A fuel cell is a chemical plant the size of an engine. Hydrogen from the tanks meets oxygen from the air across a membrane, and the reaction produces electricity, heat and water vapour. Nothing burns and there are no pistons. The electricity goes to an electric motor, so a fuel-cell car drives like a battery-electric one: instant torque, one gear, regenerative braking. A small battery sits between the stack and the motor to catch that regen and to cover the moments when the driver asks for more power than the stack can deliver quickly.
What changes is the refuelling. Hydrogen is stored as a compressed gas, usually at 700 bar, in tanks wound from carbon fibre because no reasonable thickness of steel would do the job at that pressure and weight. The BMW iX5 Hydrogen carries seven of them under the floor, holding at least 7 kg between them, and BMW quotes a full refill in under five minutes. That is the case for the technology in one sentence, and it is why fuel cells keep being proposed for the vehicles a charging stop hurts most: long-distance trucks, buses on fixed routes, and large heavy cars.
Two things have kept the segment small. One is the station network, which exists in useful density in Japan, South Korea, California and parts of Germany, and barely at all elsewhere; a fuel-cell car outside that footprint is unusable in a way a battery car never quite is. The other is the hydrogen itself. Most of what is produced today comes from natural gas, and the emissions saved at the exhaust are spent at the reformer instead. Hydrogen made by electrolysis from renewable electricity changes that arithmetic, and is a much smaller share of supply than the marketing around it suggests.
