Electrochemical insertion of sodium into graphite in molten sodium fluoride at 1025 °C

The electrochemical insertion of sodium into graphite was studied in molten sodium fluoride at 1025 °C. The results obtained evidenced two mechanisms for sodium insertion into graphite: sodium intercalation between the graphite layers and sodium sorption into the porosity of the material. Subsequent internal rearrangement of inserted sodium occurred, via transference from the pores towards the intercalation sites. In addition, the intercalation compound was found to undergo a fast decomposition process (k = 2.55 × 10⁻⁹ mol s⁻¹). X-ray diffraction analysis was used to confirm the formation of a high stage compound (Na_0.1C₈), the composition of which was consistent with compositions observed in the case of chemical vapor and electrochemical insertion of sodium, during experiments in the sodium perchlorate-ethylene cabonate electrolyte.     

Electrochemical insertion of alkaline ions into polyparaphenylene: effect of the crystalline structure of the host material

The intercalation of lithium and sodium ions into different materials derived from polyparaphenylene (PPP) (as synthesized PPP without a thermal treatment, PPP annealed for 36 h at 400 °C and PPP pyrolyzed at 700 °C for half an hour in argon atmosphere) was studied using the electrolyte composed of ethylene carbonate (EC) and propylene carbonate (PC) and MClO₄ as the alkaline salt (M=Li or Na). These materials exhibit various degrees of crystallinity: PPP is a semi-crystalline polymer with about 30% of cristallinity, whereas pyrolyzed PPP exhibits a totally disorganized structure. Spectroscopic characterization indicates that the configurations of the polymer chains are similar in these two materials. Then, we present in this work a comparative study of the intercalation of alkaline ions into these materials in order to specify the effect of the crystalline structure on the intercalation processes. The electrochemical capacities are close whatever the degree of crystallinity contrary to the potentials profiles of the galvanostatic curves. That indicates different intercalation processes into these materials. A two-step mechanism of the intercalation into PPP is proposed. First, the intercalation of the alkaline ions into the crystalline parts of the polymer occurs and secondly the insertion of Li⁺ and Na⁺ into the amorphous regions takes place at very low potentials.     

Electrochemical insertion of lithium ions into disordered carbons derived from reduced graphite fluoride

Both covalent (obtained by direct fluorination at high temperature) and semi-ionic carbon fluorides (synthesized at room temperature) were reduced in order to obtain disordered carbons containing very small content of fluorine and different physical properties according to the reduction treatment (chemical, thermal or electrochemical). After a physical characterization (X-ray diffraction, electron spin resonance and FT-IR spectroscopies), the electrochemical behaviours of the pristine carbon fluorides and of the treated samples were investigated during the insertion of lithium using liquid carbonate-based electrolytes (LiClO₄-EC/PC, 50:50%, v/v). Both galvanostatic and voltammetric modes were performed and revealed that the voltage profiles and the capacities differed according to the starting material and the reduction treatment. Semi-ionic carbon fluoride treated in F₂ atmosphere for 2 h at 150 °C and then chemically reduced in KOH exhibits high reversible capacities (the reversible capacity is 530 mAh g⁻¹ in the second cycle); in this case, the voltage profiles show a large flat portion at potentials lower than 0.3 V which is attributed to the insertion/deinsertion of lithium ions between the small graphene sheets and/or the absorption of pseudo metallic lithium into the microporosity of the sample. Nevertheless, a part of the lithium ions are removed at potentials higher than 0.5 V versus Li⁺/Li limiting the useful capacity.     

Electrochemical removal of sodium ion from fermented food composts

Fermented food composts, to be recycled into fertilizer and animal feed, require sodium chloride concentrations to be less than 1 wt% due to several toxicities. Electrochemical methods are used to remove sodium ions from fermented food composts. By washing the compost with tap water (with no electric current applied), 48% of the initial sodium ion is removed. With an electric current density of 3 mA/cm² (the distance between the electrodes is 16 cm), the removal efficiency increases to 96% for a 36 h operation. Major factors influencing the efficiency are the treatment time and the electric current density. Removal efficiency increases with energy demand to yield 96% removal at 60 Kwh/m². Due to the difference in relative ionic mobility, less than 9% of calcium is removed, during the same operation time, which supports the feasibility of this method.     

Electrochemical impedance spectroscopic study of the intercalation of lithium and sodium ions into polyparaphenylene in carbonate-based electrolytes

Electrochemical impedance spectroscopy is used to investigate the electrochemical intercalation of lithium and sodium ions into polyparaphenylene under galvanostatic conditions in carbonate-based electrolyte. The evolution of the charge transfer resistance was studied at various selected potentials both during the reduction and the oxidation processes in order to control the stoichiometry of the intercalated compounds. The reversibility of the intercalation process, the effect of the cycling and the stability of the intercalated materials in the electrolyte as a function of the time were examined. A significant decrease of the charge transfer resistance occurs during the intercalation. That is related to an increase of the conductive state especially for the richest compounds Na_0.46(C₆H₄) and Li_0.50(C₆H₄).     

Electrochemical study on the type of immobilized acetylcholinesterase inhibition by sodium fluoride

Homogeneous cholinesterases inhibition by sodium fluoride was studied by many authors, using conventional techniques. Controversial results however were reported on the inhibition kinetics and mechanism. In this work electrochemical methods were applied to identify the type of sodium fluoride inhibition of the immobilized acetylcholinesterase taking advantage of the capabilities of the electrochemical biosensors. The acetylcholinesterase inhibition was evaluated by current measurement, at constant potential, of the mediated by K₃[Fe(CN)₆] and of the non-mediated oxidation of thiocholine iodide, produced by enzymatic acetylthiocholine iodide hydrolysis. Direct amperometric thiocholine detection was preferred for further studies, being more sensitive.The biosensor transducer response to thiocholine iodide was investigated by cyclic and hydrodynamic voltammetry. The reversibility and pH dependence (E_1/2 = 1.08-0.06 pH) of thiocholine oxidation were demonstrated.The amperometric biosensor response to acetylthiocholine iodide obtained on a modified by acetylcholinesterase adsorption carbon electrode at +0.80 V/Ag, AgCl was characterised according to IUPAC recommendations. The enzyme reaction kinetic parameters: I_max, , and K_I were evaluated under kinetically controlled conditions, for various pH and temperature values. The reversibility of the inhibition was confirmed by dilution. Based on the kinetic analysis, the inhibition of the immobilized acetylcholinesterase by sodium fluoride was found to be of a competitive type.     

Electrochemical dissolution of hard metal alloys

Different components of hard metals such as TiC, TiN, pure Ti and the binder metals Fe, Co and Ni were investigated in neutral sodium nitrate solutions by cyclovoltammograms at current densities up to 35 A/cm². All combinations of TiN and TiC dissolve anodically. The surface of pure Ti is blocked by the anodic surface oxide which is not removed by cathodic current densities up to 30 A/cm². Cathodically all samples were conductive and showed a strong hydrogen evolution. The binder metals Fe, Co and Ni should be free of oxide films at cathodic potentials. Anodically, Co dissolves in an active state while Fe and Ni show a potential shift to the passive region. Our surface model developed for machining of Fe seems to be applicable to all other materials except pure Ti.     

Electrochemical lithium insertion into anatase-type TiO2: An in situ Raman microscopy investigation

Anatase type TiO₂ has been previously largely reported as a candidate negative electrode material for lithium-ion batteries. We report here for the first time the complete in situ Raman study of lithium insertion and de-insertion into three variously nano-sized TiO₂ anatase powders (Prolabo, ca. 80 nm, AK1, ca. 15 nm and MTi5 ca. 8 nm), of which AK1 and MTi5 show superior capacity and cyclability. From these measurements realized in a galvanostatic mode between 3 and 1 V versus Li/Li⁺, the phase transition from a tetragonal to an orthorhombic structure was clearly observed to take place at different quantities of x in Li_xTiO₂. These results confirm the extension of the solid solution domain as particle size is reduced. For the smaller TiO₂ nano-sized materials (AK1 and MTi5), a more pronounced decrease in band intensity when x > 0.3 for Li_xTiO₂, was observed and may be related to the decrease in the optical skin depth linked to the conductivity increase as lithiation proceeds.     

Electrochemical storage of energy in carbon nanotubes and nanostructured carbons

Possibilities of electrochemical energy conversion using carbon nanotubes and related materials in various systems, such as lithium batteries, supercapacitors, hydrogen storage, are considered. It is shown that for these applications the electrochemical properties of multiwalled (MWNTs) and single walled (SWNTs) nanotubes are essentially dominated by their mesoporous character. During lithium insertion into nanotubular materials a high irreversible capacity C_irr (from 460 to 1080 mAh/g) has been observed after the first cycle with a tendency to further decomposition of electrolyte with cycling. Penetration of solvated lithium ions in the accessible mesopores is at the origin of this phenomenon; an almost linear dependence has been found between the mesopore volume and C_irr. Reversible capacity for lithium insertion C_rev ranged between 220 and 780 mAh/g; however, a great divergence (hysteresis) between insertion and extraction characteristics was observed independently on the kind of nanotubes and oxygen content. Amount of lithium stored by electrostatic attraction is negligible in comparison to real redox reactions which for thermodynamic reasons present linear variation of potential, especially during deinsertion (pseudocapacitive effects). During positive polarization, i.e., removal of lithium, resistivity of the electrode also gradually increases. Due to the open network of mesopores formed by the nanotubes entanglement, and consequently an easily accessible electrode-electrolyte interface, nanotubular materials are quite adapted for supercapacitor electrodes in various electrolytic solutions. High values of capacitance (80 F/g) have been obtained in 6 M KOH for materials with a surface area of only ca. 430 m²/g. Capacitance values have been enhanced either by additional oxygenated functionalisation of nanotubes (130 F/g) or by conducting polypyrrole (PPy) electrodeposition where the maximum values reached 170 F/g. The next domain of energy storage in the carbon nanostructures is the accumulation of hydrogen by the electrochemical decomposition of aqueous alkaline medium on a negatively polarized carbon electrode in ambient conditions. For SWNTs only moderate values (below 0.5 wt.% of H₂) have been found, while for activated carbons with highly developed surface area of 1500 m²/g, the amount of reversibly sorbed hydrogen was ca. 2 wt.%, noticeably larger than under dihydrogen pressure (only 0.4 wt.% for the same material at 70 bar and 273 K). The enhancement observed for the activated carbon is interpreted by the formation of nascent hydrogen during water reduction which penetrates easily in the available carbon nanopores. The values obtained by this method are comparable to those of metallic alloys, such as LaNi₅ for example.     

无水硅酸钠催化酯交换合成碳酸二甲酯

以廉价的Na2Si O3·9H2O为原料,通过简单的焙烧处理,制备了系列无水硅酸钠,并将其作为固体碱催化剂应用于碳酸乙烯酯(EC)与CH3OH酯交换合成碳酸二甲酯(DMC)的反应。采用TG-DTA、XRD和Hammett指示剂法对无水Na2Si O3进行表征。结果表明,焙烧温度对无水Na2Si O3的碱强度、总碱量及催化活性没有显著影响。当焙烧温度为200℃时,样品(Na2Si O3-200)的碱强度(Ho)为15.0~18.4,总碱量为10.9 mmol/g。以Na2Si O3-200为催化剂,考察了原料配比、温度和时间对酯交换合成DMC反应的影响。当CH3OH与EC的摩尔比为10∶1,在65℃反应2 h后,EC转化率与DMC收率可分别达到89%和88%。即使在室温条件下,Na2Si O3-200也能有效地催化EC与甲醇酯交换反应的进行。此外,经过4次使用后,Na2Si O3-200的催化活性没有出现明显下降的趋势。     

海藻酸钠的浓度对聚苯胺/海藻酸钠电化学性能的影响

以海藻酸钠(SA)为软模板,采用原位氧化聚合法制备了聚苯胺/海藻酸钠(PANI/SA)电极材料,研究了SA的浓度对其结构、形貌及电化学性能的影响。利用傅里叶变换红外光谱(FTIR)和扫描电镜(SEM)对所制PANI/SA的结构和形貌进行了表征。在1 mol/L H2SO4溶液中,通过循环伏安法(CV)、恒电流充放电(CD)和交流阻抗法(EIS)测试了电极材料的电化学性能。结果表明:PANI/SA的比电容随聚合体系中SA质量分数的增大先升高后降低,w(SA)=0.01%时,PANI/SA为由纳米纤维相互交织缠绕的网状结构,其比电容最高(459.7F/g),较纯PANI提高了20.8%。     

Variation of Cell Voltage with Reaction Time in Electrochemical Synthesis Process of Sodium Dichromate

To address the problems existing in the traditional production technique of sodium dichromate, a new green technology of producing sodium dichromate with an electrochemical synthesis method was studied. Using a self‐made electrosynthesis reactor of pure titanium and stainless steel, with a multiple‐unit metal oxides combination anode, a cathode of stainless steel, and a reinforcing combination cation exchange membrane with perfluorosulfonic and perfluorocarboxylic polymers, experiments were carried out on the direct electrochemical synthesis of sodium dichromate from sodium chromate. From the experimental results and electrochemical reaction principles, it was shown that the electrochemical synthesis reaction process of sodium dichromate may be quantitatively determined from the variation of the cell voltage measured macroscopically with reaction time. Cell voltages were experimentally measured at different initial sodium chromate concentrations in the anolyte, and the dependence of the cell voltage on reaction time was discussed. The mathematical model of the variation of cell voltage with reaction time and the change rate equation of cell voltage were established, and satisfactorily formulated the change law of cell voltage in the electrochemical synthesis process of sodium dichromate.     

新方法合成十二烷基苯磺酸钠

本实验首次尝试采用氨基磺酸作为磺化剂,羧甲基纤维素钠为助溶剂成功合成了十二烷基苯磺酸钠。所得产品无色、透明、无异味。该工艺具有设备简单,对设备腐蚀性小,反应比落温和,反应速度易于控制,原料便于运输的优点。     

Macrokinetic Study of the Electrochemical Synthesis Process of Sodium Dichromate

In order to address the problems existing in the traditional production techniques for sodium dichromate, a new green technology for producing sodium dichromate using an electrochemical synthesis method was studied. Using a self‐made electrosynthesis reactor composed of pure titanium and stainless steel, with a multiple‐unit metal oxide combination anode, a stainless steel cathode, and a reinforcing combination cation exchange membrane, the kinetic experiments were carried out on the direct electrochemical synthesis reaction of sodium dichromate from sodium chromate. The kinetic data were experimentally measured at different temperatures and different initial sodium chromate concentrations of anolyte. The results show that the electrochemical synthesis process presents complicated zero‐order reaction kinetic characteristics, with the reaction rate constant depending on the initial sodium chromate concentration. The macrokinetic equation of the electrochemical synthesis reaction was established, and some kinetic parameters were calculated. The apparent activation energy is less than that of the ordinary chemical reaction, and this shows that the electrochemical synthesis of sodium dichromate has great development potential.     

Electrochemical responses from surface oxides present on HNO3-treated carbons

Carbons of different surface oxide compositions, which were prepared from HNO₃ oxidation of polyacrylonitrile-based carbon fabric followed by heat treatment at different temperatures, were subjected to electrochemical response analysis using a voltammetric method. Temperature programmed desorption was employed to analyze the surface oxide composition. A significant amount of unstable oxides that can be irreversibly removed with reduction using a cathodic potential sweep were found to be present on the oxidized carbons. About 2.6 electrons are required to remove one oxygen atom from the carbon surface, and this may be the first quantitative study ever reported. After the initial cathodic sweep, no irreversible reduction peak response was observed for the subsequent electrochemical measurements. The capacitive performance of the carbons is related to the population of stable CO-desorbing complexes that can be determined with thermal desorption after the cathodic sweep. The present work has shown that carbon capacitance increases from 170 to 190 F/g with an increase of the CO-desorbing oxides from 1.31 to 1.56 mmol/g. This means that each stable CO-desorbing oxide is able to store 0.8 electron charge per volt within the potential window employed, suggesting the effective role of CO-desorbing oxides in improving the capacitance.     

Carbothermic reduction of zinc sulfide in the presence of sodium carbonate

The carbothermic reduction of zinc sulfide in the presence of sodium carbonate has been studied using X-ray diffractometry (XRD), thermogravimetric analysis system (TGA), atomic absorption spectrometry (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), elemental analyzer (EA) and scanning electron microscopy (SEM). Experimental results of AAS indicated that the reduction in the presence of sodium carbonate proceeded significantly faster than in the presence of calcium carbonate and its sulfur fixation efficiency was acceptable. The results of XRD revealed that zinc sulfide was first transformed from β-type to α-type, then reduced to zinc vapor. The sequence of the variation of sodium containing material was found to be . A reaction mechanism is proposed to interpret the overall reaction. Results of kinetic study indicated that the rate of reduction could be increased by increasing the reaction temperature, the initial molar ratio of C/ZnS, or the initial molar ratio of Na₂CO₃/ZnS. The rate was also found to be increased with a decrease in sample height, size of C aggregate, size of Na₂CO₃ aggregate or the initial bulk density. The reduction rate, however, was found not to be influenced by the argon flow rate. An empirical expression of the initial rate of zinc yield has been determined.     

Electrochemical capacitors based on highly porous carbons prepared by KOH activation

Various coal and pitch-derived carbonaceous materials were activated for 5 h at 800 °C using potassium hydroxide and 1:4 component ratio. Porosity development of the resultant activated carbons (ACs) was assessed by N₂ sorption at 77 K and their capability of the charge accumulation in electric double layer was determined using galvanostatic, voltammetric and impedance spectroscopy techniques. ACs produced from different precursors are all microporous in character but differ in terms of the total pore volume (from 1.05 to 1.61 cm³ g⁻¹), BET surface area (from 1900 to 3200 m² g⁻¹) and pore size distribution. Very promising capacitance values, ranging from 200 to 320 F g⁻¹, have been found for these materials operating in acidic 1 mol l⁻¹ H₂SO₄ electrolytic solution. The variations in the electrochemical behavior (charge propagation, self-discharge, frequency response) are considered in relation to the porous texture characteristics, elemental composition but also possible effect of structural ordering due to various precursor materials used. Cycling investigation of all the capacitors has been also performed to compare ability of the charge accumulation for different carbon materials during subsequent cycles.     

Electrochemical reduction of cerium oxide into metal

The Fray Farthing and Chen (FFC) and Ono and Suzuki (OS) processes were developed for the reduction of titanium oxide to titanium metal by electrolysis in high temperature molten alkali chloride salts. The possible transposition to CeO₂ reduction is considered in this study. Present work clarifies, by electro-analytical techniques, the reduction pathway leading to the metal. The reduction of CeO₂ into metal was feasible via an indirect mechanism. Electrolyses on 10 g of CeO₂ were carried out to evaluate the electrochemical process efficiency. Ca metal is electrodeposited at the cathode from CaCl₂–KCl solvent and reacts chemically with ceria to form not only metallic cerium, but also cerium oxychloride.     

Chemical and electrochemical intercalation of lithium into boronated carbons

Boron-substituted carbons have been produced by chemical vapour deposition from acetylene and boron trichloride precursors at 1140°C. Li intercalation was investigated, chemically from the Li vapour and electrochemically in Li–carbon cells. In both cases, the amount of intercalated Li increases with the boron content of the carbon, up to a value of 13 at.% boron. Above this value, the intercalation of lithium is not so efficient. This behaviour is understood by assuming that boron, which acts as an electron acceptor and hence favours the intercalation of electron donors like lithium, can enter substitutionally into the carbon lattice up to a certain limit, which is close to 13 at.% for the materials deposited at 1140°C.