Local Electrochemical Potential Research Articles

New perspectives on spatial dynamics of lithiation and lithium plating in graphite/silicon composite anodes

Huge volume expansion, unstable SEI-film evolution and uncontrollable Li plating remain challenging issues for achieving high energy-density and fast charging graphite/silicon anodes. Unfortunately, less reports on Li plating in graphite/silicon anodes have been found up to now. Herein, for the first time, we report that the interfacial polarization of silicon dominates the co-lithiation process with graphite, thus determining the preferential Li-plating sites and SEI film evolution. Interestingly, the disturbance of local electrochemical potential due to consecutive volume variation of silicon upon cycling, indeed keep Li plating sites changing and deteriorate the battery performances. In a typical case of silicon, global electrochemical potential-equilibrium of electrons in silicon can be achieved by regulating Li-ion diffusion kinetic in silicon, thus delaying Li-plating on graphite. Comparing to commercial silicon, P-type photovoltaic silicon exhibits weaker disturbance of local electrochemical potential upon volume variation, resulting in smaller change in Li-plating sites and smaller re-growth SEI film. Therefore, our findings propose a new perspective on the competitive lithiation mechanism and Li plating for graphite/silicon anodes which is highly desirable for designing high-capacity and fast-charging graphite/silicon.

2021 W.R. Whitney Award: The Importance of Local Chemistry and Potential in Localized Corrosion and Stress Corrosion Cracking

The nature and rates of the chemical and electrochemical reactions that occur within the occluded regions of a given alloy are controlled by the local electrochemical potential and the local solution composition. The very small physical dimensions of these regions lead to challenges in both measurement and modeling. When performed in a coordinated and complementary way, measurements and modeling provide insights into the controlling processes of a range of localized corrosion phenomena, including crevice corrosion, pitting, intergranular corrosion, and stress corrosion cracking. Examples of attempts to overcome the measurement challenges are described for a range of corrosion scenarios, including identification of the critical ionic species in stainless steel crevice corrosion and in the corrosion of aircraft lap joints, operando measurement of chemistry and potential simultaneously within stress corrosion cracks, and monitoring of water layer thickness in salt spray testing. Examples of work addressing the challenges in modeling localized corrosion including intergranular corrosion of AA5XXX alloys, scaling laws in crevice corrosion, the extent to which the Laplace Equation can be used and applied to geometrically complex galvanic structures, and an approach to modeling localized corrosion for extraordinarily long service times. Finally, suggestions regarding future avenues of research are provided.

Performance analysis and implementation of a scanning tunneling potentiometry setup: Toward low-noise and high-sensitivity measurements of the electrochemical potential.

Scanning tunneling potentiometry allows for studying charge transport on the nanoscale to relate the local electrochemical potential to morphological features of thin films or two-dimensional materials. To resolve the influence of atomic-scale defects on the charge transport, sub-µV sensitivity for the electrochemical potential is required. Here, we present a complete analysis of the noise in scanning tunneling potentiometry for different modes of operation. We discuss the role of various noise sources in the measurements and technical issues for both dc and ac detection schemes. The influence of the feedback controller in the determination of the local electrochemical potential is taken into account. Furthermore, we present a software-based implementation of the potentiometry technique in both dc and ac modes in a commercial scanning tunneling microscopy setup with only the addition of a voltage-controlled current source. We directly compare the ac and dc modes on a model resistor circuit and on epitaxial graphene and draw conclusions on the advantages and disadvantages of each mode. The effects of sample heating and the occurrence of thermal voltages are discussed.

Scanning Electrochemical Potential Microscopy Imaging Horseradish Peroxidase on HOPG

Electrochemical scanning tunneling microscopy (EC-STM) has been successfully applied to study the electron-transfer (ET) reactions of biomolecules at solid-liquid interfaces.1–4 However, there are still limitations, including requirement of sample conductivity and potential damage to the biomolecules from strong tunneling current, to be overcome.A different type of probe microscopy, scanning electrochemical potential microscopy (SECPM), can be used as a promising alternative. SECPM shares the similar hardware setup with EC-STM, except for the current amplifier being substituted by a potentiometer to measure the local electrochemical potential as feedback signal. This feature enables SECPM to work on less conductive biomolecular samples. It has been shown that SECPM is capable of imaging horseradish peroxidase (HRP) at single molecular level under in situ conditions, with image resolution comparable or even better than EC-STM (Figure 1). 5 The electrochemical double layer (EDL) existing at the solid-liquid interface in an electrochemical cell can be investigated by SECPM using tip-distance curves. Preliminary results will be shown.In this work, we use SECPM to image HRP molecules adsorbed on HOPG under in situ conditions, with varying electrochemical potentials applied to the substrate. HRP redox reactions were controlled by varying the substrate potential, and the corresponding change of the potential distribution in the EDL covering the adsorbed HRP surface was recorded by SECPM. The experimental results could offer a novel perspective to understand how the ET reactions influences the EDL, and then help the further research on biosensors and EDL capacitors.This work is supported in part by the North-East Centre for Energy Materials (EP/R021503/1) funded by EPSRC.

Non-Invasive Nanoscale Potentiometry and Ballistic Transport in Epigraphene Nanoribbons.

The recent observation of non-classical electron transport regimes in two-dimensional materials has called for new high-resolution non-invasive techniques to locally probe electronic properties. We introduce a novel hybrid scanning probe technique to map the local resistance and electrochemical potential with nm- and μV resolution, and we apply it to study epigraphene nanoribbons grown on the sidewalls of SiC substrate steps. Remarkably, the potential drop is non-uniform along the ribbons, and μm-long segments show no potential variation with distance. The potential maps are in excellent agreement with measurements of the local resistance. This reveals ballistic transport, compatible with μm-long room-temperature electronic mean-free paths.

Substrate induced nanoscale resistance variation in epitaxial graphene

Graphene, the first true two-dimensional material, still reveals the most remarkable transport properties among the growing class of two-dimensional materials. Although many studies have investigated fundamental scattering processes, the surprisingly large variation in the experimentally determined resistances is still an open issue. Here, we quantitatively investigate local transport properties of graphene prepared by polymer assisted sublimation growth using scanning tunneling potentiometry. These samples exhibit a spatially homogeneous current density, which allows to analyze variations in the local electrochemical potential with high precision. We utilize this possibility by examining the local sheet resistance finding a significant variation of up to 270% at low temperatures. We identify a correlation of the sheet resistance with the stacking sequence of the 6H silicon carbide substrate and with the distance between the graphene and the substrate. Our results experimentally quantify the impact of the graphene-substrate interaction on the local transport properties of graphene.

Microscopic Determination of Carrier Density and Mobility in Working Organic Electrochemical Transistors

A comprehensive understanding of electrochemical and physical phenomena originating the response of electrolyte-gated transistors is crucial for improved handling and design of these devices. However, the lack of suitable tools for direct investigation of microscale effects has hindered the possibility to bridge the gap between experiments and theoretical models. In this contribution, a scanning probe setup is used to explore the operation mechanisms of organic electrochemical transistors by probing the local electrochemical potential of the organic film composing the device channel. Moreover, an interpretative model is developed in order to highlight the meaning of electrochemical doping and to show how the experimental data can give direct access to fundamental device parameters, such as local charge carrier concentration and mobility. This approach is versatile and provides insight into the organic semiconductor/electrolyte interface and useful information for materials characterization, device scaling, and sensing optimization.

In Situ Coupling of Single Molecules Driven by Gold-Catalyzed Electrooxidation.

A single-molecule method has been developed based on the scanning tunneling microscope (STM) to selectively couple a series of aniline derivatives and create azobenzenes. The Au-catalyzed oxidative coupling is driven by the local electrochemical potential at the nanostructured Au STM tip. The products are detected in situ by measuring the conductance and molecular junction elongation and compared with analogous measurements of the expected azobenzene derivatives prepared ex situ. This single-molecule approach is robust, and it can quickly and reproducibly create reactions for a variety of anilines. We further demonstrate the selective synthesis of geometric isomers and the assembly of complex molecular architectures by sequential coupling of complementary anilines, demonstrating unprecedented control over bond formation at the nanoscale.

In Situ Coupling of Single Molecules Driven by Gold‐Catalyzed Electrooxidation

AbstractA single‐molecule method has been developed based on the scanning tunneling microscope (STM) to selectively couple a series of aniline derivatives and create azobenzenes. The Au‐catalyzed oxidative coupling is driven by the local electrochemical potential at the nanostructured Au STM tip. The products are detected in situ by measuring the conductance and molecular junction elongation and compared with analogous measurements of the expected azobenzene derivatives prepared ex situ. This single‐molecule approach is robust, and it can quickly and reproducibly create reactions for a variety of anilines. We further demonstrate the selective synthesis of geometric isomers and the assembly of complex molecular architectures by sequential coupling of complementary anilines, demonstrating unprecedented control over bond formation at the nanoscale.

Scanning probe microscopy and potentiometry using a junction field effect transistor based sensor

Scanning tunneling microscopy in its conventional form relies on a steady state tunneling current of 10−12–10−6 A. However, for various applications, it is desirable to reduce the current load to a minimum. Here, we present first experiments using a cooled junction field effect transistor in open gate operation, thereby reducing the DC-current to less than 10−19 A. This enables almost ideal measurements of the local electrochemical potential on a surface. Various methods applying dynamic modes can be used to maintain a constant distance between the scanning probe and the sample surface. Here, we use an AC-bias applied to the sample and a lock-in amplifier connected to the preamplifier to evaluate the conductance of the tunneling gap.

Corrosion Driven Drying Phenomena Occurring during the Atmospheric Corrosion of Magnesium Containing Galvanized Steel

The corrosion behaviour of various Zn-Mg-Al hot-dip galvanised coatings on steel substrates were studied using a combination of time-lapse photography (TLP), the scanning Kelvin probe (SKP) and non-contact height profiling. In a standard experiment 1uL electrolytic droplets comprising varying concentrations of aqueous NaCl were deposited onto the surface of the Zn-Mg-Al alloy coating held in a humid air atmosphere maintained in isopiestic equilibrium (equilibrium with respect to the vapour pressure of water) with a large remote reservoir of aqueous electrolyte which had the same composition as the original droplet. The progress of atmospheric corrosion was then followed using TLP with photographs taken at 2-minute intervals for a period of up to 60 hours. When required, in-situ SKP was used to produce area maps of electrochemical potential if in the vicinity of the electrolytic droplets over a similar period of time. The SKP probe could also be used as a non-contact height profilometer which allowed the topography of the electrolytic droplet and the surrounding Zn-Mg-Al alloy surface to be mapped. This made it possible to study time-dependent changes in droplet volume and shape as atmospheric corrosion proceeds. In most cases the phenomenon known as cathodic secondary spreading was observed at the droplet periphery, where a thin film (microns) of electrolyte develops and advances away from the droplet as time proceeds. The presence of this thin electrolyte film changes the reflectivity of the Zn-Mg-Al alloy surface with respect to visible light and also produces a marked reduction (drop) in local electrochemical potential. The kinetics of secondary spreading could therefore be followed using a combination of TLP and in-situ SKP. Results are presented which show the dependence of spreading rate on: time, original NaCl concentration and Zn-Mg-Al alloy composition. The role of aluminium and magnesium in modifying (reducing) rates of secondary spreading is discussed in relation to their relative electrocatalytic activities for cathodic oxygen reduction and their distribution within the microstructure of the various alloy coatings. It is shown, using in-situ profilometry, that at lower initial concentrations of NaCl (and consequently higher relative humidity under isopiestic conditions) the original electrolyte droplets dry out within approximately 2-3 hours of being deposited on the alloy surface. The mechanism of this drying phenomenon is discussed in relation to the relevant mechanisms of atmospheric corrosion. It is proposed that as corrosion proceeds all the Na+ cations originally present in the droplet migrate out into the region of secondary cathodic spreading. Conversely, Cl- anions migrating towards the anodic sites at the droplet centre become sequestered as insoluble (or poorly soluble) zinc corrosion products such as Simonkolleite, Zn(OH)Cl. (The presence of Simonkollite is confirmed using grazing angle Z-ray diffraction.) As these corrosion products precipitate the activity of water in the droplet (and its associated vapour pressure) increase and water evaporates from the droplet as it tries to maintain isopiestic equilibrium with the experimental atmosphere. Corrosion-driven drying was also seen for higher initial concentrations of NaCl but at considerably slower rates. The drying effect, caused by the secondary spreading phenomena, may be an important self-limiting mechanism of atmospheric corrosion affecting Zn-Mg-Al alloy coatings.

Understanding the In Vivo Reactivity of Metal-on-Metal Implants

CoCrMo-based hip implants experienced high failure rates despite the high wear and corrosion resistance of the bulk material. Although they exhibit a lower volumetric wear compared to other implant materials, CoCrMo-based implants produced a significantly larger number of smaller wear particles. Simulation studies show that up to one trillion wear particles could be produced in one patient annually.1 Implant wear particles can trigger an immune system inflammatory response2,3 which is associated with eventual failure of the implants. In light of these failures, many devices have been retracted from the market, and class-action lawsuits were filed in the US. Additionally, one of the largest product liability hearings is currently ongoing in the UK as patients sue implant manufacturers. CoCrMo is nominally an extremely stable material with high Cr content providing passivity. However, despite the Co:Cr ratio in the original alloy being 2:1; chemical analyses of wear particles from periprosthetic tissue have found the particles to be composed predominately of Cr species, with only trace amounts of Co remaining.4 The immune system’s inflammatory response leads to a rise in local electrochemical potential via the production of reactive oxygen/nitrogen species by activated macrophages as well as local acidification. However, the mechanism eliciting the inflammatory response is uncertain. In this work, the dissolution of CoCrMo in bulk, thin film and nanoparticle form was studied in a simulated biological environment. Electrochemical potentials were applied to simulate the increasingly oxidising environment found within the macrophage cells. A multimodal in situ spectroscopy and microscopy approach was used, at high chemical and spatial resolution, to elucidate the dissolution mechanism. The results reveal that CoCrMo dissolution occurs via a non-stoichiometric, geometrically inhomogeneous mechanism similar to de-alloying, and previously unreported for this material. Taken together, the results suggest that the in vivo dissolution of CoCrMo wear debris can be understood in terms of the local environment and specific corrosion mechanisms that come into play under these conditions: not observed in conventional testing for implant safety. I. Catelas, J. D. Bobyn, J. B. Medley, J. J. Krygier, D. J. Zukor and O. L. Huk. J. Biomed. Mater. Res. A, 2003, 67, 312–327D. J. Langton, T. J. Joyce, S. S. Jameson, J. Lord, M. Van Orsouw, J. P. Holland, A. V. F. Nargol and K. A. De Smet. J. Bone Jt. Surgery, Br. Vol., 2011, 93, 164–171Y.-M. Kwon, S. Glyn-Jones, D. J. Simpson, A. Kamali, P. McLardy-Smith, H. S. Gill and D. W. Murray. J. Bone Jt. Surgery, Br. Vol., 2010, 92, 356–361A. E. Goode, J. M. Perkins, A. Sandison, C. Karunakaran, H. Cheng, D. Wall, J. a. Skinner, A. J. Hart, A. E. Porter, D. W. McComb and M. P. Ryan. Chem. Commun., 2012, 48, 8335

Cycle aging studies of lithium nickel manganese cobalt oxide-based batteries using electrochemical impedance spectroscopy

The cycle aging of a commercial 18650 lithium-ion battery with graphite anode and lithium nickel manganese cobalt (NMC) oxide-based cathode at defined operating conditions is studied by regular electrochemical characterization, electrochemical impedance spectroscopy (EIS) and post-mortem analysis. The study finds that capacity fade, impedance rise and the end-of-life of cycled cells strongly depend on the operating conditions like temperature, current rate, depth-of-discharge and mean state-of-charge. In general, the capacity fade is characterized by a slow linear decrease at first, followed by a rapid decrease. This transition point is found to correlate well to the rate of solid electrolyte interphase (SEI) resistance growth at the anode. A longer lifetime is found for cells cycled at 45 °C than at 20 °C for the same depth-of-discharge and C-rate. Effect of cycle depth on capacity fade is related to the graphite electrode volume changes and the local electrochemical potential at the electrodes. Cells cycling upto 4.2 V are found to have longer linear capacity fade but a higher total resistance. A more stable SEI at the anode and greater surface modifications on the cathode are inferred to be the reasons for this non-intuitive behaviour.

On the Mechanism of Scanning Electrochemical Potential Microscopy

AbstractScanning electrochemical potential microscopy (SECPM) is a type of probe microscopy, in which a sharp tip similar to those employed in electrochemical tunnelling microscopy is connected to a high impedance amplifier, but the tip potential instead of tip current is used as the signal in the feedback loop. SECPM has been found to provide much higher spatial resolution than would be expected on the basis of a mechanism in which the tip responds to the local electrochemical potential of the solution; in fact, it can obtain atomic resolution similar to STM, but is a superior technique for imaging electronically insulating objects such as proteins on a metal surface. We suggest a mechanism for these high‐resolution images based on electron exchange between tip and substrate coupled to faradaic processes at the tip/solution interface. This mechanism operates alongside the conventional mechanism in which the tip responds to the local potential in the diffuse layer of the substrate and allows a simple description of the sigmoidal tip potential−distance curves that have been reported.

Effects of macro-scale corrosion damage feature on fatigue crack initiation and fatigue behavior

The remaining fatigue life for aluminum aerospace alloys is greatly reduced by the presence of corrosion damage; prediction of this deleterious influence via standard fracture mechanics methods is non-trivial. Several corrosion morphologies can develop depending on the local chemistry, pH and electrochemical potential along the geometry of the structure. Previous studies demonstrate that such variations in corrosion morphology can affect the fatigue behavior. This study aims to quantify the effect of corrosion morphologies relevant to a fastener-hole geometry of AA7050-T7451 on the initiation life, propagation life and total fatigue life. Four corrosion morphologies are considered: shallow and deep discrete pits widely distributed on the corroded surface, deep fissure-like features, and general corrosion with surface recession. Corrosion morphologies are carefully characterized using optical microscope, white light interferometer and X-ray computed tomography. The samples are subjected to fatigue using loading protocols that create identifiable marks on the fracture surface at known cycle count. These marker bands are used to determine the small scale crack growth rate (da/dN) of the samples, as well as the cycles to initiate a 10μm crack. Results showed that initiation life plateaus to near constant values for varied corrosion morphologies, and crack growth rates converge to comparable values. The macro-scale corrosion metrics are considered and correlated to the location of the fatigue initiation. Severe macro-scale metrics such as pit or fissure depth, pit mouth surface area, pit volume, pit density, root mean square, peak density, maximum valley depth, do not uniquely correlate with the location of fatigue crack initiation. This suggests a strong role of micro-scale corrosion features and underlying microstructural features on the location of the fatigue crack initiation.

Current limit diagrams for dendrite formation in solid-state electrolytes for Li-ion batteries

Abstract We build upon the concept that nucleation of lithium dendrites at the lithium anode-solid state electrolyte interface is instigated by the higher resistance of grain boundaries that raises the local electro-chemical potential of lithium, near the lithium-electrode. This excess electro-chemo-mechanical potential, however, is reduced by the mechanical back stress generated when the dendrite is formed within the electrolyte. These parameters are coalesced into an analytical model that prescribes a specific criterion for dendrite formation. The results are presented in the form of current limit diagrams that show the “safe” and “fail” regimes for battery function. A higher conductivity of the electrolyte can reduce dendrite formation.

Atomistic features in the electrochemical potential drop across a graphene grain boundary

A recent publication presents a new computational approach to the local electrochemical potential in the vicinity of a graphene grain boundary subject to an in-plane electric current []. The local electrochemical potential can be measured using scanning tunneling potentiometry, a method related to scanning tunneling microscopy. The paper predicts that atomistic features should be measurable. These features reflect the local electrochemical potential drop caused by the opaque grain boundary which is non-transparent to ballistic electrons. The paper has implications not only for scanning tunneling potentiometry, but also for Kelvin probe-force microscopy which can also measure the local electrochemical potential. In addition it could help to understand electronic transport across metallic nanocontacts.

Volta Potential on Stainless Steel Under Mechanical Stress Conditions

It is well documented fact that mechanical work or surface treatment creates stresses in the metal. In corrosive environment the stress can lead to stress corrosion cracking and failure of the metal construction. It is often supposed that stress influences the properties of the surface oxide film by creating defects like metal vacancies and thus activating the anodic dissolution. On the other hand, the kinetic of the partial corrosion reactions is determined by the local electrochemical potential. Thus it is important to know how the stress influences on the surface potential.The aim of the present study was to measure the influence of stress on the electrochemical potential of stainless steel 301 grade in situ under loading using Scanning Kelvin Probe. The measurements were carried out in the laboratory air at 50%RH and the measured Volta potential was related to the potential of the steel in the passive state. Previously this kind of measurements was carried out elsewhere [A. Nazarov, D. Thierry "Application of Volta potential mapping to determine metal surface defects",Electrochimica Acta 52 (2007) 7689-7696.].In this work a constant load cell was applied on the sample with notches that concentrates the stress in a particular location. The load was measured and it was in the elastic or plastic deformation range. It was found that elastic deformation resulted in rather low potential changes whereas plastic deformation decreased the surface potential.Figure 1: A: Potential profile in the notched area, B: Impact of the compressive stress produced by indentation.Figure 1A shows the potential profile for the steel surface in the notched area under tensile stress conditions. Figure B shows the impact of the compressive stress produced by indentation.It is possible to conclude that stress decreased the surface potential of the steel by about 200-300 mV. Thus the stress decreased the level of metal passivity that can facilitate the local anodic dissolution. After unloading under residual stress conditions the surface very slowly shifted the potential to the initial level. The effect of the plastic deformation on the steel potential is explained on the basis of the formation of dislocations and defective oxide film. The surface analytical studies are in progress.

Formulation of a macroscale corrosion damage internal state variable model

A new consistent formulation coupling kinematics, thermodynamics, and kinetics with damage using an extended multiplicative decomposition of the deformation gradient that accounts for corrosion effects is proposed. The corrosion model, based upon internal state variable (ISV) theory, captures the effects of general corrosion, pit nucleation, pit growth, pit coalescence, and intergranular corrosion. The different geometrically-affected rate equations are given for each mechanism after the ISV formalism and have a thermodynamic force pair that acts as an internal stress. Pit nucleation is defined as the number density that changes as a function of time driven by the local galvanic electrochemical potential between base matrix material and second phase material. Pit growth is defined as pit surface area growth. Pit coalescence is the interaction of the pits as they grow together and is often characterized by transgranular corrosion and is mathematically constructed from Coulomb’s Law and the Maxwell stress. General corrosion is signified by thickness loss of the material and is characterized by a modified Faraday’s Law. The intergranular corrosion rate is related to the grain boundary effects so that it is characterized by the misorientation between grains. The total damage (void volume or area fraction) is the addition of the general, pitting, and intergranular corrosion. The ability of the model to predict aspects of the corrosion mechanisms and aging history effects of an engineering material are then illustrated by comparison with experimental data of an extruded AZ31 magnesium alloy.

Selective transport through a model calcium channel studied by Local Equilibrium Monte Carlo simulations coupled to the Nernst–Planck equation

We have recently introduced the Local Equilibrium Monte Carlo (LEMC) technique (Boda, Gillespie, J. Chem. Theor. Comput. 8 (2012) 824-829) in which a non-equilibrium system is divided into small volume elements and separate Grand Canonical Monte Carlo simulations are performed for each using a local intensive parameter, which, as soon as local equilibrium is assumed, can be identified with the local electrochemical potential. The simulation provides the concentration profiles of the steady-state diffuse system, where ions are transported through a membrane from one bulk compartment to the other. The dynamics of the ions is described with the Nernst–Planck (NP) transport equation. The NP equation is coupled to the LEMC simulations via an iteration procedure that ensures that conservation of mass (the continuity equation) is satisfied. We apply the method to a simple calcium channel model and demonstrate its efficiency. The computer experiments are inspired by real electrophysiological experiments for the Ryanodine Receptor calcium channel. The diffusion coefficients in the channel are fitted to results of Dynamic Monte Carlo simulations.