Principle of Operation of Fuel Cells
As stated above, a fuel cell is an electro-chemical device in which the chemical energy of fuel is continuously converted into electric energy. This conversion of energy takes place at constant pressure and temperature.
For explaining the principle of operation of fuel cells, here we shall considera hydrogen (H2)-oxygen (O2) called Hydrox fuel cell.
The main components of a fuel cell are :
- A fuel electrode (anode)
- An oxidant electrode (cathode)
- An electrolyte (a solution of H2SO4 for acidic fuel cell and KOH for alkali fuel cells).
- Additional components are container, separators, scaling’s, fuel and oxidant supply etc.
The basic feature of a fuel cell is that the fuel and the oxidant are combined in the form of ions than in form of neutral molecules.
Figure A represents the schematic diagram of a fuel cell using hydrogen as fuel and oxygen as oxidant and alkaline solution of KOH as electrolyte.
It is called alkaline fuel cell (AFC). It consists of two permeable Nickel electrodes immersed in an electrolyte of good conductivity. An electrolyte may be an alkaline solution of KOH as shown or acidic solution of H2SO4 called respectively as alkali fuel cells and acidic fuel cells.
The porous fuel electrode is anode ( negative pole ) and the other porous oxidant Electrode is cathode (positive pole).These electrodes are separated by a porous gas barrier called separator (not shown in Figure).
The anode is supplied H2 gas as fuel at a certain pressure and the cathode is supplied O2 as oxidant at a pressure. These gases pass through the respective electrodes and bubble around through the electrolyte solution. The pores provide an opportunity to gases, electrodes and electrolyte to come in contact for their electrochemical reactions.
The electrodes are connected through an external circuit as shown in Figure A. The electro-chemical reactions are generally slow and a catalyst is required in the electrodes to accelerate the reaction. Platinum is the best catalyst but costly. In general, less expensive catalysts like nickel and silver are used according to application and design.
Chemical reactions with alkaline H2 – O2 fuel cell : + 2e
The hydrogen gas is ionized at anode and it produces a free electron and H+ions. Every hydrogen molecules brought to electrode surface is dissociated into two H atoms by catalytic property of electrode. These enter into electrolyte solution as hydrogen ions leaving behind two electrons which pass through the external circuit to the cathode (positive electrode). The reaction at anode is as follows :
Anode : H2 → 2 H+ + 2e …(i)
The oxygen suplied to cathode (positive electrode) reacts with water of electrolyte the electrons transmitted to it to produce hydroxyl (OH) ions. Thus,
Cathode :
1/2 O2 + H2O + 2e → 2OH– …(ii)
These hydroxyl ions migrate from cathode to anode through electrolyte. The hydrogen and hydroxyl ions then combine in the electrolyte to produce water i.e.
2 H++ 2OH– → 2 H2O ….(iii)
Above equation shows that OH ions produced at one electrode (cathode) are involved in a reaction at the other electrode (anode). By adding the above three equations, the overall process is chemical reaction of H2 and O2 gases to form water i.e.
H2 + 1/2 O2 → H2O ….(iv)
Thus the net reaction of a fuel cell in which hydrogen and oxygen supplied is the produce water, electrical energy and hear.
Chemical reaction with acidic electrolyte (H2SO4) :
Anode : H2 → 2 H+ + 2e …(v)
Cathode : 1/2 O2 + H2O + 2e → 2OH– …(vi)
Overall reaction : H2 + 1/2 O2 → H2O ….(vii)
Thus the chemical reaction is similar to alkaline electrolyte.
H2 → O2 fuel cells are efficient in operation. The operating temperatures are generally in the range of 100°C to 200°C. The ideal c.m.f. produced by a single fuel cell is 1.23 V at one atmosphere and 25°C. Which reduces to 1.15 V at 200°C. The actual value of e.m.f. decreases with current and at rated value of current the c.m.f. lies between 0.7V to 0.8V.
The rated voltage and current of fuel cells can be increased by using number of cells in series-parallel combination. The expected life of a fuel cell is ten thousand hours.