(A) Principle :
(i) The functioning of the fuel cell is based on the combustion reaction like,
2H2(g) + O2(g) → 2H2O(g) is exothermic redox reaction and hence it can be used to produce electricity.
(ii) The reactants of this fuel cell can be continuously supplied from outside, hence this can be used to supply electrical energy for a very long period.
(B) Construction :
(i) In fuel cell the anode and cathode are porous electrodes with suitable catalyst like finely divided platinum.
(ii) The electrolyte used is hot aqueous KOH solution in which porous anode and cathode carbon rods are immersed.
(iii) H2 is continuously bubbled through anode while O2 gas is bubbled through cathode.
(C) Working (cell reactions) :
(i) Oxidation at anode : At anode, hydrogen gas is oxidised to H2O.
2H2(g) + 4OH-(aq) → 4H2O(I) + 4e-
(oxidation half reaction)
(ii) Reduction at cathode : The electrons released at anode travel to cathode through external circuit and reduce oxygen gas to OH-.
O2(g) + 2H2O(I) + 4e- → 4OH-(aq)
(reduction half reaction)
(iii) Net cell reaction : Addition of both the above reactions at anode and cathode gives a net cell reaction.
2H2(g) + O2(g) → 2H2O(I)
(overall cell reaction)
(D) Representation of the cell :
The overall cell reaction is an exothermic combustion reaction.
However in this,
H2 and O2 gases do not react directly but react through electrode reactions.
Hence,
The chemical energy released in the formation of O-H bonds in H2O, is directly converted into electrical energy.
(E) Advantages :
1. The fuel cell operates continuously as long as H2 and O2 gases are supplied to the electrodes.
2. The cell reactions do not cause any pollution.
3. The efficiency of this galvanic cell is the highest about 70% as compared to ordinary galvanic cells.
(F) Drawbacks of H2 - O2 fuel cell :
1. The cell requires expensive electrodes like Pt, Pd.
2. In practice, voltage is less than 1.23 volt due to spontaneous reactions at the electrodes.
3. H2 gas is expensive and hazardous.
(G) Applications :
1. It was successfully used in spacecraft.
2. It has potential applications in automobiles, power generators for domestic and industrial uses.
