If we must go "Carbon Free" then batteries are the least energy dense, which is why cars carry half a tonne or so of dead weight.

I was surprised at the energy density of Liquid Ammonia, it seems it can be converted in fuel cells.
Interesting article here...

https://www.frontiersin.org/articles/10.3389/fenrg.2014.00035/full

Peter,
66, 2016 Porsche Boxster S
No longer driving Tarka, the 2014 Plus 8...