Hydrogen peroxide (H₂O₂) is a crucial chemical with diverse applications, but its current industrial production via the anthraquinone process is energy-intensive and generates toxic byproducts. Therefore, developing sustainable and environmentally friendly methods for H₂O₂ synthesis is highly desirable. Artificial photosynthesis, utilizing sunlight, water, and oxygen, offers a promising alternative. While several organic semiconductor photocatalysts have been explored for H₂O₂ production (e.g., g-C₃N₄ derivatives, resorcinol-formaldehyde resins, conjugated polymers), limitations remain due to rapid charge carrier recombination and competing reaction pathways. The two-electron reduction of O₂ is sluggish, leading to electron-hole recombination before oxygen reduction and reduced selectivity for H₂O₂. To overcome these challenges, this research investigates the use of redox intermediates, inspired by natural photosynthetic processes where electrons are channeled through redox species to enhance efficiency. Previous work has explored polymer photocatalysts with anthraquinone units, but a deeper understanding of the mechanism is needed. This study leverages the redox properties of polyimides, specifically focusing on the potential of photoinduced anion radical intermediates to drive efficient H₂O₂ photosynthesis.

The literature extensively covers the limitations of the current anthraquinone process for H₂O₂ production, highlighting its energy consumption and toxicity. Several studies have explored the use of various organic semiconductor photocatalysts for artificial H₂O₂ photosynthesis. However, these studies often face challenges related to charge recombination and selectivity. The use of redox intermediates to improve the efficiency and selectivity of O₂ reduction has been explored, drawing inspiration from natural photosynthesis. Polymer photocatalysts incorporating anthraquinone units have shown some promise, but a comprehensive understanding of their photocatalytic mechanism in H₂O₂ production remains limited. This study aims to address these gaps by focusing on the under-explored potential of polyimides with photoinduced anion radical intermediates.

The researchers synthesized a covalently crosslinked polyimide aerogel photocatalyst (PI-BD-TPB) via condensation of 1,3,5-tris[4-amino(1,1-biphenyl-4-yl)]-benzene (TPB) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BD). The photocatalytic performance was evaluated under simulated sunlight illumination, measuring H₂O₂ production in O₂-saturated water. The apparent quantum yield (AQY) and solar-to-chemical conversion efficiency (SCC) were determined. The structural characteristics of the PI-BD-TPB aerogel were investigated using FTIR, solid-state ¹³C NMR, XPS, SEM, TEM, XRD, and N₂ sorption measurements. The redox properties were studied through cyclic voltammetry. In situ FTIR and Raman spectroscopy were employed to monitor the structural changes during the photocatalytic reaction under operando conditions. First-principles calculations were used to investigate the adsorption of O₂ on the catalyst surface. Isotopic labeling experiments (using ¹⁸O₂ and H₂¹⁸O) were conducted to identify the sources of oxygen atoms in the produced H₂O₂. Electrochemical measurements, including rotating disk and ring-disk electrode studies, were performed to determine the electron transfer number and H₂O₂ selectivity. The stability of the photocatalyst was assessed through long-term photocatalytic runs.

The PI-BD-TPB aerogel photocatalyst exhibited a high H₂O₂ production rate of 2.85 mM h⁻¹ and a remarkable AQY of 14.28% at 420 ± 10 nm under simulated sunlight. The SCC efficiency reached 0.92%. The material also showed excellent stability over a continuous 144-hour photocatalytic run and a scalable H₂O₂ yield of 34.3 mmol m⁻² under natural sunlight. In situ spectroscopic studies and theoretical calculations revealed a photocatalytic redox cycle mechanism. Photoexcitation reduces carbonyl groups to anion radical intermediates, which then react with O₂ to generate H₂O₂ and regenerate the carbonyl groups. This cycle enhances O₂ adsorption and lowers the energy barrier for O₂ reduction. Isotopic labeling experiments confirmed the contribution of both O₂ reduction and H₂O oxidation pathways to H₂O₂ production. Electrochemical measurements showed a high two-electron selectivity for the O₂ reduction reaction. The findings strongly support the crucial role of the anion radical intermediate in mediating the efficient H₂O₂ photosynthesis.

The results demonstrate a significant advancement in photocatalytic H₂O₂ production, surpassing the performance of many previously reported polymeric photocatalysts. The discovery of the anion radical intermediate-mediated redox cycle provides a new mechanistic understanding of solar-driven H₂O₂ synthesis, providing valuable insights for the design of more efficient photocatalysts. The high AQY and SCC, along with the excellent stability and scalability, highlight the practical potential of the PI-BD-TPB aerogel for large-scale H₂O₂ production. The findings could inspire further research into the development of advanced photocatalytic systems for sustainable chemical synthesis.

This work successfully engineered a highly efficient polyimide aerogel photocatalyst for H₂O₂ production. The unique photocatalytic redox cycle mechanism, mediated by an anion radical intermediate, offers a new strategy for enhancing the efficiency of solar-driven H₂O₂ synthesis. The high performance, stability, and scalability demonstrated in this study underscore the potential of this technology for practical applications. Future research could explore modifications of the polyimide structure to further optimize its photocatalytic activity and expand its applications in various fields.

While the study demonstrated excellent performance under both simulated and natural sunlight, further investigations are needed to optimize the photocatalyst for even higher efficiencies under various environmental conditions. The long-term stability, while impressive, could be further tested under more demanding conditions. A detailed economic analysis comparing the cost-effectiveness of this method to the anthraquinone process is also warranted.