Lithium-based batteries are a class of electrochemical energy storage devices where the potentiality of electrochemical impedance spectroscopy (EIS) for understanding the battery charge storage ...mechanisms is still to be fully exploited. Generally considered as an ancillary technique, the application of EIS should be promoted focusing on improved experimental design of experiments and advanced data analysis using physics-based models.Electrochemical impedance spectroscopy is a key technique for understanding Li-based battery processes. Here, the authors discuss the current state of the art, advantages and challenges of this technique, also giving an outlook for future developments.
Electrochemical impedance spectroscopy (EIS) using alternating currents is a widely established technique to investigate kinetic aspects of batteries and their components, though it requires an ...interruption of battery operation with extra measurement time and effort. In this work, EIS is compared with the conventional galvanostatic (constant current) technique, which is based on direct currents, being the standard operation mode of batteries. Data from constant current measurements not only are representing application conditions but also are automatically and continuously generated during routine charge/discharge processes, i.e., without extra measurement efforts, and do give kinetic insights via the characteristic overvoltage (= resistance-reasoned voltage rise/decrease), as well. In fact, distinguishing between even very similar values for ohmic (R Ω), charge transfer (R ct ), and mass transport (R mt ) resistances can be done via analysis of overvoltage data from constant current measurements, as exemplarily demonstrated in symmetric Li||Li and LiNi0.6Mn0.2Co0.2O2 (NMC622)||Li cells with poly(ethylene oxide)-based solid polymer electrolyte, finally proving their validity. From a practical point of view, direct-current methods can be beneficial for R&D of kinetic aspects in batteries, as data is directly obtained and, thus, application-oriented.
Electrochemical impedance spectroscopy (EIS) is an accurate electrochemical method able to identify various electrochemical steps that occur in complex electrochemical systems such as battery cells. ...In order to extract the maximum information from given battery system, systematic experiments that combine EIS with other (complementary) techniques are needed, as reported occasionally in the recent literature. Additionally, a proper quantitative evaluation of measured spectra has to be based on physical models which, however, tend to be quite elaborate and frequently less accessible to the wide battery community. In various cases of practical interest, however, the models can be simplified as shown in this review. One level of simplification reduces the full solution to the well-known de Levie model and is frequently used for description of the effects of porous electrodes. The ultimate level of simplification, in turn, leads to a Randles-like equivalent circuit for each insertion electrode and a pure resistor for the electrolyte phase in separator. This review shows that care has to be taken when using these simplifications in order to keep the analysis consistent and physically sound.
Iridium-based particles, regarded as the most promising proton exchange membrane electrolyzer electrocatalysts, were investigated by transmission electron microscopy and by coupling of an ...electrochemical flow cell (EFC) with online inductively coupled plasma mass spectrometry. Additionally, studies using a thin-film rotating disc electrode, identical location transmission and scanning electron microscopy, as well as X-ray absorption spectroscopy have been performed. Extremely sensitive online time-and potential-resolved electrochemical dissolution profiles revealed that Ir particles dissolve well below oxygen evolution reaction (OER) potentials, presumably induced by Ir surface oxidation and reduction processes, also referred to as transient dissolution. Overall, thermally prepared rutile-type IrO2 particles are substantially more stable and less active in comparison to as-prepared metallic and electrochemically pretreated (E-Ir) analogues. Interestingly, under OER-relevant conditions, E-Ir particles exhibit superior stability and activity owing to the altered corrosion mechanism, where the formation of unstable Ir(>IV) species is hindered. Due to the enhanced and lasting OER performance, electrochemically pre-oxidized E-Ir particles may be considered as the electrocatalyst of choice for an improved low-temperature electrochemical hydrogen production device, namely a proton exchange membrane electrolyzer.
Electrochemical impedance spectroscopy (EIS) is a widely applied non-destructive method of characterisation of Li-ion batteries. Despite its ease of application, there are inherent challenges in ...ensuring the quality and reproducibility of the measurement, as well as reliable interpretation and validation of impedance data. Here, we present a focus review summarising best metrological practice in the application of EIS to commercial Li-ion cells. State-of-the-art methods of EIS interpretation and validation are also reported and examined to highlight the benefits and drawbacks of the technique.
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•Comprehensive evaluation of EIS-based techniques for investigation of commercial LIBs.•In-depth report and analysis of a large number of papers dealing with commercial LIBs.•Critical assessment of the state-of-the-art for EIS calibration and measurement.•Critical assessment of the state-of-the-art for EIS interpretation and validation.•Critical review of both standard and new, physics-based equivalent circuit models.
Polysulfides are central compounds in lithium–sulfur battery cells. However, the fundamental redox and diffusion properties of polysulfides are still poorly understood. We try to fill this gap by ...performing an accurate impedance spectroscopy investigation using symmetrical cells consisting of two planar glassy carbon electrodes separated with catholyte-soaked separator. The catholyte contains a mixture of selected polysulfides with predetermined nominal concentrations. Impedance measurements reveal textbook shapes of spectra for most polysulfide compounds or their mixtures. This allows reliable and accurate determination of the rate constant (exchange current density) for a given redox reaction as well as the diffusion coefficient and diffusion length for the rate-determining polysulfide species. Further, it is confirmed that polysulfides tend to disproportionate with time, which significantly changes the chemistry and electrochemistry of the system. Two approaches are proposed for identification of the prevailing redox mechanism in the resulting mixtures. The values of kinetic and transport parameters obtained for different cases of interest are commented on in significant detail. The study provides a solid basis for better understanding of the complex processes in polysulfide mixtures.
A general transmission line model that is able to describe accurately the measured impedance spectra of uncycled and cycled lithium electrodes is introduced. The model has all the essential features ...that are contained in analytical solutions for determining the impedance response of porous electrodes. In addition to that, it allows easy coupling between the various phenomena met in lithium anodes in contact with a separator: transport through a solid electrolyte interphase film, transport across “live lithium dendrites”, reaction inside “live lithium dendrites”, transport across “dead dendrites”, diffusion in a porous separator, and so forth. The model is used for quantitative analysis of measured impedance spectra collected at different C-rates and after different numbers of charge–discharge cycles. Further, combining impedance spectroscopy with scanning electron microscopy, several unique correlations between the morphological development of lithium anodes and development of impedance spectra are identified and discussed. Finally, several simplified schemes that allow identification of the main degradation or failure mechanism(s) occurring in cycled lithium anodes are presented.
Combining the abundance and inexpensiveness of their constituent elements with their atomic dispersion, atomically dispersed Fe–N–C catalysts represent the most promising alternative to ...precious-metal-based materials in proton exchange membrane (PEM) fuel cells. Due to the high temperatures involved in their synthesis and the sensitivity of Fe ions toward carbothermal reduction, current synthetic methods are intrinsically limited in type and amount of the desired, catalytically active Fe–N4 sites, and high active site densities have been out of reach (dilemma of Fe–N–C catalysts). We herein identify a paradigm change in the synthesis of Fe–N–C catalysts arising from the developments of other M–N–C single-atom catalysts. Supported by DFT calculations we propose fundamental principles for the synthesis of M–N–C materials. We further exploit the proposed principles in a novel synthetic strategy to surpass the dilemma of Fe–N–C catalysts. The selective formation of tetrapyrrolic Zn–N4 sites in a tailor-made Zn–N–C material is utilized as an active-site imprint for the preparation of a corresponding Fe–N–C catalyst. By successive low- and high-temperature ion exchange reactions, we obtain a phase-pure Fe–N–C catalyst, with a high loading of atomically dispersed Fe (>3 wt %). Moreover, the catalyst is entirely composed of tetrapyrrolic Fe–N4 sites. The density of tetrapyrrolic Fe–N4 sites is more than six times as high as for previously reported tetrapyrrolic single-site Fe–N–C fuel cell catalysts.
Water electrolysis powered by renewables is regarded as the feasible route for the production of hydrogen, obtained at the cathode side through electrochemical hydrogen evolution reaction (HER). ...Herein, we present a rational strategy to improve the overall HER catalytic performance of Pt, which is known as the best monometallic catalyst for this reaction, by supporting it on a conductive titanium oxynitride (TiON x ) dispersed over reduced graphene oxide nanoribbons. Characterization of the Pt/TiON x composite revealed the presence of small Pt particles with diameters between 2 and 3 nm, which are well dispersed over the TiON x support. The Pt/TiON x nanocomposite exhibited improved HER activity and stability with respect to the Pt/C benchmark in an acid electrolyte, which was ascribed to the strong metal–support interaction (SMSI) triggered between the TiON x support and grafted Pt nanoparticles. SMSI between TiON x and Pt was evidenced by X-ray photoelectron spectroscopy (XPS) through a shift of the binding energies of the characteristic Pt 4f photoelectron lines with respect to Pt/C. Density functional theory (DFT) calculations confirmed the strong interaction between Pt nanoparticles and the TiON x support. This strong interaction improves the stability of Pt nanoparticles and weakens the binding of chemisorbed H atoms thereon. Both of these effects may result in enhanced HER activity.
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