Inducing changes in performance and observation of electrochemical processes is quite easy to do using 'button cells', which are the favorite objects in most research studies. Unfortunately, those cells may sometimes provide misleading outcomes or lead to promising results, applicable only on small scale, belittling their technological value. In this study, a large-scale SOFC with a 100 cm2 cathode active surface area was subjected to 1700 h working time tests. After the stabilization period, a series of EIS measurements was taken under various changes in operational conditions. Variations in multiple parameters such as pO2, pH2, pH2O, drawn current, or working temperature were introduced to provide an 'electrochemical fingerprint' of the large-scale cell. The impedance data was processed via equivalent circuit fitting and DRT analysis. It was possible to assign the electrochemical and transport processes to the corresponding peaks in the DRT spectra. A clear separation of the peaks originating from diffusion and charge transfer-related resistances was obtained. The single-cell tests allowed for the analysis of the actual state of the repeating units in the working 15-cell stack. The material causes of the changes in the performance of the cells were found to be mostly related to changes in the microstructure of the anode via Ni migration and unwanted diffusion of LSC-born elements such as Co segregation or the formation of the SrOZrO2 phases. Furthermore, the degradation issues of the cells in the stack differ depending on the placement due to the formation of the hotspot in the central part.

Electrochemical impedance spectroscopy (EIS) is widely used in electrochemistry, energy sciences, biology, and beyond. Analyzing EIS data is crucial, but it often poses challenges because of the numerous possible equivalent circuit models, the need for accurate analytical models, the difficulties of nonlinear regression, and the necessity of managing large datasets within a unified framework. To overcome these challenges, non-parametric models, such as the distribution of relaxation times (DRT, also known as the distribution function of relaxation times, DFRT), have emerged as promising tools for EIS analysis. For example, the DRT can be used to generate equivalent circuit models, initialize regression parameters, provide a time-domain representation of EIS spectra, and identify electrochemical processes. However, mastering the DRT method poses challenges as it requires mathematical and programming proficiency, which may extend beyond experimentalists’ usual expertise. Post-inversion analysis of DRT data can be difficult, especially in accurately identifying electrochemical processes, leading to results that may not always meet expectations. This article examines non-parametric EIS analysis methods, outlining their strengths and limitations from theoretical, computational, and end-user perspectives, and provides guidelines for their future development. Moreover, insights from survey data emphasize the need to develop a large impedance database, akin to an impedance genome. In turn, software development should target one-click, fully automated DRT analysis for multidimensional EIS spectra interpretation, software validation, and reliability. Particularly, creating a collaborative ecosystem hinged on free software could promote innovation and catalyze the adoption of the DRT method throughout all fields that use impedance data.

This work presents the results of a comprehensive study on the impact of the A-site non-stoichiometry of SrxTi0.3Fe0.7O3-δ (x=0.90, 0.95, 1.00, 1.05) ceramics on their physicochemical properties. The materials were fabricated by the conventional solid-state reaction method and their structure was determined by X-ray diffractometry, X-ray photoelectron spectroscopy and electron microscopy. Their sintering and thermal expansion properties were then evaluated. The electrical properties of the materials were determined by electrical conductivity and electrical relaxation measurements (on bulk materials) and by electrochemical impedance spectroscopy (EIS) studies of symmetrical, CGO-electrolyte based, porous electrodes. Finally, fuel cell tests with the non-stoichiometric electrodes were evaluated. To elucidate the electrochemical reaction pathways for oxygen reduction/evolution reaction, EIS measurements were carried out in different pO2 and were analysed via the distribution of relaxation times method. The results showed a dependence of materials’ properties on the A-site non-stoichiometry, which can be used to fine-tune their properties, e.g. increase the surface exchange rate or decrease the thermal expansion coefficient.

Development of environmentally friendly, high performing oxygen evolution reaction (OER) catalysts is an important research challenge. In this work, iron doped strontium titanates with a general formula SrTi1-xFexO3-delta (x = 0.35, 0.50, 0.70, 0.90, and 1.00) denoted as STFx, were synthesized via a solid state reaction technique and characterized in terms of oxygen evolution reaction electrocatalysis in an alkaline electrolyte (0.1 M KOH). The produced powders were characterized by a high specific surface area (>20 m(2) g(-1)), beneficial for OER. The evaluation of specific activity indicated the following trend of increasing performance: STF35 < STF50 < STF70 < SFO < STF90. The lowest overpotential at 10 mAcm(-2) (GEO) of 410 mV (350 mV at 25 mu A cm(OX)(-2)) was achieved by STF90 with the corresponding Tafel slope of 60 mV dec(-1). The two materials with the highest Fe content (i.e. STF90 and SFO) showed, however, poor chemical stability in alkaline solution demonstrated by the dissolution of Sr. Based on the good electrochemical performance (similar to 460 mV at 10 mA cm(GEO)(-2), similar to 405 mV at 25 mu A cm(OX)(-2)) and chemical stability for at least 30 days (no Sr dissolution) of STF50, it can be considered an interesting, working at room temperature OER catalyst based on non-toxic and abundant elements. (C) 2021 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

This work evaluates the effects of the sintering temperature (800 °C, 900 °C, 1000 °C) of SrTi 1- x Fe x O 3- δ ( x = 0.35, 0.5, 0.7) porous electrodes on their electrochemical performance as potential oxygen electrode materials of solid oxide cells. The materials were prepared by a solid-state reaction method and revealed the expected cubic perovskite structure. After milling, the powders were characterised by a sub-micrometre particle size with high sinter-activity. It was shown that the lowest area specific resistance was achieved after sintering SrTi 0.65 Fe 0.35 O 3 electrodes at 1000 °C, and SrTi 0.5 Fe 0.5 O 3 and SrTi 0.30 Fe 0.70 O 3 electrodes at 800 °C, which can be considered to be a relatively low temperature. In general, EIS measurements showed that increasing the Fe content results in lowered electrode polarisation and a decrease of the series resistance. Even though the studied materials have much lower total conductivities than state-of-the-art electrode materials (e.g. (La,Sr)(Co,Fe)O 3 ), the polarisation resistances obtained in this work can be considered low.

Iron doped strontium titanates (SrTi1-xFexO3-delta) are an interesting mixed ionic-electronic conductor model used to study basic oxygen reduction/oxidation reactions. In this work, we performed an impedance spectroscopy study on symmetrical porous SrTi0.30Fe0.70O3-delta (STF70) electrodes on a ceriabased electrolyte. The sample was measured in varying oxygen concentration: from 0.3% to 100% in 800 degrees C-500 degrees C temperature range. Low polarisation resistance (e.g. <125 mU cm(2) at 600 degrees C in the air) values were obtained, showing an overall high performance of the STF70 electrode. Impedance data analysis was assisted by the distribution of relaxation times method, which allowed an equivalent electrical circuit to be proposed comprising of two resistance/constant phase element sub-circuits connected in series. The medium frequency contribution, with a characteristic frequency of similar to 2000 Hz at 800 degrees C in air, originates most probably from possible surface diffusion followed by charge transfer reaction limitation, whereas the lower frequency contribution (characteristic frequency <10 Hz) is due to gas-phase diffusion. (C) 2020 The Authors. Published by Elsevier Ltd.

Solid Oxide Fuel Cells (SOFC) are based on electrolytes and mixed ionic and electronic conductivity (MIEC) materials. The need to reduce costs causes an increase in interest of new compounds suitable for operating temperatures between 600 °C and 800 °C. The SrTi1-xFexO3 (STF) perovskite material is a perspective material that could be used for the oxygen electrodes. In this work STF materials with different content of iron (x = 0.35, 0.5 and 0.7) have been evaluated. The paper presents synthesis, sintering properties, paste and layer preparation with preliminary electrical measurements. The results show that the electrical conductivity increases with the addition of iron, whereas the activation energy decreases. Based on these results, the applicability of STF as a potential oxygen electrode was discussed.

This work evaluates porous Sr0.86Ti0.65Fe0.35O3 (STF35) as a possible oxygen electrode material for Solid Oxide Cells. The powder synthesis was performed by solid state method. Characterization included DC electrical conductivity study of sintered bulk samples and impedance spectroscopy study of symmetrical electrodes deposited on gadolinium doped ceria substrates. Measurements were carried out in atmospheres with different pO(2) levels: 0.1%-20% O-2. Detailed equivalent circuit analysis was carried out in order to clarify the reaction pathway on porous electrode, which extends knowledge available for dense model electrodes. At 800 degrees C in 21% O-2, the DC electrical conductivity of STF35 pellet was 0.6 S cm(-1) and the polarization resistance of the electrode in the symmetrical cell was similar to 100 m Omega cm(2). Detailed impedance spectroscopy studies revealed that the largest contribution (similar to 80%) towards the polarization resistance is due to oxygen adsorption, which is limiting the oxygen reduction performance of the porous STF35 electrode. These results show the applicability of advanced impedance analysis methods (e.g. Distribution of Relaxation Times - DRT) for description of complex impedance electrode phenomena of porous electrodes. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Impedance spectroscopy is one of the most commonly performed measurements to characterize electronic and electrochemical systems. Impedance spectra have limited resolution and many different processes may overlap what could be the reason of obstructions in its proper later analysis. Up to date, there are three approaches to solve this problem: examining impedance spectra itself, fitting spectra with equivalent circuits, and calculating the distribution of relaxation times (DRT). The latter method does not require any model or further knowledge about the system. As a result of DRT calculation, a plot containing relatively narrow peaks related to relaxation process of measured system is obtained. Namely, the position of the peak is related to time constant of the process, while the area under the peak to relaxation resistance. The aim of this work is to test available fitting functions to calculate area under a peak for previously prepared model containing different equivalent circuit elements. Obtained results are validated using exact formulas describing distribution of relaxation times for mentioned elements.