Potentiostatic Hold Research Articles

Identification of Organic Species Generated at Each Electrode during Potentiostatic Hold of a Two-Compartment Lithium-Ion Cell By Liquid Chromatography-Electrospray Ionization Mass Spectrometry (HPLC/ESI-MS) Analysis

Chemical structure identification of electrolyte decomposition species formed during aging of a lithium-ion battery can help deepen our understanding of the various reaction mechanisms occurring inside the battery, which in the long run can help in a rational design and synthesis of future electrolyte formulations. In this work, a two-compartment cell was made following the design proposed earlier by different groups.1-2 Essentially, a non-porous Li+-ion conductive glass ceramic disc was used as separator instead of the traditional porous polypropylene separator. This will eliminate any cross-talk between the electrodes and identify species formed at anode and cathode individually. NMC532 (LiNi0.5Mn0.3Co0.2O2) was used as the cathode, graphite was used as the anode, and 1.2 M LiPF6 in EC/EMC (3/7 wt./wt.) was used as the electrolyte. For identification of decomposition species, we used HPLC/ESI-MS which has been shown multiple times as a viable technique for identification of decomposition species which are typically present in small concentrations.3-4 Since only molecular weights of the species can be obtained via ESI-MS, structural identification was performed using a combination of isotope-peak analysis and already proposed reaction mechanisms. The results from two-compartment cell were also contrasted with a traditional lithium-ion cell made with porous polypropylene separator and aged under similar conditions. This abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02- 06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. References Metzger, M.; Strehle, B.; Solchenbach, S.; Gasteiger, H. A., Origin of H2 Evolution in Libs: H2o Reduction Vs. Electrolyte Oxidation. Journal of The Electrochemical Society 2016, 163, A798-A809.Vadivel, N. R.; Ha, S.; He, M.; Dees, D.; Trask, S.; Polzin, B.; Gallagher, K. G., On Leakage Current Measured at High Cell Voltages in Lithium-Ion Batteries. Journal of The Electrochemical Society 2017, 164, A508-A517.Takeda, S.; Morimura, W.; Liu, Y.-H.; Sakai, T.; Saito, Y., Identification and Formation Mechanism of Individual Degradation Products in Lithium-Ion Batteries Studied by Liquid Chromatography/Electrospray Ionization Mass Spectrometry and Atmospheric Solid Analysis Probe Mass Spectrometry. Rapid Communications in Mass Spectrometry 2016, 30, 1754-1762.Laruelle, S.; Pilard, S.; Guenot, P.; Grugeon, S.; Tarascon, J.-M., Identification of Li-Based Electrolyte Degradation Products through Dei and Esi High-Resolution Mass Spectrometry. Journal of The Electrochemical Society 2004, 151, A1202-A1209.

(Invited) Direct Formic Acid Fuel Cells Anode Optimization: Flow Fields and Catalysts

Direct formic acid fuel cells (DFAFCs) offer an efficient and compact electrochemical conversion technology to power portable electronic devices ranging from sensors to unmanned surveillance vehicles. Liquid fuel cells are desirable for portable applications due to their ease of liquid refueling and the minimal incorporation of system fail safes for handling, as compared to compressed gasses. Directly compared to other liquid fuels, formic acid has minimal parasitic losses due to (i) efficient anodic overpotentials and (ii) cathodic depolarization due to low fuel crossover.[1]The anodic overpotentials are reduced in the presence of efficient catalysts that enhance the fraction of formic acid that is electro-oxidized via the direct non-strongly adsorbed reaction intermediate pathway,[2] while most liquid fuels are plagued with strongly adsorbed carbon monoxide reaction intermediates that require high parasitic overpotentials. Fig 1 compares the formic acid overpotential on a platinum-ruthenium (PtRu) alloy catalyst through the indirect electro-oxidation pathway and carbon supported platinum catalyst decorated with a submonolayer of bismuth (Pt/C-Bi) promoting the direct electro-oxidation pathway. Two short comings incurred in a DFAFC are (Part 1) durability of the catalyst and (Part 2) efficient removal of the CO2-byproduct.[3] Part 1 – The Bi, at 54% of a monolayer coverage, promotes the direct electro-oxidation pathway by the ‘third-body’ effect and adsorption in the CH-down orientation. Alternative adsorbates to Bi are being sought that are strongly adsorbed to the Pt surface and enhance formic acid electro-oxidation through the direct reaction pathway. Stability of the adsorbate during operation and idle will be a key parameter in down selection. Part 2 – The removal of CO2 is imperative to maintain elevated formic acid concentration both in the anode flow field and at the catalyst interfaces within the 3-dimensional anode catalyst layer. The limited crossover of formic acid to the cathode permits the cell to be operated in the presences of neat fuel concentrations. Previously in the Rice group, it has been shown that DFAFC performance is strongly impacted by catalyst layer porosity via either swelling of the catalyst layer[4] or using a pore former to create larger pore structures in the catalyst layer[5]. As the DFAFC operates under higher loads, additional CO2 must be removed through a typical single serpentine flow field, resulting in non-uniformed fuel distribution at the catalyst interface due to two-phase flow. Recent work on advanced electrically conductive selectively gas permeable anode flow field (SGPFF) designs have resulted in efficient removal of CO2 perpendicular to the active area near the location where it is formed in the catalyst layer.[6] Fig 2 shows the potentiostatic hold performance (0.3V) in a dead-ended flow field operation for a standard flow field vs. the SGPFF using PtRu/C as the anode catalyst. Acknowledgements: We gratefully acknowledge support of this work by the NSF-funded TN-SCORE program, NSF EPS-1004083, under Thrust 2 and the Center for Manufacturing Research at Tennessee Tech University.

Effect of Phosphorus-Based Additives on Cathode-Electrolyte Interface of Charged Ncm Surface through Potentiostatic Holds

Nickel-rich layered oxide cathode materials have seen widespread deployment due to their high gravimetric capacity and high average discharge voltage. However, achieving this high capacity requires charging voltages above the potential at which the electrolytes decompose at the cathode surface (>4.5 V vs. Li/Li+). This interfacial instability leads to impedance rise and capacity loss. One strategy to stabilize this interface is by the addition of additives to the electrolyte, in which small amounts of a new component change the interactions of the interface. Potentiostatic holds with LiNi0.5Mn0.3 Co0.2O2 (NMC) as the working electrode and Li4Ti5O12 (LTO) as the counter/reference allow a quantitative measurement of the oxidation reactions occurring at cathode. A 60 hour hold allows for concentration polarizations to relax and oxidative side reactions to dominate the current. A more stable electrolyte at the cathode/electrolyte interface would lead to a lower current during the potentiostatic hold. In this work, the performance of five phosphorus-based additive electrolyte formulations (1% wt. ratio) is evaluated against a baseline organic carbonate electrolyte in NMC (LiNi0.5Mn0.3 Co0.2O2 ) / LTO (Li4Ti5O12) full cells. These additives include triethylphosphite (TEPi), tris(2,2,2-trifluoroethyl)phosphite (TTFPi), tris(trimethylsilyl) phosphite (TMSPi), and tris(trimethyl silyl) phosphate (TMSPa). The electrochemical protocol used to evaluate these additives comprises multiple cycles up to 4.4 V vs. Li+/Li, a potentiostatic hold at 4.6 V vs. Li+/Li, followed by AC impedance spectroscopy and extended cycling. Of the formulations surveyed, only electrolytes containing TMSPa and TMSPi (aged for one week) increased the coulombic efficiency during the initial cycling. For the potentiostatic hold, only the aged TMSPi electrolyte lowered the terminal oxidation current compared to the baseline electrolyte, indicating increased stability of the cathode/electrolyte interface. AC impedance spectra indicated the aged TMSPi electrolyte had a slightly elevated high-frequency semicircle (traditionally associated with phenomena at the solid-electrolyte interphase) and a vastly decreased mid-frequency semicircle (traditionally associated with charge-transfer phenomena), compared with all other electrolytes tested. Characterization of cycled electrodes was performed with XPS and SEM will be presented. Acknowledgements The work at Argonne National Laboratory was performed under the auspices of the U.S. Department of Energy (DOE), Office of Vehicle Technologies, under Contract No. DE-AC02-06CH11357. The submitted issue has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-Eng-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, irrevocable, worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Evaluating Fluorinated Electrolyte Stabilities on Charged Ncm Cathode Surfaces with Potentiostatic Holds

Organic carbonate electrolytes are commonly used in lithium ion batteries due to their wide electrochemical stability window and high lithium salt solubility. However, the high voltage of some cathode materials is above the stability of these conventional carbonate electrolytes (>4.5V vs. Li+/Li). One method of increasing the anodic stability of the organic carbonates is through substitution of the solvents with a fluorinated carbonates. In this work, we evaluate the stability of electrolyte formulations comprising combinations of either ethylene carbonate, fluoroethylene carbonate, or difluoroethylene carbonate as cyclic carbonates, and ethyl methyl carbonate or hexafluorodiethyl carbonate as linear carbonates. Stability of these electrolytes at both positive and negative electrodes was determined by lithium/graphite half cell tests, as well as 60 hour potentiostatic hold tests at 4.6 V vs. Li+/Li with NMC (LiNi0.5Mn0.3 Co0.2O2) cathodes and either graphite or LTO (Li4Ti5O12) anodes. In potentiostatic holds with NMC cathodes, the fluorinated electrolytes show enhanced stability (lower oxidation current) with the graphite anodes, while fluorethylene carbonate containing formulations show higher oxidation currents with the LTO anodes. These results indicate the feasibility of fluorinated electrolytes in enabling high voltage cathodes through stability of the cathode/electrolyte interface. Acknowledgements The work at Argonne National Laboratory was performed under the auspices of the U.S. Department of Energy (DOE), Office of Vehicle Technologies, under Contract No. DE-AC02-06CH11357. The submitted issue has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-Eng-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, irrevocable, worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

On Leakage Current Measured at High Cell Voltages in Lithium-Ion Batteries

In this study, parasitic side reactions in lithium-ion batteries were examined experimentally using a potentiostatic hold at high cell voltage. The experimental leakage current measured during the potentiostatic hold was compared to the Tafel expression and showed poor agreement with the expected transfer coefficient values, indicating that a more complicated expression could be needed to accurately capture the physics of this side reaction. Here we show that cross-talk between the electrodes is the primary contribution to the observed leakage current after the relaxation of concentration gradients has ceased. This cross-talk was confirmed with experiments using a lithium-ion conducting glass ceramic (LICGC) separator, which has high conductance only for lithium cations. The cells with LICGC separators showed significantly less leakage current during the potentiostatic hold test compared to cells with standard microporous separators where cross-talk is present. In addition, direct-current pulse power tests show an impedance rise for cells held at high potentials and for cells held at high temperatures, which could be attributed to film formation from the parasitic side reaction. Based on the experimental findings, a phenomenological mechanism is proposed for the parasitic side reaction which accounts for cross-talk and mass transport of the decomposition products across the separator.

The Role of Additives in Improving Performance in High Voltage Lithium-Ion Batteries with Potentiostatic Holds

In this work, various electrolyte additives designed for enhanced performance at high voltages were evaluated with elevated temperature potentiostatic holds with LiNi0.5Co0.2Mn0.3/Li4Ti5O12 full cells to determine their effect on the high voltage stability. Of the additives investigated, many showed increased oxidation current through the 60 hour potentiostatic holds test, and adversely affected both the capacity retention and interfacial impedance. Improved high voltage performance was observed with two additives, vinylene carbonate (VC) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), which was attributed to two different mechanisms of improvement. This work investigates some conclusions in the available literature of an additive molecule that decomposes on the charged cathode surface and passivates the surface against electrolyte oxidation.

Evaluation of Additive Performance on Charged Ncm Surface through Potentiostatic Holds

In an effort to achieve higher energy density batteries, many higher voltage cathodes are being investigated. However, at high voltage (V>4.5 vs. Li/Li+), conventional carbonate electrolytes are unstable, leading to electrolyte oxidation and subsequent drop in battery performance. Two possible methods for mitigating electrolyte oxidation are through the use of intrinsically stable electrolytes at the elevated cathode voltage, or through the use of electrolyte additives that sacrificially decompose on the charged cathode surface, impeding further oxidation of the electrolyte. Evaluation of electrolyte or additive performance is traditionally accomplished by cycling tests or through voltammetry with platinum or carbon working electrodes, however each of these methods has intrinsic gaps. Capacity retention can be affected by electrode ‘slippage’ as well as electrode-electrode effects, and voltammetry on inert working electrodes is not evaluating reaction rates on a representative surface. Determining oxidation rates at the cathode surface is of great value in the design of passivating additives. In this work, stability of electrolytes was assessed using high voltage potentiostatic holds (4.6V vs. Li/Li+, for at least 60 hours) at elevated temperature with a NCM (LiNi0.5Co0.2Mn0.3O2 ) electrode as the working electrode and an LTO (Li4Ti5O12) electrode as a counter and reference. A 60 hour potentiostatic hold allows for the relaxation of electrode polarization such that the dominant contributor to external current is oxidation reactions, allowing a quantitative measure of electrolyte decomposition rate. Within this protocol, the passivating effect of various additives can be evaluated both during and after the potentiostatic hold by comparing oxidation current for an additive-containing electrolyte to the baseline. Upon completion of the potentiostatic hold, the capacity and impedance are measured, and cells were disassembled with components characterized by XPS, LC-MS, and IR. Acknowledgements The work at Argonne National Laboratory was performed under the auspices of the U.S. Department of Energy (DOE), Office of Vehicle Technologies, under Contract No. DE-AC02-06CH11357. The submitted issue has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-Eng-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, irrevocable, worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Modeling the Positive Electrode Side Reaction for a High Voltage Lithium-Ion Battery

Side reactions in battery electrodes lead to a decrease in battery performance and lifespan. In this work, the side reaction in the positive electrode is examined both computationally and experimentally. Computationally, a transport model was created using the finite element method to track the lithium concentration, potential, and current in each part of the battery. The transport model employs concentrated solution theory and Butler-Volmer kinetics. The side reaction was accounted for with an extra current in the positive electrode. Different functions based on physical conditions were fitted to this current and correlated to our experimental results to determine an expression for the side reaction rate. Experimentally, a lithium-ion half-cell, coin-cell was charged to 4.6 V at 45°C. The coin cell was held at this potential and temperature for 60 hours. The resulting current at this potentiostatic hold correlates to the relaxation of concentration gradients and the side reaction current. Additionally, the concentration of carbon-conductive additive in the positive electrode, the temperature, and the potential were varied to determine how each factor affects the side reaction rate. By performing both a computational and experimental study, we are able to incorporate an expression for the side reaction into our model with physical significance.

Effect of Salt Concentration on the Corrosion Behavior of Carbon Steel in CO2Environment

Effect of salt (NaCl) concentration on the corrosion rate of carbon steel in CO2-saturated solutions was explored in a wide range from 0.001 wt% to 10 wt%. Electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) were utilized to determine the instantaneous corrosion rate of C1018 carbon steel under freely corroding condition. Both EIS and LPR results indicated that corrosion rate decreases with increasing salt concentration, and the highest corrosion rate was found at the lowest salt concentration (0.001 wt%). Potentiodynamic polarization and potentiostatic hold experiments were performed to study anodic and cathodic kinetics using microelectrode technique. Cathodic polarization results showed that the cathodic reaction is under diffusion or mixed control and the limiting current increases with decreasing salt concentration. Anodic polarization results suggested that chloride adsorption prevails at high salt concentration, while hydroxyl adsorption prevails at low salt concen...

Using Physics-Based Models and Redox Shuttles to Investigate the Effects of FEC on Electrode Passivation

Chemical additives, such as fluoroethylene carbonate (FEC), can be used to improve lithium-ion battery performance by facilitating the formation of a passivation layer on the negative electrode surface. We compare passivation layers formed by electrolytes with and without FEC, using the ferrocenium-ferrocene redox couple. To quantify experimental findings, we develop a mathematical planar-electrode impedance model to extract physical parameters related to the passivation layer. Experimental cyclic voltammetry (CV) shows that the electronic passivity of the formed passivation layers is dependent upon the imposed electrochemical conditions. That is, potentiostatic holds cause surface passivation, leading to slower ferrocenium reduction kinetics. With increasing formation time, the high-frequency arc width of the impedance spectra increases for both electrolytes. However, the increase in high-frequency arc width is greater for electrolyte without FEC than that containing FEC. Formation potential influences the magnitude of electronic passivity of the solid electrolyte interphase (SEI), with more negative potentials increasing electronic passivity. In Figure 1(a), the electrode surfaces have been passivated for 30 minutes under a 0.6 V potentiostatic hold. Two CVs are measured at ν = 10 mV s-1, comparing electrolytes with and without FEC. The addition of FEC to the primary electrolyte enhances ferrocenium reduction kinetics, when comparing peak heights and position. Figure 1(b) shows experimental and simulated impedance responses, obtained after 30 minutes of passivation around the redox shuttle equilibrium potential (3.24 V). Experimental and simulated impedance responses show good agreement over the frequency range shown, suggesting that reliable conclusions can be drawn about the SEI. Particularly, impedance response simulations suggest that interfacial kinetics and porosity of the outer layer of the SEI greatly influences the high-frequency arc width. Figure 1

Evaluation of Electrolyte Oxidation Stability on Charged LiNi0.5Co0.2Mn0.3O2 Cathode Surface through Potentiostatic Holds

The electrolyte oxidation rate was measured at 4.6 V vs. Li+/Li for a NCM/LTO full cell at temperatures ranging from room temperature to 55°C. A large (∼2.5) N/P ratio was used in electrode fabrication to enable a stable reference in the Li4Ti5O12 electrode for both cycling and the potentiostatic hold regimes. LTO was used to prevent electrolyte reduction from contributing to the signal, as well as providing a stable impedance. For the alkyl carbonate electrolyte used, oxidation rate at room temperature to 45°C approximately ranged from 1–2 μA, as measured during the terminal 4.2 hours of a 60 hour potentiostatic hold. Many cells aged at 55°C indicated increasing, unstable currents. Upon the completion of the hold, the cells were cycled to evaluate the effect of the hold and analyzed with AC impedance spectroscopy. Higher temperatures during the potentiostatic hold led to lower capacities during post-aging cycling, as well as higher impedance as measured with EIS.

Reversible and Irreversible Deformation Mechanisms of Composite Graphite Electrodes in Lithium-Ion Batteries

Repeated charge and discharge of graphite composite electrodes in lithium-ion batteries cause cyclic volumetric changes in the electrodes, which lead to electrode degradation and capacity fade. In this work, we measure in situ the electrochemically-induced deformation of graphite composite electrodes. The deformation is divided into a reversible component and an irreversible component. Reversible expansion/contraction of the composite electrodes is correlated with localized changes in graphite layer spacing associated with different graphite-lithium intercalation compounds. Phase transitions between different intercalation compounds are manifested during galvanostatic cycling as peaks in the derivative of capacity with respect to voltage; these peaks correspond remarkably well with peaks in the derivative of strain with respect to voltage. Irreversible electrode deformation is correlated with deposition of electrolyte decomposition products on graphite particles during the formation and growth of the solid electrolyte interphase (SEI). Both the irreversible capacity and the irreversible strain developed during galvanostatic cycling increase with increasing electrode surface area and increasing cycling time. During a potentiostatic voltage hold at 0.5 V vs Li+/0, in which electrolyte decomposition is the dominating electrochemical reaction, both the capacity and the electrode strain increase proportional to the square root of time. Interestingly, the choice of polymer binder, either carboxymethyl cellulose (CMC) or polyvinylidene fluoride (PVdF), has a significant influence on the irreversible electrode deformation, suggesting that the formation and growth of the SEI layer is influenced by the polymer binder.

In situ electrochemical STM study of platinum nanodot arrays on highly oriented pyrolythic graphite prepared by electron beam lithography

Model electrodes consisting of platinum dots with a mean diameter of (30±5) nm and heights of 3–5nm upon highly oriented pyrolytic graphite (HOPG) were prepared by electron beam lithography and subsequent sputtering. The Pt nanodot arrays were stable during scanning tunnelling microscopy (STM) measurements in air and in sulphuric acid electrolyte, indicating the presence of “anchors”, immobilising the dots on the HOPG surface.Electrochemical STM was used to visualise potential induced Pt, carbon and Pt-influenced carbon corrosion in situ in 0.5M sulphuric acid under ambient conditions. Potentiostatic hold experiments show that the Pt dots start to disappear at electrode potentials of E>1.4V vs. SHE. With increasing time and potential a hole pattern congruent to the original dot pattern appears on the HOPG basal planes. Corrosion and peeling of the HOPG substrate could also be followed in situ.Dissolution of Pt dots appears to be accelerated for potential cycling experiments compared to the potential hold statistics.

Comparative Studies of Hardware Corrosion at High Potentials in Coin-Type Cells with Non Aqueous Electrolytes

The electrochemical stability of control and aluminum-sputtered coin cell cases was studied by chronoamperometry in 1 M LiPF6 in ethylene carbonate/diethylcarbonate solution at various potentials vs Li/Li+ and at various temperatures for extended times. Under anodic polarization, aluminum-sputtered cases demonstrated a stable behavior due to their passivation by surface films. The parasitic current developed on the control stainless steel case was about one order of magnitude larger than on the aluminum-sputtered cases at all potentials and temperatures studied. After 200 hours of potentiostatic hold, cells were opened and examined for corrosion. Al-sputtered cases showed no corrosion under any condition, while the control cases showed localized discolorations at 4.6 V vs Li/Li+. The impact of these parasitic currents on high precision coulometry experiments using coin cell hardware is discussed.

Degradation Studies of PEMFC Cathodes Based on Different Types of Carbon

In this study different accelerated degradation tests were used evaluating three different carbon supports as well as a thin model electrode. Cyclic ADTs, by 1000 cycles beween 0.6 and 1.2 V in nitrogen, did not degrade the porous electrodes to any larger extent in terms of oxygen reduction activity, whereas a significant loss of electrochemical surface area was seen, often more than 50%. Potentiostatic hold at 1.4 V during 3 h, did not permanently degrade the electrodes but instead an improved activity was obtained after rest during night. A correlation of increase in double layer capacitance and improved performance was seen and believed to be caused by the good proton conductivity of carbon surface oxides. CO-stripping peaks revealed that the humidity and wetting of NafionTM may have caused the observed temporary changes during the potentiostatic hold.

Determination of the Potentiostatic Stability of PEMFC Electro Catalysts at Elevated Temperatures

AbstractThe electrochemical stability of platinum on carbon catalyst (Hispec™ 4000, Johnson Matthey) has been investigated predominantly at constant potentials ranging from 0.95 to 1.25 V at elevated temperatures. By combining a quartz crystal microbalance (QCM) with electrochemical techniques, dynamic insight is obtained on the oxidation and corrosion of both platinum and carbon during potentiostatic hold. From the cyclic voltammetry (CV) data, it can be concluded that at all conditions, the platinum surface area decreases when Pt on carbon catalysts are exposed to a constant potential of 1.05 to 1.25 V. Under the applied conditions, this loss of surface area is primarily caused by the dissolution of platinum.Both the QCM as well as on‐line electrochemical mass spectrometry (OLEMS) experiments show that the corrosion of carbon is catalysed by the presence of platinum at 80 °C, as long as the platinum surface is not passivated by an oxide layer.

Electrochemical Evaluation of Constituent Intermetallics in Aluminum Alloy 2024-T3 Exposed to Aqueous Vanadate Inhibitors

Experiments were conducted to determine how inhibiting forms of vanadate interact with complex Al alloy microconstituent intermetallics to impart corrosion protection. Cathodic polarization experiments on Al 2024-T3 indicate a strong correlation between inhibition and the presence of tetrahedrally coordinated vanadate. Anodic and cathodic polarization curves were measured on bulk synthesized , , , and in alkaline NaCl solutions with and without . Vanadate additions generally decreased , increased , and decreased the cathodic kinetics of all tested materials. Because of decreased cathodic kinetics, open-circuit potentials (OCPs) were shifted in the active direction in aerated solutions when vanadate was present. This shift pins the OCP just below the observed pitting potential for in vanadate solution, effectively preventing breakdown and subsequent support of rapid oxygen reduction by Cu-enriched clusters. , , , , , and at data from polarization experiments were summarized in cumulative distribution plots, and averages are presented in tabulated format. Scanning electron microscopy images of Al 2024-T3 used for of OCP measurement show that vanadate greatly decreased circumferential trenching around intermetallic particles in both aerated and deaerated solutions. Potentiostatic hold experiments were used to show suppression of dissolution in vanadate solutions.

Corrosion of similar and dissimilar metal crevices in the engineered barrier system of a potential nuclear waste repository

Crevice corrosion is considered possible if the corrosion potential ( E corr) exceeds the repassivation potential for crevice corrosion ( E rcrev). In this study, potentiodynamic polarization and potentiostatic hold were used to determine the E rcrev of similar and dissimilar metal crevices in the engineered barrier system of the potential Yucca Mountain repository in 0.5 M NaCl, 4 M NaCl, and 4 M MgCl 2 solutions at 95 °C. The results were compared with data previously obtained using crevices formed between Alloy 22 and polytetrafluoroethylene. It was observed that, except for Type 316L stainless steel, all other metal-to-metal crevices were less susceptible to crevice corrosion than the corresponding metal-to-polytetrafluoroethylene crevices. Measurements of galvanic coupling were used to evaluate the crevice corrosion propagation behavior in 5 M NaCl solution at 95 °C. The crevice specimens were coupled to either an Alloy 22 or a Titanium Grade 7 plate using metal or polytetrafluoroethylene crevice washers. Crevice corrosion of Type 316L stainless steel propagated without repassivation. For all the tests using a polytetrafluoroethylene crevice washer, crevice corrosion of Alloy 22 was initiated at open circuit potential by the addition of CuCl 2 as an oxidant, whereas no crevice corrosion of Alloy 22 was initiated for all the tests using Alloy 22 or Titanium Grade 7 metals as crevice washer. However, crevice corrosion propagation was found to be very limited under such test conditions.

An investigation into factors affecting the stability of carbons and carbon supported platinum and platinum/cobalt alloy catalysts during 1.2 V potentiostatic hold regimes at a range of temperatures

To meet automotive targets for fuel cell operation and allow higher temperature operation an understanding of the factors affecting carbon and platinum stability is critical. The stability of both carbons and carbon supported platinum and platinum/cobalt alloy catalysts was studied during 1.2 V versus RHE potentiostatic hold tests using carbon and catalyst coated electrodes in a three-chamber wet electrolyte cell at a range of temperatures. At 80 °C the wt% of carbon corroded increases with increasing BET area. Surface oxidation was followed electrochemically using the quinone/hydroquinone redox couple. Increasing temperature, time at 1.2 V and wt% platinum on the carbon increases surface oxidation. Although increasing temperature was shown to increase the extent of carbon corrosion, catalysing the carbon did not significantly change how much carbon was corroded. Platinum stability was investigated by electrochemical metal area loss (ECA). Platinum catalysts on commercial carbons lost more ECA with increasing temperature. A platinum/cobalt alloy on a low surface area carbon was demonstrated to be more stable to both carbon corrosion and metal area loss at temperatures up to 80 °C than platinum catalysts on commercial carbons, making this material an excellent candidate for higher temperature automotive operation.

Corrosion potential oscillation of stainless steel in concentrated sulphuric acid: I. Electrochemical aspects

The spontaneous oscillation of corrosion potential between the active and passive states of UNS S30403 stainless steel has been studied in 93.5 wt.% sulphuric acid at 60 °C. Detailed electrochemical analysis of the kinetics of the oscillating potential has suggested the presence of a nickel salt which is stable over a limited potential range. The nickel salt is subject to dissolution at potentials more positive than −0.25 V (MSE), while at lower potentials—less than −0.35 V (MSE)—a stable salt film is formed. A critical coverage of the salt on the surface is the prerequisite for the mixed potential to be shifted into the passive range by an increase in the exchange current density for the sulphuric acid cathodic reaction. However, when the coverage exceeds a critical value, such as can be produced by a potentiostatic hold, the salt effectively blocks the formation of a passive film.