Comment by somat

I think that is the way it is headed. But you never know. Sometimes when comparing it helps me to reduce these things down to lower levels.

What is a battery? A chemical cell to store hydrogen and oxygen(true, it does not "have" to be hydrogen and oxygen but it usually is) to later get energy out of. For example lead-acid(stores the oxygen in the lead-sulfate plates and the hydrogen the the sulfuric acid liquid) or nickle-metal(charges into separate oxygen and hydrogen compounds, discharges into water) the lithium cell replaces hydrogen with lithium. Consider a pure hydrogen, oxygen fuel-cell, it could be run in reverse(charged) to get the hydrogen and oxygen and run forward(discharged) to get electricity out of it. So it is a sort of battery, a gas battery. Gas batteries are generally a bad idea, mainly because they have to be so big. Much time and effort is spent finding liquids that can undergo the oxidation/reduction reactions at a reasonable temperature. But now consider that there is quite a bit of oxygen in the air, if we did not have to store the oxygen our battery could be much more efficient, This is the theory behind free-air batteries. But what if our battery did not have to run at a reasonable temperature. We could then use a heat engine to get the energy out. And thus the Mirai. They are shipping half of the charged fluid to run in a high temperature reaction with the other half(atmospheric oxygen) to drive a heat engine that provides motive power.

As opposed to having the customer run the full chemical plant to charge and store the charged fluids to run in a fuel cell to turn a electric motor for motive power. Honestly they are both insane in their own way. But shipping high energy fluids tend to have better energy density. Perhaps the greatest problem in this case is that it is in gaseous form(not very dense) so has no real advantage. Unfortunately one of the best ways to retain hydrogen in a liquid form is carbon.