Randles Equivalent Circuit Model Research Articles

Enhancing sp3 content in diamond-like carbon thin film electrodes by pulsed laser annealing for durable charge storage performance

In this work, diamond-like carbon (DLC) thin film electrodes were grown on Si substrate by pulsed laser deposition (PLD) technique, and the implication of pulsed laser annealing (PLA) treatment on their electrochemical supercapacitor performance was examined by three electrode systems in 1 M Na2SO4 electrolyte solutions. The SEM micrographs exhibit a sheet-like wrinkled surface of DLC film on Si which later transformed into tiny hierarchical heap-like structures with enhanced surface roughness. The Raman spectra confirm signature of the D and G peaks of pristine DLC film at ∼1352 cm−1 (1356 cm−1 after PLA) and 1591 cm−1 (1589 cm−1 after PLA), and appearance of minor T2g (diamond) (1332 cm−1) peaks with an increment of the sp3 content after PLA treatment. The X-ray photoelectron spectroscopy (XPS) spectra were deconvoluted into three different contributions CC (sp2), CC (sp3), and CO, and their sp3 percentages were estimated ∼48.9 %, and 64.7 % before and after annealing. The area under cyclic voltammetry (CV) curve and galvanostatic charging discharging (GCD) performance of PLA-treated DLC film were improved and the highest areal-specific capacitance was obtained to be ∼48.25 mF/cm2 for scan rate of 5 mV/s and 36.01 mF/cm2 for current density of 2 mA/cm2, respectively. Excellent charging discharging cyclic stability of PLA treated electrode was observed over 5000 cycles, and Coulombic efficiency and capacitance retention were determined to be ∼93.5 % and ∼ 97.3 %, respectively. The electrochemical impedance spectroscopy (EIS) was performed to obtain the Nyquist plot (Z′vs.Z′′) of both electrodes before and after the stability test, and a small degradation in impedance was observed. The Nyquist plots were fitted with the well-known Randles equivalent circuit model Rs(Cdl(RctZw)) and substantial curtailment of Warburg resistance (Zw) in PLA-induced DLC electrode signifies the dominance of the diffusion-controlled region in the charging-discharging process. Minor changes in the Nyquist plot before and after the stability test manifest the enduring charge kinetics of both electrodes and PLA-treated DLC exhibits better cyclic stability. PLA techniques have a great impact on upgrading the areal-specific capacitance, capacitance retention with high Coulombic efficiency, and stability with negligible impedance loss of the DLC film electrodes.

Dielectrophoretic and Electrical Impedance Differentiation of Cancerous Cells Based on Biophysical Phenotype.

Here, we reported a study on the detection and electrical characterization of both cancer cell line and primary tumor cells. Dielectrophoresis (DEP) and electrical impedance spectroscopy (EIS) were jointly employed to enable the rapid and label-free differentiation of various cancer cells from normal ones. The primary tumor cells that were collected from two colorectal cancer patients, cancer cell lines (SW-403, Jurkat, and THP-1), and healthy peripheral blood mononuclear cells (PBMCs) were trapped first at the level of interdigitated microelectrodes with the help of dielectrophoresis. Correlation of the cells dielectric characteristics that was obtained via electrical impedance spectroscopy (EIS) allowed evident differentiation of the various types of cell. The differentiations were assigned to a “dielectric phenotype” based on their crossover frequencies. Finally, Randles equivalent circuit model was employed for highlighting the differences with regard to a series group of charge transport resistance and constant phase element for cancerous and normal cells.

Electrochemical Impedance Imaging on Conductive Surfaces

Electrochemical impedance spectroscopy (EIS) is a powerful tool to measure and quantify the system impedance. However, EIS only provides an average result from the entire electrode surface. Here, we demonstrated a reflection impedance microscope (RIM) that allows us to image and quantify the localized impedance on conductive surfaces. The RIM is based on the sensitive dependence between the materials' optical properties, such as permittivity, and their local surface charge densities. The localized charge density variations introduced by the impedance measurements will lead to optical reflectivity changes on electrode surfaces. Our experiments demonstrated that reflectivity modulations are linearly proportional to the surface charge density on the electrode and the measurements show good agreement with the simple free electron gas model. The localized impedance distribution was successfully extracted from the reflectivity measurements together with the Randles equivalent circuit model. In addition, RIM is used to quantify the impedance on different conductive surfaces, such as indium tin oxide, gold film, and stainless steel electrodes. A polydimethylsiloxane-patterned electrode surface was used to demonstrate the impedance imaging capability of RIM. In the end, a single-cell impedance imaging was obtained by RIM.

Micro‐electrodes for in situ temperature and bio‐impedance measurement

AbstractWith fast recovery time and effective in situ tumor tissue killing ability, thermal ablation has become a popular treatment for tumors compared with chemotherapy and radiation. The thermal dose measurement of current technology is usually accompanied by monitoring a large area impedance across two ablation catheters and the localized impedance measurement is difficult to achieve. In this work, thermal‐resistive sensor and impedance sensor are fabricated on the curved surface of a capillary tube with 1 mm outer diameter. The device is applied for real‐time in situ tissue impedance monitoring during thermal ablation. The calibrated thermal‐resistive sensors have an average temperature coefficient of resistance (TCR) of 0.00161 ± 5.9% °C‐1 with an accuracy of ±0.7 °C. By adding electro‐polymerized PEDOT:PSS (poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate)) on the 300 µm diameter gold electrodes, the interface impedance reduces two orders from 408 to 3.7 kΩ at 100 Hz. The Randles equivalent circuit model fittings show a two‐order improvement in the electrode capacitance from 7.29 to 753 nF. In the ex vivo porcine liver laser ablation test, the temperature of the porcine liver tissue can reach 70°C and the impedance would drop by 50% in less than 5 minutes. The integration of laser ablation fiber with the impedance and temperature sensors can further expand the laser ablation technique to smaller scale and for precise therapeutics.

Rapid Model-Free State of Health Estimation for End-of-First-Life Electric Vehicle Batteries Using Impedance Spectroscopy

The continually expanding number of electric vehicles in circulation presents challenges in terms of end-of-life disposal, driving interest in the reuse of batteries for second-life applications. A key aspect of battery reuse is the quantification of the relative battery condition or state of health (SoH), to inform the subsequent battery application and to match batteries of similar capacity. Impedance spectroscopy has demonstrated potential for estimation of state of health, however, there is difficulty in interpreting results to estimate state of health reliably. This study proposes a model-free, convolutional-neural-network-based estimation scheme for the state of health of high-power lithium-ion batteries based on a dataset of impedance spectroscopy measurements from 13 end-of-first-life Nissan Leaf 2011 battery modules. As a baseline, this is compared with our previous approach, where parameters from a Randles equivalent circuit model (ECM) with and without dataset-specific adaptations to the ECM were extracted from the dataset to train a deep neural network refined using Bayesian hyperparameter optimisation. It is demonstrated that for a small dataset of 128 samples, the proposed method achieves good discrimination of high and low state of health batteries and superior prediction accuracy to the model-based approach by RMS error (1.974 SoH%) and peak error (4.935 SoH%) metrics without dataset-specific model adaptations to improve fit quality. This is accomplished while maintaining the competitive performance of the previous model-based approach when compared with previously proposed SoH estimation schemes.

A rapid neural network–based state of health estimation scheme for screening of end of life electric vehicle batteries

There is growing interest in recycling and re-use of electric vehicle batteries owing to their growing market share and use of high-value materials such as cobalt and nickel. To inform the subsequent applications at battery end of life, it is necessary to quantify their state of health. This study proposes an estimation scheme for the state of health of high-power lithium-ion batteries based on extraction of parameters from impedance data of 13 Nissan Leaf 2011 battery modules modelled by a modified Randles equivalent circuit model. Using the extracted parameters as predictors for the state of health, a baseline single hidden layer neural network was evaluated by root mean square and peak state of health prediction errors and refined using a Gaussian process optimisation procedure. The optimised neural network predicted state of health with a root mean square error of (1.729 ± 0.147)%, which is shown to be competitive with some of the most performant existing neural network–based state of health estimation schemes, and is expected to outperform the baseline model with ∼50 training samples. The use of equivalent circuit model parameters enables more in-depth analysis of the battery degradation state than many similar neural network–based schemes while maintaining similar accuracy despite a reduced dataset, while there is demonstrated potential for measurement times to be reduced to as little as 30 s with frequency targeting of the impedance measurements.

Continuous flow system for biofilm formation using controlled concentrations of Pseudomonas putida from chicken carcass and coupled to electrochemical impedance detection

Most studies dealing with monitoring the dynamics of biofilm formation use microbial suspensions at high concentrations. These conditions do not always represent food or water distribution systems. A continuous flow system capable of controlling the concentration of the microbial suspension stream from 104 to 106 CFU ml−1 is reported. Pseudomonas putida biofilms formed using 100-fold, 1,000-fold or 10,000-fold diluted bacterial suspensions were monitored in-line by electrochemical impedance spectroscopy (EIS) and total plate counts. Randles equivalent circuit model and a modified Randles model with biofilm elements were used to fit the EIS data. In Randles equivalent circuit, the charge transfer resistance decreased as the biofilm formed. The log colony counts of the biofilm correlated to the charge transfer resistance. In the biofilm model, the biofilm resistance and the double layer capacitance decreased as the biofilm formed. The log colony counts of the biofilm correlated to the biofilm resistance.

Controllable Electrochemical Impedance Spectroscopy: From Circuit Design to Control and Data Analysis

This article describes fundamentals of controllable electrochemical impedance spectroscopy (cEIS), from circuit design to control and data analysis. In cEIS, a feedback system controls the process of injecting the excitation signal. We design a two degree-of-freedom robust control system, which guarantees tracking and stability of cEIS in the presence of model uncertainties. This article also addresses the concept of persistently exciting signals. cEIS using current driving mode (CDM) and voltage driving mode, and their differences are highlighted. An online cEIS device is designed and built based on the dc–dc buck converter for batteries online applications, where the excitation signal is superimposed on a dc level. The performance of the fabricated cEIS is evaluated through extensive experiments in CDM. The accuracy of the fabricated cEIS is tested, which results in $\text{0.002}\;\Omega$ root mean square error in the impedance spectra computation of a three-parameter Randles equivalent circuit model (ECM). The performance of the fabricated cEIS is practically verified on a battery cell at different C-rates. First-, second-, and fractional-order Randles ECMs are estimated by using system identification methods, and their impedance spectra are compared with those obtained through the fast Fourier transform.

Electrical Conductivity Based Quality Measurement of Diesel Exhaust Fluid

In an effort to reduce environmental pollution, the urea-selective catalytic reduction system is used to reduce NOx emissions with urea additive diesel exhaust fluid (DEF) as reductant in the exhaust of diesel engines. ISO 22241-1:2006 specifies the quality characteristics of the DEF in order to maintain efficiency, and hence it is essential that the concentration of urea in DEF is accurately monitored. In this paper an online, four-electrode, temperature compensated, electrical conductivity measurement system based on Van der Pauw theory is proposed for measuring the variation in urea concentration in DEF. The equivalent circuit model of the measurement system is developed using the Randles circuit and the equivalent circuit parameters are determined from impedance analyses of system. The prototype of the measurement cell is fabricated with all associated electronics and tested for 0.25% change in urea concentration. The experimental results are compared with results obtained from the Randles equivalent circuit model of the measurement system. The temperature effect on the measurement system is abridged from 28.75 to 2.575 mV/grad with the design of simple electronic circuit.

전기 자동차 가상 플랫폼용 배터리 모델 개발 및 검증

In this paper, a battery model for electric vehicle virtual platform was developed. A battery model consisted of a battery cell model and battery thermal management system. A battery cell model was developed based on Randles equivalent circuit model. Circuit parameters in the form of 3D map data was obtained by charge-discharge experiment of Li-Polymer battery in various temperature condition. The developed battery cell model was experimentally verified by comparing voltages. Thermal management system model was also developed using heat generator, heat transfer and convection model, and cooling fan. For verification of the developed battery model in vehicle level, the integrated battery model was applied in to EV(electric vehicle) virtual platform, and virtual driving simulation using UDDS velocity profile was conducted. The accuracy of the developed battery model has been verified by comparing the simulation results from EV platform with the experimental data.

Gold Nanostructured Surface for Electrochemical Sensing and Biosensing: Does Shape Matter?

ABSTRACTThe construction of an electrochemical sensor based on a gold working electrode modified with gold nanostructures was achieved by galvanic deposition into an anodic alumina nanoporous template. Two approaches were investigated to fabricate the nanostructures of various sizes and shapes. First, gold deposition was performed through a thin alumina template resulting in the production of nanorods. In the second approach, alumina pores were widened to form cup-shaped nanostructures. The detailed topography of the nanostructured surfaces was characterized by scanning electron microscopy. The lengths and the diameters of the nanostructures were anticipated to have a significant influence on the surface area and thus on the capacity to promote electron-transfer reactions. This dependence was investigated by electrochemical impedance spectroscopy and cyclic voltammetry. The functionalization of electrode surface with 11-mercaptoundecanoic acid provided a basis for further bonding with analytes. A modified Randles equivalent circuit model with a constant phase element was used to interpret the impedance spectra. The interfacial properties of the modified electrodes were also evaluated by cyclic voltammetry in the presence of the Fe(CN)63−/4− redox couple as a probe. The results showed that the voltammetric behavior of the probe was influenced by surface modification. The accumulation of 11-mercaptoundecanoic acid on the nanostructured electrode surface did not result in the expected increase in charge-transfer resistance and the decreased capacitance, unlike the cup-shaped nanostructured electrode.

Surface Impedance Imaging Technique

We demonstrate here a surface impedance imaging technique based on sensitive dependence of surface plasmon resonance (SPR) on local surface charge density. By applying a potential modulation to a sensor surface, we are able to simultaneously obtain three images: the dc component and the amplitude and phase of the ac component. The dc image measures local molecular binding activity on the surface, as found in the conventional SPR imaging technique, and the ac images are directly related to the local impedance of the surface. Our experimental data can be analyzed quantitatively in terms of the simple free electron gas model for the sensor surface and the Randles equivalent circuit model for interfacial impedance.

Comparative Evaluations of Surface Characteristics of cp Titanium, Ti-6AI-4V and Ti-15Mo-2.8Nb-0.2Si (Timetal® 21SRx)

Abstract Commercially pure titanium (cpTi), Ti-6A1-4V (Ti64), and Ti-15Mo-2.8Nb-0.2Si (21SRx), with three unique atomic, alpha, alpha-beta, and beta grain structures, respectively, were subjected to three different surface treatments: cleaning, nitric acid passivation, and heat treatment. Experiments were conducted to determine the effects of the type of material and surface modifications on the substrate microstructure, surface oxide composition and thickness, and resultant corrosion behavior. Metallography showed the cpTi groups were an equiaxed single alpha phase material, the Ti64 groups a dual-phase alpha-beta material, and the 21SRx groups an equiaxed beta material. The different surface treatments did not alter the substrate microstructures of any groups. Spectroscopic (AES) results showed typical titanium and titanium alloy spectra with dominant Ti and O peaks for all sample groups, indicative of titanium dioxide. In addition, small Al and Mo peaks were detected throughout the surface oxides of the Ti64 and 21SRx specimens, respectively. AES depth profiling showed no significant difference in the oxide thickness between all the Cleaned and Passivated groups regardless of metal or alloy group. However, all the Heat Treated groups had significantly thicker oxides. In general, corrosion results showed Passivated and Heat Treated groups to have similar corrosion properties and significantly improved corrosion resistances compared to the Cleaned groups. All impedance spectra fit into the Randles equivalent circuit model, and all sample groups exhibited near ideal capacitive behavior (φ ≅ 90°) expected for titanium and its alloys.

A New Way of Using ac Voltammetry To Study Redox Kinetics in Electroactive Monolayers

A new protocol for analyzing variable-frequency ac voltammetry (acv) data to obtain electron-transfer rate constants for redox-active groups immobilized in monolayer films on electrodes is described. The protocol involves plotting the ratio of the peak current to the background current as a function of the log of frequency and then fitting the plot to a calculated curve obtained using the Randles equivalent circuit model. The fitting procedure is particularly straightforward since it involves just two adjustable fitting parameters and can usually be performed manually in a spreadsheet. Advantages of the new method include the following: (1) it requires fewer adjustable fitting parameters than the conventional impedance analysis performed in the complex-plane format; (2) because there are fewer fitting parameters, there is less chance of the fit yielding a false convergence; (3) nonideal effects associated with a distribution of rate constants are apparently revealed in a particularly intuitive and easily...

Electrochemical behaviour of vanadium in aqueous solutions of different pH

he electrochemical behaviour of vanadium in aqueous solutions of pH covering the range 1–13 in oxygen saturated, oxygen free and naturally aerated solutions was investigated. The behaviour of the vanadium electrode in alkaline solutions ( pH > 8) is different from that in acid or neutral solutions (pH 1–8). The equilibrium electrode potential is a linear function of the solution pH. The slope of the linear relation changes from −0.039 V/decade for solution of pH ≤ 8 to −0.058 V/decade for solutions of pH > 8. EIS investigations have shown that the metal undergoes active dissolution in aqueous solutions. In solutions of pH > 8 the corrosion behaviour of vanadium can be simulated to a Randles equivalent circuit model. In oxygen saturated solutions the electrode surface is covered with an oxide film. The interaction of this film with the H + and OH − in solutions with different pH was discussed. Polarization measurements have shown that the rate of corrosion in acid solutions is not affected by the prevailing gas. In alkaline solutions, removal of oxygen or air from the solution leads to a decrease in the corrosion rate of the metal.