Step 1: A fuel cell is an electrochemical device in which the energy of oxidation (burning) of a fuel is tapped as electricity instead of being wasted as heat. Because fuel and oxidant are pumped in continuously, the cell does not run down like a dry cell; it is really a non-stop galvanic cell.
Step 2 (set-up): Picture two hollow porous carbon rods dipping into hot concentrated KOH. Each rod carries a trace of platinum catalyst. Hydrogen is forced through the left rod (this becomes the negative anode) and oxygen through the right rod (the positive cathode). A wire with a bulb/voltmeter connects the two rods outside the cell.
Step 3 (diagram description): Left electrode labelled "H2 in, anode (–)", right electrode labelled "O2 in, cathode (+)", the space between labelled "KOH electrolyte", and the external wire labelled "e– flow (anode → cathode)".
Step 4 (chemistry): On the hydrogen side, hydrogen is oxidised by hydroxide ions:
\(2H_2 + 4OH^- \rightarrow 4H_2O + 4e^-\).
On the oxygen side, oxygen is reduced and regenerates hydroxide:
\(O_2 + 2H_2O + 4e^- \rightarrow 4OH^-\).
Adding the two half-reactions cancels the electrons and OH–, leaving the overall reaction \(2H_2 + O_2 \rightarrow 2H_2O\), which is just the controlled burning of hydrogen.
Step 5: Advantages: high efficiency (~70%), continuous operation, and water as the only, harmless product, making it a clean energy source used in spacecraft.