I think IC engines are at present a good deal more efficient than 10%, at least some of them. Hard numbers are difficult to come by because of different ways of figuring efficiency.
I’ve just never understood the mania for hydrogen. If someone discovered a new law of nature whereby the molecular bonds of water could be broken using greatly reduced energy, then we’re in business. Possibly some type of catalytic reaction?
But it seems VERY likely this would run up against the laws of thermodynamics, which nobody has figured out how to break yet. Without such a legal breakthrough, it’s a dead end, and should obviously be such to anybody.
Just figured out what we need. AAA battery powering a catalytic converter using an unobtainium reaction to split the water molecule.
Fill up the tank with a garden hose!
The advantage would not be over all efficiency, it would be fuel flexibility. You can make electricity from many things other than imported oil.
I’m not a big hydrogen fan, to many obstacles to implementation.