Can't do this in an electric car

@ninalanyon A prototype fuel-cell powered railway locomotive or multiple-unit train (I'm not sure which) is already being developed in Britain, where also the agricultural and earth-moving plant maker JCB has started building hydrogen-powered versions.

I don't know if JCB uses fuel-cells or direct internal-combustion but the practical difficulty for potential customers is the scarcity of supplies of hydrogen. The gas would certainly be practical for railways and other large-scale users, though.

Though fuel supply was no doubt a similar problem a hundred+ years ago when the petrol-engine started to usurp battery-electric power for cars and small goods-vehicles....

@ArishMell Hydrogen is just a way of propping up the fossil fuel industry. At the moment the only practical source of hydrogen is steam reforming of natural gas (methane).

Here is a graphic showing an outline of the process:
https://en.wikipedia.org/wiki/Steam_reforming

As you can see even the best case releases 1.5 tonne of O2 for every 1.1 tonne of H2 produced and it requires 10.2 MWh of energy to produce.

Hydrogen has a heat of combustion of 142 MJ/kg, that's 39 MWh/tonne so we can think of that as the embodied energy of the fuel. This means that the efficiency of the process is at best about 75% on the production side. Then you have to account for the efficiencies of transport and actual combustion in a heat engine subject to Carnot's law (efficiency = (Thot - Tcold) / Thot).

The auto ignition temperature of hydrogen is 500 C (773 K) and the exhaust temperature of a typical engine is probably above 100 C (373 K) so the absolutely best efficiency is 52% which is achievable for petrol powered Formula 1 cars but typical real world ICE cars are about 20% efficient.

So the end to end efficiency is definitely under 40%. and most likely about 15%.

In fact it is likely worse because this does not account for the efficiency of production of the 10.2 MWh/tonne of input energy which most likely comes from fossil fuels too and is again produced at less than 50% efficiency.

@ninalanyon Thak you for that information. I was not commenting on the efficiency of the fuel as I didn't know the figures, but that hydogen needs so much energy to produce, however it is produced, that it it is not as "green" as often suggested.

Also, if burnt in an conventional i.e. engine the exhaust is mainly water, yes, but also still nitrous oxides as produced by petrol and diesel engines. Although modern diesel vehicles now have catalysers that decompose the NOx to oxygen and nitrogen - something the more virulent anti-Diesel people seem not to know!