Its simple enough to separate hydrogen from oxygen in water and its a common 6th grade science experiment. The problem for cars is that it takes a lot of energy to do it at the rates a car would need it on demand.
Yes we’ve all done the Brown’s gas experiment.
I don’t dislike hydrogen....but it’s production requires energy, And the materials used in a Brown’s gas experiment requires energy to produce the materials needed in such quantity and purity to achieve a yield high enough to produce usable quantities of hydrogen