[mikeash]

Hydrogen doesn't buy you that much additional lifting power.

Air is about 1.2kg/m^3 at STP, so that's the theoretical maximum you can lift with a cubic meter of gas bag. (And you can only get that value if you could somehow hold a vacuum, of course.)

Hydrogen is about 0.09kg/m^3 at STP, so you can lift about 1.1kg/m^3. Helium is about 0.18kg/m^3, so you can lift about 1kg/m^3. Helium is twice as dense, but that only costs you about 10% of your lifting power since both are so light compared to air.

I see no reason you couldn't use hydrogen, but I don't know if anyone would want to take the risk. (Specific flaws of the Hindenburg aside, hydrogen is explosive across a distressingly wide range of mixture ratios with air.) If you did use hydrogen, I think the main motivation would be avoiding the use of scarce helium, not the minor increase in lift capacity.

[ForHackernews]

If it's an unmanned drone-blimp, the safety issue because somewhat less critical.

[gozur88]

But still critical enough to make it impractical. How would you do maintenance on a hydrogen airship without risking the people doing the maintenance? And you'd never be able to fly over populated areas.