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Chemistry · Ch 5 — Electrochemistry

Hydrogen-oxygen fuel cell

5.11.1

Hydrogen-oxygen fuel cell

In the H2\mathrm{H_2} - O2\mathrm{O_2} fuel cell, the fuel is hydrogen gas and oxygen gas is the oxidising agent. The energy of the combustion of hydrogen is converted into electrical energy.

Figure 5.10Hydrogen-oxygen fuel cell in cross-section: porous carbon anode and cathode flanking the aqueous KOH electrolyte, hydrogen fed at the anode, oxygen at the cathode, and water vapour leaving at the top.
Fig. 5.10 — Hydrogen-oxygen fuel cell in cross-section: porous carbon anode and cathode flanking the aqueous KOH electrolyte, hydrogen fed at the anode, oxygen at the cathode, and water vapour leaving at the top.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your textbook's own diagram.

What this figure shows. The flat schematic of the H2\mathrm{H_2}-O2\mathrm{O_2} fuel cell: H₂ (g) enters at the left past the porous-carbon (anode) - bar, O₂ (g) enters at the right past the + (cathode) bar, the central compartment holds the aqueous KOH electrolyte, and the product water H₂O leaves through the top (up-arrow). The reactant …

Construction : The anode and cathode are porous carbon rods containing a small amount of finely divided platinum metal that acts as a catalyst. The electrolyte is a hot aqueous solution of KOH, in which the carbon rods are immersed, as shown in Fig. 5.10. Hydrogen gas is continuously bubbled through the anode, and oxygen gas through the cathode, into the electrolyte.

Cell reactions

i. Oxidation at anode (-) : At the anode, hydrogen gas is oxidised to H2O\mathrm{H_2O}:

2H2 (g)+4 OH− (aq)⟶4 H2O (l)+4 e−\mathrm{2H_2\,(g) + 4\,OH^-\,(aq) \longrightarrow 4\,H_2O\,(l) + 4\,e^-}

ii. Reduction at cathode (+) : The electrons released at the anode travel through the external circuit to the cathode, where O2\mathrm{O_2} is reduced to OH−\mathrm{OH^-}:

O2 (g)+2 H2O (l)+4 e−⟶4 OH− (aq)\mathrm{O_2\,(g) + 2\,H_2O\,(l) + 4\,e^- \longrightarrow 4\,OH^-\,(aq)}

iii. Net cell reaction : The overall cell reaction is the sum of the electrode reactions (i) and (ii):

2H2 (g)+O2 (g)⟶2 H2O (l)\mathrm{2H_2\,(g) + O_2\,(g) \longrightarrow 2\,H_2O\,(l)}

The overall cell reaction is the combustion of H2\mathrm{H_2} to form liquid water. Interestingly, the fuel H2\mathrm{H_2} gas and the oxidant O2\mathrm{O_2} do not react directly: the chemical energy released during the formation of the O-H bond is directly converted into electrical energy accompanying the above combustion reaction. The cell continues to operate as long as H2\mathrm{H_2} and O2\mathrm{O_2} gases are supplied to the electrodes.

The cell potential is given by

Ecell0=Ecathode0−Eanode0=0.4 V−(−0.83 V)=1.23 VE^0_{cell} = E^0_{cathode} - E^0_{anode} = 0.4\,\mathrm{V} - (-0.83\,\mathrm{V}) = 1.23\ \mathrm{V}

Advantages of fuel cells

i. The reacting substances are continuously supplied to the electrodes. Unlike conventional galvanic cells, fuel cells do not have to be discarded on consuming of the chemicals.

ii. They are nonpolluting, as the only reaction product is water.

iii. Fuel cells provide electricity with an efficiency of about 70 %, which is twice as large when compared with the efficiency of thermal plants (only 40 %).

Drawbacks of fuel cell

H2\mathrm{H_2} gas is hazardous to handle, and the cost of preparing H2\mathrm{H_2} is high.

Applications of fuel cells …

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