The electrochemical preparation and properties of ionic alkali metal-and NR4-graphite intercalation compounds in organic electrolytes

The preparation of graphite-alkali metal-solvent or graphite-NR₄-Solvent ternary intercalation compounds by electrochemical reduction of graphite is described. The reduction occurs stagewise and the complete formation of a defined stage can be recognized by a striking change in the potential of the graphite electrode. Under certain conditions the stoichiometry of intercalation compounds (e.g. C₄₈K(DMSO), C₂₄K(DMSO), C₁₈Li(DME) can be calculated from the coulombs consumed by a weighed graphite electrode until a step in potential occurs. The electrochemical intercalation of alkali metals into graphite is reversible. It may be reversed with a coulombic efficiency up to practically 100%, e.g. for C_nK(DMSO). A considerable co-intercalation of alkali metal salts from the electrolyte was not observed. From a preparative point of view, a great advantage over the direct chemical reduction of graphite is that a desired degree of intercalation can be specifically prepared. A coulometric determination of the intercalated cations is possible by way of an electrochemical reoxidation. Potential measurements of electrochemically prepared intercalation compounds with a well-defined degree of intercalation represent a simple method for investigating the thermodynamics of these reactions. Furthermore, by means of dynamic electrochemical methods, such as cyclic voltammetry, information about the kinetics of intercalation reactions is obtainable.     

Role of the steric effects for the progress of alkanes intercalation in CsC24

The ability of various alkanes to be intercalated into the second-stage cesium graphitide, CsC₂₄, was investigated using the two-bulb technique. The structural changes occurring during intercalation were studied by real-time neutron diffraction. At moderate pressure, the intercalation of large molecules such as n-butane, n-pentane and n-hexane simultaneously leads to a mixture of a second-stage ternary phase and a first-stage binary phase “CsC₈”. Under increased pressure a pure first-stage ternary phase is finally formed. The intercalation of cyclopentane occurs in two steps: a pure second-stage ternary phase is first observed, whereas CsC₈ only appears at about half filling. For the smallest alkane, CH₄, complete ternarization leads to a second-stage ternary phase together with a small amount of an enriched second-stage binary derivative. Owing to the formation of binary domains rich in alkali metal during the ternarization, the cesium density is smaller in the second-stage ternary phase than in the starting binary compound. The in-plane cesium concentration of the ternary phase strongly depends on the projected surface area of the alkane molecule. During invasion of the interlamellar space, large molecules induce a decrease of the average distance between cesium ions. Electrostatic repulsive energy between cations is minimized through expulsion of cesium in binary domains. A pleated-layer model with canted fronts is presented, in order to account both for the various phases existing within each grain, and for the structural transformations caused by the intercalation reaction.     

In situ FTIR study of the decomposition of N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ionic liquid during cathodic polarization of lithium and graphite electrodes

In this work we analyzed the cathodic reactions of an important ionic liquid (IL) based electrolyte solution, namely lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)/N-methyl-N-methylpyrrolidinium (BMP) TFSI. In situ FTIR spectroscopy was used for the analysis of gaseous products of the electrochemical decomposition of this IL solution during cathodic polarization of lithium metal and graphite electrodes. The main volatile product of the reductive decomposition of the anion in these BMPTFSI solutions is trifluoromethane. BMP cations decompose to mixtures of tertiary amines and hydrocarbons. The composition of the products is influenced by the nature of the anode material. Graphite possesses a catalytic activity in the electroreduction process of BMP cations which occurs along with their intercalation into the graphite structure. The liquid phase after cathodic polarization of graphite electrodes was analyzed by multinuclear NMR spectroscopy coupled with FTIR spectroscopy. ¹⁵N NMR and FTIR spectra revealed an increase in the Li cations content in the electrolyte solution, as a result of BMP cations decomposition during repeated cycling of graphite electrodes.     

The synthesis of graphene sheets with controlled thickness and order using surfactant-assisted electrochemical processes

An electrochemical route is reported for the production of graphene sheets using the following steps: electrochemical intercalation of sodium dodecyl sulfate (SDS) into graphite followed by electrochemical exfoliation of a SDS-intercalated graphite electrode. These electrochemical processes yield a stable colloidal graphene/SDS suspension. The potential value for SDS intercalation into graphite plays an important role in determining the structural order, size, and number of layers of synthesized graphene sheets. Raman spectroscopy and transmission electron microscopy results indicate that graphene sheets with the highest structural order and lowest number of layers can be obtained by using relatively high intercalation potentials. Average size and thickness of graphene sheets prepared at high potentials for SDS intercalation into graphite were measured to be about 500 and 1 nm, respectively, indicating presence of graphene sheets as thin as a monolayer. UV–vis spectra of graphene/SDS suspensions show that a large amount of the reduced form of graphene flakes is obtained after successive electrochemical intercalation and exfoliation processes.     

Lithium iron phosphate/carbon nanocomposite film cathodes for high energy lithium ion batteries

This paper reports sol–gel derived nanostructured LiFePO₄/carbon nanocomposite film cathodes exhibiting enhanced electrochemical properties and cyclic stabilities. LiFePO₄/carbon films were obtained by spreading sol on Pt coated Si wafer followed by ambient drying overnight and annealing/pyrolysis at elevated temperature in nitrogen. Uniform and crack-free LiFePO₄/carbon nanocomposite films were readily obtained and showed olivine phase as determined by means of X-Ray Diffractometry. The electrochemical characterization revealed that, at a current density of 200 mA/g (1.2 C), the nanocomposite film cathodes demonstrated an initial lithium-ion intercalation capacity of 312 mAh/g, and 218 mAh/g after 20 cycles, exceeding the theoretical storage capacity of conventional LiFePO₄ electrode. Such enhanced Li-ion intercalation performance could be attributed to the nanocomposite structure with fine crystallite size below 20 nm as well as the poor crystallinity which provides a partially open structure allowing easy mass transport and volume change associated with Li-ion intercalation. Moreover the surface defect introduced by carbon nanocoating could also effectively facilitate the charge transfer and phase transitions.     

Catalytic oxidation of CO on CuOx revisited: Impact of the surface state on the apparent kinetic parameters

Analysis of kinetic features of the copper oxides reduction by CO pulses as related to mechanism of CO catalytic oxidation by oxygen combined with monitoring the state of the surface by an electrochemical technique using a solid electrolyte—Pyrex glass and high resolution TEM data on the defect structure of CuO allowed us to suggest a partially “flexible” model of CuO surface. This model, with a due regard for the data of FTIR spectroscopy of adsorbed CO test molecules, assigns the most active surface sites able to coordinate highly reactive CO and O forms to clusters of Cu⁺ cations located at outlets of extended defects (dislocations, twins). Variation of the number, size and structure of these clusters under the reaction medium effect allows explaining the difference between quasi-steady and true steady states of copper oxides in catalytic CO oxidation reaction as well as the difference between kinetic parameters of reaction estimated at quasi-steady and constant states of the surface following Boreskov's approach. Kinetic features of the reaction agree with the Langmuir–Hinshelwood reaction mechanism operating for clustered defect centers of CuO.     

Polyaniline/Montmorillonite Nanocomposites Obtained by In Situ Intercalation and Oxidative Polymerization in Cationic Modified-Clay (Sodium, Copper and Iron)

Polyaniline/montmorillonite nanocomposites (PANI/M) were obtained by intercalation of aniline monomer into M modified with different cations and subsequent oxidative polymerization of the aniline. The modified-clay was prepared by ion exchange of sodium, copper and iron cations in the clay (Na–M, Cu–M and Fe–M respectively). Infrared spectroscopy confirms the electrostatic interaction between the oxidized PANI and the negatively charged surface of the clay. X-ray diffraction analysis provides structural information of the prepared materials. The nanocomposites were characterized by transmission electron microscopy and their thermal degradation was investigated by thermogravimetric analysis. The weight loss suggests that the PANI chains in the nanocomposites have higher thermal stability than pure PANI. The electrical conductivity of the nanocomposites increased between 12 and 24 times with respect to the pure M and this increase was dependent on the cation-modification. The electrochemical behavior of the polymers extracted from the nanocomposites was studied by cyclic voltammetry and a good electrochemical response was observed.     

Electrochemical Quartz Microbalance characterization of Ni(OH)2-based thin film electrodes

The use of the Electrochemical Quartz Crystal Microbalance (EQCM) to study the proton intercalation performance of thin film Ni(OH)₂ layers, nowadays widely used as cathode electrode material in rechargeable Ni(OH)₂-based battery systems such as NiMH and NiCd, is reviewed. In addition, the impact of incorporating foreign metals in these layers on the electrochemical performance will be highlighted.Using EQCM much information can be obtained, as both the electrochemical response and accompanying mass changes can be measured simultaneously. EQCM was extensively used to investigate the effect of the conditions on the formation of Ni(OH)₂ thin layers, the α-to-β modification changes and the details of the redox mechanism. The proposed redox mechanisms differ in whether H⁺ or OH⁻ is transferred, the reactants and/or products are hydrated and cations from the solution take part in the reaction.By incorporation of other metals in the structure, the characteristics of thin Ni(OH)₂ layers can be tuned. This affects the oxidation and reduction potential, the reversibility, the stability of the structure and the oxygen evolution side reaction. Co²⁺ and Fe²⁺ were shown to replace Ni-sites in the hydrous oxide lattice, thereby forming very dense structures with higher stability. However, structural changes still occur in most cases. Due to this inhomogeneity, the layers are usually a combination of different structures, depending on the distribution of the incorporated metal(s). Suppression of the oxygen evolution reaction is reported for Co, Pb, Pd, Zn and Mn. The effects of Co and Mn are shown to depend on the incorporated amount. Co shifts the standard redox potential for the oxygen evolution reaction towards more cathodic potentials and decreases the oxygen overpotential significantly. Light-weight rare-earth elements also catalyze the oxygen evolution reaction.     

Charge storage mechanism of sonochemically prepared MnO2 as supercapacitor electrode: Effects of physisorbed water and proton conduction

The charge storage mechanism of nanostructured hydrated manganese dioxide, as a supercapacitor electrode, was investigated with respect to the role of amount of hydrates on the electrolyte cations diffusion. The MnO₂ materials (γ- and layered types), prepared by a novel ultrasonic aided procedure. Thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy were employed to characterize the water content of the samples. The samples then were heat-treated in air for 2 h at 70, 100 and 150 °C to prepare different γ- and L-series of electrodes with various amounts of hydrates. To determine the role of the protonic conduction on the charge storage mechanism, the electrochemical properties of the electrodes were investigated in two different electrolyte pH values of 3.3 and 6.Compared to γ25, the higher specific capacitance of L25, especially in more acidic electrolytes, is attributed to the higher amount of physically adsorbed water molecules and their contribution in diffusion process. Furthermore, it is clearly demonstrated that the total electrochemical performance of the systems under consideration is also influenced by the crystalline structure of the prepared electrodes, especially when the size of the tunnels limits the intercalation of cations.Analyzing the results of cyclic voltammetry and electrochemical impedance spectroscopy for both series of the electrodes, revealed that, increasing the heat-treatment temperature makes the charge-transfer resistance increase and the Warburg impedance decrease. This effect can be attributed to the more amount of surface physisorbed water lost by the higher heat-treatment temperatures.     

Oxygen storage in O2/Pt/YSZ cell

The phenomenon of electrochemical promotion of catalysis (EPOC) was initially characterized as fully reversible, i.e. the catalyst restores its initial activity after current interruption. However, it has been recently demonstrated that after prolonged anodic polarization an unusual promoted activity is observed for a certain time after current interruption. This phenomenon has been reported as permanent electrochemical promotion of catalysis (P-EPOC).In this work the oxygen storage reported as responsible of P-EPOC has been investigated by transient electrochemical techniques using an O₂(g)Pt/YSZ cell. A model has been proposed involving place interchange of Pt and O species in Pt/YSZ system. This seems to be induced by the strong lateral interaction of Pt–O surface dipoles and by increasing electric field at the Pt/YSZ interface. Such a rearranged oxide, so-called “phase oxide” can have a lower free energy than the initial monolayer oxide. This cooperative interaction of Pt and O species can lead to further thickening of this “phase oxide” especially at high temperature and potentials (currents). Furthermore, as the charge involved in this oxide thickening shows a t^1/2 dependency, the process seems to be diffusion controlled.     

Quasi in situ XPS study of electrochemical oxidation and reduction of highly oriented pyrolytic graphite in [1-ethyl-3-methylimidazolium][BF4] electrolytes

Highly oriented pyrolytic graphite (HOPG) electrodes were electrochemically oxidised and reduced in 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF₄] electrolytes and studied by X-ray photoelectron spectroscopy (XPS). Sample preparation and transfer have been performed under inert nitrogen atmosphere in a preparation chamber directly attached to an ultra high vacuum system. After electrochemical treatment, both, the electrolyte and the electrode surface were investigated. While on the oxidised HOPG surface the core levels of the detected elements shift towards lower energies, on the reduced samples a shift towards higher binding energies is observed. These shifts refer to a Fermi level shift proving that graphite intercalation compounds were formed. Intercalation occurs together with co-intercalation of the ionic liquid. XPS analysis of the ionic liquids before and after electrochemical treatment reveals changes in electrolyte composition. The influence of impurities on electrochemical behaviour and XPS data is discussed.     

Behavior of diffusing elements from an integrated cathode of an electrochemical reduction process

The electrochemical reduction process for spent oxide fuel is operated in a molten salt bath and adopts an integrated cathode in which the oxides to be reduced act as a reactive cathode in the molten salt electrolyte cell. Heat-generating radioisotopes in the spent oxide fuel such as cesium and strontium are dissolved in the molten salt and diffuse from the integrated cathode. However, the behavior of the dissolved cations has not been clarified under an electrochemical reduction condition. In this work, the reduction potentials of cesium, strontium, and barium were measured in a molten LiCl-3 wt% Li₂O salt and their mass transfer behavior was compared with two current conditions on the cell. The concentration changes of the cations in the molten salt phase were measured and no significant differences on the dissolution behavior were found with respect to the current. However, under a continued current condition, the removal of the high heat-generating elements requires more time than the complete reduction of metal oxide due to the slow rate of diffusion.     

Electrochemical behavior of electrodes comprising micro- and nano-sized particles of LiNi0.5Mn1.5O4: A comparative study

A comparative analysis of the electrochemical behavior and Li⁺ transport characteristics of thin-layer LiNi_0.5Mn_1.5O₄ intercalation electrodes comprised of micro- or nano-sized particles in standard Li salt solutions, was carried out and is reported herein. These electrodes were free of any conductive additives and polymeric binder in order to avoid their complex impact on the electrochemical response of the active mass. It was clearly established that the electrodes prepared from nanoparticles of the active mass show faster kinetics and a more reversible electrochemical behavior compared to the electrodes comprising microparticles.

The response of the nanoparticles to electrochemical techniques such as linear sweep voltammetry and potentiostatic intermittent titration (PITT) is characterized by high resolution. Thus D versus E could be calculated very precisely. It was encouraging to realize the good performance of the electrodes comprising nano-LiMn_1.5Ni_0.5O₄, in spite of their high surface area and their high operating voltage.     

On the electric double-layer structure at carbon electrode/organic electrolyte solution interface analyzed by ac impedance and electrochemical quartz-crystal microbalance responses

ac impedance and electrochemical quartz crystal microbalance (EQCM) techniques have been applied to analyze the structure of electric double-layer formed at carbon/organic electrolyte solution interface using a sputtered carbon electrode. The mass changes caused by electrochemical adsorption (accumulation) of ions have been estimated in the solutions of propylene carbonate (PC) dissolving tetrafluoroborate (BF₄⁻) salts of lithium (Li⁺), tetraethylammonium (TEA⁺) and tetra-n-butylammonium (TBA⁺) cations. The observed mass changes during the cathodic polarization in the solutions containing TEA⁺ and TBA⁺ were well consistent with those expected by the calculation based on mono-layer adsorption of the cations with giving the consideration to the surface roughness. On the other hand, the mass change observed in the solution containing Li⁺ salt showed that the solvation of Li⁺ with three or four molecules of PC would be the charge compensation species at the interface. Comparison of the quantity of the electricity passed during the EQCM measurements with that from theoretical calculation with simple Helmholtz-layer model revealed that the major part of the double-layer capacitance would be based on the electrostatic polarization of the solvent molecule directly adsorbed at the carbon surface.     

Electrochemical Nanogravimetric Studies of Ruthenium(III) Trichloride Microcrystals

Ruthenium (III) trichlorid solid crystals have been mechanically attached to gold and paraffin-impregnated graphite surfaces and studied in the presence of aqueous solutions of M⁺Cl⁻ electrolytes, where M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and K₂SO₄ by cyclic voltammetry and electrochemical nanogravimetry at a quartz crystal microbalance (EQCM). The electrochemical reduction of the layered α-RuCl₃ microcrystals causes drastic changes in the composition and the structure of crystals. The comparison of the current—potential and surface mass change—potential functions belonging to the first reduction-reoxidation cycle with the subsequent ones reveals that the simple intercalation scheme described in the literature cannot be entirely valid. During the first reduction step at ca. 0.2 V vs. SCE the charge consumption is substantially higher than in the course of the further potential cycling, and the simultaneous rapid and intense mass decrease indicates that considerable chemical and structural transformations occurs. Although a loss of the surface mass cannot be entirely excluded, the frequency increase most likely is not related to the dissolution of the microcrystals, however, large amounts of water molecules and—to a much smaller extent—chloride ions leave the crystal phase, and in fact a new material, which remains strongly attached to the gold or graphite surface, is formed. The extremely high frequency change at the first reduction process during the first cycle is most likely related to the stress effect originating in the phase transition of the surface layer and/or the removal of the water rigidly coupled to the surface into voids of the immobilized microcrystals. Depending on the amount of microcrystals on the electrode surface and the experimental conditions (the nature and concentration of the contacting electrolyte, scan rate, and potential range) used, after the “break-in” cycle stable electrochemical and nanogravimetric responses develop. The several reduction and reoxidation pairs of waves in the cyclic voltammograms and the simultaneous mass changes are in connection with the wide variety of intercalation reactions and complex formation during the electrochemical transformations. The mass change was reversible, in general, during reduction mass increase, while during oxidation mass decrease occurred at medium electrolyte concentrations in three or more steps. The mass excursions are rather complicated, involving different mass increase/decrease regions as a function of potential and the composition of the contacting solution. Taking into account the layered structure of RuCl₃, the electrochemical reduction can be explained as an intercalation reaction in that mixed valence intercalation phases with a general formula K Ru [Ru Cl_3z-y (H₂O)_y] • dH₂O are formed from z (RuCl₃ · H₂O). The reduction/reoxidation waves are related to the redox transformations of Ru(III) to Ru(II) sites and the insertion/deinsertion of cations and water molecules, while the composition of the polynuclear complexes and the structure of microcrystals change.     

Electrochemical behaviour of the cell Pb/PEO40. Pb(ClO4)2/Pb

The electrochemical behaviour of the polymer-based cell Pb/(PEO)₄₀. Pb(ClO₄)₂/Pb has been investigated over the temperature range 20–160°C by means of impedance spectroscopy and dc polarization methods. The bulk resistance and the charge transfer resistance display a linear trend in an Arrhenius plot. The transference numbers have been evaluated by an electrochemical method, and compared with those obtained from a preliminary experiment with Tubandt's method. These measurements indicate that both cations and anions are mobile, with a transference number for Pb²⁺ lower than 0.1 between 70 and 130°C.     

Electrochemical intercalation of lithium into a natural graphite anode in quaternary ammonium-based ionic liquid electrolytes

Electrochemical intercalation of lithium into a natural graphite anode was investigated in electrolytes based on a room temperature ionic liquid consisting of trimethyl-n-hexylammonium (TMHA) cation and bis(trifluoromethanesulfone) imide (TFSI) anion. Graphite electrode was less prone to forming effective passivation film in 1 M LiTFSI/TMHA-TFSI ionic electrolyte. Reversible intercalation/de-intercalation of TMHA cations into/from the graphene interlayer was confirmed by using cyclic voltammetry, galvanostatic measurements, and ex situ X-ray diffraction technique. Addition of 20 vol% chloroethylenene carbonate (Cl-EC), ethylene carbonate (EC), vinyl carbonate (VC), or ethylene sulfite (ES) into the ionic electrolyte resulted in the formation of solid electrolyte interface (SEI) film prior to TMHA intercalation and allowed the formation of Li-C₆ graphite interlayer compound. In the ionic electrolyte containing 20 vol% Cl-EC, the natural graphite anode exhibited excellent electrochemical behavior with 352.9 mAh/g discharge capacity and 87.1% coulombic efficiency at the first cycle. A stable reversible capacity of around 360 mAh/g was obtained in the initial 20 cycles without any noticeable capacity loss. Mechanisms concerning the significant electrochemical improvement of the graphite anode were discussed. Ac impedance and SEM studies demonstrated the formation of a thin, homogenous, compact and more conductive SEI layer on the graphite electrode surface.     

Preparation and characterization of poly(vinyl chloride)/organoclay nanocomposites by in situ intercalation

In this article, poly(vinyl chloride) (PVC)–organoclay nanocomposites were prepared via in situ polymerization intercalation and melt blending intercalation, respectively. Their nanostructures were characterized by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). Differences in the morphologies of the PVC hybrids prepared by in situ intercalation and melt intercalation were investigated. In addition, three kinds of organoclay were used, in order to consider the effect of the interlayer environment on intercalation. The results show that ammonium cations have a great influence on the hybrids prepared by melt intercalation, while they have no obvious effect on the nanostructures of the composites produced via in situ intercalation due to its distinctive process and its mechanism. Copyright © 2004 Society of Chemical Industry     

X-ray diffraction studies of electrochemical graphite intercalation compounds of ionic liquids

Electrochemical intercalation studies are used to characterize a series of ionic liquids composed of a variety of cationic and anionic species. Electrochemically, the ionic liquids are characterized by cyclic voltammograms and charge–discharge experiments for the intercalation and de-intercalation of the various cationic and anionic species into graphite. X-ray structure analysis is also performed to determine the relationship between the electrochemical behaviour of the ionic liquids, and the formation of intercalated graphitic compounds. Two different types of imidazolium cations are studied, specifically the di- and trisubstituted imidazolium. These cations are paired with the following anions: tetrafluoroborate, hexafluorophosphate, bis(trifluoromethanesulfonyl)imide, bis(perfluoroethanesulfonyl)imide, nitrate and hydrogen sulfate. Results indicate stronger intercalation chemistry for the trisubstituted imidazoliums, correlating with the greater charge–discharge efficiencies found for these types of ionic liquids. Many of the anions exhibit very poor charge–discharge efficiencies, correlating to very poorly formed graphite intercalates. The exception to this is the hydrogen sulfate intercalate, which had low charge–discharge efficiencies but formed a well defined graphite intercalate. Only the imide based anions exhibited both high charge–discharge efficiencies and the formation of a clearly defined graphite intercalate.     

A study of the lithium intercalation into nanoparticles of LiCoO<Subscript>2</Subscript> from an aqueous solution

Nanoparticles of lithium cobalt oxide (LiCoO₂) were synthesized by means of a citrate sol–gel combustion route. The particles were characterized by scanning and transmission electron microscopies (SEM and TEM), energy-dispersive X-ray spectroscopy, and X-ray diffraction (XRD) measurements. Near spherical nanoparticles of around 100 nm were observed in SEM and TEM micrographs. XRD data indicated that the as-prepared nanoparticles presented pure phase of LiCoO₂ with R-3m symmetry. The kinetics of electrochemical intercalation of lithium into the nanoparticles were investigated by means of cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance spectroscopy (EIS) with special emphasis on the application potential as cathode material for aqueous rechargeable lithium batteries. CV studies of the nanoparticles at slow scan rate of 0.1 mV s⁻¹ between 600 and 820 mV versus Ag/AgCl, demonstrated that the nanoparticles represented well-defined reversible peaks. The non-linear chemical diffusion of lithium into the nanoparticles was explored by EIS. In this regards, the results were discussed based on an equivalent circuit, distinguishing the kinetic properties of lithium intercalation. The kinetic parameters of lithium intercalation were obtained using the equivalent circuit, which were in good agreement with the experimental results. The changes of kinetic parameters of lithium intercalation with potential were also discussed in detail.