The 8008 1789

You're about 40 years out of date. NASA tested hydrogen for it's explosive potential back in the 60s and found it overrated. It can be a problem in a confined space, with the right amount of air, but even so it takes a pretty decent volume before it can do any damage to something as bulky as a car. (Strategically placed catalyzers - like NASA requires for batteries - can reduce the chance even further.)

If you want an explosion, gasoline vapor is a better choice. (Or acetylene - now there's a really nasty gas.)

I said "Compared to" - which it is. (Plant-derived volatiles might be a close second - but I'm suspicious of the energy and pollution cycles involved in the increased agricultural demands.)

Depends on the process, but, basically, the containers, handling, and production facilities are the primary sources of pollution. All these apply to other fuels-energy sources as well, though. (No form of energy is pollution free, or even close, once you get beyond an activity more complex than sunbathing.)

Once you deduct out the common items you're left with the differences, which, going back to batteries, involves large amounts of toxic and corrosive chemicals as well as hard-to- find and toxic metals, all distributed widely around the country while in use. Some suggested methods of storing hydrogen also involve problem materials, but it's not intrinsic to the fuel and they might not ever be used. (And, of course, all these things require energy to process the materials, with all that implies.)

Yeppers. Several ways. (We used to fill balloons up with it, tie them in a train with a fuse on the last one... Looked like a lightning bolt in the clouds. Not much noise, though, the burn was too slow. If you want thunder, well, see my comment about acetylene above :-)

-Wm