Fuel Cells

FUEL CELLS

Definition

Fuel cell is a voltaic cell, which converts the chemical energy of the fuels directly into electricity without combustion.

Fuel cell converts the energy of the fuel directly into electricity. In these cells, the reactants, products and electrolytes pass through the cell.

Fuel + Oxygen → Oxidation products + Electricity .

Example

Hydrogen-oxygen fuel cell.

Fuel Battery

When a large number of fuel cells are connected in series, it forms fuel battery.

1. Hydrogen-Oxygen fuel cell

Hydrogen-oxygen fuel cell is the simplest and most successful fuel cell, in which the fuel-hydrogen and the oxidiser-oxygen and the liquid electrolyte are continuously passed through the cell.

Description

It consists of two porous electrodes anode and cathode. These porous electrodes are made of compressed carbon containing a small amount of catalyst (Pt, Pd, Ag). In between the two electrodes an electrolytic solution such as 25% KOH or NaOH is filled. The two electrodes are connected through the volt meter.

Working

Hydrogen (the fuel) is bubbled through the anode compartment, where it is oxidised. The oxygen (oxidiser) is bubbled through the cathode compartment, where it is reduced.

Fig 8.5 H2-02 Fuel cell

Various reactions

At Anode

Hydrogen gås, passed through the anode, is oxidised with the liberation of electrons which then combine with hydroxide ions to form water.

At cathode

The electrons, produced at the anode, pass through the external wire to the cathode where it is absorbed by oxygen and water to produce hydroxide ions.

Overall cell reaction

The emf of the cell = 0.8 to 1.0 V

Applications of H₂ - O₂ fuel cell

1. H₂ - O₂ fuel cells are used as auxiliary energy source in space vehicles, submarines or other military-vehicles.

2. In case of H₂ - O₂ fuel cells, the product of water is proved to be a valuable source of fresh water by the astronauts.

Advantages of H₂ - O₂ fuel cell

(i) It emits only water vapour and no other harmful chemicals to the environment.

(ii) Efficiency is more than 75%.

(iii) As hydrogen is the lightest element, it can be transported easily from one place to another.

(iv) Hydrogen, as a fuel, can replace the use of batteries.

(v) It causes less noise pollution.

Limitations (or) Disadvantages of H₂ - O₂ fuel cell

1. Hydrogen gas is explosive.

2. It is very expensive to be carried out.

3. As hydrogen is a gas, it is difficult to compress into liquid form.

4. Hydrogen is not present as it is, but always present in combined form with either oxygen or some other element, so it must be separated first.

5. While using H₂ - O₂ fuel cell in an automobile, a high pressure must be created inside the engine, which is risky.

 

2. Advantage and Disadvantages of Fuel cells

Advantages of Fuel cells

1. Fuel cells are efficient (75%) and take less time for operation.

2. It is pollution free technique.

3. It produces electric current directly from the reaction o a fuel and an oxidiser.

4. It produces drinking water.

Disadvantages of Fuel cells

1. Fuel cells can not store electric energy as other cells do.

2. Electrodes are expensive and short lived.

3. Storage and handling of hydrogen gas is dangerous.

 

3. Microbial Fuel Cells (MFCs)

Microbial fuel cell is a device that converts chemical energy to electrical energy by the action of micro-organisms under anaerobic conditions.

Bioelectricity is generated by the oxidation of organic waste and renewable biomass using bacteria.

Construction (or) Principle

MFCs are type of electrochemical cells, constructed using either bioanode and (or) a biocathode. A membrane separates the compartments of the anode (where oxidation occurs) and the cathode (where reduction occurs). The electrons produced during oxidation are transferred directly to the cathode. The charge balance of the system is maintained by the ionic movement inside the cell.

Organic electron donors, that is oxidized to produce CO₂, protons and electrons, are used in most MFCs. The cathode reaction uses a. variety of electron acceptors, mostly oxygen (O₂).

Components

typical MFC consists of the following two compartments.

(i) Anodic compartment

(ii) Cathodic compartment

(i) Anodic compartment

It consists of microbes suspended under anaerobic conditions in the anolyte.

(ii) Cathodic compartment

It consists of electron acceptor (oxygen)

Permeable membrane

Anionic and cationic compartments are separated by a selectively permeable, cation-specific membrane.

Fig. 8.6 Microbial Fuel cell

Working

When both the electrodes are connected, anode oxidation occurs on organic waste (biomass) and electrons released from the process are transferred to the anode. The electrons, transfered to the anode, can be accomplished by the electron mediators.

From the anode these electrons are directed to the cathode across an external circuit. For every electron, conducted, a proton is transported across the membrane to the cathode. Finally oxygen present at the cathode recombines with hydrogen and electron to produce water.

Applications

1. In waste water treatment, MFCs, generate less excess sludge as compared to the aerobic treatment process.

2. MFCs can be used in river and deep-water environments, where it is difficult to use batteries.

3. MFCs are used to convert carbon rich wastewater into methane gas.

4. MFCs are used as convenient biosensor for waste water streams.

5. MFCs are used in space, especially to operate remotely operated vehicles.

6. MFCs play an important role in the field of microbiology, soil chemistry and electrical engineering.

7. Many commercial soil based MFC kits are available for the purchase on the web and in toy stores.