Anodic Behavior Research Articles

A novel electrocoagulation process with centrifugal electrodes for wastewater treatment: Electrochemical behavior of anode and kinetics of heavy metal removal

Anodic passivation is a key problem to impair the efficiency of in the electrocoagulation (EC) process. Process intensification of EC has attracted increasingly greater attention. In this work, a novel centrifugal electrode reactor was designed and applied in EC process to enhance the treatment of simulated heavy metal wastewater using aluminum anode. Results showed that the removal efficiency of heavy metals was significantly improved by the centrifugal electrodes, compared with the stationary electrodes. Electrochemical behavior of centrifugal electrodes was analyzed by an improved rotating disk electrode system. Anodic polarization behavior of aluminium showed a typical characteristic of dissolution in centrifugal electrodes, rather than passivation in static condition. Anode dissolution was controlled by the diffusion of Cl− ion that was enhanced by centrifugal electrodes. Thus, anode passivation was reduced. In addition, the kinetics analysis indicated that the removal of heavy metals in EC by centrifugal electrodes conformed to Variable-Order-Kinetic (VOK) model based on the Langmuir adsorption.

Low Temperature Behavior of Alloy Anodes for Lithium-Ion Batteries

Lithium-ion batteries are ubiquitous in most energy storage applications, but they fail completely below about -20 ºC, which hinders their use in low-temperature applications, like electric aviation and space exploration. Materials that alloy with lithium offer high specific capacities and have been researched thoroughly at room temperature, but their low temperature behavior is not well understood, despite indications that they may exhibit superior performance. In this work, we characterize the electrochemical and structural changes of antimony, tin, and silicon anodes at temperatures down to -40 ºC using a tailored ether electrolyte. Antimony anodes exhibit reversible cycling at -40 ºC, with a first cycle specific capacity of 450 mAh g-1, and tin and silicon can be cycled at -20 ºC with first cycle specific capacities far exceeding that of graphite. In addition, three-electrode coin-cell experiments reveal strong influences of the lithium metal counter electrode on the overall galvanostatic voltage profiles; these influences become dominant at low temperature and obscure the inherent behavior of the alloy anodes. This finding highlights the importance of using an appropriate reference electrode for low-temperature experiments, especially for sensitive experiments such as the galvanostatic intermittent titration technique (GITT). GITT experiments revealed the contributions of kinetic processes and thermodynamic hysteresis as a function of temperature in these materials, and they showed that the different materials feature different kinetic and hysteretic behavior at low temperature. The low temperature electrochemical behavior of silicon is seemingly more dependent on structural changes during the lithiation/delithiation transformations, while tin and antimony are more strongly impacted by kinetic effects at low temperatures. Finally, X-ray diffraction experiments on antimony anodes reveal difference in the average crystallite size and crystallite evolution at low temperature during cycling. Overall, these results are an important for understanding the fundamental behavior of alloy materials at low temperature and are an important step for the engineering of high-energy, low-temperature lithium-ion batteries.

Benchmarking the Electrochemical Degradation Behavior of Aluminum Foil Anodes for Lithium-Ion Batteries

Aluminum is an attractive candidate for replacing graphite anodes in lithium-ion batteries because it has high specific capacity (990 mAh g-1), and directly using aluminum foil as the anode structure eliminates the need for slurry-coating processes. However, achieving highly reversible lithiation and delithiation of aluminum is challenging due to volume changes during transformation, sluggish lithium-ion transport through the surface oxide layer, and poor initial Coulombic efficiency which all lead to degradation during cycling. Previous studies have focused on understanding the fundamental electrochemical reaction and material transformation behaviors of aluminum, yet there has not been a focus on how different aluminum alloy compositions behave and degrade under electrochemical cycling conditions. In this work, we carry out comprehensive electrochemical testing to benchmark the performance of three different aluminum alloy foils under different cycling conditions. We found that for foils of constant thickness, all foil compositions exhibit a power-law dependence of cycle life on the areal capacity lithiated per cycle, revealing that degradation is significantly accelerated when high areal capacities are used each cycle. Furthermore, the composition of the aluminum alloy was found to strongly affect the Coulombic efficiency over the first 10 cycles, with higher purity foils exhibiting higher Coulombic efficiency. Finally, ex situ scanning electron microscopy and operando optical microscopy revealed different reaction mechanisms and mechanical degradation behavior amongst the different alloys. Based on understanding these various parameters, we showed how full cells with aluminum anodes can be cycled for hundreds of cycles at relatively low areal capacities. The improved understanding of the behavior of aluminum foil anodes herein paves the way for future work intended to engineer aluminum-based foils with enhanced stability.

Characteristics of Anodic Dissolution for Al-Cu Alloys in EmImCl-AlCl3 Ionic Liquid

Recycled Al obtained by re-melting process used the primary Al can be produced with 3-5% of the energy required for the electrolysis. However, the Al recycled by this method contains various elements as impurities, resulting in lower quality. After this repeated recycling, the Al eventually becomes scrap that is difficult to recycle and is landfilled. As a purification process, we propose an electrorefining method using scrap Al as an anode. Copper (Cu) is one of the major additive elements in aluminum. The Cu should be removed as far as possible in the recycling of Al scrap, since this alloy corrode from Al-Cu compounds precipitated at the grain boundaries. As Cu is a more noble element than Al, when Cu dissolves from the anode into the electrolyte due to an increase in the anode potential, the Cu ions are rapidly reduced at the cathode and electrodeposited as metallic Cu. In order to remove copper from Al scrap by electrorefining, the anodic dissolution of copper in scrap Al must be avoided. In this study, the anodic dissolution behavior of Al-Cu alloys on the metallurgical structure was investigated using Al-Cu alloys with controlled copper concentration and size of intermetallic compounds.Al-1.0, 3.0 and 5.0 Cu casting alloy plates and cold-rolled plates were used as anode specimens. The preparation of the ionic liquid and all electrochemical measurements were carried out in Ar filled glove box. The ionic liquid electrolyte was prepared by mixing 1-ethyl-3-methyl imidazolium chloride (EmImCl) and AlCl3 in a molar ratio of 1 : 2. Electrochemical measurements were carried out in a three-electrode type cell using various Al-Cu alloys as working electrodes, Pt plates as counter electrode, and , Al wires as reference electrode. The electrolyte was heated at 323 K by hot plate stirrer, and each electrode was connected to a potentiostat for anodic polarization measurement and constant potential electrolysis. Anodic polarization curve was measured from the immersion potential to 1.5 V vs. Al/Al(III) with scanning rate of 1 mV s-1. Constant potential electrolysis was performed at potentials from 0.3 to 1.2 V, and amount of electric charge densities of 0.3, 3.0, and 30 C cm-2 were applied. The specimens after the constant potential electrolysis were rinsed by distilled water and dried, then observations of appearance, XRD measurement, and FE-SEM observation were carried out.In the anodic polarization measurements of various Al-Cu alloys, peaks of anodic current density were observed at around 0.3 V and 0.8 V in casting alloys and Al-5.0Cu alloy with Al2Cu on the surface. Constant potential electrolysis was then carried out at potentials of 0.3, 0.8 and 1.2 V with electric charge density of 30 C cm-2 in order to investigate the dissolution behavior of Al-Cu alloys depending on the electrolysis potential. Al-5.0Cu casting alloy and cold-rolled plate after constant potential electrolysis were analyzed by SEM-EDS. Al in the matrix phase was preferentially dissolved then Al2Cu was appeared on the surface at 0.3 V. Dissolution of Al2Cu and Al in the matrix phase was occurred at higher than 0.8 V. In summary, the potential dependence on the anodic dissolution behavior of the metallurgical structure of Al-Cu alloys in EmImCl-AlCl3 ionic liquid has been clarified. By controlling the anodic dissolution potential, the dissolution of Cu in Al2Cu into the electrolyte can be suppressed, which is expected to further improve the purity of the electrorefined aluminum. Acknowledgement Part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO). We would like to express our gratitude to all parties concerned.

Lithiation and Delithiation Behaviors of the Li-Cu Porous Anode in Li-Metal Battery System

The demand for High-energy-density batteries is rapidly increasing due to fast evolution of portable electronic devices, electric cars and energy storage systems. Lithium ion battery system based on graphite anode has made a lot of progress, however, it has almost reached its limit with the low theoretical capacity. Lithium metal anode has the advantages of high theoretical specific capacity (3860mAhg-1) and lowest electrochemical potential voltage (-3.04V vs standard hydrogen electrode). However, the commercialization of lithium metal is still hindered by several inherent challenges such as inhomogeneous lithium dendrites and huge volume expansion during deposition and dissolution. In this study, a use of the pore between Li and Cu powder for stable Li deposition without volume changes from commercially available cathode materials like NCM is investigated. In this system, binder-free anode is facilely and simply fabricated by pressing the well-mixed two powders of similar size. The effect of pore space created by adjusting the pressure on battery performance is verified. A utilization of Li powder induces controllable and uniform dendrite growth because the current density per unit area is lowered by increasing the specific surface area. Inert Cu powder not only leads to localize Lithium deposition, but also maintains a robust skeleton without volume changes. The lithiation and delithiation behavior of Li-Cu porous anode are thoroughly studied by material characterizations and electrochemical properties

A 3D Electrochemical Model for the Si/C Composite Active Particles

SiO/Graphite (Gr) composite has attracted more and more interests in both the academic and industrial communities regarded as one of the most promising anode materials for the next generation of high-energy-density lithium-ion batteries (LIBs). This material achieves high capacity meanwhile maintaining a relatively good mechanical integrity compared to the pure Si anode. However, the heterogeneous composition of such an anode system brings in highly nonlinear and complex electrochemical behaviors compared to the single-material anode. The computational modeling provides an efficient and accurate way to explore the electrochemical behaviors of SiO/Gr composite anode. Herein, we propose a 3D model at the electrode level containing particle geometries based on a representative volume element (RVE) and study the electrochemical process of the half-cell charging. The effects of SiO proportion, charging rate, SiO distribution, and SiO particle size on the electrochemical performance are discussed. The addition of SiO in Gr anode leads to the ultra-high real-time C-rate (CR ) compared to the applied one within a specific time range, which is harmful to the active particles, especially under high C-rates. The results reveal that anode with higher SiO proportions performs a better rate capability. We also discover that moving SiO particles towards the separator and shrinking the SiO particle can benefit cell performance. Results provide an in-depth understanding of the electrochemical behaviors of the composite anode and guide the design for SiO/Gr anode materials in maximizing the theoretical capacity while maintaining better rate performance.

(Invited) The Behavior of High-Capacity Anodes at Low Temperature for Li-Ion Batteries

Lithium metal and alloy anodes offer the promise of improved capacity for Li-based batteries, and they could overcome low-temperature diffusion limitations associated with graphite. However, the electrochemical behavior of these materials at low temperatures has been seldom investigated, and most liquid electrolytes exhibit extremely low Coulombic efficiency (CE) for cycling of these high-capacity electrode materials at low temperatures. Here, we demonstrate an ether-based electrolyte system with carbonate additives that substantially improves the CE of lithium metal cycling down to -60 °C. Lithium metal deposited at low temperatures in this and other electrolytes shows a clear reduction in grain size with decreasing temperature, which is correlated with lower CE. Cryo-TEM and X-ray photoelectron spectroscopy investigation of the solid-electrolyte interphase (SEI) shows that this tailored electrolyte allows for greater inorganic content in the SEI at low temperatures, which enables a compact and conducting passivation layer. We further extend these studies to sodium metal anodes, which show improved CE at low temperatures compared to lithium. Finally, we investigate alloy anode materials and show that they can exhibit stable electrochemical cycling down to -40 °C, indicating the promise of these high-capacity materials for low-temperature batteries.

(Corrosion Division H. H. Uhlig Award) The Journey from Numerical Simulations to Impedance Analysis

Our work in corrosion has encompassed numerical simulations and electrochemical impedance spectroscopy. This presentation provides an overview of insights gained from these endeavors.Numerical simulations train insightIn collaboration with Kevin Kennelley and Lee Bone, then from Arco, our group developed boundary-element models to simulate cathodic protection (CP) of pipelines with coating defects that exposed bare steel. Designed initially for the trans-Alaska pipeline,1 the model was extended to account for multiple pipes, multiple CP systems,2 buried tanks, and tank bottoms.3 The program could be used to simulate rectifier wars in which impressed current on one CP system can induce corrosion on adjacent or crossing systems. This program was used to demonstrate the behavior of heterogeneous anodes. For example, addition of impressed current anodes to an inadequate set of sacrificial anodes runs the risk of protecting the sacrificial anodes at the expense of protecting steel exposed by coating defects.In collaboration with Richard Woollam, then with BP, and, later, with Scott Briggs, NWMO, our group developed finite-element COMSOL models that accounted for corrosion under droplets containing dissolved oxygen.4 The models include coupled, nonlinear, conservation equations for ionic species, which account for migration, diffusion, local electroneutrality, homogeneous reactions, and formation of precipitates. The presence of anodic and cathodic regions was not assumed a priori, but was rather the result of numerical simulations, which revealed galvanic coupling caused by differential aeration cells. This work showed that, while differential aeration created nonuniform corrosion for steel covered by a deposit or droplet, corrosion of copper was largely uniform.Impedance spectroscopy can provide useful parametersWhile the use of the Stern-Geary relationship to extract corrosion current is well known, this approach applies only for systems under kinetic control. More information may be extracted from the ubiquitous constant-phase-element (CPE) behavior. In collaboration with scientists/engineers from France and Italy, our group developed the power-law model, which provides an equation that relates CPE parameters to film thickness, dielectric constant, and film resistivity at the film-electrolyte interface.5,6 This model can be augmented with the measurement model, from which capacitance may be extracted.7 Thus, if the dielectric constant is known, the measurement model can yield the corresponding film thickness and the power-law model can yield the resistivity distribution in the film.8 An early version of this approach is now in commercial use.9 Acknowledgements I am grateful for all the students and collaborators who walked part of this journey with me. These include students J. Matthew Esteban and Doug Riemer (CP models), Ya-Chiao Chang and Chen You, (droplet models), and Bryan Hirschorn (power-law model). I thank collaborators Bernard Tribollet, Vincent Vivier, Marco Musiani, and Nadine Pébère, with whom I have explored the arcane arts of impedance spectroscopy. Finally, I thank the Corrosion Division awards committee for bestowing upon me the 2022 H. H. Uhlig Award. References E. Orazem, J. M. Esteban, K. J. Kennelley, and R. M. Degerstedt, “Mathematical Models for Cathodic Protection of an Underground Pipeline with Coating Holidays: 1. Theoretical Development,” Corrosion, 53 (1997), 264-272.Liu, A. Shankar, M. E. Orazem, and D. P. Riemer, “Numerical Simulations for Cathodic Protection of Pipelines,” in Underground Pipeline Corrosion: Detection, Analysis, and Prevention, M. E. Orazem, editor, Woodhead Publishing Limited, Cambridge, UK, 2014, 85-126.P. Riemer and M. E. Orazem, “A Mathematical Model for the Cathodic Protection of Tank Bottoms,” Corrosion Science, 47 (2005), 849-868.-C. Chang, R. Woollam, and M. E. Orazem, “Mathematical Models for Under-Deposit Corrosion: 1. Aerated Media,” Journal of The Electrochemical Society, 161 (2014), C321-C329.Hirschorn, M. E. Orazem, B. Tribollet, V. Vivier, I. Frateur, and M. Musiani, “Constant-Phase-Element Behavior Caused by Resistivity Distributions in Films: 1. Theory,” Journal of The Electrochemical Society, 157 (2010) C452-C457.E. Orazem, B. Tribollet, V. Vivier, S. Marcelin, N. Pébère, A. L. Bunge, E. A. White, D. P. Riemer, I. Frateur, and M. Musiani, “Dielectric Properties of Materials showing Constant-Phase Element (CPE) Impedance Response,” Journal of The Electrochemical Society, 160 (2013), C215-C225.Liao, W. Watson, A. Dizon, B. Tribollet, V. Vivier, and M. E. Orazem, “Physical Properties Obtained from Measurement Model Analysis of Impedance Measurements,” Electrochimica Acta, 354 (2020), 136747.You, A. Titov, B. H. Kim, and M. E. Orazem, “Impedance Measurements on QLED Devices: Analysis of High-Frequency Loop in Terms of Material Properties,” Journal of Solid-State Electrochemistry, 24 (2020), 3083-3090.P. Riemer and M. E. Orazem, “Impedance-Based Characterization of Raw Materials as Used in Electrochemical Manufacturing,” The Electrochemical Society Interface, Fall 2014, 23:3 (2014), 63-67.

Effect of stray current on corrosion behavior of Mg alloy sacrificial anode in buried pipeline

In this work, a self-designed stray current simulated system was employed in corrosion evaluation test, and the effect of stray current on corrosion behavior of Mg alloy was investigated. The anti-corrosion performance of Mg alloy directly related to its corrosion film. The corrosion film covered on Mg surface was incomplete, which was a mixture of porous MgO + Mg(OH)2, and the corrosion happened on the bare Mg substrate at the rupture of corrosion film. The pitting corrosion randomly occurred on α-Mg, and the anodic hydrogen evolution (AHE) further damaged the corrosion film. With the extension of time, the pits developed into severe localized corrosion and the corrosion rate was accelerated. The stray current was composed of direct current (DC) and low-frequency alternating current (AC), which promoted the rupture of corrosion film, hence the stray current significantly accelerated the corrosion rate of Mg alloy.

Understanding the Electrochemical Behavior of Multi-Component Aluminum-Based Foils As Anodes for Lithium-Ion Batteries

Alloy anodes are promising materials for next-generation lithium batteries. The intrinsic properties of aluminum, such as high capacity, light weight, earth abundance, and low cost, make it a competitive alloy anode candidate in lithium batteries. Furthermore, the ability to readily manufacture aluminum-based foils is an advantage for potentially simplifying battery manufacturing processes. However, utilization of Al anodes is not yet fully practical due to the capacity fading during charge and discharge. To improve cycling performance, here we fabricate aluminum alloys and composite foil structures and investigate their reversibility for Li storage in Li-ion batteries. Alloy foils including Al-Sn, Al-Zn, and Al-In were fabricated with contents ranging from 50 to 99 atomic % Al, and the electrochemical behavior of these materials as anodes for Li-ion batteries were investigated. With these materials, we studied the effects of composition, structure, and morphology on the cycling performance of the alloy foils using battery-relevant capacity metrics (areal capacity of > 2 mAh/cm2 with foil thickness < 50 microns). We show that the composition of the foils plays a significant role in determining the electrochemical cycling capability of these foils. Furthermore, the foil processing characteristics are also important in determining the achievable specific capacity, rate behavior, and cycle life of these foils anodes. This effort indicates the promise of foil-based alloy anodes and provides guidance toward engineering strategies to improve performance.

Rare Earth Metal Production Via Molten-Salt Electrolysis

The extraction and purification of metals such as aluminum has relied on the electrowinning process for decades. The Hall-Héroult process, developed in 1885, utilizes a molten salt electrolyte to electrochemically produce aluminum metal (Al) and carbon dioxide (CO2) from aluminum oxide (Al2O3) and carbon.1–3 Similar molten salt techniques have been developed for a variety of other metals including the rare earth metal Neodymium (Nd). Neodymium is of particular interest recently with significant increases in demand being driven by the increased production of new technologies such as electrified vehicles and magnetic data storage that require neodymium in the form of neodymium–iron–boron (Nd–Fe–B) permanent magnets. 4,5 The state of the art procedure for neodymium processing, similar to the aluminum process, utilizes a neodymium and lithium fluoride molten salt electrolyte with a sacrificial carbon anode to convert neodymium oxide (Nd2O3) and carbon to neodymium metal and carbon dioxide.4,6 As an unfortunate byproduct of this process performed in a fluoride containing molten salt, perfluorocarbons (PFCs) can also be produced simultaneously alongside carbon dioxide from the sacrificial anode. The formation of PFCs combined with the emission of a significant amount of greenhouse gas (carbon dioxide) make the current process for neodymium electrowinning undesirable from an environmental standpoint.7 An alternative molten salt process has been proposed in which the fluoride salts have been replaced with chloride salts consisting of lithium chloride (LiCl) and potassium chloride (KCl). Rather than directly converting neodymium oxide to neodymium metal, the oxide is first converted to chloride salt form by reaction with hydrochloric acid. The neodymium salt is then dissolved into the LiCl-KCl molten salt and neodymium is electroplated via the below set of reactions.8,9 Cathode: 2NdCl3 + 6e- → 2Nd(solid) + 6Cl- Anode: 6Cl- → 3Cl2 + 6e- Overall: 2NdCl3 → 2Nd(solid) + 3Cl2 This process has several distinct advantages. Utilizing the chlorine reaction eliminates the need for a sacrificial anode material as well as the production of carbon dioxide. The chloride based molten salt also eliminates the formation of PFCs. The chlorine produced could then be recycled to make more hydrochloric acid for use in converting neodymium oxide to chloride. Our work evaluates anode and cathode behavior during this neodymium chloride molten salt process in order to determine its viability. Overpotential and stability of various anode materials are investigated in order to minimize energy consumption and ensure long life of process materials. The effect of various plating conditions such as current density and substrate material are investigated to determine impact on deposit quality, coulombic efficiency, and metal purity. Additional purification techniques such as vapor distillation procedures are developed to ensure a product that is viable for industrial use. This proof of concept work aims to develop a safe, sustainable and environmentally friendly path towards large scale production of rare earth elements. Reactor design and cathode efficiency results are based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy under the Advanced Manufacturing Office, Award Number DE-EE0009434. Anode design work was supported through a subcontract from the Ames Laboratory with funding from the Department of Energy - Energy Efficiency and Renewable Energy under contract No. DE-AC02-07CH11358; Agreement No. 26110-AMES-CMI. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government. T. R. Beck, Electrochem. Soc. Interface, 23, 36–37 (2014).G. G. Botte, Electrochem. Soc. Interface, 23, 49–55 (2014).W. E. Haupin, J. Chem. Educ., 60, 279–282 (1983).M. F. Chambers and J. E. Murphy, Electrolytic production of neodymium metal from a molten chloride electrolyte.B. Sprecher, R. Kleijn, and G. J. Kramer, Environ. Sci. Technol., 48, 9506–9513 (2014).V. S. Cvetković et al., Met. 2020, Vol. 10, Page 576, 10, 576 (2020).H. Vogel, B. Friedrich, H. Vogel, and B. Friedrich, Int. J. Nonferrous Metall., 6, 27–46 (2017).R. Akolkar, J. Electrochem. Soc., 169, 043501 (2022).D. Shen and R. Akolkar, J. Electrochem. Soc., 164, H5292–H5298 (2017).

A Perspective on the Repassivation Potential and a Novel Calculation Thereof

Corrosion can occur on metallic surfaces in nearly any humid environment, leading to either early failure or costly repairs. Specifically, localized corrosion, such as pitting, can be difficult to identify at an early stage and can act as crack nucleation sites. Determining under which conditions pitting will take place is therefore important for maintenance and design, yet non-trivial from an electrochemical perspective.The pitting potential (Epit) and repassivation potential (Erp) have long been used in literature to determine a material’s susceptibility to pitting, with the latter acting as the lower threshold at which pitting can occur. A lower threshold of pitting can often be useful for conservative estimates, making Erp a popular parameter. However, a consensus on how to measure and define Erp has yet to be defined. Conventional cyclic potentiodynamic polarization (CPP) scans to measure Erp are shrouded in controversy, as values were found to be dependent on the scan rate and turn-around current density, unless sufficient charge densities had passed. Even once a plateau of Erp is reached at sufficient charge densities, there is debate as to if values are too conservative at that point. Two recent pitting frameworks have proposed alternative measurements of Erp, using both 1-D pit and 3-D bulk electrodes. Breakthroughs and limitations from both of these theories will be considered.In this work, the various methods of measuring and defining Erp will be discussed to provide clarity between different notation and to highlight possible paths going forward. In addition, an alternative calculation of Erp is proposed which is less conservative yet harmonized with the critical pit stability product (ix)crit, as both thresholds should represent the same lower limit of pitting. The proposed equation to calculate Erp includes dependencies on conventional parameters, such as the anodic Tafel behavior within a pit (ba), the transition potential (ET), and the critical percent saturation of metal salts (f). Validation is provided through both results found in literature and the use of Finite Element Method (FEM) modeling in conjunction with the maximum pit model.

(Digital Presentation) The Effect of Tartaric-Sulfuric Acid (TSA) Anodizing on the Corrosion Resistance of the AA7475-T761

The microstructure of the high strength aluminum alloy, AA7475–T761, in the as-received condition was characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy, and statistical analyses of grain size and micrometer precipitates distribution and density were performed. An anodic aluminum oxide (AAO) film was potentiostatically grown on the alloy in tartaric sulfuric acid (TSA). The anodizing behavior of the alloy and the mechanisms of localized corrosion initiation and propagation in bare (not anodized) and anodized conditions were discussed. The alloy corrosion behavior was investigated in sodium chloride solutions using electrochemical techniques (cyclic potentiodynamic polarization) and the morphology and extent of corrosion propagation was investigated through optical microscopy, optical profilometry and scanning electron microscopy analyses. It was observed that the two alloy surface conditions (bare and anodized) play different roles in the propagation of the localized corrosion process. The bare alloy presented lower localized corrosion potentials and the propagation of the localized corrosion was shallower, but more heavily distributed, than in the anodized condition. However, the alloy in the anodized condition presented lower repassivation potentials due to the deeper pits formed. In addition, the variability of pitting potentials for the anodized condition was high, due to the heterogenous structure of the AAO film formed, and dependent on the time and drying storage conditions previous to electrochemical characterizations. The electrochemical results were correlated with the microstructural characteristics of the oxide surface film of the alloy in both conditions, bare (not anodized) and anodized.

Unique Porous Structures Supported by Anodic Treatments in Liquid Ammonia (-50°C, Patm)

Since the first anodic porosification of semiconductors in the 1950s, numerous studies have reported a wide range of porous structures. The conditions used during the growth of pores can explained the shapes obtained but the formation mechanism and especially the influence of surface chemistry during porosification still raise many questions.It is in such context that the contribution of liquid ammonia (-55°C) is crucial. Indeed, this remarkable non-aqueous solvent allows, on the one hand, to obtain novel porous structures on n-InP and, on the other hand, it allows for the first time a specific and original interfacial chemistry, governed by the chemistry of nitrogen.In fact, in liquid ammonia, the dissolution of III-V semiconductors (GaAs, InP) is systematically done after a surface passivation step. This passivation step is particularly stable on InP and revealed the formation of a phosphazene type film (monolayer) resulting from the concomitant oxidation of the solvent and the semiconductor. The "multi-material" approach is a key factor, as it underlines the determinant role of the surface chemistry. In fact, at higher current densities, the porous structures of n-InP and n-GaAs show contrasting structures, which give an original behaviour compared to the aqueous medium. In order to determine the efficiency of the dissolution mechanism (at lower and higher current density), the quantity of indium (or gallium) salt will be quantified by Inductively coupled plasma optical emission spectroscopy analysis after ammonia evaporation.Moreover, the remarkable reproducibility of the anodic behaviour allowed physico-chemical characterisations of the porous surface, supported by scanning electron microscopy and the variation of the interfacial potential, photoluminescence and X-ray photoelectron spectroscopy.More specifically on n-InP, where an original structure was revealed, these analyses, during the early stages of porosification, led to the identification of a porosification mechanism in this solvent. Furthermore, at higher current densities, an amorphous film becomes thicker on the surface of the porous structure. Further porosification requires the permeability of this amorphous structure which would most probably result from the polymerisation of phosphazene since the presence of nitrogen could be clearly identified on the pore walls by Auger spectroscopy.

(Digital Presentation) Anodic Formation of Oxide Film on MoSi2 Containing Al or Nb in LiCl-KCl Molten Salt

Li and Mg metals are produced by electrolysis in a molten chloride bath. Currently carbon-based material is used as anode. However, carbon-based anode reacts electrochemically with oxide ions dissolved in the bath, which causes its undesirable consumption. In our laboratory, a new type of inert anode by forming a thin and dense oxide film has been studied. It was shown that a SiO2 film was formed on MoSi2 anode in molten LiCl-KCl melt with Li2O, and that the dissolution of MoSi2 was inhibited by the formation of oxide film. However, sufficient electrolytic current was hardly applied. In this study, the modification of the anodic properties of MoSi2 has been attempted by the addition of Nb or Al in MoSi2. A MoSi2 rod containing Nb(0.1~1.0mol%) or Al(0.1~0.5mol%) was prepared by sintering from MoSi2, Nb and Al powder. The prepared rod was pre-oxidized (1573K, 4h) to form a SiO2 film in advance, and then used as working electrode. An Al plate and Ag/AgCl were used as counter and reference electrode, respectively. The potential of the Ag/AgCl electrode was calibrated against the Li metal deposition potential measured by cyclic voltammetry. Electrolysis was conducted at 700K in an Ar atmosphere. Anodic behavior of MoSi2 was investigated by cyclic voltammetry, and potentio-static electrolysis was performed. The weight change in MoSi2 anode was measured, and the surface and cross section of the MoSi2 electrode were observed and analyzed by SEM-EDX and XRD.The shape of the cyclic voltammogram did not change significantly by the addition of Nb or Al in the MoSi2. Anodic peaks were seen around 2.5V (vs. Li / Li+, the same hereinafter) and around 3.6V. Above 3.6V, anodic current continued to flow with gas bubble formation. The current at 3.6V in the cyclic voltammogram changed with the added amount of Nb or Al as shown in Fig.1, which suggests the Nb or Al addition strongly influenced the anodic property of MoSi2.Current during potentio-static electrolysis at 4.2V increased with current oscillation. It was seen that a black film peeled periodically, and the current oscillation seemed to link the peeling. The quantity of electricity increased with Ni or Al addition in MoSi2. An oxide layer was observed on the MoSi2 surface after potentio-static electrolysis, and a dense layer was formed on MoSi2 containing 0.5~1.0mol% Nb. The weight of MoSi2 decreased by potentio-static electrolysis, and Nb or Al addition in MoSi2 clearly inhibited the weight change. The addition of 0.5~1.0mol%-Nb seemed suitable from the improvement of the anodic property of MoSi2.It was indicated that Nb or Al addition in MoSi2 influenced its anodic property, and that the addition inhibited its anodic consumption in molten LiCl-KCl. However, it was shown that the formation and peeling of the surface film occurred repeatedly, so the formation of a durable surface film should be necessary.The results of this research were obtained as a result of the commissioned work (JPNP 14004) of the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Anodic Behaviour of Ni-Fe-Cu electrode In Molten Fluoride Salts

The industrial process for Aluminium production is the Hall-Héroult process, an electrolysis process where the alumina raw material is dissolved in a cryolite based electrolyte, the aluminium is deposited at the cathode while the oxygen from the alumina reacts anodically with the carbon anodes, which are consumed in the process. The chemical energy in the carbon anodes provides part of the process energy, resulting in CO2 emission of about 1.6 kg/kg aluminium produced. In industrial cells, the electrolyte is modified from the cryolite, Na3AlF6, composition by adding AlF3 to lower the process temperature and improve current efficiency. This modification can be characterized by the Cryolite Ratio, CR, which is the molar ratio of NaF/AlF3. In modern cells, this ratio is often as low as 2.2, as compared to 3 in pure cryolite, which enables an operating temperature of around 960°C, as compared to more than 1009°C for Cryolite.One way to eliminate CO2 process emissions from aluminium electrolysis is to switch from consumable carbon anodes to inert anodes, resulting in O2 emissions instead of CO2. Significant effort has been put into achieving this, but the challenge is to find anode materials that can withstand the harsh conditions in the high temperature cryolite based electrolyte. It would help anode life to reduce the electrolysis temperature from 960 °C to 800 °C or lower. Lowering the CR to 1.3 or lower would achieve this, but for traditional electrolyte components this would lower the alumina solubility to unacceptably low levels. By replacing some of the NaF in the electrolyte with KF, it is possible to operate at this very low CR with acceptable alumina solubility.In this paper, we present a study on anode performance in potassium-rich low CR electrolyte. Both electrodes are vertical, the anodes made of homogenised CuFeNi alloy and cathodes of hot-pressed TiB2. The paper describes the effect of varying the anode composition, anode surface treatment as well as current density on anode stability. Also, the effect of varying the ratio of KR = KF/(NaF+KF), process temperature as well as alumina saturation is addressed.In general, pre-oxidation of the metallic anodes as well as the Cu content greatly affects anode stability. It is possible to operate at low temperature at lower KR, improving the industrial viability of the process, as NaO is added to the cells with the alumina, this reduces the alumina solubility which puts constraints on the process control. The aluminium obtained by electrolysis in this system had less than 0.2% impurities from the electrodes.

Anodic electrocatalytic behavior of graphite supported TiO2 towards the generation of hydroxyl radicals

The electrocatalytic mechanism of TiO2 towards anodic oxidation is mostly described as the electron-hole theory which seems limited and incomplete for its electrocatalytic application. This study focused on the anodic oxidation behavior of graphite supported TiO2 (TiO2/C). The results reveal that the TiO2/C anode generates H2O2 as the intermediate which decomposed to •OH indirectly from 1.0 V to 1.2 V. At higher potentials (1.4 V ∼ 2.0 V), the •OH generation is mainly attributed to H2O decomposition directly but inhibited by H2O2 in high concentration. The EPR analysis on oxygen defects establishes the relationship between the oxygen defect and the •OH generation experimentally. Together with the in-situ Raman experiments, the mechanisms for the •OH generation on TiO2/C anode are further correlated with oxygen vacancy and Ti-OH structure. This study provides a fundamental insight into the electrocatalytic behavior of TiO2 towards anodic oxidation, which is vital to its application in EAOPs.

Anodic Behaviour of Ni-Fe-Cu electrode In Molten Fluoride Salts

The anodic behaviour of NixFeyCuz alloy at different sodium- and potassium-cryolite based electrolyte compositions was examined by chronopotentiometry, galvanostatic polarisation, and linear sweep voltammetry at 1098 K. Steady-state anodic polarisation curves show anodic overpotential in electrolyte with molar ratio CR=(NaF+KF)/AlF3=1.4 is significantly less than in electrolyte with CR=1.3. Short-term galvanostatic polarisation curves indicate the potential difference between the anode and reference electrode (Al) was stable in electrolytes with CR=1.4 and potassium ratio KR=KF/(KF+NaF)=0.3 and CR=1.4 KR=0.35. Linear sweep voltammetry method indicates the presence of a diffusion controlled reaction at a potential below onset of oxygen evolution, which can likely be attributed to oxide formation. The diffusion coefficient obtained from linear sweep voltammetry and chronopotentiometry are within a similar range. Tafel curves confirm the presence of a reaction below the potential of oxygen evolution, but indicate a similar overpotential level for the oxygen evolving reaction as for carbon anodes.

Efficient electro-catalyzed PMS activation on a Fe-ZIF-8 based BTNAs/Ti anode: An in-depth investigation on anodic catalytic behavior

Phenanthrene (PHE), mainly released from coal tar and petroleum distillation, is an important kind of prevalent polycyclic aromatic hydrocarbons (PAHs) contamination in China (up to 2.38 ± 0.02 mg/kg in soil and 8668 ng/L in surface water) and other countries in the world. Metal–organic frameworks (MOFs) show promising application prospects in the decontamination field, however, suffering from the intrinsic fragility and fine powder forms. Therefore, macroscopic MOFs architecture-sandwich-like Fe-ZIF-8/blue TiO2 nanotube arrays (BTNAs)/Ti substrate (FBTT) anode with strong interfacial bonding (Fe-O-Ti and Fe-2-MIM-Ti coordination) was constructed using innovative in situ growth, condensation-crystallization-deposition, and pyrolysis methods, aiming at exploring the feasibility of MOFs-based anode/peroxymonosulfate (PMS) mediated PHE elimination, revealing the in-depth mechanisms, simultaneously overcoming the intrinsic drawbacks of MOFs. The FBTT-4 (doping content of 30 %) efficiently degraded PHE by 90.01 % and 74.5 % within 10 min at 350 μg/L and 3 mg/L, respectively, mediated by the ·OH compared to the SO4·-, 1O2, and O2·-. Post-optimized range of anodic potential enabled (i) anodic oxidation, (ii) activation of water and PMS molecules to produce active species, (iii) capture of electrons in reactants to reduce Fe3+/Ti4+ to Fe2+/Ti3+, maintaining the proportion of Fe/Ti with low valence and thus stable PMS activation capacity, and (iv) regulation of the Fe/Ti d-band center to modulate the anode adsorption capacity. The further increment in anodic potential could promote “dark photocatalysis” with a Z-scheme-like mechanism. Thus, it is proposed that the development of macroscopic MOFs-based anode, especially those with small band gaps, represents vast potentials in electrocatalytic contamination elimination. Simultaneously, the MOFs-based anode is expected to fully exploit their catalytic capacities and solve their intrinsic defects as well.

In Situ Growth of Se‐NiSx Pine‐Needle‐Like Nanowires on Nickel Foam: Boosting Ethanol Electrooxidation Reaction in Hydrogen Production

AbstractHydrogen production from overall water splitting assisted by organic oxidation reforming reaction is an innovative method to improve the anode behavior. Utilizing transition metal chalcogenides where the electronic states are regulated by anion modification, hexagonal Se‐NiSx pine‐needle‐like nanowires are prepared via a simple one‐pot hydrothermal reaction without any post‐processing steps. The nanowires can serve as ethanol electrooxidation reaction (EOR) catalyst with outstanding electrocatalytic activity (j100 = 1.36 V) and long‐term durability. In the practical tests of the hybrid water splitting, the catalyst required only 1.50 V to obtain 50 mA cm−2, and worked stably for over 50 h at a high current density of 200 mA cm−2 accompanied by an excellent selectivity (FE = 98.0%). These demonstrate that the introduction of Se significantly improves the catalytic performance of transition‐metal chalcogenides for EOR, indicating a new avenue for the design and improvement of other electrocatalysts.