Fuel Cells are small device used to produce sufficient amounts of power for devices of varying size.
Fuel Cells contain honeycomb shaped structure, during which chemicals are pushed few. The chemical reactions directly generate electrical charges. Fuel Cells can last much longer than mere batteries, sometimes even lifetimes.
The most common form uses simple water. In a fuel cell, water is broken down into Oxygen and Hydrogen, generating a charge as it moves through the layers. The atoms then immediately bond back together repeating the process.
A thing to note is that most forms of fuel cells are explosive.
Fuel cells are used to power small vehicles, robots and even small single or double person ships. While present in starships, they are not the main power source, serving more as emergency power at beast.
The most common variants are water based and Magnesium Fuel Cell, which both use sea water,