There is a graphene device which harvests energy from the environment to generate electricity. Here is the paper describing it. The basic idea is to create asymmetrical electrodes so that the charge carriers flow in a preferred direction. When anions come within a certain distance from the graphene, energy is transferred and current flows. The smaller the anions are in the ionic solution, the greater the probabilty they get close and the more current you get. The details are not clear yet.