imagine a very thin and very short wire - just for one electron to move a bit left and a bit right. The surrounding environment - in particular Brownian motion molecules, especially the ones with large dipole moment, and the background EM radiation - will cause the electron to move left-right. Thus we have electric current generation. Now diode-terminate the wire to allow aggregating of many of such wires without mutual cancellation of the current generated by those wires. Well, meet the Maxwell's Daemon remote cousin - Maxwell's Diode.

Basically, there are always local micro gradients even though the macro integration of those gradients gives 0. It may as well happen that graphene being few atoms thick and thus dipping into the local scale properties may bridge and aggregate without [total] mutual cancellation those properties into the macro scale (graphehe sheet vibrations may happen be [one of] such a filter, kind of piezoelectric effect). There is no violation of the 2nd law as extracted work/energy wouldn't be larger than the entropy increase resulting from the smoothing of the micro gradients (bringing the Universe's heat death closer).

Or even imagine a pond with no visible movement of water and place over it a myriad of very very small water mill wheels - they will be rotating chaotically back and forward due to small perturbances in the water (and the smaller a giving wheel the larger share of local gradient it will be extracting as the smaller share of it will be mutually cancelling with the neighboring gradients under the wheel). That chaotic rotation of that myriad of wheels can be aggregated into usable work/energy.