"Fuel cell" is really a family of technologies, not one device. They all do the same job — turn fuel and oxygen into electricity without combustion — but they run at different temperatures, use different electrolytes, and suit very different jobs.
The five you'll hear about
- PEM (proton-exchange membrane). Low-temperature, fast to start, compact. The workhorse for cars, buses, forklifts, and portable power. Needs fairly pure hydrogen and a platinum catalyst.
- SOFC (solid-oxide). Runs very hot (700–1,000 °C), which makes it highly efficient and flexible about fuel — it can even run on natural gas reformed on-site. Favored for stationary power and combined heat-and-power. Slow to start, so it likes to run continuously.
- MCFC (molten-carbonate). Also high-temperature; used in larger stationary installations and capable of high efficiency, especially when waste heat is captured.
- PAFC (phosphoric-acid). A mature, reliable mid-temperature design used in buildings and campuses for steady combined heat-and-power.
- AFC (alkaline). One of the oldest types — efficient and proven (it flew on Apollo and the Space Shuttle), but sensitive to carbon dioxide, so it needs clean gas streams.
How to think about the trade-off
The basic tension is temperature versus flexibility. Low-temperature cells (PEM) start fast and fit in vehicles but demand clean hydrogen. High-temperature cells (SOFC, MCFC) are more efficient and fuel-flexible but want to run steadily, which makes them better for buildings and grids than for stop-and-go driving.
Bottom line. There's no single "best" fuel cell — there's a best fit for each job. Matching the chemistry to the use case is most of the engineering story.