fuel cell

A fuel cell is a battery that never runs flat as long as you keep feeding it fuel — it is an electrochemical engine that converts hydrogen and oxygen straight into electricity, with only water and heat as exhaust. Hydrogen flows over one electrode where a catalyst strips off its electrons (sending them through your circuit as current) and lets the bare protons cross a membrane; on the other side they reunite with oxygen from the air and the returning electrons to make H₂O. No combustion, no moving pistons — just a quiet, continuous flow of charge.

Because it sidesteps the heat-then-spin route of an engine, a fuel cell isn't shackled by the Carnot limit and can reach 50–60% efficiency, far above a car engine's ~25%. Its appeal is range and refuel speed: a hydrogen tank refills in minutes and carries far more energy per kilogram than any battery, which is why fuel cells suit trucks, ships, and backup power. The headaches are storing hydrogen (bulky and explosive), pricey platinum catalysts, and the energy lost making 'green' hydrogen in the first place.