The Electrolyzed Water Cell is a product of Electrolyzers when placed next to an MFE or MFSU. They accept Water Cells and, depending on how full the MFE or MFSU is (70%), it will transfer some of its total storage too. When the MFE or MFSU requires power, it will turn the Electrolyzed Water Cells back to Water Cells.
They are capable of absorbing 15000 EU, or approximately 1.5 RE Batteries per cell. However, they lose 1500 EU, only giving off 13500 EU.
Electrolyzed Water Cells do stack, up to 64. This makes them a good way of storing energy easily.
They are also used in various crafting recipes for machinery. :)