Electrode


An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit. In electrochemical cells, electrodes are essential parts that can consist of a variety of materials depending on the type of cell. An electrode may be called either a cathode or anode according to the direction of the electric current, unrelated to the potential difference between electrodes.
Michael Faraday coined the term "" in 1833; the word recalls the Greek ἤλεκτρον and ὁδός.
The electrophore, invented by Johan Wilcke in 1762, was an early version of an electrode used to study static electricity.

Anode and cathode in electrochemical cells

Electrodes are an essential part of any battery. The first electrochemical battery was devised by Alessandro Volta and was aptly named the Voltaic cell. This battery consisted of a stack of copper and zinc electrodes separated by brine-soaked paper disks. Due to fluctuation in the voltage provided by the voltaic cell, it was not very practical. The first practical battery was invented in 1839 and named the Daniell cell after John Frederic Daniell. It still made use of the zinc–copper electrode combination. Since then, many more batteries have been developed using various materials. The basis of all these is still using two electrodes, anodes and cathodes.

Anode

'Anode' was coined by William Whewell at Michael Faraday's request, derived from the Greek words ἄνο, 'upwards' and ὁδός, 'a way'. The anode is the electrode through which the conventional current enters from the electrical circuit of an electrochemical cell into the non-metallic cell. The electrons then flow to the other side of the battery. Benjamin Franklin surmised that the electrical flow moved from positive to negative. The electrons flow away from the anode and the conventional current towards it. From both can be concluded that the electric potential of the anode is negative. The electron entering the anode comes from the oxidation reaction that takes place next to it.

Cathode

The cathode is in many ways the opposite of the anode. The name comes from the Greek words κάτω, 'downwards' and ὁδός, 'a way'. It is the positive electrode, meaning the electrons flow from the electrical circuit through the cathode into the non-metallic part of the electrochemical cell. At the cathode, the reduction reaction takes place with the electrons arriving from the wire connected to the cathode and are absorbed by the oxidizing agent.

Primary cell

A primary cell is a battery designed to be used once and then discarded. This is due to the electrochemical reactions taking place at the electrodes in the cell not being reversible. An example of a primary cell is the discardable alkaline battery commonly used in flashlights. Consisting of a zinc anode and a manganese oxide cathode in which ZnO is formed.
The half-reactions are:
Overall reaction:
The ZnO is prone to clumping and will give less efficient discharge if recharged again. It is possible to recharge these batteries but is due to safety concerns advised against by the manufacturer. Other primary cells include zinc–carbon, zinc–chloride, and lithium iron disulfide.

Secondary cell

Contrary to the primary cell a secondary cell can be recharged. The first was the lead–acid battery, invented in 1859 by French physicist Gaston Planté. This type of battery is still the most widely used in automobiles, among others. The cathode consists of lead dioxide and the anode of solid lead. Other commonly used rechargeable batteries are nickel–cadmium, nickel–metal hydride, and Lithium-ion. The last of which will be explained more thoroughly in this article due to its importance.

Marcus's theory of electron transfer

Marcus theory is a theory originally developed by Nobel laureate Rudolph A. Marcus and explains the rate at which an electron can move from one chemical species to another, for this article this can be seen as 'jumping' from the electrode to a species in the solvent or vice versa.
We can represent the problem as calculating the transfer rate for the transfer of an electron from donor to an acceptor
The potential energy of the system is a function of the translational, rotational, and vibrational coordinates of the reacting species and the molecules of the surrounding medium, collectively called the reaction coordinates. The abscissa the figure to the right represents these. From the classical electron transfer theory, the expression of the reaction rate constant can be calculated, if a non-adiabatic process and parabolic potential energy are assumed, by finding the point of intersection. One important thing to note, and was noted by Marcus when he came up with the theory, the electron transfer must abide by the law of conservation of energy and the Frank-Condon principle.
Doing this and then rearranging this leads to the expression of the free energy activation in terms of the overall free energy of the reaction.
In which the is the reorganisation energy.
Filling this result in the classically derived Arrhenius equation
leads to
with A being the pre-exponential factor, which is usually experimentally determined, although a semi-classical derivation provides more information as is explained below.
This classically derived result qualitatively reproduced observations of a maximum electron transfer rate under the conditions. For a more extensive mathematical treatment one could read the paper by Newton. An interpretation of this result and what a closer look at the physical meaning of the one can read the paper by Marcus.
The situation at hand can be more accurately described by using the displaced harmonic oscillator model, in this model quantum tunneling is allowed. This is needed in order to explain why even at near-zero absolute temperature there are still electron transfers, in contradiction with the classical theory.
Without going into too much detail on how the derivation is done, it rests on using Fermi's golden rule from time-dependent perturbation theory with the full Hamiltonian of the system. It is possible to look at the overlap in the wavefunctions of both the reactants and the products and therefore when their energies are the same and allow for electron transfer. As touched on before this must happen because only then conservation of energy is abided by. Skipping over a few mathematical steps the probability of electron transfer can be calculated using the following formula
with being the electronic coupling constant describing the interaction between the two states and being the line shape function. Taking the classical limit of this expression, meaning, and making some substitution an expression is obtained very similar to the classically derived formula, as expected.
The main difference is now the pre-exponential factor has now been described by more physical parameters instead of the experimental factor. One is once again revered to the sources as listed below for a more in-depth and rigorous mathematical derivation and interpretation.

Efficiency

The physical properties of electrodes are mainly determined by the material of the electrode and the topology of the electrode. The properties required depend on the application and therefore there are many kinds of electrodes in circulation. The defining property for a material to be used as an electrode is that it be conductive. Any conducting material such as metals, semiconductors, graphite or conductive polymers can therefore be used as an electrode. Often electrodes consist of a combination of materials, each with a specific task. Typical constituents are the active materials which serve as the particles which oxidate or reduct, conductive agents which improve the conductivity of the electrode and binders which are used to contain the active particles within the electrode. The efficiency of electrochemical cells is judged by a number of properties, important quantities are the self-discharge time, the discharge voltage and the cycle performance. The physical properties of the electrodes play an important role in determining these quantities. Important properties of the electrodes are: the electrical resistivity, the specific heat capacity, the electrode potential and the hardness. Of course, for technological applications, the cost of the material is also an important factor. The values of these properties at room temperature for some commonly used materials are listed in the table below.
PropertiesLithium Manganese Copper Zinc Graphite
Resistivity
Electrode potential −3.02−1.05−0.340−0.760—
Hardness <550050307–11
Specific heat capacity 2.9970.4480.3850.38980.707

Surface effects

The surface topology of the electrode plays an important role in determining the efficiency of an electrode. The efficiency of the electrode can be reduced due to contact resistance. To create an efficient electrode it is therefore important to design it such that it minimizes the contact resistance.

Manufacturing

The production of electrodes for Li-ion batteries is done in various steps as follows:
  1. The various constituents of the electrode are mixed into a solvent to produce an 'electrode slurry'. This mixture is designed such that it improves the performance of the electrodes. Common components of this mixture are:
  2. * The active electrode particles.
  3. * A binder used to contain the active electrode particles.
  4. * A conductive agent used to improve the conductivity of the electrode.
  5. The electrode slurry above is coated onto a conductor, which acts as the current collector in the electrochemical cell. Typical current collectors are copper for the cathode and aluminum for the anode.
  6. After the slurry has been applied to the conductor it is dried and then pressed to the required thickness.