General Motor’s new HydroGen4 is the latest proof that fuel-cell cars are advancing at an astonishing rate; trouble is, they have so far to go. Hydrogen-powered cars are still light years away from being affordable and practical, but this one comes closer to looking, working and driving like a conventional car than any we’ve yet driven.You fill up with compressed hydrogen from something that looks like a regular pump; it takes no longer than brimming a petrol tank. The hydrogen flows into the fuel cells, where it is split into protons and electrons, the electrons creating a current that drives the electric motor. They recombine with oxygen to create only water vapour as a by-product.