Until a couple of weeks ago, many people may not have even heard of Nyquist. However, if you are an electrochemist the name Nyquist has a meaning unknown to many. The Nyquist frequency is named after electronic engineer Harry Nyquist. Harry Nyquist was a Swedish-born American electronic engineer who made important contributions to ‘communication theory’ Read more about What is Nyquist?[…]
“Whilst electrochemistry has been around for a long time it is a powerful tool for nanotechnology because it’s so finely tuneable” A simple electrochemical method has been used to tune the electrical and mechanical properties of graphene. AsianScientist (Aug. 11, 2015) – Graphene has been called the miracle material but the single-atomic layer material is Read more about Graphene Electrochem Exfoliation[…]
Detects Diseases, Measures Biomarkers, Costs $25 In advanced industrialized nations most diagnostic tests that check body fluids for markers of disease make use of specialized sensors and devices that, although they deliver accurate results, can be very expensive. Now researchers at Harvard University have unveiled a new portable device that can perform a slew of Read more about Handheld Electrochemical Sensor[…]
Take two different metals, place an ion transporting medium between them, wire them up to be connected outside the electrolyte solution (perhaps to a voltmeter or potentiostat) and you’ve made a battery. With the ever-increasing demand for portable power, batteries are becoming more and more important. As demands on battery performance increase, so do demands Read more about Potentiostats for Battery Research Testing[…]
Ohio University Electrochemical Engineering Research Center Receives $379,000 NIST Grant .S. Department of Commerce and National Institute of Standards and Technology (NIST). The award will be used to establish a consortium to support, sustain, and enhance U.S. manufacturing capacity in the nation’s chemical industry and allied sectors through innovative electrochemical processes. Under the Advanced Manufacturing Read more about Ohio University Receives Electrochemical Engineering Grant[…]
Room temperature ionic liquids or simply ionic liquids (some even call them molten salts) can act as a great solvent for electrochemistry but they bring along a challenge of what to use for the reference electrode. Traditional saturated calomel or Ag/AgCl reference electrodes use water as a solvent which can cause trouble for ionic liquids. Read more about Reference Electrode for Ionic Liquids[…]
A Common Potentiostat Design
This style of I/E Converter is well suited to potentiostats with output currents of a few tenths of an ampere up to several amperes. This scheme has been used by Gamry, PAR, Solartron, and perhaps others.
The I/E Converter is a “passive” design
The current path through the I/E converter only traverses passive components such as wires and resistors. No active components (such as op amps or transistors) are in the current path. The current measurement resistor is connected between the Working electrode and the potentiostat’s power supply ground (or “current return”).
The Working electrode is not at Virtual Ground
This is a consequence of the passive design. The working electrode voltage (vs the potentiostat’s internal ground) depends on the current flowing. In the sketch shown to the right, the working electrode will be at (i*Rm) volts. The actual voltage may be higher due to the resistance of the cell cable connecting the potentiostat to the working electrode!
Potentiostat with differential electrometer
The Classical Potentiostat The schematic at the right is the classical potentiostat design shown in nearly every modern electrochemistry textbook. It has three basic features. The Working electrode is at Virtual Ground. The working electrode is at the same potential as the potentiostat’s electronic ground. This ground is often connected to Earth Ground. The electrometer Read more about Potentiostat Architectures – Active I/E Converters[…]
In the previous post (Electrochemical Corrosion Measurements Primer) we pointed out that Icorr cannot be measured directly. In many cases, you can estimate it from current versus voltage data. You can measure a log current versus potential curve over a range of about one half volt. The voltage scan is centered on Eoc. You then fit the measured data to a theoretical model of the corrosion process.
The model we will use for the corrosion process assumes that the rates of both the anodic and cathodic processes are controlled by the kinetics of the electron transfer reaction at the metal surface. This is generally the case for corrosion reactions. An electrochemical reaction under kinetic control obeys Equation 1-1, the Tafel Equation.
Most metal corrosion occurs via electrochemical reactions at the interface between the metal and an electrolyte solution. A thin film of moisture on a metal surface forms the electrolyte for atmospheric corrosion. Wet concrete is the electrolyte for reinforcing rod corrosion in bridges. Although most corrosion takes place in water, corrosion in non-aqueous systems is not unknown.
Corrosion normally occurs at a rate determined by an equilibrium between opposing electrochemical reactions. The first is the anodic reaction, in which a metal is oxidized, releasing electrons into the metal. The other is the cathodic reaction, in which a solution species (often O2 or H+) is reduced, removing electrons from the metal. When these two reactions are in equilibrium, the flow of electrons from each reaction is balanced, and no net electron flow (electrical current) occurs. The two reactions can take place on one metal or on two dissimilar metals (or metal sites) that are electrically connected.
Figure 1-1 diagrams this process. The vertical axis is potential and the horizontal axis is the logarithm of absolute current. The theoretical current for the anodic and cathodic reactions are shown as straight lines. The curved line is the total current — the sum of the anodic and cathodic currents. This is the current that you measure when you sweep the potential of the metal with your potentiostat. The sharp point in the curve is actually the point where the current changes signs as the reaction changes from anodic to cathodic, or vice versa. The sharp point is due to the use of a logarithmic axis. The use of a log axis is necessary because of the wide range of current values that must be displayed during a corrosion experiment. Because of the phenomenon of passivity, it is not uncommon for the current to change by six orders of magnitude during a corrosion experiment.
The Quartz Crystal Microbalance (QCM) is an exciting tool for the electrochemist. With it, the researcher can now follow not only the current that flows, but the weight changes of the electrode, too! This is a valuable tool when studying reactions which involve films, adsorbates, metal deposition, corrosion, or monolayer formation. It is sensitive enough Read more about Quartz Crystal Microbalance[…]
One guideline that I have heard recommended (although I cannot give a reference for it) is that data over a decade range of frequency is required to support each circuit component.
All curve-fitting software should report some measure of the “goodness of fit.” Often this is the chi-squared parameter ( X2 ) or a value related to it. Boukamp makes the recommendation that the value of X2 should decrease by tenfold if a new circuit element is introduced into the circuit model. The tenfold decrease provides the justification for including the new circuit element. If the inclusion of an additional circuit element does not substantially improve the goodness-of-fit (as evidenced by the decrease in the X2 value), then based on Occam’s Razor, you should keep the simpler model, or continue your search for an improved one.
The old joke about the ability to “fit an elephant” if you use enough parameters is all too true with impedance data. Each component added to the model should have a physical explanation. Adding components only because they make the fit look better (smaller X2) without a physical interpretation is the equivalent to “fitting an elephant.”