This popular article oversells it by about 3 orders of magnitude. Roughly, the effect is that moisture causes a nickel hydroxide material to expand. You can dr

This popular article oversells it by about 3 orders of magnitude. Roughly, the effect is that moisture causes a nickel hydroxide material to expand. You can drive the moisture out with light or heat and it contracts. In a humid environment, you can cycle it. The fastest cycle time they show graphs for is 0.1 Hz (a cycle every 10 seconds.) They use a thin film of about 1 micrometer -- presumably a thicker film would take a lot longer to change the moisture level. They don't try to measure the efficiency, but usually anything that involves temperature change is pretty inefficient.