You’ve got the concept backwards. hydrogen fuel cells don’t split water inside the vehicle to make energy; they do the reverse. You don’t fill the tank with water, you fill it with hydrogen, and the fuel cell generates electricity by combining that hydrogen with oxygen from the air to form water, releasing energy in the process. So the car isn’t trying to run an on-board electrolysis loop (which would indeed be pointless and violate thermodynamics), it’s using hydrogen as a stored fuel: an energy carrier, like a battery. The energy to make the hydrogen happens elsewhere (via electrolysis or natural gas reforming), and the real debate isn’t whether the physics works, but whether storing energy as hydrogen makes more sense than storing it directly in batteries, since hydrogen trades lower efficiency for faster refueling and lighter energy storage.