Improved electrooxidation of phenol at exfoliated graphite electrodes

In the presented paper, we report on electrochemical oxidation of phenol occurring at exfoliated graphite (EG) in alkaline solution. The mechanism of the electrocatalytic reaction of phenol oxidation was modified on adding methanol to the phenol-containing electrolyte. Using the voltammetry method, the influence of methanol additive on cyclic behavior of EG electrode was examined. A particular attention has been paid to the first two cycles when an abrupt decrease in electrocatalytic activity of various electrode materials has been observed. The results obtained showed that in the presence of methanol EG, electrode preserves its electrocatalytic activity for a longer time of phenol oxidation. In the absence of methanol in a phenol/KOH electrolyte, the charge of phenol oxidation peaks decreases sharply on cycling, whereas in the presence of methanol, the observed drop is considerably inhibited. The anodic charge attained for the 15th cycle of phenol oxidation in methanol-admixed electrolyte is the same as that for the third cycle recorded in methanol-free electrolyte. The thermogravimetric analysis (TG), Fourier-transformed infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) data showed that an improved electrocatalytic activity of EG can be accounted for by new chemical composition of oligomer film built on the EG surface with the participation of methanol and/or the products of its oxidation.Presented at the 4th Baltic Conference on Electrochemistry, Greifswald, March 13–16, 2005.     

Regeneration of expanded graphite electrodes by joined electrochemical and ozone treatment in liquid phase

The researches presented in this work were devoted to electrochemico-chemical regeneration of exhausted electrode made of expanded graphite (EG). The aimed process was conducted by electrochemical treatment and ozone flow performed together in wet environment. EG was covered with insoluble products of incomplete oxidation of phenol formed during cyclic voltammetry measurement. The same electrochemical technique was applied for evaluation of regeneration efficiency. To understand the process of EG regeneration, the electrode was characterized by calculating of BET surface, FTIR and XPS analysis. Moreover, SEM images of the investigated samples were also done. Obtained results have showed the success of regeneration treatment, which led to significant enhancement of electrode activity compared to original EG. The present work also revealed that the mechanism of phenol electrooxidation is changed after the regeneration treatment of electrode material. This effect is probably caused by the modification of chemical composition of EG surface due to its interactions with OH radicals intensively generated during the process of regeneration.

     

Multiple anodic regeneration of exfoliated graphite electrodes spent in the process of phenol electrooxidation

The present paper deals with the studies concerning anodic regeneration of exfoliated graphite (EG) electrodes coated with oligomer products of incomplete phenol electrooxidation. The electrochemical activity of regenerated samples depends on the concentration of regenerating electrolyte and potential applied during the regeneration. The effective method of a single-step regeneration of spent EG electrodes was found to be anodic treatment proceeded in 6 M KOH at the potential of 1.3 V vs. Hg/HgO/0.5 M KOH. During the oxidative regeneration of EG, the physical and chemical interactions between intensively generated active oxygen and/or OH· radical and oligomer covering the EG surface take place. As has been proved by FTIR and XPS analyses, active oxidative agents cause significant modification of chemical composition of oligomer that resulted in enhanced concentration of surface functionalities mainly involving C=O bonds. The results of multiple regeneration of EG electrode, considered as promising method for practical application, showed that the highest efficiency of regeneration is attained due to potentiostatic treatment carried out in 6 M KOH at the potential of 1.2 V. The electrochemical activity of EG regenerated at this potential increases gradually with the number of regenerating loops and after the third regeneration is almost fourfold higher compared to that of the original EG.     

电催化氧化技术处理苯酚废水研究

电化学氧化法可有效处理挥发酚类废水,而阳极材料性能直接制约其电氧化效率.本文分别针对不锈钢、柔性石墨和SnO2 /Ti复合材料测定了其析氧过电位,并以不锈钢、柔性石墨为阳极材料,在 5~6V直流电压下,对合成苯酚废水进行了电化学氧化处理.结果表明:析氧过电位次序为SnO2 /Ti>柔性石墨 >不锈钢,处理后水的COD值接近或小于 100mg·L-1,且出水的苯酚浓度小于 0. 5mg·L-1.     

Electrochemical properties of exfoliated graphite affected by its two-step modification

Exfoliated graphite (EG) was modified by a two-stage process consisting of electrochemical oxidation followed by the thermal treatment. Within the former one, the process of re-intercalation of H₂SO₄ into the EG by linear sweep voltammetry was carried out. Thus, obtained re-intercalated EG underwent heat treatment at 800 °C in order to synthesize re-exfoliated EG (re-EG). The electrochemical features of the re-EG were examined in the model process of phenol electrooxidation carried out by cyclic voltammetry technique in alkaline solution with and without phenol addition. Taking into account the anodic charges as a main criterion of electrochemical activity, it was found that the modification of EG caused over twofold improvement of its electrochemical activity. This behavior is related with the changes within the chemical composition of modified EG surface and on much smaller scale with the modification of its structure. The degree of electrochemical activity improvement depends on the conditions under which the processes of re-intercalation and re-exfoliation were performed. The results of Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis supported by the data of the Brunauer-Emmett-Teller (BET) surface area and scanning electron microscope (SEM) observations allow an understanding of the physicochemical properties of re-exfoliated EG and enhancement of its electrochemical activity.     

Effects of current densities on the formation of LiCoO<Subscript>2</Subscript>/graphite lithium ion battery

The activation characteristics and the effects of current densities on the formation of a separate LiCoO₂ and graphite electrode were investigated and the behavior also was compared with that of the full LiCoO₂/graphite batteries using various electrochemical techniques. The results showed that the formation current densities obviously influenced the electrochemical impedance spectrum of Li/graphite, LiCoO₂/Li, and LiCoO₂/graphite cells. The electrolyte was reduced on the surface of graphite anode between 2.5 and 3.6 V to form a preliminary solid electrolyte interphase (SEI) film of anode during the formation of the LiCoO₂/graphite batteries. The electrolyte was oxidized from 3.95 V vs Li⁺/Li on the surface of LiCoO₂ to form a SEI film of cathode. A highly conducting SEI film could be formed gradually on the surface of graphite anode, whereas the SEI film of LiCoO₂ cathode had high resistance. The LiCoO₂ cathode could be activated completely at the first cycle, while the activation of the graphite anode needed several cycles. The columbic efficiency of the first cycle increased, but that of the second decreased with the increase in the formation current of LiCoO₂/graphite batteries. The formation current influenced the cycling performance of batteries, especially the high-temperature cycling performance. Therefore, the batteries should be activated with proper current densities to ensure an excellent formation of SEI film on the anode surface.     

石墨负极电化学扫描循环过程的EIS、Raman光谱和XRD研究

运用电化学阻抗谱(EIS)、Raman光谱和XRD研究了石墨负极在1 mol/L LiPF6-EC∶DEC∶DMC电解液中的电化学循环扫描过程. EIS研究结果表明, 在电化学循环扫描4~10周范围内, SEI膜(固体电解质相界面膜)电阻随循环扫描周数增加近似线性增长, 但石墨负极/电解液界面总阻抗由于电荷传递电阻的降低而减小. Raman光谱研究结果表明, 在经历电化学循环扫描后, 活性材料表层发生粉化和无定形化, 石墨化程度降低; 但XRD研究结果显示, 石墨材料的本体结构没有发生变化, 仍然保持着完整的石墨层状结构.     

Electrochemical oxidation of ethyl 3-oxo-3-phenyl-2-phenylhydrazonopropionate

The electrochemical oxidation of ethyl 3-oxo-3-phenyl-2-phenylhydrazonopropionate has been studied in the pH range 3.0–11.0 at a pyrolytic graphite electrode by linear and cyclic sweep voltammetry, coulometry and spectral studies. The results indicate that the 2 e⁻, 2 H⁺ oxidation of this compound gives phenol and ethyl 3-phenyl-2,3-dioxopropionate as the major products of electrooxidation.     

Phenol Degradation Features in Aqueous Solutions upon Dielectric-Barrier Discharge Treatment

The formation kinetics and mechanism were studied for the intermediate and final products of degradation of aqueous phenol solutions in air and oxygen low-temperature dielectric-barrier discharge plasmas in a reactor with different geometric arrangements of electrodes. It was shown that the plasma-enhanced oxidation of phenol could be described in the general form by the well-known mechanism of enzymatic oxidation of toxic pollutants (-mechanism). The estimated effective rate constant of phenol degradation in an aqueous solution in an ₂ plasma in reactors with the parallel-plate and coaxial arrangement of electrodes was 2.2 × 10^–3 s^–1 and 5.8 × 10^–2 s^–1, respectively. It was found that the potential toxicity of solutions after treatment was 7.5 times lower than that of the untreated solutions.     

Electropolymerization of an EDOT-modified diarylethene

A diarylethene substituted with 3,4-ethylenedioxythiophene (EDOT) was synthesized to induce electrochemical anodic polymerization. Upon electrochemical oxidation of 1,2-bis(2-methylbenzo[b]thiophene-3-yl)perfluorocyclopentene (BTF)-substituted EDOT at the 6,6′-position (BTFTT), a red-purple polymeric film (PBTFTT) was deposited on a working electrode. A similar film was deposited on an electrode from the solution exposed to UV light through electrochemical oxidation. The film growth was controlled by the cycle numbers in cyclic voltammetry during the electropolymerization. The film thickness was linearly correlated to the potential cycle numbers, with a slope of 17.9 nm/cycle. The IR spectrum of the electrodeposited polymer showed characteristic CC stretching frequency at 1630 and 1481 cm⁻¹ indicating that the BTF units in the polymer are closed.     

A novel high-energy-density lithium-free anode dual-ion battery and in situ revealing the interface structure evolution

Lithium-free anode dual-ion batteries have attracted extensive studies due to their simple configuration, reduced cost, high safety and enhanced energy density. For the first time, a novel Li-free DIB based on a carbon paper anode (Li-free CGDIB) is reported in this paper. Carbon paper anodes usually have limited application in DIBs due to their poor electrochemical performance. Herein, by using a lithium bis(fluorosulfonyl)imide (LiFSI)-containing electrolyte, the battery shows outstanding electrochemical performance with a capacity retention of 96% after 300 cycles at 2C with a stable 98% coulombic efficiency and 89% capacity retention after 500 cycles at 5C with a stable coulombic efficiency of 98.5%. Moreover, the electrochemical properties of the CGDIB were investigated with a variety of in situ characterization techniques, such as in situ EIS, XRD and online differential electrochemical mass spectrometry (OEMS). The multifunctional effect of the LiFSI additive on the electrochemical properties of the Li-free CGDIB was also systematically analyzed, including generating a LiF-rich interfacial film, prohibiting Li dendrite growth effectively and forming a defective structure of graphite layers. This design strategy and fundamental analysis show great potential and lay a theoretical foundation for facilitating the further development of DIBs with high energy density.

A novel lithium-free anode dual-ion battery is fabricated based on a carbon paper anode. In situ EIS, XRD and OEMS demonstrate the multi-functional effects of LiFSI on the performance of the Li-free CGDIB.     

Electrochemical performance of graphene nanosheets as anode material for lithium-ion batteries

Graphene nanosheets (GNSs) were prepared from artificial graphite by oxidation, rapid expansion and ultrasonic treatment. The morphology, structure and electrochemical performance of GNSs as anode material for lithium-ion batteries were systematically investigated by high-resolution transmission electron microscope, scanning electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy and a variety of electrochemical testing techniques. It was found that GNSs exhibited a relatively high reversible capacity of 672 mA h/g and fine cycle performance. The exchange current density of GNSs increased with the growth of cycle numbers exhibiting the peculiar electrochemical performance.     

Reason analysis for Graphite-Si/SiOx/C composite anode cycle fading and cycle improvement with PI binder

Designed Graphite-Si/SiO_x/C composite electrodes for rechargeable lithium-ion batteries are prepared with different binder of carboxymethyl cellulose-styrene butadiene rubber (CMC-SBR) and polyimide (PI). Electrode performance of composites highly depends on the selection of binder. The Si-based/graphite composite electrode containing PI binder shows very stable cycle stability with the retention higher than 95 % after 30 cycles; however, the capacity of composite electrode with CMC-SBR binder fades to less than 80 % after 20 cycles. The improvement mechanism of PI binder is characterized by SEM, EDS mapping, adhesive strength test, and electric performance test. The surface of anode film does not show crack after several cycles, and the SEI on the surface of Si/SiO_x/C particle is characterized. It is found that anode film peeing off strength matches well with the composite cycle stability. This result is further supported with cell disassembly result. We believe that improvement of anode film adhesion strength is an effective way to get stable long cycle life.     

Electrocatalytic Properties and Voltammetric Determination of Melatonin at a Nanostructured Film Electrode

A multi-wall carbon nanotube (MWNT) film coated glassy carbon electrode (GCE) was fabricated, and the electrochemical behavior of melatonin on the MWNT film coated GCE was investigated. The oxidation peak current of melatonin increases significantly and the oxidation peak position shifts positively at the MWNT film modified GCE compared to that at a bare GCE. This indicates that MWNTs feature highly effective catalysis to the electrochemical oxidation of melatonin. A simple and sensitive electroanalytical method was developed for the determination of melatonin. The oxidation peak current is proportional to the concentration of melatonin from 8×10^–8 to 1×10^–5molL^–1. The detection limit is about 2×10^–8molL^–1 for 3min accumulation. The proposed method was demonstrated to work satisfactorily with commercial capsules.     

Investigations into the electrooxidation of guanosine-5′-triphosphate at the pyrolytic graphite electrode

The electrochemical oxidation of guanosine-5-triphosphate has been investigated in phosphate-containing electrolytes in the pH range 1.5–10.9 at a pyrolytic graphite electrode by cyclic sweep voltammetry, spectral studies, bulk electrolysis and related techniques. In this pH range, the oxidation occurred in a single well-defined peak (I_a). The peak potential of oxidation peaks (E_p) was found to be dependent on pH, concentration and sweep rate. The kinetics of the UV-absorbing intermediates was followed spectrophotometrically and the decay of the intermediate occurred in a pseudo-first-order reaction. The first-order rate constants for the disappearance of the UV-absorbing intermediate have also been calculated. The products of the electrode reaction were characterized by HPLC and GC/MS. A tentative mechanism for the formation of the products has also been suggested.     

Anode materials for lithium ion batteries obtained by mild and uniformly controlled oxidation of natural graphite

Modification of natural graphite for anode materials has been a recent focus of research and development. Here we report that a common natural graphite, whose electrochemical performance is very poor, can be modified by solutions of (NH₄)₂S₂O₈, concentrated nitric acid solution, or green chemical solutions such as aqueous solutions of hydrogen peroxide and ceric sulfate. All treatments result in marked improvement of the electrochemical performance, including reversible capacity, coulombic efficiency in the first cycle, and cycling behavior. The main reason is the effective removal of active defects in natural graphite, formation of a new dense surface film consisting of oxides, improvement of the graphite stability, and introduction of more nanochannels/micropores. As a result, these changes inhibit the decomposition of electrolytes, prevent the movement of graphene planes along the a-axis direction, and provide more passages and storage sites for lithium. They are mild and the uniformity of the product can be well controlled. Pilot experiments show economic promise for their application in industry to manufacture anode materials for lithium ion batteries.Presented at the 3rd International Meeting on Advanced Batteries and Accumulators, 16–20 June 2002, Brno, Czech Republic     

Synthesis and electrochemical oxidation of 1-aryl-3,5-diethoxycarbonyl-2,6-unsubstituted 1,4-dihydropyridines

The possibility of obtaining 1-aryl-3,5-diethoxycarbonyl-2,6-unsubstituted 1,4-dihydropyridines with electronacceptor substituents in the N-phenyl radical was demonstrated. The intermediate products of their formation, viz., 2,4-bis(arylaminomethyiene)-3-phenylglutaric acid esters, were established. The electrochemical oxidation potentials of 1-aryl-2,6-unsubstituted 1,4-dihydropyridines are 100–200 mV higher than those of the 2,6-dimethyl analogs. A linear correlation between the electrochemical oxidation potentials and the Taft ⁰ constants of the substituents in the 1-phenyl ring was established.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 373–377, March, 1991.     

Evaluating the performance of organic coatings under mechanical stress using electrochemical impedance spectroscopy

Stress in coatings originating from a mechanical load imposed during exploitation is a relatively unexplored field of investigation. Paradoxically, a number of constructions and installations seem to operate under such conditions. The purpose of this work was to investigate the impact of a cyclic mechanical load exerted on coating/metal systems. It was the authors aim to verify whether repeated mechanical stress constitutes a significant factor contributing to coating degradation. Epoxy coated St3SAl steel samples were subjected to 21,000 uniaxial strain/release cycles. The maximum force applied assured maintenance within the elastic deformation region of the metal substrate. The state of the protective film was monitored using electrochemical impedance spectroscopy. The coating response to the mechanical load occurred in a three-stage process. The system subjected to the strain/release cycles revealed signs of degradation earlier compared to a non-strained, reference sample.Contribution to the 3rd Baltic Conference on Electrochemistry GDASK-SOBIESZEWO, 23–26 April 2003.Dedicated to the memory of Harry B. Mark, Jr. (February 28, 1934–March 3rd, 2003)     

Biocatalytic anode for glucose oxidation utilizing carbon nanotubes for direct electron transfer with glucose oxidase

Covalently linked layers of glucose oxidase, single-wall carbon nanotubes and poly-l-lysine on pyrolytic graphite resulted in a stable biofuel cell anode featuring direct electron transfer from the enzyme. Catalytic response observed upon addition of glucose was due to electrochemical oxidation of FADH₂ under aerobic conditions. The electrode potential depended on glucose concentration. This system has essential attributes of an anode in a mediator-free biocatalytic fuel cell.     

C K‐edge NEXAFS study of fluorocarbon formation on carbon anodes in molten NaF–AlF3–CaF2

Operational instability from processes occurring at the anode during the production of aluminum in the commercial Hall‐Héroult process may lead to an increase in undesirable fluorocarbon emissions, higher energy use, and shorter plant life. One contribution to this instability may be the possible formation of a fluorocarbon film at the electrode interface. Here, the surface composition of graphite anodes after electrolysis in molten NaF–AlF₃–CaF₂ at 990 °C is investigated for evidence of fluorocarbon formation using C K‐edge near edge X‐ray absorption fine structure. Fluorocarbon is identified on an anode surface after prolonged anode effect (very high overpotential with increased cell resistance) and also on an anode surface after normal electrolysis without anode effect. This provides evidence that fluorocarbon formation may occur prior to anode effect lowering the surface tension of the anode and therefore resulting in dewetting to contribute to the onset of the anode effect. Confirmation that such compounds form furthers our understanding of electrochemical reactions of graphite with fluoride and of the fundamental processes that occur in an aluminum smelter cell. Copyright © 2013 John Wiley & Sons, Ltd.