Highly efficient electrosynthesis of hydrogen peroxide on a superhydrophobic three-phase interface by natural air diffusion

Hydrogen peroxide (H2O2) is a crucial clean oxidant, widely used in various industries and environmental remediation. Its industrial synthesis predominantly relies on the anthraquinone oxidation process, characterized by complex operations, high energy consumption, significant organic waste, and transportation challenges. Electrochemical oxygen reduction reaction (ORR) presents a promising green alternative, producing H2O2 via a direct two-electron reduction. This method is environmentally friendly and cost-effective, particularly vital for H2O2-based electrochemical advanced oxidation processes (EAOPs) such as electro-Fenton (EF) and photoelectro-Fenton (PEF), where efficient H2O2 production is crucial for hydroxyl radical formation and subsequent organic pollutant degradation. Efficient H2O2 production hinges on catalyst selectivity, oxygen mass transfer, and electron transfer at the cathode. Carbonaceous materials like carbon black (CB), nanotubes, and graphene, have shown promise as ORR catalysts. However, oxygen mass transfer limitations due to oxygen's low solubility in water often restrict the two-electron ORR kinetics, despite enhanced electron transfer at higher potentials. Gas diffusion electrodes (GDEs) have been employed to improve oxygen mass transfer, supplying oxygen externally to the cathode. However, GDEs suffer from extremely low oxygen utilization efficiency (OUE, typically <1%), high aeration energy consumption, and susceptibility to damage without continuous oxygen supply. While research has focused on improving GDE catalytic layers, enhancing oxygen mass transfer efficiency remains a critical challenge. This work addresses this by designing a novel Natural Air Diffusion Electrode (NADE), aiming for highly efficient H2O2 production without external aeration.

Previous studies have explored various approaches to enhance H2O2 electrosynthesis efficiency. Gas diffusion electrodes (GDEs), while improving oxygen mass transfer, suffer from low oxygen utilization efficiency (<1%) and high energy consumption due to external oxygen pumping. Researchers have attempted to improve GDE performance by modifying the catalytic layer with materials such as quinones and azo compounds. These modifications improved H2O2 yield to some extent, but still suffered from limitations in oxygen utilization and high energy consumption. Other studies have focused on modifying the cathode catalytic interface to enhance two-electron ORR activity. For example, using hydrophobic materials to improve surface waterproofing and oxygen transfer has been explored. However, achieving a stable superhydrophobic three-phase interface and understanding its effects on catalytic characteristics remain largely unaddressed.

This study designed a natural air diffusion electrode (NADE) that allows for natural air diffusion to the ORR interface without external aeration. The NADE was fabricated using carbon felt (CF) modified with polytetrafluoroethylene (PTFE) as the gas diffusion layer and substrate, with carbon black (CB) loaded on the other side to form the catalytic layer. The hydrophobicity of the catalytic layer was controlled by varying the PTFE/CB mass ratio to establish a stable superhydrophobic three-phase interface. The oxygen diffusion coefficient of the modified CF was determined using an oxygen diffusion coefficient prediction model. The electrochemical performance of NADEs with different PTFE/CB mass ratios was evaluated using linear sweep voltammetry (LSV), Tafel plots, and electrochemical impedance spectroscopy (EIS). The H2O2 production yield, current efficiency, and oxygen utilization efficiency (OUE) were measured under various conditions. The influence of catalyst loading on H2O2 generation was also investigated. The stability of the NADE was assessed through 10 consecutive H2O2 production tests. The potential of the NADE for wastewater treatment was examined by degrading various organic pollutants using electro-Fenton (EF) and photoelectro-Fenton (PEF) processes. Analytical methods used included UV-Vis spectrophotometry for H2O2 concentration, high-performance liquid chromatography (HPLC) for pollutant concentration, and total organic carbon (TOC) analysis for mineralization efficiency. An oxygen diffusion coefficient model was developed to quantify the oxygen mass transfer.

The study demonstrated that the optimized NADE achieved a significantly higher H2O2 production rate (101.67 mg h⁻¹ cm⁻²) compared to traditional GDEs. This improvement was attributed to the significantly enhanced oxygen diffusion coefficient (1.15 × 10⁻¹ cm² s⁻¹) in the NADE, which was 5.7 times higher than that in a conventional GDE. The NADE's superhydrophobic three-phase interface ensured efficient oxygen transfer and a stable reaction environment. The optimal PTFE/CB mass ratio of 0.6 yielded the highest H2O2 production and current efficiency (81.3%). Electrochemical impedance spectroscopy revealed that the NADE with the optimal PTFE/CB ratio exhibited a lower charge transfer resistance compared to hydrophilic interfaces, indicating improved electron transfer kinetics. Increasing the catalyst loading enhanced H2O2 production and current efficiency, reaching optimal performance at a loading of 13.2 mg cm⁻². The NADE demonstrated superior H2O2 production and current efficiency compared to conventional GDEs, even at high current densities (up to 240 mA cm⁻²). The OUE of the NADE was significantly higher (44.5-64.9%), substantially improving on the <1% typically observed in GDEs. Long-term stability tests showed only a 5% decrease in H2O2 production and current efficiency over 10 cycles (20 h) at a current density of 60 mA cm⁻². Finally, the NADE demonstrated excellent performance in degrading various organic pollutants using EF and PEF processes, showing high removal efficiencies and TOC reduction. The energy consumption for H2O2 production using the NADE was significantly lower than that of conventional GDEs, even at high current densities.

The findings demonstrate the superiority of the superhydrophobic NADE in H2O2 electrosynthesis compared to conventional GDEs. The significantly improved oxygen mass transfer, facilitated by the natural air diffusion and superhydrophobic three-phase interface, significantly increases the reaction rate and oxygen utilization efficiency. The results address the long-standing challenge of low OUE and high energy consumption in GDE-based H2O2 production. The NADE's excellent performance, combined with its stability and cost-effectiveness due to the elimination of aeration equipment, provides a significant advancement in the field. The successful application of the NADE in EF and PEF processes further highlights its potential for practical wastewater treatment applications.

This study successfully developed a superhydrophobic natural air diffusion electrode (NADE) for highly efficient H2O2 electrosynthesis. The NADE's unique design overcomes limitations of conventional GDEs, achieving substantially higher H2O2 production rates, current efficiencies, and oxygen utilization efficiencies. The NADE’s stability and cost-effectiveness, owing to the elimination of external aeration, makes it a promising candidate to replace conventional GDEs in industrial-scale H2O2 production and wastewater treatment. Future research could focus on exploring different hydrophobic materials for the catalytic layer, optimizing the electrode structure for even better performance, and scaling up the NADE for industrial applications.

While the NADE demonstrates significant advantages, there are some limitations. The current study primarily focuses on a specific set of organic pollutants, and further research is needed to evaluate its performance with a wider range of pollutants. Additionally, the long-term stability of the NADE under harsh industrial conditions requires further investigation. While the NADE has been proven effective at high current densities, the trade-off between current efficiency and energy consumption at extremely high current densities still requires more systematic investigation.