What was the first successful demonstration of a fuel cell in space?

Question

Hydrogen fuel cells (together with batteries) were used for electrical power in the Apollo program, where it was recombined with oxygen on board to produce electricity.

Doing chemistry in practical space environments means managing the following:

1. macrogravity; large accelerations and vibrations during launch phase
2. microgravity; weak, unpredictable accelerations in various directions, but no "up" or "down" during operational phase
3. thermal excursions
4. pressure excursions

So there must have been some serious testing efforts in early crewed spaceflight era, when there was the availability of plenty of oxygen and the only solar panels were tiny.

Question: When was the successful demonstration of a fuel cell in space? It would be great to have answers for both a Soviet and a US technology demonstration.

Answer 1

a hydrogen-oxygen fuel cells were first used on the manned spacecraft Gemini in 1965.

Image source: Smithsonian

More info: On the Shoulders of Titans Chapter 7

Soviet designers had no problems with the additional mass of energy sources and were forced to use fuel cells only in the Buran spacecraft.