thats bs

You use your gas to charge the battery,
electricity from battery use to split water molecules,
water molecules split to H2 and O2 fuel your car,
but you need MORE bacause there is battery to be charged.

Every time you convert one type of energy to another one
you loose some energy in this process because the
efficiency of such processes is not 100%.

So any such device will not only not save you any gas
but it will in effect cause you to use more gas on top
of what you spend buying parts to build it.

It is really worth it to go to school and pay attention to your
physics and chemistry teachers! You will have knowledge
important to know how to tell BS from a real thing :-)