Abstract: Pseudocapacitors have attracted more and more attention during the recent years because of their high capacitance and large energy density. As a traditional pseudocapacitive material, WO3 possesses high theoretical capacitance and good conductivity among various transition metal oxides. Herein, a nanoflower-like WO3 (NFL-WO3) electrode with perfect electrochemical performance is synthesized through a facile and effective electrodeposition method. The NFL-WO3 electrode after optimization exhibits a very high areal specific capacitance (Ca) of 658 mF cm−2, which is the highest one in pure WO3 materials to the best of our knowledge and a large gravimetric specific capacitance (Cg) of 196 F g−1 at a scan rate of 10 mV s−1. Our result demonstrates the excellent balance between Ca and Cg compared with other reported WO3 based materials. After experiencing 5000 cycles, 85% of its capacitance can still be retained. Based on the full analysis of cyclic voltammetry curves and Nyquist plots, the pseudocapacitance should be the dominate contribution to the total capacitance. In addition, it can be assembled into an asymmetric supercapacitor for powering small electronics (light-emitting diode sets and mobile phone). All of the above exhibit the potential application of the NLF-WO3 electrode in energy storage devices.

Abstract: The structural, electrical and magnetoelectric properties of SmFeO3 ceramic samples, synthesized using a soft-chemical method, were studied. A structural analysis of the material was carried out by the Rietveld refinement of room temperature x-ray diffraction data. The temperature dependence of the dielectric peaks was analyzed by fitting them with two Gaussian peaks corresponding to two phase transitions—one being electric, and the other being magnetic in nature. The depression angle of the semicircles in a Nyquist plot representing the grain and grain boundary contributions in the sample was estimated. The grain boundary effect, appearing at temperatures above 75 °C, is explained using the Maxwell–Wagner mechanism. The impedance study reveals a semi-conducting grain with an insulating grain boundary, leading to the formation of surface and internal barrier layer capacitors and resulting in a very high dielectric constant. The effect of dc conductivity on the loss tangent at low frequencies and high temperature has been analyzed. The frequency dependence of ac conductivity in the two different regions can be explained on the basis of correlated barrier hopping and quantum mechanical tunneling models. The material is found to exhibit canted antiferromagnetism and improper ferroelectric characteristics. The value of the magnetoelectric voltage-coupling coefficient (α) of a SmFeO3 ceramic is found to be 2.2 mV cm−1 Oe−1.

Abstract: In this work, polymer nanocomposites consisting of a poly(vinyl chloride) (PVC) and polyvinylidene fluoride (PVDF) polymer network with ZnO nanoparticles as a dopant were prepared by solution casting. An XRD study of the PVC/PVDF/ZnO polymer nanocomposites shows predominantly sharp and high intensity peaks. However, the intensity and sharpness of the XRD peaks decreases with further increment in loading of ZnO (wt%), which reveals a proper intercalation of ZnO nanoparticles within the PVC/PVDF polymer system. Fourier transform infrared spectroscopy has been used to verify the chemical compositional change as a function of ZnO nanoparticle loading. TGA analysis clearly describes the thermal degradation of the pure polymer and polymer nanocomposites. The complex dielectric function, AC electrical conductivity and impedance spectra of these nanocomposites were investigated over the frequency range from 10 Hz to 35 MHz. These spectra were studied with respect to the Wagner − Maxwell − Sillars phenomenon in the low frequency region. Nyquist plots of the PVC/PVDF/ZnO nanocomposites were established from impedance measurements. The temperature-dependent DC ionic conductivity obtained from the Nyquist plots follows Arrhenius behaviour. © 2016 Society of Chemical Industry

Abstract: A simple, additive-free, cost-effective and scalable successive ionic layer adsorption and reaction (SILAR) method is reported to prepare nickel–cobalt binary hydroxide (Ni–Co–BH) on a reduced graphene oxide (RGO) directing template over a macro-porous conductive nickel foam substrate. This green technique is not only considered as fundamental research interest, but also describes the commercial applications of supercapacitors to reduce the electrode fabrication cost. Three different Ni–Co–BH–G (Ni–Co–BH/RGO) composites are synthesised by tailoring the nickel–cobalt ratios. The flower-like 3D framework of Ni–Co–BH–G provides a porous nano-structure to facilitate the charge transfer and ion diffusion. The cathodic peak current density vs. square root of the scan rate slope values of cyclic voltammetry are consistent with specific capacitance (SC) retention (vs. current density) from charge–discharge curves and the diffusion time constant of the Nyquist plot of the Ni–Co–BH–G composites. Taking the advantage of 3D conductive mesoporous open framework, the Ni–Co–BH–G has provided an excellent SC of 2130 F g−1 at 2 A g−1. An asymmetric supercapacitor device is designed with the optimized Ni–Co–BH–G as the positive electrode and concentrated HNO3 treated conducting carbon cloth (CCN) as the negative electrode. An excellent energy density of ∼92 W h kg−1 and a high power density of ∼7.0 kW kg−1 with lifetime stability up to 10000 charge–discharge cycles (capacitance retention ∼ 80%) are provided by the asymmetric device. Four asymmetric devices have been assembled in series, which provided ∼5.6 V charge–discharge potential. The assembled system has powered a 5 V light-emitting diode (LED) successfully.

Abstract: In this work, perovskite solar cells (PSCs) with CH3NH3PbI3-x Cl x as active layer and spiro-OMeTAD as hole-transport media have been fabricated by one-step method. The methylammonium iodide (CH3NH3I) solution with different concentrations is used to modify the interface between mesoporous TiO2 (meso-TiO2) film and CH3NH3PbI3−x Cl x perovskite layer. Several techniques including X-ray diffraction, scanning electron microscopy, optical absorption, electrochemical impedance spectroscopy (EIS) and photoluminescence are used to investigate the effect of the interfacial modification. It is found that the interfacial modification by CH3NH3I enhance the crystallinity and increase the grain size of CH3NH3PbI3−x Cl x layer, and improve the surface wetting properties of perovskite precursor on meso-TiO2 film. The sunlight absorption and external quantum efficiency of PSCs in the visible region with wavelength less than 600 nm have been improved. The Nyquist plots obtained from the EIS suggest that the CH3NH3I modification can reduce the charge recombination rates. The photoluminescence measurement shows that the exciton dissociation in the modified devices is more effective than that in the control samples. The photovoltaic performance of the modified devices can be significantly improved with respect to the reference (control) devices. The CH3NH3I modified devices at the optimized concentration demonstrate the average power conversion efficiency of 12.27 % in comparison with the average efficiency of 9.68 % for the reference devices.

Abstract: Vertical graphene nanosheets (VGN) grown as controlled porous network are studied and demonstrated as a promising electrode material for supercapacitors. The VGN synthesized by microwave plasma enhanced chemical vapor deposition using CH4/Ar gas mixture as precursor are considered for electrochemical performance in Na2SO4, KOH, and H2SO4 to delineate the electrolyte effect. Among the electrolytes, H2SO4 exhibited excellent specific areal capacitance (188 microfarad/cm2) and good capacitance retention (96.8%). No significant change is observed in impedance spectra even after 200 cycles. An electric equivalent circuit for the system is simulated from Nyquist plot to elucidate the behavior of electrode/electrolyte interface. This potential supercapacitor electrode material is well characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and contact angle measurement. Utilization of aqueous electrolytes for potential supercapacitors is also discussed in relation to improved performance observed in H2SO4 medium.

Abstract: In this present study, we have reported the synthesis of ZnMn2O4, transitional metal oxide by sol–gel method. The structural and morphological properties are confirmed using various characterization techniques namely XRD, FT-IR and SEM with EDX. Dielectric studies of ZnMn2O4 are measured at the frequency varying from 50 Hz to 5 MHz for the temperature range of 303–573 K. The temperature dependent electrical parameters like impedance and modulus exhibit a strong correlation with the grains, grain boundaries and space charge effects in the synthesized material. Diffusion of oxygen vacancies in the dipoles and defects in the material due to oxygen vacancy complexes are investigated by the activation energy obtained from Arrhenius plot. It was found that the relaxation process was dominated by the hopping mechanism between the Mn3+ and Mn4+. Nyquist plot of impedance was attributed to the existence of space charge interface, grain boundary and grain conduction mechanism of the material.

Abstract: The internal resistance and capacitance of Si/organic hybrid solar cells (Si-HSC) based on poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) are investigated by electrochemical impedance spectroscopy (EIS). Three types of Nyquist plots in Si-HSC are observed firstly at different bias voltages, while suitable equivalent circuit models are established to evaluate the details of interface carrier transfer and recombination. In particular, the carrier transport property of the PEDOT: PSS film responds at a high frequency (6 x 10(4)-1 x 10(6) Hz) in three-arc spectra. Therefore, EIS could help us deeply understand the electronic properties of Si-HSC for developing high performance devices. (C) 2016 The Japan Society of Applied Physics

Abstract: The growth in photovoltaic installations makes characterization and condition monitoring essential. In this paper, broadband impedance spectroscopy is implemented for characterization and performance monitoring of silicon solar cells for near real-time operation. An optimized quasi-logarithmic broadband signal is designed to estimate the impedance response of the cells. Electrochemical equivalent circuits of the frequency response are then modelled from the obtained Nyquist plots and the cell parameters are extracted using complex nonlinear least squares. This procedure can be applied for direct estimation of the internal parameters of the silicon solar cells/module at different operating points. Results show that the implemented broadband characterization yields good correlation to the conventional electrochemical impedance spectroscopy at significantly reduced procedural time and equipment cost.

Abstract: Recently Co3O4-based nanostructures have received great interests owing to their potential applications as supercapacitor. In this work, novel three-dimensional (3D) starfish-like Co3O4 nanowire bundles on nickel foam (Co3O4-oNF) were successfully prepared via a facile hydrothermal method. The phase/micro structure and morphology of the sample were identified by X-ray diffraction, scanning and transmission electron microscopy, respectively, and demonstrated a unique 1D–3D combined mesoporous nanostructure. Cyclic voltammetry, chronopotentiometry, and impedance measurements were employed to characterize the electrochemical performance of the specimen in 3.0 M KOH solution, and show that the Co3O4-oNF not only has a superior specific capacitance of 1399.28 F g−1 at a current density of 1.0 A g−1 but also exhibits reliable capacitance retention of 92.26 % after 1000 cycles even at a high current density of 12.5 A g−1. The Nyquist plot reveals that the internal resistance of the specimen is merely 0.9 Ω, indicating that the starfish-like Co3O4 has a solid mechanical bonding with the nickel substrate. The synthesized 3D nanostructured Co3O4 holds great promises for applications in supercapacitor.

Abstract: Electrochemical impedance spectroscopy (EIS) is a widely-used diagnostic technique to characterize electrochemical processes. It is based on the dynamic analysis of two electrical observables, that is, current and voltage. Electrochemical cells with gaseous reactants or products (e.g., fuel cells, metal/air cells, electrolyzers) offer an additional observable, that is, the gas pressure. The dynamic coupling of current and/or voltage with gas pressure gives rise to a number of additional impedance definitions, for which we use the term electrochemical pressure impedance. It also gives rise to different experimental probing approaches. In this article we present a model-based study of electrochemical pressure impedance spectroscopy (EPIS). Possible quantifications and realizations of EPIS are discussed. The study of generic cell geometries consisting of gas reservoir, diffusion layer(s) and electrochemically active layer(s) reveals distinct spiral-shaped features in the Nyquist plot. Using the example of a sodium/oxygen (Na/O2) cell, the dynamic spatiotemporal behavior of the state variables is quantified and interpreted. Results are compared to first experimental EPIS measurements by Hartmann et al. [J. Phys. Chem. C118, 1461, 2014]. A sensitivity analysis highlights the properties of EPIS with respect to geometric, transport, and kinetic parameters. We demonstrate that EPIS is sensitive to transport parameters that are not well-accessible with standard EIS.

Abstract: Simulation and preparing predictive model of electrochemical impedance Nyquist plots based on radial basis function neural network (RBFNN) are presented in this paper. The RBFNN as a powerful predictive system predicts the real and imaginary parts of impedance as a function of time, temperature and inhibitor concentration. The mean R value of 0.9996 as regression coefficient and mean square error (MSE) value of 1.72 × 10 −3 as results show the validity of proposed method for simulation and prediction of electrochemical impedance spectroscopy (EIS) in different environmental situations.

Abstract: Using high-temperature solid state reaction technique a new polycrystalline multiferroic material of (LaLi)1/2(Fe2/3Mo1/3)O3 has been prepared. Thermo-gravimetric analysis is carried out to optimize the calcination temperature of the material. The formation of the desired compound is confirmed by preliminary X-ray diffraction analysis. The complex impedance spectroscopy technique is used to study the dielectric properties and other electrical parameters (loss tangent, impedance, modulus, conductivity) of the material as a function of frequency (1 kHz–1 MHz) at different temperatures (RT−425 °C). The nature of Nyquist plot confirmed the presence of bulk and grain boundary effect in the compound. The relaxation time obtained from the maximum frequencies corresponding to the imaginary impedance and electric modulus plots at different temperatures are found to obey Arrhenius law. The temperature and frequency dependence of ac conductivity plot obeys Jonscher’s universal power law. The P–E loop confirms the ferroelectric behavior of the compound at room temperature.

Abstract: In recent years phosphoric acid doped PBI-type fuel cells have drawn much attention as a promising candidate for energy storage and conversion applications at relatively high temperatures (100 – 200 °C). The elevated operating temperature of HT-PEMFCs (high-temperature polymer electrolyte membrane fuel cells) simplifies water and thermal management. In addition, it greatly enhances the fuel cell's tolerance against impurities (e.g. CO) in hydrogen, which is critical for operation with reformate hydrogen. However, durability and stability of high-temperature PEM-MEAs still need to improve for widespread commercialization [1], and better tools for identifying performance-loss related mechanisms are desirable. A very useful in-situ technique for performance analysis of HT-PEMFCs is electrochemical impedance spectroscopy (EIS). As previously reported, EIS can be used to characterize the impact of different parameters (e.g. stoichiometry, temperature, etc.) on cell kinetics [2, 3]. Equivalent circuit models are usually used to analyze the EIS data and identify the performance-loss related mechanisms. A major drawback of this approach is that the assumptions for the model equivalent circuits are sometimes ambiguous or even misleading due to the lack of priori knowledge about the electrochemical system under study. Furthermore, a clear separation of various physiochemical processes is difficult if the time constants of these processes overlap significantly in the frequency domain. To overcome these drawbacks, advanced mathematical methods such as Distribution of Relaxation Times (DRT) can be applied. DRT relies on the representation of the polarization impedance by its characteristic time constants and is numerically approximated by a discrete distribution function. This method has been successfully demonstrated for process identification and separation in solid oxide fuel cells (SOFC) [4].In this study, we applied this technique on high temperature PEM-MEAs for the first time. Electrochemical processes were identified and analyzed by varying cell parameters such as temperature and stoichiometry. The results offer a refined understanding of loss mechanisms and provide valuable guidance for fuel cell improvement and optimization. [1] A. Chandan, M. Hattenberger, A. El-kharouf, S. Du, A. Dhir, V. Self, B.G. Pollet, A. Ingram, W. Bujalski, J. Power Sources 231 (2013) 264 [2] J. L. Jesperesen, E. Schaltz, S.K. Kaer, J. of Power Sources 191 (2009) 289-296 [3] F. Mack, R. Laukenmann, S. Galbiati, J. A. Kerres, R. Zeis, ECS Transactions 69 (17), 1075-1087 (2015) [4] A. Leonide, V. Sonn, A. Weber, and E. Ivers-Tiffee, Journal of the Electrochemical Society 155 (1) B36-B41 (2008) Figure 1 Typical Nyquist plot of a phosphoric acid doped PBI-type HT-PEMFC at different current densities (a) and the corresponding distribution function (b). Figure 1

Abstract: Single crystals of pure lauric acid (LA) were harvested from ethanol solution by a slow evaporation technique. X-ray diffraction showed that the LA crystallized in the monoclinic system and was used to determine the lattice parameters. The Kurtz–Perry powder technique showed that the second-harmonic generation efficiency of LA was 0.87 times that of potassium dihydrogen phosphate. Fourier transform infrared spectral analysis was used to identify the various fatty acid functional groups present in the sample. Thermogravimetric analysis and differential thermal analysis revealed that the LA crystal is stable up to 45 °C. The mechanical strength of the sample crystal was estimated by the Vickers hardness test. Impedance analysis was carried out for the sample at different frequencies and a Nyquist plot was drawn to understand the electrical properties.

Abstract: Amorphous perfluoroalkenyl vinyl ether polymer devices can store a remarkably powerful electric charge because their surface contains nanometre-sized cavities that are sensitive to the so-called quantum-size effect. With a work function of approximately 10 eV, the devices show a near-vertical line in the Nyquist diagram and a horizontal line near the −90° phase angle in the Bode diagram. Moreover, they have an integrated effect on the surface area for constant current discharging. This effect can be explained by the distributed constant electric circuit with a parallel assembly of nanometre-sized capacitors on a highly insulating polymer. The device can illuminate a red LED light for 3 ms after charging it with 1 mA at 10 V. Further gains might be attained by integrating polymer sheets with a micro-electro mechanical system.

Abstract: Organic–inorganic nanocomposite was obtained after the incorporation of cupric oxide (CuO) nanoparticles in a host matrix based on pyrogallol and formaldehyde (PF) using sol–gel method. The material was subjected to heat treatment under inert atmosphere at 650 °C during 2 h to obtained PF/Cu-650 nanocomposite. The X-ray diffraction analysis exhibited the presence of two phases: metallic copper Cu and graphite C. The voltage–current V(I) characteristics present a negative differential resistance at room temperature. The evolution of the conductivity as a function of measurement temperature indicates the dominance of the three dimensions Godet-variable range hopping transport model. The alternative current conductance was investigated using admittance spectroscopy; the obtained curves show the presence of hopping conduction mechanism. The Nyquist diagrams were used to identify an equivalent circuit and the fundamental parameters of the circuit are determined with the aim to study the contributions of the grains and grain boundaries to the conductivity.

Abstract: In the present research, temperature dependence of dielectric properties of cobalt–zirconium substituted barium hexaferrites, fabricated using citric acid sol gel method, has been reported. The dielectric constant, loss tangent and A.C. conductivity were investigated on the circular pellets in temperature range 30–350 °C and frequency range 10 kHz–1 MHz using impedance analyzer. This paper also presents impedance (Z*) and electric modulus (M*) analysis of all the samples. The single semi-circular arcs, observed in impedance Nyquist plots, suggest the dominance of grain boundaries in the conduction process. Dielectric constant and dielectric loss tangent show very small variation up to 200–250 °C temperature and abrupt increase afterwards up to 350 °C. Thus, these ferrites can be successfully implemented in the practical applications like capacitors, microwave devices etc. up to 250 °C, without any significant change in properties.

Abstract: Hydroxyapatite (Ca10 (PO4)6 (OH)2, HAP) is the main inorganic component of the hard tissues in bones and also important material for orthopedic and dental implant applications. Nano HAP is of great interest due to its various bio-medical applications. In the present work the nano HAP was synthesized by using surfactant mediated approach. Structure and morphology of the synthesized nano HAP was examined by the Powder XRD and TEM. Impedance study was carried out on pelletized sample in a frequency range of 100Hz to 20MHz at room temperature. The variation of dielectric constant, dielectric loss, and a.c. conductivity with frequency of applied field was studied. The Nyquist plot as well as modulus plot was drawn. The Nyquist plot showed two semicircle arcs, which indicated the presence of grain and grain boundary effect in the sample. The typical behavior of the Nyquist plot was represented by equivalent circuit having two parallel RC combinations in series.

Abstract: We study the interaction of liquid organic solvents within the pores of n- and p-type porous silicon (PSi) interfaces. Several polar (acetone, methanol, ethanol, isopropanol, and water), borderline (chloroform), and nonpolar (toluene and isoprene) molecular solvents have been characterized on distinct n- and p-type pore structures, as analyzed using scanning electron microscopy. Fundamental to these studies has been the generation of Nyquist diagrams comparing the behavior for both dry and solvent treated interfaces for each system studied. The results of these studies on an undecorated PSi interface suggest a closer correlation with the dipole moments associated with the applied organic solvents rather than their dielectric response. This is supported by the observed interaction of the considered solvents with metal oxide (AuxO (x ≫ 1) and SnOx) decorated PSi interfaces where nanostructured metal oxide-solvent dipole–dipole interactions appear to be manifest. In correlation with the Nyquist plots and the...

Abstract: In this present study, nanocrystalline perovskite structure CaMnO3 substituted with Ce and Fe was prepared by a sol–gel technique using citric acid as a chelating agent at 800 °C. The compound was analyzed by powder X-ray diffraction technique and found to be single phased orthorhombic perovskite structure with space group Pnma. The morphology of the synthesized material study is carried out by using SEM and EDX confirmed the chemical compositions of the sample. Electrical measurements were performed to determine the conductive nature of the sample, and the conductivity increases with increasing Fe concentration. Electrode polarization effect was apparently seen in the Nyquist plot and the conductivity measurements. Dielectric relaxations were present in the sample was attributed to the existence of oxygen vacancy, migration in the ions and the Ea is calculated using Arrhenius equation.

Abstract: The internal impedances of different dye-sensitized solar cell (DSSC) models were analyzed by electrochemical impedance spectrometer (EIS) with an equivalent circuit model. The Nyquist plot was built to simulate the redox reaction of internal device at the heterojunction. It was useful to analyze the component structure and promote photovoltaic conversion efficiency of DSSC. The impedance of DSSC was investigated and the externally connected module assembly was constructed utilizing single cells on the scaled-up module. According to the experiment results, the impedance was increased with increasing cells connected in series. On the contrary, the impedance was decreased with increasing cells connected in parallel.

Abstract: In this study, graphene/TiO2 composite films at different annealing temperatures from 450 to 650 °C acted as photoelectrode of dye-sensitized solar cell (DSSC). The graphene/TiO2 composite films were characterized by scanning electronic microscopy, X-ray diffractometer, and electrochemical impedance spectroscopy. The Nyquist plot is built to simulate the redox reaction of internal device at the heterojunction by an equivalent circuit model. It is useful to analyze the component structure and promote photovoltaic conversion efficiency of DSSC. According to the experiment results, the optimal annealing temperature of graphene/TiO2 composite film was 550 °C, where the open-circuit voltage was 0.74 ± 0.01 V, the short-circuit current density was 14.17 ± 0.32 mA/cm2, the fill factor was 51.00 ± 1.95%, and the photovoltaic conversion efficiency was 5.34 ± 0.12%.