Chapter 11 ̶ Fuel Cell

Chapter 11  ̶  Fuel Cell

11.1 Introduction

Generally we use cell/battery for portable use for small power. This cell/battery is sealed. Once it is manufactured it does not require any material from outside. It may be primary or secondary cell. Primary cell cannot be charged, whereas secondary cell can be recharged. In primary cell chemical reaction takes place only in one direction, i.e., it is irreversible. In secondary cell chemical reactions are reversible; therefore it can proceed in both the direction. 

Fuel cell is an open device which needs fuel for operation. Fuel takes part in electrochemical reaction and potential difference is developed between two electrodes. Fuel cell is in operation when fuel is provided to it. It can produce high amount of power. It can produce several kW and MW electricity, so can be used as power plant.

11.2 Principle of Operation of Fuel Cell

In electrolysis electricity is used to produce hydrogen and oxygen gas at electrodes. In fuel cell the process is reversed. In this case hydrogen and oxygen gas combines in electrochemical cell to give electricity and water. A fuel cell consists of two porous electrodes and electrolyte. Here platinum coated graphite plates are used as electrodes. Hydrogen gas in form of fuel is supplied at anode. At anode hydrogen molecules are reduced to hydrogen ions which move on into the electrolyte.  

H₂ (gas) ® 2H⁺ (ion) + 2e^-                      (At anode)         

Electrons liberated at anode give negative potential to electrode. These electrons move toward cathode through external circuit. Oxygen gas is supplied at cathode where it combines with hydrogen ion to give water.

           4H⁺ + O₂ + 4e^- ® H₂O                     (At cathode)

Hence hydrogen gas supplied as fuel at anode gives hydrogen ion whose movement through electrolyte causes electricity. When external circuit is connected current flows through the load.  The overall reaction is

           2H₂ + O₂ ® 2H₂O + electric energy + heat energy

This reaction is exothermic therefore air is circulated to keep the cell cool.

Fuel Cell operation

Reference: 

D.P.Kothari, K.C.singal and R.Ranjan, Renewable Energy Sources and Emerging Technologies Second Edition 2016, Page 171, PHI Learning Pvt. Ltd. New Delhi.

Methanol can also be used as fuel in fuel cell directly or by converting it into hydrogen.

2CH₃OH + 3O₂ ® 2CO₂ + 4H₂O

11.3 Types of Fuel Cell

Fuel cells are categorized on the basis of electrolyte used. On the basis of electrolyte used, temperature process gas and catalyzed are determined.

11.3.1.  Alkaline Fuel Cells (AFCs)

Electrolyte- KOH

Electrodes- porous electrode of carbon catalyzed with nickel

Operating Temperature- 80°C

Fuel- Hydrogen and oxygen

H₂ + 2(OH)^- ® 2H₂O + 2e^-                    (At anode)

½ O₂ + H₂O + 2e^- ® 2(OH)^-                 (At cathode)

H⁺ + OH^- ® 2H₂O                            

As Carbon dioxide present in air combines with KOH to form K₂CO₃, it limits the life of cell. This cell can be best operated in spacecrafts where pure hydrogen and oxygen is used.

11.3.2.  Polymer Electrolyte Membrane Fuel Cell (PEMFC)

            It is also known as Proton Exchange Fuel Cell (PEFC).

Electrolyte- Solid electrolyte which is an ion exchange membrane

Operating Temperature- 80°C - 100° C

Fuel- Hydrogen and oxygen (air)

11.3.3.  Phosphoric Acid Fuel Cell (PAFC)

Electrolyte- H₃PO₄

Electrodes- porous electrode of graphite coated with platinum

Operating Temperature- < 200°C

Fuel- Hydrogen and oxygen (air)

H₂ (gas) ® 2H⁺ (ion) + 2e^-                      (At anode)         

4H⁺ + O₂ + 4e^- ® H₂O                     (At cathode)

2H₂ + O₂ ® 2H₂O + electric energy + heat energy    (Overall reaction)

11.3.4.  Molten Carbonate Fuel Cells (MCFCs)

Electrolyte- Molten mixture of alkali carbonates

Electrodes- nickel

Operating Temperature - 650°C

Fuel- Fuel containing CO₂ and CO

Here CO is oxidized via water-gas shift reaction to CO₂ with the production of hydrogen. The oxidizing agent for hydrogen is carbonate ions which are formed at the cathode. Thus the oxidant gas must contain the CO₂ gas.

H₂ + CO₃^2- ® H₂O + CO₂ + 2e^-                           (At anode)

CO + CO₃^2- ® CO₂ + 2e^-

½ O₂ + CO₂ + 2e^- ® CO₃^2-                                     (At cathode)

11.3.5.  Solid Oxide Fuel Cell (SOFC)

Electrolyte- Solid metal oxide electrolyte (zirconium dioxide)-zirconia

Electrodes- porous electric conductors

Operating Temperature – 800 to 1000°C

Fuel- H₂ and CO mixture

11.4 Advantage of Fuel Cell

        i.             Fuel cell plants are eco-friendly. They have no pollutant and noiseless.

      ii.            No remains are left as in coal, oil or gas power plants in form of ash or gases from chimney

    iii.            It can be installed in isolation, so no power is lost in transmission.

    iv.            High efficiency upto 55% in comparison to 30% of thermal power plant.

      v.            Many fuel cells can be added according to the requirement.

    vi.            May be operated by waste fuel.

  vii.            It can supply heat, hot water and steam. It has cogeneration capability.