These are devices which convert the energy produced during the combustion of fuels like H2, CO and CH4 directly into electrical energy. The most successful fuel cell is the H2-O2 fuel cell. It was used in the Apollo space programme and the water produced was used as drinking water for the astronauts.
The electric cells are the devices which produce electricity by converting the chemical energy to electrical energy. For example, photoelectric cell, photovoltaic cell, fuel cell etc.
"The fuel cells are the devices which are used to generate electricity. They generate electric power by chemical reaction between the source of fuel and an oxidant".
They contain two electrodes. The positive is called anode while the negative electrode is cathode. The source fuel can be hydrogen, alkanes like methane or propane, gasoline, methanol etc. Generally hydrogen is used as a basic fuel. All the source fuels have a tendency to oxidize. The chemical reaction in the fuel cell is completed at the electrodes. All the cells contain electrolyte and a catalyst.
An electrolyte carries the charged particles from one electrode to another while the catalyst works to speed up the chemical reaction on electrodes. The main advantage of fuel cells is that they produce small amounts of pollution. This is due to the formation of a single harmless byproduct, water.
The use of a single fuel cell produces a small amount of direct current (DC). So they are used by assembling many fuel cells into a stack.
In general principle, a fuel cell is like a battery. But a fuel cell does not need to recharge. It generates energy in the form of electricity and heat till fuel is supplied.
In a fuel cell, two electrodes (cathode and anode) are present around an electrolyte. Hydrogen gas enters from the anode electrode of the cell while oxygen or air enters from the cathode. The ionization of hydrogen is done at the anode in the presence of a catalyst. The ionization of hydrogen produces the proton (H+) and an electron. Both the protons and electrons move from the anode.
The protons travel through the electrolyte and react with oxygen to produce water as byproduct.
The electricity will generate till the fuel is supplied. The electrons generate a current which provides the current through the electric circuit for working. The inverter is used for the conversion of direct current to alternating current, if it is needed.
The overall chemical reaction is
2H2 (gas) + O2 (gas) → 2 H2O + energy
The role of the electrolyte is very important in fuel cells. Only the appropriate ions should be passed through the electrolyte because passing of electrons or any other ions would be the reason for disrupting the chemical reaction.
In some cells, the oxygen reacts with electrons and travels through the electrolyte to react with the protons at the anode. Sometimes a fuel reformer is used in the fuel cell. A fuel reformer can use the hydrogen from any fuel like hydrocarbon, gasoline etc.
- The efficiency of a fuel is based on measurement of maximum energy that can be obtained from the fuel by burning it.
- This is known as the heating value (HV) or calorific value (CV).
- The available energy in a fuel cell is Gibbs energy.
- The Gibbs energy shows the maximum amount of electricity which is generated from the fuel.
- The value of Gibbs energy is less than the calorific value.
- The efficiency of fuel cell and Gibbs energy are related to each other.
- Thus the efficiency is defined as the electrical energy generated divided by the calorific value of the fuel.
- The fuel efficiency is due to chemical properties of fuel. For example, the maximum theoretical efficiency of hydrogen fuel cell is 83% at 25°C but its 58% at 500°C for a combustion engine.
- The fuel efficiency decreases with increasing temperature because the Gibbs energy decreases with increasing temperatures.
- So the fuel cells at high temperature approx 800-1000°C may be used with a turbine to generate electricity with a maximum efficiency of 80%. The electricity is produced from the waste heat in fuel cells with turbine.
- But the fuel cells with low temperature or without turbine are more efficient.
Fuel cell is a device in which fuels like hydrogen, alcohol etc are converted to electricity. A hydrogen fuel cell can be made quite easily. The following things are needed to make the fuel cell.
The process of making the fuel cell consists of a few steps.
- Platinum coated wire or platinum wire (approx one foot).
- A stick or small piece of wood or plastic.
- A battery and battery clip (9 volt).
- Transparent tape.
- Water and a volt meter.
- First cut the platinum wire into two equal long pieces and give it a spring shape. These are the electrode of the fuel cell.
- The next step is to cut the leads of the battery clip in half and paste the insulation on the cutting ends. Then these battery lead wires are attached to the ends of the platinum electrode. So the battery clip and two wires will be attached to the electrodes.
- Then a stick is taped with electrodes and with the glass of water. Thus only the platinum electrodes should be dipped in water and not the wire connection.
- Then take the volt meter and connect the red and black wire with the positive and negative terminals of the volt meter respectively. This completes the formation of a fuel cell. Now for operating this cell, we also need hydrogen and oxygen. When the battery touches the clip, some bubbles are formed in water. This is due to the electrolysis of water. It makes hydrogen and oxygen from water.
The platinum acts as a catalyst and reverses the electrolysis reaction. So hydrogen and oxygen reacts to make water again and generate electricity. At the initiation of the procedure, the process is slow but as the bubbles get used up in the reaction, the voltage drops slowly. This is due to the use of hydrogen and oxygen for the reaction and to produce electricity. We can produce hydrogen and oxygen from solar power, store them in high pressure tanks of electric car and than use them for producing electricity.
- The important application of fuel cells is that they are used as a power source in various fields like in space craft, military, research areas, and communication centers etc. This is because of the light weight, non combustion nature and less moving parts of fuel cells.
- If the waste heat of a fuel cell is used in a co-generation system, then the efficiency of fuel cell is increased up to 85%. It gives better result than a cola power plant.
- Fuel cell power plants also create less amount of pollution (less than one ounce of pollution if hydrogen gas is used as the source) than other power systems.