Electrochemistry

What is Electrochemistry

Electrochemistry is the study of the movement and separation of charge in matter. As such, it is the study of the transfer of electrons. For it is the presence, absence and movement of these wonderful, little, negatively-charged quanta that provide matter with the ability to hold or transfer charge. Most chemical reactions involve charge transfer, and therefore most chemistry, certainly the most interesting chemistry, is electrochemistry. (And, naturally I am a disinterested observer, right?)

It is a curious and wonderful thing that matter may hold charge, either positive or negative. The charge can be discrete and measureable or partial and diffuse. The statement that opposites attract either emerged from our understanding of electromagnetism or was an intuitive statement of the phenomenon. Thus, it is also intriguing that charged matter can maintain separations leading to interesting effects. Electrochemistry is the study of these phenomenon and their relationship to chemical systems.

Electrochemistry has brought us batteries, semiconductors, photovoltaics, medical sensors, PVC, metal plating, anti-corrosion technology , and a better understanding of biochemical systems.

Introduction to Electrochemical Techniques > Electrochemical Reactions

Electrochemical techniques have a wide range of application, but their use in corrosion and electroplating tends to be concerned with trying to find out about properties of the metal-solution inte***ce: for example, the rate of reactions at the su***ce, the nature of films on the su***ce or the morphology of the su***ce. The basic tools available to us are voltage and current. The voltage across the inte***ce can be changed and the current recorded or vice versa. From these two parameters, we must attempt to deduce everything we can about what is happening at the inte***ce. When we immerse a metal in solution, there will be a tendency for the metal to react with the solution, either with metal atoms dissolving as cations or cations already in the solution depositing as metal atoms:



As a result of these reactions, the metal will tend to accumulate a negative or positive charge. The build-up of this charge on the metal will change its potential in such a way as to inhibit the reaction generating the charge until the potential reaches a value at which the rates of the two reactions are equal and opposite. This is known as the equilibrium potential, and is the potential the metal will adopt in the solution in the absence of any other reactions.

It is very important to appreciate that when a piece of metal is sitting in a solution at its equilibrium potential, this does not mean that the rates of the metal dissolution and reprecipitation reactions are zero. Instead it implies that the rates of the two reactions are equal. Since electrochemical reactions invariably involve a transfer of charge, we can define their rates in terms of charge/unit area/unit time or current density. When the metal dissolution and reprecipitation reactions are in equilibrium, we refer to the (equal and opposite) rates of each of the two reactions as the exchange current density.

In corroding systems other anodic reactions are possible, the two most important being the reduction of dissolved oxygen to hydroxyl ions and the reduction of hydrogen ions or water molecules to hydrogen gas:



The balance between one or other of these cathodic reactions and the metal dissolution reaction results in a rate of reaction given by the corrosion current density. One of the main applications of electrochemical methods to the study of corrosion is the estimation of the magnitude of the corrosion current density. Electrochemical techniques are also used to study the mechanisms of corrosion processes.

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy is a power tool for examining many chemical and physical processes in solutions as well as solids. For solution phase electrochemistry a complex sequence of coupled processes such as, electron transfer, mass transport and chemical reaction can all control or influence the output from an electrochemical measurement. In this section some of the fundamental principles and basic methodology for solution phase electrochemical measurements is reviewed.

Electrode Kinetics

Introduction
The rate of the electron transfer reaction at the electrode su***ce can be varied by changing the applied voltage. In this section we note the main relationships between the heterogeneous rate constants for electron transfer and the voltage, and introduce the terms reversible and irreversible in the context of electrolysis reactions.
Electron Transfer and Energy levels
The key to driving an electrode reaction is the application of a voltage (V). If we consider the units of volts


V = Joule/Coulomb

we can see that a volt is simply the energy (J) required to move charge (c). Application of a voltage to an electrode therefore supplies electrical energy. Since electrons possess charge an applied voltage can alter the 'energy' of the electrons within a metal electrode.
The behaviour of electrons in a metal can be partly understood by considering the Fermi-level (EF). Metals are comprised of closely packed atoms which have strong overlap between one another. A piece of metal therefore does not possess individual well defined electron energy levels that would be found in a single atom of the same material. Instead a continum of levels exist with the available electrons filling the states from the bottom upwards. The Fermi-level corresponds to the energy of the highest occupied orbitals.



This level is not fixed and can be moved by supplying electrical energy (see above figure). Electrochemist's are therefore able to alter the energy of the Fermi-level by applying a voltage to an electrode.
Depending upon the position of the Fermi level it may be thermodynamically feasible to reduce/oxidise a species in solution. The figure below shows the Fermi-level within a metal along with the orbital energies (HOMO and LUMO) of a molecule (O) in solution.



On the left hand side the Fermi-level has a lower value than the LUMO of (O). It is therefore thermodynamically unfavourable for an electron to jump from the electrode to the molecule. However on the right hand side, the Fermi-level is above the LUMO of (O), now it is thermodynamically favourable for the electron transfer tooccur, ie the reduction of (O. Whether the process occurs depends upon the rate (kinetics) of the electron transfer reaction.



The Rate of Electron Transfer
We begin by considering the one electron transfer reaction



where k.red and k.ox are the rate constants for the reductive and oxidative steps respectively. Assuming there are arbitary amounts of (O) and (R) in the solution the total current flowing i is the sum of the reductive i.c and oxidative i.a currents



where A is the electrode area, F the Faraday constant, n the number of electrons transferred and [ ]o the su***ce concentration of either (O) or (R). Using transition state theory from chemical kineticsit is possible to relate the free energies of activation to the rate constants k.ox or k.red. These are predicted by



The free energies of activation for the electrode reaction are related to both the chemical properties of the reactants/transition state and the response of both to potential. With a small amount of rearrangement it is possible to show that the rate constants (reductive constant shown) show the following potential dependent behaviour



the first exponential can be seen to be independent of the voltage. The second contains the term E - Ee, which is the difference between the applied voltage E and the voltage established by the mixture of (O) and (R) at equilibrium. The term alpha reflects the sensitivity of the transition state to the applied voltage. If alpha = 0 then the transition state shows no potential dependence. Typically alpha = 0.5 this means that the transition state responds to potential in a manner half way between the reactants and the products response.

By using the above approach it is possible to derive the Butler-Volmer equation, which is the fundamental relationship between the current flowing and the applied voltage.



This expression shows how the current will respond to changes in potential, the value of alpha and the quantity io which is called the exchange current (density) .
Here we wish to see how the voltage influences the current in the absence of concentration effects. To do this we will assume that the a solution is well mixed ie that the su***ce and bulk concentrations are identical which will be reasonable under condition of small current flow. Now the Butler-Volmer equation simplifies to



Without concentration and therefore mass transport effects to complicate the electrolysis it is possible to establish the effects of voltage on the current flowing. In this situation the quantity E - Ee reflects the activation energy required to force current i to flow. Plotted below are three curves for differing values of io with alpha =0.5.



For each curve when E - Ee = 0 then no current flows since the system is in total equilibrium. However as a voltage different to that of Ee is applied then different responses are observed depending upon the value of io. When io is 'large' (curve a) then a small change in E -Ee results in a large current change. Essentially there is little or no activation barrier to either of the electrolysis reactions. For this case the electrode reaction is said to be reversible since both kred and kox are large. At the other extreme whenio is very 'small' (curve c) then a large value of E -Ee is needed to alter the current. This reflects the fact that there is now a high barrier to activation and so the rates of the reduction and oxidation processes become slow. Electrode reactions of this type are termed irreversible. Intermediate behaviour is generally referred to as quasi-reversible (curve b). Not surprisingly the different rates of electrode kinetics give rise to substantially different behaviour in voltammetric and impedance analysis.