Dynamic Electrochemical Impedance Spectroscopy Research Articles

Evaluation of Carbon Support Corrosion Using Accelerated Stress Protocol and Impedance Spectroscopy

Proton Exchange Membrane Fuel Cell (PEMFC) gathering momentum again in various commercial applications has made scientists evaluate its components rigorously.[1–3] In this context, evaluation of the electrocatalyst and its supports has been vital. As widely investigated and accepted, carbonaceous materials are generally preferred as support due to their promising intrinsic characteristics like good electrical conductivity and higher surface area. The metal-support interaction plays a vital role in the catalytic activity and durability of electrocatalyst. Functionalization or inducing defects on these carbon support surface materials are much essential to achieve better impregnation of Pt nanoparticles with improved adherence. It also aids in achieving well dispersed nucleation of Pt on the support and hence exhibits enhanced electrochemical performance.[4] However, the carbon supports are susceptible to corrosion under PEMFC conditions and get accelerated when the fuel cell generates voltages > 1.2 V during start-up/shut-down and gross fuel starvation. This condition leads to deterioration of electrocatalyst layer through various mechanisms such as Pt dissolution, Ostwald ripening, coalescence via particle migration which ultimately leads to decreased electrochemical performance and durability of PEMFC. The stability of electrocatalyst and carbon corrosion resistant property of support material has an integral role in substantiating the longevity of fuel cell performance. Hence, understanding the role of carbon supports in corrosion, after functinalization becomes essential. Herein, we have attempted to evaluate the carbon supports rigoursly under functionalized and non-functionalized conditions. The protocol employed is an accelerated stress test (AST) of potential cycling between 1.0 V – 1.5 V vs. RHE for 5,000 cycles formulated by the U.S. Department of Energy.[5] The studies were also supported by carrying out dynamic electrochemical impedance spectroscopy near the oxidation potential of carbon. The corrosion behaviour of functionalized forms of carbon were found to be more corrosive than their respective non-functionalized forms due to surface oxygenated species. However, the functionalized form of acetylene black showed one order less in the corossion current compared to the conventional Vulcan carbon (Fig 1). In additon, the durability of acetylene black based carbon supported Pt electrocatalysts exhibited remarkable durability with less than 40% loss in their electrochemical surface area after the AST protocol study, satisfying the set DoE targets for 2020. Based on our results, we propose that the functionalized acetylene black will be a promising excellent corrosive resistance support for Pt electrocatlyst in PEMFCs with enhanced performance and also durability.

The Influence of Microstructure on the Passive Layer Chemistry and Corrosion Resistance for Some Titanium-Based Alloys.

The effect of microstructure and chemistry on the kinetics of passive layer growth and passivity breakdown of some Ti-based alloys, namely Ti-6Al-4V, Ti-6Al-7Nb and TC21 alloys, was studied. The rate of pitting corrosion was evaluated using cyclic polarization measurements. Chronoamperometry was applied to assess the passive layer growth kinetics and breakdown. Microstructure influence on the uniform corrosion rate of these alloys was also investigated employing dynamic electrochemical impedance spectroscopy (DEIS). Corrosion studies were performed in 0.9% NaCl solution at 37 °C, and the obtained results were compared with ultrapure Ti (99.99%). The different phases of the microstructure were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Chemical composition and chemistry of the corroded surfaces were studied using X-ray photoelectron spectroscopy (XPS) analysis. For all studied alloys, the microstructure consisted of α matrix, which was strengthened by β phase. The highest and the lowest values of the β phase’s volume fraction were recorded for TC21 and Ti-Al-Nb alloys, respectively. The susceptibility of the investigated alloys toward pitting corrosion was enhanced following the sequence: Ti-6Al-7Nb < Ti-6Al-4V << TC21. Ti-6Al-7Nb alloy recorded the lowest pitting corrosion resistance (Rpit) among studied alloys, approaching that of pure Ti. The obvious changes in the microstructure of these alloys, together with XPS findings, were adopted to interpret the pronounced variation in the corrosion behavior of these materials.

Dynamic electrochemical impedance spectroscopy of quasi-reversible redox systems. Properties of the Faradaic impedance, and relations to those of voltammograms

By analysing the electrochemical impedance spectra (EIS) of quasi-reversible redox systems, the two elements of the Faradaic impedance: charge transfer resistance and the coupled Warburg-coefficient can be obtained at a given potential. The same applies also to DEIS (dynamic EIS) measurements, when high frequency impedance spectra are measured while the potential is scanned to simultaneously accomplish cyclic voltammetry or other transient measurements. In case of DEIS both the charge transfer resistance and the Warburg coefficient depend on the applied potential program, e.g. on scan-rate. A theory is presented, yielding a transformation by which this dependence can be eliminated. The proposed procedure yields two, scan-rate independent, hysteresis-free functions, which are closely related to the EIS results, and also to the functions which are the transformed forms of the cyclic voltammograms as suggested in T. Pajkossy, S. Vesztergom, Electrochim. Acta,297 (2019) 1121. To illustrate the properties of the transformations and the functions involved, numerical simulations are also presented. The theory opens a new route for the high-accuracy, fast determination of charge transfer rate coefficients of quasi-reversible redox systems by employing DEIS.

Synergy between partially exfoliated carbon nanotubes-sulfur cathode and nitrogen rich dual function interlayer for high performance lithium sulfur battery

Confinement of lithium polysulfides (LPS) within the cathode is one of the major challenges of LithiumSulfur Batteries (LiS), which hinders its practical application. Herein, a simple strategy is demonstrated to reduce the dissolution of LPS by graphitic carbon nitride nanosheets (GCN) supported by a gas diffusion layer (GDL), which consists of high nitrogen content, acts as polar sites for LPS adsorption. The partially exfoliated multi-walled carbon nanotube is used as a sulfur host (PECNT/S), which accommodates the high amount of sulfur loading due to its high specific surface area and pore volume. PECNT/S/GCN-GDL cathode remarkably exhibits excellent cyclic stability at 1C with 92.2% of initial capacity retention even after 500 cycles due to the strong trapping of LPS by chemisorption on GCN while the electrically conducting GDL ropes the electron conduction as well as electrolyte and Li+ ion diffusion. Dynamic electrochemical impedance spectroscopy technique further provides the understanding of confinement of LPS on the resultant PECNT/S/GCN-GDL electrode during charge-discharge processes. In addition, the importance of the upper potential limits has been studied in this work. Our study thus addresses many challenges of the LiS batteries by the use of carbon-based nanomaterials.

On the physical definition of dynamic impedance: How to design an optimal strategy for data extraction

Dynamic electrochemical impedance spectroscopy (DEIS) has attracted the interest of researchers due to its capability of acquiring impedance spectra of non-stationary systems in a broad range of frequencies. Although developed in the 70's, a physical definition of dynamic impedance, and in general of dynamic transfer function, is not present in literature. In this manuscript the general concept of dynamic frequency response is given in correlation to the Volterra series expansion: this can be used to define the ideal dynamic impedance response of an electrochemical system. It is clarified that DEIS is the intermodulation of the ac signal with the dc signal. Starting from this, different strategies for the extraction of the dynamic impedance spectra are compared, which are based on different heuristic definitions of DEIS. There are two main methodologies, one based on the use of window functions and the other on quadrature filters. Limits and merits of the different methodologies are discussed in the frame of the data analysis and data handling and show that window functions suffers at recovering low-frequencies impedances while quadrature filters work best in experiments which have a periodic or quasi-periodic trend.

Superhydrophobic surface by SiO2 particle modified SiO2-Ni/a-C:H film deposition and superior corrosion protection

A superhydrophobic surface was constructed by a simple method of one-step electrochemical deposition in which methanol was used as carbon source and SiO2 nanoparticles/nickel were used as dopant. Carbon-based films doped with SiO2 nanoparticles and nickel (SiO2-Ni/a-C:H films) were fabricated for comparison experiments at various SiO2 nanoparticle concentrations. It was turned out that the maximum contact angle of SiO2-Ni/a C:H film with SiO2 nanoparticle concentration of 0.006 mg ml−1 (sample-3 film) is 162.4 degrees, and the minimum sliding angle is 1.2 degrees, showing excellent superhydrophobicity. Furthermore, dynamic potential polarization and electrochemical impedance spectroscopy (EIS) measurements indicated that the superhydrophobic film exhibited excellent anti-corrosion performance. It is very important for the industrial manufacture of corrosion-resistant superhydrophobic films and has broad prospects for expanding the application of carbon-based films.

Heterogeneous oxidation of highly boron-doped diamond electrodes and its influence on the surface distribution of electrochemical activity

The electrochemical active surface area (EASA) of polycrystalline boron-doped diamond (BDD) electrodes is heterogeneous and can be affected by numerous factors. There is a strong need for proper consideration of BDD heterogeneity in order to improve this material's range of application in electrochemistry. Localized changes in surface termination due to the influence of oxidation agent result in increased surface resistance. The observed behavior of this characteristic feature varies among individual grains, depending on their crystallographic orientation. Still, there is not much information about this key factor in terms of its influence on the electrochemical response of BDD. In this study we compared two approaches towards BDD surface oxidation, namely: anodic polarization at potentiostatic and potentiodynamic conditions. The surface impedance measurements via Nanoscale Impedance Microscopy (NIM) allowed the confirmation of diversified propensity for the modification of surface termination in BDD. We showed that the NIM studies provide a deep understanding on the electrical characterization and variation of surface resistance in BDD electrodes. In order to evaluate the actual heterogeneity of electrochemical activity distribution, voltammetry, dynamic electrochemical impedance spectroscopy (DEIS) and scanning electrochemical microscopy (SECM) studies were performed. For each investigated electrode, departure from the Randles-Sevcik equation was observed, with its level depending on the surface heterogeneity and oxidation treatment, justifying the standardization of pre-treatment procedure and development of non-standard model for diffusion transport in proximity of BDD electrode.

Improve Corrosion Resistance of Zircaloy by Copper–Graphene Nanocomposite Coatings

In this study, the corrosion behavior of Zircaloy was investigated in the presence and absence of copper–graphene nanocomposites coating. The coating was prepared employing Hummers’ and electrochemical reduction methods. The morphology of copper–graphene nanocomposites coating was studied using scanning electron microscopy (SEM). Corrosion behavior was investigated employing dynamic polarization and electrochemical impedance spectroscopy (EIS) tests in a solution containing lithium hydroxide (LiOH), boric acid (H3BO3), and deionized water. The results showed that corrosion resistance of Zircaloy increased with introduction of copper–graphene nanocomposites coatings. The lowest corrosion rate was attained in the Zircaloy with copper–graphene nanocomposites coating (corrosion rate: 0.040 mm/year). An approximately 20 times decrease in the corrosion rate was observed in the Zircaloy with copper–graphene nanocomposites coating when compared to the un-coated Zircaloy (corrosion rate: 0.831 mm/year).

Programmable dynamically changing RC model for evaluation of Dynamic EIS methods and instrumentation

Programmable dynamically changing RC model for evaluation of Dynamic EIS methods and instrumentation

Dynamic electrochemical impedance study of methanol oxidation at Pt at elevated temperatures

The methanol oxidation reaction (MOR) is studied at temperatures up to 140 °C by a combination of cyclic voltammetry, chronoamperometry, and dynamic electrochemical impedance spectroscopy (dEIS). A mechanistic analysis of the reaction is done based on the Tafel relation, the calculated activation energy, and the fitting of dEIS data. At the fuel cell relevant potentials, 0–0.80 V vs RHE, the MOR mechanism is similar at all temperatures. The rate-determining step is the adsorption of water at low overpotentials (< 0.50 V vs RHE), a combination of the methanol adsorption reaction and the surface reaction between adsorbed CO and OH at intermediate potentials (0.50–0.65 V vs RHE), and the methanol adsorption reaction at high potentials (> 0.65 V vs RHE). The shoulder on the oxidation peak at 0.60–0.65 V vs RHE corresponds well with where the CO coverage approaches zero.

Implementation of DEIS for reliable fault monitoring and detection in PEMFC single cells and stacks

Dynamic Electrochemical Impedance Spectroscopy (DEIS) was presented as novel method for diagnostic and monitoring of PEMFC stack and single cells operation. Impedance characteristics were obtained simultaneously with current - voltage characteristics for stack and each individual cell. Impedance measurements were performed in galvanodynamic mode. It allowed to compare performance of each cell and identification of faulty cell operation for activation, ohmic and mass transfer losses regions. The biggest difference in impedance value between healthy and faulty cell was registered for mass transfer losses region. The authors discussed the statistical selection of an equivalent circuit based on the course of χ2 value in the function of current.

Evaluation of the corrosion inhibiting efficacy of a newly synthesized nitrone against St37 steel corrosion in acidic medium: Experimental and theoretical approaches

A novel amphiphilic nitrone, N-phenyl-1-(4-((11-(pyridin-1-ium-1yl) undecanoyl) oxy)phenyl)methanimine oxide bromide (NP-1-4-11-PUOPMOB) has been synthesized from a fatty acid derivative as a starting material. Structural characterization of the new compound has been realized by spectroscopic techniques (FTIR, 1H NMR, and 13C NMR). The corrosion inhibition effect of the compound for St37 steel corrosion in 1 M HCl medium has been investigated using experimental (weight loss, electrochemical impedance spectroscopy, potentiodynamic polarization, dynamic electrochemical impedance spectroscopy) and theoretical approaches complemented by surface morphological examination using energy dispersive X-ray spectroscopy, scanning electron microscope, and atomic force spectroscopy. Results from both chemical and electrochemical techniques reveal that the presence of the nitrone in the acid solution impedes St37 steel corrosion. The inhibition efficiency obtained at 125 ppm and 150 ppm concentrations for all methods is found to be over 90%. NP-1-4-11-PUOPMOB behaves as a mixed type corrosion inhibitor according to the potentiodynamic polarization studies. The adsorption of NP-1-4-11-PUOPMOB molecules onto the metal surface follows Langmuir adsorption isotherm and the calculated Kads (equilibrium constant of the adsorption process) value reflects strong interaction. There is evidence of NP-1-4-11-PUOPMOB adsorption on the metal surface from SEM, EDAX, and AFM studies. Experimental and theoretical results are in good agreement.

Probing the Reaction Interface in Li-Oxygen Batteries Using Dynamic Electrochemical Impedance Spectroscopy: Discharge-Charge Asymmetry in Reaction Sites and Electronic Conductivity.

Fundamental questions concerning the reaction interface in Li-O2 batteries, including where reactions occur and discharge-charge asymmetries come from, have stimulated a flurry of investigations; nevertheless, heated debates still prevail. Dynamic electrochemical impedance spectroscopy (EIS) is employed here to probe the reaction interface in a Li-O2 battery under potentiostatic and galvanostatic modes. Two impedance semicircles are identified during discharge with the high- and the low-frequency ones related to the Li2O2 film and the oxygen reduction reaction (ORR), respectively. However, upon triggering the oxygen evolution reaction (OER), only one semicircle is observed, implying that the reaction interface changes. Combining qualitative analysis on the EIS structure and quantitative information obtained from model fitting reveals that the ORR occurs on the Li2O2-electrolyte interface during discharge and the OER occurs on the electrode surface during charge. In addition, it is found that the electronic conductivity of Li2O2 is higher at oxidative potentials (charge) than reductive potentials (discharge). Discharge-charge differences in the reaction interface and the electronic conductivity reported here expand the scope of discharge-charge asymmetries of Li-O2 batteries.

The Use of Dynamic Electrochemical Impedance Spectroscopy in Corrosion Inhibitor Studies

Dynamic Electrochemical Impedance Spectroscopy (DEIS) has been used as an effective method for determining information about corrosion and inhibitor systems. Thus, it has been successfully applied to investigate the formation mechanism of pitting corrosion, the damage mechanism occurring over time in organic coatings, the cavitation caused by corrosion detection and the monitoring of the battery discharging process. This method has been suggested as an effective and useful tool for establishing the time of the best inhibition activity. In this study, benzotriazole (BTA) was used as a corrosion inhibitor for St37 steel in a 0.1MHCl environment. Consequently, the advantage of using DEIS to determine the electrochemical changes occurring over time with the addition of an inhibitor was demonstrated.

Degradation of IrOx Nanoparticles Supported Onto Sb-Doped SnO2 Aerogel Monitored By Dynamic Electrochemical Impedance Spectroscopy and Identical-Location TEM

Supporting metal nanoparticles is a common approach in heterogeneous gas-phase catalysis to decrease the metal loading, prevent agglomeration and thus minimize the cost of a catalytic conversion. This approach proved particularly successful in proton-exchange membrane fuel cells (PEMFC) where the replacement of Pt-blacks (used in early PEMFCs) by carbon-supported Pt nanoparticles has significantly improved the Pt specific power density. Using the same material’s concepts in proton-exchange membrane water electrolysers (PEMWE) could minimize the noble metal loading especially at the anode where the oxygen evolution reaction (OER) takes place. However, high-surface area carbon supports are rapidly degraded in the operating conditions of a PEMWE anode (E > 1.6 V vs. the reversible hydrogen electrode, T = 80 °C) calling for alternative support materials. In this contribution, IrOx nanoparticles were synthesized using the polyol method and deposited onto antimony doped tin dioxide aerogel (ATO) or Vulcan XC-72 (as a reference), and tested as OER electrocatalysts. By combining classical electrochemical techniques, dynamic Electrochemical Impedance Spectroscopy (dEIS) and Identical-Location Transmission Electron Microscopy (IL-TEM), we gained some insights into the mechanisms leading to loss of OER activity. IL-TEM images (Figure 1) provide strong evidence that the activation step was detrimental to the ATO support while no major degradation was observed at higher voltages (OER region). Approximately 73 % of the initial activity was lost for IrOx/ Vulcan XC-72 after 4,000 potential cycles between 1.2 and 1.6 V vs. the reversible hydrogen electrode (RHE) compared to 16 % for IrOx/ATO. The polarization resistance measured in the OER region by dEIS remains roughly the same for IrOx/ATO while a large increase was observed for IrOx/Vulcan XC-72. Figure 1

A Dynamic Impedance Study of the Initial Stages of Nickel Oxidation

The nature of the species that form on oxidizing Ni electrode in alkaline solution has been studied for a long time. Bode summarized the formation and interconversion of these hydroxide and oxyhydroxide phases in terms of time and potential [1], but their structure and kinetics remain an active area of study [2]. The initially formed oxide is believed to be a hydrated hydroxide, α-Ni(OH)2, which irreversibly interconverts to a less hydrated β-Ni(OH)2 at higher potentials. The formation and interconversion of these hydroxide phases is studied here by dynamic electrochemical impedance spectroscopy (dEIS) [3,4], which applies a multi-frequency waveform during a cyclic voltammetry sweep or during a chronoamperometry experiment. It enables the study of non-steady states of the surface that cannot be accessed by conventional potentiostatic EIS. It is particularly suited to study the irreversible α-Ni(OH)2 to β-Ni(OH)2 interconversion. An electrochemical polishing procedure was used to remove the oxide layer in order to have a clean and oxide-free surface before doing the experiment. This step consisted of galvanostatic holding in phosphoric acid solution. Impedance data were collected during voltammograms at different slow sweep rates, with selected hold periods before, during, or after the peaks, and were correlated to charges under the voltammograms. The impedance data were fitted to equivalent circuits. In most cases the faradaic impedance approximated a charge-transfer resistance, but the use of complex capacitance plots showed evidence for additional structure at low frequencies. Comparison of the the polarization resistance determined from the slope of the voltammograms with the estimated low-frequency intercept of the impedance showed a discrepancy in the peak region, which also suggests that a slow process exists that whose kinetics is not determinable by dEIS. The sweep-hold experiments followed the conversion of the α phase to the β phase, and found that the oxidation is not complete in the potential range of 0.2-0.3 V. Voltammograms of the admittance at selected frequencies were found to be sensitive to the surface condition, and showed that the surface condition did not return to the same value after cycling. This research was conducted as part of the Engineered Nickel Catalysts for Electrochemical Clean Energy project administered from Queen’s University and supported by Grant No. RGPNM 477963-2015 under the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Frontiers Program. [1] H. Bode, K. Dehmelt and J. Witte. Electrochim. Acta., 11, 8 (1966). [2] M. Alsabet, M. Grden and G. Jerkiewicz, Electrocatalysis, 2, 4 (2011). [3] R.L. Sacci, F. Seland and D.A. Harrington, Electrochim. Acta., 131, 13 (2014). [4] R.L. Sacci and D.A. Harrington, ECS Transactions, 19, 31 (2009).

Oxygen Evolution Reaction Investigation on Pt(111) and Ir(111) Using Dynamic Electrochemical Impedance Spectroscopy in Acidic Medium

Polymer electrolyte membrane water electrolyzers (PEMWE) are promising devices for energy conversion from electricity to chemistry. They are able to deal with important and fast changes in energy production. At the cathode compartment, the hydrogen evolution reaction occurs without kinetic limitation and practically reversibly. Nonetheless, the oxygen evolution reaction (OER) which is the anodic reaction remains a scientific challenge due to kinetic limitations and stability issues. Iridium is currently the best compromise between activity and stability [1]. However, it still undergoes degradation in the harsh oxidative and acidic operating conditions of a PEMWE [2]. The development of optimized OER catalysts is an essential step to practically implement PEMWE. A better understanding of structural effects on the catalysts activity and stability is needed. This study focuses on well-defined electrocatalysts at an atomic scale. The Ir(111) surface was characterized before and after electrooxidation and OER thanks to different electrochemical methods. A Pt(111) single-crystal, widely studied in the literature [3-4], was characterized in the same way and used as a reference. Above 1.4 vs. RHE and 1.3V vs. RHE, respectively for Pt(111) and Ir(111), the initial clean surface undergoes irreversible strucutral changes, related to the formation of defects and leading to an improvement of the oxygen evolution activity. Although classical electrochemical impedance spectroscopy (EIS) is a valuable technique when applied to stable and reversible systems, it fails with irreversible ones. Therefore, we used dynamic electrochemical impedance spectroscopy (DEIS). One can see (fig. 1) that the two methods give the same result in the reversible hydrogen adsorption area (fig.1a) while they differ for oxide formation (fig.1b) and OER (fig.1c). By analyzing the whole set of DEIS spectra, we are able to provide details on mechanism parameters for oxide formation and OER.

Understanding Reaction Mechanisms Using Dynamic Electrochemical Impedance Spectroscopy: Methanol and Formic Acid Oxidation

Electrochemical impedance spectroscopy (EIS) is a suitable tool for mechanistic studies due to the amount of data that is accessible through this technique [1]. However, even if the frequency range is wide, the mechanistic information is convoluted making the interpretation complicated. Therefore, data analysis of impedance data is often limited to pattern recognition and fitting of equivalent circuits. Methods for calculating EIS spectra from a stated mechanism is readily available and easily implemented [2]. However, the system under study needs to be well understood so that realistic reaction mechanisms can be formulated. In this work, methanol and formic acid oxidation at platinum electrodes were studied at temperatures up to 140°C by using a self-pressurized autoclave [3]. Dynamic EIS (dEIS) was used, where a multisine potential signal is superimposed on a cyclic voltammogram allowing for the calculation of the EIS spectra at any point during the voltammogram [4]. This enables the study of the reactions for transient surface conditions that are not accessible in a conventional steady-state EIS measurement. These two reactions have been characterized previously by EIS and ac voltammetry, and modeling of the reaction mechanisms have been reported and discussed [5,6]. In the case of methanol oxidation, six different reaction models were proposed and fitted to the experimental voltammogram using non-linear optimization in Maple. An example of the results of this procedure are shown in Fig. 1, and we were able to distinguish between the models, for example, the fitted dEIS spectra indicated that the surface reaction between adsorbed CO and adsorbed OH was chemical, as was proposed by Kauranen and co-workers [7]. In the case of formic acid oxidation, nine different models were tested, and a ternary reaction pathway model was necessary to give satisfactory fit to the experimental data. The role of adsorbed formate (HCOO) has been controversial [8,9], and through our mechanistic modeling, we found that formate likely contributes to both the direct (no strongly adsorbed intermediates) and indirect (through adsorbed CO) reaction pathways. In many cases, the suggested reaction mechanisms could all reasonably represent the experimental cyclic voltammogram, as exemplified by Fig. 1, and the dEIS data was necessary to distinguish between the models. This demonstrates the important role that EIS and dEIS can have in mechanistic studies of electrochemical reactions. 1. D. D. Macdonald, Electrochim. Acta, 2006, 51, 1376-1388.2. D. A. Harrington, J. Electroanal. Chem., 1998, 449, 9-28.3. T. Holm, P. K. Dahlstrøm, O. S. Burheim, S. Sunde, D. A. Harrington, F. Seland, Electrochim. Acta, 2016, 222, 1792-1799.4. R. Sacci, F. Seland, D. A. Harrington, Electrochim. Acta, 2014, 131, 13-19.5. U. Krewer, M. Christov, T. Vidakovic, K. Sundmacker, J. Electroanal. Chem., 2006, 589(1), 148-159.6. F. Seland, R. Tunold, D. A. Harrington, Electrochim. Acta, 2008, 53(23), 6851-6864.7. P. S. Kauranen, E. Skou, J. Munk, J. Electroanal. Chem., 1996, 404(1), 1-13.8. K. Jiang, H.-Z. Zhang, S. Zou, W.-B. Cai, Phys. Chem. Chem. Phys., 2014, 16, 20360.9. J. Joo, T. Uchida, A. Cuesta, M. T. M. Koper, M. Osawa, Electrochim. Acta, 2014, 129, 127-136. Figure 1

Hydrothermal preparation and intrinsic transport properties of nanoscale Li2FeSiO4

Nanoscale Li2FeSiO4 cathode material is successfully prepared using a ball-milling assisted hydrothermal method with inorganic precursors. Structures, morphologies, and thermal stabilities of the samples are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric-differential scanning calorimetry (TG-DSC). The prepared Li2FeSiO4 samples are well crystallized and have an irregular cuboid morphology with particle sizes in the range of 50–300 nm. The crystal structure of pure Li2FeSiO4 is stable in air at room temperature, and no distinct new phase is observed for the sample annealed at 300 °C in argon. The temperature-dependent intrinsic transport properties of nanoscale Li2FeSiO4 are obtained using dynamic potential scanning (DPS) and electrochemical impedance spectroscopy (EIS); the ionic conductivity and electronic conductivity are 1.32 × 10−8 S/cm and 2.8 × 10−8 S/cm, respectively, at 20 °C. Nanoscale Li2FeSiO4 exhibits stable cycle performance at different rates from 0.1C to 2C, and at 0.1C its specific charge and discharge capacity remain 144.1 mAh/g and 138.8 mAh/g, respectively, after 50 cycles.

Understanding Reaction Mechanisms Using Dynamic Electrochemical Impedance Spectroscopy: Modeling of Cyclic Voltammetry and Impedance Spectra

The fitting of reaction models consisting of elementary reaction steps to experimental data constitutes a method for verification of the reaction models and the possibility of finding the kinetic parameters of a system. However, due to the complexity that easily arises due to numerous reaction steps, it is notoriously difficult to pose a problem and optimize it to a single solution. Therefore, the successful fitting of a reaction model is not proof since several models can potentially be fit successfully to the same experimental dataset. A possible solution to this problem is to fit the reaction models to one dataset and verify the results by using a complementary experimental technique under the same conditions. Dynamic electrochemical impedance spectroscopy (dEIS) is uniquely suited for such an approach. In this work, a method for fitting and optimizing kinetic parameters in a reaction mechanism to experimental data from cyclic voltammetry is described. Subsequently, the optimized kinetic parameters are used to calculate corresponding dEIS results. Methanol oxidation on Pt at elevated temperatures is used as an example of the application of this method.