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Electrochemical Cells

Electrochemical cells are devices that can produce electrical energy from chemical energy and chemical energy from electrical energy. No heat is evolved from the electrochemical cells.

The two types of electrochemical cells are:

  1. Voltaic cells: The electrochemical cells that produce electrical energy with the use of chemical energy are called voltaic cells. These are also known as galvanic cells. 
  2. Electrolytic cells: The type of electrochemical cells that produce a chemical reaction are electrolytic cells.

The electrochemical cells have a cathode and an anode. The cathode is the electrode at which electrons are accepted or consumed. The anode is an electrode at which the electrons are released or liberated. These electrodes are made from various conducting metals that are conducting or semiconducting. Other than metals, graphite and some conductive polymers are used too.

The tendency of an electrode to lose or accept an electron is described by its electrode potential. The differences between the electrode potentials of both the electrodes help to calculate the cell potential. The cell potential gives the measure of the voltage of the cell. A standard hydrogen electrode is used as a reference electrode to measure the electrode potential. The hydrogen electrode is the primary reference electrode. Another electrode known as a secondary reference electrode is calomel electrode.

Half cell

The two halves of the cells in which electrolytic solutions are present are the half cells. Each half cell contains an electrode. The electrodes can be the same or different.  

Galvanic or voltaic cells

These are named after Luigi Galvani or Alessandro Volta. He created the first electrochemical cell that generated a direct current.

When electrons shift from species to species through a spontaneous redox reaction, energy is released. This energy can be used to perform tasks when the reaction is divided into 2 half-reactions which are oxidation and reduction. These 2 reactions are carried out in 2 separate containers, and a wire is used as a bridge between the 2 containers to take electrons from one container to another. By this, a galvanic or voltaic cell is formed.

Galvanic cell

The galvanic cell mostly has 2 types of metals in each of the electrolyte solutions which are connected by using a salt bridge. It may also have half-cells with a porous membrane between them.

Salt bridge

It is a device that connects two halves of the electrochemical cells and is formed of a strong electrolyte. It maintains the electrical neutrality in the circuit. It also completes the electrical circuit.

The solution in the salt bridge must be inert and nonreactive with other solutions. This prevents a reaction between the solution and the salt bridge and allows movement of ions between the two half cells.

The two types of the salt bridge are glass tube bridge and filter paper bridge.

Glass tube bridge is a tube that has a U-shape. It is filled with electrolytes like sodium chloride and potassium nitrate.

The filter paper bridge is formed by a porous material such as filter paper that has electrolytes soaked.

The solution in the salt bridge must be inert and nonreactive with other solutions. This prevents a reaction between the solution and the salt bridge and allows movement of ions between the two half cells.

Let’s see an example.

We can make Galvanic / voltaic cells with solid copper (Cu(s)) in a silver nitrate solution (AgNO3(s)).

During this reaction, AgNO3 breaks into Ag+ and NO3- ions. Then, when the copper electrode is introduced in this solution having silver ions Ag+(aq), it will instantly oxidize Cu(s) to Cu2+(aq) and reduce itself to Ag(s). This reaction will generate energy, and the reaction has to be split into 2 separate containers, as discussed above. Otherwise, the energy released will be lost and cannot be used. Then after connecting a wire (between the two containers) to allow the floor of electrons between them, the Galvanic/ voltaic cell is ready.

These reactions take place on metal strips, known as electrodes. The electrode on which reduction takes place, i.e. the metal electrode which gains an electron(s) is called a cathode and the electrode at which the oxidation takes place, i.e. the metal electrode that loses an electron(s) is called an anode.

Considering the reaction in the above example, Cu is the anode and Ag is the cathode.

Note: Electrons always flow from anode to cathode.

Cell notation

It is the symbolic representation of the two halves of the galvanic cells by using abbreviations and symbols of the elements. Guidelines of cell notation are as follow:

  1. The two halves are represented by using symbols of the elements and chemical formulas of the compounds.
  2. The anode half is written first and the cathode half is written later. First, the reactants are mentioned within each half and then the products are mentioned.
  3. Reactions of both the halves are separated by using two vertical parallel lines in between. This double vertical line indicates the salt bridge of the galvanic cell.
  4. The phased of each of the element and compound is mentioned in parentheses as s, g and aq for solid, gas and aqueous respectively.

The cell notation of the above mentioned galvanic cell is:

Cu(s)│1MCu(NO3)2(aq)║1MAgNO3(aq)│Ag(s)

Daniel cell

These are the type of electrochemical cells that have copper sulphate solution filled in the container which is made from copper. This also contains unglazed earthenware containers that contain sulphuric acid and electrodes of zinc. The aim of the Daniel cell was to eliminate the problem of hydrogen bubbles.

This cell was invented by a British chemist and meteorologist, John Frederic Daniel, in 1836. It is an example of a Galvanic cell.

In this cell, the oxidation of zinc takes place at the anode, and the following half-reaction takes place:

Zn(s) → Zn2+(aq) + 2e

While copper undergoes reduction at the cathode and the following reaction takes place:

Cu2+(aq) + 2e → Cu

So, the entire reaction can be written as:

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

This reaction leads to the formation of copper in solid-state at the cathode electrode, and the zinc electrode undergoes corrosion into the solution to form cation of zinc.

Uses of voltaic cells

  1. The voltaic cells are used to get electrical power. These are used to make the chargeable batteries present in the laptops and cell phones.
  2. The solar cells are made from the galvanic cells, and so are rechargeable.
  3. The chargeable electric vehicles, like bikes and cars, have galvanic cell batteries. 

Advantages of voltaic cells

  1. These are easy to make and are easily available.
  2. They last for a long period of time as most of them can be charged.

Disadvantages of voltaic cells

  1. Some of the galvanic cell batteries are very heavy.
  2. They are expensive than the electrolytic cells.
  3. Some of them show rusting or spoilage very easily.

Dry cell

The EverReady cells are dry cells. It is a type of voltaic cell. These are known as dry cells because the electrolyte used is not a liquid. It is a paste. The dry cells use a paste of manganese dioxide and ammonium chloride in order to generate acidic ions and to complex the zinc ions received from the chemical reaction around the positive electrode made by carbon rods. These cells are also known as Leclanché cells. The dry cells are used in the remote controls or flashlights.

Electrolytic cells

These are the type of electrochemical cells that drive a nonspontaneous reaction using electrical energy. These can decompose chemical compounds, like water into hydrogen and oxygen. This decomposition takes place by the process called electrolysis. So, electrolytic cells need a DC power supply, two electrodes and an electrolyte to perform electrolysis.

An electrolytic cell

The three components of electrolytic cells are an electrolyte and two electrodes.

Electrolytes

Electrolytes are the substances that give an electrically conducting solution when dissolved in polar solvents, such as water. This is because when the electrolyte is dissolved in the polar solvents, it breaks into cation and anions and gets distributed uniformly throughout the solution. These cations and anions under an electrical potential in the solution move to the electrode with an abundance of electrons and a deficit of electrons, respectively. This movement of cation and anions in the direction opposite to each other generates current and forms the electrolytic cells.

While salts, acids and bases form an electrolyte, few gases under certain conditions can also behave like an electrolyte, such as hydrogen chloride at high temperature and low pressure.

So, a substance or an element that dissociates into ions (when put in a solution), has the ability to conduct electricity. Salts are most commonly used to make electrolytes. Molten salts such as molten sodium chloride also form electrolytes and conduct electricity. In fact, ionic liquids are molten salts that have a melting point below 100° C and are highly conductive non-aqueous electrolytes. These have many applications in fuel cells and batteries. Hydroxides made from alkaline metals are also strong electrolytes but do not dissolve in water after a limit. Due to which their application is restricted to certain situations only.

Uses of electrolytic cells:

1. Electroplating: This is the process of coating an electrically conductive object with a thin layer of metal using an electrical current. The electroplating with a particular element adds many properties to metal, such as protection against corrosion, abrasion resistance and wear resistance. This is also used in jewellery and other for several aesthetic reasons.

2. Batteries: Batteries are used in various appliances and machines. These can be formed by electrochemical cells.

3. Electrowinning or electro-refining: It helps to produce various pure metals such as sodium, calcium, aluminium and magnesium. Both electrowinning and electro-refining are electroplating on a large scale. Both processes are used to purify metals by removing impurities. These are an economical and straightforward process for purification of non-ferrous metals.

In electrowinning, a metal is kept in a liquid leach solution, and then a current is passed from an inert anode to the leach solution. This extracts the metal, and then the metal gets deposited on the cathode. While in the electro-refining process, the unrefined impure metals are present on the anodes and when the current is made to pass through the acidic electrolyte the anodes get corroded, and because of electroplating, the pure refined metal gets deposited on the cathode.

4. Oxygen production: The oxygen used in the spacecraft and submarines is prepared with the help of electrolytic cells.

5. Hydrogen fuel is also produced by using electrolytic cells.

Let’s see electrowinning of copper:

In the electrowinning process of copper, a bar made from impure copper will behave like the anode, and a piece of pure copper (with little impurities) will act as a cathode. The solution of aqueous CuSo4 and H2SO4 will make electrolyte. 

Diagram of electrowinning of copper Image source: NCERT

Now, when the current is passed in the cell, it results in electrolysis. Due to electrolysis, the impure copper anode is oxidised and forms Cu2+ ions, and at the cathode electrode, positive copper ions undergo reduction that produces pure copper metal. The impurities of metals other than these do not get dissolved and instead form a solid sludge at the bottom of the container.

The electrochemical cells can also be divided as rechargeable, non-rechargeable and fuel cells.

As we have seen that in fuel cells the fuel is the source of chemical energy required to generate electricity and it has to be supplied externally, in rechargeable and non-rechargeable batteries the fuel is already stored internally, i.e. inside the battery.

The non-rechargeable batteries are capable of producing electricity only till the time the fuel inside them is available to generate the chemical reaction. Once these batteries run out of charge, they cannot be reused and have to be disposed of. These are also known as Primary cells. These types of cells are considered as harmful for the environment as they need a lot of energy in manufacturing and cannot be reused.

Rechargeable batteries are used in cars, and these can be recharged and reused. These types of batteries can be used several times by recharging whenever they get out of charge as they have reversible cell reactions that allow them to recharge again. These are also known as secondary cells. These batteries can run both as a galvanic cell and an electrolytic cell.

Examples of rechargeable or secondary batteries are Lead-Acid battery, nickel-cadmium (NiCd) battery and Silver-zinc batteries.

Lead-acid/Lead storage battery

These batteries are formed by joining many electrochemical cells. These are used as a power source in the vehicles. These are known as secondary cells as these can be used for a period of time without an electric supply and can be charged by an electric supply too. Thus, it acts both as an electrolytic and galvanic cell.

Fuel cells

Fuel cells are the type of electrochemical cells that convert the chemical energy of fuel into electricity. This is done when an electrochemical reaction takes place between hydrogen fuel and oxygen or any other oxidizing agent. While in batteries chemical energy is generated from the chemicals that are already filled in it, fuel cells need a continuous flow of oxygen and supply of fuel. So, the electricity from fuel cells is dependent on the flow of oxygen and fuel.

The first fuel cell was used commercially by NASA to generate power for space capsules and satellites.

Uses of fuel cells:

  1. These are used as a primary source of power in industries and residential buildings. Fuel cells are very useful in remote areas.
  2. These are also used in vehicles, boats and submarines powered by fuel cells.
  3. With the low-quality gas coming from landfills or wastewater treatment plants, fuel-cells can be used to decrease the release of methane in the atmosphere.

A fuel-cell system that runs on hydrogen is generally very light in weight and small in size. They also have no part in moving condition in any situation and also involve no combustion in any of its parts, that’s why their reliability of up to 99.9999% is achievable.  

Summary:

  1. Electrochemical cells are capable of producing electrical energy by using the chemical energy generated through chemical reactions and chemical energy by using electrical energy. 
  2. The types of electrochemical cells are Galvanic or Voltaic cells, electrolytic cells, Fuel cells, chargeable and non-rechargeable cells.
  3. Galvanic cells use the energy generated by the chemical-filled in the cells to generate electrical energy. These cells can be recharged.
  4. Electrolytic cells are used to decompose elements, i.e. to break them into different elements. These cells cannot be recharged.
  5. Electroplating and electrowinning are the two very useful applications of electrolytic cells that are widely used in industries.
  6. Fuel cells use chemical energy from the fuel to generate electricity. Fuel cells are widely used in remote places and areas which are hard to reach. Fuel cells work only until they receive the supply of fuel and oxygen.
  7. Rechargeable cells can be used for dozens of times by recharging them whenever they get discharged.
  8. Non-rechargeable cells cannot be reused like the rechargeable cells and need to be disposed of once they stop working.
  9. Lead storage batteries and dry cells are advance types of electrochemical cells used in daily life.

Resources:

  1. Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/electrochem.html
  2. Openstax chemistry 2e
  3. Wikipedia
  4. Science direct https://www.sciencedirect.com/topics/chemistry/electrochemical-cell