction intermediates, supporting the proposed mechanism.
This work demonstrates that tailoring water oxidation active sites at the molecular level can significantly enhance photocatalytic performance. By simultaneously improving charge separation and reaction kinetics, the proposed strategy provides an effective route for efficient H2O2 production in pure water without the need for sacrificial agents. The findings offer new insights into the design of high-performance photocatalysts and may contribute to the development of sustainable chemical production technologies driven by solar energy.