Electrode Potentials and Electrochemical Cells - Commercial Applications of Fuel Cells (A-Level Chemistry)
Commercial Applications of Fuel Cells
Non-Rechargeable Batteries
Electrochemical cells are used in making batteries. The overall EMF of the cell, gives an indication of how effective the battery is.
There are three main types of battery: non-rechargeable batteries, rechargeable batteries and fuel cells.
In each case, electrons released from the oxidation reaction flow through an external circuit where they can do work and transfer energy to appliances.
How Non-rechargeable Batteries Work
The first battery type of battery ever made was a non-rechargeable battery based on Zinc/Copper.
All non-rechargeable batteries work on the principle that one species is more easily oxidised than the other. Electrons are transferred from this to the less reactive species.
The voltage of non-rechargeable batteries can be worked by calculating the difference in the electrode potentials of each half cell.
Worked example: In a zinc/chloride battery, the following half equation reactions are involved. Determine which reactions actually occur, the overall reaction for the battery and the overall EMF produced.
Answer:
1. Identify the direction of each reaction. The MnO2 reaction has the most positive potential and will occur in the forward reaction. The Zn²+/Zn reaction will occur in the reverse direction.
2. Work out the overall EMF. The MnO2 reaction is the reduction reaction and on the right hand side. The Zn cell undergoes oxidation and is on the left hand side.
EMF = E⦵ (right electrode, reduction) – E⦵(left electrode, oxidation)
EMF = +1.23 – (-0.76) = 1.99 V
3. Write the overall equation. Balance the equations so that the electrons are equal and then add the half equations. In this case, multiply the MnO2 reaction by 2 then add the two half equation, cancelling the electrons.
Examples of non-rechargeable batteries include zinc/carbon batteries and alkaline batteries.
The batteries are used for watches, cameras, toys, torches and electronic equipment.
Advantages and Disadvantages of Non-Rechargeable Batteries
Rechargeable Batteries
Rechargeable batteries are based on reversible reactions.
Lithium Cells
In the lithium cell, the following reactions occur:
The reaction with the more positive E⦵, occurs in the direction of the reduction reaction.
The more negative reaction is reversed and oxidation occurs.
The actual reactions which occur when the battery is being used (discharging) are:
The overall EMF of this type of cell is:
EMF = E⦵ (reduced) – E⦵ (oxidised)
EMF = 0.36 – (-3.04) = +3.40 V
The electrons released flow through an external circuit, where they can do work.
When the battery is being charged, the reverse reactions occur and electrons flow in the opposite direction from the electricity supply to the lithium ions.
Advantages and Disadvantages of Rechargeable Batteries
Hydrogen Fuel Cells
A fuel cell produces electricity by using a fuel (like hydrogen)on the positive electrode and an oxidant on the negative electrode. They react in the presence of an electrolyte, which remains in the cell.
Fuel cells can be used to run cars and vehicles.
As long as there is a constant supply of fuel, the cells can operate continuously.
Alkaline hydrogen-oxygen fuel cell
The reactions that occur in the alkaline hydrogen-oxygen fuel cell are
Overall the reaction is:
These are the steps which describe how this type of cell works:
1. Hydrogen enters at the negative electrode and releases electrons. Hydrogen is oxidised by hydroxide ions to water as shown in the half equation:
2. The electrons flow through the external circuit. This is where they can do work and transfer energy. The electrons flow to the positive electrode where oxygen enters.
3. The electrons are accepted and releases hydroxide ions. Electrons are accepted by the oxygen entering at the positive electrode as shown in the half equation:
4. The hydroxide ions travel to the negative electrode. The hydroxide ions travel through the semi permeable membrane to the negative electrode where they react with the hydrogen atoms and oxidation occurs. The cycle repeats itself.
Acidic hydrogen-oxygen fuel cell
Hydrogen-oxygen fuel cells also work with an acidic electrolyte.
Hydrogen gas is fed into the anode and split into protons and electrons. Only the protons can pass through the semi-permeable membrane, which forces the electrons to travel through the external circuit to the cathode. At the cathode, oxygen combines with H+ and electrons to make water.
Advantages and Disadvantages of Alkaline hydrogen-oxygen fuel cells
Electrode potentials, also known as half-cell potentials, are a measure of the relative tendency of a half-cell to either lose or gain electrons. In electrochemical cells, the electrode potential difference between two half-cells determines the direction and magnitude of electron transfer.
Electrochemical cells are devices that use chemical reactions to generate an electrical current. They consist of two half-cells, each containing an electrode and an electrolyte, which are separated by a permeable membrane. When a potential difference is applied across the electrodes, electrons flow from the half-cell with a lower electrode potential to the half-cell with a higher electrode potential, generating an electrical current.
Fuel cells are a type of electrochemical cell that convert chemical energy from a fuel into electrical energy through an electrochemical reaction. They consist of two electrodes, a cathode and an anode, separated by an electrolyte. The fuel, typically hydrogen, is fed into the anode, where it undergoes an oxidation reaction to produce protons and electrons. The protons pass through the electrolyte to the cathode, where they react with oxygen to produce water and generate an electrical current.
Fuel cells have a wide range of commercial applications, including:
Power generation for buildings and homes
Transportation, including cars, buses, and boats
Backup power for critical systems, such as hospitals and data centers
Portable power for electronic devices, such as laptops and cell phones
Remote power for telecommunications and monitoring systems.
Fuel cells offer several advantages over traditional power sources, including:
High efficiency: Fuel cells can convert chemical energy into electrical energy with efficiencies of up to 60%.
Clean energy: Fuel cells produce water as the only byproduct, making them a clean energy source.
Reliability: Fuel cells can provide continuous power for long periods of time without the need for refueling.
Quiet operation: Fuel cells produce little to no noise, making them ideal for use in residential and commercial settings.
Scalability: Fuel cells can be scaled to meet the energy needs of any application, from small electronic devices to large power plants.
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