A fuel cell is a new promising power source, because it has the least environmental impact in comparison to conventional fossil fuel. As an alternative and renewable energy source, it helps in reducing the exhaustion of fossil fuels. Fuel cells have high efficiency, longer operation, so they are widely used for different applications as portable power sources. Basically, fuel cells are electrochemical devices that convert chemical energy from a fuel into electrical energy without a combustion process. They differ from storage batteries in the sense that fuel cells need fuel supply from an external source whereas a battery stores electricity inside it.
In 1839 Sir William Grove discovered fuel cell technology, so he is known as 'father of fuel cell. However, the first fuel cell devices were invented by an engineer Francis Bacon in 1934. Fuel cells found practical application when NASA developed fuel cell technology in 1950 as a power source for space travel and also used fuel cells in its spacecrafts like Gemini, Apollo and Skylab.
In India, Bharat Heavy Electricals Limited is developing fuel cell technology in phosphoric acid fuel cells (PAFC) in capacities of 1, 5, 10, and 50 kW ratings.
TERMINOLOGY USED IN FUEL CELL TECHNOLOGY
Anode - Electrode where oxidation (loss of electrons) takes place. It is the negative terminal in the fuel cell.
Cathode - Electrode where reduction (gain of electrons) takes place. It is the positive terminal in a fuel cell.
Electrolyte - A chemical compound that conducts ions from one electrode to another.
Stack - A number of individual fuel cells connected in series to increase voltage.
Membrane - Layer separating the anode and cathode and allowing hydrogen ions (H) and oxygen ions (0) to pass through it. It acts as an electrolyte in fuel cells.
Inverter - A device which converts direct current (de) into alternating current (ac).
Ionization - Process in which loss or gain of electrons to form positive or negative ions.
Fuel cell - Device that converts chemical energy of a fuel into electrical energy without combustion.
PEM - An acronym used for Polymer Electrolyte Membrane; and also used for Proton Exchange Membrane.
MCFC - Molten Carbon Fuel Cell
PAFC - Phosphoric Acid Fuel Cell
SOFC - Solid Oxide Fuel Cell
DMFC - Direct Methanol Fuel Cell
AFC - Alkaline Fuel Cell
Reformer - Device which extracts pure hydrogen from fuel; also known as processor
Cogeneration - Generation of electricity and useful heat in a single installation
CONSTRUCTION AND WORKING OF A FUEL CELL
A fuel cell consists of two porous electrodes, i.e., an anode and a cathode which are separated by an electrolyte. In PEM fuel cells a proton exchange membrane is used as an electrolyte. The anode is supplied with hydrogen and the cathode is supplied with oxygen. At the anode the hydrogen molecules split into electrons and hydrogen ions(protons).
When the anode and cathode are electrically connected, the protons move to the cathode passing through the electrolyte. The electrons run via an external electrical circuit to the cathode while supplying an electrical load. At the cathode, electrons and protons react to form water with the supplied oxygen.
CHEMICAL REACTIONS IN A FUEL CELL
At anode:
2H2 → 4H+ + 4e-
The electrons so liberated at the anode flow through the external circuit to the cathode. H+ ions move from anode to cathode through electrolyte.
At cathode:
O2 + 4e- → 2O--
4H+ + 2O-- → 2H2O
Hydrogen ions combine with oxygen ions and produce water and beat.
e- → direction of flow of electrons
i → direction of flow of electric current
LOSSES AND EFFICIENCY
There are four types of losses occuring in a fuel cell as described below:
1. Activation losses: These losses are due to slowness of reaction taking place on the surface of the electrodes. So there is a voltage drop due to this loss to drive the chemical reaction that transfers electrons.
2. Ohmic losses: Because of the internal resistance of the fuel cell there is a voltage drop (i.r). An internal resistance (r) in the fuel cell is due to resistance of electrolyte and the contact resistance between the electrode and electrolyte.
3. Concentration losses: These losses are due to two factors:
- slow ion movement in the electrolyte, causing a change in concentration at the electrode.
- slow movement of reactants through porous electrodes.
4. Fuel cross over losses: These losses are due to the wastage of fuel passing through the electrolyte.
Maximum efficiency of an ideal fuel cell:
The efficiency of a fuel cell is given by
η = (Output energy/Input energy) ༝ 100
The efficiency of an ideal hydrogen - oxygen (H2 - O2) fuel cell is calculated as under:
output energy = 237 kJ
losses in the fuel cell = 48.7 kJ
input energy = Output energy + Losses
= 237 + 48.8
= 285.8 kJ
η = (237/285.8) ༝ 100
η = 83%
This is the maximum efficiency that can be achievable by a fuel cell under ideal conditions. However, actual fuel cell produces more heat and hence more losses giving the efficiency lower than 83%.
OUTPUT VOLTAGE OF AN IDEAL FUEL CELL
A hydrogen fuel cell has a maximum theoretical voltage of 1.23 V (which is also the minimum voltage needed for decomposition of water by electrolysis). In actual practice a fuel cell has an operating voltage of 0.6 V to 0.8 V.
For obtaining higher voltages individual cells are connected in series in stacks. Stacks are connected in parallel to get higher currents.
The maximum current which can be produed by a cell is proportional to the surface area of the electrodes.
FUEL CELL TECHNOLOGY
The main objective of fuel cell technology is the development and application of fuel cell at high conversion efficiency. This includes selection of cell materials, stack, operating temperature, etc. and to solve the main problems encountered in fuel cells. Some of these problems are:
1. Slow reaction rate leading to low current and power.
2. Hydrogen as fuel source is not readily available.
To solve these problems, different technologies have been tried on different types of fuel cells, some of which are discussed below:
1. Alkaline fuel cell (AFC): In this type of fuel cell an alkaline solution of potassium hydroxide (KOH) is used as electrolyte. The fuel cell operates at a temperature of about 80°C. The reactants are hydrogen (H₂) and oxygen (O₂) from air. They combine to form water.
Reactions:
At anode H₂ + 2 (OH) → 2 H₂O + 4e¯
At cathode
O₂ + 2H₂O + 4e¯→ 4 (OH-)
H+ + OH → H₂O
In this fuel cell, hydrogen is applied to anode, which reacts with hydroxide (OH) ion present in the electrolyte and forms water and releases electrons. These released electrons flow through external circuit. Oxygen is applied to cathode where it reacts with water present in the electrolyte and electrons picked up from anode to form (OH-) ions. These (OH) ions combine with (H+) ions to form water.
The efficiency of an alkaline fuel cell is about 70%.
2. Polymer electrolyte membrane fuel cell (PEMFC): The polymer electrolyte membrane fuel cells are also called proton exchange membrane fuel cells. In this type a solid electrolyte in the form of membrane is used. The thickness of the membrane is 0.076 cm or 0.76 mm. The membrane which is kept in between the anode and the cathode allows ions to pass through it and is impermeable for gases.
The operating temperature range is 40°C to 100°C and the efficiency is about 60%. These types of fuel cells are suitable for power output below 1 MW.
3. Phosphoric acid fuel cell (PAFC): The electrolyte used in this fuel cell is phosphoric acid (H₂PO₂). At the anode hydrogen gas is converted into hydrogen ion (H+) and electrons.
H₂ → 2H+ + 2 e
The H+ ions migrate from anode to cathode through electrolyte and electrons flow through the external circuit.
At cathode, H+ ions react with oxygen (0₂) and produce water.
O₂ + 4H+ + 4e¯ → 2H₂O
The operating temperature range is 150-220 °C. Efficiency of the phosphoric acid fuel cell (PAFC) is more than 40% and can be increased to about 80% with cogeneration plant.
4. Direct methanol fuel cell (DMFC): The main difference between PEMFC and DMFC is that of fuel source. H₂ is used in PEMFC whereas methanol is used directly as source of fuel in DMFC.
Reactions involved are:
At anode: CH₂OH + H₂O → CO₂ + 6H+ + 6e
At cathode: 30₂ + 12H+ + 12 e¯ → 6H₂0
The direct methanol fuel cell has efficiency about 40% and operating temperature in the range of 50°C to 120°C. Main advantage is that no reformer or processor is required as methanol is used directly as fuel.
High temperature fuel cells: The main types of fuel cells working at high temperature are described below:
(a) Molten carbonate fuel cell (MCFC): The molten mixture of alkali carbonates are used as electrolyte. Fuel is reformed from other sources into H₂ and CO.
It operates in the temperature range of 600°C to 700°C. The fuel cells are available in the capacity range of 300 kW to 3 MW, mainly used as combined heat and power (CHP) plant. The efficiency is of the order of 50%.
(b) Solid oxide fuel cell (SOFC): Here solid electrolyte of zirconium dioxide called Zirconia is used. The fuel source is a mixture of H₂ and CO obtained by reforming of natural gas.
The fuel cell operates in the range of 700°C to 1000°C. Capacities are available in the range of 1 kW to 2 MW and the efficiency is about 60%. It can also be used as combined heat and power (CHP) plant.
SOURCES OF FUEL CELL
Hydrogen is the main fuel for all fuel cells. As the pure hydrogen gas is not available generally, so in that case it has to be extracted from other sources. These sources are given below:
1. Hydrogen (H₂): It is an important source of fuel cells though it is not available directly in nature. It is to be produced from different raw materials and stored. However, storage of pure hydrogen is very expensive. It is generally reformed (or processed) from raw materials and used in fuel cell.
2. Hydrazine (N₂H₂): It is a liquid fuel, so it is convenient to store. In some types of fuel cells it can be used without transformation.
3. Ammonia (NH3): It is an indirect source of fuel. It is generally available in gaseous form. However, it can be stored in liquid form. It is decomposed into a mixture of hydrogen (H₂) and nitrogen gas (N₂).
This mixture is used as fuel and nitrogen is discharged.
4. Liquid hydrocarbon: Light hydrocarbons also called naphtha are reformed into H₂ and CO. The mixture of this product can be used as fuel.
5. Gaseous hydrocarbons: These are hydrocarbons in gaseous forms like methane, propane, etc. They are decomposed into a mixture of H₂ and CO at high temperature. This mixture is then used as fuel source.
6. Methanol: It is used both as indirect and direct type of fuel source. It is reformed into hydrogen (H₂) and carbon-monoxide (CO) at high temperature (200°C). Methanol can be used directly as fuel source in Methanol Fuel Cell (MFC).
ADVANTAGES OF FUEL CELL
1. There is direct conversion of fuel into electrical energy. So air pollution is practically nil as there is no combustion of fuel.
2. Fuel cells have higher efficiencies than diesel or gas engines.
3. Quiet operation, therefore no noise pollution, particularly suitable for military applications.
4. Operating time is much longer as compared with batteries.
5. Fuel cells produce high quality power.
6. No cooling water is required so it can be located at any place.
7. Output of fuel cell power plants can be increased easily by stacking fuel cells. 8. Land requirement for fuel cell power plants is much less as compared to conventional power plants.
9. Maintenance cost is low as there are no moving parts.
10. They produce only water and heat as a by-product.
11. Transmission and distribution cost is practically nil as they can be located near the load.
12. Commercial fuels such as LPG, natural gas, biogas, etc., can be used for reforming (processing).
13. Fuel cells have cogeneration capabilities.
DISADVANTAGES OF FUEL CELL
1. Production, transportation and storage of hydrogen is difficult.
2. Reforming (processing) of hydrocarbons for producing hydrogen is a technically challenging job.
3. Size of fuel cells is bigger than batteries.
4. Fuel cell technology is not yet fully developed and only few
products are available.
5. Fuel cells use expensive materials (like platinum as a catalyst).
6. Capital cost of fuel is very high.
7. Fuel cells give the output so for ac power supply additional equipment inverter is required to convert dc into ac.
USES OF FUEL CELLS
Fuel cells can be made into stacks and modules, so they can be used for all kinds of applications from portable devices to power plants. Some of the uses of fuel cells are given below:
1. Military applications: As the fuel cells operate quietly they are very useful at strategic locations. They generate electricity without emission of gasses. So they can be used as electric generators without being detected by the enemy.
2. Space applications: The fuel cells can be used as a power source in space crafts. In fact hydrogen-oxygen (H₂-O₂) alkaline fuel cells had been used in Apollo missions by NASA.
3. Automobile applications: The fuel cells in combination with batteries can be used to power vehicles without refueling. The phosphoric acid fuel cell (PAFC) with methanol as fuel can be suitably developed for transport systems.
4. Electricity generation: Fuel cells can be used for commercial generation of electrical power. The fuel cells can be used as peak load power plants as hydrogen storage could be used to meet additional demands during peak periods.
5. Off-grid power application: They can be used as off-grid power plants for small villages, remote locations and other areas inaccessible by state power grids.
6. Combined heat and power (CHP) applications: Where the fuel to electricity conversion efficiency is low, the fuel energy which is not converted into electrical energy, i.e., wastage can be utilized for heating purposes. This type of plant is also known as cogeneration plant.
7. Portable devices application: The small fuel cells are also called micro fuel cells. They are portable and are used in digital cameras, laptops, mobile phones, etc. They have longer life as compared to lithium batteries.
ENVIRONMENTAL IMPACT OF FUEL CELLS
1. Water and heat are byproducts of fuel cells. They do not cause environmental pollution so their impact on the environment is nil.
2. No cooling water is required as heat can be used in cogeneration plants or for fuel reforming processes.
3. Amount of carbon dioxide (CO₂) emission to the atmosphere is the least as compared to other conventional power plants. Other pollutants are also negligible.
4. The storage of hydrogen poses risks as it is highly inflammable. Hydrogen can be liquified and stored in large cryogenic insulated vessels. But liquifying hydrogen is very expensive.
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