The base for the FX-888 has one of those brass sponges, as well as the cellulose sponge.
The brass sponges work well too, but I really prefer the classic cellulose sponge with distilled water. Somehow the little ttssss-s-s sound when the hot tip hits the wet sponge is reassuring. It seems like the tips last longer too. And that's quite enough innuendo I think.