High-efficiency electrogeneration of hydrogen peroxide from oxygen reduction by carbon xerogels derived from glucose

Assessing the oxygen reduction reaction by a 2-electron mechanism on ceria surfaces

The 2-electron pathway of the oxygen reduction reaction is an unwanted process in the development of fuel cells. In contrast, it has gained the scientific community's attention due to its importance as a promising way of removing emergent pollutants and endocrine disruptors from water bodies and a more sustainable alternative for large-scale commercial hydrogen peroxide production. Cerium oxide has shown remarkable potential and selectivity experimentally for this mechanism, and its possible applications, exceeding the previous reference materials. In this work, we studied the 2-electron pathway for oxygen reduction on different ceria-cleaving directions (100), (110), (221), and (331) by first principles methods based on density functional theory. Our results show that the (100) surface is the most favorable for reduction, with the (331) crystallographic plane also showing potential for good catalytic activity. This fact could be essential for designing new nanostructures, with higher portions of those planes exposed, for higher catalytic activity.

Sucrose-derived N-doped carbon xerogels as efficient peroxydisulfate activators for non-radical degradation of organic pollutants

Metal-free activation of peroxydisulfate (PDS) for degrading organic pollutants in water has received increasing attention because it can prevent secondary pollution. However, most of the catalysts that are efficient are derived from non-renewable fossil resources, are very expensive and have complex preparation processes. Also, the emerging non-radical mechanism is still unclear. Herein, 3D sucrose-derived N-doped carbon xerogels (NCXs) were synthesized by a simple and sustainable hydrothermal process and then employed as novel metal-free PDS activators to degrade organic pollutants. The structure, composition and performance of NCXs were regulated by changing the carbonization temperature. The sample carbonized at 900 °C (NCX900) exhibited the best catalytic performance, completely removing bisphenol A in 60 min. Quenching experiments and linear sweep voltammograms demonstrated that PDS was activated mainly through an electron-transfer non-radical mechanism. It was found that graphitic N played a critical role in activating PDS. With this non-radical mechanism, the NCX900/PDS system could adapt well to the wide pH range (3-11) and high Cl^- concentration; it selectively oxidized organic pollutants with low ionization potentials. This work provides a sustainable approach to the low-cost and efficient metal-free catalysts for wastewater treatment.

Thermally treated candle soot as a novel catalyst for hydrogen peroxide in-situ production enhancement in the bio-electro-Fenton system

Thermally treated candle soot (TCS) was used as a two-electron (2e¯) oxygen reduction reaction (ORR) catalyst to in situ produce H₂O₂ in a bio-electro-Fenton (BEF) system. Compared with the pristine candle soot (CS), TCS showed larger Brunauer-Emmett-Teller (BET) surface area (102.54 m² g^-1 vs. 61.79 m² g^-1), higher mesoporous ratio (50.39% vs. 34.98%), and improved hydrophilicity. X-ray photoelectron spectra (XPS) results revealed that the C-O-C was the dominant oxygen-containing group of the CS, and its percentage reached at 80.55%. However, the C-O-C ratio of the TCS decreased to 48.93%, whilst it's CO and OC-O ratios significantly increased to 27.92% and 23.15%. The TCS showed a high H₂O₂ selectivity (87.5%∼97.0%) at the neutral pH condition, which was much higher than that of the commonly used carbon black (CB) catalyst. Finally, the H₂O₂ concentration maxima (C_max-_H2O2) of the bio-electro-Fenton system running with the TCS air-cathode (BEF-TCS) achieved at 32.02 mg/L, which was 6.29 times higher than that of the BEF-CB (5.09 mg/L). The removal and mineralization ratios of the SMX in the BEF-TCS reached at 83.0% and 79.0%, respectively. This paper reported a novel 2e¯ ORR electro-catalyst which was low-cost, easily available and highly efficiency.

Recent advances in electrochemical 2e oxygen reduction reaction for on-site hydrogen peroxide production and beyond

The electroproduction of H₂O₂ through 2e oxygen reduction reaction (ORR) as an alternative strategy for the conventional anthraquinone process is highly energy-efficient and environment-friendly. Different kinds of electrocatalysts with high selectivity, activity, and stability have been recently reported, and are an essential part of the whole electroproduction process of H₂O₂. In this review, we expound the ORR mechanism and introduce some methods to screen out potential electrocatalysts through theoretical calculations and experimental verifications. In addition, recent advances in reactor design for large-scale on-site production of H₂O₂ and integrated systems for electricity-H₂O₂ co-generation are mentioned. With ideal electrocatalysts and rational reactor design, different concentrations of H₂O₂ can be obtained depending on the practical applications. Utilizing the solar or chemical energy, it can promote energy efficiency and sustainability of the process. Finally, we make a brief conclusion about recent developments in electrocatalysts, device design, as well as integrated systems, and give an outlook for future research challenges, which are meaningful for advancing the electrochemical on-site production of H₂O₂via 2e ORR to the marketplace.

A flow-through electro-Fenton process using modified activated carbon fiber cathode for orange II removal

This study investigated the removal of Orange II by an electro-Fenton process using a novel recirculation flow-through reactor. The hydrogen peroxide was generated in-situ on the activated carbon fiber (ACF) modified with carbon black and polytetrafluoroethylene (PTFE). The modified ACF cathode was characterized by scanning electron microscopy (SEM) and nitrogen adsorption-desorption study. In light of the production of H₂O₂ and removal of Orange II, the optimum weight percentage of PTFE in the mixture of carbon black and PTFE was 75%. The effects of some important operating parameters such as current and flow rate were investigated. The best Orange II removal reached 96.7% with mineralization efficiency of 55.4% at 120 min under the current of 100 mA, initial pH 3, Fe²⁺ 0.3 mM and the flow rate of 7 mL min^-1. The cathode exhibited good regeneration ability and stability. OH was proved to be the main oxidizing species in this flow-through electro-Fenton system. This work demonstrated that such electro-Fenton process using modified ACF cathode was promising for the degradation of organic pollutants.