You know, of course, it takes a significant amount of energy to separate hydrogen to use as a fuel? You should also know that thermodynamically (there is no such thing as perfect conversion or perpetual motion) it takes more energy to produce hydrogen for fuel than is released when the hydrogen is used as a fuel? Hydrogen is clean, but you have to do something involving energy to produce it ... there is virtually no free hydrogen gas on earth.
Then there's the problem with storing hydrogen ... ever see pictures of the Hindenburg? It's pesky stuff being at the extreme left end of the periodic table, and being at the top means it's pretty hard to keep corralled, and that's a particularly bad combination in moving vehicles ... that sudden stopping thing when two objects meet.
Then for combustion there's the thing about byproducts ... unless you are using pure oxygen (which again takes energy to produce) and then it's just water. But if you are using hydrogen and air, there are going to be some gaseous byproducts. It's been over 50 years since I last took a chemistry course, so perhaps there's been some revisionism since then, and I'll admit I absolutely hated chemistry and have tried hard to erase it from memory, so I'm open to the thought that you could know something I don't, but these days a lot of people are making things sound a lot more possible than they are ... at least outside a lab.