What is the outcome of the reaction with hydrogen fuel cells?

2 H2 molecules ad 1 O2 molecule form 2 water molecules. This is a very exothermic reaction releasing lots of energy, here released as a flow of electrons (i.e. electrical current).

The outcome of the chemical reaction taking place in a hydrogen fuel cell is the same as that of the combustion of hydrogen in oxygen (this is generally the case for any fuel used in a fuel cell - products are simply the same as combustion products). This can be written either as a word equation:

hydrogen + oxygen ---> water

Or as a balanced symbol equation

2 H2 + O2 ----> 2 H2O

Technically this reaction produces the same amount of energy as combustion of hydrogen would, but there is one key advantage of using a fuel cell:

The chemical energy is converted directly into electricity, so a hydrogen fuel cell is a more efficient way of producing electricity than direct combustion of hydrogen which would require converting heat to electricity.