Do you have a reputable reference to suggest that there is any at that height? Do you know how much you'll need, and how much work energy it will cost to collect it? Or do you want to perform electrolysis of the condensed water vapor?
Yes, there is something, 20 km is not too far from the Earth.
I will need about 500 liters of H per 24 hours. (at that pressure) or , said 22 l =)) at 760 mm.
I don't know how much energy will cost to collect it, maybe you will help to estimate it?
It is the one way to do it, but -50 C and 2 mg/kg doing things hard.
I thought about some liquid which can react with water in air. Or maybe we have to use Na to get NaOH and after that melt it and do electrolysis of it, i don't know.