I've done extensive work in this area, eventually creating a hydrogen generator to run in my Montero a couple of years ago.
If your plan is to create the hydrogen onboard the vehicle as you use it, you should reconsider this plan. There has been no known real life success story using any type of technology that I'm presently aware of, and I spent a lot of time studying and experimenting.
Hydrogen does have energy, and would enhance your power and MPG, but hydrogen must be manufactured. This is why it's actually an energy carier, not a source. In truth, you always lose some energy whenever you convert energy from one type to another. In this case, the most efficient method is to use electrolysis to split the molecule into hydrogen and oxygen (Brown's gas). You're converting electrical energy into chemical energy. You will suffer a loss of around 50% or more energy. Then you'd need a way to store the hydrogen, and there hasn't been an efficient way designed to do that yet (while staying within the budget of a home tinkerer, that is).
Any way you cut it, this is presently a losing proposition.
'81 Rabbit N/A
'82 Audi 4000 Diesel (a vw in luxery clothing!)