CPE Element Research Articles

Electrical analysis and molecular dynamics of anion in ferroelastic [(CH3)4P]2CoBr4 crystal near high-temperature phase transitions

The recent technological advances require an aligned innovation in the materials used for the construction of electronic devices. Hybrid materials play a important role in the optoelectronic applications because they combine both the organic and inorganic components. To this end, the [(CH3)4P]2CoBr4 compound was successfully synthesized using the slow evaporation solution growth approach at room temperature. In-depth research has been conducted on the structural, morphological, vibrationnal and electrical characteristics. The [(CH3)4P]2CoBr4 compound crystallises in the monclinic system with P121/C1 space group. The examination of SEM images reveals that the majority of grains have an average size of around 4 μm with irregularly shaped and non-spherical particles. The measured Raman spectra demonstrate the coexistence of two transition phases located at 373 K and 428 K. In fact, these observations suggest that the anionic components play a significant role in influencing the phase transitions mechanism. Analysis of the complex impedance spectra reveals that the electrical characteristics of the material is strongly dependent on both frequency and temperature. That indicates the existence of a relaxation phenomenon and of a semiconductor-type behavior. One single semicircle is detectable in the Nyquist plots of the complex impedance spectra, which can be satisfactorily fitted with a combination of R//C//CPE elements assigned to the bulk response. Therefore, we can conclude from this behavior that the sample is electrically homogeneous. The dependency of s(T) on temperature also shows that the correlated barrier hopping and the non-overlapping small polar on tunneling model are the responsible mechanisms for the AC conduction in [(CH3)4P]2CoBr4.

Performance and Electrochemical Behavior of LSM Based Fuel Electrode Materials Under High Temperature Electrolysis Conditions

High-temperature solid oxide electrolysis cells (SOECs) have gained considerable attention due to more favourable thermodynamic and electrochemical kinetic conditions over low-temperature electrolysis. The SOECs can be used for the production of hydrogen from steam electrolysis, syngas (H2 + CO) from co-electrolysis, and carbon mono-oxide from pure CO2 electrolysis. Despite several advantages, the long-term durability of SOECs is still an issue. The long-term durability of SOEC depends on the stability of electrodes as well as on the operational conditions, for example current load, temperature, and fuel gas composition. The conventional cermet-based fuel electrode i.e. Ni-YSZ shows good initial performance, however, it experiences severe degradation during long-term due to Ni- agglomeration and migration away from the electrolyte. Therefore, searching for new electrode materials is crucial in order to enhance the overall performance and durability of SOECs.In this work, we have considered Lanthanum strontium manganite-based perovskite oxides as fuel electrodes i.e. La0.6Sr0.4MnO3 (LSM). LSM is very stable chemically under oxidizing atmosphere, however, it undergoes phase transformation into a Ruddlesden-Popper (La0.6Sr0.4)2MnO4±δ phase under reducing atmosphere. We have first prepared the electrolyte-supported single cells using 8YSZ electrolyte and LSM+YSZ/LSM oxygen electrodes. The single cells were then electrochemically characterized using AC- and DC-techniques under steam electrolysis, co-electrolysis, and CO2-electrolysis conditions in 800-900 °C temperature range. Moreover, the electrochemical behaviour of LSM+GDC (50:50) and LSM+YSZ (50:50) composite electrodes containing single cells were also investigated. The LSM and LSM+GDC fuel electrode containing single cells show good electrochemical performance in all three electrolysis modes. However, lower performance is observed for LSM+YSZ fuel electrode containing single cell. For example, a current density of 997, 1025, and 511 mA.cm-2 at 1.5 V, are obtained for LSM, LSM+GDC, and LSM+YSZ fuel electrode containing single cells respectively, with 50% N2 and 50% H2O feed gas mixture at 900 °C. Furthermore, the impedance spectra were also recorded for all these cells under OCV and polarization conditions, and fitted with an equivalent circuit model using an inductor, a series resistance and 4 R//CPE elements. The impedance spectra vary significantly with the gas compositions. The detailed electrochemical results will be presented and discussed in detail.

Multichannel Electrochemical Impedance Spectroscopy and Equivalentcircuit Synthesis of a Kw-Scale Vanadium Redox Flow Battery

An original multichannel Electrochemical Impedance Spectroscopy (EIS) system operating at high bias current and suitable for kW-class Vanadium Redox Flow Batteries (VRFBs) is presented. Power and signal connections, whose stray parameters affected measurements, required a careful optimization and calibration in the implementation of the measurement chain. In any cases, electromagnetic interferences set an upper limit to the applied stimulus frequency that limited the set of measurement data at each bias point. The work provides early results obtained with this EIS system, which were used to identify an equivalent circuit of the whole stack in which each cell is represented with a dynamic Thévenin equivalent, i.e. the series of a voltage source, a variable resistor and a variable RC loop. The dependence of the equivalent parameters on the operating conditions has been experimentally analyzed, confirming that only the RC-loop resistance is strongly affected by mass transport. In addition, more complex equivalent circuits have been proposed, i.e. a Randles cell modified with CPE, a Randles cell modified with CPE and Warburg element and a two-loop model of ZARC elements. The inclusion of the constant phase element constitutes a substantial improvement in the fit, Fig. 1. Conversely, the addition of the Warburg element, which aims to model the mass transfer in the electrochemical process, does not produce significant effects for the frequencies at which we have experimental data. To our knowledge, this is the first dynamic equivalent circuit of a whole stack that has been validated against EIS measurements taken on a real industrial-scale VRFB. Building on the measurement methodology here presented, advanced online state of health of industrial flow batteries can be developed and implemented.

Time-fractional approach to the electrochemical impedance: The Displacement current

• Analysis of the connection between PNP model and displacement current. • Electrical conductivity and evidence of the anomalous diffusion. • Implications of the fractional extension to the total current density. • Extensions of the PNP model and connection with CPE elements. We establish, in general terms, the conditions to be satisfied by a time-fractional approach formulation of the Poisson-Nernst-Planck model in order to guarantee that the total current across the sample be solenoidal, as required by the Maxwell equations. Only in this case the electric impedance of a cell can be determined as the ratio between the applied difference of potential and the current across the cell. We show that in the case of anomalous diffusion, the model predicts for the electric impedance of the cell a constant phase element behaviour in the low frequency region. In the parametric curve of the reactance versus the resistance, the slope coincides with the order of the fractional time derivative.

Structural phase transition, optical and electrical properties of the hybrid material [(C2H5)4N]2ZnI4

The bis (tetra-ethylammonium) tetraiodo-zincate (II) was prepared by slow evaporation an aqueous stoichiometric mixture of tetra-ethylammonium iodide and zinc iodide at room temperature. This compound has been characterized by X-ray diffraction (XRD), thermal analysis, UV–vis spectroscopy and complex impedance spectroscopy. Single crystal X-ray diffraction analysis shows that the title compound crystallizes in the tetragonal system with space group P4¯21m.The differential thermal analysis (DTA) and temperature-controlled X-ray diffraction study reveal a reversible first order phase transition at about 444 ​K between low temperature-tetragonal and high temperature-orthorhombic forms of [(C2H5)4N]2ZnI4. Below 444 ​K, the low temperature form exhibits two linear regimes of thermal expansion with a change in the slope around 373 ​K which is ascribed to a second-order phase transition. The diffuse reflectance indicates the existence of an optical direct transition with the band gap energy is equal to 3.9 ​eV. The spectra of complex impedances obtained in the temperature range from 403 to 468 ​K were modelled by a combination of R//CPE elements. . A change of conduction regime around 444 ​K is noted in the variation of the Ln (σDC.T) versus reciprocal temperature. In both regimes, the conductivity obeys the Arrhenius law with activation energies of 1.37 ​eV in regime I and 2.37 ​eV in regime II. Frequency dependence of the AC conductivity followed the universal Jonscher's power law.

Multi-pathway mechanism of polydopamine film formation at vertically aligned diamondised boron-doped carbon nanowalls

Boron-doped carbon nanowall (B:CNW) electrodes were used as a platform for studying the electropolymerisation of dopamine. Due to the unique properties of B:CNW, including the fast charge-transfer kinetics and high surface conductivity, a high degree of reversibility of redox reactions was achieved. Three separated redox peaks were observed on voltammograms and attributed to three fundamental reactions in the dopamine polymerisation mechanism: dopamine/dopamine quinone, leukodopaminechrome/dopaminechrome, and dihydroxyindole/indolequinone. The mechanism was also supported by the density functional theory calculations of the single point energy of the polydopamine structural units. Moreover, the electrochemical impedance spectroscopy experiments strongly suggest that the majority of th epolymerisation occurs only in the narrow range of potentials between +0.0 and +0.4 V vs. the silver chloride reference electrode. The concept of a resistor-like CPE element is introduced to facilitate the description of the electrochemical properties of B:CNW electrodes in a neutral electrolyte. Further, it is shown that electropolymerisation differs significantly as a function of pH. In acidic environments (pH 3–6) mostly the dopamine/dopamine quinone reaction can be seen, whereas in a more alkaline pH (8–10), the leukodopaminechrome/dopaminechrome reaction becomes dominant. The dihydroxyindole/indolequinone redox pair is seen only in a short pH interval between 7 and 8. Additionally, the kinetics of polymerisation is significantly boosted when the pH is more than 7. Together, these results provide insight into the complexity of the formation of polydopamine and can assist in better controlling the properties of electropolymerised dopamine films.

Synthesis, structural and electrical characterization of a new organic inorganic bromide: [(C3H7)4N]2CoBr4.

A new organic inorganic hybrid [TPA]2CoBr4, where TPA = [(C3H7)4N]+ (i.e., tetra-propyl-ammonium) compound has been synthesized by slow evaporation method at room temperature. Single crystal X-ray diffraction (SC-XRD), X-ray powder diffraction (XRPD), thermal analyses, vibrational and complex impedance spectroscopy have been used to characterize both structural, thermal, electrical properties. [TPA]2CoBr4 crystallizes in the monoclinic system (C2/c space group) with the following cell parameters: a = 33.145 (5) Å, b = 14.234 (3) Å, c = 15.081 (2) Å and β = 110.207 (5)°. In the crystal structure, the organic TPA cations which form layers stacked along the a-axis, are separated from each other by inorganic tetrahedral [CoBr4]2− anions. The XRPD pattern confirms both the high purity of the sample and the crystalline nature of the powder. The differential scanning calorimetry (DSC) analysis shows an endothermic peak at 394 K upon heating which is ascribed to a structural phase transition since no decomposition of the titled compound is evidenced by thermogravimetric analysis. The ac conductivity and the dielectric properties confirm the presence of the phase transition. At the structural phase transition around 394 K, a change from a quantum mechanical tunneling to a correlated barrier hopping conduction models is determined from the temperature dependence of the exponent s of the Jonscher's power law. The analysis of complex impedance spectra shows that the electrical properties of the material are heavily dependent on frequency and temperature, indicating a relaxation phenomenon and semiconductor-type behavior. One single semicircle is detectable in the Nyquist plots of the complex impedance spectra which can be satisfactorily fitted with a combination R//CPE elements assigned to the bulk response. This behavior suggests that the sample is electrically homogeneous. Capacitance analysis proves the high effective permittivity at radio frequencies in the sample.

CPE Analysis from Cyclic Voltammetry and Electrochemical Impedance Spectroscopy

The electric properties of an electrode can be evaluated by the determination of the capacitance of the interface. However, the literature reports a large panel of experimental protocols for capacitance determination, which involve either cyclic voltammetry (CV) or electrochemical impedance spectroscopy (EIS). On the one hand, CV is a routine electrochemical technique widely used for the determination of electric properties of super capacitors or energy storage devices. The capacitive charge is determined through the variation of the scan rate in a non-faradaic current domain and is often used to estimate the performances of these electrodes. On the other hand, EIS is used for the determination of interfacial processes and also provides precise information on the double layer capacitance and the oxide capacitance in the case of more complex systems. However, substantial discrepancies in the assumptions are made between both techniques as in CV, the system is usually considered as ideal (and the current-scan rate relationship is assumed to remain linear at all frequencies), whereas in EIS, the system is usually considered as non-ideal and a CPE element is introduced. Although both techniques are used to provide similar information on the capacitive characteristics of electrochemical interfaces, a clear correlation between the results obtained with each technique is not straightforward. In fact, recent work from Allagui et al. [1]revealed that the time-voltage relationship cannot be used when the system is considered as ideal.In this work, electrochemical impedance spectroscopy and cyclic voltammetry were performed on well-known systems showing capacitive behaviours on a wide frequency domain and over a wide potential range: a blocking electrode (Au) and an electrode with a thin oxide film (Al). The capacitive charge was determined using cyclic voltammetry from the analysis of the conventional current to scan rate linear relationship and EIS measurement. In CV measurements, the calculated capacitance predominantly stems from the double layer capacitance for the considered scan rate ranges (low frequency range). Additionally, it was shown that the value of the determined capacitance is scan-rate dependent. Such experimental results highlight the arbitrary character of the scan rate ranges used in the literature as they usually remain in low frequency range. In fact, careful selection of scan rate is critical and should be adapted to the electrode considered, particularly for systems involving a thin oxide film as the Cdl or the Cox response depends on the frequency range. Similarly, the EIS response shows that the capacitance determination cannot be performed without a large error when a non-ideal behaviour is experimentally obtained. From a physical point of view, such outcome in this frequency domain is the results of observations made in the selected time domain [2].

Revisiting cyclic voltammetry and electrochemical impedance spectroscopy analysis for capacitance measurements

The electric properties of an electrode can be evaluated by the determination of the capacitance. However, the literature reports a large panel of experimental protocols for capacitance determination, which involve either cyclic voltammetry (CV) or electrochemical impedance spectroscopy (EIS). In fact, substantial discrepancies in the assumptions are made between both techniques as in CV, the system is usually considered as ideal (and the current-scan rate relationship is assumed to remain linear at all frequencies), whereas in EIS, the system is usually considered as non-ideal and a CPE element is introduced. In this work, the electric properties of two electrode configurations, namely an Au blocking electrode and an Al electrode with a thin oxide film were investigated using EIS and CV. The capacitive domain was determined for both systems assuming the presence of an ideal and non-ideal system. The results collected from CV revealed a limited linearity between the current and scan rate for both electrode configurations. Finally, a CPE element was introduced to determine the capacitance from CV data and compared to EIS results.

Genome-wide analysis reveals a switch in the translational program upon oocyte meiotic resumption.

During oocyte maturation, changes in gene expression depend exclusively on translation and degradation of maternal mRNAs rather than transcription. Execution of this translation program is essential for assembling the molecular machinery required for meiotic progression, fertilization, and embryo development. With the present study, we used a RiboTag/RNA-Seq approach to explore the timing of maternal mRNA translation in quiescent oocytes as well as in oocytes progressing through the first meiotic division. This genome-wide analysis reveals a global switch in maternal mRNA translation coinciding with oocyte re-entry into the meiotic cell cycle. Messenger RNAs whose translation is highly active in quiescent oocytes invariably become repressed during meiotic re-entry, whereas transcripts repressed in quiescent oocytes become activated. Experimentally, we have defined the exact timing of the switch and the repressive function of CPE elements, and identified a novel role for CPEB1 in maintaining constitutive translation of a large group of maternal mRNAs during maturation.

Electrochemical impedance study of commercial LiNi0.80Co0.15Al0.05O2 electrodes as a function of state of charge and aging

A comprehensive electrochemical impedance study is conducted on LiNi0.80Co0.15Al0.05O2 electrode material as a function of state-of-charge and aging. Electrodes are harvested from four commercial batteries with different state-of-health conditions. Odd random phase electrochemical impedance spectroscopy and the symmetric cell approach are applied in this work in order to obtain reliable impedance results. An equivalent electrical circuit model is constructed. The parameters related to the generalized finite-space Warburg element, the CPE element and the charge transfer resistance are further interpreted. Valuable information is obtained and closely linked to the physical phenomena. The charge transfer resistance has been proved to be the most reliable parameter for the estimation of state-of-health.

Identification of CPE and GAIT elements in 3’UTR of macrophage migration inhibitory factor (MIF) involved in inflammatory response induced by LPS in Ciona robusta

Innate immune responses face infectious microorganisms by inducing inflammatory responses. Multiple genes within distinct functional categories are coordinately and temporally regulated by transcriptional ‘on’ and ‘off’ switches that account for the specificity of gene expression in response to external stimuli. Mechanisms that control transcriptional and post-transcriptional regulation are important in coordinating the initiation and resolution of inflammation. Macrophage migration inhibitory factor (MIF) is an important cytokine that, in Ciona robusta, is related to inflammatory response. It is well known that in C. robusta, formerly known as Ciona intestinalis, the pharynx is involved in the inflammatory reaction induced by lipopolysaccharide (LPS) injection in the body wall. Using this biological system, we describe the identification of two C. robusta MIFs (CrMIF1 and CrMIF2). The phylogenetic tree and modeling support a close relationship with vertebrate MIF family members. CrMIF1 and CrMIF2 possess two evolutionally conserved catalytic sites: a tautomerase and an oxidoreductase site with a conserved CXXC motif. Real-time PCR analysis shows a prompt expression induced by LPS inoculation in CrMIF1 and a late upregulation of CrMIF2 and in silico analyses of 3’UTR show a cis-acting GAIT element and a CPE element in 3’-UTR, which are not present in the 3’-UTR of CrMIF1, suggesting that different transcriptional and post-transcriptional control mechanisms are involved in the regulation of gene expression of MIF during inflammatory response in C. robusta.

Investigation of disorder and its effect on electrical transport in electrochemically doped polymer devices by current–voltage and impedance spectroscopy

We report on the analysis of temperature-dependent current–voltage characteristics and impedance measurements of electrochemically doped poly(3-methylthiophene) devices at different doping levels. The extent of doping is carefully tailored such that only the bulk-limited transport mechanism prevails. A transition from exponentially distributed trap-limited transport to trap-free space-charge-limited current is observed in current–voltage conduction upon increasing the doping. The obtained trap densities (3.2 × 1016 cm−3 and 8.6 × 1015 cm−3) and trap energies (31.7 meV and 16.6 meV) for different devices signify the variation in disorder with doping, which is later supported by impedance measurements. Impedance–frequency data for various devices can not be explained using the parallel resistance–capacitance (RC) model in the equivalent circuit. However, this was established by incorporating a constant phase element Q (CPE) instead of the capacitance parameter. It should be emphasized that low doping devices in particular are best simulated with two CPE elements, while the data related to other devices are fitted well with a single CPE element. It is also observed from evaluated circuit parameters that the spatial inhomogeneity and disorder are the cause of variability in different samples, which has an excellent correlation with the temperature-dependent current–voltage characteristics.

KLF6 cooperates with NUR77 and SF1 to activate the human INSL3 promoter in mouse MA-10 leydig cells.

Insulin-like 3 (INSL3), a Leydig cell-specific hormone, is essential for testis descent during foetal life and bone metabolism in adults. Despite its essential roles in male reproductive and bone health, very little is known regarding its transcriptional regulation in Leydig cells. To date, few transcription factors have been shown to activate INSL3 promoter activity: the nuclear receptors AR, NUR77, COUP-TFII and SF1. To identify additional regulators, we have isolated and performed a detailed analysis of a 1.1 kb human INSL3 promoter fragment. Through 5' progressive deletions and site-directed mutagenesis, we have mapped a 10 bp element responsible for about 80% of INSL3 promoter activity in Leydig cells. This element is identical to the CPE element of the placental-specific glycoprotein-5 (PSG5) promoter that is recognized by the developmental regulator Krüppel-like factor 6 (KLF6). Using PCR and western blotting, we found that KLF6 is expressed in several Leydig and Sertoli cell lines. Furthermore, immunohistochemistry on adult mouse testis revealed the presence of KLF6 in the nuclei of both Leydig and Sertoli cells. KLF6 binds to the 10 bp KLF element at -108 bp and activates the -1.1 kb human, but not the mouse, INSL3 promoter. KLF6-mediated activation of the human INSL3 promoter required an intact KLF element as well as Leydig/Sertoli-enriched factors because KLF6 did not stimulate the human INSL3 promoter activity in CV-1 fibroblast cells. Consistent with this, we found that KLF6 transcriptionally cooperates with NUR77 and SF1. Collectively, our results identify KLF6 as a regulator of human INSL3 transcription.

MicroRNA-22 Gates Long-Term Heterosynaptic Plasticity in Aplysia through Presynaptic Regulation of CPEB and Downstream Targets.

The maintenance phase of memory-related long-term facilitation (LTF) of synapses between sensory and motor neurons of the gill-withdrawal reflex of Aplysia depends on a serotonin (5-HT)-triggered presynaptic upregulation of CPEB, a functional prion that regulates local protein synthesis at the synapse. The mechanisms whereby serotonin regulates CPEB levels in presynaptic sensory neurons are not known. Here, we describe a sensory neuron-specific microRNA 22 (miR-22) that has multiple binding sites on the mRNA of CPEB and inhibits it in the basal state. Serotonin triggers MAPK/Erk-dependent downregulation of miR-22, thereby upregulating the expression of CPEB, which in turn regulates, through functional CPE elements, the presynaptic expression of atypical PKC (aPKC), another candidate regulator of memory maintenance. Our findings support a model in which the neurotransmitter-triggered downregulation of miR-22 coordinates the regulation of genes contributing synergistically to the long-term maintenance of memory-related synaptic plasticity.

Ionic mobility and attenuation of ultrasound in doped cerium trifluoride crystals

Dielectric properties of CeF3 crystals doped by aluminum, erbium, and holmium were studied in the temperature range of 290–450 K and frequency range of 25 Hz-1 MHz. Cole-Cole diagrams are constructed. An equivalent circuit consisting of a resistance, capacitance and CPE element is suggested for interpretation of the obtained results. Ionic movement parameters are determined as dependent on the temperature. An acoustooptic method was used to measure ultrasound attenuation in the frequency range of 200–800 MHz.

The dielectric behavior of a thermoelectric treated B 2O 3–Li 2O–Nb 2O 5 glass

A transparent glass with the composition 60B 2O 3–30Li 2O–10Nb 2O 5 (mol%) was prepared by the melt-quenching technique. Glass–ceramics, containing LiNbO 3 ferroelectric crystallites, were obtained by heat-treatment (HT) above 500 °C, with and without the presence of an external electric field. The dielectric properties of the glass and glass–ceramic were investigated, as a function of temperature (270–315 K), in the 10 mHz–32 MHz frequency range. The presence of an external electric field, during the heating process, improves the formation of LiNbO 3 crystallites. The rise of the treatment temperature and the applied field, during the heat-treatment, leads to a decrease in the dc electric conductivity ( σ dc), indicating a decrease of the charge carriers number. The dielectric permittivity ( ε′) values (300 K;1 kHz) are between 16.25 and 18.83, with the exception of the 550 °C HT sample that presents a ε′ value of 11.25. An electric equivalent circuit composed by an R in parallel with a CPE element was used to adjust the dielectric data. The results reflect the important role carried out by the heat-treatment and the electric field during the HT in the electric properties of glass–ceramics.

Characterization of anodic silicon oxide films grown in room temperature ionic liquids

The electroformation of silicon oxide was performed in two room temperature ionic liquids (RTIL), 1-butyl-3-methyl-imidazolium bis(trifluoromethane sulfonyl) imide (BMITFSI) and N- n-butyl- N-methylpiperidinium bis(trifluoromethane sulfonyl) imide (BMPTFSI). This phenomenon was studied by electrochemical techniques and it was observed that the oxide growth follows a high-field mechanism. X-ray Photoelectron Spectroscopy experiments have shown that a non-stoichiometric oxide film was formed, related to the low water content present in both RTILs (<30 ppm). The roughness values obtained by using AFM technique of the silicon surface after etching with HF was 1.5 nm (RMS). The electrochemical impedance spectroscopy at low frequencies range was interpreted as a resistance in parallel with a CPE element, the capacitance obtained was associated with the dielectric nature of the oxide formed and the resistance was interpreted considering the chemical dissolution of the oxide by the presence of the TFSI anion. The CPE element was associated with the surface roughness and the very thin oxide film obtained.

Electrical properties of a graphite-rich quartzite from a former lower continental crust exposed in the Serre San Bruno, Calabria (southern Italy)

In this study electrical properties of a graphite-rich quartzite from the former lower crust exposed in the Serre San Bruno, Calabria (southern Italy) have been analysed by means of complex electrical conductivity measurements in the frequency range 10 −3 to 10 6 Hz on plug samples of 25 mm in diameter and 22 mm high. The samples were identified as a former oil sandstones containing up to 12–15% flake-like graphite, which present highly metamorphosed relics of its former hydrocarbon filling. Though high concentrations of graphite were detected, the graphite quartzite is highly resistive. The spatial ordering of isolated graphite grains like tiles prevent the generation of interconnected electrical pathways. These isolated good conductors are interconnected by electrolyte bridges, thus causing the high polarizability, indicating redox reactions at the electrolyte/graphite interfaces that could be modelled using CPE elements. The type of graphite ordering was caused by the early formation of isolated oil droplets in the reservoir sandstone. After high-grade metamorphism and shearing, they appear as flat graphite grains in the foliation plane and were smeared together because of the rigidity of sillimanite and quartz, the main rock constituents. Thus this graphite quartzite is by no means a candidate to enhance the electrical conductivity of the deep continental crust. Syngenetic graphite in former lower crustal rocks is largely resistant against varying geochemical conditions during prograde and retrograde metamorphism. This had also been shown for graphite that is contained in normal metapelites as accessory mineral, or enriched in former black shales where it may cause high conductivity [Jödicke, H., Kruhl, J.H., Ballhaus, C., Giese, P., Untiedt, J., 2004. Syngenetic, thin graphite-rich horizons in lower crustal rocks from the Serre San Bruno, Calabria (Italy), and implications for the nature of high-conducting deep crustal layers. PEPI 141, 37–58; Nover, G., 2005. Electrical properties of crustal and mantle rocks—a review of laboratory measurements and their explanations. Surv. Geophys. 26 (5), 593–651].

Electrochemical dissolution and passivation of nickel powder randomly dispersed in a graphite + polypropylene matrix

Nickel powder (10% in weight) randomly dispersed in a graphite (40%) + polypropylene (50%) matrix was studied as a composite electrode in a sulphuric-sulphate acid medium by means of voltammetry and Electrochemical Impedance Spectroscopy (EIS). Under voltammetric conditions, this thermoplastic material shows an electrochemical behaviour similar to nickel metal. However, under EIS measurement conditions the electrodissolution behaviour of the nickel powder is partially limited by the diffusion of the anions in the electrolyte in order to electroneutralize the excess of electrogenerated positive charge. The low frequency range of the measured impedance spectra can be interpreted as a parallel arrangement of a capacitive and a Warburg impedance, instead of a CPE element initially considered in the fitting. Values for CPE exponents lower than 0.85 are interpreted as a competition between a capacitive/transport contribution to the overall impedance response.