Fuel cell chemistry

The fuel cell belongs to a branch of chemistry called electrochemistry. This explains how electricity can be derived from a chemical reaction.

In nature certain materials will react with one another spontaneously. Sulphuric acid will dissolve metals. The two components of an epoxy glue such as Araldite react when mixed to form a tough adhesive. Natural gas burns in air to produce heat.

The particular reaction that is important to the fuel cell is that between hydrogen and oxygen. As the Hindenburg airship disaster graphically illustrated to the world in 1937, hydrogen will burn in oxygen or air releasing a vast quantity of energy in the form of heat. In the fuel cell this energy does not appear as heat. Instead it emerges as electrical energy.

The reaction between hydrogen and oxygen can be expressed by a simple chemical formula:

Two hydrogen molecules and one oxygen molecule react to create two molecules of water. In fact the reaction can be broken down into two separate halves, one involving hydrogen and one involving oxygen. The hydrogen half involves the hydrogen molecule, H2, splitting into two hydrogen atoms, H, and each of these releasing an electron to form a positively charged hydrogen ion, a proton:

On the oxygen side, the oxygen molecule, O2, also splits into two oxygen atoms, O, and each of these absorbs two electrons released from two hydrogen atoms to produce a doubly negatively charged oxygen ion, O2~:

Now the negatively charged oxygen ion will attract two positively charged hydrogen atoms and they will coalesce to form a water molecule, H2O:

Then the reaction is complete.

When hydrogen burns in air, the various steps of the reaction occur in the same place at the same time. But in a fuel cell the hydrogen and oxygen are not allowed to mix.

In the fuel cell the hydrogen is supplied to one electrode of the cell and oxygen to the other. The two electrodes are separated by a material called the electrolyte. This electrolyte is impermeable to the gases. It will not conduct electricity either. What it will do is conduct positively charged hydrogen ions.

So at the hydrogen electrode (called the anode) the hydrogen molecules first separate into atoms and then release electrons to form positively charged ions. Only then can the hydrogen cross the electrolyte boundary and reach the oxygen at the second electrode. But at that electrode (called the cathode) it will find only oxygen atoms and molecules; these still need to pick up the electrons that the hydrogen atoms released at the anode of the cell if they are to complete the reaction.

Now electrons are what produce an electrical current. If a wire is connected between the anode and the cathode of the cell, the electrons will rush from one to the other in order to complete the reaction. Put a small electric light bulb in the circuit and it will glow, proving that there is indeed a current flowing. The fuel cell allows the hydrogen to get to the oxygen one way, but forces electrons to take a different route. That is how it works.

The electrolyte is the key to the operation of a fuel cell and the different types of fuel cell under development are normally identified by the electrolyte each uses. In the illustration below the electrolyte only allowed

Load

Depleted fuel out

H2 fuel in

Electrolyte (ion conductor)

Depleted fuel out

H2 fuel in

Air/oxygen

Depleted air/oxygen — and water out

Electrolyte (ion conductor)

Figure 7.1 The principle of the fuel cell

Air/oxygen

Depleted air/oxygen — and water out

Figure 7.1 The principle of the fuel cell charged hydrogen atoms to pass. This is the basis of several fuel cells. Others will only allow charged oxygen atoms to pass and in yet other cases the electrolyte is permeable only to a more complex charged molecule. And in every case the electrolyte must not allow electrons to pass through it from one electrode to the other. That would create a short circuit.

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