How should I store 15h cooled and then 2h cooked at 65C white soybeans? I cooked a litre of soybeans for two hours and cooled them for 15*. I should put them in the refrigerator, but in what solution? My intuition says that an acidic liquid would work. Should I use one teaspoon of citric acid to make them acidic?

*Note (Jefromi): the title said "15h cooled and then 2h cooked" soybeans. I tried not to change the meaning, but I suspect that the OP may have actually meant that the soybeans were soaked for 15 hours, then cooked.
Generally, cooked beans keep best frozen, at least if you're keeping them for more than a day or two. As for what you'd refrigerate them in to save the to use tomorrow, I'd suggest either their own liquid (if you're going to use it in the recipe) or drained (but tightly covered). Alternatively, if they're going to go into some sauce, you could let them marinate in it.

A teaspoon of citric acid (assuming anhydrous, i.e., dry) is a fair bit, especially for 1L. That will be very sour. ⅛ tsp citric acid into 1L water is notably sour.