Found this, this is what your are doing basically
The electrolysis of water is a simple method of producing hydrogen. A low voltage current is run through the water, and gaseous oxygen forms at the anode while gaseous hydrogen forms at the cathode. Typically the cathode is made from platinum or another inert metal when producing hydrogen for storage. If, however, the gas is to be burnt on site, oxygen is desirable to assist the combustion, and so both electrodes would be made from inert metals. (Iron, for instance, would oxidize, and thus decrease the amount of oxygen given off.) The theoretical maximum efficiency (electricity used vs. energetic value of hydrogen produced) is between 80–94%.[64]
From here:
Hydrogen - Wikipedia, the free encyclopedia
The scale is the probem, you would need a system the size of your car to do what your claiming.
Hydrogen fuel cells are a different story all together as they are seen to be a feasable way to power a vehical if they can make comercial quantities of hydrogen without burning to much fossil fuels in doing so.