As in a) which is more efficientb) difference in emissions
Well ideally, hydrogen would be made from electrolysis, which takes water and electricity, and gives hydrogen and oxygen. The only pollutant is oxygen (which is fine), and whatever pollution the electric power plant made. Later on, the fuel cell turns the hydrogen, plus oxygen from the atmosphere, into electricity. Here's the thing. If you're trying to move electricity around, a hydrogen fuel cell is a VERY inefficient way to do it. It wastes more energy than it transfers, and it creates all sorts of new problems and hazards. Much more efficient is simply put the electricity into a battery and carry the battery. I think there's a perception that charging batteries would be slow, and pumping hydrogen would be quick. That's not true. Anyway, straight electrics would not become the dominant technology, most likely it would be plug-in hybrids, which are mostly straight electric but also with an onboard engine for sustaining long hauls. They could run on any of a variety of fuels, but the vast majority of their miles would be on power from a wall socket.
The biggest difficulty with hydrogen fuel cell automobiles is that they do not exist. At least you can't buy one. Theoretically they emit no carbon dioxide from their tailpipes, but a good deal of CO2 is produced when the hydrogen is made; it comes out a smokestack at a chemical plant somewhere. As for the efficiency, the comparison can't be made. Hydrogen is made by extracting it from natural gas, natural gas which you could compress and burn in your present automobile's engine if you wanted. And so in making hydrogen, a great deal of energy is lost before the stuff ever gets to the fuel tank of the car. You'd have thought that adults would have thought the whole matter through a bit better, but they didn't.
