AC Electrochemical Impedance Spectroscopy Research Articles

Investigation of hydrophobic treatments with perfluoropolyether derivatives of gas diffusion layers by electrochemical impedance spectroscopy in PEM-FC

In the present work the effect of two different hydrophobic surface treatments, for gas diffusion layer (GDL) for PEM fuel cells, is reported. This hydrophobic coating consists of two new perfluoropolyether (PFPE) dispersions; the PFPE used are commercially available products (Fluorolink® P56 and Fluorolink® TLS 5007, from Solvay Solexis s.p.a., Italy). The former perfluoropolyether (P56) is an anionic, segmented polyurethane with high molecular weight, while the latter (TLS) is a phosphate ammonium salt with lower molecular weight. The GDLs obtained were tested in a single fuel cell at the lab scale. The cell testing was run at 60°C with two different relative humidities (RHs) of the feeding gases: 80/60% and 80/100%, Hydrogen/Air respectively.The new PFPE coatings guarantee good cell performances and in order to assess ohmic resistances and electrochemical parameters, AC electrochemical impedance spectroscopy (EIS) of the running cell was also performed. The EIS spectra were recorded at OCV and from low to high current density (i.e. 0.17, 0.34, 0.52, 0.7 and 0.87A/cm2).

Effect of rheology controller agent addition to Micro-Porous Layers on PEMFC performances

Formulations of carbon-based inks for doctor blade applications were considered and the use of Carboxymethylcellulose (CMC) as rheology modifier and suspension stabilizer was proposed. Rheological measurement evidenced that CMC addition guarantees a good dispersion of the carbon powder which results in a higher stability of the ink. The electrical performances of Gas Diffusion Layer (GDL) coated with Micro-Porous Layers (MPLs) prepared with and without CMC were compared. A single PEMFC was used to assess the properties of these GDLs: electrochemical performances were tested in terms of I–V curves and of AC Electrochemical Impedance Spectroscopy (EIS) of the running cell. CMC-containing GDLs showed improved performances when the cell was operated at 80°C and RH 80–60. Analysis of the EIS spectra demonstrated that, at high Current Density (CD), the CMC-based GDLs suffer for water management. Such a behavior was ascribed to a hydrophilic character of the GDL due to residual amounts of CMC in MPL coating.

Effects of Vacuum Annealing on the Charge–Discharge Characteristics of Eutectic Al–Si/Al Thin Film as Anode Material for Li-Ion Batteries

In this study, radio frequency magnetron sputtering was used to prepare eutectic Al­Si/Al bi-layered films as anode materials and the effect of vacuum annealing in the charge­discharge capacity characteristics at different temperatures were discussed. For the purpose of 400nm Al­Si film can possess the lowest crystallization temperature, the eutectic composition was adopted. The pre-sputtered 40nm Al thin film not only reduced the resistivity of the composite anode film, but also diffused to prevent peeling between the Al­Si films and Cu foils after vacuum annealing. While the annealing temperatures were elevated (RT ³ 400°C), indexes of crystalline (IOC) and resistivities of specimens were changed. The properties of materials containing ASEC-400 (at RT) and ASEC-200 (at 55°C) had outstanding charge­discharge characteristics. The morphology transformation at the surface and cross section resulted from annealing at different temperatures and cycling testing were examined by Focus Ion Beam (FIB). Besides, the relationship between cycling performances and electrochemical characteristics of Al­Si/Al film anodes were also investigated by Cyclic Voltammetry and Electrochemical AC Impedance Spectroscopy (EIS). [doi:10.2320/matertrans.M2012086]

Understanding Elemental Reactions at Electrode | Electrolyte Interface in Rechargeable Li Ion Battery by Using Electrochemical AC Impedance Spectroscopy

Understanding Elemental Reactions at Electrode | Electrolyte Interface in Rechargeable Li Ion Battery by Using Electrochemical AC Impedance Spectroscopy

La<SUB>0.3</SUB>Sr<SUB>0.2</SUB>Mn<SUB>0.1</SUB>Zn<SUB>0.4</SUB> Oxide-Sm<SUB>0.2</SUB>Ce<SUB>0.8</SUB>O<SUB>1.9</SUB> (LSMZ-SDC) Nanocomposite Cathode for Low Temperature SOFCs

Nanocomposite based cathode materials compatible for low temperature solid oxide fuel cells (LTSOFCs) are being developed. In pursuit of compatible cathode, this research aims to synthesis and investigation nanocomposite La0.3Sr0.2Mn0.1Zn0.4 oxide-Sm0.2Ce0.8O1.9 (LSMZ-SDC) based system. The material was synthesized through wet chemical method and investigated for oxide-ceria composite based electrolyte LTSOFCs. Electrical property was studied by AC electrochemical impedance spectroscopy (EIS). The microstructure, thermal properties, and elemental analysis of the samples were characterized by TGA/DSC, XRD, SEM, respectively. The AC conductivity of cathode was obtained for 2.4 Scm(-1) at 550 degrees C in air. This cathode is compatible with ceria-based composite electrolytes and has improved the stability of the material in SOFC cathode environment.

Supercritical fluid processing of Nafion® membranes: Methanol permeability and proton conductivity

AbstractNafion® membranes commonly used in direct methanol fuel cells (DMFC) are typically limited by high methanol permeability. These membranes have phase‐segregated sulfonated ionic domains in a perfluorinated backbone, which make processing difficult and limited by phase equilibria considerations. This study used supercritical fluids (SCFs) as a processing alternative, since the gas‐like mass transport properties of SCFs allow for better penetration into the membranes and the use of polar cosolvents could also influence their morphology, thus fine‐tuning their physical and transport properties. The SCF processing was performed at 40°C and 200 bars using pure CO2 and CO2 with several polar cosolvents of different size and chemical functionalities like: acetic acid, acetone, acetonitrile, cyclohexanone, dichloromethane, ethanol, isopropanol, methanol, and tetrahydrofuran. Methanol permeability measurements revealed that the SCF processed membranes reduced the permeation of methanol by several orders of magnitude, especially with the use of some small polar cosolvents. Proton conductivity measurements, using AC electrochemical impedance spectroscopy, were on the order of 0.03–0.09 S/cm, which indicates that processing with SCF CO2 plus some cosolvents maintained the high proton conductivity while reducing the methanol permeability. The results are explained using XRD and SAXS. XRD analysis of the SCF processed samples revealed an increasing pattern in the crystallinity, which influenced the transport properties of the membrane. SAXS measurements confirmed the morphological differences that led to the changes in transport properties of the SCF processed membranes. Finally, processing flow direction (parallel versus perpendicular flow) played a major role in the morphological changes of this anisotropic membrane. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

리튬 이차전지의 흑연 음극에 형성된 표면피막의 열적 안정성

투과형 전자 현미경과 전기화학 임피던스 분광법을 이용하여 리튬 이차전지의 흑연 음극에 생성되는 표면피막의 고온 안정성을 고찰하였다. 투과형 전자 현미경을 이용한 분석에 의해 <TEX>$60^{\circ}C$</TEX>에서 저장되는 동안 표면피막의 일부가 전해질 용액 중에 용해되는 것이 확인되었으며, 이 과정에서 피막의 두께 감소 및 피막의 형상이 다공성 구조로 변하였다. 한편, 이 과정에서 피막의 조성 변화에 기인하는 것으로 추측되는 표면피막의 저항 증가가 전기화학 임피던스 분광법에 의하여 확인되었다. 표면피막의 고온 안정성은 vinylene carbonate, 1,3-propane sultone, etylene sulfite와 같은 첨가제에 의해 제한적으로 개선되었다. The stability at elevated temperatures of a surface film formed on a graphite electrode in lithium secondary batteries was investigated by transmission electron microscopy (TEM) and electrochemical AC impedance spectroscopy (EIS). TEM analysis revealed that the surface film partly dissolved in the electrolyte solution during storage at <TEX>$60^{\circ}C$</TEX>, resulting in a decrease in the thickness of the surface film and a change in its morphology to a porous structure. On the other hand, an increase in the impedance of the surface film which is attributable to a change in composition of the surface film was confirmed by EIS analysis during the storage at <TEX>$60^{\circ}C$</TEX>. It was also shown that the addition of vinylene carbonate or 1,3-propane sultone or etylene sulfite, even if limited, improves the stability of the surface film at elevated temperatures.

Dielectrophoretic capture of E. coli cells at micropatterned nanoelectrode arrays

This paper reports capture and detection of pathogenic bacteria based on AC dielectrophoresis (DEP) and electrochemical impedance spectroscopy (EIS) employing an embedded vertically aligned carbon nanofiber (VACNF) nanoelectrode array (NEA) versus a macroscopic indium-tin-oxide (ITO) transparent electrode in "points-and-lid" configuration. The nano-DEP device was fabricated using photolithography processes to define an exposed active region on a randomly distributed NEA and a microfluidic channel on ITO to guide the flow of labeled Escherichia coli cells, respectively, and then bond them into a fluidic chip. A high-frequency (100 kHz) AC field was applied to generate positive DEP at the tips of exposed CNFs. Enhanced electric field gradient was achieved due to reduction in electrode size down to nanometer scale which helped to overcome the large hydrodynamic drag force experienced by E. coli cells at high flow velocities (up to 1.6 mm/s). This DEP device was able to effectively capture a significant number of E. coli cells. Significant decrease in the absolute impedance at the NEA was observed in EIS experiments. The results obtained in this study suggest the possibility of integration of a fully functional electronic device for rapid, reversible and label-free capture and detection of pathogenic bacteria.

Preparation and Characterization of Nanocomposite Calcium Doped Ceria Electrolyte With Alkali Carbonates (NK-CDC) for SOFC

The entire world’s challenge is to find out the renewable energy sources due to rapid depletion of fossil fuels because of their high consumption. Solid oxide fuel cells (SOFCs) are believed to be the best alternative source, which converts chemical energy into electricity without combustion. Nanostructure study is required to develop highly ionic conductive electrolytes for SOFCs. In this work, the calcium doped ceria (Ce0.8Ca0.2O1.9) coated with 20% molar ratio of two alkali carbonates (CDC-M: MCO3, where M=Na and K) electrolyte was prepared by coprecipitation method. Ni based electrode was used to fabricate the cell by dry pressing technique. The crystal structure and surface morphology were characterized by an X-ray diffractometer, scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM). The particle size was calculated in the range 10–20 nm by Scherer’s formula and compared with SEM and TEM results. The ionic conductivity was measured by using ac electrochemical impedance spectroscopy method. The activation energy was also evaluated. The performance of the cell was measured 0.567 W/cm2 at temperature 550°C with hydrogen as a fuel.

Electrical conduction in (La 0.97Sr 0.03) 2(Mo 1 − x Ga x) 2O 9 − α ceramics

A novel series of ceramic samples (La 0.97Sr 0.03) 2(Mo 1 − x Ga x) 2O 9 − α (x = 0, 0.01, 0.03, 0.05, 0.08) were prepared by a solid-state reaction method. All the doubly doped ceramics have a single cubic structure except (La 0.97Sr 0.03) 2(Mo 0.92Ga 0.08) 2O 9 − α . The conduction behaviors of the samples with x ≤ 0.05 were investigated using various electrochemical methods including gas concentration cells, oxygen electrochemical permeation and AC impedance spectroscopy in the temperature range of 823–1273 K. In dry oxygen-containing atmosphere, the ceramic samples were almost pure oxide-ionic conductors. In wet oxygen-containing atmosphere, the ceramic samples were also almost pure oxide-ionic conductors, but didn't possess the protonic conduction. The electrochemical oxygen permeation experiment further verified the oxide-ionic conduction nature under oxygen-containing atmosphere. The phase transition has been completely suppressed in the doubly doped samples in both dry and wet air atmospheres. The oxide-ionic conductivities were affected by the doping levels, and increased in the order: σ (x = 0.05) < σ (x = 0) < σ (x = 0.03) < σ (x = 0.01). (La 0.97Sr 0.03) 2(Mo 0.99Ga 0.01) 2O 9 − α exhibited the highest oxide-ionic conductivity of 0.103 S cm −1 at 1273 K.

Novel Ni/Ti2N composite coating for sintered NdFeB permanent magnets

The paper presents the anticorrosion characteristics of a novel Ni/Ti2N composite coating applied through electrodeposition and cathodic arc physical vapour deposition for protection of sintered NdFeB permanent magnets that are susceptible to corrosion in active environments. The composite coating was evaluated in simulated marine and industrial environments with the help of dc polarisation techniques. The rate of coating degradation was determined through ac electrochemical impedance spectroscopy. The coating morphology and surface chemistry were studied with scanning electron microscope. X‐ray diffraction was used for the identification of component phases in the coating–substrate system. The results showed that the corrosion current density iCorr of the composite coated NdFeB magnets came out to be 0·8575 and 0·3011 μA cm−2 that is about 10 and 4% of the ceramic coated NdFeB magnets in the marine and industrial environments respectively. The magnetic properties remained unaffected and were as good as for the uncoated, NiCuNi and ceramic coated NdFeB magnets.

Non-dimensional analysis of PEM fuel cell phenomena by means of AC impedance measurements

AC impedance or electrochemical impedance spectroscopy (EIS) is becoming a fundamental technique used by researchers and scientists in proton exchange membrane (PEM) fuel cell analysis and development. In this work, in situ impedance measurements are presented for a series of operating conditions in a 50 cm 2 fuel cell. The electrode charge transfer resistance was determined from the corresponding arcs of the Nyquist diagrams. The analyses were performed for H 2/O 2 and H 2/air operation at different stoichiometric factors and reactant gases humidification. Characteristic time scales of charge transfer processes at the different operating conditions were estimated from the corresponding Bode plots. These values were used for a non-dimensional analysis of the different fuel cell electrochemical and transport processes, namely electrochemical reaction versus GDL reactant transport. Fuel cell adapted Damkhöler numbers are thus presented, where the results indicate that the GDL diffusion transport is the limiting process for the cases under analysis, especially when air is used as oxidant. Additional analysis of channel convective mass transport versus GDL diffusive mass transport is also presented.

Hysteresis in the measurement of double-layer capacitance at the gold–ionic liquid interface

The capacitance of the polycrystalline gold electrode–ionic liquid BMIMTf [1-butyl-3- methyl imidazolium trifluoromethanesulfonate (triflate)] interface has been studied using ac voltammetry and electrochemical impedance spectroscopy (EIS). Slow potential scanning reveals a massive hysteresis between potential scans toward negative or positive directions and provides a distorted C( E) curve. Kinetic studies indicate that 10 min wait times are required after each potential step for system relaxation. EIS resolves the overall series capacitance into contributions from ion adsorption and double-layer capacitance, C DL . Isolation of C DL reveals that even 10 min of equilibration time is insufficient to completely remove the hysteresis. The shape of the negative-going C DL ( E) curve is camel-shaped whereas the shape of the positive-going curve is bell-shaped.

Effect of YSZ electrolyte surface modification on the performance of LSM/YSZ composite cathode

In order to understand the effect of YSZ electrolyte surface morphology on the performance of LSM/YSZ composite cathode, the YSZ electrolyte surface is structured with an interlayer of YSZ by screen printing. Three different YSZ powders are used for screen printing: the as-received original powder, the high-energy milled fine powder, and the pre-sintered coarse powder. The morphologies of different YSZ powders, the microstructures of the modified YSZ interlayers, and half cells with LSM/YSZ composite cathode are studied with scanning electron microscope. The performance of the LSM/YSZ cathodes is characterized with ac electrochemical impedance spectroscopy by using symmetric cells at 700–800 °C and OCV in air. The results indicate that the polarization resistance is decreased when pre-sintered coarse YSZ powder is screen printed onto the YSZ electrolyte surface. This is attributed to the increase of the effective surface area of the YSZ electrolyte.

Electrochemical activity and stability of Pt catalysts on carbon nanotube/carbon paper composite electrodes

This article reports a facile microwave-assisted approach to synthesize Pt catalysts on carbon nanotube (CNT)/carbon paper (CP) composite through catalytic chemical vapor deposition. The Pt deposits, with an average size of 3–5 nm were uniformly coated over the surface of oxidized CNTs. The electrochemical activity and stability of the Pt–CNT/CP electrode were investigated in 1 M H 2SO 4 using cyclic voltammetry (CV) and ac electrochemical impedance spectroscopy. The Pt catalysts showed not only fairly good electrochemical activity (electrochemically active surface area) but also durability after a potential cycling of >1000 cycles. The analysis of ac impedance spectra associated with equivalent circuit revealed that the presence of CNTs significantly reduced both connect and charge transfer resistances, leading to a low equivalent series resistance ˜0.22 Ω. With the aid of CNTs, well-dispersed Pt catalysts enable the reversibly rapid redox kinetic since electron transport efficiently passes through a one-dimensional pathway. Thus, the CNTs do not only serve as carbon support, they also charge transfer media between the Pt catalysts and the gas diffusion layer. The results shed some light on the use of CNT/CP composite, offering a promising tool for evaluating high-performance gas diffusion electrodes.

Microstructural and Electrochemical Characterization of Ni/Ti&lt;sub&gt;2&lt;/sub&gt;N Composite Coating for Sintered NdFeB Permanent Magnets

Sintered NdFeB magnets have complex microstructure that makes them susceptible to corrosion in humid or moist environments. The paper presents the anticorrosion characteristics of a novel Ni/Ti2N composite coating applied through electrodeposition and cathodic arc physical vapour deposition (CAPVD) to sintered NdFeB permanent magnets. The performance of composite coating was evaluated in simulated marine environment with the help of dc polarization techniques. The rate of coating degradation was also determined by employing ac electrochemical impedance spectroscopy (EIS). The coating morphology and surface chemistry were studied with scanning electron microscope (SEM). X-ray diffraction (XRD) was used for identification of component phases in the coating-substrate system. The results showed that the composite coating provided an adequately improved corrosion protection to the sintered NdFeB magnets in the simulated marine environment compared to the earlier reported ceramic and metallic coatings. The composite coating did not damage the magnetic properties of coating-substrate system that remained at par with the ceramic and nickel coating having copper interlayer.

High performance LiFePO 4/C cathode for lithium ion battery prepared under vacuum conditions

LiFePO 4/C with smaller particle size (0.3–0.6 μm) was synthesized via a two-step vacuum sintering method. X-ray diffraction and scanning electron microscopy were used to detect the phases presented in the composites and observe sample morphology. In addition, AC electrochemical impedance spectroscopy, cyclic voltammetry, along with constant current discharge/charge tests, were used to characterize the electrochemical properties of the composites. It was shown that LiFePO 4/C with a single olive crystal structure could deliver discharge capacity of 145.5 and 108.7 mAh g −1 at 0.5 and 6C for the fist cycle, and kept reversible capacity of 147.5 and 117.1 mAh g −1 after 100 cycles.

Study the Diffusion Behavior of Li+ in the WO3 Electrochromic Device by Electrochemical AC Impedance Spectroscopy

not Available.

Conductivity studies of sol-gel prepared BaCe0.85−xZrxY0.15O3−δ solid electrolytes using impedance spectroscopy

A systematic investigation on doped barium cerate perovskites on conductivity was performed by means of ac electrochemical impedance spectroscopy technique. BaCe0.85−xZrxY0.15O3−δ powders (x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by a modified sol-gel Pechini method and sintered at 1,250 °C–1,450 °C, depending on Zr content, to obtain good densities (93–97% of the theoretical ones). The measured total conductivities for these solid solutions in three different atmospheres were reported: in dry oxygen, in dry nitrogen and wet (0.5 bar H2O) hydrogen (5%H2/Ar) atmospheres. Arrhenius plots recorded in dry oxygen as well as in dry nitrogen showed some residual hydration which remained in the specimens upon initial heating. The compositions with x = 0.3 and 0.4 gave conductivities close to 10−2 S/cm in 5%H2/Ar/H2O atmosphere at 600 °C. The isothermal conductivities values showed a little variation for x from 0.2 to 0.4 between 500 and 800 °C.

Effects of Surface-Film Formation on the Electrochemical Characteristics of LiMn2O4 Cathodes of Lithium Ion Batteries

cathodes of Li ion batteries tend to develop native surface films in carbonate-based electrolytes. By combining dc voltammetry and ac electrochemical impedance spectroscopy, we study how these films react with an electrolyte of in ethylene and diethyl carbonates. We demonstrate that such reactions can affect the measurement of the characteristic electrochemical parameters of the cathode, namely, the intercalation capacitance, the initial capacity loss, and the solid-state diffusion coefficient of . A generalized framework for data analysis, based on the considerations of electrode equivalent circuits, is used to combine the results of the dc and ac measurements. The conducting film on the cathode introduces an ohmic drop at the electrode surface. Oxidative dissolution of the electrode’s native surface film occurs during the first charge cycle. The potential- and cycle-dependent effects of the associated faradaic reactions have been examined under voltage-controlled charge/discharge conditions at different rates.