Use of electrochemical impedance spectroscopy in the analysis of the oxygen evolution reaction

Determination of the electrochemically active surface area (ECSA) is essential in electrocatalysis to provide surface normalized intrinsic catalytic activity. Conventionally, ECSAs of metal oxides and hydroxides are estimated using double layer capacitance (Cdl) measured at nonfaradaic potential windows. However, in the case of Ni-based hydroxide catalysts for the oxygen evolution reaction (OER), the nonfaradaic potential region before the Ni(II) oxidation peak is nonconductive, which hinders accurate electrochemical measurements. To overcome this problem, in this work, we have investigated the use of electrochemical impedance spectroscopy (EIS) at reactive OER potentials to extract the capacitance that is hypothesized to arise due to reactive OER intermediates (O*, OH*, OOH*) adsorbed on the catalyst surface. This allowed the estimation of ECSA and intrinsic activity of NiFe layered double hydroxide (NiFe LDH), the most active, state-of-the-art OER electrocatalyst in alkaline media. We analyzed the OER adsorbates capacitance (Ca) on NiFe LDH and Ni(OH)2 at different electrode potentials and identified a suitable potential range for accurate ECSA evaluation. Finally, we validated our method and the choice of potential range through rigorous catalyst loading and support studies.

To achieve carbon neutrality, extensive research is ongoing to produce green hydrogen via water electrolysis combined with renewable energy sources. Understanding and developing catalysts for membrane electrode assembly (MEA) type water electrolysis systems, such as proton exchange membrane water electrolysis (PEMWE) and anion exchange membrane water electrolysis (AEMWE), are critical steps toward this goal. Therefore, this presentation will focus on catalyst development for PEMWE and AEMWE, with emphasis on cost-effective catalysts.For oxygen evolution reaction (OER) catalysis in PEMWE, the primary challenge is to minimize iridium (Ir) usage for sustainable green hydrogen production. Layered monoclinic iridium nickel oxide (IrNiOx) platelets were synthesized using a molten salt method and employed for the OER. These platelets exhibited high OER activity with reduced Ni dissolution in acidic conditions. When applied in MEA, they demonstrated enhanced interconnectivity within the catalyst layer, promoting electron transfer. Even at low Ir loading (0.2 mgIr cm-2), the platelets showed good performance with an initial cell voltage of 1.70 V at 1 A cm-2 and minimal degradation over 100 hours. This highlights the effectiveness of incorporating transition metals into Ir oxide to reduce Ir usage in PEMWE.For oxygen evolution reaction (OER) catalysis in AEMWE, attention is focused on nickel iron (NiFe) hydroxide catalysts for their high activity and stability in alkaline conditions. However, the lack of initial electrical conductivity of as-prepared NiFe LDH limits its potential as an electrocatalyst. To address this issue, a monolayer structuring approach was proposed. This method improved mass transport to allow a high energy conversion efficiency of 72.6% with exceptional stability over 50 hours at 1 A cm-2. Additionally, lack of initial electrical conductivity hinders determination of electrochemically active surface area (ECSA) of NiFe LDH using conventional double layer capacitance method. Use of electrochemical impedance spectroscopy (EIS) at reactive OER potentials to extract the capacitance that is hypothesized to arise due to reactive OER intermediates (O*, OH*, OOH*) adsorbed on the catalyst surface was thus investigated. This allowed the estimation of ECSA and intrinsic activity of NiFe LDH, validating the methodology through rigorous catalyst loading and support studies.For hydrogen evolution reaction (HER) catalysis in AEMWE, replacing platinum (Pt) with Ni-based alloys containing molybdenum (Mo) species has shown promise. Dynamic active sites of Ni catalysts with Mo species require innovative approaches to maintain initial performance, warranting material innovation or careful manipulation of operating protocol. The approaches in which this is made possible is shortly presented, together with the innovations that allows probing of such phenomenon.

Aging studies of lithium-ion batteries are essential for understanding material degradation, which impacts performance and, consequently, battery lifespan. In this paper, we propose the use of electrochemical impedance spectroscopy, differential capacity analysis, and electrochemical noise measurements to evaluate the effects of different C-rates (2C, C/2, and C/20) on a cell. We study aging up to 800 charge/discharge cycles. We demonstrate that aging is associated with a linear increase in electrode resistance, which correlates with capacity fading. Additionally, noise measurements indicate a rise in noise levels at low frequencies following a 1/fγ trend with 1<γ<2.

Electrochemical impedance spectroscopy (EIS) is an efficient tool that reveals the electrochemical characteristics of catalysts, surfaces, interfaces, coatings, and so forth. Use of EIS in different areas of energy research wherever current, potential, and charge determine the performance has become inevitable. Electrocatalytic water splitting is one of such fields focused on generating high purity hydrogen, where EIS is used to correlate the activity trends measuring charge transfer resistances (Rct). In doing so, different conventions are followed. A few perform EIS at the open circuit potential (OCP), a few perform at onset potential or at a potential before onset potential, a few perform at different potentials for different catalysts at which they deliver the same current density, and a large group of people choose a constant potential beyond onset, at which all the studied catalysts show appreciable catalytic activity. Existence of such different practices in using EIS to characterize water splitting electrocatalysts often lead to misinterpretation of the activity trends. Hence, to provide a clear view on the appropriate use of EIS in water splitting electrocatalysis, we have carried out a comparative EIS study on the oxygen evolution reaction (OER) activity trend of stainless steel 304 (SS‐304), Co, Ni, and Cu foils in 1 M KOH at all the above‐stated conditions and the results showed that the EIS carried out at constant potentials in the catalytic turnover region is appropriate.

Zero-emission hydrogen and oxygen production are critical for the UK to reach net-zero greenhouse gasses by 2050. Electrochemical techniques such as water splitting (electrolysis) coupled with renewables energy can provide a unique approach to achieving zero emissions. Many studies exploring electrocatalysts need to “electrically wire” to their material to measure their performance, which usually involves immobilization upon a solid electrode. We demonstrate that significant differences in the calculated onset potential for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) can be observed when using screen-printed electrodes (SPEs) of differing connection lengths which are immobilized with a range of electrocatalysts. This can lead to false improvements in the reported performance of different electrocatalysts and poor comparisons between the literature. Through the use of electrochemical impedance spectroscopy, uncompensated ohmic resistance can be overcome providing more accurate Tafel analysis.

This work describes the use of electrochemical impedance spectroscopy (EIS) as a means to monitor solid phase synthesis on resin beads. EIS was used to track changes during the swelling of beads in various solvents, during three typical reactions and throughout cleavage of the final product from the bead. The impedance response was investigated in a chemical reactor and was found to be faintly sensitive to the resin swelling and solvent flow. The position of the electrode within the reactor was found to be critical as polystyrene based beads float or sink dependent upon the solvent used. However, by choosing electrode position it was possible to monitor reaction progress on beads or within the bulk reactant/product mixture. Of the three typical chemical reactions studied impedance spectroscopy successfully followed two. Fitting of the impedance data to an equivalent electrical circuit provided an estimate as to the relative contribution of capacitive and resistive components to the overall response. Kinetic data from two reactions were also modelled, in both cases complex kinetics was observed, in close agreement with other studies.

On a evalue les caracteristiques de protection contre la corrosion de trois systemes de peinture dans une solution de NaCl 0,5M en se servant de la spectroscopie d'impedance electrochimique. Les systemes de peinture, une peinture hydrodiluable, une peinture aux solvants et un melange des deux, etaient prepares commercialement. Des mesures spetroscopiques sur ces systemes ont ete effectuees sur une large gamme de frequences en fonction du temps. On a obtenu des courbes de Bode, d'angle de phase et de Nyquist qui mettent en evidence des pertes progressives de la resistance du feuil et l'augmentation de sa capacitance en fonction du temps. Le deplacement graduel de l'angle de phase a partir de la capacitance vers la resistance dans le cas du systeme hydrodiluable etait plus important que celui que l'on observait pour le systeme aux solvants. On a trouve que les caracteristiques de protection contre la corrosion etaient superieures a celles du systeme hydrodiluable, et que celles de la couche de peinture mixte aux solvants et hydrodiluable etaient moderement superieures.

Despite widespread use of electrochemical impedance spectroscopy (EIS) to study multi-layer photoelectrochemical (PEC) cells, the appropriate equivalent circuits describing these devices under hydrogen evolution conditions are not commonly investigated.1 This creates ambiguity with respect to the pathways of charge recombination and ionic movement within layers of the device during operation, therefore limiting rational development of future PEC devices. In this study, amorphous TiO2 (a-TiO2) protected silicon photocathodes have been investigated using EIS to show that these devices behave according to the Maxwell circuit during hydrogen evolution, which occurs when the semiconductor is in depletion. Therefore, there are two simultaneous processes occurring in parallel and on different timescales. While the high frequency component is the result of the depletion region in the semiconductor, the low frequency component of this circuit is found to originate from the a-TiO2 itself, implying a capacitive charging process of this protective overlayer during PEC cell operation. Studies to isolate the physical mechanism of capacitive charging in the a-TiO2 from a range of possibilities such as proton intercalation and vacancy migration have been carried out. This result is important for the development of future PEC devices using emerging semiconductors which heavily rely on the a-TiO2 overlayer in acidic electrolytes, such as Cu2O and Sb2Se3, where this low frequency process has previously been observed.1 Using the Maxwell circuit for a-TiO2 protected photocathodes under hydrogen evolution conditions, one can independently determine properties of the semiconductor, such as the depletion capacitance and doping density, and the protective overlayer in these PEC devices under hydrogen evolution conditions. With this baseline, EIS can continue to aid the development of PEC cells as additional overlayers are developed and tested for further performance improvements. Yang, W. et al. Operando Analysis of Semiconductor Junctions in Multi-Layered Photocathodes for Solar Water Splitting by Impedance Spectroscopy. Adv. Energy Mater. 11, 2003569 (2021). Figure 1

Introduction Lifetime expectations and uncertainties regarding end-of-life performance hinder widespread deployment of fuel cell technology. Conventional cell performance diagnostic methodologies rely upon either ex-situ measurements, which lack an immediate connection with the physical degradation phenomena, or upon single cell in-operando electrochemical impedance spectroscopy (EIS), which often is troubled by challenges of feature identification of the spectra and uncomplete representation of the full stack operating conditions in large scale systems [1]. To address these problems, we propose to use electrochemical impedance spectroscopy techniques as diagnostic and prognostic tools to predict the cause-effect relationship between degradation phenomena and stack operating conditions. This experimental methodology can be potentially integrated into commercial systems for stack state of health monitoring during continuous operation. Experimental methods Through experimental study on a solid oxide fuel cell (SOFC) short stack, we explore the potential use of electrochemical impedance spectroscopy (EIS) for in-operando diagnosis. We identify the mechanisms contributing to this system EIS, by carrying out pristine cell/stack tests at different operating conditions (gas partial pressure, current, temperature).We collect experimental data on a 6-cell short-stack with active area of 80 cm2 per cell. Each anode-supported cell features a Ni-YSZ fuel electrode (240 μm), a Sr-doped LaMnO3 (LSM) oxygen electrode (40 μm) and an 8 mol% Y2O3 stabilized Zirconia (YSZ) electrolyte (8 μm). In our experimental setup, the first (reference electrode, i.e., cathode of cell no.1) and last (i.e., anode of cell no.6) electrodes are connected to the counter and working electrode leads of the potentiostat, respectively. This allows connecting the diagnostic system in parallel with the load and the stack, while performing floating ground galvanodynamic EIS measurements. This configuration enables the load to impose the operating current of the stack, while the potentiostat modulates the signal without any current in its main circuit. Reference and working sensing connections to individual cells in the stack are independent on the potentiostat leads.We operate the stack with mixtures of hydrogen, nitrogen and steam at the fuel electrode and mixtures of oxygen and nitrogen/helium at the oxygen electrode. We control the operating temperature via an electric furnace, while monitoring the stack temperatures with a set of six thermocouples, as reported in [2]. Discussion Figure 1 compares the measured EIS spectra for each cell in the short stack at beginning of life and after a major degrading event. With continuous stack monitoring, we are able to identify how degradation phenomena differently impact the state of health of the cells during the stack normal operation. This allows identifying the most degrading operating conditions of the system in real-time, hence informing the operational planning and lifetime maximization strategies of the system. Conclusions Preliminary results suggest that our methodology can provide meaningful insights regarding the cause and nature of the physical processes at the origin of degradation phenomena in solid oxide cells. This process is particularly interesting as a non-disruptive diagnostic and prognostic tool for commercial scale systems that need to optimize their operating conditions to either maximize their lifetime or meet a performance target at end-of-life.Figure 1 – (left) Beginning of life and (right) degraded cell plot comparison of experimental EIS for a 6-cell short stack at 1 A. Tstack=750°C, Anode flow=1.2 Nl min-1 (N2/H2=50/50%), Cathode flow=18.9 Nl min-1 (Air)[1] A. Baldinelli, L. Barelli, G. Bidini, A. Di Cicco, R. Gunnella, M. Minicucci, A. Trapananti, 2019, p. 020012.[2] L. Mastropasqua, S. Campanari, J. Brouwer, J. Power Sources 2017, 371, 225–237. Figure 1

This paper presents the potential use of electrochemical impedance spectroscopy (EIS) for the electrochemical study of stainless steel behaviour under different constant loads and for the detection and characterisation of stress corrosion cracking (SCC). The test specimens were made from sensitised stainless steel of type 304 and were immersed in 0·5M aqueous solution of sodium thiosulphate. The specimens were subjected to constant load tests. Different load levels were applied in order to provoke and evaluate different experimental conditions. The results of EIS measurements clearly showed that the initial polarisation resistance is inversely proportional to the increasing load level. The polarisation resistance also changes with time. Although EIS is generally used for the characterisation of stationary processes, basic electrochemical parameters indicated temporal breaks of the passive film. It was confirmed that under stress, these damages can initiate SCC. It can therefore be concluded that EIS can be used as a supplementary technique for monitoring the initial phases of intergranular SCC.

Chronic impedance spectroscopy of an endovascular stent-electrode array

Limiting ac frequency and dc current of electrochemical double layer capacitors

Supported lipid bilayers (SLBs) serve as essential model systems in studies of membrane biophysics, biosensing, and bioelectronic interfaces. In particular, SLBs formed on conductive polymer (CP) electrodes constitute a new platform to study ion transport across ion channels or the activity of pore-forming toxins through the use of electrochemical impedance spectroscopy (EIS). However, unavoidable pores in the SLB limit detection sensitivity in, e.g., biosensing applications. In this work, we rigorously assess the impact of such ion-conducting pathways on EIS measurements by combining an analytically derived equivalent circuit model (a-ECM) with finite-element-method (FEM) simulations. We start by considering simple, idealized conditions to build intuition regarding the pore-related resistance and capacitance contributions, directly comparing a-ECM predictions with full Poisson-Nernst-Planck (PNP)-based FEM simulations. Subsequently, we introduce additional complexities, including a thin water layer at the SLB/CP interface, SLB surface charge effects, and nonaxisymmetric pore locations, to progressively refine our model. Finally, by extending our analysis to a distribution of pores, we demonstrate how our insights can be used to estimate the pore size and density from experimental EIS data of SLBs formed on PEDOT:PSS electrodes. By bridging intuitive circuit models with accurate FEM simulations, our work provides practical guidelines for interpreting EIS spectra and extracting meaningful physical parameters associated with membrane pores.

A comparative study between the results of different electrochemical techniques (EIS and AC/DC/AC): Application to the optimisation of the cataphoretic and curing parameters of a primer for the automotive industry

PurposeThe purpose of this paper is a theoretical modeling use of electrochemical impedance spectroscopy (EIS) technique for different cases that could describe the possible electrochemical behaviour on steel coated with metallic and oxide thin films (of nickel) deposited by magnetron sputtering, and compare them to know if the theoretical analysis resembles the real case. It is extremely important to clarify that such simulations do not consider the use of the constant phase element (CPE) for the analysis. Therefore, the goal for the theoretical models should be to gain acceptance in electrochemical research.Design/methodology/approachIn order to obtain the equivalent circuits to explain the different possible behaviours of the films and their protective properties in sour media, EIS experimental data were correlated with data from the simulation software. The different nickel and nickel oxide thin films were tested after their deposition by magnetron sputtering on low‐carbon steel and after they had then been exposed to the sour media electrolyte of NaCl 3 wt% + H2S (saturated).FindingsThe EIS simulation starts from the laboratory evaluation of nickel and nickel oxide thin films as anticorrosive protection for low‐carbon steel exposed to sour media. From these results, it is found that the nickel and nickel oxide films could adopt seven different behaviours, and all are possible to occur.Practical implicationsThe equivalent circuits proposed will give an insight into the corrosion phenomena for different metals coated with thin films and exposed to sour media, because all of the simulations are made on the basis of real EIS results.Originality/valueThe electrical analysis in the simulation diagram did not consider the use of the CPE to adjust the plots. In consequence, the values of all parameters for the seven different adjustments obtained through the simulations establish a reference for the explanation of the corrosion phenomena. They are also a tool with which to predict the possible behaviour of a thin film deposited on metal and exposed to electrolytes that are as aggressive as sour media.