15 Electrochemical Applications

Electrochemical applications are applications that pertain to electrochemical reactions. An electrochemical reaction involves an oxidation reaction (such as Fe ^ Fe2+ + 2e) in which electrons are generated, and a reduction reaction (such as O2 + 2H2O + 4e- ^ 4OH) in which electrons are consumed. The electrode that releases electrons is the anode; the electrode that receives electrons is the cathode.

When the anode and cathode are electrically connected, electrons move from the anode to the cathode. Both the anode and cathode must be electronic conductors. As the electrons move in the wire from the anode to the cathode, ions move in an ionic conductor (called the electrolyte) placed between the anode and the cathode such that cations (positive ions) generated by the oxidation of the anode move in the electrolyte from the anode to the cathode.

Whether an electrode behaves as an anode or a cathode depends on its propensity for oxidation. The electrode that has the higher propensity serves as the anode, while the other electrode serves as the cathode. On the other hand, a voltage can be applied between the anode and the cathode at the location of the wire such that the positive end of the voltage is at the anode side. The positive end attracts electrons, thus forcing the anode to be oxidized, even when it may not be more prone to oxidation than the cathode.

The oxidation reaction is associated with corrosion of the anode. For example, the oxidation reaction Fe ^ Fe2+ + 2e- causes iron atoms to be corroded away, becoming Fe2+ ions, which go into the electrolyte. The hindering of the oxidation reaction results in corrosion protection.

Electrochemical reactions are relevant not only to corrosion, but also to batteries, fuel cells, and industrial processes (such as the reduction of Al2O3 to make Al) that make use of electrochemical reactions. The burning of fossil fuels such as coal and gasoline causes pollution of the environment. In contrast, batteries and fuel cells cause fewer environmental problems.

A battery involves an anode and a cathode that are inherently different in their propensities for oxidation. When the anode and cathode are open-circuited at the wire, a voltage difference is present between them such that the negative end of the voltage is at the anode side. This is because the anode wants to release electrons, but the electrons cannot come out because of the open circuit condition. This voltage difference is the output of the battery, which is a source of direct current (DC).

A unit involving an anode and a cathode is called a "galvanic cell." A battery consists of a number of galvanic cells connected in series, so that the battery voltage is the sum of the voltages of the individual cells.

An example of a battery is the lead storage battery used in cars. Lead (Pb) is the anode, while lead dioxide (PbO2, in the form of a coating on the lead) is the cathode. Sulfuric acid (H2SO4) is the electrolyte. The oxidation reaction (anode reaction) is

The reduction reaction (cathode reaction) is

Discharge is the state of operation of the battery. The PbSO4 is a solid reaction product that adheres to the electrodes, hindering further reaction. A battery needs to be charged by forcing current through the battery in the opposite direction, thereby breaking down PbSO4, i.e., making the above reactions go in the reverse direction. In a car, the battery is continuously charged by an alternator.

Another example of a battery is the alkaline version of the dry cell battery. This battery comprises a zinc anode and an MnO2 cathode. Because MnO2 is not an electrical conductor, carbon powder (an electrical conductor) is mixed with the MnO2 powder in forming the cathode. The electrolyte is either KOH or NaOH. The anode reaction is

The cathode reaction is

A fuel cell is a galvanic cell in which the reactants are continuously supplied. An example is the hydrogen-oxygen fuel cell. The anode reaction is

The cathode reaction is

4e- + O2 + 2H2O — 4OH-The overall cell reaction (the anode and cathode reactions added together) is

which is the formation of water from the reaction of hydrogen and oxygen.

During cell operation, hydrogen gas is fed to a porous carbon plate that contains a catalyst that helps the anode reaction. The carbon is an electrical conductor, which allows electrons generated by the anode reaction to flow. The porous carbon is known as a "current collector." Simultaneously, oxygen gas is fed to another porous carbon plate that contains a catalyst. The two carbon plates are electrically connected by a wire; electrons generated by the anode reaction at one plate flow through the wire and enter the other carbon plate for consumption in the cathode reaction. As this occurs, the OH- ions generated by the cathode reaction move through the electrolyte (KOH) between the two carbon plates, and then are consumed in the anode reaction at the other carbon plate. The overall cell reaction produces H2O, which comes out of the cell at an opening located at the electrolyte between the two carbon plates. The useful output of the cell is the electric current associated with the flow of electrons in the wire from one plate to the other.

Materials required for electrochemical applications include the electrodes, current collector (such as the porous carbon plates of the fuel cell mentioned above), conductive additive (such as carbon powder mixed with the MnO2 powder in a dry cell), and electrolyte. An electrolyte can be a liquid or a solid, as long as it is an ionic conductor. The interface between the electrolyte and an electrode is intimate and greatly affects cell performance. The ability to recharge a cell is governed by the reversibility of the cell reactions. In practice, the reversibility is not complete, leading to low charge-discharge cycle life.

DIY Battery Repair

DIY Battery Repair

You can now recondition your old batteries at home and bring them back to 100 percent of their working condition. This guide will enable you to revive All NiCd batteries regardless of brand and battery volt. It will give you the required information on how to re-energize and revive your NiCd batteries through the RVD process, charging method and charging guidelines.

Get My Free Ebook

Post a comment