It will be fueled by hydrogen, the most sustainable technology in development today.
Oh come on. Run some basic numbers re fuel energy density of hydrogen vs liquid hydrocarbons like petrol then do some basic calculations re volumes and storage (liquid hydrogen and/or compressed gas) and then look at the weight of the tankage..... etc. etc. Lets not even get into the weight, efficiency, and output of fuel cells; a battery system would be necessary for surge energy demands like take off, adding weight....
I have run these numbers many times and there is no way an aircraft is going to fly for any reasonable period of time using hydrogen as a fuel, especially a vertical machine; factor in endurance then just laugh. It just doesn't work, both from a weight and a volumetric perspective; let's not even talk about how hydrogen is produced, sources (probably a steam process on natural gas), energy requirements for chilling and liquefying the gas (pressure tanks would be WAY too heavy for aircraft), and the infrastructure to supply the fuel...... More grabbing headlines and money here, IMHO; buy stock early and then bail when the time is right..... The only hydrogen craft I see flying passengers are balloons, blimps, or dirigibles.