The German Type 212 class was the first submarine class to use operational hydrogen fuel-cell air-independent propulsion.
Type 212 submarines use a diesel-electric system for conventional operations and proton-exchange-membrane fuel cells for quiet underwater endurance. The fuel cells generate electricity through an electrochemical reaction rather than combustion, producing water as the main by-product.
This arrangement lets the submarine remain submerged much longer than a conventional diesel-electric boat without running its diesel engines. The design also uses a non-magnetic steel pressure hull, reducing its magnetic signature.
The class was developed for the German and Italian navies. Type 214 submarines later exported related fuel-cell technology, while Sweden’s Gotland class became famous for Stirling-engine air-independent propulsion. Those systems are all AIP, but they use different energy-conversion methods.