Using this perform a search for "water electrolysis voltage."
You'll be provided a list of web pages which will offer you some very fascinating, and very educational, documentation on various techniques of enhancing water electrolysis.
You'll be surprised at the variety of materials used and the efficiencies obtained.
You'll also find that it is possible to produce Hydrogen Peroxide by electrolysis as well.
Going down the list will keep you productively busy for quite some time as you take in some incredible possibilities.