Understand the hydrogen–oxygen fuel cell

In a hydrogen–oxygen fuel cell, hydrogen and oxygen react electrochemically:

$2H_2 + O_2 \rightarrow 2H_2O$

Unlike ordinary combustion, this reaction is carried out in such a way that the chemical energy is directly converted into electrical energy.

Electrode reactions

At the anode, hydrogen is oxidized:

$2H_2 + 4OH^- \rightarrow 4H_2O + 4e^-$

At the cathode, oxygen is reduced:

$O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$

These reactions produce a flow of electrons through the external circuit.

What is the main purpose of combustion of hydrogen here?

The oxidation (combustion) of hydrogen in the fuel cell leads to separation of charges and electron flow, which creates an electrical potential difference between the two electrodes.

Check the options

• A: generate heat
  Heat may be produced as a by-product, but this is not the main purpose of a fuel cell.

• B: remove adsorbed oxygen from electrode surfaces
  This is not the function of hydrogen combustion in a fuel cell.

• C: produce high purity water
  Water is formed, but the primary objective of the fuel cell is to produce electricity, not water.

• D: create potential difference between the two electrodes
  This is correct, since the electrochemical reaction generates electrical energy by establishing a potential difference.

Final answer

Therefore, the correct option is:

$\boxed{\text{D}}$