Interesting thread and something I know enough about to be dangerous.
Firstly, anything other than jet fuel (hydrocarbon) will not have sufficient energy density or power density to be viable for large aircraft - so PERHAPS for small aircraft hydrogen/fuel cells or batteries may work (sort of). Power and energy density are critical to aircraft range and efficiency. No so with automotive use...
Hydrogen is not a good energy vector (moving energy from one place to another) and yes if you have green hydrogen and can extract CO2 from the atmosphere then Fischer-Tropsch to make petrol, diesel or jet fuel makes most sense (BTW FT is already approved for use in aviation!!).
Hydrogen is NOT safe - very wide explosive limits and very high flame speeds (often more like explosive detonation rather than just slow burn) - so risk of explosion higher and damage can be massive compared to say a petrol/diesel explosion. Oh and one big risk (based on industry experience) hard to believe but its true - hydrocarbon fuels fires are always visible (flames, smell and smoke) hydrogen flame isnt colourless, no smake or smell - flame invisible so a risk of walking straight into a H fire without realising unless you happen to spot the heat shimmer.
Hydrogen again energy density is very low and even worse when you add the weight and volume of pressurised tanks and insulation.....
Water and Water Methanol - can be used to boost piston and indeed gas turbine engines. But the main effect is that as the water evaporates you get expansion (and a bit of cooling and slightly increased mass flow for a gas turbine) so more power, but that takes energy - so add just enough methanol (around 38% is the magic number) to add that energy (and some O2 as the methanol dissociates ) back in and you get more power. Used in older gas turbines for short periods to either make up for engine wear or boost for high altitude/hot day take off.
Just a downside that water/methanol can increase exhaust aldehydes and ketones (that sweet to pungent smell you used to get with old carburated cars when cold and on choke) - not nice or good for your health.
Lastly, batteries for aviation and particularly large commercial aircraft. BIG SNAG: when a commercial aircraft takes off there's up to say 100-150 tons of fuel on board - when they land most of thats gone. Aircraft cannot land with full load of fuel on board. Batteries weight the same at landing as at take-off - high landing speeds would be required and massive increase in strength of airframe and undercarriage = weight = loss of efficiency...
Apologies for the long post but I though it was worth weighting in with a few facts......
