Amount Of Oxygen Functional Groups Research Articles

Preparation of magnetic biochar used bioleached sludge to enhance tetracycline removal in a heterogeneous Fenton-like system

The environmental application of iron (Fe)-rich acidic sludge generated after bioleaching is still a challenge. Pyrolysis offers a promising avenue for converting this sludge into biochar. In this study, magnetic biochar (BLS-BC7) was prepared from acidic Fe-rich sludge and employed as a catalyst for tetracycline (TC) degradation by hydrogen peroxide (H2O2). Furthermore, compared with the original sludge-based biochar, BLS-BC exhibited a 2.3 times higher specific surface area, providing more active sites for the catalytic reactions. The scanning electron microscopy and Raman results indicated that BLS-BC7 was coated with a large number of particles on its surface, presenting a rough appearance with a high degree of defects, thereby providing more active sites for biochar. Characterization via photoelectron spectroscopy, elemental analysis, and Fourier transform infrared spectroscopy results indicated that BLS-BC contained a higher amount of oxygen functional groups, along with an increased content of CO and the formation of Fe–O, facilitating H2O2 activation to generate reactive oxygen species (ROS). Additionally, quenching experiments and electron paramagnetic resonance spectroscopy demonstrated the generation of multiple ROS such as 1O2, ·OH, and O2−·, in the BLS-BC7/H2O2 system. This resulted in a TC removal rate of 100 % within 120 min, with a mineralization rate of 53.4 %. The biochar exhibited easy recoverability and strong reusability, maintaining a TC removal rate of 91.3 % after five cycles of experiments. The X-ray diffraction patterns before and after the reaction showed that BLS-BC7 exhibited strong stability, with no apparent change in peak positions before and after use.

Enhancing Graphene Nanoplatelet Reactivity through Low-Temperature Plasma Modification.

Graphene-based materials have great potential for applications in many fields, but their poor dispersion in polar solvents and chemical inertness require improvements. Low-temperature plasma allows the precise modification of materials, improving the physicochemical properties of the surface and thus creating the possibility of their potential use. Plasma treatment offers the possibility of introducing oxygen functional groups simply, rapidly, and in a controlled way. In this work, a systematic investigation of the effect of plasma modification on graphene nanoplatelets has been carried out to determine the optimal plasma parameters, especially the exposure time, for introducing the highest amount of oxygen functional groups on a surface. Different gases (O2, CO2, air, Ar, and C2H4) were used for this purpose. The chemical nature of the introduced oxygen-containing functionalities was characterized by X-ray photoelectron spectroscopy, and the structural properties of the materials were studied by Raman spectroscopy. The plasma-induced changes have been shown to evolve as the surface functionalities observed after plasma treatment are unstable. The immersion of the materials in liquids was carried out to check the reactivity of carbons in postplasma reactions. Stabilization of the material's surface after plasma treatment using CH3COOH was the most effective for introducing oxygen functional groups.

High-response room-temperature NO2 gas sensor fabricated with thermally reduced graphene oxide-coated commercial cotton fabric

Electronic textile-based gas sensors with a high response for NO2 gas were fabricated using reduced graphene oxide (rGO)-coated commercial cotton fabric (rGOC). Graphene oxide (GO) was coated on cotton fabric by simply dipping the cotton into a GO solution. To investigate the relationship between the degree of reduction and the sensing response, the GO-coated fabrics were thermally reduced at various temperatures (190, 200, 300, and 400 °C). The change in the amount of oxygen functional groups on the rGOCs was observed by x-ray photoelectron spectroscopy, Raman spectroscopy, and x-ray diffraction patterns. The maximum sensing response of 45.90 % at 10 ppm of NO2 gas at room temperature was exhibited by the rGOC treated at 190 °C, which was the lowest heat-treatment temperature. The high response comes from the greater amount of oxygen functional groups compared to other rGOC samples, and the tubular structure of the cotton.

Effects of chemical aging on carbonaceous materials: Stability of water-dispersible colloids and their influence on the aggregation of natural-soil colloid

Although hydrochar and biochar have been used as soil conditioners, there is not a clear understanding of how their properties changes due to aging impacts their colloidal particles behavior on the soil system. From this premise, we produced hydrochar and biochar from the same feedstock (cashew bagasse) and aged with different chemical methods: (i) using hydrogen peroxide, (ii) a mixture of nitric and sulfuric acids, and (iii) hot water. It was analyzed the effects of aging on the stability of the carbonaceous materials (CMs) colloids in aqueous medium with different ionic strength (single systems), as well as the stability of the natural-soil colloid when interacting with biochar and hydrochar colloids (binary systems). A chemical composition (C, H, N, and O content) change in CMs due to the chemically induced aging was observed along with minor structural modifications. Chemical aging could increase the amount of oxygen functional groups for both biochar and hydrochar, though in a different level depending on the methodology applied. In this sense, hydrochar was more susceptive to chemical oxidation than biochar. The effectiveness of chemical aging treatments for biochar increased in the order of water < acid < hydrogen peroxide, whereas for hydrochar the order was water < hydrogen peroxide < acid. While the increase in surface oxidation improved the biochar colloidal stability in water medium at different ionic strengths (single systems), the stability and critical coagulation concentration (CCC) slightly changed for hydrochar. Natural-soil clay (NSC) interactions with oxidized carbonaceous material colloids (binary systems) enhanced NSC stability, which is less likely to aggregate. Therefore, the aging of carbonaceous materials modifies the interaction and dynamics of soil small particles, requiring far more attention to the environmental risks due to their application over time.

Promotion of crystallization of polyamide 6 at an interphase and a mechanical interlocking effect at an interface due to surface irregularities of carbon fibers

The effect of surface irregularities in the carbon fiber (CF) axial direction on the interfacial properties of polyamide 6 (PA6) was investigated using CFs with different amounts of oxygen functional groups on the surface, that is, chemical reactivity. The interfacial shear strength between PA6 and CF was measured using the microdroplet method. The crystal structure of PA6 at the interphase was characterized using unpolarized and polarized micro-Raman spectroscopy and polarized light microscopy. Not only the chemical interactions at the interface but also the surface irregularities of the CF were found to contribute to the formation of oriented crystals of PA6 in the CF axial direction at the interphase, and these oriented crystals of PA6 at the interphase enhanced the interfacial shear strength. The surface irregularities of the CF were found to cause mechanical interlocking effects. This study demonstrates the importance of CF surface irregularities in controlling interfacial properties.

Investigating the Effects of Nitric Acid Treatments on the Properties of Recycled Carbon Fiber.

In this study, the chemical state change of recycled carbon fiber (rCF) surfaces and the mechanism of the oxygen functional groups according to nitric acid treatment at various times and temperatures were investigated to upcycle the carbon fiber recovered from used carbon composite. When treated with nitric acid at 25 °C, the carbon fiber surface demonstrated the same tensile properties as untreated carbon fiber (CF) for up to 5 h, and the oxygen functional group and polar surface energy of C-O (hydroxyl group) and C=O (carbonyl group) increased slightly compared to the untreated CF up to 5 h. On the other hand, at 100 °C, the tensile properties slightly decreased compared to untreated CF up to 5 h, and the amount of C-O and C=O decreased and the amount of O=C-O (lactone group) started to increase until 1 h. After 1 h, the amount of C-O and C=O decreased significantly, and the amount of O=C-O increased rapidly. At 5 h, the amount of oxygen functional groups increased by 92%, and the polar surface energy increased by 200% compared to desized CF. It was determined that the interfacial bonding force increased the most because the oxygen functional group, O=C-O, increased greatly at 100 °C and 5 h.

Enhanced electrochemical performance of zinc/bromine redox flow battery with carbon-nanostructured felt generated by cobalt ions

Surface properties of graphite fibers greatly determine the performance of flow batteries. In this work, graphite felt is modified with transition metal ion (cobalt)-assisted thermal treatment process. This multi-step thermal treatment process generated well aligned carbon nanostructures as well as large amount of oxygen functional groups on graphite fiber surface. Such an electrode surface architecture is highly beneficial for the performance of Zinc-Bromine redox flow batteries, as it facilitates superior charge as well as mass transfer, associated with dense organic polybromide phase formed during charging. Consequently, the flow cell with modified felt, decorated with carbon nanostructures, operates at higher energy efficiency (84%) compared to pristine felt (68%) and conventionally thermally treated felt (68%). Postmortem analysis using scanning electron microscopy reveals the durability of carbon nanostructure on the felt, as these structures are observed to remain intact after long-term cycling (100 cycles) experiments.

Production of activated carbon with tunable porosity and surface chemistry via chemical activation of hydrochar with phosphoric acid under oxidizing atmosphere

This work highlights the chemical activation of hydrothermally carbonized biomass with phosphoric acid under static air as activating atmosphere. Argan nut shells were hydrothermally carbonized, and then chemically activated with phosphoric acid under oxidizing atmosphere. The activated carbons displayed excellent specific surface areas (from 1200 m2g−1 to 1880 m2g−1) and the material yield were in acceptable level (between 27 and 50%). The varied activation conditions provided a distinctive possibility for creating and controlling the mesoporosity of the activated carbons. At 500 °C, a mesoporosity around of 93% could be reached by an H3PO4 impregnation ratio of 4. The activating temperature 500 °C and the impregnation ratio of 3 were observed as optimum for achieving the best textural properties. The optimized material HC-500-3 exhibited a specific area of 1880 m2g−1, pore volume and average pore diameter of 1.36 cm3g−1 and 2.9 nm, as well as good amount of oxygen functional groups (2.25 mmolg−1) including carboxyl, phenol, and lactone groups. The adsorption performance of optimized material was evaluated using Bisphenol A as a target molecule. The adsorption process could be described by Weber Morris and Langmuir isotherm models. At a temperature of 25 °C, Langmuir monolayer adsorption capacity was observed to be 420 mgg−1 which is comparable or higher than those reported previously. The estimated thermodynamic parameters indicate spontaneous and exothermic adsorption process (∆H° = −19.39 kJmol−1; −5.58 kJmol−1 ≤ ∆G° ≤ −4.65 kJmol−1). The mesoporous activated carbon prepared by chemical activation of biomass-derived hydrochar under atmospheric air proved to be very potential adsorbent for Bisphenol A removal.

A Simple, Scalable, Low‐Cost Honeycomb‐Like Carbonized Corncob for Highly Efficient Solar Steam Generation

AbstractDirect solar steam generation (DSSG) is currently one of the most promising desalination methods due to its high energy conversion efficiency. The key element of DSSG is solar thermal conversion materials (STCMs), which absorb incoming solar illumination owing to their excellent solar absorption and convert light to heat. Although many types of STCMs have been applied to DSSG, there are still some existing problems such as complex preparation, high cost, and low energy conversion efficiency. In this work, carbonized corncob, as an example of inexpensive agricultural waste, is prepared as a STCM, which has porous honeycomb‐like pores, and hydrophilic functional groups, ensuring sufficient water transportation. Different pyrolytic temperatures lead to different size pores, content of amorphous carbon, and amount of oxygen functional groups, which affect the water transportation efficiency and light‐to‐heat conversion efficiency. Benefiting from developed pores, 95.7% solar absorption and thick‐hole wall, the carbonized corncob pyrolyzed at 800 °C reaches a solar‐to‐steam conversion efficiency of 86%, and its water evaporation rate is 1.62 kg m–2 h–1, higher than most biomass reported so far. Using cost‐effective materials and methods to carry out efficient photothermal conversion is of great significance for the realization of practical seawater desalination and sewage treatment.

Hydrophilicity control of laser-induced amorphous carbon-encapsulated carbon nano-onions and their application to proton exchange membrane fuel cells under low humidity

Proton exchange membrane fuel cells (PEMFCs) are operated in a range of different environments including near-dry operating conditions. When a PEMFC operates under low relative humidity (RH) conditions, performance degradation rapidly occurs because of the decreased ionic conductivity of dehydrated membrane electrode assemblies (MEAs). As such, securing additional water inside the MEA is important for achieving a high-performance PEMFC under low RH operating conditions. Herein, amorphous carbon-encapsulated carbon nano-onions (AC-CNOs) were prepared by laser pyrolysis method. The hydrophilicity of AC-CNOs varied on the basis of particle size, which affected the amount of oxygen functional groups attached to the amorphous carbon. To confirm the water retention effect of the AC-CNOs, three different catalysts were incorporated into the MEA of the anode electrode, and a Pt@AC–CNO–3 with an average AC-CNO size of ∼164 nm exhibited a significantly improved performance under low RH condition. Moreover, to achieve a synergetic effect from both the anode and cathode, an additional AC-CNO layer was introduced on the cathode catalyst layer. The dual-side MEA with the hydrophilic catalyst layer at the anode and an additional carbon covering layer at the cathode showed significantly increased performance relative to the reference MEA, i.e., >80.2% under RH44%@90 °C conditions.

The effect of plasma treated carbon felt on the performance of aqueous quinone‐based redox flow batteries

4,5-dihydroxybenzene-1,3-disulfonic acid (Tiron) and anthraquinone-2,7-disulfonic acid (AQDS) are interesting redox couples for aqueous quinone-based redox flow batteries (QRFBs) because of their high solubility and good reaction reversibility in acidic condition. Tiron is rate-determining material due to its slow reaction kinetics. To improve that, Tiron is transformed into 2,4,5,6-tetrahydroxybenzene-1,3-disulfonic acid (TironA), which is effectively formed during the first cycle of QRFB. Once TironA is formed, a desirable two-electron redox reaction with stable and reproducible charge and discharge step of QRFB occurs, although the electron transfer of TironA is still lower than that of AQDS. To further facilitate that of TironA, oxygen (O2) and nitrogen (N2) plasma-treated carbon felt (CF) electrodes are suggested. Oxygen functional groups formed onto CF by O2 plasma become the active sites for redox reaction of TironA. As the amount of oxygen functional groups formed increases, the redox reactivity of TironA is enhanced. In contrast, when N2 plasma is used, pyrrolic N and quaternary N are mainly formed. With the formation, (i) hydrogen atoms within pyrrolic N act as proton donor and interact with oxygen groups of TironA and (ii) nitrogen cations within quaternary N interact with the negatively charged atoms of TironA by electrostatic interaction. Thus, both reaction kinetic of TironA and performance of QRFB increase. Regarding the performance of QRFB using N2 plasma-treated CF, its energy efficiency (EE), discharging capacity, and state of charge are 62%, 17.3 Ahr·L−1 and 64.5% for 50 cycle, which correspond to excellent achievements.

Direct hydroxylation of 1,4‐Dichlorobenzene to 2,5‐Dichlorophenol over Activated Carbon Catalysts

AbstractThe direct hydroxylation of 1,4‐dichlorobenzene (1,4‐DCB) to 2,5‐dichlorophenol (2,5‐DCP) with green oxidant, to replace the conventional multi‐step processes attracted much attention. In this work, activated carbons were prepared by modification firstly using nitric acid, and then the resultant samples were treated with hydrogen peroxide or thermally under nitrogen flow. The prepared samples were applied as heterogeneous catalysts in the one‐step hydroxylation of 1,4‐DCB, and were characterized by XRD, BET, XPS, and Boehm titration. Acetonitrile, isopropanol, glacial acetic acid, and the mixture of them were used as the reaction solvents and EPR was carried to characterize the reaction systems in these solvents. The reaction parameters, such as the feeding speed of H2O2, moles of 1,4‐DCB used, molar ratio of H2O2 to 1,4‐DCB, mass ratio of catalyst to 1,4‐DCB, reaction temperature, and reaction time were optimized. It was found that the direct hydroxylation of 1,4‐DCB could be efficiently catalyzed by activated carbon with acetonitrile as a suitable solvent. Under the optimized conditions, the highest 2,5‐DCP yield of 14.4 % with selectivity of 86.4 % was obtained. The amount of oxygen functional groups on the activated carbon samples resulted in differences in adsorption capacity of 1,4‐DCB and decomposition rate of H2O2, the integrated effect of which might be responsible for the activity.

The effect of biochar, hydrochar particles and dissolved organic matter on the photodegradation of metribuzin herbicide in aquatic media

Biochar and hydrochar particles and/or dissolved organic matter (DOM) may participate in several decomposition and transport processes of organic pollutants, thus their effect on aqueous photolysis of metribuzin (MBZ), selected as a potential pesticide pollutant of water systems, has been studied. Chars have been characterized by several techniques in order to correlate physicochemical characteristics with observed photodegradation rates. Biochar presented a micro-mesoporous graphitic structure with high surface area, and lower amounts of oxygen functional groups while hydrochar presented coral like structure, low surface area, incomplete carbonization and higher amounts of oxygen functional groups. MBZ photolysis was enhanced in the presence of low concentration levels (50 mgL−1) of char particles and DOM while an inhibitory effect was observed for higher concentrations (100–400 mgL−1). DOM from hydrochar present a stronger optical filter effect than biochar DOM while the biochar matrix and DOM present the higher ability to generate hydroxyl radicals (•OH) and singlet oxygen (1O2), respectively. The participation of different ROS in photodegradation pathways for each matrix was studied also by scavenging experiments. The overall effect on photodegradation rate was the result of counterplay between the decrease of direct photolysis due to the light attenuation and the occurrence of indirect photolysis reaction paths via the generated ROS depending on the role of char matrix and DOM to act as source or sink for the ROS. The results demonstrate the significant and dissimilar role of char matrices and dissolved organic matter on the photo degradation rate of organic pollutants in aquatic matrices.

Surface characteristics of porous carbon derived from genetically designed transgenic hybrid poplar for electric double-layer capacitors

Wood and wood residues are attractive sources for supercapacitor electrode materials because of their abundance and sustainability. As each lignocellulosic biomass has an intrinsic chemical composition, improvements in the energy storage capability have relied on modified preparation conditions or the introduction of functional groups or foreign materials. However, owing to advances in genetic science, the influence of biomass chemical composition on the surface characteristics and electrochemical behavior of wood-derived porous carbons can be investigated toward the development of high-performance supercapacitor electrodes. Herein, we compare the electrochemical performance of porous carbons derived from genetically designed transgenic and wild poplar following chemical activation with KOH at 800 °C. The transgenic poplar-derived porous carbon shows ~80% higher capacitance than the wild-poplar-derived porous carbon at 0.2 A/g and greater performance at all test rates (0.2–20 A/g). An increased cellulose fraction or decreased lignin fraction in the biomass increases the surface area and the amount of oxygen functional groups on the porous carbon, thus enhancing the electrochemical performance. These findings are expected to advance the use of biomass-derived materials in energy storage applications.

Degradation of Carbon Electrodes in the Vanadium Redox Flow Battery

The Vanadium Redox Flow Battery (VRFB) is a promising candidate for large scale energy storage. These systems are expected to operate for long cycle life ~ 10 years of lifetime (~ 500 - 2000 charge – discharge cycles).1 The VRFB’s system includes an posolyte (VO2 +/VO2+) and negolyte (V2+/V3+) compartments with carbon electrodes, and uses acidic electrolytes (usually 3M H2SO4). The carbon electrodes in VRFB need to perform in a harsh and corrosive environment associated with the charge/discharge cycling. In this study, we used polyacrylonitrile (PAN) based carbon papers as model carbon electrodes to study changes in the electrodes during cycling operation. The carbon papers were pre-treated according to the conventional approach by heating them in air at 500 ⁰C to incorporate oxygen functional groups on their surfaces.2,3 These pre-treated electrodes were then used in flow battery for 100-cycles at a current density of 80 mA/cm2. At the single cell level, the performance of these electrodes following the operation indicated a slight decrease in voltage efficiency (from 60 % to 52 %). This decrease in performance may be attributed to the electrode degradation during operation. Characterization of the electrode prior and following 100-cycles was performed using Raman spectroscopy, XPS analysis as well as electrochemical studies. Raman analysis showed differences between the fresh and the degraded carbon electrodes, including increasing defect and electronic density distribution. XPS analysis of the degraded electrodes suggest an alteration in the amount of oxygen functional groups. Electrochemical studies showed an decrease in the electron transfer kinetics after degradation, coupled with noticeable difference between the electrodes exposed to the posolyte (VO2 +/VO2+) and negolyte (V2+/V3+). The results provide insight into the stability of the oxygen functional groups during cycling operation which could assist in developing pre-treatment methods to functionalize the VRFB’s electrodes and for other similar electrochemical systems.

Improvement of gas sensing property for two-dimensional Ti3C2Tx treated with oxygen plasma by microwave energy excitation

Two-dimensional layered Ti3C2Tx MXene was prepared through hydrothermal etching method with LiF and hydrochloric (HCl) acid. Ti3C2Tx was further treated with oxygen plasma activated by microwave energy to obtain the activated Ti3C2Tx at different temperatures ranging from 350 °C to 550 °C. The gas-sensing properties of raw Ti3C2Tx and Ti3C2Tx activated with oxygen microwave plasma were tested toward different volatile organic compounds gases. The results indicated that Ti3C2Tx activated at 500 °C exhibited excellent gas-sensing properties at room temperature (25 °C) to 100 ppm ethanol with a value of 22.47, which is attributed to the enhancement of the amount of oxygen functional groups and defects on the MXene Ti3C2Tx film, and in turn to lead to more oxygen molecules adsorption and desorption reaction in the active defect sites. The enhancement of ethanol-sensing performance demonstrated that the activated Ti3C2Tx possess great potential in gas sensing.

Enrichment of surface oxygen functionalities on activated carbon for adsorptive removal of sevoflurane

Activated carbons have been reported to be useful for adsorptive removal of the volatile anaesthetic sevoflurane from a vapour stream. The surface functionalities on activated carbons could be modified through aqueous oxidation using oxidising solutions to enhance the sevoflurane adsorption. In this study, an attempt to oxidise the surface of a commercial activated carbon to improve its adsorption capacity for sevoflurane was conducted using 6 mol/L nitric acid, 2 mol/L ammonium persulfate, and 30 wt per cent (wt%) of hydrogen peroxide (H2O2). The adsorption tests at fixed conditions (bed depth: 10 cm, inlet concentration: 528 mg/L, and flow rate: 3 L/min) revealed that H2O2 oxidation gave desirable sevoflurane adsorption (0.510 ± 0.005 mg/m2). A parametric study was conducted with H2O2 to investigate the effect of oxidation conditions to the changes in surface oxygen functionalities by varying the concentration, oxidation duration, and temperature, and the Conductor-like Screening Model for Real Solvents (COSMO-RS) was applied to predict the interactions between oxygen functionalities and sevoflurane. The H2O2 oxidation incorporated varying degrees of both surface oxygen functionalities with hydrogen bond (HB) acceptor and HB donor characters under the studied conditions. Oxidised samples with enriched oxygen functionalities with HB acceptor character and fewer HB donor character exhibited better adsorption capacity for sevoflurane. The presence of a high amount of oxygen functional groups with HB donor character adversely affected the sevoflurane adsorption despite the enrichment of oxygen functional groups with HB acceptor character that have a higher tendency to adsorb sevoflurane.

Application of Boehm Titration for the Quantitative Measurement of Soot Oxygen Functional Groups

The oxygen functional groups on diesel soot have potential effects on its oxidation reactivity, which is of great importance for diesel particulate filter (DPF) operation. For the quantitative measurement of oxygen functional groups on diesel soot, Boehm titration was adopted and procedures considering CO2 removal and sample characteristics involving sample mass as well as the interferences of soluble organic fractions (SOFs) were evaluated to ensure the titration precision possible. Titration was also conducted on samples of nitric acid (HNO3)-modified soot as well as real soot samples and the results were combined with X-ray photoelectron spectroscopy (XPS) analysis to verify the reliability of titration procedures. Results showed that heating at 105 °C for 90 min can eliminate CO2 interference to an acceptable extent. Integrated with mixing and titration error, a sample mass of 70 mg showed the best repeatability. Very little oxygen functional groups were detected for pure diesel oil. Titration with samples varied in SOF content showed a comparable amount of oxygen functional groups, indicating that SOF wrapped around soot surface had little physical and chemical effect on titration. Titration tests with HNO3-modified samples correctly reflect the changing trend of oxygen functional groups after acid treatment, which was further verified by XPS analysis. However, due to the relatively large standard deviation and error propagation, the concentration value of oxygen functional groups calculated by subtraction often showed a large error range, especially for low-concentration samples. Titration error was much lower for high-concentration samples. For real diesel soot, much higher concentrations of 941 ± 119, 376 ± 151, and 903 ± 106 μmol/g were measured for phenolic hydroxyl, lactones, and carboxyl groups, respectively. In all, the Boehm titration process conducted in this article showed the capability and reliability of the quantitative measurement of oxygen functional groups on soot surface. Coupled with XPS analysis, more information about the current state and possible transformation process of carbonyl groups and the possible distribution of functional groups between internal and external surfaces can be inferred.

Degradation of Carbon Electrodes in the Vanadium Redox Flow Battery

The Vanadium Redox Flow Battery (VRFB) is a promising candidate for large scale energy storage. These systems are expected to operate for long cycle life ~ 10 years of lifetime (~ 500 - 2000 charge – discharge cycles).1 The VRFB’s system includes an posolyte (VO2 +/VO2+) and negolyte (V2+/V3+) compartments with carbon electrodes, and uses acidic electrolytes (usually 3M H2SO4). The carbon electrodes in VRFB need to perform in a harsh and corrosive environment associated with the charge/discharge cycling. In this study, we used polyacrylonitrile (PAN) based carbon papers as model carbon electrodes to study changes in the electrodes during cycling operation. The carbon papers were pre-treated according to the conventional approach by heating them in air at 500 ⁰C to incorporate oxygen functional groups on their surfaces.2,3 These pre-treated electrodes were then used in flow battery for 100-cycles at a current density of 80 mA/cm2. At the single cell level, the performance of these electrodes following the operation indicated a slight decrease in voltage efficiency (from 60 % to 52 %). This decrease in performance may be attributed to the electrode degradation during operation. Characterization of the electrode prior and following 100-cycles was performed using Raman spectroscopy, XPS analysis as well as electrochemical studies. Raman analysis showed differences between the fresh and the degraded carbon electrodes, including increasing defect and electronic density distribution. XPS analysis of the degraded electrodes suggest an alteration in the amount of oxygen functional groups. Electrochemical studies showed an decrease in the electron transfer kinetics after degradation, coupled with noticeable difference between the electrodes exposed to the posolyte (VO2 +/VO2+) and negolyte (V2+/V3+). The results provide insight into the stability of the oxygen functional groups during cycling operation which could assist in developing pre-treatment methods to functionalize the VRFB’s electrodes and for other similar electrochemical systems.

Quinone/ester-based oxygen functional group-incorporated full carbon Li-ion capacitor for enhanced performance.

Lithium ion capacitors (LICs) are regarded as one of the most promising energy storage devices since they can bridge the gap between lithium ion batteries and supercapacitors. However, the mismatches in specific capacity, high-rate behavior, and cycling stability between the two electrodes are the most critical issues that need to be addressed, severely limiting the large energy density and long cycling life of LICs while delivering high-power density output. Herein, quinone and ester-type oxygen-modified carbon has been successfully obtained by chemical activation with alkali, which is beneficial to the absorption of PF6- together with lithium ions, which would largely improve the electrode kinetics. In particular, the cathode capacity is considerably enhanced with the increase in the amount of oxygen functional groups. Moreover, for the full carbon LIC device, an energy density of 144 W h kg-1 is exhibited at the power density of 200 W kg-1. Surprisingly, even after 10 000 cycles at 20 000 W kg-1, a capacity retention of 70.8% is successfully achieved. These remarkable results could be ascribed to the enhancement of cathode capacity and the acceleration of anode kinetics. Furthermore, the density functional theory (DFT) calculations prove that the oxygen functional groups can deliver enhanced electrochemical activity for lithium storage through surface-induced redox reactions. This elaborate study may open an avenue for resolving the issues with the electrode materials of LICs and deepen the understanding on the surface engineering strategies for incorporating oxygen-functional groups.