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 properties of graphite electrode in propylene carbonate-based electrolytes containing lithium and calcium ions

Electrochemical properties of graphite electrode are studied in propylene carbonate (PC) electrolytes containing both LiN(SO₂CF₃)₂ and Ca(N(SO₂CF₃)₂)₂ salts, and the influence of the salt concentrations on the intercalation/de-intercalation properties of graphite electrode is clarified. In the higher concentration electrolytes, reversible lithium-ion intercalation/de-intercalation at graphite electrode takes place. In contrast, only the exfoliation of graphite occurs in the lower concentration electrolytes. The effect of the salt concentrations on the electrochemical properties of graphite is discussed.     

Electrochemical intercalation of aluminium chloride in graphite in the molten sodium chloroaluminate medium

Aluminium chloride intercalation in graphite was studied by anodic oxidation of compacted graphite (rod) and graphite powder electrodes in sodium chloroaluminate melt saturated with sodium chloride at 175 °C. The studies carried out by employing both galvanostatic and cyclic voltammetric techniques had shown that the intercalation reactions take place only beyond the chlorine evolution potential of +2.2 V vs. Al on both the electrodes. The extent of intercalation reaction was directly related to the anodic potential and probably to the amount of chlorine available on the graphite anodes. In the case of graphite powder electrode, a distinctly different redox process was observed at sub-chlorine evolution potentials and this was attributed to the adsorption of chlorine on its high surface area. This finding contradicts a report in the literature that the intercalation reactions occur at potentials below chlorine evolution in the chloroaluminate melt.     

Electrochemical noise and impedance study of aluminium in weakly acid chloride solution

The electrochemical noise (EN) characteristics of pure aluminium in unbuffered potassium chloride solution and with acetic acid/sodium acetate buffer at pH 5.4 and 4.3 have been analysed to throw light on the influence of pH and of the presence of buffer at the aluminium surface on chloride ion-induced corrosion. Comparison has been made with results obtained by electrochemical impedance spectroscopy (EIS) and quantitative deductions made concerning the values of the noise resistance and the magnitude of the electrochemical impedance. Deviations between results obtained by the two experimental techniques are discussed.     

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.     

Charge cycling and impedance characterization of a polyphosphazene solid polymer electrolyte-manganese(IV) oxide intercalation cathode

The compatibility of polyphosphazene (PPZ) polymer electrolytes with MnO₂/C/SPE intercalation cathodes (IC) was investigated. Three-layered laminates of a phosphazene-based solid polymer electrolyte (SPE) film sandwiched between two MnO₂-based ICs (one preloaded with lithium) were constructed. The cathodes were fabricated by either solvent casting or compression techniques. Two different crystal forms of manganese(IV) oxide—λ-MnO₂ and γ-MnO₂—were employed, together with methoxy ethoxy ethoxy PPZ (MEEP) SPE binder material. Carbon black was employed as the electronically conductive phase. One cathode in each laminate was prepared in the ‘chemically intercalated’ form by using LiMn₂O₄ in place of MnO₂. The podand polymer, SMEP, which has better thin film mechanical properties than does MEEP, was complexed with lithium trifluoromethane sulfonate (Li triflate) and used as the SPE. Li⁺ ions were cycled galvanostatically between the two-ICs, through the phosphazene-based SPE layer. The performance of the cell was continuously monitored by electrochemical impedance spectroscopy (EIS) and by measuring the laminate thickness and voltage drop. The method of cathode fabrication (casting vs. pressing) was found to be the primary factor influencing the cycle life.     

High frequency impedance measurements of sodium solid electrolytes

Solid-state sodium batteries are currently gaining enormous interest as a lower-cost and more environmentally friendly alternative to lithium batteries. They contain significantly less critical rare elements in both the electrolyte and the active material. However, to date, there is no efficient material combination of metallic anode, cathode, and solid electrolyte for room temperature applications that does not require an additional liquid electrolyte while maintaining high energy density. NaPSiO-based glass-ceramics show high ionic conductivity at room temperature, good corrosion resistance against ambient humidity and CO₂, and stability against metallic sodium. However, the conductivity mechanisms of this promising class of materials are currently poorly understood. Herein, high frequency impedance measurements up to 10⁸ Hz shed light on the contributions of grains and grain boundaries to the total impedance, including the distribution of relaxation time constants of the fully crystallized material. In addition, analysis of the temperature dependence allows separation of electrode contributions and determination of activation energies for grain and grain boundary conductivities. Our study provides the basis for fine-tuning the stoichiometry of NaPSiO-based glass-ceramics in terms of maximizing the conductive phase fraction to optimize the performance of future solid-state sodium batteries.     

Electrochemical impedance spectroscopic investigation of the role of alkaline pre-treatment in corrosion resistance of a silane coating on magnesium alloy, ZE41

The protective performance of the coatings of bis-1,2-(triethoxysilyl) ethane (BTSE) on ZE41 magnesium alloy with different surface pre-treatments were evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 0.1 M sodium chloride solution. Electrical equivalent circuits were developed based upon hypothetical corrosion mechanisms and simulated to correspond to the experimental data. The morphology and cross section of the alloy subjected to different pre-treatments and coatings were characterized using scanning electron microscope. A specific alkaline pre-treatment of the substrate prior to the coating has been found to improve the corrosion resistance of the alloy.     

Electrochemical lithium storage of sodium titanate nanotubes and nanorods

Layered hydrated sodium titanate nanotubes are synthesized via a hydrothermal reaction in alkaline solution. The as-prepared nanotubes are calcined at different temperatures (300-600 °C) in air. The microstructure of obtained samples is characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is observed that the calcined products maintain their parent tubular morphologies below 500 °C. After calcinations at 600 °C, the hollow tubular morphology could completely be converted to the short solid nanorod morphology. In the meanwhile, the monoclinic sodium hexatitanate as a main phase is formed in nanorods, coexisted with sodium trititanate as a residual phase. The electrochemical lithium storage of obtained samples is studied by galvanostatic method and cyclic voltammetry. It is demonstrated that the nanotubes calcined at 500 °C have relatively large reversible capacity, good reversibility and excellent high rate discharge capability. The lithium intercalation process is shown to have pseudocapacitive feature caused by their layered structure and open lithium insertion tunnels, which is in favor of the high rate charge/discharge capability of sodium titanate nanotubes.     

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 impedance spectroscopy study of waterborne coatings film formation

In order to establish electrochemical impedance spectroscopy (EIS) as a viable quantitative method for characterization of latex film formation, three waterborne acrylate and styrene–acrylate polymer dispersions were periodically analyzed during a course of 2 weeks. Impedance spectra were fitted on the base of equivalent circuit consisting of a capacitor in parallel with a Warburg element representing film capacitance and the extent of ion diffusion through the film. Calculated EIS parameter values showed a decrease in Warburg diffusion over time, which is a result of particle coalescence and in agreement with the established theory of latex film formation. Atomic force microscopy (AFM) of the samples showed a smoothing of the surface and blurring of interparticle boundaries which confirmed that EIS can be used to study film formation of latex.     

Electrochemical impedance spectroscopic investigation of adhesion formation mechanism at an epoxy/aluminum interface

A theoretical method for characterizing the structure of a coating/metal interface by electrochemical impedance spectroscopy using water molecules as the probe was established. The properties of coating/metal interfaces for a series of epoxy resins with different water affinities were studied using this method. It was found that as the water affinity of the coating decreased, it became much more difficult for water molecules to reach the coating/metal interface. This suggests that, during the adhesion formation, a more hydrophobic layer is formed along the epoxy/metal interface.     

Electrochemical impedance characterization of FeSn2 electrodes for Li-ion batteries

This work reports the electrochemical characterization of a micro-scale FeSn₂ electrode in a lithium battery. The electrode is proposed as anode material for advanced lithium ion batteries due to its characteristics of high capacity (500 mAh g⁻¹) and low working voltage (0.6 V vs. Li). The electrochemical alloying process is studied by cyclic voltammetry and galvanostatic cycling while the interfacial properties are investigated by electrochemical impedance spectroscopy. The impedance measurements in combination with the galvanostatic cycling tests reveal relatively low overall impedance values and good electrochemical performance for the electrode, both in terms of delivered capacity and cycling stability, even at the higher C-rate regimes.     

Electrochemical and spectroscopic properties of dendritic cobalto-salicylaldiimine DNA biosensor

A metallodendrimer-based electrochemical DNA biosensor was constructed by a layer-by-layer assembly of cobalt(II) salicylaldiimine metallodendrimer (SDD-Co(II)) and a 21 bases oligonucleotide NH₂-5′-GAGGAGTTGGGGGAGCACATT-3′ (pDNA) on a gold electrode. The complementary oligonucleotide was 5′-AATGTGCTCCCCCAACTCCTC-3′ (tDNA). UV-visible spectra of SDD-Co(II) in 1:1 (v/v) acetone-ethanol solution showed absorption bands at 325 nm and 365-420 nm related to π-π* intra-dendrimer transitions and d-π* metal-dendrimer charge transfer transitions, respectively. Square-wave voltammetry (SWV) characterisation of the Au|SDD-Co(II)|pDNA biosensor system in phosphate buffer saline solution of pH 7.4, indicated a reversible one-electron electrochemical process with a formal potential, E°′, value of +210 mV. Electrochemical impedance spectroscopy (EIS) data confirmed that the hybridisation of the biosensor's pDNA with the tDNA to form double-stranded DNA (dsDNA) resulted in an increase of the impedimetric charge transfer resistance, R_ct, value from 6.52 to 12.85 kΩ. The limit of detection (LOD), calculated as 3σ of the background noise, and sensitivity of the sensor were 1.29 kΩ/nmol L⁻¹ and 0.34 pmol L⁻¹, respectively.     

Electrochemical impedance spectroscopy study of a surface confined redox reaction: The reduction of azobenzene on mercury in the absence of diffusion

The kinetics of azobenzene reduction on mercury electrodes in the absence of diffussional mass transport is studied by electrochemical impedance spectroscopy (EIS) in acetic acid/acetate buffered solutions at different pH values. Cyclic voltammetry experiments confirm the absence of diffusion effects and provide the values of the surface equilibrium potential. The analysis of the impedance frequency spectrums at every potential within the faradaic region conforms well the model and provides the global rate constant of the process, k_f. The potential dependence of k_f suggests the existence of an EE mechanism, with two electron transfers controlling the overall rate. The kinetic parameters of every step are obtained and their pH dependences clarify the role played by the protonation steps.     

Electrochemical behaviour of PANi/polyurethane antifouling coating in salt water studied by electrochemical impedance spectroscopy

Electrochemical behaviour of polyaniline–polyurethane (PANi–PU) antifouling coating in 3.5 wt% NaCl is studied by electrochemical impedance spectroscopy (EIS). A thick coating (∼1 mm) of 10, 15 and 20% PANi in marine grade PU, is cast over corrosion resistant aluminium alloy 2024 and its impedance characteristics are measured by EIS and compared with neat PU. On addition of 10% PANi, the impedance of the coating drastically comes down from 10⁹ to 10⁷ Ω. 20% is the maximum processable amount of PANi for the selected PU system. The coatings are exposed to 3.5 wt% NaCl and its impedance characteristics are monitored as a function of time. Changes in the impedance characteristics of the systems were found to occur as a function of the exposure time in all cases, though their evolution with time showed marked differences with PANi content. Water sorption and break down frequency are derived from the experimental results and analysed.     

Estimation of kinetic parameters of the passive state of carbon steel in mildly alkaline solutions from electrochemical impedance spectroscopic and X-ray photoelectron spectroscopic data

The unambiguous interpretation of electrochemical impedance spectra of complex systems such as passive metals and alloys in terms of an unique kinetic model is often hampered by the large number of adjustable modeling parameters. In this paper, a combination of in situ electrochemical data and ex situ surface analytical information is employed to validate the estimates of kinetic and transport parameters of the passive state of carbon steel. For the purpose, electrochemical impedance spectroscopic and X-ray photoelectron spectroscopic data for the oxidation of carbon steel in mildly alkaline solutions are quantitatively compared with the predictions of the Mixed-Conduction Model for oxide films that represent the passive oxide as an intermediate phase between magnetite and maghemite. Estimates of the kinetic rate constants at the film interfaces, as well as the diffusion coefficients and field strength in the film are obtained and their relevance for the corrosion mechanism of carbon steel is discussed.     

Electrochemical and spectroscopic evidences of corrosion inhibition of bronze by a triazole derivative

The electrochemical behavior of the bronze (Cu-8Sn in wt%) was investigated in 3% NaCl aqueous solution, in presence and in absence of a corrosion inhibitor, the 3-phenyl-1,2,4-triazole-5-thione (PTS). The inhibiting effect of the PTS was evidenced for concentrations higher than 1 mM for the cathodic process whereas its effect was clearly seen with a concentration as low as 0.1 mM for the anodic process. A significant positive shift of the corrosion potential was also observed, and its inhibiting effect increased with both its concentration and the immersion time of the sample. From voltammetry and electrochemical impedance spectroscopy experiments, the inhibiting efficiency of the PTS was found to be in the 94-99% range for 1 mM concentration. Scanning electron microscopy and X-ray energy dispersion analysis of the specimen surface show the presence of sulphur on the surface. Raman micro-spectrometry study confirms the protective effect of the PTS in aqueous solution through three types of interactions with the electrode, namely the adsorption of the inhibitor in a flat configuration, the formation of copper-thiol molecules, and when copper is released, the formation of a polymeric complex.     

Electrochemical impedance study of self-assembled layer-by-layer iron–silicotungstate/poly(ethylenimine) modified electrodes

Electrochemical impedance spectroscopy (EIS) has been used to study multilayer films containing anionic iron-substituted silicotungstate [SiW₁₁Fe^III(H₂O)O₃₉]⁵⁻ (SiW₁₁Fe) and positively charged poly(ethylenimine) self-assembled by the layer-by-layer method on glassy carbon and indium tin oxide electrodes. The effect of the charge of the outermost layer of the multilayer assembly on the electron transfer of soluble species was studied using the redox probes [Fe(CN)₆]³⁻ and [Ru(NH₃)₆]³⁺; cyclic voltammetry indicating that the surface charge has a significant effect on the process. EIS demonstrated that the electrostatic attraction or repulsion between the surface and the redox probes plays a significant role. Analysis of the impedance spectra showed that the charge transfer resistance increases with an increasing number of bilayers for both redox probes and that the porosity of the multilayer film, which varies with the electrode substrate, also has a significant effect on the electrochemical response.     

Mechanism of anodic oxidation of molybdenum in nearly-neutral electrolytes studied by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy

Anodic oxidation of molybdenum in weakly acidic, nearly neutral and weakly alkaline electrolytes was studied by voltammetric and electrochemical impedance spectroscopic measurements in a wide potential and pH range. Current vs. potential curves were found to exhibit two pseudo-Tafel regions suggesting two parallel pathways of the dissolution process. Electrochemical impedance spectra indicated the presence of at least two reaction intermediates. X-ray photoelectron spectroscopic (XPS) results pointed to the formation of an oxide containing Mo(IV), Mo(V) and Mo(VI), the exact ratio between different valence states depending on potential and pH of the solution. A physico-chemical model of the processes is proposed and a set of kinetic equations for the steady-state current vs. potential curve and the impedance response are derived. The model is found to reproduce quantitatively the current vs. potential curves and impedance spectra at a range of potentials and pH and to agree qualitatively with the XPS results. Subject to further improvement, the model could serve as a starting point for the optimization of the electrochemical fabrication of functional molybdenum oxide coatings.