Carbon-based materials in flow-through electro-fenton: Advanced catalyst design, mechanisms and perspectives

Flow-Through Electro-Fenton (FTEF) combined electro-Fenton reaction with membrane separation technology in a typical flow-through reactor is proposed to be a greener, safer, and more efficient strategy for wastewater treatment as compared to the conventional electro-Fenton process. Carbon-based materials have attracted much attention in recent decades for FTEF due to their inherent merits, such as high conductivity, expansive surface area containing abundant active sites, and numerous functional groups available for facile functionalization. However, the efficiency, selectivity, and stability of the carbonaceous materials applied in FTEF are still unsatisfactory. Therefore, there is an urgent need to revisit carbon-based FTEF from a more in-depth perspective to rationalize research approaches and provide valuable suggestions. In this review, the mechanism of the twoelectron oxygen reduction reaction (2e-ORR) to produce hydrogen peroxide (H2O2) and the subsequent electron transfer process to produce hydroxyl radical (center dot OH) are discussed. The current research progress of carbonaceous materials-based FTEF is presented and categorized into commercial membranes and complex freestanding membranes. As for the major drawbacks of current carbonaceous materials, such as low H2O2 generation, unstable activity, and selectivity of center dot OH generation, several atomic and molecular design strategies are discussed in detail based on the most advanced progress of carbon materials in the field of 2e-ORR. The advantages and disadvantages of carbon-based membranes are summarized. Additionally, the strategies to balance the generation and activation of H2O2 in carbonaceous materials-based FTEF are outlined. Moreover, a scientific perspective on future research directions for carbon-based FTEF is provided, covering four key aspects. Finally, several factors influencing the pratical scale-up application are discussed. Overall, this review is dedicated to sharpening the understanding of carbon-based FTEF and presenting promising applications in wastewater treatments.