Here's why:

* Electrochemical Reaction: Fuel cells work by converting chemical energy directly into electrical energy through an electrochemical reaction.

* Hydrogen as the Fuel: Hydrogen is a highly reactive element that readily participates in this reaction.

* Oxygen as the Oxidant: The other key component is oxygen, which acts as the oxidant in the reaction.

* Water as the Byproduct: The primary byproduct of this process is water, making fuel cells a clean and efficient energy source.

Important Note: While hydrogen is the most common fuel for fuel cells, other fuels are being explored, such as:

* Methanol

* Ethanol

* Biodiesel

* Natural gas

These fuels are often processed to produce hydrogen before being used in the fuel cell.