Some personal adventures in passivity—A review of the point defect model for film growth

A review is presented of the history of the development of the Point Defect Model (PDM) for the growth and breakdown of passive films that form on the surfaces of reactive metals in contact with corrosive, condensed phase environments. The PDM has passed through three generations, with each successive generation addressing issues that have arisen from experiment. Thus, the first Generation model (PDM-I), which was developed in the late 1970s/early 1980s, assumed that the passive film was a single defective oxide layer that contained cation vacancies and oxygen vacancies that were generated and annihilated at the metal/film and film/solution interfaces. This model was inspired by the work by Wagner on high temperature oxidation. As with gas-phase systems, the film was assumed not to dissolve. However, it soon became evident that this model could not account for the properties of the passive state on metals in contact with aqueous environments and, accordingly a Generation II model (PDM-II) was developed to address these issues. PDM-II incorporated the bi-layer structure of the film comprising a defective oxide (or hydride) barrier layer that grows into the metal and an outer layer that forms by precipitation of material from the reaction of cations transmitted through the barrier layer with species in the environment (including water, CO₃²⁻, HS⁻, etc.), introduced metal interstitials to the suite of defects, recognized barrier layer dissolution, and recognized the need to classify reactions as to whether they are lattice conservative or non-conservative. PDM-II has enjoyed considerable success and the author knows of no instance where it has been demonstrated to be at odds with experiment when confluence between experiment and theory has been demonstrated. A Generation III model (PDM-III) has been recently developed to extend the theory to those cases (e.g., the valve metals) where the outer layer is so resistive that it controls the impedance of the interface and hence the corrosion rate. A fourth generation model that will describe passivity on alloys is now under development. The experimental evidence upon which each generation is based is reviewed.     

Spectroelectrochemical Behavior of Polycrystalline Gold Electrode Modified by Reverse Micelles

The increasing demand for raising the reliability of electronic contacts has led to the development of methods that protect metal surfaces against atmospheric corrosion agents. This severe problem implies an important economic cost annually but small amounts of corrosion inhibitors can control, decrease or avoid reactions between a metal and its environment. In this regard, surfactant inhibitors have displayed many advantages such as low price, easy fabrication, low toxicity and high inhibition efficiency. For this reason, in this article, the spectroelectrochemical behavior of polycrystalline gold electrode modified by reverse micelles (water/polyethyleneglycol-dodecylether (BRIJ 30)/n-heptane) is investigated by atomic force microscopy (AFM), potentiodynamic methods and electrochemical impedance spectroscopy (EIS). Main results indicate a strong adsorption of a monolayer of micelles on the gold substrate in which electron tunneling conduction is still possible. Therefore, this method of increasing the corrosion resistance of gold contacts is usable only in conditions of long-term storage but not in the operation of devices with such contacts. In this regard, the micelle coating must be removed from the surface of the gold contacts before use. Finally, the aim of the present work is to understand the reactions occurring at the surfactant/metal interface, which may help to improve the fabrication of novel electrodes.     

Predicting pH dependencies of electrode surface reactions in electrocatalysis

It is shown how to adopt the Nernst equation to electrode potential-dependent Gibbs energies, calculated for reactants and products from density functional theory, to make predictions of reversible potentials for redox reactions on electrode surfaces in electrolytes of any pH. The theory is general because any spectator species may be included in electrochemical interface. We demonstrate its application to H and OH deposition on Pt(111).     

Direct electron transfer reactions between human ceruloplasmin and electrodes

In an effort to find conditions favouring bioelectrocatalytic reduction of oxygen by surface-immobilised human ceruloplasmin (Cp), direct electron transfer (DET) reactions between Cp and an extended range of surfaces were considered. Exploiting advances in surface nanotechnology, bare and carbon-nanotube-modified spectrographic graphite electrodes as well as bare, thiol- and gold-nanoparticle-modified gold electrodes were considered, and ellipsometry provided clues as to the amount and form of adsorbed Cp. DET was studied under different conditions by cyclic voltammetry and chronoamperometry. Two Faradaic processes with midpoint potentials of about 400 mV and 700 mV vs. NHE, corresponding to the redox transformation of copper sites of Cp, were clearly observed. In spite of the significant amount of Cp adsorbed on the electrode surfaces, as well as the quite fast DET reactions between the redox enzyme and electrodes, bioelectrocatalytic reduction of oxygen by immobilised Cp was never registered. The bioelectrocatalytic inertness of this complex multi-functional redox enzyme interacting with a variety of surfaces might be associated with a very complex mechanism of intramolecular electron transfer involving a kinetic trapping behaviour.     

金属核及氧化铈的物理化学性质对一氧化碳氧化的影响

氧化铈独特的氧化还原性能使其适合用作氧化反应中的催化剂或载体.氧化铈负载的过渡金属纳米粒子或孤立的单原子提供了金属-载体界面,从而降低了去除界面氧原子的能耗,提供了可以参与ManVanKulvian氧化过程的活性氧物种.CO氧化是测试氧化铈负载催化剂还原性的主要探针反应,并且它常见于在相对低温下消除CO的各种应用中.在过量H2中优先氧化CO(PROX)反应可控制CO浓度达到超低水平,以防止氢氧化电催化剂中毒.催化剂在CO氧化反应中的活性和在PROX反应中对CO和H2的选择性取决于金属物种的种类和分散性、CeO2的结构和化学性质以及催化剂的合成方法.在这篇综述中,我们总结了最近发表的关于CeO2负载的金属纳米粒子和单原子催化CO氧化和PROX反应的相关工作;以及不同的负载金属和同种金属在普通CeO2表面上的反应性.我们还总结了密度泛函理论计算中提出的最可能的反应机理;并且讨论了各种负载型金属在PROX反应中影响CO氧化选择性的因素.     

Electroactive conducting polymers for corrosion control

This paper reviews the literature describing the effects of conducting polymer coatings on the corrosion rate of ferrous alloys (iron, steel and stainless steel). The literature is interpreted in terms of the proposed mechanisms of corrosion protection: barrier, inhibitor, anodic protection and the mediation of oxygen reduction. The most intriguing aspect of the reported literature are the studies demonstrating corrosion protection when deliberate defects were introduced into the coating to expose the bare metal. These studies show that protection afforded by conducting polymer coatings is not due to simple barrier protection or inhibition alone. Many studies illustrate that the polymer/metal interface is modified to produce passivating oxide layers and that charge transfer reactions occur between the metal and polymer. These studies support the proposed anodic protection mechanism, as do the reports of significant ennoblism. On the other hand, there is considerable variation in the reported shift in corrosion potential and these highlight the influence of substrate preparation, coating composition and mode of application and the nature of the electrolyte on the corrosion protection provided by the conducting polymer. For example, the evidence suggests that the emeraldine base form of polyaniline is superior to the emeraldine salt in terms of corrosion protection for steel. However, the number of direct comparisons is small and the reasons for the differences are not well understood. Also not well understood are the role of the counterion release and local pH changes on pinhole protection. It is also argued that the conducting polymer reduces the likelihood of large increases in pH at the polymer/metal interface and so stabilizes the coating against cathodic disbondment. Further work is clearly needed to increase the protection period by further studies on the corrosion protection mechanism so that the polymer composition and processing methods may be optimized.     

Unveiling the High-valence Oxygen Degradation Across the Delithiated Cathode Surface

Charge compensation on anionic redox reaction (ARR) has been promising to realize extra capacity beyond transition metal redox in battery cathodes. The practical development of ARR capacity has been hindered by high-valence oxygen instability, particularly at cathode surfaces. However, the direct probe of surface oxygen behavior has been challenging. Here, the electronic states of surface oxygen are investigated by combining mapping of resonant Auger electronic spectroscopy (mRAS) and ambient pressure X-ray photoelectron spectroscopy (APXPS) on a model LiCoO₂ cathode. The mRAS verified that no high-valence oxygen can sustain at cathode surfaces, while APXPS proves that cathode electrolyte interphase (CEI) layer evolves and oxidizes upon oxygen gas contact. This work provides valuable insights into the high-valence oxygen degradation mode across the interface. Oxygen stabilization from surface architecture is proven a prerequisite to the practical development of ARR active cathodes.     

Passivity of titanium,part 1: film growth model diagnostics

Diagnostic criteria for the growth of the anodic oxide film on titanium in H₂SO₄ are reported. The criteria apply to the generalized high field model, which postulates that the electric field within the film is dependent upon the film thickness, and the point defect model, which describes the electric field as being constant during film growth. The diagnostic criteria show that the PDM more realistically models film growth than does the HFM, and we conclude that in this system the electric field strength is invariant with applied voltage and film thickness. The constancy of the electric field in the passive film on titanium, as demonstrated in this work, is attributed to band-to-band Esaki tunneling, which buffers the electric field against changes in the applied voltage and film thickness.     

Characterization of the interface dipole at organic/ metal interfaces

In organics-based (opto)electronic devices, the interface dipoles formed at the organic/metal interfaces play a key role in determining the barrier for charge (hole or electron) injection between the metal electrodes and the active organic layers. The origin of this dipole is rationalized here from the results of a joint experimental and theoretical study based on the interaction between acrylonitrile, a pi-conjugated molecule, and transition metal surfaces (Cu, Ni, and Fe). The adsorption of acrylonitrile on these surfaces is investigated experimentally by photoelectron spectroscopies, while quantum mechanical methods based on density functional theory are used to study the systems theoretically. It appears that the interface dipole formed at an organic/metal interface can be divided into two contributions: (i) the first corresponds to the "chemical" dipole induced by a partial charge transfer between the organic layers and the metal upon chemisorption of the organic molecules on the metal surface, and (ii) the second relates to the change in metal surface dipole because of the modification of the metal electron density tail that is induced by the presence of the adsorbed organic molecules. Our analysis shows that the charge injection barrier in devices can be tuned by modulating various parameters: the chemical potential of the bare metal (given by its work function), the metal surface dipole, and the ionization potential and electron affinity of the organic layer.     

Electrochemical Surface Science This manuscript is based on the Bonhoeffer-Eucken-Scheibe lectures of the Deutsche Bunsengesellschaft, given by the author at Erlangen, Berlin, and Leipzig in 1999/2000

The last 30 years have seen remarkable changes in interfacial electrochemistry, particularly in the kind of questions that were addressed in electrochemical studies. Ever since classical surface science, traditionally performed under ultrahigh vacuum conditions, has succeeded in describing surfaces and surface reactions on a molecular level, electrochemists longed for a microscopic understanding of the solid/electrolyte interface and, at the same time, searched widely for new experimental ways to reach that goal. Herein, studies are described concerning the structure and the dynamics of bare and adsorbate-covered electrode surfaces and of metal deposition as a simple, yet important, electrochemical process. In all these cases, the scanning tunneling microscope plays a pivotal role emphasizing the surface-science approach to the problems.     

Topological and Electron‐Transfer Properties of Yeast Cytochrome c Adsorbed on Bare Gold Electrodes

The redox metalloprotein yeast cytochrome c was directly self‐chemisorbed on “bare” gold electrodes through the free sulfur‐containing group Cys102. Topological, spectroscopic, and electron transfer properties of the immobilised molecules were investigated by in situ scanning probe microscopy and cyclic voltammetry. Atomic force and scanning tunnelling microscopy revealed individual protein molecules adsorbed on the gold substrate, with no evidence of aggregates. The adsorbed proteins appear to be firmly bound to gold and display dimensions in good agreement with crystallographic data. Cyclic voltammetric analysis showed that up to 84 % of the electrode surface is functionalised with electroactive proteins whose measured redox midpoint potential is in good agreement with the formal potential. Our results clearly indicate that this variant of cytochrome c is adsorbed on bare gold electrodes with preservation of morphological properties and redox functionality.     

An arbitrary Lagrangian–Eulerian model for studying the influences of corrosion product deposition on bimetallic corrosion

In this paper, a model is established to simulate the time-dependent deposition of corrosion product on the metal surface by considering mass transfer, electrochemical reactions and precipitation reaction. The model is also capable of tacking the movement of metal corrosion interface and the growing interface of the corrosion product deposits via arbitrary Lagrangian–Eulerian finite element method. The current model not only can be used to predict the time-dependent metal corrosion but also for investigating the influences of the deposits’ nature on metal corrosion. The numerical results of current density and corrosion rate are in good agreement with experiments. The presented model predicts that an exponential relationship exists between the maximum corrosion depth and the porosity of corrosion product deposits, and it is also predicted that the growth of the corrosion product layer is linear relative with the root of time, which is consistent with the existing theories.     

Determining the thermodynamic electrode potential in the presence of a mixed potential using EQCM technique

We give the theory explaining how to use Electrochemical Quartz Crystal Microbalance (EQCM) to determine the thermodynamic equilibrium electrode potential in a system where the open circuit potential is a mixed potential. The approach is applicable to electrodes of the first or the second kind in the presence of one or more soluble redox couples, at least one of which is irreversible. The key insight to be gained is that if the mass of an electrode of the first or the second kind does not change, its electrode potential is the thermodynamic equilibrium potential. This is true regardless of any other redox processes involving only soluble species that may be occurring at the electrode/solution interface. The model system used to test and confirm the theory was an electrode of the first kind, NiH_x,solid/Ni(II) undergoing active corrosion caused by hydrogen peroxide. The electrochemical quartz crystal microbalance method can be applied in fundamental and applied studies, e.g., of electroless deposition and corrosion systems containing multiple redox systems.     

Using atomic layer deposition to hinder solvent decomposition in lithium ion batteries: first-principles modeling and experimental studies

Passivating lithium ion (Li) battery electrode surfaces to prevent electrolyte decomposition is critical for battery operations. Recent work on conformal atomic layer deposition (ALD) coating of anodes and cathodes has shown significant technological promise. ALD further provides well-characterized model platforms for understanding electrolyte decomposition initiated by electron tunneling through a passivating layer. First-principles calculations reveal two regimes of electron transfer to adsorbed ethylene carbonate molecules (EC, a main component of commercial electrolyte), depending on whether the electrode is alumina coated. On bare Li metal electrode surfaces, EC accepts electrons and decomposes within picoseconds. In contrast, constrained density functional theory calculations in an ultrahigh vacuum setting show that, with the oxide coating, e(-) tunneling to the adsorbed EC falls within the nonadiabatic regime. Here the molecular reorganization energy, computed in the harmonic approximation, plays a key role in slowing down electron transfer. Ab initio molecular dynamics simulations conducted at liquid EC electrode interfaces are consistent with the view that reactions and electron transfer occur right at the interface. Microgravimetric measurements demonstrate that the ALD coating decreases electrolyte decomposition and corroborates the theoretical predictions.     

Voltammetric and drift spectroscopy investigation in dithiophosphinate-chalcopyrite system

The mechanism of dithiophosphinate (DTPI) adsorption on chalcopyrite was investigated by diffuse reflectance Fourier transformation (DRIFT) spectroscopy and by cyclic voltammetry (CV) at various pHs. CV experiments showed that the redox reactions occurred at a certain degree of irreversibility on the chalcopyrite surface in the absence of a collector due to preferential dissolution of iron ions in slightly acid solution and irreversible surface coverage by iron oxyhydroxides in neutral and alkaline solutions. In the presence of DTPI, CV experiments failed to identify the type of the adsorbed DTPI species and electrochemical processes occurring on chalcopyrite due to formation of an electrochemically passive surface layer preventing electron transfer. However, DRIFT spectroscopy tests showed this passive layer to be mainly CuDTPI + (DTPI)2. Both CV and DRIFT spectroscopy established that the activity of collector species decreased with increasing pH due to formation of stable hydrophilic metal oxyhydroxides on the chalcopyrite surface.     

Electrochemical studies for the electroactivity of amine-capped aniline trimer on the anticorrosion effect of as-prepared polyimide coatings

The electroactive polyimide consisting of various content of amine-capped aniline trimers (ATs) have been successfully synthesized and characterized by Fourier-Transformation infrared and UV-visible absorption spectroscopy. The electroactivity of as-prepared polyimides was tested by electrochemical cyclic voltammetry (CV) studies. It was noticed that the as-prepared electroactive polyimide with higher content of amine-capped ATs shows higher electroactivity (i.e., larger redox current) than that of non-electroactive polyimide, leading to enhance corrosion protection efficiency on cold-rolled steel (CRS) electrodes. This enhanced corrosion protection efficiency has been explained based on a series of electrochemical measurements such as corrosion potential, polarization resistance, corrosion current and electrochemical impedance spectroscopy (EIS) studies in 5 wt-% NaCl electrolyte. This significant enhancement of corrosion protection on CRS electrodes as compared to non-electroactive polyimide might probably be attributed to the redox catalytic property of as-prepared electroactive polyimide coatings inducing the formation of passive layer of metal oxide.     

Activation of Molecular Hydrogen by a Singlet Carbene through Quantum Mechanical Tunneling

Carbenes are among the few metal‐free molecules that are able to activate molecular hydrogen. Whereas triplet carbenes have been shown to insert into H₂ through a two‐step mechanism that at low temperature is assisted by quantum mechanical tunneling (QMT), singlet carbenes insert in concerted reactions with considerable activation barriers, and are thus unreactive towards H₂ at cryogenic temperatures. Here we show that 1‐azulenylcarbene with a singlet ground state readily inserts into H₂, and that QMT governs the insertion into both H₂ and D₂. This is the first example that shows that QMT can also be important for singlet carbenes inserting into dihydrogen.     

Qualitative spectroscopic characterization of the matrix–silane coupling agent interface across metal fibre reinforced ion exchange resin composite membranes

The characterization of novel metal reinforced electro-dialysis ion exchange membranes, for water desalination, by attenuated total reflectance Fourier transform infrared spectroscopy mapping is presented in this paper. The surface of the porous stainless steel fibre meshes was treated in order to enhance the amount of surface oxide groups and increase the material hydrophilicity. Then, the metal membranes were functionalized through a sol–gel reaction with silane coupling agents to enhance the affinity with the ion exchange resins and avoid premature metal oxidation due to redox reactions at the metal–polymer interface. Polished cross sections of the composite membranes embedded into an epoxy resin revealed interfaces between metallic frameworks and the silane layer at the interface with the ion exchange material. The morphology of the metal–polymer interface was investigated with scanning electron microscopy and Fourier transform infrared micro-spectroscopy. Fourier transform infrared mapping of the interfaces was performed using the attenuated total reflectance mode on the polished cross-sections at the Australian Synchrotron. The nature of the interface between the metal framework and the ion exchange resin was shown to be homogeneous and the coating thickness was found to be around 1 μm determined by Fourier transform infrared micro-spectroscopy mapping. The impact of the coating on the properties of the membranes and their potential for water desalination by electro-dialysis are also discussed.     

Zero-current bipotentiometric indication using two differently pretreated electrodes

A new potentiometric technique of zero-current bipotentiometry using differently pretreated platinum electrodes is described, and its application to various redox titrations discussed. The potential across the electrodes appears to be generated by differences in kinetics of the reactions occurring on the two dissimilar electrode surfaces.     

Corrosion and passivity of metals in methanol solutions of electrolytes

Electrochemical and corrosion behaviour of metals in alcohols are the subject of numerous investigations because of the application of mentioned solvents in chemical engineering, production of oxide nanoparticles (sol-gel techniques) and application of alcohols as fuels. Despite relatively rich bibliography related to electro-catalytic oxidation of alcohols on metal surface in mixed aqueous–alcohol solutions, the knowledge of the mechanism of reactions on metal/anhydrous alcohol interface is still not sufficient. Anodic oxidation of metal surface in alcohol leads to several electro-catalytic reactions with formation of surface compounds being the product of metal and alcohol oxidation. Identification of these products is very difficult. Therefore, our knowledge of the composition and structure of passive films or corrosion products on metal surface in anhydrous alcohol solvents is poor. Our paper presents the investigations of anodic behaviour of metals (Cu, Zn, Fe, Ni, Al and Ti) and semiconductors (p-Si) in methanol solutions of electrolytes, performed in our laboratory within the last 10 years. On the base of electrochemical measurements (linear sweep voltammetry, electrochemical impedance spectroscopy), spectroscopic investigations (X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and low-energy electron diffraction) and scanning electron microscopy techniques, the role of metal–alcohol intermediates in the formation of surface and soluble compounds is discussed. The practical application of electrochemical etching of metals as a method of production of micro- and nanoparticles of metals and oxides is also shown.