Re: Methanol as a store of energy for a fuel cell
Diesel fuel is liquid energy, highly concentrated and relatively inexpensive compared to other fuels. Diesel engines are also very efficient (large scale engines as high as 50%), which is why they are used EVERYWHERE when it comes to producing propulsion power on a large scale (and flexible electric power on a moment's notice, i.e. "peaking" plants). I cannot imagine the physical size of fuel cells and fuel tanks that can crack methanol JUST to get the hydrogen, compared to that of a diesel engine.
https://www.sciencedirect.com/science/article/pii/S2092678216302175
This article looks very favorably on fuel cells, but it is based on mathematical models, and not actual equipment. What it DOES say is that fuel cell systems are 40-60% efficient on HYDROGEN, or perhaps methane (I may have missed that detail scanning the article). Other fuels require some kind of 'cracking' which could dramatically lower efficiency, and the heat content (volumetrically and by weight) of "lightweight" fuels isn't as good as the heavier ones (like diesel fuel). Hydrogen gas is typically produced in large quantities using COAL, and the tanks that hold it are LARGE by comparison. Methanol tanks would ALSO need to be larger than diesel. So unless we have a spacecraft with liquid H2 and O2 available, fuel cells [from a practical standpoint] make less sense than classic diesel engines.
And In My Bombastic Opinion, re-purposing existing (proven) tech (diesel electric has been used in trains for a LONG time, for example) is better than trusting some new, shiny unproven tech too soon.
Apparently there is one fuel cell powered ship in operation in Germany, using hydrogen. My guess is that it does not scale well (like to an entire fleet) at this time.