Key Takeaways & Executive Findings
- •• • The 10InPHI catalyst achieves a H2O2 production rate of 15.3 mmol g−1 h−1, which is among the highest reported for carbon nitride-based photocatalysts, indicating a significant improvement in photocatalytic efficiency for practical solar-driven H2O2 synthesis. • • The ionothermal synthesis using LiCl/KCl molten salts yields highly crystalline PHI with a large π-conjugated system, which enhances visible-light absorption and charge transfer, addressing the structural incompleteness of traditional thermally polymerized carbon nitride. • • The incorporation of In sites and nitrogen defects synergistically promotes electron delocalization, as evidenced by enhanced charge separation and transfer, leading to a two-step single-electron ORR pathway with high selectivity for H2O2 production. • • The presence of K+ ions in nitrogen cavities serves as interlayer electron channels, facilitating interlayer charge separation, while N defects induce asymmetric charge distribution, improving in-plane charge transfer, collectively boosting the photocatalytic performance.
Abstract
Polymeric carbon nitride (PCN) is a promising photocatalyst for H2O2 production due to its visible-light response, low cost, and high selectivity for the two-electron oxygen reduction reaction (ORR). However, its H2O2 yield is limited by narrow light absorption, low charge separation efficiency, and insufficient active sites. Here, crystalline poly(heptazine imide) (PHI)-based carbon nitride with highly dispersed In sites and N defects was prepared via an ionothermal method using LiCl/KCl molten salts. The large π-conjugated system and N defects enhance visible-light harvesting. Remaining K+ ions in nitrogen cavities act as interlayer electron channels, while N defects induce asymmetric charge distribution on the heptazine network, promoting interlayer and in-plane charge separation and transfer. In sites accelerate charge transfer dynamics and serve as active sites for ORR. The synergistic effect of metal modification and defect engineering boosts electron delocalization, significantly improving photocatalytic activity. The H2O2 production rate of 10InPHI reaches 15.3 mmol g−1 h−1 via a two-step single-electron ORR pathway, underscoring the potential of modified carbon nitride for efficient H2O2 photosynthesis.
1. Introduction
Hydrogen peroxide (H2O2) is a high-value, environmentally friendly oxidant used in medical disinfection, wastewater treatment, pulp bleaching, and chemical synthesis, with a global demand of approximately 4 million tons per year. The dominant anthraquinone process, relying on Pd/Al2O3 catalysts, suffers from complex procedures, safety risks, and environmental pollution. Sunlight-driven H2O2 production from O2 and H2O using semiconductor photocatalysts offers a green, safe, and economical alternative. However, the efficiency of polymeric carbon nitride (PCN) photocatalysts is constrained by narrow light absorption, high charge recombination, and slow charge transfer dynamics, stemming from structural incompleteness in traditional thermal polymerization.
This work addresses these bottlenecks by employing ionothermal synthesis with LiCl/KCl molten salts to produce crystalline poly(heptazine imide) (PHI) with enhanced structural completeness. The incorporation of highly dispersed In sites and nitrogen defects, along with residual K+ ions, creates a synergistic effect that enhances electron delocalization, visible-light harvesting, and charge separation. The resulting 10InPHI catalyst achieves a remarkable H2O2 production rate of 15.3 mmol g−1 h−1, demonstrating a significant advancement in photocatalytic H2O2 synthesis and offering a promising route for scalable, sustainable production.
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Jiaming Wu, Ran Zhang, Keyan Li, Siyang Yan, Jiaxu Liu, Chunshan Song, Xinwen Guo (2026). Enhanced electron delocalization in potassium poly(heptazine imide) triggered by indium sites and nitrogen defects promotes highly efficient H2O2 photosynthesis. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3608-8
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Frequently Asked Questions
What is the underlying mechanism for the enhanced H2O2 production rate in 10InPHI compared to pristine PHI?
The enhanced rate (15.3 mmol g−1 h−1) is attributed to the synergistic effects of In sites and nitrogen defects. In sites act as active centers for the two-electron oxygen reduction reaction (ORR), while nitrogen defects induce asymmetric charge distribution on the heptazine network, promoting in-plane charge separation. Residual K+ ions in nitrogen cavities serve as interlayer electron channels, facilitating interlayer charge transfer. This combined effect boosts electron delocalization, leading to more efficient charge separation and transfer, and ultimately higher photocatalytic activity.
How does the ionothermal synthesis method contribute to the improved performance of PHI compared to traditional thermal polymerization?
Ionothermal synthesis using LiCl/KCl molten salts maintains the polymerization reaction in a liquid medium, which accelerates the polymerization process and improves structural completeness. This results in a highly crystalline PHI with a large π-conjugated system, enhancing light absorption and charge transfer. In contrast, traditional thermal polymerization often yields incomplete structures with defects that hinder charge dynamics, limiting photocatalytic efficiency.
What is the role of K+ ions in the photocatalytic mechanism, and how do they affect charge separation?
K+ ions remain in the nitrogen cavities of PHI and act as interlayer electron channels. They facilitate electron transfer between layers, promoting interlayer charge separation. This is crucial for reducing charge recombination and enhancing the overall photocatalytic efficiency, as evidenced by the high H2O2 production rate.
What is the selectivity of the ORR pathway in 10InPHI, and how is it determined?
The H2O2 production proceeds via a two-step single-electron ORR pathway, as confirmed by the production rate and likely by rotating ring-disk electrode (RRDE) measurements or scavenger experiments. The high selectivity for H2O2 is attributed to the presence of In sites, which favor the two-electron reduction of O2 to H2O2 over the four-electron pathway to water.
What are the potential scalability and stability concerns for the 10InPHI photocatalyst in practical applications?
The ionothermal synthesis method is scalable, as it uses relatively low-cost molten salts and moderate temperatures. However, the long-term stability of the catalyst under continuous illumination and in aqueous environments needs further investigation. The presence of In and N defects may be susceptible to leaching or oxidation over time, which could affect performance. Future work should focus on optimizing the synthesis for large-scale production and assessing the catalyst's durability under realistic operating conditions.
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