The effects of temperature on the performance of electrochemical double layer capacitors

An electrochemical double layer capacitor test cell containing activated carbon xerogel electrodes and ionic liquid electrolyte was tested at 15, 25 and 40 °C to examine the effect of temperature on electrolyte resistance (R_S) and equivalent series resistance (ESR) measured using impedance spectroscopy and capacitance using charge/discharge cycling. A commercial 10 F capacitor was used as a comparison. Viscosity, ionic self-diffusion coefficients and differential scanning calorimetry measurements were used to provide an insight into the behaviour of the 1,2-dimethyl-3-propylimdazolium electrolyte. Both R_S and ESR decreased with increasing temperature for both capacitors. Increasing the temperature also increased the capacitance for both the test cell and the commercial capacitor but proportionally more for the test cell. An increase in temperature decreased the ionic liquid electrolyte viscosity and increased the self-diffusion coefficients of both the anion and the cation indicating an increase in dissociation and increase in ionic mobility.     

Deep eutectic solvent-based supramolecular gel polymer electrolytes for high-performance electrochemical double layer capacitors

Gel polymer electrolytes (GPEs) can avoid the electrolyte leakage risk of electrochemical double layer capacitors (EDLCs). But aqueous GPEs often suffer from narrow electrochemical windows. Herein, a series of deep eutectic solvent (DES)-based supramolecular GPEs are firstly developed for carbon-based EDLCs with wide voltage windows. The as-fabricated DES-based GPE shows an ionic conductivity of ~58 mS cm^?1, which makes the stable voltage window of a carbon-based EDLC reach 2.4 V. The carbon-based EDLC exhibits a specific capacitance of 32.1 F g^?1, an energy density of 24.6 Wh kg^?1 and a capacitance retention of ~90% after 15,000 charge-discharge cycles. Moreover, when quinhydrone is added into the DES-based GPE, the specific capacitance and energy density of the corresponding EDLC can be further expanded to 60 F g^?1 and 43.6 Wh kg^?1, respectively. Therefore, our work may present a universal strategy to prepare novel supramolecular GPEs for high-performance EDLCs with wide voltage windows.     

Aging and failure mode of electrochemical double layer capacitors during accelerated constant load tests

Electrochemical double layer capacitors of the BCAP0350 type (Maxwell Technologies) were tested under constant load conditions at different voltages and temperatures. The aging of the capacitors was monitored during the test in terms of capacitance, internal resistance and leakage current. Aging was significantly accelerated by elevated temperature or increased voltage. Only for extreme conditions at voltages of 3.5 V or temperatures above 70 °C the capacitors failed due to internal pressure build-up. No other failure events such as open circuit or short circuit were detected. Impedance measurements after the tests showed increased high frequency resistance, an increased distributed resistance and most likely an increase in contact resistance between electrode and current collector together with a loss of capacitance. Capacitors aged at elevated voltages (3.3 V) exhibited a tilting of the low frequency component, which implies an increase in the heterogeneity of the electrode surface. This feature was not observed upon aging at elevated temperatures (70 °C).     

High-performance activated carbons for electrochemical double layer capacitors: Effects of morphology and porous structures

In this work, we prepare a series of activated carbons (ACs) from xylose with different combinations of hydrothermal synthesis (HTS) and chemical activation with KOH. The prepared ACs exhibit a variety of morphologies and porous structures, which depend on the conditions used in the hydrothermal synthesis and the chemical activation. The largest surface area (S_BET) of the prepared ACs, which is calculated by the Brunauere Emmette Teller (BET) method, is ~3500 m²/g, and the largest volume fraction of mesopores of the prepared ACs is 60%. The correlations between the specific electrochemical properties of the ACs and the structures of the ACs (ie, porous structures and morphologies) are discussed. Spherical ACs exhibit superior rate performance with low impedance. The ACs with three-dimensional interconnected network of large surface area and porosity possess high specific capacitance. The largest specific capacitance reaches 340 F/g at a current density of 0.5 A/g and 220 F/g at a current density of 50 A/g, which are superior or comparable to those of similar systems. The long-term capacitance retention is ~86.8% after 10 000 cycles at a current density of 50 A/g. The energy density reaches 48 Wh/kg at a power density of 13.6 kW/kg.     

Investigation of gas diffusion layer compression by electrochemical impedance spectroscopy on running polymer electrolyte membrane fuel cells

Two gas diffusion layers based on the same carbon cloth substrate, produced by an Italian Company (SAATI), and coated with microporous layers of different hydrophobicities, were assembled in a polymer electrolyte membrane fuel cell and its performances assessed. For comparison the cell mounting the carbon cloth without microporous layer was also tested. The membrane electrode assembly was made of Nafion^® 212 with Pt load 0.3/0.6 mg cm⁻² (anode/cathode). The cell testing was run at 60 °C and 80 °C with fully humidified air (100%RH) and 80%RH hydrogen feedings. The assembly of gas diffusion layers and membrane with electrodes was compressed to 30% and 50% of its initial thickness. For each configuration polarization and power curves were recorded; in order to evaluate the role of different GDLs, AC impedance spectroscopy of the running cell was also performed.The higher compression ratio caused the worsening of cell performances, partially mitigated when the operating temperature was raised to 80 °C. The presence of the microporous layer onto the carbon cloth resulted extremely beneficial for the operations especially at high current density; moreover, it sensibly reduces the high frequency resistance of the overall assembly.     

Study of PEM fuel cell performance by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy is a suitable and powerful diagnostic testing method for fuel cells because it is non-destructive and provides useful information about fuel cell performance and its components. This paper presents the diagnostic testing results of a 120 W single cell and a 480 W PEM fuel cell short stack by electrochemical impedance spectroscopy. The effects of clamping torque, non-uniform assembly pressure and operating temperature on the single cell impedance spectrum were studied. Optimal clamping torque of the single cell was determined by inspection of variations of high frequency and mass transport resistances with the clamping torque. The results of the electrochemical impedance analysis show that the non-uniform assembly pressure can deteriorate the fuel cell performance by increasing the ohmic resistance and the mass transport limitation. Break-in procedure of the short stack was monitored and it is indicated that the ohmic resistance as well as the charge transfer resistance decrease to specified values as the break-in process proceeds. The effect of output current on the impedance plots of the short stack was also investigated.     

Analytic parameter identification of proton exchange membrane fuel cell catalyst layer using electrochemical impedance spectroscopy

A finite transmission line is proposed for proton exchange membrane fuel cell reaction layer, when the faradic current is absent due to purging of Inert gas at the back of cathode and anode. Also a finite transmission line is presented when a charge transfer accrued among catalyst and electrolyte interface. The electrochemical impedances of finite transmission lines are computed using MATLAB software. Relative to the orders and types of the evaluated impedances, some relations to determine and identify the parameters of the proposed models are derived. In first model, it is shown that the electrical elements of transmission line can be extracted explicitly from the Nyquist and Bode diagrams whereas for the second one, some of the parameters cannot directly be investigated. However, using a numerical procedure, some valuable results about parameter variations are obtained.     

Mesoporous activated carbon fiber as electrode material for high-performance electrochemical double layer capacitors with ionic liquid electrolyte

Activated carbon fibers (ACFs) with super high surface area and well-developed small mesopores have been prepared by pyrolyzing polyacrylonitrile fibers and NaOH activation. Their capacitive performances at room and elevated temperatures are evaluated in electrochemical double layer capacitors (EDLCs) using ionic liquid (IL) electrolyte composed of lithium bis(trifluoromethane sulfone)imide (LiN(SO₂CF₃)₂) and 2-oxazolidinone (C₃H₅NO₂). The surface area of the ACF is as high as 3291 m² g⁻¹. The pore volume of the carbon reaches 2.162 cm³ g⁻¹, of which 66.7% is the contribution of the small mesopores of 2-5 nm. The unique microstructures enable the ACFs to have good compatibility with the IL electrolyte. The specific capacitance reaches 187 F g⁻¹ at room temperature with good cycling and self-discharge performances. As the temperature increases to 60 °C, the capacitance increases to 196 F g⁻¹, and the rate capability is dramatically improved. Therefore, the ACF can be a promising electrode material for high-performance EDLCs.     

Characterization of photoelectrochemical cells for water splitting by electrochemical impedance spectroscopy

The photocurrent-voltage characteristic of a photoelectrochemical cell for solar hydrogen production via water splitting, using undoped-hematite as photoanode, was obtained. Photoelectrochemical characteristics of the cell were also investigated by electrochemical impedance spectroscopy. Both techniques were carried out in the dark and under illumination. The analysis of the frequency spectra for the real and imaginary parts of the complex impedance allowed obtaining equivalent electrical analogs for the PEC cell operating in the dark and under 1 sun simulated illumination. Additionally, different electrode configurations were used (two and three-electrode arrangements). The two-electrode configuration allowed the study of the overall charge transfer phenomena occurring at the semiconductor, within the electrolyte and at the counter-electrode side of the cell, whereas the three-electrode configuration gave more detailed information concerning the double charged layer at the semiconductor/electrolyte interface.     

Principle and application of electrochemical impedance spectroscopy method

电化学阻抗测定技术是一种研究电极反应动力学和电化学体系中物质传递与电荷转移的有效方法,通过电化学阻抗数据所提供的信息,能够分析电极过程的特征,包括动力学极化,欧姆极化和浓差极化,为电化学过程设计,电极材料开发和电极结构研究提供基本依据.本文在介绍电化学阻抗谱技术原理的基础上,分别以液流电池,空气扩散电极过程为对象,阐述电化学阻抗谱中电荷转移,物质传递等过程以及多孔电极本身的电荷传递电阻等,并综述阻抗技术在设计电池结构,优化电极材料等方面的应用实例.     

Evaluation of polymer electrolyte membrane fuel cells by electrochemical impedance spectroscopy under different operation conditions and corrosion

Electrochemical impedance spectroscopy (EIS) was employed for in situ diagnosis for polymer electrolyte membrane fuel cells during operation. First, EIS was measured as a function of operation parameters such as applied current density, gas flow rates and gas humidification temperature. The resistance that correlated with conductivity of the membrane and the contact resistance between bipolar plate and gas diffusion layer (GDL) was set as R_m in the assumed equivalent circuit. The charge transfer resistances were considered for cathode (R_ct(C)). The value of R_ct(C) was sensitive to the parameters that affected cell voltage. Additionally, the diffusion resistance (R_d) was ascribed to the effect of oxygen supply and drainage of generated water. Second, the influence of corrosion of type 430 stainless steel bipolar plates was evaluated by EIS method during operation. Corrosion of the stainless steel bipolar plates resulted in an increase in the value of R_d.     

A study of the electrochemical processes in lithium-sulphur cells by impedance spectroscopy

The changes in the properties of lithium-sulphur cell components (electrolyte, sulphur and lithium electrodes) during cycling are studied by AC impedance spectroscopy. It is shown that during the charge and discharge of lithium-sulphur cells the conductivity of the electrolyte is changed. We believe that the observed changes in the electrolyte conductivity can be explained by the formation of soluble lithium polysulphides by electrochemical reactions. The properties of the electrolyte significantly influence the rate of the electrochemical processes which occur both on the sulphur and lithium electrodes in lithium-sulphur cells.     

Investigation of solar cell degradation using electrochemical impedance spectroscopy

The performance of solar cells reduces annually due to various unavoidable phenomena of thermal cycling, damp heat, UV exposure, and mechanical stress, etc. Generally, I-V characteristic is used to check the performance of the solar cell, but the minor stress conditions mentioned above are difficult to characterize by I-V measurement. Impedance spectroscopy is a widely used method in fuel cell and a battery that can be used to detect minor degradation in the solar cell by analyzing the change in equivalent circuit parameters. In this work, commercially available polycrystalline silicon solar cell is investigated under the condition of hotspot, mechanical stress, and disconnection of interconnection ribbon and then characterized by impedance measurement, Fourier transform (Bode plot) as well as I-V characteristic. The results show noteworthy decrease in parallel resistance (Rp) which is clearly visible in Nyquist plot in compare to the I-V characteristic. The Rp decreases in EIS from 283.60 to 234.80 Ω for mechanical stress test, from 273.0 to 187.10 Ω for hotspot and from 352.80 to 345.20 Ω for disconnection of interconnection ribbon test. The results confirm potential application of impedance measurement for solar cell characterization due to noteworthy change in equivalent circuit parameters after test conditions.     

Study on the electrochemical characteristics of quasi-solid-state electric double layer capacitors assembled with sulfonated poly(ether ether ketone)

Sulfonated poly(ether ether ketone) (S-PEEK) was synthesized by sulfonation of commercial PEEK. The S-PEEK membrane was prepared by casting from organic solution dissolving the polymer, and the polymer electrolyte was obtained by soaking the S-PEEK membrane in water or sulfuric acid solution. The effect of soaking solvent on the liquid uptake and the ionic conductivity of the polymer electrolyte has been investigated. The quasi-solid-state electric double layer capacitors (EDLCs) which consisted of activated carbon electrodes and polymer electrolyte were assembled, and their electrochemical characteristics were studied by cyclic voltammetry and charge–discharge cycle tests.     

Evaluation of the corrosion resistance of Ni(P)Cr coatings for bipolar plates by electrochemical impedance spectroscopy

In the present research, the corrosion resistance of Ni–P and Ni–P–Cr coatings on AA7075-T6 aluminum plates under simulated anodic and cathodic conditions of polymer electrolyte membrane fuel cells (PEMFC) has been studied by electrochemical impedance spectroscopy (EIS). Three Ni–P coatings 20 μm, 30 μm, and 40 μm thick applied by electroless deposition were tested. Besides, a two-layer Ni–P–Cr coating 30 μm thick was also analyzed. It was formed by an inner Ni–P layer, and an outer 10 μm thick chromium one added by electroplating. Corrosion tests were combined with interfacial contact resistance (ICR), roughness, contact angle, and SEM-EDX measurements. The best results were obtained for the 20 μm Ni–P and the two-layer Ni–P–Cr coatings, although the latter showed a high ICR value due to the high electrical resistivity of the chromium oxide surface formed. It was verified that coating degradation occurs when the electrolyte penetrates the micro-cracks and the nodular surface interfaces, reaching the base metal and causing the coating delamination. This behavior is associated with a sharp decrease in the polarization resistance (R_p) of the equivalent circuit model fitted to the EIS results.     

Multiple model impedance spectroscopy techniques for testing electrochemical systems

In recent years, a number of fundamental problems associated with impedance spectroscopy techniques for testing electro-chemical systems have come to light. This paper will briefly attempt to explain some of these difficulties and propose a possible solution.     

New insight into the discharge process of sulfur cathode by electrochemical impedance spectroscopy

In this paper, the electrochemical reactions of sulfur cathode during discharge–charge process were investigated by EIS technique combining with XRD, SEM and EDS methods. The discharge process of the sulfur cathode could be divided into two discharge regions. These are the first discharge region (2.5–2.05 V) where the reduction of elemental sulfur to form soluble polysulfides and further reduction of the soluble polysulfides occurs, and the second discharge region (2.05–1.5 V) where the soluble polysulfides are reduced to form a Li₂S solid film covered over the carbon matrix. It was found that the EIS can distinguish the individual contributions of charge transfer resistances, ion diffusion impedance and properties originating from Li₂S film in the frequency domain of 100 kHz to 100 mHz. During the upper voltage plateau, the impedance of interfacial charge transfer dominates the reduction reaction, while during the lower voltage plateau, the mass transport in the cathode is a control step. It was also proved that the solid Li₂S appeared at the beginning of the lower voltage plateau region and became denser during the following discharge process.     

Characterization of high-power lithium-ion batteries by electrochemical impedance spectroscopy. II: Modelling

A common way to model lithium-ion batteries is to apply equivalent circuit (EC) models. In this work two different EC models are build up and parameterized for a commercial 6.5 Ah high-power lithium-ion cell. Measured impedance spectroscopy data depending on temperature and state of charge (SOC) are used for parameter estimation.The first EC model consists of an ohmic resistor (R), an inductor (L) and three RC-elements (a parallel connection of a capacitor (C) and a resistor). The second EC model consists of one R, one L, two Zarc elements and a Warburg element. The estimated parameters were used to develop two empirical electrical cell models which are able to predict the voltage of the cells depending on current, temperature and SOC. Hereby the internal cell resistance Ri is based on the EC models and a Butler-Volmer adjustment. Both approaches were validated by current profiles, which cover typical automotive applications to prove the model performance at low temperatures and high dynamic operation. An accurate voltage prediction could be realized with both EC models. The second, more complex, model is able to predict cell voltage more precisely, but at the expense of up to four times higher computational effort.     

Study of current distribution and oxygen diffusion in the fuel cell cathode catalyst layer through electrochemical impedance spectroscopy

In this study, an analysis of the current distribution and oxygen diffusion in the Polymer Electrolyte Fuel Cell (PEFC) Cathode Catalyst Layer (CCL) has been carried out using Electrochemical Impedance Spectroscopy (EIS) measurements. Cathode EIS measurements obtained through a three-electrode configuration in the measurement system are compared with simulated EIS data from a previously validated numerical model, which subsequently allows the diagnostics of spatio-temporal electrochemical performance of the PEFC cathode. The results show that low frequency EIS measurements commonly related to mass transport limitations are attributed to the low oxygen equilibrium concentration in the CCL–Gas Diffusion Layer (GDL) interface and the low diffusivity of oxygen through the CCL. Once the electrochemical and diffusion mechanisms of the CCL are calculated from the EIS measurements, a further analysis of the current density and oxygen concentration distributions through the CCL thickness is carried out. The results show that high ionic resistance within the CCL electrolyte skews the current distribution towards the membrane interface. Therefore the same average current density has to be provided by few catalyst sites near the membrane. The increase in ionic resistance results in a poor catalyst utilization through the CCL thickness. The results also show that non-steady oxygen diffusion in the CCL allows equilibrium to be established between the equilibrium oxygen concentration supplied at the GDL boundary and the surface concentration of the oxygen within the CCL. Overall, the study newly demonstrates that the developed technique can be applied to estimate the factors that influence the nature of polarization curves and to reveal the effect of kinetic, ohmic and mass transport mechanisms on current distribution through the thickness of the CCL from experimental EIS measurements.     

3D-honeycomb carbons for high performance electrical double layer capacitors electrodes

 Sodium fulvic acid based hierarchical porous carbons (SFA-HPCs) with a specific surface area of 1919 m2/g and total volume of 1.7 cm3·g^–1 has been synthesized by a simple self-template method. The carbon skeleton can be formatted by the decomposition process of sodium fulvic acid (SFA) in a N₂ atmosphere. The sodium compund in SFA is used as a self-template to create the hierarchical porous structure. The unique hierarchical structure of SFA-HPCs provides an efficient pathway for electrolyte ions to be diffused into the internal surfaces of bulk electrode particles. It results in a high charge storage capacitance of 186 F·g^–1 at current load of 40 A·g^–1. The capacitance 230 F·g^–1 at 0.05 A·g^–1 and 186 F·g^–1 at 40 A·g^–1 show its high rate capability. Besides, it also achieves desirable cycling stability, 99.4% capacitance remained after 10000 cycles at 40 A·g^–1.