Electrochemical Impedance Spectroscopy Fitting Research Articles

Influence of Anode Flow-Field Channel Distance on the Performance of Polymer Electrolyte Membrane Water Electrolyzers

Polymer electrolyte membrane water electrolysis (PEMWE) is well suited for flexible and efficient hydrogen production based on electricity from renewable energy sources [1]. Of all PEMWE stack components, flow-fields (FF) or bipolar plates (BP) are currently among the most expensive parts: In an analysis from 2020, FF/BP are reported to account for more than half of current PEMWE stack costs with medium potential for cost reduction towards the next-generation [2]. Another recent study attributes only 28% of current PEMWE stack costs to FF/BP, but claims that next-generation bipolar plates will be 3 times less expensive [3]. In any case, the optimization of flow-fields represents an important lever to improve the economic competitiveness of PEMWE. Although sophisticated designs are reported in the literature, commercial flow-fields tend to follow simpler approaches [4]. For classic design patterns like serpentine or parallel channels, studies compared different channel patterns or assessed the influence of the channel width [5]. Nevertheless, systematic investigations on the influence of the channel distance (FF land width) on the PEMWE performance have not been reported so far. Recently, an optimal FF land width of 4 times the thickness of the porous transport layer (PTL) was postulated [6]. However, it remains unclear if this ratio is universally valid, because only one specific FF configuration and only one PTL type (with variable PTL thickness) was investigated in the study.In this work, we analyze the influence of the anode side flow-field channel distance (land width) on the cell performance of 5 cm2 PEMWE single-cells. Different single-channel serpentine flow fields with land widths ranging from 0.7 mm to 6.2 mm and a constant channel width of 1 mm are characterized as a function of PTL type (powder-sintered- as well as fiber-based titanium PTL). For all FF/PTL combinations, polarization curves up to 6 A/cm2 are performed with electrochemical impedance spectroscopy (EIS) measurements at each current step. The anode proton sheet resistance is evaluated by EIS fitting during blocking conditions. On one side, a voltage loss analysis reveals that mass transfer losses in PEMWE cells are very small for the best suited FF/PTL combinations (<15 mV at 6 A/cm²). On the other side, for large land widths in combination with certain PTL types, severe mass transport losses are observed at high current densities (increased cell as well as iR-free cell voltage, cf. figure 1a), accompanied by a partial dry-out of the membrane that is reflected by an increased high-frequency resistance (HFR, cf. figure 1b). For this case, the remaining water content in the membrane will be estimated. Porosity and pore structure of the PTL used in this study are analyzed ex-situ through mercury intrusion porosimetry (MIP). The PTL samples cover a broad range of pore sizes (≈15-60 µm) and porosities (≈30-70 %). Further analysis will be conducted to elucidate the contribution of PTL bulk properties such as their in-plane permeability to the observed mass transport losses for large flow-field land widths.

Corrosion Behavior of High-Strength C71500 Copper-Nickel Alloy in Simulated Seawater with High Concentration of Sulfide

The corrosion behavior of high-strength C71500 copper-nickel alloy in high concentrations of sulfide-polluted seawater was studied by potentiodynamic polarization measurements, electrochemical impedance spectroscopy (EIS), immersion testing, and combined with SEM, EDS, XPS, and XRD surface analysis methods. The results showed that the C71500 alloy shows activation polarization during the entire corrosion process, the corrosion rate is much higher (0.15 mm/a) at the initial stage of immersion, and the appearance of diffusion limitation by corrosion product formation was in line with the appearance of a Warburg element in the EIS fitting after 24 h of immersion. As the corrosion process progressed, the formed dark-brown corrosion product film had a certain protective effect preventing the alloy from corrosion, and the corrosion rate gradually decreased. After 168 h of immersion, the corrosion rate stabilized at about 0.09 mm/a. The alloy was uniformly corroded, and the corrosion products were mainly composed of Cu2S, CuS, Cu2(OH)3Cl, Mn2O3, Mn2O, MnS2, FeO(OH), etc. The content of Cu2S gradually increased with the extension of immersion time. The addition of S2- caused a large amount of dissolution of Fe and Ni, and prevented the simultaneous formation of a more protective Cu2O film, which promoted the corrosion process to some extent.

Superamphiphobic properties of fluorine doped graphene prepared by RF-PECVD: EIS and first principle study

A fluorine doped graphene (FDG) film was deposited on the surface of roving fabric by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). The micromorphology, functional group types, chemical composition, molecular structure and nanostructure image of the deposited films under different discharge power were characterized by field emission scanning electron microscope (FE-SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and transmission electron microscope (TEM). The static and rolling−water/−oil contact angle were measured by contact angle meter. The electrochemistry properties were tested using electrochemical workstation. The superamphiphobic mechanism was studied by electrochemical impedance spectroscopy (EIS) and density functional theory (DFT), and the corresponding software were Gemry Echem Analyst and Materials Studio 7.0, respectively. The results show that the obtained FDG film has the best hydrophobic and oil repellent performance under the discharge power of 80 W. F atom in the FDG film is arranged on both side and the capacitance structure (C-model) is presented in the equivalent element by EIS fitting and DFT calculation under the discharge power of 80 W. The phenomenon proves that the electron transfer of free electrons from the position of K point in the Brillouin region of graphene surface was forbidden by the F atom, which is the mechanism of superhydrophobic and oil drainage on the FDG film surface.

Correlation between electrochemical impedance measurements and corrosion rates of Mg-1Ca alloy in simulated body fluid

The corrosion behaviour of an Mg-1Ca alloy in simulated body fluid (SBF) for 14 days was investigated by electrochemical impedance spectroscopy (EIS), hydrogen evolution measurement, weight loss and observation of the corroded alloy surfaces, with the aim of establishing a clear correlation between the values of the parameter obtained from EIS fitting and corrosion rates. It was found that corrosion proceeds in three stages, characterized by differences in the values of resistances and by the weight of the relative contribution of the inductive behaviour to the overall EIS response. Among all the parameters obtained from EIS fitting, only the charge transfer resistance presented a good correlation with the corrosion rate over the whole duration of the measurement, and an apparent Stearn-Geary coefficient could be calculated. Based on the EIS results, a better understanding of the corrosion behaviour of the Mg-1Ca alloy in SBF solution was attained.

Dark photovoltaic effects of Ti-rooted TiO2 nanopillars as the anode in CdSe-sensitised solar cell

TiO2 nanopillars (NPs) rooted in Ti were cultivated hydrothermally with the strongest organic base tetramethylammonium hydroxide (TMAOH) in aqueous solution in Ti substrate. TiCl4 was used to coat TiO2 NPs with a nanoscale compact thin TiO2 layer against charge recombination with oxidised species in electrolyte. A consecutive layer of CdSe nanoparticles was induced by successive ionic layer adsorption and reaction (SILAR) mode for sensitisation by light harvesting. J–V curves surprisingly indicate that untreated TiO2 NPs photoanode rooted in Ti produces considerable currents in the dark (thermal photovoltaic power generation) even higher compared to that under intense UV illumination, signifying TiCl4–CdSe treatment does not profit. Electrochemical impedance spectroscopy fitting parameters further reveal that the parameters associated with injection and recombination issues have been improved, as a comprehensive electrochemical result of TiCl4–CdSe treatment. Thus, the current-limiting problem probably arises from the remarkable continuous absorption bands in a wide spectrum range from visible to far-infrared, which is confirmed by UV–vis absorption spectra. However, the reason for the substantial photocurrent-limiting is that the intense irradiating UV light causes the innumerable electrons distributed in the inter-band-gap states to transit to much higher levels above the CB edge of TiO2 with excessive energy, then they collide with TiO2 crystall lattices to release thermal energy eventually to raise the transport resistance.

Effects of temperature variation on Li x FePO4/C (0

LiFePO4/C was prepared via solid state reaction and characterized with X-ray powder diffraction and charge–discharge test. As-prepared LiFePO4/C has a triphylite structure and exhibits an excellent rate capability and capacity retention. Electrochemical impedance spectroscopy (EIS) was applied to investigate LixFePO4/C (0<x<1) electrode on temperature variation. The valid equivalent circuit for EIS fitting was determined which contains an intercalation capacitance for Li+ ion accumulation and consumption in the electrode reaction. The surface layer impedance needs to be included in the equivalent circuit when LiFePO4/C is deeply delithiated at a relatively high temperature. EIS examination indicates that a temperature rise leads to a better reversibility, lower charge transfer resistance, higher exchange current density J0 and greater Li+ ion diffusion coefficient for the LixFePO4/C electrode process. The Li+ ion concentration in LixFePO4/C is potential to impact the Li+ ion diffusion coefficient, and a decrease in the former results in an increase in the latter.

Novel approach to correlate degree of surface deterioration to coating impedance for laboratory test panels coated with two types of primers

Relationships between deteriorated coating surface conditions of laboratory test panels determined by image processing software and coating impedance data collected with electrochemical impedance spectroscopy (EIS) were investigated. A unique relationship was found between the log |Z|0·1 values of experimental EIS data and the corresponding deteriorated area ratios determined by the image processing method. A well defined one to one relationship was also found between log |Z|0·1 values and log (Rpo) values extracted through EIS data fitting analysis. This study demonstrates that estimation of log (Rpo) at a certain deterioration stage is feasible via field image processing alone without running in situ EIS measurements and going through time consuming EIS fitting analysis. A field coating maintenance program established by this methodology can be an effective tool for bridge maintenance engineers who need to make timely decisions on coating repair/rehabilitation work based on the quantitative ‘visual’ image scan results and ‘invisible’ coating pore resistance.

Correlationship between Microscopic Observations and Electrochemical Behaviour of Different Kind of Galvanized Steel

Zinc anodic dissolution has been studied according to the steel galvanized method by means of the electrochemical impedance spectroscopy (EIS) and microscopic observations. Relevant information on the galvanized method is provided by the analysis of experimental data. The galvanized method has no influence on the kinetics parameters of the zinc anodic dissolution process. The galvanized method only changes the surface texture of the working electrode. Thus, the EIS fitting allows to calculate the fractal dimension of the surface of the working electrode.