Hydrogen is not a primary energy, but a source of energy. Its environmental friendliness thus depends on how the energy is generated. Once it is generated, hydrogen may supply its energy to a vehicle engine, causing no CO2 emission. At the moment, unfortunately, this process still has a very unfavorable efficiency.
So far the only possibility for hydrogen propulsion in the vehicle is the internal combustion engine. Better would be the fuel cell. Depending on the degree of purity of the supplied hydrogen, here one can hope for complete absence of CO2. But this technology is still unaffordable. A disadvantage of the internal combustion engine is its relatively large performance loss to roughly half during bivalent operation. Thus, downsizing (reduction) is no option. On the contrary, BMW makes use of its largest engine as illustrated on top to prove that the driving pleasure must not be given up.
How and where the production of hydrogen should take place is not yet decided upon. In first instance, the proposition was to produce hydrogen at home. In the meantime, this idea has been abandoned: too difficult, dangerous and inefficient. Energy generated by photo-voltaic is considered too valuable for wasting, too. The old Hoffmann apparatus, with which current is generated, using electricity out of water, is no longer state-of-the-art. Processes with high temperatures are required. This is possibly, but not likely, nuclear power. Much better suited are, e.g., reactors. The light of several mirrors must be bundled onto them. Thus, temperatures of 1000 and more degrees are possible for producing hydrogen favorably.
Even more problematic is the safekeeping in the vehicle. The BMW is reduced to two seats in the rear, and as much boot space as a convertible, in spite of its large car body. Nevertheless, the hydrogen tank suffices for just approx. 200 km, but only within a short period after re-fueling. The hydrogen is energy-costly cooled down to -253°C. Unfortunately, it does not remain at this temperature without adding additional energy. After one week of not driving the car, the hydrogen is halved. This is in spite of an immense and very decent isolation. Alternatively, there are pressure tanks with at the moment 700 bar, using energy and requiring either heavy or very expensive pressure tanks. Not to mention collision damages. Such vehicles may not be parked in closed rooms, anyways.
At the end of ingenuity has helped us almost always out of trouble. Maybe this time, too. In between, there was talk of the possibility of chemical solution, a connection of the hydrogen to a metal. But since it is now gone quiet.