Nitrogen and sulfur doped mesoporous carbon cathodes for water treatment

Boosting Hydroxyl Radical Yield via Synergistic Activation of Electrogenerated HOCl/H2O2 in Electro-Fenton-like Degradation of Contaminants under Chloride Conditions

Hydroxyl radical production via catalytic activation of HOCl is a new type of Fenton-like process. However, metal-chlorocomplex formation under high chloride conditions could deactivate the catalyst and reduce the process efficiency. Herein, in situ electrogenerated HOCl was activated to •OH via a metal-free, B/N-codoped carbon nanofiber cathode for the first time to degrade contaminant under high chloride condition. The results show 98% degradation of rhodamine B (RhB) within 120 min (k = 0.036 min-1) under sulfate conditions, while complete degradation (k = 0.188 min-1) was obtained in only 30 min under chloride conditions. An enhanced degradation mechanism consists of an Adsorb & Shuttle process, wherein adsorption concentrates the pollutants at the cathode surface and they are subsequently oxidized by the large amount of •OH produced via activation of HOCl and H2O2 at the cathode. Density functional theory calculations verify the pyridinic N as the active site for the activation of HOCl and H2O2. The process efficiency was also evaluated by treating tetracycline and bisphenol A as well as high chloride-containing real secondary effluents from a pesticide manufacturing plant. High yields of •OH and HOCl allow continuous regeneration of the cathode for several cycles, limiting its fast deactivation, which is promising for real application.

Surface-catalyzed electro-Fenton with flexible nanocatalyst for removal of plasticizers from secondary wastewater effluent

Activation of H₂O₂ with metal-free catalysts is an efficient and environmentally benign alternative to electron-Fenton (EF) for organics degradation. In the present study, flexible nanocatalysts were synthesized with self-regulated metal oxide nanoparticles (FeOx NPs) for efficient removal of plasticizers from secondary wastewater effluent (SWE). Compared with NGr/EF and FeOx@Gr/EF systems, FeOx@NGr/EF could enhance the decay kinetics of plasticizers by 3.9-4.4 times and reduce 48-59% of the disposal cost. Reactive oxygen species tests and trapping experiments proved that the surface-catalyzed EF effectively broadened the range of solution pH. Density functional theory calculations coupled with electrochemical measurements indicated that the electron transfer rates between Fe-O-C atoms were enhanced with N-doping due to strong interactions between N-Fe bond. The synergistic effects of FeOx and N could improve the oxygen reduction activity for H₂O₂ generation, and accelerate electron transfer between FeOx/NGr and H₂O₂ for ^•OH generation, offering an alternative for wastewater treatment.

H2O2 production at gas-diffusion cathodes made from agarose-derived carbons with different textural properties for acebutolol degradation in chloride media

The excessive cost, unsustainability or complex production of new highly selective electrocatalysts for H₂O₂ production, especially noble-metal-based ones, is prohibitive in the water treatment sector. To solve this conundrum, biomass-derived carbons with adequate textural properties were synthesized via agarose double-step pyrolysis followed by steam activation. A longer steam treatment enhanced the graphitization and porosity, even surpassing commercial carbon black. Steam treatment for 20 min yielded the greatest surface area (1248 m² g^-1), enhanced the mesopore/micropore volume distribution and increased the activity (E_1/2 = 0.609 V) and yield of H₂O₂ (40%) as determined by RRDE. The upgraded textural properties had very positive impact on the ability of the corresponding gas-diffusion electrodes (GDEs) to accumulate H₂O₂, reaching Faradaic current efficiencies of ~95% at 30 min. Acidic solutions of β-blocker acebutolol were treated by photoelectro-Fenton (PEF) process in synthetic media with and without chloride. In urban wastewater, total drug disappearance was reached at 60 min with almost 50% mineralization after 360 min at only 10 mA cm^-2. Up to 14 degradation products were identified in the Cl^--containing medium.

Preparation of Sulfur-doped Carbon for Supercapacitor Applications: A Review

Electrochemical capacitors, also known as supercapacitors (SCs), have lately played an important role in energy storage and conversion systems due to their specific characteristics such as high strength, durability, and environmental friendliness. A wide range of materials is used as electrodes for SC applications because the electrochemical efficiency is primarily determined by the electrode materials used. Carbonaceous materials with unique surface, chemical, electrochemical, and electronic characteristics have become attractive for energy storage research, but they cannot meet the rising need for high specific energy and specific power. Besides, heteroatom-doped carbon materials have shown pseudocapacitance characteristics and improved specific energy, specific power, and conductivity. This makes them more adaptable in SC application. Among different heteroatom doping of carbon, S-doped carbon has gained considerable attention in SC applications due to its unpaired electrons and easily polarizable nature. S-doped carbon materials-based SCs have demonstrated enhanced surface wettability, improved conductivity, and induced pseudocapacitance effect, thereby delivering improved specific energy and specific power. Many reports on S-doped carbon for SC applications have been published, but there is no specific Review on the preparation of S-doped carbon for SC applications. This Review focuses on recent developments in the field of SC electrodes made from S-doped carbon materials. Herein, the preparation methods and applications of S-doped carbon for SCs were summarized following a brief discussion of different electrochemical characterization techniques of SCs. Finally, the challenges of S-doped carbon materials and their potential prospects were discussed to give crucial insights into the favorable factors for future innovations of SC electrodes. This Review aims to provide insight for further research on the preparation of S-doped carbon for electrochemical energy storage applications.

Generation of hydroxyl radicals by metal-free bifunctional electrocatalysts for enhanced organics removal

Low yields of H₂O₂ and a narrow range of appropriate pH values have been two major drawbacks for electro-Fenton (EF) process. Herein, metal-free electrochemical advanced oxidation processes (EAOPs) were developed with nitrogen and sulfur co-doped electrochemically exfoliated graphene (N, S-EEGr) electrocatalysts, which was confirmed as an outstanding bifunctional catalyst for synchronous generation and activation of H₂O₂ via (2 + 1) e^- consecutive reduction reactions. Specifically, two elements (N, S) in metal-free N, S-EEGr-CF cathode synergize to promote the formation of H₂O₂ followed by its activation. With N, S-EEGr-CF cathode, phenol of initial 50 mg L^-1 could be effectively removed within pH 3-11 and 6.25 mA cm^-2, and 100% removal efficiency could be achieved within 15-min even at neutral pH. The pseudo-first-order rate constant for phenol removal in metal-free EAOPs with N,S-EEGr-CF at neutral pH was 10 times higher than that with EF process. Detection of active species, coupled with decay kinetics with specific trapping agents, confirmed that OH was the dominant oxidizing species promoting removal efficiencies of organics (phenol, antibiotics and dyes) at pH 3 and pH 7. In the actual wastewater treatment, the synergistic effect of bifunctional catalyst would also be used for improving the degradation efficiency of organics. Thus, the metal-free EAOPs with N,S-EEGr-CF cathode may serve as an alternative in wastewater treatment with a broadened range of solution pH values and avoiding Fe²⁺ (catalyst) addition.

Carbonaceous cathode materials for electro-Fenton technology: Mechanism, kinetics, recent advances, opportunities and challenges

Electro-Fenton (EF) technique has gained significant attention in recent years owing to its high efficiency and environmental compatibility for the degradation of organic pollutants and contaminants of emerging concern (CECs). The efficiency of an EF reaction relies primarily on the formation of hydrogen peroxide (H₂O₂) via 2e^─ oxygen reduction reaction (ORR) and the generation of hydroxyl radicals (^●OH). This could be achieved through an efficient cathode material which operates over a wide pH range (pH 3-9). Herein, the current progresses on the advancements of carbonaceous cathode materials for EF reactions are comprehensively reviewed. The insights of various materials such as, activated carbon fibres (ACFs), carbon/graphite felt (CF/GF), carbon nanotubes (CNTs), graphene, carbon aerogels (CAs), ordered mesoporous carbon (OMCs), etc. are discussed inclusively. Transition metals and hetero atoms were used as dopants to enhance the efficiency of homogeneous and heterogeneous EF reactions. Iron-functionalized cathodes widened the working pH window (pH 1-9) and limited the energy consumption. The mechanism, reactor configuration, and kinetic models, are explained. Techno economic analysis of the EF reaction revealed that the anode and the raw materials contributed significantly to the overall cost. It is concluded that most reactions follow pseudo-first order kinetics and rotating cathodes provide the best H₂O₂ production efficiency in lab scale. The challenges, future prospects and commercialization of EF reaction for wastewater treatment are also discussed.

Tin dioxide decorated on Ni-encapsulated nitrogen-doped carbon nanotubes for anodic electrolysis and persulfate activation to degrade cephalexin: Mineralization and degradation pathway

In this study, bamboo-shaped carbon nanotubes exhibiting high nitrogen content and Ni encapsulation (Ni@NCNT) were effectively synthesized by a simple pyrolysis method. The catalytic peroxydisulfate activation for cephalexin (CPX) degradation was investigated using the prepared material. SnO₂ was further decorated and fabricated on the anode material (SnO₂/Ni@NCNT) for electrochemical degradation of CPX in an aqueous solution. Transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy indicated that the SnO₂ nanoparticles were uniformly distributed on the surface of Ni@NCNT. Electrochemical characterization employing cyclic voltammetry and linear sweep voltammetry demonstrated that SnO₂/Ni@NCNT displayed higher oxygen evolution potential and electrocatalytic activity than Ni@NCNT. Mineralization of CPX in wastewater was performed using electrolysis coupled with persulfate oxidation. The analysis revealed a synergistic strengthening effect. The electropersulfate oxidation resulted in higher total organic carbon (TOC) removal (70.3%) than the sum of electrooxidation (48.1%) and persulfate oxidation (9.2%) toward CPX. This phenomenon might result from the regeneration of sulfate radicals (SO₄^•-) on the anode and complementary oxidation by SO₄^•- and OH. Persulfate oxidation alone was shown to result in low TOC removal, although CPX was mostly degraded. Additionally, the CPX degradation pathway involving electropersulfate oxidation was proposed and it is indicated that CPX molecules were completed decomposed by the examination of short chain acids, mineralized ions, and ecotoxicity evolution indicated that the antibiotic was completely degraded. This study provides a new approach for the design and preparation of novel electrode materials and electrochemical degradation facilities for the removal of pollutants via persulfate activation.

Highly Graphitized Fe-N-C Electrocatalysts Prepared from Chitosan Hydrogel Frameworks

The development of platinum group metal-free (PGM-free) electrocatalysts derived from cheap and environmentally friendly biomasses for oxygen reduction reaction (ORR) is a topic of relevant interest, particularly from the point of view of sustainability. Fe-nitrogen-doped carbon materials (Fe-N-C) have attracted particular interest as alternative to Pt-based materials, due to the high activity and selectivity of Fe-Nx active sites, the high availability and good tolerance to poisoning. Recently, many studies focused on developing synthetic strategies, which could transform N-containing biomasses into N-doped carbons. In this paper, chitosan was employed as a suitable N-containing biomass for preparing Fe-N-C catalyst in virtue of its high N content (7.1%) and unique chemical structure. Moreover, the major application of chitosan is based on its ability to strongly coordinate metal ions, a precondition for the formation of Fe-Nx active sites. The synthesis of Fe-N-C consists in a double step thermochemical conversion of a dried chitosan hydrogel. In acidic aqueous solution, the preparation of physical cross-linked hydrogel allows to obtain sophisticated organization, which assure an optimal mesoporosity before and after the pyrolysis. After the second thermal treatment at 900 °C, a highly graphitized material was obtained, which has been fully characterized in terms of textural, morphological and chemical properties. RRDE technique was used for understanding the activity and the selectivity of the material versus the ORR in 0.5 M H2SO4 electrolyte. Special attention was put in the determination of the active site density according to nitrite electrochemical reduction measurements. It was clearly established that the catalytic activity expressed as half wave potential linearly scales with the number of Fe-Nx sites. It was also established that the addition of the iron precursor after the first pyrolysis step leads to an increased activity due to both an increased number of active sites and of a hierarchical structure, which improves the access to active sites. At the same time, the increased graphitization degree, and a reduced density of pyrrolic nitrogen groups are helpful to increase the selectivity toward the 4e- ORR pathway.

Recent Progress on Carbonaceous Material Engineering for Electrochemical Hydrogen Peroxide Generation

Electrochemical synthesis of hydrogen peroxide (H2O2) provides a clean and safe technology for large-scale H2O2 production. The core of this project is the development of highly active and highly selective catalysts. Recent studies demonstrate that carbonaceous materials are favorable catalysts because of their low-cost and tunable surface structures. This brief review first summarizes the strategies of carbonaceous material engineering for selective two-electron O2 reduction reaction and discusses potential mechanisms. In addition, several device designs using carbonaceous materials as catalysts for H2O2 production are introduced. Finally, research directions are proposed for practical application and performance improvement.

The oxygen reduction reaction of two electron transfer of nitrogen-doped carbon in the electro-Fenton system

Degradation mechanism of ORR for the NGO-Ti mesh cathode material in the EF process.

Mesoporous Carbon with Different Density of Thiophenic-Like Functional Groups and Their Effect on Oxygen Reduction

The metal-support interactions between sulfur-doped carbon supports (SMCs) and Pt nanoparticles (NPs) were investigated, aiming at verifying how sulfur functional groups can improve the electrocatalytic performance of Pt NPs towards the oxygen reduction reaction (ORR). SMCs were synthetized, tailoring the density of sulfur functional groups, and Pt NPs were deposited by thermal reduction of Pt(acac)₂ . The extent of the metal-support interaction was proved by X-ray photoelectron spectroscopy (XPS) analysis, which revealed a strong electronic interaction, proportional to the density of sulfur defects, whereas XRD spectra provided evidence of higher strain in Pt NPs loaded on SMC. DFT simulations confirmed that the metal-support interaction was strongest in the presence of a high density of sulfur defects. The combination of microstrain and electronic effects resulted in a high catalytic activity of supported Pt NPs towards ORR, with linear correlations of the half-wave potential E_1/2 or the kinetic current j_k with the sulfur content in the support. Furthermore, a mass activity value (550 A g^-1 ) well above the United States Department of Energy target of 440 A g^-1 at 0.9 V (vs. reversible hydrogen electrode, RHE), was determined.

Effects of N mono- and N/P dual-doping on H2O2, OH generation, and MB electrochemical degradation efficiency of activated carbon fiber electrodes

Pure N mono- and N/P dual-doped cotton-stalk-derived activated carbon fibers (CSCFs) were synthesized by steam, HNO₃(CSCF-N), NH₃(CSCF-A), and (NH₄)₃PO₄(CSCF-N/P) treatments. This study investigated how three different N/P modifiers affected the pore structure, chemical property, H₂O₂ generation ability, and electrocatalytic activity of methylene blue (MB) degradation of CSCFs in an electric-Fenton system. Results confirmed that the three employed treatments effectively doped N/P in the carbon lattice and slightly changed the pore structures. NH₃ and (NH₄)₃PO₄ were the most effective modifiers for the N mono-doping and N/P dual-doping of CSCFs, respectively. Among the fabricated CSCFs, the N/P dual-doped CSCF-N/P demonstrated the highest electrochemical activity in an electro-Fenton system, followed by the N mono-doped CSCF-A, the CSCF-N, and the raw CSCF. In contrast to the CSCF electrode, the CSCF-N/P electrode exhibited enhanced H₂O₂, OH generation, and MB degradation efficiency by 42%, 41%, and 35%, respectively. Under optimum conditions, the electrochemical decolorization efficiency of MB (initial concentration, 100 mg L^-1) of the CSCF-N/P reached 93% after 150 min and was 24.1% higher than that of the CSCF. By the tenth cycle, 82.2% of the MB could still be decomposed, suggesting the excellent stability and reusability of the N/P co-doped CSCF electrode. The outstanding electrocatalytic performance of the CSCF-N/P electrode is primarily due to the simultaneous doping of active N/P sites with low activation energy and introduction of mesopores with strong trapping forces for MB. The MB reduction catalyzed by CSCF electrodes followed pseudo-first-order kinetics, and the reaction rate depended on the modifiers.

Electrolytic ammonia removal and current efficiency by a vermiculite-packed electrochemical reactor

The ammonia removal as well as the current efficiency during electrolysis was investigated by using a vermiculite-packed electrochemical reactor under continuous mode. Experimental results showed that adsorption of ammonia by vermiculite and electrolytic desorption of ammonia simultaneously existed in the reactor, leading to 89% removal of initial 30 mg N/L ammonia and current efficiency of 25% under the condition of 2.0 A, 6.0 min hydraulic retention time with 300 mg Cl/L chloride as the catalyst. The ammonia removal capacity had a linear relationship with the products of hydraulic retention time, current and chloride concentration within experimental conditions. The treatment results of secondary effluent indicated that 29.9 mg N/L ammonia can be reduced to 4.6 mg N/L with 72% removal of total nitrogen and a current efficiency of 23%, which was 2% less than synthetic wastewater due to the reducing components in the real wastewater.