Fuel Cells – Definition, Types, Advantages, Limitations

The study of the link between electrical energy and chemical changes is the subject of electrochemistry, a chemistry subdiscipline. Electrochemical reactions are chemical processes that include the input or creation of electric currents. A fuel cell is an electrochemical cell that uses an electrochemical process to create electrical energy from fuel. To keep the processes that generate electricity going, these cells need a constant supply of fuel and an oxidising agent (usually oxygen). As a result, until the supply of fuel and oxygen is shut off, these cells can continue to generate power.

Fuel cells, while being conceived in 1838, did not enter commercial usage until a century later, when NASA utilised them to power space capsules and satellites. Many establishments, including businesses, commercial buildings, and residential structures, now employ these devices as a major or secondary source of electricity.

Fuel Cell

Fuel cells are cells that directly transform the chemical energy of a fuel cell into electrical energy. Fuels such as hydrogen (H₂), carbon dioxide (CO₂), methane (CH₄), propane (C₃H₈), methanol (CH₃OH), and others are used to create electrical energy in the cells shown below. The fuel cell is constantly supplied with fuel, while the products are continuously removed. There are a great number of fuel cells on the market. The most popular type is a hydrogen-oxygen fuel cell.

Fuel Cell used in the Apollo Space Program

Bacon invented the hydrogen-oxygen fuel cell in 1959. As a result, it’s also called the Bacon cell. It is a possible source of electrical energy that was employed as the major source of electrical energy during the United States’ Apollo space program.

Two porous carbon electrodes are impregnated with a suitable catalyst such as platinum (Pt), silver (Ag), cobalt oxide (CoO), and so on in a basic H₂–O₂ fuel cell. The electrolyte is a concentrated solution of potassium hydroxide (KOH) or sodium hydroxide (NaOH) that fills the gap between two electrodes. A porous carbon electrode bubbles hydrogen gas (H₂) and oxygen gas (O₂) into the electrolyte.

• At the anode, 2H₂ (g)+4OH^– (aq)  →  4H₂O (l)+4e^–
• At the cathode, O₂ (g)+2H₂O (l)+4e^–  →  4OH^– (aq)
• Overall reaction, 2H₂ (g) + O₂ (g)  →  2H₂O (l)

Microbial fuel cells (MFCs) are cells that use microbes to catalyse biological processes to generate power from organic or inorganic chemicals. It would be capable of achieving a 50% efficiency rate. Proteobacteria, Desulfuromonas, Alcaligenes faecalis, Pseudomonas aeruginosa, and other microorganisms have been employed in MFCs.

An anode and cathode compartment are separated by a cation-specific membrane-like potassium nitrate base membrane in a microbial fuel cell. The microorganism oxidises the fuel at the anode, creating carbon dioxide, electrons, and protons. Protons are delivered to the cathode compartment through the membrane and electrons are transferred to the cathode compartment through an external electric circuit. The cathode compartment produces water by mixing oxygen with electrons and protons.

Anode reaction: C₁₂H₂₂O₁₁+13H₂O→12CO₂+48H⁺+48e^–

Cathode reaction: 4H⁺+O₂+4e^–→2H₂O

Types of Fuel Cells

Fuel cells come in a variety of forms.

1. Polymer Electrolyte Membrane (PEM) Fuel Cell
2. Phosphoric Acid Fuel Cell
3. Solid Acid Fuel Cell
4. Alkaline Fuel Cell
5. Molten Carbonate Fuel Cell
6. Hydrogen-Oxygen fuel cell
7. Microbial fuel cells (MFCs)
8. Solid Oxide Fuel Cells (SOFCs)
9. Zinc-Air Fuel Cell (ZAFC)
10. Direct Methanol Fuel Cell (DMFC)

The Polymer Electrolyte Membrane (PEM) Fuel Cell

1. Proton exchange membrane fuel cells are another name for these cells (or PEMFCs).
2. These cells function at temperatures ranging from 50 degrees Celsius to 100 degree Celsius.
3. The electrolyte used in PEMFCs is a polymer that can conduct protons.
4. A PEM fuel cell is made up of bipolar plates, a catalyst, electrodes, and a polymer membrane.
5. Despite its environmentally benign applications in transportation, PEMFCs can also be utilised for fixed and portable power generation.

Phosphoric Acid Fuel Cell

1. Phosphoric acid is used as an electrolyte in these fuel cells to channel the H⁺.
2. These cells operate at temperatures ranging from 150-200 Celsius.
3. Because phosphoric acid is non-conductive, electrons must go to the cathode via an external connection.
4. Because the electrolyte is acidic, the components of these cells corrode or oxidise with time.

Solid Acid Fuel Cell

1. The electrolyte in these fuel cells is a solid acid substance.
2. At low temperatures, the molecular structures of these solid acids are organised.
3. At higher temperatures, a phase shift can occur, resulting in a significant increase in conductivity.
4. CsHSO₄ and CsH₂PO₄ are two examples of solid acids (cesium hydrogen sulphate and cesium dihydrogen phosphate respectively).

Alkaline Fuel Cell

1. This was the fuel cell that served as the major source of power for the Apollo space programme.
2. An aqueous alkaline solution is employed in these cells to saturate a porous matrix, which is then used to separate the electrodes.
3. These cells’ operating temperatures are relatively low.
4. These cells are quite effective. Along with power, they generate heat and water.

Molten Carbonate Fuel Cell

1. Lithium potassium carbonate salt is employed as the electrolyte in these cells. At high temperatures, this salt becomes liquid, allowing carbonate ions to migrate.
2. These fuel cells, like SOFCs, have a relatively high working temperature of 650 Celsius.
3. Because of the high working temperature and the presence of the carbonate electrolyte, the anode and cathode of this cell are prone to corrosion.
4. These cells can run on carbon-based fuels like natural gas and biogas.

Solid Oxide Fuel Cells (SOFCs)

Solid oxide fuel cells employ a hard, non-porous ceramic substance as the electrolyte and operate at temperatures between 500 and 1000 degrees Celsius. A solid oxide electrolyte is used in SOFCs to transport negative oxygen ions from the cathode to the anode. SOFCs have an efficiency of 50–60 percent.

• At the anode: 1/2O₂+2e^–→O
• At the cathode: H₂+1/2O→H₂O+2e^–
• The overall cell reaction: H₂+12O₂→H₂O

Satellites and space capsules employ SOFCs to generate electricity. It is mostly employed in big, high-power applications such as industrial generating plants.

Zinc-Air Fuel Cell (ZAFC)

The Zinc-Air Fuel Cell (ZAFC) is a kind of fuel cell that was created in the United States for use in vehicles. The electrolyte is an aqueous alkali solution such as potassium hydroxide, and the electrode reactions are as follows:

• Anode: Zn+2OH^–→Zn(OH)₂+2e^–
• Cathode: O₂+2H₂O+4e^–→4OH^–
• Overall Reaction: 2Zn+O₂+2H₂O→4Zn(OH)₂

It is used as an alternative fuel for vehicles.

Direct Methanol Fuel Cell (DMFC)

Methanol is utilised as a fuel in this subclass of proton-exchange fuel cells. The key benefit of this fuel cell is the ease with which stable liquid fuel methanol may be transported. Polymer membrane serves as the electrolyte, and the electrode reactions are as follows:

• Anode: CH₃OH+H₂O→6H⁺+CO₂+6e^–
• Cathode: 3/2O₂+6H⁺+6e^–→3H₂O
• Net reaction: CH₃OH+3/2O₂→CO₂+2H₂O

Working of Fuel Cell

A fuel cell may use the chemistry between hydrogen and oxygen to create power. This type of cell was utilised in the Apollo space program and had two purposes: as a source of fuel and as a supply of drinking water (the water vapour produced from the cell, when condensed, was fit for human consumption).

This fuel cell worked by transferring hydrogen and oxygen through carbon electrodes into a concentrated sodium hydroxide solution.

• Cathode Reaction: O₂ + 2H₂O + 4e^– → 4OH^–
• Anode Reaction: 2H₂ + 4OH^– → 4H₂O + 4e^–
• Net Cell Reaction: 2H₂ + O₂ → 2H₂O

This electrochemical process, however, has a slow response rate. A catalyst, such as platinum or palladium, is used to solve this problem. Before being inserted into the electrodes, the catalyst is finely split to maximise the effective surface area.

Fuel cells have a 70% efficiency in the generation of energy, whereas thermal power plants have a 40% efficiency. Because the creation of electric current in a thermal power plant requires the conversion of water into steam and the use of that steam to move a turbine, there is a significant variation in efficiency. Fuel cells, on the other hand, provide a platform for converting chemical energy into electrical energy directly.

Setup of fuel cells

• A fuel cell’s primary function is to generate energy, which may be used to power anything from a single light bulb to an entire city. The generation of electricity in a fuel cell is based on a basic chemical reaction that occurs within the cell. The power is then returned to the cell to complete the electric circuit.
• At the anode, hydrogen atoms are introduced to start the chemical process. At this point, a chemical process removes the electrons from the hydrogen atoms. The hydrogen atoms now have a positive electric charge on them. The cables carry the remaining negatively charged electrons, which create current. At the cathode, oxygen atoms are introduced. They combine with the electrons that the hydrogen atoms have left behind.
• The oxygen atoms, together with the negatively charged electrons, would either unite with the positively charged hydrogen ions at this point or after passing through the anode, depending on the kind of cell.

Advantages of Fuel Cell

Fuel cells are a possible source of electrical energy, and they offer an advantage over galvanic cells and other traditional techniques of generating electricity by burning fuel. The following are some of the major benefits of fuel cells:

1. High Efficiency: Fuel cells are theoretically more efficient than traditional techniques for producing electrical energy, such as burning hydrogen, methane, methanol, carbon fuels, or nuclear reactors, since they transfer the energy of a fuel directly into electrical energy. Fuel cells should theoretically be 100% efficient, but only 60–70% efficiency has been achieved thus far. The efficiency of the traditional approach, which involves burning fuel, is only approximately 40%. The thermodynamic efficiency of a fuel cell, n=ΔG/ΔA×100, where, ΔH is the heat of combustion and ΔG is the work done.
2. Pollution-free Working: The by-products produced by a fuel cell do not pollute the environment. A hydrogen-oxygen fuel cell, for example, generates just water and hence does not contribute to pollution.
3. Continuous Supply of Energy: As long as fuels are supplied into fuel cells, they can provide energy indefinitely. Unlike traditional cells or batteries, these cells do not experience a decline in voltage or current over time.

Limitations of Fuel Cells

1. Gaseous fuel is tough to handle. The fuel gas (hydrogen, oxygen, etc.) must be held as a liquid in a specifically built cylinder at a very low temperature and high pressure. This rise is due to the increased cost of the cell, which comes with a number of practical issues.
2. The catalysts required for electrode reactions, such as platinum (Pt), palladium (Pd), silver (Ag), and others, are highly costly and add to the cell’s cost.
3. The electrolytes employed in fuel cells are extremely caustic, posing a number of practical issues.

Sample Questions

Question 1: What is a fuel cell?

Answer:

Fuel cells are cells that immediately convert a fuel cell’s chemical energy into electrical energy. In the cells depicted below, hydrogen (H₂), carbon dioxide (CO₂), methane (CH₄), propane (C₃H₈), methanol (CH₃OH), and other fuels are utilised to generate electrical energy. Fuel is continually delivered to the fuel cell, while products are continuously withdrawn. On the market, there are many different types of fuel cells. A hydrogen-oxygen fuel cell is the most common form.

Question 2: What are the types of fuel cells?

Answer:

 Few types of fuel cells are Hydrogen-Oxygen fuel cell, Microbial fuel cells (MFCs), Solid Oxide Fuel Cells (SOFCs), Zinc-Air Fuel Cell (ZAFC), Direct Methanol Fuel Cell (DMFC), etc. 

Question 3: Why do we need fuel cells?

Answer:

Fuel cells are in high demand since they are a cost-effective and ecologically beneficial source of power. Because they may be manufactured individually, they can be utilised for a variety of applications.

Question 4: What are the limitations of fuel cells?

Answer:

1. Gaseous fuel is difficult to work with. The fuel gas (hydrogen, oxygen, etc.) must be kept as a liquid at a very low temperature and high pressure in a specially designed cylinder. This increase is due to an increase in the cell’s price, which comes with a variety of drawbacks.
2. The electrode catalysts, such as platinum (Pt), palladium (Pd), silver (Ag), and others, are extremely expensive and add to the cell’s cost.
3. Fuel cell electrolytes are very caustic, which poses a variety of practical challenges.

Question 5: What are hydrogen fuel cells?

Answer:

The hydrogen fuel cell is a device that directly transforms the chemical energy of hydrogen and oxygen into electricity. Water is created as a by-product of this operation.