Here is some info.
I recall reading somewhere that they can be rejuvenated by boiling a little distilled water, then turning off the heat, and dropping the cells in. The cup of the cell is zinc, the cap is carbon, and it’s sealed with a rubber disk, The rubber deteriorates with time and cracks. In hot water the air inside the cell expands,and should cause bubbling in the water. Then when the water cools some water is drawn into the cell by vacuum action. Let the water soak in for a while and then test the cell for voltage. From what I’ve read it should be 1 to 1 1/2 volts. If not the cell did not rejuvenate. Then the area of the sealing disc will have to be sealed with some type of glue or silicone.
Here is more rejuvenation info: