Hydrogen has a volume problem, though. A 1st generation Toyota Mirai contains 5 kg of H2, equivalent to 197 kWh. That would take up 55 m3 at atmospheric pressure which is why the Mirai stores it at ~700 atmospheres. That's still a 78 liter tank. AFAICT 200 kWh of petrol takes up 25 liters, i.e. a third. On top of that the high-pressure tank in the Mirai weighs 87 kg.