Millions of people lack access to clean drinking water, highlighting the need for efficient and cost-effective water purification methods. Advanced oxidation processes (AOPs) offer a promising solution by generating reactive oxygen species (ROSs) to mineralize organic pollutants. The Fenton reaction, a representative AOP, is highly efficient but requires continuous reagent supply (H₂O₂, iron salts, acids/alkalis), increasing costs. Heterogeneous photocatalysis, a greener alternative, suffers from rapid electron/hole pair recombination and high thermodynamic barriers for radical generation. Photo-Fenton reactions, combining photocatalysis and Fenton reactions, still necessitate H₂O₂ supply. This research addresses these limitations by developing a self-cycled, cost-effective AOP system.

Existing literature highlights the challenges associated with traditional AOPs. The high cost and complexity of supplying reagents for Fenton reactions are widely acknowledged. While heterogeneous photocatalysis offers a more sustainable approach, its efficiency is hampered by electron-hole recombination and the energy required for ROS generation. Studies have explored bicarbonate (HCO₃⁻) as a potential enabler for in-situ H₂O₂ generation and decomposition, along with its role in ROS formation through activation by metal ions (Co, Cu, Mn). This research builds upon these findings to create a self-cycling system.

This study designed a self-cycled Fenton-like system using a novel artificial leaf. The artificial leaf consists of a SnO₂/BiVO₄/WO₃ photoanode and a PTFE-modified Mo single-atom catalyst/mildly reduced graphene oxide-coated gas diffusion electrode (PTFE@Mo-SACs/mrG-GDE) cathode. The system operates as follows: 1. **H₂O₂ Production:** The artificial leaf efficiently produces H₂O₂ through solar-driven water oxidation at the photoanode and oxygen reduction at the cathode in a bicarbonate electrolyte. The SnO₂/BiVO₄/WO₃ photoanode was optimized to improve photocurrent density and H₂O₂ Faradaic efficiency. The WO₃ texture and BiVO₄ content were tailored, and a SnO₂ₓ overlayer was incorporated to enhance charge carrier transport and H₂O₂ selectivity. The Mo-SACs/mrG cathode was designed for highly selective and efficient H₂O₂ generation, with PTFE nanoparticles enhancing oxygen diffusion and three-phase contact. 2. **ROS Generation:** The generated H₂O₂ is in-situ activated by Mn(II) ions in the bicarbonate electrolyte, producing various ROSs (•OH, •O₂⁻, ¹O₂). The Mn(II) ions are oxidized to Mn(IV) during this process. 3. **Catalyst Recycling:** The Mn(IV) species are reduced back to Mn(II) at the cathode, completing the self-cycling process. The system's long-term stability was confirmed over a month. The researchers used various characterization techniques including electron paramagnetic resonance (EPR) spectroscopy, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and rotating ring-disk electrode (RRDE) tests to study ROS formation, Mn species redox cycling, and the performance of the photoanode and cathode. The solar-to-hydrogen peroxide efficiency (SHyE) was calculated using the H₂O₂ production rate, Gibbs free energy, and sunlight power intensity. A large-scale (70 cm²) artificial leaf was also constructed and tested for real-world wastewater treatment.

The key findings of the study include: * **Efficient H₂O₂ Production:** The artificial leaf achieved an unassisted H₂O₂ production rate of 0.77 µmol/(min cm²) under AM 1.5 G illumination, corresponding to a bias-free SHyE of 1.46%. This is significantly higher than many existing systems. * **Self-Cycling Mechanism:** EPR and CV data confirmed the efficient recycling of Mn(II)/(IV) species in the system. The generation of multiple ROSs (•OH, •O₂⁻, ¹O₂) was verified through EPR. * **Optimized Photoanode:** The SnO₂ₓ/BiVO₄/WO₃ photoanode exhibited a stable H₂O₂ FE greater than 84% over a wide potential range (0.5-1.7 V vs. RHE). * **Optimized Cathode:** The PTFE@Mo-SACs/mrG-GDE cathode demonstrated high H₂O₂ selectivity and current density, with a Tafel slope of 53.2 mV/dec. * **Scalability:** A 70-cm² artificial leaf was successfully constructed, producing 5.17 µmol/min of H₂O₂. * **Wastewater Treatment:** The large-scale artificial leaf effectively degraded 4-nitrophenol (NP) and other pollutants in a real wastewater treatment demonstration.

The results demonstrate the successful development of a self-cycled photo-Fenton-like system for efficient and sustainable wastewater treatment. The high H₂O₂ production rate and SHyE, coupled with the efficient recycling of Mn(II)/(IV) species, overcome the limitations of traditional AOPs. The successful scaling-up of the system to a 70 cm² artificial leaf shows significant potential for practical applications. The study’s findings contribute significantly to the field of photoelectrochemical water treatment and sustainable environmental remediation. The high efficiency and cost-effectiveness of this system make it a strong candidate for decentralized water treatment solutions.

This research successfully developed a self-cycled photo-Fenton-like system based on an artificial leaf for wastewater treatment. The system achieves high H₂O₂ production efficiency and demonstrates excellent scalability and long-term stability. Future research could focus on further enhancing solar utilization efficiency, continuous oxygen supply optimization, and electrolyte component refinement to improve performance and broaden its application range for industrial-scale applications.

The study's limitations include potential challenges in scaling up the system for mass production and the need for continuous oxygen supply. The dependence on sunlight might limit its applicability in locations with limited sunlight. While the system shows promising stability, further long-term testing is needed under various environmental conditions to fully assess its durability. Additionally, the optimization of the electrolyte components could be further explored to increase the efficiency.