Q 1220423311.     A fuel cell is a device to convert the energy of a fuel into electrical energy without the use of heat engine. Such conversions are possible because the combustion reactions are essentially redox reactions. Electrical energy can be obtained indefinitely from a fuel cell as along as the outside supply of fuel is maintained. The essential components of hydrogen-oxygen fuel cell are detailed in the figure below


From the figure, it can be seen that hydrogen and oxygen gases are supplied to the cell. Hydrogen molecules supplied at the anode undergo oxidation in presence of metal catalyst. The hydrogen ions migrate through the electrode while the electrons travel through the external circuit.

At the cathode, the electrons and hydrogen ions combine With `O_2` molecules to form water. The following half reactions take place at anode and cathode.

Anode : `2H_(2(g)) + 4OH_((aq))^ → 4H_2O_((l)) + 4e^-`

Cathode:`O_2(g) + 2H_2O_((l)) + 4e^ → 4OH_((aq))^-`

Overall reaction `:2H_2(g) +O_(2(g)) → 2H_2O_((l)) `

The overall reaction has a value of `ΔH° = -285.8 kJ mol-1` and `ΔG° = -237.39 k J` `mol^(-1) at 25°C`.

Calculate E° for the half reaction

`2H_2O(l) +2e^- → H_(2(g)) + 2OH_((aq))^-`

If the cell voltage of hydrogen-oxygen fuel cell is 1.23V, and the potential for the half cell reaction is as given below.

`O_(2(g)) + 2H2O(l) + 4e^-→ 4OH_((aq))^- E°= 0.40 V`