Electrochemical Cells and Electrodes

Reference electrodes are electrodes with a stable and well-defined electrochemical potential against which the potential of other electrodes in the system can be controlled and measured. They are used in three electrode systems to perform electrochemical methods such as cyclic voltammetry or linear sweep voltammetry. Reference electrodes are sometimes referred to simply as 'the third electrode'.

Importantly, reference electrodes have a stable, known and well-defined electrochemical potential. The composition of a standard reference electrode should remain effectively unchanging and constant during electrochemical measurements. The purpose of the reference electrode is to provide a stable potential for controlled regulation of the working electrode potential and in doing so allow the measurement of the potential at the working electrode without passing current through it.

An ideal reference electrode should also have zero impedance. This is determined by the resistance of its isolation junction.

The working electrode is probably the most important component of an electrochemical cell: the working electrode is where the electrically driven chemical reaction and electron transfer happens.

Platinum, gold, carbon and mercury are the most commonly used materials but being electrochemically inert and easy to be fabricated into many forms, platinum is often preferred. Gold electrodes are less tolerant to oxidation in the positive potential range, but good to form self-assembled monolayer on its surface while carbon electrodes are more tolerant to more negative potentials.

Counter electrodes, also referred to as auxiliary electrodes, are used in two or three electrode systems to complete the circuit so that electrochemical methods such as cyclic voltammetry and linear sweep voltammetry can be performed. In a three electrode electrochemical cell they do so without passing significant current through the reference electrode.

Redox reactions, also referred to as oxidation-reduction reactions, involve the loss or gain of electrons. The loss of electrons is called oxidation and the gain of the electrons reduction.

Oxidation could happen onto the working electrode if the working electrode of the electrochemical cell is driven to a relatively positive potential to the reference electrode. However, while a negative potential is applied to the working electrode, it will result into a reduction reaction. When the potential reaches to a point where an oxidation or reduction is induced, current starts to flow.

The potential of this point is referred to as the onset oxidation or reduction potential (Eonset), shown below where Epa is the anodic peak potential, Epc is the cathodic peak potential and E1/2 is the half-wave potential [E1/2 = (Epa + Epc)/2].

When the applied potential rises through the half-wave potential, oxidation becomes thermodynamically favourable as the current also continues to increase. The oxidation process, however, becomes limited by rate of diffusion process to the electrode surface, characterised by a drop of the current resembling a duck beak.

The potential sweep of the electrode is then reversed and scanned in the opposite direction until the initial potential is reached. For a chemically reversible charge transfer process, the reversed potential sweep gives the rise of the cathodic peak potential passing the half-wave potential by the reduction of the electrochemically generated species from the first half sweep.

The reversed potential sweep is mostly associated with a reductive (negative) current.

An electrochemical cell is a device that is used either to generate electricity from a spontaneous chemical reaction or that uses electricity to drive a non-spontaneous chemical reaction.

The electrochemical cell with three electrodes is an electrically driven electrolytic type of cell that is designed for the use of cyclic voltammetry.

For a better control and measurement of the current and potential going through the cell during the electricity driven chemical reaction, it is better to use a three electrode system to reduce and compensate the potential changes caused by large currents passing through the working and counter electrodes.

Apart from working electrode where chemical reactions happen and the counter electrode which serves as a current source, there is the third electrode called capillary reference electrode in the three electrode system. Not passing any current, the reference electrode is to act as a reference in measuring and controlling the working electrode potential. Ideally, the reference electrode should be placed as close as possible to the working electrode surface, i.e. the use of the Luggin capillary reference electrode to reduce the current resistance (I times R) drop since the bad conducting of organic solvents. This is also referred to as Ohmic drop which is the amount of potential lost from the reference electrode to the working electrode, caused by the Ohmic resistance between these 2 electrodes.

A photoelectrochemical cell (PEC) is a type of device that utilises a light source onto a semiconductor or photosensitizer to produce electrical energy (similar to a dye-sensitized solar cell) or to trigger chemical reactions to store energy in the form of chemical bonds, i.e. the production of the hydrogen by the splitting of water.

Photoelectrochemical cells are made up of an electrolyte and either two or three electrodes with an anode, a cathode and/or a reference electrode.

Photoelectrochemical cells are light or solar energy driven and they offer a promising potential applications in clean energy captivation, energy production and storage and light-emitting devices.

• Water splitting for hydrogen production
• To reduce CO₂
• Photoelectrochemical dye-sensitized solar cells (DSSCs)
• Photoelectrochemical perovskite solar cells (DSSCs)

Find out more about cyclic voltammetry

The critical component of the photoelectrochemical cell is the semiconductor, or rather the photosensitizer, on the working electrode. Electron-hole pairs are generated on the working electrode by the irradiation of the photons with an energy level that is equal or greater than the bandgap (E_g) of the semiconductor. When light illuminates the photoanode, electrons on the valence band (VB) get excited to the conduction band (CB), and leave a hole behind.

The photogenerated electrons are swept toward the conducting back contact, and are transported to the metal counter-electrode via an external wire. The electrical energy produced and stored in this process is similar to the a photovoltaic dye-sensitized solar cell (DSSC).

Both these excited electrons and the holes left behind in the photoelectrodes will be involved in some form chemical reactions, i.e. water splitting. At the counter electrode, the electrons reduce protons (H⁺) to form hydrogen (H₂) while the photogenerated holes at the photoanode oxidise water (or OH^-) to form oxygen (O₂).