Entropy is maximized by turning heat into electricity, so electricity appears on the device, as it gets cooler (by a tiny amount).

EDIT: I just noticed that you may be asking about the "reverse electro-dialysis" part :)

It's a process where fresh and salty water enter a machine, and they get mixed while generating electricity. It's the reverse of this:

https://en.wikipedia.org/wiki/Electrodialysis

That runs on the same conceptual machine.

(And oh, looks like it's written together.)