You can liquify it pretty easily with a few wheezeworts in an hydrogen filled room. And then you can store tons of chlorine in a very small room. - A lot of CO2 ? You can use the carbon skimmer or liquify it with the same method that is used for chlorine. CO2 is nice as a ...
Thermodynamically, methane would provide slightly more efficient internal combustion engines than gas. For energy density, no, compressed methane wouldn't be good enough. You would need to liquify it - perhaps by converting it to methanol. How much energy is released by by burning methane ...