Hydrogen peroxide (H₂O₂) is a valuable oxidant with wide applications in medicine, industry, and environmental management. Traditional production methods, like the anthraquinone process and electrochemical synthesis, suffer from high energy consumption and toxic byproducts. Artificial photosynthesis, using sunlight to drive the reaction between water and oxygen, offers a cleaner alternative for H₂O₂ production. Utilizing seawater as a reactant source not only addresses freshwater scarcity but also reduces production costs. However, challenges remain, including poor light absorption, mismatched band gaps, low electron transfer rates, and the need for sacrificial agents in most existing photocatalysts. Salt ions in seawater often deactivate catalysts or consume photogenerated carriers, hindering industrial applications. This research focuses on overcoming these limitations by designing a novel metal-free photocatalyst.

Carbon dots (CDs) have shown promise as co-catalytic active sites and electron acceptors/donors in photocatalytic systems, improving catalytic efficiency. Previous studies have explored various materials for photocatalytic H₂O₂ synthesis, but many struggle with issues such as low light absorption, inadequate band gap alignment, slow electron transfer, and reliance on sacrificial agents. The use of seawater as a reactant has also been investigated, but the deactivating effects of salt ions on catalysts remains a significant hurdle.

The researchers synthesized a metal-free photocatalyst (PM-CDs-x) using a phenolic condensation approach. This involved combining carbon dots (CDs), the organic dye procyanidins, and 4-methoxybenzaldehyde. The synthesis process involved dissolving CDs in water, mixing them with procyanidins and sodium carbonate, adding 4-methoxybenzaldehyde, and then heating the mixture to 80°C for 48 hours to form a gelatinous precursor. The precursor was washed, frozen, and freeze-dried to obtain the final photocatalyst. The structural and photoelectrochemical properties of the CDs and the composite photocatalysts were characterized using techniques such as TEM, HRTEM, FT-IR, XRD, XPS, UV-vis absorption spectroscopy, and UPS. Photocatalytic activity was assessed by measuring H₂O₂ production in pure water and real seawater under visible light irradiation (λ ≥ 420 nm). The effects of varying CDs content, rotational speed, oxygen partial pressure, reaction time, and the addition of sacrificial agents were investigated. Electron chemical impedance spectroscopy (EIS), transient photovoltage (TPV) measurements, electron paramagnetic resonance (EPR) spectroscopy, and rotating disc electrode (RDE) and rotating-ring disc electrode (RRDE) techniques were used to elucidate the reaction mechanism.

The optimized photocatalyst, PM-CDs-30, exhibited a significantly higher H₂O₂ production rate (1776 µmol g⁻¹h⁻¹) in real seawater compared to the pure polymer (PM-CDs-0, 771 µmol g⁻¹h⁻¹ in pure water and 359 µmol g⁻¹h⁻¹ in seawater), representing a 4.8-fold increase. The apparent quantum yield (AQY) reached 0.54% at 630 nm in seawater, and the solar-to-chemical conversion (SCC) efficiency was 0.21%. Characterization revealed that CDs were uniformly distributed within the polymer matrix. The addition of CDs enhanced light absorption in the visible and near-infrared regions and improved charge separation and transfer efficiency. Transient photovoltage (TPV) studies indicated that CDs acted as electron sinks, trapping photogenerated electrons and preventing electron-hole recombination. The presence of metal cations in seawater further enhanced the electron sink effect of the CDs by ionizing their surface functional groups. Active species trapping experiments showed that photogenerated electrons and molecular oxygen were crucial for H₂O₂ production, while hydroxyl radicals were not involved. EPR confirmed the formation of superoxide radicals (O₂⁻) during the reaction. The catalyst exhibited high stability, maintaining its activity over five cycles of use. A thermodynamic-kinetic model accurately predicted the influence of rotational speed and oxygen partial pressure on H₂O₂ production.

The findings demonstrate the successful design and synthesis of a highly efficient and stable metal-free photocatalyst for H₂O₂ production in real seawater. The superior performance is attributed to the synergistic effects of CDs and the metal cations present in seawater. CDs enhance charge separation, while the cations amplify the electron-trapping capacity of CDs, further promoting the two-electron oxygen reduction reaction (ORR) to generate H₂O₂. The results address the critical challenge of salt-induced deactivation in seawater photocatalysis, paving the way for more efficient and practical H₂O₂ production systems. The close agreement between the theoretical model and experimental data validates the proposed mechanism and provides a useful tool for optimizing the photocatalytic process.

This research successfully demonstrated a metal-free composite photocatalyst (PM-CDs-30) for highly efficient H₂O₂ production in real seawater. The superior performance stems from the synergistic interplay of CDs, acting as electron sinks, and the salt ions enhancing the electron trapping. The high H₂O₂ production rate (1776 µmol g⁻¹h⁻¹), superior stability, and validated theoretical model open new avenues for sustainable and cost-effective H₂O₂ generation. Future research could focus on further optimizing the catalyst structure and exploring its potential in large-scale applications.

While the study demonstrates excellent photocatalytic activity in real seawater, further investigation is needed to determine the long-term durability and scalability of the process. The model assumes uniform conditions throughout the reaction vessel; however, in a practical large-scale system, gradients in oxygen concentration and light intensity may influence H₂O₂ generation efficiency. Further research should also explore a wider range of seawater compositions and environmental factors.