The hydrogen is stored in a super-insulated, double-walled tank (usually made of carbon fiber-reinforced plastic) under high pressure of 350 or 700 bar. In the fuel cell, the hydrogen reacts with oxygen in the air to form water, generating electrical energy and heat in the process. This means that no pollutants are emitted from the exhaust, only water vapor. The electrical energy generated is then used by an electric motor to power the car. In addition, a small battery is installed to temporarily store the electricity generated.

Source: Deutsche Energie-Agentur GmbH (dena) www.pkw-label.de [Online]
[Zitat vom 04.11.2021]
https://www.pkw-label.de/alternative-antriebe/brennstoffzellenfahrzeuge-fcev