The Fuel Cell

The hydrogen fuel cell is in principle the simplest.

The basic cell consists of two porous carbon cloth electrodes bonded to a polymer electrolyte membrane. Outside the electrodes are flow field plates. These contain channels to ensure that the gases are in  contact with the whole surface of the electrodes. They also serve to remove the water which is produced.

Oxidation (loss of electrons) occurs at the anode and reduction (gain of electrons) at the cathode. The fuel – in this case, hydrogen – is oxidised at the anode and releases electrons. These electrons can flow from the anode (which therefore becomes the  negative terminal of the cell) around the external circuit to the cathode (which is therefore the positive terminal). Hydrogen ions flow through the polymer electrolyte membrane to the cathode to balance the charges.

A fuel cell can therefore supply a current in the same way as a storage cell (battery). However, unlike a battery, a fuel cell needs no recharging and its electrodes are unchanged.

The reactions occurring in the cell are:

At the anode: 2H2(g) ➝ 4H+ + 4e–

At the cathode: O2(g) + 4H+ + 4e– ➝ 2H2O(l)

Overall: 2H2(g) + O2(g) ➝ 2H2O(l)

A hydrogen fuel cell has a maximum theoretical voltage of 1.23 V (which is also the minimum voltage needed for the decomposition of water by electrolysis). In practice, because of losses caused by back-reaction, internal resistance and inefficient diffusion of the gases, the voltage obtained is 0.6 – 0.9 V, direct current (DC).

Larger voltages are obtained by connecting cells in series, in stacks. Stacks connected in parallel provide larger  currents. The maximum current which can be produced by a cell is proportional to the surface area of the electrodes. At the moment the maximum current density (current per unit surface area) obtainable is about 2 A cm–2. Reaction at the electrodes would be far too slow without a catalyst, which for hydrogen fuel cells is usually platinum.

Because fuel cells can be made into stacks and modules, they can be used for all kinds of applications – from portable devices to power plants.