I’m not an engineer, but regarding fuel cells, I have the understanding that every time you convert from one form of energy to another efficiency lost?
That is, converting natural gas or coal to electricity, then using the electricity to get oxygen and hydrogen out of water, then burning the hydrogen. Not to mention spending the energy to compress the hydrogen.
Assuming a 50% efficiency for a fuel cell battery (middle of the road design) times the 90% efficiency of a modern electric motor driveline and inverter combination, and you end up with a 45% efficient propulsion system. To put that into perspective, gas engines rarely exceed 25% energy conversion efficiency, and diesels can reach 40% but those are banned by the EPA due to NOx emissions.
The percentages noted are amount of energy per pound of fuel converted to useful torque/horsepower at the drive wheels. This wouln’t take into account frictional/parasitic/aerodynamic losses associated with a complete vehicle.