[moron4hire]

Keeping the hydrogen output separate from the water input and oxygen output is easy. You feed water in to the bottom of a U shaped container. You put the hydrogen-producing electrode in one half of the U and the oxygen-producing one in the other. The hydrogen and oxygen bubble up out of the water into separate collectors.

[pjc50]

That only works with separate electrode systems; in-panel splitting (where the panel has a sheet of water directly over a catalyst) evolve both gases in the same place.