Electrochemical behavior of graphite highly fluorinated by high oxidation state complex fluorides and elemental fluorine

Electrochemical intercalation of anions into graphite: Fundamental aspects, material synthesis, and application to the cathode of dual-ion batteries

In this review, fundamental aspects of the electrochemical intercalation of anions into graphite have been first summarized, and then described the electrochemical preparation of covalent-type GICs and application of graphite as the cathode of dual-ion battery. Electrochemical overoxidation of anion GICs provides graphite oxide and covalent-fluorine GICs, which are key functional materials for various applications including energy storage devices. The reaction conditions to obtain fully oxidized graphite has been mentioned. Concerning the application of graphite for the cathode of dual-ion battery, it stably delivers about 110 mA h g-1 of reversible capacity in usual organic electrolyte solutions. The combination of anion and solvent as well as the concentration of the anions in the electrolyte solutions greatly affect the performance of graphite cathode such as oxidation potential, rate capability, cycling properties, etc. The interfacial phenomenon is also important, and fundamental studies of charge transfer resistance, anion diffusion coefficient, and surface film formation behavior have also been summarized. The use of smaller anions, such as AlCl4 - , Br- can increase the capacity of graphite cathode. Several efforts on the structural modification of graphite and development of electrolyte solutions in which graphite cathode delivers higher capacity were also described.

Fluorinated photoreduced graphene oxide with semi-ionic C-F bonds: An effective carbon based photocatalyst for the removal of volatile organic compounds

There is much interest in developing metal-free halogenated graphene such as fluorinated graphene for various catalytic applications. In this work, a fluorine-doped graphene oxide photocatalyst was investigated for photocatalytic oxidation (PCO) of a volatile organic compound (VOC), namely gaseous methanol. The fluorination process of graphene oxide (GO) was carried out via a novel and facile solution-based photoirradiation method. The fluorine atoms were doped on the surface of the GO in a semi-ionic C-F bond configuration. This presence of the semi-ionic C-F bonds induced a dramatic 7-fold increment of the hole charge carrier density of the photocatalyst. The fluorinated GO photocatalyst exhibited excellent photodegradation up to 93.5% or 0.493 h-1 according pseudo-first order kinetics for methanol. In addition, 91.7% of methanol was mineralized into harmless carbon dioxide (CO2) under UV-A irradiation. Furthermore, the photocatalyst demonstrated good stability in five cycles of methanol PCO. Besides methanol, other VOCs such as acetone and formaldehyde were also photodegraded. This work reveals the potential of fluorination in producing effective graphene-based photocatalyst for VOC removal.

Electrochemical Introduction/Extraction of Fluoride Ions into/from Graphene-like Graphite for Positive Electrode Materials of Fluoride-Ion Shuttle Batteries

A cathode material, graphene-like graphite, was developed for all-solid-state-type fluoride-ion shuttle batteries (FSBs). Fluoride ions were electrochemically introduced/extracted into/from it, and covalent C-F bonds were formed upon electrochemical oxidation. The introduction of fluoride ions into it occurred at a lower voltage than that into graphite. While the layered structure of graphite was completely destroyed during charging, that of graphene-like graphite was still maintained to some extent. The discharge voltage was higher than 1 V versus Pb/PbF₂, which was higher than that of most of the previously reported cathode materials. The first discharge capacity (161 mAh g^-1) was larger than that of graphite (140 mAh g^-1), and the Coulombic efficiency and cyclability were much higher. This work demonstrates that graphene-like graphite prepared by thermal reduction of graphene oxide at 300 °C, GLG300, is a promising material for positive electrodes of FSBs.

Novel and Highly Sensitive Electrochemical Sensor for the Determination of Oxytetracycline Based on Fluorine-Doped Activated Carbon and Hydrophobic Deep Eutectic Solvents

Residues of oxytetracycline (OTC), a veterinary antibiotic and growth promoter, can be present in animal-derived foods; their consumption is harmful to human health and their presence must therefore be detected and regulated. However, the maximum residue limit is low, and consequently highly sensitive and accurate detectors are required to detect the residues. In this study, a novel highly sensitive electrochemical sensor for the detection of OTC was developed using a screen-printed electrode modified with fluorine-doped activated carbon (F-AC/SPE) combined with a novel deep eutectic solvent (DES). The modification of activated carbon by doping with fluorine atoms (F-AC) enhanced the adsorption and electrical activity of the activated carbon. The novel hydrophobic DES was prepared from tetrabutylammonium bromide (TBABr) and a fatty acid (malonic acid) using a green synthesis method. The addition of the DES increased the electrochemical response of F-AC for OTC detection; furthermore, it induced preconcentration of OTC, which increased its detectability. The electrostatic interactions between DES and OTC as well as the adsorption of OTC on the surface of the modified electrode through H-bonding and π-π interactions helped in OTC detection, which was quantified based on the decrease in the anodic peak potential (E _pa = 0.3 V) of AC. The electrochemical behavior of the modified electrode was investigated by cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy. Under optimum conditions, the calibration plot of OTC exhibited a linear response in the range 5-1500 μg L^-1, with a detection limit of 1.74 μg L^-1. The fabricated electrochemical sensor was successfully applied to determine the OTC in shrimp pond and shrimp samples with recoveries of 83.8-100.5% and 93.3-104.5%, respectively. In addition to the high sensitivity of OTC detection, the proposed electrochemical sensor is simple, cost-effective, and environmentally friendly.

Advantage of semi-ionic bonding in fluorine-doped carbon materials for the oxygen evolution reaction in alkaline media

Metal-free carbonaceous catalysts have potential applications for oxygen evolution reaction (OER) devices because of their low-cost and abundant supply. We report that fluorine-doped carbon black is an active catalyst for OER. Fluorine-doped carbon black (F-KB) is simply synthesized by the pyrolysis of KETJENBLACK (KB) as carbon substrate with Nafion as fluorine precursor. As a result, the OER activity of F-KB is significantly higher than that of pristine KB in alkaline media. The OER catalytic activity of F-KB is found to be dependent on the quantity and characteristics of carbon-fluorine bonding (C–F) which can be controlled by the pyrolysis temperature. It is further found that the OER activity depends on the quantity of semi-ionic C–F bonds, but not covalent C–F bonds. This result proves the importance of carbon atoms with semi-ionic C–F bonds as the active sites for OER.

Fluorine-doped carbon has a higher electrocatalytic oxygen evolution activity than pristine carbon black in alkaline media. The activity of oxygen evolution and characteristics of carbon to fluorine bond are controlled by pyrolysis temperature of Nafion.

Mesoporous Non-stacked Graphene-receptor Sensor for Detecting Nerve Agents

A novel gas sensor consisting of porous, non-stacked reduced graphene oxide (NSrGO)-heaxfluorohydoroxypropanyl benzene (HFHPB) nanosheets was successfully fabricated, allowing the detection of dimethyl methyl phosphonate (DMMP), similar to sarin toxic gas. The HFHPB group was chemically grafted to the NSrGO via a diazotization reaction to produce NSrGO-HFHPB. The NSrGO-HFHPB 3D film has a mesoporous structure with a large pore volume and high surface area that can sensitively detect DMMP and concurrently selectively signal the DMMP through the chemically-attached HFHPB. The DMMP uptake of the mesoporous NSrGO-HFHPB was 240.03 Hz, 12 times greater than that of rGO-HFHPB (20.14 Hz). In addition, the response rate of NSrGO-HFHPB was faster than that of rGO-HFHPB, an approximately 3 times more rapid recovery due to the mesoporous structure of the NSrGO-HFHPB. The NSrGO-HFHPB sensor exhibited long-term stability due to the use of robust carbon and resulting high resistance to humidity.

Effects of halogen doping on nanocarbon catalysts synthesized by a solution plasma process for the oxygen reduction reaction

Halogen (F, CI, and Br)-containing carbon materials were successfully synthesized by solution plasma process. The effects of halogen doping on chemical structure and electrocatalytic activity were investigated.

Solvothermally exfoliated fluorographene for high-performance lithium primary batteries

High-quality fluorographene (FG) was prepared by solvothermal exfoliation of fluorinated graphite (F-graphite) through intercalation of acetonitrile and chloroform with low boiling points. High-yield production of FG was demonstrated by wrinkled few-layered structures with disordered edges and poor regularity along the stacking direction. X-ray photo electron spectroscopy (XPS) spectra indicated that the intercalation of chloroform led to the partial transformation from covalent C-F bonds to semi-ionic C-F bonds. A lithium primary battery (Li-battery) using a FG cathode exhibited a remarkable discharge rate performance because of good Li(+) diffusion and charge mobility through nanosheets. FG nanosheets exfoliated using chloroform showed a high specific capacity of 520 mA h g(-1) and a voltage platform of 2.18 V at a current density of 1 C, accompanied by a maximum power density of 4038 W kg(-1) at 3 C, which is almost four times higher than that of F-graphite. The results indicate that the solvothermal exfoliation using a low-boiling-point solvent is a facile, efficient and high-yield approach to prepare high-purity FG nanosheets for high-performance Li-batteries.

A class of high performance metal-free oxygen reduction electrocatalysts based on cheap carbon blacks

For the goal of practical industrial development of fuel cells, cheap, sustainable and high performance electrocatalysts for oxygen reduction reactions (ORR) which rival those based on platinum (Pt) and other rare materials are highly desirable. In this work, we report a class of cheap and high-performance metal-free oxygen reduction electrocatalysts obtained by co-doping carbon blacks with nitrogen and fluorine (CB-NF).The CB-NF electrocatalysts are highly active and exhibit long-term operation stability and tolerance to poisons during oxygen reduction process in alkaline medium. The alkaline direct methanol fuel cell with the best CB-NF as cathode (3 mg/cm²) outperforms the one with commercial platinum-based cathode (3 mg _Pt/cm²). To the best of our knowledge, these are among the most efficient non-Pt based electrocatalysts. Since carbon blacks are 10,000 times cheaper than Pt, these CB-NF electrocatalysts possess the best price/performance ratio for ORR, and are the most promising alternatives to Pt-based ones to date.

A facile route to recover intrinsic graphene over large scale

The intrinsic properties of initially p-type doped graphene (grown by chemical vapor deposition (CVD)) can be recovered by buffered oxide etch (BOE) treatment, and the dominant factor governing p-type doping is identified as the H(2)O/O(2) redox system. Semi-ionic C-F bonding prevents the reaction between the products of the H(2)O/O(2) redox system and graphene. BOE-treated graphene field effect transistors (FETs) subsequently exposed to air, became p-type doped due to recovery of the H(2)O/O(2) redox system. In comparison, poly(methyl methacrylate) (PMMA)-coated graphene FETs had improved stability for maintaining the intrinsic graphene electronic properties.