Applications of Electrochemistry

Based on Lucarelli's Essential Chemistry Chapter 8

What is a battery?

A group of ≥2 galvanic cells, usually connected in series

What is a primary cell?

A galvanic cell that can not be recharged

What are some examples of primary cells?

*Leclanche cells: Similar to dry cells, but with a solution instead of a paste and a solid MnO₂ cathode

*Dry cells: use Zn (reducing agent, anode) and MnO₂ (oxidising agent, cathode paste). The cathode paste also includes NH₄Cl to provide protons for the reduction half-equation. A porous fibre is used instead of a salt bridge, and a graphite rod is used to conduct electrons. Max. voltage is 1.5V.

*Alkaline cells: use Zn powder (reducing agent, anode paste) and MnO₂ (oxidising agent, cathode paste). KOH is used instead of NH₄Cl, but cancels itself out in the overall reaction.

*Indicates more important cells to understand

Silver oxide button cells: use Zn (reducing agent, anode paste) and Ag₂O (oxidising agent, cathode paste). KOH is used in the anode paste, and C in the cathode paste. They contain Hg.

Lithium cells: Must be airtight and watertight because Li will react with either. Uses Li as anode (reducing agent) and MnO₂ as cathode (oxidising agent)

What is a secondary cell?

A galvanic cell that can be recharged via electrolysis (but not forever)

What are some examples of secondary cells?

Lead-Acid Cells: (6 make a car battery) the electrodes are made of lead grids, with the anode being filled with spongy Pb and the cathode with powdered PbO₂. The electrodes are separated by a porous barrier and immersed in a sulfuric acid solution. The presence of lead and sulfuric acid makes these batteries quite hazardous. Must be vented.

Lithium-Ion Cells: The anode (where oxidation occurs) is made of porous graphite with Li ions between the layers. The cathode (where reduction occurs) can be made of porous LiCoO₂. The electrodes are arranged in sheets, with a porous separator between them, and immersed in an organic electrolyte containing Lithium ions. Light.

Nickel metal hydride cells: Use a rare-earth or transition metal alloy to store hydrogen at the anode, NiOOH for the cathode, and an electrolyte of KOH for the salt bridge. The metal hydride is oxidised during discharge, producing hydrogen ions and neutral metal. The H⁺ reacts with the OH^- from the KOH, forming water, at the cathode, the water, an electron, and NiOOH form Ni(OH)₂ and OH^-, reducing the Ni.

What are some dangers associated with Lithium-ion batteries?

Short-circuit: If the porous separator between the electrodes is damaged, the battery may rapidly discharge, generating large amounts of heat.

Overcharging: Can release O₂, which can combust with the organic electrolyte, as well as increasing the chance of the battery overheating.

Electrolyte breakdown: Can release CO₂, causing a pressure buildup that may cause the battery to burst.

What is a fuel cell?

A galvanic cell that is constantly refilled with oxidant and reductant, with products being expelled as they function.

What are some examples of fuel cells?

*Alkaline Hydrogen-Oxygen Fuel Cell: hydrogen gas is used as the reductant and oxygen gas is used as the oxidant (held in separate compartments). A hot solution of KOH_(aq) is used as catalyst, while porous nickel is used for the electrodes (with Pt as a catalyst). It operates at 150-200°C.

Proton Exchange Membrane Fuel Cells: hydrogen gas is used as the reductant and oxygen gas is used as the oxidant, they must be pure in order to not contaminate the catalyst. It has a special thin, solid polymer called a proton exchange membrane (that only protons can move through) used as both electrode separator and electrolyte. The electrodes are made of porous C (with Pt nanoparticles for a catalyst).

Phosphoric Acid Fuel Cell: Hydrogen gas is used as the reductant and oxygen gas is used as the oxidant. The electrolyte is a silicon carbide matrix containing H₃PO₄. It operates at 150-220°C. The electrodes are made of porous C (with Pt particles for a catalyst).

What are some examples of fuel cells in use?

Electricity production and breathalysers (ethanol fuel cells are used to detect ethanol on your breath)

How is Copper electrorefined?

Impure copper is used an an anode while pure copper is used as a cathode. By applying a certain DC voltage, only the copper in the anode undergoes electrolysis, first being oxidised from the anode and then reduced onto the cathode. The impurities fall off the anode and become “anode slime/sludge”, or remain in the electrolyte (initially CuSO₄ and H₂SO₄).

What is electroplating?

A process that places a thin coating of one metal onto an object made of another metal. The plating metal is the anode, while the object is the cathode. The electrolyte is a salt solution of the plating metal. Application of DC voltage causes the anode (plating metal) to be oxidised and the reduce onto the cathode. For some metals such as gold, zinc, and silver, cyanide ions may need to be used as a ‘complexing ion’, to decrease the concentration of plating ions.

Corrosion

An electrochemical process where metal is oxidised in the presence of water and air.

Rusting

Corrosion of Fe.

4Fe_(s) + 3O_2(aq) + xH₂O_(l) → 2Fe₂O₃.xH₂O_(s)

Rust is hydrated Iron (III) oxide.

How does Fe rust, chemically speaking?

A spontaneous redox reaction occurs between Iron and Oxygen:

Iron is oxidised at the anodic site: Fe_(s) → Fe²⁺_(aq) + 2e^-

Oxygen is reduced at the cathodic site: O_2(aq) + 2H₂O_(l) → 4OH^-_(aq)

The full equation is: 2Fe_(s) + O_2(aq) + 2H₂O_(l) → 2Fe(OH)_2(s)

The Iron (II) hydroxide is then oxidised further: 4Fe(OH)_2(s) + O_2(aq) + 2H₂O_(l) → 4Fe(OH)_3(s)

The brown Iron (III) hydroxide is then dehydrated, forming Fe₂O₃.xH₂O_(s)

Which areas of iron are most likely to be anodic sites?

Areas with low oxygen concentration (middle of a water drop), or where the surface is stressed, e.g. a scratch or edge

Where is oxygen most likely to be reduced?

Areas with high concentration of oxygen and available electrons, (at the edge of a water drop).

Where does rust accumulate?

Fe²⁺ ions migrate to the edge of the water droplet, where O₂ is reduced, forming solid compounds

How does the presence of salts impact corrosion of Fe?

Salts accelerate corrosion, as high electrolyte concentration acts as a salt bridge and allows Fe to be oxidised easily. This is why corrosion occurs in coastal areas more quickly than inland.

How does the presence of acids and bases impact corrosion of Fe?

Low pH (acids) accelerate corrosion by increasing the reduction potential of O₂, and can directly react with Fe, while high pH (bases) can cause passivation and slow down corrosion

Passivation

When corrosion of a metal forms an oxidised surface layer that protects the rest of the metal from water/air, preventing corrosion. It occurs in metals such as Al, Cr and Zn.

How can corrosion/rusting be prevented?

Inert nonmetallic coating - Plastic, grease or paint, for example. They exclude oxygen and water from the iron so that it does not rust. It is vulnerable to scratching.

Inert metallic coating - Tin or copper, for example. They exclude oxygen and water from the iron so that it does not rust. If scratched, rusting will be accelerated due to the presence of a less reactive metal.

Galvanisation - Coating iron with zinc. Prevents the iron from contacting oxygen and water, and since zinc is more reactive, it is oxidised first, forming a protective layer with its passivation properties.

Sacrificial Anode - A more reactive metal is placed in salt bridge contact with iron, and is oxidised instead of the iron. Only works in wet environments e.g. ships, water tanks.

Direct Current Cathodic Protection: Fills the iron with electrons using a DC power source so that it becomes a cathode instead of an anode. The positive terminal of the power source is attached to an anode, which can be made of scrap iron, C, Pt, etc. A Pt anode will oxidise water instead of itself, so it can last forever (while there is a current). The anode must be in salt bridge contact with the iron so it only works in wet environments.