Low Frequency Electrochemical Impedance Spectroscopy Research Articles

Stable and Efficient Dye-Sensitized Solar Cells and Supercapacitors Developed Using Ionic-Liquid-Doped Biopolymer Electrolytes.

An ionic liquid (IL) 1-ethyl, 2-methyl imidazolium thiocyanate incorporated biopolymer system is reported in this communication for applications in dual energy devices, i.e., electric double-layer capacitors (EDLCs) and dye-sensitized solar cells (DSSCs). The solution caste method has been used to synthesize ionic-liquid-incorporated biopolymer electrolyte films. The IL mixed biopolymer electrolytes achieve high ionic conductivity up to the order of 10-3 S/cm with good thermal stability above 250 °C. Electrical, structural, and optical studies of these IL-doped biopolymer electrolyte films are presented in detail. The performance of EDLCs was evaluated using low-frequency electrochemical impedance spectroscopy, cyclic voltammetry, and constant current charge-discharge, while that of DSSCs was assessed using J-V characteristics. The EDLC cells exhibited a high specific capacitance of 200 F/gram, while DSSCs delivered 1.53% efficiency under sun conditions.

An improvement of equivalent circuit model for state of health estimation of lithium-ion batteries based on mid-frequency and low-frequency electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is a non-invasive, information-rich measurement method. The biggest advantage is that it is possible to identify and analyze the battery state using a suitable equivalent circuit model (ECM) without the need for complete knowledge of the battery's past operation. Conventional equivalent circuit models (CECMs) achieve a high degree of accuracy by identifying model parameters with relatively fixed circuit components. However, CECM method fitting process may suffer from fitting failure and fitting error, resulting in poor estimation accuracy. To solve this problem, it is crucial to establish a suitable ECM with good fitting effect and high accuracy. Accordingly, this study proposes a method of mid-frequency and low-frequency domain ECM (MLECM) based on fusion SEI film resistance and charge transfer resistance. Firstly, two model building methods are presented. Then, by fitting and analyzing the model parameters of two different types of batteries, we conclude that MLECM has the advantages of fewer parameters and better parameter fitting. Finally, a method of power battery state of health (SOH) estimation based on the improved model is proposed by MLECM and the mathematical model of SOH. Validated by two datasets of experiments with different types of batteries, the results show that the maximum RMSE of the proposed estimation method is only 1.38% in the two datasets. And the average root mean squared error (RMSE) of MLECM is reduced by 0.708% compared to CECM, while the computational load of the former is reduced by 69.57% compared to the latter. Compared with the CECM method, the MLECM has high estimation accuracy, high applicability and low computational load.

Fast capacity estimation for lithium-ion battery based on online identification of low-frequency electrochemical impedance spectroscopy and Gaussian process regression

Accurate capacity estimation is critical to improving the safety and reliability of lithium-ion (Li-ion) battery systems. Traditional capacity estimation mainly extracts capacity-related features by passively collecting the battery voltage, current, and temperature signals, which requires high integrity and regularity of charging curves. In this paper, six health indicators (HIs) are extracted through the online identification of Low-frequency electrochemical impedance spectroscopy (LEIS) by step wave, combined with Gaussian process regression (GPR) to achieve a fast capacity estimation for Li-ion batteries. The step wave is injected into the battery system during the charging process through BMS and bi-directional converter cooperation. Compared with square and multi-sine waves, the stress and response of each frequency step wave are equivalent to that of the sine wave. Moreover, HIs are resolved from the actual Warburg impedance angle instead of the empirical angle π/4. Three novel HIs related to the Li-ion diffusion coefficient are proposed: Warburg factor Wd, pseudo-Li-ion diffusion state PLDS and residual signal of empirical mode decomposition PLDSr. The volume of GPR training data is only 34% of the whole frequency band EIS data. The results show that the identified LEIS achieves 0.96 goodness-of-fit (R2) at the minimum sampling frequency of 50 Hz, and the novel HIs significantly improve the state of health (SOH) estimation accuracy with R2 above 0.95, root mean squared error below 1%, and mean absolute percentage error of about 0.9%. This method is an effective way to the active SOH detection for Li-ion battery, which is vital for the online SOH evaluation and early safety warning.

An Improvement of Equivalent Circuit Model for State of Health Estimation of Lithium-Ion Batteries Based on Mid-Frequency and Low-Frequency Electrochemical Impedance Spectroscopy

An Improvement of Equivalent Circuit Model for State of Health Estimation of Lithium-Ion Batteries Based on Mid-Frequency and Low-Frequency Electrochemical Impedance Spectroscopy

Detection of oxygen starvation during carbon corrosion in proton exchange membrane fuel cells using low-frequency electrochemical impedance spectroscopy

Proton exchange membrane fuel cells (PEMFCs) are considered a significant player in the hydrogen economy. However, before mass production is possible, significant improvements in durability are necessary. Monitoring the changes in the electrode structure is challenging without a complex measurement apparatus. Precisely, the changes in electrode properties during carbon corrosion (increase in the porosity and electrode collapse) cannot be quantified using conventional electrochemical methods. Here, we report capturing the oxygen diffusivity in the PEMFC cathode catalyst layer using low-frequency electrochemical impedance spectroscopy (0.3-0.01 Hz). The low-frequency arc is fitted with resistance, inductance, and capacitance in parallel to represent the resistance to oxygen supply, inertia to oxygen diffusion, and oxygen storage capacity in the catalyst layer, respectively. Over 600 cycles of accelerated stress test (ASTs) of carbon corrosion, the capacitance increases by 25–45% (0–150 ASTs), indicating an increase in oxygen storage capacity and electrode porosity. Then, (150–600 ASTs) the resistance and inductance increase while the capacitance decreases by 80%, highlighting a decrease of the oxygen diffusivity and storage in the catalyst layer as the electrode collapses, which causes oxygen starvation. Altogether, this low-frequency approach correlates electrochemical impedance measurements with the changes in electrode structure during carbon corrosion.

Yeast Propagation Control: Low Frequency Electrochemical Impedance Spectroscopy as an Alternative for Cell Counting Chambers in Brewery Applications

Electrochemical impedance spectroscopy is a powerful tool in life science for cell and pathogen detection, as well as for cell counting. The measurement principles and techniques using impedance spectroscopy are highly diverse. Differences can be found in used frequency range (β or α regime), analyzed quantities, like charge transfer resistance, dielectric permittivity of double layer capacitance and in off- or online usage. In recent contributions, applications of low-frequency impedance spectroscopy in the α regime were tested for determination of cell counts and metabolic burden in Escherichia coli and Saccharomyces cerevisiae. The established easy to use methods showed reasonable potential in the lab scale, especially for S. cerevisiae. However, until now, measurements for cell counts in food science are generally based on Thoma cell counting chambers. These microscopic cell counting methods decelerate an easy and quick prediction of yeast viability, as they are labor intensive and result in a time delayed response signal. In this contribution we tested our developed method using low frequency impedance spectroscopy locally at an industrial brewery propagation site and compared results to classic cell counting procedures.

Non-enzymatic glucose sensing based on hierarchical platinum micro-/nanostructures

Non-enzymatic glucose monitoring for biomedical applications asks for long-term stable and selective sensors at neutral pH. A hierarchical platinum micro-/nanostructured electrode along with a novel measurement scheme leads to increased sensitivity, selectivity and stability compared to state-of-the-art. The hierarchical electrode coating was applied by a scalable two-step process, which combined electrochemical deposition and colloidal synthesis to obtain a hierarchical structure with high surface roughness. This three-dimensional structure consists of a cauliflower-like platinum deposition, which is coated by a platinum nanowire network. Amperometric glucose measurements showed a 10,000-fold increase in sensitivity (473 μA cm−2 mM−1) compared to unmodified electrodes and linear behavior in the physiological range. The obtained sensitivities are among the highest values reported for non-enzymatic glucose sensors in neutral pH media. The hierarchical morphology provides a selectivity mechanism depending on the reaction kinetics, improving the selectivity for glucose in the presence of the interferent ascorbic acid 2000-fold. Selectivity was further enhanced by low-frequency electrochemical impedance spectroscopy (EIS). Chronoamperometric protocols were developed to achieve long-term stability and overcome the loss of sensitivity due to electrode poisoning. Using chronoamperometric protocols for both the amperometry and the EIS measurements improved stability significantly. The presented results make hierarchical platinum micro-/nanostructured electrodes a promising candidate for continuous glucose monitoring.

Low-Frequency Electrochemical Impedance Spectroscopy as a Monitoring Tool for Yeast Growth in Industrial Brewing Processes

Today’s yeast total biomass and viability measurements during the brewing process are dependent on offline methods such as methylene blue or florescence dye-based staining, and/or the usage of flow cytometric measurements. Additionally, microscopic cell counting methods decelerate an easy and quick prediction of yeast viability. These processes are time consuming and result in a time-delayed response signal, which not only reduces the knowledge of the performance of the yeast itself, but also impacts the quality of the final product. Novel approaches in process monitoring during the aerobic and anaerobic fermentation of Saccharomyces cerevisiae are not only limited to classical pH, dO2 and off-gas analysis, but they also use different in situ and online sensors based on different physical principles to determine the biomass, product quality and cell death. Within this contribution, electrochemical impedance spectroscopy (EIS) was used to monitor the biomass produced in aerobic and anaerobic batch cultivation approaches, simulating the propagation and fermentation unit operation of industrial brewing processes. Increases in the double-layer capacitance (CDL), determined at frequencies below 1 kHz, were proportional to the increase of biomass in the batch, which was monitored in the online and inline mode. A good correlation of CDL with the cell density was found. In order to prove the robustness and flexibility of this novel method, different state-of-the-art biomass measurements (dry cell weight—DCW and optical density—OD) were performed for comparison. Because measurements in this frequency range are largely determined by the double-layer region between the electrode and media, rather minor interferences with process parameters (aeration and stirring) were to be expected. It is shown that impedance spectroscopy at low frequencies is not only a powerful tool for the monitoring of viable yeast cell concentrations during operation, but it is also perfectly suited to determining physiological states of the cells, and may facilitate biomass monitoring in the brewing and yeast-propagating industry drastically.

A Novel Application for Low Frequency Electrochemical Impedance Spectroscopy as an Online Process Monitoring Tool for Viable Cell Concentrations.

New approaches in process monitoring during industrial fermentations are not only limited to classical pH, dO2 and offgas analysis, but use different in situ and online sensors based on different physical principles to determine biomass, product quality, lysis and far more. One of the very important approaches is the in situ accessibility of viable cell concentration (VCC). This knowledge provides increased efficiency in monitoring and controlling strategies during cultivations. Electrochemical impedance spectroscopy—EIS—is used to monitor biomass in a fermentation of E. coli BL21(DE3), producing a recombinant protein using a fed batch-based approach. Increases in the double layer capacitance (Cdl), determined at frequencies below 1 kHz, are proportional to the increase of biomass in the batch and fed batch phase, monitored in offline and online modes for different cultivations. A good correlation of Cdl with cell density is found and in order to get an appropriate verification of this method, different state-of-the-art biomass measurements are performed and compared. Since measurements in this frequency range are largely determined by the double layer region between the electrode and media, rather minor interferences with process parameters (aeration, stirring) are to be expected. It is shown that impedance spectroscopy at low frequencies is a powerful tool for cultivation monitoring.

Multi-Scanning Droplet Cell Microscopy (multi-SDCM) for truly parallel high throughput electrochemical experimentation

3D printing was used to realise an 8 channel measuring head that is supplying individual droplets onto a surface of interest. Each of these droplets is individually addressed using separate counter and reference electrodes and can thus be operated completely independent. In this way 8 independent electrochemical experiments can be performed simultaneously using a multichannel potentiostat, alternatively time critical measurements such as extensive potential or current transients can be probed using a quasi simultaneously time slice approach. Even low frequency electrochemical impedance spectroscopy can be parallelised. The performance is illustrated by local anodic oxidation of industrial grade titanium sheets which allows a visual inspection of the differently coloured oxide spots formed at voltages ranging from 11–18V. The reproducibility is confirmed both within the individual channels and for multiple anodisation positions.

Electrochemical impedance evaluation of aluminum/boron–epoxy composite bonds

Al/boron-epoxy double lap-shear specimens with five different surface preparations were exposed to hot, humid conditions for up to 3 years. Electrochemical impedance spectroscopy (EIS) measurements were taken using an in situ sensor throughout the exposure period. Periodically, some specimens were pulled to determine bond strength degradation. Aside from a set of deliberately poorly prepared specimens, all specimens retained at least 80% of their initial bond strength with the best specimens showing only a little degradation of strength. The low-frequency EIS impedance for measurements taken with sensor electrodes on the same composite surface decreased exponentially by two orders of magnitude during the exposures as the composite absorbed moisture. This decrease happens well before significant bond degradation, thus allowing the measurements to be used as a warning of moisture intrusion before the moisture has weakened the interface and resulted in a reduction of bond properties. One application of this bond health monitoring technology would be in bonded repair patches.

Low Frequency Impedance Studies on Covalently Modified Glassy Carbon Paste

Cyclic voltammetry and impedance studies have been carried out on covalently coupled horseradish peroxidase, hemin, reconstituted horseradish peroxidase and blank glassy carbon paste electrodes. The cyclic voltammogram of native enzyme paste showed a clear oxidation peak at −50 mV and an increase in the oxidation currents in presence of the substrate was observed. In order to increase the electron transfer kinetics the redox center (hemin) of the enzyme was directly bound onto the electrode followed by reconstitution with apoperoxidase. Hemin bound paste showed a reduction peak at −275 mV and an oxidation peak at ca. −50 mV. Upon reconstitution with apoperoxidase a shift of 175 mV in the reduction peak and ca. 100 mV in the oxidation peak was observed. We have also studied the effect of electron donors like hydroquinone and p-aminobenzoic acid on the modified electrodes. It was observed that the native enzyme showed a clear increase in the electron transfer in the presence of electron donor whereas with the reconstituted enzyme paste the effect of the electron donor was not evident. Low frequency electrochemical impedance spectroscopy was carried out in the frequency range of 10 mHz to 1 kHz in order to study the kinetics of slow processes occurring at the electrode surface. The impedance data of the system in the supporting electrolyte was simulated to an equivalent circuit model, the modified Randles cell circuit for an irreversibly adsorbed species. The data in the presence of substrate was modeled to modified Randles cell circuit with Warburg impedance. The models show a reasonably good fit in the frequency region studied. The unmodified blank paste electrode showed more or less a constant phase angle behavior. From the simulated data the parameters values of the circuit used were obtained which were used to calculate the kinetic parameters.

Characterization of vacancy transport in passive films using low frequency electrochemical impedance spectroscopy

Characterization of vacancy transport in passive films using low frequency electrochemical impedance spectroscopy