EMF response of neutral-carrier based ion-sensitive field effect transistors with membranes free of ionic sites

Response characteristics of field effect transistors covered by membranes containing neutral carriers and an organic liquid but neither a polymer nor ionic sites were determined. The commercially available 2-nitrophenyl octyl ether and dioctyl sebacate, both frequently used plasticizers for poly(vinyl chloride) ISE membranes, had impurity concentrations large enough to lead to small emf responses. Upon removal of these impurities by HPLC, no emf responses were detectable any more, which seems to be due to a negligibly small permselective uptake of primary ions into the membrane and not the high impedance of the membrane bulk. The use of membranes of the more polar nitrobenzene and 2-fluoro-2′-nitrodiphenyl ether resulted in small but transient emf responses. The absence of ionic sites led however to the failure of permselectivity and resulted in co-extraction even of the highly hydrophilic chloride ions. Liquid membranes containing neutral carriers but no ionic sites are thus not only unsuitable for ISEs due to their high impedance, but also because it is very likely that either the failure of permselectivity occurs or the concentration of primary ions permselectivity entering the membrane is too small to determine the phase boundary potential.     

Warburg type impedance in thin layer unsupported electrochemical cells containing associated salts

The Warburg type impedances in thin layer unsupported electrochemical cells containing associated salts where the electrode reaction is kinetically reversible and the equilibrium between the associated neutral species and charge carriers is fast, are derived using the approach based on bulk electroneutrality. The extension of the approach to cells with slow electrode reactions where the use of the Nernst equation is not allowed, is also demonstrated by using the Butler-Volmer equation.     

Some electrochemical phenomena in the mixed ionic-electronic conductors

The impedance of the interphase β-Ag_2+δS/Pt has been investigated. An important feature is the presence of Warburg impedance. The Warburg coefficient can be expressed in terms of the ionic and electronic conductivity of β-Ag_2+δS.     

Impedance spectroscopic determination of effect of temperature on the transport resistances of an electro-electrodialysis cell used for concentration of hydriodic acid

Effect of temperature on different transport resistances of an electro-electrodialysis (EED) cell used for concentration of hydriodic acid (HI) was found by equivalent circuit modeling of the measured impedance response of the cell. The EED cell consisted of two compartments separated by Nafion 117 membrane and each compartment had a platinum electrode. Both the compartments were filled with aqueous solution of 55?wt% HI containing 0.5?M iodine. Impedance measurements were carried out at four different temperatures in the range of 308?C353?K. Equivalent circuit for the cell consisted of a resistor for ohmic resistance of the cell, a Warburg element for the resistance due to diffusion boundary layer and a constant phase element for the resistance to transport of ions due to non-electro neutral heterogeneous transport layer at the membrane. Effect of temperature on impedance due to heterogeneous transport was lower than the Warburg impedance and the solution and membrane resistance. Effective capacitance of the heterogeneous transport layer was found to reduce with temperature. The dynamics of the heterogeneous transport layer along with the diffusional boundary layer were found to reduce with increase in the cell operating temperature.     

Influence of flash sintering on the ionic conductivity of 8 mol% yttria stabilized zirconia

The ionic conductivity of flash-sintered, polycrystalline 8 mol% yttria stabilized zirconia (8YSZ) was enhanced compared with that of conventionally-sintered specimens. Flash sintering was carried out at a furnace temperature of 850 °C with an electric field of 100 V cm^–1 to initiate flash. The current density limit was varied between 60 and 100 mA mm^–2. Post-flash impedance measurements over the range 215–900 °C showed that both bulk and grain boundary conductivities had increased with the increased current density limit which was set prior to flash. The conductivity increases post-flash were ionic, not electronic, although electronic conductivity probably occurred, in addition to ionic conductivity, during flash. The conductivity increases were not attributable to sample densification or microstructural changes. The higher ionic conductivities are attributed to a change in YSZ defect structure that led to an increased concentration of mobile charge carriers; possible explanations for this are discussed.     

二元离子液体基染料敏化太阳能电池性能的研究

对6种用于染料敏化太阳能电池的二元离子液体电解质进行了考察,电池的光电转化效率在1.39%～4.98%,其中,1-乙基-3-甲基咪唑四氟硼酸盐/碘化1-丁基-3-甲基咪唑类电解质具有最高的光电转化效率。对这种二元离子液体电解质体系进一步优化,测试了不同碘浓度下相应染料敏化太阳能电池的效率、电化学阻抗谱(EIS)和紫外-可见吸收光谱(UV-vis)。结果表明,随着碘单质浓度的增大,铂-电解质界面的传荷电阻(RPt)、TiO2-电解质表面的传荷电阻(Rct)和瓦尔堡阻抗(Zw)逐渐减小,而电解质对紫外光的吸收逐渐增大,在AM1.5的条件下,当碘单质的浓度为0.25 mol/L时电池效率最高,达到5.20%。     

Multiscale modeling of the ionic conductivity of acceptor doped ceria

The ionic conductivity of acceptor doped ceria is strongly influenced by grain boundaries and interfaces, with most experiments showing a conductivity decrease in these regions. Classical models explain this observation by the formation of space charge layers, that are depleted of mobile ionic charge carriers. However, some experiments demonstrate an increase in ionic conductivity and recent models show that the space charge layers can also be enriched in mobile ionic species. Because of these discrepancies, it is still not certain whether nanocrystalline or thin film ceria can offer superior ionic conductivity or not. Recently, we have demonstrated by means of Monte Carlo simulations that the ionic conductivity in space charge layers can indeed exceed the bulk value. In this work, we combine these Monte Carlo simulations with a continuum model to predict charge carrier concentration profiles. This multiscale approach allows for a realistic prediction of the grain boundary ionic conductivity.     

Impedance study of the charge transport at poly-o-phenylenediamine film electrodes

The measured low-frequency capacitance of the polymer film was proportional to the film thickness and showed a maximum at the formal potential of the polymer. We analyzed the relation between the capacitance and potential by taking into account the contribution of an interaction between electroactive sites. The Warburg coefficient and the width of a linear 45 ° region in an impedance diagram showed each their minima near the formal potential. These impedance data were interpreted in terms of the diffusion-migration transport of both electron and anion through the film. The results of the impedance analysis suggested that electron transport was ensured by interchain electron hopping. From the combinations of the measured kinetic parameters, we inferred that one of the two charge carriers moved much faster than the other. The obtained diffusion coefficient showed a maximum in the vicinity of a voltammetric current peak; this corresponded with the potential dependence of a coupled diffusion coefficient expected in the extreme case of electron-transport control.     

Ionic conductivities and high resolution microscopic evaluation of grain and grain boundaries of cerium-based codoped solid electrolytes

Doped CeGdO and codoped CeGdOSmO compositions were synthesized, giving rise to nanoparticulate powders. Ionic conductivities at bulk and grain boundaries of the sintered samples were determined, exhibiting increased conductivity in the samaria-codoped samples. Scanning electron microscopy (SEM) showed a significant reduction in the grain size of samaria-codoped electrolytes. This reduced grain size of the codoped samples caused a reduction in Schottky barrier height, increasing oxygen vacancy concentration in the space-charge layer of the grain boundary and culminating in greater ionic conductivity in the boundary region. For the gadolinium doped samples, high resolution transmission electron microscopy images at grains showed the presence of large cluster of defects (nanodomains), hindering the movement of charge carriers and reducing ionic conductivity. However, the samaria-codoped system displayed better homogeneity at atomic level, resulting in reduced oxygen vacancy ordering and, consequently, smaller nanodomains and higher bulk (grain) conductivity. The reduced grain sizes and smaller nanodomains caused by codoping favor the ionic conductivity of ceria-based ceramics, doped with gadolinia and codoped with samaria.     

Anodic growth of passive layers on steel rebars in an alkaline medium simulating the concrete pores

Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) techniques have been used to study passive layers anodically grown on steel rebars in an aqueous alkaline solution simulating the electrolyte of the concrete pores. Nyquist diagrams recorded by EIS at the different stabilization potentials show a diffusional tail at low frequencies. The analysis of the impedance measurements has been made by means of an equivalent circuit with a Warburg component and within the framework of the point defect model (PDM) theory. It is observed that the calculated concentration of vacancies is a function of the potential in accordance with the theoretical prediction of the PDM.     

Effect of Acceptor (Mg) Concentration on the Resistance Degradation Behavior in Acceptor (Mg)-Doped BaTiO3 Bulk Ceramics: I. Impedance Analysis

Degradation behavior of the electrical resistance of acceptor (Mg)-doped BaTiO₃ ceramics was contrasted against different acceptor concentrations. Coarse-grained specimens with uniform grain sizes and different acceptor concentrations were prepared by sintering both in air and a reducing atmosphere. The specimens sintered under both atmospheres showed similar trends in the degradation behavior with a critical dependence on the acceptor concentration. The time to degradation decreased systematically with the increase of acceptor concentration. An impedance spectroscopy study was conducted on these samples at various temperatures. An equivalent circuit analysis of these data was considered, from which bulk and grain-boundary conductivity was determined for each composition as a function of grain size. Attempts were made also to determine the ionic conductivity for the different samples as a function of temperature and doping concentration.     

Exploring point defects and trap states in undoped SrTiO3 single crystals

The defect chemistry and electronic trapping energies in undoped single crystalline SrTiO₃ were examined by electrochemical impedance spectroscopy at low (25–160 °C) and intermediate (500–700 °C) temperatures. Electronic and ionic conductivity as well as chemical capacitance values were obtained with a transmission line equivalent circuit. Impedance spectroscopy at low temperatures was used to quantify trapping energies of main ionic defects. Particularly the chemical capacitance is shown to be a highly valuable, though hardly used tool for establishing a defect model based solely on electrochemical measurements. It is very sensitive for minority charge carriers and can thus unveil otherwise hardly accessible defect concentrations. The chemical capacitance analysis yields a valence dependent acceptor concentration in the ppm range for the investigated samples. Complementary positron annihilation lifetime spectroscopy (PALS) suggests existence of Ti vacancies and both methods (chemical capacitance and PALS) agree in their quantification of the corresponding vacancy concentration (6 ppm). Beyond successfully predicting acceptor defect concentrations in undoped SrTiO₃, the method is sensitive for electronically relevant defects in sub-ppm concentrations.     

Parametric study on the electrochemical impedance spectroscopy of organic-coated steels in hydrochloric acid solutions

Electrochemical impedance technique was used to study the corrosion resistance of polymer-coated steels in hydrochloric acid solutions. The mechanism of the degradation process of the coating can be described using a Randles-type equivalent circuit model with the inclusion of Warburg diffusion impedance. The impedance parameters: coating resistance R_po, coating capacitance C_c, charge transfer resistance R_ct and double layer capacitance C_dl of the metal/coating interface, and the Warburg coefficient σ were calculated using a nonlinear least-square minimization scheme. The time dependency of impedance parameters was compared to the method of breakpoint frequency. It was found that the delamination of the metal/coating interface is more associated with the charge transfer resistance than the coating resistance. Open circuit potential measurement is indicative of the corrosion status of the coating. Department of Chemical Engineering, Neili, Taoyuan, Taiwan 32026 Fax: 886.3.4559373 email:cekenyin@saturn.yzu.edu.tw     

Dielectric,transport and thermal properties of clay based polymer‐ nanocomposites

The polymer nanocomposite films (PNC) with varying amounts of organically modified sodium montmorillonite (DMMT) clay in poly(methyl methacrylate) (PMMA) based polymer matrix were prepared by solution cast technique. Dielectric measurements were carried out on these films as a function of frequency at 30°C and 100°C. The addition of clay significantly improved the ionic conductivity. Transport parameters, such as the diffusion coefficient (D), number density (n) and mobility (μ) of charge carriers were determined using a new approach, which is based on impedance spectroscopy. The temperature‐dependent dc conductivity, relaxation and mobility plots obey the Arrhenius rule. The results suggest that the higher ionic conductivity of these PNC films at elevated temperature is not only due to increased mobility of ions, but it is accompanied by a significant increase in carrier concentration. Analysis of DSC thermogram reveals a very high percentage of amorphous content for all samples. A good correlation among dielectric permittivity, carrier concentration, mobility and ionic conductivity has also been observed. POLYM. ENG. SCI., 58:220–227, 2018. © 2017 Society of Plastics Engineers     

Electrochemical impedance spectroscopy for studying passive layers on steel rebars immersed in alkaline solutions simulating concrete pores

Present paper deals with the use of the electrochemical impedance spectroscopy to identify different processes in the passive layer growth over steel rebar surface immersed in an alkaline media simulating the concrete pore solution. Two cases have been considered: a passive layer spontaneously grown in a high alkaline media and a passive layer assisted by the application of an anodic potential in the same media. The application of electric equivalent circuits allows distinguishing between the different mechanisms occurring in this passive layer when grows in different conditions. An electric equivalent circuit with two RC loops connected in parallel is often used for fitting the EIS diagrams obtained for spontaneous growth of passive layers in the alkaline solution simulating the concrete pores. However, when the passive layer is formed under anodic polarization, a Warburg element must be introduced in the equivalent circuit. According to the Point Defect Model (PDM), this Warburg element is allocated to the transport of oxygen vacancies through the passive layer, which concentration changes with the potential.     

Breaking bubbles and the water-to-air transport of particulate matter

Bubbles collapsing at an air-liquid interface eject small, high-speed droplets into the adjacent gas phase. The composition of these aersol-sized droplets may be markedly different from the bulk solution in which the bubbles originate. This bubble-breaking/fractionation phenomenon has major implications for a variety of environmental and industrial processes. In preliminary work reported here we have measured the transfer of micron-size latex particles to jet drops produced by the bursting of one-millimeter diameter bubbles. Jet drop concentration was measured as a function of: bulk particle concentration, bulk ionic strength, and depth of bubble release. Results show the droplet concentration to be relatively insensitive to bulk concentration but strongly dependent on ionic strength and trace contaminants.     

Oxygen Ion Conduction in Bulk and Grain Boundaries of Nominally Donor‐Doped Lead Zirconate Titanate (PZT): A Combined Impedance and Tracer Diffusion Study

Oxygen ion conduction in Nd³⁺‐doped Pb(Zr_xTi_1?x)O₃ (PZT) was investigated by impedance spectroscopy and ¹⁸O‐tracer diffusion with subsequent secondary ion mass spectrometry (SIMS) analysis. Ion blocking electrodes lead to a second relaxation feature in impedance spectra at temperatures above 600°C. This allowed analysis of ionic and electronic partial conductivities. Between 600°C and 700°C those are in the same order of magnitude (10^?5–10^?4 S/cm) though very differently activated (2.4 eV vs. 1.2 eV for ions and electron holes, respectively). Oxygen tracer experiments showed that ion transport mainly takes place along grain boundaries with partly very high local ionic conductivities. Numerical analysis of the tracer profiles, including a near‐surface space charge zone, revealed bulk and grain‐boundary diffusion coefficients. Calculation of an effective ionic conductivity from these diffusion coefficients showed good agreement with conductivity values determined from impedance measurements. Based on these data oxygen vacancy concentrations in grain boundary and bulk could be estimated. Annealing at high temperatures caused a decrease in the grain‐boundary ionic conductivity and onset of additional defect chemical processes near the surface, most probably due to cation diffusion.     

Re-examination of bulk and grain boundary conductivities of Ce1−xGdxO2−δ ceramics

Powders of gadolinium-doped ceria solid solutions, Ce_1−xGd_xO_2−δ (x = 0.05, 0.1, 0.2, 0.3 and 0.4), were prepared by a freeze-drying precursor route. Dense ceramic pellets with average grain sizes in the range of several microns were obtained after sintering at 1600 °C. Cobalt nitrate was added to the powders to obtain dense ceramic samples with grain sizes in the submicrometer range at 1150 °C. The ionic conduction was analysed by impedance spectroscopy in air, to de-convolute the bulk and grain boundary contributions. The bulk conductivity at low temperature clearly decreases with increasing content of Gd whereas the activation energy increases. An alternative method is proposed to analyse the extent of defect interactions on conduction. For samples without addition of Co, the specific grain boundary conductivity increases with increasing Gd content. Addition of cobalt does not alter the bulk properties but produces an important increase in the specific grain boundary conductivity, mainly in samples with lower Gd-concentration (x = 0.05 and 0.1). Segregation of Gd and its strong interaction with charge carriers may explain the blocking effects of grain boundaries.     

Impedance studies of cathode/electrolyte behaviour in SOFC

This paper reports impedance studies of the cathode/electrolyte behaviour in solid oxide fuel cells (SOFC), based on comparative investigation of half-cells with yttria stabilized zirconia (YSZ) electrolyte and different cathode materials: lanthanum strontium manganite (LSM), and composite LSM/YSZ with low ionic conductivity as well as the electron conducting Ag, Pt and Au. For improved impedance data analysis the technique of the differential impedance analysis is applied. It ensures structural and parametric identification without preliminary assumptions about the working model. It is found that despite the low ionic conductivity of LSM, the cathode reaction of the oxide cathode materials is a two-step process including: (i) charge transfer with activation energy of the resistivity E_a increasing with the temperature and (ii) transport of oxygen ions through the bulk of the electrode (rate-limiting stage) with E_a independent on the temperature. For the metal (electron conducting) electrodes, the reaction behaviour is described with one step process with higher E_a at higher temperatures. The activation energy of the electrolyte conductivity decreases with the increase of the temperature. The observed changes in E_a for the electrolyte and the cathode reaction (the charge transfer step for the LSM-based electrodes) appear in the same temperature interval. This interesting coincidence suggests for correlation between the bulk (electrolyte) and surface conduction properties. Approaches for improvement of both the ionic conductivity and the supply with electrons in LSM should be also searched.