Electrohydrodynamic Impedance Research Articles

Analysis of Hydrogen Feeding to the Anode of a PEMFC By a Transport Impedance Technique

Hydrogen feeding to the catalyst layer of the anode of a proton-exchange membrane fuel cell (PEMFC) must be fast enough to allow for high power response and avoid starvation events. However, there are limitations to the flow rate posed by the anode architecture (manifold, inlet port, flow field), and the gas diffusion layer (GDL). Within the GDL, hydrogen transport conditions may change in-operando as a result of water generation and saturation of pores. Delays in hydrogen feeding will give rise to a decrease in power response, and transitory sub-stoichiometric conditions, that may damage the electrode and decrease fuel cell durability. Therefore, it is of high interest to probe hydrogen feeding conditions when designing new anode architectures, and during operation of the fuel cell.Mass transport conditions in the fuel cell and electrochemical systems can be probed by techniques based on the impedance concept [1,2,3]. Among them, one recently applied in our group is the current modulated H2 flow-rate spectroscopy (CH2S), which provides the transfer function H [4,5]:H(j w) = nF QH2 / I (Eq. 1)Where QH2 is the modulated hydrogen inlet flow, I the modulated cell current, n(=2) the electron exchanges per H2 molecule, and F(=96485 C mol-1) the Faraday constant. A typical response in a PEMFC with dead-end anode is shown in Fig. 1.The H function normally presents two or more semicircles in Nyquist plots, extending in the real axis from H'=0 to H'=1 (stoichiometric modulation). The high frequency semicircle is normally ascribed to the set-up time response limitation, mostly the flow meter. At higher frequencies, the H function shows characteristics of the time response of hydrogen flow up to the anodic catalyst layer.In this communication, the CH2S technique is applied in conventional single cells and in passive portable feeding PEMFCs. Some properties of H2 transport path towards the anodic catalyst layer are analyzed, like conduits length, inlet port type, anode flow field, liquid water contents, hydrogen stoichiometry, and anode hydrophobicity.Acknowledgement: The work is partially financed by the ELHYPORT project (PID2019−110896RB-I00), Spanish Ministry of Science and Innovation.[1] C. Deslouis, I. Epelboin, C. Gabrielli, P.S.-R. Fanchine, B. Tribollet, Relationship between the electrochemical impedance and the electrohydrodynamical impedances measured using a rotating disc electrode, J. Electroanal. Chem. Interfacial Electrochem. 107 (1980) 193–195.[2] A. Sorrentino, T. Vidakovic-Koch, R. Hanke-Rauschenbach, K. Sundmacher, Concentration-alternating frequency response: A new method for studying polymer electrolyte membrane fuel cell dynamics, Electrochim. Acta. 243 (2017) 53–64.[3] D. Grübl, J. Janek, W.G. Bessler, Electrochemical Pressure Impedance Spectroscopy (EPIS) as Diagnostic Method for Electrochemical Cells with Gaseous Reactants: A Model-Based Analysis, J. Electrochem. Soc. 163 (2016) A599–A610.[4] M.A. Folgado, H. Moreno, A. Molinero, J.C. Oller, J.M. Barcala, A.M. Chaparro Hydrogen Transport Impedance for the Study of Anodes in PEMFCs, European Fuel Cell Forum 2021, A0704 (Extended ). Lucerne (Switzerland).[5] A. Molinero, J.C. Oller, J.M. Barcala, H. Moreno, M.A. Folgado, A.M. Chaparro, Experimental Set-Up for Transport Studies of Anodes in PEMFCs. European Fuel Cell Forum 2021, B0207 (Extended ). Lucerne (Switzerland).Fig. 1. Nyquist plot of the H function according to Eq. 1, for a PEMFC cell with commercial electrodes (Pt/C 0.3mg·cm-2) and Nafion 212NR membrane, working with hydrogen feeding in dead-end mode, and air feeding in cathode. a) Full signal; b) Low frequency detail. Numbers are modulation frequencies. Figure 1

Investigation of the Corrosion Mechanism for Sulfate Reducing Bacteria (SRB) Using a Split-Chamber Zero Resistance Ammetry Technique

Microbiologically influenced corrosion (MIC) is one of the most aggressive forms of corrosion leasing to infrastructure and equipment damage in various industries, including oil and gas, water systems, medical devices, marine environments, nuclear waste storage facilities, and aviation fuel systems and storage. During the last 10 year, PHMSA estimates that MIC has caused 503 internal corrosion incidents at a reported property damage of $188 million and a loss of 53,000 barrels of oil. Some common bacteria associated with MIC are sulfate-reducing bacteria (SRB), iron and CO2 reducing bacteria and iron and manganese oxidizing bacteria. SRB are generally considered the most aggressive group of bacteria in pipeline systems that causes MIC and pitting, especially of carbon steel in the oil and gas industry. SRB are facultative anaerobes and thrive in anoxic environments, using sulfate as a terminal electron acceptor and producing hydrogen sulfide (H2S) as a metabolic byproduct. Furthermore, SRBs also can reduce both nitrate and thiosulfate and obtain their energy from organic nutrients, such as lactate.Electrochemical techniques to monitor for MIC focus on studying the electrochemical characteristics of the interface or mass transport properties of a system that are modified by the microbiological activities. Polarization sensors, such as the BIOX system or BioGeorge, use polarization based on a galvanic couple between a stainless steel electrode and a sacrificial anode. The measured galvanic current is proportional to biofilm that has grown on the electrode surface. Other electrochemical sensors use electrochemical impedance spectroscopy (EIS) or amperometry to measure biofilm thickness by comparing the electrochemical signatures of a reference channel (without bacteria) to a measurement channel (exposed to bacteria); while sensors based on electrochemical resistance use linear polarization measurements to determine the amount of biofilm on an electrode surface. Electro hydrodynamical impedance has also been used to measure the diffusion coefficient in a biofilm and correlate to biofilm growth and thickness. While these approaches can accurately predict the presence and/or thickness of a biofilm on a metal surface, they cannot determine the risk of MIC associated with biofilm formation, as the presence of a biofilm does not necessarily mean that a surface experiences MIC. Additionally, many of the techniques are destructive or require visual examination of the surface after analysis.This work presents a split-chamber zero resistance ammetry (SC-ZRA)-based approach to overcome the limitations to MIC monitoring described above and serve as a screening system to determine the risk of MIC associated with certain microorganisms or groups of microorganisms. Previous work using a split-chamber approach to assess MIC was used by Daumus for the study of stainless steel corrosion in the presence of sulfate reducing bacteria, and subsequently used by Miller et al to evaluate MIC under aerobic, Fe (III)- and nitrate-reducing conditions. In this approach, two identical electrochemical cells (chambers) are separated by an ion-transport membrane. Each chamber contains an identical electrode of the same material which are electrically connected through a zero-resistance ammeter. When one of the chambers is inoculated with microorganisms, the galvanic current between the two electrodes is measured through the zero ammeter. This configuration mimics the microbiologically induced development of localized anodic and cathodic patches on a metal surface that leads to corrosion. The flow of electrons (difference in corrosion current between the two chambers) depends solely on the microbiological activities of bacteria in one of the two chambers, so the influence of the microorganisms on MIC, as well as the extent of corrosion, can be quantified.Using the SC-ZRA technique we were able to characterize the mechanism and electrochemical signatures of SRB corrosion. Specifically, we found that in split-chamber incubations containing an electron donor, electrons flow from the inoculated to the uninoculated chambers. While the current direction could be interpreted as electron transfer such as in a microbial fuel cell, these systems deploy inert graphite electrodes. When used for MIC characterization, the SC-ZRA uses reactive carbon steel electrodes. Indeed, when positive current was detected, greater corrosion was detected on WE1, which is consistent with redox couples, as well as previous work to characterize MIC using SC-ZRA measurements. After depletion of an electron donor, SRB uses electrons from the metals surface as an electron donor reversing the flow of electrons from the uninoculated to the inoculated chamber. In future work, this technique can be used to provide a mechanistic understanding and a monitoring tool for corrosion of metals that are exposed to SRB under a variety of redox regimes.

Hydrogen reaction analysis in aqueous solutions containing H2S/CO2 at different pressures

The hydrogen evolution reaction (HER) in H2S and H2S/CO2 solutions at different pressures was studied using platinum and stainless steel as working electrodes, with polarization curves and electrohydrodynamic impedance. When Pt was the working electrode, the polarization curves in H2S solutions without CO2 presentedonly one current plateau, while two plateaus were seen when a mixture of H2S/CO2 was used. However, when steel was the working electrode, two plateaus always existed even in the presence of H2S without CO2. HER was interpreted by another source of H2 coming from the H2S reaction coupled to the classical direct reduction of H+.

Discussion and challenges concerning the elaboration of a dissolution reaction mechanism

This paper presents a discussion of the particularities and challenges involved in the elaboration of reaction mechanisms. The use of steady state techniques to establish reaction models is critically analysed. Indeed, this paper shows that the same Tafel slope may be associated with different mechanisms; therefore, the Tafel slope cannot be used by itself to determine a specific mechanism with reasonable certainty. Furthermore, a critical analysis of electrochemical methods frequently used to determine corrosion rate, such as the Tafel Slope and LPR, shows the necessity of knowing the mechanisms of reactions beforehand to avoid mistakes in corrosion rate predictions. Reaction mechanisms must be established using both steady state and transient techniques. Finally, in cases where the kinetics are controlled by mass transport, electrohydrodynamic impedance emerges as a very useful tool, whose use is exemplified here by copper electrodissolution in sulphate medium.

Transient Response of Chemical-Electrochemical (CE) Processes in Rotating Disk Electrode (RDE) Systems: Diffusion and Electrohydrodynamic Impedances

We present a transient treatment of chemical-electrochemical (CE) processes in rotating disk electrode systems as an extension to a steady-state analysis already initiated by our group (results have been submitted to the Journal of the Electrochemical Society). Our numerical procedure can investigate systems with different diffusion coefficients over a wide range of homogenous reaction rate constants and was used to investigate both diffusion and electrohydrodynamic impedance responses. Two sets of rate constants (termed “slow kinetics” and “fast kinetics”) were used to assess the impact of the chemical step on both impedances behavior. For diffusion impedances, slow kinetic systems displayed two impedance loops, one associated with the homogenous reaction and the other to the convective diffusion. As the value of the electroactive species rate constant grew relative to its chemical counterpart, the homogenous reaction loop started to dominate the overall impedance. For systems with fast kinetics, only the loop corresponding to the convective diffusion was visible (for frequencies up to 100 Hz) and the overall impedance decreased rapidly with an increase on the reaction rate of the electroactive species. Results for electrohydrodynamic impedance corroborated the previous findings: two time constants could be observed for slow kinetic systems, but only one in the case of fast kinetics. These findings show that impedance techniques can be of great help on establishing reaction mechanisms for complex systems, since they are sensitive to the presence of non-electrochemical activities as long as they interfere with the concentration profile of the electroactive species.

Hydrogen Evolution Reaction Evaluation in Aqueous Solutions Containing CO2 at Different Pressures

This paper addresses the hydrogen evolution reaction (HER) occurring on platinum in an aqueous solution containing dissolved CO2 at several pressures. A new setup that uses a low inertia rotating disk electrode (RDE) adapted in an autoclave is proposed. This setup allows obtaining polarizations curves and electro-hydrodynamic (EHD) impedance, the latter for the first time in pressured conditions. New equations to adjust the Levich plots are proposed and briefly discussed.

Convective mass transport in ionic liquids studied by electrochemical and electrohydrodynamic impedance spectroscopy

In this work, the role of convective mass transport in two ionic liquids (ILs) that differ by the anions 1-butyl-2,3-dimethyl-imidazolium bis(trifluoromethanesulfonyl)imide ([BMMI][Tf2N]) and 1-butyl-2,3-dimethyl-imidazolium tetrafluorborate ([BMMI][BF4]) were studied in the presence of ferrocene (Fc) as a redox tracer. A rotating disk electrode (RDE) system was used, and the use of electrochemical (EIS) and electrohydrodynamic (EHD) impedance spectroscopies allowed for a deeper analysis of mass transport and charge transfer at the surface of the electrode. The two ILs showed a viscoelastic character that was observed using a parallel plate geometry viscometer with a high storage-to-loss modulus ratio in a frequency domain where the RDE response was characterized by significant elongational flow kinematics. As a general conclusion, the observed effects were interpreted as unbalanced contributions of the elongational flow close to the electrode surface, which induced an anisotropic diffusivity that increased along the flow lines, and the effect of Fc and Fc+ as structure breakers or strengtheners in the diffusion layer. Finally, from EIS analysis, it was established that increasing the overpotential in the anodic direction yielded an increased Schmidt number, which indicated a detrimental effect on the mass transport of Fc+, whose production rate was also increased under the same conditions. EIS showed that the anion had an effect on the reaction rate of Fc oxidation and on the double-layer capacitance value.

AFM and hydrodynamic electrochemical characterization of the self-assembled 1,4-dithiane on gold surface

Abstract Polycrystalline gold electrode surface spontaneously modified by 1,4-dithiane (1,4-dt) organosulfur species was characterized by rotating disk electrode (RDE) and electrohydrodynamic impedance (EHD) using Fe(CN) 6 3 - as the electrochemical probe molecule. The gold(1 1 1) monocrystal surface was also modified with 1,4-dt to assess the surface morphology by atomic force microscope (AFM). The electrochemical measurements showed that the polycrystalline gold surface modified with 1,4-dt exhibits a typical behavior of a partially blocked surface and that the blocking effect increases with the increase of the immersion time in the precursor solution. The AFM image clearly shows the presence of defects on the modified surface. Even for a high immersion time (72 h) the electrode surface was not entirely blocked. The surface roughness of the Au/1,4-dt is approximately three times greater than the surface roughness of the bare-gold electrode.

Recent developments in the electro-hydrodynamic (EHD) impedance technique

During the last decade, the electro-hydrodynamic (EHD) impedance method has undergone many improvements from both the theoretical and instrumental points of view. This technique was applied successfully to electrodissolution, corrosion, corrosion protection, electrocrystallization, biofilm and calcareous deposit characterization. More recent developments were achieved in our group in the case of a CE mechanism or when a mediation reaction proceeds at a conducting polymer modified electrode. This paper provides a survey of the extended potential of this method illustrated by a few examples taken from the above listed topics.

Mechanistic Studies of Cu Electropolishing in Phosphoric Acid Electrolytes

Electropolishing of Cu and Cu/Ta/Si wafer samples were studied using a rotating disk electrode in a variety of phosphoric acid-based electrolytes, including several with ethanol and other species added as diluents. Diluents allow a wide range of water concentrations to be accessed and also reduce the dissolution rate during Cu electropolishing, simplifying possible applications to damascene processing. The measured limiting current densities are subject to a Levich analysis, demonstrating that water is the acceptor species involved in the rate-determining step. The effective diffusion coefficient that is determined is in almost exact agreement with that previously obtained from the electrohydrodynamic impedance, which does not require knowledge of the limiting species. © 2004 The Electrochemical Society. All rights reserved.

Characterization of calcareous deposits in artificial seawater by impedance techniques: 3—Deposit of CaCO 3 in the presence of Mg(II)

Calcareous deposits were formed on steel under cathodic protection conditions in artificial seawater at various potentials from −0.9 to −1.4 V/SCE. The deposition kinetics was analysed by chronoamperometry measurements and the calcareous layers were characterized by electrochemical and electrohydrodynamical impedance spectroscopies, scanning electron microscopy observations and X-ray diffraction analyses. At 20 °C, the deposits were composed of aragonite CaCO 3 when formed at potentials E comprised between −0.9 and −1.1 V/SCE, of brucite Mg(OH) 2 and aragonite when formed at −1.2 V/SCE, and only of brucite when formed at potentials E≤−1.3 V/SCE. However, the in situ impedance techniques demonstrated the presence of a Mg(II)-containing porous layer along with the aragonite deposit at E≥−1.1 V/SCE. In seawater enriched with Mg 2+, the deposition of aragonite was almost totally inhibited, and the behavior of the film containing Mg(II) could be described.

Chromate Conversion Coatings Formation on Zinc Studied by Electrochemical and Electrohydrodynamical Impedances

The formation of chromate conversion coatings on zinc was studied by chronopotentiometric, electrochemical, and electrohydrodynamic impedances, and interfacial pH measurements. The electrochemical experiments were performed with a rotating disk electrode of pure zinc, and the pH measurements were obtained with a zinc deposit on a gold grid electrode in a submerged impinging jet cell. The electrolyte was an industrial chromate bath. The experimental results were achieved for different immersion times, temperatures, and rotation speeds. Kinetic reactions and physical model for the chromate layer formation on zinc were proposed, and the electrochemical and electrohydrodynamic impedances were well simulated. © 2002 The Electrochemical Society. All rights reserved.

Electrodissolution of Zinc at the Limiting Current

The mass-transfer-limited region of zinc electrodissolution in alkaline electrolyte was investigated using a zinc rotating disk electrode. Pseudo-steady-state polarization measurements reflect a strong dependence of dissolution rate and shape of the limiting current plateau on composition and mass transfer rate. Electrochemical impedance spectroscopy results show that there is no appreciable dependence of the high frequency resistance on potential or rotation speed at prepassive potentials. Electrohydrodynamic impedance measurements, together with a simple model, suggest that the zincate ion is the appropriate mass transfer-limited species in this range, although the diffusion coefficients estimated from these analyses are lower than expected. © 2001 The Electrochemical Society. All rights reserved.

Electrohydrodynamic Impedance in the Presence of Nonuniform Transport Properties

A numerical model is described for electrohydrodynamic impedance spectroscopy (EHDIS) in the presence of viscosity and diffusivity gradients near the electrode. The viscosity is assumed to depend on the concentration of a diffusion-limited reaction product raised to a power. An effective diffusion coefficient measured from a Levich analysis is shown to approach the near-surface value of the diffusion coefficient when the near-surface kinematic viscosity is greater than in the bulk. Similarly, EHDIS measurements can lead to quick estimates of near-surface viscosity. The model is applied to interpret experimental data for the steady-state limiting current and electrohydrodynamic impedance of the zinc-KOH system. © 2001 The Electrochemical Society. All rights reserved.

Corrosion of mild steel in low conductive media simulating natural waters

The corrosion of a mild steel was examined in two aerated neutral aqueous solutions, defined as reference solution (0.2 g L−1NaCl) and as Σ-solution (1.3 g L−1NaCl + 0.63 g L−1NaHCO3 + 0.27 g L−1Na2SO4). Their composition was chosen on the basis of the physical and chemical properties of certain natural waters. The solutions simulated the least (reference solution) and the most (Σ-solution) aggressive waters of the Sebou river in Morocco, as determined after a four-year examination (1991–94), at 13 pump stations located along the river. Various experimental methods were used to determine the corrosion mechanism. Cathodic range voltammetry using a rotating disc allowed the kinetics of oxygen reduction process to be determined. Since the conductivity of the solutions were low, the potential was corrected for ohmic drop estimated through the high frequency limit in the Nyquist diagrams (electrochemical impedance spectroscopy) as well as the current interrupter method. After correction, the polarization curves revealed a diffusion plateau attributed to dissolved oxygen reduction. At the plateau, a two-step mechanism was derived involving oxygen diffusion through the hydrodynamic layer and through a porous inner layer formed by the corrosion products. This inner layer could not be observed by SEM, but both EIS and EHD (electrohydrodynamic impedance) confirmed the presence of a thin porous dielectric layer. At the open circuit potential, the corrosion rate was determined by the diffusion rate of dissolved oxygen in the Σ-solution, and by charge transfer in the reference solution. This shows that the corrosion mechanism strongly depends on the electrolyte and its conductivity.

AC and Electrohydrodynamic Impedances for a Chemical Preceding Electrochemical Mechanism I. Theory

Analytically accurate solutions for the ac impedance and the electrohydrodynamic (EHD) impedance have been calculated for a chemical electrochemical mechanism occurring on a rotating disk electrode. Schmidt number corrections were considered. These analytical expressions have been compared to a direct numerical integration by the finite difference method and the divergence found was less than 1% (ac impedance) and less than 4% (EHD impedance) in a wide range of rate constant values for the preceding chemical reaction. The aim of this treatment was to obtain an alternative to numerical treatments which are time consuming, suitable for fitting procedure of impedance data which require a large number of elementary calculations of the impedance value, while keeping a good accuracy compatible with the experimental errors. © 2000 The Electrochemical Society. All rights reserved.

A flow modulation study of nickel–iron codeposition

Nickel–iron codeposition was studied by the method of electrohydrodynamic (EHD) impedance spectroscopy. Experiments were conducted in both the potentiostatic and galvanostatic modes. Low-frequency asymptotes of the phase shift of the two modes are in agreement with steady-state behavior. Changes in the spectra with agitation, bath chemistry and applied current density are presented. Simulations from a previous competitive adsorption theory applied now to EHD impedance are compared to the experimental data. The dependence of the spectra on experimental parameters is in fair agreement with model predictions. The model is able to predict a unique feature of the spectra; however the simulated variations in the EHD spectra are not completely consistent with experiment.

Carbon Steel Corrosion in Natural Waters and in the Corresponding Synthetic Waters Analysed by Impedance Spectroscopy

The aim of this work is to study the possibility of using synthetic water instead of natural water for analyzing corrosion of carbon steel. The underground waters from the Southwest of Tunisia are concentrated in dissolved ions (chloride, sulphate, calcium, magnesium and more). The low-carbon steels are used as main material for the pumping and the dispatching devices of these waters. The influence of mass transport on the corrosion process is analysed, then the electrohydrodynamical impedance is used as a complementary tool of the usual ac impedance for this characterisation of the phenomena occurring at the interface carbon steel natural water.

Flow Modulation as a Means of Studying Leveling Agents

Electrohydrodynamic (EHD) impedance spectroscopy has been used to study the influence of coumarin on nickel electrodeposition. The low‐frequency limit of the phase shift of the EHD‐impedance experiments was found to be 180° this result is consistent with the observation that the nickel deposition rate decreases as the (mass‐transfer‐limited) flux of coumarin to the electrode increases. The experiments were analyzed with a model (developed previously) that is consistent with existing theories describing the influence of leveling agents. The model successfully predicts variations in spectra with disk rotation speed and current density but is less successful at predicting variations with the bulk coumarin concentration. The use of the EHD‐impedance method for the determination of interfacial‐kinetic and transport parameters relevant to models of leveling agents is demonstrated.

Influence of the sodium nitrate content on the rate of the electrodeposition of silver from thiosulphate solutions

On a pre-plated, polycrystalline silver rde, the silver electrodeposition process from solutions consisting of 1 × 10 −3 M AgNO 3 + x M NaNO 3 + 0.025 M Na 2 S 2 O 3 was studied by means of ac impedance spectroscopy, stationary polarization curves and low frequency electrohydrodynamic (EHD) impedance measurements. The sodium nitrate concentration ( x) was varied between 0.05 and 0.6 M in order to investigate its influence on the kinetic parameters in a formerly proposed CE mechanism. A catalytic effect of NaNO 3 on both the chemical and charge transfer reaction rates was observed and quantified through the combined use of the different measurements. Thus, a continuous transition was monitored from a clearly noticeable chemical reaction (0.05 M NaNo 3) to one hard to detect (0.6 M NaNO 3), possibly explained through hydration phenomena. These results highlight the important consequences of adding NaNO 3, originally intended to be a supporting electrolyte.