Electrolysation at high voltage
Electrolysis starts at around 1.3V you may not have notices because water has a high resistance.

One way to increase the current is to add sodium hydroxide to the water, this will reduce its resistance significantly (it also makes the water dangerous, so wear eye protection, and gloves, and rinse splaches promptly)

To use a higher voltage efficiently you need to have several electrolysis cells in series. 

This could be a tank with several stainless-steel dividers that divide the tank into sealed slices, each compartment being in-effect a separate electrolysis cell and the dividers doiung double duty as both both electrodes and as the end walls of each cell.

this wirks well if you want Brown's gas, but if you want the individual components it means you need many more collectors. - using a low voltage and high current is probably much easier in that case.