While looking at MHD stuff I came across this:
at around 9min-5s he says:
..."water electrolyses at around 1.23v, Eurea at 0.37 volts.... net positive..."
So!
Just take a leak in your electrolyser and you're golden!
(If there's any truth in the "net positive", you'd want to make a 38 plate dry cell
AND
you'd want to make it with separate outputs from each cell as the + side is nitrogen, which you DON'T want to feed into an engine!
Good luck "easy building" that along with an easy and discrete ...er... filling mechanism
)